A side-by-side comparison of using epoxy resin and a heat gun in a workshop, with tools and equipment on a workbench.

Why Epoxies Don’t Stick to Polypropylene: A Simple Explanation

Introduction When you need to glue things together, the type of glue you choose is really important. Epoxies are strong glues used in many projects because they’re tough and last a long time. However, they just don’t work well with a type of plastic called polypropylene. This article explains why this is the case, focusing on the nature of both the glue and the plastic.

What is Polypropylene? Polypropylene is a kind of plastic you can find almost everywhere. It’s used in car parts, containers, and even in packaging. It’s liked because it’s light, it doesn’t break easily, and it can handle a lot of wear and tear. It’s also good at resisting damage from chemicals and heat. But these features that make it so useful also make it hard for glues like epoxies to stick to it.

Why Chemicals Matter

One main reason why epoxies don’t stick to polypropylene is because of the plastic’s chemical makeup. Polypropylene is a non-polar polymer, which means it doesn’t mix well with substances that are polar, like water. This nature makes it tough for the glue to bond at a chemical level. Epoxies usually need to react chemically with what they are sticking to, but they find nothing to react with on polypropylene, so the bond doesn’t form well.

A detailed illustration of various plastic products and components, including bottles, car parts, and household items, showcasing the wide range of applications for plastic materials.
A detailed illustration showcasing various plastic products and components, from bottles to car parts, highlighting the versatility and widespread use of plastic materials.

Low Surface Energy

The stickiness of a glue also depends on the surface energy of the material. Polypropylene has very low surface energy. This means it doesn’t let other substances spread out evenly over its surface. Because epoxies need to spread out to stick properly, they can’t form a good bond with a surface that doesn’t let them spread.

Molecular Movement

Polypropylene’s molecules move around quite a bit at room temperature, which affects how the glue sets. For a glue to work, its molecules need to link up with the molecules of the surface it’s sticking to. The movement in polypropylene makes it hard for this linking process to happen, so the epoxy doesn’t set as it should.

An abstract representation of molecular structures and interactions with colorful spheres and connecting strands, depicting the chemistry behind resin formation.
An abstract depiction of molecular structures and interactions illustrating the chemistry behind resin formation with colorful spheres and connecting strands.

Surface Treatment Techniques

Treating the surface of polypropylene with special techniques like corona, flame, or plasma can make it easier for glues to stick. Here’s how each method works:

  • Corona Treatment: Uses a high-voltage electric discharge to modify the surface by introducing reactive groups, making the plastic’s surface more receptive to adhesives.
  • Flame Treatment: Involves quickly passing the plastic through a flame, which oxidizes (adds oxygen to) the surface and enhances its ability to bond with glues.
  • Plasma Treatment: Employs ionized gases to etch and modify the surface of the polypropylene, significantly improving its adhesive properties.

These treatments are designed to enhance the surface energy of polypropylene without altering its inherent properties, like color or strength, enabling better bonding with adhesives.


Understanding why epoxies can’t stick to polypropylene helps us make better choices about what glues to use for different materials. By learning about the nature of both the epoxy and the polypropylene, you can find the right method and materials for your project. This knowledge is especially important for making sure things are put together in a way that they stay together.

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