Note: Descriptions are shown in the official language in which they were submitted.
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FOUNDRY COMPOSITIONS CO~TAINING PROPYLENE GLYCOL MONOACETATE
The present invention relates to the use of propylene glycol
monoacetate as an organic ester hardener, especially for cold
silicates used for bonding sands in the production of foundry cores.
- Various hardeners may be used for hardening such silicates,
the choice of a hardener being dependent upon a number of factors,
These include water solubility, type of metal to be cast, type of
sand used in casting, type of silicate solution, the degree of oontrol
obtainable by altering the composition and quantity of the hardener
usrd and the toxicity and fire hazards of the hardeners used
The most commonly used hardeners may be placed in three
categories, These are:
(1) Powder hardeners such as dicalcium silicate and ferrosilicon,
(2) Carbon-dioxide,
(3) Ester hardeners
- 15 Because of the difficulties of adding po~Jder hardeners to sodiumsilicate, and also of obtaining good through hardening with carbon
dioxide, ester hardeners have been gaining in popularity, Ester
hardeners in general have the advantages of a high rate of growth of
strength and easy handling in continuous mixers By using blends of
esters it is possible to control the bench life of a formulation
and obtain good through hardening. Ester hardeners are thought
to operate by a combination of hydrolysls to produce hydrogen ions
and dehydration of the silicate solution during this hydrolysis.
However, even amongst organic esters, their degree of water solubility
is important because total miscibility not only enables efficient
and uniform distribution of the ester but also makes maximum use of
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the ester employed. In this respect conventionally used esters
such as triacetin, ethylene glycol diacetate and diacetin are
somewhat deficient, Triacetin is soluble in water at 20& to an
extent of 6.7g/lOOg of water, the solubility of ethylene glycol
diacetate being 16.6g/lOOg of water. Diacetin as usually produced
is a mixture of mono, di and tri acetate esters of glycerol and can
contain as little as 50~ of the desired diacetin. The water
solubility of diacetin therefore varies with the method of production
and hence with the actual diacetin oontent of the product. Further
variability is possible with diacetin depending on which two o~ the
three hydroxyl groups of the glyoerol are esterified.
It has now been found that these disadvantages may be minimised
- by using a composition containing a specific ester hardener.
Accordingly, tlle present invention is a foundry composition
comprising a silica base, a silicate and propylene glycol monoacetate.
The silica base for use in such compositions is preferably sand.
The desirable purity of sand would depend upon the end use of the
mould produced. For example, for high quality iron and steel castings
sand of very high chemical purity and hlgh packaging density is
required.
The silicate in the compositions of the present invention is
suitably an alkali metal or an aIkaline earth metal silicate.
Sodium silicate is preferred. Of these silicates the types which
have a high SiO2/Na20 molecular ratio are preferred. Thus the
silicates suitably have a SiO2/Na20 molecular ratio of between 2:1
and 3:1, most preferably between 2,5:1 and 2.8:1. The silicates
are preferably used as their aqueous solution.
The bonding reaction of sand/silicate mixtures is believed
to be due to the gelation of the soluble and colloidal silica present
which forms a glutinous but strongly cohesive film around the sand
grains. The speed with which the gelation effect is achieved can be
controlled by controlling the SiO2/Na20 ratio, the type o~ sand, the
water to solids ratio in the composition and the quanti-ty of propylene
glycol monoacetate used.
~5 Propylene glycol monoacetate is totally miscible with water and
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hence is an extremely efficient hardening agent for silicate foundry
cores, The propylene glycol monoacetate may be used as such or as
a mixture thereof with other conventional hardeners such as for
instance a hardener selected from diacetin, triacetin, ethylene
glycol diacetate and die-thylene glycol diacetate. Whether used
alone or in admixture with conventional hardeners it is preferable
that the ester hardening mixture contains at least 50% by weight
of propylene glycol monoacetate.
Apart from the solubility aspects, propylene glycol monoacetate
is superior to conventional hardeners in respect of the compressive
strength imparted to the moulding and the faster rate at which such
compressive strength is built up. For example, for identical
concentrations, propylene glycol monoacetate imparts on an average
at least twice the compressive strength to the mould when compared
with diacetin.
The amount of propylene glycol monoacetate added to the silicate
is suitably between 1 and 20~ by weight of the silicate preferably
between 5 and 15~ by weight of the silicate.
The propylene glycol monoacetate used in the co~positions of the
present invention may be produced by conventional techniques eg by
reacting in the liquid phase propylene oxide and acetic acid at an
el`evated temperaturein the presence of a catalyst,
It is normal to use a catalyst for the reaction and many
different possible catalysts would be apparent to those skilled in
the art. Different catalysts produce propylene glycol monoacetate
of different degrees of purity and material ranging from 59% to
over 90~ propylene glycol monoacetate have been prepared. However
all these products are totally miscible with water and all produce
higher compressive strengths and a faster rate of build up of
compressive strength than diacetin. The gel time is found to vary
with the propylene glycol monoacetate content, the higher the
content of propylene glycol monoacetate the shorter the gel time.
In general the higher propylene glycol monoacetate content material
is preferred and the preferred catalyst to produce such material
from propylene oxide and acetic acid is a chromium salt of saturated
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or unsaturated aliphatic carboxylic acid containing between 1 and
l0 carbon atoms. The carboxylic acid used in preparing the
chromium salt catalyst is suitably selected from formic, acetic,
butyric, pentanoic, hexanoic, octanoic, 2-ethylhexanoic and decanoic
acids and mixtures thereof. Whichever salt is used, i-t should
preferably be soluble in the reaction medium. Chromium octanoate
is a particularly preferred example of such a catalyst.
The chromium salt may be prepared in situ, by heating chromium
hydroxide with the carboxylic acid for a short time in order to
effect solution. The reaction is then continued in the usuàl manner.
The amount of chromium salt employed in the reaction may vary
between 0.1 and 5% by weight of the reactant acid employed.
Aslight molar excess of propylene oxide is preferred in the
reaction mixture. Thus, for example the molar ratio of propylene
oxide to the reactant carboxylic acid in the reaction mixture is
preferably between 1.05 and 1.2.
The reaction of the present invention may be carried out at a
temperature between 40 and 120C. The reaction pressure is
preferably above atmospheric, for example between 20-40 psig. The
process of the present invention may be operated batch-wise or
continuously. The course of the reaction may be followed by
measuring the acidity of the reaction mixture from time to time and
hence determining the content of unreacted acid,
Ester hardeners including propylene glycol monoacetate need
to have low levels of residual acidity eg less than 0.1% as acetic
acid. Such low levels of acidity can be obtained at the reaction
stage using chromium salt catalysts whilst maintaining the propylene
~ycol monoacetate content at around 90%, The product may be
separated from the catalyst by distillation which may be carried out
at reduced pressure.
When using other catalysts the reaction needs to be stopped
when the acidity is at a significant level, typically around 5%, in
order to maximise the content of propylene glycol monoacetate. The
product then needs to be separated from the catalyst and residual
acidity by fractional distillation whichmay becarriedoutunderreduced
pressure.
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One of the significant features of this invention is thatpropylene glycol monoacetate thus produced has a purity of around
90% and it is this quality of the pure product which makes
propylene glycol monoacetate a particularly suitable ester for use
5 as a hardener.
The invention is further illustrated with reference to the
following examples,
l~AMPIES
___
The gel time at room temperature was determined on the following
10 silicate/hardener solutions. All quantities referred to are parts
by weight,
Comparative ¦ E~camples
Ingredients Test ¦ 1_ 2 3
Sodium silicate solution 100 100 100 100
(wt per ml 1,5g)
Diacetin 10 - - -
PGMA Sample 1 - 10
PGMA Sample 2 - - 10
P~.qA Sample 3 - - - 10
20 Test results are given in Table 1,
The compressive strength was assessed by monitoring the strength
build-up at room temperature of silicate/sand composites, All
quantities referred to are parts by weight,
The following mixes were prepared:
Comparative t Examples
Composition Test ~ 4 5 6 7 8 9 10
New Windsor Rose ) 100100 100 100 100 100 100 100
Sod um sllicate 3,53,5 3~5 3~5 3'5 ~ 5 3
30 Diacetin 0,35 _ _ _ 0175 -
PGM~ Sample 1 - 0,35 - - _ 0175 -
PGMA Sample 2 _ _ o,~5 _ _ _ 0,175
PGI~A Sample ~5 _ _ _ 0 53 ~ 0,175
2 inch diameter sand brique-ttes were prepared by hand ramming
35 150 gms of sand/silicate mix, The briquettes were allowed to harden
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at room temperature. The compressive strength, after various timesJ
was determined using a Xowden test machine operation with a cross
head speed of 1.5 inches/minute.
Test results are given in Tables 2 and ~.
In the data above and in the following Tables the expression
~'PGMAI' refers to propylene glycol monoacetate. The references to
"Samples 1, 2 and 3" indicate that samples of PGMA produced in
three different batches by reacting in the liquid phase propylene
oxide and acetic acid in the presence of an anion exchange resin
catalyst (which was vinyl pyridine copolymerised into a cross-
linked polystyrene backbone) at a temperature be-tween 95 and 105 C
and a maximum pressure of 50 psig. The PGMA was separated from
the crude product by fractional distilla-tion,
In addition the expression "EGDA" below represents ethylene
glycol diacetate.
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The results set out in Tables 1 and 2 show that in sandcomposites a higher ultimate compressive strength was obtained
with all the experimental hardeners. At both 5% and 10% additions
PGMA Sample No 1 had the fastest strength build up with the
biggest ultimate strength.
Example 10
Preparation of PGMA
To a suitable stirred, heated autoclave were charged 360 pt
by weight of acetic acid and 5.4 pt by weight of chromium octanoate.-
The vessel was sealed and evacuated and purged with nitrogen severaltimes to ensure removal of air. The vacuum was finally broken with
nitrogen to atmospheric pressure. The vessel contents were heated
to 90 C and at this temperature the addition of 418 pt by weight
(20% excess) of propylene oxide was commenced at a rate of approxim-
ately 15 pt by ~eight per minute. The temperature was maintainedat 90-100C and the maximum pressure recorded was 28 psig. After
1 hours reaction the acidity was less than 0.5%, the reactor and
con-tents were cooled to 50& and vacuum applied for 30 min to
remove excess propylene oxide. The crude product was distilled to
give a 91% yield of a product containing 9~ propylene glycol
monoacetate and an acidity of 0.02% as acetic acid,
Example 11
Comparative Gel Times and Compressive Strengths
The gel times and compressive strengths of diacetin/~GDA mixtures
were compared with a relatively inexpensive PGMA~ GDA mixture. The
results are shown below.
(1) Gel times at 24 &
60 parts Diacetin ~ ~
40 parts E~IDA ~ 10 minutes
70 parts PGMA
~ 0 parts EGDA ~ 9 minutes 40 seconds
(2) Stren~ths - using 10~ catalyst on silicate. The figures are
kgs load to crush a 2" diameter cylindrical specimen.
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60/40 Diacetin/EGDA
After 1 hour 185 147
After 3 hours 238 299
A~ter 6 hours 316 287
5These figures indicate that the 70/30 PGMA/EGDA blend is very
similar to the d~acetin/EGDA blend in rate of strength build-up,
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