Adherend surface conditions

Adhesives must function solely through surface attachment. Therefore, the nature of the condition of the adherend surface is crucial to the formation of strong and durable bonds. By surface we usually mean that region of a material which interacts with its surroundings. There is some region of a bonded assembly where the adhesive and the adherend interact, but only rarely is this a sharp boundary. Usually it is a very diffuse, somewhat ill-defined region of interaction that has become... 

The primary amine then can react with another isocyanate to produce a urea. Thus one mole of water consumes at least two moles of isocyanate, builds molecular weight, and liberates carbon dioxide in the process. If this happens in a closed container such as a drum of adhesive, the result can be explosive, particularly as the reaction mass rises in temperature because of the exothermic reactions. On the other hand, these reactions can be useful in a bond line because under the proper conditions desirable urea linkages can be introduced into the curing adhesive through the reactions of small amounts of water normally present on the adherend surfaces. 

Adhesion Regarding the development of adhesive forces, the adhesives described differ only marginally. The most important parameter is basically the respective condition of the adherend surface to be bonded. 

Surface treatment Generic term for processes applied to surfaces to achieve a surface condition of the adherends suitable for a bonded j oint or to optimize them in view of their adhesiveness. Such processes are divided into surface preparation, pretreatment and post-treatment. 

Once the condition of wettability of the adherend surface is settled, the viscosity of the adhesive has to be considered. Low viscosity of the adhesive facilitates the spread of the adhesive, while high viscosity makes it difficult to apply the adhesive homogeneously over the surface. Viscosity decreases with increasing temperature and increases with increasing values of average molecular weight (MW). 

Besides the environmental factors mentioned above, the materials involved in a structural joint also influence bond strength and durability. The factors in the material category include the adherends the adhesive the design of the joint absence of surface contamination (including contamination with wood extractives) stability of the adherend surface the ability of the adhesive to wet the surface, and entrapment of air/volatiles. Thus, the condition of the adhesive/adherend interface becomes a decisive factor affecting the initial bond strength as well as the long-term durability of the bonded joint [31]. 

The adhesive has to spread over the adherend surface, penetrating its irregularities, displacing air and any contaminants present. The ideal conditions for this to be realised are that ... 

Discretion is advised when comparing the performance of high temperature adhesives where the evaluations were performed in different laboratories. Important parameters critical to the performance of the adhesive, such as quality of the polymer, adhesive type, adherend, surface treatment, bonding conditions, aging and test conditions, often vary between one investigation and another. 

Polymer composite materials absorb water from humid air. This water may degrade the bond, especially when elevated temperature cure is required. Water or humidity on the adherend bond surface may seriously reduce the adhesion compared to that with a dry surface condition. Therefore, the dryness of the laminates that are to be bonded has to be ensured. However, if room temperature cure is used and drying of laminates is considered... 

The term primer is used to describe any liquid preparation used to modify the surface of an adherend. While there are some primerless systems, the sealants used in construction often require primers as the surface condition may be extremely variable (See Sealant joint design, Seaiants in doubie giazing and Selection of joint sealants). 

Thus, bonds made with 6061-T6 alloy will normally last about four times longer than those prepared with the less corrosion-resistant 2024-T3 alloy, when exposed to a marine environment. Similarly, after three weeks exposure to 5% salt spray at 95°F, joints prepared with 2026-T4 alloy had failed, those prepared with 6151-T4 alloy had weakened considerably, but had not fallen apart, and those prepared with X5085-H-111 alloy had retained about 30% of their initial strength. The effects described are for bonds formed with only minimal preparation of the adherend surface, under which conditions bondline corrosion becomes the dominant factor in determining durability. The same trends may not be apparent when anodized surfaces, or corrosion-inhibiting primers, are used. Also the same order of relative durabilities may not apply to noncorrosive exposure conditions. For example, long-term tropical exposure indicates that more durable bonds are formed with 2024-T3 clad alloy than with 6061-T6 alloy. ... 

Because adhesives bond by surface attachment, the physical and chemical condition of the adherends surface is extremely important for good joint performance and durability. Immediately after preparation, all surfaces undergo an inactivation process. To achieve optimal adhesion it is recommended that no more time than necessary should be allowed to elapse between final surface preparation and bonding. The prepared surfaces should be kept covered with... 

The mechanical energy expended in rubbing an adhesive into a metal surface has been studied and correlated with measured increases in bond strength.8) jt is presumed that the likely mechanism is an increase in the displacement of contaminant film from the adherend surface. If this explanation is correct then durability could also be improved, since the effect of a potential weak boundary layer condition at the interface should be diminished. It is doubtful, however, that there would be much improvement in resistance to severe service conditions, such as a combination of significant stressing and water-soaking conditions. 

A wide range of surface pretreatment procedures has been developed for different plastic adherends available in the current market. As with other adherends mentioned above, it is the purpose of any surface pretreatment to establish a surface condition for good wetting by the adhesive. 

F. Gaillard, A. A. Roche, and M. J. Romand, The effects of surface conditioning of AISI-304 stainless steel on the interfacial properties of alloy/epoxy composite structure adherend surface characterization using X-ray emission spectroscopy, in Adhesive Joints—Formation, Characteristics, and Testing (K. L. Mittal, ed.), p. 85, Plenum Press, New York (1984). 

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