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An Overview of Structural Adhesives

Basic Adhesion Mechanisms:

Adhesives work by different mechanisms depending upon their type and the substrate to which they are applied.  It is important to realize that there are often multiple bonding mechanisms at work for any given adhesive family.

 

Mechanical Interlocking:

Most glues used on wood, for example, enlist the mechanism of mechanical interlocking.  The resins soak into the wood or paper or fabric surfaces penetrating into the fibers.  These resins then set up to a solid that is interlocked around and through the fibers.

 

Nearly every adhesive applied to porous materials works by this principle.  Even metals can be considered porous.  At a microscopic level we see this principle at work with the anaerobic Loctite adhesives used on metals wherein the low surface tension adhesive leeches via capillary action into the grain structure of the substrates and then, in the absence of air, sets up (polymerizes) into a strong resinous solid.

 

It should be readily apparent that the strength of a glued joint using this mechanism is more dependent upon the strength of the substrate than it is on the strength of the adhesive.  Marketers make much of one brand adhesive’s strength over another, but the reality is that as long as the adhesive has a strength of at least as much as the strength of the substrate, a “strong” joint will be attained.  Any tensile strength of an adhesive greater than the substrate is mostly marketing bluster and bravado.  Of more importance at that point is all the other factors including waterproofness, ease of use, temperature of cure, pot life, UV resistance, ease of cleanup, etc.  Different applications will dictate the adhesive selection based on a mix of these factors.

 

Solvent/Diffusion Bonding:

This mechanism is similar to welding.  A solvent is introduced at the interface between two materials (typically plastics) where it causes the materials to “melt” (become somewhat liquid).  These semi-liquids flow together at the molecular level (diffuse together) and when the solvent ultimately evaporates the materials solidify into a homogeneous mass.

 

This method is typical of that used to join plastics like acrylic (Poly Methyl Methacrylate (“PMMA”) commonly known as Plexiglass or Lucite, by introducing a solvent such as methylene chloride at the interface.

 

Another example is the joining of PVC plumbing parts using a solvent such as toluene or tetrahydrofuran.  Here the adhesive is comprised of PVC monomers mixed with a plasticizer such as di-octyl pthalate (“DOP”) and dissolved in a toluene carrier solvent.

 

Yet another example is the case of “model airplane glue” and its use on polystyrene parts.  Here the active solvent is usually Xylene which also carries with it styrene monomers.   The presence of these monomers gives the adhesive its gap filling ability as it hardens into a resinous mass.

 

Adsorption & Surface Reaction:

Here the adhesive molecules are attracted to specific molecular sites on the substrate.  What is called covalent bonding occurs as the result of sharing of electron pairs.  Members of the epoxy family work by this mechanism on non-porous solid materials like metals, plastics, glass and ceramics.  Additionally, the acrylic adhesive family (the cyanoacrylates) as used for labels for example, work by this mechanism.

 

Electrostatic Bonding:

This mechanism is primarily apparent in the coatings industry with paints on non-absorbent surfaces.

 

Popular Adhesive Families:

 

Poly Vinyl Acetate Emulsion (PVA):  Elmer’s Glue, Titebond Cabinet Glue, Titebond II

Also known as “white glue,” it is commonly used around the house or for cabinet making.  It is inexpensive, relatively fast drying and easy to clean up.  It is not waterproof nor does it produce a particularly strong bond joint.  Contrary to popular opinion, the “Yellow Glue” version of this adhesive, typically called “Cabinet Maker’s Glue,” is not stronger nor does it give a superior bond.  It is still PVA just with a yellow dye added.

 

Aliphatic:  Titebond III

 

Plastic Resin Glues (Urea Formaldehyde):  Weldwood Plastic Resin Glue, Urac 185 by Cytec

These are relatively inexpensive adhesives ideal for bonding porous surfaces.  They do, in fact, create a bond joint stronger than the wood itself and are commonly used in the manufacture of plywood, particleboard and are ideal for structural applications.  Weldwood was a popular boat-building adhesive at one time, however it is only water resistant and not rated as fully water proof.  This is the adhesive used by certain manufacturers of classic wood water skis.   These adhesives are activated by mixing with water into a paste that is typically applied with a brush or roller.  They can only successfully be used at work shop temperatures at 70F or above.

 

Resorcinol Resin Glues:  Weldwood Waterproof Resorcinol Glue, Aerodux 500, Penacolite G-1131 (also called “Cascophen”)—These are truly waterproof glues typically used in the assembly of structural wood components such as laminated beams for construction, aircraft frames and ribs, and classic wood boat construction.  They are typically approved by Lloyd’s of London for use in yachts and hold several mil spec ratings for aircraft and experimental aircraft construction.  They have extremely long life with bonds lasting easily forty years with no deterioration from the elements.

 

Polyurethane Adhesives:  Gorilla Glue, Probond, Excel

These are relatively new on the scene and widely and aggressively marketed.  They are able to bond many types of substrates ranging from porous (wood, paper & fabrics) to non-porous substrates including plastics and metals.  They require no mixing and can be used over a wide range of shop temperatures.  They are generally considered to be waterproof and exhibit good gap-filling properties.  Their cure is initiated through contact with moisture as is present in wood.  They do not clean up with water, however.  Clean up will require either alcohol or acetone solvents.

 

Epoxy Adhesives & Resins:  Epon, Smooth-On, Hysol/Loctite, Araldite

 

Cyanoacrylate Adhesives:  Eastman 910, Super Glue, Loctite Prism

 

Anaerobic Adhesives:  Loctite, Pacer Technologies

 

Nitrocellulose Cement:  Duco Cement

 

Solvent-based adhesives:  Testors Model airplane glue—Xylene solvent, Plexiglass bonding—Methylene Chloride solvent, PVC bonding—Tetrahydrofuran solvent

 

Contact Adhesives:  Rubatex SD-40 Neoprene Adhesive

 

The Eight Major Considerations In Adhesive Selection:

Whenever we are faced with a selection decision for the correct adhesive, we must consider the following factors:

 

Bond Strength:

With modern adhesives, this has progressively become nearly a non-issue since most quality adhesives will provide a bond line strength greater than the strength of the substrate being bonded.  Thus, if the substrate you are working with is white oak, whether your adhesive provides a 2,000 psi shear strength or a 4,000 psi shear strength is of little consequence when the inherent shear strength of the wood itself is less than 2,000 psi.  Of more importance in most applications is joint design that will provide sufficient area of contact (number of square inches of bond joint) to support the intended load.

 

Glue Line Appearance:

Since glue line appearance is a relative factor we have provided a picture of a multi-laminated glue block using various structural adhesives for direct comparison.

 

Water Resistance:

Be aware that there is a difference between water resistant and waterproof.  For a manufacturer to claim his adhesive is waterproof it must successfully pass the requirements set forth in ASTM ____ (“American Society For Testing Materials”).

 

Curing Temperature:

Workshops or work areas in various parts of the country can vary widely in ambient temperature.  Additionally, each adhesive has its own idiosyncratic cure temperature requirements.

 

Cost:

For small projects, cost is rarely a major concern but as the size of a given project increases the amounts of adhesives used, this can become a major consideration.

 

Ease of Mix & Application:

Here the decision is in regard to the need for mixing as opposed to a single constituent product as well as criticality of measurement of the parts and then sensitivity to mix methodology.

 

Pot Life:

In the case of multi-constituent adhesives, once mixed, they generally has a limited time during which they can be applied.

 

Shelf Life:

Due to the cost of some adhesives, the useful shelf life must be considered.


 

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