What determines whether plastics can be bonded successfully?
The surface, more than the adhesive. An adhesive can only bond to a surface it wets, and most plastics -- polyolefins in particular -- have surface energy too low for that without pretreatment. Surface preparation, adhesive selection and joint design are decided together, since each constrains the others.
Can low surface energy plastics such as polyethylene and polypropylene be bonded?
Yes, with the right surface preparation. Flame plasma, plasma treatment and fluorooxidation all raise the surface energy of polyolefins enough for structural bonding. Fluorooxidation in particular is used to make normally hydrophobic polyethylene wettable, and its effect is long-lasting and frequently permanent.
Is a primer needed, or is pretreatment enough?
It depends on the substrate, the adhesive and the durability requirement. Pretreatment alone is often sufficient; on some combinations a primer adds a chemical bridge that pretreatment cannot. Establishing which applies for a specific joint is done by test rather than by rule, because the answer changes with resin grade.
How is a bonded joint qualified for production?
By testing the joint under conditions that reflect service -- temperature, humidity, chemical exposure and aging -- rather than measuring initial strength alone. Initial strength is the easiest property to achieve and the least predictive of whether the joint survives its service life.
