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Substitution of Cobalt Driers and Methyl Ethyl Ketoxime

8 Results from technical evaluation

The results obtained in the technical evaluation of the alternative driers and anti-skinning agents in different air-drying coating systems were used to evaluate if there from a technical point of view, at present, exist any proper substitution alternatives to cobalt driers, to methyl ethyl ketoxime and to hydroquinone respectively.

Only the overall results/conclusions from the testing/evaluation are presented.

Running eight do-it-yourself products and four industrial coatings through the evaluation procedure described in figure 7.1 and section 7.1 and testing four printing inks according to the procedure in section 7.2 generates a high number of data, which is rather difficult to present in a clear and well-arranged way.

For one product, DIY-P3, which is a waterborne stain, all the generated data from the project is presented in Appendix A – Chapter 1 to give an overview of the outcome of running one product through the technical evaluation procedure. The example given is for an air-drying product where the possibility of substituting Co driers seems rather promising. For the other air-drying products only the number of tests performed during the technical evaluation procedure is presented in Appendix A – Chapter 2. All obtained results have been communicated to the respective manufacturers on their own products.

As the technical evaluation has been so intensive on laboratory testing, it is quite obvious that it has been impossible to work in-depth with every single system within the frame of this project optimising it completely with regard to drying time. Even though alternative drier systems with potential to substitute Co driers in specific products have been identified, the products are only developed to a certain extent due to the limited time. The manufacturers need to continue the work optimising their own products if it seems worthwhile doing so. They also need to verify the obtained results as well as perform necessary supplementary tests before they carry out any substitution in their products. The results presented are therefore only meant as guidelines if Co free alternative driers seem to be worthwhile testing in a specific type of air-drying product.

The alternative anti-skinning agents were tested in one concentration only in each of the investigated products, for which reason the results can only be taken as guidelines on whether a specific anti-skinning agent can be expected to work in a specific product or not. As it is the case with the alternative driers the manufacturers need to verify the results themselves and to perform any necessary optimising.

8.1 Substitution of cobalt driers

The results from the technical evaluation were used to evaluate if appropriate alternatives to Co driers exist for the respective oxidative drying products included in the project. The tested drier combinations for the specific products containing the alternatives were as a starting point based on suggestions from the drier manufacturers. During the initial drying time tests the drier combinations were adjusted with regard to concentrations and combinations if necessary.

Table 8.1 gives an overview of the most common concentrations and combinations that the alternatives have been tested in. Combinations containing other secondary driers have also been tested in a limited number, and with limited success.

The concentrations of the alternative driers are, where possible, given as concentration metal on solid oxidative drying binder present in the coating product. Otherwise the concentration is given as the total drier product on the solid oxidative drying matter. If an alternative has been identified as a potential Co substitute in a specific product, they are in most cases used in combination 1 (see table 8.1). In a few cases they are used in combination 2 or combination 3 (see table 8.1).

Table 8.1.
The concentrations and combinations the alternative driers typically have been used in when tested in DIY and industrial air-drying coatings.

# DA = Drying accelerator (2,2-bipyridyl product)

## Concentration of drier product on solid binder

8.1.1 Overall results for do-it-yourself products

Eight different do-it-yourself products were included in the project in which the different alternative driers have been tested. In some products it was considerable more difficult to substitute the cobalt driers than in others. If a high number of sample preparations and drying time tests was needed it is an indication of the product being particularly difficult to Co substitute (See table 8.2 and appendix A - Chapter 2). This especially accounts DIY-P1, DIY-P2, DIY-P4, DIY-P6 and DIY-P8.

In table 8.2 it is indicated for each of the air-drying do-it-your-self products, which drier that seems to be the most promising Co alternative on basis of the investigated drier combination and concentrations. For some products it is indicated that further optimising MAYBE necessary. This classification has only been given to those alternatives, which is comparable to or even better than the reference product for almost every tested property.

Table 8.2
The most promising Co alternatives are listed and it is indicated if further optimising of the alternative drier systems is needed. If no other indication is given the alternatives listed have been used in combination 1 (See table 8.1)

* The Alternative has been used in combination 2 (See table 8.1)

** The alternative has been used in combination 3 (See table 8.1)

If the most promising drier is given in brackets it indicates that the drier was promising in the screening, but turned out to be inferior compared to the reference in the further testing. As these driers in most cases in fact induced comparable drying with the reference in the initial testing, it might be a matter of optimising the system. For instance by increasing the amount of the calcium drier to obtain a more stable drier system and hereby reducing the loss-of-dry. The same comments account for the results shown for the industrial products in table 8.3.

The alternatives in table 8.2 have been chosen by evaluating and comparing the drying times with those of the reference product. The film properties, hardness, gloss and yellowing in dark places of the alternative systems have also been compared to those of the reference products before selecting the most promising alternative driers for the respective products. In Appendix A - Chapter 3 the drying times of the alternative systems are compared with the reference products. This accounts for drying times obtained both before and after storing the samples at elevated temperatures. A comparison of film hardness, gloss, viscosity and yellowing can also be seen in Appendix A – Chapter 3. Only results from the most promising alternatives are shown together with results obtained for the references.

In table 8.2 other potential alternative driers are listed as well. In general these alternative drier systems have not been included in the stability/ageing test due to having inferior drying profiles in the initial testing compared to those alternatives chosen for further testing. However, this fact should not exclude them totally as potential alternatives as there is no knowledge on how they would perform after an induced ageing. With regard to Mn4, which was received late in the technical evaluation work, it has not been possible due to the late receipt to include it in the stability test for every product, even though it gave promising drying times in the initial testing. The same comments account for the results shown for the industrial products in table 8.3.

In most cases none of the vanadium driers gave sufficient drying in the tested concentrations and combinations. If sufficient drying was actually induced by the vanadium drier system the coating film became much too soft. As it can be seen in table 8.2 there is only one case where vanadium has been pointed out as "other potential" alternative, but it was not selected for further testing due to a soft coating film.

Mn1 seems to be the alternative drier, which is most useful for all-round purposes in conventional do-it-yourself coatings. Mn1 is among the most potential alternative drier for every product except DIY-P6 where the substitution totally failed.

Mn4 was included rather late in the testing process, for which reason drier systems containing Mn4 might not have been optimised to the same degree as for the other alternatives. In several cases it has been tested without secondary driers. Improved drying and film properties would most likely be obtainable by using the right combination with Zr and Ca driers. It was realised during the project that the concentration should rather be lowered than increased when optimising a drier system containing Mn4. This should be borne in mind when reading the used concentration in table 8.1 where the optimum concentrations of Mn4 most likely have to be found in the low range.

Mn2 and Mn3 are only among the most promising and other potential alternative driers in a few cases. It could be an indication of that these driers are not as efficient for air-drying do-it-yourself products as Mn1 and Mn4, or they might be more difficult to dose. Mn2 and Mn3 are more useful in the case of industrial products, which could indicate that these driers are more efficient for speciality products.

The driers made specifically for waterborne systems, Mn5(w) and Mn6(w) could be used in DIY-P3 with some success. Both Mn5(w) and Mn6(w) gave reasonable drying times in the initial test. Mn5(w) was slightly better than Mn6(w), but as one of the other alternatives, Mn1, gave a much faster drying in the initial test, Mn5(w) and Mn6(w) were excluded for further testing.

Mn traditional, the conventional manganese carboxylate drier, was only tested in a few do-it-yourself products without much success and it seems like Mn traditional has been tested in concentration that are 10 times higher than it should have been. The reason why Mn traditional was dropped in the rest of the products was though mainly that the addition of a drying accelerator induced another adjustable parameter in the alternative drier systems compared to the systems using driers where the drying accelerator is built into the drier product. More adjustable concentrations in a system often lead to an increase in the number of systems that needs to be investigated before proper drying is obtained.

All the alternative manganese driers tend to build in viscosity in the products. As no vanadium driers were selected for further testing their tendency to build up viscosity is not known.

Alternative drier systems with some potential to substitute cobalt driers were identified in case of DIY-P2, DIY-P4 and DIY-P8, but a relatively high extent of loss-of-dry was experienced after storage of the samples at elevated temperature, as the drying times were increased considerably. In the case of DIY-P1 and DIY-P7 substitution was possible with some success and for DIY-P3 and DIY-P5 the substitutions came out quite successfully. No potential alternative drier system was identified in the case of DIY-P6.

At present it seems not worthwhile substituting cobalt driers with vanadium driers from a technical point of view. To obtain proper drying as well as proper film hardness a manganese alternative should be used in preference to a vanadium alternative. This conclusion is of course only valid on basis of the drier systems and products tested in this project. As the success of substituting cobalt driers seems very binder and product specific, vanadium driers might work in other systems.

In the used concentration and combinations it was experienced that the various manganese driers differ quite a lot in drying efficiency in the specific products. Products containing binders based on linseed oil seem in general more difficult to Co-substitute than those products containing binders based on tall oil. The higher the amount of oxidative drying matter in a product the more difficult the substitution seems to become. As only two waterborne products, DIY-P3 and DIY-P6, were included it is difficult to draw any conclusion on what influence the thinner has on the substitution process. The overall conclusion is though that Co substitution seems possible in certain cases depending on the specific alternative driers as well as the specific product.

In the cases where the substitution success was rather limited due to a high extent of loss-of-dry after the stability tests it might be a question of optimising the drier system with regard to the auxiliary drier (e.g. increasing the content of calcium driers). Within the project frame it has not been possible to change the drier systems after the stability test had been performed. As the optimum proportion between manganese drier and drying accelerator also might differ from one type of binder to another it could in certain cases be worthwhile using a traditional manganese drier combined with a drying accelerator, which makes the proportion adjustable.

8.1.2 Overall results for the industrial products

As the group of do-it-yourself products was tested and investigated before the group of industrial products, some of the prior experiences could be used, for which reason fewer drier combinations and fewer tests in general were needed for the industrial products. The number of tested systems is given in table 8.3. In Appendix A – Chapter 2 an overview of all performed tests for the industrial products in connection with substituting cobalt driers and methyl ethyl ketoxime is given.

In table 8.3 it is indicated for each of the included air-drying industrial products, which drier that seems to be the most promising Co alternative if any on basis of the investigated drier combination and concentrations. Other potential alternative driers are listed as well. For some products it is indicated that further optimising MAYBE necessary. This classification has only been given to those alternatives, which is comparable to or even better than the reference product for almost every tested property. In table 8.3 the number of tested drier systems is given. In general, the higher the number of tested drier systems the more difficult the substitution was.

Table 8.3
The most promising Co alternatives are listed and it is indicated if further optimising of the alternative drier systems is needed.

* The Alternative has been used in combination 2 (See table 8.1)

As it can be seen from the number of sample preparation IND-P10 and IND-P11 were more difficult to Co substitute than IND-P12 and IND-P13, even though potential alternative drier systems were identified for all four products. In the case of IND-P10 a too high extent of loss-of-dry was observed after storing the samples with alternative drier systems at elevated temperatures, even though the drying times were still quite low. The very fast drying is very essential to IND-P10. The initial film hardness of the samples with the alternative drier systems was also too low.

In the case of IND-P11 the film hardness of the samples with alternative drier systems were far too low. This problem might be overcome by adding Zn as an additional drier. Loss-of-dry after storage at elevated temperature was also observed.

In IND-P12 and IND-P13 the cobalt driers were substituted with much higher success than in IND-P10 and IND-P11, the alternative systems having comparable drying time with the reference products, even after the samples had been stored at elevated temperature. In the case of IND-P12 three different manganese driers, Mn2, Mn3 and Mn4, can be used as Co substitute. All of them give higher film hardness and gloss, comparable or improved yellowing properties and comparable water resistance with reference product. Only in one case, inferior water resistance was observed.

Three driers, Mn1, Mn4 and Mn5(w) could be used with success for substituting cobalt driers in IND-P13, but in this case, film properties as film hardness, gloss and water resistance were slightly inferior to the reference in most cases. All three alternative systems were comparable to the reference with regard to yellowing.

As it was the case with the do-it-yourself products the alternatives driers shown in table 8.3 have been chosen by evaluating and comparing the drying times and other film properties with those of the reference product. These comparisons are shown in appendix A - Chapter 3 as well. Only results from the most promising alternatives are shown together with results obtained for the references.

Vanadium driers were not tested in the industrial products. All manganese alternatives included in the evaluation are represented within the group of most promising alternatives, for which reason it seems as if the industrial products in fact are easier to Co substitute than the do-it-yourself products. That is probably due to the binders. The do-it-yourself products contain common alkyds, whereas the industrial products contain either modified alkyds or alkyd blended with non-oxidative drying binders, and therefore the amount of oxidative drying matter is low compared to the do-it-yourself products. The industrial products therefore have a better drying profile even without the presence of driers.

8.1.3 Overall results for printing inks

The product group of printing inks differs quite a lot from the other air-drying products investigated. They are typically applied (printed) in a very thin layer and often at absorbent substrate, and therefore they show very little set off even though the drying times are much longer than those of the paints. The need for duct stability is one of the reasons to the longer drying time.

Therefore the printing inks, also without hydroquinone, have relatively long drying times when tested on a drying time recorder, even though they were applied in a much thinner film compared to the paint products. In the case of INK-P16 and INK-P17 the drying time measurements were performed on samples without hydroquinone. This accounts for both samples with alternative drier systems as well as the reference inks.

Mn1 combined with Mn traditional gave comparable drying results to the reference in all four sheet-fed printing inks. Both Mn1 and Mn traditional are used in rather high concentrations, Mn1 being added in concentrations ranging from 0.32 – 0.45% Mn metal on oxidative drying matter and Mn traditional was added in concentration corresponding to 0.5 – 0.8% Mn metal. In Appendix A – Chapter 3 the drying time results are presented for one alternative drier system for each ink and compared to the reference inks.

Mn4 has been tested in combination with Mn traditional. In one ink Mn4 was added in concentrations of 0.03 – 0.07% Mn metal on oxidative drying matter in combination with Mn traditional added in 0.5% Mn metal on oxidative drying matter. The system gives comparable or even better drying than the reference ink. Set-off was not investigated. The drying result for Mn4 used in INK-P15 is shown in Appendix A – Chapter 3.

Mn2 and Mn3 did not induce sufficient drying in the inks in the tested concentration of 5.0 – 13.5% ^[1] of total drier product on oxidative drying matter. One vanadium alternative was tested in a single drier system in INK-P14 and INK-P15, but as no drying was induced it was not tested further.

The number of tested systems in INK-P16 and INK-P17 is much lower than in INK-P14 and INK-P15 (see Appendix A – Chapter 2) as only the two most promising alternatives, Mn1 and Mn2 from the testing of INK-P14 and INK-P15 were investigated. Mn4 has only been tested in INK-P15 due to late receipt of this drier, but it seems as a potential alternative to cobalt driers in inks.

Only one drier system for each printing ink was chosen for testing the set-off effects. The set off effect of the inks containing the alternative driers was compared to those of the reference inks and in most cases the set-off effect was comparable to the reference ink.

It seems possible to substitute cobalt driers in sheet-fed printing inks, but the test results obtained for the drying times seem less significant than for the paint products. Probably due to the very thick printing inks are difficult to apply in even layers for the drying time tests and larger variations are therefore observed making them more difficult to evaluate. However, it might in fact be easier to substitute cobalt driers in this product group than the others. Less surface drying is needed for the printing inks and if the alternative drier system induces less drying than the cobalt based it is possible to reduce the amount of antioxidant (hydroquinone) present in the printing ink.

8.2 Alternative anti-skinning agents

To get an indication of whether the alternative anti-skinning agents can be expected to work in an air-drying product or not it has been investigated how effective the different alternatives prevent skinning from occurring in open and closed containers for the do-it-yourself and industrial products. The testing has been performed in accordance with the description given in 7.1.3.8.

In the printing inks the alternatives have been evaluated by measuring auto-oxidation temperature and by investigating duct stability as described in 7.2.2.3 and 7.2.2.4. The original cobalt based drier systems were used in the inks.

8.2.1 DIY and Industrial products

The anti-skinning agents have been tested in one concentration only in each product. These concentrations are listed in table 8.4. The concentrations listed are the concentration of the entire anti-skinning product, not just the active substances. All alternatives, except acetone oxime, were tested in approximately the same concentration in all air-drying products, no matter what the original methyl ethyl ketoxime concentration had been. This means that they are added in a higher concentration than methyl ethyl ketoxime in some products.

The samples within a product series contain the same alternative drier system, chosen from the technical evaluation of the alternative driers. The manufacturer supplied IND-P9 with its original drier system.

In table 8.5 and 8.6 it is shown for how many days the different anti-skinning agents are able to prevent skinning on the surface of the different air-drying products in closed and open containers respectively.

Table 8.4
The used concentrations of the tested anti-skinning agents in
do-it-yourself and industrial products.

The results of each anti-skinning agent should be compared both with the results obtained for the product without anti-skinning agent as well as the product containing methyl ethyl ketoxime. As DIY-P8 had a high extent of loss-of-dry after the ageing test, this product was not included in the test of anti-skinning agent. Due to a limited amount of sample DIY-P4 was only tested with regard to skinning in closed containers.

Table 8.5
Number of days before skinning is observed in the DIY and industrial products for sample stored in closed containers at 23C and 50 % relative humidity.

* All systems separate within 4 days

Tests performed in closed containers show that acetone oxime is the anti-skinning agent, which is most comparable to methyl ethyl ketoxime, but amino/amido based anti-skinning agents also give comparable or better results in several cases. The phenolic based anti-skinning agents do not work in the tested concentrations. Vitamin E has only been tested in product IND-P9 in three concentration, 0.5, 1.0 and 2.0% respectively and in one concentration in DIY-P4 (0.35%). Vitamin E is not efficient in DIY-P4 and in IND-P9 it is impossible to differentiate between the different anti-skinning agents.

The results from the open containers show that the amino/amido compounds in most cases are comparable to or more efficient than methyl ethyl ketoxime in preventing skin formation in this situation. This could strongly indicate that these anti-skinning agents would have a more negative effect on the drying times than the oximes. Vitamin E prevents the skinning to some extent in product IND-P9, but is also added in rather high concentrations.

The results from the skinning tests can also be used to indicate whether more or less of a specific alternative anti-skinning agent is needed to get comparable results with the reference. Further optimisation of the concentrations is most likely necessary. For some anti-skinning agents a higher concentration would properly give better results with regard to prevent the skinning. On the other hand higher concentrations of anti-skinning agents would inevitable influence on the drying times. The effect was more pronounced for the less volatile alternatives.

Table 8. 6
Number of days before skinning is observed in the DIY and industrial products for sample stored in open containers at 23C and 50 % relative humidity.

The skin formation tests have only been performed with one concentration of the respective anti-skinning agent and the effect of the anti-skinning agents on the drying time has not been investigated. The obtained results should therefore only be used as guidance to the paint manufacturers on whether it is worthwhile trying to substitute methyl ethyl ketoxime in their products or not.

8.2.2 Printing inks

The efficiency of anti-skinning agents/antioxidant within printing inks was investigated by measuring the auto-oxidation temperature of ink samples containing different anti-skinning agents. The testing has been performed as described in 7.2.2.3.

The more the auto-oxidation temperature is increased compared to the ink without any antioxidant the more efficient can one expects the antioxidant to be. The obtained results for two sheet-feed inks are given in table 8.7. The concentrations given are for the entire anti-skin product, not just the active substances.

Compared to the inks without anti-skinning agents, all the investigated anti-skinning agents, except methyl ethyl ketoxime and phenolic type 2, have an effect on the auto-oxidation temperature. Hydroquinone, which is the antioxidant commonly used in printing ink, has the strongest effect. In the tested concentration (0.8 % w/w) it gives higher auto-oxidation temperature than the reference in the tested concentration. This was expected as the references only contain approximately 0.2 % w/w hydroquinone.

Vitamin E gives in concentrations of 2.5 % w/w some reasonable results, which are comparable to the reference inks. The effect on the auto-oxidation temperature is not as distinct for the amino/amido based compounds, but they show an effect, which probably could be increased by increasing the amount of anti-skinning agents.

Table 8.7
The measured auto-oxidation temperatures of two inks containing different anti-skinning agents.

# No distinct auto-oxidation.

An investigation of the duct stability was performed on a drying time recorder. The drying time results obtained for samples with different alternative anti-skinning agents are given in table 8.8. The samples with 0.4 % hydroquinone are used as reference. All inks contain the original cobalt drier systems. The concentrations given are the concentration of the entire anti-skinning product and not just of the active substances. The compositions of the alternatives can be found in table 6.2. The used hydroquinone product contains 49 weight-% active substance.

If an ink sample with an alternative anti-skinning agent dries faster than the ink sample with hydroquinone, the alternative is not expected to prevent drying in the ducts to the same extent as the reference ink.

Vitamin E is used in reduced amount compared to the auto-oxidation tests, whereas the amino/amido compounds are used in increased concentrations. In the tested concentration amino/amido no. 1 and no. 2 seem to give the best protection against drying, amino/amido no. 2 being slightly better than no. 1.

Vitamin E also protects from drying, but has lower set-to-touch and tack-free times than both the amino/amido compounds and hydroquinone. For INK-P16 vitamin E seems as efficient as the amino compounds.

Table 8.8.
test of duct stability. Investigated by comparing drying times of the ink samples at a drying time recorder.

The alternatives are only comparable or slightly better in the case of INK-P14 even though they are used in rather high concentration. This indicates that they are not as efficient antioxidant as hydroquinone, but that they might work in the printing inks. The influence on the set-off effect of the different alternative anti-skinning agents has not been investigated.

Footnotes

[1] The reason for these percentages being much higher compared to Mn1 and Mn4 is that they are given as total drier product and not as metal concentration.

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Version 1.0 December 2003, © Danish Environmental Protection Agency