CA1195072A - Roll compacting of polymer powders into fully dense products - Google Patents

Abstract

ABSTRACT

ROLL COMPACTING OF POLYMER
POWDERS INTO FULLY DENSE PRODUCTS

A flowable thermoplastic polymer powder is fed to the nip of a pair of cooperatively rotating com-pression rolls whereupon the polymer feed is passed between the rolls which compact and cause the particles to coalesce into a shaped article which thereupon emerges from between the rolls in the form of a sheet or film.
During the operation, the circumferential speed of the rolls is maintained essentially equal to the linear speed of the sheet or film exiting therefrom. The sheet or film is withdrawn from between the rolls under tension at a force which is adjusted so as not to exceed the elastic limit of the resulting sheet or film product. Flowable thermosetting polymer powder is also compacted into fully dense products by feeding the powder to the nip of a pair of cooperatively rotating compression rolls, whereupon he polymer feed is passed between rolls which compact and cause the particles to cold weld.

Description

56~7~

-2- l ROLL COMPACTING OF POLYMER
POWDERS INT~ FULLY DENSE_PRODUCTS

DESCRIPTION

Technical Field This invention relates to ~he formation of polymer sheets and films direc~ly from polymer powder by continuous compaction rolling/ and to the products obtained thereby.

Ba_k.~ Art Conventional methods for fabricating thermoplastic polymeric materials into shaped articles include extrusionr casting, injection molding and other hot-forming techni~ues. These techniques commonly involve three basic steps: (l) melting or softening th~
thermoplastic material; (2) shaping the molten or softened polymer with or without pressure in a mold cavity, in a press or throuyh a die; and (3) cooling the shaped article in its final shape~ However, this procedure becomes cumbersome when processin~ thick sections and is totally unsuitable when working with very viscous~ ultra-high molecular weight po:Lymers o~ those with very high melting points. On the other hand, interest in these two latter Gategories of thermoplastics is growing rapidly because of their unique thermal and mechanical properties.
Cold-forming, i.e., shaping a material below its .. - melting point, i6 a processing technique that has been well developed ln metallurgy but only recently applied in the ield o~ polymers~ Most of these recent polymer applications involve stamping and ~orging, machining~ deep drawing~ cold rollings or cold extrusion. In all these processes, a starting material in the shape of a sheet or billet of relatively thick cross-section is required/
which is itself usually prepared by hot extrusion. The combination of hot-forming followed by cold-forming and, in the case of machining, production of scrap ~hich may or not be reusable, adds to the cost of the overall shaping operation and represents a significan~ engineering and economic drawback. Nevertheless, there are considerable incentives to applying cold-forming techniques to shaping thermoplastics. For instance, parts are shaped entirely in the solid state, and since there is therefore no phase change which wou:ld otherwise cause shrinkage and distortion, adherence to strict dimensional tolerances i~
facilitated. Alsov enhancement of certain engineering properties of the material is often realized.
Generally, for a thermoplastic polymer material to be formable in the solid phase/ it must have ductility and strength. Materials of this type which have been cold formed include acrylonitrile-butadiene-styrene copo]ymers (ABS resins) t cellulose acetate-butyrate, polycarbonatesr polysulfones, polyvinylchloride (PVC) and polyolefins (e.g. t high molecular weight, high density polyethylene~
Most such forming operations take place lO-20C below the melting point or glass transition temperature oE the polymer.
Powder processing technology has been Eully developed for metals, where it has in many instances shown itself to be more a-ttractive than hot forging and melt processing, i.e~, casting. In the polymer field~ however, only a relatively few investigations, of a preliminary nature, have been made, as exemplified in the following publications:

D.M. Bigg~ l'High~Pressure Molding of Polymeric Powders'l5 7~

33rd Annual Technical Con~erence, Society of Pla~tics Engineers, p. 472 (1975);

M.A. Rudner, t'Fluorocarbons" (Reinhold 1953);

G.W. Halldin and IoL~ Kamel, "Powder Processing of Ultrahigh Molecular Weight Polyethylene, I~ Powder Characterization and Compaction', Polymer Engineering and Science/ 17(1), ~1 (1977~;
~ = .

G.W. ~alldin and I.L. Kamel, "Powder Processing of Ultrahigh Molecular Weight Polyethylene, II. Sinteringl', 35th Annual Technical Conference, Society of Plastics Engineerst 298 (1977~;

G.S. Jayaraman, J.F. Wallace, P.H. Geil and E. Baer, "Cold Compaction Molding and Sintering of Polystyrenei', Polymer Engineering and Science, ~ , 529 ~1976);

20 U-S- Pat. No. 2.,067,025 ~1937) to Schmidt for "Method of Transforming Polymerized Vinyl Chloride Into Thin Sheets and Product Obtainable Therebyi';

U.S. Pat. No. 2,528,529 (1950) to Lyon for 'Method Of and 25 Apparatus For Forming Plastic";

U.S. Pat. No. 2,920,349 (1960) to White fcr '~Polyethylene Films"; and 30 U.S. Pat. No. 2r928rl33 (19603 to Schairer for :'~ethod Of ~` Producing Sheet Material".

Specialty polymers such as ultra-high molecular weight polyethylene (UHMW-PE~, poly(tetrafluoroethylene) 35 and poly(benzimidazole) are receiving increasingly greater - s -attention because of their unique mechanical and/or thermal properties. Unfortunately, these properties also limit the processability of such polymers by conventional hot- and cold-forming techniques. On the other hand~
powder-forming techniques would seem to offer attractive alternatives to the problem of shaping such materials. As indicated previously~ conventional powder processing has been used to a very limited extent for shaping thermoplastic polymers but has not been pxoven capable of widespread commercial applicability~ A need therefore exists for improved powder processing techniques which can take full advan~age of the pxoperties of polymers in gene-cal and the unique properties of the aforesaid specialty materials in particular, to produce non-tearable, thin film and sheet at hi~h overall rates of production.
Accordingly, it is an object of ~he present invention to provide new processes for producing shaped articles in the form of films directly from thermo-plastic polymer powders.
Another object is to provide shaped thermoplastic articles in the form of films having improved properties and which have been formed directly from thermoplastic polymer powders~
Yet another object is to provide an apparatus for producing shaped articles in the form of films directly rom thermoplastic polymer powders.
These and other objects of the invention, as well as a uller understanding o the utility and advantages thereof, can be had by reference to the followingdisclosure and claims.

Disclosure of the Invention The foregoîng objects are achieved according to the present invention by the disco~ery of a process whereby powdered thermoplastic polymer material to be shaped is continuously fed from a hopper to the gap between a pair oE heated work rolls. The material is thereby compacted into a sheet or film having the desired thickness.
The process of the invention comprises eeding a flowable thermoplastic polymer powder to the nip of a pair of cooperatively rotating compression rolls whereupon the polymer feed is passed between the rolls which compact and cause the particles to coalesce into a shaped article which thereupon emerges from between the rolls in the form of a sheet or film. During the operation, the circumferential speed of the rolls is maintained essentially equal to the linear speed of the sheet or film exiting therefrom. The sheet or film is withdrawn from between the rolls under tension at a force which is adjusted so as not to exceed the elastic limit of ~he resulting sheet or film product~
~ n another aspect of the invention, ~here is provided an apparatus for producing shaped articles in the form of sheet or film directly from thermoplastic polymer powders which comprises a pair of compression work rolls adapted to rotate cooperatively to compact and coalesce the polymer powder within the nip oE the rollsO The apparatus includes a drive means for cooperatively rotating the rolls and a hopper for xeceiving and feeding the polymer powder to the nip of the rolls at a predetermined rate~ In one embodiment of the inventionV
the hopper is adapted so as to keep the polymer powder feed physically and thermally isolated from the work roll surfaces prior to delivery of the powder to the nip of ~he rolls~ Means are also provided for withdrawing the sheet or film from the nip o~ the work rolls and applying tension on said sheet or film at a force so as not to exceed the elastic limit thereof.
Polymers suitable for use in the present 5 invention are film forming ~hermoplas~ic polymers, such as linear polyolefins (e.g., polyethylene, polypropylene), polyamides~ polyhalo olefins (e.g., polyvinyl chloride)~
perfluoro polymers (e.g~, polytetrafluoroethylene~, acrylonitrile-butadiene-styrene, polycarbonates, 10 polysulfones, and cellulose esters (e.gn, cellulose acetate, diacetate, and triacetatejO The polymer powder feed is desirably of a uniform particle size~ The polymer can be a single polymer or a plurality of polymer compositions either in mutual admixture or in stratified 15 layers as described herein below. The powder m~st be free flowing, which dictates the minimum particle size; the upper limit of particle size is dictated by the thickness of the sheet or film desiredO
In another aspect of the invention, I have found 20 that under some rolling conditivns, thermosetting polymer materials can be processed to form continuous sheetsO
Unlike thermoplastic polymers, thermosetting polymers do not coalesce and resolidify after application of heat.
However, I have found that thermosetting polymer powders 25 can be cold welded by roller pressure at slightly elevated temperatures.
The process of the invention for thermosetting plastic resins is carried out at a temperature sufficiently high to aid in the cold welding between contact points of particlesn In one example, 75~ by weight of phenolic resin manufactured by Hooker Chemicals under the trademark "Durez" was mixed with 25~ by weight of yellow pine particles having a mesh si~e of ~80~ The admixture was rolled at a temperature o 255~E' labout 35 1~4C). In another example~ 75~ by weight of urea-formaldehyde manufactured by American Cyanamid under the "Bettle" trademark was mixed with 25~ by weight of yellow pine particles having a -80 mesh size. The admixture was rolled at a temperature of 240~ (about 116C).
The process of ~he invention for thermoplastics is carried out at a temperature which is below the rnelting point but high enough to ensure coalescence and opti.mal tensile strength o~ the filmO The temperature of the ~hermoplastic ~eed material is controlled by the temperature of the work rolls, which are uniformly heated so that the desired temperature is maintained uniformly throughout the polymer feed in the gap between the rolls.
The temperature at which the work rolls are heated to achieve a given temperature of the polymer feed will depend upon the circumferential speed of the drive rolls;
the faster the speed, the higher the temperature of the rolls, and vice versaO
In addition to temperature, certain okher factors determine the outcome of the process. Thus, the pressure exerted by the work rolls should be controlled so as to be surficiently high to achieve complete densification ~i~eO, a density which corresponds essentially to the maximum density of the material in bulk form) of the product and 26 optim.ize its strengthc The rate at which the polymer powder is fed to the work rolls is ~djusted to match the roll p.ressure and film thickness consistent with full densification of the product. The work rolls are operated preferably at the same circumferential speed which is adjusted to match the other parameters. ~enerally, the ~` maximum work roll speed is determined by the requirement that the polymer powder feed be heaked uniformly to the desired temperature by the time it enters the nip of the work rolls; ~he lower limit of the work roll speed is determined primarily by the production rake desired.

_g .

The invention is suitable for producing polymer film or sheets having a wide range of desired thicknesses directly rom the polymer powder. The process is especially suited to producing Eilms or sheets having a uniform predictable thickness of between 0.0025 and 0~05 inch. The product is withdrawn from the work rolls at a take-off tension which permits the establishment of a "neutral point", i.eO, a state of affairs where the linear or circumferential speed of the rolls equals the speed of the material exiting the nip of the work rolls.
For purposes of the present disclosure the "entry arc" is the arc in the circumferential portion of the work roll surface attenuated by the nip angle. The entry arc is thus a func~ion of the diameter of the work roll. It affects the amount of feed material pulled into the t'roll gap" which is the region between the work rolls immediately preceding the entry arc. The amount of material pulled into the entry arc will determine the thickness of the film or sheet and its properties~ For a given roll diameter, the amount of material that can be drawn into the entry arc will be constant. By restricting the entry arc for a given roll diameter, one can simulate the performance of a smaller diameter roll. This can be done through appropriate design of the feed hopper which 26 in effect controls the thickness of the powder material between the entry arcs Qf the work rolls. The need to regulate such feed thickness would occur, for example~ in situations where the feed material is a mixture of powders differing in density (e.g., in the case of two or more polyolefins) and it is desired to achieve the same product density.
With respect to the work roll surfaces, the coarser the surface, the more polymer power will be pul1ed into the nip of the rolls. Generally, the work roll surfaces have a degree of smoothness such that the surface irregularities vary from 1 to 10 micro inches, and preferably from 4 to 6 micro inches. The work roll surfaces can be provided with a slight convex crown, although the feature of crowned work rolls is not essential to the practice of the invention. For example, when forming a 6-inch wide 0.005-inch thick UHMW-PE sheet using 1-foot long 6-inch diameter stainless steel work rolls, a 0.0005-inch convex crown would be suitable. The choice of other suitable crowns will depend on factors such as the type of powder being rolled, the thickness of the film produced, the speed and pressure of the work rolls, and temperature. The application of these factors in choosing a crown will become apparent to those skilled in the art having the benefit of the present disclosure before them.
Brief Description of the Drawings Further details of the invention and its preferred embodiments can be had by reference to the accompanying drawings, wherein:
FIGS. 1 and 2 are partial elevational views of two different compaction rolling mills according to the invention, each of which includes a pair of work rolls and a feed hopper to supply the rolls with and continuously meter material in the manufacture of a thermoplastic polymer film;
FIGS. 3 and 4 are partial elevational views of rolling mill designs similar to those shown in FIGS. 1 and 2, illustrating different constructions of the feed hopper; and FIGS. 5A and 5B are photomicrographs of polymer sheet material formed according to the invention, and sheet material formed coventionally by skiving from a log of polymer material, respectively.

.~ q~

-In the drawings, the same structural elements are designated by the same reference numerals. Letter suffixes are added to denote specific ones of these elements where necessary.

Best Mode for Carrying Out the Invention In the following discussion of the drawings and in the subsequent examples, a powdered thermoplastic polymer material to be shaped is continuously supplied from a hopper to a gap between a pair of heated, cooperatively rotating compaction work rolls. The material is compacted by the rolls under heat and pressure into a sheet of film having the desired thickness or caliper. The material can be a polymer such as polyethylene or polypropylene whose particle size can range ~rom about 30 U.SO mesh to about a 325 U.S. mesh Preferably, the material is an ultra-high molecular weight polyethylene (U~MW-PE) powder.
Referring to FIGSD l and 2, compaction work rolls 12 and 14 are mounted for movement rotationally on parallel shaEts (not shown) and are driven ~y drive means (not shown) in the direction of the arrows toward gap 16.
Rolls 12 ancl 14 are disposed parallel to one another in a common horizontal plane to locate gap 16 vertically above the nip of the rolls~ Rolls 12 and 14 are driven preferably at the same linear rotational ~i~e , circumferential) speed~
The drive means can comprise any structure or device which is conventional in the rolling art, such drive means being connected to the shaft of each roll so as to permit relative adjustment of rolls 12 and 14 at gap 16~ Preferably, rolls 12 and 14 are located to provide a nip angle ~Cof about 7~8~
Rolls 12 and 14 are capable of exerting a predetermined compacting pressure in order to achieve a sheet or film 13 by direct compression rolling of the powder material 11O The rolls 12 and 14 can be of any convenient diameter. Howevery the diameters of rolls 12 and 14, which are preferably the same, is one of several 6 parameters which directly affects the thickness of the resulting sheet or film 13. A roll diameter of 6-inches has been used successfully in carrying out a direct compression rolli~g of polymer powder, and rolls having a larger diameter have also been used successfully. The linear speed of rolls 12 and 14, the surface characteristics of the rolls, and the type of material Eed to the region of gap 16, are additional parameters that directly relate to the thickness of the sheet or fllm 13~
For example, and as a general proposition, the greater the diameter of rolls 12 and 14, the coarser the surface eharacteristics of the rolls and the greater the volume of material supplied to the region of gap 16, then the greater will be the thickness of the sheet or film 13 obtained. With respect to the rotational speed of the rolls 12 and 14, if the speed of rotation is increased~
other parameters remaining constant, then the thickness of the sheet or film 13 is reduced.
Work rolls 12 and 14 are of a length as may be required in the manufacture oE sheet or film to comply with product wiclth considerations. For example, rolls 12 and 14 can be of a length sufficient to permit the manufacture of a sheet or ~ilm 13 having a width of about 5-6 feet. Aclclitionally~ rolls 12 and 14 or at least an outer annulus thereof, are aclvantageously ~or3ned of a material having good heat-concluctanceO A preferred material for this purpose is cast steel when the process is carried out at temperatures not exceeding 350F
(177C)~ and high temperature steel when higher temperatures are employed.

-13~

As indicated 9 the process of the in~ention includes heating the particulate feed material 11 and compression rolling it between the rolls 12 and 14. Each of the rolls 12 and 14 can be cored or otherwise formed to receive a heating element (not shown) capable of being controlled to a temperature or within a range cf temperatures~ Any form of conven~ional heating element and control can be employed, such as a heated fluid circulating within the rolls so as to maintain the surfaces of the latter at a uniform tempera~ure.
The present invention contemplates several constructions oE hopper lOa. Each hopper serves as a store for the material 11 which is supplied continuously to the region oE gap 16. Hopper lOa can be adapted to receive feed material 11 by means (not shown) at a rate substantially equal to the rate at which the material is supplied to gap 16 so as to maintain the store of material at a desired level within the hopper. In the form shown in FIG. 2, hopper lOa includes an upper surface 18 to which a structure for supply of material 11 can be attachedt and a first pair of side walls 20 and 220 These side walls extend toward rolls _ and 14 along substantially their full lengthO A second pair of side walls, including an end wall 24 and a wall at the opposite end (not shown) complete the hopper lOa, thereby defining in cross-section a more or less rectangular enclosure.
rhe last-mentioned end walls are contoured substantially to the contour of the rolls 12 and 14 to preven~ loss oE
material 11. Referring to FIGS. l and 2, side wal:Ls 20 and 22 include wall portions 20a, and 22a (~IGo 1.) and _ , 22b (FIG. 2) of contoured outline, substantially concentric wi-th the surface of rolls 12 and 14, so as to extend toward gap 160 As shown in FIG~ 2~ wall portions 20a and 22a are substantially coextensive and~ together with the end wall 24 (and the opposite end wall not shown) ~aso7~
-14~
form a metering outlet 26 at the rim. Metering outlet 26 is in the form of a rectangular slit. The length of the outlet will correspond substantially to that of rolls 12 and 14 to provide a uniEorm flow of feed material 11 to gap 16 along that length. The width of outlet 26 can be on the order of about 0.06 inch to meter the 10w of material 11~ In the form sho~n in FIG~ 1~ the material 11 exits hopper lOa in a free falling stream along a path generally following a line tangent to rolls 12 and 14.
FIG. 2 illustrates a somewhat similar hopper construction including a pair of wall portions 20b, 22b.
One of the wall portions, for example the wall portion 2')b, is longer than the other wall portion 20b, thereby forming an outlet 28 defined by the rims of each wall portion between the end wall 24 and the opposite side wall (not shown3. The outlet in the construction of hopper lOb of FIG. 2 can be dimensioned so as to duplicate substantially the dimensions of the outlet of the hopper 10 of the construction of FIG. 1 ~lowever r because of the location of outlet 28 to provide a differential opening, a metered flow of material 11 will be thrown or biased against the surface o work roll 12. The powder material 11 thus thrown will cascade against the surface of work roll 12, thereby increasing the contact timeO
The design of the hopper must provide several functions. For instance, the hopper must confine ~he feed material 11 while at the same time provîding an outlet which suitably meters the material to the gap within which the material is heated. In FIG. 1, the material 11 is heated through convection, i.e., exposing the thin falling ~- stream of material to the heat emanating from rolls 21 arid 14, while in FIG. 2, heat is acquired by direct contact between the material and the surface of the roll onto which it is ~'thrown". In these forms, the construction of the hopper not only prevents loss of material 11 rom the sides, it also serves in the manufacture of a sheet or film 13 of less thickness than would otherwise be obtained. To this end, the hoppers of FIGS. 1 and 2 meter or restrict the amount of material 11 supplied to gap 16.
The form of hopper 10b of Fig. 2 permits the rotational speed of rolls 12 and 14 to be increased since the material 11 is capable of being heated more rapidly because of direct contact between the material and the rolls 12 and 14.
FIG. 3 shows one form of generalized hopper design according to the invention for feeding polymer powder to the roll gap 16 for compacting. The hopper 10c is so designed to prevent the powder 11 from touching the rolls 12 and 14 while metering an amount of powder that will produce a thin sheet 13. The amount of powder material 11 is small enough so that there is good heat transfer from the rolls 12 and 14 to the powder. This system also allows a faster production rate for thin sheet.
FIG.4 shows a hopper design according to the invention for making thicker polymer strip. In this embodiment the powder 11 is allowed to contact work rolls 12 and 14. The surface contact area could extend up to the top of the rolls if desired. In such circumstances, the center 15 of hopper 10d does not contain powder. If the center contained powder, it would allow powder into the gap 16 that is not high enough in temperature. The rolls 12 and 14 would pull powder into the gap on the surface of the rolls which has been heated by its contact.
Powder would also be pulled into the gap from the center which has a lower temperature. By eliminating the center zone, this undesirable condition is eliminated.
The hopper as shown in FIG. 4 may be used with different powder compositions in each partitioned section to acheive a stratified or layered rolled sheet final product. Similarly, a partition (not shown) may be installed in the hopper of FIG~ 3 to achieve stratification of the final rolled sheet product~
I have found in certain applications ~hat vibrating means (not shown) attached to the hopper will facilita~e the flow of the powder or blend to the rolls.
This helps to guarantee a more uniform flow of material.
FIG. 5a is a photomicrograph of polymer sheet material formed by direct compaction rolling of U~W-PE
powder according to the invention~ The sample is illuminated from behind and, as can be readily seen~ ~he material is virtually devoid of pin hole perforations~ By contrast, FIG 5h shows the numero~s undesirable pin-hole perforations present in a sheet of the same material and thickness, and back-lit in the same way~ but produced by a conventional skiving technique.
The following examples are intended to illustrate, without limitation, the process, apparatus, and product of the invention.

Example 1 A quantity of U~MW-PE, having a melting point of about 392-428F (200-220C), (sold by American Hoechst as grade 412 under the "~OSTALEN GUR" registered trademark) is formed into a sheet of film by p~ssing the material in powdered form to the region of the gap between a pair of cooperating 12-inch long 6-inch diameter work rolls mounted for rotation about parallell horizontal axes~ The work rolls have a 0~0005-inch crown at their centers. The material is then compressed between the rolls which are positioned with a nip angle of 7-8 and driven at a speed of about 2 ft/min~ The material is passed to the region of the gap by a hopper as shown in FIG. 2, heated to a temperature of about 290 to 300F (about 143 to 149C) and compressed between the rolls so as to form a sheet or film o~ a thickness of about 0~022 inch and a density of about 0.82 g/cc (substantially fully dense) when drawn from the nip of the rolls at a tension so as to main~ain flatness of the material.

Exam~le 2 The procedure of Example 1 Produces ~ sheet of fil~ of thinner gage when the rolls are polished with 325 mesh emery cloth (44 micron grit), Example 3 The procedure of Example 1 is repeated except that the speed of the work rolls is increased to about 3~1 ~t/min. ~ sheet of film is produced having a thickness of abollt 0.011-0.013 inch.

Exam~le 4 The material of Example 1 is passed to the reyion of the gap between a pair of compacting 6~inch diameter work rolls similarly mounted, and providing a like function as the work rolls of Example 1, except that the rolls are driven at a speed of about 11 ft/minO The material is passed to the region of the gap by the hopper of FIG~ 2 having a metering outlet extending along the gap with a width of about 0~035-0.040 inch. The material is heated to a temperature of about 290 to 300F ~about 143 to 149C~, and compressed and drawn under tension from the nip of the rolls. The resulting sheet of film had a thickness of about 0.005~0~006 inchO
2~
Exam~le 5 A quankity of UHMW-PE is formed to a sheet of film by passing the material in powdered fcrm to the region of the gap between a pair of cooperating 6~inch diameter work rolls mounted ~or rotation about parallel~
horizontal axes~ The material is then compressed between the rolls which are positioned with a nip angle o- 7-8~
and driven at a ro~ation speed of about 11~0 ft/min. The material i~ passed to the region of the gap and heat~d to a temperature of about 255 to 266~F (about 124 to 130C) and compressed to form a sheet of film of a thickness of about 0.021 inch and a density of about 0u66 g/cc when drawn under tension from the nip of the rolls.

Dle 6 The material oE Example 1 is formed into a sheet or film by passing the material to the region of the gap bekween a pair of cooperating 6-inch diameter work rolls . mounted for rotation about parallel, horizontal axes having a nip angle of about 7 8. The work rolls are of a diameter of 6 inches and driven at a speed of about 11~0 ft/min. The material is heated within the region of the gap to a temperature of about 284F (about 140C) and directly roll compacted by the work rolls. The work rolls were preheated to a temperature of about 266~F (130C)o The resulting sheet had a thickness of about 0O020 inch and a density of about 0.82 g/cc when drawn from the nip or the work rolls under tension to produce a 1at sheet~

~ ç~
~Ihe feed material of Example 5 is heated to a temperature of about 266F (abou~ 130C) within the gap between compacting, cooperating work rolls, such as the work rolls of Example 7. The work rolls are preheated ~o 25 a temperatu:re of about 300F tabout 149C) The resulting sheet of film having a thickness of ahout 0.022 i.nch and a density of about 0~66 g/cc when drawn under tension to produce a flat sheetu Example 8 ~- The feed material of Example 5 is heated to a temperature of about 266F labout 130C) within ~he gap between the compacting wor~ rolls of Example 6~ The work rolls were preheated to a tempera~ure of about 300F
~about 149C). The resulting sheet of film has a ~aS07Z

thickness of about 0,022 inch and a density of about 0.66 ~/cc when drawn from the nip of the work rolls and a resulting thickness o~ about 0.015 inch and a dens.ity of about 0.94 g/cc when heated to a melt temperature o:E about 284F (about 140C)~

~xample 9 A quantity of polyethylene powder toge~her with ranging percentages of lampblack t6%, 4~ and 2~ by weight) are fed to a pair of compacting rolls according to ~he design o the embodiment shown in FIG~ 3. The opening at the nip of work rolls 12 and 14 is adjusted to a value of between 0.035 and 0.040 inch~ The rolls are operated at a linear circumferential speed of 11 feet/minute and the surfaces thereof are maintained at a temperature of 290-300F ~about 139-145C). The sheet material thus produced has a thickness of 0~0035-QO004 inch and a - density of 0O94 g/cc~ Tension is applied uniformly across the film exiting from the work rolls sufficient to afford a flat materialO
Exam~le lO
The process of Examples 1-9 can be carried out wherein reinforcing metallic screen material is simultaneously .roll compacted with the polymer powder feed material~ , The sheets of ilm can comprise "feed stock"
suitable for use in further processing operations.
Ilhe particle size of the feed material for the process of Examples 1-9 can be of uniform mesh, but .~ pre~erably the material will be comprised of a distribution of particles varying in size Erom relatively coarse to relatively fine particles~ To this end, it has been foun~ that the process may be more effec~ively carried out with material having a non-uniform particle size. A typical distribution of particle size (U.S.
mesh) can be as follows:
Sieve SizePer Cent -50+80 ~7 -80+100 10.8 -~00+1~0 35.~
-140~2~0 37.4 -200+325 13~2 -325 0.3 1~
The polymer feed material can contain additional ingredients to alter the appearance and~or properties of the productO Thus, coloring agents and opacifiers such as carbon black, wood powder (e.gO, cherry bark, box elder bark~ yellow pine, or maple), preferably of -80 mesh size, various types of metal powders (e.g., copper, aluminum~ r metal oxides (e.g~, aluminum oxide), intermetallic compounds (e.g~ aluminum silicide), intersticial compounds (e.g., silicon carbide), and ceramic powders (e-g-, powdered metal carbides such as tungsten carbide~, graphite, or molybdenum disulfide can be incorporated in various amounts with the polymer feed. The added ingredients may be linear (e~y., fibrous) structural elements. Even foaming agents can be added in order to 25 achieve novel bulking structures.
The resu]ting sheet or film is distinctly different in its properkies from extruded~ cast, or ski~ed films containing similar adjuvants l'hus, the present films contain these adjuvants uniformly dispersed 30 throughout the coalesced polymer particles whereby the material takes on the propertles of the adjuvant matexial~

Industrial Applicability The present process and apparatus provide a 35 shaped polymer product which is useful in applications which have been found for previous films~ but at high quality and lower costO In addition, the present invention makes possible the production of polymer films having incorporaked therein modifying and adjuvant materials, e.g~, metal powders~ pigments, wood powder, and the like, which would be extremely difficul~ or impossible to do by conventional film forming techniques, thereby giving rise to a host of new applications in the plastic film industryO
The foregoing description and example are presented for the purpose of illustrating the invention and its utility and advantages without intending to limit same in any way to speciEic features or embodiments. It is understood that changes and variations can be made in the product and process of the invention without departing from the scope thereof as defined in the following claims.

3~

Claims (13)

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

1. A process for producing a polymer film comprising:
collecting a supply of polymer powder;
feeding a free falling stream of the powder between a pair of spaced rollers;
metering the stream of powder fed between the rollers;
heating the powder;
rotating the rollers;
compressing the powder between the rollers to compact and to coalesce the powder into a film;
drawing the film from between the rollers;
maintaining the circumferential speed of the rollers essentially equal to the linear speed of the film drawn from between the rollers; and applying tension to the film drawn from between the rollers.

2. A process according to claim 1 wherein the feeding step includes the step of directing the powder toward the center of the space between the rollers.

3. A process according to claim 1 wherein the feeding step includes the step of directing the powder onto the sur-face of one of the rollers.

4. A process according to claim 1 wherein the feeding step includes the steps of:
feeding a first portion of the stream of powder onto the surface of a first roller; and feeding a second portion of the stream of powder onto the surface of the second roller.

5. A process according to claim 4 wherein:
the first portion of the stream of powder comprises a first powder composition; and the second portion of the stream of powder comprises a second powder composition.

6. A process according to claim 1 wherein the collecting step includes the step of vibrating the powder supply to fac-ilitate the feeding thereof.

7. Apparatus for producing a film from a polymer powder comprising:
a pair of spaced rollers defining a nip therebetween;
a hopper located above the nip, to collect a supply of the powder, and defining an opening to feed and to meter a free falling stream of the powder between the rollers;
means to heat the powder;
means to rotate the rollers to compress the powder fed therebetween into a film; and means to draw the film from between the rollers and to maintain tension on the film as it is drawn from be-tween the rollers.

8. Apparatus according to claim 7 wherein the hopper directs the powder onto the surface of a first roller.

9. Apparatus according to claim 7 wherein the hopper directs the powder toward the center of the space between the rollers.

10. Apparatus according to claim 7 wherein the hopper includes partition means separating the inside of the hopper into first and second sections, and separating the hopper opening into a first portion to direct powder from the first hopper section onto the surface of a first roller and a second portion to direct powder from the second hopper section onto the surface of the second roller.

11. Apparatus according to claim 10 wherein the parti-tion means further separates the hopper into a third, central section located between the first and second sections and the partition means keeps the powder supply out of the central hopper section.

12. Apparatus according to claim 7 wherein the hopper includes means to vibrate the powder supply.

13. Apparatus according to claim 12 wherein the hopper is located adjacent the rollers and the exterior surface of the hopper defines a contour substantially identical to the contour of the adjacent surfaces of the rollers.