September 9, 2026
A city landscape lighting project experienced failures in its outdoor LED wall-washer luminaires after two years of installation. Inspection revealed two main issues: seal failure at cable entry points causing water ingress and internal circuit short circuits during rainy seasons, and aging/cracking of PCB board fixing adhesive due to prolonged high-temperature exposure, requiring on-site repair.
Selection Requirements:
Good sealing and waterproof performance for cable entries and connectors
Ability to bond multiple substrates (aluminum housing, PC cover, PCB board)
Good adhesion to cured silicone rubber for field repair
High temperature resistance required in some areas (up to 150–180°C)
Compliance with UL94 HB flammability rating
| Application | Recommended Product | Key Features |
|---|---|---|
| Cable entry sealing, connector potting | DOWSIL™ 734 Flowable Sealant | Flowable, self-leveling, can fill fine gaps and complex structures; good adhesion to silicone rubber for field repair |
| PCB fixing, component bonding | DOWSIL™ 739 Plastic Adhesive | Non-flowing paste, suitable for plastic-to-metal bonding; UL94 HB flammability rating |
| High-temperature areas (near power modules) | DOWSIL™ 736 Heat Resistant Sealant | Continuous service at 260°C, intermittent at 315°C; non-slumping, can be applied on vertical and overhead surfaces |
All surfaces must be thoroughly cleaned and degreased. Suitable solvents include isopropyl alcohol, acetone, or methyl ethyl ketone. For aluminum housings, light abrasion with fine sandpaper is recommended to increase bonding surface area.
Cable entries: Inject DOWSIL™ 734 Flowable Sealant around cable entries and connectors; its self-leveling property fills all gaps. Recommended thickness: 0.25–0.75mm.
PCB fixing: Apply DOWSIL™ 739 by dispensing or manual application at the contact area between PCB edge and housing.
High-temperature areas: Apply DOWSIL™ 736; its non-slumping paste consistency allows application on vertical and overhead surfaces.
At 25°C and 50% RH:
734: Skin-over time approx. 7–13 minutes, cures to ~3mm depth in 24 hours
739: Tack-free time approx. 75 minutes, fully cured in 24–72 hours
736: Tack-free time approx. 17 minutes, cures to rubbery solid in 24 hours
⚠️ Note: These products are acetoxy-cure systems that release small amounts of acetic acid during curing. This may corrode sensitive metals such as copper, brass, zinc, carbon steel, galvanized iron, or magnesium, especially in confined spaces. Avoid direct contact with sensitive metals or ensure adequate ventilation.
A: 734 is a flowable/self-leveling sealant ideal for filling fine gaps, cable entries, and connectors, with excellent adhesion to silicone rubber for field repair . 732 is a non-slumping paste general-purpose sealant suitable for most sealing and bonding applications, available in multiple colors, compliant with FDA and MIL-A-46106 . In short: use 734 where flow/filling is needed, use 732 where non-slumping application is needed.
A: No. These are moisture-cure products that require exposure to atmospheric moisture to cure. In totally confined spaces (e.g., metal-to-metal lap joints), curing may not complete. If use in limited spaces is necessary, ensure at least one side is exposed to air or extend cure time accordingly. DOWSIL™ 736 is specifically not recommended for totally confined applications.
A: Poor adhesion is typically expected on non-reactive plastics such as PTFE, polyethylene, polypropylene, and highly plasticized rubber substrates. Use of DOWSIL™ 1200 OS Primer or surface modification methods (chemical etching, plasma treatment) is recommended. Acetic acid released during cure may cause corrosion on sensitive metals like copper and brass.
A: Store at or below 32°C in original, unopened containers. As the products cure by reaction with moisture in the air, keep containers tightly sealed immediately after use. A plug of cured material may form in the tip/nozzle during storage—this is easily removed and does not affect remaining contents. Refer to the "Use Before" date on the product label for shelf life .
Q6:Can heat guns be used to accelerate curing?
A: Yes. Mild heat below 60°C can accelerate curing and increase throughput [citation:739 TDS]. However, note that heating may accelerate acetic acid release and increase corrosion risk on sensitive metals. Heating above 60°C is not recommended. Also note: increasing temperature without increasing atmospheric moisture content (i.e., lowering RH) will not accelerate cure—moisture is the driving force for the curing reaction.
This project successfully addressed waterproof sealing, PCB fixing, and high-temperature bonding for outdoor LED luminaires through the appropriate selection of DOWSIL™ 734, 739, and 736 silicone sealants. Among them, 734 stands out as the preferred choice for field repair due to its flowability and excellent adhesion to silicone rubber ; 739 provides UL94 HB flame-retardant protection for plastic bonding ; and 736 delivers reliable high-temperature performance (continuous 260°C / intermittent 315°C) for demanding areas
