Under low-temperature conditions, the glass transition temperature of SBR film significantly affects the tear resistance of its composite with black T-cloth. This effect stems from changes in mechanical properties caused by phase transitions in the material, which act on the composite system through interfacial bonding and stress transfer mechanisms.
The glass transition temperature of SBR film represents the critical point at which it transitions from a highly elastic state to a glassy state. When the ambient temperature falls below this transition temperature, the mobility of the SBR molecular segments decreases dramatically, and the material exhibits brittle characteristics. This phase transition directly leads to a significant increase in the film's elastic modulus and a significant decrease in its elongation at break, making the material more susceptible to brittle fracture rather than plastic deformation under load. For SBR film composited with black T-cloth, low-temperature embrittlement disrupts the continuity of the composite interface, weakening the film's ability to wrap and restrain the fabric fibers.
The tear resistance of black T-cloth depends on the frictional resistance between yarns and the fiber pullout work. In the composite system, the SBR film impregnates the fiber surface to form a mechanical interlocking structure, evenly transferring external loads to the fabric interior. When SBR is in its glassy state, its hard-brittle properties alter the stress distribution pattern: on the one hand, crack propagation within the film itself accelerates, leading to premature failure at the composite interface. On the other hand, the brittle film cannot effectively buffer fabric deformation, causing stress to concentrate at yarn intersections and accelerating fiber breakage. This dual effect significantly reduces the tear strength of the composite.
Interfacial bonding is a key factor influencing property transfer. At low temperatures, the glass transition of SBR film degrades interfacial compatibility. Specifically, surface contraction of the film generates internal stress, reducing the contact area with the T-cloth. Simultaneously, the brittle film's adaptability to fabric surface roughness decreases, weakening the mechanical engagement. This interfacial debonding reduces load transfer efficiency, concentrating more energy on localized fibers during tearing rather than dissipating it throughout the fabric structure through the film. This ultimately results in a sharp decline in tear resistance.
From a fracture mechanics perspective, low-temperature embrittlement of SBR film alters the crack propagation path of the composite. In its elastic state, the film can induce crack deflection through local deformation, increasing the fracture surface energy. However, the brittle nature of the glassy film makes it easier for cracks to penetrate the interface in a straight line, resulting in penetrating failure. This shift in failure mode causes the composite material to transition from ductile to brittle fracture, resulting in an order of magnitude decrease in tear strength. Furthermore, fragments generated by the brittle film can act as crack initiators, further accelerating material failure.
Changes in the material's microstructure are also noteworthy. Under low-temperature conditions, the freezing of SBR molecular chains reduces the free volume and increases the material's density. While this densification effect increases the film's hardness, it sacrifices its toughness reserve. For the composite system, this means the film cannot absorb energy through plastic deformation, converting virtually all external forces into damaging effects on the fabric fibers. Simultaneously, the size of the plastic zone at the crack tip of the brittle film decreases, reducing crack propagation resistance, creating a vicious cycle.
In practical applications, this performance degradation manifests itself as a shortened service life of the composite material in low-temperature environments. For example, in sports protective equipment used in cold regions, if the SBR/T fabric composite fails to maintain sufficient low-temperature toughness, it may fracture suddenly under impact loads, losing its protective function. Therefore, adjusting the glass transition temperature of SBR to match the ambient operating temperature through molecular structure design or blending modification is an effective way to improve the low-temperature tear resistance of composite materials.
The glass transition temperature of SBR film influences the tear resistance of black T-cloth at multiple levels by altering the material's phase state, interfacial bonding, stress transfer, and crack propagation mechanisms. Understanding this influence can provide theoretical guidance for the development of low-temperature-resistant composite materials and promote their application in equipment for extreme environments.
