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Editorial archive

A new polyester resin for beta-hydroxalklamide powder coatings

Saturday, 1 May 1999

Publication: European Coatings Journal

Issue: 5/1999

Daniel Maetens, Luc Moens, Luc Boogaerts, Kris Buysens

A carboxyl terminated polyester resin, of novel molecular composition, has been developed for use in powder coatings containing beta-hydroxyalkylamide (HAA) type hardeners. The design of this new
resin enables powders to be formulated that exhibit outstanding flow without sacrificing other properties, such as acid rain resistance, or colour stability on overbake.

Safe alternatives for TGIC

For many years, triglycidylisocyanurate (TGIC) has been the standard hardener for outdoor polyester based powder coatings. Due to suspicions concerning its toxicity and consequent labelling, safe
alternatives for TGIC have been developed and promoted in recent years. beta-Hydroxyalkylamides (HAA), known under the trade name ?Primid? (Ems-Chemie), have become one of the major cross-linking
compounds replacing TGIC [1]. Typical application fields are architecture (e.g. window frames), metal industry (e.g. garden furniture) and automotive (e.g. window/door frames).
Polyester/beta-hydroxyalkylamide powder coatings exhibit strong worldwide growth and are continuing to gain market share. Though, the combination of carboxyl functionalised polyester resins and a
beta-hydroxyalkylamide type hardener has become an established curing chemistry for exterior durable powder coatings, suitable for a wide range of applications, there are still improvements which
can be made in the polyester resins.

All advantages are in one and the same polyester resin

A resin combining outstanding flow with excellent corrosion resistance when formulated with HAA was not available till now. Recently, a new type of polyester resin combining most of all these
advantageous properties in one and the same product has been designed in the UCB Chemicals Research laboratories. The high gloss paints based on the new resin and HAA exhibit

? an outstanding flow

? improved degassing properties

? excellent corrosion resistance

? excellent UV-resistance

? excellent overbaking and gasoven resistance

? excellent flexibility

The new resin can be formulated with both N,N,N?,N?,-tetrakis(2-hydroxyethyl)adipamide (?Primid XL-552?), the standard grade and already commercialised on a large scale, as well as with
N,N,N?,N?,-tetrakis(2-hydroxypropyl)adipamide (?Primid QM-1260?), a new HAA designed for improved gas oven resistance [1].

Low viscosity with an excellent hydrolysis / UV resistance is very difficult to design

Besides the particle size (distribution) of the powder and the surface tension of the melted powder [2], the flow of powder coatings depends on:

? the reactivity of the powder (gel time)

? the melt viscosity at the curing temperature

The melt viscosity of a polymer with a well defined molecular weight is mainly influenced by its basic monomer composition. Incorporation of plasticizing monomers in the polyester backbone, lowers
its melt viscosity and consequently improves flow. The available polyester resins used for outdoor applications are essentially based on aromatic diacids, such as terephthalic and isophthalic acid,
and neopentyl glycol as the diol compound. As the presence of plasticizing monomers results in a reduction of the glass transition temperature of the polymer, and consequently in a reduced storage
stability of the formulated powder, the percentage of these monomers, present in the resin, should be limited.

Besides, the introduction of these monomers in the polyester backbone was found to have a negative influence on corrosion resistance and/or UV-resistance of the final coating. In conclusion, one
can say that a resin combining a low viscosity with an excellent hydrolysis/UV resistance is very difficult to design.

Reactivity only depends on the structure of the polyester

In comparison with the esterification of normal aliphatic alcohols, where temperatures of above 225 °C or catalysts are necessary, b-hydroxyalkylamide (HAA) already reacts with acid groups at
relatively low temperatures starting at 150 °C [3, 4]. The increased reactivity can be explained by the fact that the esterification of HAA is carried out through an oxazolinium intermediate [5,
6]. This implies that, in contrast with TGIC based powder coatings, the reactivity of carboxyl functionalised polyester resins towards HAA can not be regulated using reactivity-modifying additives.
This has the advantage that reactivity is affected neither by the formulation nor by the nature of the pigmentation, but only depends on the structure of the polyester. On the other hand, as the
reactivity at 180 °C of the existing polyesters with HAA is quite high, the coating has less time to flow out properly resulting in an increased orange peel effect.

The new resin should get outstanding flow with excellent corrosion and UV resistance

The main objective of the current study is to design a resin that combines outstanding flow with excellent corrosion and UV resistance. The new polyester resin, designed to lower both the melt
viscosity and the reactivity towards HAA, is obtained from a particular monomer composition in combination with a particular process as developed and patented by UCB Chemicals [7]. The main
characteristics of powder coatings based on this new resin, formulated with either ?Primid XL-552? or ?Primid QM-1260?, and the comparison with those obtained from beneficial commercially available
polyester resins, having standard outdoor weathering resistance, are outlined below. Not only flow and weathering resistance, but also some other important properties are confirmed.

Materials

Reference polyesters and their characteristics (Table 1):

- PE1 = commercial ?Primid XL-552? resin known for its excellent corrosion resistance, excellent mechanical properties and good flow.

- PE2 = commercial ?Primid XL-552? resin having an excellent flow, improved overbaking and gasoven stability; pinhole-free in thicker films, but lower corrosion resistance.

- PE3 = commercial standard ?Primid QM-1260? resin

Formulations / Processing

Most of the tests described below are done on a standard ?White RAL9010? formulation (Table 2).

For testing corrosion and UV-resistance, a green and a brown formulation respectively are used (Table 3).

? Extrusion: Prism, 16 mm L/D 15/1 double screw extruder; Barrel temperature 80°C, 250 rpm; Torque 65-80%

? Grinding: Retsch ?ZM100?

? Powder is applied to chromated aluminium (Cr to the 6+, DIN 50939, 1 mm thickness) panels using Ransburg ?Gema PCG 1? spraygun

Flow curves

Dynamic Rheology measurements presented below are done using the ?Stresstech Rheometer? (Reologica Instruments AB) with a high temperature cell and disposable cone and plate fixtures. The real part
of viscosity or dynamic viscosity eta' is measured using the Oscillation Controlled Strain Mode (Constant frequency: 1 Hz, Strain: 2%)

The melted powders are first allowed to stabilize for one minute at a temperature of 120 °C and subsequently ramped to 130 °C. The actual measurement is started at 130 °C (time = 0) and the
temperature is then ramped up to 180°C using a heating rate of 20 °C/min.

Wave-scan measurements

In order to demonstrate the relationship between the rheological behavior of the powder upon curing and the surface smoothness of the powder coating, wave-scan measurements are done using the
?Wave-Scan? as developed by Byk-Gardner [8]. During a wave scan measurement, the apparatus is moved across the surface over a scan length of 10 cm with a data point being recorded every 0.08 mm.
The measured data of the brightness profile (modulation of reflected light) are first separated by mathematical filtering into a long-wave (structural size >0.6 mm) and a short-wave (structural
size <0.6 mm). The values of ?long waviness? (structures from 10 to 0.6 mm) result from the variance of the filtered data and can be correlated with what can be seen with the naked eye. The
measuring scale goes from 0?100, with lower values indicating a smoother surface.

Corrosion resistance

Corrosion resistance of the paints is determined in an effective and reproducible way by exposing the panels in acidic/humid conditions: Kesternich using 2.0 liter of sulphur dioxide; 45 °C during
8 h alternating with degassing at ambient temperature during 16 h. Tests are done on dark green coatings (RAL 6005) and colour change (Dela b*) is reported as the dark green, made from a blue
pigment, a yellow pigment sensitive to acids, and black pigment (see above), becomes blue upon sulphur dioxide exposure.

Weather resistance

The cured coatings are submitted to the QUV accelerated weathering tester (Q-Panel Co) according to ASTM G53-88. Panels are subjected to the alternate effects of condensation (4 hours at 50°
centigrade) as well as the damaging effects of sunlight simulated by fluorescent UVA lamps (340 nm / I = 0.77 W/m2/nm) (8 hours at 60° centigrade).

Overbaking / Gasoven resistance

Baking is done in an electrical heated, mechanically ventilated oven. Standard conditions are 180 °C during 10 minutes metal temperature (18 minutes total curing time). Overbaking conditions are
given in Figure 6 and resistance is expressed by the Yellowness Index (YI, ASTM D313). The gas oven stability test is obtained by simulation in the same oven containing NOx made from a mixture of
sodium nitrite and acetic acid; resistance is expressed by Delta YI taking curing in the NOx free oven at the given temperature as the reference. All measurements are done on films having a
thickness of 60±3 µm.

Flexibility

Flexibility of the film is evaluated by testing reverse and direct impact strength according to ASTM D2794. All tests were done on chromated aluminium (Cr6+, DIN 50939, 1 mm thickness) panels at 22
± 1 °C after a one hour relaxation period.

Visco-elastic behaviour can be described using the storage and the loss modulus

The flow of a powder coating depends mainly on the reactivity and the melt viscosity of the powder at the curing temperature and can be expressed in terms of the rheological behaviour during the
melting/curing phase [9]. In fact, during cross-linking, the melted formulation evolves from a viscous system into a system having visco-elastic properties. The visco-elastic behaviour of a
material can be described using two parameters, G'(omega) and G''(omega), the storage and the loss modulus. These are determined by the shear stress tau, the strain gamma and the phase shift delta,
the parameters measured during a rheological investigation of a material [10].

The rheological properties of ?PE1?, ?PE2? and the new resin formulated with ?Primid XL-552? on the one hand and ?PE3? and the new resin formulated with ?Primid QM-1260? on the other hand, in a
standard white formulation were compared by determining the dynamic viscosity eta' of the formulations before and during curing (Flow Curve). This parameter gives a good indication of the effective
viscosity and the reactivity of the melted formulation, altogether indicative for the flow of the final coating. The dynamic viscosity eta' can be determined on the basis of the measured parameters
tau, gamma and delta via the loss modulus G''.

eta' = G''/ omega (with omega = 2 pi ny and ny = the frequency)

Flow behaviour and general appearance of ?Primid XL-552? based powder coatings

Observations (Figure 1):

? The formulation based on PE1 proved to have the highest viscosity all over the whole curing cycle.

? Besides, PE1 is the most reactive one; as a consequence h?min (minimum viscosity just before curing starts) is considerably higher compared to the other formulations, and a faster increase of
viscosity upon curing is observed.

? The formulation based on the new resin has the lowest reactivity and h?min; even compared to the formulation based on PE2 (commercial high flow resin), an improved rheological behaviour clearly
appears.

? The low reactivity of the new polyester can also be deduced from the fact that dynamic viscosity only increases slowly once curing has started.

Flow behaviour and general appearance of ?Primid QM-1260? based powder coatings

Observations (Figure 2):

- Also in this case, the formulation based on the new resin shows a reduced reactivity, resulting in a longer time to flow out and a lower eta' min.

The best flow is obtained with the new resin

The effective flow of the final coatings, obtained after an 18 minutes (total time) at 180 °C curing cycle in a ventilated electrical oven is compared by doing wave-scan measurements (Tables 4?5).
Flow of the coatings was evaluated at film thickness of both 60 µm and 80 µm.

As appears from Table 4 and 5, the lowest value for ?long waviness?, indicating the best flow is obtained for the formulations based on the new resin, independent of whether ?Primid XL-552? or
?Primid QM-1260? is used as the crosslinker.

Degassing during curing

Powder coatings based on HAA tend to be more sensitive to pin-holes than conventional TGIC-based powders. This limitation can be greatly reduced by the selection of an optimised polyester, combined
with good formulating practice. A resin with reduced viscosity and reduced reactivity, such as the new one, has a longer time to flow out and will give better degassing properties. The use of an
optimised quantity of 0.2% of the degassing agent benzoin is used in the ?RAL 9010? formulation used for the tests. Evaluation with the naked eye is done on panels coated with a layer increasing in
thickness from 50?150 µm. The value, expressed in micron, given in Table 6 indicates up to which thickness a pinhole free film is observed.

New resin shows the lowest melt viscosity and the lowest reactivity

The wave-scan values and the smoothness of the white powder coatings evaluated above are in good correlation with the rheological behaviour of the corresponding formulations during the melting and
the curing phase. The new resin shows the lowest melt viscosity (eta' min) and the lowest reactivity compared to the standard resins both formulated with ?Primid XL-552? and ?Primid QM-1260?.

Pinholing due to degassing problems is only observed in films with a thickness above 125 µm.

New system shows hydrolytic stability

The hydrolytic resistance of polyester based powder coatings is to a great extend dependent on the polyester monomer composition. Monomers usually incorporated in the polymer chain for reducing its
viscosity are known to have a negative effect on the weathering resistance of the coating. The particular composition as used in the new polyester assures a very good hydrolytic resistance of the
final coating.

In this study, corrosion resistance is tested in a Kesternich cabinet. This type of test gives an excellent indication of the hydrolytic stability of the cured film in a reproducible way. The new
resin is tested in a green ?RAL 6005? formulation and compared with ?PE1? and ?PE2? in the same formulation (Figure 3)

Observations (Figure 3):

? ?PE2?, known as a resin giving an excellent flow with ?Primid XL-552?, degrades quite fast under the acidic / humid conditions

? The new resin shows comparable resistance as ?PE1?, known in the market as the reference for excellent corrosion resistance

Powder coatings based on ?Primid QM-1260? are known to have an increased hydrolytic resistance compared to those based on ?Primid XL-552? [10]. As expected, both the new system as well as standard
?PE3? resin, show an excellent hydrolytic stability.

Monomer composition does not prove any negative influence of UV-resistance

UV resistance is tested in a Medium Brown ?RAL 8014? formulation using accelerated conditions. UVA (340 nm) bulbs are used; this type of lamps provides up to around 350 nm a good correlation to the
solar spectrum and therefore produces quite a good correlation with natural exposure. Results in Figure 5 represent the relative gloss as a function of exposure time for the ?Primid XL-552? based
powder coatings.

As appears from Figure 5, the particular monomer composition, as for the new resin, does not prove any negative influence on UV-resistance.

Overbaking and gasoven resistance

Polyesters, such as ?PE2?, are designed for very good overbaking and gasoven stability when used in pale shade formulations with ?Primid XL-552?. In Figure 6 and 7, overbaking and gasoven
resistance respectively is compared for the new resin and ?PE2?. To further improve the gasoven stability, a new HAA-type hardener, known under the trade name ?Primid QM-1260? is used. The new
resin is compared for gas oven stability with the standard resin ?PE3?. As can be seen from the Figures 6?8, the new resin, when compared to the reference resins, performs better.

New resin is found to be as good as for the references considering mechanical properties

The standard resins used as reference have excellent mechanical properties. The flexibility of a film (at different thickness) based on the new resin / ?Primid XL-552? or ?Primid QM-1260? is
evaluated by testing reverse and direct impact strength on chromated aluminium (Cr6+). Impact resistance of the coatings based on the new resin is found to be as good as for the references (Table
7).

General conclusions

As part of our continuing research effort to improve our range of polyester resins specifically tailored for cross-linking by HAA, we have designed a resin combining outstanding flow and excellent
corrosion resistance. Moreover, the particular monomer composition of the new resin enables powders to be formulated of good colour stability on overbake and in direct-fired gas ovens. Other
properties, such as UV-resistance and flexibility are excellent, and fulfil the requirements of the architectural and metal industry. Combining all these qualities, the proposed system seems to be
an excellent alternative for TGIC containing powder coatings, which have been the standard for exterior durable powder coatings for many years.

[8 figures, 7 tables (see tiff or pdf on page 4 to 6)]

LIFELINE: Daniel Maetens, Luc Moens, Luc Boogaerts and Kris Buysens are with UCB Chemicals, R&T Powder Coatings in Drogenbos/Belgium

References

[1] A. Kaplan, European Coatings Journal, 6, 448?452 (1998)

[2] V. V. Verkholantsev, European Coatings Journal, 5, 360-365 (1998)

[3] G. Swift, H. J. Cenci, US. Patent 4076917 (1976)

[4] J. Lomax, G. Swift, J. Coat. Technology, 50 (643), 49?55 (1978)

[5] Z. W. Wicks Jr, M. R. Appelt, J. C. Soleim, J. Coat. Technology, 57 (726), 51?61 (1985)

[6] D. Stanssens, R. Hermanns, H. Wories, XVIII International Conference on Organic Coatings, Athens, July 6?10 (1992)

[7] UCB Chemicals patent; pending

[8] Byk-Gardner, Application report wave-scan, Germany (1992)

[9] M. Osterhold, F. Niggeman, Prog. Org. Coat. (1998), 33, 55?60

[10] J. D. Ferry, Viscoelastic Properties of Polymers, Wiley, New York (1980)

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