Deep influence mechanism of post - vulcanization time extension on NBR materials
Post-vulcanization is a crucial "secondary curing" step in rubber processing. Its essence is to promote the further cross-linking of molecular chains, the migration of small-molecule substances, and the stabilization of the network structure through continuous heat preservation. For nitrile rubber (NBR), extending the post-vulcanization time will significantly change its microstructure and macroscopic properties. The NBR molecular chain contains polar nitrile groups (-CN). During the cross-linking process, in addition to the formation of carbon-carbon bonds, there are also dipole interactions and hydrogen bonding between nitrile groups, which makes its cross-linked network more sensitive to time and temperature. When the post-vulcanization time is insufficient, the cross-linking density is low, the molecular chain segments have a large movement space, and the material is prone to permanent deformation. On the other hand, excessively extending the post-vulcanization time may cause the formed cross-linking bonds to break due to thermo-oxidative aging or lead to the degradation of the molecular chains, which in turn reduces the mechanical strength and elasticity of the material. Therefore, the control of the post-vulcanization time needs to be precisely matched with the cross-linking kinetic characteristics of NBR. Its influence is not simply "the longer the time, the better the performance", but there is a dynamic equilibrium range.
Sources and characteristics of small-molecule substances in NBR raw materials
The "small molecule particles" in NBR raw materials are not solid particles in the strict sense, but refer to oligomers with a molecular weight below 5000, unreacted monomers, residues of processing aids (such as softeners, plasticizers, antioxidants, etc.) and by - reaction products. The content of these small molecule substances mainly depends on the polymerization process and the purity of raw materials. For example, in emulsion polymerization, if the terminator is added in excess or the degree of polymerization is not properly controlled, the proportion of low - molecular - weight polymers will increase. In recycled rubber or low - purity NBR, due to the residues from previous processing, the content of small molecule additives (such as paraffin oil and phthalates) is usually higher. From the perspective of molecular structure, the polarity of these small molecule substances is generally lower than that of the NBR main chain (due to the low nitrile group content or no nitrile group), and the intermolecular forces (van der Waals forces, hydrogen bonds) between them and the macromolecular chains are relatively weak, so they are more likely to migrate under the action of heat.
The internal relationship between the migration and precipitation of small-molecule substances and the shrinkage of NBR
The shrinkage of NBR material is essentially a macroscopic manifestation of insufficient volume stability after vulcanization, and the migration and precipitation of small-molecule substances are the core inducements. When the post-vulcanization time is prolonged, the continuous thermal environment provides sufficient kinetic energy for small molecules, enabling them to overcome the constraints between molecular chains and migrate to the material surface or directly volatilize through the "diffusion - penetration" mechanism. Specifically: On the one hand, small molecules have high fluidity at high temperatures and can fill the voids in the cross-linked network. After cooling and solidification, if the small molecules are not completely combined with the macromolecular chains (such as plasticizers not being fully "anchored"), volume shrinkage will occur due to the decrease in temperature, or they will slowly precipitate during subsequent use, forming a "cavity - collapse" effect, which is macroscopically manifested as a reduction in the material size. On the other hand, the higher the content of small molecules, the more migratable substances there are per unit volume, and the larger the "volume deficit" left after precipitation, so the shrinkage rate is naturally higher. For example, when the proportion of small molecules exceeds 5%, the shrinkage rate of NBR can be increased by more than 30% compared with that of pure rubber, and the shrinkage direction shows anisotropy (more significant along the orientation direction of the molecular chains).
Driving factors and actual hazards of small molecule precipitation
The precipitation of small-molecule substances is not a random process. Its core driving forces are "concentration gradient" and "energy difference". During the post-vulcanization stage, the internal temperature of the material is higher than that of the surface. Under thermal motion, small molecules diffuse from the high-temperature region (inside) to the low-temperature region (surface). At the same time, the continuous densification of the cross-linked network will compress the space where small molecules exist, forcing low-molecular-weight substances to gather and precipitate at the weak parts of the network (such as defects and interfaces). In addition to causing shrinkage, this precipitation behavior will also cause multiple hazards: I. Surface precipitates (such as waxy additives and oligomers) will reduce the surface finish of NBR and even make it sticky, affecting the product appearance and assembly performance; II. The formation of internal voids will weaken the bearing capacity of the material, resulting in a 10%-20% decrease in tensile strength and tear strength; III. If the precipitates are toxic and harmful substances (such as certain plasticizers), it may also cause environmental compliance risks. Therefore, controlling the content of small molecules in raw materials and optimizing the post-vulcanization process (such as stepwise cooling and segmented heat preservation) are the key technical paths to suppress the shrinkage and precipitation of NBR.