Application of chemical fingerprinting to critical surface cleaning solvents

Abstract

A major goal among aerospace contractors has been the elimination of the use of Ozone Depleting Chemical (ODC) solvents. Where halogenated solvents such as trichloroethane (TCA) were once extensively used for critical bond lines on the Space Shuttle Solid Rocket Boosters, now various organic and aqueous based cleaners are in use or under evaluation. Prior to use in manufacturing these must be scrutinized for cleaning performance, lot-to-lot consistency and aging behavior. Specialized methods for techniques such as FTIR, NMR, HPLC, LC/MS, GC, GC/MS and a variety of chemical analyses have been developed to correlate the chemical composition with behavior. Examples of organic solvent-based cleaners that are now considered for use are PFTM Degreaser (PT Technologies, Safety Harbor, FL) and RE-ENTRY® Plus-4 (Petroferm, Inc., Fernandina Beach, FL). Both use d-limonene as a key ingredient for its solvating ability and user acceptability. PFTM Degreaser contains primarily normal hydrocarbons plus d-limonene for non-polar cleaning. RE-ENTRY® Plus-4 contains primarily d-limonene plus several polar compounds designed to reduce electro static discharge (ESD). Chemical fingerprinting has enhanced our general understanding of these solvents with specific findings critical to proper use. In particular, the susceptibility for d-limonene oxidation and formation of non-volatile residue (NVR) are important to its use, storage and analytical method development. Examples of effective analytical methods and their application to both acceptance testing and monitoring of residues on various substrates will be presented. AIAA 2001-3289 Chemical Fingerprinting Chemical Fingerprinting of PFTM Degreaser and RE-ENTRY® Plus-4 as well as many other RSRM materials has helped establish pertinent analytical methods and acceptance limits for important parameters associated with these materials. These methods and acceptance limits can, in some cases, be readily shared with the material manufacturer. This in turn may help the manufacturer produce a more consistent and optimal product. Several instrumental techniques are available for testing of many materials. PFTM Degreaser and RE-ENTRY® Plus-4 are both volatile, organic solvent mixtures and can be readily analyzed using a variety of spectroscopic instruments. Gas chromatography (GC) can be used to separate and quantify chemical components. When the GC is interfaced with a Mass Spectral Detector (GC/MS) major constituents as well as unknown, trace components and possible contaminants can be identified. GC interfaced with a Flame lonization Detector (GC/FID) is generally used to quantify the major constituents. Fourier Transform Infrared Spectroscopy (FTIR) is used to quickly generate a chemical IR fingerprint. Eventually a good database from the analysis of several acceptable lots of PFTM Degreaser or RE-ENTRY® Plus-4 can be generated. FTIR spectra of newly received lots of these materials can be compared to this database and a degree of acceptability quickly determined. Nuclear Magnetic Resonance spectroscopy (NMR) can be used to similarly generate a spectroscopic fingerprint of PFTM Degreaser and RE-ENTRY® Plus-4. NMR is generally not used as an acceptance test since the instruments are expensive and not typically used in Quality Control departments of material vendors. Traditional wet lab methods are also incorporated into Fingerprinting. Typical acceptance testing for most solvent cleaners such as Nonvolatile Residue (NVR) determination, Karl-Fischer moisture determination and flashpoint are performed. Specific gravity and refractive index determination are also performed and provide additional information for the material's