impact resistance of mid- and high-index plastic

[yzf-r1]

Does anyone know what the values are (or where i can find the values) of impact resitance of 1.60, 1.67 and 1.74 plastic lenses?

many thanks

[keithbenjamin]

Is there actually a value that exists to describe impact resistance? Or is it simply pass/fail for a specific test or standard at a given CT.

-KB

[mrba]

As Kieth said impact is a pass fail test. Are you refering to tensile strength? I have seen impact charts where poly is 4, 1.67 is 3 ish, 1.60 is 2.5 ish etc. Does anyone know what I am talking about?

[Darryl Meister]

I have seen impact charts where poly is 4, 1.67 is 3 ish, 1.60 is 2.5 ish etc.

Those numbers are probably relative to hard resin (or possibly relative to the FDA impact standard, though they look kind of low for that). For instance, if they are relative to hard resin, saying that poly is a "4" is basically saying that poly is 4 times more impact-resistant than hard resin. Generally, poly is considerably more impact-resistant than hard resin. Many 1.67 high-index materials are also very impact-resistant. Of course, impact resistance will be affected by a number of other factors, even for the same type of impact, including center thickness, power, coatings, base curve, any stress already inherent in the lens, and so on.

Impact resistance is generally measured in terms of the kinetic energy required for a projectile to fracture a lens. The SI unit of kinetic energy is the joule (J). For reference, the FDA's drop-ball test results in about 0.2 J of kinetic energy when the ball hits the lens.

Best regards,
Darryl

[keithbenjamin]

I've been digging trying to find a relative comparision of impact resistance for different lens materials but have come up empty. Do you have anything like that Darryl? or tensile strength numbers for that matter?

I did, however, find a good point made on an optometrist's web site. He suggested it was very important not to test the impact resistance of polycarbonate lenses by firing a bb or any other projectile at a lens, while on the face of the wearer. So keep that in mind.

I think he lived in Arkansas, in which case, I suppose the warning is understandable.

:D

-KB

[mrba]

You know he tried it!!!!!!!!

[Darryl Meister]

Do you have anything like that Darryl? or tensile strength numbers for that matter?

I don't know that we have any specific values "published." One of the problems is that a chart wouldn't be very meaningful, since the impact resistance of a lens material will change dramatically depending upon the configuration of the lens. And the values of the table would only apply to that particular manufacturer's lens materials, and only under specific conditions (e.g., specific power, specific coating, specific thickness, etc.).

For instance, a high-index lens material might be 5x more impact-resistant than hard resin at the same 2.0 mm center thickness. However, labs would generally surface this material down to 1.5 mm and then perhaps add an AR coating to it. The power and base curve of the lens will also affect its impact resistance. The combination of a thinner center and a brittle AR coating might drop the impact resistance down to only 2 or 3x that of hard resin. Two manufacturers may make high-index lens materials with similar refractive indices that actually perform quite differently in terms of impact resistance. Some materials may even lose more impact resistance when certain coatings are added than others.

Consequently, such a chart would be of limited value in choosing impact-resistant lens materials. Furthermore, if it is used to make decisions regarding eye safety by an eyecare professional, a manufacturer might be open to a certain level of liability because of the chart. If eye protection is a priority for a given wearer, you should probably choose a material that has been specifically advertised as offering an enhanced level of impact resistance. And even then you should be cognizant of how additional lens treatments may affect its impact resistance. Obviously, a material like polycarbonate, which has an exceptional level of impact resistance under most circumstances, would be a good choice if safety is a concern.

Moreover, unless a lens manufacturer specifically positions a particular material as having a greater level of impact resistance, they are probably not going to emphasize its impact resistance. This is particularly true for lenses that would not have an exceptional level of impact resistance in certain configurations, such as certain high-index lenses with various coating combinations and extremely thin centers. Unless the manufacturer has positioned the lens as having an added level of impact resistance, their main concern is mostly likely to ensure that the lens consistently passes the FDA's drop-ball test under typical conditions.

Best regards,
Darryl

[chip anderson]

For what it's worth, I had an artificial eye patient of mine (she had the artificial eye before the accident) in poly lenses. The impact was such that her lenses turned wrong side out. She did not have eye injury, but her lenses didn't break, they turned wrong side out.

Chip

[Darryl Meister]

Chip makes a good point; material impact resistance is only part of the consideration. Safety frames and sports goggles, for instance, are required to have eyewires that keep the lens from popping out as easily.

Best regards,
Darryl

[harry a saake]

Darryl, as you once pointed out, we have in glass lenses internal flaws known as griffith flaws, do we have this in poly, cr-39, and high index, if so how , why and what are they called.

[Jim G]

Just to add to Darryl's excellent post, the physical surfacing, edging and hard coat application to the backside are also significant variables. i.e. the same lens, surfaced to the same specs can/may have statistically significant variances in impact resistance. SRC and ARC coatings likewise...both generally detrimental.

Surprisingly, and if I remember my info correctly, a well-manufactured and tempered GLASS lens may have better impact resistance than that a CR-39 lens.

Again, generally speaking, poly is the most impact resistant and (most) mid-index (1.55/1.56) the least. We routinely ran impact tests and found that certain mid-index would not pass the test w/ a SRC and ARC coating.