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.
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.
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:
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.
Furan resin consists substantially of furfuryl alcohol 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.
The acid hardener generally consists of one or more sulphonic acids in aqueous or hydroalcohol solutions and contains varying amounts of sulphuric acid.
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.
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.
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".
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.
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.
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 replaced by an aliphatic acid and by sulphuric acid.
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).
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.
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.
The acid hardeners which are the subject of the present invention have the following composition by weight:
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%.
Arylsulphonic acid is absent.
According to a preferred aspect of the invention, the aliphatic hydroxy acid is selected from lactic acid and/or citric acid; arylsulphonic acid.
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.
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.
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.
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.
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.
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.
This resin has the following composition:
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:
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.
This list of references cited by the applicant is for the reader's convenience only. It does not form part of the European patent document. Even though great care has been taken in compiling the references, errors or omissions cannot be excluded and the EPO disclaims all liability in this regard.