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.
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 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 which 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.
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.
Adhesives Used to Bond Polypropylene
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.
Permabond 2050 is a medium-high viscosity, flexible cyanoacrylate adhesive that is known for its improved peel strength when compared to other grades. Due to its superb impact and vibration resistance, it is ideal for use in applications that are going to undergo thermal cycling or thermal shock. Additionally, the 2050 is the perfect choice for use on flexible or semi-porous substrates. It is also easy to apply and offers strong bonds very quickly.
Permabond ET515 is a semi-flexible, two-part epoxy adhesive that offers bonds with good impact strength to a wide variety of substrates. It is easy to apply and achieves full cure at room temperature in 20-30 minutes. The ET515 is ideal for bonding to substrates where differential thermal expansion is expected.
Surface Preparation Tips for Bonding Polypropylene
Due to polypropylene’s low surface energy, adequate surface treatment is required prior to bonding to ensure maximum strength bonds.
When using cyanoacrylate adhesives such as Permabond 105 and 2050, 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.
When using two-part epoxies such as Permabond ET515, use flame, corona or plasma treatment before an attempt at bonding. Flame treatment requires professional equipment and is usually more suited to flat substrate surfaces rather than complex shapes. Careful calibration of the equipment is required as an over-treatment of surfaces can give the same results as not treating the surfaces at all. Corona or plasma treatment is ideal for complex shaped components and works by activating the surface electrons of the substrate and making them more available to bonding.
For more information on surface preparations, read: What you need to know on bonding plastics.