1.How to optimize coating materials?
Select high-toughness resin substrate: give priority to resins with excellent low-temperature flexibility, such as fluorocarbon resin (PVDF) and polyvinylidene fluoride (PVF). The fluorocarbon bond energy in its molecular chain is high and it is not easy to become brittle at low temperatures. If you need to balance the cost, you can choose modified polyester to replace ordinary rigid polyester (PE).
Add tougheners and elastomers: introduce low-temperature flexible additives into the resin, such as phthalate plasticizers, nitrile rubber (NBR), polyurethane elastomer (PU) and other elastic particles (through "energy absorption" to relieve impact stress). For example, adding 5%-10% nitrile rubber to polyester resin can reduce the brittle transition temperature of the coating from -15℃ to -30℃.
Regulate cross-linking density: Reducing the cross-linking density of the coating can improve flexibility. For example, reducing the cross-linking degree of the fluorocarbon coating from 80% to 60%-70% can significantly reduce the risk of bending cracking at low temperatures.
2.How to adapt pigments and fillers?
Reduce the proportion of rigid fillers: Avoid excessive use of rigid fillers such as talcum powder and quartz powder (which can easily increase the brittleness of the coating), and use flexible fillers such as hollow glass beads instead, or add an appropriate amount of elastic pigments.
Choose low-crystallinity pigments: organic pigments are preferred to replace some inorganic pigments (such as titanium dioxide), because organic pigment particles are more compatible with resins and are not easy to form stress points at low temperatures.
3.How to choose substrate material?
Choose low-carbon and high-toughness substrates: Low-carbon steel substrates (carbon content ≤ 0.12%) are preferred, such as Q235B and SPCC (low-carbon grade), whose low-temperature impact energy (-40℃ Charpy V-notch) can reach more than 27J, which is much higher than medium-carbon steel (carbon content 0.25%-0.6%, low-temperature impact energy is often <15J); if higher toughness is required, micro-alloyed steel (such as low-temperature steel with added nickel and manganese, such as Q355ND, -40℃ impact energy ≥34J) can be selected.
Avoid brittle elements such as phosphorus and sulfur: Control the content of phosphorus (P≤0.025%) and sulfur (S≤0.015%) in the substrate, because these elements are easy to segregate at the grain boundary, resulting in low-temperature embrittlement ("cold brittleness" phenomenon).
4.How to handle the substrate rolling and heat treatment process?
Annealing after cold rolling: Through complete annealing (temperature 700-800℃, slow cooling), the substrate grains are refined (the grain size is reduced from 50μm to less than 20μm), the grain boundary defects are reduced, and the low-temperature toughness is improved (for example, the impact energy of the annealed SPCC substrate at -30℃ can be increased by 30%).
Control the rolling deformation: Avoid excessive cold rolling (deformation>80%) to cause substrate work hardening. "Multi-pass small deformation rolling" (single pass deformation <20%) can be used to reduce internal stress accumulation and reduce the risk of low-temperature embrittlement.
5.How to regulate production and post-processing processes?
Optimizing the Curing Process
Avoiding over-curing or under-curing: Controlling the curing temperature and time based on the resin type ensures the coating's crosslinking degree remains within the "toughness range" of 60%-80%.
Low-Temperature Aging Treatment
Post-production, the color-coated coil undergoes "low-temperature aging" to prematurely release internal stress in the coating and reduce the risk of sudden cracking caused by low-temperature impact during subsequent use.
Surface Microstructure Adjustment
Using the coating process to create a microscopic uneven texture or adding elastic microspheres allows the surface to deform during impact to absorb some of the energy, reducing the likelihood of cracking.