The demand for composite materials containing thermoplas- tics 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 inter- esting 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 speci- fication (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 prod- uct requirements were defined. So far, rapid analytical methods for the quantification of the mass percentages of wood flour and poly- mer 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]. Thermo- analytical 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 degrada- tion 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 sim- ilar 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 quantita- tive 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. The measurements were done under nitrogen atmosphere and two methods for heating rate parameters were chosen by Ren- neckar 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] quan- tified 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 compo- nents (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.