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Issue 02/2018

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Basics Mechanical

Basics Mechanical recycling of bioplastics Part I: recycling of production waste of biodegradable plastics Despite great efforts by plant engineers, machine manufacturers and production teams, the amount of production waste generated during the processing of plastics can only be minimized, but not completely avoided. Such production waste can be melt lumps, sprues and defective parts in injection moulding, parison waste in blow moulding or for example edge trim, cutoffs and offspec startup material in film and fibre production processes. It is obvious that the best and most economical way of treating this is avoiding it, but experience shows that still in most of the cases 2 to 10 % of the production material is lost due to process reasons. As long as this cannot be avoided, it is at least essential to recover these spoilt materials and bring them back to the production process. This can be done by means of tailor-made recycling concepts and thus significantly reduce production costs. The property profile of the recycled material may differ only slightly from that of the original granulate - this is regarded as a quality mark of an efficient recycling technology. The easiest way of recovering these materials is to shred them to chips and refeed them to the process together with the new material, but as easy as that is, it can lead to process difficulties like: • Inconsistent feeding performance of the production extruder • Air inclusions in the melt that lead to defects of the finished products • No way to remove process materials like printing inks and similar The choice of the optimal recycling technology is mainly driven by the objective to bring the material through the process without damaging its chemical and physical properties. Some conventional plastics even have to be handled with care during processing in order to avoid material degradation. When processing biodegradable plastics, special attention must be paid to low and uniform processing temperatures (avoiding any temperature peaks), increased sensitivity to shearing and oxidation. The same applies, of course, to the production of recyclates from production waste, which must not lose mechanical and chemical properties compared to the original granulate. A traditional way of feeding materials to a recycling extruder is the use of a cutter-compactor. In principle, this is a cylindrical hopper with a fast-rotating knife disk at the bottom. By this knife disk, the material is cut and agglomerated by the heat induced by the fast-rotating disk and then fed to the extruder screw by centrifugal force. Opposed to that concept, NGR recycling machines (Feldkirchen, Austria) feed the material to the extruder by an integrated cutter-feeder. The principle of the cutter-feeder-extruder combination fulfils all requirements for the design of a recycling plant for exactly this purpose. With this recycling solution, the production waste is fed into a single machine and processed into recyclate - this ONE-STEP arrangement prevents contamination (dust, unclean intermediate storage of ground material, etc.) from impairing the recycling success. Even when the material is shredded by the slow-running shredder, temperature peaks are minimized. The material to be cut is fed into the extruder in less than one minute, thus preventing oxidation of the material. Finally, a tailor-made screw and a long extruder ensure gentle melting at very low shear rates. A hot die pelletizer finally processes the plastic melt into a high-quality pellet. Experience shows that the cutter-compactor often comes to its limits when used for biodegradables, as the heat being put to the material before the extruder – when the material is still in contact with air – leads to degradation that damages the material. The NGR Cutter-Feeder-Extruder on the other hand has proven its ability to recycle most of the commonly used biodegradables, such as starch or PLA based products. Additional to the benefits of the cutter-feeder, screws, vacuum vent and melt filter can be tailored to the special requirements of biodegradables as for example low heatup of the material or the removing of printing inks. NGR expects that with the increasing use of biodegradables, recycling of processing waste will gain further importance. So, NGR see themselves well prepared for the actual and future demands of these applications. MT Professional recycling of biodegradable plastic film using a shredder-feeder-extruder combination Biodegradable film material pelletized recyclate (photos: NGR) 52 bioplastics MAGAZINE [02/18] Vol. 13

10 Years ago Published in bioplastics MAGAZINE Materials Improving heat-resistance of PLA using poly(D-lactide) by Sicco de Vos Sr. Polymer Product Development Engineer PURAC Innovation Center Market Unit Chemical & Pharma Introduction A solution for the low heat-resistance while maintaining transparency would accelerate the acceptance of PLA and widen the application window. The use of PDLA results in the formation of PLA stereocomplex crystallites (sc-PLA) that act as a so-called nucleating agent and crystallization enhancer for PLA. Six years of innovative research and development at Purac have resulted in the commercial availability of D(-)-lactic acid and D-lactide, the monomer that enables large-scale utilization of PDLA. Concept of Stereocomplexation In 1987, Tsuji & Ikada were the first to publish about the handshake of mirrored polylactides: the stereocomplexation between poly(L-lactide) (PLLA) and poly(D-lactide) (PDLA) that produces crystals with a melting temperature beyond 200°C. Many scientific papers on stereocomplex-PLA followed, but its commercial utilization had to wait until the 21st century. A decisive development for commercialization of PDLA is the start-up of a dedicated plant for D-lactic acid in 2008 by Purac. In addition, Purac plans to expand its product portfolio with L- and D-Lactide. These cyclic monomers can be polymerized in one-step to PLA, thereby eliminating several time- and cost-intensive technologies for the PLA producer. Purac PDLA is a bio-based additive for blending with PLA with the following benefits: • More efficient nucleating agent in injection molded PLA than talc at lower loading • Semi-crystalline PLA plastics with HDT B values of 100 - 150°C possible • PLA plastics with better heat resistance enable new applications, e.g. automotive parts • Reduced shrinkage of film and fiber • Bulk density of PLA unchanged Purac’s roots in PLA Materials Figure 1: PLA cup collapsed with hot coffee PLA was already developed in the 1960s for use in medical applications. Purac is a significant player in that market with its medicalgrade lactides, PLLA, and other resorbable lactide copolyesters. Fermentation is the core expertise of Purac and is used to produce lactic acid, the key ingredient for lactide. Lactide is the cyclic dimer of lactic acid that is basically obtained by dewatering lactic acid. Subsequent ring-opening polymerization of lactide is the simplest route to PLA. PLA Material Properties: Strengths and Weaknesses PLA for technical applications has conquered a promising market volume and is the strongest growing bioplastic with a favorable environmental footprint. The added value of PLA originates primarily from its unique combination of properties, such as high optical clarity, rigidity and strength, and favorable gas and water barrier properties for food packaging. These properties can be modified by value added PLA stereocomplex crystals (sc-PLA) are formed by mixing polymers resulting from separately polymerized L-lactide and D-lactide. Melt-blending PLLA and PDLA produces crystals, by association of the polymers in a 1:1 ratio, with a melting temperature of about 20°C, i.e., at least 50°C higher than common PLA. This semi-crystalline sc-PLA is a suitable polyester for melt-spun fibers and biaxially stretched film. Melt-blending of PLA (copolymer) with a few percent of PDLA as an additive, produces sc-PLA crystallites in the PLA melt by racemic crystallization of the PDLA with an equivalent amount of PLA. Upon cooling the melt, e.g. during injection molding, the presence of the sc-PLA crystals promotes crystallization. Thus, the sc-PLA crystals act as heterogeneous nucleation sites for PLA crystallization and are nucleating agents. The nucleation efficiency of PDLA is superior to that of talc, a common filler and nucleating agent. PLA can crystallize 20-0 times faster by blending with only 1-5% (w/w) of PDLA. The resulting material will exhibit a higher derystallinity, which will translate macroscopically egrity up to higher temperatures and will hat con- 22 bioplastics MAGAZINE [02/08] Vol. 3 PLA Materials Figure 2: Large PLLA crystals formed slowly upon cooling from the melt at 140°C. bioplastics MAGAZINE [02/08] Vol. 3 21 In March 2018, François de Bie, Total Corbion PLA, says: polymer technologies, such as co-polymerization, blending, modification with additives, and combining materials or films with different properties. Further recognition of PLA in specific high-end applications is currently limited by a number of material properties that need improvement to meet the material requirements in these markets: 1. weak structural integrity at elevated temperatures, expressed as the heat deflection temperature (low HDT), 2. brittleness, i.e., low impact strength, . gas barrier performance, in particular for bottle applications. The biggest issue is the low heat resistance of PLA. The material becomes soft and weak upon heating beyond temperatures of 50-60°C, which causes practical problems during storage, transportation and use of pellets and finished articles. When hot coffee is poured into a PLA cup, if collapses (Fig. 1). Clearly, amorphous – glassy – PLA loses its structural integrity completely when subjected to temperatures above its glass transition temperature. Due to the chiral (see box) nature of lactic acid, several distinct forms of polylactide exist: poly(L-lactide) (PLLA) is the product based on L(+) lactic acid or L-lactide, the major product of Purac. Likewise, polymerization of D-lactide produces PDLA. Today commercially available PLA grades are random copolymers of D- and L-lactic acid isomers with relatively slow nucleation and crystallization rates. As a result, most PLA materials will be amorphous – i.e., glassy and not crystalline – after melt processing. These materials become sticky and soft at temperatures above 60°C. Purac allows polymer producers to add value in a new way by offering L-lactide and D-lactide as solid flakes, available in bulk quantities from 2009. By combining these lactides smartly, new PLA grades with tailored physical properties – like improved heat-stability – can be made by polymer industry. • If lactides are combined in the same polymer chain by a one-pot polymerization of L- and D-lactides, polylactides with melting temperatures ranging from about 10-180°C can be made. At very low D-isomer content, semi-crystalline PLLA is obtained, while amorphous, optically clear PLA is made with D-contents higher than 10-15%. • Polymerization of only the D-lactide monomer produces PDLA. This PLA type is the mirror reflection of PLLA and can be mixed with PLA (co)polymer to improve the material’s heat resistance according to the stereocomplexation concept. Figure 3: PDLA increases the number of PLLA crystals upon cooling from the melt at 140°C, resulting in faster stallization and higher crystallinity In the last 10 years the high heat PLA story has moved from concept to reality, it’s been a 10 year journey with great success. Corbion entered into a 50/50 JV with Total to commercially bring high heat PLA to the market. Our 1 kTpa pilot plant is already up and running and our large scale PLA plant, capable of producing 75.000 tonnes of high heat PLA / year is in final construction phase. The pre-marketing activities of Total Corbion PLA have resulted in a number of commercial applications using the technology concept outlined by Chief Scientist Sicco de Vos in 2008: high heat PLA coffee cups are a commercial reality and when I go to the supermarket I can buy single use PLA cofffe capsules ; both products include our proprietary and unique PDLA technology. Also for full stereocomplex PLA we have advanced: proprietary technology will enable PLA applications able to withstand temperatures close to 200°C (HDT-A). Furthermore, the technology enables full stereocomplex morphology not only in the lab environment but also in commercial production facilities. Samples of glass fiber reinforced stereocomplex PLA will be made available to those wanting to test the new technology for their applications. bioplastics MAGAZINE [02/18] Vol. 13 53

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