Influence of process parameters on the formation of inhalable fiber dust during shredding for mechanical recycling of fiber-reinforced organo sheets

Shredding is a major process step in mechanical recycling, as the plastic scrap needs to be cut into small pieces for further processing. It will be melted and finally injection-molded to form new plastic parts. Also for fiber-reinforced thermoplastics, the mechanical recycling process is an ecofriendly process because all resources are reused. One challenge during the shredding of fiber-reinforced plastics is the fiber dust formation. The fiber dust disturbs the following production processes, and the fiber fragments can be alveolar and thus potentially toxic. Therefore, it is a major aim of this research to optimize the cutting process by investigating the effect of different shredding parameters on the alveolar fiber dust formation. Three different continuous fiber-reinforced thermoplastic organo sheets (PP with glass fibers, PA6 with glass fibers, PA66 with carbon fibers) are shredded while the parameters of throughput, rotational speed, screen size and feedstock size are varied. The results show no clear effect of the throughput either on the amount of fine particles nor on the number of short fibers after removal of the plastic matrix for all materials. Reducing the screen size in the cutting mill leads to a large increase in the amount of fine particles and on the number of short fiber fragments. Additionally, the rotational speed has a large impact on the amount of fine particles. With increasing rotational speed, the number of fine particles rises, whereas there is no major effect on the fiber length distribution. The investigations relating the rotational speed are valid for all materials, throughput, screen sizes and feedstock sizes. Decreasing the feedstock size leads to an increasing amount of fine particles with no change in fiber length distribution for the glass fiber-reinforced organo sheets. For the carbon fiber-reinforced materials, it does not lead to any change in the amount of fiber dust but to an increase in shorter fibers. Thus this parameter seems to be material dependent. Images taken by a scanning electron microscope show alveolar fiber dust fragments in the ground material of all samples.