U.S. patent application number 11/397773 was filed with the patent office on 2006-11-02 for method of producing a thermoplastically moldable fiber-reinforced semifinished product.
This patent application is currently assigned to Quadrant Plastic Composites AG. Invention is credited to Karl-Ludwig Brentrup, Harri Dittmar.
Application Number | 20060244170 11/397773 |
Document ID | / |
Family ID | 36498908 |
Filed Date | 2006-11-02 |
United States Patent
Application |
20060244170 |
Kind Code |
A1 |
Brentrup; Karl-Ludwig ; et
al. |
November 2, 2006 |
Method of producing a thermoplastically moldable fiber-reinforced
semifinished product
Abstract
A continuous method for producing a thermoplastically moldable
semifinished product of a thermoplastic material and reinforcing
fibers, comprises blending thermoplastic fibers and reinforcing
fibers together to form a nonwoven blend, consolidating the
nonwoven blend by needling or by a thermal treatment, heating the
consolidated nonwoven blend to a temperature above the softening
temperature of the thermoplastic, compressing the consolidated
nonwoven blend successively in a heated compression mold and in a
cooled compression mold at a pressure of less than 0.8 bar for at
least 3 seconds, and optionally applying functional layers to the
semifinished product. The preferred product is a thermoplastically
moldable semifinished product of a thermoplastic material and
reinforcing fibers with an average length of 20 to 60 mm and an air
pore content of 35 to 65 vol %.
Inventors: |
Brentrup; Karl-Ludwig;
(Moeriken, CH) ; Dittmar; Harri; (Battenberg,
DE) |
Correspondence
Address: |
BROOKS KUSHMAN P.C.
1000 TOWN CENTER
TWENTY-SECOND FLOOR
SOUTHFIELD
MI
48075
US
|
Assignee: |
Quadrant Plastic Composites
AG
Hardstrasse 5CH- 5600
Lenzburg
CH
|
Family ID: |
36498908 |
Appl. No.: |
11/397773 |
Filed: |
April 4, 2006 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10472530 |
Oct 24, 2003 |
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11397773 |
Apr 4, 2006 |
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10470969 |
Dec 4, 2003 |
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11397773 |
Apr 4, 2006 |
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Current U.S.
Class: |
264/122 ;
156/148; 156/62.2 |
Current CPC
Class: |
D04H 1/54 20130101; D04H
1/48 20130101; D04H 1/4218 20130101; B29B 15/12 20130101; D04H
1/544 20130101; B29C 70/506 20130101; D04H 1/485 20130101 |
Class at
Publication: |
264/122 ;
156/148; 156/062.2 |
International
Class: |
B32B 17/00 20060101
B32B017/00 |
Foreign Application Data
Date |
Code |
Application Number |
Apr 5, 2005 |
EP |
05 007 391.5 |
May 4, 2005 |
EP |
05 009 770.8 |
Claims
1. A continuous process for the preparation of a porous
thermoformable semifinished product having a porosity of from 25
volume percent to 75 volume percent from thermoplastic fibers and
reinforcing fibers, comprising the steps of: a) blending from 10 to
80 weight percent of thermoplastic fibers with 90 to 20 weight
percent of reinforcing fibers, said reinforcing fibers supplied as
individual fibers or as multi-fiber strands which are opened into
individual fibers during blending, to form a lofty, continuous
non-woven mat; b) consolidating the lofty non-woven mat of step a)
by needling, or by thermal treatment which melts the thermoplastic
fibers only partially, to form a porous and flexible but handleable
consolidated mat; c) heating the consolidated mat to a temperature
above the melting point of the thermoplastic fibers; d) compressing
the consolidated mat while still hot in a heated double band press
at a pressure of 0.8 bar or less for a period of at least 3 seconds
to form a densified but still porous product containing reinforcing
fibers and molten thermoplastic; and e) cooling said densified
porous product at a pressure of 0.8 bar or less in a cool zone of a
double band press maintained at a temperature below the melt
temperature of the thermoplastic fibers, for a time period
sufficient to allow the thermoplastic to solidify and form a porous
thermoformable intermediate product.
2. The process of claim 1, wherein prior to or during step d), at
least one functional layer is compressed onto the heated
consolidated mat.
3. The process of claim 2, wherein prior to the heated zone of the
double band press is positioned a heated roll pair which force the
functional layer(s) against the heated consolidated mat.
4. The process of claim 3, wherein the heated roll pair exert a
line pressure of from 10 to 50 N/mm.
5. The process of claim 2 wherein at least one functional layer is
a molten film of a compatible thermoplastic.
6. The process of claim 1, wherein the average length of the
thermoplastic fibers and the average length of the reinforcing
fibers differ by no more than 25% prior to needling.
7. The process of claim 1, wherein the thermoplastic fibers and
reinforcing fibers are air blended, deposited on a moving belt, and
carded.
8. The process of claim 1, wherein the reinforcing fibers are
supplied to the process as multi-fiber strands.
9. The process of claim 1, wherein the average fiber length of the
thermoplastic fibers and the reinforcing fibers lies within the
range of 20 mm to 120 mm prior to needling.
10. The process of claim 1, wherein the porosity of the porous
thermoformable intermediate product is uniformly distributed.
11. The process of claim 1, wherein the heated zone of the double
band press is maintained at a temperature greater than 80.degree.
C. and the cool zone of the double band press is maintained at a
temperature of less than 30.degree. C.
12. The process of claim 1, wherein the pressure during step d) is
between 0.05 bar to 0.5 bar.
13. The process of claim 1, wherein a dwell time in the heated zone
of the double band press is from 5 to 60 seconds.
14. The process of claim 1, wherein within the double band press
following the cool zone is a cooled roll pair which exert pressure
on the porous, thermoformable intermediate product, the line
pressure between the cooled roll pair being from 10 to 50 N/mm.
15. The process of claim 1 wherein the amounts of thermoplastic
fibers and reinforcing fibers are such that the porous,
thermoformable intermediate product has a thermoplastic content of
between 20 to 65 weight percent, a reinforcing fiber content
between 80 and 35 weight percent, the thermoplastic fibers and
reinforcing fibers each having an average length of from 20 mm to
60 mm, the porous, thermoformable intermediate product having a
content of from 35% to 65% by volume of uniformly distributed
porosity, and having an areal weight of between 250 to 1800
g/m.sup.2.
16. The process of claim 15, wherein said reinforcing fiber
comprises basalt fibers.
17. The process of claim 16, wherein basalt fibers and natural
fibers are present as reinforcing fibers, in a weight ratio of
10:90 to 50:50.
18. The process of claim 1, wherein the lofty, continuous non-woven
mat has a void content V of from 85 volume percent to greater than
99 volume percent, and the flexible but handleable consolidated mat
is consolidated by at least one needle punching consolidation step
and has a volume percent porosity of from 0.85V to 0.95V, and the
porous thermoformable intermediate product has a void content of
0.25V to 0.8V and minimally 25 volume percent porosity.
19. The process of claim 1, further comprising f) allowing a
compressed product formed in step (e) to re-expand to a greater
thickness while at a temperature at or higher than the melting
temperature of the thermoplastic and cooling under contact but at
low or no pressure such that the final thickness of the fully
cooled intermediate product is less than that in the initial
portion of the cool zone.
20. The process of claim 19, wherein said step of re-expanding is
accomplished outside said double band press by reheating the porous
thermoformable intermediate product.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a continuation-in-part of U.S.
application Ser. Nos. 10/472,530, filed Oct. 24, 2003; U.S. Ser.
No. 10/470,969, filed Dec. 4, 2003; European Patent Application No.
EP 05 007 391.5 filed Apr. 5, 2005; and European Patent Application
No. EP 05 009 770.8 filed May 4, 2005, priority to all of which are
hereby claimed.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] This invention relates to a method for producing a
thermoplastically moldable fiber-reinforced semifinished product
from a mixed nonwoven containing thermoplastic fibers and
reinforcing fibers.
[0004] 2. Background Art
[0005] Thermoplastically moldable semifinished products containing
reinforcing fibers, in particular glass fibers, are being used to
an increasing extent for the production of moldings, in particular
for automotive parts. Such "plastic panels" are characterized by
high strength and toughness. GMT semifinished products are
manufactured on a large scale industrially by combining continuous
glass fiber strand mat and molten thermoplastic films in a double
band press. This procedure consumes a substantial amount of energy,
because the viscous melt must be pressed into the mat at pressures
far above 1 bar. It is thus exceptionally difficult, in practice,
to achieve a fiber content greater than 45 wt % and an areal weight
below 2000 g/m.sup.2 by this method. Since the reinforcing fibers
in the reinforcing fibers in the glass mats are generally in the
form of fiber bundles or "strands", impregnation with thermoplastic
is never entirely complete and uniform, and therefore
microscopically heterogeneous regions are present, thus resulting
in a high standard deviation in the mechanical properties. This is
also the case with thermally expanded GMT, which, due to the
restoring forces of the glass fibers needled together, contains air
pores which are irregularly distributed within the matrix.
[0006] German Patent Application DE-A 36 14 533 describes a method
for producing molded articles of thermoplastics which contain a
reinforcing insert. Based on textile fiber technology, a nonwoven
blend of thermoplastic fibers and reinforcing fibers is produced by
carding or air-laying methods and is consolidated, for example, by
needling. Cut sections of this nonwoven blend are heated and
pressed directly to form three-dimensional molded articles without
prior consolidation into a semi-finished product. Complete
impregnation is very difficult to obtain, especially with
components having a complex shape, so that the mechanical
properties of the moldings leave much to be desired.
[0007] According to WO 98/3508, in a complex method, blended
strands of reinforcing fibers and thermoplastic fibers are first
produced and then a nonwoven is produced from them. This nonwoven
is pressed on a double band press at high temperature and high
pressure to form a semifinished product. Production of mixed
strands of reinforcing fibers and thermoplastic fibers is difficult
due to the differing tensile elongations and modulus of the
different fibers, and only a limited selection of blends is
commercially available.
[0008] WO 02/062563 describes a continuous method for producing
thermoplastically moldable thin semifinished products of a
thermoplastic and long reinforcing fibers by dry blending
thermoplastic fibers and reinforcing fibers to form a nonwoven
blend; consolidating the fiber blend by needling; heating the
consolidated nonwoven blend; compressing, employing a calender or a
pair of pinch rollers to form a semifinished product, and
optionally, laminating a functional layer thereto. Sheet products
produced in this manner are dense and have surface irregularities
such as waviness.
[0009] WO 02/076711 describes a method similar to that of WO
02/062563 for producing thick nonwoven blends, wherein the step of
compressing can also be performed by a laminating device at
pressures between 1 bar and 10 bar. However, it has been found that
at such high pressures the air pores are forced almost completely
out of the softened nonwoven blend and the melt flows apart in
length and width, resulting in an uncontrolled variation in areal
weight and in distortion of the semifinished product, with the
result that the boundaries of the semifinished product are wavy
rather than smooth and straight.
[0010] Similar problems arise in the methods according to EP-B 593
716 and U.S. Pat. No. 4,978,489 in which the nonwoven blend is
compression molded by pressure rollers facing each other. In this
process, the mat is compressed so strongly that the resulting
semifinished product contains maximally only 20 vol %, and
preferably 10 to 15 vol % of air pores. In the method according to
U.S. Pat. No. 4,948,661 a mixed nonwoven is compressed between
heated plattens or a double band press until the air is completely
eliminated from the consolidated product.
[0011] The compression of mixed nonwovens in calenders or by
pressure rollers has the further disadvantage that only low
production speeds can be used and that a bulge is formed by the
abrupt compression in the gap between rollers, which may result in
strong distortion und even the formation of holes.
[0012] WO 03/086725 describes an apparatus and a process for making
fiber-reinforced composites involving molding a mixed nonwoven mat
in a continuous compression belt at relatively high pressure, up to
30 bar. "Pseudo-foamed composite sheets" are said to result from
this process, but the air pore content is necessarily very low, and
the high pressure will cause distortion of the non-woven and
non-homogenous pore distribution. The sheet materials have a very
high thermoplastic content, and thus strength and modulus are
relatively low.
[0013] Published German Application DE-A 195 20 477 discloses a
fiber reinforced sheet or panel that is thermally expanded and thus
contains air pores. As discussed further below, these air pores are
very irregularly distributed in the panel, whereas the air pores in
the semifinished product of the present invention are uniformly
distributed. The difference can easily be recognized in SEM
pictures. Example 1, shows an expanded panel whose weight per unit
of area is much greater than 2000 g/m.sup.2. The length of the
glass fibers is 100 mm, and the content of glass fibers is 30 wt
%.
[0014] Published German EP-A-758577 discloses a stampable sheet
prepared by a papermaking process wherein reinforcing fibers,
thermoplastic fibers, and optionally non-fibrous thermoplastic
particles are deposited on a foraminous screen from dispersion in
water, dried, heated above the thermoplastic melt temperature, and
then pressed in a cold press to form a dense stampable sheet. This
sheet is then heated agin and allowed to freely expand by a factor
of 1.1 to 15. A high pressure, for instance 5 kgf/cm.sup.2 (about 5
bar) in Example 1 of the publication, is used to produce the dense
intermediate.
SUMMARY OF THE INVENTION
[0015] Therefore, an object of this invention is to provide a
simple continuous method of producing a distortion-free
semifinished product of thermoplastic material and reinforcing
fibers which contains air pores with a uniform distribution, and
which can be readily reshaped by thermoforming to provide finished
parts having excellent and highly reproducible properties in all
directions. This and other objects are achieved by the inventive
method, wherein individual thermoplastic fibers and individual
reinforcing fibers are blended to form a mat of blended fibers, the
mat is consolidated, preferably by needling, and is compressed
twice, first in a heated compression mold, followed immediately by
a cooled compression mold, in both cases at a pressure of less than
0.8 bar, to form an intermediate product which contains a uniform
distribution of reinforcing fibers, thermoplastic, and air pores,
the latter exceeding 25% by volume.
BRIEF DESCRIPTION OF THE DRAWING
[0016] FIG. 1 is an SEM of an intermediate product of the present
invention, showing a uniform porosity.
[0017] FIG. 2 is an SEM of an intermediate product of the present
invention, at higher magnification.
[0018] FIG. 3 is an SEM of a prior art intermediate product
illustrating the non-uniform porosity.
[0019] FIG. 4 is a higher magnification of the product used in FIG.
3.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S)
[0020] The process steps may be described in greater detail as
follows:
[0021] Thermoplastic fibers and individual, nonbonded reinforcing
fibers are blended together. Suitable thermoplastics include all
spinnable thermoplastics, e.g., polyolefins such as polyethylene
and polypropylene, polyamides, linear polyesters, thermoplastic
polyurethanes, polycarbonates, polyacetals, and the corresponding
copolymers and thermoplastic blends, as well as polymers having
high thermal stability, such as polyarylates, polysulfones,
polyimides, polyetherimides and polyether ketones. Particularly
preferred is polypropylene with an MFI (230.degree. C., 2.16 kp)
according to DIN 53735 greater than 20 g/10 min, in particular
between 25 and 150 g/10 Min. The thermoplastic fibers generally
have an average length (weight average) of 20 mm to 100 mm.
[0022] Preferred reinforcing fibers are glass fibers, but carbon
fibers, basalt fibers and Aramid fibers may also be used.
Furthermore, natural fibers, e.g., those made of flax, jute, hemp,
kenaf, sisal and cotton are also useful. Of special interest are
basalt fibers which, in contrast to glass fibers do not melt and
form a slag when fiber-reinforced finished parts are thermally
processed. The relatively expensive basalt fibers are preferably
mixed with natural fibers in a weight ratio of 10:90 to 50:50. In
general, the reinforcing fibers have an average length (weight
average) of 20 mm to 100 mm. In order to be readily blendable with
thermoplastic fibers, they must be substantially in the form of
individual nonbonded fibers, i.e., they must not remain bonded
together with polymer binders.
[0023] The reinforcing fibers may be supplied as precut or "staple"
fibers, or may be cut to length shortly prior to the blending
operation. In general, fibers, whether precut or cut just prior to
use, are in the form of multifilament strands. These strands must
be capable of substantial individualizing of fibers. Of course,
some fibers will generally be present in strands or partial
strands, but the substantial majority of fibers will be present in
individual form. To achieve this end, it is preferable that
polymeric binders and the like be absent, or present in minimal
amounts such that the strands may be "opened" by conventional
textile equipment. Carding, for example, is highly efficient in
opening strands of fibers. The fibers may be dry or may be in the
form of precut and only partially dried fibers.
[0024] In a preferred process, the thermoplastic and reinforcing
fibers are supplied in the form of multi-fiber strands, are blended
in an air stream, and deposited on a moving belt. The fibers, which
at this stage are in the form of strands, partially opened strands,
and fibers, are subjected to one or more carding operations.
Following carding, the number of unopened and partially opened
strands is low, and the mat appears to be relatively homogenous.
Following needling, a very homogenous appearance is achieved, with
virtually no strands observable to the eye. The mat product is
lofty and contains in excess of 75% air pores, generally greater
than 90%. A thickness or "loft" of 2.5 cm to 15 cm prior to
needling is typical, depending on the desired areal weight of the
final product.
[0025] In a preferred embodiment of the invention, the average
lengths of the thermoplastic fibers and of the reinforcing fibers
differ by maximally 25%, preferably by maximally 10% and in
particular, by maximally 3%. The preferred glass fibers are
commercially available as endless fibers or as cut fibers with
lengths of 0.5 inch (12.7 mm). In addition, cut fibers with a
length of e.g. 1 inch (25.4 mm) or 2 inches (50.8 mm) are also
available. In practice, the thermoplastic fibers are cut to
approximately the same length as the glass fibers; i.e. in a
particularly preferred embodiment of the invention both
polypropylene fibers and glass fibers which have approximately the
same length and which have an average length (weight average) in
the range of 25 mm to 55 mm are employed.
[0026] This matching of fiber lengths has the advantage that during
the production of the semifinished product, demixing of the fibers,
which would result in inhomogeneities in the semifinished product,
i.e. in glass-rich and polymer-rich domains, surprisingly does not
occur. This is especially important if the fibers are blended by
the airlay process.
[0027] The thermoplastic fibers and reinforcing fibers are used in
a weight ratio of 10:90 to 80:20, preferably 20:80 to 65:35, and in
particular 25:75 to 55:45. In the blending process the glass fibers
should be relatively dry which means that their water content
should be less than 6 wt. % preferably 0.5 to 4 wt. %. Blending is
preferably performed according to the airlay or carding processes
which are well-known in the textile technology. It has been
surprisingly discovered that the carding process is relatively
insensitive to water content, and thus the fibers can contain up to
15 wt. % of water. Blending results in a non-woven continuous mat
preferably having an areal weight of 200 to 2500 g/m.sup.2, more
preferably from 250 to 1500 g/m.sup.2. Upon mixing the fibers,
glass fiber bundles are opened to a large extent or completely so
that most or all of the glass the fibers are present as individual
filaments.
[0028] The nonwoven blend thus obtained is then consolidated,
preferably by needling on one or both sides. This may be
accomplished with felting needles on conventional needling looms.
Needling causes some breakage of the reinforcing fibers, so that
the average fiber length is reduced; on the other hand, needling
consolidates the nonwoven blend, so that it can be handled without
problems in subsequent steps of the process. It is also possible,
in principle, to perform the consolidation by thermal means, e.g.,
by IR irradiation or by means of hot air. However, in this case,
the thermoplastic fibers should not melt completely, but rather
should melt only superficially, to the extent that a semifinished
product will have sufficient cohesion for ease of handling and
transportability.
[0029] The needling operation causes a considerable decrease in the
loft of the mat. The loft may decrease by from 25% to 90% of the
loft of the material following carding, for example. However, the
product at this stage still generally contains a very high amount
of porosity. Only during the heated and cooled compression steps
described below is the porosity reduced to the desired final
product specification. For example, a blend of polypropylene fibers
and glass fibers formed into a web of ca. 1000 g/m.sup.2 by the air
lay process will have a thickness of about 100 mm, of which >99%
will be porosity. Needling reduces this thickness to about 15 mm,
however the product is still very porous, with a porosity of 93% or
more. Consolidation as described herein to a thermoformable
intermediate product of 2 mm thickness results in a boardy product
still having about 50% air voids. Upon full consolidation by
thermoforming, the air void content of the 1 mm thick product is
reduced to less than 5%. This is considered "full density," as in
most cases, complete elimination of porosity cannot be
achieved.
[0030] Thus, the subject invention is also directed to a process
for the stepwise consolidation of a composite material of
reinforcing fibers and thermoplastic fibers, comprising forming a
blended, lofty web having a void content, V, of preferably from 85%
to >99%, more preferably 90 to 99%, consolidating this web in
one or more thickness reducing operations, at least one of which is
a needle punching operation, until the void content is reduced to
between 0.85V to 0.95V, and further consolidating by heating the
web to a temperature greater than the melting point of the
thermoplastic fibers and compressing in a double band press as
described herein until the porosity of the cooled intermediate
product is in the range of 0.25 V to 0.8V, preferably 0.3V to 0.6V,
and minimally about 25% porosity.
[0031] The consolidated nonwoven blend is then heated, preferably
in a continuous oven or by IR irradiation, to temperatures above
the softening temperature of the thermoplastic. The temperature
should preferably be 20.degree. C. to 60.degree. C. above the
softening temperature. In the case of polypropylene fibers, the
temperature is preferably between 180.degree. C. and 220.degree.
C., in particular between 190.degree. C. and 210.degree. C. The
thermoplastic melt can easily "impregnate" the individual
filaments, and one obtains a substantially better and more uniform
impregnation of the glass fibers than with the above mentioned GMT
method, where the majority of glass fiber bundles are not opened
into individual filaments. This results, for example, in poor
homogeneity of the properties of finished products produced from
GMT semifinished products. At this stage in the inventive process,
the fibers are not surrounded by resin. Rather, the thermoplastic
adheres irregularly to the reinforcing fibers, often in the form of
small globules. The distribution of thermoplastic, however, is
substantially uniform.
[0032] Immediately after heating, the heated nonwoven is
compressed. A heated compression mold and a cooled compression mold
are employed in succession. The nonwoven blend is compressed at a
pressure of less than 0.8 bar, preferably 0.05 to 0.5 bar, for at
least 3 seconds. In the heated compression mold the dwell time is
preferably between 5 and 60 seconds, whereas in the cooled
compression mold it can last for more than several minutes. The
upper limit of 0.8 bar is critical; at higher pressures, the air
pore content becomes too low, and extensibility may become a
problem. Pressures, whether areal pressure or line pressure, are
measured by standard techniques. Generally speaking, the
manufacturer of the press supplies instructions for measuring or
calculating pressure, including in most cases, gauges for this
purpose. Preferably, pressure plates are used on which two
revolving fabric belts, e.g., made of teflon-coated glass or Aramid
fabric, slide along the pressure plates and thereby entrain the
nonwoven blend. The heated compression mold is preferably heated to
more than 80.degree. C., in particular to 100.degree. C. to
220.degree. C., and the cooled compression mold is preferably kept
at a temperature of less than 50.degree. C., more preferably less
than 30.degree. C., and in particular at 15.degree. C. to
25.degree. C. At higher temperatures of the cooled rolls, the dwell
time must be lengthened, as the thermoplastic matrix material must
be a solid when exiting this cool zone. Note that if not
purposefully cooled, the temperature in this second zone of the
press will increase to a relatively high level due to contact with
the heated product.
[0033] If the composite is not cooled under pressure, the higher
modulus of the reinforcing fibers will, in conjunction with the
molten or hot thermoplastic, cause severe and unpredictable lofting
of the composite sheet. Such lofted sheets contain too many air
voids, and are difficult to thermoform. In particular, heating
prior to thermoforming is difficult. Likewise, if the cooling
pressure is too high, a product with low stiffness, and which is
difficult to handle, will be produced. Too much pressure during
cooling will also damage pressure sensitive surface layers.
[0034] In a further embodiment, the cooled intermediate product
produced as described herein is reheated uniformly to the melt
temperature of the thermoplastic matrix or above, while being under
little or no pressure, and allowing the product to expand uniformly
in the thickness direction to produce a thicker and correspondingly
less dense product which may be thermoformed to finished products
which also have low density. The re-expansion of the web to form
this lower density intermediate product may take place after full
consolidation in the double band press, i.e. as a separate step, or
the expansion may be allowed in the cooling zone of the press by
lowering the pressure in that section or establishing essentially
pressureless contact. Such materials cannot be made by allowing the
hot consolidated product from the heated zone of the press to exit
the press and rise unconstrained, since expansion under these
conditions will produce an irregular and commercially unacceptable
product. The increase in thickness must be limited by contact,
either in a double band press or other type of press. For example,
cut sheets of dense intermediate product with an air void content
of 40% and a thickness of 2 mm may be heated within a press having
a 3 mm gap. Upon expansion to 3 mm, further expansion is
restrained. The product is preferably cooled in this condition,
which is why the use of a double band press is preferred. If a
platten press or the like is used, it is preferable to heat the
dense intermediate product so that the minimal thickness expansion,
e.g. in local areas, is about the thickness desired in the final
product or slightly more, and then the cool press is closed,
exerting only so much pressure that the desired final thickness is
achieved and thickness variations are eliminated.
[0035] During the compression step of the preferred process, a
heated roll pair is preferably situated upstream from the heated
compression mold and a cooled roll pair is preferably situated
downstream from the heated compression mold. These roll pairs are
preferably located within the double band press, i.e. within the
continuous bands.
[0036] The heated roll pair serves mainly to supply functional
layers and to apply them to the heated nonwoven blend. A low lineal
or "line" pressure of less than 10 N/mm is sufficient for this
purpose. The compression mold, which is preferably heated to
150.degree. C. to 200.degree. C., causes the reinforcing fibers to
be pressed into the thermoplastic melt and to be wetted to a
sufficient extent, and it also causes some of the air to be forced
out. The cooled roll pair presses the functional layers tightly
onto the partially consolidated semifinished product with a line
pressure of preferably 10 to 50 N/mm, so that the functional
layer(s) are bonded thereto. Furthermore, if desirable, the cooled
roll pairs can be set to cause a further reduction in thickness.
The thermoplastic melt is completely solidified by the cooled
compression mold so that restoring forces can no longer act, any
functional layers are firmly fused to the product, and the
semifinished product is thus consolidated.
[0037] The compression operation is performed under such gentle
conditions that the resulting semifinished product still has an air
pore ("void") content between 25 and 75 vol %, in particular 35 to
65 vol %. Due to this fact, in contrast with compact or almost
compact semifinished products, this product can be processed more
easily, e.g., by thermoforming in compression molds. Due to the
above mentioned uniform impregnation of the glass fibers with the
thermoplastic matrix, the air pores are homogeneously distributed
in the semifinished product. This is in the contrast to expanded
GMT, which has an irregular distribution of air pores, and contains
unopened glass fiber bundles and matrix agglomerates.
[0038] If necessary, functional layers are brought into contact
with one or both sides of the heated nonwoven blend simultaneously
with the compression operation and are jointly compressed. These
additional layers may be decorative layers, thin fiber nonwovens,
thermoplastic films or fabric sheeting, for example, and may be
supplied for aesthetic and/or structural purposes. Further examples
include carpeting, woven or non-woven scrim, fabrics, metal foils,
metallized plastic films, wood veneer, composite films, etc. The
preferred films are compatible thermoplastic films. By "compatible"
is means that the thermoplastic is of the same type as the
thermoplastic of the thermoplastic fibers, or can form a strong
fusion bond with the thermoplastic of the thermoplastic fibers.
[0039] The resulting flat semifinished product preferably has a
thickness of 0.5 mm to 10 mm, in particular from 1.0 to 5.0 mm. For
special applications, the thickness may also amount to more than 10
mm. The average length (weight average) of the reinforcing fibers
in the semifinished product is 15 mm to 100 mm, preferably 20 mm to
60 mm, and in particular 25 mm to 50 mm.
[0040] A further object of this invention is to provide a
thermoplastically moldable semifinished product of 25 to 55 wt % of
a thermoplastic material and 75 to 45 wt % reinforcing fibers with
an average length (weight average) of 20 mm to 50 mm, containing 35
to 65 vol %, preferably 45 to 55 vol % air pores with uniform
distribution. The semifinished product preferably has a weight per
unit of area of 250 to 1800 g/m.sup.2 and contains 25 wt % to 55 wt
% polypropylene and accordingly 75 wt % to 45 wt % glass fibers.
The glass fibers preferably have an average length (weight average)
of 25 mm to 50 mm, and the fibers of the non-woven are consolidated
by needling together. In another preferred embodiment, the
semi-finished product contains 35 to 80 weight percent of glass
fibers, the latter present predominately, preferably by more than
80%, and in particular more than 90%, as individual filaments.
EXAMPLE 1
[0041] Staple fibers of PP having a melt flow index (230.degree.
C., 2.16 kg) of 25 g/10 min and a length of about 40 mm are mixed
together with chopped glass fibers having a length of 50.8 mm and a
water content of about 1%. Mixing is carried out in a blending unit
before providing the fibers to a continuous airlay process for
further mixing, and the resulting continuous nonwoven fleece,
having an areal weight of 1200 g/m.sup.2, is needled from one side
on a conventional needle loom. The thus preconsolidated fleece is
heated in an air flow oven to about 190.degree. C. to melt the PP
and thereafter immediately conveyed to a heated double belt
laminator. There it is compressed at a pressure of 0.5 bar for
about 15 sec. The laminator temperature is about 150.degree. C., to
maintain the core of the fleece above the softening point of the PP
and to enable it to penetrate the glass fibers homogeneously. On
the other hand, due to the relatively low pressure, the
three-dimensional randomly oriented glass fibers partially resists
the pressure and keeps a certain portion of air voids within the
fleece. Subsequently the fleece is introduced into a cold double
belt laminator held at 20.degree. C., where it is compressed at a
pressure of 0.2 bar to 90 seconds to solidify the PP. The resulting
semifinished product is cut to blanks having a size of 2 m by 2 m.
Their thickness is 2.3 mm, the average glass fiber length is
slightly less than 50 mm. The thickness in case of a full
consolidation would be 1.0 mm. Consequently the calculated voids
content is 55 vol. %. The homogenous voids distribution and the
high degree of glass fiber filamentation can be seen by the SEM
picture of FIG. 1. FIG. 2 is an enlarged view (200.times.) of the
intermediate product. The high percentage of individual fibers
(>90%) is noteworthy. The blanks show a high stiffness, no
distortion and can easily be handled, e.g. by robotic handling
equipment.
COMPARATIVE EXAMPLE 2
[0042] Example 1 is repeated, except that compression is carried
out in the heated double belt laminator at a pressure of 3 bar for
40 seconds and in the cooled double belt laminator at 2 bar for 100
seconds. The resulting semifinished product has a thickness of only
1.1 mm. A fully consolidated sheet would have a thickness of 1.0
mm. Accordingly, the calculated voids content is only 9 vol. %. Due
to the high pressure neither thin films nor scrims can be applied
without damage by the glass fibers. The thin large sheets can only
be handled with difficulty due to their low structural
stiffness.
COMPARATIVE EXAMPLE 3
[0043] The same non-woven fleece (1200 g/m.sup.2, 40% GF, 60%
PP-Fibers) as in Example 1 is cut into 250 mm wide rolls to adjust
the width to a laboratory calendering machine, having a drum
dimension of 300.times.200 mm (width.times.diameter). The fleece is
heated to 180.degree. C. by passing through an air flow oven and
then introduced into the calender nip. The 2 rollers of the
calender are adjusted to 40.degree. C. surface temperature, a
linear load of 300 N/mm and a speed of 5 m/min. The compressed
fleece shows an elongation of 100%, caused by the high pressure,
and cannot be used due to extreme shape distortion and fiber
orientation.
[0044] In a further trial, the linear load is reduced to 50 N/mm
and the line speed to a very low 2 m/min. The heated (180.degree.
C.) fleece is compressed in the calender nip but is still useless
due to high distortion and inner stress. Surface layers like
PP-films of 150 micron thickness and scrims of 30 g/m.sup.2 could
be co-calendered, with good adhesion, but are useless due to the
previously mentioned distortion and orientation of the reinforcing
fibers.
[0045] By varying temperature, pressure and speed further tests are
carried out, but finally no internal stress free, flat and
sufficiently cooled blanks could be produced, setting the calender
to efficient line speeds of more than 5 m/min.
COMPARATIVE EXAMPLE 4
[0046] As in Example 1 of DE-A-19520477, two needled glass fiber
mats which together presented an areal weight of 1240 g/m.sup.2 and
a glass fiber length of 100 mm are impregnated with molten
polypropylene of areal weight 2860 g/m.sup.2 in the heated zone of
a double band press at a pressure of 3 bar. In the following
cooling zone, the laminate contacts the belt in a pressureless
manner, whereby the impregnated mat expands due to the continued
presence of molten polypropylene. Upon cooling to under 110.degree.
C., a porous, expanded intermediate product is obtained, having a
glass fiber content of 30 weight percent, a density of 0.6
g/cm.sup.3, and a porosity of 50 vol. percent. The areal weight was
4100 g/m.sup.2, and is too high for most automotive applications.
FIG. 3 is an SEM (32.times.) of the product, while FIG. 4 is a
further SEM (200.times.).
[0047] The great differences between low density intermediate
products produced by "lofting" GMT by heating and allowing the mat
to expand, as compared with an intermediate product of the subject
invention is demonstrated by the Figures. In FIGS. 3 and 4, a GMT
intermediate containing about 30 weight percent glass fibers and a
ratio of unconsolidated thickness to fully consolidated thickness
of 1.5:1 prepared in Comparative Example 4, has large areas with
virtually no porosity and other areas which are completely pore
dominated. This structure is typical of products consolidated at
high pressure. FIGS. 1 and 2, on the other hand, illustrate a
product of the subject invention with substantially the same glass
fiber content and the same loft. The porosity is uniformly
distributed.
[0048] The semifinished product produced according to the present
invention may be rolled up and stored or cut immediately into
sheets, e.g., with dimensions of 400 mm to 3000 mm.times.300 mm to
2300 mm. It may be processed thermoplastically to form
three-dimensional finished parts. To do so, appropriate cut
sections are first heated to temperatures above the softening
temperature of the thermoplastic and then are reshaped. In doing
so, the semifinished product expands due to the restoring forces of
the needled fiber nonwoven; the more it expands, the greater is the
air pore content. For example, a semifinished product that has
expanded to more than twice its original thickness, and preferably
more than three times its original thickness, can be reshaped more
easily during thermoforming than a compact sheet. In reshaping, the
semifinished product is compressed by means of the usual two-part
molds or is shaped by deep drawing.
[0049] The finished parts can be used in the transportation sector
as automotive, railway and aircraft parts, as vehicle body parts,
or as large area panels and furniture parts. In addition, they may
be used as cover layers in sandwich laminates for shell elements or
partitions.
[0050] While embodiments of the invention have been illustrated and
described, it is not intended that these embodiments illustrate and
describe all possible forms of the invention. Rather, the words
used in the specification are words of description rather than
limitation, and it is understood that various changes may be made
without departing from the spirit and scope of the invention.
* * * * *