Curing Adhesive Tack-free

Some UV adhesives (especially some of the earlier versions) are slightly anaerobic - that is they are inhibited from fully curing by the presence of oxygen in the surrounding atmosphere at the surface. This sticky or partially uncured adhesive layer is only a few microns thick but can attract dust and in some circumstances can contaminate adjacent components. 

The degree of tack-free cure is to some extent subjective and many of the later-generation UV adhesives will be less susceptible to the oxygen inhibition effect. UV cyanoacrylates and UV epoxies, for example, will give excellent tack-free finishes even under relatively low-intensity UVA lamps. 

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When formulating a silicone adhesive, sealant, or coating, based on hydrosilylation addition cure, one must consider the following properties of the uncured product pot life, dispensing technique, rheology, extrusion rate, cure performance. These characteristics directly affect the processing properties of the polymer base or crosslinker parts. The degree of cure conversion at the temperature of interest is determined by properties such as tack free time, cure profile and cure time. Once... 

Reactive diluents can be used to reduce the modulus and increase the elongation of the cured waterborne epoxy formulations just as they are often used for 100 percent solids and solvent-borne epoxy adhesives. The reactive diluents become codispersed in the formulation with mechanical and chemical stability similar to that of the base epoxy emulsion. Polyglycidyl ether of caster oil, phenyl glycidyl ether, and diglycidyl ether of neophenyl glycol are examples of mono- and difunctional reactive diluents that have been used to improve flexibility and increase the tack-free time of waterborne epoxy adhesives. 

Physical properties were evaluated using standard DIN or ASTM specifications. The sealants were filled into Teflon molds to form homogeneous test pieces of comparable thickness. The specimens were then moisture cured and conditioned at 25 °C and 50% relative humidity for 14 days before mechanical property testing. The hardness of the cured sealant samples was measured by Shore A. Shelf life at 50 °C was determined for a maximum of 21 days. Tack-free times were determined by finger touch under ambient conditions. For adhesion testing the substrates were first wiped with either methyl ethyl ketone (aluminum, steel, glass, concrete, wood) or methanol (PVC, PMMA, ABS, polystyrene), then washed with detergent, rinsed with distilled water, and allowed to air dry prior to preparation of the test specimens. Specimens were cured for 14 days at ambient conditions. 

Tests on adhesives to determine their curing or setting behaviour, for example, tack-free time, skin-over time, pot life, and so on. 

Most versatile rubber adhesive. Superior resistance to oil and hydrocarbon solvents. Inferior in tack range but most dry tack-free, an advantage in precoated assemblies. Shear strength of 150-2,000 Ib/in. higher than neoprene, if cured... 

Another way of automating assembly is to use the epoxy adhesive in the front of a silver filled or oxide filled tape. These tapes are made by impregnating the epoxy adhesive into a glass mesh support, B-staging to form a tack-free film, and then slitting the tape into rolls or diecutting the tape into custom-made preforms. In production, the user places a preform under the part to be bonded, or uses a machine to chop or punch preforms from a roll of tape. In either case, the tape provides precise control of bond thickness, eliminates voids, and avoids an excessive fillet. The tapes can be cured very quickly, at temperatures over 160°C, and the controlled thickness of the bond reduces thermal stresses due to thermal expansion mismatch between bonded part and the substrate. 

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