目的 揭示印花镍网表面交联聚乙烯醇（PVA）涂层在CO2脉冲激光作用下的去除行为及机理,明确纵向光斑搭接率对热积累、热解特征和清洗效果的影响规律。方法 结合热重分析（TGA）、有限元数值模拟与不同纵向光斑搭接率（η=15%、30%、45%、60%）激光清洗实验,研究涂层热解行为、温度场演化与脱除形式。结果 TGA显示涂层在约330 ℃与400 ℃出现两处DTG峰,分别对应脱水/主链初裂与深度裂解-碳化阶段。数值模拟表明,随η增大温度场由“孤立热点”向“连续热积累带”演化,表面基线最高温度为114~247 ℃,整体低于DTG峰值温区,说明η=30%~45%条件下涂层去除主要由失水干燥、交联网络收缩/松弛及界面热应力累积引起的中温脆化剥离主导;η=60%时重复照射更易产生局部热点并诱发碳化残留,阻碍进一步脱除。实验表明清洗机理随η呈三阶段演变：η=15%以浅层挥发为主;η=30%~45%以脆化-固态剥离为主,且综合效率最高;η=60%出现碳化阻滞。结论 提出了交联PVA涂层的分区去除机理与双模式协同机制,并确定η=30%~45%为兼顾高效去除与基材保护的优选光斑搭接率范围。
Abstract
To clarify the removal behavior and mechanism of crosslinked poly (vinyl alcohol) (PVA) coatings on printed nickel mesh under CO2 pulsed-laser irradiation, the effects of longitudinal spot overlap ratio on thermal accumulation, thermal decomposition, and cleaning performance are systematically investigated. Thermogravimetric analysis (TGA/DTG), Fourier transform infrared spectroscopy (FT-IR), laser cleaning experiments, mass-change analysis, and three-dimensional transient finite-element simulations are combined to establish the relationship among the thermal decomposition characteristics of the coating, temperature-field evolution, and removal behavior. TGA/DTG results show that the crosslinked PVA coating exhibits two major mass-loss-rate peaks at about 330 ℃ and 400 ℃, corresponding respectively to dehydration/initial chain scission and deeper cracking accompanied by carbonization. Laser cleaning experiments are carried out at longitudinal overlap ratios of 15%, 30%, 45%, and 60%. The experimental results reveal a pronounced dependence of removal mode on overlap ratio. At 15% overlap, the smoke concentration and vaporized mass are the highest, indicating that the absorbed energy is mainly consumed by superficial volatilization and moisture release, whereas the effective stripping of the coating remains limited. At 30%-45% overlap, the cleaning performance is markedly improved. In this range, repeated heating-cooling cycles promote dehydration drying, network shrinkage/relaxation, and interfacial thermal-stress accumulation caused by thermal-expansion mismatch between the coating and the nickel substrate, thereby facilitating embrittlement cracking and interfacial debonding. As a result, the coating is removed mainly by solid-phase peeling, and the overall removal efficiency is the best. At 60% overlap, the vaporized mass decreases further and the cleaning effect deteriorates, indicating that excessive overlap is unfavorable for continuous coating removal. Finite-element simulations show that, with increasing overlap ratio, the surface temperature field evolves from isolated hot spots to a more continuous thermal-accumulation band. The maximum baseline surface temperature increases from about 114 ℃ to 247 ℃, but generally remains below the DTG peak-temperature range. This indicates that large-area rapid pyrolysis is not the dominant removal mechanism under the present processing conditions. Instead, coating removal at overlap ratios up to 45% is governed mainly by moderate-temperature dehydration, embrittlement, and interfacial debonding induced by repeated thermal cycling. However, when the overlap ratio reaches 60%, local hot spots become more pronounced. Combined with the high transverse overlap in the experiments, such local overheating can promote dehydration, unsaturation, and carbonization in limited regions. FT-IR analysis further supports this interpretation. For the samples cleaned at overlap ratios of 15%-45%, the post-cleaning spectra show only limited differences, indicating that chemical decomposition is relatively weak and that removal in this range is dominated mainly by physical embrittlement and interfacial peeling. In contrast, the sample treated at 60% overlap exhibited significantly enhances C==C and C==O absorption bands, confirming dehydration- induced unsaturation and oxidative carbonization. The carbonized products form a barrier layer that hinders subsequent volatilization and peeling, leading to a carbonization-inhibition effect. Based on the combined evidence, a segmented removal mechanism and a dual-mode synergistic mechanism are proposed for laser cleaning of crosslinked PVA coatings on printed nickel mesh. Within the present parameter window, coating removal is governed mainly by moderate-temperature dehydration drying, embrittlement, and interfacial debonding, while localized pyrolysis/carbonization plays a secondary role and becomes detrimental at excessive overlap. As the overlap ratio increases, the dominant cleaning mechanism evolves through three stages: superficial volatilization, efficient embrittlement-assisted stripping, and carbonization inhibition. Therefore, a longitudinal spot overlap ratio of 30%-45% is recommended as the optimal range for balancing efficient coating removal and substrate protection.
关键词
激光清洗 /
聚乙烯醇交联聚合物 /
热重分析 /
FTIR /
数值模拟 /
去除机理
Key words
laser cleaning /
crosslinked polyvinyl alcohol (PVA) polymer /
thermogravimetric analysis (TGA) /
FTIR /
numerical simulation /
removal mechanism
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