Indicator Products: A New Tool for Lifetime Prediction of Polymeric

However, the total amount of indicator products was in correlation with the remaining .... Unaged samples of both materials were also subjected to an ...
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Biomacromolecules 2005, 6, 775-779

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Indicator Products: A New Tool for Lifetime Prediction of Polymeric Materials Minna Hakkarainen and Ann-Christine Albertsson* Department of Fibre and Polymer Technology, Royal Institute of Technology (KTH), Teknikringen 56-58, SE-100 44 Stockholm, Sweden Received August 30, 2004; Revised Manuscript Received November 16, 2004

The possible correlation between the degree of degradation in the polymer matrix and the release of indicator products was investigated. The degree of degradation was measured by following the molar mass changes by size exclusion chromatography, while indicator products were analyzed by gas chromatography-mass spectrometry. The degree of degradation in polyethylene and polyethylene vinyl acetate matrix after thermooxidation was found to be in correlation with the amount of dicarboxylic acids and lactones formed during aging, while the degree of degradation in photooxidized polyethylene could be predicted from the amount of dicarboxylic acids. The relative amount of lactones compared to the relative amount of dicarboxylic acids increased if the oxidation temperature was increased. However, the total amount of indicator products was in correlation with the remaining number average molar mass and the number of chain scissions caused by oxidation. The amount of butanedioic acid and butyrolactone correlated well with the total amount of dicarboxylic acids and lactones, respectively. Thus, instead of the whole compound classes, butanedioic acid or butyrolactone alone could be used as indicators of oxidation. The detected correlation offers a novel tool for making lifetime predictions and studying the long-term properties of polymeric materials. Introduction The accurate lifetime prediction of polymeric materials is a difficult, but import task. Susceptibility to oxidative degradation is one of the most important factors influencing the long-term properties and lifetime of polymers. Ultimately, the material will deteriorate to a point where it is no longer able to fulfill its expected function safely. Two general types of reactions take place during oxidative polymer degradation, solid-state reactions that add oxidation species to the polymer backbone and reactions that lead to the formation of volatile degradation products. The relationship between these two types of reactions is still poorly understood. If a correlation can be established between the formation of certain volatiles and the deterioration of solid-state polymer, totally new types of test methods could be developed to determine the oxidative stability or to make lifetime predictions based on the release of indicator products. We have in earlier studies seen that the chromatographic fingerprints of the emitted volatiles change depending on the degree of oxidation in the polymer matrix.1,2 We have also introduced chromatographic fingerprinting as a tool to differentiate between abiotic and biotic degradation of polymers.3,4 Chromatographic fingerprinting and volatile component analysis have also been applied for identification of different polyethylene and polypropylene samples.5 Electronic nose technology and analysis of volatiles has long been applied in the food industry to quality control food products and to determine the shelf life for various products. Some * To whom correspondence should be addressed. Phone: +46 (0)8 790 8274. Fax: +46 (0)8 10 07 75. E-mail: [email protected].

recent examples are the use of sensor arrays to differentiate milk products according to their aging times6 and the use of a solid-phase microextraction-mass spectrometry-multivariate data system to predict the shelf life of pasteurized milk.7 Different electronic nose systems have also been applied in the car industry to analyze the “new car odor”. Two MS-based devices were evaluated to analyze the compounds emitted from different plastic parts present in the car interior.8 Metal oxide semiconductor chemical sensors have been used to estimate the oxidative stability of polypropylene during processing.9 This alternative method could be applied for rapid evaluation of antioxidant additive formulations and their efficiency during multiple-pass extrusions. We have in earlier studies shown that, during thermooxidation of virgin and recycled polyamide 6.6, there was a correlation between the formation of 1-pentyl-2,5-pyrrolidinedione, the most abundant thermooxidation product, and changes in mechanical properties.10 We have also identified around 200 different degradation products after photo- and thermooxidation of polyethylene and different environmentally degradable polyethylenes.11-13 These studies preliminarily indicated that especially the amount of different dicarboxylic acids increased on prolonged aging. This work was acknowledged, and the formation of dicarboxylic acids during oxidation of polyethylene was utilized to chemically recycle polyethylene by oxidizing it to low molar mass dicarboxylic acids.14-16 The aim of the present study was to investigate if the amount of dicarboxylic acids or some other compound group formed during photo- or thermooxidation of polyethylene can be correlated to simultaneous molar mass changes. If such a correlation exists, these compounds could

10.1021/bm049483f CCC: $30.25 © 2005 American Chemical Society Published on Web 01/21/2005

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be used as indicator products to rapidly predict the degree of degradation in the polyethylene matrix and further to estimate the remaining lifetime of the product. Experimental Section Materials. The materials subjected to thermooxidation were two injection-molded starch-based blends consisting of 70 wt % corn starch and either 30 wt % polyethylene (PE) or 30 wt % ethylene vinyl acetate (EVA) copolymer. The vinyl acetate content in the EVA was 28%. Approximately 0.8 mol % maleic acid units were present in both PE and EVA. The number average molar masses (Mn) were 20 500 g/mol for PE and 23 300 g/mol for EVA. These starch/PE and starch/EVA blends were prepared according to Ramkumar et al.17 The materials subjected to photooxidation included three polyethylene films containing photosensitizing or stabilizing additives and one polyethylene film without additives. The photosensitizers/stabilizors used in the films were (1) iron dimethyldithiocarbamate (SG1), (2) iron dimethyldithiocarbamate and 0.8% carbon black (SG2), and (3) iron dimethyldithiocarbamate and nickel dibutyldithiocarbamate (SG3). The fourth film had no additives (LDPE). The film thicknesses were 27.5, 26, 21, and 30 µm, respectively. The photosensitized polyethylenes were a gift from the late Professor D. Gilead. Degradation Procedures. PE and EVA were thermooxidized in closed glass vials. A 100 mg sample of polymer was put into each vial, and the vials were kept at 190 °C (30 min or 3 h) or at 230 °C (30 min). The different polyethylene films were photooxidized for 300 h in an Atlas UVCON weatherometer equipped with a UV lamp (FS-40 fluorescent sunlamp) giving radiation with wavelengths between 280 and 359 nm. During the irradiation the temperature increased to a mean value of about 50 °C. Extraction of Low Molar Mass Compounds. The thermooxidation products were extracted according to a method described by Hakkarainen et al.1 After the thermooxidation 0.5 mL of diethyl ether was added to each vial. After 1 h the diethyl ether was separated from the remaining polymeric material and evaporated to dryness with a gentle stream of nitrogen. The hexane-soluble products were dissolved in 1 mL of hexane and subjected to solid-phase extraction (SPE). The remaining hexane-insoluble fraction containing the dicarboxylic acids was dissolved in 2% HCl in methanol (100 µL) and analyzed separately. Solid-phase extraction was used to separate the products into three fractions. The sorbent used for extraction was silica bonded to aminopropyl chains (NH2) from Varian. The column was first activated with 2 mL of hexane. After activation the hexane fraction with degradation products was passed through the column. The column was first washed with 1 mL of hexane, then with 1 mL of chloroform, and finally with 2% acetic acid in diethyl ether (1 mL). The fractions were concentrated to 50 µL and subjected to GC-MS analysis. Unaged samples of both materials were also subjected to an extraction procedure similar to that of a reference. The degradation products from the photooxidized films were extracted according to the method described by Hakkarainen et al.11 A 1 mL sample of diethyl ether was added above the films.

Hakkarainen and Albertsson

After 1 h the diethyl ether was separated from the remaining polymer films and evaporated to dryness. The products were then dissolved in 50 µL of hexane. The hexane fraction was removed, and the hexane-insoluble products were dissolved in 50 µL of 0.1% HCl in methanol. Before GC-MS analysis the methanol fraction was warmed for 15 min at 60 °C to methylize the keto and dicarboxylic acids. Gas Chromatography-Mass Spectrometry (GC-MS). The gas chromatograph used for the analysis of thermooxidized samples was a Perkin-Elmer 8500 model with a split/ splitless injector. It was connected to a Perkin-Elmer iontrap detector (ITD) mass spectrometer. The gas chromatograph was equipped with DB-1 (dimethylpolysiloxane) and DB-FFAP (nitroterephthalic acid-modified polyethylene glycol) capillary columns from J&W (30 m × 0.32 mm i.d.). The nonpolar DB-1 column was used to analyze hexane, chloroform, and methanol fractions with nonpolar and medium polar degradation products. The polar DB-FFAP column was used to analyze the ether/acetic acid fractions. The original ether fractions were analyzed with both columns. The column temperature was raised from 60 to 325 °C at 5 °C/min for hexane fractions and from 40 to 325 °C at 5 °C/min for chloroform and methanol fractions. The ether/ acetic acid fraction was analyzed with the DB-FFAP column that was programmed from 60 to 250 °C at 10 °C/min and held at 250 °C for 21 min. Helium was used as a carrier gas. The samples were introduced in the splitless injection mode at 250 °C. The photooxidized samples were analyzed with a Varian gas chromatograph coupled to a Finnigan SSQ7000 mass spectrometer. The column used was a DB-WAX capillary column from J&W (30 m × 0.32 mm i.d.). The column temperature was held for 1 min at 60 °C, raised to 240 °C at 10 °C/min, and then held for 8 min at 240 °C. Helium was used as a carrier gas. The samples were introduced in the splitless injection mode at 225 °C. The relative amounts of the different compounds were obtained by integrating and comparing the peak areas in the different chromatograms. High-Temperature Size Exclusion Chromatography (HT-SEC). A Waters 150 °C high-temperature SEC apparatus equipped with two PLgel 10 µm mixed-B columns and an RI detector was used to measure changes in molar mass and distributions. The mobile phase was 1,2,4trichlorobenzene (TCB) at 135 °C, and the flow rate was 1 mL/min. Calibration was performed according to polystyrene standards ranging between 770 000 and 2000 g/mol. Results and Discussion Degradation products were identified and their relative amounts determined after thermooxidation of starch/polyethylene and starch/polyethylene vinyl acetate and photooxidation of different polyethylenes. The results from the GC-MS analysis were compared with the molar mass changes to find a suitable marker or “indicator product” for the oxidative degradation. Indicator Products Released during Thermooxidation. Alkanes, alkenes, alcohols, ketones, aldehydes, carboxylic acids, dicarboxylic acids, and lactones were extracted and

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Figure 2. Correlation between the relative amount of all dicarboxylic acids and the relative amount of butanedioic acid after thermooxidation at 190 and 230 °C.

Figure 1. (a) Relative amount of dicarboxylic acids and lactones formed during thermooxidation of PE and EVA at 190 and 230 °C. (b) Number average molar mass after thermooxidation of the same PE and EVA samples.

identified after thermooxidation of starch/polyethylene and starch/ethylene vinyl acetate copolymer at 190 and 230 °C. The exact identity and relative amounts of the individual compounds have been given elsewhere.1 Calculation and comparison of the relative amounts of the different product classes after different aging times were performed to see if a correlation could be found between the formation of certain products or product classes and the deterioration of the polymer matrix. Especially the amount of dicarboxylic acids and lactones increased as the molar mass decreased. These compound classes were thus chosen for further studies to see if the formed amounts could be correlated to the molar mass changes. Figure 1a plots the relative amount of dicarboxylic acids and lactones extracted from PE and EVA after different aging times at 190 and 230 °C. The samples are arranged as a function of decreasing number average molar mass or increasing degree of degradation. The number average molar masses for the same thermooxidized samples are given in Figure 1b. It is clearly seen that the amount of dicarboxylic acids and lactones increases as the degree of degradation in the polymer matrix increases and that there is a correlation between the formation of these indicator products and molar mass decrease. The total amount of indicator products increased irrespective of the aging temperature as the molar mass decreased. However, the relative amount of lactones increased at higher aging temperature; i.e., the relative amount of lactones in relation to the relative amount of dicarboxylic acids was higher in the samples aged at 230 °C than in the samples aged at 190 °C. Low amounts of indicator products were formed during early stages of thermooxidation, i.e., at a low degree of

degradation as measured by the molar mass decrease. However, as the number average molar mass continued to decrease increasing amounts of indicator products were formed, and the formation rate accelerated as the degree of oxidation increased. If the relative amount of dicarboxylic acids and lactones formed in PE and EVA after different aging times at 190 and at 230 °C was plotted as a function of the remaining number average molar mass, there seemed to be a logarithmic relationship between the formation of indicator products and the remaining number average molar mass. The correlation coefficient R2 for the logarithmic curve was 0.9611. An even higher fit was obtained if the formation of indicator products was followed only at one temperature (190 °C) or for one material (PE) at two temperatures. The correlation coefficients R2 were then 0.9954 and 0.9855, respectively. Predicting the degree of degradation would be further simplified if, instead of whole compound classes, only one or two compounds could be used as indicator products. Figure 2 compares the relative amount of butanedioic acid to the relative amount of all the dicarboxylic acids to see if the amount of butanedioic acid correlates with the total amount of all dicarboxylic acids. Butanedioic acid was chosen as a suitable candidate to represent the group of dicarboxylic acids because it was the most abundant or one the most abundant dicarboxylic acids in the degradation product patterns. Figure 2 shows that the amount of butanedioic acid correlates quite well with the total amount of all dicarboxylic acids. Butyrolactone was by far the most abundant lactone, and its amount also correlated well with the total amount of lactones. In general, the relative amount of butanedioic acid after different aging times was 12-15% of the relative amount of all dicarboxylic acids, while the relative amount of butyrolactone was approximately 50% of the relative amount of all lactones. Instead of measuring the amount of all dicarboxylic acids and lactones, the degree of degradation after aging at constant temperature could, thus, be predicted from the concentration of either butanedioic acid or butyrolactone without considerable loss of accuracy. This considerably simplifies the analysis and decreases the analysis time. Indicator Products Released during Photooxidation. When polyethylene with different photoinitiators and stabilizers was photooxidized, different induction periods and degradation rates were observed depending on the used

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Figure 5. Relative amount of different indicator products as a function of chain scissions during thermooxidation at 190 and 230 °C. Figure 3. Remaining number average molar mass and relative amount of dicarboxylic acids after 300 h of photooxidation for different polyethylenes.

Figure 6. Relative amount of dicarboxylic acids as a function of chain scissions during photooxidation.

Figure 4. Relative amount of indicator products as a function of chain scissions during thermooxidation at 190 °C.

additives.13 The main low molar mass degradation products identified after photooxidation were different alkanes, ketones, carboxylic acids, and dicarboxylic acids. Again the formation of a large amount of dicarboxylic acids was connected with severe oxidation in the polyethylene matrix. In Figure 3 the number average molar mass and relative amount of dicarboxylic acids are presented for different polyethylene materials after 300 h of photooxidation. The different susceptibilities to photooxidation are clearly seen as there is a considerable difference in both the remaining number average molar mass and the amount of dicarboxylic acids formed in different materials after photooxidation. It is also clearly seen that the formation of dicarboxylic acids is in correlation with the molar mass decrease. The highest amount of dicarboxylic acids is formed in SG1, which contained iron dimethyldithiocarbamate photoinitiator. SG1 was also the material that was most susceptible to photooxidation as determined by the highest molar mass decrease. The lowest amount of dicarboxylic acids was formed in the material SG3, which contained both iron dimethyldithiocarbamate (a photoinitiator) and nickel dibutyldithiocarbamate, which is a photostabilizer. Even the lowest molar mass decrease showed that SG3 was the material that was most stable against photooxidation. Relationship between the Number of Chain Scissions and the Amount of Indicator Products. To investigate if the amount of indicator products can be correlated to the number of chain scissions during oxidation, the relative amount of indicator products formed during thermo- and photooxidation was plotted as a function of the number of chain scissions. The number of chain scissions (n) after different oxidation times was calculated from the following equation: n ) 1/Mn - 1/Mn,0. Figure 4 shows the correlation between the number of chain scissions and the relative amount of indicator products

released during thermooxidation at 190 °C. Three different cases were studied: (a) all dicarboxylic acids and lactones were used as indicator products, (b) all dicarboxylic acids were used as indicator products, or (c) only butanedioic acid was used as an indicator product. The correlation coefficients for the different cases were R2 ) 0.9732, R2 ) 0.9742, and R2 ) 0.9987, respectively. A good correlation was thus found in all cases, but the best correlation was obtained when only butanedioic acid was used as an indicator product. Figure 5 shows the relationship between indicator products and number of chain scissions during thermooxidation at 190 and 230 °C. The obtained correlation coefficients, R2 ) 0.9202 (dicarboxylic acids and lactones), R2 )0.8371 (dicarboxylic acids), and R2 ) 0.7847 (butanedioic acid), were slightly lower when values resulting from aging at two different temperatures were plotted together. It is also seen that if different aging temperatures are used then both dicarboxylic acids and lactones have to be used as indicator products, while only butanedioic acid can be used if only one aging temperature is studied. Figure 6 shows the correlation between the amount of dicarboxylic acids formed and the number of chain scissions during photooxidation. All three figures show that the formation of indicator products is in correlation with the number of chain scissions, and analysis of indicator products could thus be used to predict the degree of degradation or remaining lifetime of the product. Biodegradability of Polyethylene and Related Polymers. Biodegradation of commercial high molecular weight polyethylene proceeds slowly and it is generally accepted that abiotic oxidation is the initial and rate-determining step. The identification of dicarboxylic acids as major abiotic oxidation products of polyethylene has relevance even for the studies concerning the biodegradability of polyethylene. Dicarboxylic acids are very biodegradable and are rapidly bioassimilated by various microorganisms.18 The removal of these low molar mass compounds from the surface of polyethylene causes bioerosion, increases the surface area, and thus accelerates even the abiotic oxidation rate, which in turn accelerates the biodegradation rate. The presence of these

Indicator Products: A Tool for Lifetime Prediction

easily assimilable hydrophilic compounds on the surface of the polymer films also makes it easier for the microorganisms to get attached to the surface of the films19 and may thus even accelerate the biodegradation of higher molar mass compounds or polymer chains. It is also important from the environmental point of view to know that the formed low molar mass compounds are bioassimilated and do not accumulate in the environment. Conclusions There was a clear correlation between the release of indicator products and matrix changes during oxidation of polyethylene and polyethylene vinyl acetate. The degree of degradation in the polymer matrix after thermooxidation as measured by the molar mass decrease and number of chain scissions was in correlation with the amount of dicarboxylic acids and lactones formed during aging, while the degree of degradation after photooxidation could be predicted from the amount of dicarboxylic acids. The amount of butanedioic acid and butyrolactone correlated well with the total amount of dicarboxylic acids and lactones, respectively. Thus, for faster analysis butanedioic acid or butyrolactone alone could be used as an indicator of oxidation. The relative amount of butanedioic acid after different aging times was 12-15% of the relative amount of all dicarboxylic acids, while the relative amount of butyrolactone was approximately 50% of the relative amount of all lactones. The analysis of indicator products is a novel and attractive alternative to conventional techniques for rapid evaluation of the degree of degradation in polymeric materials. The detected correlation between the release of indicator products and matrix changes offers many possibilities in various areas of polymer science. The efficiency of different antioxidants, shelf life, long-term properties, or the remaining service life of polymer products could be predicted from the release of indicator products. The same principal could also be applied to process control and monitoring, acceptance or rejection of raw materials,

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and intermediate and final products. Further studies should be performed to verify the observed correlations on a larger scale and to study the influence of different variables such as aging temperature, sample thickness, and humidity. Development of extraction methods based on, e.g., the SPME technique would provide a portable nondestructive tool for the extraction of indicator products during processing, storage, and service life. References and Notes (1) Hakkarainen, M.; Albertsson, A.-C.; Karlsson, S. J. Chromatogr., A 1996, 741, 251-263. (2) Gro¨ning, M.; Hakkarainen, M. J. Chromatogr., A 2001, 932, 1-11. (3) Albertsson, A.-C.; Barenstedt, C.; Karlsson, S.; Lindberg, T. Polymer 1995, 36, 3075-3083. (4) Albertsson, A.-C.; Erlandsson, B.; Hakkarainen, M.; Karlsson, S. J. EnViron. Polym. Degrad. 1998, 6, 187-195. (5) Willoughby, B. G.; Golby, A.; Davies, J.; Cain, R. Polym. Test. 2003, 22, 553-570. (6) Capone, S.; Epifani, M.; Quaranta, F.; Siciliano, P.; Taurino, A.; Vasanelli, L. Sens. Actuators, B 2001, 78, 174-179. (7) Marsili, R. T. J. Agric. Food Chem. 2000, 48, 3470-3475. (8) Garrigues, S.; Talou, T.; Nesa, D.; Gaset, A. Sens. Actuators, B 2001, 78, 337-344. (9) Potyrailo, R. A.; Wroczynski, R. J.; Morris, W. G.; Bradtke, G. R. Polym. Degrad. Stab. 2004, 83, 375-381. (10) Gro¨ning, M.; Hakkarainen, M. J. Appl. Polym. Sci. 2002, 86, 33963407. (11) Hakkarainen, M.; Albertsson, A.-C.; Karlsson, S. J. Appl. Polym. Sci. 1997, 66, 959-967. (12) Hakkarainen, M.; Albertsson, A.-C.; Karlsson, S. J. EnViron. Polym. Degrad. 1997, 5, 67-73. (13) Karlsson, S.; Hakkarainen, M.; Albertsson, A.-C. Macromolecules 1997, 30, 7721-7728. (14) Pifer, A.; Sen, A. Angew. Chem., Int. Ed. 1998, 37, 3306-3308. (15) Remias, J. E.; Pavlosky, T. A.; Sen, A. Surf. Chem. Catal. 2000, 3, 627-629. (16) Partenheimer, W. Catal. Today 2003, 81, 117-135. (17) Ramkumar, D.; Vaidya, U. R.; Bhattacharya, M.; Hakkarainen, M.; Albertsson, A.-C.; Karlsson, S. Eur. Polym. J. 1996, 32, 999-1010. (18) Albertsson, A.-C.; Erlandsson, B.; Hakkarainen, M.; Karlsson, S. J. EnViron. Polym. Degrad. 1998, 6, 187-195. (19) Hakkarainen, M.; Karlsson, S.; Albertsson, A.-C. J. Appl. Polym. Sci. 2000, 76, 228-239.

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