Hardening by cooling

The basic concept with the adhesives that harden by cooling is that a thermoplastic polymer will soften and melt when heated and, if its viscosity falls to a sufficiently low value, the molten polymer may successfully wet the substrate materials to be bonded. Upon cooling, the thermoplastic polymeric adhesive will resolidify, and possibly recrystallize, and therefore develop high cohesive strength. For obvious reasons, these adhesives which harden by cooling are referred to as hot-melt adhesives. Therefore, as usually supplied they contain no water or volatile solvents, i.e. they possess a 100% solids level. 

The earliest types of hot-melt adhesives were based on natural materials such as beeswax, i.e. sealing wax, but virtually all modem hot-melt adhesives are based upon synthetic thermoplastic polymers. They are to be found in a variety of forms such as free-films, coated onto tapes, powders, rods, pellets, blocks 

One of the most widely used polymers in the formulation of hot-melt adhesives is the copolymer of ethylene and vinyl acetate (EVA). For example, such copolymers containing abut 28 to 33% vinyl acetate are widely used as the basis of hot-melt adhesives for bookbinding. They not only bond the individual pages of a paperback book together but they also attach the cover. For such an application the adhesive must have excellent flexibility, toughness and fatigue resistance, and these properties need to be maintained from about -20 to 60 °C. EVA hot-melts are also used for carton and case sealing, carpet applications and in many other industries. 

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The ice-cream is still plastic as it comes from the freezer, and it is extruded into the final sales shape - carton, tub, box, etc. It must then be hardened by cooling down to a storage temperature of - 25°C or lower, during which the other half of its heat of freezing is removed. 

Plastics can be classified as thermoplastic or thermosetting. Thermoplastics are materials that can be repeatedly softened by heat and hardened by cooling. Typical of the thermoplastic family are the styrene polymers and copolymers, acrylics, cellulosics, polyethylenes, polypropylenes, vinyls, and nylons. Thermoset polymers are those that undergo chemical reactions induced by heat, pressure, catalysts, and ultraviolet (UV) light, leading to an infusible state. Typical plastics in the thermosetting family are amines (melamine. 

Thermoplastic A resin or plastic compound that can be repeatedly softened by heating and hardened by cooling. Examples of thermoplastics are acetal, acrylic, chlorinated polyether, fluorocarbons, polyamides (nylons), polycarbonate, polyethylene, polypropylene, polystyrene, some types of polyurethanes, and vinyl resins. 

From the stabilization battery the RDX/water mixture flows to the phleg-matizer or to a continuous liltcr. If RDX is phicgmatizcd this is done in three jacketed and stirred vessels where crystals are coated with wax. Ilie molten wax is added to the tirst phicgmatizer and is hardened by cooling in the second and third vessels amt finally filtered. Moisture content (both unphlegniatizcd and phicgmalized) - ]07t. 

Thermoplastic polymers can be recycled for different purposes because they can be repeatedly softened by heating and hardened by cooling. Thermosetting plastics become hardened permanently. 

Thermoplastic a plastic that repeatedly can be softened by heating and hardened by cooling through a temperature range characteristic of the plastic, and that in the softened state can be shaped by flow into articles by molding or extrusion. 

Thermoplastic - Thermoplastics are resin or plastic compounds which, after final processing, are capable of being repeatedly softened by heating and hardened by cooling by means of physical changes. There are a large number of thermoplastic polymers belonging to various classes such as polyolefins and polyamides. Also called thermoplastic resin. 

Like the extrusion of metals, the extrusion of plastics involves the continuous forming of a shape by forcing softened plastic material through a die orifice that has approximately the geometric profile of the cross-section of the work. The extruded form is subsequently hardened by cooling. With the continuous extrusion process, such products as rods, tubes, and shapes of uniform cross-section can be economically produced. Extrusion to obtain a sleeve of the correct proportion almost always precedes the basic process of blow molding. 

Injection molding is the most important molding method for thermoplastics [7—9]. It is based on the ability of thermoplastic materials to be softened by heat and to harden when cooled. The process thus consists essentially of softening the material in a heated cylinder and injecting it under pressure into the mold cavity, where it hardens by cooling. Each step is carried out in a separate zone of the same apparatus in the cyclic operation. 

Polymers that are capable of being repeatedly softened by heating and hardened by cooling. 

Spinneret A type of extrusion die, i.e., a metal plate with many tiny holes, through which a plastic melt is forced to make fine fibers and filaments which are hardened by cooling in air or water or by chemical action. 

Spinneret Spi-no- ret (1826) n. (1) An extrusion die consisting of a plate with many tiny holes, through which a plastic melt or solution is forced, to make fine fibers and filaments. Early spirmeret holes were round and thus produced fibers of circular cross-section. Today, spinneret holes have many different shapes, even annular ones, to produce fibers of corresponding cross-sections. One purpose is to decrease the fiber-bundle density, giving added warmth, moisture permeability, and enhanced dye receptivity to the textile fabric. An important application of hollow fibers is in artificial kidneys for dialysis. Filaments emerging from the spinneret may be hardened by cooling in air or water, or by chemical action of solutions. (2) A spinneret hole. 

Adhesives that harden by cooling from the melt... 

Thermoplastic adhesives can be apphed as a molten liquid to encapsulate the fabric and then harden by cooling. These heat-activated adhesives generate joint strength almost instantly after cooling. The most common sources of heating include hot air or heated... 

Thermoplastic refers to those plastics tliat are capable of being repeatedly softened by heat and hardened by cooling. These are the types of plastic that we can use in our projects. 

In preparation for paraflSn infiltration water and fats are removed after fixation from the small blocks of tissue by consecutive extractions with alcohol and fat solvents, such as xylene. The tissue blocks are then placed in several changes of molten paraffin. After the displacement of xylene by paraffin is complete, the tissue blocks in paraffin are removed from the oven and hardened by cooling. Sections can be cut easily by a rotary microtome at a thickness of 5 to 7 microns (0.005 to 0.007mm.). When water-soluble carbohydrates are to be studied, it is important to cut and mount the paraffin sections on slides without exposure to water. In ordinary work, the paraffin ribbons are floated on water and lifted on slides for mounting. Paraffin is removed from the tissue sections prior to microchemical tests by consecutive baths in several changes of xylene and alcohols. 

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