Rheological properties of poly(vinyl chloride)/epoxidized natural rubber blends. Part I: Composition dependence and the effect of compounding conditions

Epoxidized natural rubber is a recently commercialized modified form of natural rubber. This paper is part of our continuing effort to study the responses of this new material to melt mixing and other shaping processes. The Shimadzu capillary rheometer was used to evaluate the composition dependence of miscibility of polyvinyl chloride/epoxidized natural rubber blends (PVC/ENR blends). The rheometer was also used to evaluate the effect of compounding parameters on the rheological properties of the blends. The results confirmed PVC/ENR blends as miscible systems that show a synergism in apparent shear viscosity highlighted by the positive deviation from the logarithmic additivity rule. These results based on capillary rheometry are also in very good agreement with our earlier attempt to predict optimum mixing from torque rheometry by using the Brabender Plastogram as an indicator.     

Explaining the abnormally high flow activation energy of thermoplastic polyurethanes

The rheological properties of a thermoplastic polyurethane (TPU) were studied at small and large deformation via three different types of rheometry: dynamic shear, capillary, and torque (an instrumented batch mixer). The effect of degradation during TPU processing on the melt viscosity was investigated and several factors, such as temperature, time, shear stress, and flow type that may affect the degradation were studied. Apparent activation energy of flow (E_a) was determined to be 328 kJ/mol, much larger than expected. A simple model was derived to describe the relationship of molecular weight and thermal dissociation of urethane linkages. Contributions of flow and the degradation reaction of TPU to overall activation energy were found to be additive: E_a=E_η+1.7ΔH_deg. True activation energy of flow (E_η) was estimated to be 144 kJ/mol. While the high apparent flow activation energies in dynamic shear and capillary rheometry can be explained by simple thermal degradation, melt viscosities interpreted from the instrumented batch mixer showed a much lower apparent activation energy (186 kJ/mol). This low value may be due to a combination of effects: errors in the relation between viscosity and mixer torque for TPU, side reactions resulting from air exposure, high stress level during the melting, and extensional stresses.     

Structure–property relationships in PVC compression moldings

Compression moldings were produced from two rigid PVC compounds at a range of temperatures. The tensile and impact properties of these moldings depended primarily on the level of particle fusion as assessed by extrusion rheometry. Properties were not related to the level of primary crystallization measured by X-ray diffraction, but the particle fusion process appears to be at least partly due to recrystallization. Fusion occurred more readily in the mass PVC compound than in the suspension PVC compound. Annealing the sheet produced at 200°C caused changes in crystallinity which resulted in small property changes. The maximum annealing effect occurred at 110°C.     

Biopolyester melt behavior by torque rheometry

A torque rheometer method has been developed to evaluate the melt behavior of biopolyesters. The main features of this technique include: (i) a modified equipment configuration, so as to maintain the melt temperature within ± 1°C pf the set temperature after the first few minutes of polymer melting and thermal equilibration, and (ii) a kinetic equation that can be used to determine the melt behavior from the torque-time data. Applying this methology to a poly(3-hydroxybutyrate-co-3-hydroxyvalerte) copolymer shows that the degradation kinetics follow Arrhenius behavior with temperature, that the degradation rate increases with increasing hear, and that the activation energy for random chain scission is independent of shear rate. This effect of shear on the degradation process may be a result of viscous heating. The degradation rate is increased because of the increased internal energy of the polymer chains as a result of the mechanical deformation. There is no evidence of direct mechanical degradation. Last, the viscosity values obtained with the torque rheometer are consistent with those obtained by capillary rheometry.     

A study of extrudate distortion in controlled-rheology polypropylenes

The melt fracture characteristics of controlled-rheology polypropylenes (CRPP) were studied by means of capillary rheometry experiments. CRPPs were produced through reactive extrusion of a commodity polypropylene resin using various peroxide concentrations. These materials exhibited lower molecular weights and narrower molecular weight distributions than those of the starting commodity resin. The CRPP materials studied were found to exhibit only gross melt fracture. At extremely high shear rates and relatively low temperatures, a sigmoidal flexure was observed in the flow curve of certain CRPPs. Generally, it was found that the severity of melt fracture decreased with increasing shear rate for a given material and temperature and in some cases, the extrudates exhibited completely smooth surfaces. Also, the severity of surface distortions was reduced when high L/D dies were employed at a given shear rate. The critical shear stress for the melt fracture onset was found to increase with decreasing molecular weight and polydispersity, and correlations have been developed between the critical stress values and the polymer polydispersity and shear compliance.     

Preparation and properties of self-reinforced polypropylene/liquid crystalline polymer blends

The mechanical properties of self-reinforced liquid crystalline polymer/polypropylene (LCP/PP) blends strongly depend on the viscosity ratio of the blend components in the melt. This ratio was determined for PP blends with different commercial LCPs (Vectra A950 and Vectra B950), by means of capillary rheometry, under conditions representative for the blending process during extrusion. It was found that optimal mechanical properties were achieved when the LCP/PP viscosity ratio at 285°C ranges between 2 and 4 at a shear rate of 800–1000s⁻¹. The LCP/PP viscosity ratio appears to be shear stress dependent. This creates the option of fine tuning the LCP droplet deformation process by means of the extrusion rate. This shear stress dependence is more pronounced for PP blends with Vectra B950 than for blends with Vectra A950.     

Rheology of a thermoplastic paste through the mushy state transition

The rheology of a highly filled thermoplastic paste employed in a novel continuous casting process was investigated across the melt-mushy zone temperature transition. The paste comprises of an ultrafine (or nano-particulate) zinc oxide powder (, 50 vol%) and a continuous phase consisting of a blend of organic waxes with an onset of solidification around . Data were collected over nine decades of shear rate, using controlled strain and controlled stress devices, steady shear and oscillatory modes, rotational rheometry, and capillary rheometry using the multi-pass technique. The melt rheology could be described using a Carreau-type constitutive model, with strong evidence of a low shear rate viscosity plateau and a possible high shear rate viscosity plateau, with a transition between the two described by a highly shear-thinning power-law region. Nearer the solidification temperature the material exhibited strong pseudo-plastic behaviour, and capillary flow appears to be determined by pressure-dependent slip behaviour.     

Effect of catalyst on the reactive processing of polyesters with epoxy-functional polymers

The objective this work was to determine the effects of selected polyester catalysts on the reaction of a polyester with epoxy functional polymers. Polyesters containing various catalyst metals were melt blended with either an ethylene-co-glycidyl methacrylate or a styrene-co-glycidyl methacrylate copolymer. The viscosities of the blends were monitored as a function of mixing time using torque rheometry. In addition, the molecular weight distributions of selected samples were analyzed using gel permeation chromatography. Both the torque rheometry and the gel permeation chromatography results indicate that the polyester reacts with epoxy functional polymers. This reaction occurs under conditions and at processing times which are readily obtainable in conventional melt processing equipment. Furthermore, the reaction kinetics of polyesters with glycidyl methacrylate copolymers are dramatically affected by the nature of the catalyst system used to prepare the polyester. Under the conditions used, antimony catalysts are particularly effective at promoting the reaction between polyesters and the epoxy functionality and the activity of the catalysts studied appears to decrease in the following order: antimony > gallium > tin ? titanium > germanium. Manipulation of the polyester catalyst system may offer a method to control the extent of reaction obtained in reactive processing of polyesters with epoxy functional compounds.     

A Model for Primary Normal Stress Difference of Dynamic Extrusion of a Polymer Melt through a Capillary

The dynamic extrusion process of a polymer melt through a capillary under a superimposed vibration has been was researched deeply, and a mathematical model for the primary normal stress difference of a polymer melt under the vibration force field was set up. Accordingly, the calculation steps of above primary normal stress difference were established based on a rheological measuring equipment which was disigned by the authors. The primary normal stress difference of polymer melt under a vibration force field can be calculated by measuring the instantaneous data of capillary die swell, capillary entry pressure, capillary volume flux, and their phase difference under vibration with different frequencies and amplitudes.     

Effects of the incorporation of low‐molecular‐weight diurethanes on thermal and rheological properties of thermoplastic polyurethane

In this study, two low‐molecular‐weight diurethanes were synthesized and blended with thermoplastic polyurethane (TPU). The effects of the incorporation on the thermal and rheological properties of TPU were evaluated. The diurethanes were obtained from the reaction of 4,4′‐diphenylmethane‐diisocyanate (MDI) with 1‐butanol (Additive 1) or 1‐octanol (Additive 2). Blending of the additives with TPU was carried out in a torque rheometer, and the blends obtained were analyzed by differential scanning calorimetry (DSC), torque rheometry, and capillary rheometry. The torque rheometry showed that an increase in the amount of both additives displaced the charging peaks to longer times and reduced the torque values after melting. The DSC analysis showed that the incorporation of the additives did not affect the glass transition temperature (T_g) of the flexible phase of TPU. However, an increase in the amount of Additive 1 led to a reduction in the T_g of the rigid phase, while increasing the amount of Additive 2 caused an increase in the T_g of this phase. Capillary rheometry results showed that blends with up to 2 wt % of additive led to intrinsic viscosity and melt‐flow stability values higher than those of processed TPU. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

A calibration technique to evaluate the power-law parameters of polymer melts using a torque-rheometer

In this work we develop an empirical method to calibrate the measuring head of a torque-rheometer to obtain the power-law parameters of polymer melts. This method is based on a similar analysis developed by Lee and Purdon. However, in this work, only one geometrical parameter, with well-defined physical limits, is needed for calibration, instead of the two arbitrary parameters proposed by the previous authors. Moreover, this parameter is closely related to the ordinate of the logarithmic relationship between the torque (M_T) and the angular velocity (S₁), obtained from the torque-rheometer data. This allows us to define a calibration function for this parameter, which is used to evaluate the consistency index (m) of the melt. On the other hand, experimental results confirm the equivalence between the slope of the logarithmic values of torque and angular velocity obtained from the torque-rheometer data and those of shear stress vs. shear rate obtained from capillary rheometry. This further highlights the feasibility of evaluating the melt flow index (n) and the consistency index (m) from the torque-rheometer data.     

Effects of dry blending on morphogy of PVC powder particles

Mass and suspension PVC were blended on a Fielder mixer and changes in powder morphology and additive distribution investigated. The techniques used to characterize and assess processing behavior have included microscopy, density, size analysis, torque, and capillary rheometry. The particulate structure of PVC remained intact, and there were no pronounced differences in the processability of blends discharged at different temperatures. In the absence of shear PVC particles are largely unchanged in character. Solid additives do not appear to enter into resin particles while liquid stabilizer readily does so.     

Prediction of the onset of extrudate distortion in polyethylenes from rheological data

An extensive set of rheological data, including steady shear, normal stress, and oscillatory response functions, have been obtained for several polyethylene melts, and a modified form of the Spriggs nonlinear co-rotational model has been used to represent the data. The resulting model was used to predict the transient stress response of the melts in a simplified model of the extrusion process, with a resulting criterion for the onset of melt distortion in terms of readily evaluated rheological parameters. The predicted occurrence of melt distortion based on this criterion agreed well with observations for the polymers in this study as well as for a variety of other polymers reported in the literature.     

Polyisoprene modified polystyrene

Polyisoprenes have been used to prepare modified polystyrenes in an attempt to improve impact strength. It has been determined that polyisoprene reinforces polystyrene by the formation of complex rubber phase particles, similar to those in commercial polybutadiene reinforced polystyrenes. These particles, however, are subject to breakdown by melt shearing, resulting in a significant drop in impact strength and an increase in tensile strength. This particle destruction is due to the fact that polyisoprene does not crosslink during the polymerization process. It is significant that before shearing these uncrosslinked rubber particles satisfactorily toughen polystyrene.     

4,4'/2,4'-二苯基甲烷二异氰酸酯/多元醇本体聚合流变动力学

采用转矩流变仪模拟双螺杆挤出反应条件,分别在无催化剂和铋/锌复合催化剂作用下进行4,4'/2,4'-二苯基甲烷二异氰酸酯/聚四氢呋喃醚二醇/1,4-丁二醇本体聚合,结合二维流动简化假设、黏度/分子量关联式和Arrhenius方程,建立了热塑性聚氨酯(TPU)重均分子量与反应体系转矩、温度的流变动力学方程,分别拟合得到无催化剂和催化剂作用时体系转矩与重均分子量的关联式;发现采用流变动力学方程模拟的TPU平均分子量与实时取样后采用凝胶渗透色谱法测定的结果吻合良好,采用流变动力学方法测量方便迅速,能提供整个反应过程中TPU平均分子量的变化情况。     

Effect of NBR partitioning agent on the mechanical properties of PVC/NBR blends and investigation of phase morphology by atomic force microscopy

Poly(vinyl chloride) (PVC)/acrylonitrile–butadiene rubber (NBR) blends with different types of partitioning agent were obtained through melt blending. The samples were characterized according to the viscosities properties, torque rheometry and mechanical resistance as tensile testing, tear strength, and hardness. The morphology and phase imaging were studied using an atomic force microscopy operating in tapping mode (TMAFM). It was observed that the PVC/NBR blends with PVC as partitioning agent showed an increase in the tensile stress and Young’s modulus compared to the PVC/NBR blends with calcium carbonate as partitioning agent. The morphology of the blends examined by TMAFM evidenced the effect of the partitioning agent as obtained with other techniques.     

Pumping characteristics of self-wiping twin-screw extruders—A theoretical and experimental study on biopolymer extrusion

The flow of Newtonian and non-Newtonian fluids which obeys a power law relationship between shear stress and shear rate has been modeled in the melt conveying section of a self-wiping co-rotating twin-screw extruder using a finite element analysis of an unwound channel section. Predictions of throughput against pressure gradient are compared with experimentally obtained results for maize grits which is represented as a power law material. Rheological data applicable to extrusion simulation were obtained from capillary rheometry. Comparisons are reasonable with predicted characteristic showing similar behavior.     

New approaches to the processing of rutile-filled polyolefins

The performance of a fluorocarbon elastomer processing additive in rutile-filled linear low density polyethylene (LLDPE) compounds was evaluated by capillary rheometry, and blown film extrusion. Different compounding sequences were considered and their effects on the performance of the processing additive in the presence of various rutiles examined. Lower apparent melt viscosities and higher shear rates for the onset of melt fracture were observed when using certain surface treated rutiles. The nature of the surface coating applied to rutiles was found to have a great influence on the Theological properties of the filled compounds and on the dispersibility of the solids. The acid-base characteristics of rutiles were determined by inverse gas chromatography techniques, and inherent agglomeration indexes for the pigments were measured by an application of powder rheology principles. It was found that those rutiles with high agglomeration indexes or those with highly basic surfaces interfered the most with the processing additive. Mechanisms by which rutile dispersibility and acid-base character influence the effectiveness of the fluorocarbon elastomer processing additive are discussed.     

Effects of Starting Powder Characteristics on Bulk Assembly of Titania

Seven nanoscale titanias synthesized from a sulfate process were investigated for rheological characterization. Alteration of synthesis parameters had resulted in variation of the powder surface area and soluble sulfate level from powder to powder. The powder differences were further corroborated in stress-controlled rheometry where the viscoelastic yield stress of a suspension was seen to vary strongly with processing history. Capillary and torque rheometry further showed the dependence of processing history on steady-state extrusion pressure and steady-state mixing torque. The powders were washed to determine whether sulfate equilibration through washing could be used to circumvent the synthesis parameter differences.     

Dog-legging in the melt spinning process

Experiments were performed in a continuous capillary rheometry to investigate the dog-legging mechanism in a melt spinning process. The bending angle of the spinline was measured under various spinning conditions while spinning PP, PET, and PEN fibers. Experimental results indicate that obstacles inside the capillary of the spinnerette are the primary cause of bending of the spinline. The bending angle is greatly influenced by the obstacle's dimensions and position inside the capillary. An increase in the obstacle's radius or a decrease in the obstacle's length caused the bending angle to increase. The closer the obstacle was to the exit of the capillary implied the bending angle would be the wider except for the block immediately above the exit. Also at high shear rates, PEN was found to be relatively insensitive to spinline-bending than PET polymers. In accounting for the dog-legging behavior, our results suggest that the die swell model is more realistic than the conventional vortex model.