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Substitution of Cobalt Driers and Methyl Ethyl Ketoxime

7 Technical evaluation

The testing of air-drying coating systems with alternative driers and anti-skinning agents has been performed at several stages. First, initial drying time tests were performed for the do-it-yourself (DIY) products. The experiences from these tests were used in the initial drying time test for the industrial products, which in general meant that the number of initial drying time tests for the industrial products was considerable lower than for the DIY products.

After the initial drying time test the most promising alternative drier systems were chosen for further testing of stability, viscosity, film hardness, gloss and yellowing. This accounts for both DIY and industrial products. The results obtained are used to evaluate the possibility to substitute Co driers in the specific air-drying products.

One alternative Co free drier system was chosen for each product to be used for the skinning test, except for the printing inks where cobalt based drier systems were used when testing the efficacy of the alternative anti-skinning agents.

The alternative anti-skinning agents have in most cases only been tested in a single concentration, as it was not intended to optimise the different systems. The tests were only performed to get an indication of whether an alternative anti-skinning agent can be expected to work properly or not in a specific product. Increasing the concentration of anti-skinning agents would give a better protection against skinning, but higher concentrations will inevitable mean longer drying times. No drying times and set-off effects have been investigated with the alternative anti-skinning agents.

The printing inks differ as product type from the DIY and industrial products for which reason they needed to be tested in another way. In the DIY and industrial products the alternative anti-skinning agents need to prevent in-can skinning, whereas in the printing ink they need to induce duct stability for an extended time period. For the printing inks only drying time profiles and the set-off effect were investigated along with the auto-oxidation temperature and duct stability.

All tests in the technical evaluation of the alternatives have been performed as comparative tests, meaning that the reference product containing the original Co based drier system and methyl ethyl ketoxime, or hydroquinone in the case of printing inks, was always included in the testing.

7.1 Do-It-Yourself and industrial products

7.1.1 Procedure for technical evaluation

The first step in finding an alternative drier to Co-driers was to investigate the drying profile of samples, containing alternative drier systems, and compare them to the reference products. The testing was performed by means of a drying time recorder.

The first test series was only meant to be a screening to verify whether the alternative driers seemed to work in the respective products or not. The pre-selection of alternative drier systems were therefore performed on systems without anti-skinning agents. The alternative driers were tested in different concentrations and combinations with secondary driers for each product. The tested drier systems (usually Ca and Zr and the Co alternative) have in most cases been based on suggestions from the drier manufacturers. If insufficient drying times were obtained the drier concentrations/combinations were adjusted and the new system was tested. For some of the products many adjustments and initial drying time tests were necessary before some reasonable alternative drier systems (if any) were obtained for further testing.

In the next step the most promising drier compositions were tested on the drying time recorder again. This time anti-skinning agent, preferable methyl ethyl ketoxime, was added to the solvent-borne systems to make the drying profile even more comparable to the reference product. No such testing was performed for the waterborne products.

On basis of the drying time test in the second step between 0 and 5 of the most promising alternative Co free drier systems were selected for each product for further testing. Viscosity, pendulum hardness and gloss were measured and compared with the properties of the reference products. If reasonable drying times and film properties were obtained it was investigated how the samples were influenced by storage at 40ºC for two week. Storage at elevated temperature induces an accelerated ageing in the samples and to some extent simulates prolonged storage and an increase in drying time after storage indicates a loss-of-dry of the drier systems.

After storage the samples were evaluated. If severe phase separation or sedimentation was observed for the alternative systems and this was not the case for the reference product, the alternative system was rejected, especially if it was not possible to easily mix the sample again by stirring.

Table 7.1.
Methods used for evaluating the efficiency of the alternative driers in different oxidative drying products.

Properties as viscosity, gloss and hardness of the dry film were measured before and after storage at 40ºC to verify how much the accelerated ageing had altered the products and to evaluate how comparable the alternative system was with the reference products. The drying time profile was also evaluated again after storage. In table 7.1 an overview is given of the methods used for the technical evaluation of coatings containing the Co free alternative drier systems. The overall process of the technical evaluation is described in figure 7.1.

All tests were performed as comparative tests meaning that the reference product containing the original Co based drier system and methyl ethyl ketoxime was always included in the testing.

For each solvent-borne coating product the most promising alternative system was chosen for the skin formation test.

Figure 7.1.
Schematic overview of the technical evaluation and selection process of Co free drier alternatives for a do-it-yourself product or an industrial product. Any test results obtained with the alternative systems have always been compared to those of the reference coating containing Co drier.

7.1.2 Preparation of test systems

The respective driers were added to 100 g of product sample (in some cases only 25 or 50 g of product sample was used). A balance capable of weighing to the nearest 0.001 g was used and the exact amount added of each drier was noted. The numbers of driers added to a system range from one to four. In most cases three different driers were added, a primary drier (the alternative to Co driers), a through-drier and an auxiliary drier.

Through-driers and auxiliary driers were added first. The sample was then shaken mechanically or stirred thoroughly by hand for about 5-15 minutes. The primary drier was then added and the sample was shaken or stirred again to ensure a uniform distribution of the driers in the product.

If an anti-skinning agent should be present in the system, it was added after the driers. The exact amount of anti-skinning agent was noted as well.

The sample rested for at least 20 – 24 h in a tight closed can at 23 2ºC and 50 5% relative humidity before any testing was performed.

7.1.3 Used methods

7.1.3.1 Drying time profile
The drying time profile of the coating samples has been evaluated with a straight-line drying time recorder used in accordance with ASTM D 5895 "Measuring Times of Drying or Curing during Film Formation of Organic Coatings Using Mechanical Recorders".

The samples were applied on glass plates in a well-defined film thickness using an adjustable cube applicator with 5 buckets. The DIY and industrial products were applied 80 m wet whereas the printing inks have been applied 30 m wet.

The stylus of the drying time recorder was immediately after application lowered down into the wet film. The stylus then moved across the sample on the glass plate with a constant and well-defined velocity. The measurements were performed at 23 2C and at a relative humidity of 50 5%. The drying time measurement can range from 1 hour to 50 hours depending on what stylus velocity that is chosen. All do-it-yourself products have been tested over a period of 20 hours. The industrial products have been tested over period of 2 to 20 hours depending on the product. The printing inks have been tested for periods of 50 hours.

Afterwards the track left of the stylus on the film was evaluated. The appearance of the track depends on how dry the film is. The drying of the film is divided into following stages:

Stage 1: Set-to-touch
The set-to-touch condition is reached when the film has solidified sufficiently that it no longer flows or sticks to objects that lightly touch it. The coating keeps flowing together until the set-to-touch time is reached. A pear-shaped depression appears in the film and the stylus begins to leave a visible trace in the film.

Stage 2: Tack-free time
After the set-to-touch has been reached the coating begins to polymerise for which reason the stylus leaves a visible line in the coating film. The tack-free condition is reached when the film surface has dried to an extent where the film no longer adheres to light objects placed on it. The continuous track of the stylus in the film stops and the stylus starts to tear the film or leave discontinuous cutting of the film.

Stage 3: Dry-hard time
The dry-hard condition is reached when the drying has proceeded to an extent where the film is not displaced or that no noticeable marks are left on the film when influenced with a relatively strong pressure. The stylus stops tearing or cutting the film, but leaves a visible trace on the film.

Stage 4: Dry-through time
The dry-through condition is reached when the film has solidified to an extent where a large twisting force can be applied without distorting the film. The stylus leaves no longer any visible trace on the film.

The drying time test has in general not been performed in duplicate due to the huge amount of tests performed. The drying times of samples containing alternative Co-free drier systems were always compared with the drying time of the reference product.

7.1.3.2 Hardness (elasticity) of dry film
The hardness of the dry film has been measured with a König Albert Pendulum according to ISO 1522-73 "Pendulum damping test". The hardness of the dry film is determined by registering the time (the number of pendulum swings) it takes before the amplitude of the pendulum is damped to a certain extent. The more swings observed the harder is the film.

The samples have been applied on glass plates with a baker applicator with a gap size of 90 µm for the solvent-borne products and 120 µm for waterborne products. The samples were after application stored at 23 2C and at a relative humidity of 50 5%. The hardness of the dry film has been determined after different periods of drying time.

Testing of the film hardness of a product series was continued until the number of swings did not change significantly from one drying period to another. The gap between two such periods should be at least five to seven days. The hardness of the dry film of the products containing the alternative drier systems was always compared with the film hardness of the reference product.

7.1.3.3 Gloss
The gloss of the coating systems is measured on the same samples as the film hardness. The measurements have been performed with a Picogloss gloss meter model 503 according to DS/EN ISO 2813 "Determination of specular gloss of non-metallic paint films at 20º, 60º and 85º". The measurements have mainly been performed with the 60º geometry.

7.1.3.4 Viscosity
The reference coating products as well as the coating samples containing the selected alternative drier systems have been characterised by measuring the viscosity of the samples on a rotational visco.me.ter with defined shear rate. A Bohlin VOR Rheometer with C 25 geometry and different torque bars was used for the measurements. The viscosity was measured over a large span of shear rates. The measurements have been performed in accordance with ISO 3219 "Determination of the rheological behaviour of paint and printing inks using a rotational viscometer with defined shear rate".

7.1.3.5 Yellowing
The samples were applied on Hostaphan PE sheets with a baker applicator with a gap size of 90 µm for solvent-borne systems and 120 µm for the waterborne systems. The applied film on Hostaphan was after 24 h of drying cut into two equally sized pieces. One piece was kept in natural daylight (also direct sunlight) for a month. The other piece was stored in a cupboard in complete darkness for the same time period.

After a month the two pieces were compared according to ISO 3668 "Visual comparison of the colour of paints". The colour of the two samples was compared in artificial daylight in a colour-matching booth. The observer judged the degree of yellowing of the sample kept in dark by comparing it to the sample kept in natural daylight. The observer judged the yellowing of the film by following ranking:

It has to be noted that the judging is subjective but it gives an indication of the tendency that the different systems have to yellowing in dark places. The same person has evaluated all samples.

For each samples series (each air-drying coating product) the reference product (containing the original Co-drier system) was included to verify whether the samples with the alternative drier systems were more prone to yellowing or not.

7.1.3.6 Stability/ageing
After preparation the samples were stored at ambient temperature for at least 24 hours before storage at increased temperature.

All samples were evaluated concerning appearance in the container and the viscosity was measured prior to placement of the samples in the oven at 40ºC for 2 weeks. The reference product of each product series was also stored at 40ºC for 2 weeks. When comparing the aged alternative with the aged reference product one should be aware of that most of the reference products already have been stored between ½ and 1½ year at ambient temperature before the stability/ageing test was performed.

After storage the appearance was evaluated again. If severe phase separation or sedimentation was observed for the alternative systems and this was not the case for the reference product, the alternative system was rejected, especially if it was not possible to easily mix the sample again by stirring. If a sample containing an alternative driers system passed the evaluation, the properties investigated before storage (drying time, viscosity, film hardness and gloss) were investigated again.

7.1.3.7 Water resistance
Some of the industrial products were also tested for early water resistance according to ISO 2812, "Determination of resistance to liquids – Method 2".

The products were applied on glass panel in wet film thickness ranging from 90 – 120 µm depending on the dry matter of the respective products. The applied systems were drying for 24 hours at 23 ± 2ºC and a relative humidity of 50 ± 5 % before the test for water resistance was performed.

Absorbent material of the dimension 20 x 20 mm was soaked with water. Excess water was allowed to drip of before placing the wet material on the film surface. The wet material was then covered by watch-glasses. The wet materials were left at three different spots for periods of 1, 3, and 8 hours respectively. It was evaluated, if water affected the film and whether the alternative system was affected more or less than the reference product.

7.1.3.8 Test of skin formation
Only one drier system was chosen for each product. A large sample (200 g) was prepared containing the selected alternative drier system. Different anti-skinning agents were than added to 20 g of the basis sample.

15 g of the each sample is stored in closed PE containers. The sample was inspected every second day by opening the lid and noting if the sample had skin on the surface. 5 g of same samples was kept in an open container. These were also investigated for skinning every second day.

7.2 Printing Ink

The printing inks have been evaluated in a slightly different way than the paint products. For the printing inks it is essential that the ink film does not have a high extent of set-off. After performing a number of drying time measurements one alternative drier system was selected for each ink and the ink was tested for set-off.

7.2.1 Sample preparation

7.2.1.1 Driers
The respective driers were added to 10 g of sample. A balance capable of weighing to the nearest 0.001 g was used and the exact amount added of each drier was noted. The number of driers used in the printing ink systems ranged from one to three. In most cases two driers were used. The driers were mixed in the sample one at the time, ending with the most active drier. The sample was stirred by hand. The samples rested for 20-24 h in a tight closed can at 23 2ºC and 50 5% relative humidity before the drying test was performed.

7.2.1.2 Anti-skinning agents
Ink samples containing the original cobalt based drier system were added the alternative anti-skinning agents/anti-oxidant in one or two concentrations.

7.2.2 Used methods

7.2.2.1 Drying time
See description in paragraph 7.1.3.1 as the drying time measurements are performed in the same way for paint products and printing inks.

7.2.2.2 Set-off test
Set-off effects have been evaluated using an IGT-tester, which is an apparatus for applying ink with a well-defined film thickness on paper. These test prints can be tested for set-off by running a paper against the print after different periods of drying. The set-off was evaluated by measuring the density of the printing ink on the set-off paper. The higher the density was the more set-off effect. The set-off was compared to that of the reference inks.

7.2.2.3 Auto-oxidation temperature
5 to 10 mg of sample is placed in an open 40 l aluminium crucible. The sample is then scanned by means of differential scanning calorimetry (DSC) in a temperature interval of ranging from 50 – 300 C. The heating rate is 10C/min. Air is used as a carrier gas with a flow of 100 cm3/min.

An exothermic raise in the DSC curves is observed due to oxidation in the sample. The auto-oxidation temperature is taken as the intersection of the extrapolated baseline and the tangent to the exothermic raise. The determinations are performed in duplicate.

Measurements were performed on samples without any anti-skinning agents and on samples containing different anti-skinning agents. This method is a qualitative method and can only be used to compare the auto-oxidation temperatures of samples that belong to the same product series. The higher the auto-oxidation temperature the more efficient is the anti-skinning agent/anti-oxidant assumed to be.

7.2.2.4 The roller/duct stability
The roller/duct stability of printing inks containing alternative antioxidant has been investigated by performing drying time measurements on a drying time recorder. The measurements were performed as described in paragraph 7.1.3.1

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Version 1.0 December 2003, © Danish Environmental Protection Agency