End sealants

K > Na > Li. After neutralization or removal of the catalyst, fluid polymers are used as is or to formulate grease compounds. If a reactive end blocking group was incorporated during polymerization, the fluid polymer is then useful for making a coating, a sealant, or a Hquid mbber product. 

The first step in formulating a urethane sealant is to prepare what is commonly called the prepolymer, typically by reaction of a hydroxy-terrninated polyether with a stoichiometric amount of a diisocyanate. Each hydroxy group reacts with one end of every diisocyanate molecule. 

One-part urethane sealants (Table 3) are more compHcated to formulate on account of an undesirable side reaction between the prepolymer s isocyanate end and water vapor which generates carbon dioxide. If this occurs, the sealant may develop voids or bubbles. One way to avoid this reaction is to block the isocyanate end with phenol and use a diketamine to initiate cure. Once exposed to moisture, the diketamine forms a diamine and a ketone. The diamine reacts with the isocyanate end on the prepolymer, creating a cross-link (10). Other blocking agents, such as ethyl malonate, are also used (11). Catalysts commonly used in urethane formulations are tin carboxylates and bismuth salts. Mercury salt catalysts were popular in early formulations, but have been replaced by tin and bismuth compounds. 

Urethane sealants have good inherent adhesion to most substrates, but silane adhesion promoters are often used to improve this adhesion. Epoxy-, amino-, and mercapto-functional silanes are the most common because of their dual reactive nature. The silane end can react with surface hydroxyls the epoxy, amino, or mercapto end can react with the isocyanate. 

Polybutenes enjoy extensive use as adhesives, caulks, sealants, and glaring compounds. They are used as plasticizers in mbber formulations with butyl mbber, SBR, and natural mbber. In linear low density polyethylene (LLDPE) blends they induce cling to stretch-wrap films. Polybutenes when modified at their unsaturated end groups with polar fiinctionahty are widely employed in lubricants as dispersants. Blends of polybutene with polyolefins produce semisoHd gels that can be used as potting and electrical cable filling materials. 

When metal inserts require hermetic sealing, consider coating them with a flexible elastomer such as an RTV rubber, polyurethane, or epoxy system. A second method is to design an annular space or reservoir at one end of the insert from which to dispense the flexible elastomers to effectively create a hermetic seal. Flexible sealants are also used to compensate for differences in the thermal coefficient of expansion between metal and plastic. 

The LP grades are liquid polymers used in sealant and mastic applications, and are formed by breaking down a high molecular weight polymer in a controlled manner. The liquid polymer again has mercaptan end groups. 

Specialty chemicals comprise a large number of chemicals that are used as aids to the manufacture of other major products (e.g., in paper milling, plastic production, water treatment, mining), are used as end products (e.g., pesticides in farming), or are components of consumer products (personal care products, paints and coatings, adhesives and sealants, photographic chemicals). 

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