Polyethylene/starch blends with enhanced oxygen barrier and mechanical properties: Effect of granule morphology damage by solid-state shear pulverization

Effects of Polymer Properties on Solid-State Shear Pulverization: Thermoplastic Processability and Nanofiller Dispersibility

Solid-state shear pulverization (SSSP) is an alternative polymer processing technique based on twin-screw extrusion with a continuous cooling system. In SSSP, low-temperature mechanochemistry modifies the macromolecular architecture and morphology, which in turn leads to physical property changes in the material. While a wide range of homopolymers, polymer blends, and polymer (nano)composites have been previously developed with SSSP, a fundamental understanding of how mechanochemistry affects polymer chain architecture and structure, and in turn, material properties, has not been elucidated. This paper conducts a systematic processing-structure-property relationship investigation of 10 thermoplastic polymers with varying properties, as they are subjected to consistent SSSP mechanochemical pulverization and nanocomposite compounding. Structural, mechanical, and thermal characteristics of the neat polymers are correlated to their response to SSSP by way of process covariants. Further, we investigate how SSSP processing parameters cause structural changes such as molecular weight reduction and filler dispersion level, which in turn dictate system properties like melt viscosity and thermal stability. Mechanochemical engagement with a high degree of physical contact during pulverization and compounding, characterized by the SSSP covariants exhibiting specific mechanical energy values above 4 kJ/g and an average screw temperature above 20 °C, is ensured when polymers have a glass transition temperature below the processing temperature (<50 °C) and high toughness (>40 MPa). Crystallinity and low thermal diffusivity (<0.2 mm²/s) are additional factors for engaged SSSP processing. Chain scission is an unavoidable outcome of SSSP, though the associated molecular weight reduction was <10% for 7 out of 10 polymers. The elucidated processing-structure-property relationships would allow the SSSP process for a given polymer system to be tailored to the specific needs for molecular structure alterations and performance improvements.

Polylactic Acid Chemical Foaming Assisted by Solid-State Processing: Solid-State Shear Pulverization and Cryogenic Milling

A chemical foaming process of polylactic acid (PLA) was developed via the solid-state processing methods of solid-state shear pulverization (SSSP) and cryogenic milling. Based on the ability of solid-state processing to enhance the crystallization kinetics of PLA, chemical foaming agents (CFA) are first compounded before foaming via compression molding. Specifically, the effects of the pre-foaming solid-state processing method and CFA concentration were investigated. Density reduction, mechanical properties, thermal behavior, and cell density of PLA foams are characterized. Solid-state processing of PLA before foaming greatly increases the extent of PLA foaming by achieving void fractions approximately twice that of the control foams. PLA's improved ability to crystallize is displayed through both dynamic mechanical analysis and differential scanning calorimetry. The solid-state-processed foams display superior mechanical robustness and undergo low stress relaxation. The cell density of the PLA foams also increases with solid-state processing, especially through SSSP. Additionally, crosslinking of PLA during the pre-foaming processing step is found to result in the greatest enhancement of crystallization but decreased void fraction and foam effectiveness. Overall, SSSP and cryogenic milling show significant promise in improving chemical foaming in alternative biopolymers.

The Effect of Poly (Ethylene glycol) Emulation on the Degradation of PLA/Starch Composites

As a hydrophilic renewable polymer, starch has been widely used in biocompatible plastics as a filler for more than two decades. The present study aimed at investigating the effects of polyethylene glycol (PEG), as a plasticizer, on the physicochemical properties of a hybrid composite-polylactic acid (PLA) and thermoplastic starch (TPS). A solvent evaporation process was adopted to gelatinize the starch and disparate PEG contents ranging from 3 to 15 wt.% (with respect to the sample weight) were examined. It was revealed that the increase in the PEG content was accompanied by an increment in the starch gelatinization degree. Referring to the microstructural analyses, the TPS/PLA mixture yielded a ductile hybrid composite with a fine morphology and a uniform phase. Nevertheless, two different solvents, including acetone and ethanol, were used to assess if they had any effect on the hybrid's morphology, tensile strength and thermal properties. It was found that ethanol culminated in a porous hybrid composite with a finer morphology and better starch distribution in the PLA structure than acetone. As the result of PEG addition to the composite, the crystallinity and tensile strength were decreased, whereas the elongation increased. The hydrolytic degradation of samples was assessed under different pH and thermal conditions. Moreover, the microbial degradation of the PLA/TPS hybrid composite containing different PEG molar fractions was investigated in the soil for 45 days. The rate of degradation in both hydrolytic and biodegradation increased in the samples with a higher amount of PEG with ethanol solvent.

Review of the Most Important Methods of Improving the Processing Properties of Starch toward Non-Food Applications

Starch is the second most abundantly available natural polymer in the world, after cellulose. If we add its biodegradability and non-toxicity to the natural environment, it becomes a raw material very attractive for the food and non-food industries. However, in the latter case, mainly due to the high hydrophilicity of starch, it is necessary to carry out many more or less complex operations and processes. One of the fastest growing industries in the last decade is the processing of biodegradable materials for packaging purposes. This is mainly due to awareness of producers and consumers about the dangers of unlimited production and the use of non-degradable petroleum polymers. Therefore, in the present review, an attempt was made to show the possibilities and limitations of using starch as a packaging material. The most important physicochemical features of this biopolymer are discussed, and special attention is paid to more or less environmentally friendly methods of improving its processing properties.

Recent developments towards performance-enhancing lignin-based polymers

This review examines recent strategies, challenges, and future opportunities in preparing high-performance polymeric materials from lignin and its derivable compounds.

Poly(butylene adipate-co-terephthalate)/thermoplastic starch/zeolite 5A films: Effects of compounding sequence and plasticizer content

This work investigated the effect of the compounding sequence and the glycerol content on poly(butylene adipate-co-terephthalate)/thermoplastic starch/zeolite 5A (PBAT/TPS/Z5A) composites. The composite pellets and films were prepared by an extrusion process using a PBAT:TPS ratio of 60:40, Z5A loading of 3 wt%, and glycerol contents of 35 and 40 parts per hundred parts of starch (phs). Prior to blown film extrusion, the composite pellets were produced by two compounding sequences: sequence I (SI)-mixing PBAT with Z5A prior to blending with TPS; sequence II (SII)-mixing TPS with Z5A before blending with PBAT. The SII compounding sequence provided improved mixing between PBAT and TPS, leading to increased continuous phase region and a reduced TPS dispersed phase size. Increasing the glycerol content decreased the viscosity and size of the TPS dispersed phase and gave rise to a more uniform dispersion of the TPS domains and Z5A particles. Compounding Z5A via the SII sequence with a glycerol content of 40 phs effectively improved the mixing and the performance of the PBAT/TPS blend.

From high oleic vegetable oils to hydrophobic starch derivatives: II. Physicochemical, processing and environmental properties

Medium-substituted esters of starch and higher fatty acids, structurally identified in the first part of paper were subjected to further analyses, mainly to check application potential. In order to determine the possibility of using the esters in the packaging industry, the glycerol-plasticized starch esters were extruded on a single screw extruder in the form of a film. The mechanical properties tests consisted of tensile and tear strength. Hydrophobicity, water absorption and oil absorption were checked as the processing and functional properties. Environmental tests, such as phytotoxicity on monocotyledonous and dicotyledonous plants and biodegradability in soil under strictly controlled conditions of the vegetation hall were carried out. Esterification increased the hydrophobicity of the starch and the tensile and tear strength, without losing important environmental features such as biodegradability and non-toxicity. The obtained polymer materials give hope for their use in the production of new ecofriendly and biodegradable packaging.

Reprocessable vinylogous urethane cross-linked polyethylene via reactive extrusion

Reactive extrusion of a precursor polymer and a diamine cross-linker leads to reprocessable vinylogous urethane polyethylene vitrimers with desired rheological properties.

Biodegradation study of bio-corn flour filled low density polyethylene composites assessed by natural soil

This paper illustrates the aim to introduce biodegradable vegetable filler in synthetic polymers to prepare novel biodegradable composites. Low density polyethylene/alkali treated corn flour (LDPE/ATCF) composites were prepared by reactive extrusion using a twin-screw extruder. The microstructure, thermal properties and tensile properties were evaluated and compared with virgin LDPE. The Fourier transform infrared (FTIR) spectra showed a decrease in the hydrophilic nature of corn flour (CF) after alkali treatment. Scanning electron microscopy (SEM) micrographs showed good dispersion between matrix and filler. The tensile and elongation at break decreased by increasing the filler content in the composites. However, the Young’s modulus increased with the increase in filler content. The biodegradation of composites was studied in the environment using the soil burial test for 6 months. Differential scanning calorimetry (DSC) analysis showed an increase of the melting enthalpy (ΔHm) and crystallinity of LDPE with evidence of degradation. The biodegradability of the composites was enhanced with increasing ATCF content in the matrix. This result was supported by weight loss and degraded surface of composites observed through morphological studies. From the results, we conclude that the use of ATCF as filler in LDPE reduces pollution problems. This is advantageous for both the economy and the environment.

Local demixion in plasticized polylactide probed by electron spin resonance

Improving the barrier properties to gas and organic compounds of biosourced polyesters, such as polylactides (PLAs), by increasing their crystallinity has been suggested by several authors. This paper investigates the risk of microphase separation for a technological approach that would involve a plasticization of PLA, to further its crystallization kinetics, with common plasticizers: Acetyl tributyl citrate (ATBC) and Poly(ethylene glycol) (PEG). Overplasticization effects following microphase separation were monitored along the film thickness by exposing dynamically thermo-compressed films to nitroxide spin-probes. The method enabled a scan of the local polymer mobility for different concentration profiles in spin-probes, with in particular a maximum moving continuously in time towards the geometric center. The results were interpreted as excess local temperatures that would give similar ESR spectra motion in the bulk. It was shown that measured excess temperatures could be related to local shifts in the glass transition temperature along the film thickness.

Mechanochemical Milling of PP/Ground Tire Rubber Blends in Solid-State

In this study, solid-state mechanochemical milling was investigated as a viable strategy to produce high performance blends composed of polypropylene (PP) and ground tire rubber (GTR), thereby providing a potentially new route to recycle discarded tires. The improved dispersion of the blends after mechanochemical milling was confirmed by fluorescence microscopy and polarized optical microscopy (POM) observation. After 20 cycles of milling, the tensile strength of PP/GTR (100/40) blends was enhanced by 13.8%, and the elongation at break was over 12 times than that of the unpretreated blend. The formation of smaller and less ordered spherulites of PP after mechanochemical milling as confirmed by POM observation and wide-angle X-ray diffraction analysis might be the reason for the significant enhancement of the elongation at break.