Polymer sheet, strip, or film is compression rolled continuously at ambient temperature between rollers under semiboundary or boundary lubrication conditions to effect a single pass reduction in the thickness of the polymer material of from 19/20 to 1/20 of the original thickness. Polymer films formed by this compression rolling process exhibit increased modulus (stiffness), tensile strength, and (in the case of non-opaque materials) enhanced clarity. The circumferential speed of the rollers is maintained essentially equal to the linear speed of the thermoplastic sheet material passing between the rollers and the film rewind tension is maintained in the vicinity of the elastic limit of the material exiting from the rollers.
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1. A process for compression rolling of thermoplastic sheet material comprising:
(a) passing the material between cylindrical rollers under semi-boundary or 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; (b) maintaining the circumferential speed of the rollers in step (a) essentially equal to the linear speed of the thermoplastic sheet material passing between said rollers; and (c) maintaining the film rewind tension in the vicinity of the elastic limit of the material exiting from the rollers.
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semi-boundary lubrication conditions. 9. 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 surfaces; 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 percent in a single pass; c. maintaining the circumferential speed of the roller in step a. essentially equal to the linear speed of the thermoplastic sheet material passsing between said rollers; and d. maintaining the film rewind tension so as not to exceed the elastic limit of the material exiting from the rollers. 10. A process for compression rolling of thermoplastic sheet material comprising: a. passing the material between cylindrical rollers under a dry boundary lubrication condition to effect a reduction in the original thickness of the material of between about 5 and 95 percent in a single pass, the surfaces of said cylindrical rollers being constituted of lubricative materials selected from the group consisting of polytetrafluoroethylene, polyamides, polycarbonates, polyacrylates, polymethacrylates, graphite, and molybdenum sulfide, to achieve said boundary lubrication conditions; b. maintaining the circumferential speed of the rollers in step a. essentially equal to the linear speed of the thermoplastic sheet material passing between said rollers; and c. maintaining the film rewind tension so as not to exceed the elastic limit of the material exiting from the rollers. 11. A process for compression rolling of thermoplastic sheet material comprising: a. 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% in a single pass; b. maintaining the circumferenital speed of the rollers in step (a) essentially equal to the linear speed of the thermoplastic sheet material passing between said rollers; and c. maintaining the film rewind tension so as not to exceed the elastic limit of the material exiting from the rollers. 12. A process for compression rolling of thermoplastic sheet material comprising: (a) passing the material between cylindrical rollers under boundary lubrication conditions other than completely dry lubrication to effect a reduction in the original thickness of the material of between about 5 and 95 percent in a single pass; (b) maintaining the circumferential speed of the rollers in step (a) essentially equal to the linear speed of the thermoplastic sheet material passing between said rollers; and (c) maintaining the film rewind tension so as not to exceed the elastic limit of the material exiting from the rollers. |
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optimum value of the ZN/P parameter. On the other hand, as the value of ZN/P decreases, the coefficient of friction (μ) 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 to decrease, we enter into a range in which the lubricating conditions are defined as semi-fluid or semi-boundary lubrication. (See FIG. 1). In this region, lubrication is neither hydrodynamic 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 of lubrication, a continuous fluid film no longer exists. The frictional and load bearing capabilities of the lubricant under conditions of boundary lubrication are now primarily functions of the properties of the solid surfaces involved, including the surfaces of the polymeric films, the work rolls, and the lubricant itself which is interposed between these surfaces.
Thus, it is possible to utilize higher applied work roll loads when rolling under semi-boundary and/or boundary lubrication conditions that is practicable when compression rolling under the hydrodynamic conditions taught in the prior art. In particular, as the ZN/P conditions operative with the use of fluid lubricants approaches the area of semi-boundary lubrication, such lubricants become increasingly ineffective and even inoperative. The only remedy if hydrodynamic lubrication is to prevail is that of increasing the value of ZN/P by increasing the viscosity of the inert fluid, increasing the rolling speeds, and/or by decreasing applied loads on the work rolls. These measures are counter-productive in practice, particularly when it is necessary and desirable to conduct compression rolling under conditions of high work roll loadings, and/or take advantage of heat control properties of low viscosity fluids.
The unique compression rolling technique of the present invention, which is outside the scope of hydrodynamic lubrication, provides substantially increased flexibility in the chioce choice and application of the operating parameters and in the production of better quality films. A major advantage of the invention is that it permits utilization of some of the beneficial characteristics of hydrodynamic lubrication without the attendant disadvantages, while also providing the superior virtues of semi-boundary and boundary lubrication. In this regard, a salient consideration is that of the desirability of physical contact between the roll surfaces and the polymeric film surfaces, such contact being impossible in hydrodynamic lubrication. The ability to compression 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 of 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 advantages of the invention are only realized when the circumferential speed of the work rolls is essentially equal to the linear speed of the plastic material passing therebetween. More particularly, it is important that the work roll circumferential speed be equal to the entering linear speed of the polymeric 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 phenomenon 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 polymer 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 avoidance of such slippage and the degree of reduction per pass can be enhanced by selecting a work roll surface having a coefficient 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; work rolls coated with a low-friction material such as fluorinated polymeric olefin (e.g., "Teflon") can be used for rolling the polymer film having a high coefficient of friction; hard rubber-clad work rolls whose coefficients of friction are relatively high can be advantageously 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 alloy steel work roll which can take and retain a high polish, the degree of polish required 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 aqueous solutions or inorganic silicates can be used in lieu of increasing the coefficient of friction of the work rolls.
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 is 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 a 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, which presumably result from the fact that the elements or "fibers" in the netting are oriented at about 45° angle to the direction of rolling. Cold compression rolled netting produced according to the present invention is useful, for example, in making sacks for fruits or vegetables.
In the rolling pre-formed polymer film to achieve a reduction 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-condition" 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 temperature of between about 150°-250° F. and preferably about 200° F.
The achievement of semi-boundary or boundary lubrication 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 does not affect the operating characteristics of a full fluid cold rolling process. Only when the conditions of semi-boundary and boundary lubrication are achieved do the properties of the lubricant affect the performance of the operation. Therefore, a change in the composition of the lubricant during 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 acid salts) into a lubricant under semi-boundary or boundary lubrication conditions will cause the lubricant's coefficient of friction to drop, thus necessitating a decrease in the film rewind tension. This phenomenon is not observed when operating under conditions of full-fluid lubrication.
The conversion of a given full fluid (hydrodynamic) plastic cold rolling process to the semi-boundary or 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 of 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 semiboundary 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.
Examples of non-inert fluids and materials which possess desirable properties as lubricants under conditions of semi-boundary lubrication suitable for use in the present invention are natural fats including vegetable, animal and marine compounds, long chain fatty acids alcohols, amines, amides, polyethylene glycols, esters of these and of various acids and alcohols, and the like. When used as such, they act as hydrodynamic fluids in the same fashion as any inert fluid of equivalent viscosity properties, but additionally, are effective lubricants under semi-boundary conditions.
In addition to fluid, it has been discovered that certain solids are likewise effective in the compression rolling of polymeric plastic films. In the absence of any other fluid, water can be used in conjunction with these solids for purposes of heat control. Examples of suitable solids found to be useful are polytetrafluorethylene (Teflon), polyamides, polycarbonates, polyacrylates and methacrylates. Solid films of colloidal graphite, colloidal molybdenum sulfide such or pre-applied to the work roll surfaces with suitable bonding agents are also effective under certain desirable operating conditions. It has 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 of the present invention.
The following examples are intended to illustrate, without limitation, the cold rolling process of the present invention and the advantages thereof.
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 (as 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 speed of the work rolls which permits the film 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 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 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 reduction in gauge of 75 percent (i.e., reduction to 25 percent of the entry gauge).
Compression Rolling: Boundary (Dry) Lubrication
The procedure in the preceding example is repeated except that instead of flood cooling, the work rolls are preconditioned in the following manner.
The work roll surfaces are thoroughly degreased with the aid of an organic solvent such as naphtha, methylethyl ketone, toluene, benzene and the like. The work rolls are then vapor blasted by either conventional wet or dry blasting techniques using as the preferred grit aluminum oxide 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 surfaces are coated with a dispersion of a 1:4 to 4:1 blend of finely divided MoS2 (submicron to not more than 10 micron particle size) and micronized graphite in a phenolic thermoplastic resin binder. This coating is preferably applied by spraying, e.g., with an artist's air brush or commercial spray nozzel nozzle in 2 or 3 passes 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 at a temperature of between 200° and 250° F. for a period of time to between 15 and 30 minutes.
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.
Compression Rolling: Boundary (Dry) Lubrication
The procedure in the preceding example is repeated except that the preconditioning of the work roll surfaces is carried out in the following manner to provide a dry, prelubricated surface on the work rolls.
After degreasing and grit bearing of the work roll surfaces, the latter are sprayed with an extremely find fine dispersion 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 illustrating the process of the present invention. It is understood that changes and variations can be made therein without departing from the scope of the invention as defined in the following claims.
Kipp, Egbert M., Jenks, Richard H.
| Patent | Priority | Assignee | Title |
| 11155049, | Feb 23 2016 | Subaru Corporation | Composite material shaping device and composite material shaping method |
| 4851173, | Sep 24 1984 | British Technology Group Limited | Oriented polymer films, a process for the preparation thereof |
| 4857127, | Mar 26 1986 | British Technology Group Limited | Process for preparing improved oriented polymer films and tapes |
| Patent | Priority | Assignee | Title |
| 2244208, | |||
| 3083410, | |||
| 3365352, | |||
| 3465070, | |||
| 3470288, | |||
| 3504075, | |||
| 3875284, | |||
| 27404, |
| Executed on | Assignor | Assignee | Conveyance | Frame | Reel | Doc |
| Jan 26 1983 | Revere Copper and Brass Incorporated | (assignment on the face of the patent) | / | |||
| Dec 30 1986 | REVERE COPPER AND BRASS INCORPORATED, A DE CORP | BT COMMERCIAL CORPORATION, A NEW YORK CORP | SECURITY INTEREST SEE DOCUMENT FOR DETAILS | 004659 | /0974 |
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