In-situ self-reinforcement of amorphous polylactide (PLA) through induced crystallites network and its highly ductile and toughened PLA/poly(butylene adipate-co-terephthalate) (PBAT) blends.

Crystallites of a semicrystalline polylactide (cPLA) were induced in an amorphous PLA (aPLA) and its blends with poly(butylene adipate-co-terephthalate) (PBAT) to achieve in-situ self-reinforced PLA based structures. The approach involved the melt blending of cPLA as a minor phase with aPLA and its blends with PBAT at processing temperatures below the crystal melting peak of cPLA. An injection molding (IM) process was first adopted to obtain self-reinforced PLA (SR-PLA) structures at aPLA/cPLA weight ratios of 100/0, 95/5, 90/10, 85/15, and 80/20. IM barrel and mold temperatures revealed crucial impacts on preserving the cPLA crystallites and thereby enhancing the final mechanical performance of SR-PLA (i.e., aPLA/cPLA) samples. SR-PLA samples at various aPLA/cPLA weight ratios of 100/0, 90/10, 80/20, and 70/30 were then melt blended with PBAT to produce SR-PLA/PBAT at a given ratio of 85/15. These blends were first prepared in an internal melt mixer (MM) to evaluate the rheological properties. The rheological analysis confirmed the significance of cPLA reinforcing efficiency within SR-PLA and its corresponding blends with PBAT. Similar SR-PLA/PBAT blends were also prepared using the IM process to explore their thermal and mechanical characteristics. The effect of cPLA concentrations in blends was distinctive, leading to significant enhancements in stain at break and toughness values. This was due to the increased crystallite network within the matrix, further refining PBAT droplets. Morphological analysis of the melt-processed blends through MM and IM also revealed that the PBAT droplets were further refined when the IM process was applied. The induced shear during the molding could have further elongated the cPLA crystallites towards a fiberlike structure, which could additionally cause the matrix viscosity to increase and refine the PBAT droplets.

Tuning the Structure-Property Relationships in Binary and Ternary Blends of PLA/PBAT/PHBH.

While the brittle polylactide (PLA) has a high durability among bioplastics, poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) (PHBH) with certain ductility exhibits facile compostability. The addition of polybutylene adipate terephthalate (PBAT) may also be used to improve the ductility and toughness of brittle bioplastics. Binary and ternary blends of PLA/PBAT/PHBH based on either PLA or PHBH as the matrix have been manufactured using a twin-screw extruder. The melt rheological, mechanical, and morphological properties of the processed samples were examined. Binary blends of PLA/PHBH show superior strength, with the PLA75/PHBH25 blend exhibiting a tensile strength of 35.2 ± 3.0 MPa, which may be attributed to miscible-like morphology. In contrast, blends of PLA with PBAT demonstrate low strength, with the PLA50/PBAT50 blend exhibits a tensile strength of 9.5 ± 2.0 MPa due to the presence of large droplets in the matrix. PBAT-containing blends exhibit lower impact strengths compared to PHBH-containing blends. For instance, a PLA75/PBAT25 blend displays an impact strength of 1.76 ± 0.1 kJ/m2, whereas the PHBH75/PBAT25 blend displays an impact strength of 2.61 ± 0.3 kJ/m2, which may be attributed to uniformly dispersed PBAT droplets.

Poly(glycidyl methacrylate) modified cellulose nanocrystals and their PBAT-based nanocomposites.

Melt processing of cellulose nanocrystals (CNCs) reinforced nanocomposites is still a serious challenge due to the hydrophilic nature of CNCs and their severe agglomeration tendency within the polymer melt. In this study, chemical modification of CNC through grafting poly(glycidyl methacrylate) (PGMA) with various degrees was implemented. Wettability of the modified CNCs (mCNCs) were controlled and their structure was characterized through Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), optical microscopy, X-ray diffraction (XRD), thermogravimetric analysis (TGA), and differential scanning calorimetry (DSC). The nanocomposites of polybutylene adipate terephthalate (PBAT) with 3 wt% CNC and mCNC were prepared using an internal melt mixer. To differentiate the effects of CNC and PGMA molecules on the final properties of nanocomposites, PBAT/PGMA compounds were separately prepared. To confirm the chain characterization and molecular weight of the synthesized PGMAs, 1H NMR and gel permeation chromatography (GPC) analysis were conducted. Melt rheological analysis, dynamic mechanical analysis (DMA), DSC, and atomic force microscopy (AFM) were used to monitor the mCNC dispersion quality and the effect of PGMA modification in PBAT compounds. The results revealed that grafting CNC with longer PGMA considerably improved the CNCs' dispersion quality within PBAT. Such dispersion enhancement of long-chain mCNCs and interfacial interaction of PGMA and PBAT resulted in a noticeable increase in storage modulus and complex viscosity of the final nanocomposites.

Synergistic Enhancement of Flame Retardancy Behavior of Glass-Fiber Reinforced Polylactide Composites through Using Phosphorus-Based Flame Retardants and Chain Modifiers.

Flame retardancy properties of neat PLA can be improved with different phosphorus-based flame retardants (FRs), however, developing flame retardant PLA-based engineering composites with maintained mechanical performance is still a challenge. This study proposes symbiosis approaches to enhance the flame retardancy behavior of polylactide (PLA) composites with 20 wt% short glass fibers (GF). This was first implemented by exploring the effects of various phosphorus-based FRs up to 5 wt% in neat PLA samples. Among the used phosphorus-based FRs, the use of only 3 wt% of diphosphoric acid-based FR (P/N), melamine coated ammonium polyphosphate (APPcoated), and APP with melamine synergist (APP/Mel) resulted in achieving the V0 value in a vertical burning test in the neat PLA samples. In addition to their superior efficiency in improving the flame retardancy of neat PLA, P/N had the least negative effect on the final mechanical performance of PLA samples. When incorporated in PLA composites with 20 wt% GF, however, even with the use of 30 wt% P/N, the V0 value could not be obtained due to the candlewick effect. To resolve this issue, the synergistic effect of P/N and aromatic polycarbodiimide (PCDI) cross-linker or Joncryl epoxy-based chain-extender (CE) on the flame retardancy characteristics of composites was examined. Due to the further chain modification, which also enhances the melt strength of PLA, the dripping of composites in the vertical burning test terminated and the V0 value could be reached when using only 1 wt% PCDI or CE. According to the scanning electron microscopic analysis, the use of noted chain modifiers further homogenized the distribution and refined the particle size of P/N within the PLA matrix. Hence this could synergistically contribute to the enhancements of the fire resistance performance of the PLA composites. Such incorporation of P/N and chain modifiers further leads to the enhancement of the mechanical performance of PLA composites and hence the resultant product can be proposed as a promising durable bioplastic engineering product where fire risk exists.

Foam 3D Printing of Thermoplastics: A Symbiosis of Additive Manufacturing and Foaming Technology.

Due to their light-weight and cost-effectiveness, cellular thermoplastic foams are considered as important engineering materials. On the other hand, additive manufacturing or 3D printing is one of the emerging and fastest growing manufacturing technologies due to its advantages such as design freedom and tool-less production. Nowadays, 3D printing of polymer compounds is mostly limited to manufacturing of solid parts. In this context, a merged foaming and printing technology can introduce a great alternative for the currently used foam manufacturing technologies such as foam injection molding. This perspective review article tackles the attempts taken toward initiating this novel technology to simultaneously foam and print thermoplastics. After explaining the basics of polymer foaming and additive manufacturing, this article classifies different attempts that have been made toward generating foamed printed structures while highlighting their challenges. These attempts are clustered into 1) architected porous structures, 2) syntactic foaming, 3) post-foaming of printed parts, and eventually 4) printing of blowing agents saturated filaments. Among these, the latest approach is the most practical route although it has not been thoroughly studied yet. A filament free approach that can be introduced as a potential strategy to unlock the difficulties to produce printed foam structures is also proposed.

Entirely environment-friendly polylactide composites with outstanding heat resistance and superior mechanical performance fabricated by spunbond technology: Exploring the role of nanofibrillated stereocomplex polylactide crystals.

This study aims at investigating the manufacturing and characterization of all-polylactide composites prepared by melt spunbond spinning technology. To do so, a series of asymmetric stereocomplex polylactide (SC-PLA) blends (PLLA 95 wt%/PDLA 5 wt%) was melt spun. To examine the impact of molecular structure of PDLA, the blends of linear PLLA, and low and high molecular weight as well as branched PDLAs, were subjected to a single step spunbond process. DSC thermograms of the samples showed two melting temperatures at around 170 °C and 210 °C, which were attributed to the melting of homo and stereocomplex crystals, respectively. The samples were spun at 190 °C, between the homo and stereocomplex crystals' melting temperatures, and at 230 °C, above the stereocomplex crystals' melting temperature. Morphology images showed the formation of fibers in the range of 40-50 µm. Shear rheological measurements revealed that the spun SC-PLA samples had a substantially higher viscosity and storage modulus in the low frequency region, and higher shear thinning behavior, compared to the non-spun samples. Extensional rheology measurements also showed that the spun samples demonstrated strain hardening behavior. Substantial enhancement of rheological properties was noted for the samples containing the branched and high molecular weight PDLA spun at 230 °C. After etching, the spun samples at 190 °C exhibited small spherical crystals with diameters in the range of 80-90 nm, whereas comparatively thin fibers in the size range of 60-70 nm were observed for the samples spun at 230 °C. Remarkable enhancements up to 100% and 60% was noted for the tensile modulus and strength, respectively, of the spun SC-PLA samples. The spun fibers also demonstrated a considerable reduction in boiling water and hot air shrinkage. The distinctive role of nanofibrillated stereocomplex crystals as a rheology modifier and a crystallization nucleating agent makes PLA more sustainable and paves the way for the fabricated all-PLA composites in applications requiring high heat resistance and superior mechanical performance. The present study unequivocally indicates a huge potential for the sustainable entirely all-PLA products manufactured by fiber in fiber and, indeed, unfolds unknown opportunities for PLA-based merchandises in future.

Effect of solvent type on the dispersion quality of spray-and freeze-dried CNCs in PLA through rheological analysis.

Polylactide (PLA) nanocomposites with spray-and freeze-dried cellulose nanocrystals (i.e., SCNC and FCNC) were prepared through solution casting using four different solvents: tetrahydrofuran (THF), chloroform (CHL), dimethylformamide (DMF), and dimethyl sulfoxide (DMSO). Small amplitude oscillatory shear rheological analysis was extensively employed to explore the CNC dispersion quality in PLA. Overall, the rheological properties differences of PLA/SCNC and PLA/FCNC nanocomposites were not very significant. Moreover, the use of THF and CHL did not lead to a proper dispersion of CNCs in PLA due to their low dielectric constants. On the other hand, while the use of DMF was effective on the enhancement of CNC dispersion, DMSO could more dramatically lead to such enhancement due to its higher dielectric constant. The percolation threshold in PLA/SCNC nanocomposites prepared with DMF and DMSO was predicted around 1.52 and 0.12 wt% CNC, respectively. The crystallization behavior of PLA/nanocomposites prepared with DMF and DMSO were also explored.

Effects of molecular weight and crystallizability of polylactide on the cellulose nanocrystal dispersion quality in their nanocomposites.

This study investigated how cellulose nanocrystals (CNC) dispersion quality and its percolation network formation could be influenced when using polylactide (PLA) with various molecular weights and crystallizability. In this context, systematic rheological experiments were conducted on PLA/CNC nanocomposites prepared through solution casting method using dimethylformamide (DMF) as the solvent. It was found that lower CNC percolation concentrations could be obtained when a PLA matrix possesses lower molecular weight as the shorter chains and CNCs interpenetration could be facilitated during their dissolution in the solvent. On the other hand, the CNC percolation concentration was further lowered when the PLA with higher crystallizability was used. During the solvent evaporation step that occurred at 85 °C, the isothermal heterogeneous crystallization of PLA around the dispersed CNCs could prevent the driving force of the CNCs towards their re-agglomeration. Therefore, the finest CNC dispersion was appeared in the highly crystallizable low molecular weight PLA through which the rheological properties were dramatically improved and the thermal stability was significantly extended to higher temperatures. The crystallization behavior of the prepared nanocomposites was also analyzed using differential scanning calorimeter and X-ray diffractometer. The thermal degradation behavior of the PLA/CNC nanocomposites were examined through thermogravimetric and rheological analysis.

Polylactide cellulose-based nanocomposites.

Among biopolymers, polylactide (PLA) is considered as the most appropriate substitute for the petroleum-based polymers which are widely used in various commodity and engineering applications. PLA, however, also suffers from series of shortcomings such as slow crystallization rate and low melt strength which result in poor processability, formability and foamability which substantially limit its production and usage. Recently, the use of biobased/biodegradable cellulose nanoparticles such as cellulose nanocrystals (CNC), cellulose nanofibers (CNF), and bacterial nanocellulose (BC) have been proposed to manufacture fully green PLA-based biocomposites while they could resolve some of the noted drawbacks of PLA. However, due to their high hydrophilicity and the presence of hydrogen bonding cellulose nanoparticles are not compatible with hydrophobic polymers. Therefore, the dispersion of these nanoparticles in thermoplastics still remains as the main challenge to process/develop their nanocomposites. This article reviews the studies conducted on these challenges of developing PLA cellulose-based nanocomposites including the difficulties of their processing and possible enhancements of their rheological, thermal, and mechanical properties. The investigations that have been conducted on PLA-CNC, PLA-CNF, and PLA-BC nanocomposites are separately discussed in this review article, while the studies on the development of PLA-nanocellulose blend nanocomposites and PLA-nanocellulose microcellular foams are also highlighted.

Poly (lactic acid) blends: Processing, properties and applications.

Poly (lactic acid) or polylactide (PLA) is a commercial biobased, biodegradable, biocompatible, compostable and non-toxic polymer that has competitive material and processing costs and desirable mechanical properties. Thereby, it can be considered favorably for biomedical applications and as the most promising substitute for petroleum-based polymers in a wide range of commodity and engineering applications. However, PLA has some significant shortcomings such as low melt strength, slow crystallization rate, poor processability, high brittleness, low toughness, and low service temperature, which limit its applications. To overcome these limitations, blending PLA with other polymers is an inexpensive approach that could also tailor the final properties of PLA-based products. During the last two decades, researchers investigated the synthesis, processing, properties, and development of various PLA-based blend systems including miscible blends of poly l-lactide (PLLA) and poly d-lactide (PDLA), which generate stereocomplex crystals, binary immiscible/miscible blends of PLA with other thermoplastics, multifunctional ternary blends using a third polymer or fillers such as nanoparticles, as well as PLA-based blend foam systems. This article reviews all these investigations and compares the syntheses/processing-morphology-properties interrelationships in PLA-based blends developed so far for various applications.

Lightweight polypropylene/stainless-steel fiber composite foams with low percolation for efficient electromagnetic interference shielding.

Lightweight polypropylene/stainless-steel fiber (PP-SSF) composites with 15-35% density reduction were fabricated using foam injection molding. The electrical percolation threshold, through-plane electrical conductivity, and electromagnetic interference (EMI) shielding effectiveness (SE) of the PP-SSF composite foams were characterized and compared against the solid counterparts. With 3 wt % CO2 dissolved in PP as a temporary plasticizer and lubricant, the fiber breakage was significantly decreased during injection molding, and well-dispersed fibers with unprecedentedly large aspect ratios of over 100 were achieved. The percolation threshold was dramatically decreased from 0.85 to 0.21 vol %, accounting for 75% reduction, which is highly superior, compared to 28% reduction of the previous PP-carbon fiber composite foam.1 Unlike the case of carbon fiber,1 SSFs were much longer than the cell size, and the percolation threshold reduction of PP-SSF composite foams was thus primarily governed by the decreased fiber breakage instead of fiber orientation. The specific EMI SE was also significantly enhanced. A maximum specific EMI SE of 75 dB·g(-1)·cm(3) was achieved in PP-1.1 vol % SSF composite foams, which was much higher than that of the solid counterpart. Also, the relationships between the microstructure and properties were discussed. The mechanism of EMI shielding enhancement was also studied.