screening alternative degreasing solvents using multivariate analysis
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Screening Alternative DegreasingSolvents Using Multivariate AnalysisC . T R E V I Z O , D . D A N I E L , A N DN . N I R M A L A K H A N D A N * ,
Civil, Agricultural, and Geological Engineering Departmentand University Statistics Center, New Mexico State University,Las Cruces, New Mexico 88003
Multivariate analysis was used to explore physicochemicalproperties of organic chemicals that would characterizeand identify degreasing solvents. The exploratory techniquesused in this study include cluster analysis, discriminantfunction analysis, and canonical discriminant analysis. Outof a compilation of 16 physicochemical propertiesevaluated, aqueous solubility, Henrys constant, andsurface tension were identified as relevant properties thatcould effectively screen degreasing solvents from among30 chemicals of similar chemical classes. The suitability ofthese three properties and the multivariate techniquesused in classifying degreasing solvents were demonstratedon an external testing set of 10 solvent- and nonsolvent-type chemicals. On the basis of the results of these studies,canonical discriminant analysis is recommended as apotential tool for screening purposes. The cluster analysisprocedure was informative for explorative purposes; thediscriminant function analysis procedure was not efficientin separating solvents from others.
IntroductionSolvents are a class of chemicals that can dissolve specificcomponents or break down certain chemicals in a complexmixture into more elementary forms. Because of this property,solvents have been used widely in various applicationsranging from cleaning, degreasing, coating, painting, andextracting to chemical processing, manufacturing, andequipment maintenance (1, 2). In addition to their directuse in the industry, numerous commercial formulations andproducts containing solvents are used on a daily basis in thedomestic, commercial, institutional, and military sectors.Common specific uses of solvents include mobilization ofsolids; preparation of reactants; application of particles ontoa surface for coating; extraction of oil, flavors, and fragrances;thinners for paints, oils, and ink; adhesive for plastics; cleaningprinted circuit boards and machine parts; dry cleaning ofgarments; decaffeinating coffee; etc. (3).
Over 30 different synthetic organic chemicals have beenused as degreasing solvents. It is estimated that the annualuse of the five most commonly used solvents [viz., trichlo-roethylene (TCE), tetrachloroethylene (PCE), methylenechloride, 1,1,1-trichloroethane (TCA), and trichlorotrifluro-ethane (CFC 113)] in the United States is around 800 000 t(4). Such large usage as well as improper storage and disposalof spent solvents over the past decades have resulted in their
release into the environment, contaminating soils, ground-water, and the atmosphere.
Because of their toxic, persistent, and recalcitrant nature,environmental contamination by degreasing solvents hasemerged as one of the serious problems in the industrializedworld. Recent studies have confirmed that many of thecurrent solvents are hazardous to humans and harmful tothe environment, causing (or suspected to cause) cancer,smog formation, ozone depletion, etc. As such, many of thecommon solvents are now targets of public concern andregulatory control. The Environmental Protection Agency(EPA) has included over 20 solvents in their list of 127 prioritypollutants. The Clean Air Act Amendments of 1990 have listedseveral solvents as hazardous air pollutants (HAPs). Theemissions of the most common solvents (viz., methylenechloride, PCE, TCE, TCA, carbon tetrachloride, and chloro-form) are now regulated by 40 CFR, Parts 9 and 63, underthe Toxic Release Inventory (TRI) program, whereby indus-tries are now required to report to the EPA on their productionand transfers.
In an effort to minimize the release and environmentalimpacts of solvents, industries are being forced to adaptprocess modifications, recycling, and reuse of solvents onone hand and to develop environment-friendly substitutesolvents on the other (2). In seeking substitutes or designingnew ones, it is important to identify or develop solvents thathave the desired degreasing characteristics and, at the sametime, are nontoxic and readily biodegradable and poseminimal threat to the environment. Evaluation of solventsthat are in current use in terms of their physical and chemicalproperties is the first step to characterize the desired featuresof a good solvent and to effectively develop a greener andefficient substitute solvent.
Selection of substitute solvents is not a straightforwardtask because no single physicochemical property relates tosolvent characteristics. The search for alternate solvents hasbeen characterized as Edisonian because of the trial anderror nature of the experimental evaluation of numerouspotential alternatives (5). While acknowledging this processto be a significant technical challenge, Zhao and Cabezas (2)have identified the following three steps in developingsubstitute solvents:
step 1: to determine the substitute candidates or thereplacement formulations;
step 2: to do performance and evaluation tests; andstep 3: to do the full scale test.
The first step has been recognized as the most importantand most difficult one. Efforts of previous workers in fulfillingthe first step have been classified the into three categoriesby Zhao and Cabezas (2): (i) screening of available solventdatabases for single chemical substitutes; (ii) using computer-based molecular designing tools to develop new chemicalswith the desired properties; and (iii) designing mixtures ofavailable chemicals to achieve desired properties. Severalspecial purpose computer software tools have been devel-oped and are being applied for this purpose (2, 6). The firstapproach of screening databases is a more simple approachand can also enhance the effectiveness of the other twomethods. Irrespective of the approach, identification ofdesired properties for a given application is a prerequisite inseeking substitute chemicals. One of the objectives of thisstudy was to identify physicochemical properties of goodsolvents.
The second objective of this study was to develop ascreening process based on statistical multivariate analysisof physicochemical properties of good solvents. The screening
* Corresponding author fax: (505)646-6049; e-mail: firstname.lastname@example.org.
Civil, Agricultural, and Geological Engineering Department. University Statistics Center.
Environ. Sci. Technol. 2000, 34, 2587-2595
10.1021/es9912832 CCC: $19.00 2000 American Chemical Society VOL. 34, NO. 12, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 9 2587Published on Web 05/16/2000
of substitute solvents remains a subjective process, dependingon the application and the experience of end-users. Two ofthe commonly employed methods are the weighted-sumevaluation method and the pass/fail screening method. Inthe first method, quantifiable screening criteria weighted byappropriate weighting factors are summed up and comparedfor the alternatives. The criteria used are indirect measuresof the overall effectiveness of the solvent. Some examples ofcriteria are reductions in raw material input, waste quantity,operational hazards, costs, etc. (4).
The second method involves a step-by-step evaluation ofthe alternatives against yes/no or pass/fail type of criteria.Those that satisfy all the criteria are then selected for furthertesting. Examples of criteria might be as follows: is flashpoint less than or greater than 140 C, is dielectric strengthless than or greater than 20 kV, etc. (7). Proposed solventsthat pass the necessary criteria are then evaluated furtherunder field conditions.
An expert system software named SAGE is now availableto aid in the screening process (http://clean.RTI.org/sol_alt).Users can run SAGE online over the Internet or downloadit to run on desktop computers to identify possible alternatesolvents. This software first prompts the user to specify thematerial, nature, and shape of the part or surface to becleaned; the contaminants to be cleaned; the degree ofcleaning expected; the process configuration; etc. It thenrecommends a list of possible alternate solvents and pro-cesses that best satisfy the input data.
The ultimate aim of this study was to develop and validatean alternate screening process to aid the substitute solventsearch process. A statistical exploratory approach involvingmultivariate analysis procedures is adapted in this study.The following procedures are used: cluster analysis, dis-criminant function analysis, and canonical discriminantanalysis.
Materials and MethodsA training data set of 45 common solvent and nonsolventchemicals was initially compiled as the starting point for thisstudy. The following physicochemical properties for thesechemicals were compiled from handbooks (e.g., refs 8-11)and literature (e.g., refs 12 and 13): boiling point (BoilPt),melting point (MeltPt), molecular weight (MW), octanol/water partition coefficient [log(P)], water solubility [log(S)],vapor pressure (VP), Henrys law constant [log(HC)], surfacetension (ST), solubility parameter (SolP), autoignition tem-perature (AT), excess molar refraction (R), solute dipolarity(), effective hydrogen-bond acidity (), effective hydrogen-bond basicity (R), and the characteristic volume of McGowan(MolarV). The significance of each of these parameters hasbeen discussed elsewhere (e.g., refs 2, 10, and 13). In addition,calculated values of zero-order and first-order simple andvalence molecular connectivity indexes (0, 0, and 1) werealso adapted as additional properties (14).
From the initial 45 chemicals identified, only 30 chemicalscould be evaluated in this study as a training set due to thenonavailability of all the 18 physicochemical properties. Thesolubility parameter and