OK

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. Check out:


Let’s start by understanding the benefits offered by antioxidants when used in polypropylene applications…

Need for Antioxidants in Polypropylene


TAGS:  Antioxidants    

Antioxidants Stabilizers Selection for Polyolefins (PP, PE) 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...


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.

BASF Process and Thermal Stabilizers for Polyolefins

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 MFR
Hydrolysis stability
LTTS Time to embrittlement Impact on formulation costs
No AO - (0) - (0)
AO3 + +++++ + +
AO2 + +++++ ++ ++
AO1 ++ +++++ ++++ +++
Aliphatic Phosphite PS2 +++ + + ++++
Aromatic Phosphite PS1 +++ +++ + ++++
Thiosynergist TS1 + +++++ +++ +++++
AO1/PS1 (1010/168) +++++ ++++ ++ ++++
AO1/PS1/TS1 +++++ ++++ +++++ +++++

PP Molded Grade/Starting Point Formulation


PP Appliances Applicable type: PP molding grade, 1 mm thick

Applicable base polymer: PP-homopolymer

Applicable industrial sectors: Appliance housings, housewares, automotive parts


Ingredients Parts by weight
PP homopolymer 99.2
Ca-stearate 0.1
Aromatic phosphite PS1 0.1
Phenolic AO1 0.2
Thiosynergist TS1 0.4

Processing Stability of Polypropylene Fibers


In some color sensitive applications such as PP fibers, phenolic antioxidants like AO4 (calcium phosphonate) can be replaced by hydroxylamine stabilizers (N,N-dioctadecylhydroxylamine) in processing and hindered amine stabilizers HAS 1 (Butanedioicacid, dimethylester, polymer with 4-hydroxy-2,2,6,6-tetramethyl-1-piperidine ethanol) for LTTS whilst conferring excellent light stability to substrates. These phenol-free packages Hydroxylamine/Phosphite/HAS provide superior initial color and good gas fading resistance.

Processing stability MFR
LTTS Time to embrittlement
Initial Color Gas Fading Resistance Impact on formulation costs
AO4/PS1 ++++ ++++ +++ ++ ++++
Hydroxylamine/PS1/HAS ++++ ++ ++++ ++++ ++

 »  Click Here, to Get a Complete List of Antioxidants for Polypropylene Grades!


Antioxidant Selection for Polyethylene (PE)


Polyethylenes are a known class of thermoplastic polymers having many members. They are prepared by homo-polymerizing ethylene or inter-polymerizing (e.g., copolymerizing) ethylene with one or more alpha-olefins having from 3 to about 18 carbon atoms by known polymerization reactions and conditions.

The viscosity of polyethylenes which have pendant vinyl and/or vinylidene groups tends to change during melt process operations, e.g during extrusion, molding, etc. Such thermally-induced changes in viscosity have been attributed to the changes in molecular weight and/or linearity of the polymers caused by crosslinking.

A wide variety of "stabilizers" have been developed to reduce the changes (e.g., crosslinking) that can occur during melt processing or under conditions of use. Many of the stabilizers are organic compounds which are classified in the plastics industry as antioxidants.
Antioxidant/Free Radical Interaction
Many antioxidants tend to function as free radical scavengers and they interact with free radicals that are formed during polymerization or in the presence of air or other oxidizing medium. Antioxidants are a known class of stabilizers which includes, for example, hindered phenols, triaryl phosphites, aromatic amines, hydroxylamines, and the like.

 »  Check Out the Antioxidants Used for Polyethylene Grades!

Processing Stability of HDPE


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).

The combination of a low volatile sterically antioxidant such as AO1 (Pentaerythritol Tetrakis(3-(3,5-d-tert-butyl-4-hydroxyphenyl)propionate) in combination with an aromatic phosphite such as PS1 (Tris(2,4-ditert-butylphenyl) phosphite) is particularly synergistic and and is more robust than an antioxidant alone. Cross-linking of HDPE can be efficiently suppressed. As the phosphite is consumed during processing, a minimum loading of phosphite PS1 is necessary to ensure a substantial amount in the product after multiple extrusion.

The ratio phenolic AO1/Phosphite PS1 depends on the catalytic system used to produce HDPE:

  • HDPE (Cr-catalyst) can undergo cross-linking as well as chain scission. A ratio 1:4 will protect HDPE (Cr-catalyst) against changes in molecular weight distribution during processing. Thus degradation of mechanical properties can be reduced.
  • HDPE (Ti-catalyst) shows a molecular weight decrease during melt processing. An optimal stabilization can be achieved by using a blend phenolic AO1/Phosphite PS1 with a ratio 1:1.

Discoloration of HDPE may occur because of the formation of oxidation products such as quinone methides. As phosphites and phosphonites can prevent the formation of chemical reactions of the phenolic AO during processing and formation to oxidation products, discoloration can be prevented.
Discoloration of Polymer by Oxidation
Examples of effective phosphites and phosphonites are PS3 (2,4,6-tri-t-butylphenol)2-butyl 2 ethyl 1,3-propanediol phosphite) and PS4 (Tetrakis(2,4-di-tert-butylphenyl)[1,1-biphenyl]-4,4'diylbisphosphonite) respectively.

It needs to be pointed out that phosphites and phosphonites can be sensitive to hydrolysis. Aromatic phosphites of high purity PS1 (Tris(2,4-ditert-butylphenyl) phosphite) are inherently more resistant to hydrolysis than phosphites and phosphonites for example PS3 (2,4,6-tri-t-butylphenol)2-butyl 2 ethyl 1,3-propanediol phosphite) and PS4 (Tetrakis(2,4-di-tert-butylphenyl)[1,1-biphenyl]-4,4'diylbisphosphonite) respectively.

Long Term Thermal Stability (LTTS) of HDPE


Sterically hindered phenolic antioxidants have a positive effect on the long term thermal stability LTTS of PEHD. 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.

Thiosynergists Improve LTTS of HDPEFor 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.

In addition, for LTTS of HDPE plastic articles in applications where contact with water is required (e.g, washing machines, appliances…), the stabilizers must be resistant to leaching. The phenolic antioxidant AO1 (Pentaerythritol Tetrakis(3-(3,5-d-tert-butyl-4-hydroxyphenyl)propionate) confers better resistance to embrittlement than AO2 (Octadecyl-3-(3,5-di-tert.butyl-4-hydroxyphenyl)-propionate).

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 of HDPE, thiosynergists such as TS2 (Dilauryl 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.

For polyethylene wire and cable grades in contact with the copper conductor, specific stabilizers such as metal deactivators are recommended to form stable complexes with metal ions in order to reduce the overall oxidation rate caused by these ions. The metal deactivator MD1 (2', 3-bis [[3-[3, 5-di-tert-butyl-4-hydroxyphenyl] propionyl]] propionohydrazide) combines both the phenolic antioxidant function and complexing activity.

In conclusion, a certain minimum amount of antioxidant is necessary in PE to stabilize and protect the polymer from autoxidative degradation. Primary antioxidants and thioesters are added to the polymer to provide end use product stability while phosphites or phosphonites are added to provide color and processing stability during pelletization and extrusion/molding. As the temperatures that the finished part must withstand rise, so must the level of antioxidants in the polymers to prevent long-term degradation and maintain the polymer physical properties.

Processing stability MFR
Processing stability Discoloration
Hydrolysis stability
LTTS Discoloration LLTS Leaching Resistance Impact on formulation costs
No AO - - (0) - (0) (0)
AO3 + + +++++ + + +
AO2 + + +++++ ++ ++ ++
AO1 ++ ++ +++++ ++++ ++++ +++
Phosphite PS3 +++ ++++ + + + ++
Phosphite P4 +++ ++++ + + + ++
Phosphite PS1 +++ +++ +++ + +++ ++++
Thiosynergist TS2 + + +++++ +++ ++++ +++++
AO1/PS1 +++++ ++++ ++++ ++ +++ ++++
AO1/PS1/TS2 +++++ ++++ +++++ +++++ +++ +++++

HDPE Grade/Starting Point Formulation


HDPE for washing machinesApplicable type: HDPE plaque, 1 mm thick

Applicable base polymer: HDPE, Ti-catalyst

Applicable industrial sectors: Washing machines

Ingredients Parts by weight
PEHD-Ti catalyst 99.7
Ca-stearate 0.1
Aromatic phosphite PS1 0.1
Phenolic AO1 0.1

HDPE Injection Molding Grade/Starting Point Formulation


HDPE under-hood reservoirs Applicable type: HDPE molding grade

Applicable base polymer: HDPE (Cr-catalyst)

Applicable industrial sectors: Under hood reservoirs


Ingredients Parts by weight
PEHD-Cr-catalyst 99.63
Ca-stearate 0.02
Aromatic phosphite PS1 0.20
Phenolic AO1 0.05
Thiosynergist TS2 0.1

HDPE Wire Insulation Grade/Starting Point Formulation


HDPE wire insulation Applicable type: HDPE

Applicable base polymer: HDPE containing 1% TiO2

Applicable industrial sectors: Wire insulations

Ingredients Parts by weight
HDPE 98.80
TiO2 1.0
Metal Deactivator MD1 0.20

 »  View All the Antioxidants Used for High Density PE Grades!


Antioxidant Stabilizers for LLDPE


A blend phenolic AO1/phosphite PS1 is synergistic and confers very good processing stability to LLDPE grades. An optimal stabilization can be achieved by using a blend phenolic AO1/phosphite PS1 with a ratio 1:4.

Discoloration of LLDPE may occur because of the formation of oxidation products such as quinone methides. As phosphites and phosphonites can prevent the formation of chemical reactions of the phenolic AO during processing and formation to oxidation products, discoloration can be prevented. Examples of effective phosphites and phosphonites are PS3 (2,4,6-tri-t-butylphenol)2-butyl 2 ethyl 1,3-propanediol phosphite) and PS4 (Tetrakis(2,4-di-tert-butylphenyl)[1,1-biphenyl]-4,4'diylbisphosphonite) respectively.
AO1/PS1 Blend as a Synergistic
However, it needs to be pointed out that phosphites and phosphonites can be sensitive to hydrolysis. Aromatic phosphites of high purity PS1 (Tris(2,4-ditert-butylphenyl) phosphite) are inherently more resistant to hydrolysis than phosphites and phosphonites such as:

  • TNPP (Tris (nonylphenyl)phosphite),
  • PS3 (2,4,6-tri-t-butylphenol)2-butyl 2 ethyl 1,3-propanediol phosphite) and
  • PS4 (Tetrakis(2,4-di-tert-butylphenyl)[1,1-biphenyl]-4,4'diylbisphosphonite)

The handling of liquid stabilizers can be preferred to avoid plate-out issues caused by insoluble solid additives. Such plate-out can result in potential for contamination, regular manufacturing interruption and cleaning operations, negative impact on economics (downtime), quality issues (gel formation on start up), blooming.

Migration of insoluble additive alters surface properties leading to poor aesthetics, visible powder deposits, reduced gloss, reduced printability, change of adhesion properties. The liquid phosphite TNPP (Tris (nonylphenyl)phosphite) contribute to fulfill the above mentioned requirements, but phosphites which are more stable to hydrolysis are preferred such as the liquid nonylphenol-free phosphite PS5.

Powder antioxidants such as AO2 and AO1 can also be replaced by effective liquid antioxidants such as AO4.

Processing stability MFR
Processing stability Discoloration
Hydrolysis stability
Plate out Blooming Impact on formulation costs
No AO - - (0) (0) (0)
Powder AO3 + + +++++ - +
Powder AO2 + + +++++ ++ ++
Powder AO1 ++ ++ +++++ ++ ++
Liquid AO4 ++ ++ +++++ +++++ ++++
Powder Phosphite PS3 +++ +++ ++ ++ ++
Powder Phosphite PS4 +++ +++ +++ ++ ++
Liquid TNPP ++++ ++++ + +++++ ++++
Powder Phosphite PS1 +++ +++ +++++ ++ ++
Liquid Phosphite PS5 +++++ +++++ ++++ +++++ +++++
Powder AO1 + Powder PS1 +++ +++ +++++ ++ ++
Liquid AO4 + Liquid PS5 +++++ +++++ ++++ +++++ +++++

LLDPE Grade/Starting Point Formulation


LLDPE for Lightweight Carpet Backing Applicable type: LLDPE

Applicable base polymer: LLDPE-polymer (ethylene-octene copolymer), density 0, 93 g/cc, melt index of 1.0 dg/min

Applicable industrial sectors: Lightweight carpet backing

Ingredients Parts by weight
LLDPE 99.73
Ca-stearate 0.12
Aromatic phosphite PS1 0.1
Phenolic AO1 0.05

LLDPE Grade/Starting Point Formulation


LLDPE Packaging Film Applicable type: LLDPE film

Applicable base polymer: LLDPE (C6, density 0, 918 g/cc)

Applicable industrial sectors: Packaging film


Ingredients Parts by weight
LLDPE 99.79
Ca-stearate 0.12
Liquid Phenolic AO4 0.03
Liquid phosphite PS5 0.06

 »  View All the Antioxidants Used for High Density PE Grades!


Check Out the Antioxidants Commercially Available






Polymer Application Find Out Light Stabilizers for Polymers


Polymer Application Check Latest News on Antioxidants


Channel Alerts

Receive weekly digests on hot topics

Eastman Plasticizers
Back to Top