Coefficients for Assessing the Visibility of Materials Used in Anti-Smog Face Masks

Visibility is a very broad concept in terms of its definition as well as material properties, observation conditions, and the nature of human eyesight. There is a major difference between object visibility, which is a property of materials independent of the circumstances of observation, and visibility conditions, which determine our ability to see things. Anti-smog face masks are naturally used under low visibility conditions, and so it is necessary to increase their object visibility, which mostly depends on optical properties, although other factors, such as motion, may also be at play. The fact that moving objects are more readily perceptible is attributable to the physiology of human vision.

In the case of personal protective equipment (PPE), optical properties are mostly associated with materials reflecting, scattering, and transmitting optical radiation in the visible (VIS), ultraviolet (UV), and infrared (IR) ranges, which are collectively known as optical materials. Their applications in PPE include eye protection elements and high visibility devices (background and reflective elements). The optical materials used for eye protection mostly involve polymers and mineral glass [2]. The background materials in clothing usually exhibit vivid colors (mostly yellow or orange). In turn, reflective materials are made of micro-lenses or micro-prisms [3].

Regardless of their application, transparent materials are characterised by such parameters as transmission, absorption, and reflection/scattering of UV-VIS-IR radiation. The transmission/ absorption spectral characteristics of transparent optical materials determine their colour. Similarly to transparent materials, textile fabrics also reflect/ scatter UV-VIS-IR light. In the case of selected textile materials (e.g., those used for blocking visible light), the parameters describing their optical properties are tested and analysed in great detail [3].

Most textile materials used in PPE are completely or mostly opaque to VIS due to their functions as they are not intended to be translucent. In addition to their basic barrier functions, such as the protection of the human body against external factors including heat/cold, rain, moisture, and UV radiation, textile materials may also be designed to reflect VIS to improve the user's visibility. Colour, retroreflective properties, and sometimes also phosphorescence, influence the visibility of textile materials under given conditions (day/night, varying weather, etc.).

The basic parameters describing the visibility properties of textile materials are as follows [4]: –

the luminance coefficient,

The highest levels of user visibility are provided by PPE, such as warning clothing (for both occupational and casual use) and reflective accessories. Pursuant to Regulation (EU) 2016/425 of the European Parliament and of the Council [5], such devices are classified as category II PPE, and therefore they must meet the essential requirements concerning, amongst others, photometric properties for devices used to ensure the wearer conspicuity under given conditions.

Following trends in clothing design, it can be seen that the visibility aspect is increasingly emphasised not only in the case of protective clothing but also in non-occupational casual apparel. Reflective elements are now widely used in jackets, trousers, headwear, and footwear. Such elements may be incorporated into textile materials using different methods. A novel technology of high-quality in situ phosphorescent printing with a potential for continuous roll-to-roll industrial production was proposed by Zhang et al. [6]. Phosphorescent moieties were covalently connected to a polyethylene-polypropylene nonwoven. The resulting material emitted bright blue light with a high quantum yield (~83.35%), which could be attributed to aggregation-induced emission. The high-resolution printing technology enabled the production of a wide range of phosphorescent patterns and shapes on nonwovens. The versatility of the method was demonstrated by producing phosphorescent materials with different polymeric matrices, such as Nylon 66 and a polyethylene terephthalate membrane [6].

In another paper [7], water-soluble phosphorescent flavin mononucleotide (FMN) bio-molecules derived from vitamin B2 were used to produce a glow-in-the-dark photoluminescent polyester emitting yellow-green light. The paper analysed different ways of incorporating the phosphorescent substance into the nonwoven, such as screen printing and coating with gelatin, sodium alginate, and water-soluble polyacrylate while also activating the nonwoven with air-atmospheric plasma treatment for improved spreading. The authors reported gelatin to be an ideal polymeric coating and binder for observing the phosphorescent properties of FMN in printed patterns. The deposition of such a coating on a PET nonwoven subjected to plasma treatment enhanced the resistance of the phosphorescent product to washing under harsh chemical conditions. In turn, the authors of [8] imparted phosphorescent properties to a nonwoven made of polyethylene and polypropylene fibers induced with preliminary irradiation with an electron beam. The fibers revealed intensive phosphorescence, a high quantum yield (>90%) and excellent stability after washing under harsh conditions. Moreover, the colour and intensity of the phosphorescence, quantum yield as well as phosphorescence duration may be controlled using aggregation-induced phosphorescence quenching. Due to the aggregation-caused quenching (ACQ) effect, aggregates have limited applicability in layers emitting light. On the other hand, phosphorescent fabrics prepared in this way may effectively distinguish between monocyclic aromatic hydrocarbons as a result of adsorption-induced phosphorescence changes. Therefore, such nonwovens may be used as chemical sensors, photoelectric materials, as well as for decorative purposes.

It should be noted that wearer visibility can also be increased by face masks, which may be very useful for at least two reasons. First of all, face masks can protect the respiratory system against smog pollution, which in Poland occurs mostly in the autumn and winter [9]. Secondly, due to the global pandemic of SARS-CoV-2, face mask use has become commonplace [10, 11, 12]. Given the current epidemiological data, there is no indication that the widespread obligation to cover one's mouth and nose with face masks will be lifted any time soon [13].

For a face mask to be user friendly, it must offer a high level of comfort provided by low airflow resistance. Thus, phosphorescent marking is suitable for facemasks as it substantially enhances visibility while taking up a relatively small area. Phosphorescent dyes absorb wavelengths from a certain part of the visible light spectrum, re-emitting wavelengths that are longer than the absorbed ones. Importantly, phosphorescent substances may emit light even if they are not currently illuminated, but were before.

The objective of the study was to assess the visibility of the material with a phosphorescent pattern of a half-mask protecting against smog applied using the available laboratory methods.

The study involved two samples of materials intended for anti-smog face masks. The samples were made of a non-woven printed with a yellow-green phosphorescent dye and differed in terms of the print pattern and background colour. Sample 1 was blue and featured an open star pattern, while sample 2 was white and featured a solid star pattern (see Figure 1).

Fig. 1

The nonwovens shown in Figure 1 were used as the outer layers in the anti-smog filtering-absorbent face mask models presented in Figure 2. The outer layers featured phosphorescent patterns made by means of screen printing: open and solid stars. It should be noted that screen-printed patterns cannot be arranged too densely on the surface as the dye filling the voids between fibers, can increase breathing resistance, which adversely affects user comfort. Air flow resistance for printed and non-printed material is presented in Table 1.

Fig. 2

The half-mask models presented in Figure 2 consisted of the following layers:

Model A –

blue spunbond outer layer printed with a phosphorescent pattern in the form of open stars, with a surface density of 86 g/m²,

Model B –

white spunbond outer layer with a printed phosphorescent pattern in the form of solid stars, with a surface density of 84 g/m²,

The spun-bond nonwovens with a pattern printed with phosphorescent ink constituting the external layer of the filtering half-mask had the same surface weight. The materials differed only in colour. In order to determine how the colour of the material affects its visibility, the optical properties were determined.

The phosphorescent effects on the outer layers of the two half-mask models are presented in Figure 3.

Fig. 3

Measurements of the samples with phosphorescent printed patterns described in the previous section included the reflectance coefficient, chromaticity coordinates, and luminance determined from digital images.

3.1

Determination of the reflectance coefficient and chromaticity coordinates

The reflectance (brightness) coefficient and chromaticity coordinates (colour measurements) are used for assessing high-visibility materials, including those incorporating phosphorescent dyes, used in the production of warning clothing which provides wearer conspicuity under conditions of insufficient lighting, e.g., at dusk.

Colour measurements using a reflectometer involve measuring the ratio of reflected light to incident light in the 400–700 nm range. According to publications by the International Commission on Illumination (CIE), the light beam reflected off an object (usually a vividly coloured sample) may be expressed in terms of its trichromatic components: X, Y, and Z. These components may be calculated using mathematical equations based on measurements of re-emission from the sample. Subsequently, one can use tables with data on the illuminant s(λ) and the observer x(λ), y(λ), z(λ) to produce a re-emission curve expressing the relationship between the reflection coefficient R and wavelength f(λ) = R. It should be borne in mind that the results are significantly affected by measurement geometry, i.e., the angles of incidence and reflection, the light source (illuminant D65, A), and parameters of the optical analyser (monochromator), that is, the observer (normal observer 2º or 10º).

According to the requirements of the standard PN EN 20471:2013-07/A1 [14], colour should be measured pursuant to the procedures stipulated by the International Commission on Illumination (CIE) [15] using a polychromatic light source (illuminant D65), a 45/0 geometry (the angle of reflection to the angle of incidence), and a normal observer. This produces the chromaticity coordinates x, y and Y and the luminance coefficient β, which is computed from Equation 1 on the basis of the coordinate values obtained. The colour coordinates x and y express hue and saturation, while Y stands for brightness, ranging from 0 for black to 100 for white. (1) β=100/Y \beta = 100/Y

The requirements contained in PNEN 20471:2013-07/A1 concern high visibility materials for occupational use which are yellow (β < 0.75) or orange (β < 0.40).

Figure 4 shows a photograph of the MiniScan XE Plus reflectometer (USA) used for determining the reflectance coefficient and chromaticity coordinates in the preliminary studies of face mask materials. During the measurements, the materials were placed on a black metal plate and a white metal plate (reference plates in reflectance studies), as well as grey and white nonwovens. Prior to the measurements, the samples were conditioned for at least 60 min in a room illuminated with artificial light (fluorescent lamps). The intensity of incident light was approx. 500 lx.

Fig. 4

The paper presents the possibilities of assessing the optical properties - visibility of anti-smog models of half-masks with new functionalisation.. Half-mask outer layers were printed with phosphorescent dye, increasing their visibility under smog conditions. It was also shown that the two patterns designed did not affect the deterioration of the filtration properties. Two colours of the background material of the outer and inner layers and two types of pattern: solid and open stars were used.

Two methods for objective evaluation of optical properties were proposed based on the reflectance coefficient, chromaticity coordinate determination using a reflectometer, and luminance determination using digital image analysis.

The analysis of RGB values from digital images is useful for assessing the phenomenon of phosphorescence - increasing visibility after lighting (irradiation)

Reflectance results indicate that under dark conditions better visibility can be obtained using white material with a print pattern containing solid shapes. However, it should be remembered that the more phosphorescent print on the surface of the material, the more the air flow resistance may increase. Therefore, a compromise has to be found between good visibility and air flow resistance. This conclusion is very useful for face mask designers.

Further development of the method of determining luminance from digital images proposed may constitute an element of laboratory evaluation of the potential visibility of materials with phosphorescent patterns intended for face masks. Anti-smog and other face masks with phosphorescent print on their outer layers are conspicuous after dark. The research will be continued to evaluate the properties that determine face mask visibility under real-life conditions.