Paper Coating Improved
Paper Coating Improved Compostable Paper Extrusion Coating By: Kelvin T. Okamoto Vanessa Salazar Cereplast, Inc. The first paper hot cup coated with compostable PLA resin was commercialized in 2006. Since then, several companies around the globe have introduced similar hot cups. However, the compostable paper hot cups cost much more than the typical paper hot cup coated with standard polyethylene. The additional cost is due to both increased material costs and reduced manufacturing efficiency in coating paper and in cup forming. Cost increases in compostable paper hot cups over standard paper hot cups include: • Material density: Polyethylene has a density of about 0.95 g/cm³ versus about 1.25 g/cm³ for PLA-based formulations. At the same coating thickness, this means that the PLA-based formulations have a 30 – 35% weight disadvantage. Melt Flow Rate, g / 10 min Figure1: Moisture vs. Melt Flow Rate 450 400 350 300 250 200 150 100 50 0 500 1000 1500 2000 2500 3000 3500 ppm Moisture @ 210 °C / 2.16 kg @ 230 °C / 2.16 kg • Coating thickness: For paper cups, the typical polyethylene cup coating is 20 µm thick versus the typical PLA coating today of 25 µm thick. Thus, the PLA coating is at a 25% coating thickness disadvantage and when adding in the density difference between the two materials, the material coating weight disadvantage for PLA over PE is about 60 – 65%. • Paper extrusion coating machines typically run about 25 – 50% slower with PLA coatings. • Additional disadvantages of PLA paper coating over PE coating include - Neck-in of 75 to 150 mm (3 to 6 inches) per side which may reduce maximum paper width able to be produced; - A band of thick resin along each edge (also known as hard edge) of 25 to 50 mm (1 to 2 inches) that increases scrap and reduces maximum paper width capable; - Reduced coating adhesion to paper; and - Sensitivity to moisture during processing. 38 bioplastics MAGAZINE [05/11] Vol. 6
Paper Coating Cereplast, Inc. has been working on an improved version of its paper coating resin for over a year. The new developmental grade is Compostable 4008D; a few companies have evaluated the resin and have identified several possible benefits over existing compostable PLA resins for extrusion coating of paper available on the market. One benefit includes improved thermal stability; in comparison to Cereplast’s previous paper extrusion grade Compostable 4001, Compostable 4008D maintains its melt flow rate at 190°C for five minutes longer. This should translate into the new formulation being able to be processed at higher melt temperatures to improve adhesion to paper and to reduce hard edge and coating thickness. Just as important, Compostable 4008D is nearly completed ASTM D6400 compostability testing. With only the plant toxicity testing to be completed, the new formulation has successfully passed compostability disintegration testing at up to 75 µm thick as a neat film, which is thicker than most other PLA-based paper extrusion coating resins. Cereplast will complete ASTM D6400 and, if needed, EN 13432 testing if and when the last test is passed. The one major issue that cannot be especially addressed by formulation is moisture sensitivity of the PLA material to degradation during processing. Compostable 4008D, along with competitive PLA-based paper extrusion coating resins, must be dried to less than 100 ppm moisture and ideally to less than 50 ppm moisture; the dried resin must then be fed to the extruder at the desired moisture level. The chart in Figure 1 shows the effect of moisture on the measured melt flow for Compostable 4008D at 210°C and at 230°C. It should be noted that the melt flow rate at 230°C continues to drop even as the moisture drops from 175 ppm to 57 ppm. It should also be noted that moisture has a much greater effect on melt flow rate at 230°C versus 210°C; the higher temperature is desired in actual processing to improve coating adhesion. Typically during a processing trial, much effort is spent on getting the paper coating resin dried properly but it is also essential to ensure that the resin is fed in the extruder dry also. To do this, the dried paper coating resin must be conveyed to the extruder hopper using dry conveying air if possible and must be kept dry above the extruder using either a dryer hopper or by conveying dry air or nitrogen upwards through the feed hopper. Other processing system changes may also be needed when trying to run PLA-based paper extrusion coating resins on equipment designed and optimized for running polyethylene. Cereplast, as well as many extruder manufacturers, can provide further guidance to successfully processing Compostable 4008D. www.cereplast.com bioplastics MAGAZINE [05/11] Vol. 6 39
