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Functional films are the invisible workhorses of flexible electronics, photovoltaic (PV) modules and lithium-ion energy storage. A 50 µm PET or polyimide (PI) web carries hard-coats, barrier layers, index-matching coats, encapsulants and separator ceramics; the colorant and nano-dispersion used inside those coats determines whether the final film passes haze, adhesion, weathering and regulatory audits. This article from Dongguan DENSON Functional Materials Co., Ltd. (DENSON) ties film architecture, coating chemistry and pigment engineering into one selection workflow for converters coating UV-curable and waterborne systems onto PET/PI web.
A functional film is a thin polymer web whose surface chemistry, optical index, dielectric constant or pore structure has been engineered to do one specific job: outcouple light from an OLED, block water vapour from a perovskite cell, or hold lithium ions between anode and cathode. Across the three fastest-growing end markets, the film is no longer a passive substrate—it is an active electrical and optical component.
Flexible and foldable displays: colour filter (CF) films, circular polariser stacks, hard-coated PET, TAC and colourless polyimide (CPI) substrates, plus optically clear adhesives (OCA). Even a 0.1 % change in transmittance or a haze spike above 0.5 % is visible to the eye.
Photovoltaics: EVA and POE encapsulant films laminate the cells, while the backsheet must block UV, insulate at 1500 V system voltage and survive 25 years outdoors. White TiO2 or carbon-black pigments in the backsheet and POE dispersion in the encapsulant directly influence module power and PID resistance.
Energy storage: microporous PE/PP separators (typically 7–25 µm) and ceramic-coated separators (alumina or boehmite dispersed in waterborne acrylic) require extremely clean, ash-free pigment and binder packages; any heavy-metal or ionic contamination accelerates electrolyte oxidation.
For a colorant supplier, this means every pigment grade shipped into these markets must be qualified not only for colour, but for optical purity, ionic cleanliness, UV stability and regulatory traceability.
Although the three end markets look different on paper, they share the same converting logic: unwind a polymer web, meter-coat a thin functional layer, cure it inline (UV, IR or thermal oven), and rewind under tension control. The table below summarises the dominant film stacks.
| End market | Core film layers | Typical substrate | Coating chemistry | Critical colourant role |
|---|---|---|---|---|
| Flexible / foldable OLED display | CPI substrate, barrier SiNx, OC hard-coat, circular polariser, OCA | CPI (50–100 µm) or PET (100–125 µm) | UV-curable acrylate hard-coat; waterborne anti-glare coat | Low-dose carbon-black or graphite for light-blocking bezel; near-zero haze |
| PV front encapsulant | Glass / EVA or POE / cells / EVA or POE / backsheet | EVA or POE (0.45–0.60 mm embossed) | Cross-linked EVA; POE with silane grafting | Fluorescer-free; optionally low-dose TiO2 in white POE |
| PV backsheet | PVDF / PET / PVDF (TPT) or PA / PET / PA | BOPET 250–300 µm, white TiO2-filled | Waterborne fluoropolymer or PVDF coating | Rutile TiO2 for reflectivity >85 %; carbon black in edge-seal coats |
| Battery separator | Microporous PE or PP, often ceramic-coated | PE / PP monolayer or PP/PE/PP trilayer (7–25 µm) | Waterborne boehmite / alumina + acrylic binder | No deliberate pigment; ultra-clean alumina with <50 ppm Fe, <10 ppm chloride |
| EV battery insulation film | Mylar/PET laminated to acrylic adhesive | PET 25–75 µm, sometimes mica-filled | UV or thermal acrylic | Talc / BaSO4 matting; no halogens |
Figure 1. Low-kV SEM of a commercial PE battery separator (TESCAN CLARA). The open pore network must remain open to lithium-ion transport while blocking dendrite penetration; pigment and dispersant residue in the pores is unacceptable.
The substrate fixes the upper temperature limit, the optical baseline and the surface energy the colorant must wet.
BOPET (biaxially oriented PET) dominates at 25–250 µm. It offers ~91 % total transmittance, a glass transition around 80 °C, and a corona-treated surface of 44–52 mN/m. PET is low cost, globally available, and the default web for backsheet, optical hard-coat and insulation applications. Its weakness is UV resistance: uncoated PET yellows after extended xenon exposure, which is exactly why backsheet converters coat a fluoropolymer layer on the sun-facing side.
Polyimide (PI) films such as DuPont Kapton remain amber (transmittance ~70 % in visible, near-opaque to UV) and survive 200–400 °C continuously. Amber PI is the standard substrate for flexible printed circuits, heater films and high-temperature insulation. Colourless polyimide (CPI) is the newer substrate for foldable OLEDs: it pushes transmittance above 88 % at 550 nm while retaining 100k+ bend cycles. Because CPI is already faintly yellow, the hard-coat on top must use a low-dose blue-violet optical brightener or fluorescent whitening agent carefully—any over-dose triggers blue-haze complaints in display QC.
Converters today split functional coats into two families, and the colorant must be matched to the cure chemistry.
UV-curable acrylate coats run at 20–100 m/min with low-temperature ovens, and give 1H–4H pencil hardness, 3M tape adhesion and high cross-link density. They are the default for display hard-coats, anti-fingerprint coats and index-matching layers. Pigments must be pre-dispersed in acrylate monomers (typically hexanediol diacrylate or aliphatic urethane acrylate) at low mill-base viscosity, and must not quench the photo-initiator at 365 nm. Carbon-black grades designed for UV coats are surface-treated to minimise UV absorption in the initiator band.
Waterborne coats are mandated for PV backsheet, ceramic separator coating and any market targeting low-VOC or low-halogen declarations. Waterborne acrylic or polyurethane dispersions are coated at 200–600 J/m2 dry weight and dried in 80–120 °C arch ovens. Here the pigment must be supplied as a fully water-wetted, defoamer-stable dispersion; rutile TiO2 and precipitated BaSO4 are the workhorse pigments, while boehmite (AlOOH) slurries are the functional white ceramic on separators.
A practical caution for specifiers: DENSON's FLUV and EB series are genuinely solvent-free UV systems, while the GT series is purely waterborne. The FLTY high-solids grade retains roughly 20 % PMA (propylene glycol methyl ether acetate, CAS 108-65-6) and must not be marketed as solvent-free—this distinction matters on PV and display RoHS/low-VOC declarations.
Functional-film colorants are judged on five parameters rather than on tint strength alone:
Particle size distribution. For optical films, the pigment's D90 must be below one-tenth of the visible wavelength (<40 nm for true transparency, <150 nm for acceptable haze). Larger particles scatter light and inflate haze measured per ASTM D1003.
Weathering stability. Rutile TiO2 (surface-treated with Al/Zr/Si) is preferred over anatase for outdoor PV use; anatase photocatalyses polymer chalking. Fluorescent whitening agents must be grade-approved for 25-year outdoor exposure per IEC 61730.
Ionic and heavy-metal cleanliness. Battery-facing dispersions must meet <100 ppm total ash, <50 ppm Fe and <10 ppm chloride. Cadmium, lead, chromium(VI) and mercury are excluded by RoHS 2 (Directive 2011/65/EU as amended).
Dispersant compatibility. Polymeric wetting dispersants (BYK- or Efka-type anchor resins) must match the binder chemistry: acrylate-compatible dispersants for UV coats, anionic/non-ionic acrylic dispersants for waterborne. A dispersant mismatch shows up as viscosity drift, settling in the pan, or a haze rise after 72 h.
Regulatory traceability. Every pigment lot ships with a TDS, batch-specific SDS, REACH SVHC declaration (current ECHA candidate list), RoHS test report, and (where the film touches packaging) EU 10/2011 or FDA 21 CFR parts 175–178 migration statements.
| Film / function | Preferred colorant | Typical loading | Coating vehicle | Key QC test |
|---|---|---|---|---|
| Display hard-coat (anti-scratch) | Nano-silica (10–30 nm) + trace fluorescent whitener | 10–25 wt% | UV urethane acrylate | ASTM D3363 pencil ≥3H; haze <1.0 % |
| Display light-blocking bezel | Surface-treated channel black | 3–8 wt% | UV acrylate or waterborne | Optical density ≥2.0 at 400–700 nm |
| PV backsheet white coat | Rutile TiO2 (Al/Zr treated) | 20–35 wt% | Waterborne PVDF / PA | Reflectivity >85 %; ISO 4892-3 / ASTM G154 >6000 h no chalking |
| POE encapsulant white | TiO2 masterbatch in POE carrier | 2–5 wt% | POE extrusion | Yellowing index ΔYI <3 after 1000 h UV |
| Ceramic separator coating | Boehmite γ-AlOOH (D50 ~600 nm) | 15–25 wt% slurry | Waterborne acrylic / SBR | Air permeability 150–250 s/100 mL; thermal shutdown <140 °C |
| EV insulation / Mylar tape | Precipitated BaSO4 matting | 5–10 wt% | UV or thermal acrylic | Dielectric breakdown >120 V/µm; IEC 61249-2 low-halogen |
| EMI-shielding film | Flake silver or copper, or graphite | 15–30 wt% | UV urethane or waterborne | Sheet resistance <1 Ω/sq; RoHS / REACH SVHC clear |
Figure: Glass-glass PV module production at Solarwatt's Dresden factory (Germany); EVA/POE encapsulant layers are applied and laminated before framing and final test.
A colorant qualification programme on functional films should follow this minimum test matrix. DENSON recommends running all of the following on incoming mill-batches, and the first article after any pigment lot change.
Optical: total transmittance and haze per ASTM D1003 (haze-gard II or equivalent); yellowness index per ASTM E313; optical density at 400–700 nm for light-blocking coats.
Mechanical / adhesion: cross-cut tape adhesion per ASTM D3359 (target 4B–5B); pencil hardness per ASTM D3363; steel-wool abrasion for hard-coats; flex endurance (≥200k cycles, R=3 mm) for CPI-based display films.
Weathering: fluorescent UV / condensation exposure per ASTM G154 Cycle 1, or xenon-arc per ISO 4892-2. PV backsheets are typically qualified to ≥6000 h with ΔE<3 and no chalking/cracking before module lamination.
Electrical: dielectric breakdown per IEC 60243; volume resistance; surface resistance for EMI/ESD grades; ionic conductivity soak test for separator ceramics.
Regulatory: RoHS XRF scan; ion chromatography (Cl−, SO42−, Na+, Fe); total halogens per IEC 61249-2 (chlorine <900 ppm, bromine <900 ppm, total <1500 ppm); phthalate GC-MS (DEHP, DBP, BBP, DIBP).
Figure 3. Operator loads a crystalline silicon module into a vacuum laminator. EVA/POE encapsulant and backsheet pigments must be stable through the 140–150 °C, 15–20 min lamination cycle without outgassing, bloom or yellowing.
Shipping functional films into the EU, US and Japanese markets requires the following compliance stack, and DENSON prepares it on every lot:
REACH (EC 1907/2006): SVHC candidate-list declaration updated with every ECHA publication; Annex XVII restrictions (notably the phthalate entries and certain PAHs); SDS to Regulation (EU) 2020/878 format with GHS/CLP hazard statements.
RoHS 2 / (EU) 2015/863: Pb, Hg, Cd, Cr(VI), PBB, PBDE, DEHP, BBP, DBP, DIBP all below 0.1 % homogeneous material (Cd below 0.01 %).
Low-halogen / IEC 61249-2: mandatory for electronics-grade flexible circuits and insulation films; pigments such as chlorinated phthalocyanines and brominated flame retardants must be excluded.
Food-contact / packaging: if the film touches packaging, EU 10/2011 plastic directive and/or FDA 21 CFR parts 175–178 apply; heavy-metal and primary-aromatic-amine migration limits must be reported.
CBAM (EU Carbon Border Adjustment Mechanism): for pigment feedstocks with embedded carbon (TiO2, carbon black, aluminium), importers must report embedded emissions; DENSON works with pigment suppliers to provide the necessary emissions documentation.
Product stewardship: GHS/CLP labelling, UN transport classification where relevant, and batch traceability linking every reel back to a specific pigment mill-batch.
Figure 4. White PV backsheet film on the rewinder of a converter line. The high-reflectivity rutile TiO2 coat must survive ISO 4892-2 / ASTM G154 weathering, damp-heat (85 °C / 85 % RH) and PID testing for a 25-year module warranty.
Q1: Can the same carbon-black dispersion be used in display hard-coat and PV backsheet?
No. Display-grade blacks are low-dose, UV-transparent in the initiator band and milled to D90 <100 nm; outdoor backsheet blacks must weather for 25 years and often use furnace blacks with a different surface treatment. Mixing them up causes either incomplete UV cure or premature UV chalking.
Q2: Why is rutile TiO2 preferred over anatase in PV backsheets?
Rutile is chemically stable, has a higher refractive index (2.7 vs 2.55) and much lower photocatalytic activity. Untreated anatase generates surface radicals under UV that chalk the fluoropolymer binder. Always choose Al/Zr/Si double-treated rutile for outdoor exposure.
Q3: What is the practical haze limit for a foldable-OLED hard-coat on CPI?
Display converters typically specify total transmittance ≥89 %, internal haze ≤0.5 % and yellow index b*≤2.5. Pigment D90 above 150 nm almost always blows the internal-haze budget.
Q4: Is DENSON FLTY a solvent-free product?
No. FLTY high-solids grades contain approximately 20 % PMA (CAS 108-65-6) and must be declared as high-solids solvent-borne, not solvent-free. The genuinely solvent-free UV range is FLUV/EB, and the purely waterborne range is GT.
Q5: Which standards should be cited on a functional-film colorant TDS?
At minimum: ASTM D1003 (haze/transmittance), ASTM D3359 (adhesion), ASTM D3363 (pencil hardness), ISO 4892-2 or ASTM G154 (weathering), IEC 61249-2 (low-halogen), IEC 60243 (dielectric breakdown), plus REACH SVHC and RoHS declarations.
Functional-film colorants are system materials, not commodity pigments. Particle size, ionic cleanliness and weathering stability matter as much as tint strength; every grade must be qualified against ASTM D1003, ASTM D3359 and ISO 4892 / ASTM G154 in the actual coating vehicle.
Match the dispersion to the cure chemistry. UV-curable display hard-coats need acrylate-wetted, photo-initiator-friendly nano-pigments; waterborne PV backsheet and ceramic-separator coats need fully dispersed, low-ash, RoHS/IEC 61249-2 certified water-based pigments. Never substitute a solvent-borne mill-batch into a water line.
Compliance is a documented stack, not a checklist. REACH SVHC, RoHS, low-halogen IEC 61249-2, no phthalates/heavy metals, GHS/CLP SDS and (where applicable) EU 10/2011 / FDA 21 CFR and CBAM documentation must travel with every lot. DENSON supports converters with this full documentation chain for colorants and nano-dispersions designed for PET/PI web in flexible displays, photovoltaics and energy storage.
Mr. Jia: 159-1691-0100 13360650502 Mr. Jiang：13924940045
Email：denson168@163.com
Address: Room 101, No. 1, Lane 11, Changsheng Street, Dalian Tang, Wanjiang Street, Dongguan City, Guangdong Province
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