Preparation of Resins of the “Pulver-Bakelite” Type from Hydrolysis Activated Lignin

T. A. Pershina, A. N. Novikov, M. V. Moiseenko, V. I. Komarova

(Presented by Prof. T. P. Kuleshova, Doctor of Chemical Sciences)

Of all types of technical lignins, the most chemically active is sulfate, or alkali, lignin, on the basis of which lignin–phenol-formaldehyde resins can be obtained with physicochemical indices close to those of ordinary phenol-formaldehyde resins [1,2]. Activated lignin, which is formed when hydrolysis lignin is heated with sodium hydroxide under pressure, is similar to alkali lignin in its content of functional groups.

Analysis of alkali lignin [1]: methoxyl groups—12.7%, hydroxyl groups—10.57%, carboxyl groups—3.66%.

Results of the analysis of activated hydrolysis lignin from the Krasnoyarsk plant: methoxyl groups—11.8%, hydroxyl groups—9.5, carboxyl groups—2.0%.

In the literature there are indications of the preparation of phenol-formaldehyde resins in which a small amount of phenol is replaced by activated lignin. Thus, M. I. Chudakov [3] obtained resins at ratios of phenol to activated lignin of 2 : 1 and 3 : 1.

Proceeding from economic considerations, in the production of resins of the “pulver-bakelite” type it is necessary to replace as much phenol as possible with activated hydrolysis lignin, since the cost of the latter is three times lower than that of phenol.

Experimental Part

We carried out the activation of hydrolysis lignin in a rotating autoclave at a temperature of 200–210° under a pressure of 18–20 atm for 6 hours [4].

After cooling, the contents of the autoclave were filtered, and the activated lignin was precipitated with hydrochloric acid to an acid reaction (by Congo red). The precipitate was separated from the filtrate, washed with water to a neutral reaction and to a negative reaction for the Cl′ ion, dried to a moisture content of 4.7%, ground, and sieved. The fraction passing through a 0.25 sieve was taken.

The preparation of resins of the “pulver-bakelite” type from activated lignin was carried out in two stages.

The first stage is the condensation of activated lignin with phenol; the second stage is the condensation of phenol-lignin with formaldehyde.

Into a flask with a weighed portion of phenol, equipped with a reflux condenser and stirrer, sulfuric acid \(d=1.84\) was added in an amount of 3% of the weight of phenol and lignin; then, at a temperature of 70–80°, lignin was added in small portions. The temperature was raised to 110°, and at this temperature the mixture was stirred for a definite interval of time.

Table 1

Table 2

Table 3

Table 4

To the phenol-lignin obtained at a temperature of 60—70° there was gradually added (2—5 ml at a time) 32% formalin. The temperature was then raised to 90—95°. The resins were dried by rapid heating in a porcelain dish at a temperature of 180—200°.

In the resins, the amount of free phenol [5], the softening temperature, and the tensile strength [7] were determined.

Resins were obtained under different conditions in order to determine the influence of the latter on the physicochemical and mechanical properties. Since the synthesized resins are intended for preparing a substitute for pulverized bakelite, their quality was controlled mainly by determining the tensile strength, which should be within the range 20—25 kg/cm².

Table 5

In the present work, various weight ratios of phenol to lignin were taken (1:1; 1:1.6; 1:2; 1:2.3; 1:3; 1:4). The amounts, conditions, and test results are given in Table 1.

The effect of the duration of the first and second stages on the properties of the resins is indicated in Tables 2 and 3. Having established the optimum time for carrying out the first and second stages at T=110° (stage I) and T=90–95° (stage II), the amount of formalin necessary for obtaining the best resins was determined (Table 4), as well as the effect on the quality of the resins of the catalysts used (Table 5).

Conclusions

1. With an increase in the amount of activated lignin, the tensile strength of the resins, as was to be expected, decreases, especially sharply at a phenol-to-lignin ratio of 1:4.

2. At a lignin-to-phenol ratio of 1:1, the best resins are obtained when the first stage is carried out for 3 hours and the second for 40 min. At ratios of 2:1, 3:1, and 4:1, strong resins are obtained when the first stage is carried out for 1 hour and the second for 40 minutes.

3. The effect of the amount of formalin on the strength of resins at different ratios of phenol and activated lignin has been established.

4. In all cases, the best resins are obtained when sulfuric acid is used as the catalyst.

LITERATURE

1. Sokolov A. A., Bogomolov B. D., Krupkina F. A., Brodskii R. S., Afanas’eva N. V. Scientific Reports of Higher Education Institutions, Chemistry and Chemical Technology, issue 3, 556–558, 1958.

2. Barr E. I. Technology of Synthetic Plastic Masses. L., Goskhimizdat, 1954.

3. Okun’ M. G., Skrynnik I. V., Sukhanovskii S. I., Chudakov M. I., Journal of the Hydrolysis and Wood-Chemical Industry, No. 3, 1960.

4. Tronov B. V., Dobronrazova Z. A., Journal of the Hydrolysis and Wood-Chemical Industry, No. 4, 1960.

5. Petrov G. S., Levin A. N. Thermosetting Resins and Plastics. M., Goskhimizdat, 1959.

6. Bauer K. Analysis of Organic Compounds, Foreign Literature Publishing House, 1953.

7. GOST 3552—47. Pulver-bakelite.