Helpful tips on retention chemistry, coagulants, starch, etc.

Jan 4, 2022

Retention controlMost microparticle systems are based on anionic products, silica, micropolymers and bentonite.

Coagulation - high charged cationic polymer interacts with dissolved and colloidal anionic trash to reduce charge so agglomeration of small particles occurs.

Flocculation - results in large flocs that are broken down under applied shear.

Microflocculation - formed as stock approaches the headbox. Small porous and shear insenstive microflocs release water while maintaining formation.

Most wet end starches are cationic or amphoteric, whereas surface starches are oxidized or hydroxypropylated.

Control knobs for RDF: furnish (composition, refining level, properties), wet end additive program (type, dose, sequence), headbox/former (J/W, L/b, former turbulence).

For retention, measure whitewater solids. For drainage, measure couch vacuum, couch solids, steam pressure, former boxes, etc.

When auditing the wet end, check fiber charge, cationic demand profiles, retention/drainage evaluations.

Components of a microparticle program: charge modifier in thick stock, retention aid to thin stock, microparticle as close to drainage area as possible. Microparticle prevents the formation of large flocs. Large flocs can hurt formation.

In coloidal silica microparticle systems (CSM), the cationic component provides the flexibility and fiberbonding ability of the system.

An overly anionic system will cause bridging problems. An overly cationic system will be contaminated with unadsorbed cationic components that consume microparticles for no efficient purpose and can even cause deposits. Need to find optimum charge balance.

Microparticles have the ability to reflocculate on the wire and in the whitewater loop, which is a main strength of using them.

Need to choose the right CPAM (Cationic Polyacrylamide) molecular weight and charge density when setting up a CSM system. The higher the shear rate in a system, the higher the molecular weight has to be. CPAM must stay in the "active" condition which depends on the conductivity of the system, the amount of anionic trash and the addition point relative to shear points.

Preferred polymer configuration factors: CPAM charge density, conductivity of wet end and fiber surface charge.

For good drainage on the wire, the air content in the headbox should be <0.5%.

Synthetic PCC carries a positive charge in its native form in neutral papermaking.

Early microparticles were silica based, 5 nm wide, spherical with a surface area of 500-800 m2/g, with a highly anionic surface.

Modern microparticles are structured polysilicate microgels with surface areas of 1200-1400 m2/g with an anionic surface.

Bentonite is another form of microparticle (300x100x1 nm). Smectite crystals in water have surface area of 800 m2/g with a highly anionic surface. Bentonite also has the ability to adsorb anionic and nonionic coloidal materials on its surface, such as pitch and stickies.

Retention aid systems: Coagulants, macroflocculants, microflocculants


Agglomeration phenomenon of small particles in aqueous solutions once nuetralized with a coagulant. Examples include alum (also used for pitch control), cationic starches, polyamines, polyDADMACs and polyethyleneimine (PEI).

  • Cationic starches act as coagulants (6-30 #/T) and influence retention, quaternary amine that maintains its properties at any pH.
  • Poly-aluminum chloride (inorganic) better at higher pH than alum. Dosage rate of 10-20 #/T.
  • Polyamines - low MW copolyer of dimethylamine and epichlorohydrin. Charge found on the amine group. Most used coagulant (4-10 #/T).
  • PolyDADMAC - neutralize charges and wood pitch control. Can be used as a retention aid at high MW's. Dosage 1 #/T.
  • Polyethyleneimine (PEI) - typically used in an acid system for charge neutralization at 1 #/T.


Electrostatic bridging chemical reaction between flocculant and stock components. Create macro or micro flocs. Flocculants are polymeric high molecular weight and can have positive, negative or no charges. Examples include polyacrylamides (PAM) and polyethylene oxide (PEO). [fines (2-20 um), colloids (0.01-2.0 um).]

Bridging theory most common mechanism for retention, uses electrostatic attraction to create bridges.

PAM - most commonly used flocculant. High MW linear chain provides good bridging results. Can be cationic or anionic.

PEO - nonionic in nature. Can be used alone in systems with lignosulfonate ions and relies on hydrogen bonding.

All flocculants are constrained by shear effects of headbox and foils, stock temp, stock pH, stock conductivity.

Anionic and nonionic PAM's require the surface first to be treated with a cationic coagulant. Require higher MW's. Also attracts fines and fibers via hydrogen bonding.

Mechanical pulps have higher levels of soluble and insoluble anionic contaminants, making neutralization difficult. Can use bentonite (smectite clay, composed of aluminum silicate) and an anionic PAM to form a chemical "fishing net". Bentonite swells in water and acts like an anionic trash sponge.


Also called microparticle retention, uses a cationic coagulant and a small microparticle that is negatively charged.

  • Cationic starch and/or cationic PAM with colloidal silica.
  • Cationic coagulant and/or cationic PAM with hydrated bentonite.
  • Advantages of microparticle systems: high overall retention, better z-direction retention, better flocculated, increases drainage in the forming and pressing sections.
  • Disadvantage: can cause high sheet porosity on lower basis weights.


Main purpose is to increase dry strength properties of paper. Starches come from potato, corn, wheat, tapioca, waxy maize. Most common starch comes from corn.

  • Biopolymer starch is a polyglucoside as is cellulose. H2O + CO2 -> glucose + oxygen. Glucose -> starch + H2O.
  • Amylose starch are linear structures, solubility in water is less, soluble in hot water without swelling, constitutes 20% of starch.
  • Amylopectin starches are branched structures, more soluble in water, soluble in hot water with swelling, constitutes 80% of starch.
  • Starch is insoluble in water under its gelatinization temperature. As temperature increases, granules start to swell (gelatinization) and viscosity increases, until a temperature is reached where the swollen granules start to rupture and collapse.
  • Retrogradation - on cooling, the return to a more orderly state. Normal - irreversable insolubilization of amylose molecules.
  • Native starches can be modified to change its pasting temperature, solids-viscosity curve, gelatinization and cooking, retrogradation tendencies, ionic character and hydrophilic character.

Van der Waals forces - most important attraction force is the London dispersion force, which acts upon nonpolar molecules.

Electrostatic forces - electrical charge on surface of particles created by ionizable anionic groups (carboxyl, sulfonate, silicate).

For more information on retention chemistry, contact your Valmet representative.