Isocyanate-based adhesives

A wood-based composite can be defined as a composite material mainly composed of wood elements. These wood elements are usually bonded together by a thermosetting adhesive (wood truss products could also be regarded as wood-based composites, but connected by metal connectors). The commonly used adhesives include urea-based adhesive (such as urea formaldehyde resin), phenolic-based adhesive (including phenol resorcinol adhesives), isocyanate-based adhesive, and adhesives from renewable resources (like soybean, lignin etc). The wood elements in wood composites can be in many different forms such as ... 

Applications for blocked urethane adhesives are small. However, they may be used as flocking adhesives or as crosslinkers for solvent-borne adhesives. Blocked urethane adhesives are also used as splicing adhesives for belts. Blocked isocyanates based on TDI, IPDI, and derivatives of hexamethylene diisocyanate are most commonly commercially available. 

The development of useful adhesive compositions based on the interaction of isocyanate resins with natural polymers such as lignin, proteins, and carbohydrates. 

Isocyanate-Based Adhesives. Generally diisocyanates or polyisocyanates are used in the preparation of polyurethane adhesives. The urethane link (-O-C(O)-NH-) is formed by the reaction of an alcohol with an isocyanate (-N = C = 0) group ... 

I 75 Novel Adhesion Promoters Based on Hyperhranched Polymers Table 15.2 Polyols and isocyanates used in the study. 

Most urethane adhesives are based on urethane prepolymers. A prepolymer is made by reacting an excess of diisocyanate with a polyol to yield an isocyanate-terminated urethane as shown in Fig. 11. Prepolymers may have exeess isocyanate present ( quasiprepolymers ) or they may be made in a 2 1 stoichiometric ratio to minimize the amount of free isocyanate monomer present. Most moisture-cured prepolymers are based on 2 1 stoichiometric ratios. Two-component adhesives generally are based on quasi-prepolymers, which use the excess isocyanate to react with either chain extenders present in the other component or with the substrate surface. 

Hazardous Decomp. Prods. Heated to decomp, emits CO, CO2, NOx, hydrogen cyanide Storage Air- and moisture-sensitive store under nitrogen reacts slowly with water Uses In isocyanate-based adhesives component in polyurethane elastomers high strength backbone or crosslinking intermediate... 

EPI adhesives are two-component systems. In the following the emulsion/polymer based component (the adhesive component) and the isocyanate based cross-linker component will be discussed separately. 

Schollenberger, C.S., Polyurethane- and isocyanate-based adhe.sives. In Skeist. l.S. (Ed.). Handbook of Adhesives, 3rd edn. Van Nostrand Reinhold, New York, 1990. p. 359. 

The use of adhesives/sealants in both the industrial and consumer spheres has increased dramatically in the past 20 years. In the industrial segment, both reactive and nonreactive systems are used in a wide variety of applications. The increasing use of reactive systems has, however, tended to overshadow that of the nonreactive systems. Most prominent amongst the reactive systems favored in industrial applications are anaerobic sealants (methacrylate ester based), instant adhesives (alkyl cyanoacrylate ester based), acrylic (toughened) adhesives, epoxy resin adhesives, polyurethane/isocyanate-based adhesives, silicone adhesives/sealants, and phenolic resin adhesives. 

The adhesion properties of isocyanate-based adhesives involves a combination of several chemical properties which will be examined in detail. These characteristics are summarized as follows ... 

Usually in adhesion technology strong adhesion is expected if chemical bonds between adherend and adhesive develop. Hence the ready reaction of the isocyanate group with a large number of other functional groups favours the success of isocyanate-based compositions as adhesives. 

Good strength and flexibility characteristics in the boundary film formed between the adhered members by the isocyanate-based adhesives are also considered to be favourable contributions to good adhesion. 

To explain the adhesion of isocyanate-based adhesives to impervious, apparently unreactive surfaces such as glass and metals, it is considered that isocyanate groups react with the molecular film of tightly absorbed water always present on such surfaces, the reaction of the isocyanate groups with the oxyhydrate layer on metal surfaces and isocyanate polymerization on (alkaline) glass. All of these processes provide intimate contact if not actual chemical bonds between adhesive and adherend. 

This has become the most important type of isocyanate-based adhesive system. In this method a preformed, fully reacted, high molecular weight polymer is employed as a vehicle in the adhesive formulation. The strength of the vehicle holds adherend members in exact position after assembly... 

Using heat-reactive p-alkylphenolic resins or ortho-ortho resole resins in aromatic solvent and a polymeric isocyanate in the presence of a small amount of tertiary amine, a rapidly curing adhesive system of good overlap shear for both wood and aluminum can be obtained. The best metal and wood adhesive is based on p-cresol or a straight phenolic ortho-ortho resole in combination with a polymeric aromatic isocyanate such as PAPI or Mondur MRS." ... 

Isocyanate-based adhesives are being studied as partial or complete replacements for the traditional formaldehyde-hardened adhesives, such as urea-formaldehyde and phenol-formaldehyde resins. These latter adhesives are being used for the production of wood laminates, bonded wood joints, and as binders for composition board. 

Presently, the isocyanates most commonly used in the formulation of wood adhesives are based on MDI, particularly polymeric MDI (PMDI), which has a low vapor pressure and is therefore preferred because it is less of a hazard. Commercial PMDI is a mixture of oligomers with the generalized structure (24) the value of n typically varies from zero to eight. ... 

Adhesives based on cyanoacrylic acid esters differ significantly from most other reactive adhesives because they consist in large part of pure monofunctional monomers. Epoxy, acrylic, and isocyanate based adhesives typically include di-, tri-, and tetrafunctional reactants, and usually contain significant quantities of curatives or coreactants. Cyanoacrylate adhesives rarely contain significant amounts of coreactants because they are able to homopolymerize rapidly at room temperature. Also, non-neutral additives drastically alter the cure rate of shelf life of the compounded adhesives. 

The chemistry of 2C-Polyurethane adhesives is outlined in Fig. 61. Cross-linking occurs after mixing a polyol-based, filled resin with an isocyanate-based hardener which can either be a monomeric diisocyanate or an isocyanate terminated prepolymer synthesised from a polyol with the isocyanate-containing compound. The active hydrogen groups, such as OH, NH, SH etc. react in an electrophilic manner with the isocyanate groups. 

Chapter 5 Thermosetting formaldehyde based adhesives, by Dr A. Pizzi, a detailed study of UF, PF, RF and MF adhesives and glues, their chemistry and their uses in woodworking, panels and furniture (this chapter will be followed in later volumes by a chapter on Wood Based Panels and a chapter on Isocyanate Based Adhesives ). 

Ferreira et al. [75] synthesized a HMA for medical use. Urethanes based on polycaprolactone diol (PCL) were synthesized by reaction of the molecule either with isophorone diisocyanate (IPD-isocyanate) or hexamethylene diisocyanate (HDI-isocyanate). Nies Berthold et al. [76] tried out an adhesive composition based on polymers or polymer blends consisting of caprolactone copolymers or caprolactone copolymers and polycaprolactone. The adhesive can be utilized as HMA for temporarily gluing together biological tissue and other materials in medicine. 

Wattle tannins have been used as fortifiers in starch-based adhesives for corrugated carton manufacture in order to improve the moisture resistance of boxes used in shipment of fruit, for example (144, 217). Addition of 4% wattle tannin based on resin solids substantially improved the durability of these cartons. Wattle tannin urethane-based varnishes with excellent durability have been developed by Saayman (214). Other adhesive applications based on reactions of wattle tannins with isocyanates have been developed for specialty applications such as the bonding of aluminum (181). 

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