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Asbestos-based plastic product molding compounds & mold release agents: this article describes the molding compounds & release agents used to produce asbestos-based or asbestos filled / reinforced plastic or molded products. This articles series about the manufacture & use of asbestos-containing products includes detailed information on the production methods, asbestos content, and the identity and use of asbestos-containing materials.
This article series about asbestos plastics & molded materials describes the history, manufacturing process & uses of asbestos plastics and molded materials.
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The Military Specification MIL-M-14, "Molding Plastics and Molded Plastic Parts, Thermosetting," lists twenty types along with twelve types being mineral filled (asbestos). This specification is considered one of the most important and useful specifications for both military and commercial applications.
This specification lists requirements that can be met by more than just one or two compounds so that limited procurement does not exist. This situation results in listing requirements which are lower than some of the commercially available types. The specification lists requirements on mineral filled phenolics, melamine, alkyd, diallylphthalate, and silicone resins. Table 9.8 lists some of the specification requirements.
TABLE 9.8. PROPERTY VALUES FOR BATCH-ACCEPTANCE TESTS OF DIALLYLPUTHALATE, SILICONE, AND ALIYD RESIN MOLDING COMPOUNDS ( MIL-M-14) [click to enlarge]
Other types of compounds include asbestos-tetrafluoroethylene resins. Such combinations improve mechanical properties of the base resin and permit compositions based on the asbestos content to be tailored to such a wide variety of chemical, mechanical, and electrical applications as resistance to deformation under load, resistance to wear, stiffness, thermal conductivity, compressive strength and hardness.
Originally, when molding compounds were manufactured using phenolic resin as the basic component, expensive procedures were use The ingredients were mixed with an alcoholic resol solution and were thoroughly impregnated in agitators. After this operation the solvents were evaporated and the usual procedure of grinding the mixture on ball mills followed.
The general procedure presently being used is to take the phenol resin and grind it with other ingredients. The mixtures can also be kneaded on heated rolls. For some mixtures, premastication in a Banbury mixer is required prior to the heated roll operation. During the roll operation, the resin melts and thoroughly impregnates the fillers and other ingredients.
After thorough mixing of ingredients has been accomplished, the homogeneous sheet is ground in a mill. In the case of novolacs, hexamethylene is added during the rolling operation. When the hexamethylene is added a quick reaction occurs where ammonia is liberated from the hexa and acts as a catalyst. Immediately when this reaction occurs, the mixture is removed from the rolls.
Various automatic techniques are used for mixing and preparing phenolic molding compounds. The usual technique is to take the rolled sheet and automatically convey it by means of a conveyor belt to a grinder or a mixing screw.
When difficulty exists in obtaining proper treatment of fiber fillers or other types of fillers, it is sometimes required that the fillers be impregnated with an alcohol solution of the resin. Another method is to mix the ingredients in a hollander to produce a pulp slurry. Generally, in this type of operation the slurry is made into a preform. The preform is made by vacuum suction on a sieve which produces a shape approximately similar to the final article.
During the preparation of the molding compound, it is important that the correct amount of condensation occurs. If the condensation is too high, a poor flowing product is produced, if insufficient condensation occurs, a free flowing compound will be produced.
However, excess water of condensation produced during the curing cycle in turn can result in excessive sticking of the compound in the mold cavity, producing blemished surface, poor electrical properties or requiring the part to have a long curing cycle.
The heat conductivity of the compound also contributes to the hardening velocity. Asbestos-filled mixtures heat up quicker than wood flour filled mixtures, thus giving better hardening conditions.
The time required for the compound to harden may not be sufficient to harden the part completely, as in general, the cure of a part takes place after the initial set. When thick wall sections are to be cured, the heat conductivity of the molding compound is indeed an important function. As the thickness of the wall increases, the hardening time naturally increases and this increase is in exponential relation to the wall thickness.
Premix Molding Compounds of Asbestos-Containing Plastics
These types of compounds have relatively low raw material costs. Various types of mixes are prepared which are dependent upon the specific applications. The end users of these materials are largely in the automotive, electrical, electronics, home appliance, refrigeration, and industrial equipment fields. Production runs are long.
A blend of reinforcing fibers (glass, asbestos, sisal, etc.) fillers (asbestos, gypsum, talc, clay, etc.), polyester, or phenolic resins, catalyst for polyester (benzoil peroxide) pigments, and release agent (stearic acid, lecithin, silicone, etc.), makes up a premix compound. It is important that the blends are uniform and properly mixed. They are generally prepared in a kneader, in a similar mixer such as the Banbury mixer or special screw conveyors.
TABLE 9.9. HIGH STRENGTH REINFORCED ASBESTOS PREMIX COMPOUND *
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