Biodegradation study by Pseudomonas sp. of flexible polyurethane foams derived from castor oil

https://doi.org/10.1016/j.ibiod.2013.05.019Get rights and content

Highlights

  • SEM images of polyurethane before and after 60 days biodegradation by Pseudomonas sp.

  • Biodegradation of polyurethane foams based on castor oil by Pseudomonas sp.

  • Biodegradation of polyurethane foams based on castor oil modified with maleic anhydride.

Abstract

The synthesis and biodegradation of polyurethane foams obtained from environmentally benign processes were studied.

Flexible polyurethane foams based on castor oil modified with maleic anhydride (MACO) were synthesized. The synthesis involved a single-stage process by mixing castor oil/MACO (weight ratios 75:25 and 25:75) and 2-4 toluene diisocyanate (TDI) in stoichiometric amount of OH:NCO. The biodegradability studies with cultures of a Pseudomonas sp. strain (DBFIQ-P36) involved incubation periods of 2 months at 37 °C. Polymers were characterized before and after biodegradation by Fourier Transform Infrared Spectroscopy (FT-IR), INSTRON mechanical tester, and Scanning Electron Microscopy (SEM). The results showed that the addition of MACO produces a considerable increase in the rate of degradation and an important change in the chemical and morphological structures. This is due to the presence of ester groups that are vulnerable to chemical hydrolysis and enzymatic attack. The eco-toxicity after the biodegradation was evaluated. Toxic compounds such as primary amines were identified by Gas Chromatography–Mass Spectrometry (GC–MS) in combination with Nuclear Magnetic Resonance (NMR) as degradation products.

Introduction

Due to the environmental impact caused by the growing use of polymers derived from petroleum, there is great interest in the development of new environmental-friendly polymers, with low costs and controlled life span. The materials to be designed must present both good physical properties and biodegradable characteristics. Following this idea, the use of renewable sources like vegetable oils – soy, palm, tung, castor – constitutes an interesting alternative (Guner et al., 2006, Sharma and Kundu, 2008, Aranguren et al., 2012).

Castor oil (CO) exhibits an unusual chemical composition that makes it quite valuable for many applications. It also presents advantages such as: renewability, easy availability in a large quantities, and low cost (Mutlu and Meier, 2010). The reactive hydroxyl functional group contained in its structure can be used as a polyol – polyester to create new and “green” macromolecular architectures of polyurethanes (PUs) by reaction with different diisocyanates (Hablot et al., 2008, Papadopoulon et al., 2008).

There are several reports related to the elaboration of PUs based on castor oil, such as interpenetrating polymer networks (Quipeng et al., 1990, Xie and Guo, 2002, Niranjan et al., 2009), elastomers (Yeganeh and Mehdizadeh, 2004;; Oprea, 2010), coatings (Trevino and Trumbo, 2002), adhesives (Somani et al., 2003), and some semi-rigid PU foams that have potential uses in thermal insulation (Ogunniyi et al., 1996) and other interesting applications. Wang et al. (2008) prepared a polyester-polyol (MACO) by chemical modification of hydroxyl groups with maleic anhydride. Then, biodegradable semi-rigid PU foams were synthesized by copolymerization of MACO with styrene using benzoyl peroxide (BPO) as initiator. The products exhibited mechanical properties comparable to those of a foam derived from commercial polyether. Mazo et al., 2011, Mazo et al., 2012a, Mazo et al., 2012b studied the kinetics of transesterification and condensation of castor oil with maleic anhydride using conventional and microwave heating.

PUs based on vegetable oils are polyester-type PUs that exhibit biodegradable characteristics due to the presence of ester groups. As a consequence of their chemical structures, it was reported that the biodegradation mechanism is hydrolysis by water action and by enzymatic attack (Gu 2003, Gu 2007, Wang et al., 2008, Dutta et al., 2009, Oprea, 2010, Aranguren et al., 2012). For example, Wang et al. (2008) studied the degradation rate of PU foams based on CO buried in soil under controlled conditions during 4 months. After the biodegradation the PUs showed a clear weight loss and a decrease of mechanical properties.

The impact of water, fungi, bacteria and enzymes (protease, urease, lipase and esterase) on PUs derived from petroleum has been tested in several opportunities using different techniques under controlled conditions (Darby and Kaplan, 1968, Young and Sung, 1988, Nakajima-Kambe et al., 1995, Nakajima-Kambe et al., 1997, Nakajima-Kambe et al., 1999, El-Sayed et al., 1996, Skarja and Woodhouse, 1998; Gu and Li, 2005, Gu 2007, Shah et al., 2008a, Shah et al., 2008b, Rodrigues da Silva et al., 2010). It was confirmed that PUs are susceptible to microbial attack, especially by fungi (Toward, 2002). Darby and Kaplan (1968) reported that polyester-type PUs were more susceptible to fungal attack than polyether-type PUs, due mainly to the hydrolysis of ester bonds.

The degradation mechanisms depend on abiotic and biotic (type and quantities of microorganisms and organism) factors. They are also affected by polymer characteristics, such as additives content, morphology, chemical composition, crystallinity degree, hard to soft segment ratios, hydrolysable chain extenders in the hard segment, and diisocyanate types used in the synthesis (Young and Sung, 1988; Toward, 2002, Gu 2007; Shah et al., 2008a, Shah et al., 2008b; Lucas et al., 2008, Rodrigues da Silva et al., 2010).

Some authors investigated the degradation of PUs by enzymes secreted by specific bacteria (Nakajima-Kambe et al., 1995, El-Sayed et al., 1996, Nakajima-Kambe et al., 1997, Nakajima-Kambe et al., 1999, Akutsu et al., 1998, Howard and Blake, 1998, Howard et al., 1999; Akutsu-Shigeno et al., 2006; Gu and Li, 2006; Shah et al., 2008a, Shah et al., 2008b). El-Sayed et al. (1996) studied the degradation for about 120 h of polyester-type PU coatings by five different bacterial strains isolated from soil and identified as Acinetobacter calcoaceticus, Arthrobacter globiformis, Pseudomonas cepacia and two strains of Pseudomonas sp. Other four strains were also investigated: Pseudomonas aeruginosa, Pseudomonas putida, and two A. Calcoaceticus strains provided by the U.S. Navy. All microorganisms exhibited esterase activity. Shah et al., 2008a, Shah et al., 2008b reported the degradation of polyester-type PU using five types of bacteria isolated from soil identified as Bacillus sp., Pseudomonas sp., Micrococcus sp., Arthrobacter sp. and Corynebacterium sp. Also, the presence of esterase enzyme was detected. Gautama et al. (2007) studied the biodegradation of automotive waste polyester-type PU foams using Pseudomonas chlororaphis ATCC55729.

In most cases, additional nutrients for microorganisms were incorporated in biodegradation tests. However, Nakajima-Kambe et al., 1995, Nakajima-Kambe et al., 1997 and Akutsu et al. (1988) showed that Comamonas acidovorans strain (TB-35) was able to degrade polyester-type PU synthesized from poly (diethylene adipate) (with molecular weight of 2500 and 2690) with TDI, using the PU as sole carbon source. The microorganism exhibited esterase activity. The TB-35 strain produced two different esterases, one of which was secreted to the culture broth and the other was bound to the cell surface. It was detected that the cell surface-bound esterase is vital for PU degradation, hydrolyzing the ester linkage and releasing diethylene glycol and adipic acid.

Although the term “biodegradation” indicates the predominance of biological activity in the phenomenon, in nature biotic and abiotic factors act synergistically to decompose organic mater. The biodegradation of polymeric materials in general includes four steps: biodeterioration, depolymerization or biofragmentation, assimilation and mineralization. These processes generate oligomers, dimer and monomer, and finally CO2 and H2O (Lucas et al., 2008, Shah et al., 2008a, Shah et al., 2008b. However, in the case of PUs, complete degradation was not reported and the biodegradation mechanism has not been totally elucidated.

In this work, the synthesis, characterization and biodegradation of new flexible PU foams based on CO modified with maleic anhydride are studied. The final properties of the materials are evaluated and compared with traditional PU foams based on polyethers. The degradation of the polymers by enzymatic attack with Pseudomonas sp. cultures under controlled aerobic conditions is investigated at laboratory level. The Pseudomonas sp. was chosen following previous works (Howard and Blake, 1998; Gu and Li, 2005; Gautama et al., 2007, Shah et al., 2008a, Shah et al., 2008b, Dutta et al., 2009). Also, this bacterium presents advantages such as its abundance in the environment and its characteristic property of having plasmids with high ability to degrade a variety of organic compounds which are not used as nutrients by other bacterium species. The weight loss and the tensile strength of the samples are measured in order to determine the degradation degree. Also, Fourier transform infrared spectroscopy (FT-IR), and scanning electron microscopy (SEM) are used to follow molecular and morphological changes along the degradation process. Additionally, the toxic effect after the biodegradation is evaluated by bacterial Microtox® bio-assay and the degradation products are identified by gas chromatography–mass spectrometry (GC–MS) in combination with Nuclear Magnetic Resonance (NMR).

Section snippets

Materials

CO of USB-grade (hydroxyl value 159.5 mg of KOH g−1, 930 g mol−1) and commercial-grade maleic anhydride (MA) were obtained from Merck (Whitehouse Station, NJ, USA), toluene diisocyanate (TDI, VORANATE™ T-80) from The Dow Chemical Company (California, USA), polyether polyol (Arcol F3040, 3000-molecular weight triol, OH value 56 mg of KOH g−1) from Bayer Material Science (Santa Clara Mexico), silicone L-580 (Niax*) from Momentive (Cotia, Brasil), and triethilenediamine (TEA) and stannum octoate

Results and discussions

First, a preliminary study in Petri dishes was carried out to verify the ability of the bacterial strain to degrade the polymers. After 10 days of incubation, the formation of clear zones (by hydrolysis) around of the microorganism colonies was not observed for PU foams in basal medium with agar–agar as C and N source. In contrast for PU-1 and PU-2, the experiments carried out with basal medium containing agar, glucose and ammonium nitrate as the sole C and N sources showed the formation of

Conclusions

The biodegradation by Pseudomonas sp. strain of flexible PU foams based on castor oil modified with maleic anhydride was studied by different techniques.

The bacterial growth and the mass loss were monitored along the degradation process, and the corresponding changes in the chemical structure and morphology were followed by FTIR, SEM and mechanical test.

The materials with high MACO content exhibited a considerable increase of the degradation rate associated to the hydrophilicity of the

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