Hydrophobicity and Wearer Satisfaction

By Jeff Hopkins
The advantages of hydrophobic topcoats on AR stacks are universally known by now. They provide a smooth surface on top of the relatively uneven AR stack. They also cause beads of water (and in the case of oleophobic topcoats, grease) to stand up higher on the lens, allowing the wearer to remove them with minimal effort. This combination of benefits eliminates the cleaning difficulties that caused dissatisfaction with earlier generations of AR.

Recently, hydrophobic topcoats have given way to “super-hydrophobic” topcoats. The distinction between them relates primarily to the contact angle of droplets on the hydrophobic surface: the higher the contact angle, the smaller the area of contact with the droplet on the lens. The more the droplet “stands up” on the lens, the easier it is to remove. Generally speaking, a topcoat that creates a contact angle of 110 degrees or more is considered “super-hydrophobic.”

The Durability Issue

Super-hydrophobic coatings are very easy to clean and have tremendous “curb appeal,” providing an excellent demonstration of the lens’s cleanability at the time of dispensing. But the standard “lifetime” of a lens is two years. What is the effect of months and years of rubbing and environmental exposure on the topcoat, which is, after all, only two-hundredths of a micron thick? If the topcoat fails before the patient returns for a new Rx, he or she could have the sort of cleaning problems that turned many wearers against AR in the first place.

The ability of hydrophobic coatings to continue performing well over time is, therefore, crucial to patient satisfaction and is likely to have an effect on the patient’s decision to ask for AR on his or her next pair of lenses. Fortunately, two tests have been developed to assess the ability of hydrophobic coatings to hold up over time in real-life situations.

Cotton Cloth Rub Test

In this test, various hydrophobic and super-hydrophobic coated lenses were subjected to 6,400 wiping cycles with a dry cotton cloth. This simulates two years of dry-wiping, assuming two cleanings per day, each consisting of four to five wipes per cleaning. (While the use of water or cleaning spray is always recommended for AR-coated lenses, most dispensers agree that this doesn’t represent common practice among wearers. And since the industry’s goal is to grow the AR market beyond the most conscientious wearers, it is vital to understand performance in a dry-wiping scenario.) The contact angle (hydrophobicity) of the lens was measured repeatedly over the course of the test.

Environmental Exposure Test

For this test, lenses were subjected to six weeks of continual outdoor exposure. The area in which the exposure occurred was industrial, with a relatively high incidence of pollutants and particulate matter in the air; there was also significant exposure to rain and moisture. The average UV index was four. This degree of exposure simulates two years of environmental effects under normal wearing conditions. Once again, contact angle measurements were made throughout the test period.

Conclusion

The degree of hydrophobicity, as measured by contact angle, is an important factor in assessing the cleanability of a lens, and is therefore an index of wearer satisfaction with an AR coating. However, hydrophobicity itself is not a guarantee that satisfaction will remain high for the life of the prescription. Hydrophobic coatings vary in their endurance when subjected to normal wear and tear. The ability of the topcoat to retain its hydrophobic and oleophobic properties over time will affect satisfaction toward the end of the prescription’s life, when the wearer is considering repurchase of AR. Therefore it is important for lab personnel and dispensers to understand the enduring properties of the AR coatings they recommend.

CURRENT ISSUE


August/September LabTalk 2017