Surface chemistry of paper

The surface chemistry of paper is responsible for many important paper properties, such as gloss, waterproofing, and printability. Many components are used in the

Pigment and dispersion medium

Coating components are subject to particle-particle, particle-solvent, and particle-polymer interactions.

In the papermaking process, the pigment dispersions are generally kept at a pH above 8.0.[1]

Pigments, binders, and co-binders

Binders promote the binding of pigment particles between themselves and the coating layer of the paper.^[2] Binders are spherical particles less than 1 µm in diameterr. Common binders are styrene maleic anhydride copolymer or styrene-acrylate copolymer.^[1] The surface chemical composition is differentiated by the adsorption of acrylic acid or an anionic surfactant, both of which are used for stabilization of the dispersion in water.^[4] 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 carboxymethyl cellulose (CMC), cationic and anionic hydroxyethyl cellulose (EHEC), modified starch, and dextrin.

Sizing

In

Cationic starch increases strength because it binds to the anionic paper fibers.^[6] 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.^[6]

Surface modification

Plasma surface modification

Three different plasma-solid interactions are used: etching/ablation, plasma activation, and plasma coating.^[7] 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.^[7]

Contact angle

An ideal hydrophobic surface would have a

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

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

where ${\displaystyle \gamma _{\mathrm {SL} }}$ is the interfacial tension between the solid and the liquid, ${\displaystyle \gamma _{\mathrm {LG} }}$ is the interfacial tension between the liquid and the vapor, and ${\displaystyle \gamma _{\mathrm {SG} }}$ 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.^[7]

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.^[8] 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. The ability of adhesives to bond to a paper surface is also affected by the surface chemistry.

Inkjet printing paper

Co-styrene-maleic anhydride and co-styrene acrylate are common binders associated with a cationic starch pigment in

There have been several studies that have focused on how the paper printing quality is dependent on the concentration of these binders and ink pigment. Data from the experiments are congruent and stated in Table 2 as the corrected contact angle of water,[9] the corrected contact angle of black ink,^[8] and the total surface energy.^[10]

The contact angle measurement has proven to be a very useful tool to evaluate the influence of the sizing formulation on the printing properties. Surface free energy has also shown to be very valuable in explaining the differences in sample behavior.^[8]

Filter paper

Various composite coatings were analyzed on filter paper in an experiment done by Wang et al.^[11] 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.^[11] Oil and water were 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, aqueous ethanol was also extracted from the solution when tested on the filter paper.^[11]

See also

• Paper chemicals
• Papermaking
• Sizing

References