Unsaturated polyester (PE) furniture coating — often called polyester lacquer and abbreviated PE — is a three-component reactive wood finish whose film former is an unsaturated polyester resin (UP-R), whose reactive diluent and crosslinker is styrene, and which cures at room temperature through a radical initiator–promoter system. Because its solids are very high and one pass can build a thick, level film, PE is used for high-gloss flat furniture panels, instrument shells and marble-look tops where a "piano finish" is wanted. It is a resin route in its own right, distinct from waterborne, PU and UV.
TL;DR — in one minute
- PE is "resin + cobalt accelerator + peroxide initiator"; a free-radical copolymerisation links the resin double bonds with styrene into a 3-D network, releasing almost no small molecules, so theoretical solids approach 100%.
- Gel time is the core process window: typically 18–24 min at 25 °C, shortening exponentially with heat — about 35–40% faster per 5 °C rise; dosage and resin type reshape the curve.
- High solids let PE build a 200–500 μm wet film in one pass that self-levels into a full, high-gloss coat — its key advantage over spray-applied PU/NC.
- PE cures anaerobically: the air-facing layer stays tacky from oxygen inhibition; a paraffin wax (melt ~50–58 °C) migrates to the surface to block oxygen, at the cost of wax haze, polishing, and poor release in thin films.
- Engineering boundaries are clear: hard but brittle film, edge chipping, no meaningful local repair; strong styrene odour and VOC constrained by GB 30981.2-2025 (effective 2026-06-01). Suited to automated lines, not on-site work or small batches.
Key data (verifiable)
- Hardness: PE film reaches pencil hardness of 3H or above, above conventional solvent PU clear coat.
- Solids and build: solvent-free/high-solids PE builds 200–500 μm wet in one pass; theoretical solids approach 100%, versus ~50% for most domestic PU.
- Working time (pot life): radicals start on mixing, so use typically within 20–30 min.
- Ambient gel time: ~18 min at 25 °C for an ortho-type resin with ~2% MEKP + ~1.5% cobalt (gel-point double-bond conversion ~21–23%).
- Temperature sensitivity: 15 °C ≈ 47 min, 25 °C ≈ 18 min, 35 °C ≈ 7.6 min, 45 °C ≈ 3.4 min; −35 to −40% per +5 °C.
- Compliance: solvent wood coatings follow GB 18581-2020 (PU class ≤600–670 g/L, nitro ≤700 g/L VOC); from 2026-06-01 folded into GB 30981.2-2025, adding SVOC and auxiliaries, with some solvent limits cut by over 20%.
1. Composition and naming: a three-part reactive system
Unlike coatings that dry by evaporation or addition, PE cure needs three roles working together.
1.1 What each component does
- Main agent (UP-R resin): a linear polyester made from polyol + polyacid carrying reactive double bonds, dissolved in styrene as a viscous liquid; typical resin solids 60–65%, acid value 20–35 mgKOH/g, 25 °C viscosity several hundred to 1200 mPa·s.
- Reactive diluent / crosslinker (styrene): lowers viscosity and then copolymerises into the network — the root of PE's "solvent-free, high-solids" behaviour, and also the main odour/VOC source.
- Initiator + accelerator: initiator is an organic peroxide (MEKP, cyclohexanone peroxide); accelerator is a cobalt salt (cobalt octoate, ~0.6–1% Co). The redox pair drops the peroxide decomposition energy to a room-temperature-usable level.
1.2 Resin type and standard naming
By dibasic acid the common grades are ortho-, iso- and bisphenol-A type. Naming and base parameters follow the GB/T 24148 "Plastics — unsaturated polyester resins (UP-R)" series (Part 1 naming, Part 4 viscosity, Part 7 = GB/T 24148.7-2014 ambient gel time). "General / corrosion-resistant / flame-retardant" grades come from resin choice; coatings plants then formulate PE lacquer.
2. Cure mechanism: three stages of radical copolymerisation
Double bonds on the linear resin and the styrene copolymerise under radicals into an insoluble, infusible body network. Because almost no small molecule leaves the system, the network is dense and shrinkage comes from polymerisation itself.
2.1 How initiation and acceleration start the reaction
Peroxides alone have critical temperatures mostly above 60 °C, so they cannot cure at room temperature. A reducing accelerator (cobalt) cuts the decomposition activation energy to tens of kJ/mol, sustaining radical generation at room temperature. So initiator and accelerator must be mixed fresh and metered precisely.
2.2 Gel – set – post-cure
Cure runs in three stages: gel (flow lost, elastic gel at critical crosslink density) → set (hardness and shape enough to demould without deformation) → post-cure (stable properties, high conversion). PE's short stage intervals and room-temperature contact moulding are its efficiency sources.
2.3 Shrinkage and internal stress
As styrene and resin double bonds convert from van-der-Waals spacing to covalent bonds, the volume contracts; combined with fast exotherm in thick films this stores internal stress. Symptoms are edge pull-back and thin/under-cut corners, plus higher demands on substrate wetting and intercoat adhesion. Low-exotherm formulas, controlled single-pass build, and a PE-base + tough-topcoat sandwich manage it.
3. Quantifying gel-time control (the process core)
Gel time sets the work window and takt, and is where PE most often goes wrong on site. The two tables below are published measurements per GB/T 24148.7-2014 on an ortho resin with MEKP (~9% active oxygen) + cobalt octoate, given to show magnitude and trend — always recalibrate on your own resin, temperature and humidity.
3.1 Effect of temperature
| Temperature (°C) | 15 | 20 | 25 | 30 | 35 | 40 | 45 |
|---|---|---|---|---|---|---|---|
| Gel time (min) | 47.2 | 29.6 | 18.4 | 11.7 | 7.6 | 5.1 | 3.4 |
Following Arrhenius, each 5 °C rise cuts gel time by ~35–40%. Cold shops need gentle warming or more accelerator; hot summer gel can fall under 5 min, forcing cooling or dose reduction.
3.2 Effect of initiator / accelerator dosage
| Dosage (% of resin) | 0.5 | 1.0 | 1.5 | 2.0 | 2.5 | 3.0 |
|---|---|---|---|---|---|---|
| Initiator MEKP (min) | 76.3 | 35.7 | 24.1 | 18.4 | 14.2 | 10.8 |
| Accelerator cobalt (min) | 42.3* | 26.7 | 18.4 | 12.5 | — | 9.1 |
*cobalt at 0.2% is ~88.5 min. Both initiator and accelerator shorten gel time; beyond a point cage effects reduce returns, and excess accelerator causes a hot early exotherm, stress and uneven cure.
4. High build and curtain coating: the payoff of high solids
4.1 One-pass thick film
Because styrene reacts rather than evaporates, PE can lay 200–500 μm at once without the sag, slump and pinholing that low-solids coatings suffer. Thick build plus self-levelling gives fullness and the "piano" look — why PE is hard to replace on flat high-gloss panels and tops.
4.2 Curtain coating and thixotropy
High-gloss flat mass production uses curtain (pour) coating: a continuous "curtain" of liquid falls from a slot and the part passes through once. Anti-sag at high build relies on thixotropy (fumed silica, organobentonite) balanced against viscosity and levelling — a narrower window than roller transfer efficiency.
4.3 Anaerobic tack and paraffin blocking
PE is a classic anaerobic cure: the air interface stays tacky from oxygen inhibition. The traditional fix is a paraffin wax (alkane, melt ~50–58 °C) that floats to the surface during cure and forms an oxygen barrier. Costs: a wax haze on the part, so a high-gloss surface must be sanded and polished after cure; wax also migrates to the substrate and hurts adhesion; and in thin coatings the wax cannot float out evenly.
5. Performance and system comparison
| Aspect | PE polyester | PU polyurethane | NC nitrocellulose | UV-cured |
|---|---|---|---|---|
| Cure | Radical copolymer | Isocyanate + hydroxyl | Solvent evaporation | UV-initiated polymer |
| Film hardness | High (≥3H) | Mid-high | Mid-low | Mid-high |
| Build/fullness | High, one pass | Multi-coat | Poor, multi-coat | High (one pass) |
| Toughness/impact | Brittle side | Tough | Flexible | By formula |
| Repairability | Hard to spot-fix | Repairable | Easy | By system |
| VOC/odour | High (styrene) | Mid-high | High | Low |
| Typical use | Gloss panels/tops | Indoor/outdoor wood | Antique/open grain | Line flat goods |
6. Engineering boundaries: why PE suits automated lines
6.1 Brittleness and edges
High crosslinking gives hardness but limited elongation: the film cracks under impact or substrate movement, and sharp corners/edges chip. Design out sharp angles or use a PE base + PU/UV top sandwich.
6.2 No meaningful local repair
Crosslinked PE is finicky about recoat and sand-in; spot repairs show, so full-panel rework is normal. This makes PE dependent on first-pass yield, so cleanliness, dust control and dosage management matter.
6.3 Styrene and compliance
Styrene is a reactant but also a strong-smelling, irritating VOC; ventilate and protect, keep clear of open flame. On compliance, solvent wood coatings follow GB 18581-2020 and from 2026-06-01 are folded into GB 30981.2-2025 (comprehensive industrial-coating limits; adds SVOC, brings hardener/thinner/paste and putty under control, cuts some solvent VOC by over 20%, requires C/NC hazard marking). Solvent wood coatings are also CCC-listed (CNCA-C21-01:2024).
6.4 Metering and batch consistency
Mis-metering the three parts means no cure or abnormal cure. Cure is sensitive to temperature, humidity, substrate and batch, so use metering pumps / inline blending, small-batch calibrate per shift, and retain samples for traceability.
7. Selection and process checklist
- Match form: gloss flat panels/tops suit PE; shaped parts and field-repair stock — use care or switch to PU.
- Calibrate the gel-time curve first: your resin + shop conditions fix MEKP/cobalt dosage and the work window.
- Control sag on build: balance thixotrope and levelling, verify 200–500 μm anti-sag and self-levelling.
- Pick the oxygen fix: paraffin blocking (needs later polish) or a UV/barrier top instead of wax, weighing haze vs adhesion.
- Do compliance up front: check VOC, benzene series, heavy metals and SVOC against GB 30981.2-2025 / GB 18581-2020, get a CMA/CNAS third-party report.
FAQ
Q1: What is the essential difference between PE and PU?
The cure path. PE free-radically copolymerises styrene with resin double bonds into a crosslinked network; the solvent reacts, solids near 100%, so it builds thick in one pass. PU cures by isocyanate–hydroxyl addition, commonly ~50% solids and needing multiple coats. PE is harder and fuller but more brittle and hard to repair; PU is tougher and repairable.
Q2: Why does a PE surface stay tacky and need paraffin?
PE cures anaerobically; the air-facing layer is inhibited by oxygen and cannot fully cure. Paraffin floats up during cure and forms an oxygen barrier, making the part self-seal against air. The price is wax haze, post-cure sanding/polishing, and poor release in thin films.
Q3: How is gel time actually controlled in production?
Treat temperature and dosage as the main variables. Run small samples per GB/T 24148.7-2014, plot your resin's temperature–gel-time curve; warm and raise accelerator in winter, cool and cut dose in summer, and fold pot life into takt planning. Too much accelerator risks a hot early exotherm and stress.
Q4: Is PE still compliant? What did the new standard change?
PE's odour and VOC come mainly from styrene; it is less green than waterborne/UV. Solvent wood coatings follow GB 18581-2020 and, from 2026-06-01, GB 30981.2-2025 — which adds SVOC and auxiliary limits, cuts some limits by over 20% and requires C/NC marking; solvent wood coatings must pass CCC. Judge by measured value vs limit in a third-party report.
Q5: Where is PE the wrong choice?
Carved/irregular parts, channel products needing frequent field repair, edge-rich impact-loaded pieces, and odour/VOC-sensitive indoor mass goods — PE's brittleness, no local repair and styrene release become liabilities here; prefer PU, waterborne, or a PE base + UV/PU top sandwich.
Q6: Does one-pass thick build actually save paint and money?
Split it. PE cuts coats and drying waits, and styrene reacts so volatile loss is small; but curtain lines, metering and dust control are costly, first-pass yield must be high, and edge scrap/rework adds up. For flat gloss volume, total cost can win; for varied small batches, it often loses.
Last updated: 2026-10-18 | Compiled by the technical team of Kexin New Materials (Guangdong) Co., Ltd..
References (verifiable): GB/T 24148 "Plastics — Unsaturated polyester resins (UP-R)" series incl. GB/T 24148.7-2014 (ambient gel time); GB 18581-2020 (limit of harmful substances in wood coatings); GB 30981.2-2025 (Part 2: industrial coatings, effective 2026-06-01); GB/T 23986-2009 / GB/T 23985-2009 (VOC), GB/T 23990-2009 (benzene series), GB/T 34675-2017 (radiation-cured VOC); GB/T 6739 / ISO 15184 (pencil hardness); CCC rule CNCA-C21-01:2024. Gel-time values are published ortho-system examples — recalibrate in-house.