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Description  |
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This invention relates to the production of compounds of the general
formula:
ROSO.sub.3 H
where R denotes a primary alkyl group. The compounds are primary alkyl half
esters of sulphuric acid. They are the acid form of primary alkyl
sulphate, variously known as primary alcohol sulphate, which is an anionic
detergent.
Primary alkyl sulphate (PAS) of formula:
ROSO.sub.3 M
where
R is a straight or branched alkyl chain of 8 to 20 carbon atoms, and
M is a solubilising cation such as sodium or ammonium has been known for
very many years as an anionic detergent.
Primary alkyl sulphate is customarily produced by sulphation of the
corresponding primary alcohol so as to produce the acid form of the
detergent (PAS acid).
This acid form is then neutralised to the detergent itself so that the
production route is
ROH.fwdarw.ROSO.sub.3 H.fwdarw.ROSO.sub.3 M
Sulphation of the primary alcohol can be carried out using sulphur trioxide
in conventional apparatus for sulphation/sulphonation reactions.
A known problem, however, is that the resulting PAS acid is unstable and
spontaneously reverts to the primary alcohol. If the PAS acid is stored at
ambient temperature a substantial proportion will decompose to the
starting alcohol within 24 hours. Moderate cooling below ambient
temperature reduces the amount of decomposition but it still remains a
problem. Consequently it is necessary to neutralise the PAS acid promptly
after it has been made by the sulphation reaction. This reduces
flexibility in operating the overall production process. It is moreover
inconvenient because the physical properties of PAS acid would make it
easier to handle and transport than the paste which results from
neutralisation.
When the spontaneous decomposition of PAS acid occurs it leads to the
formation of the original primary alcohol which was the starting material
for the sulphation and also to sulphuric acid. If the mixture is
neutralised and analysed it is found to contain the desired primary alkyl
sulphate, an inorganic salt such as sodium sulphate resulting from
neutralisation of the sulphuric acid which is the product of decomposition
and also the primary alcohol which results from decomposition. The
customary analysis normally reports a percentage of so-called
non-detergent organic matter (NDOM) often referred to as "free oil" (FO)
which includes the primary alcohol decomposition product and also any
primary alcohol starting material which did not react during sulphation.
As will be explained below the invention may make use of compounds
containing alkylene oxide residues--the presence of these can lead to the
introduction of trace quantities of dioxan compounds whose presence in the
product is undesirable.
The present invention aims firstly to inhibit the spontaneous decomposition
of PAS acid. Preferred forms of the invention seek to do so while also
keeping dioxan impurities at low levels.
Broadly we have now found that the spontaneous decomposition of PAS acid
can be effectively inhibited by the presence of alkylene oxide residues,
which may also be termed alkyleneoxy groups, and hydroxyalkyleneoxy
groups.
Broadly, according to a first aspect of this invention there is provided a
method of preparing and stabilising primary alkyl sulphuric acid having
the formula:
ROSO.sub.3 H
where R is a saturated straight or branched primary alkyl group of 8 to 22
carbon atoms which method comprises sulphating a feedstock comprising the
corresponding primary alcohol of formula:
ROH
to produce a sulphated reaction product, and incorporating in the sulphated
reaction product, as a stabilising agent, one or more compounds which
include alkylene oxide residues or hydroxyalkyleneoxy groups, the
proportion of the alcohol ROH in the feedstock exceeding the proportion,
if any, of compounds which include alkylene oxide residues or
hydroxyalkyleneoxy groups.
The invention leads to a mixture containing the said primary alkyl
sulphuric acid and the stabilising agent. Such a mixture represents a
further aspect of this invention.
The amount of the stabilising agent will usually be less than the amount of
the primary alkyl sulphuric acid. In particular the weight ratio of
primary alkyl sulphuric acid to stabilising agent may lie in a range from
20:1 to 2:1.
The group R may well be a saturated alkyl group having 8 to 15 carbon
atoms,
Usually the primary alkyl group R will be a mixture of saturated, straight
or branched alkyl chains averaging 11 to 18 carbon atoms, the range from
12 to 15 carbon atoms being particularly significant.
The stabilising agent may be a wide variety of compounds which can be
represented by a general formula:
Y(A).sub.n X
where A represents an alkylene oxide residue or a hydroxyalkyleneoxy group,
n is positive and Y and X can be a substantial variety of terminal groups.
Without wishing to be bound by any theory as to the mode of action of the
stabilising agent, it is believed that stabilisation is brought about
through oxygen atoms which are ether linkages and which are provided by
the alkylene oxide residues or hydroxyalkyleneoxy groups. These ether
oxygen atoms are believed to transiently bind hydrogen ions and prevent
those ions from participating in a decomposition pathway of the primary
alkyl sulphate. This does not, of course, exclude the possibility of some
other stabilising action being operative simultaneously or alternatively,
in particular in the event that Y or X is hydrogen so that the stabilising
agent has a hydroxyl group which may well participate in an alternative
mechanism of stabilisation.
It is preferred that the stabilising agent incorporates residues of an
alkylene oxide containing from 2 to 4 carbon atoms, notably ethylene
oxide.
An alkylene oxide residue can also be termed an alkyleneoxy group, of
formula:
--(OW)--
where W represents a divalent alkylene group. A hydroxyalkyleneoxy group is
of formula:
##STR1##
where
##STR2##
is a hydroxy substituted alkylene group, notably hydroxypropyl or
hydroxyisopropyl.
It is also preferred that the stabilising agent should have surfactant
properties. Then it can serve as an additional detergent in a detergent
product which incorporates the primary alkyl sulphate obtained by eventual
neutralisation its acid form.
Preferred stabilising agents therefore are compounds and mixtures of
compounds which comply with a general formula:
R.sup.1 (A).sub.n X
where R.sup.1 is a straight or branched primary alkyl group of 8 to 22
carbon atoms, A is as defined before and X is hydrogen, --OSO.sub.3 H or
an alkyl group of 1 to 8, preferably 1 to 4, carbon atoms. Where X is an
alkyl group, for example methyl dioxan formulation may be beneficially
suppressed. The average value of n may lie in the range from 0.5 to 4 or
possibly higher, e.g. 0.5 to 10 or more. More preferably the value of n is
at least 1. It may be preferred that the value of n is not over 3. A range
from 1.5 to 2.5 is particularly envisaged. R.sup.1 may well be saturated
alkyl with an average chain length of not more than 18 carbon atoms. It
may contain an average of at least 11 carbon atoms, and preferably 11 to
15 carbon atoms.
As already mentioned, the amount of stabilising agent will usually be less
than the amount of PAS acid. The weight ratio of acid to stabilising agent
will preferably lie in a range from 20:1 to 1.5:1. The amount of
stabilising agent may be sufficient that the range does not extend beyond
10:1 or even 9:1. At the other end of the range the amount of stabilising
agent may be such that the range does not go beyond 3:1 or even 4:1.
Preferably the ratio by number of PAS acid molecules to alkylene oxide
residues or hydroxyalkyleneoxy groups of the stabilising agent is greater
than 1:1, i.e. PAS acid molecules outnumber alkylene oxide residues and
hydroxyalkyleneoxy groups.
Sulphation is preferably carried out using sulphur trioxide.
The invention is not limited to any specific apparatus for carrying out the
chemical reaction by which PAS acid is formed. Conventional practice is to
effect sulphonation using sulphur trioxide in a reactor which provides a
high surface area for contact between the reactants. A falling film
reactor is normally used.
The stabilising agent may be added to the PAS acid after the sulphation has
been carried out. Alternatively or in addition, the stabilising agent or a
precursor of it may be added to the primary alcohol before sulphation
takes place, or even during the course of sulphation if the apparatus will
permit this.
It is preferred for the sake of reducing the dioxan impurities, that
stabilising agent is added after sulphation has taken place even if some
has been incorporated prior to or during sulphation. The addition
preferably occurs promptly after the PAS acid leaves the reactor in which
the sulphation occurs. Desirably it is added after not more than 15
minutes, better after not more than 5 minutes.
Preferred stabilising agents for addition after sulphation has occurred are
compounds and mixtures of compounds of the formula:
R.sup.1 (A).sub.n OX
where A denotes an alkylene oxide residue having from 2 to 4 carbon atoms,
and X is hydrogen or an alkyl group of 1 to 8, preferably 1 to 4 carbon
atoms, while R.sup.1 and n are as defined previously.
If the stabilising agent of formula R.sup.1 (A).sub.n OX is a material in
which X denotes hydrogen, some of it may be sulphated very quickly by
residual sulphur trioxide contained in the PAS acid. The resulting product
of formula:
R.sup.1 (A).sub.n OSO.sub.3 H
is the acid form of alkyl ether sulphate which can also serve as anionic
detergent following neutralisation, especially if A denotes an ethylene
oxide residue.
Stabilising agents of formula R.sup.1 (A).sub.n OX in which X denotes
hydrogen are nonionic surfactants. Stabilising agent of this type which
does not react prior to neutralisation will therefore provide a content of
nonionic surfactant in the neutralised mixture and in products made from
it.
If the stabilising agent or a precursor of it is incorporated into the
primary alcohol before sulphation, it is preferred to incorporate a
compound of the formula:
R.sup.1 (A).sub.n OH
where R.sup.1, n and A are as defined previously. This compound is then
sulphated at the same time as the primary alcohol, so that the sulphated
mixture contains as stabilising agent a compound of the formula:
R.sup.1 (A).sub.n OSO.sub.3 H
In corporation into the primary alcohol before sulphation thereof may be
brought about by treating that alcohol with alkylene oxide, so as to
convert the feedstock which contains a alcohol into a mixture of alcohol
and alkoxylated alcohol. This mixture corresponds to a general formula:
R(A).sub.p OH
where p has an average value which preferably lies in the range 0.1 to 0.5.
If alcohol ethoxylate of formula R.sup.1 (A).sub.n OH is made separately
and added before sulphation, or if an alkyl ether sulphuric acid of
formula R.sup.1 (A).sub.n OSO.sub.3 H is added after or before sulphation,
it is desirable that the eventual mixture of alkyl sulphuric acid and
alkyl ether sulphuric acid corresponds to a general formula:
R.sup.1 (A).sub.p OSO.sub.3 H
where p again has an average value of 0.1 to 0.5.
When the stabilising agent or a precursor thereof is included in the
feedstock before Sulphation, either by addition to the primary alcohol or
by exposure of the primary alcohol to an alkylene oxide, the amount of
alkoxylated compound included in the eventual feedstock is restricted by
the requirement that the proportion of alcohol in the feedstock shall
exceed the proportion of compounds which include alkylene oxide or
hydroxyalkyleneoxy groups.
It may be noted that the conventional manufacture of alkyl ether sulphate
entails the sulphation of an alkoxylated alcohol which includes a
proportion {which may be fairly small) of the alcohol itself. In such
manufacture, the resulting alcohol sulphuric acid has been only a minor
component of little importance.
If the feedstock contains only a small proportion (or none at all) of
material other than the alcohol and any compounds which contain alkylene
oxide or hydroxyalkyleneoxy groups, then the product of reaction will
contain a majority by weight of primary alkyl sulphuric acid. This is a
significant form of the invention.
Although the present invention requires that the feedstock contains the
alcohol in larger amount than compounds (if any) which contain alkylene
oxide residues or hydroxyalkyleneoxy group, it is possible that other
materials may be present in the feedstock. Moreover these may undergo
reaction.
Notably the feedstock may contain another material which undergoes
sulphation or sulphonation, simultaneously with the sulphation of the
primary alcohol. In particular the feedstock could contain alkylbenzene
which undergoes sulphonation to alkylbenzene sulphuric acid. The product
of reaction would then be a mixture of the acid forms of primary alkyl
sulphate and alkylbenzene sulphonate.
The eventual mixture of PAS acid and stabilising agent produced by this
invention may be stored for several hours, at least, before
neutralisation. The lifetime of the PAS acid before significant
decomposition has taken place is significantly extended by incorporation
of stabilising agent in accordance with this invention when storage is at
any of a range of temperatures, e.g. 0.degree. C. to 40.degree. C. If,
however, storage is at a temperature towards the lower end of this range,
preferably 0.degree. C. to 20.degree. C. more preferably 0.degree. C. to
10.degree. C., especially 0.degree. C. to 5.degree. C. the rate of the
decomposition reaction will be reduced by the lower temperature as well as
through the stabilising action. Consequently, the duration of storage in
the presence of stabilising agent may be increased relatively to the
possible duration of storage at a higher temperature with the same
stabilising agent present. The use of low temperatures is also very
valuable for inhibiting the formation of dioxan which, as mentioned
earlier is an undesirable contaminant.
Incorporation of the stabilising agent in the PAS acid at a temperature at
which the PAS acid is liquid is preferred.
In the following examples all quantities and percentages are by weight
unless otherwise stated.
EXAMPLE 1
This example commenced with a fatty alcohol of formula:
ROH
in which R was a mixture of straight and branched alkyl groups (about 60%
branched), mainly of 12 and 13 carbon atoms. This alcohol, made by the OXO
process, was commercially available as LIAL 123.
The alcohol was sulphated using sulphur trioxide and a pilot plant falling
film reactor. Three batches of the resulting PAS acid were collected from
the following film reactor. One batch was maintained at 30.degree. C. for
24 hours. A second batch was maintained at 10.degree. C. for the same
period. A third batch was immediately mixed with nonionic surfactant as
stabilising agent in an amount which formed 15% of the resulting mixture
(i.e. an 85:15 weight ratio of reaction product:stabilising agent). It was
then also stored at 10.degree. C. This nonionic surfactant was coconut
alcohol (mainly C.sub.12 and C.sub.14) ethoxylated under reaction
conditions which yield a high proportion of molecules with up to four
ethylene oxide residues and a very small proportion with six or more
ethylene oxide residues. This was a so-called "peaked nonionic" in
accordance with the general formula:
R.sup.1 (A).sub.n OH
with
R.sup.1 mostly C.sub.12 and C.sub.14 linear alkyl,
A denoting an ethylene oxide residue, and
n having an average value of 2.0.
Portions were taken from each batch of PAS acid at intervals, neutralised
with sodium hydroxide solution and analysed for content of sulphate
detergent (SD) and for sulphate-free organic matter (SFOM).
The neutralisation conditions were calculated such as to give an aqueous
liquor with a theoretical anionic detergent content of approximately 25%
by weight.
The analytical results for sulphate-free organic matter are quoted as a
percentage of the quantity of anionic detergent found by analysis.
Portions from the third batch were also analysed for dioxan content. The
amounts of this contaminant were expressed as parts per million based on
the quantity of anionic detergent found by analysis.
The following results were obtained:
______________________________________
30.degree. C. 10.degree. C.
10.degree. C. + coconut 2EO
Storage
SD SFOM SD SFOM SD SFOM Dioxan
Time (%) (%) (%) (%) (%) (%) (ppm)
______________________________________
Zero 23.1 3.9 23.1 3.5 22.1 15.2 <10
15 Mins
20.5 4.4
30 Mins
19.9 5.0 20.5 15.1 <10
45 Mins
20.1 5.5
1 Hour
19.8 7.1 22.0 3.6 20.2 14.2 10
2 Hours
19.1 6.3 21.7 3.7 20.2 14.1 23
4 Hours
16.7 13.8 20.4 14.4 18
6 Hours 20.2 14.3 27
24 Hours
16.3 18.7 20.5 5.9 20.1 14.5 41
______________________________________
It can be seen from these results, especially the rinsing values for
sulphate-free organic matter, that without the stabilising agent
spontaneous decomposition is considerable at 30.degree. C. and still
significant at 10.degree. C.
With stabilising agent added the decomposition is less. The figure for
anionic sulphate detergent (SD) starts lower because of the addition of
stabilising agent. It then decreases to a smaller extent than is the case
without stabilising agent. The figure for sulphate-free organic matter is
higher because stabilising agent appears in this. The figure drops
initially, which is attributed to small quantities of unreacted alcohol
and sulphur trioxide in the PAS acid reaction product. When the reaction
product is stirred before neutralisation some of the sulphur trioxide is
able to react with the stabilising agent and this causes the small drop in
the value for SFOM. Thereafter the value for SFOM remains almost constant.
An "ideal" value for SFOM was calculated on the basis of two assumptions:
1) the sulphation would yield a mixture containing small amounts of
unreacted sulphur trioxide and alcohol (as normally observed) so that if
neutralised prior to any decomposition of the PAS acid it would lead to a
PAS:alcohol:Na.sub.2 SO.sub.4 ratio of 100:1.5:1.5;
2) the unreacted sulphur trioxide would combine with some of the
stabilising agent, thus converting it to detergent.
The value of SFOM predicted in this way was 14.6%, close to the observed
values.
EXAMPLE 2
This example used a commercial fatty alcohol (LIAL 125) of formula:
ROH
where R was a mixture of straight and branched alkyl groups, mainly of 12
to 15 carbon atoms.
It was sulphated as in Example 1. Three batches were collected and held at
0.degree. to 5.degree. C.
No stabilising agent was added to one batch. A second of these batches was
immediately mixed with a stabilising agent which was the same as used in
Example 1. This was pre-cooled to a temperature below 5.degree. C. before
addition. It was added in an amount providing 15% by weight of the
resulting mixture, as in Example 1.
The third batch was likewise immediately mixed with a stabilising agent in
an amount providing 15% by weight of the resulting mixture. This
stabilising agent was again a peaked nonionic surfactant obtained by
ethoxylation of LIAL 125. It was in accordance with a general formula:
R(A).sub.n OH
where R is the same as for LIAL 125, A denotes an ethylene oxide residue
and n has an average value of 2.0.
Portions were taken from each batch at intervals and neutralised with
sodium hydroxide at under conditions calculated to give an aqueous liquor
with a theoretical anionic detergent content of approximately 20% by
weight.
The resulting neutralised samples were analysed and results are given below
for Content of sulphate detergent (SD). For the batches which received
stabilising agent results are also given for sulphate-free organic matter
(SFOM) and dioxan content. As in the previous example, these results are
given as percentages and parts per million of the quantity of anionic
detergent found by analysis.
__________________________________________________________________________
No Stabiliser
+ coconut 2EO + C.sub.12 -C.sub.15 alcohol 2EO
Storage Time
SD (%)
SFOM (%)
SD (%)
SFOM (%)
Dioxan
SD (%)
SFOM (%)
Diosan
__________________________________________________________________________
Zero 21.1 1.5 19.8 17.5 ND 18.8 16.2 ND
4 hours
-- -- -- -- -- 15.8 17.2 ND
4.5 hours
-- -- 19.8 14.8 ND -- -- --
5 hours
17.7 4.6 -- -- -- -- -- --
18 hours
-- -- -- -- -- 16.6 17.2 ND
22 hours
-- -- 18.7 16.6 ND -- -- --
__________________________________________________________________________
(ND means dioxan was below a detection limit of 15 ppm based on the
quantity of anionic detergent).
As can be seen from these results, with stabilising agent added at low
temperature the decomposition is reduced and dioxan formation is low.
EXAMPLE 3
This example also used a LIAL 125 fatty alcohol. It was ethoxylated to
comply with a general formula:
R(OCH.sub.2 CH.sub.2).sub.p OH
where p had an average value of 0.25.
It was then sulphated as in Example 1. Four batches were collected. One was
held at 20.degree. C. another was held at 0.degree. C. to 3.degree. C. Two
others were mixed with a further stabilising agent in an amount which was
15% of the overall mixture. This further stabilising agent was LIAL 125,
ethoxylated to contain an average of two ethylene oxide residues as used
in Example 2 and held at 20.degree. C. and at 0.degree. C. to 3.degree. C.
respectively.
Portions from each batch were removed, neutralised and analysed as in the
previous example. Results obtained are set out in the following Table:
The "ideal" value for SFOM, calculated as in Example 1, when stabiliser was
present, was 14.5%.
__________________________________________________________________________
20.degree. C. with
20.degree. C. no Stabiliser
0.degree. C. no stabiliser
stabiliser 0.degree. C. with stabiliser
added after
added after
added after
added after
Storage
sulphation sulphation sulphation sulphation
Time SD SFOM
Dioxan
SD SFOM
Diosan
SD SFOM
Dioxan
SD SFOM
Dioxan
__________________________________________________________________________
Zero 24.8
2.6 <10 25.1
3.6 <10 217
17.5
40 21.8
17.4
9
30 Min
24.7
6.0 57 -- -- -- 22.1
15.0
64 -- -- --
1 Hour
23.9
8.4 177 25.1
4.4 45 21.7
14.3
61 21.9
13.2
22
2 Hrs
22.1
8.2 196 24.7
5.3 54 22.2
14.4
60 22.5
16.0
32
4 Hrs
22.5
9.8 196 24.5
5.3 89 21.5
15.4
73 22.3
13.9
25
6 Hrs
21.6
11.3
175 24.3
6.2 123 22.3
14.8
72 21.8
13.7
25
24 Hrs
18.0
22.7
491 24.5
4.8 141 21.2
15.1
235 21.9
14.6
74
3 Days
16.1
29.2
1031
23.0
9.1 226 20.5
20.0
705 22.6
14.4
66
__________________________________________________________________________
The results in the first two columns are for a system where the stabiliser
was the alkyl sulphuric acid product of the ethoxylated alcohol present in
the starting alcohol. These results show an improvement compared with the
results without any stabiliser in the previous examples, showing the
benefit of including the alkyl ether sulphuric acid. However, the addition
of nonionic surfactant as stabiliser after sulphonation gave a
considerable further improvement.
EXAMPLE 4
LIAL 123 was sulphated to produce PAS acid as in Example 1. Stabilising
agent was then added to a batch of PAS acid no provide a weight ratio of
85:15 of reaction product to stabilising agent and maintained at a
constant temperature as listed below.
______________________________________
Temperature
Stabilising Agent
8.degree. C.
20.degree. C.
30.degree. C.
______________________________________
A 4.1 4.2 --
B 4.3 4.4 4.5
C 4.6 -- --
D 4.7 -- --
______________________________________
A: LIAL 123 having an average degree of ethoxylation of 3
B: LIAL 125 having an average degree of ethoxylation of 2
C: LIAL 125 having an average degree of ethoxylation of 3
D: Coconut oil having an average degree of ethoxylation of 3
B, C and D were narrow range ethoxylates.
Samples were then taken periodically from each batch and neutralised as in
Example 2 and analysed to determine the amount of non-detergent organic
matter (NDOM) and the level of dioxan therein. The results are listed in
the following tables and the NDOM figures are % by weight and ppm based on
the quantity of anionic detergent detected.
______________________________________
Time Batch 4.1 Batch 4.2
(Hours) NDOM Dioxan NDOM Dioxan
______________________________________
2 6 47 8.3 103
4 6.9 8 132
6 6.1 7.7 161
24 6.8 47 8.8 216
______________________________________
Batch
4.5 4.6 4.7
Time 4.3 4.4 Di- Di- Di-
(Hrs) NDOM Dioxan NDOM Dioxan
oxan oxan oxan
______________________________________
24 10.4 5 11.9 122 356 37 38
48 10.6 87 12.9 308 478 54 81
96 11.5 170
120 11.5 165
______________________________________
The above NDOM figures do not vary significantly with time thus indicating
that the PAS acid does not decompose to any significant extent. Further
the results show that dioxane formation is suppressed by storing the batch
at lower temperature and by employing a narrow range ethoxylate as a
stabilising agent.
EXAMPLE 3
LIAL 125 alcohol was sulphated to produce PAS acid of at least 97% purity,
2 batches of PAS acid were collected and mixed together with a stabilising
agent in a weight ratio of 85:15 and maintained at a temperature of
0.degree. to 5.degree. C. The stabilising agents employed were LIAL 125
having an average degree of ethoxylation of 2 (narrow range (Batch 5.1)
and a methyl end-capped nonionic surfactant having an average degree of
ethoxylation of 6.5 and available under the trade name REWOPAL MT65 from
REWO (Batch 5.2).
Samples were taken from each batch after 1 hour and 24 hours and
neutralised as in Example 2 and analysed to determine NDOM and dioxan
levels.
The results are shown below.
______________________________________
Time 5.1 5.2
(hours) NDOM Dioxan NDOM Dioxan
______________________________________
1 8.65 24 8.8 35
24 8.2 4 6 9.3 54
______________________________________
The results show that the PAS acid does not decompose significantly and
dioxan levels are kept at a low level with time. Additionally the
stabilising agent in Batch 5.2 has a higher degree of ethoxylation than
that in Batch 5.1. This would be expected to give a higher level of
dioxans. However, the results show only a marginal difference in dioxan
formation thus illustrating the beneficial effect of employing an
end-capped stabilising agent.
* * * * *
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