Evaluating the Impact of 222 nm Far-UVC Radiation on the Aesthetic and Mechanical Properties of Materials Used in Public Bus Interiors

Abstract

The use of 222 nm far-UVC radiation can be an effective means of disinfecting public buses against viruses, including SARS-CoV-2. However, it can cause degradation of the mechanical and visual properties of interior materials. The purpose of this study is to investigate the effects of 222 nm far-UVC radiation on the color and mechanical degradation of materials used to construct public bus interiors. This research work involves exposure of samples of materials commonly used in bus interiors to various levels of far-UVC radiation and measuring and evaluating changes in color and mechanical properties. The results of the study showed that far-UVC irradiation causes significant color degradation (∆E00 >5) in all the polymeric materials tested, after 290 J/cm² radiant exposure. In addition, significant changes in mechanical properties were observed when evaluating elasticity modulus, elongation at ultimate strength, elongation at break, and tensile strength. A particularly large decrease in elongation at break (up to 26%) was observed in fiber-reinforced composite materials. The results of this study can be used as a guide for the development of protocols for the use of far-UVC disinfection in public transportation, which can help limit the transmission of infections while preserving the integrity and visual properties of bus interior materials.

1. Introduction

The COVID-19 pandemic highlighted the need for effective and efficient methods of disinfecting public transport systems, which are used by millions of people every day and can be a potential source of transmission [1,2,3,4]. Traditional disinfection methods can be time-consuming and not always effective, so it is important to explore new alternatives [5,6,7]. Hessling et al. [8] noted that while the antimicrobial properties of UV radiation from mercury vapor lamps are well-established and have been used for over a century, this type of radiation is also harmful to human cells and tissues that contain DNA. The study highlights that exposure to UV radiation can result in skin irritation and potentially carcinogenic mutations, as well as eye damage, such as photokeratitis. The authors also noted that proper safety precautions should be taken when using UV radiation to disinfect surfaces.

These results suggest that this method may not be suitable for use in situations where humans are exposed to UV irradiation. However, Buonanno et al. [9] suggested and provided evidence that there is a specific wavelength range in the far-UVC region, at approximately 200 nm, that can effectively kill bacteria while minimizing damage to human cells. This proposed method of using 207 nm UV light in the presence of humans was based on the fact that UV light at a wavelength of approximately 200 nm is strongly absorbed by biomolecules, such as proteins, limiting its ability to penetrate deeper into living tissue. These findings have led to extensive research in recent years into the use of far-UVC for surface and air disinfection.

This type of UV radiation has been shown to be highly effective in killing bacteria and other microorganisms, including SARS-CoV-2 [10], the virus that causes COVID-19, while being less harmful to human cells [11]. Ma et al. demonstrated the efficacy of 222 nm far-UVC light in inactivating a wide range of common pathogens, including SARS-CoV-2 and influenza viruses. The study found that low doses of far-UVC were highly efficient in destroying all 12 pathogens tested, including SARS-CoV-2, HCoV-229 E, and H1N1 influenza A. Matsuura et al. conducted a study to investigate the inactivation ability of UV radiation at different wavelengths on the SARS-CoV-2 virus under the same experimental conditions. The results showed that the inactivation ability decreased in the order of 254 nm, 265 nm, and 222 nm UV wavelengths. This suggests that all three wavelengths have the ability to inactivate the virus [12]. Kitagawa et al. also investigated the effectiveness of 222 nm UV light in disinfecting surfaces contaminated with SARS-CoV-2. The study concluded that 222 nm UV-C could be used for infection prevention and control against COVID-19 in both unoccupied and occupied spaces. This research supported the potential utility of far-UVC as a tool to control the spread of SARS-CoV-2 in public spaces [13]. The study in [14] concluded that the 222 nm Kr–Cl excimer lamp has high potential for use as a tool to inactivate various types of pathogens in the residential settings. Buonanno et al. recently published a report demonstrating that continuous far-UVC exposure in occupied public spaces at the current regulatory exposure limit (~3 mJ/cm²/h) can achieve a significant level of viral inactivation in a relatively short period of time [11].

Despite the potential of far-UVC technology for the inactivation of SARS-CoV-2, researchers acknowledge that further efforts are needed to ensure safe disinfection with UV light that is protective of human health and the environment. Therefore, the use of 222 nm far-UVC radiation for surface disinfection and sterilization in public transportation may be a safe and effective way to reduce the risk of transmission of COVID-19. However, exposure to UV radiation can cause significant degradation of various materials, and several scientific articles have investigated the effects of UV degradation on various materials. These include metals, wood, polymers, asphalt, veneers, and silicone rubber. These studies have provided valuable insights into the mechanisms of UV degradation and have highlighted the importance of understanding the potential adverse effects of UV radiation on materials in order to develop effective strategies to mitigate these effects. These studies have shown that UV radiation can cause significant degradation of materials through a process known as photooxidative degradation. This process leads to the breaking of polymer chains, the formation of free radicals, and a decrease in molecular weight, resulting in a decrease in mechanical properties and ultimately rendering the material unusable, although the timing of this process can be unpredictable [15].

2. Related Works

A general overview of the effects of UV radiation on various materials has been given by [16]. Other authors delved deeper into specific areas of metals and wood materials, such as the photocorrosion of metals in by Burleigh et al. [17] and the surface degradation of wood by Hon and Chang [18].

The UV degradation effects in polymers was explored in another group of research articles. Clough et al. investigated the discoloration of optical polymers caused by UV radiation and their ability to recover [19]. Tworek et al. examined the properties of polyvinyl chloride (PVC) membranes containing cadmium pigments after exposure to UV radiation. The study found that UV radiation caused changes in the mechanical and barrier properties of the membranes, including an increase in tensile strength and a decrease in water vapor permeability. The study also found that the presence of cadmium pigments in the membranes increased their resistance to UV radiation [20]. Discoloration of (U)HMW-PE caused by high-dose gamma radiation and long-term thermal aging was investigated by Kömmling et al. [21]. The study found that both gamma-irradiation and thermal aging caused discoloration of the (U)HMW-PE samples. The study also found that the degree of discoloration was dependent on the radiation dose and aging time. In addition, the study found that the discoloration was caused by changes in the chemical structure of the (U)HMW-PE due to the radiation and aging. The study concluded that high-dose gamma-irradiation and long-term thermal aging can cause discoloration of (U)HMW-PE, and this may affect the performance and durability of the material. Kowalonek, J. examined the surface studies of UV-irradiated poly(vinyl chloride)/poly(methyl methacrylate) blends [22]. The study shows that UV exposure alters the surface properties of PVC/PMMA blends, resulting in increased roughness and decreased surface energy. The addition of PMMA to the blends was found to improve their resistance to UV radiation. The study suggested that changes in surface properties caused by UV radiation could affect the performance and longevity of the material and that incorporating PMMA into PVC blends could improve their performance and durability under UV exposure. Shao et al. investigated the effects of different wavelengths of UV light on the surface discoloration of dyed veneer. The study found that different wavelengths of UV light had different effects on the surface discoloration of the veneer. The study also found that the color of the veneer changed differently depending on the wavelength of the light. It was concluded that the wavelength of the light is an important factor in the discoloration of dyed veneer, and that this should be considered when selecting the light sources to be used in illuminating dyed veneer [23]. Nekeb et al. evaluated the effects of UV radiation on textured silicone rubber. The results showed that UV exposure caused changes in the mechanical and physical properties of the rubber, such as changes in its tensile strength, elongation, and surface roughness. In addition, the magnitude of these changes was found to be dependent on the duration and intensity of the UV exposure. The study concluded that UV radiation could significantly affect the properties of textured silicone rubber, potentially affecting its performance and longevity [24]. The study by Irving et al. compared the effects of UV-C and 405 nm germicidal light sources on endoscope storage. The study found that both types were effective in reducing bacterial contamination, but UV-C caused more damage to the endoscopes and had higher safety hazards and maintenance costs. The study suggested that 405 nm germicidal light sources are a more appropriate option for endoscope storage [25].

An important paper related to our research by Yates et al. examined the effects of 254 nm UV-C radiation exposure on aircraft cabin materials. The article analyzed the durability and performance of various materials commonly used in aircraft cabins when exposed to UV-C radiation, which was a new perspective solution for aircraft disinfection. Changes in the physical and chemical properties of the materials were also examined, as was the effectiveness of the materials in blocking UV-C radiation. The results of the study indicated that certain materials, such as ABS and PC, were found to be compatible with UV-C exposure, while others, such as PVC and PU, were not. The study also found that the level of compatibility was dependent on the specific type of material and the duration of exposure. The results of this study can be used to improve the design and selection of materials for aircraft cabins to ensure safety and durability [26,27].

Overall, the literature suggests that exposure to UV-C light can cause degradation of materials and that the extent of degradation depends on the type of material and the intensity and duration of UV-C light exposure. Further research is needed to understand the mechanisms of degradation and to develop methods to mitigate the effects of UV-C light on materials.

Our study aims to investigate the effects of 222 nm far-UVC radiation on the color and mechanical properties of various materials commonly used in public bus interiors, such as plastics, textiles, and coatings. This area of research has not been extensively studied in the past, making it a critical and timely investigation in the field of transportation and materials science. The research will focus on understanding the mechanisms of UV-induced color degradation and mechanical degradation of materials. The results of this study will have important implications for the transportation industry, particularly for public bus interiors, as it will help to develop automated systems to reduce disinfection costs and to develop UV-resistant materials and methods to prevent color and material mechanical degradation, which can extend the life of bus interiors and improve the overall passenger experience.

3. Materials and Methods

3.1. Materials

Bus interior materials can vary depending on the type of bus and its intended use. However, some common materials used in bus interiors include [28,29,30,31,32,33]:

• Seats: often made of vinyl or cloth and may be padded for added comfort.
• Flooring: typically made of a durable material, such as vinyl or rubber.
• Ceiling: usually made of a lightweight material, such as fiberglass or PVC.
• Walls: usually made of a lightweight material, such as fiberglass or plastic.
• Windows: typically made of tempered glass for safety.
• Handrails and grab bars: generally made of a durable material, such as stainless steel or plastic.
• Lighting: normally provided by fluorescent or LED lighting.
• Audio and visual systems: typically found on larger buses and may include PA systems, speakers, and video displays.

An in-depth analysis of interior materials for buses built with composite body materials has been conducted. These materials are becoming increasingly popular in the bus industry due to their combination of strength, durability, and light weight. This study focused on a selection of interior materials samples taken from seats, flooring, walls, and video display equipment. The locations of the tested materials in the bus interior are shown in Figure 1.

The initial specifications of the sample materials are provided in

Table 1. Samples of bus interior materials.

Sample No.	Material	Example of a Piece of Material	Location	Properties Under Investigation	Properties of Unaged Material
1.	Fiber-reinforced composite (E-glass fiber with epoxy resin)		Wall cladding trim elements, constructional wall material	Tensile strength, 0 0°/90° fiber orientation, MPa	292.67
				Tensile strength, 0 ± 45° fiber orientation, MPa	171.30
				Elastic modulus, 0 0°/90° fiber orientation, GPa	36.8
				Elastic modulus, 0 ± 45° fiber orientation, GPa	20.70
				Elongation at break, 0°/90° fiber orientation, %	0.57
				Elongation at break, ±45° fiber orientation, %	0.70
2.	Floor covering–PVC		Floor covering of public transport/city bus	Tensile strength, MPa	7.79
				Elastic modulus, MPa	149.29
				Elongation at break, %	73
3.	Velour fabric of seats		Fabric covering of passenger seats	Not applicable	-
4.	Powder coating of handhold railings		Powder coating of passenger handhold’s	Not applicable	-

The sample materials used for testing.

.

In the bus tested, the interior walls were made of FRC (fiber-reinforced composite (matrix: vinyl ester resin, fiber: E-glass fiber)). The FRC is a type of material that consists of a matrix, typically a vinyl ester resin, and fibers, such as E-glass fibers. The matrix acts as a binder that holds the fibers together and gives the material its shape and form. The fibers, on the other hand, give the material its strength and stiffness. In this case, the matrix was a vinyl ester resin and the fibers were E-glass fibers. The floors were made of polyvinyl chloride (PVC). PVC is constructed by polymerizing vinyl chloride monomer (VCM) and is composed of polymer chains containing 57–60% chloride. PVC has many useful properties, such as excellent chemical resistance, high strength, and good electrical insulation. It is a thermoplastic polymer that is widely used in a variety of industrial and consumer products. The wall cladding trim elements, structural wall material, and flooring have been tested by evaluating their tensile strength, elasticity modulus, elongation at break, and elongation at ultimate strength (ULS). All these materials and the fabric covering of passenger seats and powder coated passenger handhold were tested using colorimetric analysis.

3.2. Experimental Setup for 222 nm far-UVC Exposition

For this study, a far-UVC exposition chamber (Figure 2) was specially designed and constructed to simulate the long-term effects of 222 nm far-UVC light on various materials. It consisted of two Quantadose B-Series DF28B-B3 20-watt filtered excimer 222 nm far-UVC lamp modules, which were equipped with 222 nm bandpass filters and special enclosures to cool and to protect the lamps from damage. The lamps were powered by a special 40 W input high voltage power excimer lamp electronic power supply. The use of bandpass filters in the lamp modules ensured that only the desired wavelength of UV light was emitted, further enhancing the precision and accuracy of the far-UVC exposition chamber. These modules were installed in a specially designed box, which helped to minimize the amount of ambient light entering the chamber, further enhancing the accuracy of the far-UVC exposition process.

The exposure of the far-UVC light could be varied by adjusting the distance between the material sample and the far-UVC light source. The distance could be varied in 13 mm increments, from 18 mm to 330 mm, corresponding to an irradiance from 0.93 mW/cm² to 0.27 mW/cm². This allowed for a detailed study of the effects of different levels of far-UVC exposure on the materials under test and simulation of different far-UVC exposure and disinfection scenarios, for public transport environments.

The spectra of the lamps used in the far-UVC exposition chamber were measured using an AvaSpec ULS2048 CL spectroscope manufactured by Avantes. The spectrogram of the direct light from the lamps is shown in Figure 3. As can be seen from the graph, the lamps emit light primarily at a wavelength of 222 nm.

Radiant exposure was measured using a Gigahertz Optik, X1-5 optometer with a 222 nm calibrated sensor. In this study, far-UVC radiant exposure is conducted in 3 iterations, every 50 h. Radiant exposure values are shown in

Table 2. Radiant exposure values for each far-UVC exposition iteration.

Time, h	Radiant Exposure, J/cm2
50	96.714
100	193.428
150	290.142

for each exposition iteration.

According to Welch et al. [34], it is sufficient to expose surfaces for 2.0 mJ/cm² under far-UVC 222 nm irradiation to deactivate the COVID-19 virus; for other pathogens, such as influenza, it should be almost 4.0 mJ/cm², and to deactivate most widespread biological pathogens, up to 8.0 mJ/cm² exposure is necessary. It takes approximately 1 h for a regular city bus to travel from end to end, after which the bus driver takes a break for approximately 10–15 min. Assuming that during each such break a disinfection is performed with an exposure of approximately 10 mJ/cm² and the duty cycle of the city bus is approximately 16 h a day, 7 days a week, the annual irradiation exposure would be approximately 47 J/cm². In our case, 150 h of irradiation in a 222 nm far-UVC irradiation chamber was approximately equivalent to a radiation exposure of 290 J/cm², which corresponds to approximately 6.2 years of city bus disinfection.

Actual far-UVC lamp radiation was measured every 20 h for all exposure iterations. It decreased by approximately 0.05% with each measurement. Radiation exposure was measured every 50 h of exposure chamber operation. It was also found that within 30 min of starting the far-UVC exposure chamber, the radiation from the lamps decreased by approximately 10% until their temperature stabilized. Therefore, all radiant exposure measurements were determined taking this into account, and materials were not placed in the exposure zone of the chamber until 30 min after the chamber was turned on.

3.3. Experimental Setup for Tensile Testing

The effect of far-UVC radiant exposure on the mechanical properties was investigated on FRC 0/90°, E-glass, epoxy resin, FRC ±45°, E-glass, epoxy resin, and PVC. Five test samples were prepared for each for each mechanical property test (elasticity modulus, tensile strength, elongation at ultimate strength (ULS), and elongation at break) for each material. The samples were cut from sheets of materials used in real bus production, which are produced by a vacuum infusion molding method. The dimensions of the test samples are provided in Figure 4 and

Table 3. Dimensions of test material samples.

Type of Test Material Sample	Dimensions Lxbxh, mm
FRC 0/90°, E-glass, epoxy resin	250 × 25 × 2
FRC ±45°, E-glass, epoxy resin	250 × 25 × 2
PVC	150 × 25 × 2

.

FRC and PVC tensile tests were performed on a Zwick/Roell Z100 testing machine using a 100 kN force cell with a Zwick BTC-EXMACRO.002 extensometer at a rate of 2 mm/min and 25 mm/min for FRC and PVC, respectively. Test speed and sample dimensions for PVC were determined in accordance with the requirements of ISO 527-3:2019 and ISO 527-1:2019. Test speed and sample dimensions for FRC were determined in accordance with the requirements of ISO 527-1:2019 and ISO 527-5:2019. Two different orientations of FRC were tested. The orientation of the FRC test samples is shown in Figure 5. Fiber-reinforced composites were constructed of biaxial E-glass fiber with epoxy resin matrix. Weight of fabric was 1041 g/m², fabric was composed of 2 layers of E-glass, half of the fibers were oriented in 0-degree direction, the half of the fiber oriented in 90-degree direction. Number of samples tested was 5 of each material series.

To facilitate comparison of the results of the effect of far-UVC radiation on mechanical properties, the results were recalculated as a ratio of the properties of the original and the affected materials.

E-modulus was calculated from the test results using equation:

$$E=\frac{L_0\left(X_H-X_L\right)}{ab\left(L_H-L_L\right)}$$

(1)

where:

E—Tensile modulus, in kN/mm² or GPa;

L₀—Initial gauge length, in mm;

X_H—End of tensile modulus determination, in kN;

X_L—Beginning of tensile modulus determination, in kN;

a—Sample thickness, in mm;

b—Sample width, in mm;

L_H—Strain, in mm at X_H;

L_L—Strain, in mm at X_L;

3.4. Methods for Colorimetric Analysis

Typically, color is measured using colorimeters and color spectrophotometers. Recently, computer vision technology has emerged as a robust technique that can be implemented as an alternative to traditional color measurement methods. A computer vision system (CVS) is a system that uses cameras, image processing algorithms, and a computer to interpret and understand visual information from the world. The system captures images or video, processes the data, and then extracts information or makes decisions on the basis of that data.

A special CVS was developed for the colorimetric analysis of materials exposed to far-UVC light. The CVS consisted of basic components: the vision cabinet, illumination, camera, software, and computer. The built-in vision cabinet was a specialized device designed to provide a controlled environment for image acquisition of test samples. It consisted of a physical housing for the camera and other components, with a platform at the base for loading test samples. The dimensions of the cabinet were 0.16 × 0.10 × 0.30 m (L × W × H), and the distance between the sample and the camera lens was 0.25 m. To minimize reflections and ensure uniform light distribution, the interior of the cabinet was coated with a neutral gray paint (Figure 6). The digital camera used in the vision system was a Basler ace acA3088-57uc USB 3.0 camera equipped with a Sony IMX178 CMOS rolling shutter sensor and a 16mm F/1.4 fixed focal length lens from Ricoh Company Ltd. The camera was connected to a computer (AMD Ryzen 9 5900X, 32 GB DDR4 RAM, Windows 11 64-bit) via a USB 3.0 interface. The tested materials were illuminated by a continuous light source consisting of a 10W D65 LED ring light installed around the camera lens. The luminance was controlled by a Heylec HY3010B laboratory power supply and was set at 4018 lx (measured with a PeakTech 5086 Digital Lux Meter). A warm-up period of five minutes was used before image acquisition began to ensure stable lighting conditions.

A color camera uses multiple pixels with different color filters to reproduce the color impression of the real situation for each location in the image using three-color values. These color values depend not only on the technical details of the camera but also on the lighting and other factors. In order to capture realistic colors with a camera, color calibration is required. Color calibration at Basler is a well thought-out and standardized process consisting of four steps: white balance, gamma correction, matrix correction, and six-axis operator correction [35]. Prior to image capture, the camera was calibrated using the Basler Camera Color Calibrator tool in the Pylon Viewer using an X-Rite ColorChecker Classic Mini color chart. Figure 6 shows the measurement environment during the calibration process.

The specialized software was designed to facilitate data acquisition, color analysis, and storage of images captured by the vision system. The software was developed using the National Instruments Vision Builder for AI 2020 development version, a powerful tool for creating custom vision applications. The software was designed to work seamlessly with the vision system, providing an intuitive user interface and a wide range of image processing and analysis functions. Data analysis was performed using custom data analysis and visualization scripts developed in MathWorks MATLAB R2022b. The custom scripts were developed to specifically analyze the color of the samples captured by the vision system.

3.5. Method for Evaluating Material Color Differences

The image acquisition software was designed to acquire 80 images for a sample measurement, and frame averaging was used. This allowed the elimination of all noise sources except for the spatial non-uniformity of the digital camera response [36]. A video camera was used to capture the image of the material sample in the sRGB color space.

Since the test material samples were a single color, the average of all pixel values was calculated and divided by 255 to move to the floating-point range [0, 1]. This resulted in non-linear scalar $s{R}^{\prime }, s{G}^{\prime }, s{B}^{\prime }$values that should be converted to CIE Lab* color space values using a color conversion algorithm.

First, the $s{R}^{\prime }, s{G}^{\prime }, s{B}^{\prime }$values were transformed to linear sR, sG, sB values by [37]:

$$P=\left\{\begin{array}{cc}\frac{P^{\prime }}{12.92}& P^{\prime }\le 0.04045\\ {\left(\frac{P^{\prime }+0.055}{1.055}\right)}^{2.4}& P^{\prime }>0.04045\end{array}\right.,$$

(2)

where P = sR, sG, or sB are color space values. Conversion to the CIE XYZ color space with D65 white point was accomplished by using the color conversion matrix [37]:

$$\left[\begin{array}{c}X\\ Y\\ Z\end{array}\right]=\left[\begin{array}{ccc}0.4124& 0.3576& 0.1805\\ 0.2126& 0.7152& 0.0722\\ 0.0193& 0.1192& 0.9505\end{array}\right]\left[\begin{array}{c}sR\\ sG\\ sB\end{array}\right],$$

(3)

Next, a nonlinear transformation of the XYZ values to the CIE Lab* color space was performed using the following formulas [38]:

$$L^{*}=116f\left(\frac{Y}{Y_n}\right)-16,$$

(4)

$$a^{*}=500\left(f\left(\frac{X}{X_n}\right)-f\left(\frac{Y}{Y_n}\right)\right),$$

(5)

$$b^{*}=200\left(f\left(\frac{Y}{Y_n}\right)-f\left(\frac{Z}{Z_n}\right)\right),$$

(6)

let $t=\frac{X}{X_n},\frac{Y}{Y_n}or\frac{Z}{Z_n}$, then:

$$f\left(t\right)=\left\{\begin{array}{ll}\sqrt[3]{t}\hfill & t>\frac{216}{24389}\hfill \\ \frac{24389t+432}{3132}\hfill & otherwise\hfill \end{array}\right.,$$

(7)

where $X_n=0.950489,Y_n=1.000000,\mathrm{and}{Z}_n=1.088840$ for CIE standard illuminant D65.

A color difference metric, ∆E00 (CIEDE2000) [39], was chosen because it is considered to be the most accurate and widely accepted method for measuring the difference between two colors. Developed by the International Commission on Illumination (CIE) in 2000, this metric is based on the CIE Lab* color space, which is a perceptually uniform color space that considers the non-uniformity of the color space and the correlated color temperature of the reference white. The formula corrects for the shortcomings of the older CIE94 formula. Given a pair of color values in CIELAB space $L_1^{*},a_1^{*},b_1^{*}$ (before far-UVC exposition), $L_2^{*},a_2^{*},b_2^{*}$ (after far-UVC exposition), and parametric weighting factors $k_L$, $k_C$, and $k_H$, the ∆E00 can be calculated using the following formulas [40]:

$$\Delta E00=\sqrt{{\left(\frac{\Delta L^{\prime }}{k_L{S}_L}\right)}^2+{\left(\frac{\Delta C^{\prime }}{k_C{S}_C}\right)}^2+{\left(\frac{\Delta H^{\prime }}{k_H{S}_H}\right)}^2+R_T\left(\frac{\Delta C^{\prime }}{k_C{S}_C}\right)\left(\frac{\Delta H^{\prime }}{k_H{S}_H}\right)},$$

(8)

$$\Delta L^{\prime }=L_2^{*}-L_1^{*},$$

(9)

$$\Delta C^{\prime }=C_2^{\prime }-C_1^{\prime },$$

(10)

$$\Delta H^{\prime }=2\sqrt{C_1^{\prime }{C}_2^{\prime }}\sin \left(\frac{\Delta h^{\prime }}{2}\right),$$

(11)

$$C_i^{\prime }=\sqrt{{\left(a_i^{\prime }\right)}^2+{\left(b_i^{*}\right)}^2} i=1, 2,$$

(12)

$$a_i^{\prime }=\left(1+G\right)a_i^{*} i=1, 2,$$

(13)

$$G=0.5\left(1-\sqrt{\frac{{\overline{C}}_{ab}^{*}{}^7}{{\overline{C}}_{ab}^{*}{}^7+{25}^7}}\right),$$

(14)

$$C_{i, ab}^{*}=\sqrt{{\left(a_i^{*}\right)}^2+{\left(b_i^{*}\right)}^2} i=1, 2,$$

(15)

$${\overline{C}}_{ab}^{*}=\frac{C_{1,ab}^{*}+C_{2,ab}^{*}}{2},$$

(16)

$$\Delta h^{\prime }=\left\{\begin{array}{ll}0\hfill & C_1^{\prime }{C}_2^{\prime }=0\hfill \\ h_2^{\prime }-h_1^{\prime }\hfill & C_1^{\prime }{C}_2^{\prime }\ne 0; \left|h_2^{\prime }-h_1^{\prime }\right|\le 180°\hfill \\ \left(h_2^{\prime }-h_1^{\prime }\right)-360\hfill & C_1^{\prime }{C}_2^{\prime }\ne 0; \left(h_2^{\prime }-h_1^{\prime }\right)>180°\hfill \\ \left(h_2^{\prime }-h_1^{\prime }\right)+360\hfill & C_1^{\prime }{C}_2^{\prime }\ne 0; \left(h_2^{\prime }-h_1^{\prime }\right)<-180°\hfill \end{array}\right.,$$

(17)

$$h_i^{\prime }=\left\{\begin{array}{ll}0\hfill & b_i^{*}=a_i^{\prime }=0\hfill \\ ta{n}^{-1}\left(b_i^{*}, a_i^{\prime }\right)\hfill & otherwise\hfill \end{array}\right. i=1, 2,$$

(18)

$$S_L=1+\frac{0.015{\left({\overline{L}}^{\prime }-50\right)}^2}{\sqrt{20+{\left({\overline{L}}^{\prime }-50\right)}^2}},$$

(19)

$$S_C=1+0.045{\overline{C}}^{\prime },$$

(20)

$$S_H=1+0.015{\overline{C}}^{\prime }T,$$

(21)

$${\overline{L}}^{\prime }=\left(L_1^{*}+L_2^{*}\right)/2,$$

(22)

$${\overline{C}}^{\prime }=\left(C_1^{\prime }+C_2^{\prime }\right)/2,$$

(23)

$$T=1-0.17\cos \left({\overline{h}}^{\prime }-30°\right)+0.24\cos \left(2{\overline{h}}^{\prime }\right)+0.32 \cos \left(3{\overline{h}}^{\prime }+6°\right)-0.20 \cos \left(4{\overline{h}}^{\prime }-63°\right),$$

(24)

$${\overline{h}}^{\prime }=\left\{\begin{array}{ll}\frac{h_1^{\prime }+h_2^{\prime }}{2}\hfill & \left|h_1^{\prime }-h_2^{\prime }\right|\le 180°; C_1^{\prime }{C}_2^{\prime }\ne 0\hfill \\ \frac{h_1^{\prime }+h_2^{\prime }+360°}{2}\hfill & \left|h_1^{\prime }-h_2^{\prime }\right|>180°;\left(h_1^{\prime }+h_2^{\prime }\right)<360°;C_1^{\prime }{C}_2^{\prime }\ne 0\hfill \\ \frac{h_1^{\prime }+h_2^{\prime }-360°}{2}\hfill & \left|h_1^{\prime }-h_2^{\prime }\right|>180°;\left(h_1^{\prime }+h_2^{\prime }\right)\ge 360°;C_1^{\prime }{C}_2^{\prime }\ne 0;\hfill \\ h_1^{\prime }+h_2^{\prime }\hfill & C_1^{\prime }{C}_2^{\prime }=0;\hfill \end{array}\right.,$$

(25)

$$R_T=-\sin \left(2\Delta \theta \right)R_C,$$

(26)

$$\Delta \theta =30 e^{-{\left(\frac{{\overline{h}}^{\prime }-275°}{2}\right)}^2},$$

(27)

$$R_C=2\sqrt{\frac{{\overline{C}}^{\prime }{}^7}{{\overline{C}}^{\prime }{}^7+{25}^7}},$$

(28)

According to [41], the value of ∆E00 can be interpreted to range from normally invisible difference (<1) to very obvious difference (>5).

4. Results and Discussion

All four material samples were exposed to far-UVC light in three iterations. The initial color measurement of the samples was taken before each far-UVC exposure, and subsequent measurements were taken after each iteration of far-UVC exposure. Figure 7 shows the color changes of each sample using red, green, and blue color scales after each iteration of far-UVC exposure.

To accurately evaluate the color changes in the material samples after far-UVC exposure, the ∆E00 value was calculated for each measurement and presented in Figure 8. The figure also includes heat maps showing the color differences among the different iterations of the measurements.

According to the interpretation of ∆E00 values given in [41], the normally invisible color differences occurred for Sample 3, and a very small difference, visible only to the trained eye, occurred for Sample 4 after all stages of far-UVC exposure. An obvious difference was recorded for Samples 1 and 2. Therefore, it can be concluded that the maximum effect of far-UVC 222 nm radiation is on FRC and PVC materials (Sample 1 and Sample 2). It can be concluded that far-UVC 222 nm radiation has a significant effect on the color of the tested materials, so this should be considered when using far-UVC exposure of 222 nm wavelength for disinfection of surfaces made of these materials.

Previous research, such as the study in [42], has shown that both colored fabrics and plastics are discolored by far-UVC light of 254 nm wavelength. However, this study did not measure the quantitative difference in color (∆E00), so it is not possible to directly compare the effect of exposure to 254 nm and 222 nm far-UVC light on materials.

The change in mechanical properties (elasticity modulus, tensile strength, elongation at ULS, and elongation at break) of FRC 0/90°, FRC ±45° and PVC were further evaluated. Each mechanical property of each material was measured five times in each radiant exposure iteration. The change in elasticity modulus after exposing the samples to far-UVC radiant for different times is shown in Figure 9.

Elasticity modulus test results show that the specific elastic rigidity of the tested PVC samples increased after appropriate far-UVC radiant exposure. FRC shows a decrease in elasticity modulus due to microcrack initiation of the epoxy matrix under far-UVC radiant exposure. In addition, the elasticity modulus decreases more for FRC 0/90° than for FRC ±45°, 16% and 10%, respectively, after 290 J/cm² radiant exposure. This phenomenon can be explained by the greater effect of far-UVC radiation on the structure of the FRC 0/90° matrix.

Noticeably, as do all polymers, FRC with epoxy matrix and PVC lose some of the internal intermolecular and molecular bonds, and materials become brittle. Mainly, the decrease in plasticity can be seen in tensile test results by the decrease in elongation at break (Figure 10) and elongation at ultimate strength (ULS) (Figure 11).

The largest decrease in elongation at break and ULS is seen in the tensile test results of a ±45°-oriented E-glass FRC. The elongation at ULS and elongation at break of the ±45° FRC decreased by 26% and 17%, respectively, after 290 J/cm² radiant exposure. This result can be explained as an effect of the failure influence of the FRC matrix on the final result. The FRC matrix is uncoated, with no additional UV stabilizers. The results show that it has the highest sensitivity to far-UVC irradiation. The elongation at break of all polymers tested is close to each other.

Different degradation of mechanical properties under far-UVC 222 nm irradiation of different types of polymers may be caused by penetration depth of irradiance, polymer intermolecular chain length and bond strength, angle of incidence reflection, surface roughness, transparency of material to far-UVC irradiation, etc. The dependence of the change in tensile strength on radiant exposure is shown in Figure 12. It can be seen that after 290 J/cm radiant exposure, the lowest far-UVC effect was on PVC, and the highest was on FRC 0/90°, 2% and 17%, respectively.

The main factors affecting photoaging of polymers are a variety of photochemically reactive components, such as Cl⁻; other components in polymers can also affect the photoaging reaction. A part of PVC is a chloride, which is photochemically reactive component, which means that a part of its intermolecular bonds can be broken due to far-UVC irradiation. This causes a change in mechanical properties under far-UVC radiation exposure. FRC also has a strong absorption of UV radiation (especially at 300 nm), making epoxy structures susceptible to UV degradation. In UV light radiation, radicals, which are highly reactive and have a very short life, are formed from broken bonds of epoxy resin, causing microcracking of the resin. Due to the mentioned sensitivity to UV irradiation, degradation of physical matter of polymers also produces a decrement in its mechanical properties.

5. Conclusions

The use of far-UVC radiation with a specific wavelength of 222 nm for disinfection can have a significant negative effect on the mechanical and visual properties of the surfaces to be disinfected. The results of the study showed that far-UVC radiation at 222 nm causes significant color degradation in all the polymeric materials tested. The degree of color degradation varies depending on the type of polymeric material and the duration of exposure to far-UVC radiation. An obvious color difference was observed on FRC and PVC materials, where ∆E00 values of 6.431 and 7.194, respectively, were obtained after 290 J/cm² radiant exposure.

When evaluating the effect of far-UVC radiation on the mechanical properties of the materials, it was observed that the modulus of elasticity increased for the PVC samples tested and decreased for the FRC material samples. It was also observed that the modulus of elasticity decreased faster for FRC 0/90° than for FRC ±45°. The largest decrease in elongation at break and elongation at ULS is seen in the tensile test results of a ±45°-oriented E-glass FRC, which decreased by 26% and 17%, respectively, after 290 J/cm² radiant exposure.

The results of the study show that when designing far-UVC disinfection in public transport, it is very important to pay attention to its effect on materials. Therefore, in future work, it is important to find the optimum time for far-UVC disinfection to minimize negative impact on materials. These results can guide the development of safe and effective disinfection protocols that minimize damage to the materials and ensure the longevity of the disinfected surfaces.