The tested biocomposites clearly show better or comparable characteristic values than the ‘common’ natural fibre reinforced PP. Besides the typical increase in stiffness, a significant increase in strength (>50%) and a noticeable improvement in notch impact strength (>250%) can be observed. This striking optimization of materials and processes is an innovative contribution to composite processing; up to now, properties such as strength and notch im- pact strength have hardly been affected or only slightly improved. Especially man-made cellulose has proved to be a favourable rein- forcing fibre due to an advantageous geometry as well as its overall mechanical performance.
The SEM analysis shows strongly differing morphologies for each composite. Poor adhesion was observed for PHBV/Ecoflex composites, as the blend was partially degraded during processing and the interphase was destroyed by the shrinking matrix. For PLA, a moderately strong interphase was observed. Due to the use of maleic-acid anhydride grafted polypropylene, an improved inter- phasial adhesion in PP composites was noted. An analysis of the fi- bre-size distribution of PLA composites after processing showed a broad spectrum of abaca fibre diameter. In addition, the critical fi- bre length of abaca is much higher and the aspect ratio lower than those of jute and man-made cellulose. Amongst others, this is the reason for deteriorated mechanical parameters of abaca compos- ites. Man-made cellulose is characterised by the highest aspect ratio.
Semi-crystalline PHBV is widely comparable to PP composites and can sometimes substitute it in certain applications. But the main hindrance to overcome is still its processability. PLA compos- ites show much higher mechanical characteristic values than PP; however, it has to be kept in mind that the material loses the abil- ity to transfer stress as soon as the glass transition temperature is exceeded. Therefore, the combination of thermally untreated PLLA and NF is applicable as engineering material only at temperatures lower than +50 °C. Further possibilities are to use thermal temper- ing to increase the crystallinity of PLA or the synthesis of stereo- complexes made of PLDA- and PLLA, which enables heat distortion temperatures above 100 °C