Thermoplastic pastes

Thermoplastic pastes are polymer solutions that have a low melting point, usually lower than 65°C. When these polymer solutions are injected into the targeted body site, the polymer solution cools rapidly to body temperature... 

Hybrid microcircuits and MCMs MIL-STD-883, Method 5011-compliant, low outgassing, high-purity, reworkability Silver-filled or insulative epoxies, thermoset or thermoplastic pastes or films... 

Polyimides became popular for die-bonding adhesives because they are cleaner (in terms of ionic contaminants) than equivalent epoxy products. The market at present, however, is dominated by high-purity epoxy adhesives. Polyimides are always applied from solvent solution and require higher curing temperatures than epoxies. They are stable to higher temperatures. Solvent-borne thermoplastics pastes (or cast-film preforms) have been used for some lower reliability die-attachment applications. 

The decade 1930-1940 saw the initial industrial development of four of today s major thermoplastics polystyrene, poly(vinyl chloride) (PVC), the polyolefins and poly(methyl methacrylate). Since all these materials can be considered formally as derivatives of ethylene they have, in the past, been referred to as ethenoid plastics however, the somewhat inaccurate term vinyl plastics is now usually preferred. 

In all of the examples given so far in this chapter the product of polymerisation has been a long chain molecule, a linear polymer. With such materials it should be possible for the molecules to slide past each other under shear forces above a certain temperature such that the molecules have enough energy to overcome the intermolecular attractions. In other words above a certain temperature the material is capable of flow, i.e. it is essentially plastic, whereas below this temperature it is to all intents and purposes a solid. Such materials are referred to as thermoplastics and today these may be considered to be the most important class of plastics material commercially available. 

Over the past 40 years there have been a number of developments that have resulted in the availability of rubbery materials that are thermoplastic in nature and which do not need chemical cross-linking (vulcanisation or setting) to generate elastomeric properties (see also Section 11.8 and 31.2). This approach has been extended to the fluoroelastomers. 

Although some of the polyamides described in Section 18.10 are somewhat rubbery, they have never achieved importance as rubbers. On the other hand, the past decade and a half has seen interest aroused in thermoplastic elastomers of the polyamide type which may be considered as polyamide analogues of the somewhat older and more fully established thermoplastic polyester rubbers. 

Today the phenol-formaldehyde moulding compositions do not have the eminent position they held until about 1950. In some, important applications they have been replaced by other materials, thermosetting and thermoplastic, whilst they have in the past two decades found use in few new outlets. However, the general increase in standards of living for much of this period has increased the sales of many products which use phenolics and consequently the overall use of phenol-formaldehyde moulding powders has been well maintained. 

In terms of tonnage the bulk of plastics produced are thermoplastics, a group which includes polyethylene, polyvinyl chloride (p.v.c.), the nylons, polycarbonates and cellulose acetate. There is however a second class of materials, the thermosetting plastics. They are supplied by the manufacturer either as long-chain molecules, similar to a typical thermoplastic molecule or as rather small branched molecules. They are shaped and then subjected to either heat or chemical reaction, or both, in such a way that the molecules link one with another to form a cross-linked network (Fig. 18.6). As the molecules are now interconnected they can no longer slide extensively one past the other and the material has set, cured or cross linked. Plastics materials behaving in this way are spoken of as thermosetting plastics, a term which is now used to include those materials which can in fact cross link with suitable catalysts at room temperature. 

For the past century one successful approach is to plot a secant modulus that is at 1% strain or 0.85% of the initial tangent modulus and noting where they intersect the stress-strain curve (Fig. 2-2). However for many plastics, particularly the crystalline thermoplastics, this method is too restrictive. So in most practical applications the limiting strain is decided based on experience and/or in consultation between the designer and the plastic material manufacturer. Once the limiting strain is known, design methods based on its creep curves become rather straightforward (additional information to follow). 

DOT is used for frozen foods. They have been a major outlet for plastics. In the past thermoset plastic materials were used but it practically all went to thermoplastics such as CPET. 

We can further characterize polymers into thermoplastics and thermosets . Thermoplastics consist of linear or lightly branched chains that can slide past one another under the influence of temperature and pressure. These polymers flow at high temperatures which facilitates their molding into useful products. Thermosets consist of a network of interconnected chains whose positions are fixed relative to their neighbors. Such polymers do not flow when heated. 

Block copolymers possess unique and novel properties for industrial applications. During the past 20 years, they have sparked much interest, and several of them have been commercialized and are available on the market. The most common uses of block copolymers are as thermoplastic elastomers, toughened thermoplastic resins, membranes, polymer blends, and surfactants. From a chemist s point of view, the most important advantage of block copolymers is the wide variability of their chemical structure. By choice of the repeating unit and the length and structure of both polymer blocks, a whole range of properties can be adjusted. 

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