Rheology of PVC plastisol--VI: criteria for yielding and fracture of an immobilized layer

PVC plastisol exhibits pseudo-plastic flow in steady shear; that is, viscosity decreases with the increasing shear rate. At higher shear rates viscosity reaches a minimum and then increases, i.e., dilatant behavior. Previously, pseudo-plastic behavior was explained by a mechanism in which the suspended particles partition into an immobilized layer and a mobile phase. The development of the immobilized layer with the increase in shear rate was shown to quantitatively account for pseudo-plastic behavior. In higher shear rates dilatation of the immobilized layer was shown to be the cause of dilatacy. At even higher shear rates the immobilized layer fractures. In this paper the viscosity minimum was interpreted as the yielding of the immobilized layer. Subsequently, data in the literature were analyzed to find criteria for the yielding and fracture of the immobilized layer. Yielding was found to obey Coulomb's criterion, from which the coefficient of friction and the cohesive strength of the immobilized layer were evaluated. These properties were controlled by the nature of particle assembly in the immobilized layer and the plasticizer type had only a minor effect. The value of the coefficient of friction was on the lower side and within the range of values found in the literature for other materials. There were two modes of fracture of the immobilized layer, one with low strength, low strain to break, and the other with high strength, high strain to break. The former is analogous to the brittle fracture of solids and the latter ductile failure. The strength of brittle fracture was somewhat higher than cohesive strength, which was evaluated from yielding data. This is akin to Griffith's criterion for brittle fracture of a solid. Ductile failure occurred when the shear stress exceeded normal stress.

Rheology of PVC Plastisol--V: storage modulus and network formed by particle contact

Our previous work with PVC plastisols showed that pseudo-plastic behavior under increasing frequency of dynamic measurement was a result of the development of an immobilized layer. Subsequently, the dynamic viscosity and the storage modulus of the mobile phase were evaluated. The samples consisted of fine and coarse particles, of which the fine/coarse ratio was varied. The present work relates the storage modulus at different frequencies to the fine/coarse ratio through a model network consisting of particle-particle contacts. The contacts are of three kinds, fine-fine (f-f), fine-coarse (f-c), and coarse-coarse (c-c). The average number of contact points per particle has been evaluated for the above kinds of contacts at different frequencies. Also, a number of particles participating in the network have been evaluated as a relative measure. At lower frequency relatively larger numbers of f-f contact points exist per particle but fewer numbers of particles participate in the network. Therefore, the network is rather loose. The f-c contact shows a similar trend but fewer contact points per particle at all frequency levels. The c-c contact is limited to a pair formation, i.e., only 0.5 contact point per particle on average. However, c-c contact contributes significantly to the magnitude of the modulus, because of the size of the particle. At the higher frequency there are fewer f-f contacts, but a larger number of particles are participating in the network. The trend with the f-c contact is very similar. The c-c contact increases significantly at the higher frequencies.

Arteriovenous fistula. Cutaneous manifestations

The association between congenital large-vessel arteriovenous (AV) fistulae of the extremities with Kaposiform acroangiomatosis is well established. Based on pathogenetic considerations, it is likely that many benign, cutaneous angiomatoses represent minute or microscopic AV fistulae. Combined large vessel and small vessel congenital AV fistulae of the extremity would result in the previously mentioned syndrome.

The XYY syndrome in prisoners and outpatients with cystic acne

The XYY complement appears to be more common in individuals with nodulocystic acne than in those without acne. Consequently, nodulocystic acne may prove to be a useful adjunct to height as an easily recognizable, phenotypic feature of the XYY syndrome.