How Light-Curing Works
One can't argue that sunlight is by far the strongest and best-known source of UVA and UVB light. However, this light is filtered through the Earth's ozone, resulting in a much less potent light than the light at the original source. So, is it enough for curing your light-curable materials?
The short answer is NO. And here’s why:
Curing with light is a relatively simple process. Light-curable materials (LCMs) contain ingredients known as photoinitiators that start the curing reaction upon exposure to certain wavelengths of light. LCMs reach full cured properties immediately after exposure to light of appropriate wavelength, intensity, and duration.
To give you more details, intensity is the light energy reaching a surface per time and it is often measured in mW/cm2. When using the term intensity, it is important to define wavelength(s). Higher intensity light (of the proper wavelengths) will generally provide a faster cure. Higher Intensity = Faster Cures
Distance from a light-curing source also affects intensity. Intensity decreases with increasing distance from the light source. When curing through a substrate, light-transmission rates below 100% will reduce the intensity that reaches the LCM.
In addition, LCMs themselves absorb light and hence have a maximum cure depth. For most products, this depth is between ¼" (6 mm) and ½ (12.7 mm). Also, it may take 3-4 times longer to cure a product ½" (12.7 mm) deep than it does to cure that same product ¼" (6 mm) deep.
Consequently, the sun’s limited 3mW/cm2 of energy doesn’t provide the required depth of cure for LCMs. For those who try anyhow may find the very top of the adhesive has a slight cure on the surface, while the lower sections of the “adhesive blob” will remain wet and uncured.
On an average sunny day in Connecticut, USA, the measurable UV energy reaching earth’s surface is 3mW/cm2. Since most adhesives require a minimum of 75 mW/cm2 to cure it is obvious the sun simply won’t be sufficient. To compare to other locations on the globe check the below table.
Table 1. UV-B Irradiance[a] From Sunlight
|Source/Type of Lamp or Sunlight||Irradiance of Lamp/Sunlight in mW/cm||Time Required for 5 min of UVB Light at Noon From the Equator; Hours (min)|
|Equator, noon, SEA 0.0, (Henriksen et al., 1989)||0.265||0.083 (5.0 min)|
|Kodiakanal, India (10.14 N) sunny, April, SEA 0.0, (Sharma and Srivastava, 1992)||0.260||0.085 (5.1 min)|
|Miami, Florida (26 N) noon, June 21, summer solstice SEA 2.3, (Frederick and Snell, 1988)||0.253||0.087 (5.2 min)|
|Neuherberg, Germany (48.2 N) clear, July 13, SEA 26.3 maximum irradiance, (McKenzie et. al., 1993)||0.175||0.13 (7.8 min)|
|Longyearbyen, Norway (78.2 N), sunny, July 15 SEA 54.8, (Henriksen et al., 1989)||0.55||0.40 (24 min)|
|Fort Worth, Texas (32.8 N), shaded, diffuse, March 2, 3 pm, (Gehrmann, 1987)||0.15||1.47|
The data in the left column are: sunlight; location, time, date, weather, latitude, solar elevation angle (SEA), and literature citation.