Research and Technology - Forensic Science Communications - January 2007
January 2007 - Volume 9 - Number 1
Research and Technology
A New Approach for the Analysis of Duct Tape Backings
Andria L. Hobbs
Georgetown University School of Medicine
Preston C. Lowe
Chemist/Scanning Electron Microscopy Program Manager
Duct tape was invented in the 1930s as a waterproof medical tape. During World War II, the military adapted duct tape for its own purposes, and following the war, the tape was used for sealing ductwork in homes. Today, the use of duct tape for a variety of applications is concentrated primarily in North America. Common duct-tape construction consists of a polyisoprene-based adhesive, fabric reinforcement (scrim), and a polyethylene backing (Johnston and Serra 2005).
Despite its many positive uses, duct tape is often submitted to crime laboratories as evidence associated with abductions and homicides. The FBI Laboratory analyzes duct tape in comparative examinations and for sourcing purposes. The tape comparison examination attempts to establish an evidentiary link between a suspect and a crime or between different crime scenes. When no source tape is available for comparison, a duct tape specimen can be examined to determine class characteristics that may provide manufacturer information.
A logical first step for either comparative examinations or sourcing requests is conducting visual and microscopic examinations on the submitted samples in order to evaluate physical characteristics—such as backing and adhesive color, width, yarn count per square inch, and weave pattern. If the samples are consistent after visual and microscopic examinations, chemical composition analysis is performed on the three main components of each tape: backing, adhesive, and fabric reinforcement. According to the FBI Laboratory’s standard operating procedure, the adhesives are analyzed by Fourier transform infrared spectroscopy (FTIR) with a microscope attachment. For samples that remain consistent following FTIR examination, scanning electron microscopy/energy dispersive X-ray spectroscopy (SEM/EDS) is performed on both adhesives and backings. X-ray diffractometry (XRD) is also performed on the intact specimens and occasionally on the duct tape backings alone.
Prior to this study, duct tape backings submitted to the FBI Laboratory were inspected visually and microscopically for color and fabrication markings, measured for width and thickness, and analyzed by SEM/EDS for elemental content and by XRD for crystalline compound information. No additional examinations typically were performed on the backings, because earlier forensic publications on duct tape analysis (Bartick and Merrill 1999; Smith 1998) noted that chemical analysis of duct tape backings offered little additional discriminating value, because backings were single-layered polyethylene with an aluminum colorant. Since that time, however, while conducting examinations on a duct tape specimen for a case, FBI Laboratory examiners determined that the EDS spectra of the backing differed depending on which side (on the intact tape, either the exposed or the adhesive side of the backing) was analyzed; the exposed side included aluminum, whereas the adhesive surface did not. Through additional analysis and discussion with industry contacts, the FBI Laboratory discovered that the use of multilayered backings had become common practice in recent years. Therefore, this study was initiated to determine how frequently multilayered backings might be encountered, what types exist, and how they should be properly examined.
This study involved the analysis of 82 duct tape samples, which the FBI has purchased or obtained directly from manufacturers since 1993. Most of the tapes were purchased at discount stores or home-improvement retailers, are marketed as general purpose or economy grade, and cover a variety of U.S. and international manufacturers. Therefore, the group represents tapes that could be obtained easily by consumers and would be comparable to casework evidence submitted to the FBI Laboratory.
Prior to analysis, the adhesive layer of each of the 82 samples was removed with a suitable solvent (e.g., hexane), and the reinforcement fabric was removed manually. Three different methods were used to evaluate the duct tape backings: macroscopic examination, manual cross-sectioning, and FTIR analysis with an attenuated total reflectance (ATR) accessory. The backing was analyzed with the unaided eye to determine possible layer structure; some tape backings differ in color depending on which side is viewed. A cross section was prepared by first holding the sample between two glass slides, using liquid nitrogen or propellant from an aerosol duster to freeze the backing to make it more rigid, and then cutting a cross section with a single-edged razor blade. Very thin cross sections were required for proper examination. The cross section was examined with a compound microscope using transmitted light (Leitz Ortholux, 35–400X, Leica Microsystems, Wetzlar, Germany) in order to determine layer structure.
FTIR-ATR spectroscopy was used to evaluate the composition of both sides of each backing. ATR was chosen because it is primarily a surface technique in that the depth of analysis is approximately 2 micrometers (Merrill and Bartick 2000). Manipulating such a thin peel from a duct tape backing would be impractical for traditional FTIR microscopy analysis because typical backings range in thickness from 0.002 to 0.006 inch (~50–150 µm). Furthermore, the ATR technique requires minimal sample preparation, and instrument operation is not complicated. For this study, if the two sides of the backing differed in chemical composition as determined by FTIR-ATR, the backing was considered multilayered. If the two sides were similar, then the ATR data were considered inconclusive with regard to layer structure.
The infrared spectrometer was a Nicolet 560 Magna-IR ESP (Enhanced Synchronization Protocol) (Thermo Electron Corporation, Madison, Wisconsin) equipped with a KBr beam splitter and a DTGS detector. The ATR accessory was a SensIR Technologies (Danbury, Connecticut) DuraSamplIR–1 and a resolution of 4 cm–1. No corrections were applied to the spectra. with a ZnSe crystal. Thirty-two sample and background scans were taken in reflectance mode with a spectral range of 4000–650 cm
Initial macroscopic visual examinations indicated that 5 of the 82 samples in the reference collection had multilayered backings. One backing (Sample 55) was silver-colored on the exposed side and white on the adhesive side, one exhibited printing with product information on one side, and three samples had camouflage print on one side. For the purposes of this paper, samples with printed backings were considered multilayered.
Microscopic examination of the cross sections indicated that 20 of the 82 samples were multilayered. Sample 55 contained an additional colorless layer (see Figure 1), highlighting the need to further examine those samples that, even to the unaided eye, appeared multilayered.
Figure 1: Sample 55: Micrograph of cross section magnified 250 times. Backing appears to have three layers.
FTIR-ATR analysis revealed 19 multilayered backings (including Sample 55), 12 of which had not been detected by the other examinations. Sample 23 is an example in which the ATR spectra showed the presence of multiple layers that had not been apparent during macro- or microscopic examination. Figures 2 and 3 show the cross-section micrograph and the ATR spectra, respectively. When the tapes were examined using FTIR-ATR, an acrylate was observed in addition to the polyethylene on the adhesive side of the backing, as evident by the peaks at 1736, 1193, and 1165 cm–1 (Figure 3). An industry representative confirmed that rather than being applied as a prime coat, acrylates may be coextruded into the polyethylene on the adhesive side of some tape backings during the manufacturing process to promote adhesion between the adhesive and backing. If no fillers are added to any of the layers or if the same fillers are added to all layers, microscopic examination would not be expected to reveal a layer boundary; without filler material, polyethylene and acrylates are both colorless materials. Moreover, because the components are molten during the coextrusion process, a definitive boundary may not be present.
Figure 2: Sample 23: Micrograph of cross section magnified 250 times. Backing appears to have a single layer.
Figure 3: Sample 23: ATR spectra of backing from the exposed side (side 1) and adhesive side (side 2). Note the presence of an acrylate in side 2 (indicated by the peaks at 1736, 1193, and 1165 cm–1).
Thirteen samples were also determined to be multilayered by microscopic examinations but inconclusive by FTIR-ATR. For example, the microscopic examinations of Sample 81 revealed a three-layered backing, although no chemical differences were observed by FTIR-ATR. Figures 4 and 5 show the cross-section micrograph and the ATR spectra, respectively, of sample 81.
Figure 4: Sample 81: Micrograph of cross section magnified 250 times. Backing appears to have three layers.
Figure 5: Sample 81: ATR spectra of both sides of the backing. No differences indicated.
The results of all of the examinations revealed that 32 (39 percent) of the 82 duct tape samples had multilayered backings.
The same manufacturer did not produce all of the tapes with multilayered backings in the collection samples analyzed. In fact, each of the major manufacturers was represented in the collection and produces multilayered backings. Therefore, identifying the tape manufacturer in order to direct investigators toward a likely origin for duct tape evidence may not be possible solely by backing analysis.
Because of this study, the FBI Laboratory has modified its standard operating procedure for tape analysis. Microscopic examinations are now performed on duct-tape-backing cross sections to identify any possible layers. The protocol has been modified to include FTIR-ATR analysis of both sides of the backings. The results from these analyses may lead to discrimination between two samples that may not have been differentiated otherwise. Even when no differences are found through these examinations, the layer structure may influence the sample preparation for subsequent analyses (e.g., SEM/EDS).
Microscopic examination and FTIR-ATR analysis by themselves are not always conclusive with respect to the layer structure of duct tape backings, but together they yield additional information for a more complete evaluation. The FBI Laboratory’s reference collection also contains more than 300 additional rolls of duct tape, including those obtained directly from manufacturers, as well as some industrial-grade, flame-retardant, and other high-end duct tapes. Microscopy and FTIR-ATR will be used to elucidate the backing layer structure of these samples in order to better evaluate the various layer structures and chemical compositions that might be encountered in forensic casework.
This is publication number 06-11 of the Laboratory Division of the Federal Bureau of Investigation. Names of commercial manufacturers are provided for identification only and do not imply endorsement by the FBI.
Coauthor Jennifer Gauntt served as a Chemist in the FBI Laboratory’s Chemistry Unit before resigning to attend medical school.
Bartick, E. G. and Merrill, R. A. Advances of Infrared ATR Analysis of Duct Tape. Presented at the American Academy of Forensic Sciences meeting, Orlando, Florida, 1999.
Johnston, J. and Serra, J. The examination of pressure sensitive adhesive tapes, IAMA Newsletter (2005) 5(1):19–31.
Merrill, R. A. and Bartick, E. G. Analysis of pressure sensitive adhesive tape: I. Evaluation of infrared ATR accessory advances, Journal of Forensic Sciences (2000) 45:93–98.
Smith, J. The forensic value of duct tape comparisons, Midwestern Association of Forensic Scientists Newsletter (1998) 27(1):28–33.