摘要:

AbstractThe fabrication of poly(ethylene terephthalate), PET, into fibers, films, and containers usually involves molecular orientation caused by molecular strain, which may lead to stress‐ or strain‐induced crystallization (SIC). The SIC of PET was studied by the methods of birefringence, density, thermal analysis, light scattering, and wide‐angle X‐ray. The development of crystallinity is discussed in relation to the rate of crystallization, the residual degree of orientation, and stress relaxation. The experimental procedure involves stretching samples at temperatures above the glass transition temperature,Tg, to a given extension ratio and at a specific strain rate of an Instron machine. At the end of stretching, the sample is annealed in the stretched state and at the stretching temperature for various periods of time, after which the sample is quickly quenched to room temperature for subsequent measurements. During stretching, the stress strain and the stress relaxation curves are recorded. The results indicate that the SIC of annealed, stretched PET can proceed in three different paths depending on the residual degree of orientation. At a low degree of residual orientation, as indicated by the birefringence value, annealing of stretched PET leads only to molecular relaxation, resulting in a decrease of birefringence. At intermediate orientation levels, annealing causes an initial decrease in birefringence followed by a gradual increase and finally a leveling off of birefringence after a fairly long period of time. At higher orientation levels, annealing causes a rapid increase in birefringence before leveling off. The interpretation of the above results is made using the measurements of light scattering, differential scanning calorimetry, and wide‐angle X‐ray. The rate of the SIC of PET is also discussed in terms of specific da

ISSN:0032-3888    

DOI:10.1002/pen.760321802

出版商:Society of Plastics Engineers    

年代:1992

数据来源: WILEY

摘要:

AbstractThe dynamic creep behavior of a filled poly(dimethylsiloxane) elastomer was studied under cyclic stress. The stress level was chosen such that the increase in the internal temperature was small and that microcracks were not observed. This work has demonstrated that cyclic stress in combination with high temperature accelerates the degradation of the elastomer. The results suggest that because of the applied force, breaks in the load‐bearing chains of the network occur. These breaks, while relieving the mechanical stress, create highly reactive ionic fragments. It is believed that because of the subsequent reactions of the ionic fragments, changes in the specific gravity, storage modulus, effective crosslink density, and length of the sample (creep) are observed. The observed decrease in the storage modulus is thought to occur because of the reaction of the ionic fragments with moisture, which results in the formation of silanol chain ends that reduce the effective crosslink density. The results also show that contrary to the prediction of the Boltzmann's Superposition Principle, the rate of creep is greatly enhanced when the sample is subjected to a sinusoidally varying dynamic load as compared to a comparable static load. The polymer weight loss was found to be linear with time and strongly dependent on the level of applied dynamic and static force. In addition, the weight loss and rate of creep were also found to be strongly dependent upon temperatur

ISSN:0032-3888    

DOI:10.1002/pen.760321803

出版商:Society of Plastics Engineers    

年代:1992

数据来源: WILEY

摘要:

AbstractInterest in the development of polymeric materials for high temperature, electronic and microelectronic applications has led to an increasing number of new polymers. Many of these polymers have complex organic ring structures and semirigid backbones, characteristics that have posed some difficulties for structural analysis. The purpose of this paper is to test and compare two practical nondestructive optical techniques, polarized optical microscopy and polarized refractometry, for the determination of three‐dimensional surface and bulk anisotropy in these advanced materials. The optical techniques are first tested on a series of optically homogeneous uniaxially oriented isotactic polypropylene films and then applied to the analysis of high refractive index Kapton (PMDA‐ODA) polyimide films. The study includes a test of the validity of the compensator method, including the effect of fringe jumping, the use of polymer retarders, and the tilting technique of St

ISSN:0032-3888    

DOI:10.1002/pen.760321804

出版商:Society of Plastics Engineers    

年代:1992

数据来源: WILEY

摘要:

AbstractFor relatively soft polymer coatings on soda‐lime glass substrates the indentation load increases substantially when the indenter penetrates into the glass substrate since the glass can now directly support some of the indenter load. A model for the indentation load‐depth behavior is developed by accounting for the indentation load shared by the coating and substrate. This model accounts for the additional load supported by the coating due to the pile‐up of coating material underneath the indenter. The model predicts the indentation behavior as a function of coating and substrate hardnesses and coating thickness. Comparison of the model to experimental data for a wide range of polymer coatings (two epoxies, epoxy acrylate, and urethane acrylate) on soda‐lime glass substrates shows good ag

ISSN:0032-3888    

DOI:10.1002/pen.760321805

出版商:Society of Plastics Engineers    

年代:1992

数据来源: WILEY

摘要:

AbstractPolymer coatings are widely used to protect glass from indentation damage. A model for the strength degradation that occurs when a sharp indenter penetrates through the coating is developed by accounting for the indentation load shared by the coating and substrate. This model accounts for the additional load supported by the coating due to the pile‐up of coating material underneath the indenter. The model predicts the strength degradation as a function of indentation load, coating and substrate hardnesses, and coating thickness. Comparison of the model to experimental data for a wide range of polymer coatings (two epoxies, epoxy acrylate, and urethane acrylate) on soda‐lime glass substrates shows good agreem

ISSN:0032-3888    

DOI:10.1002/pen.760321806

出版商:Society of Plastics Engineers    

年代:1992

数据来源: WILEY

摘要:

AbstractPolymer motion in solution and in model printing inks (as measured by NMR spectroscopy) was compared to the “tack” or tensile stress developed in the splitting of thin fluid films. For model inks based on polyisobutene, at constant pigment content, increasing the polymer content caused polymer motion to slow down, and caused the tack to increase. At constant polymer content, increasing the pigment level had much less effect on polymer motion. At 20% pigment content, tack values were little different from those of the corresponding polymer solutions. This suggests that the tensile stress developed in an ink film as it splits is related to the motion of its polymeric ingredients, but is much less dependent on the concentration of dispersed solid pigm

ISSN:0032-3888    

DOI:10.1002/pen.760321807

出版商:Society of Plastics Engineers    

年代:1992

数据来源: WILEY

摘要:

AbstractThis paper presents procedures to monitor oxidation degradation of two commonly used cable insulations, crosslinked polyethylene (XLPE) and ethylene propylene rubber (EPR). Since the techniques described require only micro specimens, deterioration of cable insulation can be monitored without destroying the function of the cable. The techniques described are melting point, crystallinity, infrared carbonyl absorbance, gel content, relative hardness, and differential scanning calorimetry (DSC) oxidation induction time. The techniques were applied following accelerated aging over a period in excess of two years. The results from the micro specimens were directly related to embrittlement and the decrease in ultimate elongation. The previous thermal history of XLPE was determined using DSC melting peak analysis.

ISSN:0032-3888    

DOI:10.1002/pen.760321808

出版商:Society of Plastics Engineers    

年代:1992

数据来源: WILEY

摘要:

AbstractThe effects of the filler volume fraction and strength of adhesion on the mode of tensile failure of a particulate reinforced polypropylene (PP) are investigated using finite element simulation (FES). When there is perfect adhesion between constituents, an upper bound for tensile yield strength is found to be 1.33 times the matrix yield strength above a critical volume of particulate concentration. Utilizing Sjoerdsma's model for interacting stress concentration fields, one can determine the concentration dependence of the yield strength below the critical filler volume fraction. When there is no adhesion between constituents, a modified version of an equation by Nicolais and Narkis adequately describes a lower bound for the tensile yield strength. The particulate concentration and the matrix ductility are the prime factors in controlling the brittle failure of the composite. Upper and lower bounds for brittle failure strength are characterized using a strength‐of‐materials approach and stress concentration factors for both “perfect” and “zero” adhesion. The properties of calcium carbonate filled PP homopolymer were measured over a wide range of filler volume fractions. CaCO3was either treated with stearic acid to prevent adhesion between constituents or used as received. Maleic anhydride grafted PP (MPP) was used to promote adhesion. For filler volume fractions below 0.2, yielding of the composite by combined microcavitation and shear deformation was the principal failure mechanism. Atvfabove 0.35, a brittle failure mechanism dominated. In the range between 0.2 and 0.35, both failure modes were observed in the populations tested. Good agreement was found between the experimental results and the pro

ISSN:0032-3888    

DOI:10.1002/pen.760321809

出版商:Society of Plastics Engineers    

年代:1992

数据来源: WILEY

ISSN:0032-3888    

DOI:10.1002/pen.760321801

出版商:Society of Plastics Engineers    

年代:1992

数据来源: WILEY