Basil D. Favis

High performance LDPE/thermoplastic starch blends: a sustainable alternative to pure polyethylene

Abstract Thermoplastic starch (TPS), as opposed to dry starch, is capable of flow and hence when mixed with other synthetic polymers can behave in a manner similar to conventional polymer–polymer blends. This paper presents an approach to preparing polyethylene/thermoplastic starch blends with unique properties. A one-step combined twin-screw/single screw extrusion setup is used to carry out the melt–melt mixing of the components. Glycerol is used as the starch plasticizer and its content in the TPS is varied from 29 to 40%. Under the particular one-step processing conditions used it is possible to develop continuous TPS (highly interconnected) and co-continuous polymer/TPS blend extruded ribbon which possess a high elongation at break, modulus and strength in the machine direction. The PE/TPS (55:45) blend prepared with TPS containing 36% glycerol maintains 94% of the elongation at break and 76% of the modulus of polyethylene. At a composition level of 71:29 PE/TPS for the same glycerol content, the blend retains 96% of the elongation at break and 100% of the modulus of polyethylene. These excellent properties are achieved in the absence of any interfacial modifier and despite the high levels of immiscibility in the polar–nonpolar TPS–PE system. The 55:45 blend possesses a 100% continuous or fully interconnected TPS morphology, as measured by hydrolytic extraction. This highly continuous TPS configuration within the blend should enhance its potential for environmental biodegradation. The elongation at break in the cross direction of these materials, although lower than the machine direction properties, also demonstrates ductility at high TPS concentrations. At a glycerol content of 36% in the TPS, the blends demonstrate only very low levels of sensitivity to moisture. A high degree of transparency is maintained over the entire concentration range due to the similar refractive indices of PE and TPS and the virtual absence of interfacial microvoiding. Effective control of the glycerol content, TPS concentration and processing conditions can result in a wide variety of morphological structures including spherical, fiber-like, highly continuous and co-continuous morphologies. These various blend morphologies are shown to be the determining parameters with respect to the observed mechanical properties. This material has the added benefit of containing large quantities of a renewable resource and hence represents a more sustainable alternative to pure synthetic polymers.

Polymer blends containing poly(3-hydroxyalkanoate)s

Abstract The remarkable properties of poly(3-hydroxyalkanoate)s (PHAs) have resulted in a growing interest in these polymers. They offer a wide variety of useful mechanical properties and show excellent biodegradability. However, they are still expensive and poly(3-hydroxybutyrate) (PHB) in particular is quite brittle. Polymer blending offers interesting possibilities to prepare less expensive biodegradable materials with useful mechanical properties. In this review the literature concerning PHA-containing blends has been summarized. Blends incorporating either PHB or copolymers of 3-hydroxybutyrate and 3-hydroxyvalerate (P(HB-HV)) were discussed. The thermal and crystallization behaviour of the blends, their mechanical properties, morphology and biodegradability have been reviewed. Among the conclusions drawn from the literature, it is evident that PHAs can form miscible blends with polymers containing the appropriate functional groups via hydrogen bonding and/or donor-acceptor interactions. The crystallization behaviour of the PHAs is influenced by both the miscible and immiscible components. Immiscible PHA-containing blends show improved apparent biodegradability when compared to miscible blends. Their apparent biodegradability (weight loss) is mostly controlled by the blend morphology. Blends of PHA with other biodegradable polymers also usually show improved biodegradability when compared with pure PHAs.

Processing and characterization of thermoplastic starch/polyethylene blends

Abstract The behaviour of gelatinized starch plasticized with glycerol (also known as thermoplastic starch (TS)) is studied as the dispersed component in a polyethylene (LDPE or LLDPE) matrix. A processing technique was developed to compound the blends in one continuous process in a co-rotating twin-screw extruder fed by a single-screw extruder. The use of the single-screw as a side feeder allowed for gelatinization of the starch before feeding it into the twin-screw at controlled temperature and pressure. The screw configuration of the twin-screw extruder maintained high pressure (⩾0.9 MPa) during blending to prevent early evaporation of water. These materials displayed morphological characteristics typical of immiscible polymer-polymer blends. The number-average diameter of the dispersed phase increased from 4 μm with 8 wt% TS to 18 μm with 36 wt% TS in LDPE blends. It ranged from 3 to 8 μm in LLDPE blends containing 7 to 39 wt% TS. These results therefore indicate the possibility of achieving a level of morphological control with respect to the size and shape of the dispersed phase in these systems. Dry granular starch, on the other hand, typically is dispersed as a spherical like particle with a fixed morphology of approximately 10 μm. The blends in this study, at high TS loadings, demonstrate high elongational properties at break even without addition of an interfacial modifier. The LDPE blend containing 22% TS had 240% elongation at break and its modulus was 109 MPa. The LLDPE blend containing 39% TS had more than 540% elongation at break, while the modulus was 136 MPa.

Cocontinuity and phase inversion in HDPE/PS blends : Influence of interfacial modification and elasticity

In this work the level of continuity and cocontinuity for blends of HDPE/PS prepared on a twin-screw extruder have been studied by both morphology and dissolution studies. Addition of SEBS as an interfacial modifier results in a shift of the percolation threshold for dispersed PS to higher concentrations. The region of phase inversion, however, is maintained at 70% PS. The shift in the percolation threshold to higher values is related to reduced elongation of the PS dispersed phase after interfacial compatibilization. These results indicate that an interfacial modifier significantly influences percolation phenomena without shifting the region of phase inversion. Models based on viscosity ratio have failed to predict the region of phase inversion in this study. Elastic effects are shown to be able to describe the basic tendencies.

The relative role of coalescence and interfacial tension in controlling dispersed phase size reduction during the compatibilization of polyethylene terephthalate/polypropylene blends

The breaking thread and the sessile drop methods have been used to evaluate the interfacial tension between a polypropylene ( PP ) and a polyethylene-terephthalate (PET). An excellent correlation was found between the two. The breaking thread technique was then used to evaluate the interfacial tension of these blends at various levels of a styrene-ethylene butylene-styrene grafted with maleic anhydride (SEBS-g-MA) compatibilizer. In order to evaluate the relative roles of coalescence and interfacial tension in controlling dispersed phase size reduction during compatibilization, the morphology of PP/PET 1/99 and 10/90 blends compatibilized by a SEBS-g-MA were studied and compared. The samples were prepared in a Brabender mixer. For the 10/90 blend, the addition of the compatibilizer leads to a typical emulsification curve, and a decrease in dispersed phase size of 3.4 times is observed. For the 1/99 blend, a 1.7 times reduction in particle size is observed. In the latter case, this decrease can only be attributed to the decrease of the interfacial tension. It is evident from these results that the drop in particle size for the 10/90 PP/PET blend after compatibilization is almost equally due to diminished coalescence and interfacial tension reduction. These results were corroborated with the interfacial tension data in the presence of the copolymer. A direct relationship between the drop in dispersed phase size for the 1/99 PP/PET blend and the interfacial tension reduction was found for this predominantly shear mixing device.

Morphological stability, interfacial tension, and dual-phase continuity in polystyrene-polyethylene blends

The morphological stability of polystyrene high-density polyethylene (PS/PE) blend is investigated in the region of dual-phase continuity. The effect of the addition of a triblock SEBS copolymer to the blends on the stability of these morphologies, is examined. The results show that the morphology of the unmodified blends changes from co-continuous to droplet matrix for PS-rich blends whereas the morphology of a 50/50 blend maintains continuity but coarsened significantly upon annealing at 200°C. In the presence of the copolymer, these morphologies are much more stable. Selective solvent extraction of polystyrene in di-ethyl ether reveals that the level of PS continuity in the 50/50 blend is higher for the unmodified system than for the modified one. Upon annealing, the level of PS continuity significantly increases for the unmodified 50/50 PS/PE blend. The effect of the copolymer content in the blend on the interfacial tension between the two components is also investigated using the breaking thread method. The interfacial tension is found to be reduced from 5.6 to 1.1 mN/m by the addition of 20 parts of the copolymer to the blend.

Factors influencing encapsulation behavior in composite droplet-type polymer blends

In this study the influence of the molecular weight of the dispersed phase components on encapsulation effects in the composite droplet phase was examined for high density polyethylene (HDPE)/PS/PMMA ternary blends. Three different blends composed of various PS and PMMA materials dispersed in an HDPE matrix were prepared using an internal mixer. The morphology was studied by light and electron microscopy. Current models used for predicting encapsulation effects and composite droplet formation in ternary systems (based on static interfacial tension) predict in all cases that PS will encapsulate the PMMA. However, in one case, an unexpected encapsulation of PS by PMMA was observed. It was found that arguments based on the effect of viscosity ratio or the absolute viscosity of the different dispersed phases do not explain that discrepancy. In addition, the reversal of that latter composite droplet morphology from PMMA encapsulating PS to PS encapsulating PMMA was observed upon annealing treatment. Considering all the above, a conceptual model was developed to predict encapsulation effects in composite droplet type systems based on the use of a dynamic interfacial tension (i.e. taking into account the elasticity of the polymer components). Calculations based on the dynamic interfacial tension model, using elasticities based on constant shear stress, were able to account for all of the observed encapsulation effects in this study.

Diblock Copolymers as Emulsifying Agents in Polymer Blends: Influence of Molecular Weight, Architecture, and Chemical Composition

Interfacial agents used in the compatibilization of immiscible polymer blends often consist of block copolymers containing at least one segment compatible with each of the two phases of the blend. This work examines the influence of the molecular weight, architecture, and chemical composition of the interfacial agent on its ability to emulsify a polymer blend. The system chosen is a blend containing 80% polystyrene and 20% ethylene-propylene rubber, compatibilized by diblock copolymers of poly(styrene-hydrogenated butadiene). The emulsification curve, which relates the dispersed phase particle size to the concentration of interfacial agent added to the system, was used as a tool to characterize the efficacy of the different interfacial agents. The observed behavior is similar to that of classical emulsions: a rapid drop in phase size at low concentrations of interfacial modifier, followed by a levelling off to an equilibrium diameter value once a “critical” concentration has been reached. For systems compatibilized by symmetrical diblocks (i.e., containing approximately 50% styrene by weight), the volume average particle diameter decreased from 2.7 μm for the unmodified system to about 0.4 μm once interfacial saturation is reached. The critical concentration for emulsification decreased with increasing interfacial agent molecular weight, due to the higher interfacial area occupied by longer molecules; however, this parameter did not affect the equilibrium particle diameter. The asymmetrical diblock copolymer (30% styrene) was found to be less effective than the symmetrical ones over the entire range of concentrations studied (5 to 35% modifier, based on the volume of the minor phase). Asymmetrical diblock copolymers would tend to form micelles, whereas symmetrical copolymers are less constrained at the interface. No significant difference was observed between the emulsifying capability of tapered and pure diblocks of similar composition and molecular weight.

Properties of PETG/EVA blends : 1. Viscoelastic, morphological and interfacial properties

Abstract The linear viscoelastic properties of molten polyethylene terephthalate glycol/polyester-ethylene vinylacetate (PETG/EVA) immiscible blends were measured as a function of frequency for different compositions and temperatures. The morphology of the blends was determined by scanning electron microscopy and the rheological behaviour of the blends is described by the Palierne emulsion model. The behaviour is typical of homogeneous molten polymers except for the appearance of a shoulder in the storage modulus data at low frequencies. This corresponds to an increase in elasticity in the terminal zone and longer relaxation time compared to the matrix. The model describes correctly the experimental data, and the model predictions are used to determine the interfacial tension between PETG and EVA. The interfacial tension is shown to vary between 4.5 MnM−1 and 7 mNm−1, depending on composition and temperature. These values are in the range of interfacial tension values determined for other similar polymer systems. No comparison with literature data was possible and an attempt to use the breaking thread method to verify the interfacial tension value for this system was not successful.

The role of interfacial contact in immiscible binary polymer blends and its influence on mechanical properties

In this paper an immiscible blend comprised of a crystalline(polyethylene) and an amorphous(polycarbonate) component was studied. Depending on whether the crystalline material is the dispersed phase or matrix, high levels of voiding or good apparent contact, respectively, can be observed. It is shown that the system PC dispersed in HDPE displays a tensile modulus which mimics theoretical behaviour for perfect adhesion even in the absence of an interfacial modifier. The complementary blend of HDPE in PC with the voided interface displays all the characteristics of PC containing dispersed air. The transition from one behaviour to the other is closely related to the estimated region of phase inversion. The same pseudo-adhesion behaviour was not observed in a second amorphous/crystalline system (polystyrene and polyethylene). It is suggested that in order for this behaviour to occur a contraction of crystalline matrix must take place onto a rigid dispersed phase (below its glass transition).