Polyurethane-based binder system for the production of casting molds or cores

Producing casting molds by blending flowable, granular material such as sand with a binder system including reactants to form polyurethane, i.e., polyisocyanates and polyols, and a solvent consisting of an ester having as its acid component an aliphatic dicarboxylic acid with from six to twelve carbon atoms, or a benzene polycarboxylic acid with at least three COOH groups; and having as its alcohol component an aliphatic, cycloaliphatic, arylaliphatic, or aromatic alcohol having from six to thirteen carbon atoms.

BACKGROUND OF THE INVENTION 
In casting technology, casting molds and cores of any considerable size are 
made from material mixtures containing a grainy base (in most cases sand) 
mixed with a polyurethane-based, cold hardening binder system. Such a 
binder system is composed of polyisocyanates with at least two NCO groups 
in the molecule and polyols with at least two OH groups in the molecule as 
reaction partners, as well as a tertiary amine or in some cases a chelate 
compound as an accelerator. Depending on whether the so-called "cold 
hardening process" or the so-called "gas hardening process" is being used, 
the accelerator will be added to the mixture either (a) together with the 
other ingredients of the binder system immediately before the mixture is 
to be used, or (b) only after the mixture, made up without the 
accelerator, has been put into a casting box and the mixture in the box is 
then treated with a gaseous tertiary amine accelerator. 
A binder system of this sort generally also includes a solvent, especially 
when one or both reaction partners are present in a higher molecular form 
such as prepolymers. Thus, for example, resins of condensation made of 
phenols or phenol-related compounds with aldehydes are quite well-suited 
polyols, which regularly require a solvent on account of their relatively 
high molecular weight. 
Although the solvent does not participate in the reaction between the 
polyisocyanates and the polyols to form urethanes, it nevertheless exerts 
an influence on the course of the reaction, which is probably related to 
the fact, among others, that the two reaction partners have varying 
degrees of compatibility with the various types of solvents. In general, 
polar solvents are well suited for phenol resins and similar polyols but 
less compatible with polyisocyanates while the opposite is true of 
non-polar solvents. In practice, therefore, mixtures of polar and 
non-polar solvents are normally used, the proportions being adjusted to 
suit the particular binder system used. The individual ingredients of this 
mixture should not have too low a boiling point so that the solvent does 
not lose its effectiveness too soon through evaporation. 
For non-polar solvents, aromatic hydrocarbons which are usually in the form 
of mixtures with a boiling point above ca. 150.degree. C. (at normal 
pressure) are preferred; and for polar solvents, aliphatic and cyclic 
ketones, fatty acid esters, acetals or ketals, glycol esters, glycol ether 
esters, glycol diethers and similar types of compounds having a 
sufficiently high boiling point have been used. 
However, all of the above-named polar solvents have a serious disadvantage, 
in contrast to the non-polar solvents mentioned: they have an extremely 
unpleasant smell and thus make for unpleasant working conditions, which 
cannot generally be remedied by special hoods or the like. In this 
connection it should be pointed out that resins have been developed that 
have only a slight unpleasant odor (which might, for example, be due to a 
residue of free formaldehyde), so that the solvents are in fact the 
principal source of unpleasant smells on the job. Solvents with no odor 
and with otherwise satisfactory characteristics are, therefore, urgently 
necessary. 
A first step in this direction has been disclosed in Austrian Pat. No. 
342,794 in suggesting the use of phthalic acid dialkylesters (preferably 
o-phthalic acid), which are liquid at room temperature and have an alkyl 
radical of from one to twelve and typically from three to six carbon 
atoms. Such phthalic acid esters are quite odorless, if not completely so. 
They have the additional advantage that they are more compatible with 
polyisocyanates than, for example, isophoron (a cyclic ketone frequently 
used as a solvent) and therefore lead to casting forms with somewhat 
better characteristics. Of course, their compatibility with 
polyisocyanates is still not optimal, and they have the additional 
disadvantage that they crack easily during the casting process, which 
leads to sublimation with a lot of smoke and correspondingly strong smell. 
SUMMARY OF THE INVENTION 
The invention has the purpose of creating an odorless polar solvent for a 
polyurethane-based binder system for material mixtures used in the 
production of casting molds and cores, which will avoid the disadvantage 
of the phthalic acid dialkyl esters with regard to thermal stability 
during casting and will also improve the characteristics not only of the 
material mixture but also of the casting forms made from it. 
The invention achieves this by means of a solvent consisting of or 
containing esters having as their acid component either an aliphatic 
dicarboxylic acid with from six to twelve carbon atoms or a benzene 
polycarboxylic acid with three or more COOH groups, and having as their 
alcohol component an aliphatic, cyclo aliphatic, arylaliphatic or aromatic 
alcohol with from six to thirteen carbon atoms. 
Typical examples of the acid components of the group of esters lying in 
this range are the radicals of the aliphatic dicarboxylic acids adipinic 
acid (6 carbon atoms), suberinic acid (8 carbon atoms), azelaic acid (9 
carbon atoms), sebacinic acid (10 carbon atoms) and decandicarboxylic acid 
(12 carbon atoms), as well as the radicals of the benzene polycarboxylic 
acids trimellitic acid (3 COOH groups attached to the benzene nucleus) and 
pyromellitic acid (4 COOH groups attached to the benzene nucleus). For the 
alcohol components, typical examples are all the aliphatic alcohols with 
six to thirteen carbon atoms, i.e., from hexylalcohol to tridecylalcohol, 
as well as cyclic, arylaliphatic and aromatic alcohols like cyclohexyl 
alcohol, cyclooctyl alcohol and benzyl alcohol, and in some cases alcohols 
with additional ether bridges, like butoxyethyl alcohol. This includes all 
isomers and mixtures of isomers, which typically occur commercially, for 
the acid components as well as the alcohol components. In addition, the 
solvents specified in the invention need not consist of only one of the 
esters lying in the range of the invention, but may also be mixtures of 
different esters in this range. 
Correspondingly, the solvents defined by the invention can be chosen from 
one or several of the following particular esters, which have been shown 
to be very appropriate: bis-(2-ethylhexyl)-adipate (DOA), 
di-n-nonyl-adipate and di-isononyl-adipate, di-n-octyl-adipate (DIDA), 
bis-[methylcyclohexyl]-adipate, bis-[methyl-cyclohexylmethyl]-adipate, 
benzyl-octyl-adipate, bis[butoxyethyl]-adipate, di-n-hexyl-azelate (DHAZ), 
tetrakis-[2-ethylhexyl]-pyromellitate, trisisooctyl-trimellitate, 
tris-octyl-trimellitate, bis-2-ethylhexylsebacate, di-n-octyl-sebacate, 
di-n-hexyl-sebacate, as well as esters from alcohol mixtures like 
tri-nC.sub.8 -C.sub.10 -Tri-mellitate (TTM) and di-nC.sub.7 -nC.sub.9 
-adipate. Their boiling points are without exception above this value. So 
they all belong to the class of materials having a high boiling point. 
The solvents specified in the invention are odorless and nontoxic, so they 
fulfill the demands for environmental protection on the job. Also, their 
thermal stability is very high, and their rate of evaporation is 
practically nil. But above all, they improve the characteristics of the 
mold material and the molds made from it to a considerable degree, which 
is especially striking in the case of the gas hardening process. 
Before the advantages of the solvents specified by the invention are 
explained in detail, it should be pointed out that these advantages were 
not at all predictable, even though the esters used for solvents up to now 
were analogous in structure. Of course, the aliphatic esters which are 
familiar as solvents have either or both components with less than six 
carbon atoms, and the familiar phthalic acid esters have only two ester 
groups attached to the benzene nucleus. In contrast, the solvents 
specified by the invention, insofar as they are esters of an aliphatic 
dicarboxylic acid, have a relatively high number of carbon atoms, as well 
as a comparable number of carbon atoms in the two components; and the 
benzene polycarboxylic acids have three or more ester groups attached to 
the benzene nucleus. This difference has proved to be decisive for the 
success of the invention. The esters used according to the invention 
have-despite the consistently higher number of ester groups in the 
molecule--a strongly hydrophobic, non-polar molecular structure, in which 
the polar effect of the ester groups is largely screened off by the 
hydrophobic radicals lying on the outside. What is surprising is that they 
play something of a double role in that they function like a good polar 
solvent with good dissolving power with respect to the resin but behave 
otherwise like a non-polar solvent. That is, they are hydrophobic and 
exceptionally compatible with polyisocyanates. 
Other esters having components with long chains which are not in the range 
of the invention, like butyl stearate for example, do not have the 
positive effects of the solvents specified by the invention. Thus the 
range of solvents specified by the invention is limited below by the fact 
that success does not occur if one of the two components has fewer than 
six carbon atoms. The upper limit is established by the fact that esters 
with more than twelve or thirteen carbon atoms are as hard as wax. 
The high thermal stability of the solvents specified by the invention in 
connection with their high boiling points delays the moment of 
disintegration in casting and thus increases the thermal load the molds 
are capable of bearing. Thus not only are problems arising from 
undesirable cracking avoided, but there is also an improvement of the 
casting surface, especially in the case of cast iron. Besides that, the 
solvents specified by the invention do not evaporate so that even in 
storage of cores or other molds which are being hardened by the gas 
hardening process, the solvents remain available to take on energy when 
used in casting. 
Further, the general stability of molds produced by the use of the solvents 
specified by the invention is quite excellent and in any case better than 
that attainable with previously used solvents. In this connection, two 
other factors should be especially pointed out, however, which play an 
important role in the gas hardening process, namely, the sand-life and the 
permanence of the hardened cores. Sand-life is the length of time in which 
a material mixture which has been prepared but not yet treated with the 
accelerator can be stored and remains useful. Use of the solvents 
specified in the invention gives sand-lives of five hours or more, after 
which time the forms are not as strong as they were initially but still 
quite adequate for casting. Such long sand-lives have not been possible 
with previously known solvents. The hardened molds are also capable of 
extended storage. While with the use of previously known solvents the 
stability falls off after reaching a maximum, especially when the humidity 
is high, molds made with a solvent specified by the invention do not show 
this phenomenon. 
This superiority of the solvents of the invention over the previously known 
solvents, with regard to strength and storage capacity of the hardened 
molds and to sand-life, is a consequence of the strongly hydrophobic 
nature of the solvents specified by the invention. The solvents previously 
known (including those involving phthalic acid esters) are not hydrophobic 
enough and it is generally necessary to make the mixture more 
water-repellant by treating it with special silanes. The solvents 
specified by the invention do not require such treatment. Even without the 
addition of silanes, they yield results which are previously unattainable 
except through use of silanes. When in the use of solvents specified by 
the invention, silane treatment is applied as well, the results are even 
better. 
It has been further shown that the use of solvents specified by the 
invention drastically reduces or eliminates entirely the tendency for 
material to stick in the production of cores. This makes for efficient 
production with minimal waste of material and no need for time-consuming 
repair operations. This, too, is a clear advantage in comparison to 
previously known solvents, including phthalic acid esters. 
Another surprising advantage of this invention is that when it is used with 
mixtures for the gas hardening process, a considerably smaller amount of 
gaseous tertiary amine is required. This reduced consumption of amine can 
be as much as 50% depending on the particular resin and solvent used. 
The solvents specified by the invention can be used alone but it is 
preferable to use them mixed with non-polar solvents of the usual types, 
particularly with aromatics having a high boiling point. The portion of 
such a solvent mixture which consists of solvents as specified by the 
invention can be anything in excess of 10% by weight and it is preferably 
between 10% and 60% and is chosen to suit the particular resin in 
question. The prepared resin solution can, in any case, have a solid 
content of from 40% to 60% by weight.