In nanotechnology one very interesting theme is to combine different types of materials by inclusion of nanoparticles in polymer particles thereby enhancing for example mechanical properties, barrier properties and chemical resistance of the resulting films or foams. Several types of nanofillers exist of which an important naturally occurring one is montmorillonite (MMT) clay. MMT has a high aspect ratio and can be made hydrophobic by chemical modification. The MMT is expected to be dispersible in a polyol. In case the inorganic filler (either encapsulated to as it is) is present in one of the components of the polyurethane formation an elegant route towards hybrid polyurethanes can be envisioned. By using polyols in combination with inorganic fillers (e.g. clay, BaSO4 or CaCO3) in polyurethane foam manufacturing, cell-openness and the hardness or load bearing properties of the flexible foam are improved. It is observed that the inorganic fillers act as nucleating agents during the foaming process. By introducing the particles in the foam, a heterogeneous morphology is created and the particles can act as sites for localized stress concentration. The particle absorbs the fracture energy and therefore increases the toughness of the foam. Instead of inorganic fillers, organic fillers can be used in a similar way. The preferred method at Dow Chemicals is to synthesize the polymeric material in-situ, by a free radical dispersion polymerization in a liquid polyether polyol media. At the moment it is most common to use styrene-acrylonitrile copolymer polyol dispersions. The goal of this thesis is to encapsulate montmorillonite (MMT) clay in polymer using the polyol as the preferred continuous phase and subsequently as one of the reagents for producing polyurethane foams. The first step in the research was to explore the possibilities of exfoliating native and modified MMT in polyol liquids. Exfoliation is needed before any encapsulation can be attempted and is described in chapter 2. We found that dispersing the MMT in various polyols leads to birefringence due to formation of liquid crystals. The behaviour of the MMT in triethanol amine (TEA) appeared to be complex and maybe not fully understood. But the observation of birefringence over time and flow induced birefringence indicates that some platelets have exfoliated towards single platelets or stacks of a few platelets. From the rheological and x-ray diffraction (XRD) data and observations in the transmission electron microscope we found that the cloisite 30 B exfoliates in polyol 767. The native clay gives broad diffraction peaks in the polyol 767 and IP 3040, giving rise to the idea that the clay is intercalated. Subsequently we started exploring our envisaged encapsulation technique; dispersion polymerization in the presence of clay, the results are presented in chapter 3. We started with clay free recipes in order to explore the possibilities of free radical dispersion polymerization in polyols and also ethanol. Monodisperse polystyrene particles of 2 µm were obtained in ethanol at 70 0C. Dispersion polymerizations at 70 0C in TEA and the polyols lead to agglomeration and in the first hour control over particle formation is lost. Experiments with higher stirring rates did not result in stable dispersions. By increasing the polymerization temperature to 121 0C, colloidally stable polystyrene particles in TEA were obtained. Mixtures of ethanol and TEA led to stable polystyrene particles at 70 0C. We believe that the reduced viscosity (by the dilution with ethanol) or by increasing the temperature provides the key to the formation of stable polystyrene particles. By using 120 0C stable dispersion of polystyrene in polyols are formed. Subsequently we attempted to encapsulate the clay in polyol as a polymerization medium, as shown in chapter 4. First attempts to encapsulate native clay in the polyols led to aggregation. We modified the faces of the clay platelets by using electrostatic interaction (ion exchange) or the side of the clay platelet by a covalent bond between the hydroxyl group of the clay and a silane. Increase in d-spacing of the clay indicated that the clay was modified with TMC-10. The clay containing polymer particle sizes ranged from 500 nm- 2 µm and was broader than that of pure polystyrene. Thermogravimetric analysis (TGA) indicated that these particles were thermally more stable than the pure pSt particles. In case of the edge modified clay different morphologies were found for the same recipes. The dumbbell shaped particles resembled the morphologies found with the face modified clay. TGA and XRD results indicate that the clay is present in the pSt phase and is fully exfoliated. Now that clay containing polymer dispersions in polyols can be obtained the next step is to use these dispersions in polyurethane formation. In chapter 5 some preliminary experiments are shown using different dispersions. Polyurethane foams with clay were prepared by adding clay containing TEA and polymeric methylene difenyl diisocyanaat (MDI). It was found that reproducing the properties of foam, even without particles, seemed already very challenging. Therefore chapter 5 might be seen as a first attempt to prepare foams with encapsulated particles but no sound conclusions can be drawn yet. After adding the clay or polymeric particles to the recipe, scanning electron microscopy (SEM) images revealed a disrupted foam structure. By using dynamical mechanical thermal analysis (DMTA) an attempt was made to investigate the storage modulus of the foam. A decrease was found after the incorporation of clay or polymer particles. Also the Tg of the foam changed due to the addition of clay. This effect was less when the clay is added in the foam incorporated in polymeric particles. If the clay is still accessible during the foam formation, as in the case in the arrangement of the armoured particles, an influence on the thermal properties of the PUR foam is observed. From this exercise in producing particle containing foams it is concluded that lab scale foam production is very difficult to reproduce. With some caution however we can see that the presence of (clay containing) particles does affect the properties of the foams and that the accessibility of the clay for the reagents might be an important factor.
|Qualification||Doctor of Philosophy|
|Award date||31 Oct 2012|
|Place of Publication||Eindhoven|
|Publication status||Published - 2012|