U.S. patent number 3,800,008 [Application Number 05/169,333] was granted by the patent office on 1974-03-26 for oriented polymer strap.
This patent grant is currently assigned to E. I. du Pont de Nemours and Company. Invention is credited to Howard W. Starkweather, Jr..
United States Patent |
3,800,008 |
Starkweather, Jr. |
March 26, 1974 |
ORIENTED POLYMER STRAP
Abstract
Manufacture of strapping from resin at least 75 percent of which
is polyethylene having a density of at least 0.95 g/cc and
preferably from 0.955 to 0.967 g/cc and a melt index of from 0.2 to
1.0 and preferably from 0.4 to 0.9 by extruding a billet, roll
orienting the billet, followed by stretching at ambient
temperature, followed by stretching in a liquid bath maintained at
from 93.degree.C. to the melting point of the polyethylene and
preferably from 110.degree. to 130.degree.C. to give a total
deformation of from 10 to 18 times and preferably 11 to 13.5
times.
Inventors: |
Starkweather, Jr.; Howard W.
(Wilmington, DE) |
Assignee: |
E. I. du Pont de Nemours and
Company (Wilminton, DE)
|
Family
ID: |
22615231 |
Appl.
No.: |
05/169,333 |
Filed: |
August 5, 1971 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
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845692 |
Jul 29, 1969 |
3651196 |
|
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Current U.S.
Class: |
525/222;
526/352.2; 264/280; 528/502B; 264/178R; 264/210.7; 264/289.6 |
Current CPC
Class: |
C08L
23/06 (20130101); C08L 23/06 (20130101); B29C
55/06 (20130101); B29C 48/001 (20190201); C08L
2666/06 (20130101); B29C 48/0018 (20190201); B29C
48/08 (20190201); C08L 23/0853 (20130101); B29K
2023/065 (20130101) |
Current International
Class: |
C08L
23/00 (20060101); C08L 23/02 (20060101); C08f
029/12 () |
Field of
Search: |
;260/897
;264/210,288 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Primary Examiner: Tillman; Murray
Assistant Examiner: Seccuro; C. J.
Parent Case Text
CROSS-REFERENCE TO RELATED APPLICATION
This application is a divisional of copending U.S. Pat. application
Ser. No. 845,692, filed July 29, 1969 by Howard W. Starkweather, Jr
now U.S. Pat. 3,651,196.
Claims
I claim:
1. A uniformly oriented strap formed of at least 75 weight percent
polyethylene having a density of from 0.95 to 0.967 g per cc, a
melt index of from 0.2 to 1.0, said strap having a major melt peak
as measured by differential thermal analysis at a rate of
10.degree.C. per minute of 140.degree. to 145.degree.C., and an
angular width at the one-half maximum as determined by X-ray
corresponding to the tilting of the polymer chains from the
longitudinal direction toward the transverse direction of from
4.5.degree. to 6.0.degree..
2. The strap of claim 1 wherein the resin consists essentially of
polyethylene having a melt index between 0.4 and 0.9.
3. The strapping of claim 1 wherein the resin consists essentially
of polyethylene and a copolymer of ethylene and vinyl acetate
having a melt index from 1.0 to 3.0 and containing from 70 to 80
weight per cent ethylene and from 20 to 30 weight percent vinyl
acetate.
Description
BACKGROUND OF THE INVENTION
The conventional strapping used to bind large containers is
currently made from steel. Another type of strapping which is
widely used where the very high strengths of steel are unnecessary
is a strap formed of a series of parallel rayon cords held together
by means of a binder. This latter type of strapping is described in
U. S. Pat. No. 3,028,281, issued Apr. 3, 1962 to Thomas J. Karass.
Another type of strapping is roll oriented plastic strapping such
as described in U. S. Patent 3,354,023 issued Nov. 21, 1967 to
Gordon Beale Dunnington and Reuben Thomas Fields on Nov. 21,
1967.
SUMMARY OF THE INVENTION
This invention has as an object the manufacture of a strap from
polyethylene which has a strength high enough to act as a
substitute for steel strapping or roll oriented strapping from
other polymers.
These objects are accomplished by the following invention in which
polyethylene having a density of at least 0.95 and preferably from
0.955 to 0.967 and a melt index of from 0.2 to 1.0 and preferably
from 0.4 to 0.9 is extruded into a billet which is then rolled from
2.5 to 6.5 times its original length so as to produce a uniplanar,
axial oriented crystalline product which strap is then stretched
further at ambient temperature from 1.05 to 3.0 times its length
which strap then is stretched further in a hot oil bath from 1.6 to
3.0 times and preferably from 2.0 to 2.5 times its length to
produce a strapping having a deformation ratio of 10 to 18 times
and preferably from 11 to 13.5 times the original length of the
billet. The oil bath may be maintained at a temperature required to
heat the strapping being hot stretched at from 93.degree.C. up to
the melting point of the polyethylene with the range of from
110.degree. to 130.degree.C. being preferred. The residence time in
the oil bath and the temperature of the oil bath are interdependent
variables in controlling this temperature. The width of the final
strapping is preferably from 0.4 to 0.75 times the width of the
billet from which it is rolled. To accomplish this objective it has
been found that the uniformity of the extruded billet prior to the
roll orienting step is of extreme importance to the sucessful
production of a high strength rolled shape. This uniformity relates
both to the cross-sectional dimensions of the extruded billet and
to any orientation imposed on the billet. An irregular billet
cannot be roll oriented into a useful hgih strength strapping
because some sections will pass their maximum orientation potential
and fibrillate or become hairy before the central sections have
been oriented to their optimum. Roll-oriented polyethylene tapes
and ribbons have been made before but such prior art tapes and
ribbons have not had sufficient strength to compete with steel
strapping or other polymer strapping because it had not been
possible to impart sufficient orientation to such polyethylene
tapes and ribbons for them to have the requisite strength. The
strapping of this invention is preferably from 4 to 50 mils thick
and from 1/4 to 3/4 of an inch wide although wider widths can be
made and are desirable for some purposes such as helically wrapping
large diameter pipe, and widths as narrow as 1/8 inch are useful.
The strapping of this invention has particular utility as a tape
with an adhesive on one side thereof for use in packaging moderate
sized objects due to the "dead bend" property of the tape without
significant loss in tensile strength. That is, the straps or tapes
of the present invention may be creased to 90.degree. or even
180.degree. without any significant tendency to straighten and
without significant loss in tensile strength.
DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic side view of the entire apparatus.
In carrying out the process of this invention, pellets are fed by
means of metering feeder 1 into the extruder hopper 2, and extruded
through extruder head 3 into a quench bath 4. The billet thus
formed, is drawn out of the quench bath by rolls 7 and 8, and is
fed across dancer arm 9, into preheater 10, wherein it is passed
back and forth across rollers 11.
The temperature of the billet is from ambient temperature to
15.degree.C. below the crystalline melting point of the particular
polyethylene being roll-oriented. Although the billet can be roll
oriented at room temperature the operation is performed more
smoothly and with a substantial reduction in power consumption when
an elevated temperature is used. It should be further noted that
even though water is preferably used in the quench bath because of
its ready availability and high specific heat, the billet is
preferably in anhydrous condition as it is fed into the orienting
rolls. This is because the heat developed in the orientation rolls
by the rearrangement of the polyethylene molecules in the billet
may cause vaporization of any water or other low boiling liquid
present in the billet, and thereby, create voids or other flaws in
the final strapping. The preheated billet is then fed through one
or more pairs of orienting rolls 12 and 13 and is drawn under
tension out of the orienting rolls by means of tension rolls 14 and
15, and skew roll 16 used to enable multiple wraps whereupon it is
passed into hot oil stretching bath 17 in which it passes over roll
18 down through the hot oil around roll 19 and up and out of the
hot oil bath over roll 20 and passed between tension rolls 21 and
22 and around skew roll 23 to enable multiple wraps around the
tension rolls. The strapping is then passed through heat
conditioner 24 equipped with exhaust 25, through wash tank 26 and
finally is taken up onto spool 27.
The orienting rolls preferably are of tongue and groove
construction as shown in U. S. Pat. No. 3,354,023 referred to
above. It is to be understood that while two pairs of orienting
rolls are shown in FIG. 1, any desired number of rolls may be used.
The function of the flanges on the tongue and groove rolls is to
assist in controlling the width of the oriented strapping by
controlling the size of opening defined by the rolls. The amount of
tension on the strapping which is imposed by the speed of tension
rolls 14, 15, and 16 relative to the speed rolls 12 and 13,
controls the amount of decrease in width the strapping undergoes
after leaving orienting rolls 12 and 13. The strappings of this
invention are distinguished from films in that they have a high
uniaxial orientation. The amount of stretch or necking down of the
strapping on leaving the orienting rolls must be accurately
controlled since the width of the final strapping is preferably
within .+-.0.005 inch of the width being sought or the strapping
cannot readily be fastened with commercially available fasteners.
These fasteners generally are heavy gauge metal seals or clips
which fit around the strapping joint and are crimped with a machine
similar to that commercially used to join steel strapping, such as
those illustrated in U. S. Pat. No. 3,028,281, except preferably
with straight sides or edges. Clips or seals require width
tolerances. In order to obtain a uniform rectangular billet it is
necessary to have the cross-section near the corners of the
extrusion die somewhat oversize. By using a die of this shape the
tendency of the extrudate towards becoming round is overcome and a
billet of truly rectangular cross-section can be obtained. If a
rectangularly shaped die opening is used, the billet will have a
nearly oval cross-section, and excessive cross orientation will be
imposed by the orienting rolls, thereby lowering the amount of
length deformation which can be imposed on the strapping which in
turn lowers its ultimate strength and usefulness.
The polyethylene as used herein may be blended with up to 25
percent by weight as based on the total composition of an addition
polymer such as low density polyethylene, ethylene/vinyl acetate
copolymer, ethylene/methacrylic acid copolymer, ethylene/vinyl
acetate/methacrylic acid terpolymer, isotactic polypropylene, or
moldable styrene/butadiene rubber. Blending with 5-20 percent by
weight as based on the total composition of ethylene/vinyl acetate
copolymer containing from 70 to 80 percent ethylene and from 20 to
30 percent vinyl acetate is particularly helpful in improving
resistance to splitting. The addition of isotactic polypropylene
tends to improve surface hardness. Pigments are readily added as a
concentrate dispersed in low density polyethylene.
The melting behavior of the strapping of the present invention
provides an indication of the difference in structure between the
strapping of the present invention and prior art strappings. The
melting behavior of various high density polyethylene strappings is
studied by differential thermal analysis using a thermal analyzer
using various heating rates of from 0.5 to 100.degree.C./minute.
The slowest heating rate of 0.5.degree.C./minute apparently permits
recrystallization and a single melting peak of
134.degree.-137.degree.C. is observed for all high density
polyethylene strappings. High heating rates give evidence of
serious thermal lags and do not give any additional information. A
heating rate of 10.degree.C./minute when used on samples of
strapping which have been oriented by rolling and air stretching
alone exhibit a single melting peak near 137.degree.C. However,
samples prepared in accordance with the present invention using a
rolling, air stretch and final hot oil stretch to high deformation
ratios when examined using a heating ratio of 10.degree.C./minute
have multiple peaks with a major peak at from 140.degree. to
145.degree.C. It is believed that this higher melting peak reflects
the partial unfolding of lamellar crystals to form extended chain
crystals. Very slow heating rates would permit these extended chain
crystals to refold before melting.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
In each of the following examples the strapping was made on the
apparatus above described using one or two pairs of three-inch
diameter tongue and groove roll orienting rolls as at 12 and 13,
take-off rolls consisting of one 4 inch diameter rubber roll as at
14, one 4 inch diameter steel roll as at 15 and one skew roll as at
16 to permit about 15 wraps, a vertical hot oil bath as at 17 and
another set consisting of a 4 inch diameter rubber roll as at 22, 4
inch diameter steel roll as at 21 and a skew roll as at 23 to draw
the strapping in the hot oil bath. The melt index of the
polyethylene being used is determined in accordance with A.S.T.M.
D-1238 using a temperature of 190.degree.C. and a load of 2160 g.
on the melt indexer. The total deformation ratio is the weight per
unit length of the unoriented billet divided by that of the
oriented product. The stretch ratio in air (air str.) is the linear
speed of the take-off rolls 14, 15 and 16 divided by the linear
speed of the final rolling stage 12 and 13. The oil stretch (Oil
str.) ratio is the ratio of the linear speeds of the tension rolls
21, 22 and 23 divided by the linear speed of rolls 14, 15 and
16.
In Examples 4, 5, 7, 8, 9 and 10 the polyethylene contained 1
percent and in Example 11, 3 percent of a pigment concentrate
consisting of 75 percent titanium dioxide, 0.75 percent aluminum
stearate, and 24.25 percent of a polyethylene having a melt index
of 3.5 and a density of 0.923 g./cc. In Example 8, 5 percent; in
Example 9, 10 percent; and in Examples 10 and 11, 20 percent by
weight as based on the total composition of an ethylene/vinyl
acetate copolymer containing 25 percent by weight of vinyl acetate
and 75 percent by weight of ethylene and having a melt index of 2.0
.+-. 0.4 was incorporated in the polyethylene from which the
strapping was formed. The column headed DTA is the differential
thermal analysis as measured at 10.degree.C./minute and the value
reported is that of the largest peak. ##SPC1##
The quench bath was water maintained at temperatures of from
ambient to 50.degree.C. in all cases. The tensile strength and
modulus data were obtained in a conventional test machine equipped
with slotted mounting rolls with a one-inch per minute loading rate
and a five inch separation between rolls. The test results are all
based on the original dimensions of the strapping.
The term "uniplanar, axial orientation" employed in defining the
product of this invention may be fully understood from the
following discussion.
"Axial," "planar," and "uniplanar axial" indicate different types
of crystal orientation in high polymeric materials. "Axial"
orientation means that a given crystal axis (frequently the polymer
chain axis) is parallel to a macroscopic axis (e.g., the machine
direction in an extruded object). For example, prior art materials
which had been drawn in only one direction (e.g., fibers or one-way
stretched films) generally exhibit an appreciable degree of axial
orientation in which the polymer chain axes are aligned parallel to
the stretched direction. "Planar" orientation means that a given
crystal axis is parallel to a macroscopic level plane. Conventional
two-way stretched films for example generally exhibit a degree of
planar orientation in that the molecular chain axes lie
approximately parallel to the surface of the film although said
axes are arranged at random within this plane. "Uniplanar axial"
orientation means a given crystal axis is parallel to a macroscopic
axis and a given crystal plane is parallel to a macroscopic plane.
In the rolled, extruded shapes discussed here the molecular chain
axis is generally in the direction of rolling and a certain crystal
plane is parallel to the rolled surface. As used here the terms
"axial," "planar," and "uniplanar axial" orientation refer not only
to perfect alignment of the types discussed but also to structures
in which there is a preferred orientation even though there may be
some angular distributions about the preferred orientation.
Roll-oriented polymers generally exhibit "uniplanar, axial
orientation."
X-ray diffraction furnishes a convenient technique for observing
the type of orientation in the ogjects of this invention. A sample
is mounted on an instrument such as a Single Crystal Orienter which
has the ability to rotate the sample in the X-ray beam about two
mutually perpendicular axes. Since a crystalline material will
diffract X-rays only when the X-ray beam, the detector, and
suitable crystalline planes within the sample are arranged in the
manner described by Bragg's Law, it is possible to determine the
crystal orientation within the sample by studying the variation in
the intensity of the diffracted X-rays as the sample is rotated.
This intensity will pass through a maximum as the angular
orientation of the sample reaches a value corresponding to the most
populous orientation of the crystals within the sample. The breadth
of the distribution of crystal orientations may be characterized by
the width of a plot of X-ray intensity vs. the angular orientation
of the sample at an intensity value equal to one-half of the peak
maximum. Further aspects of the definition of the types of
orientation and of techniques for determining the distribution of
crystal orientation in synthetic polymers are described in a paper
by C. J. Heffelfinger and R. L. Burton in the Journal of Polymer
Science, Volume 47, pages 289-306 (1960).
In an extruded, rolled shape made from high density polyethylene,
the uniplanar axial orientation is such that the polymer chains
tend to be in the direction of rolling and either the (100) or the
(110) crystal planes tend to be parallel to the rolled surface. The
angular width at the one-half maximum corresponding to the tilting
of the polymer chains from the roll direction toward the transverse
direction is less than 7.degree. and preferably from 4.5.degree. to
6.degree.. These angles correspond to those obtained for uniplanar
axial orientation in high density polyethylene which has been roll
and stretched to increase its length at least ten-fold and
preferably from 11 to 13.5 fold.
It is well known in the art that controlled deformation of a
crystalline polymer results in an improvement in the physical
properties of the polymer in the direction of deformation. This is
most highly developed in the case of fiber and filaments where very
marked improvement in tensile strength and modulus with an axial
orientation is obtained by cold drawing of the extruded fiber or
filament. Attempts to obtain equivalent improvement in physical
properties in more massive polyethylene shapes with triaxial
symmetry such as tapes, straps, sheets, angles, tees, and the like
have not succeeded in producing strengths above about 40,000 psi.
as is illustrated by control Examples 1 and 6 which do not utilize
the hot stretch step.
* * * * *