CA1136367A - Process for compression rolling of polymeric films - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE A process for compression rolling of thermoplastic sheet material such as polyethylene or polypropylene which involves passing the material between cylindrical rollers having surfaces constituted of lubricative material selected from the group consisting of polytetrafluoroethylene, polyamides, polycarbonates, polyacrylates, polymethacrylates, graphite and molybdenum sulfide, under 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. The circumferential speed of the rollers is maintained essentially equal to the linear speed of the thermoplastic sheet material passing therebetween and the film rewind tension is maintained 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

~7 PROCESS FORCOMPRESSION ROLLING OF POLY~ERIC FILMS
This invention relates to the produc-tion of polymer films by the continuous cola 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, 1978.
Films made from polymer materials, and particularly from synthetic organic thermoplastic polymers such as poly-ethylene and polypropylene, have found 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 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 , .. :

~3~3~7 with regard t~ film clarity and col~r uniformity.
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 thP p~lymer mol~cules within the latter. Previous efforts toward developing prvcesses for the compression rolling of plastic are described in Williams et al., SPE Journal 17, 42-48 (1971) and in U.S. 3,504,075, ~.S. 3,194,863, U.S.
3,083,410, and Re. 27,404. These methods generally involve "full fluia 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 hyarodynamic or full fluia lubrication the surfaces ~0 in relative motion (i.e., the work roll surfaces an~ polymeric sheet material) are separated at all times by a continuous uninterrupted fluid lubricant layer so that at no time is there actual physical contact between opposing suraces. In practice, however, it is often impossible or disaavantageous to maintain a continuous plastic film rolling operation under hydrodynamic lubrication conditions. Thus, hydrodynamic lubrication is limited by the adverse effect of the applied loads or pressures.
An increase in the applied load, a frequent requirement for achieving a aesired degree of reduction in the polymeric film with resultant improvements in the film clarity and ph~sical properties, requires a compensating increase in the viscosit~

of the fluid lubricant andfor an increase in the roll~n~ speed.

~L1363~77 Inasmuch as increments 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 successfully compression rolling under high roll pressures. Thus, increasing the ro7ling 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 hydro-dynamic 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 S and 95 percent, in the thickness of the sheet in a single pass.
It is a feature of the present invention that the compression rolling operation is carried out under conditions of lubrication known as boundary (dry, non-fluid) lubrication.
The latter condition includes, as the extreme case, dry rolling.
A process of this invention involves compression rolling of thermoplastic sheet material comprising passing the material between cylindrical rollers under boundary lubrication condition to effect a reduction in the original thickness of the material between about 5 and 95% in a single pass, the surface of said cylindrical rollers being constituted of lubricative material selected from the group consisting of polytetrafluoroethylene, polyamides, polycarbonates, polyacrylates, polymethacrylates, graphite and molybdenum sulfide, to achieve said boundary lubrication condition.

~L~L3~3~

The process of the invention is characterized by actual surface-to-surface contac-t between the rollers and the plastic sheet material. The primary purpose of any liquid which may be used is to serve either as a coolant and temperature regulator - 3a -~3&~
or as a means of establishing and maintaining frictional con-tact between the film and xoll surfaces rather than as a true hydrodynamic lubricant. The compression rolling of pla6tic~
under these conditions vf lubrication has been found to elimin-ate serious disadvantages characteristic of compression rolling under conditions of hydro~ynamic 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 deter-mined empirically by plottiny yield strength versus degree of film thickness reduction. It has been discovered that by operating under the aforesaid rewin~ tension, an unexpected enhancement in physical properties such as yield str~ngth, resistance to water penetration and film clarity are realized.
In this connection, the process of the invention can be con-ducted at rewind tensions which are slightly belo~ the vici~ityof the elastic limit of the film provided tha~ the concomitant loss in the aforesaid properties or the ~ecrease in the degree Qf film reduction can be tolerated for the particular use ~or which the film is intended~ It is also important to control the adjustment of the fllm unwind tension concurrent with the rewind tension ~1 order to ensure a proper rate of feed to the work rolls.
The work rolls used in the process of the present invent-ion are preferably precision flat profile work rolls (no "crown") and in this regard they differ from conventional metal rolling work rolls which usually have a deliberate convex ~crown" where-in the aiameter of the roll increases slightly lrv. Grld~ cen ~crO

3~

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 rol.ls, the idler rolls are ad-vantageously provided with a very slight convex crown to pre-~ent 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 sur-face finish as possible which has been found to further de-crease the tendency of the polymer film to develop wrink~ing.
One way of achieving a high surface finish on .the idler rolls is to apply a coating of fluorinated polymeric olefin (e.g., n*Teflonn.). An alternative technique suitable for use in -the present invention 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 aue in part at least to the behavior of the polymer ~ilm and whatever fluid ~o may be present between the rollers. In this connçction, it is helpful to consider the essential nature of the.three primary types of lubrication, namely~ hydrodynamic, semi-boundary, and boun~ary-lubrication. In so doing, reference is made to FIGS~ 1 and 2 of the drawings wherein the qualita-tive relationships of the major tribological parameters of lubricant viscosity (Z), rolling speed (N~ and applied load or pressures between the rollers (P) are sh~wn. FIG. 1 is a plot of the dimensionless parameter, ZN/P, versus the co-efficient of friction, ~. ~IG. 2 depicts the corresponding variation of ZN/P with lubricant film thickness, h. The three factors of ZN/P, coefficient of friction, and lubricant film tnickness are relatea to and aetermine the t~ee basic ty~e~

*Trade Mark for polytetrafluoroethylene 31~3~3~
of lubrication, namely, hydrodynamic, semi-boundary, ~nd boundary.
Hydrodynamic lubrication occurs when the values ~f lubricant viscosity (Z~, rolling speed (N), and pressure be-tween the rollers (P) are such as to ~orm a fluid film which generates suf~icient pressure to separate the roller surfaces from the surfaces of the sheet of material passing therebstween.
Referring the FIG. 2, it can be seen that hydrodynamic or full fluid lubrication becomes operative when the value of ~N/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 ~riction continues to increase. Thus, it can be seen that one of the practical disaavantages of compression rolling unaer conditions of hydrodynamic lubrication is that there is only one optimum value of the ~ parameter. On the other hand, as the Yalue of ZN~P decreases, the coefficient of friction ~) is no longer a linear function of ZN/P but rather, begins to increase as the fluid film thickness (h~ decreases. As ZN/P continues tn aecrease, we enter into a range in which the lubricating ~on-aitions are defined as semi-fluid or semi-boundary lubrication.
(See FIG. 1~. In this region, lubrication is neither hydro-dynamic nor is it boundary lubrication; rather, it involves elements of both types of lubrica~ion. With a still furth~r aecrease in ZN/P, the region of boundary lubrication is attained.
In this region of lubrication, a continuous fluid ~;lm no longer exists. The frictional and load bearing capabilities o~ the lubricant under conaitions of boundary l-~rica~ion ~re now 3~

primarily functio~s of the properties of the solid surface~
invol~ed, incluaing the surfaces of the polymeric film~, the work rolls, ana the lubricant itself which i~ interposed be-tween these surfaces.
Thus, it is possible to uti:Lize higher applied work roll loads when rolling under semi--boundary and/or boundary lubrication conditions ~han is practicable when compression rolling under the hydrodynamic conditions taught in the prior art. In particular, as the ZN/P conaitions operative with the use of fluid lubricants approaches the ar~a of semi-boundary lubrication, such lubricants become increasingly ~ne~fective and even inoperative. The only remedy if hyarodynamic lub-rication is to prevail is that of increasing the value of ZN/P by increasing the viscosity o~ the inert.fluid, increas-ing the rolling speeds, by decreasing applied loaas on the work rolls. These measures are counter-proauctive in practic~, particularly when lt is necessa~y ana aesirable to conauct compression.rolling und OE conditions of high work roll loaa-ings, and~or take a~vantage of heat control properties o~ l~w viscosity flui~s. . . ..
The unigue compression rolling technique of the present invention, which is outside the scope.o~ hydrodynamic lubricat-ion, proviaes substant;ally increased flexibiiity in the choice and application.of the operating parameters and in ~he produc-tion of better quality films. A.major.advantage of the invention is that it permi~s utilization of some of the beneficial charact-eristics o~ hydroaynamic lubrication without ~he attendant 2is-advantages, while also providing the superior virtues of . boundary lubrication. In this regard, a salient consideration is that o~.the desirability of physical contact betwee~ the roll sur~aces and the polymeric ~ilm surfaces, ~uch contact being impos~ibls ;n hydrodynami~ lubricat~on. mha : . : ' ;:
: ;, 3~i~7 ability to compresslon roll satisfactorily with solid--to-solid contact between the work rolls and the polymeric film improves the smoothness and related optical properties of the polymeric film surfaces. The flow oE the polymeric film between the work rolls is also more effectively controlled in the absence of a hydrodynamic film and in such cases the polymer film surface itself provides the necessary "lubrication".
In this connection, it has been found that the advant~
ages of the invention are only realized when the circumferent-ial speed of the work rolls is essentially equal to the linearspeed o e 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 increase by an amount equivalent to the lengthening of the film by virtue of the reduction in film thickness and by an incremental amount due to the phonomenon of forward extrusion. In order to realize this state of affairs, it is necessary 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 tendency toward breakage of the polymeric film in the work roll-film contact area. The avoid-ance of such slippage and the degree of reduction per pass can be enchanced 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 average coefficient of friction; however .

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the present invention is concerned with rollers having a surface constituted of lubricative material such as a fluorinated polymeric olefin (e.g., "Teflon") and which can be used for rolling of polymer film having a high coefficient of friction.
As a desirable option, the film emerging from the work rolls can be subjected to lateral tension, e.g., by use of a "tenter frame", in order 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 the direction in which the film travels. The use of a tenter frame in the practice of the present invention i5 an attractive feature compared to conventional compression rolling of polymeric materials since a rolled film in which the physical properties are enhanced 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 which is realized through this approach is the superior physical properties of the product, both laterally and transversely, -. -- g ~3~i3~7 which presumably result from the fact that the elements or "fibers~ i~ the netting are oriented At about a 45 degree angle to the direction of rolling. Cold compression rolled netting pxoduced according to the present invention is useful, for example, in making sacks for fruits or vegetables.
In rolling pre-formed polymer film to achieve a re-duction in thickness according to the present invention, it is desirable to employ a starting polymer material ~hich meet~
~airly precise control of yage aimensions, both from front-to-back and from side-to-side. In order to realize this, it may be desirable to "pre-condition" ~he starting fi~m prior to cold compression rolling, with a light reduction xolling or conditioning pass using heated rolls such that the polymer, e.g., polyethylene or polypropylene, is sub~ected to a temp-erature of between about 150-250F. and-preferably about 200F.
The achievement of boundary lubricat-ion conditions ;n the cold rolling of plastic sheet material accoraing to ~he present invention can-be achieved in practice ~y virtue of the fac~ that the specific ~ature of the lubricant does not affect the operating characteristics of ~ full fluid cola rolling process. Only when the conditions of semi-boundary ana ~oun~ary lubrication are achievea aO the properties of the l-ubricant affect the performance o~ the operation. Therefore, a change in the c~mposition of ~he lubricant during a col~
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 noilyness agents~
or "antiwear agen~sn (e.g., long-chain fatty acid salts) into a lubricant under semi-boundary or boundary l~brication con-ditions will cause the lubricant's coefficient of ~ric~ion toarop, thus necess;tating a aecrease in ~he film rewind tension~

This phenomenon is not observed when operatlng un~er con~i ~iG

, -lD-, ~L~L3~36~7 of full-fluid lubrication.
The conversion of a given full fluid (hydro~ynamic) plastic cold rolling process to the 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 increasing 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 emerging 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 discernible to the touch on the aforesaid surfaces when the rolling is conducted under full fluid lubrication.
The use of non-inert fluids and materials which possess desirable properties as lubricants under conditions of semi-boundary lubrication is illustrated in FIG. 3 wherein it can be seen that the incorporation of additives such as long chain polar compounds into the fluid permits extension of the effect of hydrodynamic lubrication into the semi-boundary lubrication area even though the film thickness has now become thinner than that associated with full hydrodynamic lubrication.

. .

~3~.~6t7 It has been discovered that certa:in solids are effective in the compression rolling of polymeric plastic films.
In the absence of any other fluid, water can be used in con-junction with these solids for purposes of heat control.
Examples of suitable solids found to be useful are polytetra-fluorethylene (Teflon), polyamides, polycarbonates, polyacrylates, and methacrylates. Solid films of colloidal 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 fluids such as the long chain polar compounds with non-polar fluids is also effective in the practice o the present invention.
The following examples are intended to illustrate without limitation, the cold rolling process of the present invention and that of the aforesaid Application No. 294,~40 and the advantages thereof~
EXAMPLE I
Compression Rolling: Semi-Boundary Lubrication . _ , A roll of high density polyethylene film (density = 0.9 to 0.99) 23.25 inches wide and 0.016 inch thick is mounted on an unwind spool at the entry side of a 4-hi cold rolling mill.
The roll diameters are 9 inches and the face width of each roll is 27 inches. The work rolls are provided with a chrome-nickel alloy finish and have 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 under a wide range of extensive tas opposed to compressive) stresses across the entire width of the film.

The film is threaded through the work roll and taken up on a rewind spool. The rewind spool is adapted to enable the film winding speed to be varied in relation to the peripheral ,~

.

~ L13fi36r~
speed o~ 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 film.
The take up sp~ol is activated and the gear~ of the work rolls are engaged to a speed of 125 rpm. The polymeric film in the contact areas of the two work rolls is subjected to increasing vertical pressures exerted through the work roll screw-down elements. The unwind and rewind tensions on the film are simultaneously adjusted to produce a compression rolled polymeric film of the desired thickness having greater flatness (i.e., uniform gauge across the width of the film), optimum clarity and optimum physical properties. The film entering the work rolls is flooded on both the top and bottom sides with water for purposes of cooling.
Under the foregoing conditions, the exit gauge of the film is'0.004 inch, representing a single-pass reductiun in gauge of ~5 percent (i.e.', reduction to 25 percent of the entry gauge). ' - -~xampI-e II
~ompression Rolling: Bou~aary (Dr~'~ lubri'c'a't'ion The procedure in the preceding ex~mple is repeated ex-cept that instead of flood cooling, the work rolls are pre-conditioned in the ~ollowing manner.
The work roll surfaces are thoroughly degreased with -the aid of an organic solvent such as naphtha, methylethyl ketone, tolue~e, benzene and the like. The work rolls are then vapor blastea by either conventional wet or ary blasting tech-niques using as the preferred grit aluminum ox;de particles'of Tyler mesh size in the 150 to 200 range. The vapor blasting is conducted so as to produce a surface finish in the range of 20 ~~ to 30 microinches. Finally, the work roll sur~aces are coated with a a~spersion of a 1:~ to 4:1 blend of finely divi~ed MoS2 (submicron to not more than 10 micro~ particle size3' and ~cro-nized graphite in a phenolic thermoplasti.c resin hinder.
This CoAtin~ is preferably applied by spraying, e.g., with an artist's air brush or commercial spray nozzel in 2 or 3 passe~.
to produce a coating having a uniform thickness of between 0.0002 and 0.0005 inch. The applied coating is then air cured until the surface is dry to the touch or, preferably, by exposure to infrared or other heating means ak a temperature of between 200 and 250F. for a period of time of between 15 and 30 minutes.
10 The compression rolling is carried out without the use of any flood cooling fluid. The polymer film and/or work roll surfaces are sprayed only as needed with a fine spray of water for the purpose of controlling the heat generated by.the friction : between the film and the work roll surfaces.
Example III
Compression Rollihg: Boundary (Dry) ~brication . _ The procedure in.the preceding example is repeated ex-cept that the preconaitioning of the work roll surfaces is carried out in the following manner to provide a dry, pre-lubricated surface on the work xolls.
After aegreasing and grit blasting of the work roll surfaces, the latter are spxayed with an extremely find aispersion of TFE fluorocarbon.in an inorganic binder and then cured. A suitable commercial formulation is *Molykote 523 manufactured by Dow Corning.
The foregoing examples are presented for the purpose of illustr.ating the process of the present invention~ It is understood that changes and variations can be made therein with-out aeparting from the scope of the invention as defined in the following claims.

*Trade Mark for molybdenum disulfiae lubricant " .. ' :, '

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 passing the material between cylindrical rollers under boundary lubrication condition to effect a reduction in the original thickness of the material between about 5 and 95% in a single pass, the circumferential speed of the rollers being substan-tially equal to the linear speed of the thermoplastic sheet material and the surface of said cylindrical rollers being constituted of lubricative material selected from the group consisting of polytetrafluoroethylene, polyamides, polycarbonates, polyacrylates, polymethacrylates, graphite and molybdenum sulfide, to achieve said boundary lubrication condition.