COMPARATIVE PERFORMANCE OF PLA AND PET BOTTLES FOR ALCOHOL AND SUGAR ACID SOLUTIONS By Praveen Rawal A THESIS Submitted to Michigan State University in partial fulfillment of the requirements for the degree of MASTER OF SCIENCE Packaging 2011 ABSTRACT COMPARATIVE PERFORMANCE OF PLA AND PET BOTTLES FOR ALCOHOL AND SUGAR ACID SOLUTIONS By Praveen Rawal Poly (lactic acid), PLA, has gained increasing attention in the last decade as it can be obtained from renewable resources and is compostable. PLA has poor barrier properties against moisture vapor and oxygen. The barrier properties of PLA can be improved with silicon oxide (SiOx) coating. The aim of this study was to benchmark the thermal, mechanical and barrier performance of SiOx coated PLA bottles produced with the Plasmax TM process against poly(ethylene terephthlate) (PET) bottles. Four different types of bottles were used: uncoated PLA (PLAU), SiOx coated PLA (PLAC), neat PET (PETS) and 3-layer co-extruded PET (PETM) with nylon as the middle layer. Bottles containing two simulants (alcohol and sugar/ acid solution) and a control (distilled water) were stored for 4 months at 37.8 °C and 70% RH. Bottles exposed to control and sugar/acidic solution showed loss of clarity in segments of the bottles, starting after 4 weeks of exposure. The heat deflection, glass transition temperature, tensile and compression strength for both PET and PLA bottles increased until week 8. After 16 weeks, coated as well as uncoated PLA containers exposed to alcohol solution became brittle. After exposure for 16 weeks, tensile strength and barrier properties decreased and showed statistically significant difference for uncoated and coated PLA bottles exposed to two simulants and the control solution. PET containers did not show any significant change as the length of exposure increased from 8 weeks to 16 weeks. Dedicated to my family iii ACKNOWLEDGEMENTS I would like to express my heartfelt gratitude to my advisor, Dr. Rafael Auras for giving me the opportunity to work on this project. His invaluable guidance and support helped me to complete this project and claim my Masters of Packaging Science degree. I would also like to thank my committee members Dr. Susan Selke and Dr. Janice Harte; both of whom were always guiding me. I am thankful to Diageo for providing partial funding for this project, as well as to the School of Packaging for supporting me by providing teaching assistantship and giving me an opportunity to enhance my teaching skills. I would like to thank Amcor PET Packaging for providing PET and PLA bottles and SIG for applying silicon oxide coating on PLA bottles. I am extremely grateful to following people, who contributed in filling, labeling and placing approx. 2000 bottles in the environmental chamber for the study: Azhari, Hayati, Gaurav, Kang, Min Joo, Nikhil, Sukeewan, Sung wook, Turk and Wontae. Special thanks to Dr. Hazel Ann Hosein for training me on AFM and then scanning few of my samples. I am also thankful to Dr. Alicia Poster and Dr. Xudong Fan for helping me in determining silicon oxide coating thickness of PLA bottles using TEM technique. Thanks to Dr. Nora Bello, Chandni Bhan and Sumit Sinha for helping me with statistical analysis. I would also like to thank faculty and staff members who made me feel as a family member of the School of Packaging. Thanks to Rajeev, Mahesh, Bhupinder, Ajay, Dr. Brijesh tripathi, Eric, Ploy, Oh, Waree, Sanal, Shubham, Pankaj Gaur, Dhiraj, Abhishek, Apurva, Ashish and all other friends and graduate students for their help and support. I am greatly indebted to Pankaj Kumar. I am lucky to have a great friend like him. He has always helped me whenever I needed his help. I must say that he helped and encouraged me a lot iv during thesis writing. I am also thankful to my sister Prachi Kumar for her support. Last but not the least; I want to express my gratitude towards my parents for their understanding, support and love. They have always encouraged me during the entire master’s program. They have greatly contributed in whatever I have achieved in my life. I am also thankful to my brother, sister-in-law, niece and nephew for their love and support. Thanks to everyone else that I might have missed to mention for all their help and support throughout my work Praveen Rawal v Table of Contents List of Tables viii  List of Figures ix  Chapter 1- Introduction 1.1  Applications of Polylactides 1.2  Plastic Bottles 1.3  Dissertation goal and objectives 1  2  2  4  Chapter 2- Literature Review 2.1  Poly Lactic Acid (PLA) 2.1.1  Advantages of PLA 2.1.2  Polylactic acid Disadvantages 2.1.3  Manufacturing of PLA 2.1.4  Processing of PLA 2.1.5  Properties of PLA 2.2  Polyethylene Terephthlate (PET) 2.2.1  General Properties of PET 2.2.2  Manufacturing of PET 2.3  Comparison between PET and PLA 2.4  Bottle manufacturing process 2.4.1  Single Step ISBM process 2.4.2  Two Step ISBM process 2.5  Techniques to improve the barrier properties of PLA 2.5.1  Non-vacuum coating/ Liquid coating Technique 2.5.2  Vacuum Based coating 6  6  6  7  8  8  9  11  11  12  13  15  16  16  17  18  20  Chapter 3- Materials and Methods 3.1  Materials 3.2  Filling operation 3.3  Surface Analysis 3.3.1  Atomic force Microscope (AFM) 3.3.2  Optical Microscope 3.4  Visual inspection 3.5  Physical Properties 3.5.1  Heat Deflection Temperature (HDT) 3.5.2  Glass transition temperature 3.6  Mechanical Properties 3.6.1  Tensile strength 3.6.2  Compression Strength 3.7  Barrier properties 3.7.1  Water vapor transmission rate (WVTR) 3.7.2  Oxygen transmission rate (OTR) 3.8  Weight loss 24  24  24  25  25  25  25  26  26  27  27  27  28  28  28  29  29  vi 4 Chapter 4- Results and Discussion 4.1  Visual Inspection 4.2  Physical Properties 4.2.1  Heat Deflection temperature (HDT) 4.2.2  Glass transition temperature 4.3  Mechanical Properties Analysis 4.3.1  Tensile strength 4.3.2  Compression Strength 4.4  Barrier Properties 4.4.1  Water Vapor transmission rate 4.4.2  Oxygen Transmission Rate 4.4.3  Weight Change Analysis 4.5  Surface Roughness of PLA Bottles 4.6  Optical Microscope Analysis of Coated PLA bottles 30  32  45  45  49  53  53  58  62  62  66  70  74  86  Chapter 5 - Conclusion 89 References 93  vii L IST OF TABLES Table 2-1. Major Properties of PLA and PET 15  Table 3-1. Composition of solutions 25  Table 4-1. Molecular weight for PLAU and PLAC 31  Table 4-2. Heat deflection temperature, °C 47  Table 4-3. Glass transition temperature, °C 51  Table 4-4. Tensile strength, Kpsi 55  Table 4-5. Compression strength (lbs) 60  Table 4-6. WVTR values (gm/pkg/day) 64  Table 4-7. OTR values (cc/pkg/day) 68  Table 4-8. Weight change (%) 72  st Table 4-9. Roughness of the 1 PLA bottle 74  Table 4-10. Roughness in the body of PLA bottles 74  Table 4-11. Roughness of bottles made with different processes 75  Table 4-12. Roughness of the bottle 76  st Table 4-13. Roughness of the 1 coated PLA bottle 77  Table 4-14. Roughness of the body of coated PLA bottles 77  viii LIST OF FIGURES Figure 3-1. DMA samples before and after the clamp was locked. 27  Figure 4-1. Unexposed PLAC bottles 32  Figure 4-2. PLAC bottles exposed to alcohol solution after 4 weeks 33  Figure 4-3. PLAC bottles when exposed to alcohol solution at week 8 34  Figure 4-4. PLAC bottles when exposed to alcohol solution at week 16 35  Figure 4-5. Unexposed PLAC bottles 36  Figure 4-6. PLAC bottles when exposed to sugar solution at week 4 37  Figure 4-7. PLAC bottles exposed to sugar solution at week 8 38  Figure 4-8. PLAC bottles when exposed to sugar solution at week 16 39  Figure 4-9. Unexposed PLAC bottles 40  Figure 4-10. PLAC bottles when exposed to control solution at week 4 41  Figure 4-11. PLAC bottles when exposed to control solution at week 8 42  Figure 4-12. PLAC bottles when exposed to control solution at week 16 43  Figure 4-13. HDT values when exposed to alcohol solution 45  Figure 4-14. HDT values when exposed to sugar/acidic solution 46  Figure 4-15. HDT values when exposed to control solution 46  Figure 4-16. Tg when exposed to alcohol solution 49  Figure 4-17. Tg when exposed to sugar/ acidic solution 50  Figure 4-18. Tg when exposed to control. 50  Figure 4-19. Tensile strength when exposed to alcohol 53  Figure 4-20. Tensile strength when exposed to sugar/ acidic solution. 54  ix Figure 4-21. Tensile strength when exposed to control solution 54  Figure 4-22. Compression strength when exposed to alcohol solution 58  Figure 4-23. Compression strength when exposed to sugar/acidic solution 59  Figure 4-24. Compression strength when exposed to control solution 59  Figure 4-25. WVTR when exposed to alcohol solution 62  Figure 4-26. WVTR when exposed to sugar/acidic solution 63  Figure 4-27. WVTR when exposed to control solution 63  Figure 4-28. OTR of bottles when exposed to alcohol solution 66  Figure 4-29. OTR of bottles when exposed to sugar/acid solution 67  Figure 4-30. OTR of bottles when exposed to control solution 67  Figure 4-31. Weight loss of alcohol solution in bottles 70  Figure 4-32. Weight loss of sugar/acidic solution in bottles 71  Figure 4-33. Weight loss of control solution in bottles 71  Figure 4-34. Roughness of neck of the coated bottle (BN) 78  Figure 4-35. Roughness of the shoulder of the coated bottle (BS) 79  Figure 4-36. Roughness of body of the coated bottle (BB1) 79  Figure 4-37. Roughness of the body of the coated bottle (BB2) 80  Figure 4-38. Roughness of the body of the coated bottle (BB3) 80  Figure 4-39. Body roughness of the PLAC bottle exposed to alcohol for one week. 81  Figure 4-40. 3-D image of PLAC bottle body roughness exposed to alcohol for one week. 82  Figure 4-41. PLAC bottle body roughness exposed to alcohol for two week. 83  Figure 4-42. 3-D image of PLAC bottle body roughness exposed to alcohol for 2 weeks. 83  Figure 4-43. PLAC bottle body roughness exposed to alcohol at week 4 84  x Figure 4-44. 3-D image PLAC bottle body roughness exposed to alcohol at week 4 84  Figure 4-45. PLAC bottle body roughness exposed to alcohol at week 8 85  Figure 4-46. 3-D image of PLAC bottle body roughness exposed to alcohol at week 8 85  Figure 4-47. Optical micrographs of coated PLA bottles stored with alcohol 87  Figure 4-48. Optical micrographs of coated PLA bottles stored with control 88  xi 1 CHAPTER 1- INTRODUCTION PLA belongs to the family of aliphatic polyesters, and it is a rigid thermoplastic that can be extruded as semi-crystalline or totally amorphous, depending on the architecture of the polymer backbone (i.e. the stereochemical makeup of the backbone). PLA is made by fermentation of lactic acid (LA), which can be obtained from 100 % renewable resources. Lactic acid is the basic building block for PLA and is mostly produced by carbohydrate fermentation of corn dextrose [1]. Lactic acid can exist in two optically active isomers, D-lactic acid or L-lactic acid, due to its chiral nature. High molecular weight PLA of about 100,000 Daltons can be produced using three methods: (a) direct condensation polymerization; (b) azeotropic dehydrative condensation and (c) polymerization through lactide formation. Polymerization through lactide formation was developed and patented by Cargill Inc. in 1992, and it is the most widely used method for fabricating PLA resin [1, 2]. In this method, the lactic acid is produced from fermentation of dextrose. This lactic acid is then converted into an intermediate low molecular mass poly(lactic acid) by pre-polymerization of either D-lactic acid, L-lactic acid or a mixture of the two lactic acids. Under low pressure the intermediate low molecular mass PLA is catalytically converted into a mixture of lactide stereo-isomers. The lactide mixture is then purified using vacuum distillation. Lactide is polymerized further using ring opening polymerization to obtain high molecular mass PLA [1, 3]. PLA can also be considered as a unique polymer because in many ways it acts like PET and also performs a lot more like a polyolefin [4]. Previous studies have already shown that PLA is an economically feasible polymer to be used as packaging material [1]. The amount of LA that migrates from PLA is much lower than the amount of LA found in common food ingredients [1, 5]. Therefore, PLA was recognized as GRAS for food applications [5, 6]. 1 1.1 Applications of Polylactides PLA was first commercially used as fibers for resorbable sutures. Then gradually many different prosthetic devices were built up from PLA. Due to the bioresorbable and biocompatible properties of PLA it has been widely studied and used in medical applications [1]. Applications of PLA can range from biomedical implants to packaging to durable consumer goods. PLA can be fabricated into various forms from fibers to films to molded components. Now-a-days, rigid PLA is also used in durable goods. For example, NEC Corporation and Unitika are manufacturing mobile phone components from kenaf fiber-reinforced PLA composites; Samsung is also planning to use PLA blends for mobile components; and Fujitsu is using PLA-based casings for laptop computers [7]. PLA can be potentially used as hollow fiber-fill for pillows, comforters, bulk continuous filament for carpets as well as yarns for apparel. PLA can be used for a broad range of applications, due to the fact that it can be stress crystallized, thermally crystallized, impact modified, filled, copolymerized and processed in most polymer processing equipment [4]. Initially, due to its high cost, PLA was only used for manufacturing high value packaging films, rigid thermoforms, food and beverage containers and coating paper. Due to recent advances in fermentation, production costs of PLA have been dramatically reduced, so now PLA is used in the arena of fresh produce, short shelf life products like fruits and vegetables [8]. Other applications include containers, drinking cups, sundae and salad cups, laminating films and blister packaging, and it is currently used for packaging mineral water [9]. 1.2 Plastic Bottles Bottles are rigid containers which consist of a body, a neck and a mouth. Normally the neck of the bottle is narrower than the body and there is an opening at the top (mouth). The mouth of the bottle is sealed often using plastic caps. Bottles can be made with glass, plastic, and 2 recently aluminum. They are typically used to store liquids such as water, milk, soft drinks, beer, wine, cooking oil, medicine, shampoo, inks and chemicals. The food industry has almost completely replaced glass bottles with plastic bottles due to their light weight and relatively low production cost. Still wine, beer and other alcoholic drinks are still commonly sold in glass bottles due to the fact that they have high barrier properties as compared to plastics. Plastic bottles can be formed using a variety of resins like high density polyethylene (HDPE), low density polyethylene (LDPE), polyethylene terephthalate (PET), polyvinyl chloride (PVC), polypropylene (PP), and polystyrene (PS). The choice of material varies depending upon the application. For example, HDPE is used for laundry and milk bottles; PVC for edible oil, liquor and dairy products; PP for hot fill applications like pancake syrup and PS bottles for pills, tablets and capsules [10]. PET bottles are used for packaging distilled spirits, carbonated soft drinks, and noncarbonated beverages, but the largest single application is soft drink bottles [10]. All these bottles are made from non-renewable resources. 3 1.3 Dissertation goal and objectives According to some of the early theoretical studies based on solubility parameters, it has been predicted that PLA will interact with nitrogen compounds, anhydrides and some alcohols whereas it will have no reaction with aromatic hydrocarbons, ketones, esters, sulfur compounds and water [1]. Since these studies were theoretical predictions, experimental research is necessary to determine the actual compatibility of PLA with these compounds. This study will assess the interaction of PLA with two particular solvents (alcohol and sugar solution). For this, we will compare properties of PLA with PET bottles and see whether the PLA bottles can be used for packaging alcoholic, high sugar and high acidic products for which the PET bottles are already commercially used. As previously mentioned, PLA has low water vapor and oxygen barrier properties; therefore, to overcome this drawback the PLA bottles will be coated with silicon oxide. The PLA (PLAU) and PLA silicon coated (PLAC) bottles exposed to alcoholic, high sugar/acidic and control (water) solution at 37.8 °C and 70 % RH for the duration of 4 months will be compared with PET bottles (PETS) and PET multilayer bottles (PETM) exposed to the same solutions, conditions and time. Specifically, the optical, physical, mechanical and barrier properties of the PLA bottles and silicon oxide coated PLA bottles will be compared with commercially available PETS and PETM bottles used for beverage packaging. This objective is accomplished by conducting a four months shelf life study of the bottles under accelerated conditions (37.8 °C and 70 % RH), and comparing the properties of the bottles at weeks 0, 1, 2, 4, 8, 12 and 16. Specific goals of this thesis are: 1. To assess if a silicon oxide layer can be properly coated on PLA bottles. 2. To compare the optical properties of PLAU and PLAC bottles with commercially available 4 PETS and PETM bottles. 3. To determine and compare heat deflection temperature (HDT) and glass transition temperature (Tg) of PLAU and PLAC bottles with commercially available PETS and PETM bottles. 4. To assess the tensile strength and compression strength of PLAU and PLAC bottles compared with commercially available PETS and PETM bottles. 5. To compare the water vapor and oxygen transmission rate of PLAU and PLAC bottles with commercially available PETS and PETM bottles. 5 2 2.1 CHAPTER 2- LITERATURE REVIEW Poly Lactic Acid (PLA) Poly(lactic acid), PLA, is an aliphatic polyester, generally obtained from renewable resources such as corn [1, 11]. PLA polymers are considered environmentally friendly as they are biodegradable in composting conditions [12-14]. Biodegradable polymers like PLA are generally presumed to have inferior performance to hydrocarbon based polymers, but the mechanical performance of high molecular weight PLA is comparable to that of petroleum-based polymers such as PET and PS. PLA has a high modulus of elasticity and high stiffness. Further, PLA can also be formed into shapes with good definition like other petroleum based thermoplastics [11, 15-17]. Although PLA is at parity with conventional plastics on the aforementioned properties, some of the major drawbacks of PLA for practical applications are still significant, such as its brittleness and low toughness [11]. The building block of PLA is the lactide dimer which exists in three different forms: Llactide, D-lactide and meso-lactide. The distinction between these forms is based upon the rotation of polarized light. The L-lactide rotates the polarized light in the clock wise direction; the D-lactide rotates the polarized light in the anti-clock wise direction, whereas the meso-lactide is optically inactive [18]. Different concentrations of these isomers yield different grades of PLA. 2.1.1 Advantages of PLA Poly(lactic acid), PLA, polymers offer unique advantages over conventional polymers. Lactic acid is the basic building block of the polymer. It can be derived from 100% renewable resources like corn, sugar beets, etc.. PLA is compostable [1, 2, 4, 13, 19] and thus when 6 disposed appropriately in a composting facility will degrade. During composting, PLA mineralization releases CO2 in the same amount as the plant feedstock used CO2 during growth to produce the raw materials for PLA production [1, 4]. So, it can be stated that PLA is almost carbon neutral if PLA production is not included. Furthermore, currently prevalent production methods to produce PLA consume 20 to 50 % less fossil fuels compared to the processes used for production of common hydrocarbon based plastic resins [4, 20]. Thus, PLA is a relatively energy efficient polymer. All these benefits do not hinder PLA’s recyclability as it can be converted back into lactic acid simply by hydrolyzing it with boiling water or steam [1, 19]. PLA, being part of the polyester family, also offers excellent flavor barrier properties, similar to those of PET [1]. 2.1.2 Polylactic acid Disadvantages Like other polymers, PLA has its limitations. One of the major limitations of PLA is poor barrier properties against gas and water vapor as compared to the petroleum based polymers presently used in the packaging industry [19]. In terms of physical properties, PLA is brittle as compared to other polymers used for packaging. This limitation, though, can be corrected and flexibility of the polymer can be increased by adding plasticizers or blending with rubbery polymers such as low modulus polymers or elastomers with low glass transition temperature (Tg). The negative impact of these additional components in the polymer matrix is that they lead to decrease in the strength and modulus of the toughened polymer. So, it is very difficult to obtain the desired stiffness-toughness balance in the final product for processing [19]. This results in PLA having a narrow processing window. 7 2.1.3 Manufacturing of PLA Cargill Dow LLC has patented a method for PLA production. In this method, corn is broken down in the corn wet mill into starch and various other components like protein, fats, fibers, sugar and water. Recovered starch goes through bacterial fermentation and is converted into dextrose. Purified lactic acid is generated by processing the obtained dextrose through acidulation and many purification steps [21]. A continuous condensation polymerization technique is used to produce low molecular weight “PLA pre-polymer” using aqueous lactic acid (D-lactic acid, L-lactic acid or a mixture of the two). This pre-polymer is then converted into lactide stereo-isomers. Tin catalysis under low pressure is utilized for this purpose. The molten lactide mixture is processed through vacuum distillation and purified polymer grade lactide is obtained. Purified lactide is then converted into high molecular weight PLA with controlled optical purity by ring-opening polymerization [2, 19, 22-24] 2.1.4 Processing of PLA PLA in its homopolymer configuration has a very narrow processing window. For example, the L-lactide homopolymer can be processed only within 12 °C of its melting point. This happens because the melting point of homopolymer is extremely high (175 °C) and close to such high temperatures, the molecular chains start to break down. This degradation behavior of PLA is similar to that of PVC [1]. To improve the processing window, the melting point is depressed by blending it with its stereo-isomer D-Lactide component in 90:10 proportions. As a result, the process window broadens to about 40 °C [6]. The main drawback of lowering of the melting point is that it also decreases the crystallization rate of the polymer to a great extent and thus affects the ultimate crystallinity achieved in the polymer [6]. 8 2.1.5 Properties of PLA Structure Unmodified PLA is a linear macromolecule and its stereochemical makeup defines its molecular architecture [1]. The stereochemical composition and properties of the polymer can be controlled by polymerizing different ratios of D-lactide, L-lactide, D,L-lactide or meso-lactide during processing of the polymer [6]. PLA can be semi-crystalline or amorphous depending upon the percentage of L-lactic acid during polymerization. For example, PLA with 93% L-lactic acid is semi-crystalline while PLA produced with L-lactic acid content between 50% and 93% will be amorphous in nature [1, 25]. With currently available technology, PLA cannot be produced without meso-lactide impurities; therefore, most commercial PLAs are copolymers of L and L,D lactide [1, 26]. Thermal properties The proportion of D-lactide in PLA affects the thermal properties of the resultant polymer. The melting temperature of pure poly(D-lactide) or poly(L-lactide) is 207°C [6, 27, 28]. During the manufacturing of the polymer, it is normal to have copolymerization, slight racemization and impurities in the polymer matrix appear. Because of these, the melting point of PLA can range between 130 and 180 °C [1, 6]. For the same reasons, the glass transition temperature can range between 55 and 65 °C [1]. Additionally, an increase in the molecular weight leads to an increase in the melting temperature of PLA and a decrease in its crystallinity [1, 29]. PLA is not thermally stable above its melting temperature, and its thermal stability is inferior to that of PS, PP, PE and PET [1]. Like other plastics, it behaves like rubber above its glass transition temperature whereas it behaves like glass between its glass transition and β transition temperatures. Below its β 9 transition temperature PLA is a brittle polymer [30]. According to Garlotta [6], thermal degradation of PLA starts above 200 °C and the main reasons for the degradation are hydrolysis, chain scission reactions due to oxidation, lactide reformation and inter or intramolecular transesterification reactions. On the contrary, Migliaresi et al. reported the thermal degradation of PLA was due to chain splitting and not due to hydrolysis. They did not observe any oxidation in the main chain, either. Generally the literature [1, 19, 24, 31] has identified the following main reasons for PLA’s thermal instability: (1) Hydrolysis of polymeric chains by trace amount of water producing hydrolyzed lactic acid which works as a catalyst for further degradation, (2) Zipper-like depolymerization; the polymerization catalyst can initiate this degradation, (3) Oxidation reactions can cause chain scission in the main chain, (4) Intermolecular trans-esterification to monomer and oligomeric esters and, (5) Intramolecular trans-esterification resulting in the formation of monomer and oligomeric lactides of low molecular weight. Mechanical Properties Mechanical properties of all polymers depend upon factors like molecular weight, stereochemical composition, crystallinity and arrangement of crystals in the structure. For example, tensile strength increases as the molecular weight of the polymer increases [32]. When PLA is compared to commodity polymers like LDPE, HDPE, PP and PVC, PLA has higher tensile strength and flexural modulus. But when compared to PS, they both have similar properties, they are brittle, have tensile strength greater than 7000 psi, elongation at break less than 5 % and Izod impact strength less than 0.5 ft-lb/in [26]. According to Sinclair [33], tensile 10 strength of the PLA varies depending upon L-lactide content for example PLA having 95 % Llactide was reported to have tensile strength of approximately 10,000 psi [26, 33]. He also suggested that the tensile strength also varies with the percentage of residual monomer present in the polymer; for example the tensile strength PLA copolymer having 5 % residual lactide is approximately 6800 psi, whereas polymer having 0 to 2 % residual lactide is around 8000 psi. Basically, residual monomer acts as a plasticizer in the polymer [33]. 2.2 Polyethylene Terephthlate (PET) PET has established itself as the plastic of choice in various applications because of a variety of advantages that it offers. PET has excellent thermal and chemical resistance, strong mechanical properties, and clarity as well as good water and gas barrier properties [10]. The combination of all these properties makes it suitable for not just packaging applications but many other applications. It is used to make fibers for apparel and to make engineering components [34, 35]. In packaging applications, it is utilized to make films, clamshell blisters for packaging fresh produce, and bottles for beverages [34]. In the USA, PET is widely used in the manufacturing of bottles for carbonated soft drinks, but in recent years, PET for non-carbonated beverage products has grown at a rapid pace [10]. PET retains good mechanical properties at elevated temperatures [34] and that makes it suitable for hot fill applications such as juice and isotonic products [36]. Because of all of its advantages over other plastic materials used for packaging, its use has become dominant. This has allowed for dedicated recycle streams and PET has become the plastic of choice when it comes to recycling. Its universal recycling symbol is “1”. 2.2.1 General Properties of PET PET is a semi crystalline material. Total crystallinity in PET produced by solid state 11 polymerization can be as high as 55% [37]. The extent of crystallinity in a specific PET grade is driven by many factors such as molecular weight and its distribution, nucleating agents, chain orientation and the nature of the catalyst used in polymerization [38]. Crystallinity in PET components can be affected by processing conditions. For example rapid cooling of PET from melting temperature to below Tg produces an amorphous, transparent PET for film and clear bottle applications. A slow cooling injection process produces a PET bottle with opaque crystallized finish such as is used in hot fill applications [36]. Deformation under stress at elevated temperature is much less in semi-crystalline PET as compared to amorphous PET [10]. PET has higher tensile strength than many other general purpose packaging plastics like polyolefins, PVC, etc. For example, HDPE has a tensile strength between 31- 45 MPa while PET can have it between 48.2- 72.3 MPa [10]. Its superior clarity as compared to polyolefin materials makes it suitable for display oriented products. It has very good oxygen, CO2 and flavor barrier [10]. Its thermal stability is well-known and it can be formed into intricate shapes with definition. 2.2.2 Manufacturing of PET PET is a condensation polymer. The process of making the polymer is briefly described here [10, 39]. Manufacturing of PET can be divided into four steps (1) trans-esterification or direct esterification, (2) pre-polymerization, (3) melt polycondensation and (4) solid state polycondensation. PET can be manufactured through two different routes, one using dimethyl terephthalate (DMT) and the other using terephthalic acid (TPA). The basic raw materials are para-xylene and ethylene. Para-xylene is converted into either DMT or TPA. Ethylene is converted into ethylene glycol (EG). In the first step, bis(hydroxyethyl) terephthalate (BHET) is 12 obtained, when DMT and EG are polymerized using a trans-esterification process and methanol is generated as the by-product, which is continuously removed. In the alternate process, TPA and EG are polymerized using a direct esterification reaction and water comes out as the by-product. In the second step, BHET is pre-polymerized to a degree of polymerization (DP) of approximately 30. In the third step, the polymer is further polymerized using polycondensation reaction in order to increase DP to about 100. After this step, PET melt is solidified and formed into chips. These chips go through solid state polycondensation under vacuum and high temperature. The resulting PET has DP greater than 150, which is normally used to manufacture bottle grade resin [39]. Bottle grade PET thus has higher molecular weight, intrinsic viscosity and stronger mechanical properties compared to film grade PET. 2.3 Comparison between PET and PLA Although PET and PLA both are polyesters, the two polymers are vastly different in structure and somewhat different in behavior. PET is aromatic polyester with a benzene ring in each of the repeating units. These benzene units make PET chains stiffer. As a result, PET chains require more energy to crystallize and to melt. On the other hand, PLA is aliphatic polyester. It has relatively small pendent methyl groups which hinder rotation, degree of order and thus, 3 crystallization. For these reasons, the specific gravity of PLA (1.24 g/cm ) [40] is lower than that 3 of PET (1.34 g/cm ) [40]. Normally PET chains are linear, while the PLA molecule tends to form a helical structure [41]. In the case of PET, the rate of crystallization can be controlled by copolymerizing it with either diethylene glycol or isophthalic acid at low levels (1-10%) [4]. Similarly in PLA the crystallization can be controlled by incorporating D-lactic acid units into L-PLA [18]. PLA has a low impact on the environment in terms of greenhouse gas emission. This is 13 because CO2 generated during the biodegradation of the polymer is balanced by CO2 consumed during the growth of plant feed stocks. LCA studies have shown that the total greenhouse gas emission over the life cycle of PLA is about 1600 Kg CO2/ metric ton of material. This was calculated assuming that PLA will go in the compost pile. But in the case of PET incineration is considered the end of life cycle, and LCA studies have predicted that a total of 7150 Kg CO2/ metric of greenhouse gas is emitted throughout the life cycle of PET when it is incinerated [18]. PLA is quite permeable to water, and ester linkages hydrolyze quickly along the backbone of the polymer. This is because hydrolysis is autocatalytic, and with the presence of moisture and residual monomer, it becomes even faster. For PET, the inherent rate of hydrolysis is slow and it is not autocatalytic [18]. Major properties of PLA and PET are summarized in Table 2-1. 14 Table 2-1. Major Properties of PLA and PET Properties PLA Ref. PET Ref. 125-178 [6] 250-265 [42] Glass transition temperature (°C) 56-63 [6] 73-80 [10] Heat Deflection temperature (°C) 55-65 [43] 70 [43] 2.9 [43] 5.7 [43] 70 [43] 70 [43] 1.24 [40] 1.34 [40] Tensile strength (MPa) 68 [44] 57 [44] Elongation at break (%) 4 [44] 30-300 [10] Izod impact (J.m ) 29 [44] 59 [44] Flexural strength (MPa) 70 [43] 70 [43] Flexural modulus (MPa) 3700 [44] 2700 [44] 88 [43] 106 [43] 38-42 [45] 3.0-6.0 [46] 170-200 [45] 15-25 [45] 18-22 [45] 1.0-2.08 [45] Melting temperature (°C) -4 -1 -1 -1 Thermal conductivity x 10 (cal.cm .s .C ) -6 -1 Thermal expansion coefficient x 10 (°C ) -3 Density (g.cm ) -1 Rockwell hardness 2 OTR (cc-mil/100 in day.atm) @ 20 °C, 0% RH 2 CO2 (cc-mil/100 in .day.atm) @ 20 °C, 0% RH 2 WVTR (g-mil/100in .day) 2.4 Bottle manufacturing process The manufacturing process for PLA bottles is essentially the same as that for PET bottles and is known as injection stretch blow molding (ISBM). ISBM can be a single-step or a two-step process. The two step process allows for better process control and more flexibility with machine efficiencies and capability. The same preform can generally be utilized with various sized 15 containers and varying shapes. The injection cycles are usually longer than the blow cycles and this makes the 1-step machines slower. On the other hand, 1-step machines are more energy conserving and cost efficient and justify the cost for smaller scale productions [36]. The two step ISBM process becomes prohibitively expensive for such small commercial scale activities because investment is needed for both injection machine and mold and for the blow machine and mold. Some of it is alleviated if an existing preform can be utilized for the new container. Regardless of whether it is a one step or two step process, ISB molding allows for biaxial orientation of the polymer. The stretch rod provides orientation in the axial direction, and the compressed air that forms the container introduces orientation in the hoop direction. Higher orientation allows for better clarity and at the same time increases crystallinity, accounting for increased mechanical and barrier properties of the bottle [1, 10, 25, 30, 36, 46]. These processes are described below. 2.4.1 Single Step ISBM process In the single-step process, the resin is melted and formed into a preform. The preform is then cooled to 100- 120 °C, well below the melting temperature of PLA but above its glass transition temperature. This is most efficiently done in the same machine and cooling is carried out at the conditioning station. Once cooled to an appropriate temperature, the preform is stretch blown at the blow molding station [30]. 2.4.2 Two Step ISBM process The two-step ISBM process, also referred to as the cold process, is essentially the same as the single step process except the preform is cooled to ambient temperature [36] before blowing and then reheated just before blowing. This allows for maximizing production capacities since it allows for concentration on individual processes or steps while the preforms are stored in the 16 warehouse in the interim. In the first step, the preforms are made using an injection molding machine. These preforms are later blown in a separate stretch blow machine. In injection molding, the molds get clamped and the extruder nozzle moves forward to inject the PET or PLA melt into the mold cavity. In order to compensate for the material shrinkage during cooling in the mold, the screw is kept in the forward position by a holding pressure. After the holding phase, the nozzle shuts down and the screw begins to retreat to its original position to initiate the next cycle. The injected preform is cooled to ambient temperature [10]. In the second step, the preform is conveyed on a rotating spindle and passed through an infra-red bank oven, where it is heated to 85 – 95 °C, the optimum temperature for blow molding in a 2-step process [46]. The heated up preform is transferred to the blow mold and the blow nozzle moves down to make a seal on the preform neck. A stretch rod then moves inside the preform towards the tip of the preform at a speed of 1.2 - 2 m/s and stretches the preform towards the base in the blow mold [46]. Compressed air at relatively low air pressure of about 0.2 - 0.5 MPa is simultaneously blown into the preform to partially inflate the preform, so that it does not touch the stretch rod. Once the stretch rod has traveled to the base cup, the air pressure is increased to 3.8 – 4.0 MPa to form the preform into the desired shape with good definition [30]. 2.5 Techniques to improve the barrier properties of PLA As pointed out earlier, PLA has poor barrier properties against moisture vapor, oxygen and many other permeants. In previous studies, four main approaches to improve the barrier properties of PLA have been described. These are 1) fusing nano or micro fillers in the polymer matrix, 2) blending with polymers with better barrier properties, 3) using a multilayer structure 17 and 4) applying a barrier coating. Out of the four approaches, two have obtained commercial acceptance and are commonly used in the industry. One of them is using multilayer structures and the other is applying a barrier coating. When PLA is used in a multilayer structure with barrier polymers which are often conventional non-biodegradable hydrocarbon based plastics, it negatively impacts the biodegradability and compostability of PLA. This undermines the most important benefit of using PLA as a packaging material. Thus, at present, coating with barrier materials is the only practical way of enhancing barrier properties of PLA. Some of these processes are discussed and currently they are commercially available for coating PET bottles. PLA can be coated with thin layers of organic or inorganic coatings. These coatings such as silicon oxide (SiOx), aluminum oxide (Al2O3), and diamond-like coating (DLC) have been proved to improve the barrier of PET bottles against oxygen and organic vapors [36, 47, 48]. These coatings are applied either under atmospheric pressure or under vacuum. 2.5.1 Non-vacuum coating/ Liquid coating Technique Coating carried out under atmospheric pressure includes applying a liquid phase chemical solution and curing it with ultra-violet light or thermal radiation. This technique allows for coating to be carried out without vacuum, unlike traditional chemical vapor deposition technique and thus is more energy efficient. Non-vacuum or atmospheric based coatings/ liquid coatings have many advantages over vacuum coating techniques in terms of food contact regulations, recyclability and cost. As the coating is applied on the external surface of the bottles, it does not come in contact with the food and thus does not fall under food contact regulations. The coating can be easily removed by an aqueous cleaning solution in the recycling process, so it does not affect the recyclability of the base material. This coating method is considered cheaper as compared to the vacuum deposition technique since the vacuum chamber adds cost to the bottles 18 [49]. One of the disadvantages of this technique is that the coating is susceptible to scratches, scuff and physical damage during transportation. A few of these techniques are detailed here. Bairocade TM Barrier Coating TM One of the first of the barrier coatings was Bairocade (Pittsburgh, U.S.A) gas barrier coating introduced by PPG Industries [36]. This coating consists of epoxy-amine. It is electrostatically sprayed on the external surface of PET bottles under atmospheric pressure. Organic solvents are evaporated and the polymer film is cross-linked or thermoset by curing it at 65 °C in an infrared oven [50]. The cross linked coating is about 6 to 8 microns thick, enough to offer excellent CO2 and O2 barrier with PET containers [36, 51]. This coating is currently used for both carbonated soft drinks (CSD) and juice applications [50]. To apply this method to coat PLA will require modification in the curing temperature. PET has a glass transition temperature close between 73- 80 °C, [10] and so it can sustain the curing temperature of 65 °C. PLA has a glass transition temperature between 56- 63 °C [6] and will have problems if cured at 65 °C. BLOX TM TM BLOX is a barrier coating developed by Dow Chemicals [36, 49]. It is an amorphous thermoplastic epoxy resin which is clear, tough and highly adhesive. This material could be used TM as a barrier layer in multilayer structures or as a coating material for the bottles. BLOX is claimed to provide barrier to oxygen and carbon dioxide which is 10 times better than polyethylene naphthalate (PEN). It is also claimed to be more cost competitive than the other alternative barrier polymers. Dow (Midland, MI, U.S.A) and Tetrapak (Geneva, Switzerland) TM collaborated to produce PET performs with a layer of BLOX Nanolak called Sealica [36, 48, 52]. TM An exterior coating by InMat ® TM and introduced by the trade name Nanolak 19 is an aqueous suspension of nano-dispersed silicates such as vermiculite and montmorillonite dispersed in a polyester matrix. The coating can be applied to the substrate by a spray or gravure coating process. There are hundreds of nano-dispersed silicate platelets per micron of coating thickness. This dispersion creates a tortuous path for the permeating molecules such as oxygen, carbon dioxide and aromatic compounds and thus improves the barrier properties of the material. ® TM InMat has claimed that Nanolak provides a very efficient barrier option which can reduce the permeability of uncoated substrate by up to 100 times. For oxygen barrier, 1-2 microns of this coating is as effective as 12 microns of EVOH film [53]. Combustion Chemical Vapor Deposition (CCVD) Microcoating Technologies Inc., (Atlanta, GA, U.S.A) now known as nGimat also developed a nanopowder coating which employs a combustion chemical vapor deposition (CCVD) technique to coat nanopowder on polymers. The manufacturer claims that the coating provides good barrier against oxygen and carbon dioxide and can increase the shelf life of 20 oz. carbonated soft drink bottles from 10 to 30 weeks [54, 55]. 2.5.2 Vacuum Based coating In this technique the intended coating materials are heated to form gases and then deposited as a solid thin layer on the substrate The coating can be formed by condensation i.e. physical vapor deposition (PVD), or by chemical reaction to form a new compound after volatilization, i.e. chemical vapor deposition (CVD) [56, 57]. Most of this process is completed under vacuum. When coating with Al2O3 or SiOx, thin layers of oxides are formed. Oxides are chemically inert. They are stable even at high temperatures and are resistant to oxidation. Since oxygen is the most electronegative divalent element in the periodic table, the oxides formed have 20 a significant degree of ionic bonding. Thus, these coatings have characteristics of ionic crystals, which means high optical transparency, high electrical resistivity, low thermal conductivity and chemical stability [56, 57]. In order to improve the gas barrier properties of the films and bottles, mostly silicon and aluminum oxide coatings have been used in the packaging industry. Silicon oxide coating has been gaining significant ground in recent times and is discussed here. Silicon Oxide Coating Silicon oxide, commonly known as silica is a widely used industrial coating material in the optics and microelectronics sectors. Silica has a high melting point of 1610 °C and a -6 -1 coefficient of thermal expansion of 0.5 x 10 °C . There are many reasons for the popularity of silicon oxide coating (SiOx, (1