What is de-ionized water, and why use it?
De-ionized (DI) water is just that. It is water that has gone through a physical process where its mineral ions have been removed. Since the most common impurities in tap water are ions such as calcium, chlorides, and sodium, we want to clean our lenses with water that has these impurities removed as all can leave residues that appear to the eye as water stains.
To remove these contaminants, water is moved through specially prepared resin tanks that exchange positively charged ions and negatively charged ions for hydrogen and hydroxyl ions. The end
result is pure water with very low electrical conductivity (or high resistance) and few dissolved solids. With this property, we are able to measure or quantify what is “clean water.”
For the application of cleaning ophthalmic lenses, Quantum recommends that the resistivity of your DI water is in the range of 18- 12 mega ohms at 20°C. A higher resistivity value is better.
The use of such clean, pure water will prevent a lens from becoming more contaminated than it already is from the surfacing and spin coating process. Also, since DI water is missing ions, it will be
“hungry,” seeking out and absorbing ions from other sources, like from the lens. This will help to enhance the “etching” process, which is important for proper adhesion of the AR stack to the lens.
NOTE: Resistivity of your DI water should be in the range of 18-12 mega ohms at 20°C.
Purpose
Cleaning a lens prior to AR (anti-reflective) processing is arguably more important than cleaning a lens prior to spin coating. The process of AR application to a lens is vastly different than the application of a primer or lacquer to a lens, so the requirements are significantly different. The conditions required for an AR to bond to a lacquer are different than the conditions needed for a primer or lacquer to bond to a lens.
In this regard, a typical lens flowing through a surfacing lab will experience all kinds of contamination. Some examples are fingerprints, powders from slurry, ink marks, water stains, and even residual components of seemingly good cleaning products like dish washing detergent. All these contaminants will need to be removed in order to bond an AR layer stack to the lacquer.
It is quite possible to remove many of these contaminants by “hand cleaning.” However, there are some points to consider with this type of cleaning when trying to successfully process a lens through an AR coating lab. Hand cleaning does not lend itself to reproducibility and is an acquired skill. Hand cleaning is slow. Most importantly, hand cleaning does not remove enough contaminants to ensure trouble-free and consistent adhesion of the AR treatment.
One also has to keep in mind why a lens gets a scratch. The most common reason is when an abrasive object comes in contact with it (touches it) and leaves a mark in the lacquer. This is most likely to happen during hand cleaning. To successfully process a lens through an AR process and retain high quality and high yield, a systematic approach must be used.
The best way to achieve such a goal is to clean lenses with de-ionized water, an appropriate cleaning agent, and use of a high quality ultrasonic automatic cleaning machine with appropriate fixturing that eliminates the necessity to touch the lens to render it clean.
Operation
The de-ionized water purification system consists of ion exchange resin and/or other water treatment media contained in portable tanks of various sizes that are easily connected to your tap water supply. These tanks contain:
- Cation ion exchange resin
- Anion ion exchange resin
- Mixed bed resin and/or granular activated carbon
When the water quality drops below the setpoint, the DI water tanks are swapped for tanks with freshly regenerated ion exchange resins that have been processed at an ion exchange regeneration facility.
The DI water equipment comes in various sizes and can be custom configured to meet any flow rate or quality specification, no matter how demanding. See the figure on the next page for an example of the typical basic DI water purification system.
Service Deionization
For small AR lab operations, Quantum recommends contracting with a local Service Deionization (SDI) vendor to supply and maintain the DI water purification system.
This procurement option provides high purity deionized water users with a cost effective alternative to the difficult issues associated with owning and operating regenerable deionizers or demineralizers. Renting or leasing portable deionized water systems means no capital expenditure and eliminates the problems of:
- Chemical storage
- Chemical handling
- Operator training
- Waste treatment
- Environmental permitting
Quick Reference
Generally speaking, the DI water purification system requires little in the way of operating instructions. The vendor supplying the system will go over any system-specific operation not covered in this guide. It is important to understand how to properly read and record the resistivity using the vendor-supplied resistivity meter. This process will vary depending on the model used. Please refer to the documentation supplied with the meter for operational instructions.
NOTE: Quantum recommends a DI water purity of 18-14 mega ohms. The water quality should never be allowed to drop below 12 mega ohms.
Maintenance
Your SDI vendor will inform you of any on-site maintenance that falls under your responsibility.
You should contact your SDI vendor to schedule a system service when the reading on the resistivity meter reads between 13-12 mega ohms. This will allow the vendor enough time to service the system before the water purity drops below the minimum level of 12 mega ohms.
The lab staff should check the water quality of the system daily, after the unit has been running for at least 30 minutes. The resistivity readings should be recorded and charted over time for reference as to when the system is likely going to need service.
For any questions, comments, or concerns, please contact Quantum Innovations at 888.214.7932.