Set up of Fingerprint test Methods
| Author | European Union Publications Office, 2006 |
| Pages | 61-74 |
Page 61
The work on fingerprint resistance has been focused on the definition of test procedures that can be useful to obtain a quantitative evaluation of this characteristic. In order to do this, the main points among partners under study have been:
- the composition of fingerprints residues, to be simulated by a synthetic compound
- the methods to produce the fingerprint
- the methods to evaluate the damages.
Concerning the composition of fingerprints, previous studies, especially in field of forensic medicine and science, show that the composition of fingerprints is very complex and varying from person to person.
It can be considered a mixture of a water-based solution (sweat), secreted by the eccrine glands, and a water insoluble lipid complex (sebum), secreted by the sebaceous glands. Sweat is a solution of water (about 99%) with inorganic (chlorides, sulphates, phosphates, ammonia, metal ions...) and organic constituents (amino-acids, vitamins, lactic acid, glucose, urea, proteins...). Sebum is made up of lipids, mainly fatty acids, wax esters, squalene, tryglycerides, sterols. Different studies have showed that significant variations in sebum composition can be detected not only among different individuals but also in the same person at different ages [13,14]. A fingerprint can be composed of different ratios of sweat/sebum but, because of the high water content of sweat, sebum represents the main part of the permanent deposit.
The approach and the work performed by partners involved on coated stainless steels are reported in the following paragraphs.
Page 63
Stainless steels and, to a certain extent, pre-painted steel present a main drawback: during sheets handling, the surface is marked by finger prints.
The human skin is composed of three parts : epidermis, derm and hypoderm. From the skin is able to be rejected sweat and sebum.
Sweat is a solution produced by blood plasma after filtration. Its composition is :
- water (99%)
- antibody
- mineral salts
- destroyed species (urea, ...)
- lactic acid
- vitamin C
The sweat pH is normally acid between 4 and 6.
Sebum is produced by a specific glands based on all the body except on the hands and the feet. Lipids are cumulated in the glands until they break out. The complete analysis of sebum is too difficult to be given because there is not one but several sebum compositions (one for each person). But according to the ASTM standard D4265-98, it is now possible to make a synthetic composition (table 3-1).
Table 3-1: Composition of synthetic sebum according to ASTM D 4265-98
squalene (C30H50) 5%
palmitic acid (C16H32O2) 10%
stearic acid (C18H36O2) 5%
oleic acid (C18H34O2) 10%
linoleic acid (C18H32O2) 5%
paraffine 10%
olive oil (mainly composed of oleic acid (83,5%) 20%
cholesterol (C27H46O) 5%
coconut oil 15%
further components 15%
The sebum is mainly composed of carbon, in fact organic acids. We also need to keep in mind that these organic compounds are not aromatic compounds but long carbon chains aliphatic one.
Page 64
By SEM exam, we can define 3 areas (see figure 3-1 and figure 3-2):
- a virgin area, in which there is nothing ; it consists in fact in the area not reach by the finger
- a grey area, mainly composed of carbon, geometrically we recognized a finger print
- peaks located on the grey areas and composed of mineral salts (NaCl and KCl).
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Several chemical analysis have been made on fingerprint.
XPS
We used a X-ray Source, issue from a magnesium anode (energy = 1253,6eV) the area analysed is 40mm². This analysis confirms that the finger print is mainly composed of carbon (figure 3-3). It consists in organic compounds (table 3-2).
Table 3-2: Carbon chemical links (on a BA product)
| Chemical links | 1st finger print | 2nd finger print |
| C-C / C-H | 91,3% | 85,5% |
| C-O | 6,3% | 10,8% |
| C=O / O-C-O | 1,5% | 1,2% |
| O=C-O | 0,9% | 2,5% |
Page 65
The fingerprint can definitively be considered as sebum pieces left on the surface. The sweat part is insignificant.
FTIR analysis
The natural and synthetic sebums have been compared and no main difference has been raised (figure 3-4). The chemical links react with the same wave numbers (we only saw a little difference in the 1100-1200cm-1 range).
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Page 66
Fingerprint on a surface is made by the sebum deposition and we suppose that the perception is linked to the quantity of sebum laid on this surface. We tried therefore to develop a method to quantify the sebum deposition. Gas chromatography was chosen because it is very well appropriate to quantify organic compounds. The chromatograph reference is HP 6890 with a FID detector and a low polarity column Sim-Dist (diameter = 0,53um ; phase thickness = 0,17um and length = 10m). The temperature increase is:
30°C/min 5°C/min 60°C 250°C 390°C
First, we compared two chromatograms (figure 3-5):
- the one of synthetic sebum directly dissolved in hexane,
- the one of synthetic sebum laid on a coated surface and then dissolved in hexane.
The aim of this trial is to ensure us that the coating is not dissolved by hexane and that the chromatogram represent only the sebum analysis. The two chromatograms are identical. So that we can conclude that by dissolution of a fingerprint, we only remove the sebum and not the coating. We can say that the anti-fingerprint coating does not release any organic compound. Then, we tried to quantify the sebum of a fingerprint. We deposed a print of sebum on the surface and then dissolved it with hexane. The chromatogram is showed in the figure 3-6. There is a too small quantity of sebum on the fingerprint that the analysis is not sensitive enough to quantify it. We cannot quantify the fingerprint on the surface by gas chromatography.
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Page 67
It seems important to find a method to estimate the anti-fingerprint surface properties. We have tried to characterize fingerprint visual perception by using:
- AFM: but this tool is not adapted (problem because of sebum transparency)
- FTIR with optical interface: this method is quite interesting but not precise enough
- Gas chromatography: as it is said upper, this analysis is not sensitive enough.
Nevertheless, we have developed a reproducible method to make a finger print. Sebum is created according to the ASTM standard D4365-98 (see the composition in the table 3-1). Then it is deposited by hand.
In order to simulate finger print geometry, the method is following described:
- to dissolve 15% of sebum in hexane
- to clean hands with soap and ethanol
- to deposit 0,5ml of the previous solution on a polishing felt (size 2cm x 5cm)
- to press the felt (using like a pad) with the thumb
- to wait for hexane evaporation
- to press on the test specimen to make the fingerprint with the thumb.
The last step is to evaluate the fingerprint perception. We compare the test specimen behaviour with a surface reference. Aluminium with a microgranuled surface is not too much sensitive to fingerprint. It is our surface reference (quotation 0). The characterization is made by side view and top view:
- flat (F) / side view
- vertical (V) / top view
The fingerprint visibility is quoted from 0 to 4.
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Page 68
The fingerprint perception is evaluated by the test described in the previous paragraph. The following table presents the results.
Table 3-3: Fingerprint resistance of reference materials
| N° | Description | Quotation |
| - | Micro-granuled aluminium | F0 – V0 |
| 1 | 2R / Bright surface | F4 – V4 |
| 2 | Brushed surface (Mikrolon) | F4 – V4 |
| 3 | 2R / Bright surface | F4 – V4 |
| 4 | Brushed surface + clear coat | F4 – V4 |
| 5 | Brushed surface + PET film | F2 – V2 |
| 6 | HDG + primer + topcoat CH26 | F0 – V0 |
Uncoated stainless steel are very sensitive to fingerprint. We obtained the same results with either bright surface (fingerprint appears dark) or brushed surface (fingerprint appears white). PET film behaviour is better than clear coat one.
Page 69
The visibility and the easy-to-remove properties of fingerprints were tested by evaluating natural fingerprints. The fingerprints were applied on the different coatings, and after induction for four days the visibility was evaluated under different observation angles. The approximate grades of visibility are given in figure 3-7. Afterwards, it was tested if the fingerprints could be removed by dry wiping. For that purpose, the visibility after one time dry wiping and the number of necessary wiping processes until a complete removal of the fingerprint were evaluated.
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Page 70
In order to test the susceptibility of the stainless steel surfaces for fingerprints, some fingerprints were applied on the surfaces and their visibility was evaluated qualitatively.
On all surfaces the fingerprints were easily to see directly after its deposition onto the surface. After four days on the bright 1.4016 the fingerprint was weakly visible and could be removed almost completely by wiping over the surface using a dry tissue "KIMWIPE 200". On the bright 1.4301 surface the fingerprint was still good visible four days after deposition. By wiping it was not removable completely and a weak dark discoloration remained on the surface. On the brushed surface the fingerprint was moderately visible before wiping and similar to the bright 1.4301 surface after wiping.
However, in the case of the surface coated with Senocoil a complete different behaviour was detected: the fingerprints were moderately visible four days after application. By wiping using the dry tissue they could be removed easily without any residue.
Page 71
Various artificial sweat/sebum compositions are suggested by standards used in different application fields.
For instance the composition of a perspiration fluid, based on urea, salts, acetic acid and lanolin (83%), is given by ASTM for evaluating the perspiration resistance of architectural wall coatings [15]. A U.S. military standard for fingerprint removers suggests to use urea, NaCl, and lactic acid [16]. Many different composition are used to evaluate the sweat/fingerprint resistance of different type of goods in various sectors. For instance in the field of galvanic coatings the resistance to fingerprints is often intended as fingerprint visibility on the coating; artificial fingerprints are composed by vaseline and the visual effect is estimated in terms of colour variation (-ßE or -ßL) [17-19]. In other fields (e.g. in watches, jewellery, hi-fi industry) a higher attention is paid for the acid component of sweat, that can cause stains of the surfaces and releases on the skin. Salts (NaCl, K2CO3...) and acids (lactic, acetic, pyruvic...) are the main components used.
Therefore the experimental work on fingerprint resistance evaluation has been made:
- by making real fingerprints,
- according with the two standards above mentioned (table 3-4 and table 3-5),
- with the receipt proposed by Arcelor (standard ASTM D 4265) (see previous table 3-1).
Table 3-4: Procedures for fingerprint evaluation tests with "acid formulation"
Modified MIL C 15074E:
"Corrosion Preventive, Fingerprint Remover" (acid formulation)
Synthetic solution composition
NaCl 7 g
Urea 1 g
Lactic acid (85%) 4 g
Distilled water/methanol (50:50 v/v) up to 1 litre
Application method
The above solution in the amount of 1.5 ml is dropped on a pad (dimension ~ 30 x 30 mm) made of clean surgical gauze, 32-ply in thickness held on a clean glass plate.
A rubber stopper with an end roughened with 240 grit abrasive paper is used, after washing it with soap and water and rinsing with distilled water.
The roughened end of the stopper is put on the gauze and then on the test panel (application load ~ 1kg, time of contact few seconds).
Then the panel is slight dried (1 hour at 40 °C).
Page 72
Table 3-5: Procedures for fingerprint evaluation tests with "fat formulation"
Modified ASTM D 3730
"Testing High-Performance Interior Architectural Wall Coating - Perspiration Resistance" (fat formulation)
Synthetic solution composition (% p/p)
Methanol 3.7
Wetting agent 3.8
Perspiration fluid 92.5
Perspiration fluid: Urea 1.30
K2CO3 0.11
Na2SO4.10H2O 0.65
NaCl 2.71
Acetic acid (5% solution) 1.08
H2O 10.85
Anhydrous lanolin 83.30
Application method
The mixture is applied on the sample by using a 25 mm wide artist's brush. Then the panel is covered with a watch-glass and put in a oven at 49±2°C for 4 hours. After cooling till to room temperature and wiping off with a soft dry cloth, the panel is evaluated.
In particular, the main points under study have been:
- the ability of the synthetic compounds to simulate real fingerprints residues
- the applying methods
- the ability of these procedures to compare different materials behaviour
- the possibility to make a comparison by means of photographic standards.
The main results are that: - the fingerprints are less visible on coated stainless steels than on uncoated, - the procedures to realise synthetic fingerprint have a good reproducibility - their visibility is generally lower respect to real fingerprints, especially with the 2nd synthetic composition.
A better simulation and higher visibility of the stain was obtained by applying the receipt indicated in the standard ASTM D4265 with a "fingerprint stamp" (figure 3-8).
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Page 73
The application method utilised was as follows:
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1 ml of the sebum diluted at 15% in esane is dropped on a pad (dimension - 30 x 30 mm) made of clean surgical gauze, 30-ply in thickness held on a clean glass plate.
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The fingerprint stamp is washed with neutral soap (for dishes), rinsed and gently dried with hot air.
-
Then it is put on the gauze and, immediately after, on the test panel (application load - 1-2 kg, time of contact few seconds).
-
Every two tests the stamp is washed and the pad is changed.
The visibility of fingerprint made with the stamp is really very similar to that obtained manually and the reproducibility is good. This procedure has been successfully utilised also for another ECSC Project (7210-PR/383). However, with the above method some of the coated stainless steels seemed to behave in the same manner, whilst with real fingerprint some differences have been observed. But the finger print stamp can be optimised (kind of rubber, depth and width of grooves, etc.).
Page 74
The evaluations of fingerprints by visual examinations were easy with both natural and simulated fingerprints, applied with the stamp and the procedure above described. Although taking photos of fingerprint is easy , the "translation" of the visual sensation of the damages by realising photographic standards, in order to have an objective comparison ranking, resulted difficult as light orienting depends on steel finishes, because of the high reflectivity of steel surfaces.
As regards visual examination (naked eye), the chosen conditions to compare different materials was as follows:
- diffuse light
- samples in horizontal (flat) position
- observation angle of 85°, for most samples.
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