WiPull | Normalization of the single fiber pull-out test to measure fiber-matrix adhesion

The »WiPull« project, part of the WIPANO funding initiative, aims to develop a basis for standardizing a measurement method for quantifying fiber-matrix adhesion. For this purpose, a single filament is embedded in a well-defined manner in a liquid polymer droplet with the aid of an appropriate preparation system. After the drop has hardened or cooled, the filament is pulled out of the polymer drop using a suitable testing device, and time, force, and displacement are recorded. This procedure is known as the single fiber pull-out (SFPO) test.

For this purpose, the commercially available FIMATEST system of the project partner Textechno H. Stein GmbH & Co. KG is used. This consists of the embedding station FIMABOND and the testing device FAVIMAT+ (see figure).

The following figure shows the pull-out procedure: a single fiber is embedded in a polymer matrix. In the next step, the fiber can be pulled out of it and the adhesion can be determined. Thermoplastics (e.g. PP, PA, PEEK, etc.), as well as duromers (e.g. epoxy resins) and even mineral matrices (e.g. concrete), can be investigated.

One objective of the »WiPull« project, which is being worked on together with the project partners FIBRE (Fiber Institute Bremen) and Textechno, is identifying manipulated variables and influencing parameters on the sample preparation and the actual pull-out test. In addition, the most important commercial fibers and polymer systems are to be tested with the pull-out test at the project partners and the results subsequently compared with each other in interlaboratory tests. The analysis results are to be used to enable standardized testing of different materials, identify particularly challenging materials, and provide information on correct testing. From experience gained, procedures can be determined how the respective fiber-matrix combinations can be reproducibly tested for adhesion. Furthermore, the differences or the comparability of different materials will be shown. The procedure will be analyzed and evaluated for the possible influencing variables and sensitivity coefficients and repeatability and reproducibility. After the successful evaluation of the materials and determination of the test procedure, the transfer into a DIN SPEC for the description of the measurement procedure follows.

The method is frequently used at Fraunhofer IGCV for questions in the field of recycling carbon fiber plastics. It is of great use because the SFPO can also determine the adhesion of recovered and shortened recycled carbon fibers to the matrix polymers used.

An example of the use of the SFPO:

In the »CaRinA« project of Fraunhofer IGCV, the fiber-matrix adhesion of a modified polypropylene matrix to recycled carbon fibers was determined. It is clearly recognizable that the use of maleic anhydride as an adhesion promoter greatly improves the adhesion of the polymer matrix to the carbon fibers studied.