Wet end chemistry ch 4

WPS, CBNU

Sources and types of interfering substances

• Substances released during pulping and bleaching – Mechanical pulps: hemicelluloses, lignin, proteins, pectins in

process white water (anionic trash high demand of cationic additives)

• Substances released during the refining and storage of pulps

• Substances in fresh process water, additives, broke and recycled fiber – Hydrophobic resins, surface sizing starch, coating adhesives,

dispersant from broke

Summary of this chapter

• In paper mill situation – Closed up white water systems

– Use higher levels of recycled fibers

– Lead to decreased additive efficiency, lower paper equality, and poorer papermachine runnability

• Solutions – Papermaker should be recognize of the

nature and level of such materials and be able to solve problems attributed to them

Treatment of interfering substances

• neutralize with a charge neutralizing agents prior to addition of other cationic chemical additives

Wet end chemistry ch 5

WPS, CBNU

Definition of papermaking chemistry

• Table 5.1 Resident species of a papermaking furnish – Dissolve electrolytes

– Suspended fibers

– Suspended fiber fines

– Suspended filler particles

– Water

– Surface active molecules

– Dissolved polyelectrolytes

– Aggregated sizing molecules

Important interactions among furnish components

• Table: 5-2 example of important interactions among furnish components – Aggregation of fiber, fillers, and fines – Adsorption of dissolved polymers onto fibers, fines and

fillers – Aggregation of pitch and sizing molecules – Adsorption of pitch and sizing molecules onto fiber, fines

and fillers – Neutralization of negative charges on suspended and

dissolved anionic materials – Establishment of equilibria between dissolved inorganic salts

and insoluble ionic products – Development of micelles composed of surface active

molecules – Sorption of water by filler, fines and starches

Dimensions of furnish components

• Table 5.3 Smallest dimension of furnish components

– Width

– Length

– Diameter

– Colloidal range: 10.0-0.1 μm

The Colloidal state 1

• High surface area to mass ratio: lie in the interface between the particle and the dispersion medium

• These surface molecules participate in secondary bonding interactions with other molecules identical to themselves located in the bulk of the particle and with the molecules of the surrounding medium

• Two kind of interactions are unbalanced excess energy exists at the particle surface

• This excess energy: strongly influences the behavior of the entire system of dispersed particles

The Colloidal state 2

• When the surface chemical properties of a dispersed system are important colloid chemistry

• Papermaking chemistry- area of colloid chemistry

– large specific surface area involved in interactions of particle surfaces

Classification system in colloid chemistry

• Lyophobic system

• Lyophilic system

– Lyo (solvent)

• Hydrophobic system

• Hydrophilic system

– Hydro (water)

Further comments about the use of the term “hydrophobic”

• Pigments and fiber fines are known to be wettable and to interact with water

• Surface of fiber fine is covered with hemicellulose and cellulose

• Dispersion of these materials (pigments, fines, rosin size dispersion) do exhibit the characteristics of lyophobic colloids in Table 5-4

Stability of hydrophobic dispersions

• Rate vs equilibrium

• Kinetic control vs thermodynamic control

• Controlled aggregate

– A uniform distribution of materials throughout the sheet

• Uncontrolled aggregate

– Poor paper quality and deposits problems

Surface charge on hydrophobic particles

• Charges in papermaking systems: dissociated surface charge of carboxyl and sulfonic acid, retention aids, and cationic starch on surface

• Mineral filler (clay and titanium dioxide): develop surface charge by ionization of the particle/water interface and by adsorption of the other charged substances

Description of charges at solid/liquid interfaces

• Helmholz (1879)

• Gouy, Chapman, and Stern (1913-1924)

• Figure 5.1 Representation of the electric double layer existing at the surface of a hydrophobic particle suspension in water

Zeta potential

• The potential at the interface plane between the Stern layer and Gouy-Chapman region

• Common methods of measuring surface charge give values for zeta potential

• No net charge in suspension

• Local areas of positive and negative potential exist close to the surface of the particles being observed

Importance of the electric double layer

• The stability of a hydrophobic suspension –measured by its ability to remain dispersed over a period of time

• The stability depends on

– The relative magnitude of the repulsive and attractive forces that exist between the suspended particles

– How likely the particle are to collide with one another

Attractive and repulsive forces

• Van der Waals forces-attractive

• Double layer repulsive forces

• DLVO theory

– The attractive and repulsive forces that exist between two particles.

– Potential energy as a function of distance between two particle (Fig 5.2)

Aggregation and stabilization of hydrophobic dispersions

a. coagulation by charge neutralization

• a layer of strongly adsorbed counterions and a diffuse region of counter- and co-ions make up the electrical double layer of particles suspended in water

• The thickness of the double layer: decreased when electrolytes are added to the suspension due to an increase in the availability and number of counterions

• Increased counterion concentration : reduce the zeta potential

• Particles can then approach one another more easily and aggregation become more likely.

Aggregation and stabilization of hydrophobic dispersions

b. patch model

• Low molecular weight cationic polyelectrolytes with high charge density

• Closely packed aggregates that promote good formation and good drainage

Aggregation and stabilization of hydrophobic dispersions

c. flocculation by bridging polymers

• High molecular weight polyelectrolyte

• Mechanism – The polymer adsorbs onto the particle surface with a series

of loops and tails extending into the liquid phase

– These loops and tails extend well beyond the double layer

– Flocculation occurs by a adsorption of the extended loops and tails onto the negative surface of a second particles

• Bridging flocculation is dependent on the collision frequency of the two particles and repulsion between their double layer does not come into play

Aggregation and stabilization of hydrophobic dispersions

c. flocculation by bridging polymers

• Very important phenomenon in paper making

• The charge density and molecular weight of a bridging polymer are very important

• Higher molecular weight promote increased bridging

• Low MW and high charge density cationic polymer with high MW low charge density anionic polymer produce strongest aggregates, dual polymer systems

Aggregation and stabilization of hydrophobic dispersions

d. steric stabilization

• Long chain polymers are adsorbed onto the particle surface in a manner that the solvated polymer loops and tails extending away from one particle interfere with the solvated loops and tails from another particle, preventing the particles from approaching closely enough to

aggregate

Adsorption of polymers from solution by solid surfaces

• Mixing and diffusion in wet end system

• These motions lead to particle-particle or particle-polymer collisions.

• Collisions lead to attractive forces between negative and positive charged species

• Other forces – Hydrogen bonding – Van der Waals forces

Adsorption of polymers from solution by solid surfaces

• Polymer adsorption isotherms

– Rapid adsorption occur

– On the order of a few seconds or less for the major portion of the reaction

– Equilibrium retention level

Adsorption of polymers from solution by solid surfaces

• The effect of fines level on cationic starch adsorption by pulp

– Fine: high specific surface area

– Table 5.8 Effect of fiber and fines surface area on polymer adsorption

Surface science and papermaking

• Source of surface tension

• Papermaking phenomena and surface tension

• Interaction between liquids and solid surfaces

– Young equation

Association colloids 1

• Colloidal system: have a large number of small solute molecules associate together to form an aggregate which is very much larger than the individual solvent molecules and fall into the colloidal size range associate by van der Waals force: associate colloids

• Application of associate colloids in the paper industry: Table 5-11

Association colloids 2

• Surface activity

– Amphipathic association molecules: cause a dramatic, spontaneous lowering f the surface or interfacial tension with very low concentration

• Micelle formation

– Critical micelle concentration (CMC)

• Detergency

• Emulsions

• foams

Summary of this chapter

• Brief review of colloid and surface science concepts

• Adsorption of cationic polymer: critical step in the function of many additives