Technical Field

This invention relates to a monofilament of low carboxyl content for use in fabricating a paper machine dryer fabric having improved resistance to hydrolytic degradation and abrasion. The invention also relates to a method of making this filament and to a composition therefor.

Background Art

Polyester monofilaments, when woven into fabrics, have many applications in industrial filtration processes and as belts for conveying certain industrial product ^' s. Such fabrics are used extensively in the paper industry to filter the wood pulp slurry and convey the formed paper to dryer sections of the paper-making process. The dryer section also contains conveying belts made out of industrial fabrics.

In industrial applications such as those mentioned above, the onofilaments are subjected to considerable abrasion and heat in wet atmospheres. Most polyesters known to the art are easily abraded and degraded in such applications, thereby limiting the useful life of the fabric.

There are several patents relating to improving the hydrolytic and thermal stability of polyesters. A non- exhaustive list includes the following references:

FR 2,309,581

US 3,959,228

SUBSTITUTE SHEET f O PI

US 4, 071, 504 US 3 , 975 , 329 US 3 , 676 , 393 US 3, 853 , 821 US 3 , 716, 582

GER 2,020,330 US 3,193,522 Most methods are directed towards reacting the free carboxyl end-groups of the polyester molecules with a suitable additive in order to chemically deactivate these end-groups. The carboxyl end-groups in the presence of water and high temperatures catalyze chain-scission at the ester linkages of the polymer molecule. Each broken bond creates an additional carboxyl group in the polymer thereby accelerating the chain-scission process.

Most additives for end-capping are slow to react with the molten polymer, thereby causing increased expenditure in time and equipment for commercial produc¬ tion of hydrolytically stable products. Catalysts are available to accelerate the reaction for a few of the additives, such that the reaction is complete within the 1 to 6 minutes that the polymer resides in a commercial extruder. The point of addition of the catalyst is usually at the start of the polymerization reaction. This ensures a thorough dispersion of the catalyst and is also the most convenient. The additive is best added at the throat of the extruder by means of a wel1-metered controlled-volume pump. However, this obviously entails complicated and expensive modification of the production machinery. It is preferable to use a polyester that has as low a carboxyl content as possible as the feed resin. Values as low as 15 meq COOH per 10 ⁶ gms of resin and preferably a value of 10 or lower is recommended.

Disclosure of the Invention

It is an object of the present invention to provide a monofilament of low carboxyl content for use in fabri¬ cating a paper machine dryer fabric having improved resistance to hydrolytic degradation and abrasion.

It is another object of the invention to provide a process whereby such monofilaments can be easily fabri¬ cated with conventional machinery known to the art.

It is another object of the present invention to provide a monofilament of low carboxy content for use in fabricating a paper machine dryer fabric having improved resistance to hydrolytic degradation and abrasion, the monofilament comprising: a) a polyester; b) a polyester stabilizer; and c) a thermoplastic material.

It is another object of the invention to provide a composition comprising: a) a polyester; b) a polyester stabilizer; and c) a thermoplastic material. Brief Description of the Drawings

The invention is illustrated by means of the follow¬ ing drawings and tabulations in which: FIGURE 1 is a graph of the % of residual tensile strength vs. time showing a comparison in resistance to hydrolytic degradation between a commercial monofilament and the monofilament according to the invention.

Best Mode for Carrying Out the Invention Preferably, the polyester comprises polyethylene terephthalate.

In accordance with a preferred embodiment of the invention, the thermoplastic material is selected from the group consisting of polyurethanes and polyetherester block copolymers. In accordance with another preferred embodiment of the invention, the polyester stabilizer is selected from the group consisting of glycidyl ethers, carbodiimides, ketenimines, hydroxyl containing compounds, aziridines and isocyanates. The preferred thermoplastic material consists of poly { butylene terephthalate-co-(multibutyleneoxy)tere- phthalate } . This is a Dupont material known as HYTREL.

The optimum .amount of thermoplastic material to be added depends on the balance between the degree of abra- sion resistance required and the deterioration in the physical properties of the monofilament due to addition of the thermoplastic material. Amounts ranging from 1 percent to 30 percent of the compatible polymer may be used in the blend with 5 to 15 percent being the optimum for most applications.

The preferred polyester stabilizer consists of a polycarbodiimide known under the trademark STABAXOL.

Preferably, there is added about 0.1 to 5 percent by weight of said polyester stabilizer, more preferably about 1 to 2 percent by weight.

As stated above, the abrasion resistance can be improved by blending the polyester with optimum amounts of a compatible polymer which has superior abrasion resistance. Examples of compatible polymers with good abrasion resistance are polyurethanes produced by the reaction of methylene diphenyl isocyanate or tolylene diisocyanate with polyethylene adipate or phthalate or polyalkylene oxides. Elastomeric polyesters such as

polyether-ester block copolymers are also compatible with good abrasion resistance. An example of elastomeric polyester is the commercial product marketed under the trademark of HYTREL which is poly { butylene tere- phthalate-co-(multibutyleneoxy)terephthalate } . The polymers mentioned above have melt points considerably lower than those of the polyesters into which they are incorporated.

The hydrolytic stability of the polyester is improved by the addition of organic compounds such as carbodiimides which act as acid scavengers. The carboxyl end-groups in the polyester molecules catalyze chain- scission at the ester linkages, thereby degrading the physical properties of the product. Several organic compounds with groups capable of reacting with the COOH end-groups are known to the prior art and may be used as stabilizers for polyesters. Examples of such groups known to the prior art are gly- cidyl ether, carbodiimide, ketenimine, hydroxyl, aziri- dine and isocyanate. The stabilizer is normally incor¬ porated in the molten resin immediately after polymeri¬ zation or in the extruder during the fabrication process .

Other properties required of the stabilizer are good stability to high temperatures, a melting point below normal processing temperatures and lower diffusivity through the polymer. Polycarbodiimide such as those available commercially under the trademark of STABAXOL meet all those requirements and is used extensively with polyurethane and polyester products. However, since the polycarbodiimide is not readily soluble in molten polyesters, and since it has a low viscosity, it cannot be processed in a conventional single screw extruder without creating erratic flow and conveying in the feed section of the screw. Expensive modifications of the

extruder would be necessary in order to overcome the feed problems. An alternative to modifying the extruder is to compound the stabilizer in an appropriate carrier prior to incorporating into the polyester. The carrier for a compounded concentrate can be any thermoplastic material.

It is preferably to use a material having a low melt point to prevent thermal degradation of the stabilizer during compounding. It is also preferable to use a carrier that is compatible with the polyester. Poly- urethanes and elastomeric polyesters are the most suit¬ able carriers and this invention relates to their use in producing polyester monofilaments with low carboxyl con¬ tent.

The method of making a monofilament of low carboxly content for use in fabricating a paper machine dryer fabric having improved resistance to hydrolytic degrada¬ tion and abrasion according to the invention comprises: a) compounding a polyester stabilizer in a thermo¬ plastic material; b) incorporating the compounded material obtained in step a) with a polyester; and c) extruding the mixture obtained in step b) in a screw extruder so as to obtain a monofilament.

When used throughout the specification and claims, the term resistance to hydrolytic degradation means resistance to chemical degradation from the reaction of steam with the polyester. The chemical degradation results in a loss in strength of the monofilament fiber.

While there is thermal degradation at the dry end of the dryer machine, we are only concerned, in the present case, with the hydrolytic degradation at the initial stages of drying under the influence of steam.

TABLE 1, which is given below, is a summary of the

results obtained from the hydrolysis testing .

TABLE 1

Results of Hydrolysis Testing

Commercial Product Present Invention

Time, % Residual % Residual

Hours Lb/Strand Strength Lb/Strand Strength

0 18.47 100.0 17.46 100.0

24 18.27 98.9 17.09 97.9

48 17.84 96.6 16.99 97.3

72 17.63 95.4 16.65 95.4

96 17.30 93.7 — —

120 16.30 88.2 16.10 92.2

144 14.85 80.4 16.50 94.5

168 11.47 62.1 16.08 92.1

192 8.71 47.2 15.87 90.9

216 6.38 34.5 — —

240 4.73 25.6 14.38 82.4

264 12.68 72.6

288 10.39 59.5

312 7.72 44.2

TABLE 2, hich is given below, shows the results of abrasion tests conducted on the same monofilaments.

OMP

TABLE 2

Results of Abrasion Testing

Relative i-ibrasion Sample Condition Resistance (RAR)*

Before Hydrolysis 1.493

After 120 hrs. of Hydrolysis 1.695

After 192 hrs. of Hydrolysis 2.130

*RAR = % Weight Loss of Commercial Product % Weight oss of Present Invention

The tests leading to the ^' results of FIGURE 1 and TABLE 1 were carried out as follows:

Several five-foot lengths of the monofilaments were wound into 2" coils, mounted on a wire rack and auto- claved at 250°F and 15 Psig steam pressure. The autoc- lave was sampled every 24 hours. After conditioning the steam treated coils at 80°F and 50% R.H., they were uncoiled and tested for breaking strength on an Ihstron Universal Tensile Tester. Each data point in TABLE 1 and FIGURE 1 represents the average from several breaks. The tests leading to the results of TABLE 2 were carried out as follows:

Monofilaments to be tested were wrapped around a tared polyethylene rod and reweighed to obtain the weight of monofilament used in the test. The rod with the mono- filament was spun in a sand slurry for 40 minutes . The

sa ple was then removed, dried and reweighed to determine the weight loss.

As it appears in the graph and tabulations, it is obvious that the monofilaments according to the invention is more satisfactory than the commercial filament as its properties will be retained longer in an atmosphere of simultaneous abrasion and hydrolysis.

OMPI