Cracked bottle caps, broken hangers, prematurely aged cables... Sudden "death" of plastic products often lies in invisible world of molecules. Life and death of a product is determined as early as selection of raw materials! Today, we'll dissect "genetic code" of plastics and explore how three core variables determine their fate.

 

Lifespan of plastics lies in three core variables at molecular level:
1. Composition
Monomer Identity: Is it flexible PE, rigid PS, or easily hydrolyzed nylon?
Chemical Bond Type:bCarbon Chain (PE/PP/PS): Stable, susceptible to oxidation

 

▲ Relative size of side groups in olefin polymers and their impact on thermomechanical stability
Amide/ester bonds (nylon/polyester): Disintegrate when exposed to water at high temperatures, requiring rigorous drying!

 

▲ Polyester Functional Groups and Hydrolyzable Sites
Case Study:
Nylon/polyester materials exposed to 0.01% moisture and high-temperature processing → Molecular chain breakage (hydrolysis) → Plummeting strength → Product failure!
2. Molecular Weight (MW)
Polymers are never limited to a single chain length. A wide range of chain lengths can be found in a single batch—some with hundreds of repeating units, others with tens of thousands. It's like running a polymer marathon: some runners have short legs (low molecular weight), some are tall and have long legs (high molecular weight), and majority are concentrated around 1.7 meters (medium molecular weight). We use molecular weight (MV) to describe this "chain length group."

 

Higher MW = longer chains = stronger: tangled, tangled chains absorb stress and resist tearing.
Lower MW = shorter chains = more brittle: fractures under even the slightest stress.
Red Line of Life and Death: Degradation during processing (high temperature/hydrolysis) causes a drop in MW → a precipitous drop in performance.
3. Molecular Order

 

In polymer materials, like this piece of instant noodles, tightly packed, directional areas are crystalline regions; chaotic, disorganized areas are amorphous regions.
Crystalline Region: High hardness and heat resistance, but also brittle (such as HDPE bottles).
Amorphous Region: High transparency and flexibility (such as PS cups).
Critical Temperature: Glass Transition Temperature (Tg)
Below Tg → Hard and brittle like glass (easily cracked)
Above Tg → Softens and deforms
�� Life and Death Lesson: Three genes work in tandem! Wrong ingredient selection + MW degradation during processing + improper crystallization = inevitable death.

Even when main ingredients are chosen correctly, additives can be lethal:

 

Substances not listed in the formula are often ultimate killer:
Water (H₂O): "Bone-dissolving water" of nylon/polyester, hydrolyzed and broken during high-temperature processing.
Particle contamination: Impurities in high-voltage cables → Cause "water treeing" → Breakdown within 10-15 years
Masterbatch:
Crime scene: Black nylon component performance plummets
Autopsy report (DSC/TGA): PE carrier in masterbatch exceeds standard by 30% + missing glass fiber → Complete failure!

 

Metal ions/residual solvents: Catalyze degradation, reducing insulation.
Residues from previous batches: Contaminate new product formulations.
Identification tools:
DSC (Differential Scanning Calorimetry): Measures crystallinity/Tg/melting point
TGA (Thermogravimetric Analysis): Measures decomposition temperature/additive content

Raw Material Selection Tips
Verification: Confirm that monomer type, MW range, and crystallization characteristics match application scenario (temperature/stress/media).
Additive incompatibilities:
Migrating dyes are prohibited in crystalline plastics (PP/HDPE)!
Nylon/polyester must use hydrolysis-resistant pigments/carriers.
Halogen-containing flame retardants should avoid contact with metal/high-temperature equipment.
Impurity Elimination: Electrical performance/long-lasting products require ultra-clean materials.
Production Safety Clauses:
Dry! Dry! Dry! Nylon/polyester moisture content ≤ 0.02%!
Precise Temperature Control: Upper limit prevents decomposition, lower limit prevents high stress.
Forced Dispersion: High-shear mixing prevents additive agglomeration.
Mold Corrosion Resistance: Use corrosion-resistant steel or surface treatment for halogen-containing materials.
Strictly Prevent Cross-Contamination: Throughly clean machine after material changes!
For further reading, please refer to Plastic Life and Death Book, Part 2: Layout Decides! How Structural Traps Can Destroy a Product'.