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Cloth

Rubber cloth is a material usually made from cotton or other cloth materials, and either coated with a layer of rubber, or the cloth material is impregnated with rubber. There are a variety of different types of rubber cloths, depending on the application).

     There are basically three grades of the rubberized bellows cloth heavy, medium and light.

     The medium grade of cloth is usually the same as the lightest cloth doubled to itself, with the rubber in the middle, with one side dyed a color. The lightest cloth just has an airtight coating on one side.

     The heaviest rubber cloth is always a combination of two types of heavy cotton, with the rubber in the middle. The outer layer of cloth is the heavier of the two, referred to as a "drill" cloth, and which is dyed some color. The inner cloth is a cotton "twill" cloth which has been bleached and a light fuzz raised on the cotton, from a brushing action. Even though no one will see the side that will be glued, it must be bleached. The reason is that to use, what the industry calls, "greige goods", which is neither dyed or bleached, but instead a dirty, slightly yellow color, it would be disaster when you tried to glue it. Cotton shrinks when water hits it. When it has been dyed a color or bleached, the process takes out most of the shrinkage. But if two layers of cotton were joined together, with one of them still able to shrink from the water content of the glue, it would want to curl away from the glued edge almost immediately.

     The "fuzzing" of the glue side of the rubber cloth will store up more of the glue and make it easier to work with, because it won't slip around as easily or slip back to the corner where there is resistance to turning and a loop might develop. There can be too much fuzzing, so much that the cloth soaks up all of the glue that you can lay on the edge of the bellows board, with none left over to go completely through the weave of the cotton and bond with the rubber layer.

     The rubber used to produce the various grades of rubber cloth in the 1920's, and before, was and still is referred to as natural rubber. This was before the age of synthetics. Substitutes, such as Neoprene did not have a track record to verify its longevity. Some form of Neoprene compound was used as a rubber substitute in the making of replacement bellows cloth until well into the 1960's, when even the best grades of Neoprene, were shown to have a life expectancy of no more than thirty years. At this time, some of the original material, which was now going on fifty years, was still holding up. In the final summation, it could be proven that Natural Rubber could last longer than any synthetic substitute. As with the Neoprene coatings, there were different grades of quality, with the lasting grades always costing more.

     By 1950, the quality of the Neoprene coatings used in making replacement rubber cloth had sunk to an all-time low. With such poor quality, it quite often was better to leave the original material in place rather than replace it with a product that had a life expectancy of one or two years. This was largely the result of indifferent suppliers, more interested in profit and competition with others, who asked for the cheapest coating of Neoprene. Of course, the manufacturers cooperated by loading the Neoprene coating down with clay filler and making the cheapest product possible.

     In the late 1970's, some suppliers began to switch to using the best grade of Natural Rubber in all types of rubber cloth. Examination of the bellows cloth used by Aeolian Company was of particular interest, because there were many examples of Aeolian products around (now fifty to sixty years old) exhibiting the Natural rubber was still "live". Other examples of other player actions produced in the 1920's had shown that the Natural Rubber coating was crystallizing or had become totally hard. What had Aeolian done differently? It seems that in their quest for excellence, Aeolian had been willing to spend more money in the coating of their bellows cloth. Specifically, it was in the cost of the "Plasticisors" that were added to the sap of the rubber tree. Permanent plasticisors cost four times as much as the cheaper types of additive.

     Natural Rubber is recognized in the industry as having a better "hand". The term of the "hand" refers to the feel of the rubber as being soft and without undue resistance to folding. The hand is determined by what is called the "durometer" of the rubber. Durometer is the scale of the softness to hardness. All rubber must be cured after it is applied to the fabric. This is done by heat. Before the curing heat is applied, the substance is like chewing gum: sticky and without much recovery from stretching. The application of heat allows the metamorphosis into rubber. Rubber can be over-cured or under-cured. Just the right amount of heat for the product has to be predetermined.

     The total thickness of the lightest pneumatic cloth is in the range of .007" to .009" (three sheets of ordinary writing paper that you type letters on, would be equivalent to a thickness of .009") The total thickness would depend upon the thickness to the cotton cloth to start with. If Nylon fabric were used instead of cotton, the total finished thickness could be brought down to .005" using the thinnest nylon cloth to start with.

     Thickness of pneumatic cloth also is determined by the method in which the rubber is applied to the cloth. If it is "calendered" it means that the rubber coating is applied to the cloth by being transferred by a roller, such as ink is transferred by the rollers on a printing press. If the spreading method is used to apply the rubber to the cloth, it means that the cloth passes under the reserve of rubber and in effect works like a squeegee, which can be regulated down to the barest minimum, but still resulting in an airtight coating.

     The best method used to make pneumatic cloth is to use the machine known as the spreader. This method eliminates pinholes, presuming that a fine weave, first class quality of cloth is being used.

     As the pneumatic cloth is wound up from the spreader, the rubber coating is uncured and sticky, so it must be dusted with corn starch to keep it from sticking to the back side of the previous round of cloth. Remember, that at this stage, the uncured rubber is still soft and malleable, and so it leaves a cloth imprint into the surface of the rubber coating, although it will not stick because of the dusting. This is actually an advantage in the finished product because it allows a better glue bond at the lap-over on the hinge end of the pneumatic. If the surface of the rubber coating were to be perfectly flat and shiny, the glue could not adhere as well without this roughness of the cloth imprint. Goods could be produced without the need for the dusting with the corn starch, but the material would have to either pass between heated rollers to cure before winding, or else have a plastic insert inter-wound, to separate the surface of the uncured rubber from the cloth until it could be heat cured. In this case, after the curing, the plastic liner is simply backed out of the roll of the cloth, but it leaves a slick finish to the rubber. The corn starch cannot be scrubbed off the rubber coating after curing. Even though an uneven dusting of the corn starch, can be unsightly, it doesn't affect the quality of the rubber.   

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