Biodegradation of crosslinked polyurethane acrylates/guar gum composites under natural soil burial conditions

2.1 Materials

Poly(tetramethylene oxide) glycol of a molecular weight of 1400 (Terathane 1400) (Sigma-Aldrich Chemie GmbH, Buchs, Switzerland) and poly(1,4-butylene adipate) diol (PBA, Aldrich, Mw=1000 g/mol) were dried and degassed at 120°C for 2 h under high vacuum. All other chemicals were used as received. Dimethylformamide (DMF), 1,6-hexane diisocyanate (HDI), pentaerythritol triacrylate (PETA) were purchased from Sigma-Aldrich. Guar gum (GG) – a natural polysaccharide (approximate molecular weight 220,000) was supplied by Sigma-Aldrich.

2.2 Preparation of the polyurethane acrylate prepolymer

The polyurethane acrylate prepolymer (PUA) was synthesized using a two-step polymerization procedure as described in previous papers (18). These polyurethanes were synthesized in the absence of any catalyst and with molar ratios of poly(ether)-diol or poly(ester)-diol/HDI/PETA of 1/2/2. First, the isocyanate end-caped urethane prepolymer was prepared by reaction of a mixture of the required amounts of polyol and HDI, in a 250 ml glass reactor equipped with a mechanical stirrer at 80°C for 2 h. Afterwards, the free NCO groups of the obtained prepolymer were chain-ended using the stoichiometric amount of PETA and 15 ml of DMF as solvent at 80°C until all the NCO groups were reacted, which was confirmed by the disappearance of the IR peak of NCO (2260 cm^-1). Samples of the polyurethane acrylate product were cast onto cleaned glass plates and kept at 80°C for 24 h in order to obtain the crosslinked pure polyurethane acrylate film. The polyester urethane samples were marked as BG and the polyether urethane samples were marked as G.

2.3 Preparation of the polyurethane acrylate/guar gum composites

For the preparation of the polymer composites, the guar gum was uniformly dispersed into the polyurethane acrylate polymer immediately after the completion of the synthesis process. The polymers mixture was thoroughly mixed at 80°C, for a short amount of time (10 min) in order to avoid the thermal polymerization of the vinyl groups in the reaction vessel, thus obtaining a homogeneous composition. The polymer mixtures were then used in order to obtain films through the conventional casting method. After casting, the polymer films were kept in an oven for 24 h at 80°C in order for the thermal crosslinking of the acrylate end-groups to take place and in order to ensure the removal of the residual solvent.

By proceeding as described above, the guar gum remains dispersed into the network formed by the acrylate crosslinks. By changing the amount of guar gum dispersed into the composite films from 1% (G1, BG1), 3% (G3, BG3), 5% (G5, BG5) to 10% (G10, BG10), one obtains a series of guar gum/polyurethane acrylate crosslinked composite films. The films thus prepared were used for the determination of physico-mechanical properties and for surface behavior evaluation.

2.4 Soil burial degradation test

In order to evaluate the biodegradability of these materials, samples were buried in garden soil, 25 cm deep and under outdoor environment conditions within the Iasi, Romania area. The dumbbell-shaped pieces (75×12.5×4 mm; ISO 37 type 2) that were cut out from each obtained sample were dried prior to soil burial. The specimens were left buried for 1 year. There was no additional watering or any other such intervention made upon the test ground during the period of testing. After 6 and 12 month of burial, respectively, degraded samples were recovered, carefully cleaned with distilled water and then dried at 24°C for 48 h. The susceptibility to biodegradation was determined by tensile strength testing and by observation of the surface structure and of the morphological changes that had occurred during the soil burial time period.

2.5 Characterization

Fourier transform infrared (FTIR) spectra were obtained at room temperature using a Bruker VERTEX 70 Instrument equipped with a Golden Gate single reflection ATR accessory. The spectra were recorded in the range of 400–4000 cm^-1 with a nominal resolution of 4 cm^-1.

Stress-strain measurements were performed on dumbbell-shaped samples cut from the obtained films. The tensile properties were evaluated at room temperature using a Shymadzu EZTest (Japan), equipped with a 5 kN load cell. Dumbbell-shaped specimens were prepared using die (75×12.5×4 mm; ISO 37 type 2). The cross-head speed was of 50 mm/min. Four identical dumbbell-shaped specimens were tested for each sample and their average mechanical properties were reported.

The morphological changes of the sample surface after burial were evaluated through scanning electron microscopy (SEM) with a TESLA BS 301 microscope operating at 10 kV, in order to determine the biodegradation behavior of the composites. The samples were placed on glass slides which were fixed on copper supports, and then covered with a thin layer of carbon-gold.

3.1 FTIR spectroscopy

The molecular structures of the film surfaces before and after soil burial degradation were analyzed by FTIR. The FTIR spectra are used to track changes in the chemical composition of the surface of the biodegraded materials. For this, specific functional groups’ peaks were monitored for changes in wavelength, decreases in absorption values or their complete disappearance (19). The comparative observation of the variation of characteristic bands of the IR spectrum of the degraded and pure guar gum mixtures can be used to monitor the chemical structural changes and the differences between various composites.

Figure 1 shows the FTIR spectra of the polyester urethane acrylates (BG samples) and their composites with different guar gum content before and after soil burial biodegradation.

Figure 1:

FTIR spectra of polyester urethane acrylates and their guar gum composites, before and after soil burial degradation.

FTIR spectra revealed that after soil burial all guar gum composites’ degraded surface exhibited important differences in their chemical structure compared to the control composites. After 12 months of soil burial degradation, the absorbance intensity in the spectral region of 3340–3360 cm^-1, attributed to the N-H hydrogen bonds and to the O-H vibration, had increased and broadened. This is due to the increased amount of OH groups that overlap with the NH groups (20).

The intensity of the absorbance at 1724 cm^-1 attributed to the C=O groups decreased with the time span of soil burial degradation. The ether absorption band includes the peaks at 1160–1179 cm^-1, assigned to the aliphatic ether (C-O-C) stretch in the soft segment, and at 1060–1100 cm^-1, assigned to the urethane ether (N-C-O-C) stretch in the hard segment. The band at 1251–1254 cm^-1 is assigned to the C-O-C stretching vibration in the hard and soft segments (21). This band decreases in intensity after soil burial degradation.

Figure 1 shows that the FTIR spectra of polyester urethanes have a significantly reduced intensity of COO absorption bands (1408 cm^-1) after soil burial degradation, compared to the initial samples. Additionally, after soil burial degradation, the stretching vibrations at 1463 cm^-1 of the samples containing guar gum (BG1-BG10) were lost and new peaks (1576 cm^-1 and 1536 cm^-1) appeared. The characteristic absorptions of GG at 1164–1179, 1060 and 984 cm^-1 – ascribed to the stretching vibrations of the C-OH- and the band at 1620 cm^-1 – ascribed to the bending vibration of the -OH groups – exhibit a clear decrease after soil burial degradation.

This indicates that the guar gum that was dispersed in the crosslinked polyurethane acrylate network was degraded first and thus facilitated the penetration of soil microorganisms into the crosslinked polyurethane acrylate structure. It can also be observed that the chemical structure of the crosslinked polyurethane acrylates has changed after soil burial degradation. Thus, the results suggest that bacterial enzymatic attacks break the molecular chains under hydrolysis, which in turn leads to an increase in the number of urea bonds. Furthermore, the degraded samples suffered a change in color that was a result of the oxidation reactions that had occurred during soil burial.

Figure 2 shows the FT-IR spectra of polyether urethane acrylates (G samples) and their guar gum composites before and after soil burial biodegradation for 6 and 12 months, respectively.

Figure 2:

FTIR spectra of polyether urethane acrylates and their guar gum composites, before and after soil burial degradation.

The changes in the intensity of the absorbance at 3334 cm^-1 for pure ether polyurethane acrylates (G0, Figure 2) were less significant that those of the polyester polyurethane acrylate samples (BG0). The intensity of the carbonyl band at 1724 cm^-1 has weakened and a new peak (a shoulder) appears at 1683 cm^-1 after the degradation of the pure polyether urethane samples (G0). FTIR analysis revealed that the pure samples with ether (G0) are less degraded than the pure samples with ester (BG0) in their structure.

FTIR analysis of the polyurethane acrylates-guar gum composites after soil burial degradation indicated the appearance of some new groups in the range of 1400–1600 cm^-1. Some of the peaks (1446, 1367, 1179, 987 cm^-1) decreased in intensity, showing a deterioration of the sample surfaces after soil burial degradation. The peak at 1179 cm^-1 decreased and shifted to a high wavenumber (1205–1211 cm^-1) after soil burial degradation. This shift indicates that the degradation also took place within the C-O-C group of the soft segment of these composites (22).

Thus, all these changes in the FTIR spectra clearly show a severe degradation of the guar gum composite samples that were subject to soil burial, degradation mainly caused by the action of microorganisms, by hydrolysis and by oxidation.

3.2 Mechanical properties

The degree of degradation of the guar gum composite samples after 6 and 12 months of soil burial was determined by measuring the tensile strength of these composites. Figure 3 presents the mechanical analysis of the polyester polyurethane acrylates and of their composites with guar gum, before and after degradation.

Figure 3:

Stress-strain traces of polyester urethane acrylates and their guar gum composites, before and after soil burial degradation.

It can be observed that BG0 samples show an increase in tensile strength and a dramatic decrease in elongation at break values after 6 months of soil burial. This behavior can be attributed to the chemical and physical changes that occurred due to their ester content that is sensitive to hydrolysis and oxidation, thus generating more rigid and closed structures.

At the same time, due to extended degradation, guar gum composites (particularly samples with a high content of guar gum – BG 10.12, Figure 3) suffered a dramatic decrease in elongation at break values and a loss in tensile strength of up to 100% after 12 months of soil burial. The tensile strength loss is caused by the high concentration of guar gum and the possible formation of guar gum clusters that increase the hydrophilic properties of the composite matrix. This enables the growth of microorganisms in the polymer matrix which initiate the structural damage of the polymer material.

The tensile strength and elongation at break values of the polyether polyurethane acrylates and of their composites with guar gum are showed in Figure 4.

Figure 4:

Stress-strain traces of polyether urethane acrylates and their guar gum composites, before and after soil burial degradation.

The composites with guar gum filler show a dramatic decrease in elongation at break and tensile strength values.

The tensile strength values of the guar gum composite films – both those containing polyester and those containing polyether – had dropped below 50% after 6 months of burial soil degradation. Elongation at the break decreases more dramatically, with values reaching up to only 20% after 6 months of soil burial degradation. After 12 months, the tensile properties of composite films containing polyester and 10% guar gum cannot be measured because the samples had become completely degraded.

The biodegradation of the guar gum composite films begins by surface erosion under the action of the bacteria and fungi from the soil, as confirmed by SEM microscopy. The decrease in tensile strength is primarily due to the consumption of the guar gum, which leads to the destruction of the connection between the guar gum and the polymer matrix, microorganisms penetrate inside the polymer films through numerous freshly-formed microvoids and raven the synthetic polymer chains. All these morphological and chemical changes seriously affect the mechanical properties of the composite samples.

3.3 Surface morphology

The influence of burial time on the biodegradability of the guar gum composites can very well be seen on the SEM micrographs. The presence of the guar gum in the composite structure improves the adhesion of the soil microorganisms to the film surfaces.

The addition of guar gum contributes to a higher degradation rate of the composites because the guar gum is a strongly hydrophilic material, which draws moisture to the composite, enabling the development of microorganisms which process the guar gum, thus penetrating the interior of the composite matrix and destroying the integrity of the polymer film.

Once the guar gum from the composite matrix has been consumed, the level of composite porosity increases, thus creating adequate conditions for the penetration of the microorganisms throughout the polymer matrix. This leads to the accelerated destruction of the entire composite with the consequence of a sharp decrease in mechanical properties.

The SEM micrographs for the pure polymer films and their composites with different content of guar gum are shown in Figures 5 and 6.

Figure 5:

SEM images of the surfaces of polyester urethane acrylates; (A) (BG0), (b) (BG0.6), (C) (BG0.12) and their composites with 5% guar gum; (D) (BG5), (E) (BG5.6), (F) (BG5.12) and 10% guar gum; (G) (BG10), (H) (BG10.6), (I) (BG10.12), before and after 6 or 12 month of soil burial degradation.

Figure 6:

SEM images of the surfaces of polyether urethane acrylates; (A) (G0), (B) (G0.6), (C) (G0.12) and their composites with 5% guar gum; (D) (G5), (E) (G5.6), (F) (G5.12) and 10% guar gum; (G) (G10), (H) (G10.6), (I) (G10.12), before and after 6 or 12 month of soil burial degradation.

The SEM micrographs of soil-burial degraded polyester urethane acrylate-guar gum composites show a growing number of holes, cracks, cavities, surface irregularities and microbial colonization (Figure 5). This indicated that the surface of the polymer was attacked by the microorganism at the same time as the attack on the guar gum filler. This is due to the presence of the ester groups from the soft segments that are sensitive to hydrolysis and microorganism attack. With samples that contain ether in their structure, the guar gum filler is the first to be degraded, followed by the degradation of the remaining polymer structure.

Before the soil burial test, the composite films exhibited a relatively smooth and clear surface. After soil burial (6 months and particularly after 12 months), a dramatic degradation of the surface was observed, due to the action of the soil microorganisms. The degradation activity of the microorganisms was probably catalyzed by the hydrolysis of the guar gum filler. Similar results have been obtained in previous studies (23). The degree of degradation increases with the amount of guar gum and with degradation time.

The physical aspect of all guar gum composites has changed after 6 months of soil burial exposure. Apart from the change in color of the films due to morphological changes, one can also observe the growth of fungal mycelia on the film surface. After 12 months, there are large cavities in the film surface and a strong degradation of the entire surface was observed. The incorporation of guar gum in the polyurethane acrylate composites provides an initial carbon source for soil microorganisms which then enables even further degradation of the polymer matrix. When using a higher concentration of guar gum, the degradation of the samples after 12 months progresses to the point that the structural integrity of the composite becomes completely compromised.

The homogeneous dispersion of guar gum throughout the composite matrix enables the degradation to also progress in the interior of the studied composites, not only on their surface.

This results in an increased degradation rate of the polymer matrix itself. After 12 months of soil burial the composite films exhibit holes and cracks and in some cases (BG10) a total destruction of the sample occurs.

The surface of the films degraded by soil burial exhibited extensive hyphae, organized into colonies. The results of the degradation of the composite films must be assessed while taking into account the presence of complex soil conditions, involving not only fungal activity, but also the influence and presence of acting bacteria, water etc (24).