Free Formaldehyde Testing of Wood-Based Panels by the Perforator Method (EN 120 / GB/T 17657)

Sample Preparation and Conditioning

Test pieces are cut to approximately 20 mm × 40 mm × full panel thickness, with a total mass of about 105 g to 110 g per determination. Samples must be taken from representative positions of the panel, sealed immediately in airtight packaging to prevent formaldehyde loss and moisture change, and conditioned to equilibrium before testing. The moisture content of companion test pieces is determined separately, because the extraction efficiency and the final result depend on it.

Testing Items

A perforator method determination typically comprises the following measured and derived items:

• Free formaldehyde content (perforator value), mg/100 g oven-dry board, determined in duplicate
• Moisture content of the test pieces, required for normalization and correction of the result
• Classification assessment against formaldehyde emission classes, such as E1 and E2, where applicable
• Optional comparative evaluation of core board versus finished (overlaid) panel content

For classification, the widely applied reference values are: in many European national provisions, E1 corresponds to a perforator value of no more than 8 mg/100 g for particleboard and fibreboard, with E2 between 8 mg/100 g and 30 mg/100 g. Under the earlier Chinese national standard GB 18580-2001, E1 was defined as not more than 9 mg/100 g and E2 as greater than 9 mg/100 g but not more than 30 mg/100 g. The current GB 18580-2017 designates the 1 m³ climate chamber method (limit 0.124 mg/m³) as the conformity assessment method, while the perforator method is retained as a factory production control technique with an established correlation to the chamber result.

Testing Methods

Extraction Stage

The weighed test pieces are placed in the round-bottom extraction flask with approximately 600 mL of toluene. The perforator column is charged with about 1000 mL of distilled water, and the condenser is fitted above it. The toluene is brought to boiling and maintained for two hours. Formaldehyde stripped from the wood by the hot solvent vapor passes with the condensate through the water column, where it is continuously absorbed into the aqueous phase. A reagent blank is run in parallel to account for any formaldehyde background in the toluene and apparatus.

Quantification Stage

The formaldehyde concentration in the collected aqueous solution is determined by one of two standard techniques:

• Acetylacetone photometric method: formaldehyde reacts with ammonium acetate and acetylacetone to form a yellow lutidine derivative, measured spectrophotometrically at 412 nm against a calibrated formaldehyde standard series
• Iodometric titration: formaldehyde is oxidized with iodine in alkaline solution, followed by back-titration with sodium thiosulfate; this classical procedure remains valid where photometry is unavailable

Calculation and Moisture Correction

The total mass of formaldehyde in the aqueous extract is calculated from the measured concentration and solution volume, then normalized to 100 g of oven-dry board using the sample dry mass. Because extraction efficiency varies with board moisture content, the standards prescribe a correction factor when the moisture content of the test pieces deviates from the reference range, and the corrected result is reported on an oven-dry basis. The final perforator value is the mean of duplicate determinations, and acceptance of results is checked against the repeatability limits stated in the standard.

Quality Assurance

Reliable results require calibrated balances and spectrophotometers, freshly prepared standard solutions, verified blank values, and experienced operation of the glassware, which must be leak-free and scrupulously clean. Because toluene is flammable and a health hazard, the apparatus is operated in a fume hood or dedicated extraction enclosure with appropriate fire safety measures. Many manufacturers and traders engage accredited third-party laboratories to obtain independent perforator results for certification and trade purposes.

Applications

• Factory production control of particleboard, MDF, and plywood lines, enabling rapid feedback on resin formulation and pressing conditions
• Product classification and marking for E1, E2, or stricter national low-formaldehyde grades
• Compliance demonstration for panel certification schemes under EN 13986 and related harmonized panel standards
• Import and export conformity documentation where buyers specify a maximum perforator value in contracts
• Adhesive research and development, comparing urea-formaldehyde resin systems, scavengers, and curing recipes
• Correlation studies linking perforator content with climate chamber (EN 717-1), gas analysis (EN 717-2), and desiccator (JIS A 1460, ASTM D5582) results for multi-market compliance programs

FAQ

What does the perforator value actually represent?

It represents the amount of free and readily releasable formaldehyde contained in the board, extracted under aggressive boiling toluene conditions and expressed as mg per 100 g of oven-dry material. It is a content measurement, not a direct measurement of the concentration that the panel emits into room air.

How is the perforator method different from chamber, desiccator, and gas analysis methods?

Chamber methods (EN 717-1, ASTM E1333, ASTM D6007) measure emission into controlled air and give results in mg/m³, reflecting real service behavior. Desiccator methods (JIS A 1460, ASTM D5582) measure emitted formaldehyde absorbed in water over a fixed period. Gas analysis (EN 717-2) measures emission in a flowing air stream. The perforator method is unique in measuring total extractable content quickly, which makes it ideal for production control, though it does not directly simulate indoor conditions.

How much sample is needed and how should it be taken?

About 105 g to 110 g of test pieces, cut to roughly 20 mm × 40 mm × panel thickness, is required for one determination, with additional material for moisture content and a duplicate run. Samples should be taken from representative areas of the panel, immediately sealed in airtight bags, and shipped promptly to the laboratory to preserve their original condition.

Is the perforator method still accepted under current regulations?

Yes. Under GB 18580-2017 the 1 m³ climate chamber is the method of conformity assessment, while the perforator method is explicitly permitted for factory production control supported by a demonstrated correlation. In European practice, the perforator value remains a core internal control parameter for E1-class panel production even where emission methods govern final classification.

Can finished, laminated, or painted panels be tested?

They can, but the result will include the formaldehyde contribution of overlays, impregnated papers, or coatings in addition to the substrate. For assessing the raw board, surface layers should be removed before the test pieces are cut, and this deviation should be clearly documented in the test report.

How long does a perforator test take?

The extraction itself runs for two hours, and with sample preparation, moisture determination, photometric or titrimetric analysis, calculation, and duplicate confirmation, a complete determination is typically completed within one working day once the laboratory has the conditioned sample.

What accuracy can be expected?

EN 120 specifies repeatability and reproducibility limits for the method, and experienced laboratories routinely achieve duplicate agreement within these limits. Consistent sample conditioning, correct moisture correction, and clean apparatus are the main factors controlling precision.