Phenol-Formaldehyde Type Networks

The resol-type resins, richer in formaldehyde, are significantly more complex and may include, in addition to the -CHj-bridges, longer bridges such as -CH2-O-CH2-  

Solid-state carbon-13 NMR studies [453-456] indicated that, in addition to the bridging groups shown in the above schemes, bridges such as 

Despite their complexity and lack of thorough understanding, the phenolic resins find ever expanding new applications in conjuction with other crosslink-able species. A stream of new patents continues to flow along these lines. Among the latest combinations one finds, for example, phenol-aralkyl resins [458] in which some of the aromatic rings are hydroxylated and others are not  

The above oligomeric species are cured in a stepwise fashion alone or with other monomers or oligomers to yield matrices for composites and laminates having improved processing and final properties. 

---

As is the case with phenol-formaldehyde-type resins, the lack of detailed understanding of the reaction mechanisms and complete characterization of the complex polymeric products did not prevent the large scale utilization of furan based network polymers. In both instances, the synthetic chemistry and chemical technology aspects are far better understood than the physical and topological aspects of the solid systems [446, 464-466]. 

In those years, Edison had switched from the cylinder-type phonograph records to the platter type. The latter ones were made of the new phenol-formaldehyde material, just invented by Leo Baekeland. The problem with the new material was that it was extremely biitfle, hence the new platters needed to be very thick. Aylsworth s solution to the problem was to mix in natural rubber and sulfur, which on heating forms a network. Since the phenol-formaldehyde compositions are all densely crosslinked, the overall composition was a simultaneous interpenetrating network. 

Judging from the range of occurrence of an effect first observed In phenol-formaldehyde networks has led eventually to comparison with extreme cases which, pattern-wise, seem to be related only remotely. The most closely related patterns were observed In four chemical types of organic networks and, next. In... 

As indicated previously, phenol-formaldehyde polymers find practical utilization mainly in the form of network polymers. The polymerization is normally carried out in two separate operations. The first operation involves the formation of a low molecular weight fusible, soluble resin and the second operation involves curing reactions which lead to the cross-linked product. Various types of initial low molecular weight resins are produced commercially and are considered in this section. 

There are two types of phenol-formaldehyde resin. Those prepared using an excess of formaldehyde with base catalysis are known as resoles. The resole prepolymers possess many unreacted methylol groups that upon further heating react to produce the network structure. 

Crosslinking resoles in the presence of sodium carbonate or potassium carbonate lead to preferential formation of ortho-ortho methylene linkages.63 Resole networks crosslinked under basic conditions showed that crosslink density depends on the degree of hydroxymethyl substitution, which is affected by the formaldehyde-to-phenol ratio, the reaction time, and the type and concentration of catalyst (uncatalyzed, with 2% NaOH, with 5% NaOH).64 As expected, NaOH accelerated the rates of both hydroxymethyl substitution and methylene ether formation. Significant rate increases were observed for ortho substitutions as die amount of NaOH increased. The para substitution, which does not occur in the absence of the catalyst, formed only in small amounts in the presence of NaOH. 

The first type concerns aminomcthylation giving rise to the mactomolecular network starting from polyfunctional polymeric substrates and/or amines. Hardening of phenolic novolacs-- and other resins with urotropinc belongs to this type of reaction. The process may be carried out in the presence of protcic molecules, which are thus inserted into the cured material. - Similarly, the crosslinking of collagen or other proteic macromolccules with formaldehyde, or with mixtures of aldehydes, is frequently encountered (see also Chap. HI, C.2).- - ... 

Membranes which may be used in the removal of alkali metal ions by electrodialysis are those which are impermeable to anions, but which allow the flow therethrough of cations. Such cation-selective membranes should, of course, possess chemical durability, high resistance to oxidation and low electrical resistance in addition to their ion-exchange properties. Homogeneous-type polymeric membranes are preferred, for example, network polymers such as phenol, phenosulfonic acid, formaldehyde condensation polymers and linear polymers such as sulfonated fluoropolymers and copolymers of styrene, vinyl pyridine and divinylbenzene. Such membranes are well known in the art and their selection for use in the method of the invention is well within the skill of the art. 

This approach was then extended by Pizzi s group to other phenolic type adhesives such as phenol-resorcinol-formaldehyde networks [19], In this work, it was shown that the addition of 1 gave cold setting resins with performances and costs comparable to those made using formaldehyde alone. Thus, the phenol-resorcinol-furfural-formaldehyde cold sets obtained appeared to have a lower bulk shrinkage compared to those prepared without 1. Moreover, it was established that the presence of furfural did not slow down the curing rate of the resins. 

Phenolic resins are of two types, reactive and nonreactive. Nonreactive resins tend to be oligomers of alkyl-phenyl formaldehyde, where the para-alkyl group ranges from to C4 to C9. Such resins tend to be used as tackifying resins. Reactive resins contain free methylol groups. In the presence of methylene donors such as hexamethylenetetramine, crosslink networks will be created, enabling the reactive resin to serve as areinforcing resin and adhesion promoter. 

Bifunctional monomers, such as A-A, B-B and A-B, yield linear polymers. Branched and crosslinked polymers are obtained from polyfunctional monomers. For example, polymerization of formaldehyde with phenol may lead to complex architectures. Formaldehyde is commercialized as an aqueous solution in which it is present as methylene glycol, which may react with the trifunctional phenol (reactive at its two ortho and one para positions). The type of polymer architecture depends on the reaction conditions. Polymerization imder basic conditions (pH = 9-11) and with an excess of formaldehyde yields a highly branched polymer (resols. Figure 1.8). In this case, the polymerization is stopped when the polymer is still liquid or soluble. The formation of the final network (curing) is achieved during application (e.g., in foundry as binders to make cores or molds for castings of steel, iron and non-ferrous metals). Under acidic conditions (pH = 2-3) and with an excess of phenol, linear polymers with httle branching are produced (novolacs). 

Resole PF resins are produced by the reaction of phenol with excess formaldehyde (P F molar ratio 1 1.8 to 1 2.2) in the presence of an alkali catalyst (Fig. 3). Because resole resins contain reactive methylol groups, they are self-curing resins that can condense with active sites on the phenol rings to form a cross-linked network in the presence of heat even without additional hardener. Resole resins have a very branched structure (Fig. 3) and are by far the more important of the two types of PF resins for wood composites. 

Formaldehyde reacts with phenol by electrophilic substitution at the 2-, 4- and 6-positions of phenol and will subsequently condense, forming a densely cross-linked network. This reaction can be catalyzed by acid as well as base catalysts. The nature of the product obtained is largely dependent on the type of phenol, the molar ratio of formaldehyde to phenol (f p) and the catalyst used. Phenolic resins are mainly divided into two broad classes resoles and novolacs. 

If the reactant molecules have more than two functional groups, the polymerisation process results in cross-linking between different chains which serves to turn the entire substance, in some degree, into a. single three dimensional network. The most notable example of this type is the conden.sation between phenol and formaldehyde ... 

---