Selecting the Adhesive

You only have to type the words adhesives for plastics into an internet search engine and you are rewarded with thousands of websites, each one of which is trying to tempt you to visit their site for one reason or another. 

There is a bewildering range of adhesives available to the end user and this chapter will by no means attempt to answer all the questions or even give all the options available. Hopefully, however, it will provide the reader with some guidelines as to the factors that should be considered when selecting an adhesive for the bonding of engineering plastics and elastomers. 

Adhesives and sealants are a multi-billion dollar industry that serves many applications and end-markets. Adhesives are used in a huge variety of industries, from automotive to medical and from electrical switchgear to sports goods. 

Adhesives are primarily designed to bond parts together and sealants prevent leakage of fluids or gases or seal against the ingress of atmospheric components into an assembly. 

They distribute loads across the entire joint area. 

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Inside the human body, cell adhesion to foreign biomaterial surfaces is mediated by a layer of proteins found in the blood or serum. Biomaterials that are able to control the adsorption of blood proteins will be able to control selectively the adhesion of cells. This function underlies the so-called biocompatibility of... 

Based on the data shown in Figs. 1-8 and similar results obtained on the other candidate adhesives CAA (10 Volts) was selected as the surface preparation for use in the remaining studies of this program. These data were also used to select the adhesive systems for continued evaluation. 

As a wide range of adhesives exists, a clear classification is essential in selecting the adhesive most suitable for the application. The adhesives are typically classified according to the following factors ... 

Because of the sensitivity of these adhesives to temperatures in the range of 25-65 °C (depending on the adhesive formulation), the fire resistance of an adhesive-repaired joint would depend primarily on joint design and on the additional measures taken to protect the bond-line. Also, adhesives nsed in these applications can display significantly different viscoelastic responses over the temperature ranges attained normally in service. Thus, in some applications, temperature-induced creep is a risk factor that needs to be considered cautiously when selecting the adhesive for that particular application. 

And of course, the selection of adhesives and sealants will be another very important chapter, it will be studied later in this Handbook, because it is important to gain the basic knowledge of adhesives and sealants before we explain how to select the adhesives and sealants and the equipment. 

Additives. Because of their versatility, imparted via chemical modification, the appHcations of ethyleneimine encompass the entire additive sector. The addition of PEI to PVC plastisols increases the adhesion of the coatings by selective adsorption at the substrate surface (410). PEI derivatives are also used as adhesion promoters in paper coating (411). The adducts formed from fatty alcohol epoxides and PEI are used as dispersants and emulsifiers (412). They are able to control the viscosity of dispersions, and thus faciHtate transport in pipe systems (413). Eatty acid derivatives of PEI are even able to control the viscosity of pigment dispersions (414). The high nitrogen content of PEIs has a flame-retardant effect. This property is used, in combination with phosphoms compounds, for providing wood panels (415), ceUulose (416), or polymer blends (417,418) with a flame-retardant finish. 

When selecting a particular paster adhesive, coating material, and coating process for sohd fiber packaging, the fact that sohd fiberboard lacks the open-flute stmcture (to facihtate exit of moisture from internal phes of paperboard) should be considered. It is generally desirable to minimize the amount of process water added with the adhesive or coating with sohd fiber products. Moisture trapped in the filler phes can result in pin holes or bhsters in the coating and/or warp of the sheet (15). 

The web materials used to form dmms and cans can be customized to each packaging apphcation. The materials are selected based on the package properties of moisture and humidity resistance, nonstick resistance, or barrier properties required. The adhesives used to bond the phes together include sihcates, poly(vinyl alcohol), and poly(vinyl acetate) (10). Sihcate adhesives are most commonly used in the manufacture of dmm packages. 

Some applications, such as incise drapes require loading of the PSA matrix with anti-bacterial agents. Proper selection of the monomers allows the PSA to be compatible with compounds like iodine, while maintaining the adhesion to the skin. 

The dry adhesive films on the two substrates to be joined must be placed in contact to develop adequate autoadhesion, i.e. diffusion of polymer rubber chains must be achieved across the interface between the two films to produce intimate adhesion at molecular level. The application of pressure and/or temperature for a given time allows the desired level of intimate contact (coalescence) between the two adhesive film surfaces. Obviously, the rheological and mechanical properties of the rubber adhesives will determine the degree of intimacy at the interface. These properties can be optimized by selecting the adequate rubber grade, the nature and amount of tackifier and the amount of filler, among other factors. 

Plasticizers reduce hardness, enhance tack and reduce cost in rubber base adhesive formulations. A plasticizer must be easily miscible and highly compatible with other ingredients in the formulations and with the surfaces to which the adhesive is applied. The compatibility and miscibility of plasticizers can be estimated from the solubility parameter values. Most of plasticizers have solubility parameters ranging between 8.5 and 10.5 hildebrands. However, the high miscibility and compatibility also lead to easier diffusion of the plasticizer to the surface, decreasing the adhesion properties. Therefore, plasticizers should be carefully selected and generally combinations of two or more of them are used. 

The selection of the cure system in these applications is directed by constraints such as location of the adhesive in terms of confined space, speed and depth of cure, etc. The volumes of silicones typically applied are relatively small. In general, the uncured adhesive needs to be dispensed in a well-defined and limited area, and needs to stay in place without flowing during cure. No by-products of the cure reaction are acceptable as they may contaminate other sensitive areas of the devices. These constraints often direct the choice to the platinum-catalyzed hydrosilylation cure system that is relatively expensive. 

As with PSA, the phenolics are added primarily for increased cohesive strength and temperature resistance ([216], pp. 284-306). More phenolic is used in adhesives with higher strength requirements, e.g. for metal-metal bonding. Resins based on /j-/-butyl phenolics are most commonly selected ([216], pp. 284-306). They are usually present in the adhesive at 35-50 parts per 100 rubber (phr), with typical optima at 40-45 phr ([216], pp. 284-306). Significant deviation from this optimum may have drastic effects. 

We have also looked at the lap shear strength of selected EB-ciirable epoxy adhesives. Because the adhesives being developed were being used for both aluminum-to-aluminum and composite-to-composite applications the lap shear strengths for both adherends was measured. Aluminum adherends were T2024 phosphoric acid anodized according to ASTM 3933. The composite adherends... 

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