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This may sound as a very basic (kindergarten-ish) question.

How does one come up with the soft touch of plastics. At times, it almost feels like rubber. Is the compostion different for different types?

I am basically referring to the dashboard feel of a certain cars.

A wonderful topic to discuss, share and learn.. thank you and keep the info flowing... I am learning so much more...

Thanks Dot, great initiative. This is indeed very informative.

Are you also planning to add Carbon Fiber to the list?

EDIT: It would also be interesting to know how the choice of plastics may affect overall safety (fire, crash, pedestrian etc. safety), it would also be interesting to know the cost aspect - cheap vs. better looking plastics in cars, and how much relative cost difference is there between these two? Some kind of indication would be useful here like better looking dashboards generally cost say 50% more. Hope what I am saying is making sense! Thanks again.

These soft touch dashboards are typically polyurethane foam overmolded by PVC skin. However sometimes OEMs change materials, so it might just differ from one make to another make.

Yes, very briefly though.

Sure.
What I understand is that to make PP as fire retardant (FR), lots of FR-fillers/additives (20-50%)are needed which in someway, is detrimental to aesthetics. On the other hand, PC-ABS materials can be made FR compliant with much lower loading of FR additives. Aesthetics can be maintained. Just look at the quality of the plastics of your LCD TV's remote control or your mobile phone and compare that with the door trim of your car. The former is typically made from ABS family while the latter is typically from PP.

From the cost perspective, prices of PP is much lower than say, ABS. I looked at January spot prices and the prices are,

PP - 1400-1500 $/ton.
ABS - 2100-2200 $/ton.

Thats a pretty large difference. So naturally many manufacturers try to use PP wherever possible.

With regard to safety, impact properties of plastic grades vary with many parameters, so it is difficult to comment on them. What I understand that competing engineering thermoplastic formulations can be tailored to meet safety requirements.

A very interesting topic Dot and nice presentation!

Can you also touch a bit upon the strength of the different plastics?

Why don't OEM's use plastic exteriors? Can carbon-fibre fall into the family of plastics? A plastic exterior would definitely reduce the weight!

Tensile strength of some neat (unfilled) thermoplastics that we have talked about can be given by the following gradation.

Polyamides (Nylon)>PC>ABS>PP>PE

Carbon fiber as such, is a fabric, like a cloth piece. It needs a binder that gives the "plastic" like quality. These binders are many times epoxy and hardener. We will talk about them a little later.

I guess, you mean why not replace body panels by plastics? It has been tried several times. Some commercial European cars had thermoplastic fenders made out of polyamide-polyphenyleneoxide blends. But no sustainable solution has been found so far. Metal is still better. One reason is scratch resistance and refinish issues of thermoplastic panels.

Carbon fiber- epoxy based panels are a challenge from processing side as they take a long time to make when compared to metal sheets. Moreover there is lot of manual expertise involved. This can pose problems during mass production.

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We will now look at foams.

Very generally, foams are materials which have trapped air/gas in them. This makes them low on density.

In our daily lives we use lots of foam materials which are based on polymers. We use foamed polystyrene or "thermocol", foamed PVC or polyolefin shoes. Polyethylene foams are used in packaging. Mattresses have foamed Polyurethane or PU. So does refrigerator insulation and car seats.

Foams can be made from both thermoplastics and thermosets. For that one needs a foaming agent that can cause cavity formation during molding stage. Many types of foaming agents are used and these depend on application and processing conditions. The following picture shows foams made from thermoplastics and thermosets. As discussed before, many additives and fillers go into final foam formulations.



Next we will look specifically into PU foams, how they are made and where all they are used in vehicles.

Polyurethane or PU foams are extensively used in vehicles. Car seats are almost universally made from PU foams. PU are good candidates because of their combined properties of mechanical strength, dimensional stability, compression set, fatigue resistance, softness and comfort. Formulation and density can be tuned to get a wide variety of materials.



Typically PU foams are made from two major components, polyol and isocyanates. These two components react to form urethane chemical bonds which ultimately lead to polymer formation. These PU polymers eventually form net like or crosslinked structure which makes it unflowable. Blowing agents like water or pentane are utilized to create foams. Unlike thermoplastics, the polymer formation takes place in a mold. So the final part comes out after the reaction is over.

Here is a youtube video showing how polyol and iso are injected into a mold and a final PU part comes out when the reaction is over.

Reaction Injection Molding - YouTube

Formulators usually play with the polyol part to modulate the foam properties. Hard or rigid foams result if the polyol formulation has more "hard" components. Soft foams result if the "hard" components are taken out. Rigid foams find application in cars, for example, roof liners have semi-rigid foams.

Two more areas where PU finds applications in vehicles are soft touch plastic coatings and sealants. They will be covered later.

Since we are in topic of thermosets, lets take a quick look at rubbers which are also mostly thermosets except for high end elastomeric block-copolymers.

Rubbers are polymers with sub ambient glass transition temperature. That means that for rubbers to behave as plastics, we need to freeze the material below a certain temperature which is characteristic of the material. For example water or ice melts at 0degC which is its melting point. Below the melting point water is solid, above melting point water is liquid. Likewise, in many polymers, below a characteristic temperature it behaves like a glass, above it like a liquid or a soft material. Moreover rubbers have net like molecular structure which generates its elastomeric properties. Thats why many rubbers need to cured to make them "rubbery" as curing generates the cross-links or net like molecular structure.

Many types of polymers behave as rubbers. Typically they are made from isoprene, butadiene, isobutylene molecules. Isobutylene monomer generates butyl rubber. A particular rubber which has gained importance in automobiles is EPDM rubber which are made from a Ethylene Propylene Diene Monomer (EPDM). EPDM is widely used as it can be processed in many ways: Extruded, Profile extruded, Injection molded, Compression molded. It has much better weathering properties than conventional rubbers.

While rubbers are used in tyres as all of us know, they also find uses in many parts in a car. EPDM rubbers are used as door seal, serpentine belt, radiator hoses. The following illustration captures some of the applications in vehicles.



EPDM properties can be varied with Ethylene, Propylene or the Diene content. It can be made softer or harder, processable at different temperatures and conditions by changing its chemical constituents.

Another aspect which can be highlighted is color of rubbers. Rubbers are inherently colorless or white. Usually most formulations have fillers which include carbon black. These fillers increase strength and abrasion resistance of rubbers. Carbon black also renders the color of rubbers as black.


Mods: Can you kindly merge this post with the one above. Thanks.

Lets look at the sticky subject of sealants and adhesives.

I will just mention the base materials which are used for these applications, since formulations of adhesives and sealants are quite diverse and depends on,
1. material or substrates which are to be sealed
2. time of application and time to cure
3. service temperature and pressure
4. environmental conditions
5. mechanical and fatigue properties
etc.

Typically four materials are widely used. They are,
A. Silicones
B. Polyurethanes
C. Acrylics
D. Hot melts like PVC.

We have already discussed a little bit of polyurethanes and acrylics. In sealants and adhesive applications, the polymeric nature of the polyurethanes and acrylics are chemically tuned to make it suitable for flow, one pot shelf life and adhesive properties. Of course cost is another factor always taken into consideration. Urethanes and acrylics are known to have superior fatigue resistance.

Silicones are polymeric organo-silanes made from silicon sources. They are crosslinked by a couple of means like platinum catalysis or moisture curing. Silicone properties are tuned by choice of monomers and amount of silica in them which impart higher strength.

Which particular material is chosen by OEM and which ones by aftermarket refinishers are dependent on geography, regulations and cost. While I have seen that silicone are used in transmission case sealing and urethanes for glass bonding, I wont be surprised if another material is used in a different country. As per my understanding, this market does not have uniform standards across.

Here is an illustration which shows where all adhesives and sealants are used in vehicles. Body panel sealing, headlight sealing are other areas of application.



Finally a couple of video showing robots applying sealants and adhesives on body panel and headlights during production.

KUKA Robot- Automotive Vision Guided Sealant - YouTube

I like this video where the first robot flame/corona treats the headlight case then sealant is applied. Flame/corona treatment is needed to make the polymeric surface polar and more conducive for bonding.

KUKA Robot- Adhesive bonding of car headlights - YouTube

Spectacular thread dot! Thanks so much for taking the time to put this together. Did you create all the explanation diagrams yourself?


Could you give us a few examples of what automotive plastic parts need a higher level of dimensional stability?

Wow. I always assumed that was some sort of part number!

However, what is the reason to have that material grade printed on the part?

cya
R

Very Technical and highly informative thread this Dot. :thumbs up

Need a quick insight from you on this: you must have come across a thread where the foam coating at the wheel well, between the well lining and the car's body caught fire. The subject car was a Ford Figo.
I presume this foam is used to reduce the tyre noise from seeping into the cabin.
My query is, isn't this Foam supposed to Fire Retardant?

Well, yes. Had loads of fun making them. Took individual photographs from net searches though.

Parts that should have very low shrinkages due to temperature and humidity variations. For example, housing (not the casing) of ORVMs, under the hood housings.

As far as I understand, they are printed so that they can be properly segregated during disposing.

As far as my understanding goes, it meets some basic flame resistant criteria, but they cant be called flame retardant.

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Lets look at paints and coatings now.

Paints and coatings are a diverse field from material usage stand point since OEMs around the world use various materials for the same application. It depends on geographic regulations, prices, availability, tradition etc. Technology has been changing and what was practiced even 10 years back, may not be prevalent now.

I must admit, I had to read the most for this post. For example, what I understood as standard material as the binder was not so standard after all.

First lets look at the few basic building blocks used in paints and coatings. The following diagram illustrates in a simplistic manner, how crude oil, naptha, natural gas and seed oil are converted into materials that are sprayed as paint and coatings in automobiles.



Typically acrylics, polyurethanes, epoxies and alkyds are used in automobile applications. Acrylics are traced back to acrylic monomers which comes from propylene gas. Most Epoxies are made from BPA which in turn is made from phenol. Urethanes are made from polyols, esters and Isocyanates. However in coatings, many urethanes have hybrid chemical structures and can contain epoxies and acrylics as building blocks. Amines are used as reactants in several applications. Alkyds are combination of seed oil and naptha products.

All liquid coating formulations have the following generic composition

Media (solvent or water)
Binder (urethane, acrylic, alkyd)
Color (specially in base coat)
Additives of various kinds (eg, viscosity regulators, dispersants).

Next we will look at the painting process and touch upon the usage of various materials in different stages. Finally we will look at some non-paint, coating only applications.

Ah! So thats very similar to the "number" that we see printed within the "recycle" sign on most plastic items.


Image Source


R&D and Paint booth are two areas that are very heavily guarded in most automobile factories. Naturally, the reasons for hiding R&D are obvious, but can you shed some more light on why the paint booth is so closely guarded? Is it entirely due to proprietary processes, or is there some chemical magic at play too?

cya
R

Hey dot, just remembered that the next installment is not in yet!

Since heavy debates are going on the sunfilms and tinted glass, can you please touch on the finer details of laminated windscreen, tempered glass etc. (if time permits)? :)

Hi Dot,

Many thanks to you for the great information. The presentation and the easy layout makes it easier for all of us to understand/comprehend your information.

Appreciate it!!