Biopregs can in this project prove to be a better, cheaper and greener material than the glass and synthetic composites now used in the sports industry.
Three major topics in developing a high performance Biopreg material for the soleplate are the improvement of the material properties, the forming ability of the material in a pressing process and the continuous production of the soleplate. In Work Package 2 our focus is on the improvement of the material properties.
The main problems in improving the material properties and in the continuous production of Biopregs are: difficult impregnation and bad adhesion of the thermoplastics with the natural fibres, the restricted and variable length of the natural fibres, the difficult orientation without angles of the natural fibres in the yarns and the high air content or porosity after consolidation in the composites.
The problems described above were restricting the mechanical properties to achieve the ultimate potential of the Biopregs. The aim in this project is to get more insight in the improvement methods which can be economically solved with existing yarns, weaving technologies and with proved consolidation methods.
The choice of thermoplastic polymer (the matrix) first of all depends on how well the polymers can be impregnated in the natural fibres. Of the polymers, Acrylics have the best options because of their possibility that their viscosity can be adjusted to its purpose, their good mechanical and physical properties in combination with acceptable material costs. PE and PP are the cheapest thermoplastics but they have the lowest mechanical and physical properties. Besides that they need special agents to improve the adhesion to natural fibres.
Mats and compounds have weak mechanical properties to produce a sufficiently lightweight soleplate. In datasheets their properties are half of what we needed to get an equal stiff and strong soleplate like the glass fibre fabric reinforced thermoplastics.
Uni-directionals offer the best mechanical properties but miss a stable structure when they are pressed over a 3 dimensional mould. So a fabric is needed to keep the yarns and fibres from floating and spreading during hot pressing. In WP 3 and WP 4 we can experiment with different kinds of combination of fabric with UD’s gives the best mechanical results for the sole plate.
Material properties of Biopregs together with their material efficiency parameters are put in a calculation tool to estimate the potential performances of Biopreg and compare them with other composites.
Figure 1: E-modulus and tensile strength of Biopreg 50/50 with competitive materials
The strength of the Biopreg is amongst the lowest. The stiffness properties are average.
Figure 2: The Density of Biopreg 50/50 with competitive materials
The densities of the Biopreg are the lowest this will make the difference with glass fibre composites in constructing with Biopregs.
When the material indexes of the bending plates are calculated and compared to the indexes of the other materials, the relative differences compared to the Biopreg composites are as follows:
Figure 3: Comparison of properties Biopreg 50/50 with competitive materials
With data from the scientific studies we made a tool to compare different resources for composite on the most important environmental impacts.
The result of the impact calculations of the separate fibres and polymers are given below:
Figure 4: Eco-indicator 95 and energy use values of Biopregs versus alternative materials
In earlier scientific research is shown that the mechanical properties of the natural fibre reinforced thermoplastic composite are much depended on the properties of the yarn. Therefore we ordered better quality yarns and tested them at Zwartz and compared them with standard natural yarns.
The best yarn that is found has a tensile strength of almost 400 MPa that is twice the strength of the existing yarns that are used in earlier Biopregs. So it is expected that the properties can be improved with almost a factor 2.
A composite of Acrylics with the standard natural fabric with the standard natural yarn was chosen to be the material for a zero measurement. To make such a sample the fabric must be impregnated so far that the viscosity of the polymer is low enough to let the air escape out as much as possible.
Figure 5: Test setting to make Acrylics
Figure 6: Hot press for consolidating the fibres into the polymer matrix
This can then be measured by weighing the density of the fibres and the polymer upfront and after the consolidation by weighing the density of the composite. For mechanical tests the sheet has been cut to standardized test pieces (see Figure 7).
Figure 7: Material cut to test pieces.
Earlier Natural fabric/Acrylics UD tests show that the objectives of the material performances can be reached. The test results with the Natural fabric at HSMD are now below those objectives but we discovered that we had poor quality natural yarns and the preparation of the sample was not good enough. In following tests we are planning to search for better quality yarns of natural and make better samples of sheets that will show less elongation at break.
The tests from HSMD showed us that the inter laminar shear strength (ILSS) and the bending stiffness and strength are reasonable but the tensile E modulus and strength were dissatisfactory. The reason for the poor results will be investigated in the following test. According to the literature we assume the following may affect the results:
The use of low twisted yarn, fabrics with little crosses, and the pre tensing of the fabric before consolidating could improve and mixing the sheets with uni-directionals could improve the mechanical properties.
To get an indication that the natural /acrylic Biopreg is moldable enough to get the form that is required we conducted a test with a rubber pressing process. The shape was determined out of soleplates of Healus Ltd that were used in earlier experiments. The natural /acrylic laminate that is delivered by KIEM and DeWeerd is been processed resulted in a very stiff shell, very different from earlier samples with PP-Biopreg.
The research described in this report has given the SME’s the theoretical basis of making a high performance Biopreg that can be having 150% better mechanical performances than the standard fabric Biopregs. The cost price will be higher because of the use of finer yarns. None the less we think that it will still be a competitive composite when comparing it with Glass or even carbon fabric reinforced thermoplastics. This we will be further investigated in the next WP3 and WP4.
The first experiments done with a standard natural fabric delivered by Zwartz has not yet delivered the mechanical performance we expected but are mostly explainable because of the low quality of the natural yarn and fabric. Much improvement is expected when we use new finer yarns of better quality and a better composition of the layers. This composition could be a combination of a fine fabric with cheaper UD sheets. These UD sheets will be further investigated in the next WP3 and WP4.
Another breakthrough is found in a process which was never used before: the use of acrylics in the natural fabric. This matrix appeared to be giving a very good quality sheet that is good thermo formable. Because of the very good impregnation properties and the very good mechanical properties of acrylics itself it could be a very promising matrix material. Still further test have to be conducted to find the right formulas of additives for the most optimal process.
We already have produced a Biopreg soleplate with the acrylics that is, as expected, suitable for making a prototype of the HEELLESS running shoe. The soleplate is thin, light and stiff enough to replace the Twintex (glass fabric reinforced thermoplastic). Another big advantage is unlike the Twintex the Biopreg has no problems with gluing because of the good adhesive properties of the natural fibres. So no expensive and complex glues are necessary to connect the soleplate to the upper and lower shoe. Also we expected that the better adhesive properties to lead a better performing soleplate. With the new developed knowledge we now think that we can reach the objectives of the material development and we hope to prove that in the coming WP’s.
Research from KIEM, Netherlands