Textile Waste from Woollen Yarn Production as Raw Materials for Thermal Insulation Products

For the experiments, waste from the production of woollen yarn was used. Raw material was received from JSC Danspin, Lithuania. Unspun woollen fibres formed during various wool processing operations up to the spinning stage were used for the tests. According to [20], waste from the spinning of textile fibers accounts for about 11 percent. In addition, defibred woollen yarn waste was used. Three types of compositions were carded from woollen yarn waste in a carding machine. All compositions are presented in Table 1. The compositions presented were selected based on the results of previous studies [21, 21].

Polylactide (PLA) as the binding material was supplied by MAXModel s.a.s, France. The main characteristics of the PLA fibres were as follows: 4.4 dtex, length - 51 mm, and melting temperature - 130°C. PLA and wool fibres were mixed using an internal mixer. During the carding process, the mixed fibres were combed in the same direction to create composite webs. The resulting web of aligned fibres was then cross laid into several layers that overlapped. The degree of overlap depended on the desired density of the product. Part of the web produced was thermally untreated, and the other part of the prepared web was conveyed through an oven to allow the PLA to melt. The web was then hardened in an oven for 3 minutes at a temperature of 160°C. The thickness and density of the thermally treated composite were controlled by the two drums on which the material was conveyed. After compression between the drums, the material was transported out of the oven and naturally cooled at ambient air temperature to provide dimensional stability to the product. The thermally treated composite was cut to the required length and width.

The composition of the woollen yarn waste was determined by two methods. The length of fibres and their number in the raw material, as well as the composition of non-fibrous matter and the content of different fibres, were determined. In the first case, by combing the raw material by two hand carding combs, three fractions of fibres were obtained, and their mass was fixed. For that purpose, 5 samples of 40g each were taken from different parts of the wool package. In the second case, the amounts of non-fibrous matter were determined by extraction on Soxhlet apparatus according to ISO 1833-1 [23] and the different fibre content according to ISO 1833-4 [24], ISO 1833-7 [25] and ISO 1833-11 [26] standards.

Thermal conductivity measurements were carried out on specimens of 300 × 300 × 50 mm dimensions at a mean temperature of 10°C according to the requirements of EN 12667 [27] and EN ISO 8301 [28] using a device of symmetric configuration with horizontal heat flow meters and protection of the lateral surfaces of the specimens - FOX 304 (Laser Comp., USA). The direction of heat flow was upward, and the temperature difference between the cold and hot plates was 20°C. The specimens were preliminarily brought to balanced hygroscopic moisture at a temperature of 23 ±2°C and relative humidity of 50 ± 5%.

To analyse the structure of the composites, specimens of 40×40×40 mm were prepared. The specimens were inspected by a Zeiss EVO-50 EP scanning electron microscope (SEM). Additionally, the samples were coated with carbon prior to SEM measurement. The analysis was performed using the variable pressure mode at an accelerating voltage of 20 keV and working distance of 10 mm to 15 mm.

For processing of the experimental data and evaluation of their reliability, mathematical-statistical methods, along with the programme ‘STATISTICA’ were used [29].

Mathematical-statistical analysis of the experimental data led to a description of the relation between density and thermal conductivity λ₁₀, W/(m∙K) by regression equation (1): (1) λ¯10°C=b0+b1⋅ρ+b2ρ, {{\bar \lambda }_{10^\circ C}} = {b_0} + {b_1} \cdot \rho + {{{b_2}} \over \rho }, where λ¯10°C {{\bar \lambda }_{10^\circ C}} is the thermal conductivity value at the mean measurement temperature - 10°C in W/(m K); ρ is the density of the specimen in kg/m³; and b₀, b₁, b₂ are constant coefficients calculated according to the experimental data using the least-squares method [30].

The variation in the parameter examined λ¯10°C {{\bar \lambda }_{10^\circ C}} on the controlled input factor ρ is presented as the coefficient of determination R² (that is, the square of the correlation coefficient R).

The standard deviation S_r (a measure of the amount of variation or dispersion of a set of experimental values) was determined according to equation (2) [30]: (2) Sr=∑i=1i=n(λxi−λ¯xi)2n−m, {S_r} = \sqrt {{{\sum\nolimits_{i = 1}^{i = n} {{{\left( {{\lambda _{xi}} - {{\bar \lambda }_{xi}}} \right)}^2}} } \over {n - m}}} , where λ_xi and λ¯xi {{\bar \lambda }_{xi}} are the actual and i-value of the resulting characteristic calculated by Eq. (1); n is the number of test results, and m is the number of estimated constant parameters.

The main characteristics of thermal insulating materials are thermal conductivity and thermal resistance. Without the thickness of the materials, it is impossible to describe their thermal resistance. If a thermal conductivity indicator is used, the heat transfer intensity of different materials can be compared. A lower thermal conductivity shows a lower thermal transmittance of the material.

Before starting thermal conductivity studies of the composite, the thermal conductivity of combed woollen yarn waste and PLA fibres was investigated separately (Fig. 1). The thermal conductivity of the woollen yarn waste was determined in the density range of 20.7 kg/m³ to 55.2 kg/m³, respectively; the thermal conductivity in this density range ranged from about 0.0421 W/(m∙K) to 0.0330 W/(m∙K). Thermal conductivity studies showed that with an increase in the density of the specimens of about 2.7 times, a decrease in thermal conductivity of about 30.3% is observed. Since PLA fibres were used as a binder to bind the fibres, it is important to investigate the thermal conductivity of the binder as well. The thermal conductivity coefficient of PLA was determined in the density range of approximately 20.2 kg/m³ to 58.1 kg/m³, respectively; the thermal conductivity in this density range ranged from approximately 0.0404 W/(m∙K) to 0.0331 W/(m∙K). Studies of PLA fibres showed that with an increase in density of approximately 2.9 times, the thermal conductivity coefficient decreased by approximately 22.1%.

Fig. 1

Experimental research shows that the specimens of woollen yarn waste and PLA fibres have good thermal conductivity properties, thus the composition of these fibres will have a positive effect on the value of the total thermal conductivity of the composite. The experimental thermal conductivity data obtained for the woollen yarn waste and polylactide fibres specimens were described using regression equations (see Table 6. Equations (3) and (4) are derived from Equation (1)).

Test results of the thermal conductivity investigations of different compositions of the woollen yarn waste products are presented in Fig. 2.

Fig. 2

Experimental studies show that the density variation for both compositions - thermally treated and thermally untreated - ranges from ~ 15.1 to 41.2 (Figures 2a, and 2b). The experimental values of the thermal conductivity of the composites were described by regression equations (see Table 7, Equations 5–8). When comparing the thermal conductivity of the thermally treated and thermally untreated composites (Fig. 2a) at a density of ~ 15 kg/m³, the difference was 5.4%. With a corresponding increase in density to ~ 40 kg/m³, this difference was 1.6%.

When comparing the thermal conductivity of the thermally treated and thermally untreated composites with yarns (Fig. 2b) at a density of ~ 15 kg/m³, a 7.4% difference was obtained. With a density increase of ~ 40 kg/m³, this difference was 3.3%.

To summarise the experimental data, we can state that the changes in the thermal conductivity of both thermally untreated composites remain similar. After thermal treatment of specimens of both compositions, the values of the thermal conductivity increased. This increase can be explained by the fact that PLA fibres acquire a partially vitreous structure, which also increases the thermal conductivity.

A comparison of the experimental data with the results obtained by other authors was carried out. Ye et al. [32] investigated the thermal characteristics of sheep wool and sheep wool with hemp fibres. The density of sheep wool ranged from 9.60 kg/m³ to 25.9 kg/m³, while that of wool with hemp fibres ranged from 9.9 kg/m³ to 18.1 kg/m³. They found that the thermal conductivity of sheep wool ranged from 0.0665 W/(m·K) to 0.0342 W/(m·K), whereas that of sheep wool with hemp fibres ranged from 0.0644 W/(m·K) to 0.0382 W/(m·K). Comparison of the results obtained by the authors and ours shows that the difference is quite large because the authors obtained better results of the thermal conductivity at lower densities. This difference can be explained by the fact that the authors used only sufficiently high-quality wool to produce mats by carding. Meanwhile, our composites are made from wool waste with PLA binder and are thermally treated. This difference can be stated as the influence of technological parameters on the thermal conductivity coefficient.

Jiri Zach [33] conducted a study on a hemp fibre composite. The researcher found that the thermal conductivity coefficient ranged from approximately 0.0401 to 0.050 W/(m∙K), which depended on both factors: the density and the thickness of the material. We can see that the composite obtained by us has better thermal conductivity characteristics than that obtained by the Czech authors. This difference can be explained by the fact that the composite obtained by the Czech authors is not only a fibrous structure but contains hemp shives, which amount to 32% in the composite.

The authors in [34] examined the thermal conductivity of thermal insulation materials made from feather waste. They explained the low thermal conductivity as due to the low thermal conductivity of the feather fibers and to the void structure formed by air-laid processing, which effectively traps air. Asdrubali et al. [35] provided a summary of values for the thermal conductivity of different authors. Comparison of the experimental results obtained by us and other scientists shows that the differences are due to the technological parameters of material processing. It is also sometimes difficult to compare the results obtained with those achieved by other scientists due to different measurement parameters and methods [18, 36], as well as to other factors such as material density, thickness, structure, and fibre orientation.

The macro and microstructures of the composite material from the woollen yarn waste depend on several factors, the most important of which are the initial raw materials and thermal treatment parameters. Thermal treatment parameters are described by two indicators: temperature and time. Time is understood as the duration from the start of exposure of the fibres at high temperature to the start of melting of the PLA. Only by combining these two parameters can we obtain a rational structure of the composite with the best physical-mechanical properties.

The diameters of the fibres were determined by scanning electron microscopy (Fig. 3 a). The diameters of the wool fibres investigated in our work ranged from about 23 μm to 44 μm. Meanwhile, for natural fibres obtained from raw materials of vegetable origin, a large inequality in the diameters of the fibres is characteristic. The thicknesses of such fibres often varies several times in the same sample. Inequality of the fibre diameter impedes the production of composites; it is difficult to select a rational amount of binding material to ensure constant properties or products, and to select production parameters.

Fig. 3

A general view of the composite prepared is presented in Fig. 3 b. We can visually observe that contact zones are formed only in separate areas. As further analysis shows, contact zones are formed between the contacting polylactide fibres and between the polylactide and wool fibres (Fig. 3 c and Fig. 3 d). This means that the raw material must be evenly mixed to obtain a greater number of contact zones using the same amount of binder. In Fig. 3 c the structure of some polylactide fibres interconnected into a tidy network is observed. Fibres that intersect each other are bound only in their contact zones. Different types of contact zones are observed between polylactide and wool fibres (Fig. 3 d). Since the wool fibres do not melt, polylactide dissolution occurs in the contact areas of the two fibres. A drop of dissolved PLA surrounds the wool fibres and forms a contact zone.

When the temperature and time of the thermal treatment process are unbalanced, i.e., too short a duration of thermal treatment and too low a temperature, or in other cases too long a duration of thermal treatment and too high a temperature, then between different fibres, i.e., sheep wool and polylactide, unsuitable contact zones are formed. If the temperature is too low or the time too short, weak contact zones are formed between the sheep wool and polylactide fibres. When the temperature is too high or the time too long, the polylactide fibres melt suddenly and large contact zones, i.e., conglomerations of polylactide melt, are formed. In such a case, contact zones are simply formed in separate areas. Because of the large conglomerations of the binding material, the thermal properties of the composite are poor. When the thermal treatment process is proportionate, i.e., the ratio of temperature and duration of thermal treatment is rational, the best performance of wool fibres is obtained.

Plowman et al. [37] investigated the structure of sheep wool fibres. Their found that sheep wool fibres differed in diameter, which ranged from about 15μm to 50μm. The heights and patterns of the cuticle scale also revealed significant differences. In our case, the diameter of the sheep wool waste fibres ranged from about 20 μm to 45 μm.

Many authors discuss nonwoven sheep wool products without additional thermal treatment. In the production of thermal insulation products with fibrous binders, thermal treatment plays an essential role. Bulgarian researchers [38] developed mats composed of poly(L-lactic acid) (PLA) and poly(ε-caprolactone) (PCL) at various weight ratios. The thermal treatment of the fibrous materials was carried out in a vacuum oven at a temperature of 60°C. The researchers found that on increasing the PCL content in the pristine PLA/PCL mats, a denser network of interconnected fibres was formed after thermal treatment. In our case for thermal insulation materials, a denser network of interconnected fibres forms intensified heat transfer zones.