Impact Resistance

Impact Resistance

ability of a lens to resist fracture or breakage when the lens is subjected to impact

Factors Influencing Impact Resistance

coating, thickness, shape, material, frame support, Rx type

Crown Glass Index

1.523

High Index Glass Indeces

1.6, 1.7, 1.8, 1.9

CR-39 Index

1.498

Trivex Index

1.53

Polycarbonate Index

1.586

Glass Structure

rigid, strong bonds between atoms, slight imperfections can weaken lens material

CR-39 Structure

soft, flexible

Polycarbonate Structure

very soft, extremely flexible, more than high index plastics

Trivex Structure

hybrid between poly and CR-39, quasi-thermoset

Thermoset

CR39, thermostable, greater resistance to heat, cannot be remolded, less impact resistance

Thermoplastic

polycarbonate, high index, thermolabile, may be melted down and reshaped, recyclable, greater impact resistant

Title 21, CFR 801.410

FDA requirement for impact resistance lenses, GLASS lenses must pass FDA DBT

DBT

drop ball test, applicable to glass lenses only

Prior to _____, no impact testing was required.

1972

Z80.1-1972

DBT for impact resistance only

DBT Parameters

• Lens must withstand impact of 15 mm steel ball

• Weighing 15.88 gm

• Dropped from a height of 1.27 m (50 inches)

• Equivalent to 0.2 joules of impact energy

T/F: There is no required minimum thickness with current ANSI standard, jus pass DBT

True

DBT Exceptions

• raised ledge multi-focal

• prism segment

• slab off

• laminated lens

• plastic lenses → batch tested

• Non-Rx/Sunglasses → batch tested

Problems with DBT

• may reduce lens impact resistance

• tests only a small area of lens → more peripheral

• lens sits on mount → absorbs sig. energy

Impact Testing Method Changes

• vary projectile size

• change projectile height/velocity

Alternative Method of Impact Testing

ballistic testing

Static Testing

apply energy load to lens front surface continuously until lens breaks, more accurate than DBT, with glass lenses DBT is equivalent

Compressive Strength

squeezing or pressing force required to reduce or crush

Tensile Strength

elongated force required to pull apart or separate

How is compressive stress induced?

treating lens by heat or chemical

Compressive Stress

tensile stress (internal force) must overcome compressive stress (external force) for lens to fracture

T/F: heat and chemical tempering is applicable to glass lenses only

True

Heat Tempering (glass)

air or thermal tempering, heated to near melting point, then both surfaces rapidly chilled by blasts of forced air

Chemical Tempering (glass)

salt bath solution and heated, much higher impact resistance then heat tempered

Impact Resistance Ranking

polycarbonate > CR-39 > chem-temp glass > heat-temp glass > untreated glass

For large projectiles which lens types which ranking?

chem-temp glass > CR-39 > heat-temp glass

Purpose of Lens Coating

may reduce lens fracture resistance as a result of reduced tensile stress, coating will flex on impact → propagating energy through the lens could result in fracture

AR Coating

mean fracture energy 63% less than uncoated lenses

SR Coating

mean fracture energy 57% less than uncoated lenses

What patients are polycarbonate lenses made for?

pediatric, active, monocular, amblyopes, risk for injury patients

What are the drawbacks of polycarbonate lenses?

soft, highly scratchable, dust can cling to lense

Fracture Resistance and UVR Exposure

significantly reduced by UVR exposure

Fracture Resistance and Lens Curvature

flattening based urge could decrease fracture resistance

Rimlon

• no glass

• Preferred poly, trivex, 1.67

Rimless

• No CR-39 or glass

• Preffered poly or trivex