PROCESS

The invention relates to a process for obtaining a phenol-formaldehyde resin with low free formaldehyde content and to the resin obtained by this process, which is particularly used for impregnating various materials such as paper, textile, glass fibres, wood, etc.

From RO patent No. 117 536 there is known a process for obtaining a soluble adhesive by the condensation, in alkaline medium, of the phenol residues resulting from the phenol manufacturing with a formaldehyde solution having 35% concentration, at a temperature of 100 ⁰ C, for 20 ... 60 minutes. In the description there is not mentioned the free formaldehyde content of the resulting product.

In RO patent 116090 there is presented a process for obtaining phenol- formaldehyde resins by the polycondensation of phenol and/or of the derivatives thereof with formaldehyde, which comprises the stages of: i) polycondensation of a derivative or of a mixture of phenol derivatives with formaldehyde, in a molar ratio ranging from 0.5 to 3 moles, in the presence of an acid or basic catalyst, at a temperature between 35 and 25O ⁰ C; ii) polycondensation of a phenol derivative or of a mixture of phenol derivatives, which are the same as or different from the ones in stage i), with formaldehyde, in a molar ratio ranging from 0.5 to 3 moles, in the presence of an acid or basic catalyst, different from the one in stage i), at a temperature ranging from 35 to 250 ⁰ C and iii) mixing the products obtained in stages i) and ii) and the possible conditioning of the product by operations of diluting and/or evaporation and/or compounding with different conditioning products. In the description of the mentioned patent there is not presented or suggested the possibility of obtaining phenol-formaldehyde resins with low free formaldehyde content.

The problem the invention proposes to solve is that of obtaining a phenol- formaldehyde resin with a low free formaldehyde content, namely less than 0.1 %.

The invention solves the technical .problem by a process for obtaining a phenol- formaldehyde resin by the condensation reaction of phenol with formaldehyde in a. basic medium, characterized in that 35 .. : 50 parts by weight of phenol are condensed with 40 ... 60 parts by weight of formaldehyde, in an aqueous medium, by adding basic catalyst, at a temperature of 35 ... 65°C, for 2 ... 3 hours, then the reaction mass temperature is raised at 70 ... 9O ⁰ C and this temperature is maintained for 1 ... 5 hours, until the free phenol content is less than 6%, then there are added, while cooling, 0.1 ... 10 parts by weight of melamine and 0.1 ... 13 parts by weight of sulphamic acid or its salts and stirring is continued for further 20-40 minutes, thereafter, are added 5-20 parts by weight of 20 to 30 % concentration partly hydrolyzed polyvinyl alcohol solution or 1 to 10% by weight dry ethanol amine, thereafter the reaction mass is cooled down to 20 ⁰ C and the obtained desired product as clear aqueous yellow-brown coloured solution and a solid matter content of 58 ... 68% by weight is

discharged.

The phenol-formaldehyde resin as claimed by the invention is obtained by the process defined in claim 1 and has a viscosity, determined by the Ford cup, of 15 .. 40 sec, a pH ranging from 7 to 10; density of 1.190 .. 1.220 g/cm ³ , free phenol of less than 6% and free formaldehyde of less than 0.1%.

The invention has the following advantages:

- the obtained product is ecological having a formaldehyde content of less than 0.1 %;

- the product is stable in time upon storage at a temperature of 10 . 20

°C;

- the orocess is p.asv tn hp r.arriprt nut There are given hereinafter 3 embodiments having the role of illustrating the invention and not of limiting it.

The raw materials used in the examples were commercially available technical quality products.

The partly hydrolyzed polyvinyl alcohol was obtained, in the known manner, by homopolymehzation of the vinyl acetate followed by alkaline hydrolysis of vinyl polyacetate, in the mixer, there being obtained polyvinyl alcohol with a hydrolysis degree higher than 70% and which contains hydrophilic and hydrophobic blocks.

The phenol content was determined chromatographically.

. The formaldehyde content in the resin was determined by reaction of the formaldehyde and hydroxylamine hydrochloride and titration of the formed acid with a sodium hydroxide solution up to a pH of 3.5, determined by a pH-meter.

Example 1

35 g of 100% phenol are introduced into a 1L 3-neck flask provided with stirrer, refrigerating medium and thermometer, and 40 g of 100% formaldehyde as a 45% concentration solution are added thereto. After homogenization, there are added 3g of NaOH as basic catalyst in the form of 50% concentration aqueous solution, for 2 hours, in 5 portions. After completing the catalyst addition, the reaction mass is heated at 80 ⁰ C and it is maintained at this temperature for 2 hours, thereafter at 75°C for 3 hours. A sample of about 2 g is taken from the reaction mass and the free phenol content is determined by cromatography . At a value thereof of less than 6% there is started the cooling of the reaction mass and, while cooling, 7 g of melamine and 7 g of urea are added. Cooling is continued down to 3O ⁰ C and when this temperature is reached, 2 g of sulphamic acid are added and the stirring is continued for 30 minutes. A sample is taken from the reaction mass and the free formaldehyde content is determined. If the nhtainpri value is higher than Q 1 %, 2 g of partly hydrolyzed polyvinyl alcohol are then added as 20% concentration solution. Stirring is continued for further 30 minutes, then the reaction mass is cooled down to 20°C and it is discharged.

The obtained product has the following characteristics:

aspect: clear aqueous yellow-brown coloured solution;

solid matter content: 59.5% by weight;

'.- viscosity determined by the Ford cup: 17 sec; - , pH = 9;

density: 1.19 g/cm ³ ;

free phenol content: 4.8%;

free formaldehyde content: 0.08%; and

curing time at 120°C:13 min and 40 sec.

Example 2

There is proceeded similarly as in Example 1 , except that instead of partly hydrolyzed polyvinyl alcohol 2 g of triethanol amine are used.

The obtained product has the following characteristics:

aspect: clear aqueous yellow-brown coloured solution;

solid matter content' 59 5% by weight;

viscosity determined by the Ford cup. 25 sec,

pH = 8.9,

density 1.19 g/cm ³ ;

free phenol content' 4.9%;

free formaldehyde content. 0 07%, and

curing time at 120 ⁰ C: 13 min and 45 sec.

Example 3

There is proceeded similarly as in Example 1 , except that instead of was used as catalyst. The obtained product has the same physical-chemical characteristics as the ones in Example 1 , but in addition, the miscibility in water is much improved.

The resins obtained in the presented examples were used for impregnating paper meant for the melamined systems. The resulting melamined systems met the quality specifications concerning these products.