Thermal, mechanical, and rheological properties of plasticized poly(L-lactic acid)

Research progress in fully biorenewable tough blends of polylactide and green plasticizers

Plasticized PLA plastic films are being increasingly used in, among others, packaging and agriculture sectors in an attempt to address the rapid growth of municipal waste. The present paper aims to review the recent progress and the state-of-the-art in the field of fully bio-renewable tough blends of PLA with green plasticizers aimed at developing flexible packaging films. The different classes of green substances, derived from completely bio-renewable resources, used as potential plasticizers for PLA resins are reviewed. The effectiveness of these additives for PLA plasticization is discussed by describing their effects on different properties of PLA. The performance of these blends is primarily determined by the solvent power, compatibility, efficiency, and permanence of plasticizer present in the PLA matrix of resulting films. The various chemical modification strategies employed to tailor the phase interactions, dispersion level and morphology, plasticization efficiency, and permanence, including functionalization, oligomerization, polymerization and self-crosslinking, grafting and copolymerization, and dynamic vulcanization are demonstrated. Sometimes a third component has also been added to the plasticized binary blends as compatibilizer to further promote dispersion and interfacial adhesion. The impact of chemical structure, size and molecular weight, chemical functionalities, polarity, concentration, topology as well as molecular architectures of the plasticizers on the plasticizer performance and the overall characteristics of resulting plasticized PLA materials is discussed. The morphological features and toughening mechanisms for PLA/plasticizer blends are also presented. The different green liquids employed show varying degree of plasticization. Some are more useful for semi-rigid applications, while some others can be used for very flexible products. There is an optimum level of plasticizer in PLA matrices above which the tensile ductility deteriorates. Esters-derivatives of bio-based plasticizers have been shown to be very promising additives for PLA modification. Some plasticizers impart additional functions such as antioxidation and antibacterial activity to the resulting PLA materials, or compatibilization in PLA-based blends. While the primary objective of plasticization is to boost the processability, flexibility, and toughness over wider practical conditions, the bio-degradability, permeability and long-term stability of microstructure (and thereby properties) of the plasticized films against light, weathering, thermal aging, and oxidation deserve further investigations.

Degradation of Structurally Modified Polylactide under the Controlled Composting of Food Waste

The degradation of polylactide (PLA) films of different structures under conditions of controlled composting has been studied. We have demonstrated that PLA underwent degradation within one month in a substrate that simulated standard industrial composting. Regardless of the initial structure of the samples, the number-average molecular weight (Mn) decreased to 4 kDa while the degree of crystallinity increased to about 70% after 21 days of composting. Addition of an inoculant to the standard substrate resulted in the accelerated degradation of the PLA samples for one week due to an abiotic hydrolysis. These findings have confirmed that industrial composting could solve the problem of plastic disposal at least for PLA.

Effect of Polyethylene Glycol Methyl Ether Methacrylate on the Biodegradability of Polyvinyl Alcohol/Starch Blend Films

Blend copolymers (PVA/S) were grafted with polyethylene glycol methyl methacrylate (PEGMA) with different ratios. Potassium persulfate was used as an initiator. The blend copolymer (PVA/S) was created by combining poly(vinyl alcohol) (PVA) with starch (S) in various ratios. The main idea was to study the effect of different ratios of the used raw materials on the biodegradability of plastic films. The resulting polymers (PVA/S/PEGMA) were analyzed using FTIR spectroscopy to investigate the hydrogen bond interaction between PVA, S, and PEGMA in the mixtures. TGA and SEM analyses were used to characterize the polymers (PVA/S/AA). The biodegradability and mechanical properties of the PVA/S/PEGMA blend films were evaluated. The findings revealed that the mechanical properties of the blend films are highly influenced by PEGMA. The time of degradation of the films immersed in soil and Coca-Cola increases as the contents of PVA and S and the molecular weight (MW) of PEGMA increase in the terpolymer. The M8 sample (PVA/S/PEGMA in the ratio of 3:1:2, respectively) with a MW of 950 g/mol produced the lowest elongation at break (67.5%), whereas M1 (PVA/S/PEGMA in the ratio of 1:1:1, respectively) with a MW of 300 g/mol produced the most (150%). The film's tensile strength and elongation at break were improved by grafting PEGMA onto the blending polymer (PAV-b-S). Tg and Tm increased when the PEGMA MW increased from 300 to 950. Tg (48.4 °C) and Tm (190.9 °C) were the lowest in M1 (300), while Tg (84.8 °C) and Tm (190.9 °C) were greatest in M1 (950) at 209.3 °C. The increased chain and molecular weight of PEGMA account for the increase in Tg and Tm of the copolymers.

Grafted Lactic Acid Oligomers on Lignocellulosic Filler towards Biocomposites

Lactic acid oligomers (OLAs) were in situ synthesized from lactic acid (LAc) and grafted onto chokeberry pomace (CP) particleboards by direct condensation. Biocomposites of poly (lactic acid) (PLA) and modified/unmodified CP particles containing different size fractions were obtained using a mini-extruder. To confirm the results of the grafting process, the FTIR spectra of filler particles were obtained. Performing ¹HNMR spectroscopy allowed us to determine the chemical structure of synthesized OLAs. The thermal degradation of modified CP and biocomposites were studied using TGA, and the thermal characteristics of biocomposites were investigated using DSC. In order to analyse the adhesion between filler particles and PLA in biocomposites, SEM images of brittle fracture surfaces were registered. The mechanical properties of biocomposites were studied using a tensile testing machine. FTIR and ¹HNMR analysis confirmed the successful grafting process of OLAs. The modified filler particles exhibited a better connection with hydrophobic PLA matrix alongside improved mechanical properties than the biocomposites with unmodified filler particles. Moreover, a DSC analysis of the biocomposites with modified CP showed a reduction in glass temperature on average by 9 °C compared to neat PLA. It confirms the plasticizing effect of grafted and ungrafted OLAs. The results are promising, and can contribute to increasing the use of agri-food lignocellulosic residue in manufacturing biodegradable packaging.

Properties of Mosquito Repellent-Plasticized Poly(lactic acid) Strands

Poly(lactic acid) (PLA) is an attractive candidate for replacing petrochemical polymers because it is fully biodegradable. This study investigated the potential of PLA as a sustainable and environmentally friendly alternative material that can be developed into commercially viable wearable mosquito repellent devices with desirable characteristics. PLA strands containing DEET and IR3535 were prepared by twin screw extrusion compounding and simultaneously functioned as plasticizers for the polymer. The plasticizing effect was investigated by thermal and rheological studies. DSC studies showed that the addition of DEET and IR3535 into PLA strands reduced the glass transition temperature consistent with predictions of the Fox equation, thus proving their efficiency as plasticizers. The rheology of molten samples of neat PLA and PLA/repellents blends, evaluated at 200 °C, was consistent with shear-thinning pseudoplastic behaviour. Raman studies revealed a nonlinear concentration gradient for DEET in the PLA strand, indicating non-Fickian Type II transport controlling the desorption process. Release data obtained at 50 °C showed initial rapid release followed by a slower, near constant rate at longer times. The release rate data were fitted to a novel modification of the Peppas-Sahlin desorption model.

Properties and Degradation of Novel Fully Biodegradable PLA/PHB Blends Filled with Keratin

The utilization of keratin waste in new materials formulations can prevent its environmental disposal problem. Here, novel composites based on biodegradable blends consisting of poly(lactic acid) (PLA) and poly(3-hydroxybutyrate) (PHB), and filled with hydrolyzed keratin with loading from 1 to 20 wt % were prepared and their properties were investigated. Mechanical and viscoelastic properties were characterized by tensile test, dynamic mechanical thermal analysis (DMTA) and rheology measurements. The addition of acetyltributyl citrate (ATBC) significantly affected the mechanical properties of the materials. It was found that the filled PLA/PHB/ATBC composite at the highest keratin loading exhibited similar shear moduli compared to the un-plasticized blend as a result of the much stronger interactions between the keratin and polymer matrix compared to composites with lower keratin content. The differences in dynamic moduli for PLA/PHB/ATBC blend filled with keratin depended extensively on the keratin content while loss the factor values progressively decreased with keratin loading. Softening interactions between the keratin and polymer matrix resulted in lower glass transitions temperature and reduced polymer chain mobility. The addition of keratin did not affect the extent of degradation of the PLA/PHB blend during melt blending. Fast hydrolysis at 60 °C was observed for composites with all keratin loadings. The developed keratin-based composites possess properties comparable to commonly used thermoplastics applicable for example as packaging materials.

Micro-contextual identification of archaeological lipid biomarkers using resin-impregnated sediment slabs

Characterizing organic matter preserved in archaeological sediment is crucial to behavioral and paleoenvironmental investigations. This task becomes particularly challenging when considering microstratigraphic complexity. Most of the current analytical methods rely on loose sediment samples lacking spatial and temporal resolution at a microstratigraphic scale, adding uncertainty to the results. Here, we explore the potential of targeted molecular and isotopic biomarker analysis on polyester resin-impregnated sediment slabs from archaeological micromorphology, a technique that provides microstratigraphic control. We performed gas chromatography-mass spectrometry (GC-MS) and gas chromatography-isotope ratio mass spectromety (GC-IRMS) analyses on a set of samples including drill dust from resin-impregnated experimental and archaeological samples, loose samples from the same locations and resin control samples to assess the degree of interference of polyester resin in the GC-MS and Carbon-IRMS signals of different lipid fractions (n-alkanes, aromatics, n-ketones, alcohols, fatty acids and other high polarity lipids). The results show that biomarkers within the n-alkane, aromatic, n-ketone, and alcohol fractions can be identified. Further work is needed to expand the range of identifiable lipid biomarkers. This study represents the first micro-contextual approach to archaeological lipid biomarkers and contributes to the advance of archaeological science by adding a new method to obtain behavioral or paleoenvironmental proxies.

Compatibility of energetic plasticizers with the triblock copolymer of polypropylene glycol-glycidyl azide polymer-polypropylene glycol (PPG-GAP-PPG)

Glycidyl azide polymer (GAP) is well known as an energetic prepolymer, but its application as a binder in propellants is limited due to its relatively high glass transition temperature and relatively poor mechanical properties. Copolymerization of GAP with polypropylene glycol (PPG) has been shown to improve GAPs properties because of the good thermal and mechanical properties of PPG. In this research we synthesized triblock copolymer of PPG-GAP-PPG and the compatibilities of this copolymer were investigated with energetic plasticizers (20% w/w) n-butyl nitroxyethylnitramine (BuNENA), trimethylolethane trinitrate (TMETN), and butanetriol trinitrate (BTTN) by solubility parameter, differential scanning calorimetry (DSC), rheological analysis, scanning electron microscopy (SEM) and vacuum stability test (VST). The DSC results showed that BuNENA had better compatibility with the triblock copolymer in comparison to TMETN and BTTN. It reduced the T g of PPG-GAP-PPG from −58 to −63 °C. The rheological analysis was in good agreement with the DSC results obtained for the compatibility of the plasticizers. In the case of the addition of 20% w/w BuNENA, the viscosity of copolymer/plasticizer decreased from 550 to 128 mPa s, indicating appropriate compatibility of plasticizer with the copolymer. SEM images showed a better distribution of BuNENA in the copolymer matrix.

Efficiently Extracted Cellulose Nanocrystals and Starch Nanoparticles and Techno-Functional Properties of Films Made Thereof

Cellulose nanocrystals (CNC) and starch nanoparticles (SNP) have remarkable physical and mechanical characteristics. These properties particularly facilitate their application as high-performance components of bio-based packaging films as alternatives to fossil-based counterparts. This study demonstrates a time-efficient and resource-saving extraction process of CNC and SNP by sulfuric acid hydrolysis and neutralization. The yields of the hydrolyzed products were 41.4% (CNC) and 32.2% (SNP) after hydrolysis times of 3 h and 120 h, respectively. The nanoparticle dispersions were wet-coated onto poly(lactic acid) (PLA) and paper substrates and were incorporated into starch films. No purification or functionalization of the nanoparticles was performed prior to their application. Techno-functional properties such as the permeability of oxygen and water vapor were determined. The oxygen permeability of 5–9 cm3 (STP) 100 µm m−2 d−1 bar−1 at 50% relative humidity and 23 °C on PLA makes the coatings suitable as oxygen barriers. The method used for the extraction of CNC and SNP contributes to the economic production of these nanomaterials. Further improvements, e.g., lower ion concentration and narrower particle size distribution, to achieve reproducible techno-functional properties are tangible.

Grafting Modification of the Reactive Core-Shell Particles to Enhance the Toughening Ability of Polylactide

In order to overcome the brittleness of polylactide (PLA), reactive core-shell particles (RCS) with polybutadiene as core and methyl methacrylate-co-styrene-co-glycidyl methacrylate as shell were prepared to toughen PLA. Tert-dodecyl mercaptan (TDDM) was used as chain transfer agent to modify the grafting properties (such as grafting degree, shell thickness, internal and external grafting) of the core-shell particles. The introduction of TDDM decreased the grafting degree, shell thickness and the T_g of the core phase. When the content of TDDM was lower than 1.15%, the RCS particles dispersed in the PLA matrix uniformly-otherwise, agglomeration took place. The addition of RCS particles induced a higher cold crystallization temperature and a lower melting temperature of PLA which indicated the decreased crystallization ability of PLA. Dynamic mechanical analysis (DMA) results proved the good miscibility between PLA and the RCS particles and the increase of TDDM in RCS induced higher storage modulus of PLA/RCS blends. Suitable TDDM addition improved the toughening ability of RCS particles for PLA. In the present research, PLA/RCS-T4 (RCS-T4: the reactive core-shell particles with 0.76 wt % TDDM addition) blends displayed much better impact strength than other blends due to the easier cavitation/debonding ability and good dispersion morphology of the RCS-T4 particles. When the RCS-T4 content was 25 wt %, the impact strength of PLA/RCS-T4 blend reached 768 J/m, which was more than 25 times that of the pure PLA.

Effects of PLA Film Incorporated with ZnO Nanoparticle on the Quality Attributes of Fresh-Cut Apple

A novel nanopackaging film was synthesized by incorporating ZnO nanoparticles into a poly-lactic acid (PLA) matrix, and its effect on the quality of fresh-cut apple during the period of preservation was investigated at 4 ± 1 °C for 14 days. Six wt % cinnamaldehyde was added into the nano-blend film. Scanning electron microscope (SEM) analysis showed a rougher cross-section of the nano-blend films and an X-ray diffraction (XRD) was carried out to determine the structure of the ZnO nanoparticles. Compared to the pure PLA film, the nano-blend film had a higher water vapor permeability (WVP) and lower oxygen permeability. With the increase of the nanoparticles (NPs) in the PLA, the elongation at break (ε) and elastic modulus (EM) increased, while tensile strength (TS) decreased. Thermogravimetric analysis (TGA) presented a relatively good thermostability. Most importantly, the physical and biochemical properties of the fresh-cut apple were also measured, such as weight loss, firmness, polyphenol oxidase (PPO), total phenolic content, browning index (BI), sensory quality, and microbiological level. The results indicated that nano-blend packaging films had the highest weight loss at the end of storage compared to the pure PLA film; however, nanopackaging provided a better retention of firmness, total phenolic countent, color, and sensory quality. It also had a remarkable inhibition on the growth of microorganisms. Therefore, Nano-ZnO active packaging could be used to improve the shelf-life of fresh-cut produce.

Characterization of Antimicrobial Poly (Lactic Acid)/Nano-Composite Films with Silver and Zinc Oxide Nanoparticles

Antimicrobial active films based on poly (lactic acid) (PLA) were prepared with nano-silver (nano-Ag) and nano-zinc oxide (nano-ZnO) using a solvent volatilizing method. The films were characterized for mechanical, structural, thermal, physical and antimicrobial properties. Scanning electron microscopy (SEM) images characterized the fracture morphology of the films with different contents of nano-Ag and nano-ZnO. The addition of nanoparticles into the pure PLA film decreased the tensile strength and elasticity modulus and increased the elongation of breaks-in other words, the flexibility and extensibility of these composites improved. According to the results of differential scanning calorimetry (DSC), the glass transition temperature of the PLA nano-composite films decreased, and the crystallinity of these films increased; a similar result was apparent from X-ray diffraction (XRD) analysis. The water vapor permeability (WVP) and opacity of the PLA nano-composite films augmented compared with pure PLA film. Incorporation of nanoparticles to the PLA films significantly improved the antimicrobial activity to inhibit the growth of Escherichia coli. The results indicated that PLA films with nanoparticles could be considered a potential environmental-friendly packaging material.

Characterization of Active Packaging Films Made from Poly(Lactic Acid)/Poly(Trimethylene Carbonate) Incorporated with Oregano Essential Oil

Antimicromial and antioxidant bioactive films based on poly(lactic acid)/poly(trimenthylene carbonate) films incorporated with different concentrations of oregano essential oil (OEO) were prepared by solvent casting. The antimicrobial, antioxidant, physical, thermal, microstructural, and mechanical properties of the resulting films were examined. Scanning electron microscopy analysis revealed that the cross-section of films became rougher when OEO was incorporated into PLA/PTMC blends. Differential scanning calorimetry analysis indicated that crystallinity of PLA phase decreased by the addition of OEO, but this did not affect the thermal stability of the films. Water vapor permeability of films slightly increased with increasing concentration of OEO. However, active PLA/PTMC/OEO composite films showed adequate barrier properties for food packaging application. The antimicrobial and antioxidant capacities were significantly improved with the incorporation of OEO (p < 0.05). The results demonstrated that an optimal balance between the mechanical, barrier, thermal, antioxidant, and antimicrobial properties of the films was achieved by the incorporation of 9 wt % OEO into PLA/PTMC blends.

Novel super-toughened bio-based blend from polycarbonate and poly(lactic acid) for durable applications

Novel high-performance bio-based PC/PLA blends are created via high-temperature reactive blending, using targeted toughening of the brittle phase.