Vegetable Oil-Based Metal Working Fluids-A Review

Vaibhav Koushik A.V, Narendra Shetty. S & Ramprasad.C Dept. of Mechanical Engineering, Atria Institute of Technology, Bangalore-560024, Karnataka, India E-mail : [email protected], [email protected], [email protected]

product quality by reducing cutting forces and vibrations, leading to an efficient metal cutting operation as measured in terms of increased tool life and improved surface finish. This results from (1) cooling the tool, work piece and chip, (2) influence of the cutting fluid in facilitating chip formation and (3) reduction in chip compression, built up edge and toolchip seizure [5]. Also, there are secondary functions of the cutting fluid, which may be of the greatest practical importance in particular machining operations. These include cooling the work piece to reduce expansion and distortion and making handling easier, washing away the swarf and protecting the work piece and machine components from corrosion [6]. Cutting fluids are normally classified into three main groups, namely i) neat cutting oils ii) water soluble fluids iii) gases. The water soluble fluids can be classified as emulsifiable oils (soluble oils), chemical (synthetic) fluids and semichemical (semi synthetic) fluids [2, 7, 8].

Abstract – Metal working fluids are widely employed to increase the machining productivity and quality of metal cutting, but their usage poses a great threat to ecology and health of workers in the industry. Therefore, a need arose to identify eco-friendly and hazard free alternatives to conventional mineral oil based metal working fluids. Vegetable oils have become identified world over as a potential source of environmentally favorable metal working fluids due to a combination of biodegradability, renewability and excellent lubrication performance. Low oxidation and thermal stability, poor low temperature behavior, however limit their potential application as metal working lubricants and has become the thrust area of research of scientists and tribologists world over. Keywords – Environmental impact, lubricants, Metalworking fluids, vegetable oil.

I.

INTRODUCTION

A lubricant is a substance used to facilitate relative motion of solid bodies by minimizing friction and wear between interacting surfaces. In addition to the primary purpose of reducing friction and wear, lubricating oils are also required to carry out a range of other functions, including removal of heat, corrosion prevention, transfer of power, providing a liquid seal at moving contacts and removal of wear particles [1]. Metal working fluids (MWFs), otherwise known as cutting fluids come under lubricants which are extensively used in machining operations [2].There are few human activities that do not depend upon on the use of articles that at some stage in their manufacture have required the application of metal cutting techniques. It is interesting to note that the use of coolants for machining was first reported by Taylor in 1907, who achieved up to 40% increase in cutting speed when machining steel with high speed steel tools, using water as a coolant [3]. During metal cutting operation, the material that is removed from the work piece slides along the rake face of the cutting tool in the form of continuous or discontinuous chips, resulting in friction. Lubricants penetrate into the zone of contact between the tool and the freshly formed chip to influence friction [4]. Cutting fluids enhance machining productivity and

II. EVOLUTION OF METAL CUTTING FLUIDS A. Retrospects Until the mid 19th century, essentially all lubricating oils were based on natural oils, i.e. animal fats, vegetable oils and marine oils, which are triglycerides of mixed C8-C22 linear fatty acids. Industrial development during the 19th century rapidly increased the need for lubricants, which exceeded the supply of natural oils. However, the increased exploitation of petroleum as a fuel source also provided a new material possessing lubricant properties; mineral oil, which was co-produced, when crude oil was refined for gasoline production. This led to the availability of mineral oil in large quantities and at competitive prices. Mineral oils are found to be more stable than natural oils; also they are cheaper and available in a wide range of viscosities. They are extremely complex mixtures of C20-C50 hydrocarbons containing a range of linear alkanes (waxes), branched alkanes (paraffins), alicyclic, olefinic and aromatic species. They also contain a

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significant concentration of hetero atoms, mainly sulphur [1, 9].

based oils potential candidates for use in industry as MWFs. Many investigations in developing effective substitutes to mineral oil based lubricants are in progress, various vegetable oils available around the world, especially rapeseed[16], canola[2], and coconut oil[3,17,18] being some of the more promising candidates as base stocks for metal working lubricants.[2].

Most of the metal working fluids being employed today are mineral oil based (neat oils and soluble oils) [2]. If mineral oil is regarded as chemical products, the lubricant industry is responsible for approximately 8.5% of the global chemical production by weight [1]. They are ubiquitous in the machine tool industry, with estimates of consumption in North America exceeding two billion gallons in 2000 [10]. While metal working fluids have their advantages in the machining industry, they also give rise to significant problems in the immediate working environment and in waste disposal [11].Major health hazards involved in the use of mineral oil based metal working fluids include skin diseases caused by contact with nitrosamines, heavy metal compounds and bactericides present in MWFs; respiratory problems due to inhalation of toxic mists and exposure to oil fags and fumes [12]. It is reported that about 80% of all occupational diseases of operators are by skin contact with cutting fluids [14]. As a limitation in the direction of disposal of used metal working fluids, when inappropriately discharged, cutting fluids may pollute soil and water resources resulting in serious environmental impact [13]. It is learnt that EU alone consumes approximately 320,000 tons per year of metal working fluids [MFWs], out of which at least two thirds need to be disposed [14].

The use of vegetable oil in metal working applications alleviates problems faced by workers such as skin cancer and inhalation of toxic mists in the working environment. [19]. Such lubricants have the tendency to get ingested and metabolized by microorganisms, thus achieving complete biodegradability and thereby return to nature [13]. The life cycle of the products based on renewable resources is represented in fig.1. By progressively enhancing the usage of vegetable oil based metal working lubricants, energy security for the countries that use them will be obtainable and such a situation will help to create local and regional economic development opportunities [20].

The awareness of these hazardous effects caused by the use of mineral oil based lubricants has resulted in legislations to regulate the usage of lubricants in countries like Austria, Canada, Hungary, Japan, Poland, Scandinavia, Switzerland, U.S.A, and E.U. Among these countries, Austria is the only country that has a law banning the use of mineral oil based lubricants in particular applications like chain-saw oils. In this direction, environmental labels, a scheme of labeling environmental friendly products are being defined and developed by certain countries like; Blue Angel in Germany, White Swan in Scandinavian countries, Eco mark in India [12, 15].

Fig. 1: Life cycle of chemical products based on renewable resources [13].

B. Prospects - eco friendly and biodegradable lubricants

III. STRUCTURE AND LUBRICATION PROPERTIES OF VEGETABLE OILS

With this background about the MWFs of mineral oil origin in use and its accompanying limitations, lines of thinking have been in progress world over in making use of vegetable oil based lubricants, which are liquid agricultural products and are produced from plant and cash crops. These are highly attractive substitutes for petroleum based oils because, in addition to being renewable, biodegradable and non toxic, vegetable based oils possess naturally occurring high viscosity and high flash point [9]. The above reasons make vegetable

Vegetable oils comprise of primarily triglycerides, that is, tri-esters of long chain carboxylic acids (fatty acids) combined with glycerol. Most of these oils contain at least four and sometimes as many as twelve different fatty acids. The proportion of each fatty acids depend not only on the type of the plant, but also on the geo-climate and the weather available. The triglyceride structure of the vegetable oils provides desirable qualities of boundary lubrication; a high natural viscosity and viscosity index (the strong intermolecular ISSN (Print): 2319-3182, Volume-1, Issue-1, 2012

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interactions are resilient to temperature changes) and structural stability over reasonably operating temperature ranges. The flash point of vegetable oils is high, which correlates to a very low vapor pressure and volatility, thereby eliminating potential hazards during use.

acid chains produce oriented molecular films that interact strongly with the metallic surfaces, reducing both friction and wear. In general, vegetable oils have a poor oxidative and thermal stability when compared to mineral oils and this issue can be addressed by various methods such as reformulation of additives, chemical modification of vegetable oils and genetic modification of oil seed crop.[2,21,22,9].

The oxidation stability of vegetable oils depends on the level of unsaturated products present. The lower the unsaturation, the better the oxidative stability, but higher the melting point. Excess of certain poly unsaturated fatty acids leads to unfavorable oxidation behavior and resination at high temperatures. The long and polar fatty

The Rheological properties of various vegetable oils are presented in Table-1. .

TABLE I : PHYSICO-CHEMICAL PROPERTIES OF VEGETABLE OILS [22, 24] Soybean Kinematic viscosity at 40°C (cSt) Kinematic viscosity at 100°C (cSt) Viscosity index Saponification value (mgKOHgˉ1) Total acid value (mgKOHgˉ1) Iodine value (mgl gˉ1) Pour point (0°C) Flash point (0°C)

High oleic soybean

Sun Pongammia Jatropha Rapeseed Jojoba Neem Castor Coconut flower pinnata curcass

32.93

41.34

40.05

45.60

24.90

43.00

47.48

68.03

220.6

36.2

08.08

09.02

08.65

10.07

06.43

08.30

08.04

10.14

19.72

6.76

219

-

206

216

233

172

208

135

220

130

189

-

-

180

94.69

179

196.80

166

180

248-265

00.12

-

1.40

01.10

22.00

03.20

23.00

01.40

-

144

85.90

-

104

98.00

78.00

97.00

66.00

87.00

6-8

-09.00

-

-12.00

-12.00

09.00

-09.00

0.00

09.00

-27.00

20

240

-

252

240

-

-

240

-

250

240

00.61

. IV. PERFORMANCE OF VEGETABLE OIL BASED MWFS IN METAL CUTTING - REPORTS OF STUDY

output parameters chosen to measure the performance of cutting fluids were radial wheel wear and workpiece roughness. In order to compare the performance of the new cutting fluid, two other types of fluids were tested: cutting oil and a semi-synthetic fluid. The radial wheel wear values were measured using printed profile technique and ‘G-ratio’ (volume of material removed/wheel worn volume) was used to compare the different cutting fluids. They found that cutting oil caused the lowest wheel wear (high G) whereas higher wheel wear was observed using semi- synthetic cutting fluid. The new formulation containing concentration of 21% showed similar performance to the cutting oil, with high value of G. At higher concentration (32%) of the formulation under study, chip agglomeration was

Alves and Oliveira [23] experimentally determined the performance of water and vegetable oil combination cutting fluid as a grinding fluid to be applied in CBN grinding process. Parameters such as biodegradability and good mechanical performance was considered as criteria for acceptance. The grinding tests were performed in a conventional surface grinder. The workpiece material was SAE 8640. The tests were performed using a vitrified CBN wheel. The natural oil and the emulsifier used were Sulfonate castor oil 80% and polyglycol of synthetic ester respectively. The

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observed, thus increasing friction between the workpiece and wheel, leading to increase in wheel wear. Analysis of wheel wear results are presented in Fig. 2. The study of the influence of cutting fluid on the workpiece roughness revealed a high roughness value for the two cutting fluids having high viscosity (cutting oil and the new cutting fluid at 32% concentration), which is in stark contrast to the generally accepted notion that; higher the lubricant ability, lower is the roughness. The above result is considered to be due to chips agglomeration. At lower concentrations (15% and 21%), the new cutting fluid was found to cause a decrease in roughness. The variation of surface roughness for different cutting fluids is represented in fig. 3. By performing corrosion and biodegradability test, they concluded that the cutting fluid formulated possessed corrosion inhibition characteristics and was biodegradable.

tool flank wear and surface roughness were studied with respect to cutting conditions. The machine tool used was a lathe machine (PSG Company, India). The nanoboric acid powder of 50nm particle size was reported to have been obtained by mechanical milling with a high energy ball mill. The solid lubricant particles were manually mixed in SAE 40 oil and coconut oil in different weight proportions and were followed by mixing with a sonicator. For the purpose of comparison, pure coconut oil and SAE 40 oil were used, along with the suspensions under the study. The cutting tool temperature was measured using an embedded thermocouple placed at the bottom of tool insert in tool holder. The cutting tool was analysed under an optical projector to measure tool flank wear and a Talysurf was employed for measuring average surface roughness. The cooling action of the selected lubricating conditions was observed by variation in cutting temperature. Their results showed that the cutting temperature increased with cutting speed irrespective of the lubricant and were found to be less with coconut oil compared to SAE 40 oil. Cutting temperature increased with increase in feed rate at all the lubricating condition. Among the various compositions under the study, they reported that the coconut oil with 0.5% nanoboric acid particle suspension exhibited the best performance. The variation of cutting temperature with cutting speed and feed for the selected lubricating conditions can be observed in the Fig. 4 and 5 respectively. The tool flank wear was reported to have been measured under the selected lubricating conditions with cutting speed. The flank wear was observed to increase gradually with increase in speed and feed. It was observed that the solid lubricant along with oil created a thin lubricant film on the workpiece and tool leading to reduction in flank wear. The suspension containing 0.5% nanoboric acid particle in coconut oil was reported to exhibit the most significant reduction in flank wear compared to the remaining cutting oils under study.

Fig. 2 : Radial wheel wear values for different cutting fluids [23].

Fig. 3 : Roughness values (Ra) for the different cutting fluids.[23]

Vamsi Krishna et al. [18] investigated the performance of nanoboric acid powder suspension in SAE 40 and coconut oil during turning of AISI 1040 steel with cemented carbide tool (SNMG120408). Influence of solid lubricant (nanoboric acid, 50nm particle size) to oil proportions on cutting temperatures,

. Fig. 4 : Variations of cutting temperatures with cutting speed [18]

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a . S-add and P-add columns represent amount of sulphur and phosphor containing additives at the levels low (+), middle (++) and not available (-)

Fig. 5 : Variation of cutting temperature with feed [18]

Fig. 7 : Variation of surface roughness with feed [18]

The variation of tool flank wear with cutting speed is represented in Fig.6. The surface roughness was observed to increase with increase in feed rate. Among the selected lubricating conditions, they have found surface roughness to have considerably reduced with coconut oil compared to SAE 40 oil. Fig.7 represents the variation of surface roughness with feed.

Fig. 6 : Variation of tool flank wear with cutting speed [18] (feed=0.2mm/rev, depth of cut=1mm, test time= 15min.)

Conclusively Vamsi Krishna et al. observed that cutting temperature, tool flank wear and surface roughness decreased significantly with nanolubricants suspended in coconut oil and this was attributed to the lubricating action of boric acid and the better lubricating properties of the base oil.

TABLE 2 : Composition of Cutting fluids under study [16]

Name

Type

Description

Sadd

RM Mineral oil-based, Commercial (refere general purpose, oil nce oil) 20cSt at 40°C Vegetable oil-based, Commercial RV general purpose, 19 oil cSt at 40°C Blend of rapeseed Formulated oil and ester oil, A ++ oil general purpose, 20 cSt at 40°C Blend of rapeseed oil, ester oil and Formulated B meadowfoam oil, ++ oil general purpose, 20 cSt at 40°C Blend of rapeseed Formulated oil and ester oil, C + oil mild duty, 20 cSt at 40°C Blend of rapeseed oil, ester oil, and Formulated D meadowfoam oil, + oil mild duty, 20cSt at 40°C

Padd -

Belluco and De Chiffre [16], have investigated the performance of vegetable based oils in drilling austenitic stainless steel. The efficiency of six cutting oils was reported to have been evaluated in drilling AISI 316L austenitic stainless steel using HSS-Co tools by measurements of tool life, tool wear, cutting forces and chip formation. Seven tools were tested with each fluid to catastrophic failure. A commercial mineral-based oil was taken as reference product and five vegetable based cutting fluids at different levels of additivation were tested. The composition of the selected lubricant conditions are represented in table 2. The machine employed was a Cincinnati Sabre 750S CNC vertical milling centre. Tool wear measurements were reported to have been carried out on a microscope equipped with an X-Y table and electronic gauge and the cutting force measurements of each machined hole was performed using a dynamometer. The analysis of variance (ANOVA) was performed to investigate the effect of different fluids on all measured parameters. Measurements of corner wear on flank and chip tangling were found to be less reliable and thus they have reported to have restricted the study of performance of

-

++

++

+

+

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cutting fluids to tool life and drilling thrust results only. The variation of drilling tool life and drilling thrust with different cutting fluids is represented in fig.8 and fig.9 respectively.

globally. Just as any asset gets linked with a liability, so also the mineral oil asset, while meeting the industrial requirements, simultaneously exposed the workers, and the environment in a wider context, to toxic mists, oil fags and fumes causing health problems of the nature of skin and respiratory ailments and through waste disposal polluting the soil and water resources. This prompted the research communities’ world over in evolving hazard free biodegradable metal working fluids, leading to the exploitation of liquid agricultural products namely vegetable oils for the purpose of tribological applications. Several review papers have already provided indications on the scope of vegetable based oils effectively replacing the mineral oils as MWFs and further research efforts are in progress confirming this hope. In this review paper, an effort has been undertaken to provide highlights on the vegetable oils that have shown promising scope of their emergence as metal working fluids. Mention has been made of castor oil, coconut oil, rapeseed oil and canola oil as vegetable oils undergoing further evaluations.

Fig. 8 : Average drilling tool life with different fluids. Error bars represent 95% confidence interval. [16]

VI. ACKNOWLEDGEMENTS The authors are highly grateful to The Principal, Atria Institute of Technology, Bangalore, India and The Professor and Head of the Department of Mechanical Engineering of the same institution for extending all the encouragement and guidance. The authors also feel equally indebted to the Prof. Satish V. Kailas and Mr. Sathwik Chatra K. R. of the Department of Mechanical Engineering, Indian Institute of Science, Bangalore, India, for their support and guidance in the various stages of the preparation of this paper.

Fig.9. Average drilling thrust with different fluids. Force measurements until tool failure. Error bars represent 95% confidence interval. [16]

They found that all vegetable-based oils produced better results than the reference mineral oil. The best performance was reported to be an 177% increase in tool life and 7% reduction in thrust force with respect to commercial mineral oil. Owing to the good correlation observed between tool life and thrust force, they concluded that a simple and inexpensive test for cutting fluids can be devised using one tool for testing several products by comparison of cutting forces, resulting in significant cost reduction and time savings with respect to tool life testing.

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[4]

Anirudhan Pottrayil, Satish V. Kailas, S.K. Biswas, Experimental Estimation of friction force in lubricated cutting of steel. Wear 269 (2010) pp.557-564

V. CONCLUSION Cutting fluids form the most commonly used process materials in the metal working industry. They help to achieve returns in terms of tool life, surface finish, accuracy to size and make chip breaking and chip transport easier. Mineral oils formed the basestocks of metal working fluids hitherto in the industrial sector

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