Foundry moulds and cores

A foundry moulding composition comprising (a) a granular refractory material, and (b) from 0.5% to 8% based on the weight of the refractory material of a binder comprising (i) an aqueous solution of a potassium alkali phenol-formaldehyde resin, said aqueous solution having a solids content of from 50% to 75% and said resin having a weight average molecular weight (M.sub.w) of from 600 to 1500, a formaldehyde:phenol molar ratio of from 1.2:1 to 2.6:1 and a potassium hydroxide:phenol molar ratio of from 0.2:1 to 1.2:1 and (ii) at least one silane in an amount of from 0.05% to 3% based on the weight of said aqueous solution, said binder being curable by contact therewith of from 5% to 60% based on the weight of said aqueous solution of a C.sub.1-3 alkyl formate.

BACKGROUND OF THE INVENTION 
This invention relates to the manufacture of foundry moulds and cores which 
do not evolve pungent acid gases on thermal decomposition. More 
particularly it refers to a method of making moulds and cores of this type 
rapidly at ambient temperature. 
Phenol formaldehyde and phenol formaldehyde/furfuryl alcohol condensation 
products catalyzed with strong acids such as sulphuric acid, paratoluene 
sulphonic acid, are well known as binders for sand in the production of 
foundry moulds and cores. However, they have the disadvantage that pungent 
fumes of sulphur dioxide are evolved on thermal decomposition. 
The use of alkaline phenolic resins catalyzed with esters has been 
suggested in Japanese Patent Publication No. 130627/1975 and is the 
subject of co-pending U.S. Application Ser. No. 224,131, filed Jan. 12, 
1981, now U.S. Pat. No. 4,426,467, issued Jan. 17, 1984 and U.S. Ser. No. 
434,462 filed Oct. 14, 1982, now abandoned. The use of such binder systems 
enables the manufacture of foundry moulds and cores which do not evolve 
pungent acid gases on mixing or during casting. Further, by suitable 
selection of resins and ester catalysts rapid hardening at ambient 
temperature can be achieved. However, to obtain such results on a large 
scale it is necessary to use specialized rapid mixing equipment such as 
that described in British patent specification Nos. 1257181 and 1369445 of 
Baker Perkins. 
The present invention is based on the discovery that the use of esters as 
catalysts for alkaline phenolic resins in the manufacture of foundry 
moulds and cores can be adapted to a gassing system which is capable of 
rapid cure at ambient temperature. The use of gassing to promote curing of 
binders for foundry moulds and cores is known. The major systems which are 
or have been industrially used are as follows: 
(a) The "Carbon Dioxide Process" in which CO.sub.2 is passed through a 
mixture of sand and sodium silicate. However, the resultant cores or 
moulds are very sensitive to water and lose strength "damp back" on 
storage, will not accept aqueous washes and show very poor breakdown on 
casting. It is necessary to add breakdown agents such as sucrose to 
promote better breakdown. Over-gassing produces very poor strengths. 
(b) The "Sulphur Dioxide Process" disclosed by SAPIC in British Pat. No. 
1,411,975 which uses (1) a peroxide which is dangerous to store and 
dispense, particularly in a foundry environment, and (2) pungent SO.sub.2 
which has a low Threshold Limit Value (TLV) and is unpleasant to handle. 
(c) The "Isocure" process disclosed by Ashland in British Pat. No. 
1,190,644 which uses a benzilic ether phenolic polyol and methylene 
diphenyl diisocyanate. The reaction between the polyol and diisocyanate is 
accelerated by gassing with triethylamine or dimethyl ethylamine. The 
diisocyanates have very low TLV's and react with water preferentially over 
the polyol so that it is necessary to use dry sand and dry air to convey 
the sand/binder mix into core box or mould. The amines have relatively low 
TLV's and their toxicology is not well understood. Cured cores tend to 
absorb water and lose some strength on storage. Certain casting defects 
are observed with "Isocure" cores/moulds, e.g. "pinholing" caused by the 
nitrogen content of the binder which reduces to ammonia in the casting 
environment, dissolves in the molten metal, and is evolved as small blow 
holes on cooling; "graphitic defect" which is a deposit of graphite carbon 
which collects in flakes on the surface of the casting; and "finning" or 
"veining" caused by the mould or core cracking under the expansion 
stresses during casting and molten metal running into the cracks. 
The present invention enables the rapid and efficient production of foundry 
moulds and cores without the disadvantages of the prior art as described 
above. 
SUMMARY OF THE INVENTION 
Accordingly, the present invention provides a method of making a foundry 
mould or core which method comprises mixing a granular refractory material 
with from 0.5 to 8% of a binder which comprises an aqueous solution, 
having a solids content of from 50 to 75% by weight, of a potassium alkali 
phenol-formaldehyde resin having the following characteristics: 
(a) a weight average molecular weight (M.sub.w) of from 600 to 1500; 
(b) a formaldehyde: phenol molar ratio of from 1.2:1 to 2.6:1; and 
(c) a KOH: phenol molar ratio of from 0.2:1 to 1.2:1, the binder including 
from 0.05 to 3% by weight based on the weight of the resin solution of at 
least one silane, forming the mixture in a vented core or mould box and 
gassing the formed mixture with at least one C.sub.1 to C.sub.3 alkyl 
formate to cure the binder. 
DETAILED DESCRIPTION OF THE INVENTION 
The granular refractory materials used in the present invention may be of 
any of the refractory materials employed in the foundry industry for the 
production of moulds and cores, such as silica sand, chromite sand, zircon 
or olivine sand. The compositions of the invention have the particular 
advantage that the difficulties commonly associated with the bonding of 
sands of alkaline reaction such as olivine and chromite or beach sands 
containing shell fragments and which arise from the neutralization or 
partial neutralization of the acid catalyst used, are completely overcome 
since in the invention the binder is cured under alkaline conditions. The 
invention is, therefore, of particular utility where it is necessary or 
desirable to employ alkaline sands. 
The nature of the phenol-formaldehyde resin used is an important feature of 
the present invention. There are several features of the resin which are 
important. Since the present invention is directed to cold set techniques, 
the resin binder will be used as an aqueous solution of the resin. The 
solids content of the aqueous solution is in the range 50 to 75% by 
weight. Solids contents below 50% are not used because they contain too 
much water which reduces the effectiveness of the binder. Solids contents 
above 75%, are not used because the viscosity becomes too high. 
The phenol-formaldehyde resins used in this invention have a weight average 
molecular weight (M.sub.w) of from 600 to 1500. Resins with M.sub.w 
outside this range give products which are relatively weak or build up 
strength more slowly. We have, to date, obtained best results using resins 
having M.sub.w in the range 700 to 1100. 
The resins used in this invention are potassium alkaline 
phenol-formaldehyde resins by which is meant that the alkali in the resin 
is potassium alkali. This alkali will usually be present in the resin 
during manufacture but can be added to the resin subsequently as KOH, 
preferably in aqueous solution of suitable strength. The alkalinity of the 
resin is expressed in terms of its KOH content and specifically by the 
molar ratio of KOH to the phenol in the resin. Other alkalis are not 
expressly excluded and may be present in minor amounts but will not be 
specifically added because they give products having lower strength. 
The molar ratio of KOH: phenol in the resin solution is in the range 0.2:1 
to 1.2:1 preferably 0.3:1 to 1:1. Outside this range the products have 
relatively poor strength and, above the top range limit, the resin is 
hazardously alkaline. We have obtained best results using resin solutions 
having a KOH: phenol molar ratio in the range 0.4 to 0.6. 
The resins used have a formaldehyde to phenol molar ratio of from 1.2:1 to 
2.6:1, preferably 1.5:1 to 2.2:1. Lower ratios are not used because the 
resins are relatively unreactive. Higher ratios are not used because the 
resins produced contain undesirably high levels of unreacted formaldehyde 
and give products having lower strength. 
It is a subsidiary aspect of this invention that the resin used satisfies 
the following criteria: 
(a) M.sub.w from 700 to 1100; 
(b) KOH: phenol molar ratio 0.4:1 to 0.6:1 and 
(c) formaldehyde: phenol molar ratio 1.5:1 to 2.2:1 
A silane is included in the binder to improve strength. Amounts as low as 
0.05% by weight on the weight of resin solution provide a significant 
improvement in strength. 
Increasing the amount of silane gives greater improvements in strength up 
to about 0.6% by weight on the resin solution. Higher silane 
concentrations are not preferred because of added cost. Further, because 
the silane typically used is .gamma.-aminopropyltriethoxy silane which 
contains nitrogen, use of excess silane may increase the risk of pinholing 
defects and for these reasons amounts in excess of 3% by weight on the 
resin solution are not used. 
The binder and particulate refractory material can be mixed and formed by 
conventional techniques. The vented core and mould boxes used can also be 
of conventional type as are used in prior art gassing systems. The binder 
is cured, according to the present invention, by gassing with a C.sub.1 to 
C.sub.3 alkyl formate, very preferably methyl formate. The alkyl formate 
curing catalyst will not usually be used as a pure gas but as a vapour or 
aerosol in an inert carrier gas. By inert carrier gas we mean a gas which 
does not react with the formate catalyst or have an adverse effect on the 
curing reaction or the properties of the product. Suitable examples 
include air, nitrogen or carbon dioxide. 
The gassing catalyst is a C.sub.1 to C.sub.3 alkyl formate preferably 
dispersed in a carrier gas as vapour or an aerosol. Other esters e.g. 
formate esters of higher alcohols such as butyl formate, and esters of 
C.sub.1 to C.sub.3 alcohols with higher carboxylic acids such as methyl 
and ethyl acetates, are not effective as gassing catalysts. Methyl formate 
is significantly more active as a catalyst than ethyl formate which is 
better than the propyl formates. The reasons for the catalytic activity of 
the C.sub.1 to C.sub.3 alkyl formates and, within this group, the marked 
superiority of methyl formate, are not clear. The relative volatility of 
these compounds enables their use as gassing catalysts. This is especially 
true of methyl formate which is a volatile liquid having a boiling point 
at atmospheric pressure of 31.5.degree. C. At ambient temperatures (below 
31.5.degree. C.), typically 15.degree. to 25.degree. C., it is 
sufficiently volatile that passing carrier gas through liquid methyl 
formate (maintained at ambient temperature) gives a concentration of 
methyl formate vapour in the carrier gas sufficient to act as catalyst to 
cure the binder. Ethyl and the propyl formates are less volatile than the 
methyl ester, having boiling points in the range 54.degree. to 82.degree. 
C. at atmospheric pressure. 
In order to entrain sufficient of these esters in the gas phase to enable 
effective catalysis, we have found it appropriate to heat the esters to 
near their boiling point and use a stream of carrier gas preheated to e.g. 
100.degree. C. 
An alternative to true vaporization is to form an aerosol in the carrier 
gas. Methyl formate is so volatile as to make this impractical. Using 
ethyl and propyl formates it is desirable to pre-heat them to enhance even 
distribution in the core or mould during gassing. 
As is indicated above, methyl formate is the most active catalyst and, by 
virtue of its volatility, is the easiest to use. Accordingly, the use of 
methyl formate in a stream of inert carrier gas as the gassing catalyst 
forms a particular aspect of this invention. A further practical advantage 
of these formate esters, especially methyl formate is their relative low 
toxicity and the fact that their toxicity is well understood. 
The concentration of the formate catalyst in the carrier gas is preferably 
at least 0.2% by volume and typically from 0.5 to 5% by volume. The total 
amount of catalyst used will typically be from 5 to 60% preferably from 15 
to 35%, by weight on the weight of the resin solution. The time required 
for adequate gassing depends on the size and complexity of the core or 
mould and on the particular resin used. It can be as short as 0.1 secs but 
more usually is in the range 1 sec to 1 min. Longer times e.g. up to 5 
mins can be used if desired or for large moulds or cores. After gassing 
the core or mould is stripped from the box. Sufficient time must elapse to 
permit the strength of the mould or core to build up to enable stripping 
without damage. Production speed can be enhanced by purging the mould or 
core box with a suitable inert gas such as air which removes residual 
catalyst vapour and water and other by-products of the curing reaction. 
The amount of resin solution used as binder is from 0.5 to 8%, preferably 1 
to 3%, by weight on the weight of the refractory particulate material. Use 
of lower amounts of binder gives cores of poor strength. Higher amounts of 
binder give no significant advantage and give generally poorer breakdown 
on casting and increase the difficulty of sand recovery. 
The following Examples illustrate the invention. 
The techniques used in the Examples are described below: 
Manufacture of phenol formaldehyde resin solutions 
100% phenol was dissolved in 50% aqueous KOH in an amount corresponding to 
the desired KOH:phenol molar ratio (from 0.2 to 1.2). The solution was 
heated to reflux and 50% aqueous formaldehyde was added slowly, whilst 
maintaining reflux, in an amount corresponding to the desired 
formaldehyde:phenol molar ratio (1.6, 1.8 or 2.0). The reaction mixture 
was maintained under reflux until it attained a pre-determined viscosity 
corresponding to the desired value of M.sub.w. (If desired the solids 
content can be adjusted by distillation, but this is not usually 
necessary, a further advantage of the invention. In some cases minor 
amounts of KOH solution were added to adjust the KOH:phenol ratio, but 
this would not be necessary in full scale production.) The resin solution 
was cooled to 40.degree. C. and 0.4% by weight on the weight of the resin 
solution of .gamma.-aminopropyl triethoxy silane was added. 
Testing of resins 
(a) viscosity--measured using an Ostwald (U-tube) viscometer at 25.degree. 
C. 
(b) solids content--measured by heating a weighed sample (2.0.+-.0.1 g) in 
an air circulating oven for 3 hrs at 100.degree. C. 
(c) Molecular weight M.sub.w)--measured using gel permeation 
chromatography. Samples were prepared by precipating resin from the resin 
solution by adding H.sub.2 SO.sub.4 ; separating, washing and drying the 
precipitate and dissolving it in tetrahydrofuran. 
Preparation of foundry sand core mix 
1 kg of the selected sand was charged into a Fordath laboratory core mixer 
and mixed for 2 mins, with 20 g phenol-formaldehyde resin prepared as 
described above. The mix was discharged into a tin and sealed immediately 
to prevent evaporation of water. 
Preparation of test foundry cores 
5.times.5 cm cylinder compression test pieces were prepared by the standard 
procedure recommended by I.B.F. working party P but using a perforated 
bottom plate of the cylinder with a recess which could be connected to a 
source of negative pressure. The top of the cylinder was sealed with 
another perforated plate connected to a bubbler containing liquid methyl 
formate at ambient temperature. When vacuum was applied to the bottom 
plate air was bubbled through the methyl formate and the ester vapour 
conveyed in the air stream through the sand resin mix in the cylinder core 
box. Compression strength was determined on the resultant cores after 
storing at 20.degree. C., 50% relative humidity for 1 min, 5 mins, 1 hr. 2 
hrs. 3 hrs. and 24 hrs. Initial tests indicated that 30 secs. was 
sufficient time to produce the optimum strength and this was used as a 
standard in the Examples below.