Abstract
Light-weight paper products that contain less fibres, but with a maintained bulk and improved strength properties, are highly desirable due to the low cost of raw materials and improved logistics of packaged goods. In this respect, the adsorption capacity of dry strength additives onto fibres, which is affected by the surface charge of said fibres, is very important for the development of these mechanically robust paper products. The influence of the surface charge on the adsorption of strength additives was investigated for, dissolving grade fibres, kraft fibres and kraft fibres modified with carboxymethyl cellulose (CMC) with different surface charge densities, but the same fibre dimensions. The strength additives investigated were cationic starch (CS), anionic polyacrylamide (APAM) and polyelectrolyte complexes (PECs), containing CS and APAM. A linear relationship was found between the surface charge of the fibres and the saturated adsorbed amount of CS. However, when either APAM or PECs adsorbed as secondary layers onto the CS, no correlation between cellulose charge and the saturation adsorption could be observed. The adsorption of APAM was dramatically affected by the pre-adsorbed amount of CS, whereas PECs were less influenced. Moreover, the additives improved the tensile strength (60%) and strain at break (> 100%) of handsheets formed with the kraft fibres and adsorbed APAM. It was also found that CS/APAM increased the sheet density while CS/PECs lowered it. In conclusion, the gained fundamental understanding of these adsorption of additives is of significant importance to facilitate the industrial development of sustainable low-cost high-end packaging products.Graphical abstract
Highlights
IntroductionThe popularity of fibre-based packaging has increased worldwide
Due to environmental concerns, the popularity of fibre-based packaging has increased worldwide
With 10 mg/g, 15 mg/g and 20 mg/g additions of carboxymethyl cellulose (CMC), the absolute values of the surface charge of kraft birch fibres increased from 7 μeq/g to 13 μeq/g, 23 μeq/g and 24.5 μeq/g respectively
Summary
The popularity of fibre-based packaging has increased worldwide. Ideal properties of paper based packaging materials are high dry paper strength and low density; as it would be beneficial to minimise the total amount of fibres used per package. It is important that any reduction in paper density does not reduce the dry strength of the package. The dry strength of paper is affected by multiple factors; such as the joint area between the fibres and intrinsic strength of the component fibres, as well as sheet formation (Lindström et al 2005). Dry strength additives are frequently used to optimise the mechanical properties of board made from both virgin and recycled fibres. The most common dry strength additives used in papermaking are modified starches and polyacrylamide based polymers (Silva et al 2015). An emerging additive during the last decade is polyvinylamine (PVAm), which will impart both wet and dry strength to papers (Pelton 2014)
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