WET END CHEMISTRY NEWS LETTER FOR SEPTEMBER, 1998

The following schematic of the Microparticulate Process, slide 21 of the Zeta Data Technology Presentation shown on the web site, represents research done by Dr. Christian Pierre of Centre Technique in Grenoble. It is entitled "The Effect of Various Retention Aids on Flocculation and Retention." Most noteworthy is the exceptionally high combination of retention and formation obtainable with cationic starch and colloidal silica. Cationic starch alone is close behind.

Microparticles are characterized by being so small that their size is not expressed in the conventional dimensions, but in surface area, e. g. 500 m²/g. The extremely small particles serve as unique loci for a micro-flocculation that offers an extremely attractive combination of retention, drainage and formation.

The PEI (polyethylenimine) curves represent use of high charge density wet end chemicals much lower in molecular weight than PAM (polyacrylamide), and more highly charged than cationic starch. Accordingly, PEI exhibits a flocculation effect intermediate between the other two chemistries. For a given level of retention PEI shows better formation than PAM and yet significantly inferior to cationic starch and silica, or cationic starch alone.

The 'hooks' at the end of the PEI curves merit special comment. A sufficient amount of these highly charged resins has been added to cause the system to reverse charge and become increasingly positive. Conventional colloidal systems would normally start to re-disperse. Under these circumstances papermaking fibers start to disentangle and improve in formation, but the filler remains fully adsorbed and retention is unchanged!

This particular response is exploited in making decorative laminate base paper. Because of the exposed nature of its end use, in kitchen counter-tops for example, it requires an exceptionally high combination of retention and formation of opaque pigment. Typically with a headbox ash content of about 50% titanium dioxide, and high usage of wet strength resin, the headbox is often operated at a zeta potential of +10 to +15mV zeta potential because this offers the best compromise in retention and formation.

It is worthy of comment that chemistry optimization is of such critical importance to decorative laminate papermakers that they represent the only significant segment of the industry which universally uses precise zeta potential control of the wet end.

We have been privileged to supply Zeta Data technology for monitoring and controlling the decorative laminate base paper process. Parenthetically, one thing we soon learned is the importance of stopping the wet strength resin addition during a break. On one machine, this action enabled reduction of the average time from a break to sheet ash re-stabilization, by two thirds. The Zeta Data investment was rapidly repaid!

Let us review the fundamentals. The commercially available microparticles have in common a large surface area and a negative charge. Actually, the smaller the particle size, the more effective the particle. Organic microparticles are justifiably claimed to have a greater effectiveness (although not greater cost efficiency) because they are the smallest, followed by colloidal silica and smectite, commonly called bentonite.

The highly negative microparticles are compellingly attracted to a positively charged substrate. Our extensive data shows that the most efficient microparticulate process has a slightly positive final zeta potential. It makes intuitive sense that the much larger filler and fiber particles might well show a small residual final positive charge in order to have been able to attract billions of tiny negative microparticles.

Our data also shows that process efficiency drops off sharply on either side of the optimum zeta potential. Papermakers who use precise control of headbox zeta potential are able to realize the remarkable benefits, to both process and quality parameters, of true process chemistry optimization.

John G. Penniman

September, 1998

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