While many plastics are easy to bond with common adhesives, polypropylene (PP) and polyethylene (PE) plastics are notoriously difficult to bond. These plastics have found common use due to their strength and chemical resistance, but unfortunately, that same chemical resistance is what makes adhesives fail.
Special primers can allow cyanoacrylates (super glue) to stick to PP or PE plastics. Often sold as "plastic super glue" in a kit including a small container of primer and cyanoacrylate adhesive, these primers are also available on their own. The primer is applied to both surfaces, and then then the plastic parts can be glued just like any other material.
Where larger gaps must be filled, urethane adhesives specifically formulated for plastics can bond PP or PE, provided that the surface is first roughened with sandpaper. These adhesives are a two-part formula that are mixed just like epoxy. Unfortunately, the bond strength is significantly lower when bonding PP or PE. Still, there is more than enough strength for most repairs.
Unlike some materials, it is impossible to form a truly strong bond with PP or PE plastics, and therefore any item repaired using adhesives should not be expected to retain all of its original strength. Therefore, adhesives should never be used to repair items that are exposed to high mechanical stresses, extreme temperatures, or harsh chemicals.
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