Rubber footwear: applications, manufacture.

Rubber footwear has become a part of our day-to-day life, from children's cosmetic footwear to pull-ons for adults. Additionally, rubber footwear predominates today's safety footwear market. The reasons for this dominance are categorized through the following properties/applications:

* Dielectric strength. Rubber, by nature, is a poor conductor or non-conductor of electricity. Rubber has more superior dielectric strength as opposed to leather. Due to this, it is used in the manufacture of dielectric shoes or overshoes to protect the linemen working on high voltage power lines.

* Waterproofness. Vulcanized rubber, when exposed to water, does not absorb any water or does not allow water to permeate through. This property is utilized in making waterproof footwear. No stitching is involved in manufacturing of rubber boots. Inherent tack of uncured rubber is used to obtain the leakproof seams. After the vulcanization of the boots, these seams form a continuous boundary through which water cannot leak. In leather footwear, as everyone knows, the leather by nature tends to absorb water; water tends to permeate through the stitched seams making the wearer's feet wet. Another disadvantage leather presents is that of water weight gain. The rubber boot does not gain any weight due to water absorption. This increase of weight is an important factor causing fatigue to the user.

* Slip resistance (traction). This feature is also very important when working on wet or slippery surfaces. Rubber has superior slip resistance as compared to leather.

* Abrasion resistance (wear resistance). Rubber can be compounded to get good wear resistance and hence increase the service life of the footwear.

* Chemical resistance. By advent of different synthetic rubbers today, rubber can be compounded to obtain superior resistance to permeation and penetration by a variety of chemicals. Due to this, rubber footwear is widely used in the clean-up of chemical spills. (e.g. Neoprene boots were used in the Exxon Valdez oil spill.)

* Flame resistance. Rubber can also be compounded to impart flame resistance to final vulcanizate. This property of flame resistance is essential in firefighter's footwear.

* Electrical conductivity. As mentioned before, rubber by nature is a poor conductor of electricity and hence can be used in making dielectric footwear. However, rubber can also be compounded to impart electrical conductivity to the final vulcanizate.

This particular aspect is very useful in the manufacture of conductive footwear. This type of footwear is commonly used in hospitals, computer industries, explosives factories, etc., where it is necessary to discharge the static charge developed on the body to avoid arching.

In leather shoes, one has to wear an additional strap around the calf which helps dissipate the static charge, whereas in the rubber footwear the entire footwear can be made conductive.

* Maintenance. Rubber footwear is very easy to maintain. It requires much less maintenance as compared to leather footwear and is more cost effective.

Manufacturing process begins with compounding

Compounds used in rubber footwear vary greatly depending on the final properties required or the application of the particular footwear. For example, the conductive footwear is built using a conductive rubber compound, where the rubber compound is specially designed to be electrically conductive. Similarly, the slip resistance, abrasion resistance and chemical resistance properties can also be enhanced by special compounding.

In addition to the desired properties in footwear, compounds are designed to have balance between processibility, storage stability and the cost.

Because of its properties and the cost effectiveness, natural rubber is the most commonly used elastomer in rubber footwear. However, due to their chemical resistance, oil resistance and resistance to ozone cracking, the synthetic elastomers like polychloroprene, nitrile and EPDM have also achieved importance in the footwear industry. The effect of blending of natural rubber with different synthetic rubbers on its chemical resistance, oil resistance and ozone resistance is illustrated in table 1.

  Table 1
                  Chemical     Ozone           Oil
                  resistance   resistance      resistance

Natural rubber    Poor         Poor            Poor
EPDM              Fair         Excellent       Good
Neoprene          Good         Fair            -
Nitrile           Good         -               Excellent
NR/EPDM (blend)   Fair         Good/excellent  Fair
Neoprene/nitrile  Good         -               Excellent

The manufacturing process involves several operations (figure 1):

* Mixing. Several rubber compounds to suit the needs are mixed and are sheeted out.

* Calendering/milling. Compounded rubber is then processed on mills and calenders to get sheets of desired thicknesses, and rubber coated fabrics.

* Gum cutting/fabric cutting. These calendered sheets are then cut by dies according to the design specifications of particular boots.

* Boot building. Cut parts are then assembled and applied around the shoe lasts or forms to build the boot. This is done by a team of several skilled personnel on the rotary. Different parts of rubber footwear are illustrated in figure 2.

* Vulcanizing. Boots built in this fashion are then stacked on the boot cars. These boot cars are then pushed in autoclaves where boots are vulcanized under specific air pressure (28-30 psi) for a specific period of time at certain temperatures. Typically used time and temperatures are 60 minutes at 300[degrees]F. In addition to the air, 3-4 pounds of ammonia is also introduced in the vulcanizer. Ammonia cure tends to enhance the rubber surface giving gloss and resistance to scuffing and marking. The undesirable side effect of darkening from ammonia is the only negative factor in rubber footwear that is lighter in color (ref. 1).

Testing

Safety footwear thus manufactured is tested for its compliance with national and international standards such as ANSI Z-41 (ref. 2), NFPA-1974 (ref. 3), CSA Z-195 M92. (ref. 4). In many instances the third party certifications or classifications are obtained for this footwear to assure the consumer of its quality. The classified footwear generally bears the logo of certifying laboratory or agency.

Conclusion

Thus, due to today's demanding consumer, rubber footwear has gained its own place in the footwear market addressing safety. Rubber footwear makes significant contributions to the elastomer applications. Due to its good dielectric properties, negligible water absorption, good resistance to slip, abrasion, flame and chemicals, its use can be expected to increase.

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Plus Rubber World's annual directory of Independent Testing Laboratories and Instrumentation and test equipment suppliers

[Figure 1 to 2 ILLUSTRATION OMITTED]

References

[1.] C.K. Rider, "The Vanderbilt Rubber Handbook," Thirteenth Edition, R.F. Ohm Ed., R.T. Vanderbilt, Inc. Norwalk, CT, Ch. 6, p. 688, 689 (1990). [2.] American National Standard for Personal Protection- Protective Footwear. ANSIZ41 (1991). [3.] National Fire Protection Association std. on protective footwear for structural firefighting NFPA-1974 (1992). [4.] Protective Footwear, Occupational Health and Safety,. A national standard of Canada. CAN/CSA-ZI95-M92.