Processing characteristics

262 Aliphatic Polyolefins other than Polyethylene, and Diene Rubbers ll.l.S Processing Characteristics 

Attention also has to be paid to screw design. The screw metering zone should be long and shallow to ensure melt homogenisation but not so shallow that 

Film manufacture also requires special considerations in the case of polypropylene in order to ensure small crystal structures and hence high clarity. Chill-roll casting processes may be used and these give films of high clarity and minimal thickness variations at high rates of production. Blown film processes can, however, give superior mechanical properties and in addition equipment costs are lower, and in consequence the process is more economic for lower tonnage production. 

In order to reduce the tendency of the film to shrink oriented film may be annealed at about 100°C whilst under tension immediately after drawing. The film is often coated with another polymer sueh as a vinylidene ehloride-based copolymer. This both improves the barrier properties and improves the heat scalability. 

An important application for polypropylene is film tape. This is made by slitting unoriented film (cast or blown) into tapes 2 or 3.5 mm wide and stretching under heat about seven-fold. With cast film the orientation is more completely monoaxial and there is a tendency for the film to split along its length (fibrillate). Tubular film does not self-split so easily and also has a somewhat softer feel. Such tapes may be woven into sacks and these have 

Film tapes may also be made drawn up to 10 1 ratios and then fibrillated by pin rollers, embossed rollers or some other technique. Fibrillated tape has been a substantial replacement for sisal. 

Unlike the activation of the CVD-diamond deposition which is a chemical process, the activation of DLC deposition is a physical phenomenon in which the sp configuration is stabilized by ion bombau dment of the growing film instead of (or in addition to) atomic hydrogen. The process is known in general terms as physical vapor deposition (PVD). 

In the case of hydrogenated DLC (a-C H), it is believed that both ion bombardment and the presence of atomic hydrogen are the key factors in promoting the sp configuration.  

Process Classification. The DLC deposition processes are divided into two broad categories  

The purely PVD processes which include ion beam sputtering, laser or glow discharge from a solid carbon target (producing a-C). 

The PVD/CVD processes such as DC or RF plasma or ion-beam deposition from a gaseous hydrocarbon (producing a-C H). 

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The above batch process has undergone numerous refinements to improve yields, processing characteristics, purity, and storage stabiUty, but it remains the standard method of manufacture for these products. Recentiy a continuous process has been reported by Bayer AG (6) wherein the condensation is carried out in an extmder. The by-products are removed in a degassing zone, and the molten polymer, mixed with stabilizers, is subsequendy cracked to yield raw monomer. 

J. McDevitt, "Processing Characteristics of Castable PBX Explosives," in Symposium of ProcessingPropellants, Explosives, and Ingredients, ADPA, Washington, D.C., 1977, p. 3.2-1. 

The equipment for the slit-film fiber process is shown in Figure 15 (29). An olefin film is cast, and as in melt spinning, the morphology and composition of the film determine the processing characteristics. Fibers may be produced by cutting or slitting the film, or by chemomechanical fibrillation. 

Siace most fabricated elastomer products contain 10—50 vol % of filler, their physical properties and processing characteristics depend to a great extent on the nature and quaUty of the fillers. Rubber technologists manipulate the formula so as to optimize a large number of properties and keep costs down. 

The resin must be of highest purity for optimum processing characteristics and properties. Degradation results in discoloration, bubbling, and change in melt flow rate. 

A number of thermally stable polymers have been synthesized, but in general the types of stmctures that impart thermal resistance also result in poor processing characteristics. Attempts to overcome this problem have largely been concentrated on the incorporation of flexible groups into the backbone or the attachment of stable pendent groups. Among the class of polymers claimed to be thermally stable only a few have achieved technological importance, some of which are polyamides, polyimides, polyquin oxalines, polyquinolines, and polybenzimidazoles. Of these, polyimides have been the most widely explored. 

PPQs possess a stepladder stmcture that combines good thermal stabiUty, electrical insulation, and chemical resistance with good processing characteristics (81). These properties allow unique appHcations in the aerospace and electronics industries (82,83). PPQ can be made conductive by the use of an electrochemical oxidation method (84). The conductivities of these films vary from 10 to 10 S/cm depending on the dopant anions, thus finding appHcations in electronics industry. Similarly, some thermally stable PQs with low dielectric constants have been produced for microelectronic appHcations (85). Thin films of PQs have been used in nonlinear optical appHcations (86,87). 

High demands are placed on the substrate material of disk-shaped optical data storage devices regarding the optical, physical, chemical, mechanical, and thermal properties. In addition to these physical parameters, they have to meet special requirements regarding optical purity of the material, processing characteristics, and especially in mass production, economic characteristics (costs, processing). The question of recyclabiUty must also be tackled. 

For substrates of WORM and EOD(PCR) disks the industry in the future wants polymers that have a markedly improved resistance to heat softening compared to BPA-PC and, if possible, a lower water absorption and lower birefringence, but otherwise maintain the good characteristics in toughness, production, and cost (194). This goal is being approached in different ways further modification of BPA-PC, newly developed polymers, improvement of the processing characteristics of uv-curable cross-linked polymers, and development of special copolymers and polymer blends, eg,... 

Some of these isocyanates are commercially available in derivatized form, such as biurets and carbodiimides, to provide materials having improved handling or processing characteristics. 

Copper Development Association P.O. Box 1840 Greenwich, Conn. 06836 Standards for wrought and cast copper and copper alloy products a standards handbook is pubUshed with tolerances, alloy data, terminology, engineering data, processing characteristics, sources and specifications cross-indexes for six coppers and 87 copper-based alloys that are recognized as standards. 

Table 2. General Process Characteristics for Hydroprocessing Various Feedstocks...

Plasticity Retention Index. The oxidation behavior of natural mbber may affect both the processing characteristics and final vulcanizate performance, and the plasticity retention index (PRI) test can be used to give an indication of both. Natural antioxidants present in natural mbber give some protection and a measure of the efficacy of protection is given by PRI. PRI% = P q j Pq x 100, where Pq is the initial Wallace plasticity and P q is the... 

Maxillofacial polymers include the chlorinated polyethylenes, polyethemrethanes, polysiloxanes (see Elastomers), and conventional acrylic polymers. These are all deficient in a number of critical performance and processing characteristics. It is generally agreed that there is a need for improved maxillofacial polymers that can be conveniently fabricated into a variety of prostheses (218,227,228). 

Design nd Operation. The destruction efficiency of a catalytic oxidation system is determined by the system design. It is impossible to predict a priori the temperature and residence time needed to obtain a given level of conversion of a mixture in a catalytic oxidation system. Control efficiency is determined by process characteristics such as concentration of VOCs emitted, flow rate, process fluctuations that may occur in flow rate, temperature, concentrations of other materials in the process stream, and the governing permit regulation, such as the mass-emission limit. Design and operational characteristics that can affect the destmction efficiency include inlet temperature to the catalyst bed, volume of catalyst, and quantity and type of noble metal or metal oxide used. 

Since biological systems can reasonably cope with some of these problems, the intuition behind neural nets is that computing systems based on the architecture of the brain can better emulate human cognitive behavior than systems based on symbol manipulation. Unfortunately, the processing characteristics of the brain are as yet incompletely understood. Consequendy, computational systems based on brain architecture are highly simplified models of thek biological analogues. To make this distinction clear, neural nets are often referred to as artificial neural networks. 

Process Characteristics in Transfer Functions In many cases, process charac teristics are expressed in the form of transfer functions. In the previous discussion, a reactor example was used to illustrate how a transfer function could be derived. Here, another system involving flow out of a tank, shown in Fig. 8-6, is considered. 

The above comments refer to comparisons between the two compositions at the same glass-fibre level. If, however, comparison is made between a nylon 66 composition with a glass content of x% and a nylon 6 compound with a glass content of (x + 5)%, then the differences in mechanical properties become very small. At the same time the nylon 6 material will have slightly easier processing characteristics and surface quality. 

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