Contemporary wet end chemistry is complex and difficult to understand. We invoke the metaphor of the gecko tropical lizard, widespread in the US Southwest and Mexican Northwest. The inter-molecular forces that enable the gecko to run up walls are identical to those that enable simultaneously increased ash level and increased strength.

The following article, "Climbing the Walls" is excerpted from the ECONOMIST, June 10^th 2000, page 88. It originated in NATURE, June 6, 2000, pages 681-685.

"Geckos have very sticky feet. These tiny tropical lizards are able to run up walls and along ceilings extremely fast, yet they can stick to a sheet of polished glass with only one foot. The secret of their success lies in the rows of tiny hairs on the bottom of their feet. Thousands of these hairs, called setae, are arrayed like the bristles of a toothbrush across a gecko's toes. But what makes them so sticky has been unclear.

"Microscopy reveals that the tip of each seta is divided into hundreds of tiny "spatulae", each pointing in a different direction and tipped with a cone-shaped structure. This shape suggests a suction mechanism, but suction relies on air pressure---and gecko feet are known to stick to walls even in a vacuum. So Robert Full of the University of California, Berkeley, and his colleagues decided to take a look at how setae attach to surfaces. Their results are published in this week's Nature.

"Using a tiny micro-electro-mechanical force sensor, they conducted various experiments to measure the stickiness of a single seta. The maximum adhesive force that could be exerted by a single seta had already been estimated, by measuring the total force exerted by a foot and dividing by the number of setae (around 5000 per square millimetre). But to their surprise, the researchers found that a single seta can actually exert ten times as much force as this. Setae are, in other words, even stickier than expected---giving the gecko a surprisingly large safety margin.

"The researchers found that the way in which geckos place their feet on a surface may be crucial to a seta's stickiness. A single seta will adhere most strongly if the spatulae are pointing towards the surface. Indeed, the resulting adhesive force is about 600 times greater than the simple frictional force between lizard skin and the surface. And a seta will stick to a surface most firmly if it is first pushed to the surface and then pulled along it by a few millionths of a meter. These findings suggest that setae operate at a molecular level, and exploit inter-molecular forces, called van der Waals forces, for their stickiness.

"But if geckos can stick to surfaces so strongly, how do they detach their feet again? They curl up the tips of their toes before moving, forming a sort of reverse fist. This allows them to peel their feet off the surface gently at a critical angle without damage, much like peeling a sticky label off a jar without tearing it. The researchers found that setae reliably detach from the surface at an angle of about 30^o."

Papermaking stock normally carries a characteristic negative electrostatic charge. When we reduce the negative repulsive charge to a negligible value, i. e. close to zero zeta potential, its' repulsive effect is eliminated. This enables the powerful inter-molecular van der Waals forces to become effective, the same forces used by the gecko lizard. Van der Waals forces require intimate contact, and are extremely weak at greater than atomic distance gaps.

One must understand that a homogeneous web, called by the industry "good formation", is essential to maximum strength. During the last two decades anionic microparticles, such as colloidal silica and bentonite, have gained increasing acceptance as microflocculation loci, following the addition of charge-neutralizing chemicals such as cationic starch or cationic scavenger.

The powerful microflocculation force increases retention. When the particles in the resultant homogeneous web are close to zero zeta potential, the highly beneficial contribution of van der Waals forces is maximized, resulting in increased strength.

Wet end on-line sensors that measure consistency and ash provide information useful to process stabilization under non-equilibrium conditions, such as start-up, grade change, machine break or upset.

However, the procedure of controlling consistency, instead of zeta potential, as the key wet end process control parameter is open to question. The problem is that a retention aid is used to adjust consistency and/or sheet ash to the desired level. Unfortunately, retention aids promote MACROFLOCCULATION, which does not compare well with microflocculation. The usual result is reduced formation, degraded strength, and significantly impaired physical properties.

On the machine, the preferred process is to monitor sheet ash on the dry end with the scanner. Control on the wet end consists in:

1. Computer adjustment of filler flow to maintain the desired level of sheet ash.
2. Computer adjustment of cationic chemical and microparticle flow rates in tandem. This enables control of drainage over a wide range, while maintaining the optimum zeta potential, thereby maximizing process efficiency.

Benefits of the Preferred Process

The following data provides papermaking examples of the influence of van der Waals forces. It is taken from a paper that describes an extensive series of laboratory experiments with the coated free sheet (CFS) process, "Process Chemistry Optimization, Revision 1" published on the web site www.papermaking-chemistry.com

The zeta potential parameter quantitates the electrostatic charge. It is routinely measured and controlled in our lab experiments, and on the machine, in order to obtain the best possible results.

Dry precipitated calcium carbonate (PCC) was added in 5% increments from 0 to 30% to replace fiber in the stock, in order to determine its influence on process and physical property parameters. In Series II we optimized the process by adding a cationic scavenger to neutralize the repulsive charge of the particles, obtaining a final average zeta potential of -0.6mV. Series III was uncontrolled and we obtained an average zeta potential of -14.2mV.

When we quantify the difference in results, we show that an optimized process with 14% sheet ash has equivalent strength to an uncontrolled process with 4% ash. On a 10% filler for fiber substitution basis, this represents a cost reduction of $15 per ton of product. Additionally, drainage is far superior. Retention is about double. Sizing efficiency is superior by an order of magnitude.

In Series IV the optimized average zeta potential was +4.5mV and the uncontrolled was -15.2mV. The sheet ash levels ranged from 21 to 29%. In every optimized case, both the sheet ash AND the Scott Bond are higher. The data show that sheet ash in the 20% plus range can be increased by 5% with equal or superior strength. Sizing and drainage of the optimized system are superior.

The problem with obtaining testimonials is that the more successful the wet end chemist, the more reluctantly he publishes his results. An example of a highly successful wet end chemist is Dr. Takanori Miyanishi of Nippon Paper. He acquired an on-line zeta potential instrument in 1989, and starting in 1993 he has presented a paper at every TAPPI Papermakers Conference.

The first six papers were focused on wet end chemistry, and included the measurement of zeta potential. Over time, the substance of the papers has become increasingly less specific, and the last two have diverged into subjects unrelated to chemistry optimization. The last chemistry optimization paper, presented in 1998, documents savings estimated at $1,000,000 annually on a newsprint machine, and is published on the web site referenced above. Unfortunately, Dr. Miyanishi does not acquiesce to re-publication of his earlier papers. However, copies are obtainable on request to TAPPI.

The decorative laminate segment of the industry is high in wet strength resin and can support 50% titanium dioxide as headbox ash. Virtually all the decorative laminate machines control zeta potential, typically in a range +10 to +15mV, to obtain the best balance in retention and formation. No technical papers have been published, however, in the past 15 years.

Our best active reference remains the gecko lizard, and our most useful instrument measures on-line zeta potential.

John G. Penniman