surface coating – What Is Piping: All about Piping Engineering

What is Surface Coating? Surface coating decorates and protects the surface on which it is applied. It can be defined as a homogeneous mixture of pigments, binders, solvents, additives. It covers the surface completely. The success or failure of any coating is influenced by the following factor: Substrate ConditionSurface condition and method of applicationEnvironmental condition at which it is applied and expected to withstand during serviceAnd last but not least the quality of paint used Why Coat a Surface? Decorates the surface on which it is applied.Protects the surface from rustProtects from micro-organism like fungi and algae maintaining its original body. RUST NEVER SLEEPS!!! Corrosion of Steel Environmental Corrosion: Unstable condition of steelChemical Corrosion : Chemicals like acids, alkalies attack metalCorrosion Prevention: Isolation of metals from corrosive environment by using paint coatings Fig. 1: Figure showing rate of corrosion. Corrosion Control Using Inhibitive Primers Zinc phosphates/Chromate forms a passive layer with adhered rust.Using Sacrificial Primer. Indirect catholic Protection by Zinc in Zinc-rich primersBarrier Coatings. High DFT Coating System Isolates Surface from corrosive Environment Composition of Paint or Surface Coat Basic constituents of paints are Pigments 5 to 25%Binders 60 to 65%Solvents 15 to 25%Additives 1 to 5 % Relative proposition of these ingredients can be varied to produce films with any desired physical and application characteristics Pigments for Surface Coating A finely divided powder which can disperse in media of various types to produce paints. It is insoluble in the medium. Important properties are ColourTinting StrengthOpacityFastness to lightResistance to heatOil absorption of pigmentParticle size: Hiding Power, Gloss or smoothness, Rate of settling of pigment Binders for Surface Coating Binders are the heart of the paint system. Binders bind or cement the pigment particle into a coherence film which adheres to the substrate. Mechanical and resistive properties of the film are controlled very largely by binder. The durability of the paint depends on the quality and quantity of binder used!!!! Convert the liquid coating on application to a solid dry film.Provide gloss to filmMaking coating adhere to the substrateGiven elasticity to filmResistance to water, chemical, and abrasionDisperse the pigments and extendersHold the pigment in suspension. The Choice of Binder for Paint depends on the end use of the paint Type of Binders Drying oils: Vegetable oils on exposure of air, convert from liquid to solid thru a process of oxidation. Can be a sole film former but most often mixed with resinResins: Most surface coating contain a synthetic resin based film former. Most decorative paints are based on oil modified resins. Few Important Binders/Resins ALKYD Resins Largest groups of synthetic resins.They are oil modified polyesterGood exterior durabilityLow alkali and water resistance. AMINO Resins: Melamine and urea-formaldehyde Epoxy Resins: Has excellent adhesion, hardness, chemical, and corrosion resistanceCan be used to do high build paintPoor exterior durability Poly-amide Resins: Used as curing agents for epoxy resins. Polyurethane Resins: Good resistance to high temp, chemical and acid resistance, good resistance to various gases, alkali resistance.Low resistance to solvents like ketones, esters Chlorinated Rubber: It is one pack thermoplastic.Have good Chemical resistance, good acid and alkali resistance.Can be applied as high build paint.Disadvantage: Poor resistance to high temp and solvent like ketones, aromatic HC Vinyl Resins:Cellulose Resins: Widely used in auto finishingAcrylic Resins: Possess good light fastness, good adhesion and excellent durability. Solvents for Surface Coating The primary function of the solvent is to dissolve film formers, thereby consistency suitable for the application. Choice solvent influences viscosity, drying and flow, and leveling. Solvents are lost at atmosphere, so it is an economic loss. Solvents, in isolation or combination, are used in making thinner for the paints. Examples of solvent: Hydrocarbon Solvent: Aliphatic, aromatic, solvent Naptha, alcohols, ketones, esters, etc. Additives for Surface Coating Used in a small amount to give a coating one or more desirable properties. The only difference between additive and other raw materials is that the amount of additive is very small. Properties that can be controlled thru additives are: Viscosity Setting Drying Gloss Opacity Bacterial action Thickness Deodorants etc Classification of Paints and Surface Coatings Paints can be classified based on: The Physical state: Liquid Paint and Stiff PaintThe Thinner Used: Water thinnable and solvent thinnableThe End used: Decorative and protective.Modes of film formation: Thermosetting and ThermoplastsThe order of application: Undercoat and topcoatThe extent of gloss: Glossy, semi-glossy, egg-shell. mattModes of Film Formation: Film formation is either by thermosetting or thermoplasts. Thermoplastic (Non -convertibles): In these coatings when the paint is applied on a surface, the solvent evaporates living resin to its original form spread over the surface. So change is only physical and can be reversed to its original form by using thinner. E.g Chlorinated Rubber Thermosetting (Convertible): Chemical changes occur in the coating and dry film is different from its liquid state. Ex. Epoxy, alkyds, etc. Surface Preparation Surface preparation is the most important part of a coating system. The surface preparation of the coating system is what a foundation is to a building. Surface Preparation of Steel Some of various methods of steel are DegreasingHand tool cleaningPower tool cleaningFlame CleaningPicklingAbrasive Blast CleaningWet Abrasive Blast Cleaning International Standard of Blast Cleaning Fig. 2: Few International Standards for Blast Cleaning Paint or Surface Coat Application Proper application of the protective coating is an important criterion in giving the paint system. Four main application procedures: Brush ApplicationRoller ApplicationConventional SprayAirless Spray Theoretical Coverage (Sq.M/Ltr) =(%Volume Solids X100)/DFT in microns