Michel A. Huneault

Enhanced dispersion of cellulose nanocrystals in melt-processed polylactide-based nanocomposites

Dispersion and distribution of cellulose nanocrystals (CNC) in a thermoplastic matrix is one of the most important issues in the development of CNC-based high performance composites. During melt processing, agglomeration of CNC is prone to occur due to poor polymer wetting on the hydrophilic CNC surface and to strong particle–particle interactions. Because of the high temperature and intensive mixing involved in melt-processing, degradation of the CNC is also possible. To avoid these problems, solvent mixing followed by solvent casting is the main processing route used in the majority of studies on polymer–CNC composites. In this work, we have explored a novel two-step process where solvent-mixing and melt-mixing were carried out sequentially to improve the overall dispersion of the CNC. The first step consisted in forming a CNC suspension into a polyethylene oxide (PEO) aqueous solution. In the second step, water was removed by freeze-drying to form a water-free well dispersed PEO/CNC mixture. The final step consisted in melt-mixing the PEO/CNC mixture into PLA for the preparation of the composites. PEO and PLA are known to be miscible in certain molecular weight and composition ranges, thus leading to a composite where the CNC particles are well dispersed into a homogeneous mixture of PLA and PEO. Two different PEO molecular weights were investigated in this study, and several formulations were compared under the same processing conditions. Direct blending of CNC and molten PLA was also carried out for comparison purposes. CNC particles tended to agglomerate during blending but the agglomerates were smaller and their number was considerably decreased when the PEO content increased in the formulation. At the highest PEO/CNC ratio, no agglomerates were observed. Thermomechanical and rheological properties of the PLA-based nanocomposites were also investigated.

Dispersion visualization of model fluids in a transparent Couette flow cell

Dispersion mechanisms in model fluid systems of different viscosity ratios were studied in a transparent Couette flow cell. The counter-rotating concentric cylinders were driven by two independent dc motors. Drops of the minor phase were then maintained at a constant position by fixing the inner and outer cylinders’ rotational speeds. The advantage of this new setup is that visualization can be made at high shear rates without any secondary flow effects, usually observed with cone-plate or parallel plates geometry. Constant viscosity viscoelastic drops (Boger fluid) and Newtonian drops [high viscosity polydimethylsiloxane, (PDMS)], deformed at low shear rates in a Newtonian matrix (low viscosity PDMS), oriented along the flow field and drop deformation increased with shear rate, as expected. However, when a critical shear rate (characteristic of the fluid system used) was reached, the deformed drops began to contract in the flow direction. When increasing the shear rate over this critical value, drop cont...

Biaxial Orientation of Polylactide/Thermoplastic Starch Blends

Abstract The biaxial stretchability and film properties of polylactide/thermoplastic starch blends were investigated. Polylactide (PLA) and thermoplastic starch (TPS) were blended in various proportions. Blends containing 27, 42 and 60 wt.% TPS were prepared via a twin-screw extrusion process. Interfacial modification was performed by grafting the PLA with maleic anhydride. These blends were subsequently cast into sheets and biaxially drawn using a laboratory biaxial stretcher. The morphology of extruded strands and cast sheets was investigated using scanning electron microscopy. The compatibilized blends exhibited a much finer morphology as well as preferential minor phase size orientation along the machine direction. The addition of starch did not affect significantly the biaxial stretchability of the pure PLA. Even at high starch content (60 wt.%), the biaxial draw ratio at which the sample breaks is very similar to that of the pure PLA. At higher temperatures, all blends could be stretched at significantly much higher biaxial draw ratio and very thin films could be obtained. The starch content and processing variables affected the tensile properties.

Foaming of Polystyrene/ Thermoplastic Starch Blends:

This study investigates the fabrication of extruded foams from polystyrene/thermoplastic starch (PS/TPS) blends. A specially designed twinscrew extrusion process is used for starch gelatinization, PS incorporation, polymer mixing, and blowing agent incorporation. In-line rheometry is used to monitor the viscosity of the TPS/PS blends and to evaluate the plasticizing effect of 1,1,1,2 tetrafluoroethane (HFC-134a) used as blowing agent. Differential scanning calorimetry, scanning electron microscopy, density measurement, and picnometry are used to evaluate the thermal properties, the blend morphology, and the foam cell structure. Glycerol content in the TPS phase and the TPS content in the overall blend have a strong effect on the blend viscosity and, in turn, on the ability to foam the material. The foams blown with the hydrofluorocarbone alone have large open-cell content and their density cannot be reduced below 170 kg/m 3. The addition of a small amount of ethanol however results in three-fold reductions in density and much better foam cell homogeneity.

Thermomechanical and crystallization behavior of polylactide-based flax fiber biocomposites

In this work, the rheological, thermal and mechanical properties of melt-compounded flax fiber-reinforced polylactide composites were investigated. The effect of compounding on fiber length and diameter, and the relationship between fiber content and the crystallization behavior of the biocomposites, at various temperatures, were also examined. After melt-compounding, fiber bundles initially present were, to a large extent, broken into individual fibers and the fiber length was decreased by 75 %, while the aspect ratio was decreased by nearly 50 %. The crystallization half-time was found to decrease with increasing flax fiber content, and showed a minimum value at 105 °C for all systems. The elastic modulus was increased by 50 % in the presence of 20 wt% flax fibers. The addition of maleic anhydride-grafted polylactide had a positive effect on the mechanical properties of the biocomposite. This system is particularly interesting in the context of sustainable development as it is entirely based on renewable resources and biodegradable.

Foam extrusion of polystyrene blown with HFC-134a

There is much interest in developing industrial processes to manufacture extruded polystyrene foam that do not involve ozone depleting blowing agents. A popular alternate candidate is HFC-134a. It has a zero ozone depletion factor and is nearer in chemical structure to standard blowing agents (CFC-22 and HCFC-142b) than carbon dioxide. Although exhibiting main good features, HFC-134a is not used widely as a blowing agent as low foam density is not readily achieved and extruder operation is difficult. A review of past and on-going works on the use of HFC-134 will be addressed first. Then attention will be paid mainly on some processing aspects, with emphasis on the plasticization behavior of polystyrene (PS) by HFC-134a and the effect of screw design on dynamic dissolution of HFC-134a in PS during foam extrusion. Solubility efficiency during extrusion processing has been assessed for different screw configurations by an in-line ultrasonic technique. These results have also been correlated to off-line solubility and diffusivity properties.

Toughening mechanisms in interfacially modified HDPE/thermoplastic starch blends.

The mechanical behavior of polymer blends containing 80 wt% of HDPE and 20 wt% of TPS and compatibilized with HDPE-g-MA grafted copolymer was investigated. Unmodified HDPE/TPS blends exhibit high fracture resistance, however, the interfacial modification of those blends by addition of HDPE-g-MA leads to a dramatic drop in fracture resistance. The compatibilization of HDPE/TPS blends increases the surface area of TPS particles by decreasing their size. It was postulated that the addition of HDPE-g-MA induces a reaction between maleic anhydride and hydroxyl groups of the glycerol leading to a decrease of the glycerol content in the TPS phase. This phenomenon increases the stiffness of the modified TPS particles and stiffer TPS particles leading to an important reduction in toughness and plastic deformation, as measured by the EWF method. It is shown that the main toughening mechanism in HDPE/TPS blends is shear-yielding. This article demonstrates that stiff, low diameter TPS particles reduce shear band formation and consequently decrease the resistance to crack propagation.

Structural changes of Salix miyabeana cellulose fibres during dilute-acid steam explosion: Impact of reaction temperature and retention time

Dilute-acid steam explosion of Salix miyabeana has been carried out to understand the effect of processing conditions, expressed through a severity factors (SFT), on the changes in cellulose fibre structures in a perspective of using these in polymer composites. This thermo-chemico-mechanical extraction leads to the isolation of cellulose fibres as observed by SEM images. Fibre length as well as length to diameter aspect ratios decreased with the severity of the treatment. Likewise, fibre whiteness diminished with an increasing severity factor, which could be a tangible effect of physical degradation. Variations in crystallinity seemed to be dependent upon the reaction temperature, generally decreasing with regards to retention time. Above a severity threshold, a structural disorganization was observed. Overall, dilute-acid steam explosion was shown to be a valuable cellulose extraction process that can provide a variety of fibre structures.

Evaluation of the FAN Technique for Profile Die Design

Abstract 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.

Effect of interfacial modifier on single drop deformation and breakup in step increasing shear flow

This work deals with in situ visualisation of deformation and breakup of a copolymer modified single Newtonian drop immersed in a Newtonian homogenous matrix. The experiments were carried out on a model system made of poly-isobutylene as the suspending fluid and two poly-dimethylsiloxanes with different molecular weights as the drop phase with viscosity ratios 0.036 and 1.13, below and above but close to unity. Three weight concentrations 0.5%, 2% and 10% of the block copolymer laying below, close to and above the critical concentration of the total drop surface coverage were examined. Single drop deformation experiments were carried out in a home-designed Couette quartz cell connected to a home-modified Paar Physica Rheometer. The variation in the length-to-diameter ratio (L/D) versus shear rate and capillary number was measured both in steady and in transient regimes till breakup. The results indicated a weaker resistance of copolymer modified drops against hydrodynamic stresses at both viscosity ratios as compared to the clean drop. However, the drop deformation was found to be complex and depends on the copolymer concentration and the viscosity ratio.