Alkaline degreasing solution and degreasing method employing the same

An alkaline degreasing solution containing 0.01 to 3 g/l of alkali silicate calculated as Si concentration, and 0.01 to 10 g/l of a nonionic surfactant having an HLB value (=20.times.Mw/M, where Mw represents the weight of a hydrophilic group part and M represents the molecular weight of the surfactant) of 5 to 11 and a structure being expressed in the following general formula (I), with a pH value of at least 9.5: EQU R--O--(EO).sub.m (AO).sub.n H (I) where R represents alkyl group, EO represents ethylene oxide group, AO represents alkylene oxide group (propylene oxide group and/or butylene oxide group), m represents an addition molar number of EO which is an integer of 3 to 7, n represents an addition molar number of AO which is an integer of 1 to 6, and m and n satisfy the relation of n<m.ltoreq.3n.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a degreasing solution and a degreasing 
method for degreasing surfaces of metal materials such as iron, zinc and 
aluminum, and non-metal materials such as plastic. 
2. Description of the Background Art 
In the field of metal surface treatment or the like, a degreasing treatment 
is generally performed on an object for removing fat and oil such as 
mineral oil and animal and vegetable oils adhering to its surface, as a 
pretreatment for chemical conversion through phosphating or the like. A 
degreasing agent employed for such a degreasing treatment generally 
contains a builder which is mainly composed of acid or alkali and a 
nonionic or anionic surfactant as its main component. While an alkali 
builder which is mainly composed of phosphate or silicate has been 
generally employed in consideration of detergency, an alkali builder which 
is mainly composed of silicate is recently employed by preference, since 
phosphate causes enrichment of lakes and marshes or the like. An alkali 
builder which is prepared by further blending silicate with carbonate or 
the like is also employed. 
While a nonionic or anionic surfactant is employed as described above, a 
nonionic surfactant is generally employed since an anionic surfactant has 
high foamability. 
An HLB value is generally known as a value indicating the characteristics 
of the nonionic surfactant. This HLB value indicates the balance between 
the hydrophilic and hydrophobic groups of the surfactant, and the number 
of hydrophilic group which is contained in the surfactant increases as the 
HBL value increases. In case of alkaline-degreasing a metal surface or the 
like, a nonionic surfactant having a high HLB value is generally employed. 
This is because the detergency increases with the HLB value, and a 
nonionic surfactant having an HLB value of at least 12 is generally 
employed in a conventional alkaline degreasing agent. However, a nonionic 
surfactant having a high HLB value, which has a strong foaming tendency in 
general, is generally employed together with a defoaming agent having a 
low HLB value. 
In such a conventional degreasing agent, however, oil such as mineral oil, 
machine oil or spindle oil which is mixed into the degreasing solution by 
alkaline degreasing form an extremely stable emulsion, due to the action 
of the surfactant. Consequently, the degreasability of the surfactant 
disadvantageously extremely reduces when the concentration of the oil 
which is mixed into the degreasing agent increases. 
Particularly when alkaline degreasing is performed under a condition of 
sufficiently stirring and mixing the degreasing agent, a stable emulsion 
is readily formed by the surfactant and the mixed oil, to cause such a 
problem that the mixed oil is not separated but the oil contamination 
resistance reduces. An example of such a stirring method is a 
supervibration stirring method which is disclosed in Japanese Patent 
Publication No. 6-71544 (1994). This supervibration stirring method is 
adapted to set diaphragms in a degreasing treatment vessel and vibrate the 
same under conditions of prescribed vibration width, frequency etc. 
thereby vibrating and stirring a degreasing solution which is stored in 
the degreasing treatment vessel, for example. When a degreasing treatment 
is performed by such a supervibration stirring method with a conventional 
alkaline degreasing solution, however, an extremely fine and stable 
emulsion is formed and oil cannot be readily separated from water after 
alkaline degreasing. 
SUMMARY OF THE INVENTION 
An object of the present invention is to provide an alkaline degreasing 
solution having excellent degreasability and extremely low foamability 
with excellent oil contamination resistance and an alkaline degreasing 
method employing the same. 
The alkaline degreasing solution according to the present invention 
contains 0.01 to 3 g/l of alkali silicate calculated as Si concentration, 
and 0.01 to 10 g/l of a nonionic surfactant having an HLB value 
(=20.times.Mw/M, where Mw represents the weight of a hydrophilic group 
part and M represents the molecular weight of the surfactant) of 5 to 11 
and a structure being expressed in the following general formula (I), with 
a pH value of at least 9.5: 
EQU R--O--(EO).sub.m (AO).sub.n H (I) 
where R represents alkyl group, EO represents ethylene oxide group, AO 
represents alkylene oxide group (propylene oxide group and/or butylene 
oxide group), m represents an addition molar number of EO which is an 
integer of 3 to 7, n represents an addition molar number of AO which is an 
integer of 1 to 6, and m and n satisfy the relation of n&lt;m .ltoreq.3n. 
In the above general formula (I), R represents alkyl group preferably 
having a carbon atom number of 8 to 20, more preferably 8 to 12. If the 
carbon atom number of the alkyl group represented by R is small, there is 
such a tendency that the hydrophilic part reduces and surface activity as 
well as degreasability lower. If the carbon atom number of the alkyl group 
represented by R increases, on the other hand, oil-water separability 
tends to reduce. 
The HLB value of the nonionic surfactant which is employed in the alkaline 
degreasing solution according to the present invention is 5 to 11. If the 
HLB value reduces, the degreasability reduces, the surfactant strongly 
tends to shift to an oil phase, and the consumed volume of the surfactant 
increases. 
In the general formula (I), m represents the addition molar number of EO, 
which is an integer of 3 to 7. If m is less than 3, the degreasability 
tends to reduce. If m exceeds 7, on the other hand, there is such a 
tendency that the foamability increases, oil-water separability 
deteriorates and oil contamination resistance reduces. 
In the general formula (I), n represents the addition molar number of AO, 
which is an integer of 1 to 6. If n is less than 1, i.e., zero, there is 
such a tendency that the foamability increases, oil-water separability 
deteriorates, and oil contamination resistance reduces. If n exceeds 6, on 
the other hand, there is such a tendency that the degreasability reduces, 
the surfactant readily shifts to an oil phase in oil-water separation, and 
the concentration of the surfactant in a water phase reduces. Specific 
examples of AO (alkylene oxide) are propylene oxide and butylene oxide. 
In the general formula (I), m and n are in a relation satisfying n&lt;m3n. If 
m is not more than n, there is such a tendency that the degreasability 
reduces, oil-water separability deteriorates, the surfactant readily 
shifts to an oil phase in oil-water separation, and the concentration of 
the surfactant in a water phase reduces. If m exceeds 3n, on the other 
hand, there is such a tendency that the foamability increases, oil-water 
separability deteriorates and oil contamination resistance reduces. 
The concentration of the alkali silicate which is employed in the inventive 
alkaline degreasing solution is 0.01 to 3 g/l, preferably 0.01 to 1 g/l 
calculated as Si concentration. There is such a tendency that the 
degreasability is insufficient if the concentration of alkali silicate 
serving as an alkali builder is too low, while the oil-water separability 
deteriorates and oil contamination resistance reduces if the concentration 
of alkali silicate is too high. 
The pH value of the inventive alkaline degreasing solution is at least 9.5, 
and preferably in the range of 10.5 to 12.5. If the pH value is too low, 
degreasability tends to reduce. If the pH value is too high, on the other 
hand, there is an apprehension of corroding aluminum or zinc which is a 
nonferrous metal material. 
The alkaline degreasing solution according to the present invention can be 
employed in an ordinary degreasing method, and can degrease an object by 
dipping, spraying, or a solution contact method consisting of a 
combination thereof. The inventive alkaline degreasing solution is useful 
particularly in case of performing alkaline degreasing with sufficient 
stirring and mixing, in an alkaline degreasing method employing a 
supervibration stirring method of setting diaphragms in a treatment vessel 
and vibrating the same for stirring a treatment solution which is stored 
in the treatment vessel, for example. 
A degreasing method according to a first aspect of the present invention is 
adapted to dip an object in an alkaline degreasing solution which is 
stored in a degreasing treatment vessel for alkaline-degreasing the same, 
by employing the aforementioned alkaline degreasing solution according to 
the present invention and alkaline-degreasing the object while stirring 
the alkaline degreasing solution stored in the degreasing treatment vessel 
by vibrating diaphragms which are set in the degreasing treatment vessel. 
According to the alkaline degreasing method of the first aspect of the 
present invention, the diaphragms which are set in the degreasing 
treatment vessel are vibrated, thereby vibrating and stirring the alkaline 
degreasing solution stored in the degreasing treatment vessel. With such 
diaphragms, the alkaline degreasing solution is preferably stirred to 
cause a continuous wave motion having a wave height of at least 0.5 cm, 
for example, on the surface of the alkaline degreasing solution which is 
stored in the degreasing treatment vessel. The wave height of such a wave 
motion of the alkaline degreasing solution is more preferably in the range 
of 1.0 to 3.0 cm. The term "wave height" indicates the height of waves 
with respect to a reference solution surface in a still standing state, 
and the wave height of at least 0.5 cm, for example, is the height of 
waves causing a wave motion of at least +0.5 cm with respect to the 
reference solution surface. Such a wave height can be measured by setting 
a scale or the like in the vicinity of the surface of the degreasing 
solution which is stored in the degreasing treatment vessel, for example. 
In the first aspect of the present invention, the vibration of the 
diaphragms which are set in the degreasing treatment vessel can be 
provided by transmitting vibration of a vibrating motor to the diaphragms, 
for example. A vibration stirrer which is disclosed in Japanese Patent 
Publication No. 6-71544 (1994) or Japanese Patent Laying-Open No. 6-28779 
(1994) can be employed as such a vibration stirrer. The vibration 
conditions for the diaphragms, which are properly set in response to the 
shape and the dimensions of the diaphragms, the number and the positions 
of such diaphragms, the dimensions of the treatment vessel and the like, 
can be set in the ranges of a frequency of 200 to 600 vtm and a vibration 
width of 5 to 20 mm, for example. 
The inventive alkaline degreasing solution is useful in an alkaline 
degreasing method which is in a system of still-standing an alkaline 
degreasing solution after degreasing, separating oil from water, removing 
oil which is mixed into the cleaning solution by alkaline degreasing, and 
thereafter employing the solution for alkaline degreasing again. 
An alkaline degreasing method according to a second aspect of the present 
invention comprises the steps of dipping an object in a degreasing 
treatment vessel storing the aforementioned alkaline degreasing solution 
according to the present invention for degreasing the same, introducing 
the alkaline degreasing solution from the degreasing treatment vessel into 
an oil-water separation vessel after degreasing, separating emulsified oil 
from the alkaline degreasing solution in the oil-water separation vessel, 
and returning the alkaline degreasing solution to the degreasing treatment 
vessel after separation of the oil in the oil-water separation vessel. 
In the alkaline degreasing method according to the second aspect, the step 
of dipping the object in the alkaline degreasing solution for degreasing 
the same may be a step of alkaline-degreasing the object while stirring 
the alkaline degreasing solution by vibrating diaphragms which are set in 
the degreasing treatment vessel by the aforementioned supervibration 
stirring method of the first aspect. 
In the second aspect, further, the alkaline degreasing solution which is 
stored in the degreasing treatment vessel may be temporarily moved into 
another vessel such as a preliminary vessel after degreasing, so that the 
same is thereafter introduced into the oil-water separation vessel. 
The alkaline degreasing solution according to the present invention has 
excellent degreasability, and extremely low foamability. Therefore, it is 
not necessary to employ another nonionic surfactant, such as the 
conventional nonionic surfactant, having a low HLB value for serving as a 
defoaming agent. 
Further, the alkaline degreasing solution according to the present 
invention exhibits remarkably excellent oil-water separability as compared 
with the conventional alkaline degreasing solution. Therefore, oil which 
is mixed in alkaline degreasing can be readily separated and removed, 
whereby the degreasability of the alkaline degreasing solution can be 
maintained over a long period. Thus, the inventive alkaline degreasing 
solution is excellent also in oil contamination resistance. 
The alkaline degreasing solution, which has excellent degreasability and is 
excellent in oil-water separability, is useful in an alkaline degreasing 
method employing a supervibration stirring method, for example. 
In the alkaline degreasing method according to the first aspect of the 
present invention, the aforementioned alkaline degreasing solution of the 
present invention is employed for alkaline-degreasing the object while 
stirring the alkaline degreasing solution by vibrating the diaphragms 
which are set in the degreasing treatment vessel. When the object is 
alkaline-degreased with the conventional alkaline degreasing solution 
under such stirring in the supervibration stirring condition, oil which is 
mixed into the alkaline degreasing solution form a stable emulsion, which 
in turn remains in the state being mixed into the degreasing solution, 
such that the oil cannot be readily separated from water. Consequently, 
the detergency of the alkaline degreasing solution reduces in a short 
period and the degreasing solution cannot be thereafter used. On the other 
hand, the alkaline degreasing method according to the first aspect of the 
present invention employs the aforementioned inventive alkaline degreasing 
solution which is excellent in degreasability as well as in oil-water 
separability, whereby oil can be readily separated from water after 
alkaline degreasing, so that the oil which is mixed into the alkaline 
degreasing solution can be readily removed. According to the alkaline 
degreasing method of this aspect, therefore, the alkaline degreasing 
solution can be repeatedly used over a long period with a long life. 
In the alkaline degreasing method according to the second aspect of the 
present invention, the aforementioned inventive alkaline degreasing 
solution is employed so that the same is introduced into the oil-water 
separation vessel after alkaline degreasing. The aforementioned inventive 
alkaline degreasing solution is excellent in oil-water separability and 
hence the same can readily separate oil from water in the oil-water 
separation vessel. Thus, the alkaline degreasing solution from which the 
mixed oil is removed by oil-water separation can be returned into the 
degreasing treatment vessel to be reused. 
The foregoing and other objects, features, aspects and advantages of the 
present invention will become more apparent from the following detailed 
description of the present invention when taken in conjunction with the 
accompanying drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Nonionic surfactants were prepared from the following surfactants (in the 
formulas, PO represents propylene oxide): 
Example 1: C.sub.12 H.sub.25 O(EO).sub.5 (PO).sub.3.5 H 
Example 2: sec-C.sub.10 H.sub.21 O(EO).sub.5 (PO).sub.3.5 H 
Example 3: C.sub.18 H.sub.37 O(EO).sub.6 (PO).sub.2 H 
Comparative Example 1: C.sub.9 --(C.sub.6 H.sub.4)--O(EO).sub.8.5 H 
Comparative Example 2: C.sub.9 --(C.sub.6 H.sub.4)--O(EO).sub.6 (PO).sub.2 
H 
Comparative Example 3: C.sub.12 H.sub.25 O(EO).sub.10 (PO).sub.2 H 
Comparative Example 4: C.sub.12 H.sub.25 O(EO).sub.10 (PO).sub.6 H 
Comparative Example 5: (C.sub.6 H.sub.5)--C(CH.sub.3).sub.2 --(C.sub.6 
H.sub.4)--O(EO).sub.8 H 
The concentrations of the surfactants were set at 1.5 g/l. Alkali builders 
containing 0.1 g/l, 0.5 g/l, 2.0 g/l and 3.0 g/l of sodium metasilicate 
(alkali silicate), 4.0 g/l of trisodium phosphate, 6.0 g/l of disodium 
phosphate, 2.0 g/l of sodium carbonate, 3.0 g/l of sodium bicarbonate and 
1.0 g/l of sodium nitrite were employed. As to sodium metasilicate, alkali 
builders containing the same in four types of concentrations were prepared 
as described above. 
As to alkaline degreasing solution which were prepared in the 
aforementioned manner, characteristics in degreasing were evaluated on 
objects of SPCC (cold-rolled steel plates). Under conditions of a 
degreasing bath temperature of 40.degree. C. and a degreasing time of 1 
minute, the degreasing solution was stirred in a degreasing treatment 
vessel with a propeller at a rotation speed of 300 rpm. 
Initial degreasability, foamability and oil contamination resistance of the 
degreasing solution was evaluated in the following manner: 
The initial degreasability was evaluated under the following evaluation 
criteria, by degreasing the objects, thereafter showering the same with 
tap water at room temperature for 30 seconds, and measuring the 
water-wetting area percentage: 
TABLE 1 
______________________________________ 
Evaluation Water-wetting area percentage (%) 
______________________________________ 
.smallcircle. 
100 
.smallcircle..about. .DELTA. 
at least 95 - less than 100 
.DELTA. at least 80 - less than 95 
X less than 80 
______________________________________ 
The foamability was evaluated under the following evaluation criteria, by 
driving the degreasing solution which was diluted to 1/10 with a spray 
cleaning apparatus at room temperature for 5 minutes and measuring the 
heights of bubbles from the solution surfaces in an apparatus tank 
immediately after stoppage of driving. 
TABLE 2 
______________________________________ 
Height of Bubbles from 
Solution Surface in 
Evaluation Tank(cm) 
______________________________________ 
X at least 10 
.DELTA. at least 5 - less than 10 
.smallcircle. 
less than 5 
______________________________________ 
The oil contamination resistance was evaluated under similar evaluation 
criteria to the above, by mixing 10 g/l of rustproof oil ("NP Oil AR-1" 
(trade name) by Nippon Paint Co., Ltd.) into the degreasing solution, 
thereafter stirring the same by Disper at 40.degree. C. for 10 minutes, 
thereafter leaving the same at 40.degree. C. for 10 minutes, thereafter 
removing oil floating on the degreasing solution, degreasing the objects 
with the remaining degreasing solution, and measuring the initial 
degreasability levels. 
Table 3 shows the results of the aforementioned evaluation with the HLB 
values of the respective surfactants. As to the alkali silicate 
concentrations, "0.01 to 1.0 g/l" is based on data of 0.1 g/l and 0.5 g/l, 
"1.0 to 3.0 g/l" is based on data of 2.0 g/l, and "at least 3.0 g/l" is 
based on data of 5.0 g/l respectively. 
TABLE 3 
__________________________________________________________________________ 
Initial Oil Contamination 
Degreasability 
Resistance 
Alkali Silicate 
Alkali Silicate 
Concentration Concentration 
(g/l) (g/l) 
at at 
Foam- least least 
Surface Active Agent HLB ability 
0.01-1.0 
1.0-3.0 
3.0 0.01-1.0 
1.0-3.0 
3.0 
__________________________________________________________________________ 
Example 1 
C.sub.12 H.sub.25 O(EO).sub.5 (PO).sub.3.5 H 
7.2 .largecircle. 
.largecircle. 
.largecircle. 
.largecircle. 
.largecircle. 
.largecircle. 
.DELTA. 
Example 2 
sec-C.sub.10 H.sub.21 O(EO).sub.5 (PO).sub.3.5 H 
7.6 .largecircle. 
.largecircle. 
.largecircle. 
.largecircle. 
.largecircle. 
.largecircle. 
.DELTA. 
Example 3 
C.sub.18 H.sub.37 O(EO).sub.6 (PO).sub.2 H 
8.1 .largecircle. 
.largecircle..about..DELTA. 
.largecircle..about..DELTA. 
.largecircle. 
.largecircle..about..DEL 
TA. .largecircle..about 
..DELTA. 
X 
Comparative Example 1 
1 #STR1## 12.6 
X .largecircle. 
.largecircle. 
.largecircle. 
X X X 
Comparative Example 2 
2 #STR2## 8.8 .DELTA. 
.largecircle. 
.largecircle. 
.largecircle. 
.largecircle..about..DEL 
TA. .DELTA. 
X 
Comparative 
C.sub.12 H.sub.25 O(EO).sub.10 (PO).sub.2 H 
11.9 
X .largecircle. 
.largecircle. 
.largecircle. 
X X X 
Example 3 
Comparative 
C.sub.12 H.sub.25 O(EO).sub.10 (PO).sub.6 H 
9.0 .largecircle. 
.largecircle..about..DELTA. 
.largecircle..about..DELTA. 
.largecircle. 
.DELTA. 
.DELTA. 
X 
Example 4 
Comparative Example 5 
3 #STR3## 12.5 
.DELTA. 
.largecircle. 
.largecircle. 
.largecircle. 
.largecircle. 
.DELTA. 
X 
__________________________________________________________________________ 
As clearly understood from Table 3, the alkaline degreasing solution 
according to Examples 1 to 3 of the present invention are excellent in 
initial degreasability, with small foamability and excellent oil 
contamination resistance. As clearly understood from comparative examples 
shown in Table 3, further, those having low HLB values are generally 
inferior in initial degreasability, while those having high HLB values 
have foamability and attain no excellent results in oil contamination 
resistance, although the same are excellent in initial degreasability. 
Supervibration Cleaning 
A system of a supervibration cleaning apparatus shown in FIG. 1 was 
employed to perform alkaline degreasing with the alkaline degreasing 
solution according to Examples 1 and 2 and comparative examples 1 and 2. 
As shown in FIG. 1, the employed supervibration cleaning apparatus system 
comprises a degreasing treatment vessel 1, a preliminary vessel 3, and an 
oil-water separation vessel 5. Diaphragms 6 are set in the degreasing 
treatment vessel 1, so that an alkaline degreasing solution stored in the 
degreasing treatment vessel 1 can be vibrated and stirred through 
vibration of the diaphragms 6. The degreasing solution which is stored in 
the degreasing treatment vessel 1 can be moved into the preliminary vessel 
3 by a pump 2. Further, the degreasing solution can be transmitted from 
the preliminary vessel 3 to the oil-water separation vessel 5 by another 
pump 4. Oil mixed in the degreasing solution which is transmitted from the 
degreasing treatment vessel 1 into the preliminary 3 is separated from 
water to some extent in the preliminary vessel 3, and the degreasing 
solution is moved into the oil-water separation vessel 5 from a lower 
portion of the preliminary vessel 3 by the pump 4. The degreasing solution 
is further stood still and subjected to oil-water separation in the 
oil-water separation vessel 5, so that the mixed oil is removed and the 
degreasing solution is returned into the degreasing/treatment vessel 1 
again. 
The degreasing solution which is stored in the degreasing treatment vessel 
1 is subjected to oil-water separation through the aforementioned 
operation, to be repeatedly used. 
FIG. 2 is an enlarged sectional view showing a portion around the 
diaphragms 6. As shown in FIG. 2, the diaphragms 6 are mounted on a 
vibration bar 7, the upper end of which is mounted on a vibration plate 8. 
The vibration plate 8 is supported by a cushion member 11, to be vibrated 
at a prescribed frequency with a prescribed vibration width by vibration 
of a vibration motor 9. The vibration motor 9 is vibrated by power from an 
invertor 10, and this vibration is supplied to the vibration plate 8, so 
that the diaphragms 6 provided in the degreasing treatment vessel 1 are 
vibrated through the vibration bar 7. The vibration motor 9 is prepared 
from that of 150 W, for example. The supervibration apparatus shown in 
FIG. 2 is prepared from a supervibration stirrer (".alpha.-2 type Stirrer" 
(trade name) by Nihon Techno Co. Ltd.,), for example. 
The apparatus shown in FIGS. 1 and 2 was employed to perform alkaline 
degreasing with the aforementioned alkaline degreasing solution according 
to Examples 1 and 2 and comparative examples 1 and 2. Each degreasing 
solution had an alkali silicate concentration of 1.0 g/l as Na.sub.2 
SiO.sub.3. Objects were prepared from the above SPCC with application of 
large quantities of oil, and subjected to a continuous degreasing 
treatment through the aforementioned apparatus. After performing the 
degreasing treatment for a prescribed time, equilibrium concentrations of 
emulsified oil contained in the degreasing solution stored in the 
degreasing treatment vessel and degreasability values at this point of 
time were evaluated under the aforementioned evaluation criteria. Table 4 
shows the results. 
TABLE 4 
______________________________________ 
Example Example Comparative 
Comparative 
1 2 Example 1 Example 2 
______________________________________ 
Equilibrium 
2700 2800 11000 5600 
Oil 
Concentration 
(ppm) 
Degreasa- 
.smallcircle. 
.smallcircle. 
X .DELTA. 
bility 
______________________________________ 
It is clearly understood from Table 4 that the degreasing solution 
according to Examples 1 and 2 of the present invention have low 
equilibrium oil concentrations and excellent oil-water separability, as 
well as excellent degreasability. 
Although the present invention has been described and illustrated in 
detail, it is clearly understood that the same is by way of illustration 
and example only and is not to be taken by way of limitation, the spirit 
and scope of the present invention being limited only by the terms of the 
appended claims.