TREATMENT OF POLYAMIDES

This invention relates to the treatment of polyamides, and, in particular, to a process for removal of silicone coatings from polyamides. The invention has particular utility in the removal of silicones from airbags made from silicone-coated poly amide textiles.

It is becoming common to treat polyamide textile materials with silicone coatings to render those materials air-proof. The use of silicone-coated air-proof polyamide textiles for vehicle airbags is now widespread, and it is desirable to be able to remove the silicone coatings from the polyamide material in order that the polyamide be rendered available for recycling.

A process for recycling polyamide material coated with silicone resin is disclosed in published European Patent Application No. 950684 A2 in the name of F.A. Rueb Holding GmbH. The method there proposed involves the treatment of the coated polyamide at elevated temperature, with an alkaline hydroxide at a concentration of 5 to 50% by weight, preferably 10 to 30% by weight, of the alkaline hydroxide. It is indicated that the thermal treatment of the polyamide material can take place in the presence of a tenside. A disadvantage of the method proposed in European Patent Publication No. 950684 A2 is that the concentration of the hydroxide is so great as to degrade the polyamide being treated. Specifically, the strength of the alkaline hydroxide solution is such as to shorten the polymer chains to such an extent that the physical properties of the polyamide are degraded, thus producing a recycled product of reduced commercial value.

Additionally, US Patent No. 4654041 to Hansa Textil Chemie GmbH discloses a process for removal from textile materials such as cotton fabric, wool/polyacryl knitted fabric, and cotton/polyester corduroy fabrics silicones which have been applied to those materials for the purpose of providing those fabrics with a soft and pliant feel. The disclosed process comprises treating the material with an aqueous preparation containing surface active equilibration catalysts for organosiloxanes selected from the group consisting of alkyl or aryl or alkylaryl sulphonic acids, aliphatic alcohols with 3 to 15 carbon atoms, and surface-active quaternary ammonium compounds, said catalysts being present in an amount of 0.2 to 5 weight percent. The aqueous preparation may additionally contain an alkali carbonate and may also contain auxiliary solvents for the equilibration catalysts, which may be low molecular weight water- soluble alcohols such as isopropyl alcohol. That invention, however, is not at all concerned with polyamide textiles nor with airbags produced from polyamide textiles and can be thought of merely as a laundering process, taking place in conventional washing machines as commonly used in the textile industry.

According to the present invention there is provided a process for removing silicone from polyamide material which comprises treating silicone-coated polyamide material at an elevated temperature with an aqueous composition containing a surfactant, an alkaline hydroxide at a concentration of less than 2.1 percent hydroxide ion content by weight, and an alcohol miscible at that elevated temperature with the hydroxide solution, under such conditions of temperature and pressure and for such a duration of time as to dissolve the silicone coating but also to produce a lowering of the relative viscosity in formic acid of the polyamide of no more than 25.

The present invention also extends to polyamide which has been treated by the process of the present invention.

The combination of temperature, pressure and period of contact of the silicone-coated polyamide material with the treatment solution is chosen to achieve efficient dissolution of the silicone coating with minimum degradation of the quality of the polyamide material as defined by the lowering of its relative viscosity (a measure of the degree of polymer chain shortening to which the polyamide material has been subjected). Suitable temperatures may be selected from the range 90° to 180° Centigrade, e.g. 130° Centigrade.

Preferably the polyamide material is reduced to small pieces since it has been found that the process performs better in such circumstances.

Any suitable surfactant may be employed in the process of the present invention; surfactants which have been found to be suitable include cationic or amphoteric surfactants, either used alone or used in combination with non-ionic surfactants. Preferred surfactants include quaternary ammonium salts, preferably quaternary ammonium chlorides, such as ditallowdimethylammonium chloride, dimethyldidecyl ammonium chloride (DDAC) as well as alcohol ethoxylates. Specific examples of suitable surfactants are those commercially available under the trade marks BEROL 185, ARQUAD 2.10-80, ARQUAD 2T-70 and HANSA SR 142.

Any suitable alcohol may be used; examples of alcohols which have been found to be satisfactory in the process of the present invention include ethanediol, isopropanol, 2,3- butanediol and 2,5-dimethyl-2,5-hexanediol.

The alkaline hydroxides which may be used include sodium hydroxide, potassium hydroxide and ammonium hydroxide.

Preferably the proportion of the polyamide material to the aqueous composition is approximately 20: 100 by weight. Preferably the alkaline hydroxide is used at a concentration of 0.5 to 2.85 percent by weight of the treatment liquor.

Although the invention is applicable to polyamide materials in general, the remainder of the specification will concentrate particularly on nylon-6,6 and upon nylon-6, and, in particular, upon the process of treating airbags made from nylon textiles coated with silicone, in order to retrieve high-quality nylon.

Reference has been made hereinbefore to determining the degradation in quality of the nylon by determination of its relative viscosity after being subjected to the process of the present invention. Relative viscosity is determined according to standard test method ASTM D789- 04: 'Determination of Relative Viscosity of Polyamide', and is with respect to 90% formic acid.

We now set out by way of example only, embodiments of the process of the present invention. In each example the airbag pieces were present in the solution in an amount of approximately 2.25g per lOOg of solution.

Example 1

An airbag made of woven nylon-6,6 coated with a silicone was cut up into small pieces and was then treated with an aqueous solution containing 5% by weight of isopropanol and 2% by weight of sodium hydroxide for a period of one hour and at a temperature of 13O ⁰ C. The pieces were then neutralised and washed to remove the treatment solution, and the relative viscosity of the recovered nylon-6,6 measured by the ASTM method referred to above. The relative viscosity was found to be 54, whereas the relative viscosity measured by the same method of 'virgin' nylon-6,6 was approximately 60, a drop in relative viscosity of just 6.

Example 2

An airbag made of woven nylon-6,6 coated with a silicone was cut up into small pieces and was then treated with an aqueous solution containing 5% by weight of ethylene glycol and 2% by weight of sodium hydroxide for a period of two hours and at a temperature of 130°C. The pieces were then neutralised and washed to remove the treatment solution, and the relative viscosity of the recovered nylon-6,6 measured by the ASTM method referred to above. The relative viscosity was found to be 51, whereas the relative viscosity measured by the same method of 'virgin' nylon-6,6 was approximately 60, a drop in relative viscosity of just 9.

Example 3

An airbag made of woven nylon-6,6 coated with a silicone was cut up into small pieces and was then treated with an aqueous solution containing 4% by weight of ditallowdimethyl ammonium chloride, 2% by weight of sodium hydroxide and 1% by weight of 'Berol 185' (Berol 185 is a non-ionic surfactant based on a synthetic primary alcohol) for a period of one hour and at a temperature of 130°C. The pieces were then neutralised and washed to remove the treatment solution, and the relative viscosity of the recovered nylon-6,6 measured by the

ASTM method referred to above. The relative viscosity was found to be 54, whereas the relative viscosity measured by the same method of 'virgin' nylon-6,6 was approximately 60, a drop in relative viscosity of just 6.

It will be appreciated that the invention may be performed otherwise than as particularly described hereinbefore and the present invention includes within its scope all modifications and variations which would be apparent to one skilled in the art.