Some of the physical and mechanical properties of the materials are shown in Table 4. The relations of the data in this table with each other were also examined with graphics.
Table 4 Some physical and mechanical properties of the materials.
Materials
|
Physical and mechanical properties
|
MC (%)
|
ρ
(kg/m3)
|
σ10
(N/mm2)
|
σm
(N/mm2)
|
σT
(N/mm2)
|
Dimensional stability (DS)
|
Δεl
(%)
|
Δεb
(%)
|
Δεd
(%)
|
A1
|
10.40 (0.20) *
|
273.113 (11.06)
|
0.266 (0.03)
|
0.0027 (0.00)
|
0.1498 (0.01)
|
-0.203 (0.13)
|
0.086 (0.22)
|
2.897 (0.81)
|
A2
|
10.42 (0.15)
|
257.228 (2.19)
|
0.304 (0.01)
|
0.0129 (0.03)
|
0.1318 (0.01)
|
-0.393 (0.11)
|
-0.136 (0.02)
|
1.042 (0.15)
|
A3
|
11.29 (0.31)
|
262.334 (2.58)
|
0.349 (0.01)
|
0.0095 (0.07)
|
0.0756 (0.04)
|
-0.101 (0.01)
|
-0.132 (0.16)
|
-0.067 (0.21)
|
B1
|
11.99 (0.10)
|
305.425 (2.40)
|
0.793 (0.05)
|
0.0258 (0.00)
|
0.3046 (0.01)
|
-0.167 (0.04)
|
0.044 (0.09)
|
0.632 (2.31)
|
B2
|
10.79 (0.32)
|
292.223 (10.65)
|
0.722 (0.02)
|
0.0221 (0.01)
|
0.1793 (0.02)
|
-0.163 (0.10)
|
-0.108 (0.05)
|
5.180 (0.23)
|
B3
|
10.50 (0.36)
|
298.378 (6.10)
|
0.891 (0.02)
|
0.0220 (0.02)
|
0.2162 (0.01)
|
-0.107 (0.25)
|
0.335 (0.10)
|
1.716 (2.59)
|
C1
|
10.32 (0.16)
|
263.370 (9.79)
|
0.364 (0.04)
|
0.0061 (0.00)
|
0.1473 (0.02)
|
-0.217 (0.08)
|
-0.258 (0.16)
|
-0.838 (0.66)
|
C2
|
11.62 (1.79)
|
252.695 (10.07)
|
0.324 (0.02)
|
0.0068 (0.00)
|
0.1693 (0.02)
|
-0.112 (0.03)
|
-0.079 (0.04)
|
2.416 (0.44)
|
C3
|
10.33 (0.09)
|
244.485 (3.84)
|
0.315 (0.01)
|
0.0057 (0.00)
|
0.0681 (0.00)
|
-0.103 (0.10)
|
-0.042 (0.09)
|
1.837 (3.01)
|
D1
|
8.92 (2.04)
|
261.286 (8.18)
|
0.310 (0.02)
|
0.0082 (0.00)
|
0.1315 (0.01)
|
-0.156 (0.05)
|
0.028 (0.24)
|
2.039 (2.24)
|
D2
|
9.59 (0.26)
|
261.574 (9.61)
|
0.328 (0.03)
|
0.0045 (0.00)
|
0.1403 (0.00)
|
-0.116 (0.00)
|
-0.221 (0.04)
|
0.184 (0.82)
|
D3
|
9.81 (0.66)
|
259.875 (6.68)
|
0.414 (0.03)
|
0.0093 (0.00)
|
0.0897 (0.03)
|
-0.090 (0.03)
|
-0.145 (0.11)
|
-0.730 (1.20)
|
E1
|
15.96 (3.48)
|
275.383 (10.31)
|
0.739 (0.11)
|
0.0117 (0.01)
|
0.2098 (0.03)
|
-0.075 (0.06)
|
-0.055 (0.04)
|
1.800 (0.10)
|
E2
|
9.96 (0.07)
|
272.094 (9.10)
|
0.709 (0.03)
|
0.0089 (0.01)
|
0.1279 (0.01)
|
-0.007 (0.31)
|
-0.088 (0.10)
|
-4.413 (1.25)
|
E3
|
7.10 (0.51)
|
266.287 (10.06)
|
0.563 (0.05)
|
0.0026 (0.00)
|
0.1048 (0.01)
|
-0.327 (0.01)
|
-0.042 (0.13)
|
0.431 (1.11)
|
* Values in brackets are standard deviations.
3.1 Moisture content (MC)
The MC of the materials was in the range of 7.10%-15.96% (Table 4). The mean MC of all materials was 10.60% with a standard deviation of 1.82%. As seen in Fig. 4, the MC values changed in direct proportion to particle size in the 100% bark-based group A materials and inversely in the 100% cone-based group B materials. The variation of the MC values in the perlite containing materials is shown in Fig. 5. It was determined that the perlite ratio had an effect that increased the MC in the D group materials and decreased it in the E group materials. It was observed that perlite did not have the same effect in both groups due to the structural differences of the bark and the cone. The samples containing perlite were expected to have higher MC than the others. However, except for E1, the MC values of the D and E group samples were lower. This can be explained by the increased density of perlite under pressure and the decrease in hygroscopicity. Kain et al. (2012, 2013b) produced materials with particle sizes between 0-45 mm from pine (Pinus sylvestris) bark, and found a mean MC of 12.2% with a standard deviation of 0.6%. The MC of 20 mm thick boards produced from Larch (Larix decidua Mill.) bark was reported to be 15.6% with a standard deviation of 0.7% (Kain et al. 2015). Tsalagkas et al. (2019) found the MC of the boards produced from poplar (Populus sp.) bark was in the range of 7.29%-9.12%. Materials produced from 19–25.4 mm thick larch and poplar bark were reported to have MC in the range of 3%-8.6% (Tudor et al. 2020b). On the other hand, the MC of 19-21 mm thick materials produced from bark of spruce (Picea abies L.) and larch (Larix decidua Mill.) was reported to be 8%-9% (Tudor et al. 2020a). In general, although the MC results obtained in this study were similar to some of the studies mentioned above, there were differences in terms of wood type, raw material particle size, pressing technique, board form, other materials added to the board, and board thickness.
3.2 Density
Density has a significant effect on many properties of the material, such as water absorption, bending strength, and thermal insulation performance (Xing et al. 2007; Gupta et al.2011). The lowest and highest densities of the materials were 244.48 kg/m3 in the C3 group and 305.43 kg/m3 in the B1 group, respectively. The mean density was 269.70 kg/m3 with a standard deviation of 16.50. In this study, it was observed that the densities of the materials were affected by the structural properties such as raw material density, fiber, extractive substance, etc., and production parameters such as gluing, laying, additives, and compression ratio. In this context, although the expected results were obtained in some of the materials; some were not available.
It is thought that the density differences are due to the different resistance of bark and cones with different particle sizes under pressure. Figs. 6 and 7 give clues about density-particle size and density-perlite ratio relationships. Table 4 shows that there was a regular variation depending on the bark/cone mixture in the group C samples and depending on the cone/perlite mixture in the group E samples. However, in the other groups, it was observed that the density results did not have a regular variation depending on the content. It was expected that there would be an inverse proportion between particle size and material density in the group A and B samples, but it was seen that there was no such relationship. The main reasons leading to this situation are considered as non-homogeneous internal adhesion, laying and pressing errors. The density of the cone particles was higher than that of the bark. Therefore, a higher density was measured in the B and E group materials than the others due to the cone content. The group D samples consisting of shell-perlite mixture were expected to have the lowest density, but it was determined that it was not so due to the possible errors mentioned above. In general, it is preferred that the densities of insulation materials are low (ρ=10-1000 kg/m3) because the main factor that makes the insulation is the air gaps in the material. In other words, the thermal insulation properties of materials with low unit volume weights are better than materials with higher unit volume weights (Akıncı 2007). Studies with similar raw materials were examined in the literature. For example, the densities of 20 mm thick materials produced from 6-10 mm thick larch (Larix decidua Mill.) bark were between 270-540 kg/m3 (Kain et al. 2014), the densities of 20 mm thick boards produced from the bark and woods of Picea abies, Pinus sylvestris, and Abies alba were between 350-500 kg/m3 (Kain et al. 2013a), the densities of 10-20-30-40 mm thick boards produced from 1-8 mm, 8-13 mm and 13-45 mm black locust (Robinia pseudoacacia) barks were between 185.8-548.3 kg/m3 (Pásztory et al. 2017), and the densities of 30 mm thick materials produced from the 3-6 mm and 10-30 mm particles of larch (Larix decidua Mill.) barks were found to be between 191-609 kg/m3 (Kain et al. 2018). Moreover, the densities of 10 mm thick boards produced using stone pine (Pinus pinea L.) cones were between 730-760 kg/m3 (Ayrilmis et al. 2009), and the densities of 20 mm thick materials produced using poplar (Populus sp.) bark were 336.80-413.07 kg/m3 (Tsalagkas et al. 2019), and the mean densities of the boards produced with pine and larch bark were reported to be 960 kg/m3 (Rudenko 2010). When the densities of the materials in the above studies were compared with the data in this study, it was seen that the mean density was 269.717 kg/m3 and that the change interval of 244.485-305.430 kg/m3 was considerably lower than them.
3.3 Mechanical properties
There is a need for thermal insulation materials with sufficient compressive strength in horizontal or slightly inclined applications in buildings. If the thermal insulation material does not have sufficient compressive strength, the material will be deformed against the forces acting on it from the external environment and will not be able to fulfill the task expected from it (Akıncı 2007). One of the important features of thermal insulation materials is the mechanical resistance they show against loads of variable duration. In thermal insulation materials, a decrease in the thickness of more than a certain value causes an unacceptable deterioration in the performance of the material. At this time, even if the material carries a load, it cannot fulfill its main task. For this reason, in thermal insulation materials, the compressive stress at 10% deformation (that is when there is a 10% decrease in thickness) is taken as the basis. This value is called compressive stress at 10% deformation, and is shown by σ10. The highest compressive strength was measured with 0.891 N/mm2 in the B3 group material, and the lowest compressive strength was measured in the A1 group material with 0.266 N/mm2 (Table 4). The mean compressive strength value was found to be 0.493 N/mm2 with a standard deviation of 0.21. On the other hand, the highest values were recorded in the B and E group materials containing cones. It is considered that the main reasons for this variability in the data were the density differences of the raw materials (the average bulk density of the bark was 210-220 kg/m3 and the average bulk density of the cone was 220-270 kg/m3) and structural differences. In addition, the possible reason for the low compressive resistance of the perlite-containing groups may be that perlite and the other raw materials could not provide good internal bonding. On the other hand, the compressive strength increased in direct proportion to the particle size in the group A materials and in direct proportion to the perlite ratio in the group D materials. It decreased inversely with the perlite ratio in the E group materials and inversely with the cone ratio in the C group materials. No regular change was observed in the B group materials. Fig. 8 shows the particle size-density-compressive strength relationships. The general expectation is that the compressive strength increases with the increase in density. However, the findings show that there was no such a relationship between density and compressive strength; however, it showed that the particle size increase had a positive effect on the compressive strength. It is thought that gluing and pressing errors had an effect on the results, especially in the production of core layers.
Fig. 9 gives an idea about the perlite ratio-density-compressive strength relationships. As the perlite ratio increased in the D group materials containing shell, the compressive strength increased, while it decreased in the E group materials containing cones. It was determined that the compressive strength changed inversely with the density of the group D materials and directly proportional to the density of the group E materials. Fig. 10 shows the behavior of the group A materials in the compressive resistance test. According to the graph, the maximum deformation of the A1, A2 and A3 group materials occurred at the end of the application of force of 4.92 N, 7.53 N and 9.32 N, respectively. These results showed that the compressive strength increased as the particle size increased.
Fig. 11 shows the behavior of the group B materials in the compressive resistance test. According to the graph, the maximum deformation of the B1, B2 and B3 group materials occurred at the end of the application of force of 20.15 N, 17.61 N and 24.83 N, respectively. These results showed that there was an irregular relationship between particle size and compressive strength in the group B materials. Fig. 12 shows the behavior of the group C materials in the compressive resistance test. According to the graph, the maximum deformation of the C1, C2 and C3 group materials occurred at the end of the force application of 14.38 N, 10.01 N and 8.60 N, respectively. These results showed that the compressive strength of C group materials decreased as the shell ratio decreased.
Fig. 13 shows the behavior of the group D materials in the compressive strength test. According to the graph, the maximum deformation of the D1, D2 and D3 group materials occurred at the end of the force application of 14.87 N, 9.15 N and 12.20 N, respectively. These results showed that although there was no linear relationship between the perlite content and the compressive strength in the group D materials, the general trend was that the perlite ratio decreased the compressive strength. Fig. 14 shows the behavior of the group E materials in the compressive strength test. According to the graph, the maximum deformation of the E1, E2 and E3 group materials occurred at the end of the application of force of 19.34 N, 19.58 N and 14.90 N, respectively. These results showed that although there was no linear relationship between the perlite content and the compressive strength in the group E materials, the general trend was that the perlite ratio increased the compressive strength.
The force-deformation graphs showed that in general, all materials underwent a similar plastic deformation from the beginning of the test up to 2 N, and that after this point, a permanent deformation took place. In summary, the group A, C and D materials containing shells had lower compressive strength than the others; the group B materials showed the highest compressive strength; and the perlite content had a different effect depending on whether the raw material was bark or cone. It was determined that the structural properties and production parameters of the bark and cone also affected the compressive strength. The compressive stress/strength of mineral wools at 10% deformation, which is one of the materials currently widely used in the insulation sector, is given between 0.5-500 kPa in TS EN 13162+A1. The compressive stress at 10% deformation for EPS is given as 30-500 kPa in TS EN 13163:2012+A2, and the compressive stress/strength of XPS at 10% deformation is given as 100-1000 kPa in TS EN 13164+A1. According to EN 13171:2012, the compressive stress at 10% deformation required by wood fiber thermal insulation boards was reported between 5-500 kPa. The compressive strength data at 10% deformation of all material groups produced in this study met the requirements in the standards given above. (Kain et al. 2012, 2013b) reported that the panel density showed a highly significant (p<0.001) positive correlation with CR and some other mechanical properties (Dost 1971; Kain et al. 2012). It was reported that the increase in density did not directly affect the mechanical properties (Gupta et al. 2011), and that the mechanical properties of shell-based materials produced in different particle sizes weakened as the particle size increased (Kain et al. 2012). In addition to the particle size, which determines the density on mechanical properties, the glue ratio, gluing, and pressing conditions should not be ignored. Giannotas et al. (2022) measured the CRs of the composites produced from cement and Scots pine (Pinus sylvestris L.) and black pine (Pinus nigra L.) shells between 4.15-17.77 N/mm2, and they reported that the CR ratio decreased as the content of bark increased. In a study, the composites of different densities were produced using gypsum, cork (Quercus suber) and sawdust, and CRs were found between 2.27-6.58 N/mm2. It was reported that there was an improvement in compressive strength values in parallel with the increase in density (Hernández-Olivares et al. 1999). IB and TS are the mechanical properties that are expected to meet the standards in thermal insulation materials. These properties give clues as to whether the components that make up the material are sufficiently bonded to each other, and thus whether the integrity of the material can be maintained for long periods of time during use. On the other hand, it is important that the material maintains its dimensional stability during storage, transportation, and in variable environmental conditions (temperature, humidity, wind, etc.) at the place of use. Otherwise, undesirable effects, such as surface blistering, fluctuations, and pulling back of fasteners, may occur.
According to Figs. 15 and 16, the TS of the group A, B, D and E materials was higher than IB. The results are expected to be like this due to the hollow structure of the materials, the shape of the specimens in the IB and TS tests, and the test mechanisms. The IB and TS of the group B materials were higher than those of the group A materials. This result was given by the fibrous structure of the cones that made up the B group materials and thus a stronger bonding in gluing. The TS of the group A materials decreased as the particle size increased, whereas the IBs did not show a regular variation. While the TSs of the group B materials were expected to decrease in parallel with the increase in particle size, it was observed that there was no regular decrease. This result is thought to be caused by some deficiencies or errors in the production stages. The IBs of the B group materials were very close to each other. The TS of the D group materials was lower than that of the E group materials. It was found that as the perlite ratio in the material content increased, the TS values generally decreased and that the IB values were independent of these ratios. The IB values of the three-layer MDF with a density of 850 kg/m3 produced using black spruce bark in the middle layer were measured between 0.37-0.58 N/mm2 (Xing et al. 2007). (Kainet al. 2015) determined the IBs of the composite materials produced from larch shells at densities of 500-450-400-350-300-250 kg/m3 between 0.32-0.06 N/mm2. Particle boards were produced by using the bark of black spruce (Picea mariana (Mill.)) and trembling aspen (Populus tremuloides (Michx.)) in fine, medium and coarse grain sizes. It was determined that the mechanical properties weakened as the bark ratio increased, and that the materials with fine bark grain size showed higher IB (Yemele et al. 2008b, a). In addition, the IB values of the particle boards produced by (Buyuksari et al. 2010) with pine (Pinus nigra Arnold var. pallasiana) and beech (Fagus orientalis Lipsky) wood and pine (Pinus pinea L.) cones were between 0.29-0.57 N/mm2, and it was stated that as the cone ratio of the boards increased, the IB values decreased. In some studies, the bending and breaking strengths of composite materials produced using Polycaprolactone (CAPA 6800) pine (Pinus pinea L.) cones were inversely proportional to the cone ratio (Jha et al. 2018), and the mechanical properties of composites produced from polypropylene and pine (Pinus pinea L.) cones improved with the increase in cone ratio (Arrakhiz et al. 2012). In this study, the dimensional change results of the materials were found to be -0.156% (Δεl), -0.054% (Δεb) and 0.942% (Δεd). The maximum and minimum Δεl values were -0.007% and -0.393% in the E2 and A2 groups; the maximum and minimum Δεb values were 0.335% and -0.258% in the B3 and C1 groups, and the maximum and minimum Δεd values were 5.180% and -4.413% in the B2 and E2 groups, respectively. As can be seen in the findings, the dimensional changes of the materials occurred at reasonable rates, and the dimensional stability data in terms of width and length gave better results than thickness. As possible reasons for the differences, it can be suggested that the free-falling particles in the laying phase generally overlap in the horizontal position and provide better bonding, and that especially the pressure applied in the production of the middle layer is lower than that of the surface layers.