Abstract
Evolution of surface improvements on paper are driven by tech- nical needs, in respect of printing and writing, economical drivers, such as mineral versus fibre cost and market niche exploitation, runnability and aesthetic requirements. From the simple anti- feathering absorption control of hydrophobisers, such as starch, to the sophistication of digital printing, the need to modify the surface of paper is an accepted criterion for investment in our industry. Simple pond and roll application has, through increases in speed and the application of pigmentising, developed in recent times to controlled film application processes. The demands throughout these developments for the scientific understanding of the interactional processes between fluids and substrate: absorption, hydraulic penetration, basepaper fibre debonding and roughening: and the complex rheology of pigment suspensions, based on stability, surface chemical and hydrodynamic criteria, have usually been met retrospectively. The machine concept or application is developed, installed, trouble-shooting achieves compatibility in the industrial environment and only latterly does the question of optimisation through a thorough scientific analysis come to the fore. By studying the models and accepted design criteria that have emerged throughout this evolution, it is pro- posed that advances through extrapolation of the models and by seeking new untried dimensions of the models can be made which can act to fuel the development further. Notwithstanding that current knowledge in multivariate applications inevitably remains inexact, the author attempts to demonstrate how the concepts available today could be extended to build potential for develop- ment in the future. Mechanics of suspension flow, dewatering and network absorption remain the primary controlling parameters and a critical review of past and present assumptions and new research is timely in order to re-fashion the directions that can be deduced. Rates of absorption as a function of available fluid vol- ume arising from the resistance dynamics of rapid dewatering provide a key opportunity for improving surface coating holdout especially in the challenging on-machine context where the time- scale for hydrophobising is too short and the wettability and moisture profiles are at their most critical. On the basis of this review, it is proposed that high rate absorption, through the use, for example, of ultra-high surface area filler pigments in the substrate, together with rapid low solids immobilisation of pig- mentised formulations with disparate coating and basepaper permeabilities, provides a means of maximising the potential for surface treatment applications both at high speed and in the con- text of sophisticated niche product development. The roles of basepaper and pigment structure are therefore also an integral part of the review.
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