Extremely tough and resistant to chemicals

Feet are involved in around a fifth of all occupational accidents. Therefore, wearing the right footwear plays a key role in cutting the risk of foot-related injuries at work. Industrial shoes are available in three categories: occupational, protective and safety footwear. Protective toe caps are only mandatory in the latter two types of footwear. Safety shoes must meet the tough requirements stipulated in standard DIN EN 345. The standard’s specifications for toe caps are particularly strict. For this purpose, Bayer MaterialScience AG has developed Makroblend® DP 6015, a very strong, impact-modified PC+PBT blend. Among other tests, it also passes those specified in standard DIN EN ISO 12568 for toe caps. “The blend’s particular strengths are its high toughness and impact resistance, which even withstand exposure to chemicals,” explains Walter Köppchen, expert for PC+PBT blends in the Polycarbonates Business Unit of Bayer MaterialScience. The new material’s performance profile also impressed Uvex Arbeitsschutz GmbH in Fürth, Germany. The company has certified the blend for toe caps used in safety shoes of the uvex xenova® product line.

One of the tests prescribed by standard DIN EN ISO 12568 is a drop test, whereby a test object resembling a hatchet is dropped on the toe caps with a force of 200 joules at temperatures of -20 °C and 60 °C. The test simulates the impact on the toe area caused by heavy falling objects and measures the extent to which the toe caps are deformed. “The toughness and stiffness of our blend ensures that the deformation of the caps is negligible, thus protecting the toes against crushing,” explains Köppchen. The excellent mechanical property profile must also remain intact after exposure to chemicals. Therefore, the standard stipulates that the drop test must also be performed on caps that have been exposed to fuel, diluted sodium hydroxide solution and diluted sulfuric acid for 24 hours. “Despite this chemical load, caps made of Makroblend® pass the drop test with excellent results. This is what sets our PC+PBT blend apart from many other thermoplastics,” reveals Köppchen. The standard also includes a pressure resistance check for the toe caps. In this test, the caps are subjected to a static load of 1.5 metric tons, which they must be able to withstand. Makroblend® DP 6015 meets this demand too without any problems.

Plastic protective toe caps offer a number of advantages over steel caps. For instance, they are much lighter, they don’t rust, and they provide better insulation against cold and heat. They are not magnetic, which means that safety shoes fitted with plastic caps can be employed in areas where metal detectors are in use, such as entrances to airports, or on sites with electronic controls. What’s more, plastic caps cushion external impact and pressure thanks to their excellent resilience. In contrast, impact stress can deform steel toe caps, thus trapping injured toes. The design flexibility of plastics makes it easier to implement high-quality orthopedic shoe designs using the corresponding caps. This feature, coupled with the caps’ elasticity and low weight, offers wearers maximum comfort.

Bayer MaterialScience provided Uvex with extensive support in the development of shoe caps made of Makroblend® DP 6015. For example, the drop test to DIN EN ISO 12568 was reproduced using a very complex simulation model. “For the simulation we used special material parameters that reproduce the material behavior at high deforming speeds. Bayer MaterialScience can determine these crash values on a high-speed tensile testing machine,” explains Florian Dorin, expert for finite element simulations in the Polycarbonates Business Unit of Bayer MaterialScience. These parameters were then used to calculate which loads were acting on every area of a cap at any given moment in the course of the impact. “In this way, we were able to calculate the optimum wall thickness distribution and geometry for the caps, which helped keep the material as thin as possible, enabling us to make the shoe as lightweight and short as possible,” says Dorin.
In addition, the experts at Bayer MaterialScience also simulated the caps’ behavior when they are subjected to side impacts, reproducing the action of a forklift rolling over a foot, for example.