By Dr. Jin-Wook Ha and Dong-Sik Yu ^*

Department of Chemical and Environmental Eng., College of Eng.,

Soonchunhyang University, Korea

^*Department of Visual Optics, Kyungwoon University, Kumi, Korea

 

ABSTRACT

 

  We investigated photochromic coating on transparent plastic lenses and evaluation of therapeutic application for patients with light-sensitive eyes and eyestrain. This coating system was based on acryls as binder, spiropyrans and spirooxazines as photochromic material. Photochromic coating with blue, brown, yellow, green and violet were prepared on transparent plastic lenses by flow coating and cured at 80-120℃. These photochromic lenses gave a coating having good mechanical properties such as adhesion, hardness, hot water and chemical resistance. A darkened transmittance of 70% and speed of photochromic reaction in the darkening/fading cycle of 10 minutes had been obtained. The UV cut-off point was brought close to 400nm. In clinical study of 65 subjects, photochromic lenses reduced light-sensitivity and eyestrain. Photochromic lenses associated with this coating system were considered to be used for eye health and comfort.

 

1. INTRODUCTION

 

   It is widely accepted that solar radiation, especially UV radiation, can harm the eyes. The purpose of sunglass is to provide protection from solar radiation, and comfort to the eyes under all conditions of illumination, that is, sunglasses are worn to secure protection against sun-related damage such as eyelid damage, pterygium, pinguecula, photokeratoconjunctivitis, cataract, and light-induced maculopathies [1]. In theory, this ideal is more nearly attainable by photochromic lens, which darken automatically in bright sunlight and lighten automatically when the exciting radiation is removed.

The first commercial photochromic lenses were made of glass lenses containing a silver halide photochromic system activated by UV radiation in 1964 by Corning. However, increasing market for plastic lenses has resulted in a demand for photochromic systems that are compatible with the matrices used in plastic lenses. The first commercial plastic photochromic lens was the Photolite^TM lens, introduced by American Optical in 1982 [2].

The photochromic behavior in plastics materials is quite different from that in glass.  Photochromic materials such as oxazines, pyrans, fulgides and dihydroindolizines [3] are added  to the plastic lens materials such as allyl diglycol carbonate (ADC), polycarbonate (PC) etc. When photochromic materials are exposed to UV or blue wavebands, a bond of molecules is cleaved to the colored form. Removal of the exciting wavelengths allows the molecules to bleach back to their original orientations, as is illustrated in Fig. 1 [4].

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Fig. 1. Photochromic transformation of spiropyrans (above) and spirooxazines (below).

 

The photochromic compounds are applied to a plastic lens by body casting (or in mass), imbibition and coating process. Plastic lenses are softer and should therefore be hard coated. Especially, the method of body casting and imbibition process for the photochromic effect are not applicable to soft and high index polymers because of the poor mechanical properties, and then additionally hard coating process is needed. In order to overcome disadvantages, it is a surface coating process that has a scratch-resistant coating with photochromic dyes [5].

 

The objective of this work is to develop photochromic coating on transparent plastic for absorbing short wavelength blue light that can reduce the visual function of light-sensitive eyes and to evaluate therapeutic applications for patients with photophobia.

 

2. EXPERIMENTAL

 

2.1. Materials and coating procedures

   Almost chemicals such as ethanol, isopropyl alcohol (IPA), acetone, methyl ethyl ketone (MEK) and other solvents were of reagent grade and used without further purification. Spiropyrans and spirooxazines were used as photochromic materials. Tetramethyl orthosilicate (TMOS) and 3-glycidopropyltrimethoxysilane (GPTMS) as inorganic-organic hybrid materials were used. Urethanes and Acryls (type A, B, C; noted as A, B and C pending patent application) were used as binders. Allyl digylcol carbonate (ADC, or CR-39) were used as transparent plastic lenses. The transparent plastic lenses were used after washing with ethanol and dried at 50℃ before coating.

 Fig. 2 shows processing for photochromic coating. Photochromic materials and solvents were mixed in 1.84-3.00 : 62.28-61.45 (total wt. % of coating solution) and were dissolved. Inorganic-organic materials (5.06%) were added to this solution. Binders (31.65%) were added and after stirring, the solution was applied to photochromic coating for lenses. The transparent plastic lenses were flow coating and cured at 80℃ for 30 minute and 120℃ for 90 minute.

 

Fig. 2. Processing for photochromic coating.

 

 

2.2. Mechanical property tests

The mechanical property tests chosen for photochromic coating lenses were as follows. Transmittance were measured with an UV-VIS spectrophotometer (TAD-2166 from KOTRIC; Korea Testing and Research Institute for Chemical Industry). The adhesion test was determined by means of ASTM D3359 (cross-cut). For assessment of hardness, pencil hardness test according to ASTM D3363 was performed [6,7]. Abrasion test was done by means of steel wool abrasion test. Hot water resistance was investigated by immersing in boiling water for period of 10 minutes. Chemical resistance was assessed by rubbing wet paper with acetone or ethanol against lens surface.

 

2.3. Clinical study methods

 Clinical study of photochromic lenses were applied to 65 patients who had light-sensitive eyes and eyestrain by ocular problems such as photophobia, epiphora and asthenopia or by nonspecific symptoms at Department of Ophthalmology, Soonchunhayang University Hospital. Sixty five subjects, aged from 10 to 65 years old (mean age 37 years), 34 males and 31 females, were included. The mean spherical equivalent power of their spectacles was -0.75±1.00D. After wearing photochromic lenses for one month clinical effects were evaluated by light-sensitivity and eyestrain. The criteria of light-sensitivity and eyestrain were divided into none (grade 0), mild (grade 1), moderate (grade 2), severe (grade 3) and very severe (grade 4) according to the depth of subjective symptoms, and were evaluated.

 

3. RESULTS AND DISCUSSION

 

3.1. Photochromic coating

The preliminary solubility tests of photochromic materials were done in methyl ethyl ketone (MEK), toluene, tetrahydrofuran (THF), methylene chloride (MC),  methanol, isopropyl alcohol and octanol. The solubility of these materials was under wt. 1% in alcoholic solvents, 5% in MEK and MC, 8% in toluene and THF. MEK was selected as coating solvent in view of mixing with binders. Urethanes and acryls (type A,B,C) were used as binders. Acrylic binders type C had been applied most successfully for providing transparent plastic lenses with a remarkably adhesion, hardness, abrasion resistance and chemical resistance coating which exhibited good photochromicity. However urethanes and acrylic binders type B, C could not be a good coating properties, especially type B had a shorter shelf life for storage. The comparison of mechanical properties of hard coating lenses according to acrylic binders is summarized in Table 1.

 

Table 1. Mechanical property of hard coating lenses

Acrylic binders              A              B                C

Adhesiona                100/100       100/100          100/100 Hardnessb                  H             4H                3H Hot water resistance      haze           pass              pass Chemical resistance       fail            pass             pass/failc Shelf lifed                30 days        3 days            60 days

 ^aCross-cut and tape test; detached square number/total square number

 ^bPencil hardness test; 1kg loading, 5 strokes

 ^cPass in acetone, ethanol and IPA, fail in acetone.

 ^dShelf life; storage at room temperature

 

  Photochromic coating including selected solvent and acrylic binder was formed on transparent plastic lenses. By the experimental preparation as described above, photochromic coating lenses had obtained. Fig. 3 shows five photochromic lenses with yellow, green, violet, brown and blue (from left to right).

 

Fig. 3. Photochromic lenses. Colorless state (above), colored state (below).

 

  Transmittance and UV blocking of these coatings were measured with an UV-VIS spectrophotometer. A darkened transmittance of 70% and   speed of photochromic reaction in the darkening/fading cycle of 10 minutes had been obtained. The UV cut-off point was brought close to 400nm, as shown in Fig. 4.

  In Table 2 some mechanical properties of photochromic coating lenses are summarized. The adhesion test using cross-cut and tape test showed good results that the edge of the cuts were smooth, none of the squares of the lattice was detached. Hardness was shown in 4-5H by pencil hardness test. Abrasion test using steel wool #0000 was performed 7 strokes and no visible defects were detected while viewing through the naked eye. The lenses passed hot water resistance and chemical resistance using acetone and ethanol. No visible defects were detected.

 

Fig. 4. UV cut-off point of photochromic coating lens.

 

Table 2. Mechanical property of photochromic coating lenses

Item                  Test condition                       Results

Adhesion             cross-cut and tape test, 5 times         100/100 Hardness             pencil hardness, 1kg load, 5 times       4-5H Abrasion              steel wool #0000, 1kg load, 7 strokes   pass Hot water resistance  10 minutes, boiling water               pass Chemical resistance   acetone, alcohols, rubbing, 100 times    pass

 

3.2. Therapeutic applications

 Light from the sun which reaches the earth’s surface depends on the absorption and scattering characteristics of the earth’s atmosphere. The short waveband UVC (100-280nm) is absorbed by the earth’s atmosphere, UVB (280-315nm) is absorbed at the cornea but some UV band (295-380nm) can penetrate reach the retina, especially  UVA (315-380) penetrates deeply into the eyes so that crystalline lens and retinal damage can be caused by excessive exposure [3]. Sunglass filters is to protect the human eye against excessive solar radiation so as to reduce eyestrain and increase visual perception [8]. Photochromics as ideal sunglass filters are one of the most effective methods of ensuring patient eye health and comfort. Photochromic lenses absorb UV and change density depending on outdoor conditions when UV is present, and lenses are clear indoors.

 In this work, effects of photochromic coating lenses in light-sensitive eyes and eyestrain have been evaluated. As can be seen from Fig. 5 and 6, total subjective symptom improved significantly (p<0.05, t-test, SPSS for Window). This study demonstrated photochromic coating lens reduced light-sensitivity (photophobia) and eyestrain (asthenopia). The development of these photochromic coating technology will be opened promising prospects for therapeutic applications.

 

Fig. 5. Results of light-sensitivity symptoms after wearing for one month.

 

Fig. 6. Results of eyestrain symptoms after wearing for one month.

 

4. CONCLUSIONS

 

  The first commercial photochromic lenses were made of glass lenses impregnated with inorganic salts (silver halide), but in recent years organic photochromic lenses  have made an important breakthrough in the world market. Surface coating technique was chosen for the development of photochromic coatings with a great variety of colors for transparent plastic lenses. The photochromic coating lenses exhibited 70% light transmittance and darkening/fading time of color change was within 10 min.  Various color lenses were developed such as blue, green, brown, violet and yellow. Lenses had an excellent physical properties, 100% adhesion and 4-5H hardness. Clinical trials of developed lens were applied to 65 patients who had light-sensitive eyes (photophobia) by various reasons at the ophthalmology of Soonchunhyang University Hospital. Results showed that glaring was significantly reduced and that the visual health was remarkably improved. Protection of UV-B and blue light could prevent the patient from cataract and the yellowish crystalline lens at old ages. Through the result of clinical trial, photochromic lens could be a new technique for both cure and precaution of photophobia.

 

Acknowledgement

 We would like to thank Joon-Soon Kim, M.D. and Moon-Sik Cho, M.D. at Department of Ophthalmology of Soonchunhyang University Hospital for assisting in the clinical study.

 

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