THE MILLENIUM NEWSLETTER

PROPER USE OF THE MICROPARTICLE IN PAPERMAKING

Introduction

About twenty years ago, Eka Nobel introduced the use of colloidal silica with cationic starch, and named the composition "Compozil." In the late 70's Allied Colloids found it useful to employ bentonite in the newsprint process in Sweden, and introduced the technology to North America by giving a paper at the CPPA Conference in Montreal.

Over the intervening years, the industry has increasingly supported both technologies and variations have proliferated. In 1993 the author organized a seminar on the Microparticulate Process as part of the TAPPI Papermakers Conference in Atlanta. The resulting papers, as published in the Proceedings, still represent a good introduction to the subject.

Conceptually, colloidal silica and a form of bentonite called "smectite" represent colloidal particles so small that they are best described by surface area per gram (beginning in the range of about 400 square meters/gram) rather than by particle size. These particles carry a high negative (i.e. anionic) charge and are very reactive to positively charged (i. e. cationic) chemical additives.

The September '98 Newsletter on the web site www.papermaking-chemistry.com describes how well their proper use can result in excellent formation and high retention. The research was sponsored by Eka Nobel, and conducted at Centre Technique in Grenoble. The author, Dr. Christian Pierre, was one of the 1993 Seminar speakers.

Paper Chemistry Laboratory, Inc. initiated the intensive microparticulate phase of its activity with the arrival of Professor Anatoly Makhonin from St. Petersburg, Russia, in November 1991. Wet end chemistry experiments were conducted with a Laboratory Zeta Data, which measures, in one-minute cycles, zeta potential, drainage, conductance, and temperature. Over a period of six years, more than 5000 fully documented experiments were performed

How The Game Was Played in the Lab

The Lab Zeta Data Instrument applies a vacuum to draw paper stock against a screen and

form a pad of fines, fibers (and fillers) between two electrodes. The further application of vacuum draws white water through the pad. The moving fluid displaces the mobile portion of the electric double layer and creates a measurable difference in potential between the two electrodes. This "streaming potential" is applied to the Helmholtz Smoluchowsky equation together with the system parameters, including measured pad conductance and temperature, to calculate the zeta potential, in millivolts (mV).

At the conclusion of the measurement cycle, the maximum height of the drainage liquid is measured and the value applied to a software algorithm that calculates volume. The maximum drainage volume is output as the drainage, in milliliters (ml). It has been shown to correlate with Specific Filtration Resistance.

A Lab Zeta Data experiment is started after obtaining ten stable, internally consistent data points as a base line. A small positive final zeta potential is desired in order to optimize the chemistry, get the best results, and do a repeatable experiment. Therefore, we add sufficient cationic chemical to obtain a semi-final zeta potential in the range +5 to +10mV. The last chemical addition is the highly anionic microparticle, which brings the zeta potential down to an optimum range, which is typically +2 to +4mV in a clean, alkaline fine paper system.

It turns out that, as a result of the dynamics of chemical addition and mixing, maximum drainage is always achieved in the second recorded data point following microparticle addition. This ml value is recorded as the maximum drainage and the "% Increase" calculated from comparison with the base line data. We consider that "% Increase" is a pure and simple measure of microparticulate process efficiency.

In the later stages of the work, we were gratified to learn, when operating at the zeta potential that maximized drainage, physical properties such as Scott Bond and internal sizing were also maximized, as well as sheet ash. It does seem counter-intuitive to simultaneously obtain higher sheet ash, strength and sizing. Something good must be going on.

Mechanism of the Microparticulate Process

Microparticles are characterized by being so small that they serve as unique loci for a micro-flocculation that offers an extremely attractive combination of retention, drainage and formation. The resulting architectural structure not only causes excellent retention of the small particles in the system, and creates good drainage on the wire, but also carries over to good water removal in the press and dryer sections of the process.

In contrast, the prior technology consisted of adding one or two pounds per ton of a "retention aid", which can be typically described as a high molecular weight, low charge density, polyacrylamide resin. Its use results in a high level of retention. However, since this is accomplished by MACROflocculation, it also results in a commensurately poor level of formation.

It is important to recognize that both physical properties, including Scott Bond and HST sizing; as well as the key process parameters, including retention and drainage, are maximized when the process is conducted precisely at the optimum zeta potential. It is of equal importance to understand that they fall off sharply, by perhaps 20-30%, if one misses the optimum zeta potential by as little as 5 or 6mV.

Optimizing the chemistry of the Microparticulate Process is easily done by using appropriate charge-neutralizing chemicals. They include cationic starch, internal size emulsified in a cationic carrier, and cationic scavenger when appropriate. Most clean processes are so sensitive to the headbox zeta potential it becomes of critical importance to use the technology.

ROOM FOR IMPROVEMENT

The reality is that, over the past twenty years, the industry has exhibited a strange ambivalence. On the one hand, there has been recognition of the importance of high surface area of the microparticle to process efficiency,

Cytec has developed an organic microparticle smaller and more efficient than silica; we were one of the first labs to informally evaluate a research sample, and reported very favorably. Du Pont introduced an "in situ" process, to produce microparticles near the paper machine with higher surface area but greatly reduced freight costs. Again, our lab obtained excellent results.

Eka and others have increased efficiency by increasing the surface area of colloidal silica. To cite one example, Eka reduced the solids content from 12% to 7 1/2% in order to increase the surface area per unit mass. This also has the unfortunate result of increasing both the raw material cost/lb. fob as well as the freight cost, because so much more product must be shipped to deliver equivalent solids.

On the other hand, there has been a failure to recognize the importance of electrokinetics, and the seriously negative effects of a continuing use of the traditional "retention aids" to trigger the microparticulate process. The availability of more sophisticated microparticles has been nullified in far too many cases by pairing them with what I have come to term "brute force" retention aids.

For example, in one case a fifth generation silica microparticle was used at 0.5 lb/ton with a retention aid that was added at 2 lb/ton. In this circumstance, the influence of the retention aid is so powerful that adding more microparticle actually impairs formation. This is NOT what the Microparticulate Process is all about.

We should also acknowledge that certain chemical additives, notably cationic guar gum, seem to have a synergistic and remarkably beneficial effect on drainage, retention and sheet properties. This is illustrated in the web site Newsletter for January, 1999.

Because It's Broke, Let's Fix It and Optimize Drainage Instead of Retention

Thousands of experiments have taught us that a typical bentonite is about half as efficient a microparticle as the conventional 12% n. v. colloidal silica. In the lab experiments, we have become accustomed to using either 0.15% silica or 0.3% bentonite and obtaining comparable results. Silica, however, is more expensive to manufacture and far more expensive to ship. Therefore, the most cost-effective approach is NOT to use silica or one of the newer, high efficiency microparticles. The most cost-effective approach is to use the most cost-effective microparticle, namely bentonite.

In the best of all possible worlds, one increases (or decreases) retention and/or drainage, by increasing (or decreasing) the amounts of cationic chemical (usually a scavenger) and bentonite in tandem. The respective amounts are balanced in order to maintain the optimum headbox zeta potential.

This protocol enables management of retention and drainage by the operator at minimum chemicals cost over a very wide range while always maintaining maximum formation. Should a higher level of sheet ash be considered necessary, a very small amount of cationic polyacrylamide retention aid is best diluted with a much larger amount of cationic charge neutralizing chemical. This protects against the serious formation degradation caused by conventional brute force use of retention aids.

Technically, we will only serve our customers well when we learn the importance and practice the art of operating the headbox at the appropriate target zeta potential. In the case of the Microparticulate Process, it is the value that maximizes drainage. This effective use of modern technology enables bentonite to out-perform the more efficient and much more costly microparticles, and they in fact become an irrelevance.

Using this concept as the control strategy can simultaneously increase sheet ash, Scott Bond and sizing while minimizing chemical usage and stock costs.

 John G. Penniman,

Paper Chemistry Laboratory, Inc.