Polymer Degradation and Stability 92 (2007) 1599e1610www.elsevier.com/locate/polydegstab

Laboratory investigation of biodegradability of a polyurethanefoam under anaerobic conditions

Meltem Urgun-Demirtas a, Dileep Singh b, Krishna Pagilla a,*

a Department of Civil, Architectural, and Environmental Engineering, 10 West 33rd Street, Illinois Institute of Technology, Chicago, IL 60616, USAb Nuclear Engineering Division, Argonne National Laboratory, 9700 South Cass Avenue, Bldg. 212, Argonne, IL 60439, USA

Received 5 December 2006; received in revised form 23 April 2007; accepted 25 April 2007

Available online 7 May 2007

Abstract

Polyurethane (PU) foams can be used in many remediation applications as an isolation material to prevent the release of hazardous materialsinto the environment. The integrity of a PU foam was investigated in this study using short-term accelerated laboratory experiments includingbioavailability assays, soil burial experiments, and accelerated bioreactors to determine the fate of PU foam in the soil where anaerobic processesare dominant. The experimental results have shown that the studied PU foam is likely not biodegradable under anaerobic conditions. Neitherweight loss nor a change in the tensile strength of the PU material after biological exposure was observed. The FT-IR chemical signature ofthe PU foams was also nearly identical before and after biological exposure. The composition of the PU material (aromatic polyester and poly-ether PU) used in this study could have played a significant role in its resistance to microbial attack during the short-term accelerated exper-iments.� 2007 Elsevier Ltd. All rights reserved.

Keywords: Polyurethane; Biodegradation; Anaerobic conditions; Landfill applications

1. Introduction

Polyurethane (PU) foam is used in many consumer and in-dustrial applications. It has tremendous potential in environ-mental applications in pollution control and pollutionprevention. One such application is the use of high densityPU foam as a barrier to prevent direct contact between the haz-ardous pollutants and the environment. The suitability of thePU foam for this type of applications is dependent on the ag-ing behavior, degradation due to mechanical and environmen-tal effects, and structural integrity. PU barrier aging anddegradation is caused by numerous weathering factors, includ-ing heat, sunlight, moisture and temperature variations, micro-organisms, radioactivity, and chemical pollutants, etc. [1]. Thedegradation mechanisms responsible from the abiotic and bi-otic degradations of PU can occur simultaneously or subse-quently [2].

* Corresponding author. Tel.: þ1 3125675717; fax: þ1 3125678874.

E-mail address: pagilla@iit.edu (K. Pagilla).

0141-3910/$ - see front matter � 2007 Elsevier Ltd. All rights reserved.

doi:10.1016/j.polymdegradstab.2007.04.013

Polyurethane degradation in the environment is also depen-dent on the type of PU or the chemical and structural compo-sitions. Polyurethanes are synthesized from three basiccomponents: a diisocyanate, a polyglycol, and an extenderby using the various low-molecular-weight prepolymer blocks.The terminal hydroxyl group allows for alternating blocks,called ‘‘segments,’’ to be inserted into the PU chain. Blocksproviding rigid crystalline phase and containing isocyanateand the chain extender are referred to as ‘‘hard segments’’.Those yielding generally either noncrystalline or a low crystal-linity phase and containing polyester/polyether called as ‘‘softsegments’’ [3,4].

Generally, the hard segment contributes to hardness, tensilestrength, impact resistance, stiffness, and modulus. On theother hand, the soft segment contributes to water absorption,elongation, elasticity, and softness. Modification of these seg-ments might result in changes in the degree of tensile strengthand elasticity. Hence, it is possible to produce versatile PUpolymers whose properties can be easily modified by varyingtheir molecular structures of soft segment and hard segment

1600 M. Urgun-Demirtas et al. / Polymer Degradation and Stability 92 (2007) 1599e1610

[5]. Many PU foam parameters including process conditions,additives, morphology, chemical structure, crystallinity, hardsegment to soft segment ratio, etc., determine the degradabilityof PU and its fate in the environment and their effects on PUdegradation can vary [1,2,6e9].

The biodegradation of PU by microorganisms has becomean important issue among the aging and degradation mecha-nism studies on PU fate [10,11]. The biodegradation of PUcould be due to utilization of PU as a carbon and/or nitrogensource by microorganisms [12] or fortuitous biodegradation orco-metabolic biodegradation in the presence of other nutrientsand substrates. PU biodegradation in the laboratory under con-trolled conditions has been reported as mainly due to fungalattacks [13]. The results reported on PU biodegradation mostlycome from lab studies, in many cases, providing additional nu-trients to microorganisms, and use of highly concentratedenzymes to promote biodegradation.

Ester PUs are susceptible to microbial attacks, whereas pol-yether PUs are relatively more resistant to this kind of attack[14]. A study by Darby and Kaplan reported that polyetherPUs are hardly susceptible to microbial degradation [14].This difference could be due to the PU biodegradation mech-anism, which involves exo-type depolymerization in the etherPUs [15], but endo-type depolymerization in the ester PUs[16]. However, it has been shown that degradation/deteriora-tion of polyether PU can happen in many medical implantsand tissue engineering materials, which are exposed to manyhydrolytic enzymes and oxidants in the human body[17e19]. On the other hand, Brown et al. reported no signof biodegradation of polyether PU foams under accelerated an-aerobic conditions in a landfill simulator after 21 months oftreatment [11]. However, there is very limited informationon the fate and biodegradation of PU in the environment[11]. The only reported information on half-life of polyetherPU foam in the environment was estimated by hydrolysis ofPU in superheated water at w200 �C, and was predicted as400 years by extrapolation of experimental results [11,20].Field studies representing actual application conditions, never-theless, are necessary in assessing the benefits and obstaclesassociated with the use of PUs as a barrier material in remedi-ation and pollution prevention applications in the environment.

The biodeterioration of PU is highly undesirable for long-term use of PU as a barrier material during the remediation ap-plications because material weakening by microbial attackscan cause system failure [10]. When the PU material is dam-aged significantly by microbial attacks, slow release of hazard-ous contaminants to the environment can occur by diffusionover the long-term. A better understanding of PU bioavailabil-ity to microorganisms in the environment such as in soil orwater is a key issue of importance for long-term integrity ofthe PU structure for the remediation applications. Lack of in-formation on environmental fate of the PU foam could alsolimit its use in the remediation of polluted sites, such as thosecontaminated with radionuclides or heavy metals in soil andgroundwater.

In this study, the performance of PU foam was evaluated asa barrier material to restrict the mobility of the radionuclides

in a dedicated landfill environment for the disposal of steelpipes and other process gas equipment that are contaminatedby various radionuclides. The long-term integrity of PUfoam in the disposal environment was to be evaluated underplausible degradation scenarios that may lead to failure ofthe foam material by accelerated experiments. To ascertainwhat degradation mechanisms could affect the foam, the PUfoam was exposed to harsh conditions not likely to be experi-enced in real applications in a landfill. The effects of variousconditions such as mechanical stresses, heat, moisture, tem-perature cycling, biodegradation, and radiation exposure onthe aging and potential degradation of PU were studied to de-termine the applicability of the PU foam material as a contam-inant barrier in the landfill environment [21]. However, in thispaper, only the results from the lab-scale biodegradation stud-ies conducted to determine whether PU is bioavailable to soilmicroorganisms are discussed. The efforts to determine envi-ronment half-life of PU in case it was biodegradable by micro-organisms are also reported. By determining the rate constantsof the biodegradation process, it is possible to simulate foambehavior in the landfill over long periods based on relativelyshort time scale lab studies.

The integrity of the material under biodegradation condi-tions could be tested under favorable conditions to microor-ganisms and with high microbial concentrations compared tothe field to simulate accelerated scenarios. Measurement ofPU parameters such as tensile strength, weight loss, and chem-ical signature variations by techniques such as Fourier Trans-form Infrared Spectroscopy (FT-IR) could be used todetermine degradation of PU due to microbial action. Further-more, plotting of the percentage change in tensile strength,weight or absorbance in the FT-IR signature against the expo-sure time could give a straight line or some other pertinentmathematical relationship. The extrapolation of such a rela-tionship could be used to estimate the half-life of the PU inthe environment. It has been shown that biodegradability ofPU in a specified environment could be estimated based onthe tensile half-life [22].

The main objectives of this research are as follows:

� To determine bioavailability of PU as a carbon and/or ni-trogen source under anaerobic conditions in batch, se-quencing batch and packed bed reactors using anaerobicinocula from soil and wastewater sludge.� To determine degree of deterioration of PU by chemical

and physical analyses and bioassays.� In the event the PU samples used are bioavailable, to

determine the degradation rate constant of PU by using ap-propriate kinetic models that fit the experimental data.

2. Materials and methods

The experimental protocols for the assessment of PUbiodeterioration included bioavailability assays using batchshake-flask and soil reactors, continuous flow soil and sludgebioreactors, analytical and physical measurements of PU

1601M. Urgun-Demirtas et al. / Polymer Degradation and Stability 92 (2007) 1599e1610

deterioration. The microbial deterioration of PU was investi-gated by measuring the physical properties (weight loss, ten-sile strength) of PU plugs before and after bioassays,chemical structure measurements using FT-IR, and measure-ment of growth of microorganisms (microbial counts).

2.1. Tested polyurethane material

The PU material used in this study was supplied by NorthCarolina Foam Industries (NCFI) (Mount Airy, NC). The PUfoam was fabricated into cylindrical blocks dimensionally8 inches in diameter and 4 ft in length. The density was calcu-lated by taking the ratio of sample weight and measured vol-ume. The average density of the longitudinal-cut sampleswas 2.95 lb/ft3.

The chemical composition of the foam product (by volume)can be summarized as follows:

� Polymeric MDI (50%).� Polyether polyol, 700 MW, 4,5 functional propoxylated su-

crose and glycerin (19%).� Polyester polyol, 350 MW, 2,2 functional aromatic ester

(10%).� Polyether polyol, 320 MW, 3 functional propoxylated aro-

matic amine (7%).� Plasticizer, fire retardant, surfactant, catalysts and blowing

agents (14%).

Based on the chemicals used for the synthesis of this PUfoam, it can be considered as a rigid foam. It is well knownthat the blend of polyisocyanates (MDI) and polyols of higherfunctionality polyethers and aromatic polyester resulting inhigh cross-link density [23] in the PU foams studied here,can play a significant role in the PU foam’s resistance to mi-crobial attack because the degradation of PU occurs first inthe soft segments, i.e., in the amorphous region [1].

2.2. Preparation of PU plugs

The solid blocks of PU were cut into dumb-bell-shapedplugs (10.2 cm� 3.5 cm� 1.8 cm) to increase the contactarea with the microorganisms in the bioreactors. The PU plugswere pretreated by washing once in ethanol and twice in dis-tilled water. Then, the plugs were dried at 50 �C overnight toa constant weight and then cooled under desiccation. This tem-perature was chosen to prevent the loss of volatile organicmatter in PU while drying of moisture.

2.3. Microbial growth media

The composition of the basal mineral medium used wasadopted from that described by Nakajima-Kambe et al. [13]where glucose and/or ammonium nitrate is omitted when PUis supplied as a sole carbon and/or nitrogen source (Table 1),respectively.

2.4. Biodegradation experiments

During the experiments, American Society for Testing andMaterials (ASTM) methods designed for testing biodegrada-tion of plastics under simulated environmental conditions (ma-tured compost or municipal sewage sludge) [24], and othermethods currently used in the published literature were used[10]. Since the burial of dumb-bell shaped pieces in soil fol-lowed by tensile strength testing is a standard method for as-sessing the susceptibility of plastics to biodegradation [25],the PU plugs were buried in Tennessee landfill soil to stimu-late the relevant environmental conditions in the landfill.

2.4.1. Bioavailability studiesA series of batch cultures was set up to screen PU biode-

gradability by an anaerobic sludge inoculum and to monitordegradation of PU. In batch growth assays, 2 plugs(10.2 cm� 3.5 cm� 1.8 cm) (w10 g per liter) were placedin 1000 mL flasks containing 400 mL synthetic basal mediumprepared according to the composition in Table 1. Each shake-flask was inoculated with anaerobic digester sludge inoculumtaken from a local wastewater treatment plant. The sludge con-tains both hydrolytic and methanogenic microbial culturesmaintained under anaerobic conditions. The PU bioavailabilityassay was performed in the defined basal mineral medium inwhich PU and other alternative carbon and nitrogen com-pounds served as sources of carbon and/or nitrogen (Table 2).Growth tests were performed under nine different conditionswhich are summarized below:

1. PU as sole source of carbon and nitrogen (ammonium ni-trate and glucose omitted).

2. PU as sole source of carbon (alternative nitrogen source,ammonium nitrate, was added, glucose omitted).

3. PU as sole source of nitrogen (alternative carbon source,glucose, was added, ammonium nitrate omitted).

4. PU as well as added sources of carbon (glucose) and nitro-gen (ammonium nitrate) as nutrients for growth.

5. Only added nitrogen (ammonium nitrate) and carbon (glu-cose) sources were available (PU was not present) (posi-tive control).

6. Nitrogen sources were not present, but alternative carbon(glucose) source was added (negative control).

Table 1

Composition of the basal medium (pH¼ 7.2)

Component Amount (mg/L)

KH2PO4 2000

K2HPO4 7000

NH4NO3 1000

Glucose 3750

MgSO4$7H2O 100

ZnSO4$7H2O 1

CuSO4$7H2O 0.1

FeSO4$7H2O 10

MnSO4$7H2O 2

1602 M. Urgun-Demirtas et al. / Polymer Degradation and Stability 92 (2007) 1599e1610

Table 2

Experimental conditions for the PU bioavailability assays

Experiment no PU Additional C source Additional N source Inoculum

C source N source Glucose Ammonium nitrate

1 O O � � O2 O � � O O3 � O O � O4 O O O O O5 � � O O O6 � � O � O7 � � � O O8 � � � � O9 O O � � �

7. Carbon sources were not present, but alternative nitrogen(ammonium nitrate) source was added (negative control).

8. Carbon and nitrogen sources were not present, but inocu-lum was present (negative control).

9. PU as sole source of carbon and nitrogen, but no inoculum(negative control).

Flasks containing the experimental contents were purgedwith 50 cm3/min of a 99.0% N2 gas for 5 min to remove dis-solved oxygen and to maintain anaerobic conditions during theexperiments. Before the analysis, flasks were initially con-nected to a trap system to relieve any excess gas pressure inthe flasks and to trap off-gas. The flasks were shaken on a plat-form placed on a mechanical shaker. After the flasks wereshaken for the required time, the cylindrical PU plugs were re-moved and flask contents were centrifuged for further analysis.Each set of experiments were run for 2e6 weeks at room tem-perature (w25 �C) with negative and positive controls.

2.4.2. Continuous reactor experimentsTwo different accelerated continuous reactors’ setups were

employed as follows:

1. Sequencing Batch Reactor (SBR) was operated with inoc-ula taken from an anaerobic digester using PU plugs ascarbon and/or nitrogen source in the bioreactor.

2. Upflow Packed Bed Reactor (UPBR) was operated withbacterial suspension contained in the Oak Ridge, Tennes-see landfill soil. PU plugs served as packing material aswell as carbon and/or nitrogen source.

The bioreactors were made of two identical cylindricalflasks with a 2.5 L working volume and were operated underthe same conditions. The PU plugs (13 plugs) were placedin a wire basket inside each reactor. The bioreactors werepurged with 50 cm3/min of a 99.0% N2 gas for 1 h to removedissolved oxygen and to establish anaerobic conditions beforeexperiments were started. During the operation of bioreactors,the basal mineral feed was also purged with 50 cm3/min ofa 99.0% N2 gas for 10 min to remove dissolved oxygen andto maintain the anaerobic conditions in the bioreactors. Thegas outlets at the bioreactors were connected to a trap systemto relieve any excess gas pressure in the bioreactors and to trap

off-gas. The SBR was operated with sequential 24 h cycles at2 days hydraulic residence time (HRT) and 30 days mean cellresidence time (MCRT). The UPBR was also operated at thesame 2 days HRT and 30s day MCRT. The basal mineral me-dium used during the bioavailability studies was supplied tothe reactors to maintain 2 days HRT. The reactors were runat room temperature (w25 �C).

2.4.3. Soil burial experimentsSoil burial experiments were conducted at room tempera-

ture (w25 �C) by placing the PU plugs in field soils in labora-tory containers. Three sets of 10 replicate PU plugs wereburied for 10 weeks in 10 cm deep covered containers filledwith the Oak Ridge, Tennessee landfill soil to get statisticallysignificant data. The PU plugs were buried horizontally ata depth of 5 cm in soil to allow the development of anaerobicconditions and to stimulate the environmental conditions in thefield. One plug was sacrificed each week to determine biode-terioration over the 10 weeks period by determining the phys-ical and chemical properties of the PU plugs.

2.5. Characteristics of landfill area

The pH values of Tennessee soil were determined from theSoil Survey Laboratory Information Manual (USDA, 1995)[26] using 1 soil:1 H2O (one part soil to one part distilledwater by weight) pH of the soil sample that ranged from 6.5to 6.8. The measured mean annual earth temperature for Ten-nessee soil at 90 ft depth where PU will be used as a barriermaterial was around 13 �C [21]. The temperature increase inthe landfill designed for the disposal of pipes and equipmentscontaminated with radionuclides is not likely to be as much aslandfills designed for the disposal of municipal waste rich inorganic nutrients. The moisture content in the landfill soilwas determined as 21% by measuring the weight loss in sam-ple weight after drying at 105 �C for 1 day.

2.6. Analysis

The methods currently used in testing microbiological deg-radation and deterioration of a wide range of PU materialshave been recently reviewed by Gu and Gu [10] and Zhenget al. [27]. Among them the most commonly practiced

1603M. Urgun-Demirtas et al. / Polymer Degradation and Stability 92 (2007) 1599e1610

methods to assess the biodegradability of PU in different envi-ronmental conditions are tensile strength and weight loss,change in the FT-IR signature and bacterial growth.

Physical examination of PU plugs for deterioration is con-sidered as an important method for investigating biodeteriora-tion of the PU plugs [28] because physical changes in thestructure of PU are more likely to occur before complete deg-radation of PU takes place. The degree of deterioration of sam-pled PU plug was assessed each week by measuring changesin selected physical properties including tensile strength andweight loss after appropriate sample cleaning procedures.

2.6.1. Weight lossPU degradation was monitored by measuring the weight of

PU plugs before and after incubation in the soil containers,batch shake-flasks, or continuous reactors. The PU plugswere taken out and washed with distilled water and ethanolto remove debris on the surface. Then, plugs were dried toconstant weight overnight at 50 �C and weighed. The weightloss percentage was calculated as

%Weight loss¼ m0�mt

m0

� 100

where, m0 is the initial weight of the plug, mt is the finalweight of the plug after experimentation.

2.6.2. Tensile strength measurementsTensile strength is a measure of the force, generally given

in pounds per square inch (psi), required to break the polymerplug. The tensile strength at breakpoint of PU foam ‘‘dog-bone’’ plug was measured by ASTM procedure D638 [24] us-ing an Instron Universal Testing Instrument, Model 4465 ata crosshead speed of 0.1 in/min, and a constant speed of100 lbf/min on specimen, and at 73 �F and 50% relative hu-midity. The changes in tensile strength can be determined bystressestrain curves. The sample size of the PU plug was10.2 cm� 3.5 cm� 1.8 cm following ASTM guidelines.Average tensile strength values and standard deviation of un-treated PU plugs for statistical purposes are shown in Table 3.It can be seen that the standard error is approximately 12%because of heterogeneous physical structure of untreated PUplugs where circular pores as large as 1e2 mm were randomlydistributed (pictures not shown).

2.6.3. FT-IR analysisBefore and after treatment, each PU foam plug was ana-

lyzed by FT-IR spectroscopy (Nexus 470 FT-IR) equippedwith a DTGS KBr detector (ThermoNicolet, Madison, WI).Since biodegradation of PU occurs mainly at the surface, thefoam was removed from the surface (up to 1 mm) and incor-porated into IR pellets for direct absorbance measurements.Spectroscopic grade KBr (approximately w 100 mg) (Sigma-Aldrich) was used during the pellet preparation. The infraredspectra of the compounds were recorded on the IR in 400e4000 cm�1 region with a resolution of 4 cm�1 [29]. The spec-tra were evaluated using Thermonicolet’s OMNIC software.

The ranges of 3600e2600 cm�1 and 1800e700 cm�1 wereof particular interest where the major absorption bands ofPU are assigned [30]. The IR absorbance of the carbonylgroup (C]O), urethane (NH), aryl (C]C), and ether (CeO)functional groups are indicated at 1720, 1630, 1600, and1125 cm�1, respectively [31]. The ether index was definedas the ratio of the peak height of the 1105 cm�1 peak assignedCeOeC stretching in the aliphatic ether bond in the soft seg-ment to the of the peak height of the 1413 cm�1 peak C]Cstretching in the aromatic rings in the hard segment which pro-vides a measure of the ether content in each sample and servesan indicator whether the PU was deteriorated by the microbialattacks [32]. The appearance of new peaks including1175 cm�1 peak generally assigned to cross-linking of softsegment degradation products and 1713 cm�1 peak assignedto carbonyl peak as a function of exposure time was used asa measure of the degree of biodegradation [18,33].

2.6.4. Growth assayThe total number of microorganisms in the sample which

could have the ability to degrade and deteriorate PU plugswere determined by the Acridine Orange Direct Count(AODC) method [34] and expressed as number of colonyforming units (CFU) per mL. 2 mL of sample taken fromthe collected sample was stained with 0.2 mL of 0.1% Acri-dine Orange. The stained microbial mixture was allowed tostand at room temperature for 1 or 2 min for the reactionwith the stain. Then, a treated filter paper (Osmonics, Polycar-bonate black 0.22 mm filter) was placed in a vacuum filter fun-nel and the sample was filtered to get a distribution of colonieson the filter paper. Then, the damp filter was placed on a glassmicroscope slide (2 cm� 5 cm) for analysis. The number ofmicroorganisms per millimeter area is estimated from a countof at least 5e6 randomly chosen microscope fields.

3. Results and discussion

The biodegradability and long-term integrity of tested PUwere investigated under accelerated anaerobic conditionsbased on literature and ASTM methods [7,24,35]. Kim andKim [7] investigated the degradation of various types of PU

Table 3

Tensile strength of untreated PU plugs

Sample Size (in� in) Tensile strength (psi)

1 0.495 36.7

0.610

2 0.495 38.9

0.590

3 0.490 40.2

0.620

4 0.505 41.7

0.652

5 0.495 32.7

0.620

Average 38.0

Standard deviation 3.5

1604 M. Urgun-Demirtas et al. / Polymer Degradation and Stability 92 (2007) 1599e1610

under accelerated composting conditions for 45 days to deter-mine the biodegradability of PU in the environment by naturaldegradation process. Bentham et al. [35] also performed soilburial experiments to evaluate the susceptibility of differentPU materials to microbial attack for a period of 28 days’ incu-bation. The biodegradability studies and continuous experi-ments during this research were performed under similaraccelerated conditions. PU was exposed to inoculum takingfrom anaerobic digester sludge which contains large and di-verse group of biologically active anaerobic bacteria(1.5� 108e6.8� 108 CFU/mg of sludge). The bacterial countin the field soil where the landfill will be located was3.6� 103 CFU/mg of soil. In other words, the soil biologicalconditions were accelerated by 4� 104e2� 105 times duringthe lab-scale experiments.

Humidity and nutrient rich environment are other importantparameters affecting the rate and extent of decomposition oforganic materials including PU [33,36e38]. Baldwin et al.[38] reported that moisture content was a major factor affect-ing sanitary materials in the landfills after 6 years of burial be-cause the wetter the samples, the quicker is the decompositionof materials. It has been also reported in another study that theimpact of low relative humidity (decreasing RH to 45%) to mi-crobial growth is much more than that of decreasing tempera-ture from 30 to 22 �C [39]. Therefore, bioavailability andcontinuous reactor experiments were performed in the waterenvironment. The bioavailability and continuous flow experi-ments were also performed under nutrient rich conditions (Ta-ble 1) with respect to natural environment, i.e. landfillconditions, to increase the microbial activity, hence acceleratethe activity of degradation of PU foams. As it has been re-ported that amendment of soil with only glucose (1%) in-creased the cell counts one to two orders with respect tocontrol [40]. The landfill with a 90 ft of depth can possiblylimit the bacterial activity because of the limitation in nutrientsupply, low temperature (13 �C) and low moisture content(21%). Soil burial and UPBR experiments were also per-formed in parallel to the bioavailability studies and SBR ex-periments to eliminate the biases in the choice ofmicroorganisms, and to determine the significance of indige-nous bacteria on the long-term integrity of PU in the landfill.

3.1. Bioavailability studies

The PU bioavailability assays were performed in the de-fined basal mineral medium in which PU plugs and other alter-native carbon and nitrogen compounds were used to screen PUbiodegradability. Table 4 shows the weight losses in PU plugsas function of treatment time in the shake-flasks. Each datapoint in Table 4 is an average measurement of two PU plugsfrom each flask. The obtained weight loss of PU foam plugsranging from 0.46 to 2.4% can be considered as statisticallynegligible based on the weight loss of control samples duringthe experiments and also weight loss for untreated samplesused for QA/QC experiments (data not shown). Tensilestrength of the treated PU plugs at break can be considered

as a quantitative strength parameter indicative of microbialdeterioration under anaerobic conditions [22].

Degradation was assessed quantitatively by measuringchanges in tensile strength at failure of dumb-bell-shapedPU plugs prepared according to ASTM D 638 method [24].Tensile strength of the plugs (Table 5) used as carbon and/ornitrogen source during the bioavailability studies was thesame as the tensile strength of untreated plugs (Table 3)(95% confidence limits). From tensile strength analysis ofPU plugs, it can be concluded that there is no deteriorationon the surface of the PU foams during these bioassays. Thetensile strength results are important because it has been re-ported that mechanical failure of PU occurred before theweight loss [22,41]. During an initial degradation of PU mate-rial with 1.7% weight loss, the tensile strength loss was 66%,hence PU material lost its mechanical properties significantly[42]. These observations suggest that weight loss measurementalone is not a reliable end point to assess PU biodeterioration.The results from this study show that there is no biodeteriora-tion of the PU plugs based on both weight loss and tensilestrength measurements.

FT-IR spectroscopy was used to monitor changes in thecomposition on the surface of PU foams due to microbial de-terioration (Fig. 1). In the FT-IR spectrum, the percentagetransmittance of each sample is dependent on the amount ofmaterial used during the pellet preparation, and not relatedwith the chemical composition of the PU. To indicate whetherthe PU chemical composition changes due to biodegradation,it is important to monitor specified peaks at each wavelengthand ether index that constitute the chemical signature of PUas described in Section 2. It has been reported that the de-crease in the absorbance or disappearance of the peaks/valleysat the assigned wavelength is usually consistent with the

Table 4

Cumulative weight loss during bioavailability studiesa

Treatment % Cumulative weight loss

After 2 weeks After 4 weeks After 6 weeks

Control (untreated) 0.74 0.42 0.61

PU(þ)/C�/N� 2.40 1.11 0.78

PU(þ)/C�/Nþ 1.21 0.63 0.90

PU(þ)/Cþ/N� 1.15 0.75 1.22

PU(þ)/Cþ/Nþ 1.23 0.46 1.10

a Notes: PU þ or � indicates presence or absence of PU plug in the assay.

C þ or � indicates presence or absence of supplementary carbon. N þ or �indicates presence or absence of supplementary nitrogen.

Table 5

Tensile strength of treated plugs during bioavailability studies

Treatment Tensile strength (psi)

After 2 weeks After 4 weeks After 6 weeks

PU(þ)/C�/N� 39.1 37.3 41.2

PU(þ)/C�/Nþ 37.0 37.8 40.6

PU(þ)/Cþ/N� 35.2 40.1 39.3

PU(þ)/Cþ/Nþ 35.0 38.1 39.3

Control (Untreated) 38.0� 3.5

1605M. Urgun-Demirtas et al. / Polymer Degradation and Stability 92 (2007) 1599e1610

0.00.10.20.30.40.50.60.70.80.91.01.11.21.31.41.51.61.71.8

5001000150020002500300035004000Wavenumbers (cm-1)

Abso

rban

ce

Fig. 1. FT-IR spectrum of treated (dashed) and untreated (solid) PU foams after 6 weeks of bioavailability studies.

percentage of weight loss. For example, the decrease in ratioof ester bond over ether bond was approximately 50%, whichagreed with the measured amount of polyurethane degradedaccording to Zheng et al. [27].

FT-IR analysis of PU plugs showed that there is no changein the chemical signature of the PU foams and no new peakswhich usually appear as a result of microbial attack as men-tioned in the Section 2 were seen. Hence no deterioration oc-curred on the surface of the PU foams due to biologicalactivity in the bioassays. These findings are important becausedegradation of PU is initiated at the surface of PU foam and

then penetrated into the solid structure of the PU foam [16],and hence, the PU foam studied is further confirmed to benot susceptible to biodeterioration such as in a landfill or in an-aerobic environment.

Fig. 2 shows the number (AODC Method) of anaerobic mi-croorganisms (as colony forming units (CFU) per ml) duringthe 4 and 6 weeks’ bioassays. The results show that there isno significant growth of the microorganisms in the PU supple-mented medium compared to positive and negative controls.The obtained increase in number count for each bioassay is<101 for each tested condition. From Fig. 2, it can also be

0 7 14 21 28 35 42

CFU

/mL

Time (days)

0 7 14 21 28

12345678

4 week bioassay

6 week bioassay

(a)

(b)

CFU

/mL

Time (days)

0.0

5.0x107

1.0x108

1.5x108

2.0x108

2.5x108

3.0x108

0.0

5.0x107

1.0x108

1.5x108

2.0x108

2.5x108

Fig. 2. Anaerobic bacteria count as a function of time during the bioavailability studies.

1606 M. Urgun-Demirtas et al. / Polymer Degradation and Stability 92 (2007) 1599e1610

concluded that the increase in microbial population from t¼ 0to 28 days during the 4 weeks bioassay ranged from 2.8 to 8.9times of initial concentration and 5.3 times for negative con-trol which contained only inoculum in distilled water. The mi-crobial population increases during the 6 weeks bioassayranged from 1.3 to 4.5 times of initial concentration and 2.4times for negative control containing inoculum in distilled wa-ter. The growth in negative controls could be explained asgrowth due to used inoculum taken from anaerobic digestercontaining high levels of nutrients that allow continued micro-bial growth in these flasks.

From the experiments on bioavailability of PU as a sourceof carbon and/or nitrogen it can be concluded that microorgan-isms did not use PU foam as either carbon or nitrogen sourceunder anaerobic conditions. Similarly, PU was not biode-graded even when supplementary carbon and nitrogen wereadded to the growth medium. Hence, the PU foam used inthis study can be considered to be not biodegradable under an-aerobic conditions tested.

3.2. Soil burial experiments

Three sets of 10 replicate PU plugs were buried for 10weeks in containers containing the Oak Ridge, Tennesseelandfill soil to get statistically significant data on degradationof PU under anaerobic conditions in soil. The soil experimentsare more representative of the actual field conditions in termsof biological activity. The dumb-bell-shape plugs were buriedhorizontally at a depth of 5 cm in the soil. Tensile strength, FT-IR and weight loss measurements were performed on weeklyremoved test plugs.

Table 6 shows the cumulative weight loss in test PU plugsduring soil burial experiments. Dry weight analysis of 10weeks treated samples showed an average of 0.25% weightchange and standard deviation of �0.28%. The results indicateno statistically significant weight loss in PU plugs, within the95% confidence interval. It can also be seen that there is nodistinct trend in the weight loss of the plugs with time andhence it can be concluded that PU plugs are not affected bypotential biological activity in the soil. Tensile strength

Table 6

Cumulative weight loss during soil burial experiments

Treatment time (week) Cumulative weight loss (%)

Run 1 Run 2 Run 3

Control 0.18 0.18 0.18

1 0.10 0.20 0.0

2 0.54 0.18 0.34

3 0.88 0.41 0.12

4 0.63 0.21 0.32

5 0.15 0.65 0.64

6 0.00 0.46 0.71

7 0.29 0.83 0.14

8 0.27 0.13 0.20

9 0.09 0.25 0.11

10 0.57 0.05 0.13

measurements were performed on weekly removed test plugsand the results for the three replicate runs are shown in Table 7.The changes in the tensile strength of the PU plugs are notstatistically significant at 95% confidence limit.

Fig. 3 shows the FT-IR spectrum of 10 weeks treated anduntreated PU foam samples. The obtained FT-IR signatureof treated PU foams is the same as that of the control PUfoam sample at the end of the 10-week burial experiments.Similarly, no change in the FT-IR spectrum of the soil buriedsamples was detected during the intermediate weeks duringthe 10-week experimental period. This indicates that no dete-rioration of PU foam sample surface was detected during thesoil burial experiments.

Based on the results of the weight loss measurements, ten-sile strength measurements, and FT-IR spectra of the PU plugsburied in the soil and the control samples, it can be concludedthat there is no biodeterioration of PU foam due to potentialmicrobial activity in the field soil under anaerobic conditionsduring these short-term experiments.

3.3. Accelerated bioreactor studies

Bioavailability studies are a screening tool to determine thedegree of PU bioavailability as a C and/or N source to micro-organisms under anaerobic conditions. As discussed before,PU was not bioavailable neither as C nor N source to microor-ganisms. Accelerated bioreactor studies were performed in aneffort to investigate whether operating conditions includingnutrient availability, inoculum size, mixing, residence time,etc. restrict the bioavailability of PU as a C and/or N sourceto the microorganisms.

Two different continuous reactors including SBR andUPBR were operated with inoculum taken from an anaerobicdigester and Oak Ridge, Tennessee landfill soil, respectively,using PU plugs as carbon and/or nitrogen source in bioreactorsand also as packing material for UBPR to determine the bio-degradation of PU under short-term accelerated conditions.Both reactors were run with basal mineral medium containingN and C sources as described in Section 2 to increase the mi-crobial density in the reactors and provide accelerated

Table 7

Tensile strength of treated plugs during soil burial experiments

Treatment Time (week) Tensile strength (psi)

Run 1 Run 2 Run 3

1 36.2 39.0 37.4

2 37.5 37.0 39.0

3 39.2 38.7 40.5

4 36.8 36.7 36.7

5 35.2 39.4 40.9

6 35.7 37.6 36.1

7 38.4 37.3 36.9

8 39.0 37.2 36.8

9 41.7 37.4 39.6

10 38.6 37.1 39.4

Control 38.0� 3.5

1607M. Urgun-Demirtas et al. / Polymer Degradation and Stability 92 (2007) 1599e1610

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

500 1000150020002500300035004000

Abso

rban

ce

Wavenumbers (cm-1)

Fig. 3. FT-IR spectrum of treated (dashed) and untreated (solid) PU foams after 10 weeks of soil burial experiments.

biological conditions. Tensile strength, FT-IR and weight lossmeasurements were performed on 3 test plugs biweekly by re-moving them from the reactors to get statistically significantdata.

The cumulative weight loss in test PU plugs during contin-uous reactor experiments is shown in Table 8. Dry weightanalysis of 10-week treated samples showed that no statisti-cally significant weight loss (95% confidence interval) in PUplugs placed in the SBR and UPBR. As seen during the soilburial experiments, there is no distinct trend in the weightloss of the plugs with time; hence, no biodegradation of PUwas observed under short-term accelerated conditions. Table9 shows the tensile strength measurements that were per-formed on biweekly removed test plugs during the SBR andUPBR experiments under short-term accelerated conditions.Tensile strength of the plugs used as carbon and/or nitrogensource in the continuous reactors was the same as the tensilestrength of untreated plugs at 95% confidence limit.

Fig. 4 shows the number counts of the anaerobic microor-ganisms as CFU/ml during the continuous reactor experimentsunder accelerated conditions. Growth assays showed that thereis no significant growth of the microorganisms in the bothSBR and UPBR. The increase in the number of anaerobic mi-croorganisms (1.1e1.3 log increase) is due to high levels of Nand C sources in the supplemented basal mineral medium. Theincrease in obtained microbial counts is slightly higher in SBR

Table 8

Cumulative weight loss during the continuous reactor experiments

Treatment time (week) Cumulative weight loss (%)

SBR UPBR

After 6 week 0.27� 0.16 0.26� 0.17

After 8 week 0.18� 0.05 0.23� 0.21

After 10 week 0.24� 0.13 0.21� 0.10

Control (untreated) 0.22� 0.12

(1.4 log) than that of UBPR (1.1 log). This could be due to dif-ferences between the used inoculum. SBR inoculated with an-aerobic sludge digester suspension could also include morehydrolytic and methanogenic microbial cultures than that ofUBPR inoculated with Tennessee Oak Ridge soil.

The FT-IR spectra of 10 weeks treated PU plugs in contin-uous reactors and untreated PU plugs are shown in Fig. 5. Theobtained FT-IR signatures of treated PU foams in both types ofreactors are the same as that of the control PU foam sample atthe end of the 10 weeks of continuous reactor experimentsunder accelerated conditions.

The obtained results from weight loss measurements, ten-sile strength measurements, and FT-IR spectra of the PU plugsin both bioreactors were consistent with the 10 weeks soilburial experiments and bioavailability studies. From these

0 10 20 30 40 50 60 70

107

108

109

CFU

/mL

Days

SBRUPBR

Fig. 4. Anaerobic bacteria count as a function of time during the continuous

reactor experiments.

1608 M. Urgun-Demirtas et al. / Polymer Degradation and Stability 92 (2007) 1599e1610

results, it can be concluded that there is no biodeterioration ofPU foam due to microbial activity in the accelerated short-term reactors under anaerobic conditions.

Furthermore, based on the detailed investigations on agingbehavior of PU foam (details not shown), the tested PU mate-rial was also mechanically and physically stable when exposedto extreme degradation conditions [21]. Those results can be

Table 9

Tensile strength of treated plugs during the continuous reactor experiments

Treatment Tensile strength (psi)

SBR UPBR

After 6 week 37.3 36.4

After 8 week 36.8 38.5

After 10 week 37.4 38.1

Control (untreated) 38.0� 3.5

summarized as follows: (1) No thermal aging characterizedby compressive strengths’ changes was observed at 90 �Cfor 60-day period in both air and water environments. How-ever, the density of the material decreased (by as much as15%) with long-term aging. (2) The maximum deformationwas w8% during long-term testing (1000 h) of PU foam undera fixed compressive stress of 75 psi (equivalent to 90 ft soildepth). (3) Dry and wet freeze-thaw cyclings did not degradethe compressive strength of the PU foam. (4) Gamma irradia-tion of the PU foam (equivalent to a ‘‘1000-year alpha andgamma dose) resulted in no significant degradation of thefoam, and the gas generation was minimal.

4. Conclusions

The experimental results obtained during these investiga-tions show that there is no biodegradation of PU foam was

(a)

(b)

0.00.10.20.30.40.50.60.70.80.91.01.11.21.31.41.51.61.7

5001000150020002500300035004000Wavenumbers (cm-1)

Wavenumbers (cm-1)

Abso

rban

ceAb

sorb

ance

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

5001000150020002500300035004000

UPBR

SBR

Fig. 5. FT-IR spectrum of treated (dashed) and untreated (solid) PU foams during the continuous reactor experiments.

1609M. Urgun-Demirtas et al. / Polymer Degradation and Stability 92 (2007) 1599e1610

observed in this study under anaerobic conditions in both soiland liquid based bioassays. The main observations can besummarized as follows:

� No change in the tensile strength of the PU plugs afterbiological treatment.� No observed weight loss of PU plugs after biological

treatment.� No change in the FT-IR chemical signature of the PU

plugs after biological treatment.� No growth of anaerobic bacteria using PU as either carbon

or nitrogen source.

As a result of this investigation, the tested PU foam mate-rial could be considered not likely to be biodegradable underanaerobic conditions. However, long-term, field scale studiesor data collection from existing applications of PU will greatlyimprove the understanding of PU biodegradation in soil andwater environments.

Acknowledgements

This work was supported by Argonne National Laboratorythrough a sub-contract to Illinois Institute of Technology under23900-MP-0154 issued by Bechtel Jacobs Company. The au-thors gratefully acknowledge the support and valuable insightsby A. Dada, R. Seigler, E. Strassburger, R. James, and J. Gaddof Bechtel Jacobs Company.

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