title: From colloidal spheres to nanofibrils: extensional flow properties of mineral pigment and mixtures with micro and nanofibrils under progressive double layer suppression.
abstract: Suspensions of mineral pigment and cellulose fibrillar derivatives are materials regularly found in the forest products industries, particularly in paper and board production. Many manufacturing processes, including forming and coating employ flow geometries incorporating extensional flow. Traditionally, colloidal mineral pigment suspensions have been considered to show little to no non-linear behaviour in extensional viscosity. Additionally, recently, nanofibrillar materials, such as microfibrillar (MFC) and nanofibrillar cellulose (NFC), collectively termed MNFC, have been confirmed by their failure to follow the Cox-Merz rule to behave more as particulate material rather than showing polymeric rheological properties when dispersed in water. Such suspensions and their mixtures are currently intensively investigated to enable them to generate likely enhanced composite material properties. The processes frequently involve exposure to increasing levels of ionic strength, coming either from the weak solubility of pigments, such as calcium carbonate, or retained salts arising from the feed fibre source processing. By taking the simple case of polyacrylate stabilised calcium carbonate suspension and comparing the extensional viscosity as a function of post extension capillary-induced Hencky strain on a CaBER extensional rheometer over a range of increasing salt concentration, it has been shown that the regime of constriction changes as the classic DLVO double layer is progressively suppressed. This change is seen to lead to a characteristic double (bimodal) measured viscosity response for flocculated systems. With this novel characteristic established, more complex mixed suspensions of calcium carbonate, clay and MNFC have been studied, and the effects of fibrils versus flocculation identified and where possible separated. This technique is suggested to enable a better understanding of the origin of viscoelasticity in these important emerging water-based suspensions.
authors: School of Chemical Technology, Department of Forest Products Technology, Aalto University, 00076 Aalto, Helsinki, Finland. Electronic address: katarina.dimic.misic@aalto.fi.
authors: Katarina
authors: K
authors: Dimic-Misic
authors: School of Chemical Technology, Department of Forest Products Technology, Aalto University, 00076 Aalto, Helsinki, Finland.
authors: Michael
authors: M
authors: Hummel
authors: School of Chemical Technology, Department of Forest Products Technology, Aalto University, 00076 Aalto, Helsinki, Finland.
authors: Jouni
authors: J
authors: Paltakari
authors: School of Chemical Technology, Department of Forest Products Technology, Aalto University, 00076 Aalto, Helsinki, Finland.
authors: Herbert
authors: H
authors: Sixta
authors: School of Chemical Technology, Department of Forest Products Technology, Aalto University, 00076 Aalto, Helsinki, Finland.
authors: Thad
authors: T
authors: Maloney
authors: School of Chemical Technology, Department of Forest Products Technology, Aalto University, 00076 Aalto, Helsinki, Finland; Omya International AG, CH-4665 Oftringen, Switzerland.
authors: Patrick
authors: P
authors: Gane
mesh_terms: D002119:Calcium Carbonate; D002482:Cellulose / Q000737:chemistry*; D003102:Colloids / Q000737:chemistry*; D004548:Elasticity; D008903:Minerals / Q000737:chemistry*; D057139:Nanofibers / Q000737:chemistry*; D011108:Polymers / Q000737:chemistry*; D011134:Polysaccharides / Q000737:chemistry; D013535:Suspensions; D014783:Viscosity; D014867:Water
chemical_list: D003102:Colloids; D008903:Minerals; D011108:Polymers; D011134:Polysaccharides; D013535:Suspensions; D014867:Water; D002482:Cellulose; D002119:Calcium Carbonate
keywords: Coating colour suspensions; Extensional filamentation; Extensional viscosity; Flocculation and ionic strength; Micro and nanofibrillar cellulose; Nanofibril suspensions