Tons of CO 2 generated
Tons of CO 2 generated 50 45 40 35 30 25 20 15 10 5 0 ‘Material Carbon Footprint’ for million sq. mt. of coated paper Polyethylene Coating Material Use of Natur-Tec coating resin with 100% biobased carbon reduces CO 2 emissions by 100% compared to use of petroleum based Plyethylene ZERO carbon food-print Natur-Tec Figure 3: Material carbon footprint value proposition for Natur-Tec coated paper compared to PE coated paper Intrinsic value proposition for using PLA-based coatings Bio-based plastics, in which the fossil carbon is replaced by bio/renewable-based carbon, offer the intrinsic value proposition of a reduced carbon footprint and are in complete harmony with the rates and time scale of the biological carbon cycle. The carbon footprint of biopolymers has been discussed in detail in the recent MRS bulletin [2] by Narayan. Accordingly for every 100 kg usage of petroleum based resins such as polyethylene or polypropylene a net 314 kg of CO 2 is released into the environment at the end of cycle. On the other hand if this material is replaced with PLA or Natur-Tec extrusion coating resin (100% biobased modified-PLA) the net CO 2 evolved is zero as all the carbon in this material comes from renewable resources. This is the material carbon footprint of the resin. The other half of the total carbon footprint is the emissions arising from the process of converting the carbon feedstock to product, the impact during product use, and ultimate disposal – called the process carbon footprint. Although the process carbon footprint for PLA is higher than PE/PP, the overall CO 2 released to the environment, taking into account the intrinsic carbon footprint as discussed above is lower and will continue to get even lower as process efficiencies are incorporated and renewable energy is substituted for fossil energy [2]. For extrusion–coated paper if we assume that the base paper used is same and the coating thicknesses for the PE and Natur-Tec coating are 15 and 30 gsm respectively, then one can calculate the material carbon footprint. PE contains 85.71% carbon (0% is biobased) while Natur-Tec resin contains 40% carbon (100% is biobased). So for a million square meter usage one can estimate that the PE coating will give out 47.1 tonnes of CO 2 while this value will be ZERO for coating that is renewably resourced as shown in Figure 3. This is a strong value proposition that is environmentally sustainable and intrinsic to the use of biobased feedstock. Processability PE coating lines have a larger air-gap. This is because PE is non-polar and paper is polar, as such there is little or no adhesion between the thermoplastic and the paper unless the PE melt flows into the paper pores or some modification is done to the PE. A solution to this was to let the web of PE melt drop through the air for certain distance causing the hot surface to oxidize slightly creating polar groups that will help in adhesion to the paper fibers [1,9, 10]. Because the polymer melt is expected to drop through an open space before contacting the substrate, the polymer must have sufficient melt strength to support its weight. Virgin PLA has poor melt strength and considerable neck-in (greater than 10%). It is a polar polymer and does not need a larger air-gap that was initially designed for a non-polar polymer such as PE. Secondly, PLA has low elongational viscosity and cannot be stretched out on the substrate for thinner coatings making it cost prohibitive. The shortcomings of pure PLA are overcome by reactive blending and modification of the PLA to provide (1) improved resin melt strength for processing on PE coating lines, (2) improved elongational viscosity for application of thinner coatings close to 20 microns (30 gsm) and (3) reduced neck-in (close to 5%) for processing wider webs. In particular, a polymer used for extrusion coating needs to have reasonably high elongational viscosity and the melt should exhibit strain hardening at high elongational rates. Strain hardening means the resistance to deformation increases at a more rapid rate as deformation continues 100000 (a) 1000000 (b) 100000 ɳe + (Pa s) 10000 — 0.3 s-1 — 1s-1 — 3 s-1 — 10 s-1 — LVE ɳe + (Pa s) 1000 01 0.1 1 100 — 0.1 s-1 10000 — 0.3 s-1 — 1 s-1 — 3 s-1 — 10 s-1 — LVE 1000 0.01 0.1 1 10 100 Time t (sec) Time t (sec) Figure 4: Elongational viscosity comparison of extrusion coating grades of (a) polyethylene with (b) modified PLA resin from Natur-Tec ® . 36 bioplastics MAGAZINE [05/11] Vol. 6
Water uptake (gm/sq. m) [7]. This is provided by the polymer molecular architecture – branched vs. linear. Polymers exhibiting strain hardening or high extensional viscosity deform uniformly as stress is applied to the melt. Figure 3 compares the extensional viscosity over a range of strain rates for coating grades of PE and modified-PLA. The modified-PLA grade shows strain hardening at high strain rates of 10s -1 and greater and the Linear Viscoelastic Envelope (LVE) value is comparable to that of PE (at 1000 – 50000 Pa-s) over the range tested. Thus it is possible to process virgin PLA on a PE coating line with improvements to its polymer architecture. Performance Paper products are commonly coated with plastics for two major desired properties: (1) water proofing as in the case of disposable paper cups, plates, etc. and (2) greaseresistance as in the case of take out boxes, pizza-boxes, etc. Water proofing is measured using a Cobb test where amount of water absorbed by the paper in a given time indicates the relative water resistance of the paper. Lower the uptake of water better is the performance. As shown in Figure 4 the performance of modified-PLA coated paper (samples from Natur-Tec) was comparable to that of PE-coated paper and significantly improved compared to the uncoated paper. Although the coatings were applied at different weights the thickness was the same order of magnitude based on density of the two resins. Grease resistance of the papers was measured using the 3M kit test – twelve kit solutions are prepared by mixing different amounts of Castor oil, n-Heptane and Toluene and the lowest number of solution that stains the paper in 15 seconds is recorded. As shown in Table 1 both the coated papers passed all the twelve test solutions and had a high degree of grease resistance compared to the base paper. The PLA-based paper performed at par with the PE coated samples in terms of properties for both water and grease resistance. 25 20 19,1 15 10 5 0 1 min Cobb test per TAPPI T331 0m-90 0,3 0,2 paper (230 gsm) PE-coated paper Natur-Tec modified-PLA coated paper Figure 5: Water-proofing property using Cobb test – for uncoated paper, PE coated paper and modified-PLA coated paper from Natur-Tec Sample Type TAPPI T 559 cm-02 Grease Resistance/ 3M Kit test Uncoated paper (230 gsm) 1 PE coated paper 12+ Natur-Tec modified-PLA coated paper 12+ Table 1: Grease resistance property using 3M kit test – for uncoated paper, PE coated paper and modified-PLA coated paper from Natur-Tec CONCLUSIONS 1. PE coating on paper is not compatible with end-of-life composting or recycling operations. Replacing the petrofossil PE coatings with biobased and fully biodegradable PLA coatings offers the value proposition of a reduced material carbon footprint and its process carbon footprint mirrors existing PE operations. It is readily and fully biodegradable in industrial composting operations (compostable plastic) and therefore, can be easily removed from the environmental compartment in a safe and efficacious manner. PLA-based coating offers the intrinsic value proposition of ZERO material carbon footprint. 2. PLA-based coatings can be successfully processed on traditional PE lines when modifications are made to the formulation to improve melt strength and elongational viscosity such that strain hardening occurs at higher elongational rates of >1sec -1 . 3. PLA-based coated paper provides water-proofing and grease resistance that is competitive with a PE coated paper. References [1] B. A. Morris, “Understanding why adhesion in extrusion coating decreases with diminishing coating thickness, Part I & II: Penetration of porous substrates,” SPE-ANTEC, 63, 2964-2968 (2005). [2] R. Narayan, Carbon Footprint Of Biopolymers Using Biocarbon Content Analysis And Life-Cycle Assessment, MRS Bulletin, Volume 36, Issue 9, September 2011. [3] New Opportunities in Recycling and Product Manufacture Eliminate the Environmental Hazards Inherent in the Composting of Plastic-Coated Paper Products, Will Brinton, from Woods End Laboratories, Inc., Mt. Vernon, ME Cyndra Dietz, Alycia Bouyounan, Dan Matsch from Eco-Cycle, Inc., Boulder, CO, April 2011- Read the full report and find more information at www.ecocycle.org/microplasticsincompost. [4] Algalita Marine Research Foundation ; www.algalita.org/pelagic_ plastic.html [5] R. Narayan, American Chemical Society Symposium Series, 939 (2006), C. 18, pp. 282. [6] R. Narayan, in Renewable Resources and Renewable Energy, M. Graziani, P. Fornasiero, Eds. (CRC Press, Taylor & Francis Group, 2006), C. 1. [7] R. Narayan, in Science and Engineering of Composting: Design, Environmental, Microbiological and Utilization Aspects, H.A.J. Hoitink, H.M. Keener, Eds. (Renaissance Publications, OH, 2003), pp. 339. [8] ITC India Ltd. – Paperboards and Specialty Papers Division; www.itcportal.com/itc-business/paperboards-and-packaging/ paperboards-and-specialty-papers.aspx [9] R. J. Hernandez, S. E. M. Selke and J. D. Culter, Plastics Packaging – Properties, Processing, Applications and Regulations, Chapter 8, Hanser Gardner Publ. 2000. [10] B. A. Morris and N. Suzuki, “The case against oxidation as a primary factor for bonding acid copolymers to foil,” Annual Technical Conference – Society of Plastics Engineers, 59:1, 25- 35 (2001). bioplastics MAGAZINE [05/11] Vol. 6 37
