Emission of Volatile Organic Compounds from Wood and Wood-Based Materials

2. Wood and Wood-Based Materials

2.1 Wood Species
2.2 Wood-Based Materials
2.2.1 Solid Wood
2.2.2 Glued Wood Materials
2.3 Adhesives
2.4 Surface Treatments
2.5 Factors Influencing the Indoor Air
2.6 Material Selection

Cellulose, Hemicellulose and Lignin

Wood is composed of a number of different types of cells. In the wood used as material in buildings and interior furnishings, the water of the celllumen and the main part of the water in the cell walls is dried away. The cell walls of wood is primary composed of cellulose, hemicellulose, and lignin.

Cellulose constitutes 40-45% of the cell wall and is composed of linear polymers of glucose hydride units (with a degree of polymerization of 7-15,000).

Hemicellulose is mainly composed of pentosans and hexosans in chains (with a degree of polymerization of 50-250). The hemicellulose amounts to 25-30% of the cell wall.

Ligning amounts 20-30% (25-30% of coniferous wood and 20-25% of deciduous wood). Lignin is amorphous giant molecules predominantly consisting of p-coumaryl-alcohol, coniferyl alcohol and sinapyl-alcohol. Sinapyl-alcohol is mainly found in deciduous wood. Lignin acts as a stiffening substance in the cell wall.

In additioned to the three basic polymers: cellulose, hemicellulose, and lignin wood contains a number of other substances, which are often named extractives. The composition and the content of these extractives vary extensively, primary, between wood species, but also between different parts of the wood and from tree to tree. Heartwood contains more extractives than sapwood, some of the extractives are the primary reason for the dark colouring of the heartwood. The extractives of heartwood forms part of the natural defence of the wood against fungi attacks.

Terpenes and resins are among the most important extractives, both composed of isoprene units, polyphenols (such as flavonols, anthocyanines, quinons, lignans and tannins), carbonhydrates, fatty acids and inorganic substances.

The volatile substances from wood are the extractives, which is why these seen from an indoor air point of view are interesting. It is, however, also among the volatile substances that the major variations exist.

2.1 Wood Species

Approx. 12,000 wood species have been identified world-wide, approx. 1,000 are used industrially world-wide. In Denmark approx. 20-30 wood species are used industrially. Some of the most important wood species for interior purposes are: Pine (Scotts Pine), spruce (Norway Spruce), beech, oak, ash, birch, alder (Common Alder) and cherry.

2.2 Wood-Based Materials

Wood materials can be divided into two main groups - solid wood and glued wood materials.

2.2.1 Solid Wood

By solid wood is understood wood, which has been sawn, dried, planed, etc. in such a way that the original structure of the wood is easily recognised in the material. Solid wood is present in a number of products e.g., windows, panels, rafters, floors and furniture. Solid wood in products can be found in combination with glue and surface treatment. In the case of the glue it is found in significantly smaller amounts than what is the case in the wood-based panels - typically below 1%.

Indoor air-wise solid wood differs from the wood-based panels by not being exposed to so high temperatures during production as the wood-based panels. The amount of volatile extractives in the wood can be reduced by exposing the wood to heat, which will intensify the evaporation of the volatile substances. The solid wood is exposed to heat during the drying. Normally, the drying will take place at temperatures far below 100E . A possible influence of the emission of volatile substances from the wood can be to optimise the drying in order to emit the volatile substances.

2.2.2 Glued Wood Materials

The glued wood materials mainly consist of wood-based panels. The main part of the wood-based panels sold on the Danish market is used for production of interior furnishings and furniture as well as for structural purposes in buildings, e.g. floorings, walls, and ceilings.

Wood-based panels most commonly used:

Other glued wood materials comprise:

The wood-based panels contain in excess of the wood materials, glue.
Particleboards and MDF panels contain 8-10% glue, OSB 4-5% glue and plywood, glulam and laminated veneer approx. 5-7% glue. Particleboards, MDF and OSB contain furthermore approx. 1% wax.

2.3 Adhesives

Common for the wood-based materials is the use of glue in the production process. For the manufacture of wood panels in Denmark the following glues are used:

There are, however, imported panels on the market that are manufactured with other types of glue e.g. phenolic glue and polyurethane glue isocyanate glue, PUR-glue, PU glue)

Approx. 90% of the consumption of glue in the Danish wood and furniture industry is used for panel manufacture. The remaining 10% is used in the furniture companies and joineries for gluing of edges, lamination etc. and for the manufacture of glulam.

For gluing of edges and lamination mainly three types of glues are used: Polyvinlyacetate glue (PVAc-glue, PVA-glue), urea-formaldehyde glue (UF-glue) and melting glue.

For manufacture of glulam for structural purposes in buildings two types of glue are used:
Phenol-resorcinol glue and melamine-urea-formaldehyde glue.

2.4 Surface Treatments

In far the most cases both solid wood and panels are surface treated. This could either be a foil, a paint/lacquer film or an oil/wax treatment. Treatments are carried out to increase the lifetime of a product, improve the performance properties during use, make cleaning and maintenance easy, decrease the emission from the underlying material (diffusion tight coatings) and of aesthetic reasons.

At the same time as the emission from the underlying material is often decreased, the treatment itself will contribute to emission of other chemical substances.

Most surfaces made for interior use by the wood and furniture industry are: melamine coating, laminates, oil treatment, wax treatment, acid-curing lacquer, UV-curing lacquer, water-borne lacquer and polyurethane lacquer.

Content of VOC

Type and content of volatile organic compounds in oils, paints and lacquers vary extensively. Content of binders and solvents in the investigated oils and lacquers appear from the table in paragraph 2.6.

In excess of solvents other substances can emit from the different types of coating. Examples are:

	Photo initators from UV-curing lacquer
	Residue monomers (e.g. acrylics) from water-borne lacquer and poly- esters
	Acetales and formaldehyde from acid-curing lacquer
	Aldehydes from oxidative curing binders based on linseed oil (e.g. alkyds)

The examples are far from being exhaustive. Actual treatments should be evaluated with regard to potential emitting substances. Regarding the practice of the present project, reference is made to "All Chemicals List", chapter 5 and Appendix 5.

2.5 Factors Influencing the Indoor Air

Wood and wood-based materials and products used in such a way that they influence the indoor air, will appear as a combination of several materials and often including a surface treatment. It will, however, still be interesting to know with which amounts and substances the individual materials contribute to the emission.

The emission of VOC’s to the indoor air from individual materials will depend on a number of factors the most important ones being:

2.6 Material Selection

The experimental part comprised investigation of 24 examples of wood and wood-based materials representing solid wood and wood-based materials usually used in furniture, interior furnishings and building products in 1996.

The test programme comprised wood and wood-based materials of different degrees of complexity from solid wood to coated veneered wood-based panels.

Survey of investigated materials:

Examples

The selected 24 wood-based materials are to be considered as examples and cannot be considered as being representative for all wood-based materials used indoors.

Surface treatments forming part of this investigation, are usually used flooring oils and lacquers for industrial coating of wood furniture and interior furnishings.

The composition information of the investigated oils and lacquers has been taken from the safety data sheets of the materials including content of volatile organic compounds and Code Numbers (in ready for use mixture) according to the Statutory Orders of the Danish Working Environment Service, stated in Table 2.1.

Table 2.1
Investigated Oils and Lacquers

* The UV-curing lacquer contains furthermore photo initiators
# The Code Number gives safety & health information on lacquers and oils according to the regulations of the Danish Working Environment Service. The figures before the hyphen informs about the volatile substances, and states the requirement for use of respiratory protective equipment against inhalation of volatile substances.

Coating with nitrocellulose lacquer, acid-curing lacquer, polyurethane lacquer, water-borne lacquer consists all of one primer layer and one top layer of lacquer, which have been applied the beech veneered particleboards by spraying. The UV-curing lacquer was coil coated in three layers.

A close description of the test specimens investigated by emission testing in test chamber appears from Appendix 6. As the spruce variables with narrow and broad annual rings respectively (4 and 5) showed uniform emissions by the initiative screening, only the narrow annual ringed spruce example was examined closer by emission testing in test chamber.