Application method liquid adhesives

Only in this century has the art of using adhesives been based on the properties and applications of liquid adhesives other than old glue formulations from fish and animal products or cements made from inorganic slurries or solutions. The subject of adhesives assumed a new stature with the advent of synthetic polymers having readily controlled and understood properties. However,there are many unanswered questions about the principles underlying the subject and accepted methods of application. 

The first three factors are generally controlled by the rheological properties of the liquid adhesive through the application of fillers in the formulation. The final factor can be controlled through the viscosity however, other methods are also possible to control the bond line thickness. 

Films. Both structural and nonstructural adhesives are commonly available in film form. Adhesives applied in the form of dry films offer a clean, hazard-free operation with minimum waste and excellent control of film thickness. However, the method is generally limited to parts with flat surfaces or simple curves. Optimum bond strength requires curing under heat and pressure, which may involve considerable equipment and floor space, particularly for large parts. Film material cost is high in comparison to liquids, but waste or material loss is the lowest of any application method. 

The second widely used method for preparing polyurethane foam laminates is the liquid adhesive, or wet process. Here special adhesives, either in the form of water solutions, or as solutions in organic solvents, are applied to either the fabric or the foam. The equipment is conventional. The water or solvent is evaporated, and the bond may be set by drying or curing at elevated temperature (20). One of the adhesives used for this application is based on acrylic interpolymer latices. This adhesive is used in the manufacture of thermal garments and insulated bags. Carpet underlay can also be made by adhesive laminating (7). 

Basically, the system is comprised of two liquid components - an oligomeric liquid blend (usually acrylic), and a dilute solution of an often proprietary free radical initiator or activator. Application methods are similar to those used for contact adhesives the oligomeric portion is applied by brush, roller or spray to one surface, and the initiator solution is applied in a thin (quasi-monomolecular) film to the other surface. The two films are non-air inhibited and may be left open, unbonded for many hours with no adverse effect on subsequent bonding operations. 

Most acrylic adhesives are supplied as two high viscosity liquids or pastes that are mixed together, called Mix-in Accelerator systems. Some types can also optionally be cured by no-mix accelerator lacquers. Accelerators are low viscosity liquids which are brush or spray applied to one or both of the substrates being bonded. Following accelerator application, the unmixed adhesive is applied to the primed substrate. This method eliminates the need for premixing the adhesive and accelerator, which eliminates pot life concerns. 

The selection of the method of application depends on the adhesive form, whether liquid, paste, powder, film, or hot melt. Other factors influencing the choice of application method are the size and shape of parts to be bonded, the areas where the adhesive is to be applied, and production volume and rate. ... 

Physical Form. Another method of distinguishing between adhesives is by physical form. The physical state of the adhesive generally determines how it is to be applied. Liquid adhesives lend themselves to easy handling via mechanical spreaders or spray and brush. Paste adhesives have high viscosities to allow application on vertical surfaces with little danger of sag or drip. These bodied adhesives also serve as gap fillers between two poorly mated substrates. 

The method used to apply the adhesive or sealant can have as much to do with the success of the joining operation as the kind of material applied. The selection of an application or dispensing method depends primarily on the form of the adhesive liquid, paste, powder,... 

Heating the adhesive before atomization enables heavier adhesive buildup, reduces overspray losses, and minimizes contamination from atmospheric water vapor. Spray methods can be used on both small and large production runs. The liquid to be sprayed is generally in solvent solution. Sizable amounts of product may be lost from overspray. Two-component adhesives are usually mixed prior to placement in the spray gun reservoir. Application systems are available, however, that meter and mix the adhesive within the spray gun barrel. This is ideal for fast-reacting systems, but guns must be thoroughly cleaned to avoid buildup of polymerized product. 

The power of laser ablation can be extended as a popular method for trace and bulk analysis in conjunction with ICP-OES and is an invaluable tool in the study of surface behaviour particularly where sensitive surfaces are important. The common area for surface knowledge is in environment, medicines, adhesives, powders, slurries, oil-based samples and liquids. It finds application in the analysis of metallurgical samples, non-conductive polymers, ceramic materials, surface mapping, elemental migration, depth profiling, thin film coatings, biological and clinical specimens, forensic, paint chips, inks, bullets, fabrics, etc. 

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