The present invention relates to monolithic porous silico-calcium masses for gas vessels. In particular, the invention relates to monolithic porous masses free from asbestos for acetylene vessels, to a process for the preparation of said masses and to vessels, in par¬ ticular cylinders, containing said masses.

It has long been known that acetylene, due to its thermodynamic characteristics which make it unstable under certain conditions of temperature and pressure, is stored and sold in cylinders, dissolved in particu¬ lar solvents, usually acetone or dimethylformamide. In fact, the high solubility of said gas in the above men¬ tioned solvents allows its handling with high safety. Acetone, or dimethylformamide, or solvents with similar properties, are normally absorbed on porous masses ha¬ ving a monolithic structure which completely fill the acetylene cylinder. One of the main purposes of the po¬ rous mass consists in stopping possible flashbacks which can develop downstream the cylinder and prevent decomposition phenomena of acetylene, initiated by ex¬ ternal causes, such as localized overheating or others, inside the cylinder itself.

It is therefore of particular importance that said filling material be a thermal insulator without voids, cavities, interstices, which could initiate dangerous decomposition phenomena. On the other hand said mass must have high porosity in order to facilitate the ab¬ sorption and the subsequent release of acetylene when it is being used. The material must moreover be incom-

bustible, inert towards acetylene and its solvents, and light-weight, and with special mechanical strength pro¬ perties, which make it almost unalterable, namely not prone to fissuring and disintegrating to powder over the course of years.

Very good results have been obtained with masses having silico-calcium composition, claimed in different patents, with the addition of binders, among which asbestos being particularly preferred, which impart to the whole a support skeleton with the twofold purpose of improving the strength and plasticity of the final mass and of maintaining the various constituents su¬ spended in the fluid mixture, from the stage of prepa¬ ration of the mixture till the stage of setting in the subsequent oven-baking stage.

The established toxicity and cancerogenic proper¬ ties of asbestos has made necessary replacing it with other fibrous materials which exert the function of binder among the different components of the structural supporting mass as to give the mass itself the neces¬ sary mechanical characteristics.

Patent specification EP 0 056 645, in the name of N.I. Industries, describes an asbestos-free porous mass for acetylene cylinders, wherein asbestos is replaced by alkali-resistant glass fibres.

Patent specification EP 0 064 916, in the name of L'Air Liquide, describes asbestos-free porous masses wherein the problem of the stabilization of the mass in the lack of asbestos is overcome by using ultrafine synthetic silica. The toughness of the mass is further improved by adding non-reactive synthetic fibres of mi-

neral or organic origin, such as aluminium silicate, or carbon fibres or aramidic fibres.

EP 0 262 031, in the name of L'Air Liquide, tea¬ ches the use of treated cellulose fibres, instead of the costly glass fibres.

US 4 765 458, in the name of N.I. Industries, claims an asbestos-free mass containing refractory car¬ bon fibres as reinforcing material.

US 4 970 246, in the name of Coyne Cylinder Com- pany, teaches the use of artificial fibres consisting of a mixture of aramidic and oxidized polyacrylonitrile fibres.

As above mentioned, the filling porous mass for gas vessels, and in particular for acetylene cylinders, must satisfy certain requirements, such as chemical inertness, structural stability and strength, as widely discussed in the prior art. Other than the mechanical characteristics, such as uniform pore distribution, lack of fractures, minimal volume shrinkage, the mass must also be endowed with low thermal conductivity, such as to stop possible flashbacks which can develop downstream the cylinder during gas release. The func¬ tion of thermal insulator of the mass is also to pre¬ vent decomposition phenomena of acetylene inside the cylinder itself, due to, for example, localized overhe¬ ating, facilitated by the formation of thermal bridges, because of the different thermal conductivity of the various components of the mass. To this end the rein¬ forcing fibre has a critical role in giving the porous mass the necessary requirements.

It has now been found that the use of carbon fi-

bres, endowed with particular physico-chemical charac¬ teristics, as reinforcing fibres in the preparation of the porous mass, allows to obtain masses having advan¬ tageous structural characteristics in terms of mechani- cal strength, low thermal and electrical conductivity.

It is an object of the present invention a mono¬ lithic porous mass free from asbestos containing synthetic inorganic carbon fibres as binder, characte¬ rized in that said fibres have low thermal conducti- vity, hydrophilic characteristics and have rough sur¬ face. In particular, the fibres according to the inven¬ tion show: a) a carbon content between 80 and 93% w/w; b) thermal conductivity between 1 and 7 W/m. ^β K; c) hydrophilic character; and d) a surface which presents a particular rough pattern.

The dimension of the fibres is not critical for the scope of the present invention. In a preferred em¬ bodiment of the present invention, the fibres have a diameter between 3 and 12 micron and length between 2 and 20 mm, although good results may be obtained also with fibres having different diameter and length from those above indicated.

Carbon fibre content is equal or less than 12% by weight based on the dry porous mass.

The low thermal conductivity of carbon fibres uti¬ lized according to the present invention gives the mass a higher thermal homogeneity, since local heat conduc¬ tion and creation of thermal bridges are avoided. For comparison purposes, graphite carbon fibres have thermal conductivity higher than 20 W/m.°K.

The particular surface rough pattern of the fibres utilized according to the present invention allows a better clasping with the mass at the end of the drying stage, therefore increasing its strength. One of the problems which normally is encountered during the preparation of the wet mixture of the porous material relates to the dispersion and homogenization of the fibres in the silico-calcium mass. The skilled technician is well aware of the difficulties encounte- red when asbestos is substituted with other fibres such as for example glass fibres. Water repellent property of glass fibres inevitably leads to the agglomeration of the fibres during the mixing stage of the various components. A non homogeneous mass, wherein fibres are gathered in "tufts" broken up by spaces without fibres, is obtained, therefore, due to external stresses, dan¬ gerous crackings can form in the time inside the mass itself.

The carbon fibre utilized in the present invention has marked hydrophilic characteristics so that, when added into water, it uniformly disperses and this uni¬ formity remains until the completion of the process.

The preparation of the mass can be carried out ac¬ cording to conventional methods described in the prior art, as for example in the above cited patents.

According to the invention, the porous mass is prepared by mixing lime in water (slaking the lime) and, under stirring, adding inorganic synthetic carbon fibres, thickening agent, if any, and silica dioxide, preferably crystalline quartz powder.

Once it has become homogeneous, the mixture is lo-

aded into the cylinders, then subjected to baking and final drying.

In certain preferred embodiments of the invention, the ratios of the ingredients are 1:1 - 1:1.3 in re- spect of calcium oxide and quartz powder.

The above cited ratios do not place limits, and a different combination of ingredients does not fall out¬ side the scope of the present invention.

The dispersion and homogenization of the fibres in the mixture can be facilitated by optionally using di¬ spersing and thickening agents as additives. Examples of said additives are derivatives of the cellulose, such as carboxymethylcellulose or hydroxypropylcellu- lose, polyesters, such as polyethylene glycols. Additi- ves of the inorganic kind having thixotropic effect, which achieve the same result, can also be suggested.

In certain cases, particular thixotropic substan¬ ces could suitably be used. Satisfying results are ob¬ tained with mineral substances, such as colloidal ma- gnesium aluminosilicate, which, after been suitably di¬ spersed in water, form a gel, thus providing very good thickening properties. Their percent amount, as calcu¬ lated on the final dry product , is generally lower than 13% w/w. The so obtained thickening avoids the separation of the components of the mixture and assures a homoge¬ neous final composition of the porous mass.

These products, which essentially consist of hy- drated magnesium aluminosilicates, have a composition which, in some aspects, is comparable with the one of silico-calcium mass and well amalgamate with the mass

itself, giving homogeneous mixtures, which after baking, reach high consistency.

Compressive strength tests, carried out on speci¬ mens of the porous mass, prepared according to the pre- sent invention with the above defined carbon fibres, gave particularly high values, to the order of 35-50

2 kg/cm . These values are hardly reached by other porous masses of similar constitution, but with different fi¬ bres as binder. A typical preparation process according to the in¬ vention comprises the steps of: a) treatment of calcium oxide with water, at a tempe¬ rature between 20 and 60°C; b) addition of synthetic inorganic carbon fibres and subsequent homogenization; c) optional addition of a viscosity stabilizing agent; d) addition of crystalline quartz powder; e) stirring till thorough mixing; f) filling the cylinders with said wet mixture; g) baking the mass contained in the cylinders at 170-

200 ^β C under vapour pressure (autoclaving) ; h) final drying at 280°C; wherein the steps b) , c) and d) can be carried out also in different sequence.

The following examples further illustrate the in¬ vention. Example 1

In a mixer provided with a multi-speed helical stirrer kg 12.7 of quicklime, with a size lower than 6 mm, were slaked in 70 kg of water at a temperature of

20°C, with slow stirring. After 20 minutes other 30 kg of water were added and hydration was continued for a further hour.

Thereafter 1.2 kg of inorganic synthetic carbon

(τ>\ fibres Carbocem by RK Fibres Ltd, having fibre len¬ gth of 3 mm were added slowly and at small portions, and the mixture was homogenised for 15 minutes at hi¬ gher speed. Then the mixture was completed by adding, constantly under stirring, 100 g of an additive to uni- form the viscosity and 14.8 kg of crystalline quartz powder.

After a further two hours, the very fluid and ho¬ mogeneous mixture could be introduced directly into the cylinders which were immediately submitted to the sub- sequent baking stage under pressure, at a temperature of 180°C (autoclaving) .

After drying at 280°C the obtained mass resulted homogeneous with a density of 266 g/litre and a poro¬ sity of 91.4%. The compressive strength was between 30 and 45 kg/cm . Example 2

First, the milk of lime was prepared in accordance with the procedure illustrated in Example 1, by slaking 13.1 kg of quicklime in 103.2 kg of water at a tempera¬ ture of 40°C with stirring at a rate of 50 rpm. There¬ after, 1.71 kg of inorganic synthetic carbon fibres t -D )

Carbocem , having fibre length of 13 mm were added slowly in small portions, followed by 16.2 kg of quartz crystalline powder.

The stirring was continued at high speed (150

rpm), until the mixture was homogeneous. The operation in total required about 3 hours.

The porous mass obtained after the baking stage, as illustrated in Example 1, had a density of 277 g/litre and a porosity of 89.5%; the fibre resulted perfectly distributed into the mass, whose compressive

2 strength was about 40-50 kg/cm .