Flexibly Controlling the Polycrystallinity and Improving the Foaming Behavior of Polylactic Acid via Three Strategies

Preparation of tomato peel pomace powder/polylactic acid foams under supercritical CO2 conditions: Improvements in cell structure and foaming behavior

Polylactic acid (PLA) is an eco-friendly material that can help address the problems of petroleum depletion and pollution. Blending renewable biomass materials with PLA to create composite foams with a tunable pore structure, superior performance, and low cost is a green technique for improving the pore structure and mechanical characteristics of single PLA foams. PLA/TP composites were created using melted tomato peel pomace powder (TP), which has a lamellar structure, as a reinforcing agent. Then, the relationship between the vesicle structure, morphology, and properties of the PLA/TP composite foams produced through supercritical CO2 intermittent foaming were investigated. The findings revealed that TP considerably enhanced the rheological characteristics and crystalline behavior of PLA. The PLA/TP composite foam had a better cell structure, compression characteristics, and wettability than pure PLA. The expansion ratio of the PLA/TP composite could reach 18.8, and its thermal conductivity decreased from 174.2 mW/m·K at 100 °C to 57.8 mW/m·K at 120 °C. Furthermore, annealing before foaming decreased the average composite foam blister size from 110.09 to 66.53 μm, and the annealing process also improved compression performance. This study contributes to solving environmental difficulties and creating PLA foams with controlled bubble structures, uniform bubble sizes, and outstanding overall performance.

Thermally-Activated Shape Memory Behavior of Biodegradable Blends Based on Plasticized PLA and Thermoplastic Starch

Biodegradable blends based on plasticized poly(lactic acid) PLA and thermoplastic starch (TPS) have been obtained. The influence of the PLA plasticizer as a compatibility agent has been studied by using two different plasticizers such as neat oligomeric lactic acid (OLA) and functionalized with maleic acid (mOLA). In particular, the morphological, thermal, and mechanical properties have been studied as well as the shape memory ability of the melt-processed materials. Therefore, the influence of the interaction between different plasticizers and the PLA matrix as well as the compatibility between the two polymeric phases on the thermally-activated shape memory properties have been studied. It is very interesting to use the same additive able to act as both plasticizer and compatibilizer, decreasing the glass transition temperature of PLA to a temperature close to the physiological one, obtaining a material suitable for potential biomedical applications. In particular, we obtain that OLA-plasticized blend (oPLA/TPS) show very good thermally-activated capability at 45 °C and 50% deformation, while the blend obtained by using maleic OLA (moPLA/TPS) did not show shape memory behavior at 45 °C and 50% deformation. This fact is due to their morphological changes and the loss of two well-distinguished phases, one acting as fixed phase and the other one acting as switching phase to typically obtain shape memory response. Therefore, the thermally-activated shape memory results show that it is very important to make a balance between plasticizer and compatibilizer, considering the need of two well-established phases to obtain shape memory response.

Bio-based poly(lactic acid) foams with enhanced mechanical and heat-resistant properties obtained by facilitating stereocomplex crystallization with addition of D-sorbitol

The preparation of bio-based poly(lactic acid) (PLA) foams with high mechanical properties and heat resistance is of great significance for environmental protection and green sustainable development. In this paper, D-sorbitol (DS) containing six hydroxyl groups was introduced into poly(l-lactide) (PLLA)/poly(d-lactide) (PDLA) blends for first time to promote the formation of stereocomplex (SC) crystals, which could improve the foaming behavior and enhance mechanical properties and heat resistance of PLA foams. The results showed that DS could improve the formation efficiency and crystallinity of SC crystals by enhancing the hydrogen bonding between the enantiomeric molecular chains. Furthermore, the compression modulus and interactions Vicat softening temperature of the PLLA/PDLA/DS blend foam increased about 854% and 16% compared to the pure PLLA foam, respectively. Besides, when the annealing process was introduced, the compression and heat resistance of the PLA foams increased further. This study provided a feasible strategy for the preparation of bio-based and biodegradable PLA foams with outstanding compressive and heat resistance properties.

Fabrication of bio-based biodegradable poly(lactic acid) (PLA) and poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) composite foams for highly efficient oil-water separation

The open-cell bio-based biodegradable polymer foams show good application prospect in dealing with the serious environmental issue caused by oil spill and organic solvents spills, while the cell structures and hydrophobic properties of the foams limit their performance. In this work, the poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) was selected to help prepare bio-based biodegradable poly(lactic acid) (PLA) foams. Based on a two-step foaming method, the crystallization ability of different samples was regulated by the "original crystals" together with PHBV in the foaming process, where skeleton structures were provided to facilitate the open-cell structures and promote their mechanical property. As illustrated, PHBV facilitated the formation of open-cell PLA foams, where the foams displayed superior oil-water separation capacity. The maximum volume expansion ratio of the foams was 80.08, the contact angle of deionized water reached to 134.5°, the adsorption capacity for oil or organic solvents was 10.8 g/g-51.8 g/g, and the adsorption capacity for CCl4 can still maintained 83.5 % of the initial value after 10 adsorption-desorption cycles. This work not only clarified the foaming mechanism of open-cell foams, but also provided a green and simple method for preparing bio-based biodegradable foams possessing excellent oil-water separation performance.

Composites made of Ginkgo biloba fibers and polylactic acid exhibit non-isothermal crystallization kinetics

Polymer crystallization affects material microstructure and the final product quality, and the crystallization kinetics that govern this process are critical. In this study, alkali-treated Ginkgo biloba fibers (GFs) were melt blended with polylactic acid (PLA) to obtain GF/PLA blends. The non-isothermal crystallization kinetics of the GF/PLA composites were subsequently investigated using the Avrami, Jeziorny, Ozawa, and Liu-Mo methods, and the crystallization activation energies of the systems were calculated by Kissinger and Friedman models. The results showed that the GFs significantly promoted PLA crystallization, accelerated the crystallization rate, and shortened the crystallization time. The Avrami method showed some deviation from the linear relationship due to the effect of secondary crystallization, while the numeric value obtained by the Jeziorny method increased with the cooling rate. The Ozawa method could only be used in a very narrow range of temperatures, while the Liu-Mo method showed a more desirable fit. Crystallization activation energy calculations showed that the GFs promoted an increase in the crystallization capacity of the blend and a decrease in the effective potential barrier. This resulted in more selective biocomposites than pure PLA, offering greater applicability in domains including tissue engineering and 3D printing.

Crystallinity, Rheology, and Mechanical Properties of Low-/High-Molecular-Weight PLA Blended Systems

As semi-crystalline polyester (lactic acid) (PLA) is combined with other reinforcing materials, challenges such as phase separation, environmental pollution, and manufacturing difficulties could hinder the benefits of PLA, including complete biodegradability and strong mechanical properties. In the present investigation, melt blending is utilized to establish a mixture of low- and high-molecular-weight polylactic acids (LPLA and HPLA). The crystallinity, rheology, and mechanical properties of the combination were analyzed using rotational rheometry, differential scanning calorimetry, X-ray diffraction, polarized optical microscopy, scanning electron microscopy, and universal testing equipment. The results demonstrate compatibility between LPLA and HPLA. Moreover, an increase in LPLA concentration leads to a decrease in the crystallization rate, spherulite size, fractional crystallinity, and XRD peak intensity during isothermal crystallization. LPLA acts as a diluent during isothermal crystallization, whereas HPLA functions as a nucleating agent in the non-isothermal crystallization process, promoting the growth of LPLA crystals and leading to co-crystallization. The blended system with a 5% LPLA mass fraction exhibits the highest tensile strength and enhances rheological characteristics. By effectively leveraging the relationship between various molecular weights of PLA's mechanical, rheological, and crystallization behavior, this scrutiny improves the physical and mechanical characteristics of the material, opening up new opportunities.

Antimicrobial Poly (Lactic Acid)/Copper Nanocomposites for Food Packaging Materials

Composites based on polylactic acid (PLA) and copper for food packaging applications were obtained. Copper clusters were synthesized in polyethylene glycols 400 and 600, respectively, using ascorbic acid as a reducing agent, by reactive milling. Copper clusters were characterized by Scanning Electron Microscopy (SEM), Fourier Transform Infrared (FT-IR), and Ultraviolet-Visible (UV-VIS) spectroscopy. Copper/PLA composites containing Proviplast as plasticizer were characterized by FT-IR spectroscopy, mechanical tests, Differential Scanning Calorimetry (DSC), Thermogravimetric Analysis (TGA), absorption of the saline solution, contact angle, and antibacterial properties. It was observed that the concentration of Copper/PEG influenced the investigated properties. The mechanical properties of the samples decreased with the increasing of Copper/PEG concentration. We recorded the phase transformation temperatures and identified the exothermic or endothermic processes. The lowest absorption values were recorded in the case of the sample containing 1% Cu. The contact angle decreases with the increase in the concentration of the PEG 600-Cu mixture in the recipes. The increase in the content of Cu clusters favors the decrease in the temperature, taking place 15% wt mass losses. The obtained composites showed antibacterial properties for all tested strains. These materials could be used as alternative materials for obtaining biodegradable food packaging.

Fabrication of silver ions aramid fibers and polyethylene composites with excellent antibacterial and mechanical properties

Nowadays, with the continuous understanding of the pathogenic mechanism of bacterium, the demand for antibacterial plastic products had significantly increased. Besides that, many counties issued mandatory standards for plastic products, which imposed strict requirements on ash content to prevent the addition of excessive inorganic matter to plastics in order to avoid weakening the properties of plastics and deteriorating the recyclable property. Based on this, the development of composites with organic fiber-loaded antibacterial agents is of practicable value and urgency. We used an open-ring addition reaction to modified aramid fiber (AF) by utilizing epoxypropyltrimethoxysilane to react with the reactive groups on the surface of AFs. Subsequently, the modified fibers were surface loaded with silver ionic glass beads. After that, a series of high-density polyethylene composites with excellent mechanical properties and antibacterial properties were prepared using melt mixing method. It was shown that the composite had a low ash value (1.88 wt%) even at a higher filling concentration (7 wt%) and the fibers could change the crystalline properties and morphology of the composite. Because of the fiber reinforcement and crystallization induction effects, the tensile strength and elasticity module of the composites could be improved by 141% and 136%, respectively. In addition, the composites had excellent long-lasting contact antibacterial effects against the inhibition of E. coli. The proposed organic fiber loading technique and antibacterial composites will provide a method for designing and preparing eco-friendly and high-performance plastic products.

Effect of Perfluoropolymers on the Anti-Wear Properties of Carbon Fiber/Polyphenylene Sulfide Composites: A Comparative Study

In this work, several perfluoropolymers (PFP), including commercial polytetrafluoroethylene (PTFE), perfluorinated ethylene-propylene copolymer (FEP), tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer (PFA), and irradiated PTFE (iPTFE) were used as additives to lubricate carbon fiber (CF)-reinforced polyphenylene sulfide (PPS) composites. The tribological properties of the yielding composites were studied and correlated with the melt processability of PFPs. Although the neat FEP and PFA have higher friction coefficients when compared with neat PTFE, the composites filled with FEP and PFA additives were found to exhibit a lower friction coefficient compared to PTFE at PFP content below 10 wt %. Moreover, the iPTFE-filled composites also showed similar results as FEP or PFA filled ones, very different from PTFE at low additions. Based on the morphological investigation, we postulate that FEP, PFA, and iPTFE are melt-kneaded with PPS due to their melt processability at processing temperature, leading to the good dispersion in composites in the form of smaller deformed spheres and/or fibril bands. The well-dispersion of PFPs in composites promotes the formation and growth of the transfer film on the counterface during sliding.