Bonding of plastics

What are plastics?

Plastics are polymer-based materials whose properties are created not only by the polymer itself but also by fillers, fibres, plasticisers, pigments, flame retardants, UV stabilisers, processing aids and other additives.

The polymer structure determines much of the material behaviour, while additives and manufacturing history can strongly affect the actual bonding surface. Two parts carrying the same generic material symbol can therefore behave differently if one contains release agent, glass fibre, flame retardant, plasticiser or a surface coating.

Main groups of plastics

Thermoplastics: why the exact type matters

Thermoplastics include some of the easiest and some of the hardest plastics to bond. Their behaviour cannot be predicted from the word “thermoplastic” alone.

PE and PP

Polyethylene and polypropylene are low-surface-energy polyolefins. Adhesives tend to bead rather than wet them effectively, so standard epoxy or cyanoacrylate products may fail without an appropriate primer, special LSE adhesive or surface activation.

POM

Polyoxymethylene, usually abbreviated POM, is an engineering thermoplastic with low surface energy and high chemical resistance. It often requires a specialised bonding approach and validation on the actual grade.

ABS, PC, PMMA and rigid PVC

These engineered plastics are generally easier for adhesives to wet than PE or PP. That does not mean they are risk-free. Polycarbonate, PMMA, polystyrene and ABS can be susceptible to stress cracking, crazing or cosmetic damage from unsuitable solvents or adhesive chemistries, particularly when the part already contains moulding stress.

Polyamide

Polyamides can have relatively favourable surface energy, but crystallinity and moisture absorption influence their behaviour. Moisture content can affect dimensional stability and the bonding process, so conditioning and testing may be important in production applications.

Thermosets: cured crosslinked materials

Thermosets form a crosslinked polymer network during cure. Typical examples include cured epoxy, phenolic and unsaturated polyester systems. Unlike thermoplastics, a fully cured thermoset does not simply melt back into a processable liquid when reheated.

The original version of this article divided “duromers” into crosslinked and non-crosslinked groups. That division has been removed because the defining feature of a thermoset or duroplastic material is the irreversible crosslinked network.

Elastomers and thermoplastic elastomers

Elastomers are capable of large reversible deformation. Conventional rubbers such as EPDM or NBR are typically crosslinked, while TPEs obtain rubber-like behaviour through a multiphase or block structure that still allows thermoplastic processing.

For bonding, the important point is not only flexibility but also surface chemistry. Silicone elastomers and many polyolefin-rich TPEs have low surface energy and may require primers, plasma, corona, flame treatment or a dedicated adhesive technology.

How adhesive chemistry relates to polymer behaviour

The old article attempted to assign every adhesive family directly to “thermoplastic”, “duromeric” or “elastomeric” groups. This can be misleading because an adhesive family and the mechanical behaviour of its cured film are not the same classification.

A more useful engineering distinction is:

• Conventional hot-melt adhesives are usually thermoplastic and set mainly by cooling.
• Reactive epoxies, many acrylics and many polyurethane systems form chemically cured networks.
• Reactive PUR hot melts are applied as thermoplastic melts, set initially by cooling and then crosslink chemically with moisture.
• Silicone, polyurethane and MS-polymer adhesives/sealants can be formulated to remain highly flexible after cure.
• Cyanoacrylates and UV-curing adhesives cure by chemical reaction and should be selected by their product-specific properties rather than placed into one generic “plastic type” category.

Why identify the plastic before choosing an adhesive?

The plastic designation gives the first indication of:

• surface energy and wettability,
• chemical resistance,
• risk of stress cracking,
• thermal expansion,
• stiffness and flexibility,
• possible need for primer or activation.

If the part is marked, symbols such as PP, PE-HD, ABS, PC, PA6, POM or PMMA are a useful starting point. ISO 11469:2016 provides a system for generic identification and marking of plastic products using symbols and abbreviated terms from the ISO 1043 series.

If the material is not known, do not identify it only by colour, hardness or appearance. Check drawings, supplier documentation or material certificates, and validate the bonding process on the real production part.

Surface free energy and wettability

For a liquid adhesive to develop adhesion, it must make intimate contact with the substrate. This requires adequate wetting.

For a solid plastic, the relevant property is usually described as surface free energy. For the liquid adhesive, the related property is surface tension. A liquid tends to wet a surface better when the substrate has sufficiently high surface energy relative to the liquid.

3M classifies materials below about 36 dyn/cm as low-surface-energy materials that are especially difficult to bond. Polyolefins such as PP and PE and fluorinated materials such as PTFE fall into this difficult category.

Why the old surface-energy table has been removed

The previous version listed single numerical values for plastics, glass and stainless steel as if they were directly comparable “surface tensions”. That is too simplistic for engineering use.

Surface energy depends on the exact material, grade, additives, ageing, contamination and treatment. Metal and glass values can also be reported using different surface-science methods and should not be mixed casually with liquid surface-tension data.

For adhesive-process control, it is safer to use a validated test method for the actual substrate rather than rely on a generic internet table.

Water-drop test and dyne pens: what they can and cannot tell you

Water-drop test

A clean high-energy surface tends to allow a water droplet to spread more readily, whereas contamination or a low-energy surface tends to make the droplet bead. This is useful as a quick visual comparison.

However, a single water droplet does not provide a reliable numerical value of surface free energy and cannot prove that an adhesive joint will survive long-term temperature, humidity, chemicals or mechanical loading.

Dyne pens and test inks

Dyne pens provide a more structured process-control check, especially after plasma or corona treatment of PE and PP. They can indicate whether the treated surface is above or below a selected dyne level.

They still do not replace a bonding test. Surface activation can also decay with time, so the interval between treatment and adhesive application should be defined and controlled.

How can difficult plastics be prepared for bonding?

Depending on the plastic and adhesive system, the process may include:

• cleaning and degreasing with a plastic-compatible cleaner,
• controlled abrasion where appropriate,
• primer or adhesion promoter,
• corona treatment,
• atmospheric or low-pressure plasma treatment,
• flame treatment for selected polyolefins,
• special chemical treatment for very difficult polymers.

Surface treatment must be validated for the actual part. Aggressive abrasion or solvents can damage thin, transparent, coated or highly stressed plastics.

How to select an adhesive for plastic step by step

1. Identify both substrates. Record the exact plastic designation, grade and any coating or filler.
2. Define the loads. Determine whether the joint sees shear, tension, peel, cleavage, impact, vibration or fatigue.
3. Define the environment. Include temperature, humidity, water, chemicals, UV and thermal cycling.
4. Check the joint geometry. Consider bond-line thickness, overlap, gap and access for application.
5. Check the production process. Define working time, fixture time, cure time, dispensing method and throughput.
6. Select surface preparation. Use only treatments compatible with the plastic and adhesive.
7. Run representative tests. Use actual production parts and, for critical applications, include ageing and environmental exposure.
8. Standardise the process. Record cleaning, treatment, adhesive quantity, cure conditions and acceptance criteria.

Common mistakes when bonding plastics

• treating all “plastic” as the same substrate,
• selecting an adhesive before identifying the material,
• assuming a high catalogue strength guarantees adhesion to PE or PP,
• using an aggressive solvent that causes crazing or cracking,
• ignoring mould-release agents or plasticisers,
• abrading a surface and then leaving abrasive dust behind,
• touching the prepared surface with bare hands,
• treating a water-drop or dyne result as proof of long-term durability,
• testing on a random plastic coupon instead of the actual production part.

FAQ – bonding plastics

Which plastics are the most difficult to bond?

PE, PP, POM and fluoropolymers such as PTFE are among the most demanding because of low surface energy and, in some cases, high chemical resistance. They often require a special adhesive, primer or surface activation.

Which plastics are generally easier to bond?

ABS, rigid PVC, PMMA and many engineered plastics usually offer better wettability than polyolefins. The actual grade, coatings, mould-release agents and stress-cracking sensitivity still need to be checked.

Is surface tension the correct term for a plastic?

For solid materials, “surface free energy” is the more precise term. “Surface tension” is normally used for liquids.

Does a spreading water droplet mean the plastic will bond well?

It is a positive indication of wettability or cleanliness, but it is not a durability test. The actual adhesive joint still needs validation under representative loads and environmental conditions.

Can plasma or corona treatment make PP and PE easier to bond?

Yes. These treatments can increase surface energy and improve wetting, but the effect depends on the polymer, treatment parameters and time between treatment and bonding. The process must be validated and controlled.

Can I identify a plastic just by looking at it?

No. Similar-looking parts can be made from very different polymers or blends. Use markings, drawings, supplier documentation or laboratory identification where necessary.

For a more application-focused guide, see What kind of adhesive should I use for plastic? Proven types and selection guidelines.