Stabilization of polyamides

Another method of achieving the desired molecular weight is by addition of a small amount of a monofunctional monomer, a monomer with only one functional group. Acetic acid or lauric acid, for example, are often used to achieve molecular weight stabilization of polyamides. The monofunctional monomer, often referred to as a chain stopper, controls and limits the polymerization of bifunctional monomers because the growing polymer yields chain ends devoid of functional groups and therefore incapable of further reaction. Thus, the use of benzoic acid in the polyamide synthesis yields a polyamide (XI) with phenyl end groups that are unreactive toward polymerization. 

Forsstrom D (1999) Novel techniques for characterisation of the oxidative stability of polyamides. PhD Thesis, Royal Inst of Technology, Stockholm... 

FIGURE 9.7 Melt viscosity stability of polyamide 6,6 containing Firebrake ZB vs. antimony oxide (PyroChek 68PBI is a brominated PS). 

This anhydrous zinc borate is recommended for use in engineering plastics processed at temperatures higher than 300°C which is the upper limit of Firebrake ZB processing temperature. It is reported to be an effective smoke suppressant in chlorofluoropolymers for plenum cable applications.66 Recently it was claimed that this anhydrous zinc borate can replace antimony oxide (or sodium antimonate) completely in high temperature polyamide applications. Like its hydrated analog, this anhydrous zinc borate can also improve CTI, thermal stability, and the color stability of polyamide containing halogen sources.67 68... 

The use of magnesium hydroxide in polyamides is restricted by the low degradation temperature and the low hydrolytic stability of polyamides. Polyamide 6 and... 

B. M. Kovarskaya, Bliumenfel d Amine stabilizers of polyamides. Plastics (1965) No 8, 8 (in Russian). 

Stability of Polyamide 6 Against Hydrolysis During Processing. 168... 

An interesting stabilizer system that may involve a redox couple is the use of copper iodide in the stabilization of polyamides. This is often the melt stabilizer of choice, and the reaction (Scheme 1.68) to remove the main chain-carrying radical is proposed, followed by a redox reaction with peroxy radicals to regenerate the cuprous ion. 

In practice, monocarboxylic acids, such as acetic acid or lauric acid, are often used to achieve molecular weight stabilization of polyamides. 

Chaudhuri et al. [22a] in their study of the thermostability of wholly aromatic polyamides have introduced various polar groups onto the polymer, e.g. NO2, COOH, OH, SO3H, etc. Compared to methylated or chlorinated polymers, the stability of polyamides 16 is lower, and decreases in the following order NO2 > COOH, OH > SO3H... 

Aramids have been developed from the need to enhance the chemical stability of polyamides. Because of the stability of aromatic rings, aramids also have higher tensile strength and thermal resistance than aliphatic polyamides. These qualities make aramids popular for use in impact resistance applications. However, aramids - along with PA - have low UV resistance. If used in outdoor applications, they must be heavily coated to protect against UV radiation. 

Linski B. Stabilization of polyamides—I the efficiency of antioxidants in polyamide 6. Polym Degrad Stabil 1996 53(l) 89-98. 

Since the effect of temperature varies with type of polymer aud its formulations, temperatures different than those encountered in end-use environments can distort the stability rankings of materials in addition to causing unrealistic aging behavior. For example, it was shown [141] that change in air temperature from 30 to 60°C in an artilicial weathering test changed the rank order of the stabilities of polyamide, polypropylene, and polyester yam based on reduction... 

Certain compounds that improve the light stability of polyamides are also stabilizers against thermal oxidative destruction. For example, hydroxyphenylbenzoxazole, which exhibits an appreciable effect as a photostabilizer [45], is also an inhibitor of thermal oxidation [42]. 

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