Polymer stabilization long-term heat stability

Although HALS stabilizers were developed as UV stabilizer it is more and more recognized that these molecules could also impair long-term heat stability. Especially were phenolic antioxidants cannot be used due to their discoloration HALS is used to protect the polymer against long-term heat degradation [50]. 

After the introduction of the first HA(L)S in the 1970s, this class of Ught stabilizers rapidly became the most important UV stabilizer for the majority of plastics. Without the discovery of HA(L)S, the outdoor applicability of many polymers would be limited. So, the use of PP in automotive application without the use of HA(L)S would be impossible. Although HA(L)S stabilizers were developed as UV stabilizer they are more and more used as long-term heat stabilizers too [74,109]. Especially where phenohc antioxidants cannot be used due to their discoloration, HA(L)S is used to protect the polymer against long-term heat degradation [73]. 

Gijsman, P. Gitton, M. Hindered amine stabilizers as long-term heat stabilizers for polypropylene. Polym. Degrad. Stab. 1999, 66, 365— 371. 

Hindered phenols are used preferentially for processing and long-term heat aging of PO where their very low-discoloring properties are advantageous (Pospisil, 1998 Zweifel, 1998 Pospisil, 1993). Phenolic antioxidants are effective also for thermal stabilization of styrene-based polymers. Low amounts are used in PET and aliphatic PA. Common concentrations in plastics range between 0.025 and 0.3 %. 

Here we report the use of a readily prepared polymer immobilised TEMPO as a catalyst for alcohol oxidations.15 It was derived from a commercially available oligomeric, sterically hindered amine, poly[[6-[(l,l,3,3-tetramethylbutyl)amino]-l,3,5-triazine-2,4-diyl] [2,2,6,6-teramethyl-4-piperidinyl)-imino]-1,6-hexane-diyl[(2,2,6,6-tetramethyl-4-piperidinylimino]], better known as Chimassorb 944 (MW 3000 see figure 3 for structure). This compound is used as an antioxidant and a light stabiliser for plastics. It contributes significantly to the long-term heat stability of polyolefins and has broad approval for use in polyolefin food packaging.16... 

Non-discoloring, sulfur containing phenolic antioxidant and stabilizer that provides long-term heat stability by preventing thermo-oxidative degradation. Used for the process stabilization of polyethylene wire and cable resins for polyethylene during extruder compounding. Can also be applied in styrenic polymers, polypropylene, elastomers such as EPDM and SBR and for carboxylated SBR latex, polybutadiene rubber and polyisopropene rubber. 

HALS was based on the discovery that the 2,2,6,6-tetramethyl-l-piperidinyloxy, free radical (TEMPO) (1)), which already was known as an effective radical scavenger [46,47], was a very effective UV stabilizer too [48,49]. However, due to its physical and chemical properties TEMPO itself did not led to practical use. TEMPO is colored and will impart color to the to be stabilized polymer, it is thermally unstable and volatile [49]. Furthermore, it reacts with phenolic antioxidants present in many polymers leading to a reduction of processing and/or long-term heat stability. The discovery that compounds in which the /V-oxyl functionality was replaced by a N—H functionality also showed good UV stabilization activity was the key finding that led to the development of HALS stabilizers [49]. 

Low-density polyethylene (LDPE) is extensively used for the manufacture of films. During processing, which is carried out at temperatures of approximately 200°C, cross-Unking, and thus formation of gel, can occur through oxidation if the polymer is not stabilized. Such gel particles are visible in the film as agglomerates, known as fish eyes or arrow heads. The processing stabilizers used in LDPE consist of systems commonly used for polypropylene, namely, combinations of a phosphite or phosphonite and a long-term heat stabilizer (hindered phenol) in overall concentrations up to 0.1%. Concentrations seldom exceed 0.1%, since the compatibility of any additive in LDPE is considerably lower than in any other polyolefins. 

Outstanding properties effective processing and long term heat stabilizer, efficient in carbon black-filled polymer systems ... 

The origin of the talc ore used in polypropylene will have a significant influence on the color and long-term oven aging performance of the composite. Numerous tests have been performed to determine the variables in talc that contribute to this difference. It appears that talc surface chemistry can lead to talc-polymer and talc-stabilizer interactions that can cause discoloration and depolymerization under long-term heat (150-160°C) aging. High iron content in the talc can also adversely influence the level of performance. 

Besides providing effective protection of polymers against photo-degradation, some high molecular weight and oligomeric HALS (e.g. HALS-3) have been shown to act as effective long term heat stabilizers. For this reason, in some relevant literature, it is possible to find the abbreviation HATS (Hindered Amine Thermal Stabilizers) or HAS (Hindered Amine Stabilizers). A detailed overview of the various mechanisms proposed for HALS action can be found in [1]. 

Naugard Super Q is an effective processing and long term heat stabilizer for a variety of application areas in LDPE, LLDPE, HOPE, and ethylene-propylene compolymers. It is particularly efficient in carbon black-filled polymer systems such as wire and cable and geomembranes. 

Gijsman, P. The mechanism of action of hindered amine stabilizers (HAS) as long-term heat stabilizers. Polymer Degradation and Stability, 43 (1994), p. 171 176... 

Depending on the aging condition (temperature) and failure criterion HA(L)S can even outperform phenolic antioxidant [185]. In low-density polyethylene (LDPE), HA(L)S acts as a long-term heat stabilizer too, but this polymer at 100°C PAO-2 is still much more effective than several HA(L)S types [186]. 

Rotzinger B. Talc-filled PP a new concept to maintain long term heat stability. Polym Degrad Stab 2006 91(12) 2884-7. 

Naugard 10 is a versatile antioxidant capable of giving excellent processing stability and long term heat aging characteristics. It is used in a variety of polymers where elevated processing temperatures are encountered. It is particularly effective in high density polyethylene and polypropylene. Product Features ... 

The contribution of HALS to Long Term Heat Stability of polymers is already well documented in literature. The effect of HALS as Long Term Heat Stabilizers is competitive with traditional antioxidants up to 130C. This effect is particularly evident in PP Fibers in which Uvasorb HA88 FD is one of the most efficient and appreciated HALS in the market for the UV protection and also for the superior Long Term Heat Stabilization. 

Conventional techniques determine long-term heat stability of polymer articles at temperatures below melting conditions, for example, during accelerated aging in circulating air ovens until degradation effects such as discoloration and embrittlement are visible (Figure 28). 

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