Toughening of polypropylene with calcium carbonate particles

We present chain structure, phase morphology, and toughness relationships in thermoplastic polymers and polymer/rubber blends. In neat polymers, molecular aspects of craze/yield behavior are controlled by two chain parameters: entanglement density ν e and characteristic ratio C ∞ . The crazing stress is proportional to ν e 1/2 , and the yield stress is proportional to C ∞ . The dispersed rubber toughens a polymer/rubber blend mainly by promoting energy dissipation of the matrix. The toughening efficiency correlates with the rubber phase morphology and the chain structure of the matrix

Chain structure, phase morphology, and toughness relationships in polymers and blends

A theory is advanced to explain the effects of rubber particle cavitation upon the deformation and fracture of rubber-modified plastics. The criteria for cavitation in triaxially-stressed particles are first analysed using an energy-balance approach. It is shown that the volume strain in a rubber particle, its diameter and the shear modulus of the rubber are all important in determining whether void formation occurs. The effects of rubber particle cavitation on shear yielding are then discussed in the light of earlier theories of dilatational band formation in metals. A model proposed by Berg, and later developed by Gurson, is adapted to include the effects of mean stress on yielding and applied to toughened plastics. The model predicts the formation of cavitated shear bands (dilatational bands) at angles to the tensile axis that are determined by the current effective void content of the material. Band angles are calculated on the assumption that all of the rubber particles in a band undergo cavitation and the effective void content is equal to the particle volume fraction. The results are in satisfactory agreement with observations recorded in the literature on toughened plastics. The theory accounts for observed changes in the kinetics of tensile deformation in toughened nylon following cavitation and explains the effects of particle size and rubber modulus on the brittle-tough transition temperature.

Dilatational bands in rubber-toughened polymers

Toughness mechanism in semi-crystalline polymer blends: I. High-density polyethylene toughened with rubbers

Toughening mechanisms in epoxy–silica nanocomposites (ESNs)

The development of dissipative mechanisms during the fracture of toughened polyamides is investigated by SEM. A preliminary fractographic analysis is carried out on the ductile fracture surfaces of water conditioned PA 6 specimens. Extensive yielding gives rise to a wide development of shear bands and rumples, depending on the local stress conditions. Then, the stress whitened regions under the ductile fracture surfaces of rubber toughened HI-PA 6 specimens are analysed by SEM. This is made possible by means of a simple technique for the cryogenic dissection of ductile fracture surfaces: sections are cut both transversally and longitudinally with respect to the ductile crack growth direction. Under the ductile fracture surfaces extensive cavitation occurs around the rubbery particles, following preferential and definite shear directions. The presence of cavitation is correlated with the distance below the ductile surfaces. The rumpled morphologies can be directly correlated with cavitation and shear bands occurring inside the whitened regions below ductile fractures. Both the orientation of shear bands on the fracture surfaces of water conditioned PA6 and the cavitation-shear yielding morphologies developed inside the fractured PA-rubber blends comply with an octahedral shear model, common to other polymers (rubber toughened epoxies, for instance) and to ductile metals.

Plero Fabbri

Papers

Mechanisms of energy dissipation during impact in toughened polyamides: a SEM analysis