How do I bond plastics?

Finding the right adhesive for plastics

What are plastics?

I want to bond plastics, but how? Many people wonder how to bond plastics and then try a so-called “all-purpose adhesive”. But a special adhesive is required for plastics. The choice of the right adhesive, however, isn’t the only important factor.

Everyone is familiar with the term “plastic”. Plastic is a synthetic material. It is a generic term used for many different materials in our everyday lives. Plastics are polymers. The word polymer is derived from Ancient Greek and means “consisting of many parts”. Polymers are macromolecules with a chain-like structure. These molecules are made up of monomers that are strung together like individual pearls in a pearl necklace. Depending on the structure of the monomers, the polymers have different shapes and sizes. Plastics can be divided into linear, branched and cross-linked polymers.

Types of plastic

Depending on the type of plastic, its behaviour towards mechanical, thermal or chemical influences varies. Plastics can be categorised into the following types: thermoplastics, thermosets and elastomers.

What are thermoplastics?

The term thermoplastic refers to a synthetic material that becomes pliable or mouldable when heated. Thermoplastics consist of branched or unbranched polymer chains without cross-linking. These adjacent chains are held together by physical interactions and entanglements. They are made to vibrate by heating or by the addition of solvents. The stronger the heating, the greater the vibration among the chains. If the distance between them becomes too great due to the movement, the physical interactions decrease. The plastic melts.

Examples of thermoplastics are synthetic materials such as PVC (e.g. pipes or flooring), PA polyamide (e.g. air filter housings) or polyethylene (e.g. packaging or cable insulation, canisters).

What are thermosets?

Thermosets have a high temperature resistance and rigidity. They are not soluble, cannot be melted or welded and will hardly swell. Thermosets are characterised by a close-meshed polymer network with many connections. Chemical bonds ensure their strength. This makes them very resistant and durable. Thanks to their dense polymer network, water or solvents have little or no chance of penetrating. Thermosets are hardly pliable and do not creep.

Examples of thermosets are epoxies, polyurethanes and phenol-formaldehyde resins, which serve as the base material for fibre composites such as carbon, CFRP, GRP or fibreglass. They can be found in daily life in motorbike helmets or wind turbines.

What are elastomers?

Elastomers are elastically flexible, cannot be melted or welded, are non-soluble and highly swellable. They are characterised by a wide-meshed polymer network with only a few connections. The strength is primarily provided by the physical interactions and only secondarily by the chemical bonds and entanglements. Elastomers are also creeping. This means that the material deforms under prolonged load or tension, but returns to its original elastic shape thanks to the few connections between the polymer chains.

Examples of elastomers are polyurethane (as thermal insulation or foam), silicones (e.g. joint sealants) or rubber.

Selection of adhesives

So plastics differ greatly. For this reason, there is no such thing as a plastic adhesive for each and every plastic. Knowledge of the different types of plastic is essential to selecting the right adhesive: because if a bond is to work, it is important to know which plastic is to be bonded. The type of plastic is often embossed in abbreviations on objects, such as PP or LD-PE. Some types of plastic are easy to bond, others are difficult.

Plastics that are increasingly used in almost all areas of industry today are PTFE (known as Teflon), PE or polyethylene and PP or polypropylene. This is due to their specific properties such as mouldability, elasticity, breaking strength, temperature resistance and chemical resistance. However, they are difficult to glue. The reason for this is their low surface tension or surface energy. The surface must therefore be pre-treated mechanically by grinding or blasting, chemically by fluorination or physically by flame treatment. The use of a primer also ensures better wetting of the surface.

Surface energy

Why is surface energy an important factor in the bonding of plastics?

A low or low-energy surface tension leads to incomplete wetting of the adhesive surface and thus to a lower adhesive strength.

The higher the surface tension of the material, the better the adhesive strength.

Surface tension

Here is an overview of plastics and their surface tension for you. Plastics that are easy to bond are therefore PVC or polycarbonate.

Practical tips for bonding plastic – the WEICON checklist

✓ What type of plastic is it?

✓ What about the geometry and wetting of the adhesive surface?

✓ What is the environment like? Are there specific weather conditions? What are the prevailing temperatures? What load will the bond be exposed to?

You can also find more information on choosing the right adhesive in the video “Which one is the best match for you?“ or in our Adhesive Finder.

✓ Select the right product for surface pre-treatment and cross-linking, e.g. WEICON Surface Cleaner, WEICON Activating Cleaner or our Primers

✓ Select the right adhesive, e.g. WEICON Easy-Mix PE-PP 50 based on methyl acrylate, WEICON Plastic-Bond or one of our instant adhesives