Strength in unity. Quelle/Publication: European Coatings Journal. Seite/Page:

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1 1 Strength in unity A waterborne hybrid protective coating system has been developed which provides very high salt spray resistance with very low VOC levels Careful selection of the binder system was required to maximise performance, also using a zinc-free anticorrosive pigment to avoid heat degradation Mechanical properties showed a good balance between hardness and flexibility PVDC emulsion plus inorganic binder improves saltspray resistance Do-hyung (Dohn) Lee* Jae-sung Kim Sung-won Cho Recently, concern with environmental preservation has been growing worldwide, and various environmental preservation measures are being researched and proposed For this reason, the coating industry has also been trying to progressively reduce VOC emissions from coating materials and also to develop more eco-friendly coatings such as high solids, solvent-free and waterborne (WB) paints The technology that will be described here is an organicinorganic hybrid waterborne anticorrosive coating which has been improved through innovative design to surpass the performance of conventional waterborne anticorrosive coatings This has been achieved by combining a poly(vinylidene chloride) (PVDC) copolymer, a binder that has good barrier properties to both oxygen and moisture, with a nano-scale inorganic binder that has strong mechanical properties and outstanding adhesion properties on various metal substrates This coating material is eco-friendly, and it shows a striking improvement in anticorrosive properties and water resistance It also has a good durability due to improvements in toughness and scratch resistance Basic concepts of the hybrid system The basic theoretical concept of this anticorrosive coating is to combine a flexible organic material that has low oxygen and moisture permeability as a polymer matrix with a nano-scale inorganic binder that is strong and also has very strong adhesion to metal substrates By combining this hybrid binder system with an electrochemical anticorrosion mechanism, better anticorrosive performance can be achieved Over the past few decades, a considerable number of studies on blends or hybrids of organic materials based on synthetic polymers with inorganic materials based on nanoparticles of metal oxide have been actively conducted As regards the organic materials, poly(vinyl acetate), poly(methyl methacrylate), epoxy, and various other materials have been successfully incorporated into an inorganic matrix In this study, the hybrid of two materials, a PVDC copolymer organic emulsion binder having excellent barrier properties (see Table 1) and a nano-scale silica based inorganic binder having strong mechanical properties, was created Why a poly(vinylidene chloride) copolymer was chosen A binder with vinylidene chloride (VDC) as the main monomer has good impermeability to oxygen and humidity because of its high crystallisation rate and density Because of these characteristics, PVDC has been used as a barrier resin in solventborne form The organic binder matrix that will be discussed in this paper does not differ greatly from this, and it inherits these traditional characteristics However, polymers that use VDC as the main monomer and that have been copolymerised with other vinyl types of monomers are more widely used for coating A VDC homopolymer emulsion provides a barrier effect, but is not stable Due to its unusually rapid crystallisation under ambient conditions, it is very difficult to obtain a continuous coating film on the substrate Therefore, pure PVDC emulsions have hardly ever been used as a coating material In order to apply PVDC in coatings, it has been necessary to prevent crystallisation in the emulsion during storage, and induce controlled crystallisation of the polymer after coating This can be accomplished by a proper combination of co-monomers with VDC In order to obtain a satisfactory level of film-forming ability, several co-monomers, such as vinyl chloride and acrylic types (see Table 2), have been used Sometimes, a small amount of vinyl carboxylic acid or sulfonate monomer is also used to improve the adhesion and storage stability By means of experiments on the stability of film formation by polymerisation of suitable co-monomers, a more stable organic binder that has an optimum MFFT (Minimum Film Forming Temperature) and can be used under ambient drying conditions could be obtained The general properties of the experimental PVDC copolymer emulsions which were evaluated in this research are shown in Table 3 and an FTIR analysis is presented in Figure 1 Criteria for selecting a nano-scale silica binder The inorganic binder in hybrid coatings is an important material that reinforces the mechanical performance of the final coating It is spread uniformly within the organic polymer matrix, and then forms strong bonds between inorganic and organic groups This coating material can enhance long-term durability under severe environmental conditions, providing heat resistance, non-flammability, good hardness, scratch resistance and water/seawater resistance Over the past few decades, various metal oxide based composites have been researched as coating materials In particular, in this research, the most economical nanoscale silica based inorganic binder was selected However, traditional alkali silicate inorganic binders were excluded They were considered unsuitable for the experimental Vincentz Network +++ Plathnerstr 4c +++ D Hannover +++ Tel:+49(511)

2 2 coating due to their content of highly water soluble salts, which causes osmotic film degradation Thus, the chosen nano-scale silica binder has a relatively low water soluble salt content, instead of being based on alkali silicates The nano-scale silica binder is a stable inorganic binder which is stabilised by an electrostatic repulsion balance due to an electric double layer in water When the balance collapses, the particles begin to agglomerate with each other or change into gel or condense Because the film-forming ability of the nanoscale silica binder itself as a main binder was poor in the experimental coating, it was used as a subsidiary binder to reinforce the organic polymer matrix The general properties of the nano-scale silica binder used in these experiments are shown in Table 4 Achieving a stable binder mixture is not simple According to reference [2], when a film is formed by a solventborne PVDC matrix including well dispersed nanoscale silica particles, its surface structure is formed in the manner shown in Figure 2(b) When the content of nanoscale silica particles is at an optimum level, the water vapour permeability is dramatically reduced and reaches its minimum point If it exceeds a certain critical point, however, the coating becomes more porous and water vapour permeability increases, as shown in Figure 3 Based on this principle, the organic and inorganic binders discussed in the previous sections were combined for this experiment However, this hybridising work was not simple as it involved combining two materials that are unstable for several distinct reasons A low ph and the existence of chloride ion in the organic binder had a serious effect on the stability of the inorganic binder when the two were mixed In practice, some of the experimental hybrid compounds immediately changed into a gel when they were mixed Table 5 shows the results of the stability tests of the nanoscale silica binders in relation to the stabilising mechanism, with the same organic polymer As can be seen in Table 5, the mixture of PVDC copolymer emulsion and nano-scale silica binder has better stability when the silica surface has a more negative charge, a larger particle size, and a lower ph In particular, the nano-scale silica binder with most of the alkali ion removed showed the most stable tolerance to low ph and various ionic substances of the PVDC emulsion In this study, organosilane was also introduced in order to increase cohesion and particle stability at the interface of the organic emulsion and nano-scale silica particles Organosilane was adopted without regard to its specific type, but the one which was the most stable in the presence of the aqueous solution or the aqueous dispersion after hydrolysis was selected When this was added to the experimental WB protective coatings, several further advantages, such as better adhesion to metal substrates, film density and smoothness of the coating film surface, could also be obtained (see Figure 4) The results of this study on organic-inorganic hybrid WB anticorrosive coating showed significantly upgraded coating film properties compared to the corresponding conventional WB organic coating Zinc-based pigments promote heat degradation A PVDC coating film containing zinc compounds showed excellent corrosion resistance in practice, but it also showed an irreversible breakdown at high temperatures of over 90 C This therefore means that the use of anticorrosive pigments such as zinc phosphate and zinc dust, which impart a cathodic passivation effect or cathodic protection effect, is limited and they should be carefully selected Zinc compounds accelerate the degradation of the coating film caused by the dehydrochlorination phenomenon of PVDC copolymers at high temperatures Heat stability can be enhanced by using corrosion inhibiting materials that do not contain zinc compounds However, the anticorrosive performance then normally drops significantly Organic-inorganic hybrid technology offers a possible solution to this problem In the test results, the experimental hybrid coating film containing a corrosion inhibiting material such as calcium ion exchanged silica rather than zinc compounds showed good heat stability with synergistic effects on the anticorrosive properties Good corrosion resistance and physical properties These experimental results led to the conclusion that, compared to conventional WB protective coatings, improved film performance could be obtained from this WB PVDC/nano-silica hybrid protective coatingfigure 5 reveals the results of a 1000 hours salt spray test (to ASTM B117) These results showed that the new experimental WB anticorrosive coating outperformed even anticorrosive systems based on 2K solventborne epoxy coating, a conventional 1K waterborne epoxy ester and waterborne acrylic emulsion paint In particular, the corrosion-inhibiting performance in the scribed area of the test panels was significantly different from the condition of the control samples Furthermore, premature coating film failure, such as heat induced blistering and peel-off phenomena, which were noted at high temperatures above 90 C, was improved without any reduction of the anticorrosive properties This means it can be concluded that one of the major obstacles to expanding the use of WB PVDC protective coating in the industrial field has been resolved In addition to the performance described above, the new protective coating also has good mechanical performance In conventional general protective coatings, a barrier effect can be achieved by making the coating film thick, but sometimes this also makes the coating film very brittle However, by the complementary combination of the flexible characteristics of an organic emulsion binder and the reinforcing effect of an inorganic binder, this WB coating is not only tough but also flexible And it also showed barrier properties even at low film thickness This can be demonstrated through the bending test and impact resistance test results (see Figure 6) Research continues on improving weathering resistance By performing a series of general property evaluations on this new waterborne protective coating, it has been demonstrated that some technological improvements have been achieved In this study, an advanced WB protective coating technology with a good anticorrosive performance was successfully obtained by use of the ideal combination of a PVDC copolymer and a nano-scale silica binder The heat degradation of the coating film was also minimised without any loss of anticorrosive properties, and the Vincentz Network +++ Plathnerstr 4c +++ D Hannover +++ Tel:+49(511)

3 3 mechanical properties of the WB anticorrosive coating were enhanced However, the weatherability of PVDC itself still seems to be limited Improvements to the weatherability of PVDC based WB coatings have been investigated by introducing pure acrylic monomers or by blending it with other highly weatherable polymers Additionally, to overcome this weak point, further research on a new waterborne topcoat based on highly weatherable binders including silicone-acrylic emulsion is currently in progress This technology would provide a complete durable waterborne protective coating system í REFERENCES [1] Wessing R A et al, Vinylidene chloride monomer and polymers, 1997, Vol 24, John Wiley & Sons, pp [2] Hwang T et al, Synthesis and barrier properties of poly(vinylidene chloride-co-acrylonitrile)/sio2 hybrid composites by sol-gel process, Jnl of membrane science, 2009, Vol 345, pp * Corresponding Author: Do-hyung (Dohn) Lee KCC Central Research Institute T dohnlee@kccworldcokr Results at a glance A waterborne hybrid protective coating has been developed which provides a high level of salt spray resistance with very low VOC levels The basic concept employs an organic binder with low permeability (a PVDC copolymer) combined with a non-scale silica based inorganic binder that enhances substrate adhesion as well as improving the barrier properties Since anticorrosive pigments containing zinc can cause degradation of the PVDC at elevated temperatures, a zinc-free active anticorrosive pigment was chosen The final system shows good anticorrosive performance, low VOC content, a good balance of hardness and flexibility and strong mechanical properties Research is continuing on improving the weathering resistance of this system, by modifying the organic polymer and/or developing a suitable topcoat Vincentz Network +++ Plathnerstr 4c +++ D Hannover +++ Tel:+49(511)

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7 7 Figure 1: FT-IR analysis of an experimental PVDC copolymer Vincentz Network +++ Plathnerstr 4c +++ D Hannover +++ Tel:+49(511)

8 8 Figure 4: 1000 x magnification of coating films: (a) organic-inorganic hybrid surface without organosilane; (b) with organosilane Vincentz Network +++ Plathnerstr 4c +++ D Hannover +++ Tel:+49(511)

9 9 Figure 5: 1000 hours salt spray test results of protective coatings on steel substrates according to the panels Vincentz Network +++ Plathnerstr 4c +++ D Hannover +++ Tel:+49(511)

10 10 Figure 6: Heat resistance test (90 C/24 h): (A) conventional WB PVDC copolymer coating, blistering defects; (B) WB PVDC/nano-scale silica binder coating, no blistering Vincentz Network +++ Plathnerstr 4c +++ D Hannover +++ Tel:+49(511)

11 11 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 Vincentz Network +++ Plathnerstr 4c +++ D Hannover +++ Tel:+49(511)

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