Thermosets types

Polymeric Ma.teria.Is, The single-ply membranes are made from a wide variety of polymers. The following is a brief description of those polymers and their characteristics. There are three thermosetting-type elastomeric membranes as of this writing (1996) neoprene, CSPE, and EPDM. Neoprene is stiU used where oil resistance is needed. Eor instance. Hydrotech uses neoprene flashings, the base of which is hot-set in mbberized asphalt (see ElASTOL RS, SYNTHETIC-POLYCm.OROPRENE). 

Polymer-matrix materials include a wide range of specific materials. Perhaps the most commonly used polymer is epoxy. Other polymers include vinyl ester and polyester. Polymers can be either of the thermoset type, where cross-linking of polymer chains is irreversible, or of the thermoplastic type, where cross-linking does not take place but the matrix only hardens and can be softened and hardened repeatedly. For example, thermoplastics can be heated and reheated, as is essential to any injection-molding process. In contrast, thermosets do not melt upon reheating, so they cannot be injection molded. Polyimides have a higher temperature limit than epoxies (650°F versus 250°F or 350°F) (343°C versus 121°C or 177°C), but are much more brittle and considerably harder to process. 

Polyurethanes (PU). The thermosetting type of this large family of polymers is mainly used as foam. A mixture of two components with a foaming agent forms a light, hard foam, which is a superior thermal insulator. 

Conventional industrial coatings materials of the thermoset type are usually acrylic, polyester, epoxy, polyurethane or silicone resins dispersed or dissolved in organic or water/ether-alcohol coupling solvents. They are cured with gas convection or electric IR ovens. The raw materials for the polymers come from petroleum feedstocks which are processed or manufactured into a finished coating system. 

Phenolic Resin Adhesives. PF resin adhesives are low molecular weight prepolymers formed from phenol and formaldehyde, and are of the thermosetting type. The polymerization is controlled by the acid or alkaline conditions, i.e., pH, at which it is conducted. Another important reaction variable is the molar ratio of phenol to formaldehyde. By varying the time and temperature of reaction, and by choosing catalysts and phenols of varying reactivity, a wide variety of adhesives can be prepared for many applications and conditions. 

Thermoset Type Designation Max. Continuous Service Temperature Comments... 

III). The values for the thermoplastic composites can be five to ten times higher than those of thermoset type (39) ... 

Thermosetting adhesives, on the other hand, soften as the temperature is increased to the glass-transition temperature, but do not melt and resolidify. Instead, because of their highly cross-linked macrostructures, they decompose and char on reaching their decomposition temperatures. Epoxies, cyanate esters, and phenolics are generally thermosetting types. 

Basic aspects of both of the thermopolymer and thermoset types, as well as some of their combinations, will be discussed briefly in this section, focusing on the possible health hazards of their monomers and of any other possible remnants from the reactions involved. 

Poly(amide imide)s (PAI)s have been used in the late 1960s as wire coating materials.Soon afterwards this class of polymers were considered for aerospace applications." Varieties with urethane units and ester units have been discussed. Now various grades, suitable for extrusion and injection molding are available. Thermosetting types are also on the market. PAIs are in between poly(amide)s (PA)s and poly(imide) (PI) in their properties. 

To the thermoset type belong bismaleimides and bisnadimides as well as oligomeric end capped imides. End capping occurs with reactive phen-ylethyl groups. These types are used for reactive injection molding and related techiuques. The chemistry of formation of the imide moiety is quite similar for both the thermoset type and the thermoplastic type. There are several monographs on PIs. Bismaleimides are a separate subclass... 

TP Thermoplastic type GF Glass fiber reinforced type TS Thermosetting type... 

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