*3.6. Tensile Test Results*

Tensile test samples were cut using a punch, which ensured a repeatable shape and dimensions of tested samples. Five measurements were made for each composite variant (filler fraction, type of wood waste). A graphical presentation of the results is shown in Figure 9 (strain at break) and Figure 10 (stress at break).

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

Figure 9 (strain at break) and Figure 10 (stress at break).

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

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

Tensile test samples were cut using a punch, which ensured a repeatable shape and dimensions of tested samples. Five measurements were made for each composite variant (filler fraction, type of wood waste). A graphical presentation of the results is shown in

**Figure 9.** Strain at break*.*  **Figure 9.** Strain at break.

samples.

(F = 91.1, test F = 2.208).

*3.6. Tensile Test Results* 

**Figure 10.** Stress at break (MPa)*.* **Figure 10.** Stress at break (MPa).

[34] was reported.

The tensile test results indicate that a higher stress at break was observed in samples modified with XIAMETER 4234-T4 silicone. All composites showed lower stress at break, with the exception of composite 10 - B, whose value (1.98 MPa) was similar to unmodified silicone (2.02 MPa). The greatest differences between the native and filled samples were observed for composites with meringue waste and amounted to 10 wt %—30% and for 20 wt %—35%. The composites with the filler from deciduous trees had smaller differences. For 10%, the filling was on average approximately 8% (the lowest value was shown by hornbeam—1.72 MPa), while for 20% by wt filling 26%. ANOVA analysis confirmed the significant influence of the tested factors on the stress at break, F = 35.75, test F = 2.208. *3.7. Cytotoxicity Test Results*  Samples of pure XIAMETER 4234-T4 silicone showed the greatest strain at break, and all other composites showed reduced strain at break. The sample showing the most similar strain to unmodified silicone (1.269) was that with 10 wt % beech content (1.181). The lowest value was observed in composite 20 - S (0.567), but also the highest measurement error of 6% and standard deviation of 0.06, which may be a result of the preparation of the filler for introduction, the introduction technology itself or the physicochemical properties of wood. Composites containing hornbeam filling showed similar deformation values, irrespective of the percentage of the composite (0.994 and 0.985, respectively). This seems to be related primarily to the distribution of the filler in the composite, and secondly to the grain size. The one-way analysis of variance showed a significant impact on the stain values of the type and content of the filler (F = 162.19, test F = 2.208).

The cytotoxicity of the tested composites was obtained according to the standard procedures against model cell lines [29,30,32]. The viability and proliferation assays, such as an MTT assay, allowed for biocompatibility of different, natural, or synthetic agents assessments [32,33]. Figure 11 presents the cytotoxicity against NHDF cells. The cytotoxicity It can be observed that the smaller the filler particles and the proportion by weight, the better the strain at break, which confirms the adopted theories. The lowest strain at break was observed for composites with 20% spruce filling, which proves a significant influence of wood properties on the tested composite characteristics.

test results showed that neither silicon nor any of the tested composites showed any toxicity to the normal fibroblasts. After 72 h, the fraction of living cells for the XS, 10 - B, and 10 - O groups showed similar values of 100%, 98%, and 93%, respectively. The remaining The tensile test results indicate that a higher stress at break was observed in samples modified with XIAMETER 4234-T4 silicone. All composites showed lower stress at break, with the exception of composite 10 - B, whose value (1.98 MPa) was similar to unmodified

and 20 - H composites showed the most proliferative values, for which the living cell fractions were 190% and 196%, respectively. The results confirm the biocompatibility of composites and any toxicity against the NHDF cell line in a standard in vitro cytotoxicity assay silicone (2.02 MPa). The greatest differences between the native and filled samples were observed for composites with meringue waste and amounted to 10 wt %—30% and for 20 wt %—35%. The composites with the filler from deciduous trees had smaller differences. For 10%, the filling was on average approximately 8% (the lowest value was shown by hornbeam—1.72 MPa), while for 20% by wt filling 26%. ANOVA analysis confirmed the significant influence of the tested factors on the stress at break, F = 35.75, test F = 2.208.

## *3.7. Cytotoxicity Test Results*

The cytotoxicity of the tested composites was obtained according to the standard procedures against model cell lines [29,30,32]. The viability and proliferation assays, such as an MTT assay, allowed for biocompatibility of different, natural, or synthetic agents assessments [32,33]. Figure 11 presents the cytotoxicity against NHDF cells. The cytotoxicity test results showed that neither silicon nor any of the tested composites showed any toxicity to the normal fibroblasts. After 72 h, the fraction of living cells for the XS, 10 - B, and 10 - O groups showed similar values of 100%, 98%, and 93%, respectively. The remaining results showed significant cell proliferation when in contact with composites. The 20 - O and 20 - H composites showed the most proliferative values, for which the living cell fractions were 190% and 196%, respectively. The results confirm the biocompatibility of composites and any toxicity against the NHDF cell line in a standard in vitro cytotoxicity assay [34] was reported. *Polymers* **2021**, *13*, x FOR PEER REVIEW 14 of 18

in Table 5.

#### *3.8. Possibly of Using WPC on the Seabed 3.8. Possibly of Using WPC on the Seabed*

Figure 12 shows the image of composite samples before and after ageing in seawater. Figure 12 shows the image of composite samples before and after ageing in seawater. Based on the observation of plates covered with marine algae, it can be concluded that algae covered the composites with a higher wood waste content. The exception is the composite containing hornbeam waste in which higher algae content was observed on the sample containing 10 wt % wood waste. The silicone sample was also overgrown by marine algae. Most importantly, in all the samples tested, there is a larger or smaller sample of marine algae. After performing additional tests, this will enable the use of the tested composites as materials for use on the seabed. The results of this research are presented in Table 5.

Based on the observation of plates covered with marine algae, it can be concluded that algae covered the composites with a higher wood waste content. The exception is the composite containing hornbeam waste in which higher algae content was observed on the sample containing 10 wt % wood waste. The silicone sample was also overgrown by marine algae. Most importantly, in all the samples tested, there is a larger or smaller sample of marine algae. After performing additional tests, this will enable the use of the tested composites as materials for use on the seabed. The results of this research are presented

**Figure 12.** Comparison of plates with composites before and after immersion in a marine aquarium*.*

**Table 5.** Results of microscopic research: wood filler and aged samples. *Polymers* **2021**, *13*, x FOR PEER REVIEW 15 of 18 *Polymers* **2021**, *13*, x FOR PEER REVIEW 15 of 18 *Polymers* **2021**, *13*, x FOR PEER REVIEW 15 of 18 *Polymers* **2021**, *13*, x FOR PEER REVIEW 15 of 18 *Polymers* **2021**, *13*, x FOR PEER REVIEW 15 of 18 *Polymers* **2021**, *13*, x FOR PEER REVIEW 15 of 18 *Polymers* **2021**, *13*, x FOR PEER REVIEW 15 of 18 *Polymers* **2021**, *13*, x FOR PEER REVIEW 15 of 18

> film on their surface. Along with filler content, the number of marine algae increased and the color of the silicone matrix changed. At the same time, it should be noted that in the case of spruce waste, a smaller algae growth rate was observed compared with composites filled with deciduous tree waste. Samples with 10 wt % filler showed more diffuse growth than those with 20 wt % filler. Composites containing hornbeam waste were the most overgrown, while the least overgrown were those with beech filling, possibly due to the hardness of the tree (beech shows the lowest hardness among deciduous trees). This also explains the observations of the spruce composite filling. It can therefore be concluded that more algae growth occurred on harder trees (i.e., those containing more cellulose fibers). In order to select the best filler and content, a simplified multicriteria assessment tafilm on their surface. Along with filler content, the number of marine algae increased and the color of the silicone matrix changed. At the same time, it should be noted that in the case of spruce waste, a smaller algae growth rate was observed compared with composites filled with deciduous tree waste. Samples with 10 wt % filler showed more diffuse growth than those with 20 wt % filler. Composites containing hornbeam waste were the most overgrown, while the least overgrown were those with beech filling, possibly due to the hardness of the tree (beech shows the lowest hardness among deciduous trees). This also explains the observations of the spruce composite filling. It can therefore be concluded that more algae growth occurred on harder trees (i.e., those containing more cellulose fibers). film on their surface. Along with filler content, the number of marine algae increased and the color of the silicone matrix changed. At the same time, it should be noted that in the case of spruce waste, a smaller algae growth rate was observed compared with composites filled with deciduous tree waste. Samples with 10 wt % filler showed more diffuse growth than those with 20 wt % filler. Composites containing hornbeam waste were the most overgrown, while the least overgrown were those with beech filling, possibly due to the hardness of the tree (beech shows the lowest hardness among deciduous trees). This also explains the observations of the spruce composite filling. It can therefore be concluded that more algae growth occurred on harder trees (i.e., those containing more cellulose fibers). In order to select the best filler and content, a simplified multicriteria assessment tafilm on their surface. Along with filler content, the number of marine algae increased and the color of the silicone matrix changed. At the same time, it should be noted that in the case of spruce waste, a smaller algae growth rate was observed compared with composites filled with deciduous tree waste. Samples with 10 wt % filler showed more diffuse growth than those with 20 wt % filler. Composites containing hornbeam waste were the most overgrown, while the least overgrown were those with beech filling, possibly due to the hardness of the tree (beech shows the lowest hardness among deciduous trees). This also explains the observations of the spruce composite filling. It can therefore be concluded that more algae growth occurred on harder trees (i.e., those containing more cellulose fibers). In order to select the best filler and content, a simplified multicriteria assessment tafilm on their surface. Along with filler content, the number of marine algae increased and the color of the silicone matrix changed. At the same time, it should be noted that in the case of spruce waste, a smaller algae growth rate was observed compared with composites filled with deciduous tree waste. Samples with 10 wt % filler showed more diffuse growth than those with 20 wt % filler. Composites containing hornbeam waste were the most overgrown, while the least overgrown were those with beech filling, possibly due to the hardness of the tree (beech shows the lowest hardness among deciduous trees). This also explains the observations of the spruce composite filling. It can therefore be concluded that more algae growth occurred on harder trees (i.e., those containing more cellulose fibers). film on their surface. Along with filler content, the number of marine algae increased and the color of the silicone matrix changed. At the same time, it should be noted that in the case of spruce waste, a smaller algae growth rate was observed compared with composites filled with deciduous tree waste. Samples with 10 wt % filler showed more diffuse growth than those with 20 wt % filler. Composites containing hornbeam waste were the most overgrown, while the least overgrown were those with beech filling, possibly due to the hardness of the tree (beech shows the lowest hardness among deciduous trees). This also explains the observations of the spruce composite filling. It can therefore be concluded that more algae growth occurred on harder trees (i.e., those containing more cellulose fibers). In order to select the best filler and content, a simplified multicriteria assessment ta-Microscopy images of samples aged in seawater showed the growth of marine algae film on their surface. Along with filler content, the number of marine algae increased and the color of the silicone matrix changed. At the same time, it should be noted that in the case of spruce waste, a smaller algae growth rate was observed compared with composites filled with deciduous tree waste. Samples with 10 wt % filler showed more diffuse growth than those with 20 wt % filler. Composites containing hornbeam waste were the most overgrown, while the least overgrown were those with beech filling, possibly due to the hardness of the tree (beech shows the lowest hardness among deciduous trees). This also explains the observations of the spruce composite filling. It can therefore be concluded that more algae growth occurred on harder trees (i.e., those containing more cellulose fibers). Microscopy images of samples aged in seawater showed the growth of marine algae film on their surface. Along with filler content, the number of marine algae increased and the color of the silicone matrix changed. At the same time, it should be noted that in the case of spruce waste, a smaller algae growth rate was observed compared with composites filled with deciduous tree waste. Samples with 10 wt % filler showed more diffuse growth than those with 20 wt % filler. Composites containing hornbeam waste were the most overgrown, while the least overgrown were those with beech filling, possibly due to the hardness of the tree (beech shows the lowest hardness among deciduous trees). This also explains the observations of the spruce composite filling. It can therefore be concluded that more algae growth occurred on harder trees (i.e., those containing more cellulose fibers). Microscopy images of samples aged in seawater showed the growth of marine algae film on their surface. Along with filler content, the number of marine algae increased and the color of the silicone matrix changed. At the same time, it should be noted that in the case of spruce waste, a smaller algae growth rate was observed compared with composites filled with deciduous tree waste. Samples with 10 wt % filler showed more diffuse growth than those with 20 wt % filler. Composites containing hornbeam waste were the most overgrown, while the least overgrown were those with beech filling, possibly due to the hardness of the tree (beech shows the lowest hardness among deciduous trees). This also explains the observations of the spruce composite filling. It can therefore be concluded that more algae growth occurred on harder trees (i.e., those containing more cellulose fibers).

the composites with the best properties and 1 the worst. The same grade was assigned for

the composites with the best properties and 1 the worst. The same grade was assigned for

the composites with the best properties and 1 the worst. The same grade was assigned for

ble was developed (Table 6). The grading scale was adopted from 1 to 5, where 5 means the composites with the best properties and 1 the worst. The same grade was assigned for

the composites with the best properties and 1 the worst. The same grade was assigned for

the composites with the best properties and 1 the worst. The same grade was assigned for

the composites with the best properties and 1 the worst. The same grade was assigned for

the composites with the best properties and 1 the worst. The same grade was assigned for

Microscopy images of samples aged in seawater showed the growth of marine algae

Microscopy images of samples aged in seawater showed the growth of marine algae

Microscopy images of samples aged in seawater showed the growth of marine algae

Microscopy images of samples aged in seawater showed the growth of marine algae

Microscopy images of samples aged in seawater showed the growth of marine algae

Microscopy images of samples aged in seawater showed the growth of marine algae

**Figure 11.** Cytotoxicity test results (%)*.* 

0% 20% 40% 60% 80% 100% 120% 140% 160% 180% 200%

Survival fraction (%)

*3.8. Possibly of Using WPC on the Seabed* 

**Figure 12.** Comparison of plates with composites before and after immersion in a marine aquarium.

**Figure 12.** Comparison of plates with composites before and after immersion in a marine aquarium*.*

Based on the observation of plates covered with marine algae, it can be concluded that algae covered the composites with a higher wood waste content. The exception is the composite containing hornbeam waste in which higher algae content was observed on the sample containing 10 wt % wood waste. The silicone sample was also overgrown by marine algae. Most importantly, in all the samples tested, there is a larger or smaller sample of marine algae. After performing additional tests, this will enable the use of the tested composites as materials for use on the seabed. The results of this research are presented in Table 5. In order to select the best filler and content, a simplified multicriteria assessment table was developed (Table 6). The grading scale was adopted from 1 to 5, where 5 means the composites with the best properties and 1 the worst. The same grade was assigned for identical averaged test results. Assessments were made for each percentage of filling separately, based on the results of the tests carried out. The results of the cytotoxicity and ageing tests were not included in the multicriteria analysis. Cytotoxicity measured in the MTT test showed no cytotoxicity of all materials against normal cells, NHDF and more accurate interaction of materials with cells requires additional tests. In turn, the aging was assessed on the basis of macro and microscopic photos of the surface, which makes it impossible at this stage to assess changes in the mechanical and physicochemical properties of the materials.

Figure 12 shows the image of composite samples before and after ageing in seawater.

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

Materials

**Table 6.** Quality assessment of wood–silicon composites, Wc—criterion weight, E—evaluation, R—result.


#### **4. Conclusions**

The conducted research allows the following conclusions to be reached:

The density of composites changed with filler content and the type of wood. A reduction of approximately 3–5% in density was observed for the 10% fill composites and for 20% fill and an increase of up to 4%. This is mainly due to the structure of wood waste and the density of fillers. However, on the basis of statistical analysis, it was found that the examined variables are of little importance for the tested characteristic.

The introduction of wood filler into composites increased the hardness, which is not directly proportional to the hardness of trees. The highest hardness was characteristic for the composite with 10 and 20% spruce filler (density 700–850 kg/m<sup>3</sup> ), which may be related

to the particle size of 998.13 µm and the resin presence probably additionally hardening the composite. At the same time, the silicone-spruce composite shows the smallest abrasive wear by an average of approximately 30% compared to deciduous trees fillers. Both the size and type of the tree from which the waste was obtained significantly influences the hardness and abrasion.

For 20% at the higher the resilience and the lower the strain and stress at break was observed. All composites with 20 wt % filling, showed higher resilience and lower values of the characteristics determined in the static tensile test. For the 10% share, there is no unequivocal dependence of the resilience on the filler content in the composite. It seems that in this case both the type of wood and the property gradient resulting from the casting technology are important. The tested characteristics are strongly dependent on the weight fraction of the filler and the properties of the wood waste.

Conducted aging tests in the sea water environment showed that the area covered with algae increased with the increase of the filler, which was related to the type and structure of wood waste. All composites stimulated the proliferation of normal cells of the human body, demonstrating their lack of toxicity on normal fibroblasts, NHDF cells.

The multicriteria analysis proves that the best test results were obtained for composites filled with spruce waste, both with 10% by weight and with 20% by weight.

Further research will allow for a more complete characterization of WPC as materials for use in protective coatings, insulation systems, or packaging.

**Author Contributions:** M.M. and M.S. (Małgorzata Szymiczek) conceived, designed and carried out the experiments, analyzed data and wrote the paper. M.S. (Magdalena Skonieczna) was responsible for cytotoxicity analysis. All authors were involved in revising the paper's important content. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Acknowledgments:** The authors would like to thank Mateusz Lipiarz (a student doing the master's thesis) for his help in carrying out the research. Mikołaj Mrówka (AGH University of Science Technology, Poland) is acknowledged for help with Figures formatting.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**


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