Poly(Lactic Acid) Additives and Processing Aids

Poly(Lactic Acid) Additives and Processing Aids

8 Poly(Lactic Acid) Additives and Processing Aids Chapter Outline 8.1 Limitations of Poly(Lactic Acid) in Processing and Applications 8.2 Toughening...

2MB Sizes 1 Downloads 87 Views

8

Poly(Lactic Acid) Additives and Processing Aids

Chapter Outline 8.1 Limitations of Poly(Lactic Acid) in Processing and Applications 8.2 Toughening of Poly(Lactic Acid) 8.2.1 Reinforcing Fillers 8.3 Improved Heat Deflection Temperature and Heat Resistance 8.3.1 The Addition of Nucleation Agents 8.3.2 Blending of Poly(Lactic Acid) With Heat Resistance Polymers 8.3.3 Compounding Poly(Lactic Acid) Composites With Nano-Scale Natural Fibers 8.4 Flow Enhancement, Melt Strength, Faster Molding Time 8.4.1 Cross-Linking Agents 8.4.2 Chain Extenders 8.4.3 Blending With Other Polymers 8.4.4 Plasticizers 8.5 Specialty Additives: Antistatic, Impact Modifier, Fiber Compatibilizer/Coupling Agents 8.5.1 Antistatic Agents 8.5.2 Impact Modifier 8.5.3 Fiber Compatibilizer/Coupling Agents 8.6 Conclusion References Further Reading

273 276 276 286 287 290 291 292 293 294 294 295 296 296 297 299 300 301 305

8.1 Limitations of Poly(Lactic Acid) in Processing and Applications Currently, poly(lactic acid) (PLA) is one of the most commonly used biodegradable polymers replacing petroleum-based nonbiodegradable polymers for numerous applications, such as food packaging and biomedical devices. In addition, PLA has numerous advantages such as

Polylactic Acid. DOI: https://doi.org/10.1016/B978-0-12-814472-5.00008-X © 2019 Elsevier Inc. All rights reserved.

273

274

POLYLACTIC ACID

being environmentally friendly and biocompatible. PLA is biodegradable, recyclable, and can be derived from renewable sources, such as wheat, rice, and corn (Farah et al., 2016). Furthermore, the biocompatibility quality of PLA with human tissues has gained huge attention from biomedical industries due to its nontoxicity and carcinogen-free interaction with human tissues. After implantation surgery, the degradation of implanted PLA devices was also found to produce a nontoxic degradable product which did not interfere with the healing process of tissues. However, there are still some drawbacks to PLA which have restricted its application in food packaging and biomedical applications. The poor mechanical properties of PLA, such as low tensile strength and modulus, brittleness, and low elongation at break are some of the most significant limitations to its industrial application (Farah et al., 2016). PLA is a brittle polymer with very low elongation ability under strain. The poor toughness of PLA has significantly limited its application in food packaging and biomedical industries. For example, the application of PLA as a material to fabricate screws and fracture fixation plates in bone surgery has been significantly limited. This is due to the PLA biomedical components or devices, such as screws or bone fracture fixation plates, requiring high plastic deformation behavior under high stress level condition (Daniels et al., 1990). The low stiffness of PLA implant devices can cause excessive bone motion and hinder the healing process. In addition, the slow degradation rate of PLA is one of its main limitations. PLA is a synthesis polymer that poses hydrolytically unstable ester functional groups in the backbone of polymer chains. The biodegradation of PLA initially occurs through the hydrolysis of backbone ester functional groups to form soluble oligomers with a drastic reduction in molecular weight (Farah et al., 2016). Finally, soluble oligomers are metabolized by the body. The rate of PLA biodegradation is dependent on the crystallinity of PLA, molecular weight, molecular weight distribution, the permeation rate of water through the polymer matrix, and the stereoisomeric content (Rasal et al., 2010). The crystalline region of PLA polymer was found to be more resistant to degradation when compared to the amorphous region (Tokiwa and Calabia, 2006). The biodegradation behavior of PLA is the most significant characteristic which has gained a great deal of attention and interest for biomedical industry applications. This is attributed to the slow degradation time of PLA could causes the long in vivo life time of implant devices or appliances in the human body which can reach up to years in certain cases (Bergsma et al., 1996; Rasal et al., 2010). Several reports have

8: POLY(LACTIC ACID) ADDITIVES

AND

PROCESSING AIDS

275

been reported of a second surgery being conducted 3 years after implantation surgery to remove PLA-based implant devices (Incardona et al., 1996; Bergsma et al., 1996). The slow degradation rate of PLA is an obstacle to the application of PLA-based implant devices in biomedical applications due to the necessary disposal of the implants is after a few years. The poor processing effect of PLA has limited the application of PLA in some industries. PLA can be processed by several processing methods depending on the required application, including extrusion, compounding, blow molding, injection molding, etc. for large-scale production (Farah et al., 2016). However, the processing of PLA at higher temperature ($200 C) or for a longer processing time can severely reduce the thermal stability of PLA by degrading the molecular weight and physical properties of PLA by oxidative main chain scissioning (Ng et al., 2014), hydrolysis, and reforming of lactide, transesterification of inter- or intramolecular reactions. For instance, the required processing temperature of PLA homopolymers is about 185 C 190 C, which is excessive when compared to the melting temperature of 175 C (Farah et al., 2016). Auras et al. (2004) found that severe degradation of PLA occurred when PLA was processed at a processing temperature which is 10 C higher than the melting temperature. During these processing temperatures, the occurrence of chain scissioning and chain splitting in the PLA matrix can lead to thermal degradation of PLA by reducing its molecular weight (Carrasco et al., 2010; Farah et al., 2016). According to Carrasco et al. (2010), PLA processing was found to be responsible for a reduction of molecular weight, which leads to weakening of the mechanical properties. In addition, they also found that mechanical processing could lead to disappearance of the crystallite structure in a PLA matrix by conducting X-ray diffraction (XRD) analysis and DSC. This is due to the fast cooling process after extrusion, blow molding, injection molding, etc., causing the polymer chains to become unable to rearrange into a crystallite structure and to remain in a random-ordered structure (Migliaresi et al., 1991). Their observations also concluded that the processing of PLA severely reduced the crystallinity of the PLA matrix. In addition, the hydrophobic behavior of PLA could result in low affinity between cells and PLA-based materials, and lead to inflammatory responses from the living body when subjected to direct contact with biological fluids (Rasal et al., 2010). This is a major problem in the application of PLA for biomedical implantation devices and plates. This is due to the required biological activities that cannot take place in

276

POLYLACTIC ACID

PLA due to its hydrophobicity. Furthermore, the lack of functional groups in PLA also causes bulk and surface modifications to PLA with bioactive molecules (Kakinoki and Yamaoka, 2010).

8.2 Toughening of Poly(Lactic Acid) 8.2.1 Reinforcing Fillers In order to improve the mechanical properties, such as stiffness, of PLA nanocomposites, various researches have been conducted by adding various types of reinforcing fillers including montmorillonite (MMT), talc, carbon nanotubes (CNTs), into PLA nanocomposites (Balakrishnan et al., 2010; Lee et al., 2003). The incorporation of MMT was observed to effectively increase the stiffness (Young’s modulus and flexural modulus) of PLA nanocomposites. In research conducted by Balakrishnan et al. (2010), the addition of 4 phr MMT particles into PLA significantly increased the Young’s modulus and flexural modulus by 10% and 18%, respectively. According to Jiang et al. (2007), the addition of 7.5 wt.% of MMT particles into PLA nanocomposites rapidly increased the Young’s modulus by 43%. The improvement of stiffness (Young’s modulus and flexural modulus) of PLA nanocomposites was mainly attributed to the effective intercalation/exfoliation of MMT stacked layers in the PLA matrix. This is because the effective intercalated/exfoliated MMT particles in the PLA matrix could lead to a larger interfacial area which interacts with the PLA matrix. As a result, the improvement in interaction effect between MMT particles and the PLA matrix can cause PLA rigidity. The higher interfacial interacted areas could further transfer the applied stress effectively from the PLA matrix to MMT particles, subsequently improving the stiffness of PLA nanocomposites. Furthermore, the intercalated MMT particles could also further restrict the molecular mobility of PLA chains. The intercalation effect of MMT particles in the PLA matrix can be evaluated using XRD analysis. According to Balakrishnan et al. (2010) and Lee et al. (2003), the XRD curve of pristine MMT shows the presence of a deflection peak (001) at 2θ of 3.50 3.76 as shown in Fig. 8.1. In research conducted by Balakrishnan et al. (2010), the deflection peak (001) was found to disappear from the XRD curve of PLA/MMT nanocomposites (refer to Fig. 8.1A). This indicates that the MMT particles were homogeneously dispersed in the PLA matrix to form an exfoliated structure and caused the large interlayer spacing between layers of

8: POLY(LACTIC ACID) ADDITIVES

AND

PROCESSING AIDS

277

MMT particles. On the other hand, Balakrishnan et al. (2010) also found the shifting of the deflection peak (001) to lower 2θ value with smaller intensities. This also proved that the PLA matrix is effectively diffused into the interlayer galleries of MMT particles (which is known as intercalation), subsequently induce the d-spacing of the deflection peak (001). This is because the effective intercalation of MMT particles could also increase the d-spacing and thus weaken the electrostatic attraction between the MMT platelet and thus induce the PLA rigidity.

Figure 8.1 XRD curves of (A) pristine PLA, 2 phr MMT/PLA, 4 phr MMT/PLA, and pristine MMT (Balakrishnan et al., 2010) and (B) pristine MMT, pristine PLLA, and PLLA/MMT nanocomposites (Lee et al., 2003).

278

POLYLACTIC ACID

According to Balakrishnan et al. (2010), an increasing number of MMT particles in the PLA matrix gradually decreased the tensile strength and flexural strength of PLA composites by 10% and 25%, respectively. This also indicates that the increase number of MMT particles tended to agglomerate together and weakened the interfacial adhesion effect between MMT particles and the PLA matrix. This further caused the agglomerated MMT particles to act as a stress concentration point in the PLA matrix when subjected to applied stress and they were unable to evenly transfer the stress throughout the PLA matrix. This was attributed to the brittle behavior of PLA nanocomposites due to early material failure during straining. In addition, the dispersion and orientation of MMT particles in the PLA matrix also plays an important role in the tensile and flexural strength due to the different applied stress orientations between tensile straining and flexural bending (Balakrishnan et al., 2010). By referring to Balakrishnan et al. (2010) it can be seen that increasing the MMT amount up to 4 phr caused a significant detrimental effect on the impact strength of the PLA nanocomposite by lowering the impact strength by 13%. However, the addition of 2 and 4 phr MMT particles significantly improved the impact strength of PLA/LLDPE nanocomposites by 53% and 21%, respectively (Balakrishnan et al., 2010). The presence of 10 wt.% of LLDPE in MMT/PLA nanocomposites could induce the impact strength as compared to PLA nanocomposites. From this observation, the presence of LLDPE could provide better distribution and orientation of MMT particles in the PLA matrix and thus increase the energy absorbed by the polymer matrix when subjected to rapid loading. Harris and Lee (2007) found that the addition of 2 wt.% talc significantly improved the flexural strength and flexural modulus of PLA by 25%. This is attributed to the addition of talc particles inducing the crystallinity of PLA by acting as a nucleation agent and thus further enhancing the toughness of PLA. In addition, the presence of talc could also provide a reinforcement effect on the rigidity and toughness of the PLA matrix due to the structure of talc particles and orientation of talc particles in the PLA matrix by effectively transferring the applied stress from the talc particles to the PLA matrix. Yu et al. (2012) investigated the effect of increasing the talc content on the mechanical properties (flexural strength and modulus) of PLA as shown in Fig. 8.2. They also obtained similar results to by Harris and Lee (2007), in which the addition of talc significantly enhanced the flexural strength and flexural modulus of neat PLA. In addition, Yu et al. (2012) also found that the flexural strength and flexural modulus of PLA were rapidly increased

8: POLY(LACTIC ACID) ADDITIVES

AND

PROCESSING AIDS

279

Figure 8.2 Effect of talc content in wt.% on flexural strength and flexural modulus of poly(lactic acid) (PLA) (Yu et al., 2012).

when the talc content was increased from 0 to 2.0 wt.%. The improvement of flexural strength and flexural modulus of PLA by adding talc filler was mainly due to the replacement of the PLA matrix with talc filler which is highly rigid in nature and could effectively restrict the extendability and mobility of the PLA matrix when subjected to external loading. Furthermore, the good interfacial adhesion effect between the talc filler and PLA matrix provided an effective reinforcing effect and toughening effect by evenly transferring the applied load throughout the whole polymer matrix as evidenced by the scanning electron microscopy (SEM) analysis conducted by Yu et al. (2012) in Fig. 8.3. However, further increasing the talc content from 2 to 24.3 wt.% was found to only provide a small increment in the flexural strength and modulus of PLA composites. From this observation, the enhancement effect of talc on the flexural strength and modulus of the PLA composite was less significant when the talc content was further increased above 2 wt.%. According to Yu et al. (2012), the reduction in the toughening effect of talc at a higher content ( . 2 wt.%) was attributed to insufficient delamination of talc particles which caused the presence of thicker talc particles, as evidenced by SEM micrographs in Figs. 8.3C F. The poor interfacial adhesion effect between the thicker talc particles and the PLA matrix causes the applied load to be unable to effectively transfer from the polymer matrix to talc particles and thus triggers the brittle behavior of the PLA matrix at higher talc filler content. In addition, the addition of a higher content of talc particles was

280

POLYLACTIC ACID

Figure 8.3 SEM micrographs of the fractured surfaces of talc-added PLA composites with talc contents of (A) 2.5 wt.%, (B) 5.0 wt.%, (C) 10 wt.%, (D) 15 wt.%, (E) 20 wt.%, and (F) 30 wt.% (Yu et al., 2012).

also observed to decrease the orientation degree of talc particles in the PLA matrix and the orientation direction of talc layers was also found not to be parallel to the injection direction. As a result, this further caused the debonding effect of talc particles and the PLA interface, thus propagating the presence of microcracks along the fracture direction (Yu et al., 2012). Ouchiar et al. (2015) compared and investigated the effect of increasing talc and kaolin content on the properties of PLA composites. The addition of 5 wt.% talc content was found to provide a slight increment

8: POLY(LACTIC ACID) ADDITIVES

AND

PROCESSING AIDS

281

in the Young’s modulus of neat PLA from 2.4 GPa to 2.6 GPa. In addition, the increment value of Young’s modulus of PLA composites added with 5 wt.% of kaolin was found to be similar to a PLA composite with 5 wt.% of talc added. According to Ouchiar et al. (2015), further increasing the talc and kaolin content from 5 to 30 wt.% was found to gradually increase the Young’s modulus of PLA composites, as shown in Fig. 8.4. However, the increment in Young’s modulus of talc-added PLA composites was observed to be significantly higher than kaolin-added PLA composites. This is because the talc-added PLA composites exhibited an earlier start of crystallization as compared to pristine PLA and kaolin-added PLA composites, which confirms the nucleation effect of talc which induces the rigidity of PLA composites. According to Zhou et al. (2018), the increasing use of CNTs with carboxyl groups (CNTs-COOH) content up to 0.5 wt.% was found to rapidly increase the tensile strength and Izod impact strength of PLA nanocomposites, as shown in Table 8.1. This also illustrated that the addition of a small amount of CNTs could provide improvements in the tensile strength and impact strength of PLA. This may be due to the high stiffness of CNTs with high aspect ratio and surface area which could further toughen the PLA matrix by effectively interlocking in the PLA matrix. The effective interlocking effect of CNT particles could effectively transfer the applied stress from the CNT particles to the PLA matrix and thus strengthen the PLA nanocomposites. In addition, the strong chemical bonds between CNT-COOH particles and the PLA matrix also hindered the mobility of PLA macromolecular chains and thus strengthened the PLA matrix. However, Zhou et al. (2018) also reported that increasing the CNT-COOH content from 0.5 to 2.0 wt.% significantly decreased the tensile strength and Izod impact strength from 42.8 6 0.3 MPa and 27.7 6 0.5 kJ/m2, respectively, to 39.6 6 0.2 MPa and 8.8 6 0.3 kJ/m2, respectively. A similar observation was found in a study conducted by Wang et al. (2016), who also found that the addition of a higher content of CNTs ( . 3 wt.%) gradually decreased the tensile strength of PLA nanocomposites, as shown in Table 8.2. The marginal decrement in tensile strength and impact strength may be attributable to the higher content of CNTs in the PLA matrix which tended to agglomerate together into larger CNT aggregates due to the van der Waals force interaction between CNT particles (Zhou et al., 2018; Wang et al., 2016). Therefore the presence of CNT aggregates in the PLA matrix could reduce the interfacial adhesion effect between CNT particles and the PLA matrix by acting as a stress

282

POLYLACTIC ACID

Figure 8.4 (A) Young’s modulus of PLA composites when added with increasing contents of talc and kaolin fillers (Ouchiar et al., 2015); (B) tensile strength of chemically functionalized kenaf fibers (KF-OX)/multiwalled carbon nanotubes (MWCNTs)/PLA nanocomposites before and after the annealing process (Chen et al., 2017).

8: POLY(LACTIC ACID) ADDITIVES

AND

PROCESSING AIDS

283

Table 8.1 Tensile Strength and Izod Impact Strength of PLA Nanocomposites Reinforced With Various Carbon Nanotube Content With Carboxyl Groups (CNT-COOH) CNT-COOH Content (wt.%)

Tensile Strength (MPa)

Izod Impact Strength (kJ/m2)

0 0.1 0.5 1.0 2.0

39.5 6 0.2a 40.5 6 0.3a 42.8 6 0.3a 40.6 6 0.3a 39.6 6 0.2a

15.5 6 0.2a 22.6 6 0.3a 27.7 6 0.5a 20.5 6 0.4a 8.8 6 0.3a

CNT-COOH, Carbon nanotubes with carboxyl groups. a Zhou et al. (2018).

Table 8.2 Tensile Strength of PLA Nanocomposites Reinforced With Various CNT Contents Under Parallel and Vertical Extrusion Direction Tensile Strength (MPa) Content of CNTs Parallel Extrusion (wt.%) Direction 0 1 3 5 10

B51.3a B58.8a B68.1a B62.8a B57.8a

Vertical Extrusion Direction B60.5a B65.0a B67.8a B64.5a B62.5a

CNTs, Carbon nanotubes. a Wang et al. (2016).

concentration point and weakening the transferring of applied load throughout the PLA matrix. In a study conducted by Chen et al. (2017), they KF-OX by coupling with 3-glycidoxypropyltrimethoxysilane (OX-silane) to overcome the poor compatibility between kenaf fibers with hydrophilic behavior and the PLA matrix with hydrophobic behavior. Chen et al. (2017) added KF-OX and MWCNTs into the PLA matrix to investigate the effects of increasing KF-OX content on the properties of PLA nanocomposites.

284

POLYLACTIC ACID

Figure 8.5 SEM observation of (A) flax/PLA composites and (B) oxidized-TiO2flax/PLA composites (Foruzanmehr et al., 2016).

Increasing the KF-OX content up to 30 wt.% gradually increased the tensile strength of MWCNT/PLA nanocomposites before and after the annealing process. The enhancement effect in tensile strength of PLA nanocomposites was mainly attributed to the chemical reaction between KF-OX fibers and the PLA matrix. In addition, the recrystallization of PLA nanocomposites by the annealing process was observed to enhance the tensile strength of KF-OX/MWCNT/PLA nanocomposites, as shown in Fig. 8.5. The addition of a higher content of KF-OX (30 wt.%) rapidly increased the tensile strength of the PLA nanocomposite up to 91.5 MPa (which is 84% higher than pristine PLA) when subjected to an annealing process. This is because the good compatibility of KF-OC and the PLA matrix and the formation of crystalline structure at the interfaces between the PLA matrix and KF-OX fibers significantly improved the mechanical properties (tensile strength) of PLA nanocomposites. This is attributed to the presence of transcrystallinity in the PLA matrix which could provide a resistance effect against the applied external loading due to the excellent interfacial adhesion effect of KF-OX fibers and the PLA matrix (Quan et al., 2005). The excellent interfacial adhesion effect of KF-OX fibers and the PLA matrix could transfer the applied straining stress more effectively from KF-OX fibers to the PLA matrix and thus result in enhancement of tensile strength. However, a further increment of KF-OX content from 30 to 40 wt.% was observed to rapidly decrease the tensile strength of PLA nanocomposites from 91.5 to 53.6 MPa, respectively. This is due to the excessively entangled KF-OX fibers in the polymer matrix of PLA nanocomposites possibly hindering the recrystallization of PLA chains and weakening the stiffness of PLA nanocomposites (Chen et al., 2017).

8: POLY(LACTIC ACID) ADDITIVES

AND

PROCESSING AIDS

285

Wootthikanokkhan et al. (2013) investigated the effect of kenaf fiber, Cloisite 30B nanoclay, and hexagonal boron nitrile (h-BN) fillers on the properties of PLA composites. They found that the addition of 5 pph kenaf fiber, Cloisite 30B nanoclay, and h-BN slightly increased the tensile modulus of PLA composites before and after annealing treatment. However, the addition of kenaf fiber was observed to provide a smaller increment in tensile modulus of PLA composites than Cloisite 30B nanoclay or h-BN. This also indicates that the Cloisite 3B nanoclay and h-BN could provide better improvement of the tensile modulus of PLA composites due to the good compatibility between Cloisite 30B nanoclay and h-BN fillers with the PLA matrix (Wootthikanokkhan et al., 2013). This is attributed to the hydroxyl functional groups on Cloisite 30B nanoclay modified with alkyl ammonium surfactant forming a polar interaction with carbonyl functional groups of PLA chains by promoting the exfoliation effect of Cloisite 30B interlayer particles in the PLA matrix. This contributed to an increment in the tensile modulus of PLA composites. On the other hand, the good compatibility between hBN filler and PLA matrix is mainly attributed to the interaction effect between carbonyl functional groups of PLA and nitrogen’s loan pair of electrons on the surface of the h-BN filler, which can be observed with the presence of B-N-B bending vibration at 1360 cm21 on the FTIR spectrum. However, the poorer interfacial adhesion effect between kenaf fiber and the PLA matrix due to lacking of polar interaction between kenaf fiber and PLA chains resulted in lower rigidity of the PLA composites. Swaroop and Shukla (2018) investigated the effect of adding magnesium oxide particles (nano-Mg) on the properties of PLA nanocomposites. In their work, increasing the nano-MgO content up to 2 wt.% gradually increased the tensile strength and elastic modulus of PLA nanocomposites from 29.1 MPa and 1.89 GPa, respectively, to 37.5 MPa and 2.47 GPa, respectively. This is due to the smaller size of nano-MgO particles offering a higher interfacial area of MgO nanoparticles by inducing the surface/volume ratio of nano-MgO particles in the PLA matrix. The incorporation of nano-MgO particles in the PLA matrix could provide a high surface interaction between the MgO filler and PLA matrix, which promotes the transferring of applied stress from the PLA matrix to nano-MgO fillers and results in the improvement of the mechanical properties of PLA. However, the opposite observation was found when the loading level of nano-MgO was increased from 2 to 4 wt.%. According to Swaroop and Shukla (2018), the tensile strength and elastic modulus of PLA nanocomposites were gradually

286

POLYLACTIC ACID

decreased from 37.5 MPa and 2.47 GPa, respectively, to 26.2 MPa and 1.96 GPa, respectively. This is attributed to the higher content of nanoMgO tending to self-agglomeration into larger agglomerated particles which could weaken the interfacial adhesion effect between the agglomerated nano-MgO particles and the PLA matrix. Thus the agglomerated particles were phase-separated from the PLA matrix and acted as a stress concentration point in the PLA matrix, which reduced the reinforcement effect of nano-MgO in the PLA matrix.

8.3 Improved Heat Deflection Temperature and Heat Resistance The ability of a polymer material in maintaining the important properties at maximum operating temperature for a desired period of time is defined as heat resistance. The heat resistance behavior of a polymer matrix is highly dependent on the crystallization behavior and level of crystallinity of a polymer material (Ma et al., 2011). The chain segments of semicrystalline PLA are found to coexist in three forms: crystalline fraction, rigid amorphous fraction, and mobile amorphous fraction (Nagarajan et al., 2016). The chain segments in the crystalline fraction of semicrystalline PLA are arranged in an ordered crystalline structure. The crystalline chains coexist with random long molecular chains of amorphous fraction. When the temperature of the PLA polymer reaches its glass transition temperature, Tg, the PLA molecular chains in the crystalline region are not likely to move due to intermolecular bonding, while the PLA molecular chains in the amorphous region tend to move freely (Nagarajan et al., 2016). There are some rigid PLA chain segments within the amorphous region which could hinder the mobility of the entire long molecular chain, known as the rigid amorphous fraction. On the other hand, the remaining chain segments in the amorphous region which pose high mobility at Tg are referred to as the mobile amorphous fraction. The mobile amorphous fraction poses very low heat resistance at a deflection temperature close to Tg due to the presence of a random long molecular chain arrangement structure. In addition, heat resistance is commonly evaluated by detecting the softening point when subjected to a fixed load. Two measuring techniques are commonly applied to quantify the heat resistance by measuring the Vicat softening temperature (VST) and the heat deflection temperature (HDT). The temperature at which the sample is punctured to 1 mm

8: POLY(LACTIC ACID) ADDITIVES

AND

PROCESSING AIDS

287

depth with a 1 mm2 cross-sectional area flat-ended needle is referred to as the VST. The HDT is defined as the temperature when a specimen distorts for 250 μm under a certain load (commonly either 0.46 or 1.80 MPa) and thickness at a heating rate of 2 C/min. In order to improve the heat resistance of PLA, nucleation agents are added, blending PLA with heat resistance polymers, and compounding PLA composites with nano-reinforcement fillers and natural fibers.

8.3.1

The Addition of Nucleation Agents

As discussed earlier, the heat resistance of PLA is significantly affected by the crystallization behavior of the PLA matrix. The addition of nucleating agents can significantly modify the crystallization behavior of the PLA matrix and thus have a significant effect on the heat resistance of the PLA. In a study conducted by Yu et al. (2012), they investigated the effect of an increasing concentration of talc modified with coupling agent on the mechanical and thermal properties of PLA composites. They found that increasing the talc content up to 30 wt.% significantly increased the glass transition temperature, Tg, and crystallinity, Xc, of PLA composites, as tabulated in Table 8.3. In addition, the increased talc content was also found to marginally reduce the cold crystallization temperature, Tcc, of PLA (Yu et al., 2012). The significant effect of talc content on Tg, Tcc, and Xc also suggested an improvement in the crystallization ability of PLA by talc filler due to its strong nucleating effect. The increment in Tg is mainly attributed to the strong interfacial adhesion effect between PLA and talc particles, which can reduce the free volume in the PLA matrix and restrict the mobility of PLA chains. In other words, a higher Tg also indicates that the random PLA molecular chains in a mobile amorphous fraction are required to approach a higher temperature to move freely and thus induce the heat resistance of PLA. In addition, higher crystallinity, as tabulated in Table 8.3, also indicates that the increasing higher intermolecular bonding of PLA chains in the PLA matrix contribute to increasing the crystalline fraction in the PLA matrix. On the other hand, the melting temperature, Tm, was observed to marginally decrease with increasing talc content in the PLA matrix, as determined by Yu et al. (2012). This also suggested that the crystals nucleated at the surface of talc particles were generally not perfect and tended to rupture and melt at lower temperature. Also, the HDT of PLA composites was observed to gradually increase with increasing talc content up to 30 wt.%, as tabulated in Table 8.3. This also indicates that the addition of talc could increase the

Table 8.3 Effect of Nucleating Agents on Tg, Tcc, Tm, Xc, and HDT of PLA

PLA

Nucleating Agent

Tg ( C)

Tcc ( C)

Tm ( C)

Xc (%)

HDT ( C)

References

PLA of industrial grade: Revode 201 Purchased from Zhejiang Hisaiv Biomaterials, China

Talc (2500 mesh, aspect ratio of 6) purchased from Sichuan Serpentine Mineral Factory, China Treated with 0.3 wt.% of 3-amino propyltriethoxysilane coupling agent 0 wt.% 57.7 129.7 146.8 3.1 49 Yu et al. (2012) 2.5 wt.% 58.7 120.8 145.5 15.3 49 6 0.6 5.0 wt.% 58.8 120.8 145.6 15.2 49 6 0.6 10.0 wt.% 58.5 114.6 143.7 20.1 50 6 0.6 15.0 wt.% 59.0 113.6 143.0 21.0 51 6 0.6 20.0 wt.% 59.0 109.8 142.5 24.5 52 6 0.6 30.0 wt.% 59.2 108.4 142.9 25.1 52 6 0.6

PLA of grade PLE003 Purchased from NaturePlast, France

Talc (Grade of Luzenac 00) purchased from 0 wt.% 57.5 121.5 NP 5 wt.% 57.1 100.6 NP 10 wt.% 56.5 103.6 NP 20 wt.% 56.7 100.0 NP 30 wt.% 56.6 98.0 NP

NP, Not provided; NT, not tested.

Barrisurf LX, Imerys, France NP NT Balakrishnan et al. (2010) NP NT NP NT NP NT NP NT

8: POLY(LACTIC ACID) ADDITIVES

AND

PROCESSING AIDS

289

service temperature for the PLA composites to be able to withstand an increased load and thus improve the heat resistance of PLA composites. Quchiar et al. (2015) also investigated the effect of clay mineral, talc, and kaolinite as nucleating agents on the thermal behavior of PLA composites. They found that by increasing the talc and kaolinite content, the glass transition temperature, Tg, of PLA composites was found to remain at around 57 C. In addition, the cold crystallization temperature, Tcc, of PLA composites was observed to gradually decrease with increasing talc content. The reduction of Tcc is attributed to the talcadded PLA composites having already crystallized during the cooling process. However, they also observed that the addition of kaolinite did not provide a significant effect on the heat resistance behavior of PLA composites. Balakrishnan et al. (2010) discovered the absence of crystallization temperature during cooling of PLA composites when MMT was added as a nucleating agent. This is believed to be due to the very slow PLA crystallization rate during the cooling process. The decrease in Tg after addition of MMT is mainly attributed to the plasticizing effect caused by the surfactant of MMT particles in PLA matrix. This is because the alkyl ammonium surfactant used to modify the MMT particles could facilitate the intercalation effect of the PLA molecular chain into the interlayer galleries of MMT particles. Thus this further caused the PLA chains in the mobile amorphous fraction of PLA matrix to move freely at lower Tg. They also observed that the increasing MMT content significantly decreased the crystallization temperature, Tc. They also suggested that the presence of MMT particles could act as a nucleating agent by promoting the initial crystallization of the PLA matrix. Wang et al. (2016) conducted an investigation into the effect of increasing CNT content on the crystallization and thermal properties of PLA nanocomposites. Increasing CNT content significantly enhanced the crystallization ability of PLA by increasing the crystallinity from 4.7% to 12.46%. This may be due to the high aspect ratio and specific surface area of CNT particles inducing the interfacial adhesion effect between CNT particles and the PLA matrix. Furthermore, the strong interfacial interaction between OH and COOH groups of CNT particles with the PLA matrix significantly induced the formation of numerous crystal nuclei and thus increased the crystallinity of the PLA matrix. They also found that the crystallization temperature, Tc, of all CNT-added PLA nanocomposites (Tc 5 B59 C) was observed to be slightly lower than pristine PLA (Tc 5 59.7 C). This also indicates the promotion of initial crystallization of PLA by the presence of CNTs as a nucleating agent. On the other hand, Zhou et al. (2018) conducted an investigation into

290

POLYLACTIC ACID

the effect of adding CNT with carboxyl groups (CNT-COOH) as a nucleating agent on the thermal properties of PLA composites. They found that increasing the amount of CNT-COOH significantly increased the initial degradation temperature and glass transition temperature of PLA/CNT-COOH nanocomposites. The increment in the initial degradation temperature and glass transition temperature may be attributable to the formation of strong interfacial bonds between the CNT-COOH and PLA matrix which could hinder the mobility of the random molecular chains of PLA. According to Tang et al. (2012), the addition of ethylenebishydroxystearamide (EBH) as a crystal nucleating agent was found to rapidly increase the crystallinity of PLA from 19% to 42% with an annealing time of 20 minutes at a melt-crystallization temperature of 105 C. This is due to the partially melted amorphous region in the PLA undergoing a continuous recrystallization process into more perfect and thicker lamellae. Furthermore, the addition of EBH rapidly enhanced the HDT of neat PLA from 51% to 93%. The HDT of EBH-added PLA composites started to increase when the crystallinity of PLA increased beyond 20%. In addition, they found that the significant improvement in HDT of PLA composites was mainly attributed to the increment in crystallinity of PLA.

8.3.2 Blending of Poly(Lactic Acid) With Heat Resistance Polymers The blending of poly(ε-caprolactone) (PCL) and poly(ε-caprolactone/ P(CL/L-LA) with poly(L-lactic acid) (PLLA), was observed to significantly decrease the glass transition temperature, Tg, and melting temperature, Tm, of PLLA. The reduction of Tg for PLLA blends indicates the promotion of the crystalline structure in the polymer matrix of PCL/PLLA blends and P(CL/L-LA) blends at lower temperature, thus increasing the degree of crystallinity in PLLA blends. In addition, they also found that the PLLA is miscible with P(CL/L-LA) with presence of one Tg of P(CL/L-LA)/PLLA blends proven miscibility. However, Tg was not found when undergoing copolymerization of P(CL/L-LA) which the Tg disappeared from the differential scanning calorimetry (DSC) curves. However, PCL was found to be immiscible with PLLA due to it having two melting temperatures. In a study conducted by Cock et al. (2013), DSC analysis revealed that the blending of PCL with PLA could rapidly increase the rate of crystallization of PLA blends. In addition, the blending of PCL with PLA also shifted the Tg of pristine PLA to a lower value, which is attributed to the molten L-lactide),

8: POLY(LACTIC ACID) ADDITIVES

AND

PROCESSING AIDS

291

temperature of PCL at about 55 C. This also decreased the heating crystallization by supporting the nucleating effect of PCL during postmelt crystallization of the PLA matrix. D’Amico et al. (2016) found that the blending of poly(3-hydroxybutyrate) (PHB) with PLA improved the heat resistance of PLA by increasing the rate of crystallization of PLA. The increasing PHB content in the PLA matrix rapidly reduced the Tg of the PLA blends by promoting earlier crystallization of the PLA/PCL matrix at a lower Tg value. Furthermore, the blending of PHB up to 70 wt.% with PLA also greatly increased the crystallinity of pristine PLA from 2.7% to 35.2%. This is attributable to the blending of PHB with PLA which promoted the formation of crystallites in the PLA matrix by increasing the crystalline fraction and thus reducing the amorphous mobile fraction. The blending of polyamide (PA) microfibrils with PLA under an isothermal crystallization process significantly improved the crystallization kinetics of PLA due to the heterogeneous crystal nucleation effect of PA microfibrils (Kakroodi et al., 2017). This was due to the constraining effects of a large amount of small PA crystallites. According to Hashima et al. (2010), increasing the polycarbonate (PC) content significantly increased the HDT of PLA-blended styrene butadiene styrene block copolymer, which is attributed to the higher crystallinity and Tg of PC. On the other hand, Guo et al. (2015) found that blending of 50 wt.% of polyoxymethylene (POM) (polymer with a high heat resistance behavior) greatly increased the HDT of PLA from 65 C to 133 C. In addition, the increasing POM content slightly shifted the Tg of the PLA/POM blends to lower values, indicating that the presence of POM could promote an earlier crystallization process of PLA.

8.3.3 Compounding Poly(Lactic Acid) Composites With Nano-Scale Natural Fibers The addition of natural fibers such as kenaf fiber and wood fiber into PLA has gained great interest from various researchers to improve the properties of PLA, including heat resistance behavior. According to Renstad et al. (1998), the incorporation of 20 pph kenaf fiber significantly increased the HDT of annealed PLA from 99.7 C to 128 C. The improvement in the HDT value of PLA may be caused by the fibrous shape of kenaf fiber with a high aspect ratio promoting the isothermal crystallization of PLA. This can be further confirmed with the increasing crystallinity of PLA when 20 pph kenaf fiber was added from 33.9% to 37.4% (Renstad et al., 1998). On the other hand, Huda et al. (2008) found that the addition of 40 wt.% untreated kenaf fiber into

292

POLYLACTIC ACID

PLA composites could greatly increase the HDT value from 64.5 C 6 1.2 C to 170.3 C 6 1.0 C under the applied load of 0.46 MPa and heating rate of 2 C/min. Furthermore, they also investigated the effect of silane and alkali treatments on the HDT of PLA composites. The silane-treated kenaf fiber-reinforced PLA composites exhibited the highest HDT value of 174.8 C 6 1.1 C when compared to untreated kenaf fiber-reinforced PLA composites (170.3 C 6 1.0 C), alkali-treated kenaf fiber-reinforced PLA composites (HDT 5 172.8 C 6 0.9 C), and alkali silane treated kenaf fiber-reinforced PLA composites (173.4 C 6 1.0 C). The tremendous enhancement of the heat resistance of all kenaf fiber-reinforced PLA composites may be attributable to the kenaf fiber being able to restrict the deformation of kenaf fiberreinforced PLA composites and thus promote the crystallization of the PLA phase in the composites. Furthermore, the high aspect ratio and long kenaf fibers could improve the PLA matrix and kenaf fiber interfacial adhesion effect by increasing the interfacial bonding strength. Awal et al. (2015) investigated the effect of adding wood pulp and wood pulp/bioadimide into PLA on the thermal behavior of PLA biocomposites via thermogravimetry analysis and HDT test. They reported that the temperature at 5% weight loss, T5wt.%, of pristine PLA is 304 C, while the T5wt.% of wood pulp is 208 C. The T5wt.% of both wood pulpreinforced PLA biocomposite and wood pulp/bioadimide-reinforced PLA biocomposite was found to occur at 274 C. The addition of wood pulp alone and wood pulp/bioadimide into PLA significantly increased the temperature at maximum weight loss of pristine PLA from 350 C to 450 C. On the other hand, the addition of wood pulp into PLA slightly induced the HDT of pristine PLA from 54 C to 56 C. The incorporation of bioadimide with wood pulp into PLA greatly increased the HDT of the PLA biocomposite up to 61 C. This indicates that the presence of bioadimide could provide better wettability of wood pulp fiber in PLA matrix and lead to strengthening of the interfacial adhesion strength between the wood pulp fiber and the PLA matrix (Awal et al., 2015). As a result, the addition of wood pulp with bioadimide would increase the service temperature of PLA composites in material engineering fields.

8.4 Flow Enhancement, Melt Strength, Faster Molding Time The melt strength of a polymer is one of the most important properties used to identify the processability of a particular polymer,

8: POLY(LACTIC ACID) ADDITIVES

AND

PROCESSING AIDS

293

especially for application in high melt drawing or stretching flow, such as film and mold molding. In industrial applications, the low melt strength of PLA could cause poor processability and end-products with poor quality, such as the presence of unstable bubbles in the blow filming process and sinking phenomena during the mold blowing process (Liu et al., 2013; Field et al., 1999). In order to improve the melt strength and processability of PLA, various modification methods such as the incorporation of cross-linking agents, plasticizer, chain extender, and blending with a polymer with higher melt strength have been intensively investigated and developed (Liu et al., 2013).

8.4.1

Cross-Linking Agents

The conventional route of lactic acid polymerization was found to be unable to produce polylactic acid with high molecular weight for a limited polymerization time. The low molecular weight of PLA results in poor melt strength and processability which could cause inferior PLA end-products. Targeting to induce the melt strength of PLA, suitable cross-linking agents or high-energy electron methods were introduced in processing the PLA blends. Nijenhuis et al. (1996) prepared cross-linked PLA by adding a cross-linking agent of dicumyl peroxide at high curing temperature to increase the melt strength of PLA. In addition, various researchers (Liu et al., 2013; Yu et al., 2013) used Luperox and lauroyl peroxide as cross-linking agents to increase the melt strength and melt viscosity of PLA by introducing the formation of a cross-linking structure in PLA. According to Liu et al. (2013), the addition of Luperox and lauroyl peroxide significantly increased the melt strength of PLA, however it caused a reduction in melt strain due to the presence of the cross-linked structure which restricted the slippage effect of PLA melts. They also found that the large molecular size (Mz) is related to the melting strength and flex life which are attributed to the intermolecular interaction restricting the deformation of the PLA matrix. Yu et al. (2012) also improved the melt strength and processability of PLA by adding 0.3% of Luperox and 0.3% of lauroyl peroxide, respectively, into PLA. The addition of 0.3% Luperox increased the melt strength of pristine PLA from (10.5 6 2.1) 3 1022 N to (18.3 6 1.4) 3 1022 N, while the addition of 0.3% lauroyl peroxide increased the melt strength of PLA up to (15.1 6 2.1) 3 1022 N. This is due to the formation of long-branched chains induced by the crosslinking reaction enhancing the tension-stiffening behavior of the elongation flow and restricting the mobility of the PLA. Thus the melt strength and melt viscosity of cross-linked PLA were found to be significantly

294

POLYLACTIC ACID

improved. On the other hand, increasing the cross-linking agent, benzoyl peroxide (BPO), content up to 1 wt.% significantly increased the melt strength and melt viscosity of PLA by promoting the formation of three-dimensional networks (Zhang et al., 2017).

8.4.2 Chain Extenders The application of various chain extenders such as diisocyanate and 1,4-butanediol with PLA has been discovered to improve the melt strength and processability of PLA by increasing its molecular weight (Liu et al., 2013). Di et al. (2005) modified PLA with the application of chain extenders (1,4-butanediol and 1,4-butane diisocyanate) to improve the properties of PLA in terms of melt strength, melt viscosity, and processability. The addition of 1,4-butanediol and 1,4-butane diisocyanate significantly enhanced the melt strength and melt viscosity of modified PLA by producing a PLA foam with smaller cell size and larger cell density. On the other hand, Liu et al. (2013) and Yu et al. (2013) also investigated the effect of chain extenders (pyromellitic dianhydride or PMDA and oxazoline) on the melt strength, melt viscosity, and processability of PLA. The addition of PMDA and oxazoline significantly increased the melt strength, melt velocity, and melt viscosity. The enhancement of melt strength, velocity, and melt viscosity is mainly corresponding to the increasing of molecular weight such as Mz, which the presence of larger intermolecular bonding could restrict the chain deformation ability (entanglement occurs) resulted higher melt strength. Importantly, such observation was also reported by Yu et al. (2013).

8.4.3 Blending With Other Polymers Blending of polymers is one of the most commonly applied methods for improving the melting strength of a polymer with low melt strength and poor processability. In the work conducted by Liu et al. (2013) and Yu et al. (2013), the blending of PLA with biodegradable polyester Bionolle (a copolymer of polybutylene succinate adipate) and Ecoflex (an aliphatic aromatic copolymer) slightly increased the melt strength and processability. The enhancement effect of Bionolle and Ecoflex on the melt strength and processability was mainly attributed to the partial compatibility of these polymers with PLA increasing the physical intermolecular interaction in polymer blends and resisting the deformation of polymer chain. In addition, the blending of PLA with Biomax (elastomer) significantly increased the melt strength, melt strain, and

8: POLY(LACTIC ACID) ADDITIVES

AND

PROCESSING AIDS

295

processability due to good compatibility of PLA and Biomax, and thus induced an intermolecular interaction in the polymer blending system (Liu et al., 2013). According to Gu et al. (2008), the blending of PLA with poly(butylene adipate-co-butylene terephthalate) (PBAT) was observed to improve the melt strength, viscosity, and processability due to the stronger complex shear-thinning tendency of polymer blend melts of PLA/PBAT. Zhang et al. (2017) blended cross-linked PLA (crosslinked with 0 1 wt.% BPO) with PBAT to further improve the melt strength, viscosity, and processability. Their results showed that the incorporation of PBAT could effectively enhance the melt properties such as melt strength and viscosity, of cross-linked PLA by chain entanglement, which resists polymer chain deformation (Zhang et al., 2017).

8.4.4

Plasticizers

Plasticizers are commonly added to polymer blending systems to improve their processability. For PLA, the addition of plasticizers could efficiently reduce the glass transition temperature, Tg, and depress the melting temperature and crystallinity of PLA, which is a semicrystalline polymer (Farah et al., 2016). Various plasticizers such as poly(ethylene glycol) (PEG), acetyl-tri-n-butyl citrate (ATBC), glucose monoesters, partial fatty acid esters, and epoxidized soybean oil (ESO), have been used and investigated in improving the processability and toughness of PLA (Arrieta et al., 2014; Jacobsen and Fritz, 2004; Xu and Qu, 2009; Liu et al., 2013). According to Xu and Qu (2009), the increasing of ESO content as plasticizer gradually increased the melt strength of PLA and reached a maximum at a content of 6 wt.%. The improvement in melt strength is due to the long-branched molecular chains of PLA increasing the intermolecular interactions in PLA melt and thus restricting polymer chain deformation. In addition, the interaction between ESO and PLA may also contribute to the existence of hydrogen bonding between the epoxy groups of ESO and carbonyl groups in the ester linkage of PLA. Tee et al. (2014) investigated the effect of adding two plasticizers, epoxidized palm oil (EPO) and ESO, to improve the properties and processability of PLA. The addition of EPO and ESO was found to significantly reduce the torques (maximum and end), stock temperature, and required time to reach a homogeneous blend. This indicates that the incorporation of both EPO and ESO could provide a better lubrication effect and thus improve the processability of PLA. The EPO plasticizer with lower viscosity than ESO was found to have a better effect in aiding PLA processing.

296

POLYLACTIC ACID

Li et al. (2014) synthesized a plasticizer, lacti-glyceride, for the purpose to improving the thermal processability and melt properties of PLA/PVA blends. The addition of lacti-glyceride improved the processability of PLA/PVA by enhancing the compatibility between PLA and PVA due to the presence of both lactic acid ester groups and hydroxyl groups on lacti-glyceride. Maiza et al. (2015) investigated the effect of adding two different plasticizers, triethyl citrate (TEC) and ATBC, to PLA. The addition of TEC and ATBC decreased the Tg and melt viscosity of PLA and enhanced the processability of PLA. This is because the low molecular weights of TEC and ATBC allow the molecules of TEC and ATBC to occupy the space between the polymer chains and increase the mobility of PLA chains. Saravana et al. (2018) investigated the effect of plasticizers with polyethylene glycol 1500 (PEG1500) and polyethylene glycol 6000 (PEG6000) grades on the properties of talc-reinforced PLA composites. The increasing of both PEG1500 and PEG6000 increased the Tg of talc-reinforced PLA composites. This indicates the good miscibility of PLA and these plasticizers could enhance the mobility of PLA chains and thus improve their processability. Furthermore, the addition of both PEG1500 and PEG6000 could increase the processability by overcoming the agglomeration of talc in the PLA matrix.

8.5 Specialty Additives: Antistatic, Impact Modifier, Fiber Compatibilizer/Coupling Agents 8.5.1 Antistatic Agents For hydrophobic polymers, such as PLA, the electric discharge phenomenon during the molding process can attract dirt or dust and cause malfunction of office machines. In order to overcome this problem, antistatic agents are commonly added to prevent a static electricity charge from forming on the surface of these polymers. The effectiveness of antistatic agents is measured by determining the surface resistivity of the polymer blends (Niaounakis, 2015). The addition of antistatic agents into hydrophobic polymers such as PLA could significantly reduce the surface resistivity of the polymer by 109 1012 Ω (Niaounakis, 2015). In order to reduce the surface resistivity or, in other words, to increase the conductivity of polymer, the addition of conductive fillers such as carbon black, CNTs, carbon fibers, conductive ceramics, and powdered metal into hydrophobic polymers is necessary

8: POLY(LACTIC ACID) ADDITIVES

AND

PROCESSING AIDS

297

(Moon et al., 2005; Silva et al., 2019). According to Silva et al. (2019), the addition of carbon black up to 15 wt.% significantly increased the electrical conductivity and reduced the electrical resistivity of PLA composites. These properties have allowed carbon black-added PLA composites to be use as antistatic packaging for the transportation and storage of electronic devices (Silva et al., 2019). On the other hand, the addition of semiconducting behavior of CNTs can increase the electrical conductivity of polymer composites, as stated by Moniruzzaman and Winey (2006). The addition of CNTs into the PLA matrix significantly decreased the surface resistivity of PLA composites by improving the effectiveness of electromagnetic wave shielding (Moon et al., 2005).

8.5.2

Impact Modifier

As discussed earlier, PLA can be processed by using conventional melt-processing techniques such as extrusion and injection molding. In addition, it also has good biocompatibility, biodegradability, and tensile properties. However, the poor toughness (high brittleness) and low ductility of PLA have limited the application of PLA in various applications. In order to improve the toughness of PLA, impact modifiers can be applied into the PLA blending system to reduce the brittleness of PLA without weakening its stiffness. A few companies have introduced impact modifiers (as tabulated in Table 8.4), which are specially designed for PLA applications. Notta-Cuvier et al. (2014) added 10 wt. % Biomax Strong (BS) 100 impact modifier into PLA and significantly increased the elongation at break and tensile properties of PLA. Furthermore, the addition of 10 wt.% of BS into plasticized Cloisite 25A/PLA composites achieved good ductility while maintaining the rigidity and strength of PLA composites. According to Mat Taib et al. (2012), increasing the BS impact modifier content up to 50 wt.% provided a significant improvement in the notched impact strength and elongation at break of PLA. This also indicates that the addition of a BS impact modifier could increase the toughness of PLA. However, the tensile modulus and yield stress of PLA were found to decrease with an increasing amount of BS impact modifier. This is due to the toughening effect of BS impact modifier decreasing the crystallinity of PLA by increasing the plastic deformation of the PLA matrix. Barletta et al. (2017) added 1.8 wt.% of Paraloid BPM-515 impact modifier into PLA/ talc composites to increase the compatibility effect between PLA and talc fillers and thus improve the toughness. In a study conducted by Diaz et al. (2016), the addition of Paraloid BPM-515 impact modifier

Table 8.4 Different Grades of PLA Impact Modifier Produced by Various Companies Company

Grade

Application

Properties

DuPont

Biomax Strong 100 (an ethylene copolymer impact modifier)

Packaging and industrial application

Biomax Strong 120 (an ethylene copolymer impact modifier) Paraloid BPM-515 (an acrylic impact modifier) Biostrength Strong 150 (high-efficiency core shell impact modifier) Biostrength Strong 280 (acrylic core shell impact modifier)

Food packaging applications

Improves toughness properties, impact DuPont Biomax Strong 100 strength, maintains clarity similar to (2014) polypropylene at a blending level ,5 wt.% Increases toughness, flexibility, impact DuPont Biomax Strong 120 strength, good contact clarity (2014)

Dow Chemical Arkema

Automobile, medical, and electronics industries Opaque application which does not require high transparency Clarity applications

References

Increases toughness and maintains clarity, lower cost

Dow Chemical (2010)

Increases impact strength, effective in durable sheet extrusion, injection molding applications

Arkema (2008); Biostrength 150-Opaque Impact Modifier (2014)

Increases toughness of PLA and maintains clarity

Arkema (2008); Biostrength 280-Transparent Impact Modifier (2014)

8: POLY(LACTIC ACID) ADDITIVES

AND

PROCESSING AIDS

299

into PLA slightly increased the elongation at break. In addition, the impact strength of PLA was observed to rapidly increase with the incorporation of Paralois BPM-515 impact modifier, which also indicates the toughening effect of PLA. Choochottiros and Chin (2013) investigated and synthesized two different transparent impact modifiers for PLA. They synthesized poly(butadiene-co-methyl methacrylate-co-butyl methacrylate-co-butyl acrylate-co-hydroxyethyl methacrylate) (known as BMBH copolymer) and poly(butadiene-co-lactide-co-methyl methacrylate-cobutyl methacrylate) (known as BLMB copolymer) as PLA impact modifiers to improve the toughness and impact strength while maintaining the clarity of PLA by increasing compatibility with the PLA matrix. However, all commercially available PLA impact modifiers as described earlier are nonbiodegradable and they are usually used at 10 wt.% for industrial application (Notta-Cuvier et al., 2014). Due to the nondegradable commercially available impact modifiers mentioned earlier, various types of biodegradable polymers have been used as biodegradable impact modifiers for PLA applications. PCL is one of the most commonly used biodegradable polymers as an PLA impact modifier (Wang et al., 1998; Broz et al., 2003; Notta-Cuvier et al., 2014). According to Wang et al. (1998), the reactively compatibilized PLA/ PCL blends achieved synergistic compatibility and toughness at a PLA/ PCL weight ratio of 80/20. In addition, the biodegradable polymers such as poly(propylene carbonate), poly(butylenes succinate), poly (para-dioxanone), poly(butylenes adipate-co-terephthalate), and poly(tetramethylene adipate-co-terephthalate) have also been used as biodegradable impact modifiers for PLA applications (Notta-Cuvier et al., 2014). Odent et al. (2012) conducted an investigation into the application of poly(ε-caprolactone-co-δ-valerolactone) (a random aliphatic copolyster) as a biodegradable impact modifier to improve of toughness of PLA while maintaining the transparency of PLA.

8.5.3

Fiber Compatibilizer/Coupling Agents

A number of researches have investigated the reinforcement effect of natural fibers (such as kenaf fiber and flax fiber) on the mechanical properties of biodegradable polymers such as PLA and polyvinyl alcohol (John and Anandjiwala, 2008; Foruzanmehr et al., 2016; Lee et al., 2009). Natural fibers with biodegradability, high toughness, high specific strength, low cost, low density, and renewable qualities are a very promising alternative to conventional reinforcing fillers such as calcium carbonate in PLA composites (Foruzanmehr et al., 2016). However, the

300

POLYLACTIC ACID

low compatibility of hydrophobic PLA with hydrophilic natural fibers had significantly weakened the mechanical physical properties of PLA composites and limited the applications of PLA composites (Kumar et al., 2010; Lee et al., 2009). In a study conducted by Foruzanmehr et al. (2016), the interfacial adhesion between flax fibers and the PLA matrix was significantly improved by coating an oxidized TiO2 film on the surface of flax fibers with a solution gel dip-coating technique. The tensile strength and elongation at break of flax/PLA composites were significantly increased after the addition of oxidized-flax/PLA composites. This is mainly attributed to the better interfacial adhesion between fibers and the PLA matrix by effectively transferring the stress between the fibers and the PLA matrix. Lee et al. (2009) investigated the effect of using 3-glycidoxypropyl trimethoxy silane as a coupling agent on the mechanical properties of kenaf fiber PLA composites. The treatment of kenaf fibers with 3-glycidoxypropyl trimethoxy silane significantly improved the interaction between kenaf fibers and the PLA matrix (Lee et al., 2009). Wang et al. (2011) investigated the effect of adding wood flour with a surface treatment with four different coupling agents, vinyltrimethoxysilane (vinyl silane), γ-aminopropyl triethoxysilane (amino silane), γ-glycidoxypropyltrimethoxy silane (epoxy silane), and γ-methacryloxypropyltrimethoxysilane (allyl ester silane), on the mechanical properties of wood flour/PLA composites. The addition of amino silane, epoxy silane, and allyl ester silane significantly increased the tensile strength, elongation at break, and impact strength of wood flour/PLA composites. This is because the silane coupling agents improved the interfacial interaction between the PLA matrix and the wood fibers by forming a “bridge” between the PLA matrix and the wood flour.

8.6 Conclusion The poor properties of PLA, such as brittleness, poor toughness, low thermal stability, and poor processability, can be improved by the incorporation of various additives including reinforcement fillers, etc. Various researches have been carried out by adding suitable additives to modify the existing properties of PLA in order to broaden its applications. In addition, numerous reinforcement additives have been used to improve the mechanical properties of PLA in order to achieve the targeted properties required by various industries. The addition of processing aids into PLA blends was also targeted at improving the processability of PLA during melt-processing. Generally, the incorporation

8: POLY(LACTIC ACID) ADDITIVES

AND

PROCESSING AIDS

301

of additives and processing aids is aimed at improving the mechanical properties, processability, thermal properties, and other specialty properties of PLA, while maintaining its biodegradability and biocompatibility.

References Arkema, 2008. Arkema presents modifier range for PLA. Addit. Polym. 2008, 4 5. Arrieta, M.P., Lopez, J., Rayon, E., Jimenez, A., 2014. Disintergrability under composting conditions of plasticized PLA-PHB blends. Polym. Degrad. Stab. 108, 307 318. Auras, R., Harte, B., Selke, S., 2004. An overview of polylactides as packaging materials. Macromol. Biosci. 4, 835 864. Awal, A., Rana, M., Sain, M., 2015. Thermorheological and mechanical properties of cellulose reinforced PLA bio-composites. Mech. Mater. 80, 87 95. Balakrishnan, H., Hassan, A., Wahit, M.U., Yussuf, A.A., Abdul Razak, S.B., 2010. Novel toughened polylactic acid nanocomposites: mechanical, thermal and morphological properties. Mater. Des. 31, 3289 3298. Barletta, M., Pizzi, E., Puopolo, M., Vesco, S., 2017. Design and manufacture of degradable polymers: biocomposites of micro-lamellar talc and poly(lactic acid). Mater. Chem. Phys. 196, 62 75. Bergsma, J.E., De Bruijn, W.C., Rozema, F.R., Bos, R.R.M., Boering, G., 1996. Late degradation tissue response to poly(L-lactide) bone plates and screws. Biomaterials 16, 25 31. Biostrength 150-Opaque Impact Modifier, 2014. Available from: ,www. palmerholland.com.. Biostrength 280-Transparent Impact Modifier, 2014. Available from ,www. palmerholland.com.. Broz, M.E., Vanderhart, D.L., Washburn, N.R., 2003. Structure and mechanical properties of poly(D,L-lactic acid)/poly(ε-caprolactone) blends. Biomaterials 24 (23), 4181 4190. Carrasco, F., Page`s, P., Ga´mez-Pe´rez, J., Santana, O.O., Maspoch, M.L., 2010. Processing of poly(lactic acid): characterization of chemical structure, thermal stability and mechanical properties. Polym. Degrad. Stab. 95, 116 125. Chen, P.Y., Lian, H.Y., Shih, Y.F., Chen-Wei, S.M., Jeng, R.J., 2017. Preparation, Characterization and Crystallization Kinetics of Kenaf Fiber/ Multi-walled Carbon Nanotube/ Polylactic Acid (PLA) Green Composites. Mater. Chem. Phys. 196, 249 255. Choochottiros, C., Chin, I.J., 2013. Potential transparent PLA impact modifiers based on PMMA copolymers. Eur. Polym. J. 49, 957 966. Cock, F., Cuadri, A.A., Garcia-Morales, M., Partal, P., 2013. Thermal, rheological and microstructural characterisation of commercial biodegradable polyesters. Polym. Test. 32, 716 723.

302

POLYLACTIC ACID

D’Amico, D.A., Montes, M.L.I., Manfredi, L.B., Cyras, V.P., 2016. Fully biobased and biodegradable polylactic acid/ [oly(3-hydroxybutirate) blends: Use of a common plasticizer as performance improvement strategy. Polym. Test. 49, 22 28. Daniels, A.U., Chang, M.K.O., Andriano, K.P., 1990. Mechanical properties of biodegradable polymers and composites proposed for internal fixation of bone. J. Appl. Biomater. 1, 57 78. Di, Y., Iannace, S., Maio, E.D., Nicolais, L., 2005. Reactively modified poly (lactic acid): properties and foam processing. Macromol. Mater. Eng. 290 (11), 1083 1090. Diaz, C.A., Puo, H.P., Kim, S., 2016. Film performance of poly(lactic acid) blends for packaging application. J. Appl. Packag. Res. 8 (3), 43 51. Dow Chemical, 2010. Dow introduces impact modifier for polylactic acid. Addit. Polym. 2, 2 3. DuPont Biomax Strong 100, Product Data Sheet. August 2014. Available from ,www.dupont.com.. DuPont Biomax Strong 120, Product Data Sheet. September 2014. Available from ,www.dupont.com.. Farah, S., Anderson, D.G., Langer, R., 2016. Physical and mechanical properties of PLA, and their functions in widespread applications—a comprehensive review. Adv. Drug Deliv. Rev. 107, 367 392. Field, G.J., Micic, P., Bhattacharya, S.N., 1999. Melt strength and film bubble instability of LLDPE/LDPE blends. Polym. Int. 48, 461 466. Foruzanmehr, M., Vuillaume, P.Y., Elkoun, S., Robert, M., 2016. Physical and mechanical properties of PLA composites reinforced by TiO2 grafted flax fibers. Mater. Des. 106, 295 304. Gu, S.Y., Zhang, K., Ren, J., Zhan, H., 2008. Melt rheology of polylactide/ poly(butylene adipate-co-terephthalate) blends. Carbohy. Polym. 74, 79 85. Guo, X., Zhang, J., Huang, J., 2015. Poly(;actic acid)/ polyoxymethylene blends: Morphology, crystallization, rheology, and thermal mechanical properties. Polymer 69, 103 109. Harris, A.M., Lee, E.C., 2007. Improving mechanical performance of injection molded PLA by controlling crystallinity. J. Appl. Polym. Sci. 107, 2246 2255. Hashima, K., Nishitsuji, S., Inoue, T., 2010. Structure-properties of supertough PLA alloy with excellent heat resistance. Polymer 51, 3934 3939. Huda, M.S., Drzal, L.T., Mohanty, A.K., Misra, M., 2008. Effect of fiber surface-treatments on the properties of laminated biocomposites from poly (lactic acid) (PLA) andkenaf fibers. Compos. Sci. Technol. 68, 424 432. Incardona, S.D., Fambri, L., Migliaresi, C., 1996. Poly-L-lactic acid braided fibers produced by melt spinning: characterization and in vitro degradation. J. Mater. Sci. Mater. Med. 7, 387 391. Jacobsen, S., Fritz, H.G., 2004. Plasticizing polylactide—the effect of different plasticizers on the mechanical properties. Polym. Eng. Sci. 39 (7), 1303 1310.

8: POLY(LACTIC ACID) ADDITIVES

AND

PROCESSING AIDS

303

Jiang, L., Zhang, J., Wolcott, M.P., 2007. Comparison of polylactide/ nano-sized calcium carbonate and polylactide/montmorilonite composites: Reinforcing effects and toughening mechanisms. Polymer 48 (26), 7632 7644. John, M.J., Anandjiwala, R.D., 2008. Recent developments in chemical modification and characterization of natural fiber-reinforced composites. Polym. Compos. 29, 187 207. Kakinoko, S., Yamaoka, S., 2010. Stable modification of poly(lactic acid) surface with nuerite outgrowth-promoting peptides via hydrophobic collagenlike sequence. Acta Biomaterialia. Available from: https://doi.org/10.1016/j. actbio.2009.12.001. Kakroodi, A.R., Kazemi, Y., Nofar, M., Park, C.B., 2017. Tailoring poly(lactic acid) for packaging applications via the production of fully bio-based in situ microfibrillar composite films. Chem. Eng. J. 308, 772 782. Kumar, R., Yakabu, M.K., Anandjiwala, R.D., 2010. Effect of montmorillonite clay on flax fabric reinforced polylactic acid composites with amphiphilic additives. Manufacturing 41 (11), 1620 1627. Lee, J.H., Park, T.G., Park, H.S., Lee, D.S., Lee, Y.K., Yoon, S.C., et al., 2003. Thermal and mechanical characteristics of poly(L-lactic acid) nanocomposite scaffold. Biomaterials 24, 2773 2778. Lee, B.H., Kim, H.S., Lee, S., Kim, H.J., Dorgan, J.R., 2009. Bio-composites of kenaf fibers in polylactide: role of improved interfacial adhesion in the carding process. Compos. Sci. Technol. 69, 2573 2579. Li, H.Z., Chen, S.C., Wang, Y.Z., 2014. Thermoplastic PVA/PLA blends with improved processability and hydrophobicity. Ind. Eng. Chem. Res. 53 (44), 17355 17361. Liu, X., Yu, L., Dean, K., Toikka, G., Bateman, S., Nguyen, T., et al., 2013. Improving melt strength of polylactic acid. Int. Polym. Process. 28 (1), 64 71. Ma, Q., Georgiev, G., Cebe, P., 2011. Constraints in semicrystalline polymers: using quasi-isothermal analysis to investigate the mechanisms of formation and loss of rigid amorphous fraction. Polymer 52 (20), 4562 4570. Maiza, M., Benaniba, M.T., Quintard, G., Massardier-Nageotte, V., 2015. Biobased additive plasticizing polylactic acid (PLA). Polı´meros 26, 1 15. Mat Taib, R., Ghaleb, Z.A., Mohd Ishak, Z.A., 2012. Thermal, mechanical, and morphological properties of polylactic acid toughened with an impact modifier. J. Appl. Polym. Sci. 123, 2715 2725. Migliaresi, C., Cohn, D., Lollis, A.D., Fambri, L., 1991. Dynamic mechanical and calorimetric analysis of compression-molded PLLA of different molecular weight: effect of thermal treatments. J. Appl. Polym. Sci. 43, 83 95. Moniruzzaman, M., Winey, K.I., 2006. Polymer nanocomposites containing carbon nanotubes. Macromolecules 39, 5194 5205. Moon, S., Jin, F., Lee, C., Tsutsumi, S., Hyon, S., 2005. Novel carbon nanotube/poly(L-lactic acid) nanocomposites; their modulus, thermal stability, and electrical conductivity. Macromol. Symp. 224, 278 295.

304

POLYLACTIC ACID

Nagarajan, V., Mohanty, A.K., Misra, M., 2016. Perspective on polylactic acid (PLA) based sustainable materials for durable applications: focus on toughess and heat resistance. ACS Sustain. Chem. Eng. 4, 2899 2916. Ng, H.M., Bee, S.T., Ratnam, C.T., Sin, L.T., Phang, Y.Y., Tee, T.T., Rahmat, A.R., 2014. Effectiveness of trimethylopropane trimethacrylate for the electron-beam-irradiation-induced cross-linking of polylactic acid. Nucl. Inst. Meth. Phys. Res. B 319, 62 70. Niaounakis, M., 2015. PDL Handbook Series, Biopolymers: Processing and Products, vol. 5. Elsevier, Oxford, pp. 239 240. Nijenhuis, A.J., Grijpma, D.W., Pennings, A.J., 1996. Crosslinked poly(L-lactide) and poly([epsilon]-caprolactone). Polymer 37 (13), 2783 2791. Notta-Cuvier, D., Odent, J., Delille, R., Murariu, M., Lauro, F., Raquez, J.M., et al., 2014. Tailoring polylactide (PLA) properties for automotive applications: effect of addition of designed additives on main mechanical properties. Polym. Test. 36, 1 9. Odent, J., Raquez, J., Duquesne, E., Dubois, P., 2012. Random aliphatic copolyesters as new biodegradable impact modifiers for polylactide materials. Eur. Polym. J. 48 (2), 331 340. Ouchiar, S., Stoclet, G., Cabaret, C., Georges, E., Smith, A., Martias, C., et al., 2015. Comparison of the influence of talc and kaolinite as inorganic fillers on morphology, structure and thermomechanical properties of polylactide based composites. Appl. Clay Sci. 116 117, 231 240. Quan, H., Li, Z.M., Yang, M.B., Huang, R., 2005. On transcrystallinity in semi-crystalline polymer composites. Compos. Sci. Technol. 65 (7 8), 999 1021. Rasal, R.M., Janorkar, A.V., Hirt, D.E., 2010. Poly(lactic acid) modifications. Prog. Polym. Sci. 35, 338 356. Renstad, R., Karlsson, S., Sandgren, A., Albertsson, A.C., 1998. Influence of Processing Additives on the Degradation of Melt-Pressed Films of Poly (ε-Caprolactone) and Poly(Lactic acid). J. Envir. Polym. Degrad. 6 (4), 209 221. Saravana, S., Bheemaneni, G., Kandaswamy, R., 2018. Effect of polyethylene glycol on mechanical, thermal, and morphological properties of talc reinforced polylactic acid composites. Mater. Today Proc. 5, 1591 1598. Silva, T.F.D., Menezes, F., Montagna, L.S., Lemes, A.P., Passador, F.R., 2019. Preparation and characterization of antistatic packaging for electronic components based on poly(lactic acid)/carbon black composites. J. Appl. Polym. Sci. 136, 47273. Available from: https://doi.org/10.1002/APP.47273. Swaroop, C., Shukla, M., 2018. Nano-magnesium oxide reinforced polylactic acid biofilms for food packaging applications. Internat. J. Bio. Macromol. 113, 729 736. Tang, Z., Zhang, C., Liu, X., Zhu, J., 2012. The crystallization behavior and mechanical properties of polylactic acid in the presence of a crystal nucleating agent. J. Appl. Polym. Sci. 125, 1108 1115.

8: POLY(LACTIC ACID) ADDITIVES

AND

PROCESSING AIDS

305

Tee, Y.B., Talib, R.A., Abdan, K., Chin, N.L., Basha, R.K., Yunos, K.F.M., 2014. Toughening poly(lactic acid) and aiding the melt-compounding with bio-sourced plasticizers. Agric. Agric. Sci. Proc. 2, 289 295. Tokiwa, Y., Calabia, B.P., 2006. Biodegradability and biodegradation of poly (lactide). Appl. Microbiol. Biotechnol. 72, 244 251. Wang, L., Ma, W., Gross, R.A., McCarthy, S.P., 1998. Reactive compatibilization of biodegradable blends of poly(lactic acid) and poly(ε-caprolactone). Polym. Degrad. Stab. 59, 161 168. Wang, L., Qiu, J., Sakai, E., Wei, X., 2016. The relationship between microstructure and mechanical properties of carbon nanotubes/polylactic acid nanocomposites prepared by twin-screw extrusion. Compos. Part A 89, 18 25. Wang, Y., Qi, R., Xiong, C., Huang, M., 2011. Effects of coupling agent and interfacial modifiers on mechanical properties of poly(lactic acid) and wood flour biocomposites. Iran. Polym. J. 20 (4), 281 294. Wootthikanokkhan, J., Cheachun, T., Sombatsompop, N., Thumsorn, S., Kaabbuathong, N., Wongta, N., et al., 2013. Crystallization and thermomechanical properties of PLA composites: effects of additive types and heat treatment. J. Appl. Polym. Sci. 129 (1), 215 223. Xu, Y.Q., Qu, J.P., 2009. Mechanical and rheological properties of epoxidized soybean oil plasticized poly(lactic acid). J. Appl. Polym. Sci. 112 (6), 3185 3191. Yu, F., Liu, T., Zhao, X., Yu, X., Lu, A., Wang, J., 2012. Effects of talc on the mechanical and thermal properties of polylactide. J. Appl. Polym, Sci. 125 (S2), E99 E109. Yu, L., Toilla, G., Dean, K., Bateman, S., Yuan, Q., Filippou, C., et al., 2013. Foaming behaviour and cell structure of poly(lactic acid) after various modifications. Polym. Int. 62 (5), 759 765. Zhang, R., Cai, C., Liu, Q., Hu, S., 2017. Enhancing the melt strength of poly (lactic acid) via micro-crosslinking and blending with poly(butylene adipate-co-butylene terephthalate) for the preparation of foams. J. Polym. Environ. 25 (4), 1335 1341. Zhou, Y., Lei, L., Yang, B., Li, J., Ren, J., 2018. Preparation and characterization of polylactic acid (PLA) carbon nanotube nanocomposites. Polym. Test. 68, 34 38.

Further Reading Hiljanen-Vainio, M., Varpomaa, P., Seppala, J., Tormala, P., 1996. Modification ofpoly(L-lactides) by blending: mechanical and hydrolytic behaviour. Macromol. Chem. Phys. 197, 1503 1523. Janorkar, A.V., Metters, A.T., Hirt, D.E., 2004. Modification of poly(lactic acid) films: enhanced wettability from surface-confined photografting process. Macromolecules 37, 9151 9159.