Hot melt systems

To improve on penetration and heat resistance (properties normally in opposition when formulating hot melts), reactive hot melt systems are being developed. Moisture curing urethanes have already gained wide acceptance in a number of wood and automotive applications. UV cure is slowly gaining acceptance in PSA, labeling, and other markets. 

Many thermoplastic materials are available commercially and it might seem at first sight that all of them could be considered for hot-melt systems. However,... 

Not all solvent adhesives will he replaced with latexes per se. For some applications, hot melt adhesive systems are heing developed. However, just as ethylene-vinyl acetate copolymers produced by emulsion polymerization are a major component of many hot melt systems, suitably designed emulsion polymers to meet specific requirements will probably he present in these systems, too. 

Most systems function with paste but Young et al. (10) described the use of a process whereby the extrusion was with a hot-melt system, the extrudate cut in a pelletizer (Randcastle RCP-2,0, Randcastle Inc. Cedar Grove. New Jersey, U.S.A.) into cylindrical pellets, which were then spheronized on a conventional spheronizer (Caleva Model 120, AC Compacting LLc, North Brunswick, New Jersey, U.S.A.) heated with a heat gun to 65-70 C while dusting with microcrystalline cellulose (MCC) to prevent agglomeration. The picture of the pellets shown to illustrate the product appears round but the process times of 45 and 80 minutes are considerably longer than those involved with the paste systems. 

High-pressure extrusion i.e. jetting—widely used for hot melt systems where a jet is fired by a pressurised gun. 

The coating must also be solidified very rapidly once it is applied to the fiber so that it will offer protection when the capstan is reached. This requirement rules out solvent-containing formulations for all but the thinnest coatings, because solvent removal is a slow process. Solvent-free coating formulations that are rapidly cross-linked by thermal activation or by UV radiation can be used very successfully. Thermoplastic hot-melt systems that solidify quickly upon cooling are also viable. The material systems of choice for this application depend upon both coating-application and coating-performance considerations. 

One of the newer developments in adhesives is the growing use of ultraviolet light or electron beam radiation to cure adhesives. Adhesives designed for UV- or E-beam curing are usually pressure sensitive or hot-melt systems based on acrylates, functional rubbers, or epoxidized rubbers, and use special UV or EB lamps to provide the cure. These systems can provide greatiy improved heat resistance compared to hot melts, and avoid the soivent emission problems of some of the solvent-based systems with which they compete. 

Uses Resin in nat. rubber, styrene-butadiene rubber (SBR), nitrile-butadiene rubber, block polymers, ethylene-vinyl acetate for hot-melt systems, and coatings... 

Chem. Descrip. Aliphatic resin, stabilized with 0.05% antioxidant (tetrakis[methylene(3,5-di-t-butyl-4-hydroxyhydrocinnamate)]methane) Uses Tackifier in pressure-sensitive and hot-melt systems, pkg. adhesives, coatings... 

Zonatac 115L Zonestei 85 tackifier, hot-melt systems Piccotac 1020... 

Solvent recovery and recycle has limitations in that it still depends on solvents and requires expensive equipment. If complex mixtures of solvents are used, they must be separated and cleaned up before reuse. Workers may still be exposed to hazards, and insurance rates where solvents are involved are increasing. One-hundred percent solids systems may be the wave of the future, however, at this time the technology is limited. Current hot melt systems have thermal stability problems which inhibit their use in various applications. And though problems and disadvantages remain to be solved with water-based systems, the current technology is practical and presents advantages the other alternatives cannot achieve at this time. 

This is a low viscosity liquid, which dissolves many amorphous plastic resins, with high TgS enabling their ready incorporation into hot melt systems. It is compatible with cyanate ester prepolymers, epoxy resins and bismaleimides. 

Adhesive joints may be subjected to a variety of adverse service conditions, including elevated temperature, organic solvents, water and stress (see Durability fundamentals. Durability creep rupture). Solvent-based, emulsion and melt adhesives are normally based on thermoplastics with fairly low softening temperatures. If a loaded joint is subjected to elevated temperature, failure may occur because of Creep unless the adhesive is cross-linked. Likewise, attack by organic solvents can be minimized by cross-linking. Solvent-based, emulsion and hot melt systems are available that cross-Unk after the initial bonding has been carried out. These systems provide improved in-service performance. 

For optimum adhesion, the wetting capability of the hot-melt system must be good. Wetting is related to viscosity and is again largely influenced by tackifying resin selection and quantity. 

Several new approaches have been introduced into the garment industry primarily in response to the need for faster-setting adhesives. These include reactive hot-melt systems that cross-link on exposure to relative humidity in flie ambient environment and solventless liquid adhesives that cure on exposure to ultraviolet (UV) light. 

Figure 8. Hand gun for hot-melt (system lang) a) Adhesive stick b) Feeding c) Retaining ring d) Seal e) Melting chamber f) Nozzle g) Wedge heater or heating wedge h) TVigger...

Considering the product groups, the highest growth rates are expected for hot-melt systems and polymer dispersions and emulsions. A continuing decrease will occur for adhesives based on natural polymers (animal and vegetable), solvent-based adhesives, and systems based on water-soluble polymers. 

Hot melt adhesives are also high solid systems which can meet environmental requirements. Thus, the growth of hot-melthas been projected to double by 1980 (Fig. 6). It is likely that new hot-melt systems will be introduced to the market. To demonstrate the promising features of hot-melt, we choose the ethylene-vinyl-acetate system (EVA) as an example. EVA can be formulated with a tackifier and a wax. The formulation can be facilitated with the use of computer to plot the viscosity-composition profile. 

Hot-melt systems are usually flexible and tough. They are used extensively for sealing applications involving paper, plastic films, and metal foil. Table 7.29 offers a general comparison of hot-melt adhesives. Hot melts can be supplied as (1) tapes or ribbons, (2) films, (3) granules, (4) pellets, (5) blocks, or (6) cards, which are melted and pressed between the substrate. The rate at which the adhesive cools and sets is dependent on the type of substrate and whether it is preheated. Table 7.30 lists the advantages and disadvantages associated with the use of water-based, solvent-based, and hot-melt thermoplastic adhesives. 

The ability to speed up packaging lines to achieve much greater productivity without increasing space requirements, by simply converting to hot melt adhesives from cold glues, was an economic bonus to many companies. Hot melt application equipment interchangeable with aqueous dispensers was developed by several companies, permitting relatively inexpensive conversions to the hot melt system. 

Hot melt adhesives used in diaper construction are of two general types high viscosity systems based on atactic polypropylene for the end seal and lower viscosity, faster setting ethylene-vinyl acetate based adhesives for the side seam. With the evolution of the construction to a multi-line laminating system, manufacturers have adopted one or the other hot melt system with which they feel most comfortable. The composition of an atactic polypropylene based hot melt adhesive is illustrated in Table 12. 

A more comprehensive listing may be obtained from the various trade and educational associations concerned with hot melt systems, which could provide information regarding manufacturers who are associated with their organizations. 

Pressure sensitive adhesives based on a variety of elastomers and applied from either latex, solvent, or hot melt systems have shown rapid growth in recent years. In addition, the development of hot melt assembly adhesives based on the styrenic thermoplastic elastomers is a key factor in the production of disposable diapers and other sanitary products. Even though the current emphasis of elastomer-based adhesive development is on pressure sensitive adhesives, large volumes of solvent cements, latex cements, and mastics are still produced. 

Hot Melt - Hot melt adhesives are based on diermoplastic polymers that are solids at room temperature. Hot melts are appUed at high temperatures, under which they melt and wet the substrate surface. On cooling, the polymer returns to the solid state providing good eohesive strengfli. Two major advantages of the hot melt systems are that they set quickly after being applied (seeonds to minutes) and require no cure and hence have no by-products. 

Plasticizers (A.S.C., 1986 Dick, 1987) - Plasticizers are widely used in elastomer based adhesives and sealants and hot melt systems to improve processability and final cured properties. Typical use levels are 10 to 20 weight percent. Plasticizers are low molecular weight, low volatility molecules which are soluble in die base polymer. They have several functions ... 

In hot melt systems, plastieizers lower the melt viscosity and can improve the wetting of the adherend, but may lower adhesion if the plastici2er level is too high. When added to emulsion adhesives or sealants the plasticizer migrates to the polymer droplets, swelling them and hence increasing the viseosity of the emulsion and helping to reduce slump. 

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