car foams - 27

21
Elvis Ayroso (A007496H) Edmund de Souza (A0118668R)

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Page 1: Car Foams - 27

Elvis Ayroso (A007496H)

Edmund de Souza (A0118668R)

Page 2: Car Foams - 27

Characteristic carbamate functional group

Formed typically by an isocyanate group reacting with an alcohol group through step polymerization

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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

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Isocyanates◦ Methylene diphenyl isocyanate (MDI)

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• Isocyanates Toluene

diisocyanate (TDI)

• Methylene diphenyl

isocyanate (MDI)

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Polyols◦ Determines flexibility of the polyurethane material

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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.

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Highly exothermic

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The cross-linking between polymer chains:

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CO2 is produced by the reaction mixture when water reacts with the isocyanate group.

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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).

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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

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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

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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

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Choice of mixer◦ Impinging mixers are recommended, since turbulence is

maximized through this form of mixing

◦ Turbulence leads to uniformity of PU material

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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.

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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.

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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.

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