Adhesive bond operation

The importance of quality assurance in any industry is widely acknowledged. More stringent customer expectations with regard to quality, together with the realisation that continued improvements in quality are often necessary for a company to sustain good economic performance, mean that some level of quality assurance is mandatory for all those associated with adhesive bonding operations. The use of technical specifications and quality control procedures go some way towards fulfilling the requirements of a quality system, and these measures are outlined here. 

The limitations of hot-melt adhesives are restricted toughness at usable viscosities, low heat resistance, and poor creep resistance. All properties are affected by the polymer and diluents used in the formulation. The application conditions, including the amount of adhesive and the pressure applied to the bond line, also affect end-use properties. In the hot-melt adhesive-bonding operation, a minimum amount of pressure must be applied until the hot-melt becomes solid or sufficient tack develops to hold the substrates in place. 

Four primary safety factors must be considered in all adhesive bonding operations toxicity, flammability, hazardous incompatibdity, and equipment. 

Critical to the success of an adhesive bonding operation is the type of surface pretreatment that is used. If the pretreatment (or lack of it) is not matched to the application and the substrate(s). 

Figure 8.1 shows the schematic of a general adhesive-bonding operation. Table 8.1 describes each step in the bonding line briefly. 

The additional assembly proeess risk, Up, is eonsidered in isolation after the handling and fitting operations, questioning the assembly situation of a eomponent after initial plaeing, and whieh requires further proeessing, for example welding or adhesive bonding. Therefore, beeause additional assembly proeesses are performed after initial part plaeement, simply defaults to ap when it is eonsidered in the assembly sequenee ... 

Primers can be used to protect both treated metal and nonmetal substrates after surface treatment. The use of a primer as a shop protectant may increase production costs, but it may also provide enhanced and more consistent adhesive strength. The use of a primer greatly increases production flexibility in bonding operations. Usually primer application can be incorporated as the final step in the surface preparation process. The primer is applied as soon as possible after surface preparation and usually no more than a few hours later. The actual application of the adhesive may then be delayed significantly. 

Bonding operations frequently require the mechanical or chemical removal of loose oxide layers from iron and steel surfaces before adhesives are applied. To guard against slow reaction with environmental moisture after the bond has formed, iron and steel surfaces are often phosphated prior to bonding. This process converts the relatively reactive iron atoms to a more passive, chemically stable form that is coated with zinc or iron phosphate crystals. Such coatings are applied in an effort to convert a reactive and largely unknown surface to a relatively inert one whose structure and properties are reasonably well understood. 

Four primary factors must be considered in all adhesive bonding or sealing operations toxicity, flammability, hazardous incompatibility, and equipment. The adhesive or sealant must be carefully considered because not only can it create health and safety issues within the factory but also it can provide issues within the community relative to release of volatiles and waste disposal. 

The Bonding Process. Before the actual assembly operation, the cleanliness of the shop and tools should be verified. The shop atmosphere should be controlled as closely as possible. Temperature in the range of 18 to 32°C and relative humidity from 20 to 65 percent are best for almost all bonding operations. All parts should be fitted together without adhesive or sealant to indicate possible production problems due to fit. The suitability of fit is established by either visual inspection or direct measurement with gauge or shim. It is desirable that the extremes in mechanical tolerances also be noted and that test specimens be made with the worst possible fit to ensure that the bonding process will always provide reliable joints. 

In view of these suggestions and examples, it is evident that the wood products industry retains its faith in natural adhesives and their capabilities even in this era of synthetics. It is likely that the industry will continue to turn to them without restraint as their price or performance dictates. The desire, of course, is to keep the various wood-bonding operations functioning as efficiently as free market conditions and competitive innovation will permit. 

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