How does the low thermal expansion coefficient of borosilicate glass prevent thermal stress cracking in street light lenses?
Publish Time: 2025-09-04
In urban lighting systems, street lights, as outdoor equipment operating 24/7, ensure the stability of their optical performance, directly impacting road safety and energy efficiency. Lenses, as the core optical component of street lights, are exposed to complex and changing environmental conditions over long periods of time. Especially in regions with large temperature swings between day and night, high summer temperatures, and freezing winters, thermal expansion and contraction can easily lead to internal stress accumulation, resulting in cracking, deformation, and even failure. Traditional lenses made of soda-lime glass or plastic are vulnerable to this problem. However, street light lenses made of borosilicate glass, with its extremely low thermal expansion coefficient, have become a key technological breakthrough in addressing the problem of thermal stress cracking.1. Thermal Stress Formation Mechanism and DangersWhen street lights are operating, the LED light source generates a significant amount of heat, causing the internal temperature of the lens to rise rapidly. When turned off at night or in rainy weather, the ambient temperature plummets, causing the lens to cool rapidly. This cyclical temperature fluctuation causes the material to expand and contract repeatedly. If a material has a high thermal expansion coefficient, its volumetric change will be significant. Since the lens is typically fixed to a metal lamp body, the differential expansion rate between the metal and glass generates shear stress at the interface. When this thermal stress exceeds the material's tensile strength, microcracks initiate in stress-concentrated areas (such as edges, mounting holes, or areas of sudden curvature). These cracks gradually expand over time, eventually leading to lens breakage. Once cracked, this not only affects light distribution but can also cause cascading failures such as light leakage, water ingress, and short circuits, seriously impacting the lifespan and safety of the streetlight.2. Borosilicate Glass's Low Expansion: Suppressing Stress at the SourceThe core advantage of borosilicate glass lies in its extremely low coefficient of linear expansion, which is only one-third that of ordinary soda-lime glass and close to that of some metals. This means that with the same temperature change, borosilicate glass experiences minimal dimensional change and excellent volume stability. For example, when the temperature rises from -20°C to 80°C (ΔT = 100°C), the length of borosilicate glass changes by only 0.033%, while that of ordinary glass can exceed 0.09%. This slight deformation significantly reduces the thermal stress differential within the material and between it and the metal support, fundamentally suppressing crack formation.3. High Thermal Shock Resistance: Withstanding Sharp Temperature ChangesThe low coefficient of expansion directly imparts to borosilicate glass its exceptional thermal shock resistance. It can withstand temperature fluctuations of over 150°C without cracking, far exceeding the 50–70°C limit of ordinary glass. This property ensures that street light lenses maintain structural integrity even when exposed to the summer sun at noon and then suddenly cooled by heavy rain, or suddenly illuminated in extreme winter temperatures. Experiments have shown that lenses made of borosilicate glass experience no cracking or optical degradation after 1,000 cycles of temperatures from -40°C to 120°C, demonstrating exceptional environmental adaptability.4. Material Homogeneity and Internal Stress ControlBorosilicate glass is manufactured using high-purity raw materials and a precision melting process to ensure uniform composition, minimal bubbles, and minimal impurities. Its internal structure is dense and free of significant internal stress. During the subsequent molding and annealing process, residual stress is eliminated through slow cooling, leaving the finished lens in a "zero stress" or "low stress" state. This high uniformity further enhances its resistance to thermal shock, preventing cascading failures caused by localized weaknesses.5. Optimizing Compatibility with Metal Lamp BodiesStreet light lenses are typically secured to aluminum alloy lamp bodies using press rings or adhesives. While higher than borosilicate glass, the difference is relatively small. Properly designed mounting gaps and buffer structures (such as silicone seals) can effectively absorb residual stress and avoid stress concentration caused by rigid constraints. In contrast, using PC plastic, which has a higher expansion coefficient, is more susceptible to warping or cracking due to expansion mismatch with metal.6. Long-Term Reliability and Maintenance Cost AdvantagesBecause borosilicate glass lenses are less susceptible to cracking due to thermal stress, their service life can reach over 10 years, matching the lifespan of LED light sources. This not only reduces replacement frequency and overhead maintenance costs, but also improves the overall reliability of urban lighting systems.Due to its extremely low thermal expansion coefficient, borosilicate glass effectively suppresses the accumulation of thermal stress in street light lenses under frequent temperature changes due to its material nature, fundamentally solving the problem of cracking and becoming an ideal choice for high-performance, long-life outdoor lighting optical components.
