Dip Hardcoating Overview - Quantum Innovations

This document will answer frequently asked questions about dip hardcoating, such as:

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• How does a dip hardcoating process work?
• What are the main parameters of the process?
• How is the best quality achieved?
• What kind of performance can be expected on different types of lenses (CR-39, Polycarbonate, Trivex, 1.6, etc.)?

Main Process Parameters

A successful dip coating process involved many different parameters:

• Lens material
• Lens type
• Surface preparation
• Regeneration rate of hardcoating
• Filter: flow rate, pore size
• Solution temperature (°C)
• Viscosity: % of solids, solvent balance
• Ambient temperature (°C) and relative humidity (RH %)
• Lift-out speed
• Mechanical stability
• Pre-cure
• Finale cure
• Quality control

Lens Material

There are many different lens materials in use today. New lens materials are introduced frequently. Each may require a specific surface preparation, as well as a specific coating (index matching).
The main ones, by increasing refractive index, are listed below:

• CR-39 (1.49)
• Trivex (1.53)
• Mid-index materials (up to 1.56)
• Polycarbonate (1.58)
• MR-6 (1.6)
• MR-7 (1.66)
• MR-8

Lens Type

Surfaced lenses are different from single vision, bifocal, trifocal and progressive, in that they are subjected to several specific steps that increase the risks for surface defects in the hardcoating, while possibly adding debris, such as cerium oxide, to the surface of the lenses. These lenses may require a specific process before they join other lenses in the dip-coating process.

Surfaced to Factory Lens Comparison

Surface Preparation

Surface treatments are used to prepare the lenses so that the required wettability needed to get an even layer of hardcoating is achieved. Wettability can be characterized by the contact angle between the surface of the lens and a drop of pure water. The lower the contact angle, the better the wettability.

Regeneration Rate of Hardcoating

• Hardcoat solution has a limited shelf life.
• Shelf life can be extended by cold storage (0°F).
• Typical pot life in a dip hardcoating system is 10-16 weeks.
• In order to avoid problems, dip hardcoating systems are designed so that the contents of the primer and hardcoating tanks are replaced at least once every 25 days.

Filter: Flow Rate and Pore Size

• Flow rate should allow at least 20 times the volume of the tank to be recycled per hour.
• Mechanically, a high recirculation flow rate works against small pore size.
• Maximum pressure applied to the filters should be kept under 30 psi so that the gels generated by the coating cannot pass through the structure of the filters.

Solution Temperature

When the temperature increases, the viscosity of the coating decreases, in turn decreasing the thickness of the hardcoat layer. Temperature increase also causes a faster aging/deterioration of the
hardcoating solution. It is important to keep this parameter as stable as possible. See manufacturer’s specifications.

Viscosity: Percentage of Solids (Density), Solvent Balance

• As soon as the percentage (%) of solids changes, the thickness of the hardcoating layer changes. This affects the performance of the hardcoat.
• Is it vital that the % of solids is correct in order to get a high quality hardcoating on the lenses.
• The correct % of solids is maintained by adding the proper solvent to the primer and hardcoating.

Ambient Temperature (°C) and Relative Humidity (RH %)

Lift-out Speed

The lift-out/withdrawal speed of the lens from the hardcoating solution, combined with the % solids ratio, determines the thickness of the hardcoat on the lens.

• A controlled lift-out speed helps to create an even hardcoat thickness.

Mechanical Stability

• No stability: Variations in lift-out speed cause waves in the hardcoat, resulting in optical power aberrations.
• Stability: Tightly controlled lift-out speed across the profile of the lens helps create an even hardcoat thickness on the lens.

Pre-cure

During this phase, the solvents contained in the hardcoat are evaporated, so that the top layer of the hardcoat is dry.

• This prevents the hardcoat from being damaged while being handled.
• Most systems utilize infrared lamps to perform this function.
• The surface of the lenses must be gradually heated up, and special care must be taken with tinted, high-plus and high-minus lenses in order to avoid overheating and cracking.

Final Cure

Final curing in an oven develops the hardcoat to its final hardness.

• Attention must be paid to the temperature and the length of time the lenses are exposed to the curing heat. See manufacturer’s specifications for correct settings.
• An undercured hardcoat will not deliver the expected scratch resistance and adhesion.
• An overcured hardcoat may cause a yellowing of the lens.

Quality Control

Many quality control tests can be performed on hardcoated lenses:

• Adhesion:
  • Salt water boil with crosshatch
• Thickness:
  • Measurement using a spectrophometer
• Scratch Resistance:
  • Steel wool
  • Taber abrasion
  • Bayer
  • Tintability
  • AR compatibility
• Aging:
  • Xenon lamp
  • QUV

These tests are well known and widely used throughout the industry. Quantum offers these tests at no charge to our process contract customers.

Additional reading:
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Troubleshooting

Is a Lighter Color Aluminum Hard Coat Thinner than a Darker One?

I have been asked this question often: “Is it always the case that a lighter color hard coat is less thick than a darker one?” While the answer is not simple, I usually respond by saying, “Maybe, but not necessarily.” To explain, let’s start with some basic information.

Some Basics of Hard Coat Aluminum Anodizing

Normally, natural hard coat appears as a dark grey-green (some say, brown) color when .002” is created on a alloy with a low-temperature sulfuric acid bath cooled to approximately 32 degrees Fahrenheit. This is what is known by Sanford Process Corporation—known worldwide for its hard coat expertise—as “classic” hard coat. The color is not a mystery and there are scientific reasons why the aluminum takes on color during the hard coat anodizing process. The depth of color is not always the best way to judge thickness and never is the best way to judge the performance characteristics the coating will have. I’ll get more into performance in our follow up to this blog, but for now let’s stick with thickness and color.

Classic Hard Coat Color and Alloys

With classic hard coat, color is created by entrapment of alloying materials within the aluminum as the anodic coating forms and grows. In general terms, the thicker the coating the darker it becomes, because more of these alloying constituents will be trapped in the oxide. But, since each alloy is different from a metallurgical perspective, they will appear much different. Therefore, it is not a good idea to correlate color with thickness.

Even within an alloy, there are allowable amounts of other metals or silicon that are acceptable for that alloy. This variance alone can show different color, even at like thicknesses. Plus, there are other outside influences on color. Mechanical surface preparation methods, manufacturing method, and temper also play a role. This is in addition to a number of operating parameters that the anodize house controls. Bath temperature, current density, voltage, acid concentration, etc., will also effect the final color.

Quick Answer

The quick answer is that hard coat color on anodized aluminum—while a good litmus test—should not be used to make final judgement on thickness or performance.

There are also methods of hard coating that can reduce the entrapped alloying constituents and create an anodic coating that is much more of a clear coating. Ask us about this unique technology.

Contact us to discuss your requirements of Metal Packaging Coatings Manufacturer. Our experienced sales team can help you identify the options that best suit your needs.