Mineral Filler for PP in drinking water use
Mineral Filler for PP in drinking water use
(OP)
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.
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





RE: Mineral Filler for PP in drinking water use
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 (www.multibase.com) or www.wacker.com as that's a great lubricant that lasts.
Chris DeArmitt PhD FRSC CChem
www.phantomplastics.com
Consultant to the plastics industry
RE: Mineral Filler for PP in drinking water use
Regards
Pat
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RE: Mineral Filler for PP in drinking water use
RE: Mineral Filler for PP in drinking water use
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
RE: Mineral Filler for PP in drinking water use
Contact me if you want proof.
Chris
Chris DeArmitt PhD FRSC CChem
www.phantomplastics.com
Consultant to the plastics industry
RE: Mineral Filler for PP in drinking water use
Once again I learned something new today.
Regards
Pat
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RE: Mineral Filler for PP in drinking water use
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
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RE: Mineral Filler for PP in drinking water use
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
www.tynevalleyplastics.co.uk
RE: Mineral Filler for PP in drinking water use
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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RE: Mineral Filler for PP in drinking water use
RE: Mineral Filler for PP in drinking water use
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
www.phantomplastics.com
Consultant to the plastics industry
RE: Mineral Filler for PP in drinking water use
RE: Mineral Filler for PP in drinking water use
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
RE: Mineral Filler for PP in drinking water use
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
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RE: Mineral Filler for PP in drinking water use
Chris DeArmitt PhD FRSC CChem
www.phantomplastics.com
Consultant to the plastics industry
RE: Mineral Filler for PP in drinking water use
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.
RE: Mineral Filler for PP in drinking water use
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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RE: Mineral Filler for PP in drinking water use
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
RE: Mineral Filler for PP in drinking water use
We used that product along with Kaolin, TiO2 and Carbon Black to enhance the properties of Nylon 66
http://