1、 The essence of weather resistance: synergistic aging of materials and structures
Outdoor weather resistance is not a single indicator, but a comprehensive reflection of materials from multiple regions, structural design, and processing technology. Research has confirmed that even with the same modified polypropylene raw material, the weather resistance life can differ by 3-5 times depending on the structural design.
2、 UV radiation degradation and its prevention technology
Damage mechanism:
The UV-B (280-315nm) and UV-A (315-400nm) bands in sunlight have sufficient energy to break the C-C bonds in polypropylene molecular chains. The molecules that break the chain generate free radicals, triggering a chain degradation reaction, manifested as surface pulverization (white powder when wiped by hand), color fading (red is the most sensitive, gray is more stable), and a decrease in impact strength.
Quantitative indicators:
After one year of outdoor use (equivalent to approximately 800-1000 hours of xenon lamp aging), the material's tensile strength retention rate should not be less than 80%.
The color difference Δ E value should be controlled within 5 (visible to the naked eye but acceptable).
Technical countermeasures:
Hindered amine light stabilizer system: HALS (such as Tinuvin 770, 944 series) is added to the raw materials. The mechanism is to capture the free radicals generated by degradation, rather than absorbing ultraviolet radiation. The usual addition amount is 0.3% -0.5% by weight. Attention: HALS has antagonistic effects with certain flame retardants and needs to be verified by compounding.
UV absorber complex: Combine benzophenone or benzotriazole absorbers with HALS. The former converts ultraviolet light into heat energy, while the latter captures residual free radicals, producing a synergistic effect. Typical compounding ratio UVA: HALS=1:2.
Carbon black filling modification: Adding 2% -3% high structure carbon black (particle size 20-30nm) can absorb more than 90% of UV radiation. But carbon black will reduce the impact toughness of the material and is only suitable for dark colored floors (light colored ones cannot be used).
Surface fluorocarbon coating treatment: A few excellent products use co extrusion technology to form a 0.1-0.2mm thick fluorinated polymer layer (such as PVDF) on the surface. The fluorocarbon bond energy can reach up to 485 kJ/mol, much higher than the 413 kJ/mol of C-H bond, and the outdoor weather resistance life can reach more than 10 years.
Detection and verification:
Conduct xenon lamp aging test according to GB/T 16422.2 or ASTM G155. Typical cycle: irradiance of 0.51W/m ² 340nm, blackboard temperature of 65 ℃, relative humidity of 50%, water spraying cycle of 18min/102min. After continuous aging for 2000 hours, it is required to have no powdering, no cracks, and a color difference Δ E ≤ 5.
3、 High temperature thermal oxidative aging and dimensional stability
Damage mechanism:
When exposed to outdoor sunlight, the surface temperature of the floor can reach 65-75 ℃ (even exceeding 80 ℃ for dark colors). High temperature accelerates the thermal oxidation reaction, leading to cross-linking or breakage of molecular chains. Crosslinking makes the material brittle, while fracture reduces its strength. Meanwhile, the linear expansion coefficient of polypropylene is about 8-12 × 10 ⁻⁵/℃, which means that the theoretical expansion and contraction of a 10 meter long site can reach 4-6mm under a temperature difference of 50 ℃.
Technical countermeasures:
Antioxidant system: The main antioxidant (hindered phenols such as Irganox 1010) is compounded with auxiliary antioxidants (hypophosphite esters such as Irgafos 168). The main antioxidant captures peroxide free radicals and assists the antioxidant in decomposing hydroperoxides. Add 0.1% -0.2% each.
Mineral filler modification: Add 20% -30% talc powder or calcium carbonate. The filler significantly reduces the coefficient of linear expansion (which can be reduced to 5-6 × 10 ⁻⁵/℃), while increasing the thermal deformation temperature. However, excessive filling will reduce the impact strength and surface glossiness.
Fiberglass reinforcement: Add 10% -15% short cut fiberglass (length 3-4.5mm). Fiberglass significantly increases its thermal deformation temperature (from 120 ℃ to over 150 ℃), but it can cause anisotropic shrinkage (with significant differences in flow direction and non flow direction), and exposed fiberglass can cause surface roughness.
Structural compensation design: The lock buckle reserves a sliding gap of 0.2-0.3mm, allowing each board to independently expand and contract due to heat. At the same time, leave sufficient 10-15mm expansion joints around the site.
Detection and verification:
Thermal aging test: Place the floor in a forced ventilation oven at a temperature of (100 ± 2) ℃ for 7 or 14 days. After the test, the retention rate of impact strength is ≥ 70%, and the retention rate of lock pulling force is ≥ 80%.
Vicat softening temperature: B50 method (50N load, heating rate 50 ℃/h) should be ≥ 120 ℃.
Size change rate: According to GB/T 8814, when the floor is placed at (80 ± 2) ℃ for 24 hours, the shrinkage rate in the length direction is ≤ 0.8%.
4、 Low temperature brittleness and impact resistance performance
Damage mechanism:
The glass transition temperature (Tg) of polypropylene is approximately -10 ℃ to 0 ℃. When the Tg is below, the molecular chain segments in the amorphous region freeze and the material transitions from ductile fracture to brittle fracture. At -20 ℃, the impact strength may only be 10% -20% of the room temperature value. Common failure modes: brittle fracture at the root of the lock buckle and fracture of the support pillar.
Technical countermeasures:
Elastic toughening: Add 5% -15% POE (ethylene octene copolymer) or EPDM (ethylene propylene diene monomer rubber). Elastic particles are dispersed in the PP matrix, which can induce silver lines and shear bands in excellent cracks and absorb impact energy. Attention: POE will reduce hardness and thermal deformation temperature, and the amount added needs to be balanced.
β crystal nucleating agent: Add a small amount of amide or rare earth β nucleating agent to induce the formation of β crystal in PP. The impact strength of the β crystal form can reach 2-3 times that of the α crystal form, and the ductile brittle transition temperature is reduced by about 15 ℃. However, the β crystal form will transform into the α crystal form during thermal aging, and the long-term effect needs to be verified.
Structural toughness design: The root of the lock buckle needs to be designed with an R-angle (rounded radius ≥ 0.5mm) to eliminate stress concentration. The connection between the pillar and the surface layer adopts a gradual wall thickness (decreasing thickness from the root to the end) to avoid abrupt changes in cross-section.
Detection and verification:
Low temperature ball impact: The floor is adjusted at (-20 ± 2) ℃ for 24 hours, and a 1kg steel ball is freely dropped from a height of 1 meter. Require at least 9 out of 10 test points to be free of fractures or penetrating cracks.
Low temperature lock strength: Under the same conditions, the pulling force of the lock should not be less than 60% of the normal temperature value.
5、 Damp heat aging and hydrolytic stability
Damage mechanism:
In outdoor environments, the bottom of the floor is in long-term contact with damp ground (especially during the southern rainy season). Although polypropylene itself has excellent water resistance (water absorption rate& lt;0.01%）， But certain additives (such as flame retardants, electrostatic agents) may have hydrophilicity. Moisture infiltration can cause the migration and precipitation of additives (surface whitening), or react with fillers (calcium carbonate decomposes in acidic and humid environments).
Technical countermeasures:
Hydrophobic multiple area design: Avoid using hydrophilic additives. If needed, a compatibilizer (such as maleic anhydride grafted PP) should be added to enhance the interfacial bonding.
Bottom ventilation structure: The suspended floor naturally grows with a gap of 5-8mm, forming air microcirculation. But it is necessary to confirm that the foundation ground has a drainage slope to avoid long-term immersion in water.
Mold prevention (based on actual reports) treatment: Add 0.1% -0.3% of specific mold prevention (based on actual reports) agents (such as silver ion carriers or thiazole compounds) to prevent mold growth at the bottom of the floor.
Detection and verification:
Damp heat aging: According to GB/T 2423.4, place the floor in an environment of (65 ± 2) ℃ and relative humidity (93 ± 3)% for 21 days. After the experiment, there should be no mold, bubbles, or precipitates. The retention rate of impact strength is ≥ 70%.
Boiling water test: Immerse the floor in boiling water for 2 hours and observe that there are no bubbles, delamination, or discoloration on the surface.
6、 Freeze thaw cycle failure
Damage mechanism:
This is the most severe damage scenario for outdoor flooring. Water enters the gaps or microcracks between the floor and foundation, and freezes at minus zero temperatures (with a volume expansion of 9%). Repeated freeze-thaw cycles result in cumulative fatigue damage, leading to floor warping, loose locks, and cracked pillars.
Technical countermeasures:
Low water absorption rate: Confirm floor water absorption rate& Lt; 0.05% (can be achieved through closed cell foaming or dense surface layer). The harder it is for water to enter, the lower the risk of freezing and thawing.
Elastic deformation capability: The floor still needs to maintain a certain elastic deformation capability at low temperatures (achieved through POE toughening or β crystal structure) to absorb the stress generated by ice crystal expansion.
Drainage design: The foundation slope shall not be less than 0.5%, the floor hollowing rate shall not be less than 30%, and it shall be confirmed that rainwater will not be retained.
Detection and verification:
Freeze thaw cycle test: After immersing the floor in water for 2 hours, freeze it at (-20 ± 2) ℃ for 4 hours, and then melt it in room temperature water for 2 hours. This is one cycle. After 50 cycles, it is required to have no cracks, no delamination, and no warping (warping height ≤ 1mm/m).
