Millable polyurethanes for athletic footwear.

Millable polyurethane elastomers are a specialty type of synthetic rubber made by reacting a dihydroxy polyether or polyester with chain extender and diisocyanate. The nearly equal molar amounts of reactants results in a high molecular weight hydroxy terminated polymer. These MPEs are of interest to the rubber industry because they can be mixed, extruded, calendered, compression or injection molded on rubber processing equipment. Vulcanization is carried out' using sulfur or peroxide cure systems. MPEs offer excellent abrasion resistance, oil and fuel resistance, ozone and weathering protection, good load bearing and damping properties, and high tensile and tear strength.

Applications for MPEs are in many markets besides footwear including automotive, hose and belting, marine and oil field products. Many large industrial rolls, copier rolls, O-rings, seals, gaskets and many other mechanical goods requiring abrasion resistance are made using these specialty elastomers.

Abrasion resistance of millable polyurethanes

We began our experiment by comparing the abrasion resistance of three millable polyurethane elastomers using three different laboratory abrasion tests. We will compare the effect of increasing durometer on abrasion resistance. The three different laboratory abrasion tests are: Taber, Akron and DIN abraders.

The Taber abrader is a laboratory tester which uses a flat molded sheet as a test sample. The wearing action of the Taber abrader is produced by contact of the test sample, turning on a vertical axis against the sliding rotation of two abrading wheels. The wheels are driven by the sample in opposite directions. About 30 square centimeters are abraded in a series of arcs. A 1,000 gram weight is applied to the sample and a H-18 wheel is used.

An exclusive feature of this abrader is that the wheels traverse a complete circle of the specimen surface causing abrasion at all angles relative to the test specimen. A weight loss is measured and its volume loss is determined.

The Akron abrader has a different method for testing abrasion resistance. A wheel shaped specimen is prepared by molding a sample 13 mm thick, having an inside diameter of 13 mm and an outer diameter of 63 mm. About 50 square centimeters of the sample are abraded. This sample is rotated at 36 rpm against an abrasive wheel (abrasive A grain size 4) at a 15 degrees angle. A weight of 6 lbs. is applied to the wheel. As with other abrasion tests a volume loss is determined.

The third abrasion tester was the DIN. This test consists of a roller driven at a constant speed of 40 rpm. A sheet of specified abrasive paper is secured around the roller and a cylindrical test piece (16 mm diameter, 6 mm thick), travels over the abrasive paper under a constant load of iON at a speed of 0.32m/sec. After traveling a total distance of 40 m, the test piece is removed and a volume loss is determined. An abrasive index can be determined by relating results to those of a control compound.

A description of the MPEs tested can be found in table 1. MPE-A and MPE-B are general purpose millable polyurethane elastomers while MPE-C is a special polyether NIPE made with an aliphatic diisocyanate. MPE-A and MPEB are TDI/polyether and TDI/polyester respectively and were both cured with sulfur. MPE-C can only be vulcanized by peroxide.

All of the MPEs tested contained a precipitated silica with silane coupling agents and small amounts of process aids and plasticizer. Curatives were added in normal amounts to obtain a cure time 11' or less at 149 degrees C. No ingredients were added that would influence test results favoring one or another sample or test. Formulations and physical properties can be examined in tables 2-4.

All of these MPEs are commercial products. MPE-A and MPE-B have been used in many commercial applications for a number of years. MPE-C is a newer product and has been used exclusively in the footwear industry, but now is finding use in seals for computer hardware and pharmaceutical closures.

In figure 1, Taber abrasion resistance is measured against durometer for MPE-A, MPE-B and MPE-C. Results are measured from volume loss in mm3 against increasing durometer 60 to 80 A. From figure 1, it appears that MPE-C has superior abrasion resistance compared to MPE-A and MPE-B, especially at 70 to 80 durometer. However, it was found that the abrasion action of the Taber abrader caused a gummy buildup on the test surface of MPE-C which self lubricated the sample allowing for a lower volume loss. Knowing this, we have to discount the test results of MPE-C, and look carefully at the other MPEs which may give better performance. We must point out that abrasion is most important in the hardness range of 60 to 65 A at which most outsoles for athletic footwear are made. In this range both MPEA and MPE-B have very good abrasion resistance but MPEA is quite a bit better than MPE-B as measured by Taber.

Other information gathered from figure 1 is that MPE-A also has better abrasion resistance than MPE-B at the higher durometers. We can conclude that sulfur cured PTMEG/TDI could out-perform polyester/TDI in resistance to abrasion. However, it may be concluded that polyether MPEs test better on the Taber abrader than polyester MPEs.

In the next series of tests, the abrasion resistance of the MPEs was measured by the Akron abrader. The results can be compared in figure 2. In this test, we see a reversal of results compared to those of the Taber abrader. MPE-B has the best abrasion resistance and MPE-C has the worst. The Akron test seems to give better results with polyester urethanes than polyether urethanes.

One important fact observed from the data and in field testing is that as durometer increases, abrasion resistance decreases. The reason for the decrease in abrasion is the increased amount of filler needed to obtain 80 durometer A. This dilutes the polyurethane and thus lowers the abrasion resistance.

So far in our analysis, we have some conflicting data. We decided to test these MPEs on the DIN abrader. The results can be seen in figure 3.

The DIN abrader is a rapid test compared to the Taber and Akron and we did not see the gummy build-up which occurred with MPE-C on the Taber abrader. The DIN results at low durometer indicate that MPE-C was best, while MPEB was better than MPE-A. At hardness of 70A, all MPEs gave equal results. At high durometer, DIN test results were identical for the polyethers MPE-A nd MPE-C, while the polyester MPE-B had much poorer abrasion resistance.

In the 60 to 65 durometer range which is most important in athletic footwear, we find some test agreement between Akron and DIN abrasion. MPE-B is better than MPE-A. That is polyester/TDI MPE has better abrasion resistance than polyether/TDI.

What can we conclude from these test results? First, we see that results are widely scattered over the range of hardness especially with the Taber. We also are looking at a potential bias of test apparatus for a particular type of MPE. From our practical experience, we know that all of these MPEs exhibit excellent abrasion resistance in many applications. The action of the Taber abrader may have generated sufficient heat build-up to melt the MPE-C samples causing the gummy condition. Therefore, Taber would not be the best choice for screening urethane outsole formulations.

As previously explained, abrasion resistance of MPE decreases as the amount of filler increases. Both Akron and DIN abraders show this trend. We believe these test methods should be used to evaluate millable polyurethane footwear outsoles.

We know that no one test will be able to predict the abrasion resistance in the fidd for all conditions. These laboratory tests are meant to be for quality control of abrasion resistant compounds. Our laboratory has more experience with the Taber but will in the future rely more on the Akron abrader and the DIN test.

But which data should we believe? Which test should be performed? We decided to look at other physical properties, especially coefficient of friction.

Comparison of physical properties Abrasion resistance, as measured by Taber, Akron and DIN, is not the only laboratory criterion on which to evaluate MPEs for use in athletic footwear outsoles. Let's compare these millable gums on original stress strain properties and tear strength, and look carefully at coefficient of friction values.

Upon examining table 2, it is evident that as hardness increases, so does tensile strength, tear strength and modulus. Elongation and coefficient of friction decrease, as hardness increases. MPE-B has higher modulus, tensile and tear strength than the polyethers MPE-A and C.

Of special interest to the footwear industry is tear strength. Tear properties are very important in demolding shoe soles and in stitching operations. Comparing the tear strength of MPE-B with MPE-A and MPE-C, MPE-B has much greater tear strength than these other MPEs. The combination of high abrasion resistance and high tear strength should be useful for aggressive designs of outsoles for hiking boots.

A comparison of coefficient of friction can be examined in figure 4. It is evident that MPE-C has a higher coefficient at each hardness and should provide better traction than MPE-A, or MPE-B.

MPE-A also has a higher coefficient of friction than MPE-B. Because the abrasion resistance and traction were very good, MPE-A was picked for use in tennis shoes. The use of MPE-A in the toe area provided excellent resistance to abrasion from toe drag while still providing good traction.

In addition to having excellent traction, MPE-C also has very good abrasion resistance. An outsole compound with a high coefficient of friction gives a squeaky sound heard on all indoor basketball courts;

On this basis, MPE-C was selected for indoor basketball and aerobic outsoles.

There exists great interest in transparent and brightly colored outsoles. The use of transparent parts of the shoe sole is to highlight a 1ogo or to show a feature of the midsole such as an energy recovery device. In both cases, the transparent outsole should have excellent abrasion resistance, good traction and very good light stability.

Such a combination of physical properties is possible only with millable polyurethane elastomers like MPE-C. Only MPEs made with aliphatic diisocyanate can be light stable and produce transparent compounds. MPEs made with TDI or MDI would turn yellow rapidly.

Transparent soling compounds must be cured with liquid peroxides in order to achieve transparency. Never use those peroxides which contain filler, such as calcium carbonate or magnesium silicate, or transparency will be lost.

Hardness, tensile strength and tear properties are improved by the addition of fumed silica. The use of other types of fillers or precipitated silica will cause toss of transparency or diminish the brightness of the color.

An example of transparent compounds and their physical properties can be examined in table 6.

Summary

Several types of millable polyurethane elastomers exist, i.e., polyethers and polyesters, in combination with diisocyanates like TDI and aliphatic. These MPEs can be cured with sulfur or peroxide. The resultant chemical structures produce products with differing physical properties and abrasion resistance.

MPE-A has the right combination of abrasion resistance and high coefficient of friction which provides excellent performance in the toe area of tennis shoes. Laboratory tests indicate that MPE-B may provide improved performance against tearing and abrasion in hiking boots. Abrasion testers may have a bias toward polyester or polyether depending on how much heat build-up is generated on the sample. As hardness increases, so does modulus, tensile and tear increase, but coefficient of friction and abrasion resistance decrease. Polyethers exhibit higher coefficient of friction (better traction) than do polyester MPEs.

Fumed silica offers better reinforcement, better abrasion and tear resistance than precipitated silica. Fumed silica is also used to prepare transparent outsoles. MPE-C has been used for indoor basketball shoes and aerobic shoes because of its high abrasion resistance; very high coefficient of friction giving a squeaky noise; and because of the aesthetic value of transparency.