EP1944394A1

EP1944394A1 - Process for the production of flame-retardant cellulose acetate fibres

Info

Publication number: EP1944394A1
Application number: EP08100240A
Authority: EP
Inventors: Francesco Tozzi-Spadoni; Andrea Seghizzi; Giovanni Patritti
Original assignee: Bio-Tex Srl
Current assignee: Bio-Tex Srl
Priority date: 2007-01-11
Filing date: 2008-01-09
Publication date: 2008-07-16
Anticipated expiration: 2028-01-09
Also published as: ITMI20070035A1; ATE459737T1; EP1944394B1; DE602008000720D1

Abstract

The present invention relates to a process for the production of continuous fibres or filaments based on cellulose acetate resistant to combustion and textile articles obtained therefrom, comprising the dissolution in acetone of an organic halogen-derivative additive, the dosing and mixing of said solution in the acetone solution of cellulose acetate and spinning under suitable conditions.

Description

The present invention relates to a process for the production of flame-retardant cellulose acetate fibres.
The present invention derives from the field of fireproof cellulose materials.
In particular, the present invention relates to a process for producing threads and/or a continuous multifilament cellulose acetate fibre having flame-retardant properties.
Cellulose acetate fibres fall within the category of artificial fibres and are obtained starting from granules of cellulose acetate. Cellulose acetate can, in turn, be obtained starting from cellulose from wood, cotton and/or other similar products, through a chemical reaction called acetylation. Cellulose acetate is consequently of a vegetable origin and is easily flammable.
Cellulose acetate fibres are obtained by the dissolution of cellulose acetate granules (called flakes) in an organic solvent, typically acetone, and subsequent dry spinning.
The production process is developed through the following phases:

Transportation and mixing of the flakes

The raw material (cellulose acetate flakes) is transported from silos in which it is stored in the weighing scales where the mixture of various components (up to a maximum of 4 types of different flakes), is formed.

- Dissolution

The flake is discharged into specific dissolvers called "turbo-dissolvers" together with a pre-established quantity of acetone and a small quantity of water. The cellulose acetate is soluble in said mixture and therefore forms a solution having a high viscosity (about 100,000 cPs) containing approximately 27% of cellulose acetate.

- Filtration

The solution thus obtained (hereinafter called "dope") undergoes three successive filtration passages. The first filtration is obtained by means of sand filters called "Funda", the two subsequent filtrations by means of press filters. The purpose of the filtration is to remove the particles with a diameter higher than 10 µm which, if not removed, could hinder the subsequent spinning phase.

- Spinning

The spinning process used for the production of cellulose acetate fibres is a dry process, i.e. the fibre is spun in air without the help of solvents.
The dope coming from the filtration is heated by heat exchange with hot water in an appropriate exchanger called lantern, at this point it is fed through a gear pump into a suitable die. The die is a metallic plate equipped with a suitable number of calibrated holes. The number of holes is equal to the number of fibrils which form the fibre. The diameter of the holes varies from a few tens of microns to a maximum of about a hundred microns.
As it is extruded through said holes, the dope undergoes a stretching process which aligns the molecules of the cellulose acetate polymer providing characteristics of mechanical resistance.
At the outlet of the die, the dope also undergoes a rapid loss of acetone due to the low boiling point of this solvent (56°C) and relatively high temperature to which it has been preheated in the lantern. Hot air is also insufflated into the spinning cell in countercurrent with respect to the descent direction of the thread. This hot air completes the evaporation process of the acetone and coagulation of the fibre. The air containing acetone is then fed to a recovery plant of the solvent.
The fibre is subsequently collected on cardboard supports forming bobbins having a weight ranging from 3 to 8 Kg. The collection is effected with single heads at a rate ranging from 400 to 750 m/min.

- Acetone recovery

The air containing the acetone vapours is fed to suitable activated carbon exchangers. The activated carbon fixes the acetone molecules allowing the passage of purified air. Once the exchange capacity of the carbon has been exhausted, the exchanger is subjected to regeneration. The regeneration process is effected by insufflating vapour into the exchanger. This vapour "strips" the acetone adsorbed in the exchanger regenerating it and making it available for a recovery cycle. The stripped acetone is condensed obtaining a mixture of water and acetone called borlanda. This borlanda is then fed to a distillation column at whose head the more volatile solvent (acetone) is recovered, whereas the water is collected on the bottom of the column.
The recovery percentage of acetone with respect to the circulating fluid is extremely high (>95%) and consequently the production process is practically a closed circuit.
In addition also the thread scraps and wastewater coming from the washing of the filters are recovered by dissolution in acetone.
The cellulose acetate fibres thus obtained are applied in numerous fields which vary from the textile industry to furnishing and upholstery materials. The flammability of cellulose acetate fibres, however, still represents one of the main obstacles for their diffusion and wider range of use.
In order to overcome or at least mitigate this disadvantage, attempts have been made to provide acetate fibres with flame-retardant properties.
These attempts, which have so far proved to be not completely satisfactory, are essentially based on the direct addition to the fibres, or indirect addition to the dope, of substances which inhibit or delay combustion, so-called flame-retardants. The addition of these substances to the fibres, which are typically in the form of powders having a reduced particle-size, is still not sufficient for preventing, or significantly delaying, combustion.
The production of fire-resistant cellulose fibres, which according to the conventional techniques envisages the addition of a flame-retardant additive to the mass of dope to be spun, causes various drawbacks in the processing of the fibre, above all during the spinning phase.
In most cases a considerable reduction in the physical characteristics of the thread has also been observed. This disadvantage is not at all negligible considering that, as cellulose acetate, within the range of continuous fibres, is one of those having lesser physical characteristics, a further reduction in these properties makes the fibre unusable.
A production technology of fire-resistant cellulose fibres which is based on the addition of liquid flame-retardant additives, such as organic phosphates, is also known.
This technology, however, also has drawbacks. Once the liquid flame-retardant product has been added to the fibre, it does in fact tend to be released from the fibre itself during the washing cycles of the fibre end-product causing a considerable reduction in the desired flame-retardant effect.
Furthermore, liquid flame-retardant additives have a plasticizing effect on the dope mass and, if added in high percentages, as is necessary for providing an adequate flame-retardant effect, they make spinning difficult.
One of the general objectives of the present invention therefore consists in providing a process for the production of continuous flame-retardant acetate fibre which substantially reduces the drawbacks described above.
A further objective of the invention consists in providing a process for the production of fibres based on cellulose acetate having fire-resistance wherein the presence of the flame-retardant component does not substantially reduce the mechanical resistance properties of the same fibres.
Another objective of the present invention consists in providing a cellulose acetate fibre having flame-retardant properties and end-products based on said fibre which preserve the flame-retardant properties even after washing.
According to a first aspect of the invention, the Applicant has selected flame-retardant additives in solid form, soluble in acetone and almost insoluble in water, which can be used for producing flame-retardant fibres based on cellulose acetate without substantially jeopardizing the mechanical properties.
Conveniently, the flame-retardant additives used within the scope of the invention are solid at room temperature, so as to remain entrapped in the matrix of the fibre, they have a melting and boiling point which are such that they do not evaporate or sublimate during the spinning phase and do not colour the cellulose acetate fibre.
As the flame-retardant additives selected are soluble in acetone, they allow the extrusion process of the fibre to be effected maintaining the mechanical characteristics.
The additive exerts its flame-retardant function, inside the fibre, by poisoning the flame.
In the combustion process of a solid, the heat developed by the flame itself causes the formation of volatile products (vapours/gases) which, by reacting with the oxygen contained in the atmosphere, produce heat which feeds the process. When the additive in question burns together with the textile substrate, it decomposes emitting products deriving from bromine which interfere with the combustion mechanism preventing its self-feeding.
This type of mechanism is commonly called "flame poisoning" and falls within the group of mechanisms which act in "gas phase".
In particular, the flame-retardant additives used within the scope of the invention are organic halogen-derivatives.
Halogen-derivatives having a more efficacious flame-retardant effect are organic bromo-derivatives, i.e. organic compounds containing bromine, preferably in a percentage of 50-80%. Their flame-extinguishing effectiveness can be attributed to the facility with which they decompose during combustion.
Some substances exist which have a synergic effect with respect to these mechanisms, typically derivatives of antimonium, i.e. substances which, although not having a flame-retardant effect in themselves, when combined with halogen-derivatives, enhance their effect. This is due, again during combustion, to the formation of compounds between antimonium and bromine which are extremely volatile and consequently rapidly pass into gas phase.
In this case, the proponent has preferred not to include antimonium compounds in the fibre as they are insoluble in acetone and cause the deterioration of the performances of the fibre.
Among organic compounds of bromine, those containing aromatic bromine (bromine atoms bound to aromatic rings) are generally more stable to heat and therefore decompose more slowly with a consequent lower flame-retardant effect. Their stability, however, makes them preferable in the field of additives for plastic materials when a high heat-resistance is required during the moulding and extrusion phases (temperature > 200°C). Aliphatic organic bromo-derivatives (where the bromine atoms are bound to linear carbon chains) are less stable to heat and consequently have a greater flame-retardant effect. The lower stability to heat does not represent an obstacle in this specific use as the temperature does not exceed 90°C during the processing.
Particularly suitable organic bromo-derivative additives are tribromoneopentyl alcohol (TBNPA, CAS Number 36483-57-5) and tetrabromobisphenol A (TBBPA, CAS Number 79-94-7), and mixtures thereof.
The Applicant has observed that the incorporation of the additives TBNPA and TBBPA into the acetate fibre takes place without causing a significant denaturalization of the fibre itself, thus preserving the mechanical resistance properties of the fibre also after continuous thread spinning.
The Applicant has in fact also observed that by using the flame-retardant additives described above, it is possible to produce continuous fire-resistant fibres or filaments based on cellulose acetate
According to another aspect of the present invention, a process is provided herein for the production of flame-retardant fibres based on cellulose-acetate comprising the following phases:

A) dissolution of the flame-retardant additive in acetone,
B) dispersion of the additive in the dope,
C) dry spinning of the dope.

According to an embodiment of the process of the invention, said flame-retardant additive is dissolved in acetone and then added to the acetone solution of cellulose acetate (dope) already filtered, resorting, for example, to the use of a suitable dynamic mixer capable of producing a homogeneous dispersion of the additive inside the solution of cellulose acetate.
Typically, in phase a) of the process, said flame-retardant additive is dissolved in acetone until a solution is formed which contains it in a concentration ranging from 50-80%, preferably from 60 to 70%.
In order to facilitate the operations, for example, TBNPA or TBBPA are dissolved in a mixer until a concentration of additive equal to about 65-70% is obtained. The solution containing the additive is then added to the dope in suitable quantities so that the percentage of additive with respect to cellulose acetate preferably ranges from 10 to 20%.
The dope containing the additive solution is then mixed (phase b) until a homogeneous solution containing the additive is obtained, which is subsequently spun (phase c)).
It has been observed that with the process of the invention, it is possible to spin titers up to a minimum of 100 deniers.
In general, the process of the invention limits:

coagulation phenomena and the formation of solid clots,
the formation of a non-homogeneous dispersion which is incompatible with the subsequent spinning phase;
lowering of the viscosity of the dope due to the addition of high percentages of non-polymeric additives;
also possible undesired effects due to compounds with a plasticizing effect.

The characteristics and advantages of a process for the production of flame-retardant acetate fibres according to the present invention will appear more evident from the following illustrative and non-limiting description, referring to the enclosed schematic drawing which illustrates a five-phase embodiment of the process of the invention.
With reference to the enclosed figure, this shows an embodiment of the process which comprises the following phases:

a - dissolution of the flame-retardant additive in acetone
b - dispersion of the acetone solution of the additive in the dope
c - dry spinning of the dope containing the additive under thermodynamic and winding conditions which take into account the lower viscosity of the dope with respect to a standard dope, without additive.

In particular, in phase a) of the process, the solid flame-retardant additive of the type previously described, is added to a pre-established quantity of acetone typically in a mixer used as a dissolver. The mixture is then mixed until a limpid solution is formed, possibly adding small quantities of acetone to complete the dissolution.
In the subsequent phase b) the solution of the additive is mixed with a pre-established quantity of dope, typically using dye producing machinery.
In phase c) spinning is effected adapting the thermodynamic and spinning conditions to the modified characteristics of the dope.
In practice, the visco-elastic conditions of the dope, modified by the presence of the additive, are brought to those necessary for spinning the material. The collection is effected with single heads at a rate conveniently ranging from 400 to 500 m/min and spinning temperatures preferably ranging from 58°C to 70°C. The draw applied is defined "high" for the "high draw down spinning condition" acetate technology. The spinning conditions of a 150 den, for example, are the following:

high draw conditions
number of filaments analogous to the standard product.

The fibre obtained can have titers which typically vary from 100 to 600 deniers in the form of multifilament fibres. The yarn is shiny and has a shine almost analogous to the standard product. It is possible with further expedients to produce dyed fibres in mass, opaque and black. It has been observed that opaque fibres containing a flame-retardant additive have a behaviour to flames which is even higher than raw fibres (shiny).
The mechanical properties of the continuous thread obtained fall within the limits established for standard products.
An index which measures the facility with which a material burns is LOI (limiting oxygen demand), this represents the minimum percentage of oxygen which is sufficient for maintaining the combustion of the substance examined. The greater the index, the lower the tendency is to burn.
Standard acetate has a LOI index of about 18, the product obtained with the process of the invention has a LOI index conveniently around 30 for opaque yarns and > 24 for shiny yarns.
As the flame-retardant additive remains dispersed in the mass and as it is almost insoluble in water, the properties described do not deteriorate during the normal treatment to which the fibres are subjected during their processing and during the life of the end-product produced with these (washing resistance).
The continuous fibre obtained with the process of the invention is applied in the textile industry for producing clothes, linings, outdoor knitwear and furnishing, for example in the production of mattress-covers, curtains and drapery, also mixed with other flame-retardant fibres.
According to another aspect of the invention, a flame-retardant fibre or thread based on cellulose acetate is provided, as indicated in claims 9-15.
In accordance with another aspect of the invention, a textile article having fireproof properties is provided, as indicated in claims 16 or 17.
The fabric obtained with the process of the invention also has excellent transpirability and antistatic properties together with a good mixability with other fibres such as viscose, polyester, cotton, wool, polyamide, etc.
The following example is provided for purely illustrative purposes and should not be considered as limiting the disclosure scope as specified in the enclosed claims.

EXAMPLE 1

A sample (sock), in titers higher than 100 deniers, made with cellulose acetate thread containing the flame-retardant additive TBNPA in a quantity of 15% by weight was tested to control flame-development resistance.
In particular, the LOI (limiting oxygen demand) was determined, i.e. the index value which measures the facility with which a material burns which represents the minimum percentage of oxygen sufficient for maintaining the combustion of the substance examined.
The conditions, the instruments used and the values revealed are indicated below.
Method used: UNI EN ISO 4589-2:2003.
Instrument used: Oxygen Index Apparatus (LOI)
The test was carried out on test-tubes after washing the sample with a detergent at 40°C.
The test-tubes used:

TYPE V form;
Dimensions 38 x 140;
Thickness < 10.5;
Ignition used: Type B;
OXYGEN INDEX (LOI): 30.3%;
Standard deviation: 0.19.

EXAMPLE 2

A second sample (sock), in titers higher than 100 deniers, made with cellulose acetate thread containing the flame-retardant additive TBBPA in a quantity of 15% by weight was tested to control flame-development resistance.
In particular, the LOI was determined.
The conditions, the instruments used and the values revealed are indicated below.
Method used: UNI EN ISO 4589-2:2003.
Instrument used: Oxygen Index Apparatus (LOI)
The test was carried out on test-tubes after washing the sample with a detergent at 40°C.
The test-tubes used:

TYPE V form;
Dimensions 38 x 140;
Thickness < 10.5;
Ignition used: Type B;
OXYGEN INDEX (LOI): 24.1%
Standard deviation: 0.24.

Claims

A process for the production of a flame-retardant cellulose acetate fibre, or thread comprising the following phases:
A) dissolution of the flame-retardant additive in acetone,

B) mixing of the acetone solution containing the flame-retardant additive with a dope,

C) dry spinning of the mixture obtained containing the additive,
said process being characterized in that said flame-retardant additive is an organic halogen-derivative in solid form, soluble in acetone and insoluble in water.
The process according to claim 1, characterized in that said organic halogen-derivative is a bromo-derivative.
The process according to claim 2, characterized in that it is an organic bromo-derivative with a bromine content of 50-80%.
The process according to any of the claims 1-3, characterized in that said flame-retardant additive is selected from tribromoneopentyl alcohol (TBNPA) and tetrabromobisphenol A (TBBPA), and mixtures thereof.
The process according to any of the claims 1-4, characterized in that in the dissolution phase A), an acetone solution is obtained, containing said flame-retardant additive in a quantity ranging from 50 to 80%.
The process according to any of the claims 1-5, characterized in that the flame-retardant fibre or thread obtained contains said flame-retardant additive in a quantity not lower than 10%.
The process according to claim 6, characterized in that the flame-retardant fibre or thread obtained contains said flame-retardant additive in a quantity not lower than 15%.
A process for the production of a continuous flame-retardant thread based on cellulose acetate comprising the following phases:
- dissolving an organic flame-retardant halogen-derivative additive in acetone to obtain an acetone solution having a concentration of said additive of 60-70% by weight;

- mixing the acetone solution of flame-retardant additive obtained in an acetone solution of cellulose acetate (dope) in such a quantity as to obtain a fibre or thread with a content of additive not lower than 10% with respect to the dry fibre/thread;

- dry spinning the acetone mixture containing the additive.
A continuous flame-retardant fibre or thread of cellulose acetate containing a flame-retardant additive characterized in that said additive is an organic halogen-derivative in solid form, soluble in acetone and insoluble in water.
The continuous fibre or thread according to claim 9, characterized in that said flame-retardant additive has a melting point and boiling point which are such that it does not evaporate or sublimate during the spinning phase and is colourless.
The continuous fibre or thread according to claim 9 or 10, characterized in that said flame-retardant additive is an aliphatic or aromatic bromo-derivative.
The continuous fibre or thread according to claim 11, characterized in that said bromo-derivative contains bromine in a quantity of 50-80%.
The continuous fibre or thread according to any of the claims 9-12, characterized in that said flame-retardant additive is selected from tribromoneopentyl alcohol (TBNPA) and tetrabromobisphenol A (TBBPA), and mixtures thereof.
The continuous fibre or thread according to any of the claims 9-13, characterized in that said flame-retardant additive is present in a quantity ranging from 10 to 20% by weight.
The continuous fibre or thread according to any of the claims 9-14, characterized in that it has a LOI index ranging from 24 to 30.
A textile article with fireproof properties comprising a fibre or a thread according to any of the claims 9-15.
A textile article according to claim 16, characterized in that it is selected from clothes, linings, furniture or mattress covers or padding, drapery, curtains.