Adhesives and glues are designed to adhere materials together. Technically, glues are considered natural, and adhesives are considered synthetic. There is no universal glue that will adhere anything to everything under all conditions. There are several general-purpose and special-purpose glues and adhesives. The first questions to ask when deciding which glue to use are:
■ What materials are you bonding?
■ Under what conditions do you need them to maintain their bond (waterproof, freeze/thaw)?
■ How quickly must the bond reach full strength?
■ How strong must the bond be?
■ What properties (clear, sandable, etc.) must the glue have when it is dry or cured? Animal Glues
Hide glue is made by boiling animal bones, hoofs, and skins, which results in a hard cake that is then soaked in water and heated. This produces a gelatinous mass. When reheated, this mass becomes liquid, giving it an instant "grab" when applied warm. Hide glue was once used for cabinetmaking purposes and hand veneering processes. Its set time, however, takes up to 24 hours, and it is odorous. Warming the glue with applied moisture will reverse the adhesive process. Hide glue is a good adhesive for repairing antiques and has good workability. Its bonding strength, however, is not as strong as that of PVA glues, and even the heat of an attic space can cause hide glue to fail.
Nearly 5,000 years ago, the Egyptians used hide glue to make furniture.4 During the seventeenth century, the best glues were pure hide or fish glues. In the early twentieth century, animal glues were still used for secondary or assembly gluing without the application of heat for a speedy turnaround time. Hide glue was generally used to glue wood joints and to lay down veneers, but it is only occasionally used today. Today, synthetic glues have replaced animal glues.
Vegetable glues are made from a range of starch granules found in tapioca, rye flour, natural rubber, and soybean dissolved in a solution of caustic alkali. Starch glues were used primarily for plywood lamination and veneer work. Special rubber-latex vegetable glues have been formulated and used for contact adhesives to bond plastic laminates, upholstery, and foam work.
Synthetic (also known as white, yellow, or carpenter) glues are a category of special-purpose glues developed during the second half of the twentieth century. These glues respond to heat and special accelerants and therefore can have vastly accelerated setting times.
■ Polyvinyl acetate (PVA) is a white or yellow synthetic polymer that was first discovered in Germany by Dr. Fritz Klatte in 1912. It is a thermoplastic, water-based glue that sets as water diffuses into the materials being glued. It is prepared by the polymerization of vinyl acetate. Partial or complete hydrolysis of the polymer is used to prepare polyvinyl alcohol. PVA is sold as an emulsion in water and used as an adhesive for porous materials, particularly wood. It is the most commonly used wood glue.
■ Contact adhesives: Ideal for paper-backed veneers and laminating sheet materials together. Contact adhesive is not recommended for wood-on-wood conditions but is fine for phenolic-backed wood veneering. These adhesives are typically thinner based and caustic, but newer water-based contact adhesives are available and are far less caustic.
■ Epoxy: Extremely tough and durable synthetic resins that consist of two parts that, when mixed together, bond a wide variety of dissimilar materials under relatively harsh conditions. Used for securing metal or stone to wood. Both the resin and the hardener are irritants, and their vapors are toxic.
■ Instant glue: An extremely fast-bonding adhesive. The chemical name is ethyl cyanoacrylate. Cyanoacrylate is the generic name for substances such as methyl-2- cyanoacrylate, which is typically sold under the brand names Super Glue and Instant Krazy Glue.® 2-Octyl cyanoacrylate is used in medical glues such as Dermabond and TraumaSeal. Cyanoacrylate adhesives are sometimes known as instant adhesives. They work best on smaller surfaces. Accelerators are available to speed their curing and bonding times. The bond is instant, colorless, transparent, and strong (except in shear). It was originally used for nonporous surfaces. Gel versions are now available for more porous surfaces.
■ Plastic cement: Used to join polystyrene plastic. It works by dissolving the areas that will be joined together, and in the dissolved areas, molecules from the two parts fuse together. It is very volatile and high in VOCs.
■ Polyurethane glues: These glues bond all wood, especially dense woods or those with high levels of oil such as teak. They expand as they cure and are messy to work with, but they hold materials together very well. Mortise-and-tenon joinery is excellent for polyurethane glues because they fill the voids and fully bond the joint. Gorilla Glue is a brand name of polyurethane glue, but many companies manufacture polyurethane glues.
■ Pressure-sensitive adhesives: Adhesives that bond on initial contact to most surfaces. Paper-backed veneers need the application of significant pressure (typically in refinishing cabinetry using pressure-sensitive veneers). The advantage of pressure-sensitive adhesives is that there is no drying or curing time. The strength of the bond varies with the formulation. Examples include no-lick stamps and envelopes, various tapes, and pressure-sensitive veneers.
■ Rubber cement: Polymers mixed in a solvent such as acetone or benzene to keep them fluid enough during application make this glue one of the class of drying adhesives. As the solvent evaporates, a strong yet flexible bond remains. An advantage is the ease of rubbing to remove excess cement. It is used for bonding two – or three-ply paper-backed veneers and laminated components or components with a plywood backing. The fumes are toxic, and protective measures are needed when using rubber cement.
■ Silicon: One hundred percent silicon is a good adhesive for bonding glass or metal to wood. The process of curing can take up to 48 hours. Silicon is a relatively clean, transparent adhesive that works well when PVA and epoxy glues cannot be used.
■ UVglue: UV glue uses UV light for curing. The result is a permanent and clear bond. One or both of the materials have to be clear, as the process needs UV rays from sunlight or artificial light sources. UV glues are especially useful for bonding glass furniture, as the process does not leave an unwanted "fog."
■ White glue: Nontoxic, odorless, nonflammable, and dries clear in less than an hour. Used for bonding paper, wood, cloth, and pottery.
■ Yellow glue: Used for bonding wood furniture. It is a higher-quality derivative of white glue that dries stronger and is more resistant to moisture.