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پاییز  95  برشما عزیزان  تبریک و تهنیت باد



پروکسیلین pyroxylin (سلولز نیترات )
ارسال شده توسط سیدمهدی شمس تاریخ ارسال : شنبه ۱۳۸٧/۱٠/٧

pyroxylin, partially nitrated cellulose (see nitrocellulose ). It is used in lacquers, plastics, and artificial leathers. Pyroxylin lacquers are made by dissolving pyroxylin in a mixture of volatile solvents and adding a plasticizer and a pigment or dye. Pyroxylin plastics are made by colloiding pyroxylin with large amounts of a plasticizer such as camphor; such plastics (e.g., celluloid) are highly flammable. Collodion is a solution of pyroxylin in ether and ethanol.

IIenri Braconnot

discovered in 1832 that nitric acid, when combined with starch or wood fibers, would produce a lightweight combustible explosive material, which he named xyloidine. A few years later in 1838 another French chemist Theophile-Jules Pelouze (teacher of Ascanio Sobrero and Alfred Nobel) treated paper and cardboard in the same way. He obtained a similar material he called nitramidine. Both of these substances were highly unstable, and were not practical explosives.

However, Christian Friedrich Schonbein, a German-Swiss chemist, discovered a more practical solution around 1846. As he was working in the kitchen of his home in BasIc, he spilled a bottle of concentrated nitric acid on the kitchen table. He reached for the nearest cloth, a cotton apron, and wiped it up. He hung the apron on the stove door to dry, and, as soon as it was dry, there was a flash as the apron exploded. His preparation method was the first to be widely imitated - one part of fine cotton wool to be immersed in fifteen parts of an equal blend of sulfuric and nitric acids. After two minutes, the cotton was removed and washed in cold water to set the esterification level and remove all acid residue. It was then slowly dried at a temperature below

1 aacc. Schonbein collaborated with the Frankfurt professor Rudolf Bottger, who had discovered the process independently in the same year. By a strange coincidence, there was even a third chemist, the Braunschweig professor F. J. Otto, who had also produced guncotton in 1846 and was the first to publish the process, much to the disappointment of Schon be in and Bottger. (Itzehoer Wochenblatt, 29 October 1846, columns 1626 f.)

The process uses the nitric acid to convert the cellulose into cellulose nitrite and water:

The sulfuric acid is present, as a catalyst, to protonate the nitric acid to form the nitronium ion.

The power of guncotton made it suitable for blasting. As a projectile driver, it has around six times the gas generation of an equal volume of black powder and produces less smoke and less heating. However the sensitivity of the material during production led the British, Prussians and French to discontinue manufacture within a year.

Jules Verne

viewed the development of guncotton with optimism. He referred to the substance several times in his novels. His adventurers carried firearms employing this substance. The most noteworthy reference is in his From the Earth to the 1\4oon, in which guncotton was used to launch a projectile into space.

Further research indicated that the key was the very careful preparation of the cotton:

Unless it was very well cleaned and dried, it was likely to explode spontaneously. The 

British, led by Frederick Augustus Abel, also developed a much lengthier manufacturing process at the Waltham Abbev Royal Gunpowder Mills, patented in 1865, with the washing and drying times each extended to 48 hours and repeated eight times over. The acid mixture was also changed to two parts sulfuric acid to one part nitric.

Guncotton remained useful only for limited applications. For firearms, a more stable and slower burning mixture would be needed. Guncotton-like preparations were eventually prepared for this role, known at the time as smokeless powder.

Guncotton, dissolved at approximately 25% in acetone, forms a lacquer used in

+ preliminary stages of wood finishing to develop a hard finish with a deep luster. It is normally the first coat applied, sanded, and followed by other coatings that bond to it.

Production

 

Nitrocellulose is made using either concentrated sulfur~/nitric acid or sulfuric acid/potassium nitrate. In general, cotton is used as the cellulose. The cellulose is added to the acid mix to nitrate. After the cellulose has finished nitrating, it is washed and dried. Nitrocellulose is stored wet so it cannot be accidentally lit or explode.

Nitrate film

 

Nitrocellulose was used as the first flexible fi 1m base, beginning with Eastman Kodak products in August, 1889. Camphor is used as plasticizer for nitrocellulose film. It was used until 1933 for X-ray films (where its flammability hazard was most acute) and for motion picture film until 1951. It was replaced by safety fi 1m with an acetate base

    

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