The retro reflective material has good retro reflection effect and is widely used in road traffic, special operation clothing, firefighting, mining area and other fields. Its bright reflection effect can effectively enhance the visibility of objects in the dark environment, and ensure safety, especially in recent years, the application of reflective fabrics is more and more extensive.
Retro reflective properties are the most important properties of reflective fabrics. The diameter of glass beads, the number of microspheres on the surface of the reflective fabric, and the covering area of the surface microspheres have great influence on the retro reflection. When the diameter of the glass bead is larger than the surface of the surface, the retro reflection coefficient increases, and the retro reflection properties of the reflective fabric can be improved.
Retroreflective materials or reflective materials, with high refractive index glass beads or prism lattice as the core material, using optical refraction and reflection principle; the incident light in a certain range of incident and reflected light along the opposite direction of incident light returned, and when the direction of the incident light in a large range, can still keep this characteristic.
Therefore, research on reflective material core element and the reflective material preparation process, provide guidance on improving the retroreflective property of reflective material, raise awareness of the glass bead reflecting material, the key is to improve the reflective retroreflective fabric presents a practical application.
Core element of reflective material
The history of the reflective materials contains a part of the development of raw materials. Since the practical application of reflective materials, according to the principle of retro reflection, the core components of the reflective material can be divided into two categories.
Glass bead
Glass bead is a new type of material developed in recent years. It is a widely used and uniquely developed glass (or ceramic) sphere with hundreds of microns to a few millimeter diameter, including hollow, solid and porous glass beads. Glass microsphere is made of borosilicate. It has good optical performance, spherical lens characteristics, strong impact resistance, good rolling property, low thermal conductivity and light weight as filler material.
The reflective material uses solid glass beads, which has the characteristics of wear resistance, acid resistance, high hardness, smooth surface and symmetry. The microbeads are colorless and transparent, so their products are semitransparent, which does not affect the choice of color. In order to achieve eye-catching effect, usually using glass or plastic is composed of TiO₂、BaO、ZrO₂、ZnO、SiO₂ beads. The solid microspheres can be divided into ordinary refraction microspheres, high refraction glass microspheres and ultra-high refraction glass microspheres according to the refractive index. In the preparation of reflective fabrics, a solid glass bead with high refractive index is generally chosen. At present, the reverse reflection performance of the glass bead type reverse reflection material remains to be improved. In recent years, gradient refractive index has also been used instead of high refractive index or ultra-high refractive index glass microspheres to study the technological parameters of this new type of antireflective material. The retroreflector made by this condition greatly reduced the requirements of the original high refractive index microspheres and improved the reverse reflection performance.
Microprism
A prism contains more than two transparent bodies of non-parallel planes, the main function of which is to change the direction of the light by reflection. According to the shape classification, there are main angle prism, rhombic prism, five angle prism, trapezoid angular prism and prism. The reflection prism and the splitter prism are divided according to the type of reflection. The reflection prism has the above 5 shapes, and the most representative is the full reflection prism, which is changed to 90 or 180 from the direction of a prism light. The microprism, now used as a reflective material, is transformed from a cubic right angle prism to a better triangular prism with a reverse reflection effect.
Classification and structure of reflective materials
According to the core elements of the reflective material, the reflective materials can be divided into glass bead and microprism. The glass bead can be divided into the homogeneous glass microsphere and the variable refractive index ball lens reflector.
Homogeneous glass microbead type reflective material
The glass bead in the optical system as a thick lens surface micro spherical lens of the incident light converged on the glass beads, when the incident light is parallel to the optical axis, the inverse better reflect performance. As shown in Figure 1-1, the reflected light in the opposite direction of incident light is returned, I0 through the glass beads of incident light, I is an inverse ray of light.
The retroreflective material is made by using high refractive index glass beads to concentrate the incident light in a narrow light cone angle, and silver plated or coated aluminum as the retroreflector to make the light return to the light source. It has a strong retro reflection function, and produces bright visual reflection effect.
This kind of reflective material mainly consists of two kinds of reflective film and retro-reflective fabrics. The surface of the retro reflection film is densely covered with glass beads with a particle size of 30~ 100 μm. It is usually divided into waterproof layer, microbead layer, dyeing layer, viscose layer and wax paper layer. The dense glass bead plays a key role in the reverse reflection. The light reflects the retro reflective film through the transparent layer, and a small part of the light scatters. Most of the light passes through the refraction, reflection and refraction of the glass bead, and returns in the direction of the light source.
The retroreflective fabric is a composite fabric with a large number of microspheres on the surface. The incident light is refracted to the back of the glass bead, passing through the reflection layer and returning to the opposite direction of the incident light. Its structure is shown in Figure 1-2, including microbeads, reflectors, adhesives, and base cloth. Antireflective fabrics are generally formed by the arrangement of glass beads with a particle size of dozens or hundreds of microns, and a glass bead coated on the back of the reflective layer.
Variable refractive index spherical lens reflector
The variable refractive index spherical lens is based on the classical refractive index optics Luneburg sphere lens to fabricate gradient index glass beads, and the metal layer is plated to improve the retroreflective characteristics, as shown in Figure 1-3.
Any parallel light incident to this gradient index (Gradient Index, GRIN) distribution can be aggregated on the other side of the sphere. The refractive index gradient distribution of glass beads is the following formula
n(r)=√(2-(r/a)²)
In the formula, n (r) indicates the refractive index of the distance of the center point of the microsphere at the center point of r, and the a indicates the diameter of the microsphere.
The rays move along the curve in gradient index materials, which makes the action of the direction change more intense, reduces the high demand for the refractive index and improves the retroreflective performance. Therefore, gradient refractive index microspheres can be used to prepare retroreflective materials.
Microprism reflective material
The microprism type reflective material usually uses the three prism microcrystal structure to form its core. As shown in Figure three, the reflection of the lattice reflections highlighted, the reflective surface is inlaid in the strip, effectively prevent the dust and rain to make it more durable, wear-resistant and easy to clean. The principle of reflection is: after the incident light is refracted by the three plane of the prism, it returns to the direction of the incident light source. The gloss is shining, which is three to four times higher than that of the traditional products. The plastic micro prism has the same specifications. It can be arranged on the substrate accurately and without gaps, and can be well adhered to the substrate by mechanical compression. However, due to the high reflectivity of the micro prism, it will cause glare and other problems. In the actual application process, the contrast between the pattern and the background is required, and the manufacturing is extremely complex. Therefore, the micro prism reflective material can not completely replace the glass bead reflective material.
Reflective fabric processing technology
According to the coating method of the glass bead on the base cloth, the reflective fabric was prepared by the bead method, the planting bead method and the transfer method.
Coating method
The coating method is to mix the glass beads and adhesives to prepare the coating liquid and apply it to the fabric. In the process, it includes multi – process and one – way processing.
- Multichannel processing method
Base cloth, coating layer, coated reflection layer, coated glass bead layer and post treatment (coated with transparent resin, waterproof, etc.). The base cloth is coated on the ground to make the surface of the base cloth smooth and can increase the reflection of the reflection layer, thus improving the reflective intensity of the reflective fabric.
- One way processing method
All the glass beads, reflective layer materials, pigments and adhesives required for the preparation of the retroreflective fabric are made into printing liquid or coating solution, and then the retroreflective fabric is made on a base cloth through a coating or printing. The coating process is difficult to process, and the retro reflective fabric has a rough hand feeling, which is mainly used for civilian use.
Method of planting beads
Planting beads were divided into electrostatic planting beads and sprinkling bead method. The electrostatic planting bead method uses the high voltage electric field to adsorb the glass beads on the fabric. Sprinkling bead method is the glass beads evenly in the adhesive coated fabric. The process is as follows: base cloth, bottom coating or dip coating, coated reflecting layer, coating adhesive, planting bead and post treatment.
The retroreflective fabric prepared by the method of planting beads is of general brightness, with a uniform appearance but a rough hand feeling. It is mainly made in clothing and serves as a warning to protect life and safety.
Transfer method
The transfer method is to apply glass beads first to a specific carrier material, after multichannel processing, and finally transfer the glass beads to the textile. The process is as follows: carrier, pretreatment, pearl, reflective layer treatment, coating or printing, hot melt coating, composite material, back protection plastic film products, hot transfer to the fabric. The luminance of the reflective fabrics prepared by the transfer method is controllable, and the appearance is uniform and delicate, but the process is complicated and the processing is difficult. This method is used flexibly. It is processed into cloth or computer and is carved into various substrates, such as raincoat, clothing, skin and shoes and hats.
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