Investigation of Transcrystalline Interphases in Polypropylene/Glass Fiber Composites Using Micromechanical Tests

Hanna Brodowsky,Edith Mäder

doi:10.3390/fib6010016

Hanna Brodowsky, Edith Mäder

Open Access

https://doi.org/10.3390/fib6010016

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Abstract

In composites, a strong interphase between the components is essential for mechanical properties. By using a suitable sizing (i.e., surface modification) of the fiber, the interphase may be varied, e.g., by suppressing or promoting heterogeneous nucleation of a thermoplastic matrix. In the latter case, three-dimensional transcrystallized interphases with properties differing from those of the bulk matrix are formed. Polypropylene-glass fiber composites are prepared as single-fiber model composites with (a) sizings either inducing or suppressing a transcrystalline interphase, (b) different amounts of modifier maleic acid anhydride grafted polypropylene, and (c) different molecular weights of the matrix polymer. These are studied in quasi-static or cyclic load tests. Static tests permit insights in the interfacial characteristics such as critical interface energy release rate, adhesion strength and frictional stress. Cyclic tests on these model composites can be used to study the nature of dissipative processes and the damage behavior. Atomic Force Microscopy (AFM) investigations of the fiber fracture surfaces provide supplementary information. The transcrystalline layer can indeed improve the mechanical parameters (a 70–100% increase of strength and a 25 or 125% increase in toughness, depending on the molecular weight (MW) of the matrix polymer at low modifier concentration). However, the effect is partially neutralized by an opposing effect: high nucleation in the bulk in samples with commonly used concentrations of modifier.

Highlights

The nucleation may occur by heterogeneous nucleation at the surface and can be significantly higher than in the bulk
A transcrystalline interphase has in the past been observed to improve ]mechanical properties, but contradictory results have been observed
It may be reduced due to the effect of additives such as MaPP, which is added as a coupling agent between fiber and PP matrix

Summary

Introduction

The nucleation may occur by heterogeneous nucleation at the surface and can be significantly higher than in the bulk In this case, the impedation by neighboring crystallites occurs almost immediately in the lateral directions, leaving only the outward direction for crystal growth. The thickness of the TC layer is determined by the different rates of the two nucleation processes in the bulk and at the surface, in relation to the growth rate [5,8,9]. These three quantities depend on a number

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