Outdoor lighting needs to withstand the test of snow and ice sun, wind and rain thunder and lightning for a long time, and the cost is high, and because the use of the outer wall is more difficult to dismantle and repair, it must meet the requirements of long-term stable work. The LED is a delicate semiconductor component, if damp, there will be a chip moisture absorption phenomenon, damage LED, PcB and other components, therefore, LED is suitable for working in dry and low temperatures. To ensure the long-term stable work of 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 structural waterproof is that after the combination of various structural components of the product, it has waterproof function. The material waterproof means that when the product is designed, the position of potting glue to seal the electrical components is set aside, and the glue material is used to achieve waterproof 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
1. Ultraviolet light
Ultraviolet rays have destructive effects on wire insulation, shell protective coating, plastic parts, potting glue, sealing rubber ring strips and adhesives exposed outside the lamps.
After the aging and cracking of the insulation layer of the wire, the water vapor will penetrate into the interior of the lamp through the gap in the wire core. After the coating of the luminaire shell is aging, the coating on the edge of the shell is cracked or peeled off, and cracks will appear. The plastic casing will deform and crack as it ages. Electronic potting colloid will crack when aging. The rubber strip of the sealing rubber ring is aging and deformed, resulting in gaps. The adhesive between the structural parts aging, reducing the adhesive force will also appear gaps. These are the damage of ultraviolet light to the waterproof ability of lamps.
2. High and low temperature
The temperature outside varies greatly from day to day. In summer, the surface temperature of lamps during the day can rise to 50 ~ 60 ° C, drop to 10 ~ 20 qC at night, and the temperature of winter ice and snow can fall to zero, and the temperature difference is greater throughout the year. Outdoor lamps in the summer of the high temperature environment, the material accelerated aging deformation. When temperatures drop below zero, plastic parts become brittle and crack under the pressure of snow and ice.
3. Heat expands and cold shrinks
Lamp shell thermal expansion and contraction: changes in temperature lead to thermal expansion and contraction of lamps, different materials (such as glass and aluminum profiles) of the linear expansion coefficient is different, 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 air tightness of the lamp.
Internal air thermal expansion and cold contraction: The water droplets on the glass of the buried lamp can often be observed on the ground of the square, and how do the water droplets penetrate into the lamp filled with potting glue? This is the result of respiration when heat expands and cold 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= 1 0.18 atm= 1 1.86 m water column
As the temperature rises, under the action of huge negative pressure, the humid air penetrates into the interior of the lamp body through the tiny gap on the material of the lamp body, encounters the lamp shell with lower temperature, 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. Repeat the breathing process of temperature change every day, and the water inside the lamp is more and more.
The physical change of thermal expansion and cold contraction makes the waterproof and airtight design of outdoor LED lamps a complex system engineering. The following two kinds of lighting waterproof system technical characteristics are analyzed in order to understand its advantages and disadvantages.
4, about the structure waterproof
Based on the structural waterproof design of the lamp, it is necessary to closely match the silicone sealing ring waterproof, the shell structure is more precise and complex, usually suitable for large size lamps, such as strip floodlights, square and circular floodlights and other medium and high-power lamps.
Structural waterproof lamps only do pure mechanical structure assembly, simple to use tools, less assembly processes and processes, short assembly cycle, and convenient repair on the production line. The lamps pass the electrical performance and waterproof test, and can be packed and shipped, which is suitable for engineering projects with short supply cycle.
However, the machining requirements of the lamps with structural waterproof design are higher, and the dimensions of each component must be precisely matched. Only the appropriate material and structure can ensure its waterproof performance, the following design points.
(1) Design silicone waterproof ring, choose the right hardness of the material, design the right pressure, its cross-section shape is also very critical. The cable inlet line is the channel of water seepage, waterproof wire should be selected, and the use of strong cable waterproof fixed head (PG head) can prevent water vapor from penetrating from the gap in the cable core, but the premise is that the wire insulation layer does not age and crack under the long-term strong extrusion of the PG head.
(2) At normal temperature, the linear expansion coefficient of glass is about 7.2×10 ~ m/(m·K), and the aluminum alloy is about 23.2× 101 m/(m·K), and the difference between the two is large. When the external size of the lamp is larger, it must be carefully considered. Assuming that the length of the lamp is 1 000 mm, the shell temperature is 60℃ during the day, the temperature drops to 10℃ in rain or at night, and the temperature drops by 50℃, the glass and aluminum profile will shrink 0.36 mm and 1.16 mm respectively, the relative displacement is O.8 mm, and the sealing element will be pulled repeatedly during the repetitive displacement process, affecting the air tightness.
(3) Many medium and high-power outdoor LED lamps can be installed waterproof ventilation valve (respirator) using the waterproof and breathable function of molecular sieve in the respirator, balance the internal and external air pressure of the lamp, eliminate negative pressure, prevent human water vapor absorption, and ensure the internal drying of the lamp. This cost-effective waterproof device can improve the waterproofing ability of the original structure design. However, the respirator is not suitable for buried lights, underwater lights and other lamps that often soak 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, processing accuracy and assembly technology. If the weak link deformation and water seepage, the LED and electronic devices will cause irreversible damage, and this situation is difficult to predict in the factory inspection process, with sudden. Therefore, to improve the reliability of structural waterproof lamps, it is necessary to continue to improve waterproof technology.
5, about the material waterproof
The lamps with waterproof material design use filling potting glue to insulate and waterproof, and use sealing glue to seal the joints between structural parts, so that the electrical parts are completely airtight and achieve the waterproof role of outdoor lamps.
6. Potting glue
With the development of waterproof material technology, various types and brands of special potting adhesives for lamps continue to appear, such as modified epoxy resin, modified polyurethane resin, modified organic silicone and so on. The physical and chemical properties 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, the elastic modulus is small, and the adaptability is better. Among them, the elastic modulus of modified organic silica gel 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, without aging and cracking. Among them, modified organic silicone is the most stable.
Adhesion: Strong adhesion is not easy to peel off, among which the adhesion of modified epoxy resin is the strongest, but the chemical structure stability is poor, easy to aging and cracking.
Hydrophobicity: indicates the ability of colloid to resist water seepage. Among them, the modified organic silica gel has better hydrophobicity.
Insulation: insulation is related to product safety indicators, the above several materials of special filling glue are good material of special potting glue are good.
From the above physical and chemical properties, the performance of modified silicone materials is very good.
7, sealing adhesive
Sealant is usually tubular packaging, suitable for glue construction, generally used for wire ends, shell structural parts between the joint bonding and sealing. Commonly used single group distributor, reaction with air water vapor at room temperature, natural solidification.
Special attention: Some lighting production plants use neutral curtain wall glue for construction, rather than professional electronic sealant, which is easy to decompose harmful substances and damage the lamps.
Some types of potting glue and sealant in the solidification process, will decompose a small amount of chemical liquid or gas, such as the colloidal decomposition next to the lamp bead damage to the lamp bead phosphor, resulting in color temperature drift, or damage to the LED chip, or decomposition and transparent PC plastic chemical reaction, damage the PC structure of the substance, and so on. This is a potential hazard in colloidal applications, and the design must be fully understood from the colloidal manufacturer about its chemical and physical properties, and tested to verify.
In the adhesive seal of the lamp shell structure, the effect of thermal expansion and contraction is very large, especially large lamps, the linear expansion coefficient of different materials is very different, and the thermal expansion and contraction are constantly pulled, which is easy to crack. Therefore, the waterproof ability of the material waterproof design mainly depends on the circuit board potting.
Material waterproof production process is long, 1 glue solidification cycle needs 24 hours, some product design is more complex, and even need 2 to 3 glue filling cycle, resulting in a longer shipment cycle, a large number of production sites, and the production environment is dirty. It is very troublesome to repair the product after colloid solidification.
The structural design of the material waterproof lamp does not need to be too precise, as long as the design reserves the colloidal potting area, the liquid can 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 mould products. Such as soft lamp belt, small strip lamp, buried lamp and other small lamps.
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