Technical Bulletin Low Oil Absorption Talc for High Solids Coatings

Transcription

1 Technical Bulletin 1206 Low Oil Absorption Talc for High Solids Coatings

2 Contents Content Abstract Introduction Talc as mineral filler Talc grades used in the study Description of experiments Results and discussions First test series Second test series Conclusions References Abstract High solids coatings are one solution of the paint industry to solve the emission problem of volatile organic compounds (VOC) in the paint formulations. A lot of attention has been given to develop new binder systems that have low solvent emissions (VOC) with good application and protection properties. New generation binders are low molecular weight and low viscosity resins with low pigment binding power and so they set new requirements for pigments and extenders. Traditionally talc is the preferred extender in protective coatings due to its good barrier and anti-corrosion properties. The high oil absorption value of a conventional type of talc makes it difficult to get low enough VOC-content in modern protective coatings. New, low oil absorption, talc extender with controlled top size has been developed in response to this challenge. This new talc product is based on pure, macro-crystalline talc ore which means good barrier and corrosion protection due to high platyness and hydrophobic character of the talc. What is unique is the optimized particle size distribution for high solid coatings. The coarse fraction is sharpened is such a way that the fineness of grind requirements are met. The fines content is also reduced to lower the resin demand to the level that enables the production of VOC-compliance coatings. This paper demonstrates how the solid content and pigment volume concentration (PVC) in the solvent based protective coating can be increased without increasing the viscosity and at the same time anti-corrosion properties of coating are also improved. 2 Mondo Minerals B.V.. Technical Bulletin 1206

3 Introduction One of the most important development targets in the paint and coating industry for many years has been to reduce the volatile organic compounds (VOC) in the formulations. The development of high solids and water based coatings has been an attempt to reduce VOC emissions. In the movement towards low VOC solvent borne coatings, little attention has been given to pigmentation. With a new generation of polymer binders adequate pigmentation is absolutely necessary to provide a paint film with necessary optical and protective properties. In these new developments extenders became more specific and more functional than before. In the sectors of metal protection, maintenance and marine coatings, all developments tendencies are towards low VOC systems. However, official international VOC limits for the products for such applications are absent [1]. The European Unions (EU) paint product directive, 2004/42/EC, concerns decorative paints, varnish, wood stains and vehicle repair paints, but not other industrial coatings. The current VOC-legislation that controls the VOC emissions of industrial protective coatings is a solvent emission directive 1993/13/EC, that concerns the manufacturing plants and work using the coatings, but not directly the coating products themselves. The talc is the preferred extender in traditional protective coatings due to its good barrier and anti-corrosion properties. However the high oil absorption value of pure and platy talc with conventional particle size distribution (PSD) makes it difficult to get sufficiently low enough VOC-content in modern high solid coatings. A new, low oil absorption, talc with specially designed PSD has been developed to response this challenge. This new type of talc product, Plustalc D30E, is based on pure, macro-crystalline talc ore, however the both ends of PSD curve is cut. The coarse fraction is sharpened is such a way that the fineness of grind requirements are met. The fines content is also reduced to lower the resin demand to the level that enables the production of VOC-compliance coatings. The purpose of this study was to compare the performance of the new low oil absorption talc, Plustalc D30E, with the performance of three other commonly used talc types. The talc grades were tested in a commercial solvent based 2 pack epoxy primer whose pigment volume concentration (PVC) and solid content were relatively high. Talc as mineral filler Talc is a natural mineral that is mined from the earth s crust. Talc stones are often referred to as soapstone. These stones are crushed, refined and fine milled to PSD that is suitable for various paint and coating applications. The fine structure of the talc is presented on the Figure 1 [2]. Chemically, talc is a magnesium silicate, Mg 3 Si 4 O 10 (OH) 2, with a sandwich type of crystal structure. Its theoretical chemical composition is 31,88 % MgO, 63,37 % SiO 2 and 4,75 % H 2 O. Si +4 O -2 OH - Mg +2 Figure 1:. The fine structure. of talc. Mondo Minerals B.V.. Technical Bulletin

4 In nature talc does not exist in its theoretical form or purity, but a small fraction of Mg and Si atoms can be replaced by iron (Fe), aluminium (Al) or nickel (Ni). Typically, the colour of talc stones is pale green or white, but the colour can vary from pinkish to dark green. The surface of talc stone has slippery feeling and pearly lustre. The unique fine structure of talc makes it ideal filler for protective coatings: Talc s surface is electrically neutral, which makes talc highly hydrophobic, thus water repellent. Talc s particle form is platy, which is important for an optimum filler packing in the paint film. The platy particles also create a good barrier against unwanted substances such as water, CO 2 and O 2. The platyness helps also to design the right structural viscosity for the paint to reduce the settling of solid particles, improve application properties (sprayability and brushablity) and improve the sag/levelling balance. The platy particle form of talc reduces the shrinkage of binder system during curing and so promotes the adhesion of paint film to substrate. Talc is softest known mineral. The softness comes from the platy structure. The talc plates slide across each other when the plate pack is stressed by the force, for instance when shearing the talc powder between the fingers. The slippery feeling comes also from this phenomena. The softness together with the platy particle form improve the sandablity of primers and improve the flexibility of paint film. Talc is highly inert. The talc does not dissolve in acids or in bases, which is very important property for the filler in many paint applications. The particle form of talc is not always platy; it can be also blocky. Blocky talcs are also called as a microcrystalline. The oil absorption of micro-crystalline talc is lower than that of macro-crystalline (platy) talc at the same Hegman fineness, even if the specific surface area as measured by nitrogen adsorption is higher with micro-crystalline talc. There is no ideally pure talc deposit existing in the nature, but the mineralogical composition of commercial talc grades varies widely. The typical by-minerals for the talc are carbonates like calcium carbonate, dolomite or magnesite. The other typical by-minerals are chlorite (Mg-Al-silicate) and mica. To be able to utilise the protective functionality properties of talc it should be pure and platy. The purity secures the good water repellency and the platyness good barrier properties. Talc grades used in the study The properties of talc grades used in this study are given in table 1. All the talc grades had the same fineness of grind (Hegman fineness), but they varied in mineralogical composition and shape of PSD curve. The oil absorption value of the talc used in protective coating is very important. The oil absorption of talc is effected by its mineralogical composition, morphology and PSD. Pure and platy talc has quite high oil absorption at a given fineness. When talc contains by-minerals such as magnesite (MgCO 3 ), dolomite (Ca,Mg-carbonate) or chlorite (Al-Mg-silicate) its oil absorption is lowered. The platyness of talc varies from deposit to deposit and it has big impact on the oil absorption value. The talc grades that consist of small plates have lower oil absorption than the talcs with big plates. The less platyness is beneficial for oil absorption point of view, but the protective properties are reduced compared with platy talc. The best protection is therefore achieved by pure platy talc whose oil absorption value is reduced by modifying its PSD. The figure 2 shows how the PSD and mineralogical composition affect the oil absorption value of talc product at the constant fineness of grind. 4 Mondo Minerals B.V.. Technical Bulletin 1206

5 Property Unit Method Composition: Talc % Pure, platy, standard talc Magnesite rich talc Chlorite rich talc Plustalc D30E MgCO3 % XRD + LOI + 1,5 37 Chlorite % Acid solubles 1, Dolomite % Loss on Ignition % 1000 C/0,5 h 5,8 20,6 8,2 7,3 Acid solubles % 1 M HCL, 100 C 3,0 36,9 6,7 7,0 Particle size: Hegman fineness ASTM D Average PS, D50 µm Sedigraph ,5 4,5 5,2 9,8 Top cut, D98 µm Sedigraph Oil absorption g/100g ISO 787/ Specific Surface Area m 2 /g BET, ISO ,1 6,6 4,2 5,1 Whiteness, Ry % DIN Table 1:. The properties. of talc grades. used in the study. Figure 2 shows very clearly that if the talc contains by-minerals like magnesite (MgCO 3 ) and/or chlorite (Al-Mg-silicate) the oil absorption value is lowered at certain fineness of grind. Figure 2 shows also how the reduction of fines of pure, platy talc reduces the oil absorption value. The oil absorption can be reduced from 43 to 28 g/100 g when the finest fraction of talc particles is removed. This permits a remarkable reduction in resin and solvent amounts in the coating formulations. PSD by Sedigraph OA = 28g/100g OA = 43g/100g OA = 36g/100g OA = 29g/100g Figure 2:. Particle size. distribution of traditional types of talcs and low oil absorption talc at the same fineness of grind (Hegman 4) ,1 Conventional talc Magnesite rich talc Plustalc D30E Chlorite rich talc Mondo Minerals B.V.. Technical Bulletin

6 When the oil absorption of the talc is reduced, it is possible to optimise the filler loading in a coating. Figure 3 illustrates the viscosity profiles of the talc grades used in this study. The viscosity was measured by ICI-viscometer after the talc was dispersed gradually increasing amounts in a linseed oil. As figure 3 shows the viscosity of standard type of talc increases exponentially while the low oil absorption talc (Plustalc D30E) demonstrates clearly moderate viscosity build up. Figure 3:. The viscosity. profiles of. different. talc grades in. linseed oil. ICI-viscosity of talcs tested Talc mixed in 100 g of linseed oil 3,0 2,5 2,0 1,5 1,0 0,5 0, Talc load in g s Pure, platy, standard talc Chlorite rich talc Magnesite rich talc Plustalc D30E 6 Mondo Minerals B.V.. Technical Bulletin 1206

7 Description of experiments The paint formulation used was solvent based 2 pack epoxy primer that was based on solid epoxy resin and a polyamide hardener. The components were mixed in the stoichiometric ratio 4:1 by volume. The starting formulation was the commercial general purpose anticorrosion primer for steel protection. Two series of paints were evaluated. In the first series each talc grade was milled to the PVC and solid content that had the same application viscosity as the starting formulation where pure, platy standard talc was used. The target viscosity was cp measured by Brookfield RVDV-II+ viscometer (spindle 6, 50 rpm). In practise the amount of talc and solvent content were changed in the formulations. In the second series the PVC was kept constant (PVC= 51 v%) and the solids content was changed in such a way that the same application viscosity was achieved as in the first test series. In this series the amount of solvent was changed and other components in the formulation were kept constant. The magnesite rich talc was not used in the second test series because it had worse loadability than the standard talc. The starting formulation: Raw material w% Solid epoxy 19,47 Auxiliary resin 1,80 Acrosolv PM 3,00 Thickener 0,93 Wetting agent 1 0,26 Wetting agent 2 0,26 Talc, pure, platy standard type 24,88 BaSO 4 18,93 TiO 2 7,03 Pigment 1,08 Xylene 15,68 Iso-butanol 6,71 Total, w% 100 PVC tot, v% 51 Solid content by weight, w% 68,9 Solid content by volume, v% 45,1 Density, g/ml 1,514 VOC, g/l 471 The anticorrosion properties of the paints were studied according to ISO-standards: continuous neutral salt spray, 381 h (ISO 7253) water condensation, 240 h (ISO 6270). The paints were applied on sand blasted steel panels (Sa 21/2) by using high pressure airless spray gun. The panels were 150 x 75 x 2 mm in size. The dry film thickness was µm. After the application the panels were left to be dried at room temperature for one week before the start of tests. Three parallel test panels were prepared for each paint. The visible defects on the test panels (degree of rusting and blistering) were evaluated immediately after end of the test according to ISO 4628 standard. The adhesion tests were carried out according ISO 4624 standard by using hydraulic pull-off tester. The diameter of the pull-off button was 20 mm. Mondo Minerals B.V.. Technical Bulletin

8 Results and discussions First test series The anticorrosion performance of different the talc grade in the first test series is given in table 2 and figure 4. In this series PVC and solids content were adjusted to give the same application viscosity as the reference paint had by differing the talc and solvent loads. These types of adjustments were done to be able to have the truly comparable formulation on each talc type. There were no surface defects (blistering or rust dots) in any of the paints after humidity test for 240 hours. The adhesion was also good for all the paints after the humidity test. However the drop of adhesion was least with Plustalc D30E. After the salt spray test (381 h) the blisters with the size of class 2 and some rust dots appeared on all test panels. The rusting degree is classified from 0 (best= no rusting) to 5 (worst= % of coated area was rusted) according to the ISO 4628 standard. The reference talc and magnesite talc had a few small rust dots all over the panels, and the rusting class of these talc grades was between 0 and 1. The reference talc had also a few blisters all over the panels. Other panels contained a few blisters and some rust dots only at the scratch site. The amount of rust dots and blisters were lowest with Plustalc D30E. It must be noted that the paint with Plustalc D30E had very high PVC (56 v%) and yet anti-corrosion properties were better than the reference paint of lower PVC (PVC= 51 v%). Also the solids content was 4 v%-units higher with Plustalc D30E than in the reference paints. So the anti-corrosion properties can be improved with increased PVC by using low oil absorption talc. The anti-corrosion properties were improved until approaching the critical PVC, after which the coating film becomes too porous and the anti-corrosion properties collapse. Table 2:. Anti-corrosion. properties of. different talc. types in solvent based 2 pack. epoxy primer at constant appli-. cation viscosity ( cp. by Brookfield RvDV-II). Pure, platy, standard talc = Ref. Magnesite. rich talc Chlorite. rich talc Plustalc D30E PVC tot, v% Solids by volume, v% 45,1 44,3 46,9 49,3 Solids by weight, w% 68,9 68,0 70,6 72,7 Talc content, w% 24,9 22,8 27,3 31,2 Density of paint, g/ml 1,514 1,495 1,547 1,595 VOC, g/l Adhesion (ISO 4624) before tests. in MPa and type of breakage 5,6 (100 % B) 5,6 (100 % B) 6,5 (100 % B) 6,0 (100 % B) Condensation (ISO 6270, 240 h): Visible surface defects (ISO 4628) Adhesion after test, MPa 4,8 (100 % B) 4,8 (100 % B) 6,3 (100 % B) 6,5 (100 % B) Salt spray test (ISO 7253, 381 h): Degree of rusting (ISO 4628) Blistering (ISO 4628) Ri = 0-1; a few rust dots all over the panel. 2 (S2); blister all over the panel. Ri = 0-1; a few rust dots all over the panel. 2 (S2); blisters at scratch. Ri = 0-1; a few rust dots at scratch. 1 (S2-3); a few blisters at scratch. Ri = 0-1; a few rust dots at scratch. 1 (S2); a few blisters at scratch. Adhesion after test, MPa 4,8 (100 % B) 6,9 (100 % B) 6,2 (100 % B) 5,8 (100 % B) Overall performance Adequate Adequate Good Very good B = cohesion type breakage of coating film. 8 Mondo Minerals B.V.. Technical Bulletin 1206

9 Adhesion by Pull-Off Test PVC= 51 v% Solids= 45,1 v% PVC= 50 v% asolids= 44,3 v% PVC= 56 v% Solids= 49,3 v% PVC= 53 v% Solids= 46,9 v% Figure 4: Adhesion of different talc types after water condensation (ISO 6270, 240 h) and neutral salt spray tests (ISO 7253, 381) in solvent based 2 pack epoxy primer at constant application viscosity ( cp by Brookfield RvDV-II). Pure, platy, standard talc= Ref. Magnestic rich talc Chlorite rich talc Plustalc D30E Beforere tests After water condensation test After salt water test Second test series Anti-corrosion performance of different talc grades in the second test series is given in table 3 and figure 5. In this series the PVC was kept constant and the amount of solvents was reduced to get the same application viscosity as the reference paint had. The highest solid content was achieved by Plustalc D30E; 51,1 v % (reference had 45,1 v %). The VOCcontent could be reduced from 471 g/l of the reference paints to 420 g/l with Plustalc D30E. There were no visible surface defects on the paints films after the humidity test (ISO 6270, 240 h). The reduction in adhesion was lowest with Plustalc D30E after humidity testing. A few blisters with size of class 2 and some small rust dots appeared on the scratches of chlorite rich talc and Plustalc D30E panels and the rusting degree (Ri) was 0. The reference panels contained blisters and rust dots all over the panels (Ri=0-1). The adhesion after the salt spray test was best with Plustalc D30E. Mondo Minerals B.V.. Technical Bulletin

10 Table 3:. Anti-corrosion properties of different talc types in solvent based epoxy primer at constant PVC. (51 v%) and at constant application viscosity ( cp by Brookfield RvDV-II). Pure, platy,. standard talc = Ref. Chlorite. rich talc Plustalc D30E PVC tot, v% Volume based solids, v% 45,1 49,0 51,1 Mass based solids, w% 68,9 72,2 73,8 Talc content, w% 24,9 26,1 26,7 Density of paint, g/ml 1,514 1,575 1,603 VOC, g/l Adhesion (ISO 4624) before tests. in MPa and type of breakage Condensation (ISO 6270, 240 h): 5,6 (100 % B) 6,6 (100 % B) 6,9 (100 % B) Visible surface defects (ISO 4628) Adhesion after test, MPa 4,8 (100 % B) 5,5 (100 % B) 7,1 (100 % B) Salt spray test (ISO 7253, 381 h): Degree of rusting (ISO 4628) Blistering (ISO 4628) Ri = 1; a few rust dots all over the panel. 2 (S2); blister all over the panel. Ri = 0; a few rust dots at and below scratch. 2 (S2); blisters at scratch. Ri = 0; a few rust dots at scratch. 1 (S2); a few blisters at scratch. Adhesion after test, MPa 4,8 (100 % B) 6,4 (100 % B) 7,0 (100 % B) Overall performance Adequate Good Very good B = cohesion type breakage of coating film. Figure 5: Adhesion of different talc types after water condensation (ISO 6279, 240 h) and neutral salt spray tests (ISO 7253, 381) in solvent based epoxy coatings at constant PVC (51 v%) and at constant application viscosity ( cp. by Brookfield RvDV-II). Adhesion by Pull-Off Test PVC= 51 v% Solids= 45,1 v% Pure, platy, standard talc= Ref. PVC= 51 v% Solids= 41,0 v% Chlorite rich talc PVC= 56 v% Solids= 49,3 v% Plustalc D30E Beforere tests After water condensation test After salt water test 10 Mondo Minerals B.V.. Technical Bulletin 1206

11 Conclusions The anti-corrosion performance of new low oil absorption talc (Plustalc D30E) was compared with three other commercial talc types in solvent borne general purpose epoxy primer. The formulation was based on solid epoxy resin and a polyamide hardener. The components were mixed in the stoichiometric ratio 4:1 by volume. The starting formulation had high pigment volume concentration (PVC= 51 v%) and relatively high solid content also (solids by volume= 45,1 v%). The talc types tested were: pure, platy standard talc (reference) magnesite rich talc chlorite rich talc pure, platy, low oil absorption talc (Plustalc D30E) All the talc grades had the same top cut: Hegman fineness 4. The study showed that by pure, platy and low oil absorption talc (Plustalc D30E) the solid content and PVC in the paint formulations can be increased without increasing the viscosity compared with traditional high solvent containing formulations and at the same time the anti-corrosion properties are improved. This means that Plustalc D30E-type talc helps to develop VOC-complaint paints with reduced formulation costs and with improved coating performance. The benefits of low resin and diluent demand of Plustalc D30E can be utilised in all the paint and coating systems were an increased solids content is desired. The performance of different talc types can be summarised as follows: Water. repellency Anticorrosion properties (blistering & rusting) Adhesion Load-. ability Viscosity Solids content Reduction of VOC Pure, platy standard talc Magnesite talc Chlorite talc Pure, platy low oil absorption talc (Plustalc D30E) negative impact 0 no effect + positive impact ++ very positive impact. References [1]. CEPE Presentation Proposal for Inclusion of PC into Product Directive 2004/42/EC, December [2] Mondo Minerals B.V.. Technical Bulletin

12 MONDO MINERALS B.V.. Kajuitweg 8. NL AR Amsterdam. Phone Fax info@mondominerals.com The information contained in this Technical Bulletin relates only to the specific tests designated herein and does not relate to the use of products in combination with any other material or in any process. The information provided herein is based on technical data that Mondo Minerals believes to be reliable, however Mondo Minerals makes no representation or warranty as to the completeness or accuracy thereof and Mondo Minerals assumes no liability resulting from its use for any claims, losses, or damages of any third party. Recipients using this information must exercise their own judgement as to the appropriateness of its use, and it is the user s responsibility to assess the materials suitability (including safety) for a particular purpose prior to such use.