Patent Application
20080251212
The present invention relates to apparatus for the production of wide polymeric sheet and more particularly to apparatus for the manufacture of wide ultra high molecular weight polyethylene sheet.
U.S. Pat. No. 6,951,685 issued Oct. 4, 2005 describes a method for the manufacture of ultra high molecular weight polyethylene (UHMWPE) materials in the form of slit film fibers, tapes and narrow sheets. Such materials are described as having, among other useful properties ballistic resistance of a very high order.
As is apparent to the skilled artisan, the equipment and processing techniques described in this patent require significant capital investment and the application of relatively stringent processing conditions. Both of these requirements increase virtually exponentially as the UHMWPE product width is increased from a fiber to a tape and upwards to a sheet. Thus, in order to contain the additional cost of equipment required to make such wider materials, i.e. sheet as defined herein, it would be desirable to have a method for their manufacture that minimizes such costs and process control requirements.
U.S. Pat. No. 4,931,126 describes an apparatus for increasing the width of a fiber reinforced thermoplastic sheet or tape product, which apparatus increases such width by longitudinally joining parallel sheets or tapes of the fiber reinforced thermoplastic material in an overlap or butt configuration and melting the overlapping or abutting areas of the parallel tapes.
European Patent Publication No. EP 1 627 719 A1 describes a multilayered UHMWPE material comprising a plurality of “monolayers” of UHMWPE in the absence of any adhesive wherein the each monolayer is laid at an angle to any adjacent monolayer. The term “monolayer” as used in this publication is defined as comprising “a plurality of high-strength unidirectional polyethylene strips, oriented in parallel in one plane, next to one another”. According to one embodiment the strips partially overlap. The “monolayers” are formed by subjecting the overlying strips to conditions of temperature and pressure in the ranges of 110-150° C. and 10-100 N/cm2 These conditions produce a “sheet” having joint areas that are inadequate to maintain even a modicum of integrity and their properties are grossly inferior to those of the sheets of the present invention, as will be demonstrated in the discussion and examples that follow.
There thus remains a need for a method of producing wide strips or sheets of substantially pure and highly oriented UHMWPE from narrower tapes or strips of these materials, and for the products produced by such a method.
It is therefore an object of the present invention to provide apparatus for the production of wide sheets of any custom width from narrow strips or tapes of a polymer, which wide sheets exhibit properties equal or superior to those of the parent strip materials from which the wide sheets were fabricated.
It is another object of the present invention to provide apparatus for the fabrication of a wide sheet of substantially pure and highly oriented UHMWPE, which wide sheets exhibit strength and modulus properties equal or superior to those of the parent strip materials from which the wide sheets were fabricated.
According to the present invention there is provided an apparatus for the continuous fabrication of wide polymer sheet from a plurality of polymer strips of indeterminate length abutting or partially overlapped longitudinally to define joints between adjoining strips.
As used herein, the terms “substantially flat”, “essentially flat” and “substantially pure” are meant and intended to have the following meanings: “substantially flat” refers to sheet material in accordance with the present invention wherein a joint between two adjoining strips of material has a thickness that is not greater than 80% of the combined thicknesses of the adjoining/overlapping/abutting strips; “essentially flat” refers to sheet material in accordance with the present invention wherein a joint between two adjoining strips is essentially the same thickness as that of the strips being joined with little if any thickness difference therebetween; and “substantially pure” refers to UHMWPE that contains no foreign materials or substances that negatively affect the properties of the UHMWPE except as artifacts of the UHMWPE production process such as catalysts, etc.
The starting material UHMWPE strips of the present invention are those prepared in accordance with the methods described in the following U.S. Pat. Nos. 6,951,685; 4,879076; 5,091,133; 5,106,555; 5,106,558; and 5,578,373 the teachings of which are all incorporated herein by reference in their entireties. Particularly preferred as the starting materials in the process described herein are the UHMWPE materials prepared as described in the aforementioned U.S. Pat. No. 6,951,685. Such material comprise highly oriented UHMWPE of high purity.
According to the present invention, wide UHMWPE sheet is produced by a process that comprises calendering an array of overlapping or abutting strips of indeterminate length prepared as just described at a temperature below the melting point of the UHMWPE, generally in a range of between about 120° C. and about 155° C. (depending upon the tension applied to the strips during bonding as described below) at a pressure above about 300 pounds per lineal inch (pli) and under a tension of between about 0.3 grams/denier and about 5 grams/denier. The arrangement of the array and the resulting final product sheet is depicted in
Referring now to accompanying
As depicted in
Referring now to
Individual rolls of material 01 and 01′ (shown as element 302 in
As material 302 exits feedstock payoff zone 10 it is passed through a series of bars 20 (best seen in
Upon exiting zone 20 material 302 enters zone 30 which comprises two sets of offset rolls 31 and 31′ that include flanges 32 and 32′ mounted upon adjustable shafts 33 and 33′ that serve to direct the flow of material 302 into subsequent zone 40 and control the amount of overlap of material 302 as it enters this subsequent zone.
Zone 40 comprises a series of vertically offset rolls 40 and 40′ that pull material 302 from feedstock rolls 01 and through zones 20 and 30. A motor 42 is provided to drive rolls 41 and 41′. Rolls 40 and 40′ can be utilized to develop either tension or pulling energy as will be clearly understood by the skilled artisan.
Zone 50 comprises a final set of guide rolls 31 including flanges 32 mounted on a shaft 33 which serve to provide final guidance of overlapped material 302 into calender zone 50. The overlapped materials at this point in the process and in accordance with this embodiment are shown generally in
After exiting zone 50 wide ballistic sheet 300 enters zone 60 which comprises an offset set of pull rolls 61which serve to draw material through apparatus 100 under tension as described elsewhere hererin. A motor 62 is provided to drive rolls 60.
In zone 70 wide ballistic sheet 300 is taken up and rerolled onto a shaft 71 driven by motor 72.
Referring now to
The processing conditions described herein, temperatures below the melting point of the UHMWPE strips, tensions in the range of from about 0.3 and about 5 grams/denier and pressures above about 300 pli, define an operating window whose parameters of temperature and tension are intimately interrelated. As is well known in the art of producing UHMWPE, as tension increases on a fiber or strip of UHMWPE, the “melting point” i.e. the temperature at which the onset of melt can be detected, increases as tension increases. Thus while at a tension of 0.3 grams/denier a temperature of about 120° C. may be below the melt point of the UHMWPE strips, at a tension of 5 grams/denier a temperature of 154° C. may still be just below the melt point of the UHMWPE strips. Thus, this interrelationship of tension and temperature must be carefully considered and maintained in order to obtain the enhanced products of the present invention. The pressure element of the processing conditions, is largely independent of the tension and temperature relationship just described. According to various preferred embodiments of the processing conditions of the present invention, temperatures in the range of from about 125° C. and 150° C. and tensions in the range of from about 0.4 and about 4.5 grams/denier are specifically preferred. The speed at which the process can be operated successfully is dependent solely upon the rate at which heat can be imparted to the UHMWPE strips. As long as the strips can be brought to the proper temperature prior to introduction into the calender rolls, and maintained at such temperature during their brief dwell in the nip of the calender rolls, the process will be effective. Such more rapid heating could be through the use of a preheating oven, the use of a larger calender rolls, multiple calender rolls, etc.
While the foregoing discussion centers about the use of a “pair of opposed calender rolls”, it will be readily understood that such language is meant and intended to mean that at least one pair of opposed calender rolls is utilized and that a plurality of calender rolls defining multiple sequential pairs of opposed calender rolls or a plurality of calender rolls defining a plurality of nips, i.e. zones of pressure contact between opposing calender rolls through which the abutting/overlapping UHMWPE material passes could also be successfully utilized. For example, a four roll calender could provide three distinct nips. Any and all such configurations are intended to be included in the language of the claims attached hereto.
UHMWPE wide sheet produced in the apparatus and in accordance with the process described herein exhibit a remarkable degree of transparency, in excess o 30%, while those of the prior art prepared as described below exhibited the opacity of the parent strip materials. This is undoubtedly due to either the fact that at low temperatures the process of the prior art does not produce well consolidated or intimately commingled structures, thus, exhibiting the transparency of the parent material, while at higher temperatures melting occurs, as discussed in greater detail below, leading to the presence of voids in the melted areas that serve to diffuse light and result in increased opacity.
In order to demonstrate clearly the distinctions between the products produced by the apparatus of the present invention and the far inferior products of the prior art, samples of wide UHMWPE fabricated in accordance with the present invention and in accordance with the process described in European Patent Publication No. EP 1 627 719 A1 were produced and subjected to SEM study to clearly observe the structural differences between the joint areas/volumes in each of the products. The results of these studies are shown in
The SEM images presented in
The SEM photograph labeled
In further evaluation of the teachings of the prior art, samples were prepared according to the teachings of the reference at temperatures of 140° C. and 150° C. and pressures of 145 psi and 14.5 psi respectively. The SEM photographs of these samples are shown in
In order to make the comparison of the products produced using the processing conditions of the apparatus of the present invention and those of the prior art easier, it has been calculated that the pressures utilized in the present invention are above about 17,000 psi and about 85,000 psi at the upper end of evaluated pressures. These are considerably higher than the 14.5-145 psi pressures indicated as useful in the prior art European Patent Publication.
While the apparatus described herein has been described largely in connection with the manufacture of wide UHMWPE sheet, it will be readily apparent that similar apparatus could be used for the manufacture of wide polymeric sheet from strips of virtually any suitable polymeric material under appropriate conditions of temperature, pressure and tension.
As the invention has been described, it will be apparent to those skilled in the art that the same may be varied in many ways without departing from the spirit and scope of the invention. Any and all such modifications are intended to be included within the scope of the appended claims.
