Use of Hydrolysis Lignin for the Production of Resins of the Pulver-Bakelite Type

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

(Presented by Prof. L. P. Kulev, Doctor of Chemical Sciences)

Hydrolysis lignin is a waste product from the production of alcohol and other products obtained from wood. In 1960, the amount of lignin in the Soviet Union was expected to be approximately 500 thousand tons. The problem of using lignin is connected with the economics of hydrolysis production as a whole and therefore requires urgent solution.

Despite numerous recommendations for its use as a chemical raw material for the production of a number of products, it is still not being processed, but is mainly hauled to dumps or partly burned together with coal in boiler furnaces.

By its chemical nature, pure hydrolysis lignin consists of polymers of a fatty-aromatic character, containing phenolic hydroxyls. Technical air-dry hydrolysis lignin contains from 15 to 20% of difficultly hydrolyzable carbohydrates, fats, waxes, and mineral substances. The difficulty of obtaining it in pure form poses the task of using technical lignin directly.

The most promising direction in the use of lignin is its modification as a high-molecular-weight substance, imparting to it a certain reactivity for subsequent polymerization or condensation with active monomers or polymers. In the coming years, apparently, lignin will be used chiefly for the partial replacement of phenol and formaldehyde in the production of pulver-bakelite, which is used in the machine-building industry for shell precision casting [1].

Pulver-bakelite is prepared on the basis of phenol-formaldehyde resins, the disadvantages of which include the use of comparatively expensive and scarce phenol and the release of a considerable quantity of harmful gaseous products when hot metal is poured into molds.

In the Federal Republic of Germany, hydrochloric-acid lignin (a waste from glucose production) is currently used completely for the production of pulver-bakelite [2]. In this process up to 30% of phenol is saved. Work on the use of hydrolysis lignin for producing pulver-bakelite is being carried out in the USSR at VNIIPS; as a result, the lignophenol-formaldehyde resin obtained at a phenol-to-formalin ratio of 1.1:1 mol and a phenol-to-lignin ratio of 2:1 (by weight)

parts), is recommended for the preparation of pulverbakelite. According to M. I. Chudakov et al., with hydrolysis lignin one can save 30% of the phenol and obtain lignophenol-formaldehyde resins whose physicochemical and mechanical properties are close to those of analogous phenol-formaldehyde resins, also prepared at a phenol-to-formaldehyde ratio of 1.1 : 1 mole.

In our opinion, when lignin is used, less than 1 mole of formaldehyde should be taken per 1.1 moles of phenol, since lignin is not merely mechanically admixed with the resin, but enters into reaction with phenol and occupies one of its reactive sites; at the indicated amount of formaldehyde, apparently, a resin should be obtained that is close in properties to thermosetting resins and is of little use for pulverbakelite. We carried out syntheses of lignophenol-formaldehyde resin under the conditions of M. I. Chudakov and obtained pulverbakelite with a tensile strength of only 20–25 kg/cm² (under our conditions for determining strength, the tensile strength of pulverbakelite from phenol-formaldehyde resin is 85 kg/cm²). We then planned to carry out a study that would make it possible to establish the optimum amounts of formaldehyde and phenol necessary for the synthesis of sufficiently strong lignophenol-formaldehyde resins.

The quality of phenol-formaldehyde resins depends on the catalysts used, the ratio of the components, the time and temperature of condensation, and the drying conditions.

By varying these conditions and using the regularities derived for the condensation reactions of phenol with formaldehyde, it is possible to ensure the production of a resin with predetermined properties. The introduction of a third component, lignin, does not allow the known regularities to be used fully and therefore requires additional studies to clarify the influence of the listed conditions on the quality of the polycondensation product. This was the subject of the investigation whose results are reported in the present work.

Experimental Part

Hydrolysis lignin from the Krasnoyarsk plant was used, with the following characteristics: methoxyl groups—9.1%, hydroxyl groups—5.2%, carboxyl groups—1.0%, sulfuric acid—1.0%, residual moisture (after drying to the air-dry state)—7.9%, ash content—1.1%, carbonyl groups—1.7%.

The lignin was preliminarily ground and sieved through a sieve with an aperture diameter of 1 mm. The fraction with particle size of 1 mm and smaller was selected. Phenol was placed in a round-bottom flask with a stirrer, a reflux condenser, and an opening for adding formalin. The flask was heated in a glycerol bath, and at the specified temperature lignin and hydrochloric or sulfuric acid were gradually added over 1–6 hours. After a homogeneous liquid mixture had been obtained (4–12 hours), formalin (32%) was added in portions at 90–95°, and the synthesis was completed after the established time had elapsed. The resulting resin was dried by short-term heating to 200°, and also, under other conditions, was ground and sieved through a sieve with an aperture diameter of 0.06–0.08 mm. Free phenol and formaldehyde [5], softening temperature, polymerization rate [6], and tensile strength [7] were determined in the resins.

Conditions for the Synthesis of Resin 79 (lignophenol-formaldehyde) with a tensile strength of 22.5 kg/cm²

Into a three-necked flask equipped with a stirrer and reflux condenser, 300 g of phenol were introduced at a temperature of 110° (glycerol bath)

Table 1

Effect of the lignin-to-phenol ratio on the mechanical strength of resins

Note. Time of the first stage 10 hours, temperature 110°; time of the second stage 2 hours, temperature 95°.

Table 2

Effect of the amount of formalin on the mechanical strength of resins

Note. Time of the second stage 2 hours, temperature 90–95°.

and, with stirring, 13 ml of hydrochloric acid (d 1.17) and 100 g of lignin were added. The remaining hydrochloric acid (37 ml) and lignin (200 g) were introduced into the reaction flask in three portions at intervals of 1 h 20 min after each charge. The total time of the first stage of synthesis was 10 hours. The amount of free phenol in the lignophenol was 21%, the drop temperature-

Table 3

Effect of condensation time on the mechanical strength of resins

Note. The time of the second stage was 2 hours; temperature 90–95°.

Table 4

Effect of the nature of the condensation-reaction catalysts on resin strength

Note. The time of the second stage was 2 hours; temperature 90–95°.

[[unclear: beginning of word]] 65°. After the cooking of the phenol-lignin resin was completed, the temperature was lowered to 90–95°, and, with the stirrer running, 40 ml of 32% formalin was slowly added over 20 minutes. Stirring was then continued for another 1 h 40 min. The resin obtained was a mobile dark mass. Free phenol: 16.3%; dropping point: 107°. Then, in the reaction flask, the resin was dried with brief heating to 180°. For this purpose, the temperature was raised over 30 minutes and held for 40 min at 180° with mechanical stirring. Temperature

Table 5

Effect of condensation temperature on the mechanical strength of resins

Note. The time of the second stage was 2 hours, temperature 90—95°.

The drop point of the dried resin was 148—149°. The gelation time with 10% urotropine at 150° was 25 seconds. Free phenol, 4.1%; formaldehyde, 0.7%.

Tables 1—5 present data characterizing the influence of the conditions of synthesis and drying of the resins on their quality (mechanical strength).

It follows from the tables that, with an increase in the amount of lignin at the same amount of phenol, the tensile strength of the resins decreases (Table 1). In experiment 79 a different ratio of formalin to phenol was taken as compared with experiments 64—71; the strength of the resin proved to be higher. The optimum amount of formalin required to obtain a resin with maximum strength depends on the ratio of lignin to phenol and, at a ratio of 1 : 1, amounts to 0.15 mole of formaldehyde per 1 mole of phenol (Table 2, resin 79). When the amount of formaldehyde is changed in either direction, a resin of poorer quality is formed. The time of the first stage of resin synthesis (condensation of lignin with phenol) is 12 hours (Table 3, resin 79). Under these conditions a resin with a minimum content of free phenol is formed. From Table 4 it is seen that resins obtained in the presence of sulfuric or hydrochloric acid are of almost identical quality. Raising the condensation temperature of lignin with phenol above 110° is undesirable, since it does not increase the mechanical strength of the resins (Table 5). Resins obtained in the presence of hydrochloric acid are best dried by brief heating to 180—200°. Those prepared using sulfuric acid as catalyst are dried at 110° for 40 minutes.

The synthesized lignophenol-formaldehyde resins were tested in the foundry shop of the Tomsk Cutting Tools Plant. The mechanical strength of hollow shell molds made from resins 67 and 79 was sufficient and made it possible to use them for producing various castings weighing up to 30 kg. The surface finish of the cast parts was the same as that of castings obtained using pulver-bakelite.

The encouraging positive test results make it possible to undertake broader studies of these resins in order to determine the feasibility of introducing them into foundry production.

Conclusions

A procedure has been developed for cooking a phenol-lignin-formaldehyde resin (FLF-79), which, in mixture with urotropine, we recommend as a substitute for pulver-bakelite used in precision casting. In the production of FLF-79, 30% of the phenol and 90% of the formalin can be replaced by hydrolysis lignin.

References

1. M. I. Chudakov. Use of lignin and lignin-containing wastes abroad, Moscow, 1950.

2. S. I. Sukhanovskii, M. I. Chudakov. Chemical Science and Industry, 2, No. 4, 1957.

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

4. F. I. Smirnov. Improvement of shell-mold casting technology, Leningrad, Exchange of Advanced Experience. Series—Foundry Production, issue 9, 1952.

5. K. Bauer. Analysis of Organic Compounds, IL Publishing House, Moscow, 1953.

6. K. Andrianov, D. Kartashov. Practical Work on Synthetic Resins and Plastics, Goskhimizdat, 1946.

7. GOST 3552—47 for pulver-bakelite.