Phenol-formaldehyde resins Linear

The same chemical mechanisms and driving forces presented for phenol-formaldehyde resins apply to resorcinol resins. Resorcinol reacts readily with formaldehyde to produce resins (Fig. 2) which harden at ambient temperatures if formaldehyde is added. The initial condensation reaction, in which A-stage liquid resins are formed, leads to the formation of linear condensates only when the resorcinol/formaldehyde molar ratio is approximately 1 1 [119]. This reflects the reactivity of the two main reactive sites (positions 4 and 6) of resorcinol [120]. However, reaction with the remaining reactive but sterically hindered site (2-positiori) between the hydroxyl functions also occurs [119]. In relation to the weights of resorcinol-formaldehyde condensates which are isolated and on a molar basis, the proportion of 4- plus 6-linkages relative to 2-linkages is 10.5 1. However, it must be noted that the first-mentioned pair represents two condensa-... 

Crosslinking in phenol-formaldehyde resins is carried out on essentially linear prepolymers which have been formed by having one of the components in sufficient excess to minimise crosslinking during the initial step. These prepolymers may be one of two kinds the so-called resoles or the so-called novolaks. 

Phenol-formaldehyde resin. Phenol-formaldehyde resin is prepared from phenol by reaction with formaldehyde. Phenol is an enol, the 2-, 4-, and 6-positions of which are activated for reaction with an electrophile. Phenol is sequentially hydroxymethylated approximately as illustrated below. Dehydration of the phenolic hydroxymethyl groups affords a benzyl cation, a new electrophile that can react with another substituted or unsubstituted molecule of phenol. Both linear polymerization and cross-linking are possible, depending on the ratio of the reactants and the polymerization conditions. 

In contrast to the linear thermoplastic polymers, which are soluble and fusible, the cross-linked network polymers are insoluble and infusible. They are formed from polymerizing systems containing monomers or prepolymers with a functionality of three or more. A good example is the phenol-formaldehyde resin systems. The cross-linking reaction takes place in the bond under applied pressure and heat, and the whole adhesive bond might consist of only one super giant molecule. Such resins are, therefore, called thermosetting resins. 

Nonconventional Bonding with Acid Activation. Concentrated or spray-dried, spent sulfite liquor, with or without previous ultrafiltration, was used as a binder for waferboards. The press temperatures used were between 210 and 230 °C and the press times between 5 and 10 min, with the amount of resin between 4 and 5%. The boards produced were better or comparable to the boards made using phenol-formaldehyde resin according to IB, MOE, MOR, hardness, and linear expansion tests the cost of the binder was twofold or threefold less (187). 

Figure 7.13 demonstrates the accelerating effect of increasing nitrogen content on the activity in aging of activated carbons from phenol-formaldehyde resin, in which part of the phenol was substituted by aniline [164]. The surface area was from 700 to 840 m /g. The oxidation rate increased almost linearly with the nitrogen content. Conversion to carbonate proceeds much more slowly at room temperature reaction times of one to two months were necessary to obtain well-measurable results [163]. 

Despite the similar chemical nature, there is no evidence for cross-reactivity between phenol-formaldehyde resin and PTBP-FR (Geldof et al. 1989). Some authors have attributed test reactions to PTBP-FR to free formaldehyde however, most patients are actually not allergic to formaldehyde. Schubert and Agatha (1979) performed patch-test studies with the chroma-tographically extracted ingredients of commercially available PTBP-FR. They found two linear condensates named 2-hydroxy-5-tert.-butylbenzylalcohol and 2,2 -dihydroxy-3,3-di-(2-hydroxy-5-tertiary butyl)-benzyl-5,5-ditertiary butyl-diphenylmethan to be the real allergens. 

Resins, especially those containing epoxy, colophony, and PTEP-FR, are used as adhesives and glues in sports equipment. Reports of contact allergy include facial dermatitis from epoxy resin glue inside a helmet (Malanin and Kalimo 1985), erythematous facial dermatitis from colophony in a bowlsgrip (Elair 1982), and a well-demarcated linear erythematous vesicular patch on the back of a leg from the PTEP-FR and/or phenol-formaldehyde resin in a knee-guard (Vincenzi et al. 1992). 

Other well known condensation polymers include phenol-formaldehyde resins, the prototype of which is Bakelite (Figure 13.15 C). Such structures were known as early as the 1870s, and in the early 20th century these tough, durable thermosets were among the first synthetic polymers of commerical importance. More modern versions of this type of polymer are known as Novolac. This chemistry is also that which make.s calixarenes (Chapter 4), which are cyclic tetramers rather than linear polymers. 

The cyclic phenol-formaldehyde resins were naturally produced in low yield as by-products of linear resins [17[. In the cyclic phenol—aldehyde resin, the end valences are joined to form a cyclic structure, for example. 

Paracyclophane 93, 97, 101-102 Percolation theory 140-142 Phenol formaldehyd resins 72-75, 77, 78 Phenol maleimide resins 75 Phenolic triazine 87 Piezoelectricity 159-160 Poisson ratio 153, 179 Polyacrylates 19, 21, 22, 23 Polyamide, aromatic, linear 2, 30, 31, 127, 170-174, 184... 

It should be possible to form linear noncross-linked polymers of melamine—formaldehyde or phenol—formaldehyde by reaction of one mole of the patent with one mole of formaldehyde, but this is generally not the case. The melamine crystal itself is very insoluble in water and only becomes soluble as the formaldehyde molecules add on. If much less than 1.5 moles of formaldehyde pet mole of melamine ate used, the aqueous resin solution is very unstable. 

Baekeland found that a relatively stable resole prepolymer could be obtained by the controlled condensation of phenol and formaldehyde under alkaline conditions. These linear polymers of phenol-formaldehyde (PF) may be converted to infusible cross-linked polymers called resites by heating or by the addition of mineral acids. As shown in structure 4.80, the initial products obtained when formaldehyde is condensed with phenol are hydroxybenzyl alcohols. The linear resole polymer is called an A-stage resin, and the cross-linked resite is called a C-stage resin. 

Baekeland recognized that the trifunctional phenol would produce network polymers and therefore used difunctional ortho- or para-substituted phenols to produce linear paint resins. Linear thermoplastic products are formed by alkaline or acid condensation of formaldehyde with phenol derivatives such as /r-cresol (structure 4.81). 

Data of low-temperature nitrogen adsorption were used to evaluate the parameters characterizing the pore structure of the obtained polymeric materials in dry state. The BET specific surface area, Sbet, and the total pore volume, V, were estimated by applying the standard methods Sbet from the linear BET plots and F/ from adsorption at relative pressure p/po=0.975) [7]. The mesopore structure was characterized by the distribution function of mesopore volume calculated by the Barret-Joyner-Halenda (BJH) method [27]. In Table 2 the values of these parameters are given for both synthesized polymers. The melamine-formaldehyde resin MEA has a more developed pore structure (5 B 7=220mVg, F,=0.45cm /g) and narrower mesopores (D=7.3nm) in comparison to the phenolic-formaldehyde polymer PHD. 

The first completely synthetic plastic, phenol-formaldehyde, was introduced by L. H. Baekeland in 1909, nearly four decades after J. W. Hyatt had developed a semisynthetic plastic—cellulose nitrate. Both Hyatt and Baekeland invented their plastics by trial and error. Thus the step from the idea of macromolecules to the reality of producing them at will was still not made. It had to wait till the pioneering work of Hermann Staudinger, who, in 1924, proposed linear molecular structures for polystyrene and natural rubber. His work brought recognition to the fact that the macromolecules really are linear polymers. After this it did not take long for other materials to arrive. In 1927 poly(vinyl chloride) (PVC) and cellulose acetate were developed, and 1929 saw the introduction of urea-formaldehyde (UF) resins. 

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