The change in flexibility of kraft silicone paper over time is a dynamic process influenced by multiple factors, the core mechanism of which can be attributed to the synergistic aging effect of the silicone layer and the kraft paper substrate. As a flexible functional layer, the molecular chain structure of the silicone layer gradually relaxes and rearranges over time, while the fiber network of the kraft paper substrate experiences strength decay due to environmental factors. Together, these factors determine the evolution trend of the material's flexibility.
Molecular chain relaxation of the silicone layer is the primary factor affecting flexibility. In freshly prepared kraft silicone paper, the silicone molecular chains are tightly intertwined, giving the material initial flexibility and elasticity. However, during long-term storage or use, the molecular chains gradually untangle due to thermal motion, leading to a decrease in the viscosity and an increase in fluidity of the silicone layer. While this process can improve the material's ductility in the short term, excessive relaxation weakens the interaction forces between molecular chains, causing the silicone layer to gradually harden and thus reducing flexibility. For example, at high temperatures, the thermal motion of the silicone molecular chains accelerates, significantly shortening the flexibility decay period; while at low temperatures, molecular chain motion is hindered, prolonging the time the flexibility is maintained.
Fiber aging in kraft paper substrates is another key variable. Kraft paper uses natural fibers as its framework, and its strength and flexibility depend on the hydrogen bonding and interweaving density between these fibers. Over time, moisture, oxygen, and ultraviolet radiation in the environment disrupt the hydrogen bond network between fibers, leading to fiber breakage and increased porosity. This process not only weakens the tear resistance of the substrate but also affects the flexibility of the silicone layer through interfacial stress transfer. For example, in humid environments, moisture penetrates into the fiber gaps, triggering hydrolysis and accelerating fiber degradation; while in dry environments, fibers shrink due to water loss, causing the substrate to become brittle, which also limits the deformation capacity of the silicone layer.
Environmental factors have significantly different effects on the flexibility of kraft silicone paper. Temperature is one of the most significant environmental parameters: high temperatures accelerate the relaxation of silicone molecular chains and fiber aging, leading to a rapid decrease in flexibility; low temperatures may cause embrittlement of the silicone layer, especially prone to cracking during low-temperature bending. The influence of humidity is also significant: high humidity promotes fiber hydrolysis and may cause the silicone layer to absorb moisture and expand, leading to interfacial stress concentration; low humidity may cause fiber shrinkage due to water loss, exacerbating substrate embrittlement. Furthermore, ultraviolet radiation induces photo-oxidative aging of the silicone layer, generating free radicals that attack molecular chains, causing cross-linking structure damage and further reducing flexibility.
Mechanical stress has a cumulative effect on the flexibility of kraft silicone paper. Long-term repeated bending or stretching can cause fatigue cracks in the silicone layer and lead to fiber network breakage, resulting in irreversible damage. This damage is particularly noticeable in stress concentration areas, such as creases or edges, significantly reducing the material's local flexibility. For example, under frequent folding conditions, creases in kraft silicone paper gradually show whitening, a direct manifestation of fiber breakage and silicone layer delamination, signifying a permanent loss of flexibility.
The interfacial bonding state between silicone and kraft paper plays a regulatory role in the evolution of flexibility. An ideal interfacial bonding ensures uniform stress transfer, avoiding premature failure caused by localized stress concentration. However, if defects exist at the interface (such as bubbles, impurities, or uneven adhesive distribution), stress transfer efficiency will decrease, accelerating the separation of the silicone layer from the substrate. For example, during the preparation process, if the silicone coating is uneven or the curing temperature is not properly controlled, a weak boundary layer may form at the interface, becoming the starting point for flexibility degradation.
The flexibility of kraft silicone paper exhibits a typical characteristic of "initial stability - mid-term degradation - late-term embrittlement" over time. In the initial stage of use, the material properties are stable, and flexibility is mainly determined by the molecular chain structure of the silicone layer and the fiber network of the substrate. In the mid-stage, under the combined effects of environmental factors and mechanical stress, molecular chain relaxation and fiber aging accelerate, and flexibility begins to decline. In the late stage, the hardening of the silicone layer and the embrittlement of the substrate work synergistically, and the material gradually loses its flexibility, ultimately becoming easily broken and difficult to bend. This process can be slowed down by optimizing the material formulation (such as adding anti-aging agents), improving the preparation process (such as controlling coating uniformity), and improving storage conditions (such as avoiding light and moisture).
