Operational limits of blade coating associated with high aspect ratio pigments: Part II—cylindrical laboratory coater

Lisa Weeks, P. Gane, Tony Lyons, D. Bousfield
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Abstract

Process issues such as blade deposits, scratches, spits, and defects on the coating surface develop during blade coating of paper at high solids content and fast machine speeds. Coatings that contain high aspect ratio pigments display these issues at lower solids concentrations and slower web speeds than spherical or block shaped pigments. The shape of the pigment particles influences the characteristics of the coating formulation after drying, and high aspect ratio pigments can lead to high gloss surfaces that have good printing properties and are of interest in barrier coatings. The rheology, particle packing, and water retention are all functions of the pigment shape and size distribution. The base sheet properties of porosity, surface roughness, and permeability interact with the coating and the blade coater operating conditions. The possible causes of poor runnability are part of an interrelated and complex system. A cylindrical laboratory coater (CLC) was utilized to determine the speed at which operational and quality issues begin at varying solids concentration for two pigments with different shape factors on two different paper surfaces with varying absorption rates. An operational window was developed for both pigments. Characteristics of the pigment suspensions were explored in order to find a relationship with the onset of operational problems. The high shear rate viscosity of these suspensions did not explain the operational windows. The difference between the solids concentration and the immobilization solids, which is a function of the shape factor, has a linear relationship to the runnability limits. This could be ascribed to the tight packing structure of the particles as the solids content approaches the immobilization solids.
与高纵横比颜料相关的叶片涂层的使用限制。第2部分-圆柱形实验室涂层机
在高固体含量和快速机器速度下,纸张的刀片涂布过程中会出现刀片沉积、划痕、喷口和涂层表面缺陷等工艺问题。含有高纵横比颜料的涂料在较低的固体浓度和较慢的网速下比球形或块状颜料显示这些问题。颜料颗粒的形状影响干燥后涂料配方的特性,高宽高比颜料可以导致具有良好印刷性能的高光泽表面,这是阻隔涂料的兴趣所在。颜料的流变性、颗粒堆积和保水性都是颜料形状和尺寸分布的功能。基片的孔隙率、表面粗糙度和渗透性与涂层和叶片涂层机的操作条件相互作用。可运行性差的可能原因是一个相互关联的复杂系统的一部分。使用圆柱形实验室涂布机(CLC)来确定在不同固体浓度下,两种不同形状因素的颜料在不同吸收率的两种不同纸张表面上产生操作和质量问题的速度。为这两种颜料开发了一个操作窗口。探讨了颜料悬浮液的特性,以便找到与操作问题发生的关系。这些悬浮液的高剪切速率粘度并不能解释操作窗口。固相浓度与固相浓度之差是形状因子的函数,与可操作性极限呈线性关系。这可以归因于颗粒的紧密堆积结构,因为固体含量接近固定化固体。
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