Cured adhesives

The polymers of the 2-cyanoacryhc esters, more commonly known as the alkyl 2-cyaiioacrylates, are hard glassy resins that exhibit excellent adhesion to a wide variety of materials. The polymers are spontaneously formed when their Hquid precursors or monomers are placed between two closely fitting surfaces. The spontaneous polymerisation of these very reactive Hquids and the excellent adhesion properties of the cured resins combine to make these compounds a unique class of single-component, ambient-temperature-curing adhesives of great versatiUty. The materials that can be bonded mn the gamut from metals, plastics, most elastomers, fabrics, and woods to many ceramics. 

Elastomeric Modified Adhesives. The major characteristic of the resins discussed above is that after cure, or after polymerization, they are extremely brittie. Thus, the utility of unmodified common resins as stmctural adhesives would be very limited. Eor highly cross-linked resin systems to be usehil stmctural adhesives, they have to be modified to ensure fracture resistance. Modification can be effected by the addition of an elastomer which is soluble within the cross-linked resin. Modification of a cross-linked resin in this fashion generally decreases the glass-transition temperature but increases the resin dexibiUty, and thus increases the fracture resistance of the cured adhesive. Recendy, stmctural adhesives have been modified by elastomers which are soluble within the uncured stmctural adhesive, but then phase separate during the cure to form a two-phase system. The matrix properties are mosdy retained the glass-transition temperature is only moderately affected by the presence of the elastomer, yet the fracture resistance is substantially improved. 

Adhesive tapes (electron-beam-cured adhesives)... 

Plasticizers and Processing Aids. Petroleum-based oils are commonly used as plasticizers. Compound viscosity is reduced, and mixing, processing, and low temperature properties are improved. Air permeabihty is increased by adding extender oils. Plasticizers are selected for their compatibihty and low temperature properties. Butyl mbber has a solubihty parameter of ca 15.3 (f /cm ) [7.5 (cal/cm ) ], similar to paraffinic and naphthenic oils. Polybutenes, paraffin waxes, and low mol wt polyethylene can also be used as plasticizers (qv). Alkyl adipates and sebacates reduce the glass-transition temperature and improve low temperature properties. Process aids, eg, mineral mbber and Stmktol 40 ms, improve filler dispersion and cured adhesion to high unsaturated mbber substrates. 

DET-glycidyl adduct 25 10 min room temp. yes 75 fast cure adhesives laminating... 

Non-chromate and low-VOC containing primers are capable of excellent performance. Table 4 shows the results of a comparison of various environmentally acceptable primers with 250°F curing adhesive with the standard BR 127 solvent-based chromate inhibited primer [42]. 

In this reaction, no by-products are evolved, and the crosslinking reactions occur within the bulk of the material. A typical hydrosilylation crosslinking system is depicted in Scheme 11, where n and y can vary depending on the required viscosity of the uncured formulation and the final targeted properties of the cured adhesive. 

For all its benefits, the water/isocyanate reaction can be troublesome as well. All raw materials that go into a one-component moisture-cured adhesive must have very low moisture content, usually less than 0.05% water. If higher water levels are present, the adhesive can start curing in the reactor, causing an increase in viscosity, or, in extreme cases, gelation. 

The amorphous polyesters are becoming increasingly important for one-component 100% solid moisture-curing adhesives. These materials are usually viscous, amorphous liquids. Poly(2-methylpropane adipate), an example of an amorphous polyester, is a liquid even at a molecular weight of 2000. The amorphous polyesters are usually asymmetrical in structure. In the poly(2-methylpropane adipate) example, the pendant methyl group would be expected to interfere with chain packing, thereby preventing crystallization [27]. 

The soft segments made from asymmetrical (amorphous) polyols are important for two-component structural adhesives and one-component moisture-curing adhesives. These materials are applied and usually cured at room temperature. 

The blocked isocyanate systems (with curative present) are latent cure systems. In order to create a latent curing adhesive, the blocked isocyanate is added to a catalyzed polyol component without a reaction occurring at room temperature. In theory, the blocked adhesive system is relatively stable at room temperature. When this system is heated to the unblocking temperature, the chemical reaction, which... 

The two-component urethane structural adhesives are among the most difficult to characterize, simply because of the widely varying properties that are possible. These adhesives may be rigid plastics similar in modulus to standard epoxy adhesives, with glass transition temperatures of the cured adhesive being approximately 60°C. 

Blooming is the phenomenon of the formation of a white residue of cured adhesive near a bond line. It is the result of the evaporation of a small amount of the adhesive monomer from where it was applied onto the adjoining surfaee and its subsequent polymerization. The use of higher molecular weight monomers, such as the alkoxy esters, eliminates this problem. 

Huber, H.F., Radiation cured adhesives. Beta-Gamma, 1, 3 (1992). 

Figure 24 (a) and (b) Fluorescence spectra from extrinsic probes in cured adhesive under... 

Another well-established use of UV/visible lighf curing adhesives is glass, a plastics bonding in the automotive industry. Typical applications are lamination of safety glass, fastening of rear-view mirrors to windshields, and assembly of headlights. ... 

Compared to the chemically cured adhesive systems in common use today, this enzymatic binder has certain advantages ... 

Composites Both UV and EB cures are employed for the production of wood composite materials and in fiber-reinforced composites for aircraft and aerospace applications. The EB technology has been successful in the manufacture of large structures that exceed the size of autoclaves, and in curing adhesive joints in cases where uniform radiation can be provided more easily than uniform heat. In industrial and consumer applications, multiple combinations of different reinforcing fibers can be co-cured in one cycle by EB with considerably lower residual stresses than those introduced by thermal cure.16... 

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