Additives/Masterbatches
Additives/Masterbatches Enhancing biopolymer performance Polycaprolactone technology offers tailored degradation The demand for fully biobased, biodegradable materials – in contrast to the traditional use of petroleum-based feedstocks - represents a major challenge for the plastics industry. Renewable materials like polylactic acid (PLA) and starch often cannot meet exacting performance or biodegradation criteria expected by customers. Polycaprolactone (PCL) thermoplastic technology, which has been used commercially for over 40 years, has become increasingly valuable due to the ability of these polyesters to offer tailored degradation at the end of the final product’s useful life. Using PCL allows for the production of durable, flexible materials that can biodegrade rapidly under aerobic and anaerobic conditions and has a minimal impact on formulation cost and renewable material content. This means formulators can benefit from performance characteristics previously achievable only with the use of petroleum-based feedstocks. PCL structure and properties PCL is an aliphatic polyester consisting of five methylene units separated by an ester group, a structure that imparts little steric hindrance and renders it semi-crystalline in nature. Such polymers are typically manufactured via ring-opening polymerization of ε-caprolactone. Because the manufacture of PCL uses only small amounts of catalyst and yields products with remarkably low acid values, formulators can create high-performance end-use products that better resist degradation during their functional lifetime. In bioplastic applications, PCL is most effective when used as the minority component (10-30 %) in compounded products, improving the properties of renewable components [1]. PCL itself has a low melting range (50-60 °C), making it easy to use and shape while having a minimal impact on the melting point of compounded products. PCL-based products like Ingevity’s Capa ® offering are available in pellet form that can be fed directly into extrusion equipment without predrying. The mechanical properties of PCL are dependent upon molecular weight, but high elongation and low modulus are common across the range of molecular weight. The low glasstransition temperature (Tg -60 °C) means that PCL remains flexible even at low temperatures. Enhancing biopolymer performance Existing biopolymer materials can be made more environmentally friendly by combining different polymer types in order to customize desired mechanical and biodegradation properties. This approach eliminates the long waiting process to qualify a new polymer technology. And the tailored materials still meet challenging performance specifications while also being competitively priced when compared to conventional plastics. Many biopolymers are brittle and have lower mechanical strength when compared to PCL (Table 1). When blended with such materials, PCL behaves as a polymeric plasticizer, improving flexibility and giving the resulting material higher impact toughness. The low melting point, relative to competitive materials such as polybutylene adipate-co-terephthalate (PBAT), facilitates the dispersion of PCL in the blend but also enables lower processing temperatures. This is critical for many biopolymers that tend to be thermally sensitive and start to degrade at temperatures close to the melting point. PCL also acts as a lubricant, which is beneficial for biopolymers that are sensitive to the effects of shear during processing. PLA PLA is a rigid, brittle and transparent material based on monomers that are manufactured from the fermentation of carbohydrates, such as corn or sugar cane, and is a commonly used biopolymer. PLA is typically used in bottles and packaging due to its transparency and suitability for food-contact applications. However, the brittleness and low impact strength of PLA tend to limit its more widespread use. The combination of PLA and PCL can impart improved mechanical properties to materials (Fig. 1), creates materials that are home compostable, and has minimal impact on the overall renewable content and formulation cost [2]. Table 1 – Comparison of mechanical properties of biopolymers. (tests performed on 250 µm films). Property Method Capa 6500 Capa 6800 PBAT PLA LDPE Molecular Weight (g/mol) GPC 50000 80000 > 100,000 - - Density (kg/m3) ISO 1183 1.15 1.15 1.26 1.24 0.9 Melt Flow Index @ 190 °C, 2.16 kg (g/10 min) 28 7.3 3.7 3.7 1.0 Melting Point (°C) DSC 60-62 60-62 110-115 152 111 E Modulus*, Mpa ISO 527-3 315 260 80 >1800 240 Tensile Strength * (MPa) ISO 527-3 55 65 48 61 > 26 Ultimate Elongation * (%) ISO 527-3 >1300 >1600 >1500 7 >300 *: Mechanical tests performed on 250 µm films 24 bioplastics MAGAZINE [03/20] Vol. 15
Additives/Masterbatches By: Scott Phillips, Technical Market Development Manager Anthony Maher, DEVELOPMENT Chemist Ingevity North Charleston, South Caroline, USA Tensile Strength, MPa Tensile Strength, MPa Fig. 1 – The mechanical properties of blends of PLA (Ingeo 2003D, NatureWorks) and Capa thermoplastic. 80 70 60 50 40 30 20 10 0 0 0 20 40 60 80 100 Capa 6500, % 80 2000 70 Tensile Strength Ultimate Elongation 1800 1600 60 1400 50 1200 40 1000 30 800 600 20 400 10 200 0 0 0 20 40 60 80 100 Capa 6800, % Thermoplastic starch Tensile Strength Ultimate Elongation 2000 1800 1600 1400 1200 1000 800 400 200 Starch is a low-cost, renewable raw material, readily available as a byproduct of the food industry. However, starch presents several challenges when being considered for use as a functional polymer: It is brittle, sensitive to moisture, difficult to process, and has poor barrier properties. Many of the shortcomings of starch-based polymers can be improved by adding PCL, which has shown to improve hydrophobicity, increase flexibility, and reduce the thermal sensitivity of starches during processing [3,4,5]. PCL also enhances barrier properties, which allows starch-based polymers to be used in packaging and paper coatings because of their ability to retard the penetration of water and other chemicals [1]. Polyhydroxyalkanoates (PHAs) PHAs like polyhydroxybutyrate (PHB) have emerged as promising biopolymers due to their high rate of biodegradation. Such materials can be produced by bacterial fermentation of biobased feedstocks, including industrial waste. PHB is a linear aliphatic polyester capable of biodegrading rapidly in composting conditions but is also thermally sensitive, brittle and rigid - which limits its usage. PCL acts as a polymeric plasticizer in blends with PHB, enhancing its processability, as 800 Ultimate Elongation, % Ultimate Elongation, % well as improving mechanical performance and maintaining its impressive biodegradability profile [6,7]. Biodegradability and stability The potential negative impacts associated with traditional plastics on the environment continue to underscore the importance of more innovative, biodegradable options. PCL is readily biodegradable in aerobic and anaerobic conditions, which is a key requirement in many bioplastic applications, and biodegrades rapidly under composting conditions in comparison to many other polymers. Combining PCL with other biopolymers allows the biodegradation rate of materials to be tailored precisely for the intended application [8]. Aerobic biodegradation The rapid biodegradation of PCL is possible due to its semicrystalline nature and the low melting point of the crystalline regions. Under home and industrial composting conditions (Fig. 2 and 3), the polymer backbone is sufficient enough in the amorphous phase that it can be digested by microorganisms through process of enzymatic cleavage. In competing systems, such as polybutylene adipate-co-terephthalate (PBAT), the rate of degradation is thought to be retarded by the presence of more crystalline aromatic regions. Due to the crystalline nature and higher melting point of PLA, biodegradation is very slow (with chemical hydrolysis initially dominant over enzymatic degradation) [9]. Ingevity’s PCL-based products are certified as OK Compost Industrial” and OK Compost Home by TÜV Austria. Anaerobic biodegradation Recently, the industry has become increasingly interested in anaerobic digestion because it provides the added benefit of biogas production. PCL has been shown to be biodegradable under anaerobic conditions, achieving complete biodegradation within 90 days (Fig. 4). Stability PCL is unique among biopolymers due to its ability to degrade under aerobic and anaerobic conditions, yet also to remain stable during its functional lifetime. PCL has been shown to outperform both PLA and PBAT in hydrolytic stability tests (Table 2). The growing use of PCL PCL-based compounds are becoming increasingly valued in paper coatings because of their ability to enhance the flexibility of rigid biopolymers and act as an effective barrier for moisture and grease. Particularly for extruded coatings, PCL can increase the melt flow of materials like PLA, requiring the use of less material to fully coat the substrate. bioplastics MAGAZINE [03/20] Vol. 15 25
