Foundry binder system with low emission of aromatic hydrocarbons

Description

[0001] The present invention relates to furan resin-based binder systems to be used as binders mixed with sand or similar granular substances for the production of moulds or cores for use in foundries.

STATE OF THE ART

[0002] The above-mentioned binder systems for foundries normally consist of two or more components which are mixed with sand or similar granular substances so as then to form the cores or moulds around which or into which the molten metal will then be poured.

[0003] Typical binder systems are those of the polyurethane type, for example, normally consisting of a hydroxyl component, a polyisocyanate and a catalyst. Such binder systems are described in the following documents, for example: US-3,409,579; US-3,676,392; US-3,485,797; WO 97/05447 and WO 00/40351.

[0004] Widespread adoption of no-bake furan binder systems dates from the 1960's and they are still used extensively, being described, for example, on pages 30-34 of the publication "No-bake core & molds" issued in 1980 by the American Foundrymen's Society, incorporated here for reference purposes. These binder systems consist of two liquids (furan resin and acid hardener) which, added to the sand, produce without the use of heat cores and moulds suitable for the production of ferrous and nonferrous metal castings.

[0005] Furan resin consists substantially of furfuryl acid condensed to a greater or lesser degree with formaldehyde, modified where appropriate with urea, phenolic resins and various additives; furan resins which can be used in binder systems of the no-bake type are described, for example, on pages B320-6 to B320-15 of the publication "Manual of chemically setting sands", issued by the Centre Technique des Industries de la Fonderie in 1993, incorporated here for reference purposes.

[0006] The acid hardener generally consists of one or more sulphonic acids in aqueous or hydroalcohol solutions and contains varying amounts of sulphuric acid.

[0007] The sulphonic acids most frequently used are benzenesulphonic acid, toluenesulphonic acid, xylenesulphonic acid and cumenesulphonic acid. Of these, toluenesulphonic acid is by far the most widely used.

[0008] Sulphuric acid, generally present in small amounts in sulphonic acids as a residue of the sulphonation process, is in some cases added intentionally in very large amounts to the hardener to increase the acid energy and therefore accelerate the reaction of hardening of the binder system.

[0009] The hardeners just described, coming into contact with the molten metal during casting, break down releasing aromatic hydrocarbons and sulphurous anhydride. This phenomenon has been monitored by BCIRA (British Cast Iron Research Association), which as early as 1981 published the experimental data collected in the report "Environmental pollution arising from cold-setting sands catalysed by different sulphonic acids".

[0010] The report shows that the sulphonic acids evolve not only the aromatic hydrocarbon from which they are derived (for example toluene in the case of toluenesulphonic acid) but also undergo partial dealkylation of the aromatic ring with the formation of benzene. For benzene, classified as a "recognized human carcinogen", the limits in force for concentrations in the workplace and for emissions into the atmosphere are far more restrictive than those of the other aromatic hydrocarbons.

[0011] Foundries are therefore faced with the problem of not exceeding benzene emission limits, a problem which is not easy to solve since bringing down the benzene contained in the air is much more complex than bringing down the sulphurous anhydride, for example.

DESCRIPTION OF THE INVENTION

[0012] The invention on which the present patent application is based addresses and solves the problem just described with a binder system for foundry sands which can be used in processes of the no-bake type consisting of a furan resin and an acid hardener in which the arylsulphonic acid is completely or partially replaced by an aliphatic acid and by sulphuric acid.

[0013] Since the simpler aliphatic carboxylic acids (formic acid, acetic acid, propionic acid) are not usable in practice because of their pungent or unpleasant odour, the invention provides for the use of aliphatic hydroxy acids and in particular of 2-hydroxypropionic acid (lactic acid) and/or of 2-hydroxypropane-1,2,3-tricarboxylic acid (citric acid).

[0014] As with the conventional hardeners described in the introduction, the presence of sulphuric acid serves to increase the acid energy of the hardener which is the subject of the present invention.

[0015] The hardener which is the subject of the invention, coming into contact with the molten metal, does not evolve aromatic hydrocarbons or evolves a reduced amount of them, depending on the quantity of arylsulphonic acid which it may contain.

[0016] The acid hardeners which are the subject of the present invention have the following composition by weight:

a)	water	10-50%
b)	sulphuric acid	0.5-40%
c)	aliphatic hydroxy acid	10-60%
d)	arylsulphonic acid	0-20%

where the sum of the percentages of components a), b), c) and d) is 100 and the components are assumed to be pure.

[0017] Preferably, the water content is between 25 and 50% by weight, the sulphuric acid content is between 10 and 35% by weight and the aliphatic hydroxy acid content is between 20 and 50% by weight; according to a preferred aspect, the aliphatic hydroxy acid content is greater than 20% by weight and in any case not less than 22%.

[0018] Arylsulphonic acid is preferably absent. If present, it must as far as possible not exceed 20% by weight so as not to negate the advantages deriving from the present invention.

[0019] According to a preferred aspect of the invention, the aliphatic hydroxy acid is selected from lactic acid and/or citric acid; arylsulphonic acid, if present, is preferably toluenesulphonic acid.

[0020] The hardener which is the subject of the invention is mixed with the sand together with the furan resin at ambient temperature to give the cores and moulds suitable for the production of ferrous and nonferrous metal castings; preferably, 100 parts by weight of silica sand or other refractory material are mixed with 0.2-0.8, preferably approximately 0.4-0.6 parts by weight of acid hardener and with 0.6-1.5, preferably 0.8-1.2 parts by weight of furan resin.

[0021] By the term ambient temperature is meant a temperature between roughly 0° and +40° C, preferably between +15 and +30° C, depending on the geographical position (latitude, altitude etc.) in which the foundry is situated.

[0022] The furan resins which are preferably used in the embodiment of the present invention are those already described on pages 30-34 of the publication "No-bake core & molds" already cited and on pages B320-6 to B320-15 of the "Manual of chemically setting sands" already cited, both incorporated here for reference purposes.

[0023] Preferably, the furan resin will have a content by weight of furfuryl alcohol of 60-80%, a nitrogen content of 1-8% and a water content of 5-20%; particularly preferred is a furan resin containing approximately 75% by weight of furfuryl alcohol, approximately 10% of urea-formaldehyde resin (corresponding to about 2% of nitrogen) and about 15% of water.

[0024] These and other aspects of the invention will become clear from the following examples provided purely by way of non-limiting examples of the invention.

Examples

[0025] 

Note 1: the percentages indicated in the examples are by weight.

Note 2: the components used for the examples have the following characteristics:

• the 62% sulphuric acid is commercial 50° Bé sulphuric acid (approximately 62%);
• the 80% lactic acid is a commercial aqueous solution, approximately 80%;
• the monohydrate citric acid is commercial monohydrate citric acid;
• the 65% toluenesulphonic acid is an aqueous solution containing 65% of toluenesulphonic acid with prevalence of the para isomer and approximately 1.2% of sulphuric acid, marketed for example by Cavenaghi SPA with the designation "H 60 hardener";

Hardener 1 (according to the invention)

[0026] 

Sulphuric acid 62%	40%
Lactic acid 80%	60%
Total	100

Hardener 2 (according to the invention)

[0027] 

Water	25%
Monohydrate citric acid	25%
Sulphuric acid 62%	50%
Total	100

Hardener 3 (according to the invention)

[0028] 

Sulphuric acid 62%	40%
Lactic acid 80%	30%
Toluenesulphonic acid 65%	30%
Total	100

Hardener 4 (according to the invention)

[0029] 

Water	25%
Monohydrate citric acid	25%
Sulphuric acid 62%	30%
Toluenesulphonic acid 65%	20%
Total	100

Hardener 5 (for comparison)

Toluenesulphonic acid 65% 100%

[0030] 

Control mixture
100 parts by weight	silica sand LA32
0.4 parts by weight	hardener
1.0 parts by weight	GIOCA NB 2F resin

[0031] GIOCA NB 2F resin is the commercial designation of a furan resin obtained by condensation of furfuryl alcohol, formaldehyde and urea, marketed by Cavenaghi SPA.

[0032] This resin has the following composition:

• furfuryl alcohol approximately 75%
• nitrogen approximately 2%
• water approximately 15%

[0033] Standard +GF bars were prepared with the above-mentioned mixtures. The flexural strength of these bars was measured after 3, 5 and 24 hours from the time of discharging the mixture from the mixer; the results obtained are given in the following table:

Hardener	Setting		Flexural strength in kg/cm2 after
	Start	Finish	3 hours	5 hours	24 hours
1	30'	49'	9	17	23
2	18'	29'	9	11	16
3	17'	29'	14	16	17
4	25'	37'	13	17	19
5	17'	23'	20	23	25

[0034] The results obtained demonstrate that, as well as offering the benefits claimed, the hardeners according to the invention also offer performance characteristics comparable with those of the control hardeners.

Claims

1. An acid hardener which can be used in the preparation of no-bake foundry binder systems having the following composition by weight:

a) water	10-50%
b) sulphuric acid	0.5-40%
c) aliphatic hydroxy acid	10-60%
d) arylsulphonic acid	0-20%

2. An acid hardener according to claim 1 characterized in that the water content is between 25 and 50% by weight.

3. An acid hardener according to any one of the preceding claims characterized in that the sulphuric acid content is between 10 and 35% by weight.

4. An acid hardener according to any one of the preceding claims characterized in that the aliphatic hydroxy acid content is between 20 and 50% by weight.

5. An acid hardener according to any one of the preceding claims characterized in that the aliphatic hydroxy acid content is greater than 20% by weight.

6. An acid hardener according to any one of the preceding claims characterized in that the aliphatic hydroxy acid content is not less than 22% by weight.

7. An acid hardener according to any one of the preceding claims characterized in that said aliphatic hydroxy acid is selected from lactic acid and/or citric acid.

8. An acid hardener according to any one of the preceding claims characterized in that said arylsulphonic acid is toluenesulphonic acid.

9. A no-bake foundry binder system composed of a furan resin and an acid hardener according to any one of the preceding claims.

10. A binder system according to claim 9 composed of 0.2-0.8 parts by weight of said acid hardener and 0.6-1.5 parts by weight of furan resin.

11. A binder system according to claim 10 composed of approximately 0.4-0.6 parts by weight of said acid hardener and approximately 0.8-1.2 parts by weight of furan resin.

12. A binder system according to claims 9-11 characterized in that the furan resin has a furfuryl alcohol content of 60-80% by weight, a nitrogen content of 1-8% by weight and a water content of 5-20% by weight.

13. Foundry cores and/or moulds containing 0.2-0.8 parts by weight of an acid hardener according to claims 1-6, 0.6-1.5 parts by weight of furan resin and 80-120 parts by weight of sand or other refractory material.

14. Cores and/or moulds according to claim 13 containing approximately 100 parts by weight of sand or other refractory material, approximately 0.4-0.6 parts by weight of said acid hardener and approximately 0.8-1.2% parts by weight of furan resin.

15. A method for the production of foundry cores and/or moulds characterized in that an acid hardener according to claims 1-8 is mixed with sand or other refractory material at ambient temperature together with a furan resin.

16. A method according to claim 15 characterized in that 80-120 parts by weight of sand are mixed with 0.2-0.8% parts by weight of said acid hardener and with 0.6-1.5 parts by weight of furan resin.

17. A method according to claim 16 characterized in that approximately 100 parts by weight of sand or other refractory material are mixed with approximately 0.4-0.6 parts by weight of said acid hardener and approximately 0.8-1.2 parts by weight of furan resin.

18. Foundry cores and/or moulds obtainable according to the process in claims 15-17.