Oriented strand board (OSB) is made from thin, rectangular-shaped wood strands, adhesive (commonly referred to as “resin”) and a small amount of wax, which helps prevent moisture absorption into board during brief periods of wetting. Relatively small amounts of resin and wax are blended with wood strands and formed into a mat. The mat is usually composed of three layers of aligned strands, with each layer oriented perpendicular to the adjacent layers. A heated press is used to cure the resin and consolidate the loose mat into a rigid panel.
This document addresses some commonly asked questions about the resins used to make OSB:
The answers to these questions can be presented through a simplified explanation of how the resins “work” – considering some of the potential health-related aspects at the same time – and a brief description of the two most commonly used resin types. Specific information about the potential for formaldehyde emission is presented in the discussion of phenol-formaldehyde resins.
What are the resins?
The resins used to make OSB and plywood are part of a group of materials called polymers (a word meaning “many units”), which are long chain-like molecules. Polymers can occur naturally (as cellulose and lignin in trees, for example) or as man-made materials (like polyethylene milk jugs). Small building blocks, analogous to the individual links that form a chain, react with each other to form long chain-like molecules; or polymers. Some polymers melt when they are heated. They are called thermoplastics, a term we commonly shorten to “plastic”. We encounter plastics in daily life as bottles and bags, some of the fibers used in clothing, as well as the carpets we walk on. Another type of polymer is known as a thermoset. While some of the precursors to thermosetting polymers can melt and flow upon heating, they eventually cure to a rigid form that is unaltered by subsequent heating. We commonly encounter these materials as the plates that cover electrical outlets and switches, many of the handles and knobs on stoves, and as adhesives (like epoxy).
Physical links between the individual units that make up a polymer is formed by a chemical reaction between two molecules. These reactions are usually chemically stable under conditions of normal use for the object, meaning that the bonds don’t “come apart” on their own. This particular characteristic is one of the things that allow man-made polymers, and some natural polymers, to be such unique durable materials. While polymerization reactions can occur at room temperature, the resins used to make OSB require the application of heat (energy) in order to make the reactions occur quickly.
The resins used to make OSB and plywood are supplied to wood products manufacturers as short polymer chains, which are not capable of functioning as an adhesive without further polymerization. They may be applied to wood as water-based solutions, non-aqueous solutions, or as a powder. The liquids are sprayed onto the wood, whereas the powders are physically blended with the wood. Since the resins are chemically reactive before they are heated, they are applied to the wood in a controlled environment in the factory. Workers are protected by adequate ventilation and emissions are controlled by air monitoring equipment.
The term curing is used to describe the conversion of the short polymer chains into a large three-dimensional polymer. Adjacent molecules in a liquid resin droplet, or powder particle, quickly react with each other to form a three-dimensional, cross-linked network. Cured resin particles bond with wood wherever they contact it, effectively sticking adjacent flakes together throughout the board. These droplets of adhesives are often visible as tiny red/brown spots on the surface of OSB panels. In plywood, where the adhesive is applied uniformly to the veneers, a continuous red/brown glue line is visible. Once curing is complete, the molecules that make up the adhesive are no longer reactive under normal use conditions. One of the main benefits of thermosetting adhesives is the relative chemical stability of the glue bonds, as opposed to those formed by solvent loss adhesives [such as the common white glues, based on poly vinyl acetate (PVA)]. PVA glues are typically used for products that are not exposed to moisture.
What resins are commonly used to make OSB?
Two types of resins dominate OSB production; they are phenol-formaldehyde (PF) and poly (diphenylmethane diisocyanate), abbreviated pMDI.
1. Phenol Formaldehyde (PF)
Phenol formaldehyde is one of the dominant resins used in oriented strand board (OSB) and plywood production. Cured PF is considered “waterproof”, and resin is considered the benchmark of comparison when determining the water resistance of other adhesives for wood products. Although the cured PF resin is unaffected by exposure to water, panels bonded with the resin are predominately intended for only occasional, short-term exposure to moisture (Exposure 1 bond classification). PF is supplied to manufacturers in 2 forms: liquid or solid, both forms having advantages and disadvantages as addressed below. Cured adhesive is typically reddish brown in color. On OSB, cured PF is visible on the flakes as small reddish/brown spots. These resin spots form a series of tiny “spot welds” that hold the OSB together.
Resin manufacturers produce PF adhesives by reacting phenol and formaldehyde (in combination with proprietary additives and extenders) in a high pH (alkaline) water solution. Reactions are not carried to completion, because the resulting thermoset polymer would be cured and, therefore, no longer useful as an adhesive. Instead, the reaction is stopped at a low degree of conversion. The resulting resin can be made to suit an OSB producer’s needs. The short chain PF molecules can penetrate into the wood cell walls or hollow spaces inside wood cells. Curing the resin converts the soluble, short chain molecules into an insoluble three-dimensional network, and primarily forms mechanical bonds between the wood and resin.
Liquid PF Resins
What is referred to as “liquid PF resin” is really an alkaline water-based solution of low molecular weight PF chains. The amount of solid PF remaining after the water is removed is referred to as the “percent solids content” of the resin. Typically, liquid PF used in OSB production ranges from 40, to approximately 60% solids content. It is shipped directly from the resin manufacturer to the plant, where it is sprayed onto dried flakes during blending operations. During hot pressing, most of the water is vaporized, allowing the resin to cure.
Advantages to using liquid PF include lower cost, greater ease of handling and application, and better flake coverage at normal loadings, when compared to powdered PF. The liquid resins also adhere to flakes better than the dry powdered resins.
Liquid PF is more difficult to use successfully when the intended panel applications require high resin contents, since the addition of more water accompanies the addition of more resin. Excessive moisture can lead to the generation of relatively large amounts of steam pressure during pressing. When the steam pressure exceeds the tensile strength of the adhesive bonds, defects known as “blows” or steam blisters occur.
Solid PF Resins
Solid PF is produced by spray-drying PF solutions, yielding a powder or flake. This resin is more expensive to manufacture, but costs less to transport (per unit weight of solids) and can be stored for much longer periods of time, provided that it is kept cool and dry. Because the resin is not fully cured when it is dried, it (briefly) melts when it is heated in the press. The resulting liquid resin can interact with the wood, penetrate, and form the basis for a mechanical bond after the resin cures. While not a part of the resin formulation, moisture is needed to help transfer heat in the mat, as steam. Since no moisture accompanies the application of resin during the blending operation, all of the moisture in the mat must come from the wood. When powdered PF is used, the wood does not have to be as dry as it does when liquid PF is used. Reduced drying time can result in energy savings for a mill. Although the adhesion of dried resin powder to wood flakes can be problematic when producing higher resin content OSB, bonds formed using low resin loadings tend to be stronger, since solid PF does not tend to overpenetrate into the flakes. When powdered PF is used, it is often applied during or after wax application. This helps facilitate even resin distribution on the flakes.
Concerns Regarding Potential Formaldehyde Release from OSB
There has been some concern regarding the emission of formaldehyde from wood-based composites bonded with formaldehyde-containing resins. There are two main potential sources of formaldehyde emission from wood composites: unreacted free formaldehyde and formaldehyde resulting from breakdown of the resin.
The release of unreacted formaldehyde is greatly limited by controlling the ratio of formaldehyde to phenol in the resin and by several other factors, when it is manufactured. It is generally desirable to have a slight excess of formaldehyde present in a resin, so that OSB producers do not need to add an additional formaldehyde source to the resin – cure is initiated by simply raising the temperature during pressing. The small amount of unreacted free formaldehyde is lost from structural composite products soon after pressing.
A potentially greater source of formaldehyde emission comes from the chemical breakdown of cured resin through the addition of water (called hydrolysis). Formaldehyde emission resulting from hydrolysis of the cured resin affects products bonded with amino/phenolic resins, such as urea formaldehyde (UF). UF resins are less chemically stable than PF resins in their cured state, and can hydrolyze under appropriate conditions of elevated temperature and humidity. The chemical structure of cured PF resin is less attractive towards water (less hydrophilic) than the structure of cured UF resin. Phenolic-based compounds also tend to be more chemically stable. Both of these characteristics make cured PF resin much less susceptible to hydrolysis than UF. This is one of the reasons that PF resins are considered to be waterproof (and suitable for use in Exposure 1 grade panels), while UF is not. Tests have shown that formaldehyde emission from phenolic bonded plywood and OSB are not significant enough to be regulated.
More information on this subject can be found in TECO’s Technical Tip entitled, “Formaldehyde Emissions from Wood-Based Panels”.
2. Poly(diphenylmethane diisocyanate), pMDI or MDI
MDI has become a common resin used in OSB production, despite costing significantly more than PF. Like PF, it produces waterproof bonds suitable for use in Exposure 1 classified panels. In fact, the nature of its adhesion to wood makes its performance better than PF when exposed to moisture. Unlike PF, MDI does not primarily form mechanical bonds with the wood substrate; it is also capable of forming covalent chemical bonds with wood. These chemical bonds are stronger and more stable than mechanical linkages, so manufacturers can potentially use less resin to achieve similar, or greater, performance with lower adhesive loadings than PF. Lower resin loading saves money, which can help to offset the increased cost per unit of adhesive.
The surface of wood is rich in chemical functional groups called hydroxyl groups (–OH). MDI resins are terminated in isocyanate groups (–N=C=O), which can react with the hydroxyl groups on wood, forming urethane linkages. A combination of factors such as the non-polar, aromatic component of MDI resins, and the existence of the urethane linkages as part of a cross-linked network help to make cured MDI resins resistant to hydrolysis.
Some advantages associated with using MDI adhesive include:
As discussed regarding the use of powdered PF, greater tolerance for higher moisture content wood and lower press temperatures can result in energy savings. The combination of reduced costs (energy savings and lower resin usage) and increased productivity (reduced press cycle time) can help offset the additional cost of the adhesive. Because of the chemistry involved, MDI-bonded products can be used in more demanding applications where increased water resistance is required.
Potential disadvantages associated with MDI use include:
Conclusions
The information presented above was intended to be a sketch of adhesives, and some of the issues surrounding their use. It was not meant to be a complete discussion of the topic. Much of the specific information about the subject is proprietary information held by resin producers and the manufacturers of wood-based composites.