Innovative Application Of PC In The Field Of LED (Part I)

- Dec 18, 2018-

General Manager of Zhanglin Dongguan Yinxi Photoelectric Materials Technology Co., Ltd.

Material Consultant for Aladdin Lighting Industry Research White Paper 2018

Chief Engineer of Fuyi Dongguan Yinxi Photoelectric Materials Technology Co., Ltd.

Material Consultant for Aladdin Lighting Industry Research White Paper 2018

I. Preface

PC (polycarbonate) was first synthesized by Alfred Einhorn, a German scientist, in 1898, because it has not been found a suitable application field, and for more than half a century no one knows it. But gold always shines on that day. In 1955, Hermannschnell, a Bayer scientist, synthesized a PC and applied for a patent that year. In the same year, Bayer formally named its PC Makrolon. Throughout the history of plastic science and technology, we have to say that the 1950s and 1960s were the era of great development of plastic science and technology. At the same time, Daniel Fox, a scientist at GE in 1953 (later Jack Welch, the CEO of GE), independently synthesized a PC and filed a patent application with the U.S. Patent Office in 1955.

Finally, the U.S. Patent Office ruled that the technology patents belonged to Bayer because they filed applications a week earlier than GE. In just one week, Bayer got a lot of patent fees from GE. Time is money, which is perfectly reflected here.

In 1958, Bayer began mass production and commercialization of Makrolon. Two years later, after paying a "protection fee", GE began to mass produce PCs. The name of GE's PC is "Lexan". Since then, PC, as an engineering plastics, has been on the stage of history. Because of its excellent optical properties, mechanical properties and flame retardant properties, it has attracted much attention.

II. Structure and Properties of PC Materials

Polycarbonate is a kind of polymer whose macromolecular chain is composed of carbonate repetitive structural units. Polycarbonate, or PC for short, is the second largest general engineering plastics. According to the specific composition, PC can be divided into three categories: aliphatic, alicyclic and aromatic, but only aromatic PC has practical application value in engineering.

PC is a colorless transparent (or yellowish transparent), rigid and tough material, non-toxic, tasteless, similar to the appearance of plexiglass. The properties of PC are closely related to the structural characteristics of its macromolecules. The main chain of PC macromolecule is a linear macromolecule composed of isopropylene group and carbonate group interacting with benzene ring. Molecular structure is symmetrical, regular and repetitive unit length. Benzene ring is rigid. Carbonate group is a polar water absorbent group. Although it has flexibility, it forms a conjugate system with two benzene rings, which increases the rigidity and stability of the main chain. Isopropylene group is a nonpolar water absorbent group. The symmetrically distributed methyl steric hindrance decreases, providing the main chain with flexibility. So PC macromolecule is rigid and flexible. The polarity of polycarbonate group is affected by aromatic group, and its water loss rate is not high (about 0.05%), but it still has the disadvantage of easy hydrolysis under high humidity. PC has regular structure and macromolecules can crystallize. But in fact, the crystallinity of PC is very low, and it is basically amorphous polymer, which may be related to its molecular rigidity and long repetitive unit.

General properties: PC is transparent, yellowish or white, hard and tough resin. When burning, it emits odor of flowers and fruits, self-extinguishing from fire, yellow flame and melting bubbles.

Mechanical properties: The mechanical properties of PC are excellent, with the advantages of rigidity and toughness. Its impact performance is the best one of thermoplastics, which is three times higher than PA and POM, and close to the level of PF and UP FRP. The tensile strength and flexural strength of PC are both good, and are less affected by temperature. The creep resistance of PC is better than that of PA and POM, and its dimensional stability is good. Thermal properties: PC has good resistance to high and low temperatures. It can be used in the temperature range of - 130 - 130 C. The thermal deformation temperature can reach 130 - 140 C. It is a self-extinguishing material with low thermal conductivity and linear expansion coefficient under load.

Electrical properties: PC is a weak polar polymer, its insulation performance is general. However, its electrical properties change little in a wide range of temperature and temperature, such as the dielectric constant and the tangent value of dielectric loss angle are almost unchanged in the range of 23-125 C. However, it should be noted that the volume resistivity increases with the increase of crystallinity of PC products.

Environmental performance: PC can resist organic acids, dilute inorganic acids, salts, oils, aliphatic hydrocarbons and alcohols, but not chlorinated hydrocarbons, dilute alkali, Australian water, concentrated acids, amines, ketones and lipids. PC can be soluble in dichloromethane, dichloroethane and cresol solvents. PC can not withstand hot water above 60 C. Long-term contact will lead to stress cracking and loss of toughness. The ultraviolet resistance of PC is not good. Ultraviolet absorbent should be added, but the air and ozone resistance of PC is better.

Optical properties: PC is one of the mechanical plastics with excellent strength. Its transmittance can reach 93% and refractive index is 1.587. It is suitable for lens materials. The shortcomings of PC as a high-grade optical material are as follows: first, low hardness and poor wear resistance; second, high birefringence, which is not easy to be used in high-precision products such as optical instruments [3].

III. Application Types and Performance Characteristics of PC Materials in the Field of LED

In the field of optical materials, with the development of optoelectronic products toward "light, thin, short and small", the comprehensive performance requirements of some important optical components such as optical lenses, brazing, optical discs, light emitting diodes and so on are becoming higher and higher, and the optical components are becoming smaller and smaller. Compared with inorganic glass, PC not only has the advantages of light weight, high strength, high impact resistance and easy processing, but also has the characteristics of high transmittance (transmittance up to 90%), high refractive index and excellent dimensional stability. All kinds of optical lenses fabricated by optical grade PC are incomparable with traditional inorganic glass in both impact resistance and processing performance, so they occupy an increasingly important position in the field of optical materials.

3.1 Healthy Lighting-LED Antireflective and Resistant Blue Light Diffusion Technology

3.1.1 Effects of LED Blue Light on Health

As the fourth generation of green light source, LED (Light-Emitting Diode) has the advantages of high luminous efficiency, small size, long life, energy saving and environmental protection. Since the first practical white light-emitting diode (WLED) came out in 1998, it has been used in lighting, liquid crystal display, outdoor display and other fields. At present, the main scheme of WLED fabrication is to generate white light by high-energy blue light exciting yellow phosphor (YAG: ce3+) at about 450 nm.

LCD, lighting and other WLED light sources are ubiquitous in life, which brings people comfortable living environment and visual enjoyment. At the same time, due to the long-term dependence on artificial light, people have gradually recognized and paid more attention to the health hazards of HEV stimulated by LED. Excessive blue light from LED lamps may cause structural damage and visual fatigue of the retina. Blue light penetrates through the lens, leading to macular lesions or cataracts, especially in children whose lenses are clear and unable to resist blue light effectively, which is more likely to lead to macular lesions and cataracts. Many studies have shown that excessive blue light can affect the circadian rhythm. There is a third type of photoreceptor cells in the human retina, intrinsic photosensitive retinal ganglion cells (ipRGCs), which are responsible for regulating non-visual effects beyond the body's vision, such as the function of managing time, coordinating and controlling the rhythm and amplitude of people's activities in different periods. The wavelength of blue LED is about 450 nm, which coincides with the rhythm of human body's circadian rhythm. Blue light stimulates ipRGcs to produce signals to prevent the release of melatonin. Melatonin is closely related to our circadian rhythm and is an important hormone affecting sleep. Many studies have shown that blue light can lead to sleep quality decline, insomnia and depression.

In 2012, IEC of the International Electrotechnical Commission (IEC) brought the blue-light hazard of LED lamps into safety requirements, thus making the blue-light hazard one of the parameters that must be considered. The light radiation safety of LED lamps should meet the requirements of EN62471 "Photobiological Safety of Lamps and Lamp Systems". Then IEC/TR62778 was introduced to evaluate the blue light hazards. It was cited by the revised IEC60598-1, IEC62031 and other major light sources and lamps safety standards in the same year. The latest national standard GB7000.1-2015 "Lighting Part 1: General Requirements and Tests" officially implemented in China on January 1, 2017 also stipulates that lamps with integrated LED or LED modules should be evaluated according to IEC/TR62778 for blue light hazard. Blu-ray hazard has become a parameter that must be considered in lamps and lanterns.

3.1.2 Solutions in the current market:

At present, light diffusion technology is mainly used in the market to solve the dazzling problem of light source. Light diffuser converts LED point light source and line light source into line light source and surface light source to achieve "non-dazzling" effect. It can be added to transparent resin substrates such as PC, PVC, PS, PMMA, PET and epoxy resin to increase light scattering, refraction and transmission, so that the whole resin emits more soft, beautiful and elegant light, achieves the comfortable effect of transparent and opaque, and also at the visual angle. Degree increases the brightness of light, but the light scatterer absorbs little blue light and cannot shield blue light.

Aiming at the blue-rich phenomenon of white LED light source, PC substrate was modified by blue-light absorber. High-energy blue light was absorbed by blue-light absorber and released into thermal energy or harmless low-energy radiation to eliminate the high-energy blue-light band in white LED spectrum. Results On the surface, the blue light transmittance at each wavelength can be effectively reduced by the modification of PC material with blue light absorber, and the blue light absorption effect will be enhanced with the increase of the proportion of blue light absorber [4]. Although this method can effectively reduce the proportion of blue light and reduce the health hazards caused by blue light, there are also some shortcomings.

A Product Function Angle:

The poor absorption selectivity of the color powder can effectively prevent HEV, but also absorb a large number of high wavelength blue light and some yellow-green light. Therefore, compared with the original light diffusion plate, the light flux decreases, the light efficiency decreases, and the transmittance decreases.

B Appearance Performance Angle:

Due to excessive absorption of blue spectral band, the appearance of the product is yellowish and biased, and the absorption of some green and yellow light causes the decrease of light transmittance.

C Processing Technology Angle:

This method needs to mix the toner powder with the base material powder, and the toner is easy to agglomerate, and it is difficult to disperse uniformly in the base material, resulting in uneven distribution of toner, low efficiency of blue resistance, resulting in product color difference and optical instability.

D. Economic Value Perspective:

Because of the poor dispersion effect of toner, the dosage will increase, resulting in higher cost.

3.1.3 Antireflective and Resistant Blue Technology

In order to provide consumers with healthier LED lighting, a new generation of anti-reflective and anti-blue technology is developed to solve the problems existing in the current anti-blue technology. By using the latest quantum light diffuser, high-energy blue HEV can be effectively blocked. Without changing the original technology, the harmful proportion of blue light can be reduced, yellowing can be eliminated, light flux can be increased and light transmittance can be improved.

Quantum light diffuser is a hybrid nanocomposite microsphere with pomegranate core-shell structure. It combines the functions and advantages of nano-scale quantum dot materials and micro-scale light diffuser. Quantum materials give high-energy blue light absorption and light conversion compensation functions, and the microspheres have light diffusion effect. The technical characteristics of the antireflective and resistive blue are as follows:

A Product Function Angle:

Quantum light diffuser is designed for 450 nm excitation light of GaN. Blue-blocking has specificity. It can only block high-energy blue light. It is beneficial for all blue light transmission. Quantum dots absorbing high-energy blue light can excite green light and yellow light. Therefore, the second generation anti-reflective blue light diffuser not only achieves the effect of healthy lighting, but also increases the light flux, reduces the light efficiency and improves the transmittance even compared with the original light diffuser.

B Appearance Performance Angle

Because of the selective absorption of blue spectral band and the excitation of visible light such as long wave blue, green and yellow light, the appearance of the product will not be yellowish and biased.

C Processing Technology Angle

As a spherical powder with micro-nano structure, quantum dot photodiffusive agent has excellent dispersion and compatibility. It can be directly blended with the base material powder and then injected. The processing technology is the same without changing any process parameters.

D Economic Value Perspective

Because the amount of light diffuser itself is small, and nano-scale quantum dots can be uniformly dispersed in microspheres. At the same time, light waves are reflected and refracted repeatedly in the light diffuser, the optical path is increased, the utilization ratio of each quantum dot is increased, and the efficiency of Antireflective and anti-blue is improved. Therefore, only a small amount of quantum material is needed to obtain ideal performance, optical quality is more stable, the amount of light diffuser is reduced, and the cost is reduced.

Test Report of 3.1.4 Quantum Antireflective and Resistant Blue PC Light Diffusion Plate

Under the same experimental conditions, the test parameters of quantum antireflective blue PC light diffuser, ordinary light diffuser and LED bare light were compared.

Fig. 1 is a comparison of the spectra of bare light, ordinary light diffusion plate and transparent and barrier blue diffusion plate. The absolute spectral value of antireflective blue diffusion plate at 450 nm of blue excitation peak is 14.96 mW/nm, which is lower than that of bare light (19.13 mW/nm) and common plate (17.7 mW/nm). The high-energy blue light spectral region (less than 450 nm) is significantly reduced, while other visible regions are enhanced, and the proportion of beneficial spectra to high-energy blue light is significantly increased. It can be seen that the product can obtain ideal Antireflective and anti-blue performance.

Fig. 1 Spectral comparison of bare LED, ordinary PC and transparent and resistive blue PC light diffusion plates

The three spectra measure the parameters of similar data, and compare the data of different parameters. From Table 1, we can see that the test data of many parameters, such as color rendering index, P (W), PF, half-wave width, CRI, are basically consistent with those of bare and ordinary PC.

The absolute spectrum of this product is smaller than that of bare light and ordinary PC, which indicates that the passage of high-energy blue light is blocked, while the luminous flux of E (mW) is similar to that of ordinary PC, but is larger than that of bare LED. This is because the product can stimulate more beneficial visible light and compensate for the reduction of high-energy blue light. Yes, and long-wave blue light is compensated, so it won't cause yellowing. The light efficiency is 95.95 lm/W close to bare light, which is higher than 92.57 lm/W of ordinary PC light diffusion plate. The transmittance of antireflective blue light diffuser is 98.4%, which is 3.7% higher than that of ordinary PC light diffuser 94.7%.

The absolute spectral data were used to plot, and the performance of ordinary PC and anti-transparent blue PC were compared based on bare-light data. As can be seen from Figure 2, ordinary PC boards can not block blue, and the visible part will weaken. Anti-reflective and anti-blue PC light diffusion plate can not only effectively prevent blue, but also provide a beneficial light compensation mechanism to improve the light transmittance.

Figure 2 Spectral comparison of ordinary PC and transparent blue PC light diffusion plates

Figure 3 compares the blue light barrier capability of ordinary PC and blue PC diffuser. It can be seen from the figure that the product has high blue resistance, especially in the range of 420-460 nm, which is the most harmful to human health.

Figure 3 Comparison of HEV Barrier Capacity

Fig. 4 shows the beneficial light compensation mechanism of Antireflective and barrier blue technology. It can be seen that the added beneficial light and the blocked blue light cancel out, and the total e remains unchanged. At the same time, due to the addition of long-wave beneficial light, the light efficiency is higher than that of ordinary PC boards.

This compensation mechanism can partially supplement the white light of LED lamp, which is more continuous and closer to the visible light of the sun than the missing part of the solar spectrum.

Figure 4 Spectral Compensation Mechanism

3.1.5 summary

Referring to ordinary optical diffusion PC board, the anti-reflective and anti-blue technology has obvious advantages and disadvantages compared with the current anti-blue technology.

Current blue-blocking technology can block blue light, but it also reduces a lot of other visible light. Compared with ordinary light-diffused PC boards, each index is obviously reduced, the light efficiency is reduced, the transmittance is reduced, and the color is yellowing.

Anti-reflective and anti-blue technology takes advantage of the excellent barrier performance of quantum light diffuser to HEV, and uses beneficial light compensation mechanism to achieve a healthy illumination effect of anti-reflective and anti-blue, which is closer to the solar spectrum. Compared with ordinary optical diffusion PC board, it can not only block half of the high-energy blue light, but also increase the luminous flux by 18.3 LM (3.7%) and light efficiency by 95.95 lm/W by 3.38 lm/W (3.7%) and the transmittance by 98.38% and 3.67%.