Instruction manual for powder paints handling

composed by Kazan State Technological University

              laboratory head PhD in Chemical Sciences

                                                         V.N. Naumov

1998  

1. Selection of powder paint type

There are two main types of powder paints (PP): thermosets and thermoplastics.

When the thermoset PPs are heated, chemical reaction occurs (chemical crosslinking) that results in high performance properties of the coat: high hardness, thermal stability, adhesion to base, protective and decorative properties. Only physical processes occur during thermoplastic coating formation, namely melting and bodying. Thermoplastic coatings are softer and more flexible (e.g. polyethylene, polyamide coats), generally with poor adhesion, and can not be used under high temperatures (more than 100-120^oC). Thermosetting PPs are mainly employed nowadays for application of protective and decorative coatings.

According to film-forming base thermosetting PPs can be divided to: epoxy, polyester-epoxy, polyester, polyurethane, polyacrylic, etc.

Film-forming base is crucial factor that determines coat properties.

Epoxy PPs are widely used for application of protective and decorative coats. The possess good mechanical strength, adhesion and solvent resistance. However epoxy powder paints get yellow when overheated. This peculiarity explains necessity of thorough heat curing for such coats. Failure to strictly maintain thermal mode of hardening indicated in certificate leads to colour change, deterioration of adhesive behaviour and, as a result – performance data. Being exposed to UV rays (sunlight) upper layer gets destroyed (chalks). This results in worsening of aesthetic appearance only, usually not affecting protective characteristics of the coating.

Epoxy-polyester PPs comprise epoxy and polyester filming agents reacting with each other when heated. Today epoxypolyester PPs are dominating among powder paints. This type of PC has lower yellowing tendency and is more resistant to overheating in furnace (overexposure in drying chamber).

Polyester PPs are primarily applied on outdoor wares and in cases when yellowing is intolerable during overheating. Polyester powder paints do not chalk outdoors that enables their use in coating façade elements, aluminum window profiles etc. As distinct from other PPs, it is less resistant to solvents.

Polyurethane PPs are relatively expensive and are used to a limited extent. However they are more resistant to weather and abrasion.

Acrylic PPs are relatively expensive coatings used primarily for wares exposed outdoors. Polyester PPs have similar service characteristics and are more economically feasible.

According to appearance, PPs are divided to glossy, high-gloss, mat and semimat.

There are different kinds of PPs (powder paints) by decorative effect. They are distinguished to: hammer paints; paints providing textured surface appearance – from orange skin to glass-paper; metallic paints; low-friction paints; unpigmented varnishes for unpainted metallic surfaces protection, as well as for making “depth” effect on painted surfaces and for application of top layer on some metallic coatings; paints with high zinc content – powder primers for corrosion protection of steel; heatproof special polyester PPs; paints for porous surfaces (e.g. for aluminum alloy, hot-galvanized materials or porous welding seams) preventing blistering and swelling; the multicoloured PPs being special for bringing depth effect and similarity with natural material.

Specific type of PP is selected from above mentioned list depending on: 1. Service conditions (indoors, outdoors, in air, soil, aggressive media); 2. Metal type (steel, aluminum, zinc etc.); 3. Surface type (continuous, porous, welded etc.); 4. Desired decorative or protective effect.

Wide choice of paints complying with requirements mentioned in items 1-4 intended for specific usage conditions is provided by «Herberts Powder Coatings» company being a part of German group of chemical companies Hoechst AG.

2. Preparing wares for coating

Before applying protective powder coating metal surface shall be cleaned of rust and dirt.

Preparation for coating includes mechanical cleaning, brushing, abrasive treatment, etching and bead-blasting.

Mechanical treatment method is defined by article purpose, coating service conditions, required surface smoothness.

Highest protective characteristics may be achieved after bead-blasting that creates developed active surface. Sand-blasting is used for parts that are intended for use under atmospheric impact and in aggressive media.

Brushing and abrasive treatment are used at relatively low corrosion. Rust and calx are removed to clean metal.

If mechanical methods are ineffective, e.g. in narrow slots and gaps, etching is used. Generally etching is employed for removing rust and calx, formed during heat treatment and welding.

Solutions to be used for etching are: 10% H₂SO₄ , 0,5% thiocarbamide or1% formaldehyde, at 20⁰С; 20% HCl, 0,5% urotropine at 20-25⁰С; 55% H₃PO₄(density of 1,4-1,6), 25% ethanol, 20% Н₂О at 18-25⁰С. Etching is followed by flushing and drying.

After removal of corrosion products (and before etching, in case of strong soiling with oil deep-seated in rust) part shall be defatted.

Defatting may be conducted with organic solvents of water alkaline solutions.

Denaturated alcohol, isopropanol and acetone being most suitable, cheap solvents like 646 can also be used for defatting (though the last is more toxic than alcohols and acetone). Turpentine is tolerable. Petrol and white-spirit possess worse dissolving power and another drawback is low ignition temperature. All operations with organic solvents shall be held in compliance with fire safety measures with at air vent on. Chlorous hydrocarbons possess very good defatting capacity, however only dichloromethane with MAC of 50 mg/m³ may be used for defatting.

For iron, titanium and nickel alloys defatting may be done with alkaline solutions.

Solution composition, g/L: NaOH 10-20, sodium carbonate 25-50, liquid glass 3-5; temperature 60-80⁰С, exposure (submersion) time 10-45 min.

For jet defatting these components shall be taken in 15:15:3 ratio in 1 L of water at 70-80⁰С for 0,75-1 min.

Defatting solution for aluminum (% mass):

butyl alcohol 40, isopropyl alcohol 30, orthophosphoric acid 10, water 20. Alcohols may be replaced with butyl cellosolve with addition of 1-2% (mass) of anion active surfactant (e.g. sulfonic acid).

After alkaline defatting article is flushed with large amount of warm water, dried and then immediately painted. When it's to be stored defatted, the article shall be passivated.

Solution for defatting and passivation of ferrous metals with insignificant pollution in washing machines or with other means (g/L):

Soda ash     8-10;

Surfactant (ОП-7, ОП-10, synthanol АЛМ)      1;

Sodium nitrite 2-3 at 60-70^oС for 3-5 minutes.

Defatting and depreservation of heavily polluted ferrous metal wares are held in jet washing machines at 70-90^oС for 5-15 minutes with following solution (g/L):

Soda ash 50;

Sulfonic acid 50

One of the solutions suitable for simultaneous defatting and etching is (g/L):

Phosphoric acid (density 1,8) 100-200;

Synthanol DC                                        10 3,7;

Trisoduim phosphate                                20-50

Process duration is 3-30 minutes, depending on fat pollution level and presence of rust. After removal of pollution parts are water-flushed and passivated with 2-3 g/L sodium nitrite and 2-3 g/L soda ash at 60-70^oC.

For removal of emulsol film from ferrous metal wares, following solution is used (g/L):

Trisodium phosphate   8-10;

Liquid glass                  2-4;

Caustic soda                 3-5;

Sodium nitrite                6-8

All above described solutions for acid etching and alkaline defatting carry a defitine threat for personnel. So all operations with solvents are held in separate room equipped with washing machines and/or jet washers in compliance with all applicable safety measures. Worked acid and alkaline solutions and flushing water shall be properly disposed. Discharge to sewage shall be done only in compliance with all sanitary regulations.

Environmental harm may be decreased and sanitary regulations may be fulfilled by using synthetic detergents (soap powder or paste), even defective batches that can be bought at low price at the factory. Defatting is conducted with 1-5% detergent solutions at 70-80^oC for 10-20 minutes or by jet spray for 1-2 minutes. If above mentioned is unavailable, it's admitted to treat parts with rags, moistened with warm detergent solution with subsequent flushing with hot water, drying and additional defatting with available solvent.

Material surface degreasing quality inspection

Quality inspection of material surface degreasing includes several steps. 2-3 drops of benzene are applied on tested surface and kept for not less than 15 seconds. Then the sample is pressed by pieces of filter paper and held down till the solvent is soaked in paper completely.

Other sheet of filter paper is exposed to 2-3 drops of benzene and held down till the solvent evaporates. Then both sheets are compared by sight under outdoor lighting by their outwards. Degreasing is considered satisfactory if oil spot on the first sheet has disappeared.

Proper degreasing of surface is enough before painting articles for indoor usage. If coated part is intended for operating outdoors, it is necessary to carry out chemical treatment - phosphate coating for ferrous and zinc surfaces, and chromate coating for aluminum parts.

Phosphate coating is carried out with 15-20% solution of phosphoric acid with 1-3% of tannin as an additive.

Oak or willow extracts in aqueous solution forms are used as tannin. These solutions are applied with brush or hydrospray.

Chromate coating is carried out using the following composition (g/l):

Phosphoric acid    100;

Chromic anhydride  20;

Sodium fluoride         5

In order to ensure quality chromate film, the surface should be treated with this composition not less than 3 times at 20-25^oС. All operations with acid compositions are carried out with observance of safety precautions and industrial sanitary.

3. Coating application and formation process

Coating application and formation process includes following steps:

1.     Preparation of powder paint;

2.     Preparation of coated ware;

3.     Coating application;

4.     Coating formation.

Preparation of powder paint

In high humidity conditions indoors (e.g. in summer time) powder gets clumped. This results in blocking of spray (PP not fed to nozzle) or in periodic emission of PP ("spitting") and, consequently, in coating defects. To avoid this, PP shall be dried and sifted. Drying is carried out in oven at 40-50^oC for 1-2 hours. Then paint is evenly scattered on the pan and mixed at a regular intervals. After it cools down to room temperature, PP is sifted through 0.12-0.16 sift. Ready paint is brought to coating stage.

PP application is carried out with electrostatic or tribostatic spray gun. Unlike electrostatic, tribostatic gun does not require external power supply and powder is charged by friction on the surface of special dielectric material.

Tribostatic guns have higher safety in terms of electric current and electric discharge affection risk and provide complete fire safety during coating application.

Tribostatic gun is specific for lacking Faraday screen effect and inverse corona that cause bad penetration of paint particles to corners and closed spaces, recharging and repelling of particles from ware surface, all this resulting in appearance deterioration of the ware.

Electrostatic spray guns usually have higher capacity and are suitable for all kinds of powders. «Start» gun used in powder coating area has the following characteristics:

Power voltage 200 V;

Voltage at discharge electrodes up to 50 kV;

Excess pressure required for powder coating process is 0.2-0.4 atmospheres.

Compressed air fed on spray must necessarily pass through air preparation unit including water and oil separator and drier (silica gel tank). Excess pressure is regulated by valve with manometer in 0.2-0.4÷0.5 atm. range, depending on required production capacity. Powder paint is filled in spray bunker and closed with a lid. According to article type, flow of air and powder from bunker may be regulated with respective taps on spray gun handle.

Special nozzles are used for coating large surfaced articles, while long and narrow articles may be coated without special nozzle. If paint is "blocked", gun shall be shaken up. All wares to be coated shall be properly earthed; it affects coating quality directly. Colour heterogeneity shall be avoided as it may deteriorate article appearance after drying. Spray nozzle shall be held at the same distance from ware surface, best in 15-25 cm range. Electronic system of spray acts so that effective charge decreases until full disappearance when electrodes built in the gun approach article surface and increases to the maximum level (50 kV at discharge electrode) when moving them apart.

After application of PP article is exposed to drying oven for a period indicated in paint certificate. Optimal operation mode: loading painted articles to pre-heated oven. Temperature indicated in the passport shall be maintained with ±5^oС accuracy. It shall be made sure that temperature gradient in various parts of drying chamber is constant and does not vary much from top to bottom.

If temperature difference throughout the height is high, exposure time shall be corrected. Exposure time shall be counted from full fusion of PP provided required temperature is achieved. Drying time varies for massive articles, depending on their feature of construction and is defined experimentally. It may be 10 to 30 minutes in addition to drying time indicated in paint certificate.

4. Probable defects and their prevention

Powder paint application defects.

Powder paint application defects are revealed visually. Surface colour heterogeneity may be an evidence of defect appeared after coat setting. Not completely painted areas shall be repainted. Deficient paint application defined by color heterogeneity shall be eliminated by additional passage with spray gun. Foreign particles sticking to the article shall be blown away but not be brushed off together with the paint as in this case appearance will be deteriorated even after additional spraying. In obvious cases of not completely painted areas and application defects unset paint shall be washed away with water and the article shall be dried and re-painted.

Defects emerging after coating setting are harder to eliminate. Not completely painted areas may be painted by spraying powder on hot surface (immediately after extracting article from drying chamber) with second exposure at given temperature following. Unpainted dots may be eliminated employing НЦ enamel of required hue, which shall be selected after dissolving in suitable solvent (acetone, 646, 648) of appropriate colour gamma. Paint is applied with a brush, article is dried at room temperature and finally exposed to drying chamber for required period of time.

If its appearance does not satisfy due to coating defect or wrong colour, article is completely repainted. This can be conducted employing «Start» fluid that creates electroconductive layer on the surface of painted substrate. Painted article is wiped with Elektro-moistened rag, exposed to air for 2-3 minuted, grounded and then coated with powder. Paint curing is conducted according to orders mentioned in the certificate.

5.     Tests of physical and mechanical properties of coating

Physical and mechanical properties of coating shall conform with those indicated in technical specification for Russian PPs or in product certificate for foreign PPs. Manufacturer guarantees declared physical and mechanical properties if curing procedures are duly held. In definite cases (PP out of date, poor treatment of substrate etc.) necessity may arise to define coating adhesion and film flexibility of set paint.

It is carried out along with GOST 15140-78 State Standard (Paint Materials, adhesion determination methods). Adhesion determination is conducted visually and is evaluated on a four-point scale, or according to number of squares flaked from substrate. Lattice cuts are made with a device having working blade. Not less than 5 parallel and 5 perpendicular cuts to substrate shall be made 1 mm from each other forming a grating of 1x1 mm squares.

After cutting lattice surface is cleaned from flaked film pieces and adhesion is evaluated on a four-point scale.

Best mark – 1 point – smooth cut edges and no flaked pieces. 2, 3 and 4 points for 5, 35 and more per cent of pieces flaked from each lattice.

Second method derives from the first one. After making 5 parallel cuts, sticky tape (10x100 mm) is applied perpendicularly across the cuts, leaving one end loose. Then the tape should be quickly pulled away. Adhesion is evaluated as follows: 1 if cut edges are smooth; 2 for slight peeling of the film (within 0.5 mm); 3 for peeling of whole stripes.

Coating flexibility is defined according to GOST 6806-73 State Standard (considering amendments №1 dated 01.05.1998).

Tested polymer paint is applied on polished sheet steel (thickness 0.25-0.31mm; width 20-50 mm; length 100-150 mm). After curing at given temperature and 2 or more hour stand at room temperature sheet is bent over shank of definite diameter (1, 2, 3, 4, 5, 6, 8, 10, 12 mm) starting from the smallest. Bending value of coating is taken as smallest diameter of shank on which paint coating stays intact (no cracks).

6.     Safety measures for powder paints handling

According to class of hazard powder paints are classified as low toxic polymer materials. Paint toxicity is defined by structure of its constituent substances. LD (severe oral intoxication) for powder paints is found in 1350-1390 mg/kg range.

PPs may cause skin irritation and long contact with PP leads to allergy and eczema. It's necessary to avoid inhaling dust and prevent PP from getting on mucous membrane during work with PP. If it hits into the eye, it is necessary to bathe them with large quantities of water. Maximum allowed concentration of dust in workshop air is 3 mg/m³.

All works with PP should be carried out in rooms equipped with forced local and general supply-and-exhaust ventilation according to GOST 12.4.021-75 state standard providing air conditioned according to GOST 12.1.005-88.

Flammability characteristics.

Lower flammability limit 400^oC

Lower combustion energy 5 mJ

Lower explosion limit of air mixture 30 g/m³

Room floor shall be well grounded. Equipment used shall be explosion proof. Grounding required when pouring paint from one tank to another as static electricity may emerge while mixing different powder paints. Working personnel shall wear antistatic treated shoes and clothes. Paint shall be stored in properly shut tanks away from fire, heat, sparkles etc. Smoking and eating at the workplace are forbidden. It's necessary to keep dust content in air below lower explosion limit and maximum allowed concentration indicated in sanitary terms.

Workplace is equipped with fire extinguishing appliances in accordance with GOST 12.3.005-75 and GOST 12.3.002-75 state standards.

Wet cleaning without scattering of the paint must be conducted in rooms where it is used. Spilled powder should be cleared with a vacuum cleaner running on compressed air. Spilled paint accumulation in working area shall be avoided!

Persons are admitted to painting works that have learned working instructions, instructions for handling PPs, safety and fire safety rules and passed medical inspection.

Workers are provided individual safety means complying with GOST 12.4.011-87, GOST 12.4.068-79 and GOST 12.4.202-83 state standards.

Those directly involved in work with PP should wear eyes protecting glasses if there is risk that paint hits the eye and gloves if he contacts PP directly or for a long time. In case of insufficient ventilation masks or semimasks with antiduct filter or РПП-57, ШБ-1 (ГОСТ 12.4.028-76) respirators shall be worn to protect face.

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