The use of atomic force microscopy to determine intermolecular adhesive forces in wood based composite materials

Thesis (PhD)--Stellenbosch University, 2017.

ENGLISH ABSTRACT: 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. 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.

AFRIKAANSE OPSOMMING: Suid-Afrika het 'n aansienlike hoeveelheid indringende hout spesies wat jaarliks skade ter waarde van miljoene dollers aan die nasionale ekonomie veroorsaak. Hierdie spesies dek meer as 8% van oppervlakte en moet uitgerooi word vanaf openbare grond. Potensiële gebruike vir hierdie goedkoop, onontginde en oorvloed beskikbare grondstowwe wat as afval beskou word en inkorporering daarvan in hout plastiek samestellings (WPCs) as hout vullers kan aansienlike waarde toevoeg tot 'n afvalmateriaal, tesame met die vervaardiging van produkte met goeie eienskappe. Die doel van die studie was om die tussenvlak interaksie kragte en eienskappe van LDPE-hout samestellings met verskillende versoeningsmateriale en verskillende hout spesies volledig te karakteriseer. Die eerste deel van die studie bespreek deeglik die kwessies van WPCs, asook die atoomkrag mikroskopie (AFM) en hoe laasgenoemde gebruik kan word om die individuele komponente van die samestelling te karakteriseer en 'n beter begrip te kry van wat goeie tussenvlak adhesie beïnvloed en hoe hierdie gemaksimeer kan word; veral die gebruik van chemiese krag mikroskopie (CFM) as die basiese konsep vir die studie. In die tweede deel is die punt-oppervlak interaksie kragte tussen twee verskillende hout spesies en AFM punte, gemodifiseer met drie verskillende versoeningsmateriale gekarteer om die verskillende versoenbaarheid tussen die komponente van WPCs te wys. Kragkaarte en histogramme was gebruik om potensiële versoeningsmateriaal bindingsplekke op die hout substrate te identifiseer en te wys. In die derde deel was chemies gefunksionaliseerde punte gebruik om die adhesiekrag tussen versoeningsmateriaal bedekte AFM punte, die polimeer en die verskillende hout substrate te kwantifiseer en die resultaat gekorrileer met makroskopiese eienskappe van WPCs in 'n poging om die meganiese eienskappe te verstaan en verduidelik asook om die haalbaarheid van die gebruik van uitheemse hout spesies te bepaal vir die produksie van WPCs met die mees geskikte versoeningsmateriaal. Die finale deel van die studie het gefokus op die fisiese en meganiese eienskappe van WPCs bestaande uit LDPE, ses indringende houtsoorte en drie verskillende versoeningsmateriale. Die voginhoud, digtheid, trek modulus, treksterkte, verlenging tydens breek en impaksterkte is ontleed en die resultate vergelyk met kommersiëleWPCs om die tegniese haalbaarheid van die fisiese en meganiese eienskappe van WPCs vervaardig van die indringersoorte te bepaal. Kwantitatiewe analise met behulp van eenrigting ANOVA met Tukey post hoc toets en regressiemodel is gebruik om betekenisvolle verskille en verwantskappe te bepaal. Die studie het bewys dat CFM in staat is om die oppervlak struktuur en chemiese funksionaliteite van die verskillende komponente te karakteriseer asook die kwantifisering van funksionele groepe en dus 'n aanduiding van hul adhesie kragte op 'n molekulêre vlak gee. Die inkorporering van PE- g-MA en dPE versoeningsmateriale het gelei tot beter adhesie en dus ‘n verbetering in die trek eienskappe van die samestelling. Die gemete eienskappe het baie goed met dié van kommersiële WPCs vergelyk. Ten slotte kan die indringer hout spesies wat bestudeer is in WPCs inkorporeer word deur gebruik te maak van die regte versoeningsmateriaal. Verder bied termies gedegradeerde LDPE 'n nuwe en goedkoper versoeningsmateriaal wat tradisioneel gebruikte duur versoeningsmateriale kan vervang, in baie gevalle met uitstaande eienskappe.

Doctoral