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

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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