Phosphorescent (Glow-in-the-Dark) Silicone Mold Formulations and Mixing Control

Glow-in-the-dark silicone is a phosphorescent pigment in polysiloxane, not a luminescent gum. Two families dominate: zinc sulfide (ZnS, usually copper-activated) and strontium aluminate (SrAl2O4 with Eu/Dy). SrAl is brighter and afterglows longer. ZnS is older, dimmer, and cheaper. Both are dense minerals. Both settle.

The usual failure is a stratified cup: bright sludge on the bottom, dead rubber on top. Mixing control is the work. A loading percent in an email is not a formulation. Platinum RTV is the common pour; tin will glow and keep shrinking; compounded LSR/HCR exists for production parts. A glow jacket and a glow part are different jobs.

SrAl versus ZnS is a pigment choice

Zinc sulfide phosphors are the older glow. Afterglow is shorter (minutes-class in ordinary talk, not a lab curve you should invent). Color is often green-yellow. Particles can be smaller than coarse SrAl grades, which helps surface finish and is why some thin coatings still use ZnS. Brightness after a strong charge is lower than modern aluminates. Some ZnS packages and some sulfide-containing leftovers are unfriendly to platinum cure. Test the lot. Do not assume a craft pigment is platinum-safe because it is “for resin.”

Strontium aluminate is the current default when someone wants a part that still shows in a dark room after the lights have been off for a while. Green is the efficient color; aqua and blue exist and are typically dimmer for the same charge. Particles are often larger and harder. They read as grit on a polished mold face or on a thin cosmetic wall. Many SrAl grades are coated because the host can hydrolyze with moisture; a wet bag is a dead bag and a platinum risk. SrAl is denser than the gum and wants to fall out of a thin RTV.

Neither pigment is a dye. You cannot “tint to glow” with a drop. Phosphorescence is particle loading, particle size, and a charge (UV and short-wave visible). A yellow daytime color does not prove a long afterglow. Charge the coupon the way the user will (room light versus 365 nm), then look in the dark after a stated wait. Do not publish a fake “hours of glow” number from memory.

Radioactive paints (old radium, some tritium) are a different technology. Do not confuse them with SrAl/ZnS shop pigments.

Settling is the failure mode

Phosphorescent powders are minerals. In a still, low-viscosity platinum RTV they sediment. A 20-minute rest after a good mix can undo the mix. Symptoms:

• Gradient glow: feet bright, logo dead
• A sandy layer that under-cures or tears
• Clear streaks that look like flow lines in the light and like holes in the dark
• A cup leftover that glows like a lantern while the part does not

Control, not slogans:

• Pick a thixotropic RTV if the pour can tolerate it. Treated fumed silica already in the base is what holds pigment. A water-thin clear platinum is the worst carrier for SrAl.
• Mix pigment into one component first if the TDS allows, wet thoroughly, then combine. Scraping the bottom of the cup is part of the mix, not optional.
• Vacuum after wetting. Dry powder is air. Vacuum on a dry dump foams.
• Shorten the time from last stir to gel. Do not degas for half the pot life and then pour a stratified mix.
• For a block mold, consider whether a long gel will re-settle in the cavity. Face coats of pigment-rich rubber, then a backfill of unfilled or lightly filled rubber, are a known art-mold trick. They are a two-layer part. Spec them if you use them.
• Production HCR milling and compounded LSR exist so you are not fighting Stokes’ law in a bucket.

A rotating fixture during gel is a prototype stunt. It is not a production control plan.

Particle size, surface, and the mold face

Coarse SrAl on a Class-A cosmetic part is texture, whether you wanted VDI grain or not. It also sands the master if you are pouring a glow jacket over a soft pattern. If the silicone is the mold, pigment at the cavity face copies into every casting as pits or sparkle. Keep glow pigment out of a precision mold rubber unless the jacket is allowed to be gritty. If the silicone is the product, decide whether glow is through the volume or only in a skin.

Dispersion quality is visible in the dark as speckle versus a even field. Agglomerates are bright dots with dark halos. A three-roll mill or a compounded masterbatch beats a stick in a cup. Shop RTV will never match a milled HCR sheet for uniformity.

Abrasiveness: SrAl is a ceramic. It is kinder than quartz flour at modest pigment levels and still unwelcome in a micro-gate LSR tool as a dump-in. Use a glow LSR from a compounder if you are injecting.

Mixing order, color, and cure

Daytime color and night color are different specs. Pigment plus a tint (especially carbon or heavy white) can kill perceived afterglow even when the phosphor is present. If you need a body color, test the tint on a charged coupon. Optical-clear unfilled platinum shows the pigment as a suspension; that can look speckled in the light and impressive in the dark.

Platinum inhibition: untreated ZnS, some sulfide residues, some moisture-damaged SrAl, and random craft “glow-in-the-dark” lots with unknown binders have all produced sticky faces. Run a small plaque with the production mix ratio. Include a heavily pigmented smear. Tin-cure will often crosslink through a dirty pigment and then move dimensionally for months. Use tin for a dirty one-off master, not for a glow part that must fit next quarter.

Food, skin, and toys: phosphorescent silicone is not food-contact by default. FDA 21 CFR 177.2600 does not attach to a craft pigment. ISO 10993 and toy rules (including CPSIA for children’s products in the US) are finished-article problems. Glow teething toys are a regulatory file, not a mix ratio. Keep SrAl dust out of a culinary mold shop the same way you keep other minerals out.

UV charging and outdoor use: afterglow needs a charge. A part that lives in a dark housing will not glow. Some SrAl grades handle weathering better than ZnS; that is still a compound and coating question, not a reason to skip a weather test if the part is outdoor safety gear.

Write the mix without a fake loading

“Carrier: platinum RTV [or named glow LSR/HCR]. Pigment family: strontium aluminate or zinc sulfide, supplier and lot, coated/uncoated as stated. Charge and inspect: [light source], [wait], visual or a customer method, on coupons from top and bottom of the shot. Homogeneity: no sludge line, density check if the part is thick. Thixotropic base required for pours. Mechanicals and tear on the filled mix. Contact/regulatory on the finished formulation. Ban shop percents and ‘hours of glow’ claims without a method.”

Refuse a PO that says “make it glow like the competitor” with no pigment family, no charge condition, and a photo taken with a long exposure. Long exposure is not afterglow.

FAQ

Is strontium aluminate always better than zinc sulfide in silicone?

SrAl is brighter and afterglows longer after a charge. ZnS can be finer and cheaper, and it still shows up in thin coatings. SrAl particles are often coarser and settle hard. “Better” is afterglow versus surface finish, cost, and whether the lot is platinum-safe. Test the lot; do not assume a craft jar.

Why is the bottom of my glow pour bright and the top dead?

Settling. Both pigments are dense minerals. A thin RTV lets them fall during pot life and during a long gel. Use a thixotropic base, wet then vacuum, pour soon after the last stir, and inspect glow on both ends of the part. Mixing longer and then resting the cup undoes the mix.

Can glow pigment inhibit platinum RTV?

Yes. Some ZnS packages, sulfide residues, wet or hydrolyzed SrAl, and unknown craft binders can leave a sticky face, especially in the pigment-rich sludge. Plaque-test the actual pigment in the actual base. Tin-cure may crosslink and then shrink in storage; it is not a free pass.

Will glow powder ruin the finish of a silicone mold jacket?

Coarse SrAl at the cavity face is texture and can pit the master. Keep phosphorescent pigment out of a precision mold rubber unless grit is allowed. For glow parts, decide volume fill versus a pigmented face coat plus a plain backfill, and spec that stack.

How much pigment do I add to get a long afterglow?

Do not invent a percent. Afterglow depends on pigment family, particle size, loading the mix can still suspend, thickness, tint, and charge. Specify SrAl versus ZnS, a charge method, a wait, and homogeneity. Use a compounded glow LSR/HCR when a cup cannot hold the suspension.