One-component adhesive systems

The concept of utilizing a blocked isocyanate in a one-component adhesive system is shown below ... 

When formulated into one-component adhesive systems, the product is stable when stored for 6 months to 1 year at room temperature. It will then cure when exposed to 145 to 160°C for about 30 to 60 min. Since the reaction rate is relatively slow at lower temperatures, the addition of 0.2 to 1 percent benzyldimethylamine (BDM A) or other tertiary amine accelerators is common to reduce cure times or cure temperatures. Other common accelerators are imidazoles, substituted urea, and modified aromatic amine. 

Dicyandiamide is a true latent catalyst for epoxy resin curing. It is also considered to be the workhorse of one-component adhesives due to its ease of use, excellent performance properties, long shelf stability, and low toxicity. In certain admixtures with DGEBA, it has demonstrated a room temperature storage life in excess of 4 years. Dicyandiamide is usually added to the solid epoxy resin in concentrations of about 3 to 6 pph. It melts at about 150°C. Cures can be conducted in the range of 120 to 175°C but are very slow at the lower temperatures. As a result, it is common practice to add accelerators such as benzyl-dimethylamine (BDMA) and mono- or dichlorophenyl substituted ureas to these systems. 

Paste adhesives are supplied as either one- or two-component adhesive systems. They can be used in applications where pressure cannot be applied. Some two-part pastes cure at room temperature after the appropriate proportions are mixed. Epoxy, urethane, and acrylic adhesives are all available as paste adhesives. 

The next major advance in curing of silicones was the discovery and development of systems which cured at room temperatures, the so-called RTV, room temperature vulcanizing, systems. The first of these were two component systems based on a variety of silanol terminated polyslloxanes crossllnked with various silanes or alkoxysilanes. A major step forward was the development of one component RTV systems using silanol terminated polydlmethylslloxanes and acytoxysllanes. These systems which cure when exposed to moisture in the air [23] provided the backbone for the rapid growth of this type of material as both sealants and adhesives. 

In one component RTV systems the cure can result by methyltriacetoxy-silane reacting with moisture with tin soaps added to control the cure rate. These resins were used for adhesive and sealant applications [32]. 

A hardener is a substance added to an adhesive formulation to initiate the curing reaction and take part in it. Two-component adhesive systems have one component, which is the base, and a second component, which is the hardener. Upon mixing, a chemical reaction ensues that causes the adhesive to solidify. A catalyst is sometimes incorporated into an adhesive formulation to speed the reaction between base and hardener. 

Commereial adhesives are sold as one or two part systems. One component adhesives may be 100% non volatile liquids, solvent solutions, pastes, sticks, powders, supported or imsupported films. Two part adhesives have the resin and hardener (curative) in two separate containers as Hquids or paste these are mixed just before use. These systems normally cure at room temperatures. 

One or two-part adhesives are dominantly applied, although there are multiple component adhesive systems in use as well. 

Elastic adhesives are usually produced on a polyurethane (PUR) basis, which have elongations up to 400%. One-component polyurethane systems cure with atmospheric moisture relatively slow and are therefore less sensitive to moisture. Due to its simple processing, 1-component polyurethane systems have gained market share increasingly (Brunner 1996). 

Polyurethane sealant formulations use TDI or MDI prepolymers made from polyether polyols. The sealants contain 30—50% of the prepolymer the remainder consists of pigments, fiUers, plastici2ers, adhesion promoters, and other additives. The curing of the sealant is conducted with atmospheric moisture. One-component windshield sealants utili2e diethyl malonate-blocked MDI prepolymers (46). Several polyurethane hybrid systems, containing epoxies, siUcones, or polysulfide, are also used. 

A number of systems are being evaluated to reduce levels of cobalt in the complexes with resins. Organic borates and post vulcanisation stabilisers are being examined. Chlorotriazine-based bonding systems can reduce the adhesion system from three components to one. 

The catalyst does not make up part of the final epoxy network structure or have a significant effect on the final properties of the cured resin. Thus, the final cured properties of the epoxy system are primarily due to the nature of the epoxy resin alone. Homopolymerization normally provides better heat and environmental resistance than polyaddition reactions. However, it also provides a more rigidly cured system, so that toughening agents or flexibilizers must often be used. In adhesive systems, homopolymerization reactions are generally utilized for heat cured, one-component formulations. 

Reactivity can also be increased by externally heating the epoxy formulation to a preselected curing temperature. Epoxy resin reactions roughly obey Arrhenius law that for every 10°C rise in temperature, the reaction rate doubles. Certain epoxy resin systems must be heated for any reaction to take place at all. This is beneficial in that these latent adhesive formulations are one-component products that do not require metering or mixing yet have long, practical shelf lives. 

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