Phenolic resins, novolac-type

An alternative copolymerization is illustrated by the method of Blasius. In this preparation, a phenol-formaldehyde (novolac) type system is formed. Monobenzo-18-crown-6, for example, is treated with a phenol (or alkylated aromatic like xylene) and formaldehyde in the presence of acid. As expected for this type of reaction, a highly crosslinked resin results. The method is illustrated in Eq. (6.23). It should also be noted that the additional aromatic can be left out and a crown-formaldehyde copolymer can be prepared in analogy to (6.22). ... 

There are many types of epoxide resin the Atlas for the Coating Industry, produced by the Federation of Societies for Coating Technology, cites 47 IR spectra of different resins [77]. Some typical example structures are provided below for Novolac-type phenol (and Novolac-type o-cresol) epoxides, aromatic glycidyl ester-type epoxides, alicyclic epoxides (18-4), and polyalcohol glycidyl epoxides (polyglycol and polyol based). The IR spectra of some epoxides are show in Fig. 54. 

Most processors of fiber-reinforced composites choose a phenol formaldehyde (phenoHc) resin because these resins are inherently fire retardant, are highly heat resistant, and are very low in cost. When exposed to flames they give off very Htde smoke and that smoke is of low immediate toxicity. PhenoHc resins (qv) are often not chosen, however, because the resole types have limited shelf stabiHty, both resole and novolac types release volatiles during their condensation cure, formaldehyde [50-00-0] emissions are possible during both handling and cure, and the polymers formed are brittle compared with other thermosetting resins. 

The formation of a phenolic resin is often formally separated into two steps, though it probably should be three. If we use a three-step model, the first step is activation of the phenol or aldehyde. The second step is methylolation, and the third is condensation or chain extension. In addition to the clarity provided by the formalism, these steps are also generally separated in practice to provide maximum control of exothermic behavior, with the strategy being to separate the exotherm from each step from that of the others as much as possible. As there are significant differences in the activation step and in the details of the methylolation and condensations steps of novolacs and resoles, we will treat the two types separately. 

Phenolic oligomers are prepared by reacting phenol or substituted phenols with formaldehyde or other aldehydes. Depending on the reaction conditions (e.g., pH) and the ratio of phenol to formaldehyde, two types of phenolic resins are obtained. Novolacs are derived from an excess of phenol under neutral to acidic conditions, while reactions under basic conditions using an excess of formaldehyde result in resoles. 

This chapter emphasizes the recent mechanistic and kinetic findings on phenolic oligomer syntheses and network formation. The synthesis and characterization of both novolac- and resole-type phenolic resins and dieir resulting networks are described. Three types of networks, novolac-hexamethylenetetramine (HMTA),... 

From this result on MRS, we expected that a combination of phenolic-resin-based resist and aqueous alkaline developer would lead to etching-type dissolution and non-swelling resist patterns. In this paper, we report on a new non-swelling negative electron beam resist consisting of an epoxy novolac, azide compound and phenolic resin matrix (EAP) and discuss the radiation chemistry of this resist. 

A similar strategy consists in the liquefaction of biomass with phenol under acidic conditions to obtain phenolic monomers. The type of monomers obtained vary greatly (Lin et al, 2001). These monomers can be reticulated with the help of temperature and formaldehyde to obtain novolac- or resol-type resins. 

Finally, crosslinked structures of type VII may result from the hardening, for instance, of phenolic resins (503) through reaction of novolacs with hexamethylenetetramine, which is a preformed aminomethylating agent (see Table 31, Chap. III). 

Novolac (Resin). The raw materials used for novolac-type resins are phenols and aldehydes just as for resol-type resins. These materials... 

Studies of novolac-type resins (phenolic polyethylene oxide blends) show by C NMR that a blend of 30 70 composition leads to a ca 2 ppm high freqnency shift compared to a pure phenolic resin. This is ascribed by the anthors to increased hydrogen bonding . 

Some of the factors identified in determining the final properties of these resins are the phenol-formaldehyde ratio, pH, temperature and the type of catalyst (acid or alkaline) used in the preparation of the resin. The phenol-formaldehyde ratio (P/F) (or formaldehyde to phenol ratio, F/P) is a most important factor as it leads to two different classes of synthetic polymers, namely Novolacs and resoles. The first class of resins, Novolacs, is produced by the reaction of phenol with formaldehyde with a P/F > 1 usually under acidic conditions (Scheme 2a). Resoles are produced by the reaction of phenol and formaldehyde with a P/F <1 usually under basic conditions (Scheme 2b). 

The commercial importance of phenol-formaldehyde resins has resulted in extensive studies of these systems, with the aim of identifying the reaction mechanisms and intermediates that occur during subsequent polymerization reactions. However, the complexity of Novolac-type systems has made a detailed understanding of the subsequent chemical processes and their relationship to the physical properties of the final polymerized product difficult. Thus, it is necessary to simplify the system in order to more readily unravel this complexity. Model compounds are frequently used to understand complicated chemical systems and their application to phenol-formaldehyde systems has been well documented . ... 

Although many types of compounds have been tested as sensitizers in phenolic host resins (Novolacs, Resols, etc.) (S), all commercial positive resists employ aromatic diazoquinones of some type which photochemically generate base soluble products via Wolff rearrangement initiated by the loss of nitrogen (6). A staggering variety of diazoketones have been synthesized and evaluated for lithographic purposes, but derivatives of J[ and 2 are most commonly employed (5). 

Both resole and novolac types of phenolic resin are used in aerospace applications. The resoles are generally made by using alkaline catalysts and an excess of formaldehyde. The resole structure is highly complex and involves both methylene and dimethyl ether bridges between the phenolic moieties. During cure both water and formaldehyde are evolved. 

The novolac resins are prepared by using acidic catalysts and a deficiency of formaldehyde. Because this type of resin is less reactive, cross-linking is accomplished by the addition of a curing agent or catalyst. The most common is hexamethylenetetramine or "hexa." The curing agent serves as a latent source of formaldehyde. As in the case of the resoles, volatiles are emitted during the cure. The chemistry of the phenolic resin is old but complex and well documented in the literature (10). 

Novolac type phenol formaldehyde resins are generally used as binders in these applications. This mechanism is shown in the scheme ... 

Phenolic resins are a generic name given to a wide range of crosslinked polymers produced by phenol and formaldehyde. There are two types of phenolic resins. Resol and Novolac. The type of resin being made depends on the pH of the catalyst and the ratio of phenol to formaldehyde. 

Another miscible semicrystalline polymer/amorphous polymer blend SMP is a polyethylene oxide (PEO)/novolac-type phenolic resin blend [24]. The blend was found to be completely miscible in the amorphous phase when the phenolic content is up to 30 wt%, and the crystalline melting temperature (T,f) of the PEO phase working as a transition temperature can be tuned. 

With 0 5-1 0 molecules of formaldehyde to one of phenol, an acid catalyst is used. As can be seen, a short-chain linear polymer is produced. The group R- may be hydrogen, an alkyl group or even an aromatic substituent. It need not necessarily be para- to the phenolic hydroxyl, but the lightest colour, best solubility in oils and greatest reactivity are obtained with the substituent in this position. This type of phenolic resin is called a novolac. 

Choi and Chung [16] were the first to prepare phenolic resin/layered sihcate nanocomposites with intercalated or exfoliated nanostructures by melt interaction using linear novolac and examined their mechanical properties and thermal stability. Lee and Giannelis [10] reported a melt interaction method for phenolic resin/clay nanocomposites, too. Although PF resin is a widely used polymer, there are not many research reports on PF resin/montmorillonite nanocomposites, and most of the research investigations have concentrated on linear novolac resins. Up to now, only limited research studies on resole-type phenolic resin/layered silicate nanocomposites have been published [17-19] and there is still no report on the influence of nano-montmorillonite on phenolic resin as wood adhesive. Normally H-montmorillonite (HMMT) has been used as an acid catalyst for the preparation of novolac/layered silicate nanocomposites. Resole resins can be prepared by condensation reaction catalyzed by alkaline NaMMT, just as what HMMT has done for novolac resins. 

A more complicated system exists if a two nucleus phenolic compound is used for the synthesis of phenolics. In 4,4 -(l-methylethylidene)bisphenol, the para positions of the phenolic nuclei are substituted. Therefore, methanal can only react with the ortho positions relative to the hydroxy group. A uncatalyzed reaction of methanal with 4,4 -(l-methylethylidene)bisphenol gives novolac type resins (equation 41). 

SiC nanofibers by melt-spinning of polymer blends have been prepared from PCS as a SiC ceramic precursor and a novolac-type phenol-formaldehyde resin [121]. These nanofibers were amorphous, about 100 nm in diameter, more than 100 (un long, and were rich in oxygen. 

A second type of epoxy resin is made by reaction of phenol-formaldehyde novolacs with epichlorohydrin (Fig. 3.22). Using novolacs of DP 2-6 gives solid resins and permits much higher cross-linking, giving cured products of higher heat and chemical resistance. 

Adhesives of the aminoplastic (see Step polymerization) and phenol formaldehyde (see Phenolic adhesives single-stage resoles and Phenolic adhesives two-stage novolacs) types are most widely used. Although basically similar, an adhesive for plywood manufacture will require a different formulation to one for particle board, or medium-density fibre board (MDF) since methods of application and processing differ. Thus, in plywood, large sheets of veneer must be uniformly coated with adhesive, usually by a roller or curtain coater in particle board, chips or wafers must be coated with very fine adhesive droplets, while small bundles of wet fibres must be sprayed with adhesive in the manufacture of MDF. Hence, formulation and production of resins has become a mixture of art and science, with resin manufacturers able to produce resins tailored for use in a particular board-manufacturing plant, or with a particular species of timber. 

Although resol types are also used in the manufacture of wood chipboard, other forms of particle board, such as waferboard and oriented strand board (OSB) are usually made using novolac types. These resins are produced by reacting excess phenol with formaldehyde in the presence of an acid catalyst. The resin is converted to a fine powder, which is usually sprayed on to the large wafers along with molten wax, which helps the dry resin powder adhere to the wafers until it is cured under elevated temperatures of up to 200 °C. With this method, very small quantities of adhesive - as low as 2.5 g solid resin per 100 g dry wafers - can be used, while still achieving satisfactory bonding. 

---