Colloids and Surfaces A: Physicochemical and Engineering Aspects
Predicting the adhesion strength of thermoplastic/glass interfaces from wetting measurements
Graphical abstract
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, ) 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
References (47)
A variational mechanics analysis of the stresses around breaks in embedded fibers
Mech. Mater.
(1992)- et al.
Characterisation of interphase nanoscale property variations in glass fibre reinforced polypropylene and epoxy resin composites
Compos. Part A Appl. Sci. Manuf.
(2002) - et al.
Mechanical behaviour and practical adhesion at a bamboo composite interface: Physical adhesion and mechanical interlocking
Compos. Sci. Technol.
(2015) - et al.
Evaluation of mechanical interlock effect on adhesion strength of polymer–metal interfaces using micro-patterned surface topography
Int. J. Adhes. Adhes.
(2010) - et al.
Effect of sizing on carbon fiber surface properties and fibers/epoxy interfacial adhesion
Appl. Surf. Sci.
(2011) - et al.
Mechanical properties of flax fibre/polypropylene composites. Influence of fibre/matrix modification and glass fibre hybridization
Compos. Part A Appl. Sci. Manuf.
(2005) - et al.
Surface molecular characterisation of different epoxy resin composites subjected to UV accelerated degradation using XPS and ToF-SIMS
Polym. Degrad. Stab.
(2009) - et al.
Surface OH group governing wettability of commercial glasses
J. Non. Solids
(1999) - et al.
Wetting behaviour and surface properties of technical bamboo fibres
Colloids Surf. A Physicochem. Eng. Asp.
(2011) - et al.
Control of surface energy of glass by surface reactions: contact angle and stability
J. Colloid Interface Sci.
(1984)
Wetting Properties and Stability of Silane-Treated Glass Exposed to Water, Air, and Oil
J. Colloid Interface Sci.
Contamination of silica surfaces: impact on water–CO2–quartz and glass contact angle measurements
Int. J. Greenh. Gas Control.
Wetting behavior of flax fibers as reinforcement for polypropylene
J. Colloid Interface Sci.
The effect of types of maleic anhydride-grafted polypropylene (MAPP) on the interfacial adhesion properties of bio-flour-filled polypropylene composites
Compos. Part A Appl. Sci. Manuf.
Characterization of fiber/matrix interface strength: applicability of different tests, approaches and parameters
Compos. Sci. Technol.
A review of interphase formation and design in fibre-reinforced composites
J. Adhes. Sci. Technol.
A criterion for optimum adhesion applied to fibre reinforced composites
J. Mater. Sci.
The science of adhesion
J. Mater. Sci.
Improved polymer–glass adhesion through micro-mechanical interlocking
J. Micromech. Microeng.
Polymer Permeability
Polypropylene/Cellulosic Fiber Composites Chemical Treatment of the Cellulose Assuming Compatibilization Between the Two Materials
J. Macromol. Sci. Part A- Pure Appl. Chem.
The nature of adhesion in composites of modified cellulose fibers and polypropylene
J. Appl. Polym. Sci.
Effect of physical adhesion on mechanical behaviour of bamboo fibre reinforced thermoplastic composites
Colloids Surf. A Physicochem. Eng. Asp.
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