Using Reactive Extrusion To Manufacture Greener Products: From

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Chapter 1

Using Reactive Extrusion To Manufacture Greener Products: From Laboratory Fundamentals to Commercial Scale Preetam Giri,1 Chetan Tambe,2 and Ramani Narayan*,1 1Department of Chemical Engineering and Materials Science, Michigan State University, East Lansing, Michigan 48824, United States 2Cardolite Corporation, 11 Deer Park Dr., Suite 124, Monmouth Junction, New Jersey 08852, United States *E-mail: [email protected].

Reactive extrusion (REX) offers a fast, facile, solvent-free, and cost-effective route towards the adoption of green technologies in the commercial space, thus advancing the cause for sustainable industrial practices. In the following work, we present a brief summary of the use of REX in our group to develop value-added biobased or biodegradable products from starch, polylactic acid and other polyesters. We have successfully implemented REX for both the bulk polymerization of monomers, as well as the reactive blending of polymers with suitable modifiers. We have utilized REX for the ring opening polymerization and copolymerization of ε-caprolactone (CL), and 1,4-dioxan-2-one (PDX). REX has been used for the simultaneous maleation and plasticization of starch to produce maleated thermoplastic starch, and its subsequent transesterification reaction with poly(butylene adipate-co-terephthalate). We have also produced starch foams through the REX of starch with water as a blowing agent for insulation and packaging applications. Furthermore, maleation, free-radical branching, and reactive compatibilization of polylactide (PLA) with other polyesters have been carried out using REX. The effect of varying the REX process parameters including temperature profile, feed rate, and screw speed, have been extensively studied. Several of these technologies have

© 2018 American Chemical Society

been commercialized by companies engaged in the biobased materials sector.

Introduction Extrusion-based systems offer a continuous and inexpensive route for the processing of polymers. Specifically, reactive extrusion (REX) is an extremely versatile method that can be well-adapted for a wide-range of applications, including polymerization reactions, chemical modifications of polymers through grafting, branching and functionalization reactions (1), and also physical modifications such as reactive blending and compatibilization (2, 3). Figure 1 shows the schematic of a basic REX process (1). Solid and liquid reactants are fed through a feeder and an injection pump, respectively. The barrel which consists of individually heated blocks, also houses the screw elements, where the mixing and conveying occurs. The final product exits the extruder through a die at the end of the barrel. The motor at the front of the assembly powers the screws by providing the necessary torque. Extruders are usually of two major types: single-screw or twin-screw, and in the case of twin-screw, they are available in two major configurations, i.e. co-rotating and counter-rotating. As evident, in the co-rotating twin-screw extruder, the two screws rotate in the same direction, whereas, in the counter-rotating configuration, the rotation of the screws are in opposite directions to each other (4). The different components of an extruder give rise to several controllable process parameters including the screw configuration, the screw speed, the feed rate and the temperature profile. Through the manipulation of these parameters, we can create a range of unique operating conditions. The screw configuration, is composed of a combination of conveying elements that carry the reaction mixture forward, and the kneading elements, which apply a shear force on the reaction mixture thereby providing the thorough mixing needed.

Figure 1. Schematic representation of a reactive extrusion system. Reproduced with permission from Ref. (1). Copyright 2008 John Wiley and Sons. 2

Reactive extrusion offers several advantages over conventional polymer processing, such as (5): a.

b.

c.

d.

e.

Continuous process with significantly low reaction time compared to batch processing. This prevents prolonged exposure to elevated operating temperatures that could easily lead to the undesired thermal degradation of the polymer being processed. Being a solvent-free process, it eliminates the added costs of having solvent inlet and recycling streams, and also avoids the usage of potentially harmful solvents, thus ensuring a safer approach. The shear forces produced by the screw elements spread out the reaction mixture and create thin layers of the material in the area between the screws. This exposes a high surface area that is available for the reaction to occur. Furthermore, this minimizes the temperature gradients developed due to drastic increase in viscosity during polymerization reactions; thus, ensuring efficient heat and mass transfer within the system. It offers efficient downstream devolatilization of the reaction mixture to remove gaseous byproducts drives the reaction forward, thus resulting in a higher efficiency. Its modular buildup allows for control over several processing parameters. This along with the fact that multiple input streams can be incorporated into the system makes the REX ideal for a diverse range of applications.

REX has been extensively used for the polymerization and property modifications of several biobased and biodegradable plastics (6, 7). This review article specifically covers the work that the Biobased Materials Research Group (BMRG) at Michigan State University has carried out in terms of using reactive extrusion as an efficient process to synthesize value-added biobased products for commercial usage. The review is divided into three brief sections – first, the chemical modification of starch, followed by the chemical modification of polylactide (PLA), and lastly, the ring-opening polymerization of cyclic monomers, using REX.

Chemical Modification: Starch Starch is a naturally occurring anhydroglucose polymer that is typically derived from plant-based biomass. Its use as a biobased platform for the production of polymeric materials has been extensively studied and documented (8, 9). Since it is inexpensive, easily available from biobased sources, and also biodegradable, starch makes for an ideal choice of raw material for the development of sustainable commodity plastics. Starch exists as a mixture of two isomers, amylose and amylopectin, in varying ratios depending on its source. As shown in Figure 2, amylose exists as a linear chain of α D-glucose units linked to each other through α(1→4) linkages, whereas amylopectin forms a branched 3

structure with α(1→4) linear linkages and α(1→6) linkages at the branching nodes. The ratio of the amylose to the amylopectin in starch leads to different physical and mechanical properties (10).

Figure 2. Isomers of starch: amylose and amylopectin. Despite its merits, the two major issues that starch poses in terms of being used as a substitute for conventional plastics are its poor melt processability and its hydrophilicity. The first arises from the fact that starch is not a thermoplastic material and hence does not melt, while the latter owes it to the presence of multiple hydroxyl groups on the starch backbone which leads to extensive hydrogen bonding in the presence of water. The issue with melt processing is addressed by the addition of a plasticizer, most commonly water or glycerol, which breaks down the semi-crystalline structure of starch in the presence of elevated temperatures and shear forces in an extruder (11). Thermoplastic starch, the resultant product can then undergo further processing to produce injection molded articles or blown films. The hydrophilicity of starch is usually reduced through its melt-blending with more hydrophobic polymers or reacting it with a dibasic acid or acid anhydride in the presence of a plasticizer (12). REX has been successfully used to carry out extensive chemical modification of starch in order to functionalize it for various applications (13). Our group has expressly worked on the production of starch foams, the maleation and plasticization of starch and its subsequent transesterification with biobased polyesters to make blown films. Foams Starch foams were produced using hydroxypropylated high amylose cornstarch (70% amylose) and water, with the latter acting as both a plasticizer and blowing agent. For experimental purposes, a lab-scale twin-screw corotating 4

extruder with a 30mm screw diameter and a L/D of 32 was used. This was later scaled up to an industrial scale extruder with an 80mm screw diameter and a L/D of 16. Since effective plasticization of the starch, and even mixing of the blowing agent is critical for the formation of the foams, a combination of conveying and kneading blocks was used in terms of screw configuration of the extruder. Figure 3 shows the screw configuration used for the production of the starch foams. In this study, poly(hydroxyl aminoether) (PHAE), as shown in Figure 4, was used as an additive to impart flexibility and resilience, thus addressing one of the major drawbacks of starch foams. Additionally, talc was used as a nucleating agent for the experimental study. The starch, PHAE and talc were fed using individual feeders, whereas the water was injected using a positive displacement pump (14). Table 1 shows the process parameters used for making the starch foams and the resultant density of the foams produced.

Figure 3. Extruder screw configuration for producing starch foams. Reproduced with permission from Ref. (15). Copyright 2006 John Wiley and Sons.

Figure 4. Chemical structure of PHAE and PHE. The foams produced were characterized for density, compressive strength, resiliency, moisture sorption, thermal resistance. Environmental scanning electron microscopy (ESEM) was used to image sections of the foam. Further, a temperature hold time test and a dynamic cushion curve test was performed for thermal insulation and cushioning applications, respectively (14). 5

Table 1. Process parameters used for the production of starch foams and the resultant density of the foams. Reproduced with permission from Ref. (14). Copyright 2006 John Wiley and Sons.

The compressive strength of the foams was studied as a function of their density. The compressive strength, as plotted in Figure 5, was found to have a power-law type dependence on the density. This can be explained in terms of denser foams having thicker cell walls and thus being able to resist deformation better. The addition of PHAE, at a loading level of 7% of the starch, resulted in almost a ~24% improvement in the resiliency, and a reduction in the weight gained due to moisture sorption by up to 5% of their original weight, thus improving the hydrophobicity of the foams. Thermal insulation properties of the starch foams were compared to those of expanded polystyrene (EPS) and polyurethane foams as shown in Figure 6. Starch foams were found to perform better than the EPS foams.

Figure 5. Logarithmic plot of compressive strength as a function of the density of the foams. Reproduced with permission from Ref. (14). Copyright 2006 John Wiley and Sons.

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The ESEM imaging showed that the addition of the PHAE gave rise to a finer microstructure thus accounting for the improvement in resiliency. Furthermore, the dynamic cushioning test showed that the starch foams had good shock absorption properties (14).

Figure 6. Comparison of expanded polystyrene (♦), starch (□), and polyurethane (▲) foams in terms of thermal insulation. Ambient temperature (■). Reproduced with permission from Ref. (14). Copyright 2006 John Wiley and Sons. An additional study was carried out to evaluate the effect of the addition of poly(hydroxyl ether) (PHE), shown in Figure 4, as compared to PHAE (16). It was found that the addition of this hydrophobic ether substantially improved the moisture sensitivity of the foams. Figure 7 shows the time taken for the penetration of water for the various additives used and a visual of the foam sheets after being kept in contact with water for 5 hours. As evident from the data, the PHE performed better than PHAE in terms of improving the resistance to moisture uptake, while poly(vinyl alcohol) (PVOH) was found to have a negligible effect on the hydrophobicity of the foams (16). As a follow-up to the previous work, several biodegradable polymers including poly(caprolactone) (PCL), poly(butylene adipate-co-terephthalate) (PBAT), cellulose acetate (CA), methylated pectin (MP), were added to the foam formulations to investigate their effect (15). The formulations were optimized to produce foams with the least density. It was observed that the foams with PCL and PBAT in them showed a ~4% lower weight gain when exposed to a relative humidity of 95±5% and a temperature of 38±5°C, as opposed to a starch control foam. Also, the dimensional stability in presence of moisture was improved twofold over the control foam. The improved hydrophobic behavior 7

was attributed to the migration of the polyesters to the surface thereby restricting the uptake of moisture. The other additives were found to have an insignificant effect on the foam properties (15). Furthermore, the maleation of PBAT and its subsequent addition to the starch foam formulations was carried out (17). It was found that the maleation significantly improved the interfacial adhesion of PBAT with the starch, thus leading to a lower foam density and an improvement in resiliency by ~11%. The resultant foams were also found to have improved hydrophobicity characterized by lower weight gain and greater dimensional stability after exposure to moisture (17).

Figure 7. Time taken for water penetration in starch foam sheets extruded with different modifiers, and a picture showing extruded starch foam sheets after contact with water for 5 hours, A: PVOH and B: PHE. Reproduced with permission from Ref. (16). Copyright 2012 John Wiley and Sons. The work done on starch foams at BMRG is commercialized by KTM Industries Inc. (www.greencellfoam.com), located at Lansing, Michigan, to produce biodegradable foam for packaging applications (18). It is currently sold under the trade name of Green Cell Foam. Plasticization and Maleation Plasticization of starch using glycerol, and simultaneous plasticization and maleation of starch using glycerol and maleic anhydride (MA), were carried out using REX. The extrudate of the former process is thermoplastic starch, while that of the latter is maleated thermoplastic starch (MTPS) (19). A corotating twin-screw extruder with a screw diameter of 30mm and a L/D of 42 was used for the processing. The MA was ground to a powder and premixed with the starch before being fed. The glycerol was pumped using a peristaltic pump, at a starch to glycerol compositional ratio of 80:20. Vacuum was applied downstream to remove the unreacted MA and excess moisture. The amount of MA was varied keeping the glycerol content and the processing temperature fixed. The resultant TPS and MTPS were characterized using Soxhlet analysis, Fourier-transform infrared spectroscopy (FTIR), nuclear magnetic resonance (NMR), thermogravimetric analysis (TGA) and X-ray diffraction using a wide-angled X-ray (WAXS) 8

diffractometer. The Soxhlet extraction using acetone determined the amount of glycerol grafted onto the starch backbone. It was found that with the addition 2.5% by weight of the MA, the grafting percentage increased significantly. This was attributed to the covalent grafting of the glycerol on the starch backbone during the maleation. As shown in Figure 8, the maleic anhydride cleaves the starch backbone by hydrolysis, following which the new chain ends react with glycerol to form hemiacetals (12).

Figure 8. Maleation, hydrolysis and glucosidation reactions undergone by starch during the REX process to produce MTPS. Reproduced with permission from Ref. (12). Copyright 2013 Elsevier. The FTIR and NMR analyses establishes the occurrence of the hydrolysis and glucosidation reactions brought about by MA grafted onto the starch. Thermal stability of MTPS was found to be slightly higher than TPS. The WAXS diffraction patterns revealed that the granular structure of the starch was completely disrupted in the MTPS formulations. Also, the MA esterification reactions led to the formation of MTPS with a free carboxylic acid group at the C6 position on the starch. This was further explored in additional studies for promoting acid-catalyzed transesterification reactions (19). 9

A second study looked at the effect of adding a free radical initiator, Luperox 101 in quantities of 0.1 to 0.5% by weight to the MTPS formulations produced using REX. The hydrodynamic radius of the MTPS was found to decrease with the addition of the Luperox. This could be attributed to the fact that the freeradical was causing increased cleaving of the starch backbone giving rise to smaller moieties (20). Transesterification of MTPS with PBAT Based on the findings of our prior work that the free carboxylic group at the C6 position on MTPS would enhance its grafting with polyesters via an acid-catalyzed transesterification mechanism (19), a subsequent study of the REX of PBAT with MTPS was carried out (12).

Figure 9. Proposed reaction mechanisms between MTPS and PBAT. Reproduced with permission from Ref. (12). Copyright 2013 Elsevier. Also, as per the optimization performed in the two previous studies (19, 20), MTPS was synthesized using a 80:20 ratio of starch to glycerol by weight, along with 2% of MA and 0.25% of Luperox via REX. The MTPS and PBAT were fed at a compositional ratio of 60:40 by weight. Soxhlet extractions using dichloromethane (DCM) as the solvent were done to establish extent of grafting 10

of the PBAT onto the MTPS. Since PBAT is soluble in DCM whereas starch is not, the soluble fraction of the Soxhlet extraction was analyzed using FTIR. This confirmed the presence of starch in the soluble fraction, thus proving that the graft copolymer of PBAT and MTPS was definitely being formed. It was also established using acetone based Soxhlet extractions of the final PBAT/MTPS resin that MA enhanced the transesterification reaction between the glycerol on MTPS with PBAT. Figure 9 shows the proposed transesterification reaction pathways between the PBAT and MTPS. Furthermore, the PBAT/MTPS blends were used to produce biodegradable blown films (21). To conclude, the group has extensively worked on the chemical modification of starch using REX, eventually leading to a number of patents being issued for the technology developed (22–28). A few of these patents have been licensed by Ingredion Inc., for the manufacture of value-added products from starch.

Chemical Modification: Poly(lactic acid) Poly(lactic acid) or polylactide (PLA) is a biobased and biodegradable aliphatic polyester derived from lactic acid, a naturally occurring α-carboxylic acid that can be derived through the bacterial fermentation of carbohydrates. Since lactic acid possesses an asymmetric carbon atom, it is an optically active molecule, and can exist either in the L(+) or D(-) enantiomeric form. Currently, all of the commercially available PLA is manufactured through a two-step process where the lactic acid is used to first produce oligomeric poly(lactic acid) chains, which are then depolymerized to generate lactide, a cyclic di-ester of lactic acid which exists in three different stereoisomeric forms as shown in Figure 10. Lactide undergoes ring-opening polymerization in the presence of a tin catalyst to generate high molecular weight PLA. This technology was pioneered by NatureWorks LLC (formerly a part of Cargill Inc.) and forms the basis of their manufacturing plant in Blair, Nebraska with an annual production capacity of 330 million pounds (29, 30).

Figure 10. Stereoisomers of lactide. The thermal and mechanical properties of PLA are strongly dependent on the stereochemical makeup of the PLA chain. The mechanical behavior of PLA, including tensile strength, modulus and elongation at break have been found to be similar to those of polystyrene (PS), demonstrating the case for PLA as a 11

viable biobased alternative to PS. The barrier properties of PLA have been found to be better than that of PS in terms of oxygen, carbon dioxide and moisture permeability. Apart from the performance benefits of PLA, it has also been approved as food contact safe by the Food and Drug Administration in the United States. The biocompatibility of PLA in the human body has also been well documented. Moreover, PLA has been well established as a biodegradable and compostable material. Owing to this unique combination of properties, PLA is used in a variety of applications including manufacture of disposable commodity items, packaging materials, and also biomedical implants. However, there still exist a few issues with PLA that need to be addressed in order for it to be more widely adopted commercially. These include its inherently low toughness, a low heat deflection temperature, and a slow rate of crystallization, that in combination severely restrict its applications (31–37). The group has worked on several projects in order to improve upon the major drawbacks of PLA by utilizing the REX approach, including maleation to improve compatibility with blends (38), free-radical and epoxy-group induced branching for better melt strength (39–41), and grafting of moisture-curable moieties to toughen the PLA matrix (42). Majority of the work on PLA has been carried out using a co-rotating twin-screw extruder with a 30mm screw diameter and a L/D ratio of either 30 or 42.

Maleation Maleic anhydride (MA) was grafted onto the PLA backbone through a free-radical initiated grafting mechanism using REX. 2,5-dimethyl-2,5-di-(tertbutylperoxy)hexane or Lupersol 101, was used as the free-radical initiator. A 2% by weight of MA was used by varying the initiator concentration between 0 to 0.5% by weight. Molecular weight measurements and melt-flow index were used to characterize for the properties of the maleated product. Figure 11 shows the proposed reaction scheme for the grafting of the MA onto the PLA.

Figure 11. Proposed reaction mechanism for the maleation of PLA. Reproduced with permission from Ref. (38). Copyright 1999 John Wiley and Sons. 12

At 0.5% by weight of Lupersol 101, 0.672 weight % of MA was being grafted. It was found that increasing the concentration of the initiator resulted in an increase in the grafting percentage and a simultaneous decrease in the molecular weight of the PLA (38). Subsequent melt blends with native corn starch shows that the maleation process promotes a strong interfacial adhesion of the PLA which would be beneficial to produce biodegradable molded articles (3).

Free-Radical Branching Lupersol 101 was used as a free-radical initiator to induce branching within the PLA matrix using REX. The initiator concentration was varied in between 0 to 0.5% by weight. Also, the process temperature was kept in the range of 160°C to 200°C.

Figure 12. Proposed reaction mechanism for the free radical branching of PLA. Figure 12 shows the proposed reaction mechanism for the free-radical initiated branching of the PLA, where, the methine hydrogen radical abstraction leads to the subsequent branching. An alternate mechanism for the chain scissions could be the intramolecular transesterifications and hydrolysis in the presence of moisture and elevated temperatures inside the extruder. It was reported that the molecular weights and MFI of PLA showed a drastic decrease without the addition of the Lupersol. On the contrary, at temperatures of 170~180°C, and 0.1~0.25% of the Lupersol, a high molecular weight PLA with a low MFI was obtained (39). 13

Epoxy-Based Branching of PLA Joncryl 4368F, a multifunctional epoxy polymer (MEP), was reacted with PLA to obtain epoxy-functional-PLA (EF-PLA) via REX. The MEP was used at loading levels of 1, 5 and 10% by weight. The 5% by weight masterbatch was subsequently diluted to formulations with 0.25, 0.5 and 1% of MEP. A model compound study using stearic acid and poly(ethylene glycol) (PEG) at a weight ratio of 1:1 with MEP was performed by using DSC as an analytical tool. The EF-PLA was characterized using Soxhlet extraction with DCM, gel-permeation chromatography (GPC), and complex and extensional viscosity measurements.

Figure 13. Chemical structure of EF-PLA modifier at low (left) and high (right) MEP concentrations. Figure 13 shows a schematic representation of the EF-PLA structures at low (~5%) and high (~10%) concentrations of the MEP. Using the model compound study, it was shown that the MEP reacts only with the carboxylic groups selectively and does not react with the hydroxyl functionality. This was evidenced by the presence of an exothermic peak in the DSC scan of the stearic acid/MEP mixture, while there was no such effect seen with the PEG/MEP mixture. The subsequent reactive blending of the EF-PLA with neat PLA was carried out. It was observed that this showed a significant improvement over neat PLA in terms of increasing extensional and complex viscosity, and also strain hardening characteristics which are critical especially for blown film applications. Figure 14 shows the extensional viscosity as a function of time for the various blends at different strain rates. The marked increase in the extensional viscosity was quite evident for the formulation with 1% MEP. Blown films using the EF-PLA in neat PLA were successfully produced at much higher blow-up ratios as compared to just neat PLA (40).

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Figure 14. Extensional viscosity at 180°C for PLA and EF-PLA modified PLA at varying strain rates. Reproduced with permission from Ref. (40). Copyright 2015 John Wiley and Sons.

Further, the efficacy of the EF-PLA to compatibilize the blends of PLA and PBAT was investigated (41). Blown films were prepared by the reactive blending of PLA, PBAT and the EF-PLA (5 and 10% by weight of MEP in PLA). The processing parameters including the blow-up ratios were kept constant for all runs. The resultant films were characterized for tensile, dart impact, molecular weight and epoxy content. SEM images were also obtained using fractured cross-sections of the films. Figure 15 shows the reaction between the three components resulting in a highly branched copolymer. Due to the high bubble stability observed with the addition of the EF-PLA, films with higher incorporation of PLA were successfully produced. As a comparison, only 40% PLA could be incorporated into the films without the EF-PLA, whereas 70% of it could be used when EF-PLA was added. Since the EF-PLA was found to react with both the PLA and PBAT chains, an overall compatibilized system was observed, which was characterized by the improvement in the dart impact properties of the films. SEM images confirmed this by revealing a more diffused surface. The epoxide content analysis using titration gave further conclusion that since the EF-PLA with 10% MEP had more residual unreacted epoxy groups than the one at 5% MEP, it could be used as the compatibilizer, whereas the 5% formulation was more suited for use as a rheology modifier (41).

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Figure 15. Schematic representation of the reaction between PLA and PBAT with EF-PLA-10% giving a highly branched copolymer. Reproduced with permission from Ref. (41). Copyright 2016 John Wiley and Sons.

Moisture-Cure Based Toughening of PLA A one-step approach was used to toughen the PLA matrix through the use of REX. Vinyltrimethoxysilane (VTMOS) was grafted onto the PLA backbone, which was followed by the hydrolysis of the methoxy groups on the VTMOS due to residual moisture. Further, these hydroxyl groups condense in the presence of moisture or silanol-terminated polydimethylsiloxane (OH-PDMS) to induce crosslinking among the PLA chains. Figure 16 shows the reactions that lead up to the resultant crosslinked PLA. The crosslinking via VTMOS was found to improve tensile modulus, strength and impact toughness but a simultaneous reduction in the ductility. However, the incorporation of the OH-PDMS led to longer siloxane linkages thus resulting in an increase in modulus, impact and tensile toughness, and the overall ductility of the sample (42). The work done by the group on PLA modification was performed predominantly in collaboration with Natur-Tec®, located in Circle Pines, Minnesota. They currently use several of our modified PLA resins in their commercial products (43).

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Figure 16. Schematic showing the grafting of the VTMOS on the PLA backbone, followed by the hydrolysis of the methoxy groups of the VTMOS, and their subsequent condensation to give a siloxane crosslinked PLA. Reproduced with permission from Ref. (42). Copyright 2016 Budapest University of Technology and Economics, Department of Polymer Engineering.

Ring-Opening Polymerization of Cyclic Monomers The following section covers the work done by the group involving the ringopening polymerization (ROP) of cyclic monomers leading to the formation of biodegradable polymers. The following polymers are not biobased, however, they are biodegradable and still offer an opportunity for a viable end-of-life option resulting in a more environmentally-friendly product.

ε-Caprolactone The ROP of ε-caprolactone was carried out using aluminum tri-sec butoxide (ATSB) as an initiator via the REX route, with a residence times of less than 5 minutes. A screw configuration comprised of all conveying elements was used to prevent the unnecessary scission of the resultant PCL chains.

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Figure 17. Extruder setup for the ROP of ε-caprolactone, also showing the all-conveying screw configuration. Reproduced with permission from Ref. (44). Copyright 2005 John Wiley and Sons. Figure 17 shows the typical extruder setup used to prepare the PCL. The use of ATSB as an initiator gave rise to a novel three-arm PCL structure as shown in Figure 18. The ROP was hypothesized to propagate through a coordinationinsertion mechanism initiated by the aluminum alkoxide.

Figure 18. Schematic representation of the ROP of ε-caprolactone to PCL. Reproduced with permission from Ref. (1). Copyright 2008 John Wiley and Sons. The ε-caprolactone was purified before being fed into the extruder using a peristaltic pump. A 5% by weight solution of the ATSB was prepared in anhydrous toluene and was injected using a pump. The pumps were calibrated to specific feed rates to maintain a constant feed ratio of the monomer to the initiator. The monomer to feed ratio was used to theoretically calculate the molecular weight of each of the arm (45). Table 2 shows the percent conversion of the purified monomer to PCL at a given feed rate. Molecular weights in excess of 100,000 g/mol were obtained at a monomer conversion of 92% (46). Figure 19 shows the comparison between 18

the theoretical and observed molecular weights for the resultant PCL. As shown, the predicted values were quite similar to the molecular weights obtained. The reaction kinetics and thermodynamic aspects of the REX process were also studied and published.

Table 2. Percent conversion of purified monomer to polymer at a feed rate of 75 gm/min. Reproduced with permission from Ref. (46). Copyright 2006 John Wiley and Sons.

Figure 19. Observed vs. theoretically calculated values for number average molecular weight of PCL. Reproduced with permission from Ref. (45). Copyright 2004 John Wiley and Sons. 1,4-Dioxan-2-one The ROP of 1,4-dioxan-2-one (PDX) to produce poly(1,4-dioxan-2-one) (PPDX) was carried out using a REX setup similar to the one used for the ROP of ε-caprolactone. As a second step, copolymers of PCL and PDX were synthesized using a mixture of the -caprolactone and PDX monomers as the feed. ATSB was used as the initiator for all of the reactions. 19

Table 3. Polymerization yields for PPDX by varying temperature, and initial [Monomer]/[Initiator] molar ratio at a screw speed of 130 rpm and residence time ~ 2 minutes. Reproduced with permission from Ref. (44). Copyright 2005 John Wiley and Sons.

The polymerization yields for the various monomer feeds were measured and were found to be as shown in Table 3. Yields of up to ~80% were achieved for the PDX polymerization reactions. Further, an interesting phenomenon was observed for the copolymers of PCL and PPDX. PPDX has a tendency to undergo thermal degradation and depolymerize, however, upon adopting the copolymerization route, this effect was significantly minimized, even by the addition of just 8 mol % of ε-caprolactone to the feed. The work done on the ROP of the cyclic monomers and their subsequent blends with various biodegradable polymers have been extensively studied by the group and patented (47, 48).

Conclusion This chapter provides a broad overview of the work done to date by our group in developing value-added biobased and compostable polymer materials using REX as a facile, “just-in-time” manufacturing tool. Several of these technologies have been successfully commercialized. To summarize: •

•

•

Starch foams were extruded using water as a blowing agent. Several additives were used and their effect on the resultant foam properties were evaluated. The technology is currently under license to KTM Industries Inc., Lansing, MI, for the production of biodegradable packaging foam. Maleated-thermoplastic starch was produced through the REX of starch with glycerol and maleic-anhydride. Subsequent transesterification with PBAT resulted in increased grafting due to the acid-catalysis of maleic anhydride. This produces a starch-polyester blend that has been used for making biodegradable blown films for packaging applications. Maleation and free-radical branching of PLA was carried out using maleic anhydride and Lupersol 101 respectively. The former promotes strong interfacial adhesion of the PLA in PLA-corn starch blends, whereas the latter induces extensive branching within the PLA matrix leading to a high molecular weight and a low melt-flow index of the PLA. 20

•

•

•

An epoxy-functionalized modifier was developed to improve upon the melt-strength of PLA. Furthermore, the modifier was found to act as a compatibilizer for blends of PLA with PBAT for blown film applications. This technology is being currently used by Natur-Tec®, a division of Northern Technologies International Corporation, to produce biodegradable films for packaging. A toughened PLA matrix was obtained by grafting an alkoxysilane onto the PLA backbone, followed by hydrolyzing it with ambient moisture and hydroxyl-terminated polydimethylsiloxane. The silanol groups formed undergo a moisture-cure mechanism to give a resultant PLA matrix with improved toughness. Ring-opening polymerization of ε-caprolactone and 1,4-dioxan-2-one were successfully carried out. Moreover, copolymers of PCL and PPDX were developed through the simultaneous ROP of a mixture of both the monomers.

Acknowledgments The authors would like to thank the following former members of BMRG for their invaluable contribution in the form of their graduate or post-graduate work: Dr. P Dubois, Dr. J-M Raquez, Dr. Y Nabar, Dr. M Srinivasan, Dr. S Balakrishnan, Dr. M Krishnan, Ms. D Carlson, Dr. J Stagner, Dr. E Hablot, Dr. Z Yang, Dr. S Dewasthale and Dr. J Schneider. The authors also acknowledge the financial support provided by KTM Industries Inc. (www.greencellfoam.com), and Natur-Tec® (www.natur-tec.com).

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