Biopolymers

Polyester that changes into CO2

The range of products derived from Bionolle offer maximum biodegradability and can be used for a vast array of applications – plastic film, sheet and bottles

The film covering the surface of plastic sheets is made of aliphatic polyester – a natural material based biopolymer – however its mechanical characteristics are no less impressive because of it. On the contrary, considering its infinitesimal thickness, this coating is hard wearing and keeps its shine. All in all we would be justified in comparing its strengths to those of a spider’s web, which consists of a single woven thread made from of water-based solution of polypeptides that the creature ejects via a tiny spinneret, and elongating it rapidly to form its web. Once this has been achieved the substance loses its solubility in water. Only by solving the riddle of how spiders manage to produce their thread in the water and make their webs, can we learn how to make nylon using the same system without damaging the environment.

If we examine closely what lies around us, we discover that this is just one of a number of examples of nature-based polymers with features that easily equal those of manmade materials, leading to the conclusion that compared to Mother Nature, mankind still has a lot to learn about polymers.

Material properties

When we look at types and methods of industrial production, one of the pioneers in the field of biopolymers is the Japanese chemical engineering firm, Showa Denko K. K., that 20 years ago in 1993 started mass producing Bionolle, in the polybutylene succinate (PBS) and polybutylene succinate adipate (PBSA) versions, which is a biodegradable and compostable aliphatic polyester (OK Compost and EN13342 certified). Currently the source of the polymers used is non-renewable, but beginning next year a version produced using succinic acid from renewable sources will be available.

This material is processed with a method similar to that used for polyolefins and as such can be used with the technology and machinery already in use for conventional polymers. Its principal properties are as follows:
• excellent thermal stability;
• 93 °C melting point for PBSA and 115 °C for PBS;
• mechanical properties comparable to polyolefins;
• transparency;
• compatible with starch and other biopolymers, for example PLA;
• excellent ability to disperse vegetable fibres;
• suitable for moulding;
• suitable for welding;
• suitable for bubble extrusion;
• shiny in appearance and silky to the touch.
It biodegradability improves when the level of adipic acid in the co-polymer is increased, but the melting point falls and crystallisation worsens.

Structure and adaptability to processing

At a molecular level, the polymer has two forms: a linear structure and a long chain branching structure (known as LCB). The LBC type structure offers greater tension of the melt and a higher crystallisation temperature and was developed to increased its adaptability for the production of film and foam products. Other adaptations of this product are available for extrusion, blowing and injection moulding. In figure 1 we can see the correlation between the melt flow rate (MFR) – measured at 190 °C under a load of 2.26 kg – and the average molecular weight (MW) between the linear grade and the #1903 branching grade. The latter has a higher molecular weight at the same melt flow rate and is therefore easier to process while also offering improved mechanical properties in bubble film extrusion.

1 Melt flow rate (MFR) measured at 190 °C and 2.16 kg as a function of molecular weight for linear and LCB Bionolle

Bionolle can be used in compounds with other biopolymers, such as PLA, PBTA and PHA, in different proportions to obtain, for example, specific hardness, elasticity, stretch and biodegradability values, or with other renewable materials such as starch. Starcla, a compound made with starch and PLA specifically for film manufacturers, has recently been launched.

Figure 2 shows some examples of the improvements that have been obtained in adaptability for processing, by using polymer alloy technology. Bionolle 3001 (PBSA) used mainly in the production of compost bags, has a particularly soft texture and biodegrades rapidly, however it is more difficult to process because of its low crystallisation temperature (Tc) and crystallisation speed.

2 Crystallisation temperatures of various grades of Bionolle

Instead of using nucleating agents additional grades can be added to the polymer which will then crystallise at a higher temperature. Figure 2 also illustrates the Tc of the various Bionolle grades measured using the differential scanning calorimetry (DSC) method. The crystallisation temperature of the 3000 series (PBSA) is about 50 °C, the PBS 1000 series 80 °C and the #1903 (LCB) around 87 °C. By adding 16% in weight of grade #1903 to grade #3000, a material with a crystallisation temperature of 73 °C (an increase of 23 °C on the initial crystallisation temperature of the #3000) can be obtained. In other words, the addition of #1903 to # 3001 increases the crystallisation temperature. This means that by playing around with the molecular structure – linear or branching – the molecular weight and the combination of different grades we can control the adaptability of the material for processing, its mechanical properties and its biodegradability. Other variables which could be taken into account are the optimisation of additives and, perhaps, the addition of other biopolymers.

Excellent biodegradability

All Bionolle grades are biodegradable. Figure 3 shows the biodegradability level of the polymer according to the ISO 14855 standards. This property is influenced by crystallinity and the molecular structure. Figure 4 shows an example of biodegradability in relation to the crystallinity of PBS.The crystallinity is controlled via the temperature and the time required to anneal the heat-pressed sheets. The degree of crystallinity is measured using an NMR spectroscope. In anaerobic conditions the degree of degradation of the polymer is very slow and this leads to the conclusion that it would not be suitable for use in the production of methane or hydrogen in anoxic conditions.

3 Biodegradation curve for Bionolle according to ISO 14855
4 Biodegradability of Bionolle depending on crystallinity

Before releasing a new substance into the atmosphere it is first necessary to verify its impact on the environment, as well as establishing the substances which may be released during decomposition, since such product might be ingested or absorbed by living organisms. Research began from the ground itself. Soil samples were collected after the Bionolle decomposed and they showed an absence of organic fractions: indeed it would appear that low molecular weight fragments are absorbed directly into the ground. Biodegradability was also evaluated using a microbial esterase decomposition model. As illustrated in figure 5, in the conditions examined, the main products are monobutyl succinate and dibutyl succinate; this shows that in the initial stages of decomposition the process takes place via extra cellular enzymes such as esterase.

5 Molecular weight distribution of decomposition products of Bionolle
6 Biodegradation of Bionolle and residual fragments

The next step was to compare the biodegradability and decomposition of residual products using the OECD301C method. As we can see in figure 6, when biodegradation reaches 80% it is almost impossible to detect degradation products: this means that Bionolle has been converted into carbon dioxide and assimilated into living organisms as a source of carbon.

Organic testing

As mentioned previously, the degradation products can be absorbed by plants, animals and micro-organisms. For this reason, tests were also carried out on plant growth. Bionolle in powdered form was spread on the land and seeds were sown when the degradation process had reached 30%. No difference was found in plant growth rates or in the number and weight of the leaves when compared to the control group (grown in non Bionolle exposed soil). This result, together with those obtained from experiments carried out to evaluate the possible toxic effects on laboratory animals, show that the polymer has little or no effect on plants and animals.

The experiments on microorganisms were carried out in soil prepared with Bionolle and glucose and the reaction of organisms to exposure to both substances was analysed. It was observed that the colonies of bacteria grew constantly when the soil have been treated with the polymer; growth which was halted when the polymer was no longer added to the soil. This means that the land in question adapted to the degradation of Bionolle.

As regards the length of time required for degradation, it was observed that the biopolymer degrades by 50% in six weeks, therefore the possibility that the substance might accumulate in the soil is low even when the film is being used in agriculture.

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