Wood modification: thermal, chemical and densification routes

The idea: change the wood, not coat it

Modification attacks the cause of swelling and decay at the cell-wall level, without a biocide. Untreated wood takes up water from the air, which causes swelling and shrinkage and keeps moisture high enough for decay fungi. The Finnish ThermoWood handbook puts the critical threshold at 20 % MC. Modification routes:

• Thermal: water-binding hemicelluloses are partly destroyed.
• Acetylation: hydroxyl groups are chemically blocked.
• Furfurylation and resin impregnation: the cell wall is bulked with a polymer.
• Densification: the wood is compressed (it addresses hardness more than moisture).

Every route trades something: stability and durability against strength, brittleness or process cost. Modified wood is not a drop-in replacement for a pressure-treated post.

Thermal modification

Wood is heated in a low-oxygen atmosphere, commonly at about 160-240 degrees C (European). The ThermoWood handbook (Finland, 2021) describes three phases: high-temperature drying (up to near-zero moisture), modification at the class temperature, and cooling and conditioning back to 4-7 % moisture. It defines two classes: Thermo-S for stability (maximum 190 degrees C for Nordic softwoods) and Thermo-D for durability (212 degrees C for Nordic softwoods). The handbook assigns Thermo-D pine and spruce to durability class 2 for outdoor weathered use such as cladding; it does not recommend ground or water contact. Svenskt Trä says thermally modified wood turns brown, absorbs less moisture and moves less, but is weaker and more brittle, and should not be used in load-bearing structures.

Effects, with their sources:

Strength losses depend on species, temperature, duration and test moisture, so do not transfer one number to another process.

Acetylation and furfurylation

From a 2017 review (international commercial figures, partly manufacturer-reported, so dated):

• Acetylation reacts wood with acetic anhydride. With an acetyl gain of 16-19 % (commercial about 20 %), swelling and shrinkage fall by 70-75 % and the review rates the wood durability class 1. Radiata pine and alder are named species.
• Furfurylation impregnates furfuryl alcohol and cures it by heat. Weight gain is about 20-35 %; at 30-35 % decay resistance over nine years of tests was comparable to CCA-treated pine; durability class 1 is stated. The review is not explicit on brittleness.
• Output in 2017 was about 40,000 m3 (acetylated) and 22,000 m3 (furfurylated). These are not current capacities.

Densification in brief

Densification compresses wood across the grain in a hot press. In a Scots pine study, density roughly doubled and modulus of rupture rose about 64 %. The weakness is set recovery: the wood can spring back when wet and warm, so fixation by heat or resin is essential.

What the sawmill delivers

Modification plants buy a controlled input. The ThermoWood handbook takes Nordic softwood from heartwood, names certification (PEFC, FSC, OLB) and gives typical lengths of 2.7-5.7 m for softwoods.

Worked example: size allowance. A nominal 25 x 125 mm board ends up around 24 x 121 mm after thermal modification and finishing, roughly 3 % smaller. Width: (125 - 121) / 125 = 3.2 %. Saw to a green size that leaves room for this, plus tolerances, plus planing. Minimum finished thicknesses in the handbook are 19 mm for outdoor wall and ceiling cladding and 26 mm for outdoor patios and floorboards.

Checklist for supplying a modification plant

• Obtain the written intake specification: species, heartwood or sapwood, moisture, thickness, length, knot limits.
• Sort to it and keep lots separate by species and heart/sap.
• Saw oversize and agree planing allowance in writing.
• Provide chain-of-custody documents if required.
• Never sell thermally modified timber for structural use.
• Ask for feedback on rejects after each run.

Key points

• Modification changes the cell wall to reduce water uptake and raise durability, always at a cost.
• Thermo-S (190 degrees C) targets stability and Thermo-D (212 degrees C) durability; both are Nordic industry classes and not structural.
• Thermal modification roughly halves EMC in some species but can cut bending strength strongly (42 % in one spruce test).
• Acetylation and furfurylation reach class 1 in the 2017 review but are done in chemical plants and figures are dated.
• Densified wood needs fixation to avoid set recovery.
• The sawmill's value is uniform, specified, sorted, oversized input.