An adhesive is a specific substance capable of adhering to a surface, or of facilitating the attachment of two surfaces or substances in varying degrees of permanence or application.
TYPES OF ADHESIVES
Adhesive materials are widely used in binding and finishing for adhering pages together and in adhering pages to covers, and in packaging for the forming, sealing, and labeling of boxes, cartons, bags, and other packaging materials. There is a wide variety of adhesives available, the specific variety for a particular end-use being dictated primarily by cost, the nature of the substrate, and the specific end-use requirements. There are three general classes of adhesive:
'Water-Based Adhesives'. The most prevalent type of adhesive are those that are water borne, their popularity due to their lack of toxicity, their low cost, and their strength. Natural water-based adhesives include those based on starches, in particular corn starch. Such adhesives are derived from raw flour or starch, or, more commonly, the acid hydrolysis of the starch molecule, in which it is broken down into smaller units. At this point, it can be combined with other materials, such as tackifiers (primarily borate salts, sodium hydroxide, sodium silicate, and plasticizers and other fillers). Starch-based adhesives are often used for the forming and sealing of corrugated board used in shipping cartons, the winding of cardboard tubes, the forming and sealing of bags, and the adhesion of labels on cans. A variety of starch-based adhesive called jelly gum is used to affix labels on bottles. Jelly gums are particularly compatible with high-speed labeling equipment.
Casein, derived from precipitated cow's milk, is also used in some types of adhesive, in particular beer-bottle labeling, as it is especially resistant to cold-water immersion during beer production, and the adhesive is easily removed during glass-bottle reclamation. Casein adhesives are also used (with special synthetic elastomers) in the adhesion of aluminum foil to paper.
Other infrequently used natural adhesives include animal proteins and animal glues, the latter derived from collagen extracted from animal tissues. Once quite popular, these animal-based adhesives are used only in certain specialty applications, as the advancement of synthetic adhesives far surpasses their performance characteristics.
Natural rubbers and latexes are also used, especially in self-seal applications for candy wrappers and envelopes, among other uses.
Synthetic water-based adhesives are the most widely-used in packaging. Synthetic adhesives are derived from resin-based emulsions such as a suspension of polyvinyl acetate in water. Additives such as protective colloids and plasticizers, fillers, solvents, preservatives, and other materials are often added to impart a desired set of properties, such as water resistance or adhesion to metals and plastics as well as to paper and paperboard. The cost-effectiveness of such adhesives has also ensured their proliferation in the packaging industry. Recent developments in the use of acrylic polymers with vinyl acetate copolymers have also improved the adhesion properties of these adhesives.
'Hot-Melt Adhesives'. Hot-melt adhesives are based on thermoplastic polymers (such as the copolymers of ethylene and vinyl acetate) which melt when heated, are applied in the molten state, and form a strong bond when cooled. Waxes are often added to these adhesives to lower the viscosity of the melt, facilitating the application of the adhesive, and to regulate the speed at which the material sets. Hot-melt adhesives based on polyethylene are not useful in as broad a range of applications as ethylene-vinyl acetate-based adhesives, but are widely used in the sealing of bags and cases. Amorphous polypropylene is also used in some applications—specifically in the bonding of paper to paper to produce water-resistant wrappings or shipping tape—but the weakness of these adhesives makes them unsuitable for many other uses.
Newer hot-melt adhesives based on copolymers of styrene and butadiene have emerged in recent years, and have found application in pressure-sensitive adhesives for tape and labels, as well as in plastic soft-drink bottles.
The advantages of hot-melt adhesives—adhesion by simple cooling, coupled with their ability to flow into gaps and join ill-fitting surfaces—easily outweigh their disadvantages, which include reduced strength at elevated temperatures. And the wide variety of materials used for hot-melts imparts the ability to adhere to nearly any surface. Hot-melt adhesives are the most widely used adhesives in binding, especially perfect binding.
'Solvent-Based Adhesives'. Due to a variety of cost and environmental factors, solvent-borne adhesives are only used in very specialized applications where water-borne or hot-melt adhesives are inappropriate or ineffectual. Solutions of rubber-resin materials are often used for pressure-sensitive labels and tapes, but other less expensive and environmentally safer types of adhesives are gradually edging out such formulations. Similarly, solvent-borne polyurethane adhesives—now used in the sealing of plastic films such as plastic bags, snack wrappers, and boil-in-bag dinners—are also being edged out by other types of adhesive systems. Acylic polymers, for example, are now de rigeur in potato chip bags. A type of "warm-melt" polyurethane (containing no solvent vehicle) is now being used in some food-packaging applications.
As there are many different types of adhesives and surfaces to which adhesive is applied, so too is there a large number of devices and configurations for applying the adhesive. There are generally several categories of adhesive applicating systems.
'Circulating vs. Non-Circulating'. The most common system is the non-circulating system, which is used to apply both cold water-borne and hot-melt adhesives. A non-circulating system essentially comprises a parallel series of guns, each one attached to the main tank by individual hoses. The adhesive is pumped to each gun though the hose, and the adhesive is applied by the nozzle of the gun. In a circulating system, one single hose connects all the guns in series, the hose running in a loop out of the main tank, though the guns, then back into the tank. (A kind of hybrid system is known as an internally circulating system, in which a single hose circulates the adhesive within the device, but each gun is at the terminal of offshoot hoses which form dead ends.)
'Cold-Glue vs. Hot-Melt Systems'. In a cold-glue system, hoses pump the fluid adhesive to applicator heads which apply the adhesive to the substrate in bead, spray, or droplet form (depending upon the requirement) by means of attachments or special configurations on the applicator heads.
In a hot-melt system, an additional requirement is needed: melting the adhesive. The most common configuration for melting devices is a tank melter, or essentially a large pot or tank in which cold adhesive can be added and melted into liquid form. Small tank melters hold about eight pounds of adhesive, while larger units can hold up to several hundred pounds. The sides of the tank are lined with a thermally conductive substance, and the adhesive melts first along the sides of the tank. Internal circulation from the pumping mechanism works to distribute the heat throughout the interior of the tank. A variation on the tank melter is a grid melter, in which dimensional patterns are used to increase the surface area of the heated surface. Grid melters provide a much greater melt rate than straight tank melters. Various other configurations can also be utilized to keep the adhesive cool or warm, depending upon the chemistry of the adhesive. Some adhesives need to be "premelted" and kept in a liquid form before application, while some degrade quickly when melted and need to be kept cool until just prior to application. The delivery of the molten adhesive to the dispenser can be effected either using piston-driven or gear-driven pumping devices. The dispensing mechanism itself can be some variety of extrusion gun. A conventional extrusion gun essentially pumps the molten adhesive through a nozzle onto the substrate. Automatic extrusion guns use timers and other controllers to dispense adhesive at set rates. Manual systems require human intervention. Web-extrusion guns use the same basic principle of the extrusion gun coupled with patterned blades to apply a film of adhesive to a moving substrate. The thickness of the film and the pattern in which it is applied can be adjusted. Labels, tapes, envelopes, and other such continuous substrates commonly utilize web-extrusion guns. A third type of dispensing device utilizes rotating wheels and rollers to apply adhesive to a substrate.
'Delivery Rate'. In the application of an adhesive, it is necessary to determine the MIDR, or the maximum instantaneous delivery rate (also called the IPDR, or instantaneous pump delivery rate') or the amount of adhesive that a pump needs to feed to the dispensing guns, assuming the guns were continuously extruding adhesive. The commonly-used units for MIDR are pounds per hour (lb/h) or grams per minute (g/min). The MIDR is calculated using the size of the bead that a gun is required to lay down, and the speed with which this can be accomplished.