Resole adhesives

The phenoHc resins used for particle board are NaOH-catalyzed resoles of low viscosity and high water miscibility, similar to the Hquid resole adhesives used in plywood manufacture. The higher resin and caustic content of the board frequently necessitates the addition of hydrophobic agents such as wax emulsions to increase the barrier properties of the board. The adhesive is appHed to the particles in thin streams using high agitation to maximize material usage. Boards are cured in presses for 5—10 min at 150—185°C. 

Figure 10 The infrared spectrum of a phenol-formaldehyde (resol) adhesive.

Although unmodified phenolic resole adhesives can give bonded structures that will exhibit excellent durability - both thermal oxidative resistauce aud resistance to harsh environments such as oil, sea water, natural weathering - they are inherently brittle. Much research work has been carried out over the years to impart some degree of toughness into these systems and at the same time to reduce the shrinkage on cure and, thus, provide some stress relief. However, toughening, in the sense understood with epoxy adhesives, is not really possible with these resins. 

Most phenoUc resole adhesives are supplied in liquid form where the adhesive is dissolved in snitable solvent(s). However, there are some exceptions where the adhesive can be supplied as a readily handleable film. Notable examples are Redux 775 Film (resole/PVF) aud the 3 Ms range, which includes Scotch-Weld AF 6, AF 10, AF 31, AF 32, and so on (resole/NBR). 

As has already been indicated, phenolic resole adhesives are renowned for their excellent environmental resistance. Both this and their good thermal oxidative resistance can, at least in part, be attributed to their high aromaticity. They can also contribute a potential... 

Phenolic Resins. Phenohc resins (qv) are formed by the reaction of phenol [108-95-2] C H O, and formaldehyde [50-00-0] CH2O. If basic conditions and an excess of formaldehyde are used, the result is a resole phenohc resin, which will cure by itself Hberating water. If an acid catalyst and an excess of phenol are used, the result is a novolac phenohc resin, which is not self-curing. Novolac phenohc resins are typically formulated to contain a curing agent which is most often a material known as hexamethylenetetraamine [100-97-0] C H22N4. Phenohc resin adhesives are found in film or solution... 

Early phenoHc resins consisted of self-curing, resole-type products made with excess formaldehyde, and novolaks, which are thermoplastic in nature and require a hardener. The early products produced by General BakeHte were used in molded parts, insulating varnishes, laminated sheets, and industrial coatings. These areas stiH remain important appHcations, but have been joined by numerous others such as wood bonding, fiber bonding, and plywood adhesives. The number of producers in the 1990s is approximately 20 in the United States and over 60 worldwide. 

With a bulk process, resole resins, in neat or concentrated form, must be produced in small batches (ca 2—9.5 m ) in order to maintain control of the reaction and obtain a uniform product. On the other hand, if the product contains a large amount of water, such as Hquid plywood adhesives, large reactors (19 m ) can be used. Melt-stable products such as novolaks can be prepared in large batches (19—38 m ) if the exotherms can be controlled. 

Neoprene—phenohc contact adhesives, known for thein high green strength and peel values, contain a resole-type resin prepared from 4-/-butylphenol. The alkyl group increases compatibiHty and reduces cross-linking. This resin reacts or complexes with the metal oxide, eg, MgO, contained in the formulation, and increases the cohesive strength of the adhesive. In fact, the reactivity with MgO is frequently measured to determine the effectiveness of heat-reactive phenoHcs in the formulation. 

Different phenoHc resins are used for different types of wood for example, plywood adhesives contain alkaline-catalyzed Hquid resole resins. Extension with a filler reduces cost, minimizes absorption, and increases bond strength. These resins have an alkaline content of 5—7% and are low in free phenol and formaldehyde. Because many resins have a high water content and limited storage stabiHty, they are frequently made at or near the mill producing the plywood product. The plywood veneers are dried, coated with resin, stacked for pressing, and cured at 140—150°C. 

The manufacturing instructions and formula for a typical commercial resole to be used in a plywood adhesive application are shown in Table 2. Such resin... 

The lower cost of the urea-modified PF resins is a combination of PF solids extension by lower cost urea and improved adhesion and distribution capabilities. The improvements in storage stability stem from the thinning and dilution effects as well as from the formaldehyde scavenging. Liquid PF resoles with high free formaldehyde contents tend to be less stable in storage. 

Many applications of novolacs are found in the electronics industry. Examples include microchip module packaging, circuit board adhesives, and photoresists for microchip etching. These applications are very sensitive to trace metal contamination. Therefore the applicable novolacs have stringent metal-content specifications, often in the low ppb range. Low level restrictions may also be applied to free phenol, acid, moisture, and other monomers. There is often a strong interaction between the monomers and catalysts chosen and attainment of low metals levels. These requirements, in combination with the high temperature requirements mentioned above, often dictate special materials be used for reactor vessel construction. Whereas many resoles can be processed in mild steel reactors, novolacs require special alloys (e.g. Inconel ), titanium, or glass for contact surfaces. These materials are very expensive and most have associated maintenance problems as well. 

The PVF is made by acidic reaction between poly(vinyl alcohol) (PVA) and formaldehyde. The poly(vinyl alcohol) is, in turn, made by hydrolysis of poly(vinyl acetate) or transesterification of poly(vinyl acetate). Thus, residual alcohol and ester functionality is usually present. Cure reportedly occurs through reaction of phenolic polymer hydroxyls with the residual hydroxyls of the PVA [199]. The ester residues are observed to reduce bond strength in PVF-based systems [199]. This does not necessarily extend to PVF-P adhesives. PVF is stable in strong alkali, so participation of the acetals in curing is probably unimportant in most situations involving resoles. PVF is physically compatible with many phenolic resins. 

Standard-grade PSAs are usually made from styrene-butadiene rubber (SBR), natural rubber, or blends thereof in solution. In addition to rubbers, polyacrylates, polymethylacrylates, polyfvinyl ethers), polychloroprene, and polyisobutenes are often components of the system ([198], pp. 25-39). These are often modified with phenolic resins, or resins based on rosin esters, coumarones, or hydrocarbons. Phenolic resins improve temperature resistance, solvent resistance, and cohesive strength of PSA ([196], pp. 276-278). Antioxidants and tackifiers are also essential components. Sometimes the tackifier will be a lower molecular weight component of the high polymer system. The phenolic resins may be standard resoles, alkyl phenolics, or terpene-phenolic systems ([198], pp. 25-39 and 80-81). Pressure-sensitive dispersions are normally comprised of special acrylic ester copolymers with resin modifiers. The high polymer base used determines adhesive and cohesive properties of the PSA. 

The condensation reaction of resorcinol with formaldehyde, on an equal molar basis and under identical conditions, also proceeds at a rate which is approximately 10 to 15 times faster than that of the equivalent phenol-formaldehyde system [16-18,123]. The high reactivity of the resorcinol-formaldehyde system renders it impossible to have these adhesives in resol form. Therefore, only resorcinol-formaldehyde novolaks, i.e. resins not containing methylol groups can be produced. All the resorcinol nuclei are linked together through methylene... 

In the manufacture of pure resorcinol resins, the reaction can be violently exothermic unless controlled by the addition of alcohols. Because the alcohols perform other useful functions in the glue mix, they are left in the liquid adhesive. PRF adhesives are generally prepared firstly by reaction of phenol with formaldehyde to form a PF resol polymer, that has been proved to be in the greatest percentage, and often completely, linear [95], In the reaction step that follows the resorcinol chemical is added in excess to the PF-resol to react it with the PF-resin -CH2OH groups to form PRF polymers in which the resorcinol groups can be resorcinol chemical or any type of resorcinol-formaldehyde polymer. 

Resol resins thermoset on heating and are used for adhesives. Novolacs require a further source of formaldehyde in the form of hexamethylenetetramine to produce molding powders. Phenolic moldings are resistant to heat, chemicals, and moisture with good electrical and heat insulation qualities. Complex phenols from, e.g., cashew-nut shell liquid, are used in making brake... 

The route to crosslinked phenol-formaldehyde resins via resoles corresponds to that used by Baekeland in his original commercial technique. They now tend to be used for adhesives, binders, and laminates. The resole... 

Cure Rate of the Phenolated SEL Resins. 13C NMR spectra of the phenolated SEL formaldehyde-treated resins revealed the formation of methylol groups. A similar cure reaction to resole type phenolic resins is expected to occur with the phenolated lignin-based resins. Since cure rate normally determines production capacity of a board mill, it is important that new types of adhesives have at least the same cure rate as the conventional phenolic adhesives. Cure analysis of resins has usually been examined by... 

Various attempts have been made to prepare adhesives from lignin. The preparation of resol resin adhesives has been studied especially extensively. The introduction of phenols into the a or /2-position of the sidechain of the phenylpropane unit (phenolation of lignin) has been considered a key reaction for the formulation of these types of adhesives with adequate glu-ability. 

Resol-Type Phenol Resin Adhesives from Kraft Lignin... 

There have been many attempts to prepare resol resin adhesives from kraft lignin (23). Phenolation of lignin can, in these cases, also be considered a key reaction. That is, effective modification of lignin with phenol can enhance its reactivity, resulting in good gluability. 

Lignin was chemically modified prior to resinification, and the effect of phenolation was examined on the gluability of the resol resin adhesives (23). The phenolation was performed either with HC1 (80°C, 60 min) or without catalyst (200°C, 60 min). Prior to adhesive testing with three-ply plywood, 5 parts of coconut husk powder were mixed with 100 parts of the resin. 

In order to prepare better adhesives, the effect of phenolation time, phenolation temperature, reaction time for resol resinification, and degree of lignin purity were examined on the adhesive properties. The results indicated that optimum conditions for phenolation involve 200°C for 60 min and 60 min resol resinification at 90°C and at pH 9. The conditions for resol resinification correspond well to those of the conventional manufacturing method (24). 

For comparison, a conventional resol resin adhesive without lignin was prepared (24), and its gluability was examined. This resin was found to require a hot-pressing rate of at least 1.5 min per 1 mm plywood thickness before a satisfactory wet-bond adhesion strength was achieved at the low hot-pressing temperature of 120°C. This indicates that replacing a part of the phenol with lignin does not imply a mere extender addition, but that a positive role is achieved which enhances the reactivity of the adhesive. 

Figure 1. Comparison of wet-bond adhesion strengths of lignin-resol resin adhesives phenolated with and without acid catalyst. Legend phenola-tion with acid catalyst at 80°C for 60 min Q phenolation without catalyst at 200°C for 60 min. Note Numerical values in parentheses are percentages of wood failure hot-press temperature 120°C.

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