Thermosetting wood adhesives

The major disadvantage associated with urea-formaldehyde adhesives as compared with the other thermosetting wood adhesives, such as phenol-formaldehyde and polymeric diisocyanates, is their lack of resistance to moist conditions, especially in combination with heat. These conditions lead to a reversal of the bond-forming reactions and the release of formaldehyde, so these resins are usually used for the manufacture of products intended for interior use only. However, even when used for interior purposes, the slow release of formaldehyde (a suspected carcinogen) from products bonded with urea-formaldehyde adhesives is observed. 

Van der Klashorst, G. H Cameron, F. A Pizzi, A. The Utilization of Soda Bagasse Lignin in the Manufacture of Thermosetting Wood Adhesives. Paper presented at the International Symposium on Wood and Pulping Chemistry, April 1987. 

So, in this chapter the interest and importance of the polymer gel theory approach to the formulation of adhesives will be briefly shown and, in particular, we will limit this to thermosetting wood adhesives obtained by polycondensation such as phenol-formaldehyde-, urea-formaldehyde-, melamine-formaldehyde- and resorcinol-formaldehyde-based adhesives. The same approach is, however, very valid for other polycondensation resins, and also for adhesives obtained by routes other than polycondensation. 

The need to substitute synthetic thermosetting wood adhesives with more environmentally acceptable resins has led to intense research on adhesives derived from natural, non-toxic materials. Extensive reviews on the subject exist [4,5]. It is sufficient here to state that lignin is one of the materials at the forefront of these studies. Numerous wood adhesive fomulations based on lignin have been published over the years [6]. 

EN 12765 2002, Classification of thermosetting wood adhesives for non-siructural applications. 

UF resins are the most important and most used class of amino resin adhesives. " They constitute more than 80% of all thermosetting wood adhesives in Europe, and around 50% in North America. The advantage of UF adhesives are their (1) initial water solubility (this renders them eminently suitable for bulk and relatively inexpensive production), (2) hardness, (3) non-flammability, (4) good thermal properties, (5) absence of colour in cured polymers, and (6) easy adaptability to a variety of curing conditions. 

Transition of adhesives in thermoset wood composites to thermoplastic polymeric materials and realization that a key point would be a uniform mixing of cellulose fiber with the plastic. 

The debate over urethane formation in the wood/pMDI bondline may soon yield to studies of the peculiar morphology of this interphase. Recall that pMDI is a low viscosity, low molecular weight, low surface tension organic liquid. Consequently, pMDI wood binders readily wet and deeply penetrate into wood, as demonstrated by Shi and Gardner [27]. In fact the deep penetration of pMDI into wood contradicts traditional views on wood adhesion. The truly polymeric wood binders sueh as phenol formaldehyde (PF) and urea-formaldehyde (UF) are formulated for only moderate levels of wood penetration overpenetration is undesirable with these resins. By traditional standards, pMDI wood binders overpenetrate and yet they perform as well or better than other wood binding thermosets. One then wonders what becomes of the resin that does not span the gap between bonded wood particles Does it polymerize into a bulk phase within wood cell lumens, providing no benefit Or does the deep penetration provide some performance gain How deep is the penetration ... 

On the basis of PF resin, PUF resin has been developed to decrease the cost and to accelerate curing. Similar to the standard PF resin, PUF resin is prepared under alkaline conditions. Application of nanoclays to urea-formaldehyde (UF) resins has recently shown that nanoclays can improve the properties of thermosetting resins for interior-grade wood adhesives [20], and urea in PF resin could possibly be useful for the preparation of PUF resin/MMT composites, too. 

The water-based component is the main adhesive component in EPI adhesives. Generally, it consists of water, poly(vinyl alcohol) (PVA), one or more water-based emulsions, filler(s) and a number of additives such as defoamers, dispersing agents and biocides [1, 4, 5, 8, 9]. As with traditional thermosetting and thermoplastic wood adhesives, properties such as viscosity and solids content vary with the intended application. In the European market the typical viscosities are 2000-8000 mPa s at 25°C and the solids content is normally 50% or more. The adhesives are normally neutral with a pH in the range of 6-8 [3]. The storage stability of the EPI adhesive component is typically half a year when stored at a temperature between 10 and 30°C. 

Bondable substrates for aerobic and second-generation acrylics include clean as well as oily or "as received" metal polyurethanes, filled polycarbonates, filled nylon, phenolics, thermosets, wood, concrete, ceramics, glass, and other common materials of manufacture. Some thermoplastic materials are bonded better by using a second-generation acrylic adhesive. Examples are unfilled nylon, ABS, and rubber. 

For a general introduction, see Wood adhesives - basics. Aminoplastic resins are polycondensation products of the reaction of aldehydes with compounds carrying amine or amide groups. Formaldehyde is by far the primary aldehyde used. The name amino-plastic is traditional, but is a misnomer these are thermosetting, irreversibly hardening resins. Two main classes of these resins exist urea-formaldehyde (UF) resins and melamine-formaldehyde (MF) resins. Today, for wood adhesives, pure MF resins are not used anymore because of cost, and only melamine-urea-formaldehyde (MUF) adhesives are used. The main use of these resins is in composite products such as particle board, plywood, medium density fibreboard (MDF), oriented strand board (OSB), glu-lam/flngerjointing and furniture. 

As 50% of the success of wood adhesives is in its application parameters, these need to be carefully considered even an excellent wood adhesive will give very poor bonding if the correct applications parameters are not respected. Foremost about these is the wood percentage moisture content. Generally, thermosetting resins for panels are used at moisture contents of 10-12% for particleboard or similar panels and at 7-9% for plywood, with some notable exception such as tannin adhesives, which can reach 25%... 

A wide range of adhesive types and chemistries are used to bond wood elements to one another (Table 2), but relatively few adhesive types are utilized to form the composites themselves. The vast majority of pressed-wood products use synthetic thermosetting adhesives. In North America the most important wood adhesives are the amino resins (qv), eg, urea-formaldehyde (UF) and melamine-formaldehyde (MF), which account for 60% by volume of adhesives used in wood composite products, followed by the phenolic resins (qv) eg, phenol-formaldehyde (PF) and resorcinol-formaldehyde (RF), which account for 32% of wood composite adhesives (12,13). The remaining 9% consists of cross-linked vinyl (X-PVAc) compounds, thermoplastic poly(vinyl acetates) (PVA), soy-modified casein, and polymeric diphenylmethylene diisocyanate (pMDI). Some products may use various combinations of these adhesives to balance cost with performance. 

The greatest volume of wood adhesives for structural applications are thermosetting phenol-formaldehyde (PF) or urea-formaldehyde (UF) polymers or their derivatives. PFs are the commodity for exterior applications, while UFs are the commodity for interior applications. Because each composite product and the associated process may require a somewhat different resin, many variations of PF and UF resins are available. Isocyanate crosslinking resins are used to a lesser extent than PFs or UFs in manufactured board. Although these resins impart a durable, water resistant bond suitable for exterior application, the cost per pound of adhesive is greater than for PFs or UFs. 

Amino Resins. Amino resins (qv) include both urea- and melamine—formaldehyde condensation products. They are thermosets prepared similarly by the reaction of the amino groups in urea [57-13-6] or melamine [108-78-1] with formaldehyde to form the corresponding methylol derivatives, which are soluble in water or ethanol. To form plywood, particle board, and other wood products for adhesive or bonding purposes, a Hquid resin is mixed with some acid catalyst and sprayed on the boards or granules, then cured and cross-linked under heat and pressure. 

Phenolic Resins. PhenoHc resins [9003-35 ] (qv) are thermosets prepared by the reaction of phenol with formaldehyde, through either the base-cataly2ed one-stage or the acid-cataly2ed two-stage process. The Hquid intermediate may be used as an adhesive and bonding resin for plywood, particle board, ftberboard, insulation, and cores for laminates. The physical properties for typical phenoHc laminates made with wood are Hsted in Table 1. 

Medium-density fiberboard (MDF) a flat pressed wood composite panel composed of randomly oriented wood fibers obtained by thermomechanical wood pulping and bonded by hot-pre.ssing by using thermosetting adhesive resins. The panel has generally a density of approximately 850 kg/m- and the average amount of resin solids in the board core section is between 11 % and 14% on dry wood. 

---