Technical analysis of waterproofing of outdoor LED lighting materials

Time：2021-06-10

External lighting fixtures need to withstand the test of snow and ice, wind and lightning, and the cost is high. Because it is difficult to be repaired on the outer wall, it must meet the requirements of long-term stable work. The LED is a delicate semiconductor component. If it is wet, the chip will absorb moisture and damage the LED, PcB and other components. Therefore, the LED is suitable for drying and low temperature. To ensure long-term stable operation of LEDs under harsh outdoor conditions, the waterproof structure design of lamps is extremely critical.

When the material is waterproof, it is the position where the sealing glue seals the electrical components, and the glue material is used for waterproofing during assembly.

Factors affecting the waterproof performance of lamps

1, ultraviolet light

Ultraviolet rays have a destructive effect on the wire insulation, the outer protective coating, the plastic parts, the potting glue, the sealing ring rubber strip and the adhesive exposed to the outside of the lamp.

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 housing is aged, the coating on the edge of the casing is cracked or peeled off, and a gap may occur. After the plastic case ages, it will deform and crack. The aging of the electronic potting gel causes cracking. The sealing rubber strip is aging and deformed, and a gap will occur. The adhesive between the structural members is aged, and a gap is also formed after the adhesion is lowered. These are all damage to the waterproof ability of the luminaire by ultraviolet light.

2, high and low temperature

The outdoor temperature varies greatly every day. In summer, the surface temperature of the lamps can rise to 50-60 °C, and the temperature drops to 10-20 qC in the evening. The temperature in winter and snow can drop to below zero, and the temperature difference changes more throughout the year. Outdoor lighting in the high temperature environment in summer, the material accelerates aging deformation. When the temperature drops below zero, the plastic parts become brittle, under the pressure of ice and snow or cracking.

3, thermal expansion and contraction

Thermal expansion and contraction of the lamp housing: The temperature changes cause the thermal expansion and contraction of the lamp. Different materials (such as glass and aluminum profiles) have different linear expansion coefficients, and the two materials will be displaced at the joint. The process of thermal expansion and contraction is repeated continuously, and the relative displacement is repeated continuously, which greatly damages the airtightness of the lamp.

Internal air thermal expansion and contraction: The condensation of water droplets on the buried lamp glass can often be observed on the square floor, and 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 is lowered from 60 ° C to 10 ° C, the internal pressure change of the lamp is about: 1 (273 + 60) K / (273 + 10) K = 0.18 atm = 1.86 m water column

When the temperature rises, under the action of huge negative pressure, the humid air penetrates into the inside of the lamp body through a small gap in the material of the lamp body, and then encounters the casing of the lamp with a lower temperature, condenses into water droplets and gathers. After the temperature is lowered, the air is discharged from the lamp body under the action of the positive pressure, but the water droplets remain attached to the lamp. The breathing process of temperature changes is repeated every day, and there is more and more water inside the lamp.

The physical changes of thermal expansion and contraction make the waterproof and airtight design of outdoor LED lamps become complex system engineering.

The waterproof design of the material is insulated and waterproofed by filling potting glue, and the joint between the closed structural parts is bonded by the sealing glue, so that the electrical components are completely airtight and the waterproof effect of the outdoor lighting is achieved.

1 potting glue

With the development of waterproof material technology, various types and brands of special potting glues have appeared continuously, for example, modified epoxy resin, modified polyurethane resin, modified organic silica gel, and the like. Different chemical formulas, the physical and chemical properties of potting rubber, such as elasticity, molecular structure stability, adhesion, anti-uV, heat resistance, low temperature resistance, water repellency and insulation properties, are different.

Elasticity: The gel is soft and the modulus of elasticity is small, so the adaptability is better. Among them, the modified organic silica gel has the smallest elastic modulus.

Molecular structural 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 no cracking. Among them, modified organic silica gel is the most stable.

Adhesion: When the adhesion is strong, it is not easy to be peeled off. Among them, the modified epoxy resin has the strongest adhesion, but the chemical structure stability is poor, and it is easy to age and crack.

Water repellency: Indicates the ability of the colloid to resist water penetration. Among them, the modified organic silica gel has good water repellency.

Insulation: Insulation is related to product safety indicators. The special filling glues of the above materials are all good materials.

From the above comprehensive physical and chemical properties, the modified silicone materials perform best.

2 sealing adhesive

The sealant is usually a tubular package, suitable for glue application, and is generally used for bonding and sealing the indirect seam of the wire end and the outer structural part. It is a commonly used one-component formula that reacts with air vapor at room temperature and naturally solidifies.

Special attention: Some lighting manufacturers use neutral curtain wall glue for construction instead of professional electronic sealant, which easily decomposes harmful substances and damages the lamps.

Some types of potting and sealant will decompose a small amount of chemical liquid or gas during solidification. For example, the colloidal decomposition product next to the lamp bead damages the lamp phosphor, causing color temperature drift, or invading the LED chip, or Decomposes substances that chemically react with transparent PC plastics, destroy PC structures, and so on. This is a potential hazard in colloidal applications, and the colloidal manufacturer must be fully aware of its chemical and physical properties and tested for compliance.

In the adhesive sealing of the lamp housing structure, the sealant is most affected by thermal expansion and contraction, especially for large lamps. The coefficient of linear expansion of different materials is quite different, and the thermal expansion and contraction are continuously pulled, which is prone to cracks. Therefore, the waterproof ability of the material waterproof design is mainly based on the circuit board potting.

The production process of material waterproofing is long, and the curing cycle of one filling glue takes 24 hours. Some products are more complicated in design, and even require 2 to 3 filling cycles, resulting in a long shipping cycle, occupying a large number of production sites, and the production environment. Dirty. Product rework after solidification of the colloid is very troublesome.

The structural design of the material waterproof luminaire does not need to be too precise. As long as the design is reserved for the gel potting area, the liquid does not leak, and the waterproof performance is very intuitive. Therefore, the material waterproofing process is more suitable for small outdoor lighting, indoor moisture-proof lamps. It is commonly used in low-end and low-cost male mold products. Such as small lamps such as soft light strips, small strip lights, and buried lights.