Views: 2 Author: Site Editor Publish Time: 2026-05-07 Origin: Site
When you notice your extruded products turning yellow, smell pungent odors, find "gels" and black spots on transparent films, or experience a sudden drop in the tensile strength of your pipes, do not just rush to adjust the temperature panel. By the time these symptoms appear, the "invisible killer" inside your extruder—polymer degradation—has already done its damage.
An extruder is not just a melting furnace; it is a complex thermo-mechanical reactor. To achieve high-quality pelletizing and extrusion, operators must understand the two core culprits of material breakdown: Thermal Degradation and Mechanical Shear Degradation, along with the hidden accomplices lurking in the machine's "dead zones."
1. Thermal Degradation: When Heat Tears the DNA of Polymers
Thermal degradation occurs when the polymer's carbon-carbon backbone breaks down under excessive heat, even in the absence of oxygen and moisture. Different plastics react to extreme temperatures in highly distinct ways:
Random Chain Scission (e.g., Polypropylene - PP): High temperatures cause the long molecular chains to snap at random locations.
Symptom: The molecular weight of PP drops sharply, melt viscosity plummets (becoming watery), and the final product loses its impact strength entirely.
Depolymerization or "Unzipping" (e.g., Polystyrene - PS, PMMA): Vinyl polymers degrade in a unique way. The molecular chain begins to detach from the ends, shedding original monomers one by one, much like unzipping a jacket.
The Physics of Ceiling Temperature: In thermodynamics, there is a balance point between a polymer and its monomer known as the "Ceiling Temperature." For PS, this is theoretically 310°C, but in a real extruder, slow depolymerization begins above 200°C. This releases a pungent styrene gas at the die head and causes a brownish discoloration.
Side-Group Elimination (e.g., PVC, PVDC): When overheated, the chlorine atoms on the polymer's side chains combine with adjacent hydrogen atoms to break away, generating highly corrosive Hydrogen Chloride (HCl) gas.
Symptom: The stripped polymer backbone cross-links into extremely hard, black carbonized particles. Not only does this cause fatal black spots in the product, but the HCl gas will severely corrode your screws and barrels.
2. Mechanical Shear Degradation: Chains Torn by Violence
In an extruder, pure thermal degradation is rare. The more fatal issue is mechanical degradation—the forced rupture of molecular chains caused by extreme mechanical shear and tensile stress.
When the main motor drives the screw through highly viscous polymers, the chains are severely stretched. According to polymer physics (such as Bueche’s theory), linear polymer chains align with the flow direction in strong shear fields. Because of heavy physical "entanglements" between chains, the maximum tension concentrates exactly in the middle of the chain. Therefore, mechanical degradation is not random; the chains often snap cleanly in half.
High-Risk Zones for Chain Snapping: High-molecular-weight resins are the most sensitive to mechanical stress. The highest risks occur in:
Flight Clearances: The gap between the screw flight and the barrel wall is often less than 0.15 mm. The small amount of plastic dragged into this gap experiences terrifying shear rates, causing instantaneous local temperature spikes and severe chain scission.
Barrier Screws and Kneading Blocks: Designed to force melt through narrow gaps for better mixing, these elements are hotspots for extreme localized stress.
3. The Hidden Culprit: Oxygen and "Gel" Formation
As plastic pellets enter the hopper, a significant amount of air (oxygen) enters the barrel with them. Under the dual action of heat and mechanical shear, broken polymer chains form highly reactive "free radicals." Oxygen attacks these radicals, triggering thermo-oxidative degradation.
This explains why Polyethylene (PE) and Polypropylene (PP) behave completely differently:
PP Scission: Oxygen radicals attack the tertiary hydrogen atoms in PP, causing chain scission and a rapid drop in molecular weight.
PE Cross-linking (The Birth of Gels): Conversely, when oxygen attacks PE, the loose chains bind together to form complex 3D networks. These cross-linked PE clumps can no longer be melted. When extruded into films, they appear as stubborn, hard flaws known in the industry as "gels" (晶点), ruining optical and mechanical properties.
The Only Cure: Resin manufacturers add antioxidants to absorb these free radicals. However, antioxidants are consumable. If you use a high percentage of recycled materials (multiple heat histories), the antioxidants will be depleted, leading to an avalanche of gels and black spots.
4. The Fatal "Dead Zones": Time is the Catalyst
Understanding degradation kinetics boils down to a simple formula: Degradation = Temperature × Residence Time.
In a normal extrusion process, plastic travels from hopper to die in just a few minutes. Antioxidants can easily protect the material for this duration. However, if there are "stagnation areas" (dead zones) inside the extruder, the residence time turns from minutes to hours or even days.
Common Extruder Dead Zones:
Unfilled Screw Channels: If solid feeding is unstable, the metering section may not fill completely, leaving melt stagnant near the pushing flight.
Sharp Flight Radii: Poorly machined screw roots with sharp corners trap plastic, preventing flow.
Adapters and Connectors: Non-streamlined pipes or 90-degree elbows between the extruder and the die head create stagnant pools.
Improper Shutdowns: Leaving the barrel fully heated for over 30 minutes while the screw is stopped guarantees disastrous thermal degradation.
Material trapped in these dead zones burns into carbonized black specks. Later, a minor fluctuation in screw speed or pressure will wash these deposits out, resulting in an endless stream of black spots in your product.
Conclusion: Engineering the Perfect Flow
Thermal degradation warns us never to push limits; mechanical degradation reminds us that tighter clearances are not always better; and thermo-oxidative degradation highlights the dangers of dead zones and depleted antioxidants.
At Nanjing Haisi Extrusion, our twin-screw extruders are engineered to eliminate these risks. Through precision-machined screw profiles with optimized flight radii, streamlined die heads, and superior self-wiping capabilities, we ensure a strict "First-In, First-Out" (FIFO) melt flow. By eliminating dead zones and offering customized shear configurations, we protect your polymer's molecular integrity, ensuring maximum yield and flawless pellet quality.
