Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Mixing of two or more solid polymers; mixing of solid...
Reexamination Certificate
2000-07-20
2002-10-08
Short, Patricia A. (Department: 1712)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Mixing of two or more solid polymers; mixing of solid...
C524S539000
Reexamination Certificate
active
06462145
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a polymer blend consisting essentially of from about 50% to about 95% by weight, based on the total weight of the polymer composition, of polytrimethylene terephthalate and of from about 5% to about 50% of an elastomeric polyester. The polymer blend has particular utility in the fabrication of industrial fabrics that are particularly suitable for use in rigorous environments, where the dimensional stability and mechanical properties of the fabrics are important.
BACKGROUND OF THE INVENTION
Modern industrial fabrics are commonly assembled by weaving, braiding, knitting, knotting and other known methods from polymeric monofilament or multifilament yarns. It is also known from EP 802280 to assemble such fabrics from a plurality of extruded polymeric strips or panels. The chosen polymers are most frequently polyesters, copolyesters, polyamides, polyphenylene sulfides, polyphenylene oxides, fluoropolymers or polyketones. Selection of any particular polymer for a specific application will generally be dictated by the physical and mechanical properties desired, the cost of the polymer, and the prevailing environmental conditions of the end use.
The present invention is primarily concerned with a polymer blend for use in fabricating industrial fabrics intended for environments where the dimensional stability and resistance to repetitive compressive stress of the fabrics are important. The invention is thus particularly relevant to papermaking fabrics which are used to form, drain, dewater and convey a paper web as it is created within a paper making machine.
For the purposes of this application, the term “fabric” is taken to mean an assembly of components. The term “component” is taken to mean any of the components from which a fabric can be assembled, such as yarns(including monofilament, multifilament, staple and spun forms) or extrusions. The components forming the fabric can be arranged by interlacing, entangling or engagement so as to form an integrated cohesive structure, such as nets, cloth, felts, textiles and the like, which are created by weaving, knitting, knotting, joining, felting, needling, spiral winding, bonding, or similar methods. Typical components include individual monofilaments, multifilaments, spiral coils, and profiled plastics extrusions such as strips, tiles or panels. These components are generally fabricated by an appropriate method such as melt extrusion, melt spinning, casting or slitting from an extruded film. The fabricated components are then joined to form an integrated cohesive structure.
In a papermaking machine, a paper web is created in three stages. In the forming section, a water based stock of papermaking components is discharged onto a moving continuous forming fabric. As the fabric conveys the stock through the forming section, it is drained and agitated to provide a somewhat self supporting wet paper web. Drainage of the stock is augmented by various stationary elements with which the forming fabric is in moving contact. The web is then transferred to the press section where a major proportion of the remaining water is removed by mechanical pressing in a series of high pressure nips between opposed press rolls. Press fabrics are used both to convey the web, and to receive expelled water. The web then passes to the dryer section, in which it is conveyed on at least one dryer fabric over a series of heated cylinders where the remaining water is removed by evaporation. The resulting paper is then calendered, slit and wound onto reels.
Papermaking fabrics must ideally possess multiple characteristics simultaneously:
(a) they must be resistant to abrasive wear caused by their passage over the various stationary elements in the paper making machine, and by contact with solids in the stock which they are to convey;
(b) they must be structurally stable, so as to function as designed within the range of stresses imposed during their use;
(c) they must resist dimensional changes in the plane of the fabric due to moisture absorption over a wide range of moisture contents;
(d) they must resist stretching under the tension imposed by the powered rolls which drive the fabrics on the machine;
(e) they must be resistant to degradation caused by the various materials present in both the fiber-water slurry and in the materials used to clean the fabrics, at the prevailing temperatures of use.
In addition, some industrial fabrics, such as press felts used in the press section of a papermaking machine, must be resistant to compaction and repetitive cyclic compressive stress.
Of the various polymers available for industrial fabrics applications, those most commonly used in paper making are:
polyesters, in particular polyethylene terephthalate (PET) and various copolymers thereof, and
polyamides, particularly polyamide-6 (also known as polycaprolactam), and polyamide-6/6 (also known as polyhexamethylene adipamide).
Although yarns and extrusions formed from both polymer types offer certain advantageous characteristics, there are essentially two difficulties associated with their use:
(i) while PET yarn generally has adequate chemical resistance and dimensional stability, and is also amenable to weaving, having good crimpability and heatsetting behaviour, its abrasion and compaction resistance is not always adequate, especially when used in fabrics for higher speed paper machines; and
(ii) although yarns of both polyamide-6 and polyamide-6/6 have adequate abrasion and compaction resistance, they do not possess adequate dimensional stability in the moisture range found in the paper making environment, and the mechanical properties of fabrics made from them are known to change.
U.S. Pat. No. 5,137,601 to Hsu discloses papermaking fabrics, in particular press felts, whose component fibers and filaments are fabricated from polypropylene terephthalate, herein after referred to as PPT. In view of the use of “propylene” in the polymer name, this appears to be a polymer of terephthalic acid and 1,2-propanediol. The fabrics are alleged to have chemical resistance properties similar to polyester, and physical properties comparable to polyamide-6. There is no disclosure of suitable intrinsic viscosities for the PPT, no identification of suitable grades, no discussion of the possibility of blending PPT with a second polymer, nor are there any teachings to suggest that this polymer may be suitable to withstand repetitive cyclic compressive stress.
Best, in EP 844320, discloses monofilaments for use in paper making machine clothing whose principle component is polytrimethylene terephthalate (described by Best as PTMT, and stated to be a polymer of terephthalic acid and 1,3-propanediol). In a preferred embodiment, the PTT may be blended with up to 45% by weight of polyurethane so as to improve the abrasion resistance of the monofilaments. Best in EP 0 965 665 also discloses monofilaments of the same polyester with up to about 50% of a polyamide. There is no disclosure in either of these applications of the appropriate grade or intrinsic viscosity of a suitable PTT, nor does the disclosure teach that blending of PTT with any polymer other than either polyurethane or polyamide will improve the ability of the components to withstand repetitive cyclic compressive stresses and therefore increase the service life of the components. Specifically, there is no disclosure or suggestion that any other polymers than polyurethane or polyamide may provide satisfactory results.
Despite these innovations, and for various other reasons including the cost of the raw materials, neat polyamide yarns are still preferred for many industrial fabric applications. The term “neat” as used herein refers to a polymer system containing only one polymer, e.g. polyamide-6, and nothing else, other than conventional additives such as stabilizers, plastic processing aids, colourants, and inhibitors of oxidative, hydrolytic or thermal degradation. The compaction and abrasion resistance of these polyamide based yarns is useful in physically demanding applicati
Baker Samuel M.
Fleri Paul C.
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