The propagating reaction of autoxidation creates hydroperoxides (ROOH), relatively unstable species which must be reduced by AOs into more stable alcohol (ROH) forms. Secondary AOs decompose these species by allowing themselves to becoming oxidized (taking the oxygen from the ROOH). Used in combination with primary AOs, secondary AOs are often referred to as “synergists,” because their interaction with primary AOs greatly enhances the protection the polymer receives. Secondary AOs become cost effective when they can lower the required amount of more expensive primary AOs. And some high-performance, phosphite-based secondary AOs have even been shown to maintain a resin’s color and melt flow properties in PP when used as the sole processing stabilizer—without any primary phenolic AO in the formulation [1-1, 3-3, 3-4, 3-12][1-1][3-3][3-4][3-12].
Phosphite-based AOs support melt processing stability by accepting oxygen atoms from hydroperoxides, becoming themselves phosphates and leaving behind stable alcohol species. However, they are susceptible to reactions with water (hydrolysis) to form acids, causing melt flow changes, black specks, corrosion, and breakdowns of other additives or materials encountered in service or in processing. They also break down into smaller molecules that are able to migrate through the matrix, something obviously unwanted in food packaging, water pipes, and similar applications. Accordingly, different kinds of phosphites are available with bulky molecular structures that hinder their phosphorous atoms and resist hydrolysis. Generally, phosphites with higher phosphorous content are more active and provide better process stability than lower phosphorous grades [1-1, 3-2, 3-5, 3-12, 3-33][1-1][3-2][3-5][3-12][3-33].
In combination with primary AOs in polyolefins, phosphites help retain the melt flow properties and color stability through repeated processing passes better than each AO can do alone. This helps limit the amount of primary AO that is consumed in processing dramatically. Moreover, phosphites and hindered phenol AOs can be combined as dry blends to simplify handling and feeding [3-5].
Tris(nonylphenyl) phosphite (TNPP) is one of the oldest and most often used phosphite stabilizers for polymeric materials. In liquid form, it is economical to use even at over twice the loading of alternative dry phosphites, and its liquid form can have some advantages in handling and metering. It is used in LLDPE (linear low-density polyethylene) bags and food packaging and HDPE blow-molded drums and box wrapping film [3-5, 3-6][3-5][3-6].
Even though TNPP has been approved for food contact uses by the US FDA and European Food Agency and other agencies, studies have been directed at determining the potential toxicity of its nonylphenol content. Though not conclusively found to be a direct threat to human health, nonylphenol’s potential toxicity to aquatic life has raised concerns about its accumulation in the environment. Excess nonylphenol is stripped from TNPP during its production, cutting its concentration down to as low as 0.1% or lower. And although TNPP generally resists hydrolysis, a small amount of hydrolysis during polymer processing and in the environment will create nonylphenol as a product of TNPP. Thus the material was reevaluated in a risk assessment under the European Union’s Existing Substance Regulatory (ESR) program. In response to this negative attention, a range of alternative phosphites in liquid and solid forms have become available, including those that are able to brand themselves as inherently “100% nonylphenol free” [3-6, 3-7][3-6][3-7].
Most phosphites have a tendency to hydrolyze in the presence of moisture, causing clumping, handling, and feeding problems, and potentially black specks in the resin. However, pentaerythritol-based phosphites are effective for stabilizing polyolefins, and these reportedly have overcome some problems with hydrolysis [3-2, 3-5, 3-9, 3-12][3-2][3-5][3-9][3-12].
Thioester-based AOs are sulfur-based secondary AOs that are often called thiosynergists when combined with primary AOs. Like phosphites, thiosynergists transform reactive peroxide groups into alcohol groups, supporting long-term thermal stabilization. For example, distearyl thio dipropionate (DSTDP) fights long-term heat aging when used in combination with a hindered phenol primary AO. However, due to the odors they create, sulfur-based AOs tend to be used less than phosphites, when possible [3-2, 3-3, 3-4][3-2][3-3][3-4].