## 1 Introduction  
The production of plastics has risen from two million tons in 1950 to 380 million tons in 2015 and is still increasing worldwide, causing serious environmental pollution [1; 2; 3; 4; 5]. The EU requirements and strategies encourage the reuse of plastics and promote high-quality plastics recycling processes [6; 7]. Despite the fact that this area is an extremely hot topic and that much has been done, there is still room for specific plastics and a challenge for further advanced research.  
Polypropylene (PP) is one of the most useful and manufactured plastics [8; 9; 10] and thus, a high treat for the environment, especially after the usage. Polypropylene's physical, chemical and mechanical properties, such as high stiffness, good tensile strength, chemical inertness to most diluted alkalis,acids and solvents make this material extremely attractive for the use in different areas in the plastics and polymer materials industry. The growth market of PP includes industries such as appliance and consumer products, automotive, agriculture, construction, leisure, outdoor, sports, medical and healthcare, packaging, and fibers [1; 11; 12; 13]. Due to its low density, it offers added value in weight-sensitive applications. Higher temperature resistance makes it suitable for objects such as trays, funnels, buckets, and bottles [14; 15]. To reduce the environmental impact of PP, the focus of many studies is given toward the recycling and reuse of the PP. In the Society of the Plastics Industry, resin was defined so that PP could be recycled five times without changing its structural and mechanical properties [8].  
Another interesting approach to PP sustainable use is the development of biodegradable PP composites. The biodegradability of PP composites can be increased by adding animal or mineral-based natural fibers to the neat polymer [3; 4; 5; 7; 8; 9; 10; 16; 17]. It has already been reported that, compared to animal fibers, the natural plant fibers are more suitable for the preparation of polymer composites, since they provide much higher strength and stiffness to the final material [4]. Natural fibers are easily recyclable, have low density and high specific strength and stiffness. The use of natural fibers as polymer reinforcement improves the flexural and tensile properties of pure polymers [18; 19; 20; 21; 22; 23]. The fibers are prepared from renewable sources and therefore require less energy for the production. They are produced at lower costs as plastics and emit low quantities of toxic fumes. Using natural fibers is also beneficial in the manufacturing process since fibers tend to be less abrasive and cause less damage to processing equipment over time. Another benefit of using fibers is their low price, e.g., the price of wood flour is 0.09-0.18 \(\  
benzoylates, urethanes, (meth) acrylates, maleated esters, isocyanates, ester resins and are mainly environmentally unfriendly and costly.  
However, the presented research was focused on wood polymer composites prepared without modifications of WF or polymer, with the goal of obtaining good interfacial bonding and improved properties of PP material. Such an approach could be considered as \"green\", since only sustainable and biodegradable materials were used, which have less impact on the environment. Increased use of WF in the production of composites could also give a boost to the paper industry by providing an important new use of wood pulp, as WF can be derived from paper.  
The goal of this paper was, therefore, to compare the chemical, thermal and rheological behavior of PP materials filled with wood fiber at different loadings without the use of additives. For the investigation, two PP matrix materials were used--recycled (R-PP), and unrecycled (PP) and their ability to form strong interfacial bonding between the wood fibers and polymer matrix was investigated.