Freeze-Thaw and Age Resistant Plastic 2

[oldstuff]

What plastic / reinforcement would be suggested for a structural (shear / pullout loaded) application able to perform over 50+ years and 3,000+ freeze-thaw cycles. The plastic will be protected from UV and must be injection moldable at low cost.

 

[Demon3]

Fiber is a type of filler and in fact the one that makes the plastic most brittle, i.e. very low elongation to break. That means it will shatter when you screw into it.

Chris DeArmitt PhD FRSC CChem

http://www.phantomplastics.com

Consultant to the plastics industry

 

[patprimmer]

Glass fibre increases energy at break but decreases elongation at break. This normally results in a more brittle nature, especially in instances where elongation is forced by insertion of a screw or nail.

Nylon rail track insulators have been around for over 40 years. Some of them are in very cold climates. I will be talking with the manufacturer tomorrow to see how long they can last without replacement.



Regards
Pat
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[oldstuff]

Seems to me that if screw pullout is the primary mode of failure the best approach would be to go with fibers (well-bonded with the plastic resin). Hopefully their strength would also allow us to drop back to a more reasonable factor of safety.
I'm imagining that a 1/8" screw will sever some fibers and put the adjacent plastic in significant tension. Can the immediately adjacent fibers stop creep and prevent tensile cracks?
FYI, the load on the screw is nearly all shear and will be about 3 lbs. But the screw provides a perfect pathway in bringing temp extremes and water to the critical area.

 

[Compositepro]

All the responses regarding fiber were referring to milled fiber which is commonly available in thermoplastics. Continuous fiber reinforcement behaves very differently. It is far stronger but it is far more expensive. Thermoplastic parts with or without milled fiber in not generally considered a "structural" material. Structural plastic materials are generally thermoset resins reinforced with continuous fibers. These are very different materials and different industries with little in common.

 

[MikeHalloran]

Construction typically goes on in all sorts of weather, and even unreinforced plastics get brittle at lower temperatures.

In light of that, using self-piercing screws may not be the best idea you ever had.







Mike Halloran
Pembroke Pines, FL, USA

 

[oldstuff]

True. But cost is a driver and I doubt there'd be interest in stepping up to Thermosets unless we come to the conclussion that the less expensive options won't work.

 

[btrueblood]

"Don't add too much filler, it will make the plastic brittle and it will crack when you screw into it. "

Fiber also does allow multiple pathways for water to ingress during freeze-thaw cycles, i.e. it makes the base plastic material more porous. Dunno if typical GF ratios (30% - ish) create significant difference in a freeze thaw material test...but do know that less porosity of any kind (fibers, fillers, or foam) is better at preventing water ingress across a sealing surface exposed to freeze/thaw cycles.

 

[oldstuff]

Ugh! I miss the predicatability of concrete and steel already.
The "sure" things at this point are thermoplastics (for cost), lowest possible water absorption, and best over-all strength, creep, and fatigue characteristics. Polypropylene, Polyethylene are fielded and should be tested (with and without fibers). Other suggestions? Acetal?

 

[patprimmer]

My next suggestion would be PET copolymer.

I do not have embrittlement temperatures at hand.

PE will not take fillers well at all.

Class coupled PP would be the better of those two if you wanted a filler.

PC might be good but will be to expensive and is sensitive to many common chemicals.

Regards
Pat
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[oldstuff]

Just to refresh on the loads I'm talking about here, our load is about 3 pounds. The screw will create a hole in the plastic about the size of its 1/8" shaft diameter. The (outside) thread diameter is 3/16" and will spread this shear / tension load over a circumference of 1.18". Doesn't this provide enough info to compute the thickness of a low-cost material (HDPE, PP) required to permanently (by going up as high as 20 on a factor of safety)support the load?
Perhaps part of the solution is to develop a self-tapping shaft (to remove plastic rather than spread it) and a wider thread diameter to bear on a larger surface.

 

[MikeHalloran]

Self-drilling screws have already been invented.

Please review how you arrived at the stated circumference.



Mike Halloran
Pembroke Pines, FL, USA

 

[oldstuff]

Well, I got that figure by making the foolish mistake of using a diameter as a radius. I believe the correct circumference should be .59 inches. Thanks.