Better mechanical properties with microfibrils
Fibriltex: Microfibrillar reinforced textiles & composites
Microfibrillar reinforcements offer interesting possibilities to improve the properties of yarns and composites. Microfibrillar reinforcements have already been studied in injection moulding but their application in textile materials is quite new.
Microfibrils are formed by processing a blend of two immiscible polymers such as polypropylene (PP) and polyethylene therephthalate (PET). When this blend is heated above the melting temperature of both polymers, one polymer phase will form droplets in the matrix of the other one. When this blend is processed during melt spinning, these droplets transform into fibrils, as schematically shown in the figure below.
Transformation of droplets into fibrils of immiscible polymer blend during extrusion
Direct blends of PP and PET have been processed into monofilaments and multifilaments. Monofilament extrusion of PP/PET blends was very smooth. Blends of PP with up to 30% PET showed to be easily processable on a standard monofilament extrusion line. Except for an elevated extrusion temperature (270 °C) no further adjustments to the processing conditions compared to neat PP were needed. The maximum draw ratio of the filaments, indicating their drawability, was similar to the maximum draw ratio of neat PP. Moreover, a modest increase in tenacity compared to pure PP was observed at higher draw ratios for blends with 10 and 20% PET. Furthermore, addition of 30% PET did not result in a decrease of mechanical properties (see figure below).
Tenacity as a function of draw ratio for microfibrillar reinforced PP filaments
Microscopy demonstrates the presence of PET microfibrils in the PP matrix. The first picture clearly illustrates the formation of fibrils inside the monofilaments - spheres are elongated into fibrils and merged together to form longer fibrils - whereas the second picture shows the resulting PET fibrils in the PP filament.
Formation of PET fibrils in PP matrix
PET fibrils in PP matrix
The thermal stability of the monofilaments was determined via shrinkage and creep tests. Filaments with 30% PET show a significantly decreased shrinkage and creep (decrease with minimum 50%) and thus an increased thermal stability compared to neat PP monofilaments.
Multifilament extrusion of PP/PET blends was more critical. Care has to be taken to a proper selection of polymer types (principally Melt Flow Index of PP), spinplate geometry (L/D ratio) and compatibilisation. The addition of a compatibiliser improving the interaction between PP and PET is essential to process the blend in multifilament extrusion. Once all parameters are well chosen, improved mechanical properties (mainly the modulus) are obtained for PP with 10% PET compared to neat PP. Addition of 20 and 30% PET is possible, but in that case the speeds have to be drastically lowered.
The mono- and multifilaments can be applied as such for applications in technical textiles. In addition, it is possible to process the microfibrillar reinforced filaments into composites, as schematically represented in the figure below.
Both unidirectional composites, via filament winding, and bidirectional composites, out of woven fabrics, have been produced. The resulting mechanical properties were significantly better compared to neat PP. Bidirectional composites from PP filaments with 30% PET reached a flexural modulus of 3.9 GPa, which is in the same range as CURV (self-reinforced PP having 3.5 GPa flexural modulus).
The project Fibriltex has been finished, but the work on polymer blends is continued in the CORNET project Blends4Innovation.
This work was supported by the CORNET program, IWT (grant No. 120272) and AIF in the framework of the research project Fibriltex.