CA1137725A - Process for compression rolling of polymeric films - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE A process for compression rolling of thermo-plastic sheet material such as polyethylene or polypropylene which involves passing the material between cylindrical rollers under semi-boundary lubrication conditions to effect a reduction in the original thickness of the material of between about 5 and 95 percent in a single pass while maintaining the circumferential speed of the rollers essentially equal to the linear speed of the thermoplastic sheet material passing therebetween and maintaining the film rewind tension in the vicinity of the elastic limit of the material exiting from the rollers. The process is useful in that it can be carried out by continuous cold rolling, i.e., at ambient temperature.

Description

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PROCESS FORCOMPRESSION ROLLING OF POLYMBRIC FILMS

This invention relates to the production of polymer films by the con-tinuous co:Ld rolling of thermoplastic polymer in sheet form. This application is a division of our co-pending Canadian Patent Application No. 294,940 filed January 13, 197~.
Films made from polymer rnaterials, and particularly from synthetic organic thermoplastic polymers such as polyethylene and polypropylene, have Eound widespread utility in such diverse fields as packaging, construction, magnetic tape recording and photography. However, there has been a long-felt need among film processors and users for polymer films having improved physical properties such as strength, stiffness and clarity.
Various methods have been developed in the past for enhancing the physical properties of already-formed (e.g., cast, extruded or skived) films. For example, the film may be fed into a quenching bath immediately after having been formed by melt-extrusion or casting. In addition, films may be stretched in one or more directions or calendered at 7 temperatures above the softening point or range of the polymeric - material by means of heated rollers. However, these post-- formation procedures for improving the characteristics of polymer films have drawbacks which limit their usefulness in many cases. Thus, stretching methods tend to enlarge any pin holes or voids which may be present in the polymer film as originally prepared, thereby decreasing the moisture- ;
barrier properties of the film and diminishing its usefulness for many packaging purposes. Moreover, calendering often fails to achieve the degree of property enhancement desired, particularly with regard to film clarity and color uniformity.
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Another method which has been investigated with a view toward the processing of pre-formed polymer films, and to which the present invention is directed, involves the compression roll-ing of thermoplastic sheet material which in effect extends and orients the polymer molecules within the latter. Previous efforts toward developing processes for the compression rolling ~; of plastiF are described in Williams et al., SPE Journal 17, 42-48 (1971) and in U.S. 3,504,075, U.S. 3,194,863, V.S.
3,083,410, and Re. 27,404. These methods generally involve i nfull fluid rolling" (i.e., rolling with the use of a layer of lubricant between the film and roller surfaces~. In this tech-nique, the surfaces of the plastic sheet material and the rollers at the ~nip~ (i.e., the point at which compression actually takes place) are separated by the lubricant which forms a "hydrodynamic wedge" between the rollers and the sheet material in front of the nip as the material passes between ' the rollers.
In hydrodynamic or full fluid lubrication the surfaces ' 20 in relative motion ~i.e., the work roll surfaces and polymeric sheet material) are separated at all times by a continuous uninterrupted fluia lubricant layer so that at no time is there actual physical contact between opposing surfaces. In practice, ' however, it is often impossible or disa'dvantageous to maintain a continuous plastic ~ilm rolling operation under hydrodynamic lubrication-conditions. Thus, hydrodynamic lubrication is limited by the aaverse effect of the applied loads or pressures.
An increase in the applied load, a freguent reguirement for achieving a desired degree of reduction in the polymeric film with resultant improvements in the film clarity and phys;cal properties, reguires a compensating increase in the ViSCosIty of the fluid lubricant andJor an incre~se in the roll~ny speed.

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., Inasmuch as lncrements of applied load may require proportion-ately much larger increases in the fluid viscosity and/or rolling speed, the compression rolling of polymeric films under conditions of hydrodynamic lubrication imposes serious practical limitations in successfu]ly compression rolling under high roll pressures. Thus, increasing the rolling speed causes the generation of unwanted heat as a result ; of the additional work done on the fluid film. On the other hand, for a given rolling speed and applied load, there is only one optimum value of the lubricant viscosity under hydrodynamic conditions.
The foregoing problems are solved according to the present invention by the cold compression rolling (i.e., rolling at ambient temperature) of pre-formed polymer sheet or film material between rollers which exert a pressure on the sheet at the "nip" or roll contact area which is sufficient to effect a substantial reduction, between about 5 and 95 percent, in the thickness of the sheet in a single pass. A fluid can be used in the manner described hereinbelow to facilitate the passage of the sheet between the nip of -the rollers. This fluid, which is not a "lubricant" in the normal sense of the word, but will be termed a semi-boundary lubricant, can be applied to either the polymer sheet material directly or placed on the rollers so as to transfer it to the material as it passes between the rollers, or both.
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More particularly, the present invention provides a process for compression rolling of thermoplastic sheet material comprising:
' 30 a. passing the material between cylindrical rollers with a semi-boundary lubricant ` applied to the surfaces of the material to :1, ~ - 3 -~, l3~;2 5 establish and maintain frictional contact between surfaces; and b. adjusting the circumferential speed of the rollers and -the pressure between the rollers to achieve and maintain a semi-boundary lubrication condition and to effect a reduction in the oriyinal thickness of the material ~ of between about 5 and 95% in a single pass.
; The process of the invention is characterized by actual surface-to-surface contact between the rollers and the plastic sheet material. The primary purpose of the semi-boundary lubricant is to serve either as a coolant and tem-perature reyulator , ' i~ ~

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or as a means of establishing and maintaining frictional con-tact between the film and roll surfaces rather than as a true hydrodynamic lubricant. The compression rolling of pla~tic~
under thPse conditions of lubrication has been found to elimin-ate serious disadvantages characteristic of compression xolling under conditions of hydrodynamic :Lubrication and, additionally, ;~ results in important processing benefits.
It is a further feature of the present invention that the "film rewind tension", i.e., the tension which the film rewind rollers exert on the film emerging from the mill or work rolls, is kept as high as possible without exceeding the elastic limit or yield strength of the polymer film. The optimum rewind tension for a particular sample of polymer film can be aeter-. ~ , ................. .
mined empirically by plotting yield strength versus degree of .. . . .
film thickness reduction. It has heen discoverea that by : .
operating unaer the aforesaid rewind tension, an unexpectedenhancement in physical properties such as yield strength, resistance to water penetration and film clarity are realized.
In this connection, ~he process of the invention can be con-` 20 aucted at rewind tensions which are slightly belo~ the vicinityof the elastic limit of the film provided that the concomitant ; loss in the aforesaid properties or the decrease in the ~egree of film reduction ca~ be tolerated for the particular use for i which the fi~m is intended. It is also important to control the adjustment o~ the film unwind tension concurrent with the rewind tension in order to ensure a proper rate of feed to the work rolls.
y~ The work rolls used in the process of the p~esent invent-ion are preerably precision flat profile work rolls (no ~crown") : 30 and in this regaxd they differ from conventional metal rolling ~- - work rolls which usually have a deliberate convex ~crown" where-., . . - .
! in the aiameter of the roll increa~es slightly from end-to-center.

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~i - ~3'^~5 In some cases, it may be desirable in the practice of the present invention to employ work rolls having a concave crown, wherein the diameter of the roll decreases slightly from end-to-center.
In contrast to the work rolls, the idler rolls are advantageously provided with a very slight convex crown to prevent film wrinkling; the degree of crown depending on the width of the film being rolled. In addition, the idler rolls, with the exception of the rewinds, should have as high a surface finish as possible which has been found to Purther decrease the tendency of the polymer film to develop wrinkling. One way of achieving a hi~h surface finish on the idler rolls is to apply a coating of fluorinated polymeric olefin (e.g., "*Teflon"). An alternative technique suitable for use in the present inyention is the use of crowned "herringbone" idlers.
Without wishing to be bound to theory, it is believed that the success of the present invention is due in part at least to the behaviour of the polymer film and the fluid present between the rollers. In this connection, it is helpful to consider the essential nature of the three primary types of lubrication, namely, hydrodynamic, semi-boundary, and boundary lubrication. In so doing, reference is made to FIGS. 1 and 2 of the drawings wherein the qualita-tive relationships o~-the major tribological parameters of lubricant viscosity (z), rolling speed (N) and applied load or pressures between the roller (P) are shown. FIG. 1 is a plot of the dimensionless parameter, ZN/P, versus the co-efficient or friction, ~. FIG. 2 depicts the corresponding variation of ZN/P with libricant film thickness, h. The three factors of ZN/P, coefficient of friction, and lubricant film thickness are related to and determine the three basic types _ 5 _ *Trade Mark for polytetrafluoroethylene æs of lubrication, narnely, hydrodynamic, semi-boundary, and ; boundary.
~ydrodynamic lubrication occurs when the values of lubricant viscosity ~Z), rolling speed (N), and pressure be-tween the rollers (P) are ~uch as to form a fluid film which generates sufficient pressure to separate the roll~r surfaces from the surfaces of the sheet o~ material passing therebetween.
Re~exring the FIG. 2~ it can be seen that hydroaynamic or full fluia lubrication becomes operative when the value of ZN/P is sufficiently large so as to produce a fluid film of maximum thickness. These are also the conditions under which the coefficient of friction is at a minimum value, as shown in FIG. 1. Further increases in the value of ZN/P have no further beneficial effects in terms of increased lubricant film thickness. Instead, the thickness of the lubricant film remains approximately constant while the coefficient of friction continues to increase. Thus, it can be seen that one of the practical disadvantages of compression rolling under conditions of hydrodynamic lubrication is that there is onl~ one optimum value of the ~ parameter. On the other hana, a5 ~he value ! of ZN~P decrease~, the coefficient of friction (~) is no longer a linear ~unction of ZN/P but rather, begins to increase as the fluid film thickness ~h) ~ecreases. As ZNjP continues to s decrease, we enter into a range in which the lubricating con-ditions are defined as semi-fluid or semi-bounaary lubrication~
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(See FIG. 1)~ In this regionJ lubrication is neither hydro-dynamic nor is it boundary lubrication; rather, it involves elements of both types of lubrication. With a still further decrease in ZN/P, the region of boundary lubrication is attained.
In this region o~ lubrication, a continuous *luid film no longer exists. The ~xictional and load bearing capabilities of ~he lubricant unaer con~itions of bounaary ll~ri~a~ion ~re now , ;

3~'7~5 primarily functions of the properties of the solid sllrfaces involved, includiny the ~urfaces of the polymeric films, the work rolls, and the lubricant itself which is interpo~ed be-~ween these surfaces.
Thus, it iB possible to utilize higher applied woxk roll loads when rolling under semi-boundary and/or boundary lubrication conditions than is practicable when compression rolling under the hydrodynamic conditions taught in the ~rior art. In particular, as the ZN/P conditions operative with the use of fluid lu~ricants approaches the area of semi-boundary lubrication, such lubricants become increasingly ineffective and even inoperative. The only remedy if hydrodynamic lub-rication is to prevail is that of increasing the value of ZN~P by increasing the viscosity of the inert fluid, increas-ing the rolling speeds, by decreasing applied loads on the work rolls. These measures are counter-pro~uctive in practice, particularly when it is necessary and desirable to conduct compression rolling under conditions of high work roll load-ings, and~or take advantage of heat control properties of low viscosity fluids.
- The unigue cQmpression rolling technique of the present ~, , .
invention, which is outside the scope of hy~ro~ynamic lubricat-ion, provides substantially increased flexibility in the choice ; and application of the operating parameters ana in the produc-tion of better guality films. ~ majox advantage o~ the invention is that it permits utilization of some of the beneficial charact-eristics of hydro~ynamic lubrication without the attendant aiS-advantages, while also provi~ing the superior vir~ues of semi-boundary lubrication. In this re~ard, a -~alient csnsideration is that of the aesirability of physical contact --- between the roll surfaces and the polymeri~ film surfaces, such contact being impo~sible in hy~rodyn~mic lu~r;cat~on~ T~

ability to compression roll satisfactorily with ~olid-to-solid contact between the work rolls and the polymeric film Lmprove~
the smoothness and related optical properties of the pol~eric film surfaces. The flow of the po'lymeric film between the work rolls is also more effectivel~y controlled in the absence of a hydrodynamic film and in such cases the pol~ner film surface itself provides the necessary "lubricationn.
In this connection, it has been ound that the advant-ages of the invention are only realized when the circumferent-; 10 ial speed of the work rolls is essentially e~ual to the linearspeed of the plastic material passing therebetween. More . particularly, it is important that the work roll circumferent-; ial speed be equal to.the entering linear speed of the poly-~........ meric sheet material plus an incremental amount resulting from . the reduction in gage as the film exits from the work rolls.
This is due to the fact that as.the polymeric material passes through.the "neutral" roll contact area, the speed of the polymeric film exiting from that' area will incxease by an amount equivalent to the lengthening of ~he film ~y virtue of the reauction in film thickness.and by an incr~mental amount due to the phenomenon of forward extrusion. In oraer to ' realize this.state of affairs,.it is necessar~ to prevent any slippage between the surfaces of the work rolls and poly-mer film, since work.roll speeds which are either excessive or significantly less than the other mill operating parameters, '' will greatly increase the tendenc~ toward ~reakage of the polymeric ~ilm in the work' roll-film contact area. The avoia-ance of such slippage and the degree of reduction per pass can be enhanced by selecting a work roll surface having a coeffic-ient of friction appropriate to that of the polymer being rolled.
Thus, conventional alloy tool steel work roll surfaces can be used to roll plastics of-~verage coeffici~nt of friction; wor~

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s rolls coated with a low-friction m~terial s~lch as a fluorin-ated polymeric olefin ~e.g., "Teflonn) can be used for roll-; ing of polymer film having a high coefficient of friction;
hard rubber-clad worX rolls whose coeffecients of friction are relatively high can be aavantageously employed to roll more slippery films such as those made from polyolefins.
Relatively high coefficient of friction work roll surfaces can also be achieved by the use of highly polished chrome plated, nickel plated or any conventional a~loy steel work roll which can take an~ retain a high polish, the degree of ¦ polish re~uired to achieve a desired coefficient of friction being determinable on a case-by-case basis~ In addition, so-, called "non-lubricant~ or "anti-lubricant" fluids such as i aqueous solutions of inorganic silicates can be used in lieu - of increasing the coefficient of friction of the work rolls.
As a desirable option, the film emerging fro~ the work rolls can be subjected to l~teral tension, e.g., ~y use of a ntenter frame", in o~der to improve the properties of the film in this direction, since the work rolls ordinarily contribute to the properties of the rolled film primarily in ~he airection in which the film travels. The use of a tenter frame in the practice o~ th~ present invention is an attracti~e feature com-pared to conventional compression rolling of polymeric materials since a rollea film in which the physical properties are en-hanced in the lateral as well as in the direction of rolling has greater applicability in a wider range of uses than a film with only unidirectional improvement in properties.
It is also possible to use the cold compression rolling process of the invention to produce polymeric netting from plastic sheet netting material. An unexpected advantage wh;ch is realized through this approach is the superior physical ~~~properties o~ the~proauct, both later~lly and transversely, _9_ .

.3~7Z~5 which presumably result from the fact that the elements or "fibers" in the netting are oriented at about a 45 degree angle to the direction of rolling. Cold compression rolled netting produced according to the present invention is useful, for example, in making sacks for fluits or v~getables.
In rolling pre-formed polymer film to achieve a re-auction in thickness according to the present invention, it is desirable to employ a starting polymer material which meets fairly precise control of gage dimensions, both from front-to-back and from side-to-side. In order to realize this, it may be desirable to ~pre-con~ition~ the starting film prior to cold compression rolling, with a light reduction rolling or conditioning pass using heated rolls such that the polymer~
e.g., polyethylene or polypropylene, is subjected to a temp-erature of between about 150-250F. and preferably about 200DF.
The achievement of semi-boundary lub~icat-ion conditions in the cold rolling of plastic sheet material according to the present invention can be achieved in practice by virtue of the fact that the specific nature of the lubricant ? aoes not affect the operating characteristics of ~ full flui~
cold rolling process. Only when the conditions of semi-boundary and houndary lubrication are achieved do the properties of the lubricant affect the performance of the operation. Therefore, a change in the composition of the lubricant auring a cold rolling process will serve as an indicator of whether or not the process conditions of the present invention have been - realized. Thus~ the incorporation of so~called "oilyness agents"
or "antiwear agents" (e.g., long-chain fatty acia saltsj into a lubricant under semi-boundary or boundary l~brication con-aitions will cause the lubriçant7s coeEficient of friction to arop, thus necessitating a decrease in the film rewind tension.
This phenomenon is not observed when operating under condi~iol.s ,: . , . . . . . ~ . .... . ... . ~ . . ...

of full-fluid lubrication.
The conversio~ of a given full fluid ~hydrodynamic) plastic cold rolliny process to the semi-boundary lubrication method of the invention is conveniently brought about by increasing.the unit load on the rollers, decreasing the linear speed of the plastic sheet material through the rollers, decreasing the diameters of the rollers, or in~
creasing the rewind tension on the sheet emerging from between the rollers. Under conditions of boundary lubrication, the surfaces of the work rolls and the rolled plastic film emerg-ing from the roll nip are dry to the touch even when the operation is accompanied by the use of a fluid coolant or "non-lubricant". In contrast, a layer of fluid.is clearly ais-cernihle to the touch on the aforesaid suraces when the rolling is conducted under .full fluid lubrication.
The use of non-inert fluids and materials which possess desirable propertie~ as lubricants under conditions of semi-~ boundary lubrication is illustrated in FIG. 3 wherein it can i be seen that the incorporation of additives such as long chain polar compoun~s into the fluid permits extension of the effectof hydrodynamic lubrication into ~he semi-boundary lubrication area e~en though the film thickness has now.become thinner than that associated with full hydrodynamic lubrication.
Examples on non-inert ~luids and materials which possess : - desira~le properties as lubricants unaer conaitions o~ semi-boundary lubrication suitable~ for use in the present invention are natural fats including vegetable/ animal and marine com-pounds, long chain fatty acids, alcohols, amines~ amiaes, poly-ethylene gylcols,.esters of these and o~ ~arious acids and.
alcohols, and the like. When used as such, they act as hydro--- . dynamic fluias in the same fashion as any inert fluid of e~iv21e~t viscosity properties, but additionally, are eff~tiYe 3~7;~

lubricants under semi-boundary conditions~ I
: In addition to fluid, it has been discovered that cer-: tain solids are likewise effecti~e in the compression rolling . .
.~ of polymeric plastic films. In the absence of any othex fluid, ~ water can be used in conjunction with these s~lids for purposes : of heat control~ Examples of suitable solids found to be use-ful are polytetrafluoreth~lene (Tefl~n?., poly~mides, polycarbon-~ ates, polyacr~lates and methacrylates. Solid films o collQid-;. al graphite, colloidal molybdenum sulfide as such or pre-applied to the work roll surfaces with suitable bonding agents are also .
effective under certain desirable operating conditions. It . has been further discovered that the combined.use of flui~s such .
;~ as the long chain .polar compounds with non-polar fluids is al50 effective in the practice o~ the present invention.
The following example is intended to illustrate, without limitationf the cold rolling process of the present- ..
invention and the advantages thereofO
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~ ompression ~olling- ~emi-Bounda~ L~brication . _ . .
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A roll of high density polyethylene film (~ensity = 0.9 ~ to 0.99) 23.25 inches wide and 0.016 inch thick is moun$ed on an unwind spool at the entry side of a 4-hi cold rolli~g mill.
The roll diameters are 9.inches.and the face width of each roll is 2.7 inches~ The work rolls are provided with a chrome-nickel alloy finish and ha~e a precision flat profile (no crown)~ The unwind spool is equipped with a brake or clutch-whereby the polymeric film can be fed to the work rolls unaer a wide range of extensive (as opposed ~o compressive) stresses across the ,. . . :
entire width of the film.
The film i.s threaded through.-the work roll and taken up ~ -- . on a rewind.spool. The rewind spool is adapted to enable the film~winaing~speea to ~ variea in relation to the per.ph~lcil ~ , .
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speed of the work rolls which permits the ilm exiting from the work rolls to be subjected to a range of uniform extensive stresses across the full width of the ~
The take up spool is activated and the gears of the work rolls are engaged to a speed of 125 rpm. The polymeric film in the contact areas of the t~wo work rolls is subjected to increasing vertical pressures e.xerted through the worX roll screw-down elements. The unwind and rewind tensions on the film are simultaneously adjusted to produce a compression -10 rolled polymeric film of the desired *hickness ha~ing greaterflatness (i.e.j uniform gauge across the width o the film3, optimum clarity and optimum physical properties. The film entering the work rolls is flooded on hoth the top and bottom si~es with water for purposes of cooling.
Under the foregoing conditions, the exit gauge of ~he film is 0.004 inch, representing a single-pass reduction in gauge of ~5 percent (i.e.~ reduction to 25 percent of the entry gauge).

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Claims

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:-

1. A process for compression rolling of thermoplastic sheet material comprising:
a) passing the material between cylindrical rollers with a semi-boundary lubricant applied to the surfaces of the material to establish and maintain frictional contact between the surfaces of the sheet material and cylindrical rollers; and b) adjusting the circumferential speed of the rollers and the pressure between the rollers to achieve and maintain a semi-boundary lubrication condition and to effect a reduction in the original thickness of the material of between about 5 and 95% in a single pass, said circumferential speed of the rollers being substantially equal to the linear speed of the thermoplastic sheet material.