Did you know that more than half the substrates we coat are plastic? Often chosen for their low weight or durability, plastics are being used in an increasing number of medical, imaging, military, and virtual reality applications. Coating on plastic requires specialized cleaning, handling, and deposition processes in order to match the optical performance and durability possible on glass – and not everyone can do it. In Part 1 of this two-part series, we share how AccuCoat has overcome these challenges, and some of the surprising MIL-SPEC tests we've passed.
The Challenges of Coating Plastics
Deposition of optical coatings on plastics is not nearly as straightforward as on glass, and requires specialized processes to ensure adequate adhesion. Without these, coated plastic optics are prone to peeling and flaking, particularly when subjected to harsh environmental conditions. Coating parameters must also be modified to obtain equivalent optical performance, as plastic is unable to withstand the extended high temperature exposure used during a typical glass coating run.
At AccuCoat, we’ve put a tremendous amount of effort into process development for plastic optics – from substrate preparation to coating – as this is the key to producing durable high optical quality coated optics. Through nearly 20 years of experience on 1000+ projects, we’ve optimized our processes to achieve a high degree of adhesion and excellent optical performance on a variety of plastic materials and form factors.
Most plastics simply can’t withstand the typical 250°C coating temperatures used for glass without warping or deformation, so we’ve redesigned our deposition processes to work at lower temperatures for plastics. Careful customization of the coating parameters, materials, and layer design not only allows us to control the heating of the substrate to prevent damage, but also enables us to deposit more layers to achieve more challenging spectral profiles. We’ve learned that many of the coating parameters are substrate specific, and thus we draw on our experience to fine-tune the design and deposition controls for each unique combination of coating, substrate material, and form factor.
“When it comes to coating, 50% of your success lies in the coating design – the other 50% comes from your ability to correctly clean a part in a cost-effective way without damaging it. Cleaning is half the battle, so we really work hard on our ability to clean optics.”
- Paul Meier-Wang, President
Cleaning & Handling
Creating a good quality coated optic from a plastic substrate depends heavily on proper cleaning and handling of the parts prior to deposition. Many plastics scratch very easily as compared to glass, and without proper cleaning, it’s impossible to get good coatings that adhere well. AccuCoat has invested significant time and effort into developing a combination of ultrasonic and hand cleaning methods for plastic optics, and into learning the chemistry that works best for each plastic type.
Most lenses, prisms, and windows are cleaned ultrasonically using a few select chemicals and surfactants, followed by a hot rinse in deionized water, and a hot air dry to avoid damage. Our experience has taught us which detergent solutions are needed to correct specific cleaning or adhesion issues, and we work with different detergent vendors to find the best solution for a particular project. Sheet materials require their own unique handling methods to avoid damage and ensure cleanliness prior to coating, as well as specialized fixtures.
Many optical parts are required to meet a specific MIL-SPEC or similar set of environmental test requirements to guarantee adequate performance and reliability for a specific application. Typical tests include abrasion, chemical resistance, humidity and multiday temperature cycling.
Ability to withstand abrasion and chemicals depends primarily on applying a durable final coating layer; we typically use oxides for this purpose, which can handle most chemical tests. Severe temperature cycling, however, can be more challenging. Plastic substrates expand very differently from typical coating materials, increasing the stress on the interface. This makes it all the more important to ensure that parts are extremely clean, as any type of contamination can result in adhesion failure.
At AccuCoat, we have adapted our coating materials and layer designs to minimize stress on the substrate/coating interface, and apply the same level of attention to detail in our cleaning processes. We fine-tune all aspects of the coating process for each project, carefully considering the interplay of substrate material, form factor, and optical specifications to meet the environmental test requirements.
This is key to providing quality coated optics, as a coating which passes a specific environmental test on one substrate is not guaranteed to pass on another substrate simply by using the same process, nor can every coating necessarily pass all tests. At AccuCoat, we carefully consider the MIL-SPEC or other environmental requirements that must be met in volume, even when coating prototype parts. This ensures that the optical performance delivered initially can still be met once a product scales to volume, and reduces program management risk for our customers.
Our standard and custom coatings have been shown to pass a variety of challenging MIL-SPEC and ISO specifications, including tests such as those shown at right*.
Sample Coating Durability Tests*
Operating environment tests:
- 24 hour humidity exposure
- 10 day humidity exposure
- Temperature cycling: -62 °C to +85 °C
- Temperature shock: +23 °C/-57 °C/+71 °C/+23 °C
- Moderate abrasion
- Salt solubility (not appropriate for metals)
- Salt Fog (not appropriate for metals)
Chemical exposure tests:
- Insect Repellent
- Penetrating oil
- Carbon removing compound
- Lubricating oil
- Rifle bore cleaning compound
- Vacuum pump oil
[*Note: Not all tests will be passed by all coatings on all substrates. Please consult an AccuCoat engineer to discuss your specific requirements.]
Choose an Experienced Partner
Many variables influence adhesion of a coating to a plastic substrate, from the coating design, process and temperature to the cleanliness of the substrate (and the substrate material itself). Changing even one of these variables can significantly impact a coated plastic optic’s ability to pass MIL-SPEC or other environmental testing – putting your project schedule or product launch at risk.
At AccuCoat, we’ve put in the time to really understand the interplay of these various factors and adapt our processes accordingly. No one else has coated more plastic! Let our experience reduce your risk - contact us to discuss your project today.
In Part 2 of this series, we'll discuss some of the reasons for choosing plastic optics and the many coatings we can provide on plastic substrates. We'll also share case studies of a few successful projects, including laser sighting optics, mylar sheets, and hot mirrors for virtual reality. Send us an email at email@example.com to sign up for our newsletter and hear about more case studies and application notes!