Tuesday, April 10, 2007

THIXOTROPIC ALKYD RESIN

New thixotropic alkyd resin family based on polyamide technology

Description

Introduction In the growing market for higher solid systems for decorative and ?Do-it-Your self? paints reduction in the amount of solvents whilst keeping paint performance on the same or higher level is the main objective. The options to obtain HS paints are to design alkyd resins with lower molecular weight, longer oil length or with reactive diluents. As a consequence the existing thixotropy technology often fails due to too weak structures. Additionally, HS alkyd paints are more sensitive to tinting systems based on water, surfactants and often glycols or similar solvents; the thixotropic structure is, therefore, not stable anymore.
This is the reason why a combination of thixotropic resins and rheology modifiers such as Bentones, clays, aerosil or hydrogenated castor oils and waxes are used to obtain the right flow-sag balance, resulting in a stepwise formulation of a complex paint. 1-4 Another shortcoming of these traditional paints and lacquers is the fact that they lose their structure-forming ability at elevated temperature e.g. above 45?C. The reason is the poor association capability of the hydrogen bonding network of the polyamide based modifiers or the too strong or permanent network building of urea based agents. 5-6 In some cases stabilizers, additional rheology agents or surface active liquids may help to obtain the desired rheological properties in the final paint formulation but such paints could become sensitive to changes in weather conditions, resulting in a number of unpredictable variations in thixotropy when thinning prior to application and may not remain very stable over a period of time. 7-13 The answer is simple: Incorporate the rheological properties into the resin instead of the paintformulation. Obviously the incorporation of proper rheological properties into the resins would make life a lot easier for paint manufacturers.
All these aspects support our opinion that there is a need for stronger and more stable thixotropic structures. Akzo Nobel Resins supplies a novel aramide based thixotropic resin family that fulfils these wishes. In this article a novel generation of thixotropic alkyd resins without the shortcomings of traditional resins is presented. State of the art The three basic building blocks causing thixotropy are the amide, the urethane and the urea group. Figure 1. During the past 50 years the main technology of thixotropic alkyds has been based on the reaction product of ethylene diamine and dimeric fatty acids. The amide bonds in this resin lead to shear thinning and thixotropy. These ?amide?-systems are often combined with isocyanates and mono-amines resulting in a mixture of urethane- and urea- based thixotropic resins. The urea blocks in particular lead to more permanent network formation resulting in tough gels having excellent thixotropic behaviour in polar solvents and are also more stable at higher temperatures. However these urea blocks also cause handling problems during paint production. Novel technology.
Recently it was found that if a polyamide resin based on aromatic amide moieties was reacted with a dimeric fatty acid it could be used to develop alkyd resins which show superior gel forming properties compared to those known in the market. From this technology a new family of thixotropic alkyd resins for decorative applications has been developed and introduced. This new family, based on aramide chemistry, exhibits a number of important features which make it eminently suitable for use in high solid and high gloss decorative paint- and stain applications. They may simply be used as let down resins in the final paint formulation. Such a paint also shows superior properties when compared to traditional polyamide based types or those based on urethane or urea chemistry 14-15 . Rheology measurements Rheology measurements were performed on a Bohlin CS-50 controlled stress rheometer at 25?C. All measurements were done after a pre-shear period of 10 minutes at a shear rate of 6500 s-1. Immediately after the pre-shear period the structure recovery at rest is monitored by so called oscillatory measurements; the sample is subjected to a sinusoidal shaped deformation as a function of time with an amplitude of deformation small enough to prevent distortion of the structure recovery (frequency of 1 Hz and a strain of 0.04 Pa).
In this way the linear visco-elastic properties can be measured, reflecting the viscous (G") and elastic (G?) response of the forming gel at rest. The maximum structure is a force which may be expressed in two parameters: the storage modulus (G?) for the elastic part of the liquid and the loss modulus (G??) for the viscous part of the liquid. The quotient (G"/G?) is well known as loss factor and describes the energy loss during deformation of a polymer. In this article the inverse of the loss factor: the storage factor (G?/G") is defined, because the structure is build up during heating and the thixotropy is formed in the resin after a certain period of time. The storage factor will increase during heating until a maximum has been reached due to strong associations of the hydrogen atoms in the poly-aramide molecules. Prolonged heating permits molecular chain growth while freedom of rotation diminishes, resulting in a loss of hydrogen bonding and a decrease of the storage factor again. It is important to stop this thixotropy forming process on the right moment because the condensation reaction will later predominate and will end in a chemical gel, so continuous monitoring of the reaction process is necessary. As thixotropy is a time dependent function the most important parameter is the recovery time defined as ?the time needed to restore the resin or paint structure after total destruction? and must be in the order of brush time e.g. between 1 and 15 minutes. During this period the resin flows initially while sagging must be limited. This flow-sag balance of the thixotropic resin or paint is dependent on solvent type, temperature and on the concentration of active amide material used. Experimental results.
Formation of the thixotropic structure Firstly, an aramide resin is synthesised based on pure raw materials, thus a well defined molecular distribution is formed. Several aramide molecules having different side chains onto the aromatic ring are suitable and can be used as long the nitrogen atom is directly attached to the aromatic ring. The so formed polymer has a very high softening point and must be dissolved in a suitable solvent like Xylene or White Spirit. In the next step the aramide resin is heated with a long oil alkyd at high temperature and the rate of gel formation is followed using a light transmission measuring method. After a certain period of time the alkyd has formed its maximum gel structure. From the four chemical routes described above some rheological measurements have been done including G?, G" in different solvents and in dependence of the temperature. The same experiments have been done in the polar solvent butyl glycol. In figure 5 graphs of the different gel types, 25% of the thixotropic alkyd dissolved in butyl glycol are shown. The aramide based resin starts gel forming structure recovery at a lower G?-level compared to the urea system having a weaker recovery speed in the elastic region. After a period of flow G? increases rapidly crossing the curve of the urea gelkyd. The urethane based gelkyd stays flowable for much longer time while the classic amide and the polyester modified aramide gelkyd lose their structure completely. The dependence of temperature has also been measured. Here again the aramide based thixotropic alkyd shows a much stronger resistance against higher temperature than the other technologies. The urea curve however, starting with a low G? value at room temperature shows an increasing structure at elevated temperature, which is paradoxically non melting but possesses increased entanglement of linear polymeric chains. In fact it exhibits an extra strong structuring ability which might be explained by the fact that this molecule is twice as likely to form hydrogen bridges between the amide-amide bonds. In practice this phenomenon leads to handling problems in industrial use. 15 . The polyamide on the contrary loses structure completely at higher temperature, say above 45°C, under normal dispersion conditions.
While the polyester modified aramide exhibits an intermediate structure resulting in good handling, it still has sufficient thixotropic structure left By monitoring the amount of polyester in the aramide formulation any thixotropic strength can be predicted. Using molecular modelling and minimisation of energy by the Monte Carlo method at absolute zero point temperature (0°K) it is possible to explain the phenomena found above. In a pair of aramide molecules the most probable conformation is formed showing strong association forces between the hydrogen from the oxygen atom and the nitrogen atom of the neighbouring amide group. Discussion All these effects may be understood by the nature of the aramide molecule: The rigid aromatic molecule has less freedom of torsion with respect to the amide bonds and so the hydrogen bonds are stabilised by mesomeric effects of the aromatic ring in interaction with the nitrogen of the amide bonds directly attached onto the ring. When increasing the temperature absolute zero point, i.e. 0°K to 300°K (room temperature) it is calculated by molecular modelling experiments that the H-bond bridges in the pair of aromatic diamide molecules is still intact while a comparable pair of molecules based on Butane-diamide with a comparable molecular distance of four carbon atoms between the active Nitrogen atoms loses its structure at one side of the complex. It follows, therefore, that the aliphatic amide polymers lose their structure easily because no stabilising groups are available nor can effective hydrogen bridges be formed.
It is noted once again that the aramide based resin has the strongest ability to form an elastic structure due to very efficient hydrogen bridge formation of the amides attached on the aromatic ring. The Hydrogen bond strength is easily calculated using the method of bond order calculation defined by L.Pauling. 17 This method is further correlated in modelling experiments with crystal X-ray diffraction by J.Aerts and G.King 18 . A simple relation has been presented which is also valid below 1: n = 10 exp(1-D(n)) where n = bond order and D(n) is bond distance in ?. For the poly aramide complex the PPD diamide pair of molecules was used to calculate the hydrogen bond order. In butylglycol at 25°C the value of 0,12 was found to increase slightly to 0,13 at 125°C. For the Polyurea a pair of butane di-urea molecules was used as a model. In butylglycol at 25°C a high value of 0,29 was calculated remaining more or less the same value at 125°C.

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