Phenolic curing resin

Cured phenolic resins have outstanding heat resistance, resistance to cold flow, good electric (insulation) properties, and good dimensional stability. Phenolic resins have good adhesive properties and are employed in the production of sandpaper, abrasive wheels, and brake linings. These resins are also used as casting resins. 

Superior toughness and mechanical properties The strength of properly formulated epoxy resins usually surpasses other types of casting resins. The cured epoxy resins [Structure (4.35)] are seven times tougher than the cured phenolic resins [Structure (4.36)] as is evident from Figure 4.15. The relative toughness is attributed to the distance between the crosslinking sites and presence of internal aliphatic chains. 

Strucure (4.36) Cured Phenolic Resin Figure 4.15 Cured epoxy and phenolic resins. 

A differentiation between the two types of phenyl ring carbons (quaternary and those carbons which have protons directly bonded to them) in cured phenolic resins... 

Based on alkali-soluble methylolated kraft lignin which was precipitated by acidification and blended with an acid-curing phenolic resin. 

Phenolic Foams. Cured phenolic resins have good thermal stability and high tendency to char in an intensive fire. Even after the removal of the ignition source the foams often smolder and char until they are almost completely consumed. This phenomenon, called punking, is claimed to be overcome by the addition of boric acid/oxalic acid and ferric/aluminum chloride as the foaming catalysts. The addition of antimony compounds is also reported to decrease punking (38). 

Modification of the phenol ring can affect both the kinetics and the properties of the phenol-formaldehyde system. Substitution at the ortho and/or para position of the phenol will limit the extent of cross-linking and hence the mechanical properties of the cured phenolic resin. A summary of current work on these modified-classical phenolic resins is given. 

A recent review of addition-cure phenolic resins provides extensive information on the work in this areaJ In this entry, parts of the review will be highlighted to provide a view of the importance of the work and its relationship to the classical and modified-classical phenolic resins. 

The cross-linked polymers obtained from the addition-cure approach are often a complex arrangement of atoms bonded in heterocyclic and carbocyclic rings. However, the objective in the preparation of these systems is not simplicity. It is to obtain systems with the desirable properties of phenolic resins retained and the undesirable properties improved or removed. Voids are an undesirable result in the synthesis of both resols and novolacs. Hence, addition-cure phenolic resins are designed to avoid this result. Ease and flexibility of processing are also sought in the addition-cure systems. 

A well-studied addition-cure phenolic resin utilizes the Diels-Alder reaction for its preparation. The result is a complex polymer that originates from a diallyl phenol and a diimide (for example, see structures 2 and 3, respectively). Initially, compounds 2 and 3 react across the double bond of the imide ring and the allylic group to generate a diene [see Eq. (6)]. Compound 2 then serves as the dienophile. The polymer forms from the reaction of the diene with 2. The amount of 2 relative to the diene affects the properties of the resulting polymer, as does the cure temperature. Some of these properties will be summarized... 

The modified-classical phenolic resins are particularly noteworthy for their effect on the mechanical and thermal properties of the cured resin. The processing of these systems is very similar to the classical systems. The nonclassical phenolic resins utilize phenol, but in many cases give a cured product with a chemical structure having little resemblance to the classical system. In future articles, it would be best to develop independent grouping for addition-cure phenolics resins. This redefinition of polymer types is not within the scope of this entry and instead, this interesting class of polymers has been viewed based on current designations. 

Nair, C.P.R. Advances in addition-cure phenolic resins. Prog. Polym. Sci. 2004, 29, 401 98. 

Another reason for predicting low emissions is that the small amount of residual formaldehyde that might be present in the prepared resin is diminished even farther by reactions which occur when the resin cures. Phenolic resins are cured under heat and pressure in a hot-press, usually under highly alkaline conditions. Curing temperatures are usually in the range of 130-220 C. Under these conditions, unreacted formaldehyde continues to react with phenol to form larger phenol formaldehyde polymers. Also, some formaldehyde reacts with various chemical constituents in the wood. Moreover, some formaldehyde is probably converted to methyl alcohol and formic acid by way of the Cannizzaro reaction (J ). ... 

Toughness - cured epoxy resins are approximately seven times tougher than cured phenolic resins. The relative toughness has been attributed to the distance between crosslinking points and the presence of integral aliphatic chains ... 

Macromolecular substances occur in an exceptionally large profusion of forms in the condensed state. Household utensils of poly(ethylene) are waxy to the touch and appear turbid. Films of the same material are transparent, but not as clear as films of poly(ethylene terephthalate). Poly(styrene) beakers are brittle, but beakers made from polyamides are not. Some products, such as leather, for example, are pliable, others, such as cured phenolic resins, are very rigid. 

Decomposition of Cured Resoles and Novolaks. Above 250°C, cured phenolic resins begin to decompose. For example, dibenzyl ethers such as 9 disproportionate to aldehydes (salicylaldehyde) and cresols (o-cresol). The aldehyde group is rapidly oxidized to the corresponding carboxylic acid. In an analogous reaction in hexa-cured novolaks, tribenzylamines decompose into cresols and azome-thines, which cause yellowing. 

XA and HM carbon fibres are Courtaulds products note that Courtaulds have ceased making carbon fibres but information on their material has been included here since other manufacturers produce fibres with similar properties, see Figure 3.1, and the thermal properties of Courtaulds materials may be relevant to these other fibres. Aramid fibres used were Kevlar 29, Kevlar 49, Kevlar Hm (du Pont). E-, R- and D-glass fibres were used and Tyranno SiC and Nicalon SiC ceramic fibres (Ube Industries and Nippon Carbon, respectively). 934 (Fiberite) and MY720/HT976 (Ciba Geigy) epoxy resins were used (among others), as was polyester resin and acid cure phenolic resin. 

Reactive Contact Adhesives. The heat resistance of contact adhesives can be increased by using curing phenolic resin systems. Table 16 compares two formulations, one with and one without a reactive resin system. 

Fyfe CA, Rudin A, Tchir WJ. Application of High-Resolution C NMR Spectroscopy Using Magic Angle Spinning Techniques to the Direct Investigation of Solid Cured Phenolic Resins. Macromolecules 1980 13 1320. 

Since phenols are trifunctional in nature, i.e. two ortho and one para positions are reactive, the network structure of the cured phenolic resins is controlled by using substituted or unsubstituted phenols. For example, para-substituted phenolic resins give rise to linear polymeric structures on... 

THERMAL CHARACTERISTICS OF ADDITION-CURED PHENOLIC RESINS... 

The thermal cure characteristics, kinetics of thermal degradation and pyrolysis of four different addition cured phenolic resins were investigated using various techniques, including DSC, DMA, FTIR spectroscopy, TGA and X-ray diffraction. Resins investigated were propargyl ether resins and phenyl azo-, phenyl ethynyl-andmaleimide-functional resins. A comparison of the data obtained for all the phenolic resins was made as a function of molecular structure. 12 refs. 

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