Mineral Filler for PP in drinking water use 6

[cpretty]

As the advice is so useful on this forum, I am using this early into my investigation into a different material for our product.

We currently manufacture a ball for a ball valve out of PTFE filled polypropylene for our 'premium' valve. We are required to machine the ball to obtain a perfectly spherical surface for sealing.

We are now investigating production of a 'value' version. Natural PP is OK, but shrinkage is significantly different as stability is decreased by the removal of the PTFE. This means some of our fits are not as stable as we would like. As we are using the same tooling for both materials, tuning the tooling is not an option.

We are debating the use of mineral filled PP - talc, mica, calcium carbonate etc. This should in theory improve the material stability to similar to PTFE but at a lower cost.

There are a number of questions that I haven't been able to find with regards to our specific application that this forum may be able to help with.

How does the additive of these fillers affect machinability of the part? I understand that they improve scratch resistance etc, so would be concerned that they then make it harder to machine. Does cutter wear become an issue or is the filler small enough/soft enough to not be abrasive?

As our application is for potable water applications will fillers be a problem? From my research I can find that these fillers are OK for incidental contact with food, but in this case I would be concerned that due to being machined, there is exposed filler, which may/may not interact with the water.

Any comments or advice would be appreciated, as I am quite keen to get advice from people that have been there and done it, rather that polymer suppliers that just like to sell their product.



Craig Pretty
Tru-Design Plastics

 

[Demon3]

The first filler that comes to mind for that application is talc as it's lubricious, safe for food contact and does not cause any wear at all. It's thousands of times less hard than the machining tool.

Bear in mind that fillers have 10 fold less CTE than polymers so a mineral filled PP is unlikely to match the shrinkage of a PTFE filled PP. Instead I would suggest you look into filling the PP with UHMWPE which is cheaper than PTFE, a great lubricant and should have similar shrinkage to the system you have now. Also try UHMW silicone (

http://www.multibase.com)

or

http://www.wacker.com

as that's a great lubricant that lasts.

Chris DeArmitt PhD FRSC CChem

http://www.phantomplastics.com

Consultant to the plastics industry

 

[patprimmer]

I think the PTFE filler will not go through a phase change during moulding of PP whereas UHMWPE will go through a phase change and will therefore shrink a lot more through that phase change temperature range. I have no data to back this up. It is just my expectation.

Regards
Pat
See FAQ731-376 for tips on use of eng-tips by professional engineers &

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[Compositepro]

I think Pats reasoning is sound. The important point is that ptfe is a polymer and not a mineral filler and its function is as a lubricant. If you are seeing a shrinkage reduction with ptfe filler compared to straight polypropylene it is probably that the ptfe is already crystallized prior to molding and PP crystallizes during molding. As Chris said, any mineral filler will have a lower CTE than ptfe. Graphite is also a lubricating filler. Perhaps a lower loading of talc or graphite will give equivalent shrinkage to ptfe. The question then is will you get adequate lubrication?

 

[cpretty]

Thanks for all of your replies. I must have asked a good question to get all 3 gurus replying so promptly.

I had been thinking of the PTFE as an inert 'filler' so hadn't really thought about shrinkage differences due to polymer versus mineral. Checking out the data sheets, I should have spotted it.
PTFE grade - 1.6-1.9%, Talc @ 20% - 1.1-1.4%.

So now I have a handle on whether machining and food contact are likely to be OK - confirmed here, and I have now found some food grade filled PP from one of our suppliers.

Unfortunately my thoughts on how to control shrinkage were slightly off.

Unfilled PP is sufficiently lubricating for this application. Any lubrication added is just a bonus.

I think the low loading to tweak the shrinkage may be a solution to our problem.

I have had a bit of an investigation into specific heats (leading on from Pat's comments), thinking that maybe the cooling required and hence cooling rates are significantly different. As PTFE is already crystalised, the cooling required would be significantly less than for unfilled PP (dependent upon loading %). As the part has thick wall sections (worst case - 3" sphere with 2" hole through it) maybe the cooling is inadequate leading to shrinkage issues.

I think it is time to experiment on the factory floor and trial some materials.





Craig Pretty
Tru-Design Plastics

 

[Demon3]

Specific heat of all solids is the same on a per unit volume basis, which is what counts. This in wrong even in books which state fillers reduce specific heat capacity. That's because the units are wrong, they look up specific heat per unit weight and see that fillers are lower by a factor of three but they forget that the density is also three times higher so it cancels out.

Contact me if you want proof.

Chris

Chris DeArmitt PhD FRSC CChem

http://www.phantomplastics.com

Consultant to the plastics industry

 

[patprimmer]

Chris

Once again I learned something new today.

Regards
Pat
See FAQ731-376 for tips on use of eng-tips by professional engineers &

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[patprimmer]

If you tweak the filler and maybe the base resin grade a bit you should get your shrinkage.

Fillers can also act as nucleating agents as can colours.

At 1/2" wall section reduction of shrinkage via temperature control will be difficult as the thick sections will cool slow enough for a high level of crystallisation no matter how you cool it.

A hot mould really kills cycle times but gives higher but more uniform crystallisation.

Eastman Kodak used to make a product called Tenite Blowing Agent in masterbatch form for PP.

It can produce a very good surface with much improved dimensional stability at much reduced cycle times in thick section.

Regards
Pat
See FAQ731-376 for tips on use of eng-tips by professional engineers &

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[Pud]

product called Tenite Blowing Agent in masterbatch form for PP.

Nah - just buy a 25Kg bag of sodium bicarbonate and bung some of that in! (or get a regular PP blowing agent!)

Iirc, Tenite was found to be carcinogenic. Good stuff though - low % required.
H

http://www.tynevalleyplastics.co.uk

 

[patprimmer]

I have not seen it for years.

It was exceptional for blowing a very fine foam structure in the core but leaving no sign of blowing anywhere near the surface. I have seen 20mm thick mouldings with the absolutely perfect gloss surface and no sinks or voids over a fraction of a mm in size with 70% reduction in cycle time vs not blown. It was expensive, but cost effective when add rates where considered.

Regards
Pat
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[cloa]

Rather than specific heat by volume - you should say volumetric heat capacity. Specific heat is by mass.

 

[Demon3]

Hi Pat,

Thanks! That's a compliment indeed. Remind me and I will e-mail you the proof. I tabulated values for many polymers and fillers to make sure I was getting it right. A search actually turned up a rule of thumb that all solids have the same specific heat per unit volume. It makes sense when you think about it.

Cheers,

Chris

Chris DeArmitt PhD FRSC CChem

http://www.phantomplastics.com

Consultant to the plastics industry

 

[btrueblood]

Ditto Pat's comment Chris.

 

[cpretty]

I am have had a bit of a think about this and am thinking about the phase change of the polymer material and how that affects the cooling.

I found the following graph

http://www.gammadot.com/Techzone/nexus/PP/PPcp.htm

and was thinking about the classic boiling water scenario where energy is being added that is not increasing temperature but is changing phase.

The polymer requires energy to heat the material (related to specfiic heat) as well as to change phase (latent heat of fusion).
A filler would require only heating of the material.

In terms of cooling this would suggest that there is less material requiring energy to be lost to crystalise (fusion), then the amount of cooling required would reduce.

In terms of Chris's comment on some texts - "fillers reduce specific heat capacity", this is a use of the incorrect terminology, but is intended to say - adding a filler (assume no phase change) reduces the cooling required.

Craig Pretty
Tru-Design Plastics

 

[patprimmer]

I think it does both.

Also material is injected well above it's melting point and is not ejected untill well below its freezing point so there is considerable influence from both specific heat and latent heat components.

Regards
Pat
See FAQ731-376 for tips on use of eng-tips by professional engineers &

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[Demon3]

I see what you mean but even if that effect is there, it won't be large because of the density of fillers. For example 40 weight % filler is only maybe 15 volume % so you have only replaces 15 volume % of plastic with filler. Then remember that plastics are often around 50% or less crystalline so you've replaces only 7-8% of crystalline materials. Thus the effect you mention is small even at high filler loadings.

Chris DeArmitt PhD FRSC CChem

http://www.phantomplastics.com

Consultant to the plastics industry

 

[Fridgeworthy]

There is no need to inject the material well above it's melting point and eject well below, in fact a processor tries to play close to these, to save on time. The higher the melt temp, the longer time to cool.

As Chris mentioned, the difference will not be huge. Take the total heat to be removed going from 230 to 100 deg C. For PP (homopolymer) this is 370 J/cm^3, including the heat of fusion. For pure talc 297 J/cm^3, pure calcium carbonate 294 J/cm^3 and for pure e-glass 271 J/cm^3. So at normal filler loadings not a big change.

Filler will increase conductivity and often raise crystallization temp. If the mold has good cooling, the increased conductivity will reduce cooling time and the part will become solid at a higher temp, so it can also be ejected earlier.

 

[patprimmer]

If you inject at its melting point it freezes off in the spru.

If you eject at its freezing point it is still so soft the ejector pins go straight through the moulding.

Also there will be considerable time between the surface skim freezing and the moulding freezing all the way through.

Regards
Pat
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[cpretty]

A little feedback from my trials on this. I am happy to report that sizing and shrinkage were as expected.

Largest shrinkage - Natural PP
Slightly less than above shrinkage - Blue coloured PP
Control size - PP with PTFE
Less shrink than control size - PP with 40% talc

From analysis of the PP with 40% talc and the Blue PP trial samples I was able to determine the correct ratio of these to obtain the same sizing on critical areas to match the PP with PTFE control. It would appear that a 20% talc filled material (although it is a mix of PP grades) very closely matches the PTFE blend.

Now onto machining.

Craig Pretty
Tru-Design Plastics

 

[unclesyd]

Here is a Mica based one to add to the list.

We used that product along with Kaolin, TiO2 and Carbon Black to enhance the properties of Nylon 66

http://www.pfillers.com/index.php?pg=catalog&catID=29