Plastic Part Failure Analysis - Using Thermal Analysis (DSC) to Estimate the Anti-Oxidant Level in Polymers

Hello and welcome to a new blog post. Let me start today with the following question: How do you ensure the long-term performance of polyolefin materials in demanding applications?

Understanding and measuring oxidative stability is key. The following post explores why oxidative stability matters for polyolefins like polypropylene, and how Differential Scanning Calorimetry (DSC) provides valuable insights into material durability, processing effects, antioxidant performance, and ultimately prevent plastic part failure. Dive in to learn how this classic yet often overlooked test method can help you make informed decisions about material selection and process optimization.

DSC Testing for Oxidative Stability

DSC measures the heat absorbed or released by a material as temperature or time changes. While commonly used for phase transitions (melting, recrystallization, glass transition), it is also effective for detecting exothermic events like oxidation.

How does a typical test procedure for oxidative stability look like?

• A sample (raw material or molded part) is placed in the DSC.
• The sample is heated in a nitrogen atmosphere to a set temperature (commonly 200°C/392°F, which melts PE or PP).
• After reaching the target temperature, air or oxygen is introduced.
• The antioxidant in the polymer protects it until it is depleted; then, oxidation occurs, shown by a sharp increase in the DSC baseline.
• The time from oxygen introduction to oxidation onset is called the Oxidation Induction Time (OIT).
• Also, the test can be used to access the oxidation onset temperature (OOT).

Example PP raw material with standard antioxidant package vs. PP raw material with improved antioxidant package

Figure 1 shows the result for a tested polypropylene (PP) raw material and the OIT was measured at 2.29 minutes. A second PP raw material, which contains an improved antioxidant package, showed a higher OIT (6.42 minutes), indicating better resistance to oxidation under the same test conditions [1].

Figure 1: Using DSC to estimate the anti-oxidant level in Polyolefins - Example PP [1].

Interpreting OIT Results

The OIT value alone is not meaningful, but comparing OITs of materials with similar antioxidant chemistries provides a relative measure of oxidative stability. AS a rule of thumb, higher OIT indicates better oxidative stability and, typically, higher antioxidant content.

DSC offers a quick and practical way to assess oxidative stability compared to more complex antioxidant quantification methods.

Applications of OIT measurements

Raw Material vs. Molded Part:

• Processing (molding) consumes some antioxidants, so molded parts usually have a lower OIT than raw materials. 
• Changes in processing conditions (temperature, screw speed, backpressure) affect OIT and thus the remaining antioxidant content.

Post-Processing and Environmental Effects:

• Sterilization (gamma or E-beam) can significantly reduce OIT, leading to loss of material toughness.
• Long-term heat aging also reduces OIT over time.

Conclusion

DSC-based OIT testing, despite its limitations in perfectly simulating real-world conditions, remains a valuable and practical method for comparing the oxidative stability of polyolefins. It is particularly useful for evaluating the impact of processing and post-processing on antioxidant depletion and for comparing materials with similar stabilizer chemistries.

Thanks for reading & #findoutaboutplastics

Greetings, 

Herwig Juster

🔎I review your polymer material selection and plastic part design - contact me here

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Literature:

[1] https://www.ptonline.com/articles/the-importance-of-oxidative-stability-in-polyolefins-part-2

The Melting Point Mystery: Identifying Polyamide 6.6 & 6 with DSC (thermal analysis)

Hello and welcome to a new blog post. Today we are diving into a powerful technique for polymer identification: Differential Scanning Calorimetry (DSC). Specifically, we will explore how DSC can help you distinguish between two common polyamides, Polyamide 6.6 (PA 6.6) and Polyamide 6 (PA 6).

What is DSC?

Differential Scanning Calorimetry (DSC) is a thermal analysis technique used to observe thermal transitions in polymers. This includes identifying key characteristics such as:

• Glass Transition Temperature (Tg): The temperature at which an amorphous polymer transitions from a rigid, glassy state to a more flexible, rubbery state.
• Melting Points (Tm): The temperature at which crystalline regions of a semi-crystalline polymer melt.
• Crystallization and Crystallization Rate: For semi-crystalline polymers, DSC can also reveal information about how they crystallize upon cooling.

The Polyamide Puzzle: PA 6.6 vs. PA 6

Imagine you have a failed injection molded part made of polyamide, but you are unsure if it is PA 6.6 or PA 6. As part of your failure analysis you need to identify the polyamide type. This is where DSC becomes incredibly handy. You can take a small piece of the part, perform a DSC analysis, and the results will provide a clear answer.

Looking at a typical DSC diagram, which plots heat flow over temperature, we can observe distinct differences between PA 6.6 and PA 6 (Figure 1):

Polyamide 6.6 (PA 6.6): This polymer typically exhibits a melting point around 268°C (approximately 270 °C). On the DSC curve, this appears as a clear, sharp peak in the higher temperature range.

Polyamide 6 (PA 6): In contrast, Polyamide 6 has a lower melting point, typically around 220 °C. Its peak will appear distinctly at this lower temperature on the DSC curve.

While both PA 6.6 and PA 6 have similar glass transition temperatures (around 50°C to 70 °C), making them less ideal for differentiation, their melting points provide a robust and clear identification.

Figure 1: Material identification using DSC - Example PA 6.6 vs PA6.

Conclusion

Using DSC to identify PA 6.6 and PA 6 by their distinct melting points is a very practical and effective method. While it requires a laboratory equipped with a DSC instrument, the clarity it provides in material identification can be invaluable for quality control, material verification, and troubleshooting in the plastics industry.

Thanks for reading & #findoutaboutplastics

Greetings, 

Herwig Juster

🔎I review your polymer material selection and plastic part design - contact me here

🔥 There is a problem keeping you awake - My personal website for Polymer Consulting

📊 Discover your Polymer Material Selection score by taking quick test here

Literature:

[1] Schadensanalyse an Kunststoff-Formteilen, VDI Verlag

Polymers and the Lindy Effect (Rule of Thumb Plastic Selection and Part Design)

Hello and welcome to a new Rule of Thumb post with focus on the Lindy Effect.

What is the Lindy Effect?

The Lindy Effect, when applied to polymers, suggests that the longer a polymer material or its application has been in use, the longer it is likely to continue to be used in the future. This effect highlights the idea that longevity in the face of time and competition implies robustness and adaptability, making older technologies or materials potentially more durable and relevant than newer, less tested alternatives. 

Examples - Polymers and the Lindy Effect

1. Established Polymer Applications: Consider the long-standing use of polyethylene (PE) in packaging or polyvinyl chloride (PVC) in construction as flame retardant flooring material as well as window frame polymer. The Lindy Effect would imply that these established uses are likely to continue for many years to come, as they have already demonstrated their longevity and reliability (Figure 1). 

Figure 1: Polymers and the Lindy Effect - Examples PVC and PE.

2. Mature Polymer Materials: Similarly, materials like natural rubber or certain types of thermosetting plastics, which have been used for a considerable time, are likely to remain relevant due to their proven track record. 

In essence, the Lindy Effect in the context of polymers suggests that the longer a polymer or its application has been around, the more likely it is to persist into the future. This is because its longevity indicates a degree of robustness and adaptability that makes it a reliable choice in the face of new innovations and changing needs. This brings me to one of my top 10 rules for plastic part design and selection (Rule Nr. 10): Past performance can be guarantee of future results. It can pay of to revisit past application designs to understand what worked in the past (materials selection and design).

Check out more Rules of Thumb in the "Start Here" section.

Thanks for reading & #findoutaboutplastics

Greetings, 

Herwig Juster

🔎I review your polymer material selection and plastic part design - contact me here

🔥 There is a problem keeping you awake - My personal website for Polymer Consulting

📊 Discover your Polymer Material Selection score by taking quick test here

Literature:

[1]  Nassim Nicholas Taleb (2012). Antifragile: Things That Gain from Disorder

High Performance Thermoplastic Selection - Polyether (PPE, PAEK, PEEK, PEKK) [Part 2C]

Hello and welcome to the Part 2C of our High Performance Thermoplastics selection blog series. Today we discuss Polyphenylene Ether (PPE) and PPE blends, their chemistry and production processes, their main properties, processing methods, and applications.

Overview - 6 major high performance thermoplastics families (“the magnificent six”) 

In this blog post series we discuss six major high performance thermoplastics families (“the magnificent six”) which are outlined in the following enumeration

1. Introduction to High Performance Polymers

2. Short profile of the "magnificent six" families:

-Part 2A: Polysulfides (Polyphenylene sulfide - PPS), Polysulfones (PSU, PESU, PPSU), and Polyarylates (PAR)

-Part 2B: Imide-Based Polymers (PEI, PAI, PESI, TPI, PI) and Polybenzimidazoles (PBI, PBI+PEEK, PBI+PEKK)

-Part 2C: Polyether (PPE, PAEK, PEEK, PEKK)

-Part 2D: Liquid Crystal Polymers (LCP) and High-performance Polyesters (Polycyclohexylene terephthalate - PCT)

-Part 2E: Semi- and Fully Aromatic Polyamides (PARA, PPA, Aramid)

-Part 2F: Polyhalogenolefins (PTFE, PCTFE, FEP, PVDF, ECTFE)

Polyphenylene Ether (PPE) and its Blends with Polystyrene (PS) and Polyamide (PA)

Polyphenylene ether (PPE), also known as polyphenylene oxide (PPO), is a high-performance thermoplastic polymer renowned for its excellent thermal, mechanical, and electrical properties. While pure PPE exhibits some processing challenges due to its high melt viscosity, blending it with polystyrene (PS) significantly improves its processability while retaining many of its desirable characteristics. These PPE/PS blends have become commercially significant engineering thermoplastics.   

Jack Welch's team at General Electric faced a challenge with Polyphenylene Oxide (PPO): its extremely high glass transition temperature (208°C) made it difficult to process without degrading the material (the methyl groups of PPE are expected to undergo autoxidation). An important key to future success was the research team around Dan Fox, Allan S. Hay, and E. M. Boldebuck, who found out the miscibility of PPO with PS first. To overcome this, they decided to blend PPO with polystyrene (PS). This clever solution allowed them to maintain many of PPO's desirable properties, lower the glass transition temperature,  while also making the material easier to process at lower temperatures and more cost-effective.

Chemistry and Production Process

1. Chemistry of Polyphenylene Ether (PPE)

The base polymer, PPE, is typically synthesized through an oxidative coupling polymerization of substituted phenols, most commonly 2,6-dimethylphenol (also known as 2,6-xylenol). This reaction is catalyzed by a copper-amine complex in the presence of oxygen. The general chemical structure of PPE can be represented as (Figure 1):

Figure 1: Chemical structure of the polymer Polyphenylene Ether (PPE) [1].

The ether linkages (-O-) in the polymer backbone, along with the aromatic rings and the methyl substituents, contribute to PPE's stiffness, thermal stability, and chemical resistance.

2. Production of PPE/PS Blends

The production of PPE/PS blends primarily involves melt blending the two polymers. This process is typically carried out in extruders or other intensive mixing equipment. The key steps include:

• Raw Material Preparation: PPE and PS resins are typically received in pellet form. They may be dried to remove any moisture before blending.
• Melt Blending: The PPE and PS pellets are fed into an extruder, where they are heated and mechanically mixed. The screw design and processing conditions (temperature, screw speed) are crucial for achieving a homogeneous blend. Compatibilizers, such as styrene-butadiene-styrene (SBS) or styrene-ethylene/butylene-styrene (SEBS) block copolymers, are often added to improve the compatibility between the relatively non-polar PS and the more polar PPE. These compatibilizers help to reduce interfacial tension and prevent phase separation, leading to enhanced mechanical properties.
• Pelletizing: The molten blend exiting the extruder is then cooled and cut into pellets, which are the final product form for subsequent processing by manufacturers.

The ratio of PPE to PS in the blend can be varied to tailor the properties of the final material to specific application requirements. Higher PPE content generally leads to better thermal and mechanical performance, while higher PS content improves processability and reduces cost (Figure 2).   

Figure 2: Changing the glass transition temperature of PPE by changing the ratio of PPE and PS [1].

Main Properties of PPE/PS Blends

PPE/PS blends exhibit a combination of properties derived from both constituent polymers, often enhanced by the presence of compatibilizers. 

Key properties include:

• Excellent Thermal Stability: PPE inherently possesses high glass transition temperatures (Tg ) and heat deflection temperatures (HDT). Blending with PS can reduce these values compared to pure PPE, however the blends still offer good high-temperature performance compared to many other engineering thermoplastics.   
• Figure 3 shows the DMA of PPE+PS blend in comparison to High Impact Polystyrene (HIPS) and Polycarbonate (PC). PS has a glass transition temperature of about 100°C and up to this temperature PPE+PS can match PC in thermal performance (PC drops sharply at Tg of 147°C). There is no sharp drop in modulus observed with PPE+PS. 

Figure 3: Dynamic Mechanical Analysis (DMA) of PPE+PS vs HIPS vs PC. 

• Good Mechanical Strength and Stiffness: PPE contributes to the blend's rigidity and strength. The impact strength can be tailored depending on the blend ratio and the use of impact modifiers.  
• Excellent Electrical Insulation Properties: PPE is an excellent electrical insulator with a low dielectric constant and high dielectric strength, which are largely retained in the blends.   
• Good Chemical Resistance: PPE offers good resistance to many chemicals, including acids, bases, and detergents. The chemical resistance of the blend is generally good but can be influenced by the PS content, which is more susceptible to certain solvents.   
• High Stability towards Hydrolysis: hydrolysis resistance of PPE is superior when compared to other engineering plastics such as PBT and PA.
• Improved Processability: The addition of PS significantly lowers the melt viscosity of PPE, making the blends easier to process using conventional methods like injection molding and extrusion.   
• Dimensional Stability: PPE contributes to low water absorption and excellent dimensional stability, which is important for applications requiring tight tolerances.   
• Flame Retardancy: PPE is inherently flame retardant. Blends often exhibit good flame retardant properties, especially when combined with flame retardant additives.   

Apart from blending PPE with PS; blends out of PPE with PA are possible too. PPE/PA blends have the following advantages: 

• Combines Key Strengths: Blends the excellent dimensional stability, low water absorption, and heat resistance of PPE with the superior chemical resistance and flow of PA.
• Enhanced Performance: The resulting material is exceptionally chemically resistant and boasts the stiffness, impact resistance, and heat performance needed for demanding applications like on-line painting.
• Significant Weight Savings: Unfilled PPE/PA blends can offer part-weight reductions of up to 25% compared to glass or mineral-filled resins, thanks to their low density.

Outperforming properties of PPE/PS blends:  

• Low specific gravity: 1.1 g/cm3. Most engineering polymers such as PC, PBT, and POM have a specific density of 1.2 g/cm3 and more. 
• High self-extinguishing property: PPE has a high oxygen index (27-29) and it is easy to add flame resistancy. 
• Excellent dielectric properties: PPE has a dielectric constant of 2.8 and a dielectric tangent of 6x1E-3. The dielectric breakdown strength (110 MV/m @ 0.5 mm thickness) is the highest among engineering plastics. 
• High dimensional accuracy: PPE has one of the lowest coefficient of linear thermal expansion among engineering polymers (5.8 x 10E-5 1/°C).

Processing Methods

PPE/PS blends can be processed using various thermoplastic processing techniques, including:

• Injection Moulding: This is the most common method for producing complex shaped parts from PPE/PS blends, leveraging their improved flow properties compared to pure PPE.
• Extrusion: These blends can be extruded into profiles, sheets, and films for various applications.  
• Blow Moulding: Certain grades of PPE/PS blends can be blow molded to produce hollow parts.
• Thermoforming: Sheets extruded from PPE/PS blends can be thermoformed into various shapes. 

Applications of PPE/PS Blends

The unique combination of properties makes PPE/PS blends suitable for a wide range of applications across various industries:

• Construction and Plumbing: Applications include pump housings, impellers, and other components requiring good mechanical and chemical resistance.
• Automotive Industry: Interior and exterior components such as instrument panels, door panels, wheel covers, and electrical connectors benefit from the blends' thermal stability, dimensional stability, and impact resistance.   
• Electrical and Electronics: Housings for electrical connectors, circuit breakers, switchgear, and other electrical components utilize the excellent electrical insulation properties and flame retardancy of PPE/PS blends.   
• Household Appliances: Components for washing machines, dishwashers, microwave ovens, and other appliances benefit from the blends' heat resistance, chemical resistance, and good mechanical properties.
• Business Equipment: Housings and internal components for computers, printers, and other office equipment utilize the blends' dimensional stability, electrical properties, and aesthetic appeal.   
• Healthcare: Certain grades of PPE/PS blends can be used in medical devices and equipment due to their sterilizability and chemical resistance.   

Trade Names

Several companies market PPE/PS blends under various trade names. Some well-known examples include:

• Noryl™: Formerly a trademark of GE Plastics, now owned by SABIC Innovative Plastics. This is one of the most widely recognized families of PPE-based resins, including various PPE/PS blends and other modifications.
• XYRON™ from Japanese chemical company Asahi Kasei.
• Various manufacturers such as Global Acetal (Lemalloy PPE) produce generic PPE/PS blends with specific property profiles.   

Economic Aspects

The market for PPE/PS blends is significant within the broader engineering thermoplastics market. The demand is driven by the increasing need for high-performance materials in automotive, electronics, plumbing and appliance industries.   

• Cost Factors: The cost of PPE/PS blends is influenced by the price of the base polymers (PPE and PS), the concentration of PPE in the blend, the type and amount of compatibilizers and additives used, and the manufacturing process. Generally, higher PPE content leads to higher costs.
• Market Trends: The trend towards lightweighting in the automotive industry and miniaturization in electronics continues to drive the demand for materials like PPE/PS blends that offer a balance of performance and processability. Growing environmental concerns are also pushing for more sustainable material solutions, which may influence the development of new PPE-based blends with recycled content or improved recyclability.
• Regional Variations: The demand and market dynamics for PPE/PS blends can vary across different geographical regions based on industrial activity and specific application needs.

In conclusion, polyphenylene ether (PPE) and its blends with polystyrene and polyamide represent a versatile class of engineering thermoplastics offering a compelling combination of high-performance properties and improved processability. Their wide range of applications across various industries underscores their economic and technological significance in the world of polymer engineering. As a polymer engineering student, understanding the nuances of these materials will undoubtedly be valuable in your future endeavors.   

In the upcoming Part 2C we will continue to discuss the Polyether high performance polymers such as PAEK, PEEK, and PEKK.

Thanks for reading & #findoutaboutplastics

Greetings, 

Herwig Juster

🔎I review your polymer material selection and plastic part design - contact me here

🔥 There is a problem keeping you awake - My personal website for Polymer Consulting

📊 Discover your Polymer Material Selection score by taking quick test here

Literature: 

[1] Allan S. Hay: Polymerization by Oxidative Coupling: Discovery and Commercialization of PPO and Noryl Resins

[2] https://www.sabic.com/en/products/specialties/noryl-resins

[3] https://www.gpac.co.jp/en/product/lemalloy/

[4] https://www.findoutaboutplastics.com/2016/09/jack-welch-and-his-uprise-in-ge.html

[5] https://www.findoutaboutplastics.com/2024/08/high-performance-thermoplastic.html