粒子のブレード前配向 : 高アスペクト比顔料塗工における均一塗工構造開発のための新設計概念

抄録

The physical properties of a coated sheet are strongly related to the coating structure developed through the interactions between the coating formulation, the basepaper and the coating machine during application of the coating layer. Control of coating structure and basepaper coverage is a complex phenomenon generally confined in discussion to a consideration of the dewatering and subsequent immobilisation properties of the colour, the particle packing characteristic of the mineral components in the colour and basesheet surface profile. The influences of coating colour solids, machine speed and the flow characteristic at the coating head (and roll applicator if applicable) are manifest particularly by the observed pressure penetration of fluid coating colour into the basepaper surface voidage, the instantaneous rheological properties of the pigment-based formulation and the runnability at the blade. Poor particle packing in the region immediately upstream of the blade tip is seen as a major contributing factor to blade bleeding and stalagmite formation. Many of the demands for superior coverage in LWC papers, and uniformity of coating structure in general, depend upon a highly stable packing configuration at the crucial region entering the blade nip. Film split effects in roll applicators, circulation instabilities in short dwell coating and sensitivity to blade geometry conspire to create non-uniformities in the coating structure. Amongst coating pigments, highly anisometric particle forms, such as platey kaolin, are particularly susceptible to orientational instabilities arising from the many complexities of these interactions.
A novel approach to stabilising the flow upstream from the doctor blade is afforded by using a pre-metering blade unit which initiates a relatively small shear-related pressure zone under which the coating colour interface boundary with the basepaper is established and the colour is sheared prior to the final metering to the desired coat weight at the doctor blade. The effect of initiating coating colour dewatering and immobilisation of the colour in contact with the basepaper promotes contour coverage, and the pre-shearing serves to orient high aspect ratio particles with resultant significant improvements in runnability and coating quality including gloss and coverage. The primary design discussed refers in particular to the ECC International HARPPOC (High Aspect Ratio Particle Pre-Orienting Coater) as applied to the ECC International Helicoater® and published in the embodiment of the European Patent Specification EP 0198622 B 1.