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The surface chemistry of paper is responsible for many important paper properties, such as gloss, waterproofing, and printability. There are many components that are used in the paper-making process. The interactions between these components are responsible for creating the properties of the paper. The types and quantities of ingredients used can be adjusted to reach the desired properties. In order to understand why paper has certain properties, one must first understand the chemistry of the paper.

Paper Components and Their Interactions

Pigment and Dispersion Medium

Coating components are subject to particle-particle, particle-solvent, and particle-polymer interactions. Van der Waals forces, electrostatic repulsions, and steric stabilization are the reasons for these interactions. It is important to point out the characteristics of adhesion and cohesion between the components that form the base coating structure. Calcium carbonate and kaolin are commonly used pigments in paper production. Pigments support a structure of fine porosity and form a light scattering surface. The surface charge of the pigment plays an important role in dispersion consistency. The surface charge of calcium carbonate is negative and not dependent on pH, however it can decompose under acidic conditions. Kaolin has negatively charged faces while the charge of its laterals depend on pH, being positive in acidic conditions and negative in basic conditions with an isoelectric point at 7.5. The equation for determining the isoelectric point is as follows:

                                            $pI={{pKa}+{pKb} \over 2}$

In the papermaking process, the pigment dispersions are generally kept at a pH above 8.0.

Pigments, Binders, and Co-binders

Binder spheres coated with acrylic acid (a) and anionic surfactant (b).

The molecular structure of cationic starch. The repeating unit of starch is glucose. It has a carbohydrate monomer composition of C6H12O6. In the polymer, each unit has three -OH groups, and the units are linked together with flexible alpha-1-4 glycocidic bonds.

Binders are used in paper formulations to promote the binding of pigment particles between themselves and the coating layer of the paper. Binders are spherical particles with a diameter that is less than 1 µm. Common binders are made of styrene maleic anhydride copolymer, styrene-acrylate copolymer. The surface chemical composition is differentiated by the adsorption of acrylic acid or an anionic surfactant, which are used for stabilization of the dispersion in water. Co-binders, or thickeners, are generally water soluble polymers that influence the paper's color viscosity, water retention, sizing, and gloss. Some common examples are carboximetilcellulose (CMC), catioinic and anionic hydroxyetil cellulose (EHEC), and starches.

Sizing

A droplet of liquid on a paper surface. The contact angle is depicted.

The molecular structure of co-styrene acrylate

The molecular structure of co-styrene-maleic anhydride

With regards to paper, sizing is the use of chemicals to increase desired traits in paper. The main reason for paper sizing is to act as a binding agent increasing the strength and printability of paper products. Sizing also improves other characteristics of paper such as hydrophilic character, liquid spreading, and affinity for desired ink. Starch is the most common agent used, but in recent years a combination of cationic starch and synthetic copolymers has been increasingly used to improve the effects of surface sizing.

Cationic starch increases strength because it binds to the anionic paper fibers, but it must be added in moderation for the same reason. The amount added is usually between ten and thirty pounds per ton. When starch exceeds the amount the fibers can bind to, it causes foaming in the production process as well as decreased retention and drainage. Another problem with adding too much cationic starch is that the effluent has increased biological oxygen demand (BOD) which is an environmental hazard as well as an increased waste expense for paper makers. To make cationic starches more effective the paper can be made at a higher pH. By doing this the fibers tend to become more anionic increasing the affinity to bond to the starch and creating a stronger product.

Surface Modification

Plasma Surface Modification

One goal of plasma surface modification is to make paper hydrophobic and also oleophilic. This combination allows ink oil to penetrate the paper, but prevents water dampening absorption which increases papers printability.

There are three different plasma-solid interactions that can be used to make surface modifications; etching/ablation, plasma activation, and plasma coating. Etching or ablation is when material is removed from the surface of the solid. Plasma activation is where species in the plasma like ions, electrons, or radicals are used to chemically or physically modify the surface. Lastly, plasma coating is where material is coated to the surface in the form of a thin film. Plasma coating can be used to add hydrocarbons to surfaces which can make a surface non-polar or hydrophobic. The specific type of plasma coating used to add hydrocarbons is called plasma enhanced chemical vapor deposition process or PCVD.

Contact Angle

An ideal hydrophobic surface would have a contact angle of 180 degrees to water. This means that the hydrocarbons lie flat against the surface creating a thin layer and preventing dampening water absorption. However, in practice it is okay or even preferred to have a low level of dampening water absorption because of a phenomenon that occurs when water settles at the surface of paper. This phenomenon is when ink is unable to transfer to the paper because of the water layer at the surface. The contact angle value for hydrocarbons on a rough pigment-coated paper can be found to be approximately 110° through a contact angle meter.

The Young’s equation can be used to calculate the surface energy of a liquid on paper. the Young equation is:

                                       $\gamma _{\mathrm {SL} }+\gamma _{\mathrm {LG} }\cos {\theta _{\mathrm {c} }}=\gamma _{\mathrm {SG} }\,$ 

where $\gamma _{\mathrm {SL} }$ is the interfacial tension between the solid and the liquid, $\gamma _{\mathrm {LG} }$ is the interfacial tension between the liquid and the vapor, and $\gamma _{\mathrm {SG} }$ is the interfacial tension between the solid and the vapor.

An ideal oleophilic surface would have a contact angle of 0° with oil, therefore allowing the ink to transfer to the paper and be absorbed. The hydrocarbon plasma coating provides an oleophilic surface to the paper by lowering the contact angle of the paper with the oil in the ink. The hydrocarbon plasma coating increases the non-polar interactions while decreasing polar interactions which allow paper to absorb ink while preventing dampening water absorption.

Applications

Printing quality is highly influenced by the various treatments and methods used in creating paper and enhancing the paper surface. Consumers are most concerned with the paper-ink interactions which vary for certain types of paper due to different chemical properties of the surface. Inkjet paper is the most commercially used type of paper. Filter paper is another key type of paper whose surface chemistry affects its various forms and uses.

Inkjet Printing Paper

Co-styrene-maleic anhydride and co-styrene acrylate are common binders associated with a cationic starch pigment in Inkjet printing paper. Table 1 shows their surface tension under given conditions.

Compound	Monomer Proportion	pH	Surface Tension (mN/m)
Cationic Starch	-	5.0	32.9
Co-styrene-maleic anhydride	3:1	7.6	38.51
Co-styrene acrylate	3:4	4.3	49.99

There have been several studies that have focused on how the paper printing quality is dependent on the concentration of these binders and pigment. Data from the experiments are very similar and are stated in Table 2 as the contact angle of water, the contact angle of black ink, and the total surface energy.

Sample	Sizing Formulation (% w/w)	Contact Angle of Water (˚)	Contact Angle of Black Ink (˚)	Total Surface Energy (mN/m)
1	no surface treatment	103.1	81.7	39.5
2	100% cationic starch	39.2	36.1	51.25
3	80% cationic starch/ 20% co-styrene-maleic anhydride	80.5	65.2	38.39
4	80% cationic starch/ 20% co-styrene acrylate	60.2	60.5	42.39

The contact angle measurement has proven to be a very useful tool to evaluate the influence on the sizing formulation on the printing properties. Surface free energy has also shown to be very valuable in explaining the differences in sample behavior.

Filter Paper

Various composite coatings were analyzed on filter paper in an experiment done by Wang et al. The ability to separate homogenous liquid solutions based on varying surface tensions has great practical use. Creating superhydrophobic and superoleophilic filter paper was achieved by treating the surface of commercially available filter paper with hydrophobic silica nanoparticles and polystyrene solution in toluene. Oil and water was successfully separated through the use of the filter paper created with an efficiency greater than 96%. In a homogenous solution the filter paper was also successful in separating the liquids through differentiating for surface tensions. Although with a lower efficiency, when aqueous ethanol was tested on the filter paper the ethanol was extracted from the solution.