Spectacle frame materials
Closing Date: 16/04/2017
When looking at supplying a frame to a patient, not only must we take into account the fitting, the style and the cosmetic effects, we must look very carefully at the material that is used. It may be that we can use a so called ‘cheap and cheerful’ material if the frame is to be used short term, or it may be that the patient is somewhat heavy handed, and so to avoid unnecessary chair time for minor repairs and adjustments, a stronger more flexible material is needed.
The art of ensuring the best pair of spectacles is dispensed does not stop with the lens choice; it must be extended to the design and the material of the frame. It is imperative, particularly with many modern day materials being notoriously difficult to adjust, that the patient is appropriately advised and measured to ensure a good fitting frame at the end of the dispensing process. It is therefore as important to assist with the choice of frames as it is to assist with the lenses.
When offering advice on lenses, we do not give the patient a catalogue and say take your pick. Should we then say to a patient, ‘Here are the ladies’ frames. Try some on and let me know when you have found one’?
The most desirable properties of a material1 used for the manufacture of a spectacle frame are:
- Ease of production
- Lightweight and strong
- Easily glazed and adjusted
- Good lens and shape retention
- Inert to external agents and body fluids
- Cosmetically acceptable
There are three main categories that need consideration and are outlined in this article;
- Naturally available materials
- Man-made plastics
Strictly speaking, precious metals such as pure gold and silver fall into this category and have been employed as frame materials. These will be dealt with under metals. There is a wide array of non-metallic natural materials, some undergoing something of a renaissance as manufacturing and stabilising techniques improve. Here are just a selection;
- Wood (figures 1 and 2) – several companies are now looking to use wood as high fashion frames
- Ivory, tortoiseshell – strict rules about trade in rare animal-sourced materials (such as CITES) have left these as museum curiosities
- Horn – materials such as buffalo horn are used in some fashion frames (figure 3)
- Casein – this early milk-based polymer is rarely found now and might be remembered for its unfortunate smell
Man-made (Synthetic) plastics
There are two main types of man-made plastics;
- Thermoplastic: substances (especially synthetic resins) that become plastic on heating and harden on cooling, and are able to withstand a repeat of these processes.
- Thermosetting: substances (especially synthetic resins) which set permanently when heated.
Materials used for the manufacture of spectacle frames are therefore generally thermoplastic. Some materials such as the epoxy resins (Optyl) are said to be thermoelastic. Other plastics materials used in frame manufacture include nylons, composite materials and silicon.
Cellulose nitrate (figure 4)
This was one of the first plastics developed and was widely used in frame manufacture until its use was prohibited in many countries, including the UK, due to flammability issues. It is characterised by its toughness, high shine and the ability to retain its shape and stability in hot, humid climates. The material can be softened and adjusted but with a flash point of 90oC, just above its softening point, extra care has to be taken. It yellows with age and some practitioners claim that it smells of moth balls when heated. Although practitioners may still make adjustments to frames manufactured using cellulose nitrate, the dispensing, glazing or re-glazing of this material is now prohibited.1
Cellulose acetate (figure 5)
This is a non-flammable thermoplastic polymer. It was developed in response to the problem of flammability with the nitrate. It is comprised of a base of cotton fibres, mixed with acetic acid (figure 6). It is used for spectacle frames in one of two forms:
- block acetate (or slab acetate), which is used for frames that are cut out from a template
- extruded acetate which is acetate that is forced in a semi- molten state into a mould. This latter method gives a stronger frame, as the fibres, or molecules, are not cut through, but bend around curves.
The colour of the frame is a solid colouring that is achieved by water-based dyes (figure 7). Thicker materials therefore have a denser colour to them. Light colours can be assisted by the use of two sheets of acetate, one coloured and one crystal, being fused together. This is a common technique used on sides, with the two parts grooved ready for the reinforcement wire such that when they are fused together the wire is contained within.
After the frame is constructed, it is polished using a barrelling technique. Modern frames are treated with a high gloss polyurethane lacquer to give an increased resistance to scratching. The frame is not hypoallergenic, with many patients experiencing a mild allergic reaction. This is usually due to a reaction between the metals of the reinforcements and/or joints and the acids contained in perspiration. These responses may cause a minor rash or irritation, and lead to frame discolouration.
Prolonged wear of cellulose acetate frames can cause the frame to whiten, and appear cracked and rough to the touch. This will happen most frequently where there is direct contact between the skin and the frame, typically over the bridge and along the inner surface of the sides.
The material, if used without the reinforcement, is fairly soft and can warp quite easily, especially if maltreated or left on window sills or car dashboards. This is due to the lack of shape-memory. The older the frame becomes, the easier it will warp. This warpage needs correcting and this will required more frequently the older a frame is. An older frame, say beyond 12 to 18 months old will need regular maintenance. The reshaping of acetate frames should be done at a temperature of 58° to 62°C, with melting taking place at 138° to 230°C. It will not burn.
The reinforced sides of this material give a clue to its identity. The end of the side that rests on the front is rough to the touch, due to the side being cut to fit, and the colour can be seen all the way through the side. The density of this frame is 1.27 and so is slightly lighter in weight than nitrate.
Cellulose Propionate (cellulose acetate-propionate)
This is a thermoplastic material made by mixing cellulose flakes with propionic acids. In its raw state this material is in the form of small transparent beads that are heated in order to fuse them together. It is then poured into a mould, which usually has the joints and reinforcements pre-placed so giving added strength. The material is therefore classed as injection moulded.
The colouring is by dying but the material will only accept the dye to a depth of a few microns so it is only a surface dye. When folding the side down, and looking at the end which touches the front, the side can appear clear. The colouring technique enables intricate designs and patterns to be added. When the side is folded down, the area that is rough on an acetate frame is smooth on the propionate frame due to the lacquering taking place after the frame has been cut and shaped.
This material often has ‘CP’ printed on the sides which, along with the colouring techniques, metal reinforcements, and the smooth feel of the end of the side that makes contact with the front gives, makes it easy to identify.
It offers greater stability than acetate and is much more resistant to warping and fading than acetate, so ages far less rapidly. It is less allergenic than acetate, but not entirely so due to the nickel content of the reinforcements and joints. The hypoallergenic properties are assisted by the lacquer. If this coating is cracked during adjustment, then allergies may increase.
Propionate has a density of 1.21 and a lower specific gravity than acetate, usually offering a 6% reduction in weight in comparison. When this is coupled with the thinner frame designs that are available due to the higher tensile strength of propionate, a saving of 25% to 30% of the weight is often achieved. Adjustment and reshaping should take place at a temperature around of 67°C. It is often referred to as a ‘cold glaze’ as the material will shrink with too much heating causing it to cleave if stretching is attempted. The melting point is around 190°C. When working with this material, the initial fit of the frame needs to be very good, as there is little useful adjustment that can be made later.
Acrylic resin or Perspex (figure 8)
This was used to produce lightweight, transparent or dyed frames that were stable and yet flexible. With a relatively high softening point the material was not as easy to work with and shrank or distorted if overheated. This material was popular in the 1950s and 1960s when it was used to manufacture supra frames.
Optyl (figure 9)
This thermosetting material is probably the most versatile of all frame materials but, alas, one that is out of fashion as it is wrongly perceived as one that is difficult to work with. It is a form of epoxy resin that is made by injection moulding during which the polymerisation takes place. Some will tell you that the reinforcement is only applied for the first 25mm of the side, whilst others argue that it has no reinforcements at all. In truth, the material does not need reinforcement, but some frames have this as a decorative addition.
The main advantages of this material are its hypoallergenic properties, making it dermatologically safe, and its thermal memory. This is an extremely useful aspect of the material as, once Optyl has been correctly adjusted, it should never need readjusting. It holds the shape at which it has been set.
To adjust Optyl, the material needs a temperature in the range of 80° to 120°C depending on the age of the material and how much reshaping is needed. Heat renders the material extremely pliable. If the adjustment is carried out and the frame held in position whilst cooled rapidly, then the new shape is ‘remembered’ by the frame. If, however, the frame is reheated to the same degree and allowed to cool naturally and without being held in place, it will return to its original shape. Quite useful if you make a mistake!
The coloured dye is a few microns deep on the surface which then has a lacquer applied for a highly polished look such that, when you look down the length of the side, no colour can be seen.
This lightweight material, with a density of 1.18, does not shrink, is very pliable when hot, but very brittle when cold. It is quite resistant to scratching. Optyl should not be cleaned using spirit based materials as these can and often do break down the lacquering. Optyl is the only material that was designed specifically as a spectacle frame material prior to being used in other areas.
This thermoplastic material is mainly used in the production of sun-spectacles and industrial safety spectacles. The main production is by injection moulding and the colour can either be added into the base material or painted after the frame is finished. Dyeing the base material will have a longer lasting effect. The finished frame is then lacquered for a high quality sheen.
The material has good tensile strength and pliability. Pure nylon is usually only available in single solid colours and with a limited shine due to the nature of the material. However, a mixture of nylon and carbon fibre can be used to improve the surface finish and also assist with tensile rigidity. Metal sides are preferable to give easier adjustment which will be longer lasting than trying to adjust nylon which springs back into shape too easily.
It is not advisable to heat nylon due to excessive shrinkage that will result, at the very least, in the material undergoing a breakdown of the fibres so reducing strength considerably. Nylon has a good plastic memory and this also reduces the chance of getting a good adjustment. A density of only 1.1 makes this frame very light indeed and it has a high melting point 180 to 300°C. All forms of nylon will shrink and lose stability if overheated.
Nylon, along with any material that can be powdered, can be used for 3D printing of frames (figure 10).
This is a co-polyamide nylon derivative, which is exclusive to Silhouette. However, not all Silhouette frames are SPX. It is an injection moulded material with a very limited adjustment range of around 100° to 110°C. Any temperature below this means the material is too cold and becomes brittle. Above this temperature, the molecular structure starts to degrade. This starts with the molecules separating, and elongating into strands. The material takes on a furry appearance as it starts to shrink and will eventually pull apart. Therefore, the vast majority of companies label this as ‘cold glaze only’. However, like Optyl, it is hypoallergenic and it neither corrodes nor erodes.
It has great impact resistance and, when this is coupled with the extreme reduction in weight in comparison to acetate typically in the range of a saving of 40%, SPX becomes a desirable material. The nylon aspect of the material makes it very flexible and also hard to scratch. The colour is introduced by dyeing the base material followed by painting and lacquering the outer surface.
In the early days of production, SPX fronts often had SPX sides, which resulted in some awful adjustments and uncomfortable spectacles. Sprung steel later replaced this as a side option, but not everyone wants a plastics/metal combination. Extruded acetate and cellulose propionate are both now used.
This material was originally called Grilamid, and is a derivative of amorphous co-polyamide. It is another lightweight material with a specific gravity of 30% that of acetate. Due to its high tensile strength, a sofyl frame requires less material and is therefore thinner and subsequently lighter. The material needs to be heated slowly and with care to avoid breakages, and again many companies label this as ‘cold glaze necessary’. It is flame resistant and hypoallergenic.
Carbon Fibre (figure 11)
This is another material that can boast of being close to a perfect frame material. Again, however, it is a material that is somewhat misnamed. Carbon fibre consists of long thin, very strong, crystalline filaments that primarily are used as strengthening agents in resins and ceramics.
A carbon fibre frame is only around 20% carbon fibre, with the rest usually comprised of around 60% nylon, 5% other co-polyamide and 15% associated chemicals, bonding agents and rigiditers. Carbon fibre is also highly elastic, so can take impact shock well, and when this is coupled with its high tensile strength, breakage is rare. This makes the frame material very durable with excellent longevity.
Due to the matrix that forms this material, and the fact that it is moulded and not slab cut adding to the strength, such frames can be made very thin. This therefore produces a lightweight frame that does not warp, bend or stretch out of shape. The frame should be supplied with metal sides as it is very difficult to adjust once made.
The colouring technique for carbon fibre is quite unique. Carbon fibre is a natural matt black that cannot be dyed. Paints and lacquers tend not to adhere and can become patchy. As a result, a polymer sheet of a few microns’ thickness is laid out and painted. A mottled effect is usually used so that the matt black is inconspicuous. This is then tightly wrapped around the base material and fused in place. The melting point is very high, as is the softening point, so it is a cold glaze material, although a closing block joint is often used to secure the lenses.
Metalwork is used extensively in the manufacture of frame parts or complete frames with a wide variety of pure metals, plated and alloy metals being utilised. Metal frames usually consist of:
- Base metals (the structural metal of the frame)
- Plating (usually several layers)
- Lacquer coating
- Plastic side tips and nose pads (usually cellulose acetate or silicon)
This inert, hypoallergenic material is rarely used in the manufacturing of spectacle frames. It is a precious metal that can be highly decorative in design and can take on virtually any shape and style needed. However, it is a relatively soft material, especially in the purest forms, so it does bend really easily. This is not good for a spectacle frame, and the chair time would be considerable to maintain adequate fitting. To overcome this, we would generally increase the thickness which would make it harder to bend. Unfortunately, this would render the frame unbearable to wear due to the excessive weight that would accompany the extra thickness. Usually, 18 or 24 carat gold would be used. This would carry a hallmark, and it would be very yellow in colour.
Gold filled or rolled gold (figures 12a and 12b)
Gold filled or rolled gold is essentially a skin of carat gold rolled onto a core of base metal. The quality of the gold used is stamped as a fraction or parts per thousand. Gold filled frames display good resistance to corrosion but may cause a dermatological reaction if in constant contact with the skin.
A very thin plate of gold is deposited onto a base metal using electrolysis, which is then protected by a lacquered varnish to protect the material. It is by far the most cost effective method of gold colouring using real gold as opposed to gold colouring, or paint. Once the frame has undergone construction it is placed in a bath of cleaning chemicals. It is then passed through a series of vats each containing one of the many plating materials to be used which are in a fluid state. An electric current is passed through the fluid which causes adherence to the base material.
The layers are built up as follows:
- Base material
- 0.06 to 0.08mm of plate 24 carat gold.
- 1 to 2.2mm of palladium nickel.
- 0.06 to 0.08mm of plate 24 carat gold.
- 0.4 to 0.65mm of plate 22 carat gold
The gold plating can be worn away over a period of time due to constant wearing of the frame, and the initial sign of this is a slight fading of the colour, especially where the frame touches the side of the head. In very old frames, the gold can totally wear away to leave the bare base material.
This material is often confused with nickel silver, and to be honest, if a monel frame was held in one hand and a nickel silver frame in the other, even the most expert of frame handlers would struggle to tell the difference. Ironically, there is more nickel in monel than nickel silver. Monel usually comprises of around 70% nickel, 15% copper, 10% iron and 5% magnesium. It is sturdy, robust, easy to adjust and easy to solder. So on the face of things, it seems a good material. However, a high proportion of population is allergic to nickel to some degree, with reactions varying from an itchy sensation to a rash. It is a relatively cheap material to work with and we can use electroplating to protect against the nickel effects to a limited degree.
Nickel Silver (figure 13)
Also known as German silver, the name of this material is somewhat of a misnomer. It has absolutely no silver in it and in fact has less nickel than monel. Nickel silver is a silvery white material that gains its colour from the bleaching process caused by the combining of the nickel and the zinc elements of the alloy. The material is usually around 60% copper, 20% nickel, 5% magnesium, 10% zinc and 5% miscellaneous trade secrets.
The magnesium gives the material its corrosion resistance, but the electroplating uses a pure nickel plate which negates any advantage the frame material has due to the extreme allergic reaction some people will suffer from the direct contact with the skin. A large proportion of the population are allergic to some degree to this material but having said that the material is very tough and has good elasticity, making it spring easily back into shape when knocked. It is used a great deal in the construction of temples and bridges of combination frames and, if electroplated, will offer some protection against allergic reactions.
This metal is extremely strong, with excellent flexibility which makes it virtually unbreakable, although there will be someone somewhere who will attempt, and, no doubt, succeed in proving otherwise. It is naturally a silver grey colour, but plating using an ion plating technique may allow various colours. The material is naturally hypoallergenic, corrosion resistant, perspiration acid resistant, and is up to 10 times more resistant to atmospheric and abrasion attack than gold plate, and up to four times more resistant than pure gold.
The frame can be made quite thin in comparison to comparative materials due to a 20% greater elasticity. In pure form, the specific gravity of titanium is 4.5 g/cm3, which is 50% lighter than that of nickel. This means that, like for like, there can be a considerable reduction in the weight of the frame. However, when thinner frames are produced exploiting the greater tensile strength of titanium, then much greater weight reductions should be found.
Although the material is quite easily found and is relatively abundant in the natural world, the refining needed is extreme, and is also very costly, so immediately the expense is transferred to the purchaser of the material, who then passes it on to their customers, and so on. However, the durability of the frame coupled with the reduced need for adjustments due to non-warping of this material, may be seen as a counter balance. A further downside of titanium is that it cannot be soldered.
The construction of the frame is generally done by laser spot welding that is computer controlled, hence the accuracy and neatness of the construction. To solder like this would need a temperature of the region 1280° C. Therefore, it is deemed cheaper to replace a broken frame than repair it.
When dispensing this material, due to its thinness and flexibility, it is important that the lenses chosen can fit into the frame. For example, a glass lens would have a tendency to flake, or chip, if the frame flexed. Lenses of extreme thickness would look unsightly and would be difficult to keep in place, as would lenses of very little edge thickness.
There are three main types of titanium frame materials. They are:
(a) Pure – fully titanium (titanium alloy) which is the strongest, lightest (up to 48% lighter) and most flexible type.
(b) Clad – a core of titanium surrounded by chromium nickel made by a process almost identical to that of rolling gold, except the heat has to be far higher. It is still extremely strong lightweight (25 to 35% reduction) and flexible
(c) Partial – a mixture of materials for different parts. For example, the front may be titanium but the sides are sprung steel.
Stainless steel (figure 14)
This is manufactured by a wire drawing technique where a metal rod is heated to white hot temperature and then drawn through smaller and smaller apertures. This way, the molecular structure of the metal remains unaltered and stays strong. This makes the resultant material both very springy and very flexible. It is also very lightweight, and ideal for rimless frames, especially for the Ilford rimless. It does not corrode, and is impervious to perspiration acid. Unfortunately, it cannot be soldered. The highly flexible nature of the material renders it unsuitable for
- Glass lenses especially photochromic
- High myopic and hypermetropic corrections
- E-line bifocals.
This overview of frame materials, I hope, will serve to remind readers of the importance of careful selection and advice if a successful pair of spectacles is to be dispensed.
Liam Kite is senior lecturer and university teaching fellow at Anglia Ruskin University.
1 Keirl A, Payne R. Spectacle Frames – Materials and Finishes. Optician 13.10.08