摘要:

AbstractThe viscoelastic energy dissipations within the bulk of the test specimen are not taken into account by classical fracture mechanics. In this study it is shown that on approaching the glass transition temperature, the amount of energy dissipated in the bulk of the specimen relative to that required to create two new surfaces can be substantial, and it may be estimated from measurements carried out on specimens with different gauge lengths. Furthermore stress relaxations can take place at the crack tip, even in absence of yielding. These can be sufficiently pronounced to eliminate stress intensification and give rise to constant fracture stress conditions.

ISSN:0032-3888    

DOI:10.1002/pen.760211002

出版商:Society of Plastics Engineers, Inc.    

年代:1981

数据来源: WILEY

摘要:

AbstractA quantitative “Round Neck” (RN) model based upon volume balances and considerations of surface tension phenomena is proposed for simulation of coalescence of two spherical polymeric particles; during sintering. This model and an early model developed by Frenkel are compared and evaluated against experimental data to show their suitability to simulate geometrical variations of a particle pair during sinter

ISSN:0032-3888    

DOI:10.1002/pen.760211003

出版商:Society of Plastics Engineers, Inc.    

年代:1981

数据来源: WILEY

摘要:

AbstractWeld or knit lines result wherever two or more polymer flow fronts unite. This results in a region of a different level of molecular entanglements than the bulk material. Consequently, weld regions have been observed to have inferior mechanical properties compared to the bulk. Although this phenomenon occurs in almost all the commercially important polymer processes, there has been little systematic investigation. The effects of melt temperature, mold temperature, injection speed and injection pressure on the tensile properties of commercial grades of polystyrene (GPS), high impact polystyrene (HIPS) and polypropylene (PP) are examined. The most important processing parameters seemed to be melt and mold temperature; injection speed and pressure had little effect on the tensile properties of any of the samples. A higher melt temperature increased both the strain and stress at break considerably in GPS. In HIPS increased melt temperature increased only the elongation to break substantially. Increased mold temperature improved the stress and elongation to break in GPS but not as much as melt temperature. Polypropylene showed improved weld yield strength with increased mold temperature. Under the conditions examined, injection pressure and injection speed showed no effect on the tensile properties of any of the materials investigated.

ISSN:0032-3888    

DOI:10.1002/pen.760211004

出版商:Society of Plastics Engineers, Inc.    

年代:1981

数据来源: WILEY

摘要:

AbstractStudies on structural changes in the surface layer of plastics during internal sliding friction (friction upon seizing) of metalthermoplastic systems have been carried out. These structural changes were carried out by means of polarized light microscopy, density measurements, X‐ray diffraction and IR absorption spectroscopy (ATR). They were carried out for high density polyethylene (HDPE), polypropylene (PP) and nylon 6 (PA6). As a result it was found that the formation of completely different structures was caused by internal sliding frictio

ISSN:0032-3888    

DOI:10.1002/pen.760211005

出版商:Society of Plastics Engineers, Inc.    

年代:1981

数据来源: WILEY

摘要:

AbstractThe fracture toughness of polypropylene was increased tenfold by the addition of 30 volume percent of chopped poly(ethylene terephthalate) rovings without any appreciable sacrifice in strength or modulus. The resulting fracture toughness was independent of temperature to −40°C. For maximum effectiveness the rovings must be integrated so that the component filaments cannot disperse during the processing and injection molding stages. The three different methods employed to measure fracture toughness showed good correlation with each other and confirmed the general utility of the Izod test (with slight modification) as a measure of fracture energy. The increased toughness was attributed almost entirely to frictional energy losses during fiber pullout. Polyester fibers can be useful additives for increasing the fracture toughness of brittle resins where processing temperatures do not preclude their u

ISSN:0032-3888    

DOI:10.1002/pen.760211006

出版商:Society of Plastics Engineers, Inc.    

年代:1981

数据来源: WILEY

摘要:

AbstractDie drawing, a new process for the production of oriented thermoplastic rods with an ultrahigh modulus of elasticity, involves drawing an isotropic workpiece through a converging die at temperatures below the polymer melting point. Die drawing of polypropylene is described, but the process is applicable to any polymer which can be drawn by conventional means. The considerable advantages of die drawing compared with conventional tensile drawing and hydrostatic or ram extrusion are discussed.

ISSN:0032-3888    

DOI:10.1002/pen.760211007

出版商:Society of Plastics Engineers, Inc.    

年代:1981

数据来源: WILEY

摘要:

AbstractSuspensions, consisting of glass beads about 1 μm in diameter and a 20 wt percent polystyrene solution in diethyl phthalate, were prepared at concentrations up to 13 vol percent. The viscoelastic properties were measured by means of a conventional dynamic method and by the raised cosine pulse method. The relaxation intensity and relaxation times of suspensions increased with increasing particle concentration. However, at concentrations>13 vol percent, the stress against the raised cosine pulse strain did not approach zero. It was found that concentrated suspensions were complicated by a combination of viscous, elastic, and plastic effects. The experimental results have been qualitatively explained on the base of the network structure of particles

ISSN:0032-3888    

DOI:10.1002/pen.760211008

出版商:Society of Plastics Engineers, Inc.    

年代:1981

数据来源: WILEY

摘要:

AbstractEnvironmental stress cracking (ESC) measurements for various impact polystyrenes were performed using a constant load technique with the specimens in contact with a 50/50 solution of cotton seed oil and oleic acid. It was shown that ESC in impact polystyrene is controlled by the transport of the aggressive liquid through a pre‐established dry craze structure where capillary pressure is the driving force. At moderate stress levels just above the critical stress for environmental cracking, there is an apparent incubation time for the dry craze formation. The craze incubation time is strongly influenced by thermal stresses induced by the gel particles. As a consequence, ESC is two‐stage process involving both an incubation time and actual crack growth. Control of the craze structure to maximize fibril content is essential for good ESC resistance. The craze fibril content can be altered by variables such as gel particle size, matrix molecular weight, plasticizer content, and rubber cont

ISSN:0032-3888    

DOI:10.1002/pen.760211009

出版商:Society of Plastics Engineers, Inc.    

年代:1981

数据来源: WILEY

摘要:

AbstractThe conservation equations of momentum, energy, and mass are numerically solved for the flow of thermosets through the nozzle of an injection molding machine. The following working hypotheses are stated: perfect mixing of reactants, validity of the lubrication approximation, steady state (based on previous results for thermoplastic materials), negligible diffusional mass transfer, adiabatic boundary condition, and wall slip. Numerical results for a particular case‐show that the value of the slip friction coefficient is very important for defining the process feasibility. The apparent viscosity of the material is also relevant to the process. Materials of low viscosity gel at the wall of the nozzle inlet section; high viscosity materials may gel at the outlet section, requiring great pressure gradients and showing high wall temperatures at the exit. These high temperatures may cause degradation or undesirable side reaction

ISSN:0032-3888    

DOI:10.1002/pen.760211010

出版商:Society of Plastics Engineers, Inc.    

年代:1981

数据来源: WILEY

ISSN:0032-3888    

DOI:10.1002/pen.760211001

出版商:Society of Plastics Engineers, Inc.    

年代:1981

数据来源: WILEY