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01 | 2010

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Foam Fig. 3: Magnified

Foam Fig. 3: Magnified view of the cell structure of PLA foamed with HYDROCEROL 1% CT 3108 shows a coarse cell structure, resulting in a poor surface quality with many collapsed cells As biopolymers have found applications in food packaging, medical and many other applications, there is increasing interest in foaming these materials. The green image of biodegradable polymers like polylactidacid (PLA), starch-based polymers or copolyesters make them attractive to supermarkets and consumers. High raw-material cost have been one of the limitations of these materials and so PLA foaming – and hence weight and material-cost savings – offers an option to push these polymers further into the market. Foaming Agents and Chain Extenders for PLA Foam Article contributed by Jan-Erik Wegner and Mirco Gröseling, Clariant Masterbatches (Deutschland) GmbH, Ahrensburg, Germany Fig. 4: Photo shows smaller and more uniform cells in PLA foamed with 2% Hydrocerol CT 3108 in and with 1% CESA-extend BLA0025505 Although foaming can be accomplished using direct-gas injection, Clariant’s chemical foaming agents (CFAs) in masterbatch form are increasingly preferred, particularly in food packaging applications. The benefits of this technology include • Solid decomposition residue acts as a nucleator creating a finer cell structure and a better solubility of the gas in the polymer melt; • Decomposition reaction takes place in a defined temperature range; • Easy mixing and uniform dispersion; • High gas yield; • Approved for food-contact applications. In general, there are two kinds of chemical foaming agents characterized by whether they generate heat during decomposition (exothermic) or absorb heat during the reaction (endothermic). Exothermic foaming agents can cause odor during production and in the finished product, and their solid byproducts often are undesirable and even toxic. Therefore the exothermic CFAs are banned from use in products that must have food approval. The endothermic CFAs offered by Clariant, on the other hand, are acceptable in food packaging materials, but they have one important limitation when used in ester-based polymers like PET, polycarbonate and PLA – moisture. A byproduct of most endothermic chemical foaming agents is water, which is generated during the converting process at high temperatures. The resulting hydrolytic reaction can destroy a part of the polymer chains, resulting in a lower viscosity (increased melt flow rate, MFR), which makes the process difficult to handle. Specifically, proper die pressure, vital for foaming, cannot be maintained and the foaming process runs out of control. The melt strength drops and the film starts sagging. The dispersion of gas in the polymer is not optimized and will create surface 22 bioplastics MAGAZINE [01/10] Vol. 5

Foam MFR and Density of PLA in relation to the let down rate MFR and Density of PLA in relation to the let down rate 35 1,4 30 1,4 MFR (210°c/2,16 kg) [g/10 min] 30 25 20 15 10 5 1,2 1,0 0,8 0,6 0,4 0,2 Density [g/cm 3 ] MFR (210°c/2,16 kg) [g/10 min] 25 20 15 10 5 1,2 1,0 0,8 0,6 0,4 0,2 Density [g/cm 3 ] 0 PLA Nature 1% CT3108 1,5% CT3108 MFR (210°C/2,16 kg) [g/10 min] Density [g/10 min] 0 0 PLA Nature 1% CT3108 1,5% CT3108 2% CT3108 + 1% CESA exend + 1% CESA exend + 1% CESA exend BLA0025505 BLA0025505 BLA0025505 MFR (210°C/2,16 kg) [g/10 min] Density [g/10 min] 0 Fig. 1: Weight reduction and melt flow ratio (MFR) of PLA foam are plotted as a function of the addition of HYDROCEROL CT 3108 chemical foaming agent Fig. 2: Weight reduction and melt flow ratio (MFR) of PLA foam are plotted as a function of the addition of HYDROCEROL CT 3108 chemical foaming agent together with1% chain brancher CESA-extend BLA0025505 defects when extruded sheets are thermoformed. Due to the lower melt index the finished articles, like food-trays, can become brittle. Fortunately, certain additives, when used in combination with CFAs, can reconnect short or broken polylactid acid chains and restore them to a higher level. These additive masterbatches (tradenamed CESA ® -extend) are based, for instance, on multifunctional additives that react with the functional groups of the polymer. There are two types: chain extenders, which are designed for linear chain extension only; and chain branchers, which achieve both linear extension and cross-chain branching. Of the two, the cross-chain branching type – which actually accomplishes both chain extension and chain branching – are preferred for use in PLA along with endothermic chemical foaming agents. First, they are less sensitive to water because they do not react as fast and thus have free functional groups available to react with the polymer. Another advantage of the multifunctional additives is that the partial chain branching enhances the melt strength, therefore stabilizes the extrusion conditions and leads to a better dispersion of the blowing gas, which yields a finer and more homogeneous foam structure. Recently, lab trials were conducted to investigate the potential for density reduction in cast PLA film (NatureWorks ® 2002D) and to confirm how chain-branching additives can improve the extrusion process and the quality of the end product. HYDROCEROL ® CT 3108 was the chemical foaming agent used and the chain-branching additive masterbatch was Cesa-extend BLA0025505. Both products are manufactured by Clariant Masterbatches. The first trial extruded PLA with Hydrocerol at let down rates of 0%, 1% and 1.5% and the density reduction and melt flow rate were measured. As shown in fig. 1, density of the PLA extruded without CFA was 1.25 g/cm 3 . Adding CFA at 1% reduced the density to 1.08 g/cm 3 and a let down rate of 1.5% reduced it further to 0.94 g/cm 3 , effectively reducing material weight by 25%. At the same time, however, meltflow rate (g/10 min @ 210°C/2.16 kg) increased dramatically from 6.0 without CFA to almost 30 with 1% Hydrocerol and to almost 27 with 1.5% CFA. The foamed film had a coarse cell structure (see fig 3), and poor surface quality with many collapsed cells. Next, the PLA was foamed with Hydrocerol CT 3108 at 1%, 1.5% and 2% let down rates, and Cesa-extend BLA0025505 chain-branching agent added at a rate of 1% in all three cases (see fig. 2). At 1% CFA and 1% chain brancher, the density was reduced to 1.0 g/cm 3 . With 1.5% CFA, density was 1.05 g/cm 3 , while 2% CFA reduced density dramatically to 0.7 g/cm 3 , for an overall weight reduction of 44%. With the addition of 1% Cesa-extend, the foam structure was significantly improved despite the higher loadings of Hydrocerol. Smaller and more uniform cells are evident in fig. 4. This, even though the melt flow rate remained roughly the same as in the first test. Clearly, Cesa-extend chain brancher provides higher melt strength and allows for higher let down rates of foaming agent. Without the use of the Cesa-extend, it would be difficult to achieve the kind of density reductions required to help make PLA a more competitive option for food packaging. bioplastics MAGAZINE [01/10] Vol. 5 23

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