High-Solids Alkyd Strategy

This article presents new possible approaches for alkyd resin architecture in order to comply with market constraints related to VOC emissions. Examples and performance of alkyd concepts are presented with regard to VOC content and performance for “interior/exterior trim and cladding paints for wood and metal”.

Table 1

Background

New regulations related to emissions/VOC content in coatings and increasing oil prices strongly recommend the shift to waterborne and high solids for coating manufacturing technology.

In Directive 2004/42/CE, from the European Parliament and the Council, limitations are established for the maximum VOC in decorative paints to be used within the EU. The products covered by the Directive are for use on buildings, trims, fittings and structures associated with buildings. Specific sub-categories are listed in the Directive with different limitations for maximum VOC in g/L of the ready-to-use product with two sets of limits for each sub-category. The first set of limit values will apply from 1 January 2007 and the second, stricter set, from 1 January 2010. Limits are established for both solventborne and for waterborne paints. The target levels are presented in Table 1.

To reach these targets, along with the continuous development of waterborne systems is the possibility of finding new ways for high-solids technology.

Table 2

Definitions

There is no general consensus for defining high-solids systems. However we list below some of the most commonly used.

A system is named “high solid” when it contains a max 350 grams VOC/litre of coating.10
There are different regulations in the United States, e.g., max 420 g/L and max 250 g/L depending on application and state.10
A paint is named “high solid” when it has a VOC content of max 420 g/L at application viscosity.9
The phrase “high solid” sometimes is used for all kinds of paint systems with a non-volatile content higher than average. This is very misleading because the average non-volatile content differs from application to application.9
A high-solids coating typically contains greater than 60% solids by weight or 80% solids by volume.8
Lacquer with very high non-volatile content (>70%).11

Figure 1

Specific Approach for High Solids

Formulation of high-solids systems requires attention with respect to the choice of all involved components. Here we have listed some of the factors that influence the selection:

binders of high reactivity;
binders of low viscosity/low molecular weight;
molecule design;
pigments/fillers of low oil index;
wetting additives; and
solvents that reduce the number of H-bonds.

Table 2 shows the comparison of high-solids systems with conventional systems.

Alkyds are type B polymers that exhibit high functionality but generally have a low glass transition temperature. The Tg and hardness are built up in the drying process.

In the oxidative process, the difficult part is that crosslinking does not occur by reaction of the alkyd with another component (crosslinker) and by subsequent removal of functional groups that have a plasticizing effect, but through polymerization of double bonds (delivered by the fatty acids). Hence, the final molecular weight is not as high as a thermoplastic polymer, and the hardness development is not always as high as expected. The phenomenon occurs as the air/oxygen kicks off the autoxidation of the alpha position from the double bond on the fatty acids in a complex process of radical polymerization assisted by metal dryers. This process results in the linkage between the fatty acids whose bonds may be a carbon-carbon bond, an ether bond or a peroxide bond. It is commonly accepted that driers only assist in the process of hydroperoxide decomposition and that higher temperatures and highly conjugated fatty acids favour the formation of carbon-carbon bonds.

Figure 2

Low viscosity may be obtained by using groups that may act as built-in plasticizing moieties. However this approach has a negative impact on the Tg of the alkyd (physical drying) and may increase the water sensitivity.

Basically an alkyd binder has a drying diagram as shown in Figure 1, while a high-solids alkyd has a much longer drying time due to a very low Tg, which is built by the oxidative reaction as depicted in Figure 2.

Considering the drying process itself, and defining drying as an expression of viscous behaviour, according to the Williams-Landel-Ferry equation (Figure 3), the dry-to-touch performance at 25 ºC (viscosity measurement temperature) is achieved at a Tg as low as -29 ºC.

Figure 3

This should correspond to a viscosity of 10⁶mPas (cP) while the theoretical T_gfor a coating that should pass the blocking resistance at 25 ºC (viscosity measurement temperature) should be +4 ºC.³

The viscosity of an alkyd resin is a function of free-volume availability. The free-volume of a material is the summation of the spaces or holes that exist between molecules of a material resulting from the impact of one molecule or molecular segment striking another. These holes open and close as the molecules vibrate. Above the glass-transition temperature (T_g) the holes are large enough and last long enough for molecules or molecular segments to move into them leading to low viscosity. Free volume increases as temperature increases; the rate of volume increase is higher above T_g or in case we use a solvent.

Figure 4

So the major problem is how to deal with the almost non-existent physical drying required by the desire for a larger free-volume and with the conditions of keeping the molecular weight as high as possible!

The answer may be the design of the alkyd binder itself. Basically this means moving from low free volume to higher/high free volume (Figure 4) in an almost 100% non-volatile content.

Allowing the alkyd binders to have a reasonable molecular weight and at the same time shielding a segment that has a reasonable high T_g, the hardness build up will be transmitted to the coating in the drying process. The shielding is necessary to provide the low viscosity, while the architecture itself must allow a sufficiently reasonable free volume, leading to a reasonable application viscosity in conditions complying to the high-solids definitions.

Figures 5 & 6

The immediate response to this problem may be found in designing the alkyd binder. It is well known that dendritical structures give lower viscosity, but also low hardness development. This may be due to the fact that the core of the dendrimer must be a soft segment in order to secure good mobility/reactivity for the groups intended to build further layers. However, even in the case where the core is sufficiently hard, due to the symmetry of the dendrimer and the fact that the dendrons are built by soft segments, the hard core is so strongly shielded by the branching arms that physical drying is almost insignificant.

Figure 7

Considering a dendritical structure as shown in Figure 5, the core segment is not visible in the drying process, in order to exhibit the physical drying properties.

The pendulum is measuring oscillations in the soft shell of the molecule. Therefore the molecule should be shaped as much as possible, as in Figure 6, in order to allow the hard core of the molecule to be visible in the drying process.

Therefore, the alkyd binder should be built as a discoid structure (Figure 6), allowing a higher mobility and a lower shielding of the hard core of the molecule. The core may be shaped in order to control, by raw materials and length, the T_g. A shape as shown in Figure 7 may be very suitable to comply with these requirements and is easy to synthesize using the usual raw materials for alkyds, using semi-fabricates thereof, or loaded and reacted in a suitable order.

Considering the architecture shown in Figure 7, the molecular weight may be grown on the dendritical side of the molecule, trying to preserve the architecture as shown in Figure 8. This may be performed by reacting hydroxyl-functional dentritical moieties of low OH number on anhydride-functional moieties as will be further explained.

Figure 8

Thus the segment to be shielded may be generated from usual raw materials such as di-basic acids and diols, preferably 1,3-diols. The most common segment having a T_gof +26 ºC may be built of phthalic anhydride and neopentylglycol as in Figure 9.

Figure 9

This segment is further shielded by esters from vegetable fatty acids and highly functional polyols. This results in low-T_gmoieties that will provide the oxidative drying. The T_gof such moieties is lower than -32 ºC, depending on the acids involved.

Figure 10

The example in Figure 10 is a building block based on di-pentaerythritol and tall oil fatty acids. These building blocks may be esters of pentaerythritol, trimethylolpropane, di-trimethylolpropane, etc.

Figure 11

The oil length of the alkyd may be controlled by exchanging the fatty acids for other acids. However these acids should be selected in such a way that they do not affect the desired T_gof the building block. Thus the use of benzoic acid is not recommended, while the use of an adduct from dicyclopentadiene with a diacid (Figure 11) has a positive result in terms of preserving the T_g, reducing the oxygen inhibition and speeding up the drying, as well as not requiring any reduction of the spacer as defined above as offering the same shielding properties as a fatty acid (Figure 11).

Figure 12

The oil length of the structures shown in Figure 10 are related to the length and T_gof the spacer as described above (Figure 9).

The further growth of molecular weight according to the architecture in Figure 8 is performed by reacting maleic anhydride on the fatty acid moieties in the dendritical structure as in Figure 10 and further reaction of the said dendritical polyol fatty acid esters to those moieties. The resulting moieties from the reaction of maleic anhydride on the fatty acids are presented in Figure 12.

Figure 13

This allows a serious increase in molecular weight without a significant impact on the final viscosity.

Figure 14

Experimental and Discussion

In order to make a proper evaluation of the idea, a typical long oil alkyd was prepared and compared to alkyd binders obtained according to the concept. Furthermore, in order to maintain some functional reactive groups on the binders according to the concept, it was necessary to fulfil other requirements related to the coating formulation such as adhesion, pigment wetting, etc. For the 1,3-diol in the hard segment, the choice was to use BisMPA (dimethylol propionic acid) to control the synthesis in such a way as to preserve the larger part of the carboxylic group unreacted in direct or side reactions.

Table 3

The typical alkyd resin was synthesized according to the generally accepted cooking protocol for alkyds, at a temperature of max 235 ºC. See Table 3.

The concept alkyds may be synthesized in steps: the first step is to generate the strong segment as described in Figure 9, which is further loaded to the branched ester moieties of polyol and fatty acids. However it is possible to run the synthesis in one shot by carefully observing the correct order of loading the raw materials in order to lead to the desired structure at temperatures close to, or slightly higher than, the melting point. Concept alkyds have been synthesized according to the following protocol.

Figure 15

Charge oil fatty acid, di-pentaerythritol, xylene (4% on raw materials), and Fascat 4100 (0.1% on raw materials).

Start heating. Raise the temperature with 4 ºC /min to 160 ºC.

Raise the temperature to 220 ºC with 1 ºC /min. Hold until the final acid number is about 3 mg KOH/g.

Cool down to 180 ºC and charge the phthalic anhydride. Wait until the anhydride ring opens on the core ester.

In case the concept alkyd is according to Figure 8, drop the temperature to 120-130 ºC and then add maleic anhydride.

Start heating and when the alkyd is clear at about 140-150 ºC add Bis-MPA under stirring in small portions so it does not fall down to the bottom.

Raise the temperature to 200 ºC. Hold until the final acid number is 15-25 mg KOH/g.

Vacuum at 160-170 ºC to remove the xylene.

Table 4

Drying Performance Evaluation of the Concept

Clear coatings were prepared as shown in Table 4; performance properties are shown in Figures 14 and 15.

Figure 16

In addition, the solidity of the approach has been tested and proven by AFM (Atomic Force Microscopy) as shown in Figures 16 and 17.

The particles in the AFM images have a large size distribution. A likely explanation is that the samples were too concentrated. Several polymer molecules could then merge forming larger agglomerates. The large particles in Figures 16 and 17 could be such agglomerates. However, when the sample was further diluted, contact between the AFM tip and the sample could not be achieved. Areas with few or no big particles could be identified, Figure 16, and size analysis of these areas was performed and is shown in Figures 18-20. The section analysis of the height images gives vertical sizes ranging from ca 1.8 µm to 4 µm for those small particles.

Figure 17

The horizontal distances as well as the vertical distance were obtained from the section analysis of the height images. These distances were a lot longer than the vertical ones, approximately 16 times longer in most cases, Figures 18-20. This indicates that the particles/molecules are flat. But there is a possibility that this effect is caused by a large pressure from the tip of the AFM onto the sample, which causes the particles/molecules to flatten a bit.

Figure 18

Conclusions

Considering the drying performance and the VOC content in formulations that are aromatic-free we may assume that the approach has been successful.

The drying performance of the typical alkyd is lower than the performance of both concept alkyds as described above. The blends of the typical alkyd with any of the concept alkyds may impart both hardness and drying performance. The approach may comply with the request of VOC emissions of 2007 and even 2010. Some trimming of the formulations may be necessary and is left to the customer discretion, but this may be considered a reasonable starting point. The binders as presented here are filling the drying behaviour gap between typical alkyds and general high-solid alkyds or simple dendritical structures. The concept binders may be used as such or in combination with typical alkyds as reactive solvents without the risk of altering the drying behaviour or other general properties.

Figure 19

The AFM results may show that it is a reasonable assumption that the discoid shape as in Figure 6 for a concept as illustrated in Figure 7 may have been achieved with a good impact on free volume/viscosity and typical alkyd comparable drying properties.

For more information, visit www.perstorp.com.

Figure 20

Acknowledgements

The authors would like to thank the Market & Sales people (Per-Erik Velin, Kent Hamacek) from Perstorp Specialty Chemicals AB for encouraging this project, as well to Stefan Lundmark and Anna Andersson, from the University of Lund, Sweden.