Weathering of adhesive joints

Adhesive joints and coatings not uncommonly fail in an aqueous environment (see Durability fundamentals, Weathering of adhesive joints and Wedge test). One of the... 

Further information is given in articles on Pre-treatment of metals prior to bonding, Durabiiity creep rupture, Durabiiity subcritical debonding. Weathering of adhesive joints and Weathering tests. 

Simple adhesion tests will always be informative. They can show, for example, whether or not a surface is sufficiently firm to be painted satisfactorily, or allow the progressive effects of weathering to be monitored (see Weathering of adhesive joints. Durability). 

The practical importance and theoretical complexity of environmental deterioration of adhesive bonds has resnlted in extensive research on many aspects of the problem of Weathering of adhesive joints. 

Weathering of adhesive joints J SARGENT Exposure in temperate and tropical sites... 

H. W. Eickner, Weathering of adhesive bonded lap joints of clad aluminum alloys-Part I, Technical Report 54-447, Forest Products Lab. (1955). 

Epoxy adhesives and sealants are generally not significantly affected by simple outdoor weathering. However, there are certain circumstances that could affect the permanence of joints exposed to outdoor service, ft is important that these be considered early in the design of the adhesive joint and selection of materials. 

Extensive information on the outdoor durability of bonded aluminum joints is available in the reviews of Minford.61,62,63 Hartshorn has also provided a catalog of references to outdoor weathering of structural adhesives by adhesive class and exposure conditions.64... 

Adhesives exposed outdoors are affected primarily by heat and humidity. Thermal cycling, ultraviolet radiation, and cold are relatively minor factors. Structural adhesives, when exposed to weather, rapidly lose strength during the first six months to a year. After two or three years, the rate of decline usually levels off, depending upon the climate zone, polymer, adhesive, and stress level. The following are important considerations when designing an adhesive joint for outdoor service ... 

Adhesives of this type were developed for repair and maintenance in transport facilities, plants, pipelines, ships, etc. in the open air. In-situ conditions require development of adhesives usable at all ambient air temperatures, in wet weather, in deep waters, and in oil product media and other liquids. Smfaces to be glued together may be covered with corrosion layers, and glue line thickness cannot be limited. There is no need for any additional alteration of the joints. 

The adhesives to be used in making joints between timber members are specified in BS 5268 Part 2 (24). Permissible adhesive types based on the exposure conditions of the joint are reproduced here as Table 7.4. Weather and Boil Proof (WBP) products such as resorcinol and phenol-formaldehyde are stipulated in damp conditions whereas melamine and urea-formaldehyde adhesives are permitted in permanently dry situations. Natural adhesives such as casein are now only allowed in the least hostile exposure category. 

Weathering studies after exposures up to one year were also made by Picatinny Arsenal on the adhesives covered in Table 10.8, along with several additional adhesives. The results are given in Table 10.9 as percent retention of original joint strength. In addition to controls, the following climates were used ... 

Table 10.9 Percent Retention of Original Adhesive Joint Strength After Weathering One Year (2024-T3 Aluminum Alloy) ... 

The investment needed in terms of cost and time means that few such comprehensive programmes are undertaken in the short-term culture of contemporary industry. Still important are classic reports of such weathering trials undertaken by UK government laboratories. These involved bonded aluminium double overlap joints, stressed and unstressed, peel and honeycomb specimens (see Peel tests). These were exposed at the Royal Aircraft Establishment (RAE), Famborongh (temperate), and at the Joint Tropical Research Unit (JTRU) sites at hmisfail (hot-wet) and Cloncurry (hot-dry), Aushalia. Periods of up to 6 years were employed, exposing a variety of adhesive systems to the different climates. 

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