An excimer UV-cured soft-touch wood coating is a radiation-curable topcoat in which short-wave excimer light (172 nm vacuum ultraviolet or 254 nm UVC) cures only the uppermost layer of the film. The difference in shrinkage between that skin and the still-uncured layer below produces 1-3 micrometre surface wrinkles, so low gloss and a silky hand feel are obtained without any matting powder. Unlike the traditional "add matting powder" logic, gloss here comes from a controllable physical surface profile rather than from particle scattering.
TL;DR
- Gloss comes from diffuse reflection off 1-3 micrometre surface wrinkles, not from matting powder, so settling, mottling and haze over dark stains do not occur.
- Each of the three curing stages has its own duty: LED pre-cure controls how far the lower layer has cross-linked, the excimer lamp controls surface shrinkage, and the mercury lamp completes bulk cross-link density. Drifting outside any window shows up as gloss rebound, weak velvet feel, cracking or intercoat separation.
- 172 nm is vacuum ultraviolet and needs a nitrogen blanket (120-200 m3/h) because 160-240 nm converts oxygen into ozone; 254 nm UVC works in air, which is why most flat-panel lines use the 254 nm route.
- Acceptance has to be numeric: pick the geometry per GB/T 9754-2025 (85 degrees below 10 GU), specify the dynamic friction coefficient (target below 0.1) and judge durability by gloss rebound after abrasion plus cross-cut and pencil hardness.
1. Key figures
- Photon energy and penetration: 172 nm excimer light about 7.2 eV with 100-500 nm penetration; 254 nm UVC about 4.89 eV and deeper, so it passes through a thicker coat in one pass.
- Wrinkled skin thickness 1-3 micrometres; recommended total dry film 20-30 micrometres; flat-panel roller application rate 30-45 g/m2.
- Energy window (254 nm route): 395 nm LED pre-cure 100-200 mJ/cm2 (production range 1000-4000 mJ/cm2); 254 nm surface dose 50-400 mJ/cm2 at 5-20 mW/cm2 with 10-120 s surface reaction; rear high-pressure mercury UVA 600-2000 mJ/cm2 at 50-500 mW/cm2.
- Gloss shift: from about 100 GU down to 1-20 GU at 60 degrees; production super-matte targets are usually 2-5 GU, and premium panel buyers commonly ask for no more than 8 GU.
- Hand feel and anti-fingerprint: dynamic friction coefficient below 0.1; palmitic acid, the main sebum fatty acid, has a surface energy near 38 mN/m, so the coating must fall below roughly 25 mN/m before oils stop wetting out.
- Typical mechanical and chemical resistance: pencil hardness 2H-4H (GB/T 6739), cross-cut grade 0 (GB/T 9286), more than 300 RCA cycles without a shiny track, 500 wipes with 99.7% ethanol without film damage, and no anomaly at 60 degrees C / 95% RH for 72 h with organic acid, sunscreen and hand cream spots.
- Equipment limits: 172 nm excimer lamp life 1000-3000 h; a 254 nm tube needs 10-12 min warm-up before power stabilises.
- Measurement precision: GB/T 9754-2025 gives a repeatability limit of 1 GU and reproducibility limit of 2 GU at 85 degrees (2 GU and 3 GU at 60 degrees); condition samples at (23+/-2) degrees C and (50+/-5)% RH for at least 16 h and measure at five positions minimum.
2. How the film forms gloss and feel
2.1 Division of labour in the three stages
The system does not rely on one lamp to "dry the paint". Curing is split: a low-dose 395 nm LED pre-cure builds a gel skeleton while leaving the lower layer partially unreacted; a 254 nm or 172 nm dose then acts on the very thin surface, which polymerises and shrinks while the layer below does not follow, pulling a continuous micro- and nano-scale wrinkle profile; finally high-pressure UVA floods the middle and bottom layers to restore hardness, adhesion and chemical resistance. The three doses are coupled. Too much pre-cure locks the bottom early and gloss rebounds; too little lets the skin collapse, so the panel feels tacky and the velvet effect disappears.
2.2 Where 172 nm stops and 254 nm starts
172 nm vacuum ultraviolet carries about 7.2 eV per photon and penetrates only 100-500 nm, so the surface-first effect is strong and gloss can be pushed below 2 GU while a light scratch still heals thermally. The price is a nitrogen atmosphere: the 160-240 nm band turns oxygen into ozone, and flow must hold 120-200 m3/h, which raises capital and running cost. 254 nm UVC can be run in air with a much lower investment, but surface-shrinkage uniformity then depends on formulation, so resin functionality and the photoinitiator package have to be tuned to deliver 2-5 GU together with a smooth glide.
2.3 Functionality: matte, abrasion and adhesion in one triangle
Published formulations describe a mass-ratio waterborne excimer UV coating built from 20-30% difunctional waterborne polyurethane acrylate, 25-35% multifunctional (functionality 4-16) polyurethane acrylate, 35-40% reactive monomer, 3-6% photoinitiator and 0.5-1% additives, with the di- to multifunctional ratio recommended at (4-6):(5-7). Raising multifunctional resin sharpens shrinkage, gloss and abrasion resistance but consumes polar groups and weakens bonding to the substrate; raising difunctional resin relieves internal stress and improves adhesion while gloss climbs back. Treat this curve as a constraint in formulation review, not as something a single sample can settle.
3. Process window
| Parameter | Typical window | Symptom when off | Correction |
|---|---|---|---|
| Topcoat pick-up | 30-45 g/m2 (roller) | Too thin: dry feel, high gloss. Too thick: cracking, flat velvet | Fix pick-up first, then tune energy; never two variables at once |
| LED pre-cure | 100-200 mJ/cm2, or 20-40% of machine power | High: no matting. Low: tacky and later gloss rebound | Read 85-degree gloss plus touch test, converge in 10% steps |
| 254 nm surface dose | 50-400 mJ/cm2 at 5-20 mW/cm2 | High: ice-flower, bright spots, delamination. Low: insufficient wrinkle | Slower lamp pass raises dose, but keep line speed linked |
| Rear mercury UVA | 600-2000 mJ/cm2 | Under-cure: chemical and hardness loss, whitening after wiping | Back-calculate from pencil hardness and solvent rub count |
| Levelling and panel temperature | Levelling zone 45-50 degrees C, topcoat about 30 degrees C | Cold panels retain water and gloss varies across the sheet | Flash-off must remove water before the excimer zone |
| Nitrogen flow (172 nm) | 120-200 m3/h | Flow drift causes centre-to-edge gloss difference on one panel | Log flow every shift and put it under SPC control |
3.1 Pre-drying and oxygen inhibition
Waterborne excimer systems are sensitive to residual water: if flash-off is short, vapour lifts the skin into an irregular profile and the panel shows local bright spots plus a gritty feel. Free-radical UV curing is also oxygen sensitive, and a surface stays tacky where oxygen quenches radicals. The nitrogen blanket at 172 nm serves both to avoid ozone and to reduce surface inhibition; running 254 nm in air instead demands more initiator or a three-lamp sequence, which brings extra demands on yellowing resistance and thermal stability.
3.2 Should your line add an excimer station
Decide on three constraints rather than on "can it get mattier": whether the roller line has lamp positions and space for a nitrogen retrofit, whether the parts are mainly flat panels, and whether after-sales can absorb the recoat cost once thermal healing is exhausted. Profiled and three-dimensional parts are usually sprayed, where wrinkle uniformity is hard to hold, so self-matte polyurethane dispersion blended with a little matting powder remains the safer route.
4. Against the matting-powder route
| Aspect | Waterborne/UV plus matting powder | Excimer physical matting |
|---|---|---|
| Gloss stability | 3-7 GU, batch and panel mottling | 0-5 GU, uniformity set by lamp sequence |
| Clarity | Powder haze, weak black depth | No particles, full wood-grain and dark-tone recovery |
| Hand feel | Wax-driven, dry and gritty | Continuous wrinkle, glide below 0.1 friction coefficient |
| After abrasion | Particles flatten and shiny patches appear | Profile fixed by the cross-linked skin, little gloss change |
| Investment | None beyond normal let-down | Excimer station, nitrogen (172 nm) and lamp-life management |
| Typical failures | Settling, gloss drift, uneven feel | Cracking and delamination, ice-flower, chemical under-performance |
5. Verification and acceptance
5.1 Gloss angles and panel uniformity
GB/T 9754-2025 (identical to ISO 2813, effective 1 January 2026, replacing the 2007 edition) sets the three geometries: above 70 GU use 20 degrees, 10-70 GU use 60 degrees, below 10 GU use 85 degrees. Super-matte panels live in the 85-degree band; reading them at 60 degrees means the 2 GU repeatability limit is comparable to the whole target, so half of the decision is instrument noise. Use 85 degrees as the acceptance angle, record 60 degrees for reference, and state the five- or nine-point layout and allowed range in the technical agreement.
5.2 Feel, anti-fingerprint and soiling
No national standard prescribes hand feel, so the industry fixes load, felt and stroke in a friction test (for example 1 kg, lint-free cloth, 1800 mm/min) and adds blind panel scoring. Anti-fingerprint reduces to surface energy: once the film falls below about 25 mN/m against sebum fatty acids near 38 mN/m, oils sit as isolated dots and wipe off easily; report water and hexadecane contact angles together. Chemical and stain resistance follow GB/T 4893 and EN 12720 protocols, with coffee, lipstick, cleaner and ethanol as standing reagents.
5.3 Gloss rebound is the real durability test
Write the post-abrasion gloss delta into the contract instead of only the initial value. Run 300 or 500 RCA cycles or ethanol-wetted wipes, then remeasure 85-degree gloss and look for whitening or shiny bands. Furniture panels should also pass 60 degrees C / 95% RH for 72 h plus sunscreen and hand cream dwell tests, so a board that looks good on day one does not turn patchy after three months.
6. FAQ
Q1: How is a stable 2 GU reached without matting powder?
By surface shrinkage rather than particle scattering. Lock pick-up, pre-cure and excimer dose as a set, run single-factor trials before response surface, and screen the window with both 85-degree gloss and roughness.
Q2: Can 254 nm fully replace 172 nm?
It is cheaper to run, but 254 nm penetrates deeper and its surface-first effect is weaker, so the same resin wrinkles differently under each lamp sequence. Changing lamps is not changing recipes; functionality and initiator must be re-matched.
Q3: Why is the middle matte and the edges glossy on the same batch?
Usually lateral lamp or nitrogen-flow non-uniformity on the 172 nm route, sometimes excess pick-up at both edges. Map nine-point gloss and weigh pick-up together before deciding between reflector adjustment and roller gap.
Q4: Can a soft-touch panel be cleaned with alcohol?
Occasional wiping with 99.7% ethanol is acceptable; frequent hard rubbing and abrasive pads are not. If a wiped area turns reflective, the wrinkles there have been polished flat, which is gloss rebound, not dirt.
Q5: What should be checked first when cracking or delamination appears?
Check pick-up against the window and rear UVA dose first, then primer-to-topcoat compatibility and intercoat wetting, then the number of passes. Cracking follows excessive shrinkage stress; delamination is usually an adhesion problem, and the two point in different directions.
Q6: Does a waterborne excimer system mean VOC-free?
No. It is a water-dispersed system that still contains coalescent and some co-solvent, and VOC is determined and limited under GB 18581-2020 for wood coatings. Solvent-free dispersion routes lower VOC sharply, but declare only what a third-party report supports.
Q7: Can mouldings and shaped doors take a soft-touch finish?
Flat panels on roller or curtain coaters hold the wrinkle profile best. For shaped parts, spray a self-matte resin and accept 5-10 GU for the sake of uniformity, or use a foil-press route and re-assess weathering and repairability.
Sources
- GB/T 9754-2025, Paints and varnishes - Determination of gloss at 20, 60 and 85 degrees; issued 30 June 2025, effective 1 January 2026, replacing GB/T 9754-2007, identical to ISO 2813.
- GB 18581-2020 limit of harmful substances in wood coatings; GB/T 6739 pencil hardness; GB/T 9286 cross-cut test; GB/T 4893 series and EN 12720 for furniture surface chemical resistance.
- Luo Jinrong, Chang Shanshan, Liu Gonggang et al., Study on factors affecting the sensory performance of wood matte films cured by 254 nm excimer UV, Furniture and Interior Design, No. 2, 2024 (single-factor and response-surface results for pick-up, LED pre-cure energy and 254 nm dose against roughness and 85-degree gloss).
- Chinese patent application 202411427437, Excimer UV soft-touch coating and its film preparation (published 2 January 2025): 395 nm LED 1000-4000 mJ/cm2, 254 nm 50-400 mJ/cm2 at 5-20 mW/cm2, mercury UVA 1000-2000 mJ/cm2, 1-3 micrometre surface wrinkles, friction coefficient below 0.1.
- Chinese patent application 202610751251, Excimer UV coating and its preparation and use (published 23 August 2026): difunctional and multifunctional waterborne polyurethane acrylate ratios and the adhesion balance.
- Publicly available industry process data: nitrogen flow 120-200 m3/h for 172 nm, excimer lamp life 1000-3000 h, roller soft-touch topcoat pick-up 30-45 g/m2 and the three-stage energy settings used in volume production.
Last updated: 2026-10-20
Signed: Kexin New Materials (Guangdong) Co., Ltd. | Figures are taken from published standards, patent documents and industry test data for B2B technical exchange; they are not a batch quality warranty. Contracted parameters prevail, supported by third-party test reports. Supply terms EXW / FOB.