Views: 5 Author: Site Editor Publish Time: 2025-02-06 Origin: Site
The whitening (frosting) issue in rubber products is a complex, multifactorial problem that involves rubber materials science, processing technology, and chemical-physical changes, among others. To better understand and solve this issue, it is necessary to explore the following areas: the microscopic mechanisms, influencing factors, testing and analysis methods, and more specific solutions.
The essence of whitening (frosting) is the migration and crystallization of certain components from the interior of the rubber to the surface. This process is primarily driven by the following mechanisms:
Solubility Balance Disruption:
The additives in rubber (such as vulcanizing agents, accelerators, antioxidants, plasticizers, etc.) have a certain solubility in the rubber matrix.
When temperature, pressure, or the physical state of the rubber changes, this balance is disrupted, leading to the migration of additives from the rubber matrix to the surface.
Migration and Diffusion:
Additives diffuse in the rubber matrix at the molecular level, migrating to the surface.
The migration rate is affected by the molecular weight, polarity of the additives, the molecular structure of the rubber, and environmental conditions (such as temperature and humidity).
Surface Energy Effect:
Rubber has low surface energy and tends to adsorb small polar additives (such as paraffin, plasticizers, etc.).
When these additives migrate to the surface, surface energy leads to the formation of a thin film or powder.
Aging Reactions:
Rubber undergoes aging processes (oxidation, hydrolysis, etc.) during storage or use, generating low molecular weight products (such as carboxylic acids, alcohols, etc.), which migrate to the surface and form frosting.
Based on the composition and formation mechanism of the frosted material, whitening (frosting) can be classified into several types:
Sulfur Frosting:
When sulfur is used as a vulcanizing agent and the amount is excessive or vulcanization is incomplete, unreacted sulfur migrates to the surface, forming yellow or white powder.
Common in sulfur-cured systems (e.g., EPDM, natural rubber, styrene-butadiene rubber products).
Accelerator Frosting:
Accelerators (such as MBT, CBS, TMTD, etc.) used excessively or poorly compatible with the rubber tend to precipitate on the surface.
Accelerator frosting is typically a white or gray-white powder.
Antioxidant Frosting:
Antioxidants (such as 4010NA, RD, etc.) when used excessively or with a high migration rate, form white or pale yellow powder on the surface.
Antioxidant frosting is usually accompanied by the phenomenon of the product becoming hard and brittle.
Filler Frosting:
Fillers (e.g., calcium carbonate, talc powder) that are poorly dispersed or have improper surface treatment can aggregate on the surface, forming frosting.
Filler frosting is typically a white powder that feels gritty when rubbed.
Plasticizer Frosting:
Plasticizers (such as paraffin, aromatic oils, etc.) used excessively or poorly compatible with the rubber can migrate to the surface, forming oily or waxy substances.
Plasticizer frosting is usually accompanied by a sticky surface phenomenon.
Aging Product Frosting:
Low molecular weight products generated during rubber aging, such as carboxylic acids and alcohols, migrate to the surface, forming white powder.
Aging product frosting is usually accompanied by performance degradation of the product.
To accurately identify the cause of whitening (frosting), the following testing and analysis methods can be employed:
Fourier Transform Infrared Spectroscopy (FTIR):
Analyzing the infrared spectrum of the frosting material to determine its chemical composition (such as sulfur, accelerators, antioxidants, etc.).
Thermogravimetric Analysis (TGA):
Analyzing the thermal weight loss curve of the frosting material to determine its thermal stability and composition.
Scanning Electron Microscopy (SEM) and Energy Dispersive Spectroscopy (EDS):
Observing the microstructure of the frosting material and determining its elemental composition through energy dispersive analysis.
Solubility Testing:
Testing the solubility of additives in rubber to assess the risk of frosting.
Migration Rate Testing:
Conducting simulated experiments to test the migration rate of additives in rubber and evaluate their tendency to form frosting.
When whitening (frosting) occurs in rubber products, the following temporary measures can be taken:
Physical Wiping:
Use a clean cloth or sponge soaked in an appropriate solvent (such as alcohol, gasoline, etc.) to gently wipe the surface and remove the white powder.
Chemical Treatment:
Use specialized rubber surface treatment agents to remove the frosted material through chemical reactions.
Secondary Vulcanization:
For mild frosting, secondary vulcanization can be attempted to re-incorporate the precipitated additives into the vulcanization process.
Formula Optimization:
Control Additive Usage: Accurately calculate the amount of additives based on rubber type and product performance requirements to avoid excessive use.
Select High-Efficiency Additives: Use more efficient vulcanizing agents, accelerators, and antioxidants to reduce the amount used and improve compatibility.
Add Dispersing Agents: Add dispersing agents (such as zinc stearate, PE wax, etc.) to improve the dispersion of fillers.
Use Pre-dispersed Masterbatches: Use pre-dispersed masterbatches to enhance the dispersion and stability of additives in the rubber.
Production Process Improvement:
Optimize Mixing Process: Use a multi-stage mixing process to ensure uniform dispersion of additives.
Control Vulcanization Conditions: Optimize vulcanization temperature and time according to rubber type and product thickness to ensure complete vulcanization.
Post-vulcanization Treatment: Conduct post-vulcanization treatment to relieve internal stresses and reduce the risk of frosting.
Surface Treatment: Apply coatings or plasma treatment to the product surface to reduce surface energy and minimize the migration of additives.
Storage and Environmental Control:
Control Temperature and Humidity: Store products in an environment with temperatures between 20-30°C and humidity below 50%.
Avoid Light Exposure: Use light-blocking packaging or store in light-proof environments.
Periodic Rotation: Regularly rotate products stored for long periods to avoid localized frosting.
Whitening (frosting) is a common quality issue in rubber product manufacturing and use. A comprehensive approach, including formula design, strict control of the production process, improved storage conditions, and selecting appropriate rubber materials, can effectively prevent and address this problem, improving product quality and extending service life.
