MAXIMIZE RETENTION, DRAINAGE, STRENGTH AND PRINTABILITY

 

Use the Most Cost-Efficient Chemistry in the Most Effective Way

 

 

Hydrogen Bonding

 

The strongest papermaking forces are unseen and act at the inter-molecular level.  Hydrogen bonding is the strongest, and represents the attraction between a bound hydrogen atom and an oxygen atom on an adjacent molecule. It is enhanced by the presence of fibrils, which help by creating an interlocking effect and exposing more surface area to hydrogen bonding.

 

In the simplest form of papermaking, a low solids slurry of papermaking fibers is shaken on a screen so that the water flows through, leaving a wet web.  When the web has dried, it can be removed from the screen and called "paper."  Hydrogen bonding is an ionic phenomenon that is completely disrupted by moisture.  It represents the difference in magnitude of strength between the wet web and the dry sheet, and is very large. 

 

The addition of filler, such as calcium carbonate or clay, to the papermaking slurry may present a number of advantages, increased brightness, whiteness, opacity, printability and reduced cost among them.  The presence of filler, however, interferes with hydrogen bonding and may decrease strength properties below an acceptable level.  Cationic starch presents an inexpensive means of counteracting this, and increasing sheet strength.

 

A long lost secret is that natural gums enhance hydrogen bonding because they have a structure more similar to cellulose than is starch.  In fact, only a small percentage of gum is sufficient to act as a coupling agent between the cellulose and the starch, resulting in a synergistic increase in strength.  Additives of this nature must be thoroughly blended with stock to obtain homogeneity.         

 

 

Macroflocculation

 

In the practical world of papermaking, "retention aids" have been used since mid-century to bridge between particles, helping to retain them on the screen.  The retention phenomenon, however, has two less obvious disadvantages.  It creates a macroflocculation that impairs homogeneity of structure, or formation.  It simultaneously reduces particle-to-particle contact, resulting in a decrease in hydrogen bonding and strength.

 

We first began hearing about microparticles in the late 70's.  As the technology has evolved, many kinds of anionic nanoparticles of commercial significance have arrived.  Our laboratory has done more than 5000 experiments in this area, over a period of six years.  During the latter part of this period we controlled the electrostatic charge, so that the experimental results would be directly comparable.

 

The general industry practice is NOT to practice carefully controlled charge neutralization of the microparticle system.  Rather, it is customary to add a cationic polyacrylamide resin, creating macroflocculation.  Drainage is clearly improved, but at the great cost of impairing hydrogen bonding and irretrievably losing the influence of van der Waals force.  

 

 

The Microparticle Process

 

By addition of a cationic scavenger, the papermaking stock is brought to a uniform cationic state, for example a zeta potential of +7 to +8mV.  A sufficient amount of anionic microparticle, such as bentonite or colloidal silica, is then added to reduce the zeta potential to zero at the point of sheet formation.  Drainage is measured on the Lab Zeta Data immediately after microparticle addition.  A hand sheet is made.  It turns out that these circumstances simultaneously maximize drainage, retention, strength and sizing.

 

The highly anionic microparticle acts as a locus for flocculation of the positively charged stock, in a process called microflocculation.  Because it occurs on a nano-scale, it enhances rather than degrades hydrogen bonding and strength properties.  It electrostatically maximizes homogeneity of structure, or formation.

 

It gets even better.  It is feasible to continuously adjust drainage over a 70% range or more, by simultaneously increasing or decreasing the flow rates of cationic scavenger and anionic microparticle in parallel while maintaining zero zeta potential at the point of sheet formation.  This is a truly enormous benefit for a process concerned with recycle or high yield fiber, high filler content, and/or high basis weight.

 

 

Van der Waals Force       

 

The second (and last) of the invisible papermaking forces increases inversely with the 6^th power of the inter-molecular distance between particles.  We must come as close as we can to eliminating the repulsive negative charge in order to realize significant benefit from van der Waals force.  Clear evidence of the force exists in the fact that a papermaking process controlled to form a sheet close to zero zeta potential has both higher sheet ash and greater strength than an uncontrolled process.  

 

The papermaker is blessed with the best of all possible worlds:  achieving maximum process parameters simultaneously with maximum physical property parameters.  In the case of Coated Free Sheet (CFS), which our laboratory has extensively studied, we maximized drainage, retention, sizing and strength.

 

In reflecting on the mechanism underlying this happy confluence of events, it becomes evident that it is the outcome of eliminating macroflocculation in favor of charge neutralization and microflocculation, thereby maximizing both hydrogen bonding and van der Waals force. 

 

 

The Most Cost-Efficient Chemicals

 

We have discussed the use of chemicals in the most effective way.  It is of paramount importance that we use the most cost-efficient scavenger.  This is accomplished by taking a large portion of the stock component likely to be most offensive, and using it to screen the candidate scavengers.  The most efficient is the candidate that maximizes drainage at least cost.

 

Should performance of the best candidate be insufficient in resistance to deflocculation, and retention on the machine, a small amount of cationic retention aid may be thoroughly incorporated in the scavenger; or anionic retention aid blended with the microparticle, or both. Masking the retention aid in this manner minimizes its adverse effect.

 

 

Conclusion

 

We have briefly outlined the physical chemistry of papermaking nanotechnology.  The papermaker is now able to simultaneously maximize both process parameters and physical properties.  He can accomplish this in a completely adaptable manner, using computer control of chemical feed rates, with full confidence that the most cost-efficient chemistry is being used in the most effective way.

 

 

John G. Penniman