When choosing the best adhesive for your application, you will come across two types; adhesives with solvents, and solvent-free adhesives. 

But how can you tell which is better for you? 

As adhesive manufacturers here at Forgeway, we have helped hundreds of our customers decide on the best adhesive for their applications. We want to help you too, so we wrote this article.

In this article, we’ll discuss the difference and the pros and cons of both options. By the end of the article, you will know which adhesive is the better option for you.

What is the difference between solvent-based and solvent-free?

Before describing the difference, it’s critical to explain why people often compare the two. The debate between solvent-based and solvent-free is usually when deciding on the best contact adhesive. 

Whilst this article will focus on contact adhesives, the majority of the points we make will apply to structural adhesives and sealants too (unless we state otherwise).

When it comes to the definition of the word ‘solvent,’ it can get quite vague. Google’s definition is literally “anything that can dissolve other substances.” That means water is technically classed as a solvent. 

However, when it comes to adhesives being solvent-based or solvent-free, it’s slightly different. 

Solvent-free means there are less than 5% harmful solvents in the adhesive. Whereas any adhesive that has more than 5% harmful solvents would be classed as solvent-based.

I would guess that you are currently thinking “harmful solvents? What is the definition of a harmful solvent and who decides what is and isn’t harmful?”

Without getting too technical, harmful solvents are any solvents that are likely to cause harm to humans or the environment. So water is not harmful, therefore a water-based adhesive would be solvent-free. 

An example of a harmful solvent that is often used in adhesives is methyl ethyl ketone (MEK). Any adhesive containing more than 5% MEK would qualify as a solvent-based adhesive.

Regulatory bodies (like REACH and COSHH) are the ones responsible for defining harmful solvents. They determine whether a solvent could cause harm to humans or the environment.

What are the pros and cons of solvent-based adhesives?

The pros of using solvent-based adhesives

1. The solvent in the adhesive is able to etch the substrate. This means you won’t always have to prepare the surface to achieve a strong bond.

2. The solvent can break down the adhesive into a paste. This makes it easier to spread the adhesive across the surfaces of the substrate as you apply them.

3. Solvent-based adhesives have better temperature stability. This means they can withstand colder temperatures than solvent-free adhesives. Solvent-based also tend to be better at withstanding weathering conditions too.

4. They can achieve faster cure times. Some see this as a disadvantage because it decreases the open time significantly. However, most see faster cure time as an advantage. The solvents in the adhesive evaporate the moisture even at low temperatures. This means solvent-based are also less dependent on having the right conditions to cure.

The cons of solvent-based adhesives

1. They can be hazardous to health. This is the obvious reason most people steer well away from solvent-based adhesives. The solvents will almost always carry a health risk because they can omit volatile organic compounds (VOCs). Some solvent-based adhesives are obviously more harmful than others, but you should be aware of the risks before using them.

2. They can contribute to global warming. The VOCs we discussed earlier can also have a big impact on the environment. 

3. They will often be flammable. The fumes that solvent-based adhesives tend to discharge can be flammable. When there is a high concentration of these fumes, the risk of these fumes igniting is significantly increased.

4. They can damage the substrate they are bonding. This damage is also known as ‘witnessing’. It’s a problem because witnessing can damage the aesthetics of the substrate you are bonding. However, this will only be for some substrates.

What are the pros and cons of solvent-free adhesives?

Pros of solvent-free adhesives

1. You don’t need to worry about potential health risks. Whilst solvent-free doesn’t mean the adhesive is completely free of harmful solvents, it does mean there is less than 5%. This low percentage means you don’t need to worry about the adhesive causing health risks.

2. Pose no threat to the environment. The lack of harmful solvents also means that they will not discharge as many VOCs that are harmful to the environment.

3. You don’t need to worry about regulations and restrictions. Some countries have banned the use of certain harmful chemicals and solvents. And you will have to check that solvent-based adhesives do not contain anything that is banned.

4. You will not need to provide extraction and ventilation. The solvents used in solvent-based adhesives are often very unpleasant to work with. You will have to cope with the, sometimes pungent, smell of the solvents. The VOCs they omit may even require you (by law) to put extraction and ventilation in place. However, there is very little odour associated with solvent-free adhesives.

Cons of solvent-free adhesives

1. They are more susceptible to temperature variations. You must store the adhesive at the correct temperature and environment. Incorrect storage can severely impact the performance of solvent-free adhesives. The product will tell you how you should store it, although we would usually say that you shouldn’t store it below 0℃.

2. The cure time is longer and inconsistent. Solvent-free adhesives will almost always have a longer cure time. This is because the raw material used to replace the solvent will require elevated temperatures to evaporate and start the curing process. So when there are lower temperatures, solvent-free adhesives will take longer to cure.

3. The weathering resistance is poor. Solvents are more able to withstand harsh exposure to varying temperatures. The raw material in place of the solvent will usually have poor performance in harsh conditions and their performance will struggle when there are large variations in temperature.

How can you decide between a solvent-based or a solvent-free adhesive?

We know how difficult it is to decide between a solvent-based and a solvent-free adhesive. Here at Forgeway, we have seen the shift away from solvent-based adhesives over the years because of the health and environmental risks associated. 

Solvent-free adhesive technologies are advancing though. Their improvements mean you could find a solvent-free adhesive option for most applications. Typically, solvent-free contact adhesives are used to bond flooring and upholstery in vehicles, boats, and trains.

Nonetheless, we know that sometimes you just cannot get away from using a solvent-based adhesive. Like when you need the solvent to cut through surface contamination on the surface of the upholstery before bonding. In this case, you will have to take the necessary steps to keep the user safe.

We always recommend looking for a solvent-free adhesive first before looking at a solvent-based alternative. 

If you want some more information on adhesives, we recommend you download our Ebook. It will go through the different types of adhesives and help you come to a decision about the other adhesives you could use.

Or, you may want help from a member of our team. Here at Forgeway, we help companies decide the best adhesive for them. Click on the banner below to get in touch and discuss which adhesive is best for you.