Surface Defects of TPO Injected Foam Parts for Automotive Applications

Benefits of reduced vehicle weight can have an important environmental impact since there is a 6 to 8% improvement in fuel efficiency for every 10% in weight reduction. In this work, foaming technology is used to produce injection molded TPO parts (thermoplastic olefin compounds) for the automotive industry with a minimum of 20% weight reduction while retaining a glossy surface quality. It was found by Guo et al. (2006, 2007) that the best strategy to raise the surface quality was to decrease the shot size, the cycling time and the temperature of the injected melt. However, the targeted objective of a minimum of 20% weight reduction was not achieved in these preliminary experiments. Therefore, in this work a blend of TPO and maleic anhydride modified polypropylene (PP-g-MA) was used in order to promote TPO foamability and improve the surface quality of injected parts. Under these conditions, we managed to produce injected molded TPO samples with a 24% weight reduction and good surface quality. Moreover, we investigated the effect of PP-g-MA in the TPO system by performing rheological measurements and photoacoustic Fourier transform infrared (PA-FTIR) analysis to characterize the PP-g-MA physico-chemical interactions with TPO.

The Effect of Nanosilicates on the Performance of Polyethylene Terephthalate Films Prepared by Twin-Screw Extrusion

Polyethylene terephthalate (PET) films were prepared by cast extrusion using a twin-screw extruder with a severe screw profile. The effect of an organically modified montmorillonite on thermal, mechanical, optical, and barrier properties of the PET films were investigated. Morphological characterization of the nanocomposite films was performed by employing wide angle X-ray diffraction (WAXD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM) followed by image analysis. The results unfold a mixed morphology for the nanocomposite films with more than 95% exfoliated and intercalated silicate layer structures, depending on the screw rotation speed. The remarkable dispersion of the organoclay particles at the nano-level is discussed in terms of solubility parameter and favorable interactions between PET macromolecules and organic modifier of the nanoclay. The crystal content of the nanocomposite films and their cold and hot crystallization temperatures confirmed the role of silicate nanolayers as a heterogeneous nucleating agent. While all nanocomposite films exhibit higher haze values in comparison to the neat PET samples, incorporation of 2 wt% nanoclay brought about 25% increase in tensile modulus and barrier properties. A range of screw rotation speeds with optimized properties in terms of haze, morphology, thermal, mechanical, and barrier properties is suggested.

Nanoclay Migration and the Rheological Response of PBAT/LDPE Blends

Blends of a poly(butylene adipate-co-terephthalate) (PBAT) and a low density polyethylene (LDPE) (80 wt%/20 wt%) were prepared through a twin screw extruder while incorporating 3 wt% Cloisite 30B (C30B) nanoclay that possessed a much higher affinity with PBAT. The blends were processed through three melt mixing strategies: ( i) direct mixing of all three components, (ii) mixing C30B and PBAT followed by mixing with LDPE, and (iii) mixing C30B and LDPE followed by mixing with PBAT. The rheological properties of each system were determined in small amplitude oscillatory shear (SAOS) experiments. The migration of C30B nanoparticles from the LDPE minor phase towards the PBAT matrix was then monitored in the blend nanocomposites prepared through strategy (iii) via SAOS time sweep experiments. It was firstly understood that the C30B migration could be detected during time sweep SAOS experiments. The migration time was observed to be frequency dependent due to the smaller length scales probed at larger frequencies. Such migration occurred even faster when the SAOS time sweep experiments were conducted at a higher temperature due to the viscosity reduction.

Carbon Nanotube Conductive Networks through the Double Percolation Concept in Polymer Systems

We investigated the electrical conductivity and percolation behavior of binary and ternary nanocomposites based on multiwalled carbon nanotubes (MWCNs) using polypropylene (PP) and a blend of PP with cyclic butylene terephthalate (CBT). The nanocomposites were prepared by diluting a commercial 20 %wtMWCNT PP masterbatch using optimized melt-mixing conditions. The concentration of carbon nanotubes in the diluted PP samples was as low as 0.5 % and as high as 15 % in weight. For the PP/CBT blend CBT concentration was varied up to 40 %wt while the loading of CNT was from 0 to 5 %wt. SEM and TEM techniques were used to examine the quality of the dispersion and the formation of nanotube networks within the polymer matrix. TEM and Raman spectroscopy results showed that for the diluted PP/MWCNT composites the nanotubes are well aligned in samples obtained the microinjection molding process, although the level of alignment is less with crystalline PP than in an amorphous matrix such as polycarbonate (PC). FTIR and XRD results revealed that the orientation of both polymer chains and crystals decreased with the incorporation of nanotubes into PP. The electrical conductivity was also significantly altered by the nanotube alignment in a PP matrix, as was previously observed for PC/MWCNT composites; the conductivity decreased and the percolation threshold rose in highly sheared samples; however, the presence of a crystalline phase improved the conductivity even for high shear conditions through the phenomenon of double percolation threshold. This last concept refers to the requirement that the filler-rich phase be continuous and conductive and not to the existence of two percolation thresholds at two different CNT concentrations. The electrical conductivity of PP/CBT blends was also improved through a double percolation that is the basic requirement for the conductivity of the ternary nanocomposites.

In Situ Polymerization of PET in the Presence of Pristine and Organo-modified Clays

In this work two types of montmorillonite, a pristine and an organo-modified clay (Cloisite 30B), were employed to produce (polyethylene terephthalate)-based nanocomposites via in situ polymerization. Using water as an intermediate medium, a stable dispersion of pristine clay in ethylene glycol was achieved. However, after polymerization, no significant gain was obtained in terms of delamination of silicate platelets. The polycondensation reaction when using Cloisite 30B was carried out at a temperature as low as 250°C in order to retain a considerable portion of the organo-modifier. The results showed that Cloisite 30B was successfully intercalated by the polymer chains, resulting in a d-spacing increase from 1.9 nm to about 3.6 nm. Although some mono and double silicate layers were observed in microscopy images, the major part of the organoclay remained in a tactoid intercalated form. Sampling during the polycondensation reaction revealed that, in this process, clay was first swelled efficiently by the monomer, and this structure was preserved in the early stage of polycondensation. However, the silicate platelets collapsed with time as the polycondensation progressed and larger molecular weight oligomers were formed. An investigation on the type of impeller used in the polymerization process showed that a slight improvement was achieved in terms of aggregate size and distribution when the anchor impeller was replaced by a helical one.

On-Line Birefringence Measurement in Film Blowing of a Linear Low Density Polyethylene

Blown film properties depend on the thermo-mechanical history experienced by molten polymer during biaxial deformation. In this study on-line birefringence measurements along the length of the bubble in film blowing of a linear low density polyethylene (LLDPE) were carried out in order to assess the stress level in the melt zone and total orientation in the solid zone. Bubble temperature measurements were carried out to find out the onset and the end of crystallization. Strain rates were also determined from bubble diameter and axial velocity measurements. We have focused on the effects of key processing parameters on the thermo-mechanical history of polymers. The relations between the birefringence and temperature profiles are described. The birefringence value is shown to be very small in the molten zone and increases rapidly as crystallization proceeds. The birefringence of the solidified film is strongly dominated by the crystalline phase contribution. Stresses in the molten blown film were calculated using the data of birefringence and pressure inside the bubble. The birefringence technique appears to be a promising but limited tool to determine stresses occurring in film blowing.

Investigation of Bubble Instabilities in Film Blowing Process

This paper describes an original on-line video device developed in order to study bubble instabilities occurring in the film blowing process, taking into account their three-dimensional behavior. For a linear low-density polyethylene, two forms of instabilities and combination have been observed: draw resonance and helical instability. These instabilities could be quantitatively described and differences in behavior could be assessed using real objective measurements and criteria. The influence of key processing conditions was investigated and the results showed that the instabilities are enhanced by increasing the draw ratio, blow up ratio and frost line height. These first results are in agreement with the majority of the results reported in the literature, but allow for a more accurate analysis of the phenomena.

Evaluation of the FAN Technique for Profile Die Design

Die design is a daily concern in the manufacturing of extruded profiles. There are very few available Computer Aided Design (CAD) tools for profile die designers and considerable time and resources must be devoted to trial and error design procedures. This paper examines the applicability of the flow analysis network (FAN) to the design of profile extrusion dies. The FAN method is found to provide useful design information at a fraction of the computing cost required for 2.5 and 3D finite element methods. Experimental flow distribution obtained by extruding PVC compounds and polyethylene through simple profile dies are compared to model predictions. The numerical and experimental results provide evidence about the significant contribution of transverse flow to the flow distribution at die exit.

Drawing Behavior of Solution Modified Polyethylene

The relationship between draw ability of high density polyethylene (HDPE) and entanglement density in the polymer has been studied by solution modifying the polymer. This involved precipitating the polyethylene from solutions in trichlorobenzene. Entanglement densities of solution modified and control (as received) HDPE were estimated from low-shear melt viscosity data. One and two stage drawing techniques at closely controlled temperatures were applied to form drawn film specimens. It was shown that solution modified versions of HDPE permitted drawing to much higher ratios than the control, and resulted in films with elastic moduli in the 40 GPa range. A relationship is thereby suggested linking polymer entanglement density with mechanical properties attained in the drawing process. Optimization of drawing performance appears to be attainable by controlling entanglement density, through appropriate choices of solvent and of initial polymer concentration in the solutions.

A Model for Single-screw Plasticating Extruders

A mathematical model has been developed to describe the steady-state behavior of single-screw plasticating extruders. The model is capable of predicting the solid bed profile, pressure, and polymer temperature profiles along the screw channel. The model is based on concepts developed by Tadmor and Klein [1], but a modified model for plastication rate is proposed and the pressure profile is calculated using a unidirectional power-law flow model, in which correction factors for flight walls, curvature, and varying channel height have been incorporated. A finite difference technique (Crank-Nichols on method) is used to compute the polymer temperature profile. The overall extruder performance (pressure, temperature and solid bed profiles) is obtained with short computer times on a personal computer (typically one minute on IBM AT). A series of menus and graphics allow for selective simulation and rapid visualization of effects of key parameters, making the model attractive for screw design and optimization of operating conditions. The predictions are in good agreement with experimental data obtained for a polyethylene on a 45 mm diameter screw extruder.

Effects of a Multifunctional Polymeric Chain Extender on the Properties of Polylactide and Polylactide/Clay Nanocomposites

A multifunctional polymeric chain extender (Joncryl) was used in the melt processing of a neat polylactide and polylactide/clay nanocomposites. The effects of Joncryl on morphology, rheology, thermal and mechanical properties, barrier properties and biodegradability were investigated. Three Joncryl loadings (0.1, 0.3 and 0.5 wt%) were used in this study, and the 0.5 wt% loading induced a long-chain branched structure in the PLA matrix, as indicated by the melt rheology results. It is believed that the property variations are all related to the long-chain branched structure as well as on the molecular weight recovery. The use of Joncryl did not change the intercalated and partially exfoliated clay structures in the PLA/clay nanocomposites, as observed by X-ray diffraction (XRD) and transmission electron microscopy (TEM). The tensile modulus and maximum strength slightly increased with Joncryl loading. The oxygen barrier properties were also improved by adding Joncryl. However, the addition of Joncryl prevented the formation of large spherulitic crystals and decreased the creep resistance at low stress level. Joncryl could not only control the thermal degradation of PLA during processing, but also affected its biodegradation in compost: higher Joncryl loading led to slower biodegradation and less molecular weight reduction with time.

Effect of Processing Conditions on Properties of PET/Clay Nanocomposite Films

Polyethylene terephthalate (PET) nanocomposite films (with 3 wt.% Cloisite 30B) were prepared by cast extrusion followed by uniaxial stretching, using chill rolls. Two screw profiles with different mixing elements under different screw speeds (N) and feeding rates (Q) were used to prepare PET/clay nanocomposite (PCN) films. Transmission electron microscopy (TEM) and wide angle X-ray diffraction (WAXD) showed that the clay layers were aligned in the machine direction (MD). XRD patterns depicted that the interlayer distance of clay platelets in the state of intercalation is somehow independent of the processing conditions, but the macro-scale characterization, including barrier and mechanical properties, showed that the level of clay layer delamination was affected by processing conditions. The results reveal that the applied strain has stronger effect than residence time on the barrier and mechanical properties. At the highest screw speed (N = 250 min−1), 27% reduction in oxygen permeability and 30% improvement in tensile modulus were obtained for the more severe screw profile.

Experimental Study and Modeling of Flow Behavior and Orientation Kinetics of Layered Silicate/Polypropylene Nanocomposites in Start-up of Shear Flows

Effects of organoclay contents on the startup flow properties of layered nano-scale particles in the simple shear mode are investigated. The addition of small amounts of nanoclays to polypropylene melts was found to dramatically change the flow characteristics and creates stress overshoots at large shear rates. A rheological model, initially developed for studying the motion of a group of symmetric ellipsoid particles in viscoelastic fluids was used to describe the orientation state of the uniformly dispersed suspensions of layered silicate in polypropylene melts. The effects of shear, particle loadings, particle interactions, flow reversal and rest time after cession of shear are studied and discussed according to our experimental observations and model predictions. It is shown that another diffusion term in the governing equation for the particles can be used to predict the properties by applying the rest time which was found to change the orientation of particles and shifts it to more isotropic microstructures. The experimental results of the startup viscosity are reasonably well predicted by the model at the three shear rates tested.

Biodegradability and mechanical properties of poly-(beta-hydroxybutyrate-co-beta-hydroxyvalerate)-starch blends

Wheat starch granules and poly-(beta-hydroxybutyrate-co-beta-hydroxyvalerate) [P(HB-co-HV), (19.1 mol% HV)] were blended at 160 degrees C. Increasing the starch content from 0 to 50% (wt/wt) decreased the tensile strength of P(HB-co-HV) from 18 MPa to 8 MPa and diminished flexibility as Young's modulus increased from 1,525 MPa to 2,498 MPa, but overall mechanical properties of the polymer remained in a useful range. A mixed microbial culture required more than 20 days to degrade 150-microns-thick samples of 100% P(HB-co-HV), whereas samples containing 50% (wt/wt) starch disappeared in fewer than 8 days. Starch granules degraded before P(HB-co-HV) did. Aerobic degradation proceeded more rapidly than anaerobic degradation.