Phenolic adhesives cure conditions

Table X. Phenolic Adhesive Cure Conditions and Properties ... 

CCA preservatives interfere with the curing of any phenolic adhesive, but particularly so with tannin cold-sets. This is due to the higher capability of complexation Cu and Cr with ortho-diphenols (such as the catecholic and pyro-gallolic B rings of flavonoids). The problem is not grave under normal ambient gluing conditions, but it becomes more evident at ambient temperatures of 30 °C or higher. The problem, of course, lies with CCA as a wood preservative rather... 

Foundry core binders are isocyanate-cured alkyd or phenolic adhesives used as binders for sand used to produce foundry sand molds. These sand molds are used to east iron and steel parts. A fast, economical cure of the sand mold is required under ambient conditions. 

There are three main polymers presently used for structural adhesive bonding and they are phenolics, epoxies and urethanes. We can also include in our classification, the initial physical state of the uncured adhesive since it governs the type of application and curing conditions. Structural adhesives are manufactured in the form of films or pastes. The films are one-part adhesives, i.e., they contain a latent catalyst which requires heat for activation. Structural adhesive films are made in a number of different thicknesses and can either be supported (containing a scrim) or unsupported. In general, film adhesives require the application of pressure during cure in order to obtain ultimate properties. Paste adhesives are either one- or two-part materials. That is, pastes can either contain a latent heat activatable catalyst, or they can be a system which is separated into two parts, one of which contains the cur a t i ve/ca talys t. Two-part pastes cure at room temperature. In this section the chemistry of one-... 

Many adhesives cure by step polymerization. Structural phenolic adhesives are based on resoles, which are made by the step polymerization of phenol with excess formaldehyde under basic conditions (see Phenolic adhesives single-stage resoles). They contain... 

Typical cure conditions for the different types of phenolic adhesives are shown in Table X. Also included are representative shear and peel values, as well as an indication of the temperature performance. 

Phenolic. Phenolic adhesives are based on phenol-formaldehyde resins and are one of the earliest groups of structural adhesives to be developed. They are available in a variety of forms but usually suffer the disadvantage that they often release significant quantities of water on curing. For this reason heated presses are usually required and their employment almost inevitably involves the use of capital equipment and some degree of procedural complexity. Despite this, they and the related resorcinol-formaldehyde resins remain popular wherever structural integrity must be maintained under severe environmental conditions. 

Phenolic materials go back to the patents of Baekeland and are based upon the reaction of phenol and formaldehyde to yield a lower molecular weight resinous material that can be formulated with other resins and sometimes other curatives to yield a paste adhesive or a film adhesive. When phenol and formaldehyde are reacted in the presence of an acidic catalyst with an excess of phenol versus formaldehyde, they yield what are known as novolac resins. These materials are soluble in organic solvents and do not react further with themselves thus, they need to have a curative added. That curative is most often hexamethylene tetraamine. Another type of phenolic resin can be generated with an excess of formaldehyde and under basic conditions. These materials, known as resole phenolics, will react with themselves to yield a ftilly cured phenolic and thus must be stored frozen in order to limit this reaction. Thus, resole phenolics do not need an external CTOsslinker. Resole phenolics are widely used in the binding of paper products and the bonding of wood. Adhesives based upon these materials were brittle and could only be used to bond wood, where it is still widely used. A more widely usable adhesive was developed during World War II which modified the very brittle phenolic adhesive with poly(vinyl formal) resins. These materials were... 

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... 

The high temperatures required for phenolic cure are generally viewed as positive characteristics, as they insure that the phenolic does not cure at normal room-temperature storage conditions. This keeps the adhesive soft so that it retains the desired adhesive properties. 

Resorcinol differs from other phenols in that it reacts readily with formaldehyde under neutral conditions at ambient temperature. To make stable adhesives, which can be cured at the point of use, they are prepared with less than a stoichiometric amount of formaldehyde. About two thirds of a mole of formaldehyde for each mole of resorcinol will give a stable resinous condensation product. The resin is formed into a liquid of convenient solids content and viscosity. Such solutions have infinite stability when stored in closed containers. Glue mixes formed at the point of use from these solutions, on addition of paraformaldehyde-containing hardeners, will have a useful life of several hours due to two principal factors (1) the paraformaldehyde depolymerizes to supply monomeric formaldehyde at a slow rate, as determined by the pH (2) the availability of the formaldehyde is also controlled by the kind and amount of alcohol in the solvent. Formaldehyde reacts with the alcohol to form a hemiacetal. This reaction is reversible and forms an equilibrium which exerts further control on the availability of the formaldehyde. 

An emulsion polymer-isocyanate adhesive, a crosslinked polyvinyl acetate adhesive, a resorcinol-formaldehyde adhesive, a phenol-resorcinol-formalde-hyde adhesive, and an acid-catalyzed phenolic-formaldehyde adhesive developed bonds of high shear strength and wood failure at all levels of acetylation in the dry condition. A neoprene contact bond adhesive and a moisture-curing polyurethane hot-melt adhesive performed as well on acetylated wood as untreated wood in tests of dry strength. Only a cold-setting resorcinol-formal-... 

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