The Fiber Wire

Plugged in and turned on. All paper. All the time.

Refine and Bleach That Flax

The flax saga continues! We pulled the fiber from the cooler and continued the pulping procedure, combining it with water and running it in the hydropulper, a sort of giant blender that agitates and hydrates the fibers. After ten minutes we transferred the flax to the double disc refiner where it passed between two disks, one rotating and one stationary. We screened for rejects (i.e. fibers that were too big for our liking). After determining the residual lignin we continued into the bleaching sequence.

All bleaching processes work to brighten the pulp and you measure success by preserved yields and low levels of lignin remaining. One method of bleaching works to remove residual lignin. The other method of bleaching leaves that lignin in the paper but works to remove chromophoric groups present on the lignin. When these groups are present they result in darker color paper. This method brightens the paper but the brightness will not last. If oxygen and light reach the paper it will become discolored over time. You’ve probably seen this happen with newspapers. Want more on this topic? Check out “Why Newspapers Turn Yellow Over Time.”

We did two methods of bleaching in this lab. The first bleaching sequence was hydrogen peroxide (H202) with sodium hydroxide (NaOH). This form of bleaching continues the delignification process begun in the soda cook process. Peroxide has low selectivity, meaning it removes lignin but can also remove hemicellulose and possibly cellulose. The added alkali (NaOH) helps dissolve lignin and blocks O2 by swelling the cellulose fibers. It also reduces the peroxide reaction rate.

flax bleaching stages

The second bleaching method executed was Chlorine Dioxide (ClO2) bleaching. This method has replaced bleaching with elemental chlorine (more on this topic next Friday!). When ClO2 is used as a second stage in a bleaching sequence it brightens the pulp by destroying chromophoric groups mentioned earlier. Some delignification may occur but this is not the primary action of this bleaching stage. When used as a second stage, less ClO2 is needed to achieve additional brightening because a lot of the residual lignin removal occurred in the first stage.

Through the bleaching we established a trend to support this information as our lignin content decreased throughout the stages of the sequence. Cooking, refining and bleaching flax fiber this way was new to me. For starters it was straight from the field. The flax I’ve worked with before was already stripped of its outer layer and we did not cook it, we only soaked it. The method of cooking in a batch digester under steam pressure is not exactly the stuff of home paper studios either, although I’ve heard of people cooking fiber in their pressure cooker! I’ve also never bleached my papers before.

I’m interested to hear your way of processing flax. I have a bag of the stuff from the mill and supplier Twinrocker. Any favorite methods out there? I’ve been told that it shrinks when it’s over beaten which could make for some interesting sculptural work. Any images or links to paper art made with fantastic flax?

The technical information in this post was summarized from chapters in the Handbook For Pulp and Paper Technologists, 3rd ed by Gary A. Smook. In class the text is simply referred to as ‘Smook.’ 



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