The prediction of the crystallinity and microstructure that develop in injection molding is very important for satisfying the required specifications of molded products. A novel approach to the numerical simulation of the skin-layer thickness and crystallinity in moldings of semicrystalline polymers is proposed. The approach is based on the calculation of the entropy reduction in the oriented melt and the elevated equilibrium melting temperature by means of a nonlinear viscoelastic constitutive equation. The elevation of the equilibrium melting temperature that results from the entropy reduction between the oriented and unoriented melts is used to determine the occurrence of flow-induced crystallization. The crystallization rate enhanced by the flow effect is obtained by the inclusion of the elevated equilibrium melting temperature in the modified Hoffman–Lauritzen equation. Injection-molding experiments at various processing conditions were carried out on polypropylenes of various molecular weights. The thickness of the highly oriented skin layer and the crystallinity in the moldings were measured. The measured data for the microstructures in the moldings agree well with the simulated results. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 95: 502–523, 2005

Flow‐induced crystallization in the injection molding of polymers: A thermodynamic approach

(1990). STRUCTURE/PROPERTY RELATIONSHIPS IN TEXTILE FIBRES. Textile Progress: Vol. 20, No. 4, pp. 1-43.

Structure/property relationships in textile fibres

The evolution of structure and properties in uniaxially stretched polybuty- lene terephthalate (PBT) films has been explored. The stretch temperature has pro- nounced influences on the development of PBT polymorphism; the mechanisms of its formation involve levels of both applied stress as well as chain relaxation during the stretch period. The stretch rate, on the other hand, tends to perfect the structure. PBT films produced under different conditions develop different levels of crystallinity and orientation in the individual a and b phases, in which annealing under tension induces a partial crystal transition from the b to a phase that leads to an increment of orientation in the a phase. The mechanical behavior of the films is related to the polymorphic structure as well as to the overall chain orientation. © 2000 John Wiley &

Formation of polymorphic structure and its influences on properties in uniaxially stretched polybutylene terephthalate films

Degradation of poly(butylene terephthalate) (PBT) induced by blending time at 260°C leads to an increase of both carboxylic end groups and melt viscosity index and a decrease of the molecular weights. Crystallization kinetics of pure PBT and PBT blended with and epoxide, which was used to reduce the free carboxylic acid concentration, were compared. Lower carboxylic acid concentrations were obtained, but the molecular weights still decreased. The crystallization kinetics of PBT blended with pigments and/or mineral fillers were also studied. Dyes, some of which are known for their nucleation activity, were used for differentiating extruded PBT tubes in optical fiber loose cables. Post-shrinkage appeared to depend on the type of pigment used. Pigments Concentration, particle size and chimical nature have been correlated to the PBT crystallization kinetics. An increase of the nonisothermal crystallization temperature was observed, more or less important according to the pigment used. Talc and mica were also tested in PBT blends, with and without pigments. Their mucleation activity parameters were measured and compared with those for the pigments. An average value of the crystallization temperature, nearly constant and independent of the Pigment, was obtain controlled shrinkage on industrial products with strict dimension requirements.

Crystallization kinetics of poly(butylene terephthalate) (PBT): Influence of additives and free carboxylic acid chain ends

The wetting properties of poly(butylene terephthalate) (PBT) melt blown fibers were tuned by alkaline hydrolysis and subsequent fluorination. Fiber mats were exposed to a NaOH methanol solution for controlled periods of time at several temperatures, resulting in surface hydrolysis (h-PBT). Subsequent simple solution chemistry was applied to the h-PBT fibers, leading to fluorination of the surface (f-PBT) and the transformation of the wetting properties of the material. Electron microscopy revealed that hydrolysis leads to a textured surface that is retained in the fluorinated product. Sessile drop wetting measurements demonstrated superhydrophilicity for the h-PBT fiber mats and sticky superhydrophobicity with the f-PBT fiber mat.

Tuning Surface Properties of Poly(butylene terephthalate) Melt Blown Fibers by Alkaline Hydrolysis and Fluorination

An on-line study of structure development during poly(butylene terephthalate) melt spinning was carried out. Two polymers with different molecular weights (intrinsic viscosities of 0.75 and 1.0 dL/g) were used. The range of take-up velocities studied was 1500 to 4500 m/min. On-line measurements included diameter, temperature, birefringence, and tension. The phenomenon of diameter thinning (necking) was observed for both polymers at take-up velocities of 3500 and 4500 m/min with a mass throughput of 4 g/min. At a constant mass throughput, the distance from the spinneret at which the necking occurred varied with take-up velocity and molecular weight of the polymer. Increasing the take-up velocity at constant mass throughput caused an increase in cooling rate and a slight increase in the rate at which the temperature decreased with distance from teh spinneret. A small but detectable change in the rate of temperature decrease was observed at a position near or just beyond the formation of the neck. It is suggested that this effect is due to the increased heat transfer caused by the rapid increase in filament velocity and increased surface to volume ration in the neck. Increased take-up velocity also caused necking to occur at higher temperature, as did an increase of polymer molecular weight. Birefringence increased with distance from the spinneret and indicated substantial molecular orientation was developed in the filament prior to the necking zone. A sharp increase of birefringence in the necking zone was observed for take-up velocities of 3500 and 4500 m/min. A discussion of the mechanism of neck formation is presented, and it was concluded that necking is intimately associated with stress-induced crystallization in PBT. An increase of spinline stress resulting from either an increase of take-up velocity or an increase of molecular weight can cause stress-induced crystallization and, hence, necking to occur nearer the spinneret and at higher temperature. For a given polymer this leads to filaments with higher levels of crystallinity, crystalline orientation, and crystalline perfection (greater crystal size). These changes in morphology result in changes in the filament mechanical properties. The effect of molecular weight change on the structure and properties is complicated by the fact that the development of crystallinity seems to be affected by the molecular weight independent of the spinline stress.

On‐line studies of structure development during melt spinning of poly(butylene terephthalate)

Filaments of poly(butylene terephthalate) were prepared by melt spinning with take-up velocities in the range 1000–5600 m/min. Two polymers with different molecular weights were used (intrinsic viscosities of 0.75 and 1.0 dL/g). The filaments were characterized using measurements of density, birefringence, shrinkage, thermal properties (differential scanning calorimetry), crystal size, crystalline orientation and phases present (wide angle X-ray diffraction), and tensile mechanical properties. Filaments spun from the 0.75 IV polymer with a mass throughput of 6 g/min at 1000 m/min have essentially amorphous structures, while higher take-up velocities result in α-form crystals or, at the highest take-up velocity, a mixture of α-form and β-form crystals. Only α-form crystals were detected in the higher IV polymer. Crystal size varied with crystallographic direction but generally increased as take-up velocity increased. At the lowest take-up velocities the filaments increased in length during thermal shrinkage measurements. With increasing take-up velocity the shrinkage became positive and continued to increase until reaching a maximum in the range of the highest sprinning speeds. This behavior correlates with the variation of the orientation factors of the amorphous phase. A plateau was observed in stress versus strain curves corresponding to strain-induced transformation from α-form to β-form crystals. The length of this plateau increased with increase of take-up velocity and the α-form crystal content in the sample. Both morphology and physical properties varied with polymer molecular weight and melt spinning conditions.

Shida Chen

Papers

On‐line studies of structure development during melt spinning of poly(butylene terephthalate)

Structure and properties of high-speed melt-spun filaments of poly(butylene terephthalate)