The intumescent flame‐retardant biocomposites of poly(lactic acid) containing surface‐coated ammonium polyphosphate and distiller's dried grains with solubles (DDGS)

This work aims to develop the poly(lactic acid) (PLA) biocomposites with high flame‐retardant performance, which can be applied in electronic and electrical devices as well as automotive parts. First, an intumescent flame retardant composed of ammonium polyphosphate (APP) as the acid source and the blowing agent, and the distiller's dried grains with solubles (DDGS) as the natural charring agent was designed. The surfaces of DDGS and APP were coated by degradable polymeric flame‐retardant resorcinol di(phenyl phosphate) (RDP), and the coating effects were analyzed. And then the flame‐retardant biocomposites of PLA with RDP‐coated DDGS (C‐DDGS) and RDP‐coated APP (C‐APP) were prepared. The limited oxygen index value of the biocomposites with loading of 15 wt% C‐DDGS and 15 wt% C‐APP reached 32.0%, and UL‐94 V‐0 was attained. The biocomposites also had good mechanical properties and the tensile strength of this sample reached about 57 MPa. Finally, the char residues after burning were analyzed and the flame‐retardant mechanism was discussed.     

Synergistic effect of MXene on the flame retardancy and thermal degradation of intumescent flame retardant biodegradable poly (lactic acid) composites

The effect of Ti₃C₂ MXene nanosheets on the intumescent flame retardant (IFR) poly (lactic acid) (PLA) composites was investigated among a series of PLA/IFR/MXene, which were prepared by melt blending 0-2.0 wt% MXene, 10.0 wt%-12.0 wt% IFR and PLA together. The results of limiting oxygen index (LOI) and vertical burning (UL-94) discover that the combination of 0.5 wt% MXene and 11.5 wt% IFR synergistically improves the fire safety of PLA to reach UL-94 V-0 rating with LOI value of 33.0%. The PLA/IFR/MXene composites perform an obvious reduction in peak of heat release rate (HRR) in cone calorimeter tests (CCTs). Furthermore, the carbon residues after CCTs were characterized by scanning electron microscope (SEM), laser Raman spectroscopy (LRS), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). It is demonstrated that both the titanium composition of the MXene structure and the characteristics of the two-dimensional material enhance the PLA/IFR/ MXene composite materials' ability to produce a dense barrier layer to resist burnout during thermal degradation.     

Mechanical,thermal and combustion properties of intumescent flame retardant biodegradable poly (lactic acid) composites

ABSTRACT

Despite extraordinary mechanical properties and excellent biodegradability, poly (lactic acid) (PLA) still suffers from a highly inherent flammability, restricting its applications in the electric and automobile fields. Although a wide range of flame retardants have been developed to reduce the flammability, they normally compromise the mechanical strength of PLA. In this study, a series of composites based on PLA, have been prepared by melt-blending with intumescent flame retardants (IFRs). The morphology, thermal stability and burning behaviour of the composites were investigated using a scanning electron microscope–energy dispersive spectrometer (SEM–EDS), thermogravimetric analysis (TGA), the limiting oxygen index (LOI), vertical burning (UL-94) and the cone calorimeter test (CCT). The LOI value reached 38.5% and UL-94 could pass V-0 for the PLA/IFR composite containing only 12 wt-% IFR. The dispersion of IFR in PLA was observed using SEM–EDS. A significant improvement in fire retardant performance was observed for the PLA/IFR composite from the CCT (reducing the heat release rate and the total heat release). More importantly, compared to pure PLA, the addition of IFR did not seriously deteriorate the mechanical properties of the material.     

Simultaneously enhanced fracture toughness and flame‐retardant property of poly(l‐lactic acid) via reactive blending with ammonium polyphosphate and in situ formed polyurethane

The preparation of poly(l ‐lactic acid) (PLLA) with high mechanical and ideal flame‐retardant properties is a great challenge. Herein, a simultaneous toughness and flame‐retardant PLLA composite was successfully fabricated by using a one‐step process which introduces 4,4′‐methylenediphenyl diisocyanate and ammonium polyphosphate (APP) into PLLA/poly(ε‐caprolactone) blends. SEM, Fourier transform infrared spectroscopy and TGA were adopted to confirm that APP participated in the in situ reaction during the melt process. The impact strength was increased to 13.5 kJ m^?2 from 1.0 kJ m^?2 for L8P2A5 composite, indicating the toughening effect of reactive blending. The cone calorimeter test, limiting oxygen index and vertical burning test results indicate that the flame‐retardant properties of the composites are enhanced with increasing APP content. This work provides a method to prepare PLLA with high mechanical properties and enhanced flame retardancy. © 2020 Society of Chemical Industry     

Improving the flame retardance and melt dripping of poly(lactic acid) with a novel polymeric flame retardant of high thermal stability

A polymeric flame retardant containing phosphorus and nitrogen (PCNFR) was synthesized and characterized by Fourier transform infrared spectroscopy, nuclear magnetic resonance and gel permeation chromatography. The thermal decomposition temperatures at 10% weight loss (T_{10 wt%}) of PCNFR were around 358 °C, and the char yield at 600 °C reached about 60 wt% both in nitrogen and air by thermogravimetric analysis. The flame retarded poly(lactic acid) (PLA) composites with PCNFR were prepared. The thermogravimetric analysis results showed that PCNFR could improve the thermal stability of the flame retarded PLA composites with low loading (≤10 wt%) and at high temperature zone (≥390 °C). The condensed products from the decomposition of the flame retarded composites at 380 °C and 450 °C for different intervals were analyzed by Raman spectroscopy, and the results showed that time and temperature influenced the structure of the char residue evidently. When incorporating 30 wt% PCNFR into PLA, the limited oxygen index of the flame retarded composites reached 25.0%, and V‐0 rating was achieved. The char residues were analyzed by scanning electron microscopy, Fourier transform infrared spectroscopy and X‐ray photoelectron spectroscopy in detail. Copyright © 2016 John Wiley & Sons, Ltd.     

Reactable versus soluble DOPO derivatives in poly(lactic acid)/poly(butylene adipate-co-terephthalate) composites: Flame retardance,mechanical properties and morphology

Flame retardant poly(lactic acid)/poly(butylene adipate-co- terephthalate) (PLA/PBAT) composites containing 9,10-dihydro-9-oxa-10- phosphaphenanthrene-10-oxide (DOPO) derivatives (phosphorus-containing diol compound of DOPO-HQ, and bis DOPO phosphonates of DIDOPO) were systematically and comparatively investigated. Results showed that the different structures of the two derivatives with reactable or soluble characteristics display different effects. DIDOPO endows a higher limiting oxygen index and a better UL-94 rating for PLA/PBAT composites compared with DOPO-HQ. Compared with that of PLA/PBAT, the peak heat release rate of PLA/PBAT/DIDOPO-12.5 is 8.4% lower and that of PLA/PBAT/DOPO-HQ-12.5 is 30.6% lower. The flame retardant mechanism of the main gaseous and minor condensed phases is evident for the flame retardant PLA/PBAT composites. In comparison, DIDOPO displays a greater flame inhibition effect, and DOPO-HQ shows better barrier and protective functions in PLA/PBAT composites. Besides, the elongation at break of the composites with DOPO-HQ is slightly superior to that of PLA/PBAT/DIDOPO. After the introduction of flame retardant, the blends show dispersed particles with size reduction relative to those of PLA/PBAT. This work provides a guidance to design PLA composites with simultaneously improved flame retardancy and toughness.     

Relationship between the crystallization behavior of poly(ethylene glycol) and stereocomplex crystallization of poly(L‐lactic acid)/poly(D‐lactic acid)

Poly(l ‐lactic acid) (PLLA) is a good biomedical polymer material with wide applications. The addition of poly(ethylene glycol) (PEG) as a plasticizer and the formation of stereocomplex crystals (SCs) have been proved to be effective methods for improving the crystallization of PLLA, which will promote its heat resistance. In this work, the crystallization behavior of PEG and PLLA/poly(d ‐lactic acid) (PDLA) in PLLA/PDLA/PEG and PEG‐b‐PLLA/PEG‐b‐PDLA blends has been investigated using differential scanning calorimetry, polarized optical microscopy and X‐ray diffraction. Both SCs and homocrystals (HCs) were observed in blends with asymmetric mass ratio of PLLA/PDLA, while exclusively SCs were observed in blends with approximately equal mass ratio of PLLA/PDLA. The crystallization of PEG was only observed for the symmetric blends of PLLA_39k/PDLA_35k/PEG_2k, PLLA_39k/PDLA_35k/PEG_5k, PLLA_69k/PDLA_96k/PEG_5k and PEG‐b‐PLLA_31k/PEG‐b‐PDLA_27k, where the mass ratio of PLLA/PDLA was approximately 1/1. The results demonstrated that the formation of exclusively SCs would facilitate the crystallization of PEG, while the existence of both HCs and SCs could restrict the crystallization of PEG. The crystallization of PEG is related to the crystallinity of PLLA and PDLA, which will be promoted by the formation of SCs. © 2017 Society of Chemical Industry     

Development of ductile green flame retardant poly(lactic acid) composites using hydromagnesite&huntite and bio-based plasticizer

Turning brittle poly(lactic acid) (PLA) to ductile form via plasticizer inclusion is an effective option in the case of processing with high amounts of additives. Additionally, the integration of natural flame retardants to PLA involving bio-based plasticizer enables to use of environmentally friendly composites in conditions where fire resistance performance is required. In the current study, ductile green fire retardant PLA composites were manufactured using hydromagnesite&huntite (HH) as a natural fire retardant additive and acetyl tributyl citrate as a bio-based plasticizer. The influences of plasticizer and HH contents on the fire retardant, thermal and mechanical performances of the composites were explored. According to test results, the limiting oxygen index (LOI) value of PLA reduced from 29.2 to 28.0 and the UL-94 V rating changed from V2 to BC with the addition of 20 wt% plasticizer owing to the reduction in melt viscosity. The peak heat release rate (pHRR) and average heat release rate (avHRR) values increased steadily as the concentration of plasticizer increased due to the formation of a more porous residue structure stemming from the increased transportation rate of gases. In order to produce ductile flame retardant material, the plasticizer content was required to 20 wt% of HH. The highest LOI value (36.2) and UL-94 rating of V0 were achieved with the inclusion of 70 wt% HH in the presence of 20 wt% plasticizer. Improvement in impact resistance and reduction in tensile strength were observed as the added amount of plasticizer increased.     

聚乳酸阻燃改性研究进展

聚乳酸是目前最具前景的生物基可降解材料之一,具有诸多优点而被广泛应用于生物医疗与家纺服饰等领域。然而,聚乳酸材料仍存在着易燃烧的不足,限制了其在阻燃要求较高领域的应用与发展,因此聚乳酸阻燃改性迫在眉睫。为更好地掌握聚乳酸阻燃改性现状与发展趋势,本文首先简要介绍了聚乳酸的燃烧过程与阻燃机理,为聚乳酸阻燃改性提供了理论指导。其次,全面综述了聚乳酸阻燃改性的最新进展,包括物理共混、化学共聚与表面修饰等方法,重点阐述了聚乳酸物理共混阻燃改性现状,同时分析总结了不同添加型阻燃剂的优缺点。最后,结合聚乳酸结构特点与阻燃材料发展态势,提出绿色环保、多功能性、高效稳定等阻燃聚乳酸材料将成为未来发展趋势。     

Effect of different thermal treatments on the mechanical performance of poly(L‐lactic acid) based eco‐composites

PLLA‐based eco‐composites reinforced with kenaf fiber and rice straw and containing red or yellow pigments have been studied. The mechanical behavior of the composites was tested by DMTA at two different annealing temperatures (65°C and 85°C) and times (15 min and 120 min) as well as at two preparation conditions: vacuum drying and long time at room temperature. A decrease of microhardness was observed during the water absorption tests. Moreover, the rice straw‐based composites absorbed more water than the kenaf‐ones. Generally, the dyed NFs composites presented better water resistance than undyed ones. The pigments improved the adhesion and led to better mechanical performance. The natural fibers favored the cold crystallization process of PLLA and shifted the cold crystallization peak temperature to lower values, as it was confirmed by DSC measurements. The values of tensile storage modulus obtained after different preparation condition were strongly affected by the process of physical ageing. According to, tan δ parameter, the samples stored at room temperature for a long time showed the highest amorphous content. The PLLA eco‐composite reinforced with kenaf fibers, dyed with the red pigment, and annealed at 85°C for 2 h displays the best mechanical properties. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

Core‐shell structure of carbon nanotube nanocapsules reinforced poly(lactic acid) composites

In this study “core‐shell” structure of carbon nanotube (CNT) nanocapsules, which aimed at toughening poly(lactic acid) (PLA) were designed by a synthetic strategy consisting of two reaction steps. The first step was to produce reactive chemical bond to bridge CNTs and PLA. So coupling agent KH570 was used to modify CNTs (CNTs‐KH570). The second step involved ring open polymerization of lactide (LA). Lactide polymerized into PLA under catalysis and meanwhile grafted onto CNTs via KH570 (CNTs‐KH570‐PLA). Thus, the CNTs nanocapsules were constructed. Fourier transform infrared spectroscopy (FTIR) showed coupling agent KH570 succeeded in linking CNTs and PLA during LA polymerization. In addition, scanning electron microscopy (SEM) and transmission electron microscope (TEM) indicated CNTs dispersed homogeneous in PLA matrix and the compatibility between them was excellent. The mechanical test also suggested the designed nanocapsules had good effect on toughening PLA composites. This research found one economical and simple way to improve PLA mechanical properties and further broaden its application in many fields. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 44919.     

Influence of ammonium polyphosphate on the flame retardancy and mechanical properties of ramie fiber‐reinforced poly(lactic acid) biocomposites

Fully degradable natural fiber/degradable polymer composites have received much research attention and have various applications such as in automotive components. But flammability limits their application; it is important to improve the flame retardancy of fully degradable composites with environmentally friendly flame retardants. Flame‐retarded ramie fiber‐reinforced poly(lactic acid) (PLA) composites were prepared using three processes: (1) PLA was blended with ammonium polyphosphate (APP), and then the resulting flame‐retarded PLA was combined with ramie fibers; (2) ramie fibers underwent flame‐retardant treatment with APP, which were then compounded with PLA; and (3) PLA and ramie, both of which had been flame‐retarded using APP, were blended together. The APP in the composites is shown to be very effective in improving flame retardancy according UL94 test and limiting oxygen index measurements. Thermogravimetric analysis shows that the improved flame retardancy is due to increased char residue at high temperature. The loading of APP disturbs the compatibility between PLA and fibers, which can be directly observed using scanning electron microscopy. Furthermore it has an influence on the dynamic mechanical properties and mechanical properties according dynamic mechanical analysis and mechanical measurements. The results show that composites produced using the third process not only have the best flame retardancy but also comparatively better mechanical properties. Copyright © 2009 Society of Chemical Industry     

Dynamic mechanical and thermal properties of plasticized poly(lactic acid)

The blends of low molecular weight triacetin (TAC) and oligomeric poly(1,3‐butylene glycol adipate) (PBGA) were used as multiple plasticizers to lubricate poly(lactic acid) (PLA) in this study. The thermal and mechanical properties of plasticized polymers were investigated by means of dynamic mechanical analysis and differential scanning calorimetry. Atomic force microscopy (AFM) was used to analyze the morphologies of the blends. Multiple plasticizers were effective in lowering the glass transition temperature (T_g) and the melting temperature (T_m) of PLA. Moreover, crystallinity of PLA increased with increasing the content of multiple plasticizers. Tensile strength of the blends decreased following the increasing of the plasticizers, but increased in elongation at break. AFM topographic images showed that the multiple plasticizers dispersed between interfibrillar regions. Moreover, the fibrillar crystallite formed the quasicrosslinkings, which is another cause for the increase in elongation at break. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 101: 1583–1590, 2006     

Compatibility,crystallinity and mechanical properties of poly(lactic acid)‐poly(ether‐block‐amide) based copper nanocomposites

Binary and ternary composites of poly(lactic acid) (PLA), poly(ether‐block‐amide) (PEBAX) and copper nanoparticles were prepared by melt blending in an internal mixer. Compatibility and molecular interactions between the three components of the nanocomposites were evaluated using scanning electron microscopy and Fourier transform infrared spectroscopy. It was found that the carbonyl groups of the PLA and copper nanoparticles interact. Also, PLA and PEBAX are compatible and develop molecular interactions between the C=O of PLA and the C=O and NH of PEBAX, forming dipole–dipole bonds and hydrogen bonds. The compatibility and molecular interaction between PLA and PEBAX are reduced by copper nanoparticles. The reduction of compatibility between PLA and PEBAX produced a lower storage modulus and lower strain at break in the ternary systems than in the blend PLA‐PEBAX. Copper nanoparticles enhanced the crystallinity of PLA. PLA responded more strongly to the nucleating effect of copper when PEBAX was added indicating a synergistic effect. The strain at break of PLA was enhanced by the addition of PEBAX but was severely reduced by the presence of nanoparticles. © 2020 Society of Chemical Industry     

聚乳酸的阻燃改性研究进展

综述了近年来国内外聚乳酸（PLA）阻燃改性的研究进展,对磷系、磷氮系、硅系、膨胀型、无机纳米粉体等各种环境友好型阻燃体系在PLA树脂中的应用现状进行了概述,探讨了各自的阻燃机理,并展望了PLA阻燃改性的发展前景。     

The difference of equilibrium melting point between poly(l‐lactic acid) and poly(l‐lactic acid)/poly(d‐lactic acid) blends: cases with three molecular weights

In order to explore the origin of the higher melting point of poly(lactic acid) (PLA) stereocomplex crystal (SC) than that of homo‐crystal (HC), the equilibrium melting point () differential between SC and HC was determined using the Hoffman–Weeks method. The results showed that, for PLA samples with M_n around 16, 20 and 65 kg mol^?1, the differential between SC and HC is around 36, 42 and 55 °C, respectively. Thus, the higher melting point of SC compared to HC does not stem from differential only. For PLA samples with lower M_n, the supercooling differential between poly(l ‐lactic acid) (PLLA)/poly(d ‐lactic acid) (PDLA) blends and PLLA is smaller than that with higher M_n, which means chain diffusion behavior is crucial for SC formation in PLLA/PDLA blends. The fact that the SC adopts the intermolecular parallel arrangement rather than the adjacent chain folding is verified by the greater slope of the melting point of SC versus crystallization temperature fitting curve when M_n is relative higher. © 2018 Society of Chemical Industry     

Preparation and characterization of double-layered microencapsulated red phosphorus and its flame retardance in poly(lactic acid)

A novel double-layered microencapsulated red phosphorus (DMRP) has been prepared through chemical precipitation of aluminum trihydrate (ATH) and in situ polymerization of melamine formaldehyde (MF) resin on the red phosphorus (RP) powder surface, and its structure was characterized by Fourier transform infrared (FTIR) spectroscopy and scanning electron microscopy (SEM). The influence of DMRP on flame retardance and thermal stability of poly(lactic acid) (PLA) was thoroughly investigated by means of X-ray diffractometry (XRD), limiting oxygen index (LOI), vertical burning test, and thermogravimetric analysis (TGA). With an optimum mass ratio of RP/ATH/MF = 72.25%/12.75%/15%, it has been found that PLA with the addition of DMRP at 25 wt % loading level shows good flame retardance compared to plain RP as well as the conventional microencapsulated red phosphorus (CMRP), and can achieve UL94 V-0 rating along with an LOI increase from 20.5 to 29.3. The TGA and XRD studies indicate that the interaction occurs among all three components: RP, ATH, and MF resin. © 2012 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Biodegradable polymer blends of poly(lactic acid) and poly(ethylene glycol)

Poly(lactic acid) (PLA) and poly(ethylene glycol) (PEG) were melt-blended and extruded into films in the PLA/PEG ratios of 100/0, 90/10, 70/30, 50/50, and 30/70. It was concluded from the differential scanning calorimetry and dynamic mechanical analysis results that PLA/PEG blends range from miscible to partially miscible, depending on the concentration. Below 50% PEG content the PEG plasticized the PLA, yielding higher elongations and lower modulus values. Above 50% PEG content the blend morphology was driven by the increasing crystallinity of PEG, resulting in an increase in modulus and a corresponding decrease in elongation at break. The tensile strength was found to decrease in a linear fashion with increasing PEG content. Results obtained from enzymatic degradation show that the weight loss for all of the blends was significantly greater than that for the pure PLA. When the PEG content was 30% or lower, weight loss was found to be primarily due to enzymatic degradation of the PLA. Above 30% PEG content, the weight loss was found to be mainly due to the dissolution of PEG. During hydrolytic degradation, for PLA/PEG blends up to 30% PEG, weight loss occurs as a combination of degradation of PLA and dissolution of PEG. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 66: 1495–1505, 1997     

Enhancing the durability of poly(lactic acid) composites by nucleated modification

Biodegradable poly(lactic acid) (PLA) composites were prepared using an innovative combination of wood fiber (WF) and 1,3,2,4‐bis(3,4‐dimethylobenzylideno)sorbitol (DMDBS). DMDBS acted as an effective nucleating agent, which improved the mechanical properties and slowed down the degradation of the WF/PLA composites. The enzymatic degradation of the composites was examined by immersing in proteinase K or cellulase buffer. The presence of DMDBS resulted in a 26.7% increase of the crystallinity compared to the WF/PLA composites. The increase in crystallinity enhanced the thermal stability and tensile strength of the WF/DMDBS/PLA composites by 8.5%. The durability of the WF/DMDBS/PLA composites after nucleated modification was enhanced after enzymolysis. After nucleated modification, the surface of the WF/PLA composites showed clear cracks due to degradation, while these appeared about 2 weeks later in the case of the WF/DMDBS/PLA composites. The results revealed that the introduction of cellulase degraded WF in the composites, which increased hydrolysis or enzymolysis sites. The combination of nucleated modification and enzyme buffer gave an expanded downstream application of WF/PLA composites in packaging and agricultural materials. © 2019 Society of Chemical Industry     

Poly(para‐dioxanone) and poly(L‐lactic acid) blends: thermal,mechanical, and morphological properties

Blends of two semicrystalline polymers, poly(L ‐lactic acid) (PLLA) and poly‐p‐dioxanone (PPD) have been prepared by solvent casting in different compositions. Thermal, morphological, and mechanical properties of the blends were studied using modulated differential scanning calorimetry, wide‐angle X‐ray diffractometry, scanning electron microscopy (SEM), polarizing light microscopy (PLM), and tensile tests. Thermal analysis showed two glass transition temperatures nearly constant and equal to the values of the homopolymers and constant values of melting temperature (T_m) for all blend compositions, suggesting that both polymers are immiscible. The PLM and SEM observations validated these results, and showed the different morphology obtained by changing the composition of the blend. The blends 40/60, 50/50, and 60/40 presented a clearly macroseparated system, while the 20/80 and 80/20 blends presented better homogeneity, probably due to the low amount of one component in the other. It was found by PLM that PPD is able to crystallize according to a spherulitic morphology when its content is above 40%. Under this content, the crystallization of PPD is hardly observed. The blend 20/80 is more flexible, and tough material and neck formation during elongation is also observed, due to PPD, which may act as a plasticizer. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 12: 2744–2755, 2003