Abstract
Poly(3-hydroxybutyrate) [P(3HB)] nonwovens were obtained from polymers enriched with nucleants using the melt-blown technique. The most important physico-mechanical parameters, susceptibility to hydrolytic degradation (in neutral and alkaline medium) and to biodegradation, were analysed for the obtained nonwovens. It was determined that P(3HB) nonwovens, compared to popular polypropylene (PP) nonwovens, are characterized by elementary fibres with several times greater average diameter, greater mass per unit area and greater air permeability value. P(3HB) nonwovens are, on average, seven times more susceptible to breakage, and their elongation at maximum force is more than 50 times smaller than that for PP nonwovens. Hydrolysis of P(3HB) nonwovens is faster in an alkaline than in a neutral medium, and the observed relationships led to the conclusion that, at the start, short chains are subject to hydrolysis. Analysis of the weight loss associated with the degradation in bioreactors showed that P(3HB) nonwovens are more susceptible to biodegradation under anaerobic than under aerobic conditions.
1 Introduction
The increase in environmental pollution has resulted in the search for materials that, after a short service life, will be subject to decomposition. Another trend observed in both research and industrial technologies is doing away with raw materials derived from mineral resources (e.g., petroleum). The most widely used polymers [e.g., polyethylene, polypropylene, polyvinyl chloride, polystyrene and poly(ethylene terephthalate)] do not exhibit the above characteristics; thus, research work is being conducted to replace them with other polymers.
Polyhydroxyalkanoates (PHA) mostly consist of 3-hydroxyalkanoate acids and belong to aliphatic polyesters (1). Poly(3-hydroxybutyrate) [P(3HB)] is the most popular type of PHA and the first one to be studied. Copolymers such as poly(3-hydroxybutyrate-co-3-hydroxyvalerate) [P(3HB-co-3HV)], poly(3-hydroxyvalerate), poly(3-hydroxyhexanoate) and poly(4-hydroxybutyrate) are also well known. At present, more than 150 different PHA monomers are known (2).
PHA are a large group of polyesters with different chemical and physical properties, but some features are characteristic of the entire polyhydroxyalkanoate group. One of the most important features of PHA – because of which they have become very popular, widely studied and widely applied – is biodegradability. This feature means that PHA decompose as a result of the action of living organisms, especially bacteria, fungi and algae (3). Biodegradation can occur in the presence of oxygen (aerobic condition); the resulting by-products of which are carbon dioxide, water and a certain amount of biomass. If biodegradation occurs without oxygen (anaerobic condition), methane is produced as a by-product (4). The scientific literature includes research works about the degradation of PHA conducted under different conditions: in activated and synthetic sludge (5, 6), in the compost from municipal wastes (7), in the soil (8, 9), in organic solvents (chloroform, methanol) (10) and in water at different pH values (usually pH 7 or 10) (10, 11).
PHA are obtained mostly by bacterial biosynthesis (12). They are stored by bacteria, owing to the limited access to the elements necessary for growth – nitrogen, phosphorus, sulphur, magnesium or oxygen – but they have unrestricted access to the carbon-containing compounds (13, 14). Currently, studies are focused on the biosynthesis of PHA to understand better the mechanism of polyhydroxyalkanoate synthesis by bacteria and to direct it according to the needs (1, 15–19). Besides biological methods, it is also possible to produce PHA using traditional chemical methods (20–27). However, biosynthesis is more convenient than chemical synthesis; the basic problem with the latter method is the formation of by-products and the difficulty in obtaining PHA with high molecular weights (28).
The processing of PHA by heat treatment reduces the sensitivity to increased temperature in the molten state; melting point [for PHA, it is in the range from ca. 40°C to ca. 180°C (1)] is a little lower than the decomposition temperature. To reduce the thermal decomposition of PHA during processing, different techniques are used, e.g., polymer is cooled immediately after melting; PHA are processed below the melting point; polymer chains are extended to minimize the loss of molecular weight; substances that accelerate crystallization and, at the same time, limit the thermal decomposition of PHA are added, etc. (29, 30).
Another limitation in the application of PHA is their high degree of crystallinity and the resulting hardness, stiffness and brittleness; in the case of P(3HB), the degree of crystallinity is as high as 80% (31). In order to reduce it, P(3HB) copolymers and terpolymers with other types of PHA are produced (32). It was found that copolymer P(3HB-co-3HHx) containing 25 mol% of HHx units is characterized by a degree of crystallinity of 18% (33). Excessive lowering of the degree of crystallinity of PHA can impede the crystallization after the thermal processing [PHA exhibit slow crystallization after melting (4, 34, 35)]. In order to solve this problem, nucleants or other types of polymers are added to process PHA; also, composites are produced from two different types of PHA, one with a lower melting point and the other with a higher one (acting as a nucleant) (34, 35).
Items made from PHA are used in the medical field, such as hygiene products, as well as in toys, bottles and packaging for food. Nevertheless, the processing of PHA is still a novelty in the scientific literature and there are relatively few research works devoted to this subject. There is also a lack of reports on melt-blown nonwoven products obtained from PHA.
The aim of this research work was to examine the possibility of obtaining nonwovens made from P(3HB) according to the melt-blown technique and to perform a preliminary analysis of the physico-mechanical parameters and the susceptibility to degradation of the obtained nonwovens.
2 Materials and methods
2.1 Materials
Applied chemical reagents were phosphate buffer (pH 7, NaH2 PO4 0.0071 mol/dm3+Na2 HPO4 0.0129 mol/dm3) and tetraborate buffer (pH 10, Na2 B4 O7 0.013 mol/dm3+NaOH 0.018 mol/dm3). All chemical reagents were Chempur products (Piekary Śląskie, Poland).
Poly(3-hydroxybutyrate) (P(3HB)) enriched with nucleants (Biomer P209F) was purchased from Biomer (Krailling, Germany).
2.2 DSC analysis of poly(3-hydroxybutyrate)
Differential scanning calorimetry (DSC) analysis of P(3HB) was carried out using a DSC 6200 Exstar SII NanoTechnology apparatus (THASS); samples (ca. 5 mg) were heated in nitrogen atmosphere to 200°C and then cooled to 25°C; heat flow rate was 5°C/min.
2.3 Obtaining nonwovens by the melt-blown technique
Nonwovens were obtained using the melt-blown technique, which is an integrated nonwoven technology consisting of combining a fibre-forming process with a web-forming process. The strings of a molten polymer come out from an extruder head through a multi-hole nozzle where they are blown up by a stream of hot compressed air and set as fine fibres on a collecting drum. In this technique, the thermal properties of the polymer are the most important. The following instruments were used: a laboratory one-screw extruder (Axon) with a head, a compressed air heater and a collecting drum. A temperature of 180–170°C the in extruder zones and compressed air of 105°C were applied. Compressed air consumption was 9 m3/h, and polymer consumption was 2 g/min. P(3HB) was dried for 2 h at 80°C before processing.
2.4 Physico-mechanical parameter analysis
Analysis was performed in the accredited Laboratory of Testing Textile Raw Materials and Fabrics of TRI (Polish Centre for Accreditation no. AB 164). The following physico-mechanical parameters for the obtained nonwovens were determined:
elementary fibre diameter, according to the PN-86/P-04761/08 standard (with the exception of the number of measurements and preparation of samples), using a light microscope (Projectina, Heerbrugg, Switzerland) combined with a camera and a personal computer; applied magnification, 500×; number of elementary fibre diameter measurements, 400.
mass per unit area, according to the PN-EN 29073-1:1994 standard.
air permeability, according to the PN-EN ISO 9237:1998 standard, using a air permeability tester (Tex Test FX3300, Zurich, Switzerland).
maximum breaking force and elongation at maximum force, according to the PN-EN 29073-3:1994 standard, using a strength testing machine (Hounsfield H5KS, Salfords, UK); analysis speed, 100 mm/min; distance between clamps, 100 mm; parameters were determined at the warp and weft direction.
2.5 Hydrolytic degradation
A methodology for the hydrolytic degradation analysis of P(3HB) nonwovens (according to the PN-EN ISO 10993-13:2002 standard) was developed based on the methodology for the hydrolytic degradation analysis of PP nonwovens described by Kałużka et al. (36). Nonwoven samples with a mass of ca. 1 g (weighed with a precision of up to 0.0001 g) after dividing into small pieces and drying to constant mass (40°C, 50 mbar) were placed in a 250-ml Erlenmeyer flask to which 100 ml of phosphate buffer (pH 7) or tetraborate buffer (pH 10) was added. Samples were thermostated at 65°C and shaken for 1 h a day. After finishing the hydrolysis process, the samples were filtered using a weighted analytic filter, repeatedly washed with water, dried to constant mass (40°C, 50 mbar) and weighed with a precision of up to 0.0001 g. Loss of sample weight (in percent) was defined based on the difference between the initial sample mass and the sample mass after the hydrolysis. Changes in the mass of the samples were analysed after 21, 42, 63 and 84 days of hydrolysis. Three samples were used for each buffer and each hydrolysis time.
2.6 Biodegradation
Studies on the biodegradation of P(3HB) nonwovens were carried out in bioreactors at the Faculty of Process and Environmental Engineering, Lodz University of Technology. Samples were inserted into plastic covers with pores of about 0.5 mm and stiffening nets in order to protect their surface against mechanical damage and potential solid contaminants without blocking the access of microorganisms. Experiments were performed under aerobic and anaerobic conditions.
A simulation of an aerobic process was carried out in a continuously stirred (90 rpm) tank bioreactor with a working volume of 7 dm3. The air flow rate was controlled by a mass flow meter and kept at 50 dm3/h. The average concentration of dissolved oxygen was around 4.1 mgO2/dm3. The pH of the reaction mixture was about 8. A surplus activated sludge taken from the municipal wastewater treatment plant in Łódź (Poland) was used as inocula for the bioreactor. The process was carried out for 28 days. The samples (weighing from 0.6 to 0.7 g) were placed in the bioreactor in such a way that microorganisms had unlimited access to the surface of the nonwovens.
A concentrated synthetic wastewater solution [composition (g/dm3): casein peptone (1.56), dry broth (1.05), NH4 Cl (0.20), NaCl (0.07), CaCl2·6H2 0 (0.075 g), MgSO4·7H2 O (0.02), KH2 PO4 (0.20) and K2 HPO4 (0.50)] supplemented with glucose was used as a source of carbon and nitrogen. The bioreactor worked as a semi-batch-fed system. The concentrated synthetic wastewater was supplied every 24 h at a quantity that can achieve a C/N ratio of 20:1 at the beginning of the cycle (the optimal conditions for the activated sludge). Between feedings, the carbon concentration decreased gradually, which forced the microorganisms to utilize the nonwoven samples as the carbon source. Liquid from the bioreactor was not removed; the volume of the feed sufficiently compensated for the evaporation of water (the process temperature was 22°C).
In the second part of the experiments, anaerobic biodegradation with and without the additional carbon source (in the form of the concentrated synthetic wastewater solution) was examined. A fermented sludge taken during the methane fermentation stage from the municipal wastewater treatment plant in Łódź (Poland) was used as inocula for the bioreactors. The processes were carried out in two bioreactors with a working volume of 0.8 dm3. The pH of the reaction mixture oscillated was around 7; redox potential was -400 mV. The samples (weighing from 0.2 to 0.3 g) were placed in the bioreactors to ensure free access of the microorganisms to the surface of the tested nonwovens. Both processes were carried out for 28 days. The feed to the bioreactor was supplied every 6 days (the appropriate volume of liquid was removed). The process was carried out under mesophilic conditions (37°C).
In order to control the biological processes, the following parameters were determined: dissolved oxygen (WTW STIRR OX G-meter), pH (WTW pH meter), redox potential (using a SenTix ORP electrode), chemical oxygen demand (standard dichromate method; HACH), total organic carbon (TOC; LCK 386 and LCK 387 methods, according to HACH-LANGE procedures, IL 550 TOC TN apparatus), total nitrogen (according to HACH-LANGE procedures, IL 550 TOC TN apparatus).
Biodegradation of the nonwovens was evaluated on the basis of the following methods: visual observations – optical microscopic analysis of the samples [using a light microscope (BX40 Olympus) equipped with a camera allowed pictures to be taken) and scanning electron microscopy of the nonwoven surface before and after the biological processes [scanning electron microscope (FEI Quanta 200F); ultrasound scans were taken under low vacuum (100 Pa); the samples were dried prior to analysis for 24 h in an oven at 80°C]; weight loss measurements – indirect modified dry matter analysis, taking into account the biomass accumulation in the nonwoven fabrics using the elemental content analysis of the polymer after and before the biological treatment, combined with the elemental content of biomass (NA 2500 elemental analyzer, CE Instruments).
3 Results and discussion
3.1 DSC analysis of poly(3-hydroxybutyrate)
DSC thermograms of poly(3-hydroxybutyrate) before and after processing are shown in Figure 1. On heating thermograms (Figure 1A and C), two signals for granulate P(3HB) (at 160°C and 168°C) and one for nonwoven (at 166°C) were present. These signals could be attributed to the melting process, and the two signals for the granulate indicated that it consists of two crystalline forms. On cooling thermograms (Figure 1B and D), signals ascribed to the crystallization process were present: for the granulate at 107°C and for the nonwoven at 109°C. The changes in signals attributed to the melting and crystallization processes of P(3HB) granulate and nonwovens were the consequence of the thermal decomposition of P(3HB) during the melt-blown process.
P(3HB) DSC thermograms; heat flow rate, 5°C/min; sample mass, ca. 5 mg. (A) Heating thermogram of granulates, (B) cooling thermogram of granulates, (C) heating thermogram of nonwovens and (D) cooling thermogram of nonwovens.
3.2 Physico-mechanical parameters analysis
Results of the physico-mechanical parameter analysis of the obtained P(3HB) nonwovens are presented in Tables 1 and 2.
Elementary fibre diameter and mass per unit area of P(3HB) nonwovens.
| Elementary fibre diameter (μm) | Mass per unit area (g/m2) | ||||
|---|---|---|---|---|---|
| Average | Minimum | Maximum | Average | Minimum | Maximum |
| 26 | 14 | 45 | 111 | 81 | 137 |
Air permeability, maximum breaking force and elongation at maximum force of P(3HB) nonwovens.
| Air permeability (mm/s) at pressure decrease | Maximum breaking force (N) | Elongation at maximum force (%) | |||
|---|---|---|---|---|---|
| 100 Pa | 200 Pa | Warp direction | Weft direction | Warp direction | Weft direction |
| 3525 | 5449 | 4.3 | 5.0 | 2.1 | 2.7 |
These results were compared with results concerning polypropylene (PP) nonwovens. PP nonwovens were obtained from polypropylene characterized by a melt flow rate of 450 g/10 min at a polymer consumption rate of 2 g/min and a compressed air consumption rate of 9 m3/h (36). It was stated that, in comparison to PP nonwovens, the average elementary fibre diameter of P(3HB) nonwovens was about seven times greater; their minimum fibre diameter was about 31 times greater and their maximum fibre diameter was about three times greater (36). The mass per unit area of P(3HB) nonwovens was more than 1.5 times greater than that for PP nonwovens (36). The observed differences between the aforementioned nonwovens were mainly due to the need for processing P(3HB) at the lowest possible temperatures (to reduce the thermal degradation of the polymer); blowing of thicker P(3HB) with hot air resulted in nonwovens with coarser fibres.
The range of elementary fibre diameters (14–45 μm) of P(3HB) nonwovens was compared with the range obtained by Kann and Whitehouse (37) (0.1–50 μm). Kann and Whitehouse (37) prepared P(3HB) nonwovens using the melt-blown technique, but they used P(3HB) enriched with huge amounts of plasticizers (5–15%) and they applied different thermal conditions of processing: temperature in the extruder zones was generally lower, but the temperature of compressed air was higher than that applied in our experiments.
The coarser fibres of P(3HB) nonwovens (in comparison with PP nonwoven fibres) show that P(3HB) nonwovens are characterized by more than eight times greater air permeability value than PP nonwovens (36).
Analysis of the strength parameters (maximum breaking force and elongation at maximum force) led to the conclusion that P(3HB) nonwovens are, on average, seven times more susceptible to breakage and that the elongation at maximum force is 52 times smaller than that for PP nonwovens (36). These characteristics of P(3HB) nonwovens result from the fact that P(3HB),with its high degree of crystallinity, is harder, stiffer and more brittle than PP (31).
3.3 Hydrolytic degradation
Hydrolytic degradation of the obtained P(3HB) nonwovens was conducted in phosphate buffer solution (pH 7) and in tetraborate buffer solution (pH 10). A pH value of 7 was chosen according to the PN-EN ISO 10993-13:2002 standard and alkaline medium was chosen on the basis of a technical literature to examine the influence of pH on hydrolysis rate (11). Results of the hydrolytic degradation of P(3HB) nonwovens are shown in Figure 2.
Hydrolytic degradation of P(3HB) nonwovens in phosphate (pH 7) and tetraborate (pH 10) buffer solutions.
After analysing the above results, it can be stated that hydrolysis of P(3HB) nonwovens in alkaline medium (weight loss of ca. 30% after 84 days) is faster than in neutral medium (weight loss of ca. 19% after 84 days). It is due to the reaction mechanism of hydrolysis of organic esters; in acid and neutral medium, hydrolysis of esters is a reversible process and the by-products (alcohol and carboxylic acid) may combine to become an ester molecule again. In alkaline medium, ester decomposes into alcohol and carboxylic acid salt molecules; these products do not react again, so the alkaline hydrolysis of esters is not reversible and hence is more efficient. The appearance of the samples after hydrolytic degradation confirms the more efficient hydrolysis of P(3HB) nonwovens in alkaline medium; in this case, the nonwovens were more fragmented than after the hydrolysis in neutral medium.
Another important observation resulting from the analysis of the aforementioned figures is the fact that, in both media, the initial weight loss of P(3HB) nonwovens (after 21 days) was significant, but further hydrolysis resulted in smaller weight loss. The same observation was made by Renard et al. (11) during an alkaline hydrolysis of blends consisting of P(3HO) and P(3HB-co-3HV). The authors explained that, like other polyesters, the short chains undergo hydrolytic degradation at the beginning, followed by the longer chains but at a much slower rate (11). The initial significant weight loss of P(3HB) nonwovens was especially noticeable in the case of more efficient alkaline hydrolysis.
3.4 Biodegradation
After analysing the results of the weight loss associated with the degradation of P(3HB) nonwovens in the bioreactors (Figure 3), it was concluded that the highest biodegradation rate was found in anaerobic conditions: 32.6% without and 29.8% with additional carbon source. In aerobic conditions, weight loss of nonwovens was significantly smaller and reached 18.3%.
Biodegradation of P(3HB) nonwovens under different conditions.
Microscope scans of nonwovens confirmed the biodegradation: microorganisms disturbed the fibres, which led to their partial stratification and to changes in their structure (Figure 4B and C). Restrain biomass was visible on all scans after the biological processes.
SEM micrographs of P(3HB) nonwovens (magnification 500×): (A) before the biodegradation, (B) after the aerobic process and (C) after the anaerobic process with additional carbon source.
The fast biodegradation of P(3HB) nonwovens in anaerobic conditions suggests that they will degrade well in landfills, where municipal wastes remain mostly under anaerobic conditions.
Acknowledgments
The study was financed by the Polish Ministry of Science and Higher Education as part of a statutory research work carried out in 2011 at the Textile Research Institute, Łódź, Poland.
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