Codes and Standards

IV. ZINC-RICH PRIMERS

Zinc, employed in coating films at loadings that insure the film conductivity, will form an efficient anode of a galvanic couple with steel, sacrificially corroding itself, and overriding local cell activity on the steel which becomes entirely cathodic and protected.

The concept is easily adaptable to practical coating systems, and such primers are the most efficient of all. Zinc.rich primers based on both organic and inorganic vehicles are widely and successfully employed. They are discussed in a separate chapter.

A. ORGANIC ZINC-RICH PRIMERS

The organic zinc-rich primer may be considered a special case of a high pigment volume concentration (p.v.c.) paint. It must maintain zinc particle to zinc particle contact within its continuum and contact between pigment and substrata to ensure electrical conductivity within the film end across the interface. These requirements translate to a paint formulated at a pigment volume concentration slightly above the CPVC. The film must also display sufficient adhesion at these loadings to anchor the system to the steel. Because of cathodic alkali generation at the interface, the vehicle must resist alkalis. Chlorinated rubber, epoxy/polyamides, high molecular weight linear epoxies and epoxy ester systems are used as binders, Epoxy esters do not have quite the alkali resistance of other vehicles, but certain specific vehicles (high epoxy content) offer¢ acceptable compositions, Primer films will vary and reflect the properties of the vehicle type.

In adjusting the p.v,c, to levels slightly higher than c.p.v.c,, the primer achieves its tightest zinc to zinc packing, and zinc encapsulation is minimized. Judicious mixing of zinc dust of different particle sizes will also assist here to provide more uniform packing, resulting in better particle contact and ultimate galvanic protection. Too high a p.v.c., produces a coating having poor physical and application properties. Were zinc the only pigment, the p.v,c. fixation of a zinc primer (and, therefore, its optimum zinc loading) would be simple. Formulations are complicated by pigment anti-gassing agents, thixotropes, anti*settling agents, extenders, etc. Small amounts of highly oil absorbent materials markedly depress the c,p.v.c., but not the p.v.c., and the coating becomes porous. Used in controlled amounts such materials may be employed to reduce zinc levels, and maintain a p.v.c.: c.p.v.c, ratio, thus obtaining a strong film. This provides enough film and film/substrate conductivity for good protection. Up to 15% mica has been effectively used in this way¹⁹. The use of conductive extenders (di-iron phosphide) is not related to p,v.c, effects, and high zinc replacements have been achieved with this type of pigment²⁰.

Careless application of any organic zinc-rich primer can severely change the p.v.c.: c.p.v.c, ration in the applied film. Even with the best thixotropes, zinc settlement is possible, particularly in single package coatings. In a non* homogenous film, some areas must surely be underbound, and some Iow in pigment. Poor physical properties, or zinc encapsulation (resulting in a nullification of cathodic pro* tection) are the result. Zinc encapsulation has ruined many jobs at the outset, although the coating system itself may have been quite satisfactory, Application of organic zinc-rich paints must involve continuous agitation throughout the application.

B. INORGANIC ZINC-RICH PRIMERS

Inorganic zinc.rich paints, unlike the organics, depart radically from conventional paint technology. These vehicles (generally silicates) do not bind zinc as do the organics, but chemically react with zinc ions on zinc parti* cie surfaces forming primary bonded zinc silicate matrices.

As presented in the SSPC*Paint 20 classification, vehicles may generally be classed as either alkaline silicates (water solutions of sodium, potassium, lithium, or quaternary ammonium silicates) or alkyl silicates which may be ethyl silicate (the most common) or higher alkyl or alkoxy homologues.

1. Alkali Silicates
Film formation of the alkali silicates involves water evaporation followed by neutralization of the silicate alkalinity (either by post curing solutions or by acids derived from the atmosphere such as carbonic acid) to form silicic acid. This then reacts with zinc ions to form the three dimensional zinc-oxygen-silicon (zinc silicate) matrix. Further atmospheric neutralization of residual alkali with consequent tightening of the matrix will Occur with aging²¹.

2. Alkyl Silicates
The alkyl silicates form their matrix by an analogous route (evaporation of solvent, and hydrolysis of the vehicle to silicic acid by atmospheric moisture with evolution of the pertinent alcohol)22, The silicic acid forms a matrix with the zinc similar to that described above23. Again, hydrolysis continues for some time after initial cur* ing. The chemical nature of fully cured zinc silicates is (theoretically, at least) identical, irrespective of the silicate used.

3. Single.package
Single-pack inorganic primers of the polyol silicate type involving ester exchange or interester exchange reactions with alkyl, alkoxy and hydroxy alkoxy silicates to form mixed esters pursue the same basic chemistry to provide similar film matrices". Organic moieties remain within these films and subtract from their wholly inorganic nature. Other routes to single package inorganic zinc-rich primers include the use of amine-initiated hydrolysis of alkyl polysilicates,^25.26 and the use of alkali metal alkoxide catalysis of hydroxyl free alkyl silicates27. Single pack inorganic silicate vehicles are now generally available. Some are modifications of silanes.

4, Inorganic vs. Organic
With inorganic primers the p.v.c, concepts must be modified. Also, zinc levels lower than those necessary for the organics are possible with little immediate loss of performance. Zinc levels of 70% of film weight can give acceptable performance, and even levels of 50% if enhanced by other con* ductive pigments. Reduced zinc level products do not have as good a performance in the long term as do the 85% loaded primers28.

As discussed in the chapter on zinc-rich paints, the inorganic films show better perform-anoe than most organics. Their films are strong, hard, and resistant to impact and abrasion. They are quite resistant to heat. The matrix of the inorganic primer film is not subject to age-related deterioration as are organic primers. Weathering may actually improve its physical properties.

Adhesion is of an exceptionally high order and has led to speculation on the formation of primary valency linkages with the substrate as well as the zinc. The mechanism of adhesion at this point is unknown. Surface preparation requirements are exacting (particularly with the alkaline silicate systems), a commercial blast being the very minimum acceptable, and a white or near white blast with a typical surface profile of 1 to 2 mils being more usual and often mandatory. Alkyl silicates are rather more tolerant of poorly blasted substrates than the water*based type, probably reflecting their higher organic content and lower surface energies.

Despite their incompatibility with poorly prepared surfaces, the inorganics may be considered safer coatings than the organics. Not only are the organics subject to encapsulation, but also they can mislead the applicator by adhering initially to poorly prepared substrates. But adhesion is not protection, which is possible only through intimate contact of iron and zinc (the more contact, the better the protection). Inorganics require such contact (through good surface preparation) not only for protection, which may be initially difficult to determine, but also for good initial adhesion, which is easy to determine. If an inorganic sticks, it should protect. If an organic sticks, protection is still an open question.

5. Secondary Mechanisms
If cathodic protection were the sole mechanism of zinc-rich Primers, they would rapidly break down as zinc was consumed. In practice, this is not the case. As zinc corrodes, its corrosion products (depending upon the environment) tend to polarize the reaction, coating the zinc and bridging the voids within the film, thereby packing them so that the primer is sealed from the environment. The film is slowly transformed from a zinc-rich primer to a barrier primer, and, in this state, it is maintained until the zinc is again exposed by some abuse. The zinc will then corrode again and be healed with corrosion product (providing that the agents of physical or chemical abuse have been removed). These phenomena are primarily responsible for long term zinc.rich protection.

Zinc-rich primers are normally applied at approximately 3 dry mils. Some compositions form good films at thicknesses up to 6 mils, while others may severely mud-crack at these high builds.

The porosity of zinc-rich films (particularly the inorganic) can lead to problems because of air occlusion on top coating. The resultant bubbling and pinholing of applied top coats may necessitate the use of mist coats, thinned finish coats or tie coats (such as the WP-1 wash primer, SSPC-Paint 27) before finish coat application". Careful formulation of solvent system and pigmentation of the finish coat can mitigate this problem, and many manufacturers carefully tailor finish coats for bubble-free application over the zinc-rich primers³⁰.

6. Pre. Construction Primers
Single component inorganic zinc-rich films make good pre-construction primers, protecting steel during storage and fabrication. In thin films, they allow easy cutting of steel and good weldability, particularly when modified with the conductive extender, di-iron phosphide^31.32, After fabrication, pre-construction primers may be recoated with a suitable primer and top coat. Further considerations on zinc-rich primers are discussed in a separate chapter.

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V. INTERMEDIATE AND FINISH COATS FOR STEEL

A total coating system cannot be considered without regard for its external interface. While the primer mitigates metallic corrosion, the finish coat must counteract malignancies specific to the environment in which the system must function.

As finish coats are designed at Iow p,v.c./c.p.v.c. ratios, they are dense, highly dielectric, and are applied in film thicknesses as high as possible, They are much tike barrier primers, and play no inconsiderable part in assisting the primer in its anti*corrosive function. Unlike barrier primers, which are designed to be re.coated and thus protected from the environment, the finish coat must contend with the environment from which it must shield the lower elements of the total system.

Oleoresinous, varnish-type finish coats had to be built up of successive coats to the required film thickness, because of drying time difficulties inherent in thick, oxidizing films, High-build thermoplastics and high solids ther-mosets have enabled high film builds to be applied in single coats. While thinner coats applied successively give better solvent release; fewer pinholes and voids, and better continuity throughout the film, high*build systems have economic advantages and careful formulation can minimize their shortcomings. Statistically, more coats increase the risk of intercoat failure, although adhesion at any coating/coating interface is usually better than at a coating/metallic interface.

In any coating system, compatibility between coats is critical, Compatibility can prevent solvent attack on primer films by the finish coat, solvent induced bleeding of organic pigments from one coat to a subsequent coat, and other pitfalls. Less appreciated are effects of poorly matched viscoelastic properties, which may become obvious only after aging, inflexible finishes applied over flexi* ble primers can eventually lead to cracking on aging. Too flexible a finish coat can actually pull an inflexible primer from the substrate. Cohesive failures (cracking, checking, etc,), and adhesive failures (flaking, popping, blistering) can have grave consequences in anti*corrosive coating systems. Ali elements of the coating system must be matched to one another and to the substrata, For instance, thin steel siding requires a more flexible system than one applied to structural members, as does aluminum with twice the coefficient of expansion of steel.

Flexibility can ;be built into a system by p.v.c, adjustments, or by vehicle-changes including blending, copolymertzation, and plasticization. Primers are generally kept less flexible than succeeding coats.

Finish coat selection is dictated by the environment, although there are considerations with regard to the primer and intermediate coat that may influence this. Environments vary widely, ranging from exposure to weather and UV (ultraviolet light), to chemical immersion, high temperatures, and physical abuse. They may be simple or complex involving intermittent immersion, chemical attack, large temperature differentials, and extreme abrasion. All elements of the environment must be considered and evaluated in terms of their relative importance to provide the best compromise system.

The vehicle binder of the finish must bear the brunt of the environmental attack. Most design decisions should be based on the polymer chemistry of the vehicle involved. An empirical awareness of the effects of UV, moisture, oxygen, chemical attack, microbiological attack, highland Iow temperatures, abrasion and impact, etc. on individual finish coat polymers will often suffice, but in-depth experience and understanding of the effects of such phenomena on molecular structure may be essential when resin systems must be mixed or synthesized to attain the desired results.

An in-depth discussion of the wide ranging characteristics of each polymer type is quite beyond the scope of this chapter and is presented in a separate chapter. Table 1 presents a summary of the properties of finish coat materials. The following is no more than a brief discussion of those polymers commonly used in maintenance finishes.

A. LACQUERS: THERMOPLASTIC COATINGS

A lacquer is simply a coating that forms its film by solvent evaporation alone. Vinyls are the most common lacquers used in anti.corrosive maintenance finishes. Solutions of high molecular weight, vinyl chloride/vinyl acetate copolymers or terpolymers with vinyl alcohol or maleic acid, are used. Vinyls are distinguished by excellent acid and alkali resistance (their backbone being exclusively carbon/carbon bonds), good abrasion ?resistance and, when pigmented, excellent exterior durability. Vinyl films are attacked by the solvents from which they were cast (ketones, esters, etc.), also by concentrated organic acid, and softened by aromatics. They have Iow water and oxygen transmission rates, and are suitable for water immersion service.

Acrylic films have even better resistance to ultraviolet light than the vinyls, and show long-term gloss and color retention as well as good weatherability in exterior environments. They are more suitable for polymer modiftca-tion than the vinyls, and copolymers of both acrylates and methacrylates are possible. Acrylics are also copolymer-ized with styrene and vinyl toluene. The introduction of such aromatics may slightly upgrade the acid and alkali resistance of acrylics, although the UV resistance is decreased.

Styrene is also copolymerized with butadiene. These lacquers form films with better chemical resistance than the acrylics (ail carbon-carbon backbones with the advantage of aromaticity), but aromaticity and unsaturation in certain species of these copolymers give poorer UV resistance.

Chlorinated rubber coatings have perhaps even lower moisture and oxygen transmission properties than the vinyls, and the absence of pendent ester groups provides better chemical resistance than the acrylics. Solvent resistance is not as good as the vinyls, but chlorinated rubber has better compatibility with other film formers than have the unmodified vinyls³³.

In any lacquer, care must be taken with selection of additives and modifiers (such as plasticizers) to suit the requirements of the environment. Where high alkali resistance is required, hydrolizable plasticizers (phthalates, etc.) Are best avoided. The inert chlorinated paraffins are widely used with thermoplastics. Tricresyl phosphate is often used with vinyl systems.

Because of the finite viscosity limitations of high molecular weight thermoplastics, high-solids lacquers are not possible. By judicious formulation with slow to medium evaporating solvents and efficient thixotropes, however, high-build thermoplastics (vinyls, chlorinated rubbers) are quite possible and widely used.

Numerous other thermoplastics which can be employed in maintenance systems are beyond the scope of this chapter, although reference to their chemical structure will give a good general guide. Further data and case histories may be obtained from the respective resin manufacturers.

All thermoplastics display a glass transition temperature (Tg), and will flow at high enough temperatures, becoming soft and tacky. Heat resistance may be somewhat limited, and at Iow enough temperatures, the coatings will become brittle with reduced physicals.

B. LATEX

Latex systems show every possibility of expanding in. to the maintenance painting area as technologies develop. Unlike lacquers, latexes are dispersions (not solutions) of thermoplastic polymers in water. Molecular weights are not restricted by solution viscosities, and much higher solids of very high molecular weight polymers are possible. Film formation involves evaporation of water followed by coalescence of discreet particles of polymer (micelles) dispersed in the water. Total coalescence has never been achieved,³⁴ and systems give higher moisture and oxygen transmission rates than their lower molecular weight analogues cast from solution. Great strides are being made and their excellent durability and mechanical properties indicate a bright future for latex maintenance systems in moderate environments. The PACE program of SSPC has included the evaluation of many water-based systems.

C. OXIDIZING SYSTEMS

Oxidizing systems are thermosets which convert to three-dimensional polymeric networks by absorption of atmospheric oxygen. Such systems are based on fish and vegetable oils (esters of glycerol and vegetable oil, fatty acids) or modification of such materials with other species. Unmodified oils are rarely used now except in certain specialized primers. They are stow drying and susceptible to alkalis, but have excellent low surface energies and are perhaps the best vehicles where surface preparation is poor.

Alkyds are oxidizing systems, the polycondensation products of multi-functional polyols and di-functional acids which are generally oil modified to give a wide variety of vehicles. Alkyds may also be copolymerized with phenol, silicones, styrene, acrylics and other resins. Still the backbone of the coatings industry, alkyds have limited applications in heavy-duty maintenance. As with oils, the ester groups in the alkyd backbone are easily cleaved by alkalis. Chemical resistance is poor, ,and they are not suitable for immersion service, cementitous substrates, or for use directly over zinc-rich primers. They lend themselves well to polymer modification and may be used with certain thermoplastics to provide increased gloss and adhesion.

Alkyds make an excellent choice of vehicle in light-duty environments. Thirty percent silicone modification provides finish coats that have excellent ultraviolet light resistance and exceptionally good weathering properties.

More alkali-resistant oxidizing vehicles are obtained with the phenolic varnishes and epoxy esters (epoxy resins esterified with oil fatty acids). Both vehicles have better chemical resistance than either the unmodified oil or the analogous alkyd, and, as phenolic or epoxy content of such resins increases, so does their chemical resistance. Unfortunately, as with all epoxies, epoxy esters yellow and chalk markedly during exterior exposures, and while deterioration is not progressive, chalking restricts usage on aesthetic grounds. Epoxy esters are hard, resistant to abrasion and soluble in aromatic and even aliphatic hydrocarbons.

Replacing the dibasic acid with a polyisocyanate, the alkyd becomes an oil-modified urethane or uralkyd, Ester linkages are replaced by urethane linkages, and chemical and physical properties are again upgraded. Like epoxy esters, uralkyds both yellow and chalk on exterior exposure, and are difficult to recoat.

D. CHEMICALLY CURING THERMOSETS

Oxidizing systems are a special case of thermosetting vehicle, where the activator is supplied by the environment. In other cases, water from the air will cure the vehicle. In baking finishes (e.g. alkyd/amino) both reactive oligomers are added to one can, but are selected so that the reaction will only occur when the system is applied as a film and subjected to high temperatures (180-400°F depending on vehicle type).

More commonly, maintenance painting systems are designed so that reactions occur at room temperature. Reactive vehicles are packaged separately and combined in the field just before application. Of these two-package systems, the two most important are epoxies and polyurethanes. The properties of both systems are related to the chemical constitution of the reactants. Epoxies are less complex than polyurethanes because of the more limited choice of reactants. Epoxies are available in molecular weights ranging from materials which are liquid at room temperature to high molecular weight materials which may themselves be used without crosslinking as lacquers. The epoxy resin is most commonly cured in the field with polyamines or polyamides. With a given epoxy, polyamides produce a tighter crosslinked film having greater chemical resistance, hardness, and cure response than the same resin cured with a polyamide. The polyamides give better flexibility, water resistance and exterior durability. As a molecular weight (distance between reactive oxirane groups) of the epoxy resin' increases, the cured material becomes more flexible, but poorer in solvent resistance.

In general, epoxies show excellent adhesion, good chemical resistance, especially to alkalis and good solvent resistance. Epoxies have less acid resistance than vinyls and chlorinated rubbers, but show good abrasion and impact resistance. Exterior durability is hampered only by a tendency to chalk and yellow, which is not progressive and does not affect resistance properties. Chalk resistant epoxies with good color retention are now availabte³⁵. Because of the variety of possible epoxy resins and curing agents, performance capabilities will vary widely from one product to the next³⁶.

With certain coal-tar pitches, epoxy systems give synergistically improved coatings, which, at 16-20 dry mils or so, give good protection against moisture and oxygen transmission, chemical attack and physical abuse. They are ideal coatings for areas with restricted access after application.

Polyurethane systems have an even wider variety of possible reactants. While polyisocyanates are generally limited to adducts of toluene diisocyanate or hex-amethylene diisocyanates, polyols used in the formulation are exceptionally varied. Almost any resinous material having di- or poly-functionality based on a hydrogen donor will react with isocyanates, although - OH functionality is most widely used in coatings. The opportunities for molecular engineering with the urethane (and its associated groups, ureas, substituted ureas, allo-phanates, biurets, etc.) are much greater than with other types of vehicles. Hydroxy-terminated polyesters and acrylics, epoxies and other polyethers, phenolics, polycaprolactams, cellulosics, and even alkyds are all possible hydrogen donors. The urethanes may range from hard, chemically resistant finishes, to soft rubbery finishes having good abrasion resistance. Toluene diiso-cyanate coatings give exceptional chemical and abrasion resistance, while hexamethylene diisocyanate systems have exceptional light and weather resistance, and despite high costs are finding ever increasing markets. Urethanes show better curing properties at Iow temperatures than do most epoxies, but specific properties will depend greatly upon the type of modifier selected³⁷.

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