Comparison of extraction methods for sampling of low molecular compounds in polymers degraded during recycling

European Polymer Journal 44 (2008) 1583–1593

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European Polymer Journal journal homepage: www.elsevier.com/locate/europolj

Review

Comparison of extraction methods for sampling of low molecular compounds in polymers degraded during recycling Johanna Möller, Emma Strömberg, Sigbritt Karlsson * School of Chemistry and Chemical Engineering, Fibre and Polymer technology, The Royal Institute of Technology (KTH), SE-100 44 Stockholm, Sweden

a r t i c l e

i n f o

Article history: Received 18 August 2007 Received in revised form 29 February 2008 Accepted 30 March 2008 Available online 8 April 2008

Keywords: Extraction Recycled polymers Accelerated solvent extraction (ASE) Microwave assisted extraction (MAE) Head-space solid phase micro extraction (HS-SPME) Head-space stir bar sorptive extraction (HSSE)

a b s t r a c t The demand for mechanical recycling of plastic waste results in an increasing amount of recycled polymeric materials available for development of new products. In order for recycled materials to find their way into the material market, high quality is demanded. Thereby, a complete and closed loop of polymeric materials can be achieved successfully. The concept of high quality for recycled plastics imply that besides a pure fraction of e.g. polyethylene (PE) or polypropylene (PP), containing only minor trace amount of foreign plastics, knowledge is required about the type and amount of low molecular weight (LMW) compounds. During long-term use (service-life), products made of polymeric materials will undergo an often very slow degradation where a series of degradation products are formed, in parallel, additives incorporated in the matrix may also degrade. These compounds migrate at various rates to the surrounding environment. The release rate of LMW products from plastics depends on the initiation time of degradation and the degradation mechanisms. For polymers the formation of degradation products may be initiated already during processing, and subsequent use will add products coming from the surrounding environment, e.g. fragrance and aroma compounds from packaging. During recycling of plastics, emissions which contain a series of different LMW compounds may reach the environment leading to unwanted exposure to additives and their degradation residues as well as degradation products of polymers. Several extraction techniques are available for sampling of LMW compounds in polymers before chromatographic analysis. This paper reviews and compares polymer dissolution, accelerated solvent extraction (ASE), microwave assisted extraction (MAE), ultrasound assisted extraction (UAE), super critical fluid extraction (SFE), soxhlet extraction, headspace extraction (HS), head-space solid phase micro extraction (HS-SPME), and head-space stir bar sorptive extraction (HSSE) as appropriate sampling methods for LMW compounds in recycled polymers. Appropriate internal standards useful for these kinds of matrices were selected, which improved the possibility for later quantification. Based on the review of extraction methods, the most promising techniques were tested with industrially recycled samples of HDPE and PP and virgin HDPE and PP for method comparison. Ó 2008 Elsevier Ltd. All rights reserved.

* Corresponding author. Tel.: +46 8 790 85 81; fax: +46 8 20 88 56. E-mail address: [email protected] (S. Karlsson). 0014-3057/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.eurpolymj.2008.03.027

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Contents 1. 2.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Experimental . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1. Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2. Internal standards . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3. Analytical equipment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.4. Polymer dissolution. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.5. Accelerated solvent extraction (ASE) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.6. Microwave assisted extraction (MAE) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.7. Ultrasound assisted extraction (UAE). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.8. Head-space extraction (HS) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.9. Head-space solid phase micro extraction (HS-SPME) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3. Results and discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1. Solid–liquid extraction methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2. Head-space extraction methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3. Method evaluation of extractions of virgin and recycled polyolefins . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4. Conclusions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

0.4

1. Introduction

HDPE/PP

0.3

A

The sources of low molecular weight (LMW) compounds in polymeric materials are many. Both polymer and additives are needed to form the final plastics product. The additives are a disparate group of usually low molecular compounds; some are passive and act as fillers in the product, while others render the product stable against oxidation, heat, photolysis etc. In addition, some compounds are traces of the polymerisation per se, such as initiators and/ or catalysts. Due to the permeability of polymeric materials, they have an ability to absorb LMW compounds from their surroundings. These compounds act as contaminants in products made of recycled plastic material. The same ability holds for the opposite migration of compounds from the polymer to the environment. Hazardous components, flavourings, odours, monomers, oligomers, degradation products and flame retardants are examples of compounds found in recycled materials [1]. Some compounds might deteriorate the material properties or enhance degradation of the polymer. Coloured metal salts give visual defects on the recycled fraction. The presence of printing inks, paint residues, surfactants and fatty materials can lead to enhanced degradation of the polymer [2]. Additive degradation and loss of volatile compounds during extraction and sample handling procedures might influence the quantification of these compounds in the polymer sample. Fractions obtained after material recycling of hard plastic packages consists mainly of polypropylene (PP) and highdensity polyethylene (HDPE). Materials of such similar physical properties are difficult to separate with conventional separation methods. It is usually convenient to use Fourier transform infra red spectroscopy (FTIR) to do initial scanning for identification of polymers in a recyclate [3]. Fig. 1 shows an example of a recycled plastic fraction where FTIR was used to demonstrate the presence of HDPE and PP. One of the limiting steps in extraction is diffusion to the surface of the polymer. The particle size or film thickness is

1584 1585 1585 1585 1585 1585 1585 1585 1585 1585 1585 1586 1586 1587 1589 1591 1592 1592

HDPE/PP

0.2 HDPE/PP PP

PP

0.1

HDPE

PP

HDPE

PP

0.0 3600

3000

2400

cm

1800

1200

600

-1

Fig. 1. FTIR analysis of recycled HDPE–PP mix.

extremely important [4–9]. The diffusion coefficient of additives in polymers at 40 °C is typically about 10 10 cm2 s 1. The rate of diffusion (s 1) is proportional to D/L2, where L is the length of the shortest dimension. Therefore, grinding of the polymer is often an essential step in the analysis. An exception to this is the extraction of thin films and foams, for which the shortest dimension is small. Loss of volatile additives is possible owing to the heat generated by grinding of polymers. Therefore, the polymer must be frozen, usually with liquid nitrogen, before grinding [10]. In this study a series of extraction methods were reviewed for their potential to separate LMW compounds from recycled polymers. The following extraction techniques were reviewed: polymer dissolution, accelerated solvent extraction (ASE), microwave assisted extraction (MAE), ultrasound assisted extraction (UAE), super critical fluid extraction (SFE), Soxhlet extraction, head-space extraction (HS), head-space solid phase micro extraction

J. Möller et al. / European Polymer Journal 44 (2008) 1583–1593

(HS-SPME), and head-space stir bar sorptive extraction (HSSE). In addition, a selection was done of the appropriate internal standards for both head-space and solid–liquid methods. Extractions of virgin HDPE, PP and a mixture of recycled HDPE and PP were performed.

2. Experimental 2.1. Materials Three materials were used in the initial testing, virgin HDPE (Borealis, MG9621), virgin PP (Borealis, HD120MO) and a recycled mix of HDPE and PP (SWEREC AB). Virgin materials were used for developing a good method of extracting additives, and later apply the method on recycled materials. The problems of inhomogenity and unknown analytes could thereby be evaded. The materials were ground in liquid nitrogen to approximately 0.5 mm before extraction. 2.2. Internal standards To all liquid samples, 200 ll of 0.2 mg/l naphthalene-d8 and 10 ll of 10 mg/ml orto-terphenyl were added as internal standards. In head-space extractions, 0.7 lg of toluened8 and dichlorobenzene-d4 were added as internal standards respectively. The retention times of the internal standards are 7.95 min for naphthalene-d8, 15.21 min for ortho-terphenyl, 21.05 min for toluene-d8 and 32.45 min for dichlorobenzene-d4, which lie in the area of interest of the analytes to be examined. 2.3. Analytical equipment For analysis of extracts from UAE, MAE and HS-SPME a Varian 3400 series GC was used. The injector temperature was set at 250 °C and the detector temperature at 275 °C. The injector was used in a splitless mode and the injection volume was 10 ll. The column temperature was held at 50 °C for 1 min and then ramped at 10 °C/min to 250 °C with a final hold time of 10 min. The carrier gas was N2. Extracts from polymer dissolution, ASE and HS was analysed by GC–MS at AnalyCen.

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with enough sand to fill a thimble. The polymer/sand mix was transferred to the extraction thimble in a cell, and the cell was closed with a cell cap. The cell was thereafter put in the accelerated solvent extractor (ASE 200). Extraction was performed at 140 °C under high pressure using a 2.5:97.5 cyclohexane/isopropanol solvent mix. After extraction, internal standards were added to the collected solvent and the solution was concentrated with a rotor evaporator to approximately 10 ml. Two milliliter of this solution was filtered with a 0.5 lm syringe filter and put in a GC vial for further analysis. 2.6. Microwave assisted extraction (MAE) A polymer sample of 0.5 g was weighed in a PTFE extraction cell. 10 ml of 50:50 cyclohexane/isopropanol solution and internal standards were added and the cell was closed, shaken for 5 s, and placed in the microwave. Extraction was performed at 70–100 °C for 30–60 min, with three sample cells in each extraction. After extraction, the vials were cooled to approximately 20 °C before opening. The vial contents were thereafter rapidly poured into a clean beaker. Three milliliter of the solution was filtered with a 0.5 lm syringe filter and transferred to a screw cap vial for storage and further analysis. 2.7. Ultrasound assisted extraction (UAE) Polymer sample (0.5 g) was placed in a 15 ml screw cap vial. 10 ml of 75:25 cyclohexane/isopropanol solution and the internal standards were added, and the vial was closed and placed in an ultrasonic bath. The bath had a preset temperature of 50 °C and the extraction time was 15– 60 min. After extraction, approximately 3 ml of the solution was filtered with a 0.5 lm syringe filter and poured into a screw cap vial for storage and further analysis with GC. 2.8. Head-space extraction (HS) An exact amount of polymer sample in the range of 1– 3 g was placed in HS-vials of 20 ml. The vials were shut and thereafter internal standard was added by syringe through the septum of the cap. The vials were run in a HS–GC–MS at 110 °C.

2.4. Polymer dissolution 2.9. Head-space solid phase micro extraction (HS-SPME) Approximately 5 g of polymer material was weighed into a 100 ml glass jar. Ten milliliter of 50:50 cyclohexane/ethyl acetate solution and the internal standards were added to the jar. It was shut with a screw cap and put on a rocking table for one hour at room temperature. This was followed by 5 min of centrifugation and thereafter the upper phase was separated and transferred to a vial containing siccative. Two milliliter of the liquid phase in the vial was transferred to a GC vial for further analysis. 2.5. Accelerated solvent extraction (ASE) Cellulose extraction thimbles were placed inside 22 ml ASE extraction cells. 0.5 g of polymer sample was mixed

SPME fibres, with carbowax/divinyl benzene (CW/DVB) and poly(dimethyl siloxane) (PDMS) coating, were conditioned for 30 min at 220 °C and 250 °C, respectively in the GC injector. Before each sample, a blank run was performed to reassure low background interference. Extraction was performed by weighing 0.5 g of a polymer sample into a HS-vial of 20 ml with a poly(tetra flour ethylene) (PTFE) septum. The vial was fixed on a stand over a hot water bath of a consistent temperature of 80 °C. The fibre was held in the head-space above the polymer sample for 60 min and then moved to the GC injector. The GC run was started with the SPME fibre held in the injector for 5 min.

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3. Results and discussion 3.1. Solid–liquid extraction methods Polymer dissolution, or shake flask extraction is a method of complete dissolution of the polymer. A British standard method [11] describes the dissolution followed by re-precipitation of polymers in a two-step combination of solvents. First the polymer is dissolved in refluxing toluene and in a second step precipitated by addition of ethanol. For dissolving PE and PP, decalin has been used at elevated temperatures. The high molecular weight polymers precipitate as the solvent is cooled down and the supernatant containing the LMW is filtered and analysed [12,13]. Polymeric additives can be re-extracted from the polymer precipitate and high extraction efficiency can be achieved by means of repeated extraction [14]. There is often a considerable amount of oligomers in the solution, which may need to be removed before further analysis. Some workers have considered this too time-consuming and prefer other solid–liquid extractions [15]. One of the most efficient solid–liquid extraction techniques is ASE, sometimes also called pressurised liquid extraction (PLE), as the solvent is kept under high pressure during extraction. The extraction temperature is kept above the normal atmospheric boiling point of the solvent to accelerate extraction kinetics, and pressure is applied to maintain the solvent in the liquid phase. High pressure facilitates extraction by forcing the solvent into the matrix pores [16,17]. Normally the temperature range is 100–200 °C and the pressure is around 100 atm. Practically any aqueous or organic solvent can be used with ASE [17]. Selecting a proper solvent for extraction can be difficult, as the solvent cannot be chosen based on the atmospheric pressure properties because solubility data for polymers at high temperatures are hard to find [10]. Extraction is performed by heating the sample and solvent in a closed stainless-steel cell placed in an oven. Soxhlet-type cartridge vessels are used to hinder solubilised oligomers from leaving the extraction cell. After a predetermined time period, called purge time, the solvent is collected in a vial for further analysis. With a fully automated system, several samples can be run sequentially [18]. Some advantages compared to traditional solvent extraction techniques are: less extraction time (approximately 15 min per sample), less solvent consumption, increased extraction yields and higher reproducibility [16]. The extraction efficiency is increased with increasing temperature and by use of good swelling agents, as solubility and diffusion rates are increased. Drawbacks with this method are the expense and the insufficient body of evidence for extraction of polymers [10]. A more commonly used method is MAE, which consists of heating the extracting solvent or sample with electromagnetic radiation. Microwave heating leads to a homogenous and rapid heating of the sample/solvent system. The heating is more efficient the higher the dielectric constant of the solvent. In MAE, weak hydrogen bonds are broken, and dissolved ions facilitate solvent penetration into sample-matrix, where both enhance solubility of analytes. Any mixture of solvents can be used, as long as it contains at

least one microwave-absorbing unit, i.e. with a permanent dipolar moment. If MAE is applied to a system with a nonpolar solvent, the sample is heated but the solvent remains cold, which could be an advantage when working with thermo sensitive compounds [16,17]. MAE can be performed under multimode or focused-radiation and in open or closed-vessels [19,20]. In closed vessel MAE, the temperature and pressure can be controlled. The temperature is dependent on the microwave power, the vessel volume and the boiling point of the solvent settle the pressure of the solvents used. The solvents can be heated above their atmospheric boiling point, which enhances extraction speed and efficiency. This type of extraction is preferable when working with digestion, acid mineralization or highly flammable solvents. Several samples can be run simultaneously, and they are normally placed on a rotating frame in the microwave oven, to provide a homogenous field over the samples during extraction. An important fact is cooling the samples to room temperature before opening the vessels to prevent the problems related to overpressure when working with volatile compounds. Occasionally an additional filtration or centrifugation is necessary to remove the solid residue. When open vessel MAE is used, the pressure equals the atmospheric pressure and therefore the maximal temperature is the boiling point temperature of the solvent used. Focusing the microwaves on the sample in the vessel, whilst the heated solvent is refluxed through it, provides a homogenous and efficient heating. The additional filtration step can be avoided by use of Soxhlet-type cartridge vessels. Open vessel MAE can handle larger sample volume than closed vessel MAE and provides a safer sample handling [16]. MAE offers a fast and effective extraction method that requires only low solvent use and can handle several samples in one extraction. The equipment is however quite expensive and some problems with reproducibility have been shown. One of the easiest techniques of extraction is UAE that works by agitating the solvent/sample system with ultrasonic waves and thus enhances rate transfer over the polymer/liquid boundary layer. Diffusion inside the polymer sample is not affected [10]. The ultrasonic waves create an expansion, compression cycle that leads to the movement of molecules. Expansion in a liquid can create cavitations, i.e. small bubbles, at pre-existing weak points in the liquid, such as small contaminants or already existing micro bubbles. When the bubbles collapse they produce a rapid adiabatic compression that leads to extremely high local temperature and pressure. Even so, the surrounding media is not noticeably affected, as the volume of the bubbles is very small compared to the total volume. At the points of extreme conditions, free radicals and other compounds are formed which leads to thermal dissociation of pure water. A high temperature also increases solubility and diffusivity of analytes. When bubbles collapse close to a solid surface, the implosion is not symmetrical, and small jet streams of liquid are created that have a high impact on the surface of the solid. The impact reveals a highly reactive surface and increases penetration and transport. UAE is very effective in extracting organic and inorganic compounds, due to very high efficient temperatures and pressures, and oxidative power of free radicals [21]. Extrac-

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tion of additives and other small compounds from polymers is fast, normally less than 1 h, if stirred every 10 min. For some polymers the extraction is very fast, low density polyethylene (LDPE) or PP extraction can take less than ten minutes [10]. UAE can provide a very efficient extraction of additives from polyolefins without noticeable degradation or need for pre-concentration of the analytes. Extraction conditions are highly dependent on the structure of the desired additive, and have to be carefully adjusted for each extraction [22]. Another solid–liquid extraction method is SFE, which has many similarities to ASE. The extraction is performed with a supercritical fluid, normally carbon dioxide (CO2), under controlled temperature and pressure, to maintain the supercritical state [17]. Super critical CO2 is an inorganic, non-polar, chemically inert and non-toxic solvent, used for extraction of non-polar to moderately polar compounds. To extract more polar compounds, a polar chemical modifier, usually methanol is added to the solution. The modifier increases the polarity of the solvent, hence the solubility of polar compounds. Modifiers also act by increasing swelling of the polymer, and thereby also increasing the extracting ability of the non-polar compounds [10,18]. CO2 dissolves in polymers, which leads to swelling and plasticizing. The degree of swelling is directly connected to the extraction efficiency, as the swelling increases diffusion of molecules throughout the polymer. Swelling of the polymer also lowers the glass-transition temperature; Tg. Amorphous polymers are more susceptive to swelling than semi-crystalline polymers, as CO2 is not soluble in crystalline regions of a polymer. Therefore, extraction at temperatures above the Tg increases the extracting ability. The extraction rate increases with temperature until it is limited by solubility in the supercritical fluid. For optimal extraction, the temperature should be high to increase diffusion, and the pressure should be high to increase solubility and plasticize the polymer. SFE is a fast method which has a low solvent use and can be automated. The method can however be difficult to optimize and the equipment is quite expensive [10]. The Soxhlet method is the most commonly used semicontinuous method applied for extraction of polymeric additives. Soxhlet has been a standard extraction technique during more than one century and is the main reference to which the performances of newer techniques are compared. According to the Soxhlet procedure, analytes from solid material are extracted by repeated washing with an organic solvent under reflux in special glassware [23]. The most important advantage of Soxhlet extraction is the reflux of solvent through the sample, displacing the transfer equilibrium. Other advantages are those of high temperature and the simplicity of the method. Since the heat applied to the distillation flask is spread to the whole system, the temperature remains high during extraction. The method is easy to conduct and requires no filtration after extraction. The sample is placed in a cellulose thimble which efficiently collects dissolved polymer. Several samples can be run simultaneously and the equipment is inexpensive. This method has a possibility to extract more sample mass than most other modern extraction methods (MAE, SFE, etc.) and is non-matrix dependent [23]. Times

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for extraction typically vary from 6 h to 48 h [24–26]. The choice of solvent significantly influences extraction times and efficiency [27]. Soxhlet extraction has been shown to be efficient, but as mentioned often very slow and requires large amounts of solvents. This results in dilute solutions requiring further concentration before analysis, which is time-consuming and may result in the loss of volatile compounds. The use of large volumes of often toxic solvents is environmentally hazardous, as well as expensive [10]. Samples extracted at the boiling point of the solvent and for a long period of time, run the risk of thermal decomposition of the analytes, especially thermo-labile analytes. Further, the conventional Soxhlet cannot provide agitation which would decrease extraction times and the technique is restricted to solvent selectivity. Facing the advantages and disadvantages of the Soxhlet method, improvements of the method have been made. Different types of high pressure, automated or microwave-assisted Soxhlet extractors may be found on the market. For example, several types of ‘‘Soxtec” systems including automated or semiautomated analyzers, which extract analytes rapidly and accurately, have been developed. These instruments perform boiling, rinsing and solvent recovery. The sample to be analyzed is weighed into cellulose thimbles and inserted in the extraction device. Almost any solvent may be used (about 15 ml per sample), with a 75% recovery of the solvent after the extraction, which is completed in 30– 60 min, depending on the application. Other Soxhlet types of systems use microwaves to heat the solvent, and have means to avoid thermal decomposition of the analytes [23]. 3.2. Head-space extraction methods HS extraction is normally coupled online with gas chromatography (HS–GC). The method is used to identify and quantify volatile organic compounds (VOX) from liquid or solid samples. The sample is kept in a sealed vial and heated until equilibrium of VOX:s between the sample and the gaseous phase above the sample is achieved. An aliquot sample of gas phase is injected in a GC column, where VOX:s can be identified and quantified with further analysis, for example mass spectrometry (MS). HS–GC provides an inexpensive, easily automated, reproducible, reliable method, which requires no complex equipment [28,29]. Interferences are relatively small because only gas phase is considered, which eliminates sample-matrix effect on measurements [30]. However, the volatility of the analyte affects the extraction efficiency. Another well-known technique is SPME, where analytes from a sample are collected on a fused silica fibre covered with an absorbent polymer or in a tube with the absorbent coating on the inside which is lowered into the extracting solvent [31]. The adsorbent material can be of different polarity or have ionic abilities. This, in addition to a proper choice of solvent, provides a highly selective extraction. The sorbent polymer is usually a synthetic porous material, for example styrene-divinylbenzene copolymers, acrylic ester polymers or bonded porous silica. Bonded porous silica coating can provide a functional surface of the fibre by end capping reactions of some of the silanol groups.

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Hereby, the sorbent can be made ionic or of different polarity [32]. In head-space-SPME (HS-SPME), analytes are adsorbed to the coating material from the head-space above a liquid or solid sample in a closed vial. The fibre concentrates volatile or semi-volatile components from liquid, solid or gaseous samples. Analytes can be adsorbed selectively by choice of adsorbent material. The fibre can be directly inserted into an analytical instrument, for example GC, for identification and possibly quantification of the solutes. HS-SPME is an equilibrium analytical method. The adsorption of analytes on the fibre is limited by the equilibrium between the phases of the system (samplematrix, HS and fibre), which normally takes 2–15 min [33]. HS-SPME combines sampling, extraction, pre-concentration and GC introduction in one step, hereby eliminating any complicated sample preparation, and use of solvents. There are many advantages with HS-SPME; low cost, simple sample handling, low or no use of solvents, possibility to work online with GC, portable equipment and fast extraction to mention a few. It is a sensitive and precise method with low detection limits [33–35]. However, an important drawback is connected with quantification when a large number of analytes with different polarities and volatilities are being examined, or when chemisorption of analytes in the solid matrix limits the recovery of native analytes [33,35]. A method sprung from SPME extraction methodology is stir bar sorptive extraction (SBSE). The extraction mechanisms are the same, but SBSE provides an increased sensi-

tivity due to the larger surface of coating material. A normal SPME fibre (100 lm film thickness) only gives a coating volume of approximately 0.5 lL, whereas a SBSE stir bar can have coating volumes of up to 300 lL. This means a larger number of analytes can be extracted and the problems connected with competing analytes are circumvented. SBSE can therefore be used for quantification of analytes [36,37]. SBSE was originally developed for analysis of water samples. Extraction is performed by stirring a liquid sample with a poly(dimethyl siloxane) (PDMS) coated stir bar for a predetermined time. The stir bar is subsequently removed from the sample and analysed with GC–MS or desorbed by means of a liquid and analysed with liquid chromatography mass spectrometry (LC–MS). GC analysis provides a higher sensitivity but the LC method gives an improved selectivity and avoids loss of volatiles [36]. As in SPME the SBSE bar can be used for extraction of volatiles from solid samples, also called head-space sorptive extraction (HSSE) [38]. The bar is hung in the head-space over the sample in a closed vial for a predetermined time and thereafter analysed with GC–MS as described above. It can therefore be applied to extraction of LMW compounds from polymer samples, and should provide a higher sensitivity than HS-SPME. There is a choice to be made between inexpensive, simple equipment giving long extraction times and more expensive but rapid techniques. Important factors are extraction efficiency, reproducibility and detection limits. Table 1 shows an overview of the most important para-

Table 1 Comparison of extraction methods and parameters for analysis of LMW compounds in recycled plastics Method

Polymer dissolution

ASE

MAE

UAE

SFE [10,39]

Soxhlet [10,39]

HS

HS-SPME

Solvent

Any

Any

Any

Usually CO2

Any

No

No

Solvent usage Sample size

>10 ml >1 g

30–60 ml 0.5–10 g

Must contain microwave absorbing component 10–50 ml 0.5–5 g

10–50 ml 0.5–5 g

50–100 ml 1–5 g

No 1–5 g

No 0,5–1 g

Analysis time

>60 min

15–30 min

30–60 min

15–60 min

6–24 h

60 min

2–60 min

Advantages

No analyte remaining bound in the polymer (high efficiency), easy and inexpensive

Short extraction time, low solvent consumption, reproducible, enables sequential extraction

Fast and effective, low solvent use, several samples in one extraction, controlled pressure and temperature

Inexpensive, easy to use, high efficiency, good for polyolefins, reproducible, multiple extractions

10 ml 10– 100 mg 20– 120 min Low solvent use, fast, can be automated

Inexpensive, widely accepted, no sample reprocessing

Inexpensive, easily automated, reproducible, no complex equipment, small interferences, online with GC, low detection limits

Low cost, easy sample handling, low detection limits, online with GC, portable, fast extraction, enables selective extraction

Disadvantages

Timeconsuming sample reprocessing, dissolved oligomers

Expensive, timeconsuming sample preparation and reprocessing

Not always effective

Expensive, difficult to optimize

Timeconsuming extraction, large solvent use

Not efficient for less volatile compounds

Difficult to quantify analytes with different polarities and volatilities, chemisorption

Extraction yieldb

High

Very high

Expensive, some problems with reproducibility, timeconsuming sample treatment High

Medium high

Medium high

High

Medium

Lowa

a b

Background interference from the variety of compounds exhibited in a recycled plastic limits the extraction yield. LMW in recycled plastics.

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meters of the above described extraction methods for an easier comparison of the methods. HSSE is not included in the overview, since there is insufficient information about the method for extractions of polymeric samples. ASE is showing promising features for extraction and so is MAE. UAE is fast, reproducible, inexpensive and easy to accomplish. SFE has shown to be fast and effective for polymer extraction, but the equipment is expensive and the extraction parameters can be difficult to optimize. Soxhlet extraction is inexpensive and well established, but too time-consuming. HS and HS-SPME are both fairly simple, solvent free, inexpensive and effective methods of analysing VOX:s from polymer with low matrix interference,

A2

6

4x10

6

Intensity

3x10

F1

6

2x10

F2

6

1x10

0 5

10

15

20

25

30

t R [min] Fig. 2. Polymer dissolution of recycled HDPE–PP mix.

35

giving low detection limits. HSSE require more complex equipment and was not used in this study. Therefore, the methods chosen for further examination were ASE, MAE, UAE, HS and HS-SPME. 3.3. Method evaluation of extractions of virgin and recycled polyolefins Polymer dissolution was used for initial screening of the material content. Fig. 2 shows a polymer dissolution chromatogram for the recycled mix of HDPE and PP. Peak identification is listed in Table 2. The GC method was hereby optimized and an internal standard could be chosen to match the peaks of the chromatogram given. When working with recycled materials it is important to use an internal standard that can be easily separated in the mass-spectra from the multitude of peaks of the contaminants. The response of each peak towards an internal standard differs hence the peak-response value of a specific compound must be calculated in respect to the internal standard being used. When the extracted amount of a specific compound has been established, a quantitative comparison can be made between the methods. For HS methods, a mix of toluene-d8 and 1,4-dichlorobenzened4 was used. Deuterated chemicals were easily found in mass-spectra and the compounds were known to have good chromatographic properties as well as high volatility. These compounds were therefore suitable for HS techniques but not for solid–liquid extraction techniques. For the latter, o-terphenyl and naphthalene-d8 were used, as they showed retention times of interest and good chromatographic properties, as well as being easy to discover in the mass-spectra.

Table 2 List of compounds identified by MS No.

Type

No.

Type

A A1 A2 A3 A4 B B1 B2 B3 C C1 C2 C3 C4 C5 C6 C7 C8 C9 C10 C11 C12 C13 C14 C15

Internal standards Naphthalene-d8 Ortho-terphenyl Toluen-d8 Dichlorobenzene-d4 Solvents Chloroform Cyclohexane Isopropanol Alifatic Hydrocarbons Hexane Octane Decane Dodecane Tetradecane Hexadecane Octadecane Eicosane Docosane Tetracosane Hexacosane Octacosane Triacontane Dotriacontane 5-eicosene

D D1 D2 E E1 E2 F F1 F2 F3 F4 F5 G G1 G2 H I I1 I2 I3 J J1 J2 J3

Alcohols Branched alcohol Eicosanol Carboxylic acids Acetic acid Octadecanoic acid Fragrances, flouvours, terpenes Terpene Limonene Alpha-pinene Beta-pinene Camphene Phthalates Dibutylphthalate Phtalate (instrumental) Fatty acids Siloxanes Cyclotetrasiloxane-octametyl Decamethyl tetrasiloxane (vial septum) 2,4-bis-(trimesiloxy)benzaldehyde Other Pyren-d10 Perylen-d12 Quinoline

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For ASE, an extraction method for additives from polyolefins has been developed by DIONEX [40]. The ground polymer sample is blended with fine grain sand to prevent agglomeration of melted polymer at high temperatures. The polymer/sand mix is transferred to an ASE extraction cell with a cellulose thimble that hinders dissolved and melted polymer to leave the extraction cell. The extraction temperature should be high enough for an efficient extraction but lower than the melting point of the polymer, in this case 140 °C. Extraction of additives from polymers is limited by diffusion and several extraction cycles are therefore used in ASE to improve the extraction efficiency. When choosing a solvent for the extraction, it is important to use a solvent that is not likely to dissolve the polymer sample, such as isopropanol. The solvent is mixed with a swelling agent to improve the efficiency of extraction, but at a low enough ratio to prevent the polymer from swelling out of the cell. The method was validated by reextracting a sample, and the extraction efficiency was shown to be excellent (Fig. 3). The most important parameters in MAE are choice of solvent, time of extraction and extraction temperature.

a 2.0x10

G2

8

G2

A2

8

Intensity

1.5x10

8

1.0x10

C7 C6

C8 C9 C10

A1

7

5.0x10

C5

C11 J1

J3

0.0 0

I1

C4

5

10

C12

G

C13 J2

15

20

25

30

35

t R [min] 8

b 2.0x10

A2 8

Intensity

1.5x10

Starting with solvent choice, the most commonly used solvent combinations for MAE extraction of polyolefins are shown in Table 3 [1,10,16,18,20,39,41–43]. Since only chlorine free solvents can be used in industrial laboratories and carcinogenous solvents are non-preferable in a laboratory environment, solvent choices lie between heptane/acetone, cyclohexane/isopropanol and heptane/isopropanol. Since acetone can give problems due to rapid evaporation, and thereby cause reproducibility disturbances and difficulties with quantification, it was excluded. The cyclohexane/isopropanol combination was chosen for further investigation. The temperature is ramped to the boiling point and from there to the final extraction temperature in a 2 step process to avoid unnecessary stress on the sample. The boiling points of cyclohexane and isopropanol are 80.7 °C and 82.4 °C. The total extraction time is determined by changing the time the sample is held at the desired extraction temperature, in this case 100 °C. Based on experience with the UAE extraction, a 75:25 cyclohexane/isopropanol solution was used, but did not show enough polar strength to heat the liquid to 100 °C in the microwave without causing the polymer sample to collapse. Solvent with an increased polar fraction at different ratios were therefore tested and the combination 50:50 cyclohexane/isopropanol showed the desired results. To find the most suitable temperature and extraction time, double sample test with varying time and temperature were carried out. In previous extractions with cyclohexane/isopropanol solvent, the extraction times used have been 30–60 min for an efficient extraction [1,10,22,43]. In addition to time, three extraction temperatures, both below and above boiling points were analysed; 70 °C, 100 °C and 130 °C. All extractions at 130 °C caused the samples to collapse and were excluded from the study. The extraction was more efficient at 100 °C than at 70 °C, and was therefore chosen as extraction temperature. A commonly used solvent for UAE is chloroform and it is very efficient for extraction of additives from polyolefins. Ruling out chlorinated and carcinogenous solvents, a different solvent had to be chosen for this experiment. The goal was to find a solvent that was efficient for extraction of polyolefins with a combination of a nonpolar and a polar solvent for efficient swelling and extraction of polar compounds. The choice of solvent was based upon solubility parameters and comparisons with other extraction techniques such as ASE and MAE. The most commonly used non-chlorinated solvents in ASE and MAE and their corresponding solubility parameters are listed in Table 4 [44].

8

1.0x10

Table 3 MAE solvent combinations

G2 A1 7

5.0x10

G2 G2

0.0 0

5

10

15

20

25

t R [min] Fig. 3. ASE extraction (a) and re-extraction (b).

30

35

Nonpolar

Polar

Hexane Heptane Cyclohexane Heptane Dichloromethane Dichloromethane Chloroform

Acetone Acetone Isopropanol Isopropanol Isopropanol Xylene Isopropanol

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J. Möller et al. / European Polymer Journal 44 (2008) 1583–1593 Table 4 Solubility parameters of solvents commonly used in ASE and MAE

a

200

½

d (MPa )

Nonpolar solvents

d (MPa )

Acetone Isopropanol Ethyl acetate

20.3 23.5 18.6

Cyclohexane n-Hexane n-Heptane

16.8 14.9 15.1

Since both HDPE and PP were analysed in this study, a nonpolar solvent with good swelling ability for both types of polymer had to be chosen. The solubility parameters for HDPE and PP are in the range of 16-17 and 17.5-19 respectively [44]. Cyclohexane has a solubility parameter in between both polymers, with a somewhat larger solubility for HDPE. It was therefore chosen as nonpolar component. The polar solvent aids in extraction of polar compounds and should be non soluble in the polymer matrix. Isopropanol was chosen for its high solubility parameter, i.e. none likely to dissolve the HDPE or PP matrix. An experiment was carried out with UAE of polymer samples at different solvent ratios to test the best composition of the two solvents. The swelling agent was chosen in a larger proportion, since good swelling ability is essential in UAE. As could be predicted by solubility parameters, PP showed no visible swelling in any solvent combination, but in HDPE the swelling increased with the fraction of cyclohexane, with no visible swelling in a 50:50 solution. After extractions the extracts were analyzed with GC and the chromatograms showed a larger number of peaks when adding polar solvent to the system for both materials (Fig. 4). Since extraction with UAE is highly diffusion dependent, the time of extraction is very important. A short extraction time could give a more cost efficient and less time-consuming extraction, but also lower extraction yield. A longer extraction time increases the extraction yield, but could also be the source of compound degradation leading to misjudgements in the obtained results. In previous studies the extraction time when using UAE has been between 15 and 60 min [10,19,21]. Double sample screening tests was therefore performed with the extraction times 15, 30, 45 and 60 min. The result was clear. The longer the extraction time, the more efficient was the extraction. The chromatograms showed no degradation products at 60 min, which would have been shown as many smaller peaks at short retention times. Thus 60 min was chosen as extraction time. The temperature is another important parameter and can influence the degree of swelling, which in turn influences the extraction efficiency. Extractions where carried out at 50 °C and room temperature where the higher temperature was found to be more efficient. HS extraction was carried out by weighing a polymer sample into a HS-vial. The exact HS volume could not be established, therefore the result of the GC–MS chromatogram are given as weight percent of the original sample. The internal standards were also added in exact weight. Two temperatures were tested, 70 °C and 110 °C, where 110 °C gave a much better extraction. As it is demonstrated in Fig. 5, only alkanes smaller than 14 °C are detectable in

B2

150

Intensity

Polar solvents

C7

100

C8 C9 C10

C5

C6

C11

50

C12 C13

0 0

5

10

15

20

25

30

35

30

35

t R [min]

b

200

150 B2 B3

Intensity

½

100 C7 C8 C5

50

C9

C6

C10

G C11 G C12 C13

G

0 0

5

10

15

20

25

t R [min] Fig. 4. UAE extraction of HDPE in pure cyclohexane (a) and in a 75:25 cyclohexane/isopropanol solution (b).

HS extraction, and only VOX and other LMW compounds are extracted. For SPME two fibres were compared, a CW/DVB coated and a PDMS coated fibre. Both fibres were conditioned just before sample extraction. After extraction the fibres were analyzed with GC and showed no significant difference. However, the background interference due to the complex sample composition was too great for an efficient comparison. This, in combination with the long extraction time needed for an efficient extraction, resulted in the exclusion of SPME extraction from the study. SPME is in general a very convenient method to perform initial screening for LMW in polymers [45,46]. Care must, however, be taken to the maintenance of the fibre as loss of sensitivity may occur towards the end of the life-time of a SPME-fibre. 4. Conclusions ASE definitely showed the greatest extracting ability, but the equipment is also the most expensive. ASE has

1592

a

J. Möller et al. / European Polymer Journal 44 (2008) 1583–1593

40 B1

Mass fraction

30

20 C1

C2

10

F2 C3

B B2 D1

F5 F3 F4

A3

0 5

10

15

20

25

A4

C I2

H

30

35

40

45

t R [min]

b

40

B1

C1

F2 C2

Mass fraction

30

C3

Soxhlet extraction generally extracts all additives, but extraction times can be as long as 48 h. During such long extractions and subsequent concentration steps, there is a possibility of loss of volatile or thermally labile components. The selection of extraction solvent can make a large difference to the extraction time. However, the equipment is inexpensive, and once set up requires little ‘hands-on’ attention. The new generation of ‘‘Soxtec” extractions shows a more promising future. HS-SPME showed great problems with quantification and background interference and is therefore not recommended for quantitative analysis. HS can provide a better method for VOX analysis, since the analytes are quantifiable, but might not be useful in analysis of less volatile compounds. HS, HS-SPME and HSSE are solvent free extraction methods. In polymer extraction the head-space method of extraction of volatile compounds is applicable and here HSSE should provide a greater efficiency than HS-SPME. The efficiency of HS sampling compared to the other methods should be examined further. For very dirty matrixes, a head-space extraction has better ability to avoid large background interferences and therefore gives lower detection limits than compared to a liquid-solid extraction. Liquid-solid extractions are however efficient over a broader range of compounds, i.e. not only VOX:s. Acknowledgements

20 B2 C

A4

Part of this work was done as a project together with Lantmännen AnalyCen AB, Lidköping, Sweden. Per Ivarsson and co-workers are gratefully acknowledged for their contribution to the work by good ideas and discussions of results.

10 B

F5

D1 A3

F3

F4

H

I2

0 5

10

15

20

25

30

35

40

45

References

t R [min] Fig. 5. HS–GC–MS extraction of HDPE–PP mix at 70 °C (a) and 110 °C (b).

been found to be effective in extraction from environmental samples and has great potential for extraction from polymers. The main problem is likely to be selection of extraction solvent which does not dissolve the polymer at high temperature. Although ASE uses liquid solvents, the total solvent usage may not be higher than through the use of modifiers with SFE. MAE showed excellent extraction efficiencies, but requires a lot of sample treatment and can therefore be time-consuming. MAE offers a rapid and efficient method for the extraction of up to 12 samples simultaneously. Temperature, pressure and extraction time is easily monitored. The equipment is however much more expensive than that for Soxhlet extraction, and some problems with reproducibility have been shown. UAE was found to be a very easy and cheap method, with an extraction efficiency close to the one of MAE. SFE has been shown to provide a, in most cases, rapid and quantitative extraction using ground samples or thin films, but the method needs to be optimised for pressure, temperature and modifier. Equipment exists which can extract several samples simultaneously and which can be programmed to extract up to 28 samples consecutively.

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