Adhesive binders for wood composites: Key considerations for formulators

Before selecting a binder system, it is useful to understand the structural requirements of the target composite. Wood composites are normally produced by mixing wood fibers with adhesive binders/resins and pressing or molding under high pressure and temperature. They are typically made from the same hardwoods and softwoods used to manufacture lumber. 

Wood composites have seen a growing demand despite a decline in the housing market. This increased demand is primarily related to the following: 

• An expanded array of wood composite products designed for application-specific performance
• More efficient use of wood resources (e.g., large panels of wood composites can be made from fibers of small-diameter trees, wood that has defects, and wood waste)
• Improved adhesive systems that provide even greater sustainability

Types of wood composites

Wood composites are used for a number of structural and nonstructural applications, ranging from fiberboard to laminated beams. Applications include panels for interior and exterior use, furniture, and architectural trim. The composite type determines the performance envelope your binder must meet. The most common types of wood composites are summarized below:

1. Plywood: Made from glued layers of wood veneer. Major uses are wall paneling, door skins, and framework.
2. Medium-density fiberboard (MDF): Consists of cellulosic fibers combined with a resin. It is a hard composite, much like a typical lumber. Available in moldings and sheets. Generally used for insulation and furniture. It is not moisture-resistant and can only be used in dry, interior locations. A solid wood appearance can be achieved by layering with wood veneer.
3. Oriented strand board (OSB): Manufactured from fast-growing trees. Comparable in strength to regular wood panels (e.g., plywood) but less costly due to the trees used. Generally made by coating wood chips with isocyanate and/or phenol formaldehyde.
4. Long strand lumber (LSL): Also known as oriented strand lumber (OSL). Similar to OSB, except the strands are longer and arranged in a parallel fashion. This produces a very strong product along its length. Serves as a substitute for lumber.
5. Laminated veneer lumber (LVL): Created by layering dried and graded wood veneers with waterproof adhesive into blocks of material known as billets. Cured in a heated press. It can often outperform conventional lumber.

Formulator note on limitations: Wood composites present several processing and end-use constraints that binder selection can directly address:

• Burns more quickly than solid lumber - Relevant for fire-rated applications
• Formaldehyde and isocyanate adhesives - Drive worker exposure considerations during cutting and manufacturing
• Adhesives can be toxic - Drives bio-based binder development
• More prone to humidity-induced warping than solid woods (unless sealed or coated) - Drives moisture resistance requirements

Many of these disadvantages can be minimized by the proper selection of adhesive or the development of new adhesives that are specifically intended for wood composite manufacture.

With the performance requirements of wood composites established, the following section reviews the conventional binder systems that define industry standards.