Using Polymers Below Glass Transition Temperature 3

[ziptron]

Afternoon Everyone,

I have a general question about Tg (Glass Transition Temperature) and using it as a design criteria.

I come from the metals world and I tend to specific/use metals in temperatures where they will not be brittle. Metals, like plastics, tend to have a temperature above which they are ductile and below which they are brittle. Although every application is different, generally as a rule you want to be in the ductile state and you would design your metal to ensure this.

Are there any general rules regarding using plastics in environments below their Tg? Is it ok to do so, or is everyone typically striving to be above the Tg temperature during service? I recently looked up the Tg of Nylon, and it seems to be 40C-50C. However we often use Nylon at room temperature and thus we are using it in its brittle state. Am I missing something? Or are most Nylons outfitted with a tonne of plasteciser which brings down its Tg to below room temperature making Nylon components usable at room temperatures?

Any insights would be helpful,

Thanks!

 

[Compositepro]

It depends entirely on the application. In aerospace composites and some structural applications, the upper use temperature is considered to be 50F below the Tg. Above Tg polymers are considered to be in their rubbery phase. Yield strength is low and they creep. On the other hand you want the Tg of your garden hose or the vinyl upholstery in you car seats to be below the use temperature.

 

[rickfischer51]

A polymer below its Tg is not necessarily brittle, although they are certainly less ductile that when above the Tg. PE, PP and elastomeric polymers are above Tg at RT, but the rest are generally below Tg. PE and PP are useably stiff due to the high level of crystalinity, but still have elongation values way over 100%. Elastomers are lightly crosslinked, making them "solid" but flexible. The rest of the non-elastomeric polymers (PS, PVC, SAN, PMMA, PA, PC, POM PET, etc) are all below Tg at Rt, and have elastic moduli >300 ksi. I think all these materials have Tg between something like 50C and 150C. PS, SAN and PMMA are brittle. Pure PVC is brittle, but is usually used with modifiers which results in a fairly ductile material. (I have personally tested a custom blended unplasticized PVC that rgeularly went to at least 50% elongation. We stopped the test there. Once I took off the extensometer and kept pulling. It went to 100%, where I ran out of stroke). There rest are all fairly ductile.Tthe presence of fillers may change that. Above the Tg, the elastic moduli drop by several orders of magnitude and, as pointed out by Compositpro, they creep readily, and are virtually un-useable.

The Tg represents the point where the free volume (the space between the molecules) falls below a critical level needed for good molecular mobilty and cooperative motion between the molecular chains. The motions still take place, just at a longer time scale. As the free volume goes down (temperature, physical aging, plasticizer loss, etc), the time scale increases and eventually the material gets brittle.

Nylons are usually not intentionally plasticized, but moisture will effectively plasticize nylons. Look at the difference in elastic modulus for a 6-6 Nylon DAM and conditioned at 50% RH.

Rick Fischer
Principal Engineer
Argonne National Laboratory

 

[Demon3]

Excellent answer from Rick Fisher.

Tg is not a fixed property and depends greatly on the testing method. Like Silly Putty, when tested slowly you may get flow and when tested quickly you can get brittle fracture. So that's worth bearing in mind. A Tg by NMR will be far higher than one measured by DSC.

Amorphous polymers lose most of their stiffness over Tg so they can't be used for load bearing applications in hot environments.

Semi-crystalline polymers are reinforced by the crystals so, even though the amorphous (non-crystalltine) regions are soft over Tg, there is still some load bearing capacity (stiffness and strength).

Interestingly, the effect of fillers is wildy different between amorphous and semi-crystalline polymers. Adding a high aspect ratio mica or glass fiber will take the heat distortion temperature (HDT/Vicat) of a semi-crystalline polymer way upward to almost the melting point. Adding fillers to amorphous polymers does very little for HDT. There's a great figure showing that in Plastics Materials, 7th Edition by JA Brydson.

As mentioned, nylons are plasticized by moisture from the air. An as molded nylon will have double the modulus but much less impact resistance than nylon that has been allowed to absorb some moisture.

Chris DeArmitt PhD
President - Phantom Plastics LLC

Consulting, ideas and training on plastic materials

http://www.phantomplastics.com

 

[ziptron]

Thanks everyone for your replies!

From what I gather, plastics can be used below their Tg so long as they are tested to withstand the conditions they will experience. Its a common sense finding in the end, however it is a little different from my metals experience.

 

[CoryPad]

Try not to equate Tg with DBTT. Think of Tg as Tm, or a creep limit temperature.