U.S. patent application number 10/725649 was filed with the patent office on 2004-06-10 for polyester core materials and structural sandwich composites thereof.
Invention is credited to Feichtinger, Kurt, Ma, Wenguang.
Application Number | 20040109993 10/725649 |
Document ID | / |
Family ID | 29779217 |
Filed Date | 2004-06-10 |
United States Patent
Application |
20040109993 |
Kind Code |
A1 |
Ma, Wenguang ; et
al. |
June 10, 2004 |
Polyester core materials and structural sandwich composites
thereof
Abstract
High-strength, chemically and thermally stable, closed-cell
foams, useful as structural core materials in sandwich composites.
The core materials of the invention display anisotropic properties.
The core materials of the invention are amenable to vacuum-mediated
resin bonding to composite skins to provide lightweight,
high-strength structural sandwiches suitable for use in a variety
of applications, such as marine applications, construction,
aviation, rapid transit, and recreational vehicles.
Inventors: |
Ma, Wenguang; (Township of
Washington, NJ) ; Feichtinger, Kurt; (Warwick,
NY) |
Correspondence
Address: |
RATNERPRESTIA
P O BOX 980
VALLEY FORGE
PA
19482-0980
US
|
Family ID: |
29779217 |
Appl. No.: |
10/725649 |
Filed: |
December 2, 2003 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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10725649 |
Dec 2, 2003 |
|
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|
10183841 |
Jun 27, 2002 |
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Current U.S.
Class: |
428/317.9 ;
428/314.4; 428/480 |
Current CPC
Class: |
B32B 2250/20 20130101;
Y10T 428/249981 20150401; B29C 44/22 20130101; B32B 5/245 20130101;
Y10T 428/31786 20150401; Y10T 428/249989 20150401; B29C 44/468
20130101; Y10T 428/249986 20150401; B32B 5/18 20130101; B32B
2266/0264 20130101; Y10T 428/24999 20150401; C08J 2367/02 20130101;
B32B 3/26 20130101; B32B 2262/101 20130101; B32B 2266/08 20130101;
B32B 2307/73 20130101; Y10T 428/249976 20150401; Y10T 428/249992
20150401; B32B 2262/106 20130101; Y10T 428/249991 20150401; B32B
7/05 20190101; E04C 2/296 20130101; B32B 2262/103 20130101; C08J
9/365 20130101; E04C 2/292 20130101; C08J 9/04 20130101 |
Class at
Publication: |
428/317.9 ;
428/480; 428/314.4 |
International
Class: |
B32B 005/22 |
Claims
What is claimed is:
1. A composite comprising a foamed polyester core material bonded
to one or more structural skins, wherein the core material is
anisotropic.
2. The composite of claim 1, wherein the core material is bonded
between two structural skins.
3. The composite of claim 1, wherein the one or more structural
skins comprises a thermoplastic polymer, a thermosetting polymer,
wood, an inorganic material, or a metallic material.
4. The composite of claim 3, wherein the thermoplastic or
thermosetting polymer comprises glass fibers, metallic fibers,
inorganic fibers, or carbon fibers.
5. The composite of claim 1, wherein the one or more structural
skins comprise a structural sandwich composite.
6. The composite of claim 1, further comprising a resin bonding at
least one of the skins to the core material.
7. The composite of claim 1, wherein the foamed polyester core
material comprises foamable polyethylene terephthalate, foamable
polybutylene terephthalate, foamable polyethylene naphthalate, a
foamable copolymer of polyethylene terephthalate, a foamable
copolymer of polybutylene terephthalate, a foamable copolymer of
polyethylene naphthalate, or a mixture thereof.
8. The composite of claim 1, wherein the foamed polyester core
material comprises foamable polyethylene terephthalate.
9. The composite of claim 1, wherein the core material comprises a
nucleating agent, a fire retardant, or a reinforcing agent.
Description
[0001] This is a division of application Ser. No. 10/183,841, filed
Jun. 27, 2002, which application is hereby incorporated by
reference in its entirety.
1. FIELD OF THE INVENTION
[0002] The invention is directed to chemically and thermally-stable
structural core materials comprising compressed foamed polyester
strands and methods for their preparation. The invention is also
directed to structural sandwich composites constructed from such
core materials and methods for their preparation.
2. BACKGROUND OF THE INVENTION
[0003] Structural sandwich composites--which are sandwich-like
arrangements of a relatively low-density core material bonded
between comparatively thin, high-strength and high-stiffness
skins--are used in a wide variety of applications that require
lightweight, yet structurally strong materials. To name but a few
applications, structural sandwich composites are used in boating,
construction, aviation, rapid transit, and recreational vehicles.
Structural sandwich composites are useful because of their high
strength and low weight per unit area. When bonded between skins,
the low-density core provides a large strength and stiffness
enhancement over the skins alone, but adds only a comparatively
small weight. To illustrate the benefits of structural sandwich
composite construction, consider that dividing a material (e.g.,
aluminum or fiberglass) into two skins and bonding a core material
that is twice the original material's thickness in between them,
results in a composite having a stiffness 7 times greater and a
strength 3.5 times greater than the original material's while
having a density only 1.03 times that of the original material.
ANDREW C. MARSHALL, COMPOSITE BASICS 3-1 (5th ed. 1998).
[0004] How well a sandwiched core material functions in real-world
applications can be predicted from laboratory measurements of its
compression strength and modulus, tensile strength and modulus, and
shear strength and modulus.
[0005] The properties of the core material are of great importance.
Desirable properties include high strength, low density, rigidity,
high chemical and heat resistance, and low cost. The most common
core materials are wood, honeycomb structures, and foams comprising
both thermoplastic and thermosetting compositions. Wood core
materials suffer from variations in properties and are susceptible
to fungal decay, especially in marine use. Honeycomb cores are of
an open structure, i.e., comprised of contiguous, connected, and/or
interlocked cells, and are typically constructed from rigid
materials, such as thermoplastics, fiberglass, aluminum, and
stainless steel. While honeycomb-core materials provide strong,
high-quality, chemically resistant composites, they are difficult
to manufacture. The connected nature of the cells precludes
composite manufacture by vacuum-mediated resin techniques because
the vacuum draws the resin into the individual cells. Furthermore,
honeycomb cores are not suitable for marine applications because a
crack in the composite skin can lead to the entire composite
filling with water. Closed-cell thermoplastic or thermosetting
foams avoid some of these problems, but generally are thermally and
chemically sensitive; thus, their composites cannot be used in
certain higher-temperature applications. A further disadvantage of
thermoplastic- or thermosetting-foam core materials is that certain
resin-type adhesives can significantly degrade them, both
chemically and via the heat evolved during the cure process.
[0006] Skins can be attached to core materials by a variety of
methods. One of the most popular methods, because of the high shear
strength of the resulting composite, is bonding the skins to the
core with a resin (the resin-cure method). The resin-cure method
provides structural sandwich composites with excellent skin-core
adhesion and delamination resistance. In the resin-cure method, an
uncured resin is applied to the contacting surfaces, the core and
the skins are contacted, and bonding results upon resin cure.
Often, a reinforcing material such as a glass-fiber fabric or mat
is combined with the uncured resin to improve strength and
stiffness in the resulting joint. During resin cure, substantial
heat is generated.
[0007] Vacuum-bagging and vacuum-injection-molding techniques are
used commercially to introduce the resin between the skins and
suitable cores see, for example, U.S. Pat. No. 6,159,414 (issued
May 18, 1999); U.S. Pat. No. 5,316,462 (issued May 31, 1994); and
U.S. Pat. No. 5,834,082 (issued Nov. 10, 1998). In this process,
vacuum is used to draw the uncured resin between the core and skin.
Advantageously, the vacuum removes resin fumes as well as shields
the uncured resin from air.
[0008] With some core materials, however, such as honeycomb
structures, vacuum-mediated resin application is difficult or
impossible. And unfortunately, in these cases, open-air resin
application is proscribed because the hazardous resin fumes are not
contained and resin curing can be inhibited by air and moisture.
Thus, thermoplastic or thermosetting foams are ideal in that they
do not suffer from the biodegradability of wood cores and are
amenable to vacuum-mediated resin application. But a serious
drawback with thermoplastic- or thermosetting-foam cores is that
the heat evolved during resin cure and the chemically corrosive
properties of the resin can degrade them, resulting in weaker
composites.
[0009] Thermoplastic polyester resins, such as polyethylene
terephthalate (PET) and polybutylene terephthalate (PBT) that have
been pre-treated with branching agents (hereinafter "branched
polyesters") yield closed-cell foams having excellent strength and
mechanical properties, low density, and high chemical and thermal
resistance. The branching agents, which have multiple
chemical-reaction sites, function by chemically condensing two or
more polyester chains ("branching"). This branching gives the
pre-foam polyester melt viscoelastic properties more suitable for
foaming, leading to higher quality foams. Polyester foams, prepared
from branched polyesters, such as branched polyethylene
terephthalate, have been disclosed in U.S. Pat. No. 5,000,991
(issued Mar. 19, 1991); U.S. Pat. No. 5,229,432 (issued Jul. 20,
1993); U.S. Pat. No. 5,340,846 (issued Aug. 23, 1994); U.S. Pat.
No. 5,362,763 (issued Nov. 8, 1994); U.S. Pat. No. 5,422,381
(issued Jun. 6, 1995); U.S. Pat. No. 5,679,295 (Oct. 21, 1997);
U.S. Pat. No. 5,681,865 (Oct. 28, 1997); U.S. Pat. No. 6,342,173
(issued Jan. 29, 2002), each of which eight patents are hereby
incorporated by reference herein. These foams are closed-cell
structures with low densities, excellent mechanical properties, and
high thermal and chemical resistance. Regrettably, because the
process used for their manufacture leads to irregular surfaces,
such foams make mediocre to poor core materials. The irregular
surfaces promote weak bonding to the composite skin and wide
cell-size distribution and, therefore, poor mechanical properties.
To explain more fully, polyesters are generally foamed by extruding
a pressurized mixture of a branched-polyester melt and a volatile,
organic expanding or "blowing agent" through an annular or slit
die. Upon entering ambient pressure, the blowing agent evaporates
and the polyester foams. This process suffers in that if the die
opening size surpasses a critical limit, extruder pressure cannot
be maintained. Furthermore, as the die opening is enlarged to the
size required for use as a core material, blowing-agent evaporation
throughout the material becomes non-uniform leading to erratic
cell-size distribution, oversized cells, and an irregular
surface.
[0010] Coalesced-strand polyester foams are more suitable as core
materials because they can be produced in thicker size with a
uniform distribution of small cells. Coalesced-strand polyester
foams are disclosed in U.S. Pat. No. 5,475,037 (issued Dec. 12,
1995). Generally, coalesced-strand thermoplastic foams are prepared
by melting a thermoplastic resin, mixing the melt with a blowing
agent, and extruding the resulting gel through a multi-orifice die.
The orifices are so arranged such that some contact between
adjacent strands occurs during foaming, and the contacting strand
surfaces adhere to one another resulting in a coalesced-strand
structure. These strand foams, however, are not used as core
materials. Tenacious, tough thermoplastic resins such as
polypropylene or polyethylene which generally exhibit lower
stiffness, may be advantageously used for some applications, such
as shock absorbers (see, e.g., U.S. Pat. No. 6,213,540 (issued Apr.
10, 2001)) but they offer poor performance as composite core
materials for which high strength and stiffness are desirable.
[0011] In view of the above, there is a need for low-density
closed-cell core materials that are rigid, strong, chemically and
thermally resistant, and amenable to vacuum-mediated resin
application.
3. SUMMARY OF THE INVENTION
[0012] The invention provides high-strength, chemically and
thermally stable, closed-cell foams, useful as core materials in
composites. The core materials of the invention comprise multiple
foamed polyester strands, compressed together (or shaped) to form a
unitary, closed cell foam displaying anisotropic properties.
[0013] A further feature of core materials of the invention is
that, although they are manufactured by extruding through a
multi-orifice die, there are substantially no voids in between the
strands (no inter-strand voids). This is a result of the special
shaping process and shaping apparatus described in more detail
herein. The shaping process can be adjusted to completely remove
the strand appearance of the core material. That is, if the core
material is cut perpendicular to the strand direction, strands are
no longer visible to the human eye. The strands have coalesced to a
degree wherein the core material appears to be completely unitary.
Thus, to the human eye, the core materials of the invention appear
identical to a conventional foam board manufactured by extruding
through a single-orifice, standard slot die. However, the core
materials of the invention display improved properties over
conventional slot-die produced foam boards. And, in contrast to
such conventional foam boards, the core materials of the invention
are significantly anisotropic in character and have an unusual cell
size distribution. The cell-size distribution of core materials of
the invention defines a plurality of "discrete volumes", running
parallel to the strand direction. These "discrete volumes" comprise
an interior section running parallel to the strand direction and a
"jacket" surrounding the "interior section of the discrete volume".
The "interior section of the discrete volume" has closed cells of
average cell size relatively larger than the average cell size of
the cells in the surrounding "jacket". In other words, the
average-cell diameter is smaller where the strands have intersected
and merged than at the original strand's interior. This can be
described as a pseudo honeycomb structure.
[0014] Because of their high thermal and chemical resistance, the
core materials of the invention are amenable to vacuum-mediated
resin bonding to composite skins to provide lightweight,
high-strength, buoyant, and watertight structural sandwich
composites suitable for use in a variety of applications, such as
construction, boats, ships, and other marine applications,
aviation, rapid transit, and recreational vehicles. In fact, the
core materials of the invention are compatible with just about all
resins, and can accept high molding temperatures and pressures.
They can be processed with nearly all composite fabrication
techniques including contact molding, vacuum bagging, resin
infusion, autoclave, RTM, match metal molding, pre-preg and
others.
[0015] In one embodiment, the invention is directed to a method for
making a composite comprising: extruding a foamable gel comprising
a blowing agent and a foamable polyester through a multi-orifice
die to give a plurality of strands; foaming the strands to form a
multi-stranded foamed article; shaping the multi-stranded foamed
article to give a core material; and bonding the core material to
one or more structural skins.
[0016] In another embodiment, the invention is directed to a
composite comprising a foamed polyester core material bonded to one
or more structural skins, wherein the core material comprises a
plurality of discrete volumes, each discrete volume comprising an
interior section and a corresponding jacket, wherein the average
cell size in the interior section is larger than the average cell
size in the jacket.
[0017] In still another embodiment, the invention relates to a
composite comprising a foamed polyester core material bonded to one
or more structural skins, wherein the core material comprises a
plurality of foamed strands, wherein there are substantially no
inter-strand voids.
[0018] In yet one more embodiment, the invention is directed to a
composite comprising a foamed polyester core material bonded to one
or more structural skins, wherein the core material is
anisotropic.
[0019] In another embodiment, the invention relates to a core
material comprising foamed polyester, wherein the foamed polyester
comprises a plurality of discrete volumes, each discrete volume
comprising an interior section and a corresponding jacket, wherein
the average cell size in the interior section is larger than the
average cell size in the jacket. Preferably, the core material is
anisotropic.
4. BRIEF DESCRIPTION OF THE FIGURES
[0020] These and other features, aspects, and advantages of the
present invention will become better understood with regard to the
following description, appended claims, and accompanying drawings
where:
[0021] FIG. 1 is a flowchart outlining the general steps that may
be used to extrude a foamable polyester gel to obtain a core
material of the invention;
[0022] FIG. 2 is a drawing representing a unitary multi-stranded
foamed core material of the invention prior to conversion into a
core material of the invention by compression shaping to remove the
voids between the strands;
[0023] FIGS. 3A to 3C are perspective, side, and front view
drawings respectively of a shaper of the invention;
[0024] FIG. 4 is a drawing of a cross-sectional view of a core
material of the invention;
[0025] FIGS. 5, 6, and 7 are graph plots of shear strength versus
density (FIG. 5); shear modulus versus density (FIG. 6); and shear
elongation at break versus density (FIG. 7) conducted in a
direction end-strand, transverse, and longitudinal to the strand
direction respectively of PET core materials of the invention;
[0026] FIGS. 8 and 9 are graph plots of tensile strength versus
density (FIG. 8) and tensile modulus versus density (FIG. 9) of PET
core materials of the invention conducted in a direction end-strand
and transverse to the strand direction respectively;
[0027] FIGS. 10 and 11 are graph plots of compression strength
versus density (FIG. 10) and compression modulus versus density
(FIG. 11) of PET core materials of the invention conducted in a
direction end-strand and transverse to the strand direction
respectively; and
[0028] FIGS. 12, 13, and 14 are graph plots comparing the
properties of shear strength; shear modulus; and shear elongation
at break, respectively, of PET core materials of the invention
versus conventional, slot-die extruded PET foam boards over a
density range.
5. DETAILED DESCRIPTION OF THE INVENTION
[0029] The core materials of the invention may be prepared as
follows. A foamable polyester, of suitable melt rheology, is heated
above its melt point to form a polyester melt. The heated mixture
is pressurized in an extruder and a blowing agent is blended into
the melt to form a foamable gel. During blowing-agent addition and
mixing, the pressure is maintained above the blowing agent's
equilibrium vapor pressure at the operating temperature of the
foamable gel. The gel is cooled and extruded through a
multi-orifice die of desired design. Upon entering ambient
pressure, the blowing agent boils and expands thereby foaming the
strands. As the strands foam, they coalesce resulting in
multi-stranded polyester foam. At this point, the foam has voids or
channels running parallel to the strands. The voids are removed by
compressing the foam in a special shaping process. Temperatures,
pressures, and extrusion rates will depend upon the specific
polyester, additives, blowing agents, equipment, die design, and on
the properties desired in the final foam product. The core
materials of the invention may be bonded to skins to form
composites by well-known methods in the art, such as
vacuum-mediated resin application.
[0030] 5.1 Definitions
[0031] 5.1.1 Structural Sandwich Composite
[0032] As used herein, the phrase "structural sandwich composite"
means an article comprising a core material integrally bonded to
one or more structural skins. Preferably a "structural sandwich
composite" means a sandwich-like article comprising a core material
integrally bonded between structural skins. The phrase "Integrally
bonded" means that the skin is bonded to the core material
substantially throughout the skin's entire contact area. A
"structural sandwich composite of the invention" means a structural
sandwich composite comprising a core material of the invention.
Examples of suitable structural skins include, but are not limited
to, thermoplastic polymers and thermosetting polymers, optionally
reinforced with glass fibers, metallic fibers, inorganic fibers, or
carbon fibers; wood; inorganic materials, such as fiberglass; and
metallic materials, such as aluminum and stainless steel and many
others, which are well known to those of skill in the art.
[0033] 5.1.2 Core Materials of the Invention
[0034] As used herein, the phrase "core materials of the invention"
means a foamed article prepared by extruding a melt comprising a
foamable polyester and one or more blowing agents through a
multi-orifice die, according to the methods described more fully
herein, to give a multi-stranded foamed core material. Preferably,
the coalesced multi-stranded foamed core material is further
compressed (via shaping) to remove substantially all the voids
between the individual strands. Thus, the multi-stranded foamed
core material is transformed via shaping into a unitary closed-cell
structure, wherein certain foam-cell walls (those corresponding to
the surface where the exterior of the strands were compressed
together during shaping) are of increased (generally double)
thickness over those of the strand interior. The strand density of
the multi-stranded foamed core material prior to shaping will
govern the relative number of cells having such increased
thickness. Such a structure is referred to herein as a
"compressed-strand structure" gives the core materials of the
invention advantageous structural and mechanical properties over
traditional foam core materials.
[0035] 5.1.3 Pre-Shaped and Shaped Core Materials of the
Invention
[0036] As used herein, the phrase "pre-shaped core material" means
a "core material of the invention" prior to its being shaped by a
shaper of the invention, which shaper and shaping process is more
fully described in Section 5.3.2 below. In some cases, pre-shaped
core materials of the invention may have gaps or air pockets
between the strands. The shaping process, which results in a
"shaped core material of the invention", compresses the strands
thereby removing the gaps or air pockets. Preferably, core
materials of the invention are shaped.
[0037] 5.1.4 Strand Direction
[0038] As used herein, the phrase "strand direction" means the axis
along which a core material of the invention was extruded during
production. This is an important reference point since, in some
cases, the strand character of core materials of the invention is
not visible to strand compression and merger during the special
shaping disclosed herein (see Section 5.3.2 below). The "strand
direction" influences the anisotropic physical properties of core
materials of the invention. For example, the core materials of the
invention exhibit greater compressive strength in the "strand
direction". This is discussed more fully in Section 5.4.2
below.
[0039] 5.1.5 End Strand Direction in Composites of the
Invention
[0040] The phrase "end-strand direction" is used in reference to
composites of the invention wherein the composite's skins are
bonded to a core material of the invention perpendicular to the
strand direction. In such circumstances, the "end-strand direction"
means a direction or axis perpendicular to the strand direction
(therefore, also parallel to the composite skins).
[0041] 5.1.6 Longitudinal Direction in Composites of the
Invention
[0042] The phrase "longitudinal direction" is used in reference to
composites of the invention wherein the composite's skins are
bonded to a core material of the invention such that the skins are
parallel to the strand direction. In such circumstances, the
"longitudinal direction" means a direction or axis parallel to the
strand direction.
[0043] 5.1.7 Transverse Direction in Composites of the
Invention
[0044] The phrase "transverse direction" is used in reference to
composites of the invention wherein the composite's skins are
bonded to a core material of the invention such that the skins are
parallel to the strand direction. In such circumstances, the
"transverse direction" means a direction or axis perpendicular to
the strand direction.
[0045] 5.1.8 Foamable Polyester
[0046] As used herein, the phrases "foamable polyester" or
"foamable polyester resin" mean any thermoplastic or thermoplastic
mixture comprising polyester, a branched polyester, a polyester
co-polymer, or a branch polyester co-polymer that can be
effectively foamed to yield a core material of the invention.
[0047] 5.1.9 Polyester Co-Polymer
[0048] As used herein, the phrase "polyester co-polymer" means a
polyester prepared, according to well-known methods, by
co-polymerizing an ester monomer and one or more other
monomers.
[0049] 5.1.10 Branched Polyester
[0050] As used herein, the phrase "branched polyester" means a
polyester or a polyester co-polymer that has been condensed with
one or more branching agents. Preferably, branched polyesters are
foamable.
[0051] 5.2 Foamable Polyesters for use in the Invention
[0052] Although polyesters have excellent chemical and thermal
stability and structural properties, foaming is often difficult due
to their melt rheology (e.g., low melt strength and low melt
viscosity). In some cases, this can be overcome by using special
processing equipment. But the more common way to improve polyester
foaming characteristics is to alter the melt rheology by
pre-treating them with branching agents, for example,
polyfunctional carboxylic acids, polyfunctional anhydrides, and
polyhydroxyl compounds according to well-known methods.
[0053] Preferably, foamable polyester resins in the invention have
a crystallinity of from about 5% to about 100%, more preferably, of
from about 10% to about 60%, most preferably, of from about 25% to
about 45%, and even more preferably, of from about 28% to about
39%, as measured by differential scanning calorimetry.
[0054] Preferably, foamable polyester resins have the following
melt rheology and properties: (1) a melt strength of from about 1
to about 60 centinewtons, preferably, of from about 5 to about 30
centinewtons; (2) a melt viscosity of from about 30,000 to about
500,000 poises, preferably, of from about 200,000 to about 300,000;
and (3) an inherent viscosity of from about 0.5 to about 1.95 dl/g,
preferably, of from about 0.7 to about 1.2 dl/g.
[0055] 5.2.1 Crystallinity
[0056] The crystallinity of foamable polyester resins for use in
the invention may be measured by differential scanning calorimetry.
This involves absorbed-energy measurement as a polyester-resin
sample is heated to and beyond its crystalline melting point. The
crystallinity is defined as the ratio of the energy absorbed per
unit mass to the theoretical heat of fusion. For polyethylene
terephthalate resin, the theoretical heat of fusion is 26.9
kJ/g-mole of repeat units, or more conveniently 140 Joules per gram
of resin. 3 B. WUNDERLICH, Crystal Melting, in MACROMOLECULAR
PHYSICS (1980).
[0057] As is well known in the art, a differential scanning
calorimeter can measure very small quantities of energy that are
absorbed or released from a sample material. In a typical
procedure, a 5 mg to 10 mg polyester resin sample is sealed in an
aluminum pan and placed in the calorimeter's sample cell.
Typically, the sample is purged with an inert gas, such as nitrogen
to preclude sample oxidation. Then, the sample temperature is
ramped, for example, 10.degree. C. per minute while the
differential scanning calorimeter measures the energy absorbed or
released. The enthalpy of melting or heat of fusion is determined
automatically by the differential scanning calorimeter with
software that integrates the area under the melting peak. For
quality assurance, the foamable polyester's crystallinity should be
measured by differential scanning calorimetry prior to foaming. For
references relating to differential scanning calorimetry, see Dole
et al., 20 J. CHEM. PHYSICS 781 (1952); Wunderlich et al., 24 J.
POLYMER SCI. 201 (1957); Quinn et al., 80 J. AM. CHEM. SOC. 3178
(1958); Wunderlich et al., Part 2A J. POLYMER SCI. 987 (1967);
Atkinson et al., 65 TRANS. FARADAY SOC. 1764 (1969); Richardson,
Part C J. POLYMER SCI. 251 (1972).
[0058] 5.2.2 Melt Strength
[0059] The melt strength may be measured according to the procedure
set forth in American Society for Testing and Materials ("ASTM")
D3835 "Standard Test Method for Determination of Properties of
Polymeric Materials by Means of a Capillary Rheometer" measured at
280.degree. C. This test method describes measurement of the
rheologic properties of polymeric materials at various temperatures
and shear rates common to processing equipment. It covers
measurement of melt viscosity, sensitivity, or stability of melt
viscosity with respect to temperature and polymer dwell time in the
rheometer, die swell ratio (polymer memory), and shear sensitivity
when extruding under constant rate or stress. The techniques
described permit the characterization of materials that exhibit
both stable and unstable melt viscosity properties. Other suitable
methods are set forth in U.S. Pat. No. 5,362,763 (issued Nov. 8,
1994); U.S. Pat. No. 6,350,822 (issued Feb. 26, 2002); or U.S. Pat.
No. 6,251,319 (issued Jun. 26, 2001) which patents are hereby
incorporated by reference herein using, a Rheotens Melt Strength
Tester, Type 010.1, supplied by Gottfert Werkstoff-Prufmaschinen
Gmbh of Buchen, Germany. This test involves drawing an extruded
strand of polymer vertically into the nip between two
counter-rotating nip rollers using piston rate of 0.2 mm/sec. The
strand is extended using a Brabender Plasticorder single screw
extruder of screw diameter 19 mm and length to diameter ratio (L/D)
of 25. The extruded material exits through a right angle capillary
die of length 30 mm and 1 mm diameter at a rate of 0.030 cc/sec to
a length of 41.9 mm at 270.degree. C. The strand is then stretched
at a constant acceleration while measuring the elongation. The
temperature profile used was uniform along the length of the barrel
of the extruder and the die and was set at 280.degree. C. The nip
rollers are mounted on a balance arm, which allows the force in the
drawing strand to be measured. The velocity of the nip rolls is
increased at a uniform acceleration rate. As the test proceeds, the
force increases until eventually the strand breaks. The force at
breakage is termed the "melt strength".
[0060] 5.2.3 Melt Viscosity
[0061] The melt viscosity may be measured according to the
procedure set forth in ASTM D4440-01 "Standard Test Method for
Plastics: Dynamic Mechanical Properties: Melt Rheology", hereby
incorporated by reference herein. ASTM test procedures are
compiled, reviewed, and published by the American Society for
Testing and Materials, which is a voluntary standards development
organization operating out of West Conshohocken, Pa. This test
method describes the use of dynamic mechanical instrumentation for
gathering and reporting the rheologic properties of thermoplastic
resins. It may be used as a test method for determining the complex
viscosity and significant viscoelastic characteristics of polyester
thermoplastics as a function of frequency, strain amplitude,
temperature, and time. Rheometric calculations from data obtained
by ASTM D4440-01 may be performed according to ASTM D4065-01
"Standard Practice for Plastics. Dynamic Mechanical Properties.
Determination and Report of procedures", hereby incorporated by
reference herein. This report recites laboratory practice for
determining dynamic mechanical properties of specimens subjected to
various oscillatory deformations on a variety of instruments of the
type commonly called dynamic mechanical analyzers or dynamic
thermomechanical analyzers.
[0062] 5.2.4 Inherent Viscosity
[0063] The inherent viscosity may be measured according to ASTM
Test Method D4603-96 "Standard Test Method for Determining Inherent
Viscosity of Poly(Ethylene Terephthalate) (PET) by Glass Capillary
Viscometer", hereby incorporated by reference herein (see e.g.,
U.S. Pat. No. 5,422,381, hereby incorporated by reference herein).
This test method is for the determination of the inherent viscosity
of polyethylene terephthalate soluble at 0.50% concentration in a
60/40 phenol/1,1,2,2-tetrachloroethane solution by means of a glass
capillary viscometer. Some highly crystalline forms of polyethylene
terephthalate may not be soluble in this solvent mixture and will
require a different procedure The inherent viscosity values
obtained by this test method are comparable with those obtained
using differential viscometry described in ASTM Test Method
D5225-98 "Standard Test Method for Measuring Solution Viscosity of
Polymers with a Differential Viscometer", hereby incorporated by
reference herein.
[0064] 5.2.5 Polyesters
[0065] Polyesters that are foamable or can be converted to foamable
polyesters are suitable for use in the invention. Preferably, the
polyester is polyethylene terephthalate (PET), polyethylene
naphthalate (PEN), or polybutylene terephthalate (PBT), more
preferably, polyethylene terephthalate. The polymer's molecular
weight is not critical, generally molecular weights of 100,000 D to
1,000,000 D are suitable depending on the polymer type.
[0066] Preferred polyesters include, but are not limited to,
foamable polyesters derived from reaction of aromatic dicarboxylic
acid and a dihydric alcohol, such as those polyesters described in
U.S. Pat. No. 5,110,844 (issued May 5, 1992), hereby incorporated
by reference herein. Specific examples of preferred foamable
polyesters include, but are not limited to, foamable polyethylene
terephthalate (PET), foamable polybutylene terephthalate (PBT),
foamable polyethylene naphthalate (PEN), foamable copolymers of
PET, foamable copolymers of PBT, foamable copolymers of PEN,
foamable liquid-crystalline polyesters.
[0067] Foamable blends of polyesters and other thermoplastics are
also suitable for use in the invention. Suitable blends include
foamable polyester/polycarbonate blends as described in U.S. Pat.
No. 4,833,174 (issued May 23, 1989) and U.S. Pat. No. 4,462,947
(issued Jul. 31, 1984), both of which are hereby incorporated
herein by reference. Suitable blends also include foamable
polyester/polyolefin blends as described in U.S. Pat. No. 4,981,631
(issued Jan. 1, 1991) and U.S. Pat. No. 5,128,202 (issued Jul. 7,
1992), both of which are hereby incorporated by reference herein.
Suitable polyolefins include, but are not limited to, those listed
in column 4, line 33 through column 4, line 35 of U.S. Pat. No.
4,981,631, which disclosure is hereby incorporated by reference
herein.
[0068] Foamable co-polymers of polyesters and other thermoplastics
are also suitable for use in the invention. Suitable polyester
co-polymers include, but are not limited to, those listed in column
3, line 3 through column 4, line 41 of U.S. Pat. No. 5,475,037
(issued Dec. 12, 1995), which disclosure is hereby incorporated by
reference herein and poly-2-hydroxy-6-naphthoic acid and
polynaphthalene terephthalate, which is a copolymer of
2,6-dihydroxynaphthalene and terephthalic acid.
[0069] 5.2.6 Sources of Foamable Polyesters
[0070] Polyesters for use in the invention are readily available
commercially or can be synthesized by well-known literature
methods, for example see, GEORGE ODIAN, PRINCIPLES OF
POLYMERIZATION 97-100 (3d ed. 1991); FRED W. BILLMEYER, JR.,
TEXTBOOK OF POLYMER SCIENCE 63, 149, 225-227, 434, 452-454(2d ed.,
1971), both of which are hereby incorporated herein by reference.
In addition, foamable polyethylene terephthalate and other foamable
polyesters are available commercially, for example, from Mossi
& Ghisolfi Polymers (Houston, Tex.) and Du Pont (Wilmington,
Del.).
[0071] Suitable procedures for preparing foamable polyesters by
condensing polyesters with branching agents are disclosed in U.S.
Pat. No. 3,553,157 (issued Jan. 5, 1971), U.S. Pat. No. 4,132,707
(issued Jan. 2, 1979); U.S. Pat. No. 4,145,466 (issued Mar. 20,
1979); U.S. Pat. No. 4,462,947 (issued Jul. 31, 1984); U.S. Pat.
No. 4,999,388 (issued Mar. 12, 1991); U.S. Pat. No. 5,000,991
(issued Mar. 19, 1991); U.S. Pat. No. 5,110,844 (issued May 5,
1992); U.S. Pat. No. 5,128,383 (issued Jul. 7, 1992); U.S. Pat. No.
5,134,028 (issued Jul. 28, 1992); U.S. Pat. No. 5,288,764 (issued
Feb. 22, 1994); U.S. Pat. No. 5,362,763 (issued Nov. 8, 1994); U.S.
Pat. No. 5,422,381 (issued Jun. 6, 1995); U.S. Pat. No. 5,482,977
(issued Jan. 9, 1996); U.S. Pat. No. 5,696,176 (issued Dec. 9,
1997); U.S. Pat. No. 5,229,432 (issued Jul. 20, 1993); and U.S.
Pat. No. 6,350,822 (issued Feb. 26, 2002), all of which are hereby
incorporated by reference herein.
[0072] Suitable polyfunctional carboxylic acid branching agents
have three or more carboxylic acid functions per molecule and
include, but are not limited to, trimesic acid; pyromellitic acid;
benzophenonetetracarboxylic acid; 2,3,6,7-napthalenetetracarboxylic
acid; 1,2,5,6-napthalenetetracarb- oxylic acid;
1,2,3,4-cyclobutanetetracarboxylic acid;
tetrahydrofuran-2,3,4,5-tetracarboxylic acid;
2,2-bis(3,4-dicarboxyphenyl- )propane; and
3,3',4,4'-biphenyltetracarboxylic acid. Such branching agents are
commercially available or prepared by well-known methods.
[0073] Suitable polyhydroxyl compounds and equivalents (i.e., a
compound that can be converted in situ to a polyhydroxy alcohol,
e.g., esters of polyhydroxyl compounds) have at least three
hydroxyl groups or hydroxyl-group equivalents per molecule and
include, but are not limited to, glycerol; trimethylolpropane;
trimethylolethane; pentaerythritol, dipentaerythritol,
tripentaerythritol, and esters thereof; 1,2,6-hexanetriol;
sorbitol; glycerol tripropylate; glycerol tribenzoate;
1,1,4,4-tetrakis(hydroxymethyl)cyclohexane;
tris(2-hydroxyethyl)isocyanur- ate; ethylene oxide; and propylene
oxide. Polymers or copolymers having polyhydroxyl groups are also
suitable and include, but are not limited to,
poly(ethylene-co-vinyl alcohol) and poly(ethylene-co-vinyl
acetate). Preferably, the polyhydroxyl compound is pentaerythritol,
dipentaerythritol, tripentaerythritol, or an ester thereof;
trimethylolpropane; trimethylolethane; glycerol; or any mixture
thereof. Such branching agents are commercially available or
prepared by well-known methods.
[0074] Polyfunctional acid anhydride branching agents are the most
preferred class of branching agents for preparing suitable foamable
polyesters. Suitable polyfunctional acid anhydride branching agents
have at least one anhydride group and one or more additional
carboxylic, hydroxyl, or anhydride groups per molecule and include,
but not limited to, pyromellitic dianhydride;
1,2,3,4-cyclobutanetetracarboxylic acid dianhydride;
benzophenonetetracarboxylic acid dianhydride; diphenylsulfone
tetracarboxylic dianhydride; bis(3,4-dicarboxyphenyl)ethe- r
dianhydride; bis(3,4-dicarboxyphenyl)thioether dianhydride;
bisphenol-A bisether dianhydride;
2,2-bis(3,4-dicarboxyphenyl)hexafluoropropane dianhydride;
2,3,6,7-napthalenetetracarboxylic acid dianhydride;
bis(3,4-dicarboxyphenyl)sulfone dianhydride;
1,2,5,6-napthalenetetracarbo- xylic acid dianhydride;
2,2',3,3'-biphenyltetracarboxylic acid dianhydride; hydroquinone
bisether dianhydride; 3,4,9,1 0-perylene tetracarboxylic acid
dianhydride; tetrahydrofuran-2,3,4,5-tetracarboxylic acid
dianhydride; 3,3',4,4'-biphenyltetracarboxylic acid dianhydride;
and 4,4'-oxydiphthalic dianhydride. Polymers or copolymers
containing an acid anhydride component are also suitable. Preferred
polyfunctional acid anhydride branching agents are pyromellitic
dianhydride; 1,2,3,4-cyclopentanetetracarboxylic acid dianhydride;
1,2,3,4-cyclobutanetetracarboxylic acid dianhydride; and
tetrahydrofuran-2,3,4,5-tetracarboxylic acid dianhydride, more
preferably, pyromellitic dianhydride. The preferred polyfunctional
acid anhydride branching agent is pyromellitic dianhydride, which
is commercially available, for example, from Aldrich.
[0075] Mixtures of polyfunctional carboxylic acids, polyhydroxyl
compounds; and/or polyfunctional acid anhydrides, are also suitable
branching agents, for example, a mixture comprising a polyhydroxyl
alcohol and a polyfunctional acid anhydride.
[0076] 5.3 Preparation of Core Materials of the Invention
[0077] Well-known methods can be used in the initial,
multi-stranded foaming process to produce coalesced multi-stranded
foamed articles, which are precursors to the unitary core materials
of the invention. Such procedures are described in detail in U.S.
Pat. No. 3,573,152 (issued Mar. 30, 1971); U.S. Pat. No. 4,122,047
(issued Oct. 24, 1978); U.S. Pat. No. 4,462,947 (issued Jul. 31
1984); U.S. Pat. No. 4,824,720 (Apr. 25, 1989); U.S. Pat. No.
4,833,174 (issued May 23, 1989); U.S. Pat. No. 4,981,631 (Jan. 1,
1991); U.S. Pat. No. 5,246,976 (issued Sep. 21, 1993); U.S. Pat.
No. 5,254,400 (issued Oct. 19, 1993); U.S. Pat. No. 5,340,846
(issued Aug. 23, 1994); U.S. Pat. No. 5,360,829 (Nov. 1, 1994);
U.S. Pat. No. 5,362,763 (issued Nov. 8, 1994); U.S. Pat. No.
5,391,582 (issued Feb. 21, 1995); U.S. Pat. No. 5,399,595 (Mar. 21,
1995); U.S. Pat. No. 5,422,381 (issued Jun. 6, 1995); U.S. Pat. No.
5,458,832 (issued Oct. 17, 1995); U.S. Pat. No. 5,527,573 (issued
Jun. 18, 1996); U.S. Pat. No. 6,197,233 (Mar. 6, 2001); U.S. Pat.
No. 6,213,540 (Apr. 10, 2001); and U.S. Pat. No. 6,350,822 (issued
Feb. 26, 2002), all of which are hereby incorporated herein by
reference.
[0078] 5.3.1 Production of Pre-Shaped Core Materials of the
Invention
[0079] A typical process is outlined in FIG. 1. The polyester and
additives ("feed mixture") A are dried B, blended C, and fed into
an extruder D via the feed section of the extruder screw. The
composition of the feed mixture is described in more detail in
Section 5.3.3 below. Preferably, an extruder hopper (not shown) is
used to funnel feed mixture A into extruder D. Feed mixture A may
be blended, according to well-known methods, in a separate blender,
in the extruder hopper, or in the extruder itself. Feed mixture A
may be dried, according to well-known methods, in a separate drier
or in the extruder hopper. The drying step may be omitted, for
example, if dry raw materials are used. Extruder D, extruder drying
equipment B, and extruder cooling equipment F are well known in the
art. Any basic thermoplastic screw-type extruder can be used.
Suitable extruders are described in U.S. Pat. No. 3,573,152;
(issued Mar. 30, 1972); U.S. Pat. No. 5,340,846 (issued Aug. 23,
1994); U.S. Pat. No. 6,254,977 (issued Jul. 3, 2001); and U.S. Pat.
No. 6,350,822 (issued Feb. 26, 2002), all of which patents are
hereby incorporated by reference herein. Preferably, extruder D
comprises a single-screw, which preferably, has a length to
diameter ratio of 44, for example, about 6 cm in diameter and 280
cm in length. Preferably, the extruder has a two-stage foaming
screw. Dryer B can be any standard dryer, such as a AEC Whitlock
brand dryer (Wood Dale, Ill.) for example, Models WD-25MR and
WD-50MR desiccant-bed dryers, which have a capacity of 3.0 and 6.0
cubic feet, respectively. Preferably, cooling apparatus F is
located at the end of the extruder barrel to cool the melt such
that a melt strength and viscosity suitable for foaming is
achieved. Once in extruder D, the feed mixture is pressurized
(typically at a pressure of from about 25 atmospheres to 200
atmospheres depending on the blowing agent's properties) and melted
by heating above the foamable polyester's melting point in the
extruder screw. Then one or more blowing agents E are transferred
into the extruder, for example, via a liquid injection pump or gas
cylinder with injector valves, and blended with the feed mixture
under pressure resulting in a polyester/blowing agent mixture
("foamable gel"). Suitable blowing agents are described in more
detail in Section 5.3.4 below. The foamable gel is pressurized to
prevent blowing-agent evaporation. The foamable gel is cooled and
metered by the screw through multi-orifice die plate G. The die
plate is described in more detail in Section 5.3.5 below.
Optionally, the mixture may be cooled F, for example, in a melt
cooler, in a "cooling stage" of the extruder, in a die adapter, or
in a secondary extruder. If the foamable gel is too hot or its
viscosity too low, upon extrusion, the foam cells will expand too
rapidly, leading to cell-wall rupture and foam collapse. If, on the
other hand, the foamable gel is too cool, foaming will be
suppressed. Parameters, adjustment, and methods for temperature and
pressure control at the foaming stage are well known in the art.
Die body and multi-orifice die plate G are fastened to the forward
end of the extruder barrel or, if a separate cooling unit is used,
then at the discharge end of the extruder cooler. Upon exiting the
die into ambient pressure, the foamable gel foams as a result of
blowing-agent evaporation and expansion. Upon foaming, the strands
coalesce to give a coalesced multi-stranded pre-shaped core
material H. The extruded strands require time and space to obtain
sufficient expansion, which are readily determined by one of skill
in the art. The temperature range from die plate to shaper
including the foaming land (the "land" region of the die is the
smallest diameter of the orifice) should be controlled based on the
blowing agent, nucleating agent, melt viscosity, and melt strength
and the foam's desired physical characteristics according to
well-known methods and parameters.
[0080] FIG. 2 is an illustration of pre-shaped core material's 1
strand pattern 2 prior to shaping. The pre-shaped core material
comprises coalesced strands 3 and may comprise inter-strand voids 4
running parallel to the strand direction.
[0081] An alternative foaming extrusion process, which is well
known in the art, employs tandem extruders. According to this
process, the polyester and other components are mixed, melted,
pressurized, and homogenized with the blowing agent(s) in a first
extruder, as described above, resulting in the foamable gel. The
foamable gel is transferred by means of a conventional transfer
tube or static mixer, optionally assisted by a gear-type melt pump,
to the feed section of the second extruder, which conveys the
foamable gel to the die and adjusts the temperature and pressure
necessary for optimum foaming. Typically, the two extruders are of
different sizes. The well-known tandem-extruder process allows
excellent control of process variables. See e.g., JAMES L. THRONE,
THERMOPLASTIC FOAMS 191 (1st ed. 1996); K. -D. Kolossow, Extrusion
of Foamed Intermediate Products With Single-Screw Extruders, in
PLASTICS EXTRUSION TECHNOLOGY 456 (F. Hensen ed.1988), both of
which titles are incorporated by reference herein.
[0082] 5.3.2 Shaping the Extruded Pre-Shaped Core Materials of the
Invention
[0083] The special shaping process described below serves three
functions: (a) to compress the pre-shaped core material's coalesced
strands and eliminate inter-strand voids between and running
parallel to the strands; (b) To mold the core material of the
invention to the desired shape; and (c) to provide the core
material with a smooth surface. The shaper of the invention
basically comprises a shaping conduit of specified shape, having an
entrance, an exit, and a length, with the conduit's top having a
degree of decline, while the angle of the sides and bottom remain
straight (i.e., the sides and bottom are fixed in a parallel
relationship). As the recently extruded multi-stranded core
material is forced or pulled through the shaper, it is compressed.
The compression removes the inter-strand voids resulting in the
high-strength core materials of the invention (see FIG. 4 showing a
cross section of a core material of the invention having no
inter-strand voids). The shaping process can be tailored by
adjusting the entrance and exit size, length, and the degree of
decline to completely remove the strand appearance of the core
material. That is, if the core material is cut perpendicular to the
strand direction, strands are no longer visible or defined (at
least to the human eye). The strands have coalesced to a degree
wherein the core material appears to be unitary. But surprisingly,
this shaping results in a plurality of discrete volumes. These
discrete volumes are distinguished from one another in that each
comprises a core having closed cells of average cell size
relatively larger than the average cell size of the cells in a
surrounding jacket. This phenomenon is believed to contribute to
the anisotropic properties of the core materials of the invention
and is more fully discussed in Section 5.4.1 below.
[0084] In any case, in next step of FIG. 1, the pre-shaped core
material H is conveyed to shaper I, for example, via a puller (not
shown) or similar machine. Shaper I compresses the pre-shaped core
material, to give a core material of the invention J. Shaper I is
placed adjacent to die plate G, the distance between the die plate
and shaper may be adjusted according to the strength, temperature,
and expansion of foaming strands.
[0085] FIGS. 3A and 3C illustrate a shaper 10, suitable for use in
the invention. FIG. 3A illustrates a perspective view; a side view
is shown in FIG. 3B; and a front view is shown in FIG. 3C. The
shaper comprises a fixed channel-shaped bottom 12 connected via a
plurality of evenly spaced spring bolts 14 to adjustable top cap
15, having angle of decline 16. As shown in FIG. 3C, top cap 15 is
shaped for a close fit into the channel defined by bottom 12
thereby defining shaping conduit 17, having entrance 18 and exit
19. The position of top cap 15 and its angle of decline 16 can be
adjusted with spring bolts 14. Upon reading the Specification, one
of skill in the art will understand that the parameters of exit
size, entrance size, shaping conduit length, and the shaping
conduit top's angle of decline depend on the dimensions and pattern
of the die plate, the extrusion parameters, blowing agent,
polyester resin etc. Preferably, these parameters are adjusted such
that all the voids between the strands are substantially removed.
Substantially removed means that the inter-strand voids are
eliminated to the extent that they are not visible to the human eye
or are of diameter smaller than the average cell-size diameter.
Preferably, the cross area of entrance 18 is of from about 3.0
times to about 1.2 times the cross area of exit 19, more
preferably, of from about 2.5 times to about 1.5 times, even more
preferably, of from about 2.0 times to about 1.5 times. The length
of the shaper is adjusted based on foam residence times. Thus,
shaper length is based on speed of processing, which in turn is
dependent on other variables, such as polyester identity, cell
size, blowing agent, etc., all of which variables are readily
determined and adjusted by one of ordinary skill in the art.
Preferably, the length of the shaper is about 1 ft. and the foam
has a residence time in the shaper of about 10 to about 30 seconds.
Preferably, the angle of decline 16 of the shaping conduit's top 15
is of from about 20.degree. to about 2.degree., more preferably, of
from about 15.degree. to about .sub.5.degree., even more
preferably, of from about 10.degree. to about 5.degree.. Bottom 12
is fixed to support 20, such as a bench, by strips 22. Preferably,
strips 22 are constructed of steel.
[0086] As shown in FIGS. 3A and 3B, the distance between the top
and bottom of shaping conduit 17 gradually decreases while the
width remains fixed, thus, when a pre-shaped core material is
pulled through shaping conduit 17, it is compressed in a direction
perpendicular to its length. The distance between the top and
bottom of shaping conduit 17 is controlled by the position of top
cap 15. The exterior force required to adjust top cap 15 can be
applied by well-known methods, such as gravity, spring-loaded bolts
14, or air pressure from a self-adjusting system. Preferably,
shaper 10 is equipped with oil lines 24 parallel to the shaper's
length, situated in the lower wall of bottom 12. The oil
temperature is adjusted by a heat-exchange system and pumped
through lines 24 to control the shaping temperature. One of skill
in the art will adjust the shaping temperature considering the
identity and dimensions of the foamed strands and the blowing
agent's properties. The shaper can be constructed of stainless
steel, aluminum, copper, or graphite plates, preferably, aluminum.
Preferably, the interior surfaces of shaping conduit 17 are coated
with a lubricant or other non-stick coating to reduce the
refraction force between the pre-shaped core material and the
shaper walls, for example, but not limited to, fluoropolymers
(e.g., Lubricating Spray Coating, a product of Saint-Gobain
Performance Plastics, Wayne, N.J.).
[0087] FIGS. 3A-3C illustrate a shaper of the invention that gives
a rectangular core material of the invention. By changing die-plate
pattern and the shaper's configuration, different configurations of
the core materials of the invention can be achieved, for example,
U-channel, I-beam, V, trapezoid, rod, and pipe shapes. The distance
from the shaper entrance to the die plate can be adjusted by one of
skill in the art depending on the foamable polyester resin, blowing
agent(s), and die-plate configuration. Preferably, the shaped core
material of the invention has a smooth surface and substantially no
voids between the strands.
[0088] FIG. 4 illustrates a cross sectional view of a typical core
material 30 of the invention after the special shaping process. As
indicated in FIG. 4 by the use of dotted lines, the strand
interface lines 32 are barely visible, or depending on the
extrusion and shaping parameters, not visible at all and no
inter-strand voids are present. The shaped core materials of the
invention can be cut into desired dimensions by well-known
methods.
[0089] 5.3.3 Feed Mixture, Nucleating Agents and Other
Additives
[0090] Preferably, the feed mixture comprises foamable polyester
and the other additives if they will be used. Preferably, a
nucleating agent is included in the feed mixture to promote even
evaporation of the blowing agent, thereby controlling the size and
number of cells and cell-size distribution. Nucleating agents are
well known in the art. Suitable nucleating agents include, but are
not limited to, inorganic substances, such as calcium carbonate,
talc, clay, titanium oxide, silica, barium sulfate, diatomaceous
earth, and carbon dioxide that is generated by including a mixture
of a basic salt (e.g., sodium-, potassium-, or ammonium carbonate
or bicarbonate) and an inorganic or organic acid (e.g., boric acid,
citric acid, and tartaric acid) in the feed material. Finely
pulverized inorganic substances, such as calcium carbonate and talc
are preferred. The particle size of nucleating agent is from 0.1 to
20 microns, preferably, from 1 to 3 microns. Preferably, the
nucleating agent is present in an amount of from about 0.01% to
about 5% by weight of polyester, more preferably, of from about
0.1% to about 3% by weight. In general, more nucleating agent gives
a smaller average cell diameter. If the amount exceeds 5% by
weight, agglomeration or insufficient dispersion of nucleating
substance occurs, which results in over-expanded cells and often
cell collapse. On the other hand, if the nucleating agent is
present in an amount of less than about 0.01% by weight, the
nucleating action is negligible.
[0091] In another embodiment, the polyester can be non-foamable
polyester having one or more branching agents included as an
additive to render the polyester foamable in situ. Upon
feed-mixture heating and melting, the polyester and branching agent
react, in situ, to give foamable polyester, which is extruded as
above. Such a process is described in U.S. Pat. No. 5,340,846
(issued Aug. 23, 1994) and U.S. Pat. No. 6,254,977, (issued Jul. 3,
2001) both of which are hereby incorporated by reference
herein.
[0092] Optionally, the feed mixture may comprise further additives,
as well known in the art, depending on the desired properties of
the core material. Examples of other additives include, but are not
limited to reclaim polymer generated in manufacturing, flame
retardants, colorants or pigments, anti-static agents,
antioxidants, ultraviolet ray absorbents, and reinforcement by
short fibers, etc.
[0093] 5.3.4 Blowing Agents
[0094] The blowing agents suitable for use in the invention
generally have a boiling point temperature range of about
-90.degree. C. to about 130.degree. C. Suitable blowing agents
include, but are not limited to, aliphatic hydrocarbons, such as
octane, heptane, hexane, cyclohexane, pentane, cyclopentane,
isopentane, neo-pentane, isobutane, butane, propane, and ethane;
alcohols, such as methanol, ethanol, isopropanol, and butanol;
non-fully chlorinated chlorohydrocarbons, partially or fully
fluorinated fluorohydrocarbons, and non-fully halogenated
fluorochlorohydrocarbons, such as 1-chloro-1,1-fluoroethane;
1,1,1,2-tetrafluoroethane (HFC 134a); 1,1-difluoroethane (HFC
152a); 1-chloro-1,1-difluoroethane (HCFC 142b);
1,1,1,3,3-pentafluorobutane (HFC-365mfc);
1,1,1,3,3-pentafluoropropane (HFC-245fa); gases, such as carbon
dioxide, argon, and nitrogen; and compounds that decompose in situ
to release a blowing-agent gas, such as azobisformamide or
azodicarbonamide. Preferably, the blowing agent is hexane, carbon
dioxide, or 1,1,1,2-tetrafluoroethane or 1,1-difluoroethane.
[0095] The amount of blowing agent depends on the desired foam
density and such amounts are readily determined by well-known
methods, such as those described in U.S. Pat. No. 5,681,865 (issued
Oct. 28, 1997), hereby incorporated herein by reference. The amount
of blowing agent should not exceed an amount that causes separation
between the polyester melt and blowing agent in the extruder, that
is exceeds the solubility of said blowing agent in the melt at
operating temperatures. The preferred amount of blowing agent is of
from about 0.5% by weight to about 15% by weight of the feed
mixture, more preferably, of from about 1% to about 5%, most
preferably, of from about 1% to about 3%. Mixtures of blowing
agents can be employed, such as the mixtures described in U.S. Pat.
No. 5,679,295 (issued Oct. 21, 1997) according to the procedure
described therein, hereby incorporated by reference herein. For
example, the following combination of blowing agents could be used:
(1) of from about 50 mole percent to about 99.9 mole percent of a
first blowing agent having a boiling temperature at STP (Standard
Temperature and Pressure) of greater than 310.degree. K, such as
heptane, octane, or cyclopentane; (2) of from about 0.1 mole
percent to about 50 mole percent of a second blowing agent having a
boiling temperature at STP of less than 310.degree. K, such as
butane, tetrafluoroethane, carbon dioxide, or pentane.
[0096] 5.3.5 Die Plate
[0097] Foam strands that exit from the orifice generally expand to
about 3 to about 6 times the orifice diameter depending on the
density, thus the ratio of total orifice cross-sectional area to
that of the interior of the die just ahead of the
orifice-containing faceplate should be less than 10 percent, more
preferably, less than 5 percent.
[0098] The die plate comprises a plurality of orifices, in the
shape of holes, slits, or any other desired shape, such as square
saw tooth or triangular saw tooth wave pattern. The size of the
holes can be calculated based on a method described in U.S. Pat.
No. 6,197,233 (issued Mar. 6, 2001), hereby incorporated by
reference herein. The ratio of the theoretical diameter of an
individual strand to the distance between the orifices is,
preferably, greater than or equal to 1, more preferably, greater or
equal to 1.2. The theoretical diameter of each strand can be
calculated based on the foam volume expansion and
extrusion-direction speed of the product. The foam volume can be
calculated from mass balance and foaming temperature. The orifice
size and overall open area are determined also by considering
extrude throughput and foaming pressure. The big overall open area
can increase throughput and foam cross section area, but reduce the
die pressure and cause foaming in die. The only limitation on
special arrangement of the die orifices is that contact and
coalescence of adjacent strands or profiles after extrusion from
the die faceplate must be achieved.
[0099] Merely by way of example, a suitable die plate has the
following construction: a diameter of orifice is about 1.5 mm,
wherein the orifices are arranged in a triangular pattern with a
distance of about 6.5 mm between the center of the orifices. In
another embodiment, a preferred die-faceplate pattern comprises
evenly spaced circular orifices of a diameter of from about 0.8 mm
to about 4 mm, preferably, of from about 1.5 mm to about, more
preferably, of from about 1 mm to about 2 mm. For example, a
suitable die is a circular carbon steel plate of a diameter of 165
mm comprising about 34 equally spaced circular-shaped holes,
contained in an area of 7 cm.times.2 cm, arranged in four rows,
wherein the holes are about 0.065 inches (1.65 mm) in diameter and
spaced approximately 0.25 inches (6.35 mm) between the centers. In
another example, a suitable die is a circular carbon steel plate of
a diameter of 165 mm comprising about 34 equally spaced
circular-shaped holes, contained in an area of 7 cm.times.2 cm,
arranged in four rows, wherein the holes are about 0.052 inches
(1.32 mm) in diameter and spaced approximately 0.25 inches (6.35
mm) between the centers. In yet another example, a suitable die is
a circular carbon steel plate of a diameter of 165 mm comprising
about 78 equally spaced circular-shaped holes contained in an area
of 10 cm.times.2.5 cm arranged in five rows, wherein the holes are
about 0.065 inches (1.62 mm) in diameter and spaced approximately
0.25 inches (6.35 mm) between the centers. Special alloys of more
highly conductive metals, such as beryllium-copper, and having
heat-transfer fluid passages or electric heaters, would be
advantageous in the precise control of temperature of the die
faceplate.
[0100] 5.4 Characteristics of Core Materials of the Invention
[0101] Core materials of the invention are chemically and thermally
stable by virtue of the polyesters from which they are derived.
Furthermore, by virtue of the preparation methods described herein,
the core materials of the invention are low-density, relatively
small-cell sized, closed-cell foams, of uniform cell size with a
low open-cell count. The core materials of the invention are
further characterized by excellent mechanical properties, such as
high compression strength and compression modulus, high shear
strength and shear modulus, and high tensile strength and tensile
modulus. Strength refers to the maximum load per unit of area
transverse to the loading direction, that the material can resist.
Modulus is precise measurement of how much deformation the material
will exhibit under a given load. High modulus means that the
structure deforms relatively little under large applied forces.
[0102] 5.4.1 Inter-Strand Cell-Size Distribution
[0103] As discussed above, in the shaping process, as the recently
extruded multi-stranded core material is forced or pulled through
the shaper; it is compressed, thereby removing the inter-strand
voids, such that the core materials of the invention appear to be
unitary (the strand structure is not visible to the human eye). The
properties, however, of the unitary polyester core materials of the
invention are much improved over traditional unitary block foams
produced by extruding through a single-aperture die or unshaped
multi-stranded foams. The origins of these improved properties,
such as strength, are not fully understood, but may be due to the
change in cell-size distribution that accompanies extrusion and
shaping of multi-stranded polyester foams. Upon extrusion and prior
to shaping, the core material comprises a multitude of coalesced
foamed strands having inter-strand voids running parallel to the
strand direction. Upon shaping, the inter-strand voids are removed,
and the core materials of the invention often appear unitary,
although, in some cases, the strand intersections can be seen.
[0104] Surprisingly, however, a plurality of "discrete volumes",
running parallel to the length of the core material results. These
"discrete volumes" comprise an "interior section" running parallel
to the strand direction and a "jacket" surrounding the "interior
section of the discrete volume". This interior section has closed
cells of average cell size relatively larger than the average cell
size of the cells in the surrounding "jacket". In other words, the
average-cell diameter is smaller where the strands have intersected
and merged than at the original strand's interior. Accordingly, as
used herein, the phrase "interior section of the discrete volume"
means a section within a core material of the invention running in
the strand direction that is surrounded by a "jacket". For example,
the "jacket" could be a donut shaped cylindrical volume and the
interior section of the discrete volume a cylindrical core inside
the cylindrical donut. It follows that the sum value of the volume
of the "interior section of the discrete volume" and the volume of
the "jacket" equals the "discrete volume", which discrete volume
approximates the volume of the corresponding strand before
shaping.
[0105] Where the interior section of the discrete volume
approximates a cylinder shape, such a cylinder has a
cross-sectional area of .pi.(x)(1/2d).sup.2 where "d" is the
diameter of the discrete volume. Preferably "x" is a value of from
about 0.99 to about 0.01, more preferably, of from about 0.95 to
about 0.5, even more preferably, of from about 0.90 to about 0.75.
The volume and shape of the "jacket" surrounding the "interior
section of the discrete volume" is the difference between the
discrete volume and the volume of the interior section of the
discrete volume. Suitable extrapolations can be made by one of
skill in the art when the interior section does not approximate a
cylinder. Preferably, the average cell diameter of cells in the
jacket of the discrete volume is of from about 5% to about 98% of
the average cell diameter of cells in the interior section of the
discrete volume, more preferably, of from about 20% to about 80%,
even more preferably, of from about 30% to about 70%. This
inter-strand cell-size distribution can be viewed by any suitable
technique, such as optical microscope techniques so as to
distinguish cell diameters within the plurality of discrete
volumes.
[0106] 5.4.2 Anisotropic Properties of Core Materials of the
Invention
[0107] The core materials of the invention are significantly
anisotropic in character; meaning that they exhibit different
physical properties depending upon which axis (direction) the
property is measured. For example, the core materials of the
invention exhibit different shear properties, tensile properties,
and compressive properties depending upon the axis along which the
property was measured. This character is related to the fact that
the core materials of the invention are derived from compressed
strands. Although the strand character of the core materials of the
invention may not be visible, the anisotropic character is readily
apparent through standard testing, for example, testing for
differences in shear properties versus applied-force direction,
tensile properties versus applied-force direction, and compressive
properties versus applied-force direction. For example, in a core
material of the invention, the shear strength in the end-strand
direction differs from the shear strength in the transverse
direction, which both differ from the shear strength in the
longitudinal direction.
[0108] In core materials of the invention, the shear strength
differs, the shear modulus differs, and the shear elongation at
break differs depending on the relationship between the
applied-force direction and the strand direction. Preferably the
shear strength difference ranges from about 10% to about 400%
higher in the end-strand over the transverse direction, more
preferably, from about 20% to about 200%, even more preferably,
from about 40% to about 150%. Preferably, the shear modulus
difference ranges from about 10% to about 400% higher in the
end-strand over the transverse direction, more preferably, from
about 20% to about 200%, even more preferably, from about 40% to
about 150%. Preferably the shear elongation at break point
difference ranges from about 10% to about 600% higher in the
end-strand over the transverse direction, more preferably, from
about 20% to about 400% even more preferably, from about 50% to
about 200%.
[0109] In core materials of the invention, the tensile strength
differs and the tensile modulus differs depending on the
relationship between the applied-force direction and the strand
direction. Preferably the tensile strength difference ranges from
about 100% to about 3000% higher in the end-strand over the
transverse direction, more preferably, from about 500% to about
2000%, even more preferably, from about 800% to about 1100%.
Preferably the tensile modulus difference ranges from about 100% to
about 3000% higher in the end-strand over the transverse direction,
more preferably, from about 500% to about 2000%, even more
preferably, from about 900% to about 1500%.
[0110] In core materials of the invention, the compressive strength
differs and the compressive modulus differs depending on the
relationship between the applied-force direction and the strand
direction. Preferably the compressive strength difference ranges
from about 50% to about 1000% higher in the end-strand over the
transverse direction, more preferably, from about 100% to about
500%, even more preferably, from about 120% to about 300%.
Preferably the compression modulus difference ranges from about 10%
to about 500% higher in the end-strand over the transverse
direction, more preferably, from about 20% to about 200%, even more
preferably, from about 30% to about 100%.
[0111] 5.4.3 Density
[0112] The core materials of the invention are characterized by low
density, preferably, of from about 1 kg/m.sup.3 to about 400
kg/m.sup.3, more preferably, of from about 50 kg/m.sup.3 to about
300 kg/m.sup.3, and most preferably, of from about 60 kg/m.sup.3 to
about 250 kg/m.sup.3 as measured according to ASTM Test Method
D1622-98 "Standard Test Method for Apparent Density of Rigid
Cellular Plastics", hereby incorporated by reference herein. This
publication sets forth procedures for the determination of both the
apparent overall density and the apparent core density of cellular
plastics.
[0113] 5.4.4 Average Cell Size
[0114] The core materials of the invention are characterized by
small average cell size, preferably, of from about 0.05 mm to about
1.5 mm, more preferably, of from about 0.1 mm to about 0.5 mm, as
measured according to ASTM Test Method D3576-98 "Standard Test
Method for Cell Size of Rigid Cellular Plastics", hereby
incorporated by reference herein. This publication sets forth
methods for determination of the apparent cell size of rigid
cellular plastics by counting the number of cell-wall intersections
in a specified distance. Procedure A requires the preparation of a
thin slice, not more than one half the average cell diameter in
thickness, that is mechanically stable. For most rigid cellular
plastics this limits the test method to materials with an average
cell size of at least 0.2 mm. Procedure B is intended for use with
materials whose friable nature makes it difficult to obtain a thin
slice for viewing.
[0115] 5.4.5 Open-Cell Content
[0116] The core materials of the invention are characterized by a
low open-cell content, preferably, of from about 5 to about 20,
more preferably, of from about 5 to about 10 as measured according
to ASTM Test Method D2856-94 (1998) "Standard Test Method for
Open-Cell Content of Rigid Cellular Plastics by the Air
Pycnometer", hereby incorporated by reference herein. Cellular
plastics are composed of the membranes or walls of polymer
separating small cavities or cells. These cells may be
interconnecting (open cell), non-connecting (closed cell), or any
combination of these types. This test method determines numerical
values for open cells. It is a porosity determination, measuring
the accessible cellular volume of a material. The volume occupied
by closed cells is considered to include cell walls. This test
method consists of three procedures: procedure A, designed to
correct for cells opened during sample preparation, by measuring
cell diameter, calculating, and allowing for surface volume;
procedure B, designed to correct for cells opened in sample
preparation, by cutting and exposing new surface area equal to the
surface area of the original sample dimension; and procedure C,
which does not correct for cells opened during sample preparation
and gives good accuracy on predominantly highly open-celled
materials. The accuracy decreases as the closed-cell content
increases and as the cell size increases.
[0117] 5.4.6 Compression Strength and Compression Modulus
[0118] High compression strength and medium compression modulus in
the strand direction characterize the core materials of the
invention. For example, core materials of the invention,
preferably, have a compression strength of from about 0.20 Mpa to
about 1.5 Mpa, more preferably, of from about 1.0 to about 1.5 Mpa
at a density of 80 kg/m.sup.3. At a density of 140 kg/m.sup.3,
preferably, the core materials have a compression strength of from
about 0.8 Mpa to about 2.5 Mpa, more preferably, of from about 1.5
Mpa to about 2.5 Mpa. Preferably, core materials of the invention
have a compression modulus of from about 10 Mpa to about 30 Mpa,
more preferably, of from about 15 Mpa to about 25 Mpa at a density
of 80 kg/m.sup.3. At a density of 140 kg/m.sup.3, preferably, the
core materials have a compression modulus of from about 25 Mpa to
about 65 Mpa, more preferably, of from about 45 Mpa to about 55
Mpa. Compression properties can be measured according to the
procedure set forth in ASTM Test Method C365-00 "Standard Test
Method for Flatwise Compressive Properties of Sandwich Cores",
hereby incorporated by reference herein. This publication describes
determination of the compressive strength and modulus of composite
cores. These properties are usually determined for design purposes,
in a direction normal to the plane of facings as the core would be
placed in a structural sandwich construction. The test procedures
pertain to compression in this direction in particular, but also
can be applied with possible minor variations to determining
compressive properties in other directions.
[0119] 5.4.7 Tensile Strength and Tensile Modulus
[0120] The core materials of the invention are characterized by
medium tensile strength and high tensile modulus. Tensile strength
of the core material provides and indication of its composite's
resistance to skin buckling upon edgewise loading. For example,
core materials of the invention, preferably, have a tensile
strength of from about 0.5 Mpa to about 2.5 Mpa, more preferably,
of from about 1 Mpa to about 2 Mpa at a density of 80 kg/m.sup.3.
At a density of 140 kg/m.sup.3, the core materials, preferably,
have a tensile strength of from about 1.5 Mpa to about 3.5 Mpa,
more preferably, of from about 2 Mpa to about 3 Mpa. Preferably,
core materials of the invention have a tensile modulus of from
about 50 Mpa to about 200 Mpa, more preferably, of from about 100
Mpa to about 150 Mpa at a density of 80 kg/m.sup.3. At a density of
140 kg/m.sup.3, the core materials, preferably, have a tensile
modulus of from about 100 Mpa to about 300 Mpa, more preferably, of
from about 150 Mpa to about 250 Mpa. The tensile strength and
modulus can be measured according to the procedure set forth in
ASTM Test Method C297-94 "Standard Test Method for Flatwise Tensile
Strength of Sandwich Constructions", hereby incorporated by
reference herein. This publication illustrates the determination of
the core flatwise tension strength, or the bond between core and
facings of an assembled sandwich panel. The test consists of
subjecting a sandwich construction to a tensile load normal to the
plane of the sandwich, such load being transmitted to the sandwich
through thick loading blocks bonded to the sandwich facings or
directly to the core.
[0121] 5.4.8 Shear Strength and Shear Modulus
[0122] The core materials of the invention are characterized by
medium shear strength and high shear modulus. For example, core
materials of the invention, preferably, have a shear strength of
from about 0.3 MPa to about 1.5 Mpa, more preferably, of from about
0.6 MPa to about 1.2 Mpa at a density of 80 kg/m.sup.3. At a
density of 140 kg/m.sup.3, the core materials, preferably, have a
shear strength of from about 0.5 MPa to about 3 Mpa, more
preferably, of from about 1 Mpa to about 2 Mpa. Preferably, core
materials of the invention have a shear modulus of from about 10 to
about 35, more preferably, of from about 20 MPa to about 30 MPa at
a density of 80 kg/m.sup.3. At a density of 140 kg/m.sup.3, the
core materials, preferably, have a shear modulus of from about 20
Mpa to about 60 MPa, more preferably, of from about 30 MPa to about
50 Mpa. The shear strength and modulus can be measured according to
the procedure set forth in ASTM Test Method C273-00el "Standard
Test Method for Shear Properties of Sandwich Core Materials". This
publication illustrates the determination of shear properties of
sandwich construction core materials associated with shear
distortion of planes parallel to the facings. It describes
determination of shear strength parallel to the plane of the
sandwich, and the shear modulus associated with strains in a plane
normal to the facings. The test may be conducted on core materials
bonded directly to the loading plates or the sandwich facings
bonded to the plates.
[0123] 5.5 Composites of the Invention
[0124] Composites of the invention can be prepared according to
well-known methods by integrally bonding the core materials of the
invention to standard composite skins. Examples of such methods are
described in U.S. Pat. No. 6,206,669 (issued Mar. 27, 2001); U.S.
Pat. No. 6,156,146 (issued Dec. 5, 2000); U.S. Pat. No. 6,117,519
(issued Sep. 12, 2000); U.S. Pat. No. 6,013,213 (issued Jan. 11,
2000); U.S. Pat. No. 5,916,672 (issued Jun. 29, 1999); U.S. Pat.
No. 5,904,972 (issued May 18, 1999); U.S. Pat. No. 5,580,502
(issued Dec. 3, 1996); and U.S. Pat. No. 5,316,462 (May 31 1994),
all of which are hereby incorporated by reference herein. Just
about any composite skin suitable for use with thermoplastic foam
cores can be used. Such composite skins are commercially available,
for example, from M. C. Gill (El Monte, Calif.), DFI Pultrude
Composites, Inc. (Erlanger, Ky.); and Gordon Plastics (Montrose,
Calif.). Examples of suitable composite skins include, but are not
limited to, thermoplastic polymers and thermosetting polymers,
optionally reinforced with glass fibers, metallic fibers, inorganic
fibers, or carbon fibers; wood; inorganic materials, such as
fiberglass; and metallic materials, such as aluminum and stainless
steel and many others, which are well known to those of skill in
the art. The composites can be built up in layers. When building
composites in layers one or more structural skins can comprise
another structural sandwich composite.
[0125] A preferred composite-manufacture method is resin infusion
molding, which comprises vacuum suction to pull liquid resin into a
dry lay up (composite skins and core materials) with almost no
hazardous-vapor emissions. The chemically and thermally resistant
core materials of the invention are especially suited to such
processing.
[0126] In some applications where it is necessary that the core of
the laminate be conformed to a curved surface, such as a boat hull
or a cylindrical storage tank, the core materials of the invention
can be processed into a contourable blankets or web-like
structures, wherein the core material of the invention is divided
into an array of smaller blocks or tiles. Such a blanket may be
adhered to a fabric scrim or common carrier whereby the blanket can
be conformed to a contoured surface. A process for producing
contourable blankets from foamed articles is disclosed in U.S. Pat.
No. 5,798,160 (issued Aug. 25, 1998), hereby incorporated herein by
reference. The contourable blanket structures are thereafter bonded
to skins to form contoured composite materials of the
invention.
[0127] To produce contourable core materials of the invention,
several standard slabs are stacked and bonded together by thermal
fusion or by a suitable adhesive to create a large multi-slab
block. The block is then sliced transversely in parallel planes
normal to the lines of adhesive to yield a plurality of panels. A
saw can be used for slicing. Each panel is composed of a series of
interconnected foam-plastic sections derived from respective slabs
of the block whereby the density of the sections is evenly
distributed throughout the panel and the mechanical properties of
the panel are therefore predictable and satisfy structural laminate
criteria.
[0128] In the basic process for producing composite materials of
the invention, the core materials of the invention (in rigid,
contourable-blanket form, or any other form) and skins are laid up
in a tool while dry then a vacuum bag is placed over the lay up and
sealed to the tool. Resin is introduced via a resin inlet and
distributed throughout the laminate. The pressure differential
provides the driving force for infusing the resin into the lay up.
The procedure is described in detail in U.S. Pat. No. 4,902,215
(issued Feb. 20, 1990); U.S. Pat. No. 5,052,906 (issued Oct. 1,
1991); U.S. Pat. No. 5,721,034 (issued Feb. 24, 198); U.S. Pat. No.
5,904,972 (issued May 18, 1999); U.S. Pat. No. 5,958,325 (issued
Sep. 28, 1999); and U.S. Pat. No. 6,159,414 (issued Dec. 12, 2000),
all of which are hereby incorporated herein by reference.
Vacuum-bag lay-up products are commercially available, for example,
from Airtech International (Carson, Calif.); Hawkeye Enterprises
(Los Angeles, Calif.); National Aerospace Supply Co. (San Clemente,
Calif.); Richmond Aircraft Supply (Norwalk, Calif.) and Taconic,
Process Materials Division (Santa Maria, Calif.). Resins for
bonding the skins to the core materials of the invention are
commercially available, for example, from Aircraft Spruce &
Specialty Company (Corona, Calif.); CMI/Composite Materials Inc.
(Santa Fe Springs); E. V. Roberts (Culver City, Calif.); Gougen
Brothers, Inc. (Bay City, Mich.); and National Aerospace Supply Co.
(San Clemente, Calif.).
6. EXAMPLES
6.1 Example 1
[0129] A mixture of polyethylene terephthalate resin (COBIFOAM 0,
purchased from M&G Polymers, Patrica, Italy) having an
intrinsic viscosity of 1.25 dl/g; and a melting point of
251.degree. C. and nucleating agent was dried for 5 hours at
330.degree. F. (165.degree. C.) by a dehumidified dryer. The
mixture was charged into a two-stage extruder having a 2.5" (6.35
cm) single-screw, length to diameter of 44 to 1, with a SMR melt
cooler heated to a temperature of 280.degree. C.; and pressurized
to 10 Mpa, a polyethylene terephthalate melt. Blowing agent (3
weight percent of the total weight of the melt) was injected by a
injection pump and the mixture homogenized in the extruder by
mixing section, static mixer and SMR melt cooler to give a foamable
gel. The temperature was reduced to 250-255.degree. C. at a
pressure of 3.5-5.0 Mpa and the foamable gel was extruded through a
rectangular multi-orifice die plate that was fastened on the die
body, which the other end of the die body was attached to the melt
cooler. The extrusion rate was 20 kg/hr. The die plate was 7.0
cm.times.2.0 cm and comprised 34 equally spaced circular-shaped
holes arranged in four rows. The holes were approximately 0.065
inches (1.65 mm) in diameter and spaced approximately 0.25 inches
(6.35 mm) between the centers.
[0130] The following parameter values were used in the
extrusion.
1 Parameter Value diameter of screw 6.35 cm screw-length to
diameter 44:1 temperature of melting zone 280.degree. C.
temperature of injection zone 280.degree. C. temperature of cooling
zone 270-275.degree. C. head temperature 255-260.degree. C.
temperature of the melt 255-260.degree. C. pressure of the melt
3.5-5.0 Mpa runs of the screw 15 rpm average residence time in the
extruder 15 min
[0131] The resulting coalesced multi-stranded pre-shaped core
material was conveyed through a shaper of the invention,
constructed of aluminum plate, having a width of 7 cm and a length
of 30 cm. The ratio of the opening area to the parallel
cross-sectional area was 2.3. The first 2.5 cm range of shaper has
a 12.degree. angle of decline so that the expanded strands can be
squeezed and compressed into foam board without voids between
strands. The distance of the cap plate to the bottom of shaper was
adjusted from 3 cm to 4 cm based on output of extrusion and speed
of take off equipment.
[0132] The resulting core material of the invention had a cross
section of 7 cm by 3.4 cm; uniform cell size (<0.2 mm); no voids
between the strands; and a foam density of 5.4 pcf (86.5
kg/m.sup.3) pounds per cubic foot. The core material so produced
had the following characteristics:
2 Characteristic Value Density 86.5 kg/m.sup.3 Average cell size
0.1 mm Open-cell content <10%
6.2 Example 2
[0133] The procedure of Example 1 was followed using 4 weight
percent blowing agent and a die plate of 7.0 cm.times.2.0 cm
comprising 34 holes each of 0.052 inches (1.32 mm) in diameter. The
holes and rows were equally spaced at approximately 0.25 inches
(6.35 mm) between the centers. The coalesced multi-stranded
pre-shaped core material was shaped as described in Example 1. The
resulting core material of the invention had a cross section of
2.75 inches (7.0 cm) by 1.45 inches (3.68 cm); uniform cell size
(<0.3mm); no voids between the strands; and a foam density of
4.7 pounds per cubic foot (75 kg/m.sup.3). The core material so
produced had the following characteristics:
3 Characteristic Value Density 75 kg/m.sup.3 Average cell size 0.2
mm Open-cell content <10%
6.3 Example 3
[0134] The procedure of Example 1 was followed, with 0.15 weight
percent talc as a nucleating agent and 2 weight percent blowing
agent. The resulting core material of the invention had a
cross-section of 7.0 cm by 3.18 cm; a uniform cell size (<0.2
mm); no voids between the strands; and a foam density of 7.56
pounds per cubic foot (121 kg/m.sup.3). The core material so
produced had the following characteristics:
4 Characteristic Value Density 121 kg/m.sup.3 Average cell size 0.1
mm Open-cell content <10%
6.4 Example 4
[0135] Using the procedure of Example 1, a foamable gel comprising
a mixture of polyethylene terephthalate polyester resin (CRYSTAR
5067, available from Du Pont Polyester) having a melting point of
251.degree. C. and 1.5 weight percent 1,1,1,2-tetrafluoroethane
(HFC R134a) as a blowing agent was cooled to a temperature of
255.degree. C. and extruded through the die plate. The SMR melt
cooler was not used. The die plate was connected directly to the
end of extruder; was 10 cm.times.2.5 cm; and comprised 78 equally
spaced circular-shaped holes arranged in five rows. The holes were
about 0.065 inches (1.65 mm) in diameter and spaced about 0.25
inches (6.35 mm) between their centers.
[0136] The resulting coalesced multi-stranded pre-shaped core
material was conveyed through an aluminum-plate shaper of the
invention having a width of 11 cm and a length of 30 cm. The ratio
of the opening area to the parallel cross-sectional area was 1.6.
The first 2.5 cm range of shaper has a 12.degree. angle of decline
so that the expanded strands can be squeezed and compressed into
foam plank without voids between strands. The distance of the cap
plate to the bottom of shaper was adjusted from 3 cm to 4 cm based
on output of extrusion and speed of take off equipment.
[0137] The resulting core material of the invention had a cross
section of 4.4 inches (11.2 cm) by 1.35 inches (3.4 cm), a uniform
cell size (<0.01 mm), and no voids between the strands. The
density was 9.5 pounds per cubic foot (152 kg/m.sup.3). The core
material so produced had the following characteristics:
5 Characteristic Value Density 152 kg/m.sup.3 Average cell size 0.3
mm Open-cell content <20%
6.5 Example 5
[0138] A mixture of polyethylene terephthalate resin (COBIFOAM 0,
purchased from M&G Polymers, Patrica, Italy) having an
intrinsic viscosity of 1.25 dl/g and a melting point of 251.degree.
C. and talc powder 0.3 weight %, was dried for 6 hours at
165.degree. C. by a dehumidified dryer. The mixture was charged
into a co-rotating twin-screw extruder, model BC 132 PET-F (BC
Foam, Volpiano, Italy) equipped with a melt cooler and homogenizer.
The extruder screw was 13.2 cm in diameter and 220 cm in length.
The mixture was heated in the extruder to a temperature of
285.degree. C. and pressurized to 5 Mpa, then 1,1-difluoroethane
(R152 A) (1.4 weight percent of the total weight of the melt) was
introduced via an injection pump. The mixture was homogenized to
give a foamable gel. The temperature of the foamable gel was
reduced to about 250-255.degree. C. and the pressure was adjusted
to about 3.5-4.0 Mpa. The foamable gel was extruded at a rate of
180 kg/hr through a multi-orifice die plate of 42 cm.times.5 cm
comprising 580 equally spaced circular-shaped holes arranged in
eight rows. The holes were approximately 1.65 mm in diameter and
spaced approximately 6.35 mm between centers.
[0139] The following parameter values were used in the
extrusion:
6 Parameter Value diameter of screw 13.2 cm screw-length 220 cm
temperature of melting zone 285.degree. C. temperature of injection
zone 285.degree. C. temperature of cooling zone 265-270.degree. C.
head temperature 250-255.degree. C. temperature of the melt
250-255.degree. C. pressure of the melt 3.5-4.0 Mpa runs of the
screw 15 rpm average residence time in the extruder 15 min
[0140] The resulting coalesced multi-stranded pre-shaped core
material was conveyed through an aluminum-plate shaper of the
invention having a width of 43 cm and a length of 30 cm. The ratio
of the opening area to the parallel cross-sectional area was 1.6.
The first 2.5 cm range of shaper has a 12.degree. angle of decline
so that the expanded strands can be squeezed and compressed to
remove inter-strand voids. The distance of the cap plate to the
bottom of shaper was adjusted from 5 cm to 6 cm based on output of
extrusion and speed of take off equipment.
[0141] The resulting core material of the invention has a cross
section of 42 cm by 5.7 cm; uniform cell size (<0.2 mm); no
voids between the strands; and a foam density of 135 kg/m.sup.3.
The core material so produced had the following
characteristics:
7 Characteristic Value Density 135 kg/m.sup.3 Average cell size 0.2
mm Open-cell content <10%
6.6 Example 6
[0142] The procedure of Example 5 above was followed using 0.5
weight percent nitrogen as blowing agent. The resulting core
material of the invention had a cross section of 40 cm by 3.80 cm;
uniform cell size (<0.1 mm); no voids between the strands; and a
foam density of 350-400 kg/m.sup.3.
[0143] The core material so produced had the following
characteristics:
8 Characteristic Value Density 350-400 kg/m.sup.3 Average cell size
<0.1 mm Open-cell content <10%
6.7 Example 7
[0144] The procedure of Example 1 was followed except that: (1) the
shaping process and shaper of Example 4 was used; (2) 0.3 weight
percent talc was included in the foamable gel as a nucleating
agent; and (3) 0.9 weight percent CO.sub.2 was included in the
foamable gel as the blowing agent. The resulting core material of
the invention had a cross-section of 420 cm.times.2.3 cm, a uniform
cell size (<0.2 mm); no voids between the strands; and a foam
density of 125 kg/m.sup.3. The core material so produced had the
following characteristics:
9 Characteristic Value Density 125 kg/m.sup.3 Average cell size 0.2
mm Open-cell content <10%
6.8 Example 8
[0145] Anisotropic Properties of Core Material of the Invention
[0146] This Example demonstrates the pronounced anisotropic
properties of core materials of the invention. Samples were
selected from a density range of about 80 kg/m.sup.3 to about 150
kg/m.sup.3. The PET core materials of the invention were prepared
according to Example 1 and had an average cell size 0.1 mm and an
open-cell content <10%. Test samples were prepared by binding
core materials of the invention (7 cm by 3.5 cm planks) into a
block then cutting 1/2" panels in the grain direction according to
the procedure set forth in U.S. Pat. No. 4,536,427 (issued Aug. 20.
1985), hereby incorporated by reference herein. The test samples
were cut from the block as end-strand panels or as flat-strand
panel as appropriate depending on whether the test would be
performed by applying force in the end-strand, transverse, or
longitudinal directions.
[0147] 6.8.1 Shear Tests
[0148] The shear tests were conducted according to the procedure
set forth in ASTM C-273 as discussed in Section 5.4.8 above. The
results are shown in FIGS. 5-7, which are respectively graphs of:
shear strength versus density (FIG. 5); shear modulus versus
density (FIG. 6); and shear elongation at break versus density
(FIG. 7). The shear tests were conducted in the end-strand
direction (represented by a ".diamond-solid." in FIGS. 5-7),
transverse direction (represented by a ".box-solid."), and
longitudinal direction (represented by a ".DELTA.")
respectively.
[0149] FIG. 5 shows that the shear strength of core materials of
the invention differs depending on the relationship between the
applied-force direction and the strand direction. The most evident
difference is between shear strength in the end-strand direction
versus shear strength in the transverse direction. As shown in FIG.
5, the shear strength difference ranges from about 60% higher in
the end-strand over the transverse direction at lower densities to
about 130% higher in the end strand over the transverse direction
at higher densities.
[0150] FIG. 6 shows that the shear modulus of core materials of the
invention differs depending on the relationship between the
applied-force direction and the strand direction. The most evident
difference is between shear modulus in the end-strand direction
versus shear modulus in the transverse direction. As shown in FIG.
6, the shear modulus in the end-strand direction is roughly 100%
greater than that in the transverse direction over the density
range.
[0151] FIG. 7 shows that the shear elongation at break point of
core materials of the invention differs depending on the
relationship between the applied-force direction and the strand
direction. The most evident difference is between shear elongation
at break point in the end-strand direction versus shear elongation
at break point in the longitudinal direction. As shown in FIG. 7,
the shear-elongation-at-break-point in the end-strand direction is
roughly 170% greater than in the longitudinal direction over the
density range.
6.8.2 Tensile Properties
[0152] The tensile tests were conducted according to the procedure
set forth in ASTM C297-94 as discussed in Section 5.4.7 above. The
results are shown in FIGS. 8-9, which are respectively graphs of
tensile strength versus density (FIG. 8) and tensile modulus versus
density (FIG. 9). The tensile tests were conducted in the
end-strand direction (represented by a ".diamond-solid.") and the
transverse direction (represented by a ".box-solid.")
respectively.
[0153] FIG. 8 shows that the tensile strength of core materials of
the invention differs depending on the relationship between the
applied-force direction and the strand direction. As shown, the
tensile strength in the end-strand direction is roughly 900%
greater than that in the transverse direction over the density
range.
[0154] FIG. 9 further shows that the tensile modulus in the
end-strand direction is roughly 1100% greater than that in the
transverse direction over the density range.
[0155] 6.8.3 Compression Properties
[0156] The compression tests were conducted according to the
procedure set forth in ASTM C365-00 as discussed in Section 5.4.6
above. The results are shown in FIGS. 10-11, which are respectively
graphs of compression strength versus density (FIG. 10) and
compression modulus versus density (FIG. 11). The compression tests
were conducted in the end-strand direction (represented by a
".diamond-solid.") and the transverse direction (represented by a
".box-solid.").
[0157] FIG. 10 shows that the compression strength of core
materials of the invention differs depending on the relationship
between the applied-force direction and the strand direction. As
shown, the compression strength in the end-strand direction is
roughly 400% greater than that in the transverse direction over the
density range.
[0158] FIG. 11 further shows that the compression modulus in the
end-strand direction is roughly 50% greater than that in the
transverse direction over the density range, with a more pronounced
effect at higher densities.
[0159] In sum, this example demonstrates that the core materials of
the invention are significantly anisotropic in character.
6.9 Example 9
[0160] Improved Properties of Core Materials of the Invention Over
Standard Pet Foam Boards Produced by Extruding Foamable PET through
a Conventional, Rectangular Slot Die
[0161] This Example compares the shear properties of PET core
materials of the invention against those of standard PET foam
boards produced by extruding foamable PET through a conventional,
rectangular slot die ("conventional PET foam boards"). The shear
properties of the PET core materials of the invention were tested
in the longitudinal direction.
[0162] The conventional foam boards and core materials of the
invention used in this Example ranged in density from about 80
kg/m.sup.3 to about 215 kg/m.sup.3. Test sample of core materials
of the invention were prepared as in Example 8
(6".times.2.5".times.0.5"), and the conventional PET foam boards
(also 6".times.2.5".times.0.5") can be obtained from M&G
Polymers, Patrica, Italy. Both the conventional foam boards and the
core materials of the invention had an average cell size of about
0.1 mm and an open-cell content of about <10%. The shear tests
were conducted according to the procedure set forth in ASTM C-273
as discussed in Section 5.4.8 above.
[0163] The results are shown in FIGS. 12-14, which are respectively
graphs of shear strength versus density (FIG. 12); shear modulus
versus density (FIG. 13); and shear elongation at break versus
density (FIG. 14). The core materials of the invention are
represented by ".box-solid." and the conventional foam boards are
represented by ".diamond-solid.".
[0164] These graphs show: (1) that the shear strength of the PET
core materials of the invention are about 50% higher than that of
conventional foam boards over the entire density range; (2) the
core materials of the invention have a higher shear modulus by
about 25%-85% over that of the conventional foam boards over the
density range; and (3) the shear elongation at break of core
materials of the invention is about 100%-200% higher than that of
the conventional foam boards over the density range. This Example
illustrates that the core materials of the invention are far
superior to conventional foam boards comprising the same
composition.
[0165] Although the present invention has been described in
considerable detail with reference to certain preferred embodiments
and versions, other versions and embodiments are possible.
Therefore, the spirit and scope of the appended claims should not
be limited to the description of the versions and embodiments
expressly disclosed herein. The references discussed in Background
Section 2 are not admitted to be prior art with respect to the
invention.
* * * * *