Basics [°C] 160 140 120
Basics [°C] 160 140 120 100 80 60 40 20 0 45 40 35 30 25 20 15 10 5 0 90 80 70 60 50 40 30 20 10 0 PLA VSTA (10N), 120°C/h) Starch blend Copolyester HDT A (1.85 Mpa, 120°C/h) Fig.2: Influence of the test method used to determine temperature resistance (incomplete, just as an example) Moisture content [%] Copolyester blend Tensile strength (Mpa) PCL Fig.3: The effect of conditioning and storage on the tensile strength of a PLVA based polymer Storage time not exceeded Storage time exceeded (about 9 months) PHB Storage period: 4 months (23°C/ 50% RH) Storage period: 4 months (23°C/ 50% RH) - before testing dried to about 1% moisture content Storage period: 24 hours (23°C/ 50% RH) Melt Mass Flow Rate (190°C, 2.16kg) in [g/10min] Tensile strength (in Mpa) Fig. 4: Effect of extended storage period on the material (23°C, 50% RH) For the values obtained using the VST test it is thus necessary to clearly distinguish between results obtained using, for example, VST A 50 (load applied to the needle = 10 N and temperature gradient = 50°C/h), VST A 120 (10 N @ 120°C/h), VST B 50 (50 N @ 50°C/h) and VST B 120 (50 N @ 120°C/h) [1]. Without these data mistakes are often made in the practical application of biopolymers, such as PLA, due to a lack of understanding of this problem and directly comparing temperature resistance values that have been obtained using different test methods and/or under different test parameters. As shown in the table of temperature resistance figures obtained using Vicat A and HDT A for various biopolymers (Fig. 2), these results are not at all comparable. Alongside the often inadequate data concerning the test parameters there are other factors (such as storage time and/or conditioning/ drying) that are not given with regard to the biopolymers being tested. The chart in Fig 3 uses as an example the tensile strength of a polyvinyl alcohol (PVAL) based biopolymer to demonstrate the significant effect that humidity and/or length of storage may have on the mechanical properties of the material. It is important, when testing in line with an international standard, to supply information on the storage and conditioning of the sample as well as how much time elapsed between preparation of the sample and the actual test. Fig. 4 also shows that with biopolymers it is not only conditioning and the age of the finished components that have a significant impact, but that also the effect of exceeding recommended storage times of the resins before processing is a factor not to be underestimated. The following chart shows the impact on a starch based biopolymer of exceeding the storage times. The starch based polymer was tested immediately on delivery and then after a clearly excessive storage period. The almost quadruple melt flow index points to a reduction in the length of the molecular chain as a result of the polymer degradation. The same applies to the tensile strength. Here again the material was tested immediately upon delivery and again after an extended storage period. The significant drop of the mechanical specification also points clearly to a molecular breakdown. Barrier Properties of Films Further examples of a lack of data when evaluating biopolymers is also seen in the area of biopolymer films. This can be testing oxygen permeability in line with DIN 53380 for example. In this process a permeation cell is separated by a sample of the film. The test gas, i.e. the oxygen, is introduced into one half of the cell. It will permeate to a greater or less degree through the film and into the other half of the cell where it is perceived by a carrier gas. A sensor and appropriate software are used to measure the amount of oxygen in the carrier gas and so determine the oxygen permeability of the film. In addition to temperature, the relative humidity of the oxygen and the carrier gas can also be regulated. When stating the barrier property of a film, i.e. the coefficient of permeation, the temperature and relative humidity parameters often fail to be supplied, but as can be seen in Fig. 5 the moisture content of the oxygen (or other gases being tested), and the carrier gas have a significant influence on the permeability especially of biopolymers. 40 bioplastics MAGAZINE [06/09] Vol. 4
Basics Film Thickness Another difficulty with biopolymer film lies in the presentation of performance data without mentioning the film thickness. With barrier performance in particular it is important to state the film thickness concerned or to adhere to a recognised standard with a unified film thickness. In some published data we still find barrier properties of film being quoted without any mention of the thickness. Test Speed A further shortcoming with regard to biopolymer film lies in the testing of its mechanical performance and in particular the tensile test. With regard to the speed applied during the tensile test there is no specific standard laid down by DIN EN ISO 527; several speeds (1, 2, 5, 10, 20, 50, 100, 200, 500 mm/min) may be applied by the tester. In practice a speed of 1, 2 or 5 mm/min is chosen to determine the secant modulus. For other mechanical values (e.g. tensile strength) higher speeds are usually selected. The chart in Fig. 6 shows the effect of test speed on the secant modulus of a regenerated cellulose film. As is clear from the illustration, the secant modulus measured at 1 mm/min lies well below that of the modulus measured at the higher speed by almost 1000 MPa. At the lower speeds the molecular chains have more time to change shape and orient themselves. Hence the film is less resistant to elastic deformation. The effect of testing speed on mechanical values is also seen with injection moulded parts, but the effect on films, due to their much reduced thickness, is more significant. Hence, when tensile testing films in particular, it is very important to have information on the test speed in order to be better able to assess and compare data on different materials. For the future it can be assumed that the development of biopolymers will move ahead swiftly and more and more materials will be presented to the market. It is however important at this stage that the performance characteristics published for new types of biopolymers are comparable and meaningful. Help on this whole topic is available from a freely accessible database at www.materialdatacenter.com, assembled by the authors of this article in collaboration with the company M-Base GmbH and with support of the German Federal Ministry of Food, Agriculture and Consumer Protection (the BMELV). All commercially available polymers are tested under standardised conditions in line with the published norms here, and are can find all of the necessary information regarding the relevant test parameters in parallel with the numerical specifications of biopolymers. More information about biopolymer testing can be found in the book ‘Technical Biopolymers’ [1]. This book can be ordered via the bioplastics MAGAZINE website. It is available in German language, an English version is expected for spring 2010. www.fakultaet2.fh-hannover.de www.materialdatacenter.com [cm³/(m²*d*bar)] Fig.5: Oxygen permeability in relation to the relative humidity of the gas being tested (oxygen) and the carrier gas. Secant modulus (Mpa) 400 350 300 250 200 150 100 50 0 6200 6000 5800 5600 5400 5200 5000 4800 4600 Starch based film PLA based film (standardised to 100 μm) (standardised to 100 μm) Test speed 1 mm/min Test speed 5 mm/min Test speed 1 mm/min 23°C/0% RH 23°C/50% RH Test speed 1 mm/min [1] Endres, H.-J.; Siebert-Raths, A.: Technische Biopolymere, Carl Hanser Verlag, München 2009 Fig.6: Effect of test speed on the secant modulus bioplastics MAGAZINE [06/09] Vol. 4 41
