Adhesive bonds preparation

Although Bikerman s position would not be supported today, it stimulated developments in the understanding of theories of adhesion, which can, to some extent, be regarded as his legacy. The potential for the formation of weak boundary layers (1) is well established, and much of the technique of adhesive bond preparation is designed to avoid them. 

Fitzpatrick et al. [41] used small-spot XPS to determine the failure mechanism of adhesively bonded, phosphated hot-dipped galvanized steel (HDGS) upon exposure to a humid environment. Substrates were prepared by applying a phosphate conversion coating and then a chromate rinse to HDGS. Lap joints were prepared from substrates having dimensions of 110 x 20 x 1.2 mm using a polybutadiene (PBD) adhesive with a bond line thickness of 250 p,m. The Joints were exposed to 95% RH at 35 C for 12 months and then pulled to failure. 

Rider and Amott were able to produce notable improvements in bond durability in comparison with simple abrasion pre-treatments. In some cases, the pretreatment improved joint durability to the level observed with the phosphoric acid anodizing process. The development of aluminum platelet structure in the outer film region combined with the hydrolytic stability of adhesive bonds made to the epoxy silane appear to be critical in developing the bond durability observed. XPS was particularly useful in determining the composition of fracture surfaces after failure as a function of boiling-water treatment time. A key feature of the treatment is that the adherend surface prepared in the boiling water be treated by the silane solution directly afterwards. Given the adherend is still wet before immersion in silane solution, the potential for atmospheric contamination is avoided. Rider and Amott have previously shown that such exposure is detrimental to bond durability. 

The surface preparation must enable and promote the formation of bonds across the adherend/primer-adhesive interface. These bonds may be chemical (covalent, acid-base, van der Waals, hydrogen, etc.), physical (mechanical interlocking), diffusional (not likely with adhesive bonding to metals), or some combination of these (Chapters 7-9). 

ASTM D3933, Standard Guide for Preparation of Aluminum Surfaces for Structural Adhesives Bonding, Phosphoric Acid Anodization, A.STM, West Conshohocken, PA. El-Mashri, S.M., Jones, R.G. and Forty, A.J., Philo.s. Mag. A, 48, 665 (1983). 

Wegman, R.F., Surface Preparation Techniques for Adhesive Bonding. Noyes Publications, Park Ridge, NJ, 1989. 

Mazza, J.J. and Kuhbander, R.J., Grit blast/silane (GBS) aluminum surface preparation for structural adhesive bonding, WL-TR-94-4111. Materials Laboratory, Air Force Materiel Command, September 1999. 

Diem, H., Fritsch, R., Lehnert, H., Matthias, G., Schatz, H. and Wittmann, O., German patent DE 3125874. Process for the preparation of cocondensates which form weatherproof adhesive bonds. Assigned to BASF AG, 1981. 

Thrall, E.W. Jr., Failures in adhesively bonded structures. AGARD Lecture Series No. 12. Bonded Joints and Preparation for Bonding. North Atlantic Treaty Organization Advisory Group for Aerospace Research and Development, London, 1979. 

The final section in this volume deals with applications of adhesion science. The applications described include methods by which durable adhesive bonds can be manufactured by the use of appropriate surface preparation (Davis and Venables) to unique methods for composite repair (Lopata et al.) Adhesive applications find their way into the generation of wood products (Dunky and Pizzi) and also find their way into the construction of commercial and military aircraft (Pate). The chapter by Spotnitz et al. shows that adhesion science finds its way into the life sciences in their discussion of tissue adhesives. 

Effect of Surface Preparation on the Durability of Structural Adhesive Bonds... 

Although numerous studies (1-3) have described work aimed at establishing criteria for the durability of adhesive joints, a thorough understanding of effects of the chemical and mechanical properties, on the durability of adhesive bonds is lacking. More specifically, the effects of surface preparation and dynamic loading, especially under environmental service conditions, has not been explored in detail for automotive structures. In this paper, a description of the effects of environment on the durability of adhesive bonds is presented. Particular attention is given to... 

Adhesive Bonding Technique. Standard procedures for preparing adhesive bond specimens were used. The composite was initially sanded with 240 grit emery paper and then thoroughly rinsed with methylene chloride. Steel substrates were rinsed thoroughly with methylene chloride. Bonds were prepared as one inch overlap shear specimens bond thickness was 0.75 mm. Bond thickness was defined using 1.5 mm long wires of the appropriate thickness. 

In a specific example of adhesive bonds between cold rolled steel and SMC adherends (Table II) an adhesive based on hydrolysis resistant epoxy chemistry (i.e., adhesive E) was compared with an adhesive based on hydrolysis prone urethane chemistry (i.e., adhesive C) in composite to cold rolled steel bonds. After corrosion testing, a significant difference in both retention of initial bond strength and locus of failure was observed. For bonds prepared with adhesive E, little if any reduction of the initial bond strength was observed after corrosion testing. The locus of failure for both the tested and untested bonds was largely in the... 

The Effect of Adhesive Primers. In practice, adhesive bonds involving metal adherends often use primers as pretreatments of the metal surface prior to bonding. Table IV shows the durability of composite-metal bonds prepared with adhesive C over a series of primers (of varying corrosion resistance) in 240 hour salt spray test. The results indicate that the performance of bonds is directly related to the corrosion resistance of the primer used to prepare the adherend surface. In general, the adhesion of the primer to the steel adherend, rather than the adhesive chemistry. 

The two predominant mechanisms of failure in adhesively bonded joints are adhesive failure or cohesive failure. Adhesive failure is the interfacial failure between the adhesive and one of the adherends. It indicates a weak boundary layer, often caused by improper surface preparation or adhesive choice. Cohesive failure is the internal failure of either the adhesive or, rarely, one of the adherends. 

SURFACE PREPARATION TECHNIQUES FOR ADHESIVE BONDING by Raymond F. Wegman... 

Dukes, W. A., Kinloch, A. J. Preparation of surfaces for adhesive bonding. Explosive Research and Development Establishment, Watham Abbey, England, 1976... 

With the development of adhesion science and technology, it has become relatively straightforward to produce adhesive bonds of very reasonable strength between most materials which are in general use. An informed choice of adhesive together with appropriate preparation of the surface will produce a strong bond. 

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