Volume 21, Issue 1

The clay-containing polymeric nanocomposites (CPNC) can be visualized as binary mixtures of strongly interacting, inorganic, plate-like molecules dispersed in a polymeric matrix. To be successful, one must ascertain the thermodynamics, which controls CPNC structure on the molecular level. In this work dispersion of organoclay (Cloisite 15A, C15A) in polyamide 6 (PA6) or in polypropylene (PP) is discussed. The PA-based CPNC's contained two components: polymer and organoclay, whereas those based on PP in addition contained a mixture of two maleated polypropylene's (PP-MA), as a compatibilizer. The melt compounding was carried out either in a single-screw extruder (SSE), or a twin-screw extruder (TSE). Both compounding lines were used with or without the extensional flow mixer (EFM). Furthermore, two versions of EFM were evaluated – one commercial, designed for polymer homogenization and blending, and the other designed for dispersing nano-particles. It was found that addition of EFM significantly improved clay dispersion. Better dispersion was found compounding the CPNC's in a SSE+EFM than in TSE with or without EFM. The best results were obtained using SSE with the new EFM having a relatively small gap between the convergent-divergent plates. C15A was fully exfoliated in PA6 matrix. The results in PP/PP-MA matrix were less spectacular, but again the highest degree of dispersion was obtained using SSE+new EFM with a small gap. Tensile, flexural and impact properties were measured and evaluated.

Our ultrasound in-line monitoring studies revealed that dispersion or dissipation melting mechanism played a dominant role during melting of unplasticized polyvinyl chloride (uPVC) in an intermeshing counter-rotating twin-screw extruder. It is found that calendering effect between two screws contributes to the melting of PVC particles, and results in dispersion or dissipation melting. Thus, in this paper, a dispersed melting model in calender gaps is considered to predict polymer melting length in terms of numbers of calender gaps. Effects of screw geometries (channel depth, calender gap clearance) and processing conditions including screw speed and leakage flow on melting length are discussed for particle-filled fluid models. The calculated results are compared with experimental observations by both “screw pulling out” method and ultrasound in-line monitoring method. Ultrasound in-line monitoring provides a solution to overcome the time delay occurred in screw-pulling out method.

Polymer degradation in the extrusion process decreases quality and productivity. For this reason, it is necessary to prevent polymer degradation. In the extrusion process, polymer degradation is caused by oxidation. It depends on the processing temperature and the amount of dissolved oxygen in the molten polymer. Therefore, a quantitative analysis of these factors is required. As for the degradation characteristics of the material used in this study, temperature and oxygen concentration dependency of the oxidation rate could be quantitatively characterized with an apparatus to evaluate polymer degradation which utilized chemiluminescence generated by an oxidation reaction. Moreover, an online measuring apparatus to analyze dissolved gas in the extruded molten polymer was developed. With this apparatus, the volume ratio of dissolved gases (N2, O2 etc.) to the extruded molten polymer could be quantitatively analyzed and the quality of the extruded molten polymer evaluated. With this apparatus, dissolved nitrogen (an index of entrained air) was analyzed with a full-flight screw and a barrier screw. Furthermore, observation of cross sectional views in the screw channel obtained from the cooling experiment under the operating conditions was carried out for the full-flight screw and the barrier screw. With the full-flight screw, break up phenomenon (collapse of solid polymer) occurred in the screw channel and the amount of nitrogen increased. With the barrier screw, the amount of nitrogen decreased because of prevention of the break up phenomenon in the screw channel. Consequently, it is shown that the use of the barrier screw is suitable for oxygen reduction in the molten polymer, which is a factor in causing polymer degradation.

Two typical flow instabilities of “Shark Skin” and “Melt Fracture” for high density polyethylene were investigated in this report. Experimental measurements were carried out methodically in a wide flow region from the reservoir (entry region) to the die land with a specific slit die. Especially the flow pattern was visualized with a high speed camera and the frequency of the flow instability was analyzed. Corresponding to the visualization analysis, the pressure fluctuation in the slit die and the surface roughness at the die exit were measured simultaneously and these frequency characteristics were analyzed with a FFT (Fast Fourier Transform) analyzer. As for “Shark Skin” region, the irregularity of the surface roughness in the broad frequency band was recognized at the die exit. In spite of this surface roughness, the pressure fluctuation and the fluctuation of the flow pattern at the wall vicinity in the die land were not observed. These results indicate that “Shark Skin” occurs at the die exit. As for “Melt Fracture” region, the periodical oscillation of the flow pattern and the periodical pressure fluctuation which corresponded to the periodical distortion of the extrudate was recognized in the die land. However the periodical oscillation of the flow pattern and the periodical pressure fluctuation could not be recognized in the reservoir (entry region of the die land). These results indicate that “Melt Fracture” is initiated at the die entry and occurs in the die land. Consequently it is clarified that “Shark Skin” occurs at the die exit and the flow instability which lead to “Melt Fracture” is initiated at the die entry and occurs in the die land.

A quenching technique is scrutinized, where the surface of a polymer melt is brought into contact with a metal wall of a well defined lower temperature at time zero. Usually, this wall temperature has approximately been kept constant with the aid of a streaming heat transfer fluid. It turns out, however, that an exactly invariable wall temperature or a finite Biot number (finite heat transfer coefficient to the cooling fluid) are inadequate assumptions. However, an exact interface temperature is to berealized, if the crystallization kinetics of the polymer is to be investigated painstakingly at this interface. The reason for this requirement lies in the fact that the said kinetics is extremely sensitive to small temperature changes. The present contribution shows that a water cooled metal wall of small thickness is inadequate for the purpose. A rather thick metal wall, which is preheated to the required temperature and thermally insulated at the outside, is clearly preferable. Copper is better than steel.

A novel approach for the numerical simulation of the development of crystallinity and birefringence on the spinline was proposed. The approach was based on the calculation of elastic recovery and crystalline and amorphous orientation functions without making any assumption about freezing. To model crystallization, the amorphous orientation function and the flow effect on the equilibrium melting temperature elevation due to the entropy reduction between the oriented and unoriented melts were incorporated. The crystalline orientation function was calculated from the frozen-in elastic recovery. The entropy change and frozen-in elastic recovery calculation were based on a nonlinear viscoelastic constitutive Eq. with the crystallinity and temperature dependent viscosity and relaxation time. The crystalline and amorphous contributions to the overall birefringence were obtained from the crystalline orientation function and the flow birefringence, respectively. The temperature, diameter, density, and birefringence profile in both low- and high-speed spun PET fibers were predicted and compared with the experimental data from literature. Theoretical predictions were found to be in agreement with the published experimental data.

High-speed melt spinning of biodegradable aliphatic polyester poly(butylene succinate) (PBS), and aliphatic-aromatic copolyester poly(butylene terephthalate-co-succinate-co-adipate) (PBTSA) was carried out. The effects of spinning conditions on the structural development of PBS and PBTSA fibers were investigated through the measurements on on-line velocity profile, birefringence, wide-angle X-ray diffraction (WAXD), differential scanning calorimetry, and tensile test. Both PBS and PBTSA fibers showed well-developed crystal structure in WAXD patterns, though the deformation of the spinline continued down to the position of winding suggesting that substantial orientation induced crystallization did not occur in spinline of both cases. Increasing take-up velocity or decreasing extrusion temperature led to the enhancement of molecular orientation, crystal structure and mechanical property of both PBS and PBTSA fibers, but no distinct effect on the thermal property of those fibers. The melting temperature of PBTSA fibers was at ca.115°C, same as that of PBS fibers. Elastomeric property of hard-soft units consisting PBTSA fibers leads to its good recoverability; however, this behavior lowers the initial modulus and tensile strength compared with those of the corresponding aliphatic PBS fibers.

In polyamide 12 (PA12) tube extrusion, calibration is crucial for the major final properties such as elongation at break or burst pressure. In external calibration, the extruded tube is pulled through a cylindrical calibrator located in a water tank under vacuum. A water flow rate is applied at the calibrator inner side, creating a lubricating water layer at the polymer outer surface. A combination of a quenching and a mechanical drawing was highlighted during calibration by on-line measurements. A subsequent high molecular orientation in the outer tube layers was also featured by X-Ray diffraction and birefringence evaluation in light microscopy. Besides, influence of lubrication level in the sizing-sleeve was investigated by an estimate of the lubricating water layer and a characterization of the tube final surface state in different calibration conditions. Firstly, we quantitatively showed that rising the water layer thickness leads to a diminution of the draw ratio in the calibration tank, DrCAL (ratio between line velocity and velocity at the calibrator entrance). Excellent correlation has been found between tube superficial orientation and DrCAL: a reduced elongation of the polymer in the calibrator leads to a lower level of superficial orientation. Secondly, two kinds of surface defects were detected on the tubes by light microscopy and microtopography. At last, several correlations were featured with mechanical properties. Elongation at break was found to strongly depend on the molecular orientation resulting from calibration. Moreover, origin of rupture was investigated during tensile testing in light microscopy. We emphasized that surface defects alignments perpendicular to the extrusion initiate the rupture by creating a significant crack via coalescence of initial defects. Thus, microstructure, surface state and tensile properties can be controlled by fitting the calibration parameters to improve lubrication and reduce the draw ratio in calibrator.

The bubble stability of tubular film extrusion in metallocene linear low density polyethylene(m-LLDPE) and metallocene linear low density polyethylene/linear low density polyethylene(m-LLDPE/LLDPE) blend was investigated. Unstable bubble characteristics in m-LLDPE were found at low draw down ratio (DR) with a symmetric periodic bubble instability at low blow up ratio (BUR) and a helical instability at high blow up ratio. However, m-LLDPE showed better operation windows in tubular film extrusion than that of LLDPE, especially in bubble instability at low BUR. Both bubble instability and helical instability in LLDPE tubular film extrusion were enhanced by the blending with m-LLDPE. m-LLPE possesses higher melt elasticity as compared to m-LLDPE and LLDPE due to the long chain branching introduced by 1-octene co-monomer during polymerization. Blending resulted in broadening of molecular weight distribution (MWD) and thus increase in activation energy.

In the market, a film adapted to environmental and barrier problems is desired. The previous report discussed easy tear and gas barrier PA6 film. This report discusses the multilayer PA6 film having high barrier EVOH layer. A biaxially oriented PA6 film produced by double bubble tubular film process has high strength, but does not have enough gas barrier property compared with K-coated PA6. In the following a multilayer film with gas barrier layer is studied. A high gas barrier layer of EVOH is added to the center layer in the outer layers of PA6 films. The gas barrier ability increases with increasing the content of EVOH and decreasing ethylene content of EVOH, but at the same time the mechanical properties and processability get worse. From this point of view, it is found that the optimum content of EVOH is between 20% and 43%. In a biaxially oriented multilayer film composed of PA6 layers and EVOH one keeps high strength and high gas barrier. The material and production technology of a PA6 film can solve environmental and barrier problems.