Vulcanizing reactive adhesives

Before setting, reactive adhesives consist predominantly of reactive low molecular mass monomers and/or oligomers that, during curing, are converted by chemical reactions into high molecular mass, often three-dimensionally cross-linked polymers. Reactive adhesives can be divided into polymerization, polyaddition, and polycondensation adhesives. Vulcanizing reactive adhesives are a special group. 

Because of the metals insolubility in solvents, rubber-metal joints can only be produced with solvent-free reactive adhesives or even contact adhesives after suitable pretreatment. In the case of rubber-metal joints used as shock-absorber, respectively, oscillating elements in car manufacturing, the development of the bonded joint occurs during vulcanization. The term vulcanization dates back to the method developed by Goodyear around 1840 for the crosslinking of natural rubber with the simultaneous impact of sulfur and heat, which were known as byproducts of the volcanism . 

Curing In the curing process, the adhesive molecules are cross-linked to create a strong adhesive joint. To solidify the adhesives, several methods are utilized such as heat, pressure, time, catalyst, vulcanizing, reactivation, and radiation based on characteristics of adhesives (Fig. 6) (Cognard 2006). 

Their main field of application are sealing compounds (silicone rubber) for which they are sold in the form of reactive one-component systems (mainly in cartridges) (RTV-l-systems, room-temperature vulcanization). Just like the one-component polyurethanes described in Section 4.2.2, they cure under the influence of moisture from the ambient air. In certain formulations, this reaction leads to the separation of acetic acid perceptible by its characteristic odor. Adhesive and sealing layers on a silicone basis show the following characteristics ... 

Thermoplastic rubber is a relatively new class of polymer. It has the solubility and thermoplasticity of polystyrene, while at ambient temperatures it has the toughness and resilience of vulcanized natural rubber or polybutadiene. These rubbers are actually block copolymers. The simplest form consists of a rubbery mid-block with two plastic end blocks (A-B-A), as shown in Figure 5.7. Examples of commercial products are Kraton and Solprene . These materials are often compounded with plasticizers to decrease hardness and modulus, eliminate drawing, enhance pressure-sensitive tack, improve low-temperature flexibility, reduce melt and solution viscosity, decrease cohesive strength or increase plasticity if desired, and substantially lower material costs. Low levels of thermoplastic rubbers are sometimes added to other rubber adhesives. These materials are used as components in the following applications PSAs, hot-melt adhesives, heat-activated-assembly adhesives, contact adhesives, reactive contact adhesives, building construction adhesives, sealants, and binders. Two common varieties of thermoplastic rubber adhesives are styrene-butadiene-styrene (S-B-S) and styrene-isoprene-styrene (S-I-S). ... 

NR is normally blended with ethylene-propylene-diene rubber (EPDM) to improve the ageing resistance of the former without losing its good mechanical properties. However, due to the difference in unsaturation level between these components, a mutual incompatibility can exist, which decreases the mechanical performance. In addition to the poor interfacial adhesion caused by the thermodynamic incompatibility, these blends usually present cure rate incompatibility because of the differences between the reactivity of the elastomers with the curing agents and/or differences in solubilities of the curatives in each phase. In the case of NR/EPDM blends, the curing system can be consumed by the vulcanization of the NR phase, which is more rapidly vulcanizable because of the higher unsaturation level. ... 

The proportions of these configurations determines the reactivity to vulcanization (1, 2 addition) and the amount of crystallization (trans-1, 4). Neoprene is more e q)ensive than natural rubber or SBR but it is used more frequently for several reasons. Neoprene adhesives based on polymers with high trans-1,4, content crystallize quickly giving high bond strengths. 

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