Since then, with the development of chemical industry, especially synthetic resin, various R&D institutions have been constantly researching and innovating, and developed a series of high-quality reflective products by using glass bead technology, synthetic resin technology, membrane technology and coating technology.
Since the 1940s, this initially manufactured reflective film has been labeled as "engineering-grade" reflective film and widely used in road traffic signs. Since then, with the emergence of synthetic resin and the needs of social development, a series of products such as reflective films used in personal safety protection fields such as clothing have been developed one after another. Since then, with a series of research achievements in materials science and optical technology, especially the appearance of micro-prism reflective materials, this reflective material, which was originally mainly used for traffic signs, has gradually been replaced by newer and better reflective materials.
Reflective films can be classified in many ways. Among them, the recognized classification principle is based on the basic structure of retroreflective units, and the ranking method is mainly based on the retroreflective coefficient of the front photometric performance of retroreflective films. However, considering the different technology of reflective film, some of them are specially designed to solve the brightness of non-positive retroreflection, some are designed to give consideration to both aspects of performance, and some are designed to meet the requirements of visual recognition under bad weather conditions, so this classification method also has shortcomings. Therefore, it is necessary to be familiar with and master the application conditions and design functions of various reflective films.
In traditional customs, according to the structure of reflection unit, reflective films are divided into two types, glass bead reflective films and microprism reflective films. Each kind of reflective film also contains many kinds, such as microprism reflective film. Due to the adoption of more advanced technology, its material selection and prism structure have changed greatly, which can meet more traffic demand. According to the forms and technical characteristics of prisms, microprism reflective films can be divided into truncated prism reflective films with good long-distance retroreflection ability, truncated prism reflective films with good short-distance and large-angle retroreflection performance, full prism reflective films with all aspects in mind, fluorescent full prism reflective films with good performance in daytime and bad weather conditions, prism reflective films that meet traditional engineering-grade retroreflection parameters, and so on.
Glass bead reflective film appeared earlier, but the technology changed little. There are two main types: one is lens embedded reflective film, which is customarily called engineering-grade reflective film; One is sealed capsule type, which is usually called high-strength reflective film. For the needs of application, it should be noted that in the embedded reflective film of lens, due to its long history, various manufacturers have made many kinds of reflective films, such as super engineering reflective films, by taking advantage of the differences in diameter, density and thickness of the weather-resistant coating of lens during the long production and manufacturing process. Commonly known as economic grade reflective film, it is mainly produced in China, and is basically realized by reducing the number and density of lenses (glass beads) on the basis of engineering grade reflective film technology. These two kinds of reflective films, economic reflective films, can not meet the needs of traffic safety, and are mostly used in commercial fields, and are rarely included in traffic safety standards.
When it comes to the scientific classification method of reflective films, it is inseparable from the reflective film standard, which has great guiding significance for the application of reflective films. Among the world reflective film standards, the standards of American Society of Materials and Testing, Australia and New Zealand and the guidance of the Federal Highway Administration on reflective materials for traffic signs have played a positive role in guiding the research and application of reflective materials to make traffic signs and improve traffic safety. Below, we will introduce them one by one.
American Society for Materials and Testing is an international material testing standards organization with a long history. The English full name is ASTM. It was established to provide a series of material testing standards for the scientific community and industry, thus realizing the definition of new materials and providing a technical platform for the scientific community all over the world to communicate with each other. It provides technical support for the testing standardization of retroreflective materials and various materials in various industrial fields such as petroleum, natural gas and chemical industry.
In view of the traceability of this technology, ASTM test standards for reflective materials have also accumulated with the invention and use of reflective materials. Every time a new material appears, as long as the material appears for a period of time and its manufacturer applies to ASTM for joining, its committee will authorize the classification of the material and establish testing standards. It is precisely for this reason that in the standard of reflective film in ASTM4956, there are as many reflective films as 1 1, and it is still going on; On the other hand, however, the ASTM standard is more like a catalogue of retroreflective materials than a standard that helps to understand the application methods and problems of retroreflective films, because when ASTM first classified these materials, it did not consider the performance and needs of drivers.
For this reason, in order to provide more effective technical support and guidance for domestic traffic engineering construction units, developed countries in the world have formulated their own national technical standards instead of directly using ASTM to classify reflective materials.
Reflective film is a layered structure composed of multiple layers of materials with different properties. Different reflective films have different layer structures. Fig. 3 is the basic structure diagram of the earliest glass bead reflective film. As can be seen from the figure, the reflective film is generally a film structure object composed of a surface layer (protective film), a reflective layer (functional layer), a base layer (bearing layer), an adhesive layer and a bottom layer (protective layer). The surface layer of reflective film generally adopts resin film with good light transmittance and weather resistance. The reflective layer adopts different materials according to different types of reflective films, such as tiny glass beads, microprisms or metal reflective coatings. The base layer is mostly resin organic compounds, the adhesive layer is generally epoxy resin glue, and the bottom layer is a protective layer made of thick paper.
Table 1 is the structural diagram of various reflective films. It can be seen that different types of reflective films have different composition materials and structures.
Table 1 Schematic diagram of main structures of various reflective films. The primary function of reflective films is to improve the surface performance of traffic signs, make them meet the needs of all-weather traffic and improve the safe operation conditions of roads.
Because there are differences in the reflective performance of different kinds of reflective films, it is necessary to make corresponding specifications according to the setting function and use of the signs when they are applied to the production of traffic signs. The science of studying this application standard is regarded as an important part of traffic control and safety technology by safety engineering professionals all over the world.
Traffic control and safety technology has been developed for hundreds of years. Since 1908, the first artificial traffic sign standard came out in Britain, many countries in the world have continuously invested a lot of scientific research and technical resources to analyze and master the role and value of reflective technology in the field of traffic safety. In this respect, developed countries such as Europe and America are at the forefront, and their research results have helped China to experience a process from scratch in a short period of 10 years. The research on reflective technology of traffic signs in China began in the late 1980s. The main technical specifications are GB5768, the national standard of reflective materials for traffic signs, and GB 18833. These standards are still in the stage of perfection and development in many aspects, and the related scientific application methods and effect research conclusions still need a lot of time and practice. Lens embedded reflective film, commonly known as "engineering grade", is the original product of glass bead reflective film. The industry is used to calling it "engineering grade" reflective film, which was invented in 1937. The name of "engineering grade" comes from the registered English product name "engineering grade", which is the name of the product invention company. Later, many scientific research institutions directly used this product name to represent the name of experimental materials, so this name became an idiom in the field of traffic engineering, and its front brightness (0.2? /-4 ? ) generally below 100 CD/LX/m, until June 2008, 165438+ 10, according to the reflective brightness characteristics of engineering-grade reflective film, a new prism-type engineering-grade reflective film (also called super-engineering grade, EGP in English) was developed, which was broken through and enriched again by scientific and technological innovation.
The traditional engineering-grade reflective film was introduced to China in 1980s. In 1990s, some manufacturers began to appear in China to produce this kind of reflective film.
Adhesives for engineering-grade reflective films are generally divided into pressure-sensitive and heat-sensitive types, which can be pasted. Using the same ink and screen printing technology, you can also print various patterns on it. The bottom plate suitable for engineering-grade reflective film is aluminum plate, and the construction operation temperature is generally required to be above 18 degrees Celsius. If the temperature is too low, it will affect the performance of the adhesive and lead to the damage of the service life of the mark. Fig. 4 is a schematic structural view of a reflective film embedded with a lens. The life of engineering-grade reflective film is generally 3 ~ 7 years, and the front of white film is twice as long (0.2? /-4 ? ) is generally around 100cd/lx/m, depending on the manufacturer. Some manufacturers only provide reflective films for 7 years, and the brightness retention value after 7 years is at least 50% of the initial brightness value. Some manufacturers only provide 3-year and 5-year warranty. This is mainly caused by the different weatherability of reflective films. Reflective films made of the same raw materials have different life spans when used in different geographical and climatic conditions.
It should be noted that the brightness stability, brightness intensity and weather resistance of engineering-grade reflective films are some important basis for investigating the production quality of such reflective films. In these links, cutting corners in any link can reduce the product cost, but its quality will also be greatly reduced, especially the difference in weather resistance and photometric parameters, which can obviously reflect the advantages and disadvantages of engineering-grade reflective films. Lens sealing reflective film is a durable glass bead reflective film, which is commonly called "high strength" reflective film in the industry. 1972 was successfully developed. "High strength grade" comes from English High Intensive Beads (HIB for short), which was originally the special name of product research and development company. Until 1985, starting from Japan, enterprises in some countries and regions began to make this kind of reflective film one after another, so the word "high intensity grade" began to be used by other factories one after another and gradually became the unified name of this kind of reflective film with special structure. Considering that the readers of this book are mostly people in the industry, the name of Gao Qiang Class has become a common name in the industry. Therefore, for the convenience of readers, in the following chapters, "Gao Qiang Class" is also the main title.
The reflective coefficient of high-strength reflective film manufactured by qualified technology and materials is at least twice as high as that of engineering reflective film. The internal vacuum bracket structure also solves the problem of dew condensation of signboards due to temperature changes, further improving the reflective ability of materials. When this material came out in the 1970s, it complied with the needs of technological progress to improve the speed and road conditions, and was successfully used to make traffic signs, saving many lives. Compared with the engineering-grade reflective film, the high-strength reflective film makes the sign more visible even in the case of large angle and bright area, and effectively predicts the road danger in front of the driver.
High-strength reflective film adopts glass bead reflective technology. Because of its innovation in product structure, it has incomparable brightness and angle performance of engineering reflective film, but at the same time, it also leads to some insurmountable product defects, such as fragile and easy to tear, wrinkle, bubbles, surface honeycomb protrusions, high production energy consumption and large emissions. The limitation of glass bead technology also hinders the improvement of high strength to higher brightness and better angularity.
High-strength reflective film is also a material with glue, which is generally divided into pressure-sensitive type and heat-sensitive type. Various patterns can be made by using the same ink and screen printing technology. High-strength reflective film generally adopts resin film with good light transmittance and weather resistance as the surface layer, the second layer is vacuum layer, the third layer is embedded tiny glass beads, the fourth layer is metal reflective coating, the fifth layer is resin bearing layer, the sixth layer is adhesive, and the seventh layer is paper-lined protective layer. Fig. 5 is a schematic structural diagram of the high-intensity reflective film, and fig. 6 is a typical appearance of the high-intensity reflective film. High-strength reflective film is mainly used to make traffic signs such as guiding signs, prohibition signs, warning signs and indicating signs. After the advent of high-intensity reflective film, the time for drivers to recognize traffic signs is shortened, and the distance between the front signs and obstacles is significantly advanced, which greatly increases the time for taking safety precautions, reduces the incidence of road traffic accidents at night, and improves traffic safety. According to empirical research, the brightness of high-intensity reflective materials is much higher than that of engineering-grade reflective materials. Since 1990s, this kind of high-strength reflective material has been widely used on expressways in China.
Since then, with the improvement of motor vehicle performance and road construction technology, great changes have taken place in the urban environment, with a large number of expressways and high-speed vehicles, complex urban light sources, and endless wide roads and sharp bends, which have new requirements for drivers' visual sight distance. Some shortcomings of high-intensity reflective materials, especially in large-angle reflective performance, processing technology and cost, can not be compared with the emerging prism technology, and gradually began to be replaced.
In the second half of 1990s, especially in 2 1 century, the United States and Europe have begun the process of replacing high-strength materials with prism-grade materials. Especially in 2004, the "super-class" retro-reflective materials used prism technology, which not only improved the reflective performance, processing methods, energy saving and emission reduction, but also was not inferior to high-strength materials in price and cost. Since then, as the birthplace of high-strength materials, the United States has stopped producing such materials, making China the only producer of high-strength reflective materials.
The life of high-quality and high-strength reflective film is generally 10 year, and the front brightness of white film is (0.2? /-4 ? ) Generally above 250 CD/LX/m, under normal use, the brightness retention value after 10 years is at least 80% of the initial brightness value. The suitable substrate of high-strength reflective film is aluminum plate, and the working temperature is usually required to be above 18 degrees Celsius. The retroreflection principle of microprism reflective film is different from engineering-grade (lens embedding) and high-strength (lens sealing) reflective films. Both engineering-grade and high-strength reflective films adopt the principle of glass bead reflection, while the reflective principle of microprism reflective film is to use the refraction and reflection of microprism. According to the characteristics and structure of retroreflection, the main representative products of microprism reflective films can be divided into four categories: truncated prisms that focus on long-distance identification, truncated prisms that focus on short-distance large-angle identification, full prisms that give consideration to long-distance identification and short-distance identification, and new prism reflective films that combine these prism technologies with new material technologies. In recent years, with the diversification of application levels, they are new reflective materials to meet the needs of different levels.
Long-distance truncated microprism reflective film is the first generation of microprism reflective film that came out in the early 1980 s. Its English name is Long Distance Prismatic, and the first generation of diamonds, crystals and starlight products that can be seen on the market are all like this. The front brightness of this reflective film is very high, and the front brightness of the white film (0.2? /-4? ) generally at 800cd/lx/m, generally above 800, the distribution of retroreflected light has no directionality. Whether the reflective film is attached horizontally or vertically, there is little difference in reflective effect. However, at large incident angle and large viewing angle, the reflected brightness will be greatly attenuated. As shown in fig. 7, the structure of this reflective film is displayed under a microscope. This kind of reflective film highlights the positive retroreflection luminosity and is more suitable for road signs, warning posts and so on. Not suitable for reading traffic signs that require more visual brightness within a distance. This early prism reflective film was a phased achievement of design and research at that time. The prism structure at that time could not solve the problem of retroreflection brightness in angle measurement in grand view.
After the first generation of microprism reflective film came out, people found a problem. When the motor vehicle really enters the reading distance of the sign, that is, when the observation angle is large, the brightness of the sign decays to the point that the contents of the sign cannot be read within the reading distance, or the reading time will be longer. Therefore, people use the large-angle truncated microprism structure to make the large-angle truncated microprism reflective film to solve the problem of keeping the mark twice within the reading distance. Therefore, this kind of large-angle reflective film is also a special prism reflective film described from the aspect of reflective performance.
Compared with the long-distance truncated micro-prism reflective film, the front brightness of the large-angle truncated micro-prism reflective film is relatively low, but its reflective brightness will not be greatly attenuated at large incident angle and large viewing angle. Large angles correspond to places with many lanes and bends, as well as signs with complex contents and long reading time, so this kind of reflective film is suitable for traffic signs on urban roads and wide roads. Although its front reflective brightness is average at a long distance (only compared with the prism level at a long distance, it can still reach more than twice the high intensity level), at a short distance (the distance to read the logo content), its reflective brightness is much higher than that of the long-distance reflective film. Its directivity is stronger than that of long-distance reflective film, and it can be adjusted according to the position and direction of the sign to meet the needs of reading. Fig. 8 is a structural diagram of VIP large-angle truncated microprism under microscope. VIP (visual impact prism), translated as visual impact prism, came out in the late 1980s and was once widely used. After the full prism technology appeared, it was discontinued.
All-prism retroreflective film is a prismatic retroreflective material made of all-prism structure, that is, the part that can't reflect light in the traditional micro-prism structure is removed, so that the retroreflective film is all composed of prism structures that can realize total reflection. It combines the two characteristics of long-distance and large-angle micro-prism reflective films, and improves the reflection brightness at large incident angle and observation angle of 50-250 meters while maintaining high front brightness and being easy to find at a long distance.
The advent of this kind of all-prism reflective film breaks through the academic barrier that prism reflective film can not give consideration to both long-distance reflection ability and short-distance reflection ability at the same time. According to the path and mode of vehicle light propagation, it finds the angles (incident angle and observation angle) needed for sign recognition within an ideal distance, then determines the non-reflective areas on the traditional truncated microprism, and then removes these non-reflective areas, so as to realize the reflective structure area per unit area 100% on the reflective film, which is called "total reflection".
Of course, this is only the theoretical reflection efficiency 100%. In actual production, due to the limitation of materials and other conditions, the brightness of reflective headlights cannot reach 100%. At present, the best reflection efficiency is 58%, which is much higher than other types of reflective films, such as high-intensity reflective films, only 23%. And from the observation angle of 0.2? From the beginning to 2 o'clock? Its retroreflection efficiency is always above 50%. Fig. 9 is an electron micrograph of a full prism reflection film.
At present, after each microcrystalline cube is connected and arranged according to certain rules, there will be more than 930 units in a square centimeter of material area to control the path of light entering and reflecting. The lower layer of the microcrystal cube corner is sealed to form an air layer, and the incident light is totally reflected inside by the diffraction phenomenon of light, and the best reflection effect can be achieved without the help of a metal reflection layer. Compared with the traditional engineering-grade high-strength reflective film, the reflective film made of wear-resistant high-hardness polycarbonate material and microcrystalline cubic technology not only doubles the reflective performance, but also greatly improves the large-angle reflective performance. The front brightness of this full prism reflective film is more than 6 times that of engineering grade, and the front brightness of white film (0.2? /-4? ) is generally above 600 cd/lx/m, which is more than twice the high-strength grade, while under the measurement of the grand view angle (0.5? And second? When), the retroreflective performance is about two to four times higher.
All-prism reflective film is a kind of traffic sign material suitable for all grades of highways and urban roads. The application in the west has gradually replaced the investment and consumption of sign lighting. When making road signs, if the long-term investment benefit and safety benefit are considered, the full prism reflective film can replace any grade of reflective film. Under normal use conditions, after ten years of use, the retained brightness of the full prism is at least 80% of the initial brightness, that is, after ten years, it can still greatly exceed the retroreflection performance of the brand-new high-strength and engineering-grade reflective film, which is a more economical choice from the perspective of scientific development. At the same time, if the same ink is used, combined with screen printing technology, various traffic signs with patterns can be made.
All-prism reflective film is mainly used for guiding signs, prohibition signs, warning signs and indicating signs, especially for signs that need to be read for a long time, signs with complex visual environment, wide roads and high-grade highways, and its performance is particularly outstanding. The suitable bottom plate of diamond-grade reflective film is aluminum plate, and the processing temperature is generally required to be above 18 degrees Celsius.
Figure 10 shows the comparison of retroreflection brightness values of engineering-grade reflective film, high-intensity prism, truncated prism and full prism at various angles. With the progress of science and technology, the photometric performance of all-prism reflective film has been significantly improved at all angles.
In recent years, under the condition that the structure of prism-type reflective film has not changed greatly, the focus of innovation has shifted more to achieve richer light control effect and rich material characteristics through different material processing technologies, so as to achieve different reflective ability and flexibility to meet the needs of different levels. The reflective films commonly known as "super", "super" and prism engineering grade (new super engineering grade) in the market are all new forms of prism reflective films. The truncated prism structures of these reflective films are basically the same, but the processing techniques of the materials are different, resulting in different reflection effects, superior weather resistance and processing adaptability, which meet different application requirements.
Among them, especially the super-strong reflective film, because it conforms to the market demand, it quickly became popular after it came out at the beginning of 2 1 century. Its original design intention is to give full play to the advantages of prism structure, and on the basis of ensuring that it can surpass all functions of high-strength reflective film, it can also have better retroreflective performance and superior cost performance under multi-angle conditions.
These new prism reflective films have very high strength and thickness, which eliminates the defects of easy tearing, wrinkling, bubbles and surface honeycomb protrusions in logo processing, greatly simplifies the construction difficulty, makes the logo processing process easier to control and reduces the losses caused by poor processing. At the same time, due to the large surface brightness factor of the reflective film, the retroreflection performance is greatly improved. It not only has superior retro-reflection coefficient at a long distance, but also can keep the sign bright at a large observation angle at a close distance under general visual requirements, so that the driver can find the sign earlier and read the sign content more clearly at a close distance. Fig. 1 1 is the structural schematic diagram of these prism-structured reflective films. Different retro-reflection effects can be formed by the material treatment difference between the resin layer and the cubic crystal surface.
The surface of this kind of reflective film is mostly made of polycarbonate, which is not only more wear-resistant and scratch-resistant, but also can be used with screen printing ink, and can also be used for thermal transfer to make color traffic signs. At the same time, due to the improvement of the surface brightness coefficient, the signboard is more eye-catching and bright during the day, and it also has better weather resistance.
It is worth mentioning that in the 2008 Beijing Olympic Games, which had strict requirements on all aspects of traffic signs, Beijing traffic management authorities used this reflective film to complete the preparation task of the event with high quality and high speed, making China the first country in the history of the Olympic Games to use this reflective film to make special lane warning signs. This also shows from one side that the production technology of traffic signs in China has rapidly approached the international advanced level. See figure 12. Figure 12(a) shows the ultra-high reflective film sign being installed, and the color part on it is printed by the printer. Figure (b) shows the super-reflective film being printed. The biggest difference on the surface of super reflective film is the unique stripe pattern, as shown in Figure (c). This is a characteristic that other reflective films do not have.
The prism engineering-grade reflective film that came out in 2008 is also a brand-new product concept. While ensuring the front brightness performance of traditional engineering-grade reflective film, it has made great progress in large-angle reflective performance, and its retroreflection ability even exceeds the parameters of high-intensity reflective film. At the same time, due to the use of polycarbonate, this kind of reflective film has the ability of hardness and high weatherability, which can greatly improve the construction efficiency and provide more technical choices for the application and popularization of retroreflective materials.
After the full prism structure, there is no breakthrough in the structure of the reflective film. However, there is still a lot of room for development in terms of cost, materials and chemical coating. Fluorescent reflective film is a typical case, in which the coating technology has been improved to further optimize the function of reflective film. Fluorescent full prism reflective film is a kind of reflective film with special optical effect, which combines special fluorescent materials with excellent weather resistance (general fluorescent materials have poor weather resistance) with full prism technology. There is a unique weather-resistant fluorescent factor in the fluorescent reflective film, which can increase the activity after absorbing the energy of visible light and part of invisible light in the spectrum, thus converting the energy of invisible light into the energy of visible light, making the chromaticity and luminosity of the reflective film stronger during the day, thus increasing the distinctiveness of the sign.
Because the fluorescent reflective film can absorb the energy of invisible light in the spectrum and convert it, it can have better chromaticity and luminosity, which is called more vivid. This fluorescent reflective film is much brighter than ordinary colors in bad weather conditions and when the sun is not so strong, and it is easier to attract people's attention. The use of this fluorescent reflective film in traffic safety facilities is of great significance to ensure driving safety at dawn, dusk or in bad weather such as rain, snow and fog. At present, the application of fluorescent full prism reflective film has been very common in foreign countries, such as fluorescent warning signs, fluorescent linear delineator, and fluorescent signs in road construction areas. Yellow-green fluorescent full prism reflective film has been approved by the Federal Highway Bureau as a traffic sign for pedestrians, non-motor vehicles and school areas. Orange fluorescent full prism reflective film is mostly used for building area signs. Countries all over the world have also issued corresponding standards and specifications and technical conditions for fluorescent reflective films. Figure 13 shows the comparison between fluorescent and non-fluorescent reflective films.
Fluorescent yellow reflective film and fluorescent yellow-green reflective film have been used in China since 2006. The fine acceptance and application of this new technology can be seen in the rainy and foggy section of Sichuan-Emeishan Expressway, the accident-prone section of Badaling Expressway in Beijing and the Olympic lane on the Fifth Ring Road in Beijing. See figure 14 and figure 15. Figure 14 The warning signs on the sidewalks near the water arena of the Beijing Olympic Games are made of fluorescent yellow-green full prism reflective materials to improve the visual recognition effect of the warning signs. Pay attention to the difference in luminosity and chromaticity of warning signs using ordinary reflective films next to them. In order to ensure the Olympic traffic, a speed warning device with fluorescent yellow-green full prism reflective film is being installed on the Fifth Ring Road (Figure 15). It is worth noting that other traffic signs are not good in chromaticity and luminosity under backlight, but the area of fluorescent yellow-green full prism reflective film is very eye-catching.
It should be noted that the fluorescent reflective film is a combination of weather-resistant fluorescent factor and prism reflective film. Advertising materials printed with lemon yellow do not belong to this technical category. Although the surface looks similar in chromatography, it does not have all the technical characteristics of fluorescent reflective films.