For attaching wood to wood, wood glue isn’t always the ideal option. A PUR adhesive may be a preferable alternative in some situations. Understanding why this is the case necessitates a closer examination of the chemistry of these adhesives as well as the bonding process. The advantages and disadvantages of each will become evident once you have this understanding.
The Fundamentals of Wood Glue
Water-based formulations of polyvinyl acetate, or PVA for short, are commonly referred to as “wood glue.” This comes in the form of a white or yellow liquid that is sprayed on the surfaces to be connected.
The water content of PVA adhesives aids in the formation of bonds with porous materials. Water penetrates the surface, dries, and the glue is left to hold the connection together. When using a PVA adhesive to attach wood, it’s critical to keep the pieces held together until the water has dried. Any attempt to relocate them may cause the link to weaken and possibly dissolve before it has fully formed.
The Fundamentals of PUR Adhesives
Polyurethane reactive (PUR) adhesives, like PVA adhesives, but unlike epoxies, are one-part adhesives. That is, they do not need to be mixed before use. PUR adhesives, on the other hand, work in a very different way.
PUR is applied as a liquid to a porous surface such as wood as a hot melt. The initial binding is formed through solidification, following which the PUR adhesive begins to react with moisture. This process, which takes between 24 and 48 hours to complete, slightly stretches the adhesive to create a firm bind between the surfaces.
PVA Wood Glues: Advantages and Disadvantages
Because wood is a porous substance, PVAs function well with it. This ensures adequate penetration and a strong relationship. It also has no odor and dries completely clear. The PVA formulation can be modified for industrial-scale applications to offer the requisite open time (the amount of time the adhesive remains active after application) and set time (the time needed to form a bond.) It does, however, have some restrictions. These are the following:
- While the adhesive dries, the parts must be clamped in place.
- PVA keeps some flexibility and can break apart, thus it should be used in conjunction with other fasteners.
- It’s difficult to sand and doesn’t take stain well.
- Doesn’t always protect a joint from dampness.
- Because of its water soluble nature, it can’t be used in wet or moist environments.
- High temperatures (160°F (77°C) and above) will cause it to soften.
PUR’s Advantages and Disadvantages in Wood Joining
PUR’s strength – an eagerness to react with moisture – is also a flaw, as it implies the substance has a limited shelf life. But there’s more to this adhesive than meets the eye. Consider the following benefits of PUR adhesives when selecting a wood glue:
- Creates exceptionally strong bonding that don’t require the use of additional fasteners.
- Because of the quick set time (as little as 15 seconds, depending on the formulation), minimum clamping-in-place is necessary.
- There are no VOC or solvent emissions.
- Expansion fills holes and seals out moisture.
- It may be sanded and stained.
- At temperatures as high as 3000F (1500C), bond strength is maintained.
- Moisture has no effect on effectiveness.
The following are some things to keep in mind when using PUR adhesives:
- The product has a 12-month shelf life.
- The curing reaction begins when the product is exposed to air.
- Can’t be cleaned with water; instead, a solvent is required.
How Do Adhesive Characteristics Affect Woodworking Applications?
When the joint is maintained firmly in place during the curing phase, PVA wood glues can produce satisfactory results. It works well with other fasteners in this regard. Where it falters is in instances where no other clamping is available. Perhaps the most obvious example is edge bonding.
It’s also useful for post-joining cleanup because apparent surplus may be easily wiped away.
PUR is particularly useful when it is the only bonding method available. As a result, it’s useful in structural applications. It can also be used for edge bonding, with the caveat that precise control over the amount of adhesive applied is required to avoid any cleanup.
Another benefit of PUR in edge bonding is that it prevents water from getting into the joint.
Formulating Formulations to Meet the Needs of the Task
PUR adhesive manufacturers customize features such as viscosity, open time, and set time to meet specific needs and application types. As a result, there is a large range of PUR adhesive products available in various forms and dispensing methods.