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双重固化机制的胶粘剂

来源：林中祥胶粘剂技术信息网2011年03月23日

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Adhesives with Dual Curing Mechanisms (More Than Twice as Nice)

To really maximize their response to the customers demands, certain formulators have developed systems that have dual curing mechanisms. These respond to several curing mechanisms - generally one to achieve high handling strength and another for optimal final strength and durability. One example of a dual curing system is a sealant composition, which provides fast light curing through acrylic functions while having room temperature curing capability (and greater crosslinking) through moisture curing of silicone functions.

The advantage to such dual cure adhesives is that handling strength is immediate, and ultimate strength can continue to develop without the need for fixtures, clamping, or outside energy sources. Also, for UV cured systems, cure depth can be enhanced or cure can be achieved in shadow areas.

Polymer System	Developments
Polyurethanes	Reactive hot melt PU adhesives have been developed that provide immediate handling strength on cooling to a gel and then continue to cure via moisture curing mechanisms.
RTV Silicones	Recent developments have focused on generating dual cure (light and moisture) RTVs.
Cyanoacrylates	Light cured cyanoacrylates have been developed to speed cure and to solidify adhesive not contained within a joint.
Acrylics	Structural adhesive have been developed that cure quickly with a surface primer or via a “honeymoon effect” where the accelerator is coated on one substrate and the base polymer on the other. Acrylate / polyurethane hybrids have been developed as coatings and have the potential for dual cure adhesives.

Table 1: Recent Progress and Immediate Goals in "Performance on Demand" Adhesives and Sealants

Table 1 summarizes some of the other innovative developments that have occurred over the last decade or so in the quest for adhesives that perform on demand. Through the imaginative combination of chemistries, future development activity will likely result in even more significant products.

One definite future trend that can be foreseen with hot melt adhesives is dual reaction mechanisms. These adhesives go a step beyond conventional hot melt adhesives. In addition to melting, cooling, and bonding, they provide added value to the end-user through the use of piggy-backing reactions.

The earliest and most commercial forms are reactive (moisture curing) hot melt adhesives. A reactive hot melt adhesive is any thermoplastic sealant that can be applied at elevated temperatures as a liquid melt, cools to become a solid at room temperature, and then subsequently reacts to become a thermosetting polymer with enhanced physical properties.

For certain applications requiring unusually high bond strength and fast set time, reactive polyurethane hot melt sealants have been developed. ^1,2 More recently reactive silicone hot melt adhesives and sealants have also been introduced. ³ These systems are applied like a hot melt, and the parts can be rapidly assembled within seconds. As a hot melt, this applied formulation has a high degree of initial handling strength. The unique feature for these materials, however, is that they will continue to cure and crosslink by reacting with the moisture in the air so that when completely cured they have the strength and durability typical of polyurethane or silicone thermosetting sealants.

Another advantage of these moisture reactive hot melts over conventional hot melts is that the melt temperature is much lower. This allows application of the molten sealant to heat sensitive substrates. Melt temperature of conventional ethylene vinyl acetate (EVA) polyamide, or polyester hot melt assembly adhesives is generally well over 175°C; whereas the melt temperature for polyurethane or silicone reactive hot melt systems is no greater than 120-150°C.

Other novel dual cure reaction me