Towards a mechanism for surface hydrophobization of paper - Effect of combinations of polyelectrolytes and polymer particles

Paper materials are cost effective and light weighted, 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. Packaging paper materials are often made from recycled fibers. The constitution of the paper matrix can therefore vary a lot and the addition of hydrophobic compounds to the pulp in the paper production process is difficult to optimize. Therefore the recent development in paper hydrophobization has been towards Surface modification, so-called surface sizing. 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 aim of this licentiate project, which can be regarded as the first part of a doctoral thesis work, is to explore and identify which physicochemical properties of the formulation used for surface sizing are governing the efficiency. In surface sizing the particle suspension is first mixed with starch in solution. Starchparticles and starch is the subject of one study described in this thesis. In this study the interactions between starch and three types of particles, differing in the type of stabilizer used, are explored. The different stabilizers rendered the particles cationic, anionic or amphoteric. It was found that the cationic particles formed aggregates with the starch and that it is mainly the high molecular weight, highly branched amylopectin fraction of the starch that participates in the aggregation. The aggregate formation, as well as the relaxation kinetics, are also investigated and it was concluded that the amylopectin chains give rise to steric stabilization even at the mostdestabilized state, i.e. at maximum aggregation. The relaxation kinetics is found to be molecular weight dependent while the equilibrated state is not, leading to a proposed aggregation mechanism based on patchwise flocculation. Finally the efficiency in reducing the water uptake of test paper sheets is assessed. The cationic particles are the most efficient in decreasing the water uptake and the efficiency is enhanced by aggregation

Parameters influencing hydrophobization of paper by surface sizing

Paper surface hydrophobization, also referred to as "surface sizing", is often necessary for printing and packaging purposes. Typically, hydrophobic polymeric nanoparticles, in combination with starch are applied on the paper surface at the dry-end of the paper machine. In the surface sizing process, the nature of the nanoparticles, starch type, starch concentration and ratio between starch and nanoparticles, paper quality, ionic strength, and application as well as drying temperature are parameters that influence the result. The aim of this work was to systematically evaluate these parameters in order to create knowledge to be used for optimization of the process. Laboratory scale surface sizing trials were performed and the results from the trials showed that cationic particles gave superior performance compared to anionic particles. Both the starch type and the concentration of the oxidized starch had an impact on the performance. The effect of the ionic strength was found to depend on the particle charge: for cationic particles, the addition of salt was detrimental while for the anionic particles it was beneficial. An increase of the application or the drying temperature was found to enhance the performance up to a temperature around the glass transition temperature of the polymer

Formation and relaxation kinetics of starch-particle complexes

© The Royal Society of Chemistry.The formation and relaxation kinetics of starch-particle complexes were investigated in this study. The combination of cationic nanoparticles in suspension and anionic starch in solution gave rise to aggregate formation which was studied by dynamic light scattering, revealing the initial adsorption of the starch molecules on the particle surface. By examining the stability ratio, W, it was found that even in the most destabilized state, i.e. at charge neutralization, the starch chains had induced steric stabilization to the system. At higher particle and starch concentrations relaxation of the aggregates could be seen, as monitored by a decrease in turbidity with time. This relaxation was evaluated by fitting the data to the Kohlrausch-Williams-Watts function. It was found that irrespective of the starch to particle charge ratio the relaxation time was similar. Moreover, a molecular weight dependence on the relaxation time was found, as well as a more pronounced initial aggregated state for the higher molecular weight starch. This initial aggregate state could be due to bridging flocculation. With time, as the starch chains have relaxed into a final conformation on the particle surface, bridging will be less important and is gradually replaced by patches that will cause patchwise flocculation. After an equilibration time no molecular weight dependence on aggregation could be seen, which confirms the patchwise flocculation mechanism

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

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.\ua0In 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.\ua0\ua0In 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

Towards a mechanism for surface hydrophobization of paper

Surface hydrophobization of paper, also referred to as surface sizing, is often needed for printing and packaging purposes. Typically, hydrophobic nanoparticles in combination with starch are applied on the paper surface. In this study two types of hydrophobic nanoparticle suspensions, one cationic and one anionic, were investigated in order to gain deeper knowledge about the mechanism behind surface sizing. A study was performed where the interactions between the particles and anionic starch were investigated and this study revealed that on addition of starch the cationic particles formed aggregates while the anionic particles were unaffected. This aggregation behaviour was found to be beneficial for the surface sizing performance. The impact of salt was also investigated and it was found that by increasing the ionic strength with NaCl or with Na 2 SO 4 larger agglomerates were formed. The surface hydrophobization efficiency of such aggregates was found to be lower, however

Role of the aggregation behavior of hydrophobic particles in paper surface hydrophobation

Three types of hydrophobic particles with different charge, i.e. cationic (SP+), anionic (SP-) and amphoteric (SPA) have been synthesized by emulsion polymerizations and evaluated in paper surface hydrophobation performance (frequently referred to as surface sizing). The surface sizing evaluation was done according to a well-established process, i.e. mixing of the particles with negatively charged starch followed by application in a conventional puddle size press using a fine paper grade containing calcium carbonate filler as model system. Prior to the application, the particles were characterized by light scattering and the surface charge was determined by particle charge density titrations and ζ-potential measurements. The SP+ particles were determined to be 30. nm in diameter while SP- and SPA particles were around 65. nm. Their colloidal behavior in the presence of anionic starch differed. The SP+ particles formed aggregates via bridging flocculation up to a charge ratio of 1:1 of starch:particles. At higher starch content the aggregates were partly redispersed. The SP- and SPA particles showed no sign of aggregation in the presence of anionic starch. In addition, the sizing performance of the different particles was evaluated by assessing the decrease of water uptake in a surface treated paper as well as water contact angle measurements on the paper surface. All three types of particles decreased the water penetration. However, the SP+/starch mixtures showed superior performance, which was attributed to a stronger sensitivity to the high electrolyte concentration usually found in the vicinity of the paper surface when the semi-soluble minerals composing the filler are exposed to water

Parameters influencing hydrophobization of paper by surface sizing

Paper surface hydrophobization, also referred to as "surface sizing", is often necessary for printing and packaging purposes. Typically, hydrophobic polymeric nanoparticles, in combination with starch are applied on the paper surface at the dry-end of the paper machine. In the surface sizing process, the nature of the nanoparticles, starch type, starch concentration and ratio between starch and nanoparticles, paper quality, ionic strength, and application as well as drying temperature are parameters that influence the result. The aim of this work was to systematically evaluate these parameters in order to create knowledge to be used for optimization of the process. Laboratory scale surface sizing trials were performed and the results from the trials showed that cationic particles gave superior performance compared to anionic particles. Both the starch type and the concentration of the oxidized starch had an impact on the performance. The effect of the ionic strength was found to depend on the particle charge: for cationic particles, the addition of salt was detrimental while for the anionic particles it was beneficial. An increase of the application or the drying temperature was found to enhance the performance up to a temperature around the glass transition temperature of the polymer

Surface Treatment by Hydrophobic Particles: Influence of Starch and Ionic Strength

This paper deals with the colloidal behavior of mixtures of cationic polymer particles and anionic starch. Such mixtures are commonly used for the hydrophobization of paper. The effect of the concentration of anionic starch and of the presence of different electrolytes was assessed. In a previous study, oxidized starch had been found to induce aggregation of cationic hydrophobic nanoparticles, and in the study reported here it is demonstrated that this aggregated state is beneficial for the performance, enabling a substantial reduction of the amount of polymer nanoparticles needed to reduce the water uptake. It was found that when the particles were in a highly aggregated state, the water uptake by the paper surface was very small. The effect of mono- and divalent ions on the colloidal stability was also investigated. In general, it was found that an increased ionic strength gave a less hydrophobic paper surface. Na₂SO₄ was more detrimental than NaCl and CaCl₂, which is explained by the valence of the anion. This implies that the hydrophobization can be tuned by controlling the aggregation of the polymer particles

Coated formulations: New insights into the release mechanism and changes in the film properties with a novel release cell.

The effect of the blend ratio of water-insoluble ethyl cellulose (EC) and water-soluble hydroxypropyl cellulose (HPC-LF), on the properties of sprayed films and on the drug release mechanism of formulations coated with the material was investigated. When the original HPC-LF content exceeded 22%, both the amount of HPC-LF leached out and the water permeability of the films increased drastically when they were immersed in a phosphate buffer solution. The release mechanism of potassium nitrate through EC/HPC-LF films containing 20, 24 and 30% HPC-LF was elucidated in a new release cell equipped with a manometer to measure the pressure build-up inside the cell. A lag phase in the release accompanied by a pressure build-up was observable in all the experiments showing that all the films were initially semi-permeable to KNO(3). However, pressure data revealed that films with 30% HPC-LF became permeable to KNO(3) during the release process due to HPC-LF leaching. Importantly, the blend ratio influenced not only the release rate (which increased as the amount of HPC-LF increased), and the lag time (which increased as the amount of HPC-LF decreased), but also the release mechanism, which changed from osmotic pumping to diffusion as the amount of HPC-LF increased

Influence of Different Polymer Types on the Overall Release Mechanism in Hydrophilic Matrix Tablets

The effect of three different types of polymer chain structures on the polymer release from hydrophilic matrix tablets was investigated by comparing a synthetic semi-crystalline linear polymer (PEO), a branched amorphous polysaccharide (dextran) and an amorphous substituted cellulose derivative (HPMC). The polymer release rates for tablets containing mixtures of high and low molecular weight grades in different ratios were determined by using a modified USP II method and a SEC-RI chromatography system. The results showed that independent of polymer type: (i) plots of the release versus time had similar shapes, (ii) the release of long and short polymer chains was equal and no fractionation occurred during the release and (iii) the release rate could be related to the average intrinsic viscosity of the polymer mixtures. This confirms the hypothesis that the release rate can be related to a constant viscosity on the surface of the hydrophilic matrix tablet and that it is valid for all the investigated polymers