COMMENTARY 0 Forensic Science Society 1987

The evidential value of fabric car seats and car seat covers

KG WIGGINS and JE ALLARD

The Metropolitan Police Forensic Science Laboratory, 109 Lambeth Road, London, United Kingdom SE1 7LP

Abstract The project reported here was initiated in response to the increasing number of cases in which the forensic scientist is asked to demonstrate contact between people and cars. The demonstration of contact by means of fibre transfer is made easier today by the increased occurrence of fur-fabric seat covers and fabric seats. Surveys have been carried out of the major manufacturers of seat covers and fabrics, in order to assess the value of this type of evidence. Numerous samples were collected from industry and casework, and their fibre composition and dyes were compared. The results indicated that fibre evidence obtained from either source can often be very valuable. The implications for the court-going officer are discussed. Key words: Car: seat covers; Car: fabric seats; Fabric composition; Dyes; Dye batch variation; Evidence.

Journal of the Forensic Science Society 1987; 27: 93-101 Received 22 July 1986

Introduction After a crime has been committed, it is sometimes necessary to demonstrate contact between a person and a car. This could be important, for example, in the case of a simple car theft, an armed robbery or an assault. One means of establishing contact is to look for a cross-transfer of textile fibres between the person's clothing and the car seats. The recent popularity of fabric covered seats, and the older practice of covering seats with loose covers, has greatly increased the chances of finding a transfer of fibres. This paper is designed to help the forensic scientist assess the evidential value of any fibres found in such cases.

Initially, a survey of the major manufacturers of loose seat covers was conducted. The manufacturers of the cars most likely to be encountered in criminal cases in the United Kingdom were also contacted. The car manufacturers gave us details of their fabric suppliers for the integral coverings of car seats. Questionnaires were then sent to the firms,

requesting details of their manufacturing processes and samples of their fabrics. Further information was obtained by personal visits to a company producing cloth for loose covers and also to the dye-house of a company producing fabrics for car seats.

The samples obtained from these companies, together with samples of all seat covers received in criminal cases submitted to the Metropolitan Police Forensic Science Laboratory during 1982, were compared with regard to sheddability, composition and dyeing.

Methods The samples obtained from manufacturers and casework were compared using a selection of the following methods: thin-layer chromatography (TLC) [I-41; polarising microscopy (PM) [5,6]; infrared spectroscopy (IR) [I]; comparison microscopy (CM) [I] and microspectrophotometry (VS).

Car seat covers All samples were compared by TLC, PM and IR. If any were indistinguish- able using these methods, bright field CM and VS were introduced. This unorthodox order of comparison was employed because of the large number of samples involved. A preliminary bright field microscopic comparison would have been very time-consuming. Thin-layer chromatography was carried out on tufts of twenty to thirty fibres, each 1-2 centimetres in length. These were placed in Durham tubes, covered with a minimal volume of formic acid and water (1: 1) and placed in a sandbath at 90°C for 15 minutes. The extracts obtained were spotted one centimetre from the end of DC-Alufolien Kieselgel 60F254 TLC plates. This process was carried out on a hotplate [7] in order to aid solvent evaporation and the chromatogram was placed for a further ten minutes in a 100°C oven to complete drying. The dyes were eluted to a distance of 1.5 centimetres in a solvent mixture of water, 880 ammonia, chloroform and methanol (1 : 1 : 11 : 7).

Approximate birefringence measurements were made on all man-made fibres using a Leitz Orthoplan polarising microscope. This was performed on small tufts of fibres which were mounted on standard glass slides in Xam neutral medium improved white (Searle Diagnostic). Some fibres could be identified in this way but confirmation using IR Spectroscopy was necessary in the case of polyamide and polyacrylonitrile fibres.

Infrared spectra were recorded using a Perkin-Elmer 157 spectrophoto- meter fitted with a beam condenser (Research and Industrial Instruments Co.) The latter gives a 6: 1 reduction in bzam size. A few fibres were squashed together to produce a flat film using ten tons pressure in a standard IR press (Perkin-Elmer). The resultant film was attached with double-sided sellotape across the central orifice of a one millimetre thick lead disc.

Comparison microscopy was carried out on the mounted samples prepared for PM. The comparison microscope consisted of two Leitz Orthoplan instruments connected by an optical bridge with a binocular head. Quartz iodine lamps in a transmitted light mode provided the white light illumina- tion. All comparisons were carried out at x 100 magnification.

Visible spectra were recorded from the fibres already prepared for microscopy using a Nanospec 10s microspectrophotometer. This is attached at the phototube to a Leitz Ortholux I1 microscope and linked to a Nanometrics SDP 2000 microprocessor. Fibres were aligned using the scanning slit at a magnification of ~ 2 2 0 and their visible absorbance spectrum (390-750 nm) was recorded on a Tekman TE200 flat bed recorder.

Fabric car seaB These samples were compared using TLC and PM only. IR was not performed as most companies had stated the fibre types used, and PM was only used as a check that their information was correct.

Thin-layer chromatography was carried out on the same number of fibres used for car seat covers but the lengths were considerably shorter-between 1.5 and 4.0mm. The fibres were placed in Durham tubes with a minimal volume of pyridine and water (57:43) and heated in a sandbath at 90°C for thirty minutes. The extracts were then spotted onto TLC plates and eluted to a distance of 2.0 centimetres in the same solvent mixtures as for car seat covers.

Results

Car seat covers Sixty-five samples were collected from casework and forty-four from seat cover manufacturers. Many different colours were obtained, all of which were either fur fabric or imitation "sheepskin". Both types of fabric shed fibres very readily.

Questionnaires were completed by 8 of the 9 major manufacturers of covers, who had been in production for between 3 and 35 years. All of them bought their material from other firms who were either in England or Italy, and their outlets ranged from market stalls, chain stores, car accessory shops through to hypermarkets. Approximately 2 million covers were sold annually, individually and in sets, the price ranging from £3 per cover to £455 per set (1982 figures).

The common covers were acrylic or acrylic/polyester blends and all of the cover samples received were made entirely from fur fabric. (Some older styles of cover were made with a nylon backing panel to reduce costs.) Other products were made from the same material (eg, underblankets, coat linings, slippers and toys).

95

Colours varied widely; some changed annually but others remained con- stant. The number and size of rolls of fabric also varied considerably. Between 20 and 60 covers could be cut per roll of fabric, but some sets of covers were cut from more than one roll. Most companies could distinguish their own seat covers even if the labels were missing, usually by the cut of the material and/or the stitching.

Examples of dye batch variation were revealed by TLC (unpublished work) in the products from some companies. Also important was the finding, on two occasions, that different companies had used fibres and/or dyes from the same source. The comparison of loose seat covers collected from casework showed one example where covers from two different cases appeared to originate from the same supplier. Only one casework sample was found to be identical (by CM, VS and TLC) to a sample we obtained from a company.

Mostly the covers from one car were indistinguishable. However, there were a few examples where the dye components differed between visually identical covers in the same car.

Approximately 75% of the covers collected consisted of acrylic fibres composed of either acrylonitrile/vinyl acetate copolymer or acrylonitrile/methyl acrylate copolymer, or a mixture of these. The remain- ing covers consisted of either acrylic fibres composed of acrylonitrile/methyl methacrylate copolymer, acryliclpolyester mixtures, modified acrylic fibres, or complex mixtures (wool and man-made) of 'waste' fibre. The latter were all imitation sheepskin fabrics.

A visit to one of the seat cover manufacturers provided much useful information. They described three main methods for preparing fibres for use in car seat cover fabrics.

New ready-dyedfibres (prime fibre) were used either alone or in blends. The latter was an economy measure; for example, a blend of black and white fibres will produce a grey shade and save on dyeing costs.

Prime fibre was dyed by the firm itself. For example, one dye lot may consist of 200 kilograms of prime acrylic fibre. Four of these dye lots may be blended to produce a uniform overall colour and thus avoid shade variation between batches. The resulting 800 kilograms of fibre would produce about 2000 seat covers.

Waste fibre may be used. This would be either waste fibre from the floor of knitting mills or faultily-dyed material, which after careful blending would be overdyed. This method is used mostly by the Italian companies and is considerably cheaper than the first two.

The dyes used throughout were generally the cheapest available to achieve adequate dyeing and hence would be bought from many different sources.

96

Fabric car seats Four of the five car manufacturers contacted returned questionnaires and fabric samples. A total of 154 fabric samples were received which were either of a woven tweed-type or a velvet-type pile fabric. Some samples from one company had an unusual pile. They had a woven backing with threads of a "pipe-cleaner" type construction in one direction. These consisted of a central core into which were woven short pile fibres which shed readily as small fragments. (This construction has also been encoun- tered from one other company not included in this survey.) All of the fabrics shed fibres well, those with a pile more readily than the woven ones.

The questionnaire revealed that each manufacturer used several firms for its supply of fabrics and that different car models (from the same manufac- turer) normally had different designs of fabric. Each design was available in a small range of colours (up to 6) and these colours changed every one or two years. Colours were often the same for different designs of fabric. Seats within one car were not always covered from the same roll of fabric and two fabric designs might be used within one seat, eg., the body of the seat and side edging could be contrasting designs and might be supplied from different firms.

Nineteen companies were responsible for supplying fabric to these four car manufacturers and further information was provided by them. The automo- tive industry sets very high standards for their fabrics-they attempt to equal their engineering precision! Precise colour matching and a high level of light fastness are demanded. One firm might supply several different car manufacturers. Yarns used for car seat fabrics may sometimes be used for drapery, upholstery and apparel. Most of the fabric suppliers used other firms to dye the yarn or fabric. However, this was generally commission dyeing, i.e, detailed instructions are given by the fabric supplier and the product will therefore be exclusive (Figure 1).

Dyes are purchased from a wide range of sources, and one supplier will generally use the same dye for a particular colour as long as the price is not prohibitive. Fabric suppliers tend to buy one large batch of dye and use it over several months, and batch sizes varied from 200 to 1000 kg (i.e, 600 to 2000 m, sufficient to furnish a few hundred cars). All suppliers admitted to using 'topping-up' procedures during dyeing and a combination of three or four dyes is usually employed to obtain any particular end colour. Pale colours are often used for car interiors to create a feeling of spaciousness and overdyeing of pale fibres may occur. Dye manufacturers are allowed up to 5% variation between dye batches, i.e, a particular dye may contain up to 5% of 'shading colour' in order to maintain its defined shade. (This variation, however, may not always be detectable using our TLC methods.)

There are only 10 to 12 dyes which can be used for dyeing polyester for

DY

E

sells

dye

D

YE

R

MA

NU

FA

CT

UR

ER

se

lls d

yed

fabr

ic o

r ya

m

(com

mis

sion

dye

d or

no

n-co

mm

issi

on d

yed)

FA

BR

IC -,

L

fabr

ic

dyes

ow

n SU

PP

LIE

R

sells

fabr

ic

MA

NU

FA

mR

ER

C

AR

se

at w

ith f

abri

c

fabr

ic o

r ya

m

FIG

UR

E 1

D

yein

g pr

oced

ures

for

car

seat

fabr

ics.

automotive products. There is a further limitation on the number of premetallized dyes available for use with nylon. Nylon 66 is used in a variety of cross-sectional shapes in order to produce different finishes. Titanium dioxide is used as the delustering agent. Nylon 6 is only used for roof trims. The laminate backing of the fabric has printed details of the specific dye batch, though the car seat would normally have to be dismantled in order to obtain this information.

The dyes of the fabric samples obtained were compared using TLC. This revealed a large amount of variation as expected from the above informa- tion. Some fabric suppliers showed consistent dyeing between the same colour fabric of different designs, others showed variation. Occasionally the same combination of dyes were used to produce different shades, e.g., beige and brown or grey and black. This resulted from changes in the concentra- tions of the different components of the dye mixture. Similarities were also seen in dyes from different firms, indicating a possible common dye or dye source.

The fabrics from Company D appeared to have more consistent dyes, apparently one dyer or dye source for several fabric producers. These dyes were also more complex than those from other firms and could be used as a guide to the identification of Company D fabrics. Dye batch variation was detected in the samples from Company A. It should occur in other companies, as they all admit to using topping-up procedures.

Car seat fabrics consist of a laminate backing with a "face" fabric. This face fabric is uppermost on the seat and will shed fibres. The composition of the face fabric for the various cars is shown in Table 1.

TABLE 1 Fibre composition of the range of car seat 'face' fabrics from various car manufacturers

Car manufacturer Fibre composition of 'face' fabrics

A Polyester Polyester/nylon Polyester/viscose B Polyester C Polyester Polyester/nylon Nylon 66 D Wool* Nylon?

* top range models only. t 'pipe-cleaner' construction with a non-shedding viscose core.

Discussion The manufacturing processes involved in the production of loose covers and seat fabric are very variable. Fibres shed from them, therefore, should provide useful forensic evidence because of a combination of the following factors: the use of many different sources of dye; the production of a wide

range of fabrics and colours; the occurrence of batch variation in dyeing processes; and the regular use of blends and mixtures of fibres.

The use of pale colour fabrics in cars to create a spacious effect is an obvious disadvantage to forensic fibre examinations. Manufacturers of loose seat covers often copy one another's designs, but they will almost certainly use different combinations of dye. In a few cases it was obvious that different companies had been supplied with cloth from the same dyer. Therefore it is rarely possible to predict the manufacturer of a seat cover by examination of its dyes, although one of the car companies used fabric which could almost certainly be identified by its fibre construction and the complexity of its dye mixtures.

The forensic scientist must be aware that the seat covers or seat fabric in one car may well look identical but that different seats could originate from different rolls of cloth. Dye batch variation does occur and it is therefore essential to sample all seats in a car. Again, this can be advantageous in increasing the value of fibre evidence. The value of the evidence may also be enhanced by finding fibres matching the clothing of a victim or suspect on the car seats. A two-way fibre transfer has more significance than the sum of the two individual transfers. This is well illustrated in the following two casework examples.

The investigation of an armed robbery required that an abandoned getaway car be examined for possible links to four suspects. A two-way transfer of fibres was found between the car seat fabrics and the suspects7 clothing. Fibres matching the seat fabric were also found on a mask left at the scene and in a second getaway car which belonged to one of the suspects (the latter was presumably a secondary transfer via the suspects' clothing). In order to assess the val'ue of this fibres evidence, background information was obtained from the car manufacturer and their fabric supplier. The manufacturer gave details of the number of cars (17,000) containing this particular fabric. The supplier was able to state that this cloth was commission-dyed and therefore exclusive to them, and he also supplied the forensic scientist with fabric samples from 18 consecutive dye batches. Dye batch variation was seen by TLC of bulk fibre samples, but could not be detected on the single fibre samples in the case. Representatives from the car manufacturer and the fabric supplier were called to court to give evidence, along with the forensic scientist. A combination of this and other police evidence resulted in four convictions.

In another example, strong evidence to connect three suspects to a car was provided by the examination of fibres from the seat covers. While a rent collector was on his rounds, he became suspicious of four men who appeared to be following him. He contacted the police who arrived on the scene and questioned these men, one of whom tried to escape, and fired a

shot at a police constable during the chase. We were asked to connect three of these suspects to a car found nearby in which it was thought they had travelled to the scene.

The car had fur-fabric seat covers on all seats. The covers appeared brown in colour although they were made from a blend of at least twelve different colours of acrylic fibre. The colours included beige, orange, blue, green and black and each colour could be distinguished microscopically. Trousers from three of the suspects were examined for these fibres. On the first pair, 7 fibres of 4 microscopically different types were found, all matching the fibres from the seat covers. Examination of the dyes showed two different mixtures of dyes and these were indistinguishable from those of the seat covers. On the second pair of trousers 15 fibres of 8 microscopically different types were found, once again matching the covers in all respects. Finally, on the trousers from the third suspect, 19 fibres of 6 microscopically different types were found. The dyes revealed three different mixtures. It was concluded that the fibres on the trousers came from these particular covers or from seat covers of exactly the same composition.

The high degree of variability involved in the production of both seat covers and seat fabrics should produce valuable fibre evidence, particularly when combined with a transfer of clothing fibres onto the car seats.

Acknowledgements

We should like to thank the many companies who provided information and samples. This paper is based on a presentation made at the Tenth Triennial Meeting of the International Association of Forensic Sciences, at Oxford, England, in September 1984.

References 1. Cook R and Paterson MD. New techniques for the identification of microscopic samples

of textile fibres by infrared spectroscopy. Forensic Science International 1981; 12: 237-243.

2. Macrae R and Smalldon KW. The extraction of dyestuffs from single wool fibres. Journal of Forensic Sciences 1979; 24: 109-129.

3. Beattie IB, Roberts HL and Dudley RJ. Thin layer chromatography of dyes extracted from polyester, nylon and polyacrylonitrile fibres. Forensic Science International 1981; 17: 57-69.

4. Home JM and Dudley RJ. Thin layer chromatography of dyes extracted from cellulosic fibres. Forensic Science International 1981; 17: 71-78.

5. Heyn ANJ. Observations of the birefringence and refractive index of synthetic fibres with special reference to their identification. Textile Research Journal 1952; 22: 513-522.

6. Culliford BJ. A multiple entry card index for the identification of synthetic fibres. Journal of the Forensic Science Society 1963; 4: 91-97.

7. Wiggins K, Russell J and Salter M. A simple hot plate drying system for thin layer chromatography. Laboratory Practice 1983; 32: 72.