摘要:

AbstractMicrocellular foam is a polymeric foam with bubble sizes of 10 microns or less that is produced by saturating a polymer with gas and then utilizing the thermodynamic instabilities that result when the polymer is heated and the pressure is reduced to nucleate the cells. A model for the nucleation of microcellular foam in amorphous polymers with additives has been developed. The nucleation process depends on the solubility, concentration, and interfacial energy of any additives present. At very low levels, additives in solution act to increase the free volume of the polymer, resulting in homogeneous nucleation within the free volume Well above the solubility limit, heterogeneous nucleation dominates, as it lowers the activation energy for nucleation to levels below that for homogeneous nucleation. In the vicinity of the solubility limit of the additive, these two nucleation mechanisms compete. The polystyrene‐zinc stearate system has been chosen for experimental evaluatio

ISSN:0032-3888    

DOI:10.1002/pen.760270702

出版商:Society of Plastics Engineers    

年代:1987

数据来源: WILEY

摘要:

AbstractExperiments were performed to validate the model for the nucleation of microcellular foams in amorphous thermoplastic polymers. The polystyrene‐zinc stearate system was chosen as the model system. Other additives such as stearic acid and carbon black were also investigated. Molecular weight and orientation effects were studied. Nitrogen and carbon dioxide were used to produce the microcellular bubbles. Results show that amounts of soluble additives at levels just below their solubility limit and high gas saturation pressures yield the most acceptable foams—ones with a large number of uniform small bubbles. In this region, the bubble number is sensitive to both the gas saturation pressure and the concentration of solutes. Increasing the concentration of soluble additives above the solubility limit has little effect on bubble number and almost eliminates the dependence on saturation pressure. Molecular weight and orientation had no effect on the number of bubbles produced. Similarly, carbon black, which is insoluble in and which bonds well to polystyrene, produced no effect on bubble numbers. The agreement between theoretical predictions and experimental results is reasonably g

ISSN:0032-3888    

DOI:10.1002/pen.760270703

出版商:Society of Plastics Engineers    

年代:1987

数据来源: WILEY

摘要:

AbstractMicrocellular polymer foams exhibit greatly improved mechanical properties as compared to standard foams due to the formers' small bubble size. Microcellular foams have bubbles with diameters on the order of 10 microns, volume reductions of 30 to 40 percent, and six or seven times the impact strength of solid parts. They are produced through the use of thermodynamic instabilities without the use of foaming agents. This method leads to a very uniform cell size throughout a part's cross section. A theoretical model for the nucleation of microcellular foams in thermoplastic polymers has been developed and experimentally confirmed. This model explains the effect of various additives and processing conditions on the number of bubbles nucleated. At levels of secondary constituents below their solubility limits, an increase in the concentration of the additive or the concentration of gas in solution with the polymer increases the number of bubbles nucleated. Nucleation in this region is homogeneous. Above the solubility limit of additives, nucleation is heterogeneous and takes place at the interface between second phase inclusions and the polymer. The number of bubbles nucleated is dependent on the concentration of heterogeneous nucleation sites and their relative effect on the activation energy barrier to nucleation. In the vicinity of the solubility limit, the two mechanisms compete.

ISSN:0032-3888    

DOI:10.1002/pen.760270704

出版商:Society of Plastics Engineers    

年代:1987

数据来源: WILEY

摘要:

AbstractThe prediction and measurement off the developing radial temperature profile in a single‐screw extruder is essential since excessive extrudate temperature and poor temperature uniformity at the die affect the quality of coextrusion and film/sheet extrusion processes. A finite‐difference scheme is used to predict the development of the radial temperature profile in the metering section of a single‐screw extruder for power‐law fluids. A finite‐element analysis of the flow around the screw‐tip transposes this temperature profile onto an on‐line temperature measuring device consisting of 10 thermocouples for a comparison of numerical results with experimental data on p

ISSN:0032-3888    

DOI:10.1002/pen.760270705

出版商:Society of Plastics Engineers    

年代:1987

数据来源: WILEY

摘要:

AbstractA systematic, experimental study was conducted on the elementary process of devolatilization (DV) of volatile contaminants from polymer melts. Emphasis was placed on foam‐enhanced DV from rolling melt pools, which are present in rotating machinery. All experiments were conducted in a specially designed apparatus at room temperature using poly(dimethyl siloxane) to simulate the melt, and methyl chloride together with several chlorofluorocarbons as the contaminants. The special apparatus was devoid of the flow complexities present in industrial equipment and permitted independent control of all pertinent operating parameters. Physical properties and process parameters examined include feed composition and vapor pressure of contaminant, applied vacuum level, rotational speed of the pool, viscosity of the melt, and the effect of air entrainmen

ISSN:0032-3888    

DOI:10.1002/pen.760270706

出版商:Society of Plastics Engineers    

年代:1987

数据来源: WILEY

摘要:

AbstractThe process described as “roll‐drawing” has been applied to commercial extruded sheets of isotactic polypropylene (M̄n= 70,900). Preheated billets were drawn into thin, clear, transparent sheets in a single pass, producing uniaxial orientation of the polymer molecules in the draw direction. At the maximum draw ratio of 20, the ultimate tensile strength and Young's modulus in the draw direction were 0.5 GPa and 20 GPa respectively. The mechanical properties transverse to the draw direction were virtually unchanged. The theory of fiber reinforcement for unidirectional anisotropic plates was applied to interpret the orientation dependence of the stress‐strain behavior of the drawn sheets. From these results, it was estimated that the mechanical properties of biaxially laminated polypropylene sheets equaled the performance of aramid and carbon fiber composites, The roll‐drawing process appears to be economically attractive for the production of ultra‐high modulus crystalline thermoplastics in sheet form having excellent uniaxial or biaxia

ISSN:0032-3888    

DOI:10.1002/pen.760270707

出版商:Society of Plastics Engineers    

年代:1987

数据来源: WILEY

摘要:

AbstractSome engineering thermoplastics cannot be easily processed by injection molding because of their high viscosity. This is the case for Ultra‐High Molecular Weight Polyethylene (UHMWPE). Parts are either compression molded or machined from extruded shapes. Over the years, forging has been increasingly used to produce more or less complicated parts. Forging is a process in which a preform is subjected to bulk deformation in constraining dies, by applying heat and pressure, but with the polymer remaining in the solid (semi‐crystalline) phase. To fabricate acceptable mechanical components such as gears and sprockets, a close control of the operating conditions and a good prediction of shrinkage are necessary, For a better understanding, a study has been conducted on forged UHMWPE disks and gears. Parameters such as material and mold temperatures, dwell time, speed of forging and dwell pressure were investigated. Results show that material and mold temperatures are the most important parameters. For gears, optimization of processing conditions led to a shrinkage of ∼0.2 percent, measured on pitch diameter. The effect of post‐annealing at various temperatures is also r

ISSN:0032-3888    

DOI:10.1002/pen.760270708

出版商:Society of Plastics Engineers    

年代:1987

数据来源: WILEY

摘要:

AbstractThe viscosity, the activation energy of flow, and the exchange reactions of bisphenol A 50/50 isophthalic/terephthalic acid and poly(butylene terephthalate) blends are studied by means of an extrusion capillary rheometer, covering a range of 10 s−1to 300 s−1shear rate and 280°C to 300°C temperature. The results are interpreted in terms of compatibility and free volume additivity. The decrease in viscosity with time is explained as a result of transesterification rather than degrad

ISSN:0032-3888    

DOI:10.1002/pen.760270709

出版商:Society of Plastics Engineers    

年代:1987

数据来源: WILEY

ISSN:0032-3888    

DOI:10.1002/pen.760270701

出版商:Society of Plastics Engineers    

年代:1987

数据来源: WILEY