Tg and curing temperature 2

[niksmith]

Can someone please clarify to me whether there is a relationship between Tg( the glass transformation temperature of the resin) and the the curing temperature of the resin ?Are they the same .Is it possible that a thermosetting resin has a Tg of 50 degC and which cures at 130 degC ?

Best Regards,
Nik Smith

 

[CoryPad]

Yes, it is possible.

The curing temperature is the temperature needed to form bonds (for example, cross-linking a liquid resin and liquid hardener to form a solid polymer).

The T_g is the temperature at which the solid polymer has a change in properties (like elastic modulus).

Here is a definition from the ASM International Engineered Materials Handbook:

glass transition temperature (T_g):
The temperature at which an amorphous polymer (or the amorphous regions in a partially crystalline polymer) changes from a hard and relatively brittle condition to a viscous or rubbery condition. In this temperature region, many physical properties, such as hardness, brittleness, thermal expansion, and specific heat, undergo significant, rapid changes.


Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[Demon3]

Cory is right. I would like to add something that complicated the case and will probably help you. I have worked on curing of epoxies and it is important to realise that there is a fundamental relationship between the curing temperature and Tg under certain conditions. I can give an example.

As cure proceeds the cross-link density increases and therefore the mobility of the polymer decreases so the Tg increases as the cure reaction occurs.

First let's take the case where the Tg of the fully cured polymer is -80°C. In that case the molecules are very flexible and can cure fully no matter what the cure temperature.

In the second case (some epoxies) the Tg of the monomer may be say 0°C and the fully cured polymer 230°C. Let's say you set the cure temperature to 180°C. That means that as you increase the cure time the cross link density increases until the polymer chains are no longer able to move around at 180°C. At this point cure will stop because the epoxy groups cannot twist to meet the amine groups due to the high Tg. So, even though the theoretical Tg is 230°C, you will never reach it if your cure temperature is only 180°C. To get a final Tg of 230°C you would need a cure temperature close to 230°C.

A good way to test whether you have full cure is to take your sample and do a DSC on it under nitrogen at 300°C. Any uncured groups will then cure and you can quantify it by the size of the exotherm (heat given out).

 

[niksmith]

Thank you Demon 3 and Cory for the replies .Demon 3 ,if I understood correctly then a resin can for example can have a Tg of 50 degC but be cured at 180 degC and once the curing is completed the Tg will be higher compared to what it was, as a result of the cross-linking . Now if I increase the cured body to above the new Tg this will result in a reduction of the hardness and also a reduction of other properties .
Please correct me if I have understood wrongly .

Best Regards,
Nik Smith

 

[Demon3]

Yes, you understood wrongly.

When you cure, the cross-linking (hardening) will stop when the Tg reaches the cure temperature. So if you cure at low temperature you will never get a hard material. You need to cure at a high enough temperature to get close to complete reaction. However, if you cure at extremely high temperature (e.g. over 300°C) then you will destroy the polymer. Read the instructions for cure from the manufacturer for advise on appropriate cure temperature for your resin.

 

[Demon3]

OK, sorry niksmith, I misread your reply. A polymer above it's Tg is soft so yes, if you raise the polymer above it's Tg it will get soft. However, once above it's Tg it will continue to cure and it's Tg will increase until it equals the new cure temperature or until the cure reaction is complete, whichever comes first.