Development of Sustainable Metalworking Fluid Systems

An education module submitted to CSE Electronic library

April 25, 2008

Fu Zhao

School of Mechanical Engineering

Purdue University

585 Purdue Mall

West Lafayette, IN 47907

Tel: 765-4946637

Fax: 765-4940539

Email: fzhao@purdue.edu

Summary

This module demonstrates how to apply sustainable engineering and sustainable manufacturing

principles to re-design metalworking fluid (MWF) systems for simultaneous improvements in

economic, environmental, and health dimensions. The proposed sustainable MWF system is a

combination of an environmentally benign MWF chemical formulation and an appropriate

control system for this formulation that maximizes the MWF lifetime on the shop floor. Research

advances toward developing this sustainable MWF system are reviewed. Toward the end, the

approach adapted for sustainable MWF system development is generalized in order to be applied

to other manufacturing systems and processes, and other engineering systems in general.

Key words:

Sustainable Manufacturing; Metalworking Fluids; Bio-based Formulation; Membrane Filtration;

Microbial Sensor.

Prerequisite: Graduate standing

1. Introduction

Metalworking fluids (MWFs) are engineering materials that optimize the metalworking processes i.e. metal cutting and metal forming (McCoy, 2006). The primary function of MWFs is to provide lubricating and cooling. Additionally, MWFs deliver some secondary functions

such as chip transport, corrosion protection, and tool/work-piece cleaning (Childers, 2006). Over

the past century, metalworking fluids (MWFs) have usually been formulated as either straight-

oils or as a combination of water, oil, surfactants, and additives. Worldwide, manufacturers

currently consume over 2 billion liters of water-based and straight-oil MWFs each year creating

a significant demand for non-renewable feedstock (Glenn and van Antwerpen, 2004). In use,

MWFs are highly susceptible to contamination by microorganisms, leading to potential health

risks for workers from infection, inhalation of bio-aerosols, or dermal contact with biocides

utilized to control the biological growth (NIOSH, 1998). This biological growth, along with

physicochemical changes of the MWF and the buildup of metal particles and oils, also

deteriorates manufacturing performance and ultimately necessitates disposal. This disposal is

challenging and costly, which is serving to drive change in the industry. The large volumes of

aqueous waste carry toxic metals from manufacturing (e.g., cobalt and lead) into the environment,

along with a host of chemicals such as corrosion inhibitors, defoaming agents, surfactants,

chlorinated fatty acids and chelating agents that pose environmental risks. MWF treatment and

release to the environment can also lead to significant oxygen depletion and nutrient loading in

surface waters further posing environmental risks.

Sustainable manufacturing adds value to materials, components, or products while maintaining

the availability of natural resources and environmental quality for future generations. In this

module, we will demonstrate how to apply sustainable manufacturing principles to re-design

MWF systems for simultaneous improvements in economic, environmental, and health

dimensions.

2. Design Approaches for Sustainable Metalworking Fluid Systems

2.1 Design targets

The overarching objective of a sustainable MWF system is to meet, if not improve upon, existing

manufacturing capability while maintaining as-new performance for as long as possible with minimal eco-inputs, outputs, and health hazards. This requires resistance to microbial

degradation as well as maintenance of appropriate concentrations of active MWF constituents in

the presence of dynamic factors such as evaporation, circulation, and ingredient depletion.

Sustainability also implies that the MWF is robust to destabilization caused by contaminants

such as leak oils and particulates. As these factors also facilitate microbial proliferation, the

protection of human health and safety is a synergistic benefit resulting from contaminant control

in sustainable MWF systems. While improving both environmental and health performance, a

sustainable MWF system must also have a lower total life cycle cost than traditional systems to

be sustainable itself in practice. As a result, a sustainable MWF system is developed based on

multi-dimensional criteria including manufacturing capability, health risks, system longevity,

and total eco- and financial loading. Figure 1 conceptualizes the targets of a sustainable MWF

system.

Figure 1. Target objectives for sustainable MWF systems (adapted from Skerlos et al., 2001).

2.2 Design approach

Due to their higher cost, smoke and fire hazards, operator health problems, and limited tool life

through inadequate cooling, straight-oil MWFs only account for a small fraction of MWFs used

(Iowa Waste Reduction Center, 2003). Therefore, focus here is on water-based MWFs. A typical

water-based MWF will contain water, oil, surfactants and approximately 10 other specialty

chemicals. These MWFs require maintenance technologies such as depth filtration,

centrifugation, and biocide application to delay their inevitable deterioration over time. It is this

deterioration that leads to microbial growth and health risks, in-process fluid failure, and

eventual disposal, all factors that negatively impact the sustainability of MWF systems.

The deterioration of the MWF arises from many sources: the fundamental incompatibility of oil

and water, the susceptibility of emulsions to microbial growth, the evaporation of water, the

capability of hardwater ions to destabilize emulsions, and the susceptibility of surfactants to

foam when mechanically agitated. Therefore increasing the lifetime of a water-based MWF is

determined by its resistance to microbial growth as well as its resistance to fluctuations in the

concentrations of active MWF components which may be consumed in-process (or evaporate in

the case of water). Furthermore the MWF must also be robust to destabilization caused by

contaminants such as leak oils, ion accumulation and the buildup of metal particles from the cut

parts. As these factors also facilitate microbial proliferation, the protection of human health and

safety is a synergistic benefit resulting from contaminant control and stability of water-based

MWF systems. From this discussion it becomes clear that improving the sustainability of a

water-based MWF requires a two pronged design approach: 1) to select an environmentally

benign MWF chemical formulation, and 2) to deploy an appropriate control system for this

formulation that maximizes the MWF lifetime on the shop floor. A conceptualization of the

design approach is illustrated in Figure 2.

Figure 2. Approaches to improving the sustainability of water-based MWF systems (Adapted

from Skerlos et al., 2001).

2.3 Critical components of a sustainable MWF system

The system input to Figure 2 is a MWF designed for maximum stability and minimum life cycle

environmental impact and health risks. For water-based MWFs that are emulsions of oil droplets

stabilized in water using surfactants, the MWF formulation and chemistry must be selected so

that the oil-in-water emulsion does not destabilize under field conditions. This is a matter of

selecting the appropriate surfactants for the oil under consideration (e.g., petroleum oil, soybean

oil, etc.) as well as applying the surfactants in the correct concentrations.

One approach to designing more sustainable MWFs is to use bio-based oils and chemicals. Due

to their inherently higher biodegradability, bio-based formulations have the potential to reduce

the waste treatment costs required to meet the new MWF effluent limitation guidelines and

standards published by the US EPA in the Metal Products and Machinery Rule (US EPA, 2003).

Also, bio-based formulations could reduce the occupational health risks associated with

petroleum oil based MWFs due to their lower reported toxicity (Raynor et al., 2005) while

providing a renewable feedstock alternative that has been shown to perform better in

manufacturing operations such as thread cutting (Clarens et al., 2004). The principal technical

limitation of vegetable oil lubricants, their low oxidative stability, has been addressed by genetic

alternation, chemical modification and use of various additives (Rose and Rivera, 1998). The

principal economic limitation of vegetable based lubricants, their high cost relative to petroleum

oils, is diminishing as petroleum prices increase (Ash and Dohlman, 2005).

Once the formulation is selected, the MWF is then maintained using a control system comprised

of a coordinated system of contaminant detection and contaminant removal technologies. This

control system involves detection of physical, chemical, and biological contamination levels in

regular and short enough time intervals to enable an appropriate engineering response that avoids

the need to dispose the MWF. This use of sensors to acquire process information is also critical

to minimizing the environmental impact of the contaminant control technology itself. Since all

control technologies have their own environmental footprint, including in some cases

consumable media that must be disposed (e.g., filter media in depth filtration), it is desired to use

any contaminant control technology only at the minimum required rates derived from sensor

readings.

Sensing of microbial species and total biomass levels present in water-based MWF systems is

particularly important. In the absence of biologically inhibiting chemicals water-based MWFs

are essentially nutrients for bacterial and fungal growth. This growth leads to metabolic

destruction of active ingredients and reduces MWF performance. While ordinary microbes pose

minimal infection risk for workers, particularly hazardous species such as Mycobacteria sp. and

Legionella sp. have caused worker illness (NIOSH, 1998). Thus water-based formulations

require bio-inhibiting ingredients to keep microbial populations at bay. While generally effective,

there is a desire in the industry to minimize biocide application volumes since excessive

concentrations carry their own health risks such as acute dermatitis and more severe chronic

issues. Heat pasteurization and ultraviolet irradiation have been proposed as alternatives to

biocide use, but their use is not widespread for economic reasons. Alternatively, membrane

filtration has proven capable of dramatically extending the life of MWFs in pilot-studies (Wentz

et al., 2007).

The size, cost, and application rate of membrane filtration systems designed for biological

control depends on the biological growth rate in the system (Skerlos and Zhao, 2003). This

means that microbial sensors are important not only for detecting harmful bacteria but also for

properly maintaining of the system. Currently available microbial detection strategies, however,

are only order-of-magnitude accurate and require up to 48 hours to yield information. This

response time is too long for dynamic on-line control of MWF systems, or to prevent a microbial

outbreak from running its course. Clearly, a microbial detection approach suitable for application

to MWF systems that can in real-time quantify microbial contamination and identify specific

species is highly desirable.

3. Research Advances Toward Sustainable MWF Systems

An environmentally benign MWF chemical formulation, an online sensor for contaminant level

detection, and a high efficiency contaminant removal method are the three major components

required for a sustainable MWF system. Here we will summarize our research efforts on the

design of bio-based MWFs, detection of microorganisms using flow cytometry, and optimization

of MWF formulations for membrane filtration. Approximately 40% of the North American

market for MWFs is comprised of semi-synthetic MWF formulations (Byers, 2006). These

MWFs are comprised of water, a base oil, surfactants, and specialty additives. In general, semi-

synthetic MWFs are sold as concentrates with 10%-30% oil and are diluted 10-20 times with

water before use as metalworking fluids. The diluted fluids are stable, translucent and often

called microemulsions, with emulsified oil droplet sizes less than 100 nm. Our research focuses primarily on semi-synthetic MWFs, although the general concepts apply to all water-

based MWFs.

3.1 Design of bio-based MWFs

The selection of surfactants to disperse the oil and other hydrophobic additives in water is critical

for producing a stable microemulsion. Unfortunately, this has long been carried out using

empirical rules developed through trial and error experience (Childers, 2006), as well as using

the HLB (hydrophile-lipophile balance) method (Myers, 2006). Recently, Zimmerman et al.

(2003) designed both canola oil and petroleum oil based semi-synthetic MWFs by screening

binary mixtures of ethoxylated alcohol surfactants and sulfonate surfactants. It was found that

stable vegetable oil microemulsions can be achieved with a wide range of HLB values (from 6 to

18), while stable petroleum formulations required a narrower HLB range (from 6 to 12). This

suggests that the HLB method may not provide sufficient insight to facilitate surfactant selection

when designing vegetable oil MWFs. It was also pointed out that emulsion stability correlated

with nonionic surfactant head group size (Zimmerman et al., 2003). Therefore, we start by

identifying the fundamental structure-stability relationships driving surfactant selection for

vegetable oil semi-synthetic MWFs.

Oil-in-water microemulsions can be viewed either as a specific class of emulsion, with the oil

dispersed as nano-scale droplets in water (Shinoda and Lindman, 1987), or they can be viewed as

swollen micelles formed by micelle solubilization (Hiemenz and Rajagopalan, 1997). Shah et al.

(1977) noted that microemulsions with a ratio of dispersed phase molecules to surfactant

molecules less than 2 are likely to be swollen micelle systems. In semi-synthetic MWF dilutions,

the molar ratio of oil to surfactant is about 1:1 and the volume fraction of oil can be as low as

0.5%. Therefore our research is based on the hypothesis that semi-synthetic MWFs behave like

swollen micelle systems.

It has been long understood that the molecular structure of a surfactant such as its head group

size and tail length can greatly affect its surface active properties including its ability to

Table 1. Surfactants used in MWF formulation re-design.

solubilize hydrophobic constituents (Rosen, 2004; Adamson and Gast, 1997). For a given

surfactant, the molecular structure of the hydrophobic substance such as its carbon chain length

also has a significant effect on micelle solubilization (Weass et al., 1997). After reviewing

existing literature on the micelle solubilization of alkane hydrocarbons, halogenated

hydrocarbons, and polycyclic hydrocarbons, we hypothesize that 1) a combination of two

surfactants, one nonionic and one water soluble co-surfactant (either nonionic or anionic) is

preferred over a single surfactant; 2) the nonionic surfactant should have a carbon tail length

greater or equal to the nominal carbon chain length of the fatty acids in the oil and a head group

that is not excessively small or large; 3) the difference in tail lengths between the surfactant and

the co-surfactant should be less than 6 carbon units to maximize the feasible range of oil to

surfactant ratios yielding stable emulsions (Zhao et al., 2006).

To validate these hypotheses, screening experiments are performed for surfactants and

combinations of surfactants selected from all major classes of commercially available anionic

and nonionic surfactants, i.e. fatty acid soaps, alcohol sulfates, alcohol ether sulfates, alkane

sulfonates, alkyl aryl sulfonates, sulfo-carboxylic esters, ethoxylated alcohols, ethoxylated

glyceryl esters, polysorbitan esters, and alkyl polyglucosides. Experimental data support the

above hypotheses which hold for all three bio-based oil i.e. canola oil, soybean oil, and TMP

ester tested. Since among all the surfactants tested, only the anionic surfactants of the sulfonate

class are currently manufactured exclusively from petroleum feedstock (Hauthal, 2004; Patel,

2004). All other surfactants can be manufactured without using petroleum although most are still

at least partially derived from petroleum. To replace sulfonates, one can use an anionic surfactant

from the classes of fatty acid soap, alcohol sulfate, alcohol ether sulfate, or sulfo-carboxylic ester.

Alternatively, one can use a suitable nonionic surfactant available from any of the chemical

classes considered. For other additives that are generally present in semi-synthetic MWF

formulations such extreme pressure (EP) additives, corrosion inhibitors, and chelating agents,

research is ongoing to develop bio-based alternatives (Susantandy et al., 2004; Pedisic et al.,

2003; Rao and Johnson, 1997). The combinations of vegetable oil and bio-based surfactant

packages serve as a starting point for the development of 100% petroleum-free formulations.

3.2 Optimization of MWF formulations for membrane filtration

Some form of contaminant control and chemical addition is almost always performed in

recirculating MWF systems, driven by costs of MWF acquisition and disposal. In an ideal MWF

system, such control systems would achieve a perfect separation of contaminants and return the

MWF to its as new state (Brandt, 2006; Dick, 2006; Foltz, 2006). However, contaminant removal on its own cannot address issues such as accumulation of hardness ions, evaporation, pH

reduction due to microbial growth, and loss of surfactants. Therefore, even under ideal separation,

direct chemical maintenance and biological control is required to extend the life of the MWF.

Microfiltration is a membrane-based separation technology that can not only remove

microorganisms, but can also remove contaminant particles and free oil from MWF to produce a

high quality recyclate (Skerlos et al., 2000). A major difference between conventional filtration

and membrane filtration is that conventional filters operate by capturing particles within a filter

matrix, and the filters cannot be regenerated after use. As shown in Figure 3, membrane filtration

is typically performed with filtration tangential to the channels of bulk fluid flow. This crossflow

mode of operation discourages the accumulation of particles within the filter matrix, and the

separation takes place at the surface Membranes can be cleaned and re-used for long periods of

time, and essentially indefinitely for ceramic membranes. Therefore, rather than filter clogging

with contaminants, the principal limitation to high filtration rates in microfiltration and

ultrafiltration is the physical-chemical interaction of MWF ingredients with the membrane

surface (Skerlos et al., 2000).

Membrane filtration processes, while challenging, have proven to be able to restore the MWF to

good as new condition (Rajagopalan et al., 2004). The primary limitation to wider application of membrane filtration technology is its sensitivity to MWF formulation design, particularly to

the selection of surfactants and their application concentrations. In order to achieve a flow rate

high enough for profitable contaminant removal, MWFs have to be re-designed to minimize the

fouling interactions between MWF ingredients and membrane surface.

Figure 3. Principle of cross-flow membrane filtration for advanced MWF recycling (Adapted

from Skerlos and Zhao, 2003).

With the help of ESEM, we identify that pore constriction and pore blocking are the two major

fouling mechanisms that dominate flow rate decline during MWF microfiltration. We further

develop a mechanistic model that establishes governing relationships between MWF surfactant

characteristics and microfiltration recycling performance. The model, which is based on

surfactant adsorption/desorption kinetics, queueing theory, and coalescence kinetics of emulsion

droplets, can be calibrated using flow rate data collected from microfiltration experiments. An

analysis of the model and supporting experimental evidence indicate that the selection of

surfactant packages which 1) weakly adsorb to membranes and 2) lead to a high activation

energy of coalescence results in a higher MWF flux through microfiltration membranes. The

model also yields mathematical equations that express the optimal concentrations of anionic and

nonionic surfactant for which microfiltration flux is maximized for a given combination of oil

chemistry, oil concentration, and surfactant types. This model based approach is applied to bio-

based MWFs developed above and an improvement on flow rate up to 300% is achieved (Zhao

et al., 2007).

For any newly formulated MWFs to have practical significance, the manufacturing performance

of the fluids has to be maintained, if not improved relative to the state of the art. Here we use the

modified tapping torque test ASTM D 5619 developed by Zimmerman et al (2003) to evaluate

and compare the performance of bio-based MWFs with a representative petroleum oil MWF.

Interestingly, as shown in Figure 4 the bio-based fluids have a small, but statistically significant

improvement on tapping performance. This is consistent with previous observations by Belluco

and De Chiffre (2001 and 2004) and Clarens et al. (2004) and follows from the higher lubricity

of vegetable oils. Moreover, hardwater stability tests indicate that MWF re-formulation efforts

directed towards increasing microfiltration flux have the beneficial effect of increasing MWF

robustness to deterioration and flux decline in the presence of elevated concentrations (up to

800ppm which is much higher than the concentration found in field applications) of hardwater

ions (Zhao et al., 2007).

Figure 4 Tapping torque efficiency of vegetable oil MWFs with bio-based surfactants.

3.3 Microorganism detection using flow cytometry

The susceptibility of MWFs to biological contamination and the potential for this to lead to

health risks associated with bio-aerosols and infection from hazardous microbial species calls for

new technology to detect microbial growth in real-time. The fast response is especially important

since microbial outbreaks can occur faster than currently used dip-slides can detect harmful

microbial species. Our research focuses on the applications of flow cytometry to conduct real-

time analysis of microbial species and total biomass load.

Flow cytometry is widely used in clinical diagnosis and molecular biology research. As shown

in Figure 5, flow cytometry works by fluorescence detection. A specific fluorescent dye that

binds specifically only to the microorganisms of interest (which also allows for the possibility of

detecting all microorganisms present) is mixed with the MWF sample and introduced to the flow

cytometer. The hydrodynamic focusing design of the system encourages bacterial and fungal

cells to line up individually so they can be interrogated by a laser as suggested by Figure 5. An

optical filter then separates fluorescent emissions from laser scatter allowing an accurate count of

the specific microbial population of interest.

Figure 5. Flow cytometer principle of operation for detecting bacteria and fungus.

For semi-synthetic MWFs, we find that flow cytometry can quantify microbial concentrations

(approximate 108 microogranisms/mL) and the results are in good agreement with traditional

Petri dish method. Moreover, using a specific labeling method i.e. PNA probe flow cytometry

can detect harmful Mycobacteria sp. in MWFs (Skerlos et al., 2003). However, even a portable

flow cytometer costs around $50,000 which is prohibitively expensive for MWF field

applications. Research is ongoing to reduce the size and cost of flow cytometry by at least an

order of magnitude, enabling future applications of the technology on the shop floor.

4. Summary of Case Study: A Generalized Approach for Sustainable Engineering System

Development

Sustainable manufacturing adds value to materials, components, or products while maintaining

the availability of natural resources and environmental quality for future generations. In this

educational module we describe research advances toward the development of a sustainable

metalworking fluid system, which forms an integral part of a sustainable manufacturing strategy.

Although the focus here is on metalworking fluid system, the approach adapted does have the

potential to be applied to other manufacturing system and process, and other engineering systems

in general.

As demonstrated in the module, the first step in developing a sustainable engineering system is to

identify challenges in achieving sustainability and define design targets. This can be done by

conducting a survey on state-of-the-art practice and reveal issues from economic, environmental,

and social perspectives. In the case of metalworking fluid, the survey suggests that the current

application of metalworking fluids is far from sustainable. That is, from economic perspective,

MWFs account for a significant portion of total manufacturing cost since biological growth and

the buildup of hardwater ions, metal particles and tramp oils deteriorate manufacturing

performance and lead to frequent but premature disposal; from environmental perspective, this

frequent MWF disposal present significant resource consumption and environmental burden

since MWFs are made of petroleum oil products and waste MWFs carry toxic metals from

manufacturing (e.g., cobalt and lead) and other chemicals such as fat, oil, grease, corrosion

inhibitors, defoaming agents, surfactants, chlorinated fatty acids and chelating agents that all

pose environmental risks; from social perspective, since MWFs are highly susceptible to

contamination by microorganisms, there exist health risks to workers from infection, inhalation

of bio-aerosols, or dermal contact with biocides utilized to control the biological growth. Given

the state-of-the-art, developing a sustainable MWF system should have targets as to address all

of these identified issues and achieve simultaneous improvements in economic, environmental,

and social dimensions.

With the design targets defined, the next step in developing a sustainable engineering system is

to propose a conceptual design for the new system with major technologies identified, which has

the potential to meet all the design targets. In almost all the cases, this can only be performed by

a interdisciplinary team. In the case of metalworking fluid systems, a two pronged design is

proposed: 1) to select an environmentally benign MWF chemical formulation, with all major

components made from bio-based feedstock, and 2) to deploy an appropriate control system for

this formulation that maximizes the MWF lifetime on the shop floor, which is achieved by

combining membrane filtration with fast and cost-effective microbial sensing technology.

Researchers with expertise in the areas of manufacturing, wastewater treatment, molecular

biology, green chemistry, membrane science, and microelectromechanical systems are all

involved in the concept development.

The final, but probably the most important step is to advance technologies for the specific

engineering applications. This again requires close collaboration within the interdisciplinary

team formed in the second step. In the case of metalworking fluid systems, technology

breakthroughs are made on the development of molecular structure based MWF formulation

design, high flow rate membrane filtration for MWF microemulsions, and flow cytometry based

microbial detection. It should be kept in mind that any technology under development should

maintain compatibility with other technology components of the system. Also, besides economic,

environmental, and social considerations, it is critical that the new system has the same, if not

improved, technical performance when compared with the current system. For metalworking

fluid systems, this means that the manufacturing performance cannot be sacrificed when

applying all the technologies developed to the system.

Acknowledgements

The research advances toward sustainable MWF systems presented here are the results of years

efforts conducted by a research group lead by Professor Steve Skerlos of Mechanical

Engineering and Professor Kim Hayes of Civil and Environmental Engineering at the University

of Michigan. The author would like to express his appreciation to graduate students and

undergraduate students who made significant contribution to the research: Andres Clarens (now

Assistant Professor at University of Virginia), Julie Zimmerman (now Assistant Professor at

Yale University), Shu-Chi Chang (now Assistant Professor at National Chung Hsing University),

Doug MacLean, Ye Eun Park, Yi-Chung Tung, Carlos Aguilar, Ashley Murphree, and Marcy

Urbance. The research was supported by various grants and contracts from the US National

Science Foundation (DMII-0084796 DMII-0093514, BES-0607213), the US Environmental

Protection Agency (R831457 and EPA STAR Graduate Research Fellowship Program), Ford

Motor Company, and Boeing, Inc.

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