Pressure-sensitive and contact bonding

While specific methods of joint assembly may differ in the details, they can be classified into one of four basic types wet assembly, pressure-sensitive and contact bonding, solvent activation, and heat activation. 

Adhesives and sealants are manufactured from a variety of polymers. Their selection and their combinations used impact solvent selection. Most solvent systems are designed to optimize the solubility of the primary polymer. Adhesives can be divided into ones which bond by chemical reaction and ones which bond due to physical processes. Chemically reactive adhesives are further divided into three more categories for those that bond through polymerization, polyaddition, or polycondensation. Physically bonding adhesives include pressure sensitive and contact adhesives, melt, or solution adhesives, and plastisols. Polymerization adhesives are composed of cyanoacrylates (no solvents), anaerobic adhesives (do not contain solvents but require primers for plastics and some metals which are solutions of copper naphthenate), UV-curable adhesives (solvent-free compositions of polyurethanes and epoxy), rubber modified adhesives (variety solvents discussed below). 

Nitrile rubber is used preferably for contact adhesives with improved plasticizer resistance. Polyisobutylene is used in pressure-sensitive adhesives. Butyl rubber is sometimes added to pressure-sensitive and hot-melt adhesives, although it is mainly used in sealing compounds. Epoxy resins and reactive (meth)acrylate adhesives (reactive adhesives) are modified with polychloroprene, butyl, and nitrile rubber. Chlorinated rubber is added in small quantities to contact adhesives and also to rubber-to-metal bonding agents for improving the adhesion properties. 

After some early uncertainty in the literature about the nature of the pressure sensitive bond, Dahlquist [5,6] related modulus data to tack-temperature studies and observed that the compression modulus of the adhesive had to be less than about 3 X 10 dyne/cm (3 x lO Pa) before any adhesive tack was observed. This was explained as the highest modulus that still allowed the adhesive to be sufficiently compliant to wet out or come into molecular contact with the substrate and form dispersive bonds. As other investigators [7-9] accepted this requirement it was termed the Dahlquist Criterion . 

Propyhdyne formed from propene on lr4 supported on y-Al203 was observed by IR and NMR spectroscopies [38]. When ethene or propene was brought in contact with oxide-supported lr4 [39,40], Ire [39,40], or Rhe (A.M. Argo and B.C. Gates BC, impubhshed results) in the presence of H2, hydrocarbon hgands were formed (namely, alkyls and /r-bonded alkenes), which have been inferred from IR spectra to be intermediates in hydrogenation to make alkanes, as discussed later. The population of these hydrocarbon ligands on the supported clusters depends sensitively on the conditions, such as reactant partial pressures and temperature. 

National legislation on adhesives includes German BfR Recommendation XXVIII (Components of adhesives) and US FDA Title 21 Part 175 on adhesives. Part 175.105 deals with adhesives overall and Part 175.125 deals with pressure-sensitive adhesives. In addition, FDA Part 177.1390 permits the use of certain high temperature laminates that may be safely used for food contact at temperatures up to 275 °F, given that only the specified adhesives are used to bond the layers that make up the laminate. Part 177.1395 permits certain laminates that may be safely used at temperatures between 120 °F and 250 °F. However, it does not specify permitted adhesives. 

Pressure-sensitive adhesives are the essential components of adhesive tapes and labels. They are polymers with permanent tack, usually applied on substrates (plastic/metal films, siliconized papers). To enhance their tack, compounds with high inherent tack are added, for example, resins, plasticizers. Pressure-sensitive adhesives reach their adhesion on the material to be bonded by contact pressure, from which the term pressure-sensitive adhesive (PSA) derives. Apart from electron radiation, also UV-radiation curing described in Section 4.3.2 is applied in adhesive tape manufacturing. The monomer molecules to be polymerized are applied, in liquid form, to the substrates to be coated by rolling and are continuously cured to a polymer layer within seconds under a UV-radiation source. Depending on their composition, predetermined adhesion values can be adjusted. The adhesive tapes can be subsumed under the systems shown in Figure 5.4 ... 

Tip When bonding with pressure-sensitive adhesives, first degrease the surface of the material, then warm the adhesive area and the pressure-sensitive adhesive layer with hot air (hair-dryer) and fix it immediately under pressure. This procedure renders the adhesive layer more flexible and enables better levelling out of the surface structures. Moreover, the contact surface will be extended. 

Adhesive - A material, usually polymeric, capable of forming permanent or temporary surface bonds with another material as-is or after processing such as curing. Used for bonding and joining. Some of the classes of adhesives include hot-melt, pressure-sensitive, contact, UV cured, emulsion, etc. 

Contact-Bond Adhesives. Another group of glues, referred to collectively as contact cements or contact-bond adhesives, bear some similarities to the pressure-sensitive adhesives in that they do not rapidly achieve full hardening. Until a few years ago, contact-bond formulations were mainly organic solvent solutions of elastomers, compounded with tackilying resins and antioxidants. However, with recent concerns over energy, environmental, and fire safety questions, interest has turned to water-based systems. 

Variety of bonding methods The numerous forms and types of rubber-based adhesives and sealants also provide for numerous mechanisms for developing bonds to surfaces. Contact bond, pressure sensitive, wet bond, heat reactivation, and solvent reactivation are all feasible modes of product assembly with these products. 

High initial tack Even without tackifying resin, the inherent tack of certain grades provides for excellent contact bond and pressure sensitive adhesives. 

Solvent solutions and latex cements require the removal of the solvent from the adhesive before bonding can take place. This is accomplished by simple or heat-assisted evaporation. Some of the stronger or more environmentally resistant rubber-based adhesives require an elevated-temperature cure. Only slight pressure is usually required with pressure-sensitive adhesives (PSAs) to obtain a satisfactory bond. These adhesives are permanently tacky and flow under pressure, thus they provide intimate contact with the adherend surface. 

These methods are usable only when the adhesive retains some tack when dry. The only difference between the two techniques is that in pressure-sensitive bonding only one adherend is coated versus both surfaces in contact bonding. In both techniques, joint assembly consists of permitting the adhesive coating to dry completely, then aligning the parts and pressing them together to form the bond. 

Standard tests used to characterize the adhesion properties of tapes are for the assessment of shear strength (see Shear tests) (the ability of a tape joint to resist a load applied in the shear mode), peel strength (see Peel tests) (the resistance of a tape joint to peeling under specified conditions) and Tack (the ability of a pressure-sensitive adhesive to form a bond immediately on contact with another material). There are many standard test specifications laid down by different authorities to assess these properties and many differences in detail between them (e.g. see Appendix). No attempt will be made to describe them comprehensively, but the principles of the tests will be discussed separately. 

Adhesives may also be classified by the way they are applied or cured. Hence, anaerobic adhesives are adhesives that set only in the absence of air, for instance, when confined between plates or sheets. A contact adhesive is one that is apparently dry to the touch but will adhere to itself instantaneously on contact (also called contact bond adhesive and dry bond adhesive). A heat-activated adhesive is a dry adhesive film that is made tacky or fluid by application of heat or heat and pressure to the assembly. A pressure-sensitive adhesive is a viscoelastic material that in solvent-free form remains permanently tacky. Such materials will adhere instantaneously to most solid surfaces with the application of very slight pressure. Room-temperature setting adhesives are those that set in the temperature range of 20-30°C. These are usually two-component adhesives that must be mixed before application. A solvent adhesive is an adhesive that has a volatile organic liquid as a vehicle and sets or becomes tacky after the solvent has evaporated. A solvent-activated adhesive is a dry adhesive film that is rendered tacky just before use by applicafion of a solvent. 

Resins based on para-substituted phenols can be either one-step or two-step, but they cannot cure to a thermoset state. In the manufacture of phenolic resins, smaller quantities of acetaldehyde and furfuraldehyde are used in addition to formaldehyde. Furthermore, resorcinol, bisphenol A, and p-alkylphenols are employed, in addition to phenol, when special properties are desired. Formaldehyde concentrations of 37-50 weight % in aqueous solutions are most commonly employed. The catalysts most frequently used are acids such as oxalic, hydrochloric, sulfuric, p-toluenesul-fonic, and phosphoric and bases such as sodium, calcium, and barium hydroxide. In the weakly acidic range metal carboxylates are employed. Thermoset phenolic resins are employed as structural adhesives for laminating and bonding applications. Para-alkyl-substituted resins are employed as tackifiers in contact adhesives, pressure-sensitive adhesives, and hot-melt adhesives. 

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