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Development of a thermogravimetric analysis (TGA) method for quantitative analysis of wood flour and polypropylene in wood plastic composites (WPC)
Thermochimica Acta 543 (2012) 165–171
Contents lists available at SciVerse ScienceDirect
Thermochimica Acta journal homepage: www.elsevier.com/locate/tca
Development of a thermogravimetric analysis (TGA) method for quantitative analysis of wood flour and polypropylene in wood plastic composites (WPC) Helene Jeske a,1 , Arne Schirp b,∗ , Frauke Cornelius b,2 a b
Structural Engineering and Construction, Fraunhofer Institute for Wood Research, Wilhelm-Klauditz-Institute WKI, Bienroder Weg 54 E, 38108 Braunschweig, Germany Technology for Wood Based Materials, Fraunhofer Institute for Wood Research, Wilhelm-Klauditz-Institute WKI, Bienroder Weg 54 E, 38108 Braunschweig, Germany
a r t i c l e
i n f o
Article history: Received 21 February 2012 Received in revised form 11 May 2012 Accepted 19 May 2012 Available online 28 May 2012 Keywords: Thermogravimetric analysis (TGA) Wood plastic composites (WPC)
a b s t r a c t A special thermogravimetric analysis (TGA) method was developed to quantify the mass percentage of wood flour and polypropylene copolymer in wood plastic composites (WPC). Step separation of TGA curves was used for quantitative analysis. Four thermal degradation steps were identified and allocated to the wood and polymer fractions, respectively, based on the TGA curves for the individual components of WPC. TGA curves of the additives, maleic-anhydride-modified polypropylene and lubricant, were assigned to the polymer fraction. The heating rate was fitted to the thermal degradation of the single components by using dynamic and isothermal segments. The results show that the wood flour and polymer fractions can be quantified by TGA. For the final developed method, the deviations from the expected wood flour and polymer fractions were not exceeding 5.4% and 14.3%, respectively. Amounts of wood flour and polymer of WPC with known formulation can be rapidly quantified using TGA, therefore, this method is a useful tool for production control. © 2012 Elsevier B.V. All rights reserved.
1. Introduction The demand for composite materials containing thermoplastics and lignocellulosics, including wood plastic composites (WPC), is steadily growing. These composites are used for non-structural components like decking, fencing, siding, railing, window and door profiles [1]. Wood plastic composites display a variety of interesting characteristics, such as high modulus of elasticity (MOE) and durability, low water uptake and swelling, 3-D-formability and recyclability. A wide range of lignocellulosic materials such as wood particles (flour and fibers) or other natural fibers (hemp and flax) and thermoplastic polymers can be processed and designed into formulations and products for individual usage. The most used thermoplastic materials are polyvinyl chloride (PVC), polyethylene (PE) and isotactic polypropylene (iPP) [2]. With regard to European standardization, a technical specification (DIN CEN/TS 15534, parts 1–3, August 2007) has been published [3–5]. Additionally, several European WPC producers have created a quality seal for decking based on WPC in which product requirements were defined. So far, rapid analytical methods for
∗ Corresponding author. Tel.: +49 531 2155 336; fax: +49 531 351587. E-mail addresses: [email protected] (H. Jeske), [email protected] (A. Schirp), [email protected] (F. Cornelius). 1 Tel.: +49 531 2155 426; fax: +49 531 2155 309. 2 Tel.: +49 531 2155 422; fax: +49 531 351587. 0040-6031/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.tca.2012.05.016
the quantification of the mass percentages of wood flour and polymer in WPC for production quality control and for compositional analysis of samples with unknown formulation are missing. Up to now, several methods for compositional analysis were tested, for example, extraction in solvents using a Soxhlet apparatus, tagging of components, solid state NMR or FTIR spectroscopy [2]. Thermoanalytical methods which have been used are differential scanning calorimetry (DSC), thermogravimetric analysis (TGA) and analytical pyrolysis (Py) [2,6–11]. DSC can be used to identify a polymer based on its melting point whereas TGA is useful to determine thermal stability and degradation behavior of a polymer or composite [12]. TGA compared to DSC offers the advantage to quantify the wood flour and polymer contents in WPC without the need to establish calibration curves, however, knowledge about the composition of the formulation is required. TGA measurements of individual WPC components provide information regarding their thermal degradation behavior, therefore, TGA methods can be tailored for individual WPC formulations. Reichert and Korte [6] used DSC measurements and showed that on the basis of the peak area of the melting point, quantification of the polymer fraction is possible. However, DSC cannot be used for quantification of the polymer if a polymer and additives with similar melting points are used in WPC. Jeske et al. [8] reported that a specific calibration test series for each polymer in WPC is necessary for DSC. Renneckar et al. [2] developed a method for the quantitative determination of WPC by using the derivative curve of the TGA curve (DTG). The area of DTG peaks corresponded to the amount of polyolefin – here, PE was used, and no additives were included.
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Table 1 Composition of WPC test samples used in this work. Sample term
Wood flour (wt.%)
PP copolymer (wt.%)
MAPP (wt.%)
Lubricant (wt.%)
WPC 1 WPC 2 WPC 3
70 70 50
28.5 25.5 45.5
0 3 3
1.5 1.5 1.5
The measurements were done under nitrogen atmosphere only, and two methods for heating rate parameters were chosen by Renneckar et al. On the one hand the heating rate was constant and on the other hand a high resolution method (Hi-ResTM ) was used. The quantification was accomplished for the polymer content only. By using a single set of calibration parameters, the error range for the polymer was determined to be 12%. Phillips and Blazey [9] quantified the composition of starch-filled thermoplastics with TGA by switching the atmosphere and by using a dynamic method. First, nitrogen was chosen until 550 ◦ C was reached, and then oxygen was used. A quantification of the components polymer and starch is possible with that technique because the temperature range of degradation differs for each component. Thermal decomposition of WPC depends on wood species, amount of wood, particle size, moisture content, quantity and type of polymers, coupling agents, lubricants and other additives. Hemicelluloses, cellulose and lignin primarily decompose between 150–350 ◦ C and 250–500 ◦ C whereas the thermal degradation of PP and HDPE begins at 472 ◦ C respectively 517 ◦ C [13]. Therefore, TGA can be used for quantitative determination of WPC components (wood and thermoplastic) if the components are known. For this purpose, a TGA method was developed in this project for three WPC formulations with 50% and 70% wood filler, respectively, by using inert and reactive purge gases and by optimizing heating rate, dynamic and isothermal heating and duration of isothermal steps. 2. Experimental 2.1. Materials and methods Polypropylene fibers with ethylene as copolymer were obtained from Asota (Ges.m.b.H, Linz, Austria; 6 mm length; melt flow index (MFI) of 10–12 g/10 min), wood flour (Lignocel® BK 40/90) from Rettenmaier & Söhne GmbH + Co. KG (Rosenberg, Germany), maleic anhydride grafted polypropylene (MAPP, Licocene PPMA 6252) from Clariant (Gersthofen, Germany) and lubricant (ester wax, Licomont ET 141) from Clariant. Generally, the processing of WPC samples was a two-stage process. In the first processing step, the raw materials were compounded in a heating mixer (TSHK 100, Loedige) followed by cooling in a plough-blade mixer (FM 130 DS, Loedige). Compounds were extruded into a tape profile with the dimensions of 4 mm × 40 mm using a conical, counter rotating twin-screw extruder with 54 mm screw diameter (Battenfeld Minibex 2-54C). Tapes were cut into small segments and ground using a laboratory mill to obtain homogeneous samples for analysis. TGA measurements were performed using a TGA851e (METTLER TOLEDO, STARe SW 9.30) and 70 L aluminum oxide pans. The measurements were done under nitrogen and/or oxygen (quality 5.0) atmosphere and a gas flow of 50 mL/min for both gases. The heating rates were varied between 5 ◦ C/min and 50 ◦ C/min, and the dynamic steps were interrupted at different temperatures for set periods. At first, all components were measured separately using TGA either in nitrogen or in oxygen atmosphere. Afterwards, method development was accomplished with WPC test pieces of known composition (Table 1). Initially, all components were measured un-dried. However, it could be observed that the content of
moisture in wood floor and WPC scattered strongly (Figs. 3 and 5). Because of that, quantification tests of WPC with the optimized test methods no. 3–5 were done with well-dried samples. Samples were dried at 200 ◦ C for 10 min under inert atmosphere in the TGA instrument and immediately measured. Specific methods are described in the respective results section. Each WPC sample was measured three times. For evaluation, TGA curves were normalized based on starting weight. Step separation was achieved by using METTLER TOLEDO, STARe Software which was additionally supported by using DTG curves (first derivation of thermal gravimetry) which describe the mass loss rate (dm/dt). The SDTA curve (single differential thermal analysis) is the curve of a simultaneous differential thermal analysis and represents the heat flow (endothermic or exothermic reactions) during a measurement. The beginning of thermal degradation can be determined before it is seen in TGA curve. 3. Results and discussion 3.1. TGA–DTG of single components Initially, the components of the WPC formulations were analyzed with two dynamic methods under oxygen or nitrogen atmosphere (Table 2). The thermal degradation behavior of PPcopolymer is shown in Figs. 1 and 2. In contrast to results under exposure of PP-copolymer to oxygen atmosphere, the main thermal degradation of PP-copolymer (99.3 wt.%) under nitrogen atmosphere occurs in a narrow temperature region between 300 ◦ C and 500 ◦ C (Fig. 1; black curve). In addition, it can be seen in Fig. 1 that thermal degradation of PP-copolymer begins later under nitrogen atmosphere than under oxygen atmosphere. In Fig. 2, TGA-, DTG- and SDTA-curves of the PP-copolymer are shown. The first peak in the SDTA curve at 142 ◦ C represents the melting point of the PP-copolymer. The onset of the next peak in the SDTA curve is at approximately 310 ◦ C and represents the starting point of the thermal degradation of the polymer. At this temperature, however, no significant mass loss is visible yet in the TGA curve. The area under the second peak in the SDTA curve at 459 ◦ C represents the energy required for thermal degradation and evaporation of released gases. This second peak of the STDA curve almost coincides with the maximum mass loss rate which can be seen as the peak in the DTG curve at approximately 462 ◦ C. For a good step separation and quantification of single components in WPC it is important that the degradation of wood flour occurs in a different temperature region than that of the polymer. The thermal degradation behavior of wood flour is shown in Fig. 3.
Table 2 The dynamic methods under oxygen or nitrogen atmosphere in a temperature range between 25 ◦ C and 800 ◦ C used for TGA of individual WPC components. No.
Segment I
Segment II
1
O2 -atm. 10 ◦ C/min 25 ◦ C until 800 ◦ C
Not applicable
2
N2 -atm. 10 ◦ C/min 25 ◦ C until 600 ◦ C
O2 -atm. 10 ◦ C/min 600 ◦ C until 800 ◦ C
H. Jeske et al. / Thermochimica Acta 543 (2012) 165–171
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Fig. 1. TGA curves of PP-copolymer determined with method no. 1 (red curve) and no. 2 (black curve). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)
The thermal degradation of wood flour occurs over a wide temperature range in both oxygen (250–430 ◦ C) and nitrogen atmosphere (250–600 ◦ C). But in contrast to the PP-copolymer, some mass loss of wood flour in nitrogen occurs in the temperature regions between 25 ◦ C and 150 ◦ C, 150 ◦ C and 250 ◦ C and between 400 ◦ C and 600 ◦ C while the mass loss of the PP-copolymer occurs mainly between 300 ◦ C and 500 ◦ C. Wood degradation in the
temperature range from 200 ◦ C until 350 ◦ C is assigned to hemicellulose and cellulose degradation and from 250 ◦ C until 500 ◦ C to lignin degradation [14]. Wood and PP-copolymer degradation in method no. 1 overlap between 200 ◦ C and 350 ◦ C which means that step separation for wood and PP-copolymer could not be achieved using method no. 1. However, when method no. 2 was used, first wood flour was degraded between 200 ◦ C and ca. 390 ◦ C, then
Fig. 2. TGA (black curve), DTG (blue curve) and SDTA (red curve) of PP-copolymer determined with method no. 2. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)
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Fig. 3. TGA curves of wood flour determined with methods no. 1 (red curve) and no. 2 (black curve). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)
PP-copolymer was degraded between ca. 390 ◦ C and 500 ◦ C, and finally, by changing the inert atmosphere to oxidative atmosphere, the char of wood flour burnt residue-free. With the atmosphere change from nitrogen to oxygen for both ingredients wood flour and polymer, a complete thermal degradation is ensured. In the literature, mainly inert atmosphere is used because only one of the ingredients (wood or polymer) is to be quantified [2,6,8,10]. The TGA curves of all individual WPC components determined with method no. 2 are shown in Fig. 4. It can be seen that the thermal degradation of MAPP and wax is more similar to the TGA and SDTA curves of PP-copolymer than to the TGA curve of wood flour. Consequently, the MAPP and wax curves were assigned to the polymer fraction because step separation for the additives used is not possible in the TGA curves. 3.2. TGA–DTG of WPC 1 and 2 Next, WPC 1 and 2 were measured with method no. 2 to determine the amounts of the polymer and wood flour fractions (Fig. 5). The two curves can each be divided into four steps of mass loss [15]. Within each step, a specific weight loss occurs (Table 3) so the sum of all weight losses should theoretically add up to 70% for the wood flour in WPC 1 and 2. However, this theoretical weight loss value is not reached because a 100% separation of individual components cannot be achieved due to the overlap of the curves (Fig. 4). Step separation between steps 2 and 3 is not possible because these steps
Table 3 Polymer and wood flour fractions of WPC 1 and 2 determined using method no. 2 and step separation (expected results: wood flour content 70% and polymer fraction, including MAPP and lubricant, 30%).
WPC 1 Wood flour fraction Polymer fraction WPC 2 Wood flour fraction Polymer fraction
Step 1
Step 2
5.6%
43.2%
Step 3
Step 4
Sum
10.8%
60% 40%
11.3%
62% 38%
40.0% 6.0%
45% 37.6%
overlap. Each peak of the DTG-curve should reach the baseline for a detailed analysis (Fig. 5). Using dynamic heating, at 400 ◦ C no exact step separation is possible. Therefore, an isothermal step between 300 and 500 ◦ C was introduced. This was done next in Section 3.3. In Table 3, steps 1, 2 and 4 were assigned to the mass loss of wood flour while step 3 was assigned to the polymer fraction, based on the TGA-measurements of the single components (wood flour, PPcopolymer, MAPP and wax; Fig. 4). It can be seen that the results deviate from the expected amounts for the wood and polymer fractions strongly.
3.3. Optimization of the TGA-method for WPC 1–3 Three additional methods were developed with dynamic heating steps at 340 ◦ C and 350 ◦ C which were interrupted by an isothermal segment for 15 min and with heating rates set at 5, 10, 20 and 50 ◦ C/min (Table 4). The final developed method consists of four segments (Table 4, no. 5). The first step starts with a dynamic heating rate (50 ◦ C/min) from 25 ◦ C and ends with 350 ◦ C. The second step is an isothermal phase at 350 ◦ C for 15 min. The third step is a dynamic heating phase with a heating rate of 50 ◦ C/min from 350 ◦ C until 550 ◦ C. Finally, the atmosphere is switched from the inert gas nitrogen to the reactive gas oxygen. The last step is performed with a heating rate of 50 ◦ C/min until 700 ◦ C by burning the residue. For method no. 5, the thermal degradation of WPC 1–3 is shown in Fig. 6. The thermal degradation of all WPC samples differs which can be well seen in the DTG-curves at approximately 500 ◦ C whereas the TGA curves show that only WPC 1 and 2 are similar. In addition, step results analysis shows the differences in composition between all three formulations well. In Table 5 the results of the step separation and weight loss measurements for methods no. 3–5 (Table 4) were compared. In the left column the actual contents (wt.%) of wood flour and polymer are shown and in the remaining columns values of the analyzed average contents and standard deviations are represented. Additionally, the deviations in relation to analyzed and actual contents are listed.
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Fig. 4. TGA curves of wood flour (black curve), PP-Copolymer (blue curve), MAPP (green curve) and wax (red curve) determined with method no. 2. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)
Thermal degradation (thermodynamic, kinetic and reaction mechanisms) is complex and depends on time, temperature, concentration and migration within the sample. Small changes in one of these factors may cause various effects in thermal degradation and consequently, different ratios for each method and sample. Using TGA, the influence of these various factors on thermal degradation mechanisms cannot be determined on a molecular
level. However, TGA can be used for overall quantification of the wood and polymer contents. Research regarding the thermodynamic, kinetic and reaction mechanisms are desirable for a deeper understanding of WPC thermal degradation and for analytical method development. In Table 5 the results show that method no. 3 can be used for WPC 1 and 2 but not for WPC 3. Results for method no. 4 are very
Fig. 5. TGA–DTG-curves of WPC 1 (black curve) and 2 (blue curve) determined with method no. 2. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)
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Table 4 Overview of optimized test methods with different segments. No.
Segment I
Segment II
Segment III
Segment IV
Segment V
3
N2 -atm. 20 ◦ C/min 25–350 ◦ C
N2 -atm. Isothermal 30 min 350 ◦ C
N2 -atm. 5 ◦ C/min 350–500 ◦ C
O2 -atm. 10 ◦ C/min 500–550 ◦ C
O2 -atm. Isothermal 3 min 550 ◦ C
4
N2 -atm. 20 ◦ C/min 25–340 ◦ C
N2 -atm. Isothermal 15 min 340 ◦ C
N2 -atm. 50 ◦ C/min 340–550 ◦ C
O2 -atm. 50 ◦ C/min 550–700 ◦ C
Not applicable
5
N2 -atm. 50 ◦ C/min 25–350 ◦ C
N2 -atm. Isothermal 15 min 350 ◦ C
N2 -atm. 50 ◦ C/min 350–550 ◦ C
O2 -atm. 50 ◦ C/min 550–700 ◦ C
Not applicable
Table 5 Results of TGA curve step separation for WPC 1–3. For a description of methods no. 3–5, please refer to Table 4. Actual contents
No. 3
WPC 1 Wood flour fraction (70) Polymer fraction (30)
66.2 ± 0.5 34.2 ± 0.2
5.4 14.0
WPC 2 Wood flour fraction (70) Polymer fraction (30)
71.1 ± 2.4 29.1 ± 3.5
WPC 3 Wood flour fraction (50) Polymer fraction (50)
64.3 ± 7.4 37.0 ± 7.4
a
Deviation for no. 3
No. 4
Deviation for no. 4
No. 5
Deviation for no. 5
n.d.a n.d.
n.d. n.d.
66.5 ± 0.5 34.3 ± 0.5
5.0 14.3
1.6 3.0
66.2 ± 0.9 33.7 ± 0.6
5.4 12.3
68.0 ± 0.5 34.0 ± 0.2
2.9 13.3
28.6 26.0
50.7 ± 1.2 50.1 ± 1.1
1.4 0.2
53.8 ± 1.5 48.4 ± 1.8
7.6 3.2
n.d., not determined.
good for WPC 3, but less for WPC 2 in comparison to method no. 3 and no. 5. For method no. 4 the deviation values are smaller for WPC 3 but bigger for WPC 2. In relation to all three WPC samples method no. 5 shows good agreements between analyzed and actual fractions of wood flour and polymer. Renneckar showed 12% deviation of polymer in WPC by using the high resolution method [2]. The differences between Renneckar’s work and this work are, that on the one hand different
polymer were used – Renneckar used PE and here PP-copolymer was applied and on the other hand the polymer fraction in this work contains additives like MAPP and wax, whereas Renneckar used no additives. Additionally, with the introduced method in this work fractions of polymer and wood flour can be calculated. The degradation of polymers PP and HDPE begins at 472 ◦ C respectively 517 ◦ C [13]. The step separation in TGA curve of the thermal degradation of wood flour and PE is connected with a smaller deviation because
Fig. 6. TGA–DTG-curves of WPC 1 (red curve), WPC 2 (green curve), and WPC 3 (blue curve) determined with method no. 5. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)
H. Jeske et al. / Thermochimica Acta 543 (2012) 165–171
the degradation of wood flour is approximately zero during the degradation of PE polymer. It is not for PP and PP-copolymer (see Fig. 4). Fuad et al. [16] used also thermogravimetric analysis for determination of filler content in rice husk ash and wood-based composites based on polypropylene. They showed good agreements for the component oil palm wood flour with a deviation of 5.8% between analyzed and actual filler contents by using a dynamic method (heating rate 20 ◦ C/min, 25 ◦ C until 550 ◦ C, air). In Fuad’s work, the thermal degradation of used filler and polymer showed small overlapping areas. The calculation of contents in overlapping degradation areas was corrected by a coefficient. In our work, no coefficient was used because nonlinear effects are possible which means that the component ratio of the formulations influences the thermal degradation by generating different decomposition products. In this context Sharypov et al. [10] determined a nonlinear dependence between the quantity of the individual formulation components and their effect on thermal degradation of the composites. 4. Conclusion The developed thermoanalytical method for WPC shows that it is possible to quantify the mass percentage for wood flour and polypropylene copolymer in wood plastic composites (WPC) by using thermogravimetric analysis (TGA–DTG). The developed method shows good agreements between analyzed and actual fractions of wood flour and polymer in spite of the complex thermal degradation behavior of wood plastic composites. The TGA method can be tailored according to the formulation composition. Maxima deviations from the actual wood and polymer contents were 5.4% and 14.3%, respectively. It is possible to optimize the deviation of analyzed fraction by switching the isothermal step between 340 and 350 ◦ C, changing the heating rate and adjusting the isothermal period. An accurate determination of individual components in WPC by using TGA is limitedly possible, because of the wide range of continuous thermal degradation of wood between 200 and 700 ◦ C in nitrogen and oxygen and the various thermal decomposition patterns of polymers. WPC with known composition can be rapidly quantified with TGA measurements, therefore, TGA is a useful tool for WPC production control. TGA measurements of WPC with unknown composition are difficult to interpret. For determination of composition of WPC with unknown formulation, other analytical methods such as Soxhlet extraction, infrared spectroscopy and DSC in combination should be used.
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Acknowledgements We thank the German Institute for Standardization DIN (Deutsches Institut für Normung e.V.), Berlin, for contribution of funding from the program “INS – Innovation mit Normen und Standards”. References [1] C. Clemons, Wood–plastic composites in the United States: interfacing of two industries, Forest Prod. J. 52 (2002) 10–18. [2] S. Renneckar, A.G. Zink-Sharp, T.C. Ward, W.G. Glasser, Compositional analysis of thermoplastic wood composites by TGA, J. Appl. Polym. Sci. 93 (2004) 1484–1492. [3] D.C.T. 15534-1, Wood–plastics composites (WPC), Part 1: Test methods for characterisation of WPC materials and products, 2007 (German version CEN/TS 15534-1). [4] D.C.T. 15534-2, Wood–plastics composites (WPC), Part 2: Characterisation of WPC materials, 2007 (German version CEN/TS 15534-2). [5] D.C.T. 15534-3, Wood–plastics composites (WPC), Part 3: Characterisation of WPC products, 2007 (German version CEN/TS 15534-3). [6] A. Reichert, H. Korte, Methoden zur Bestimung der Masseanteile in Verbundwerkstoffen aus thermoplastischen Polymeren und Lignocellulosen, Holztechnologie 50 (2009) 38–43. [7] J.R. Araujo, W.R. Waldman, M.A.D. Paoli, Thermal properties of high density polyethylene composites with natural fibres: coupling agent effect, Polym. Degrad. Stab. 93 (2008) 1770–1775. [8] H. Jeske, A. Schirp, F. Cornelius, Quantifizierung des Massenanteils von Polypropylen (PP)-Copolymer in WPC (Wood Plastic Composites) mittels DSC (dynamic scanning calorimetry), Holztechnologie 52 (2011) 51–53. [9] E.A. Phillips, S.D. Blazey, Quantitative thermogravimetric analysis of starch contents in several polymer systems, Thermochim. Acta 192 (1991) 191–197. [10] V.I. Sharypov, N. Marin, N.G. Beregovtsova, S.V. Baryshnikov, B.N. Kuznetsov, V.L. Cebolla, J.V. Weber, Co-pyrolysis of wood biomass and synthetic polymer mixtures. Part I: influence of experimental conditions on the evolution of solids, liquids and gases, J. Anal. Appl. Pyrolysis 64 (2002) 15–28. [11] M. Windt, D. Meier, R. Lehnen, Quantification of polypropylenw (PP) in wood plastic composites (WPCs) by analytical pyrolysis (Py) and differential scanning calorimetry (DSC), Holzforschung 65 (2011) 199–207. [12] G.W. Ehrenstein, G. Riedel, P. Trawiel, Praxis der Thermischen Analyse von Kunststoffen, second ed., Carl Hanser Verlag, München, 2003. [13] A. Kaboorani, Effects of formulation design on thermal properties of wood/thermoplastic composites, J. Compos. Mater. 44 (2010) 2205–2215. [14] R. Bodîrlâu, C.-A. Teaca, I. Spiridon, Thermal investigation upon various composite materials, Rev. Roum. Chim. 52 (2007) 153–158. [15] W.F. Hemminger, H.K. Cammenga, Methoden der Thermischen Analyse, first ed., Springer-Verlag, Berlin, 1989. [16] M.Y.A. Fuad, M.J. Zaini, M. Jamaludin, Z.A.M. Ishak, A.K.M. Omar, Determination of filler content in rice husk ash and wood-based composites by thermogravimetric analysis, J. Appl. Polym. Sci. 51 (1994) 1875–1886.
Contents lists available at SciVerse ScienceDirect
Thermochimica Acta journal homepage: www.elsevier.com/locate/tca
Development of a thermogravimetric analysis (TGA) method for quantitative analysis of wood flour and polypropylene in wood plastic composites (WPC) Helene Jeske a,1 , Arne Schirp b,∗ , Frauke Cornelius b,2 a b
Structural Engineering and Construction, Fraunhofer Institute for Wood Research, Wilhelm-Klauditz-Institute WKI, Bienroder Weg 54 E, 38108 Braunschweig, Germany Technology for Wood Based Materials, Fraunhofer Institute for Wood Research, Wilhelm-Klauditz-Institute WKI, Bienroder Weg 54 E, 38108 Braunschweig, Germany
a r t i c l e
i n f o
Article history: Received 21 February 2012 Received in revised form 11 May 2012 Accepted 19 May 2012 Available online 28 May 2012 Keywords: Thermogravimetric analysis (TGA) Wood plastic composites (WPC)
a b s t r a c t A special thermogravimetric analysis (TGA) method was developed to quantify the mass percentage of wood flour and polypropylene copolymer in wood plastic composites (WPC). Step separation of TGA curves was used for quantitative analysis. Four thermal degradation steps were identified and allocated to the wood and polymer fractions, respectively, based on the TGA curves for the individual components of WPC. TGA curves of the additives, maleic-anhydride-modified polypropylene and lubricant, were assigned to the polymer fraction. The heating rate was fitted to the thermal degradation of the single components by using dynamic and isothermal segments. The results show that the wood flour and polymer fractions can be quantified by TGA. For the final developed method, the deviations from the expected wood flour and polymer fractions were not exceeding 5.4% and 14.3%, respectively. Amounts of wood flour and polymer of WPC with known formulation can be rapidly quantified using TGA, therefore, this method is a useful tool for production control. © 2012 Elsevier B.V. All rights reserved.
1. Introduction The demand for composite materials containing thermoplastics and lignocellulosics, including wood plastic composites (WPC), is steadily growing. These composites are used for non-structural components like decking, fencing, siding, railing, window and door profiles [1]. Wood plastic composites display a variety of interesting characteristics, such as high modulus of elasticity (MOE) and durability, low water uptake and swelling, 3-D-formability and recyclability. A wide range of lignocellulosic materials such as wood particles (flour and fibers) or other natural fibers (hemp and flax) and thermoplastic polymers can be processed and designed into formulations and products for individual usage. The most used thermoplastic materials are polyvinyl chloride (PVC), polyethylene (PE) and isotactic polypropylene (iPP) [2]. With regard to European standardization, a technical specification (DIN CEN/TS 15534, parts 1–3, August 2007) has been published [3–5]. Additionally, several European WPC producers have created a quality seal for decking based on WPC in which product requirements were defined. So far, rapid analytical methods for
∗ Corresponding author. Tel.: +49 531 2155 336; fax: +49 531 351587. E-mail addresses: [email protected] (H. Jeske), [email protected] (A. Schirp), [email protected] (F. Cornelius). 1 Tel.: +49 531 2155 426; fax: +49 531 2155 309. 2 Tel.: +49 531 2155 422; fax: +49 531 351587. 0040-6031/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.tca.2012.05.016
the quantification of the mass percentages of wood flour and polymer in WPC for production quality control and for compositional analysis of samples with unknown formulation are missing. Up to now, several methods for compositional analysis were tested, for example, extraction in solvents using a Soxhlet apparatus, tagging of components, solid state NMR or FTIR spectroscopy [2]. Thermoanalytical methods which have been used are differential scanning calorimetry (DSC), thermogravimetric analysis (TGA) and analytical pyrolysis (Py) [2,6–11]. DSC can be used to identify a polymer based on its melting point whereas TGA is useful to determine thermal stability and degradation behavior of a polymer or composite [12]. TGA compared to DSC offers the advantage to quantify the wood flour and polymer contents in WPC without the need to establish calibration curves, however, knowledge about the composition of the formulation is required. TGA measurements of individual WPC components provide information regarding their thermal degradation behavior, therefore, TGA methods can be tailored for individual WPC formulations. Reichert and Korte [6] used DSC measurements and showed that on the basis of the peak area of the melting point, quantification of the polymer fraction is possible. However, DSC cannot be used for quantification of the polymer if a polymer and additives with similar melting points are used in WPC. Jeske et al. [8] reported that a specific calibration test series for each polymer in WPC is necessary for DSC. Renneckar et al. [2] developed a method for the quantitative determination of WPC by using the derivative curve of the TGA curve (DTG). The area of DTG peaks corresponded to the amount of polyolefin – here, PE was used, and no additives were included.
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Table 1 Composition of WPC test samples used in this work. Sample term
Wood flour (wt.%)
PP copolymer (wt.%)
MAPP (wt.%)
Lubricant (wt.%)
WPC 1 WPC 2 WPC 3
70 70 50
28.5 25.5 45.5
0 3 3
1.5 1.5 1.5
The measurements were done under nitrogen atmosphere only, and two methods for heating rate parameters were chosen by Renneckar et al. On the one hand the heating rate was constant and on the other hand a high resolution method (Hi-ResTM ) was used. The quantification was accomplished for the polymer content only. By using a single set of calibration parameters, the error range for the polymer was determined to be 12%. Phillips and Blazey [9] quantified the composition of starch-filled thermoplastics with TGA by switching the atmosphere and by using a dynamic method. First, nitrogen was chosen until 550 ◦ C was reached, and then oxygen was used. A quantification of the components polymer and starch is possible with that technique because the temperature range of degradation differs for each component. Thermal decomposition of WPC depends on wood species, amount of wood, particle size, moisture content, quantity and type of polymers, coupling agents, lubricants and other additives. Hemicelluloses, cellulose and lignin primarily decompose between 150–350 ◦ C and 250–500 ◦ C whereas the thermal degradation of PP and HDPE begins at 472 ◦ C respectively 517 ◦ C [13]. Therefore, TGA can be used for quantitative determination of WPC components (wood and thermoplastic) if the components are known. For this purpose, a TGA method was developed in this project for three WPC formulations with 50% and 70% wood filler, respectively, by using inert and reactive purge gases and by optimizing heating rate, dynamic and isothermal heating and duration of isothermal steps. 2. Experimental 2.1. Materials and methods Polypropylene fibers with ethylene as copolymer were obtained from Asota (Ges.m.b.H, Linz, Austria; 6 mm length; melt flow index (MFI) of 10–12 g/10 min), wood flour (Lignocel® BK 40/90) from Rettenmaier & Söhne GmbH + Co. KG (Rosenberg, Germany), maleic anhydride grafted polypropylene (MAPP, Licocene PPMA 6252) from Clariant (Gersthofen, Germany) and lubricant (ester wax, Licomont ET 141) from Clariant. Generally, the processing of WPC samples was a two-stage process. In the first processing step, the raw materials were compounded in a heating mixer (TSHK 100, Loedige) followed by cooling in a plough-blade mixer (FM 130 DS, Loedige). Compounds were extruded into a tape profile with the dimensions of 4 mm × 40 mm using a conical, counter rotating twin-screw extruder with 54 mm screw diameter (Battenfeld Minibex 2-54C). Tapes were cut into small segments and ground using a laboratory mill to obtain homogeneous samples for analysis. TGA measurements were performed using a TGA851e (METTLER TOLEDO, STARe SW 9.30) and 70 L aluminum oxide pans. The measurements were done under nitrogen and/or oxygen (quality 5.0) atmosphere and a gas flow of 50 mL/min for both gases. The heating rates were varied between 5 ◦ C/min and 50 ◦ C/min, and the dynamic steps were interrupted at different temperatures for set periods. At first, all components were measured separately using TGA either in nitrogen or in oxygen atmosphere. Afterwards, method development was accomplished with WPC test pieces of known composition (Table 1). Initially, all components were measured un-dried. However, it could be observed that the content of
moisture in wood floor and WPC scattered strongly (Figs. 3 and 5). Because of that, quantification tests of WPC with the optimized test methods no. 3–5 were done with well-dried samples. Samples were dried at 200 ◦ C for 10 min under inert atmosphere in the TGA instrument and immediately measured. Specific methods are described in the respective results section. Each WPC sample was measured three times. For evaluation, TGA curves were normalized based on starting weight. Step separation was achieved by using METTLER TOLEDO, STARe Software which was additionally supported by using DTG curves (first derivation of thermal gravimetry) which describe the mass loss rate (dm/dt). The SDTA curve (single differential thermal analysis) is the curve of a simultaneous differential thermal analysis and represents the heat flow (endothermic or exothermic reactions) during a measurement. The beginning of thermal degradation can be determined before it is seen in TGA curve. 3. Results and discussion 3.1. TGA–DTG of single components Initially, the components of the WPC formulations were analyzed with two dynamic methods under oxygen or nitrogen atmosphere (Table 2). The thermal degradation behavior of PPcopolymer is shown in Figs. 1 and 2. In contrast to results under exposure of PP-copolymer to oxygen atmosphere, the main thermal degradation of PP-copolymer (99.3 wt.%) under nitrogen atmosphere occurs in a narrow temperature region between 300 ◦ C and 500 ◦ C (Fig. 1; black curve). In addition, it can be seen in Fig. 1 that thermal degradation of PP-copolymer begins later under nitrogen atmosphere than under oxygen atmosphere. In Fig. 2, TGA-, DTG- and SDTA-curves of the PP-copolymer are shown. The first peak in the SDTA curve at 142 ◦ C represents the melting point of the PP-copolymer. The onset of the next peak in the SDTA curve is at approximately 310 ◦ C and represents the starting point of the thermal degradation of the polymer. At this temperature, however, no significant mass loss is visible yet in the TGA curve. The area under the second peak in the SDTA curve at 459 ◦ C represents the energy required for thermal degradation and evaporation of released gases. This second peak of the STDA curve almost coincides with the maximum mass loss rate which can be seen as the peak in the DTG curve at approximately 462 ◦ C. For a good step separation and quantification of single components in WPC it is important that the degradation of wood flour occurs in a different temperature region than that of the polymer. The thermal degradation behavior of wood flour is shown in Fig. 3.
Table 2 The dynamic methods under oxygen or nitrogen atmosphere in a temperature range between 25 ◦ C and 800 ◦ C used for TGA of individual WPC components. No.
Segment I
Segment II
1
O2 -atm. 10 ◦ C/min 25 ◦ C until 800 ◦ C
Not applicable
2
N2 -atm. 10 ◦ C/min 25 ◦ C until 600 ◦ C
O2 -atm. 10 ◦ C/min 600 ◦ C until 800 ◦ C
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Fig. 1. TGA curves of PP-copolymer determined with method no. 1 (red curve) and no. 2 (black curve). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)
The thermal degradation of wood flour occurs over a wide temperature range in both oxygen (250–430 ◦ C) and nitrogen atmosphere (250–600 ◦ C). But in contrast to the PP-copolymer, some mass loss of wood flour in nitrogen occurs in the temperature regions between 25 ◦ C and 150 ◦ C, 150 ◦ C and 250 ◦ C and between 400 ◦ C and 600 ◦ C while the mass loss of the PP-copolymer occurs mainly between 300 ◦ C and 500 ◦ C. Wood degradation in the
temperature range from 200 ◦ C until 350 ◦ C is assigned to hemicellulose and cellulose degradation and from 250 ◦ C until 500 ◦ C to lignin degradation [14]. Wood and PP-copolymer degradation in method no. 1 overlap between 200 ◦ C and 350 ◦ C which means that step separation for wood and PP-copolymer could not be achieved using method no. 1. However, when method no. 2 was used, first wood flour was degraded between 200 ◦ C and ca. 390 ◦ C, then
Fig. 2. TGA (black curve), DTG (blue curve) and SDTA (red curve) of PP-copolymer determined with method no. 2. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)
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Fig. 3. TGA curves of wood flour determined with methods no. 1 (red curve) and no. 2 (black curve). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)
PP-copolymer was degraded between ca. 390 ◦ C and 500 ◦ C, and finally, by changing the inert atmosphere to oxidative atmosphere, the char of wood flour burnt residue-free. With the atmosphere change from nitrogen to oxygen for both ingredients wood flour and polymer, a complete thermal degradation is ensured. In the literature, mainly inert atmosphere is used because only one of the ingredients (wood or polymer) is to be quantified [2,6,8,10]. The TGA curves of all individual WPC components determined with method no. 2 are shown in Fig. 4. It can be seen that the thermal degradation of MAPP and wax is more similar to the TGA and SDTA curves of PP-copolymer than to the TGA curve of wood flour. Consequently, the MAPP and wax curves were assigned to the polymer fraction because step separation for the additives used is not possible in the TGA curves. 3.2. TGA–DTG of WPC 1 and 2 Next, WPC 1 and 2 were measured with method no. 2 to determine the amounts of the polymer and wood flour fractions (Fig. 5). The two curves can each be divided into four steps of mass loss [15]. Within each step, a specific weight loss occurs (Table 3) so the sum of all weight losses should theoretically add up to 70% for the wood flour in WPC 1 and 2. However, this theoretical weight loss value is not reached because a 100% separation of individual components cannot be achieved due to the overlap of the curves (Fig. 4). Step separation between steps 2 and 3 is not possible because these steps
Table 3 Polymer and wood flour fractions of WPC 1 and 2 determined using method no. 2 and step separation (expected results: wood flour content 70% and polymer fraction, including MAPP and lubricant, 30%).
WPC 1 Wood flour fraction Polymer fraction WPC 2 Wood flour fraction Polymer fraction
Step 1
Step 2
5.6%
43.2%
Step 3
Step 4
Sum
10.8%
60% 40%
11.3%
62% 38%
40.0% 6.0%
45% 37.6%
overlap. Each peak of the DTG-curve should reach the baseline for a detailed analysis (Fig. 5). Using dynamic heating, at 400 ◦ C no exact step separation is possible. Therefore, an isothermal step between 300 and 500 ◦ C was introduced. This was done next in Section 3.3. In Table 3, steps 1, 2 and 4 were assigned to the mass loss of wood flour while step 3 was assigned to the polymer fraction, based on the TGA-measurements of the single components (wood flour, PPcopolymer, MAPP and wax; Fig. 4). It can be seen that the results deviate from the expected amounts for the wood and polymer fractions strongly.
3.3. Optimization of the TGA-method for WPC 1–3 Three additional methods were developed with dynamic heating steps at 340 ◦ C and 350 ◦ C which were interrupted by an isothermal segment for 15 min and with heating rates set at 5, 10, 20 and 50 ◦ C/min (Table 4). The final developed method consists of four segments (Table 4, no. 5). The first step starts with a dynamic heating rate (50 ◦ C/min) from 25 ◦ C and ends with 350 ◦ C. The second step is an isothermal phase at 350 ◦ C for 15 min. The third step is a dynamic heating phase with a heating rate of 50 ◦ C/min from 350 ◦ C until 550 ◦ C. Finally, the atmosphere is switched from the inert gas nitrogen to the reactive gas oxygen. The last step is performed with a heating rate of 50 ◦ C/min until 700 ◦ C by burning the residue. For method no. 5, the thermal degradation of WPC 1–3 is shown in Fig. 6. The thermal degradation of all WPC samples differs which can be well seen in the DTG-curves at approximately 500 ◦ C whereas the TGA curves show that only WPC 1 and 2 are similar. In addition, step results analysis shows the differences in composition between all three formulations well. In Table 5 the results of the step separation and weight loss measurements for methods no. 3–5 (Table 4) were compared. In the left column the actual contents (wt.%) of wood flour and polymer are shown and in the remaining columns values of the analyzed average contents and standard deviations are represented. Additionally, the deviations in relation to analyzed and actual contents are listed.
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Fig. 4. TGA curves of wood flour (black curve), PP-Copolymer (blue curve), MAPP (green curve) and wax (red curve) determined with method no. 2. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)
Thermal degradation (thermodynamic, kinetic and reaction mechanisms) is complex and depends on time, temperature, concentration and migration within the sample. Small changes in one of these factors may cause various effects in thermal degradation and consequently, different ratios for each method and sample. Using TGA, the influence of these various factors on thermal degradation mechanisms cannot be determined on a molecular
level. However, TGA can be used for overall quantification of the wood and polymer contents. Research regarding the thermodynamic, kinetic and reaction mechanisms are desirable for a deeper understanding of WPC thermal degradation and for analytical method development. In Table 5 the results show that method no. 3 can be used for WPC 1 and 2 but not for WPC 3. Results for method no. 4 are very
Fig. 5. TGA–DTG-curves of WPC 1 (black curve) and 2 (blue curve) determined with method no. 2. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)
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Table 4 Overview of optimized test methods with different segments. No.
Segment I
Segment II
Segment III
Segment IV
Segment V
3
N2 -atm. 20 ◦ C/min 25–350 ◦ C
N2 -atm. Isothermal 30 min 350 ◦ C
N2 -atm. 5 ◦ C/min 350–500 ◦ C
O2 -atm. 10 ◦ C/min 500–550 ◦ C
O2 -atm. Isothermal 3 min 550 ◦ C
4
N2 -atm. 20 ◦ C/min 25–340 ◦ C
N2 -atm. Isothermal 15 min 340 ◦ C
N2 -atm. 50 ◦ C/min 340–550 ◦ C
O2 -atm. 50 ◦ C/min 550–700 ◦ C
Not applicable
5
N2 -atm. 50 ◦ C/min 25–350 ◦ C
N2 -atm. Isothermal 15 min 350 ◦ C
N2 -atm. 50 ◦ C/min 350–550 ◦ C
O2 -atm. 50 ◦ C/min 550–700 ◦ C
Not applicable
Table 5 Results of TGA curve step separation for WPC 1–3. For a description of methods no. 3–5, please refer to Table 4. Actual contents
No. 3
WPC 1 Wood flour fraction (70) Polymer fraction (30)
66.2 ± 0.5 34.2 ± 0.2
5.4 14.0
WPC 2 Wood flour fraction (70) Polymer fraction (30)
71.1 ± 2.4 29.1 ± 3.5
WPC 3 Wood flour fraction (50) Polymer fraction (50)
64.3 ± 7.4 37.0 ± 7.4
a
Deviation for no. 3
No. 4
Deviation for no. 4
No. 5
Deviation for no. 5
n.d.a n.d.
n.d. n.d.
66.5 ± 0.5 34.3 ± 0.5
5.0 14.3
1.6 3.0
66.2 ± 0.9 33.7 ± 0.6
5.4 12.3
68.0 ± 0.5 34.0 ± 0.2
2.9 13.3
28.6 26.0
50.7 ± 1.2 50.1 ± 1.1
1.4 0.2
53.8 ± 1.5 48.4 ± 1.8
7.6 3.2
n.d., not determined.
good for WPC 3, but less for WPC 2 in comparison to method no. 3 and no. 5. For method no. 4 the deviation values are smaller for WPC 3 but bigger for WPC 2. In relation to all three WPC samples method no. 5 shows good agreements between analyzed and actual fractions of wood flour and polymer. Renneckar showed 12% deviation of polymer in WPC by using the high resolution method [2]. The differences between Renneckar’s work and this work are, that on the one hand different
polymer were used – Renneckar used PE and here PP-copolymer was applied and on the other hand the polymer fraction in this work contains additives like MAPP and wax, whereas Renneckar used no additives. Additionally, with the introduced method in this work fractions of polymer and wood flour can be calculated. The degradation of polymers PP and HDPE begins at 472 ◦ C respectively 517 ◦ C [13]. The step separation in TGA curve of the thermal degradation of wood flour and PE is connected with a smaller deviation because
Fig. 6. TGA–DTG-curves of WPC 1 (red curve), WPC 2 (green curve), and WPC 3 (blue curve) determined with method no. 5. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)
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the degradation of wood flour is approximately zero during the degradation of PE polymer. It is not for PP and PP-copolymer (see Fig. 4). Fuad et al. [16] used also thermogravimetric analysis for determination of filler content in rice husk ash and wood-based composites based on polypropylene. They showed good agreements for the component oil palm wood flour with a deviation of 5.8% between analyzed and actual filler contents by using a dynamic method (heating rate 20 ◦ C/min, 25 ◦ C until 550 ◦ C, air). In Fuad’s work, the thermal degradation of used filler and polymer showed small overlapping areas. The calculation of contents in overlapping degradation areas was corrected by a coefficient. In our work, no coefficient was used because nonlinear effects are possible which means that the component ratio of the formulations influences the thermal degradation by generating different decomposition products. In this context Sharypov et al. [10] determined a nonlinear dependence between the quantity of the individual formulation components and their effect on thermal degradation of the composites. 4. Conclusion The developed thermoanalytical method for WPC shows that it is possible to quantify the mass percentage for wood flour and polypropylene copolymer in wood plastic composites (WPC) by using thermogravimetric analysis (TGA–DTG). The developed method shows good agreements between analyzed and actual fractions of wood flour and polymer in spite of the complex thermal degradation behavior of wood plastic composites. The TGA method can be tailored according to the formulation composition. Maxima deviations from the actual wood and polymer contents were 5.4% and 14.3%, respectively. It is possible to optimize the deviation of analyzed fraction by switching the isothermal step between 340 and 350 ◦ C, changing the heating rate and adjusting the isothermal period. An accurate determination of individual components in WPC by using TGA is limitedly possible, because of the wide range of continuous thermal degradation of wood between 200 and 700 ◦ C in nitrogen and oxygen and the various thermal decomposition patterns of polymers. WPC with known composition can be rapidly quantified with TGA measurements, therefore, TGA is a useful tool for WPC production control. TGA measurements of WPC with unknown composition are difficult to interpret. For determination of composition of WPC with unknown formulation, other analytical methods such as Soxhlet extraction, infrared spectroscopy and DSC in combination should be used.
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Acknowledgements We thank the German Institute for Standardization DIN (Deutsches Institut für Normung e.V.), Berlin, for contribution of funding from the program “INS – Innovation mit Normen und Standards”. References [1] C. Clemons, Wood–plastic composites in the United States: interfacing of two industries, Forest Prod. J. 52 (2002) 10–18. [2] S. Renneckar, A.G. Zink-Sharp, T.C. Ward, W.G. Glasser, Compositional analysis of thermoplastic wood composites by TGA, J. Appl. Polym. Sci. 93 (2004) 1484–1492. [3] D.C.T. 15534-1, Wood–plastics composites (WPC), Part 1: Test methods for characterisation of WPC materials and products, 2007 (German version CEN/TS 15534-1). [4] D.C.T. 15534-2, Wood–plastics composites (WPC), Part 2: Characterisation of WPC materials, 2007 (German version CEN/TS 15534-2). [5] D.C.T. 15534-3, Wood–plastics composites (WPC), Part 3: Characterisation of WPC products, 2007 (German version CEN/TS 15534-3). [6] A. Reichert, H. Korte, Methoden zur Bestimung der Masseanteile in Verbundwerkstoffen aus thermoplastischen Polymeren und Lignocellulosen, Holztechnologie 50 (2009) 38–43. [7] J.R. Araujo, W.R. Waldman, M.A.D. Paoli, Thermal properties of high density polyethylene composites with natural fibres: coupling agent effect, Polym. Degrad. Stab. 93 (2008) 1770–1775. [8] H. Jeske, A. Schirp, F. Cornelius, Quantifizierung des Massenanteils von Polypropylen (PP)-Copolymer in WPC (Wood Plastic Composites) mittels DSC (dynamic scanning calorimetry), Holztechnologie 52 (2011) 51–53. [9] E.A. Phillips, S.D. Blazey, Quantitative thermogravimetric analysis of starch contents in several polymer systems, Thermochim. Acta 192 (1991) 191–197. [10] V.I. Sharypov, N. Marin, N.G. Beregovtsova, S.V. Baryshnikov, B.N. Kuznetsov, V.L. Cebolla, J.V. Weber, Co-pyrolysis of wood biomass and synthetic polymer mixtures. Part I: influence of experimental conditions on the evolution of solids, liquids and gases, J. Anal. Appl. Pyrolysis 64 (2002) 15–28. [11] M. Windt, D. Meier, R. Lehnen, Quantification of polypropylenw (PP) in wood plastic composites (WPCs) by analytical pyrolysis (Py) and differential scanning calorimetry (DSC), Holzforschung 65 (2011) 199–207. [12] G.W. Ehrenstein, G. Riedel, P. Trawiel, Praxis der Thermischen Analyse von Kunststoffen, second ed., Carl Hanser Verlag, München, 2003. [13] A. Kaboorani, Effects of formulation design on thermal properties of wood/thermoplastic composites, J. Compos. Mater. 44 (2010) 2205–2215. [14] R. Bodîrlâu, C.-A. Teaca, I. Spiridon, Thermal investigation upon various composite materials, Rev. Roum. Chim. 52 (2007) 153–158. [15] W.F. Hemminger, H.K. Cammenga, Methoden der Thermischen Analyse, first ed., Springer-Verlag, Berlin, 1989. [16] M.Y.A. Fuad, M.J. Zaini, M. Jamaludin, Z.A.M. Ishak, A.K.M. Omar, Determination of filler content in rice husk ash and wood-based composites by thermogravimetric analysis, J. Appl. Polym. Sci. 51 (1994) 1875–1886.