Rigid Packaging Clearing
Rigid Packaging Clearing the Way for Wider PLA Use Modifier that reduces brittleness, quiets film noise and reduces processing cost combats major growth restraints of the bio-based polymer By Carol M. Casarino Global Technology Manager Packaging and Consumer DuPont Packaging and Industrial Polymers Wilmington, Delaware, USA Driven by consumer and industry demand for renewably sourced materials, consumption of bio-based polylactic acid (PLA) polymers has grown rapidly over the past decade. Applications have opened up in thermoformed packaging, cold drink cups, films for packaging and other uses, molded cutlery and other fields. To meet surging demand, polymer plants have sprung up in North America, Europe and the Asia-Pacific region. Yet for all its recent growth, PLA holds but a tiny fraction of total material use in packaging and other key application areas. The relatively short commercial history of PLA, the need for processors to learn how to handle it and the higher cost of PLA compared with petrochemical-based plastics are certainly important factors in PLA’s small market share. But inherent limitations of unmodified PLA polymers impose more telling constraints on its growth. Without modification, PLA polymers are brittle. Thermoformed packages, for example, can shatter, splinter or produce shards during die cutting from the web, or handling in the store or at home. In addition, films made of PLA are ‘noisy’, producing a crinkly sound when handled that consumers find objectionable. Other limitations arise in processing in the areas of thermal stability and energy consumption. Fig. 2. Modifier maintains excellent contact clarity while greatly increasing toughness of thermoformed PLA packages. Approaches to overcoming the shortcomings of PLA include the use of additive modifiers and modification of the polymer itself. This article focuses on an ethylene copolymer modifier that DuPont introduced in 2007 with the primary mission of enhancing PLA’s toughness but that has proven to reduce processing costs, too. Called DuPont Biomax ® Strong 120 modifier, the ethylene copolymer works extremely well as a toughener. Just a 1 % loading produces a nine-fold improvement in the flexural fatigue resistance of amorphous PLA sheet (Figure 1). In crystallized sheet, dramatic reductions in brittle behavior are also seen. Oriented PLA sheet also gains in toughness with the ethylene copolymer modifier. In addition, the modifier produces major improvements in elongation and impact strength with loading levels of just a few percent. Shattering and shards rarely occur in PLA packages made with 1 to 5% of DuPont Biomax Strong. The ethylene copolymer produces outstanding toughening effects while maintaining excellent contact clarity. A leading manufacturer of thermoformed produce containers is taking 16 bioplastics MAGAZINE [02/11] Vol. 6
Rigid Packaging Reducing Brittleness Fig. 1. Biomax strong sharply improves flex crack resistance of PLA sheet. Tests were conducted with 305 to 380 mµ thick cast PLA sheets. 1800 1600 1400 1200 (b) AMORPHOUS CRYSTALLIZED SHEET 1000 800 advantage of that feature in containers such as that shown in Figure 2. Recent tests have proven that this additive overcomes another PLA limitation specific to packaging films, namely the crinkly noise that has limited its acceptance for applications such as snack bags. Preliminary testing of monolayer PLA films shows that the ethylene copolymer reduces such noise even at just a few percent loading (Figure 3). Furthermore, by combining PLA containing the modifier with layers of flexible materials, it’s feasible to design PLA-based packaging structures that produce no more noise than conventional packages. Processing advantages Processing benefits of DuPont Biomax Strong 120 come in two areas. One is that during extrusion, it melts quickly and acts as a lubricant in the solids conveying sections of the screw. That can result in an increase of up to 21 % in extruder energy efficiency with only 2 % by wt. loading (Figure 4). Another processing benefit is enhanced thermal stability. The addition of just 2 to 5 % modifier to PLA produces major improvements in thermal stability (Figure 5). That can pay off with greater use of regrind during extrusion. Improved thermal stability combined with the increase in elongation provided by the toughening modifier could also help expand the use of PLA in extrusion coating, which uses relatively high processing temperatures. There are also potential improvements in heat-seal performance and reduced neck-in during extrusion coating, but further testing is required for proof. Another processing plus for DuPont Biomax Strong is ease of use. Supplied as pellets, it is readily blended with PLA at the extruder or molding machine using conventional metering equipment, without the need for a separate masterbatch compounding step. In summary, the use of DuPont Biomax Strong 120 overcomes limitations in toughness, noise and processing efficiency that are holding back widespread replacement of conventional petrochemical-based plastics by bio-based PLA in packaging. 600 400 200 0 35 30 25 20 dB 15 10 5 0 Specific Energy (kg/hr-kW) Shear Viscosity at 55 1/s [Pa-s] 0% 1% 5% 17% Loading of Biomax Strong Noice Reduced Fig. 3. Modifier tones down crinkly sound of monolayer PLA film. 0% 2% 5% 7% Biomax Strong Loading Less energy for processing Fig. 4. Modifier reduces PLA processing cost with lower energy consumption. 10 4 10 3 9 8 7 6 5 4 3 2 1 0 Improved thermal stability Fig. 5. Modifier sharply reduces thermal degradation of PLA during processing. PLA Control 2% Biomax Strong 5% Biomax Strong BIOMAX STRONG 100 in PLA2002D 0% 2% 5% http://biomax.dupont.com 10 2 10 2 10 3 10 4 Residence Time (sec) bioplastics MAGAZINE [02/11] Vol. 6 17
