Brown wrapping paper is paper produced by what is called the kraft process from wood pulp. It is very strong and relatively coarse. Kraft paper is usually brown in colour. It is used for brown paper bags, wrapping, envelopes and other packaging. Kraft paper is widely available in both sheet and roll form but rolls are most commonly used.
Kraft paper is manufactured using a technique called the kraft process, kraft pulping or sulfate process and describes the technology for converting wood into wood pulp consisting of almost pure cellulose fibers. The process involves treatment of wood chips with a mixture of sodium hydroxide and sodium sulfide that break the bonds that link lignin to the cellulose. The process name is derived from German word kraft which means strength or power.
The kraft process was developed by Carl Dahl in 1879 and a pulp paper mill using this technology started in Sweden in 1890. The invention of the recovery boiler by G.H. Tomlinson in the early 1930s, was a milestone in the advancement of the kraft process. It enabled the recovery and reuse of the inorganic pulping chemicals such that a kraft mill is almost closed-cycle with respect to inorganic chemicals, apart from those used in the bleaching process. For this reason, in the 1940s, the kraft process surpassed the sulfite process as the dominant method for producing wood pulp.
Woodchips are fed into vessels called digesters that are capable of withstanding high pressures. Some digesters operate in a batch manner and some in a continuous process, such as the Kamyr digester. Digesters producing 1,000 tonnes of pulp per day and more are common with the largest producing more than 3,500 tonnes of pulp per day. Wood chips are impregnated with the cooking liquors. The cooking liquors consist of warm black liquor and white liquor. The warm black liquor is the spent cooking liquor coming from the blowing. White liquor is a mixture of sodium hydroxide and sodium sulfide, produced in the recovery process. In a continuous digester the materials are fed at a rate which allows the pulping reaction to be complete by the time the materials exit the reactor. Typically delignification requires several hours at 130 to 180 °C. Under these conditions lignin and some hemicellulose degrade to give fragments that are soluble in the strongly basic liquid. The solid pulp (about 50% by weight based on the dry wood chips) is collected and washed. At this point the pulp is quite brown and is known as brown stock. The combined liquids, known as black liquor, contain lignin fragments, carbohydrates from the breakdown of hemicellulose, sodium carbonate, sodium sulfate and other inorganic salts.
Kraft paper is nowadays not only used in roll and sheet form but also in paper bags.
About the Author
Kevin Thomas works for Davpack, a uk packaging supplier. Their friendly staff are waiting to help you choose the right packaging for your business.
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