Cap recycling takes off
Circularity for polyolefin bottle caps
“Bottle made of 100 per cent rPET.” It’s an increasingly common message on plastic packaging, especially beverage bottles. But what about the cap? PET recycling has been an established business line for quite some time now. But demand for recycled polyolefin (PO), the material commonly used for making caps and closures, has only recently taken off, driven largely by a growing awareness of sustainability among consumers and companies alike and by tighter regulatory requirements such as mandatory recycled content laws.
Photo top: In the density separator, the lighter polyolefin flakes float on the water’s surface and can be drawn off for further processing.
“The market is seven times larger”
Because of their high strength and durability, PP and HDPE plastics are widely used in making containers for household cleaners and shampoo products as well as storage bins. The same properties also make them the material of choice for bottle caps and closures. Because, of course, the containers need to be stable and, most importantly, tightly sealed. When it came to recycling, though, polyolefins were long undervalued compared with PET. But they’re now gaining ground, for good reason. PET and the other polymers now each make up around 50 percent of the throughput of recycling plants.
At Krones Recycling, too, a turnkey provider of recycling systems, more than half of new lines ordered are designed to process polyolefin plastics. The growing demand for PO recycling comes as no surprise to Matthias Dawartz, Head of Project Execution and Process Engineering at Krones Recycling GmbH. As he puts it, “It was only a matter of time before recyclers recognised its potential. After all, the market for PO is roughly seven times the size of the PET market. Plus, most of the processes and systems used for recycling polyolefins like rigid PP and HDPE are similar to the ones we’re already using for PET.”
One process, multiple possibilities
Plastics recycling entails many steps, in which the materials are sorted, shredded and washed. The process involves two product flows, which are separated based on their density, in a sink-float tank. Matthias Dawartz explains: “The density separator uses water to separate the plastic flakes into two fractions: those that sink and those that float. Depending on the desired end product, one fraction is processed and the other discharged.” If it’s PET that is to be recycled, then the sinking fraction is used. The less dense polyolefin plastics float on the surface of the water and are carried off, packaged and sold to other recyclers. If the aim is to recycle only the polyolefins, then the opposite is true. Newer recycling concepts seek to unite the two – recycling PET and polyolefin – under one roof. That is, any polyolefin materials that are discharged from a PET recycling system are passed directly, as pre-sorted material, to a PO recycling line and processed further.
Food-grade quality
“In order to obtain high-quality recycled material, it’s essential that the flakes be cleaned thoroughly once they’ve been separated by density. This involves several washing stages, especially in the caustic washer that remove all contaminants and residuals,” says Dawartz. In the case of caps and closures, this step is important for another reason as well: “Most caps are made of HDPE, which is rigid but not brittle. It’s well suited for recycling, but it’s also more susceptible to picking up odours. Krones Recycling uses a hot washing process, which does a great job of reducing those odours.” Once the flakes have been washed, they are sorted once again, this time by colour and polymer type, by way of optical and sensor technologies. Additional deodorisation in a thermal-physical extraction process removes any remaining odours.
The final product is claimed to meet high quality standards and has been approved for food contact by the FDA in the United States.
