Zdeněk Kruliš

Degradation and aging of polymer blends I. Thermomechanical and thermal degradation

The commercial importance of polymer blends implies interest in knowledge of their degradation behaviour under environmental stresses. Processes accompanying thermomechanical and thermal degradation of blends of commodity polymers are reviewed. Thermomechanical degradation, characteristic of melt processing, includes reactions between component macromolecules and relevant macroradicals. Cross-reactions giving grafted copolymers occur in some blends. Thermal degradation in an inert environment includes cross-reactions between macromolecules and low-molecular weight molecules or free radicals migrating over phase boundaries. As a consequence, thermal stability of the component polymers is either reduced or increased, according to the reactivity of involved species.

Compatibilization as a Procedure for Recycling of Commingled Polyolefin Waste

An efficient reactive compatibilization system based on the combination of liquid polybutadienes (both without and with functional groups) and dialkyl peroxide has been developed for polyolefin blends. Practical applications of the reactive blending procedure for recycling of commingled polyolefin waste were proposed. Tensile impact strength was used as the main criterion of compatibilization efficiency. Liquid polybutadiene without functional groups is an effective compatibilizer for the blends composed of virgin polyolefin components, while the properties of blends prepared from a control damaged polyolefin or from real polyolefin waste are more affected by maleinized liquid polybutadiene. Incorporation of a basic inorganic admixture and a small amount of elemental sulfur can increase the compatibilization efficiency. Uncompatibilized and reactive compatibilized LDPE/HDPE/PP model blends and blends of real polyolefin waste were studied by capillary rheometry. The flow behavior of reactive-compatibilized systems is close to that of uncompatibilized ones; these systems can be processed using current procedures of plastic industry.

Reactive compatibilization of polyolefins using low molecular weight polybutadiene

Compatibilization of polyolefins using a combined system consisting of a low molecular weight liquid polybutadiene (l-PB) and a free radical initiator has been investigated. Binary blends LDPE/HDPE and LDPE/PP as well as ternary blends LDPE/HDPE/PP were prepared by reactive blending in the chamber of a Brabender Plasticorder. Transmission electron microscopy has been used for determination of phase structure of the final blends. Tensile impact strength of these materials, as the main criterion of compatibilization efficiency, was determined by a standardized method. A significant improvement in tensile impact strength of the compatibilized blends and considerable changes in their morphology have been found. The increase in tensile impact strength was apparently attained due to formation of effective linkages between main polymer components with the help of polybutadiene chains, providing good adhesion at the interface. Because the permanent thermoplasticity of these materials has been proved by repeated processing, any stable network based on covalent bonds in these blends cannot be considered. The used procedure shows to be a promising way for the recycling of commingled polyolefin waste as well as for the preparation of special polyolefin blends from virgin polymers. Die Kompatibilisierung von Polyolefinen mit einem kombinierten System bestehend aus einem niedrigmolekularen, flussigen Polybutadien und einem Radikalinitiator wurde untersucht. Durch reaktives Blenden in der Knetkammer eines Brabender-Plasticorders wurden sowohl binare Blends LDPE/HDPE und LDPE/PP als auch ternare Blends LDPE/HDPE/PP hergestellt. Die Phasenstrukturen der Blends wurden mittels Transmissionselektronenmikroskopie bestimmt. Die Zugschlagzahigkeit als Hauptkriterium fur der Kompatibilisierung dieser Materialien wurde nach einer normierten Methode gemessen. Eine signifikante Verbesserung der Zugschlagzahigkeit und erhebliche morphologische Anderungen der kompatibilisierten Blends wurden beobachtet. Der Anstieg der Zugschlagzahigkeit wird offensichtlich durch die Bildung wirksamer Bindungen zwischen den polymeren Hauptkomponenten unter Mitwirkung von Polybutadienketten erreicht, was eine gute Oberflachenadhasion zur Folge hat. Weil die Blends auch nach mehrmaligem Verarbeiten noch thermoplastisch waren, kann die Bildung eines stabilen Netzwerks mit kovalenten Bindungen ausgeschlossen werden. Das hier vorgestellte Verfahren ist ein vielversprechender Weg sowohl zur Aufarbeitung von Polyolefin-Mischabfallen als auch zur Herstellung spezieller Polyolefin-Blends aus Neumaterial.

Strong synergistic effects in PLA/PCL blends: Impact of PLA matrix viscosity

Blends of two biodegradable polymers, poly(lactic acid) (PLA) and poly(ϵ-caprolactone) (PCL), with strong synergistic improvement in mechanical performance were prepared by melt-mixing using the optimized composition (80/20) and the optimized preparation procedure (a melt-mixing followed by a compression molding) according to our previous study. Three different PLA polymers were employed, whose viscosity decreased in the following order: PLC ≈ PLA1 > PLA2 > PLA3. The blends with the highest viscosity matrix (PLA1/PCL) exhibited the smallest PCL particles (d∼0.6μm), an elastic-plastic stable fracture (as determined from instrumented impact testing) and the strongest synergistic improvement in toughness (>16× with respect to pure PLA, exceeding even the toughness of pure PCL). According to the available literature, this was the highest toughness improvement in non-compatiblized PLA/PCL blends ever achieved. The decrease in the matrix viscosity resulted in an increase in the average PCL particle size and a dramatic decrease in the overall toughness: the completely stable fracture (for PLA1/PCL) changed to the stable fracture followed by unstable crack propagation (for PLA2/PCL) and finally to the completely brittle fracture (for PLA3/PCL). The stiffness of all blends remained at well acceptable level, slightly above the theoretical predictions based on the equivalent box model. Despite several previous studies, the results confirmed that PLA and PCL could behave as compatible polymers, but the final PLA/PCL toughness is extremely sensitive to the PCL particle size distribution, which is influenced by both processing conditions and PLA viscosity. PLA/PCL blends with high stiffness (due to PLA) and toughness (due to PCL) are very promising materials for medical applications, namely for the bone tissue engineering.

Effect of dynamic crosslinking on rheological properties of molten polypropylene/ethylene-propylene elastomer blends

Abstract Dynamic mechanical properties of uncrosslinked and dynamically crosslinked polypropylene/ethylene-propylene elastomer blends in the melt were compared. It was found that the dependence of dynamic viscosity and storage modulus on frequency reflects the type of phase structure of uncrosslinked blends. Dynamically crosslinked blends with higher contents of elastomer show rheological properties which are typical of systems where the physical network of dispersed particles exists. Rheological properties of dynamically crosslinked blends at low frequencies and low shear rates were explained as a consequence of long-lived entanglements among touching crosslinked elastomer particles.

Compatibilization of polyethylene/poly(propylene)/polystyrene blends

The compatibilization of mixtures of polyolefins or of polyolefins with polystyrene using either liquid polybutadiene (l-PB)/organic peroxide or styrene-butadiene-styrene (SBS) block copolymers was investigated. Tensile impact strength was chosen as a measure of compatibility. Binary blends LDPE/high-impact polystyrene (HIPS) and LDPE/poly(propylene) (PP) as well as LDPE/HDPE/PP/HIPS blends were prepared by blending in the chamber of a Brabender Plasticorder. Composition of the blends corresponds to real commingled plastic waste. It was found that l-PB-based compatibilizer enhanced the impact strength of LDPE/HIPS blends with LDPE contents higher than 60 wt.-% only. Also SBS copolymer enhanced the impact strength of LDPE/PP blends with LDPE contents higher than 40 wt.-%. Both the compatibilizers substantially increased the toughness of LDPE/HDPE/PP/HIPS blends with composition similar to the municipal plastic waste. Die Kompatibilisierung von Mischungen von Polyolefinen oder Polyolefinen mit Polystyrol unter Verwendung von entweder flussigem Polybutadien (l-PB) und organischem Peroxid oder Styrol-Butadien-Styrol-Blockcopolymeren (SBS) wurde unterucht. Die Schlagzugzahigkeit wurde als Mas fur die Vertraglichkeit herangezogen. In einem Brabender-Plasticorder wurden binare Blends aus LDPE und schlagzahem Polystyrol (HIPS) sowie LDPE und Polypropylen und LDPE/HDPE/PP/HIPS-Blends hergestellt; die Zusammensetzung der Blends entsprach derjenigen von Kunststoff aus Hausmull. Vertraglichkeitsvermittler auf der Basis von l-PB steigerten die Schlagzahigkeit der LDPE/HIPS-Blends mit LDPE-Anteilen uber 60 Gew.-%. SBS erhohte die Schlagzahigkeit der LDPE/PP-Blends mit LDPE-Anteilen uber 40 Gew.-%. Beide Vertraglichkeitsvermittler verbesserten wesentlich die Zahigkeit der LDPE/HDPE/PP/HIPS-Blends mit einer der von Hausmull entsprechenden Zusammensetzung.

The role of lubricants in Reactive compatibilization of polyolefin blends

Reactive compatibilization using liquid polybutadienes and dialkyl peroxides was studied in model low-density polyethylene/polypropylene (4/1) blends and the commingled waste of composition similar to these blends. The influence of three types of lubricants (Ca stearate, stearic acid - Loxiol G20 and paraffin - Loxiol G22) on the structure and toughness of these blends was determined. In spite of the fact that in the waste material, a coarse morphology and poor toughness were found in comparison with the blend of virgin polyolefins, reactive compatibilization has approximately the same effect in both types of the blends as far as the structure parameters and mechanical behaviour are concerned. This effect is enhanced by addition of lubricants, the most efficient being the paraffin in the model blends, probably due to its partial miscibility with LDPE. In the commingled waste, liquid polybutadienes supported on precipitated SiO 2 appear to be quite efficient. No influence of the reactive compatibilization on both the crystal modification and the crystalline content was observed in both types of these blends.