Bleaching plant foliage

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Multi-step bleaching

Individual bleaches have particular characteristics and optimum bleaching efficiency can be obtained by using a sequence of bleaches rather than a single bath. Benefits include less yellowing, less brittleness and lower cost.

Hypochlorite and peroxide are cheap and may remove a considerable amount of colour before causing fibre damage. A follow-up bleach with chlorite or sulphite will give good whiteness at minimal cost.

A three-step process using hypochlorite followed by chlorite followed by peroxide is used to give maximum strength and whiteness to linen, a fabric notoriously difficult to bleach. Since linen is made from plant material, it is most probable that a three-step bleaching process would give similar good results with the whole plant.

Alternating an oxidative bleach with a reductive bleach takes advantage of the differing modes of action of each and results in greater permanence of the final colour.

Types of bleaching products

Hypochlorites (NaOCl, Ca(OCl)2)

Hypochlorite bleaching is usually done cold to minimise cellulose fibre damage. For the same reason, concentrations seldom exceed 4%.

Hypochlorites are available from swimming pool chemical suppliers as either solid calcium hypochlorite or liquid sodium hypochlorite (12% solution). Supermarkets sell formulations of about 5% strength.

Hypochlorites in solution are stabilized at pH 11. They can be used to bleach at this pH but will act relatively slowly (12 to 15 hours). Acid is added to reduce the pH.

Hypochlorites release acid as they decompose so the initial pH of the batch must be set high (usually around 10.0, depending on concentration) to make allowance for the fall in pH during bleaching. Alternatively, a pH buffer such as borax can be used to hold the pH steady. Bleach decomposes too rapidly for maximum efficiency if the bath pH falls below 8.5.

To prevent the alkalinity from damaging plant material, hypochlorite bleaches are usually followed by a weak acid wash.

Sodium chlorite (NaCl02)

Chlorite bleaching is normally done between the temperatures of 50 and 100°C, the optimum being 70°C.

Sodium chlorite is available either as a 40% solution or as a solid in 125kg drums. It is expensive but is the most efficient bleach for lignin without damaging fibre. Chlorite-bleached material is not usually susceptible to yellowing with age. Chlorite is stable above pH 7 and optimum release of bleaching agent occurs between pH 4.5 and 3.5, so acid must be added. Below pH 3.5 the chlorite decomposes too rapidly for efficient bleaching.

Various chemicals (hydroquinone, aldehydes) activate chlorite bleaching power. It is common to impregnate the material with bleach at pH 5 (where chlorite is stable) and then transfer to another tank containing the activator. This practice conserves bleach because only the bleach in contact with the plant material is activated to decompose.

Peroxide (H2O2, NaH2O2 )

Hydrogen peroxide is available commercially as a 50% solution. Typically, peroxide will remove about half the lignin before cellulose fibres are damaged, hence it is an ideal first bleach in a multi-step process. However, a light peroxide bleach is often used after chlorite bleaching to increase resistance to yellowing.

Peroxides are extremely sensitive to trace metal contamination, which causes rapid decomposition. Add sodium silicate (1%) to chelate any metal and to adjust the pH to the correct value. Peroxide bleaches best at pH 10.5. Above this point it becomes unstable and decomposes too rapidly, whereas a lower pH results in a slow rate of bleaching.

Hydrosulphites (NaS2O4, ZnS2O4)

Hydrosulphite is widely used for paper bleaching. It is available commercially as either sodium or zinc hydrosulphite (also sold as dithionite) and is a cheap but smelly reductive bleach. Hydrosulphite has maximum bleaching power at pH 5.5 to 6.0. It is stable in alkaline solution. Hydrosulphite can be made in situ by alkaline treatment of sulphur dioxide gas.

Borohydride (NaBH4)

Borohydrides are not presently used for commercial bleaching because of their high cost, although they may have specific applications in the future.

Control of yellowing

The extent of yellowing upon aging of bleached material depends on the thoroughness of the original bleaching regime. Generally, multi-step bleaching especially when it alternates an oxidative bleach with a reductive bleach, creates a product that yellows much less than a product from a single-stage bleaching.

A final wash in a 2% solution of barium hydroxide, calcium hydroxide, sodium bicarbonate or aluminium sulphate prevents yellowing.

Incorporation of optical brighteners (from dye manufacturers) or pigments, such as titanium dioxide or zinc oxide, 'masks' a slightly yellow product.