Other Structural Adhesives

In contrast to the attention given to the bonding of aluminum and wood, very few studies have been reported dealing with the durability of other structurally bonded systems. A limited amount of sustained load durability data is available for steel bonded with epoxies and aluminum bonded with urethanesin recent years, reports have appeared dealing with the environmental resistance of modified acrylics and cyanoacrylates. Brockmann has also reported on the durability of steel joints exposed to hostile environments.  

In a study of nine commercially available acrylic adhesives, Wilkinson and Tyler found a considerable variation in the resistance to environmental exposure. The tests employed were 1,000-h water soak, 30 days at 125°F/100% RH, and 1,000-h 5% salt spray at 95°F aluminum adherends were used. The nature of the alloy was also a factor in determining joint durability. The authors concluded that excellent durability can be obtained with the proper selection of adhesive and alloy. A similar study by Zalucha using recently developed high-performance acrylic adhesives also indicates excellent performance for many acrylic-bonded assemblies. Minford has conducted weathering tests on both the early formulations and the more recently introduced second generation acrylics. He found that the durability of the earlier products was generally poor in both water soak and 

In a more detailed investigation of the effect of moisture on cyanoacrylate-bonded steel joints. Drain et al. concluded that the loss of strength is due to the formation of an oxide layer at the adhesive-metal interface and possibly to concurrent surface hydrolysis of the polymer. The adhesive appears to be relatively impermeable to water which they suggest most likely enters the bondline by wicking along the interface. Polycarbonate joints exposed to the same aging conditions remained virtually unaffected. 

The results of a program to evaluate high-temperature adhesives for extended service have been reported by Hendricks et al They used titanium lap shear coupons, prepared with either a polyphenylquinoxaline adhesive or a thermoplastic polyimide adhesive, to study the effects of thermal and humidity exposures. The polyphenylquinoxaline was affected by exposure 

---

Epoxy adhesives are generally more resistant to a wide variety of liquid environments than other structural adhesives. However, the resistance to a specific environment is greatly dependent on the type of epoxy curing agent used. Aromatic amine (e.g., metaphenylene diamine) cured systems are frequently preferred for long-term chemical resistance. 

In North America the standards ASTM D 2559 and D 3535 give the requirements for adhesive systems used for structural purposes. EPI adhesives formulated for structural purposes meet the requirements in these standards and have the same performance properties as other structural adhesives [1,5]. 

Henriksson et al. [33] show that gluing of thermally modified wood by EPI adhesives gives the same strength as for other structural adhesives. The study did, however, show that thermal treatment of posts glued before treatment reduced the... 

The properties of cyanoacrylate adhesives have been discussed in detail in the previous sections of this chapter. The evaluation of these adhesives is complemented in this section by comparisons with other structural adhesives. A specific comparison has been made of cyanoacrylate adhesives with a two-part epoxy, EC 2216 B/A, and a two-part polyurethane, EC 3549 B/A, all sold by 3M. The substrates on which the adhesives were tested were aluminum, polystyrene, and chloroprene. The results for room temperature water immersion, 90°F/90% relative humidity, and 150°F oven aging are shown in Tables XVI and XVII. 

The results for AVI 19 and Hysol in the temperature range 20-90°C are shown in O Fig- 30.11. From the manufacturer s data, AVI 19 is expected to have a Tg of around 120°C and the results demonstrate a smooth curve through the transition region with an expected peak around 120°C. The other structural adhesive, Hysol, indicates the existence of a Tg at around 60°C from the trend curve. Manufacturer s data suggests a Tgof 85°C. 

Thermal and chemical stability not as good as with certain other structural adhesives... 

When subjected to tensile stress, the two substrates pull away from each other perpendicularly to their surfaces. In contrast, compressive stresses occur when the substrates are forced toward each other. Structural adhesives typically have tensile strengths of 55.2 MPa to 82.8 MPa and compressive strengths of 69 MPa to more than 276 MPa. 

Once formed, crystals can interact with or adhere to other components. Adhesion can occur directly between crystals to form larger structures which may act as the nidus for further mineralization. Crystals can also interact with metabolites and/or tissues to form such structures. These interactions can and do alter the stereochemical course of crystal assembly, and molecular recognition probably accounts for the striations and intricate geometries adopted by biomineralized structures. The role and composition of metabolites comprising the matrix may be useful to understanding the local and large-scale structure of biominerals. 

In this chapter we first describe the composition of cellular membranes and their chemical architecture— the molecular structures that underlie their biological functions. Next, we consider the remarkable dynamic features of membranes, in which lipids and proteins move relative to each other. Cell adhesion, endocytosis, and the membrane fusion accompanying neurotransmitter secretion illustrate the dynamic role of membrane proteins. We then turn to the protein-mediated passage of solutes across membranes via transporters and ion channels. In later chapters we discuss the role of membranes in signal transduction (Chapters 12 and 23), energy transduction (Chapter 19), lipid synthesis (Chapter 21), and protein synthesis (Chapter 27). 

Other bacterial coats. Archaebacteria not only have unusual plasma membranes that contain phytanyl and diphytanyl groups (Section A,3)608 but also have special surface layers (S-Iayers) that may consist of many copies of a single protein that is anchored in the cell membrane.609 The surface protein of the hypothermic Staphylothermus marius consists of a complex structure formed from a tetramer of 92-kDa rods with an equal number of 85-kDa "arms."610 611 S-layers are often formed not only by archaebacteria but also by eubacteria of several types and with quite varied structures.612 14 While many bacteria carry adhesins on pili, in others these adhesive proteins are also components of surface layers.615 Additional sheaths, capsules, or slime layers, often composed of dextrans (Chapter 4) and other carbohydrates, surround some bacteria. 

The three principal forces to which adhesive bonds are subjected are a shear force in which one adherend is forced past the other, peeling in which at least one of the adherends is flexible enough to be bent away from the adhesive bond, and cleavage force. The cleavage force is very similar to the peeling force, but the former applies when the adherends are nondeformable and the latter when the adherends are deformable. Appropriate mechanical testing of these forces are used. Fracture mechanics tests are also typically used for structural adhesives. 

Constrained-Layer Treatments. Constrained-layer damping treatments consist of a thin layer (/zm) of viscoelastic material sandwiched between a base material and an outer constraining layer of sheet metal or other structural material. Some of these treatments are available with self-adhesives on both sides of the viscoelastic material and act as a bonding agent between the base and constraining layers others have the constraining layer already bonded to the inner layer so they need only be applied to the base material. 

---