Semi-structural adhesives

Toughened acrylic adhesives are also classified as structural adhesives for limited exterior service environments, while polyurethane and polyesters (unsaturated) are considered as semi-structural adhesives for limited exterior service environments [3]. 

This procedure has been used for several years to prepare aluminium surfaces for painting. Even in an unsealed state, it was not generally thought that the oxide film produced, which is extremely dense, was suitable for either structural or semi-structural adhesive bonding although its corrosion resistance was known to be exceptional. It is a modem adaptation of the standard procedure which is now of interest to stractural adhesive bonders. 

Failure following crazing is not the inevitable consequence of yielding under stress. Another possible response is a period of continuous deformation. Under conditions of shear, the only situation that has been studied for adhesive in detail, continuous creep is preceded by a time delay before the continuous deformation commences. Most attention has been paid to the linear polymers such as polyethylene, plasticized as well as unplasticized polyvinyl chloride and polymethylmethacrylate none of which are used even as semi-structural adhesives. It is, in fact, the absence of cross-linking which allows for extensive, continuous deformation and, at the same time is one of the major reasons for neglecting them. The only cross-linked material for which creep has been extensively studied is natural rubber and, being above its transition temperature it, too, is not of interest in the present context. 

Semi-structural adhesives Polyurethane Polyvinyl acetate Hot melt adhesives such as ... 

The section on chemistry of adhesives evolves from rubber-based adhesives to semi- structural and finally to structural adhesives. Everaerts and Clemens provide a thorough description of chemistry and applications of pressure sensitive adhesives and Kinning and Schneider describe an enabling technology for pressure... 

Even though the strength was lower than that of recent high-performance structural adhesives, we might say that the dismantlable adhesive has enough strength for semi-structural applications. 

The adhesive used was a model semi-structural PU, similar to one described in an earlier work [7]. The two component formnlation consisted of methylene bis-4-cyclohexyl isocyanate prepolymer and trifunctional polyols with molecnlar masses in the range 500-900. A small amount of tin catalyst (UL-28, Witco Chemicals) was also present. The PU was used as control without additives and was modified by the following silanes ... 

Several applications utilise adhesives in a semi-structural manner in which the stresses to be resisted are dominantly compressive. These include ... 

The possibilities for making bonded structural sandwich elements in a variety of materials are very real. However, whilst there exist structural examples such as aluminium honeycomb panels (used in aircraft and transport applications) and metal skinned foam sandwich panels (used as the monocoque chassis in refrigerated transport applications), these composite constructions are normally utilised in non- or semi-structural ways. Typical skin materials are steel, aluminium, GRP and plywood, and common core materials are rigid foam polystyrene, polyurethane, polyisocyanurate, PVC, and honeycombed aluminium. In some instances the foam core is injected between the skins and adheres to them in others, adhesives are used to bond the separate components together. The nature of the manufaeturing process depends on the type of structure to be made, and the degree of investment in produetion maehinery. Both flat and eomplex eurved forms ean be made by a hand lay-up process as well as in an automated way. 

Selected examples have been cited to demonstrate that areas of acrylic, anaerobic and radiation-curable adhesive developments have begun to consider telechelic polybutadiene/acrylonitrile liquids as useful formulating ingredients either in direct admixture or in the preparation of rubber-modified oligomers serving as an adhesive formulating base. These cited adhesive examples cover structural, semi-structural, gap-filling and pressure-sensitive types. 

This large market includes residential housing and commercial building, in addition to roads, bridges, and other elements of the infrastructure. The largest volume of adhesives presently used is for nonstructural applications however, the use of structural adhesives is expected to increase in the future. While some end uses will involve primary structural bonds, most will be semi-structural, in which the adhesive will be used in combination with a mechanical fastener. The use of adhesives is also expected to increase as the trend towards the manufacture of prefabricated sections continues. Laminated beams and trusses are one established use for structural adhesives, typically phenolic, but there is now interest in urethanes for this purpose. Epoxies are also widely used, mostly for the bonding and repair of concrete. 

The last chapter. Flexible Bonding and Sealing is quite an innovative work which explains why flexible polyurethane adhesives may be sometimes a better choice, even for structural or semi-structural bonds, because the work required to break the bond may be higher than with a rigid epoxy structural adhesive. This is demonstrated in Eig. 5. 

Manufecturers Comments RT cure. Semi-rigid structural adhesive. ... 

In the past, for structural and semi-structural bonding, adhesives technologists favored the strong adhesives which had very high tensile shear resistance, for... 

This explains that adhesives suppliers first developed in the seventies the so-called toughened epoxies, flexibilized by addition of a small amount of cross-linkable rubber, and then recently the company SIKA pioneered the so-called flexible structural bonding in which they use PU adhesives derived from their experience in PU sealants, to get semi-structural bonds, for instance, for transportation and automotive or naval uses. 

Fig. 43 shows that a flexible, semi-structural PU adhesive giving only 6 MPa tensile strength may require greater work to break the bond than a high-tensile strength structural epoxy. The reader should refer to the chapters Flexible structural bonding by SIKA, and Epoxy adhesives by 3M, in following volumes of this Handbook. 

To obtain the best balance between waU thickness and electrical performance many radomes currently utilise simple honeycomb sandwich structures in their construction. The composite skins are quartz, glass or aramid (generally Kevlar) reinforced epoxy mouldings carbon reinforcement is not used within the window area of a radome because of the poor dielectric constants associated with these structures. Epoxy-based film adhesives are used to bond these skins to the Nomex honeycomb core and/or semi-structural foam. 

Two semi-structural bonding applications should also be mentioned. The magnets used in some of the electronic actuators are bonded into place often with an epoxy film adhesive. Amino silanes such as Scotch-Weld 3901 Primer are used in the bonding of plastic components. 

There is as yet no published information on the use of the semi-structural, toughened acrylic formulations in mixed metal-wood structures. Depending on the stress level required, these adhesives may be perfectly satisfactory in situations where exposure to weathering is not required. 

Bearing in mind that a polyester-based GRP is unlikely to be used for the highest structural duty, the semi-structural toughened acrylics would make an easily used, non-demanding, two-part adhesives which should suffice most applications. 

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