Evaluation of Insulation against Contact Heat, Radiant Heat and Sensory Comfort of Basalt Fabric-Based Composites with Parylene C Coating

Basalt is a fine-grained extrusive igneous rock that arose as a result of the rapid cooling of low-viscosity lava rich in magnesium and iron [1,2]. Basalt has a relatively homogeneous chemical structure and, hence, the ability to form fibers in a molten state. Basalt fibers’ chemical composition was determined [3] and quantitatively is as follows (in %wt.): 51.6–59.3 SiO₂, 14.6–18.3 Al₂O₃, 5.9–9.4 CaO, 3.0–5.3 MgO, 9.0–14.0 FeO + Fe₂O₃, 0.8–2.3 TiO₂, 0.8–2.3 Na₂O + K₂O, and 0.09–0.13, among others. Basalt is non-toxic and does not contain any toxic impurities or heavy metals. Therefore, it has many potential applications [4]. Basalt is an alternative raw material for the production of textile materials. Basalt materials have good mechanical properties [5–7]. They are characterized by good thermal properties (low thermal conductivity, good thermal resistance) and low moisture absorption [1,6,8,9]. In addition, basalt fabrics are non-flammable and can be used in the temperature range -260÷800 ^oC. They are resistant to UV radiation, corrosion, and the action of microorganisms, and also do not emit toxic substances in reaction with water or air.

Basalt fabrics are exceptional materials that fit into a new range of composite materials and products. A particular area of application is protective equipment [10–16]. Protective clothing is an effective barrier between the human body and the environment [17,18]. Basalt fabric poses no risk to human beings but is unsuitable for direct use. Breaks into long thin pieces occur during handling in the fabric [9,19]. Basalt fibers can cause redness and irritation through splinters of filament entering the skin or other body areas, eyes, and the respiratory tract. Basalt yarns are slippery. It is recommended to tape the cut line to keep it from fraying. The magnetron sputtering method is often used for surface modification of basalt fabrics with metals and ceramics [20,21]. One-side modification enables to improve the thermal properties of the composites from the point of view of use as a protective material. Moreover, the coating improves basalt fiber adhesion in the fabric. Basalt fabrics can also be used as a layer of multi-layer packaging protecting the user in a hot work environment [22–24]. It was noticed that composites with a modified outer layer reached a certain level of protection against contact and radiant heat [24].

Parylene (poly-para-xylylene-based polymer) is a crystal-clear, polycrystalline, and amorphous linear polymer discovered by Szwarc [25] in 1947. Gorham [26] patented the parylene coating process in 1967. Parylene is formed from granular powdered material called dimer (di-paraxylylene). Dimer is converted under heat and a vacuum to a gaseous polymerizing monomer suitable for deposition on substrates at room temperature [27–30]. The poly-para-xylylene-based polymer can be used with metals and plastics as a coating for electrical components and assemblies. This polymer has low surface friction. It also has excellent chemical and dielectric barrier properties. Parylene hardly absorbs any water and is resistant to water diffusion. It shows high corrosion resistance. The most widely used parylenes are Parylene N, C, and D [29]. Parylene N has the best crevice-penetration capability, the best dielectric strength, the lowest coefficient of friction, the lowest elongation, and the lowest dissipation factor. However, there is a problem connected with preventing film deposition in areas that are to remain uncoated. Comparing Parylene N and C, the second one has more thermal stability and chemical and moisture resistance. Parylene C is the best moisture barrier of all the parylenes mentioned. It can be deposited relatively quickly on the sample surface, with reasonable thickness control. The dielectric strength of Parylene C is slightly less than that of Parylene N, as is its crevice-penetration ability. Parylene D has the lowest crevice-penetration ability of the parylenes described. The parylene forms the hardest coating and undergoes the least elongation. The polymer shows the greatest temperature stability. Parylene C and Parylene N have medical applications as a coating. Parylene D is more suited to industrial applications where basic requirements are physical toughness, chemical resistance, and thermal stability. Parylene coating is very thin, transparent, smooth, chemically inert, and has dielectric properties. Parylene can perfectly conform to the substrate because it has good adhesion [31]. Parylene coatings provide maximum barrier protection due to their lightweight. This polymer can form protective and insulating coatings on electronic devices and generate extremely thin, pinhole-free films [32,33]. Parylene C is widely used in medicine, for example, to implant covering [34] or as an encapsulation material for implanted neural prostheses [35]. Parylene C shows high biocompatibility, biostability, and the possibility of producing a thin, continuous, and inert coating [36]. In general, Parylene C is a versatile supporting material suitable for application in flexible materials. Parylene coating reproduces the surface texture of substrate material very well [37]. The poly-para-xylylene-based polymer has very low thermal conductivity [38]. The thermal conductivity of parylene coating is dependent on the coating thickness, as was shown for Parylene C [39]. Parylene C is also used as a coating of silk fabric to protect it against exposure to daylight and ultraviolet light, which cause the aging of the material [40]. Parylene coating makes silk fabrics somewhat stiffer and smoother.

It was noticed that the modified basalt fabrics used separately or in a multi-layer textile composite are a promising solution from the point of view of the user’s thermal comfort in a hot environment. However, the inner layer in the composites was unmodified basalt fabric in direct contact with the skin. Due to the irritating influence of basalt fabric, it was found that fabric with the basalt composite should be modified. To provide user comfort, the skin contact side of basalt fabric must be coated. Parylene C coating was used due to its high biocompatibility, as well as its conformity, penetration, and purity. The advantages result from the diffusion mechanism of adding monomers to the surface of the coating, which eliminates the presence of the liquid phase and the related problems caused by surface tension in wet methods. Also, the process in the so-called “vacuum” makes it possible to thoroughly clean the surface of the adsorbed moisture of gases just before the process, which makes the applied coating even better adhere to the coated elements.

The main aim of the research work is to improve the properties of basalt fabrics from the user’s comfort point of view and obtain a skin-friendly material that can be used as an inner layer in the multi-layer textile composite in direct contact with human skin.

Fabrics made of basalt fibers were selected for tests. The basic parameters of the basalt fabrics are given in Table 1.

Optical microscopy images of tested woven fabrics using the optical microscope Delta Optical Smart 5MP PRO (Delta Optical, Warsaw, Poland) and the software Delta Optical Smart Analysis Pro 1.0.0 are shown in Figure 1.

Fig. 1.

The breaks into long thin yarns during handling in basalt fabric are shown in Figure 2. As a result of breaks and snaps of fibers, protruding parts appear and irritate the user’s skin.

Fig. 2.

Basalt fabrics as the inner layer in the multi-layer textile composite and in direct contact with the skin were modified.

Using X-ray micro-computed tomography, morphometric analysis of the modified basalt fabrics (composites BP1, BP2, BP3, BP4) was conducted. The results obtained are given in Table 2. Additionally, weft and warp flattering coefficients and the density of yarns were calculated and juxtaposed in Table 2.

A 3D reconstruction of the composites is shown in Figure 5.

Fig. 5.

Research results of the resistance to contact heat and radiant heat of the unmodified basalt fabrics and modified fabrics (composites) are presented in Tables 3 and 4, respectively. Measurements were repeated three times, and mean values of parameters t_t and RHTI₂₄ were calculated. Additionally, the expanded uncertainty was obtained, assuming k_p = 2 (Eq. 2).

As shown in Table 3, none of the samples reached the efficiency level in the case of all the unmodified basalt fabrics B1, B2, B3, B4, and two composites BP1 and BP2. The first efficiency level (F1) of protection against contact heat was obtained for composites BP3 and BP4. As shown in Table 4, all the samples reached the first efficiency level (C1) of protection against radiant heat.

Uncertainty analysis was conducted to assess differences between unmodified and modified basalt fabrics. Intervals [y-U, y+U] covering the values of the measurands at a 95% confidence level, i.e. the threshold time and radiant heat transfer index, are shown in Figures 6 and 7, respectively. The interval for each sample is presented separately for better visibility of the results.

Fig. 6.

Fig. 7.

The lower and upper ends of the first efficiency level, F1min, and F1max, for the resistance to contact heat are shown in Figure 6. The uncertainty intervals of threshold times partially and also totally overlapped were obtained for all unmodified basalt fabrics (B1, B2, B3, B4) and two modified fabrics BP1, and BP2. Thus, it was found that there are no significant differences between the threshold time values. Moreover, the samples showed no satisfactory resistance to contact heat. The interval obtained for fabric B2 partially coincides with that of threshold time values required for the first efficiency level of protection against contact heat. It was found to be insufficient. Thus, sample B2 showed no resistance to contact heat. Similar to the case of B2, it was found that composite BP4 did not reach the second efficiency level of protection.

Significant differences between threshold times were observed for the remaining composites BP3 and PB4. The composites reached the first efficiency level of protection. Composite BP3 is thinner than BP4. Composite BP3 is characterized by a high total porosity, exceeding 60% (Table 2). Air generally is an excellent thermal insulator, but it can transmit heat through convection. The air-filled pores inside the fabric are typically separated from each other. The heat flow from one pore to another is not easy. In the case of composite BP4, the total porosity is 48% (Table 2). The composite is the thickest and shows the best resistance to contact heat among the tested samples.

The lower and upper ends of the first efficiency level, C1min, and C1max, for the resistance to radiant heat, are shown in Figure 7. The uncertainty intervals of the radiant heat transfer index, obtained for all basalt fabrics and composites, partially overlap or fully overlap. Thus, it was found that there are no significant differences between the radiant heat transfer index values.

Color measurements were conducted to assess the surfaces of the modified basalt fabrics. The color difference between the unmodified and modified basalt fabrics was measured. Measurement results are presented in Table 5.

The value of ΔE₀₀ is in the range [1,2), which means that there is a very small difference between the samples’ surface colors. Perception of color difference is slight. Thus, it can be considered that the colors of the sample pairs do not differ significantly.

Additionally, a remission measurement of the basalt fabrics and composites was conducted in the spectrum of 400-700 nm. The spectral remission factor R was in the range [10,24]% for all the samples. An example remission curve obtained for BP1 is shown in Figure 8.

Fig. 8.

The low stream of reflected light from the tested surface confirmed the low-efficiency level of the protection sample against radiant heat. It means that the basalt fabric modified with Parylene C coating should be an inner layer, not an outer layer of a multi-layer textile composite protecting the user in a hot work environment.

Analysis of the effect of the porosity and thickness of the unmodified and modified basalt fabrics on parameters t_t and RHTI₂₄ was carried out. The results are shown in Figure 9. It was found that all fabrics (unmodified and modified) were characterized by a comparable value of the RHTI₂₄ parameter (the maximum difference is 4% between B4 and BP1). There was also no dependence of the RHTI₂₄ parameter on the thickness of the fabrics and their porosity. In the case of unmodified and modified fabrics, an increasing dependence of the t_t parameter on the thickness was observed, however, for unmodified fabrics, the t_t increase is milder - 15% (values increase from 3.9 s to 4.5 s), while for modified fabrics the t_t increase is 216% (values increase from 4.5 s to 9.7 s). The biggest difference in the t_t parameter between the modified fabrics and those coated with Parylene C was observed for two fabrics (B3 and B4). This difference may be due to the fact that in the case of fabric B3 (the most porous) and fabric B4 (the thickest, the only one with a different weave from the other three fabrics), Parylene C formed a thicker heat-insulating layer than in the case of fabrics: B1 and B2.

Fig. 9.

The Parylene C coating was identified on the basalt fabric surfaces (Figure 3). By coating the fabric surface with a thin conformal layer of Parylene C, the fibrous structure was protected against damage (due to fraying). Thanks to the penetrating capacity of the process, each fiber and each node separately were stabilized, resulting in strengthening the stability of the entire structure. The Parylene C coating increased the usability of basalt fabrics, eliminated the effect of skin irritation, and contributed to the creation of skin-friendly material.

The following conclusions can be drawn as a result of the research. –

The outcomes of the contact heat method showed the Parylene C coating presence improved the thermal insulation of all composites. In the case of BP3 the threshold time t_t was 6.7 s, while for BP4 it was 9.7 s. It means that the first efficiency level of protection against contact heat was reached.