06 05 23: Wood Glues

by Franklin International and ITW TAAC - The Adhesive and Sealant Council

Last updated: 10-17-2008

Introduction

Photo of construction worker applying wood glue to a frame

Used with permission from ASC.

Adhesives for bonding wood have been around for literally thousands of years. The very first ones were natural gums and plant resins. Native Americans used adhesives derived from buffalo hoofs and animal fat. Some of the earliest wood adhesives used in the modern wood assembly market were made of animal renderings. The first glue factory was founded in Holland in the 1700's and made hide glue. This was a staple in the adhesive industry until new products were invented using bones, starch, fish and casein and rubber. In the 1950's polyvinyl acetate, or PVA white glues, were introduced and quickly became prominent in the market. That technology gradually expanded and allowed the introduction of faster setting and more durable yellow glues, and recently cross-linking PVA's, which provide water resistance along with superior bond strength. Other types of products, such as polyurethanes, hot melts, and epoxies have also entered the market and solved adhesion issues in traditional and innovative applications. More viscous products in the form of construction adhesives have also evolved for situations where the fit or bonding conditions would not allow traditional products.

No matter what the technology, the keys to selecting the best adhesive product remain the same. In that regard, the properties and characteristics of the materials being bonded, and the configuration and process by which the bond will be made dictate the performance required and the type of product that will best meet the needs of the given application.

Description

There are many types of products for bonding wood on the market and some serve a particular function or niche better than others. So, before we go much further, let's go over the different types of glues and adhesives and their roles in the markets they serve. First, it may be important to comment briefly on terminology. While the word "adhesive" broadly includes any product used for bonding, one may encounter the term "glue" more frequently. For purposes of this publication, the term "glue" refers to a subgroup of adhesives, which are fairly thin and fluid and generally are designed for use in tightly fitting joints, which are clamped or fixtured as the glue begins to build strength.

Types of Adhesives

Natural Product

The first type of glue is a natural product, such as animal hide glue. These glues are capable of high strength and are often both heat and creep resistant, although they may also be very sensitive to moisture.

PVA

Photo of wood glue being applied to a bird house projecct

Used with permission from ASC.

Polyvinyl acetate emulsions, or PVA's, are the most common example of adhesives that build strength as they dry. As the liquid carrier evaporates, or is absorbed in to the surface involved, the glue hardens or cures, thus bonding the two surfaces together. In woodworking, PVA's dispersed in water are commonly known as carpenter's glues and have become one of the largest categories of products for bonding wood. The fact that they are water-based makes them easy to use and, by using this family of products, chemical concerns are often eliminated. These products offer the potential to create bonds stronger than the wood itself, but may introduce the potential for creep in certain situations. Cross-linked PVA's provide water resistance to allow this category of products to now serve applications in which substantial moisture exposure is expected.

Reactive

Another broad class of adhesives are those that are reactive. These products cure, rather than merely dry, and they build strength as that reaction proceeds. They may be two part products like epoxies that begin to react as they are combined, or may be one part products like many polyurethanes triggered by moisture; or even cyanoacryalates triggered by the absence of air. Because these products cure rather than dry, they generally show little shrinkage. The thicker bodied products are capable of gap filling and are sometimes used for that reason.

Hot Melt

Hot melt products are also available for some applications. These products are heated and delivered in molten form, and quickly cool to fixture the joint. Products in this category vary widely in strength, but offer speed and the ability to fill gaps.

Contact Cements

Contact cements are a special class of adhesives that are applied to two surfaces and allowed to dry, then bonded with momentary pressure after careful alignment. Because these products are applied in thin films and depend on good mating pressure, contact cements are only used when at least one of the surfaces involved is somewhat flexible.

Construction Adhesives

Construction adhesives are a broad group of products designed for bonding wood and other materials in applications where the large size of the materials being bonded, inconsistent fit and adverse weather conditions may prevent the use of most traditional products. This category may include water-based and solvent-based products as well as some reactives, but the products are thick enough to bridge gaps of perhaps 1/16" and many are capable of successful bonding in situations involving wet or frozen lumber, or in applications where the bonds may subsequently be subjected to excessive moisture.

Special Purposes

Finally, there are a large number of other products available that are appropriate for bonding wood in certain circumstances. These other products include two-part glues like resorcinols or urea-formaldehydes that are recommended for their ability to provide both moisture resistance and creep resistance. There are also glues for bonding the face of plastic laminate in cabinet assembly and adhesives for installing wood floors or wood paneling.

Ultimately, then, selecting the right product begins with knowing the product choices available, and also understanding the materials being bonded and the demands of the job.

Applications

Used with permission from ASC.

When it comes to choosing the right family of products for a given job, the specifics of the job often dictate the glue or adhesive properties that are most important. For instance, framing and subfloor installations are often undertaken in adverse weather conditions and involve ill-fitted joints, while cabinet assembly or handrail production typically involve well conditioned materials and tightly fitted joints. With this in mind, the issue becomes matching the properties needed for a particular operation with the type of adhesive or glue best able to meet those needs. Thus, this section is designed to review common wood gluing or bonding situations and identify the adhesive properties required.

Framing and Subfloor

Framing and subfloor applications involve the bonding of wood, but these operations are often performed before the walls and/or roof has been assembled on the structure. Such situations involve wide variations in moisture and temperature and large substrates that are not well mated. For these applications, the ability to make successful joints in cold or wet conditions and the ability to bridge gaps are necessary properties, and the best product is a construction adhesive. For subfloor applications in particular, products that meet the American Plywood Association Specification AFG-01 or the parallel ASTM D 3498 are preferred. When these applications include adhesives, the number of required fasteners can be substantially reduced, while the assemblies actually increase in strength and rigidity.

Structural Laminated Beams

Structural laminated beams are composed of tightly bound layers of wood and require creep resistant products such as urea formaldehyde or resorcinol based glues. These two part products can introduce some toxicity issues, but their rigidity serves to protect against long term failure possible with assemblies made using less rigid products. Roof trusses are sometimes produced using these same products, but are also fashioned using construction adhesives or water resistant glues in conjunction with mechanical fasteners and various metal plates or straps which protect against creep.

Exterior Windows and Doors

Exterior doors and windows are, by design, made using moisture controlled stock and tightly fitted joints. For these applications, as well as for other outdoor projects such as shutters or railings, exterior rated or moisture resistant glues are the right choice. For such applications, glues that are rated Type I or Type II are required. Among those candidate products, urea formaldehyde or resorcinol glues are the most heat resistant and creep resistant and would generally be favored over cross-linking PVA's or polyurethanes for bent laminations or projects subject to meaningful heat, while the cross-linking PVA's are the most user friendly and are often selected for flat laminations. Despite the capabilities of these glues, it is important to remember that the wood remains subject to the effects of moisture and the project is more likely to remain successful if the scope of the project include painting or sealing of the finished assembly.

Interior Cabinetry

Interior cabinetry is usually produced using PVA based wood glues, with specialty products used if that assembly involves the bonding of the melamine face of plastic laminate. Such products are low enough in viscosity to facilitate easy application and to allow the formation of invisibly thin glue joints as pressure is applied in clamping. Similar products are used in most furniture manufacture and are appropriate for virtually all well fitted, interior wood gluing operations.

Interior Staircases and Handrails

Interior staircases and handrails can involve a variety of products for the various operations involved. PVA based wood glues are often chosen for assembling the treads and producing wooden newel posts and balusters, and for setting those components into the stairs. When metal posts are involved, epoxies, which can bond well to metal and can accommodate some lack of fit between the components being bonded, are often used. For actual assembly of the treads and risers to the stringers, wood glues are used in well fitted joints, while construction adhesives and their gap filling capabilities are preferred where the fit is not so precise. Again, construction adhesives are often for attaching additional blocks for support beneath the stairs. Wood handrails are often made using PVA glues, but bent handrails, where the wood remains routinely under stress, benefit from specialty PVA's, or products like urea formaldehyde that are more creep resistant. Other options for this type of assembly may include some epoxies or polyurethane glues, each selected for their combination of strength and creep resistance.

Trim Woodworking

Trim woodworking, including baseboards and moldings may be produced using PVA based wood glues, but are installed with a variety of products. PVA based wood glues are sometimes used to secure the ends of adjoining pieces, while construction adhesives may be used to assist in mounting the trim to the wall to minimize the fasteners required and reduce the amount of filling and touch up that are needed after that installation. Hot melts or cyanoacrylates are also sometimes used in this type of operation, serving as a temporary fixative while the slower setting construction adhesive develops strength.

Wood Flooring

The installation of wood flooring typically involves products designed for that purpose. Such products are thick like construction adhesives to accommodate modest unevenness in the subfloor, but are designed to provide more working time, more stretch and less strength than products for subfloor installation. Molding strips and reducer strips, used to transition between the wood floor and surrounding areas are often installed over either concrete or wood subfloors using construction adhesives or faster setting hot melts, or some combination of the two.

Butcher Block Countertops

Butcher block countertops are made using water resistant wood glues, selecting from cross-linking PVA's, polyurethane glues or two part products like urea formaldehyde or resorcinol. If the butcher block surface will not be sealed, it is important to note that such adhesives must conform to FDA regulation 175.105 for indirect contact with food. Traditional countertops often consist of a plywood or composite wood core that is covered with high pressure laminate. While this operation may be done in shop using wood glue and a large hydraulic press to provide pressure, the application of high pressure laminate is generally done in the field using a contact cement. Using this type of product allows the operation to be completed in a short time frame without special equipment and, after a momentary rolling, creates a bond with enough strength to allow immediate routing of the edges and completion of the countertop. When selecting a contact cement, it may be helpful to recognize that a solvent based product will dry faster and produce as good a bond, but may introduce health and safety issues that might be better avoided. More progressive types of countertops include granite, concrete and stone/polymer blends. These are typically very heavy materials and are secured to the cabinet holding base with a construction adhesive to accommodate unevenness and provide stability.

In summary, choosing the right adhesive or glue for a particular project requires some knowledge of the demands of the application, and that understanding serves to aid in selecting the best product. For applications for which a product choice is not clear, an inquiry directed toward an adhesive manufacturer would serve as a good starting point to determine what product type might work best.

Emerging Issues

There are many emerging issues in the wood adhesives market. One of the biggest of these is technology growth. Wood Glues have been steadily improving over the years and new technologies, along with advanced engineering, are opening up more application possibilities. We have come a long way from plant resins and animal hides to new reactive chemistries that can literally bind almost anything. The introduction of crosslinking PVA's brought a new level of water resistance to glue bonds and added new applications for the industry. Polyurethanes, epoxies, and cyanoacrylates have advanced applications from just wood-to-wood to the increased ability to bond plastic to metal and do it with less assembly and clamp time. New substrates, or material technology, have also added challenges for adhesion. Exotic woods from overseas create innovative products, but the challenges of bonding with some of the high oil content in these types of wood can be difficult for traditional water-based wood glues. Also, construction adhesives are being asked to bond new composites made of wood and plastic with the same strength that they currently perform on traditional wood. For some of these issues, reactive technologies have been the solution. Another issue is the impact of regulations and codes on the industry. In some cases, this has been a driver in advancing technology. With a growing emphasis on clean air and the remittance of volatile organic compounds (VOC's), the solvent emulsions of several years ago have given way to newer water-based technology. New reactive chemistries that emit very little during cure are also becoming more attractive to the industry. In addition to VOC's, the building and fire safety codes have pushed many in the door, window, cabinet and furniture industry to comply with new heat and fire resistance codes. Doors and window manufactures must now past rigorous flame testing in order to comply. This trend is expected to continue for many years to come.

Relevant Codes and Standards

Perhaps the most talked about standards for wood adhesives are those that regulate the level of volatile organic compounds (VOC's) in a product. California leads the way here, with the strictest limits in the country. For products in consumer size packaging (16 ounces or less), the VOC limits are 15% by weight for construction adhesives and 10% by weight for general purpose glues. Effective January 1, 2009, VOC limits for some construction adhesives in California will be lowered to 7% by weight. Many states along the east coast and Midwest have chosen these same limits. The Federal EPA is working on a ruling to make these levels a national standard, however the OTC and EPA are currently at 15% and will stay there for the next few years. Building codes are incorporating NFPA 252, Standard Methods of Fire Tests of Door Assemblies or ASTM E 84, Standard Test Method for Surface Burning Characteristics of Building Materials and ASTM E 1537 Standard Test Method for Fire Testing of Upholstered Furniture.

Major Resources

Major resources for information on wood adhesives include:

Educational

Ahesives.org—an online educational portal providing unbiased information about adhesives and sealants to those that are relatively unfamiliar with the technology.