Reflective film is a new functional composite developed in recent 50 years. Micro prism reflective film was developed in the late 1980s. Because of its high brightness and good durability, it overcomes the shortcomings of previous prism reflective materials, such as small effective incidence angle, small observation angle and asymmetric reflected beam. Therefore, it is more and more widely used in highway traffic requiring high brightness.
1. Reflective principle and structural composition
1.1 principle
The reflective principle of micro prism reflective film is based on the light incident from a certain angle in three-dimensional space, reflected three times on three mutually perpendicular planes, and then reflected back along the original incident direction, as shown in Figure 1.
According to different manufacturing processes, there are two kinds of reflections on the prism surface: one is the (total) reflection of the air resin interface, and the other is the reflection of the aluminum resin interface.
1.2 classification by structure
According to different reflection types, the structure of micro prism reflective film can be divided into two types (see Fig. 2 and Fig. 3): coated reflective film (called type V sheeting in astm4956) and uncoated reflective film (called type IV sheeting in astm4956). Due to the gray color of coated reflective film, the market prospect is bleak.
The structural differences between the two reflective films are:
(1) There is air behind the pyramid of uncoated reflective film, so the reflection is mainly total internal reflection, which requires that there must be a stable air layer behind the pyramid. Therefore, there is a sealing layer in the uncoated reflective film (see Fig. 2) to prevent impurities such as moisture and dust from entering the air layer, resulting in the decrease of retroreflection coefficient.
(2) Coated reflective film pyramid is coated with a layer of metal, usually aluminum. Reflection mainly occurs at the resin aluminum interface. The coated reflective film has no sealing layer.
(3) Since the uncoated reflective film needs to be closed, there is a support wall. From the front, it is a diamond or hexagonal lattice. The coated reflective film does not.
2. Performance of micro prism reflective film
2.1 optical properties
The micro prism reflective film is much better than the lens reflective film in terms of reflective intensity and wide angle. Fig. 4 is a comparison of the initial values of reflective intensity of three types of products (0.2 ° viewing angle).
The reflecting light cone of the previous micro prism reflective film is asymmetric and the effective incident angle is small, but these problems have been basically solved through the design of the reflecting unit and its array.
2.2 weather resistance
Although the retroreflection coefficient of micro prism reflective film decreases by 50% after reaching the service life, its retroreflection coefficient can still be maintained at 400 CD · lx-1 · m-2, which is much higher than the other two types of reflective films, as shown in Figure 5 (0.2 ° observation angle, - 4 ° incidence angle).
3. Manufacturing technology of micro prism reflective film
3.1 key technologies
Although the basic principle of micro prism reflective film is simple, the manufacturing technology involves multi-disciplinary and high-tech fields such as materials science, optics, precision machinery manufacturing and so on. To sum up, there are four key technologies for manufacturing micro prism reflective film.
3.1.1 cell structure and its array design
The traditional prism retroreflective materials mainly have the following disadvantages:
(1) The effective incident angle is small. When the incident angle is greater than 20 °, the total internal reflection disappears and the reflective brightness is almost lost.
(2) The observation angle is small, that is, people can see the reflected light in the distance, but not in the near.
(3) Due to the three side properties of the reflection unit, the retroreflective light cone has an unsatisfactory asymmetric shape. As a result, there are different retroreflective brightness when observed from different scanning angles.
The method to solve these problems mainly depends on the design of retroreflective element structure and array, such as changing the included angle of three planes to make them have a certain distribution, or arranging the element array according to a certain law. However, due to the complex structure and array design, it is very difficult to make, copy and demould the motherboard, so it is not easy to implement.
3.1.2 fabrication and replica of motherboard
Because the height of the micro prism reflection unit is 0.003 ~ 0.015in (i.e. 0.078 2 ~ 0.381mm), the shape is irregular, the arrangement on the plane is regular, and the requirements for the smoothness of the forming surface are very high, so the motherboard needs precision machining. The plate itself is not directly involved in forming, the processing price is expensive, the area is limited, and the use cycle of the die is not high. The duplicate mold can be produced by using the master plate replica and spliced into any size to facilitate industrial production.
3.1.3 determination of base material
As a micro prism reflective film, its main material is engineering plastics. Different plastic sheet bases are selected according to the different requirements of its service performance. The main indexes considered and problems solved are: weather resistance of materials, preheating performance and rheological properties of plastic sheet base, etc. At present, TID mainly uses PVC, PC, PMMA and pet according to different product series.
3.1.4 embossing technology
Because the micro prism reflective film has high requirements for optical performance, its embossing accuracy is also very high. In the embossing process, defects such as poor flow, bubbles and scratches are not allowed, and the requirements for process conditions such as temperature and pressure are also very harsh.
4 application prospect
Due to the extremely high reflective intensity and good wide-angle performance of micro prism reflective film, with the continuous improvement of motor speed, the continuous growth of high-grade highway and the continuous complexity of road conditions, the application market of micro prism reflective film will be more and more broad. However, other problems caused by high intensity, such as glare and the contrast requirements between pattern and background in practical application, determine that the micro prism reflective film can not completely replace the glass micro bead reflective film.
