Properties and fungal biodegradation of the different cellulose derivatives structure included into castor oil-based polyurethane composites

Abstract

The requirement to protect the environment involves the synthesis of new polymers, blends, and composites with a high content of biodegradable components. This work studies the synthesis and biodegradation of castor oil-based polyurethanes filled with different cellulose derivatives (hydroxyethyl cellulose and hydroxypropyl cellulose). The polyurethane composites present a slight decrease in tensile strength and thermal stability with the increase in cellulose derivatives content. Scanning electron microscopy results showed a good dispersion of the cellulose derivatives within the polyurethane matrix. The degradation of polyurethane–cellulose derivatives by the soft rot fungus Chaetomium globosum was investigated by scanning electron microscopy, Fourier transform infrared spectroscopy, and behavior of mechanical properties. The results showed that the inclusion of cellulose derivatives results in an increase in the rate of degradation. The obtained polyurethane–cellulose derivative composites have preserved their processability and mechanical resistance and exhibited enhanced biodegradability becoming useful eco-friendly polymeric materials.

Keywords

Castor oil polyurethane composites cellulose derivatives fungal biodegradation

Introduction

Under the pressure of the need to reduce waste materials that degrade with difficulty, the use of biocompatible and biodegradable renewable resources in the synthesis of polymers has become more widely researched over the past years.^1–3

Polyurethanes have wide industrial applications, including the production of medical devices, due to their biocompatibility, degradability, nontoxicity, and chemical versatility. The degradable nature of components, such as polycaprolactone diol (PCL) within the polyurethane structure improves the biodegradation of these products by microorganisms.⁴

Vegetable oil is one of the natural products mostly used in the synthesis of polyurethanes and their composites with other renewable resources such as cellulose, lignin, starch.^5–9 Castor oil (CO) is a plant oil with free secondary hydroxyl groups and can be used as a polyol in the synthesis of polyurethanes.^10–13

Besides polyols derived from vegetable oils, polyurethane structure can also include other renewable resources such as cellulose and starch. These nanocomposites exhibit both improved environmental protection and shape-memory properties¹⁴ or high mechanical properties and thermal stability.^15,16 In order to obtain useful complex polyurethane structures, cellulose has to be integrated into the polymeric matrices by extrusion or compression injection,^17–19 and is obtained from lignocellulosic materials resulted from the liquefaction of wood^20,21 or from the liquefaction of starch.²²

In medicine (tissue engineering and pharmacy) and the food industry, cellulose has often been used in the form of cellulose ethers, such as hydroxyethyl cellulose (HEC)^23–26 or hydroxypropyl cellulose (HPC)^27–29 due to their water solubility, biodegradability, and nontoxic properties. The different structure of their side-chains determines their differing behavior in forming intramolecular and intermolecular hydrogen bonds that influence the different affinity for water (HPC < HEC) and their interaction with the host polymer matrix.³⁰

Polyurethane materials require a balance between the stability of these polymers for the duration of their use and reduced resistance of these materials to microorganisms when the products reach the landfill. Several studies have shown that ester or urethane bonds are vulnerable to enzymatic attack. Thus, many fungi have proven capable of biodegradation by polyurethane materials.³¹ The Chaetomium globosum fungus has been proven to be suitable for the biodegradation of PEG/CO-based polyurethane biomaterials.³² In addition, it was also found that polyurethanes containing modified vegetable oil, such as acrylated epoxidized soybean oil, have higher biodegradability rates compared to polyurethanes without vegetable oil when exposed to C. globosum,³³ under a process of soil-burial³⁴ biodegradation. The flexible polyurethane foams derived from CO are biodegradable under the action of Pseudomonas sp.³⁵ The introduction of wettable and biodegradable groups within the structure of the hard segment can improve the biodegradation rate of polyurethanes by the fungus Pseudomonas aeruginosa.³⁶ Cellulose and its derivatives have high water retention capacity and thus can be completely biodegraded by microorganisms.^37,38

The aim of this paper is to elaborate on the synthesis of new polyurethane composites which contain within their structure various natural polymers such as CO and cellulose derivatives (HEC and HPC). The property–structure relationships will be discussed in detail and the influence of the composition on the biodegradation process under the action of C. globosum fungus will be studied. The biodegradation rates of the polyurethane composite films have been characterized by chemical structure analysis (Fourier transform infrared (FTIR) spectroscopy), while mechanical properties and morphological characterization were performed using scanning electron microscopy (SEM). The goal was to provide new information on the biodegradation response as a function of the type and structure of the cellulose derivatives compared with cellulose filler.

Experimental

Materials

PCL (Mn = 2000), hexamethylene diisocyanate (HDI, purum, > 98.0%), CO, and N,N-dimethylformamide (DMF) were purchased from Sigma-Aldrich Corporation and were used as received without further purification.

The cellulose derivatives—HEC and HPC—were purchased from Sigma-Aldrich Chemie GmbH (Steinheim, Germany). The cellulose derivatives have two types of side-chains: hydroxyethyl and hydroxypropyl groups, respectively (Figure 1).

C. globosum is part of Chaetomiaceae molds fungi family. Biodegradation under the action of C. globosum was done at the Department of Microbiology of the Biological Research Institute, Iasi, Romania. This fungus, as a cellulolytic fungus species, contributes to the biodegradation of cellulose-rich substrates in outdoor and indoor environments.

Synthesis of CO-based polyurethane

To obtain cellulose composites, we first obtained CO-based polyurethanes to be used as a host polymer matrix. This polyurethane matrix was prepared by a two-step procedure in DMF solution, which has been already described in previous work.³⁹

The isocyanate-terminated pre-polymer was synthesized by reacting degassed and dried PCL with HDI given a 1:2 molar ratio at 80℃ for 2 h, in a 250 ml glass reactor fitted with a mechanical stirrer and a drying tube connected to a vacuum pump, which was put in an oil bath to ensure uniform heating. The urethane pre-polymer was then chain extended with CO, in keeping with a general molar ratio of [NCO]:[OH] of 1:1. Under continuous stirring, the obtained polymer was mixed with 15 ml DMF, then cast onto cleaned glass plates and kept at 80℃ for 24 h under vacuum, in order to obtain CO-based polyurethane films (P0). Some of the obtained samples were used as control material for comparison with the composite samples.

Preparation of cellulose derivative composites

Cellulose composites were obtained by dispersing different amounts of cellulose derivatives (HEC or HPC) (Table 1) into the fresh polymer solution under continuous stirring until homogeneous mixtures were obtained.

Table 1.

Formulations of the composites obtained with hydroxyethyl cellulose and hydroxypropyl cellulose.

Samples	PCL₂₀₀₀/HDI/chain extender molar ratio	Chain extender	Type of filler
P0	1:2:1	Castor oil	–
HEC1	1:2:1	Castor oil	Hydroxyethyl cellulose 5%
HEC2	1:2:1	Castor oil	Hydroxyethyl cellulose 10%
HEC3	1:2:1	Castor oil	Hydroxyethyl cellulose 15%
HPC1	1:2:1	Castor oil	Hydroxypropyl cellulose 5%
HPC2	1:2:1	Castor oil	Hydroxypropyl cellulose 10%
HPC3	1:2:1	Castor oil	Hydroxypropyl cellulose 15%

HDI: hexamethylene diisocyanate; HEC: hydroxyethyl cellulose; HPC: hydroxypropyl cellulose; PCL: polycaprolactone diol.

These composites containing cellulose derivatives were turned into films by the conventional solvent-casting method from DMF solution onto glass plates. After casting, the cellulose derivative composites were left for 24 h at 80℃, under vacuum, in order to cure and for the solvent to be removed. By using this method, the cellulose derivatives are dispersed into the wide mesh of the network formed by the CO-based polyurethane.

Biodegradation of the cellulose derivative composites

For the antifungal resistance test, ASTM G-21,⁴⁰ we have used a nutrient malt extract agar (MEA) medium that provides all the nutritional elements needed for the support of fungi growth. MEA, based on the formula recommended by Thom and Church,⁴¹ contains the proper formulation of carbon, protein, and nutrient sources essential for yeast and mold growth. Dextrose is added to the medium to provide a carbon and energy source for the fungi. This medium was chosen because it is commonly used to culture filamentous fungi.^42,43 The sterilized test pieces were placed into Petri dishes on the surface of solidified MEA before the top surface was sprayed with a C. globosum spore suspension in distilled water. The test items were then incubated at 30℃ and maintained at a humidity greater than 85% for one week. The fungal exposure was continued at 28℃ for 120 days. After fungal exposure, each sample was cleaned by repeated washing with distilled water, followed by the samples being conditioned at room temperature for 48 h.

Characterizations

FTIR spectra were recorded on a Bruker VERTEX 70 instrument equipped with a single-reflection attenuated total reflection (ATR) accessory. Spectra were recorded in an ATR mode in the wavenumber range from 600 to 4000 cm⁻¹ at a nominal resolution of 4 cm⁻¹, averaging 64 scans. FTIR spectral results were collected before fungal exposure and then after 60 and 120 days of fungal degradation, respectively.

Thermogravimetric measurements were performed using an STA 449F1 Jupiter device (Netzsch, Germany). Measurements were carried out in the temperature range of 20–700℃ under a nitrogen flow (50 ml min⁻¹) at a heating rate of 10℃ min⁻¹.

Differential scanning calorimetry (DSC) measurements were conducted on a DSC 200 F3 Maia device (Netzsch, Germany) at a heating rate of 10℃ min⁻¹ from −100 to 100℃, under an inert nitrogen atmosphere at a flow rate of 50 ml min⁻¹.

Mechanical properties were determined using a Shimadzu EZ Test (Japan), equipped with a 5 kN load cell. Dog-bone-shaped specimens were cut from the composite films using dies (ISO 37 type 2). The specimens had an overall length of 75.0 mm, a width of 12.0 mm in the clamping zone, and a width of 4.0 mm in the stretching zone. Tests were performed at room temperature (23℃) at a cross-head speed of 50 mm min⁻¹. For each product, five samples were tested, and the average values were reported. The standard deviation of the values obtained from all the performed tests was lower than 10%.

The morphological characteristics of the composite surfaces were evaluated by SEM using a TESLA BS 301 microscope operated at 20 kV. The samples were placed onto glass slides which were fixed onto copper supports and covered with a thin layer of carbon-gold for subsequent observation and photography.

Results and discussion

FTIR analysis before fungal degradation

The structural characteristics of the obtained polyurethane elastomers and of the various cellulose derivative-based composites are dependent on the hydrogen bonds that can be created between the polar groups. Hydrogen links can be formed between proton donor groups (urethane N–H) and groups that can accept these protons (C=O urethane, C=O ester from CO, C=O ester from PCL). Additionally, the OH groups from the cellulose derivatives can also create strong bonds with the polyurethane matrix. All these linkages can be monitored by tracking the FTIR spectral results in the ranges from 3600 to 3100 cm⁻¹ (N–H stretching region) and 1600 to 1760 cm⁻¹ (C=O stretching region). FTIR analysis was also performed in order to detect the changes that might occur in the chemical structure of the composites after fungal exposure. Figure 2 shows the FTIR spectra of the polyurethane composites filled with different amounts of the two cellulose derivatives.

The N–H stretching region of each spectrum exhibits a shift to higher wavelengths from 3378 cm⁻¹ (P0) to 3395 cm⁻¹ (HEC3) with the increase in HEC content. This behavior can be attributed to the fact that a higher content of cellulose derivative is interposed between the polyurethane chains so that the hydrogen bonds between polyurethane chains decrease in number.

These samples exhibit behavior similar to that of cellulose-filled samples,³⁵ indicating that the hydrogen bonds are made with the cellulosic chain. In the C=O stretching region there is a peak at around 1724 cm⁻¹ which was assigned to the disordered hydrogen-bonded C=O groups. This region also exhibits peaks corresponding to the C=O bonds from CO.¹⁵

The FTIR spectra analysis has shown that, with the increase in cellulose derivatives content, the peaks at around 1260 cm⁻¹ which were ascribed to the stretching vibrations of the C–O–C of the composite ester groups (–C–(C=O)–O–C–)⁴⁴ have increased in intensity. This behavior is similar to that of the peaks at around 1240 cm⁻¹, which are attributed to the amide vibration (asymmetrical stretching vibration of –C–O–C–). This behavior is different to that of composite samples obtained with cellulose which present an intensification of the band at 1240 cm⁻¹ regardless of cellulose content.³⁹ The FTIR spectrum between 800 and 400 cm⁻¹ is associated with the amide IV, V, and VI.^45,46

Thermal analysis

Figure 3 shows the thermograms of TGA measurements for the CO-based polyurethane (P0) and its composites with different cellulose derivatives (HEC and HPC).

The decomposition of the polyurethane matrix (P0) begins to take place around 319℃ while the maximum decomposition temperature is 415℃ leaving behind a residual mass of 1.88% (at 500℃). The samples with a low content of HEC (HEC1) present a higher onset decomposition temperature of up to 330℃ and a residual mass of 1.4%. Thus, the dispersion of a low amount of HEC throughout the composite enhances the thermal stability of the composite due to the hydrogen bonds created between the HEC and the polyurethane matrix as well as due to the rigid and bulky structure of the cellulose derivative. By increasing the HEC content, the thermal stability of the obtained composites decreases, so that in the case of HEC3 the onset decomposition temperature was 303℃ and the maximum decomposition temperature was 397℃. Thus, the increase in cellulose derivative content reduces the cohesion of the polyurethane matrix because of the large volume of the derivatives which stretches the crosslinked structure and causes the disruption of many of the hydrogen urethane bonds. Additionally, the cellulose filler exerts a plasticizing role that facilitates the elimination of degradation products.

In the case of the HPC-based composites, even the samples with a low content of cellulose derivatives (HPC1) exhibit lower thermal stability. HPC1, for instance, has its first-step decomposition temperature at around 317℃ and its maximum decomposition temperature at 412℃. In samples with a higher amount of HPC content, the decrease in thermal resistance is even more pronounced so that HPC2 has its onset decomposition temperature at 303℃ and its maximum decomposition temperature at 405℃. This can be explained by the fact that the bulkier structure of the hydroxypropyl groups in the HPC structure prevents a better ordering within the matrix of the composite compared to that allowed by the hydroxyethyl groups in the HEC structure. The presence of HPC thus hinders the formation of a considerable number of hydrogen bonds between the urethane matrix and the cellulose derivative polymer.

The DSC thermograms for all the cellulose derivative-based composite films, including those of the polyurethane matrix (P0), are shown in Figure 4.

Figure 4 shows that the glass transition temperature of the soft segment increases from −55.5℃ (P0) to −47 to 49℃ for the samples which include cellulose derivatives into their composite structures. The same trend was also observed for the endothermic peaks which are attributed to the soft segment melting temperature. Thus, these endothermic temperatures increase from 36.5℃ (P0) to 48–50℃ in samples filled with HEC and to 46–50℃ for samples filled with HPC. This behavior shows that the presence of cellulose derivatives in the polyurethane matrix increases the ordering of the microphase domains determined both by hydrogen bonding between cellulose derivatives and the urethane groups as well as by the rigidity of the cellulose polymer matrix. We have observed no difference from the behavior of cellulose-filled samples.³⁹

Mechanical properties

The stress–strain curves of the CO-based polyurethane films and of their composites with different cellulose derivatives are presented in Figure 5.

Figure 5 shows that the cellulose-derivative filler has an important effect on the tensile properties and elongation at break values of the composite films compared with CO-based polyurethane films. While the pure polyurethane films (P0) have high tensile strength (15 MPa, stdev 1.2 MPa) and high elongation at break (800%, stdev 50%), their composites with a low content of HEC (HEC1) show a decrease in tensile strength to 13 MPa (stdev 0.8 MPa) and in the strain at break values to 670% (stdev 48%), yet the latter samples also exhibited a higher Young modulus (35 MPa (stdev 1.8 MPa) for HEC1). The higher Young modulus in the case of samples with a low content of HEC proves that the cellulose derivative filler also bears a stiffening effect on the polyurethane matrix. For sample films with a high content of HEC (HEC3), the mechanical properties drop significantly, as far as values of 5 MPa (stdev 0.4 MPa) for stress at break and 370% (stdev 32%) strain at break.

The mechanical properties of the HPC-based composite films significantly and continuously decrease with the increase in HPC content. Thus, HPC1 has stress at break values situated around 7.5 MPa (stdev 0.67 MPa) and strain at break up values of to 550% (stdev 48%), while HPC3 has stress at break values of 6 MPa (stdev 0.4 MPa) and strain at break of 210% (stdev 18%). This behavior indicates that the side-chains of the cellulose derivatives prevent the dispersion of the filler into the polyurethane matrix because they bear a steric hindrance effect due to their chemical structure. By comparison, cellulose filler does not have the same effect in decreasing mechanical properties.³⁹ Nonetheless cellulose derivative composites still retain suitable mechanical properties to make them usable in many industrial fields.

Biodegradation study

We have previously reported on the poor degradation rate of CO-based polyurethane elastomers during fungal degradation tests, due to the hydrophobic nature of CO.²⁹ Other research¹¹ underlines the significant role of the moistening of the polyurethane surface in the biodegradation of the CO-based polyurethanes in the case of soil burial tests. These studies show the importance of the presence of the components which attract moisture to initiate and propagate the biodegradation process into the polymer matrix. The biodegradation of the cellulose derivative-based composites was studied by means of examining the changes that occur in the chemical structure, surface morphology, and mechanical strength of the composites by FTIR, SEM, and mechanical tests.

FTIR spectroscopy after fungal degradation

FTIR spectra, after fungal exposure, revealed chemical structure changes of the cellulose derivative-based composite surfaces, as shown in Figure 6.

There appears to be no significant changes of the peak at 1724 cm⁻¹ attributed to the carbonyl groups in the composite structure, due, probably, to the large number of ester groups in the cellulose derivative structure that do not suffer a degradation following exposure to the C. globosum fungus.

Instead, the characteristic peaks for the NH stretching vibrations become wider over time of exposure to the fungus and shift to smaller wavelengths. Thus, in the case of composites with a low content of cellulose derivatives (HEC1), the NH stretching vibration peaks shifted from 3392 cm⁻¹ in the case of untreated samples to 3380 cm⁻¹ for samples exposed to the fungus for 120 days. In the case of composites with a high content of cellulose derivatives (HEC3), the shift was from 3395 to 3374 cm⁻¹, and for HPC3, from 3392 to 3339 cm⁻¹. This can be explained by the large number of free hydroxyl groups from the cellulose derivative and by the degradation of many of the urethane groups followed by the formation of new hydrogen bonds between the degradation products and the cellulose derivative.^36,39

We can see changes in the structure and intensity of the absorption bands at 2900–2970 cm⁻¹ indicating a degradation of the CH₂ linkages with a local recombination of the resulting products. Additionally, one can notice changes that have occurred in the area of the absorption bands from 1240 to 1260 cm⁻¹ after the biodegradation, especially for HEC3 and HPC3 samples. These changes are the result of the degradation—by hydrolysis and oxidation—of the samples under the enzymatic action of the fungus.³⁵ The signals observed at 800 cm⁻¹, which are attributed to amide IV⁴⁶ and the C–C and C–H bond vibrations of the composite structure, have decreased in intensity, especially in the case of HEC-filled composites.

The composites obtained with cellulose derivatives exhibit peaks of higher intensity in the urethane domain compared to the cellulose-based composites.³⁹ This is the result of the specific structure of the cellulose derivatives, which achieve less dense structures, facilitating the penetration of moisture and fungi into the polyurethane matrix.

Mechanical behavior after fungal biodegradation

Figure 7 shows the changes in mechanical properties after 60 and 120 days of exposure to C. globosum fungus of the composite samples.

It can be seen that the tensile strength at break decreases for HEC1 from 13 MPa (stdev 0.8 MPa) to 7 MPa (stdev 0.55) after 120 days of exposure to the C. globosum fungus. Elongation at break also decreases from 680% (stdev 48%) (HEC1) to 340% (stdev 28%) (HEC1.120). In samples with a higher HEC content (HEC3), the tensile strength remains close to the initial values, but elongation at break decreases from 370% (stdev 32%) (HEC3) to 50% (stdev 4%) (HEC3.120).

For HPC-containing composites, a stiffening of the samples was observed, with higher Young modulus values but with a dramatic decrease in elongation at break after 60 days of exposure to fungi. Thus, elongation at break decreases after 60 days of biodegradation from 550% (stdev 48%) (HPC1) to 100% (stdev 8%) (HPC1.60).

The changes in mechanical properties after fungal exposure are consistent with the changes observed in the FTIR measurements and are related to the physical changes that have occurred in the matrix of composites. Due to fungal degradation, the macromolecular chains can be broken into much shorter chain segments. These shorter segments have more freedom of movement and can thus be better packaged and perform a greater number of hydrogen bonds. This results in the stiffening of the composite films which in turn decreases the elongation at break values of these polyurethane composites.

Morphology of composite films after fungal degradation

The SEM measurements showed that hyphae of C. globosum fungus appeared on the composite surface or have penetrated the composite. The SEM images of the composites before and after fungal exposure are shown in Figure 8.

The micrographs of the composites reveal a smooth surface of the composites with a good dispersion of the cellulose derivatives (HEC1), before fungal exposure. After 60 days of fungus exposure, the composite surfaces became rough and irregular and mycelium begins to appear on the composite surface. After 120 days of fungus exposure, HEC1.120 film surfaces reveal many holes. In samples with a higher HEC content (HEC3) after 60 days of exposure to fungus, numerous cracks, holes, and loose fragments of material appear on the surface of the film, and after 120 days of biodegradation, the surface of the composite films was almost broken into by the attack of fungal straining to reach the nutrients medium.

After 60 days of exposure to fungi, the surface of samples filled with HPC (HPC1) shows a sponge-like structure with multiple cracks and large holes. After 60 days of fungal exposure, HPC3 presents a strong biodegradation of the surface with traces of fungus hyphae inside the matrix of the composite. After 120 days of biodegradation, the surface is very rough and presents many holes.

The amount of penetration and cavities is correlated with the structure and amount of cellulose derivative on which fungi thrive. This is because the cellulose provides the necessary level of moisture for the development of microorganisms and thus improves the poor biodegradability of CO-based polyurethanes.³²

Conclusions

Polyurethane composites based on renewable raw materials (CO and cellulose derivatives) were biodegraded by exposure to C. globosum fungus. The results suggest that the degree of fungal biodegradation is dependent on the structure and content of the cellulose derivative included in the polyurethane composite matrix. The inclusion of cellulose derivatives increases the moisture on the polyurethane surface that allows the fungus to grow and penetrate inside the CO-based polyurethane matrix, thereby accelerating the degradation of these materials. This demonstrates that very small variations in cellulose structure produce large changes in biodegradability behavior.

Footnotes

Declaration of Conflicting Interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) received no financial support for the research, authorship, and/or publication of this article.

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, et al. Influence of NCO/OH and transesterified castor oil on the structure and properties of polyurethane: synthesis and characterization. Mater Express 2015; 5: 377–389.