Coating processes – With post-treatment of coating or coating material – Chemical agent applied to treat coating
Patent
1997-05-09
1998-10-13
Lusignan, Michael
Coating processes
With post-treatment of coating or coating material
Chemical agent applied to treat coating
264557, 264558, 264559, 264561, 264562, 427356, B05D 304, B05D 310
Patent
active
058209344
DESCRIPTION:
BRIEF SUMMARY
The invention relates to the production of fibre-reinforced cellulose casings. Such fibrous casings are produced, for example, by coating a fibrous non-woven fabric with viscose, by regeneration of the viscose into cellulose in a precipitation operation, by plasticising and drying the casing.
The web of non-woven fabric formed into a tube with overlapping seam is coated with viscose on one side or on both sides.
In this process it is important to achieve the embedding of non-woven fabric as quickly as possible in order to be able to initiate the following process step of precipitation.
An embedding of the non-woven fabric is achieved particularly rapidly and completely as a result of application of viscose on both sides if use is made to this end of a coating device such as is presented in EP-0 267 489 B1. One-sided coating has the disadvantage of a long saturation-time. In addition, the tube does not attain the uniform embedding and consequently the high degree of firmness of the casing that is coated with viscose on both sides.
The xanthate viscose that is conventionally used for the coating consists of around 85 wt-% water, and only 7 to 9 wt-% remain in the form of solids after the conversion into regenerated cellulose. Despite the extremely high viscosity, on account of the high water content viscose hardly offers any resistance to a long displacement in the case of a low application of force, which is important in connection with initiation of the precipitation operation. This is because very small changes in pressure within the tube in the course of the precipitation process result in changes in the structural configuration of the regenerated cellulose.
In the case of complete embedding of non-woven fabric a very delicate entity leaves the viscose nozzle if, with a view to production of non-woven fabric, use is made of the Manila hemp fibre conventionally employed and it is completely embedded in aqueous viscose.
In the case of the standard methods the tube leaving the viscose-coating device passes vertically downwards through an air gap before it is immersed in the precipitating bath.
The precipitating bath is located in a container having a depth of 1-4 meters with a deflection roller in the sump of the container, with the aid of which the running direction of the tube is reversed. While the precipitating liquid is passing through, an attempt is made largely to neutralise the lye that is contained in the viscose (eg, soda lye) in a quantity amounting to about 6 wt-% with acid (eg, sulphuric acid, optionally also organic acids). This neutralisation on both sides results in pronounced shrinkage stresses which are difficult to control. In this process a preliminary stiffening of the coagulation occurs first of all with subsequent conversion into cellulose.
Regeneration is effected on the outside and the inside of the tube almost simultaneously. This neutralisation on both sides results in pronounced shrinkage stresses which are difficult to control.
The bath that is required for the inner precipitation is continually supplied and exchanged by means of a system of rods for feeding and drainage.
In addition to the acid--mostly sulphuric acid--the precipitating baths also contain salts, mostly derived from the acid employed, that buffer the course of regeneration. By way of buffer salts in sulphuric-acid baths use is preferably made of sodium, ammonium and zinc sulphates.
This buffering-off is necessary, since the neutralisation between sulphuric acid and lye takes place very quickly and in the process a regenerated-cellulose skin forms that is only slightly acid-impermeable and particularly gas-impermeable.
As a result, the further course of the process of conversion from viscose into regenerated cellulose is greatly retarded: gas collects at the separation point between regenerated cellulose and coagulated viscose, said gas being capable of penetrating the layer of regenerated cellulose only in a limited quantity. These former separation points with collection of gas remain behind as light-reflecting
REFERENCES:
patent: 4164536 (1979-08-01), Bentley
patent: 4317794 (1982-03-01), Gord et al.
patent: 4808439 (1989-02-01), Basfeld
Lusignan Michael
Wolff Walsrode AG
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