Outdoor lighting fixtures need to withstand the test of scorching sun, wind, rain, thunder and lightning for a long time, and the cost is relatively high. Because it is difficult to dismantle and repair the exterior wall, it must meet the requirements of long-term stable work. LEDs are delicate semiconductor components. If they are exposed to moisture, the chips will absorb moisture and damage the LED, PCB and other components. Therefore, LEDs are suitable for working in dry and low temperature working environments. To ensure the long-term stable operation of the LED in harsh outdoor conditions, the waterproof structure design of the lamp is extremely critical.
At present, the waterproof technology of lamps is mainly divided into two directions: structural waterproof and material waterproof. The so-called waterproof structure means that after the combination of various structural components of the product, it already has the waterproof function. The material waterproofing is to leave the position of the potting glue to seal the electrical components during product design, and use the glue material to achieve waterproofing during assembly. The two waterproof designs are suitable for different product routes, each with its own advantages.
Factors affecting the waterproof performance of lamps
Ultraviolet rays have a destructive effect on the wire insulation layer exposed on the lamp, the protective coating of the shell, plastic parts, potting glue, sealant rubber strip and adhesive.
After the wire insulation layer is aged and cracked, water vapor will penetrate into the interior of the lamp through the gap of the wire core. After the coating of the lamp shell is aged, the coating on the edge of the shell is cracked or peeled, and a gap will appear. After the plastic casing ages, it will deform and crack. The aging of the electronic potting colloid will cause cracking. Sealing rubber ring rubber strip aging deformation, there will be gaps. The adhesive glue between structural parts is aging, and there will be gaps after reducing the adhesive force. These are the damage of ultraviolet light to the waterproof ability of the lamp.
B. High and low temperature
The outdoor temperature changes greatly every day. The surface temperature of the lamp can rise to 50～60℃ during the summer and 10～20℃ at night, and the temperature can drop to below zero during the winter. The temperature difference changes even more throughout the year. Under the high temperature environment in summer, the materials of outdoor lamps accelerate aging and deformation. When the temperature drops to minus zero, plastic parts become brittle, or crack under the pressure of ice and snow.
C. Thermal expansion and contraction
Thermal expansion and contraction of the lamp housing: Changes in temperature cause the thermal expansion and contraction of the lamp. Different materials (such as glass and aluminum) have different linear expansion coefficients, and the two materials will shift at the junction. The process of thermal expansion and contraction is repeated continuously, and the relative displacement is also repeated, which greatly damages the air tightness of the lamp.
The internal air expands and contracts: the water droplets on the glass of underground lamps are often observed on the ground of the square. How does the water droplets penetrate into the lamp filled with potting glue? This is the result of respiration during thermal expansion and contraction.
For example, when the temperature drops from 60°C to 10°C, the internal air pressure change of the lamp is approximately: 1-(273+60)K/(273+10)K=-1.18 atm=-1.86 m water column
As the temperature rises, under the effect of huge negative pressure, the humid air penetrates through the tiny gaps in the lamp body material and penetrates into the lamp body. When it encounters the lower temperature lamp housing, it condenses into water beads and gathers. After the temperature decreases, the air is discharged from the lamp body under the action of positive pressure, but the water droplets still adhere to the lamp. The breathing process of temperature changes is repeated every day, and more and more water accumulates inside the lamps.
The physical changes of thermal expansion and contraction make the design of outdoor LED lamps waterproof and airtight become a complicated system engineering. The following is an analysis of the technical characteristics of the two lamp waterproof systems in order to understand their advantages and disadvantages.
D. About structural waterproofing
Luminaires based on structural waterproof design need to be closely matched with silicone seals for waterproofing. The shell structure is more precise and complex. It is usually suitable for larger-sized lamps, such as strip floodlights, square and round floodlights, and other medium and large Power lamps.
Structural waterproof lamps are only assembled with purely mechanical structures. The tools are simple, the assembly procedures and processes are few, the assembly cycle is short, and the repair on the production line is convenient and fast. The lamps and lanterns pass the electrical performance and waterproof test, and can be packaged and shipped. It is suitable for engineering projects with short supply cycles.
However, the mechanical processing requirements of structural waterproof design lamps are relatively high, and the size of each component must be precisely matched. Only suitable materials and constructions can guarantee its waterproof performance. The following design points.
(1) Design silicone waterproof ring, choose materials with suitable hardness, design appropriate pressure, and its cross-sectional shape is also very critical. The cable entry line is a channel for water seepage, and you need to choose a waterproof wire, and use a strong cable waterproof fixed head (PG head) to prevent water vapor from penetrating through the gap of the cable core, but only if the wire insulation is squeezed in the PG head for a long time. No aging or cracking when pressed.
2) At room temperature, the linear expansion coefficient of glass is about 7.2×10～m/(m·K), and that of aluminum alloy is about 23.2×10 0 m/(m·K), the difference between the two is quite large. Careful consideration should be given when the external dimensions of the lamps are large. Assuming that the length of the lamp is 1000 mm, the temperature of the housing is 60°C during the day, the temperature drops to 10°C during rain or night, and the temperature drops by 50°C, the glass and aluminum profiles will contract by 0.36 mm and 1.16 mm, the relative displacement is 0.8 mm, and the sealing element Repeated pulling in the process of repetitive displacement affects air tightness.
(3) Many medium and high-power outdoor LED lamps can be installed with waterproof and breathable valves (respirator). The waterproof and breathable function of the molecular sieve in the respirator is used to balance the internal and external air pressure of the lamp, eliminate negative pressure, prevent the absorption of moisture, and ensure the interior of the lamp is dry. This economical and effective waterproof device can improve the waterproof capability of the original structural design. But the respirator is not suitable for buried lamps, underwater lamps and other lamps that are often soaked in water.
The long-term stability of the waterproof structure of the lamp is closely related to its design, the performance of the selected lamp material, the processing accuracy, and the assembly technology. If the weak link is deformed and seeps, it will cause irreversible damage to the LED and electronic devices, and this situation is difficult to predict during the factory inspection process and has a sudden nature. Therefore, to improve the reliability of structural waterproof lamps, it is necessary to continue to improve the waterproof technology.
E. About material waterproofing
The lamps designed with waterproof materials are filled with potting glue to insulate and waterproof, and the joints between the structural parts are sealed with sealant to make the electrical components completely airtight and achieve the waterproof effect of outdoor lamps.
F. Potting glue
With the development of waterproof material technology, various types and brands of special potting adhesives for lamps have emerged, for example, modified epoxy resin, modified polyurethane resin, modified organic silicone, etc. Different chemical formulas have different physical and chemical performance indexes such as elasticity, molecular structure stability, adhesion, uV resistance, heat resistance, low temperature resistance, water repellency, and insulation performance of the potting compound.
Elasticity: The colloid is soft and the modulus of elasticity is small, the adaptability is better. Among them, the modified elastic modulus of silicone is the smallest.
Molecular structure stability: Under the long-term effect of UV, air and high and low temperature, the chemical structure of the material is stable, without aging and cracking. Among them, modified organic silica gel is the most stable.
Adhesion: Strong adhesion is not easy to peel, and the modified epoxy resin has the strongest adhesion, but the chemical structure stability is poor, easy to aging and cracking.
Hydrophobicity: Indicates the colloid’s ability to resist water penetration. Among them, the modified organic silica gel has better hydrophobicity.
Insulation: Insulation is related to product safety indicators. Special filling glues for the above materials are good. Special filling glues for materials are good.
From the comprehensive view of the above physical and chemical properties, modified silicone materials perform best.
The sealant is usually a tubular package, suitable for glue construction, and is generally used for the bonding and sealing of the indirect seams of wire ends and shell structural parts. Commonly used single-component formula, reacts with water vapor at room temperature, and naturally solidifies.
Special Note: Some lamp manufacturers use building neutral curtain wall glue instead of professional electronic sealant, which is easy to decompose harmful substances and damage lamps.
Some types of potting glue and sealant will decompose a small amount of chemical liquid or gas during the solidification process, such as the damage of the colloidal decomposition product beside the lamp bead to the lamp bead phosphor, resulting in color temperature drift, or damage to the LED chip, or Decompose substances that chemically react with transparent PC plastics, destroy the structure of the PC, etc. This is a potential hazard in the application of colloids. It is necessary to fully understand the chemical and physical properties of colloid manufacturers when designing, and test and verify.
The sealant is most affected by thermal expansion and contraction in the bonding and sealing of the lamp housing structure. Especially for large lamps and lanterns, the linear expansion coefficients of different materials vary greatly, and the thermal expansion and contraction are constantly pulled, which is prone to cracks. Therefore, the waterproof ability of the material waterproof design mainly depends on the circuit board potting.
The production process for waterproofing of materials is long. One grouting cycle takes 24 hours. Some products are more complex in design, and even require 2 to 3 grouting cycles. This leads to a longer delivery cycle, which occupies a lot of production space and the production environment. Dirty. It is troublesome to repair the product after the colloid is solidified.
The structural design of the material waterproof lamp does not need to be too precise, as long as the design reserves a gel potting area, the liquid does not leak, and its waterproof performance is very intuitive. Therefore, the material waterproof process is more suitable for small outdoor lamps and indoor moisture-proof lamps. Usually used in a large number of low-end and cheap male mold products. Such as soft light strips, small strip lights, underground lights and other small lamps.
Regardless of whether the structure is waterproof or the material is waterproof, for the long-term stability and low failure rate of outdoor lamps, a single waterproof design is difficult to achieve extremely high reliability, and the potential hidden danger of water leakage still exists.
Therefore, for the design of high-end outdoor LED lamps, it is recommended to use waterproof technology flexibly to combine the advantages of structural waterproofing and material waterproofing technology, and to avoid weaknesses to ensure long-term stable operation of LED circuits. If the material is waterproof, a respirator can be added to eliminate negative pressure. For structural waterproof design, we can also consider adding potting and double waterproof protection to improve the long-term stability of outdoor lamps and reduce the rate of moisture failure.
Upward Lighting, founded in 2009, focuses on outdoor landscape lighting projects, customizing various non-standard outdoor LED lamp manufacturers, main LED flood lights, LED wall washer lights, LED line lights, LED underwater lights, LED buried Lights, LED corrugated lights, LED explosion-proof lights, etc., provide effect design, installation guidance, all-day technical support, after-sales tracking and other one-stop services.