Application of定向 Reflective Glass Bead Materials in Traffic Facilities

Traffic signs play an important role in traffic management and road safety. Although there are many different types of traffic signs, in the past, most of them were painted using conventional paints, which resulted in poor visibility at night and made it difficult to effectively prevent traffic accidents. Nowadays, in many countries abroad, directional reflective materials have become widely used for traffic sign surfaces. These signs, illuminated by vehicle headlights, enable drivers to clearly see the patterns on the signs from hundreds of meters away, greatly enhancing driving safety. In addition, reflective police uniforms, reflective barriers, reflective road markers, reflective lane markings, and reflective license plates are also being increasingly adopted. This article provides a brief introduction to the basic principles, structure, and various types of directional reflective materials.

1. Specular reflection, diffuse reflection, and retroreflective directional reflection

When a beam of parallel light strikes a smooth surface at a certain angle, part of the light is reflected. The reflected light is measured relative to the normal; the angle of incidence equals the angle of reflection. This is the fundamental principle of specular reflection.

When a beam of parallel light rays strikes a rough surface at a certain angle, the directions of the reflected rays become random and no longer follow the law of reflection. This phenomenon is known as diffuse reflection.

When a beam of parallel light strikes a surface with a special structure at a certain angle, a significant portion of the light is reflected back toward the source in the form of a conical beam. Moreover, the axis of this conical beam of reflected light is parallel to the direction of the incident beam. As a result, people will observe an exceptionally bright, reflective surface near the light source. This phenomenon is known as retroreflective directional reflection, or simply “directional reflection.” What we need, then, are reflective materials that exhibit precisely this kind of reflective behavior.

II. Structure of Directional Reflective Materials

The primary structure of directional reflective materials consists of glass beads, each only tens of micrometers in diameter. Due to the difference in refractive indices between glass and air, light passing through the glass beads is focused onto a single point behind the bead. If a reflective surface is attached at this focal point, incoming light beams can be reflected back along their original paths of incidence. Reflective films made using this principle are referred to as “lens-type reflective materials.”

The reflective layer of overseas reflective materials typically uses silver-coated glass microbeads, or first applies a surface protective layer and then performs vacuum aluminum plating. Their refractive index ranges from 2.25 to 2.50. Currently, domestically produced glass microbeads, due to inferior production equipment and technological levels, have a refractive index of only 1.90 to 2.00, and also exhibit a large scattering angle, leaving room for improvement and enhancement.

Secondly, the binder is also an important component of reflective materials. Here, the binder refers to the material used to bond the glass microspheres to the reflective surface. Abroad, polyurethane-based binders are commonly employed. However, domestically produced polyurethanes in China tend to discolor and become brittle under exposure to sunlight and rain, causing the reflective materials to lose their effectiveness and change color after only about two years of use. Recently, we have been testing M-64 acrylic pressure-sensitive adhesive manufactured by Shanghai Zhenhua Paint Factory, which boasts a service life of up to three years.

Moreover, the performance of the focal-point resin and the surface protective layer material also determines the quality of the reflective film. These materials must possess characteristics such as high transparency, excellent oil resistance, UV resistance, and flexibility with good fold endurance.

III. Types of Domestic Reflective Materials

1. Lens-type directional reflective film

This reflective material is made by coating an acrylic pressure-sensitive adhesive onto a polyester film that has been vacuum-aluminized on both sides. Glass microbeads with a refractive index of 1.9 are then adhered to the coated film. After drying, a thin layer of polyvinyl butyral resin is sprayed on top as a protective coating.

The advantages of this reflective material are:

(1) Since the glass microbeads are not covered, their reflectivity is relatively high.

(2) The diameter of glass microbeads is not strictly limited and has low cost;

(3) The process is simple and easy to master.

Its drawbacks are:

(1) When the rainwater film (refractive index 1.33) smooths the surface glass beads, the reflectivity decreases.

(2) The reflective layer has a small reflection angle;

(3) The surface is rough and loses its reflective ability once dust adheres to it;

(4) The glass beads are prone to falling off and are relatively difficult to wipe clean.

2. Flat-top directional reflective film

The flat-top reflective mold incorporates a layer of light-transmitting, focal-index resin (an intermediate layer) between the glass microbeads and the reflective layer, thereby altering the refractive path of light as it strikes the reflective layer. As a result, the incident light can be focused on the surface of the light layer and then reflected back inward.

The advantages of this reflective film are:

(1) Easy to clean, prevents water accumulation, and maintains brightness even on rainy days;

(2) Good anti-aging performance;

(3) Raw materials can be produced domestically;

(4) The process is relatively simple and can be made by hand.

Its drawbacks are:

(1) Vacuum-aluminized polyester film has high costs;

(2) The thickness of the focal resin layer is subject to strict limitations and has stringent requirements.

(3) The glass microbeads are required to have uniform particle size, which accordingly increases the cost.

IV. Bonding Issues Between Reflective Films and Signage

The bonding of reflective films to signboard surfaces is a crucial step in the manufacturing process. As for domestically produced reflective materials, the technical challenges associated with direct bonding have yet to be fully resolved (requiring constant high temperatures of 90 degrees Celsius and pressure application during adhesion). Moreover, these materials tend to age rapidly when exposed to wind and rain, making them prone to peeling off. Practical experience has shown that after about two years, they suffer severe fading and eventually become completely ineffective. Currently, imported reflective materials are beginning to be used on road signs; among those already adopted are directional reflective films from Japan and the United States. These films can be directly adhered to signboard surfaces without the need for heating, featuring a simple application process, excellent durability, and a service life of over five years. Furthermore, products from the U.S. company 3M possess a certain degree of flexibility, allowing them to be molded into various types of numerals and fonts with raised characters—without damaging the reflective film itself. These materials are currently undergoing trial use.