The present invention relates to paper production and, in particular, to the production of cellulose pulp for use in papermaking.

Nowadays, paper production is based fundamentally upon the use of plants and trees as the source of fibrous raw materials and upon the processing of recycled paper materials, whereas the use of rags, which for centuries were used as the basic raw material, has now in practice become obsolete.

Fibrous materials means vegetable fibres, that is, the thread-like cells constituting the body of the toughest plant tissues which are destined to form the framework of a future sheet of paper. The criterion by which the suitability of a plant or tree for the production of fibrous material for paper may be judged is very complex and has to take account of both quantitative and qualitative parameters, the most important of which is the amount of cellulose it contains. Moreover, extraction should be easy, the fibres should have suitable technical characteristics and the plant or tree should not have intense coloration or unpleasant odours; amongst the technical characteristics, the tensile index, the tear factor, the bursting index and the freeness are of fundamental importance.

Plants and trees used for the production of paper and paper materials can be divided into three main, large categories:

• 1. Conifers: these include various types, amongst which the best known and the most used are the genus pinus (pine) and the genus abies (fir); conifer wood contains approximately 50% cellulose, 30% lignin, 15% hemicellulose, and 5% numerous components such as resin, terpene compounds, fatty acids, etc.; conifers are the trees which are used most and which are most suitable for paper production, providing the best "technical cellulose", which is characterized by fibres several millimetres long which impart considerable mechanical strength to the sheet and have a considerable tendency to form fibrils so that they impart considerable compactness and homogeneity to the future sheet of paper.
• 2. Broad-leaved trees: until about thirty years ago, these were considered by paper-makers as secondary to conifers since the papers thus produced were not distinguished by good mechanical strength; only recently, as a result of improvements in working techniques, have broad-leaved trees started to be used in the papermaking industry and, currently, they constitute one of the main sources of cellulose.
  The broad-leaved trees normally used in the papermaking industry are: poplar, birch, chestnut and beech.
• 3. Annual plants: annual plants, amongst which straw has mainly been considered in the past (today, straw seems to have been practically abandoned) and which include plants such as, for example, kenaf, bamboo, flax and hemp, generally have a lower cellulose content than the woods of coniferous and broad-leaved trees, in most cases, no more than 35%. Annual plants are actually considered a poor substitute for wood as far as their use in the papermaking industry is concerned; this is due not only to their low cellulose content but, in particular, to problems connected with harvesting difficulties and transportation costs and, moreover, with the fact that the papers produced therefrom are distinguished by particularly poor technical characteristics.

However, continual and incessant usage of plurannual woody plants and trees such as coniferous and broad-leaved trees is causing increasing environmental problems since, although deforestation is carried out rationally and systematically, it now greatly exceeds the spontaneous increase in the sprouting-up and development of trees, which is leading to alteration of the ecosystem not only in the regions affected directly but also in the entire planet. In order to solve this worrying and disturbing problem, counter-measures of various kinds have been adopted; whereas, on the one hand, rational planting is continuously carried out on deforested land and on other, suitably prepared land, on the other hand, efforts are being made by means of suitable initiatives such as, for example, advertising campaigns and the like, to sensitize public opinion towards a usage of paper and of cellulose derivatives more generally, aimed at and limited to the absolutely necessary, and towards an increasing use of "recycled paper" and consequent differentiated waste disposal so as to enable paper to be recycled conveniently and economically. In addition, research is now directed ever more towards the establishment of working techniques which can increase yields and thus reduce wood consumption and towards the finding of alternative sources of cellulose to plurannual plants and trees. However, these factors have to take account of the increasing requirement for "highly technical" papers which anyway makes it practically impossible to use annual plants which, precisely because of their annual or even seasonal cyclicity, could constitute an optimal solution to the problem.

It has now surprisingly been found and constitutes the subject of the present invention that, the processing of the annual plant commonly known as Luffa cylindrica (or Luffa aegyptiaca) produces a cellulose pulp with analytical characteristics decidedly better than those of normal annual plants and, in some aspects, even better than those of coniferous and broad-leaved trees, rendering its use in the paper industry particularly advantageous.

Luffa cylindrica is an annual herbaceous plant belonging to the Cucurbitaceae family; Luffa is a creeping, climbing, monoecious plant with unisexual flowers and with stems which may reach lengths of 4-8 m; the leaves are petiolate, simple, alternate, from tri- to hepta-lobate, and 10-15 cm long; the flowers are conspicuous, large (5-10 cm) with a corolla formed by five yellow petals. The fruit is a pepo, dry when mature, cylindrical or only slightly angular, of very variable dimensions (typically 30-60 cm long and 10-20 cm diameter), and straight or curved; when mature, it has a dry and papery epicarp with a fibrous and spongy endocarp owing to the disappearance of the pulp and the persistence solely of the fibro-vascular bundles. A fruit contains on average 200-500 seeds about 10-15 mm long and each weighing about 0.1g; the fibrous reticulum of the mature fruit is composed of cellulose (60%), hemicellulose (29%) and lignin (11%). Luffa is an annual plant with a relatively long cycle; in a mediterranean habitat, from four to five months elapse from germination to the maturation of the fruits, with a first flowering in July and a second flowering in August.

The warmth requirements of Luffa are high: a lethal minimum of 0-2°C, minimum and maximum temperatures for growth of 12-14 and 20-35°C, respectively, with an optimum situated at around 25°C; the water content is 8.12% at 105°C. Luffa is widespread in tropical and subtropical regions where it is generally the subject of domestic horticultural cultivation. The Luffa fruit, dried after removal of the epicarp and the seeds, is commonly sold as a body sponge in shops specializing in natural body-care products, department stores, perfumeries, etc. One of the most interesting mechanical characteristics of Luffa fibres is in fact their so-called "shape memory" owing to which the fruit can be compressed up to 10-20 times and then returns to its original shape and size if immersed in water or subjected to the effect of steam. The Luffa fruit is also used industrially, practically unchanged, for producing thermally-insulating and sound-absorbent panels, packaging and padding materials, and arch-supports for footwear.

Like all fibres of vegetable origin used in papermaking, Luffa has to be subjected to processing with the object of liberating the fibres from the useless and harmful components, that is, to the processing which, in the papermaking field, takes the name of digestion when chemical agents are used or, on the other hand, grinding when the processing is mechanical. A digestion process is described below, by way of a non-limiting example.

The epicarp and the seeds were removed from the fruit which was then dried, after which it was subjected to a soda digestion process normally used for annual plants, with the use of a discontinuous boiler with a capacity of 10 litres, with manual stirring, under the following conditions: a) Reagents NaOH 18% Concentration of fibre in the boiler 8.5% N.B. all of the values relate to the dry fibre. b) Operative conditions Preheating (20-110°C) 20 min Impregnation 20 min Heating (100-170°C) 30 min Digestion 300 min Operating pressure 6 kg/cm²

Upon completion of the digestion, the liquor was separated for the analysis and the cellulose pulp was washed with running water to eliminate as much of the exhausted liquor as possible. With regard to the purification of the fibre from undigested matter or the like, this was not carried out since the presence of undigested matter or knots was negligible. The fibre yield was 57.82% which itself is an astounding parameter placing Luffa at the same level as a conifer (the fibre yield of a conifer is normally 50-60%, whereas that of an annual plant is 15-20%).

A simplified beating curve was then performed on the crude cellulose pulp in order to assess the behaviour of the fibre and the curves of the physical-mechanical characteristics upon beating. Moreover, a typical bleaching process was carried out on a portion of beaten pulp in two successive stages which, with regard to the reagents used, are represented below:

Chemical, physical and mechanical determinations were carried out on the crude, beaten and bleached cellulose pulp by means of the apparatus listed below: Apparatus used Analytical parameter FRANK dynamometer breaking index KORPUT burst meter bursting index ELMENDORF tear tester tear factor ELREPHO white brightness RIETH hollander beating curve TONIOLO sheet form analysis slips

The methods used were the ATICELCA methods and, where these were lacking, the SCAN methods were used.

The results obtained are given in Table 1 and are compared with those relating to a common annual plant (in this particular case, sorgum): Crude Luffa Bleached Luffa Crude Annual Bleached Annual Degree of beating (°S.R.) 18 untr.* 37 37 34 untr. 45 34 untr 45 Tensile index (kNm/kg) 44.1 56.0 64.3 54.65 68.2 48.8 65.5 Bursting index (MN/KG) 4.84 5.05 5.06 3.38 3.77 2.60 3.86 Tear factor (Nm²/kg) 12.12 10.7 10.2 5.2 4.2 - 3.60 Porosity (m"/100cc) 3 16 15 80 540 62 780 Ash % 2.4 - - 4-6 - - - Brightness G.E. 48 40.5 71.5 45.5 - 73 - *untr. = untreated cellulose pulp, not beaten.

As can be seen from the data relating to the degree of beating, the freeness of the pulp produced from Luffa is unexpectedly better than that of the best annual plants and similar to that of a broad-leaved tree. Similarly, the bursting index and the tear factor are clearly better than those of the most common annual plants, whereas the silica content is a great deal lower, as can be seen from the different percentages of ash; only with regard to the tensile index can Luffa be considered equivalent to a good annual plant.

The data given above are also confirmed by the images of Figures 1, 2, 3 and 4 which represent, respectively, the fibres (enlarged one hundred times) of crude Luffa cylindrica, of beaten Luffa cylindrica, of a coniferous-broad-leaved mixture (fir-beech), and of an annual plant (sorgum), from which the similarity between the fibres of Luffa cylindrica and those of the coniferous-broad-leaved mixture is clear.

The results relating to the use of Luffa are also particularly advantageous with regard to the ecological balance of the process, as can be seen from the data given below: Ecological Balance 1) Exhausted liquor C.O.D.: 20400 ppm C.O.D. (kg per ton of pulp): 10.5 Ratio C.O.D./organic matter: 435 2) Bleach C.O.D. (kg per ton of bleached pulp - 2-stages): 60.2

In fact, the C.O.D. was in practice less than 50% of that normally necessary as far as annual plants are concerned, with a consequent reduction in disposal costs and, in particular, in possible ecological damage.

On the basis of these considerations, Luffa cylindrica is clearly particularly suitable as a starting material for methods for the production of cellulose pulp for use in papermaking; these methods need not be limited solely to digestion processes with soda but may also include all methods in which a vegetable starting material is reduced to fibres or fibre fragments.