Technical Analysis of Structure Waterproofing and Material Waterproofing of Outdoor LED Lamps

Outdoor lighting fixtures need to withstand the test of ice, snow, scorching sun, wind, rain, and lightning for a long time, and the cost is relatively high. However, because they are difficult to dismantle and repair when used on external walls, they must meet the requirements of long-term stable work. The LED is a delicate semiconductor component. If it is damp, the chip will absorb moisture and damage the LED, PCB and other components. Therefore, the LED is suitable for working at a dry and low temperature. To ensure that LEDs can work stably for a long time under harsh outdoor conditions, the waterproof structure design of lamps is extremely critical.

At present, the waterproof technology of lamps and lanterns is mainly divided into two directions: structural waterproof and material waterproof. The so-called structural waterproof means that after the combination of the structural components of the product, it already has the waterproof function. As for the waterproofing of the material, it means that when the product is designed, the position of the potting glue is reserved to seal the electrical components, and the glue material is used to achieve waterproofing during assembly. The two waterproof designs are suitable for different product routes, and each has its own advantages.

Factors affecting the waterproof performance of lamps and lanterns

1. Ultraviolet rays

Ultraviolet rays have a destructive effect on the wire insulation layer, shell protective coating, plastic parts, potting glue, sealing rubber strip, and adhesive exposed outside the lamp. After the insulation layer of the wire is aged and cracked, water vapor will penetrate into the interior of the lamp through the gap in the wire core. After the coating of the shell of the lamp is aged, the coating on the edge of the shell is cracked or peeled off, and gaps will appear. After the plastic shell ages, it will deform and crack. The electronic potting colloid will crack when it ages. The sealing rubber strip is aging and deformed, and gaps will appear. The adhesive glue between the structural parts is aging, and gaps will appear after the adhesive force is reduced. These are the damages of ultraviolet rays to the waterproof ability of lamps and lanterns.

2. High and low temperature

The outdoor temperature changes greatly every day. In summer, the surface temperature of lamps can rise to 50-60 ℃ during the day, and drop to 10-20 ℃ at night. In winter, the temperature can drop to below zero in ice and snow days, and the temperature difference changes even more throughout the year. Under the high temperature environment of outdoor lamps and lanterns in summer, the material will accelerate aging and deformation. When temperatures drop below freezing, plastic parts become brittle, under the stress of ice and snow or crack.

3. Thermal expansion and cold contraction

Thermal expansion and contraction of the lamp shell: changes in temperature lead to thermal expansion and contraction of the lamps. Different materials (such as glass and aluminum profiles) have different linear expansion coefficients, and the two materials will be displaced at the junction. The process of thermal expansion and contraction is repeated, and the relative displacement is also repeated, which greatly damages the airtightness of the lamp. The internal air expands with heat and contracts with cold: On the ground of the square, water droplets on the glass of the buried lamp can often be observed to condense, but how does the water drop penetrate into the lamp filled with potting compound? This is the result of respiration when heat expands and contracts. For example, when the temperature drops from 60°C to 10°C, the change in air pressure inside the lamp is about: 1-(273+60)K/(273+10)K=-0.18 atm=-1.86 m water column temperature rises, at Under the action of huge negative pressure, the humid air penetrates into the interior of the lamp body through the tiny gaps in the material of the lamp body, and when it encounters the lower temperature lamp shell, it condenses into water droplets and gathers. After the temperature is lowered, under the action of positive pressure, the air is discharged from the lamp body, but the water droplets are still attached to the lamp. The breathing process of temperature changes is repeated every day, and more and more water accumulates inside the lamp. The physical changes of thermal expansion and contraction make the waterproof and airtight design of outdoor LED lamps a complex system engineering. The technical characteristics of the two waterproof systems for lamps are analyzed below in order to understand their advantages and disadvantages.

4. Regarding structural waterproofing

Luminaires based on structural waterproof design need to be closely matched with silicone sealing rings 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 spotlights, etc., with medium and high power lamps. Structural waterproof lamps are only assembled with purely mechanical structures, using simple tools, less assembly procedures and processes, short assembly cycle, and convenient and quick repair on the production line. After passing the electrical performance and waterproof test, the lamp can be packaged and shipped, which is suitable for engineering projects with a short delivery period. However, the mechanical processing requirements of lamps with waterproof structure design are relatively high, and the dimensions of each component must be precisely matched. Only suitable materials and structures can guarantee its waterproof performance, the following design points. (1) To design a silicone waterproof ring, choose a material with a suitable hardness, design a suitable pressure, and its cross-sectional shape is also very critical. The cable lead-in wire is a channel for water seepage, so it is necessary to choose a waterproof wire, and the use of a strong cable waterproof fixing head (PG head) can prevent water vapor from penetrating through the gaps in the cable core, but the premise is that the wire insulation layer is strongly squeezed by the PG head for a long time. No aging or cracking under pressure. (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-m/(m·K). There is a big difference between the two. When the outer size of the lamp is large, it must be carefully considered. Assuming that the length of the lamp is 1 000 mm, the temperature of the shell during the day is 60°C, it rains or the temperature drops to 10°C at night, and the temperature drops by 50°C, the glass and aluminum profiles will shrink by 0.36 mm and 1.16 mm respectively, and the relative displacement is 0.8 mm , the sealing element is repeatedly pulled during the repetitive displacement process, which affects the airtightness. (3) Many medium and high-power outdoor LED lamps can be installed with waterproof and breathable valves (respirators). Using the waterproof and breathable function of molecular sieves in respirators, they can balance the internal and external air pressure of the lamps, eliminate negative pressure, prevent inhalation of water vapor, and ensure that the interior of the lamps is dry. This economical and effective waterproof device can improve the waterproof ability of the original structure design. However, 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 water 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 it is sudden. Therefore, to improve the reliability of structural waterproof lamps, it is necessary to continue to improve waterproof technology.

5. Regarding material waterproof

Lamps designed for waterproof materials use filling and sealing glue to insulate and waterproof, and use sealant to bond and close the joints between structural parts to make electrical components completely airtight and achieve the waterproof effect of outdoor lighting.

6. Potting glue

With the development of waterproof material technology, various types and brands of special potting adhesives for lamps are emerging, such as modified epoxy resin, modified polyurethane resin, modified organic silica gel, etc. With different chemical formulas, the physical and chemical performance indicators of potting adhesives such as elasticity, molecular structure stability, adhesion, UV resistance, heat resistance, low temperature resistance, hydrophobicity, and insulation performance are different. Elasticity: The colloid is soft and the elastic modulus is smaller, so the adaptability is better. Among them, the elastic modulus of modified silicone is the smallest. Molecular structure stability: Under the long-term action of UV, air and high and low temperature, the chemical structure of the material is stable, and it will not age or crack. Among them, modified silicone is the most stable. Adhesion: Strong adhesion means that it is not easy to peel off. Among them, modified epoxy resin has the strongest adhesion, but its chemical structure is less stable, and it is easy to age and crack. Hydrophobicity: Indicates the ability of the colloid to resist water seepage. Among them, the modified organic silica gel has better hydrophobicity. Insulation: Insulation is related to product safety indicators. The special filling glue of the above materials is good, and the special filling glue of the material is good. From the comprehensive view of the above physical and chemical properties, the modified silicone material performed best.

7. Sealing glue

Sealants are usually packaged in tubes, suitable for glue construction, and are generally used for bonding and sealing seams between wire ends and shell structural parts. The commonly used one-component formula reacts with air moisture at room temperature and solidifies naturally. Special attention: Some lamp manufacturers use neutral curtain wall glue for construction, not 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 colloid decomposition product next to 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 PC, and so on. This is a potential hazard in the application of colloids. When designing, it is necessary to fully understand its chemical and physical properties from the colloid manufacturer, and test and verify. The sealant is most affected by thermal expansion and contraction in the bonding and sealing of the lamp shell structure, especially for large lamps, the linear expansion coefficients of different materials are quite different, and the thermal expansion and contraction are constantly pulled, and cracks are very easy to appear. Therefore, the waterproof ability of the material waterproof design mainly depends on the circuit board potting. The production process of material waterproofing is long, and one glue pouring and solidification cycle takes 24 hours. Some products are more complicated in design, and even require 2 to 3 glue pouring cycles, resulting in a long delivery cycle, a large amount of production space is occupied, and the production environment Dirty. It is very troublesome to repair the product after the colloid is solidified. Materials The structural design of waterproof lamps does not need to be too precise, as long as the design reserves a colloid potting area, the liquid will not leak out, and its waterproof performance is very intuitive. Therefore, the material waterproof process is more suitable for small outdoor lamps and indoor moisture-proof lamps. It is usually widely used in low-end and cheap male mold products. Such as flexible light strips, small strip lights, buried lights and other small lamps.

epilogue

Regardless of whether the structure is waterproof or the material is waterproof, for the long-term stable operation and low failure rate of outdoor lighting, it is difficult to achieve high reliability with a single waterproof design, and the potential hidden danger of water seepage still exists. Therefore, in the design of high-end outdoor LED lamps, it is recommended to use waterproof technology flexibly, combine the advantages of structural waterproof and material waterproof technology, and maximize strengths and 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 the waterproof design of the structure, we can also consider adding potting and double waterproof protection to improve the stability of outdoor lighting for long-term use and reduce the failure rate due to moisture.