To determine whether rubber has completed vulcanization (i.e., the cross-linking reaction has reached the ideal state), the key is to detect whether its physical and chemical properties meet the characteristics expected after vulcanization. The following are common methods and operation logic, covering different scenarios from simple to professional:
Appearance and feel judgment
Appearance observation
* The surface of unvulcanized rubber (raw rubber) is usually sticky, with low gloss, soft texture, and obvious plasticity. It will leave deep marks after pressing with fingers, and the recovery speed is slow.
* The surface of vulcanized rubber is smoother and non-sticky. The gloss may be matte or glossy depending on the formula. The texture is tough, the marks are shallow after pressing, and the recovery speed is fast.
Touch
* Raw rubber feels like plasticine, which is easy to deform and may break when stretched.
* The elasticity of vulcanized rubber is significantly enhanced; it can rebound quickly after stretching, it is not easy to break, and it feels more resilient.
Physical properties test
Tensile properties test
* Unvulcanized rubber: low tensile strength, high elongation at break, and prone to permanent deformation during stretching (e.g., it cannot completely recover to its original shape after stretching).
* Vulcanized rubber: tensile strength is significantly improved, elongation at break is relatively reduced, elasticity is good during stretching, it can quickly recover to its original shape after unloading, and permanent deformation is small.
Hardness test
Measured with a hardness tester (such as a Shore hardness tester): the hardness of unvulcanized rubber is extremely low (close to 0 Shore hardness), and the hardness after vulcanization will increase significantly depending on the formula (e.g., the hardness of common vulcanized rubber is between 30 and 90 Shore hardness).
Heat resistance test
Unvulcanized rubber has poor heat resistance and is easy to soften, become sticky, or even melt after heating; vulcanized rubber has significantly improved heat resistance due to its cross-linked molecular structure, and its hardness and elasticity change little after heating, and it is not easy to soften.
Chemical property judgment
Solvent dissolution test
Unvulcanized rubber will gradually swell or even dissolve in organic solvents (such as toluene and gasoline); vulcanized rubber will only swell to a limited extent in solvents and will not completely dissolve due to the cross-linking of molecular chains to form a three-dimensional network structure.
Observation of combustion characteristics
Unvulcanized rubber has a strong flame when burning, produces a lot of black smoke, and leaves sticky substances after burning; vulcanized rubber has a relatively stable flame when burning, with less black smoke, and the combustion residue is mostly brittle coke-like substances (the specific phenomenon varies depending on the type and formula of rubber).
Summary: To determine whether the rubber is vulcanized, you can preliminarily screen it through appearance and touch, and then quantify and verify it through physical tests such as hardness and stretching. In professional scenarios, it is necessary to combine the swelling method or vulcanizer to analyze the cross-linking density, and finally ensure that the rubber reaches the ideal physical properties and use requirements. The performance of different types of rubber (such as natural rubber, styrene-butadiene rubber, silicone rubber, etc.) after vulcanization varies greatly. When judging, it is necessary to combine the rubber type and formula characteristics for comprehensive analysis.
