Yamamoto Chemical Thermosensitive Dye (Non-Electronic/EL Grade)

Yamamoto Chemical supplies the Thermosensitive Dye (Non-Electronic/EL Grade) as a microencapsulated reversible leuco dye system. The non-electronic/EL designation separates the product from electrically active colourants: it is not formulated as a conductive ink component, it does not require an electric field to change state, and it does not introduce conductive pigment residues into printed dielectric layers. The product is supplied as an aqueous dispersion and as a solventborne dispersion; the specification code is completed by the nominal colour-change temperature and the carrier system. Nominal activation temperatures can be selected from -5 °C to 65 °C in 5 °C increments for the standard series. When ordering, the grade is identified as Yamamoto Chemical Thermosensitive Dye, Non-Electronic/EL Grade, followed by the transition-temperature suffix and delivery form. A single alpha-numeric model number does not cover the full matrix.

In the coloured state, the microcapsule contains a leuco dye–developer complex dissolved in a fatty-acid or ester co-solvent. Heating above the activation band melts the co-solvent and dissociates the dye–developer complex, producing decolourisation; cooling restores the coloured state. The transition is reversible but hysteresis is intentionally present. The difference between melting and recrystallisation onset is commonly 2–6 °C in neat dispersion testing. The transition bandwidth is controlled to ±2 °C around the nominal point when measured by differential scanning calorimetry according to ISO 11357-4:2021 at 10 K/min under nitrogen.

How Does the Non-Electronic/EL Grade Compare with Conductive Ink and Electronic Display Colorants?

The main difference is the absence of conductive filler and the absence of an electrical drive requirement. A conductive ink or electronic thermochromic film contains silver flake, carbon, or conductive polymer and changes state only when bias is applied. The non-electronic/EL grade is electrically inert owing to the crosslinked polymer shell around each leuco dye core. In printed electroluminescent lamp construction, the grade can be placed as an adjacent colour layer without adding measurable leakage current, provided the binder is fully cured and the layer is not printed directly beneath a pinhole-prone phosphor dielectric. However, the product has low hiding power compared with opaque thermochromic composites, and it does not generate luminance. The colour change is observed by reflectance contrast, not emission.

Compared with liquid-crystal thermochromic sheets, the non-electronic/EL grade has a broader temperature operating range, lower viewing-angle dependence, and no requirement for a black background. Liquid-crystal systems rely on helical pitch changes and exhibit iridescent colours only within a narrow band, whereas the microencapsulated leuco system produces a reversible solid colour change. The trade-off is a less precise response for clinical thermometry: liquid-crystal strips can resolve 0.5 °C gradients, while the microencapsulated grade is specified for ±2 °C transition tolerance. Published data for direct colourimetric comparison under a 10° observer are limited for this grade.

Incoming release certificates for a standard aqueous dispersion list pH, non-volatile content, particle size distribution, and transition temperature. The pH is tested by ISO 787-9:2019; typical target is 7.0–9.0. Non-volatile content is determined by ISO 3251:2019 at 105 °C for 2 h, with target 42–48 mass%. Particle size distribution is measured by laser diffraction according to ISO 13320:2020; the standard dispersion usually shows D50 2.0–5.0 µm, D90 8.0 µm maximum. The thermal transition point is verified by differential scanning calorimetry at 10 K/min under nitrogen using ISO 11357-4:2021; batch acceptance is typically ±2.0 °C of the nominal suffix. Thermogravimetric analysis per ISO 11358-1:2022 is used to check decomposition onset; the supplier's quality certificate for this non-electronic grade reports less than 1.5 % mass loss at 200 °C after correction for residual moisture.

The aqueous dispersion should be stored at 5–30 °C and must not be frozen. Because ice crystals impose mechanical stress on the microcapsule shell, a freeze-thaw excursion below 0 °C can rupture capsules and cause a rise in free leuco dye in the supernatant. After any such excursion, particle size distribution should be rechecked according to ISO 13320:2020; a D90 increase above 8.0 µm indicates agglomeration or capsule rupture. The as-supplied aqueous dispersion has a shelf life of 6 months when stored in its original container at 20 °C; viscosity drift greater than 20 % from the release value is an out-of-specification condition.

In aqueous screen-ink manufacturing, the dispersion is introduced during the letdown stage with slow sweep agitation at 300–500 rpm. A Cowles blade may be used only for binder premix, not for direct dispersion of the microcapsules. High-shear media milling is incompatible because bead impact ruptures the crosslinked capsule wall; tip speeds above 10 m/s produce immediate loss of chromic intensity. The finished ink is typically filtered through a 200 µm mesh to remove agglomerates without retaining individual microcapsules. Viscosity is adjusted with a polyurethane associative thickener to 800–1500 mPa·s at 25 °C and 10 s⁻¹ shear rate on a cone-plate rheometer per ISO 2884-2.

Ammonia-based neutralising agents are preferred over amine-functional additives because residual amines can form complexes with the leuco dye and depress colour density. The aqueous vehicle must be buffered to pH 7.0–9.0; below pH 3.0, the melamine-formaldehyde capsule shell may swell, causing premature colour loss. Cationic wetting agents should not be used above 0.2 mass% because they can displace the dispersant and induce microcapsule flocculation.

In water-based flexographic ink, the grade is not suitable for thermostatic press chambers above 40 °C because the colour state of a 31 °C grade begins to decolourise in the ink tray before printing. Press operators maintain ink temperature at least 10 °C below the nominal activation point. Viscosity, pH, and foam control are adjusted before letdown; defoamers containing silicone are acceptable only at 0.1 mass% maximum because higher concentrations can spread on the capsule surface and reduce overprint adhesion.

The practical colour-loading window is narrow. Below 5 mass% total microcapsule content in a dry ink film, the colour change is visible only under controlled lighting; above 30 mass%, the dried film becomes brittle and ink transfer efficiency drops. Formulators typically set the microcapsule addition between 10 % and 20 % of total wet ink weight, but this must be adjusted for the dispersion's non-volatile content. In a laboratory proofing drawdown at 12 µm wet film, a 15 % addition of a 45 mass% dispersion delivers a colour contrast ΔE*ab of 8–14 depending on the paper substrate and the underlying white reflective layer.

Repeated thermal cycling is a known fatigue mode. Laboratory cycling between 4 °C and 40 °C at 0.5 K/min shows gradual loss of colour contrast after 5000 cycles; the supplier's technical bulletin for this product class does not specify a fatigue limit for all formulations. For high-cycle applications, the formulator validates cycle life on the finished printed piece rather than on the raw dispersion. This limitation is independent of the non-electronic/EL designation and applies to microencapsulated leuco systems generally.

In printed electroluminescent lamp construction, conventional conductive silver and dielectric layers must be fully cured before the thermosensitive layer is applied. Residual solvent in the dielectric can diffuse into the non-electronic grade and shift the transition temperature. The grade is not intended to be printed directly between the front and rear EL electrodes, because it is not a dielectric replacement. Dielectric strength testing according to ASTM D149 on a 15 µm dry thermosensitive film is not part of the release specification; the material is not classified as an insulating barrier.

When a Screen-Printed Ink Layer Exceeds 20 µm Dry Film Thickness in Abrasion-Prone Packaging

Film thickness and capsule protrusion are the main failure variables in packaging rub applications. A 77 T mesh PET screen with 68 µm emulsion thickness deposits a wet film of 30–40 µm and a dry film of 14–19 µm from a 45 mass% solids ink. At dry films above 20 µm, capsules protrude from the binder surface and are fractured under concentrated load. In rub testing with a Sutherland tester according to ASTM D5264-98(2019), unmodified formulations at 25 µm dry film show visible capsule fracture after 50 cycles under 1.8 kg weight. Flexible urethane–acrylic hybrid binders at an ink-to-microcapsule volume ratio not exceeding 1:3 retain measurable colour contrast to 200 cycles but increase haze. The correct remedy is to reduce dry film thickness to 15 µm or to apply a UV-cured overprint at 5–8 µm to distribute surface shear.

Thermoplastic masterbatch processing is constrained by the thermal stability of the microcapsule wall. A 25:1 L/D co-rotating twin-screw extruder with temperature profile 150/155/155/150 °C and screw speed 250 rpm is used for low-density polyethylene carrier resin; the grade is fed downstream by side stuffer to limit residence time. At 190 °C melt temperature and residence time above 3 min, the colour-change intensity after cooling is reduced because the co-solvent leaks through the shell, not because the leuco dye itself decomposes. Injection molding with a 90-ton clamp force and barrel set 180 °C produces reversible colour transitions in polypropylene closures; barrel set 220 °C is above the upper processing limit and should be avoided for cycle times exceeding 45 s.

Lightfastness, solvent migration, and sterilization cycle boundaries

The product is intended for short-term indoor and cold-chain use. In accelerated xenon-arc exposure according to ISO 105-B02:2014, the aqueous dispersion printed on coated paper shows a blue wool scale rating of 3–4 at 20 AATCC fading units. Long-term outdoor use is not specified; ultraviolet absorber overprint can extend contrast retention but increases dry film thickness and changes the activation temperature. Solvent exposure must be limited to aliphatic hydrocarbons and low-polarity vehicles; methyl ethyl ketone, ethyl acetate, and N-methyl-2-pyrrolidone at greater than 5 mass% of the formulation are incompatible because the capsule shell is thermoset and not solvent-resistant. Sterilization by gamma irradiation at 25 kGy per ISO 11137-1:2016 has been reported to shift the transition temperature by less than 1 °C in a polypropylene closure, but electron-beam irradiation above 30 kGy can degrade the developer and cause incomplete decolourisation. Published data for this specific configuration is limited for high-dose gamma irradiation above 50 kGy.

In screen-printing on glass and metal substrates, the non-electronic/EL grade is paired with an adhesion promoter and is not subjected to thermal post-cure above 120 °C because capsule wall permeability increases with temperature. Adhesion testing on glass using a crosshatch method per ISO 2409:2020 is recommended after 24 h conditioning at 23 °C and 50 % relative humidity.

Cold-chain indicator labels use the product at 8 °C and 15 °C transition points. In a typical label construction, a screen-printed thermochromic patch is overlaid with a UV-cured overprint. The colour change from blue to colourless is evaluated against a colour difference meter; ΔE*ab is greater than 10 when measured with a D65 illuminant and 10° observer per ISO 11664-4:2008. Deployment below -5 °C is not recommended because co-solvent crystallisation can alter the transition bandwidth. For medical skin-contact devices, the grade must be encapsulated behind a non-migrating barrier layer; cytotoxicity testing per ISO 10993-5:2009 is the responsibility of the finished device manufacturer, and the supplier's raw dye dispersion is not supplied as a sterilised medical-grade product.