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Substitution of cobalt driers in wood coatings

2 Parameters with importance to drying

• 2.1 Solvent
  • 2.1.1 Theoretical evaluation of Initial drying mechanisms
• 2.2 Alkyds
  • 2.2.1 Oil type
  • 2.2.2 Oil length
  • 2.2.3 Molecular weight
  • 2.2.4 Ortho- or isophthalic acid
  • 2.2.5 Hydroxyl- and acid value
• 2.3 Driers
  • 2.3.1 Primary driers
  • 2.3.2 Secondary driers
  • 2.3.3 Drying accelerators

Drying of air-drying coatings consist of two steps. The first part is a physical drying where the solvent(s) evaporates. In the second part of the drying the paint film cures due to the introduction of oxygen from the air into the molecules, where a three dimensional network is formed.

Thus many parameters influence on the drying of an air-drying coating. Here the main focus will be on the binder (alkyds), the drier systems and the solvent. Other raw materials, which are not directly involved in the drying process, can also have an influence. During application and drying, factors like film thickness, relative humidity, temperature and air velocity will affect the drying process.

2.1 Solvent

The amount and type of solvent is important for the physical drying, where the solvent(s) evaporates.

The evaporation rate of the solvent will influence on the coating drying rate. The faster the evaporation rate, the faster an initial oxidative drying of the coating. The oxidative drying of the surface of the coating will influence on the evaporation of solvent, as the solvent in the film has to be transported up through the surface layer. Due to this transport phenomena the evaporation rate will to some extent vary with different drier systems.

2.1.1 Theoretical evaluation of Initial drying mechanisms

As the binder system contains solvents the first step of drying will be a physical drying were the evaporation of the solvent is dependent of temperature as well as air velocity. The humidity is a secondary parameter as the solvents used are not hygroscopic.

To describe the mechanisms of this first drying step modelling of the evaporation was done with a computer programme called dG /3/.

The solvent used for thinning the binders in the screening was Exxsol D60, which is a dearomatised high boiling naphtha (aliphatic hydrocarbons and cycloparaffines). Boiling interval 180-217 ^oC and flash point 62 ^oC.

As a model for Exxsol D60 a combination of undecane (90 wt-%) and butylcyclohexane (10 wt-%) was used. This composition represents a calculated boiling point of 194 ^oC and a calculated flash point of 62,3 ^oC, which is rather close to the commercial raw material. Still it should be noted that the composition is not at all accurate and should be regarded as an example to evaluate the climatic parameters in connection with evaporation.

In the calculations it is assumed that approximately 50 gram of solvent is applied on 1 m². In table 2.1 is the evaporation rate for a simplified solvent phase calculated under different conditions.

Table 2.1 Calculated evaporation rates for a solvent phase comparable to Exxsol D60 under different conditions.

It is obvious that both the temperature and the air velocity has a significant effect on the evaporation rate and thus on the drying time of the solvent. This also shows that it is necessary to have the same climate during drying tests when drying of paint films is compared. It is though assumed that air velocity has a much higher influence on solvent evaporation than on oxidative drying. The temperature will of course have an influence on both types of drying mechanisms.

2.2 Alkyds.

Alkyds are condensation products of polyols, polybasic acids and vegetable oils or fatty acids. The properties of the alkyd are dependent on the type and amount of the oil/fatty acid and on the used acid anhydride as well as the processing conditions.

Several parameters in an alkyd have influence on the oxidative drying properties of an air-drying paint. Many of these parameters are dependent on each other for which reason changing one of them will affect the other parameters as well.

2.2.1 Oil type

The oil part of the alkyd provides the air-drying properties, due to the presence of double bonds. Both the content and type of double bonds in the oil part influence on the drying. Higher content of double bonds and more conjugated double bonds reduces the drying time.

2.2.2 Oil length

The oil length expresses how large the oil content in percent w/w is in an alkyd. Shorter oil length reduces drying time of the alkyd but also the final cross linking as the oxidative drying is quite limited. The durability of the coating is thus also reduced. In decorative air-drying coating the alkyds usually have an oil length that exceeds 45 % w/w.

2.2.3 Molecular weight

Increasing molecule size promotes high viscosity and quick drying because less cross linking is necessary to form a coherent film.

2.2.4 Ortho- or isophthalic acid

The choice of ortho- or isophthalic acid leads to alkyds with different properties. The latter promotes longer molecule chains and thus higher viscosity. If the two is exchanged on a one to one basis the isophthalic acid gives a shorter drying time.

2.2.5 Hydroxyl- and acid value

The lower the acid value, the higher the degree of esterification and a low value is therefore preferred. The hydroxyl value is fixed for any given acid value by the hydroxyl excess employed at the beginning of the condensation process of alkyd manufacture.

A higher excess of OH groups in the manufacturing process results in shorter alkyd oil length but a noticeable effect on the drying time is not observed unless the residual hydroxyl value exceeds 60-70 mg KOH/g while the residual acid value is below 5 mg KOH/g.

2.3 Driers

Driers, also known as siccatives, are metallic soaps containing either alkaline earth metals or heavy metals combined with monobasic carboxylic acids.

The main influence of the acid is to ensure solubility in the coating medium and the binder itself. Different acids are used commercially and this could influence the drying properties due to compatibility issues with specific binders. Below a short general description is given of the different type of driers. For a more thorough description of driers consult the report “Substitution of Cobalt Driers and Methyl Ethyl Ketoxime” /2/.

2.3.1 Primary driers

Primary driers are soaps of metals, which exist in several oxidative states, and which undergoes redox reactions. The most common are Cobalt, Manganese, Vanadium, Iron and Cerium. The primary driers are essential for the drying of air-drying products as they act as catalysts for the drying process, but they differ in their ability to undergo oxidation from a lower to a higher state. Co-driers are far the most active of these driers promoting a rapid surface drying of the coating. The driers do also, in varying degree, possess some through-drying properties.

Iron and cerium driers are not efficient at ambient conditions and can therefore in most cases not be considered as alternatives to Co-driers. This means that the most probable alternatives are manganese and vanadium driers, which also in their traditional form are less active driers than Co-driers, especially with regard to surface drying.

2.3.2 Secondary driers

These are soaps of metals that exhibit no catalytic effect on their own while they only exist in a single oxidation state. However together with a primary drier they assist in the polymerisation process by the formation of coordination compounds with a consequent increase in the drying rate and an eventual improvement in drying properties.

The most important functions of secondary driers in coatings are better through drying and minimizing loss of dry. There is a spread of secondary driers for different purposes calcium, zirconium, zinc, aluminium and barium being the most important.

2.3.3 Drying accelerators

Drying accelerators are organic ligands, which are able to increase the activity of primary drier metals causing a more rapid drying of the coating film. They function by complexing with the metal atoms forming chelates.

The use of drying accelerators have been utilised by the drier manufactures to make a new generation of manganese and vanadium driers, combining the traditional salt with a drying accelerator in one product, and hereby increasing the activity of the driers making them more suitable alternatives to cobalt driers. Alternatively can the users themselves combine a traditional primary drier with a drying accelerator to obtain an increased drying effect.

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Version 1.0 August 2006, © Danish Environmental Protection Agency