Adhesives, applications components

About 8,000 metric tons of peroxides were consumed in 1972. This consumption was strongly stimulated by the rapid growth in reinforced plastics (Ref 23). The largest volume product is benzoyl peroxide which is used in polystyrene and polyester markets for such items as toys, automobiles, furniture, marine, transportation and mil requirements. Also, methyl ethyl ketone peroxide is used in large volumes to cure (as a catalyst) styrene-unsatur-ated polyester adhesive resins used in mil ammo adhesive applications, as well as in glass fiber reinforced plastic products such as boats, shower stalls, tub components, automobile bodies, sports equipment, etc. The monoperesters are growing slowly because of some substitution of the peroxydicarbonates and azo compds (Refs 8,9 23)... 

Adhesion often can be improved, however, by using a primer, which must be applied and dried before applying the top coat. A more convenient and economical process would be the application of a single coat containing an adhesion-promoting component. 

In hot mixing or elevated-temperature curing of an epoxy system, vapor pressure could also be of concern relative to the quality of the adhesive bond. If the components in an epoxy system become too hot, boiling can occur, resulting in gas bubbles. If gas bubbles become trapped in the cured adhesive film, they can lead to reduction of cohesive strength and stress risers. For many adhesive applications, particularly those in the electrical and electronic industries (due to possible ionization of air voids), complete removal of any gas bubbles from the epoxy is essential. 

Of the two groups, the reactive diluents are generally preferred for structural adhesives applications because they do not degrade end properties as much as the unreactive diluents do. Note that diluents, either reactive or unreactive, do not necessarily result in an increase in elongation of the cured epoxy. They may only reduce modulus and hardness. Their enhancement regarding flexibility will depend on the component s functionality and molecular nature. 

A typical starting formulation for a two-component epoxy-polyamide emulsion is shown in Table 14.3. This formulation can be used as either a coating or an adhesive. In either application, once the mixed emulsion is applied to the substrate, the water must be evaporated. In the case of the adhesive application, this must be completed before nonporous... 

A typical product has the following characteristics number average molecular weight, — 1800 carboxyl assay, — 1.1 meq./gram color, amber bulk viscosity (Brookfield synchrolectric viscometer) 70°F., 8000 poises, 100°F., 2000 poises, 150°F., 250 poises, and 250°F., 15 poises. The product can be cured with polyfunctional epoxides, aziridines, and isocyanates for applications such as thermoset and pressure sensitive adhesives, electronic component encapsulation, rocket propellant binders, epoxy flexibilizers, etc. Detailed vulcanization data can be obtained for the 1800 molecular weight material, EMD-590, from the Enjay Chemical Co., Cranford, N. J. 

Eor adhesive applications, it is more convenient to have the two components of the epoxy resin system react at a reasonable rate at room temperature. Replacing the anhydride component by a suitable, more reactive polyfunctional amine enables this to occur at room temperature. Several aliphatic and aromatic polyfunctional amines are used for this purpose (Eq. 21.28). 

The single largest use for the phenol-formaldehyde resins is in adhesive applications for the production of plywood, chipboard, and particle board. The resin can comprise as much as one-third of the weight of the board, particularly of particle boards, which contributes to a total demand for phenolics in the U.S.A. of over half a million metric tonnes per year. They are also used as the matrix adhesives for the production of several types of grindstones. In combination with paper, woven cotton, glass fiber, etc., components, phenolics contribute to the production of engineering and decorative laminates in the form of rods, tubes, and sheets. The sheet products Arborite and Formica are familiar as the finished surfaces of furniture, bathroom, and kitchen counter tops and other areas where attractive patterns and water resistance are important characteristics. Molded products from phenolics are also important where heat or electrical resistance is required, such as saucepan and toaster handles, switches, and the printed circuit boards used in computers. Recent phenolics production in the U.S.A. has totaled over 500,000 metric tonnes per year, not including fillers [38]. 

Since these one-component polyurethane adhesives cure through reaction with water, one talks of moisture curing one-component polyurethane adhesives. Their characteristic feature is their excellent adhesiveness. Apart from adhesive applications, these formulations are applied to a great extent as sealing compounds, for example, as fitting foam glue or PU-foam (Section 4.9). 

Solvent-based adhesives, two-component epoxy resin adhesives, two-component methacrylates for assembly purposes (priming, filling, smoothing, sealing, etc.) For these applications, a multitude of so-called building/ construction adhesives is commercially available. 

Polymers are also used in adhesives and Avalon [11] noted that TG can be used in combination with a database of information about the components of the product and the product itself to compare the maximum mass loss temperature and profile of the TG curve to provide a quantitative analysis of the product. The database of information can be either a strength or weakness of this approach, depending on the completeness of the data available for the comparative work. The use of DSC, DMA/TMA and TG were also discussed [12] relative to adhesive applications. Among the properties examined were % crystallinity, Tg, cure state, environmental stability, blend ratios, modulus and damping, thermal expansion and shrinkage, composition, volatiles and plasticizer levels. 

Adhesion properties of epoxies, complete reactivity with no volatiles during cure, and minimal shrinkage make the materials outstanding for adhesives, particularly in structural applications. The most commonly known adhesive applications involve the two-component liquids or pastes, which cure at... 

Major benefits of this type of application are as follows weight reduction augmentation of mechanical fit reduction of mechanical fit with subsequent reduction of hoop stress ease of machining of gear teeth of separate components as opposed to machining cluster cheaper capital outlay, for example, adhesive application assembly machine costs 100,000 compared to electron beam welding machine, which costs 500,000 easier replacement of gears. 

Apart from their use as tie layers in coextension, the modified polymers can find other adhesive applications such as improving adhesion of extrusion coatings, thermal lamination interplies, and as dispersions or powder coatings. As coatings, the modified polymers applied to metal surfaces can act as an adhesive layer to bond plastic components. A particularly useful version is to prepare a coextended film of a non-modified base polymer such as polyethylene or polypropylene, forming the bnlk of the structure, and apply a thin layer of the modified polymer to one or both external surfaces. This steucture may now be used to thermally bond to a metal substrate. 

An acceptable static mixer should develop 30,000-40,000 laminar deviations with a pressure drop not to exceed 200 psi, should have low shear, and should be short in overall length. Static mixers are located on extruders, mix-meter-and- dispense equipment, adhesive applicators, and other equipment where two or more components must be quickly blended together. Many major equipment manufacturers supply static mixers with their Unes. 

AdhesiVBS and Sealants. Most industrial adhesives contain surface-active components and additives, and air entrainment during their mechanical application can significantly reduce joint strength. Defoamers are usually formulated into adhesives to protect users against such difficulties. Additional benefits such as improved uniformity of products, increased throughput, and reduced labor costs can also result from the use of defoamers during adhesive application. The footwear and nonwoven fabric industries are extensive users of defoamers in this way. 

One-component polyurethane adhesive Adhesive containing isocyanates ernes on exposure to moisture or to heat, when latent hardener is included Primer Special coating designed to improve adhesion between adhesive and substrate - applied before adhesive application Reactive adhesives Adhesives that cure or set when exposed to heat, moisture, radiation, etc. 

---