PAPER CHEMISTRY LABORATORY, INC.
NEWSLETTER JANUARY 2003
THROUGHNESS OF MIXING QUANTIFIED
Led by companies like Motorola and GE, American industry has been adopting the 'Six Sigma' quality doctrine, except for the paper industry, which has ignored it. One major failure of the chemical supplier is that of not ensuring the chemicals are mixed thoroughly with the stock. (Sounds pretty fundamental, doesn't it?)
On-line measurement of headbox zeta potential standard deviation provides an index of the 'Six Sigma' target for mixing efficiency. In a wood-free product, using on-line Zeta Data instrumentation for measurement, we have determined it to fall in the range 0.12mV to 0.20mV, depending on chemistry.
Multiplying by 6 to obtain the 'Six Sigma' target provides a range of 0.72 to 1.2mV zeta potential. Typical standard deviation values obtained on a modern paper machine are all in excess of that value: 1.5-2.0mV for tissue, 2.5-3.0mV for alkaline fine paper, and consistently higher for a 3-headbox-board machine, actually as high as 4-5mV with endless breaks and poor productivity.
For example, a particular Northwest alkaline fine paper machine was experiencing 6-10 breaks/day. We observed that the alkaline size was being added just before the headbox, with obviously insufficient time for mixing, and we measured a headbox zeta potential standard deviation of 2.9mV. The industry is far removed from an optimum level of mixing efficiency.
An important purpose in adding process chemicals (as opposed to functional chemical additives) is to neutralize the repulsive negative charge so that the particles may approach as closely as possible on an intermolecular scale, thereby invoking van der Waals' powerful attractive Force. Properly executed, this action can enable excellent microparticulate control of formation, significant increase in fine paper ash level and (contrary to conventional wisdom) increased strength at the higher ash level.
Charge neutralization is the only way to maximize cost efficiency, yet not a single chemical supplier employs it consistently or well. Instead, they measure "cationic demand" which is easy to do but has a correlation with surface charge of 0.17, so low it is negligible.
Maximizing van der Waals Force requires the ultimate in thoroughness of mixing; it requires a homogeneous stock. It is a horrible indictment, but the paper machine is too unstable a platform for the 'Six Sigma' doctrine to function.
Because the art of wet end chemistry has grown haphazardly, rather than as a discipline with accepted principles, everybody feels comfortable in imposing their personal opinion, often based on little data. A result is the current Wild West wet end mentality, with everybody and nobody right.
Until we learn how to properly mix the chemicals with the stock, NOBODY will be right.
John G. Penniman