Diisocyanate adhesives

In some experiments with nonemulsifiable diisocyanates, adhesive squeezeout was observed during pressing. It could be reduced by using fillers like gypsum, chalk, or wood flour. All in all, the use of fillers increased the binding qualities of the glue formulations (Table VI). 

Isocyanates [polymeric MDI (PMDI)] as a fortifier for phenolic resins have only been used in the past in rare cases. Deppe and Ernst [41] reported a precuring reaction between the isocyanate and the phenolic resin, even if both components had been applied separately to the particles. Hse et al. [36] also found good results with an isocyanate and a PF resin added separately to wood particles. Pizzi and Walton [191] reported on the reactions and their mechanisms of PF resins premixed in the glue mix with nonemulsifiable water-based diisocyanate adhesives for exterior plywood. Pizzi et al. [192] reported on the industrial applications of such systems (PF -I- PMDI -I- sometimes tannin accelerator UF + PMDI)... 

Resorcinol Toluene-2,6-diisocyanate adhesive mfg., contact Hexane... 

Toluene-2,4-diisocyanate adhesive/sealant, anaerobic Tetrahydrofurfuryl methacrylate adhesive/stiffener, surgical dressings Dextrin... 

The various reasons for polyurethane and diisocyanate adhesion characteristics will now be discussed. 

For metal adhesion, polyisocyanates are best mixed into a solution of a synthetic or chlorinated rubber. Table 8.3 compares the adhesion of natural rubber tyre tread to mild steel using a polyisocyanate priming cement with that obtained using a chlorinated rubber cement. PVC also adheres well to most substrates treated with diisocyanate adhesion primers. 

The use of hydroxyethyl (also hydroxypropyl) methacrylate as a monomer permits the introduction of reactive hydroxyl groups into the copolymers. This offers the possibility for subsequent cross-linking with an HO-reactive difunctional agent (diisocyanate, diepoxide, or melamine-formaldehyde resin). Hydroxyl groups promote adhesion to polar substrates. 

R is an organic moiety, which can be aliphatic or aromatic. Most commonly used in adhesives are the aromatic isocyanates, e.g., methylene diphenylisocyanate (MDI) and toluene diisocyanate (TDI). These polyisocyanates and others will be discussed in Section 3. 

Blocked isocyanate, for our purposes, will refer to the reaction product of a diisocyanate or isocyanate-terminated prepolymer in which the isocyanate functionality has been reacted with a blocking agent . Once blocked , the diisocyanate can be added to polyols or certain chain extenders, and these materials will not react at room temperature. The concept is shown in the sixth item of Fig. 1. An adhesive formulated with a blocked isocyanate is basically a two-component adhesive that does not react until heated to the activation temperature. When an adhesive is made with a blocked isocyanate together with hydroxyl-containing curatives, the adhesive has a good long shelf life at room temperature. However, once heated... 

In certain niche applications, aliphatic isocyanates, such as isophorone diisocyanate (IPDI), hexamethylene diisoeyanate (HDI), methylene 4,4 -biscyclo-hexylisocyanate (H12MDI), and polymeric versions of these diisocyanates, are used, e.g., in instances where light stability or reduced reactivity is needed. These isocyanates usually cost more than the aromatic diisocyanates. Thus, they are used in adhesive areas that can Justify the higher costs. 

Polyurethane dispersions (PUD s) are usually high-performance adhesives based on crystalline, hydrophobic polyester polyols, such as hexamethylene adipate, and aliphatic diisocyanates, such as methylene bis(cyclohexyl isocyanate) (H12MDI) or isophorone diisocyanate (IPDI). These PUD s are at the more expensive end of the waterborne adhesive market but provide excellent performance. 

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. 

Liquid organic rubbers with reactive functionality can be prepared by several methods. End-functional oligomers are preferred. Chains attached to the network at only one end do not contribute as much strength to the network as those attached at both ends [34], Urethane chemistry is a handy route to such molecules. A hydroxy-terminated oligomer (commonly a polyester or a polyether) can be reacted with excess diisocyanate, and then with a hydroxy methacrylate to form a reactive toughener [35]. The methacrylate ends undergo copolymerization with the rest of the acrylic monomers. The resulting adhesive is especially effective on poIy(vinyl chloride) shown in Scheme 2. 

Adhesives based on isocyanate (especially PMDl, polymethylene diisocyanate, more exactly polymeric 4,4 -diphenylmethane diisocyanate) have been used for more than 25 years in the wood-based panel industry [88], but still have a low market value in the wood-working industry compared to systems based on UF-, MUF- or PF-resins. The main application is the production of waterproof panels, but also the production of panels from raw materials that are difficult to glue, like straw, bagasse, rice shells or sugar cane bagasse. They can be used as adhesives for wood-based products like particleboard, oriented strandboard (OSB), laminated strand lumber (LSL), medium-density fiberboard (MDF) or... 

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. 

Some other groups such as ester, ether, amide, or urea are present in the Polymer chain of commercial polymers. In 1937, O. Bayer found that reaction of diisocyanates with glycols fields polyurethanes which are useful as plastics, fibres, adhesives, rigid foams and surface coatings. 

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