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nonionic surfactant mixture
Journal of Materials Processing Technology 152 (2004) 215–220
Cutting fluid emulsions produced by dilution of a cutting fluid concentrate containing a cationic/nonionic surfactant mixture H. Bataller, S. Lamaallam, J. Lachaise, A. Graciaa, C. Dicharry∗ Laboratoire des Fluides Complexes, Centre Universitaire de Recherche Scientifique, Université de Pau et des Pays de l’Adour, 64000 Pau, France Received 27 April 2003; accepted 10 March 2004
Abstract A cutting fluid concentrate has been formulated from a paraffinic oil, a hard water, MonoEthanolamine Borate (MEAB) and a cationic/nonionic surfactant mixture. Its dilution with hard water yields very stable oil-in-water emulsions, the so-called cutting fluid emulsion, with an average droplet diameter of 50 nm, independently of both the hardness of water and the weight ratio of hard water to concentrate. The high stability of these emulsions results from the existence of a strong repulsive potential between droplets which is caused in particular by the presence of the cationic surfactant at their surface. From a practical point of view, the ability to produce stable cutting fluid emulsions for a wide range of dilution, whatever the hardness of water, might allow the user to fine-tune the lubricating and cooling abilities of the fluids and therefore optimize their performances during the metal working process. © 2004 Elsevier B.V. All rights reserved. Keywords: Cutting fluid concentrate; Dilution; Hard water; Cutting fluid emulsion; Stability
1. Introduction Heat dissipation and lubrication are common problems that faces the metal processing industry. In machining processes such as metal/turning, milling, drilling and in particular when the metal removal operations are conducted at high speeds and low pressures, the regulation of heat generation and the lubrication of the contact point are achieved by pouring an oil-in-water emulsion, the so-called cutting fluid emulsion, over the cutting surface. Such a fluid has the particular advantage that it combines the cooling property of water and the lubrication property of oil. It is usually obtained by diluting a cutting fluid concentrate with mains waters at a weight ratio of water to concentrate ranging from approximately 98/2 to 95/5 depending on the machining operation [1,2]. A cutting fluid concentrate usually contains a mineral oil, a surfactant mixture, in some cases water and various additives which are included to meet the specifications of commercial concentrates such as resistance to bacterial growth and low corrosion capacity [1,2]. For easiness of use, the concentrate must be stable and not viscous. Its appearance has to be monophasic and its dilution with water
∗ Corresponding author. E-mail address: [email protected] (C. Dicharry).
0924-0136/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jmatprotec.2004.03.027
associated with gentle mixing must produce an oil-in-water emulsion. An emulsion is a nonequilibrium system which will finally separate in two phases. A short time of separation will restrict the use of the emulsion as a cutting fluid because lubrication capacity decreases with stability. The stability of an emulsion can be increased by using ionic surfactants because their adsorption at the surface of the oil droplets yields electric repulsion between them [3]. A commercial cutting fluid concentrate generally contains a mixture of anionic and nonionic surfactants [4]. Mixed surfactants increase the solubilization capacity of surfactants [5] and in some cases may facilitate the spontaneous formation of an emulsion when the system is brought into contact with water [4]. The presence of an anionic surfactant in the surfactant mixture gives a negative electric charge to the oil droplets of the emulsion [6]. However, the stability of this emulsion will often depend on the metallic ion (Ca2+ and Mg2+ ) contents of the water used for dilution. When the ion concentration is high enough, the anionic surfactants may form insoluble compounds with them and then precipitate [7]. This causes the decrease of the surface charge of droplets and the emulsion may be destabilized. The total concentration of calcium and magnesium ions in water defines the hardness of water. A water is said hard if the concentration is greater than 1.5 mmol/l [8]. The hardness of water may vary from one region to another
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of a same country. Therefore, the stability of a cutting fluid emulsion containing an anionic surfactant may drastically vary depending on the region in which it is used. Under these conditions and in order to maintain the lubricating and cooling capabilities of the fluid, it may be necessary to regenerate the fluid [9]. As a consequence, the amount of fluid required for the operations increases as well as the amount of waste water to stock and treat [10]. It is possible to reduce the risk of the surfactant precipitation with the Ca2+ and Mg2+ ions by using anionic surfactant molecules in which some oxyethylene groups have been introduced [7]. Another possibility is to use a cationic surfactant. We have chosen this second option. Therefore, the aim of this work is to formulate a cutting fluid concentrate from an aqueous phase, an oily phase and a cationic/nonionic surfactant mixture and to test its ability to form stable cutting fluid emulsions for a large range of hardness of water. For this purpose, we have first determined the phase diagram of these systems in order to locate the area containing the cutting fluid concentrates, i.e. the systems having a monophasic aspect. Afterwards, one of them has been diluted with different hard waters, and then it has been diluted at different weight ratios with a hard water of fixed hardness. The stability of the emulsions issued from these dilutions has been investigated by analysing the evolution of their droplet size distribution.
2. Materials and methods The concentrates formulated in this work are simplified systems compared to commercial concentrates. Nevertheless, they contain the main ingredients of commercial concentrates, i.e. oil, water, a pH regulator, and surfactants.
MEAB was prepared from 1/1/1 by weight of boric acid, 2-aminoethanol (from Aldrich with a purity >99%) and purified water. MEAB performs an antibacterial action by maintaining the pH greater than 9. The weight ratio of MEAB to the aqueous phase was written as XMEAB : XMEAB =
MEAB MEAB + hardwater
(1)
• The surfactant mixture. This is a mixture of a cationic surfactant (ethoxylated N-tallow ammonium methylsulfate, trade name: Noxamium S11M) and a lipophilic amine (ethoxylated N-tallow amine, trade name: Noramox S2) manufactured by CECA. These surfactants have some polydispersity in the poly-(oxyethylene) chain length distribution with an average of 11 for the former and 2 for the latter. The length of their chain is distributed as follows: 3 wt.% of C14 , 30 wt.% of C16 , 40 wt.% of C18 =, 26 wt.% of C18 , 1 wt.% of C20 . In basic dilute solution, which our experiments were based upon, Noramox S2 behaves like a nonionic surfactant. The weight ratio of Noxamium S11M to the surfactant mixture was written as XS11M : XS11M =
S11M S11M + S2
(2)
The weight ratio of the aqueous phase to the oily phase was written as WOR: aqueous phase WOR = (3) oily phase Finally, the weight ratio of the surfactant mixture to the system was written as XS : XS =
S11M + S2 S11M + S2 + oily phase + aqueous phase
(4)
2.1. Materials
2.2. Phase diagram
• The oily phase. It is a paraffin base oil (100 Neutral Solvent) manufactured by TotalFinaElf. • The aqueous phase. This is a mixture of hard water and MonoEthanolamine Borate (MEAB). Hard water was obtained by adding 0.76 g/l of calcium chloride hexahydrate and 0.132 g/l of magnesium sulphate heptahydrate (both from Aldrich with a purity of 98%) to water purified by the Millipore milli-Q 185 E system (conductivity less than 10−1 S/cm). Under these conditions the CaCl2 and MgSO4 molar concentrations were 3.47 and 0.536 mmol/l respectively, which correspond to those required for the normalized tests (NF T 60-185, 60-186 and 60-187) used in France for the characterization of aqueous cutting fluids. Therefore, the total concentration of Ca2+ and Mg2+ in water is CT (=[Ca2+ ] + [Mg2+ ] = 4.006) mmol/l. This water can be considered as very hard since CT > 1.5 mmol/l. It was used both for the formulation of the concentrates and for their dilution.
The phase diagram was obtained as follows: the samples were prepared in test tubes by mixing together and in the following order the aqueous phase (MEAB and hard water), the cationic surfactant, the oil and the nonionic surfactant. WOR was fixed at 3. The samples were shaken for homogenization and stored in a thermostat regulated water bath at 25.0 ± 0.5 ◦ C for 2 days, after which time the phase behaviour was determined visually. 2.3. Preparation of the cutting fluid emulsions The cutting fluid emulsions were prepared in test tubes by adding, quickly and in one step, the required amount of hard water to the concentrate, at 25.0 ± 0.5 ◦ C. These samples were then gently homogenized by just three successive turnings of the test tubes. The translucent aspect of the emulsions have shown that they contained very small droplets, which consequently
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made it possible to carry out dynamic light scattering measurements. 2.4. Droplet size measurement The size of the emulsions droplets was determined by laser light scattering. The apparatus was made up of a 15 mW He–Ne laser (λ = 632.8 nm) and a Malvern 7032 Multi-8 Correlator. All experiments were performed at the scattering angle θ = 90◦ , and the sample temperature was maintained at 25 ± 1 ◦ C. In order to limit the problems of data analysis linked to the multiple particle light diffusion of concentrated systems [11], the emulsions underwent a new dilution with hard water to obtain a volume fraction of dispersed phase equal to 0.01% which made it possible to take the measurements under “infinite” dilution. The droplet size distributions were calculated using the algorithm CONTIN [12–14]. It should be noted that the time elapsed between the first dilution of an emulsion and the measuring of the droplet size was about 10 min. 3. Results 3.1. Phase diagram We showed in a previous study [15] that the phase behaviour of the systems containing a paraffinic oil, the same aqueous phase and surfactant mixture than those used in this work depends on the values of XS11M and XMEAB . We specifically showed that increasing XMEAB shifts the interactions between the surfactant mixture and the aqueous and oily phases from hydrophilic, for low XMEAB , to lipophilic, for high XMEAB . Preliminary investigations showed that XS11M = 0.40 makes it possible to obtain many systems having a monophasic aspect for XS ≤ 0.10, which typically corresponds to the surfactant weight content of commercial cutting fluid concentrates. Fig. 1 shows the phase behaviour of the systems in the diagrams (XMEAB , XS ). Three different phase behaviours can be distinguished. For low enough values of XMEAB , systems noted WI are formed. These diphasic systems contain an aqueous microemulsion in balance with a phase in excess that contains mainly oil. For high enough values of XMEAB and XS , systems noted WII are formed. These systems are also diphasic but they contain an oily microemulsion in balance with a phase in excess that contains mainly water. The boundary that separates the WI systems from the WII systems was not determined for XS < 0.02. The zone NE locates the systems for which no excess of oil and water was noticed at the same time. Their aspect is monophasic. Therefore, this zone locates the potential cutting fluid concentrates. 3.2. Dilution of a concentrate with hard water The concentrate diluted with hard water is shown by a triangular symbol in Fig. 1. Its composition is reported in
Fig. 1. System: 100 Neutral Solvent/(hard water CT = 4.006 mmol/l, MEAB)/(Noxamium S11M, Noramox S2). Phase behaviour of the system in the diagram (XMEAB , XS ) at the weight ratio of Noxamium S11M to the surfactant mixture XS11M = 0.40, the ratio of the aqueous phase to the oily phase WOR = 3 and the temperature T = 25 ◦ C. The triangular symbol locates the concentrate diluted with hard water in this work.
Table 1. This system was selected because its high surfactant content might favour the stability of the emulsion once formed. Moreover, it also contains a high amount of MEAB which might preserve the cutting fluid emulsion from any bacterial degradation. 3.2.1. Influence of the hardness of water (CT ) added to the concentrate Waters of different hardness were added to the concentrate in the weight ratio 95/5. These hard waters were obtained by diluting with purified water an hard water for which CT = 8.016 mmol/l ([CaCl2 · 6H2 O] = 1.52 g/l and [MgSO4 · 7H2 O] = 0.264 g/l). The mass mean diameters of the droplets of the cutting fluid emulsions measured immediately after the dilution of the concentrate and 80 days later are reported in Fig. 2. The temperature of the samples was hold at 25.0 ± 0.5 ◦ C during this time. Considering the weak agitation applied to mix the concentrate and hard water, it is worthwhile noticing the nanometric size of the droplets of the emulsions. The value of the mass mean diameter does not depend on the hardness of the water added to the concentrate and no appreciable evolution has been observed during 80 days. Therefore, the stability of the cutting fluid emulsions does not depend on the hardness of water. Letter A (CT = 4.006 mmol/l) in Fig. 2 locates the cutting fluid emulsion whose stability is discussed thereafter. Table 1 System: 100 Neutral Solvent/(hard water CT = 4.006 mmol/l, MEAB)/(Noxamium S11M, Noramox S2). Composition of the cutting fluid concentrate XMEAB XS11M XS WOR
0.55 0.40 0.10 3
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Fig. 2. Influence of the total concentration of Ca2+ and Mg2+ of the hard water added to the concentrate in the weight ratio 95/5 on the mass mean diameter of the emulsions; T=25 ◦ C. Letter A (CT = 4.006 mmol/l) locates the cutting fluid emulsion whose stability is discussed thereafter.
3.2.2. Influence of the amount of hard water added to the concentrate Water of fixed hardness (CT = 4.006 mmol/l) was added to the concentrate in different weight ratios of hard water to concentrate (97.5/2.5, 95/5, 90/10, 80/20, 60/40 and 50/50). The mass mean diameters of droplets measured immediately after the formation of the cutting fluid emulsions and 7 days later are reported in Fig. 3. The temperature of the samples was hold at 25.0 ± 0.5 ◦ C during this time. The initial mass mean diameter of droplets is 50 nm whatever the weight ratio of hard water to concentrate. The cutting fluid emulsions obtained for the ratios 97.5/2.5, 95/5 (cutting fluid emulsion A), 90/10, 80/20, 60/40 are stable at least for 7 days. After this time, only the mass mean diameter of the droplets of the emulsion obtained for the ratio 50/50 (cutting fluid emulsion B) changes appreciably. The droplet size distributions of emulsion B at 0 and 7 days after its formation are shown in Fig. 4. It can be noticed the
Fig. 3. Influence of the amount of hard water (CT = 4.006 mmol/l) added to the concentrate on the mass mean diameter of the droplets of the emulsion; T = 25 ◦ C. Letters A (weight ratio 95/5) and B (weight ratio 50/50) locate the cutting fluid emulsions whose stability is discussed thereafter.
Fig. 4. Droplet size distributions of B (weight ratio of hard water (CT = 4.006 mmol/l) to concentrate 50/50) at 0 and 7 days after its formation; T = 25 ◦ C.
broadening of the distribution with time and its shift towards greater diameters.
4. Discussion It has been shown in Figs. 2 and 3 that the dilution of the concentrate with hard water yields droplets of 50 nm in diameter, independently of both the hardness of water and the weight ratio of hard water to concentrate. The great efficiency of the emulsion formation, whatever the conditions of dilution, is probably the consequence of two phenomena occurring simultaneously: • The sudden shift of affinity of the surfactant mixture in favour of the aqueous phase. • The passage of the interfacial tension between the aqueous and oily phases by a very low value. The first phenomenon is the consequence of the sudden decrease of the concentration of MEAB in the aqueous phase caused by the addition of hard water. Actually, the dilution of the concentrate decreases XMEAB from 0.55 to 0.02 for emulsion A and from 0.55 to 0.22 for emulsion B. As it is shown in Fig. 1, for XMEAB lower than about 0.30, the mixture of surfactants favours the formation of a WI system at equilibrium, and then, this type of phase behaviour leads to the formation of an oil-in-water emulsion when the system is mixed [16]. The low interfacial tension is directly the consequence of the utilization of a surfactant mixture [17]. Almost all the cutting fluid emulsions are stable for a long period. Those obtained for the weight ratio of hard water to concentrate 95/5 have a mass mean diameter unchanged at least for 80 days, whatever the hardness of water. Only the mass mean diameter of those obtained for the weight ratio 50/50 (cutting fluid emulsion B) changes appreciably in 7 days of observation.
H. Bataller et al. / Journal of Materials Processing Technology 152 (2004) 215–220
In order to highlight the high stability of these emulsions, we have compared the so-called coagulation time of their droplets, i.e. the time required for the total number of droplets to decrease to half the initial number, with the theoretical one obtained by applying the Smoluchowski’s theory of rapid coagulation [18]. In this theory, it is assumed that there is no interaction between droplets, that is to say that droplets are not covered with surfactant. According to the theory, the normalized number of droplets in function of time can be written as: n(t) 1 = n(0) 1 + t/τ
(5)
where n(0) is the number of droplets per volume determined immediately after the formation of the cutting fluid emulsion, n(t) the number of droplets per volume at the time t and τ the coagulation time. From the droplet size distribution, it is possible to calculate the number of droplets per volume by using the following equation: n(t) =
π
n
6φ
(6)
0 3 i=1 ωi (di (t))
where φ is the volume fraction of dispersed phase, di the mass mean diameter of class i and ωi0 the number frequency of droplets in class i. Assuming that the dispersed phase contains the surfactants and the oil, φ can be written as: Vo + Vs φ= (7) Vo + V s + V w where Vo , Vs , and Vw are the volume of oil, the volume of surfactants and the volume of the aqueous phase, respectively. Provided the droplet size distributions of the cutting fluid emulsion at the times 0 and t are known, Eqs. (5)–(7) give the corresponding coagulation time. The coagulation times of A and B calculated at t = 0 and 7 days are reported in Table 2. It is noteworthy that, according to Eq. (5), the coagulation time of cutting fluid emulsion A is infinite. However, this result does not mean that A is infinitely stable but it means that the time necessary to observe coagulation is greater than 7 days (actually, it is greater than 80 days, according to Fig. 2). The decrease of the coagulation time observed for cutting fluid emulsion B is due to a great extend to the increase of the volume fraction of dispersed phase φ in the emulsion. In fact, for a given mass mean diameter, increasing Table 2 Coagulation time τ calculated for the cutting fluid emulsions A and B. Hardness of water (CT = 4.006 mmol/l). Temperature T = 25 ◦ C
219
the volume fraction of dispersed phase increases the number of droplets in the emulsion and therefore their probability to collide and coagulate. Consequently, the time of coagulation decreases. According to the Smoluchowski’s theory, the coagulation time can be written as: 3η τSmol = (8) 4kTn(0) where k, T, η and n(0) are the Boltzmann constant, the absolute temperature, the viscosity of the continuous phase and the initial number of droplets per unit of volume. Assuming that the continuous phases of A and B contain the hard water and MEAB, their MEAB weight contents are 1.89 and 22 wt.%, respectively. Under these conditions, the viscosity of the continuous phase of A is 0.935 mPa s and that of the continuous phase of B is 1.4 mPa s. From the viscosity of the continuous phases and the initial numbers of droplets n(0) (Table 2), Eq. (8) gives the theoretical coagulation times, τ Smol (Table 3). These theoretical coagulation times appear “infinitely” lower than the experimental ones. Therefore, the droplets of our cutting fluid emulsions coagulate very slowly compared to those of the theoretical system in which droplets would be deprived of surfactant. This result shows that there is a strong repulsive potential between droplets, and therefore, that they are covered with surfactants. This repulsive potential probably has a strong electric contribution, considering the presence of the cationic surfactant in the system. Because the cationic surfactant molecules do not form precipitates with the ions present in solution (contrarily to anionic surfactants), they remain adsorbed on the surface of droplets and they contribute to stabilize the emulsions thanks to electric repulsion. However, it is also likely that this potential has a strong steric contribution. This hypothesis is supported by the fact that the mass mean diameter of the droplets of the cutting fluid emulsions remains unchanged for a long period, whatever the hardness of water (Fig. 2). It is known that, the higher the hardness of water is, i.e. the higher the salt concentration in the aqueous phase is, the weaker the electric repulsion between droplets is [3]. It results from the screening of electric repulsion that the droplets can collide and coagulate more easily. Consequently, without the existence of steric interactions, the cutting fluid emulsions formed Table 3 Theoretical coagulation time τ Smol calculated for the cutting fluid emulsions A and B. Hardness of water (CT = 4.006 mmol/l). Temperature T = 25 ◦ C
Emulsion
φ
n(t = 0) (×1019 m−3 )
n(t = 7 days) (×1019 m−3 )
τ (days)
Emulsion
XMEAB
η (mPa s)
n(t = 0) (×1019 m−3 )
τ Smol (ms)
Aa Bb
0.02 0.19
0.51 5.09
0.51 0.97
∞ 4.25
Aa Bb
0.02 0.22
0.935 1.40
0.51 5.09
33 5
a b
Weight ratio of hard water to concentrate 95/5. Weight ratio of hard water to concentrate 50/50.
a b
Weight ratio of hard water to concentrate 95/5. Weight ratio of hard water to concentrate 50/50.
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in the hardest waters should have probably broken more quickly. 5. Conclusion A cutting fluid concentrate was formulated from a paraffin oil, a hard water, MonoEthanolamine Borate and a mixture of cationic/nonionic surfactants. Its dilution with hard water produced very stable cutting fluid emulsions with oil droplets of sub-micrometric size, whatever the hardness of water and the range of dilution investigated in the study. The efficiency of the emulsion formation probably results from both the steep decrease of the concentration of MEAB in the aqueous phase yielding hydrophilic conditions for the surfactant mixture, and the occurrence of a very low interfacial tension between oil and water. The high stability of the cutting fluid emulsions is probably due to a great extent to the utilization of a cationic surfactant which does not form precipitate with the metallic ions present in hard water. Therefore, its presence at the surface of droplets gives them an electric protection which hinders the coagulation phenomenon. Moreover, the high stability of these emulsions whatever the hardness of water also makes it possible to consider a steric protection of the droplets. The concentrate developed in this work meets some of the specifications of commercial concentrates such as a monophasic aspect, an easiness of dilution with water and a formation of stable cutting fluid emulsions. However, many other properties have not been checked as for examples, the performance of the cutting fluid emulsions in metal working operations, their foaming tendency or their corrosive property. The satisfaction of all of the specifications of commercial concentrate would probably make the formula of this concentrate more complex. Nevertheless, this formula constitutes a good basis of work while showing that the utilization of a cationic surfactant in the surfactant mixture makes it possible to do not care about the hardness of water used for dilution. Moreover and from a practical point of view, the ability of such a concentrate to form stable cutting fluid emulsions for a wide range of mixing might allow the user to fine-tune the lubricating and cooling abilities of the emulsion and therefore optimize its performances during the metal working process.
References [1] M.A. El Baradie, I. Characterization, J. Mater. Process. Tech. 56 (1–4) (1996) 786–797. [2] V. d’Hollander, Les fluides aqueux pour la coupe des métaux, in: La Lubrification Industrielle, Tech. Ed. Paris, 1984, pp. 483–553. [3] J.N. Israelachvili, Intermolecular and Surface Forces, 2nd ed., Academic Press, San Diego, 1992, pp. 213–254. [4] G.W.J. Lee, Th.F. Tadros, Formation and stability of emulsions produced by dilution of emulsifiable concentrates. Part I. An investigation of the dispersion on dilution of emulsifiable concentrates containing cationic and non-ionic surfactants, Colloids Surf. 5 (1982) 105–115. [5] H. Kunieda, A. Nakano, M. Akimaru, The effect of mixing of surfactants on solubilization in a microemulsion system, J. Colloid Interf. Sci. 170 (1) (1995) 78–84. [6] G. Rios, C. Pazos, J. Coca, Destabilization of cutting oil emulsions using inorganic salts as coagulants, Colloids Surf. A: Physicochem. Eng. Asp. 138 (2–3) (1998) 383–389. [7] K. Shinoda, K. Fontell, Ionic surfactants capable of being used in hard water, Adv. Colloid Interf. Sci. 54 (1995) 55–72. [8] J. Rodier, L’analyse de l’eau, Eaux naturelles, eaux résiduaires, eau de mer—interprétation des résultats de l’analyse physico-chimique, Ed. Dunod Paris, 1984, pp. 989–1121. [9] M. Ferreti, Utilisation rationnelle des fluides de coupe, Maintenance Enterprise 463 (1992) 27–31. [10] M.A. El Baradie, Cutting fluids. Part II. Recycling and clean machining, J. Mater. Process. Tech. 56 (1–4) (1996) 798–806. [11] D. Langevin, J. Rouch, in: P. Kumar, K.L. Mittal (Eds.), Handbook of Microemulsions Science and Technology—Light Scattering Studies of Microemulsions Systems, Marcel Dekker, New York, 1999, pp. 387–410. [12] S.W. Provencher, Inverse problems in polymer characterization: direct analysis of polydispersity with photon correlation spectroscopy, Makromol. Chem. 180 (1979) 201–209. [13] S.W. Provencher, A constrained regularization method for inverting data represented by linear algebraic or integral equations, Comput. Phys. Commun. 27 (1982) 213–227. [14] S.W. Provencher, CONTIN: a general purpose constrained regularization program for inverting noisy linear algebraic and integral equations, Comput. Phys. Comm. 27 (1982) 229–242. [15] H. Bataller, C. Dicharry, J. Lachaise, A. Graciaa, Formulation of model cutting-oil water emulsions using paraffinic oil and ionic/nonionic surfactant mixture, J. Disper. Sci. Technol. 21 (5) (2000) 571–588. [16] J.L. Salager, in: F. Nielloud, G. Marti-Mestres (Eds.), Pharmaceutical Emulsions and Suspensions—Emulsion Properties and Related Know-How to Attain Them, Marcel Dekker, New York, 2000, pp. 74–122. [17] Z. Hou, Z. Li, H. Wang, Ultra-low interfacial tension in oil–water-mixed surfactant systems, J. Disper. Sci. Technol. 22 (2–3) (2001) 255–259. [18] Th.F. Tadros, B. Vincent, in: P. Becher (Ed.), Encyclopedia of Emulsion Technology: Basic Theory—Emulsion Stability, Marcel Dekker, New York, 1983, pp. 129–285.
Cutting fluid emulsions produced by dilution of a cutting fluid concentrate containing a cationic/nonionic surfactant mixture H. Bataller, S. Lamaallam, J. Lachaise, A. Graciaa, C. Dicharry∗ Laboratoire des Fluides Complexes, Centre Universitaire de Recherche Scientifique, Université de Pau et des Pays de l’Adour, 64000 Pau, France Received 27 April 2003; accepted 10 March 2004
Abstract A cutting fluid concentrate has been formulated from a paraffinic oil, a hard water, MonoEthanolamine Borate (MEAB) and a cationic/nonionic surfactant mixture. Its dilution with hard water yields very stable oil-in-water emulsions, the so-called cutting fluid emulsion, with an average droplet diameter of 50 nm, independently of both the hardness of water and the weight ratio of hard water to concentrate. The high stability of these emulsions results from the existence of a strong repulsive potential between droplets which is caused in particular by the presence of the cationic surfactant at their surface. From a practical point of view, the ability to produce stable cutting fluid emulsions for a wide range of dilution, whatever the hardness of water, might allow the user to fine-tune the lubricating and cooling abilities of the fluids and therefore optimize their performances during the metal working process. © 2004 Elsevier B.V. All rights reserved. Keywords: Cutting fluid concentrate; Dilution; Hard water; Cutting fluid emulsion; Stability
1. Introduction Heat dissipation and lubrication are common problems that faces the metal processing industry. In machining processes such as metal/turning, milling, drilling and in particular when the metal removal operations are conducted at high speeds and low pressures, the regulation of heat generation and the lubrication of the contact point are achieved by pouring an oil-in-water emulsion, the so-called cutting fluid emulsion, over the cutting surface. Such a fluid has the particular advantage that it combines the cooling property of water and the lubrication property of oil. It is usually obtained by diluting a cutting fluid concentrate with mains waters at a weight ratio of water to concentrate ranging from approximately 98/2 to 95/5 depending on the machining operation [1,2]. A cutting fluid concentrate usually contains a mineral oil, a surfactant mixture, in some cases water and various additives which are included to meet the specifications of commercial concentrates such as resistance to bacterial growth and low corrosion capacity [1,2]. For easiness of use, the concentrate must be stable and not viscous. Its appearance has to be monophasic and its dilution with water
∗ Corresponding author. E-mail address: [email protected] (C. Dicharry).
0924-0136/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jmatprotec.2004.03.027
associated with gentle mixing must produce an oil-in-water emulsion. An emulsion is a nonequilibrium system which will finally separate in two phases. A short time of separation will restrict the use of the emulsion as a cutting fluid because lubrication capacity decreases with stability. The stability of an emulsion can be increased by using ionic surfactants because their adsorption at the surface of the oil droplets yields electric repulsion between them [3]. A commercial cutting fluid concentrate generally contains a mixture of anionic and nonionic surfactants [4]. Mixed surfactants increase the solubilization capacity of surfactants [5] and in some cases may facilitate the spontaneous formation of an emulsion when the system is brought into contact with water [4]. The presence of an anionic surfactant in the surfactant mixture gives a negative electric charge to the oil droplets of the emulsion [6]. However, the stability of this emulsion will often depend on the metallic ion (Ca2+ and Mg2+ ) contents of the water used for dilution. When the ion concentration is high enough, the anionic surfactants may form insoluble compounds with them and then precipitate [7]. This causes the decrease of the surface charge of droplets and the emulsion may be destabilized. The total concentration of calcium and magnesium ions in water defines the hardness of water. A water is said hard if the concentration is greater than 1.5 mmol/l [8]. The hardness of water may vary from one region to another
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of a same country. Therefore, the stability of a cutting fluid emulsion containing an anionic surfactant may drastically vary depending on the region in which it is used. Under these conditions and in order to maintain the lubricating and cooling capabilities of the fluid, it may be necessary to regenerate the fluid [9]. As a consequence, the amount of fluid required for the operations increases as well as the amount of waste water to stock and treat [10]. It is possible to reduce the risk of the surfactant precipitation with the Ca2+ and Mg2+ ions by using anionic surfactant molecules in which some oxyethylene groups have been introduced [7]. Another possibility is to use a cationic surfactant. We have chosen this second option. Therefore, the aim of this work is to formulate a cutting fluid concentrate from an aqueous phase, an oily phase and a cationic/nonionic surfactant mixture and to test its ability to form stable cutting fluid emulsions for a large range of hardness of water. For this purpose, we have first determined the phase diagram of these systems in order to locate the area containing the cutting fluid concentrates, i.e. the systems having a monophasic aspect. Afterwards, one of them has been diluted with different hard waters, and then it has been diluted at different weight ratios with a hard water of fixed hardness. The stability of the emulsions issued from these dilutions has been investigated by analysing the evolution of their droplet size distribution.
2. Materials and methods The concentrates formulated in this work are simplified systems compared to commercial concentrates. Nevertheless, they contain the main ingredients of commercial concentrates, i.e. oil, water, a pH regulator, and surfactants.
MEAB was prepared from 1/1/1 by weight of boric acid, 2-aminoethanol (from Aldrich with a purity >99%) and purified water. MEAB performs an antibacterial action by maintaining the pH greater than 9. The weight ratio of MEAB to the aqueous phase was written as XMEAB : XMEAB =
MEAB MEAB + hardwater
(1)
• The surfactant mixture. This is a mixture of a cationic surfactant (ethoxylated N-tallow ammonium methylsulfate, trade name: Noxamium S11M) and a lipophilic amine (ethoxylated N-tallow amine, trade name: Noramox S2) manufactured by CECA. These surfactants have some polydispersity in the poly-(oxyethylene) chain length distribution with an average of 11 for the former and 2 for the latter. The length of their chain is distributed as follows: 3 wt.% of C14 , 30 wt.% of C16 , 40 wt.% of C18 =, 26 wt.% of C18 , 1 wt.% of C20 . In basic dilute solution, which our experiments were based upon, Noramox S2 behaves like a nonionic surfactant. The weight ratio of Noxamium S11M to the surfactant mixture was written as XS11M : XS11M =
S11M S11M + S2
(2)
The weight ratio of the aqueous phase to the oily phase was written as WOR: aqueous phase WOR = (3) oily phase Finally, the weight ratio of the surfactant mixture to the system was written as XS : XS =
S11M + S2 S11M + S2 + oily phase + aqueous phase
(4)
2.1. Materials
2.2. Phase diagram
• The oily phase. It is a paraffin base oil (100 Neutral Solvent) manufactured by TotalFinaElf. • The aqueous phase. This is a mixture of hard water and MonoEthanolamine Borate (MEAB). Hard water was obtained by adding 0.76 g/l of calcium chloride hexahydrate and 0.132 g/l of magnesium sulphate heptahydrate (both from Aldrich with a purity of 98%) to water purified by the Millipore milli-Q 185 E system (conductivity less than 10−1 S/cm). Under these conditions the CaCl2 and MgSO4 molar concentrations were 3.47 and 0.536 mmol/l respectively, which correspond to those required for the normalized tests (NF T 60-185, 60-186 and 60-187) used in France for the characterization of aqueous cutting fluids. Therefore, the total concentration of Ca2+ and Mg2+ in water is CT (=[Ca2+ ] + [Mg2+ ] = 4.006) mmol/l. This water can be considered as very hard since CT > 1.5 mmol/l. It was used both for the formulation of the concentrates and for their dilution.
The phase diagram was obtained as follows: the samples were prepared in test tubes by mixing together and in the following order the aqueous phase (MEAB and hard water), the cationic surfactant, the oil and the nonionic surfactant. WOR was fixed at 3. The samples were shaken for homogenization and stored in a thermostat regulated water bath at 25.0 ± 0.5 ◦ C for 2 days, after which time the phase behaviour was determined visually. 2.3. Preparation of the cutting fluid emulsions The cutting fluid emulsions were prepared in test tubes by adding, quickly and in one step, the required amount of hard water to the concentrate, at 25.0 ± 0.5 ◦ C. These samples were then gently homogenized by just three successive turnings of the test tubes. The translucent aspect of the emulsions have shown that they contained very small droplets, which consequently
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made it possible to carry out dynamic light scattering measurements. 2.4. Droplet size measurement The size of the emulsions droplets was determined by laser light scattering. The apparatus was made up of a 15 mW He–Ne laser (λ = 632.8 nm) and a Malvern 7032 Multi-8 Correlator. All experiments were performed at the scattering angle θ = 90◦ , and the sample temperature was maintained at 25 ± 1 ◦ C. In order to limit the problems of data analysis linked to the multiple particle light diffusion of concentrated systems [11], the emulsions underwent a new dilution with hard water to obtain a volume fraction of dispersed phase equal to 0.01% which made it possible to take the measurements under “infinite” dilution. The droplet size distributions were calculated using the algorithm CONTIN [12–14]. It should be noted that the time elapsed between the first dilution of an emulsion and the measuring of the droplet size was about 10 min. 3. Results 3.1. Phase diagram We showed in a previous study [15] that the phase behaviour of the systems containing a paraffinic oil, the same aqueous phase and surfactant mixture than those used in this work depends on the values of XS11M and XMEAB . We specifically showed that increasing XMEAB shifts the interactions between the surfactant mixture and the aqueous and oily phases from hydrophilic, for low XMEAB , to lipophilic, for high XMEAB . Preliminary investigations showed that XS11M = 0.40 makes it possible to obtain many systems having a monophasic aspect for XS ≤ 0.10, which typically corresponds to the surfactant weight content of commercial cutting fluid concentrates. Fig. 1 shows the phase behaviour of the systems in the diagrams (XMEAB , XS ). Three different phase behaviours can be distinguished. For low enough values of XMEAB , systems noted WI are formed. These diphasic systems contain an aqueous microemulsion in balance with a phase in excess that contains mainly oil. For high enough values of XMEAB and XS , systems noted WII are formed. These systems are also diphasic but they contain an oily microemulsion in balance with a phase in excess that contains mainly water. The boundary that separates the WI systems from the WII systems was not determined for XS < 0.02. The zone NE locates the systems for which no excess of oil and water was noticed at the same time. Their aspect is monophasic. Therefore, this zone locates the potential cutting fluid concentrates. 3.2. Dilution of a concentrate with hard water The concentrate diluted with hard water is shown by a triangular symbol in Fig. 1. Its composition is reported in
Fig. 1. System: 100 Neutral Solvent/(hard water CT = 4.006 mmol/l, MEAB)/(Noxamium S11M, Noramox S2). Phase behaviour of the system in the diagram (XMEAB , XS ) at the weight ratio of Noxamium S11M to the surfactant mixture XS11M = 0.40, the ratio of the aqueous phase to the oily phase WOR = 3 and the temperature T = 25 ◦ C. The triangular symbol locates the concentrate diluted with hard water in this work.
Table 1. This system was selected because its high surfactant content might favour the stability of the emulsion once formed. Moreover, it also contains a high amount of MEAB which might preserve the cutting fluid emulsion from any bacterial degradation. 3.2.1. Influence of the hardness of water (CT ) added to the concentrate Waters of different hardness were added to the concentrate in the weight ratio 95/5. These hard waters were obtained by diluting with purified water an hard water for which CT = 8.016 mmol/l ([CaCl2 · 6H2 O] = 1.52 g/l and [MgSO4 · 7H2 O] = 0.264 g/l). The mass mean diameters of the droplets of the cutting fluid emulsions measured immediately after the dilution of the concentrate and 80 days later are reported in Fig. 2. The temperature of the samples was hold at 25.0 ± 0.5 ◦ C during this time. Considering the weak agitation applied to mix the concentrate and hard water, it is worthwhile noticing the nanometric size of the droplets of the emulsions. The value of the mass mean diameter does not depend on the hardness of the water added to the concentrate and no appreciable evolution has been observed during 80 days. Therefore, the stability of the cutting fluid emulsions does not depend on the hardness of water. Letter A (CT = 4.006 mmol/l) in Fig. 2 locates the cutting fluid emulsion whose stability is discussed thereafter. Table 1 System: 100 Neutral Solvent/(hard water CT = 4.006 mmol/l, MEAB)/(Noxamium S11M, Noramox S2). Composition of the cutting fluid concentrate XMEAB XS11M XS WOR
0.55 0.40 0.10 3
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Fig. 2. Influence of the total concentration of Ca2+ and Mg2+ of the hard water added to the concentrate in the weight ratio 95/5 on the mass mean diameter of the emulsions; T=25 ◦ C. Letter A (CT = 4.006 mmol/l) locates the cutting fluid emulsion whose stability is discussed thereafter.
3.2.2. Influence of the amount of hard water added to the concentrate Water of fixed hardness (CT = 4.006 mmol/l) was added to the concentrate in different weight ratios of hard water to concentrate (97.5/2.5, 95/5, 90/10, 80/20, 60/40 and 50/50). The mass mean diameters of droplets measured immediately after the formation of the cutting fluid emulsions and 7 days later are reported in Fig. 3. The temperature of the samples was hold at 25.0 ± 0.5 ◦ C during this time. The initial mass mean diameter of droplets is 50 nm whatever the weight ratio of hard water to concentrate. The cutting fluid emulsions obtained for the ratios 97.5/2.5, 95/5 (cutting fluid emulsion A), 90/10, 80/20, 60/40 are stable at least for 7 days. After this time, only the mass mean diameter of the droplets of the emulsion obtained for the ratio 50/50 (cutting fluid emulsion B) changes appreciably. The droplet size distributions of emulsion B at 0 and 7 days after its formation are shown in Fig. 4. It can be noticed the
Fig. 3. Influence of the amount of hard water (CT = 4.006 mmol/l) added to the concentrate on the mass mean diameter of the droplets of the emulsion; T = 25 ◦ C. Letters A (weight ratio 95/5) and B (weight ratio 50/50) locate the cutting fluid emulsions whose stability is discussed thereafter.
Fig. 4. Droplet size distributions of B (weight ratio of hard water (CT = 4.006 mmol/l) to concentrate 50/50) at 0 and 7 days after its formation; T = 25 ◦ C.
broadening of the distribution with time and its shift towards greater diameters.
4. Discussion It has been shown in Figs. 2 and 3 that the dilution of the concentrate with hard water yields droplets of 50 nm in diameter, independently of both the hardness of water and the weight ratio of hard water to concentrate. The great efficiency of the emulsion formation, whatever the conditions of dilution, is probably the consequence of two phenomena occurring simultaneously: • The sudden shift of affinity of the surfactant mixture in favour of the aqueous phase. • The passage of the interfacial tension between the aqueous and oily phases by a very low value. The first phenomenon is the consequence of the sudden decrease of the concentration of MEAB in the aqueous phase caused by the addition of hard water. Actually, the dilution of the concentrate decreases XMEAB from 0.55 to 0.02 for emulsion A and from 0.55 to 0.22 for emulsion B. As it is shown in Fig. 1, for XMEAB lower than about 0.30, the mixture of surfactants favours the formation of a WI system at equilibrium, and then, this type of phase behaviour leads to the formation of an oil-in-water emulsion when the system is mixed [16]. The low interfacial tension is directly the consequence of the utilization of a surfactant mixture [17]. Almost all the cutting fluid emulsions are stable for a long period. Those obtained for the weight ratio of hard water to concentrate 95/5 have a mass mean diameter unchanged at least for 80 days, whatever the hardness of water. Only the mass mean diameter of those obtained for the weight ratio 50/50 (cutting fluid emulsion B) changes appreciably in 7 days of observation.
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In order to highlight the high stability of these emulsions, we have compared the so-called coagulation time of their droplets, i.e. the time required for the total number of droplets to decrease to half the initial number, with the theoretical one obtained by applying the Smoluchowski’s theory of rapid coagulation [18]. In this theory, it is assumed that there is no interaction between droplets, that is to say that droplets are not covered with surfactant. According to the theory, the normalized number of droplets in function of time can be written as: n(t) 1 = n(0) 1 + t/τ
(5)
where n(0) is the number of droplets per volume determined immediately after the formation of the cutting fluid emulsion, n(t) the number of droplets per volume at the time t and τ the coagulation time. From the droplet size distribution, it is possible to calculate the number of droplets per volume by using the following equation: n(t) =
π
n
6φ
(6)
0 3 i=1 ωi (di (t))
where φ is the volume fraction of dispersed phase, di the mass mean diameter of class i and ωi0 the number frequency of droplets in class i. Assuming that the dispersed phase contains the surfactants and the oil, φ can be written as: Vo + Vs φ= (7) Vo + V s + V w where Vo , Vs , and Vw are the volume of oil, the volume of surfactants and the volume of the aqueous phase, respectively. Provided the droplet size distributions of the cutting fluid emulsion at the times 0 and t are known, Eqs. (5)–(7) give the corresponding coagulation time. The coagulation times of A and B calculated at t = 0 and 7 days are reported in Table 2. It is noteworthy that, according to Eq. (5), the coagulation time of cutting fluid emulsion A is infinite. However, this result does not mean that A is infinitely stable but it means that the time necessary to observe coagulation is greater than 7 days (actually, it is greater than 80 days, according to Fig. 2). The decrease of the coagulation time observed for cutting fluid emulsion B is due to a great extend to the increase of the volume fraction of dispersed phase φ in the emulsion. In fact, for a given mass mean diameter, increasing Table 2 Coagulation time τ calculated for the cutting fluid emulsions A and B. Hardness of water (CT = 4.006 mmol/l). Temperature T = 25 ◦ C
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the volume fraction of dispersed phase increases the number of droplets in the emulsion and therefore their probability to collide and coagulate. Consequently, the time of coagulation decreases. According to the Smoluchowski’s theory, the coagulation time can be written as: 3η τSmol = (8) 4kTn(0) where k, T, η and n(0) are the Boltzmann constant, the absolute temperature, the viscosity of the continuous phase and the initial number of droplets per unit of volume. Assuming that the continuous phases of A and B contain the hard water and MEAB, their MEAB weight contents are 1.89 and 22 wt.%, respectively. Under these conditions, the viscosity of the continuous phase of A is 0.935 mPa s and that of the continuous phase of B is 1.4 mPa s. From the viscosity of the continuous phases and the initial numbers of droplets n(0) (Table 2), Eq. (8) gives the theoretical coagulation times, τ Smol (Table 3). These theoretical coagulation times appear “infinitely” lower than the experimental ones. Therefore, the droplets of our cutting fluid emulsions coagulate very slowly compared to those of the theoretical system in which droplets would be deprived of surfactant. This result shows that there is a strong repulsive potential between droplets, and therefore, that they are covered with surfactants. This repulsive potential probably has a strong electric contribution, considering the presence of the cationic surfactant in the system. Because the cationic surfactant molecules do not form precipitates with the ions present in solution (contrarily to anionic surfactants), they remain adsorbed on the surface of droplets and they contribute to stabilize the emulsions thanks to electric repulsion. However, it is also likely that this potential has a strong steric contribution. This hypothesis is supported by the fact that the mass mean diameter of the droplets of the cutting fluid emulsions remains unchanged for a long period, whatever the hardness of water (Fig. 2). It is known that, the higher the hardness of water is, i.e. the higher the salt concentration in the aqueous phase is, the weaker the electric repulsion between droplets is [3]. It results from the screening of electric repulsion that the droplets can collide and coagulate more easily. Consequently, without the existence of steric interactions, the cutting fluid emulsions formed Table 3 Theoretical coagulation time τ Smol calculated for the cutting fluid emulsions A and B. Hardness of water (CT = 4.006 mmol/l). Temperature T = 25 ◦ C
Emulsion
φ
n(t = 0) (×1019 m−3 )
n(t = 7 days) (×1019 m−3 )
τ (days)
Emulsion
XMEAB
η (mPa s)
n(t = 0) (×1019 m−3 )
τ Smol (ms)
Aa Bb
0.02 0.19
0.51 5.09
0.51 0.97
∞ 4.25
Aa Bb
0.02 0.22
0.935 1.40
0.51 5.09
33 5
a b
Weight ratio of hard water to concentrate 95/5. Weight ratio of hard water to concentrate 50/50.
a b
Weight ratio of hard water to concentrate 95/5. Weight ratio of hard water to concentrate 50/50.
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in the hardest waters should have probably broken more quickly. 5. Conclusion A cutting fluid concentrate was formulated from a paraffin oil, a hard water, MonoEthanolamine Borate and a mixture of cationic/nonionic surfactants. Its dilution with hard water produced very stable cutting fluid emulsions with oil droplets of sub-micrometric size, whatever the hardness of water and the range of dilution investigated in the study. The efficiency of the emulsion formation probably results from both the steep decrease of the concentration of MEAB in the aqueous phase yielding hydrophilic conditions for the surfactant mixture, and the occurrence of a very low interfacial tension between oil and water. The high stability of the cutting fluid emulsions is probably due to a great extent to the utilization of a cationic surfactant which does not form precipitate with the metallic ions present in hard water. Therefore, its presence at the surface of droplets gives them an electric protection which hinders the coagulation phenomenon. Moreover, the high stability of these emulsions whatever the hardness of water also makes it possible to consider a steric protection of the droplets. The concentrate developed in this work meets some of the specifications of commercial concentrates such as a monophasic aspect, an easiness of dilution with water and a formation of stable cutting fluid emulsions. However, many other properties have not been checked as for examples, the performance of the cutting fluid emulsions in metal working operations, their foaming tendency or their corrosive property. The satisfaction of all of the specifications of commercial concentrate would probably make the formula of this concentrate more complex. Nevertheless, this formula constitutes a good basis of work while showing that the utilization of a cationic surfactant in the surfactant mixture makes it possible to do not care about the hardness of water used for dilution. Moreover and from a practical point of view, the ability of such a concentrate to form stable cutting fluid emulsions for a wide range of mixing might allow the user to fine-tune the lubricating and cooling abilities of the emulsion and therefore optimize its performances during the metal working process.
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