Views: 4 Author: Site Editor Publish Time: 2022-02-22 Origin: Site
Wood glue isn't always the best choice for joining wood to wood. In some applications, PUR adhesives may be a better choice. Understanding why requires a closer look at the chemistry of these adhesives and the process by which they form bonds. Once you have this knowledge, the pros and cons for each become clear.
The adhesive known as "wood glue" is usually a water-based formulation of polyvinyl acetate (PVA for short). This comes in the form of a white or yellow liquid that is applied to the surfaces being joined.
The water content helps the PVA binder form bonds with porous materials. The water seeps into the surface and then dries, leaving the glue to hold the joints together. When gluing wood with PVA adhesive, the parts must be clamped together until the water dries. Any attempt to reposition them weakens or even destroys the link before it is fully formed.
Less commonly, EVA holt melt adhesives are sometimes used to bond wood. They work similarly to PVA adhesives, but use a solvent instead of a water base. Check out our EVA Adhesive Guide for more information.
Like PVA adhesives, unlike epoxy resins, polyurethane reactive (PUR) adhesives are one-part adhesives. That is, they do not need to be mixed before application. However, the bonding method of PUR adhesives is very different.
PUR is used as a hot melt adhesive, applied in liquid form to porous surfaces such as wood. There, curing forms an initial bond, after which the PUR adhesive begins to react with moisture. This process, which takes 24 to 48 hours to complete, allows the adhesive to expand slightly, creating a strong bond between the surfaces.
Readers who want to learn more about this family of adhesives should check out our complete guide to PUR adhesives.
PVA is suitable for wood because it is a porous material. This ensures good penetration, resulting in a strong bond. Also, it has no odor and is transparent when dry. For industrial-scale applications, PVA formulations can be tailored to provide the desired open time (the time the adhesive remains active after application) and set time (the time it takes to form a bond). However, it also has some limitations. these are:
Parts need to be clamped in place while the adhesive dries
PVA retains a degree of flexibility and can decompose, so should be considered a complement to other fasteners
Not easy to sand and not easy to stain
The joints are not necessarily sealed against moisture
Water solubility limits use in wet or humid conditions
Softens at high temperatures (160°F (77°C) and higher.)
For PUR, its strength - the desire to react with moisture - is also a weakness, as it means that PUR has a limited shelf life. However, this adhesive does much more than that. When choosing wood glue, first consider these advantages of PUR adhesives:
Produces an extremely strong bond without the need to supplement other fasteners
Short set times (as short as 15 seconds, depending on recipe), so minimal in-situ clamping is required
No VOC/solvent emissions
Expands to fill voids and form a waterproof seal
Can sand and accept stains
Maintains bond strength at temperatures up to 300⁰F (150⁰C)
Efficacy is not reduced by moisture
Important points to note about PUR adhesives are:
Shelf life is about 12 months
Curing reaction begins when exposed to air
Cannot be washed with water - solvent is required instead
PVA wood glue can produce acceptable results where the joint is held firmly in place through the curing process. In this regard, it complements other fasteners. Where it struggles is in applications where there is no other clamping. Edge bonding is probably the most obvious example.
It's also useful in situations where post-attach cleanup may be required, as visible excess can be easily wiped off.
PUR is especially useful when it is the only bonding mechanism. This makes it valuable in structural applications. It is also suitable for edge bonding, but the limitation is that tight control of the amount of adhesive dispensed is important to minimize any cleanup.
Another advantage of PUR in edge bonding is that it seals the joint against water ingress.
Companies that produce PUR adhesives can tailor properties such as viscosity, open time, and set time to meet specific requirements and application types. Therefore, there are many forms of PUR adhesive products with different dispensing options.
Our application experts can help you choose the best product for your operation. Contact us for advice on PUR adhesives or to help test PUR adhesives for your application.