“How to improve the reflection coefficient of hot-melt markings”

Several factors affecting the reflective coefficient of hot-melt marking paint

In the industry of road marking, there is a saying that “three parts paint, seven parts construction”, which indicates that the level of construction technology has a great impact on the quality of road marking. So, what factors actually affect the height of the reflection coefficient for hot melt road markings? The author has consulted a large amount of relevant materials, but unfortunately no one has systematically organized this section. Now, the author is trying to organize and explain these factors. If there are any inappropriate aspects, please feel free to supplement and correct them.

Generally speaking, there are several factors that can affect the reflectivity: 1. Paint type 2. Glass microbead type (refractive index, roundness, particle size distribution) 3. Glass microbead distribution 4. Glass microbead settling rate 5. Paint application thickness, etc. Below are coordinate graphs to explain the impact of these factors on reverse reflection:

• Coating thickness

When the coating thickness reaches around 2.0mm, the reflection coefficient tends to stabilize and no longer increases. During the process of coating from thin to thick, a portion of the light shining on the coating will pass through the coating and scatter into the interior of the coating. After passing through the coating, a portion of the light will reflect back to the glass microbeads. Therefore, when the coating is thin, the vast majority of the light will pass through the coating and reach the base layer, which is absorbed. When the coating reaches a certain thickness, most of the light cannot reach the base layer and will scatter back to the glass microbeads, But as the coating thickness continues to thicken, all the light that needs to be scattered back has already been scattered back. Increasing the thickness does not improve the reflection coefficient. Generally, when the coating reaches 2.0mm, the reflection coefficient approaches its maximum.

• Distribution amount of glass microbeads

The reflection coefficient increases with the increase of glass bead distribution. The highest reflection coefficient occurs when the distribution reaches around 450g/㎡. However, when the distribution is further increased, the reflection coefficient actually decreases. This is because the excessive accumulation of glass beads on the coating causes the light reflected back to be obstructed by the accumulation of glass beads, and some of the light is scattered out, resulting in a decrease in the reflection coefficient.

• Variety of glass microbeads (roundness rate)

The reflection coefficient increases linearly with the increase of the roundness of glass microspheres.

• Variety of glass microbeads (refractive index)

The reflection coefficient increases linearly with the increase of the refractive index of glass microspheres.

• Glass microbead variety (particle size)

The reflection coefficient decreases with the increase of glass microbead particle size. Everyone must be very surprised that in practical use, large particle glass beads are indeed higher than small particle beads in reverse! This is an illusion caused by not comparing under the same conditions. 1. The roundness rate of the large particle glass beads currently used is higher than that of ordinary beads. 2. The settling rate of large particle glass beads is much easier to control than that of small particle beads. So in practical use, it may give the illusion that the thicker the artificial glass beads, the better their reflection. In fact, under the same spreading quality, small particle glass microspheres will be more densely distributed and compact than large particle beads. Under the conditions of Note 5, small particle beads will be higher than large particle beads in reverse. Here, we need to correct a misconception from the perspective of motivation. The main purpose of introducing large particle glass beads is to reflect light in wet conditions rather than increasing the coefficient of retroreflection. Of course, when using large particle glass beads in combination, higher requirements are also placed on the coating (strong adhesion ability, improved resistance to pressure, cracking, and pollution). We believe that ordinary coatings should not use large particle glass microspheres.

• Paint variety (amount of titanium dioxide added)

The reflection coefficient increases with the increase of the amount of titanium dioxide added to the coating. When the amount of titanium dioxide added reaches 18%, the reflection coefficient reaches its maximum. However, increasing the amount of titanium dioxide added does not significantly improve the reflection coefficient, as there are complex relationships and unknown reasons that cannot be explained here. To understand why it is necessary to increase the content of titanium dioxide to improve the reverse reflection coefficient, an explanation can be given in conjunction with Figure 4. Currently, the vast majority of glass microbeads on the market belong to low refractive index glass microbeads, which have a refractive index of 1.5, and their reflected light focuses on the back of the beads, If a glass microbead is suspended, the light will penetrate the bead and scatter without any regression reflection (reverse reflection), so it needs a reflective layer to be placed on the back of the glass bead to allow the light to reflect back. The coating of hot melt marking paint is the reflective layer on the back of the beads, and good regression reflection can be achieved by making the refractive index of the coating as much as possible greater than that of the beads. At present, the refractive index of rutile type titanium dioxide is the highest among all white pigments, about 2.7. Therefore, the simplest and most crude method to improve the reflection coefficient is to add more titanium dioxide. However, for the reflection layer of glass microspheres, the best materials are aluminum, silver, and other materials. If we can use less aluminum, silver, and less titanium dioxide to make high reflection coatings, it is also a way to reduce costs and increase efficiency.

Finally, there are two concepts that need to be explained, namely “pigment volume ratio” and “critical pigment volume ratio”. For hot-melt marking coatings, there is a critical point in the process of coating from rough to smooth, which is the critical pigment volume ratio. Generally speaking, during the construction of hot-melt marking coatings, the viscosity decreases with the increase of temperature. During this process, a critical point will appear, and when this critical point is reached, the reflection coefficient is highest. So, during the cooking process, we need to observe where the critical viscosity of the coating is? The maximum performance of the coating can only be achieved when the critical viscosity is reached. Of course, it can be difficult to find the critical viscosity in practical operation. It is generally recommended that the coating be boiled to just the right level during construction, and should not be too high or too low.