Adhesive systems, applications

J. Hebling, E.M. Giro, C.A. Costa, Human pulp response after an adhesive system application in deep cavities, J. Dent. 27 (1999) 557-564. 

Rubber base adhesives, also called elastomeric adhesives, are widely used in industrial and household applications. In fact, about one-third of the adhesives used in the World are made from natural or synthetic rubbers. Some of the elastomeric adhesive systems showing industrial importance in recent years are the following ... 

In general, fully compatible resin are desirable. However, there are many applications where borderline compatibility is tolerated, and even in some cases, borderline compatibility or controlled incompatibility may enhance tack in adhesive systems. On the other hand, a resin with a borderline compatibility in combination with an oil or plasticizer in an adhesive formulation, will result in phase separation and therefore the migration of the oil or plasticizer to the adhesive surface is favoured. 

One key consideration in developing radiation curable adhesive systems is the thermal stability and volatility of any photoinitiators used. These chemicals are designed for liquid systems where these issues do not arise. Few of the commercial photoinitiators have adequate thermal stability at the highest hot melt temperatures (180-200°C) and many are too volatile. Reduced application temperatures and special antioxidant packages are often required. 

PVA in, 25 617 setting speed of, 25 579-580 smectites application, 6 697t solution, 1 532-534 structural, 1 534-545 styrenic block copolymers in, 24 714 use of latex in, 14 711-712 vinyl acetate polymers in, 25 578-583 viscosity of, 25 581 water-borne, 25 475 Adhesive systems, microencapsules in, 16 460... 

This paper discusses how tightening environmental regulations in the USA are encouraging a shift from solvent-based to waterborne adhesives for use in flexible packaging applications. It also looks at how new adhesive systems have a relatively easy path to Food Drug Administration clearance for food packaging uses. 

Uses. The. motivation for first recovering NBA in the 1920s was its use as a lacquer solvent. That application is even stronger today. The NBA vapors from lacquer drying are nontoxic and virtually nonflammable. Other fast growing uses for NBA are plasticizers and chemical intermediates, mostly for esters and ethers used in water-based coatings and adhesives systems. 

Much of the focus of the following work was aimed at the previously mentioned Chemlok adhesive system. This was because it had been used extensively in a familiar marine application, we were acquainted with its long term performance so it could be used as a baseline for comparison with other commercial adhesive formulations. All of the commercial adhesives used, including Chemlok 205/220, are proprietary formulations, thus the exact functional nature of each adhesive is not available. 

At least one conclusion can be drawn from all of this. Primers that experience large increases in swelling in the presence of hydroxide ion, despite stability and considerable bond strength in salt water, would probably perform poorly if a cathodic potential was applied. Thus, the qualification of an adhesive system for a marine application that includes exposure of the metal substrate to a cathodic potential, should Include examination of the... 

The advanced applications for nitrocellulose plastisol propellants require that they be integrally bonded to the motor case. Successful case bonding for the multiyear storage life of a rocket calls for special adhesives and liners which are completely compatible with these highly plasticized propellants. Best results have been obtained with a combination of an impervious rubber liner and a crosslinked adhesive system with a limited affinity for the plasticizers used in the propellants. Examples of effective liners are silica-filled butyl rubber and chlorinated synthetic rubber. Epoxy polyamides, isocyanate-crosslinked cellulose esters, and combinations of crosslinked phenol-formaldehyde and polyvinyl formal varnishes have proved to be effective adhesives between propellant and impervious liners. Pressure curing of the propellants helps... 

In order to accurately determine the locus of failure of adhesion systems, the chemisti y of the fracture surfaces must be analyzed using surface-sensitive characterization techniques. Many surface analysis techniques are presently available and each technique is based on an intrinsic property of the surface atoms or molecules. Lee155 , Czanderna 156) and Park 157) have reviewed these techniques. However, they suggest that one be aware that new techniques and applications are continually being introduced. 

Primer - A coating applied to the surface of a material, prior to the application of an adhesive sometimes considered as a part of the adhesive system. 

The adhesive used in virtually all softwood plywood has a phenol—formaldehyde (PF) base to provide an exterior-grade, durable, waterproof bond. Thus, most grades of plywood can be used in structural applications. A very small percentage of softwood plywood is made using interior-grade adhesive systems, and this material is used in interior cabinetry, furniture, and shelving. 

Formulators in the adhesives industry do not normally manufacture epoxy resins. Generally, formulators buy epoxy resins, modify them with other materials, do similar compounding to the curative, and then package the product as a complete adhesive system ready for the end user. There are many excellent textbooks6-8 available giving information about the preparation, chemistry, and use of epoxy resins in general applications. It is not the intention here to go into such detail but to focus only on epoxy adhesive systems. 

Over the past several decades, significant advances have been made in developing epoxy-based adhesives having improved performance over these early adhesive systems. These improvements were made possible by (1) the incorporation of toughening additives into epoxy resin formulations and (2) the use of multifunctional epoxy resins primarily for high-temperature applications. These innovations are discussed in later chapters. 

As shown in Table 1.7, the epoxy resin is rarely used unmodified as an adhesive system. Rather, it is used in the form of a compound containing various modifiers and additives to improve properties, such as strength, flow, and heat resistance, and to add or advance other properties that are demanded by the specific application. 

These are essential considerations for a practical adhesive system. The adhesive composition must have sufficient reactivity to cure under conditions that are convenient and practical to the end user. It must have viscosity that allows easy mixing and application. Once on the substrate, the adhesive must be able to flow over the substrate and come into intimate contact with it—a process called wetting. 

In certain cases it is necessary for the adhesive formulator to reduce the viscosity of the adhesive system to achieve better application and wetting characteristics. Wetting (see below), as measured by the contact angle that the adhesive makes on the surface, is not governed by the viscosity of the adhesive. However, the rate and ease with which the adhesive wets the surface of the substrate and fills in the peaks and valleys on the surface are a function of viscosity. Viscous adhesives could require an impractical amount of time to adequately wet the surface of a substrate. 

Since slower-curing epoxy adhesives systems flow over and wet high-energy surfaces very well, there is little chance for air to become trapped at the interface. As a result, mechanical abrasion is often recommended as a substrate surface treatment prior to application of the epoxy adhesive. The added surface area and the mechanical bonding provided by the additional peaks and valleys on the surface will enhance adhesive strength. If the adhesive does not wet the substrate surface well, such as in the case of epoxy resin on polyethylene, mechanical abrasion is not recommended since it will only encourage the probability of gas voids being trapped at the interface. 

The syntheses of commercial epoxy resins that are commonly used in many applications were discussed in Chap. 2. Additional information is provided in this chapter with regard to the physical and chemical properties of certain epoxy resins relative to their use in adhesive systems. 

Solid epoxy resins are usually formulated as solvent solutions and blends with lower-MW resins for the production of liquid adhesive systems. However, solid epoxy resins are also often employed in the manufacture of adhesive systems having solid form. There are several forms of solid epoxy adhesives that find application. The most common are supported or unsupported film, powder, and solder stick. Formulations for these adhesives are detailed in Chap. 13. 

In adhesive systems, vinyl esters impart low-viscosity, flexibility, and superior wetting characteristics to DGEBA-type epoxy resins. Because of their good chemical resistance they are often found in construction applications such as flooring and grouting. However, their shrinkage is greater than that of any conventional epoxy resins, and often the formulator will have to counteract this. 

To lower the viscosity to provide easier mixing of multicomponent adhesive systems and dispensing at the application stage... 

When used in epoxy adhesive systems, solvents are generally employed for reducing the viscosity for formulation or application purposes. Once the adhesive is applied to the substrate, the solvent must evaporate prior to cure. Otherwise, bubbles or vapor pockets could form in the bond line, causing a physically weak joint with poor adhesion. The solvent in the adhesive formulation must not adversely affect the substrate to which it is applied. Plastics, elastomers, and polymeric foams are especially sensitive to certain solvents used in epoxy adhesives. 

In certain applications, such as in the electrical and electronic industries, adhesive systems must have a degree of electrical and/or thermal conductivity. Electrical conductivity is, of course, important in adhesives that must make electrical interconnection between components and in adhesives that must provide electromagnetic or radio-frequency interference (EMI and RFI) functions. 

Thermal conductivity is also important in highly integrated electronic applications where the heat generated by components must be transferred to a heat pipe or by some other means outside the electronic package. Thermal conductivity within adhesive systems is also a means of reducing exotherm and stresses that could develop during the curing cycle or other excursions to elevated temperatures. 

Thermal Conductivity. Many of today s electronic products feature miniaturization. In these applications, higher thermal conductivity and better dimensional stability are required from adhesive systems. Thermal management has become a significant area of development, and thermally conductive adhesives provide a way to transfer heat away from sensitive electronic components. 

These adhesives have found their way into several niche markets where their advantages, such as fast setting speed, are highly valued. They have not seen application in the more ordinary markets because of their materials and equipment cost. However, it is expected that these epoxy adhesive systems will grow at a faster annual rate than the average market as their advantages become more widely known. 

One of the most popular uses of radiant curing is the advancement (viscosity increase) or crosslinking of pressure-sensitive adhesives. These applications have been satisfied mostly with acrylate-based adhesive systems. With epoxy-based adhesives, the main applications are electrical and electronic components, the bonding of large aerospace structures such as composites, and the bonding of transparent substrates such as glass and plastic. 

Many of the attributes of solvent-borne epoxy coatings could be carried over to the waterborne epoxy coatings. These same attributes are useful in the application of waterborne epoxies as adhesive systems. They include good adhesion to a variety of substrates such as metals, wood, concrete, glass, ceramics, and many plastics chemical resistance low shrinkage toughness and flexibility and abrasion resistance. 

Many epoxy dispersions are compatible with most types of latex emulsions including acrylic, urethane, styrene butadiene, vinyl chloride, and polyvinyl acetate. The epoxy dispersion can be used as a modifier for these emulsions to alter handling and application characteristics such as emulsion rheology, foaming tendencies, pH sensitivity, wetting properties, and coating coalescence. They can also be reacted into the latex resin either by reacting the epoxy with a functionalized latex or by use of an epoxy with a coreactant. In this way adhesive systems can be formulated that are cured at room or elevated temperatures. 

Induction Curing Adhesives. Typically one-component epoxy adhesive systems are used for induction bonding of structural parts. However, two-component adhesives have also been used. The properly designed induction cured adhesive should provide the following application and performance properties ... 

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