Antioxidants Stabilizers Selection for Polyolefins (PP, PE)

Polymer degradation is a natural phenomenon that cannot be totally stopped. It tends to deteriorate the physical & mechanical like molecular weight, melt flow rate, appearance, processing and thermal stability properties of the polymer. To reduce such damages that occur during melt processing or under conditions of use, antioxidant stabilizers are developed. Explore, in detail, about the need for antioxidants, and how they impact the processing & long term thermal stability of polyolefins (PP, PE). Also, select the right combination of antioxidant stabilizers & explore formulation examples for your polymer applications.

Need for Antioxidants in Polypropylene (PP)

Polypropylene (PP) in their natural state (without additives) are inherently unstable and degrade when exposed to oxygen. The polymers change color to yellow-brown and begin to flake away until the material becomes useless.

When PP degrades, chain scission takes place. The physical properties of the polymer deteriorate and its average molecular weight (chain length) decreases, melt flow rate increases and a powdery surface eventually forms. Polymer degradation is a natural phenomenon that cannot be totally stopped.

Instead, resin producers seek to stabilize the color and physical properties of their polymers for a reasonable life span, which varies depending on the end use requirements.

Polypropylene can be processed by virtually all thermoplastic-processing methods. Most typically PP products are manufactured by:

  • Extrusion Blow Molding,
  • Injection Molding, and
  • General Purpose Extrusion

Additives are needed to stabilize polypropylene during melt processing and protect plastics against thermo-oxidative degradation during service life:

  • Melt processing stability of polypropylene can be quantified by measuring molecular weights distribution, melt flow or viscosity, and discoloration before and after processing in an extruder for example.
  • As most polypropylene articles are exposed to oxygen, elevated temperatures, light, and moisture during their service lives, thermo-oxidation occurs. The thermo-oxidative stability of a PP plastic part during services is determined by aging the part at elevated temperatures in a circulating air oven for example. The property called Long Term Thermal Stability (LTTS) is measured. Mechanical properties such as embrittlement on bending, elongation, tensile impact are determined as a function of aging time.

Let’s read about the effective combination of antioxidants & other additives used by the formulators…

Antioxidant Stabilizers for PP Extrusion & Molding Grades

Processing Stability of PP Extrusion & Molding Grades

The sterically hindered phenolic antioxidant AO1 (Pentaerythritol Tetrakis(3-(3,5-d-tert-butyl-4-hydroxyphenyl)propionate) is particularly effective for the melt processing of polypropylene. This additive has a higher number of phenolic groups that serve as H-donors than other antioxidants such as AO2 (Octadecyl-3-(3,5-di-tert.butyl-4-hydroxyphenyl)-propionate) or AO3 (Butylhydroxytoluène).

AO1 provides effective Melt Processing & LTTS than AO2 & AO3

The combination of a low volatile sterically antioxidant such as AO1 (Pentaerythritol Tetrakis(3-(3,5-d-tert-butyl-4-hydroxyphenyl)propionate) with a phosphite such as PS1 (Tris(2,4-ditert-butylphenyl) phosphite) is particularly synergistic and is more robust than an antioxidant alone. It needs to be pointed out that phosphites and phosphonites can be sentitive to hydrolysis. Aromatic phosphites of high purity PS1 (Tris(2,4-ditert-butylphenyl) phosphite) are inherently more resistant to hydrolysis than aliphatic phosphites PS2.

Long Term Thermal Stability of PP Molding & Extrusion Grades

Sterically hindered phenolic antioxidants have a positive effect on the long-term thermal stability (LTTS) of polypropylene. However, the molecular weight of these additives and their structural properties confer different effects. Phenolic antioxidants such as AO3 (Butylhydroxytoluène) are too volatile and are physically lost in short time.

In addition, phenolic antioxidants act as H-donors. The stability of the phenoxyl radical is provided by the sterical hindrance of the substituent in the 2,6-position. The efficiency of sterically hindered phenolic antioxidants used for long term exposure of polymers at temperatures higher than 120°C decreases in the order: 2,6 di-tert.butyl > 2 –tert, butyl-6-methyl > 2,6-dimethyl groups as substituents.

For example, antioxidants such as AO1 (Pentaerythritol Tetrakis(3-(3,5-d-tert-butyl-4-hydroxyphenyl)propionate) exhibit better performance with respect to oven aging than less hindered phenolic antioxidant such as Bis[3,3-bis-(4’-hydroxy-3’-tert-butylphenyl)butanoicacid]-glycol ester.

AO1 (Pentaerythritol Tetrakis(3-(3,5-d-tert-butyl-4-hydroxyphenyl)propionate) confers a better LLTS than AO2 (Octadecyl-3-(3,5-di-tert.butyl-4-hydroxyphenyl)-propionate) because of its higher number of phenolic groups that serve as H-donors.

Though phosphites provide the best protection against polypropylene during melt processing, they do not contribute to LLTS. The phosphites protect the phenolic antioxidant during processing, thus leaving the phenolic structure practically intact which contributes to LTTS.

To improve LTTS, thiosynergists such as TS1 (Dioctadecyl 3,3′-thiodipropionate) as hydroperoxide decomposers in combination with a phenolic antioxidant are recommended. The ratio 1:2 or 1:3 of phenolic antioxidant to thiosynergist provides the best results in terms of cost and performance.

Processing stability MFRHydrolysis stabilityLTTS Time to embrittlementImpact on formulation costs
No AO(0)(0)
Aliphatic Phosphite PS2+++++++++
Aromatic Phosphite PS1+++++++++++
Thiosynergist TS1++++++++++++++
AO1/PS1 (1010/168)+++++++++++++++

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