Organic Roll Coating to Form Interference Layers

                        Organic Roll Coating to Form Interference Layers

                                    Ken McCarthy
                        Multilayer Coating Technologies LLC

Abstract

This presentation will discuss an anti-reflection layer coated from a solution of organic polymers. The optical properties of this layer will be delineated, along with the required uniformity down-web, cross-web, roll-to-roll, and batch-to-batch.

Introduction

The primary purpose of an interference layer is to reduce glare reflecting from the surface of a film, thereby enhancing a customer’s visual experience. The coverage of this layer is vital to its being able to function as an interference layer. In this paper, the reasons for tight coverage control and some of the potential issues will be discussed.

Interference Layer

To provide anti-reflection properties, the interference layer is coated on the outside of an imaging structure. The substrate under this layer is PET. Without an interference layer, there can be a significant glare issue that makes the image difficult to see in areas of bright light. Any reflection off the outside surface detracts from the image. The structure is shown in Figure 1.

                          Figure 1

The organic interference layer operates on the principle of thin film interference, that is, light rays from the top surface of the coating destructively interfere with light rays from the lower surface of the coating. Practical limitations reduce but do not eliminate all reflection. For example, the system is optimum for only one wave length, nominally 550nm. The thickness of the organic interference layer and its index of refraction determine its effectiveness in reducing glare. From optics considerations, the design of an organic interference film is to have an index of refraction of 1.41 and a thickness of 0.0975 u to provide optimum antireflection properties at 550nm.

Control of the organic interference layer thickness is important for consistent performance, as may be seen in Figure 2. If the coating is too thick, optimum performance slides towards the red part of the visible spectrum, degrading effective performance in the green and blue parts of the spectrum. If the coating is too thin, optimum performance moves to the blue part of the spectrum, degrading effective performance in the green and red parts of the spectrum.

            

Figure 2

A practical way to measure the organic interference layer thickness is to measure the reflectance density using 358 nm light. Figure 3 shows a theoretical curve and an empirical curve. A measured reflectance density range of 1.20 to 1.30 will keep the organic interference layer thickness within 2.5% of nominal. The spectral effect on the organic interference layer performance will be minimal in this range.

Actual reflectance density data from a recent production run are shown in Figure 4. In this run every other roll was measured at three cross-web locations. The data show that all measurements fall between 1.20 and 1.30, verifying that the process delivers 2.5% variation.

                  

                       Figure 3

       Figure 4

Summary

In this discussion, we have described an anti-reflection layer generated by coating a polymer layer of a determined index of refraction and thickness. This organic interference layer enhances the viewing experience by greatly reducing glare. The effects of changes in coverage on the effectiveness of this anti-reflection layer were calculated and shown in graphical form. Reflectance data demonstrates this process delivers 2.5% variation for a 0.0975u thick coating.

Acknowledgement

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

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