I am participating in a competition to identify viable new markets for approximately 2,000 tonnes of recycled HDPE collected annually from used oil containers. These include:

• 19-litre pails (injection molded) – 23% of the total
• 1-litre and 5-litre jugs (blow molded) – 77% of the total

The injection-molded and blow-molded containers are separated at some stage before being used in the manufacturing of new products. Currently, about 65% of the recycled material is used to manufacture new containers by blending recycled pellets with virgin resin during the molding process. The remaining HDPE flakes are used in various plastic lumber products, including:

• Chock blocks
• Curb stops
• Decks
• Fence posts
• Garden planters
• Hot tub steps
• Outdoor benches and chairs
• Pallets
• Tables
• Various sizes of plastic lumber

Cleaning Trials & Contamination

I initially tried cleaning the HDPE flakes to remove contaminants on the surface and within the polymer matrix using two solutions:

• Caustic Wash: A solution of 0.5 wt% NaOH and 0.3 wt% polysorbate 80, heated to 75°C. HDPE flakes (25% of the water's weight) were stirred in the solution for 10 minutes and then dried.
• Solvent Wash: A separate sample was cleaned using a mixture of 22.5 g ethyl acetate and 25 g distilled water, stirred at 480 rpm for 10 minutes at room temperature, then dried.

Unfortunately, there was no quantitative method to assess cleaning efficiency at the batch level. Most techniques analyze individual flakes and don’t provide insight into the polymer composition. As a result, we have to accept contamination. This led me to conclude that application testing is the only practical way to determine the impact of contamination on mechanical performance but this essentially means we must choose an end-use before knowing if the material is even suitable.

Irrigation Pipes: Why I Ruled Them Out

Initially, I considered irrigation pipes as a potential end-use. However, I read that recycled HDPE cannot be used in any pipe requiring a hydrostatic design pressure rating. Over time, brittle, slow-growing cracks may develop under stresses (soil stress and hoop stress from water flowing) that are only a small fraction of the short-term yield stress. This is especially problematic because wetting agents reduce the energy needed to form new surfaces, aggravating crack formation. Water can then diffuse into the craze, weakening the structure and accelerating fracture growth.

Another issue is that the starting HDPE likely includes regrind of regrind, this severely affects oxidative stability and leads to failure at fusion welds due to poor thermal performance.

The starting plastic resin ultimately dictates the final application, as properties are fixed by the catalyst technology and manufacturing process used to produce the original resin. Recycled HDPE should only be used in applications tolerant of inevitable variations and contamination. HDPE from oil containers cannot be used in applications involving constant stress or exposure to fluids.

I'm now considering:

• Roof tiles – low-stress, outdoor, semi-structural
• Intermediate Bulk Containers (IBCs) – though I’m unsure if they’re suitable for this type of recycled resin

Would really appreciate any input on whether these are viable or if there are better-suited, low-stress applications for this type of recycled HDPE especially those that can tolerate contamination and inconsistent variable resin grades of feedstock.