PAPER CHEMISTRY LABORATORY, INC. Fall Newsletter 2003
‘SIX SIGMA’ PAPER QUALITY
A Control Strategy to Maximize Cost Efficiency
INTRODUCTION
Papermaking chemists are universally aware that mixing of chemicals with stock on the machine is less than perfect. What we have not known is either how important or how bad it is. ‘Six Sigma’ quality doctrine provides both.
Scientists have reached the profound understanding that a paramount papermaking objective is to maximize intermolecular contact of all stock components. Physical strength properties can increase exponentially.
There are two issues: obtaining stock homogeneity so that intermolecular contact is possible and then neutralizing the repulsive negative surface charge to maximize the frequency and closeness of that contact. In addressing the task, we need to bear in mind the phenomenon of cationic decay and engineer the mixing dynamics to eliminate it as a variable. ‘SIX SIGMA’ HOMOGENEITY CRITERIA Zeta potential standard deviation of a normal stock varies in the range 0.12mV to 0.2mV. Multiplying by a factor of 6 provides a ‘Six Sigma’ target range of 0.72 to 1.2mV zeta potential. Our investigation of industry practice indicates that the usual quality of mixing is much worse. We have measured tissue processes in the range of 1.5 to 2.0mV, and alkaline fine paper in the range 2.5-3.0mV, often with several breaks/day. A recent coated free sheet (CFS) standard deviation measurement was 3.5mV. Multiple headbox machines are the worst performers, often in the range 4-5mV with multiple adverse symptoms, for example: little relationship of functional chemical addition rate to performance, and poor (often unacceptable) runnability. In the case of multiple headbox machines, foreign white water is added at the stuff box, but insufficient down-stream mixing effort is available to obtain homogeneity. Actually, an identical problem is raised with any common white water usage. Traditional multiple headbox machine design concepts are working so badly they need radical surgery. HOMOGENEITY vs INDUSTRY PRACTICE As an example on the affirmative side: when internal size is dissolved in hydrocarbon with a surface tension of 24 dynes/cm2, and applied by spray to a papermaking web, the required amount of size is reduced by two orders of magnitude. Preferential wetting, generated by a hydrocarbon surface tension 1/3
that of water, is clearly a superior mechanism to that which occurs on a CFSpaper machine using emulsified AKD size, with Hercules Sizing Test
(HST) results ranging from 100 to 500 seconds.
The example shown is an eminently fair representation of the real world, because an appropriate zeta potential range is maintained, thereby creating a level playing field. (In the example, HST sizing has a 0.71 correlation with zeta potential.) Despite this, the nominal sizing excess ranges from 3X to 15X a target value of 30 HST seconds. In summary, whereas perfect mixing could result in a sizing reduction of 99%, current paper machine mixing practice increases sizing usage by a factor ranging from 3X to 15X. A CONTROL STRATEGY TO MAXIMIZE COST EFFICIENCY The Trial Zeta Data enables the lab process chemistry development effort necessary to accomplish stock homogeneity and charge neutralization. Machine trials to meet ‘Six Sigma’ criteria can be monitored by the Trial Zeta Data. The final step is installation of a 7th Generation On-Line Zeta Data, to additionally measure drainage. It can close the loop and operate unattended for 6 week or longer periods. Addition of the cationic component and the microparticle at increased, balanced rates can enable a drainage increase up to 70%, while maintaining zero zeta potential.
This permits less expensive fiber usage or an increase of 5-10% in sheet ash without loss of strength. It also permits more precise control of sensitive water removal processes, such as through air dry (TAD) newsprint and tissue.