Plastics (aka polymers or polymeric materials) and adhesives are relatively new technologies for industrial applications. Going back 40 years, only a handful of industrial manufacturers used adhesives or plastics in their processes. These days, the vast majority of industrial manufacturers are investigating how they can use adhesives for joining plastic components.

Trust has grown in these technologies over the past decade or so. Automotive, aerospace, and product assembly manufacturers (among many others) all use polymeric materials and bonding in some form or another. 

Nowadays, these industries are looking to use adhesives for bonding polymeric materials. They expect adhesives to last as long as, if not outlast, the life expectancy of the product. This shows how much trust has grown in these technologies. 

Here at Forgeway, we manufacture industrial adhesives. We help industrial manufacturers solve their complex adhesive bonding challenges. Bonding plastics is often a challenge we help our customers solve.

Unfortunately, there is no one-stop solution to bonding plastic or polymeric components.

So that is why we wrote this article. In collaboration with industry experts, we want to share the experience we have gained over the years. By reading the article, you will become aware of the mistakes other industrial manufacturers have made when bonding plastics.

By the end of the article, you will know what aspects to consider when bonding plastics from design to assembly. You’ll know how to avoid common mistakes we see other manufacturers often make. We can help you achieve bonding success.

Why are plastics becoming increasingly popular for industrial use?

As touched on in the introduction, polymers and adhesive bonding are two relatively new technologies. Traditionally, metals were the go-to material for any structural purposes. Even non-structural materials were usually metal.

Just 40 years ago, for example, you would see very little plastic inside cars. It would be even rarer to see a polymeric material used for structural purposes. From small components to large and important components, plastic use is very widespread. Even car bumpers are polymers.

On top of that, experts predict a further rise in plastic use. Some say that plastic production will have to double by 2040 to keep up with demand.

So what’s the cause for polymers becoming so popular?

We recently put this question to Professor John Watts of the University of Surrey. He is a Professor of Materials Science and has vast experience in surface analysis and adhesives. He mentioned that there were two main reasons for the increasing popularity of plastics in industrial manufacturing.

1. Plastics are often easier to shape and mould than other materials. The main alternative to polymeric materials is metal. However, most metals are significantly more difficult to shape and mould. This is mostly due to plastics having a significantly lower temperature resistance (typically around 150℃). It’s easier and more energy efficient to soften polymers to mould and shape them.

2. Plastics have an incredible range of properties. There is a huge variety of monomers used in plastics. The characteristics of these monomers vary. All these variations mean that a blend of monomers can create a polymer best suited to the required properties. Acrynotrile butadiene-styrene (ABS) is a typical example of this. It combines various monomers to create a plastic that is strong, heat-resistant, and impact-resistant. Making it ideal for manufacturing parts that experience movement and impact.

In summary, John Watts said that plastics’ increasing popularity is here to stay. In fact, it’s highly likely the popularity will increase with the development of more sustainable versions of this material type. More industrial manufacturers are becoming aware of the benefits of using plastics in their processes.

Despite the environmental concerns about the pollution of waste plastics and the low-temperature resistance, industrial manufacturers are increasingly using plastics in place of metals. Expect to see a lot more growth in the polymeric materials industry in the coming years. 

However, with the growth of adhesives too (which are also polymeric by nature), the paths of these two new technologies will inevitably cross. As industrial manufacturers here at Forgeway, we are witnessing this crossing of paths and the cross-fertilisation between the two technologies.

So, in collaboration with Professor John Watts, we’ve collated the top tips on how you can ensure your plastic bonding doesn’t end in failure.

What should you consider before choosing a glue for bonding plastics?

Before you start choosing the optimum plastic glue, you should understand what we mean by the term ‘plastic glue’. Plastic glue is a term used by people to indicate that adhesive is suitable for bonding plastics. Plastic glue does not refer to an adhesive that is made out of plastic.

Now that we clarified that, it’s important to remember that adhesives are very different to traditional joining methods. If using a different joining method such as welding or mechanical fasteners and you want to bond a polymeric material, you’ll need to adjust the joint design to ensure a strong bond with adhesives.

Poor joint design will set the adhesive joint up to fail. You could choose the perfect plastic glue, but still experience bond failure if you don’t get the joint design right.

There are two main important things to remember when designing joints for adhesive bonding:

1. Maximise the contact area between the surface and adhesive. The more contact area you have between the surface and the adhesive, the higher strength the bond will be.
2. Minimise peel force. There are several different types of force an adhesive could experience. However, most adhesives cannot withstand much peel force. It’s important to try and reduce exposure to peel force where possible.

Joint design isn’t the only thing you need to consider. Substrate compatibility is also very important. Some grades of plastics are prone to causing adhesion problems.

We wrote a separate article dedicated to this topic. But in summary, materials with low surface energy will cause ‘wetting’ issues between the adhesive and the surface because they are hydrophobic. 

The hydrophobic surface repels the adhesive. So plastics that have low surface energy will repel the adhesive and cause weak bonds. Polypropylene, polyethylene, and polytetrafluoroethylene (PTFE familiar by the name Teflon™) are all examples of polymers with low surface energy (LSE).

If the material to be bonded has a low surface energy, there are three possible solutions for creating a strong bond.

The first solution is to use an alternative material that has a more hydrophilic surface. This will help the adhesive to ‘wet’ and create a strong bond. Plastics like ABS, PVC, or acrylic (among others) will have higher surface energy making them more hydrophilic and easier to bond.

The second solution is to use a specialist plastic adhesive. There are several options available with varying characteristics. Here at Forgeway, we manufacture a structural adhesive for polymers that requires no substrate surface preparation. You can find out more about our structural plastic adhesive Purok VX90 here.

The third solution is to alter the nature of the plastic’s surface. If you can’t find a more hydrophilic material or a specialist plastic adhesive that meets your requirements, you can still alter the surface of the plastic to improve compatibility. This is known as surface preparation and can come in various forms. Surface preparation is covered in more detail later in the article.

In summary, before you can choose an adhesive for bonding plastic, you should consider the joint design and the surface energy of the plastic. If you don’t, you may struggle to achieve a strong bond.

Correct surface preparation is essential for plastic bonding

Surface preparation is one of the most important steps in the bonding process. Plastic bonding is reliant on effective surface preparation to ensure a strong bond.

There are two main reasons why you might need to prepare the polymer before bonding. You may need to increase the surface energy and you may need to remove contaminants from the plastic’s surface.

Preparing the plastic to increase the surface energy

As we have already mentioned, the surface energy of the plastic plays a massive role in how receptive it will be to the adhesive. If you find that the surface energy is low and you don’t want to use a specialist adhesive, you’ll need to prepare the surface.

Nonetheless, ‘preparing the surface’ isn’t simple. When is it ever simple? 

This is because you’ll find yourself evaluating three surface preparation options. All with their advantages. All with their disadvantages. Let’s take a deeper look into each surface preparation type.

Physical Surface Treatment chemically changes the surface of the plastic

‘Physical surface treatment’ involves chemically changing the surface by exposing it to flame, corona discharge or plasma. This method of treatment will increase the surface energy of the polymer by grafting oxygen-rich groups to the surface, therefore, increasing the surface energy. 

It is very effective. 

A range of industries use this method. However, there are a few drawbacks to be aware of. The process will degrade with time and a corona-treated polymer surface such as polyethylene will recover its hydrophobic state in minutes if not seconds.  For this reason, the treatment is generally used as an in-line treatment by industries producing printed or coloured polymer foil (e.g packaging).  Flame treatment is very long-lived and can be used as a batch process.

A plastic surface that previously repelled an adhesive will almost always become a bondable surface after the correct choice of physical surface treatment.

Mechanical pretreatment increases the surface area available to bond

As we mentioned earlier in the article, a key objective of adhesive joint design is to maximise the surface area available to bond. Mechanical surface treatment is one way of increasing the surface area of the plastic.

Mechanical pretreatment is when the surface is abraded. Sanding, grinding or sandblasting are the main ways to abrade a surface.

It takes the surface from a flat surface to a rough surface. So what was previously flat, now has peaks and troughs. In most cases, an increased available bonding area leads to a stronger joint.

However, mechanical pretreatment doesn’t alter the chemical structure of the surface. Whilst there is a greater surface area available to bond, the surface still has low surface energy making it hydrophobic. The adhesive will not be able to form a strong bond with that surface.

Mechanical pretreatment is also prone to damaging the surface and can also be inconsistent. It may improve the performance of the adhesive due to the increase in surface area. But this isn’t guaranteed.

Chemical pretreatment creates a new bonding interface

Chemical pretreatment involves applying a special chemical that will bond to the hydrophobic surface of the plastic and will create a new layer of ‘bondable’ surface.

Chemical pretreatment is often called ‘priming’ the surface. Primers diffuse into the surface of the plastic, intermingle with the polymer chains, and will create a new layer on the surface that the adhesive can bond.

However, primers are very polymer specific. They are not a ‘one-size-fits-all’ solution. They only work with certain types of plastics.

For example, if you wanted to prime polypropylene before bonding, you would have to use a primer that works with it. The primer needs to be able to intermingle with polypropylene to create a strong base foundation.

But that’s not all. The primer also needs to be compatible with the adhesive. Some adhesives will not work with certain primers. 

Therefore, before using the primer, it’s important to make sure that it is compatible with the surface and the adhesive. For this reason, it is always best practice to use the primer recommended by the adhesive manufacturer.

Preparing the plastic to remove contaminants

After a plastic moulding process, there can be residual release agents left on the plastic’s surface. These release agents are contaminants invariably of low surface energy. They will interfere with the adhesive’s ability to bond.

Whilst there are other potential contaminants (like poor quality cleaner sprays), release agents are the most common cause of contamination on plastic. They can act as a weak boundary layer (WBL). This is a vanishingly thin contaminant layer of low cohesive strength. The joint strength is then simply a function of the strength of this WBL.

Not all plastics will have release agents. But most moulded (as opposed to blown or extruded) polymeric materials will. It can be difficult to determine their presence and chemical nature. One way to determine if there are release agents is by asking the supplier. They can help you understand what release agents they used in their process.

Once you have determined what release agents are on the plastic surface, you can then choose a method of cleaning. There are two main methods to clean contamination from an adhesive’s surface:

1. Use physical surface treatment. As we touched on above, methods such as plasma, flame, or corona treatment will physically alter the surface of the plastic. This will remove any contamination and ensure a readily bondable surface on the plastic.
2. Use solvent cleaners. Solvent cleaners will dissolve or displace the contamination and ensure the surface is clean and ready for bonding. Hexane is an example of a solvent used for this purpose.

There are some adhesives (like acrylics) which can permeate through some surface contamination. But this causes an exothermic reaction and also cannot permeate through heavy contamination.

Using a surface analysis technique to ensure the plastic is ready to bond

Whenever a customer has any concerns about the bondability of their plastic, we always recommend they test the surface. Here at Forgeway, we have a laboratory with testing equipment that can easily determine if a surface is bondable.

You don’t necessarily need to send us pieces of your substrate. We can visit your facilities to check if the plastic’s surface is going to cause bonding problems.

Watch the video below to get a better understanding of whether your surface is bondable or not.

How to choose the right plastic glue to help you avoid bond failure

Now that you have established the bondability of your plastic and any surface preparation methods required, it’s time to choose the right plastic glue for your application. You may be surprised to know that strength is one of the least important characteristics of a plastic glue.

There are three things you should know when choosing a plastic glue; firstly, strength isn’t very important. Secondly, flexibility is one of the most important characteristics of a plastic glue. And thirdly, resistance to environmental factors will determine how durable the bond will be.

Why is strength not important in a plastic glue?

Strength isn’t important in a plastic glue because plastics themselves aren’t very strong. If you use a very high-strength structural adhesive, it will increase the likelihood of substrate failure.

We’ve discussed adhesive strength in a separate article. There’s one important thing to remember when evaluating the strength of an adhesive. It’s not important how ‘strong’ the adhesive is. It’s more important to ensure the adhesive is ‘strong enough’. Otherwise, you run the risk of using an over-specified (and often more expensive) adhesive.

Why is flexibility more important than strength in a plastic glue?

With any bond, we consider flexibility to be just as important as strength from an adhesive. However, when bonding plastics, we’ve found that flexibility is often more important.

This is because a rigid adhesive will not absorb any stresses or forces. Any stress will transfer to the plastic and cause substrate failure. 

What you need to consider when assessing the flexibility needed is the type of force and type of load the bond will experience in its end-use. This will help you determine if you need a rigid or flexible adhesive.

There are four types of force the adhesive could experience:

1. Tensile force
2. Shear force
3. Peel force
4. Cleavage force

Tensile and shear force are the most common types of force. However, peel and cleavage forces can still occur on a bond. You should try to eliminate these two types of force where possible. Most adhesives will struggle to withstand them.

There are also two types of load the bond could experience:

1. Continual load: Where the adhesive is continually under force and there are no fluctuations from impact or vibration. A rigid adhesive is best in this situation because it helps prevent too much creep.
2. Occasional load: Where the force on the adhesive fluctuates. It will often have to withstand impact and vibrations. Some people refer to this as ‘fatigue load’. A flexible adhesive is best for this as it will deform and stretch depending on how much load it needs to withstand. 

Understanding what type of force and load the adhesive will experience is going to help you understand how much flexibility you need from your adhesive. 

If you aren’t sure of the difference between flexible and rigid adhesives, take a read of our article by clicking the button below. It will help you understand the different adhesive types.

Why is resistance to environmental factors important from a plastic glue?

A big benefit of using plastic instead of metal is that you don’t need to worry about corrosion. Water is a killer in metal-to-metal joining. It can displace the adhesive from the metal oxide substrate surface. However, water doesn’t affect plastic bonding in the same way as polymeric materials don’t corrode.

Water can affect a bonded plastic joint in different ways though. Water will permeate an adhesive to a certain degree and may bring about plasticisation (where the adhesive becomes more flexible and less strong).  Continual immersion in water may raise the need for a sealant bead to protect joint edges.

You’ll need to consider what other types of chemicals the bond will experience. Some adhesives have very poor chemical resistance. If the plastic bond is likely to experience exposure to chemicals, you should make sure the adhesive can withstand it.

Temperature resistance isn’t something people usually associate with an adhesive for bonding plastic. This is because plastics themselves usually have a low operating temperature (around 150℃). Nonetheless, some adhesives cannot withstand temperatures above 60℃. 

It’s important to understand what temperature the adhesive will face. Then you can ensure the plastic glue you choose will have sufficient temperature resistance. High-temperature adhesives are available but are mainly for bonding metals.

It’s not just about temperature resistance. Exposure to ultraviolet (UV) light (sunlight) can also affect some plastic glues. Most people don’t consider the impact of UV on an adhesive as they think that UV can’t get to it. Usually, the substrates will cover the adhesive and stop any UV from passing through to the bond itself. 

But there are instances where the adhesive will face exposure to UV. If the plastic is transparent or the adhesive is also being used as an external sealant, you’ll need to consider the UV resistance of the plastic glue you are using.

If an adhesive manufacturer promotes a ‘plastic-bonding glue’ or a ‘glue for bonding polymers’, it may be suitable to bond plastics but have very little resistance to the environmental factors discussed above. You must make sure the adhesive you choose can withstand these factors and remain durable. Adhesive durability is obviously crucial to the longevity of a bond.

How to make sure you have a strong and durable plastic bond

Now you know the different things to consider when bonding plastics, it’s time to put it into practice. You’ll be aware that plastic bonding is no walk in the park. In fact, it’s often one of the most difficult processes for industrial manufacturers to get right.

Here at Forgeway, we’ve helped industrial manufacturers achieve bonding success with their plastic applications. There is never a simple answer. We always take the manufacturer through the different steps we outlined above. This helps them progress towards a solution for their plastic bonding challenge.

Now, it’s time for you to reach ‘problem solved’. If you’re stuck on what you should do next, we’re here to help. Click on the button below to learn more about this topic. The Plastic Bonding Masterclass Webinar will help you understand this topic in greater detail.

Or, if you’d like to research the solution yourself, you can head over to our knowledge centre. There are a wealth of articles and content that will help you find answers to your adhesive and bonding questions.

We’d like to thank Professor John Watts for his contribution to this article. We recommend you head over to his page and view his work.