car foams - 27
TRANSCRIPT
Elvis Ayroso (A007496H)
Edmund de Souza (A0118668R)
Characteristic carbamate functional group
Formed typically by an isocyanate group reacting with an alcohol group through step polymerization
Polyurethane properties can be dictated by chemistry of the polymer
Straight chain monomers can form soft blocks
Aromatic monomers can form hard blocks
Attributed to steric hindrance
Isocyanates◦ Methylene diphenyl isocyanate (MDI)
• Isocyanates Toluene
diisocyanate (TDI)
• Methylene diphenyl
isocyanate (MDI)
Polyols◦ Determines flexibility of the polyurethane material
Flexible polyurethane foams (FPFs) have some important
properties that make them ideal for car seats:
Lightweight
Retain their structure well
Easily shaped and cut
They also have a variety of other uses:
upholstering furniture and mattresses
packaging material for fragile objects
The global market for PFs estimated to be worth USD $33bn
in 2010, and produced about 13,650 kilotonnes of material.
Highly exothermic
The cross-linking between polymer chains:
CO2 is produced by the reaction mixture when water reacts with the isocyanate group.
CO2 (the primary agent) and methylene chloride are used as blowing agents, which cause bubbles to form within the polymer, leaving cavities within the plastic.
Methylene chloride is used because it has a low reactivity and a low boiling point (~40.7C).
Choice of additives◦ Tin based additives stabilize foam
◦ Amine based additives control rate of reaction and curing
◦ Choice of dyes might affect dispersity
◦ Surfactants and catalysts may also be used
Mixing◦ Raw materials are mixed and adequate dispersion is achieved
through high pressure impellers
Foam Forming◦ Polyurethane will then be injected into molds and foam forms
as reaction progresses
Curing◦ Crosslinks in polyurethane foam forms during this period
Choice of mold structure◦ PU is lightweight and mold material may not have to be very
strong
◦ Strategic placement of vents is crucial
◦ Amount of volume in mold can also dictate final rigidity
◦ Orientation of mold will dictate direction of PU expansion
Choice of mixer◦ Impinging mixers are recommended, since turbulence is
maximized through this form of mixing
◦ Turbulence leads to uniformity of PU material
Reaction is exothermic, safety precautions must be placed to prevent runaway reaction in the delivery system
Storage conditions of raw materials must be at an adequate temperature as not to solidify.
Polyurethane foams have a high ignition point (>350C)
Toluene diisocyanate is a confirmed carcinogen. To minimise exposure to this, most of the process equipment is shielded.
Industry has shifted away from the use of CFCs as blowing agents. Methylene chloride is unreactive, so emissions are less likely to have a significant environmental impact.
Polyurethane foams can be easily recycled – “offcuts” and unwanted foam are shredded and used to produce lower grade foams, which are mostly used in packing materials.
Even after refining the manufacturing process, as much as 30% of all flexible polyurethane foams can become scrap.