Process Considerations in Organic Roll Coating to form Interference Layers

            Process Considerations in Organic Roll Coating to form Interference Layers

                                            Ken McCarthy
            Multilayer Coating Technologies LLC

Abstract

Manufacturing techniques to form interference layers by organic roll coating will be discussed. A related paper described the important product characteristics of an organic interference layer and the product parameters that need to be tightly controlled (Organic Roll Coating to Form Interference Layers). Process considerations for the necessary control of the manufacturing activities include chemical batch preparation, dispensing, coating, web speed control, and drying. These five areas are covered in this paper

Introduction

For many imaging systems, an organic interference layer can be added to the outside surface of these systems. The primary purpose of this layer is to reduce glare reflecting from the outside surface, thereby enhancing the customer’s visual experience. Controlling the coverage of this layer is vital to its being able to function as an interference layer.

Chemical Make

Control of the chemical make process of the organic fluid is critical in order to get reproducible results from batch to batch and from run to run. A computer controlled recipe driven batch process is a useful way to insure accurate batches.

Each recipe is loaded into the batch control system. Operators then interface with the batch control system through process views as they make each batch. A typical computer screen for such a system is shown in Figure 1, Verification points can only be entered locally at the reactor where the batch is being made. This procedure ensures operator safety and that the batch is being observed as it is being made.

Figure 1

Dispensing

The next important step is control of the dispensing system that delivers fluid to the coating applicator. It is vital to have this system deliver a uniform flow to the applicator. High frequency pulsation in pressure will lead to variations in coverage, even if they do not appear in the flowmeter output signal. If an on-line measurement of coating uniformity can be employed, variations in coverage can be found quickly and corrected. An on-line measurement for the organic interference layer is not available, but for other coatings, on-line measurements are possible. Figure 2 is an eight second record of such a measurement.

Figure 2       

Figure 3

A pressure monitor at the slot coating application shows a similar pattern in frequency and form, as seen in Figure 3. By modifying the pressure source and reducing the viscosity of the fluid, the pressure variation is eliminated, as shown in Figure 4.

                    Figure 4

Note that the P-P variation in Figure 3 is about 0.6 psi, whereas the variation in Figure 4 is about 0.015 psi. Measurements of coating uniformity after the changes were flat lines. For slot coating, having uniform delivery of fluid to the applicator is necessary to achieve uniform product.

Coating

For the organic interference layer coating, control of cross web coating coverage is vital. From Transport Phenomena, the equation for ideal flow in a slot is:

            Q = 2/3 (P1 – P2) B3 W/ mL 

where Q is flow, P is pressure, B is slot height, W is slot width, m is viscosity, and L is slot length. Substituting (B0+d) for B, where B0 is the nominal slot height and d is the deviation from nominal, the sensitivity of coverage to slot height uniformity can be derived. It can be shown that for a six mil slot, it is required that d be < 0.06 mils (1.5 microns) to achieve coverage uniformity within 3%.

Using the expression above for an applicator, the cross web coverage variation can be estimated from slot height measurements. Again, with a model fluid, Figure 5 shows the estimated cross web variation and actual cross web coverage measurements.

Figure 5

Web Speed

Coverage variations can also occur due to web speed variations as the web goes by a slot applicator. Again, using a model fluid on a pilot coater, down web variation is shown in Figure 6. By using an FFT analysis of this data, two key frequencies are found, with

                

                    Figure 6

repeat distances of 5.04 inches and 1.12 inches, as shown in Figure 7. These frequencies were related to ripple torque in the motor which is directly coupled to the backing roll.

                              

                    Figure 7

This problem is minimized by retuning the drive. Once found, it was relatively easy to correct this issue.

The above is representative of a typical continuous variation in web speed. It is also possible to have singular events that cause variations in web speed that lead to coverage variations. An example is shown in Figure 8, again for a model system. Here an on-line measurement, shown in red, indicated a gross change in coverage that was occurring during certain winder operations. The blue curve represents web speed measurements taken during these same winder operations, and the close correlation is obvious. By changing the winder sequencing, this variation was eliminated.

                                       

                    Figure 8

Drying

Several considerations must be reviewed when determining the drying conditions for an organic interference layer. The primary one is safety as related to LFL limits when coating solvent layers. Since most of the solvent is removed in the first part of the oven, the biggest determining factor is the flow rate of the coating and the air flow in the oven. The obvious constraint here is on web speed.

A second issue that may occur is blushing due to the solvent leaving too rapidly, cooling the web below the dew point of the room or oven air. Drying extenders such as higher molecular weight homologues of the primary solvent can eliminate this problem.

Other drying issues that need to be considered are non-uniformities such as orange peel and Benard cells. Usually for coatings that are as thin as organic interference layers, these problems do not arise.

Summary

To deliver constant coverage of the organic interference layer, several manufacturing parameters must be carefully controlled. Chemical batches need to be consistent from batch to batch and a recipe driven, computer controlled batch system facilitates that control. Dispensing flow has to be even. The slot coating applicator should be designed and built carefully with minimum variation in the slot height. Variations in web speed must be kept as low as possible. Drying parameters need to be set to give safe, uniform and defect free coating. Paying attention to these considerations will provide consistent anti-reflection coatings using organic polymers. As demonstrated in the related paper, an  organic polymer interference layer of 0.0975 u can be controlled to within 2.5%.

Acknowledgement

The author wishes to acknowledge the useful assistance of John Cabral, John Gabriel, and especially Jim Slack.

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