Further Activities and Characterisations

Author:European Union Publications Office, 2006
Pages:75-83
SUMMARY

0.1. Set-up of a specification list. 0.2. Thickness measurement. 0.3. Vapour resistance. 0.4. Draw forming tests. 0.5. Resistance to food and detergents.

 
INDEX
CONTENT

Page 75

The evaluations of the performances of the new products were made by means of many different tests. A part from scratch-mar and antifingerprint resistance, special attention was paid to adhesion and formability of the coatings. Durability and resistance to different environmental conditions have been evaluated taking in mind the possible uses of these products (e.g. mainly home appliances, but also internal building).

The approach of the partners involved on stainless steel (Arcelor, DOC, CSM) was sometimes different, as regards both test methods and wished properties. This fact can be in part explained by the following reasons: each customer in these sectors has his own specifications, that have been developed for post or prepainted carbon steel and not for stainless steels. A specification list has been set up by DOC (see table 4-1), but for the reasons above mentioned this one was not fully shared by the other partners. An example of the high variety of possible tests is given in the table 4-3 resulting from an investigation regarding only the methods to evaluate the resistance to food and detergents.

Some of the further tests considered by the partners are reported in the following paragraphs.

Page 76

0.1. Set-up of a specification list

A specification list has been set up at DOC in order to quantify the properties to be fulfilled by a coating system before it can be introduced into the market. For this purpose, possible costumers of clear coated stainless steel were addressed by ThyssenKrupp Nirosta. The resulting list is represented in table 4-1. This specification list has been taken as a base to evaluate the tested coatings. During the project, not all properties were studied with the same intensity, but the emphasis was set on those properties which are the essential basis for a coating applied in household industry: adhesion, formability, scratch resistance, repulse of fingerprints and chemical resistance. At DOC, single experiments were also performed with respect to heat and UV resistance, weldability and printability.

Table 4-1: Specification list set up at DOC for scratch resistant anti-fingerprint coatings on stainless steel


Property Specification
visual appearance almost no changes in comparison to uncoated stainless steel
adhesion no deadhesion after 3T bend; GT0 after 5 mm Erichsen indentation
formability no cracks after 3T bend
scratch resistance gloss change after 50 double strokes:
repulse of fingerprints weak visibility, easy to remove by dry wiping
chemical resistance resistance against lemon juice, vinegar, alkaline oven cleaner (Sidol®), ketchup, mustard
heat resistance (2 h, 200 °C) no colour change, no deadhesion
UV resistance (QUV-A-Test, 1000 hours) colour change Ε*
long term resistance (outdoor exposure) no specification defined yet
weldability no colour change, no deadhesion nearby the welding seam
printability good adhesion of the ink, possibly after plasma pre-treatment
adhesion of protection foil temporary protection foil should be easily to apply and later to remove
feasibility of paint repairs It should be possible to repair defects invisibly and steady.

Page 77

0.2. Thickness measurement

At Arcelor this measurement is made with a permascop. Two conditions are necessary:

- to have a magnetic substrate

- to have a zone without treatment to adjust the zero (0).

The figure 4-1 shows the measurement principle.

[ FIGURE ARE NOT INCLUDED ]

The measurement is made in six steps:

  1. measurement of magnetic induction current of the substrate

  2. adjustment of 0

  3. measurement of substrate current with a standard coating

  4. adjustment of the value of the standard. we have to be careful with ranges: we need to have the same for the standard and the unknown sample

  5. measurement of sample current

  6. reading of the thickness value

Page 78

0.3. Vapour resistance

This test has been developed at Arcelor to simulate an exposition of the coating to hot vapour. It consists in putting the test specimen over boiling water during two hours. At the end of the test, the surface has to be intact. The results regarding the reference materials are given in the table 4-2.

[ FIGURE ARE NOT INCLUDED ]

Table 4-2: Vapour resistance of reference materials


Description Quotation
- Micro-granuled aluminium OK
1 2R / Bright surface OK
2 Brushed surface (Mikrolon) OK
3 2R / Bright surface White Marks
4 Brushed surface + clear coat Complete whiteness of the surface
5 Brushed surface + PET film OK

Coated surfaces are more sensitive to vapour than uncoated stainless steel. Coil coated steel panel seems to be inert (but the white surface is here a real advantage).

Page 79

0.4. Draw forming tests

In aim to follow the aptitude of the coating to be formed, at Arcelor draw forming trials were made. The aim was to test the adherence and the behaviour of the coating during draw forming.

The trials were made with a blank, which has a diameter of 62,5-70mm. The cup diameter was 33mm, so that the ratio between blank and cup diameters was 1.89 or 2.12.

[ FIGURE ARE NOT INCLUDED ]

The characterization of the samples is made visually by looking to a whitening of the coating or to areas of non-adherence.

Page 80

0.5. Resistance to food and detergents

To individuate the most common specifications regarding the stain resistance to food and to other products, an investigation at CSM has been begun and a big variety of products & test-conditions & evaluating methods has been found. In the table 4-3 the results of this investigation, regarding the home appliance sector, are shown.

Table 4-3: Products and conditions in stain resistance tests


Foods and substance simulating foods Commercial detergents and further products
Commercial detergents and further products
butter
citric acid solution, at 1% or 8% or 10%
coffee
egg
fats substances
lactic acid solution 8%
lard
lemon juice
margarine
marmalade
milk
mustard
olive oils
onion
seed oil
spinach
sulfuric acid solution 2%
tea
tomato sauce
tomato sauce, concentrated
vinegar
CIF crema
CIF liquido
Aiax antistatico
Aiax gel 2 in 1
denaturized Alcohol
Bref antipolvere
Shoe polish (black)
Lysoform liquido
Red lipstick
Prodotti anticalcare
Tergitutto crema
Smack brilla acciaio









Conditions Evaluation after:
spot test open
spot test closed (under watch glass)
room temperature, for 24 hours or more
cyclic conditions (various combinations of time and temperature)
an elapsing variable time or suddenly rinsing with water washing with neutral soap and rinsing with water etc.

Tests made at CSM

The resistance to stains due to contact with different kinds of food was evaluated according with ECCA T18 on the reference materials. Being the possible tests so many, the characterizations have been restricted to few more aggressive foods. The tests, performed for 24 and 72 hours, consisted in putting a controlled quantity of food (~2 ml) on the samples which were then maintained at room temperature under watch glass (covered spot test conditions).

At the end of the test the samples were washed with tap water and neutral detergent (for dishes) and observed.

The results (table 4-4) show that with all the kinds of food considered the coated stainless steel do not get stained.

Page 81

As regards the un-coated stainless ones, all of them have problems with mustard, whilst the stain resistance to tomato sauce, lemon juice and vinegar is not good when the finishing of the surface is brushed.

[ FIGURE ARE NOT INCLUDED ]

Tests made at Arcelor

In order to characterise the chemical and food products resistance we followed the ISO 2812 standard. This consists in putting 3ml of each product to be tested on the surface and keep the contact during 72h at ambient temperature.

The products tested are :

- lemon

- coffee

- tea

- milk

- ketchup

- mustard

- meat

- blackberry

- vinegar

- Mr Propre (washing liquid).

The anti-soiling properties are summarized in the table 4-5.

Page 82

[ FIGURE ARE NOT INCLUDED ]

An organic coating (film or clear coat) improves stainless steel behaviour towards acid resistance. The PET film is more chemically inert than clear coat. As far as anti-soiling properties are concerned, the PET film is the best product.

Tests made at DOC

In order to determine the chemical resistance of the different coatings against food and detergents, defined areas on the specimens (spots of 1.5 cm in diameter) were covered with different test agents: tomato puree, mustard, vinegar, lemon juice, tea and Sidol-. The latter one is a severe alkaline oven cleaner. The surfaces were exposed to the different substances at room temperature for 24 hours or at 60 °C for 1 hour. Then the substances were removed by washing and wiping them under flowing water. The results of the visual inspection of the samples after rinsing are given in table 4-6. The uncoated stainless steel surfaces showed tarnishing discoloration after the test with mustard. The Senocoil coating showed a stain after the exposure of mustard at 60 °C. The impact of Sidol- on Senocoil resulted in deadhesion of the coating.

Table 4-6: Results after testing the chemical resistance of the coatings. Scale: 0 = no stain visible, 1 = weak stain visible, 2 = stain visible, 3 = partly deadhesion of the coating 4 = completely deadhesion of the coating


Exposure Test agent
tomato puree mustard vinegar lemon juice tea Sidol ®
1.4016, bright 25 °C/24 h 0 1 0 0 0 0
60°C/1 h 0 1 0 0 0 0
1.4301, bright 25 °C/24 h 0 1 0 0 0 0
60°C/1 h 0 1 0 0 0 0
1.4301, bright 25 °C/24 h 0 1 0 0 0 0
60°C/1 h 0 1 0 0 1 0
Senocoil 25 °C/24 h 0 0 0 0 0 3
60°C/1 h 0 2 0 0 0 4

Page 83

[ FIGURE ARE NOT INCLUDED ]