SPECTACLE FRAME MATERIALS
A variety of materials are available for making frames. An ideal spectacle frame material should have the following properties:
• It should be rigid enough.
• It should be flexible enough to give it different shapes.
• It should be economical.
• It should be long-lasting.
• It should be resistant to corrosion from sweat.
• It should be easily convertible into different shapes.
• It should be cosmetically attractive and soothing.
• It should be lightweight.
• It should not cause any allergic reaction to tissues it comes in contact with.
Metal frames usually consist of a number of different materials: the structural metal of the frame called the base metal and the plating
material, frequently an organic lacquer (coating), and the plastic side tips and nose-pads.
Before coating the wire is drawn through rollers to change it to the desired configuration.
This hardens most alloys.
Joints between the components of metal frames can either be made by soldering or by welding. Both soldering and welding weaken the heated area.
Many alloys cannot be soldered or welded satisfactorily with the simple brazing torches used in practice.
This is because an oxide layer forms very rapidly and prevents adhesion.
Many metals are toxic and great care should be taken not to inhale any dust from these alloys.
Even safe metals such as titanium and aluminum can be dangerous if the particle size is such as to cause lung damage.
Following types of materials are available for manufacturing of spectacle frames
Stainless steel:
This is a durable material with flexibility.
Spring effect can be given in the frame.
It is suitable for patients who are prone to develop allergic reactions from ordinary frames.
It needs to be electroplated or painted to prevent corrosion.
Aluminium:
It is very economic, lightweight, cosmetically attractive and resistant to corrosion.
It is a soft material so sides are made thick.
It is possible to add decorative finish to the material.
It is a very good conductor of heat hence becomes very cold in winters and hot in
summers.
So the temple (two long sides) needs to be covered with plastic
sleeves.
There are fewer chances of allergic reactions from this material.
German silver:
It is corrosion resistant and flexible but needs
electroplating with nickel and is quite allergenic hence can cause allergic reactions in susceptible individuals.
Titanium:
It is most lightweight of all frames, 40 percent lighter than normal frame materials, highly corrosion resistant from sweat, heat
resistant and non-allergic.
Different colors can be added in it to make it
more attractive. But they are very costly.
It has very good memory, i.e. it retains its original configuration.
It is available as pure titanium, clad titanium and a combination of both.
Rolled gold:
This is a precious metal frame.
The process by which these frames are made makes the product become harder and springier.
This metal occupies the upper end of the gold frame market.
9 carat gold
It is also used to make frames.
It is very resistant to corrosion, easily adjustable and convenient to work with.
Nickel silver:
This is 12 to 25 percent nickel but mostly copper.
It is mechanically quite a good material for spectacle frames but it becomes dull very quickly if not plated or coated and rapidly turns green in contact with body fluids.
It is easily worked and soldered and is one of the most common materials for spectacle frames.
Nickel silver is commonly used for the joints and side reinforcement of plastic frames. Allergy to nickel and its alloys is common but it only presents a problem where the metal
is in contact with the skin.
However this can be prevented by using plastic side-tips and bridges of inert metal or polymer coatings.
Memory metals:
This is a group of alloys which has the common property of being able to return to their original shape after considerable distortion.
They do snap if repeatedly flexed through sharp angles.
Frames are seldom made from memory metals alone.
These materials are commonly used for temples and bridges with more conventional materials used for the eye wire screws and other parts.
This is because of cost factor and they are so flexible that they cannot be adjusted satisfactorily.
The best known memory metal is nickel, titanium but it is often sold as a titanium alloy with no mention of the nickel.
There are also other memory effect metals like–aluminium, titanium-cobalt and aluminium-vanadium-titanium etc.
Combination of eyewear frame materials:
Sometimes one part of a frame is made
from one material and the other part is made from a different material for example metals and plastics.
This does not include a plastic front with metal joints, or a metal front with plastic nose-pads. It also does not include composites.
In practice, almost all frames are made from combinations of materials – plastic fronts commonly have metal sides
and vice versa.
Similarly plastic sides usually have metal reinforcement while metal sides usually have plastic tips.
Plastics:
Hawksbill turtle, the source of natural plastic has been declared endangered species by World Wildlife Fund. Hence natural plastics like
turtle shell, bone, horn, ivory, leather and wood have become obsolete now
a day.
Synthetic plastic is of two types:
1. Thermoplastics
2. Thermosetting.
Thermoplastics can be re-formed with heat and thermosetting can’t be reformed with heat because the material breaks up on exposure to
heat.
Plastic frames can be made from different materials like cellulose acetate, cellulose nitrate, cellulose propionate, cellulose acetate butyrate,
PMMA, epoxy resins, polyamides (nylons), polycarbonate, silicone rubbers, carbon fiber, Kevlar and composite materials.
Cellulose acetate:
This is one of the best materials available and probably the most common plastic spectacle frame material.
It is very light, strong, and mechanically stable at normal temperatures, easily worked
and relatively inert.
It is cosmetically attractive and very transparent material.
It tends to whiten where in contact with patient’s body fluids particularly at the bridge and temples.
It is attacked by some common
solvents and dissolves in acetone.
Acetate sides usually have full-length
wire reinforcement and any color is usually throughout the material.
It softens at around 50°C temperature so prone to damage by excess heat.
Cellulose nitrate:
Cellulose nitrate is very similar to cellulose acetate but it catches fire at a temperature little above required to adjust it.
It is strong, convenient to work with and its surface can be polished brightly.
It becomes dark “urine-yellow” color and very brittle with age.
Polyamides (Nylons):
These are very rarely called “Nylons”.
The materials are used in sunglasses, sports spectacles, safety spectacles and temporary aphakic spectacles.
They are very strong, but have a very soft
surface, can be very flexible and often cannot be adjusted with ordinary frame heaters.
Frames Carbon Fiber graphite:
It is very light, heat resistant, durable and strong. It is one of the favored materials for manufacturers of frames.
Carbon fiber graphite:
It is very light, heat resistant, durable and
strong. It is one of the favored materials for manufacturers of frames.
Polycarbonate:
This is best known as a lens material and is very strong. Its use as a frame material is uncommon, other than for sports and safety
spectacles.
Silicone rubbers:
These are soft, flexible materials, used for bridges, side-tips, rim-liners etc.
They are extremely stable and typically retain
their elastic properties from –50 to 200°C.
Cellulose acetate butyrate (CAB):
It is occasionally used for safety spectacles.
Little information is available on its properties as a spectacle frame material.
Some of the plastic side tips of metal frames may be made from it.
Polymethyl methaacrylate/ Acrylic resins:
This is remarkably transparent, cosmetically good looking, highly stable but has low resistance to impact and not convenient to work with as it is very brittle.
This is almost obsolete as a frame material.
Epoxy/epoxide resins:
Epoxy frames are usually translucent, but opaque colors have also been available (in this case the body of the material is usually white, and is again colored with a surface dye).
These frames have been claimed to be hypoallergenic. They are usually coated with a transparent lacquer both to protect them and reduce their allergenic potential.
Kevlar:
Kevlar is most common and very popular material for making bullet proof protection.
The Kevlar is stronger than steel and all other type of fiber just becouse of that chemical structure of Kevlar is comprised of several repeating inter-chain bonds. These chains are cross-linked with hydrogen bonds, providing a tensile strength 10X greater than steel on an equal weight basis.
Kevlar material is expensive becouse production is expensive of the difficulties arising from using concentrated sulfuric acid, needed to keep the water-insoluble polymer in solution during its synthesis and spinning.
Kevlar is a type of aramid fiber. It is woven into textile materials and is extremely strong and lightweight, with resistance toward corrosion and heat.
This is another material now in occasional use. It is what “bullet-proof vests” are made from.
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FRAMES