ophthalmic plastic lens manufacturing, their history, merits, demerits and specifications, difference between CR-39 and Polycarbonate lenses and their durability

1. PLASTIC LENS
MANUFACTURING
SURAJ SHIL
B.OPTOM 2nd YEAR
RIDLEY COLLEGE OF OPTOMETRY, JORHAT

2. Introduction
 A plastic material is defined as a “polymeric material” of large
molecular weight which can be shaped by flow
 At present times if plastics are processed properly and
tailored for appropriate applications, they may offer many
advantages compared with the materials they replace
 Most plastics are synthetic materials formed by combining
various organic ingredients with inorganic materials such as
carbon, hydrogen, oxygen, nitrogen, sulphur, chlorine etc.

3. Types
 On the basis of physical properties, plastic lenses can be
classified as:
i. Thermoplastic materials, which soften when heated
and therefore can be remoulded
ii. Thermosetting materials, which once hardened cannot
be softened even at high temperatures

4. Thermoplastic Manufacturing: Injection
molding
 Thermoplastic materials have their molecules arranged in long
chains
 They are usually supplied in pellet, granular or sheet form
 The material softens when heated and can be stretched,
pressed or moulded into complex shapes with no appreciable
changes in its chemical structure
 When cooled, the material hardens and shrinks and has the
same configuration as the mould in which it was heated

5.  Since no chemical change transpires, the softening and
hardening cycle maybe repeated indefinitely
 Thermoplastic materials are less dimensionally stable than
thermosetting materials and they can withstand less heat
without deformation
 Examples of thermoplastic materials are polymethyl
methacrylates, cellulose acetate, cellulose nitrate,
polycarbonate, polystyrene etc.

6. Thermosetting Manufacturing
 Thermosetting materials are supplied in liquid monomer form
and cast into moulds
 Plasticizers, dyes, binding monomers and catalysts may be
added to the basic chemical compound, after which
polymerization occurs
 The molecules form a three dimensional cross-linked, lattice
pattern rather than the two dimensional structure of
thermoplastic materials
 This process transforms a liquid monomer first into a syrup,
then into a gel and finally into a solid

7.  The relationship between the time and temperature required
in the hardening process is known as the curing cycle
 Once hardened the material cannot be softened(even at high
temperatures)
 If the thermosetting materials are subjected to high
temperatures, the material decomposes without melting or
substantially softening
 Examples: allyl diglycol carbonate{ Columbia Resin 39
(pittsburg plate glass)}, melamine compounds etc.

8. Development of optical plastics
 Although plastics are available for a long time, plastic lenses
are relatively new compared with glass lenses
 Just as World War 1 served as the impetus for the
development of optical glass industry, World War 2 served as
the impetus for the development of the plastics industry
 Plastic material developed during the second world war were
PMMA(thermoplastic), CR-39(thermosetting) for use in
military aircrafts

9. Development of optical plastics
 In 1947 Robert Graham, formed the Armorlite Lens Company
in Pasadena, California, later described the first ophthalmic
lenses made from CR-39(scratches more easily than glass, is
much more scratch resistant than PMMA)
 In 1957 General Electric developed a new plastic material, a
polycarbonate resin, called Lexan, which has great
mechanical strength and high service temperatures
 In 1982 corning glass works announced the development of a
lens called Corlon, a two layered ophthalmic composite
material consisting of a glass lens backed by a thin layer of
polyurethane film

10. Manufacture of CR-39
 Ophthalmic lenses made of CR-39 are cast from allyl diglycol
carbonate monomer
 Supplied as yellow viscous liquid
 Some add a copolymer or other additives, such as anti
yellowing agents, ultraviolet absorbers and mold releasers
 These copolymers may reduce lens shrinkage, make casting
easier and make lens tougher and lighter
 After catalyst and other ingredients are added, the liquid resin
is poured into a glass mould having a concave and a convex
surface

11. Manufacture of CR-39
 The inside surface of the mould are highly polished to produce
finished lens surfaces of high quality
 The entire mould is then placed in an oven and subjected to a
controlled time/temperature relationship : the cure cycle
 After completion of the cure cycle, the moulds are removed
from the oven, dismantled and separated from the finished
lens
 Lower temperatures and longer curing times result in lenses
having superior rigidity, dimensional stability, impact
resistance and scratch resistance

12. Manufacture of Polycarbonate lenses
 Polycarbonate is a thermoplastic material so it begins as solid
and is melted down and is then injected into a mould at a
temperature of 320
o
C
 In the injection process, the polycarbonate conforms, under
pressure, to the highly polished metal surfaces of the injection
mould
 A device will squeeze the lens to prevent lens shrinkage and
ensure optical accuracy
 After removal from the moulds, the lenses are inspected and
processed through a coating machine

13. Manufacture of Polycarbonate lenses
 Because the surface hardness of polycarbonate is much
softer than CR-39, all polycarbonate lenses receive hard
coating
 After the coating process is completed, a heat curing process
produces polymerization and cross linking of the coating
 The coating solvents evaporate, leaving behind a silica based
solid coating approximately 4-6 µm thick

14. Optical and physical properties of CR-
39
 Refractive index = 1.498
 Abbe value = 58
 Specific gravity = 1.32

15. Optical and physical properties of CR-
39
 Other desirable characteristics include :
i. Lightness
ii. Impact resistance
iii. Chemical inertness
iv. Resistance to fogging
v. Tintability
vi. Versatility in optical design: examples include aspheric
surfaces for cataract lenses, magnifiers with aspheric
surfaces, progressive addition multifocal lenses etc.

16. Optical and physical properties of CR-
39
 Disadvantages include:
i. Surface abrasion
ii. Warpage upon glazing
iii. Increased thickness
iv. Inferior photochromic properties
v. Index variability with temperature

17. Optical and physical properties of
polycarbonates
 Refractive index = 1.586
 Specific gravity = 1.20

18. Optical and physical properties of
polycarbonates
 Advantages are :
i. Impact resistance far superior than CR-39
ii. All polycarbonate lenses absorb ultraviolet radiation upto
380nm without additional treatment

19.  Disadvantages are :
i. Polycarbonates are soft and scratches very easily
ii. Having a higher index of refraction may produce more
reflections than ophthalmic crown glass or CR-39
iii. Low abbe number, so more chromatic abberations

20. Impact Resistance of CR-39
 With ballistics tests using a 1/8 inch steel ball, it was found
that a 3mm CR-39 lenses were more impact resistant than air
tempered glasses
 Welsh et al tested untreated glass, air tempered glass,
chemically tempered glass and CR-39 plastic lenses with a
ballistic device using a 3/8 inch missile and found the CR-39
to be the most impact resistant and untreated glass to be the
least

21. Impact Resistance of Polycarbonate
 Polycarbonate is impact resistant because its molecules are
extremely long chains of atoms that can slide back and forth
on each other, with the result that the material can be flexed
and even deformed without breaking
 Stephens and Davis showed that polycarbonate has 21 times
the impact resistance of CR-39 for ¼ inch and 1/8 inch steel
balls fired at high speed

22. References
 Clinical optics, Page no. 11-19

23. THANK YOU