Epoxy adhesives substrates

Epoxy resins are also used in special appHcations, such as an overlaying procedure requiring a durable, heat-resistant bond of a difficult-to-bond overlay on a wood-base panel substrate. Metal sheets used as overlays, for example, often require an epoxy adhesive. 

Although the above experiments involved exposure to the environment of unbonded surfaees, the same proeess oeeurs for buried interfaces within an adhesive bond. This was first demonstrated by using electrochemical impedance spectroscopy (EIS) on an adhesive-covered FPL aluminum adherend immersed in hot water for several months [46]. EIS, which is commonly used to study paint degradation and substrate corrosion [47,48], showed absorption of moisture by the epoxy adhesive and subsequent hydration of the underlying aluminum oxide after 100 days (Fig. 10). After 175 days, aluminum hydroxide had erupted through the adhesive. 

Many types of chemical treatment are used in industry. Chromic, permanganic, sulphuric, and chlorosul-phonic acids are often used as the oxidants. It has been shown that the adhesion of polyethylene to substrates, such as cellophane, steel, aluminium, and epoxy adhesives, improves upon pretreatment with any of the etchants mentioned previously. 

Adhesives and sealers can be an important part of a total corrosion protection system. Structural bonding procedures and adhesives for aluminum, polymer composites, and titanium are well established in the aerospace industry. Structural bonding of steel is gaining increasing prominence in the appliance and automotive industries. The durability of adhesive bonds has been discussed by a number of authors (see, e.g., 85). The effects of aggressive environments on adhesive bonds are of particular concern. Minford ( ) has presented a comparative evaluation of aluminum joints in salt water exposure Smith ( ) has discussed steel-epoxy bond endurance under hydrothermal stress Drain et al. (8 ) and Dodiuk et al. (8 ) have presented results on the effects of water on performance of various adhesive/substrate combinations. In this volume, the durability of adhesive bonds in the presence of water and in corrosive environments is discussed by Matienzo et al., Gosselin, and Holubka et al. The effects of aggressive environments on adhesively bonded steel structures have a number of features in common with their effects on coated steel, but the mechanical requirements placed on adhesive bonds add an additional level of complication. 

The corrosion resistance and polymer-bonding compatibilities of the lonizable organophosphonates and the neutral organo-silanes are directly related to their inherent chemical properties. Specifically, NTMP inhibits the hydration of AI2O2 and maintains or Improves bond durability with a nitrile-modified epoxy adhesive which is cured at an elevated temperature. The mercaptopropyl silane, in addition to these properties, is compatible with a room temperature-cured epoxy-polyamide primer and also exhibits resistance to localized environmental corrosion. These results, in conjunction with the adsorbed Inhibitor films and the metal substrate surfaces, are subsequently discussed. 

N-type Hgo.79Cdo.21Te semiconductor strips 1 are secured to a sapphire substrate 2 by a layer of epoxy adhesive. Biasing electrodes 6 and 7 are formed using ion-etching and metal lift-off technique (see EP-A-0007667). At the area of the read-out means (8 and 6) and (8 and 7) each of the strips 1 branches into two parts separated from each other by a slot 13 which extends in a direction parallel to the strip. One part provides the continuation of the ambipolar drift path and the other part supports a metal strip connection which forms the read-out electrode 8. The metal strip may not extend beyond the inner end of the slot 13 as shown in the figure or may extend right across the strip so as to form the read-out electrode beyond the slot 13. The continuation of the ambipolar drift path is narrower than the part of the drift path before the... 

The silicon substrate is removed while the temporary substrate provides mechanical support. Next, the substrate is bonded to a final carrier substrate 54 with a layer 52 of epoxy adhesive. The bonding layer 48 is removed, which also removes the temporary carrier substrate 50. The final carrier substrate is chosen according to the same criteria as described in the first embodiment. Processing continues by forming indium interconnects 34b for later hybridization of the read-out integrated circuit assembly with a detector array. 

Epoxy adhesives are chemical compounds used to join components by providing a bond between two surfaces. Epoxy adhesives were introduced commercially in 1946 and have wide applications in the automotive, industrial, and aerospace markets. Epoxies are probably the most versatile family of adhesives because they bond well to many substrates and can be easily modified to achieve widely varying properties. This modification usually takes the form of... 

Cured concrete can be bonded to cured concrete, as in the installation of precast buttons to a highway surface. Steel bridge railings can also be bonded to the concrete surface of a bridge sidewalk. In the case of deteriorated concrete, the adhesive can be used to rebuild the structure to its former line and grade. Epoxy adhesives are also commonly used on other roadway materials, such as asphalt and brick however, the predominant application is concrete substrates. The most frequent combinations of substrates that are bonded with adhesives in this market segment are... 

Both room temperature and heat-curable epoxy adhesives are used in medical applications. The room temperature curing systems require metering and mixing, and the cure time is generally slow (several hours). Elevated-temperature cures could affect temperature-sensitive substrates. 

Epoxies can cure in deep sections and are useful in potting and deep-section sealing applications. They adhere well to different substrates and therefore are used in the general assembly of many medical devices. A clear, medical-grade, low-viscosity epoxy adhesive has proved useful in the fabrication of access ports that are implanted beneath the skin of patients who require multiple infusions.21... 

Physical chemistry is an important factor, which leads to the exceptionally good performance properties of epoxy adhesives. Physical chemistry deals with the physiochemical and surface chemistry aspects of the system. Here system is defined as the adhesive, its constituents, the substrates, and the interface region of the joint. The physical chemistry can determine the success or failure of an epoxy adhesive application. 

TABLE 3.3 Surface Tension of Several Liquids Including Epoxy Adhesive Formulations (Top) and Critical Surface Tension of Various Substrate Materials (Bottom)... 

Most common adhesive liquids readily wet clean metal surfaces, ceramic surfaces, and many high-energy polymeric surfaces. However, epoxy adhesives do not wet low-energy surfaces such as polyethylene and fluorocarbons. The fact that good wetting requires the adhesive to have a lower surface tension than the substrate explains why organic adhesives, such as epoxies, have excellent adhesion to metals, but offer weak adhesion on many untreated polymeric substrates, such as polyethylene, polypropylene, and the fluorocarbons. 

FIGURE 3.4 Contact angle of an uncured epoxy adhesive on four substrates of varying, critical surface tension. Note that as the critical surface tension of the substrates decreases, the contact angle increases, indicating less wetting of the surface by the epoxy adhesive.4... 

The viscosity increase in a epoxy resin-curing agent system could result in poor wetting of the substrate surface, resulting in suboptimal adhesion. Several reaction mechanisms can also occur to an epoxy adhesive once it is mixed and applied to a substrate but before the substrates are mated. These mechanisms can result in a weak boundary layer, which will prevent optimal wetting and reduce the strength of the adhesive. 

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 base epoxy resin can be either liquid or solid. As molecular weight increases, the epoxy equivalent weight and the number of hydroxyl groups available for reaction increase. Waterborne epoxy adhesives provide excellent adhesion to metals and other high-energy substrates. Modified waterborne epoxy adhesives can also provide good adhesion to substrates such as vinyl and flexible plastic film. Characteristics of these epoxy dispersions are summarized in Table 4.6. 

Epoxy adhesive formulations demand a great variety of solvents and diluents with a wide range of evaporation rates, solvent strengths, and dispersion powers. These variations are required due to (1) the many types of epoxy resins, curing agents, and possible organic additives that can be used within a formulation and (2) the many different possible methods that can be used to apply the epoxy to the substrate (brush, spray, trowel, etc.). 

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. 

FIGURE 7.1 The effect of temperature on the tensile shear strength of modified epoxy-phenolic compared to other hybrid adhesives (substrate is aluminum).4... 

Not only is low tensile shear strength noticed on moisture aging, but also the mode of failure changes from one of cohesion to adhesion. Table 7.6 shows the effect of humidity and water immersion on an epoxy-nylon adhesive compared to a nitrile-phenolic adhesive. Substrate primers have been used with epoxy-nylon adhesives to provide improved moisture... 

Aluminum powder, in particular, is frequently employed at relatively high concentrations in high-temperature epoxy adhesive formulations. The filler provides improvement in both tensile strength and heat resistance, and it increases the thermal conductivity of the adhesive. Aluminum powder fillers also reduce undercut corrosion and, hence, improve adhesion and durability of epoxy adhesive between bare steel substrates. It is believed that this is accomplished by the aluminum filler providing a sacrificial electrochemical mechanism.27... 

FIGURE 9.9 The effect of mineral extenders on the tensile shear strength of epoxy adhesive (the substrate is aluminum).29... 

The degree of tensile strength improvement is often in the 50 to 100 percent range. The effect of various fillers and loading ratios on the strength properties of epoxy adhesive formulation is indicated in Fig. 9.9. The effect of different fillers loaded at a constant 100 pph is indicated in Table 9.12 for shear strength on phenolic laminate and aluminum substrates. 

---