Effects of aging on the thermomechanical properties of poly(lactic acid)

In this study, we examined the role of environmental parameters and physical structure in the aging process of poly(lactic acid) (PLA). The role of heating history on the aging behavior of the material was also investigated. PLA samples with a D ‐content of 4.25% were exposed to a relative humidity of 80% at three different temperatures, 20, 40, and 50°C (below the glass‐transition temperature of the material), at various aging periods of 30, 60, 80, 100, and 130 days. Selected samples were subjected to two consecutive heating runs. The stability of PLA was monitored by a number of techniques, including size exclusion chromatography, differential scanning calorimetry, dynamic mechanical analysis, and tensile measurements. The initial thermal processing (150°C) of the material resulted in an overall molecular weight reduction. A substantial lowering of properties was observed for PLA samples aged at 20°C for 30 days. No further loss of properties was observed for samples aged up to 40°C for several time intervals. A major portion (80–90%) of the induced changes in the tensile properties could be reversed after drying. At 50°C and 100 days of aging, a sharp decrease in the overall properties was noticed. The results seem to confirm the earlier finding that PLA degradation driven by hydrolysis needs a higher temperature (>50°C) in combination with ample time to take place. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Improvement of mechanical properties of polylactic acid adhesion joints with bio‐based adhesives by using air atmospheric plasma treatment

The packaging industry generates a high volume of wastes; so that, there is a high demand of biodegradable materials, which do not damage the environment. Nowadays, there is an interesting consumption of polylactic acid (PLA) due to its biodegradable features. This work focuses on the improvement of mechanical properties of PLA adhesion joints for uses in the packaging industry. In order to achieve that purpose, atmospheric plasma treatment is used to selectively modify PLA surface properties. The obtained experimental results show that the atmospheric plasma treatment is suitable to increase the mechanical performance of PLA–PLA adhesive joints. Optimum conditions for the atmospheric plasma treatment were obtained with a nozzle–substrate distance of 10 mm and an advance rate in the 100–300 mm s^?1 range; for these particular conditions, the effectiveness of the surface modification is the highest. The main plasma‐acting mechanisms are microetching together with the insertion of polar groups which lead to an interesting synergy that causes a remarkable increase in mechanical properties of adhesion joints. In particular, the shear strength of untreated PLA–PLA adhesion joints is close to 50 N cm^?2 and this value is increased up to values of about 168.7 N cm^?2 with optimum plasma treatment conditions. This indicates that atmospheric plasma treatment is both a technical and an environmental friendly technique to improve mechanical performance of PLA adhesive joints. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 42391.     

Properties of poly(lactic acid) blends with various starches as affected by physical aging

Poly(lactic acid) (PLA) and starch are both biodegradable and renewable polymers derived from agricultural feedstock. A previous study showed that a small amount (0.5%) of methylenediphenyl diisocyanate (MDI) could enhance the mechanical properties of starch and PLA blends by improving the interfacial interaction. In this study, blends of PLA (1/1, w/w) and starch with or without MDI were evaluated for thermal and mechanical properties as well as morphology, as affected by physical aging when stored up to 12 months at 25 °C and 50% relative humidity. The blends were prepared by thermally blending PLA with wheat starch, corn starch, and/or high amylose corn starch, with or without MDI. All samples exhibited phenomena of physical aging. The samples with MDI aged more slowly, showing a slower reduction rate of excess enthalpy relaxation, than those without MDI. The mechanical properties decreased slowly as aging proceeded. Microstructure showed a reduced interaction between starch and PLA around the interface with aging. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 90: 3683–3689, 2003     

用扩链法制备的聚乳酸基热熔胶及其性能研究

用聚乳酸-聚己内酯扩链共聚法制备了可生物降解的热熔胶粘剂。采取凝胶色谱(GPC)、热分析(DSC)、红外光谱(FTIR)等测试手段对聚合物基体树脂的形态结构进行了表征,研究了NCO／OH摩尔比对基体树脂性能的影响;增粘剂用量的影响。研究结果表明以扩链法制备的热熔胶粘剂性能优异,其搭接剪切强度最优值可达到6．788 MPa。     

Chain extension and mechanical properties of unsaturated aliphatic copolyesters based on poly(L‐lactic acid)

Chain extension of poly(L ‐lactic acid) (PLLA) with unsaturated groups (PLBM) was attempted using benzoyl peroxide (BPO) and the resulting variation in molecular weight and mechanical properties was explored. Bulk copolymerization of L ‐lactic acid (LA)/1,4‐butanediol (BD)/maleic acid (MA) (100/1/1) isomerized some of the cis‐structured maleate units into trans‐structured fumarate units. The optically active LA promoted isomerization during the condensation polymerization. Chain extension of PLBM with BPO did not bring about a discernible increase in the molecular weight when the chain extension was carried out in various solvents with different radical abstraction abilities. In contrast, the hot pressing of PLBM containing BPO increased the molecular weight and sometimes produced chloroform‐insoluble gels depending on the BPO concentration and temperature. The chain extension at low temperatures increased the flexibility of PLBM considerably. However, PLBM lost the flexibility precipitously as the chain‐extension temperature increased above 120°C. The biodegradation rate of PLBM was much slower than that of PLLA. The biodegradation rate was further lowered by the chain extension. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 90: 1802–1807, 2003     

Improvement of microstructures and properties of poly(lactic acid)/poly(ε‐caprolactone) blends compatibilized with polyoxymethylene

Incompatibility of poly(lactic acid)/poly(?‐caprolactone) (PLA/PCL) (80:20) and (70:30) blends were modified by incorporation of a small amount of polyoxymethylene (POM) (≤3 phr). Impact of POM on microstructures and tensile property of the blends were investigated. It is found that the introduction of POM into the PLA/PCL blends significantly improves their tensile property. With increasing POM loading from zero to 3 phr, elongation at break increases from 93.2% for the PLA/PCL (70:30) sample to 334.8% for the PLA/PCL/POM (70:30:3) sample. A size reduction in PCL domains and reinforcement in interfacial adhesion with increasing POM loading are confirmed by SEM observations. The compatibilization effect of POM on PLA/PCL blends can be attributed to hydrogen bonding between methylene groups of POM and carbonyl groups of PLA and PCL. In addition, nonisothermal and isothermal crystallization behaviors of PLA/PCL/POM (70:30:x) samples were investigated by using differential scanning calorimetry and wide angle X‐ray diffraction measurements. The results indicate that the crystallization dynamic of PLA matrix increases with POM loadings. It can be attributed to the fact that POM crystals have a nucleating effect on PLA. While crystallization temperature is 100 °C, crystallization half‐time can reduce from 9.4 to 2.0 min with increasing POM loading from zero to 3 phr. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018 , 135, 46536.     

Effects of thermoplastic poly(ether-ester) elastomer and bentonite nanoclay on properties of poly(lactic acid)

This work presented the influence of thermoplastic poly(ether-ester) elastomer (TPEE) and bentonite (BTN) on improving the mechanical and thermal properties of poly(lactic acid) (PLA). PLA was initially melt mixed with TPEE at six different loadings (5–30 wt%) on a twin screw extruder and then injection molded. The mechanical tests revealed an increasing impact strength and elongation at break with increasing TPEE loading, but a diminishing Young's modulus and tensile strength with respect to pure PLA. The blend at 30 wt% TPEE provided the optimum improvement in toughness, exhibiting an increase in the impact strength and elongation at break by 3.21- and 10.62-fold over those of the pure PLA, respectively. Scanning electron microscopy analysis illustrated a ductile fractured surface of the blends with the small dispersed TPEE domains in PLA matrix, indicating their immiscibility. The 70/30 (wt/wt) PLA/TPEE blend was subsequently filled with three loadings of BTN (1, 3, and 5 parts by weight per hundred of blend resin [phr]), where the impact strength, Young's modulus, tensile strength and thermal stability of all the blends were improved, while the elongation at break was deteriorated. Among the three nanocomposites, that with 1 phr BTN formed exfoliated structure and so exhibited the highest impact strength, elongation at break, and tensile strength compared to the other intercalated nanocomposites. Moreover, the addition of BTN was found to increase the thermal stability of the neat PLA/TPEE blend due to the barrier properties and high thermal stability of BTN.     

Thermal and mechanical properties of poly(lactic acid)/starch/methylenediphenyl diisocyanate blending with triethyl citrate

Triethyl citrate (TC) was added as a plasticizer to a blend of poly(lactic acid) (PLA) and starch in the presence of methylenediphenyl diisocyanate (MDI). As expected, TC improved the elongation at break and toughness and, at the same time, decreased the tensile strength and modulus. However, TC did not significantly affect the coupling effects of MDI on starch and PLA. The tensile strength of the blend with MDI was much greater than the tensile strength without MDI at the same TC level. The tensile properties of the blend changed dramatically as the TC concentration increased from 5 to 12.5%. At a TC concentration of 7.5%, the blend produced desirable elongation and toughness with fairly good tensile strength. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 88: 2947–2955, 2003     

The effect of poly(ethylene glycol) as plasticizer in blends of poly(lactic acid) and poly(butylene succinate)

The effect of polyethylene glycol (PEG) on the mechanical and thermal properties of poly(lactic acid) (PLA)/poly(butylene succinate) (PBS) blends was examined. Overall, it was found that PEG acted as an effective plasticizer for the PLA phase in these microphase‐separated blends, increasing the elongation at break in all blends and decreasing the T_g of the PLA phase. Significant effects on other properties were also observed. The tensile strength and Young's modulus both decreased with increasing PEG content in the blends. In contrast, the elongation at break increased with the addition of PEG, suggesting that PEG acted as a plasticizer in the polymer blends. Scanning electron microscope images showed that the fracture mode of PLA changed from brittle to ductile with the addition of PEG in the polymer blends. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 43044.     

Effects of moisture content and heat treatment on the physical properties of starch and poly(lactic acid) blends

Starch, a hydrophilic renewable polymer, has been used as a filler for environmentally friendly plastics for about 2 decades. Starch granules become swollen and gelatinized when water is added or when they are heated, and water is often used as a plasticizer to obtain desirable product properties. The objective of this research was to characterize blends from starch and poly(lactic acid) (PLA) in the presence of various water contents. The effects of processing procedures on the properties of the blends were also studied. Blends were prepared with a lab‐scale twin‐screw extruder, and tensile bars for mechanical testing were prepared with both compression and injection molding. Thermal and mechanical properties of the blends were analyzed, and the morphology and water absorption of the blends were evaluated. The initial moisture content (MC) of the starch had no significant effects on its mechanical properties but had a significant effect on the water absorption of the blends. The thermal and crystallization properties of PLA in the blend were not affected by MC. The blends prepared by compression molding had higher crystallinities than those prepared by injection molding. However, the blends prepared by injection molding had higher tensile strengths and elongations and lower water absorption values than those made by compression molding. The crystallinities of the blends increased greatly with annealing treatment at the PLA second crystallization temperature (155°C). The decomposition of PLA indicated that PLA was slightly degraded in the presence of water under the processing temperatures used. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 81: 3069–3082, 2001     

Crystallization behavior and morphology of poly(lactic acid) with a novel nucleating agent

To improve the crystallization ability of poly(lactic acid) (PLA), a novel nucleating agent with a benzoyl hydrazine compound was used in this study. The crystallization behaviors of PLA/talc and PLA/bibenzoylhydrazinepropane (BBP) with or without poly(ethylene glycol) (PEG) were investigated with differential scanning calorimetry (DSC) and polarized optical microscopy. The DSC curves showed that the crystallization temperature and crystallinity of PLA/BBP (PBBP) was higher than that of PLA/talc. With the addition of PEG, a synergistic effect was found. According to the results of nonisothermal crystallization kinetics, the values of F(T) of PBBP_0.5PEG₅ were usually smaller than those of PTa₃PEG₅, so the nucleation efficiency of BBP was much better than that of talc. From a polarized optical microscopy photo, it was easy to determine that the nucleation density of BBP was higher than that of PTa₃PEG₅, and the spherulitic diameter increased linearly with the crystallization time no matter the impingements. The spherulitic growth rate of PBBP_0.5PEG₅ was faster than that of PTa₃PEG₅, and the induction time of PBBP_0.5PEG₅ was the shortest among all of the samples. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 41367.     

Effect of a chain extender on the rheological and mechanical properties of biodegradable poly(lactic acid)/poly[(butylene succinate)‐co‐adipate] blends

Biodegradable polymer blends based on poly(lactic acid) (PLA) and poly[(butylene succinate)‐co‐adipate] (PBSA) were prepared with a laboratory internal mixer. An epoxy‐based, multifunctional chain extender was used to enhance the melt strength of the blends. The morphology of the blends was observed with field emission scanning electron microscopy. The elongational viscosities of the blends, with and without chain extender, were measured with a Sentmanat extensional rheometer universal testing platform. The blends with chain extender exhibited strong strain‐hardening behavior, whereas the blends without chain extender exhibited only weak strain‐hardening behavior. Measurements of the linear viscoelastic properties of the melts suggested that the chain extender promoted the development of chain branching. The results show that PBSA contributed to significant improvements in the ductility of the PLA/PBSA blends, whereas the chain extender did not have a significant effect on the elastic modulus and strain at break of the blends. The combined blending of PLA with PBSA and the incorporation of the chain extender imparted both ductility and melt strength to the system. Thus, such an approach yields a system with enhanced performance and processability. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

Radiation‐induced crosslinking and mechanical properties of blends of poly(lactic acid) and poly(butylene terephthalate‐co‐adipate)

Blends of poly(L ‐lactic acid) (PLLA) and poly (butylene terephthalate‐co‐adipate) (PBTA) were prepared at ratios of 50 : 50, 60 : 40, and 80 : 20 by melt blending in a Laboplastomill. Improved mechanical properties were observed in PLLA when it was blended with PBTA, a biodegradable flexible polymer. Irradiation of these blends with an electron beam (EB) in the presence of triallyl isocyanurate (TAIC), a polyfunctional monomer, did not cause any significant improvement in the mechanical properties, although the gel fraction increased with the TAIC level and dose level. Irradiation of the blends without TAIC led to a reduction in the elongation at break (E_b) but did not show a significant effect on the tensile strength. E_b of PBTA was unaffected by EB radiation in the absence of TAIC. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

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     

Blends of poly(ε‐caprolactone‐b‐lactic acid) and poly(lactic acid) for hot‐melt applications

The condensation reaction product of poly(lactic acid) (PLA) and a hydroxyl‐terminated four‐armed poly(ε‐caprolactone) (PCL) was studied by size‐exclusion chromatography, DSC, and NMR. The use of both L ‐lactic acid (LLA) and rac‐lactic acid (rac‐LA) was studied and the use of two different catalysts, stannous 2‐ethylhexanoate [Sn(Oct)₂] and ferrous acetate [Fe(OAc)₂], was also investigated. The thermal stability and adhesive properties were also measured for the different formulations. The characterization results suggested the formation of a blend of PLA and a block‐copolyester of PLA and PCL. The results further indicated partial miscibility in the amorphous phase of the blend showing only one glass‐transition temperature in most cases, although no randomized structures could be detected in the block‐copolymers. The polymerization in the Fe(OAc)₂‐catalyzed experiments proceeded slower than in the Sn(Oct)₂‐catalyzed experiments. The discoloring of the polymer was minor when Fe(OAc)₂ was used as catalyst, but significant when Sn(Oct)₂ was used. The ferrous catalyst also caused a slower thermal degradation. Differences in the morphology and in the adhesive properties could be related to the stereochemistry of the poly(lactic acid). © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 91: 196–204, 2004     

Mechanical and thermal properties of wood fibers reinforced poly(lactic acid)/thermoplasticized starch composites

Thermoplasticized starch (TPS) filled poly(lactic acid) (PLA) blends are usually found to have low mechanical properties due to poor properties of TPS and inadequate adhesion between the TPS and PLA. The purpose of this study was to investigate the reinforcing effect of wood fibers (WF) on the mechanical properties of TPS/PLA blends. In order to improve the compatibility of wood with TPS/PLA blends, maleic anhydride grafted PLA (MA‐g‐PLA) copolymer was synthesized and used. TPS, TPS/PLA blends, and WF reinforced TPS/PLA composites were prepared by twin‐screw extrusion and injection molded. Scanning electron microscope and crystallinity studies indicated thermoplasticity in starch. WF at two different weight proportions, that is, 20% and 40% with respect to TPS content were taken and MA‐g‐PLA at 10% to the total weight was chosen to study the effect on mechanical properties. At 20% WF and 10% MA‐g‐PLA, the tensile strength exhibited 86% improvement and flexural strength exhibited about 106% improvement over TPS/PLA blends. Increasing WF content to 40% further enhanced tensile strength by 128% and flexural strength by 180% with respect to TPS/PLA blends. Thermal behavior of blends and composites was analyzed using dynamic mechanical analysis and thermogravimetric analysis. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018 , 135, 46118.     

Morphology,mechanical, and rheological properties of poly(lactic acid)/ethylene acrylic acid copolymer blends processing via vane extruder

This work aimed to study, for the first time, the melt blending of poly(lactic acid) (PLA) and ethylene acrylic acid (EAA) copolymer by a novel vane extruder to toughen PLA. The phase morphologies, mechanical, and rheological properties of the PLA/EAA blends of three weight ratios (90/10, 80/20, and 70/30) were investigated. The results showed that the addition of EAA improves the toughness of PLA at the expense of the tensile strength to a certain degree and leads the transition from brittle fracture of PLA into ductile fracture. The 80/20 (w/w) PLA/EAA blend presents the maximum elongation at break (13.93%) and impact strength (3.18 kJ/m²), which is 2.2 and 1.2 times as large as those of PLA, respectively. The 90/10 and 80/20 PLA/EAA blends exhibit droplet‐matrix morphologies with number average radii of 0.30–0.73 μm, whereas the 70/30 PLA/EAA blend presents an elongated co‐continuous structure with large radius (2.61 μm) of EAA phase and there exists PLA droplets in EAA phase. These three blends with different phase morphologies display different characteristic linear viscoelastic properties in the low frequency region, which were investigated in terms of their complex viscosity, storage modulus, loss tangent, and Cole‐Cole plots. Specially, the 80/20 PLA/EAA blend presents two circular arcs on its Cole‐Cole plot. So, the longest relaxation time of the 80/20 blend was obtained from its complex viscosity imaginary part plot, and the interfacial tension between PLA and EAA, which is 4.4 mN/m, was calculated using the Palierne model. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 40146.     

Effect of surface acetylated‐chitin nanocrystals on structure and mechanical properties of poly(lactic acid)

In this work, the miscibility between chitin nanocrystals (ChNs) and poly(lactic acid) (PLA) was expected to be improved by surface acetylation of ChN. The reaction of acetic anhydride onto the ChN surface was confirmed by FTIR and ¹³C NMR, while XRD and TEM proved the crystalline structure and rod‐like morphology were maintained. The acetylated ChN (AChN) was incorporated into a PLA matrix by solution blending, and resulted in an increase of tensile strength and Young's modulus and they reached to the maximum value as 45 and 37% higher than neat PLA film, respectively, with the loading level of AChN reaching to 4 wt %. The enhancement could be attributed to that acetylation improved dispersion of AChN in the PLA matrix and interfacial adhesion between AChN and PLA. The performances of the nanocomposites based on PLA and chitin nanocrystals derived from renewable resources have good potential for industrial applications. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 39809.     

Effects of radio frequency and heat on wood bonding with a poly(vinyl acetate) dispersion adhesive

Water‐based poly(vinyl acetate) dispersions are widely used as wood adhesives with dielectric heating systems. However, little is known about the effects of radio frequency (RF) exposure or heat on the adhesive characteristics and on the performances of bonded joints. In this study, the properties of bonded joints exposed to RF were compared with hot pressing and with standard drying, and the observed behavior was explained. Joint characteristics were evaluated by means of both conventional (e.g., shear strength in both dry and wet conditions, etc.) and unconventional (e.g., bondline temperature, moisture content at interface, etc.) procedures, and also selected properties of polymeric film were measured (e.g., water absorption, analysis of the fraction dissolved in water, glass transition temperature [T_g], etc.) to explicate the observed differences. Results evidenced that the effect of both RF and heating was to appreciably speed up the drying process. However, when high values of energetic impulse (owing to both hot pressing and RF) were given to the assemblies, permanent changes were induced into the polymeric glueline. This occurrence was a time‐driven process and reflected mainly on the mechanical performance in wet conditions, which improved appreciably in comparison to the standard reference series. The reason of such behavior was connected to the poly(vinyl alcohol) phase present inside the polymer, and a role was also assigned to AlCl₃, used as complexing agent of the polymeric protective colloid. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

Thermal and mechanical properties of plasticized poly(L‐lactic acid)

Acetyl tri‐n‐butyl citrate (ATBC) and poly(ethyleneglycol)s (PEGs) with different molecular weights (from 400 to 10000) were used in this study to plasticize poly(L‐lactic acid) (PLA). The thermal and mechanical properties of the plasticized polymer are reported. Both ATBC and PEG are effective in lowering the glass transition (T_g) of PLA up to a given concentration, where the plasticizer reaches its solubility limit in the polymer (50 wt % in the case of ATBC; 15–30 wt %, depending on molecular weight, in the case of PEG). The range of applicability of PEGs as PLA plasticizers is given in terms of PEG molecular weight and concentration. The mechanical properties of plasticized PLA change with increasing plasticizer concentration. In all PLA/plasticizer systems investigated, when the blend T_g approaches room temperature, a stepwise change in the mechanical properties of the system is observed. The elongation at break drastically increases, whereas tensile strength and modulus decrease. This behavior occurs at a plasticizer concentration that depends on the T_g‐depressing efficiency of the plasticizer. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 90: 1731–1738, 2003