Determining Cure Profile and Post-Cure Shrinkage of Photopolymers Using UV Accessory on a Rotational Rheometer
Application Note Aug 19, 2015
Malvern Instruments Limited
A photopolymer usually consists of monomers, oligomers and photo-initiators which cross-link to form a network structure when exposed to light, often in the ultraviolet or visible region of the electromagnetic spectrum.
Photocuring is a relatively rapid process compared to thermal curing and therefore the process can be used for selective curing using high energy light sources, which makes the process suitable for printing circuit boards and fabricating microchips. Photo-polymers are widely used in medical, 3D-printing, coating, adhesive and proto-resist technologies.
Rheological measurements are commonly used to characterise the progression of viscoelastic properties of photopolymers during photo-curing. By measuring the change in complex modulus (G*) it is possible to estimate the rate of crosslinking. In addition, photopolymers tend to show significant post-cure shrinkage depending on the monomer concentration. Normal force control capability on the rheometer allows the vertical shrinkage to be measured during curing from the change in gap under a constant applied force. This can be used to calculate the percentage of post-cure shrinkage.
Cross-linking kinetics of photopolymers tend to have a very strong dependency on the intensity of the UV light and the length of exposure time. It is also important to bear in mind that the intensity of the light beam reduces with the distance from the irradiative surface.