Thermoset condensation polymers

A condensation polymer is one in which the repeating unit lacks certain atoms which were present in the monomer(s) from which the polymer was formed or to which it can be degraded by chemical means. Condensation polymers are formed from bi- or polyfunctional monomers by reactions which involve elimination of some smaller molecule. Polyesters (e.g., 1-5) and polyamides like 1-6 are examples of such thermoplastic polymers. Phenol-formaldehyde resins (Fig. 5-1) are thermosetting condensation polymers. All these polymers are directly synthesized by condensation reactions. Other condensation polymers like cellulose (1-11) or starches can be hydrolyzed to glucose units. Their chemical structure indicates that their repealing units consist of linked glucose entities which lack the elements of water. They are also considered to be condensation polymers although they have not been synthesized yet in the laboratory. 

This section focuses on polycondensation reactions to synthesize thermoset polymers. Specific condensation chemistries are studied here a more extensive treatment of the subject of thermoset polymers can be found in Chapter 28 of this handbook. 

Phenol-formaldehyde resins have been widely used for brown and black electrical fittings, and they represent a thermosetting condensation polymer. Depending upon the ratio of phenol to formaldehyde and the pH of the reaction mixture a resole or a novolac is formed by the substitution of formaldehyde molecules at different positions around the benzene ring. In the case of resoles, phenol groups are... 

Diffusion and Penetration. Thermosetting condensation polymers such as phenol-formaldehyde and urea-formaldehyde resin systems generate water as a byproduct of cure. If water also is the solvent, it is a requirement that the solvent water diffuse into the wood to lower the concentration of water at the interface which might otherwise inhibit cure. Water, or other solvent(s) if present, will cany mobile lower molecular weight polymer fractions into the cell interstices and cell walls. This chromatographic effect is the initiation of penetration. 

Solution polymerizations are often used for the production of thermosetting condensation polymers, which are carried to a conversion short of the gel point in the reactor. The crosslinking is later completed in a mold. Such reactions may be carried out in a refluxing organic solvent. The water of condensation is carried overhead along with the solvent vapors. When the vapors are condensed, the water forms a second phase that is decanted before the solvent is returned to the reactor. Not only does this drive the reaction toward higher conversions but also the amount of water evolved provides a convenient measure of conversion (see Example 8.1). 

Thermoset condensation polymers - Thermosetting polymers contain unsaturated backbones and/or reactive substituents. With heat, these polymers crosslink into an irreversible network, forming a hard infusible plastic. The major thermoset polymers are polyesters and polynrethanes. For polyesters, at least one of the reactants is imsatiu ated, as shown in Figure 4. 

Nippon Shokubai and U.K. Seung are producing a fluorescent polymer claimed to be made from a co-condensation of ben2oguanamine and formaldehyde. Fine highly thermoset particles are manufactured in solution and later dried. It is useful in a wide range of appHcations, specifically plastics, and markets where bleed is a problem. 

Most processors of fiber-reinforced composites choose a phenol formaldehyde (phenoHc) resin because these resins are inherently fire retardant, are highly heat resistant, and are very low in cost. When exposed to flames they give off very Htde smoke and that smoke is of low immediate toxicity. PhenoHc resins (qv) are often not chosen, however, because the resole types have limited shelf stabiHty, both resole and novolac types release volatiles during their condensation cure, formaldehyde [50-00-0] emissions are possible during both handling and cure, and the polymers formed are brittle compared with other thermosetting resins. 

Most network polymers (the epoxies and the polyesters, for instance) are made by condensation reactions. The only difference is that one of the two reacting molecules is multifunctional (polyester is three-functional) so the reaction gives a three-dimensional lacework, not linear threads, and the resulting polymer is a thermoset. 

Acid catalysts, such as metal oxides and sulfonic acids, generally catalyze condensation polymerizations. However, some condensation polymers form under alkaline conditions. For example, the reaction of formaldehyde with phenol under alkaline conditions produces methy-lolphenols, which further condense to a thermosetting polymer. 

Phenol-formaldehyde resins are the oldest thermosetting polymers. They are produced by a condensation reaction between phenol and formaldehyde. Although many attempts were made to use the product and control the conditions for the acid-catalyzed reaction described by Bayer in 1872, there was no commercial production of the resin until the exhaustive work by Baekeland was published in 1909. In this paper, he describes the product as far superior to amber for pipe stem and similar articles, less flexible but more durable than celluloid, odorless, and fire-resistant. ° The reaction between phenol and formaldehyde is either base or acid catalyzed, and the polymers are termed resols (for the base catalyzed) and novalacs (for the acid catalyzed). 

The second step is the condensation reaction between the methylolphe-nols with the elimination of water and the formation of the polymer. Crosslinking occurs hy a reaction between the methylol groups and results in the formation of ether bridges. It occurs also by the reaction of the methylol groups and the aromatic ring, which forms methylene bridges. The formed polymer is a three-dimensional network thermoset ... 

Amino resins are condensation thermosetting polymers of formaldehyde with either urea or melamine. Melamine is a condensation product of three urea molecules. It is also prepared from cyanamide at high pressures and temperatures ... 

Highly cross-linked condensation materials form the basis for a number of important adhesives and bulk materials, especially phenolic and amino plastics. Most of these products have formaldehyde as one of their starting reactants. These materials are thermosets that decompose prior to melting, and are therefore more difficult to recycle than most condensation polymers that are thermoplastics and do melt prior to decomposition. 

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. 

There are two types of cure reactions of thermosetting resins step (e.g., epoxies, phenolics, urethanes) and chain (e.g., unsaturated polyesters, vinyl esters, acrilates) polymerizations. In the first type, the size of the polymer chain increases because of the linking of the oligomers (e.g., monomers, dimers, etc.) to themselves. Short chains can be linked to long ones in a condensation reaction. In the second case, the size of the polymer chain increases because of... 

One of the oldest known thermosetting synthetic polymers is made by condensation of phenols with aldehydes using basic catalysts. The resins that are formed are known as Bakelites. The initial stage is the base-induced reaction of benzenol and methanal to give a (4-hydroxyphenyl)methanol, and this reaction closely resembles an aldol addition and can take place at either the 2-or the 4-position of the benzene ring ... 

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