- March 01, 2000, Dr. Peter M. Schweizer
Curtain coating belongs to the class of premetered coating methods. The term "premetered" indicates that only the exact amount of fluid to be coated onto the web is fed to the coating head. This feature, and the associated lack of excess fluid at the coating head, leads to significant advantages in terms of operational procedures and product quality when compared to other coating methods.
In particular, the web film thickness in the machine direction (MD), Hwet, does not depend on physical fluid properties nor geometrical process parameters but solely on the ratio of volumetric flow rate/width, Q, to web speed, U, according to the equation below:
(See text for formula)
Therefore, film thickness variations in MD can easily be kept below 1% (+/-0.5%) with the help of standard process control procedures for the web speed and the flow rate (pump speed).
For the most part, curtain coating has been developed to a high degree of sophistication in the photographic industry in order to explore its simultaneous multilayer capabilities and the resulting enormous advantages in productivity. Now that the basic patents have expired, this method also is available to other industries.
Following is a description of the main features of curtain coating in terms of attractiveness, limitations, and requirements.
The Process and the Hardware
Liquid distribution in the transverse direction (TD) is accomplished with a die, which is designed either as a slot or a slide type.
For physical reasons, a slot die allows a maximum of three layers to be coated simultaneously. In contrast, a slide die can be expanded at will, and products with more than ten layers can be coated in one pass.
When the coating fluid detaches from the die lip, an unsupported liquid sheet or curtain is formed, which, driven and accelerated by gravitational force, typically falls between 50 and 300 mm before impinging on the moving web.
The cross section of the flow field in the curtain impingement zone resembles the shape of a foot characterized by the presence of a heel. Hc and V are the thickness and impingement velocity of the curtain, respectively, and a is the impingement angle.
An important feature of this flow field is the dynamic wetting line located at the bottom of the rear curtain surface where the fluid first strikes the web. The position of the dynamic wetting line is subject to process optimization.
The curtain must be guided along its edges in order to prevent necking-in as a result of surface tension forces. If the curtain, and therefore, the film to be coated, is narrower than the web (inboard edging), the edge guides must terminate a short distance above the web. In this configuration, the curtain impinges onto a web that is supported by a backing roll.
On the other hand, if the curtain is wider than the web (coating over the edges), the edge guides are longer than the curtain, and the resulting excess fluid must be collected and handled properly. In this configuration, the curtain typically impinges onto an unsupported web. Moreover, while this setup is still a premetered coating method, it is characterized by excess fluid, which is associated with operation problems.
In most cases, the curtain impinges onto a web whose orientation is more or less perpendicular relative to the gravity vector. However, it may be advantageous or necessary to drop the curtain onto a web with an uphill or downhill orientation, for example, if the coating machine has an arched dryer.
The uncoated web approaching the impinging curtain will drag along a boundary layer of air, which deforms the curtain (balloon effect) and tends to be entrained between the web and the liquid film, particularly if the web speed is high and the curtain is wide.
Such undesirable effects can be prevented by mounting a passive (mechanical blockage) or active (suction) baffle in close proximity to the web a short distance upstream of the impinging curtain.
In curtain coating, the primary functions of the die are liquid distribution in the TD direction and formation of the curtain. These functions physically are separated by the height of the curtain from the coating function (i.e., the application of the liquid sheet to the moving web).
For inboard coating, the die, with its peripheral equipment, typically is mounted onto a coating stand, an important feature of which is a movable platform. The platform carries the die and its base and allows the die to be moved from the coating position over the backing roll to a start/stop position above a trough, where the die can be properly filled and cleaned.
The hardware components described above must be complemented with adequate components of a fluid conditioning and delivery system, including a metering pump, flow meter, filter, degassing system, etc.
Limits of Application
Exact limits of application cannot be quantified, because the range of admissible values for relevant process parameters (web speed, for example) always depends on thevalues of all other relevant process parameters, e.g., wet film thickness, rheological properties, surface tension, curtain height, application angle, etc.
Regarding rheological fluid properties, a strong shear thinning behavior is beneficial for high-speed curtain coating, because, owing to the very high shear rates in the curtain impingement zone, the effective viscosity in the curtain heel will be low, which is necessary to postpone air entrainment to high web speeds. Coating speeds in excess of 1,000 mpm are possible.
Unlike any other coating method, the volumetric flow rate for curtain coating must be above a minimum value in order to establish and maintain a stable curtain.
The minimum flow rate depends on the design of the curtain edge guide, on the level and type of disturbances in and around the curtain, and on the ability of the curtain to withstand the action of the disturbances, i.e., on the dynamic surface tension.
For all practical purposes, however, the minimum flow rate/width should be >1 cm2/s. According to the equation, these requirements makes curtain coating a high-speed method, particularly if the wet film thickness is low.
The web speed must be >10 m/s (600 mpm) if the wet film thickness is to be 10 microns and the curtain is to be stable.
The Operating Window
The operating window of curtain coating is characterized by three boundaries, namely the minimum flow rate required to establish and maintain a stable curtain as explained above; the onset of air entrainment; and the onset of vortex formation in the curtain heel.
Criteria for optimizing the curtain coating process for any specific operating conditions are well known qualitatively and quantitatively. Therefore, process optimization allows one to place the boundaries of the operating window so the desired operating point (web speed, wet film thickness, etc.) comes to lie somewhere in the middle of the operating window and sufficiently away from its boundaries. This means the process is robust relative to external perturbations, and the process can be run with great latitude with regard to uncontrollable fluctuations of process parameters.
Advantages Over Other Methods
The "premetered" feature and the associated lack of excess fluid offers great operational advantages and savings in manufacturing costs over other coating methods such as roll, blade, bar, air knife, and spin coating.
In particular, problems related to recycling of excess fluid, replenishing of evaporating solvents, formation of foam, and crusts in fluid containers, etc., are eliminated.
In addition to the superb film thickness uniformity in MD, the TD uniformity also is excellent. In most cases of single-layer coatings and in all cases of simultaneous multilayer application, dies with a fixed slot geometry are used.
For such dies, the TD uniformity depends primarily on the geometry of the die internals (distribution cavities and metering slots); rheological fluid properties; volumetric flow rates/width; deflection of die plates due to internal fluid pressures; degree of isothermal operating conditions; degree of variation of coating width; and mechanical precision of the metering slots.
Dies with fixed geometry can be optimized for specific operating conditions, thereby satisfying specified performance criteria for the maximum allowable TD uniformity, the avoidance of internal contamination, ease of cleaning, maximum allowable fluid pressure, maximum residence time, etc.
For dedicated dies, total TD nonuniformity can be kept below +/-1%. For dies that are used for different operating conditions, the resulting TD nonuniformity depends on the ranges for rheological parameters and flow rate/width.
Curtain coating often is referred to as contour or conformal coating because of its ability to evenly coat over uneven substrate surfaces such as paper and board. Moreover, butt splices of good quality can be overcoated without generating any loss of product.
One feature that distinguishes curtain coating from any other method is the virtual absence of sharp lines and streaks, which results in considerable gains in base yield. The explanation lies in the fact that near the curtain impingement zone, there is no rigid equipment part such as a blade, roll, or die lip that interferes with the actual coating process.
Compared to blade coating, curtain coating generates much smaller fluid pressures at the application point. This feature is very welcome in many paper coatings because of the much gentler treatment of the often delicate paper substrate. Obviously, the result is fewer web breaks and, hence, savings in manufacturing costs.
The views and opinions expressed in Technical Reports are those of the author(s), not those of PFFC editors. Please address comments to author(s).