Effect of clay type and loading on thermal, mechanical properties and biodegradation of poly(lactic acid) nanocomposites

Epoxidized Ionic Liquids as Processing Auxiliaries of Poly(Lactic Acid) Matrix: Influence on the Manufacture, Structural and Physical Properties

In this study, we set out to modify poly(lactic acid) (PLA) by incorporating epoxidized ionic liquids (ILs) that were specifically designed with imidazolium-NTf₂ moieties. First, we synthesized di-, tri- and tetra-epoxidized ILs, which were incorporated into a PLA matrix at 3, 5, and 10 wt% through a melt extrusion process. We investigated the relationship between the structure and properties of the resulting materials in terms of thermal, mechanical, rheological, and surface properties. The results showed the potential of ILs to impact these properties. Notably, the tri- and tetra-epoxidized ILs enhanced the thermal stability of the PLA matrix as well as the crystallinity while reducing the glass transition temperature and melting point, which is promising for reactive extrusion processing. Overall, this research opens new routes for using reactive ILs to improve the processing and properties of PLA polymers.

Functional Filaments: Creating and Degrading pH-Indicating PLA Filaments for 3D Printing

With the rapid pace of advancements in additive manufacturing and techniques such as fused filament fabrication (FFF), the feedstocks used in these techniques should advance as well. While available filaments can be used to print highly customizable parts, the creation of the end part is often the only function of a given feedstock. In this study, novel FFF filaments with inherent environmental sensing functionalities were created by melt-blending poly(lactic acid) (PLA), poly(ethylene glycol) (PEG), and pH indicator powders (bromothymol blue, phenolphthalein, and thymol blue). The new PLA-PEG-indicator filaments were universally more crystalline than the PLA-only filaments (33-41% vs. 19% crystallinity), but changes in thermal stability and mechanical characteristics depended upon the indicator used; filaments containing bromothymol blue and thymol blue were more thermally stable, had higher tensile strength, and were less ductile than PLA-only filaments, while filaments containing phenolphthalein were less thermally stable, had lower tensile strength, and were more ductile. When the indicator-filled filaments were exposed to acidic, neutral, and basic solutions, all filaments functioned as effective pH sensors, though the bromothymol blue-containing filament was only successful as a base indicator. The biodegradability of the new filaments was evaluated by characterizing filament samples after aging in soil and soil slurry mixtures; the amount of physical deterioration and changes in filament crystallinity suggested that the bromothymol blue filament degraded faster than PLA-only filaments, while the phenolphthalein and thymol blue filaments saw decreases in degradation rates.

One-Pot Reactive Melt Recycling of PLA Post-Consumer Waste for the Production of Block Copolymer Nanocomposites of High Strength and Ductility

Post-consumer waste recycling is a crucial issue for building a sustainable society. However, mechanical recycling of poly(lactic acid) (PLA) often reduces the performance of the recycled material because PLA has a strong tendency to degrade during reprocessing. Therefore, it is of great interest to develop an effective recycling method to improve the mechanical performance of this material. This paper presents a one-pot melt process for turning PLA waste into a biodegradable block copolymer and its high strength and ductility composite. The process was conducted in a melt-mixer through a transesterification of PLA with poly(ethylene glycol) (PEG) or poly(propylene glycol) (PPG) as a soft component and clay as reinforcement. Effects of soft component content and sequence of clay addition on the mechanical performance of the prepared materials were focused. The results showed the successful preparation of PLA-based multiblock copolymers of high molecular weights (~100 kDa). Both virgin PLA and recycled source could serve as the starting material. PEG was more efficient than PPG in providing an intense improvement of PLA ductility. The nanocomposite of intercalated structure yielded nearly 100 times higher elongation at break (E_b = 506%) than the starting PLA (E_b = 5.6%) with high strength of 39.5 MPa and modulus of 1.4 GPa, considering the advantages of clay addition. Furthermore, the products with a broadened range of properties can be designed based on the ratio of PLA and soft component, as well as the organization and spatial distribution of clay in the copolymer matrices.

Centrifugally spun alginate-poly(lactic acid) microbeads: A promising carrier for drug delivery and tissue engineering

A facile and high yield centrifugal spinning technique known as Forcespinning® (FS) was used to develop unique microstructures consisting of PLA microbeads along alginate fibers. Morphological variation and structural features appeared in the field-emission scanning electron micrographs for the PLA-alginate composites and dried PLA-alginate films from precursor emulsions at constant PLA and varied alginate contents. Shrunk and deflated microbeads were observed for composites whilst spherical beads were evident for the PLA control. Furthermore, PLA was found surrounding the alginate when the alginate was present at 0.24 wt% or lower, while alginate (mushroom-like structures), were seen protruding through the PLA layer at ≥0.34 wt% alginate. Rheological characterization of the composite emulsions revealed that the filler (alginate) provided shear thinning properties including pseudoplasticity, desirable for printing and other related applications in contrast to the Newtonian flow shown by the PLA control. Along with infra-red spectroscopy, the nanocomposites were further characterized using thermal gravimetry and differential scanning calorimetry featuring reversible events influenced by heat capacity and irreversible kinetic/thermodynamic counterparts. The work provides a comprehensive investigation of biocompatible networks of PLA-alginate microbeads embedded in nano-sized fibers and the prospective application of these microbeads as a drug delivery system.

Characterization of Optimized Ternary PLA/PHB/Organoclay Composites Processed through Fused Filament Fabrication and Injection Molding

The aim of this study was to investigate the structure-properties relationship of ternary blends of polylactide/polyhydroxybutyrate (PLA/PHB)/organo-modified layered silicate (OMLS). Morphological, thermal, rheological, and mechanical characterizations were performed to understand the influence of OMLS on PLA/PHB (70/30 wt%) formulations optimized through modifications with an epoxy-based chain extender, the use of a plasticizer, as well as the influence of the type of processing route: injection molding or fused filament fabrication. The addition of OMLS allowed the blend compatibility to be improved, with the appearance of a single melting peak on DSC thermograms at 146 °C, as well as the reduction in the size of the nodules for the injected molded specimens. Concerning the printed samples, AFM analysis revealed a coalescence of the PHB minor phase due to its degradation. This phenomenon was dramatically enhanced in the presence of OMLS and has been ascribed to the degradation of both the organo-modifier and the PHB minor phase in the blend. Rheological and mechanical tests (17% decrease in Young's modulus and 13% decrease in elongation at break) confirmed this degradation that would have occurred during the manufacturing of the filaments and the printing of specimens due to additional thermal and cooling steps.

Polylactic acid/Gemini surfactant modified clay bio-nanocomposites: Morphological, thermal, mechanical and barrier properties

To improve PLA's properties and overcome its drawbacks such us poor thermal stability, resistance and gas barrier properties, several studies have been performed using different nanofillers. In this work, PLA nanocomposites reinforced by three organoclays, OMt(8-4-8), OMt(10-4-10) and OMt(12-4-12) at various weight percentages (1 and 3 wt%) were prepared by melt mixing using a twin-screw extruder. The organoclays were obtained from sodium montmorillionite and gemini surfactants bearing different hydrophobic chain lengths. The resulting nanocomposites have been characterized in terms of composition and morphology by X-ray diffraction analysis (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The thermal stability and cold crystallization behavior were accessed by thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). The effect of clay composition and concentration on the mechanical and rheological properties of the nanocomposites as well as their water vapor permeability has been also investigated. The resulting nanocomposites exhibit a significantly reduced permeability as compared to unfilled PLA and an improved young modulus and toughness at the detriment of ductility.

Facile Synthesis of Phosphorus and Cobalt Co-Doped Graphitic Carbon Nitride for Fire and Smoke Suppressions of Polylactide Composite

Due to the unique two-dimensional structure and features of graphitic carbon nitride (g-C₃N₄), such as high thermal stability and superior catalytic property, it is considered to be a promising flame retardant nano-additive for polymers. Here, we reported a facile strategy to prepare cobalt/phosphorus co-doped graphitic carbon nitride (Co/P-C₃N₄) by a simple and scalable thermal decomposition method. The structure of Co/P-C₃N₄ was confirmed by scanning electron microscopy (SEM), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). The carbon atoms in g-C₃N₄ were most likely substituted by phosphorous atoms. The thermal stability of polylactide (PLA) composites was increased continuously with increasing the content of Co/P-C₃N₄. In contrast to the g-C₃N₄, the Polylactide (PLA) composites containing Co/P-C₃N₄ exhibited better flame retardant efficiency and smoke suppression. With the addition of 10 wt % Co/P-C₃N₄, the peak heat release rate (PHRR), carbon dioxide (CO₂) production (PCO2P) and carbon oxide (CO) production (PCOP) values of PLA composites decreased by 22.4%, 16.2%, and 38.5%, respectively, compared to those of pure PLA, although the tensile strength of PLA composites had a slightly decrease. The char residues of Co/P-C₃N₄ composites had a more compact and continuous structure with few cracks. These improvements are ascribed to the physical barrier effect, as well as catalytic effects of Co/P-C₃N₄, which inhibit the rapid release of combustible gaseous products and suppression of toxic gases, i.e., CO.

Modern Biodegradable Plastics-Processing and Properties: Part I

This paper presents a characterization of a plastic extrusion process and the selected properties of three biodegradable plastic types, in comparison with LDPE (low-density polyethylene). The four plastics include: LDPE, commercial name Malen E FABS 23-D022; potato starch based plastic (TPS-P), BIOPLAST GF 106/02; corn starch based plastic (TPS-C), BioComp®BF 01HP; and a polylactic acid (polylactide) plastic (PLA), BioComp®BF 7210. Plastic films with determined geometric parameters (thickness of the foil layer and width of the flattened foil sleeve) were produced from these materials (at individually defined processing temperatures), using blown film extrusion, by applying different extrusion screw speeds. The produced plastic films were tested to determine the geometrical features, MFR (melt flow rate), blow-up ratio, draw down ratio, mass flow rate, and exit velocity. The tests were complemented by thermogravimetry, differential scanning calorimetry, and chemical structure analysis. It was found that the biodegradable films were extruded at higher rate and mass flow rate than LDPE; the lowest thermal stability was ascertained for the film samples extruded from TPS-C and TPS-P, and that all tested biodegradable plastics contained polyethylene.

Effect of Organic Modifier and Clay Content on Non-Isothermal Cold Crystallization and Melting Behavior of Polylactide/Organovermiculite Nanocomposites

In clay/polymer nanocomposites, the crystallization behavior and kinetics of the polymer can be affected by the presence of clay, its content and the degree of miscibility between the clay and the polymer matrix. The effect of two different organomodified vermiculites on the non-isothermal cold crystallization and melting behavior of polylactide (PLA) was studied by differential scanning calorimetry (DSC). In the presence of vermiculites, the cold crystallization of PLA occurred earlier, particularly for the highest content of the most miscible organovermiculite with PLA. The cold crystallinity of PLA decreased at low heating rates, notably at high organoclay loadings, and increased at high heating rates, especially at low vermiculite contents. According to the crystallization half-time, crystallization rate coefficient (CRC), and crystallization rate parameter (CRP) approaches, the cold crystallization rate of PLA increased by incorporating vermiculites, with the effect being most noteworthy for the vermiculite showing better compatibility. The Mo model was successful in describing the non-isothermal cold crystallization kinetics of the PLA/vermiculite composites. The melting behavior was affected by the heating rate and the type and content of clay. The nucleating effect of the most compatible clay resulted in the less perfect crystallites. The activation energy was evaluated using the Kissinger and Takhor methods.

High-Strength, Low-Permeable, and Light-Protective Nanocomposite Films Based on a Hybrid Nanopigment and Biodegradable PLA for Food Packaging Applications

Herein, a multifunctional filler, a dye (organic)-clay (inorganic) hybrid nanopigment (DCNP), was embedded into polylactic acid (PLA) to fabricate a colored biodegradable/biocompatible film, which is explored as a high thermomechanical resistant as well as a superior light and mass transport barrier for food packaging applications. In this respect, the DCNP was synthesized through a wet chemical process with a reaction yield of 76% and incorporated into a PLA matrix at various concentrations using a solution casting method. The morphological characterizations revealed partially intercalated/exfoliated structure for PLA-DCNP films. The samples filled with DCNP showed up to 20% and 12 °C improvements in storage modulus (E') and glass transition temperature (T g), respectively, compared to those with neat PLA. Incorporation of a small amount of DCNP into the PLA matrix significantly declined the water vapor and gas permeability of PLA by 36 and 54%, respectively. The optical property investigations indicated remarkable color characteristics and light protection performance of the samples containing DCNP. The results also showed better performance of the PLA-DCNP film compared to that of the PLA-Cloisite 20A sample at an identical filler loading. In conclusion, the desirable properties of the resulting films highlighted the potential application of such nanocomposite films as a promising option for food packaging materials.

The Strengthening and Toughening of Biodegradable Poly (Lactic Acid) Using the SiO2-PBA Core-Shell Nanoparticle

The balance of strengthening and toughening of poly (lactic acid) (PLA) has been an intractable challenge of PLA nanocomposite development for many years. In this paper, core-shell nanoparticles consisting of a silica rigid core and poly (butyl acrylate) (PBA) flexible shell were incorporated to achieve the simultaneous enhancement of the strength and toughness of PLA. The effect of core-shell nanoparticles on the tensile, flexural and Charpy impact properties of PLA nanocomposite were experimentally investigated. Scanning electron microscopy (SEM) and small-angle X-ray scattering (SAXS) measurements were performed to investigate the toughening mechanisms of nanocomposites. The experimental results showed that the addition of core-shell nanoparticles can improve the stiffness and strength of PLA. Meanwhile, its elongation at break, tensile toughness and impact resistance were enhanced simultaneously. These observations can be attributed to the cavitation of the flexible shell in core-shell nanoparticles and the resultant shear yielding of the matrix. In addition, a three-dimensional finite element model was also proposed to illustrate the damage processes of core-shell nanoparticle-reinforced polymer composites. It was found that, compared with the experimental performance, the proposed micromechanical model is favorable to illustrate the mechanical behavior of nanocomposites.

Nucleating and Plasticization Effects in Drawn Poly(Lactic Acid) Fiber during Accelerated Weathering Degradation

Changes in the polymer properties of poly(lactic acid) (PLA) fibers during drawing and degradation processes were analyzed using solid-state NMR, with the goal of elucidating morphological changes that influence fiber tensile properties. Combination of X-ray diffraction (XRD) and differential scanning calorimeter (DSC) indicated that the drawn PLA fibers consisted of different proportions of α crystalline and amorphous forms. ¹³C CP-MAS NMR spectra showed amorphous-like broad singlet signals, of which the full width at half maximum (FWHM) decreased with increasing crystallinity and crystal orientation. The T₁H value decreased by interaction with additives and increased with increasing crystal orientation. The interaction with additives also reduced T₁C values, which increased with increasing crystallinity. Use of organic clay enhanced the crystallization of high draw-ratio PLA fibers due to nucleation, which increased tensile strength; this effect gradually decreased with time during accelerated weathering. In contrast, the plasticization due to the addition of flexible polymers increased fiber elongation, which rapidly dropped during the degradation. Changes of FWHM, T₁H, and T₁C values indicated that the degradation occurred at sites within the amorphous portions of the PLA fibers containing organic clay, while the flexible polymers were preferentially degraded if they were present in the PLA fibers.

Bio-Based Polymers with Potential for Biodegradability

A variety of renewable starting materials, such as sugars and polysaccharides, vegetable oils, lignin, pine resin derivatives, and proteins, have so far been investigated for the preparation of bio-based polymers. Among the various sources of bio-based feedstock, vegetable oils are one of the most widely used starting materials in the polymer industry due to their easy availability, low toxicity, and relative low cost. Another bio-based plastic of great interest is poly(lactic acid) (PLA), widely used in multiple commercial applications nowadays. There is an intrinsic expectation that bio-based polymers are also biodegradable, but in reality there is no guarantee that polymers prepared from biorenewable feedstock exhibit significant or relevant biodegradability. Biodegradability studies are therefore crucial in order to assess the long-term environmental impact of such materials. This review presents a brief overview of the different classes of bio-based polymers, with a strong focus on vegetable oil-derived resins and PLA. An entire section is dedicated to a discussion of the literature addressing the biodegradability of bio-based polymers.

Thermal behavior of modified poly(L-lactic acid): effect of aromatic multiamide derivative based on 1H-benzotriazole

The goal of this work was to synthesis a novel aromatic multiamide derivative based on 1H-benzotriazole (PB) as an organic nucleating agent for poly(L-lactic acid) (PLLA), and investigate the effect of PB on the non-isothermal crystallization, melting behavior and thermal decomposition of PLLA. Here, PB was firstly synthesized through 1H-benzotriazole aceto-hydrazide and terephthaloyl chloride, then PB-nucleated PLLA was fabricated via melt-blending technology at various PB concentration from 0.5 wt% to 3 wt%. Finally, the thermal performances were evaluated through differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). The high thermal decomposition temperature of PB indicated that PB possessed possibility as a nucleating agent for PLLA, and the non-isothermal crystallization behavior confirmed the crystallization accelerating effectiveness of PB for PLLA. Upon optimum concentration of 2 wt%, the onset crystallization temperature, the crystallization peak temperature and the non-isothermal crystallization enthalpy increased from 101.4°C, 94.5°C and 0.1 J·g-1 to 121.3°C, 115.8°C and 35.1 J·g-1, respectively. In addition, the non-isothermal crystallization behavior was also affected by the cooling rate and the final melting temperature. The melting behavior further evidenced the advanced nucleating ability of PB, and the competitive relationship between PB and the heating rate, the nuclear rate and crystal growth rate. Thermal stability measurement showed that PB with a concentration of 1 wt%–2 wt% could slightly improve the thermal stability of PLLA.

Biodegradation assessment of PLA and its nanocomposites

Poly(lactic acid) nanocomposites containing Cloisite 15A, Cloisite 30B, and Dellite 43B were prepared by melt-mixing in a batch mixer and were exposed to UV radiation, temperature, and microorganism in solution and in a compost. Exposed samples, collected along the time, were characterized by several techniques. While the addition of organoclays had a positive effect on thermal stability, the degradation rate of nanocomposites increased when exposed to UV radiation and microorganism. Moreover, the degradation rate depends on the organoclay type. Even though the degradation rate is higher for nanocomposites, Fourier transform infrared spectrometry and gel permeation chromatography results demonstrated that the degradation mechanism is the same.