EP3056296A1

EP3056296A1 - Foundry binder mixture

Info

Publication number: EP3056296A1
Application number: EP16154978.7A
Authority: EP
Inventors: Giorgio Zennaro; Gianni Claudio STROPPOLO
Original assignee: Cavenaghi SpA
Current assignee: Cavenaghi SpA
Priority date: 2015-02-13
Filing date: 2016-02-10
Publication date: 2016-08-17

Abstract

The present invention relates to synthetic resin-based systems, such as furan resins, to be used, in combination with suitable curing agents, as binders in mixtures with silica sand or other granular masses, for the production of foundry moulds and cores. In particular, one aim of the invention consists of the use of at least one alkyl ester of phosphorous acid and/or phosphoric acid and/or a salt thereof as an acidic catalyst for the manufacture of foundry binder mixtures, for example those of the 'no-bake' type. A further aim of the invention also consists of a mixture usable as a foundry binder, for example of the no-bake type, containing: a) monomeric furfuryl alcohol in quantities below 25% by weight of the mixture; b) formaldehyde in quantities no higher than 0.5% by weight of the mixture; c) at least one alkyl ester of phosphorous acid and/or phosphoric acid and/or a salt thereof.

Description

The present invention relates to synthetic resin-based systems, such as furan resins, to be used, in combination with suitable curing agents, as binders in mixtures with silica sand or other granular masses, for the production of foundry moulds and cores.

State of the art

The aforesaid foundry binder systems normally consist of two or more components which are mixed with sand or other granular masses in order to subsequently produce the shapes or cores into which or around which the molten metal will be poured.
The systems which took hold in the 1960s and are still widely used today were the furan no-bake binder systems, such as, for example, those described in pages 30-34 of the publication "No-bake cores and molds", published in 1980 by the American Foundrymen's Society, and incorporated herein for reference. Such binder systems consist of two liquids (furan resin and acidic curing agent) which, when added to the sand, produce, with no heating needed, cores and moulds which are suitable for the production of ferrous and non-ferrous metal castings. The furan resin is substantially constituted of furfuryl alcohol varyingly condensed with formaldehyde, and - if necessary - modified with ureic, melamine, and phenolic resins and various additives. The condensation reaction normally takes place in the presence of an acidic catalyst, such as, for example, acetic acid. Furan resins used in the no-bake systems are described, for example, in pages B320-6 to B320-15 of the publication "Manuel des sables à prise chimique", published by the Centre Technique des Industries de la Fonderie in 1993, incorporated herein for reference. Furan resins are also described in US2343972 , US4634723 , and US 5459183 , incorporated herein for reference.
The acidic curing agent has the function of creating the strongly acidic environment (usually below pHl) needed to cause the sand/resin mixture to cure.
The acidic curing agent usually consists of a hydroalcoholic or aqueous solution of one or more sulphonic acids, if necessary, mixed with other organic or inorganic acids. The most frequently used sulphonic acids are benzenesulphonic acid, toluenesulphonic acid, xylene sulphonic acid, cumene sulphonic acid, phenolsulphonic acid, and methanesulphonic acid, or mixtures thereof. Among these, toluenesulphonic acid is by far the most used.
In some cases the curing agent consists of orthophosphoric acid, either alone or mixed with sulphonic acids. Orthophosphoric acid, however, brings drawbacks related to the formation - as a result of the casting heat - of non-volatile compounds, which accumulate in the sand and lower the refractoriness during subsequent reuse. Another component of the inorganic curing agents is sulphuric acid, which is almost always present in sulphonic acids in small amounts, as the residue of the sulphonation process. In some cases sulphuric acid is intentionally added to curing agents to increase the strength thereof. Sulphuric acid, however, is an often undesired component for both environmental reasons (unpleasant odours) and for reasons of quality in relation to highly critical castings.
With the entry into force of European Union Regulation n. 1272/2008/EC (1st December 2010 ), furan resins with a furfuryl alcohol content of 25% or over are classified as toxic.
Patent application WO 2012/080454A1 relates to a low-emission cold-curing resin for the foundry industry with a furfuryl alcohol content of below 25%, a formaldehyde conversion product content of 40% or above, and an assumed free formaldehyde content of below 0.5%, which uses an organic acid with a pKa of 2.5 or above, and/or the salts thereof, as an acidic catalyst for condensation.

Description of the invention

The need for continual improvement in the technological performance and environmental impact of foundry binder systems has led to testing on condensation catalysts for other furan and phenol-furanic resins than those which are well known and commonly used in industrial production.
Thus it has been surprisingly found that the alkyl esters of phosphorous acid and/or phosphoric acid, together with the salts thereof, make it possible to obtain furan resins with a furfuryl alcohol content below 25%, with a lower level of free formaldehyde and mechanical resistance levels which are either greater than or equal to those of the resins obtainable as described in WO 2012/080454A1 .
The aforesaid alkyl esters of phosphorous acid and/or phosphoric acid may have a pKa below 2.5. They are preferably C₁-C₈ alkyl esters, even more preferably diesters, such as dimethyl phosphate, diethyl phosphate, dipropyl phosphate, dibutyl phosphate, bis(2-ethylhexyl) phosphate, dimethyl phosphite, diethyl phosphite, dipropyl phosphite and dibutyl phosphite; particularly preferred esters have proved to be dibutyl phosphite, dibutyl phosphate, and bis(2-ethylhexyl) phosphate.
In particular, one aim of the present invention consists, therefore, of the use of at least one alkyl ester of phosphorous acid and/or phosphoric acid and/or a salt thereof as an acidic catalyst for the manufacture of foundry binder mixtures, for example those of the 'no-bake' type.
A further aim of the invention also consists of a mixture usable as a foundry binder, for example of the no-bake type, containing:

a) monomeric furfuryl alcohol in quantities below 25% by weight of the mixture;
b) formaldehyde in quantities no higher than 0.5% by weight of the mixture;
c) at least one alkyl ester of phosphorous acid and/or phosphoric acid and/or a salt thereof. According to one aspect of the invention, the said mixture has a formaldehyde conversion products content of at least 35% by weight and, preferably, of at least 45% by weight.

The term "formaldehyde conversion products" is understood as the products resulting from the condensation between formaldehyde and furfuryl alcohol and, optionally, components other than furfuryl alcohol and/or products deriving from the condensation between formaldehyde and one or more compounds other than furfuryl alcohol. The aforesaid compounds other than furfuryl alcohol can be: urea, melamine, phenol or derivatives thereof; the phenol derivatives are preferably C₆-C₂₅ organic compounds containing at least one phenolic radical or at least one benzene radical with two, three or four hydroxyl groups, such as diphenols (for example resorcinol), alkylphenols (for example ortho-cresol, meta-cresol and/or para-cresol), bisphenols (such as 4,4'-isopropylidenediphenol).
According to one aspect of the invention, the said mixture has a water content no greater than 20% by weight and, preferably, no greater than 15% by weight.
According to one aspect of the invention, the mixture has the following composition by weight:

a) monomeric furfuryl alcohol in quantities below 25% of the mixture;
b) formaldehyde conversion products in quantities of at least 35% of the mixture;
c) water in quantities no higher than 20% of the mixture;
d) formaldehyde in quantities no higher than 0.5% of the mixture;
e) at least one alkyl ester of phosphorous acid and/or phosphoric acid and/or a salt thereof.

According to one aspect of the present invention, the one alkyl ester of phosphorous acid aaid/or phosphoric acid and/or a salt thereof is present in the mixture in quantities ranging from between 0.1% and 5.0% by weight, and preferably between 0.3% and 2.0% by weight, with respect to the weight of the mixture.
According to a further aspect, the alkyl ester of phosphorous acid and/or phosphoric acid may be chosen from dibutyl phosphite, dibutyl phosphate and bis(2-ethylhexyl phosphate). According to one aspect of the invention, the mixture may contain further components, such as, for example:

compounds able to increase adhesion between the binder system and the grains of sand, such as for example silanes, preferably including 3-aminopropyl triethoxysilane, 3-(Diethoxymethylsilyl)propylamine), 3-glycidoxy propyl trimethoxy silane;
organic compounds able to slow down the curing speed of the sand/resin/curing agent mixture, such as glycols, preferably C₂-C₁₂ glycols;
compounds capable of reducing the viscosity of the resin, such as C₁-C₄, alcohols, preferably ethanol, or C₅-C₇, aromatic alcohols, preferably benzyl alcohol;
products of furfuryl alcohol conversion with aldehydes other than formaldehyde, such as acetaldehyde, propionaldehyde, butyraldehyde, acrolein, crotonaldehyde, benzaldehyde, salicylaldehyde, cinnamaldehyde and glyoxal, preferably furfuryl alcohol and glyoxal conversion products;
organic compounds comprising one or more amide and/or amine groups, such as, for example, urea;
phenol derivatives, in particular C₆-C₂₅ organic compounds containing at least one phenolic radical or at least one benzene radical with two, three or four hydroxyl groups, such as diphenols (for example resorcinol), alkylphenols (for example ortho-cresol, meta-cresol and/or para-cresol), bisphenols (such as 4,4'- isopropylidenediphenol).
aldehydes with 2 or more carbon atoms, such as acetaldehyde, propionaldehyde, butyraldehyde, acrolein, crotonaldehyde, benzaldehyde, salicylaldehyde, cinnamaldehyde and glyoxal or mixtures thereof, preferably glyoxal;

A further aim of the present invention consists of a process for the production of the aforesaid mixture usable as a foundry binder, wherein a mixture containing:

from 50 to 90 parts by weight of monomeric furfuryl alcohol, preferably from 60 to 85 parts by weight;
from 11 to 45 parts by weight of formaldehyde, preferably from 15 to 30 parts by weight;
from 0.1 to 5.0 parts by weight of at least one alkyl ester of phosphorous acid and/or phosphoric acid and/or a salt thereof, preferably from 0.3 to 2.0 parts by weight;
from 0.5 to 1.5 parts by weight of water, preferably from 0.5 to 1.2 parts by weight;

According to one aspect of the invention, formaldehyde is used in the form of paraformaldehyde, in particular paraformaldehyde with a 89-91% by-weight content of formaldehyde. The mixture thus obtained is then cooled to a temperature ranging from 55 to 65°C and subjected to a further treatment to lower the free formaldehyde content to a value below 0.5%. This occurs by adding an organic compound containing one or more amide or amine groups, such as, for example, urea; or by subjecting the mixture thus obtained to supplementary condensation with a phenolic compound, preferably chosen from among C₆-C₂₅ organic compounds containing at least one phenolic radical or at least one benzene radical with two, three or four hydroxyl groups, such as diphenols (for example resorcinol), alkylphenols (for example cresoles or xylenols), bisphenols (such as 4,4'-isopropylidenediphenol).
Upon reaching a free formaldehyde content below 0.5% by weight, the mixture is cooled to below 35°C, additivised with compounds able to increase adhesion between the binder system and the grains of sand, such as - for example - silanes, preferably including 3-aminopropyl triethoxy silane, 3-(Diethoxymethyl-silyl)propylamine), 3-glycidoxypropyltrimethoxysilane and any additives capable of reducing viscosity (for example, ethanol or benzyl alcohol) or the reactivity of the sand/resin/curing agent mixture (for example glycols).
A further aim of the present invention consists of a foundry binder system, preferably of the no-bake type, comprising or consisting of a binder mixture compliant with the description set out above and an acidic curing agent.
The acidic curing agent may be an aqueous or hydroalcoholic solution with a pH below 2, preferably below 1, at ambient temperature. The aforesaid solution may contain one or more sulphonic acids, such as - for example - benzenesulphonic acid, toluenesulphonic acid, xylene sulphonic acid, cumene sulphonic acid, phenolsulphonic acid, methanesulphonic acid, if necessary, mixed with other organic or inorganic acids. The aforesaid solution may also contain orthophosphoric acid, either alone or mixed with sulphonic acids.
The binder system may be composed of 0.2-0.8 parts by weight acidic curing agent and 0.6-1.5 parts by weight binder mixture, preferably from 0.3-0.6 parts by weight acidic curing agent and 0.7-1.2 parts by weight binder mixture.
One aim of the invention is also represented by the foundry cores and/or moulds containing sand and/or another refractory material and a binder mixture compliant with the description set out above. The cores and/or moulds according to the present invention may be composed of 0.2-0.8 parts by weight acidic curing agent, 0.6-1.5 parts by weight binder mixture, and 80-120 parts by weight sand and/or other refractory material; preferably, such cores and/or moulds contain 100 parts by weight sand and/or other refractory material, 0.3-0.6 parts by weight acidic curing agent, and 0.7-1.2 parts by weight binder mixture.
One aim of the present invention also consists of the procedure for the production of foundry cores and/or moulds, wherein an acidic curing agent is mixed with sand and/or another refractory material, at ambient temperature, and with a binder mixture compliant with the description set out above. Preferably, 80-120 parts by weight sand are mixed with 0.2-0.8 parts by weight said acidic curing agent and with 0.6-1.5 parts by weight binder mixture; still more preferably, approximately 100 parts by weight sand and/or other refractory material are mixed 0.3-0.6 parts by weight acidic curing agent and 0.7-1.2 parts by weight binder mixture. The aims of the present invention also include the foundry cores and/or moulds obtainable according to the said procedure.
The term 'ambient temperature' is understood as a temperature ranging approximately from 0 ° to + 40 °C, preferably from +15 to + 30 °C, depending on the season and the geographic position (latitude, altitude, etc.) of the foundry.
The following examples are provided as non-limiting illustrations of the present invention.
In all the examples, the free formaldehyde content was determined using the AN006.6 method, which is annexed hereto, and based on the reaction of formaldehyde with sodium sulphite. This method is taken from the text by Walker, J. Frederic, Formaldehyde, Third Edition, Robert E. Krieger Publishing Corporation and the EN ISO 11402:2004 method, incorporated herein for reference.
Examples 1 and 2, which are not compliant with the present invention, were obtained as described in WO 2012/080454A1 .

Example 1: Resin NS-1A (not compliant with the invention).

Recipe:

a) Furfuryl alcohol	384.40 g
b) Paraformaldehyde (89-91% by-weight formaldehyde)	91.50 g
c) Water	10.50 g
d) Sodium hydroxide, 30% by-weight aqueous solution	1.10 g
e) Lactic acid, 80% by-weight aqueous solution	12.50 g
f) Urea	37.50 g
g) Sodium hydroxide, 30% by-weight aqueous solution	7.50g
h) 3- (Diethoxymethylsilyl)propylamine	1.15 g

Processing instructions:

Load the raw materials a) b) c) d) e) into a reactor equipped with a reflux condenser
Start the agitator and keep it operating throughout the duration of the production
Turn on heat and bring to reflux condition (120-125 °C) in 40-90'
Maintain at reflux (pH = 4.2-6.5) until a free furfuryl alcohol content of between 22 and 25% is reached (gas chromatographic control)
Cool to 60°C
Load the raw material f)
Adjust the pH so that it is above 5.5 with the raw material g)
Keep at 40-60°C for 15'
Load the raw material h)
Cool to 35°C

The resin thus obtained is clear and has the following characteristics:

pH at 25°C: 5.9
Viscosity a 20°C: 140 mPa.s
Nitrogen (Kjeldahl): 3.20%
Free furfuryl alcohol (G.C.): 22.4%
Water (Karl Fischer): 9.6%
Free formaldehyde (AN006.6): 0,60 %
Products of formaldehyde conversion: 67.40%

Example 2: Resin NS-1B (not compliant with the invention)

Recipe:

a) Furfuryl alcohol	384.40 g
b) Paraformaldehyde (89-91% by-weight formaldehyde)	91.50 g
c) Water	10.50 g
d) Sodium hydroxide, 30% by-weight aqueous solution	1.10g
e) Lactic acid, 80% by-weight aqueous solution	12.50 g
f) 4.4'-isopropylidenediphenol	17.00g
g) Potassium hydroxide, 48% by-weight aqueous solution	11.70g
h) Urea	10.60 g
i) 3- (Diethoxymethylsilyl)propylamine	1.20 g

Processing instructions:

Load the raw materials a) b) c) d) e) into a reactor equipped with a reflux condenser
Start the agitator and keep it operating throughout the duration of the production
Turn on heat and bring to reflux condition (120-125 °C) in 40-90'
Maintain at reflux (pH = 4.2-6.5) until a free furfuryl alcohol content of between 22 and 25% is reached (gas chromatographic control)
Cool to 80°C
Load the raw materials f) and g) and keep at 80°C for 1 hour
Cool to 45°C
Load the raw materials h) and i)
Keep at 40-45°C for 15'
Cool to 35°C

The resin thus obtained is clear and features the following characteristics:

pH at 25°C: 7.2
Viscosity a 20°C: 140 mPa.s
Nitrogen (Kjeldahl): 0.91%
Free furfuryl alcohol (G.C.): 22.6%
Water (Karl Fischer): 9.4%
Free formaldehyde (AN006.6): 0.76 %
Products of formaldehyde conversion: 67.24%

The two resins were used as the basis for the synthesis of the resins according to the invention. In the latter resins, solely the catalyst was changed [(raw material e)] and - where necessary - the type and amount of the raw material d) was adjusted to maintain the pH during condensation at a value above 4.2, as was the quantity of water c), in order to maintain the reflux temperature at 120-125°C. The processing instructions remain unchanged.

Example 3: Resin NS-2A (compliant with the invention)

Recipe:

a) Furfuryl alcohol	384.40 g
b) Paraformaldehyde (89-91 % by-weight formaldehyde)	91.50 g
c) Water	2.74 g
d) Sodium hydroxide, 30% by-weight aqueous solution	3.07 g
e) Dibutyl phosphate	5.13 g
f) Urea	36.51 g
g) Sodium hydroxide, 30% by-weight aqueous solution	1.36 g
h) 3- (Diethoxymethylsilyl)propylamine	1.12 g
i) Water	28.04 g

Processing instructions:

Load the raw materials a) b) c) d) e) into a reactor equipped with a reflux condenser
Start the agitator and keep it operating throughout the duration of the production
Turn on heat and bring to reflux condition (120-125 °C) in 40-90'
Maintain at reflux (pH = 4.2-6.5) until a free furfuryl alcohol content of between 22 and 25% is reached (gas chromatographic control)
Cool to 60°C
Load the raw material f)
Adjust the pH so that it is above 5.5 with the raw material g)
Keep at 40-60°C for 15'
Load the raw material h)
Cool to 35°C
Load the raw material i)

The resin thus obtained is clear and has the following characteristics:

pH at 25°C: 7.7
Viscosity a 20°C: 110 mPa.s
Nitrogen (Kjeldahl): 3.1%
Free furfuryl alcohol (G.C.): 23.2%
Water (Karl Fischer): 11.8%
Free formaldehyde (AN006.6): 0,36 %
Products of formaldehyde conversion: 64.64%

Example 4: Resin NS-4A (compliant with the invention)

Recipe:

a) Furfuryl alcohol	384.40 g
b) Paraformaldehyde (89-91% by-weight formaldehyde)	91.50 g
c) Water	4.83 g
d) Triethanolamine **	0.67 g
e) Dibutyl phosphite	4.56 g
f) Urea	36.45g
g) Sodium hydroxide, 30% by-weight aqueous solution	0.60 g
h) 3- (Diethoxymethylsilyl)propylamine	1.14 g
i) Water	9.16 g

** Triethanolamine was used instead of 30% sodium hydroxide in order to obtain a more gradual adjustment of the pH.

Processing instructions:

Load the raw materials a) b) c) d) e) into a reactor equipped with a reflux condenser
Start the agitator and keep it operating throughout the duration of the production
Turn on heat and bring to reflux condition (120-125 °C) in 40-90'
Maintain at reflux (pH = 4.2-6.5) until a free furfuryl alcohol content of between 22 and 25% is reached (gas chromatographic control)
Cool to 60°C
Load the raw material f)
Adjust the pH so that it is above 5.5 with the raw material g)
Keep at 40-60°C for 15'
Load the raw material f)
Cool to 35°C
Load the raw material i)

The resin thus obtained is clear and has the following characteristics:

pH at 25°C: 8.02
Viscosity a 20°C: 80 mPa.s
Nitrogen (Kjeldahl): 3.1%
Free furfuryl alcohol (G.C.): 22.2%
Water (Karl Fischer): 11.7%
Free formaldehyde (AN006.6): 0.14%
Products of formaldehyde conversion: 65.96%

Example 5: Resin NS-2B (compliant with the invention)

Recipe:

a) Furfuryl alcohol	384.40 g
b) Paraformaldehyde (89-91% by-weight formaldehyde)	91.50 g
c) Water	2.74 g
d) Sodium hydroxide, 30% by-weight aqueous solution	3.07g
e) Dibutyl phosphate	5.16 g
f) 4.4'-isopropylidenediphenol	16.55 g
g) Potassium hydroxide, 48% by-weight aqueous solution	11.39g
h) Urea	10.37 g
i) 3- (Diethoxymethylsilyl)propylamine	1.12 g
j) Water	20.29 g

Processing instructions:

Load the raw materials a) b) c) d) e) into a reactor equipped with a reflux condenser
Start the agitator and keep it operating throughout the duration of the production
Turn on heat and bring to reflux condition (120-125 °C) in 40-90'
Maintain at reflux (pH = 4.2-6.5) until a free furfuryl alcohol content of between 22 and 25% is reached (gas chromatographic control)
Cool to 80°C
Load the raw materials f) and g) and keep at 80°C for 1 hour
Cool to 45-50°C
Load the raw materials h) and i)
Keep at 45-50°C for 15'
Cool to 35°C
Load the raw material j)

The resin thus obtained is clear and features the following characteristics:

pH at 25°C: 9.82
Viscosity a 20°C: 130 mPa.s
Nitrogen (Kjeldahl): 0.89%
Free furfuryl alcohol (G.C.): 24.0%
Water (Karl Fischer): 11.9%
Free formaldehyde (AN006.6): 0.16 %
Products of formaldehyde conversion: 63.94%

Example 6: Resin NS-4B (compliant with the invention)

Recipe:

a) Furfuryl alcohol	384.40 g
b) Paraformaldehyde (89-91% by-weight formaldehyde)	91.50 g
c) Water	4.83 g
d) Triethanolamine **	0.67 g
e) Dibutyl phosphite	4.56 g
f) 4.4'-isopropylidenediphenol	16.52g
g) Potassium hydroxide, 48% by-weight aqueous solution	11.37g
h) Urea	10.30 g
i) 3- (Diethoxymethylsilyl)propylamine	1.12 g
j) Water	21.77 g

** Triethanolamine was used instead of 30% sodium hydroxide in order to to obtain a more gradual adjustment of the pH.

Processing instructions:

The resin thus obtained is clear and features the following characteristics:

pH at 25°C: 9.54
Viscosity a 20°C: 120 mPa.s
Nitrogen (Kjeldahl): 0.87%
Free furfuryl alcohol (G.C.): 22.2%
Water (Karl Fischer): 11.6%
Free formaldehyde (AN006.6): 0.21%
Products of formaldehyde conversion: 65.99%

Technology-based comparison tests

Composition of the mixtures:

100 parts by weight French silica sand LA 32
0.4 parts by weight p-toluenesulphonic acid, 65% aqueous solution
1.0 part by weight resin
The mixtures were prepared in the laboratory using a Kitchen Aid Artisan rapid mixer.

With the aforesaid mixtures, standard + GF + bars were prepared with a 5 cm² rupture section. The bending strength of the aforesaid bars was measured after 3, 5, and 24 hours of stay at a temperature of 18-19°C from the time the mixture is unloaded from the mixer using a BENET device (COE model). The results obtained are shown in the table below:

Resin	Relative humidity (%)	Curing time		Bending strength (N/cm²) after:			% CH₂O
Resin	Relative humidity (%)	start	end	3 hours	5 hours	24 hours
NS-1A	48	17'	26'	270	350	395	0.60
NS-2A	47	18'	25'	300	420	415	0.36
NS-4A	47	11'	17,5'	360	385	445	0.14
NS-1B	47	10'	15'	280	360	440	0.76
NS-2B	47	11'	17.5'	340	390	450	0.16
NS-4B	46	11'	18'	360	400	445	0.21

The results show that the resins obtained using catalysts in accordance with the present invention (NS-2A , NS-4A, NS- 2B, and NS-4B resins) have a considerably lower free formaldehyde content than the non-compliant resins (NS-1A resins and NS-1B) and develop mechanical strength levels which are greater than or at least equal to those of the non-compliant resins (NS-1A and NS-1B resins); this second aspect constitutes great technical progress because, resins with very low free formaldehyde usually develop significantly lower strength levels than those of resins richer in formaldehyde.

Claims

Mixture usable as a foundry binder containing:
a) monomeric furfuryl alcohol in amount less than 25% by weight of the mixture;

b) formaldehyde in quantities no higher than 0.5% by weight of the mixture;

c) at least one alkyl ester of phosphorous acid and/or phosphoric acid and/or a salt thereof.
The mixture according to Claim 1, characterised in that said alkyl ester of phosphorous acid and/or the said alkyl ester of phosphoric acid has a pKa of less than 2.5.
The mixture according to Claim 1, characterised in that said alkyl ester of phosphorous acid and/or the said alkyl ester of phosphoric acid is a C₁-C₈ alkyl ester.
The mixture according to any one of the preceding claims, characterised in that said alkyl ester of phosphorus acid and/or said alkyl ester of phosphoric acid is a diester, preferably dimethyl phosphate, diethyl phosphate, dipropyl phosphate, dibutyl phosphate, bis(2-ethylhexyl) phosphate, dimethyl phosphite, diethyl phosphite, dipropyl phosphite or dibutyl phosphite, even more preferably, dibutyl phosphite, dibutyl phosphate or bis(2-ethylhexyl) phosphate.
The mixture according to any of the previous claims, characterised by having a formaldehyde conversion products content of at least 35% by weight.
The mixture according to any of the previous claims, characterised by having a water content not exceeding 20% by weight.
A process for producing a mixture according to any one of the previous claims, wherein a mixture containing:
• from 50 to 90 parts by weight of monomeric furfuryl alcohol, preferably from 60 to 85 parts by weight;

• from 11 to 45 parts by weight of formaldehyde, preferably from 15 to 30 parts by weight;

• from 0.1 to 5.0 parts by weight of at least one alkyl ester of phosphorous acid and/or phosphoric acid and/or a salt thereof, preferably from 0.3 to 2.0 parts by weight;

• from 0.5 to 1.5 parts by weight of water, preferably from 0.5 to 1.2 parts by weight;
is kept under stirring following the addition of a base at a temperature ranging from 110 to 125°C.
The process according to claim 7, characterised in that the said stirring is maintained for a period ranging 3 to 7 hours.
A foundry binder system, comprising an acidic curing agent and a binder mixture according to any of Claims 1 to 6.
Foundry binder system according to Claim 9, characterised in that the said curing agent is an aqueous or hydroalcoholic solution with a pH lower than 2, preferably lower than 1.
Foundry binder system according to Claim 9, characterised in that such system is composed of 0.2-0.8 parts by weight acidic curing agent and 0.6-1.5 parts by weight binder mixture, preferably from 0.4-0.6 parts by weight acidic curing agent and 0.8-1.2 parts by weight binder mixture.
Foundry cores and/or moulds containing sand and/or a refractory material and a binder mixture according to any of Claims 1 to 6.
Cores and/or moulds according Claim 12, containing 0.2-0.8 parts by weight acidic curing agent, 0.6-1.5 parts by weight binder mixture and 80-120 parts by weight sand and/or other refractory material, preferably approximately 100 parts by weight sand and/or other refractory material, 0.4-0.6 parts by weight acidic curing agent and 0.8-1.2 parts by weight binder mixture;
A process for the production of foundry cores and/or moulds according to either Claim 12 or Claim 13, wherein the binder mixture, the acidic curing agent and the sand and/or other refractory material are mixed together at room temperature.
Use of at least one alkyl ester of phosphorous acid and/or phosphoric acid (preferably with a pKa of less than 2.5) and/or a salt thereof as an acidic catalyst for the manufacture of foundry binder mixtures.
Use according to Claim 15, characterised in that said alkyl ester is a C₁-C₈ alkyl ester.
Use according to Claim 15, characterised in that said alkyl ester is a diester, preferably dimethyl phosphate, diethyl phosphate, dipropyl phosphate, dibutyl phosphate, bis(2-ethylhexyl) phosphate, dimethyl phosphite, diethyl phosphite, dipropyl phosphite or dibutyl phosphite, and still more preferably, dibutyl phosphite, dibutyl phosphate, or bis(2-ethylhexyl) phosphate.