losing strength over time - any suggestions to improve?

[billraz]

Hi, I'm using a polymide (nylon) 6 (Akulon K-222,

http://www.matweb.com/search/SpecificMaterial.asp?bassnum=PDSMEU048)

and having a problem with the part losing 20% strength over time when exposed to air/sun/humidity.

We make a product that joins rebar at their junctions prior to pouring concrete. When pouring, the clips need to withstand the load. We've created a pull test and the product can hold 100 pounds when first opened from a sealed box (it's been sitting for at least a month sealed). If the part is left out of the box in our warehouse for a few weeks, it drops to 80 pounds. I have a few questions:

Does anyone know how much time Nylon 6 takes to fully set?

Is there an alternative material for the same price or lower that would be an alternative? FYI, we've tested Polypropylene with 10% glass and it's relactively close to holding 100 pounds. Will this have any loss in strength over time?

Thanks in advance for any help.

 

[Theophilus]

Just throwing a possibility out there of adding glass to your nylon. I think you'll get a decent increase in strength returned, but it will be a tad more brittle (depends how much you add).

Perhaps the nylon is absorbing water and losing strength? Are you in a humid environment? If in the warehouse, UV degradation is probably not a factor.

Only a guess. ..



Jeff Mowry

http://www.industrialdesignhaus.com

Reason trumps all. And awe transcends reason.

 

[patprimmer]

The problem is almost certainly water absorbtion.

Akulon K222D is a very good quality unfilled nylon 6.

Like all unfilled nylon 6, it will absorb water from the atmosphere and lose rigidity, but gain impact strength.

The difference in some properties can be as much as 300% change between dry as moulded and fully conditioned.

Adding glass fibre will give a substantial increase and result in a lesser change due to conditioning.

Acetal might be a good substitute.

Glass coupled polypropylene at 20 to 30% glass might work.

PBT might work but might be brittle. ABS might work.

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[billraz]

Thank you Jeff and 'patprimer.'

I'm in Oregon with a little humidity, so that's probably the culprit. Would wrapping the product in plastic be beneficial?

Adding glass is a great idea, but there are two problems; the costs are higher and more importantly, it becomes too brittle.

Oh yeah, I remember the 300% difference in K222, thanks for the reminder. Our molder also told us about this and it could be major factor.

Acetal was our next option and I'll test this.

Thanks for all your help!
bill

 

[patprimmer]

The GF nylon probably wont be to brittle AFTER it is moisture conditioned to a reasonable degree. I know 25% GF nylon 6 is used for this application in Australia. Some are made from recycled milk bottles (blow moulding grade HDPE) but they are quite bulky mouldings. I just use cable ties or old coat hangers to tie steel reinforcement bars in place.

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[Demon3]

PP homopolymer with ~25 weight % glass fibre will have the same tensile yield strength as unfilled nylon 6,6 (see matweb.com that you already mentioned). PP is insensitive to moisture and should work well for you plus filled PP may be cheaper than the nylon you are using. As mentioned by another poster, glass fibre in the nylon will work too.

The trick in any application is usually not to find a material that works but to find the cheapest one that will work. For you that is probably glass fibre filled PP homopolymer.


There is not any memory with less satisfaction than the memory of some temptation we resisted.
- James Branch Cabell

 

[billraz]

Thanks 'patprimer.'

We had problems with the clips breaking, being brittle. We're finding out the nylon was not conditioned properly. We're conditioning the Nylon now, but my concern is they keep absorbing water and losing strength. We've changed other Nylon products to Acetal (Celcon M-90) material and have been very happy with the results.

Our molder is going to try Delrin 500p and Celcon M-90.

I enjoyed your last statement about using 'cable ties or old coat hangers.' The benefit to our product is the use of an applicator (gun) that eliminates bending over and discharging 30 ties per minute (time savings of 1/4 to traditional wire). Zip ties? That's a good question but our main competition is wire ties.

Truly appreciate all your help!
bill

 

[billraz]

Thanks James,
We have tried PP w/10% glass and it seemed to work well, just not as strong. We'll test w/25% and see what happens.

Thanks,
bill

 

[patprimmer]

Nylon absorbs water until it is at equilibrium, or in balance with it's surroundings.

For nylon 6 that is about 3% water content in a typical atmosphere and about 9% if permanently submerged or in an atmosphere of 100% RH.

It is only the nylon content of a compound that absorbs water, so the weight gain will be proportionally less for glass filled grades.

I agree with Demon3 that 25% Glass filled PP is probably the best value. It will mould a bit slower, but will be lighter (giving more parts per Kg of resin) and will be cheaper per Kg.

Coupling agents substantially improve the performance of glass filled PP. Homopolymer will be best.


Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[Demon3]

To couple the glass fibre to PP you would use aminosilane (like A1100) and maleated PP for example from Exxonmobil (Exxelor PO 1020).


There is not any memory with less satisfaction than the memory of some temptation we resisted.
- James Branch Cabell