High-Solids Alkyd Strategy
by Anders Clausson
Mircea Manea
Kent Sörensen
Susanne Stigsson
August 1, 2007
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”.
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.
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
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.
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.
This
should correspond to a viscosity of 10 6 mPas (cP) while the
theoretical T g for 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 (Tg) 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 Tg or in case
we use a solvent.
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 Tg,
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.
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.
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 Tg. 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.
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 g of +26 ºC may be built of phthalic
anhydride and neopentylglycol as in Figure 9.
This
segment is further shielded by esters from vegetable fatty acids and highly
functional polyols. This results in low-T g
moieties that will provide the oxidative drying. The T g
of such moieties is lower than -32 ºC, depending on the acids involved.
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.
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 g of 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).
The
oil length of the structures shown in Figure 10 are related to the length and
T g of 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.
This
allows a serious increase in molecular weight without a significant impact on
the final viscosity.
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.
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.
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.
Drying Performance Evaluation of the Concept
Clear coatings were prepared as shown in Table 4;
performance properties are shown in Figures 14 and 15.
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.
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.
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.
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.
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.
Bibliography
1. Hubert, J.C, Vanderbosch, RAM, Muizebelt, Klaasen, RP,
Zabel, KH –J. of Coatings Tech., vol 69 | 
