Lack of trust deters use of recycled materials in high-end applications

While the use of recyclates in packaging is becoming ever more commonplace, when it comes to higher value-added applications brand owners and companies are more hesitant.

Batch variations have, in the past, made it difficult to obtain consistency regarding such aspects as strength, odour and appearance. Material properties not only differ from recyclate batch to recyclate batch, but also from those of newly made plastic.

As a result: “There is still a lack of trust in recycled materials,” said Dominik Spancken, a scientist at Fraunhofer Institute for Structural Durability and System Reliability LBF, in Darmstadt.

Seeking to demonstrate the potential of recyclates for use in higher-end applications and to generate more trust in these materials,  researchers at the institute collaborated with Bosch GmbH and Bosch-Siemens-Hausgeräte GmbH (BSH) to investigate what recycled plastic could actually achieve, said Spancken.

Bosch first optimised recyclate derived from car starter battery housings, using additives to enhance the visual properties and increase the material’s strength. The research team then analysed whether the material was suitable for use in manufacturing dishwasher case bottoms, a component, weighing approximately two kilograms, forming the base structure of a dishwasher. It holds the side walls and housing ancillary components, such as the pump, status sensors and salt reservoir.

Based on a production figure of three million dishwashers per year, using recyclate instead of virgin plastic would save around 2500 tons of crude oil annually – equivalent to the amount of oil transported on a 110-meter-long barge. However, said the researchers, it needs to be processed before it can achieve properties that match those of virgin plastic.

Testing

“At Fraunhofer LBF, we initially investigated how the optimised recyclate behaved under mechanical stresses,” explained Spancken. In an automated process, a test specimen was subjected to a set force around 100,000 times, reflecting the typical use and load range for large white goods and establishing a limit for the amount of stress the materials could withstand without becoming damaged. In accordance with these specifications, the experts at Fraunhofer LBF, Bosch and BSH established parameters for the design.

The 100,000 cycles were chosen with a specific component in mind: a finger-wide pin on the dishwasher case bottom. This pin undergoes mechanical stress every time the dishwasher door is opened and closed, and is the area subject to the greatest load on the dishwasher case bottom.

Assuming a dishwasher door is opened 15 times a day and that the appliance has a lifespan of around 18 years, the door will be opened around 100,000 times throughout the dishwasher’s entire service life. Can a pin made from recyclate hold up?

“While the recyclate is less strong – 15 percent weaker – it has similar stiffness properties to that of virgin plastic. More importantly, however, both materials behave exactly the same way in terms of plastic deformability. In summary, it is fair to say that the recyclate can handle the same stresses as virgin material,” Spancken asserted.

Next to testing the material alone, the researchers also conducted tests on the component itself. “We calculated material characteristics from the material tests. Using these characteristic values, we then developed an assessment methodology to check whether the pin is durable,” he said. This involved clamping pins made from virgin material in a test bench and applying loads simulating the use of the dishwasher, and specifically those to which the pin is subject when opening and closing the machine door. Given that underfloor heating and radiant heat from the dishwasher could cause pin temperatures to rise, Spancken and the team adjusted its temperature to 50 degrees to reflect this.

Then, using the experimental data, the researchers created a validated calculation and design methodology – and susequently “fed” this methodology with the recyclate’s material characteristics.

“We were able to extrapolate the behavior of a pin made from recyclate using the tests on the virgin material and the calculation methodology,” says Spancken. And with encouraging results: The resilience values recorded for the pins made from recyclate differ only very slightly from those made from virgin material.

What this feasibility study shows is that, yes, a dishwasher case bottom can be made from recyclate and that switching from virgin plastic to recyclate would make a significant contribution to the creation of more sustainable appliances. And that it is time for brand owners and manufacturers to start placing more faith in the use of recycled materials.