PLA Bead Research Articles

Enhancing foamability and flame‐retardancy of polylactic acid bead foams through inter‐beads bulk polymerization and continuous phase dispersion of LDHs

AbstractPoly (lactic acid) (PLA) bead foam has a promising application because of its renewable and naturally degradable nature. However, its processing is greatly limited by inherent shortcomings such as the complex polycrystals‐inducing strategy. Herein, we developed a strategy of bulk polymerization reaction of polyvinyl acetate (PVAc) to sinter PLA beads to prepare EPLA foams accompanied by supercritical CO2 foaming technology. In order to enhance the sintering behavior and flame‐retardancy of EPLA foams, two‐dimensional nanolayered double hydroxides (LDHs) were introduced and dispersed in the continuous phase of the sintering layers. The formation of unique dispersion of LDHs and sintering structure of PVAc generated substantial increase in the crystallinity, melt elasticity, and sintering strength of EPLA foams, which facilitated the growth and stabilization of cells. Thus, the cell‐density and expansion ratio could be increased to 8.62 × 106 cell/cm3 to 9.31‐fold, respectively. Moreover, the mechanical properties of the EPLA foams were improved. The tensile strength and the compression strength increased to 2.96 and 62.5 MPa. Additionally, with adding 7 wt.% LDH, the EPLA foam reached UL‐94 V‐0 rating with high limiting oxygen index value of 29.1% and char residue of 20.4%. This study provides a novel strategy for the preparation of flame‐retardant EPLA foams with low density, three dimensional complex shapes, as well as excellent mechanical properties.

Investigation of morphology and structure of drug-loaded PLA-b-PEO-b-PLA polymeric micelle: A dissipative particle dynamics simulations study.

The dissipative particle dynamics (DPD) simulation was used to study the morphologies and structures of the paclitaxel-loaded PLA-b-PEO-b-PLA polymeric micelle. We focused on the influences of PLA block length, PLA-b-PEO-b-PLA copolymer concentration, paclitaxel drug content on morphologies and structures of the micelle. Our simulations show that: (i) with the PLA block length increase, the self-assemble structure of PLA-b-PEO-b-PLA copolymers with paclitaxel vary between onion-like structure (core-middle layer-shell) to spherical core-shell structure. The PEO shell thins and the size of the PLA core increases. The onionlike structures are comprised of the PEO hydrophilic core, the PLA hydrophobic middle layer, and the PEO hydrophilic shell, the distribution of the paclitaxel drug predominantly occurs within the hydrophobic intermediate layer; (ii) The system forms a spherical core-shell structure when a small amount of the drug is added, and within a certain range, the size of the spherical structure increases as the drug amount increases. When the drug contents (volume fraction) cdrug = 10%, it can be observed that the PLA4-b-PEO19-b-PLA4 spherical structures connect to form rod-shaped structures. With the length of PLA block NPLA = 8, as the paclitaxel drug concentrations cdrug = 4%, PEO has been insufficient to completely encapsulate the PLA and paclitaxel drug beads. To enhance drug loading capacity while maintaining stability of the system in aqueous solution, the optimal composition for loading paclitaxel is PLA4-b-PEO19-b-PLA4; the drug content is not higher than 4%; (iii) The paclitaxel-loaded PLA4-b-PEO19-b-PLA4 micelle undergo the transition from onionlike (core-middle layer-shell) to spherical (core-shell) to rod-shaped and lamellar structure as the PLA4-b-PEO19-b-PLA4 copolymer concentration increases from ccp = 10% to 40%.

Extruded polylactide bead foams using anhydrous supercritical CO2 extrusion foaming

AbstractPolylactic acid (PLA) is a bio‐based and biodegradable polymer. It is challenging to prepare PLA bead foams at a large scale in an efficient manner. Herein, beads of modified PLA are foamed by one‐step anhydrous supercritical CO2 extrusion. This dramatically shortens the production time, prevents the PLA from pyrohydrolysis, and avoids wastewater generation. The melt strength of the PLA is increased from 0.8 to 16.7 cN using a chain extender. The morphologies and microstructures of bead foams are analyzed. In the case of the PLA bead foam obtained by extrusion foaming combined with wind‐cooling pelletizing technology, the expansion ratio is higher than 13, the pore morphology is typical cellular structures, and the surface is glossy. The advantages and shortcomings of the PLA beads are also investigated. It is beneficial to promote the continuous preparation of PLA bead foams and other polymer beads like PS, and PP, which can be used in the fields of green packing, sound absorption, cold‐chain logistics, and so forth.

A facile and clean strategy to manufacture functional polylactic acid bead foams

Expanded poly (lactic acid) (EPLA) is a promising bio-material with ultra-low density, thermal insulation, cushioning properties, as well as the ability to be fabricated products in complex shapes. However, the time consuming of inducing double melt peaks of PLA and the complex two-steps process of steam-sintering assisted supercritical CO2 foaming is inevitable since PLA belongs crystalline polymer. To develop a facile and simple method to produce EPLA foams with functions, we used the bulk polymerization of liquid reactants to form a sintering layer between PLA beads. Meanwhile, carbon nanotubes (CNT) were synchronously introduced and distributed in the sintering layer. And a unique EPLA foam with functional sintering layer was produced using supercritical CO2 foaming technology. The reported method could prepare EPLA foam with high expansion ratio of 8.5 times and high cell density of 8.25 × 108 cell/cm3. Compared with other methods, the novel EPLA foams exhibited excellent tensile strength and compressive strength, superior electrical conductivity of 0.12 S/m, electromagnetic shielding ability of 99% in the X-band at CNT content of 2.68 wt.%, as well as sensitive and accurate stress-sensing function. It facilitates the development of novel manufacturing technology and the expansion of applications fields of EPLA foam.

Converging orifice used to control the discharge rate of spherical particles from a flat floor silo

The effect of the converging orifice geometry in a model silo on the discharge rate of monosized spherical particles was studied experimentally and numerically. The cylindrical container was equipped with interchangeable inserts with converging discharge orifices of various upper diameters in the upper base and a constant lower diameter in the lower base. Plastic PLA beads and agricultural granular materials: wheat, rapeseeds, and linseeds were tested. A series of discrete element method simulations corresponding to the performed experiments was conducted with a largely extended set of experimental discharge conditions. In the case of the constant thickness of the insert, the discharge rate initially increased with an increase in the half cone angle of the converging orifice and then the tendency reversed. In the majority of cases, the discharge rate through the converging orifice was higher than through the hopper with the same orifice diameter.

Intrinsic flame retardancy of poly(lactic acid) bead foams

AbstractLinear economy models are no longer acceptable for the plastic industry and a change to a sustainable circular plastic economy must take place. In the field of thermoplastic foams, the biopolymer poly(lactic acid) (PLA) is a suitable alternative for fossil based foams like polystyrene. However, the production of PLA bead foams is still a challenge. In regards to circular plastic economy products, a reduction of the needed polymer additives is aspired to simplify the union of end‐of‐life plastic streams. Flame retardants (FR) are required in many applications and are often the largest proportion of additives. It turns out that the removal of FRs, for example the removal of the REACh registered FR hexabromocyclododecane for polystyrene foams, requires a great effort. This paper shows that PLA bead foams require no FR to achieve a class E classification for construction products. Therefore, nine PLA types are analyzed by differential scanning calorimetry, thermogravimetric analysis, gel permeation chromatography, and rheotens measurements. From five types, PLA bead foams could be produced with two different densities. In addition, PLA bead foams containing 1.5 wt% alkoxy amine FR were produced. The flammability of the PLA bead foams was investigated by LOI, DIN‐4102‐1‐B2, and cone calorimeter tests.

Superhydrophobic hierarchical fiber/bead composite membranes for efficient treatment of burns

One of the current challenges in burn wound care is the development of multifunctional dressings that can protect the wound from bacteria or organisms and promote skin regeneration and tissue reconstitution. To this end, we report the design and fabrication of a composite electrospun membrane, comprised of electrospun polylactide: poly(vinyl pyrrolidone)/polylactide: poly(ethylene glycol) (PLA:PVP/PLA:PEG) core/shell fibers loaded with bioactive agents, as a functionally integrated wound dressing for efficient burns treatment. Different mass ratios of PLA:PVP in the shell were screened to optimize mechanical, physicochemical, and biological properties, in addition to controlled release profiles of loaded antimicrobial peptides (AMPs) from the fibers for desirable antibacterial activity. Fibroblasts were shown to readily adhere and proliferate when cultured on the membrane, indicating good in vitro cytocompatibility. The introduction of PLA beads by electrospraying on one side of the membrane resulted in biomimetic micro-nanostructures similar to those of lotus leaves. This designer structure rendered the composite membranes with superhydrophobic property to inhibit the adhesion/spreading of exogenous bacteria and other microbes. The administration of the resulting composite fibrous membrane on burnt skin in an infected rat model led to faster healing than a conventional product (sterile silicone membrane) and control detailed herein. These composite fibrous membranes loaded with bioactive drugs provide an integrated strategy for promoting burn wound healing and skin regeneration. Statement of SignificanceTo address an urgent need in complex clinical requirements on developing a new generation of wound dressings with integrated functionalities. This article reports research work on a hierarchical fiber/bead composite membranes design, which combines a lotus-leaf-like superhydrophobic surface with drugs preloaded in the core and shell of fibers for effective burn treatment. This demonstrates a balance between simplified preparation processes and increased multifunctionality of the wound dressings. The creation of hierarchically structured surfaces can be readily achieved by electrospinning, and the composite dressings possessed a considerable mechanical strength, effective wound exudate absorption and permeability, good biocompatibility, broad antibacterial ability and promoting wound healing etc. Thus, our work unveils a promising strategy for the development of functionally integrated wound dressings for burn wound care.

Effect of stereocomplex crystal on foaming behavior and sintering of poly(lactic acid) bead foams

AbstractPoly(lactic acid) (PLA) bead foams with stereocomplex (Sc)/α crystals were prepared by melt mixing and solid‐state foaming methods, independently. A systematic method was applied to evaluate the effect of Sc/α crystals on rheological properties and foaming behavior of PLA. The results indicated that the presence of Sc/α crystals affected the foaming behavior and the melt elasticity of PLA. Hence, the enhanced rheological properties of PLA had a significant effect on controlling the foaming behavior. As a result, PLA bead foam with an expansion ratio of 24‐fold was developed. And, the presence of Sc/α crystals could also facilitate the sintering behavior and broaden the sintering process window. Sintered PLA bead foam with finer cellular morphology and strong sintering effect was obtained by inducing an appropriate Sc/α crystalline structure. © 2018 Society of Chemical Industry

Molecular simulation and experimental studies of the miscibility of polylactic acid/polyethylene glycol blends

Computer simulation and experiments were performed to investigate the miscibility of PLA/PEG blends with different PEG concentrations. Flory-Huggins interaction (χ) parameter used to predict the miscibility for the blends was estimated by molecular dynamic simulation of fully atomistic model. The calculated χ parameter and radial distribution function suggest that the PLA and PEG blends are likely miscible at low PEG concentrations (10–30 wt%), but they become apparently immiscible at higher PEG content (>50 wt%). This result is consistent with density distribution of PLA and PEG beads calculated from dissipative particle dynamics simulation of coarse-grained model. To support the computational results, experiments based on differential scanning calorimetry (DSC) and rheometry were also performed. The DSC thermograms of 90:10, 80:20, and 70:30 (wt/wt) of PLA/PEG blends showed a single glass transition and PLA melting peak, indicating PLA/PEG is miscible over this composition. In rheometry, frequency (ω) dependence of storage moduli (G′) at low frequencies for 75:25 and 70:30 blends indicate that these samples are near the phase separation point.

Fundamental Influences of Induced Crystallization and Phase Separation on the Foaming Behavior of Poly(lactic acid)/Polyethylene Glycol Blends Blown with Compressed CO2

A sinterable poly(lactic acid) (PLA) foam is critical to the creation of strong interface bonding for the production of large-size PLA bead foam products. To fabricate such a foam, we blended a polyethylene glycol (PEG) that has a low melting temperature and high CO2 solubility with PLA. Solid-state foaming technology was used to foam the PLA/PEG blends, and CO2 was the blowing agent. The PEG was miscible with the PLA and enhanced the crystallization of PLA at a loading of lower than 10 wt % under a CO2 saturation process. Consequently, the PLA/PEG blends did not foam well because of the matrix’s improved stiffness. The CO2 saturation process induced the PEG’s phase separation because its content was higher than 20 wt %. This weakened the PEG’s plasticization effect on the PLA, but it significantly improved the PLA’s foaming behavior. This was characterized by both the foam expansion ratio, which increased from 4.6 to 11.8–24.1, and the decreased cell size. We discuss the effects of crystallization and th...

Measuring adhesion forces between model polysaccharide films and PLA bead to mimic molecular interactions in flax/PLA biocomposite

Natural fiber-reinforced polymers or biocomposites are becoming increasingly popular as an environment friendly alternative to traditional glass fiber-reinforced thermoplastics. The mechanical properties of reinforced biocomposites, such as flax/polylactic acid (PLA), are largely governed by the level of interfacial interactions between the two constituents apart from their intrinsic properties. The hierarchical organization of various polysaccharides present in natural fibers results in complex mechanisms at the interface which are still poorly understood and difficult to analyze through a traditional approach that rely on indirect assessments. The possibility of measuring direct adhesion force between individual particles using the colloidal force microscopy has been exploited here by developing an experimental set-up in which a micrometer colloidal PLA bead is brought into close contact with molecularly smooth polysaccharide surfaces that mimic the main constituents of flax fibers, cellulose, hemicellulose, and pectins. Adhesion force measurements performed under ambient and low relative humidity conditions indicate that cellulose/PLA is the weakest interface in the biocomposite. Moreover, the results emphasize the important role of water molecules for the more hydrophilic polymers in flax fibers that takes place in the fundamental forces involved in the adhesion phenomena at the biocomposite interface.

Expanded polylactic acid - an eco-friendly alternative to polystyrene foam

A commercially viable process to manufacture low-density expanded polylactic acid (E-PLA) products with environmental-friendly blowing agents such as carbon dioxide has been developed. The resulting material is a sustainable alternative to expanded polystyrene (EPS). The E-PLA has thermal conductivity and mechanical properties comparable to EPS and can be used in many similar applications. The process involves impregnation of PLA beads with carbon dioxide and fusing with heat, including steam heating. Gel permeation chromatography indicated that the PLA polymer molecular weight was not degraded by the foaming or steam heating process. Visual characterization showed that under some circumstances, PLA precursor beads exhibited voids, which disappeared during CO2 impregnation indicating some form of rearrangement or realignment of the polymer morphology or structure. Due to the effect of CO2 on the glass transition temperature of the PLA, a linear reduction of 5.5°C per wt% of absorbed CO2 was observed.

Interfacing Sca‐1pos Mesenchymal Stem Cells with Biocompatible Scaffolds with Different Chemical Composition and Geometry

An immortalized murine mesenchymal stem cell line (mTERT-MSC) enriched for Linneg/Sca-1pos fraction has been obtained through the transfection of MSC with murine TERT and single-cell isolation. Such cell line maintained the typical MSC self-renewal capacity and continuously expressed MSC phenotype. Moreover, mTERT-MSC retained the functional features of freshly isolated MSC in culture without evidence of senescence or spontaneous differentiation events. Thus, mTERT-MSC have been cultured onto PLA films, 30 and 100 μm PLA microbeads, and onto unpressed and pressed HYAFF-11 scaffolds. While the cells adhered preserving their morphology on PLA films, clusters of mTERT-MSC were detected on PLA beads and unpressed fibrous scaffolds. Finally, mTERT-MSC were not able to colonize the inner layers of pressed HYAFF-11. Nevertheless, such cell line displayed the ability to preserve Sca-1 expression and to retain multilineage potential when appropriately stimulated on all the scaffolds tested.

In Vivo Evaluation of Antibiotic Impregnated Beads in a Rat Osteomyelitis Model

The biocompatibility, release and efficacy of gentamicin released from poly(lactic acid) (PLA) beads in an osteomyelitis model were evaluated. Microsphere particles of PLA containing gentamicin were compressed into 2.5 mm beads. Osteomyelitis was induced in the tibia by a gentamicin resistant Staphylococcus aureus in 32 Sprague-Dawley rats. Following debridement at 8 weeks, rats were randomized into four groups: 1) gentamicin-impregnated PLA bead implant, 2) PLA bead without gentamicin, 3) once-daily gentamicin injections (5 mg/kg) subcutaneously for 14 days, and 4) no treatment (control). Rats were sacrificed at 7 weeks, and the tibias were sampled for quantitative culture, histologic scoring and gentamicin concentration. Radiography and culture in 31 of the 32 rats confirmed osteomyelitis. A low level of gentamicin was measured in the serum of group 1 rats up to day 42. Gentamicin was not detectable in bone samples. There was no significant difference in radiographic scores, histologic scores or quantit...

Synthesis and Characterization of Photo-Cross-Linked Networks Based on l-Lactide/Serine Copolymers

New lactide-based polydepsipeptide polymer networks and cross-linked beads have been prepared by UV photopolymerization of acrylated poly(l-lactic acid-co-glycolic acid-co-l-serine) [PLA(Glc-Ser)]. These materials have been developed for use as polymer scaffolds in tissue engineering, cell encapsulation, and injectable drug delivery, which have ligand-immobilizable and biodegradable characteristics. The acrylated PLA(Glc-Ser) cross-linked polymer networks obtained were glassy and transparent, and the gel content was approximately 90%. The networks showed relatively low swelling in water, due to their cross-linked nature, but are easily swelling in chloroform and in dimethyl sulfoxide. The acrylate polymers on copolymerization with 2-hydroxyethyl methacrylate (HEMA) resulted in cross-linked networks [PLA(Glc-Ser)/HEMA], which were swelling in water and in DMSO showing the potential of the polymer in hydrogel applications. The modified PLA(Glc-Ser) beads prepared by UV-initiated suspension polymerization we...

Preparation and in Vitro Characterization of Gentamycin-Impregnated Biodegradable Beads Suitable for Treatment of Osteomyelitis

□ A new method for preparing poly(L-lactide) (PLA) biodegradable beads impregnated with an ionic aminoglycoside, gentamycin, is described. The process employs hydrophobic ion pairing to solubilize gentamycin in a solvent compatible with PLA, followed by precipitation with a compressed antisolvent (supercritical carbon dioxide). The resulting precipitate is a homogeneous dispersion of the ion-paired drug in PLA microspheres. The microspheres are approximately 1μm in diameter and can be compressed into beads (3-6mm in diameter) strung on surgical sutures for implantation. The bead strings exhibit no significant change in release kinetics upon sterilization with a hydrogen peroxide plasma (Ster-Rad). The kinetics of gentamycin release from the PLA beads are consistent with a matrixcontrolled diffusion mechanism. While nonbiodegradable poly(methyl methacrylate) (PMMA) beads initially release gentamycin in a similar manner, the drug release from PMMA ceases after 8 or 9weeks, while the PLA beads continue to release drug for over 4months. Moreover, only 10% of the gentamycin is released from the PMMA beads, while PLA beads release more than 60% of their load, if serum is present in the release medium. The PLA system displays improved release kinetics relative to PMMA, is biodegradable, is unaltered by gas sterilization, can be used for a range of antibiotics, and can be manipulated without disintegration. These are all desirable properties for an implantable drug delivery system for the prevention or treatment of osteomyelitis.