Thermoplastics states

The binder system of a plastic encapsulant consists of an epoxy resin, a hardener or curing agent, and an accelerating catalyst system. The conversion of epoxies from the Hquid (thermoplastic) state to tough, hard, thermoset soHds is accompHshed by the addition of chemically active compounds known as curing agents. Flame retardants (qv), usually in the form of halogens, are added to the epoxy resin backbone because epoxy resins are inherently flammable. 

All thermoplastic pol)aners and elastomers, with the exception of silicones, are carbon-based. They are made up from the linking of one or more monomers into long molecular chains. Many of the same monomers are foimd in both thermoplastic and elastomeric polymers. Typical examples include styrene, acrylonitrile, ethylene, propylene, and acrylic acid and its esters. An elastomer is in a thermoplastic state prior to vulcanization. 

Thermoplastics are uncrosslinked plastics up to their decomposition temperature. Flow or melting (Fig. 2) occurs above the softening point of the amorphous structure in amorphous thermoplastics and above the melting temperature of semicrystalline thermoplastics. In this thermoplastic state, the viscous liquid can be processed. Form strength is achieved by cooUng. Meltdown, solidification, and crystallization can be repeated any number of times. 

Injection molding is one of the primary processing methods. It is carried out in the thermoplastic state, i.e. above the flow temperature range (7/) or crystalline melting temperature (Ij ) and below the decomposition temperature (Ij/). 

In the injection molding process, the polymer exists in a thermoplastic state. It flows in a viscous manner. The viscosity of a liquid is a measure of its flow resistance. For a Newtonian liquid, such as water, the viscosity is defined as [t]], (shown in Figure 1-7). 

SINCE the discovery of liquid crystalline phenomenon for low molecular weight liquid crystals (LMWLCs) more than 100 years ago, anisotropic ordering behaviors of liquid crystals (LCs) have been of considerable interest to academe [1-8], In the 1950s, Hory postulated the lattice model for various problems in LC systems and theoretically predicted the liquid crystallinity for certain polymers [1-3], As predicted by the Hory theory, DuPont scientists synthesized lyotropic LCPs made of rigid wholly aromatic polyamide. Later, Amoco, Eastman-Kodak, and Celanese commercialized a series of thermotropic main-chain LCPs [2]. Thermotropic LCPs have a unique combination of properties from both liquid crystalline and conventional thermoplastic states, such as melt processibility, high mechanical properties, low moisture take-up, and excellent thermal and chemical resistance. Aromatic main-chain LCPs are the most important class of thermotropic LCPs developed for structural applications [2,4-7]. Because they have wide applications in high value-added electronics and composites, both academia and industry have carried out comprehensive research and development. 

Thermotropic main-chain LCPs have unique combination of properties from both LC and conventional thermoplastic states these include melt processibil-ity, high mechanical properties, low moisture take-up, and excellent thermal and chemical resistance. With the successful development of these LCPs and recognition of their imique properties, comprehensive research and development have been carried out by both academia and industry (3,5,9,11,14,16-26). Among various R D directions, the synthesis of new LCPs (3,14,16,17,19-22,24,26), their rheology behavior (27-31), morphology, compatibility and processing of LCPs and blends (32-34) have received most attention. 

Dry Blending. Phenol-formaldehyde resin still in its resole thermoplastic state is cold-pulverized and then perfectly mixed with fillers and other additives in a two-roll mill heated 80-115°C. Once homogenized, the blend stays on the warmer roll. It is then possible to cut it, since during homogenization, resin condensation continues going partially from resole to resite, and this forms a cohesive layer. Once cold, the blend can be pulverized. 

The rule of thumb ( ... the reinforcing effect is directly related to the viscosity decrease of the blends compared to the neat thermoplastics ) stated by Fekete et alP is intriguing but of course too simple and limited. There is a need for a more general theory or predictive model of LCP blends which would allow input of rheological parameters, parameters related to adhesion, orientability, degradation temperature, etc. and of course the cost of the components, and allow one to compare the expected material to commercially available grades of glass-filled thermoplastics, etc. 

In practice, synthetic polymers are sometimes divided into two classes, thermosetting and thermo-plMtic. Those polymers which in their original condition will fiow and can be moulded by heat and pressime, but which in their finished or cured state cannot be re softened or moulded are known as thermo setting (examples phenol formaldehyde or urea formaldehyde polymer). Thermoplastic polymers can be resoftened and remoulded by heat (examples ethylene polymers and polymers of acrylic esters). 

Hot Plate, Infrared, and Hot Gas Welding. These processes involve external means to heat thermoplastic polymers to a viscous state in... 

This type of adhesive is generally useful in the temperature range where the material is either leathery or mbbery, ie, between the glass-transition temperature and the melt temperature. Hot-melt adhesives are based on thermoplastic polymers that may be compounded or uncompounded ethylene—vinyl acetate copolymers, paraffin waxes, polypropylene, phenoxy resins, styrene—butadiene copolymers, ethylene—ethyl acrylate copolymers, and low, and low density polypropylene are used in the compounded state polyesters, polyamides, and polyurethanes are used in the mosdy uncompounded state. 

Since the early 1980s olefin plants in the United States were designed to have substantial flexibiHty to consume a wide range of feedstocks. Most of the flexibiHty to use various feedstocks is found in plants with associated refineries, where integrated olefins plants can optimize feedstocks using either gas Hquids or heavier refinery streams. Companies whose primary business is the production of ethylene derivatives, such as thermoplastics, tend to use ethane and propane feedstocks which minimize by-product streams and maximize ethylene production for their derivative plants. 

Usage of phosphoms-based flame retardants for 1994 in the United States has been projected to be 150 million (168). The largest volume use maybe in plasticized vinyl. Other use areas for phosphoms flame retardants are flexible urethane foams, polyester resins and other thermoset resins, adhesives, textiles, polycarbonate—ABS blends, and some other thermoplastics. Development efforts are well advanced to find appHcations for phosphoms flame retardants, especially ammonium polyphosphate combinations, in polyolefins, and red phosphoms in nylons. Interest is strong in finding phosphoms-based alternatives to those halogen-containing systems which have encountered environmental opposition, especially in Europe. 

Modified ethylene—tetrafluoroethylene copolymers are commercially available ia a variety of physical forms (Table 6) and can be fabricated by conventional thermoplastic techniques. Commercial ETFE resias are marketed ia melt-extmded cubes, that ate sold ia 20-kg bags or 150-kg dmms. In the United States, the 1992 price was 27.9—44.2/kg, depending on volume and grade color concentrates are also available. 

Stabilization of the Cellular State. The increase in surface area corresponding to the formation of many ceUs in the plastic phase is accompanied by an increase in the free energy of the system hence the foamed state is inherently unstable. Methods of stabilizing this foamed state can be classified as chemical, eg, the polymerization of a fluid resin into a three-dimensional thermoset polymer, or physical, eg, the cooling of an expanded thermoplastic polymer to a temperature below its second-order transition temperature or its crystalline melting point to prevent polymer flow. 

Poly(vinylchloride). Cellular poly(vinyl chloride) is prepared by many methods (108), some of which utili2e decompression processes. In all reported processes the stabili2ation process used for thermoplastics is to cool the cellular state to a temperature below its second-order transition temperature before the resia can flow and cause coUapse of the foam. 

The two primary types of plastics, thermosets and thermoplastics, are made almost exclusively from hydrocarbon feedstocks. Thermosetting materials are those that harden during processing (usually during heating, as the name implies) such that in their final state they are substantially infusible and insoluble. Thermoplastics may be softened repeatedly by heat, and hardened again by cooling. 

Applications. Among the P—O- and P—N-substituted polymers, the fluoroalkoxy- and aryloxy-substituted polymers have so far shown the greatest commercial promise (14—16). Both poly[bis(2,2,2-trifluoroethoxy)phosphazene] [27290-40-0] and poly(diphenoxyphosphazene) [28212-48-8] are microcrystalline, thermoplastic polymers. However, when the substituent symmetry is dismpted with a randomly placed second substituent of different length, the polymers become amorphous and serve as good elastomers. Following initial development of the fluorophosphazene elastomers by the Firestone Tire and Rubber Co., both the fluoroalkoxy (EYPEL-F) and aryloxy (EYPEL-A) elastomers were manufactured by the Ethyl Corp. in the United States from the mid-1980s until 1993 (see ELASTOLffiRS,SYNTHETic-PHOSPHAZENEs). 

Early phenoHc resins consisted of self-curing, resole-type products made with excess formaldehyde, and novolaks, which are thermoplastic in nature and require a hardener. The early products produced by General BakeHte were used in molded parts, insulating varnishes, laminated sheets, and industrial coatings. These areas stiH remain important appHcations, but have been joined by numerous others such as wood bonding, fiber bonding, and plywood adhesives. The number of producers in the 1990s is approximately 20 in the United States and over 60 worldwide. 

The fiaal state of a phenolic resin varies dramatically from thermoplastic to thermoset and from soHd to Hquid, and includes solutions and dispersions. 

Properties have been determined for a series of block copolymers based on poly[3,3-bis(ethoxymethyl)oxetane] and poly [3,3-bis(methoxymethyl)oxetane]- (9-tetrahydrofuran. The block copolymers had properties suggestive of a thermoplastic elastomer (308). POX was a good main chain for a weU-developed smectic Hquid crystalline state when cyano- or fluorine-substituted biphenyls were used as mesogenic groups attached through a four-methylene spacer (309,310). Other side-chain Hquid crystalline polyoxetanes were observed with a spacer-separated azo moiety (311) and with laterally attached mesogenic groups (312). 

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