Adhesives play a prominent role in wood construction. They are used for:
- The manufacture of laminated products
- As a means of increasing the structural rigidity of sheathing/joist combinations in floor and of affixing non-structural panel products
- End joining dimension lumber
Adhesives Used for Laminated Products
Structural composites such as plywood, oriented strandboard (OSB) and wafer-board, prefabricated wood I-joists, laminated parallel strand lumber (PSL), laminated veneer lumber (LVL) and glulam are dependant upon adhesives to transfer the stresses between adjoining wood fibre.
Interior use wood products such as particleboard, which is used for furniture and for some structural applications such as flooring underlay, and hardwood plywood, which is used for furniture and decorative panelling, also rely on adhesives for laminating wood material.
The selection, application rate, and curing conditions for adhesives for these products is controlled at the point of manufacture. A brief discussion of the principal adhesives used in these products is presented to address questions which some times arise about permanence of bond, reliability, resistance to environmental factors, and emission of volatile chemicals into buildings.
There are two principle types of adhesive used for the manufacture of Canadian wood products. These are urea-formalde-hyde (UF) which is suitable only for interior use products and phenol-formaldehyde (PF) which is used for exterior applications.
Interior Wood Products
Urea-formaldehyde adhesive is a thick creamy syrup which cures to a colourless solid.
UF adhesives are very economical and fast curing but are not suitable for damp conditions. For this reason, (UF) glues are used for panels intended for nonstructural use such as particleboard and hardwood plywood.
UF adhesives are non-staining and therefore have the further advantage of not blemishing the high quality expensive face veneers used for hardwood panels for interior finish applications.
The raw materials for UF adhesives are derived fom natural gas through the intermediates of ammonia for urea and methanol for formaldehyde.
Exterior Wood Products
Phenol-formaldehyde (PF) adhesives are a dark purple-brown colour and give the dark glue lines associated with products such as plywood and OSB.
Known as the phenolics, they are a derivative of crude oil and the principle resins approved for the manufacture of wood products intended for exterior applications.
PF adhesives are used for the manufacture of glulam, PSL, LVL, plywood, OSB/waferboard and for fingerjoining stress graded lumber.
PF adhesives are somewhat more expensive than UF adhesives and exhibit lower levels of formaldehyde emissions.
Various types of extenders such as walnut shell flour, Douglas fir bark flour, alder bark flour, and wood flour are used to moderate the cost of PF glues, control penetration into the wood fibre, and moderate strength properties to suit the materials being bonded.
Resorcinol-formaldehyde (RF) adhesive is a phenolic substance which is more reactive than the PH adhesives. Being more reactive means that curing is faster and takes place at room temperature and below. Otherwise these glues have the same basic properties as the PF adhesives. However, high cost of the resorcinols means in practice that they are often blended with the PF adhesives to moderate the cost.
Emissions from Wood Product Adhesives
Formaldehyde is a chemical used in the manufacture of many contemporary products including, for example, upholstry, permanent press clothing, and carpet. It is also a component of the adhesives used to manufacture most wood panel and composite products. Formaldehyde is an allergic irritant to some people when the time and level of exposure is high. Effects are compounded when a building has air change rates below accepted standards.
The level of formaldehyde emission from any new product is time dependant. Emission level is highest when the product is new, and decreases steadily as the time in service increases.
Housing and Urban Development (HUD) in the United States has set limits on the amount of allowable formaldehyde which may be emitted for building materials and contents. These levels are recognized in Canada and in the US.
Wood products made with PF adhesive wood products are substantially below the HUD limit for formaldehyde emissions. Emission standards for products made from UF adhesive products are also below the limits established.
Wood products made with PF adhesives have a low level of formaldehyde emission because the phenol resins chemically fix the formaldehyde.
Since the formaldehyde component of UF adhesives is not completely chemically fixed by the urea, some is free to dissipate. Adhesive manufacturers have met HUD standards by reducing the formaldehyde content and consequently increasing the amount of adhesive and the curing time.
Wood products containing formaldehyde meet or surpass present HUD standards. With standards for formaldehyde emissions expected to become increasingly stringent, chemists will be challenged to formulate acceptable low cost adhesives for the wood and other products which house and furnish modern living and work spaces.
Sheathing and Panel Application
There are many adhesives available to improve the structural performace of building elements or to apply non-structural panels in a way that does not leave surface blemish on the panel, as wood nailing.
These field applied tube type adhesives are available in many types suitable for both interior and exterior use. The recommended application temperature can range between -10 and 40C (15 and 105F). These fast setting adhesives can be used to bond wood and panels to metal, gypsum wallboard, concrete and foam insulation.
Where floor sheathing is affixed to joists with a field applied elastomeric adhesive in addition to nails or screws, improved vibration and deflection can be gained. In this situation, the joists and sheathing act as a single composite section and increases in spans of 5 to 10 percent when compared to nailed or screwed floors are possible.
Repairs to Wood Members
As with all building materials, wood is subject to damage if exposed for a long period of time to adverse conditions, if original design was faulty, or if over-stressed due to loadings beyond design specifications.
Damage assessment requries the analysis of a structural engineer. In some cases, on site repair rather than replacement may be possible.
Where a repair is recommended by a structural engineer qualified in designing and supervising repairs, it may require the use of epoxy with putty-like properties which is trowelled over cracks and holes to contain the epoxy repair material. The putty is also used to embed injection and vent port hoses to accept pressure injection equipment.
A low viscosity two-part epoxy is then pumped into the injection port until all areas or the cract or defect are filled as evidenced by escape of epoxy through the vent port.
Once the epoxy repair has cured, the excess epoxy can be chipped and sanded away to restore the appearance of the member.
With the drive for increased thermal efficiency, homes have become more air-tight. This can mean that odors and emissions generated by products such as building materials and cleaning solvents can accumulate and can affect the health of occupants.
Volatile organic compounds (VOCs) are a broad family of chemicals that in general should be minimized in the living environment. Simple speaking, VOCs are chemicals that evaporate from materials. Perfume and air-fresheners emit VOCs and while acceptable to some, can cause adverse health affects for others.
Generally speaking, that ‘new-car’ or ‘new-house’ scent that evokes a fresh start is the scent of VOCs. In a new home, VOC emissions can come from hundreds of products, and the pattern of emissions are complicated by the fact that some materials, such as gypsum wallboard, can absorb VOCs and emit them later as the general level in the house decreases.
Although VOC emissions have been an increasingly important issue over the past 15 years, they are measured in parts per million, and it is only recently that comprehensive test procedures have been established so that the control of VOCs can move from being intuition-based to science-based.
Test development and product testing done by the National Research Council of Canada shows that emissions from “wet materials” like paints and caulks have an emmision rate about ten times higher than “dry materials” like wood products and ceiling tile. However, wet materials tend to give off the VOCs for only a short period of time. Other products known to emit VOCs include wood products, formed concrete, carpets and other floor coverings, and upholstry. As the range of products tested grows, it is expected that other products will also be shown to be sources. For example, lubricating oils used in the forming of sheet-metal studs could make these framing materials VOC sources.
Some testing was done on plywood, OSB, particleboard and solid wood products. The natural resins in wood products are definately sources of VOCs. However the emission rates were fairly low relative to other home products and furnishings. More testing is needed to make the results statistically valid, to correlate the relationship between moisture content and emmisions, and to determine the effectiveness of moisture barriers in separating structural wood products from the indoor occupied environment.
The research tested individual materials but much more research is needed to determine how materials behave in assemblies, and how effective vapor barriers are at keeping VOCs from framing materials from affecting the living areas.
While hyper-sensitive people do not need to take special precautions, the general public needs only to heed some basic common-sense rules.
- During the construction process, exercise good housekeeping. Drop sheets should be used to keep paint, chaulk and compound spillage from being absorbed by floor sheathing.
- When construction has finished, ventilate the house well and if possible, allow two weeks before occupancy to give time for the VOC level to dissipate.
- During the first year of occupancy a higheer-than-usual ventilation rate should be used. Although heating or cooling cost may increase as a result, this practive will allow stored VOCs to be vented.
- During renovations such as hardwood floor refinishing, occupants should plan on vacating the house during the work, and allow three days of high ventilation after completions of the work before returning.
- For home decorating, select low-emitting paints and stains and increase ventilation during the curing period