CS210760B1 - Foundry core and molding mixtures - Google Patents

Abstract

The aim of the invention is to improve the durability and increase the strength after hardening of sand foundry mixtures. This is achieved by using as binders dehydrated ketone aldehyde, especially acetone formaldehyde resins with a pH in the range of 4 to 5.5, containing 15 to 30 wt. % hydroxyl groups, in the form of 15 to 30% solutions in a non-reactive organic solvent.

Description

The present invention relates to foundry core and molding compositions comprising dehydrated ketonaldehyde resins as a binder.

For the production of foundry cores and molds, a mixture of foundry sand and binder is used, which is a curable synthetic resin, such as phenolic, urea, furan, etc. The resin is then cured by the action of heat or a catalyst.

More recently, anhydrous phenol-formaldehyde resins whose hydroxyl groups react with isocyanates containing at least two isocyanate groups per molecule have been used successfully to produce foundry cores.

The resulting polyurethane resins combine sand grains into a compact mass. The binder is cured at room temperature by the action of a catalyst supplied to the core in the form of a mist, patent Sp. st. am. No. 3,409,579. A suitable catalyst may also be added to the sand mixture immediately prior to imparting the shape in the core box, GDR Patent No. 6, 79,846.

In addition to the anhydrous phenol-formaldehyde resins, ketone-formaldehyde condensates, NSC No. 2,039,330, can also be used for this purpose. Their advantages are inexpensive raw materials and lower decomposition temperature binders, making the cores prepared therefrom particularly suitable for aluminum and non-ferrous metal castings. The drawbacks of these ketone-formaldehyde condensates are the very short lifetime of the sand mixtures prepared therefrom and a large decrease in the initial strength after curing within a few hours.

This is because the ketone-formaldehyde condensation products used have an alkaline pH, a low content of reactive hydroxyl groups and their solutions in organic solvents are too concentrated, as further detailed below.

These drawbacks are overcome by the cold-hardening foundry core and molding compositions of the invention. They consist of 100 wt. parts of dry foundry sand, 0.5 to 1.5 wt. 0.5 parts by weight of a solution of a binder based on the ketonaldehyde condensation product; % of a 60 to 95% solution of an aromatic polyisocyanate in an organic inert solvent; and 0.01 to 1 wt. part of a tertiary amine catalyst. The present invention is characterized in that the compositions comprise a dehydrated ketonaldehyde resin, in particular a .alpha.-formaldehyde resin, having a pH in the range of 4 to 5.5, containing 15 to 30% by weight, as filler. hydroxyl groups in the form of a 5 to 30% solution in a non-reactive organic solvent.

It has been shown experimentally that the pH of the ketonaldehyde resin, which is adjusted during neutralization of the acidic polycondensation alkaline catalyst, has a great influence on the durability of sand foundry mixtures and the strength of the cores produced and cured. Resins containing an alkaline catalyst, i.e. having an alkaline pH, provide particularly unsatisfactory results.

Conversely, good results are obtained by adjusting the resin to a pH of 4 to 5.5, in particular to a pH of 5 - Figs. 1 and 2, Example 1. It has also been found that a significantly better durability of the sand mixture and higher post-cure strength can be achieved If the starting acetone-formaldehyde resin is high in hydroxyl groups, i.e. a resin with a dry matter content of about 70% by weight. and a hydroxyl group content of 15 to 30%, preferably 20 to 25%, in a less concentrated solution in an organic solvent - Figs. 3, 4, Table 1, Example 2. Another advantage of such mixtures is a substantially lower initial strength or even an increase in strength 24 hours after curing.

These excellent properties of sand mixtures are not merely a function of the hydroxyl group content or the ratio of hydroxyl groups to isocyanate groups. Two low and high hydroxyl group acetone formaldehyde resins may be prepared so that the first solution with a dry matter content of 40 wt. and a second solution having a dry matter content of 20 wt. both contained 6 to 7% hydroxyl groups. Two sand mixtures, prepared from 00 wt. parts of quartz210760 sand, 1 wt. 1 part by weight of a solution of polyisocyanates and 1 wt. parts of the first and second acetone-formaldehyde resins, have the same content of hydroxyl and isocyanate groups.

However, both mixtures differ substantially in their properties. A sand mixture prepared from a more concentrated acetone-formaldehyde resin solution having a lower hydroxyl group content prior to dissolution exhibits the above-mentioned drawbacks, i.e. the life of the sand mixture is low, the initial strengths after curing are lower and still drop considerably within 24 hours. A sand mixture prepared from a less concentrated acetone-formaldehyde resin solution having a high hydroxyl group content before dissolution has substantially improved properties - Figs. 3 and 4, Table 1, Example 2.

For the production of foundry cores and molds according to the invention, quartz foundry sands with a minimum content of dust particles of organic matter and water can be used. The binder consists of dehydrated ketonaldehyde curable resins. They are formed by the alkaline condensation of ketones, most often acetone and cyclohexanone, then methyl ethyl ketone, methyl isobutyl ketone, o-, m-, and p-methylcyclohexanone, acetophenone and diphenylketone, with aldehydes, especially formaldehyde or compounds cleaving it like paraformaldehyde and trioxal; crotonaldehyde and fural. Only resins containing 15 to 30% hydroxyl groups and adjusted to a pH of 4 to 5.5 are suitable.

They are used in the form of 15 to 30% solutions in organic solvents or mixtures thereof which are not reactive with isocyanates. Suitable hydrocarbons are benzene, toluene and xylene, ketones such as acetone and cyclohexanone, esters, for example ethyl acetate, butyl acetate, ethyl acetate, 2-ethoxyethylene glycol acetate and ethers, especially dibutyl ether, dioxane and tetrahydorfuran.

Polyisocyanates of the general formula R (N = C = O) _n , where n is most often 2 or 3 and R is an aliphatic, cycloaliphatic or aromatic residue, are used to react with the ketonaldehyde resins. These include hexamethylene diisocyanate, methylcyclohexylene, dicyclohexylmethane diisocyanate, 2,4- and 2,6-toluene diisocyanate, diphenyl-4,4 ^/ diisocyanate, 3,3 ^<4,4 * Tdimetoxy -difenyldiizokyanát, diphenylmethane-4,4'-diisocyanate , 3,3'-dimetyldifenylmetan-4,4-diisocyanate, 4,4'-difenylizopropylidendiizokyanát, diphenylsulfone-4,4 &apos; diisocyanate, 1,5 -naftylendiizokyanát, triphenylmethane-4,4 ^', 4-triisocyanate, polyfenylmetanpolyizokyanát , and other liquid or solid products.

They are added in the form of 60 to 95 wt. solutions in non-reactive organic solvents, which are also used in the dissolution of ketonaldehyde resins, especially aromatic hydrocarbons. The polyadocyanate polyadocyanates to these resins, which are already at room temperature, are accelerated by the addition of tertiary amine catalysts such as trimethylamine, dimethylethylemine, triethylamine, benzyldimethylamine, N, N-diethylaniline, pyridine, 4-alkylpyridine and N-alkylmorpholine. Their amount ranges from 0.01 to 5% by weight. sand mixtures. The volatile amines can preferably be fed to the formed mixture under pressure in the form of a gas or steam.

The production of the foundry cores and molds according to the invention is generally carried out as follows. The foundry sand is mixed with an appropriate amount of a solution of dehydrated ketonaldehyde resin and polyisocyanates in a mixer. The cores are made from the resulting mixture within 2 hours, usually using compressed air. By feeding the gasified catalyst to the core, the formed sand mixture hardens within seconds. Once cured, the cores are removed and can be molded in as little as 1 hour. For large cores it is also possible to use sand mixtures to which the catalyst has already been added in a rapid mixer. The mixture is then immediately packed into the core box and cured in minutes, eg 5-15.

The foundry core and framing compositions of the invention are further exemplified by embodiments where parts and percentages are units by weight.

Example 1

160 parts of acetone and 2136 parts of 36% formaldehyde are condensed together in the presence of 27 parts of sodium carbonate at 65 ° C. After completion of the condensation, the reaction mixture was cooled and divided into 4 parts. The catalyst is neutralized by acid so that the pH of the parts is 3, 4, 5 and 6. Thereafter, the acetone-formaldehyde resin is freed from the volatile constituents by vacuum distillation, the resulting salt is filtered off, and the remaining water is distilled off by azeotropic distillation with toluene. Finally, toluene is distilled off in vacuo. The finished dehydrated acetone-formaldehyde resins contain 75% dry matter and 9-12% hydroxyl groups. Dissolve to solutions in cyclohexanone with a dry matter content of 40%.

To prepare the test specimens, 100 parts of quartz sand, part of acetone-formaldehyde resin solution and 1 part of 80% xylene technical diphenylmethane-4,4'-diisocyanate solution were mixed in a wheel mixer. The resulting mixture is introduced under pressure into the core box and the test bodies are cured by feeding 0.4 parts of gasified triethylamine in compressed air. The individual bodies are prepared and cured immediately after mixing the mixture - Figs. 1 and 2, curve J. and further at half-hour intervals, ie after 30 minutes - curve 2, after 60 min - curve J and after 90 min - curve 4 ·

The flexural strength is then determined after curing of the body both immediately - Fig. 1 and once in 1 h - Fig. 2. The best results, ie the flexural strength and their loss depending on the time of preparation of the specimens after mixing the mixture, are obtained with acetone formaldehyde resin at pH 5.

Example 2

522 parts of acetone and 1500 parts of 36% formaldehyde are condensed together in the presence of 17 parts of sodium carbonate at a temperature of 70 ° C. After completion of the condensation, the catalyst is neutralized and the pH of the reaction mixture is adjusted to a value of 5. The acetone-formaldehyde resin is then freed of volatile components and water as in Example 1. The prepared acetone-formaldehyde resin contains about 70% dry matter and about 23% hydroxyl groups. The solutions are prepared in cyclohexanone with a dry matter content of 40% and 20%. Test specimens were prepared as in Example 1 from 100 parts quartz sand, 1 part acetone-formaldehyde resin solution and 1 part 80% technical diphenylmethane-4,4'-diisocyanate solution.

Figures 3 and 4 compare the flexural strength of cured compositions prepared from the resin of this example - 20% solids, curve 2, and the resin of pH 5 of Example 1 - curve 1, depending on the storage time of the compositions. Fig. 3 shows the flexural strength values measured immediately after curing, and Fig. 4 shows 1 hour after curing. In addition, Table 1 contains the strengths of the bodies prepared from the resins of this example - 40% dry matter with 1 part and 1.2 parts of the diisocyanate solution.

Table 1

Acetoneformaldehyde resins Strength immediately bending after 1 h MPa po 24 h Example 1, low OH, pH 5, 40% dry matter 1, 9 1,8 1.5 Example 2, high OH content, 40% dry matter 1.0 0.7 0.6 Example 2, high OH content, 40% dry matter, 1.2 parts of isocyanate solution 0.9 1.3 1.3 Example 2, high OH content, 20% dry matter 2.2 2.3 2.1

It can be seen from Figures 3, 40 of Table 1 that the best results are obtained according to the invention using acetone-formaldehyde resins with a higher hydroxyl group content in a lower concentration solution.

Example 3

464 parts of acetone, 196 parts of cyclohexanone and 1665 parts of 36 # formaldehyde were condensed together at 65 ° C in the presence of 15 parts of sodium carbonate. After condensation, the resin preparation is completed as in Example 1. The resulting ketone-formaldehyde resin contains 70 # dry matter and 24 # hydroxyl groups. A resin solution is prepared in a mixture of solvents, i.e. cyclohexanone, acetone and ethylhexyl acetate, with a dry matter content of 20 #. To prepare the cores, 100 parts of quartz sand as in Example 1, 1 part of a ketone-formaldehyde resin solution, 0.2 parts of pyridine and 1 part of a 80 # toluene solution of a mixture of diphenylmethane-4,4'-diisocyanate and triphenylmethane-4,4 'were mixed in a rapid mixer. 4 ^/ '; -triisocyanate, 2: first The mixture is quickly packed into the core boxes where they cure within 10 minutes.

Claims (1)

Cold-hardening foundry core and molding mixtures consisting of 100 parts by weight of dry foundry sand, 0.5 to 1.5 parts by weight of a ketonaldehyde condensation binder solution and 0.5 to 1.5 parts by weight of 60 to 95 # polyisocyanate solution in an organic inert solvent and 0.01 to 1 part by weight of a tertiary amine catalyst, characterized in that they contain as a binder a dehydrated ketonaldehyde resin, in particular acetonformaldehyde, having a pH in the range of 4 to 5.5, containing 15 to 30% by weight hydroxyl groups in the form of a 15-30% solution in a non-reactive organic solvent.