“Considerations for Selecting a Dome Coating”

With hundreds of dome coating options available on the market, selecting the right dome coating for your application can seem like a daunting task. When selecting the best coating for your application, it is important to consider five main criteria: viscosity, substrate flexibility, substrate adhesion, hardness, and environmental durability.

• Viscosity. How thick does the coating need to be? Decorative coatings are clear and usually thin. Dome coatings have curved domes, which optically magnify and enhance the appearance of a label. Higher-viscosity, thicker resins generally produce taller domes.
• Substrate Flexibility. Flexible substrates, like thin polyester labels, paper, or soft plastics, may require careful selection of coating and enhanced curing equipment. Flexible substrates may bow or warp when thick coatings cure. Soft coatings shrink less and are designed to reduce or eliminate warpage during cure. Rigid substrates don’t bend when stressed, so they can be coated with all of the products.
• Substrate Adhesion. Individual formulations may have excellent adhesion to some inks (or substrates) and little adhesion to others. The right coating should have good adhesion. Adhesion should be tested after product lifetime testing, as well as after cure. Inks and substrates from second source suppliers may improve or worsen adhesion. Some plastics require surface treatment to enhance adhesion.
• Hardness. Both soft and hard coatings can be scratch resistant. Hard coatings resist scratching because of their hardness. Soft coatings resist scratching because they momentarily dent and then spring back when the scratching object is withdrawn. Hard coatings are measured on the D-hardness scale. Soft coatings are measured on the hardness scale. On both scales, higher numbers imply harder coatings.
• Environmental Durability. Coatings should be lifetime tested on your product. Some properties, like UV weatherability, provide a relative guideline to distinguish endurance of clarity and durability among the available products offered.

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“Guidelines for Protecting Light-Sensitive Adhesives from Curing in Dispensing Equipment in Manufacturing Environments”

Unfortunately, manufacturing facilities that use standard fluorescent lighting sometimes have a problem with the polymerization of UV/Visible light-curable adhesives in dispenser tips and lines. The chemistry of these products make them extra sensitive to light, so extra measures must be taken to insure that unwanted polymerization does not occur.

To prevent this problem, manufacturers should shield needle tips and lines from light exposure. Incorporating lines that are black, opaque, polyethylene plastic will completely block light from the adhesive and eliminate the chances of unwanted polymerization. Dymax provides light-blocking materials for syringe needle tips. If these measures are insufficient and a process can’t be shielded through use of light-blocking materials, the fluorescent lamps in the facility could be retrofitted for existing incandescent and fluorescent fixtures.

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“I need to find a UL-listed conformal coating. Can you tell
me a little bit about the UL designation and do you have any conformal coating materials with this rating?”

Underwriters Laboratories^® (UL) is a global independent safety science company offering expertise across five key strategic businesses: Product Safety, Environment, Life & Health, University, and Verification Services. UL has been working for a safer world since 1894 and partners with manufacturers to provide safe products to consumers through UL testing, certification, and follow-up audits.

DYMAX has many conformal coatings that are UL recognized; they can be found on the DYMAX website and are denoted with the UL marking. To retrieve the most up-to- date information on DYMAX conformal coatings, log onto the Underwriter Laboratories website www.ul.com, click on Certifications, and input DYMAX’s UL File Number, QMJU2.E140512.

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“What is Micro Coating?”

Micro coating is a process in which durable coatings are selectively applied to small areas on an electronic assembly that require protection from elements in their operating environment, which commonly include moisture and liquids. Micro coating was first adopted by consumer devices to reduce device thickness. It is now beginning to catch on in other markets including aerospace and automotive. Micro coating applications are often very similar to encapsulation, but the materials are typically applied over the surface of leads with a high-accuracy automated application system. Many of DYMAX’s conformal coatings and encapsulation materials can be used as micro coatings.

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"I am trying to determine the proper cure time for the Loctite 3106 using a Dymax PC-5 Light Welder. Can you help?"

To determine the proper cure time of any light-curable adhesive when exposed to light from any light source, there are a couple of different approaches that can help. The greatest tool is a radiometer, which will tell you how much intensity you have at the bond line. The PC-5 is an older model flood lamp, with an intensity of 50-150 mW/cm² over a 5" x 5" area. The different approaches depend on how you are using the adhesive. If you are using the adhesive between two substrates in a bond-line thickness of 0.002-0.006 inches, then measuring the fixture time should be sufficient. Per the Loctite TDS, fixture time at this intensity should be <5 seconds. If you are potting a deeper section, then depth of cure is important, and you can reach a depth of 2 mm in approx 12 seconds. The Loctite TDS plots the depth of cure at an intensity of 50 mW/cm². If the adhesive bond line has some squeeze out, or has a surface exposed to air, then a tack-free surface cure may be important. Tack-free time is the point when the adhesive is sufficiently cured that you will not get smearing or residue transfer onto a gloved finger.

With any of the three described situations, measuring this yourself is the best way to figure out the proper cure time, whether looking at fixture time, depth of cure, or tack free time. Set the bond line up at the lowest intensity you can use – say 50 mW/cm². Do this by increasing the distance away from the lamp until the radiometer measures 50 mW/cm². (You will want to manufacture your parts at a higher intensity to start, and within a window of intensity and time. This will control your process.) After setting a constant intensity, cure the adhesive for different times. You will see the tensile strength, burst pressure, tack-free time, depth of cure, durometer, or other datapoint climb to a max value and then plateau. Once you have identified the start of the plateau, add a safety margin, and you have the foundation for your process. You can also set the time constant, vary the intensity, and record the same datapoints. You want to define your process by knowing the minimum and maximum intensity and time needed to cure the adhesive.

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I see smoke coming off the light-curable adhesive…what is it?

Before we can answer this question we have to first understand what it’s not.

By definition¹, it’s not smoke or vapors or outgassing.

What you really are seeing coming off of the curing adhesive are fumes generated by the light-curing process. This phenomenon is the result of a very rapid polymerization or chemical reaction that occurs when the liquid adhesive is exposed to the correct wavelength of light. Both heat given off during the reaction (at the molecular level) and heat from the absorption of light energy can, in some instances, result in a small amount of adhesive fumes being emitted before the product has a chance to completely polymerize or cure.

Essentially, this phenomenon may emit trace amounts of some of the ingredients (or fractions of the ingredients) contained in the formulation. Please note that the volatilization may or may not be noticeable, but is almost always a very small amount.

Are the fumes hazardous? Always consult the MSDS to answer this question. However, if the liquid itself poses a risk to the user, then good manufacturing practices for the particular process may suggest incorporating an exhaust system in the bonding area to remove the fumes during the light-curing step.

Definitions¹:
Vapor: The gaseous state of a substance that is solid or liquid at temperatures and pressures encountered. NIOSH (National Institute for Occupational Safety and Health) Definition
Fume: A solid condensation particulate, usually of a vaporized metal. NIOSH Definition (This could also be generated from curing our adhesives)
Outgassing: The release of absorbed or occluded gases or water vapor, usually by heating in a vacuum. (Web definition)
Smoke: The vaporous system made up of small particles of carbonaceous matter in the air, resulting mainly from the burning of organic material. (Web definition)

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