10 сентября, 2015 
Pokraskin The same classification used in Chapter 1.5 is used here. Information on the uses of the adhesives discussed here can be found in Chapter 1.5.
1.6.1. Adhesives That Set Without a Chemical Reaction
1.6.1.1. Solvent-Free Adhesive Systems
Hot-melt adhesives are 100 % solids that, in the broadest sense, include all thermoplastic polymers. Polymers that are primarily used as hot-melt adhesives include ethylene-vinyl acetate copolymers (EVA), polyvinyl acetates (PVA), polyethylene (PE), amorphous polypropylene, block copolymers (thermoplastic elastomers), polyamides, and polyesters. The oldest hot-melt adhesive, which has been in use since early times, is sealing wax. In principle, glutins and glue jellies also may be regarded as hot-melt adhesives. However, modern hot-melt adhesives are primarily synthetic products. The simplest hot-melt adhesives are rosin-wax mixtures. But these products have limited strength and thermal stability.
In general, hot-melt adhesives are solid below 80 °С. Ideally, as the temperature is increased beyond this point, the material rapidly melts to a low-viscosity fluid that can be easily applied. Upon cooling, the adhesive sets rapidly. Because these adhesives are thermoplastics, the melting/resolidification process is repeatable. Typical application temperatures of hot-melt adhesives are 150-190 °С with melt viscosities in the range of 500-3000 mPas.
Hot-melt adhesives do not undergo a chemical reaction because they simply cool from the melted state to form a solid film at the bond line. These adhesives are available in forms such as pellets, slabs, bars, slugs, and films that allow convenient handling by a variety of application equipment. Because hot-melt adhesives are applied in the molten state, melt viscosity is an important property. The limitations of the application equipment often influence the viscosity range that is selected for a hot-melt adhesive formulation.
Although hot-melt adhesives consist of 100 % solids, they are rarely 100 % polymers in composition. This is due to the limited adhesion of pure thermoplastics like EVA copolymers and their lack of melt properties such as tack and wettability. The components of a hot-melt adhesive can be roughly divided into two categories: polymers and diluents. Typically, diluents are waxes, plasticizers, tackifiers, stabilizers, extenders, and pigments. The functions of the diluents are:
— Lowering the viscosity for easy application
— Enhancing wettability
— Enhancing adhesive strength
— Increasing rigidity (extenders) or flexibility (plasticizers)
It is desirable to have a hot-melt adhesive with high strength. But this property is generally accompanied by a high melt viscosity. A low melt viscosity is desirable for application purposes. Therefore hot-melt adhesive formulations must include diluents that decrease the polymer melt viscosity and increase the overall adhesive strength.
Special dispensing equipment must be used to apply hot-melt adhesives. Typical application methods include the use of rollers, screw extruders, and squirting pumps or nozzle applicators.
The polymers are generally of high molecular mass and impart strength and high viscosity to an adhesive. Waxes such as paraffin wax can function as both diluents and antiblocking agents. In addition, waxes promote surface wetting. Similarly, plasticizers such as phthalates, mineral oils, and glycolates provide both surface wetting and adhesive flexibility. Typical tackifiers include rosin, modified rosin, terpenes, modified terpenes, hydrocarbons, and chlorinated hydrocarbons. These materials provide tack and flexibility while promoting surface wetting and adhesion. Stabilizers such as hindered phenols help to maintain adhesive melt viscosity while functioning as antioxidants. Extenders such as talc, clay, and barites lower the cost of hot-melt adhesives and simultaneously help control melt flow.
The following properties of hot-melt adhesives are important:
Melt viscosity is one of the most important properties of a hot-melt adhesive. In general, as the temperature of a polymer increases, its viscosity decreases. Therefore, in a hot-melt adhesive formulation, the melt temperature controls the viscosity, which greatly influences the extent of surface wetting. The temperature of the melt and the application equipment should be maintained as constant as possible.
The bond formation temperature is the minimum temperature below which surface wetting is inadequate. A hot-melt adhesive is applied at a running temperature at which the viscosity is sufficient to wet surfaces properly. Running temperatures that are too high allow more time for surfaces to be wet properly. However, if low penetration is required and if the substrates are porous, this longer time may be detrimental and may ’ result in a bond that contains insufficient adhesive. A running temperature that is too close to the bond-formation temperature may result in a too-rapid solidification of the adhesive bond. As a result, overall properties such as strength may not be optimized.
The heat stability of the adhesive is a very important property. Running temperatures that are greater than the adhesive degradation temperature result in charring and decreased overall properties. The addition of stabilizers to the adhesive formulation contributes to the stability of the material by hindering the acceleration of degradation due to the presence of oxygen.
The tack of the adhesive indicates the stickiness of the hot-melt as it changes from a liquid to a solid state. This property affects the ability of the adhesive to hold the substrates together.
The open time is the lapse of time between the application of the hot-melt to one substrate and the loss of its wetting ability on the second substrate due to solidification.
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.
Manufacture. In the manufacturing of hot-melt adhesives, the components are melted in heated stirring vessels and subsequently formed in pelletizing or granulating machines. The melt is allowed to drop onto a cooling belt or to form a film on a cooling belt and subsequently cut. Another possibility is extrusion from an extruder followed by underwater granulation. For manufacturing hot-melt adhesives with higher viscosity and higher filler contents, it is preferable to use heatable kneaders or extruders followed by specialized processing units. Hot-melt sticks (slugs, rods) for application by handguns are produced by injection molding or by cutting hot-melt profiles.
Surface-tacky hot-melt adhesives are marketed in containers or in the form of blocks or cubes in readily removable packs (silicone-coated paper) [33] or in meltable polyethylene bags.
Hot-Melt Chemical Families. EVA copolymers are the cheapest hot-melt adhesives and are used in the greatest quantities. Typical properties of these adhesives include sufficient strength between ca. 30 and 50 °С, but limited upper service temperature of 60 — 80 °С, low hot-melt viscosity, and low creep resistance under load with time. Most EVA-based hot-melt adhesives are used to bond paper, fabrics, wood, and some thermoplastics.
PP polymers based on amorphous polypropylene have better properties than EVA hot-melt adhesives. In particular, higher heat resistance and better adhesion properties can be obtained.
Polyamides and thermoplastic polyesters are classified as high-performance hot-melt adhesives. These adhesives have greater strengths than EVA-based hot-melt adhesives and exhibit adequate bonding from approximately -40 to 70 °С. Some formulations have upper service temperatures up to 185 °С for applications that do not involve loads. One disadvantage of the higher service temperatures is the higher hot-melt application temperature, which may require special equipment. Both polyamides and polyesters are sensitive to moisture absorption during application. This can result in hot-melt foaming, which leads to voids in the solidified adhesive and decreased strength. Nonetheless, polyamides exhibit good strength and flexibility. Polyesters are even stronger, but are more rigid and are usually highly crystalline. This results in a sharp melting temperature, which is desirable for high-speed bonding. Polyamide and polyester hot-melt adhesives are used to bond plastics, glass, wood, leather, foam, fabric, rubbers and some metals, such as aluminum, copper, and brass.
Polybutene-based hot-melt adhesives are tough, partially crystalline, and their slow crystallization rates lead to long open times. Copolymers of butene result in softer and more flexible adhesives. In general, polybutene and its copolymers have low temperatures for recrystallization from the melt. This permits stress release in the adhesive bond, which may have been applied to cold surfaces. Polybutene and its olefinic copolymers exhibit good bonding to nonpolar surfaces but poor compatibility with polar substances. These hot-melt adhesives have been used on rubbery substrates and are available as pressure-sensitive adhesives.
Thermoplastic elastomers that are formulated into hot-melt adhesives include polyurethane and block terpolymers, such as styrene-butadiene-styrene; styrene-iso — prene — styrene; and styrene — olefin — styrene, in which the olefin component is typically ethylene, propylene, and/or butylene. The saturation in the mid-segment of these terpolymers results in better UV and thermooxidative resistance than that of unsaturated butadiene and isoprene midsegments.
In general, thermoplastic elastomer hot-melt adhesives are not as strong as polyesters. However, they exhibit good flexibility, high elongation, toughness, and vibration resistance. Polyurethanes and block terpolymers are used as hot-melt pressure-sensitive adhesives in tapes and labels and for nonwoven applications.
Pressure-sensitive hot-melt adhesives are surface-tacky and can be used by the application of pressure alone. These adhesives have infinite open times and are used at room temperature. Typically, pressure-sensitive hot-melt adhesives are supplied in form of blocks or cubes in silicone coated packaging units or preferably in polyethylene bags, which after melting form part of the adhesive.
Foamable hot-melt adhesives have been commercially available since 1981. Nitrogen or carbon dioxide is introduced into the hot-melt adhesive and results in a 20-70% increase in adhesive volume. Foaming increases hot-melt spreading and open time. This method is usually used with polyethylene hot-melt adhesives that are applied to selected metal, plastic, paper, porous, and heat-sensitive substrates.
Low-melt adhesives are hot-melt adhesives with an application temperature in the. range of 100-120 °С instead of 160-190 °С. They are used for bonding heat-sensitive materials.
Plastisol adhesives are solvent-free adhesives that require temperatures of 120-200 °С for setting. They consist essentially of a dispersion of finely divided polyfvinyl chloride) or polymethacrylate [34] in plasticizers and low molecular mass, heat-reactive substances that act as adhesion promoters, for example, epoxy resins, polyfethylene glycol) dimethacrylates, phenolic resins; in some cases these are even used in conjunction with corresponding hardeners. Standard heat stabilizers for polyfvinyl chloride) are also added to prevent decomposition of the polyfvinyl chloride) and evolution of HC1 during hardening or in the event of subsequent exposure to heat. Depending on the end use, varying amounts of fillers, pigments, or thixotropizing agents may be added to modify the color and fixing behavior. At a certain setting or hardening temperature, the polyf vinyl chloride)/polymethacrylate plasticizer mixture
Production and Storage. Because of their relatively high viscosities, these pasty to highly viscous compositions must be produced in kneader-type stirring vessels capable of applying considerable forces. It is important to ensure that stirring and kneading do not generate excessive temperatures, which can result in gelation of the product. For the same reason, the products must be stored under cool conditions (below 30 °С).
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