Basics Do biopolymers
Basics Do biopolymers need additives? „Plastics without additives are not viable“. This pithy phrase opens the “Plastics Additives Handbook” which is a reference book in this field. Therefore, there should be reasons for this valuation. When talking about plastics people normally think about a well-designed serviceable material, e. g. a plastic bag, a detergent container or – from an industrial point of view – a sealable food container, a heat resistant engine block cover or an impact resistant smart phone housing. What we don´t keep in mind is that the raw material, based on polymerised monomers, as obtained from (bio)chemical reactors is not a plastic. These synthetic materials have inherent properties, based on their atomic linkages, molecular structures and the associated interactions between the chains. This applies to fossil based polymers as well as to biobased polymers. Therefore, as an example, polyethylene will always be softer than a polyester like PET due to fewer interactions between the chains. However, the inherent mechanical properties of materials are only one side of the medal. Synthetic polymers alone are, in general, not a processable plastic. Processable means that the materials must not stick at the surfaces of the plastic processing machinery, must have the right viscosity and melt strength for molding and should not be changed in their molecular structure as a result of thermomechanical treatment. Therefore, additives reducing stick and slip effects, tailoring melt flow and strength as well as stabilisers are a must in processing. PVC, independent of the source (fossil or biobased), is an outstanding example for heat stabilisation. Due to self-catalyzed mechanisms which eliminate hydrogen chloride from PVC well below processing temperatures, resulting in a coloured material, heat stabilisers are needed to interrupt the catalytic cycles. Often, the viscosity of the raw material is too high leading to increased shear energy input. Plasticisers may then be needed. Moreover, due to production conditions, water (even in small quantities like moisture of the surrounding atmosphere adsorbed on the material) and oxygen can degrade the material by chemical reactions. This process may be slow at ambient temperature, but not in thermoplastic processing. Depending on atmospheric humidity and material dryness, polyesters especially need a sophisticated temperature control and additivation to maintain molecular weight and structure and therefore properties. Let us now move one step further, from processing to use. Will we have a serviceable plastic after being able to control the processing using appropriate additives? Not at all! The materials will become exposed to the environment and a serviceable plastic material should not be susceptible to rapid degradation caused by ultraviolet and visible radiation, oxidation or hydrolysis. Therefore, several classes of additives have been developed which intervene in the underlying polymer-type depending molecular processes. Furthermore, a serviceable plastic should have the right morphology which should not change during usage. This is very important for semi-crystalline polymers like PE, PP, PET and PLA as well as for blends, which comprise a large market share in the field of biobased plastics. Therefore nucleating additives and clarifiers as well as compatibilisers have been developed. Permeability of gases through a PLA/PHBV (3:1) blend film (100 µm) Permeability of gases through a PLA/PHBV (3:1) blend film (100 µm) 18 110 16 Compatibiliser (reducing surface tension) Reactive compatibilisation PLA/PHBV stat. blockcopolymer 100 H 2 O / gm -2 d -1 and N 2 / cm 3 m -2 d -1 bar -1 14 12 10 8 6 4 90 80 70 60 50 40 O 2 and CO 2 / cm 3 m -2 d -1 bar -1 Yellowing due to sunlight from window Original white 2 30 0 H 2 O 23 °C, 85 % humidity N 2 O 2 23 °C, 0 % humidity CO 2 20 Example for poor UV-stabilizer Specific synthesis of the compatibilizer (PLA/PHBV stat Blockcopolymer) gives (slightly) better results than general types of commercialised surface reducing agents 42 bioplastics MAGAZINE [04/16] Vol. 11
Basics Bioadditives? By: Rodion Kopitzky Fraunhofer UMSICHT Oberhausen Germany Last but not least, there are product depending requirements which are independent of the selected material-type like toughness, strength, elasticity, color, flame retardance, electrostatic behavior and so on. Nevertheless, cost is also an important factor. Fillers and reinforcing agents, plasticisers and impact modifiers, antifogging additives and whitening agents, pigments and dyes, antistatic and antimicrobial additives and so on can be used for tailoring the overall properties to get a serviceable plastic. To make the right choice of additives, which can a) influence each other and b) may have opposite effects on properties is often a difficult task for a developer. To focus on biobased plastics, there are in principle, no particularities concerning additives. If the biobased polymer is a drop-in material like biobased PE, formulations with additives can be transferred directly. In the case of starch, appropriate plasticisers must be chosen to get thermoplastic starch. Figuratively it is the same as choosing the right plasticiser for PVC depending on the overall requirements. PLA is known to be susceptible to water to some degree. Therefore scavengers or chain extenders which react with water or with the polyester degradation product can be used. The molecular chemical basis of these additives is the same for fossil and biobased polyesters and sometimes the commercialised products only have different names. Concerning reactive additives like chain extenders, there may be differences between fossil and biobased plastics. But this is due to the processing conditions which alter the kinetics of the reactive process. A branching chain extender like glycidyl based polyepoxides (Joncryl) reacts very quickly at 280 °C, the processing temperature of PET, but much slower at 190 °C, the processing temperature of most polyesters in biobased formulations. Therefore, not all additive types are transferable to other formulations when changing the polymer and polymer-type specific additives have to be used or developed. This is evident in the field of reactive compatibilisers for fiber filled plastics or in blends. These additives must be designed according to the molecular basis of the components and the polymer-type depending on mechanisms of compatibilisation. Do bioplastics need additives? Yes they do! Do they need Bio-Additives? Summing up the previous paragraphs the reason for an additive should not be the material basis but rather the achievable overall properties of the final plastic material formulation. Additives based on vegetable oils or fatty acids, for example, have been used as plasticisers and lubricants for fossil based polymers for several decades. The coplasticiser and acid scavenger epoxidised fatty acid ester is, on a volume basis, one of the biggest additives, used predominately with PVC. From a sustainable point of view biobased additives often have an advantage in short use application like packaging, due to low carbon footprint (if the footprint is not destroyed by inefficient energy use during production, due to lower production amounts). Biobased additives can also raise the biobased carbon content in blends with biobased and fossil based polymeric components. In respect to regulations (using the term bio or biobased or biodegradable) they may be a must. However, the terms biodegradation and biobased should not be confused. Additives disregarding their material basis should not have an effect on the degradation process. An acid scavenger like the above mentioned epoxy would be contraproductive for use with PLA in short use applications because it will decelerate the first step of biodegradation. Nowadays, due to the discussion of the raw material basis beyond fossil resources and the industrial availability of new building blocks like succinic acid, new additives are under development or are in the market entrance phase. Long term development of the biorefinery concept to provide new biobased chemicals might even initiate the synthesis of special additives like the UV-absorbers on a biobased basis in sufficient amount and at acceptable costs. Nevertheless, until then, competitive cost will be a critical factor in many cases. www.umsicht.fraunhofer.de Tiplock ® the world’s first compostable ziploc packaging Tiplock is a joint development of and Bio4Pack. NEW Bio4Pack GmbH • PO Box 5007 • D-48419 Rheine • Germany T +49 (0) 5975 955 94 57 • F +49 (0) 5975 955 94 58 info@bio4pack.com • www.bio4pack.com bioplastics MAGAZINE [04/16] Vol. 11 43 Bio4pak-adv-BioPlastick-Magazine105x148_5.indd 1 18-05-16 11:04
