To evaluate compatibility between a substrate and a thermoplastic polymer, the established methodology is to estimate their surface composition in terms of surface energy components, utilizing the results of contact angle measurements of probe liquids onto substrate and polymer surfaces at room temperature. Using this methodology, polymer surfaces are studied in solid state, however, during spreading of polymers on a substrate, polymers are in molten state and at high temperature, having different surface energies and more complex polymer/substrate interactions due to polymer chain mobility. This paper presents a model study with practical relevance to predict polymer/substrate compatibility including contact angle measurements at high temperature directly performed between molten thermoplastics; polypropylene (PP), polyvinylidene fluoride (PVDF) and maleic anhydride-grafted polypropylene (MAPP), on smooth glass fibres and plates. The values of total surface energy of thermoplastics at high temperature (260 °C) are down to 57% of that measured at room temperature, which has a strong influence on the wetting prediction. Surface energies of both the polymer and the substrate were found not to be the only factor controlling the wetting behaviour of molten polymers and the level of adhesion with the substrate, but also some intrinsic characteristics of the polymer melt play a role. We also observed that the wetting behaviour of molten MAPP is affected by the maleic anhydride (MA) content, demonstrating dramatically different results to room temperature measurements, which is suspected to be due to the formation of covalent bonds of MA groups with the glass surface enhancing the interface strength beyond the shear strength of MAPP.@en

Direct contact angle measurements were performed between different molten thermoplastics, polypropylene (PP), polyvinylidene fluoride (PVDF), and maleic anhydride-grafted polypropylene (MAPP), on smooth glass fibres and smooth glass plates. The matrices were selected as model systems for the investigation of the fibre–matrix interphase, based on the difference in surface energies between PP and PVDF (physical interactions) and the effect of chemical bonding between PP and MAPP. In this way, physical and chemical adhesion were studied independently. The mechanical strength of the interfaces was then assessed by single fibre pull-out tests. The interfacial strength and the wettability of molten thermoplastics correspond well to the predictions based on the calculation of the theoretical work of adhesion between the matrices and the fibre surfaces for PP and PVDF; however, the wetting behaviour of molten MAPP is affected by the MA content if compared with the surface energy analysis made at room temperature. The hypothesis is that the wetting behaviour of molten MAPP is mainly related to chemical interaction of MA groups with the glass surface rather than a variation in surface energy.

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