*Article* **The Impact of Wood Waste on the Properties of Silicone-Based Composites**

**Maciej Mrówka <sup>1</sup> , Małgorzata Szymiczek 1,\* and Magdalena Skonieczna 2,3**


**Abstract:** The impact of wood waste on the mechanical and biological properties of silicone-based composites was investigated using wood waste from oak, hornbeam, beech, and spruce trees. The density, abrasion resistance, resilience, hardness, and static tensile properties of the obtained WPC (wood–plastic composites) were tested. The results revealed slight changes in the density, increased abrasion resistance, decreased resilience, increased hardness, and decreased strain at break and stress at break compared with untreated silicone. The samples also showed no cytotoxicity to normal human dermal fibroblast, NHDF. The possibility of using prepared composites as materials to create structures on the seabed was also investigated by placing samples in a marine aquarium for one week and then observing sea algae growth.

**Keywords:** wood–plastic composite (WPC); silicone; mechanical properties; cytotoxicity; casting; ageing

**Citation:** Mrówka, M.; Szymiczek, M.; Skonieczna, M. The Impact of Wood Waste on the Properties of Silicone-Based Composites. *Polymers* **2021**, *13*, 7. https://dx.doi.org/10.3390/ polym13010007

Received: 30 November 2020 Accepted: 17 December 2020 Published: 22 December 2020

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## **1. Introduction**

Polymeric fillers are very popular and include natural fillers, which can be derived from both vegetables (maize husk, nutshell, ground coffee, bran, and starch) and animals (milled bird feathers). Such fillers are introduced primarily into thermoplastic polymer materials [1–4]. An important feature of this type of material, apart from the ability to change their physical and mechanical properties, is the ability to modify their environmental properties. Wood–plastic composite (WPC) with a thermoplastic matrix, which are mainly obtained by extrusion and injection, have found applications in the construction industry (boards, furniture, and lagging) [2,4–6]. The introduction of biofillers for duroplasts, along with changes in the physical and mechanical properties, allows resin shrinkage to be minimized, but it also increases the resin viscosity, making it difficult to saturate fabrics. A separate issue is the introduction of silicone biofillers, which are used for both sealing and construction elements [2,5,6]. The main problem in obtaining polymer–wood composites is the lack of compatibility between the hydrophilic wood waste filler and hydrophobic polymer matrices. To ensure proper adhesion between the filler and matrix, it is important to treat the surface of wood waste to make it hydrophilic, for example by using acetic anhydride and thermal treatment [2,6–9]. Thermal treatment of wood waste causes changes in color, regrouping of polymers, and in the case of resinous trees, modification and redistribution of wood extracts [9,10]. The result is increased measurement stability and resistance to biodegradation, and lower mechanical properties. Previous research has shown that increasing the strength and stiffness is affected by the shape of the waste and not its size [11]. Wood–polymer composites may also prove to be alternatives to traditional materials used for facade panels, concrete fillers, packaging, and protective and

anticorrosion coatings. These materials are especially interesting due to environmental concerns [1,2,6,12]. Silicone-based composites are one of the most important elastic technical materials produced industrially, e.g., glass-fiber-reinforced silicone composites [13]. In a high-power white light emitting diode (LED) package, the phosphor-silicone composite is typically used for photometric and colorimetric conversions, ultimately producing the white light [14]. Ceramifiable silicone rubber composites play important roles in the field of thermal protection systems (TPS) for rocket motor cases due to their advantages [15]. There are ceramizable (ceramifiable) silicone-based composites commonly used to increase flame retardancy of electrical cables and to ensure integrity of electricity network during fire by their ability to create a continuous ceramic structure [16]. In other paper silicone composites filled with different-sized nickel particles. The samples with particles showed larger improvements in shear storage modulus than those without particles [17]. By using an appropriate filler, composites with the desired properties can be obtained. The silicone-based membrane with 0.36 wt % of graphene oxide showed excellent antifouling performance, and is promising in practical applications [18]. There are no articles in the literature devoted to the introduction of fillers of natural origin into silicones. The authors took up this topic due to the need to create new materials based on waste products of natural origin.

This work aimed to assess the impact of wood waste from deciduous (beech, oak, and hornbeam) and coniferous trees (spruce) on the mechanical and biological properties of silicone composites used for structural element protection. Such a goal required the use of several physical and mechanical tests (static tensile test, hardness, abrasiveness, and density) and biological tests, which included cytotoxicity and aging tests in a replicated seawater environment.

#### **2. Materials and Methods**

#### *2.1. Materials*

The research was carried out on silicon matrix composites XIAMETER 4234-T4 (additive silicone) from Dow Corning (Table 1), in which the filler was wood waste from deciduous trees: oak (*Quercus robur*), beech (*Fagus sylvatica*), and hornbeam (*Carpinusbetulus*), and coniferous spruce (*Piceaabies*). Tree properties are summarized in Table 2. To identify the fillers, microscopic photos were taken on a stereo discovery Zeiss stereoscopic microscope (Carl Zeiss AG, Oberkochen, Germany), and single particles were imaged on a Zeiss Supra 35 scanning electron microscope, (Carl Zeiss AG, Oberkochen, Germany). The particle size distribution was tested on the Fritsch Analysette 22 Micro Tec Plus (FRITSCH GmbH, Idar-Oberstein, Germany).


**Table 1.** Properties of XIAMETER 4234-T4 silicone [19]. A—Xiameter RTV-4234-T-4 BASE; B— Xiameter T-4 curing agent.


**Table 2.** Properties of wood fillers [20–23].

#### *2.2. Methodology*

#### 2.2.1. Composites

The composites were prepared by gravity casting with a content of 10 and 20 wt % wood waste. The mark of the composites is shown in Table 3.



Before introducing wood waste into the silicone matrix, it was gradually heat-treated at 180◦C for 180 min, until a constant weight was obtained. The dried waste was sieved to obtain particles smaller than 1 mm. Unification of the size of wood waste helped evenly distribute the filler in the obtained composites. Smaller waste sizes also promoted homogenous functional properties throughout produced composites. The silicone component A was mixed with the fillers on a high shear mixer, then the catalyst was added. The mixing speed was 500 RPM.

The prepared compositions were deaerated in a vacuum oven for 45 min at 0.78 bar and then cast into previously prepared, leveled molds to ensure a constant plate thickness of 5 mm. Seventy-two hours after being poured into molds, samples were cut by punching. The sample preparation scheme is shown in Figure 1.

The obtained samples were subjected to mechanical and biological tests. Mechanisms were examined using hydrostatic weighing tests, radiation resistance tests, Schopper abrasion resistance tests, and Shore type A hardness tests and static tensile tests. Toxicity to normal human cells was also evaluated using the MTT test. The possibility of using the received WPC to obtain cable covers, which are placed on the seabed was also examined. All tests were carried out at the temperature in the room where the test was performed, which was 22 ◦C with a humidity of 50%.

**Properties Oak Beech Spruce Hornbeam** 

The composites were prepared by gravity casting with a content of 10 and 20 wt %

Before introducing wood waste into the silicone matrix, it was gradually heat-treated at 180°C for 180 min, until a constant weight was obtained. The dried waste was sieved to obtain particles smaller than 1 mm. Unification of the size of wood waste helped evenly distribute the filler in the obtained composites. Smaller waste sizes also promoted homogenous functional properties throughout produced composites. The silicone component A was mixed with the fillers on a high shear mixer, then the catalyst was added. The mixing

The prepared compositions were deaerated in a vacuum oven for 45 min at 0.78 bar and then cast into previously prepared, leveled molds to ensure a constant plate thickness of 5 mm. Seventy-two hours after being poured into molds, samples were cut by punch-

wood waste. The mark of the composites is shown in Table 3.

ing. The sample preparation scheme is shown in Figure 1.

After drying density (kg/m3) 900–1150 820–1270 700–850 660–1200 Flexural strength (MPa) 74–105 74–210 49–136 58–200 Flexural modulus (GPa) 10–13.5 10–18 7.3–21.4 7–17.7 Compressive strength (MPa) 48 41–99 30–79 54–99 Tensile strength (MPa) 50–180 55–180 21–245 24 Impact strength (J/cm2) 1–16 3–19 1–11 8–12 Brinell's hardness (HBW) 34 34 12 29–36

**Figure 1.** Scheme of preparation samples. **Figure 1.** Scheme of preparation samples.

**Table 2.** Properties of wood fillers [20–23].

*2.2. Methodology*  2.2.1. Composites

speed was 500 RPM.

#### The obtained samples were subjected to mechanical and biological tests. Mechanisms 2.2.2. Density Testing by Hydrostatic Weighing

were examined using hydrostatic weighing tests, radiation resistance tests, Schopper abrasion resistance tests, and Shore type A hardness tests and static tensile tests. Toxicity to normal human cells was also evaluated using the MTT test. The possibility of using the received WPC to obtain cable covers, which are placed on the seabed was also examined. All tests were carried out at the temperature in the room where the test was performed, which was 22 °C with a humidity of 50%. Densities were measured on a scale in accordance with EN ISO 1183-1:2006 using 5 samples from each system [24]. The method of determining the density by the method of hydrostatic weighing of composite polymer materials consisted in weighing the test sample with the use of an OHAUS Adventurer-Pro (OHAUS Europe GmbH, Nänikon, Switzerland) analytical balance with a density measurement kit. The sample was weighed twice. The first measurement is carried out with the sample placed on the pan and surrounded by air. The second measurement is carried out for a sample immersed in a liquid of known density. During the tests, water was used as a liquid with a known density *d* = 0.997 g/cm<sup>3</sup> .

Density was determined using Formula (1):

$$d = d\_{H20} \frac{m\_1}{(m\_1 - m\_2)} \tag{1}$$

where:


#### 2.2.3. Rebound Resilience with Schober's Test

The resilience test was carried out in accordance with the EN ISO 4662:2017 standard for five samples with dimensions of 30 mm × 30 mm × 5 mm [25]. Before conducting basic tests, the samples were mechanically conditioned (2 impacts). The measurement of the resilience of composite materials consists in hitting the sample with a weight placed on a pendulum. The sample is held in an anvil attached to a metal body. The measurement consisted in reading the value indicated by the pointer on the value axis (%).

#### 2.2.4. Abrasion Resistance Tests

The abrasion resistance tests were carried out on a Schopper–Schlobach apparatus (APGI) in accordance with the ISO 4649:2007 standard [26]. In the research, sandpaper (60 grit) was used, wound on a roller with a diameter of 150 mm, which was rotating at a speed of 40 RPM. Abrasion resistance was determined for 3 cylindrical samples with a 16 mm diameter and 10 mm height. Due to the thickness of spilled boards (5 mm), the samples were glued with cyanoacrylate adhesive (according to ISO 4649: 2007). Abrasion resistance (abrasive wear), i.e., the volume loss relative to a standard sample, was determined based on the Formula (2):

$$
\Delta V = \frac{m\_1 - m\_2}{d} \tag{2}
$$


#### 2.2.5. Hardness Test

The shore A hardness test was carried out in accordance with ISO 7619-1:2010 [27]. The measurements were made with a hardness durometer Shore A type (Etopoo). Five measurements were taken on each of the composites, maintaining a distance of at least 10 mm from the sample edge.

#### 2.2.6. Tensile Test

Tensile strength tests were performed in accordance with EN ISO 527-1 [28]. The measurements were carried out according to EN ISO 527-1 [28] for 5 samples (type 5-B) cut from each composition and native samples. The test was carried out on the Instron 4465 tensile test machine. The test speed was 50 mm/min. The stress at break and strain at break were determined.

#### 2.2.7. Cytotoxicity Testing of Composite Samples

Cytotoxicity testing of composite samples according to the procedure described earlier [29,30]. Cell viability of normal human dermal fibroblasts (NHDF, Lonza) was assessed using an MTT (3-[4-5-dimethylthiazol-2-yl]-2,5-diphenyltetrazolium bromide) test. Cells were seeded in Petri plates with a concentration of 10<sup>5</sup> cells per well. Cell cultures were seeded on tested materials and incubated for 72 h at 37 ◦C in a humidified atmosphere with 5% CO2. Then, the culture medium was removed and replaced with a trypsin solution for cell collection. After trypsin neutralization, cell suspensions were centrifuged (2000 RPM, 3 min, room temperature) and cell pellets were resuspended in the MTT solution (50 µL, 0.5 mg/mL in RPMI 1640 without phenol red, Sigma, (Saint Louis, MO, USA). After 3 h of incubation, the MTT solution was removed, and the acquired formazan was dissolved in isopropanol:HCl system. Finally, the absorbance at 570 nm was spectrophotometrically measured with a plate reader Epoch, BioTec, (Winooski, VT, USA). The results were expressed as a survival fraction (%) in comparison to the untreated controls (100%), from 3 separate experiments, ±SD.

#### 2.2.8. Ageing in Seawater Conditions

Due to the potential application of the prepared materials for the covers of structures installed in the seabed, tests were carried out in the sea water environment. The possibility of surface settling of obtained composites was investigated using marine algae. The tests were carried out in an aquarium with samples mounted on a wooden plate. Seawater was prepared in accordance with the standard ASTM D 1141-52 [31]. The chemical composition of the substitute seawater is shown in Table 4.


**Table 4.** Chemical composition of substitute sea water [31].

Marine algae was added to the aquarium, which was intensively illuminated for 30 days. After this time, the sample plate was removed. The aging was carried out at the temperature of 30 ◦C, while illuminating the aquarium with a reflector placed 1 m from the aquarium. Marine algae was added to the aquarium, which was intensively illuminated for 30 days. After this time, the sample plate was removed. The aging was carried out at the temperature of 30 °C, while illuminating the aquarium with a reflector placed 1 m from the aquarium.

#### **3. Results and Discussion 3. Results and Discussion**

#### *3.1. Characteristics of Fillers 3.1. Characteristics of Fillers*

Wood waste fillers are shown in Figure 2. Images of wood fillers were taken on a stereo discovery Zeiss stereoscopic microscope (Figure 2a,c,e,g) and single filler particles were imaged on a Zeiss Supra 35 scanning electron microscope (Figure 2b,d,f,h). Particle size tests were performed on a Fritsch Analysette 22 Micro Tec Plus equipped with a wet dispersing unit. Measuring range 0.08–2000 microns. The particle size distribution is shown in Figure 3. The frequency curves were developed on the basis of 5 measurements for each wood waste. As can be seen, the smallest filler particles were observed for beech and hornbeam. Wood waste fillers are shown in Figure 2. Images of wood fillers were taken on a stereo discovery Zeiss stereoscopic microscope (Figure 2a,c,e,g) and single filler particles were imaged on a Zeiss Supra 35 scanning electron microscope (Figure 2b,d,f,h). Particle size tests were performed on a Fritsch Analysette 22 Micro Tec Plus equipped with a wet dispersing unit. Measuring range 0.08–2000 microns. The particle size distribution is shown in Figure 3. The frequency curves were developed on the basis of 5 measurements for each wood waste. As can be seen, the smallest filler particles were observed for beech and hornbeam.

(**c**) (**d**)

**Figure 2.** *Cont*.

**Figure 2.** Microscopic image oak (**a**,**b**), beech (**c**,**d**), spruce (**e**,**f**), and hornbeam (**g**,**h**). **Figure 2.** Microscopic image oak (**a**,**b**), beech (**c**,**d**), spruce (**e**,**f**), and hornbeam (**g**,**h**).

*Polymers* **2021**, *13*, x FOR PEER REVIEW 8 of 18

**Figure 3.** The frequency curve for the tested fillers. **Figure 3.** The frequency curve for the tested fillers.

**Figure 4.** Cumulated curves of the filler's size.

*3.2. Density Test Results* 

Cumulative number of

 particles

displayed in Figure 5.

of individual trees.

The size of the most common particles was 747.78 µm for beech (8.2%), hornbeam (7.8%), and oak (9.6%), and 825.9 µm for spruce (9.3%). As can be seen, the smallest particles are characteristic of beech (90%—940.03 µm) and hornbeam (90%—917.03 µm), while Analyzing the images of fillers presented in Figure 2, it can be noticed that the structure of spruce differs from that of other trees. Spruce has a tubular structure. The structure of spruce is characterized by visible tracheas and pits, which, for example, increases the

the largest range was noted for oak (90%—998.13 µm), while for spruce 90% of the parti-

The density of composites produced using wood waste filler were determined by measuring five samples. The average values and their standard deviations are graphically

oak beech spruce hornbeam

0 500 1000 1500 2000 2500

Filler size (μm)

mechanical adhesion between the matrix and the filler. However, the release of resin without proper preparation of the spruce filler may adversely affect some properties. Microscopy images of fillers used in this study showed no significant differences in shape, but differed in surface image. These differences can primarily be seen between deciduous tree waste and spruce. **Figure 3.** The frequency curve for the tested fillers.

*Polymers* **2021**, *13*, x FOR PEER REVIEW 8 of 18

The size of the most common particles was 747.78 µm for beech (8.2%), hornbeam (7.8%), and oak (9.6%), and 825.9 µm for spruce (9.3%). As can be seen, the smallest particles are characteristic of beech (90%—940.03 µm) and hornbeam (90%—917.03 µm), while the largest range was noted for oak (90%—998.13 µm), while for spruce 90% of the particles have the size below 971.77 µm (Figure 4). This is due to the structure and properties of individual trees. The size of the most common particles was 747.78 µm for beech (8.2%), hornbeam (7.8%), and oak (9.6%), and 825.9 µm for spruce (9.3%). As can be seen, the smallest particles are characteristic of beech (90%—940.03 µm) and hornbeam (90%—917.03 µm), while the largest range was noted for oak (90%—998.13 µm), while for spruce 90% of the particles have the size below 971.77 µm (Figure 4). This is due to the structure and properties of individual trees.

**Figure 4.** Cumulated curves of the filler's size. **Figure 4.** Cumulated curves of the filler's size.

#### *3.2. Density Test Results 3.2. Density Test Results*

The density of composites produced using wood waste filler were determined by measuring five samples. The average values and their standard deviations are graphically displayed in Figure 5. The density of composites produced using wood waste filler were determined by measuring five samples. The average values and their standard deviations are graphically displayed in Figure 5. *Polymers* **2021**, *13*, x FOR PEER REVIEW 9 of 18

*3.3. Rebound Resilience Results* 

shown in Figure 6.

Samples from XIAMETER 4234-T4 silicone showed a density of 1.069 g/cm3; the specified density was 1.1 g/cm3. The introduction of 10% filler, regardless of the type of wood waste, reduced the density of the obtained composites, and the lowest density was obtained using spruce wood waste. The density of the spruce-silicone composite decreased Samples from XIAMETER 4234-T4 silicone showed a density of 1.069 g/cm<sup>3</sup> ; the specified density was 1.1 g/cm<sup>3</sup> . The introduction of 10% filler, regardless of the type of wood waste, reduced the density of the obtained composites, and the lowest density was

the structure of spruce waste as shown in Figure 2f. This is probably the result of tracheas (Figure 2f) distributed throughout the wood, which is why spruce is considered to be a light tree (range 410–500 kg/m3, and after drying 700–850 kg/m3) and the measurement method. The highest density at 10% filling was recorded for hornbeam, approximately 3% in relation to spruce. This is mainly due to the density and structure of the wood. The hornbeam has a uniform and fine structure, which affects its properties. Hornbeam, beech, and oak belong to heavy and very heavy trees, whose densities after reach 1200 kg/m3 (Table 2). Buk contains numerous wood rays, which serve as conductive channels. Oak wood, unlike beech wood, is characterized by wide conductive channels but in much smaller quantities. Composites in which the share of wood waste was 20% had a similar or slightly higher density than native silicone. Composites with 20% filling with hornbeam waste were characterized by the highest density by approximately 4% compared to XS. The conducted one-way analysis of variance showed a significant influence of the exam-

The average resilience values and standard deviations of the tested materials are

ined factors on the density of composites (F = 15.71, test F = 2.208).

obtained using spruce wood waste. The density of the spruce-silicone composite decreased by approximately 5% compared to the native sample (XS). It was most likely caused by the structure of spruce waste as shown in Figure 2f. This is probably the result of tracheas (Figure 2f) distributed throughout the wood, which is why spruce is considered to be a light tree (range 410–500 kg/m<sup>3</sup> , and after drying 700–850 kg/m<sup>3</sup> ) and the measurement method. The highest density at 10% filling was recorded for hornbeam, approximately 3% in relation to spruce. This is mainly due to the density and structure of the wood. The hornbeam has a uniform and fine structure, which affects its properties. Hornbeam, beech, and oak belong to heavy and very heavy trees, whose densities after reach 1200 kg/m<sup>3</sup> (Table 2). Buk contains numerous wood rays, which serve as conductive channels. Oak wood, unlike beech wood, is characterized by wide conductive channels but in much smaller quantities. Composites in which the share of wood waste was 20% had a similar or slightly higher density than native silicone. Composites with 20% filling with hornbeam waste were characterized by the highest density by approximately 4% compared to XS. The conducted one-way analysis of variance showed a significant influence of the examined factors on the density of composites (F = 15.71, test F = 2.208).

#### *3.3. Rebound Resilience Results*

The average resilience values and standard deviations of the tested materials are shown in Figure 6. *Polymers* **2021**, *13*, x FOR PEER REVIEW 10 of 18

**Figure 6.** Resilience values of tested materials (%)*.*  **Figure 6.** Resilience values of tested materials (%).

Introduction 10 wt % wood waste caused a reduction in elasticity for composites with oak by 27% and beech by 20% in relation to native samples, but no change was observed in composites with spruce filling, while a slight increase was observed in the resilience of samples with hornbeam filling. Such a reduction may be related to the size of the grain introduced into the composite and the structure of the wood. The beech particles were characterized by the smallest dimensions, which are confirmed by the normal distribution—Figure 3. Although the literature reports [11] a greater impact on the properties of composites, the shape of the particle has a greater impact, and in the examined case the size is also important. It seems that this is also the result of obtaining the filler from the waste and the properties of the tree itself. Introduction 10 wt % wood waste caused a reduction in elasticity for composites with oak by 27% and beech by 20% in relation to native samples, but no change was observed in composites with spruce filling, while a slight increase was observed in the resilience of samples with hornbeam filling. Such a reduction may be related to the size of the grain introduced into the composite and the structure of the wood. The beech particles were characterized by the smallest dimensions, which are confirmed by the normal distribution—Figure 3. Although the literature reports [11] a greater impact on the properties of composites, the shape of the particle has a greater impact, and in the examined case the size is also important. It seems that this is also the result of obtaining the filler from the waste and the properties of the tree itself.

In all types of composites containing 20 wt % filling the wood, resilience increased, the highest of which was observed in samples with beech filling (41%). In composites with hornbeam filling, the resilience results were relatively similar (34.6 and 36.6%). Hornbeam In all types of composites containing 20 wt % filling the wood, resilience increased, the highest of which was observed in samples with beech filling (41%). In composites with hornbeam filling, the resilience results were relatively similar (34.6 and 36.6%). Hornbeam

wood is very hard and difficult to break, hence the resilience remained at a similar level,

tional filler improved the flexibility in all systems containing 20 wt % filler. It can therefore be concluded that the introduction of 20 wt % of the wood filler into the systems, regardless of the wood type, its structure, and the particle size, improved the resilience of the obtained composite. That is why the samples recovered their original shape after applying more force than in the unmodified system. The carried out one-way analysis of variance showed that both the content and the type of filler had a significant impact on the change

The results of abrasion resistance tests were compared to a standard sample made of silicone XIAMETER 4234-T4 without filler. The average abrasion resistance values are shown in Figure 7. The test results show that the introduction of filler into the matrix reduced abrasion. The composite containing 10 wt % beech waste (10 - O) showed a loss of 0.51 cm3 and had the highest loss of volume. In the case of fillers obtained from deciduous trees, abrasion is approximately 20% lower than that of the native sample. Spruce waste causes a loss of volume by 65% compared to the native sample. The introduction of 20% beech and hornbeam fillers did not affect the abrasion resistance. The differences observed fall within the potions of errors. However, it can be assumed that the distribution of filler particles in the structure of the composite resulted in no changes observed. These fillers

of relativity (F = 38.47, F = 2.208 test).

*3.4. Abrasion Resistance Results* 

wood is very hard and difficult to break, hence the resilience remained at a similar level, regardless of the content. Regardless of the wood's cleavage and its hardness, the additional filler improved the flexibility in all systems containing 20 wt % filler. It can therefore be concluded that the introduction of 20 wt % of the wood filler into the systems, regardless of the wood type, its structure, and the particle size, improved the resilience of the obtained composite. That is why the samples recovered their original shape after applying more force than in the unmodified system. The carried out one-way analysis of variance showed that both the content and the type of filler had a significant impact on the change of relativity (F = 38.47, F = 2.208 test).

#### *3.4. Abrasion Resistance Results*

The results of abrasion resistance tests were compared to a standard sample made of silicone XIAMETER 4234-T4 without filler. The average abrasion resistance values are shown in Figure 7. The test results show that the introduction of filler into the matrix reduced abrasion. The composite containing 10 wt % beech waste (10 - O) showed a loss of 0.51 cm<sup>3</sup> and had the highest loss of volume. In the case of fillers obtained from deciduous trees, abrasion is approximately 20% lower than that of the native sample. Spruce waste causes a loss of volume by 65% compared to the native sample. The introduction of 20% beech and hornbeam fillers did not affect the abrasion resistance. The differences observed fall within the potions of errors. However, it can be assumed that the distribution of filler particles in the structure of the composite resulted in no changes observed. These fillers are characterized by the smallest particles: beech 90%—940.03 µm and hornbeam 90%—917.03 µm (Figure 4). The lowest abrasion (approximately 50% of XS abrasion) was observed for spruce waste, similarly to composite materials with 10% filling. It seems that the related abrasion is determined mainly by the type of wood and particle size. The smaller the particle, the greater the loss in volume. Samples filled with deciduous tree waste are characterized by worse abrasion resistance than composites with spruce waste. The conducted one-way analysis of variance confirms the strong relevance of the studied factors for the abrasion of composites (F = 202.9, test F = 2.51). *Polymers* **2021**, *13*, x FOR PEER REVIEW 11 of 18 are characterized by the smallest particles: beech 90%—940.03 µm and hornbeam 90%— 917.03 µm (Figure 4). The lowest abrasion (approximately 50% of XS abrasion) was observed for spruce waste, similarly to composite materials with 10% filling. It seems that the related abrasion is determined mainly by the type of wood and particle size. The smaller the particle, the greater the loss in volume. Samples filled with deciduous tree waste are characterized by worse abrasion resistance than composites with spruce waste. The conducted one-way analysis of variance confirms the strong relevance of the studied factors for the abrasion of composites (F = 202.9, test F = 2.51).

**Figure 7.** Abrasion resistance of composites (cm3)*.*  ).

**Figure 8.** Hardness value of samples (ShA)*.* 

#### *3.5. Shore A Hardness Test 3.5. Shore A Hardness Test*

0

10

20

30

40

Hardness (ShA)

50

60

70

80

Five samples of each composite variant were tested for hardness tests. Graphical presentations of hardness test results and standard error are presented in Figure 8. Materials **Figure 7.** Abrasion resistance of composites (cm3 Five samples of each composite variant were tested for hardness tests. Graphical presentations of hardness test results and standard error are presented in Figure 8.

XS 10 - O 20 - O 10 - B 20 - B 10 - S 20 - S 10 - H 20 - H

Five samples of each composite variant were tested for hardness tests. Graphical

XS 10 - O 20 - O 10 - B 20 - B 10 - S 20 - S 10 - H 20 - H

Materials

presentations of hardness test results and standard error are presented in Figure 8.

are characterized by the smallest particles: beech 90%—940.03 µm and hornbeam 90%— 917.03 µm (Figure 4). The lowest abrasion (approximately 50% of XS abrasion) was observed for spruce waste, similarly to composite materials with 10% filling. It seems that the related abrasion is determined mainly by the type of wood and particle size. The smaller the particle, the greater the loss in volume. Samples filled with deciduous tree waste are characterized by worse abrasion resistance than composites with spruce waste. The conducted one-way analysis of variance confirms the strong relevance of the studied

factors for the abrasion of composites (F = 202.9, test F = 2.51).

**Figure 7.** Abrasion resistance of composites (cm3)*.* 

*3.5. Shore A Hardness Test* 

0

0.1

0.2

0.3

Abrasion resistance

0.4

0.5

 (cm3)

0.6

0.7

0.8

In each modified system, the obtained hardness was higher than the unmodified sample. In systems containing 10 wt % XIAMETER 4234-T4 silicone, the obtained composites had comparable hardness values. Both heavy woods like beech, hornbeam, oak, and light spruce showed a hardness of around 57 ShA. Such a high value in the case of spruce precipitation may indicate the content of the resin inside the waste. As in previous systems, when 20 wt % filler was used, the highest value was obtained in samples containing spruce waste. The obtained value of 69.6 ShA was 11 ShA higher than hard oak waste samples.

This may prove that the separated spruce wood resin additionally increased the hardness of the composite, which was also confirmed by the results of resilience. Relatively soft spruce wood did not differ from harder beech, hump, or oak, even taking into account the particle size. As can be seen, as in previous studies, both the type of wood waste and the size of the particle had a significant impact on the obtained hardness results (F = 91.1, test F = 2.208).
