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.
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, aromaticamines, hydroxylamines, and the like.
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.
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.
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.
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.