As a woodworker, one of the aspects that you will probably find confusing is wood glue. If you go to a hardware store, you will see all varieties of wood glue on sale. You can easily get overwhelmed with the full range of products on offer. How do you know which type of adhesive will serve you best for your woodworking projects?
The best approach is to categorize wood glue into types. This post discusses two significant types of wood glue, polyvinyl acetate (PVA) and polyurethane. Once you understand these two types of wood glue better, you can decide which type would be best suited for your project. You can then focus on the different brands available and choose one that suits your requirements and budget.
PVA vs. Polyurethane Glue
To get the best out of using glue for your woodworking projects, you need to gain a better understanding of both these types of adhesives. Here, we take a close look at the various aspects of PVA glue, and then we study the features of polyurethane glue.
We call PVA glue wood glue and also yellow glue. It is one of the first types of glue to be used for sticking wood together. PVA glue is the backbone of any carpentry shop. The bonding of PVA is sometimes more durable than the wood that it bonds. It means that in a joint bonded with PVA, the wood surrounding the joint can fail before the glue does.
You can get various brands and types of PVA glue. Most companies will offer three categories of this glue, based on the level of water resistance. The three categories of PVA glue are as follows:
Type-I PVA glue, as you may guess, has a high resistance to water. It contains polymer strands that sustain its adhesive properties even when exposed to the elements. This category of glue contains a low percentage of water. The entanglement and chemical bonding of the polymer strands in type-I PVA glue is the strongest, thereby imparting the highest waterproof properties.
Type-II PVA glues, contain a higher percentage of water than type-I PVA glue, and the polymer strands also bond tightly but not as much with type-I glue. The result is that this category of PVA glue comes under the classification of “water-resistant.” It can withstand exposure to moisture, but only for a limited time.
Type-III (not water-resistant)
Type-III PVA glue does not possess any resistance to moisture at all. That is not to say that it lacks bonding strength, but joints bonded with type-III PVA glue should not be exposed to moisture. The water content of this category of PVA glue can be up to 50%. An advantage of using type-III PVA glue is that it dries pretty fast. Bonding takes place within an hour, but depending on atmospheric conditions can take up to 24 hours.
Note: Apart from the water-resistance factor, all types of joints bonded with PVA glue have similar strengths. A typical PVA joint can withstand a force of up to 3,500 PSI to 4,000 PSI. Once dry, you can get rid of the excess glue by sanding it down or scraping it off with a knife or chisel.
You can get different brands of polyurethane glue in the market. This glue category is useful in many various applications for bonding concrete, stone, ceramics, and foam, in addition to joining wood.
The main advantage of polyurethane glue is that it is not a water-based glue. It will not soak into the pores of the material that it bonds, especially with wood. It results in being able to stain and finish wood joints that use polyurethane glue effectively. You can even glue pieces together whose surfaces are already finished.
The curing process of polyurethane glues involves the inclusion of moisture. Thus, these glues will even perform on wood that is oily or moist, which puts this glue at an advantage over PVA glues. Polyurethane glues have a relatively short curing time – about six to eight hours. You will need to clamp the joints for adequate bonding, but you can remove the clamps after about four hours.
The strength of polyurethane glue after curing is slightly lower than PVA glue at about 3,000 PSI. Before the glue dries, you can remove by applying solvents, but not water. Once the glue dries, like PVA glue, you can use sandpaper or a sharp blade to remove the excess adhesive.
PVA Vs. Polyurethane Glue: Pros And Cons
Both types of glue have their advantages and disadvantages. Having this information is essential because it will help you decide which kind of glue to use for a particular project and application.
- Easy to Apply
- Edge grain to edge grain joints on wood are very strong
- Longer working time of approximately 30 minutes
- You can get different types of waterproof PVA glue
- Water-soluble glue
- Readily available
- You can use PVA glue on multiple materials
- Weak end grain connections in wood
- Certain varieties cannot be used outdoors or in wet conditions
- PVA glue dries slowly
To summarize, PVA glue is suitable for use in all types of woodworking projects for bonding joints. Whether you need to join sides or faces, you can use PVA glue effectively. It provides the most durable bond on the market, and you will find it simple to use.
For best results, apply PVA glue with a brush on both the surfaces to be bonded. Then, you need to clamp the joint and not touch it for 24 hours. Once done, you can remove the excess glue by sanding or a sharp blade like a chisel or knife.
Polyurethane Glue Pros
- Versatile material
- Suitable for making secure end grain connections on wood
- On curing it expands to fill joints (makes tighter joints)
Polyurethane Glue Cons
- Requires moisture to cure
- Shorter working time of approximately 15 mins
- You need a volatile solvent like thinner to clean it up
- Polyurethane glue has a maximum shelf life of 1 year
- Expands on curing (can cause deformities)
A unique feature of polyurethane glue is that it requires moisture for curing instead of relying on air. You need to apply a film of moisture by spraying or brushing on the joints before applying the glue.
Also, you need to consider the expansion property of polyurethane glue. It can contribute to tighter joints because it expands as it dries. The downside is that it can cause deformities in the wood due to the uneven expansion.
You may wonder which glue of both these types is the best choice for using on wood. The answer to this question is that there is no best among these two types of glue. You have to consider the pros and cons of each type and select your glue accordingly.
If you keep a stock of both types of glue in your woodworking workshop, you will find that you need either of these types from time to time. It depends on the scenario. Now that you know both these glue types, you will know what you can expect from each type. Experience will teach you which one to use for a particular job.