Predicting the adhesion strength of thermoplastic/glass interfaces from wetting measurements

https://doi.org/10.1016/j.colsurfa.2018.08.052Get rights and content

Abstract

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.

Introduction

How an interface is formed at high temperature between molten thermoplastics and hot substrate surfaces are yet to be studied in detail. Although the mechanical behaviour of the interface depends on the properties of both the substrate and the polymer, e.g. thermal expansion coefficients, strength, degree of crystallization, to name a few [1,2], the final adhesion strength of the substrate/polymer interface is highly dependent on their physical and chemical interactions. As the load distribution efficiency at the interface is determined by the degree of adhesion between the components [3,4], the polymer/substrate interface becomes an important design consideration in many structures that use adhesive bonding, such as fibre reinforced composites. A strong substrate/polymer adhesion is obtained through interfacial interactions, including mechanical interlocking, chemical bonding, such as covalent bonds, and physical mechanisms of adhesion, i.e. Van der Waals interaction, dipole interactions or hydrogen bonds [[5], [6], [7]].

If the molten polymer is not able to fill irregularities at the substrate surface, the area of contact between the substrate and the polymer melt will be reduced, producing in turn, a reduction in adhesion. On the other hand, if the polymer melt can fully wet the rough surface, mechanical interlocking and increased contact area will lead to increased adhesion [8,9].

Chemical adhesion also depends on the degree of wetting that provides intimate contact between both phases. Covalent bonds (further referred to as chemical bonds) can be formed across the interface when atoms at the fibre surface share electrons with polymer atoms, producing bonds with very high strength. Regularly, chemical modification of both the substrate surface and the polymer are used to promote covalent bonding at the interface by chemical reactions [[10], [11], [12], [13]]. Techniques as X-ray Photoelectron Spectroscopy (XPS), Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS) and Fourier transform infrared spectroscopy-attenuated total reflectance (FTIR-ATR) are typically used to identify chemical groups at the substrate surface to evaluate the effects of chemical reaction [10,14,15].

The substrate surface is also able to interact with the matrix without undergoing covalent bonding. These interactions arise from physical forces and predominantly control the wettability and physical adhesion of the liquid polymer on the substrate surface [16]. The study of physical interactions is crucial for obtaining a better interface, since the other mechanisms of adhesion depend on a good physical interaction. Chemical bonding forces occur over very small distances of typically 0.1 to 0.2 nm so that the chemical groups present at the substrate surface and the reactive groups of the polymer need to be brought very close together. Therefore, the polymer must spread on the substrate, penetrating the surface irregularities [5,6,8,9] for the intimacy of contact needed for chemical bonding.

The common procedure to evaluate these physical interactions is to estimate the substrate and polymer composition in terms of surface energy components, utilizing the results of contact angle measurements of probe liquids on both the substrate and the polymer in solid state at room temperature [4,7,14,17]. The direct imaging of drop profiles and the Wilhelmy balance method are currently the two principal methods used to measure contact angles [18,19]. However, the wetting analysis with probe liquids characterizes polymer and substrate surfaces in solid state, whereas during spreading, polymers are in molten state and substrates at high temperature. Thus, both materials potentially have different surface energies than at room temperature, and more complex polymer/substrate interactions may occur. After spreading and cooling down, the surface properties, in solid state, should eventually control the interfacial mechanical properties, while surface energies of both the polymer and the substrate at high temperature should control the wetting behaviour of molten polymers. This is especially relevant for cases where physical adhesion is dominant, which is usually the case for thermoplastic polymers.

In this study, contact angles of molten polyvinylidene fluoride (PVDF), polypropylene (PP) and molten maleic anhydride-grafted polypropylene (MAPP), with different maleic anhydride contents, on smooth glass fibres and smooth glass plates, were measured as a direct indication of the level of adhesion that could potentially be obtained. These values were then contrasted with the conventional analysis based on i) measuring contact angles with different reference probe liquids using the Wilhelmy technique on solid polymer films and substrates, and ii) by applying the acid-base theory for calculating the surface energy components. The thermoplastic polymers were selected as model systems for the investigation of the polymer/glass interphase, based on the difference of surface energies between PP and PVDF and the effect of chemical bonding between PP and MAPP.

In this way, physical and chemical adhesion were studied independently to systematically investigate the influence of both adhesion mechanisms on the adhesion strength of a polymer/glass interface. The total surface energies of the molten thermoplastics were determined by the pendant drop method and the interface composition was analysed by FTIR spectroscopy. Finally, to correlate the real strength of the interfaces with the theoretical work of adhesion, the polymer/glass interphase bond strength was characterised by performing single fibre pull-out tests.

Section snippets

Methodology

In this study, soda-lime silicate glass slides and optical glass fibres with similar surface composition were selected as substrates. Slides were used to study the wetting process of molten polymer drops on a glass flat surface, while fibres were used to estimate the practical adhesion using pull-out tests. By assuring that both substrates have a similar surface chemical composition, and by a consistent cleaning of both substrate surfaces, it could be guaranteed that the same surface chemistry

Surface properties of substrates (contact angle and roughness)

Table 1 shows the surface composition determined using XPS for cleaned glass slides and glass fibres after 2 days storage in ultra-pure water. Both slides and fibres have a comparable surface composition, with similar C, O, and Si content. The O/Si ratios for the slide and the fibre were 1.8 and 1.5 respectively. For a pure glass sample, the O/Si ratio should be 2 (2 oxygen for 1 silicon in SiO2), and if the surface were only constituted of SiOH bonds, the O/Si ratio should be 1. This latter

Conclusions

When chemical bonding is excluded and only physical interactions are evaluated (PP and PVDF systems), a good correlation between practical adhesion (critical interfacial shear strength, τd) and the theoretical work of adhesion, Wa, at room temperature is found. However, the analysis of the spreading of molten polymers on glass substrates at high temperatures as a direct indication of the level of adhesion at the solid polymer/glass interfaces marginally corresponded with practical adhesion

Acknowledgements

We acknowledge the support from Emmanuel Gosselin, Université de Mons, and Alex Bian, University of Applied Sciences and Arts Northwestern Switzerland, for their work on FTIR-ATR analysis and melt-blending of PP/MAPP respectively. Swiss partners were supported by the Swiss Competence Center for Energy Research (SCCER), unit Efficient Technologies and Systems for Mobility, funded by the Commission for Technology and Innovation (CTI), project grant 15091.1 PFIW-IW. Our thanks to Pierre Eloy and

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