Why choose PTFE as the reflective material of UV LED?

Since the outbreak of the new coronavirus pneumonia, deep ultraviolet products have attracted attention due to their sterilization and disinfection functions, and the UV LED industry has also been further promoted and developed. How to effectively focus the ultraviolet light emitted by the UV LED chip and ensure that the package body is not degraded by the ultraviolet light has become one of the pain points in the selection of packaging materials.

Recently, Porex’s diaphragms and molded parts are made of 100% pure polytetrafluoroethylene (PTFE)-a very special microporous structure of PTFE-which claims to have high reflectivity, high-temperature resistance, and high UV resistance. Receptivity and other advantages have attracted much attention from the UV LED packaging industry. Is this material really an excellent answer for UV LED reflective materials? It is not easy to jump to conclusions before the industry has really applied on a large scale. But you can use the test and analysis of this material to get a glimpse.

PTFE sheets

First, draw an important point:

1) Porex’s PTFE has a reflectivity of 97%, which is the highest reflectivity among currently commonly used packaging materials. Such a high reflectivity theoretically means an extremely low light transmittance, avoiding the 20-30% light leakage problem of most current LED lamp beads. And it belongs to diffuse reflection, which is ideal for light mixing in theory.

2) Porex’s PTFE has a temperature resistance range from -40 °C to 260 °C. Compared with PET (85°C) and PC (~130°C), PTFE is more resistant to high temperatures.

3) Porex’s PTFE’s high UV resistance characteristics mean that it is more suitable for UV LEDs than other materials. Most common plastics, such as acetals, PC, ABS, polyamides, polyolefins, polyesters, etc., have chemical bonds similar to the quantum energy in ultraviolet radiation. Long-term exposure to UV radiation will cause the carbon-hydrogen bond to break, and the physical properties of the material And the mechanical characteristics have changed, so it is not suitable for UV LED packaging materials. PTFE is composed of carbon-fluorine bonds, which are about 30% stronger than carbon-hydrogen bonds. The fluorine bonds of PTFE surround the spiral carbon backbone of the polymer to provide PTFE with chemical resistance and light stability.

In theory, PTFE with the above advantages is an ideal choice for UV reflective materials. So what factors are the key points to improve the reflectivity of PTFE?

It is found through experiments that the thickness of the material has a significant effect on the reflective performance of PTFE. As shown in Figure 1 and Figure 2, under the same experimental conditions, the thickness is positively correlated with reflectance, and negatively correlated with transmittance. The reflectance of the PTFE sheet with a thickness of 0.5mm is about 92%, the reflectance of the PTFE sheet with a thickness of 1mm exceeds 95%, and the total reflectance of the 3mm microporous PTFE sheet in UV or visible light exceeds 99%, and it has nearly 100 % Diffuse reflectance. It can be seen that as the thickness increases, the reflectivity of the PTFE sheet increases.

PTFE for UV LED

PTFE for UV LED

This is because the thickness is a key parameter in the diffuse reflection mechanism. Thicker materials will cause more reflection of UV or visible light in the PTFE structure before the light can pass through the material completely. Therefore, thinner materials have higher light transmittance and lower reflectance than thicker materials.

In addition, because the porous structure will repeatedly reflect UV and visible light inside the PTFE, increasing the thickness of the porous PTFE will increase the reflectivity. After calculation, the reflectivity of porous PTFE is greater than the reflectivity of 85-95% of solid non-porous PTFE.

For the UV curing of polymers, porous PTFE reflectors can solve the problem of UV intensity uniformity, because uneven UV irradiation will cause part of the adhesive to cure and the other part to have weak bonding strength. UV LED applications (such as architectural lighting, water disinfection or medical disinfection) can also benefit from the highly diffuse reflection characteristics of porous PTFE, which can convert narrow directional light sources into wider illuminance.

In addition, the relationship between reflectivity and pore size can be found through experimental data. The figure below shows the linear relationship between aperture and reflectivity. The effect of pore size on reflectivity is not obvious, and as the material becomes thicker, the effect of pore size on the reflectivity of the material will be covered by the effect of thickness.

Why choose PTFE for UV LED

In summary, Porex’s PTFE is not easily degraded due to its strong carbon-fluorine bond and is theoretically an excellent material for UV LEDs. The key factor to improve the reflective performance of PTFE is to increase the thickness of PTFE or increase the number of pores, and the pore size has little effect on the reflective performance of PTFE.