摘要:

AbstractUnlike fibers, planar reinforcements, such as flakes and ribbons, provide reinforcement in two directions. If such reinforcements are arranged parallel to their principal plane in a composite material, they thus provide a distinctly higher performance than fiber reinforcements for two‐dimensional loading conditions. This higher performance amounts to about a factor three for the Young's modulus and a factor two for the tensile strength. However, in spite of this obvious advantage, composites with planar reinforcements are as yet relatively unknown. This is mainly due to the fact that planar reinforcements are not as readily available as fiber reinforcements and therefore not much work has been done on them. The present article gives first a short outline of the theory of the elastic and tensile properties of composites with planar reinforcements. Then, a non‐exhaustive review is presented of the work on composites with planar reinforcements, with particular attention given to recent developments. A final aim of this article is that by showing the merits of planar reinforcements as compared to presently existing fiber reinforcements, it may contribute to their use in the design of composite structu

ISSN:0032-3888    

DOI:10.1002/pen.760190102

出版商:Society of Plastics Engineers, Inc.    

年代:1979

数据来源: WILEY

摘要:

AbstractWhen certain molten thermoplastics containing glass fibers up to 20 mm in length are extruded through an orifice, the extrudate has a frothy texture and solidifies into an irregular open‐cell structure. The effect is apparently caused by the recovery of the fibers from distortions arising in the region of converging flow near the entry to the orifice. The effect is particularly pronounced in systems with melt viscosities between 50 and 500 Ns/m2. Polypropylene, as an example, containing weight fractions of about 0.25 of glass fibers can be extruded through a zero‐length die 2 mm in diameter to give “ropes” that can be readily formed into flat sheets or three‐dimensional shapes because of adhesion between the still molten polymer in the newly extruded rope. Such sheets can then be bonded to other materials to make a variety of laminates. A range of bonding techniques is available and the rough surface texture of the ropes provides a useful mechanical key for bonding with grossly dissimilar materials. Fiber‐foam is highly resistant to fracture and has a modulus in the range expected by a simple model f

ISSN:0032-3888    

DOI:10.1002/pen.760190103

出版商:Society of Plastics Engineers, Inc.    

年代:1979

数据来源: WILEY

摘要:

AbstractThe yield behavior during cold drawing of commercially spun poly(ethylene terephthalate) (PET) filament yarn was investigated. Microscopic examination revealed the presence of inherent flaws within the spun filaments; these act as points for localized stress concentration. These inhomogeneities appear to be either internal cracks or crazes developed during the fiber melt spinning process. During elongation, stress magnification at these flaws results in shear band formation, indicating the onset of inhomogeneous yielding. At the yield bend in the load‐elongation curve a circumferential crack propagates within these shear band regions. This yield crack develops into the classical neck geometry which further localizes additional plastic deformation within the sample at the nec

ISSN:0032-3888    

DOI:10.1002/pen.760190104

出版商:Society of Plastics Engineers, Inc.    

年代:1979

数据来源: WILEY

摘要:

AbstractThe thermochemical and flammability properties of some thermally stable polymers considered for use in aircraft interiors are described. The properties studied include: (a) thermomechanical properties such as glass transition and melt temperature; (b) dynamic thermogravimetric analysis in anaerobic environment; (c) flammability properties such as oxygen index, flame spread and smoke evolution; and (d) selected physical properties. The thermoplastic polymers evaluated included polyphenylene sulfide, polyaryl sulfone, 9,9‐bis(4‐hydroxyphenyl)‐fluorene polycarbonate‐poly(dimethylsiloxane) and polyether sulfone. The thermoset polymers evaluated included epoxy, bismaleimide, a modified phenolic and polyaromatic melamine resin. These resins were primarily used in the fabrication of glass reinforced prepregs for the construction of experimental panels. Test results and relative rankings of some of the flammability parameters are presented and the relationship of the molecular structure, char yield, and flammability properties of these polymers are di

ISSN:0032-3888    

DOI:10.1002/pen.760190105

出版商:Society of Plastics Engineers, Inc.    

年代:1979

数据来源: WILEY

摘要:

AbstractThe use of fillers is an established method of modifying the properties of plastics. The recent increase in prices of plastic raw materials and the new developments in compounding technology have heightened the interest of the plastic industry in compounds of fillers and polyethylene. This article discusses briefly the literature dealing with the effect of fillers on the properties of polyethylene, and research carried out on the effects of different mineral fillers, talcs, micas, clays, wollastonite and dolomite on the many properties of low density polyethylene films. The fine grade talcs have proved to be better than the other fillers with respect to most film properties. The platy talcs decreased the oxygen permeability by as much as 80 percent and the water vapor transmission rate by as much as 70 percent. With talcs, clays and dolomite the yield strength increased but with all fillers the tensile strength at break weakened with increasing filler content. A low elongation at break and a low tearing, resistance indicate the brittleness of the films especially at high filler contents (over 30 wt percent). The coefficients of friction decreased considerably with all fillers and a significant improvement in printability was achieved. However, none of the fillers had more than a very minor effect on the heat sealability of low density polyethylene.

ISSN:0032-3888    

DOI:10.1002/pen.760190106

出版商:Society of Plastics Engineers, Inc.    

年代:1979

数据来源: WILEY

摘要:

AbstractThe methods of small systems thermodynamics have been applied to the calculation of the entropy barrier to the growth of polymer crystals. The height of this barrier at the equilibrium melting point of polyethylene molecules of typical lengths (X = 4,000 to 10,000 segments) is estimated to be of the order of 4 to 5 Kcal per molecule. This barrier increases linearly with ln X if the crystal is very large but retains the approximate values mentioned for crystals with the lamella thicknesses usually found. This entropy change causes the free energy to go through a maximum as in a nucleation phenomena but no new surface formation is involved. The existence of such a barrier explains why crystallization (or melting) never occurs at the equilibrium melting temperature.

ISSN:0032-3888    

DOI:10.1002/pen.760190107

出版商:Society of Plastics Engineers, Inc.    

年代:1979

数据来源: WILEY

摘要:

AbstractStatistical mechanical methods can be used to relate not only structure and thermodynamics but also structure and kinetics. Methods are developed for calculating the crystal growth rate kinetic models. When the usual macroscopic thermodynamics are applied, the resulting surface energies, σ and σeextracted from the experimental growth rates are quite comparable to those obtained from kinetic models. However, when one applies small system thermodynamics, a substantially smaller σ is obtained. This latter assumption has the advantage of explaining why extended chain seed crystals do not promote crystal growth and why folded chain crystals grow on such seeds. The statistical mechanical methods in both of these cases predict the observed decrease in the rate constant,Kgwith decreasing molecular length without the necessity of a separate theory for varying σewith molecular len

ISSN:0032-3888    

DOI:10.1002/pen.760190108

出版商:Society of Plastics Engineers, Inc.    

年代:1979

数据来源: WILEY

摘要:

AbstractThe statistical mechanical methods developed in Parts I and II in this series permit the postulation of a thermodynamic criterion for the molecular fractionation which occurs during crystallization. Using this criterion we define a “local equilibrium” melting temperature as that temperature at which a polymer molecule (considered as a small thermodynamic system) has the same free energy when crystallized into the lowest possible free energy conformation on a given crystal surface (or surfaces) as it does in a completely molten state but still in contact with the same surfaces. This temperature will be a function not only of molecular length but also of the nature of the crystal surfaces to which it is exposed. Lowest “local equilibrium” melting temperatures occur on large flat crystal surfaces (secondary nucleation sites), higher temperatures result from the intersection of two crystal surfaces (tertiary nucleation sites). A number of such potential tertiary nucleation sites have been investigated and the resulting temperatures satisfactorily cover the range over which molecular fractionation has been o

ISSN:0032-3888    

DOI:10.1002/pen.760190109

出版商:Society of Plastics Engineers, Inc.    

年代:1979

数据来源: WILEY

摘要:

AbstractThe results of dynamic mechanical and broad‐line proton magnetic resonance studies carried out over a broad, temperature range on a series of aromatic polyamides are presented. At temperatures lower than 300 K, the NMR data show no relaxation for the all‐meta linked benzene rings polyamide, whereas processes for the other polyamides are found, the magnitude of which is dependent on the polymer structure. Between 400 and 500 K further decreases in the second moment take place for all materials. Two secondary mechanical loss peaks are evident in these polymers, one in the 200‐380 K region (β) and one in the 380‐500 K region (β*), at about 104cps. In particular, the relaxation strength of the β*peak depends on the conditions of isolation of the polymers from solution. The results are discussed in terms of possible motional processes of these polymers characterized by a long range intramolecular uniaxial order. An attempt is made to correlate the relaxational properties of aromatic polyamides to the existence of a phase of low regularity such as the nematic or smectic

ISSN:0032-3888    

DOI:10.1002/pen.760190110

出版商:Society of Plastics Engineers, Inc.    

年代:1979

数据来源: WILEY

ISSN:0032-3888    

DOI:10.1002/pen.760190101

出版商:Society of Plastics Engineers, Inc.    

年代:1979

数据来源: WILEY