Molecular level aging mechanism: damage begins with chemical bond breakage
The core raw materials of spunbond nonwoven fabrics are mainly polypropylene (PP) and polyester (PET), and their aging essence is the destruction of molecular chain structure and the deterioration of functional groups. It mainly occurs through the synergy of three pathways: photo oxidative aging, thermal oxidative aging, and mechanical degradation, and ultimately manifests as macroscopic failures such as decreased fracture strength, brittleness, and yellowing.
Photooxidative aging: the main outdoor aging trigger (molecular chain “ultraviolet sniping”)
Direct chemical bond breakage: UVA/UVB (especially around 300nm) with wavelengths of 280-400nm can reach energies of 5.8-6.4eV, far exceeding the bond energies of C-C bonds (3.6eV) and C-H bonds (4.3eV) in PP molecules. It can directly break through the molecular chain skeleton, leading to main chain breakage (depolymerization) or side chain detachment. For example, under UV irradiation, the molecular chain of PP breaks from “- CH ₂ – CH (CH3) – CH ₂ -” to short chain free radicals (· CH ₂ – CH (CH3) -), and the relative molecular weight decreases from the order of 10 ⁵ to below 10 ⁴, resulting in a sharp decline in mechanical properties.
Free radical chain reaction outbreak: UV first excites residual impurities (such as catalyst residues, trace amounts of water) or additives in the material to generate initial free radicals (· OH, · O ₂⁻), which attack weak sites on the molecular chain (such as tertiary carbon atoms), triggering a chain reaction:
Initiation stage: RH (molecular chain)+· OH → R · (alkyl radical)+H ₂ O
Proliferation stage: R ・+O ₂ → ROO ・ (peroxyl radical), ROO ・+RH → ROOH (hydroperoxide)+R ・ (new radical)
Termination stage: ROOH is unstable and decomposes into strong oxidative free radicals such as RO and OH under UV/heat, further accelerating chain breakage and forming a vicious cycle of “breakage generation free radical re breakage”.
Generation and yellowing of chromophores: Broken molecular chains are prone to form conjugated double bonds (such as quinone structures and polyene structures), which can absorb visible light at 400-500nm, leading to material yellowing (increase in yellowing index Δ E); At the same time, the generated carbonyl group (- C=O) will absorb 260-340nm UV and become a new “photosensitive center”, continuously triggering photochemical reactions.
Thermal oxidative aging: “invisible damage” in processing and high-temperature environments
Intensifying thermal motion of molecular chains: During the processing (screw extrusion temperature 210-235 ℃) or in outdoor high-temperature environments, the thermal motion energy of molecular chains exceeds the chemical bond binding energy, resulting in C-C bond cleavage and the generation of alkyl radicals; At the same time, high temperature reduces the activation energy of oxidation reaction, making it easier for oxygen molecules to penetrate the interior of fibers and combine with free radicals to form peroxide free radicals, accelerating the generation of hydrogen peroxide.
Accumulated damage during processing: The screw shearing and stretching during spinning can cause mechanical breakage of molecular chains, forming a large number of terminal free radicals. These free radicals react with oxygen at high temperatures, posing a “hidden danger” for subsequent aging – experiments have shown that PP non-woven fabrics without antioxidant treatment can cause a molecular chain breakage rate of 15% -20% and a decrease in fracture strength retention rate of more than 10% during processing alone.
Mechanical degradation and environmental co aging: accelerating molecular chain failure
Spunbond non woven fabric fibers have a fine diameter (usually 1-10 μ m), a large specific surface area, and a much higher contact efficiency with oxygen, ultraviolet radiation, and pollutants than block materials. Mechanical stresses such as stretching and bending during outdoor use can cause local stress concentration in molecular chains, leading to preferential fracture of weak sites (such as near formed carbonyl groups); At the same time, ozone and acidic pollutants in the air will react with unsaturated bonds on the molecular chain, damaging the integrity of the molecular structure and forming a synergistic effect with photo oxygen and thermal oxygen aging, increasing the aging rate by 2-3 times.
Molecular level delay strategy: from “blocking damage” to “strengthening structure”
The core logic of delaying aging is to construct a “protection repair strengthening” system at the molecular level, which precisely intervenes in the key links of aging, blocks molecular chain breakage, inhibits free radical reactions, and enhances structural stability. It is specifically divided into four technical paths:
（1） Blocking Free Radical Chain Reaction: Antioxidant Synergistic System (Molecular “Firefighter”)
The core delay strategy is to use a combination of “main antioxidant+auxiliary antioxidant” to terminate the oxidation chain reaction from the source
Main antioxidant (free radical scavenger): mainly hindered phenols (such as 1010, 1076), the hydroxyl group (- OH) in the molecular structure can actively provide hydrogen atoms, combine with active free radicals such as R, ROO, etc., generate stable antioxidant free radicals (A), and terminate the proliferation stage of the chain reaction. For example, the quaternary hindered phenol structure of 1010 can cyclically capture 4 free radicals, with a thermal decomposition temperature of ≥ 320 ℃, and can still maintain activity at high processing temperatures. Adding 0.1% -0.3% can extend the thermal oxidative aging life of PP non-woven fabric by 3-5 times.
Auxiliary antioxidant (hydrogen peroxide decomposition agent): mainly composed of hypophosphite esters (such as 168) and thioethers (such as DSTDP), it can decompose the aging intermediate product ROOH into harmless alcohols and esters, avoiding the generation of new free radicals during its decomposition; Meanwhile, sulfide based auxiliary antioxidants can provide hydrogen atoms to reduce A · to AH (primary antioxidant), achieving cyclic regeneration of the primary antioxidant and increasing synergistic efficiency by 3-5 times. A typical compound such as AO-1010/168 (mass ratio 1:2) can increase the retention rate of fracture strength of QUV from 30% to over 85% after 1000 hours of aging by adding 0.2% -0.5% to PP non-woven fabric.
（2） UV shielding and energy conversion: light stable system (molecular “umbrella”)
Targeting the source of photooxidative aging, the triple mechanism of “absorption+quenching+shielding” is used to reduce UV attacks on molecular chains:
Ultraviolet absorbers (UVA): such as benzotriazoles (UV-327) and benzophenones, molecules can efficiently absorb 280-400nm UV energy, convert high energy into low energy through intramolecular proton transfer, and release it, avoiding energy transfer to the molecular chain. When the addition amount is 0.2% -0.5%, the UV shielding rate is ≥ 99%, which can extend the photo aging induction period of PP non-woven fabric from 100 hours to over 500 hours.
Hindered amine light stabilizers (HALS), such as HALS-770, do not directly absorb UV, but instead capture free radicals through a regeneration mechanism, while repairing slightly broken molecular chains (such as converting terminal free radicals into stable structures). They are known as “free radical scavenging vehicles”. When combined with UVA, it can form a synergistic effect of “absorbing UV+removing residual free radicals”, which can further increase the light aging life by 2-3 times, especially suitable for geotextiles, bird nets and other products for long-term outdoor use.
Inorganic shielding modification: adding activated nano TiO ₂ ZnO， Its nanoparticles can physically reflect UV while inhibiting the generation of free radicals; Experiments have shown that adding 5% -8% nano TiO ₂ to PP non-woven fabric reduces the UV degradation rate by 60%, but attention should be paid to dispersion uniformity to avoid fiber defects caused by agglomeration.
（3） Molecular chain structure strengthening: substrate modification and cross-linking (constructing a “rigid skeleton”)
By optimizing the molecular structure of the substrate, enhancing its own fracture resistance and reducing aging sensitivity:
Co polymerization modification: Introduce a small amount of ethylene, acrylonitrile and other copolymer units into the PP molecular chain to form branched or cross-linked structures, increase the steric hindrance of the molecular chain, and reduce the probability of C-C bond cleavage. For example, PP-PE copolymer non-woven fabric has a 15% -20% increase in molecular chain branching degree, and the molecular chain breakage rate after 300 hours of UV irradiation is 40% lower than that of pure PP.
Crosslinking modification: By irradiation (such as electron beam) or chemical crosslinking agents (such as diisopropylbenzene peroxide), PP molecular chains are formed into a three-dimensional network structure to enhance intermolecular forces. The crosslinked nonwoven fabric is not easy to depolymerize after the molecular chain breaks, but forms “cross-linking point containment”. The breaking strength retention rate is increased by 30% -50%, but the crosslinking degree (gel content 20% -30%) needs to be controlled to avoid the increase of brittleness.
Inorganic powder filling modification: 10% -30% surface activated inorganic powders such as calcium carbonate and talc powder are added, and the powder particles can be dispersed between the molecular chains, playing a “physical support” role, reducing the thermal motion and UV induced vibration of the molecular chains, and reducing the diffusion rate of free radicals. For example, PP non-woven fabric with 20% calcium carbonate (titanium ester coupling) has a relative molecular weight retention rate 25% higher than pure PP after 500 hours of thermal oxidative aging.
（4） Optimization of Processing Technology: Reducing Congenital Damage to Molecular Chains
Control the generation of free radicals in the processing stage to reduce aging hazards:
Low temperature processing control: Control the temperature of the screw extrusion melting section at 210-225 ℃ (avoid exceeding 235 ℃) to reduce the thermal degradation of molecular chains; The cooling air temperature is controlled at 8-20 ℃ to rapidly solidify the fibers and reduce the reaction time of free radicals at the end of the molecular chain.
Screw shear optimization: adopting a low shear screw design to reduce mechanical breakage of molecular chains during processing and lower initial free radical concentration; At the same time, 0.05% -0.1% of processing antioxidants (such as AO-1024) are added in advance to the raw materials to capture the free radicals generated during the processing.
Fiber morphology optimization: By designing the spinneret, the fiber diameter is made uniform (coefficient of variation ≤ 10%), reducing local stress concentration sites; At the same time, increasing the fiber orientation degree (≥ 85%), aligning the molecular chains along the fiber axis, and enhancing the tensile and UV fracture resistance.
The key principle of collaborative protection system: the molecular logic of 1+1>2
Full chain coverage: The antioxidant system blocks the chain reaction of thermal oxygen/photo oxygen, the photo stable system shields UV, and the substrate modification strengthens the molecular chain structure. The three work together to cover the entire process of “UV attack free radical generation molecular chain breakage”, avoiding the limitations of a single strategy.
Compatibility matching: All additives must have good compatibility with the molecular chain of the substrate (such as a solubility parameter difference of ≤ 1.0 between hindered phenols and PP), to avoid migration and precipitation and ensure long-term protective effect.
Accurate dosage control: Excessive additives can easily lead to “adverse effects” (such as yellowing caused by HALS addition exceeding 0.5%), which need to be adjusted according to the application scenario (outdoor high-intensity UV scenario: antioxidant 0.3% -0.5%+UVA 0.4% -0.6%+HALS 0.2% -0.3%).
Dongguan Liansheng Non woven Technology Co., Ltd. was established in May 2020. It is a large-scale non-woven fabric production enterprise integrating research and development, production, and sales. It can produce various colors of PP spunbond non-woven fabrics with a width of less than 3.2 meters from 9 grams to 300 grams.
Post time: Feb-27-2026