Guideline for the Forensic Examination
of Pressure-Sensitive Tapes
Scientific Working Group for Materials Analysis (SWGMAT)
Scope
| Referenced Documents | Terminology
| Summary of Guideline |
Significance and Use | Tape
Construction and Classes | Sample Handling |
Methods | References
Introduction
1.0. Scope
This document is intended as an introductory guide for the forensic
scientist in the examination and comparison of pressure-sensitive
tapes. Detailed analytical aspects of tape analysis will be addressed
in separate documents. The methods and practices described have
been peer-reviewed and are generally accepted within the forensic
community.
2.0. Referenced Documents
2.1. ASTM International Standards
ASTM International. ASTM E1492-05 Standard Practice for Receiving,
Documenting, Storing, and Retrieving Evidence in a Forensic Laboratory.
ASTM International, West Conshohocken, Pennsylvania, 2005.
2.2. Scientific Working Group
for Materials Analysis
Scientific Working Group on Materials Analysis (SWGMAT). Trace evidence
quality assurance guidelines: January 1999 revision, Forensic
Science Communications [Online]. (January 2000). Available:
http://www.fbi.gov/hq/lab/fsc/backissu/jan2000/swgmat.htm.
Scientific Working Group on Materials Analysis (SWGMAT) Evidence
Committee. Trace evidence recovery guidelines: January 1998 revision,
Forensic Science Communications [Online]. (October 1999).
Available: http://www.fbi.gov/hq/lab/fsc/backissu/oct1999/trace.htm.
2.3. Pressure Sensitive Tape Council
Pressure Sensitive Tape Council. Glossary of terms used in the
pressure sensitive tape industry. In: Test Methods for Pressure
Sensitive Adhesive Tape. 15th ed., Pressure Sensitive Tape
Council, Northbrook, Illinois, 2007.
3.0. Terminology
Adhesive: A material that will hold two or more objects
together solely by intimate surface contact.
Additives: Materials that are included in adhesive or
backing formulations to increase overall volume, impart color, or
provide other desired properties.
Backing: A thin, flexible material to which adhesive is
applied.
Backsizing: A layer applied to the top side of the backing.
Its purpose is to coat and fill a porous-surfaced backing with a
material that is inert to the adhesive formulation to be used.
Calendering: The use of a multiroll device to produce
adhesive tape by applying pressure-sensitive adhesive to various
backings using heat and pressure.
Cellophane: Form of regenerated cellulose. A thin, transparent
film manufactured from wood pulp. Used as a backing material in
tape products.
Cellulose acetate: A transparent film that is used for
tape backings. A matte-surface version is used for write-on tapes.
It is more moisture-resistant than cellophane.
Creped: Paper that has small folds in it, giving it high-stretch
and high-conformability. Used in masking tape (saturated paper tape).
Elastomer: A material that can be deformed but when the
forces are removed will return to its original form. Serves as the
base material for pressure-sensitive adhesives (PSAs).
Fill yarns: Fibers in the scrim fabric of reinforced tape
that run crosswise, perpendicular to the warp direction. Also called
weft yarns.
Flatback: Smooth paper backing sometimes used in masking
tapes.
Migration: The movement over a period of time of an ingredient
from one layer to another. This often occurs in PVC (polyvinyl chloride)
tapes where plasticizer in the PVC backing “migrates”
into the adhesive.
Plasticizer: Material added to plastics to impart flexibility
by creating spaces between the polymer chains and lowering the inter-
and intra-chain attractive forces, allowing freer movement of the
chains. Used in pressure-sensitive backings (particularly PVC),
as well as some adhesives to lower glass-transition temperatures
and allow use at subambient temperatures.
Pressure-sensitive adhesive (PSA): Consists of a polymeric
base usually with appropriate plasticizers and tackifiers. It can
form an adhesive bond with no physical or chemical change and with
no more than slight pressure.
Pressure-sensitive tape: Consists of a flexible backing
and PSA, which when applied to a surface, bonds immediately at room
temperature with slight pressure. The bond can be broken (usually)
without damage to the surface and without leaving a residue.
Prime coat: A coating of adhesive-like material that serves
as a bonding agent between the tape adhesive and backing.
Scrim: A loosely woven, gauze-type cloth added to duct
tape for reinforcement and strength.
Reinforcement: Cloth, scrim, glass filaments, or plastic
filaments added to tape for stability and strength.
Release coat: A coating applied to the backing on the
side opposite the adhesive that provides ease of unwind and prevents
delamination or tearing.
Tack: Property of an adhesive achieved by the addition
of a low-molecular-weight organic component that allows the elastomer
to form a bond immediately with a surface under low pressure.
Tackifier: Material added to the adhesive base polymer
to impart tack.
Thickness: Distance from one surface of either a tape,
backing, or adhesive to the other, usually expressed in mils or
thousandths of an inch.
Warp yarns: Fibers in scrim fabric of reinforced tape
that run lengthwise, in the machine direction.
4.0. Summary of Guideline
4.1. The information contained in this guideline
is intended to assist the examiner in characterizing and comparing
evidentiary tape samples. The forensic examination of pressure-sensitive
tape encompasses the determination of physical construction and
chemical composition of tape products. General information on product
variability, construction, and composition is provided. This guideline
provides an overview of techniques applied to the analysis of tape
components.
4.2. Methods for the analysis of tape include
examinations of physical characteristics, polarized light microscopy
(PLM), Fourier transform infrared spectroscopy (FTIR), pyrolysis
gas chromatography (py-GC), scanning electron microscopy with energy
dispersive spectroscopy (SEM-EDS), X-ray fluorescence spectrometry
(XRF), inductively coupled plasma (ICP) techniques, and X-ray powder
diffraction (XRD). These different procedures provide complementary
information and should be selected and employed in an order that
will obtain the most discriminating information consistent with
the laboratory’s capabilities. It is assumed that the forensic
examiner has a basic familiarity with instrumental techniques used
in the methods described.
4.3. Typically, a tape examination involves the
comparison of samples to determine if they could share a common
origin. The goal is to determine if any significant differences
exist between the samples. The evaluation of tapes for class characteristics
can associate known and questioned tapes to a group but not to a
single, individual source. A physical end match of two tape ends
provides individualizing characteristics that associate the two
tapes to each other to the exclusion of all other tapes.
4.4. Questioned tape samples may be submitted
with a request to identify possible product information, manufacturing,
and retailing sources. Sourcing a questioned tape can provide valuable
investigative lead information. Physical characteristics and compositional
data are useful for technical inquiries to tape-manufacturing companies,
comparisons with various brands of tape purchased at local commercial
outlets, and searches of reference databases.
5.0. Significance and Use
5.1. Because of the variability of tape products,
the analysis and comparison of tape evidence in the forensic science
laboratory can provide valuable information. However, some classes
of tape exhibit more variability than others. In general, the more
complex the product (e.g., duct tape), the more variable it is.
The common tape classes and their components are described further
in Section 6.0. Studies have shown differences between randomly
selected rolls of tape, but because of the ever-changing tape markets,
suppliers, and economics, it is not feasible to establish the statistical
probability that a given sample would have the same physical and
chemical characteristics as a randomly selected tape.
5.2. While tapes within a specific class may appear
similar on a macroscopic level, differences may be found on closer
analysis of the physical and chemical characteristics. Differences
are readily observed in tapes manufactured in different plants.
5.3. Differences may also be found between batches
of tape products within the same plant, because of changes in raw
materials and processing that occur over time. Also, the many components
that comprise a given tape product are subject to supply-and-demand
fluctuations in the market. For example, a lower bid for some minor
component may lead to its substitution from one batch to the next,
resulting in compositional changes that can be detected in the forensic
laboratory. Although it is less likely to find differences in tape
rolls produced by the same production line, the probability of finding
differences between batches increases with time between batches.
5.4. It may be feasible to detect physical differences
between rolls of tape produced in the same batch. For example, one
batch of duct tape produced in a large sheet may be slit into nominal
two-inch-wide (~50.8 mm) individual rolls. Numerous cutters are
spaced along the width of the sheet and can result in slightly different
roll widths within the same batch. Differences in the warp yarn
offset from the machine edge also may be found in rolls from the
same batch.
5.5. Within-roll variability has been assessed
using different analytical instruments. No significant within-roll
variations have been reported.
5.6. In the comparison of tape samples, much information
can be obtained from macroscopic and stereomicroscopic examinations.
Exclusions at this stage preclude additional analysis. When samples
are found to be similar at this stage, the examiner should proceed
with other examinations that are available and practical to adequately
address the chemical, compositional, and physical properties of
the tapes before rendering a conclusion. At that point, if no significant
differences are found, the tapes are consistent and could have come
from the same source. Only in rare circumstances can a stronger
statement be supported.
6.0. Tape Construction and
Classes
6.1. Backings
The pressure-sensitive tape backing, or film, provides a support
material for the adhesive. A wide range of materials are used for
tape backings, depending upon the commercial end use. These include,
but are not limited to, polyethylene, polypropylene, polyvinyl chloride,
saturated paper, cellulose acetate, cloth, and polyester. Furthermore,
fillers, colorants, plasticizers, release coats, primer coats, and
preservatives also may be added to tape backings.
6.2. Adhesives
The formulation of PSAs consists of an elastomer to which tackifer
resins and inorganic materials are added.
6.2.1. Elastomers
Pressure-sensitive adhesives may contain one elastomer or a blend
of several different elastomers. The following list includes elastomers
commonly used in PSAs:
- Natural rubber (polyisoprene).
- Synthetic polyisoprene.
- Polybutadiene.
- Polyisobutylene.
- Styrene-butadiene random copolymer.
- Styrene-isoprene-styrene (SIS).
- Styrene-butadiene-styrene (SBS).
- Styrene-ethylene-butylene block copolymer.
- Ethyl or butyl acrylate.
- Silicones.
- Polychloroprene.
Tackifying resins are blended with elastomers to lower the glass-transition
temperature, allowing freer movement of the polymer chains and thus
giving PSAs their “sticky” adhesive property. The tackifying
resin is typically a C-5 (5-carbon hydrocarbon component). Silicone
and acrylic PSAs do not require a tackifier. More costly silicone-based
adhesives may be found in adhesive formulations of tapes that are
geared for high-temperature applications and chemical resistance.
6.2.2. Additives
Inorganic materials are added to an adhesive formulation to either
increase the overall volume or to impart color. Such materials include
calcite, dolomite, iron oxide, kaolinite, talc, titanium dioxide
(rutile or anatase), and zincite. In addition, zincite also can
function as an “accelerator,” or cross-linker for a
rubber-based adhesive. Other materials may be added to provide resistance
to extremes in temperature and UV exposure.
6.3. Tape Classes
6.3.1. Polycoated cloth tape
Commonly referred to as duct tape, polycoated cloth tape consists
of three basic components: the backing, the reinforcement fabric,
and the PSA. These components in concert determine a duct tape’s
appearance, strength, and end use. The final product will be designed
for specific end usage, whether it is for general commodity use,
construction, etc.
6.3.1.1. Duct tape backing
The backing, which is polyethylene, is available in various colors.
Duct tapes that are silver or gray commonly contain a small amount
of aluminum to impart the silver color. Other colored backings are
achieved by adding colored pellets to the molten polyethylene. Inorganic
materials such as talc may be added to the backing, which improves
water repellency and tear strength. The backing may consist of a
single layer or multiple layers of polyethylene and can range in
thickness from about 1.5 mil to 4 mil (1 mil = 0.0010 in). The backing
also may exhibit characteristics imparted during the manufacturing
process, such as calendering marks and striations. Additionally,
lettering or designs also may be imparted on the surface or the
underside of the polyethylene.
6.3.1.2. Duct tape adhesive
The PSA formulation for duct tapes consists of an elastomer to
which tackifying resins and inorganic materials are added. The elastomer
is typically natural rubber (polyisoprene) but could also be a mixture/blend
of synthetic and/or natural elastomers. Other materials used as
elastomers include styrene-butadiene copolymer and styrene-isoprene
copolymer.
The tackifying resin is typically a C-5 that is used to make the
elastomer “sticky” or to impart tack.
Inorganic materials are added to an adhesive formulation to either
increase the overall volume or to impart color. In duct tape adhesives
any of the following may be found: calcite, dolomite, kaolinite,
talc, titanium dioxide, and zincite.
6.3.1.3. Duct tape reinforcement fabric
The scrim is commonly constructed of cotton, polyester, or a blend
of these two materials. Reprocessed cellulose also may be found.
The scrim is generally manufactured as either plain weave or weft-insertion
(having knit warp yarns and texturized fill yarns). Yarns in both
the warp and fill directions can be twisted (spun), texturized,
or filament. Variations of these can be seen.
6.3.1.4. Other components found in duct tape
Two additional layers that may be present within a duct tape product
are a release coat and a primer coat.
6.3.2. Vinyl tape
A vinyl tape, also referred to as an electrical tape, finds use
in applications that require heat resistance, electrical retardance,
and/or insulating properties. The two main components are the backing
and the PSA.
6.3.2.1. Vinyl tape backing
Polyvinyl chloride (PVC) is the most common material used to construct
the backing. Plasticizers, typically phthalate or adipate compounds,
are added to this material to impart flexibility to the PVC. Other
plasticizers may include alkyl/aryl phosphate compounds and dialkyl
tin compounds. Backings range in thicknesses of 4.5 to 7.5 mil and
are commonly black in color, imparted by the addition of carbon
black. However, a variety of colored backings are produced and available.
In addition to plasticizers, inorganic materials such as lead stearate,
lead carbonate, antimony oxide, kaolinite, calcite, and titanium
dioxide also may be found.
6.3.2.2. Vinyl tape adhesive
The adhesive can be formulated in several ways, depending on the
intended end-use market, and can be either colorless or black, through
the addition of carbon black. Commonly available vinyl tapes consist
of acrylic-based PSAs or highly cross-linked rubber-based PSAs.
The adhesive layer also may exhibit plasticizers, either intentionally
added by the manufacturer or as a result of migration from the backing
layer.
6.3.2.3. Other components found in vinyl tapes
As with duct tape, two additional layers—a
release coat and a primer layer—may
be used in vinyl tapes.
6.3.3. Polypropylene packaging tape
Polypropylene packaging tape has been designed as a general-purpose
tape used to seal packages. The two main components are the polypropylene
backing and the adhesive.
6.3.3.1. Polypropylene packaging tape backing
Packaging tape backings are typically clear but also can be found
in various shades of tan or brown. The polypropylene, which is in
the isotactic form, can be subdivided into two distinct types based
upon their tear-resistant properties: monoaxially oriented polypropylene
(MOPP) and biaxially oriented polypropylene (BOPP). A monoaxially
oriented backing is formed into a thin film by stretching the polypropylene
material in one direction only (lengthwise) as it is slowly cooled,
prior to introducing it into the tape-manufacturing process. A biaxially
oriented backing is manufactured by stretching the film in two directions
(lengthwise and width-wise). There is a distinct end-use or consumer
difference between a MOPP and a BOPP tape: a MOPP tape is marketed
as a “hand-tearable tape,” and BOPP tapes require a
cutting tool such as a dispenser. Total tape thicknesses are on
the order of 1.5 to 2.0 mil. The thickness of the film alone typically
varies from 0.9 to 1.0 mil but can range from 0.8 to 2.0 mil.
6.3.3.2. Polypropylene packaging tape adhesive
Packaging tape adhesives are typically clear but are available
in shades of tan or brown. Generally, when the backing is colored,
the adhesive will be clear and vice versa. Although clear adhesives
contain no inorganic material, the colored adhesive may contain
inorganic material such as iron oxide and titanium dioxide. Adhesive
formulations typically are isoprene-based, SIS-based, or acrylic-based.
6.3.3.3. Other components found in polypropylene packaging
tape
Two additional layers that may be present within a packaging tape
product are a release coat and a primer coat.
6.3.4. Saturated paper tape
“Masking tape” consists of a paper backing, a saturant,
and an adhesive. This type of tape is used as a masking material
for paint applications and other general-purpose applications.
6.3.4.1. Saturated paper tape backing
The backing of a paper tape is either flatback or creped paper
that has been saturated with carboxylated butadiene-styrene, acrylonitrile-butadiene,
or a similar material. The purpose of a saturant is to fill porous
material and boost the strength of the backing. The paper alone
typically exhibits weak internal and external strengths, and the
saturant fills the voids between the paper fibers, adding strength
to the product and minimizing absorption of paint products.
6.3.4.2. Saturated paper tape adhesive
The adhesive for saturated paper tapes typically is an isoprene-based
PSA or a styrene-butadiene block copolymer, either of which may
contain inorganic filler. Acrylic-based adhesives have been used
as well, but for outdoor or “clean release” formulations.
These adhesives for saturated tapes are formulated with less tack,
because strong adhesion to a surface is less desirable in masking
applications. As with most tapes, if the product is designed to
endure exposure to high heat or chemical reagents, the formulation
will be cross-linked to provide the needed strength.
6.3.4.3. Other components found within saturated paper
tape
The backsize layer is applied to the side of the backing opposite
of where the adhesive will be applied. The main purpose of this
layer is to coat and fill the porous surface of the backing with
a material that is inert to the adhesive formulation to be used.
A variety of materials, such as acrylic and polyvinyl acetate, can
be used for this purpose, and the material used will depend on the
adhesive formulation. In conjunction with the adhesive formulation,
a primer coat may be present.
6.3.5. Other tapes
The previous sections have discussed the more common types of
tapes encountered within forensic casework. Numerous other types
of tape may be found less frequently. These types include, but are
not limited to, filament/strapping tape, cloth/medical tape, and
office tape.
6.3.5.1. Filament/strapping tape
Filament tapes are similar to packaging tapes in construction
with the addition of reinforcement material. The backing for this
type of tape typically is constructed of oriented polypropylene
(low-cost) or polyester (high-cost). The reinforcement filaments
can be glass, polyamide fibers, or polyester fibers running in the
machine direction. Adhesives found on such tape products can be
colored, dependent upon inorganic material content, or colorless.
The elastomer can be either isoprene or styrene-isoprene block copolymer.
6.3.5.2. Cloth tape
Cloth tapes are used most frequently for medical and athletic
purposes. Common cloth materials include natural and synthetic woven
fabrics (e.g., cotton, polyester). Traditionally, adhesives were
natural rubber-based but have been largely replaced by acrylic copolymers
and other synthetic elastomers.
6.3.5.3. Office tape
Office or stationery tape is composed of a backing and a PSA.
The most common tape backings include cellulose acetate, cellophane,
and polypropylene and can range in appearance from clear glossy
or matte to a translucent yellow. The PSA can be isoprene-based,
acrylic-based, or styrene-isoprene copolymer-based. As mentioned
in the previous tape discussions, a release coat and a primer layer
also may be present.
7.0. Sample Handling
7.1. Because of concerns with handling tape as
physical evidence, each laboratory must develop appropriate procedures
concerning sample size, collection, packaging, preservation, and
order of examinations.
7.2. Different forensic disciplines may be called
upon to examine the same item of evidence. The order in which the
examinations will be conducted needs to be resolved on a case-by-case
basis. The order of examinations should be selected and conducted
to preserve the most transient evidence and provide the greatest
discrimination and most valuable information. Examiners must be
aware or make the submitting agency aware of the effects that some
disciplines’ processing and examinations may have upon other
specific examination requests. If another discipline is chosen before
the tape examination, the examiner should consider obtaining an
unadulterated representative sample.
7.3. When the amount of a tape specimen present
for comparison purposes is adequate in size—as deemed by the
examiner—bulk or lot sampling is the sampling method of choice.
Considerations involved with bulk sampling should include where
the sample is taken, how much sample is taken, and if the sample
is considered representative of the whole. The examiner must be
able to explain how the samples were taken and why the sampling
technique was used. Nondestructive methods should be exhausted before
subjecting the sample to any destructive tests.
7.4. Techniques to untangle tape specimens should
be chosen with care to minimize alterations in the chemical or physical
properties. Methods include mechanical separation using warm air,
liquid nitrogen, or appropriate solvents.
7.5. The item of evidence should be preserved
in a manner that does not interfere with future testing.
7.6. Tape samples submitted as evidence may be
degraded by environmental exposure or subjected to physical damage.
The strength of an association between a damaged piece of tape and
a more pristine sample might be weakened depending upon the degree
of damage. In some cases, the damaged tape may be unsuitable for
comparison purposes.
8.0. Methods
This section provides an overview of the suggested flow of analytical
techniques to be used for the analysis of tape. The selection of
methods is at the discretion of each examiner on a case-by-case
basis and will vary depending upon sample size or condition, availability
of laboratory instrumentation, and examiner training. Subsequent
SWGMAT documents will address these methods in more detail specific
to tape analysis
8.1. Physical Characteristics
Macroscopic and stereomicroscopic observations (e.g., color, thickness,
width, and reinforcement construction) provide initial and discriminating
information for tape comparisons. Physical end matches can provide
individualizing associations.
8.2. Polarized Light Microscopy
Characterization of inorganic materials and other tape additives
are accomplished with the use of PLM. Polarized light microscopy
is a useful adjunct to FTIR and elemental analysis. Optical properties
of oriented polymers such as polypropylene (MOPP and BOPP) and polyester
also can be determined. Polarized light microscopy is also used
to evaluate and differentiate the reinforcement fibers of tapes
(e.g., duct tape and strapping tape).
8.3. Fourier transform infrared spectroscopy
Organic and some inorganic constituents may be evaluated with
the use of infrared spectroscopy. These components include the backing
polymer, adhesive elastomer, plasticizers, additives, and reinforcement
fibers. The use of a bench ATR (attenuated total reflectance) accessory
is particularly useful for surface analysis of a larger area of
the adhesive and backing.
8.4. Elemental techniques
Common analytical techniques that can be used for the characterization
of the inorganic constituents of tapes include SEM-EDS, XRF, ICP
techniques, and XRD. SEM/EDS, XRF, and ICP provide elemental profiles
of analyzed specimens, whereas XRD provides crystalline structure
information. Additionally, SEM has imaging capabilities to evaluate
surface topography of tape backings.
8.5. Pyrolysis gas chromatography
Organic constituents may be further characterized by py-GC. This
technique separates the formulation into its individual organic
components. This is particularly useful when inorganic fillers in
the tape obscure the FTIR interpretation. Pyrolysis gas chromatography
can be coupled with mass spectrometry to obtain molecular information.
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