Fibrillar material as a cobinder in coating colors formulations

Abstract

The micro-fibrillated cellulose (MFC) is a potential material which will at least partly substitute the synthetic co- binders, such as carboxymethyl cellulose (CMC), in paper coating color formulations. Co-binders play an important role in controlling both the flow properties and the dewatering rate of coating colors during the application process as well as during the subsequent film immobilization [25, 19, 02]. In this study, MFC fibers are used to substitute standard, synthetic co-binder material, CMC, affecting both dewatering and rheological properties of coating colors. This study was partly attempting to establish standard measurement procedures that can give overall picture of complex rheological behavior of MFC coating colors. Elastic effects of coating color in low shear rate influence both the flow and blade load [23]. By influencing leveling, elasticity, substitution of CMC with MFC influences coating color application and immobilization process, as well as the uniformity and optical properties of the coating film [06] . It has been demonstrated that coating colors which contained MFC fibers as a co-binder had pronounced shear-thinning characteristics which is a desirable property for paper coatings. However, a complete substitution of CMC with MFC fibers in paper coatings induced low retention properties, longer shear recovery time and fast immobilization of coating colors, which can have a negative influence on leveling and final coating layer uniformity. Coating suspension rheology of the coating influences the coating performance at high speeds. The quality of the coated and printed papers is also affected by the rheology of the suspension [06,26]. Co-binders play an important role in controlling both the flow properties and the dewatering rate of coating colors during the application process as well as during the subsequent film immobilization [23,06]. The micro fibrillated cellulose (MFC) is a potential material which will at least partly substitute the synthetic co-binders, such as carboxymethyl cellulose (CMC), in paper coating color formulations [08,22]. It was expected that the introduction of the MFC material into the coating formulation affects the coating color rheology since the MFC fibers are highly flocculated and have reactive groups on their surface [08].Micro-fibrillated cellulose material (MFC), can be produced through several pre-treatment and refining routes, each giving products with very different morphological and chemical properties [18].It is expected that refining and treatment route of MFC influence traditional coating layer properties as those designed for special purposes [18]. The observation that MFC could be used in paper coating formulations, as a co-binder, since it is biodegradable and has good shear thinning properties, raises a question about the processability of MFC coatings in a high speed coating process(above 1000m/min) [12]. This work focuses on determining general rheological and dewatering behavior of coating colors that contain MFC fibers used as co-binders. A thickener is added to prevent an excessive loss of water from coating color into the base paper and to adjust the rheological properties of the color, a thickener is usually added [03,28]. The physical and chemical properties of the thickeners differ, and they can be roughly divided into synthetic and natural polymers [01]. Water retention and immobilization are the key properties for successful paper coating formulations [21,05]. The task of research is to evaluate how the replacement of CMC co-binder with MFC material influences the viscoelastic and dewatering properties of the coating color [07]. Correlation of the data matrix obtained from dewatering, low-shear viscoelastic and immobilization time measurements show that there is a pattern providing general understanding of the MFC fiber performance in coating suspensions. It is important to understand what the typical behavioral pattern of all MFC coatings would be once they are in the coating process. A key characteristic of the response of a viscoelastic material, as are coating colors, to its deformation during dynamic process, is its ability to recover after cessation of the force which causes deformation [13,04]. Pigment reactivity with binders and co binders has an important effect in viscoelastic behavior of coating color. It has been proved through many previous research papers that more elastic structures of clay coatings yield larger elastic modul than the carbonate coatings [05,14].