The natural colour of wood pulp is influenced predominantly by the amount of lignin it contains.
Lignin is a natural adhesive that binds cellulose fibres together to form wood and imbues the pulp with a natural tint, which varies from light to fairly dark brown.
Quite obviously, while paper made from unbleached pulp serves well as packaging material, it is unacceptable for most printing and writing applications.
Bleaching is the term used for a series of process steps where the pulp is exposed to various chemicals that either remove or alter the colour substances it contains, so that it appears whiter. The chemistry involved in bleaching depends on the type of pulp being processed.
Around 70% of the virgin fibre used in making paper is derived from wood pulp manufactured by the ‘kraft’ process.
In use since 1879, this process has been proven to produce the highest valued market pulp at lower cost than any alternative process.
Kraft pulp is naturally a light brown colour, due to the presence of about 5% of residual lignin. In this unbleached form, kraft pulp is used to manufacture packaging products like wrapping paper, paper bags and corrugated cardboard.
Paper products to be used for writing and high quality printing require the pulp to be bleached to an acceptable standard of ‘brightness’.
Kraft pulp may also be referred to as ‘chemical pulp’ to differentiate it from mechanically produced pulp.
Bleaching processes are applied to chemical pulp that are designed to remove as much of the residual lignin as possible, without degrading the strength of the cellulose fibres and are therefore also called delignification.
Mechanical pulp is made either by directly grinding logs of wood into papermaking fibres, or by defibrating wood chips in a pressurized refiner.
These mechanical processes intentionally retain most of the lignin present in wood and this results in pulp yields as high as 95%. Mechanical pulp also has properties that are ideally suited to the production of printing paper and board.
Bleaching of mechanical pulp is alternatively called ‘brightening’ because it only removes the chromophores or colour-causing groups.
The quest for brightness
Papers made from mechanical pulp do not permanently retain the brightness produced by bleaching, because exposure to light and air can produce new chromophores from the residual lignin.
Mechanical papers like newsprint tend to turn yellow with age. Brightness has always been a desirable characteristic of paper, even for applications where it adds little value, and is an important selling feature.
Brightness is arbitrarily defined as the percentage reflectance of blue light with the specific wavelength of 457 nm. This standard is used throughout the pulp and paper industry.
Brightness is not the same thing as whiteness, but the brightness values of the pulps and pigments going into the paper, give an accurate indication of the maximum whiteness that can be achieved with proper tinting.
Whiteness refers to the extent that paper diffusely reflects light of all wavelengths throughout the visible spectrum.
Chemicals & processes
Elemental chlorine is a very powerful oxidising agent and was used almost exclusively for the selective degradation of lignin in pulp bleaching, until environmental concerns curbed this practice.
Sodium hypochlorite is also useful for removing lignin and was often made onsite by reacting chlorine gas with sodium hydroxide.
An unstable gas, chlorine dioxide is water soluble and usually made onsite by reacting sodium chlorate with sulphur dioxide for immediate use - because it decomposes rapidly and is explosive in higher concentrations.
Elemental Chlorine Free (ECF) bleaching sequences substitute chlorine dioxide for chlorine, to virtually eliminate the output of toxic chlorinated compounds in the plant wastewater.
Most of the bleaching agents used to delignify chemical pulp, break the lignin down into smaller molecules that contain oxygen and are water soluble under alkaline conditions.
A solution of sodium hydroxide is therefore employed to wash out these breakdown products between stages.
Water is used in most pulping and bleaching processes and for this reason, pulp mills are always sited close to an adequate water resource.
Oxygen delignification requires conditions that promote the formation of free radicals and is therefore applied under very basic conditions of pH>12.
This process is facilitated by the presence of transition metal compounds especially those of iron, manganese and copper.
Hydrogen peroxide is used for brightening mechanical pulp and under conditions of higher pH and temperature, also for delignification of chemical pulp.
The chemistry in terms of radical reactions is very similar to oxygen bleaching and the two agents are sometimes used together in the same stage.
Ozone is widely used today in ECF bleaching sequences in combination with chlorine dioxide and is the primary oxidising agent used in Total Chlorine Free (TCF) sequences.
In general, the use of ozone decreases the consumption of other process chemicals and therefore the overall cost of bleaching. Ozone cannot be stored and transported like other industrial gases, as it quickly decays into diatomic oxygen and must be produced onsite.
Modern gas technologies
The development of technologies employing both oxygen and ozone were accelerated by the imperative to reduce the emission of toxic chemical by-products, and these have been shown to enable reduced chemical consumption in the production of bleached kraft pulp.
Oxygen delignification minimises the chemical oxygen demand of bleach plant effluent, by reducing the amount of lignin entering the process that is described as Enhanced Elemental Chlorine-Free (EECF).
Today oxygen delignification is a standard stage, with production cost benefits apparent in both ECF and TCF bleaching.
Both ECF and TCF bleaching of kraft pulps are considered to be Accepted Modern Technology in the European Union and in North America.
ECF bleached pulp is preferred by the market for its greater fi bre strength, higher yield and ease of recycling.
Over the last 10 years, ECF has become the dominant bleaching technique, used to produce approximately 75% of the world’s bleached kraft pulp, while TCF holds only a minor and declining market share.