Networks elastomer adhesion

SS-LINKED POLYMERS ARE USED in a wide variety of aerospace, automotive, building construction, and consumer product applications. Not all paints, adhesives, composites, and elastomers are cross-linked, but cross-linking systems are often used in these applications when resistance to solvents, resistance to high temperatures, and high mechanical performance are required. These important properties can be traced directly to the three-dimensional interconnected molecular network that is characteristic of cross-linked systems. 

Adhesion Analysis in Swollen Elastomer Blended Networks... 

An interpenetrating polymer network (IPN) consisting of an epoxy and an elastomer has been developed by Isayama.29 This is a two-component adhesive-sealant where the components are simultaneously polymerized. It consists of the MS polymer, developed in Japan by Kanegafuchi and commonly used in sealant formulations, with the homopolymerization of DGEBA using a phenol catalyst and a small amount of silane as a graft site to connect the MS polymer and epoxy homopolymer networks. 

Tiquid rubbers are polymeric products used primarily as adhesives, sealants, castable rubbers, and rocket propellant binders. The name liquid rubber comes from the properties of flowing at room temperature and curing to rubbery networks. These materials offer advantages over conventional elastomers in their ease of handling and processing as they can be readily pumped and mixed in low power (relative to normal rubber) equipment with resultant savings. 

These characteristics indicate in particular that one can improve the adhesion between the brush or the pseudo-brush and the elastomer by decreasing the number of monomers between crosslinks, Nc. This result, which is a direct consequence of the de Gennes analogy between a network and a melt (see Sect. 4.1), cannot be correct for too small values of Nc, that is for too highly reticulated networks. For pseudo-brushes, it has been conjectured [107] that Eqs. (20) and (21)... 

Blends of elastomers are routinely used to improve processability of unvulcanized rubbers and mechanical properties of vulcanizates like automobile tires. Thus, cis-1,4-polybutdiene improves the wear resistance of natural rubber or SBR tire treads. Such blends consist of micron-sized domains. Blending is facilitated if the elastomers have similar solubility parameters and viscosities. If the vulcanizing formulation cures all components at about the same rate the cross-linked networks will be interpenetrated. Many phenolic-based adhesives are blends with other polymers. The phenolic resins grow in molecular weight and cross-link, and may react with the other polymers if these have the appropriate functionalities. As a result, the cured adhesive is likely to contain interpenetrating networks. 

Epoxy structural adhesives which employ carboxylic polybutadiene/acrylonitrile solid and liquid (CTBN) elastomers as modifiers have increased in number and proliferated in use since their introduction in the mid- 60 s. Such adhesive systems are now used in aircraft, electronics, automotive and industrial bonding operations. In the mid- 70 s, amine-reactive versions of the liquid polymers (ATBN) were issued, thereby offering another way to introduce rubber modification into a cured epoxy network. References are cited which provide detailed discussions of nitrile rubber, carboxylic nitrile rubber and both carboxyl- and amine-terminated nitrile liquid polymers (1-4). ... 

Cross-linked elastomers have been studied with regard to their moduli (particularly plateau values), effects of peroxide cross linking, adhesive interactions with atomic force microscopy tips, and the effects of phenyl-group modifications. Investigations on networks containing fillers include the effects of silica or polysilicate nanoparticles, zero... 

The cross-link density of the polymer network, as well as its properties, depend on the functionality, the length and the chemical nature of the polyene (R ) and thiol (R) prepolymer chains, and it can thus be tailored as desired. Low-modulus polymers suitable for adhesive applications were obtained by using aliphatic prepolymer chains, in particular with polybutadiene-based elastomers which were cross-linked very efficiently by UV-irradiation in the presence of a tri- or tetrathiol [45-48]. As only a few cross-links need to be formed between the polymer chains to make the rubber insoluble, low concentrations of thiol (2 wt%) proved to be sufficient to achieve an effective and fast cross-finking. Hardening was found to hardly occur upon UV-curing (increase of the Persoz hardness from 40 to 55 s), which is essential to ensure outstanding adhesion. At the same time, the shear adhesion failure temperature (SAFT) increased from 80 to 160°C, due to the formation of the chemical network (Fig. 4). 

Gent and co-workers (83,84) and Lake and Thomas (85) examined the molecular aspects of both cohesive and adhesive failure in elastomeric networks. One of the most interesting results is that the work of fracture per unit area in the cohesive failure of elastomer, is about 30 to 100 J/m, which compares with only a few J/m expected for the theoretical value of a plane of C— C covalent bonds. 

As a result of these reactions the materials eventually crosslink and become set] that is, they lose the ability to flow or to be dissolved. Cure most often is thermally activated hence the term thermoset, but network-forming materials whose cure is light- or radiation-activated are also considered to be thermosets (see Section 2.11, on differential photocalorimetry). Some thermosetting materials, such as certain adhesives, crosslink by a dual-cure mechanism, that is, by either heat or light activation. In contrast to the values for crosslinked elastomers or rubbers, the glass transition temperature of thermosets is generally above room temperature. 

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