The ophthalmic lenses Essay

1. Introduction

Ophthalmic lenses are coated for assortment of grounds. For illustration, ophthalmic lenses have to defy scrape and clash. There are besides some applications necessitating particular optical belongingss such as UV- resistant and better transmission. Other demands include the ability to be easy cleaned and to accommodate assortment of environing. None of these demands is able to be attained utilizing individual lenses without surfacing.

Surface analysis techniques such as Auger electron spectrum ( AES ) , X-ray photoelectron spectrometry ( XPS ) , and Secondary Ion Mass Spectrum ( SIMS ) are rather suited for characterizing thin solid movies such as lenses surfacing. First, they are sensitive within a thickness of several monolayers to 10s of nanometers, which is mensurable to the graduated table of lenses surfacing. Second, assortment of information about solid movies such as the thickness, chemical province and composing are provided by these techniques. Furthermore, they are besides rather accurate and well-established techniques.

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The consequence of surface analysis is of great significance to the development of proficient processing. This instance analyze give a series of research about the lenses coating, taking at demoing how assortment of surface analysis techniques are used to develop and better the processing of the coating. These illustrations besides help to understand the constellation of surfacing system, and presently methods used for processing.

2. The lenses coatings

2.1. Glass and polymeric lenses

Conventionally, the ophthalmic lenses are made of silicate glass because of their particular optional belongings. However, there are some apparent disadvantages. They are excessively brickle. Many people like me one time had the memory to interrupt an eyewear glass accidentally. The clip and money ingestion for lenses fabrication is besides singular, which is non favoured for the enterprisers who purchase high productiveness.

These jobs encountered by glass ophthalmic lenses are overwhelmed by polymeric lenses. In China, more than 60 per centum of market is occupied by polymeric lenses, and the market fraction is increasing [ 1 ] . However, there are several disadvantages for the application of plastics. For illustration, the composing of plastic lenses varies from one batch to another. The thermic enlargement coefficient for plastic is besides much big, intending plastic lenses might wrap or distorted during treating affecting heat. The lenses are excessively soft, and is rather vulnerable to mechanical amendss.

The most normally used thermoplastic for oculus wear applications is thermosets such CR39 or allyl diglycol carbonate. There is little fraction of lenses made utilizing thermoplastics, for illustration, PMMA and polycarbonate. In comparing with thermosets, those thermoplastic stuffs are much less immune to heat.

2.2. multilayer coating

The coating for ophthalmic lenses has a multi-layer constellation. For glass substrate which performs high abrasion opposition, the coating system is an antireflective bed protected by top bed. The antireflective belongings of the lenses is improved by the coating bed. For polymeric lenses substrates, the coating is composed of an a difficult bed, an antireflective bed, and a top bed.

A typical multilayer constellation for the coating system on plastic glass wear is shown in the Figure 2.1. It can be found that antireflective bed besides involves more than two beds, which is favoured for cut downing the brooding loss of visible radiation in lenses. A individual bed decrease the brooding loss from 4.2 % to 1.5 % , while a dual bed decrease the loss somewhat more than that. Although there is important lessening of brooding loss in dual bed system at mediate wavelength. However, the cut downing consequence of a dual bed system is mostly limited within narrow wavelength set scope, intending a coloring material displacement for the range of oculus glass wearer. In contrast to individual and dual beds systems, a multilayer system decrease the brooding loss from 4.2 % to less than 0.5 % in much broader wavelength scope. This means a much better betterment for the public presentation of the lenses.

A important job encountered by the ophthalmic lenses surfacing is the internal emphasis between the beds due to multilayer constellation. The internal emphasis tends to originate cleft at the difficult bed which is of highest crispness. This job is mostly solved by utilizing a two beds of different mechanical belongings alternatively of a individual difficult 1. In the past an adhesive bed of Cr2O3 was used between the difficult bed and substrate. Yet it has been integrated into the processing of difficult beds, which is traveling to be introduced in ulterior session.

3. The processing of multilayer coating

3.1. The preconditioning of adhesion

In order to obtain good adhesion, the surfaces of substrates are usually pre-treated utilizing plasma or ions. A plasma is formed by bear downing gases by AC resources in a low force per unit area chamber filled with Ar and O atmosphere. The electromotive force and force per unit area should be controlled. More item about plasma would be given in ulterior session about plasma polymerization. Normally a combination of Ar and O are used to bring forth plasma. It has been proved that a plasma utilizing blend gasses is more effectual than that of individual composing [ 5 ] . For CVD and PVD procedure, the application of plasma and ions are integrated into the coating phase of the procedure, which is called plasma enhanced CVD ( PECVD ) or PEPVD.

3.2. Difficult beds

One important disadvantage of thermoplastic ophthalmic lenses, deficiency of abrasion opposition, is mostly overcome by the coating of difficult beds. Silicon oxide is an thought stuff as difficult coating because of its high strength covalency bond and hardness. Therefore, assortment of attempts has been made in order to do silicate atoms embedded into the substrate matrix to organize the coating movie, including the chemical vapor deposition ( CVD ) , sol-gel procedure, and plasma polymerization.

3.2.1. Processing method

Chemical Vaporous Disposition ( CVD )

The most mentionable procedure used to doing difficult bed in ophthalmic lenses surfacing is chemical vapour deposition, in which the coating bed is formed through bluess reaction of precursors incorporating Si. The most apparent advantage of coating obtained through this method is the accurate control to the composing of the bed.

In order to halt cleft extension and to diminish internal emphasis, two or three beds of different mechanical belongingss should be processed by commanding the composing of precusor. For illustration, in the work by Martin-Palma and colleagues, the SiOxCy: H coating was formed utilizing vapours reaction between O and hexamethyldisiloxane ( HMDSO ) as precursor. Through commanding the ratio between HMDSO and O, adhesion bed, flexible difficult bed and hardening bed are formed outside the substrate by commanding the O content in the plasma, and therefore comparative content to the precursor at different degree. By the same item, utilizing plasma assisted chemical diaphanous temperament, Kreissig et Al besides produced difficult coating of multilayer construction on the surface of polymeric substrate.

Liquid stage processing

Another technique worth mentioning is the liquid stage deposition processing, which is based on the reaction of liquid solution. One disposed exemplifying illustration involve the research by Hozumi, Kato and Takai who produced two layer difficult coatings on PMMA and polycarbonate substrates through bring arounding liquid movie obtained through spin dipping. The beneath bed is cured by ultraviolet radiation, while the upper bed is cured by heat, and the linear might be added into the solution to doing beneath bed. Coating movie formed during the reaction of the liquid movie after dunking. The function of the solution allows to be concluded as the precursor for the Si oxide movie.

One disadvantage which limits the application of liquid stage temperament is its demand for the type of polymeric substrate. For polymer types of high mutual opposition such as PMMA and polycarbonate, which are mostly used in ophthalmic lenses, the substrate has to be coated in progress, or certain types of pre-conditioning is needed. This means excess procedure to be carried out in the fabrication of the ophthalmic lenses, which is non favoured by the enterprisers.

Plasma polymerization

Plasma polymerization is a possible technique which can be used for the bring forthing of difficult bed on ophthalmic lenses. The rule of the plasma polymerization and its application in ophthalmic lenses surfacing has been mostly introduced by Wohlrab and Hofer [ 9 ] . In this procedure, the gas induced in a low force per unit area chamber was ignited to organize plasma in which gas molecules decompose into positive ions and negatrons. Since the negatrons have much smaller chance to be scattered, the energy of negatrons can be so high that they are able to originate the polymerization of monomers at room temperature is applied. Such reaction of appropriated monomer would organize a coating bed which is a combination of SiOx and polymer. In comparing with liquid processing, plasma polymerization has several advantages, such as less toxic chemicals involved and more easy multilayer surfacing. Using plasma polymerization, it is possible to extinguish some unsought optical belongingss.

3.3. he antireflective bed ( AR )

3.3.1. Function of anti-reflective bed

Anti-reflective bed benefit clients in many facets. For illustration, after the intervention of light conveying through multilayer coating, the refraction index of the lenses is more similar to the medium. Drivers would acquire better vision in the dark with the aid of antireflective bed, and antireflective bed aid to heighten contrast sensitiveness and ocular public presentation. The antireflective bed has parts far more than optical public presentation. The deposited Ti oxide atoms lead to self-cleaning ability of the lenses [ 10 ] . Many lenses coated by individual antireflective bed have certain grade of ultraviolet opposition [ 11 ] . The composing of antireflective surfacing involves a assortment of metallic oxides such as TiO2, ZrO2, Hf2O3, Al2O3 of different refractile index in laminate construction. Mg2F is besides used in the antireflective coating for glass substrate [ 1 ] . The design rule for antireflective bed was reviewed by Schallenberg.

3.3.2. The processing of antireflective bed

Antireflective coating is deposited on either polymeric or glass lenses substrate utilizing physical vapor deposition ( PVD ) method, in which a stoichiometric composing of the coating is obtained. In this processing, the liquefied metallic oxide evaporates quickly in high vacuity, and so is sputtered to the surface of the polymeric or glass substrate at high rate. However, one large job for such procedure is the decomposition of the oxide during sputtering. The O in some oxide tends to go forth the molecule under high vacuity, ensuing in non-stoichiometric composing of the coating. Therefore, a low force per unit area of O is normally used to counterbalance oxygen loss during the procedure. Similar to the processing of difficult coating, plasmas or UV irradiation are employed for preconditioning of the substrate, taking at good adhesion of the coating bed. For polymeric substrate, the application of ion has more significance. The aid of ion helps to avoid high temperature of the substrate. More item about instruments and preconditioning has been mostly reviewed by Pulker.

There are some other methods to lodge Ti oxide antireflective surfacing upon on polycarbonate substrates, such as sol-gel. In the procedure of sol-gel, a colloidal suspension of Ti or Si contained precursor organizing a movie on pre-treated surface of substrate, and loss H2O during the hardening of the movie, bring forthing solid movie which is composed of metallic oxide. Relevant illustrations involve the work reported by Yaghoubi, Taghavinia and Alamdari [ 10 ] . There is besides research utilizing chemical vapor deposition to bring forth antireflective bed.

3.4. Top coating

A bed of hydrophobic top coating is besides deposited upon the substrate. The chief ground to make so is to heighten the ability to be easy cleaned, and to protect the substrate from abrasion to some extent. Top coating is processed through vaporization or wet chemical method, and chemical vapor deposition is besides used. Nitrogen are sometimes added in order to let go of residuary emphasis in the coating [ 15 ] .

4. Application of surface analysis techniques

4.1. Compositional word picture

There is a batch research in which elastic kick sensing analysis ( ERDA ) is used for compositional analysis. For illustration, in the research cited in anterior session by Martin-Palma and colleagues, they used RF-PECVD method to bring forth difficult surfacing upon polymeric substrate, and ERDA was used for compositional analysis of the coating. The composing consequence shows that there are at there are disparage chemical province of C when high and low RF power is used. When high power is used, the C is wholly dissociated from the -CH3 group. Similarly, the compositional analysis consequence as a map of gas force per unit area shows that higher force per unit area lead to diminish sum of C and addition sum of O in movie. The content of Si is independent with the content of C in plasma [ 4 ] . The compositional information aid to command the processing parametric quantity of coatings.

Another disposed illustration demoing the application of ERDA involves the deepness profile of compositional analysis. In this survey, the deepness profile of heavy ion ERDA was carried out on a three beds surfacing with gradient of silicon oxide content. It was indicated by the deepness profile that a thin bed organizing next to the substrate, in which the content of C and H is reasonably high, is extremely polymeric feature. The polymeric composing of the adhesive bed means that silicon oxide atoms adhered, and entangled the molecular ironss of substrates. The consequence obtained utilizing heavy ion ERDA provided convince account for the adhesive mechanism.

In amount, it can be found that surface techniques such as elastic kick sensing analysis are rather utile technique for the development of surfacing method, because the information aid to explicate the development during processing, and to optimise the processing variables.

4.2. Determination of refined construction

Although there are some microscopy techniques like atomic force microscope ( AFM ) able to demo the raggedness and morphology on the surface, intending that the alteration raggedness become visiible, surface analysis techniques show their advantages in some facets such as refined construction of the coating. These techniques include Rutherford Backscattered Spectrometry ( RBS ) , neutron coefficient of reflection and sum-frequency coevals ( SFG ) .

For illustration, in the work by Batagllin et Al, individual TiO2 beds on glass substrate were formed through reactive DC magnetron sputtering in Ar and O, and several samples of multilayer coating of TiO2//SiO2 were obtained through industrial temperament. Rutherford Back Scattering was used to bespeak the thickness of each coating and the consequence is expressed in footings of the sum of monolayer within the coating. Another surface analysis technique, neutron coefficient of reflection, was used to polish the coating construction for complex coating system. Using this technique the thickness and raggedness of each coating were got, and the consequence is correlated to the optical belongings of the coating [ 16 ] . The refined construction of surfacing beds lead to new manner to measure different processing method.

Another technique in which the coating constellation and refractile index are characterised at the same time is spectroscopic ellipsometry. The research by Latella et al justify disposed illustration. Spectroscopic ellipsometry were used in combination with contact angle measuring and XPS. The all right construction of the coating is sometimes of import for the development of surfacing engineering. In this instance, the surface of as-received sample was treated utilizing H2O plasma. After preconditioning, the contact angle of the sample surface have changed a batch, which is reflected in XPS spectrum form demoing there was more C-OH bond. The raggedness of the surface, however are still comparable, taking to the decision that enhanced adhesion derived from chemical development instead than mechanical mechanism.

In the research by Miyamae and Nozoye, a coating of Si oxide is deposited upon PMMA substrate utilizing electron beam beginning. The surface morphology experience really negligible alteration after surfacing of Si oxide, as is indicated by atomic force microscope. Using surface analysis engineering of sum-frequency coevals ( SFG ) , the alteration of molecular stereotype constellation during silicon oxide coating is the investigated. The comparing between the sps and ppp signal in C=O and C-H part indicates the molecular Reconstruction on the surface. The ester methyl pendent groups tend to put analogue to the surface after surfacing.

The instances given above might demo the fluctuation of structural development might be discernable merely when techniques every bit delicate as SFG and RBS are employed. Therefore, in order to understand the refined construction of surfacing beds, assortment of surface analysis techniques are about the lone picks.

4.3. Solution for possible job

In add-on to movie word picture, which is of significance for the development of the processing path, surface analysis techniques are besides used work out possible jobs for lenses surfacing.

In the research by Kuhr et Al, it was reported that both antireflection and difficult bed are formed on PMMA substrate. Since the silicon-contained organic compounds are ever added during the injection modeling procedure, the PMMA substrate was cleaned warm acidic bath with the aid of supersonics. Secondary ion mass spectrum ( SIMS ) was used to place residuary Si oil on the surface, demoing that the Si oil has been mostly eliminated [ 19 ] . The consequence of SIMS proves that the rinse to PMMA is a utile method to acquire rid of the taint. This is of significance for the application of PICVD pattern, since injection molding is the most promising method to bring forth articles whose dimension is accurately required, and many additives used during injection modeling affect the coating adhesion negatively. The research imply possible solution for the job due to injection molding of polymeric lenses, which help to better the public presentation of surfacing on plastic lenses.

By the same item, the consequence of O plasma on injection moulded Bisphenol A polycarbonate ( BPA-PC ) home base was besides investigated using a combination of techniques including SIMS, XPS and ellipsometry. Using a combination of XPS and ellipsometry, the comparative content of C and O in samples tend to stay changeless after a threshold treating clip, which is the optimum treating clip in patterns. Using chief constituent analysis ( PCA ) manner of SIMS, it become clear that aromatic construction within polymer molecules is extremely vulnerable, which is the chief ground for its debasement when excessively long clip for hardening was used. The formation of some little molecular weight stuff besides occurred at the surface bed of the stuff during the bring arounding [ 20 ] . The consequence suggests that inordinate pre-treatment might be counterproductive in footings of adhesive sweetening.

The hazard for polymer debasement was besides proved by consequence of Electron spectrometry for chemical analysis ( ESCA ) , a technique rather sensitive to depth less than 20. Using ESCA, it was found that during ion barrage to CR39, the O contained groups was destroyed, while freshly formed C groups contain no O. Such consequence is responsible to the worsened adhesive ability when the remedy clip is long. The deepness profile of AES reveals the formation of blending bed between the coating and the substrate, which is the adhesive mechanism.

4.4. Surface analysis for adhesive mechanism

Although XPS consequence provides information about composing, in more fortunes the XPS is used to observe the alteration of chemical province, which indicate underlying mechanism during processing. This is besides of import to find the optimum processing variables.

4.4.1. Difficult beds

In the processing of difficult bed through CVD, it is possible to utilize ammonium hydroxide and other nitrogen-contained compounds as precursor to alleviate the internal emphasis brought by multilayer constellation. Both XPS and XAES are carried out on samples made utilizing different fraction of ammonium hydroxide to find the chemical province of each component. It was found that the peak place and the FWHM did non alter, while the place and FWHM of Si 2p and Si KLL extremums changed. These phenomena are attributed to the formation of many silicon tetrahedrical bonds, which is verified by many the alteration of O 1s characteristic form [ 22 ] . The surface analysis consequence aid to understand implicit in doctrine during procedure.

Liquid stage deposition for Hard bed coating is besides improved by XPS consequence. In the research by Hazumi, Kato and Takai, dual bed of the coating are obtained through liquid stage deposition. The XPS consequence shows that O contained in the additives was present on interfaces between two beds, which is the ground for enhanced inter-coating adhesion. The apprehension for the development during procedure make it clear that it is possible to utilize similar method to better public presentation of solid movie fabricated through liquid stage deposition.

There is some other research utilizing XPS to explicate adhesion of movies obtained from liquid stage deposition. For illustration, in order to acquire good adhesion of the surfacing upon polymeric substrate of high mutual opposition in liquid solution deposition, one method of pre-conditioning is surface oxidation of the substrate. Using X-ray photoelectron spectrum upon ZEONNOR substrate, it was found the surface oxidization arise increasing sum of OH- groups, which is the ground for fabulous adhesive.

4.4.2. AR bed

The research carried out by Battiston and colleagues further illustrate the application of XPS. Antireflective bed of Ti oxide was coated upon silicate glass through PE-CVD procedure. Titanium tetraisopropoxide was used as precursor, and blend of O and N was used to bring forth plasma. The substrate was cleaned in progress, and etched utilizing plasma for preconditioning. A assortment of treating parametric quantities are used, such as the substrate temperature, power and ambiance for plasma. The XPS consequence shows that with increasing processing temperature, there is more nitrogen contained in the coating. In this instance, the XPS consequences indicate non merely the alteration of chemical province, but besides the alteration of composing. The consequence shows the significance to command the ambiance for pre-treatment.

Other method such as ion beam might besides be used to pre-treat substrate surface. In the research by Lee, Chob, Kohb, and Kimathe, polycarbonate sample was subjected to Ar ion irradiation utilizing a cold-cathode ion beginning in vacuity for preconditioning, followed by deposition of SiO2/TiO2 multilayer coating. It was shown that during ion conditioning the reaching angle of the sample become smaller, and higher ion drowse consequence in smaller contacting angle, which is the grounds for more effectual pre-conditioning. Such phenomenon is explained by XPS consequence on treated and untreated polycarbonate substrate surface, .showing addition sum of C=O bond after intervention.

4.4.3. Drumhead: the significance of mechanism

In session 4.4, a batch of research relevant to mechanism during procedure in introduced. The research focuses on betterment to adhesion of surfacing upon substrate during procedure, stoping up with description to possible mechanism of development. The decision of the research can be concluded to be that an inducement of O contained group on interface would beef up the adhesion, and any possible method would bring on O to the surface would be favoured. These decisions help to develop new method of adhesion sweetening.

5. Decision

a. How is surfacing system deposited?

In order to obtain good surface adhesion, the substrate are rinsed for cleaning first, and plasma or ion beams are employed for pre-treatment.

The chief processing method of each surfacing bed is tabulated in Table 5.1.

Layer

Processing Method

Referee

Composition

Top bed

Vaporization,

Nitrogen induced polymer

CVD

[ 19 ]

Anti-reflective bed

PVD

[ 14 ] [ 16 ]

Al2O3, HfO2, ZrO2, TiO2, MgF2 ( for glass ) , SiO2 ( for polymer )

Sol-gel

[ 11 ]

Hard bed

CVD

[ 5 ] [ 6 ] [ 10 ]

SiO2 embedded in polymer

Liquid stage deposition

[ 8 ]

Plasma polymerization

[ 9 ]

Table 1. The chief processing method and composing for each coating bed

B. How does development of the technique benefit from surface analysis technique

Through qualifying solid movies or treated substrates prepared in research lab, applied scientists are able to measure the influence of treating parametric quantities toward the sample, and to understand the alteration occurred during a procedure, in the promise that the experimental method is of certain similarity with the existent status of industrial pattern. The mechanisms during each procedure, which is suggested by surface analysis techniques, are besides of significance for the technique development, . It is possible to develop new method to heighten public presentation based on mechanism revealed by surface analysis consequence. Furthermore, the surface analysis consequence suggest possible job during procedure.

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