Pigment stabilizers in powder coatings

Pigment stabilizers in powder coatings

F O C U S carrying out extended trials on various substrates, ranging from metals to paper, but they have also engineered a method that is capable of ...

34KB Sizes 2 Downloads 20 Views

F O C U S carrying out extended trials on various substrates, ranging from metals to paper, but they have also engineered a method that is capable of high speed application of continuous smooth films at thicknesses around ten microns. The paper presented at PCE 2002 has since been complemented by demonstrations to invited audiences in the DSM workshop at Zwolle. This is a much needed breakthrough in the can coating market and will further extend the influence of powder coatings in market outlets which were, for many years, considered to be unattainable. Industry news continues to be dominated by reports of further restructuring in the coatings industry as the major producers seek to reduce the losses of the past two years. It is apparent that liquid coatings interests are the first targets for divesting or rationalizing their presence in the coatings markets. Powder coatings, however, remain largely unaffected by these moves and, in many instances, this area of coatings technology is being strengthened. Raw material suppliers are certainly showing renewed interest in powder coatings. It does seem that powder coatings will emerge largely unscathed from the current setbacks in the industrial coatings industry. Sid Harris

TECHNICAL Pigment stabilizers in powder coatings Appearance and mechanical properties of pigmented coatings are largely influenced by the state of dispersion of pigments and extenders. In general terms, dispersion of pigments in coatings involves: wetting of pigment agglomerates; break-down of these agglomerates into primary 2

O N

POWDER

particles; and the colloidal stabilization of the dispersed primary particles. In highly viscous systems such as powder coatings, dispersion of pigments is more difficult than in solvent containing coatings, due to the slower wetting of pigments by the binder and the relatively short mixing time during hot melt extrusion due to thermal reactivity of the thermosetting systems. Often the pigment dispersion is incomplete and this causes roughening of the coating surface giving coatings with poor flow, low gloss and increased haze. Powder coatings containing coloured pigments do not always develop the colour strength of the pigment, and reproducibility between different batches is poor. A recent development is the use of pigment masterbatches containing pigments that are predispersed at high concentration in a low molecular weight resin. However, the use of block copolymer dispersants in powder coatings represents a new approach to resolving this problem. Work carried out at the Eindhoven University of Technology examined four poly(2vinylpyridine)-b-poly(εcaprolactone) copolymers as pigment dispersants for titanium dioxide in a polyester powder coating resin. In these dispersions the poly(2-vinylpyridine) block (P2VP) is the anchor block and the poly(ε-caprolactone) block (PCL) is the buoy block. The four copolymers differed in buoy block length ratio and anchor/buoy block length ratio. The initial laboratory work was based on a commercial carboxylic acid functional polyester resin (Uralac P3400 from DSM Resins), titanium dioxide (Kronos 2160), flow agent (Byk 360P) and benzoin degassing agent. To exclude specific effects from curing agents these were not included in the trials. P2VP-b-PCL copolymers were prepared by sequential anionic polymerization.

C O AT I N G S The powder coatings were prepared by extrusion of dry blends containing 30wt% TiO2 (11vol%), 1% flow agent and 0.5% benzoin using a laboratory twin screw extruder. After grinding and sieving at 100 mm mesh, the powder was sprayed electrostatically and placed vertically in an oven and heated for 10 minutes at 150°C and 200°C. The block polymer dispersants were employed in two ways. In the first set of powder coatings, the block polymers were mixed with the pigments prior to extrusion, by dispersing ultrasonically the TiO2 in a solution of the block polymer in dichloromethane. After removal of the solvent, the pre-treated pigments were added to the dry mix. A control was produced by dispersing the TiO2 in solvent only, since this was known to have little effect on the dispersibility of the pigment. In the second set of powder coatings the block polymer was mixed with the resin in the absence of TiO2 pigment prior to the extrusion of the dry blend. These were prepared by extrusion at 120°C and subsequently ground prior to dry mixing. A wide range of testing procedures were employed to check dispersion, rheology, gloss and haze, and these are described in the article. It was also decided to examine the influence of pre-blending on pigment dispersion without the aid of dispersants, since it is already known that the degree of dispersion of pigments in a powder coating depends largely on the extent of mixing of the dry blend prior to extrusion. The effect of using a non-pulverized resin in the dry blend compared to the use of a pulverized resin was examined by preparing powder coatings with both types of resin, and after stoving the sprayed panels at 150, 200 and 240°C for 10 minutes the films JANUARY 2003

F O C US were assessed by scanning electron microscopy. Large differences were found in the overall distribution of the pigments on examination at low magnifications that revealed pigment rich and pigment poor areas in the films based on nonpulverized resins. These rich and poor areas were practically absent when the pulverized resin was used although the boundaries between the individual powder particles were still visible in the coating surfaces. Flocculated pigment particles were observed in the films stoved at 200 and 240°C which were more pronounced at 240°C and were absent in films stoved at 150°C. The viscosity of a filled polymer is known to be increased by pigment flocculation as the result of a pigment network structure and the occlusion of polymer material within the pigment flocculates. This network formation can be monitored by measuring the evolution of the elastic modulus G’. It is also noticed that the gloss levels of the two systems seem to be independent of the stoving temperature for both methods while the haze is increased with increasing temperature, agreeing with the observed flocculation at higher temperatures. Coatings prepared from the non-pulverized resin were lower in gloss than those of the coatings formed from pulverized resins. The effect of pigment dispersion on the leveling of the powder coating was studied by evaluating the waviness of the coatings. The results showed that the waviness was not significantly influenced by the heating temperature, and the unexpected high values of waviness at 200 and 240°C indicate that the leveling of the coatings was affected by the observed pigment flocculation at these temperatures. The above tests showed that TiO2 dispersions are not colloidally stable at 200 and JANUARY 2003

O N

POWDER

240°C in the absence of dispersants. Further tests were, therefore, carried out to examine the influence of P2VP-b-PCL copolymer dispersants on the pigments’ degree of dispersion. Four different dispersants were evaluated. These differed in the molecular weights of the anchor and buoy blocks and the asymmetry of the block copolymers, and the maximum adsorbed amount of dispersant onto the TiO2 was calculated by the method described in the article. The first trials were based on pigments that had been contacted with dispersants in solution and dried prior to mixing and extrusion. Dispersants 1, 2, and 3 were used at three different concentrations, 0.5 wt%, 1.5 wt% and 2.5 wt% on pigment. Dispersant 4 was used at 1.5 wt% only. Similar results to the preblending experiments showed that the overall distribution of the pigments at the coating surface was inhomogeneous as the consequence of poor mixing due to the large resin flakes. This confirms the earlier findings that the block copolymer dispersants do not influence the dispersive and distributive mixing of the pigments during extrusion under these experimental conditions. The dispersions of the pigments pretreated with 0.5 wt% of block copolymer appeared to be slightly flocculated because the surface is not fully covered with dispersant at this concentration. At the higher levels the pigments did not flocculate at all. Addition of the dispersants significantly improved the gloss and reduced the haze of the coatings, and stable figures for elastic modulus indicated that formation of a pigment network is prevented, and lower levels of waviness were recorded. Additional experiments were carried out in which the blocked copolymer dispersants were first blended with the resin by extrusion, cooled, ground, and then re-extruded together with the

C O AT I N G S other components. Dispersants 1, 3 and 4 were tested at 1.5 wt% and 2.5 wt%. In these tests the dispersants have to diffuse to the pigments during extrusion through the viscous resin melt in order to adsorb onto the pigment surface. No pigment flocculation occurred in the coatings at either dispersant concentration, consequently the elastic modulus and viscosity of the dispersions remained constant in time while the leveling of the coated surfaces remained undisturbed. The tabulated results showed that the coatings had excellent gloss and extremely low haze levels at both 200 and 240°C. Absence of flocculation indicates that the block copolymers had moved to the pigments during extrusion, by diffusion and convection, and had completely covered the pigment surface. The article concludes with a brief discussion on the influence of block copolymer composition on steric stabilization of the dispersions and assesses a steric layer thickness of 2.4-5.0 nm as the minimum required to impart steric stability. Article entitled “Use of Poly(2-vinylpyridine)b-poly(ε-caprolactone) Copolymers as Pigment Stabilizers in Powder Coatings” by F L Duivenvoorde, K Jansen, J Laven, and R van der Linde of Eindhoven University of Technology, Journal of Coatings Technology, Aug 2002, 74(931), 49-57

GMA acrylics in powder coatings The global acrylic powder coating market is estimated at 10,000 tonnes, with 67% usage in North America. In Europe the estimated total is 1,200 tonnes, representing no more than 0.3% of the overall regional market for thermosetting powder coatings. In the Far East the current usage is 2,000 tonnes, mainly in Japan. The annual growth rate predicted for Europe is greater than 25% from 2002-2007, driven by automotive tier 1 topcoat. Current end use applications 3