Choice of adhesive

The seam closure on a folding carton is typicahy made using a latex, poly(vinyl acetate), vinyl acetate copolymer, or hot-melt adhesive (27). The choice of adhesive depends on a number of factors, including the nature of any coating used on the package and the production speeds required. 

The choice of adhesive can be made only after thorough research and preliminary tests. The parts should not be subjected to high stresses. 

In essence, the durability of metal/adhesive joints is governed primarily by the combination of substrate, surface preparation, environmental exposure and choice of adhesive. As stated earlier, the choice of the two-part nitrile rubber modified epoxy system (Hughes Chem - PPG) was a fixed variable, meeting the requirement of initial joint strength and cure cycle and was not, at this time, examined as a reason for joint failure. Durability, as influenced by substrate, surface preparation, and environmental exposure were examined in this study using results obtained from accelerated exposure of single lap shear adhesive joints. 

With the development of adhesion science and technology, it has become relatively straightforward to produce adhesive bonds of very reasonable strength between most materials which are in general use. An informed choice of adhesive together with appropriate preparation of the surface will produce a strong bond. 

The types of adhesives suitable for laminating beams are restricted by the conditions of application and by their end-use requirements. A wider choice of adhesives for plywood depends on whether softwoods or hardwoods are used, whether they are required for internal or external exposures, or whether they are to be used for ornamental or structural purposes. Thus phenol-formaldehyde types would be used for marine or exterior construction uses urea-formaldehyde types would be advantageous for cold pressing, or melamine-urea adhesives might be preferred for hardwood plywood, or lumber-core panels used in furniture production. 

In addition to joining, adhesives in electrical applications may be required to conduct heat, conduct or isolate electricity, provide shock mounting, seal, protect substrates, etc. Thermal and chemical resistance, weathering, and structural compatibility must also be considered in diverse electrical and electronic applications. Of course, the choice of adhesive will also be governed by application methods, cure temperature, processing speed, and overall economic cost. 

Once the fluoropolymer sheet, film, etc., has been etched, it is an adherable surface. The choice of adhesive depends on the required service conditions and, to some extent, the substrate. Some of the considerations in the selection of adhesive include chemical resistance, flexibility, and temperature resistance. 

Choice of adhesive. Use neoprene adhesive for applications where vibration, bumps, etc., are encountered such as over-the-road trailers. For higher temperatures epoxy is preferred. Neoprene-based adhesive should be allowed to dry to touch before the sheets are applied over it. Neoprene adhesive is used for application temperature not exceeding 107°C while epoxy adhesive is used for up to 121°C. It is important to note that adhesive can become a weak link in an otherwise strong chain. 

Structural adhesives can and do provide an excellent alternative to traditional assembly methods. Various structural systems can offer low VOC emissions, which, in turn, can reduce the overall emissions for the customer. Many products can offer little or no offgassing, which makes them safer to use in both indoor and closed environments. The choice of adhesive is critical, and a balance between performance and human health will often be determined by the customer. Adhesive manufacturers are looking to develop products that can address this balance and make the customer s choice an easy one—with no compromise on product performance or employee welfare. 

Proper Choice of Adhesive Good Joint Design Cleanliness Wetting... 

Surface cleaning is critical and depending on the choice of adhesive, can include solvent cleaning, application of a primer, surfaee abrasion and peel of the ply embedded in the laminate. 

Latex adhesives, of which there are many types (Nitrile rubber adhesives (NBR), SBR, Ethylene-vinyl acetate copolymers, acrylics, polyvinylidene chloride, etc.), should ideally be cross-linkable (for wash and dry-clean resistance), preferably with a low cure (reaction) temperature (see Reaction setting adhesives), as some fibres, such as polypropylene, may be damaged by high temperatures. The binder (adhesive) type can markedly affect physical properties and performance the amount of hard and soft polymer in the binder controls the sofmess of the finished product. Most binders impart adequate dry-state adhesion and so the choice of adhesive is usually governed by secondary requirements such as the method and conditions of application and costs. 

Adhesive bonds will always be susceptible to environmental attack, and it is essential that any such attack should not reduce the strength of the bond to an unacceptable level. The most important factor here is the correct choice of adhesive, but design of the joint can make a significant difference. Thus, a thick bond line offers a ready path for access... 

Finally, care must be taken, either in the choice of adhesive or in the assembly technique, to ensure that the adhesive is not wiped from the surface as the parts are placed together. This is likely where, for example, one part is slid into position over another the sliding action tends to wipe the adhesive from the joint area, forming pockets and reducing overall performance. In this context, co-axial assemblies must of course be slid together because of the very nature of the joint. This can cause severe problems for some adhesives but presents no difficulties for the anaerobic varieties, which are formulated specifically to cope with this type of work. 

For achieving passive safety, other elements are important besides the choice of adhesives. It is important to mention that the windshield is also designed to have crash-suitable behaviour. 

Many of the common rubber-based adhesives may be used to bond parts made of MAGNUM ABS resins to a variety of other materials such as metal, wood, and glass. Care should be taken in the choice of adhesive to prevent premature failrme of the composite due to stress cracking. Excellent resrdts are also obtained by rdtrasonic welding of MAGNUM resins to 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. 

Plastics are bonded to one another or to other materials. They differ widely in their affinity for bonding. Some plastics are easy to bond (e.g., PVC-U, ABS, PS), whereas others can be bonded only after special preparation (e.g., PTFE). On account of the low polarity and surface tension of certain plastics (e.g., polyolefins) there are still no adhesives for pipe joints which are durably resistant to shear strain and pressure. The choice of adhesive is determined by the substrate and the mechanical, thermal, and chemical requirements [69] - [75]. 

Service Temperature - the maximum temperature at which the cured adhesive can be used continuously. At higher temperatures, adhesives progressively lose strength. For this reason test specifications normally define this as the temperature at which a certain level of Lap Shear Strength (LSS) is maintained, usually (and for the purposes of this book) this strength figure is 6.9 MPa (1000 psi). Service temperatures may also be quoted as a range. Most adhesives have a lower service temperature below 0°C. For joints in very cold applications (below >55 C) the choice of adhesive is very much reduced. 

From the viewpoint of the adhesive technologist, plastics are an anomaly bonded joints can be made in which the plastic fails before the adhesive, but certain plastics are very difficult to bond. Although plastics often present difficulties in adhesive bonding (Table 4.2), all can be joined successfully with the correct choice of adhesive, joint design, and surface preparation [28] ... 

In this introductory chapter, we focus on two factors relating choice of adhesive to the substrate critical surface tension and solubility parameter. 

I am sure that after reading this chapter, with its long list of some 75 different technical characteristics and properties, our readers are now able to read and understand any paper, book or the subsequent chapters written on adhesives and sealants. I regret that these things may look complicated, but remember that for any given bonding problem or any choice of adhesives, you will probably need to know only 15 to 25 of these characteristics ... 

For composite materials, both mechanical fastening and adhesive bonding would be considered. In the case of composites, adhesive bonding would be the norm. In these cases, the composite would be hand abraded prior to bonding and the choice of adhesive would depend on the hondline thickness tolerances and the temperamre capability of the component itself. Thus, dependant on the component either ambient curing paste or elevated curing film adhesives can be used. 

---