The LED Outdoor lighting fixtures are usually used for and installed in outdoor applications, so the outdoor lighting luminaires are required to withstand the test of ice, snow, scorching sun, wind, rain, lightning, water, or even drive over, or corroded by sea water or sea wind and so on for a long time. and the cost of outdoor lighting fixtures are relatively high. However, because they are difficult to dismantle, remove and repair when these LED outdoor lighting lamps are used on external walls, building, or underground or underwater and other severe environments, so the LED outdoor lighting luminaires must meet the requirements of long-term stable work. And LED diode is a delicate semiconductor component. If the inner of LED outdoor lamps especially the LEDs and other components are affected with damp, it will lead to the LED chip absorb moisture, and LED, PCB and other components are damaged. 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 for outdoor lighting fixtures.
Factors affecting the waterproof performance of LED outdoor lamps:
1. Ultraviolet rays
Ultraviolet rays have a destructive effect on those exposed outside the LED outdoor lamp, such as: the wire insulated rubber, lamp housing protective coating, plastic parts, sealing glue, sealing rubber ring, and adhesive and etc.
After the insulated rubber 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 housing of the lamp is aged, the coating on the edge of the lamp housing will be cracked or peeled off, or with gaps. After the plastic housing ages, it will deform and crack. The electronic potting colloid will crack when it ages. The sealing rubber ring is aging and deformed, and gaps will appear. The adhesive 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 outdoor luminaires.
2. High and low temperature difference
The outdoor temperature changes greatly every day, especially In summer, the surface temperature of outdoor lamps can rise to 50-60℃ during the day time, and drop to 10-20 ℃ at night. In winter or ice snow days, the temperature can drop to below zero, and the temperature difference changes even more. For outdoor lamps, the aging and deformation of materials are accelerated in the high temperature environment in summer; in winter, when the temperature drops below zero, the plastic parts become brittle, or crack under the pressure of ice and snow.
3. Thermal expansion and cold contraction
Luminaire housing expands with heat and contracts with cold: Changes in temperature lead to thermal expansion and contraction of the luminaire. Different materials (such as glass and aluminum profiles) have different linear expansivity or linear expansion coefficients, and the two different materials will be displaced at the junction. The process of thermal expansion and contraction is repeating and happening every day, and the relative displacement is also repeating and happening, which greatly damages the airtightness of the outdoor lamps.
The internal air expands with heat and contracts with cold: It is often seen that water droplets condense on the inner of glass cover of the LED underground lights in the ground of the public square or road and etc. How does the water drop penetrate into the underground lamp which sealed and filled with gel completely? This is the result of “The Siphon effect” 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.
The temperature rises, and under the action of huge negative pressure, the humid air passes through the tiny gaps in the material of the lamp body. After the humid air penetrates into the lamp body and encounters the lamp housing with lower temperature, 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 makes the waterproof and airtight design of outdoor LED lamps a complex system engineering.
The physical changes of thermal expansion and contraction make the waterproof and airtight design of outdoor LED lamps a complex system engineering. The following is an analysis of the technical characteristics of the two lighting waterproof systems (one is structural waterproofing and the other one is Sealing Materials waterproofing) to understand their advantages and disadvantages.
I. About structural waterproofing Technology:
Outdoor luminaires based on structural waterproof design need to be closely matched with silicone sealing rings for waterproofing. The lamp housing structure is more precise and complex. It is usually suitable for larger-sized lamps with medium and high power, such as linear floodlights, square flood light and round flood light or spotlights and etc.
Structural waterproof lamps are only assembled with purely mechanical structures by simple tools, less assembly procedures and processes, and taken short assembly time, and it is convenient and quick to be repaired.
However, the outdoor lighting fixture with structural waterproofing have higher requirements for machining, and the dimensions of each parts must be precisely matched. Only suitable materials and structures can guarantee its waterproof performance. The following are several key design requirements:
(1) Silicone waterproof ring:
Choose a material with a suitable hardness and design a suitable pressure for silicone waterproof ring, and its cross-sectional shape is also very critical for the waterproof silicone ring. The wire lead 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 it is required that the wire insulation layer or rubber will be not age or crack under long-term strength squeeze of waterproof PG head.
(2) Temperature difference:
At ordinary 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 taken into consideration carefully. Assuming that the length of the lamp is 1 000 mm, the temperature of the lamp housing is 60°C during the day, its temperature drops to 10°C at night or when raining, and the temperature difference is around 50°C, the glass and aluminum profiles will contact by 0.36 mm and 1.16 mm respectively, and the relative displacement is 0.8mm, the sealing elements or parts is repeatedly pulled during the repetitive displacement process, which affects the airtightness for the outdoor lighting fixtures very much.
(3) Breather valve:
Many medium and high-power outdoor LED lamps can be equipped with waterproof breather valve (or vacuum pressure valves). The function of molecular sieves of waterproof breather valve can help to balance the internal and external air pressure of the lamp and eliminate negative pressure, prevent water vapor from being absorbed, and ensure that the interior of the lamps is dry. This economical and effective waterproof device (breather valve) can improve the waterproof ability of the original structure design. However, the breather valve is not suitable for underground light, in-ground lights, buried lamps, underwater lamps and other lamps that are often submersed 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 materials, the processing accuracy, and the assembly technology. If the weak parts of outdoor lighting fixture are deformed and water seeps, it will cause irreversible damage to the LED and electronic devices, which it is difficult to predict during the factory inspection process, and it is sudden happen to the lighting fixtures. Therefore, to improve the reliability of structural waterproof outdoor lamps, it is necessary to continue to improve waterproof technology.
II. About sealing Materials waterproofing
What is About material waterproofing for outdoor lighting fixtures?
The outdoor lighting fixture designed by materials waterproof, which is used filling and sealing glue to insulate and waterproof, and used glue or gel to seal the joints or gaps between structural parts to make electrical components completely airtight and achieve the waterproof effect for outdoor lighting.
With the development of waterproof material technology, various types and brands of special sealing for outdoor lamps continue to appear, such as modified epoxy resin, modified polyurethane resin, modified organic silica gel, etc. With different chemical formulas, the physical and chemical performance indicators of sealing adhesives such as elasticity, molecular structure stability, adhesion, UV resistance, heat resistance, low temperature resistance, hydrophobicity, and insulation performance are different.
The softer of colloid and the smaller elastic modulus of colloid is, so the better of adaptability will be. Among them, the elastic modulus of modified silicone is the smallest.
Molecular structure stability:
it is required the chemical structure of the material is stable, and it will not age or crack under the long-term action of UV, air, and high& low temperature. Modified silicone is the most stable in these materials.
If the adhesion is strong, it is not easy to peel off. The modified epoxy resin has the strongest adhesion, but its chemical structure is less stable, and it is easy to age and crack.
It is Indicator of the ability of the colloid to resist water seepage. The modified organic silica gel has better hydrophobicity in above several materials mentioned.
Insulation is one of indicators for the safety of outdoor lighting products. The special sealing glue/gel of the above mentioned materials are all good.
From the comprehensive view of the above physical and chemical properties, the modified organosilicon material performed best for outdoor lighting products.
The sealant is usually packaged in a tube, suitable for glue construction, and is generally used for bonding and sealing the joints between wire ends and shell structural parts. The commonly used one-component formula reacts with air moisture at room temperature and solidifies naturally.
Special note: Some manufacturers use neutral curtain wall glue for construction instead of professional electronic sealant, which is easy to decompose harmful substances and damage lamps.
Some types of sealing glue and sealant will decompose a small amount of chemical liquid or gas during the solidification process, for example: the phosphor of LED is easy to be damaged by the colloid decomposition product which on surrounding of the LED diodes, and resulting in color temperature shifted, or damage LED chips; or the colloid 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. It is necessary to fully understand its chemical and physical properties from the colloid manufacturer, and test and verify when design lighting fixtures and select the sealing materials.
The sealant is most affected by thermal expansion and contraction in the bonding and sealing of the shell/housing structure of the outdoor lamps. Especially for large outdoor luminaires, the linear expansion coefficients of different materials are quite different, and the pulled and cracks constantly caused by the phenomenon of thermal expansion and contraction to the outdoor lighting fixtures. Therefore, the waterproof ability of the material waterproof design mainly depends on the circuit board sealing.
The production process of waterproof materials is relatively long, and it takes 24 hours for a glue-filling and solidification cycle. Some products are more complicated in design, and even require 2-3 glue-filling cycles, that is why the delivery time of outdoor lighting fixtures is long, and a large amount of production space is occupied and required, and the production environment Dirty. Meanwhile, it is very troublesome to repair the outdoor lighting products (such as IP67 LED in-ground lights and IP68 Underwater lights ) after the glue/gel is solidified.
The structural design of sealing materials waterproofs type of outdoor lamps does not need to be too precise for, as long as there is space for sealing materials and the liquid does not leak out, and its waterproof performance is very good. Therefore, the material waterproof process is more suitable for outdoor lamps (underground lights or underwater lights) and indoor moisture-proof lamps, such as flexible light strips, small strip lights, buried lights, in-ground lights, pool lights, underwater lamps and etc.