Prior art The anti-scaling agents normally used in the polymeri- zation of vinyl chloride and/or vinyl acetate are prod- ucts which can be obtained by condensation of formalde- hyde and 1-naphthol, such as, for example, those de- scribed in European patent application EP-52412.

European patent application EP-942936, incorporated herein by reference, describes anti-scaling agents which comprise a product of condensation of an aldehyde, a phenolic compound and an aromatic carboxylic acid hydroxylated at the aromatic nucleus. Preferably the aldehyde is formaldehyde, the phenolic compound is 1-naphthol and the aromatic car- boxylic acid hydroxylated at the aromatic nucleus is selected from salicylic acid, 4-hydroxybenzoic acid, a, P, or y-resorcylic acid, 2, 3-dihydroxybenzoic acid, 2,5-dihydroxybenzoic acid, 2-hydroxy-5-methoxybenzoic acid or 6-hydroxy-2-naphthoic acid; of these, the one greatly preferred is P-resorcylic acid (that is, 2,4- dihydroxybenzoic acid).

Statement of invention An anti-contaminating agent (or anti-scaling agent), in order to be effective and economically suitable for the production of PVC in suspension (PVC/S), must satisfy the following conditions: it must be stable, readily meterable and readily applicable with vapour under pressure (the polymeriza- tion reactor must in fact be kept closed in order to avoid very dangerous contaminations owing to the emis- sion of gaseous vinyl chloride); 'it must have an elevated capacity for coagulating rapidly upon variation in the pH-value at the moment when it comes into contact with the vapour (which is -normally at a pH-value of 9-10); 'it must be able to form a coagulum having a ge- latinous appearance and, if possible, be colourless (to avoid contaminating the finished product).

The coagulum must preferably be of the"tacky gel"type in order to adhere to the internal walls of the reac- tors and to form a protective film (coating) owing to the action of the heat of the vapour.

The gel deposited on the walls must further be able to penetrate uniformly into the micropores of the surface of the steel in order to prevent the formation of mi- croparticles of PVC which can give rise to the forma- tion of the deposits.

Furthermore, it must not be necessary to heat the in- ternal walls of the reactor in order to promote the formation of the protective film; such an operation would in fact involve a delay in the production cycle; after the application, the protective film must be, among other things, strong enough to allow washing with water (pH-value 7) for the removal from the reactor of contaminating residues; the total application time must finally be as short as possible and require simple ap- paratuses which ensure reliable and homogeneous appli- cation of the product.

Results of this type are normally achieved by applying, with vapour under pressure, an aqueous solution con- taining 4-8% by weight anti-contaminating agent, at least 12-18% of which must remain on the internal walls of the reactor in the form of a protective film (such a method is described, for example, in"Fine Chemicals for PVC/S", Chapter 5,1999, published by CIRS Spa, in- corporated herein by reference); the method in question in fact allows extended and economical application cy- cles whereas mechanical or gas-operated spray appara- tuses are not presently capable of achieving the above- mentioned objectives.

The present invention relates to a novel method for the application of the anti-scaling agent known as EVERCLEAN 104NS. The preparation of this anti-scaling agent, currently marketed by the owner 3V Sigma Spa, is described in detail in Example 25 of already-mentioned European patent application EP-942936; it refers to a basic aqueous solution having a brown colour and a pH- value of between 11 and 12.4 (normally 11.54) which, as is known in the art, is applied to the internal walls of the reactors for polymerizing vinyl chloride and/or vinyl acetate by spraying in a vapour stream using an apparatus which is similar to that illustrated in Fig- ure 1.

However, this application method allows the desired anti-scaling effect to be achieved only in part. In fact, when operating in such conditions, a coagulum which adheres sufficiently to the internal walls of the reactors, thereby forming the necessary protective film, is not obtained; on the contrary, the great ma- jority of the anti-scaling agent applied in this manner (approximately 99%) is collected unchanged at the bot- tom of the reactor before the start of the polymeriza- tion reaction, thus necessitating further applications.

It has now been surprisingly found that, by acidifying before application the solution of EVERCLEAN 104NS to a pH-value of less than 5 and, preferably, in the range between 2.5 and 4.5, there is obtained coagulation of the anti-scaling agent which can therefore be applied with greater effectiveness to the walls of the reactor.

The problem addressed by the present invention is therefore to provide a method for applying an aqueous solution containing an alkali metal and/or alkaline earth metal salt, preferably a sodium salt, of an anti- scaling agent comprising a product of condensation of an aldehyde, a phenolic compound and an aromatic car- boxylic acid hydroxylated at the aromatic nucleus, characterized in that the pH-value of this aqueous so- lution is changed to a pH-value of less than 5 and, preferably, in the range between 2.5 and 4.5.

Preferably the aldehyde is formaldehyde, the phenolic compound is 1-naphthol and the aromatic carboxylic acid hydroxylated at the aromatic nucleus is 2,4- dihydroxybenzoic acid; the aqueous solution usually contains 4-8% by weight of the alkali metal and/or al- kaline earth metal salt of this anti-scaling agent and has a pH-value in the range between 11 and 12.4.

The method according to the present invention provides for contact on the wall (that is, on the internal walls of the polymerization reactor), or in the mixer, be- tween the solution of the anti-scaling agent and a so- lution of an organic and/or inorganic acid, for exam- ple, citric, oxalic, tartaric, hydrochloric, nitric and/or sulphuric acid, which in the presence of vapour ensure a sufficient reduction in the pH-value to allow the coagulation and the formation of the protective film. This aqueous solution preferably contains from 1 to 10% by weight citric acid, preferably 5%.

According to the greatly preferred embodiment of the present invention, the salt of the anti-scaling agent and the aqueous solution of the organic and/or inor- ganic acid are mixed with each other before being ap- plied to the internal walls of the polymerization reac tor; the mixture of gel and condensate obtained in this manner is applied to the internal walls of the polym- erization reactor in a stream of vapour. A suitable ap- paratus for the application according to the present invention is, for example, illustrated in Figure 2 (wherein the valves are designated using the letter V).

The pressure of the vapour is not binding for the pur- poses of the present invention and can vary according to the type of installation; the pressures normally used are, however, in the range between approximately 2 and 20 bar; the application time is normally in the range between 15 and 45 seconds.

As will be appreciated from the Examples reproduced be- low, which must not be taken to limit the present in- vention, the method claimed here has the advantage of providing the anti-scaling agent corresponding to Euro- pean patent application EP-942936 in the form of a co- agulate product. This product can therefore be readily applied to the internal walls of a reactor for polymer- izing vinyl chloride and/or vinyl acetate, thereby forming a protective film which adheres to the walls at a rate greater than 12% by weight relative to the quan- tity loaded. In this manner, it is thus possible ade- quately to protect the reactor by eliminating almost completely the formation of deposits even after long periods of activity, thereby minimizing the necessity of interrupting the production cycle to allow the in- stallation to be cleaned; the large quantity of anti- scaling agent which remains attached to the wall fur- ther allows a reduction in the quantity of anti-scaling agent to be used, with obvious economic advantages.

Examples Example 1 There was loaded into a reactor, measuring 10 m3 with walls of stainless steel, provided with an application apparatus of the type schematically depicted in Figure 1,1 kg of an aqueous solution containing 6% by weight sodium salt of the anti-scaling agent which is obtained as described in Example 25 of European patent applica- tion EP-942936. The product was sprayed for 30 seconds on the inside of the reactor with vapour at a pressure of 5 atmospheres.

After the application, the condensate was collected at the bottom of the reactor. The condensate was analyzed in order to determine the pH-value and the possible presence of coagulate. The pH-value was 10 and the con- densate contained no precipitate gel.

The condensate was further treated once more with hy- drochloric acid until a pH-value of 3.8 was reached and there was finally observed a coagulate of dark brown/black appearance which was found to correspond to more than 99% by weight of the anti-scaling agent ini- tially loaded.

Example 2 The procedure described in Example 1 was repeated and the apparatus schematically depicted in Figure 2 was used (wherein 1 litre citric acid at a concentration of 5% by weight was loaded into tank B).

After spraying in the reactor, the condensate was col- lected and it was observed that the condensate had a pH-value of 3.2 and contained 85% coagulated anti- scaling product (while the remaining 15% was left de- posited on the internal walls of the reactor as a pro- tective film).

Examples 3-4 The procedure of Examples 1 and 2 was repeated with similar results when operating with vapour at a pres- sure of 18 atmospheres.

Examples 5-8 The procedure of Examples 1-4 was repeated with similar results using EVERCLEAN 104NS, which is commercially available, in place of the product obtained according to Example 25 of European patent application EP-942936.