Synthetic thermosets

Similar to all commercial plastics, the traditional and more commonly used thermosetting resins are considered as petrochemicals, having been manufactured from petroleum. Some of the primary distillation products of crude oil, which can be classified either as olefins or aromatics, serve as precursors for the synthesis of thermosets. For example, epoxy resins are manufactured by the reaction of epichlorohydrin, a chloro-oxirane, and a derivative of propylene, with bisphenol A, which is a derivative of cumene. Another example would be the unsaturated polyesters (UPs), which are derivatives ultimately originating from ethylene (ethylene glycol) and benzene (maleic acid) [6]. Epoxies and polyesters constitute more than 95% of the thermoset composite market of the two, polyester-based systems predominate in volume by about 10-fold [6, 7]. Other thermoset resins used in reinforced form are phenolics, vinyl esters, and polyimides. Details of the properties and applications of these thermoset systems will be further discussed in the following section. 

Owing to their petrochemical nature, production of synthetic thermosets is largely dependent on the status of the oil industry. Currently, the market for 

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Apart from the synthetic thermosetting polymer adhesives, other polymers such as polyvinyl acetate and polyamides are used in... 

Synthetic thermosets which cure on heating to solids, e.g., amino plastics, epoxides, phenolic resins, unsaturated polyesters, polyaromatics, and furanes. 

The first synthetic thermosets used as adhesives were phenol-formaldehyde resins produced at the end of the nineteenth eentury, historically linked to Baekeland s process which attained industrial status at the beginning of the twentieth century [4], Furanic condensates appeared mueh later as a result of the marketing of 2. They were first used as foundry binders by Quaker Oats in 1960. The use of furanic resins in the aerospace industry began ten years later. Although furanic resins represent a mere 1 % of the total thermoset produetion, the high added-value of these materials amply justifies their use. In fact, furan-based adhesives and binders are fire-, solvent-, and acid- or alkali-resistant. They are known, however, to display two main drawbacks related to their sensitivity to shrinkage and oxidation. 

The need to substitute synthetic thermosetting wood adhesives with more environmentally acceptable resins has led to intense research on adhesives derived from natural, non-toxic materials. Extensive reviews on the subject exist [4,5]. It is sufficient here to state that lignin is one of the materials at the forefront of these studies. Numerous wood adhesive fomulations based on lignin have been published over the years [6]. 

These are dark-colored synthetic thermosetting resins containing the chemical group known as the furane ring (Figure 5.4). 

These synthetic thermosetting resins are condensation products of unsubstituted melamines and formaldehyde. They are equivalent in durability and water resistance to phenolics and resorcinols. Melamines are often combined with ureas to reduce cost. Melamines have higher service temperatures than those of ureas. 

For a general discussion of these high-temperature adhesives, compared with PBI, see Section 5.5. These adhesives are synthetic thermosetting resins formed by the reaction of a diamine and a dianhydride. As with PBIs, they were developed specifically for high-temperature aerospace applications. PI adhesives are superior to PBIs for long-term strength retention, as shown in Figure 5.4. ... 

Dielectric curing—The use of a high-frequency electric field through a joint to cure a synthetic thermosetting adhesive. A curing process for wood and other non-conductive joint materials. Curing results from the heat generated by the resonance of the molecules within the adhesive due to the imposed field. 

Name given to synthetic, thermosetting resins processed from polyhydric alcohols and polybasic acid or anhydrides. These unsaturated polyesters are prepared by esterification of a polyfunctional alcohol (e.g., glycerin) with phthalic anhydride in combination with fatty acids or rosin acids (molecular weight about 2,000 to 5,000). These resins are frequently modified by incorporation of, e.g., nitrocellulose, NC, or phenolics. AUcyds are used mainly as lacquers. 

Phenol formaldehyde, a synthetic thermosetting resin obtained by the reaction of phenols with aldehyde, is used for making plywood, particle board, medium density fiber boards and other wood- and lignocellulose-based panel and wood joinery. Phenolic... 

Phenol-formaldehyde was reported as the first commercially synthetic polymer (1899) which was introduced as BakeliteT by Baekeland in 1909. This was the period which marked the dawn for the production of commercial synthetic thermosetting polymers. Other advances in the field included the discovery of urea-formaldehyde resins in 1884 and the beginning of their commercialization as Beetle moldable resin in 1928, followed by thiourea-formaldehyde (1920), aniline-formaldehyde (Cibatine by Ciba, 1935) and melamine-formaldehyde (1937) moulding powders. The year 1909 marked the discovery of epoxy compounds by Prileschaiev, which were not used until World War 2. The first thermoset polyesters, invented by Ellis, date back to 1934 and in 1938 was reported their first use in the forms of glass-reinforced materials [1]. 

Phenol-formaldehyde resin (PF resin, phenolic resin) n. The most important of the phenolic resins. Made by condensing phenol with formaldehyde, these were the first synthetic thermosetting resins to be developed (Baekeland, 1907) and were marketed under the trade name Bakelite. Salamone JC (ed) (1996) Polymeric materials encyclopedia. CRC Press, Boca Raton, FL. 

The second synthetic thermoset resin discovered in early 1940 (after phenolic resin) was unsaturated polyester (UPE) resin. UPE consists of an unsaturated polyester, a monomer, and an inhibitor. UPE gained wide industrial applications due to their low viscosity, which offers easy processability, low cost and rapid cure schedules. 

Synthetic thermosetting polymers used in the construction industry are polyester, vinylester and epoxies these materials are generally used to manufacture parts of the machines that produce sustainable energy generators. In addition, thermoplastic resins, such as polyetheretherketone (PEEK), polyethersulphone (PES) and various liquid crystal polymers (LCP) are also used. The latter high performance polymers also meet stringent out-gassing (relevant to space environments) and flammability requirements. 

Condensation products of urea (or melamine) with formaldehyde that yields to a synthetic thermoset resin. It is also known as urea resin. 

A wide range of adhesive types and chemistries are used to bond wood elements to one another (Table 2), but relatively few adhesive types are utilized to form the composites themselves. The vast majority of pressed-wood products use synthetic thermosetting adhesives. In North America the most important wood adhesives are the amino resins (qv), eg, urea-formaldehyde (UF) and melamine-formaldehyde (MF), which account for 60% by volume of adhesives used in wood composite products, followed by the phenolic resins (qv) eg, phenol-formaldehyde (PF) and resorcinol-formaldehyde (RF), which account for 32% of wood composite adhesives (12,13). The remaining 9% consists of cross-linked vinyl (X-PVAc) compounds, thermoplastic poly(vinyl acetates) (PVA), soy-modified casein, and polymeric diphenylmethylene diisocyanate (pMDI). Some products may use various combinations of these adhesives to balance cost with performance. 

Historically, phenol-formaldehyde resins (phenolic resins, phenoplasts) are the oldest synthetic thermoset materials as early as in 1910 the first ones ( bakelite to their inventor Baekeland) were introduced. Who does not remember the black telephones and light switches that were popular in the 1930s and 1940s Phenolic resins are based on hydroxy-aromatic compounds (phenol and phenol derivatives) and aldehydes [2]. Furfural is occasionally used as aldehyde but by far the most widely used aldehyde is formaldehyde. Formaldehyde can be administered as an aqueous solution (formalin), polymeric solid (paraformaldehyde) or solid adduct with ammonia (hexamethylenetetramine, HMTA). The resins are formed by a step-growth mechanism in aqueous solution. 

The most common synthetic thermosetting matrices used for composites preparation via photochemical crosslinking are epoxy-, epoxy acrylates, siloxanes, vinyl esters. 

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