Anyone with a coating background have some insight on this? I thought "crazing" mainly happens to AR since the layer is so thin. What woud cause a hardcoat to break down like this? Patients are adamant they aren't doing anything harmful to the lenses
That looks like it was cleaned with harsh chemicals rather than crazing, or the coating was bad to start with.
This looks like an insufficient curing of the hard coat. This is most likely a uv cure coating which are prone to this problem. The uv source degrades over time and user neglects to replace it. The assumption is at the time of curing if the coating is dry it is cured that absolutely not the case.
I get both patients who are obviously lying and those that I genuinely believe or not mistreating the lenes with this same issue.Originally Posted by Kwill212
Interesting. I will have to poke the lab about this further. thank you.
Who ever does the hard coating must do a cross hatch test every day. This involves cutting lines in the coating in a cross hatch similar to a tic tac toe pattern with the lines 2mm apart. A tool is available with blades already separated by the correct distance so you just pass once horizontally and once vertically, then you apply a special tape let stay on the lens for 2 minutes and pull it off rapidly. If the coating is not cured correctly some of the little squares you created with the tool will release the coating on to the tape. Time to change uv source.
Is UV cure used for dip coatings? I thought it was only spincoat?
Uv is for spin coat only the cross hatch test is universal as well as a haze test to check the hardness of the coating. Check if your lab has the capacity to do both. If you apply AR coating these tests along with several others are critical to maintain consistency and integrity of your coatings.
...Oh the things that I was never taught...
Have I told you today how much I hate poly?
Always learning.
I've added this to the Hall of Fame Forum dealing with AR; #30.
Poly lens?
Probably, but what lensman11 was discussing has to do with backside spin coating, and will affect any substrate that would require a backside hard coat.
In this case, by reviewing the picture posted, this is a backside issue, as the front is not affected.
Adding to lensman11's post, humidity/moisture is also a factor that can affect the proper cure of the coating. Again, an adhesion test will assist in verifying that the cure is satisfactory.
I bend light. That is what I do.
I bend light. That is what I do.
First question is material? Next question type of coater? Last question have lenses been exposed to extreme heat?
As far as cure time and coating thickness, Ultra Optics coaters use a timed system for best results. With respect to coating thickness, two actions control this. The amount of time coating is dispensed / applied to backside then spin rpm and duration. UV lamp is also set based on time yielding a cured lens.
With some 1.67 backside crazing can be an issue especially if left in extreme heat. I never backside 1.70 and above as dip coat is the preferred method.
Poly is the problem. I've never seen such problems on any other material.
Wow I was thinking high index.
Perhaps just a thin backside coat. Was this Rx stock or grind?
I’ve seen more hard coats break down lately due to cleaning with harsh chemicals. All because of COVID!
I’m thinking semi-finished based on the pic. Front side looks fine. This does look look to be an adhesion related failure due to the coating being too thick.
I bend light. That is what I do.
Would you mind defining extreme heat? Are we talking mid-summer in Arizona in a car with the windows up?
Hard coats are solvent resistant so I don’t think chemical cleaning is the culprit. You might see the lens material being destroyed by a cleaner most likely poly. How important is the thickness of the hard coat, what is the method for measuring the thickness and how do you maintain the desired thickness. Is there any data to show hardcoat thickness has an adverse effect on either the adhesion or abrasion resistance.
Colts Laboratory does use "dashboard" temp to define extreme heat as one of their adhesion tests.
I wouldn't say I've done a huge amount of 1.70 and 1.74, but I spin coated them all. (No dip option available in my lab.) I've never encountered issues from it. Is that something you've experienced a lot of?
Have I told you today how much I hate poly?
When 1.70/1.74 was first available from Vision Ease we did have a few hard coat issues. Warpage from cure cycle was another issue.
Since AR should always be paired with 1.70 and above we moved to finish stock and ordered uncuts direct from VE.
Agreed. I only made those without AR when it was a same day/ emergency pair. I definitely had issues with warpage if made as thin a possible, due to the curing. I'm glad I haven't had any come back with backside coating issues, but I will keep your experience in mind.
Have I told you today how much I hate poly?
Yes. A lot of people are cleaning their glasses more than ever because they fog up on them and I've seen some instances where they wore off a small area of hard coat, then they think its a smudge on their lens and they try to remove it which makes it worse.
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