1. Introduction
The progressing depletion of fossil fuels as well as increasing environmental awareness and green consumerism result in the development of new products such as, e.g., wood/polymer composites (WPC). For over twenty years, composites of thermoplastic polymers with natural components have been applied in various branches of industry, including the automotive and construction industries [
1,
2]. Lignocellulose fillers exhibit many advantageous properties such as weight saving, good acoustic and thermal insulation, nonabrasive effect, and good availability [
3]. However, these fillers have one important disadvantage, high hydrophilicity, causing poor adhesion to non-polar polymers such as, e.g., polypropylene, as well as sparse dispersion of filler particles in the polymer matrix. For this reason, it is essential to introduce certain modifications in order to improve interface interactions in the composite system. Various methods are used to enhance compatibility between lignocellulose fillers and polymer matrices, e.g., chemical modification [
4,
5,
6], surface grafting of polymers onto fillers [
7], introduction of compatibilisers such as maleated polymer [
8], and treatment with coupling agents [
9,
10,
11,
12].
Silanes are effective coupling agents, which are extensively used to enhance polymer–filler interactions [
13,
14,
15,
16,
17]. These compounds enter condensation reactions with hydroxyl groups of wood, at the same time causing entanglement of polymer matrix chains with xylem fibers. The mechanism results from the fact that hydrolysable groups of silane may hydrolyze, as a result forming silanol, which in turn may react with hydroxyl groups of wood, while organofunctional groups may react with polymer chains [
11]. The selection of an organofunctional group is determined by the need to ensure good compatibility with the polymer.
Impregnation of wood with silicon compounds improves its dimensional stability and resistance to weather conditions [
18,
19,
20,
21]. Kim et al. (2011) [
22] observed that silane treatment significantly improved tensile, flexural, and impact strength of polypropylene composites. Moreover, Ichazo et al. (2001) [
23] found an increase in the moduli and tensile strength of wood/polypropylene composites modified with silanes, which was explained by improved dispersion of filler particles in the polymer matrix. Wood treatment using silanes also caused decreased water adsorption, increased thermal stability of the PP matrix, and more homogeneous morphology [
3,
24]. In turn, Cichosz et al. (2019) reported increased thermal resistance and particle size of cellulose fibers, as well as reduced mass loss as a result of modification with silane coupling agents.
Another factor causing limitations in the application of WPC composites is related with the relatively low resistance of wood to biological factors, including fungi. Therefore, in recent years, literature sources have reported growing interest in wood impregnation applications of natural products and synthetic compounds characterized by low impact on human health and the environment, including essential oils, chitosan, and terpenes [
20,
25,
26,
27,
28]. Another example in this respect may be provided by propolis, a natural substance with various biological properties, such as antifungal, antibacterial, antioxidant, and antiviral [
29,
30,
31,
32,
33]. Compared to untreated wood, the material treated with propolis extracts exhibits activity against decay fungi, such as
Coniophora puteana,
Trametes versicolor, and
Neolentinus lepiseus [
32,
33]. Literature data indicated that the extract of Polish propolis at a concentration above 12% limited the decay of pine wood caused by
C. puteana [
33]. Propolis extract has also been used as a constituent of wood impregnation preparations. These formulations consisting of propolis extract, silver nanoparticles, and chitosan limited decay of pine wood caused by
T. versicolor when compared to untreated wood [
34,
35]. In turn, wood impregnated with two propolis-silane formulations, namely, EEP-VTMOS/TEOS (propolis extract (EEP) with vinyltrimethoxysilane and tetraethyl orthosilicate) and EEP-MPTMOS/TEOS (EEP with 3-(trimethoxysilyl)propyl methacrylate and tetraethyl orthosilicate), limited the activity of
C. puteana even when the wood samples were subjected to leaching with water [
36]. In addition, chemical analyses (AAS, XRF, and NMR) confirmed that constituents of the propolis-silane formulations formed permanent bonds with wood [
36]. Moreover, the propolis extract was a component of wood protection formulation containing an ethanolic extract of Polish propolis together with caffeine and silicon compounds (methyltrimethoxysilane and octyltriethoxysilane), which inhibited growth of a brown-rot fungus
C. puteana [
37].
The aim of this study was to determine and characterize the supermolecular structure, phase changes, and selected physicochemical properties in composites of polypropylene with pine wood treated using propolis-silane dual component modifiers. To the best of our knowledge, propolis extract and propolis-silane dual modifiers have not been used as wood modifiers applied to composites materials. Therefore, the originality of this study consists in the evaluation of the effect of propolis-silane modifiers of a wood filler with a difunctional action (antifungal activity, compatibility action) on the structure as well as thermal and mechanical properties of wood/polypropylene composites.
2. Materials and Methods
2.1. Materials
In this study, Scots pine sapwood (Pinus sylvestris L.) was used in the form of sawdust with a grain size of 0.5 mm and wood veneer samples of 20 mm× 20 mm× 0.6 mm.
A commercially available polypropylene Moplen HP456J produced by Basell Orlen Polyolefins (Płock, Poland) with the melt flow index of 3.4 g/10 min (at 230 °C and 2.16 kg), isotacticity of 95%, and Tm = 163–164 °C was used as the polymeric matrix.
Silicon compounds (tetraethyl orthosilicate, vinyltrimethoxysilane, and octyltriethoxysilane) were purchased form Sigma Aldrich (Darmstadt, Germany). The ethanolic extract of Polish propolis was provided by PROP-MAD (Poznań, Poland). Ethanol was purchased from Avantor Performance Materials (Gliwice, Poland). Nitric acid and KBr were purchased from Sigma Aldrich (Darmstadt, Germany).
2.2. Wood Treatment
The first formulation used for wood impregnation (TEOS/VTMOS) consisted of 5% tetraethyl orthosilicate (TEOS) and 5% vinyltrimethoxysilane (VTMOS). The other formulation (TEOS/OTEOS) contained 5% tetraethyl orthosilicate (TEOS) and 5% octyltriethoxysilane (OTEOS). The solvent used to prepare silane formulations was 70% ethanol. In the next stage of this study, 70% ethanol was replaced with the ethanolic extract of Polish propolis (EEP) at a 15% concentration. The first propolis-silane formulation consisted of 15% EEP, tetraethyl orthosilicate (TEOS), and vinyltrimethoxysilane (VTMOS) at a 5% concentration (EEP-TEOS/VTMOS), while the other formulation contained 15% EEP, tetraethyl orthosilicate (TEOS), and octyltriethoxysilane (OTEOS) at a 5% concentration (EEP-TEOS/OTEOS).
The homogenous pine wood material in the form of sawdust was treated with EEP, silanes, and the propolis-silane formulations (1/25 w/v). The reaction was run at room temperature with simultaneous stirring, using a magnetic bar stirrer for 2 h. The wood samples were then filtered and dried in air flow at room temperature.
2.3. Characterisation of Treataed Wood
2.3.1. Fourier Transform Infrared Spectroscopy (FTIR)
Wood samples were mixed with KBr at a 1/200 mg ratio. Spectra were registered using a Nicolet iS5 spectrophotometer by Thermo Fisher Scientific (Waltham, MA, USA) with Fourier transform at a range of 500–4000 cm−1 and a resolution of 2 cm−1, registering 64 scans.
2.3.2. Atomic Absorption Spectrometry (AAS)
Wood samples (0.5 g) were mineralized with nitric acid (8 mL) in the microwave mineralization system (CEM Corporation, Matthews, NC, USA) and after cooling, the solutions were filtered and diluted to 50.0 mL with deionized water. The concentration of silicon in wood samples was determined using flame atomic absorption spectrometry in an AA280FS spectrometer (Agilent Technologies, Santa Clara, CA, USA).
2.3.3. X-ray Powder Diffraction (XRD)
The supermolecular structure of treated wood was analyzed by means of wide angle X-ray scattering (TUR-M62 diffractometer, Carl Zeiss, Jena, Germany). The diffraction pattern was recorded between 5 and 30° (2θ-angle range) in the step of 0.04°/3 s. The wavelength of the Cu K
radiation source was 1.5418 Å and the spectra were obtained at 30 mA with an accelerating voltage of 40 kV. Peak deconvolution was performed by the method proposed by Hindeleh and Johnson [
38], improved and programmed by Rabiej [
39]. After separation of X-ray diffraction lines, the degree of crystallinity (X
c) was determined by comparing the areas under crystalline peaks and the amorphous curve. The changes in the supermolecular structure of wood were analyzed in the function of the chemical modification process.
2.3.4. Biological Activity of Treated Wood
The analyses were carried out on wood veneer samples of 20 mm × 20 mm × 0.6 mm prepared from pine sapwood. The samples were impregnated by soaking in the solutions of modifying formulations. The samples were soaked for 20 min and next conditioned to constant weight at the relative humidity of 65 ± 5% and the temperature of 20 ± 1 °C. The mycological test of treated wood veneer samples was based on the PN EN ISO-846:2019 standard [
40]. The tested samples in 5 replicates were evaluated for resistance against
Aspergillus niger van Tieghem BAM 4 (ATCC 6275),
Chaetomium globosum Kunze BAM 12 (ATCC 6205),
Penicillium funiculosum Thom (ATCC 11979),
Paecilomyces variotii Bainire BAM 19 (ATCC 18502),
Trichoderma virens (ATCC 9645) and
Ulocladium atrum. The samples were placed under sterile conditions on the previously prepared and sterilised Petri dishes filled with the agar substrate (potato dextrose agar, Sigma Aldrich, Darmstadt, Germany) and infected using an aqueous solution of spores of the test fungus. The Petri dishes with samples were incubated for 21 days at a temperature of 28 ± 1 °C and relative humidity above 95%. The degree of fungus colonisation on the sample surface was visually observed at 3, 7, 10, 14 and 21 days of the experiment and individually rated using a four-degree scale (
Table 1).
2.4. Preparation of Composite Materials
The wood-polypropylene composites containing untreated and treated wood were obtained by the extrusion method. The mixture of polypropylene and 30% wood was mixed in a drum blender for 30 min. This mixture was then conveyed to the feed hopper of a single-screw extruder (Fairex, Le Bourget, France). The length-to-diameter ratio L/D of the extruder was 25. During extrusion the temperatures of the four processing zones were adopted as 140, 180, 190 and 195 °C, respectively, at the die temperature of 190 °C. The extrusion speed was 25 to 30 rpm. Extrusion temperature was kept at less than 200 °C to avoid decomposition and degradation of wood. The extrudate was cooled with 20 °C water after exiting the die and then pelletised into granules. Next, the granules were dried in an oven for 24 h at 60 °C.
The samples used to investigate the material structure and to conduct mechanical strength tests were prepared using an Engel injection moulding machine at the mould temperature of 30 °C. After moulding the specimens for structure analyses were immediately sealed in a polyethylene bag and placed in a vacuum desiccator for a minimum of 24 h prior to structural testing.
2.5. Characterisation of Composite Materials
2.5.1. Differential Scanning Calorimetry (DSC)
Differential scanning calorimetry was used to characterize thermal properties of composites. The tests were carried out under dynamic conditions using the Netzsch DSC 200 calorimeter (Netzch Group, Selb, Germany) in argon atmosphere. First, the samples were heated up to 220 °C at a rate of 20 °C/min. In order to remove their previous thermal history they were kept at this temperature for 3 min. In the next stage the samples were cooled down at 5 °C/min to 40 °C. The entire cycle was repeated two times and for calculations the data from the second run were used. The crystallization parameters of WPC with unmodified and modified wood fillers, such as crystal conversion (α), half-time of crystallization (t0.5) and crystallization temperature (Tc) were determined.
2.5.2. Morphological Analysis
The nucleation ability of the polypropylene matrix in the presence of wood fillers was assessed applying the hot-stage polarized light microscopy. For this purpose a Labophot-2 microscope (Nikon, Tokyo, Japan) coupled with a Panasonic CCS camera (Panasonic, Kadoma, Japan) and equipped with a Linkam TP93 heating stage (Linkam, Tadworth, UK) was used. Composite film placed on the microscope slide was heated to a temperature of 200 °C at a rate of 40 °C/min. The sample was heated for 3 min to erase the thermal memory of the polymer. The material was cooled to 136 °C at a rate of 20 °C/min while isothermal crystallisation was run. In the course of the experiment the induction time for the crystallisation process and the transcrystalline growth rate were determined.
2.5.3. Structural Investigations (XRD)
The wide-angle X-ray diffraction was used to determine polymorphic changes as well as changes in the supermolecular structure of isotactic polypropylene in composite materials. Diffractograms were analysed applying the method proposed by Hindeleh and Johnson [
38], while the content of the polymorphic β-phase (k) was calculated according to the formula proposed by Turner Jones et al. [
41].
2.5.4. Mechanical Properties
Tensile strength properties of WPC were determined using a Zwick Z020 universal mechanical testing machine (Zwick/Roell, Ulm, Germany) and evaluated according to the PN EN ISO 527–3: 2019–01 standard [
42]. The tests were performed with a load cell capacity of 20 kN at a cross-head speed of 5 mm/min. The basic strength parameters were determined: Young’s modulus (YM), tensile strength (TS) and elongation at break (EB).
4. Conclusions
This paper presents the effect of pine wood treated using novel propolis-silane formulations on properties of the resulting wood/polypropylene composites. In the first stage of the study the chemical and biological characteristics of wood treated with silanes (TEOS/OTEOS and TEOS/VTMOS), the propolis extract and the propolis-silane formulations (EEP-TEOS/OTEOS and EEP-TEOS/VTMOS) were determined. The chemicalanalyses confirmed presence of silanes and constituents of propolis in wood structure. The bands of Si–C and Si–O originating from silicon compounds and the bands associated to propolis components were observed in the spectra of treated wood. Treatment of wood using the propolis extract and propolis-silane formulations caused changes in the structure of wood, including e.g., an increased degree of crystallinity. Moreover, veneer samples impregnated with the propolis extract and the propolis-silane formulations exhibited resistance against moulds, such as A. niger and T. virens compared to untreated samples or those treated with silanes without the propolis extract.
In the second stage of the study changes in the supermolecular structure as well as thermal and mechanical properties of the composites containing polypropylene and wood treated with the propolis extract and the propolis-silane formulations were determined. Differential scanning calorimetry (DSC) showed that the presence of fillers had a considerable effect on the course of polypropylene crystallisation. The introduction of a wood filler treated with propolis-silane formulations caused an increase in crystallisation temperature and the degree of conversion, which indicates a high nucleation ability of applied modifiers, particularly EEP-TEOS/OTEOS. A high nucleation activity of the filler treated with EEP-TEOS/OTEOS was also confirmed by the results provided by polarised light microscopy (PLM), which showed that the polypropylene composite with wood treated using this preparation exhibited the highest efficacy in the modification of transcrystalline structures, which was manifested in markedly higher values of TCL growth rate compared to the other composite systems. The composite system with the EEP-TEOS/OTEOS treated wood also exhibited the best efficacy of β-PP formation, as well as very good strength properties compared to the other systems.
Summing up, wood modification using propolis extract and propolis-silane formulations affected the structure, thermal and mechanical properties of wood/polypropylene composites. Obtained fillers with the bifunctional action (antifungal activity and compatibility action) may be added to the polymer matrix in order to prepare green and bio-friendly composites for various applications.