Troubleshooting Antioxidant Blooming and Migration in Polymers

Have you noticed a hazy or weird skin-like formation on your plastic product's surface? If yes, you are witnessing the phenomenon of additive blooming, specifically, antioxidant blooming. This is when stabilizing antioxidants migrate out of the polymer bulk to the surface.

Blooming shows up as discoloration, haziness, weepiness, or skin formation. This gives the polymer surface a sweaty and gross look. While not immediately detrimental, unchecked blooming can worsen with time. So to preserve product performance, it's advisable to catch this issue early.

Once you identify the problematic antioxidant, potential solutions open up. These can include reformulating, optimizing processing methods, or improving storage conditions.

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Let’s understand what antioxidant blooming is and what factors should you be aware of to prevent it!

Why does antioxidant blooming and migration happen?

Additives can be more likely to migrate to the surface when:

• The additive has poor compatibility/solubility with the polymer matrix.
• The additive-incorporated formulations are subjected to mechanical stress or pressure during the process.

Over an extended period of time, a low migration tendency additive can move towards the surface or neighboring materials. Other factors include molecular size, free volume of the polymer, and the nature of additives and polymer.

Additive blooming and migration can be due to several factors which include:

Concentration of antioxidants

High concentrations of additives within the plastic may increase the likelihood of blooming and migration. When the concentration of the additive is below the solubility threshold, all antioxidants are solubilized in the polymer matrix.

As the concentration of antioxidants increases, the polymer totally saturates in additives. If more additives are incorporated into the polymer matrix, the excess of additives can no longer be dissolved and thus, a phase separation occurs.

Molecular weight

Low molecular weight species (oligomers, degradation products of the polymer, and additives) can migrate more readily when exposed to elevated temperatures because of increased mobility.

Environmental conditions

Environmental conditions can affect the migration behavior of additives. These factors include:

• Humidity
• Exposure to solvents or other chemicals

More than acceptable levels of antioxidant migration may pose a risk to health, safety, product quality, regulatory requirements, and industry standards for specific applications.

Techniques to Analyze Blooming

Antioxidant blooming can be confirmed using different techniques.

1. Bloomed species can be separated from the polymer surface by:
   • Surface washing or
   • Mechanical constraint (wiping, scraping)
   The residue is then analyzed by chromatographic or spectroscopic (FTIR) techniques.


2. Liquid extraction surface analysis mass spectrometry detects antioxidants and polymer degradation products.


3. FTIR, contact angle, and microscopic analysis are excellent techniques for surface characterization.
   • FTIR provides the chemical composition of the surface.
   • Contact angles give information about changes in surface free energy.
   FTIR and contact angle measurements are useful for analyzing blooming and surface properties.


4. Microscopes confirm additive migration and optical quality deterioration. Some examples of microscopes include:
   • Optical Microscopy
   • Scanning Electron Microscope (SEM), and
   • Atomic Force Microscopy (AFM)
   
   Figure 1 shows very thin deposits (30 to 4 nm thick) were identified by imaging the additive's infrared absorbance band. This was done using AFM with infrared spectroscopy.
   
   
   
   Figure 1. Microscopic images of cast film surface with (left) and without (right) antioxidant blooming¹
   
   Other analytical methods to analyze the blooming species include:
   
   • X-ray Photoelectron Spectroscopy (XPS)
   • Static Secondary Ion Mass Spectrometry (SIMS)

Measures to Prevent or Minimize Antioxidant Blooming

Different strategies can be employed to overcome blooming:

High molecular weight antioxidants

• A high molecular weight (>1500 g/mol) antioxidant can eliminate issues like:
  
  • Bloom and plate-out,
  • Volatility, and
  • Migration
  This is because of a lower diffusion rate¹. However, increasing the additive molar mass can decrease solubility. This is true for the majority of phenolic antioxidants.


• A high molecular weight hindered phenolic antioxidant (e.g., ADK STAB 328) is suitable for:
  
  • Polyphenylene ether,
  • Polybutylene terephthalate,
  • Acrylonitrile butadiene styrene, and
  • Polyester


• Milliguard™ AOX-1 is an amine-free liquid stabilizer package for polyurethane and thermoset polymers. It is a polymeric lactone that acts as an antioxidant to eliminate surface bloom and migration.


• Anti-bloom antioxidants (e.g., Dovernox® D-9411T) are an option to reduce the blooming.

Using blends

Antioxidants are mainly based on phenolics, phosphates, thioesters, amines, lactones, and blends. Among them, phenolics, phosphates, amines, and blends of antioxidants are more widely used. This is because of their efficiency in different polymer systems.

Combinations of antioxidants can provide synergistic effects by complementing each other. This is because they can work with different chemistries that can’t be achieved with a single antioxidant. For example, ANOX® BB 011 is a blend of phenolic and phosphite antioxidants developed by ADEKA. It provides a synergistic effect for PP, HDPE, LDPE, LLDPE, ABS, PC, and PA.

Appropriate use of antioxidants in a specific polymer system can minimize the migration. For example, less volatile antioxidants are less likely to migrate to the surface. Selecting antioxidants with compatible polymers can also minimize the migration to the surface.

Predicting compatibility & stability

• Compatibility of additives and polymers can be predicted using Hansen solubility parameters (HSP). The Relative Energy Difference (RED) number can be calculated from the difference in HSP. It allows evaluation of the compatibility relations. If the RED number is <1.0 it suggests good compatibility⁴.
  
  
  Check out the complete range of antioxidants with HSP values


• The concentration of the antioxidant loading depends on:
  
  • The chemical character of the additive and
  • The type of polymer matrix used
  A higher concentration of additive in the bulk is associated with a higher diffusion rate.
  
  For example, if tris(di-t-butylphenyl) phosphite is used below 1000ppm in cast film, compatibility issues will be minimal for most applications. However, performance may suffer when lower levels of phosphite are used⁵. Phenolic antioxidants are used in low amounts compared to phosphites. Polyolefins (LLDPE, HDPE) require the lowest amounts of antioxidants compared to HIPS and ABS.
  
  


• To ensure prolonged stability, the antioxidant must have:
  
  • A low rate of diffusion (kinetic parameter) and
  • A high solubility (thermodynamic parameter)
  For example, the diffusion coefficient for phenolic antioxidants in LDPE at 50°C is reported 10^-8 cm²s^-1 for butylated toluene (BHT), the smallest molecule^2,3.


• Lowering the loading level of antioxidants below a certain threshold may prevent plate-out and bloom issues.

Molecular morphology

Achieving good dispersion at the molecular level can help to minimize additive migration. Antioxidant migration can be influenced by the crystallinity of the polymer.

• The diffusion rate decreases with polymer crystallinity in a complex way. This is because of the notion of tortuosity of the diffusion path.
• The diffusion rate will be high in the amorphous structure.

Optimizing the crystallinity of the polymer could help to minimize the additive migration.

Physical conditions

Antioxidant-incorporated products should be stored in controlled environmental conditions to minimize migration. Some factors that can accelerate antioxidant migration include:

• Excessive heat,
• Humidity, and
• Light

These strategies can vary widely based on the type of polymer, antioxidant, and processing conditions. Thus, vigorous process optimization and proper selection of antioxidants can help minimize the bloom and migration of antioxidants in plastics. This thereby preserves the quality and performance of the products.

Conclusion

The emergence of hazy patches, discoloration, or skin-like formations on a polymer's surface should raise a suspicion of blooming. A visible defect that signals the migration of antioxidants from the bulk material. Over time, if left unattended, blooming tends to worsen.

Troubleshooting this issue involves analyzing the chemical composition of the bloomed layer to pinpoint the additive causing the problem. Addressing blooming requires strategic solutions, such as:

• Replacing problematic additives
• Optimizing processing methods, or
• Adjusting storage and operating conditions

Early detection of blooming provides the best opportunity to troubleshoot the root cause and implement solutions before functionality is affected or performance diminishes. Thus, regular inspection of polymer products is crucial, and swift action can solve blooming before it becomes a significant problem.