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In recent years there has been a significant increase in the clandestine production and supply of illicit drugs within the UK. Last year, a record 241,091 drug seizures were made by the Police and UK border agencies, 6% up on the previous year (1). With this increase in drug trafficking the need to establish connections between drug dealers and drug users has become of increasing importance (2). As suspected illicit drugs are commonly seized in the materials in which they are packaged (3), one possible method that can be utilised to aid in tracing their origin is to carry out a thorough examination of the packaging itself (4).
Drug packaging analysis can provide extremely strong evidential value when used appropriately (5). However, the evidential strength often varies depending on both the material in which the packaging is manufactured and the quantity of packaging that is seized. In the past, literature articles have highlighted the variety of materials that are often used to package and smuggle illicit drugs. This extensive list has included: the linings of suitcases, pineapple tins, condoms, paper, and foil (4). Nonetheless, undoubtedly the most common encountered packaging material in the forensic laboratory is plastic.
The usefulness of plastic packaging analysis for forensic purposes was primarily documented in the 18th Century (9). Researchers discovered that plastic materials were not completely homogenous (6) and as a result often contained impurities that could introduce unique characteristic features. By developing methods in which these characteristic markings could be visualised, it became possible to compare plastic materials seized from different sources. This fundamental principal revolutionised the field of packaging analysis as strong evidential connections between items of plastic could now be established and used in Court (5).
In order to fully understand and appreciate how these characteristic manufacturing marks arise, and also how they are exploited forensically, it is necessary to comprehend both the principles that underlie plastic chemistry, and the processes involved in plastic manufacture.
Aim of this review
Plastic is a by-product of the petroleum industry that was originally developed as a cheaper alternative to natural materials such as wood and metal. The principal ingredients that are required for plastic manufacture are organic monomers and plasticisers. A monomer is a low molecular weight molecule that has the ability to chemically bind to other monomers via strong covalent bonds, forming a chain of monomer units, known as a polymer (5).
In the manufacture of plastic packaging there are four main polymers that are in use today, these are: polyethylene, polypropylene, polyvinyl chloride, and polyethylene terephthalate (4).
Polyethylene terephthalate (PET) is a high strength short-chain polymer that is most frequently used in the production of soft drink bottles. This is mainly due its ability to resist bleaches, detergents, and heat as well acids and alkalis (5).
Polyvinyl chloride (PVC), a polymer formed from many vinyl chloride monomers and naturally exists as a rigid structure. As a result it is often necessary for the addition of various plasticisers, like dioctyl adipate (DOA), to make the polymer more flexible. Due to the requirement of plasticiser addition this type of polymer is now seen less frequently in food packaging materials as research has shown that these plasticisers may leach into the food products (5).
Polypropylene (PP) is a simple polymer that is formed from the polymerisation of propylene monomers. It is mainly used in the manufacture of clear films and microwave packaging (5).
Polyethylene (PE), commonly known as polythene, is an ethane polymer from which most plastic bags and films are manufactured. As the majority of plastic drug packaging analysis is performed on plastic bags and films, polyethylene is of major interest in forensic science. The polymer is very versatile and can be categorised under various grades depending on its density (5). Low density polyethylene (LDPE), of density range 0.916 to 0.925 grams per cubic centimetre, exists with a more flexible structure and is mainly used in the manufacture of cling films and bin bags (5). On the other hand, High density polyethylene (HDPE), of density range 0.941 to 0.965 grams per cubic centimetre, is composed of smaller branched monomer chains and thus exhibits a more rigid structure. HDPE is commonly used in the manufacture of milk bottles, freezer bags and shopping bags (5).
With such a large diverse range of polymers and additives that are now available to the plastics industry the chances of two separate factories manufacturing identical products is reduced. Knowledge of this is an extremely useful tool for the forensic scientist as it opens up the possibilities for establishing connections between plastic samples based on their chemical composition (14). Details of current chemical comparison methods will be covered in full later in this review.
Plastic bag and film manufacture
At the beginning of the manufacturing process virgin or reprocessed resin pellets are positioned in a cylindrical extruder where they are mixed with pigments and other additives. The mixture is then heated to approximately 250oC to produce a homogenous, viscous fluid (5). Before the viscous fluid undergoes any further processing it is filtered in order to remove any large impurities that may be present. Once this is achieved there are two possible extrusion processes that the polymer can undergo, these are known as Cast or Blown extrusion (5).
In this simple process the viscous polymer fluid is fed through a straight die. As the die is in the form of a horizontal slit, a flat sheet of polymer film emerges that is of equal thickness to the die slit (5). The flat polymer sheet is then cooled and hauled off through a series of rollers before being wound up on to a roll (Figure 1).
Figure 1: Schematic diagram of a standard Cast extrusion process.
The Blown extrusion process is more common than Cast extrusion and is generally the method of choice for the production of plastic bags (5). In this process the viscous polymer will enter a ring-shaped die where it is forced around a mandrel (a rod found inside the die), shaped into a sleeve and extruded through the die opening in the form of a plastic tube. The tube, while still in the molten state, is then further expanded to a "bubble" or hollow cylinder by blowing pressurised air through the centre of the mandrel to the inside of the "bubble", thus allowing the plastic to be stretched to the desired width. As the hollow plastic tube is forced upwards it cools down and is finally collapsed by a series of "nip rollers" before being wound on to a roll (Figure 2).
Figure 2: Schematic diagram of a standard Blown extrusion process.
Upon completion of both Cast and Blown extrusion there are a number of possible outcomes for the plastic sheet. Quite often it is simply the case that the plastic is wound up on to a roll and stored. However, sometimes it can be transported to a different location to undergo further manufacture. Additional manufacture procedures may include printing, folding, gusseting, addition of handles, heat sealing and cutting. Depending on the type of bag desired by the customer will determine which of these processes occurs.
Distinguishing features of plastic bags
As a direct result of the manufacturing process many distinguishing features are permanently incorporated in the plastic material. It is these manufacturing marks that are then compared in order to establish whether or not two plastic packaging seizures have arisen from a similar batch or brand of manufactured plastic. Examples of such marks include, striations, pigment bands and stress bands.
Striations on the plastic arise as a result of extruding the plastic film through the die, mandrel and to a certain extend over the rollers. This type of physical characteristic is often observed on both the outer and inner surfaces of the plastic film. Striation marks may be further divided into die lines and weld lines (5). Weld lines (knit lines) arise when impurities get trapped in the die and are less commonly encountered in forensic science. Die lines however, are comprised of a continuous series of longitudinal indentations or protrusions and have been shown to be present throughout the plastic manufacturing run (8). Die lines are thought to arise as a direct result of the extrusion process. If the circular die, through which the plastic is extruded, exhibits imperfections in, or also, a build up of plastic on its surface, these marks will be incorporated onto the molten plastic as it flows over the die surface (8). As a result of this, it is thought that plastic bags which present similar die lines must therefore have originated from the same blown extrusion apparatus on the same manufacturing run (8).
During blown extrusion, pressurised air is blown through the mandrel to form a hollow plastic tube. As this occurs the molten plastic is stretched. This stretching of the plastic is thought to give rise to unique horizontal streaks that run across the plastic film. These streaks are known as stress bands and they are extremely useful manufacturing characteristics. By comparing the stress bands present on different plastic materials it is possible to determine whether or not they have originated form the same manufacturing run, and also to some extent, the proximity at which the bags have been manufactured to each other.
Sometimes dyes or pigments may be added along with the plastic pellets at the beginning of the manufacturing process. When incomplete homogenisation of these substances occurs, pigment bands may arise in the plastic. Pigment bands are commonly orientated in the same direction as film production, and they are very useful in connecting large numbers of plastic items with a common source (5).
Besides the above mentioned commonly observed physical features in plastics, many other unique identifying characteristics may arise by other means. Sometimes the plastics used in the manufacturing process can contain random impurities such as carbon, resin, or grit (5). When these impurities are not isolated during the filtering process they become incorporated in the film and can present as either dark or light spots of plastic (5). In industrial terms these are referred to as "fisheyes". "Fisheyes" often appear together with "arrowheads", which are vertical streaks in the plastic that radiate out from the "fisheye" (5). Together these unique features can be used to categorise plastic bags that have been manufactured together on the same manufacturing run. However, these types of manufacture marks are of limited value as they are not often seen by the forensic scientist. If these marks are present in the plastic it is generally not of sufficient quality to be released to the customer and is normally scrapped.
Aside from stress bands, pigment bands and striations, some unique identifying marks may be introduced to the plastic as it undergoes further manufacture. Together these marks are known as construction characteristics and they must not be overlooked. Such features can be used in conjunction with physical characteristics to determine the origins or relationships of differing plastic bags that are under investigation. Many common construction features that are often examined by the forensic scientist include: the colour of the bag, any printing on the bag, the type of seal on the bag, whether heat sealers have been used to cut or heat seal the bags, and also any perforations that may be present on the bag.
The forensic examination of plastic bags and films
Like any other forensic examination the evaluation and comparison of plastic bags and films will follow a stepwise protocol. Generally this involves the logical progression from a simple, non-destructive analysis through to the use of more detailed and often more destructive techniques. It is not actually possible to describe the exact methodology used in the examination, as in practice this differs according to the resources available, the content to be examined and also the personal preferences within each of the forensic providers (5). However, it is possible to give a brief description of the current methodologies that have been developed for the soul purpose of differentiating between various plastic materials.
One of the simplest methods, yet one of the strongest in terms of evidential value is that of demonstrating a mechanical fit between the cut edges, or between the stress and die bands of two plastic bag samples. Direct mechanical fits can conclusively establish whether or not two plastic bags where manufactured sequentially on the same manufacturing run.
Using this straightforward, non-destructive apparatus, it is possible to visualise some of the surface manufacturing characteristics described above. An interference grid simply consists of illuminated, alternating black and white lines. When the grid is placed behind the sample in question, any imperfections in the plastic will obstruct the visualisation of the grid and as a consequence will make the distinguishing features of the plastic visible. Following this, the features can be easily photographed and a comparison made (12).
Incident, transmitted and oblique lighting
By altering the positioning of a light source it is possible to use incident, transmitted and oblique lighting to illuminate manufacturing marks on plastic. Transmitted light is particularly useful for visualising characteristic features on pigmented plastic, such as pigment bands and "fisheyes". Incident lighting is best for visualising scratches and printing on the surface of plastic, and finally oblique lighting is ideal for visualising manufacturing marks on transparent plastic.
Additionally, incident and transmitted light microscopy can be used to study any printing that is present on the plastic and also the texture of the plastic itself (5). This form of microscopy is using carried out on a stereomicroscope.
As plastic is extruded and then strained over a series of rollers the polymers within each layer become orientated in line with the direction of extrusion. This makes the layers of plastic birefringent (13). As a consequence, differing coloured patterns present on the plastic may be viewed. This can be achieved by using crossed polarised filters, in which two linear polarising filters are placed at right angles to each other. If the plastic packaging material is placed between the two filters at an angle of 45o, coloured patterns will be observed on the plastic that expose the manufacturing history of the packaging (4). This method can also be utilised to visualise other manufacturing marks including scratches or roller marks (5). When using this method it is also possible to photograph the coloured patterns for examination at a later date.
There are many types of photography that may be used to visualise characteristic features on plastic bags and films. These range from simple methods such as taking a photography of the packaging to more complication methods like Shadowgraph and Schlieren photography.
Shadowgraphy is simple optical technique that may be utilised to visualise and compare extrusion marks, heat-sealed edges, and cut edges on different samples of transparent polythene bags and sheets (5). In Shadowgraphy all that is required are a single point light source and a screen onto which the shadow may be projected. The shadowgraph is produced by passing the light rays through the transparent plastic. Any impurities present in the plastic will cause a localised change in refractive index and thus will scatter any light rays hitting this point. As the light is no longer permitted to pass through this section of the plastic a shadow will be cast on the viewing screen (11). The resultant shadowgraph may then be recorded by either manual photography, electronically, or developed on photographic film or paper (5).
As Shadowgraphy is a non destructive method, the sample will not be damaged during the procedure and other visualisation methods may be used to complement this technique (4).
As an alternative to Shadowgraphy, Schlieren photography may be used. Although the principles or this technique are similar to Shadowgraphy the main difference between the two processes is that Schlieren photography makes use of lenses, mirrors and a knife edge filter to produce an image (11). Similar to before, a point light source is used, however this time the light is passed through two sets of lenses. The plastic sample under investigation is placed between these lenses, in an area known as the Schlieren field (5). Additionally, at the focal point of the second lens, a knife edge filter is incorporated that directs scattered light rays towards the imaging screen and diverts uninterrupted light, thus creating a shadow. The captured images may then be recorded in a similar way to that in Shadowgraphy (5).
Magna Jet Black fingerprint powder
It is possible to use fingerprint powder to visualise characteristic features on plastic. However, this method has been shown to disrupt any further chemical analyses being carried out on the plastic. Consequently, it is advised to be used only as a last resort when other non-destructive methods have been trialled first (10).
In addition to the above discussed physical methodologies there exists a diverse range of chemical techniques that can be utilised to characterise the make-up of plastic packaging. Chemical analysis is an extremely effective tool as it allows the identification of the base polymer, additives, as well as any pigments or dyes to be identified (5).
Wide angle X-ray diffraction (WAXD)
Fourier Transfrom Infrared Spectroscopy (FTIR)
Fourier transform infrared (FTIR) spectroscopy is a measurement technique that allows one to record infrared spectra. Infrared light is guided through an interferometer and then through the sample (or vice versa). A moving mirror inside the apparatus alters the distribution of infrared light that passes through the interferometer. The signal directly recorded, called an "interferogram", represents light output as a function of mirror position. A data-processing technique called Fourier transform turns this raw data into the desired result (the sample's spectrum): Light output as a function of infrared wavelength (or equivalently, wavenumber). As described above, the sample's spectrum is always compared to a reference.
Thermal desorption capillary gas chromatography
x-ray fluorescence spectroscopy
thin layer chromatography
neutron activation analysis
isotope ratio mass spec (IRMS)
Differential scanning calorimetry
Gas chromatography - mass spec
It is uncommon for a single test to uniquely identify a sample of plastic packaging with its manufacturing batch, plant, or geographical location, or to associate one sample with (or distinguish it from) another. To build a conclusive view of the provenance of a sample of plastic drug packaging that will stand up as evidence in a court of law, it will generally be necessary to compare a range of physical and chemical properties of the sample with those of similar plastics of known origin
Sections still to complete:
The value of plastic bag evidence
Conclusions/ my work