PHOSPHATING & POWDER COATING

PHOSPHATING

SURFACE CLEANING

CHEMICAL PRE-TREATMENT (PHOSPHATING)

*Pretreatment- The preparation of a part prior to the application of a coating in order to improve adhesion & corrosion resistance.

• Phosphating is a surface treatment process by which the virgin steel surface is converted to metallic phosphate & is widely used for preparing metal surface before painting. (Thickness lies between 3 to 50 micron)

• Phosphate coatings are produced by the chemical reaction of base metal with aqueous solution of phophoric acid and phosphates (Ions of Bivalent metals…Zn+2, Mn+2, Fe+2).

• Such coatings do not offer complete resistance to atmosphere & principally used as an adherent base/primer coat for paints.

MECHANISM OF PHOSPHATING

• The free phosphoric acid in phosphating solution reacts with the Iron surface undergoing treatment in the following manner: -

Fe + 2H3PO4 Fe (H2PO4)2 + H2 ..(1) • Thus producing soluble primary phosphate and liberating

Hydrogen. Local depletion of phosphoric acid occurs at metal-solution interface. As the primary phosphates of Zinc dissociate readily in aqueous solution, the following reactions takes place: -

Zn (H₂PO₄)₂ Zn HPO₄ + H₃PO₄ ……..(2) 3 Zn HPO₄ Zn₃ (PO₄)₂ + H₃PO₄ ……(3) 3 Zn (H₂PO₄)₂ Zn₃ (PO₃)₄ + 4 H₃PO₄ .....(4) • The neutralisation of free phosphoric acid by reaction (1)

alters the position of equilibrium of equations (2), (3) & (4) towards the right and there by leads to the deposition of sparingly soluble secondary phosphates and insoluble tertiary phosphates on the metal surface.

PHOSPHATING • The conventional seven-tank Hot Zinc

Phosphating system is used. • The following are the different steps involved in

the process:-

– Alkali degreasing

– Cold swilling

– De – rusting

– Cold swilling

– Hot phosphating

– Cold swilling

– Sealing/Passivation

PHOSPHATING EQUIPMENT

ALKALI DEGREASING

• Tank:…………………………………… Mild steel

• Capacity: ……………………………..7000 L

• Chemicals used:

–Sodium hydroxide………6 – 8%

–Tri sodium phosphate….2-3%

• Operating temp: ………………….60 – 700C

• Duration:…………………………….. 15 – 20 mts.

*continuous air agitation is provided for effective cleaning.

COLD SWILLING

• Tank:…………………………….......... Mild steel • Capacity:………………………………. 7000 L • Cold swilling medium:…………… Static water • Operating temp: …………………….Room temp. • Duration:………………………………. 2-3 times *Continuous agitation is must for effective cleaning

(It brings fresh chemicals in the bath to the substrate & cleaning is very effective ).

DE- RUSTING

• Tank:………………………………………… Mild steel • Capacity: …………………………………..7000 L • Chemicals used:

– Phosphoric acid……………………25-30% • Operating temp:………………………. Room temp • Duration: ………………………………….5 - 7 mts. *Oxide films & rust stains are dissolved in acid. *The tank is lined with FRP to avoid corrosion of the

tank.

COLD SWILLING

• Tank: …………………………………………..Mild steel • Capacity: …………………………………….7000 L • Cold swilling medium: ………………..Static water • Operating temp: …………………………Room temp. • Duration: …………………………………….2-3 times *continuous air agitation is provided for effective

cleaning.

HOT PHOSPHATING

• Tank: ………………………………………….Mild steel • Capacity: ……………………………………7000 L • Chemicals used:

– Phosphoric acid & Zinc phosphate..30% – Accelerators (Sodium nitrite, an oxidising agent)

• Operating temp:………………………..60 – 700 C • Duration:……………………………………10 - 15 mts. *Iron dissolves in phosphate solution & hydrogen is

evolved due to chemical reaction & adheres to the metal surface. Sodium nitrite reacts with hydrogen & forms water.

*The tank is lined with stainless steel

COLD SWILLING

• Tank: …………………………………………..Mild steel • Capacity: …………………………………….7000 L • Cold swilling medium:………………… static water • Operating temp: ………………………….Room temp. • Duration: …………………………………… .2-3 times *continuous air agitation is provided for effective

cleaning.

SEALING (PASSIVATION)

• Tank: …………………………………………….Mild steel • Capacity: ………………………………………7000 L • Chemicals used:

– Chromium tri oxide…………….. …..0.025 – 0.05% • Operating temp: ……………………………60 – 70oC • Duration: ………………………………………25 -30 sec. * Phosphate coating is porous. Therefore, oil or other

sealers such as Chromium tri oxide, are used to achieve corrosion resistance.

POWDER COATING

INTRODUCTION

• An advanced method of applying a decorative and protective finish to a wide range of materials and products that are used by both industries and consumers.

• The youngest & fastest growing surface coating techniques

• First introduced in USA in the mid of 1950 & used first in LLH(W) under ER of IR

• Environment friendly (not emitting any VOC’s & HAP’s)

• Over sprayed powder can be recycled

• Uniform, durable, high quality and attractive finish

A QUICK OVERVIEW

• Very simple process

• More jobs can be tackled at a time

• No significant skill is essential

• Lasts for a longer period

• Material cost is very less

WHY POWDER COAT ?

• Produces a hard, abrasion resistance & tough coating

• Can be applied over a wide range of thicknesses (20-25 mils/500-600 microns)

WHAT IS POWDER COATING

“A mechanical mixture of resin, pigments & hardening agents in powdered form, when applied on to a surface as a thin/thick layer, forms an adherent film after curing”.

CONSTITUENTS OF POWDER COATING

• Powder coating is composed of finely ground particles of:

• Resins: key component

Thermosetting, Most suitable for thin coating. (Epoxy, Polyester, urethane, Acrylics, etc) & Thermoplastic, Most suitable for thick coating. (PVC, Polyehylene, Polypropylene, nylon, etc)

– Good resistance to chemicals

– Low shrinkage during curing

CONSTITUENTS OF POWDER COATING

• Pigment:

Titanium dioxide, Carbon black, Zinc oxide, etc.

– Provides desired colour & opacity

– Protects from UV-light

– Improves impermeability to moisture

• Fillers:

Barytes, French chalk, Talc, Whiting, etc

– Fills voids

– Reduces the cost

CONSTITUENTS OF POWDER COATING

• Degassing agent: – Eliminates gas bubbles

• Accelerators: Copper salts, etc

– Enhances the rate of slow reaction • Dry flowing agents: Poly acrylates, silicone, fludised alkyl esters, etc

– Improves free flow of powders • Waxes:

– Provides slip

MANUFACTURING POWDER PAINT • The dry ingredients in exact amounts are blended at

high speed to create a homogeneous mixture & heated to the resin melting temp in an extruder.

• The extruded mixture is then cooled & rolled before being broken into small chips.

• The chips are milled to a very fine size ( Average diameter of powder particle varies in between 30-50 micron) & packed.

• Each individual powder particle contains the entire coating formulation.

COMPRESSED AIR DATA

• Requires good quality of compressed air, free of oil & moisture.

• Pressure ------------------- --4 to 5 kg/cm2

• Max. moisture content----1.4 gm/m3

• Max. oil content-----------0.1 ppm

POWDER COATING PROCESS

• Basic steps:-

– Surface preparation

– Application of powder

– Curing

– Recovery of powder

SELECTION OF GUN ATTACHMENT & RIGHT POWDER

• No perfect tip for all applications.

• Extension lances help to get into recessed areas.

• Unwanted areas to be covered with high temp tape specially designed for powder coating.

• Right powder to be chosen as per the end use of the component.

POWDER COATING APPARATUS

Powder coating gun Powder coating machine

APPLICATION OF POWDER

BASIC POWDER COATING PROCESS

Four basic processes:-

• Electrostatic spraying

• Fluidised bed

• Electrostatic fluidised bed

• Flame spray

*Electrostatic spraying is the most commonly used

OPERATIONAL PROCEDURE OF POWDER COATING

• Ensure proper earthing to the equipment.

• Put powder into the hopper.

• Connect the equipment to the main air supply.

• Switch on the power supply.

• Direct the gun into the spray booth & press the trigger.

• Adjust the fluid bed air pressure to 4-5 kg/cm² till the powder in the hopper appears to ‘boil’.

• Don’t keep higher pressure than necessary otherwise powder may ‘puff’ out the gun.

*By increasing dosing air, powder speed will increase, cloud density is reduced. By reducing dosing air, powder speed will reduce, cloud density will increase.

ELECTROSTATIC SPRAYING

• An electrical charge is applied to the dry powder particles while the component to be painted is electrically grounded.

• The charged powder & grounded work piece create an electrostatic field that pulls the paint particles to the work piece.

• The coating deposited on the work piece retains its charge, which holds the powder to the work piece.

• The powder coated work piece is placed in a curing oven, where the paint particles are melted on to the surface & the charge is dissipated.

* The gun gives +ve electrical charges on the powder particles by frictioning between them & non- conductive material (air).

POWDER SPRAY BOOTH

ADDITION OF VIRGIN POWDER

• Virgin powder should be added frequently to the powder hopper because, recycled powder will always have properties that differ from virgin powder.

• In order to maintain a constant composition, always keep the volume of re-used powder to minimum.

• To reduce variations in particle size distribution due to recycling.

CURING

• The curing process requires a certain degree of temp for a certain length of time in order to reach a full cure.

– Temp…………….180-2000C

– Curing time……45 minutes.

• The application of energy to the product to be cured can be accomplished by:-

– infrared cure ovens (direct exposure to light in the range of IR-region)

RECOVERY OF POWDER

• Powder coating booth is equipped with powder recovery cyclone and filter.

• It recovers as much as 98% of powder exhausted from the booth.

• Recovered powder is progressively re-used.

• After filter, filters fine powder of powder recovery cyclone and virtually no powder goes to atmosphere.

COMPONENTS BEING CURED

COMPONENTS READY FOR DESPATCH

POWDER COATING STORAGE CONDITION

• Powder coating must be:-

– flowing easily

– capable of maintaining a good electrostatic charge.

• Proper storage is essential as it:-

– prevents particles agglomeration

– ensures satisfactory application.

FACTORS AFFECTING POWDER STORAGE CONDITIONS

• Factors:-

– Temperature

– Humidity

– Contamination

– Direct sunlight

Recommended optimum conditions:-

• Temperature ………………………….250C (Approx).

• Relative humidity…………………….50 to 65 %.

• Away from direct sunlight.

SAFETY & HEALTH

• Powder & air mixtures can be a fire hazard when an ignition source is introduced.

• The concentration of powder in air must be controlled to maintain a safe working.

• Any finely divided organic material, such as dust, can form an explosive mixture in air.

• Personal cleanliness should be maintained at all times (soap & water clean up to remove powder from the skin)

• Don’t use air blast nozzles to blow off powder from the skin.

SAFETY & HEALTH

• Disconnect the main power supply before cleaning or maintaining the gun

• Operator should hold the gun in his bare hands.

• Avoid inhalation of the powders.

• Use proper PPE

ADVANTAGES OF POWDER COATING

• Releases zero or negligible amounts of VOC into the atmosphere, resulting in minimal air pollution.

• Easy recycling of over sprayed powder for reuse

• Provides a superior finish, greater durability, improved corrosion resistance.

• Lower raw material (powder) costs.

• Unlimited colour selection with high & low gloss.

• Easy to apply thick coatings

• Requires minimum operator training & supervision

• Parts can be racked closer together.

DISADVANTAGES OF POWDER COATING

• Application of large areas not feasible.

• Coating touch up not possible.

• Difficult to apply intricate shapes & assembled components

• Can be more prone to orange peel.

• Difficult to apply thin coats (< 1.0 mil) .

REMOVING POWDER COATING

• Methylene Chloride is most effective.

NB: Most recently the suspected human carcinogen Methylene Chloride is being replaced by benzyl alcohol with great success.

*(Organic solvents such as Acetone, Thinners, etc, are completely ineffective).

• Can be removed by abrasive blasting.

• Can be removed by using 98% commercial grade Sulphuric acid.

COST EVALUATION

ITEM

MLY. CONSUMP.(Kg)

UNIT COST(Rs.) TOTAL COST(Rs.)

Degreasing Chemical

10 26/- 260/-

De-rusting chemical

10 30/- 300/-

Phosphating chemical

8 31/- 248/-

Total cost per month

Rs. 808/- only

Calculation of cost of powder coating against each coach on the basis of 25 nos. of coaches per month:-

Cost of phosphating chemicals:-

COST EVALUATION

• Raw material cost per coach for phosphating

= 808/25=Rs.32/- -------------------

• Powder required per coach=1.67 Kg

• Cost of powder per Kg = Rs. 270/-

• Total cost of powder for each coach

=Rs. 451/- ----------------------------------

• Total raw material cost for powder coating of each coach= A+B=32+451= Rs.483/- --------------------------------

• Labour cost=7.5X 13 Man days=97.5 hrs.

• Hourly rate= Rs. 39.60/-

• Total labour on cost for each coach= 97.5x 39.6=Rs.3861/--

• Total Cost for each coach= C + D= 483+3861=Rs. 4344/- only

A

B

C

D

POWDER COATING OF RAILWAY COACHING COMPONENTS

• Side wall panel

• Roof panel

• Partition frame

• Chair pillar

• Kitchen berth frame

• Push plate

• Spring cover plate

• Basin cover

• Soap dish

• Flushing valve

• Curtain rod • Bottle opener • Guard rail bearing • Tissue paper box • Shutter • T- bolt assembly • Towel rods • Long hinge. etc

Done at Liluah workshop:------