Making Biopolymers Processable Using Thermal Analysis and Rheology

Biopolymers are an attractive alternative to polymers derived from fossil fuel and are mainly used by the packaging industry today. However, their crystallization behavior makes it more difficult to process these new materials. Learn how thermal analysis and rheology provide solutions to study the relevant material properties.

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Biopolymers are an attractive alternative to polymers derived from fossil fuel and are mainly used by the packaging industry today. Never before was the plastics industry as driven by sustainability as it is now. The mounting pressure from society and legislation is weighing particularly heavy on the packaging industry demanding more sustainable alternatives.

What are biopolymers?

The term biopolymers includes bio-based polymers, biodegradable polymers, which could be oil-based, as well as the combination of both: bio-based and biodegradable at the same time. Bio-based polymers have a low carbon footprint that can be even further improved if the materials are recycled. Biodegradable plastics are sometimes criticized, because they often do not decompose in the environment, but rather under very controlled conditions in composting plants.

Therefore, materials like polyhydroxybutyrate-hydroxyvalerate (PHBV) are particularly interesting as they are bio-based and biodegradable at room temperature. For example, it will decompose in soil over the duration of just a few weeks to a month. Polyhydroxybutyrate (PHB) is generated by specific bacteria as a form of energy storage. The pure material has a high crystallinity of up to 80%, which makes it rather brittle and difficult to process conventionally. However, copolymerization within the bacteria produces PHBV with good mechanical properties.

Challenge #1: Secondary crystallization at room temperature

Unfortunately, these properties change during the service life of the manufactured products because of continued crystallization and thus embrittlement. This often happens within a few days and makes the material unsuitable for even short-term use. One solution is adding other polymers or oligomers that reduce or even hinder the secondary crystallization at room temperature. Ideally, the added material is also bio-based.

One such suitable plasticizer for PHBV is polyethylene glycol (PEG) [1]. In a study performed at the University of Birmingham in the AMCASH and Jenkins’ laboratories, Dr. Kelly1,2 investigated the miscibility of this blend. The researchers produced various mixtures of PHBV and low-molecular weight PEG and studied the material behavior using a NETZSCH Kinexus Pro+ rotational rheometer. To study the miscibility, typically frequency sweeps are performed in oscillation and the measured storage moduli plotted over the corresponding loss moduli, on log scales, to obtain a Han plot. Han et al. stated that any miscible blend would exhibit a straight line comparable to the pure material and deviations from that line indicate immiscibility [2].

However, the PHBV-PEG blends studied here degrade during the measurements and therefore, this method can not be readily applied. Therefore, a modification used for thermally unstable systems was used, which was first proposed by Yamaguchi and Arakawa [3]. Time sweeps were performed at specific frequencies. The measurement conditions are summarized in Table 1 and the results of the time sweeps are shown in Figure 1 for the storage modulus.

Table 1: Measurement conditions

Measurement ModeTime sweeps in oscillation
Geometry20 mm parallel plates
Temperature185°C
Gap1 mm
Strain0.5%
Frequencies0.25 – 25 Hz
Premelt time5 minutes
Figure 1: Measured storage modulus as a function of time for frequencies of 0.25, 1, 5 and 25 Hz (from bottom to top)

After the measurements and data collection were finished, both the storage and loss modulus data were plotted against frequency for each 60 second interval. A master curve was then generated by superimposing the data. These calculated master curves were used to compute the corrected storage and loss modulus at time t0 and to generate the Han plots, Figure 2. For all investigated blends, their miscibility was proven by a straight line comparable to that of the pure PHBV.

Figure 2: Schematic of the Han plot with the classification as miscible and immiscible depending on the linearity of the results

More details about the analysis as well as the use of the rheological data to compute the degradation rates can be found here!

Challenge #2: Processability into thin films

In another study performed at the Institut für Kunststofftechnik at the University of Stuttgart by Silvia Kliem, MSc3, bio-based citrate was studied as a plasticizer for use in film blowing. Due to the low viscosity and melt strength of pure PHBV, a suitable biodegradable additive is needed to improve its processability into thin films. The researchers blended the PHBV with different amounts of citrate (5 and 10 wt%) as a plasticizer as well as low amounts of polylactide (PLA). A NETZSCH DSC 204 F1 Phoenix® was used to study the effect of the additive on the crystallization behavior of the blend. The measurement conditions are summarized in Table 2.

Table 2: Measurement conditions

PanAl, pierced lid
Sample weightabout 11 mg
AtmosphereN2
Temperature-20°C to 200°C at 10 K/min (1. + 2. heating and cooling)
Figure 3: Dynamic DSC measurements of PHBV-PLA Blends with (blue is 5 wt%, pink is 10 wt%) and without the plasticizer citrate (green)

Figure 3 shows the heating and cooling curves of the PHBV-PLA blend with and without citrate. It can be seen that the melting and crystallization enthalpy is comparable for all three compositions when normalized for the citrate weight content (analysis results omitted in graph for better clarity). The peaks at 175°C and 120°C are for the melting and crystallization of the PHBV, respectively. The much smaller peak at 150°C shows melting of the PLA component. Comparing the different curves further, it can be observed that the additive citrate shifts the melting and crystallization peaks to lower temperatures; in the case of 10 wt% citrate by almost 4 K. This has a significant effect on the degradation of the material during processing, as the extrusion temperature can be lower due to the plasticizer.

These analysis results were validated by film blowing trials. While the PHBV-PLA blends without plasticizer could not be expanded, the extrusion was improved with 5 wt% citrate. Only with 10 wt% was it possible to keep up a steady extrusion process and to reach a film thickness < 25 µm.

The whole study can be found here!

Rheology and Thermal Analysis suitable to analyze biopolymers

These two studies show examples of bio-based plasticizers for bio-based PHBV to create a fully degradable packaging material. It can be seen that both plasticizers have advantages for different applications that require different processing as trays compared to thin films. It was found that both rheological and thermoanalytical techniques can be applied to analyze the properties of biopolymers such as PHBV and especially their processability. It is especially helpful that both rheological and thermoanalytical methods require very little amounts of material compared to processing trials, but can give valuable information about their properties. Using the right techniques will help increase our understanding of this still relatively new class of materials and allow the steady improvement and market maturity that we so urgently need.

1About AMCASH at the University of Birmingham

The AMCASH project, which is a part-funded ERDF program, is co-ordinated through the School of Metallurgy & Materials at the University of Birmingham. The project offers regional SME organizations technical assistance of typically a 2-day duration, within materials science related projects. Learn more here!

2About the Jenkins’ laboratory at the University of Birmingham

Activity mainly concerns the relationship between chemical structure, processing, microstructure and the physical properties of thermoplastic polymers (numerous polymers, blends and thermoplastic composites), and furthermore, how the properties can be influenced by each of these aspects. Learn more here!

3About the Institut für Kunststofftechnik at the University of Stuttgart

The expertise of the Institut für Kunststofftechnik under the direction of Prof. Dr.-Ing. Chrsitian Bonten comprises the entire field of plastics technology: material engineering, processing technology (mechanical and process engineering) and product engineering. Learn more here!

Sources

[1] Kelly AC, Fitzgerald AVL, Jenkins MJ. Control of the secondary crystallisation process in poly(hydroxybutyrate-co-hydroxyvalerate) through the incorporation of poly(ethylene glycol), Polymer Degradtaion and Stability. 2018; 148: 67-74, https://doi.org/10.1016/j.polymdegradstab.2018.01.003

[2] Yang H, Han CD, Kim JK. Rheology of miscisble blends of poly(methylmethacrylate) with poly(styrene-co-acrylonitrile) and with poly(vinylidene fluoride), Polymer. 1994; 35(7): 1503-1511

[3] Yamaguchi M,Arakawa K. Effect of thermal degradation on rheological properties for poly(3-hydroxybutyrate). Eur. Polym. J. 2006;42(7):1479-86

[4] https://www.kunststoffe.de/kunststoffe-zeitschrift/archiv/artikel/citrate-ermoeglichen-die-blasfolienextrusion-von-phbv-ohne-die-abbaubarkeit-zu-beeinflussen-11292093.html

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