Inorganic Chemistry as an Option for Formulating High Solids, Low and Zero - VOC Architectural and Industrial Maintenance Coatings

Transcription

1 (The work described in this paper was not funded by the U.S. Envi Agency. The contents do not necessarily reflect the views of the Agency and no official endorsement should be inferred.) Christine Stanley Ameron Protective Coatings Division 201 North Berry Street, Brea, CA Raymond E. Forante Ameron Protective Coatings Division 201 North Berry Street, Brea, CA n\ltroduction Inorganic Chemistry as an Option for Formulating High Solids, Low and Zero - VOC Architectural and Industrial Maintenance Coatings -y,05.*.1;6 ZYLYW Architectural and lndushial Maintenance (AIM) Coatings are rhquired to protect a wide range of substrates from degradation by a very wide range of environments. Coating systems pr+ently used rely almost entirely on organic polymers. These chemistries are well established and give proven protection to stationary structures in many environments. Inorganic polymers have also been used in the form of inorganic zincs coatings, but these products have limitations in that they are used only on ferrous substrates, have poor aesthetic properties, are not very compatible with organic topcoats and do not give performance in acidic and caustic immersion service. However, their performance in these limited applications exceed their organic counterparts. Further, the performance properties of silicone and silicate resin in baked coatings is well documented and they provide superior properties in heat, Wand chemical resistance. Silicone resins have also been used to improve the performance of organic polymer coatings. They can improve such characteristics as heat and sunlight W resistance. Until recently, with the exception of inorganic zincs and some organic/inorganic.copolymers, inorganic polymers were not feasible for stationary structuresbecause they required baking. A breakthrough in curing technologies has led to polymers that will film form under ambient conditions to give inorganic backbones. Further, these coatings can be formulated to very low Voc's either in solvent, 100% =lids or water based options. The performance of these products, in general, exceeds those of organic based coatings. DISCUSSION Terminology For clarity, Table 1 gives definitions of the chemical terminology used in the following discussion. In coating compositions, the typical resins using the siliconoxygen bond as the repeating unit in the backbone are silicones and silicates. The term polysiloxane can include silicones, but it is u.4 herein in its broadest sense, that is, any polymeric structure that contains repeating siliconoxygen groups in the backbone, side chains, segments or cross links regardless of substitution on the silicon atom. The presence of certain organic groups bonded to the silicon atom in silicones and polysiloxanes moderates physical, mechanical and chemical properties, typically in an advantageous fashion'. Oxysilane refers to a silicon based structure in which the silicon is bonded to up to four alkoxide or hydroxyl groups thereby ren-, dering that structure reactive to certain condensation reactions; the oxysilane may be monomeric, polymeric or a pendant group of a larger molecule. Comparison of Inorganic and Organic Binders To understand the performance difference of organic and inorganic products, we must first mmment on comparative chemical properties of the binders (!%e Table 2). The high bond strength of the Si-0 bond compared to a typical C-C bond gives the inorganic structure strength making them more durable in comparison to carbon based structures. Further, this structure is more inherently heat stable leading to heat resistance up to 2000 F. By comparison, epoxies and polyurethanes are limited to the 20@300'F 273 PDF

2 range. Inorganic siliconeoxygen bonds are unaffected by sunlight and ulhaviolet attack. By comparison, organic binders such as epoxies and alkyds typically show early chalking and fading a d polyurethanes and acrylics will show fading and gloss loss in a 3-5 year period. Since Si-0 is already oxidized, polymers based on such a backbone are not affected by atmosphere oxygen and most oxidizing chemicals. In contrast, organic polymers will eventually oxidize and degrade. An inorganic structure is not combustible, organic polymers will burn and generate smoke and toxic fumes. Inorganic Binder Chemistry Traditional silicone and oxysilane curing involves a process called hydrolytic polycondensation in which an alkoxide silane is first hydrolyzed and thc resulting silanols are condcnsed to a polymer network (See Figure I). The innovations being described involve catalysis of this process or direct condensation and the selection of appropriate silicone and oxysilane precursors to produce binders of use in AIM coatings (See Table 3). Coating Formulation: Silicones Silicone copolymers (containing either alkyd, acrylic or polyester coresins) are well known. The amount of silicone resin incorporated in the copolymer determines the properties of the coating. The binders are manufactured by condensation at high temperatures. The AIM coatings curing and.drying properties are dominated by those properties of the organic component. Their solids and VOC content are also dominated by the organic component2. Coating Formulation: Polysiloxane The chemistry described above has been successfully applied to create pure polysiloxane network binders. These binders have been formulated into pure polysiloxane AIM coatings having maximid thermal, chemical and UV resistand. 'Further, polysiloxane/organic "hybrids" AIM coatings have been formulated that significantly enhance the properties of the selected traditional organic resin b ad coating. As well as high performance properties, these formulationsoffer advantages in VOCcontent. This chemistry increases molecular weight of the binder during the curing of the film to produce solid polymer networks. The silicone and oxysilane precursors used are selected not only for performance but for their very low viscosity allowing for low VOC content formulations. Pure solvent based polysiloxane formulas have been produced with volume solids contents of 8&95%. These precursors in hybrid system arealso selected for their compatibility and diluting effect on theorganic componentsgiving coatings a volume solids content of %. Water bome inorganic binders have also been formulated into coatings. The pure inorganic versions use silane and silicates as precursors and these systems require no rosolvents and have 0 VOC. Water bome organic hybrids can also be formulated at low VOC, however, these do require some cosolvent to form a film with acceptable performance. EXAMPLES The following examples are offered to demonstrate the unique properties provided by utilization of polysiloxane chemistry in AIM coatings. Polysiloxane Topcoats Topcoats have been formulated with both pure polysiloxane and 'polysiloxane/organic hybrid binders. Table 4 describes the formulations that exploit the ultraviolet resistance of the siloxane bond. These formulas have similar appearance to polyurethanes. Table 5 outlines the formulas' characteristics. The UV resistance of both these formulas has proved during accelerated weathering tests to be more resistant than traditional organic based topcoats (See Figure 2). Another interesting property of these polysiloxane coatings is their inherent compatibility with inorganic zinc silicate (102) primers. As dis- 274

3 .. cussed previously, IO2 primers have superior corrosion prfonnam properties, but limited compatibility with organic topcoats. Traditionally, a 3 coat system utilizing an IO2 primer has been used as a high performance system in the protection of steel in aggressive industrial atmospheres (See Table 6). The epoxy midcoat is used to tie the IO2 primer to weatherable topcoats that cannot beapplied directly to this primer. The compatibility of the polysiloxane topcoats with the IO2 primers allows a 2 coat system with obvious advantages (See Table 7). Water Based System The above system describes solvent borne topcoats. An alternative to the 2 coat system above is a 0 VOC water based system. IO2 primers are available for both systems that contain 0 VOC. This technology has been used to formulate a 0 VOC pure polysiloxane, water based topcoat. As with the previous system, the topcoat is very compatible with the 102 primer. This system will provide corrosion protection equivalent to the organic and solvent based polysiloxane system. The finish of this topcoat is flat and so is not usable where a gloss finish is requested. However, this product will maintain its finish and not chalk or discolor much longer than organic finishes. Table 8 shows the coating characteristics of this product. Heat resistant Polysiloxane Coating Compositions that contain pure polysiloxane binder networks have been formulated that provide maximum heat and/or chemical resistance Heat resistance in excess of 1100 C (2OOO"F) can be achieved. Typical formulations will contain micaceous iron oxide (MiOx) as the major filler component; the most successful formulations have the binder to filler ratio as near as possible to the CPVC. Table 9 gives a description of this type of formulation, Figure 3 shows thermogravimehic anaylsis for this type of formulation and Table 10 gives the coatings' characteristics. The weight loss over the temperature range is around 10%; this accounts for the loss of the organic substituent groups, absorbed water and residual solvent. me remaining film maintains mechanical integrity and continues functioning as a barrier coat even after high temperature exposure. Table 11 is a summary of representative properties of this typeof formulation'. Typical applications would include stacks, the exterior of reactors and on piping under insulation. There are no equivalent organic coatings with this level of performance. The closest heat resistant ambient cure products are silicone alkyds. Their temperature resistance can be as high as. 1OOO"F but their VOC content is generally above 400grams/liter. Chemically Resistant Polysiloxane Coating The same binder system can be used with an optimized pigment package to achieve chemical resistance of a rope not given by organic systems (See Table 11). Indeed, because this type of formulation is essentially inorganic, it behaves likea zinc silicate without theacid exposure and chemical reactivity limitations. Table 12 describes the coatings characteristia. Polysiloxane tanklining prototypes are resistant to virtually all solvents, organic acids and mineral acids in certain concentration ranges. However, pure polysiloxanes are not resistant to alkali. Table 13 is a summary of the representative chemical resistance of this type of formulation. Tank linings based on organic binders are available. Ambient temperature cured organic systems generally have good alkali resistance, but only moderate to poor resistance to solvents and acids. These products are available in low VOC content formulations, however, to approach the resistance of the polysiloxane to solvents and acids, force cure of bake systems are necessary. These systems are generally higher in VOC content and often include aromatic amine hardners that present toxicity problems during application. In ongoing formulation work, epoxy/polysiloxane hybrids havebeen developed thal cure at room temperature and combine the resistance of polysiloxane to acids and solvents with epoxies resistance to alkali. These coatings have shown the best combination of chemical resistance. 275

4 CONCLUSIONS In summary, it should be emphasized that a new formulation chemistry has been described. me products used as examples typify the first generation of oxysilane and polysiloxane based systems. This chemistry offers the opportunity for quantum improvements in such performance areas as heat. chemiyl. ultraviolet resistance and durabiliu. This chemistry allows the retention of desirable properties in existing systems while enhancing those areas needing improvement. The use of this chemistry is consistent with the need for developing hieh wrfonnance AIM coating systems that offer reduced environmental. health and s afetvw. REFERENCES 1. Brown, L.H.; 'Treatiseon Coatines"; Myers, R.R.; Long IS.. Ed.; Marcel Dekker, Inc.: New York, 1972; Vol. 1, Part 111, Chapter Finzel, W.A.; "Prow rties of Hieh Temwrature Silicone Coatines: - Joumal of Protective Coatings and Linings, 1987,4, Law,C.H.;Gysegem,A.P.;USPatent Casmena, R.L.; Brea, Calif., Oct 1991, Ameron Technical Report 99/

5 TABLE 1 Terminology Silicon Silane Oxysilane Silicate - Silica Siloxane Polysiloxane Silicone Organic The element (Si) Substituted silicon compounds Silicon compounds with at least one substituent an alkoxide, hydroxide or aryloxide Metal salt of siliconoxygen anion Sand; siliconoxygen compound Compounds with 2 or 4 oxygens bonded to silicon Polymer with siliconoxygen backbone Polysiloxane with organic substituents on each silicon, typically 2 Carbon based compounds; polymers with carbon-carbon units with backbone TABLE 2 Comparative Properties Inorganic and Organic Binders 1. Binder Backbone \ I -C-C- / \ \ / 83Kcal/mole - Si KcaVmole 2. Si-0 is W resistant 3. Si-0 is already oxidized 4. Si-0 is not combustible 271

6 TAB= 3 Advances in Siloxane Coating Chemistry Low/Ambient Temperature Curing Silicone Intermediates Oxysilane Prepolymers Formulation Technique Inorganic/Organic "Hybrids" TABLE 4 Pure Polysiloxane Coating Topcoat Formula Desmiption 1. Binder 2. Pigment 3. Curing Pure Polysiloxane Backbnne Silicon Substituents Selected/Balanced for W Resistance and Film Properties Cross Link Density Balanced for Film Properties Selected for Appearance Full Gloss Range High, Semi, Rat Ambient Conditions Single Package Catalyzed Hydrolytic Polycondensation Hybrid Polysiloxane Coating Topcoat Formula Description 1. Binder 2. Pigment 3. Curing Mixed Polysiloxane - Acrylic Backbone Silicon Substituents Selected/Balanced for W Resistance and Film Properties Acrylic Selected for Appearance, Physical, Mechanical Properties Cross Link Density Balanced for Film Properties Selected for Appearance Full Gloss Range Full, Semi, Rat Ambient Conditions Catalyzed Polycondensation 278

7 TABLE 5 Characteristic Pure Polysiloxane Hybrid Polysiloxane Topcoat Topcoat Number of Components 1 1 Volume Solids, VOC, gramdliter Dry to Touch, hrs at 700F 2 2 Dry Through, hrs at 70 F 6 8 Application Spray, Brush and Roller Spray, Brush and Roller A. Inorganic Zinc Silicate Primer ' Corrosion Control 8. Inorganic Zinc Silicate Primer I I Epoxy Midcoat Adhesion Promoter; Tie coat TABLE 6 I I 1 Topcoat Polyurethane Appearance Weatherability Three coats required to make the organic coatings, notably polyurethane, compatible with IOZ Corrosion Control I Polysiloxane Topcoat Barrier Appearance Weatherability Two coats because polysiloxane is inherently compatible with 102 No need for epoxy midcoat to make system compatible Epoxy film not needed for corrosion control; I02 is sufficient 279

8 TABLE 7 IOZ/Polysiloxane System Advantages Better weatherability Same corrosion resistance as three coat Less manufacturing touch-up/repair Faster manufacturing turn-around Lower VOC emissions Reduced waste generation/disposal TABLE 8 Coating Characteristics of Pure Polysiloxane Water Based Topcoat Components 1 Volume Solids. % 43 VOC, grams/liter 0 Dry to Touch, hrs at 70 F 0.25 Dry Through, hrs at 7OOF 12 Application Spray 280

9 TABLE 9 Polysiloxane Coatings Heat and Chemically Resistant Example Description Binder Pigment Pure Polysiloxane Background Phenyl and Mathyl Substituents to Balance Properties Oxysilane Cross Linking to Maximize Heat and Chemical Resistance Micaceous from Oxice Level to Maximize Heat Resistance or Silica/filler Blend to Maximize Chemical Reistance 3. Curing Ambient Conditions No Baking Hydrolytic Polycondensation TABLE 10 Coating Characteristics for Heat Resistant Polysiloxane Characteristic Components 2 Volume Solids, Sb 90 VK, grams/liter 96 Pot Life, hn 6 Dry to Touch, hrs at 70 F Dry Through, hn at 7OoF 1 24 Application spray 281

10 TABLE 11 Polysiloxane Coatings Heat Resistant Formula 1. Saltspray 5000 Hrs Blistering (ASTM D714) Corrosion (AVM D1654) Scribe (ASTM D1654) Adhesion 2. Heat Resistance E A - Total Weight Loss, to Io0o"C 10% Torch 'C. Discoloration 3. Condensing Humidity Hrs Blistering Corrosion Adhesion 4. Atlas Cell (Salt and Deionized Water) 1000 Hrs Vapor Phase Liquid Phase 5. Chemical Resistance (Representative) Immersion at 25 C. (Test time, hrs.) Acetic Acid 50% Sulfuric Acid 19% Hydrochloric Acid Acetone Mathylene Chloride JP4 Xylene Excellent 10 9 Excellent Pass, No Blisters Pass, No Blisters 282

11 ~~... (... r 1 Pdyiloxane 2 Acrylic Polysiloxsne 3 Acrylic Polyurethane 4 Silicone Alkyd 5 SG Acrylic Latex z Alkyd 6 Epoxy Figure 2. Comparison of the accelerated ultraviolet resistance (QW) of various generic classes of coatings 205

12 ( ew Y2 Ol.2Wt * U (wo moo a000 4wa roao ooo.4 m.0 (oo0.0 Temperafure ( c) Figure 3. lhemmgravimetric analysis of heat resistant Polysiloxane 286

13 ,.... TABLE 12 Coating Characteristics for Chemically Resistant Polysiloxane Characteristic Components 2 Volume Solids, % Bo VOC, grams/liter 99 Pot Life, hrs 6 Dry to Touch, hrs at 700F Dry Through, hrs at 70 F 1 24 Application spray TABLE 13 Pdysiloxane Coating Immersion Formula Representative Chemical Resistance Acetone Ketones Methanol Alcohols Xylene Aromatics Methylene Chloride Chlorinated Hydrocarbons Fatty Acids Acetic Acids Organic Acids Triethanol Amine SO% Sulfuric Acid 83% Phosphoric Acid 10% Nitric Acid 10% Hydrochloric Acid Not Resistant to Alkali 283

14 \ \ -Si -OR + H@ -Si-OHtROH / -/ \ \ / 2 -Si - OH -) -Si - 0 -Si - + H20 / / \ Figure 1. Hydrolytic Polycondensation of Oxysilane 204