摘要:

AbstractCommercial samples of high density, linear low density, and low density polyethylene were modified by injection of low concentrations of free radical initiator during extrusion. Molecular properties monitored included molecular weight distribution, degree of unsaturation, and branching. When the polyethylene teed to this reactive extrusion process had similar values ofMw, but varying polydispersity, degree of branching and degree of unsaturation, the magnitude of the change in molecular weight distribution was found to be in the following order: HDPE 1>LDPE2>LLDPE. In general, terminal vinyls enhanced molecular weight increase, and branching promoted degradation. However, for a second high density polyethylene sample withMw= 154,000 (rather than the previous sample'sMwof 85,600), the change in molecular weight distribution was small and located at the lower molecular weight end. This work provided data for the kinetic model development detailed in Part II.

ISSN:0032-3888    

DOI:10.1002/pen.760332402

出版商:Society of Plastics Engineers    

年代:1993

数据来源: WILEY

摘要:

AbstractIn Part I of this work, experimental data showed that the effect of low concentrations of free radical initiator injected into polyethylene during extrusion depended upon the degree of unsaturation and branching in the feed as well as the molecular weight. This paper shows attempts to quantitatively explain these reactive extrusion results through development of two kinetic models based upon the rate equations for reactions considered dominant. The first model developed incorporated unsaturation via consideration of simultaneous crosslinking‐endlinking reactions. It contains two variable parameters: the overall initiator efficiency and a ratio of two rate constants reflecting the reactivity of the unsaturated bonds. The model was able to fit the changes in molecular weight distribution of both the low density and linear low density polyethylene but not the high density polyethylene samples. In addition to fitting the molecular weight distribution, this model also provided reasonable values of initiator efficiencies for crosslinking, endlinking, and chain extension reactions, as well as the number of terminal vinyls of the products. The second model is a special case of the first: it neglects the presence of unsaturation in the feed. This second model is actually the usual “crosslinking” model widely known from a derivation based upon statistical arguments. It was not able to fit most molecular weight distributions obtained. However, the model was shown to be useful for accounting for observed molecular weight distribution for a high density polyethylene sample of low initial unsaturation. Also, it was able to explain the amount of gel formed as a function of initiator concentr

ISSN:0032-3888    

DOI:10.1002/pen.760332403

出版商:Society of Plastics Engineers    

年代:1993

数据来源: WILEY

摘要:

AbstractThe toughening mechanisms of rubber‐dispersed polymer alloys were investigated with respect to two factors: (i) characteristic ratio,C∞, as a measure of chain flexibility of the polymer matrix and (ii) the rubber particle size in high impact polystyrene/poly(2,6‐dimethyl‐ 1,4‐phenylene ether) blend systems. Measurements of the specific gravity, synchrotron radiation small angle X‐ray scattering (SR‐SAXS) tests, and finite element analysis were carried out to gain understanding of plastic deformation (crazing and shear yielding) of these materials. Shear yielding was found to be enhanced when theC∞value of the matrix polymer was relatively low and the rubber particles were small (submicron). From these results, we presumed the impact energy absorption mechanisms to be governed by th

ISSN:0032-3888    

DOI:10.1002/pen.760332404

出版商:Society of Plastics Engineers    

年代:1993

数据来源: WILEY

摘要:

AbstractHigh velocity jet impingement mixing is a unique operation of the reaction injection molding process. The quality of mixing depends on processing parameters for a given system, and can have significant effect on the properties of the product. We have compared three methods used for characterization of mixing quality reported in the literature for a cross‐linking polyurethane system processed on an industrial scale high pressure RIM machine, with an L‐type mixhead. The methods are based on, (i) visual appearance of a molded sample, (ii) adiabatic temperature rise during reaction and, (iii) flexural modulus of elasticity of a molded sample. The visual appearance test was made quantitative by using a color matching optical system. The adiabatic temperature rise increases rapidly with nozzle Reynolds number (Renzl) for polyol, up to a critical value (Renzl= 380) beyond which there is a slow increase. The latter is attributed to the geometry of the L‐type mixhead. A simple analysis is presented to explain the limiting extent of mixing at highRenzl. The flexural modulus and the visual appearance of molded samples are found to improve untilRenzl= 500, implying a greater sensitivity to mixing as compared to the adiabatic temperature

ISSN:0032-3888    

DOI:10.1002/pen.760332405

出版商:Society of Plastics Engineers    

年代:1993

数据来源: WILEY

摘要:

AbstractKinetics of phase separation of poly(p‐phenylenebenzobisoxazole) (PBO) in methanesulfonic acid are examined by the time‐resolved small angle light scattering and turbidity measurements. Thin films of polymer solutions are spin‐coated on microscope slides and observed in a controlled environment light scattering cell. These solutions appear to undergo a spinodal decomposition‐like phase separation, following Cahn‐Hilliard type phase separation kinetics upon exposure to water vapor. An exponential increase in the scattered light intensity at short times is found. Transmittance measurements are used to determine the effects of molecular weight and coagulation medium on the rate of co

ISSN:0032-3888    

DOI:10.1002/pen.760332406

出版商:Society of Plastics Engineers    

年代:1993

数据来源: WILEY

摘要:

AbstractThe causes of void content and its resulting effect on the material properties of structural reaction injection molded (SRIM) composites were investigated. Plaques were molded under different combinations of five two‐level factors, and the resulting effect of each factor on void content was determined. The geometry of the glass reinforcement had the largest effect, as woven mats caused double the void content of random mats. Evacuating the mold cavity proved to greatly reduce void content as well. Varying the injection rate and shot size had little effect. Material property testing was conducted on groups of specimens with similar void content. Tension testing revealed no decrease in strength with increasing void content. One million cycles of tension‐tension fatigue at 20% of the ultimate strength caused no decrease in strength at any void content, while fatigue at 30% caused premature failure. Impact testing showed that only very high void contents had a substantial effect on performance. Environmental conditioning showed moisture absorption proportional to void content and furthermore significantly reduced the residual tensile strength after a single thermal cycle to 0°C. These results suggest that void content in conjunction with a weak interface may have been responsible for limiting material proper

ISSN:0032-3888    

DOI:10.1002/pen.760332407

出版商:Society of Plastics Engineers    

年代:1993

数据来源: WILEY

摘要:

AbstractThis study deals with the quantification of the residual stress profiles of injection molded rectangular bars with polycarbonate of various molecular weights. Determinations via the layer removal technique indicated that these bars can possess compressive stress of up to 10 MPa at the surface and up to 9 MPa in tension in the interior. The residual stress profile showed a shape similar to that observed in thermal quenching experiments, i.e., parabolic, for locations far away from the gate of the cavity. At locations near the gate of the cavity and for injection molding at high speed, the residual stress profile changed sign, going from compressive to tension at the surface and vice versa in the interior. These residual stress profiles were very dependent on gating geometry and location.

ISSN:0032-3888    

DOI:10.1002/pen.760332408

出版商:Society of Plastics Engineers    

年代:1993

数据来源: WILEY

摘要:

AbstractThe effects of kinetic parameters Hatta number Ha and ϕ, and transport parameters Biot number Bi and ψ on the migration of a reactive additive have been modeled. The convective boundary layer can be considered inert to fully reactive for Ha → 0 to 2, respectively. The migration is controlled by diffusion, reaction rate, equilibrium partitioning of the migrant between the plastic film and the solvent, and boundary layer resistance, depending on the time scale and values of the above parameters. For a nonreactive boundary layer, at high Bi, migration is initially diffusion‐controlled but becomes reaction‐rate controlled at later times. However, under similar conditions, a slight increase in the reactivity of the boundary layer immediately changes the transport process from diffusion‐controlled to reaction rate‐controlled. With further increase in reactivity, migration spontaneously reaches equilibrium. At low Bi, equilibrium partitioning and reactive depletion of the migrant in the fluid phase change predominantly to boundary layer‐controlling mass transport phenomenon. For a given sampling time (τ = 1.0), the migration increases with the increase in Bi only for a reactive boundary layer. At low Bi and (τ = 1.0), migration decreases only up to ϕ ≅ 0.3 for nonreactive to partially reactive boundary layer, but for intermediate and high Bi, ϕ and Ha do not

ISSN:0032-3888    

DOI:10.1002/pen.760332409

出版商:Society of Plastics Engineers    

年代:1993

数据来源: WILEY

ISSN:0032-3888    

DOI:10.1002/pen.760332401

出版商:Society of Plastics Engineers    

年代:1993

数据来源: WILEY