Phenol thermal cross-linking

In the case of highly cross-linked material, water is not released until above 400°C, and decomposition starts above 500°C as confirmed using differential thermal analysis (DTA).55 The amount of char depends on the structure of phenol, initial cross-links, and tendency to cross-link during decomposition. The main decomposition products may include methane, acetone, CO, propanol, and propane. 

In an analogous reaction, bis-(pentafluorophenyl)-sulfone is used for the preparation of poly(arylene ether sulfone) (PAES). Bis-(pentafluoro-phenyl)-sulfone is prepared by the oxidation of bis-(pentafluorophenyl)-sulfide. In the next step, 3-ethynylphenol or 4-(phenylethynyl)phenol is attached to the PSI, or PAES. These groups are capable of thermal cross-linking. Fluorinated PSI or PAES can be used for optical waveguide applications. 

Cross-Linker. It is well known that polyfunctional benzylic alcohols act as good crosslinkers for poly(4-hydroxystyrene) (11). This acid-catalyzed cross-linking reaction was studied in detail, and the reaction was proposed to proceed via a direct C-alkylation as well as an initial O-alkylation, followed by a subsequent acid-catalyzed rearrangement to the final alkylated product. Furthermore, both a thermal cross-linking and an acid-catalyzed cross-linking process were proposed for this alkylation (72). Thus we decided to use 4,4 -methylenebis[2,6-bis(hydroxymethyl)phenol] (MBHP) and 2,6-bis(hydroxy-methyl)phenol (BHP) in conjunction with 1 and 2, respectively, on the basis of its avail-... 

Thermal cross-linking of phenol polymers was also achieved by copolymerization of two different functional phenols. A copolymer of 4-hydroxyphenyl-N-maleimide (57, Scheme 12) and a furanosyl derivative (N-(4-hydroxy-phenyl)-2-furamide (64), Scheme 13) was subjected to irreversible cross-linking by thermally induced [4 + 2] cycloaddition (Diels-Alder cycloaddition). The copolymerization behavior of these two phenols was studied by following the monomer concentrations by HPLC, the increasing molecular weight by GPC, and the polymer composition by MALDI-TOF mass spectroscopy. It was found that the more electron rich and sterically less demand-... 

Scheme 13 Thermal cross-linking of phenol polymers...

Polyphenols. Another increa singly important example of the chemical stabilization process is the production of phenoHc foams (59—62) by cross-linking polyphenols (resoles and novolacs) (see Phenolic resins). The principal features of phenoHc foams are low flammabiUty, solvent resistance, and excellent dimensional stabiUty over a wide temperature range (59), so that they are good thermal iasulating materials. 

Hyperbranched polyurethanes are constmcted using phenol-blocked trifunctional monomers in combination with 4-methylbenzyl alcohol for end capping (11). Polyurethane interpenetrating polymer networks (IPNs) are mixtures of two cross-linked polymer networks, prepared by latex blending, sequential polymerization, or simultaneous polymerization. IPNs have improved mechanical properties, as weU as thermal stabiHties, compared to the single cross-linked polymers. In pseudo-IPNs, only one of the involved polymers is cross-linked. Numerous polymers are involved in the formation of polyurethane-derived IPNs (12). 

Phenol-formaldehyde resins. These arc used to cure butyl rubber forming thermally stable carbon-carbon cross-links. 

Network properties and microscopic structures of various epoxy resins cross-linked by phenolic novolacs were investigated by Suzuki et al.97 Positron annihilation spectroscopy (PAS) was utilized to characterize intermolecular spacing of networks and the results were compared to bulk polymer properties. The lifetimes (t3) and intensities (/3) of the active species (positronium ions) correspond to volume and number of holes which constitute the free volume in the network. Networks cured with flexible epoxies had more holes throughout the temperature range, and the space increased with temperature increases. Glass transition temperatures and thermal expansion coefficients (a) were calculated from plots of t3 versus temperature. The Tgs and thermal expansion coefficients obtained from PAS were lower titan those obtained from thermomechanical analysis. These differences were attributed to micro-Brownian motions determined by PAS versus macroscopic polymer properties determined by thermomechanical analysis. 

The first widely used synthetic polymer was phenol formaldehyde (Bakelite). It is made by heating phenol (C6H5OH—hydroxybenzene) together with formalde-hyde (H2CO).These react to yield a three-dimensionally cross-linked polymer. To reduce the brittleness of Bakelite, it is usually filled with fibers or platelets of an inert solid. It is a good electrical insulator, relatively hard, and thermally stable to a few hundred degrees Centigrade. Its hardness is 50-60 kg/ mm2 (Mott, 1956). 

Hydrophobic monolithic methacrylate capillary columns have been introduced by copolymerization of butyl methacrylate and EDMA as cross-linking agent. The polymerization, however, was not thermally or photochemically but chemically initiated ammonium peroxodisulfate [154]. The resulting monolithic columns were applied to RP separation of small analytes like uracil, phenol, or alkylbenzenes. Reasonable results have been obtained under isocratic conditions, delivering typical values for theoretical plate height ranging between 40 and 50 pm. 

One such process involves the thermal decomposition of a diazo compound to give an acid that cross-links phenol formaldehyde resins upon heating, similar to the conventional UV initiated plates used in the industry (Figure 4.3), but other sensitisation methods are also used (see section 4.5). It is also possible to produce plates in a dry resin process by ablation or phase change methods. 

Uses. Furfuryl alcohol is widely used as a monomer in manufacturing furfuryl alcohol resins, and as a reactive solvent in a variety of synthetic resins and applications. Resins derived from furfuryl alcohol are the most important application for furfuryl alcohol in both utility and volume. The final cross-linked products display outstanding chemical, thermal, and mechanical properties. They are also heat-stable and remarkably resistant to acids, alkalies, and solvents. Many commercial resins of various compositions and properties have been prepared by polymerization of furfuryl alcohol and other co-reactants such as furfural, formaldehyde, glyoxal, resorcinol, phenolic compounds and urea. In 1992, domestic furfuryl alcohol consumption was estimated at 47 million pounds (38). 

Commercially available antioxidants include phenols and amine derivatives the latter, though generally more effective, have the drawback of alteriiig the coloration of dyed products. These additives are necessary to prevent, to some extent, the process of thermal oxidation of rubbers, though it has to be borne in mind that the stability of rubbers is primarily determined by the chemical nature of the chains as well as by the cross-links that define network structure. 

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