Polyesters cross-links, condensation polymer

Uses. About 35% of the isophthahc acid is used to prepare unsaturated polyester resins. These are condensation products of isophthahc acid, an unsaturated dibasic acid, most likely maleic anhydride, and a glycol such as propylene glycol. The polymer is dissolved in an inhibited vinyl monomer, usually styrene with a quinone inhibitor. When this viscous hquid is treated with a catalyst, heat or free-radical initiation causes cross-linking and sohdification. A range of properties is possible depending on the reactants used and their ratios (97). 

The polymers can be categorised as formaldehyde containing and formaldehyde free and as thermoset or thermoplastic resins. Typical formaldehyde containing resins are melamine formaldehyde sulfonamide resins, where the sulfonamide is ortho and para toluenesulfonamide. The sulfonamide, which is a solvent for the dye, undergoes a condensation polymerisation with formaldehyde and melamine, the latter acting as a cross-linking agent. Non-formaldehyde, thermoplastic resins are usually polyamides and polyesters. 

Phenolic, epoxy, urea, melamine, and polyester (alkyd) polymers are cross-linked (thermoset) plastics. They are solvent-resistant and are not softened by heat. Unlike the thermoplastic step reaction polymers, which are produced by the condensation of two difunctional reactants, these network polymers are produced from reactants at least one of which has a degree of functionality higher than two. 

The photo-cross-linkability of a polymer depends not only on its chemical structure, but also on its molecular weight and the ordering of the polymer segments. Vinyl polymers, such as PE, PP, polystyrene, polyacrylates, and PVC, predominantly cross-link, whereas vinylidene polymers (polyisobutylene, poly-2-methylstyrene, polymethacrylates, and poly vinylidene chloride) tend to degrade. Likewise, polymers formed from diene monomers and linear condensation products, such as polyesters and polyamides, cross-link easily, whereas cellulose and cellulose derivatives degrade easily. ... 

Glyptal is also a polyester condensation product, but glycerol (HOCH2CHOHCH2OH) produces a cross-linked thermosetting resin. In the first stage a linear polymer is formed with the more reactive primary OH groups. 

Typical condensation polymers, such as polyester and nylon, often exhibit these properties. If the fiber is to be ironed, its Tg should be above 200 °C if it is to be drawn from the melt, its Tg should be below 300 °C. Branching and cross-linking are undesirable because they disrupt crystalline formation even though a small amount of cross-linking may increase some physical properties if effected after the material is suitably drawn and processed. 

The principal feature that distinguishes thermosets and conventional elastomers from thermoplastics is the presence of a cross-linked network structure. As we have seen from the above discussion, in the case of elastomers the network structure may be formed by a limited number of covalent bonds (cross-linked rubbers) or may be due to physical links resulting in a domain structure (thermoplastic elastomers). For elastomers, the presence of these cross-links prevents gross mobility of molecules, but local molecular mobility is still possible. Thermosets, on the other hand, have a network structure formed exclusively by covalent bonds. Thermosets have a high density of cross-links and are consequently infusible, insoluble, thermally stable, and dimensionally stable under load. The major commercial thermosets include epoxies, polyesters, and polymers based on formaldehyde. Formaldehyde-based resins, which are the most widely used thermosets, consist essentially of two classes of thermosets. These are the condensation products of formaldehyde with phenol (or resorcinol) (phenoplasts or phenolic resins) or with urea or melamine (aminoplastics or amino resins). 

The THM reaction linked to GC, GC/mass spectrometry (MS), and MS has been successfully applied to the chemical characterization of a number of synthetic and natural products, including resins, lipids, waxes, wood products, soil sediments, and microorganisms. This technique is also very effective for the detailed characterization of the synthetic polymeric materials, especially the condensation polymers, such as polyesters and polycarbonates, because many simplified pyrograms are usually obtained that consist of peaks of methyl derivatives from the constituents of the polymer samples almost quantitatively. In this chapter, the instrumental and methodological aspects of Py-GC in the presence of the organic alkali are briefly described, and then some typical applications to the precise compositional analyses and microstructural elucidation inclusive of the intractable cross-linking structures for various condensation type polymeric materials are discussed. 

As stated in Section 2.4, the condensation reaction may be the basis for the mechanism of stepwise polymerization. As long as the functionality equals 2 (di-acid and di-alcohol in polyester or di-amine in polyamide), only linear chains are obtained. However, once polyfunctionality prevails (appearance of tri-alcohol like glycerol, or tri-acid), reactive branches are formed that may interact and lead to a three-dimensional structure, called cross-linked (gelation). This is the basis for thermosetting polymers on one hand, or for stabilizing the elastomeric chain on the other hand (replacing vulcanization). 

Thermoplastic elastomers exhibit physical properties that are similar to those of cast and millable elastomers at ambient temperatures. These materials, however, are not cross-linked and flow at elevated temperatures. They are fabricated like other thermoplastic polymers, are high in molecular weight, and are hydroxyl-terminated. Such polymers form from linear hydroxyl-terminated polyester or polyethers that are condensed with diisocyanates and glycols. Strict stoichiometry must be maintained to achieve high molecular weights. 

In this experiment, the syntheses of two polyesters (Experiment 46A), nylon (Experiment 46B), and polystyrene (Experiment 46C) will be described. These polymers represent important commercial plastics. They also represent the main classes of polymers condensation (linear polyester, nylon), addition (polystyrene), and cross-linked (Glyptal polyester). Infrared spectroscopy is used in Experiment 46D to determine the structure of polymers. 

Linear and cross-linked polyesters will be prepared in this experiment. Both are examples of condensation polymers. The linear polyester is prepared as follows ... 

Condensation polymers. A. Polyester/polyamide/polyimide formation by combining A2 and 82 monomers or from a single AB monomer. Several common polymers of this class are shown. B. The diisocyanate route to polyurethanes and polyureas. C. Bakelite synthesis. Note that the final product is heavily cross-linked because of the presence of bis and tris adducts in the initial reaction with formaldehyde. 

There are two important distinctions to be made between the S—B—S systems considered in the previous section and the poly ether-polyesters. In the first instance the latter materials are prepared by the techniques of condensation polymerization as opposed to the anionic double-bond polymerization used with the S—B—S systems. Secondly, whereas both components of the S—B—S polymer are amorphous one of the components in the industrially important polyether-polyesters is crystalline. The two groups of materials do however have the common feature that there is some separation of the systems into hard and soft (rubbery) regions with the hard zones effectively acting as multiple cross-links. 

Unsaturated polyester resins are low MW condensation polymers which are transformed to a cross-linked network via radical initiated polymerization. The double bonds in the backbone copolymerize with unsaturated monomer (reactive diluent) present in the system. During polymerization, relatively short low MW polyester chains are cross-linked by short bridges consisting of, on average, around two to three styrene units, forming a densely cross-linked polymer network (see Figure 2.17). 

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