The Sticky Truth About Gluing Plastics: Why Surface Energy Matters (Plastics Processing Rule of Thumb)

Hello and welcome to todays post focusing on gluing plastics.  Ever tried to glue two pieces of plastic together, only for the bond to fail miserably? You're not alone! Gluing plastics can be surprisingly tricky, and a key factor often overlooked is something called surface energy. Think of surface energy as how "eager" a material's surface is to bond with something else. And when it comes to adhesives, eagerness is a good thing!

The Rule of Thumb: High Surface Energy = Stronger Bonds

Here's the golden rule for gluing: the higher the surface energy of a material, the greater the strength of the adhesion you can achieve.

Imagine a tiny water droplet on a surface. On a highly energetic surface, the water spreads out, trying to maximize its contact. On a low energy surface, it beads up, shrinking away. Adhesives behave similarly. They want to spread out and "wet" the surface completely to form a strong bond.

Diving Deeper: High vs. Low Surface Energy Plastics

Plastics are generally categorized into two main groups when it comes to surface energy:

• High Surface Energy (HSE) Plastics: These plastics are generally easier to bond. Their surfaces are more receptive to adhesives, allowing for better "wetting" and stronger molecular interactions.
• Low Surface Energy (LSE) Plastics: These are the notorious "difficult-to-glue" plastics. Their surfaces are less receptive, causing many common adhesives to bead up and struggle to form a lasting bond.

A Quick Look at Surface Energy Values (mJ/m²)

To give you a better idea, below is a small table (Table 1) with approximate surface energy values for some common plastics and other materials. Remember, these values are guides, and specific formulations can impact them.

Table 1: Overview of surface energy values of plastics [1].

As you can see, materials like PTFE (often known as Teflon) have very low surface energy, making them incredibly challenging to bond without specialized adhesives or surface treatments. Another example is the high performance polymer PPS. The surface energy of untreated polyphenylene sulfide (PPS) is typically around 38 mJ/m2. This relatively low surface energy makes PPS challenging to bond to other materials without surface treatment. However, PPS can be treated to increase its surface energy, such as through plasma treatments, which can raise the surface energy to 38 mJ/m2 or higher, depending on the treatment method. In contrast, materials like Polyimide are much more accommodating.

What Does This Mean for Your Next Plastics Gluing Project?

When you're facing a plastic gluing challenge, keep this rule of thumb in mind:

1. Material selection: list gluing as a post-processing operation during part requirement analysis.
2. Identify the Plastic: If you know the type of plastic, you can often infer its surface energy.
3. Opt for HSE Plastics When Possible: If you have a choice of materials, pick higher surface energy plastics for easier and more reliable bonds.
4. Specialized Adhesives for LSE Plastics: For low surface energy plastics, don't reach for your all-purpose super glue. You'll likely need specialized adhesives designed for LSE materials, or consider surface preparation techniques (like primers or plasma treatment) to temporarily increase the surface energy.

Conclusiones

Understanding surface energy is a game-changer for anyone working with adhesives and plastics. By keeping this simple principle in mind, you'll significantly improve your chances of achieving strong, long-lasting bonds and avoid those frustrating gluing failures. Happy gluing!

More Rule of Thumbs can be found in the section "Start here".

Thanks for reading and #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]https://www.3m.com/3M/en_US/bonding-and-assembly-us/applications/material-bonding/engineered-plastics/

[2] https://www.findoutaboutplastics.com/2025/04/nature-is-built-on-5-polymers-modern.html

Design Data for Engineers: Thermal Aging of PPS Compounds

Hello and welcome to today’s blog post on thermal aging of PPS compounds. During material selection, focus on gathering and understanding all the requirements of the application is essential. Key questions to answer are: What is the service environment of your part? And also: What types of load at which service temperature and time need to be fulfilled?

Thermal aging of Polyphenylene sulfide (PPS) compounds (glass; glass/mineral filled)

High performance polymers such as Polyphenylene sulfide (PPS) offer excellent performance during high heat exposure. PPS mouldings, both filled and unfilled, maintain inherent flame resistance and excellent chemical resistance due to the base resin.

Long-term heat aging results align with the polymer's thermal stability. PPS has a UL 746B continuous use temperature (CUT) of 220°C. It is in a similar range with PPSU which has a CUT of 210°C and PTFE is even above 230°C. 

Figure 1 highlights the good retention of tensile properties in glass and glass/mineral-filled compounds over long-term exposure. An aging test was performed at two temperatures (175°C and 230°C) for a maximum duration of 10,000 hours [2]. 

Figure 1: Thermal aging of PPS compounds (175°C and 230°C; 10,000 hours).

Due to the curing characteristics of cross-linked PPS, aging at temperatures above those in Figure 1 can enhance property retention, attributed to a "case hardening" effect from high-temperature air exposure.

At elevated temperatures, PPS compounds show classical deterioration beyond their glass transition temperature (Tg). Despite crystallinity effects, strength loss is gradual, with significant integrity retained even at 200°C. PPS compounds filled with glass and mineral can retain the tensile properties at both temperatures at a higher level (80% retention rate) compared to glass fiber reinforced PPS compounds (60% retention rate).

Generally, 40% glass-filled mouldings retain about 80% of their original strength at 100°C, 60% at 160°C, and 40% at 200°C.

Conclusion

In conclusion, Polyphenylene sulfide (PPS) compounds demonstrate exceptional performance under high heat exposure, making them a reliable choice for applications requiring thermal stability. Both filled and unfilled PPS moldings exhibit inherent flame resistance and chemical resistance, aligning with the polymer's robust thermal properties. The long-term heat aging results confirm PPS's ability to retain tensile properties, even at elevated temperatures. With a continuous use temperature of 220°C, PPS stands out among high-performance polymers, maintaining significant strength and integrity over time. This makes PPS an excellent candidate for applications demanding durability and reliability in challenging thermal environments. When selecting materials, it is crucial to thoroughly understand the application's requirements, including service environment, load types, and service temperatures, to ensure optimal performance and longevity.

Thanks for reading and #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] https://www.findoutaboutplastics.com/2024/09/high-performance-thermoplastic.html

[2] Don Brady: Polyphenylene sulfide (PPS), Phillips Petroleum Company

Glass-Fiber Filled PET vs. PBT: Choosing the Champion for Your Application

Hello and welcome to a new blog post. In this post we are comparing glass-fiber filled PET vs. PBT, especially heat distortion properties (HDT). We dive into why PET's higher stiffness, heat resistance, and lower water uptake can be a game-changer for E&E and metal replacement applications. It can help you to elevate your material selection process - let us get started!

PET compounds vs PBT compounds

When it comes to high-performance engineering plastics, glass-fiber reinforced compounds of PET (Polyethylene Terephthalate) and PBT (Polybutylene Terephthalate) are often top contenders. While both offer enhanced strength and rigidity, their distinct properties can make one a clear winner over the other for specific demands. 

One of PET's significant advantages lies in its higher stiffness, which directly translates to reduced creep under sustained loads. This superior mechanical integrity ensures long-term performance where dimensional stability is critical. Furthermore, PET compounds boast a notably higher heat distortion temperature (HDT). Specifically, when measured with HDT/B, PET can exhibit a delta of up to 20°C higher than PBT, making it more resilient in high-heat environments (Figure 1).

Figure 1: Comparison of HDT/A and HDT/B of PET-GF30 and PBT-GF30

Another crucial differentiator is PET's lower water uptake. This property directly contributes to better dimensional stability, as the material is less prone to swelling or warping in humid conditions. In the realm of electrical and electronic (E&E) applications, glass-fiber filled PET truly shines due to its good dielectric properties and the potential for achieving high UL RTI (Relative Thermal Index) values. These characteristics ensure reliable insulation and performance at elevated temperatures.

These combined attributes—superior stiffness, higher heat resistance, better dimensional stability, and excellent electrical properties—make glass-fiber filled PET compounds particularly compelling for demanding applications in the E&E industry. They are also an excellent choice for metal replacement, offering a lighter, often more cost-effective, and corrosion-resistant alternative without compromising on critical performance metrics. When designing your next component, understanding these nuances can lead to superior product design and longevity.

Thanks for reading and #findoutaboutplastics

Greetings

Herwig Juster

🔥 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] https://www.findoutaboutplastics.com/2025/05/4-tips-for-effective-polymer-material.html

[2] https://www.celanese.com/products/rynite-pet

[3] https://www.findoutaboutplastics.com/2025/05/pet-grade-selection-for-diverse.html

[4] https://www.polymermaterialselection.com/selection-examples-blog

When Childhood Crumbles: Understanding Plastic Part Failure in LEGO® Bricks

Hello and welcome to a new post. For many of us, LEGO® bricks are synonymous with durability and endless creative possibilities. They withstand countless builds, deconstructions, and even the occasional barefoot step. So, when a childhood treasure, carefully preserved for decades, suddenly crumbles in your hands, it's a stark reminder that even the most robust plastics have a lifespan. This is precisely the experience my friend Dirk Moses recently encountered with his 1986 LEGO® "Mobile Recovery Vehicle" set, and it offers a fascinating case study into the world of plastic part failure, particularly in ABS.

Dirk's story is a familiar one: a nostalgic journey with his son, building a cherished set from his own youth, only to be met with the disheartening sound of "crack!" as original, seemingly well-preserved parts disintegrated. No sunlight, no moisture exposure – just the relentless march of time. This isn't just a frustrating experience; it's a prime example of how polymers, even when seemingly inert, undergo subtle changes that can lead to catastrophic failure over extended periods.

Figure 1: Broken ABS bricks of the LEGO® "Mobile Recovery Vehicle" set from 1987 [1].

The Material Culprit: ABS and Its Vulnerabilities

The iconic LEGO® bricks are predominantly made from Acrylonitrile Butadiene Styrene, or ABS. ABS is a terpolymer, meaning it's made from three different monomers (check my detailed review on ABS here):

• Acrylonitrile (A): Provides chemical resistance, rigidity, and hardness.
• Butadiene (B): A rubbery component that contributes toughness and impact strength.
• Styrene (S): Offers rigidity, processing ease, and gloss.

This combination gives ABS its desirable balance of properties – stiffness, impact resistance, and a good surface finish – making it an ideal choice for durable toys like LEGO®. However, it's the "Butadiene" component that often holds the key to its long-term degradation.

The Invisible Enemy: Thermal Oxidation of Butadiene

While Dirk meticulously stored his LEGO® set away from sunlight and moisture, he couldn't escape the effects of thermal oxidation. This is a common degradation mechanism in polymers, where oxygen from the air reacts with the polymer chains, often accelerated by even ambient temperatures over extended periods.

The butadiene component in ABS, with its unsaturated double bonds, is particularly susceptible to oxidation. Here's a simplified breakdown of what happens:

1. Initiation: Heat and oxygen initiate the formation of free radicals on the butadiene backbone. These free radicals are highly reactive.
2. Propagation: These free radicals react with more oxygen and then abstract hydrogen atoms from adjacent polymer chains, creating new free radicals. This forms a chain reaction, leading to the formation of hydroperoxides and other oxidized species.
3. Chain Scission and Cross-linking: The hydroperoxides are unstable and can decompose, leading to further free radical formation. This ongoing process results in two primary detrimental effects:

• Chain Scission: The polymer chains break down into smaller fragments. This leads to a reduction in molecular weight and a loss of mechanical strength, making the material brittle and prone to cracking.
• Cross-linking: In some cases, the free radicals can also react with each other to form new covalent bonds between polymer chains, leading to excessive cross-linking. While some cross-linking can increase stiffness, excessive cross-linking makes the material extremely rigid and brittle, also reducing its ability to absorb impact.

In the case of Dirk's 40-year-old LEGO® bricks, it's highly probable that the butadiene in the ABS underwent significant oxidation. This would have caused the once tough and resilient material to become brittle, explaining why parts that felt solid suddenly "cracked" under minimal stress. The internal structure of the plastic had been fundamentally altered at a molecular level, even without external stressors like UV light or repeated use.

The Takeaway for Polymer Engineers and Consumers Alike

Dirk's experience is a valuable lesson for polymer engineers and consumers alike. For engineers, it underscores the critical importance of understanding long-term material degradation mechanisms, even in seemingly stable polymers. Accelerated aging tests are crucial, but real-world shelf-life data, like Dirk's, provides invaluable insights. Developing new additives that can better inhibit thermal oxidation in ABS and other polymers is an ongoing area of research.

For consumers, it's a gentle reminder that even our most cherished plastic possessions have a finite lifespan. While storing items in their original packaging and away from harsh conditions can certainly extend their life, it cannot stop the slow, inexorable march of molecular degradation. During material selection, understanding all the requirements and transforming them into proper part specifications is key to prevent part failure at a later stage.

Dirk's spontaneous shift to a new LEGO® Dreamzzz set for his son, and the "priceless" joy it brought, highlights a beautiful truth: while the physical bricks may crumble, the spirit of play and connection that LEGO® fosters endures, bridging generations even when the plastic sometimes falls apart. And perhaps, for Dirk, his new mission to "quality control" his "Wrinkle Knights" castle will offer further valuable data in the ongoing saga of plastic longevity!

Many thanks to Dirk for sharing with us his experiences! 

Check out my plastics pioneer series - LEGO®-founder Ole Kirk Christiansen & family here

Thanks for reading and #findoutaboutplastics

Greetings

Herwig Juster

🔥 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] https://www.linkedin.com/posts/dirk-moses-215402191_lego-kindheitserinnerungen-dreamzzz-activity-7332042307308851200-JQfm?utm_source=share&utm_medium=member_desktop&rcm=ACoAABCkmMcBev71cuhh4-jzEaiPHFO5VFb4aO0

[2] https://www.findoutaboutplastics.com/2018/11/reviewing-key-engineering-plastics.html

[3] https://www.polymermaterialselection.com/selection-examples-blog/blog