Thermoset polymers, networks

Thermoplastics are the largest class of engineering polymer. They have linear molecules they are not cross-linked, and for that reason they soften when heated, allowing them to be formed (ways of doing this are described in Chapter 24). Monomers which form linear chains have two active bonds (they are bifunctional). A molecule with only one active bond can act as a chain terminator, but it cannot form a link in a chain. Monomers with three or more active sites (polyfunctional monomers) form networks they are the basis of thermosetting polymers, or resins. 

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. 

Thermoplastics are more suitable for recycling than elastomers or thermosetting polymers. Thermoplastics can be heated above their melting temperatures and then recast into new shapes. Elastomers and thermosets, on the other hand, have extensive cross-linking networks that must be destroyed and then reformed in the process of recycling. Processes that destroy cross-linking, however, generally break down the polymer beyond the point at which it can be easily reconstituted. 

A thermoset polymer does not flow when it is heated and subjected to pressure. Thermoset polymers consist of an interconnected network of chains that are permanently chemically connected to their neighbors, either directly or via short bridging chains, as shown in Fig. 1.4. We refer to such networks as being crosslinked. Thermoset polymers do not dissolve in solvents, but they can soften and swell. 

Traditionally, we create thermoset polymers during step growth polymerization by adding sufficient levels of a polyfunctional monomer to the reaction mixture so that an interconnected network can form. An example of a network formed from trifimctional monomers is shown in Fig. 2.12b). Each of the functional groups can react with compatible functional groups on monomers, dimers, trimers, oligomers, and polymers to create a three-dimensional network of polymer chains. 

Thermoset polymers (sometimes called network polymers) can be formed from either monomers or low MW macromers that have a functionality of three or more (only one of the reagents requires this), or a pre-formed polymer by extensive crosslinking (also called curing or vulcanisation this latter term is only applied when sulfur is the vulcanising or crosslinking agent.) The crosslinks involve the formation of chemical bonds — covalent (e.g., carbon-carbon bonds) or ionic bonds. 

Classifying polymers in their crosslinked state according to end-use properties, polymer networks include vulcanized rubbers, crosslinked thermosetting materials, protective coatings, adhesives, polymeric sorbents, microelectronics materials, soft gels, etc. Polymer networks in contrast to uncrosslinked polymers,... 

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). 

Recent systematic studies on the relation between network structure and substituents in kraft lignin, steam exploded, have shown that the lignin containing networks can be modified in new ways, cf. e.g. (80). Also the toughening of glassy, structural thermosets can be achieved by incorporating a variety of polyether and rubber-type soft segment components in the polymer network structure. 

The ionic clusters act as sites of cross-linking at low temperatures. The interchain forces resulting from this ionic bond produces properties normally associated with a cross-linked thermoset polymer. The association in ionomers can be partially overcome through application of heat and pressure allowing processability while truly" cross-linked network polymers cannot be remelted, dissolved or reshaped. Thus, ionomers are often referred to as processable thermosets. 

Figure 29-2 Schematic representation of the conversion of an uncross-linked thermosetting polymer to a highly cross-linked polymer. The crosslinks are shown in a two-dimensional network, but in practice three-dimensional networks are formed.

Thermosetting polymers normally are made from relatively low-molecular-weight, usually semifluid substances, which when heated in a mold become highly cross-linked, thereby forming hard, infusible, and insoluble products having a three-dimensional network of bonds interconnecting the polymer chains (Figure 29-2). 

Thermosetting space-network polymers can be prepared through the reaction of polybasic acid anhydrides with polyhydric alcohols. A linear polymer is obtained with a bifunctional anhydride and a bifunctional alcohol, but if either reactant has three or more reactive sites, then formation of a three-dimensional polymer is possible. For example, 2 moles of 1,2,3-propane-triol (glycerol) can react with 3 moles of 1,2-benzenedicarboxylic anhydride (phthalic anhydride) to give a highly cross-linked resin, which usually is called a glyptal ... 

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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