TO DEFINE COMPOUNDING in PLASTICS TERMINOLOGY – “The intimate mixing of the PVC resin with its associated additives necessary prior to convert into a thermoplastic melt”.

PVC compounds are based on the combination of the PVC polymer RESIN and additives that give the formulation necessary for the end-usage (Pipes or Rigid Profiles or Flexible Profiles or Sheets). The compound is formed by intimately mixing together the ingredients, which is subsequently converted into the “gelled” article under the influence of heat and shear force. Depending on the type of PVC and additives, the compound prior to gelation can be a free-flowing powder (known as a dry blend) or a liquid in the form of a paste or solution.

PVC compounds when formulated, using plasticizers, into flexible materials, usually called PVC-P.

PVC Compounds when formulated without plasticizer for rigid applications are designated PVC-U.

PVC Compounding can be summed up as follows:

The rigid PVC dry blend powder (called the Resin), which also contains other materials like stabilizers, additives, fillers, reinforcements, and flame retardants, must be intensively mixed in the compounding machinery. The dispersive and distributive mixing is critical, and all in compliance with well defined temperature limits.

According to the formulation, the PVC resin, plasticizer, Filler, stabilizer and other auxiliaries are put into hot mixer mixing. After 6-10 mins discharge into the cold mixer (6-10 mins) for premixing. PVC compound must use the cold mixer to prevent material stick together after the hot mixer.

The mixture material after plasticizing, mixing and dispersing evenly at around 155°C-165°C is then fed to the Cold mixture. The melting PVC compounding is then pelletised. After pelletizing, the granules temperature can be dropped to 35°C-40°C. Then after the wind-cooled vibrating sieve, the particle temperature drops below room temperature to be sent to the final product silo for packaging.

THE MAIN INGREDIENTS OF THE PVC COMPOUND ARE -

PVC formulations include different types of additives which assist in imparting a large range of physical and chemical characteristics. This versatility is the main reason why PVC has been so successful as a commodity thermoplastic, from medical applications (like blood bags) to long life applications such as Curtain Wall Profiles or Thermal Break Profiles or window frames). The unique polar characteristics of PVC permit a wide range of appropriate additives to be incorporated within the polymer. The main groups of additives are:

PVC RESIN, Plasticizer, Heat stabilizer, Impact modifier, Lubricant, Filler, Flame retardant, Smoke Suppressant, Pigment, Blowing agent, Biocide, Viscosity modifier, Antistatic agent, UV absorber, Antifogging agent and Bonding Agent.

POLYVINYL CHLORIDE RESIN

PVC is a product based on two of the earth’s natural resources, salt and oil. Salt water electrolysis yields chlorine (in addition to caustic soda and hydrogen). Ethylene can be derived from naphtha when oil is refined. Chlorine and ethylene can be combined to form the monomer, vinyl chloride (VCM). PVC results from the polymerization of vinyl chloride.

PVC cannot be processed on its own due to its very low thermal stability and high melt viscosity. Therefore, it is necessary to combine with the polymer a number of suitable additives to give a wide and varied range of properties to satisfy many different end-use applications.

PLASTICIZERS

Plasticizers are added to PVC to achieve flexibility and workability. The flexibility is determined by the type of plasticizer and level used in relation to the base PVC . They also operate as an internal lubricant between the PVC molecules. Normally, they are based on organic esters,e.g., phthalates, adipates, trimellitates, phosphates etc.). The main factors which influence plasticizer choice are determined by the specification requirement of the finished item and depend on - Formulation cost, Migration/permanence, Plasticisation effect & Solvating efficiency of the plasticizer, Volatility, Plastisol viscosity and Extraction. The ease with which plasticizer is combined with PVC is a measure of processing characteristics important in the dry blend mixing operation.

HEAT STABILIZERS

Normally, Lead-Based Stabilisers (Lead Stearate) are primarily used for Rigid PVC applications viz. pipe, fittings, and Rigid profiles. These Lead based stabilizers are very cost effective heat stabilizers. With good insulation resistance, lead stabilizers have also been used in PVC-P in wire and cable across the world.

Some other Heat Stabilizers used in Solid Form are Bisphenol A/Alkylphenols, Epoxidised Soya Bean Oil (ESBO), and Calcium Zinc (Ca Zn) Based stabilizers.

Liquid Stabilisers are also available based upon the end usage of the material. Some of these are Organotin Compounds (based on alkyl tin such as methyl, butyl or octyl derivatives, usual mixtures of di-alkyl and mono-alkyl); Mixed Metal Compounds heat stabilizers (which are a blend of the metal soaps or salts in combination with organic phosphites). These materials are used almost exclusively in PVC-P applications.

FILLERS

For most PVC-U and PVC-P applications, fillers are added primarily to reduce formulation cost, but, many a times it is also used to enhance properties and performance. There is a relative balance between the cost benefits and any acceptable deterioration in physical properties that could result by adding these fillers. Another important aspect of adding fillers is the influence on processing with respect to output and surface finish.

Wood Fillers/Fibres/Flour Composites has expanded recently.

Other Fillers used are- ground marble fillers,Ground dolomite (calcium magnesium carbonate) and limestone fillers are also used. Talc is also used in calendered PVC compounds as filler.

FLAME RETARDANTS/SMOKE SUPRESSANTS

PVC-U formulations have low flammability due to the chlorine content. The addition of plasticiser in PVC-P formulations necessitates the use of flame retardant and smoke suppressant additives. These additives are known as functional fillers and a correct balance is necessary to achieve all the end-use specification requirements. They are predominately used in cable, conveyer belting and roofing membrane formulations to give resistance to fire initiation and propagation. It is also important to reduce dripping in a fire situation and that as little smoke as possible is generated. Antimony trioxide has been used extensively, usually in combination with phosphate ester plasticizers, giving excellent fire performance and mechanical properties.

The FR mechanism is activated by the formation of antimony oxychloride which acts as a radical scavenger and flame poison. However, antimony trioxide is a suspected carcinogen and work is ongoing to replace or reduce the levels used. The use of zinc sulfide has been suggested.

PIGMENTS

Pigments for PVC must be thermally and light stable,have good dispersibility and be compatible with in the formulation. Inorganic pigments are the most common type. Titanium dioxide (TiO2) pigments are used to give‘bright’ whiteness and opacity. Specific titanium dioxide grades are used in PVC-U applications and contribute to outdoor weathering performance. Their influence on photo degradation, and on the kinetics of weathering have been studied. Reversible discoloration effects linked to the photochemical degradation of titanium dioxide pigmented PVC, have been shown, after a period of storage of the aged material in the dark. This has been attributed to the formation of particular polyenic sequences, with the screening effect of the pigment protecting these polyenes against photo-oxidation, so permitting these polyenes to accumulate in the degraded polymer.

BIOCIDES

PVC-P materials, such as flooring and roofing material, can be prone to microbiological attack in humid or damp conditions. This is due to the fungi using the plasticizer at the surface of the article as a food source. This can lead to partial discoloration (pink color or black specks) which can further cause a tacky surface where dirt can accumulate. Unpleasant odors may also be a consequence. Biocides function by becoming active on the surface of the material to destroy the fungi. Plasticizer transfer to the surface is limited by the process of diffusion of the plasticizer within the material, the fungus also acting as a leaching solvent.

BLOWING AGENTS

Solid blowing agents are materials which decompose to release gases at particular temperatures matching the appropriate melt viscosity necessary to retain the foam structure.

There are two main types –

  • Azodicarbonamide, which is exothermic in nature and decomposes to give primarily nitrogen gas
  • Sodium bicarbonate, which is endothermic and so absorbs heat as it releases carbon dioxide gas.

ANTIOXIDANTS & LIGHT SUPPRESSORS

Primary antioxidants, such as hindered phenols, operate as effective radical scavengers to protect the PVC material during processing and in use. Phosphites and thiosynergists are also used as secondary antioxidants to extend the efficiency of the primary antioxidant by reduction of oxidation intermediates. Light stabilizers also prevent photo degradation. UV absorbers such as hydroxybenzophenone or hydroxyphenyl triazole types operate by absorbing and dissipating UV radiation prior to potential degradation of the polymer. No permanent chemical change occurs, so activity is retained.

Plastic Extrusion is one of the most widely used manufacturing processes in the manufacturing domain of Plastic Products. Thermoplastic extrusion in its current form remains a powerful tool for high-volume production of continuous profile parts.

The Plastic Extrusion Process :

To understand the plastic extrusion process, it is useful to get a picture of what is an extruder and how it works.

Typically, an extruder contains the following parts:

Rigid PVC Profiles Extrusion
  • A Hopper (used for storage of PVC Compound during the process)
  • A feeder (where the plastic compound enters the barrel from the Hopper)
  • A Metal Barrel, duly heated and contains a “Screw” driven by motor that forces material to the feedpipe
  • A Breaker Plate, fitted with screens to filter the material and maintain pressure
  • A feedpipe, where the molten material exits the barrel and is delivered to the die
  • A Die, which shapes the material into the desired extrusion profile
  • A cooling system, which helps the extrusion solidify evenly
  • A set of Calibration Dies to ensure homogeneity and get accurate dimensions of the Profiles

The plastic extrusion process starts with filling the hopper with PVC Compound in the form of pellets or flakes. The material is gravity fed through the throat of the hopper into the barrel of the extruder.

Once the material enters the barrel, it begins to be heated via heat zones. (The heat zones may be cooler near the throat and hotter near the die, to allow for gradual melting.) As it is being heated, the material is simultaneously pushed towards the die end of the barrel by a reciprocating screw, which is driven by a motor. The screw and pressure also generate heat, so the heat zones themselves do not have to be as hot as the required extrusion temperature.

The molten plastic exits the barrel through a screen reinforced by the breaker plate. This screen removes contaminants from the material and maintains even pressure within the barrel. The material passes through a feedpipe into the custom-built die, which has been machined with an opening shaped like the desired extrusion profile to create a custom plastic extrusion.

When forced through the die, the material assumes the shape of the die opening, finishing the extrusion process. Once fully through the die, the extrusion profile is cooled in a water bath or via a set of cooling rolls, causing it to solidify.

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