The use of Atomic Force Microscopy to determine intermolecular adhesive forces in wood based composite materials

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Effah, Bernard
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Stellenbosch University
South Africa has considerable invasive wood species that are causing damage worth millions of dollars to the national economy every year. These species cover over 8 % of surface area and needs to be cleared from public land. Finding potential use for this inexpensive, unexploited and abundantly available raw material regarded as waste and incorporating them into wood plastic composites (WPCs) as wood fillers can provide substantial value adding to a waste material, whilst producing products with good performance properties. The aim of the study was to comprehensively characterise the interfacial interaction forces and properties of LDPE - Wood composites with different compatibilisers and different wood species. The first part of the study discussed thoroughly issues of WPCs, as well as the Atomic Force Microscopy (AFM) and how the latter can be used to characterise the individual components of the composite to gain a better understanding of what affects good interfacial adhesion and how that could be maximised; most especially the use of chemical force microscopy (CFM) as the basic concept for the study. In the second part, the tip-surface interaction forces between two different wood species and AFM tips modified with three different compatibilisers were mapped to show the varying compatibility between the components of WPCs. Force maps and histograms were used to identify and show potential compatibiliser binding sites on the wood substrates. In the third part, chemically functionalized tips were used to quantify the adhesive force between compatibiliser coated AFM tips, the polymer and the different wood substrates and the result related to macroscopic properties of WPCs in an attempt to understand and explain the mechanical properties as well as to determine the feasibility to use alien invasive wood species for the production of WPCs with the most suitable compatibiliser. The final part of the study focused on the physical and mechanical properties of WPCs made from LDPE, six invasive wood species and three different compatibilisers. Thus, the moisture content, density, tensile modulus, tensile strength, elongation at break and impact strength were analysed and the results compared to commercial WPCs to ascertain the technical feasibility of the physical and mechanical properties of WPCs made from the invasive species. Stellenbosch University iii Quantitative analysis using one-way ANOVA with Tukey post hoc test and regression model were used to determine significant differences and relationships. The study proved that CFM is capable to characterise the surface structure, chemical functionalities of the different components and localise as well as also quantify functional groups and therefore give an indication of their adhesive forces on a molecular scale. The incorporation of PE-g-MA and dPE compatibilisers improved adhesion and thus enhanced the tensile properties of the composite. The measured properties compared very well to those of commercial WPCs. Finally, the invasive wood species that were studied can be incorporated into WPCs, by using the right compatibiliser. Furthermore, thermally degraded LDPE presents a new and inexpensive compatibilizer that can replace traditionally used expensive compatibilisers, in many cases with superior properties.
Effah, B. (2017). The use of Atomic Force Microscopy to determine intermolecular adhesive forces in wood based composite materials (Doctoral dissertation).