Brightness is a measure of how much light is reflected by paper under specified conditions and is usually reported as a percentage of how much light is reflected, so a higher number represents a brighter or whiter paper. In the US, the TAPPI T 452 or T 525 standards are used. The international community uses ISO standards. The following table shows how the two systems rate high brightness papers, but there is no simple way to convert between the two systems because the test methods are so different. Note that the ISO rating is higher and can go above 100.
|TAPPI Brightness||ISO Brightness|
Newsprint ranges from 55-75 ISO brightness. Writing and printer paper would typically be as bright as 104 ISO.
While the results are the same, the processes and fundamental chemistry involved in bleaching chemical pulps (like kraft or sulfite) are very different from those involved in bleaching mechanical pulps (like stoneground, thermomechanical or chemithermomechanical). Chemical pulps contain very little lignin while mechanical pulps contain most of the lignin which was present in the wood used to make the pulp. Lignin is the main source of color in pulp due to the presence of a variety of chromophores naturally present in the wood or created in the pulp mill.
Alkaline hydrogen peroxide is the most commonly used bleaching agent for mechanical pulp. The amount of base such as sodium hydroxide is less than that used in bleaching chemical pulps and the temperatures are lower. These conditions allow alkaline peroxide to selectively oxidize non-aromatic conjugated groups responsible for absorbing visible light. The decomposition of hydrogen peroxide is catalyzed by transition metals, and iron, manganese and copper are of particular importance in pulp bleaching. The use of chelating agents like EDTA to remove some of these metal ions from the pulp prior to adding peroxide allows the peroxide to be used more efficiently. Magnesium salts and sodium silicate are also added to improve bleaching with alkaline peroxide
Sodium dithionite (Na2S2O4), also known as sodium hydrosulfite, is the other main reagent used to brighten mechanical pulps. In contrast to hydrogen peroxide, which oxidizes the chromophores, dithionite reduces these color-causing groups. Dithionite reacts with oxygen, so efficient use of dithionite requires that oxygen exposure be minimized during its use.
Chelating agents can contribute to brightness gain by sequestering iron ions, for example as EDTA complexes, which are less colored than the complexes formed between iron and lignin.
The brightness gains achieved in bleaching mechanical pulps are temporary since almost all of the lignin present in the wood is still present in the pulp. Exposure to air and light can produce new chromophores from this residual lignin. This is why newspaper yellows as it ages.
Delignification of chemical pulps is rarely a single step process and is frequently comprised of four or more discrete steps. These steps are given a letter designation, and these are given in the following table:
|Chemical or process used||Letter designation|
|Extraction with sodium hydroxide||E|
|Alkaline hydrogen peroxide||P|
|Chelation to remove metals||Q|
|Enzymes (especially xylanase)||X|
|Peracids (peroxy acids)||Paa|
|Sodium dithionite (sodium hydrosulfite)||Y|
A bleaching sequence from the 1950s could look like: CEHEH . The pulp would have been exposed to chlorine, extracted (washed) with a sodium hydroxide solution to remove lignin fragmented by the chlorination, treated with sodium hypochlorite, washed with sodium hydroxide again and given a final treatment with hypochlorite. An example of a modern totally chlorine-free (TCF) sequence is OZEPY where the pulp would be treated with oxygen, then ozone, washed with sodium hydroxide then treated in sequence with alkaline peroxide and sodium dithionite.
The main objection to the use of chlorine for bleaching pulp is the large amounts of soluble organochlorine compounds produced and released into the environment.
Chlorine dioxide is sometimes used in combination with chlorine, but it is used alone in ECF (elemental chlorine-free) bleaching sequences. It is used at moderately acidic pH (3.5 to 6). The use of chlorine dioxide minimizes the amount of organochlorine compounds produced.
Enzymes like xylanase have been used in pulp bleaching to increase the efficiency of other bleaching chemicals. It is believed that xylanase does this by cleaving lignin-xylan bonds to make lignin more accessible to other reagents. It is possible that other enzymes such as those found in fungi that degrade lignin may be useful in pulp bleaching.
Delignification of chemical pulps releases considerable amounts of organic material into the environment, particularly into rivers or lakes. Pulp mills are almost always located near large bodies of water because of they require substantial quantites of water for their processes.
Bleaching with chlorine produced large amounts of organochlorine compounds, including dioxins. Increased public awareness of environmental issues, as evidenced by the formation of organizations like Greenpeace, influenced the pulping industry and governments to address the release of these materials into the environment . The amount of dioxin has been reduced by replacing some or all of the chlorine with chlorine dioxide. The use of elemental chlorine has declined significantly and as of 2005 was used to bleach 19-20% of all kraft pulp. ECF (elemental chlorine-free) pulping using chlorine dioxide is now the dominant technology worldwide (with the exception of Finland and Sweden), accounting for 75% of bleached kraft pulp globally.
The promise of complete removal of chlorine chemistry from bleaching processes to give a TCF (totally chlorine-free) process, which peaked in the mid-1990s, did not become reality. The economic disadvantages of TCF, the lack of stricter government regulation and consumer demand meant that EFC has not been replaced by TCF. As of 2005 only 5-6% of bleached kraft is made using TCF sequences, mainly in Finland and Sweden. This pulp and paper goes to the German market, where regulations and consumer demand for TCF pulp and paper makes it viable.
A study based on EPA data demonstrated that TCF processes reduce the amount of chlorinated material released into the environment, relative to ECF bleaching processes which do not use oxygen delignification. The same study concluded that "Studies of effluents from mills that use oxygen delignification and extended delignification to produce ECF and TCF pulps suggest that the environmental effects of these processes are low and similar." The energy needed to produce the bleaching chemicals for an ECF process not using oxygen delignification is about twice that needed for ECF with oxygen delignification or ECF processes. The environmental impact differences between TCF and ECF process however are not fully understood. Some recent studies have pointed out that no difference in acute or chronic toxicity is to be found when comparing well-treated ECF and TCF effluents breaking the paradigm that TCF is the most environmental friendly process. In fact some relevant analysis in field have been pointing out that mills which previously ran with TCF and migrated to ECF have reduced significantly their NOx air emissions.