Nylon 12 v's Nylon 6.6 1

[NZscout]

Would appreciate your comments please on the difference between Nylon 12 and GFNylon 6.6 in particular to the properties of 1:Chemical resistance, 2:Impact resistance 3:Creep.

I have an injection moulded component that is used in the dairy, it is fastened with two CSK 316SS screws into a product that cows will stand on, it gets very wet and has disinfectant overspray land on it. Two typical disinfectants sprayed are dilute Iodine and Chlorhexadine.

I am already aware of the difference in price - have been quoted the two materials already. The existing material (a blend of PC/PBT - called LUPOX TE5000LZ) has failed over 6month time due to poor weld lines around the screw holes and chemical attack. See attached image ballooned item #3

 

[patprimmer]

Presuming the type12 is not glass filled, the 6.6 will be much more resistant to creep even after it absorbs a lot more water than type 12 does.

The type 12 will have better impact, particularly where a ductile nature helps, although in some tests the GF6.6 might have more energy at break but at almost zero elongation. Depending on glass orientation vs a notch, glass fibre can greatly reduce notch sensitivity as the fibres reinforce across the notch.

I'm not sure for iodine, but nylons in general are borderline for chlorine and it depends on time and temperature. Iodine will have reasonably similar reactions to chlorine. My experience with chlorine is that you need to test in the most severe conditions likely encountered. There was a critical temperature, I think around 80 deg C where the degradation accelerated rapidly.

I don't have data for the propitiatory disinfectant, but nylon is pretty much only attacked by acids, strong oxidising agents and some phenols and of course slowly by boiling water.

Glass probably makes chemical attack worse by wicking effect.

Polyester might be the go if you can find someone who can really mould it properly.

Regards
Pat
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[Demon3]

If chemical resistance is key I would look into glass fiber filled polypropylene. I didn't see anything in your list of requirements that would preclude that and it's cheap too.

Nylons and polyesters have reactive bonds in their backbones meaning that the polymer chains break in contact with water and chemicals. Polyolfins like PP don't have that issue, just carbon-carbon bonds in the backbone and impervious to water and most chemicals.

Chris DeArmitt - PhD FRSC CChem
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[NZscout]

Thanks guys,

We had previous tested a Glass Bead PP on a related product recently and found that it did not have the stiffness or strength required. Fresh mouldings of GFPP were weaker than the aged lupox (PC/PBT) after the lupox had been soaked in concentrate iodine for 3 months. We like the look, feel and initial properties of Lupox, it just fails over time. The 3 month soak test proved to reduce strength ~30%

Pat are you able to briefly explain the issues I should look for if wanting to mould in Polyester? Is there a brand name or grade that you would recomend.

 

[Demon3]

Glass beads don't reinforce but glass fiber PP will have similar room temperature strength and modulus to the nylons plus that great chemical resistance.

Moldiing polyesters is tricky because they crystalize unpredictably and because you need to dry the resin carefully. I'd not recommend polyesters. Ester bonds break more easily than amides and those more easily than PP bonds. In chemists terms the hydrolytic stabiltiy of polyesters is ver poor, amides fair and polyolefins great.

Chris DeArmitt - PhD FRSC CChem
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[patprimmer]

Chris.

I am not sure the failure is hydrolysis based, but more oxidising or maybe solvent stress crack.

Nylon is very good to most solvents but is attacked by some phenol types. I know the solvent adhesive used for nylon 11 pipes is based on a chemical that is closely related to the active ingredient in Detol. This adhesive is a phenol based compound and does not attack polyester.

Even GF, PP is pretty crappy to creep. I agree PP is excellent to most chemicals.

I also agree that GF far exceeds GB for reinforcement, but note it is highly anisotropic.

I believe that polyester sometimes outperforms nylons in some chemical tests is because the polyesters are not so hydrophilic as nylons and therefore the attack is restricted to the surface.

OP

Chris is absolutely correct re moulding polyester. Extreme care and very good drying equipment is required and crystallisation can be quite varied which influences shrinkage, distortion and moulded in stress leading to stress cracks.

Regards
Pat
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[NZscout]

Ok Guys.. I think from a factor of safety point of view I might stay away from the Polyester, that sounds too dificult for our simple moulder to handle.

We really dont want this product to relax over time. It need to screw tight and keep the tension over time on as best as possible. So I think I am tending towards the Nylons. I believe our moulder has access to the NY12 in 30%glass fibre . It is not in the country yet but from your comments it sounds like this could be the best choice at present.

Thanks

 

[Pud]

We make PA12 parts (GF and unfilled) for pipeline pigs. This is because PA12 has outstanding resistance to stuff that could be found in pipes. Mainly hydrocarbons I guess.

Never had a complaint yet, but it's not cheap. Ok to mould though - not aggressively hygroscopic like polyesters, and wide processing window.

If PA12 does not work, try PPS - although that has it's own processing foibles (e.g. 135C Min tool temp)

Cheers

H

http://www.tynevalleyplastics.co.uk

Why be happy when you can be normal?

 

[NZscout]

Hi people, thought you might be interested to see some data from tests just done on actual moulded components. We placed our part in a 20T shop press with a 90degree cone shapped plunger attached. The cone pushed into the corresponding countersunk recess hole in the moulding.

I aatched a Dial indicator to the piston to see how much displacement/downwards movement I was causing and took readings at each 0.1mm increment. We had a digital readout pressure transducer also.

As expected, the 30% glass filled Nylon was the most resistant to the applied force. However this material ultimately fractured, where as the Impact modified Nylon66 and Lupox (PC/PBT) just yielded and deformed. I had 2 out of 7 samples of the Nylon 12 that also shattered.

 

[Pud]

corresponding countersunk recess hole in the moulding.

There's your first problem - c/s holes are (or should be!) a no-no in plastics. The "wedging" effect acts as stress raiser. Counterbores and pan-head screws for fixing.

H

http://www.tynevalleyplastics.co.uk

Why be happy when you can be normal?

 

[btrueblood]

NZ,

Are the load and resulting deflections you found in lab testing a condition that can be achieved in the field?

I'm cautioning you that it seems like you are taking the tensile properties of the parts...when in your OP you said you wanted to select parts based on "1:Chemical resistance, 2:Impact resistance 3:Creep." Basically, it seems like you are doing a test that has little if any bearing on your desired material properties.

One last thing, in my experience, glass fiber can itself corrode if exposed to strong alkaline conditions, so its benefit for #1 is debatable in a dairy/manure environment.

 

[NZscout]

Pud, I inherited this design from the previous engineer, but your comment is valid and has been discussed about possibly changing the fastener and modifying the tooling to suit.

 

[NZscout]

btrueblood, I also agree with you that the loads are far in excess and the method appears to be obscure relating to my original request. The only test equipment we possess is a torque wrench and a 20T press with pressure transducer so I was limited in what I could test for. My thought was to try and induce a cracking or splitting of the CSK screw hole as we had seen from the field failures.

Later discussion with some of the other team members changed my point of view regarding the requirement for impact. When assembled and in place on the milking platform the cows will stand on the components but it is unlikely kick directly onto or cause a blow to the clamp plate in question.

Im curious about your comment that Glass fiber itself corroding: do you have any link or anything further I could read regarding this?

 

[btrueblood]

Scout,

There's info. out there on google. I've run into problems in high alkalinity before, but with a pretty porous fiber/matrix part. Apparently, according to some texts, acid can cause the same problems. Basically, fibers have a lot more surface area per unit of cross section than monolithic glass containers, and this can cause fairly rapid etching/cracking of the vibers, faster than you would expect based on experience with glass containers.

From the sound of things - the fastest test method for your parts may be to try different materials and see what works in the field?

 

[patprimmer]

Glass is slowly attacked by alkali I think. Like BT says, fibres have a greatly increased surface area.

Also, the coating on the glass can be attacked by some chemicals that might not attack the glass or the resin, but it will still reduce the bond between the two and provide accelerated access of chemicals to the bulk of the resin.

Regards
Pat
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[btrueblood]

Ah, yes, thanks for that reminder Pat. I think most reinforcing glass does have some kind of surface treatment to promote wetting and adhesion of the resin to the fiber. FWIW, we tried switching from the standard "Eglass" fibers to an S-glass (high silica content), but it was still rapidly attacked. I have no idea if your part will be subjected to the conditions ours was (a hypochlorite and lye solution, pH of about 12 or more), or what the pH of a manure covered part might be (or even if your part would see that environment), just that manure=ammonia=basic...so you might want to look into it.