Journal of Polymers and the Environment

, Volume 20, Issue 1, pp 53–62 | Cite as

Biodegradation of Bloodmeal-Based Thermoplastics in Green-Waste Composting

  • Casparus J. R. Verbeek
  • Talia Hicks
  • Alan Langdon
Original Paper

Abstract

Polymers that are compostable and manufactured from renewable resources have gained significant importance in recent years. The objective of this work was to assess the biodegradability of bloodmeal-based thermoplastics in a commercial green-waste composting situation. Materials plasticised with tri-ethylene–glycol lost about 45% of their original mass after 12 weeks composting while unplasticised samples lost 35%. Degradation appeared to have been in two phases; an initial loss of soluble, low molecular compounds in the mesophilic phase followed by degradation of high molecular compounds as the temperature exceeded about 40 °C in the thermophilic phase. It was found that as degradation proceeded materials became more soluble. In addition, plasticised and unplasticized samples contained about 60 wt% moisture after 4 weeks of composting conditioning at 50% relative humidity resulted in approximately 8–10 wt% moisture, unaffected by the extent of degradation. FTIR revealed that proteins underwent hydrolytic cleavage resulting in the formation of primary amines. A significant reduction in combustion temperature was observed, indicative of a reduction in covalent bonding, likely due to shorter chains lengths or less cross-linking.

Keywords

Bloodmeal Compositing Thermoplastic 

Introduction

Bioplastics have had attention since the early 1900s, but production figures were small. For example, Henry Ford tested soy plastics for automobile parts in the 1930s [1]. A wide range of proteins have been trialed as possible bioplastics, such as, casein, corn gluten meal, sorghum meal, wheat, zein, egg albumin, feather meal and fish meal [2]. Many of these can successfully be extruded and injection moulded into various plastic products [3] and several aspects of the technology behind protein processing are presented in recent reviews [2, 4].

In previous studies it has been shown that bloodmeal can be successfully converted into a thermoplastic material with good mechanical properties [5]. The objective of this paper was to evaluate the biodegradability of bloodmeal-based thermoplastics during composting. Biodegradation was assessed in terms of mass loss, solubility, thermal stability and changes in chemical structure.

Composting

The main contributors to the breakdown of organic materials during composting are bacteria and fungi. Composting occurs in three phases; the mesophilic phase, thermophilic phase and maturation phase and is classified by the type of bacteria present as well as physical and chemical properties of the compost during that stage [6]. Dissolved organic compounds are broken down by micro-organisms. Degradable compounds typically comprise carbohydrates (such as cellulose, hemicelluloses and pectin), proteins, lipids and lignin. These molecules are mostly insoluble, but some micro-organisms are able to solubilise these molecules by producing exo-enzymes that break down these compounds. These molecules can then be catabolised within microbial cells [7].

During the mesophilic phase easily oxidisable compounds such as monosaccharides, starch and fats are broken down by hydrolase enzymes and is an exothermic process, raising the compost temperature [6, 7, 8, 9]. Due to the increasing population of micro-organisms, the temperature rises quickly and can last from several hours to a few days [10] and also leads to growth of thermophilic bacteria.

During the initial stages of the thermophilic phase, organic acids are decarboxylated to form carbon dioxide [6] and proteins are enzymatically degraded into the various amino acids, which then undergo deamination to form corresponding α–keto acids and ammonia [7]. As the temperature rises above 45 °C, thermophilic bacteria and fungi take over degradation entirely, characterised by oxidation and microbial breakdown leading to a further temperature rise [6, 10]. Thermal equilibrium is reached at about 70 °C and will remain at this temperature until all easily oxidisable compounds are consumed [11]. However, in commercial facilities, the temperature is ideally maintained between 55 and 65 °C by mixing and aeration, to ensure there is diversity of the thermophilic species of bacteria present, optimising the degradation process [6]. After the easily degradable carbon sources have been consumed, less reactive compounds such as cellulose, hemicelluloses and lignin are degraded and partially transformed into humus [7], which marks the onset of the mesophilic phase. New mesophilic species of bacteria and fungi further humify the compost and the compost pile now contains a large variety of crawling insects and worms [10].

Under ideal conditions, the composting process can be completed within a month [9, 11]. However, industrial composting typically takes between 6 and 8 months. The degradation process is limited by factors affecting the biological cycles of micro-organisms such as aeration and moisture content. Complete aeration is often complicated by odour restrictions in the facility’s resource consent (odour arising from aerobic composting is often deemed offensive). This may limit turning or mixing the compost in intervals just frequent enough to prevent the onset of anaerobic conditions. However, when anaerobic conditions are established, degradation of organic material is slowed, and the odour generated is much more offensive.

Natural Polymer Degradation

Natural polymers are able to undergo chemical and microbial degradation to form carbon dioxide and water, and in the case of proteins, ammonia also. Degradation is initiated by degrading the polymer into it’s monomer units, which can then be mineralised, such processes occur during composting. However, not all natural polymers degrade at the same rate, and some compounds such as lignin may take years to degrade [12, 13].

Most international standards measure the rate of degradation by measuring CO2 generated during controlled composting in sealed reaction vessels inoculated with bacteria from mature compost [14]. Only one standard is commonly used for determining whether a plastic can be deemed biodegradable in an industrial composting facility. According to ATSM D6400-04 “Standard Specification for Compostable Plastics” a plastic sample must lose 60% of its mass within 6 months (24 weeks), degrading at a rate comparable with cellulose. Quantifying biodegradation using mass loss data alone can be inaccurate. Sample disintegration makes it difficult to distinguish between polymer fragments and compost material and cleaning the retrieved samples may also be a concern [15]. Careful sieving of the compost material is required to ensure the entire sample is recovered, however this is not a guarantee that very small pieces will be found. This means that mass loss data should preferably seen in conjunction with other evidence of degradation.

Mass loss alone does not indicate that a material has undergone chemical degradation. To determine the extent to which it has occurred can be achieved by analysing changes in solubility, thermal properties and chemical structure [15].

A polymer’s behaviour under thermal conditions is determined by the strength of intermolecular interactions and bonding within the material. There are four major degradation steps typically observed during thermogravimetric analysis (TGA) of plasticised proteins [16]: vaporisation of water, decomposition of additives, weak bond cleavage and finally degradation of strong covalent bonds. The onset temperature of thermal degradation for polymers will increase with the increasing number of cross-links. This is due to the increased stability caused by strong intermolecular interaction. When a protein is degraded or undergoing peptide bond cleavage, disulfide bond cleavage and changes in tertiary structure, it becomes less thermally stable. A strong indicator of degradation occurring in a polymer is an observed decrease in the temperature at which strong chemical bond degradation will occur. When the molecular mass of a polymer is decreased due to bond cleavage, it becomes easier to oxidise, resulting in a decrease of the temperature required for oxidation.

Experimental

Materials and Equipment

Materials used are listed in Table 1.
Table 1

Materials used

 

Supplier

Grade

 

Bloodmeal (BM)

Taranaki byproducts (New Zealand)

n/a

ρ = 1,300 kg/m3 Sieved to 700 μm

Sodium dodecyl sulfate (SDS)

Biolab

Technical

Sodium sulfite (SS)

BDH lab supplies

Analytical

Urea

Balance agri-nutrients (New Zealand)

Agricultural

Thermoplastic Preparation

Bloodmeal based thermoplastic (BMT) has been developed earlier and has been patented by Novatein Ltd, New Zealand [17]. Thermoplastic protein was prepared by blending 100 parts (by mass) bloodmeal with three parts sodium dodecyl sulphate (SDS), three parts sodium sulphite and 10 parts urea dissolved in 60 parts water. Ten parts per hundred parts bloodmeal tri-ethylene glycol (TEG) was used as plasticiser. These blends were stored for at least 12 h prior to extrusion.

Extrusion trials were performed in a ThermoPrism TSE-16-TC twin-screw extruder at a screw speed of 150 rpm using a temperature profile and screw configuration shown in Fig. 1. Actual melt temperatures were within 2–5 °C of the set temperatures. The extruder had a screw diameter of 16 mm, an L/D ratio of 25 and was fitted with a single 10 mm circular die. A relative torque of 50–60% of the maximum allowed in the extruder was maintained (12 Nm per screw maximum), by adjusting the mass flow rate of the feed. The extruder was fed by an oscillating trough and the extruded material was granulated using a tri-blade granulator from Castin Machinery Manufacturer Ltd., New Zealand. Samples were injection moulded directly after extrusion and granulation, without further conditioning.
Fig. 1

Extruder screw configuration and corresponding temperature profile

Specimens for tensile test were produced using a 22 mm screw diameter BOY 15 S Injection Moulding Machine. Specimens were injected through a cold runner into a water heated mould. The shape of the tensile test specimens was in accordance with ASTM D638. Specific settings used are shown in Table 2. For each experiment five plasticized and five unplasticised samples were tested.
Table 2

Injection moulding parameters

Parameter

Value

Temperature profile (°C)

70 (Feed zone); 115; 120

Injection pressure

1,200 bar

Backpressure

400 bar

Screw speed

150 min−1

Cooling time

20 s

Locking force

30 kN

Composting

To evaluate the compostability of BMT, 30 specimens with and without plasticiser were prepared by injection moulding. Prior to testing the specimens were conditioned at 23 °C, 50% relative humidity for 1 week. Compositing trials were performed in a commercial green-waste facility located in Hamilton, New Zealand (Hamilton Organic Centre). A fresh compositing pile was started for this study of approximately 25 m3 (4 × 4 m wide and 1.5 m high) using only plant material.

The compost pile was turned for aeration once per month and approximately 0.5 tonne of water was added to the compost pile every 2–4 weeks as deemed appropriate by the facilities manager to maintain a moisture content between 40 and 55 wt%. Composting was done over summer when ambient temperature was roughly 20–25 °C. Temperature just below the samples was measured prior to turning the compost pile using a thermometer. In the first week of degradation, the temperature of the compost surrounding the samples was 65 °C and settled to 69 °C for the remainder of the 12 weeks.

To expose the samples to the composting process, a wooden frame (540 × 500 × 150 mm) with 2 mm mesh on the bottom was used to prevent loss of disintegrated material. The box was covered with 15 mm hexagonal mesh to ensure the contents of the box remained in place when being removed from the pile and to provide aeration. The samples were then placed in the box lined and covered with green-waste as shown in Fig. 2, before placing in the centre of the composting heap.
Fig. 2

Box containing BMT samples for exposure to composting process

Over 12 weeks, five samples with and without plasticiser were removed fortnightly for further analysis. After removing the samples they were photographed and weighed. Samples were subsequently conditioned at 23 °C and 50% relative humidity for 1 week and reweighed in order to reduce uncertainty as a result of moisture uptake during degradation. Immediately following, a part of each sample was used to determine moisture content. Remaining parts of the samples were freeze dried and analysed using FTIR and TGA.

Analysis

Moisture Content

Granulated samples were weighed into aluminium dishes and dried in an air-circulating oven at 80 °C for 12 h. These measurements were done in triplicate for each experiment.

Mass Loss

Mass changes as a result of composting were tracked based on the initial mass of a sample (m) as well as sample mass after different treatments. Because samples contained water as part of their formulation, samples were also dried and conditioned to determine the initial dry mass (mid) and initial mass after conditioning (mi). A significant amount of water may be absorbed during composting that may obscure mass loss as a result of degradation. To avoid this, dry mass (mfd) and conditioned mass (mi) after compositing were also considered.

Five samples (with and without plasticizer) were removed every 2 weeks and weighed after various stages, as outlined below:
  • Initial dry mass (mid) measured after drying at 100 °C for 24 h

  • Conditioned mass (mi); conditioned at 23 °C and 50% relative humidity for 1 week

  • Mass directly after removal from composting (mf)

  • Mass after composting and conditioning (mre)

  • Dry mass after composting (mfd).

These were used to calculate various quantities, as outlined in Table 3.
Table 3

Equations used for mass changes after composting

Description

Equation

Initial equilibrium moisture content

\( {\text{MC}}_{\text{i}} = \frac{{m - m_{i} }}{m} \)

1

Total mass change after composting

\( \Updelta M_{t} = \frac{{m_{f} - m_{i} }}{{m_{i} }} \)

2

Mass change, based on equilibrium water content

\( \Updelta {\text{M}}_{\text{eq}} = \frac{{m_{re} - m_{i} }}{{m_{i} }} \)

3

Moisture content after composting

\( {\text{MC}}_{\text{f}} = \frac{{m_{f} - m_{fd} }}{{m_{fd} }} \)

4

Equilibrium moisture content after composting

\( {\text{MC}}_{\text{ac}} = \frac{{m_{f} - m_{re} }}{{m_{re} }} \)

5

Change in dry mass after composting

\( \Updelta {\text{M}}_{\text{dm}} = \frac{{m_{id} - m_{fd} }}{{m_{id} }} \)

6

Solubility

Solubility was measured by immersing 0.5–1.5 g of granulated sample in 20 mL of distilled water for 24 h. Material left after dissolution was filtered and dried in an oven at 80 °C for 24 h. The change in mass between the dry mass (mid) and the dry mass after immersion (msd) was used to determine solubility according to Eq. 7. Solubility was determined once every 2 weeks for each specimen removed, presenting the result as an average, with one standard deviation as the error.
$$ {\text{S}} = \frac{{m_{id} - m_{sd} }}{{m_{id} }} $$
(7)

Thermogravimetric Analysis (TGA)

Thermogravimetric analysis was carried out using a DTA-TGA analyser (SDT 2960, TA Instruments, New Castle, Delaware, USA). The DTA-TGA analyser measures thermogravimetric changes and differential thermal analysis (DTA) simultaneously, giving changes in mass as temperature increases and calorimetric information so that phase transitions can be determined [18]. The analysis was carried out from room temperature to 800 °C at a rate of 10 °C/min using air. A sample size of 5–10 mg was used throughout.

Fourier Transform Infrared Spectroscopy (FTIR)

Freeze-dried and powderised samples were prepared as KBr discs (using 1 mg sample to 100 mg KBr). Approximately two-thirds of the KBr mixture was pressed at 10 tons pressure using vacuum to minimise water absorption during pressing. FTIR analysis was performed using a Perkin Elmer spectrophotometer, employing 16 scans at 4 cm−1 resolution from 4,000 to 400 cm−1. The background was scanned prior to each sample scan and automatically subtracted from the spectrum by the FTIR software.

There are nine characteristic IR absorption regions for amides observed in peptides as shown in Table 4. These regions typically require deconvolution of the FTIR spectrum due to complex peak overlapping. The amide A band is unaffected by the conformation of the polypeptide backbone [19] while amide I vibration does not significantly change with the type of amino acid side chain. The amide I band frequency depends on the secondary structure of the peptide backbone enabling it to be used for secondary structure analysis of proteins. The amide I band arises primarily from the C = O stretching vibration with minor contribution from the out-of-phase C–N stretch, the C–C–N deformation and the N–H in-plane bend [19, 20].
Table 4

Characteristic infrared regions of peptide linkages [21]

Designation

Approximate frequency (cm−1)

Description

Amide A

3,300

NH stretching

Amide B

3,100

NH stretching

Amide I

1,600–1,700

C=O stretching

Amide II

1,480–1,575

CN stretching, NH bending

Amide III

1,229–1,301

CN stretching, NH bending

Amide IV

625–767

OCN bending

Amide V

640–800

Out-of-plane NH bending

Amide VI

537–606

Out-of-plane C=O bending

Amide VII

200

Skeletal torsion

FTIR is a suitable technique for the analysis of dehydrated proteins to determine secondary structure from the Amide I and II regions in the IR spectrum [20]. FTIR also yields information regarding polymer degradation over time by observing changes in the absorbance and peak shifts in the spectrum which are caused by deterioration of the proteins and formation of oxidation products, both which can be investigated using conventional peak allocations.

Results and Discussion

During the first 4 weeks, samples containing plasticiser (TEG) immediately showed signs of surface pitting, shape deformation and rounded edges. Despite their dry appearance, all samples felt damp to touch. Samples without plasticiser started showing minor surface cracking but no surface pitting after 2 weeks, having very minor shape deformation (Fig. 3).
Fig. 3

BMT samples with (top row) and without plasticiser (bottom row) removed throughout the composting process (weeks 0–12, left to right in two weekly intervals)

After 6 weeks plasticised samples had begun to disintegrate with only one sample intact. The samples showed major surface cracking, pitting and shape deformation. The edges were also rounded and easily disintegrated when rubbed. In contrast, the samples without plasticiser were removed intact; showing minor surface cracks, surface pitting, minor shape deformation and smoothed off edges and corners.

After 8 weeks the plasticised samples were removed in pieces, with major surface cracking and pitting. The edges of the samples were rounded and the ends of the samples were thinner. The samples without plasticiser were removed intact, with surface cracks and pitting but little shape deformation. The edges of the samples were smoother and corners more rounded than those removed at Week 6.

After 10 weeks plasticised samples were removed in smaller pieces, showing major deterioration. Samples without plasticiser were still intact, showing little change in appearance. By the end of 12 weeks samples with plasticiser were removed in small pieces (1–3 cm2) the samples were spongy in texture and the edges were easily fragmented when touched. However, samples without plasticiser were still intact showing signs of deterioration similar to that for plasticised samples after 10 weeks exposure.

Mass Loss Due to Biodegradation

Measuring the change in mass of a sample during composting is the simplest means of determining the extent of which degradation has occurred. However, changes in mass may be observed as a result of absorption, hydrolysis or water diffusion. In addition, low molecular mass compounds such as urea and tri-ethylene glycol (TEG) may leach out and will be observed as a mass loss. As a result, various differences in mass were considered, as outlined in Table 3, of which the change in dry mass (Eq. 6) would be most indicative of biodegradation.

The molecular mass of proteins will decrease during degradation due to metabolic processes causing peptide bond cleavage. The lower molecular mass fragments formed as a result of degradation may diffuse out or be metabolised by micro-organisms to form carbon dioxide, ammonia and water, leading to an observed mass loss in the sample.

During the initial 8–10 weeks of the composting process an increase in mass was observed after the samples were removed from the compost pile, shown in Fig. 4 (calculated using Eq. 2). Plasticised samples increased their initial mass by a maximum of 17% as observed after 2 weeks whereas samples without plasticiser increased in mass by up to 41% as observed after 4 weeks.
Fig. 4

Mass change of BMT removed immediately after composting as a percentage of the original mass (Eq. 2)

The observed increase in mass was attributed to the absorption of water and hydrolysis reactions occurring during the composting process. As the proteins are hydrolysed and the plastic is further plasticised, there is an increase in the number of sites available for water to be adsorbed. The increased number of sites available for adsorption is caused by protein chains swelling as well as a result of the material becoming more porous due to enzymatic catabolism.

The large difference in the amount of water physically adsorbed between samples with and without plasticiser, may be caused by the difference in number of sites available for adsorption. Samples containing plasticiser would have had fewer sites available for sorption due to the presence of TEG between protein chains.

Based upon the change in mass after composting, degradation appeared to have occurred in two phases (Fig. 4). The initial loss of easily solubilised compounds probably occurred in the first 4 weeks of the composting process, carried out by mesophilic and thermophilic bacteria, accounting for 15% of the mass for samples containing plasticiser and 13% of the mass for samples without plasticiser.

Between weeks four and six of the composting process samples appear to undergo a rapid increase in the rate of degradation, resulting in a rapid decrease in mass.

Comparison of the samples with and without plasticiser indicated that during weeks’ ten to twelve external factors probably increased the rate of degradation, such as moisture content and the micro-organism’s population size. Therefore, the rapid increase in degradation rate is likely to be caused by the population of thermophilic bacteria present in the compost pile reaching its maximum size leading to a maximum rate of catabolism.

Despite the mass loss after week 4, moisture content (Eq. 4) seemed to have stabilised at about 60 wt% after 4 weeks (Fig. 5). Although there was some scatter in the data, it would suggest that mass loss after week 4 would be due to a loss of polymer mass, rather than water. The equilibrium water content (Eq. 5) of the same samples was also determined, but no discernible trend could be observed. Using Eq. 5, it was found that all samples had an equilibrium moisture content between 8 and 10 wt%.
Fig. 5

Moisture content directly after composting (Eq. 4)

Solubility of a protein derived bioplastic is a good indicator for the extent of cross-linking [5], but will also include soluble additives required for processing, such as urea and triethylene glycol. Solubility in water can indicate the type of interactions that are present; peptide and protein moieties that are un-associated will dissolve which is a characteristic observation for reduced molecular interaction due to increased protein degradation.

Solubility data will aid determining the extent to which protein oxidation has occurred during composting. As the amount of protein being converted to peptides and free amino acids increases, so too should the solubility of the material, however, the composting process requires water to dissolve the nutrients so the bacteria can assimilate them, so any product resulting from polymer degradation may no longer be present in the material being analysed.

During the composting process, easily solubilised compounds will be dissolved in the moisture content of the compost pile to be assimilated by the bacteria present. From Fig. 6, a dramatic decrease in solubility (Eq. 7) is observed after 2 weeks of composting. This is caused by the loss of urea and other soluble compounds such as tri-ethylene glycol. The decrease in solubility for plasticised materials is higher than that observed for unplasticized samples due to the initial loss in TEG in addition to unreacted urea. After 2 weeks the trend observed for changes in solubility is the same for plasticised and unplasticised samples. Despite a large standard deviation, the graph does suggest a slight increase in solubility that would indicate some degradation has taken place. There is, however, a clear trend between dry mass change and solubility. From Fig. 7 it can be observed that as solubility increased, the change in dry mass (indicative of extent of biodegradation) increased exponentially. There was no obvious difference between plasticised and unplasticized samples. This would indicate that an increase in solubility is directly linked to increased biodegradation, most likely due to progressive chain fragmentation over the course of biodegradation.
Fig. 6

Solubility of BMT throughout the composting process (Eq. 7)

Fig. 7

Solubility versus change in dry mass

Although water present in the sample will significantly influence the degradation of the bioplastic, for the purpose of determining total degradation, the mass loss of the bioplastic is discussed in terms of the change in dry mass (Eq. 6).

From Fig. 8 it was observed that plasticised BMT showed 47 ± 4% dry mass loss during 12 weeks of composting, comparatively BMT without plasticiser showed 36 ± 4% dry mass loss. It may be possible that the approximate 10% higher dry mass loss observed for plasticised BMT is could be due to the loss of the 10 wt% TEG used in the BMT as there is no other significant difference within the error limits.
Fig. 8

Percentage dry mass loss for BMT with and without plasticiser (Eq. 5)

Significant deterioration in BMT samples was observed throughout the 12 week composting process and was confirmed by mass loss data. To determine the extent of degradation of the proteins, FTIR and TGA data were also evaluated.

Fourier Transform Infrared Spectroscopy (FTIR)

The FTIR spectra obtained for plasticised and unplasticised BMT samples showed three significant changes after 12 weeks of composting (Fig. 9 and Fig. 10):
  • Appearance of secondary peak at 3,340 cm−1 (labelled ‘a’)

  • Increase in the absorbance of the peak at 1,552 cm−1 (labelled ‘b’)

  • Loss of definition and absorbance of peaks in the region 1,050–1,190 cm−1 observed in TEG samples (labelled ‘c’).

The appearance of a shoulder peak at 3,340 cm−1 suggests the formation of primary amine stretches, shown by point ‘a’ in Fig. 9. Increases in absorbance and broadening of the peak in the region 3,500–3,100 cm−1 can indicate the presence of water, however, because the sample was freeze-dried and the KBr disc prepared under vacuum to prevent water being incorporated, the possibility of the peak change being caused by water can be discounted. In a proteinous material, primary amines are formed as a result of hydrolytic cleavage of its peptide bonds to give amine groups. This peak is also observed for BMT without plasticiser, confirming that during composting, the protein backbone has undergone hydrolytic cleavage.
Fig. 9

FTIR spectrum of BMT containing TEG after exposure to industrial composting for 12 weeks

Fig. 10

FTIR spectrum of BMT without plasticiser after exposure to industrial composting for 12 weeks

There was also a substantial increase in the absorbance of the peak at 1,552 cm−1, shown by point b. This peak increase may be due to increased number of amide N–H in-plane bending vibrations, a result of peptide cleavage forming primary amine groups. Generally, this would also result in a corresponding peak at 1,590–1,650 cm−1, however, a specific peak in this region was not observed in the spectra of BMT due to the complex nature of infrared absorption in proteinous materials, for this change to be observed deconvolution would be required.

Another observation was the loss of the definition and absorbance of peaks in the region 1,050–1,190 cm−1, as indicated by point ‘c’. In the TEG sample prior to composting, the cause of the multiple peaks in this region could be due to peak overlay with the C–O stretches of triethylene glycol. Therefore, the loss of peaks in this region, leaving no specific bands may be due to loss of tri-ethylene glycol in the plasticised samples due its high solubility, as these peaks are absent in BMT samples without plasticiser.

In summary, the FTIR results indicated BMT undergoes hydrolytic peptide cleavage during the composting process and the plasticiser had diffused out from BMT.

Thermogravimetric Analysis

There are four zones representing the various stages of thermal decomposition for protein-based thermoplastics; vaporisation of water (1), decomposition of additives (2), weak bond cleavage (3) and strong bond cleavage, yielding inorganic matter (4).

Microbial metabolism of BMT during composting leads to changes in the intra- and intermolecular bonding of proteins and loss of low molecular mass compounds that are soluble. These changes in BMT can be observed with TGA, as they result in changes in of the material’s thermal stability.

Both samples with and without plasticiser both show several major changes in their thermal behaviour as a result of exposure to the composting process (Figs. 11 and 12):
  • Low molecular mass compounds such as triethylene glycol and urea are not present after 6 weeks (labelled ‘a’)

  • Smaller chain length peptides may have been removed by degradation, leading to increased thermal stability in zones 3 and 4 after 6 weeks exposure (labelled ‘b’)

  • Protein degradation to smaller peptides has occurred, observed by a reduction in combustion temperature as shown by the shift in exothermic peak in zone 4.

It is evident from zones 2 and 3, that after 6 weeks exposure to composting, a smaller proportion of low molecular mass compounds are present in the remaining sample (labelled a). This is observed for BMT with and without plasticiser, suggesting urea, in addition to triethylene glycol, has dissolved or been metabolised within the first 6 weeks of composting.
Fig. 11

Thermogravimetric traces of plasticised BMT after 0, 6 and 12 weeks exposure to industrial composting. The primary y-axis represents the percentage sample weight remaining as the temperature is increased and the secondary y-axis shows the exotherm expressed as temperature difference

Fig. 12

Thermogravimetric traces of BMT without plasticiser after 0, 6 and 12 weeks exposure to industrial composting

The thermal stability in zone 3 is initially lower for BMT prior to composting (labelled ‘b’) this is consistent with the lower % mass loss observed for BMT at the beginning of zone 4. After degradation the thermal stability has increased, possibly caused by the microbial metabolism removing the shorter peptide chains from BMT in the thermophilic stages of the composting process.

A decrease in exothermic peak temperature is observed for BMT as composting time increased. The temperature at which combustion occurs is influenced by the strength and number of covalent bonds present. Less covalent bonding is involved in lower molecular mass compounds such as polypeptides, making them less thermally stable than the protein they originated from. Therefore, the reduction in combustion temperature with increasing composting time indicates the proteins in BMT are being degraded to lower molecular mass compounds, confirming that hydrolytic cleavage of peptide bonds and/or disulfide bonds has occurred due to microbial metabolism.

Conclusions

Composting causes many changes to the physical and chemical structure of BMT. Within weeks, physical deformation and deterioration of the samples is observed, followed by cracking, pitting and within 12 weeks, disintegration.

After 2 weeks exposure to composting conditions there is a large decrease in the solubility of BMT, followed by a slight increase. This is indicative that the plasticiser and urea have dissolved or been metabolised by the mesophilic bacteria and are no longer present.

This is confirmed by TGA, after both 6 and 12 weeks’ exposure a dramatic reduction in the quantity of low molecular mass compounds present in the remaining sample. After 12 weeks exposure, the apparent loss of tri-ethylene glycol from BMT observed through changes in solubility and thermal stability is supported by the disappearance of ether peaks, from TEGs C–O stretching vibrations (1,050–1,190 cm−1), in the FTIR spectra of plasticised BMT. In addition, the loss of 10 wt% TEG may explain the approximately 10% lower observed mass loss for the remaining dry mass of BMT without plasticiser.

Degradation of the components in BMT appeared to have occurred in two stages. Mass loss results indicated the possibility for two different reaction rates, the first rate corresponding to the loss of low molecular mass compounds, either being dissolved or metabolised, followed by a more rapid rate occurring for the loss of higher molecular mass compounds. This model is consistent with literature concerning composting where soluble, low molecular mass compounds are consumed first by mesophilic bacteria and fungi, followed by hydrolysis and deamination of higher molecular mass proteins by thermophilic bacteria as the temperature of the compost exceeds 45 °C.

Hydrolytic cleavage of proteins in BMT was shown to occur after 6 weeks of exposure. FTIR results showed two peaks that have been altered after composting, 3,340 and 1,552 cm−1, caused by the formation of primary amines, a result of peptide bond cleavage. This interpretation is supported by TGA which showed a reduction in the combustion temperature after composting. A decrease in the temperature at which combustion of an organic material occurs can only be caused by a reduction in covalent bonding. In a protein, thermal stability could be reduced due to peptide bond cleavage, however, cleavage of disulfide bonds cannot be ruled out as possible contributing factor.

After 12 weeks exposure to composting conditions, plasticised BMT had undergone 47 ± 4% dry mass loss, and BMT without plasticiser had undergone 36 ± 4% dry mass loss. This reduction in mass can be attributed to disintegration, reduced molecular mass of proteins and loss of low molecular mass soluble compounds such as urea and plasticiser.

References

  1. 1.
    Mohanty AK, Liu W, Tummala P, Drzal LT, Misra M, Narayan R (2005) Natural fibers, biopolymers, and biocomposites. In: Mohanty AK, Misra M, Drzal LT (eds) Soy protein-based plastics, blends, and composites. Taylor and Francis, Boca Raton, p 875Google Scholar
  2. 2.
    Verbeek CJR, van den Berg LE (2009) Recent Pat Mater Sci 2:171CrossRefGoogle Scholar
  3. 3.
    Calvert FE (1947) US patent, 2424383Google Scholar
  4. 4.
    Verbeek CJR, van den Berg LE (2009) Macromol Mater Eng 295:10CrossRefGoogle Scholar
  5. 5.
    Verbeek C, van den Berg L (2010) J Polym Environ 1:171Google Scholar
  6. 6.
    Som MP, Lemée L, Amblès A (2008) Bioresour Technol 100(2008):4404Google Scholar
  7. 7.
    Tuomela M, Vikman M, Hatakka A, Itavaara M (2000) Bioresour Technol 72:169CrossRefGoogle Scholar
  8. 8.
    Haug RT (1993) The practical handbook of compost engineering. Lewis Publishers, Boca RatonGoogle Scholar
  9. 9.
    de Bertoldi M, Vallini G, Pera A (1985) Composting of agricultural and other wastes. In: Gasser JKR (ed) Technological aspects of composting including modelling and microbiology. Elsevier Applied Science Publishers, EssexGoogle Scholar
  10. 10.
    Rudnik E (2008) Compostable polymer materials. Elsevier, AmsterdamGoogle Scholar
  11. 11.
    Woodard F (2001) Industrial waste treatment handbook. Butterworth-Heinemann, WoburnGoogle Scholar
  12. 12.
    Copyright Dow Corning Corp (1997) Available from: http://www.dowcorning.com/content/publishedlit/01-1112-01.pdf. Retrieved June 2010.
  13. 13.
    Ratajska M, Boryniec S (1998) React Funct Polym 38:17CrossRefGoogle Scholar
  14. 14.
    Standard Test Method for Determining Aerobic Biodegradation of Plastic Materials Under Controlled Composting Conditions ASTM D5338 -98 ASTM International (2003)Google Scholar
  15. 15.
    Czichos H, Saito T, Smith L (2006) Springer handbook of materials measurement methods. Springer, WurzburgCrossRefGoogle Scholar
  16. 16.
    van den Berg LE (2009) Development of 2nd generation proteinous bioplastics. School of Engineering, University of Waikato, HamiltonGoogle Scholar
  17. 17.
    Verbeek CJR, Viljoen C, Pickering KL, van den Berg LE (2009) NZ patent, NZ551531Google Scholar
  18. 18.
    Evans Analytical Group (2009) Thermogravimetric analysis (TGA)/differential thermal analysis (DTA). Available from: http://www.eaglabs.com/techniques/analytical_techniques/tga_dta.php
  19. 19.
    Barth A (2007) Biochim Biophys Acta 1767:1073CrossRefGoogle Scholar
  20. 20.
    Golovina EA, Wolkers WF, Hoekstra FA (1997) Comp Biochem Physiol Part A Physiol 117:343CrossRefGoogle Scholar
  21. 21.
    Kong J, Yu S (2007) Acta Biochimica Et Biophysica Sinica 39:549CrossRefGoogle Scholar

Copyright information

© Springer Science+Business Media, LLC 2011

Authors and Affiliations

  • Casparus J. R. Verbeek
    • 1
  • Talia Hicks
    • 1
  • Alan Langdon
    • 1
  1. 1.School of EngineeringUniversity of WaikatoHamiltonNew Zealand

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