Thermoset polymer structure

Cross-link density, 10 415-416, 417-418 direct measurement of, 10 426 427 Cross-linked copolymers, 7 6 lOt Cross-linked high amylose starch, 13 742 Cross-linked hydrogels, 13 729-730 Cross-linked polymers, internal stresses and, 10 423 424 Cross-linked starches, 4 721 Cross-linked thermoset polymer structure, 10 418... 

Figure 1.1 Representations of thermoplastic and thermosetting polymer structures.

Amino resins are those polymers prepared by reaction of either urea or melamine with formaldehyde. In both cases the product that results from the reaction has a well crosslinked network structure, and hence is a thermoset polymer. The structures of the two parent amino compounds are shown in Figure 1.1. 

The final physical properties of thermoset polymers depend primarily on the network structure that is developed during cure. Development of improved thermosets has been hampered by the lack of quantitative relationships between polymer variables and final physical properties. The development of a mathematical relationship between formulation and final cure properties is a formidable task requiring detailed characterization of the polymer components, an understanding of the cure chemistry and a model of the cure kinetics, determination of cure process variables (air temperature, heat transfer etc.), a relationship between cure chemistry and network structure, and the existence of a network structure parameter that correlates with physical properties. The lack of availability of easy-to-use network structure models which are applicable to the complex crosslinking systems typical of "real-world" thermosets makes it difficult to develop such correlations. 

A thermosetting resin converts to an infusible, cross-linked plastic which is insoluble in any solvent after curing. Because of this irreversible cross-linking reaction, excellent physical properties such as heat resistance, creep resistance, mechanical strength, etc., are obtainable through design of the polymer structure. 

Membranes with extremely small pores ( < 2.5 nm diameter) can be made by pyrolysis of polymeric precursors or by modification methods listed above. Molecular sieve carbon or silica membranes with pore diameters of 1 nm have been made by controlled pyrolysis of certain thermoset polymers (e.g. Koresh, Jacob and Soffer 1983) or silicone rubbers (Lee and Khang 1986), respectively. There is, however, very little information in the published literature. Molecular sieve dimensions can also be obtained by modifying the pore system of an already formed membrane structure. It has been claimed that zeolitic membranes can be prepared by reaction of alumina membranes with silica and alkali followed by hydrothermal treatment (Suzuki 1987). Very small pores are also obtained by hydrolysis of organometallic silicium compounds in alumina membranes followed by heat treatment (Uhlhom, Keizer and Burggraaf 1989). Finally, oxides or metals can be precipitated or adsorbed from solutions or by gas phase deposition within the pores of an already formed membrane to modify the chemical nature of the membrane or to decrease the effective pore size. In the last case a high concentration of the precipitated material in the pore system is necessary. The above-mentioned methods have been reported very recently (1987-1989) and the results are not yet substantiated very well. 

This paper rerports an investigation of the yield behavior of several amine and anhydride cured DGEBA resin systems. The Argon theory is used to assess the controlling molecular parameters from the experimental results. Such parameters are then compared with the known chemical structures of the resins. The mechanisms of plastic flow in thermoset polymers such as epoxies is demonstrated. 

Heating for an extended period of time under acidic conditions will give a complex thermoset polymer of poorly defined structure including ring formation, of which the following may be typical. 

The crosslinking reaction is an extremely important one from the commercial standpoint. Crosslinked plastics are increasingly used as engineering materials because of their excellent stability toward elevated temperatures and physical stress. They are dimensionally stable under a wide variety of conditions due to their rigid network structure. Such polymers will not flow when heated and are termed thermosetting polymers or simply thermosets. More than 10 billion pounds of thermosets are produced annually in the United States. Plastics that soften and flow when heated, that is, uncrosslinked plastics, are called thermoplastics. Most of the polymers produced by chain polymerization are thermoplastics. Elastomers are a category of polymers produced by chain polymerization that are crosslinked (Sec. 1-3), but the crosslinking reactions are different from those described here (Sec. 9-2). 

The chemical structures of thermosets are generally much more diverse than the commodity thermoplastics. The most common types of thermosets are the phenol-formaldehydes (PF), urea-formaldehydes (UF), melamine-formaldehydes (MF), epoxies (EP), polyurethanes (PU), and polyimides (PI). Appendix 2 shows the chemical structure of these important thermosetting polymers. 

Melmac is a thermoset polymer discussed in Chapter 12. How are the chemical structures of Bake-lite and Melmac similar to each other How are they different ... 

VINYL ESTER RESINS. The vinyl ester resins are a relatively recent addition1 to thermosetting-polymer-chemistry. Superficially, they are similar to unsaturated polyester resins insofar as they contain ethylmic lmsaturation and are cured throngh a free-radical mechanism, usually in the presence of a vinyl monomer, such as styrene. However, close examination of the chemistry and structure of the vinyl ester resins demonstrates several basic differences which lead to their unique characteristics. 

The subject of thermosetting polymers receives very brief consideration in most books covering the fundamentals of polymer science. Usually the chemistry is represented by the structure of a phenolic network of the resol type, and some statistical calculations based on Flory s derivations are presented. Therefore, anyone trying to get a first approach to the subject finds only books with chapters written by different authors and aimed at specialists in the field. 

Thermosetting polymers are usually amorphous because there is no possibility of ordering portions of the network structure due to the restrictions imposed by the presence of crosslinks. Exceptions are networks obtained from rigid monomers exhibiting a nematic-isotropic transition. In these cases, a polymer network presenting a nematic-isotropic transition may be obtained, provided that the concentration of crosslinks is kept at a low value. 

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