Recycling aspects of natural fiber reinforced polypropylene composites

Abstract

Natural fibers nowadays play a very important role in the composites industry due to their several attractive characteristics especially energy saving potential which addresses the most vital industrial problem. Hence, this increasing demand of natural fiber thermoplastic composites in the market requires a technique to be established for handling the waste products out of these composites. The limitation of literature regarding the effect of recycling these for composites is the motivation for this work. This research focuses on recycling pre-consumer polypropylene compounds reinforced with sisal and hemp fibers up to five cycles. These two fibers are selected because they cover two main types of natural fibers: straight leaf fibers for sisal and branched bast fibers for hemp. Knowledge gained from these two types serves as a basis which could be implemented in future on other fibers from the same type. Natural fibers – being natural products – unlike synthetic fibers, offer unusual scatter of values, which in turn leads to the difficulty of assessing the effect of several reprocessing cycles on the compound. To be able to set a good foundation for this research, a thorough investigation on sisal and hemp fibers in both dry and compounded (with polypropylene) states is performed in the first section of this research followed by recycling of sisal and hemp compounds. A thorough investigation for determining the evolution of fiber shape and morphology is conducted on extracted fibers at selected cycles using dynamic image analysis in addition to FESEM and digital microscopy. Followed by rheological, flowability and thermal tests, the effect of number of cycles on the resulting properties could be analyzed based on the understanding of fibers morphology. Finally, mechanical tests are performed to assess the change in the compounds’ properties after recycling regarding the gained knowledge from the previous analysis. Understanding the difference in nature and consequently the difference in behavior between sisal and hemp fibers in the polypropylene melt gives an insight into comprehending the rheological and mechanical behavior of the compounds under study. Sisal with its straight and stiff fibers is less likely to agglomerate and build fiber clusters and is homogeneously distributed in the PP matrix. On the contrary, hemp fibers which are thin, branched and flexible tend to tangle up and build agglomerations of fiber clusters which hinder the homogenous distribution of the fibers in the matrix PP. This behavior affects the development of the viscosity along recycling cycles where sisal compound proves higher viscosity than hemp compound in the first cycles. Afterwards, due to recycling, hemp agglomerations disentangle and hemp fibers are shortened and distributed homogenously in the PP matrix causing increase in the overall compound viscosity. Flowability values show a three-stage behavior where compounds start at relatively low values (long sisal fibers / agglomerated hemp fibers). After the first and second cycles, a step in the flowability values could be observed for both hemp and sisal compounds due to shortening of sisal and hemp fibers which cause the de-agglomeration of fiber clusters. Amna Ramzy Recycling Aspects of Natural Fiber Polypropylene Composites IV Starting the third cycle after fibers have reached the shortest length and could no longer be affected by the screw action of the injection molding machine, flowability values reach a relatively stable plateau. Evolution of fiber shape as a function of recycling cycles proves to influence the mechanical properties in a remarkable course where mechanical properties improve for certain number of cycles before they start to degrade. This certain number of cycles depends on the measured property (E-modulus, strength, impact strength and elongation) Recycling of sisal and hemp compounds proves no critical degradation of thermal properties where hemp compound shows less sensitivity to recycling than sisal compound. On the other hand, melting points increased after recycling 3 cycles before they started to decrease again. In addition to influencing rheological, thermal and mechanical properties, this research shows that fiber shape also influences the degree of crystallinity of the compounds where compounds at late cycles prove higher degree of crystallinity than original compounds. This also reflects on thermomechanical properties where an improvement of the storage modulus is detected as a result to the restriction of segmental motion of the molecular chains. This statement is confirmed by the higher measured glass transition temperature of the recycled samples. Modulus of retention ratio is calculated where sisal compound proves a higher efficiency in reinforcing the composite after structural transitions. Based on the previous comprehension of the effect of recycling on the characteristics of sisal and hemp compounds enlightened by the special effect of fiber shape, an analytical model is developed to describe the flowability in terms of cycle number for hemp and sisal compounds. The model describes the three assumed stages using two power equations where conditions are set for applying the equations according to the fiber type. Similarly, non-linear functions of symmetrical sigmoidal type (4 PL) are assumed to correlate between fiber aspect ratio and selected mechanical properties such as E-modulus and tensile strength. Keywords: Recycling, Natural fiber, Polypropylene, Structure, Mechanical properties.