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How to Bond Polypropylene

In this article, we explore the best adhesives used to effectively bond polypropylene, alongside an explanation of what polypropylene is and how it’s used around the world.

Table of Contents

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What is Polypropylene?

Polypropylene is a thermoplastic polymer made up of chains of the monomer propylene, giving the chemical formula (C3H6)n – with ‘n’ representing the number of links in the polymer chain.

polypropylene molecule

Polypropylene is a semi-rigid, tough, lightweight and flexible material. Although it is naturally opaque in colour, it can also be made translucent. As a consequent of its versatile properties, it is used in many different applications. Due to not absorbing water, unlike other plastics, and not deteriorating in the presence of bacteria or mould from having a high chemical resistance, it’s a preferred material for making plastic chairs. As a result of its high melting point, it is often used in reusable food containers which are microwave and dishwasher proof. It can be coloured and reshaped easily, making it a good material for carpet fibers that offer strength, durability and good aesthetics. More generally, polypropylene is found in virtually every application where plastics are used in both the automotive industry and laboratory equipment.

polypropylene in car bumpers
Polypropylene is a prevalent plastic used in car bumpers
polypropylene wipe
Polypropylene Cleaning Wipes

As of 2020, the global demand for polypropylene is estimated at around 62 million metric tons per year. It is currently one of the cheapest plastics available in the global market.

Welding is not typically suitable for bonding polypropylene to other substrates, in addition to being expensive and not fit for use on intricately shaped components.

Properties of Polypropylene

Surface energy: 29 mJ/m2

Maximum service temperature: 135°C

Melting temperature: 160°C

Wettability – Why Polypropylene Can Be Difficult to Bond

Polypropylene can be challenging to bond due its very low surface energy or “wettability”. A surface is called “wettable” when liquids such as water can be spread across its length without the bunching of droplets. High surface energy materials, such as phenolics, are easier to bond than low surface energy materials as the adhesive is able to flow across and “wet out” the entire substrate, ensuring consistency in the bond strength across the whole surface. Generally, it is accepted that a material needs to have a surface energy greater than 36 mJ/m2 to be bondable. A material’s surface energy can be improved by using surface treatment methods such as primers, flames, etching and plasma.

The chart below shows a range of materials alongside their surface energies.

surface energy chart
Surface energy chart from Permabond

Methods of Bonding Polypropylene

Common methods used to bond polypropylene include:

  • Plastic welding: where two plastics are heated together to their melting point, clamped and left to fuse and dry. Easy process, but susceptible to weakening the plastics through heat and damaging the structural integrity of the product.
  • Mechanical fasteners: rivets, bolts and screws are used to join different plastics together. Simple process, but can damage the plastic structural integrity and lead to corrosion problems.
  • Adhesives: adhesives are applied between two sheets of plastic and left to cure. Lightweight, corrosion-resistant, no effect on product strength, minimised safety hazards, and improved aesthetics. The most economically viable and high-quality solution for polypropylene bonding.

Adhesives Used to Bond Polypropylene

Permabond TA4610 is a two-part, toughened acrylic adhesive that can be used to bond polypropylene surfaces with no pretreatment. It’s rapid-curing and provides good impact strength and chemical resistance. Additionally, it is non-hazardous, making it easy for shipping and storage. It is our top recommended adhesive solution for bonding polypropylene surfaces.

Permabond 105 is a low viscosity cyanoacrylate adhesive that is naturally best suited for hard-to-bond plastics such as polypropylene. It is a high purity adhesive which is 100% reactive and capable of developing strong bonds extremely quickly on a wide variety of substrates. It is also easy to use, requiring no mixing or heat curing. When using Permabond 105, apply a layer of Permabond POP Primer to substrate surfaces before an attempt at bonding. The POP is the best available option for an instant bonding solution when used with cyanoacrylate adhesives.

For more information on surface preparations, read: What you need to know on bonding plastics.

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