Performance of Thermal Insulation Material Produced from Lithuanian Sheep Wool

ABSTRACT In the current paper, the thermal insulation material from the wool of sheep grown in Lithuania is analyzed. Three types of thermal insulation material were produced: horizontal orientations, corrugated from the individual layer obtained, and corrugated from individual mats. The 50 mm thick products were prepared and their main performance characteristics were evaluated: thickness changes, thermal conductivity, tensile strength, short-term water absorption, and flammability. Changes in the thickness of the materials were determined under different loads, and dependences on the density of the material were obtained. Thermal conductivity was measured for products of different densities, evaluated separately for horizontal orientation and corrugated products, and density dependences were obtained. Tensile strength was determined along and across the direction of formation for relatively different types of products. For all types of products, the short-term water absorption of the specimens was determined by partial immersion in water, and the results were compared. During the flammability tests, the flame propagation time on the specimen surface and the fact of incineration after removal of the flame source were determined for all types of specimens.


Introduction
In the last two decades, much attention has been paid to the development of thermal insulation materials from renewable resources.The spectrum of raw materials used for the creation of such materials is very wide, from various plant straws or their processing waste, wood processing waste, and raw materials of animal origin, feathers, wool, etc. Sheep wool is commonly used because it is available in many parts of the world.In most cases, sheep are reared for meat, so large amounts of coarse wool are not suitable for the clothing industry and are recycled as waste.The authors (Patnaik et al. 2015;Report (National Weekly Wool Market Report), 2014) pointed out that there is a large amount of poor quality sheep wool in South Africa at a cost of less than 0.3 US dollars per kilogram.
Wool thermal insulation has been produced in Australia, New Zealand, UK, Austria, and other countries for more than 20 years (Symons, Clarke, and Peirce 1995;Thermafleece 2022;Ye et al. 2006;Sheep's wool insulation, 2022).Although sheep wool thermal insulation has been produced and used for several decades, the properties of sheep wool products are not yet fully explored.Huson (2018) noted that wool is a material whose properties are highly susceptible to changes in humidity.Zach et al. (2012) drew attention to the positive ecological and health impact of thermal insulation products made from sheep wool fibers.The authors identified the following main advantages of sheep wool insulation: it is easily renewable, recyclable, and an environmentally friendly source of raw material.
A wide range of insulation products can be made from sheep wool: soft or semi-rigid boards, mats, or loose-fill thermal insulation (Bosia et al. 2015;Rabbat et al. 2022).Italian researchers (Bosia et al. 2015) have divided sheep wool insulation products into two categories: soft mats made of 100% sheep wool and semi-rigid boards made of sheep wool (70-80%) and polyester fibers (20-30%).The authors stated that in the production of semi-rigid boards through the partial fusion of polyester fibers, the stiffness of thermal insulation products is sufficient for use in walls.
The aim of this work is to produce a thermal insulation material suitable for thermal insulation of building partitions from low-quality wool.This article examines three types of wool products: horizontally oriented fibers, finely corrugated, and coarsely corrugated.In the article, changes in the thickness of the obtained products under the influence of loads, thermal conductivity of products with different density and fiber orientation, tensile strength, short-term absorption, and flammability were evaluated.

Materials and their preparation
Sheep wool from Lithuanian farms was used for the experiments.Wool of four sheep breeds was used in the current research: Skudde, German black-heads, Lithuanian black-heads, and German Merinolandschaf.The raw wool was washed in order to obtain wool with a fat content of less than 3%.During washing, the wool was first soaked and then washed with various detergents.A detailed description of sheep wool, its processing methods, and production process is provided in the manuscript (Vėjelis et al. 2022). 2 tons of washed wool were prepared from sheep's wool for the production of thermal insulation material.Wool production was carried out at the textile company UAB Neaustima (Šiauliai, Lithuania) using carding technology.Before carding, all wool was additionally cut to a length of 40-60 mm so that it does not get tangled in the carding machine.The diameter of the wool fibers varied from 22 to 35 μm, depending on the breed of sheep.The carded and prepared mat was cured in a curing chamber at a temperature of 140°C for 3 min.To obtain the desired thickness of corrugated products, individual layers of carded matt were laid on top of each other with the help of a crosslapper in a set mode.In the case of coarse corrugation, the mat already carded and obtained with the help of the crosslapper of the desired thickness was again passed through the crosslapper, but the crosslapper mode was set so that one conveyor of the crosslapper moved forward and backward, thus laying the mat in waves.All prepared mats were pressed with rollers to maintain their shape and obtain the intended thickness of the product during curing.During production, thermal insulation materials made of sheep wool with a density range of 16 to 30 kg/m 3 were obtained.Three different types of products were prepared for testing (see Figure 1).The first type consists of separate layers of horizontally oriented fibers.The second type is finely corrugated products obtained from separate layers of horizontally oriented fibers.The third type is coarse corrugated products obtained from full-thickness material with horizontally oriented fibers.The fine corrugation was about 80 waves/m and the coarse corrugation was about 23 waves/m.
Horizontally oriented sheep wool fibers intertwine only in the horizontal plane (Figure 1a). Figure 1b shows a corrugated product made from the single carded layer, but only a very small portion of the layer is overlapped by subsequent layers.The width of the corrugation waves of these products is 10 to 15 mm.When the corrugation is performed from a full-thick material, the width of the corrugation waves is 40 to 45 mm (see Figure 1c).
In the current study, repeated tests were not performed.Each type of mat was prepared at the factory by a single mixing and produced one product sample.In our case, about 5 m 2 of the mat were consumed for all tests of one type, and this amount according to the standardized methodology is sufficient to ensure the required accuracy of the tests.
In current work more attention is paid to the preparation of products, the evaluation of technological factors and the search for trends, for example: whether corrugating the product has an impact on any characteristics; whether other characteristics change as the density changes; is short-term impregnation dependent on density and corrugation; and so on.

Thickness changes analysis under short-term load
When different loads are applied to loose-fill thermal insulation materials, a change in the thickness of the material is determined and a critical density is determined based on the changes in thickness, after which the material no longer decreases in thickness (Vėjelis et al. 2016).Similar principles may be applied to very low-density soft thermal insulation boards or mats.
We established a thickness changes analysis of sheep wool thermal insulation in specimens of 500 ± 5 mm.For the tests, the loads were selected in ascending order, 50, 100, 250, 500, 1000 and 2000 Pa.Every time after loading the thickness of the specimen was fixed and the density was calculated.A loading time was 5 min.Three measurements were performed for each type of product at each load point.

Thermal conductivity measurement
Test specimens with dimensions of (300 × 300) mm and thickness from 40 to 50 mm were prepared for the thermal conductivity coefficient tests.Before the test, all specimens were conditioned for 72 hours at a temperature of 23 ± 2°C and a relative humidity of 50 ± 5. The thermal conductivity tests of the materials were performed using the constant heat flow method according to the requirements of EN 12667 (2002).The thermal conductivity coefficients are determined at an average temperature of 10°C.Each specimen was pressed by the vertically moving top plate of the device to the selected specimen thickness, thus ensuring the intended specimen density.Thermal conductivity was determined in the density range from 16 to 71 kg/ m 3 .To avoid too much compression of the specimen, stacking of 1 to 4 specimens was used, maintaining a thickness from 40 to 60 mm of the measured specimen and minimizing the dispersion of the results due to the thickness of the specimen.160 measurements were made.Measurements were carried out in such a way that it was possible to see the trends of thermal conductivity as the density changed and to describe them with sufficient accuracy using regression equations.

Tensile strength across and along the direction of formation
The tensile strength of the specimen parallel to the surfaces was determined according to the methodology specified in the EN 2013EN :2013EN (2013) ) standard.Before the test, all specimens were conditioned for 24 hours at a temperature of 23 ± 2°C and a relative humidity of 50 ± 5. According to the test methods, the test directions for the tensile tests are parallel to the surface of the specimen.In addition, it is common practice for tensile tests on parallel surfaces to be performed in parallel and perpendicular to the direction of product formation.In our case, we did the horizontal orientation of products, and for corrugated products, we only perpendicular to the direction of corrugation because otherwise we would have determined the de-corrugation strength of the product rather than the strength of the product.
To determine the effect of the forming directions of the specimens on the tensile strength parallel to the surfaces, the specimens with horizontal fiber orientation were loaded across and along the forming directions, and the corrugated specimens were loaded perpendicular to the corrugation direction.Three specimens were prepared for each type of product.

Short-term water absorption
Short-term water absorption was determined according to method A of EN ISO 29,767 (2019);EN 12667 (2001).Before the test, all specimens were conditioned for 24 hours at a temperature of 23 ± 2°C and a relative humidity of 50 ± 5 and then placed in the empty water tank.According to the test methods, the test directions for short-term immersion are perpendicular to the surface of the specimen.The specimens were loaded with sufficient load to partially submerge them when water was added.The bottom surface of the specimen was 10 ± 2 mm below the surface of the water during the test.After immersion in water for 24 hours, the specimens were removed, weighed and the short-term absorption was calculated.For the tests two density specimens of horizontal fiber structure and corrugated fiber structure of 16.5 kg/m 3 and 31 kg/m 3 were used.Three samples were prepared for each type of product.For classic fibrous thermal insulation materials, the amount of short-term absorption allowed in an operational structure is 1 kg/m 2 (EN 13162:2012+A1:2015(EN 13162:2012+A1: 2012)).In order to achieve this value, usually all fibrous materials are additionally hydrophobized.In our study, the samples were additionally not coated with hydrophobizers.

Flammability
Combustibility tests were performed according to the requirements of ISO 11925-2 (2020).The method for determining flammability is based on the direct exposure of vertical samples to a small flame in the presence of zero external energy illumination.During the test, it was recorded whether the specimen ignites, how the flame spreads above the flame point, and the fact of smoldering is recorded.Three samples were prepared for each type of product.
In order to reduce the flammability of wool, we used Flovan CGN flame retardant.During previous studies (Stapulionienė et al. 2016) this agent was found to protect the thermal insulation material of fibrous hemp when a solution concentration of 45 g/l is used.In these studies, we chose a solution concentration of 35, 45 and 55 g/l.

Changes in thickness
In Figure 2 curves of the relationship between the density and load of wool products with different fiber orientations are presented.All tested specimens have characteristic curves.All curves were described by empirical equations with corresponding standard deviations (see Equations 1-4): (1) (3) where ρ i is the density of the material, F is the applied load and S r is the standard deviation, R 2 ρ�load is determination coefficient.
The trends of the curves are determined by the ratio of horizontal-vertical fibers and the density of the products.Studies of changes in the thickness of horizontally oriented sheep wool thermal insulation material have shown that there is a sharp increase in density in the initial zone as the load increases (see Figure 2, curve 1).This zone continues until a load of approximately 250 Pa is reached.When a load of 250 Pa is reached and the density is about 24 kg/m 3 , a sudden transition of the curve to the horizontal position is observed.Similar trends are observed when the material density of horizontally oriented fibers is increased by 2 times (see Figure 2, curve 2).The only difference is in the density of the specimen when the curve is horizontal.In this case, the density is approximately 34 kg/ m 3 .This means that in both the first and second cases, when the load increases, the fiber structure is compacted, during which compressive stresses act.When the level of compaction is reached, the fibers connected by the contact zones begin to stretch each other, tensile stresses are applied to the material, which change the direction of the curve and ensure the stability of the products.Curves 3 and 4 show different trends (see Figure 2).Corrugated products achieve stability at very low loads.Furthermore, the corrugation procedure results in a higher initial density of the specimens.
For products that are not subjected to a load during operation (except self-weight), stability is assessed by conducting dimensional stability studies under specified conditions or simply determining the thickness of the product under a load of 50 or 250 Pa [EN 823].According to the recommendations for the use of classical fibrous thermal insulation materials, we can assume that 10 kg/m 3 wool products are intended for the construction of frames in vertical structures and 30 kg/m 3 wool products are intended for the construction of frames in horizontal structures.The research we received shows that products with a density range from 24 to 31 kg/m 3 can be used for horizontal structures, and the density will mostly depend on the type of product chosen, horizontal fiber orientation or corrugation.

Thermal conductivity
The results of the thermal conductivity tests are presented in Figure 3.The test results are described by the empirical Equations ( 5) for products with horizontal structure and (6) for corrugated products with determination coefficients and standard deviations: and where λ 10 � C is the thermal conductivity of the material.Analysis of the results showed that the thermal conductivity is not affected by the structure of the product or that this influence is insignificant.Thermal conductivity varied from 0.032 to 0.048 W/ (mK) throughout the density range.In the lower density range, a larger scatter of results is observed, which is possibly due to the heterogeneity of the materials.When compressing a material, the areas of lowest density are compressed first, so as the density increases, the spread of results decreases because the density differences in the material also decrease.Products with a density range of 50-70 kg/m 3 are characterized by the lowest thermal conductivity.At this density, the products reach the limit where heat transfer through the air approaches zero.Meanwhile, in the low density range from 16 to 30 kg/ m 3 , a high coefficient of thermal conductivity is visible and heat transfer through air is the main component of thermal conductivity.
As shown by our research and the scientific works of other authors, the thermal conductivity of thermal insulating material from sheep wool varies within very wide limits.Changes in thermal conductivity are determined by many criteria, such as material density, product manufacturing method, binding material, humidity, thickness, etc.In our investigation, the thermal conductivity of the sheep wool thermal insulation material varied from 0.032 to 0.048 W/(mK).Very similar data were provided by Dénes et al. (2022).The thermal conductivity coefficient obtained by them ranged from 0.033 to 0.044W/(mK).However, the authors mentioned showed that the density of the tested specimens was significantly higher, from 32 to 248 kg/m 3 .Meanwhile, in our research, the highest thermal conductivity coefficient was obtained when the density was the lowest -from 16 to 25 kg/m 3 .
In the work of other authors (Zach et al. 2012), the density of the specimens varied from 20 to 40 kg/m 3 and the thermal conductivity from 0.034 to 0.040 W/(mK).In our studies, within this density range, the average values of the thermal conductivity ranged from 0.035 to 0.045 W/(mK).The higher value of the thermal conductivity coefficient of the specimens we measured could be due to different production technologies.The Czech researchers did not use a binding material, which prevented the formation of additional contact zones between the sheep's wool fibers.

Tensile strength
The results of the tensile strength of the specimens are presented in Figure 4.The different structure, density, and tensile direction of the specimens led to a large dispersion of the results.Analysis of the results shows that the greatest influence on tensile strength is not the contact zone between the fibers, but the orientation of the fibers themselves in the specimen.When the tensile load acts transversely to the direction of product formation, contact zones are affected, as well as interlaced fibers in the horizontal plane.For this reason, the transverse tensile strength in the product-forming direction is more than twice as high as the longitudinal tensile strength in the specimen-forming direction (see Figure 4, columns 1-4) at different densities.Comparing the tensile strength of the specimens with the horizontally oriented fibers according to the density shows that when the density is almost doubled, the tensile strength also increases slightly more than twice.Meanwhile, corrugation affects the tensile strength of the products differently.Finely corrugated specimens did not exhibit higher tensile strength than specimens with the same density of horizontally oriented fibers, or the increase in strength was insignificant.Meanwhile, the tensile strength of the coarsely corrugated specimens increased almost twice.This can be explained by the formation of high-density zones in individual areas of the product during corrugation.When corrugating a 50 mm thick mat at the wave point during folding, not only the density increases, but also the fibers on the inner side are very strongly compressed, so during tensile, not only the contact zones and interwoven fibers work, but also a large part of the fibers compressed together.Meanwhile, when the product is finely corrugated, the degree of compression of the fibers is low and the tensile strength is affected only by the individual fibers and not by their array.
Tensile strength tests are important when products are used to insulate vertical structures.Fibrous materials must have a tensile strength of at least two product weights.In this case, the double thickness of the product is calculated for a product width of 1 m.If we have a weight of 1 m 3 product of 30 kg, then the weight of a board of 5 cm is 1.5 kg and it must withstand a tensile force of 0.589 kPa.Our results show that this requirement is fulfilled in all cases.The lowest tensile strength was obtained at 3.3 kPa when the product has a horizontal fiber orientation, and the tensile test was performed along the direction of product formation.The result obtained in this way is more than 10 times higher than the intended requirement and more than 45 times higher for coarse corrugated specimens.Results of the tensile strength of the specimens: 1 -specimens with horizontally oriented fibers when the tensile direction across the direction of product formation and the density is 16 kg/m 3 ; 2 -when tensile direction along the direction of product formation and the density is 16 kg/m 3 ; 3 -by the tensile direction across the specimen and the density is 30 kg/m 3 ; 4 -when the tensile direction along the product and the density is 30 kg/m 3 ; 5 -of a finely corrugated specimen, the tensile direction perpendicular to the corrugation direction and the density is 31 kg/m 3 ; 6 -of a coarsely corrugated specimen, the tensile direction perpendicular to the direction of corrugation and the density is 31 kg/m 3 .
The analysis performed showed that the average results of the tensile strength of the obtained specimens are as follows:  kPa.The statistic of the F criterion is equal to 113.083, and p < .00000001150shows that there is a statistically significant difference in the tensile strength averages tested.
The coefficient of determination R 2 = 0.979 and the adjusted coefficient of determination R 2 = 0.971 represent the part of the variance of the dependent variable explained by the factor.The fiber orientation factor explains 97.9% of the variance in tensile strength in the specimen, and the population estimate of the explained variance is 97.1%.
Repeated analysis showed (see Figure 4) that there is no difference between specimens 1 and 4 (F = 0.0408, p = .849),and 3 and 5 are also not different (F = 0.496, p = .520).Meanwhile, specimen 2 has the lowest mean value and specimen 6 has the highest mean value.

Short-term water absorption
Figure 5 presents the results of the short-term water absorption.During the tests, the value of short-term water absorption ranged from 0.39 to 0.97 kg/m 2 .The lowest absorption is characterized by the lower-density specimens with a horizontal fiber structure, and the highest absorption is characterized by the higher-density specimens with a coarse corrugated structure.It can be argued that both density and structure have a significant influence on the absorption of the specimens.According to the results obtained, it can be concluded that the wool fiber itself does not tend to absorb or retain a lot of water, and a larger amount of water accumulates in the structure of the product itself.
Romanian researchers (Dénes et al. 2022) produced two types of thermal insulation material from sheep wool, for which they used natural rubber latex and acrylic polyurethane resin to bond.In the first case, the short-term water absorption ranged from 0.75 to 0.97 kg/m 2 and the results are very similar to our results.In the second case, absorption increased several times and ranged from 4.16 to 5.52 kg/m 2 .This shows that the properties of the binding material have the greatest influence on the absorption values.The structure of the wool fiber itself is very complex and consists of a whole series of micro-and macro-particles, so it is likely that they easily absorb water vapor from the air, but those particles are too small for water to enter the wool structure in a liquid state.Statistical analysis of short-term water absorption results showed that the averages of the results of the obtained specimens are as follows: 1-0.39 kg/m 2 ; 2-0.52 kg/m 2 ; 3-0.65 kg/m 2 ; 4-0.97 kg/m 2 (see Figure 5).The statistic of the F criterion is equal to 72.59, and p < .0000382,which shows that there is a statistically significant difference in the averages of the short-term water absorption studied.
The coefficient of determination R 2 = 0.965 and the adjusted coefficient of determination R 2 = 0.951 represent the part of the variance of the dependent variable explained by the factor.The fiber orientation factor explains 96.5% of the variance in partial water absorption in the specimen, and the population estimate of the explained variance is 95.1%.

Combustibility
Figure 6 (a and b) shows the propagation of the flame on the surface of the thermal insulation material made of sheep wool.Flame spreads very quickly on the surface of specimens that are not covered with flame retardants.Although the flame is quickly extinguished when a charred layer forms on the surface of the specimen, further smoldering of the specimen begins.
When the specimens are covered with flame retardant with a solution concentration of 35, then a slight spread of flame on the surface of the specimen is still observed (Figure 6c).However, when using a flame retardant solution concentration of 55, the flame spread over the surface of the specimen does not occur at all (Figure 6d).More detailed results of the flammability tests are presented in Table 1.
The results of the flame spread tests are presented in Figure 7.The test results are described by the empirical equations (7) for products with different content of flame retardant with determination coefficients and standard deviations: where H is the spread of the flame, mm, c is the concentration of flame retardant, g/l.There are not many scientific studies on the flammability of sheep wool.The authors (Zach et al. 2012) indicate that sheep wool has a self-extinguishing property, the fibers do not support combustion but are charred at high temperatures.Other scientists (Forouharshad et al. 2011) state that the natural flame-resistant properties of wool are related to its relatively high nitrogen content, high moisture content, high ignition temperature, low combustion heat, low flame temperature, and high limiting oxygen index.Scientists also note that wool in some cases needs a flame-resist treatment to pass a particular flammability specification and test method.The flame retardant wool was prepared by the authors using zirconium oxychloride with various acids.In another study (Horrocks 2001), the author states that wool has a high ignition temperature of 570-600°C due to its higher natural moisture content (8-16% depending on relative humidity), high nitrogen content (15-16%) and sulfur (3-4%) and low hydrogen content (6-7%) by mass.The author also states that thin samples ignite easily compared to thick ones because they have a high specific volume and oxygen accessibility.

Figure 1 .
Figure 1.Differently oriented wool fibers in the product: a) horizontal orientation; b) finely corrugated; c) coarsely corrugated.

Figure 3 .
Figure 3. Relationship between the coefficient of thermal conductivity and the density of the thermal insulation material made from sheep wool: • -products with horizontal structure; ○ -corrugated products.

Figure 4 .
Figure 4. Results of the tensile strength of the specimens: 1 -specimens with horizontally oriented fibers when the tensile direction across the direction of product formation and the density is 16 kg/m 3 ; 2 -when tensile direction along the direction of product formation and the density is 16 kg/m 3 ; 3 -by the tensile direction across the specimen and the density is 30 kg/m 3 ; 4 -when the tensile direction along the product and the density is 30 kg/m 3 ; 5 -of a finely corrugated specimen, the tensile direction perpendicular to the corrugation direction and the density is 31 kg/m 3 ; 6 -of a coarsely corrugated specimen, the tensile direction perpendicular to the direction of corrugation and the density is 31 kg/m 3 .

Figure 6 .
Figure 6.Flammability of the specimens: a -specimen not coated with flame retardants before the flame source starts to act; b -specimen not coated with flame retardants after the flammability test; c -specimens coated with flame retardant Flovan CGN, when solution concentration, g/l − 35; d -specimens coated with flame retardant Flovan CGN, when solution concentration, g/l 55.

Table 1 .Figure 7 .
Figure 7. Flame spread of the specimens with different content of flame retardant solution.