The problem of frosting and whitening of rubber products is a complex multi-factor problem involving multiple fields such as rubber material science, processing technology, chemical and physical changes. In order to understand and solve this problem more deeply, we need to conduct a deeper discussion from the aspects of microscopic mechanism, influencing factors, test analysis methods and more specific solutions.
1. Microscopic mechanism of frosting and whitening
The essence of frosting and whitening is the process in which certain components in the rubber system migrate from the inside to the surface and precipitate. This process is mainly driven by the following mechanisms:
1). Solubility balance destruction:
- The compounding agents in rubber (such as vulcanizers, accelerators, antioxidants, softeners, etc.) have a certain solubility in the rubber matrix.
- When the temperature, pressure or physical state of the rubber changes, the solubility balance is broken, causing the compounding agent to precipitate from the rubber matrix.
2). Migration and diffusion:
- The compounding agent diffuses in the rubber matrix in the form of molecules and migrates to the surface.
- The migration rate is affected by the molecular weight and polarity of the compounding agent, the molecular structure of the rubber and environmental conditions (such as temperature and humidity).
3). Surface energy effect:
- The surface energy of rubber is low, and it is easy to adsorb compounding agents with low polarity (such as paraffin, softener, etc.).
- When the compounding agent migrates to the surface, a film or powder will be formed on the surface due to the effect of surface energy.
4). Aging reaction:
- During storage or use, rubber undergoes aging reactions such as oxidation and hydrolysis to generate low molecular weight products (such as carboxylic acids, alcohols, etc.), which are easy to migrate to the surface to form frosting.
2. Classification and characteristics of frosting whitening
According to the composition and formation mechanism of frosting substances, frosting whitening can be divided into the following categories:
1). Sulfur frosting:
- When sulfur is used as a vulcanizing agent, if the amount is too much or the vulcanization is insufficient, the unreacted sulfur will migrate to the surface to form yellow or white powder.
- Commonly found in sulfur vulcanization systems (such as EPDM, NR, and SBR products).
2). Accelerator bloom:
- When the accelerator (such as MBT, CBS, TMTD, etc.) is used in excessive amounts or has poor compatibility with rubber, it is easy to precipitate on the surface.
- Accelerator bloom is usually white or off-white powder.
3). Antioxidant bloom:
- When the antioxidant (such as 4010NA, RD, etc.) is used in excessive amounts or the migration rate is too fast, white or light yellow powder will form on the surface.
- Antioxidant bloom is usually accompanied by the phenomenon of hardening and brittleness of the product.
4). Filler bloom:
- When the filler (such as calcium carbonate, talcum powder, etc.) is unevenly dispersed or the surface treatment is poor, it is easy to gather on the surface to form bloom.
- Filler bloom is usually white powder, and there is a clear granular feel when wiping it with hands.
5). Softener bloom:
- When the amount of softener (such as paraffin, aromatic oil, etc.) is too much or the compatibility with rubber is poor, it is easy to migrate to the surface to form oily or waxy substances.
- Softener bloom is usually accompanied by the phenomenon of stickiness on the surface of the product.
6). Aging product bloom:
- Low molecular weight products such as carboxylic acids and alcohols generated by rubber during the aging process migrate to the surface to form white powder.
- Aging product bloom is usually accompanied by the phenomenon of product performance degradation.
3. Test and analysis methods for whitening of bloom
In order to accurately determine the cause of whitening of bloom, the following test and analysis methods can be used:
1). Infrared spectroscopy analysis (FTIR):
- By analyzing the infrared spectrum of the blooming substance, its chemical composition (such as sulfur, accelerator, antioxidant, etc.) is determined.
2). Thermogravimetric analysis (TGA):
- By analyzing the thermal weight loss curve of the blooming substance, its thermal stability and composition are determined.
3). Scanning electron microscope (SEM) and energy spectrum analysis (EDS):
- Observe the microscopic morphology of the frosting material and determine its elemental composition through energy spectrum analysis.
4). Solubility test:
- Test the solubility of the compounding agent in rubber and evaluate its frosting risk.
5). Migration rate test:
- Test the migration rate of the compounding agent in rubber through simulation experiments to evaluate its frosting tendency.
4. Temporary measures for frosting and whitening
When rubber products have frosting and whitening, the following temporary measures can be taken to remedy:
- Physical wiping: Use a clean cloth or sponge to dip in an appropriate amount of solvent (such as alcohol, gasoline, etc.) and gently wipe the frosting surface to remove the white powder on the surface.
- Chemical treatment: Use a special rubber surface treatment agent to remove the frosting material on the surface through chemical reaction.
- Secondary vulcanization: For products with slight frosting, secondary vulcanization can be tried to allow the precipitated compounding agent to re-participate in the vulcanization reaction.
5. Long-term solution for frosting
1). Formula design optimization
- Control the amount of compounding agent: According to the rubber type and product performance requirements, accurately calculate the amount of compounding agent to avoid excessive use.
- Select high-efficiency compounding agent: Use high-efficiency vulcanizers, accelerators and antioxidants to reduce the amount and improve compatibility.
- Add dispersant: Add dispersant (such as zinc stearate, PE wax, etc.) to the formula to improve the dispersibility of fillers.
- Use pre-dispersed masterbatch: Make the compounding agent into pre-dispersed masterbatch to improve its dispersibility and stability in rubber.
2). Production process improvement
- Optimize the mixing process: Use a multi-stage mixing process to ensure that the compounding agent is evenly dispersed.
- Control the vulcanization conditions: According to the rubber type and product thickness, optimize the vulcanization temperature and time to ensure sufficient vulcanization.
- Post-vulcanization treatment: Post-vulcanize the product to eliminate internal stress and reduce the risk of frosting.
- Surface treatment: Coating or plasma treatment of the product surface to reduce surface energy and reduce the migration of compounding agents.
3). Storage and use environment control
- Control temperature and humidity: Store the product in an environment with a temperature of 20-30 and a humidity below 50%.
- Avoid light: Use light-shielding packaging or store in a light-proof environment.
- Turn over regularly: Turn over products that have been stored for a long time regularly to avoid local frosting.
6. Conclusion
1). Recap of Key Points
Rubber product frosting and whitening result from multiple factors. Solubility balance disruption, migration and diffusion, surface energy effects, and aging reactions all contribute to the emergence of this issue. Frosting can be classified into various types, including sulfur, accelerator, antioxidant, filler, softener, and aging product bloom, each with distinct characteristics.
To diagnose the root cause, several test and analysis methods are available, such as FTIR, TGA, SEM-EDS, solubility tests, and migration rate tests. When rubber products exhibit frosting, temporary measures like physical wiping, chemical treatment, and secondary vulcanization can be employed. However, long-term solutions are essential for sustainable improvement. These involve optimizing formula design, improving the production process, and controlling the storage and use environment. By addressing these aspects comprehensively, manufacturers can enhance product quality and reduce the occurrence of frosting and whitening.
2). Future Outlook and Research Directions
Future research in reducing or eliminating rubber product frosting and whitening could focus on several areas. Developing new materials with enhanced compatibility between compounding agents and rubber matrices could be a promising avenue. For example, novel polymers or additives that are less prone to migration and have better solubility characteristics could be explored.
Nanotechnology may also play a significant role. Nanoparticle-based fillers or surface treatments could potentially modify the rubber's surface and internal structure at a microscopic level, reducing the likelihood of compounding agent migration. Additionally, research into smart materials that can self-regulate the migration of additives in response to environmental changes could be an exciting area of exploration.
Futhermore, advanced simulation techniques could be developed to predict frosting behavior more accurately during the product design phase. This would enable manufacturers to optimize formulations and processes before production, saving time and resources. By investing in these research areas, the rubber industry can move closer to eliminating the persistent problem of frosting and whitening, improving the quality and durability of rubber products.
Frosting is a common quality problem in the production and use of rubber products, which requires comprehensive control from the aspects of formula design, production process, storage environment, etc. By optimizing formula design, strictly controlling production process, improving storage environment, and selecting suitable rubber varieties, the problem of frosting of rubber products can be effectively prevented and solved, and product quality and service life can be improved.
