PAPER MACHINE DRYER FABRICS

Abstract: A paper machine dryer fabric in which at least a portion of the component yarns are monofilaments comprised of a copolymer of dimethylolcyclohexane, terephthalic acid, and isophthalic acid, and an effective amount of a hydrolysis stabiliser. The molar ratio of terephthalic acid to isophthalic acid used in preparing the copolymer is about 83:17, and the copolymer has an intrinsic viscosity which is greater than about 0.75 dL/g. The copolymer imparts to the monofilaments an increased resistance to hydrolytic degradation and improved knot properties, in comparison to other known polymers used in dryer fabrics. Low density polyethylene may optionally be added to the copolymer to increase the resistance of the monofilaments to surface contamination and soiling.

Description: PAPER MACHINE DRYER FABRICS Field of the Invention The present invention relates to paper making machine dryer fabrics in which at least a portion of the component yarns are monofilaments of a high molecular weight copolyester and a hydrolysis stabilizer which provide monofilaments with an increased resistance to hydrolysis and contamination, and improved knot properties. The monofilaments can also contain low density polyethylene. Background of the Invention Dryer fabrics are commonly manufactured from monofilaments of polyethylene terephthalate (PET) , using woven or spiral construction. PET imparts reasonably good physical properties to the fabrics, and they are relatively easy to keep clean. But PET is prone to hydrolytic degradation in the hot, moist environment of the dryer section. To overcome this problem, various hydrolytic stabilizing additives have been proposed, including the aromatic carbodiimides described by Barnewall et al, US 3,975,329. Aromatic polycarbodiimides (PCDI) have become the preferred additives to stabilize PET for use in dryer fabrics . Modern dryer sections operate at higher temperatures and moisture levels than in the past. These harsher environments cause even stabilized PET to degrade, shortening the effective life of the dryer fabric. Increasing the amount of carbodiimides in the PET, and the use high molecular weight PET having low concentrations of free carboxyl end groups have resulted in only marginal improvements. In addition to hydrolysis resistance, other desirable physical properties of dryer fabric monofilaments include resistance to abrasion, to thermal degradation, to contamination, and to chemical attack. Thermal shrinkage and knot elongation of the monofilaments are also important. Other polymers proposed for dryer fabrics include polyphenylene sulphide, by Baker et al, US 4,786,554, and by Ballard, US 4,610,916, and poly (etheretherketone) , by DiTullio, US 4,359,501. Both polymers possess excellent heat, chemical and hydrolysis resistance properties. In addition, the abrasion resistance of poly (etheretherketone) monofilaments is superior to PET. However, both of these polymers are expensive and difficult to process into fabrics. Pol (cyclohexylene dimethylene terephthalate) (PCT) has better resistance to hydrolysis than PET, and stabilisation by aromatic PCDI significantly improves PCT, but its processibility into monofilaments, and into dryer fabrics, is difficult. The use of a mixture of terephthalic and isophthalic acids results in a copolyester known as "acid- modified PCT", or PCTA copolyester. Polymers of this type are disclosed by Kibler et al, US 2,901,466, for use in wearing apparel and films. The Kibler et al. polyesters appear to provide a good degree of hydrolytic stability and possess a relatively high melting point (see especially col. 2, lines 10- 18 & 52-56, col. 7, lines 13-20 and col. 14, lines 25-26) . But Kibler et al . do not disclose any relationship between hydrolysis resistance, knot elongation properties or thermal shrinkage of the disclosed polyester fibers and either molecular weight, ratio of terephthalic to isophthalic acids, or the addition of a hydrolysis stabiliser. Several copolymers of 1, -dimethylolcyclohexane, terephthalic acid, and isophthalic acid are available from the Eastman Chemical Company under the trade name "KODAR THERMX copolyester". "KODAR" is a registered trademark of the Eastman Kodak Company. A PCTA copolyester resin identified by Eagles et al, US 5,169,499, as "KODAR THERMX copolyester 6761" and stated to be "generally, a copolymer comprised of terephthalic acid, 1, -dimethylolcyclohexane, and isophthalic acid" (Col 4, lines 27-29 and elsewhere) is suggested for use in "an article of paper machine clothing" (see Claim 1) . Eagles et al. describe fabrics in which the monofilaments are the above noted PCTA copolyester, and may optionally include from 0.5% to 10% by weight of a hydrolysis stabilizer such as "STABAXOL P" or "STABAXOL P-100" available from Rhein-Chemie GmbH of Rheinau, Germany. "KODAR THERMX copolyester 6761" is noted to be a particularly suitable copolyester. Monofilaments made from the copolyester are said to have a melting point greater than 260°C, preferably more than 265°C, and allegedly provide a fabric that is more resistant to hydrolytic degradation than PET. However, both the monofilament knot elongation properties and abrasion resistance are less than is desirable. The monofilaments are also difficult to weave, and their strength when used in forming a pin seam to join the fabric ends is poor. Eagles et al. focus on only one measurable physical property of the claimed monofilaments, which is filament melting point. Eagles et al . do not disclose two important measurable physical properties of their copolymer, which are the ratio of the terephthalic and isophthalic acids in the copolymer, or the intrinsic viscosity of the copolymer, both of which affect the final properties of the monofilament. Kibler et al . indicate a relationship between acids molar ratio and monofilament melting point. Eagles et al . are also silent on the sensitivity of the monofilaments to either hydrolytic degradation or dry heat stability when changes are made to the acids molar ratio, and make no reference to the importance of the knot elongation properties of the monofilaments. Gardner et al, US 5,283,110, describe high temperature copolyester monofilaments suitable for use in dryer fabrics. These are produced from a blend of a PCTA copolyester resin, a melt extrudable fluoro- or chloroflouropolymer resin and, optionally, thermal and hydrolysis stabilizers. A particularly suitable copolymer is another Eastman Chemical Company PCTA copolymer identified by the trade name "KODAR THERMX copolyester 13319". The monofilaments are extruded from a polymer blend consisting of copolyester resin, and from 1 to about 25% by weight of the fluoropolymer resin, examples of which are TEFZEL 210, a product of E.I. du Pont de Nemours & Co., and HALAR 500, a product of Ausimont USA, Inc. The monofilaments allegedly exhibit improved knot tenacity, weavability, and hydrolysis resistance properties when compared to Eagles et al . , as well as an improved resistance to soiling and surface contamination. However, these fluoropolymer resins have the disadvantages that they are expensive, corrosive to the extruder in manufacture, and adversely affect the abrasion resistance of the monofilaments. Thus, a need exists for a dryer fabric monofilament that is resistant to hydrolytic degradation and dry heat, and provides excellent knot elongation properties in combination with a reasonable level of thermal shrinkage of at least 1%, and preferably of about 4%, so as to impart stability to the fabric and close up the mesh interstices following a heatsetting process, and which may be produced at a low cost. In addition, the resulting fabrics should ideally be resistant to contamination such as is caused by the increased use of recycled fiber in papermaking stock. Summary of the Invention The present invention seeks to provide a paper machine dryer fabric in which at least a portion of the yarns are melt extruded monofilaments of a copolymer of 1,4- dimethylolcyclohexane, terephthalic acid and isophthalic acid, wherein in the monofilaments: i) the molar ratio of the terephthalic acid to the isophthalic acid component is about 83:17; ii) the copolymer has an intrinsic viscosity, when measured by the specified procedure, which is greater than about 0.75 dL/g; iii) the copolymer contains at least 0.4% by weight of a hydrolysis stabilizer; iv) the knot elongation, measured by the elongation to break of a knotted monofilament sample in which a simple overhand knot has been formed, expressed as a percentage, is at least about 10%; and v) the thermal shrinkage of the monofilament at 200°C is at least about 1%. Whilst both thermal shrinkage and knot elongation can be measured as physical properties of a monofilament, their importance lies in their combined effect on fabric construction. In making dryer fabrics, a weaver seeks a fiber with maximum knot elongation associated with enough thermal shrinkage to tighten the fabric during heat setting. For PCTA copolymer based monofilaments a compromise has to be reached, since thermal shrinkage is inversely related to knot elongation. Experience with weaving dryer fabrics from PCTA copolymer monofilaments indicates that the knot elongation should be as high as possible, and the thermal shrinkage no less than about 1%. Preferably, the thermal shrinkage should be in the range of from about 1.5% to about 10%; the most desirable range appears to be from about 3% to about 5%. As reported herein, all intrinsic viscosity measurements of both the various polymer resins and the finished monofilaments are made at 30°C in a 60:40 parts by weight mixture of phenol and 1, 1, 2,2-tetrachloroethane in accordance with ASTM Standard Procedure D-4603-86. Although the molecular weight of a polymer is related to its measured intrinsic viscosity, no attempt has been made to determine the molecular weights of the polymer systems discussed herein. As used herein, the term "processability" refers to the amount of flexibility available to the manufacturer to adjust extrusion parameters without detrimentally affecting the physical and thermal properties of the monofilament. As used herein, the terms "knot elongation" and "normalised knot strength" refer generally to the brittleness of monofilaments when subjected to bending stresses. Both of these properties have a direct impact on weavability, or ease with which the monofilaments may be woven into dryer fabrics. "Knot elongation" is measured by forming a simple overhand knot in a monofilament sample, subjecting the sample to tension, and then measuring the elongation to break. The elongation to break of a knotted specimen under tensile stress is defined as the "knot elongation" and is expressed herein as a percentage of the original length of knotted monofilament. "Normalised knot strength" is determined by measuring the tensile break strength of both a monofilament sample including a simple overhand knot and an unknotted sample using a suitable tensile strength tester. The ratio of these two figures is referred to as the "normalized knot strength", and is expressed herein as a percentage. As used herein, the term "dry heat stability" refers to the ability of the monofilament to resist degradation in hot dry air. This is expressed as the percent tensile strength retained in a specimen heated in a hot air circulating oven at a constant temperature over a predetermined period of time as compared to an unheated specimen. As used herein the term "thermal shrinkage" refers to the amount of contraction in length of a monofilament sample, expressed as a percentage of its original length, that is obtained when the sample is placed in a hot air circulating oven, in an unconstrained condition. This property provides a quantitative measure of the propensity of the material to shrink during processing of woven fabrics made therefrom such as by a heatsetting process. The fabrics of this invention may be of woven or spiral construction, and exhibit improved properties, particularly with regard to their thermal shrinkage, dry heat stability and resistance to hydrolytic degradation, when compared to known fabrics. In addition, the weavability of these fabrics is improved as a direct result of the enhanced knot elongation of the monofilaments. These monofilaments can also exhibit a relatively high shrinkage of at least 4% at 200°C, which, together with their improved knot elongation, also makes them suitable for the manufacture of the helical coils and hinge yarns used in the manufacture of spiral dryer fabrics as described by Leuvelink, US 4,345,730. The monofilaments are also suitable for use as weft yarn material in low air permeability dryer fabrics, wherein the weft is required to shrink and close up the mesh interstices during the heatsetting process. In an alternate embodiment of this invention, an effective amount of low density polyethylene, or LPDE, is incorporated into the monofilaments to improve their resistance to surface contamination and soiling which may be caused in part by the use of recycled furnish. The LPDE is typically added into the monofilament in amounts ranging from about 0.5% to about 15% by weight during the melt extrusion process. We have found that both the 83:17 terephthalic to isophthalic acid molar ratio in the PCTA copolyester, and the copolyester intrinsic viscosity, play an important role. The molar ratio of 83:17 imparts to the copolymer a broader processability during extrusion without any known detrimental effect on the end-use performance. By treating the PCTA copolyesters to increase the intrinsic viscosity prior to extrusion, significant improvements in certain physical properties of the monofilaments may be obtained. A common means of increasing polymer intrinsic viscosity, and also, presumably, molecular weight, is to expose the polymer resin as received in an amorphous state to a reduced pressure and elevated temperature process usually referred to as "solid stating" . A PCTA copolyester which has been found to be particularly suitable for use in the manufacture of monofilaments for the dryer fabrics of this invention is "KODAR A150 PCTA copolyester" which has been solid stated prior to monofilament extrusion. This Eastman Chemical Company product has a terephthalic to isophthalic acid unit molar ratio of 83:17 as determined by nuclear magnetic resonance techniques, a melt point of about 260°C as defined by the temperature of the highest peak on the endotherm of the plot produced via Differential Scanning Calorimetry according to the method described by Eagles et al . in WO 90/12918, an intrinsic viscosity prior to solid stating of from about 0.75 to about 0.81 dL/g, and provides processing characteristics similar to PET. The addition of aromatic PCDI stabilizer greatly improves the hydrolysis resistance of dryer fabrics made from monofilaments of solid stated "KODAR A150 PCTA copolyester". A threshold value for the stabilizer content in the copolyester seems to be about 0.4%, and significant improvement is obtained at levels above about 1%. Beyond about 5% there is no significant improvement in the hydrolysis resistance of the monofilament. A suitable commercially available aromatic PCDI containing 2, 4, 6-triisopropyl -phenylene diisocyanate, is "STABAXOL P-100" produced by Rhein-Chemie GmbH, of Rheinau, Germany. When equal quantities in excess of 1% by weight of this PCDI hydrolysis stabilizer are incorporated into solid stated "KODAR A150 PCTA copolyester", solid stated "KODAR PCT 3879 copolyester" or "KODAR THERMX copolyester 13319", the hydrolytic stability of the solid stated "KODAR A150 PCTA copolyester" monofilaments is unexpectedly found to be from about 1.5 to 2 times greater than that of comparable monofilaments comprised of either the "KODAR THERMX copolyester 13319" or solid stated "KODAR PCT 3879 copolyester". The knot elongation of the monofilaments utilized in the dryer fabrics of this invention is significantly better than those obtained from monofilaments comprised of either solid stated "KODAR PCT 3879 copolyester", "KODAR THERMX copolyester 13319", or solid stated PET. At equal levels of PCDI, the dry heat stability of the solid stated PCDI-stabilized "KODAR A150 PCTA copolyester" monofilaments is superior to that of monofilaments of either "KODAR THERMX copolyester 13319" or "KODAR PCT 3879 copolyester", as well as solid stated PET. The knot elongation of the solid stated "KODAR A150 PCTA copolyester" monofilaments, combined with their relatively high thermal shrinkage, not only facilitates the manufacture of dryer fabrics, but also facilitates twisting and heatsetting the monofilaments into multifilaments and cabled yarns. By increasing the intrinsic viscosity of "KODAR A150 PCTA copolyester", to a value greater than 0.75 dL/g in the finished monofilaments by means of a solid state treatment prior to extrusion, the resistance to abrasion is increased, and is comparable to monofilaments of either "KODAR THERMX copolyester 13319" or "KODAR PCT 3879 copolyester". An upper limit to the intrinsic viscosity of the copolyester appears to be about 1.5 dL/g in the finished monofilament. At higher values difficulties arise during the monofilament extrusion process. Addition of from 0.5% to 15% by weight of LPDE to the solid-stated "KODAR A150 PCTA copolyester" during monofilament extrusion improves the resistance of finished fabrics to soiling and contamination caused in part by the use of recycled furnish. A commercially available suitable LPDE is identified by the trade name "DOW 5004 IM LDPE", and is available from Dow Chemical Company of Midland, Michigan. This LPDE has a melt index of 4 g/10 min. (ASTM D-1238), and a density of 0.923 g/cc (ASTM D-792) . Monofilament Examples The following experimental results are presented to illustrate the invention. In the Figures : FIG. 1 shows the percent retained tensile strength for monofilaments in Table 2; FIG. 2 shows the effect of PCDI stabilizer content on the tensile strength of the monofilament samples in Table 2; and FIG. 3 shows the relationship between knot elongation and thermal shrinkage. The following describes the procedures used to prepare the polymers and extrude the monofilaments for use in the fabrics of this invention, as well as the comparison monofilaments. The comparative samples tested are comprised of solid stated PET, "KODAR PCT 3879 copolyester" and "KODAR THERMX copolyester 13319" which had been solid stated by the manufacturer, Eastman Chemical Company. A. Preparation and Extrusion of Monofilament Samples "KODAR A150 PCTA copolyester" having an acids molar ratio of 83:17 and having a melt point of about 260°C, was "solid stated" by the following procedure. 272 kg of PCTA copolyester, in the pelletized amorphous state as received, was charged to a 0.84 m 3 . Patterson Kelley double-cone tumble- dryer. The dryer was sealed, vacuum applied to a value of less than 5 mm Hg, and the dryer rotated at about 1 RPM for 4 hours while the temperature of the oil in the dryer shell was maintained at 177°C. The oil temperature was then increased to 232°C while the rotation and vacuum continued for several more hours. Samples were taken periodically until the measured intrinsic viscosity had increased from the as-received value of 0.75-0.81 dL/g to be greater than 0.93 dL/g. Increasing the intrinsic viscosity of the "KODAR A150 PCTA copolyester" to a value greater than about 0.93 dL/g prior to monofilament extrusion is critical to the success of the invention. The intrinsic viscosity of the copolyester decreases during extrusion; the intrinsic viscosity of the solid stated resin should be sufficiently high such that the intrinsic viscosity of the finished monofilaments is greater than about 0.75 dL/g when measured according to the procedure described. The copolyester as prepared above was then divided into four separate batches to allow for comparative testing. Varying quantities of a commercially available PCDI hydrolysis stabilizer, "STABAXOL P-100" were physically blended into three of the batches so as to provide final PCDI concentrations of 0.55%, 1.45% and 2.3% by weight respectively. The four batches were redried in a dehumidified hot air hopper dryer and immediately extruded at a rate of 1.82 kg/hr. using a 1.9 cm single screw extruder. The extruder barrel zones were set to 291°C while the melt pump and spinneret were set to 287°C and 279°C, respectively. The molten extrudate travelling vertically downwards was quenched in water at 66°C prior to stretching and winding onto spools. The air gap between the spinneret face and the water surface was set at 7.62 cm. The first oven was maintained at 104°C where the monofilament was continuously stretched to 3.2 times its original length. The monofilaments, after being wrapped around the second roll, passed through the second oven maintained at 207°C and stretched to 1.14 times their length. The strands were next subjected to a continuous controlled shrinkage of 11.3% in the last oven which was maintained at 238°C, prior to winding on to spools at 23.77 m/min. under a controlled tension of 600 g. The monofilament strands thus obtained were rectangular in cross-section, having dimensions of 0.043 x 0.066 cm, and are identified as Samples R-0393H, R-0260F, R-0260G and R-0260H in Table 2. Comparable monofilaments of "KODAR PCT 3879 copolyester" were also prepared according to the same procedure. The amorphous polymer resin was first solid stated, then divided into four batches; amounts of "STABAXOL P-100" hydrolysis stabilizer were added to three to provide final PCDI concentrations of 0%, 0.6%, 1.48% and 2.1% by weight respectively, and the intrinsic viscosities measured. Monofilaments were then extruded using the same technique and having the same dimensions. These monofilaments are identified as Samples R-0393B, R-0330C, R-0330D and R-0330E in Table 2. Monofilament samples comprised of "KODAR THERMX copolyester 13319" were also prepared according to the extrusion method described above. This material was not solid stated prior to extrusion as were the other copolymers because it was obtained from the manufacturer in a solid stated form. The polymer resin was divided into four batches and appropriate amounts of "STABAXOL P-100" hydrolysis stabilizer were added to three to provide final PCDI concentrations of 0%, 0.32%, 1.31% and 1.93% by weight, and the intrinsic viscosities measured. These monofilaments are identified as Samples R- 0393E, R-0260A, R-0260B and R-0260C in Table 2. Monofilament samples with varying levels of thermal shrinkage were also prepared from each of the three solid stated polymers: "KODAR PCT 3879 copolyester", "KODAR THERMX copolyester 13319" and the "KODAR A150 PCTA copolyester" by the extrusion and set-up procedures described above. The thermal shrinkage of the monofilament samples was adjusted by controlling the amount of shrinkage allowed in the last oven. No PCDI was blended into these samples prior to extrusion. These monofilaments are identified as samples R-0393A through R-0393I in Table 3. For comparative purposes, a sample of solid-stated PET was also included as a control, and is designated as Sample No. R- 0393J in Table 2. This sample comprised a physical blend of solid-stated Type 26 PC polyester obtained from Hoechst Celanese Corp. of Spartanburg, SC, and STABAXOL KE7646 masterbatch from Rhein-Chemie in the ratio of 90:10 percent by weight respectively so as to produce a PCDI content in the finished PET monofilament of 1.5%. The PET was first solid stated according to the procedure described above, then extruded at a rate of 1.8 kg/hr through a 4-hole slotted spinneret. The extruder barrel zones were maintained at 277°C, the melt pump at 282°C and the spinneret at 285°C. The molten extrudate, after travelling 7.62 cm in air, was quenched in the water tank maintained at 66°C. The first set of rollers then transported the strands at 8.53 m/min. through the first stretching oven maintained at 102 ϋ C. The strands were continuously stretched to 4.28 times their original length and then transported to the second stretching oven maintained at 232°C where an additional stretching of 1.32 times was accomplished. After a continuous controlled shrinkage of 5% in the third oven maintained at 288°C, the strands were wound on spools at a speed of 45.7 m/min. under a tension of 600 gms . The intrinsic viscosity of the PET, "KODAR PCT 3879 copolyester" and the "KODAR A150 PCTA copolyester" was measured prior to and following the solid stating process, and the results obtained are given in Table 1. The solid stating procedure is an effective means of increasing the intrinsic viscosity of the "KODAR A150 PCTA copolyester" resin to a value acceptable for monofilaments for use in this invention. TABLE 1 : CHANGE IN INTRINSIC VISCOSITY (I.V. OF POLYMER RESINS WITH TIME Polymer Type Initial Final I.V. Percent Time I.V. Change (dL/g) (dL/g) (hours) KODAR A150 0.78 0.96 23 13 PCTA PET control 0.55 0.7 27 20 KODAR 3879 0.76 0.81 6.5 20 PCT B. Comparative Testing of Monofilament Samples Following sample preparation, the physical properties of the monofilaments were then tested. Each Sample was subjected to specific tests so as to quantify the following properties. a) Resistance to hydrolysis, as measured by the number of hours required to reduce the tensile strength of the monofilament sample to 50% its original value. In this test, the sample is exposed to saturated steam in a pressure vessel at 121°C and 203 kPa. b) Knot elongation. c) Normalized knot strength. d) Tensile strength. e) Dry heat stability, measured by using dry air at 177°C for 19 days. The results of these tests are displayed in Table 2. Comparative Test Results TABLE 2A: POLYMER IDENTIFICATION PCDI Level Sample # Polymer Type I.V. (dL/g) Molar Ratio (%) R-0393J SS PET 100/0 1.5 R-0393B SS PCT 3879 0.796 100/0 0.0 R-0330C SS PCT 3879 0.716 100/0 0.6 R-0330D SS PCT 3879 0.791 100/0 1.48 R-0330E SS PCT 3879 0.774 100/0 2.1 R-0393E THERMX 13319 0.874 95/5 0.0 R-0260A THERMX 13319 0.868 95/5 0.32 R-0260B THERMX 13319 0.831 95/5 1.31 R-0260C THERMX 13319 0.871 95/5 1.93 R-0393H SS KODAR A150 0.885 83/17 0.0 R-0260F SS KODAR A150 0.794 83/17 0.55 R-0260G SS KODAR A150 0.792 83/17 1.45 R-0260H SS KODAR A150 0.794 83/17 2.3 TABLE 2B: MONOFILAMENT TEST RESULTS Hydrolysis Normalized Retained (Hrs. to Knot Knot Tensile Tensile 50% Elongation Strength Strength at 177°C Sample # Tensile) (%) (%) (kg) (%) R-0393J 264 20.6 59.7 16.01 75 R-0393B 420 6.5 43 7.18 37.8 R-0330C 588 22.4 73.1 7.64 51.9 R-0330D 912 19.2 63.4 7.59 56.1 R-0330E 1015 15.2 56.1 6.77 59.5 R-0393E 396 9.6 49 6.64 18 R-0260A 607 17.3 66 6.41 41.3 R-0260B 972 20 70.5 6.45 60.4 R-0260C 1200 16.5 63 6.68 59.2 R-0393H 360 24.2 70.8 6.95 17.1 R-0260F 487 26.4 79.6 6.32 50.5 R-0260G 1188 25.4 76.9 6.45 76 R-0260H 1860 24.1 75.5 6.86 85.4 In Table 2, the heading "molar ratio" refers to the percentage molar ratio of terephthalic to isophthalic acid monomers in the polymers tested, and "PCDI Level (wt.%)" refers to the amount of the hydrolysis stabilizer "STABAXOL P-100" in the polymer as a percent by weight. Hydrolysis Resistance The hydrolysis test results in Table 2 show that solid- stated "KODAR Al50 PCTA copolyester" containing at least 1.45% by weight of PCDI (Samples R-0260G and R-0260H) , exhibited a tensile strength half-life that was between 1.2 and 1.6 times greater than the half-life exhibited by the "KODAR THERMX copolyester 13319", and from 1.3 to 1.8 times greater than that obtained using solid-stated "KODAR PCT 3879 copolyester". The solid-stated "KODAR A150 PCTA copolyester" monofilaments containing 2.3% by weight of PCDI are 4.5 times more hydrolysis resistant than the solid stated PET control. The data displayed in this column of Table 2 is reproduced in Figure 2. FIG. 1 shows the percent retained tensile strength of the monofilament samples R-0393J, R-0330D, R-0260C and R-0260H from Table 2 as a function of the number of hours spent in a hydrolyzing environment at 121°C and 203 kPa. The time required for the monofilaments of solid stated "KODAR A150 PCTA copolyester" to reach 50% retained tensile strength is much greater than the time required for any of the other monofilaments to reach this point. The hydrolysis resistance of the monofilaments of solid stated "KODAR A150 PCTA copolyester" containing 2.3% by weight PCDI is more than 7 times greater than that of the solid stated PET control (R- 0393J), twice as great as solid stated "KODAR PCT 3879 copolyester" (R-0330D) and approximately 1.5 times greater than the monofilament sample comprised of "KODAR THERMX copolyester 13319" (R-0260C) . The solid stated PET control, sample R-0393J, and the "KODAR PCT 3879 copolyester", sample R-0330D, which provided the shortest tensile strength half-lives of the monofilaments tested in this experiment, contain no isophthalic acid; the "KODAR THERMX copolyester 13319" has a terephthalic to isophthalic acids mole ratio of 95:5, while the mole ratio of the solid-stated "KODAR A150 PCTA copolyester" is 83:17. For PCDI concentrations greater than 1% by weight, as the isophthalic acid content in the copolyester increases, the hydrolysis resistance of the monofilaments increases as well. FIG. 2 clearly shows that, as the concentration of PCDI in the polymers tested increases, resistance to hydrolytic degradation increases. Further, the hydrolytic resistance of monofilaments of solid stated "KODAR A150 PCTA copolyester" containing at least 0.6% by weight PCDI is significantly greater than that of all the others at equal concentrations of stabilizer. Knot Elongation and Knot Strength Tables 2 and 3 also show that the knot elongation and normalized knot strength of the monofilaments of solid stated "KODAR A150 PCTA copolyester" are superior to all of the other monofilaments tested. At comparable PCDI levels, the knot elongation values obtained from monofilaments of solid stated "KODAR A150 PCTA copolyester" are approximately 25% greater than those obtained from all of the other samples. The normalized knot strength values of the monofilaments of solid stated "KODAR Al50 PCTA copolyester" are from 8% to 40% greater than those obtained from the other monofilament samples at comparable levels of stabiliser. Tensile Strength Table 2 also shows that the tensile strength of the monofilaments of solid stated "KODAR A150 PCTA copolyester" is equivalent to that of the comparable copolyester monofilament samples. Retained Tensile Strength After Exposure to a Thermal Environment Table 2 shows that monofilaments made from solid-stated "KODAR A150 PCTA copolyester" containing at least 1.45% by weight of stabilizer retained a maximum of 1.4 times more tensile strength than either of the comparative polymers, and 1.14 times more than the PET control at comparable PCDI levels. Thermal Shrinkage The thermal shrinkage of monofilament samples R-0393A through R-03930I was determined by placing them in a hot air circulating oven maintained at 200°C for 3 minutes. The amount of contraction in length of an unconstrained sample following exposure, expressed as a percentage of its original length, is the thermal shrinkage of the sample. The knot elongation and normalised knot tensile strength were also determined. The results are in Table 3 below. TABLE 3: THERMAL SHRINKAGE AT 200°C IN AIR (%) AND KNOT TOUGHNESS OF MONOFILAMENT SAMPLES Normalized Knot Knot Sample No. Polymer Type Shrinkage Tensile Elongation (%) (%) (%) R-0393J SS PET 4.6 59.7 20.6 (control) R-0393A SS PCT 3879 1.0 45.6 10.0 R-0393B SS PCT 3879 5.5 43.0 6.5 R-0393C SS PCT 3879 10.4 31.4 3.6 R-0393D THERMX 13319 1.1 50.9 14.2 R-0393E THERMX 13319 5.3 49.0 9.6 R-0393F THERMX 13319 11.0 32.9 3.8 R-0393G SS KODAR A150 1.4 73.4 29.3 R-0393H SS KODAR A150 4.6 70.8 24.2 R-0393I SS KODAR A150 9.5 49.3 10.6 Comparison of the knot elongation and tensile strength data displayed in Table 3, shows that there is an inverse relationship between these parameters for each of samples R- 0393A through R-0393I. This inverse relationship is shown graphically in FIG. 3. For a given thermal shrinkage value, the solid stated "KODAR A150 PCTA copolyester" maintains a higher knot elongation and normalized knot strength than the copolyesters tested, as well as the solid stated PET sample. This combination of properties of monofilaments made out of solid stated "KODAR A150 PCTA copolyester" makes them ideal for use in both woven and spiral fabrics . The inverse relationship between thermal shrinkage in the monofilament as used to create a fabric and knot elongation are of direct interest to the weaver. By comparing the preparation methods details for samples R-0393A through R-0393I given earlier with the data in Table 3, it can be seen that the thermal shrinkage is also related to the amount of shrinkage allowed to occur in the last oven during monofilament preparation. It is therefore necessary to balance these factors to provide a PCTA copolyester monofilament in which an adequate level of shrinkage is matched with an acceptable knot elongation. Experience with weaving dryer fabrics from these PCTA copolyesters shows that the knot elongation should be as high as possible, and the thermal shrinkage of the monofilament in the fabric should be at least 1%, and preferably at least 1.5%. At lower levels of shrinkage, knot elongations in excess of 25% are obtainable, whilst even at 10% shrinkage an acceptable knot elongation of at least 10% is obtainable. From a practical standpoint, a thermal shrinkage of around 4%, that is from about 3% to about 5%, which indicates a knot elongation of more than 20%, is preferred. When used as weft material in a woven fabric, monofilaments having a thermal shrinkage of at least about 4%, that is in the range of from 3% to 5%, tend to close up mesh interstices during the heatsetting process, thereby lowering the air permeability of the fabric. When used to form the coils in a spiral fabric, monofilaments having a this level of thermal shrinkage allow the coils to shrink and form around the mandrel during manufacture, thereby providing a stable coil. The relatively high normalized knot strength and knot elongation properties of the solid stated "KODAR A150 PCTA copolyester" monofilaments ensure that both woven and spiral fabrics will be durable and can withstand the rigors of the dryer section. The knot elongation properties of these monofilaments will improve the weavability of fabrics, while providing greater flexibility in fabric weave design as well as a dimensionally stable woven or spiral fabric product . D. Polyethylene Content The effect obtained by the addition of small amounts of low density polyethylene, LDPE, to "KODAR A150 PCTA copolyester" formulations was also determined. (i) The effect obtained by the addition of small amounts of LDPE to the solid stated "KODAR A150 PCTA copolyester" monofilaments of this invention is shown in Table 4. In this experiment, from 1.84 to 6.72% by weight of "DOW 5004 IM LDPE" was incorporated into the solid stated PCTA copolyester during the extrusion process. The resulting monofilaments are identified as Samples A, B, C and D. The PCTA copolyesters used were as follows: (a) in Samples A and C, soild stated "KODAR A-150 PCTA copolyester", and (b) in Samples B and D "KODAR THERMX 13319". The intrinsic viscosities of Samples A, B, C, and D were measured according to the method previously described, and the values adjusted so as to remove the effect of the LDPE and be indicative of the intrinsic viscosity of the PCTA copolyester component only. Data for the PET control, Sample R-0393J, and for the solid stated "KODAR A150 PCTA copolyester", Sample R- 0260G, are taken from Table 2. TABLE 4 : EFFECT OF ADDITION OF POLYETHYLENE ON PHYSICAL PROPERTIES OF KODAR A150 PCTA COPOLYESTER MONOFILAMENTS Sample A B C D PET R-0260G Control PE Content 1.84 3.68 6.52 6.72 0.0 0.0 (% by weight) I.V. (dL/g) 0.758 0.821 0.789 0.815 n/a .792 PCDI Level 0.46 0.92 1.63 1.68 1.5 1.45 (%) Hydrolysis 264 708 1296 1380 264 1188 (Hrs . to 50% Retained Tensile) Knot 34.25 17.05 27.77 15.01 20.6 25.4 Elongation (%) Normalized 85.6 64.4 80.8 60.1 59.7 76.9 Knot Strength (%) Shrinkage @ 1.1 3.3 2.7 3.65 4.6 3.2 200°C Retained 65.5 46.3 81.3 69.1 75 76.0 Tensile at • 177°C (%) (ii) In this experiment, 23 kg. of "DOW 5004 IM LPDE" polyethylene pellets were physically blended with 454 kg. of crystallised and dried KODAR A150 pellets. This blend was melt extruded in a 4.0 cm commercial corotating twin screw extruder. The molten strands (0.05 cm in diameter) were quenched in water to solidify and then chopped into 0.05 cm. long cylindrical pellets . This precompounded mixture was then solid stated according to the procedure described earlier. The presence of LPDE was not found to affect the solid stating process. 1.5% by weight of "STABAXOL P-100" was added via a master batch to the precompound prior to extrusion. The solid stated precompound was then extruded into monofilaments using a commercial 6.0 cm. single screw extruder at a rate of 40 kg/hr. The extruder barrel zones were set at 282°C while the melt pump and spinneret were set at 260°C. The presence of polyethylene reduced the extruder motor current from 26 amps to 15 amps indicating its lubricating effect on the extruder barrel. This prevents excessive temperature rise in the melt which could be detrimental to the polymer molecular weight as well as cause degradation of the additives. The quenching and orientation conditions of the strands thus produced were identical to those described earlier. The resulting strands are identified as Sample R-055 . Relevant test data of Sample R-0554 is presented in Table 5. Data for the PET control, Sample R- 0393J, and for solid stated "KODAR A150 PCTA copolyester", Sample R-0260G are taken from Table 2. TABLE 5 : EFFECT OF ADDITION OF POLYETHYLENE ON PHYSICAL PROPERTIES OF KODAR A150 PCTA COPOLYESTER MONOFILAMENTS Sample # R-0554 PET Control R-0260G PE Content (% by 4.4 0 0 weight) I.V. (dL/g) 0.849 0.792 PCDI Level (%) 1.24 1.5 1.45 Knot Elongation (%) 21.4 20.6 25.4 Normalized Knot 65.7 59.7 76.9 Strength (%) Shrinkage @ 200°C 4.4 4.6 4.1 Relative Contamination 1.07 1.0 0.93 Resistance Tables 4 and 5 show that monofilament properties are only marginally compromised by the addition of LDPE. When compared to the PET Control and sample R-0260G, the monofilament properties of Samples A and C, even though Sample A has a very low stabiliser content, are comparable, with the exception of the Thermal Shrinkage at 200°C. The thermal shrinkage data implies that reaching a compromise between knot elongation and thermal shrinkage is more difficult with these PCTA copolyester monofilaments containing LPDE. Sample R-0554 in Table 5 shows that this problem can be resolved. The properties of Samples B and D in Table 4 also show that the molar acids ratio affects the properties of a monofilament that contains LDPE. The main benefits provided by the addition of LPDE to solid stated "KODAR A150 PCTA copolyester" monofilaments are an increased resistance to contamination from the use of recycled materials in the papermaking furnish. In laboratory tests, polyethylene containing copolyester monofilaments exhibited a reduced tendency to retain the tacky materials found in recycled stock, as shown by the "Relative Contamination Resistance" data in Table 5. This data was obtained by the following testing procedure. Monofilament samples are woven to provide a fabric sample; a control fabric sample is also prepared. A tacky material such as adhesive tape having a known and relatively constant peel force is then affixed to both the sample and control fabric. The amount of force required to peel the tape from the test fabric surface by pulling parallel to this surface at a constant rate provides an indication of the ability of the fabric to resist adherence when compared to the control. In this test, the peel force of the sample is compared to that of a standard, in this case a fabric woven from PET monofilaments. A "relative contamination resistance" number is calculated by dividing the standard sample peel force by the test sample peel force. A value greater than one indicates greater contamination resistance than the standard, while a relative contamination resistance number less than one indicates less contamination resistance than the standard. The monofilaments comprised of solid stated "KODAR Al50 PCTA copolyester" containing 4.4% by weight of LDPE exhibit a relative contamination resistance greater than 1, and hence resist tacky materials. From about 0.5% to about 15% by weight of LPDE is effective for this purpose. In addition, the weavability of the LPDE-containing monofilaments, as demonstrated both by manufacturing experience and quantifiable properties such as knot elongation, are similar to monofilaments of PCTA copolyester which did not contain LPDE. Other properties of the LPDE-containmg monofilaments, such as their thermal shrinkage and hydrolysis resistance, are only marginally compromised when compared to monofilaments of this invention which do not contain LPDE. It is also contemplated within this invention that the monofilaments will have uses in addition to incorporation into a paper making machine dryer fabric. For such other uses it is contemplated that the monofilament will be provided in any of the known conventional forms in which monofilaments are used, including both monofilaments which are directly useable, spun yarns, multifilament spun yarns, braided yarns and the like. It is also contemplated that such yarns may include other fiber materials. The monofilaments may also be extruded having any of the cross-sectional shapes commonly used for such monofilaments, which in addition to substantially circular, includes elliptical, square, rectangular, hollow and sundry lobed shapes such as a star.

Claims: The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows: 1. A dryer fabric in which at least a portion of the yarns are monofilaments comprised of a copolymer of 1,4- dimethylolcyclohexane, terephthalic acid and isophthalic acid, wherein in the monofilament: i) the molar ratio of terephthalic to isophthalic acid in the copolymer is about 83:17, ii) the intrinsic viscosity of the copolymer, when measured at 30°C in a 60:40 parts by weight mixture of phenol and 1, 1, 2, 2-tetrachloroethane in accordance with ASTM Standard Procedure D-4603-86, is greater than 0.75 dL/g, iii) the copolymer contains an effective amount of a hydrolysis stabilizer, iv) the knot elongation, as measured by the elongation to break of a knotted monofilament sample in which a simple overhand knot has been formed, expressed as a percentage, is at least about 10%, and v) the thermal shrinkage of the monofilament at 200°C is at least about 1% . 2. A fabric according to Claim 1 wherein the intrinsic viscosity of the copolymer is from about 0.75 to about 1.5 dL/g. 3. A fabric according to Claim 1 wherein the amount of stabilizer incorporated into the copolymer is from about 0.4% to about 5% by weight. 4. A fabric according to Claim 1 wherein the thermal shrinkage is from about 1.5% to about 10%, and the knot elongation is at least about 10%. 5. A fabric according to Claim 4 wherein the thermal shrinkage is from about 3 s . to about 5%. 6. A fabric according to Claim 5 wherein the thermal shrinkage is about 4%. 7. A fabric according to Claim 1 in which the component monofilaments are woven so as to form the fabric. 8. A fabric according to Claim 1 which is assembled from coils and hinge yarns comprised of the copolymer. 9. A dryer fabric in which at least a portion of the yarns are monofilaments comprised of a mixture of a copolymer of 1, 4-dimethylolcyclohexane, terephthalic acid and isophthalic acid together with an effective amount of low density polyethylene, wherein in the monofilament: i) the molar ratio of terephthalic to isophthalic acid in the copolymer is about 83:17, ii) the intrinsic viscosity of the copolymer, when measured at 30°C in a 60:40 parts by weight mixture of phenol and 1, 1, 2, -tetrachloroethane in accordance with ASTM Standard Procedure D-4603-86, is greater than 0.75 dL/g, iii) the copolymer contains an effective amount of a hydrolysis stabilizer, iv) the knot elongation, as measured by the elongation to break of a knotted monofilament sample in which a simple overhand knot has been formed, expressed as a percentage, is at least about 10%, and v) the thermal shrinkage of the monofilament at 200°C is at least about 1%. 10. A fabric according to Claim 9 including from about 0.5% to about 15% by weight of polyethylene. 11. A fabric according to Claim 10 including from about 1% to about 7% by weight of polyethylene. 12. A fabric according to Claim 9 wherein the intrinsic viscosity of the copolymer is from about 0.75 to about 1.5 dL/g. 13. A fabric according to Claim 9 wherein the amount of stabilizer incorporated into the copolymer is from about 0.4 to about 5% by weight. 14. A fabric according to Claim 9 wherein the thermal shrinkage is from about 1.5%- to about 10%, and the knot elongation is at least about 10%. 15. A fabric according to Claim 14 wherein the thermal shrinkage is from about 3% to about 5 s ? . 16. A fabric according to Claim 15 wherein the thermal shrinkage is about 4%. 17. A fabric according to Claim 9 in which the component monofilaments are woven so as to form the fabric. 18. A fabric according to Claim 9 which is assembled from coils and hinge yarns comprised of the copolymer. 19. A monofilament comprising a mixture of a copolymer of 1, -dimethylolcyclohexane, terephthalic acid and isophthalic acid together with an effective amount of low density polyethylene effective to improve the knot properties of the fiber, wherein in the monofilament: i) the molar ratio of terephthalic to isophthalic acid in the copolymer is about 83:17, ii) the intrinsic viscosity of the copolymer, when measured at 30°C in 60:40 parts by weight mixture of phenol and 1, 1, 2, 2-tetrachloroethane in accordance with ASTM Standard Procedure D-4603-86, is greater than 0.75 dL/g, iii) the copolymer contains an effective amount of a hydrolysis stabilizer, iv) the knot elongation, as measured by the elongation to break of a knotted monofilament sample in which a simple overhand knot has been formed, expressed as a percentage, is at least about 10%, and v) the thermal shrinkage of the monofilament at 200°C is at least about 1%. 20. A monofilament according to Claim 19 including from about 0.5% to about 15% by weight of polyethylene. 21. A monofilament according to Claim 19 including from about 1% to about 7% by weight of polyethylene. 22. A monofilament according to Claim 19 wherein the intrinsic viscosity of the copolymer is from about 0.75 to about 1.5 dL/g. 23. A monofilament according to Claim 19 wherein the amount of stabilizer incorporated into the copolymer is from about 0.4% to about 5% by weight. 24. A monofilament according to Claim 19 in the form of a spun yarn, multifllament yarn, or braided yarn. 25. A spun yarn, multifllament yarn, or braided yarn including a proportion of monofilaments according to Claim 19.

Inventors: BHATT GIRISH M, BHATT, GIRISH, M.
Applicants: BHATT GIRISH M, JWI LTD, BHATT, GIRISH, M., JWI LTD.
Application year: 1995
Main classification: D21F1
IPC codes: D01F1, D01F6, D03D1, D21F1, D21F7