Adhesive, selection durability

Morris, C. E. M., Strong, Durable Adhesion Bonding Some Aspects of Surface Preparation, Joint Design, and Adhesive Selection, Materials Forum, vol. 17, 1993, pp. 211-218. 

In most cases, the ability of the adhesive to support the design loads under service conditions for the planned life of the structure is considered first. Thus the mechanical properties, durability, and environmental resistance of the bonded structure are of obvious importance. But of equal concern are the nature of the adherends, the application technique, the cure conditions, the handling requirements, and, perhaps, the cost. A list of some of the factors considered in the selection process, together with the properties of the major generic classes of adhesives, are given in Table VI." It is important to remember that, even within a given class of adhesives, the performance may vary considerably, so that it is essential to consider the properties of each individual adhesive. It is apparent from this brief discussion that adhesive selection is far from simple. A computer-based selection program has become available recently." ... 

The ability of the adhesive to support the design loads under service conditions for the expected life of the structure is the first thing to be considered in adhesive selection. Thus, the mechanical properties, durability, and environmental resistance of the bonded stmcture are also important. 

Joints based on mechanical interlock have predictable strength and durability since these properties are related to the bulk properties of the substrate materials and the adhesive layer. If the substrate materials and the adhesive selected are suitable for the environment, no trouble is expected with joint durability. 

Optimization of surfece treatment and adhesive selection for bond durability in Ti-15-3 laminates. J Adhes 71 115... 

As already stated, the durability or permanence of a bonded assembly is dependent on the intended use and service conditions to which the bond will be exposed. However, the joint design, choice of substrates, adhesive selection, substrate preparation, and primer selection, where appropriate, plus the method of application and assembly all have significant impact on the service life of adhesively bonded materials. Most or all of these considerations are interdependent, for example, the joint design and substrates chosen will limit the range of suitable adhesives that can be employed. In a similar way, the durability of a sealed joint is only as good as the adhesion of the sealant (and primer) to the surfaces forming the joint. Primers and/or sealants will adhere to surfaces only if those surfaces are properly prepared. A very large proportion of all sealant joint failures result from poor or inadequate surface preparation. 

Important decisions must be made based on the substrate material, such as finding the coating material and method that best addresses factors like surface adhesion, appearance, durability, surface properties, and solvent migration. A detailed description of this selection process is covered elsewhere in this text. 

Figure 3 241 illustrates that the durability of metal/polymer adhesion systems can greatly be influenced by the metal pretreatment chosen 1 K Therefore, it is very important to select the best pretreatment for a given system. 

Over the past fifteen years a number of different approaches have been taken in an attempt to increase the durability of the metal/polymer interfacial region in the presence of water. These attempts have met with varied degrees of success. However, to date adhesion scientists are still searching for a means of achieving sufficient wet environment durability, so that the enormous potential of metal/polymer adhesion systems can soon be utilized effectively. The authors have selected to discuss a few of the more promising durability-enhancing methods. 

This paper reviews the status of the art of adhesion to wood. The term "adhesion" here means the forming or the result of a durable interface, or zone of "intimate" contact, between one piece of wood and a second material, whether it be adhesive, coating, or another piece of wood. The review will focus primarily on solid wood, but will refer to selected literature on wood fiber or fiber-wall components if the concepts presented apply to solid wood. Several specific topics will be considered in detail mechanisms of adhesion to wood techniques for predicting whether or not adequate adhesion will occur or has occurred wood properties affecting adhesion and, finally, techniques for enhancing adhesion. 

It is impossible to avoid a discussion on prebond surface preparation since it is one of the most important factors in the fabrication of a durable and consistent epoxy adhesive joint. Selection of a proper surface preparation is not an easy task, and the actual implementation of the surface treating process in production is equally daunting. 

The adhesive should be selected on the basis of durability as defined by slow cyclic testing in a hot and humid environment. 

Devine, A. T., Adhesive Bonded Steel Bond Durability as Related to Selected Surface Treatments, U.S. Army Armament Research and Development Command, Large Caliber Weapon Systems Laboratory, Technical Report ARLCD-TR-77027, December 1977. 

Other properties that are heavily influenced by the choice of monomer include cure speed (in general higher functional monomers cure more rapidly), viscosity, and durability of the film. Table 1 lists some monomers, their viscosities, and the properties that they enhance (reprinted with permission from Sartomer). it is important to note several trends on the chart. Cure speed increases with an increase in functionality (all of the recommended monomers in that column are at least trifunctional and several are tetra- or penta-functional). Viscosity also increases as the functionality of the monomer is raised (all of the low viscosity diluents are diacrylates). The adhesion promoting monomers are all di- or mono-functional. Most formulas contain several different monomers and sometimes also oligomers as there is often a balancing act that must be performed when selecting materials that will provide the required performance properties while still maintaining the correct viscosity and surface tension. 

Because the fracture toughness depends both on cure time and temperature, the arbitrary selection of time and temperature for accelerated tests may not be appropriate for reliable prediction of longterm service life of joints (J7). In order to reduce test variability and improve the durability prediction of adhesive joints, it would be necessary first to control the cure temperature and time required to produce a level of fracture toughness that does not change further (14). The study is thus an excellent example of a multidisciplinary approach combining chemistry, fracture mechanics, and wood science in the investigation of the adhesive bonding of wood. 

The chemical literature reveals relatively new and varied interests for nitrile elastomer-modification of epoxy resins in diverse areas of coatings and primers. Desirable properties such as impact resistance, mandrel bend and adhesion improvement are attainable with little or no sacrifice in critical film properties. In some instances, it is documented that proper elastomer modification of select epoxy coatings will enhance corrosion and moisture resistance. This may relate to better film/substrate adhesion durability. The combined literature, journal and patent, has led to continuing study of nitrile elastomer modified epoxy coatings. 

Specifications for the selection of the repair material should be based on tests that can be carried out prior to the application. Each test should be suitable to assess the performance of the materials with regard to the specific property that it is intended to evaluate. Several standards cover tests to be used for the evaluation of workability or mechanical properties (compressive, tensile or flexural strength, elastic modulus, adhesion) of the repair material. However, as far as durability performance is... 

In contrast to aluminum and titanium structural bonds where performance can be optimized for most aerospace applications, steel bonds are often designed to minimize cost as long as certain performance standards are met [47]. If feasible, many manufacturers prefer to select adhesives or primers that provide adequate strength and durability with untreated steel rather than to prepare the surface for bonding. 

A. T. Devine, Adhesive bonded steel. Bond durability as related to selected surface treatments. Technical Report ARLCD-TR-77027, ARRADCOM, Dover, NJ, December 1977. 

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