Characterization of Coated Ophthalmic Lenses Introduction: Anti Resistant Coating (ARC) is a popular method of coating conducted on surface of spectacle lenses in order to prevent reflection of superfluous light on the surface of lenses and to protect the eyes of wearer against harmful effects of UV rays. This not only improves visibility but also provides better aesthetic appeal for the eye glasses. Disabilities of plastic lenses: With plastic lenses now being very much in vogue in place of traditional glass lenses, ARC has assumed additional significance and importance.
The main issue with plastic lenses is that it is made of soft, pliable material, is not conducive to water treatment, is not always scratch resistant, and more importantly, cannot withstand the high temperature pre-treatment needed to apply the surface coating on it. Thus, ways and means have been experimented on how to achieve surface coating on ophthalmic lenses without losing adhesion and inherent properties of the lenses. We would first need to consider the Plasma Polymerization Deposit Process (PPDR). “Moreover, the plasma polymerization process allows deposition of optical films at room temperature, essential for plastics.
The energetic ions in plasma processes provide similar effects as in ion assisted physical deposition processes to produce hard coatings, without requiring sophisticated ion sources. ” (Srivatsa, Bera, Basu, & Bhattacharya, 2008). Plasma Oxidization process: It has been found in empirical tests that the strength of bonding is reinforced through use of plasma. “Plasma oxidation is likely to result in stronger ionic bonding between polymer and coating resulting in greater durability against abrasion. ” (Surface analysis of medical devices, 2009). Secondary Ion Mass Spectrometry: The use of Secondary Ion Mass Spectrometry has caused a lot of benefits for the cause of surface analysis.
“During SIMS depth profiling, primary ions used to sputter the surface become implanted in the target. As the surface recedes these are exposed and themselves sputtered, whilst more ions are implanted. At some time after bombardment begins the mixture of target and primary atoms in the surface reaches an equilibrium concentration, and has a well-defined thickness. ” (Ultra low energy Secondary Ion Mass Spectrometry, n.d. ). X-Ray photo –Electron Spectroscopy (XPS): Coming to X-Ray photo–Electron Spectroscopy (XPS), it “provides quantitative compositional information from the top 10 atomic layers of a sample surface for the elements lithium to uranium.
Furthermore, information regarding the chemical states of any elements present can also be obtained. ” (X-Ray Photoelectron Spectroscopy, n.d. ). Some of the main characteristics of coated ophthalmic lenses are that it becomes scratch resistant and impervious to light and harmful UV rays. Besides, it could clean dirt, dust and grease substances which may stick on to its surface more easily than conventional glasses. However, the procedure is trickier in the case of plastic lenses since these lenses are much softer, less heat resistant and is impervious to water, or is hydrophobic.
Thus, the challenge for ARC is higher in the case of plastics than any other surface applications. ARC coating on plastic surfaces: However, there have been experiments conducted that have provided successful results of ARC coating on plastics, which is explained below. A procedure has been locally carried out, using domestically designed and modelled deposition system for coating tough, long lasting ARC on polycarbonate substances. Firstly, the surface is wetted and then a layer of surface is hardened with passing of Carbonyl Silica.
Finally, a deposit of surface handling four layered anti reflexion coating, shifting between layers of silicon-dioxide and titanium dioxide of predetermined thickness is passed. It has been observed that the bonding meets all parameters of strength and toughness needed for ophthalmic purposes. Which surface analysis technique could be used? : Coming to the next aspect of which surface analysis technique could be used for determining the composition and thickness of layers present in the coating, it could be said that the X-Ray photo –Electron Spectroscopy (XPS) model could be used.
The following table is suggestive of the various kinds of uses the XPS is capable of performing. (X-Ray Photoelectron Spectroscopy, n.d. ). Which surface preparation could yield good results? : The surface preparation that may be carried out on lenses for getting a well adhered coating could be an initial coating of Carbonyl Silica and finally, deposit of surface handling four layered anti reflexion coating, shifting between layers of silicon-dioxide and titanium dioxide of predetermined width. It is seen that by adopting this procedure the precondition with regard to bonding of the coated materials, tensile strength, light and heat resistance, scratchproof qualities, etc. , all could be compounded.
Moreover, it is also seen that these toughened coated surfaces are more long lasting with greater performance value when compared to ordinary non-coated glasses. The use of secondary ion mass spectrometry (SIMS) emission microscope could provide extremely sensitive surface analysis, high-resolution imaging, and chemical analysis of the ingredients in the surfaces of many items. There are reasons to believe that SIMS could play an increasingly important role especially in the field of high precision optical field, among others.
Coming to XPS, it is believed that through the charging of ions, the surface materials could be spluttered away and detailed technical information could be made available regarding the lower levels of any materials. Thus, in the case of ophthalmic coating, these instruments could be able to analyse its composition and other features with a remarkable degree of accuracy and estimation, which may not perhaps be possible under any other kind of technical or scientific analysis. Besides the main advantages that this surface analysis could render could be in determining the coating consistency and the determination of integrity of the applied chemical coatings, evaluation and analysis of impurities, if any, and reporting on the consistency and strength of the bonded coating with reference to its technical properties, etc.
Conclusions: Thus, it could be ascertained that while the challenge of coating plastic lenses is indeed daunting, modern technology with the use of highly sophisticated and precision tools and instruments like XPS and SIMS are very much in place to accurately determine its chemical consistency and bonding and to carry out the processes with a great deal of efficacy and successful results.
However, it must also be seen that the appropriateness of these analyses is also of importance, in that where a more basic and fundamental analysis is sufficient to achieve desired results; it would not be worthwhile to go for unwanted highly sophisticated experiments and analyses. Reference List Srivatsa, K.M. K., Bera, M., Basu, A. & Bhattacharya, T.K. , 2008. Antireflection coatings on plastics deposited by plasma polymerization process: abstract. Bulletin of Material Science, [Online] 19 Oct. , 31 (4), pp. 673-680.
Springer Link. Available at: http: //www. springerlink. com/content/y53l38735k04w863/ [Accessed 10 December 2009]. Surface analysis of medical devices using Secondary Ion Mass Spectrometry (SIMS) and X-Ray Photoelectron Spectroscopy (XPS)-supplier data by CSMA Ltd. , 2009. AZom. com: The A to Z of Materials. [Online] Available at: http: //www. azom. com/details. asp? ArticleID=2664 [Accessed 10 December 2009]. Ultra low energy Secondary Ion Mass Spectrometry, n.d. CERAM: Surface and Materials Analysis. [Online] Available at: http: //www. csma. ltd. uk/techniques/ule-sims. htm [Accessed 10 December 2009]. X-Ray Photoelectron Spectroscopy, n.d. CERAM: Surface and Materials Analysis. [Online] Available at: http: //www. csma. ltd. uk/techniques/xray_photoelectron. htm [Accessed 10 December 2009].