PSE 322 - Chapter 19: Applications of Aqueous Coatings

why coat paper?

higher gloss, high printed gloss by increasing ink hold-out

what criteria does an aqueous coating formulation have to meet?

uniformity of coverage, gloss, brightness, opacity, smoothness, coverage, adhesion to base stock, high-speed runnability, rheology, blade-scratch tendency, solids level, binder migration tendency

what is the benefit of coating with high solids content?

minimizes the cost of evaporating water from the coating, helps coating immobilize after application

what is the benefit of rapid immobilization?

it tends to keep the coating from penetrating the base stock

what happens if immobilization is too rapid?

scratches

newtonian

fluid has approximately the same viscosity coefficient over the whole range of shear rates considered

what is typically on the x-axis of a rheogram?

torque

what is typically on the y-axis of a rheogram?

shear rate

what is typically the slope of a rheogram?

viscosity

dilatent

the viscosity coefficient of a fluid increases with increasing velocity of laminar flow

pseudoplastic

the viscosity coefficient is higher when the mixture is at rest, and it gets lower as flow increases (common for coating colors)

thixotropic

the fluid tends to set up or “thicken” at decreasing flow rates or when at rest (pseuodoplastic but involves time as a factor)

What are examples of coating pigments?

kaolin clay, calcium carbonate, titanium dioxide, plastic pigments

what is different about pigments used in coatings?

tend to be smaller than typical filler particles

what are examples of coating binders?

latex, starch, protein, polyvinyl alcohol

how many parts do pigments make up in coatings?

100

how many parts do binders make up in coatings?

8-18

what are examples of coating latex?

styrene-butadiene, polyvinylacetate, acrylic

what are examples of coating starches?

hydroxyethylated, enzyme converted, oxidized

what is distinct about coating binders?

latex would be too tacky for PM dryers, coating starch is too low in MW and uncharged

how many parts do additives make up in coatings?

0.1-10

what are examples of coating additives?

dispersants, water retention agents, lubricants, crosslinkers, NaOH, anti-foam

what are examples of coating dispersants?

Na4 polyphosphate, Na hexametaphosphate, acrylate copolymers

what are examples of water retention agents?

CMC, alginates, synthetic polymers

what are examples of coating lubricants?

calcium sstearate, polyethylene emulsion

what are examples of coating cross linkers?

cyclic amid condensates, ammonium Zr carbonate, glyoxal resins

what PCC is most used in coating applications?

rhombohedral or prismatic

Will a distribution of different particle sizes flow better at a given high-solids level?

yes

how does hydroxyethylated starch compare to underivatized starch?

it does not retrograde, it develops strong non-brittle films, it can be delivered to the mill at various visocisties

how are latex binders formed?

free radical polymerization

what should you do to SBR binder for a stiffer latex?

increase proportion of styrene

what does stiffer latex provide?

higher glass, lower strength

what is the benefit of higher cross-link density in SBR?

higher binding strength

what is the benefit og a lower cross-link density SBR?

greater resistance to coating delamination

what provides outstanding gluability?

protein binder (casein)

what is the common printing method for coated paper?

offset lithography

how does pigment size and binder need relate?

smaller pigment particles require more binder (related to surface area)

What is a negative impact of increased binder?

coating is more translucent

what is the purpose of dispersants?

keep the particles separated

what do excess coating dispersants cause?

increase coating viscosity

when are cross-linkers needed in coatings?

when coatings need to stand up to moisture

what is the purpose of coating lubricant?

minimize calendering dust, improve gloss development, minimize cracking during folding

what is the optimal coating pH?

neutral to slightly alkali