Description of UV Surface Treatment based on Practical Examples(R1)

introduction,
mechanism of UV surface processing method(1)

1. Introduction

There are a number of process technologies to enhance the functions of solid surface, from chemical applications to dry processes.
The surface processing method in use of ultraviolet radiation (hereinafter abbreviated as "UV") is basically a dry process applicable in the atmosphere to give modifying and cleaning effects, although some types using a chemical liquid have recently been proposed as photo sensitizer.
Modification directly enhances the adhesion power of solid surface, while cleaning gets rid of the organic contaminant that forms a soft adhesion layer of the surface, which indirectly contributes to more adhesion and quality stabilization.

Fig 1. UV pre-coating processor of mudguards for passenger cars

The UV process has been a technology known since over 50 years ago, but has been regarded to be less practical from industrial viewpoints for many years.
Even now there is a tendency that a specialized researcher rather shows no interest from such viewpoint.
However, along with the high integration of liquid crystal display elements, the UV surface processing method was adopted for cleaning of liquid crystalline glass in early 1980s.
The UV surface treating method has now been fostered and developed to be an indispensable technology to manufacture liquid crystal products.
In early 1990s the modification technology was employed in the pre-coating process of mudguards for passenger cars (Fig. 1) and the track coil core for the magnetic levitated super-train on the Kofu Test Line (Fig. 2, 3).
The track coil is working even now.


Fig2. The experimental UV surface processor in the pre-coating process.	Fig3. The track coil core for the magnetic levitated train

Then it was followed by applications adopted such as for enhancement of adhesion power of engineering plastics which are used in devices around automobile engines and electronic equipment.
When it comes to the cleaning technology, we came across the unique product of liquid crystal display elements originated in Japan and therefore it fortunately became popular extensively free from any containment in industrial secrets.
The modification technology, on the other hand, suffered a heavy drag on its growth in diffusion because of end users' containment of the technology as secrets related to their confidential production methods.
UV/Ozone technologies have been recognized after all as useful technologies in the age of micro-metric accuracy, though they were almost regarded as of no use in the age of accuracy in the millimeter order.
The technologies are still under development even now, but we are sure that they will be developed to be indispensable as many products are destined to get more miniaturized into the nanometer order in the future.

2. Mechanism of UV Surface Processing Method

2-1. Modification

UV Surface Processing allows us to make modification or cleaning of solid surface.
It depends on the material which one of the reactions will take place.
Only a cleaning reaction takes place for glass or ceramics, while both modification and cleaning reactions for plastics or metals.
If a higher energy than molecular bond is imparted to an organic compound, the bond will be broken apart.

When C-H molecules are broken apart, the extremely light hydrogen atom will be extracted easily.
Where an oxygen atom reacts then, oxygen-enriched functional groups will be formed such as C-O, -COO- or C=O.
Chemical reactions on polymer surfaces can be analyzed by X-ray photoelectron spectroscopy (XPS) or IR spectrum.
Fig. 4 shows a C1sXPS spectrum 2) when the surface of liquid crystal polymer (LCP) was irradiated by a 200W low-pressure mercury lamp for three minutes.
A new peak attributed to carbonyl group of oxygen-enriched functional group came into being by the UV irradiation. Particularly a large number of carbonyl groups were introduced.