PROCESS FOR OBTAINING NITROCELLULOSE, PROCESS FOR NITRATING WOOD CELLULOSE TO OBTAIN NITROCELLULOSE, AND NITROCELLULOSE OBTAINED FROM SUCH PROCESS

The present invention relates to the process for obtaining nitrocellulose from cellulose short fibers. Nitrocellulose is a product employed as resin in formulating inks, being responsible for the properties that deal with the transfer and formation of film formed. It is also largely used in the segments of automotive repainting, sealers, and wood finish, printing inks using rotogravure and flexography, nail enamels and leather finishes only as illustration for some applications.

Prior Art

Nitrocellulose is available for the purpose of civil use in two types: High Nitration or HN (soluble in esters), and Low Nitration or LN (soluble in alcohols). It is normally used in conjunction with other resins. The product is environmentally correct. The main raw-material, cellulose is derived from renewable sources: wood and cotton. The production of nitrocellulose does not emanate toxic byproducts that degrade the environment or the ozone layer, it is non-toxic, biodegradable, and allows for the formulation of inks and varnishes that meet the U.S. legislation applicable to VOC (Volatile Organic Content).

The most common industrial cellulose sources are wood and cotton. Cotton fiber is probably the richest natural cellulose source having more than 95%. In wood, agricultural waste, foliage, and others, cellulose is associated with other substances, such as lignin and hemicellulose, both in substantial quantities [NEVES, J. M. O Papel, dezembro, (1985); OLIVEIRA, E. R. Alcool e Agύcar, 4, 10, (1982)].

Currently, the chemicals derived from cellulose are produced from the cellulose originated from the linter found in cotton and wood, such as the Pinus, with long fibers. The use of these raw-materials has contributed significantly with the high industrial processing costs resulting from the high commercial value of the linter and of the complex wood processing.

Review of the conventional manufacturing process

Cellulose is a straight-line polymer having a high molecular weight with a repeating unit β-D-glucose (C6H10O5)n along its chain, having from 1 ,500 to 10,000 units or more, and a molecular weight up to 300,000.

The cellulose structure is formed by joining β-glucose molecules through β-

1.4-glucosidic bonds, which makes it water insoluble. It is a hexosan formed by hydrolysis of glucose. Cellulose is a long-chain polymer having a variable molecular weight, with an empiric formula (C6H1005)n, and minimum value of n=200.

Cellulose has a linear or fibrous structure, in which multiple hydrogen bridges are formed between the hydroxyl groups of the different juxtaposed glucose chains, making it waterproof and originating compact fibers that constitute the cell wall of vegetables.

Use and applications

Industrially, cellulose is extracted from the wood of trees such as pine (Pinus), Eucalyptus, or other herbaceous plants with large quantities of cellulose in their stem, such as sugarcane, various grasses and blackgrasses. Pure cotton is formed by 99.8% of cellulose. Other textile fibers, such as jute, hemp, ramie, and linen have also a large ratio of this polysaccharide. Depending on the type of species, the cellulose will either have long or short fiber. This feature makes the paper more absorptive or more resistance, respectively.

Long fiber cellulose has the property of providing the paper with mechanical strength, a feature necessary in transportation and distribution packages, such as corrugated cardboard boxes and multi-foliated bags, while the short fiber cellulose provides a paper with a best formation, more suitable when an excellent printing surface is required and for lamination purposes.

When the Eucalyptus fibers appeared in the market over thirty years ago, paper manufacturers verified that this new cellulose extracted from such a noble wood was very special.

This is a short fiber. In fact, it is the shortest fiber among the species of hardwoods found all over the world.

Depending on the manner of calculation and weighting (through optical microscopy or optical-electronic devices), its mean length can be of only 0.65 mm. Fibers of birch, poplar, beech, and oak, for instance, are 15% to 40% larger. The fibers of conifers have mean length much above the minimum of 2 mm.

The grain size distribution is reduced; again, the fiber has the smallest grain size distribution among the fibers most commonly found in the cellulose market. The grain size distribution is equal to the weight of the fiber divided by its length. For the sake of example, the thinner the fibers, the smaller the grain size distribution.

The number of fibers per gram is high. This occurs due to the fact of being a short fiber with small grain size distribution. The values commonly found for the Eucalyptus species are about 20 million. For instance, the pine tree found in the Southern United States is in the opposite end, with 1 million of fibers. Its internal architecture is also different: the basic components of the fiber's wall, also called microfibrils, show a small angulation around the fiber's axle in comparison with other hardwoods.

Consequently, the Eucalyptus fiber is rigid, providing a strong paper structure, highly opaque.

The characteristics of the paper, carton, and cardboard do not depend only on the type of the fiber used, but also on its manufacturing, additivation, and finishing processes, as represented by SCHEME 1 , as follows:

SCHEME 1

Each β-D-glucose unit contains a primary hydroxyl and two secondary hydroxyls playing an important role in chemical conversion from cellulose into nitrocellulose. These functional groups react partially with nitric acid forming the

nitrocellulose. The primary hydroxyl groups show more reactivity than the secondary ones, being the first to react during the nitration. FORMULA 1 below represents structural formula for cellulose:

FORMULA 1 - Cellulose Molecule

Cellulose is nitrated through immersion with a sulphonitric mixture, originating the nitrocellulose. FORMULA 2 below represents its structural formula:

FORMULA 2 - Nitrocellulose Molecule

The manufacturing process consists of the purification of the raw linter for obtaining cellulose, nitration of the cellulose with sulphonitric mixture, stabilization and boiling for the extraction of residual acid occluded in the NC fibers. Industrial Process:

There are three hydroxyl groups per fundamental unit of β-Dglucose unit, which can be esterified by nitric acid represented by the following reaction:

C6H7O2(OH)3 + 3HNO3 + H2SO4 -» C6H7(ONO2)3 + 3H2O + H2SO4

It is possible to reach a theoretical level of Nitrogen near 13.5%. This is the highest level found among the commercially obtained products.

The cellulose prepared is stored in silos for supply and transported to a scale. After weighted, the cellulose is placed into the nitrator with the sulphonitric mixture (SNM), having a composition of approximately 63% of sulphuric acid, 21% of nitric acid, and 16% of water. This SNM is cooled in heat exchangers, hull-tube type at a temperature next to 30 ⁰ C. The nitration reaction occurs at this temperature for approximately 25 minutes. The content of the nitrator is unloaded into a centrifuge, where the residual acids are separated from the cellulose nitrate (CN).

In the specific case of wood, the size of the fiber shall also be taken into account, since once the content of the nitrator is unloaded into a centrifuge, and due to the reduced size of the fiber, it makes it difficult to separate the CN from the residual acids, thus significantly reducing the efficiency of the process. The acids used are partially concentrated for reutilization, or are sent for denitrification and concentration of the sulphuric acid.

The nitrated product goes to the depressured reboilers, where water is added to remove the acid. In these reboilers, depending on the nitration level, the product can sit from 2 hours (low nitration) to 70 hours (high nitration). The temperature inside the reboilers is around 97 ⁰ C.

Every nitration reaction leads to a sulphonation, reason why it is necessary to remove the sulphonated products. The nitrocellulose found in the depressured reboilers is sent to the autoclaves, where such sulphonated products are removed to ensure the stability thereof. This autoclave works at high temperatures (around 142 ⁰ C) and at pressures above atmospheric pressure. Hydrolysis also occurs to reduce the molecular weight of cellulose. After the removal of the sulphonated products, there is still a little nitric acid that needs a new washing process, which can be performed hot, cold, or alkaline (using sodium hydroxide, or sodium or calcium carbonate) at ambient pressure. Alkaline washing is done to the nitrocellulose that will be used in the manufacturing process of powder. For the production of colloid, cold water is used to remove the acidity. After this stage, the product is then submitted to a centrifuge with a 100-mesh sieve, where the final humidity control will take place (approximately 28%).

The U.S. Document 2,517,918 relates to the nitrocellulose manufacturing process comprising of granulating the sheets or coils (board) of non-disintegrated cellulose paste and nitration thereof with mixture of acidic composition between 40

and 80% of nitric acid, in conjunction with sulphuric acid and water, as to avoid gelatinization of the paste. The cellulose sheets or coils are subdivided into pieces with approximately one-eight of square inch, with density not smaller than 240 grams per liter and not greater than 360 grams per liter. The U.S. Document 2,554,179 relates to the process for improving nitrocellulose's solubility property in nitroglycerin, under which occurs the washing process of nitrocellulose using non-wetting agent, such as oils, for instance, of rosin, schist, castor oil, among others.

The U.S. Document 2,822,362 relates to the manufacturing process of nitrocellulose from sheets or coils of wood pulp sulfite subdivided into small pieces, preferably of cotton linters.

The U.S. Document 2,517,918 relates to an apparatus for washing and centrifuging nitrocellulose after its obtainment.

Objectives or Advantages of the Invention

The invention relates to a process for obtaining nitrocellulose from

Eucalyptus 100% alpha-cellulose, for it regards to the finest of short fibers, in addition to the following factors:

• Excellent physical-chemical properties, allied to the facility of refinement, further allowing to develop the final properties desired, with reduction of the long fiber content and more power economy during the refinement process;

• Excellent physical-mechanical properties, allied to the facility of refinement;

• Lower cost in comparison to that of the long fiber, as the source of fibrous raw-material.

Brief Description of the Invention

The present invention relates to a process for obtaining cellulose, as well as the product obtained employing short fibers with mean length equal to or shorter than 0.85 mm, originating from subdivided coils or sheets of alpha-cellulose, whose coils or sheets present density between 0.7 and 1.0 g/cm3 and viscosity above 300 cP, which react with water, sulphuric acid, and nitric acid (sulphonitric mixture or SNM) in a ratio of sulphonitric mixture/cellulose mass varying between 1 :7 and 1 :45.

Detailed Description of the Invention

The traditional process uses cellulose from cotton fibers and long wood fiber, but the use of cellulose from 100% short-fiber wood for nitration is inexistent from the industrial point-of-view. Furthermore, the size of the short fiber generates losses in the traditional process, which makes its use unfeasible and limited as a blend for small quantities.

First of all, in the processes of flocked or ground cellulose, the size of the fiber practicably makes it impossible to separate the nitrocellulose formed from the residual acid, which leads to substantial losses. In the case of cellulose agglutinated in small pieces, the absorption of acid is slower than that occurred in long fiber cellulose, in a manner that it only allows for good results in conditions strictly controlled, with thicknesses smaller than 0.850 mm.

Thus, the testing carried out with temperature of reaction between 45 ⁰ C and 52 ⁰ C and reaction time between 40 and 70 minutes showed partial conversion of cellulose into nitrocellulose for thicknesses greater than 0.850 mm.

The present process provides a means for using short fiber wood, in the case Eucalyptus Cellulose, through the treatment thereof with a mixture of nitric and sulphuric acids, transforming it into cellulose nitrate. The cellulose of the wood used is supplied in coils with density between 0.7 and 1.0 g/cm3, preferably 0.85 g/cm3, and viscosity above 300 cP. The Sulphonitric Mixture used has density between 1.5 and 1.8 g/cm3.

The composition for the initial sulphonitric mixture is represented in TABLE 1 below:

TABLE 1

The mixture range described above allows for the production of nitrocellulose with Nitrogen content varying between 10.7 and 13.70%. The ratio of sulphonitric mixture/cellulose in mass varies between 7 and 45. Due to the compacting level and high density of cellulose, it is possible to use a low ratio. Initially the cellulose coil is cut in the shape of different geometric figures, or cut in pieces with different shapes, with width and length between 3 mm and 20 mm, and thickness equal to or smaller than 1 mm, preferably rectangles or squares. For safety reasons, all the powder is removed before the nitration, since the presence of nitrous vapors in the lower atmosphere of the reactor could react with the cellulose powder, thus causing a violent deflagration. Therefore, the reactor shall count on a control of the limit of atmospheric explosiveness. A vent can be provided as to keep the reactor at constant pressure and to remove nitrous vapors at the end of the process.

The risk of explosion is minimized in the case of inertization of the reactor's internal atmosphere, case in which the use of Nitrogen or carbonic dioxide is recommended.

The pressurization of the reactor with the use, or not, of inert gas reduces the release of nitrous vapors and can be implemented.

The ratio of the sulphonitric mixture is also associated with the elevation of temperature, i.e., the smaller the ratio, the greater will be the elevation of temperature in the mean of reaction. The project for the reactor shall include this exhaustion of energy as to keep the control of temperature.

The sulphonitric mixture can be pre-heated or not, and its exothermicity shall be taken into account for the purposes of the temperature of reaction. The cellulose shall be added to the sulphonitric version, and not the opposite, as to avoid the possibility of both fire and explosion. Pieces containing alpha-cellulose shall be added to the sulphonitric mixture.

The agitation process is extremely important in the beginning of the reaction, since the cellulose tends to float in the mixture of reaction due to the difference of density. If this occurs, i.e., if the nitrocellulose eventually floats, it may be subject to fire, with the possibility of explosion of the reactor.

Due to the difference of density between the cellulose and the sulphonitric mixture, it is necessary to take special care with the agitation system. However, since the cellulose is less mechanically resistant than the nitrocellulose, caution is recommended in the type of agitation system, since the impact of the blades with

the nitrocellulose could generate a large amount of small fragments, difficult to process, thus resulting losses in the process.

Temperatures above 55 ⁰ C allow for a possible violent decomposition of the nitrocellulose in the reactor, provoking fire and eventually the explosion thereof. Therefore, the temperature during the nitration reaction shall preferably be equal to or lower than 55 ⁰ C.

It is recommended the use of a breaking seal for the exhaustion of increase of the internal pressure.

The reactor can be cooled through an internal coil, by jacketing, or with the addition of a colder sulphonitric mixture.

The removal of nitrocellulose from the reactor at temperatures above 4O ⁰ C, followed by rinsing with cold water, will cause the denitration of the internal fibers. The removal of the product obtained shall preferably occur at temperature equal to or lower than 4O ⁰ C. The water content of the cellulose shall be below 11 % in order to avoid an internal disorder in the reactor's temperature.

The material of the reactor shall be resistant to corrosion by the sulphonitric mixture, being recommended stainless steel or glass.

In order to best explain the scope of the invention the following examples are provided, which shall not be taken for limitative purposes of the invention.

Example 1 :

Synthesis of nitrocellulose using 1 ,000 g of Eucalyptus cellulose cut in cubes with approximately 5 mm x 5 mm x 0.78 mm of dimensions, presenting

90.93% of dry mass of alpha-cellulose.

The nitration reaction occurred in a 20-liter internal volume reactor with agitation system. The sulphonitric mixture used presented the mass composition indicated below:

Mass of sulphuric acid 98% 4,029 g

Mass of nitric acid 98% 5,817 g

Water 1 ,122 g

The initial internal temperature of the reactor was 29 ⁰ C. The fragments of cellulose described above have been introduced for 22 minutes, obtaining a final temperature equivalent to 37.O ⁰ C.

The mass of reaction inside the reactor was then heated for 34 minutes until it reached the temperature of 5O ⁰ C. The reaction lasted approximately 55 minutes and the temperature was kept constant at 5O ⁰ C, with 0.7 ⁰ C to more or less. After the alpha-cellulose nitration reaction, the temperature of the product obtained inside the reactor was lowered from 50.5 ⁰ C to 37.O ⁰ C, which took approximately 10 minutes to occur. The end product after the cooling was rinsed in water for 1 hour, until it presented pH 1.5, then boiled in water for 1 hour.

The nitrocellulose obtained presented excellent properties, with a final Nitrogen content of 12.15%, with 0.15% to more or less in mass in relation to the total mass of the product obtained, and final nitrocellulose mass corresponding to 1 ,345 g.

Example 2:

Synthesis of nitrocellulose using 1 ,200 g of Eucalyptus cellulose cut in cubes with approximately 5 mm x 5 mm x 0.78 mm of dimensions, presenting 90.93% of dry mass of alpha-cellulose.

The nitration reaction occurred in a 20-liter internal volume reactor with agitation system. The sulphonitric mixture used presented the mass composition indicated below:

Mass of the Sulphonitric mixture: 10,994 g Composition of the Sulphonitric mixture:

36.18% Sulphuric Acid

50.32% Nitric Acid

0.87% Nitrous Acid

13.50% Water

The initial internal temperature of the reactor was 29 ⁰ C. The fragments of cellulose described above have been introduced for 16 minutes, obtaining a final temperature equivalent to 36.2 ⁰ C.

The mass of reaction inside the reactor was then heated for 27 minutes until it reached the temperature of 5O ⁰ C. The reaction lasted approximately 45 minutes and the temperature was kept constant at 5O ⁰ C, with 0.7 ⁰ C to more or less. After the alpha-cellulose nitration reaction, the temperature of the product obtained inside the reactor was lowered from 50.1 ⁰ C to 36.0 ⁰ C, which took approximately 55 minutes to occur. The end product after the cooling was rinsed in water for 1 hour, until it presented pH 1.5, then boiled in water for 3 hours.

The nitrocellulose obtained presented excellent properties, with a final Nitrogen content of 11.21%, with 0.15% to more or less in mass in relation to the total mass of the product obtained, and final nitrocellulose mass corresponding to 1 ,612 g.