Materials for occupational eye protectors
Dr Stephen J Dain, Optics & Radiometry Laboratory, School of Optometry and Vision Science, University of New South Wales, Sydney NSW 2052, AUSTRALIA, E-mail: email@example.com
The selection of lens materials for non-prescription personal protective equipment has been a relatively simple process and has its origins in many studies around the 1970s. The viable materials available at that time were tempered glass, hard resin (n = 1.50) and polycarbonate. The modern spectacle non-prescription eye protector of choice is inevitably hard coated polycarbonate, which has exemplary impact resistant properties. In the prescription lens area, there is a bewildering array of materials of various refractive indices with a variety of coatings. The selection of an ophthalmic lens has optical and cosmetic considerations ahead of impact resistance. In complying with the Australian/New Zealand standard on prescription eye protection, adequate impact resistance must rate as the foremost requirement, with optical and cosmetic considerations as important but lesser considerations. In this review, the evidence on impact resistance of the available materials is presented, the standards set for testing impact resistance are detailed and some guidance is provided for the selection of prescription eye protection materials.
Eye injuries that result in attendance at hospital have been extensively studied around the world.1–5 Mechanical agents are consistently shown to be the main hazard. Injuries not requiring hospital attendance are rarely studied and the necessary protection to avoid the chronic effects of exposure to, for instance, chemicals and radiation are rarely included. As a consequence, the total incidence of eye- and face-related accidents is not known. Work and the home are both implicated as locations of eye hazards but their relative contribution varies between studies, which might be due to the geographical location of the study site and the demography of the public accessing the care. For the most part, eye injuries are avoidable. The proportion of avoidable eye injuries (where appropriate eye protection was not worn correctly) seems little changed with country or time from 88 per cent in 19726 to 78 per cent in 2001.4 It is predominantly a male problem; however, this cannot be accounted for fully by the higher occupational exposure and might also be related to eye protection wearing habits.7 The wearing of eye protection reduces injuries.8–12 Spectacles that are not eye protectors might fail to prevent eye injury and might become a secondary hazard to the eye.13–16 Studies consistently show that mechanical damage to the eye from flying objects is the predominant injury, typically representing over 70 per cent of all cases. The evidence is divided on whether the impacting object is large and slow moving or small and fast moving but the tendency seems to be that the former are the predominant hazards in the home and the latter are predominant in the workplace.
This paper will address the issues of selection of lens materials for protection against mechanical hazards in the workplace, which is the issue highlighted by the need to comply with AS/NZS1337.6.17 Not only is this the highest risk but also it is the most difficult to address satisfactorily. In the search for eye protectors that also correct ametropia, it is often forgotten that it is neither necessary nor mandatory to combine the two functions in a single lens. The issue of other means of achieving optical correction and eye protection will be addressed.
Protection against chemical, thermal and similar hazards can be achieved with quite simple barriers. Protection against optical radiation other than solar is normally achieved by the use of two elements in eye protection, low or medium impact spectacles to provide protection against mechanical hazards and a second in the form of, for instance, a welding hood or helmet. Solvent resistance might be an issue in some environments and will be addressed.
Sports eye protection is also an issue and there are a number of standards worldwide. This merits consideration in its own right and will not be addressed here.
Eye protection against lasers is also a topic outside the scope of this paper. The choice of material is totally dictated by the protective needs. The filter must not only absorb the energy before it reaches the eye but must also dissipate that energy in a way that it continues to provide protection, that is, the laser beam must neither fracture the filter nor burn a hole in it. This is an application where the use of glass is still evident.
THE NEED FOR OCCUPATIONAL EYE PROTECTION
Until relatively recently, the industry supplying non-prescription personal eye and face protective equipment and the prescription spectacle industry lived in essentially separate universes.
The eye and face protection industries have been accustomed to having standards enforced and standards certification is almost universal. Occupational health and safety laws in each of the Australian states and many countries contain similar requirements for an employer to provide employees with ‘appropriate’ personal protective equipment, for instance, Clause 15 (1)(a) of the NSW Occupational Health and Safety Regulations, as follows:18
15. Provision by an employer of personal protective equipment
(1) If measures taken by an employer under clause 11 (2) to control a risk include the use of personal protective equipment, the employer must provide each person at risk with personal protective equipment and ensure that:
(a) The equipment provided is appropriate for the person and controls the risk for that person, and
Compliance with the appropriate product standard by the presence of a standards mark is the easiest option for the employer by which to demonstrate appropriateness. It remains for them to show that the category of eye protector selected is appropriate to the risk and reference to the appropriate recommended practice for selection is also necessary. In the case of eye and face protection this is AS/NZS1336.19
There has been a non-prescription eye protection standard since 195120 and the current version was issued in 2010.21
The prescription ophthalmic lens industry has not been subject to the same legislated imperative to comply with a product standard. Any application of the standard to ophthalmic lenses has been self-imposed by the ophthalmic manufacturing industry, invoked by some optical dispensers and optometrists or ignored.
The first Australian spectacle lens standard dates back to 196922 and this eventually became a two-part standard that also addressed spectacle frames.23,24 In 2011, Part 1 (Spectacle lenses) was superseded by the adoption of several associated International Organization for Standardization (ISO) standards as Australian/New Zealand standards (AS/NZS), most notably the standard on mounted spectacle lenses.25
The eye and face protection industry is at the stage where, judging by the eye and face protection submitted to the Optics & Radiometry Laboratory, the only material that is ever used for spectacle-like eye protectors is hard-coated polycarbonate. Tri-acetate is still used in some goggles, eye shields and face shields, when solvents that attack polycarbonate (mainly hydrocarbons) are in the workplace. Tri-acetate does not match the impact resistant properties of polycarbonate.26 For this industry, the need to provide effective impact resistance in a low cost product inevitably means polycarbonate.
ASSESSING PROTECTION NEEDS
Occupational health and safety law18 and the recommended practices for personal protective equipment19 generally set down a hierarchy of action in response to the identification of a risk. This requires an understanding of the hazard and the risk. The hierarchy sets personal eye protection as the last resort, preceding it with measures that are not subject to non-compliance by the worker by failing to wear the eye protection.
In AS/NZS practice19 this means:
- 1elimination of eye hazards and hazardous practices by substitution with non-hazardous materials, practices and processes (Section 2)
- 2control and minimisation of the eye hazards by isolating and confining them (Section 3).
Only then is personal eye protection addressed (Section 4). Table 4.1 in the Standard19 links the hazardous processes with the hazard that they comprise. The typical methods of control are then given along with the suitable eye protector type. In advising the appropriate eye protector type against flying fragments and particles, there are three categories: low, medium and high impact. Other standards use similar categories. It is the systematic examination and categorisation of the hazard that is important rather than the actual names of the categories. In most standards, the highest category (in this example, high impact), which represents a hazard to the face as well as the eyes, is reserved for face shields. For the middle category (in this example, medium impact), eye shields, goggles and wide-vision spectacles are added. Wide-vision spectacles have larger dimension requirements and are required to have protection against lateral impact. If this is in the form of side shields, then they must be permanently attached. Low impact eye protectors have smaller dimension limits and side shields are recommended, where the hazard is not limited to frontal impact only. There are hazards other than impact but these are not being addressed here.
Protection is then provided by the appropriate grade of impact resistance. The recommended practice standard19 is yet to catch up with the adoption in the product standard21 of the optional use of the letter ‘S’ to indicate low impact, as in European practice.27 The European designations are not the same as AS/NZS but the compliance requirements are. European practice also has a lower category of impact protection. The letters ‘I’ or ‘F’ indicate AS/NZS medium impact. ‘I’ has been in AS/NZS use for decades; ‘F’ is the European marking and has been added to maintain the technical equivalence with international standards or de facto international standards that Standards Australia policy requires. The letters ‘V’ or ‘B’ are used to indicate AS/NZS high impact. Again, the latter letter is from European practice. The letter ‘A’ is not yet in the recommended practices standard19 but has been adopted from the European practice and designated ‘Extra high impact’ in the product standard.
It has always been possible to provide all levels of impact protection and correct ametropia and presbyopia; however, it has been achieved with a two-part solution. Provision of high and extra high impact protection, dust protection, gas tightness, splash resistance et cetera will always necessitate both an eye and face protector, in the form of an eye protector in front of prescription lenses. There are several ways of achieving low and medium impact protection, where the primary eye protector provides the impact protection and the prescription lenses provide the optical correction.
These can only provide low impact protection and are generally not very satisfactory. The recommended practice19 highlights the disadvantages, namely: ‘In the event of a severe blow to the area of contact, the clip-on may be inadequate to prevent shattering of the prescription lens(es) with a resultant risk of eye injury from lens fragments’; ‘Clip-ons are of necessity of light construction and are easily broken or damaged when not in use’; and ‘The contact between the clip-ons and prescription lenses may cause scratching and abrasion of the prescription lenses’.
Non-prescription personal protective equipment over prescription lenses
Spectacle type personal protective equipment and goggles that are specifically designed to fit over prescription lenses are widely available. They are not well accepted by the users as they have poor cosmesis and are larger and heavier and create extra reflecting surfaces in front of the eye.
A non-prescription eye protector with a carrier for prescription lenses
These are better accepted than the fit-over solution as the eye protector can be indistinguishable from a regular eye protector, which is acceptable in the workplace; however, the vertex distance of the prescription lenses, being behind the eye protector lens, is relatively short and may not provide adequate clearance for the eyelashes. They still involve extra optical surfaces in front of the eye.
Prescription eye protection
At the time that the present recommended practice standard19 was being written by Committee SF-006, there was a strong push to make some provision for prescription eye protection. The client companies of the eye and face personal protective equipment suppliers were happy with the options for the emmetrope but were left with undefined or poorly defined options for the ametrope or presbyope.
In the provision of prescription spectacles, impact protection should be a consideration but may have to be tempered by other considerations, such as optical factors, weight, thickness, abrasion resistance and cosmesis, and hence might not be a priority. Polycarbonate became available as a prescription lens material around 1978, having been available in non-prescription eye protectors for some years before, but was not used in any quantity for quite some years. Spectacle lenses are also available in an extensive range of materials and surface coatings, which, as will be seen, modify the impact resistant properties of the lens.
A pair of spectacles is a unique combination of materials, prescription, thicknesses et cetera that is quite unlike a mass-produced non-prescription eye protector. In the non-prescription case, compliance with a standard might be assured by knowledge of the materials used, type testing and production quality control using sampling. In the prescription case, the potentially damaging testing process means that testing and then supply of a spectacle is not an appropriate option. The option to manufacture two identical pairs and to test and discard one is not financially sensible.
Committee SF-006 responded to this need by writing Section 7. This was the first attempt by any standards writing organisation to address this issue. As the writing of performance criteria that would have required testing was not an option, the committee had to discard one of the central principles of standards writing, namely, that a standard should be performance restrictive not design restrictive. For instance, impact resistance is assessed by using a designated impact test not by the specification of materials, minimum thicknesses or similar parameters. At the time, glass was still quite widely used for prescription lenses, polycarbonate was little used, hard resin lenses with refractive index 1.50 were the norm and the urethane-based lenses were yet to be released.
Relying on the expertise of the committee members, Table 7.1 was prepared with four grades of guidance for the selection of lens materials for low impact prescription eye protectors. The materials were rated as ‘Preferred’, ‘Acceptable’, ‘Not Recommended’ and ‘Unsatisfactory’. The preferred were the plastic lenses, for which minimum centre and edge thicknesses were nominated. The proposal to designate polycarbonate as ‘Preferred’ and hard resin as ‘Acceptable’ was not agreed by the committee. Standards are generally about minimum standards not optimal practice. In an appropriate thickness, hard resin is capable of passing the low impact test. The understanding of the impact protective properties of glass, polycarbonate and hard resin was extensive due to a large body of research that will be discussed later in this paper.
Medium impact eye protection was explicitly excluded. At the time, polycarbonate was the only material for medium impact. Its flexibility meant that there was concern about assessing retention of the lens in the frame under impact without actually impacting the eye protector and rendering it unsuitable to supply to the worker. The options for medium impact eye protection in AS/NZS1336 for prescription wearers are still valid and will be included in this review.
There were principally two responses to the prescription eye protection provisions in AS/NZS1336. The eye protection industry still wanted standards for medium impact prescription eye protection. Given the ubiquity of polycarbonate, low impact non-prescription eye protectors are almost unknown and employers have been happy to supply medium impact eye protection where only low impact might be indicated. The most common response heard from optometrists and optical dispensers was in the form ‘AS/NZS1336 is only a recommended practice, we don't have to do it’. The resistance was mainly on the cosmetic issues of the thicker lenses and the allegation that the need to mark the lenses with ‘R’ to indicate prescription eye protection and the manufacturer's mark would weaken the lens. Since non-prescription tempered glass, polycarbonate and hard resin lenses have been marked in this way for decades and have passed low impact requirements, this allegation is clearly without basis.
The two universes of non-prescription and prescription finally collided when, in 2007, the prescription eye protection standard was published.17 AS/NZS1337.6 has requirements for both low and medium prescription eye protection. In essence, the requirements are that the eye protector should comply with the safety requirements of AS/NZS1337 (in the current form of AS/NZS1337.1) and the prescription requirements of AS2228.1. Committee SF-006 is in the process of amending this to AS/NZS ISO 21987 in a revision of AS/NZS1337.6 that is currently, January 2012, at the public review stage. Untempered glass and high index glass (even when chemically tempered) are explicitly excluded. The use of tempered crown glass is explicitly excluded from medium impact eye protectors. Minimum thicknesses are to be obtainable from the lens manufacturer along with other lens information set out in section 6. In common with AS/NZS1337.1, the prescription medium impact eye protectors must have the necessary lateral coverage. If this is in the form of side shields, these must be permanently attached (Clause 3.2.3).
What AS/NZS1337.6 did, for the first time, was allow the standards mark to be placed on a prescription eye protector. The standards mark is the best assurance of compliance available. By the end of 2011 there were 10 companies holding licenses for AS/NZS1337.6 covering 616 models (Donarski R, 2011, personal communication).
Certification is the best assurance of compliance but it is not mandated by the standard.
The objective of this standard (AS/NZS1337.6) is to provide regulatory authorities, manufacturers, importers, distributors, retailers, employers, employees and other users with minimum requirements for protection against low and medium impact to minimise safety risks and occupational hazards by the use of appropriate prescription eye protectors.17
The standard does require that ‘Risk based testing shall be adopted by considering worst-case combinations of material, eye size and refractive power/thickness’ and notes that ‘This will normally include type testing and ongoing periodic testing of production samples to monitor compliance’ (Clause 4.1). This does not mandate third-party testing. It does represent a much lower level of assurance for the end-user and makes justification of the appropriateness of the eye protection more difficult for the employer.
The ‘Standards Mark’ licensed products contain only polycarbonate and polyurethane lens materials. In the same way that the exclusive use of polycarbonate in non-prescription eye protection is an indicator, so the restriction of materials chosen thus far in the prescription eye protection area is a clear indication of what can be easily justified.
Prior to the 1997 publication of AS/NZS1336, the ophthalmic lens and non-prescription eye protection industries mainly relied on glass, hard resin and polycarbonate with the occasional tri-acetate goggle lens or eye shield/face shield visor. Up to that time, there was a substantial number of studies on the impact resistance properties of these materials. Comparative studies of the more modern materials are few and far between and the selection must rely much more on manufacturer's claims. Accordingly, the review of the materials will be separated into these two eras to ensure completeness but avoid complicating the comparison by an over emphasis on glass.
Mechanisms of lens failure on impact
Understanding the issues of impact resistance of lens materials is very much aided by an understanding of the mechanisms of failure coupled with knowledge of the characteristics of the material. It also goes some way to explaining the reasons why different tests might indicate different relative performance among materials.
Brandt28 details four mechanisms of failure, to which a fifth will be added.
- 1The first is a Hertzian fracture on the front surface of the lens initiating from a micro-defect (also called a Griffith flaw). This is the mode of failure of brittle materials. It applies to glass even when it is supported and cannot flex.29,30 The smaller the impacting or compressing object, the less energy needed to initiate failure. Classically, the failure is seen as a hole in the lens, although this might initiate catastrophic failure in a tempered lens.
- 2The second mechanism occurs when the lens flexes and the centre of the back surface comes under tension and failure again initiates from a micro-defect and a crack propagates through the lens. How and where the lens is held affects the amount of flexing and, as a consequence, the pressure needed to initiate failure.31 The size of the impacting or compressing object is much less significant.32 The fracture radiates in a star-like form from the impact/compression point.33
- 3The third mechanism occurs when the lens flexes and flattens and the edge of the lens comes under tension as the lens is flattened. Failure initiates from a micro-defect in the bevel (which is full of micro-defects unless polished). As a consequence, the quality of the bevel is reported as affecting the impact resistance.32 The origin of the fracture is at the edge of the lens with cracks radiating from this point.33
- 4The fourth Brandt mechanism occurs only with impact, not slow compression. The energy from the impact is reflected in the lens and travels as an elastic wave to the bevel where it causes a piece to flake off the lens as the result of a brittle fracture. With a tempered lens, this will normally lead to a catastrophic failure of the whole lens.
- 5The fifth mechanism is that of plastic deformation, in which the projectile simply passes through the lens, which deforms to form a hole. The velocity of the projectile is markedly reduced. There is no fracture and no secondary hazard from the fragments produced. This is the failure mechanism of fully plastic materials.
Understanding the mechanisms of failure can lead to an understanding of how to choose test methods to give different apparent relativities among materials.
Standard methods of assessing impact resistance
Assessments of impact protection are generally carried out using a dropped ball test (typically for the low impact category) and a ballistic test for the higher categories. Table 1 summarises the requirements of typical standards.34–40 What drove the choice of heights and ball size/mass is not clear except that the 16 mm and 1.27 metre combination effectively distinguishes 2.2-mm-thick tempered from untempered crown glass.34
Table 1. Ball size and drop height for the impact testing eye protection et cetera. For simplicity, any tolerances on ball diameter, velocity and mass have not been included.
|USA dress lenses35 and sunglasses36||15.9||16||1.27||5.0||0.20|
|Europe sunglasses (optional)38||16||16||1.30||5.1||0.21|
|Europe occupational27 and ski goggles39||22||43||1.30||5.1||0.56|
The arrangements for ballistic testing are equally confusing (Table 2). These practices have changed little since the 1970s.41,42
Table 2. Ball size and impact velocity for the impact testing eye protection. For simplicity, the tolerances on ball diameter and velocity and details of mass have not been included. Where the mass is not specified in the standard, the value is from measurements of 10 samples.
|AS/NZS low impact21†§||6.00||0.881†||13||0.074|
|AS/NZS medium impact21§||6.35||1.046†||40||0.868|
|Europe low energy impact27||6||0.86||45||0.870|
|AS/NZS medium impact21§||6.00||0.881†||45||0.892|
|Welding helmets and spectacles ANSI/ISEA40||6.35||1.06||45.72||1.11|
|AS/NZS automotive eye protection42||6.00 or 6.35‡||0.881† or 1.046†||50||1.10 or 1.31|
|Face shields ANSI/ISEA40||6.35||1.06||91.44||4.43|
|Europe medium energy impact27||6.00||0.86||120||6.19|
|AS/NZS high impact21§||6.35||1.046||110||6.33|
|AS/NZS high impact21§||6.00||0.881†||120||6.34|
|Europe high energy impact27||6.00||0.86||190||15.5|
|AS/NZS extra high impact21§||6.00||0.881†||190||15.9|
|AS/NZS extra high impact21§||6.00||1.046†||175||16.0|
Exactly what drove the adoption of these ball size and these velocities is not known, especially when there was evidence that 3.0 mm diameter balls were more representative of what might be expected in the workplace.
In addition, AS/NZS21 and USA practice40 include a penetration test for plastics lenses that comprises a weighted sewing machine needle (44.2 g) dropped from 1.27 metres. The American National Standards Institute (ANSI) also has a high mass impact, which is a pointed projectile of 500 g dropped from 1.27 metres.40
Eye protection is required in some sports and the impact protection might be tailored to the sport. The European standard for eye protection in skiing is included in Table 1, as it is a drop ball test. The USA standard has the high mass test43 for ski and snow boarding goggles.
Other testing methods
The methods outlined above are intended or should be intended to be reasonably representative of hazards present in occupational settings. Inevitably, they have been criticised as being unrepresentative. In addition, the different mechanisms of fracture mean that manipulating the impacting object mass and the impact velocity can result in different relative apparent merits among materials and it is not surprising that companies will advocate test methods that result in their product being well rated. Third, the ballistic methods are destructive, or at least damaging, so there is no way to test an eye protector and then supply it. Finally, without the use of many lenses, the ballistic tests are not quantitative but simply pass/fail.
A proposed pressure testing method44 works well when the fracture mechanism is the same;44–47 however, when different materials with different fracture mechanisms are compared the relativities of materials can be manipulated by the choice of the diameter of the supporting annulus and impressing object.32 In another study,46 the effects of base curve were studied in three materials. Within each material type, there is a linear relationship between the base curve and fracture load or fracture velocity. The rank order of material performance in the pressure testing and impact velocity results are the same but the fracture load measure makes the polyurethane material appear to perform only approximately 25 per cent below polycarbonate and approximately six times hard resin while the fracture energy results show a much wider gap, the polycarbonate resisting a fracture energy about three times that of polyurethane.
The smooth profile of the steel ball bearing used in the ballistic tests is probably not representative of impacting objects in real life.48 A pointed object is used for testing in the USA40 and AS/NZS21 and the concept had been developed into a quantitative test.49,50 The impact of a bristle from a rotating wire brush is an extreme example,51 as are the hazards in ordnance operations.52
The 6.0 mm or one quarter inch ball bearings are probably not representative of the impacting objects in occupational environments. The use of 3.0 mm diameter26,53 or smaller54,55 balls has been proposed as being more representative.
The issue becomes more complex when sports eye protection is included, where the impacting objects vary greatly in mass, size and speed. The range can be very large.8,33,56–59
The early days: glass versus hard resin versus polycarbonate
From the 1940s, glass (heat tempered and later chemically tempered) and hard resin (n = 1.50) were the only materials offered for both optical correction and spectacle-type eye protectors. Other materials were available for eye protectors but not with prescription lenses. In the 1970s, polycarbonate became available first for non-prescription and then for prescription lenses. There are many studies comparing different forms of glass34,45,60,61 and comparisons with hard resin and/or polycarbonate,33,56,57,62–65 These studies give a fuller understanding of the potential of polycarbonate,66,67 how its poor resistance to solvents might be improved by blending it with other materials68 or how its poor abrasion resistance might be improved by coating.69
What all these studies clearly point to is the extremely high level of impact protection afforded by polycarbonate and the irrefutable advantage over the two other materials of the time, glass and hard resin. Glass (whether untempered, heat tempered or chemically tempered) is a rigid and brittle material with a fracture pattern that represents a secondary hazard to the eyes and face. Hard resin, being a cross-linked thermosetting polymer, is less rigid but still retains brittle characteristics and a fracture pattern with sharp edges that are also secondary hazards to the eyes and face. Hard resin lenses are cast by injecting a mould with the liquid monomer and a catalyst and allowing polymerisation to take place in the mould. Polycarbonate is a thermoplastic polymer without cross-linking that deforms rather than fractures. The heated liquid polymer is injected into the mould and allowed to cool.
The need to add a hard coat to polycarbonate does reduce its impact resistance.65,69 The data of all these studies might be typified by combining the data of two studies that used a 6.35 mm ball into Table 3.58,65
Table 3. Fracture velocities of ophthalmic materials to a quarter inch ball
|Heat tempered glass||2||12|
The poor abrasion resistance of polycarbonate has been offset by the use of hard coatings69 to the extent that coated polycarbonate was, at one stage, the most abrasion resistant of the ophthalmic lens options.70 The issue of abrasion resistance is also complicated by the various methods available and these yield different relative results among materials.70 The hard coating process is subject to a number of factors, which give rise to variability in the finished product.71,72 The harder coatings might be especially prone to thermal shock, due to the different thermal expansion coefficients of the lens and its coating.73
Modern materials and coatings
The present day range of materials and coatings, as seen in trade publications such as that published by Eye Talk Consultants,74 is bafflingly large. There is no database of systematic comparative tests of materials and coatings and the impact resistance claims are not required to be based on an internationally accepted test method. The technology seems to be more the subject of patents, in which effectiveness is not a criterion, rather than technical reports and peer-reviewed publications. What information is available for selection is often phrased in general terms. Searching on the web with proprietary names and ‘impact resistance’ as key words leads to quotes such as:
‘. . . up to 60 times more impact resistant than glass and 12 times more impact resistant than ordinary plastic’75
which is unhelpful if the thickness and test method are not specified. What constitutes an ‘ordinary’ plastic is also not defined.
‘. . . meets all international standards for high-impact resistance’76
which is not true, not least because the AS/NZS and European standards limit high impact personal protective protectors to being face shields21 or goggles and face shields.27
‘. . . is formulated to pass the extreme impact testing normally reserved for polycarbonate.’77
Polycarbonate material passes the test of a 6.0 mm ball at 120 m/s (AS/NZS high impact21 or Comité Européen de Normalisation (CEN) medium impact27) or 190 m/s (AS/NZS extra high impact21 or CEN high impact27). There is no evidence that other ophthalmic lens materials can do this. Perhaps the confusion is because the USA standard40 sets the velocity of the one quarter-inch steel ball at 300 ft/s (91.4 m/s).
The evidence from the earlier studies using small object (6.5 mm diameter or less) ballistic testing (ballistic testing being the best representation of occupational hazards) is that the thermoplastic polymer without cross-linking (typified by polycarbonate) will provide a flexible lens that is highly impact resistant. The thermosetting cross-linked polymer hard resin lenses are more rigid and more brittle and as a consequence, less impact resistant in a ballistic test. Having a higher Abbé number, the hard resin lenses give better optics.
The need to accommodate the higher prescriptions has led to a number of higher index materials being developed up to ne= 1.74 and in a variety of hard and anti-reflection coatings.74
In pursuing the need to provide both excellence in optics and impact protection, it is a logical progression that manufacturers have looked for materials that have a higher Abbé number than polycarbonate but retain as much of the polycarbonate's impact protective qualities as possible. This has led to the polyurethane-based lens materials,78 which have their origins in military needs.73,79,80 Polyurethane compounds have a higher Abbé number and less cross-linking than polycarbonate. In some forms, they might be thermoplastic and in other forms thermosetting. One source describes them as ‘quasi-thermoset/thermoplastic’81 or ‘neither thermosetting nor thermoplastic’.82 In ophthalmic lens manufacture they are polymerised in the mould like hard resin lenses.
Only polycarbonate and polyurethane appear to have been put forward as lens materials for personal protective equipment, if the lists of certified models with the certification bodies is to be taken as a guide,83 but hard resin is still the industry leader in the ophthalmic field.
The second variable is that of the coatings. It has been known from the beginning that a coating will significantly reduce the impact resistance of polycarbonate65,69 and 1.50 index hard resin lenses.84,85 With hard resin lenses, the hard coat is not necessarily detrimental to impact resistance.86
Of course, glass is the most solvent resistant of the lens materials. The hard resin lenses are resistant to nearly all solvents,87 including acetone, benzene and petrol,88 and resistant to chemicals except highly oxidising acids.88 The polyurethanes are ‘unaffected by most common chemicals and solvents’.89 Polycarbonate is soluble in aromatic and chlorinated hydrocarbons and softened and cracked by ketones and esters.87 Tri-acetate is soluble in ketones and esters, and soluble or slightly softened in alcohols. It is not greatly affected by hydrocarbons, which makes it an alternative to polycarbonate for medium impact applications where hydrocarbons are used.87 The coatings on polycarbonate are solvent resistant but the cut edge on a visor or the bevelled edge on a lens is exposed polycarbonate.
FRAMES FOR EYE PROTECTORS
The frame of an eye protector is a vital element in the whole appliance. Not only must they provide the same or similar physical and chemical-resistant properties but they must also retain the lenses under impact. No detailed attention has been paid to frames in this respect in the eye protection literature. The non-prescription eye protection industry has adopted materials that are effective and cheap but do not have the attractiveness of ophthalmic frames. Nylon and polypropylene are examples of strong and solvent-resistant materials that are injection moulded to give effective eye protector frames. In eye protector form they tend to lack the quality of finish that is expected of an ophthalmic frame.
In addition, the design of the frame bevel for effective retention of the lens in the frame under impact is important.90
The issue of allergens in spectacle frame materials has been identified.91 Allergy to nickel is a common cause of dermatitis,92 the European Community has a directive regarding nickel release93 and there is an ISO test for assessing nickel release from spectacle frames.94 The paper91 presents a comprehensive review of other potential allergens in both metal and plastics frame materials.
A SUGGESTED BASIS FOR THE IMPACT STANDARDS
The reasons for the adoption of the parameters in the various test methods have, in the main, been lost in history. As far as can be seen, they were not linked directly to studies of actual hazards in the workplace. Standards are intended to be performance specific rather than design specific. That is, it is not acceptable to nominate a material as appropriate or inappropriate, not least because the writers cannot anticipate developments in the market, but a test may well be devised that achieves the same ends without limiting future materials. The experience of testing using these criteria has led to some opinions on why they might have been chosen, in light of the limited materials available at the time of their writing, which might be helpful in understanding their basis as a hierarchy.
In selecting the materials for eye protection, it is the impact test that will define what is acceptable and unacceptable. Given the present range of materials available, the licensed product information,83 the recommended practice19 and the literature referred to, some guidance is possible. It should be noted that standards are generally minimum rather than best practice. As a consequence, Tables 4 and 5 are an account of what will, as a general guide, comply with a category without reference to any additional capability.95 The categories are those of AS/NZS.
Table 4. Test methods, materials that comply and a suggested function for the test
|USA ophthalmic lenses95||16 mm||1.27 m||Untempered glass normally fails||Distinguish untempered from tempered glass|
|Sunglasses36–38||Thin acrylic may fail|
|Lowest occupational impact grade21,27,40||22 mm||1.80 m||Needs thicker tempered glass or hard resin||Create a higher category of protection|
|25 mm||1.27 m|
|Intermediate grade impact21,27||6.0 mm or ¼″||45 or 40 m/s||Hard resin lenses generally fail||Exclude hard resin|
|Urethane lenses pass|
|Higher impact grade21,27,40||6.0 mm or ¼″||91.4–120 m/s||Only polycarbonate (coated or uncoated) will pass||Exclude all but polycarbonate including tri-acetate|
|Highest impact grade21,27||6.0 mm or ¼″||190 m/s||Only uncoated polycarbonate will pass||Restrict category to uncoated polycarbonate|
Table 5. Impact categories, lens materials and compliance guidance
|Low impact||Heat or chemically tempered crown glass||3.0||2.5||Single vision, fused bifocal or progressive addition lenses outside -4 to +5|
|Chemically tempered high index||3.0|| ||Single vision only|
|Hard resin||2.5||2.0|| |
|Medium impact||Hard resin (n = 1.50)||3.0||3.0||Coatings may prevent compliance|
|Hard resin (n > 1.50)|| || ||Some may comply. Case-by-case basis|
|Insufficient information available|
|Polyurethane||2.0||2.0||Very hard coats may be a problem|
|Polycarbonate||2.0||2.0||Very hard coats may be a problem|
|High impact||Polycarbonate||2.0||2.0||Very hard coats may be a problem|
|Being a visor, lesser thicknesses may be acceptable|
|Extra high impact||Polycarbonate||2.0||2.0||Uncoated only|
|Being a visor, lesser thicknesses may be acceptable|
I acknowledge the valued assistance, however inadvertent and unintentional, of the members of Standards Australia/Standards New Zealand committees SF-006 Eye and face protection, CS-053 Sunglasses and fashion spectacles and CS-084 Eye protection for racquet sports and the International Standardization Organization committee TC95/SC6 Eye and face protection. The members of the eye and face protection and sunglass industries have always been most helpful. Glyn Walsh of Glasgow Caledonian University provided valuable advice on the draft manuscript. Finally, my thanks to the staff of the Optics & Radiometry Laboratory who are the ones who have the responsibility for the actual hands-on testing of eye and face protection and sunglasses.