Curtain coating is also being used in several speciality applications in the paper industry. This technology in principle is also suitable for the coating of printing papers or cardboard packaging materials, applications which are currently being tested.
Parameter
Minimum
Web speed [m/s]
<1
Wet film thickness (total) [µm]
> 10
Dry film thickness (single layer) [µm] < 1
Viscosity [Pas]
0.010
No. of layers [-]
> 10
Just like all other coating methods, curtain coating has its own window of application. Table 1 shows the application limits of the technology in terms of: web speed; wet and dry film thickness; viscosity, surface tension, specific flow rate and the number of layers which can be applied. The values in the table are just ballpark figures, more detailed figures are necessary to consider a multitude of further variables.
– Minimum deviation of application profiles; that is to say: <±0.5% in MD, <±1% in CD)
– Film thickness depends on neither the physical properties of the liquid nor on geometry
– No excess coating liquid – Short product changeover time is possible – Contactless application process – Multiple layer coating possible within just one pass – Possibilities for two-component applications, for example, coating colour and cross-linking agent
In the premetered category, curtain coating is a particularly attractive coating method. Because it applies a contour coating, the process also creates an even coating on uneven substrates. In addition, there is a virtual absence of coating defects, such as the sharp lines and streaks which can be observed with coating methods that use narrow gaps. The latter includes blade coaters and slot die coaters.
帘式涂布 MACHINERY AND PROCESS
用于产生多层结构的帘式涂布
多层帘式涂布机只需经过一次涂布操作就能够赋予喷墨打印纸表面三层结构(底层、吸收层 和顶层)，并能够在不均匀的纸张表面上获得均匀的涂层
Curtain coating for multilayer structures
A multilayer curtain coater can apply three layers – primer, absorption layer and top coat – to inkjet paper in one pass and provides an even coating for uneven substrates
Curtain coating belongs to the so called pre-metered coating methods, which are characterised by the following main advantages:
– A simple calculation of coat weights, that is to say, conservation of mass
Pulp&Paper Technology • 纸业技术 73
如：锐线和斑纹，这些缺陷在狭缝涂布方式中可以观测到，例如， 刮刀涂布机和狭缝冲模涂布机。
在造纸工业领域，帘式涂布可能会很陌生和稀奇，但是自20世 纪70年代以来，在照相行业单层和多层结构产品生产方面，帘式涂 布已经被证明是最有价值方法。
和其他所有加工工业一样，在造纸工业领域，想要在低成 本的条件下提高产品性能的思想已经逐渐成为主流。 对涂布产品，这就意味着所生产出来的涂层必须具 备以下两个条件：一、能够正确地包含维持一定产品功能性所需的 涂料量；二、能够在纸机纵向和横向上获得均匀分布。
新产品经常需要多层结构才能提高整个涂层自身的功能。多层 结构有助于生产过程的控制。
Curtain coating Curtain coating is an attractive method for both improving the
functionality of the coating and the process. It achieves excellent profiles in cross-web and down-web direction and creates the possibility of coating multiple layers simultaneously in one pass.
New products, often requiring multilayer structures, are able to improve the function of the entire coating itself. The multilayer possibility also helps to manage the production process.
For coated products, this means that coatings shall be produced: i) with exactly the amount of coating liquid needed to maintain functionality and ii) with even distribution in both machine and in cross-machine direction.
In the Paper Industry, similar to almost all other process industries, the requirement to manufacture products with increased performance at a lower cost level is becoming more and more dominant.
Curtain coating may be rather unknown and new in the paper industry, but it has been a proven and valuable method for coating single and multiple layer products in the photographic industry since the 1970s.
多层涂布 对于多层帘式涂布，通常采用滑动式结构，示意图见图1。在滑
动式冲模上，用于形成多层结构的不同涂料出口从上到下按一定顺 序排列， accuracy in CD is determined mainly by the design, dimensions and manufacturing tolerances of dies
Surface tension [mN/m]
–
Specific flow rate [cm3/cm*s]
>1
Maximum
> 35 – – 10 –
< 40 –
图1：滑动结构的多层帘式涂布冲模 Figure l: Dies in slide configuration for multilayer curtain coating
参数
纸幅运行速度(m/s) 湿膜厚度(整体)(µm) 干膜厚度(单个)(µm) 粘度(Pas) 层数(-) 表面张力(mN/m) 比流动速率(cm3/cm*s)
最小值
<1 > 10 <1 0.010 > 10
– >1
表1：帘式涂布应用方面的限制
最大值
> 35 – – 10 –
< 40 –
在纸幅运行速度一定的情况下，要想获得稳定的加工过程和生 产出非常薄的薄膜厚度，比流动速率是一个非常重要的参数。例 如，在高纸幅速度条件下，能够获得由低到高的不同涂布量；但是 在低纸幅速度条件下，常常会由于落帘稳定性不够而导致薄涂层形 成失败。这个限制对落帘总体流动速率也起作用。在多层结构中， 可以获得非常薄的单个涂层。