Application and Curing of Adhesives

Important factors to consider when choosing dispensing equipment include  

Viscosity and rheology are particularly important if applying adhesives to a substrate. Adhesives have two distinct phases they must flow with low viscosity to thoroughly coat the substrate surfaces and then harden into a cohesively strong solid for in-service use. 

The flow period must be long enough and at a viscosity sufficiently low for excellent wet-out of the substrate surface. However, if the flow period is too long, the adhesive will run beyond the edges and the bond line will be starved of adhesive. The optimum is to have a brief period of high flow, and then the adhesive must gel to hold its position. That is why most adhesives are formulated to be thixotropic (time-dependent shear thinning imder stress). 

In general, single-component materials are the easiest to handle but there are several potential problems that may arise, as listed below  

Two-component adhesives and sealants have similar potential handling problems  

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Cost reductions are also frequently possible because the application and cure of adhesives can often be automated. The resulting savings in labor charges frequently return the cost of investment for new equipment in a matter of months. 

It appears that both the Hitachi and DuPont polyimide films, when cured, significantly impede the drift of sodium ions at normal device operating temperatures. There is, however, evidence that underlying device oxides can be contaminated by sodium and/or moisture or other polar molecules during the application and curing of the polyimide films. Qeaner resins and adequate device stabilization may control this problem. Further work will be required to characterize contamination levels associated with specific aspects of the processing, such as the adhesion promoter and the polyimide resins themselves. 

Rubber/cement (RC) failures indicate that the weakest point in the bonded part is at the interface between the rubber and adhesive. These failures are characterised by a relatively glossy and hard bonded surface with little or no rubber present. Common causes of RC failure are the incorrect choice of adhesive, insufficient dry film thickness of adhesive, failure to properly agitate the adhesive to achieve a uniform dispersion prior to application, pre-cure of adhesive caused by excessive dwell time in the mould cavity before introducing rubber, low mould pressure, undercure of the part, migration of plasticisers and other ingredients from the body of the rubber to the rubber/adhesive interface or contamination of the surface of the adhesive coated part. 

Therefore, it is a mistake to describe epoxy adhesives in a generic manner as if all these formulations had similar properties. Depending on the type of resin and curing agent used and on the specific formulation, epoxy adhesives can offer the user an almost infinite assortment of end properties as well as a wide diversity of application and curing characteristics. 

Epoxy adhesives have achieved their commercial success due in no small way to their processing capabilities. Epoxy chemistry is compatible with (1) a variety of formulating techniques and processes accessible to the adhesive manufacturer and (2) a variety of application and curing methods accessible to the end user. 

Mechanical loss also varies with the cord construction variables such as denier per yarn strand, number of strands and number of twists per unit length ( 2 ). Significant changes In mechanical loss are also observed during the post treatments of grelge tire cords. These Include the steps of cord ten-slllzatlon, application of the adhesives and curing of tires. 

Small spots of the adhesive (typically 5-10 mm in diameter) are placed on the substrates using application and curing techniques comparable to those expected in service or specified by the manufacturer. 

In recent years the importance of the marked colloidal nature of tannin extract solutions has come to the fore [27,36-45]. It is the presence of both polymeric carbohydrates in the extract as well as of the higher molecular fraction of the polyphenolic tannins which determines the colloidal state of tannin extract solutions in water [26,36]. The realization of the existence of the tannin in this particular state affects many of the reactions that lead to the formation and curing of tannin adhesives, to the point that reactions not thought possible in solution become instead not only possible but the favored ones [26,36], while reactions mooted to be of determinant importance when foimd on models not in the colloidal state have in reality been shown to be inconsequential to tannin adhesives and their tannin applications [43,44]. 

The engineer will be concerned with the behaviour and performance of the selected adhesive from the time he first purchases it from the manufacturer, through the mixing, application and curing phases to its properties in the hardened state within a joint over the intended design life. Thus the properties of interest in approximate chronological order are likely to include ... 

There are three main polymers presently used for structural adhesive bonding and they are phenolics, epoxies and urethanes. We can also include in our classification, the initial physical state of the uncured adhesive since it governs the type of application and curing conditions. Structural adhesives are manufactured in the form of films or pastes. The films are one-part adhesives, i.e., they contain a latent catalyst which requires heat for activation. Structural adhesive films are made in a number of different thicknesses and can either be supported (containing a scrim) or unsupported. In general, film adhesives require the application of pressure during cure in order to obtain ultimate properties. Paste adhesives are either one- or two-part materials. That is, pastes can either contain a latent heat activatable catalyst, or they can be a system which is separated into two parts, one of which contains the cur a t i ve/ca talys t. Two-part pastes cure at room temperature. In this section the chemistry of one-... 

Testing is important in all aspects of materials science and engineering, but it is especially so in adhesives. Such tests evaluate not only the inherent strength of the adhesive, but also the bonding technique, surface cleanliness, effectiveness of surface treatments, etchings of surfaces, application and coverage of the adhesive, and the curing cycle. 

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