The effectiveness of S‐LEC™ solar control film L in laminated glass for reducing the burning sensation caused by near‐infrared radiation

The aim of this study was to examine the potential for S‐LEC™ Solar Control Film L, a heat insulating interlayer film for laminated glass developed by Sekisui Chemical Co., Ltd., to reduce the burning sensation caused by near infrared (NIR) radiation.


| INTRODUC TI ON
In recent years, the earth's exposure to solar radiation has increased as a result of the expansion of the ozone hole. 1 Skin disorders caused by sunlight exposure include acute sunburn and the chronic effects of photoaging, such as pigmented spots and wrinkles. 2These changes in the skin are mainly caused by ultraviolet (UV) rays. 3Sunlight also contains visible light and infrared radiation (IR), but little is known about the damage they cause to human skin.
Attention has recently been focused on the beneficial and detrimental effects of skin exposure to IR (the highest energy in solar spectrum), in particular the effects of near IR (NIR), at wavelengths between 800 and 3000 nm.Wrinkles, skin fitness, and sagging skin can be improved with the use of NIR devices that cause heat denaturation in the dermis. 3NIR irradiation can improve photoaging under appropriate management; however, NIR skin exposure also produces free radicals that may cause inflammation, 4 and induces the production of matrix degrading enzymes, such as MMP-1 and MMP-9, in the dermis. 5NIR is highly permeable, and uncontrolled exposure can cause deep skin damage, resulting in deep wrinkles and sagging. 6To prevent photoaging, it is recommended that we avoid periods of high UV exposure, seek shade, use parasols and hats, wear clothing with sleeves and collars, wear sunglasses, and make use of sunscreen. 2wever, these approaches are mainly for UV protection, and are insufficient to prevent skin disorders caused by NIR exposure.Since the skin is always exposed to sunlight, even while in a room or vehicle, it is important to develop technologies that protect the skin by blocking the penetration of both UV and IR radiation through window glass.
Automotive laminated glass is composed of an interlayer film of polyvinyl butyral (PVB) sandwiched between two glass plates.
Laminated glass with a PVB interlayer film is widely used for the front and side windows of cars as a shatterproof safety function and for crime prevention.Recently, new interlayer films with various properties have been developed, such as those with sound insulation to prevent noise pollution, or with UV protection to prevent photoaging.S-LEC™ Solar Control Film L, an interlayer film developed by Sekisui Chemical Co., Ltd., is designed to block IR while maintaining conventional safety and UV blockage.This product is expected to prevent temperature increases within a car and decrease the incidence of passengers experiencing a burning sensation on exposed skin.In this study, the NIR blocking performance was compared among laminated glass with a S-LEC™ Solar Control Film L interlayer, and the laminated glass types currently used in the Japanese and North American markets.

| Irradiation device
NIR radiation was delivered with a device developed by Sekisui Chemical Co., Ltd.(Figure 1A), equipped with an infrared lamp (IR100/110V250WRH, Iwasaki Electric Co., Ltd).The wavelength range of NIR is 700-2500 nm with a peak around 1000 nm.Prior to the studies performed on human subjects, preliminary tests were performed using the glass test products (see below) placed between the IR lamp and the intended position of the hand.The irradiation intensity was measured using FieldMaxII-TOP Laser Power/Energy Meter (Coherent).The instrument was placed at hand level, and the irradiation intensity was measured for each glass.A temperature-measuring device was also placed at hand level, and the temperature was measured for each product every 10 s for 60 s from the start of IR exposure.

| Subjects
Male and female volunteers between the age of 18 and 70 years were recruited from a volunteer panel.Applicant eligibility was checked against criteria including sex, age, and the presence or absence of complications such as photosensitivity.

| Background of the subjects
A total of 49 subjects provided informed consent to participate in the study, and included 22 males and 27 females with a mean age of 43.6 and 41.4 years, respectively.F I G U R E 1 Irradiation device and temperature increase in the irradiator after NIR irradiation.

| Test procedures
The subjects were randomly divided into six groups to evaluate the three test products in the following order: 1→2→3, 1→3→2, 2→1→3, 2→3→1, 3→1→2, and 3→2→1.Tests were conducted with an interval of 30 min between each to avoid any spillover effects from a previous test.The dorsal surface of the right hand was irradiated with NIR in all subjects (Figure 1B).

| Time until the onset of a burning sensation
The dorsal surface of the right hand was exposed to NIR for 30 s and the subjects were asked to immediately inform the examiner if they felt a burning sensation during irradiation.The subject was told to remove their hand from the device if they could not tolerate the heat.

| Degree of the burning sensation
The degree of the burning sensation measured 30 s after irradiation with NIR was evaluated based on responses from the subjects to the following question: "Assuming the highest degree of burning sensation under midsummer sunlight is 10 and no feeling of heat is 0, how would you score the burning sensation you felt 30 s after irradiation on a scale from 0 to 10?"

| Adverse events
Adverse skin reactions, including burn injury, were monitored during the tests and followed up until the completion of the test.

| Irradiation intensity and temperature of the device
The irradiation intensity of the IR lamp was approximately 700 mW cm −2 .The temperature in the irradiator was measured and compared among the three samples.The temperature increase with samples 1, 2, and 3 were 3.5, 2.7, and 1.5°C, respectively, after 30 s NIR irradiation, and 5.8, 4.5, and 3.5°C, respectively, after 60 s (Figure 1C).Based on the temperature changes at 30 s, the integrated irradiated light intensity was estimated as 8400, 6000, and 2700 mJ cm −2 for Samples 1, 2, and 3, respectively.

| The onset and degree of the burning sensation
The time (mean ± standard deviation) until the onset of a burning sensation during exposure to NIR for 30 s was 20.0 ± 8.10, 24.0 ± 6.64, and 28.0 ± 4.75 s with test products 1, 2, and 3, respectively (Figure 2A).The onset time with test product 3 was significantly longer than for test products 1 and 2 (1 vs. 3: p < 0.001, 2 vs. 3: p < 0.01).If the subjects did not feel a burning sensation after 30 s exposure, this result was recorded as 30 s.The number of the subjects who did not feel a burning sensation throughout the 30 s exposure was 14, 20, and 36 for test samples 1, 2, and 3, respectively.
No adverse events related to the study were observed.

| DISCUSS ION
Laminated glass with an interlayer film is required by law for automotive safety to control breakage.Noise reducing and UV blocking properties have also been added to glass with interlayer films.
Recently, interlayers have been developed that do not affect visible light transmittance while using thermal control technology to limit the temperature increase inside the vehicle during the summer season.
S-LEC™ Solar Control Film L developed by Sekisui Chemical Co., Ltd., is an interlayer film that blocks more than 99% of UV and contains fine metal particles which also has blocking function to NIR through absorbing it.Evaluations by Sekisui Chemical Co., Ltd., have shown that using this interlayer film in automobiles reduces the ambient temperature and power consumption of the air-conditioning system compared with standard interlayer films.However, the potential for S-LEC™ Solar Control Film L to decrease the burning sensation that can be experienced upon exposure to NIR has not been examined.
In a survey of car owners on their glass needs, 59.1% and 38.6% answered "glare" and "burning sensation caused by sunlight", respectively, as discomfort factors in a car. 7In addition, a survey conducted by Sekisui Chemical Co., Ltd., after informing 190 general consumers that the burning sensation was caused by NIR in sunlight, 51% answered "definitely yes" and 42% answered "yes" (total: 93%) to the question, "Do you want to block near infrared rays?".
Evidently, consumers are highly interested in NIR blockage; however, few studies have been focused on the effect of reducing the burning sensation caused by NIR exposure.
When the temperature change inside the NIR irradiator was examined prior to testing on the subjects, the temperature increase was greatest with Sample 1, followed by Samples 2 and 3.This result indicated that test product 3 was better at reducing the temperature increase caused by NIR.Test products 1 and 2 were both a combination of glass with an interlayer film commonly used in the Japanese and North American markets, while test product 3 was a combination of the most widely used glass in the Japanese market and a thermal barrier interlayer designed to block NIR (S-LEC™ Solar Control Film L).
In this study, subjects were randomized according to sex, and three types of glass with interlayers (Sample 1, Sample 2, and Sample 3) were evaluated to determine the onset time and degree of a burning sensation caused by NIR.The time taken to experience a burning sensation with test products 1, 2, and 3 was 20.0 ± 8.10, 24.0 ± 6.64, and 28.0 ± 4.75 s, respectively, indicating the superiority of test product 3 for delaying the onset of a burning sensation.These data included subjects who did not feel any burning during the 30 s NIR exposure and were recorded with an onset time of 30 s.
Among the subjects, 36 did not feel a burning sensation with test product 3 during the 30 s exposure, which was significantly greater than the 14 and 20 subjects who did not feel a burning sensation with test products 1 and 2, respectively.Based on these results, if the irradiation time were to be extended, the time until the onset of a burning sensation might be longer with test product 3, compared with the other products.
The onset of a burning sensation was faster for females than males, and the degree of the burning sensation felt by females tended to be higher than in males.Inoue et al. also found no differences with age and a higher sensitivity in females in a study regarding the sensing of hot or cold temperature stimuli. 8Similar observations have been reported in other studies, 9 but further research is required to draw conclusions relating to age and sex differences.The degree of the burning sensation with test products 1, 2, and 3 was 4.27 ± 2.85, 3.31 ± 2.80, and 1.20 ± 1.79, respectively, confirming that the degree of burning experienced with test product 3 was lowest.

| CON CLUS ION
This study demonstrates that automotive laminated glass with S-LEC™ Solar Control Film L, an interlayer film developed by Sekisui Chemical Co., Ltd., extends the onset of a burning sensation caused by NIR and reduces the degree of this sensation compared with F I G U R E 2 Time and degree of a burning sensation.conventional products.These results suggest that this newly developed laminated glass may suppress the temperature increase inside a vehicle and could make the automotive environment more comfortable.Importantly, it may also help prevent skin disorders such as photoaging caused by the long-term exposure to NIR.
Three types of glass with an interlayer film were used in the study: test product 1, common laminated glass used for cars in Japan (Glass A + S-LEC™ Clear Film); test product 2, common laminated glass used for cars in North America (Glass B + S-LEC™ Clear Film); and test product 3, laminated glass with an NIR-blocking interlayer film (Glass A + S-LEC™ Solar Control Film L).
This study was performed in accordance with the Helsinki Declaration and Ethical Guidelines for Medical and Biological Research Involving Human Subjects, after approval (September 13, 2022) by the ethics committee of the Non-Profit Organization Health Institute Research of Skin (No: 19000025).The study is registered in the Clinical Trials Registry of the University Hospital Medical Information Network (UMIN) (ID: UMIN000049159).