Paper Coating Formulas

1. Paper Coating Formulas • Currently dominated by aqueous, pigmented systems. • Pigment - 96 to 50% of total weight • Calcium Carbonate and Kaolin dominate • synthetic (poly-styrene) pigments • Talc / Satinwhite / Aluminum Tri-hydrate etc…. • Binder - 4 to 50% of total weight • Emulsion (Latex) polymers dominate • starches (corn, potato, tapioca) • milk (casein) and soy protein (isolate)

2. Coating Application / Metering • Most common metering processes • roll coating • blade coating • air knife coating • low impact coating • spray coating • wet / dry • curtain / slide / slot

3. Coating Application / Metering • Current technology emphasizes random, uniform dispersion of all coating components prior to application • “condenses” as water removed • drainage to basestock • evaporation (active drying) • rheology optimized for metering processes • minimize particle - particle interactions • how to induce order (decrease entropy)

4. Binder as a “necessary evil”(Here’s the necessary part) • Cohesion of coating is primary function • resist disruption by mechanical / physical (solvation) processes • Binder content contributes to properties • opacity / brightness (scattering) • permeability (air and liquid) • calender response • ink setting / print quality

5. Binder as a “necessary evil” • Expense • can be 10X versus mineral pigments • represent as much as 50% of total formula cost • handling / storage / preservation • “natural binders” • starches • proteins • environmental impact • latexes • non-renewable • slower to degrade

6. “Ain’t Nature Grand” • XXXXX rigid, cohesive structures in which adhesive materials are present at very low levels (< 2% by weight) • self-assembly • spatially specific deposition of adhesives • only where needed to reinforce structure

7. “Binder Demand” • expression of adhesive required to hold structure together • often considered a function of (proportional to) pigment surface area • influenced by processes of migration and depletion • non-uniformity in coating composition • spatially / temporally

8. “Binder Demand” • expression of adhesive required to hold structure together • Latexes • physical entrainment within matrix • average diameter of discrete particles sets limit to pore size accessible to binder • coalescence during consolidation / drying • increases strength • at high binder levels, behavior like continuous film

9. “Binder Demand” • expression of adhesive required to hold structure together • solution polymers (starches, PvOH) • deposition results from “precipitation” out of continuous phase (water) as menisci recede through pore throats1 • at high binder levels, behavior like continuous film

10. “Binder Demand” • expression of adhesive required to hold structure together • colloidal polymers (soy protein / casein) • cationic regions form strong electrostatic bonds to pigments • effective hydrated volume is large • image: Umaine photo?

11. Where can Nano help? • induction of selective deposition through mechanical / thermodynamic processes • binders deposited at points of contact between pigment particles • pigment surfaces modified to heighten attraction • binders placed on pigment surface prior to coating preparation / dispersion