ABSTRACT:The processing of poly(vinyl chloride) (PVC) consists of fusion, flow, and solidification processes. In this study, we examined how gelation in the fusion process affected the processability in the following flow process. We prepared sheets with adjusted degrees of gelation (Gs) by rolling a rigid-pipe-formulation PVC compound with a degree of polymerization of 1050 by changing the milling temperature and time. The rheological properties, including viscosity, die swell, melt fracture, and entrance pressure loss (⌬P ent ), of the sheets were measured with a capillary rheometer, and the effect of gelation on the flow processability was studied. The viscosity slightly increased as the milling temperature and time increased, and hence, G increased. The flow activation energy showed a maximum at a shear rate around 100 s Ϫ1 and increased with increasing gelation. The die swell was larger when a sheet with a more advanced gelation was extruded at a higher temperature. Melt fracture easily occurred when a sheet with an advanced gelation was extruded at a low temperature. The critical shear rate at the onset of melt fracture changed by over 3 decades, depending on G. ⌬P ent increased linearly with G. The dependence of the rheological properties on the gelation was in the following order: melt fracture Ͼ die swell Լ ⌬P ent Ͼ viscosity: The effect was larger for the elasticity than for the viscosity. The change in these rheological properties with increasing gelation was caused by a transition from particle flow to uniform molecular chain flow.
Maleic anhydride was grafted onto a polypropylene-type thermoplastic elastomer PER by reactive processing with a screw extruder, and a maleated PER (MPER) was prepared. Aiming at ionic crosslinking, magnesium 12-hydroxy stearate (MgStOH), zinc oxide (ZnO), and zinc sulfide (ZnS), and aiming at covalent crosslinking, melamine as an amino compound, and Epocizer and GRYCI-ALE as epoxy compounds, were added to the MPER, melt-mixed by use of the screw extruder, and crosslinked compounds were obtained. The rheological properties such as capillary flow properties and dynamic viscoelasticities of the compounds were measured and their melt processabilities were evaluated. The degree of crosslinking was in the order of epoxy compounds Ͼ MgStOH Ͼ melamine Ͼ ZnO, ZnS Ͼ MPER (blank). In the case of the compound with MgStOH of a moderately high degree of crosslinking, the nonNewtonian behavior is remarkable and the die swell ratio is low and suitable for extrusion. However, in the case of compounds with epoxy compounds of an excessive degree of crosslinking, fine extrudate cannot be obtained due to the lack of fusion and the control of degree of crosslinking is necessary.
ABSTRACT:The effect of degree of polymerization (DP) on the gelation and flow processability of poly(vinyl chloride) (PVC) was studied. Sheets with adjusted degree of gelation were prepared by rolling rigid pipe formulation suspension PVC compounds with DPs of 800, 1050 and 1300 by changing the milling temperature. Their degrees of gelation were measured with DSC and their capillary flow properties were measured with a capillary rheometer at 150, 170 and 190°C and the effect of DP on the relation between gelation and flow processabilities was studied. Because of the higher shearing heat during milling, the sample with the higher DP had a higher history temperature and thus tended to show a higher degree of gelation. The viscosity increased as the gelation increased. The dependency of viscosity on DP was higher at higher milling and extrusion temperatures and thus at a higher degree of gelation and a lower shear rate. This was assumed to be attributed to the more prominent uniform molecular flow as against the particle flow. The die swell increased with increasing the milling and extrusion temperatures and hence with increasing the gelation. A sample with a lower DP tended to show a larger die swell and this tendency was even more pronounced at the higher extrusion temperature. The melt fracture easily occurred when a sample with advanced gelation was extruded at low temperature. Whereas at low milling temperatures a sample with the lower DP showed a lower critical shear rate at onset of melt fracture, and thus easily generating melt fracture, at high milling temperatures it showed a higher critical shear rate and hence scarcely generated melt fracture. These experimental results were explained by the fact and concept that a sample with a lower DP shows a higher increase in the gelation during extrusion and/or the slighter feature of particle flow as against the uniform molecular flow at the same gelation level.
The increase in radome loss, sidelobe levels, and cross‐polarization level in the rain plays an important role in determining antenna characteristics. These effects have been investigated at 10 GHz ∼ 30 GHz with a reflector antenna having a planar radome, and a new method has been propsoed for deriving the radome loss in which a water‐shedding property of the radome is taken into account. Raindrops on the radome form water streams that flow down onto the surface thereby increasing the loss of the vertical polarization wave. It is also found that the cross‐polarization level generated by a wet radome is less than −40 dB. The sidelobe level is not increased and the paraxial radiation pattern is not changed by a wet radome but it is lowered as a whole. In the present theory, the contact angle of a raindrop surface with a radome surface determines the condition of the wet radome surface and this angle was predicted theoretically by using measured radome loss.
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