Basics383 Evaluating
Basics383 Evaluating Quantity, Quality and Comparability of Biopolymer Materials Article contributed by Hans-Josef Endres, Andrea Siebert-Raths, and Maren Bengs, all University of Applied Sciences and Arts, Hanover, Germany Rather than biodegradability the focus of current material development in the field of biopolymers is increasingly on a biobased raw material input to produce durable products, i.e. the use of resistant biopolymers in technical applications. And the properties required of the materials are increasing in parallel with the number of these different applications. As a result of this current development more and more manufacturers are publishing material specifications. At first glance this can be seen as a positive move from the point of view of technical marketing support, however, the quantity, quality and comparability of available material data are still very unsatisfactory. When establishing such product data it is often the case, for example, that different standards are used for the tests, as well as different testing conditions, such as the prevailing environment when the sample was taken, the temperature conditions or humidity before and during the test, or the period of time over which the test was conducted. A further problem area lies in the fact that too little experience has been gained with new types of biopolymer to be able to lay down the optimum test conditions. Furthermore many of the published test results do not specify any standard test methods or conditions, or do not adequately define the selected conditions. Unfortunately consequence it is often in the case that the material performance specifications published until now have limited informative value. The intention of this article is, with the help of various concrete examples, to point out some of the common mistakes made when attempting to ascertain the performance characteristics of biopolymers and to increase the understanding of testing of biopolymers. Melt Index An important value for plastics processors is, for example, the melt flow index (Melt mass flow rate = MFR [g/10 min]) as specified in DIN EN ISO 1133. Without quoting a temperature and the pressure applied as the significant parameters for the test, the readings cannot be evaluated. These data, which complement the values quoted, are therefore essential but are left out by many manufacturers and are missing from numerous published documents. In addition, with biopolymers there is often the problem that, unlike conventional plastics, the MFR of these new polymers no recommendations are given with regard to the test parameters when measuring. This leads to different companies choosing different test parameters, hence making it even more difficult to compare readings. Temperature Resistance Another very sensitive figure that should be known for practical application of a biopolymer is its resistance to temperature. In many documents published about biopolymers, or in press releases, we more and more often read, for 38 bioplastics MAGAZINE [06/09] Vol. 4
Basics instance, about PLA/PLA blends with a temperature resistance of around 100°C. Since the low temperature resistance of PLA often seriously limits its use, this increase from the figure of about 60°C (which is normally quoted for this material) to values around 100°C is extremely significant. Unfortunately it has emerged that this impressive figure is not supported by the facts but can largely be traced back to widely varying test methods that are not really comparable. Here too it is absolutely essential that one is given details of the test method and conditions with regard to heat resistance values. To measure heat resistance the following two different standard test methods are generally used: Measuring HDT (Heat Deflection Temperature or Heat Distortion Temperature) in accordance with DIN EN ISO 75 and measuring the VST (Vicat Softening Temperature) in accordance with DIN EN ISO 306. For the HDT test a standard sample is placed in an oil bath and subjected to a defined and constant bending force under a constantly increasing temperature (120°C/h). The HDT is reached when the outer fibre distortion of the material reaches 0.2 %. In the Vicat test the sample is also placed in an oil bath with a defined temperature gradient. However the Vicat test is not based on bending but on point load deflection. The Vicat softening temperature is reached when a flat-ended needle of a defined geometry, penetrates 1 mm into the sample under a defined pressure [1]. Both methods permit variations of the load and temperature gradient within the norm. With the HDT method the central bending load can be chosen from the following values: 1.85 MPa (HDT A), 0.45 MPa (HDT B) and 8.0 MPa (HDT C). This means that even within one method there can be significant variations in the value depending on the chosen loading, which is often not specified in the quoted results, as can be seen in Fig. 1. If, for example, the temperature resistance of polyhydroxyalkanoates (PHA) is published it may seem high, returning a value of 140°C, or, with a greater loading, be as much as 60°C lower at about 80°C. The situation is similar with the VST temperature resistance test. Here too the piercing needle force can be selected from either 10 N (VST A) or 50 N (VST B). In the VST method A represents a lower loading and hence higher resistance values, whilst method B, uses higher loading and hence a lower resistance value in contrast to method A. When comparing the temperature resistance of biopolymers the two methods can return figures that vary by as much as 100°C. Furthermore when testing temperature resistance either of two temperature gradients may be selected; either 50°C/h or 120°C/h. At the faster rate the thermodynamic loading time of the biopolymer before reaching a certain temperature is less than at a lower temperature gradient. Hence the resulting values at the higher temperature gradient are likewise correspondingly higher. It is therefore essential that the exact and full methodology used when measuring temperature resistance is specified. Where adequate data on the test methods is not supplied the temperature resistance cannot be properly evaluated. [°C] 160 140 120 100 80 60 40 20 0 HDT A (1.85 Mpa) PLA Starch blend HDT B (0.45 Mpa) Copolyester blend Fig.1: The influence of different bending loads on the measured temperature resistance using the HDT test. Temperature gradient in each case = 120°C/h (incomplete, just as an example) PHA PHB PCL bioplastics MAGAZINE [06/09] Vol. 39
