Strength in unity - European Coatings€¦ · Strength in unity A waterborne hybrid protective coating system has ... conventional 1K waterborne epoxy ester and ... on this new waterborne

Quelle/Publication: European Coatings Journal

Ausgabe/Issue: 09/2010

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Strength in unity

A waterborne hybrid protective coating system hasbeen developed which provides very high salt sprayresistance with very low VOC levels. Careful selectionof the binder system was required to maximiseperformance, also using a zinc-free anticorrosivepigment to avoid heat degradation. Mechanicalproperties showed a good balance between hardnessand flexibility.

PVDC emulsion plus inorganic binder improves saltsprayresistanceDo-hyung (Dohn) Lee*Jae-sung KimSung-won ChoRecently, concern with environmental preservation hasbeen growing worldwide, and various environmentalpreservation measures are being researched andproposed. For this reason, the coating industry has alsobeen trying to progressively reduce VOC emissions fromcoating materials and also to develop more eco-friendlycoatings such as high solids, solvent-free and waterborne(WB) paints.The technology that will be described here is an organic-inorganic hybrid waterborne anticorrosive coating whichhas been improved through innovative design to surpassthe performance of conventional waterborne anticorrosivecoatings. This has been achieved by combining apoly(vinylidene chloride) (PVDC) copolymer, a binderthat has good barrier properties to both oxygen andmoisture, with a nano-scale inorganic binder that has strongmechanical properties and outstanding adhesion propertieson various metal substrates.This coating material is eco-friendly, and it showsa striking improvement in anticorrosive properties andwater resistance. It also has a good durability due toimprovements in toughness and scratch resistance.

Basic concepts of the hybrid systemThe basic theoretical concept of this anticorrosive coatingis to combine a flexible organic material that has lowoxygen and moisture permeability as a polymer matrixwith a nano-scale inorganic binder that is strong andalso has very strong adhesion to metal substrates. Bycombining this hybrid binder system with an electrochemicalanticorrosion mechanism, better anticorrosive performancecan be achieved.Over the past few decades, a considerable numberof studies on blends or hybrids of organic materialsbased on synthetic polymers with inorganic materialsbased on nanoparticles of metal oxide have been activelyconducted. As regards the organic materials, poly(vinylacetate), poly(methyl methacrylate), epoxy, and variousother materials have been successfully incorporated into aninorganic matrix.In this study, the hybrid of two materials, a PVDC copolymerorganic emulsion binder having excellent barrier properties

(see Table 1) and a nano-scale silica based inorganic binderhaving strong mechanical properties, was created.

Why a poly(vinylidene chloride) copolymer waschosenA binder with vinylidene chloride (VDC) as the mainmonomer has good impermeability to oxygen and humiditybecause of its high crystallisation rate and density. Becauseof these characteristics, PVDC has been used as a barrierresin in solventborne form. The organic binder matrix thatwill be discussed in this paper does not differ greatly fromthis, and it inherits these traditional characteristics.However, polymers that use VDC as the main monomerand that have been copolymerised with other vinyl typesof monomers are more widely used for coating. A VDChomopolymer emulsion provides a barrier effect, but isnot stable. Due to its unusually rapid crystallisation underambient conditions, it is very difficult to obtain a continuouscoating film on the substrate.Therefore, pure PVDC emulsions have hardly ever beenused as a coating material. In order to apply PVDC incoatings, it has been necessary to prevent crystallisationin the emulsion during storage, and induce controlledcrystallisation of the polymer after coating. This can beaccomplished by a proper combination of co-monomerswith VDC.In order to obtain a satisfactory level of film-forming ability,several co-monomers, such as vinyl chloride and acrylictypes (see Table 2), have been used. Sometimes, a smallamount of vinyl carboxylic acid or sulfonate monomer is alsoused to improve the adhesion and storage stability.By means of experiments on the stability of film formationby polymerisation of suitable co-monomers, a more stableorganic binder that has an optimum MFFT (Minimum FilmForming Temperature) and can be used under ambientdrying conditions could be obtained. The general propertiesof the experimental PVDC copolymer emulsions which wereevaluated in this research are shown in Table 3 and an FT-IR analysis is presented in Figure 1.

Criteria for selecting a nano-scale silica binderThe inorganic binder in hybrid coatings is an importantmaterial that reinforces the mechanical performance ofthe final coating. It is spread uniformly within the organicpolymer matrix, and then forms strong bonds betweeninorganic and organic groups. This coating material canenhance long-term durability under severe environmentalconditions, providing heat resistance, non-flammability,good hardness, scratch resistance and water/seawaterresistance.Over the past few decades, various metal oxide basedcomposites have been researched as coating materials.In particular, in this research, the most economical nano-scale silica based inorganic binder was selected. However,traditional alkali silicate inorganic binders were excluded.They were considered unsuitable for the experimental



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coating due to their content of highly water soluble salts,which causes osmotic film degradation.Thus, the chosen nano-scale silica binder has a relativelylow water soluble salt content, instead of being based onalkali silicates. The nano-scale silica binder is a stableinorganic binder which is stabilised by an electrostaticrepulsion balance due to an electric double layer in water.When the balance collapses, the particles begin toagglomerate with each other or change into gel orcondense. Because the film-forming ability of the nano-scale silica binder itself as a main binder was poor in theexperimental coating, it was used as a subsidiary binder toreinforce the organic polymer matrix. The general propertiesof the nano-scale silica binder used in these experimentsare shown in Table 4.

Achieving a stable binder mixture is not simpleAccording to reference [2], when a film is formed by asolventborne PVDC matrix including well dispersed nano-scale silica particles, its surface structure is formed in themanner shown in Figure 2(b). When the content of nano-scale silica particles is at an optimum level, the watervapour permeability is dramatically reduced and reachesits minimum point. If it exceeds a certain critical point,however, the coating becomes more porous and watervapour permeability increases, as shown in Figure 3.Based on this principle, the organic and inorganic bindersdiscussed in the previous sections were combined for thisexperiment. However, this hybridising work was not simpleas it involved combining two materials that are unstablefor several distinct reasons. A low pH and the existence ofchloride ion in the organic binder had a serious effect on thestability of the inorganic binder when the two were mixed.In practice, some of the experimental hybrid compoundsimmediately changed into a gel when they were mixed.Table 5 shows the results of the stability tests of the nano-scale silica binders in relation to the stabilising mechanism,with the same organic polymer.As can be seen in Table 5, the mixture of PVDC copolymeremulsion and nano-scale silica binder has better stabilitywhen the silica surface has a more negative charge, a largerparticle size, and a lower pH. In particular, the nano-scalesilica binder with most of the alkali ion removed showedthe most stable tolerance to low pH and various ionicsubstances of the PVDC emulsion.In this study, organosilane was also introduced in orderto increase cohesion and particle stability at the interfaceof the organic emulsion and nano-scale silica particles.Organosilane was adopted without regard to its specifictype, but the one which was the most stable in the presenceof the aqueous solution or the aqueous dispersion afterhydrolysis was selected.When this was added to the experimental WB protectivecoatings, several further advantages, such as betteradhesion to metal substrates, film density and smoothnessof the coating film surface, could also be obtained (seeFigure 4). The results of this study on organic-inorganichybrid WB anticorrosive coating showed significantlyupgraded coating film properties compared to thecorresponding conventional WB organic coating.

Zinc-based pigments promote heat degradationA PVDC coating film containing zinc compounds showedexcellent corrosion resistance in practice, but it also showed

an irreversible breakdown at high temperatures of over90 °C. This therefore means that the use of anticorrosivepigments such as zinc phosphate and zinc dust, whichimpart a cathodic passivation effect or cathodic protectioneffect, is limited and they should be carefully selected.Zinc compounds accelerate the degradation of the coatingfilm caused by the dehydrochlorination phenomenon ofPVDC copolymers at high temperatures. Heat stability canbe enhanced by using corrosion inhibiting materials thatdo not contain zinc compounds. However, the anticorrosiveperformance then normally drops significantly.Organic-inorganic hybrid technology offers a possiblesolution to this problem. In the test results, the experimentalhybrid coating film containing a corrosion inhibiting materialsuch as calcium ion exchanged silica rather than zinccompounds showed good heat stability with synergisticeffects on the anticorrosive properties.

Good corrosion resistance and physical propertiesThese experimental results led to the conclusion that,compared to conventional WB protective coatings,improved film performance could be obtained from thisWB PVDC/nano-silica hybrid protective coating.Figure 5reveals the results of a 1000 hours salt spray test (to ASTMB117). These results showed that the new experimentalWB anticorrosive coating outperformed even anticorrosivesystems based on 2K solventborne epoxy coating, aconventional 1K waterborne epoxy ester and waterborneacrylic emulsion paint. In particular, the corrosion-inhibitingperformance in the scribed area of the test panels wassignificantly different from the condition of the controlsamples.Furthermore, premature coating film failure, such as heatinduced blistering and peel-off phenomena, which werenoted at high temperatures above 90 °C, was improvedwithout any reduction of the anticorrosive properties. Thismeans it can be concluded that one of the major obstaclesto expanding the use of WB PVDC protective coating in theindustrial field has been resolved.In addition to the performance described above, the newprotective coating also has good mechanical performance.In conventional general protective coatings, a barrier effectcan be achieved by making the coating film thick, butsometimes this also makes the coating film very brittle.However, by the complementary combination of the flexiblecharacteristics of an organic emulsion binder and thereinforcing effect of an inorganic binder, this WB coatingis not only tough but also flexible. And it also showedbarrier properties even at low film thickness. This canbe demonstrated through the bending test and impactresistance test results (see Figure 6).

Research continues on improving weatheringresistanceBy performing a series of general property evaluationson this new waterborne protective coating, it has beendemonstrated that some technological improvements havebeen achieved. In this study, an advanced WB protectivecoating technology with a good anticorrosive performancewas successfully obtained by use of the ideal combinationof a PVDC copolymer and a nano-scale silica binder.The heat degradation of the coating film was also minimisedwithout any loss of anticorrosive properties, and the



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mechanical properties of the WB anticorrosive coating wereenhanced.However, the weatherability of PVDC itself still seems tobe limited. Improvements to the weatherability of PVDCbased WB coatings have been investigated by introducingpure acrylic monomers or by blending it with other highlyweatherable polymers.Additionally, to overcome this weak point, further researchon a new waterborne topcoat based on highly weatherablebinders including silicone-acrylic emulsion is currentlyin progress. This technology would provide a completedurable waterborne protective coating system. í

REFERENCES[1] Wessing R. A. et al., Vinylidene chloride monomerand polymers, 1997, Vol. 24, John Wiley & Sons,pp 882-923 [2] Hwang T. et al, Synthesis and barrierproperties of poly(vinylidene chloride-co-acrylonitrile)/SiO2hybrid composites by sol-gel process, Jnl. of membranescience, 2009, Vol. 345, pp 90-96.

* Corresponding Author:Do-hyung (Dohn) LeeKCC Central Research InstituteT +82 31 [email protected]

Results at a glanceA waterborne hybrid protective coating has been developedwhich provides a high level of salt spray resistance withvery low VOC levels. The basic concept employs anorganic binder with low permeability (a PVDC copolymer)combined with a non-scale silica based inorganic binderthat enhances substrate adhesion as well as improving thebarrier properties. Since anticorrosive pigments containingzinc can cause degradation of the PVDC at elevatedtemperatures, a zinc-free active anticorrosive pigmentwas chosen. The final system shows good anticorrosiveperformance, low VOC content, a good balance of hardnessand flexibility and strong mechanical properties. Researchis continuing on improving the weathering resistance ofthis system, by modifying the organic polymer and/ordeveloping a suitable topcoat.



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Bild zu Strength in unity



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Figure 1: FT-IR analysis of an experimental PVDC copolymer



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Figure 4: 1000 x magnification of coating films: (a) organic-inorganic hybridsurface without organosilane; (b) with organosilane



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Figure 5: 1000 hours salt spray test results of protective coatings on steelsubstrates according to the panels.



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Figure 6: Heat resistance test (90 °C/24 h): (A) conventional WB PVDCcopolymer coating, blistering defects; (B) WB PVDC/nano-scale silica bindercoating, no blistering



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Figure 6: Mechanical properties of WB PVDC/nano-scale silica binder coating:(A) bending test, 2 mm mandrel pass; (B) impact resistance test, 120 lb-in pass



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Strength in unity - European Coatings€¦ · Strength in unity A waterborne hybrid protective coating system has ... conventional 1K waterborne epoxy ester and ... on this new waterborne