METHOD OF PRODUCING AN ABRASION RESISTANT SHEET AND APPARATUS FOR PRODUCING IT

Abstract: A method and apparaus is provided for forming an abra­ sion resistant sheet from a sheet (S) of orientable thermoplas­ tic polymer material in one continuous process which can be effected by a single continuous production line apparatus. A method and apparatus is also provided for treating the sur­ face of an oriented sheet with heated rolls (50, 54) for creating a fused surface layer on each side of the sheet. A method and apparatus is also provided for slitting a wider sheet into a plurality of strip (s) at elevated temperatures with the strap under tension with a staggered array of fixed blades (72a, 72a′). Further, a method and apparatus is provided for treating an oriented strip having a non-uniform thickness so as to conform the surfaces of the strip in a substantially uniformly flat and parallel configuration

Description: Technical Field This invention relates to the production of a sheet form of thermoplastic polymer, and includes, among other things, the working of the material in a sheet to obtain an orientation of the macromolecular chains, the surface treatment of the oriented sheet, and the cutting of the sheet to form smaller sheets. Background of the Invention Techniques for producing improved strength sheet material that has an orientation of the macromolecular chains, such as a uniplanar, axial oriented crystalline structure, are known. -For example, see the U.S. Patent Nos 3,354,123, 3,394,045 and 3,651,196. The inventors of the invention described and claimed herein know of commercially practiced processes which are subsequently applied to such oriented sheet, on a separate process line and apparatus, to treat the surface of the product in sheet form so as to increase the abrasion resistance of the sheet. The inventors are also aware of commercial processes that subsequently slit the sheet material into a plurality of narrower sheets or strips suitable for use as strap for binding articles. The inventors of the invention described and claimed herein are also aware of commercial processes for fabricating a single narrow strip of polypropylene material having an oriented structure suitable for use as strap for binding articles. However, such strap produced by some manufacturers is not of sufficiently uniform thickness throughout the length of the strap as is desired in some applications. For example, it is sometimes desired to use such strap in high speed automatic strapping machines that feed the strap around an article, tension the strap tight about the article, and then heat weld or fuse the overlapping strap ends together. But, a strap with a non-uniform thickness across its width and/or throughout its length is more likely to be improperly fed by the machine and the fusing of the overlapping strap ends is more likely to result in an unsatisfactory joint. Summary of the Invention In one embodiment of the method of the present invention, an abrasion resistant sheet is produced from a sheet of unoriented, thermoplastic polymer material, such as a polyester or linear crystallizable polypropylene material. The sheet is pro duced in one continuous process which can be effected by a single, compact, highly efficient, and low cost production line. According to one form of the apparatus of the present invention, a polyester or crystallizable polypropylene sheet material which has been mechanically worked to obtain an orientation of the macromolecular chains is contacted by novel, heated, surface treatment rolls to heat the sheet for a sufficient time to cause fusion in the side surfaces of the sheet to a depth of at least 0.00254 wm but insufficient to cause fusion in the entire thickness of the sheet. This increases the abrasion resistance of the sheet. According to another aspect of the invention, a sheet of an oriented thermoplastic polymer material, such as polypropylene, is slit at a temperature greater than 37.80Cby a plurality of staggered, fixed, inexpensive, cutting blades in a direction substantially parallel to the sheet feed direction to produce a plurality of narrower sheets or strips. In accordance with another aspect of the invention, a method and apparatus are provided for treating an oriented#thermoplastic polymer, such as polypropylene strap, which has a non-uniform thickness. The undesirable non-uniform thickness is eliminated according to the present invention by providing a pair of spaced-apart, oppositely rotating, heated cylindrical rolls and by feeding the strap in a continuous path partially around and in contact with each roll. One surface of the strap is thus contacted with one roll and the other surface of the strap is thus contacted with the other roll. The rolls are maintained in contact with the strap while maintaining the strap under a tension whereby the surfaces of the strap become substantially uniformly flat and parallel throughout the length of the strap. Numerous other advantages and features of the present invention will become readily apparent from the following detailed description of the invention and of embodiments thereof, from the claims and from the drawings. Brief Description of the Drawings In the accompanying drawings forming part of the specification and in which like numerals are employed to designate like parts throughout the same, Figure 1 is a simplified, schematic view of apparatus for effecting the method of the present invention; Figure 2 is an enlarged, fragmentary, side view of an unsatisfactory method for slitting a sheet of an oriented polypropylene material; Figure 3 is a bottom view taken generally along the plane 3-3 of Figure 2; Figure 4 is an enlarged, side view of an apparatus for slitting the sheet material in accordance with the teachings of the present invention; and Figure 5 is a bottom view taken generally along the plane 5-5 of Figure 4. Description of the Preferred Embodiment While this invention is susceptible of embodiment in many different forms, there are shown in the drawings and will herein be described in detail preferred embodiments and process parameters of the invention. It should be understood, hos;ever, that the present disclosure is to be considered as an exemplification of the principles of the invention and is not intended to limit the invention to the embodiments illustrated or to the process parameter values listed. The apparatus described herein has certain conventional mechanisms, such as drive mechanisms, control mechanisms, tension sensing mechanisms, etc., the details of which, though not fully illustrated or described, will be apparent to those having skill in the art and an understanding of the necessary functions of such mechanisms. A novel method and apu < atus for manufacturing an orientable thermoplastic polymer sheet, such as a polyester or polypropylene sheet, in a single continuous and compact line is illustrated by means of a greatly simplified schematic representation in Figure 1. The method and apparatus is directed to fabricating a sheet of the orientable thermoplastic polymer, mechanically working the sheet to obtain an orientation of the macromolecular chains, treating the surface of the sheet to increase abrasion resistance, and slitting the sheet to provide narrower sheets. Portions of the line may also be used to treat plastic strapping that has a non-uniform cross sectional thickness throughout its length so as to orient the opposing side surfaces of such a strap in a substantially uniformly parallel configuration throughout the length of the strap. The method and apparatus will. be explained with reference to a specific example involving polypropylene. There is no intention that the invention be limited to the material or specific conditions set forth. According to the present invention, polypropylene sheet material may be formed or extruded from the raw materials under the conditions described in detail in the aforementioned Patent No. 3,394,045. For a full understanding of the nature and operation of the formation of such a sheet material, reference may be had to that patent. In this description, only such information as is relevant to the present invention will be described, and only in sufficient detail as to afford an understanding of the present invention. However, the entire disclosure of the U.S. Patent No. 3,394,045, insofar as it is consistent with the present disclosure, is hereby incorporated in and-made a part of this application by reference thereto. Such modifications as are necessary to adapt the teachings set forth in that patent to the present invention are described in detail hereinafter. With reference to Figure 1, a conventional plastic extruder 10 is fed a suitable raw material, such as isotactic polypropylene granules. The granules are rendered plastic or flowable by heating to a sufficient temperature, say approximately 2540 C. Preferably, the material is extruded through a conventional sheet die 12 at a pressure of approximately 140,61 kg/cm2 to extrude a web, workpiece, or sheet S having a suitable width, for example 7.62 cm, ¯ and having a suitable thickness, for example, 0.419 cm. These dimensions are suitable for producing an ultimate sheet product that can be used as strapping. The sheet S is passed through a conventional casting roll assembly 14 comprising an upper roll 16, a middle roll 18 and a bottom roll 20. Preferably, the rolls are maintained at about 65:60C, For this purpose, the rolls may contain internal passages allowing the circulation of a heat transfer of fluid therethrough. After exiting frown the casting roll assembly 14, the sheet S is trained over a guide roll 22 and down into a tank 24 containing a bath of liquid, such as water, for reducing the temperature of the sheet to a substantially uniform value throughout its transverse cross-section. The sheet is trained around a guide roll 26 in the tank 24 and conveyed upwardly therefrom out of the tank 24 between draw rolls 28. The sheet is then conveyed to a rolling mill assembly 30 which incorporates, on its upstream side, an adjustable tension speed controlling dancer roll 32. The dancer roll 32 is preferably at ambient temperature. The dancer roll speed control may be of conventional design and may be operated through conventional controls to establish desired speed in that region of the sheet conveying path. The rolling mill assembly 30 includes a pair of opposed, oppositely rotating compression rolls 34 and 36. The compression rolls may be of conventional design for causing flow of the material in the sheet S in the direction of the feeding of the sheet in the path thereby causing production of a sheet of reduced cross-sectional thickness as the sheet exits from the nip of the rolls. The compression rolls 34 and 36 are preferably maintained at a temperature in the range of between 37.8 C and 104.4 C. The sheet S exiting from the rolling mill assembly 30 is next trained about a pair of oppositely rotating chill rolls 40 and 42 which are preferably maintained at a temperature of about 29.40C. The chill rolls are rotated at a higher surface velocity than the upstream compression rolls 34 and 36. The chill rolls 40 and 42 are operated at a higher speed to create a further orientation of the sheet between the nip of the compression rolls 34 and 36 and the chill rolls 40 and 42. The increased tension is preferably maintained in the range of between 1.79 and 62.56 kg/cm#6f sheet width to create an orientation of the macromolecular chains in the polypropylene material. The orientation may be a generally planar orientation. The sheet may have a uniplanar, axial oriented crystalline structure of increased strength or some other increased strength structure resulting from a tension induced orientation. Although it is not necessary to convey the sheet through the compression rolls 34 and 36 ahead of the chill rolls 40 and 42 to reduce the crosssectional thickness of the sheet during the application of higher tension by the chill rolls, it has been found that the use of such compression and the resulting sheet thickness reduction decreases the degree of brittleness of the formed sheet. That is, if the sheet is not compressed and reduced in cross sectional thickness by the rolls 34 and 36, the oriented sheet produced is more brittle than a sheet that is first compressed by the rolls 34 and 36. The resulting brittleness in the final sheet product is dependent upon, among other things, the type of polypropylene feed material that is used. It has been found that more expensive types of polypropylene feed material will produce an oriented sheet that is not as brittle as sheet produced with less expensive types of feed material. However, it has been found that if the less expensive feed material is used, the degree of brittleness in the formed sheet can be substarti?lly reduced if the sheet is first compressed, as with rolls 34 and 36. This permits the use of less expensive feed materials for producing oriented sheet with an acceptable brittleness level that was only heretofore possible by using the more expensive feed materials. The oriented polypropylene sheet is fed from the chill rolls 40 and 42 to a surface treatment station which includes 1) a first, heated, surface treatment roll 50 adapted to contact one side surface of the sheet, 2) a guide roll 52, and 3) a second, heated, surface treatment roll 54 adapted to contact the other side surface of the sheet. The surface treatment rolls 50 and 54 function to raise the temperature of the surface layer of the sheet to a fusion temperature, generally in the range of between about 1770c and about 2740C. The surface layer may be fused for a depth of between about 0.00254 mm and about 0 00762mum but generally the fusion layer obtains a thickness of about 0.00254 mm. This fusion treatment of the surface of the sheet increases abrasion resistance. The guide roll 52 is preferably maintained at about 29.40C between the two surface treatment rolls 50 and 54. The tendency of oriented polypropylene sheet to abrade is more fully disclosed in the aforementioned U.S. Patent No. 3,394,045 and attention is directed thereto for a complete discussion of the problem and for a complete discussion of how fusing the surface layer of the sheet mitigates this problem. The aforementioned U.S. Patent No. 3,394,045 suggests that the sheet of oriented polypropylene material may be heated and the surface fused by contact with a gas flame-heated roll. In accordance with the present invention, the novel rolls 50 and 54 are heated and maintained in contact with the strap in a manner sufficient to form the fusion layer in the sheet. Specifically, each roll 50 and 54 is preferably cylindrical in shape, fabricated from aluminum or cold rolled steel, has a diameter of about 7.62 cm,- and contains passages permitting the circulation of a heat transfer fluid, such as oil, at a temperature of between about 1770 C and 3710 C. Each roll 50 and 54 is in contact with the sheet so as to effect, in combination with the other rolls, a total tension in the sheet along the feeding direction of greater than 1.79 kg/cm of sheet width section and preferably between about 1.79 and62.5o#'kg/cm of sheet width for sheet having a thickness of between 0.0127 and 0.0889 cm. The sheet is fed in a path partially around the circumference of each roll for an arc length sufficient to produce the desired surface fusion layer for the selected sheet speed, roll temperature and roll diameter. Within the parametric ranges described above a contact arc angle of between 400 and 800 with a sheet conveying speed in the range of about 15.24 and 106.68 m/min has been found to function satisfactorily. It is believed that lower and higher speeds, including speeds of 182.88 m/min or more, will also function satisfactorily. After exiting from the surface treatment station, the sheet is trained around a plurality of heated rolls 60 which function to stress relieve the sheet to stabilize the oriented polypropylene material. The heat treatment of oriented polypropylene material to effect the stabilization is well known in the art and is not discussed in detail here. However, in orie form of the heat treatment, each of the stress relieving rolls 60 is maintained at about 1490 C. In the stress relieving section of the line, the tension on the strap may be suitably controlled, by conventional techniques, as necessary to effect proper stress relieving without substantially deleteriously effecting the orientation of the polypropylene material in the sheet. Oriented polypropylene sheet material is difficult to slit along the feeding direction to form a narrower sheet for strapping. According to one aspect of the present invention, a simple, inexpensive, and novel method and apparatus for slitting such sheet material is provided. Preferably, the slitting means is located in the stress relieving or stabilization region of the process line, and in Figure 1, is located between the last two stress relieving rolls 60 where it is designated generally by numeral 70. The inventors of the present invention have developed a method to slit oriented polypropylene sheet with stationary cutters or blades, since such stationary cutters are simpler and less expensive than the conventional rotary cutters in common use. One approach to attempting to slit oriented polypropylene sheet is illustrated in Figures 2 and 3. A plurality of blades 72 is provided with each blade 72 mounted in a separate holder 74. The blades 72 may be of conventional razor edge design and may be about 1.587 min thick. The blades 72 extend into the sheet feed path in the plane of the sheet S which is moving in the direction indicated by the arrow 76. The sheet material between the blades is cut into strips s and forced on either side of each blade toward an adjacent blade. This causes a small amount of transverse buckling of the sheet and can jam the sheet between the blades. This can overheat the blades and can lead to the production of rough and uneven edges along the resulting narrower sheets. Further, the jamming of the sheet between the blades can create excessive back tension in the sheet on the process line. Also, the transverse buckling of thesheet can lead to an undesired variation in the width of each individual narrower sheet sr strap that is thus formed. Additionally, when attempting to slit a sheet of oriented polypropylene material at high speeds, there is a tendency for the sheet to crack in the direction upstream from the edge of the knife or blade parallel to the sheet feed direction. The inventors of the present invention have found that the above-described undesirable problems can be substantially reduced or eliminated altogether if the sheet to be slit by cutting means is maintained at a temperature greater than 37.80 C and preferably at a temperature of above 82.2 C. Further, some of the above-described undesirable problems are substantially eliminated if the sheet is slit under a tension in the feed direction in the range of between 0.89 and 62.56 kg/cm of transverse width of the unslit sheet. Preferably, under such tension conditions, the unslit sheet has a thickness in the range of between 0.0127 and .0889 cm. Also, it. has found that some of the above-described problems are eliminated or substantially mitigated if the slitting means includes a plurality of spaced-apart knife or razor-type blades adapted to extend into the feed path in the plane of the sheet with the adjacent blades being staggered relative to a line in the sheet plane that is transverse to the direction of the feed path as best illustrated in Figures 4 and 5 for blades 72a. Specifically, with reference to Figure 5, the blades are preferably disposed in a V-shaped array across the path of the sheet with one of the blades, 72a', forming the vertex of the angle defined by the V-shaped array and with the one vertex blade 72a' being disposed substantially in the center of the transverse width dimension of the sheet in the path. Further, the vertex blade 72a' is preferably located furthest upstream of the remainder of the array of blades relative to the feeding of the sheet. With the above-described staggered blade configuration, the sheet material on either side of an individual knife blade is free to "flow" transversely a small amount without impinging upon an adjacent blade. Thus, the tendency for the sheet to buckle transversely or jam between the blades is substantially reduced or eliminated. Configurations of the staggered blades other than the illustrated V-shape may be employed so long as each blade is not in direct transverse alignment across the sheet relative to adjacent blades. Following the slitting of the sheet by the slitting means 70 at the location illustrated in the schematic diagram of Figure 1, the narrower sheets or strips s are guided around the last stress relieving roll 60 and around guide roll 82 through a tank 80 containing a heat transfer fluid, such as water, for cooling the strips to a desired temperature or to ambient temperature. The strips are withdrawn from the tan a along the feed path between draw rolls 84 which may be maintained at a suitable temperature at ambient or below, and preferably at about 100 C. From the draw rolls 84, the strips may be wound on separate storage spools (not illustrated) or otherwise removed as may be desired. It has been found that the above-described surface treatment rolls 50 and 54 may function, in addition to providing the fused surface layer, to treat previously fabricated oriented polypropylene strap that has a non-uniform thickness across its width and/or along its length. Specifically, it has been found that contacting such non-uniform thickness strap with the rolls 50 and 54 under the process parameters described above for those rolls, causes the surfaces of the strap to become substantially uniformly flat and parallel throughout the length of the strap. Such improved strap is less likely to jam an automatic plastic strapping machine and is more likely to form a satisfactory fusion weld after being looped and tensioned about an article. From the foregoing, its will be observed that numerous variations and modifications may be effected without departing from the true spirit and scope of the novel concept of the invention. It is to be understood that no limitation with respect to specific apparatus illustrated herein is intended or should be inferred. It is, of course, intended to cover by the appended claims all such modifications as fall within the scope of the claims.

Claims: WHAT IS CLAIMED IS: 1. The method of producing an abrasion resistant sheet from a sheet of orientable thermoplastic polymer material comprising the steps of: (a) feeding said sheet along a continuous path; (b) bringing said sheet to a substantially uni form temperature throughout its transverse cross section; (c) orienting said sheet through the application in the feeding direction of at least a ten sioning force to create an orientation of the macromolecular chains in the polypropylene material; (d) heating at least one side surface of said sheet to a temperature for a sufficient time to cause fusion in said one surface of said sheet to a depth less than the full thickness of the sheet; and (e) heating said sheet in a region of said conti nuous path to stress relieve said sheet and produce a stabilized sheet. 2. Apparatus for producing an abrasion resistant sheet from a sheet of orientable thermoplastic polymer material, said apparatus comprising; (a) means for conveying a sheet along a continuous path; (b) heat transfer means (22,24,26) for bringing the temperature of said sheet to a substantially uniform value throughout its transverse cross section; (c) means (34,36,40,42) for stretching said sheet through the application in the feeding direction of a tensioning force to create an orientation of- the macromolecular chains in the polypropylene material; (d) means (50,54) for heating at least one side surface of said sheet to a temperature for a sufficient time to cause fusion in said one surface of said sheet to a depth less than the full thickness of the sheet; and (e) means (60) for heating said sheet in a re gion of said continuous path to stress relieve said sheet to stabilize the material comprising said sheet. 3. The method of producing an abrasion resistant po lypropylene sheet from a sheet of crystallizable polypropylene material which has been mechanical ly worked to obtain an orientation of the macro molecular chains, said-method comprising the steps of: (a) feeding said sheet along a path; ; (b) contacting at least one side surface of said sheet in said path with at least a first, heated, surface treatment roll (50) with a force to effect a tension in said sheet along the feeding direction greater than 1,79 kg/cm of sheet transverse width and maintaining successive segments of said sheet=in-such con tact with said roll (50) to heat each said segment to a temperature in the range of between 1770C and 2740C for a sufficient time to cause fusion in the one side surface of said sheet to- a depth of at least 0,00254 mm, but insufficient to cause fusion in-the en#tire thickness of the sheet; and (c) cooling at least the heated one side surface of said sheet to solidify the fused one side surface. 4. The method of producing a plurality of narrow sheets (s) from a wider sheet (s) of orientable thermoplastic polymer material which has been mechanically worked to obtain an orientation of the macromolecular chains, said method com prising the steps of: (a) feeding said wider sheet along a continuous path; (b) maintaining the wider sheet at a temperature greater than 37,80C in a slitting region of said path; and (c) s-litting said wider sheet with cutting blades (72) at spaced locations across its width in said slitting region in the direction sub stantially parallel to the wider sheet feed di rection to produce a plurality of narrower 'sheets (s) 5. The method in accordance with claim 4, in which step (c) includes disposing said blades (72,#2a) in a V-shaped array across said sheet with one of said blades (72a') forming the vertex of the angle defined by V-shaped array and with said one vertex blade (72a') being disposed substantially in the center of said sheet. 6. The method of producing a plurality of narrow sheets (s) from a wider sheet (S) of orientable thermoplastic polymer material which has been mechanically worked to obtain an orientation of the macromolecular chains, said method comprising the steps of: (a) feeding said wider sheet along a continuous path; (b) maintaining the wider sheet under tension greater than 0,89 kg/cm of the wider sheet transverse width; and (c) slitting said wider sheet at spaced locations across its width in said slitting region in the direction substantially parallel to the wider sheet feed direction to produce a plu rality of narrower sheets (5). 7. The method of producing a plurality of narrow sheets (s) from a wider sheet (S) of orientable thermoplastic polymer material which has been me chanically worked to obtain an orientation of the macromolecular chains, said method comprising the steps of: (a) feeding said wider sheet along a continuous path; (b) disposing a plurality of spaced-apart cutting blades (72a, 72a') in said feeding path in the plane of said sheet with adjacent blades (72a, 72a') being staggered relative to a line in said sheet plane that is transverse to the direction of the feed path; and (c) slitting said wider sheet with said blades (72a, 72awl) at spaced locations across its width in said slitting region in the direction substantially parallel to the wider sheet feed direction to produce a plurality of narrower sheets (s). 8. Apparatus for producing a plurality of narrow sheets (s) from a wider sheet (S) of orientable thermoplastic polymer material which has been mechanically worked to obtain an orientation of the macromolecular chains, said apparatus cQm- prising: (a) means for conveying said wider sheet along a continuous path; (b) heat transfer means (60) for maintaining said wider sheet at a temperature greater than 37,80C in a slitting region of said path; and (c) slitting means (70) for s#litting said wider sheet at spaced locations across its width in said slitting region in the direction sub stantially parallel to the wider sheet feed direction to produce a plurality of narrower sheets (s) 9. The method of treating orientable thermoplastic polymer strap which has been mechanically worked to obtain an orientation of the macromolecular chains but which has d non-uniform thickness, said method comprising the steps of: (a) providing a pair of spaced-apart oppositely rotating, cylindrical, heated rolls (40,42); and (b) feeding said strap in a continuous path par tially around and in contact with each roll to contact one side surface of the strap with one roll and to contact the other side surface of the strap with the other roll while maintaining the strap under a tension of greater than 0,89 kg/cm of the strap transverse width whereby the opposing side sur -faces of said strap become substantially flat and uniformly parallel throughout the length of the strap.

Inventors: GOULD, RUSSELL J., MATSUNAGA, YUKIO A., VAN ERDEN, DONALD L.
Applicants: SIGNODE CORPORATION
Application year: 1981
Main classification: B29C71
IPC codes: B29C61, B29C55, D01F6, B29L7, B29C49, B26D1, D01D5, B29D7, B29C71, B29K23, B65D63