by R.W. Stowe, Director of Applications Engineering, Heraeus Noblelight America LLC
Question: What does “optical thickness” mean, and how can it be measured in the lab?
Answer: Optical thickness, along with the UV exposure factors of irradiance profile and UV wavelength, is one of the most important characteristics affecting depth of cure, speed and adhesion in a UV-curing application. It is the optical character of a coating, ink, adhesive or paint, and it combines two factors: spectral absorbance and physical thickness of a curable film.
Most UV-curable films are “optically thick,” and much more radiant energy is absorbed near the surface of the material. Also, absorbance varies wildly with wavelength. The reduction of light energy as it passes into or through an absorbing material can be described by the Bouguer-Lambert law:
Io is the incident irradiance (flux rate) at wavelength γ, Ia is the flux rate at depth d, Aγ is absorbance at wavelength γ and d is the depth from the surface or film thickness.
Here is a comparatively simple method of measuring the photon flux rate at the top of a curable film and at the bottom. The attenuation in a UV wavelength band associated with the optical thickness can be expressed in terms of the UV flux at the bottom as a percent of the flux at the top. Rather than a full spectral display, the method utilizes the UV bands available in a multi-band UV radiometer.
To determine the spectral absorption of a material:
- Place a clean quartz plate over a multi-band radiometer (or spectroradiometer).
- Pass the radiometer and plate under a laboratory lamp that has the same spectral distribution as one that will be used in production.
- Apply a coating or ink to the quartz plate in exactly the same film weight as the intended application.
- Repeat the second step, at precisely the same speed.
- Calculate the attenuation of the film by the ratio of these two measurements (power or energy) in several wavelength bands. These two measurements give the ratio of power or energy in each wavelength band at the top surface of the film and at the bottom (or adhesion layer) of the film.
Here are examples of results:
In principle, the effects of the quartz plate (including transmission and reflection) are assumed to have approximately the same effect on coated and uncoated measurements, so are disregarded.
These results illustrate the attenuation of UV flux within the ink or coating – and particularly the extreme attenuation of the short wavelengths – making an understanding of the combination of spectral opacity and film thickness critical to curing success. Short wavelengths (UVC) may have little effect on depth of cure – except for “optically thin” clear coats or varnishes – and depth of cure must depend on the longer wavelength UV (UVA). This may also explain why, in a curing exposure specification, we prefer to see both UVA (or UVB) and UVC quantified. Clearly, the UV spectral band(s) selected depend on the spectral response of photoinitiators.
This quick-check method can be used to explore adhesion and depth-of-cure issues of optically thick films that may be related to UV exposure parameters.
As we noted in an earlier column in UV+EB Technology, technical data sheets are often unclear regarding precise exposure requirements. This is simply because the formulator/supplier has no control over film weight applied or over the spectral distribution or irradiance of the lamp(s) to be used.
R.W. Stowe is director of applications engineering for Heraeus Noblelight America LLC. He can be reached at dick.stowe@heraeus.com.
