Curing hot melts

In most cases, the allophanate reaction is an undesirable side reaction that can cause problems, such as high-viscosity urethane prepolymers, lower pot lives of curing hot-melt adhesives, or poor shelf lives of certain urethane adhesives. The allophanate reaction may, however, produce some benefits in urethane structural adhesives, e.g., additional crosslinking, additional modulus, and resistance to creep. The same may be said about the biuret reaction, i.e., the reaction product of a substituted urea linkage with isocyanate. The allophanate and biuret linkages are not usually as thermally stable as urethane linkages [8]. 

Monomeric MDI Pure 4,4 -MDI 2.0 Solid (MP = 37 C) Flexible prepolymers Liquid 1-K adhesive Thermoplastic adhesives Curing hot melts Solvent-borne adhesives... 

Crystalline polyesters are highly important as adhesive raw materials. They are normally crystalline waxes and are highly symmetrical in nature, which can aid the crystallization process [26]. Poly(hexamethylene adipate) and poly(caprolactone), shown in Table 2, are only two of the many crystallizable backbones. Poly(ethylene adipate) and poly(letramethylene adipate) are also commonly used in urethane adhesives. The crystalline polyesters are used in curing hot melts, waterborne polyurethanes, thermoplastic polyurethanes, and solvent-borne urethane adhesives. The adipates are available mostly as diols. The poly(caprolactones) are available as diols and triols. 

Most moisture-curing hot-melt adhesives utilize a crystallizable backbone and are based almost exclusively on monomeric MDI at NCO/OH ratios of 1.5 to 2.2. Poly(hexamethylene adipate) polyol is the workhorse of the curing hot-melt adhesives. 

Moisture-curing hot melts are a small but fast growing segment of the urethane adhesive market. They are used mostly in construction and furniture assembly applications. Recent applications include RV sidewall assembly and other OEM automotive applications. Smaller applications include bookbinding and footwear. A typical adhesive is shown below ... 

These adhesives differ from normal hot-melt adhesives, such as the standard ethylene vinyl acetate hot melts. Standard hot-melt adhesives like EVA have no curing mechanism. They are heated above the crystalline melting point and applied as a low-viscosity liquid in the same manner as is the curing hot melt. The bond is closed in the same manner and strength is developed upon crystallization. 

Curing hot melts usually have less fixturing strength than do standard hot-melt adhesives but eventually develop structural strength. Once the moisture cure is complete, the curing hot melt has good structural properties and has better elevated heat strength than does a traditional hot melt such as EVA. 

Although not in wide use, a fast crystallizing TPU adhesive can be used in the shoe industry as an alternative to solvent-borne urethane adhesives. The TPU adhesives have good holding strength soon after crystallization, which can be a distinct advantage over curing hot-melt adhesives. TPU adhesives normally have... 

Good aging resistance The disadvantages of curing hot melts are ... 

Two component curing hot melts consist, for example, of (i) polyamide- -epoxy, or (ii) a polyol component -I- isocyanate. After the mixing of the two components, they possess only limited pot life. 

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