Tackiness of elastomer

Barquins, M. and Maugis, D., Tackiness of elastomers. J. Adhes., 13, 53-65 (1981). Creton, C. and Lakrout, H., Micromechanics of flat-probe adhesion tests of soft vi.scoelas-tic polymer films. J. Polym. Sci. B Polym. Phys., 38(7), 965-979 (2000). 

While there are a large number of elastomers that can be formulated into pressure sensitive adhesives, the following list is intended to focus on commercially significant materials. Two subsets are differentiated in Table 1 those polymers that can be inherently tacky, and those that require modification with tackifiers to meet the Tg and modulus criteria to become pressure sensitive. 

Working with less dilute solutions of elastomers one cannot fail to notice the influence (the stiffer the greater the effect) of molecular structure on the onset and course of non-Newtonion flow, on gelation and swelling, and the influence of the solvent as expressing itself by virial coefficients, molecular dimensions in solution, spinnability, and film forming. The sensitivity with which the tack of adhesives, demonstrated by pressure sensitive tapes which at that time reached the market, depends on the structure and composition of the elastomer was similarly striking and raised the question, which molecular structure or state was best suited to exhibit tacky adhesion, or adhesion per se. 

Pressure-sensitive adhesives (PSAs) are a class of elastomer-based materials that have the foUowing characteristics they are aggressively and permanently tacky, they adhere without the need of more than finger pressure, they require no heat or activation, they adhere well, and they can be... 

Among the different pressure sensitive adhesives, acrylates are unique because they are one of the few materials that can be synthesized to be inherently tacky. Indeed, polyvinylethers, some amorphous polyolefins, and some ethylene-vinyl acetate copolymers are the only other polymers that share this unique property. Because of the access to a wide range of commercial monomers, their relatively low cost, and their ease of polymerization, acrylates have become the dominant single component pressure sensitive adhesive materials used in the industry. Other PSAs, such as those based on natural rubber or synthetic block copolymers with rubbery midblock require compounding of the elastomer with low molecular weight additives such as tackifiers, oils, and/or plasticizers. The absence of these low molecular weight additives can have some desirable advantages, such as ... 

Besides the higher volume pressure sensitive adhesives discussed above, the industry also uses other synthetic elastomers as the base component for PSA formulation. Most of these elastomers require some form of tackification to make the materials tacky. However, a few materials are low enough in Tg and sufficiently compliant to be useful without requiring compounding with tackifiers. 

This mbber is very tacky in nature and contains acrylic group, which makes it polar in nature. Nanocomposites have been prepared based on this elastomer with a wide range of nanohllers. Layered silicates [53-55] have been used for this preparation. Sol-gel method [56,57], in situ polymerization [58], and nanocomposites based on different clays like bentonite [59] and mica [60] have been described. The mechanical, rheological, and morphological behaviors have been investigated thoroughly. 

The formulated RTV silicone is usually cured at room temperature for 16 hours and then at 120°C for 4 hours to ensure the complete removal of organic solvent. A rubbery and non-tacky elastomer is usually obtained after the curing cycle. 

A good elastomer should not undergo plastic flow in either the stretched or relaxed state, and when stretched should have a memory of its relaxed state. These conditions are best achieved with natural rubber (ds-poIy-2-methyl-1,3-butadiene, ds-polyisoprene Section 13-4) by curing (vulcanizing) with sulfur. Natural rubber is tacky and undergoes plastic flow rather readily, but when it is heated with 1-8% by weight of elemental sulfur in the presence of an accelerator, sulfur cross-links are introduced between the chains. These cross-links reduce plastic flow and provide a reference framework for the stretched polymer to return to when it is allowed to relax. Too much sulfur completely destroys the elastic properties and produces hard rubber of the kind used in cases for storage batteries. 

Self-diffusion is the exchange of molecules in a homogeneous material by a kind of internal flow. It has a direct bearing on tackiness, which depends on interpretation by diffusion of polymer molecules at the interface. This effect is well known in elastomers. 

From an environmental standpoint, the use of water as a solvent is desirable. Unfortunately, many of the monomers of interest are insoluble in water, but suspension polymerization offers a way to utilize water. Suspension polymerization is performed by mechanically dispersing a monomer in an incompatible solvent, most often water. The system is heterogeneous and when polymerization is complete the polymer is collected as granular beads. This method is not suitable for tacky materials, such as elastomers, as the beads will tend to clump together.31... 

The concept of traditional thermoset elastomers was pioneered by Goodyear s discovery in 1839 that heating natural rubber with some sulfur converted the material from one that was tacky when warm and brittle when cold into a vulcanized rubber that was conveniently useful over a wide temperature range. Crosslinking of the macromolecules of rubber with sulfur bonds endowed the naturally occurring material with some elastic memory and caused it to behave as we have come to expect elastomers to behave. Excessive sulfur crosslinking converts the stretchable, compressible, bouncy rubber into hard rubber such as the material found in the heads of mallets used in machine shops to pound sheet metal into desired shapes. A small dose of crosslinking prevents the macromolecules of natural rubber to crystallize at low temperatures and turn into a brittle solid and to become a tacky, sticky semifluid at elevated temperatures. 

Film adhesives require an outside means such as heat, water, or solvent to reactivate them to a tacky state. Among the film types are some hot melts, epoxies, phenolics, elastomers, and polyamides. Film adhesives can be die cut into complicated shapes to ensure precision bonding of unusual shapes. Applications for this type of adhesive include bonding plastic bezels onto automobiles, attaching trim to both interiors and exteriors, and attaching nameplates on luggage. 

The process of vulcanization was discovered independently by Goodyear (in 1839) in the U.S. and Hancock (in 1843) in the U.K. Both found that when natural rubber was heated with sulfur, the undesirable properties of surface tackiness and creep under stress could be eliminated. The chemical reaction involves the formation of interchain links, composed of two, three, or four sulfur atoms, between sites of unsaturation on adjacent chains. It has been foimd that about three parts sulfur per hundred parts rubber produces a useful elastomer, capable of reversible extensions of up to 700%. Increasing the sulfur, up to 30 parts per hundred, alters the material drastically and produces a hard, highly cross-linked substance called ebonite. The actual mechanism of the cross-linking reaction is still unclear, but it is thought to proceed via an ionic route. 

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