Pulp transition - bleached pulp production
Twenty years ago there was a major change in the way bleached pulp for paper and board packaging was produced. Ian Davis, Packaging Consultant with IMD Consulting, Scottish Boarders, UK, explains what prompted the change and the effect on fibres used today.
The change in bleached pulp production came about due to a family of polychlorinated chemical compounds called dioxins and furans. These were a possible by-product from the bleaching process that used elemental chlorine, or chlorine gas in the 1970s and 80s.
The problem started when dioxins were linked with cancer, despite only one being toxic – 2,3,7,8-tetrachloro-p-dibenzo-dioxin. Hysteria was created and many old pulp mills were shut down because they would have been too costly to rebuild with the necessary new technology. This, in turn, brought about a world shortage of bleached pulp and the inevitable price rises, which created a big dipper effect with market instability.
There has never been any proof that dioxins and furans are carcinogenic in humans. The only well known event regarding dioxins was a factory fire in Seveso, Italy in 1976. A cloud of chemical smoke containing dioxins covered the whole town. Analysis of health records 35 years later were inconclusive regarding increased incidence of cancers.
All bleached pulp mills had their pulps analysed to assess dioxin levels. Analytical technology had moved from parts per million (ppm) to parts per trillion (ppt). Most mills did not have access to this level of analysis, so new ppt analysis were undertaken. This found minute traces of dioxins in some processes, but reducing the use of elemental chlorine in the bleaching process virtually eliminates the dioxin and furan compounds. New bleaching stages – chlorine dioxide and ozone bleaching – were subsequently introduced.
Marketers in the pulp sector developed new grade identifiers, which are still in place today – ECF pulp, is ‘elemental chlorine free’. The other grade is TCF pulp, or ‘totally chlorine free’, which means that the pulp has been bleached primarily using the ozone bleaching process, ozone being made entirely of oxygen (O³). No chlorine dioxide or elemental chlorine is used.
Packaging boards and papers
Elemental chlorine has little effect on the cell walls of the fibres, however, ozone bleaching takes material away from the cell wall making it thinner. Even the ECF pulp processes usually have one or more ozone stages in their bleaching sequence, so the paper and board mills, at the time of the changes, noticed differences in the strength properties of their products.
Packaging boards are sold to a specific stiffness or rigidity. If the board is below the stiffness specification, you have to increase the basis weight until you achieve the target specification. This resulted in mills struggling to achieve target strength properties and stay in the basic weight specification. Many mills sold sheets of cartonboard on ‘theoretical count’, so the customers knew they were getting the correct square meterage of material, even if it weighed more than expected.
Something had to change
Cartonboard in the 1980s and 90s used bleached pulps on the top side, and sometimes on the reverse. The centre plies were made from mechanical softwood pulps. At the time, the mechanical pulps had a good bulk factor. With the moves towards ECF and TCF bleached pulps, the cartonboard sector also moved towards packaging boards which looked more like solid bleached boards, similar to those coming from the USA.
The mechanical pulping processes developed rapidly. ‘Thermo-mechanical-pulp’ (TMP) had already been established. In this process, steam is used to soften the softwood chips before they are refined into pulp. It worked best in integrated mills where the pulp is produced and stored wet before being made into paper or board. If the pulp was dried for sale as a market pulp, then a process called ‘irreversible hornification’ took place, whereby, on re-wetting, the fibre would never swell to its original size or bulk.
Developments moved on so that chemicals were introduced to help the softening process as well as producing a brighter pulp. These were called chemi-thermo-mechanical-pulp (CTMP). In terms of brightness, the original TMP might have had a range of 55 to 65% and CTMP 65 to 75%, but the markets were always asking for more.
Finally, bleached chemi-thermo-mechanical- pulps (BCTMP) were introduced using a bleaching sequence of predominantly chlorine dioxide. These pulps had good brightness (80 to 90%) and bulk.
Fibres used for manufacturing packaging boards in the mid 1990s were ECF or TCF on the outer layers and BCTMP/CTMP on the inner layers, enabling mills to achieve stiffness targets with reduced basis weights.
These changes in bleaching have had many effects, such as the co-refining of different wood species is now more feasible. Eucalypt fibres originally had thick cell walls that needed a lot of refining, however, less energy is now needed to achieve the necessary fibrillation for formation and strength.
Today, the hardwood species that are available as market pulps are mainly eucalypt, birch, beech, maple and acacia. There are even differences within a wood type. For example, there are two kinds of eucalypt – eucalyptus globulus and eucalyptus grandes. Globulus is the predominant type in the Iberian Peninsula, while the grandes is the South American variety. They each have their own merits – one is easier to refine, the other is a stronger fibre.
Acacia has a bulky fibre with good natural brightness, so less bleaching is necessary. It refines easily, uses less energy and gives good formation. Birch is a traditional northern European hardwood added to bleached softwoods for improved formation. The ‘northern mixed’ hardwoods from Canada include birch, beech and maple.
When selecting a furnish for a packaging board, it is important to identify the critical parameters, such as stiffness, foldability, crease stiffness, burst and internal bond strength (delamination). Often a compromise has to be reached. One UK mill in the late 1980s developed its products to overcome weak plybonding. It was so successful that the creases on cartons did not work. Creases rely on partial delamination of the plies, so the mill had to reduce its plybond strength to allow creases to form properly.