Philips Research - Technologies

 

Photorheology

Photorheology is a convenient technique for monitoring changes in mechanical properties during UV-curing. Within Philips, many manufacturing processes involve the desired and controlled change of liquids into (semi-)solids using thermal or radiation curing. Examples of such processes are the sealing of polymer LEDs and LC displays and the UV-curing of photopolymerizable monomers against glass bodies for aspheric lens replication. All these applications have one drawback in common, which is the shrinkage of polymerization resulting in a change of the shape or the thickness of the material cured in a mold or on a surface.

 

Following a chemical reaction in terms of the conversion from a monomer into a rigid polymer network can quite easily be done using IR measurements or by microcalorimetry. The information obtained from both methods can be used to study the kinetics and the time to reach final conversion at a certain light intensity.

 

In process studies it is often desired to get information on the development of mechanical properties like viscosity and stiffness with time during the chemical reaction. This is enabled by photorheology.

Working principle

A rheometer applies a shearing force in a material by controlled rotary or oscillatory movement. It yields torque and displacement as output variables. The quantities of interest that can be obtained are the viscosity and the visco-elasticity (stiffness) resulting from polymerization. The symbols for these quantities are h* [Pa.s], G′ [Pa] and G″ [Pa]. h* is the complex viscosity, G′ and G″ are the real and the imaginary part of the complex shear modulus G*.

 

In the actual setup there is an upper plate with radius R that is moved in an oscillatory way at a set frequency and a set gap h from the lower plate. Since the lower plate is made out of a UV transparent bottom plate, exposure can be carried out while the rheological measurement is being performed. A comparison of the response of an acrylate vs. an epoxide demonstrates the differences in the kinetics of the build-up of the modulus.
What follows is that the epoxide monomer showing delayed increase of stiffness enables much better copies of the mold, because the shrinkage that develops during polymerization was realized in the liquid state. In this way material transport through the thinnest section can still take place. At the moment that the material started stiffening, the chemical reaction (the driving force for shrinkage) has completed for over 50% of the total attainable conversion. For the acrylate almost the entire reaction takes place in the gelled state, leading to large stresses and deformations.

Feedback to process results

Lenses are made of an acrylate and an epoxide monomer, and their shapes are measured with a profilometer. The spherical body is afterwards subtracted to yield the replicalayer thickness as a function of the angle of rotation. The wet layer thickness is calculated using the known mold shape and the known body radius. These are about 0.7 μm apart at the thinnest section (~ 31°).

Back