High speed microcompression of paper coatings

A novel high speed microcompression platen tester was developed in order to measure the out-of-plane compressive modulus of thin materials. The instrument is capable of subjecting a sample of thickness 20 μm or greater to a transverse compressive pulse over a time interval ranging from approximately 2 ms to several seconds, and can therefore be used to collect data under conditions representative of those in a high speed calender nip. In this study, free layers of coating formulations normally used to coat paper were prepared and tested using the microcompression platen tester described above. Tests were conducted at high speeds, with a pulse duration of 2 ms during the compressive stroke, and at 23 °C to simulate room temperature calendering conditions. The compressive modulus of the coating did not correlate strongly with the modulus of its constituent latex. Latex content, however, strongly affected coating compressive modulus. A sharp increase in the compressive modulus was observed at the coating critical pigment volume concentration (CPVC)—essentially the latex concentration at which the coating layer porosity is reduced to zero. Pigment size distribution and pigment morphology also affected the compressive modulus of coating in a manner consistent with packing theory.