Towards an understanding of surface hydrophobization of paper - Exploring the effect of polymer nanoparticles, starch, ionic strength and process parameters

Doctoral thesis, 2017

Paper materials are cost effective and lightweight, they can easily be recycled and their use as an alternative to plastics is advantageous from an environmental and sustainability perspective. However, competing with plastics for packaging applications is a challenge for cellulosic products. The material needs to be strong and stiff also when exposed to liquids or moisture during transportation and storage. To achieve this for paper materials, which are intrinsically hydrophilic due to the nature of the cellulose, they need to be hydrophobized.  In the paper industry the hydrophobization process is referred to as sizing and sizing can be achieved in two ways: adding the hydrophobic compound to the pulp (internal sizing) or on the formed paper surface (surface sizing). Recent development in paper hydrophobization has been towards surface sizing since this method gives better retention of the hydrophobic compound and is also more effective on recycled fibers. There is a plethora of surface sizing products and these products are very efficient in making the paper surface more water resistant, but there is a lack of fundamental knowledge on how they work. The type of surface sizing products studied in this thesis is hydrophobic nanoparticle suspensions and four different particle types have been explored. They have the same polymeric core but different surface charges and chemistries. In the surface sizing application, the suspension is mixed with a starch solution where the starch is added to increase the surface strength of the paper. However, in this thesis it is shown that the starch can have a more active role in controlling the degree of hydrophobization. In the study of the colloidal systems it was found that cationic particles form aggregates with the starch and by maximizing this aggregation a paper surface takes up significantly less water. The aggregation behavior was thoroughly studied and the aggregation could be tuned by amylopectin content, temperature and time. An increased ionic strength had a pronounced effect on the electrosterically stabilized aggregates. Larger aggregates were formed at intermediate ionic strength and when the ionic strength was high enough the system collapsed and large flocs were formed. Both these effects were detrimental for the performance.    In the surface sizing procedure the particle/starch mixture is applied on the paper surface and the liquid penetrates into the paper matrix due to external pressure and capillary forces. The distribution of the hydrophobic polymer on and in the paper was evaluated with time of flight secondary ion mass spectroscopy (TOF-SIMS). The surface distribution did not correlate with the water uptake results, indicating that it is not merely the outermost surface that needs to be hydrophobized in order to have a sufficiently low water uptake. Cross-section analyses revealed that a deeper penetration of the nanoparticles was needed to achieve a water resistant paper.