Module: 11 Lecture: 45 PAINT INDUSTRIES (continued) MANUFACTURE Raw material Paint Formulations White Gloss Enamels (Solvent Based) Ingredients % by Weight 1 Titanium Dioxide (Rutile) 29.3 2 Calcium Carbonate 1.4 3 Long Oil alkyd (70% NV) 52.2 4 Mineral Turpentine Oil (MTO) 13.3 5 Dipentene 2.1 6 Methyethylketoxime 0.1 7 Cobalt octoate (6%) 0.1 8 Zirconium octoate (18%) 0.3 9 Calcium octoate (3%) 1.2 Red oxide Primer (Solvent Based) Ingredients % by Weight 1 Red Iron Oxide 35.9 2 Zinc Chrome 10 3 Talc 3 4 Whiting 1.5 5 Lecithin 0.4 6 Medium Oil Alkyd (70% NV) 36.4 7 Mineral Turpentine Oil (MTO) 11.4 8 Cobalt Octoate (6%) 0.4 9 Lead Octoate (18%) 1 N P T E L 279
Exterior Emulsion Paint (Water Based) Ingredients % by Weight 1 Water 5.2 2 Non Ionic surfactant 1.1 3 Anti-foam 0.2 4 Ammonium polyacrylate solution (2%) 9.6 5 Titanium dioxide (Rutile) 20.5 6 Talc 8.4 7 Whiting 9.2 8 Propylene glycol 2.1 9 Pine oil 0.3 10 Preservative 0.2 11 Acrylic emulsion (46%) 43 12 Ammonia 0.2 Manufacture Pigment Other raw material Vehicle Mixer Mixer Batch tinting Ball mill Packaging & storage Figure: Manufacturing of Paint Block diagram of manufacturing process Diagram with process equipment Animation N P T E L 280
Required amount of ingredient along with pigments and vehicles are mixed in mixer which is usually high speed disperser. The basic operation in the manufacture of paint is the dispersion of pigment particles (often mixed with extenders) in a paint vehicle or medium to produce uniform, stable system. The process of pigment dispersion essentially consists of wetting, dispersion and stabilization of pigments and extenders in vehicle. The dispersion involves breaking down bigger aggregates and agglomerates to smaller units, wetting of these units and particles by the paint medium and stabilization of the resulting dispersion. After grinding, the mill base is mixed with other paint ingredients, i.e. vehicle and other additives and if necessary with tinting agents in mixer. Tests such as degree of dispersion (fineness of grind), viscosity etc. are carried out for finished liquid paint. The straining of paint is done to remove contaminants and it generally utilize metal or synthetic fiber gauge (screen). The paint passes through the hopper of the filling machine where it is filled into cans or drums, labeled and packed. Grinding mill All the grinding mills generally utilize application of shear, attrition and impact to effectively break down pigment agglomerates and aggregates and provide subsequent dispersion of smaller units. The grinding mills widely used in paint industry are Ball mill Pebble mill Attritor Sand mill Bead mill Basket mill High speed disc disperser etc. Ball mill Ball mills are primarily used for fine grinding is consists of a cylinder mounted on a horizontal axis and rotated at specific speed. Steel balls are used as grinding media which is placed inside the cylinder. The grinding action of ball mill embodies combination of impact, shear and attrition. Pebble mill Ceramic lining is provided inside the cylinder where ball mills are steel lined. Pebble mill are charged with balls of steatite, alumina or porcelain. Pebble mills are slower than the all steel mills in reaching degree of dispersion. N P T E L 281
Attritor Attritor is faster than ball mill and utilizes less space and is high efficient. Attritor consist internally agitated media in which the grinding chamber is static and the grinding media is stirred or agitated by help of rotating shaft to achieve dispersion where grinding media are steatite balls, steel balls and natural pebbles. Sand Mill Conventional vertical sand mills were invented to get around the batch size limitations of ball mills. The sand mill consists of a high speed rotor (impeller) with disks mounted on it at regular intervals in a cylinder. The space between the rotor and cylinder is filled with grinding media. The pre mixed pigment slurry is pumped in at the bottom of the cylinder and rises through the grinding media which is vigorously agitated due high speed rotation of the impeller. Dispersion of pigment takes place as a result of shear as it rises through the cylinder. Bead Mill It operates on the same principle as the sand mill using beads. (generally glass beads). Basket mill The Basket mill is comprised of two shafts. The main shaft is the basket or media agitator shaft and the second is the batch agitator shaft. The basket mill is a submersible milling unit where it will achieve particle size reduction without the use of hard to clean pumps, hoses, and tanks. The basket mill allows a greater amount of material to pass through the milling chamber. High speed disperser High speed disperser consists of a vertical shaft having high shear disc mounted at the end of the shaft. The disc rotates at very high speed (up to 5000 rpm) and creates a radial flow pattern within a stationary mix vessel. The disc creates a vortex that pulls in the contents of the vessel to the blades sharp edges. The disc surfaces then mechanically tear apart pigment particles thereby reducing their size, and at the same time dispersing them. High speed dispersers are normally used for pre mixing process, as dispersers for soft pigments and as thinning mixers. SETTING OF PAINT When the paint is applied on the surface of the metal or wood, the oil present in it forms a protective film of dried oil. The film is formed through oxidation in presence of air and polymerization of the unsaturated constituents of drying oil. The drier present in the paint accelerates the drying of the protective film through oxidation and polymerization and thus acts as a catalyst for these reactions. Pigments strengthen the film and protect it by reflecting the destructive ultraviolet light, while extenders increase the covering power and weathering of pigments and N P T E L 282
thus improve the consistency, levelling and setting. The cracking aspects are reduced by adding some oils as plasticizers. REQUIREMENT OF A GOOD PAINT Pigment should be opaque to ensure good covering power. Should be chemically inert to secure stability and hence long life. Should have a good colour and high hiding power. Should be weather resistant. Should have good washability and metal anti corrosive property. Its consistency should be suitable to appreciation by the types of application such as brushing or roller coating. The individual requirements are met by proper choice of pigments, extetenders and drying oils. Pigment volume concentration Various requirements, such as gloss, washability, durability and reflectance, rheological properties are largely controlled by pigment volume concentration (PVC), which is defined as, PVC = The PVC range for various paints as follows Paint PVC range 1 Flate paint 50 70% 2 Semigloss paints 35 45% 3 Gloss paints 25 35% 4 Exterior house paints 28 36% 5 Metal primers 25 40% 6 Wood primers 35 40% The gloss decreases as the PVC increases. This is due to the fact that when volume of pigment increases relative to the nonvolatile vehicle, gloss decreases until the finish or gloss of the paint becomes flat. With increase in PVC, adhesion as well as durability both decreases. If volume of pigment increases as compared to the volume of binder, the film will lose cohesion. The paint will be in powdered form and obviously will have little durability. When extenders are added, the PVC increases and gloss decreases. N P T E L 283
PAINT FALIURE The failure of paints may be due to various causes. Various terms used to explain the paint failure are as follows. Chalking It is progressive chalking or powdering of the paint film and is used by destructive oxidation of the oil after drying of the paint on the surface. Erosion Very rapid chalking is called as erosion. Flaking or peeling Poor attachment of the paint on the surface to be coated is called flaking or peeling and caused by the presence of dirt of grease on the surface or water entering from below the paint. Alligatoring If the center portion remains attached to the surface and the portion around the center peels off, a term alligatoring is employed. Checking Fine surface cracking is called as checking and is due to the absence of plasticizers in the paint. Blistering Appearance of blisters on the coat applied to wood is generally known as blistering. Blooming Appearance of dull patches on the surfaces called blooming. Blinding Discoloration of the film is known as blinding. Cissing Refusal of some portions of the surface to be painted is called cissing. Paint failures can be avoided by Careful mixing of the constituents or ingredients in specified proportions. Proper processing of the surface to be coated before the paint is applied. Using a primer coat before the application of the paint. N P T E L 284
PROPERTIES Property Example 1 Appearance Gloss/ Matt/ Semi-gloss 2 Application Method By Brush, Roller or Sprayer 3 Drying Time Fast dry/ Slow dry 4 Adhesion Adhesion to substrates/ Existing coating/ Intercoating 5 Mechanical Characteristics Hardness/ Flexibility 6 Resistance Ultra-violet/ Chemical/ Abrasion/ Fungus/ Algae 7 Outdoor Durability Gloss retention/ Color / Ultra-violet 8 Storage Stability Settling tendency/ Viscosity stability Special applications of the paints Paints are extensively used as acid resisting coats. Oil bound paints or distempers are widely used for interior decoration of walls. Coal tar products dissolved in mineral spirits have been used as protective coatings of pipes under the name bituminous paints. Ship bottoms are protected by antifouling paints which are prepared by mixing iron oxide, mercuric oxide and copper resinate dispersed in tung oil (vehicle). Paint with damp resisting properties can be prepared by mixing paraffin wax, rosin. Bitumen and gutta parcha dispersed in tung oil (vehicle). N P T E L 285