PLASTIC FRAMES

Spectacle frame materials

Although cellulose acetate was first prepared in 1865 by the French chemist Paul Schützenberger, it was not until 1894 that the first industrial process for its manufacture was patented in the UK by Charles Cross and Edward Bevan. At about the same time, Little in the US made cellulose acetate filaments experimentally, as did Bronnert in Germany. However, this material was essentially cellulose triacetate, a rather intractable polymer, not readily soluble in commonly available solvents. In 1904 George Miles, an American chemist, discovered that if the polymer was partially hydrolysed, it became soluble in acetone.
The Swiss brothers Henri and Camille Dreyfus used this procedure for lacquer and film production at Basle in 1910, and at the outbreak of World War I set up a factory in Spondon, Derbyshire, England to make acetate 'dope' for waterproofing and stiffening fabric covered aeroplane wings. They set up a similar plant at Cumberland, Md., for the US army in 1917.
After the war they concentrated their efforts in England, and in 1919 introduced the first acetate yarn Celanese.
In 1924, they switched entirely to the US where acetone and acetic anhydride were cheaply available. In the same year, rayon became adopted as a generic term for all 'artificial silk'.
Cellulose acetate (CA) in combination with plasticizers, such as diethyl and dimethyl phthalate, produces a plastics material which can be heat softened and forced under pressure into a cool mould. This injection moulding process was developed by Dr Arthur Eichengrün of Celonwerke to exploit the moulding properties of the plasticized CA moulding materials he had produced. His first injection moulding machine had a maximum shot weight of about 8 gm provided by a hand-operated plunger mechanism, a far cry from today's machines, some of which have a maximum moulding size in excess of 100 kg.

CA plastics are tough with deep gloss and high transparency. They possess a 'feel' which is different to other plastics and which is often described as more 'natural'. This may explain why CA has retained its popularity for making items which are handled frequently such as spectacle frames and tool handles, many transparent tool handles are still made from cellulose acetate and its sister material cellulose butyrate. Other items made from CA included combs, fashion accessories, pen barrels and toys, but these are now more likely to be moulded from more modern thermoplastics.
Early spectacle frames were cut from sheet material, mostly in imitation tortoiseshell which was often referred to as 'optical shell'. Reinforcing nickel wires for the side arms were forced into heat-softened strips of CA sheet. Nowadays, frames are generally moulded into shape - a more economic process. However, certain high-class frames are still made using the old process, especially to achieve special colour effects not possible using injection moulding.
Despite being much less flammable than cellulose nitrate, acetate film did not become established for photographic use until after World War II because of the technical excellence of celluloid and the vested interests of film manufacturers. There was, however, a demand for transparent sheet material in laminated safety glass, especially car windscreens before toughened glass became available for this purpose.
Ironically, cellulose triacetate which was unsuccessful initially, returned to favour when a suitable, relatively non-toxic solvent (dichloromethane ) became available in the 1940s. Since then photographic film has been almost entirely based on cellulose triacetate and Tricel cellulose triacetate fibres were introduced in 1954.
Few plastics can boast such a long pedigree as cellulose acetate and, being made entirely from renewable resources, CA may yet have a long way to run.

Commonly used plastics include Cellulose Acetate and Cellulose Propionate. There are other materials such as Nylon and Optyl™ which are used. These can be better for people with allergy problems.

Cellulose Propionate – CP
Polymer Type
Thermoplastic

Advantages

Relatively lower plasticiser migration than Cellulose Acetate or Cellulose Acetate Butyrate.  Stiffer than Cellulose Acetate or Cellulose Acetate Butyrate.  Transparent and glossy with better low temperature impact properties Cellulose Acetate or Cellulose Acetate Butyrate.

Disadvantages

Lower weathering resistance than Cellulose Acetate or Cellulose Acetate Butyrate.  More expensive than Cellulose Acetate or Cellulose Acetate Butyrate.  Attacked by acids & alkalis and swollen by chlorinated and aromatic hydrocarbons, alcohols and ketones.

Typical Properties

Applications

Spectacle frames, goggles, tool handles, covers for television screens, cutlery handles, lampshades, drawing aids, packaging, toys, blister packaging.

If you want the colors of the rainbow, then zyl (zylonite, or cellulose acetate) is your material. Zyl is a very cost-effective and creative option for eyewear and is extremely lightweight. Particularly popular right now are laminated zyl frames that have layered colors.

Some manufacturers are also using propionate — a nylon-based plastic that is hypoallergenic. It's lightweight and has a different look and feel than other plastics.

Eyeglasses made of nylon were first introduced in the late 1940s. Because of brittleness and other problems, eyeglass manufacturers switched to blended nylon (polyamides, co-polyamides and gliamides). Today's blended nylon frames are both strong and lightweight.

Nylon is also a premier material for sports and performance frames — typically made of gliamides, which are very resistant to hot and cold and are more flexible, yet also stiff. Nylon is also easily molded into today's popular wraparound styles, as well as other shapes that are difficult to produce.

Plastic frames do have some drawbacks. They are easier to break than metal frames, they will burn (but are not easily ignited), and aging and exposure to sunlight slightly decrease their strength but do not affect color.

Plastic frames are durable, can accommodate just about any lens prescription, and are available in a wide range of prices. They are also offered in a variety of plastics (including acrylic, epoxy, cellulose acetate, cellulose propionate, polyamide, and nylon) and in different colors, shapes, and levels of resistance to breakage. Epoxy frames are resilient and return to their original shape after being deformed, so they do not need to be adjusted as frequently as other types. Nylon frames are almost unbreakable. They revert to their original shape after extreme trauma and distortion; because of this property, though, they cannot be readjusted after they are manufactured.

Nylon

Nylon, a synthetic fiber, was first put on the market in 1938 for the manufacture of toothbrush bristles and at that time was dubbed "artificial silk." It would soon find its niche, however, in women's nylon stockings, which were introduced in 1940. Today, it is used for everything from parachutes and umbrellas to car tyres.

Frames made of nylon were first introduced in the late 40s. Because of brittleness and other problems, frame manufacturers switched to blended nylon (polyamides, co-polyamides and gliamides). Today's blended nylon frames are both strong and lightweight.

Nylon is also a premier material for sports and performance frames — typically made of gliamides, which are very resistant to hot and cold and are more flexible, yet also stiff. Nylon is also easily molded into today's popular wraparound styles, as well as other shapes that are difficult to

Polyamide (PA) is a synthetic plastic especially suitable for the manufacture of high-quality injection molded frames. The material is used to make thin-rimmed, staple fronts and feather-light, flexible temples with a core of stainless steel. Although lightweight, PA is extremely resistant to breakage and highly scratch resistant. It has excellent shape retention and very good resistance to alcohol cleansing agents, cosmetics, and perspiration. PA is also hypoallergenic.

Spectacle frame materials

Raw Materials

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