Functional Additives: Additives for controlling reaction rate, protecting against ageing, increasing flame resistance, for foaming, colouring, filling, reinforcing, etc.
Catalysts : Catalysts include tertiary amines, salts of weak acids, organometallics, phosphorus compounds, etc.
Inhibitors: Less well known are inhibitors of the isocyanate reaction which include hydrochloric acid, benzoyl chloride and p-toluene sulfonic acid.
Cross-linking agents and chain extenders: Used in cold curing, rigid and integral foams. OH- and NH-functional cross-linking agents and chain extenders are of major importance particularly diols and amines.
Surfactants: It ensure that the water polyisocyanate reaction leads to a uniform blowing reaction during the manufacture of PU foams.
Foam Stabilizer: The main foam stabilizers are water-soluble polyether siloxanes.
Cell regulators: Act mainly as defoaming agents. The most important are based on methyl polysiloxanes.
Blowing agents: They are required to foam polyurethanes. Several methods can be used
- CO2 is generated by reacting isocyanate with water; this serves as a chemically generated blowing gas;
- Low boiling liquids (e.g. the chlorinated fluoroalkanes and methylene chloride) are vaporised and physically foam the exothermic reaction mixture;
- A foam is mechanically produced by blowing or whipping-in air.
- The first method is preferred for flexible foams, the second for rigid foams and the third for foam coatings.
Flame retardants: Halogen and phosphorus compounds, Aluminium hydroxide control flammability and flame spread
Anti-ageing agents, Sterically hindered phenols and secondary aromatic amines are used.
UV Stabilizers: Light resistant polyurethanes can be manufactured using aliphatic isocyanates.
Bis-benzo-oxazoles, cumarene derivatives and bis(steryl)bisphenyls are used as UV stabilizers
Pigments and colourant: Inorganic pigments titanium dioxide, iron oxides, chromium oxides, cadmium sulfide, carbon black .etc.,
Organic pigments originate from the range of azo and diazo compounds and phthalocyanines
Antistatic Agents: The addition of antistatics tetraalkylammoniumalkyl sulfate to the polyol component reduce the surface resistance to 10(8) ohm.
FILLERS
- Fillers used are carbonates (chalk) and these represent about 50% of the total requirement in polyurethanes.
- Organic extenders used to lower costs include wood, straw, shells and plant materials
Reinforcements
- Glass fiber is becoming increasingly important because of "high modulus" RIM products.
- Glass (hollow) spheres of 5 to 300 m m diameter improve the properties of rigid foams
POLYURETHANE CASTING RESINS
- Liquid di-isocyanates (reactive resins) cure after mixing with polyols (curing agent) into moulded PU with various physical properties.
- Ready to use casting compound also contains fillers, plasticizers and colourants
STRUCTURE AND PROPERTIES
- The reactive resin reacts with the curing agent as follows.
- Mixing ratios are calculated according to this equation
- Further components are added to the cross-linkable PU casting compound to obtain specific processing characteristic and properties in the moulded material.
The characteristic properties of cast resins are
- Can be processed down to 0 ° C,
- Low exotherm during curing,
- Same casting resin formulation for small and large mouldings,
- Rapid curing with catalysts,
- Low shrinkage,
- Low shrinkage effect on inserts,
- Good adhesion on all materials,
- High resistance to chemicals,
- Low water absorption and permeability to water vapor,
- Safe handling,
- Low material costs.
PHYSICAL PROPERTIES
- Specific properties vary widely with composition, but urethane castable elastomers generally have excellent tear, abrasion, impact and wear resistance.
CHEMICAL PROPERTIES
Resistance to Chemicals
- PU casting resins are resistant to weak acids, alkaline solutions, greases, oils, aliphatic hydrocarbons. They are not resistant to strong acids and alkaline solutions, aromatics, alcohols and hot water.
Resistance to High Energy Radiation
- At a dosage of 1012 J kg-I/108 Mrad, tensile strength and elongation at break fall by about 20 %.
Weathering Resistance
- PU casting resins are weather resistant
Flammability
- Chlorine-containing curing agents and additives such as aluminum trihydrate (ATH) act as flame retardants.
Toxicological Assessment
- Vapors of all isocyanates are extreme irritants for eyes
- Greatest damage is caused by inhaling vapors and dust impurities
- MAC (maximum allowed concentration) in the work place has been reduced from 0.1 ppm to 0.02 ppm.
Polyester polyol : are non-toxic.
Cured Resin: Cured PU is non-toxic
PROCESSING
Casting
- Polyol, dehydrating components and other additives are stirred at temperatures of 100 to 120°C under a vacuum of 1 to 10 mbar until no more bubbles rise.
- Isocyanate components contain no additives and are used as supplied.
- Mixture is stirred for three minutes and cooled to ambient temperature.
- Insufficient isocyanate leads to soft moulded material, excess to greater hardness
- Curing time at room temperature is 8 to 10 hours, full cure a further 8 days. Increasing the temperature to 40 and 80O C typically reduces curing time to 90 and 10 minutes, respectively.
PU STRUCTURAL FOAMS
- Structural foams are manufactured mainly from polyether polyols with butandiol, ethylene glycol, aminopolyols or diamines as cross-linkers. Tertiary amines and organotin compounds are used as catalysts.
- The formation of PU foam in the mould can be controlled so that moulded articles with a cellular core and almost non-cellular surrounding region can be produced
- The outer zone reaches the density of the basic polymer. Moving towards the middle the density decreases gradually and reaches a minimum in the centre.
RIM quasi - pre polymer process
SECONDARY PROCESSING
Joining: PU structural foams are joined using reactive adhesives based on PU, EP and UP resins. PU structural foams to be screwed together.
AVAILABILITY
- There is a large variety of solid encapsulating resins and elastomer casting resins.
- Casting resin components are supplied in liquid form. The isocyanate resin is always unmixed.
- Polyol and isocyanate stocks must be protected from moisture.
APPLICATIONS
Cast PU Resins
- Casting of cable fittings , bonding and sealing of battery cases, encapsulation of transformers, ignition coils, encased capacitors tops, foundry binders, wear-resistant floor coatings.
Flexible PU Structural Foams
- Soft-elastic PU integral foams of low density (200 to 300 kg m-3 ) for bicycle saddles or safety components in the interior of cars, e.g. head rests and steering wheel covers; also for moped saddles and motorbike seats.
- Flexible PU structural foams of moderate density (400 to 600 kg m-3) for shoe soles.
- Tough and resilient PU structural foams of high density (700 to 1000 kg m-3), the so-called micro-cellular elastomers, used for (e.g.) car bodywork such as fenders, external mirrors.
- Solid RIM polyurethane (density around 1100 kg m-3) for external car.
Rigid PU Structural Foams
- Window and dome light frames, loudspeaker, radio and tape recorder casing, tennis-racket composites, cable sockets, consoles and housings for telex machines, copy and computer-equipment, typewriter casing, seating, beds, tables, laboratory furniture and shop installations.
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