Manufacture of multilayer polymer films

A method for making multilayer polymer films with involves the use of preferably embossed rollers which oscillate oppositely with respect to each along their respective rotational axis. Multilayer film is made by passing two or more molten thermoplastic films through the nip of these rollers, thereby laminating the individual films together. The method is particularly useful for improving the adhesion between film layers, and/or when at least one of the layer is a liquid crystalline polymer. Such films are useful in packaging, multilayer containers and for circuit boards.

FIELD OF THE INVENTION 
This invention concerns a process for the preparation of a multilayer 
polymer film by feeding two or more films of molten polymer to rollers 
which oscillate along their axis with respect to another and whose 
surfaces are preferably slightly embossed, and wherein the temperature of 
the rollers is such that the film solidifies on one roller and forms a 
molten bead on the other roller. The resulting multilayer films have 
improved adhesion between layers. 
TECHNICAL BACKGROUND 
Multilayer films are important items of commerce, being particularly useful 
in packaging, where differing properties offered by various polymers of 
the film layers are important to the overall functioning of the film in 
use. The most common method of forming films from thermoplastics is 
extrusion of the polymer through a film die, and multiple layer films are 
typically formed by extruding each polymer needed separately, and either 
joining the individual films in the die or just outside the die, or 
extruding the various layers through individual dies and then laminating 
the layers together, or some combination of the two. One concern in the 
formation of such multilayer films is the adhesion between various layers, 
particularly when adjacent layers are made from polymers that are unlike 
each other. Various solutions, such as the use of separate tie 
("adhesive") layers have been used, but these involve extra expense. 
Therefore better methods of making multilayer films are desired. 
When LCPs are extruded into films (singly or in multilayer structures), the 
polymer usually is highly oriented in the machine (extrusion) direction 
(MD), and is weak and brittle in the transverse direction (TD). Special 
methods have been developed to produce LCP films (or thin tubes which can 
be slit into films) with more balanced MD/TD properties, thus improving 
the TD properties of the film. However, such methods, which for instance 
are described in U.S. Pat. Nos. 4,384,016, 4,820,466, 4,963,428, 
4,966,807, 5,156,785, 5,248,305,288,529, 5,312,238, and 5,326,245 and G. 
W. Farrell, et al., Journal of Polymer Engineering, vol. 6, p. 263-289 
(1986), usually require the use of intricate, expensive equipment which 
may be difficult to operate reliably, produce tubes which may not lay flat 
as films, and/or require labor intensive lay-up methods. One of these 
methods is moving in the TD an extrusion die surface which contacts the 
molten LCP. Thus better methods of preparing improved LCP films are 
needed. 
SUMMARY OF THE INVENTION 
This invention concerns, a process for the production of a multilayer 
thermoplastic film, comprising, feeding a molten first film of a first 
thermoplastic polymer and a molten second film of a second thermoplastic 
polymer to a pair of rollers which have a gap between them which is 
approximately equal to the total thickness of said first film and said 
second film, and passing said first film and said second film through said 
gap, provided that: 
said rollers oscillate relative to one another and parallel to their 
rotational axis at a frequency of about 20 to about 200 Hz; 
said rollers are at such a temperature or temperatures that said first film 
or said second film freezes against one roller, and on the other roller a 
rolling bank of molten polymer is formed; 
and provided that at said temperature or temperatures and said frequency a 
multilayer thermoplastic film is formed.

In all of the Figures, motors and drives for rotating the rollers 5 and 6 
are not shown, and neither are supports or bearings for 5 and 6 (except 
for 15 and 16), or means for heating 5 and 6. 
In FIG. 2, removal of one of the pins 19 or 20 would clearly cause only one 
of 5 or 6 to oscillate. Similarly, in FIG. 3, removal of one of cams 23 or 
24 would cause oscillation of only one of rollers 5 and 6. Oscillation of 
only one roller is also useful in this invention, since the rollers are 
still oscillating with respect to one another parallel to their axis of 
rotation. 
DETAILS OF THE INVENTION 
The polymers used herein are fed to the oscillating rollers in the form of 
molten films. By molten is meant the polymers are above their glass 
transition temperature and essentially free of crystallites (but still 
liquid crystalline in the case of an LCP). They will usually be above 
their melting point (if any). The molten films may be provided by any 
expedient means, for instance melting preexisting films and feeding them 
to the rolls, or more conveniently melting the polymers in individual 
extruders and extruding the polymers through separate ordinary film dies. 
It is most convenient to vertically extrude the films downward, so that 
they "fall" by gravity towards the rollers. Alternatively, the multilayer 
film may be extruded as a "single" entity by using the standard 
coextrusion method for making multilayer film, and the molten multilayer 
film fed to the oscillating rollers. 
The rollers employed can preferably be heated (see below). The axis of 
rotation of both rollers will usually be parallel to each other, and the 
gross surface of each roller will usually be parallel to the axis of 
rotation of that roller, and at a constant distance from that axis. 
Typically the rollers will be of metal construction. For convenience the 
gap between the rollers should preferably be adjustable so that films or 
sheets of different thicknesses may be readily produced. The rollers are 
of course driven so that the molten polymers are drawn into the gap 
between the rollers. The speed of the rollers is preferably adjustable so 
that the rate that the multilayer film exits the rollers is preferably 
approximately equal to the total rate of molten polymers fed to the 
rollers, except as noted below. The rollers are both preferably the same 
diameter and/or are both driven at the same surface speed through the gap. 
The surfaces of the rollers may be embossed with a pattern that is designed 
to put at least some transverse shear on the polymer as it goes through 
the nip of the rollers which are oscillating with respect to one another. 
The angle of the embossing with respect to the axis of oscillation should 
be greater than 0.degree.. Generally speaking as this angle goes from 
0.degree. to 90.degree. the amount of transverse shear imparted to the 
polymer film increases. Likewise, the deeper the embossing and/or the 
sharper the ridges of the embossing the greater the transverse shear 
imparted to the polymer film. Typical depths of embossing are about 0.02 
to about 0.15 mm, but this of course is dependent on the angle of 
embossing and sharpness of the ridges. Useful embossing patterns are 
readily ascertainable with minimal experimentation by the artisan, and 
some useful patterns are described in the Examples. 
The temperature of at least one the rollers should be such that the surface 
of the polymer which contacts that roller freezes or solidifies rapidly as 
the polymer film contacts that roller. This will normally be somewhere 
below the melting point of the polymer used, or if the polymer is 
amorphous at ambient temperature, below the glass transition temperature. 
The temperature of the second roller is such that a relatively small 
rolling bank of polymer which is molten or at least mobile is formed in 
the nip on the second roller's side of the films as they enter the nip, 
and in the nip at least part of the polymer is molten or at least mobile. 
As the multilayer film exits the nip both surfaces are essentially solid, 
although some of the polymer in the interior of the film may yet be 
molten. 
In addition, the temperature of the rollers should be below the point at 
which the film exiting the rollers sticks to the rollers. It has been 
found that at least in some cases both rollers may be at the same 
temperature. A suitable temperature range for each or both rollers is 
determined in part by the process conditions, such as the speed at which 
the rollers operate, the thickness of the film, the temperature of the 
polymers coming into the rollers, the tendency of the polymers to stick to 
the particular surface of the rollers used, and other factors. The 
operable temperature range for the rollers may be readily determined by 
minimal experimentation, and such temperatures are illustrated in the 
Examples. In actual operation, the temperature of the two rollers may be 
(nominally) the same or somewhat different. 
Heating of the rollers can be accomplished by a number of methods known to 
the art, such as by hot oil or electrically. It is preferred that the 
roller temperatures can be controlled relatively accurately (e.g., within 
1 to about 2.degree. C.) so that uniform film may be produced. 
Preferred total film thicknesses, both entering and exiting the rollers are 
about 0.02 to about 0.25 mm. Generally speaking tie layers will tend to be 
thinner than the layers they bind together. 
The rollers are oscillated with respect to one another parallel to their 
axis of rotation. One or both rollers may actually move in this direction 
(oscillate), or just one roller may oscillate and the other may be fixed 
in this respect (but still rotate). It has been found that a frequency of 
oscillation of about 20 to about 200 Hz is a useful range, preferably 
about 30 to about 150, more preferably about 60 to about 100 Hz whether 
one or both rollers is actually moved. The amplitude of oscillation can be 
about 0.5 to about 8 mm, preferably about 1.5 to about 6 mm, this 
amplitude being the total motion of the two rollers with respect to each 
other parallel to their axis of rotation. 
The multilayer film made herein has at least two layers, but may contain 
more than two layers. For each layer desired it is convenient to have a 
separate extruder and film die feeding a molten thermoplastic to the 
rollers, as illustrated in FIG. 1. These additional layers may be tie 
layers, or may have other purposes in being in the multilayer film, such 
as certain barrier properties, abrasion resistance, chemical resistance, 
appearance, etc. 
The thermoplastics used herein may be filled or otherwise mixed with 
ingredients normally found in thermoplastics, such as filler, reinforcing 
agents, dyes, pigments, antioxidants, antiozonants, lubricants, etc. Each 
layer may contain its own distinctive additive(s), or contain no additive 
at all. 
Often when two different polymer are laminated together either when "solid" 
or in the molten state, after the laminating pressure is removed (and the 
polymers solidified if applicable) it is found that the adhesion of one 
polymer to the other is poor. It is believed in the art that the basic 
cause of this problem is that different polymers tend not to wet each 
other, resulting in weak bonds between them. To overcome this problem, 
adhesives have been developed bond different polymers together. In forming 
multilayer thermoplastic films by extrusion this adhesive often takes the 
form of a so-called tie layer, which is itself a polymer with improved 
adhesion to each of the polymers it will be bonding. To obtain improved 
adhesion to both polymers the tie layer typically is composed of a polymer 
which is similar in some way to each of the polymers to be adhered and/or 
contains functional groups which react with other groups in both polymers 
to be adhered. 
However tie layers are expensive to include in multilayer films because 
their polymers tend to be expensive, and another layer must be extruded, 
making the process more complicated. It has been found that even without a 
tie layer the oscillating rolls increase the adhesion between two polymer 
layers. The reason for this is unclear, but it is suspected that the 
movement of the polymer in the roller nip may mix the polymers to an 
extent that they are somewhat physically interlocked at the polymer 
interface leading to a better bond between polymer layers. 
It has also been found that if more improved adhesion is desired a tie 
layer may be a simple blend of the two polymer being adhered. This blend 
may be made by simply mixing the two polymers in the extruder which is 
used to form the film, but the blend is preferably made by melt blending 
the two polymers beforehand, as in a twin screw extruder. Although the use 
of this tie layer does require more equipment for the multilayer 
extrusion, the polymers in the tie layer are often relatively inexpensive, 
and they don't introduce more ingredients into the multilayer film, an 
important consideration when the film is to be used for food contact or 
medical purposes. 
If the two polymers to be adhered are A and B, the weight ratio of A:B can 
be 1:99 to 99:1, preferably about 80:20 to about 20:80, more preferably 
about 65:35 to about 35:65. 
The process of forming multilayer films with the oscillating rollers is 
particularly useful when at least one of the films is a liquid crystalline 
polymer (LCP). By an LCP is meant a polymer that is anisotropic when 
tested in the TOT Test described in U.S. Pat. No. 4,118,372. As mentioned 
above, when LCPs are extruded through a simple slit film die, they tend to 
be oriented in the machine (extrusion) direction (MD), which makes them 
weak in the transverse (perpendicular) direction (TD). As more and more 
transverse orientation is induced in the LCP, the physical properties in 
the transverse direction, such as tensile strength, tensile modulus and 
tensile strain to break will also increase. This increase is often, 
although not necessarily, at the expense of the machine direction 
properties. The oscillating rollers can be used to induce more TD 
orientation in the LCP layer. 
Numerous variables may affect the degree of transverse orientation of the 
LCP in the final film. Among these are roller oscillation frequency, 
roller rotational speed, roller temperature, type of roller surface (for 
instance smooth or embossed), roller oscillation frequency, LCP melt 
temperature, LCP viscosity, and the film thickness. 
It is believed that in many cases, as the following are increased, the TD 
orientation is affected as noted: 
increasing roller oscillation frequency--increases TD orientation (up to a 
point) 
increasing rotational roller speed--decreases TD orientation 
increasing roller temperature--decreases TD orientation 
increasing roller oscillation amplitude--increases TD orientation 
increasing LCP melt temperature--decreases TD orientation 
increasing LCP viscosity--increases TD orientation 
increasing film thickness--decreases TD orientation 
In addition, embossing affects the orientation of the resulting LCP film. A 
roller with a relatively smooth surface may be used, but in this instance 
the temperature must be controlled very closely, so that this viscosity of 
the LCP at the roller nip is quite high, but not so high as to prevent the 
polymer from passing between the rollers. If the rollers are embossed, 
such tight temperature control is not necessary. Generally speaking the 
deeper the embossing, or the closer to perpendicular to the oscillation 
direction the embossing is the embossing is, the more the LCP will be 
oriented in the TD. Also, if the embossed "lines" have steep walls, as 
opposed to gently sloping walls, the TD orientation will be increased. 
One preferred form of embossing is a diamond knurled patter (see Roll C 
below), and a double diamond knurled pattern is especially preferred. By 
double diamond knurled is meant there are two independent diamond knurled 
patterns present, which leads to diamonds of different sizes embossed on 
the surface. Many of the above factors will be illustrated in the 
Examples. 
The multilayer LCP films formed by this process have improved transverse 
direction properties compared to films that are extruded through a simple 
slit die. It is preferred that the maximum tensile strength in the TD is 
at least 50 percent of the maximum tensile strength in the MD, more 
preferably the TD is at least 75 percent of the maximum tensile strength 
in the MD. Similarly, it is preferred that the tensile strain at break in 
the TD is at least 50 percent of the strain at break in the MD, more 
preferably the TD is at least 75 percent of the tensile strain at break in 
the MD. Also it is preferred that the tensile modulus (Young's modulus) in 
the TD is at least 50 percent of the strain at break in the MD, more 
preferably the TD is at least 75 percent of the tensile modulus in the MD. 
In simple extrusion through a slit die, these properties are typically 
much better in the MD than the TD. 
LCPs are notoriously difficult to adhere to other polymers, even other 
LCPs. However when the oscillating rollers are used improved adhesion of 
an LCP to another polymer has been found. This is especially true if a 
blend of the LCP and the other polymer to which it is to be bound is used 
as a tie layer. In this instance it is preferred that the weight ratio of 
LCP:other polymer is about 80:20 to about 20:80, preferably about 65:35 to 
about 35:65. 
Any thermotropic LCP may be used in this process. Suitable thermotropic 
LCPs, for example, are described in U.S. Pat. Nos. 3,991,013, 3,991,014 
4,011,199, 4,048,148, 4,075,262, 4,083,829, 4,118,372, 4,122,070, 
4,130,545, 4,153,779, 4,159,365, 4,161,470, 4,169,933, 4,184,996, 
4,189,549, 4,219,461, 4,232,143, 4,232,144, 4,245,082, 4,256,624, 
4,269,965, 4,272,625, 4,370,466, 4,383,105, 4,447,592, 4,522,974, 
4,617,369, 4,664,972, 4,684,712, 4,727,129, 4,727,131, 4,728,714, 
4,749,769, 4,762,907, 4,778,927, 4,816,555, 4,849,499, 4,851,496, 
4,851,497, 4,857,626, 4,864,013, 4,868,278, 4,882,410, 4,923,947, 
4,999,416, 5,015,721, 5,015,722, 5,025,082, 5,086,158, 5,102,935, 
5,110,896, and 5,143,956, and European Patent Application 356,226. Useful 
thermotropic LCPs include polyesters, poly(ester-amides), 
poly(ester-imides), and polyazomethines. Preferred thermotropic LCPs are 
polyesters or poly(ester-amides), and it is especially preferred that the 
polyester or poly(ester-amide) is partly or fully aromatic. 
After passing through the gap in the oscillating rollers the LCP film may 
be wound up. Before being wound up film may go through rolls which may 
accomplish other functions, such as cooling the film, or calendering the 
film to obtain a smoother surface. 
The multilayer films described herein are useful in many applications, such 
as for packaging films, bottles and other containers, for encapsulating 
electronic components, and for use in heat exchangers. 
EXAMPLE 
Polymer A was a liquid crystalline polymer which was an aromatic polyester, 
and was a copolymer of (molar ratios in parentheses): 
4,4'-biphenol(26.3)/hydroquinone(26.3)/1,6-hexanediamine(47.4)/terephthali 
c acid(36.8)/2,6-naphthalene dicarboxylic acid(63.2)/4-hydroxybenzoic 
acid(89.5)/6-hydroxy-2-napthoic acid(36.8). 
Polymer B was a nylon 6/Sclair.RTM. 11K1.sup.a /Fusabond.RTM. D226.sup.b /a 
functionalized synthetic rubber.sup.c (72.5/13.6/6.8/6.8 weight percent 
(.sup.a LLDPE available from Nova Chemicals, Calgary, Alberta, Canada; 
.sup.b A maleic anhydride grafted LLDPE available from E. I. duPont de 
Nemours & Co., Wilmington, Del. U.S.A.; .sup.c A maleic anhydride 
functionalized EPDM.) 
Polymer C was a blend prepared in a twin screw extruder of 40 percent by 
weight of Polymer A and 60 percent by weight of Polymer B. 
The apparatus used included a 3/4" (1.91 cm) Brabender (Type 2003, C. W. 
Brabender Instruments, Hackensack, N.J., U.S.A.), a 1" (2.54 cm) Wilmod 
extruder, and a 3.8 cm NRM extruder. Polymer A was extruded from the 
Wilmod extruder running at 45 rpm and the melt temperature was 280.degree. 
C. Polymer B was extruded from the NRM extruder running at 20 rpm and at a 
melt temperature of 280.degree. C. Polymer C was extruded from the 
Brabender extruder running at 60 rpm and with a melt temperature of 
270.degree. C. The output of each of these extruders was fed to 15.2 cm 
wide film die which was configured to handle three separate feeds. 
Polymers A and B were the outside layers and Polymer C was the inner 
layer. 
The molten film fell by gravity on the oscillating rollers, which were 
arranged as shown in FIGS. 1 and 2, except the 3 extruders fed the 3 
polymer films to a single film die. The rollers were 8.9 cm in diameter 
and 20.3 cm wide, and the surfaces were faced with stainless steel which 
were embossed with a diamond or knurled pattern, about 50-75 m deep, with 
a 90.degree. included angle for the sides, with the knurling lines at an 
angle of 30.degree. to the axis of rotation of the roll. The rotational 
speeds of the rollers were manually controlled using a variable speed 
drive motor and was set so the surface speed of the rolls was 6 m/min. The 
rate of oscillation was also manually controlled by a variable speed drive 
motor and was 50 Hz, while the amplitude of oscillation could be varied by 
changing the cam 17, and was 1.3 mm. Each roller was individually heated 
by Calrod.RTM. electrical heaters, which were in turn automatically 
controlled by digital controllers. It is believed that the roller 
temperatures could be maintained to about .+-.1.degree. C., and the roller 
temperatures were 177.degree. C. After passing through the oscillating 
rollers the film was passed through a set of cooling rolls and then rolled 
up on a roll. 
In the beginning of the run, the speed of extrusion and rotation of the 
oscillating rollers were adjusted so that a rolling bank of the polymer 
built up on the oscillating rollers, and then the speed of the oscillating 
rollers was set as closely as possible to maintain a constant sized 
rolling bank. Occasionally some manual adjustments were needed. 
In the resulting film, the Polymer A layer was about 0.051 mm thick, the 
Polymer B layer was about 0.10 mm thick and the Polymer C layer was about 
0.051 mm thick.