Phenol formaldehyde structural adhesive

Phenolics or phenol-formaldehyde structural adhesives are chemically reactive systems that cure to form thermosets. In one-component systems, meltable powders (resols) are used as binders for particle board or as alloys (including nitrile-phenolics, vinyl - phenolics, and epoxy-phenolics), which are used in the structural bonding of metals. In two-component systems, the resin and catalyst are mixed and then heated to initiate curing. Both systems cure by a condensation reaction that produces a byproduct. 

Further condensation can be obtained by heating the resole again to continue the reaction of the methylol groups. This further condensation, however, results in the evolution of gas which could result in a porous bondline. This evolution of small molecules limits the utility of resole phenolics in structural adhesives. Novolac phenolic resins have a structure similar to that shown in Pig. 1 and they are obtained by heating formaldehyde in the presence of excess phenol. As one can see, the novolac structure contains no residual methylol groups. Novolacs will not cure with themselves as do resoles, but they can be cross-linked by means of hexamethylene tetramine ( HEXA"). This is also shown in Pig. 1. 

The adhesive used in virtually all softwood plywood has a phenol—formaldehyde (PF) base to provide an exterior-grade, durable, waterproof bond. Thus, most grades of plywood can be used in structural applications. A very small percentage of softwood plywood is made using interior-grade adhesive systems, and this material is used in interior cabinetry, furniture, and shelving. 

Phenol-formaldehyde adhesives are produced by a condensation polymerization reaction between phenol and formaldehyde. The phenolics used for exterior particleboard are made at a formalde-hyde/phenol ratio greater than 1.0 i.e., they are classified as resoles and additional formaldehyde is not required to complete the curing reaction to a highly cross-linked network structure. Many characteristics can be incorporated into the adhesives by changes in the F/P ratio, condensation pH, and condensation time. The reactive solids content is normally between kO and 50 percent since the stability and viscosity are adversely affected at higher solids. 

The types of adhesives suitable for laminating beams are restricted by the conditions of application and by their end-use requirements. A wider choice of adhesives for plywood depends on whether softwoods or hardwoods are used, whether they are required for internal or external exposures, or whether they are to be used for ornamental or structural purposes. Thus phenol-formaldehyde types would be used for marine or exterior construction uses urea-formaldehyde types would be advantageous for cold pressing, or melamine-urea adhesives might be preferred for hardwood plywood, or lumber-core panels used in furniture production. 

Phenolic adhesives are structural adhesives with specific applications, e.g. where wide gap bonding is required and where large structures need to be bonded. Phenolic resins are the product of a special reaction ratio of a phenol and formaldehyde in the presence of an organic catalyst. There are two main types of phenolic resins phenol or methylol terminated. The phenol terminated are called novalacs, while the methylol terminated are called resoles (one step resins). Modem phenolic resins are prepared in the presence of metal carboxylates and these resins contain a large number of benzylic ether linkages and have open para positions which have good temperature stability and are usually of low viscosity. 

Strands are coated with a waterproof structural adhesive, e.g. phenol-resorcinol formaldehyde, and laid-up using special equipment to ensure proper orientation and distribution. The microwave continuous press both densifies the material and cures the adhesive. LVL is commonly produced in 0.28 by 0.48 m (11 by 19 in.) section -much thicker than that of LVL. Final product can be sawn into smaller dimension, if desired, while the length is limited only by freight and handling restrictions. 

Both the published literature and previously unpublished information obtained by the structural panel industry indicate that formaldehyde levels associated with panel products glued with phenol formaldehyde adhesives are extremely low. Large dynamic chamber tests which simulate conditions that might be found in tightly sealed residences indicate consistently that formaldehyde levels associated with freshly manufactured phenolic panel products are less than 0.1 parts per million. The data, as well as theoretical considerations, also indicate that the amount of formaldehyde contributed to the environment by phenolic panel products should rapidly approach zero as the small quantity of formaldehyde initially present in the products is released. 

Phenol formaldehyde, on the the other hand, is used to make the waterproof adhesives that are used in structural wood panel products such as softwood plywood, oriented strand board, waferboard and exterior (phenolic) particleboard. These products are commonly used for roof, floor and wall sheathings, exterior sidings, concrete forms and in pallets and numerous other products. 

Note that Figure 2.9 shows the transitions in terms of (1) onset of phase separation, (2) gelation, (3) fixation of the dimension of phase-separated structure, (4) the end of phase separation and (5) vitrification. Lu and Pizzi (1998) investigated the curing of phenol and urea formaldehyde wood adhesives in a TTT diagram. Special mention is made of the effect of the wood substrate on the cure of this composite system. 

Waferboard is a structural board made of wood wafers cut to predetermined dimensions randomly distributed and bonded with phenolic adhesives." It is estimated that by 1981, 80 million pounds of adhesives on a solid basis will be used in waferboard and other types of particleboard bonding. Currently either solid or liquid phenolic resins are being used in these applications. Formaldehyde and phenol fumes and phenol formaldehyde powder dust,... 

BRJ-473. [Schenectaify] Phenol-formaldehyde one-s resin sol tt banding agteit for use in NBR structural adhesives. 

When the aldehyde in this reaction is formaldehyde, the product is poly(vinyl formal). This polymer is, however, made directly from poly(vinyl acetate) and formaldehyde without separating the alcohol. The product with low hydroxyl (5-6%) and acetate (9.5—13%) content (the balance being formal) is used in wire enamel and in structural adhesives (e.g., Redux). In both applications the polymer is used in conjunction with phenolic resins and is heat cured. 

Recently, adhesives that include lignin in their formulation have shown several properties similar to phenol-formaldehyde resins [11, 12] and are extensively utilized in the plywood, particleboard, fiberboard and laminated wood industries. The chemistry of the lignin-based adhesives has been studied from an empirical point of view [13] however, the molecular mechanism and the interactions of the structural cross-linked 4-hydroxyphenylpropanoids of lignin that drive the adhesion phenomenon remain unclear. 

PVAc-based commercial wood adhesives are evaluated using standard tests for non-structural applications, as reported in EN 205 [8], and they are classified in agreement with the standard EN-204 [9]. This standard allows to classify wood adhesives in 4 categories from D1 to D4. D1 adhesives show a good resistance only in dry conditions D2 adhesives should withstand a rather low water presence, such as in occasional exposure in kitchens and bathrooms D3 adhesives are suitable to come in contact with cold water, such as for outside windows and doors, kitchen and bathrooms furniture D4 adhesives are suitable to be used in extreme conditions (resistance to hot water). Vinyl acetate homopolymer can be used to formulate D1 or D2 adhesives. Vinyl acetate based adhesives cross-Unked with hardeners and urea-formaldehyde (UF) adhesives belong to class D3. Only the phenol-formaldehyde (PE), resorcinol-formaldehyde (RF) and melamine-formaldehyde (MF) adhesives, some special 2-component polyurethanes (PUs), and cross-linking vinyl adhesives belong to class D4. 

Most of the glulam beams produced in Europe for load-bearing timber structures use melamine-urea-formaldehyde (MUF) and phenol-resorcinol-formaldehyde (PRF) adhesives. Nevertheless, PRF consumption has been decreasing continuously for several years because of the colour of the joints obtained, increased productivity requirements and environmental issues. 

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