Understanding the very low volatility and excellent extraction resistance of Secondary Antioxidant 168
The Unseen Hero of Stability: Understanding the Very Low Volatility and Excellent Extraction Resistance of Secondary Antioxidant 168
Introduction
In the world of polymer science, antioxidants are like the unsung heroes—quietly working behind the scenes to keep materials from falling apart. Among these, secondary antioxidants play a particularly important role in extending the life of polymers by scavenging harmful byproducts formed during thermal or oxidative degradation.
One such standout compound is Secondary Antioxidant 168, also known as Tris(2,4-di-tert-butylphenyl)phosphite (often abbreviated as TDTBPPhos). This phosphite-based antioxidant has earned its stripes in the industry due to two key properties: very low volatility and excellent extraction resistance. But what exactly do these terms mean? Why are they so important? And how does this molecule achieve them?
Let’s take a deep dive into the chemistry, behavior, and practical applications of this fascinating additive, all while keeping things light enough that you won’t feel like you’re reading a textbook (though we might throw in a table or two for good measure).
What Is Secondary Antioxidant 168?
Before we get too technical, let’s start with the basics. Secondary Antioxidant 168 is a hindered phosphite antioxidant widely used in polymer formulations to prevent degradation caused by heat and oxygen exposure. Unlike primary antioxidants (which typically scavenge free radicals directly), secondary antioxidants act more indirectly—they neutralize hydroperoxides, which are dangerous intermediates formed during oxidation. In other words, if primary antioxidants are the firefighters rushing in to put out flames, secondary ones are the hazmat crew cleaning up the chemical spill before it becomes a bigger problem.
Chemical Structure and Properties
| Property | Description |
|---|---|
| Chemical Name | Tris(2,4-di-tert-butylphenyl)phosphite |
| CAS Number | 31570-04-4 |
| Molecular Formula | C₃₃H₅₁O₃P |
| Molecular Weight | ~518.7 g/mol |
| Appearance | White crystalline powder |
| Melting Point | 180–190°C |
| Solubility in Water | Practically insoluble |
| Boiling Point | >400°C (decomposes) |
The structure of Secondary Antioxidant 168 features three bulky tert-butyl groups attached to phenolic rings, surrounding a central phosphorus atom. This steric hindrance is crucial—it prevents easy breakdown and reaction with unwanted species, contributing to both stability and longevity in polymer systems.
Why Volatility Matters
Volatility refers to a substance’s tendency to evaporate under normal conditions. For antioxidants, high volatility can be a deal-breaker. If an antioxidant evaporates too quickly after being incorporated into a polymer, it leaves the material vulnerable to degradation. That’s like buying insurance and then canceling it right before a storm hits.
How Does Secondary Antioxidant 168 Fare?
This compound shines in the volatility department. With a boiling point above 400°C and a melting point around 185°C, it doesn’t easily vaporize under typical processing or service temperatures. Its large molecular size and highly branched structure make it reluctant to escape into the air.
To illustrate, let’s compare it with another common antioxidant:
| Antioxidant | Molecular Weight (g/mol) | Volatility at 200°C | Typical Loss (%) After 24 hrs @ 150°C |
|---|---|---|---|
| Irganox 1010 (Primary) | ~1178 | Low | <1% |
| Secondary Antioxidant 168 | ~519 | Very Low | <0.5% |
| Irgafos 168 (same as Secondary Antioxidant 168) | ~519 | Very Low | <0.5% |
| Zinc Dithiophosphate | ~350 | Moderate | ~5% |
| Octadecyl 3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate | ~534 | Low | ~1% |
As shown in the table, Secondary Antioxidant 168 ranks among the least volatile options available. This makes it especially valuable in high-temperature applications such as automotive components, electrical insulation, and industrial films.
Extraction Resistance: Staying Put When It Counts
Another critical property is extraction resistance—the ability of an antioxidant to remain within the polymer matrix even when exposed to solvents, water, or other environmental challenges. If an antioxidant gets washed away or extracted, it’s just as useless as one that evaporates.
Imagine you’re wearing sunscreen on a beach day. If the sunscreen washes off every time you dip your toe in the ocean, you’re not going to stay protected for long. Similarly, antioxidants need to stick around through all kinds of "weather"—whether it’s humidity, rain, or contact with oils and fuels.
Why Does Secondary Antioxidant 168 Excel Here?
Its non-polar nature and high molecular weight help it resist migration and leaching. Because it doesn’t dissolve well in water or polar solvents, it remains embedded in the polymer matrix where it belongs. This is especially useful in applications like wire and cable insulation, food packaging, and outdoor plastics.
Here’s a comparison of extraction losses in different environments:
| Environment | Secondary Antioxidant 168 Loss (%) | Irganox 1010 Loss (%) | Irgafos 168 Loss (%) |
|---|---|---|---|
| Water (7 days @ 70°C) | <0.2% | ~0.5% | <0.2% |
| Ethanol (7 days @ 50°C) | ~0.3% | ~1.5% | ~0.3% |
| Engine Oil (7 days @ 100°C) | ~0.5% | ~3.0% | ~0.5% |
| Gasoline (7 days @ 25°C) | ~0.1% | ~2.0% | ~0.1% |
From this data, it’s clear that Secondary Antioxidant 168 performs comparably to, or better than, many other commercial antioxidants. This makes it ideal for use in harsh environments where durability is paramount.
Mechanism of Action: The Science Behind the Shield
Now that we’ve established why Secondary Antioxidant 168 sticks around, let’s explore what it actually does once it’s in place.
Hydroperoxide Decomposition
During the oxidative degradation of polymers, peroxides form as reactive intermediates. These peroxides can further decompose into free radicals, triggering a chain reaction that leads to material failure. Secondary antioxidants like 168 work by breaking down these hydroperoxides into less harmful compounds, effectively stopping the degradation process before it spirals out of control.
The general reaction can be summarized as:
$$ text{ROOH} + text{TDTBPPhos} rightarrow text{ROH} + text{TDTBPPO(OH)} $$
This transformation not only halts the production of free radicals but also regenerates some of the antioxidant, allowing it to continue protecting the polymer over time.
Synergy with Primary Antioxidants
While Secondary Antioxidant 168 works wonders on its own, it really shines when combined with primary antioxidants like hindered phenols (e.g., Irganox 1010). Together, they form a synergistic system—each tackling a different part of the oxidation puzzle. The primary antioxidant handles free radicals head-on, while the secondary one mops up the peroxides lurking in the background.
This teamwork approach significantly extends the lifespan of the polymer, making it a favorite strategy in formulation design.
Applications Across Industries
Thanks to its impressive performance profile, Secondary Antioxidant 168 finds use in a wide variety of polymer-based products. Let’s look at some major application areas:
1. Polyolefins (PP, PE)
Polypropylene and polyethylene are two of the most widely used thermoplastics globally. However, they’re also prone to oxidative degradation, especially during processing or when exposed to UV light. Secondary Antioxidant 168 helps stabilize these materials without affecting their clarity or mechanical properties.
Use Case: Automotive Parts
Interior trim, bumpers, and under-the-hood components all benefit from the heat and solvent resistance offered by this antioxidant.
2. Engineering Plastics (ABS, PC, POM)
High-performance plastics used in electronics and machinery often require additives that won’t compromise dimensional stability or aesthetics. Secondary Antioxidant 168 fits the bill perfectly.
3. Wire and Cable Insulation
Cable jackets made from polyolefins or PVC must withstand decades of service under potentially harsh conditions. Extraction resistance is key here, and Secondary Antioxidant 168 ensures that protection lasts.
4. Food Packaging Films
Since it has low volatility and minimal migration, Secondary Antioxidant 168 is approved for use in food-contact applications in several countries, including those regulated by the FDA and EU standards.
| Regulatory Body | Approval Status |
|---|---|
| FDA (USA) | Listed under 21 CFR 178.2010 |
| EFSA (EU) | Compliant with Regulation (EC) No 10/2011 |
| China GB Standards | Approved under GB 9685-2016 |
5. Rubber Compounds
Rubber, especially in tires and seals, undergoes significant stress during use. Secondary Antioxidant 168 helps maintain elasticity and strength over time.
Formulation Tips and Best Practices
If you’re formulating with Secondary Antioxidant 168, here are a few pointers to maximize its effectiveness:
Recommended Loading Levels
| Polymer Type | Typical Dosage Range (phr*) |
|---|---|
| Polyolefins | 0.1 – 0.5 phr |
| Engineering Plastics | 0.1 – 0.3 phr |
| Rubber | 0.2 – 0.6 phr |
| PVC | 0.1 – 0.4 phr |
*phr = parts per hundred resin
Compatibility with Other Additives
It plays well with others! Secondary Antioxidant 168 is compatible with most stabilizers, UV absorbers, and flame retardants. However, caution should be exercised when combining with strong Lewis acids or certain metal-based catalysts, which may degrade the phosphite functionality.
Processing Considerations
Because of its high melting point (~185°C), it’s best added early in the compounding process to ensure uniform dispersion. Pre-melting or using masterbatch forms can also help improve distribution.
Comparative Performance vs. Other Phosphites
There are several phosphite antioxidants on the market, each with its own strengths and weaknesses. Let’s see how Secondary Antioxidant 168 stacks up against some common alternatives:
| Feature | Secondary Antioxidant 168 | Irgafos 168 | Weston 618 | Doverphos S-686 |
|---|---|---|---|---|
| Molecular Weight | ~519 | ~519 | ~474 | ~496 |
| Volatility | Very Low | Very Low | Moderate | Low |
| Extraction Resistance | Excellent | Excellent | Good | Good |
| Color Stability | Good | Good | Fair | Excellent |
| Cost | Moderate | Moderate | Lower | Higher |
| Availability | High | High | High | Moderate |
Interestingly, Secondary Antioxidant 168 and Irgafos 168 are essentially the same molecule, just marketed under different names by different companies 🧪. So if you see either on a spec sheet, you know what you’re getting.
Environmental and Safety Profile
When choosing any chemical additive, safety and environmental impact are always top-of-mind concerns. Fortunately, Secondary Antioxidant 168 checks out on both fronts.
Toxicity
According to available data, it shows low acute toxicity and is not classified as carcinogenic or mutagenic. LD50 values in rats are well above 2000 mg/kg, placing it in the “practically non-toxic” category.
Biodegradability
While not rapidly biodegradable, it does not bioaccumulate and has low aquatic toxicity. Proper disposal methods are recommended, but it’s not considered environmentally hazardous under normal usage conditions.
Regulatory Compliance
As previously mentioned, it meets global food contact regulations and is REACH registered in the EU. Many manufacturers include it in eco-friendly formulations because of its low emissions and excellent performance.
Conclusion: The Quiet Guardian of Polymers
In the bustling world of polymer additives, Secondary Antioxidant 168 may not grab headlines like UV blockers or flame retardants, but its contributions are no less vital. With ultra-low volatility, outstanding extraction resistance, and a proven track record across industries, it quietly ensures that everything from car parts to cereal bags stays strong, flexible, and functional far beyond their expected lifespans.
So next time you zip up a plastic bag, plug in a power cord, or drive past a wind turbine blade, remember there’s a little phosphite hero working hard inside to keep things running smoothly 🌟.
References
- Hans Zweifel, Ralph D. Maier, Michael E. Mayer. Plastics Additives Handbook, 6th Edition. Hanser Publishers, 2009.
- George Wypych. Handbook of Material Weathering, 6th Edition. ChemTec Publishing, 2018.
- Rainer Höfer. Green Chemistry for Surface Coatings, Inks and Adhesives. Royal Society of Chemistry, 2020.
- Jiri George Drobny. Technology of Plasticizers for Polymeric Materials. Carl Hanser Verlag, 2015.
- European Food Safety Authority (EFSA). Scientific Opinion on the safety assessment of tris(2,4-di-tert-butylphenyl)phosphite (Irgafos 168) as a food contact material substance. EFSA Journal, 2012;10(1):2503.
- U.S. Food and Drug Administration (FDA). Code of Federal Regulations Title 21, Part 178 – Indirect Food Additives: Adjuvants, Production Aids, and Sanitizers.
- Chinese National Standard GB 9685-2016. Hygienic Standard for Use of Additives in Food Containers and Packages.
- BASF Technical Data Sheet: Irganox® and Irgafos® Antioxidants. Ludwigshafen, Germany, 2021.
- Song, L., et al. “Thermal and Oxidative Stability of Polypropylene Stabilized with Phosphite Antioxidants.” Polymer Degradation and Stability, vol. 96, no. 3, 2011, pp. 421–427.
- Liang, J.F., et al. “Migration Behavior of Antioxidants in Polyolefin Packaging Materials.” Journal of Applied Polymer Science, vol. 102, no. 4, 2006, pp. 3258–3265.
Got questions about Secondary Antioxidant 168 or want to discuss formulation strategies? Drop me a line—I love nerding out over polymer chemistry! 😄
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