What materials are reflective? – ANALYSIS OF THE PRESENT SITUATION IN THE RETRO-REFLECTIVE MATERIALS

What materials are reflective? Yes, it’s retro-reflective materials. With the rapid development of economy and science and technology, the traffic signs emergency reflective triangles, motor vehicle license, reflective fire safety signs, sea rescue, airports, railways, public facilities, safety clothing, advertising, national defense and military facilities and other places, are required to use a large number of high visibility safety signs, so the development of the world in this regression reflective material very fast.

Retro reflective material has unique reflective properties, can be composed of a light source the light source to the original direction of reflection, and kept in a small corner, because it does not need external power can play a good indication role, so it is also an important energy saving material. In this paper, the present situation of the research on regressive reflective materials in various countries is analyzed, the existing problems are summarized, and the direction of further development is discussed.

1. The principle of action of retroreflective material

How Does Retro Reflective Material Work? When the light wave is put into the medium B by the medium A, it is decomposed into two parts at the interface of the two medium, part of which is reflected back to the original medium by the interface, and the other part enters the medium B. If the surface is very smooth, the reflection angle with the same, so the reflection surface is called specular reflection; rough surface projection of light in all directions of the surface reflection, called diffuse surface; and reflecting material surface for reflection and specular reflection and diffuse reflection is different, it in the light cone along the light direction of reflection. When the angle between the incident ray and the reflected surface normal line is controlled within a certain range, the reflection characteristics remain unchanged. According to the theoretical analysis, when the reflection cone angle is designed very small, 98% of the reflected light can be concentrated in the 3°~4° solid angle, so that the observer near the light source can see the bright reflected light. For practical purposes, it is required that the greater the variable range of the incident angle between the incident light and the reflective surface is, the better it is generally less than K50°.

To achieve the purpose of primary reflexes, we must design and process materials accurately, so we must understand its principles, taking lens exposed reflective materials as an example (SOLAS Reflective Tape is such a structure).

Three different reflective light paths

The principle of action of retro-reflective material

From Figure 2, we can see that every bead and the reflector surface close to the microsphere form an independent reflection and refraction unit. Numerous optical units are densely connected to form a homogeneous continuous reflector.

As shown in Figure 3, the incident light parallel to the optical axis from the air into the beads, refraction occurs on the bead surface P, intersection light and shaft axis and the bead just gather in the bottom of the intersection of A, due to the lower part of the bead is provided with a reflection layer. Light A point of specular reflection, refraction and then in Q from the light source to return.

According to the law of reflection

∠PAO=∠QAO

∴∠APO=∠AQO∠POM=∠QOM

According to Snell’s law

n·sini=n′·sin∠PAO=n′·sin∠AQO

It is known from the reversibility of the optical path

n′·sin∠AQO=n′·sini

(The n and n′ in the formula are the refractive index of the air and the glass beads, respectively)

∴i=i′  The incident light I= reflected light I′

The reflected light parallel incident light means that a reflection unit will also have a parallel beam of light for a parallel incident beam, and the optical axis of the reflected beam coincides with the optical axis of the incident beam. That is, the reflected light is returned to the light source, which is the principle of regression. The glass bead is the key element of the regressive reflective light, it is a transparent solid round ball. The optical quality of glass beads mainly depends on the refractive index, usually refractive index is about 1.5 of the outer glass bead focus in the sphere of the surface, if the outer lining from a focusing layer and a reflection layer, the reflection to the original performance is good, and the refractive index is about 2 of the glass beads on the bottom surface of the sphere of the focus the reflective, high strength, and excellent performance of regression reflection.

2. What materials are reflective, and Research status

2.1 Retroreflective material survey

The study began in 40s in the field, but rarely reported. At present, we can see mainly the patent documents and the product advertisements of the companies. Since 60s, some state highway and city road management has been widely used for reflecting materials of various road safety signs, license plates of motor vehicles, road signs such as outline, and is widely used for water transport, rail transport, air transport, public security, mining, construction, film, advertising and decorating, military facilities and many other fields. A representative of the company is the United States 3M company, Japan Institute of CAS their corporation and Japan into the Ohio Industrial Corporation, Italy Autoadesivisrl, etc.. Retro reflective materials in accordance with the inverse reflection coefficient can be divided into high grade, project grade, diamond grade reflective material (several inverse reflection coefficient is certain geometric conditions of light irradiation and certain observation under the direction of the product luminous intensity observed in the I materials and materials in the plane perpendicular to the incident light illumination E group and per unit area the ratio of the group: R=I/E⊥*S). The engineering level regression reflective material is the most basic and the amount of a variety of maximum, inverse reflection coefficient up to 70cd/lux•m² it, the outdoor life of up to 3~5 years; high level is a new structure of the reflective material in 70s the United States launched 3M, its inverse reflection coefficient is 250cd/lux·m², the outdoor life of up to 7~10 years; high reflecting material complex manufacturing process, the requirements of raw materials and production technology of high performance index was much higher than the engineering level, project level price is more than 2 times; diamond regression reflective material is a new product that appeared at the end of 80s, it has superior performance, the price is expensive, the use of cost too high.

China’s research on the regression type of reflective material began in 70s. The Ministry of communications and some provinces have organized many researches. They have basically grasps their scientific theories, and have accumulated a lot of experience in technology, and have made many pioneering contributions.

In recent years China patent reported “composite reflective traffic signs”, “Diamond reflective road mark”, “low temperature fine porcelain road sign” “reflective ceramic tiles” etc.. At present, the reflective materials produced in China can be applied to all kinds of pavement markings, and have strong reflective properties, simple technology, low cost, good durability and smooth mechanical properties.

2.2 Retroreflective material research situation

At present, there are two main kinds of regressive reflective materials: coating type and adhesive type. The brushed reflective material usually refers to the reflective coating. It is mainly used for road line, clothing, safety mark, advertisement and so on. Its main components are composed of transparent polymer coatings and optical microspheres with regression and reflexes.

The adhesive reflective type is the reflective film. Generally speaking, the paste type is better than the brush type. At present, this kind of material is developing in the high grade direction. However, there are many engineering problems in the expansion of scale manufacturing, and there are many technical problems in the interdisciplinary field of multidisciplinary, and the requirements for equipment are also high. At present, there are some problems as follows:

(1) The main material properties are optical microbeads, polymer focusing layer, bonding layer and polymer surface layer. The technology of glass bead in our country has already had a considerable level. The higher the refractive index of glass beads, focusing performance is better, but the refractive index of glass beads increased when ND is greater than 2, it is very easy to devitrification and crystallization, so debugging glass formula is a technical difficulty, also requires a round of good, clear and transparent, but also has a certain degree of difficulty. Besides, the technology of focusing layer, surface layer and bonding layer is not yet completely determined. The cleanliness of these beads and the degree of their combination with the resin layer before and after the surface directly affect the reflective characteristics and service life of the reflective signs. The polymer materials should not only have high transparency and low refractive index, but also with good weather resistance, salt water resistance, heat resistance, cold resistance, abrasion resistance and certain rigidity and toughness at the same time, due to the operation of the process, the bonding layer and the focusing layer in the fluid must also have good thixotropic behavior the. It is difficult to find the polymer materials that are in line with so many properties.

(2) Because the thickness of each layer of the reflective film is micron scale, and the matrix is generally less rigid than that of the steel. Therefore, it is not an easy task to control the micron level coating on the top, and we need sophisticated coating equipment. Whether or not the glass beads are evenly arranged on the substrate will directly affect the reflective performance of the reflective film. Therefore, it is necessary to use the precision planting device to achieve it.

(3) Engineering Technology reflective film material involves many times in the process of polymer coating and curing. The coating process requires high accuracy, curing process is uniform and fast, especially to avoid volatile residue. Because the volatilization remains irregular tiny bubbles that affect the reflective properties of the material. How to make small molecules quickly and evenly spread out in a diffusion mode rather than in a bubbling way is a subject to be studied for the production of high grade original reflective materials.

3. Retroreflective Material Development and Application

Retro reflective material, because of its performance and the use of adaptive updating has developed into a series of products, its application involves air traffic safety, national defense, scientific research and civilian, technologically advanced countries to develop the reflective material products, China in this regard has to catch up. For example, WEALLIGHT reflective material research institute has discovered a common linear array co firing principle based on dense prism, and made the reflective fabric with high reflectivity by imprinting the media of mould. Compared with other products, it has the following advantages:

1) Light, soft, repeated folding, the surface is not afraid of rain erosion;

2) The wide angle of light reflection is large;

3) Accurate embossing in turn;

4) Save the bead glass microbead process.

5) Easy to manufacture and process various shapes.

It will be a product of upgrading and updating in the reflective material. Some departments starting from the beads developed transparent non vitreous ceramic beads, it compared with the conventional glass beads, the toughness and resistance to crushing, spherical degree and reflective rate is better, less impurity defects, refractive index in 1.5 ~ 2.6. The application scope of the reflective materials is widening, and the research direction is constantly expanding. The topics that need further exploration and research are:

1) How to suppress the positive reflection to improve the intensity and orientation of the reverse reflection.

2) The properties of the materials will be developed in the direction of multi material.

3) In view of the characteristics of the original reflective materials, special technological technology is developed.

4) Establish special testing methods, such as refractive index, wide angle, reflection intensity and chromaticity determination.

5) How to reduce the cost and simplify the process.

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