Synergies among the self-assembled β-nucleating agent and the sheared isotactic polypropylene matrix
Abstract The impacts of both shear temperature and shear rate on both the self-assembled β-nucleating agent (β-NA) in the hosting isotactic polypropylene (iPP) matrix and the subsequent crystallization of the hosting iPP matrix were investigated by polarized optical microscopy (POM) and wide-angle X-ray diffraction (WAXD), and small-angle X-ray scattering (SAXS) measurements, respectively. Under static condition, β-NA could self-assemble into snowflake-like aggregate. Once shear was applied, three kinds of self-assembled aggregates could be developed prior to the crystallization of matrix depending on the applied shear temperatures, that is, snowflake-like self-assembled aggregate at 200 °C, deformed snowflake-like one with smaller branches at 180 °C, tiny and dense needle-like one if shear temperature was further decreased to 160 °C. Since β-NA with various self-assembled aggregates can act as nucleating templates for the subsequent nucleation of iPP matrix, it is conceivable that the crystallization behavior of the iPP matrix can be altered. As expected, β-phase crystallinity, long period and lamellar thickness are steadily dependent on the shear temperature and shear rate. That is, when shear was applied at 200 °C, β-phase crystallinity, long period and lamellar thickness keeps almost constant regardless of the shear rate. If melt was sheared at 180 °C, β-phase crystallinity was decreased while long period and lamellar thickness remained unchanged with elevating shear rate. At lower shear temperature (i.e., 160 °C), β-phase crystallinity, long period and lamellar thickness were further decreased with increasing the shear rate. These results are of practical significance to understand the long-term ignored issue: apart from the loading of β-NA, different self-assembled aggregates formed in flow field are another crucial factor in determining the microstructure development of matrix.
Fracture toughness of α- and β-phase polypropylene homopolymers and random- and block-copolymers
Abstract The fracture and failure mode of α- and β-phase polypropylene homopolymers (PP-H), block- (PP-B) and random-type (PP-R) copolymers with ethylene were studied in high speed (1.2 m/s) flexural tests and compared. The crystallinity of the α- and β-modifications was assessed by wide-angle X-ray scattering and differential scanning calorimetry. The linear elastic fracture mechanical parameters, viz. fracture toughness (Kc) and fracture energy (Gc), were determined at room temperature and T=−40 °C on notched Charpy specimens. β-Phase PP-H and PP-B showed superior toughness to the α-versions. On the other hand, Kc and Gc were similar for PP-R in the temperature range studied for both α- and β-modifications. Fracture surfaces of the broken specimens were inspected in scanning electron microscopy and the related failure mode concluded. A model was proposed to explain the toughness improvement via β-crystallinity by considering all proved experimental findings.
Control of reflective cracking
The first is 'fatigue' cracking of the layer. This type of cracking is the result of the application of loads on the pavement and is controlled by appropriate thickness design which 'ill not be discussed in detail here. Thickness design includes not only the design strength and thickness of the stabilised, but other design elements such as delaying reflection cracking, or limiting stresses in the stabilised layer, by adopting a minimum thickness of overlying material. This may include an unbound layer utilising the 'upside down' pavement approach. The actual thickness required of the overlying material varies but for example PIARC (Ref.1) recently reported when the thickness of coated materials exceeds 20 cm the emergence of shrinkage cracks at the surface is generally uncommon; when it is 15 cm thick, the cracks emerge at the surface, but do not shorten the life of the surface course, especially if they are sealed. This experience has been gained 'with legal axle loads of 130 kN, considerably greater than the Australian legal axle load.