Cross-linked adhesive

Westwood G, Horton TN, Wilker JJ. Simphfied polymer mimics of cross-linking adhesive proteins. Macromolecules 2007 40 3960-3964. 

Addition of typical crosslinking agents,such as 20% diallyl phthalate, 10% ethylene glycol dimethacrylate, 1% maleic anhydride or 1% itaconic anhydride, to the isobutyl 2-cyanoacrylate so that after curing a more rigid,insoluble, hydrolytically stable polymeric adhesive might be formed,does not increase the strength of the dentin-poly(methyl methacrylate) joint. Perhaps the cross-linked adhesive possesses decreased resistance to the thermal shock encountered by the test specimens. 

Many interior fittings (e.g., instrument panel, door and sidewall panels, package trays, seat linings, boot carpetings, rear window shelves, etc.) are often manufactured by vacuum drawing and laminating the cover sheet material (leatherette, textile, leather, etc.) onto the trim panel. The use of hot-melt (also cross-linking) adhesives and water-based polyurethane adhesives (with hardener) is state of the art for this type of component production. 

Newer adhesives of the acrylic, anaerobic or radiation-curable types must, if they are structural, have a relatively high degree of toughness and durability if they are to compete with or challenge epoxy adhesive systems. Likewise, newer radiation-curable, pressure-sensitive adhesive systems must exhibit the properties of permanence largely associated with cross-linked adhesive masses deposited from an acrylic solution polymer base. Epoxy resin structural adhesives largely define the existing area on the one hand the cross-linked acrylics deposited on plastic or metallic films the other. 

The major benefit with this approach is that some degree of toughness can be given to inherently brittle cured epoxy matrices designed to operate at service temperatures above about 135 to 150°C, that is, highly cross-linked adhesives. Further, as most of the thermoplastics used also have TgS at these levels or considerably above, should some polymer remain in the continuous phase after cure, the effect on the final thermal properties will not be as deleterious as with RLP or elastomeric toughening. 

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... 

Figures 2 and 3 show the dependence of the peel strength on the rate of separation. With the non-cross-linked adhesives, copolymer C (-COOH) has high values at the wide range of the peel rates whereas the peel strength does not vary so markedly with the incorporation of hydroxyl groups.

A two-stage process of a permanently soluble primer with a cross-linking adhesive may ensure reversibility for the process (Podany et al 2001). The use of adhesion promoters such as coupling agents should be considered only in extreme conditions of use, e.g. on externally exposed glass, as the chemical bonds with the adherend can be broken only by dissolving away a bit of its surface. 

Which hardener will result in a more cross-linked adhesive ethylene diamine (H2N—CH2—CH2— NH2) or diethylene triamine (HjN—CH2—CH2—NH—CH2—CH2—NH2) ... 

Adhesion is enhanced further by contact time and contact pressure. Both increase the area of the wetted spots kinetically by flow of the adhesive. This is especially important for rough surfaces and cross-linked adhesives. 

Chemical Bonding. Covalent chemical bonds between wood fiber and adhesive through the sharing of electrons is probable in cross-linking adhesive systems. However, the presence of such bonds may not be essential to waterproof adhesive bonds. Intermolecular physical attractions are sufficient for waterproof adhesive bonds. 

For good bond strength and creep resistance at elevated temperatures, the Tg of a cross-linked adhesive or sealant should be above the upper end use temperature. However, when Tg is considerably above the upper end use temperature the peel strength is lowered. 

Cross-linking adhesives which can cure at room temperature may take 2-3 days to achieve 50% of the ultimate strength. Moderate heat has, in these cases, a good accelerating influence and an oven at, say may enable a good cure to be reached within a day. 

Cross-linked adhesives are frequently characterized by conventional solvent swelling methods. The adhesive is placed in a suitable solvent where it swells in volume and would dissolve if not for chemical cross-links. This is often the solvent from which it is applied. The amount the material swells is an inverse measure of the cross-link density and proportional to its extensibility. The soluble fraction is also then collected, dried, and weighed. For conventional acrylics about one third of the material remains soluble after cross-linking. 

Covalent chemical bonds can form across the interface and are likely to occur in cross-linked adhesives and thermoset coatings. This type of bond is usually the strongest and most durable. However, it requires that mutually reactive chemical groups exist. Some surfaces, such as previously coated surfaces, wood, composites, and some plastics, contain various functional groups that under appropriate conditions can produce chemical bonds with the adhesive material. There are ways to intentionally generate these conditions, such as by surface treatment of plastics with techniques like corona or flame treatment. 

FIGURE 11.13 Cure of cross-linked adhesive in a torque rheometer. 

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