Forming and Consolidation of Coating Layers and Their Impact on Coating and Print Uniformity-an Overview
The state of the art and new coating techniques for the forming of the coating layer in pigment coating of paper,pre-calendering of the paper prior to the coating and post-calendering of the final coated paper are briefly reviewed as an introduction to a discussion on the relationship between coating mass distribution,porosity distribution in coating layers and mottle in lithographic offset prints.Drying is also considered.With respect to coating mass,distribution the forming(coating)techniques are ranked as follows: blade coating>MSP-coating>curtain coating.Spray coating also leads to a uniform mass distribution,but the coating tends to be grainy due to incomplete levelling of the droplets.Because paper is rough,a uniform mass distribution (contour coating)leads to a rough coated surface and vice versa.This means that the porosity distribution becomes uneven in coating layers with a uniform mass distribution if the pressure distribution on the coating surface in the calender nip is uneven.This is true for soft-nip calendaring,and the unevenness increases with increasing temperature and pressure.In extended-nip calendering,however,where the pressure distribution is even,the porosity distribution becomes more even after the calendering.This suggests that the coating layer,prior to the calendering already exhibits an uneven porosity.This porosity distribution is governed by the drying strategy.To obtain a coating layer with a uniform mass distribution and simultaneously a coated paper with a smooth surface,the base paper beneath the coating must be smooth.This can be achieved by pre-calendering the base paper using a technique which not only smoothens the paper surface but also stabilizes it.A promising new technique,in which the paper is calendered at a high moisture content between two hot steel bands,has recently been introduced onto the market.Common pre-calendering does not stabilize the surface,and there is a risk that the calendering effect is lost in conjunction with the coating operation when the paper is exposed to the water in the coating colour.The mottle in the offset print is governed by the evenness of the porosity distribution in the coating layer and,as already said,this can be affected both by the drying and by the calendering.An uneven binder distribution in the coating surface is another cause of mottle.In LWC,the risk of mottle in the print is higher with blade coating than with MSP-coating.(The latter technique is only used for the coating of LWC-paper).What the situation is with regard to curtain coating is too early to say because there is as yet no industrial installation.From experiments on a pilot-scale for woodfree paper,less mottle was reported for curtain coating than for blade coating.
coated papers coating techniques calendering techniques drying print mottle
GUNNAR ENGSTROM
Karlstad University,SE-651 88 Karlstad,Sweden
国际会议
第二届国际造纸与环境学术会议(The Second International Papermaking & Environment Conference)
天津
英文
84-102
2008-05-14（万方平台首次上网日期，不代表论文的发表时间）