Process for preparing improved oriented polymer films and tapes

A postforming process for an oriented film or tape of polymer material, which process comprises subjecting at least one layer of such an oriented film or tape to a pressure greater than ambient pressure while heating the oriented film or tape at a temperature at which partial melting thereof occurs but which does not exceed 145.degree. C.

This invention relates to improved oriented polymer films; more 
particularly, this invention relates to processes for the continuing 
preparation of improved oriented films of polymer materials; to improved 
oriented films of polymer materials so prepared and to certain oriented 
films as new products; and to composite materials comprising the oriented 
films. 
In UK Patent Specification No. 2,164,603A which corresponds to U.S. 
application Ser. No. 06/878,971, a process for the continuous preparation 
of anoriented film of a polymer material is disclosed, which process 
comprises: 
(i) shearing at least one film of a solution and/or gel of the polymer 
material between at least one set of two surfaces which are in motion 
relative to one another, at least one of which surfaces is endless, and 
(ii) continuously removing the or each oriented film of polymer material 
from the surface on which it is formed. 
Also described herein is a composite material the dispersed phase of which 
comprises said oriented film of polymer material and wherein the 
continuous phase may comprise a cement such as a Portland or pozzolanic 
cement, a thermosetting polymer matrix such as an epoxy resin, or a 
thermoplastic matrix such as polyolefin, for example polyethylene, 
polypropylene or a blend thereof. 
In accordance with one aspect of the present invention there is provided a 
process for the continuous preparation of an oriented film or tape of a 
polymer material, which process comprises: 
(i) shearing at least one film of a solution and/or gel of the polymer 
material between at least one set of two surfaces which are in motion 
relative to one another, at least one of which surface is endless, 
(ii) continuously removing the or each oriented film or tape of polymer 
material from the surface on which it is formed; and 
(iii) subjecting at least one layer of the oriented film or tape to a 
pressure greater than ambient pressure while heating the oriented film or 
tape at a temperature at which partial melting thereof occurs but which 
does not exceed 145.degree. C. 
Films and tapes prepared in accordance with this process are found to have 
greater translucency; higher density (greater than 0.65 g cm.sup.-3 for 
example 0.7 g cm.sup.-3); and greater toughness and tear resistance. 
In accordance with a further aspect of this invention, there is provided a 
postforming process for an oriented film or tape of polymer material 
preparable by the process claimed in UK Patent Specification No. 
2,164,603, which process comprises subjecting at least one layer of such 
an oriented film or tape to a pressure greater than ambient pressure while 
heating the oriented film or tape at a temperature at which partial 
melting thereof occurs but which does not exceed 145.degree. C. 
The process of the present invention is applicable to any thermoplastic 
organic polymer material, preferably a crystallisable such polymer 
material, provided only that the polymer material comprises at least a 
fraction of sufficiently high molecular weight and that this latter 
material can form a solution and/or gel. Examples include unsubstituted or 
mono- or poly-halo-, for example chloro- or fluoro-, substituted vinyl 
polymers such as polyolefins, preferably linear polyolefins, polyvinyl 
chloride and polyvinylidene difluoride; polyalkylene oxides such as 
polyethylene oxide; polyacetals such as polyoxymethylene and 
polyacetaldehyde; polyamides, preferably aliphatic polyamides; 
unsubstituted or hydroxy-substituted polyesters such as PHB and 
polyethylene terephthalate; and polyetherketones. 
The polymer material is suitably an addition homo- or copolymer, desirably 
a polyolefin. Preferably, the polyolefin comprises polyethylene, 
especially inert linear polyethylene, polypropylene, a 
polyethylene-polypropylene copolymer or a blend comprising at least one 
polyolefin therefrom. 
It is desirable that the polymer material has a weight average molecular 
weight (M.sub.w) greater than about 500,000, desirably greater than about 
600,000. Below this value there is not sufficient of the polymer material 
of sufficiently high molecular entanglement and/or relaxation time to 
enable highly oriented films, in which the polymer material is believed to 
exist as extended chain fibrils, to be formed by the process of the 
present invention with commercial viability, if at all. Preferably, 
M.sub.w is greater than about 750,000, desirably greater than about 
850,000, especially greater than about 1,00,000. 
The process of the invention is carried out by shearing a film of a polymer 
solution and/or gel. 
By `film of solution and/or gel` is meant herein a thin layer, typically of 
a thickness not greater than 3 mm. 
The process of the present invention is applicable to any thermoplastic 
organic polymer material, preferably a crystallizable polymer material 
provided only that the polymer material comprises at least a fraction of 
sufficiently high molecular weight and that this latter material can form 
a solution and/or gel. Examples include linear vinyl hydrocarbon polymers, 
polyethylene oxide, polyacetals such as polyoxymethylene and 
polyacetaldehyde, aliphatic polyamides, polyesters such as polyethylene 
terephthalate and fluorinated polymers such as polyvinylidene difluoride. 
Preferably, the polymer material comprises a polyolefin, for example 
polyethylene, polypropylene, polyethylene-polypropylene copolymer or a 
polyolefin blend comprising at least one thereof, especially linear 
polyethylene. 
It is desirable that the polymer material has a weight average molecular 
weight (M.sub.w) greater than about 500,000, desirably greater than about 
600,000. Below this value there is not sufficient polymer material of 
sufficiently high molecular entanglement and/or relaxation time to enable 
highly oriented films, in which the polymer material is believed to exist 
as extended chain fibrils, to be formed by the process of the present 
invention with commercial viability, if at all. Preferably, M.sub.w is 
greater than about 750,000, more preferable greater than about 850,000, 
most preferably greater than about 1,000,000. 
The film of solution and/or gel of the polymer material is preferably less 
than 3 mm thick, more preferably less than 2 mm thick. It is particularly 
preferred that the film is 1 mm thick or less; for example, no greater 
than 0.5 mm. 
It is highly desirable, and usually necessary, to maintain the solution 
and/or gel of polymer material at an elevated temperature. For example, it 
is desirable to maintain the solution and/or gel at a temperature not less 
than 50.degree. C., preferably not less than 30.degree. C. below the 
melting temperature of the polymer material. In the case of linear 
polyethylene, it is desirable that the solution and/or gel is maintained 
at standard pressure at a temperature greater than 90.degree. C., 
preferably greater than 100.degree. C., more preferably at a temperature 
greater than 110.degree. C., most preferably greater than 120.degree. C., 
such as up to 130.degree. C. The solution and/or gel temperature should 
not exceed the melting temperature of the polymer material. The tensile 
properties of the oriented film are found to increase as the solution 
and/or gel temperature increases. However, the mass of oriented film 
produced per unit time is found to decrease with increasing solution 
and/or gel temperature but to increase with increasing shear. To obtain 
maximum mass per unit time of the oriented film it is therefore necessary 
to use as low a solution and/or gel temperature as is consistent with the 
required mechanical properties in conjunction with high shear. The mass 
growth rates exhibited are much greater than those found in equivalent 
fibre preparations. It is believed that the presence of a second surface 
enhances the mass growth rate (at a given temperature) by defining and 
increasing the effective width over which growth can occur and possibly 
also by providing enhanced shear between the surfaces. 
The solvent used will depend on the nature of the polymer material. In the 
case of polyolefins, such as linear polyethylene, a hydrocarbon, 
particularly an aromatic hydrocarbon such as xylene that boils at standard 
pressure above 140.degree. C. may be used. For polyethylene oxide polar 
solvents, for example, water or chlorinated hydrocarbons may be used. 
Solution and/or gels of the polymer material having a concentration for 
example of from 0.1 wt% to 10 wt%, preferably from 0.2 wt% to 5 wt% are 
desirable. 
Preferably, the surface on which the oriented film of polymer material is 
formed is at least in part wetted and adhered to by the solution and/or 
gel of the polymer material. However, the oriented films so formed should, 
in general, be sufficiently mobile to facilitate ready stripping therefrom 
during its removal. The surface may be of the same or different polymer 
material, for example PTFE, or of a metal. The texture of the surface is 
also an important factor in determining whether the above mentioned 
functional requirements are met. Thus, where a surface of fabric, such as 
"leno" woven cotton is used it is usually found that the oriented film is 
irremovably impregnated thereon. At the other extreme, where the surface 
is substantially completely not adhered to and smooth, such as a PTFE 
sheet, no film is usually formed. These properties can be utilized to a 
beneficial effect in accordance with a preferred embodiment of the 
invention wherein a portion of, preferably the majority of the, or one of 
the surfaces on which the oriented film of the polymer material is formed 
is smooth and not adhered to while a portion, preferably the minority, is 
adhered to by the solution and/or gel. Specifically, a smooth sheet, for 
example of PTFE, which is not adhered to can comprise a plurality of 
lines, for example two lines of stitches such as of cotton thread which 
extend in the machine direction. It is preferred that at least the or each 
surface on which the oriented film of the polymer material is formed is in 
motion. 
It has also been found that a relatively coarsely woven fabric such as 
woven nylon fabric may be used as a surface from which to form a 
harvestable oriented film of polymer material. The material known as 
"VELCRO" (manufactured in accordance with U.S. Pat. Nos. 2,717,434 and 
3,009,235 and GB Patent Nos. 1,289,825, 1,295,069, 1,299,897, 1,345,607 
and 2,009,257; "VELCRO" is a Registered Trade Mark) has been found to be 
particularly suitable. The continuous oriented film so produced show 
little or no tendency to lateral shrinkage on drying. 
The invention also provides a process wherein a plurality of lines of 
oriented films of a polymer material may be continuously prepared. 
The oriented film of polymer material may be continuously removed in a 
manner known per se; for example by using the haul-off shown in FIG. 5. It 
has been found that haul-off load is a good indicator of film thickness, 
i.e., that growth occurs at constant stress. 
According to a further aspect of this invention, an apparatus for the 
continuous preparation of an oriented film of a polymer material is 
provided which comprises at least one set of two adjacent surfaces, at 
least one of which surfaces is endless, movable relative to the other; and 
means for continuously removing the or each oriented film of polymer 
material from one of the surfaces on which it is formed. Preferably, such 
apparatus additionally comprises one or more baths in which the or each 
set of adjacent surfaces is contained. 
Desirably, the endless surface comprises a belt or cylinder. An increase in 
diameter of the cylinder will increase the defined contact area and 
possibly also the shear and thus the mass of oriented film produced per 
unit time. The other surface may comprise a chisel-edged seed-rod of the 
polymer material or blade of metal. However, it is preferred that the two 
surfaces are endless. 
The machine direction of the oriented film of the polymer material, the 
direction of one surface, for example the belt and the shear axis of the 
other surface such as the cylinder in a set are independently variable. 
When none of these is normal to any other, the oriented film is oriented 
in a direction different from the machine direction and the degree of 
orientation changes continuously across the film. When the machine 
direction of the oriented film of polymer material is normal to one 
surface but not normal to the shear axis of the other surface, the 
oriented film is oriented in the machine direction but the degree of 
orientation changes continuously across the film. When the machine 
direction of the oriented film of polymer material is normal both to one 
surface and to the shear axis of the other surface, the oriented film is 
oriented in the machine direction and the degree of orientation does not 
substantially change across the film. Preferred apparatus as herein 
described is that wherein one surface comprises a belt and the other 
surface comprises a right cylinder rotatable about its axis and bounded, 
at least in part, by the belt. 
There five modes of operation of the apparatus are as follows: 
(a) a moving cylinder and stationary belt; 
(b) a moving cylinder and a belt moving in the opposite hand; 
(c) a moving cylinder and a belt moving in the same hand but slower; 
(d) a moving cylinder and a belt moving in the same hand but faster; 
(e) a stationary cylinder and a moving belt. 
In practice, (a) does not appear to produce continuous lengths of oriented 
film; indeed, it has been found that, using a VELCRO surface, a belt 
tension of at least 20 N, and preferably greater than 25 N is required to 
produce an oriented film. 
In accordance with a particularly preferred aspect of this invention, there 
is provided a process as aforesaid wherein a plurality of layers of the 
oriented film or tape is stacked prior to the application of pressure and 
heat. While the machine directions of the oriented film or tape in the 
layers of the stack may be parallel (resulting in a single-laminated 
self-reinforcing composite), it is preferred that the machine directions 
of the oriented film or tape in alternate layers of the stack are 
non-parallel. It is particularly preferred that the machine directions of 
the oriented film or tape in alternate layers of the stack are at an angle 
from 75.degree. to 105.degree. (resulting in a cross-laminated 
self-reinforcing composite). 
With particular (but not exclusive) reference to linear polyethylene it is 
found, by scanning electron microscopy (SEM) and differential scanning 
calorimetry (DSC), that the oriented films or tapes prepared in accordance 
with UK Patent Specification No. 2,164,603 comprise oriented fibrils 
embedded in a matrix of unoriented material, the proportion of the latter 
increasing with throughput rates. It is found (by DSC) that the oriented 
films or tapes melt in two easily distinguished stages: thus, melting 
begins at about 118.degree. C. while the oriented fibrils melt at much 
higher temperature range from 147.degree. to 151.degree. C. We have 
perceived that by heating such oriented films or tapes at a temperature 
from 110.degree. to 140.degree. C., preferably from 125.degree. to 
135.degree. C., we can, by partially melting the unoriented material under 
pressure (for example, 2500 psi) but with retention of the oriented 
fibrils, either improve the mechanical properties of a single layer of the 
oriented film or tape or prepare a self-reinforcing composite from a 
plurality of such layers. 
For other polymer materials, it is to be expected that the temperature 
range will differ. 
By "self-reinforcing composite" is meant herein a material which, aside 
from any added fillers, is chemically homogeneous in the bulk but which 
comprises a dispersed phase of different morphology (but of the same 
chemical composition; in this case, the oriented fibrils) which is 
disposed to reinforce the continuous phase. 
This invention also provides a self-reinforcing composite prepared as 
herein described. In particular, this invention provides a 
self-reinforcing composite of linear polyethylene having a sole melting 
point peak (DSC) at about 147.degree. to 151.degree. C. 
Such materials are found to have a Young's modulus from 15 to 50 GPa, for 
example from 20 to 40 GPa; and a tensile strength from 0.75 to 1.50 GPa.

In the drawings, the apparatus represented in FIGS. 1 and 4 comprises a 
bath 1 in which a cylinder 2 having a cylindrical surface of PTFE 
rotatable about a horizontal axis is contained. A chisel-shaped seed-rod 3 
of polyethylene is maintained in sliding contact with the cylinder. 
In FIG. 2 the seed-rod is replaced by a scraper blade 4 of sprung steel. 
In FIGS. 3 and 5 the seed-rod is replaced by a continuous belt 5 of PTFE 
which comprises two lines 6 of stitched cotton thread which is in sliding 
contact with the cylinder and which is friction drivable by passage over 
drivable roller(s) 7. In FIG. 3 there is also represented belt-tensioning 
spools 8 over which the belt is laced and a film wind-off spool 9. 
In use, the bath 1 is substantially filled with a dilute (less than 10 wt%) 
solution and/or gel of high molecular polyethylene in xylene which is 
equilibrated at about 110.degree. C. by heating means (not shown). The 
cylinder is then caused to rotate by drive means (not shown). Thereafter 
the seed-rod, scraper blade or belt is maintained in sliding contact (the 
belt optionally being driven) with the cylinder. As film 10 is formed it 
is laced to a wind-off spool. 
EXAMPLE 
Four strips of polyethylene oriented tape produced in accordance with the 
process of UK Patent Specfication No. 2,164,603 were stacked with their 
machine directions parallel. The stack was then carefully placed between 
two highly polished, flat, stainless steel discs and inserted in a 
hydraulic press, fitted with heating plates which were heated to a 
temperature in the range from 130.degree. to 136.degree. C. A pressure of 
2,500 psi was then applied to the heated press for a period of 6 minutes 
whereupon the sample was water-cooled and removed from the press to 
produce a single laminated self-reinforcing composite. 
A cross-laminated self-reinforcing composite was prepared, in essentially 
the same manner, by stacking three strips of the oriented tape, the 
central strip of the stack having its machine direction at right angles to 
the other two. 
DSC experiments on the composite showed that the low melting point peak had 
disappeared but that the high melting point peak had remained at 
150.degree. C. (A totally melted and cooled specimen exhibited only the 
customary peak at 133.degree. C. on reheating). This indicated that the 
original structure of the oriented fibrils had been retained. Preliminary 
results indicated a Young's modulus of 20 GPa and a strength to break of 
0.86 GPa.