Plasticizers natural rubber adhesives

A variety of plasticizers can be used in adhesives and sealants as to their primary resin type. Paraffinic oils, phthalate esters, and polybutenes are typical plasticizers (Dostal 1990). Plasticizers for natural rubber adhesives, such as mineral oil or lanolin, are used to reduce the cost of the adhesive mass, and have a depressing effect on the peel adhesion (Satas 1999). Phthalates, chlorinated hydrocarbons, and aliphatic hydrocarbons are commonly used as plasticizers in urethane sealants (Dostal 1990). Most of sealants, except for silicones, contain plasticizers in their formulations. Silicone sealants can be plasticized only by low molecular weight silicone oils (Petrie 2000). 

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 ... 

Other polymers used in the PSA industry include synthetic polyisoprenes and polybutadienes, styrene-butadiene rubbers, butadiene-acrylonitrile rubbers, polychloroprenes, and some polyisobutylenes. With the exception of pure polyisobutylenes, these polymer backbones retain some unsaturation, which makes them susceptible to oxidation and UV degradation. The rubbers require compounding with tackifiers and, if desired, plasticizers or oils to make them tacky. To improve performance and to make them more processible, diene-based polymers are typically compounded with additional stabilizers, chemical crosslinkers, and solvents for coating. Emulsion polymerized styrene butadiene rubbers (SBRs) are a common basis for PSA formulation [121]. The tackified SBR PSAs show improved cohesive strength as the Mooney viscosity and percent bound styrene in the rubber increases. The peel performance typically is best with 24—40% bound styrene in the rubber. To increase adhesion to polar surfaces, carboxylated SBRs have been used for PSA formulation. Blends of SBR and natural rubber are commonly used to improve long-term stability of the adhesives. 

Acrylonitrile-butadiene rubber (also called nitrile or nitrile butadiene rubber) was commercially available in 1936 under the name Buna-N. It was obtained by emulsion polymerization of acrylonitrile and butadiene. During World War II, NBR was used to replace natural rubber. After World War II, NBR was still used due to its excellent properties, such as high oil and plasticizer resistance, excellent heat resistance, good adhesion to metallic substrates, and good compatibility with several compounding ingredients. 

Plasticizers can be classified according to their chemical nature. The most important classes of plasticizers used in rubber adhesives are phthalates, polymeric plasticizers, and esters. The group phthalate plasticizers constitutes the biggest and most widely used plasticizers. The linear alkyl phthalates impart improved low-temperature performance and have reduced volatility. Most of the polymeric plasticizers are saturated polyesters obtained by reaction of a diol with a dicarboxylic acid. The most common diols are propanediol, 1,3- and 1,4-butanediol, and 1,6-hexanediol. Adipic, phthalic and sebacic acids are common carboxylic acids used in the manufacture of polymeric plasticizers. Some poly-hydroxybutyrates are used in rubber adhesive formulations. Both the molecular weight and the chemical nature determine the performance of the polymeric plasticizers. Increasing the molecular weight reduces the volatility of the plasticizer but reduces the plasticizing efficiency and low-temperature properties. Typical esters used as plasticizers are n-butyl acetate and cellulose acetobutyrate. 

Finally, the solubility parameter of the adhesive and the substrate must be close. Without getting too teehnieal, the solubility parameter is a rough estimate of polarity. The old saying like dissolves like can be extended to like bonds like. More aeeurately, the solubility parameter is the ealeulated potential energy of 1 em of material for eommon solvents. Polymers are assigned solubility parameters of solvents in which they are soluble. Table 19.3 lists solubility parameters for various solvents and polymers. As an example of how to use this table, butadiene-acrylonitrile rubber with 6= 9.5 bonds natural rubber (6= V.9-8.3) to phenolic plastics (6= 11.5). Note that its solubility parameter is between that of the two substrates. 

Natural rubber - [RUBBERCOMPOUNDING] (Vol 21) - [ELASTOMERS, SYNTHETIC - SURVEY] (Vol 8) -in adhesives [ADHESIVES] (Vol 1) -cellular forms [FOAMED PLASTICS] (Vol 11) -compared to polyisoprene [ELASTOMERS SYNTHETIC - POLYISOPRENE] (Vol 9) -fluonnahon of [FLUORINECOMPOUNDS,ORGANIC - DIRECTFLUORINATION] (Vol 11)... 

With increasing frequency, the permittivity of dielectric decreases. A major factor in the selection of insulation is the ability of the insulation to resist the absorption of moisture. Moisture, of course, can greatly lower resistivity. For wire insulation, synthetic polymers and plastics essentially have replaced the use ol natural rubber. Usually, prior to coaling a wire with a plastic material, (lie wire must he treated to assure good contact and adhesion of the insulating material. Copper wire, for example, is treated with hydrogen fluoride, which creates a coating ol clipper fluoride in the... 

Latex An emulsion of rubber or resin particles dispersed in an aqueous medium. Latex can be used to make plastic films and adhesives. Natural latex is derived from the sap of a rubber tree. Synthetic latex is manufactured in a number of different varieties from basic chemicals. 

The major use of Heveaplus MG is in adhesives. It gives good bond strength for natural rubber to PVC and therefore, it is particularly useful in shoe manufacturing. It is used as a compatibilizer in plastic-rubber blends. It is also used in applications such as automobile bumpers due to its self-reinforcing nature. 

Cold-seal adhesives are members of a general category sometimes termed coadhesive substances that have a great tendency to stick to themselves, but often not to much else. Cold-seal adhesives are typically based on natural rubber, which has been applied in a latex (suspension in water) form. One major application is in plastic packaging for chocolate candy. The seal between the two parts of the wrap can then be activated with pressure, as an alternative to heat-sealing, which is problematic with the low-melting temperature candy. 

Solution adhesive is obtained from solid rubber obtained by coagulation of latex as acquired from the tree. The coagulation is effected by dilute aqueous solution of organic acids. The solid rubber can be graded in terms of dirt content, ash content, nitrogen content, and volatile matter. It can also be categorized by plasticity retention index and Mooney viscosity. To maintain uniformity it is necessary to choose solid natural rubber of known characteristics. 

In addition to the benefits of low odor and redueed fogging, these adhesives form stronger bonds to low-energy substrates sueh as EPDM rubber, natural rubber, and other difficult-to-bond plastics. This property seems to be a funetion of the solvent action of the uncured adhesive, so care must be taken to avoid stress eraeking when the adhesive is used on sensitive substrates sueh as polyearbonates and polyaerylates. 

Chlorinated rubber resins are produced by the chlorination of synthetic and natural rubbers. The chemical structure shown in Fig. 13.3 is synthesized by the addition of chlorine to unsaturated double bonds until the resin contains 65% chlorine. These resins have similar properties to those of vinyl resins. Plasticizers are added to increase elasticity and resin adhesion. These coatings are resistant to water but have poor resistance to sunhght. 

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