What is the reason of the blooming, the occurrence of the blooming cream and its preventive measures?
Spray blooming, also known as spraying, is the phenomenon that the complex (solid or liquid) contained in the interior of the unvulcanized rubber or vulcanizate is precipitated by the migration of the surface. Frosting is a common quality problem in rubber production.
Analysis of causes: 1 The solubility of various additives in rubber is excessively varied. The smaller the solubility of the additives in the rubber is, the more likely it is that excess additives (ie, the content of additives in rubber exceed its solubility in rubber). Causes blooming. When spraying with excessive amounts of cream, the other components are often sprayed together (this phenomenon is called passive blooming), although these passive blooming materials are far from saturated in rubber.
2 Temperature change The solubility of additives in rubber varies with temperature. Under normal circumstances, the solubility is high when the temperature is high, and the solubility is decreased when the temperature is decreased. Since rubber products are usually used at room temperature, once the outside temperature is lower than room temperature, the content of some additives in the formulation will be close to the solubility and precipitated, resulting in blooming. For example, the rubber shoes produced in summer are qualified at the time of factory inspection, but they are found to be blooming in the winter.
3 The solubility of the under-sulfur additive in rubber is affected by the curing conditions. Taking NR as an example, under the conditions of the vulcanization, the crosslink density is the largest, the free sulfur is reduced, the sulfur injection probability is reduced, and the opportunities for other additives to shuttle to the three-dimensional network are also reduced, thus reducing the probability of blooming; otherwise, in the state of sulfur Below, the network cross-link density is relatively small and the probability of blooming increases accordingly.
4 Aging aging means that the three-dimensional network structure of the vulcanizate is partially damaged due to bond breakage, thereby weakening the adsorption of the network structure and fixing the auxiliary additives. The ability of additives to migrate to the surface leads to blooming.
5 When the unevenly stressed rubber is subjected to an external force, it often causes stress concentration and breaks the surface, accelerating the original superabsorbent particles of the auxiliary agent, and forming a spray on the surface of the crack. Frost and extending to the periphery"
6 Uneven kneading of the kneading results in uneven dispersion of the compounding agent in the rubber, and there is a localized additive that exceeds the solubility and causes blooming.) Precautionary Measures
(1) Doping with SR: SR The solubility of the auxiliaries is higher than that of NR, so the use of some SRs helps prevent NR blooming.
(2) Accelerators and anti-aging agents are used together: if only one additive is used, the amount is too small to achieve the effect, and if the amount is too large, the cream is liable to appear. Therefore, several auxiliary agents can be used together to achieve the effect and reduce the amount.
(3) The use of different additives in the spray on the mutual interference constraints: When used together with different additives, sometimes there will be mutual interference and help inhibit the blooming, such as softener, ointment, reclaimed rubber, etc. have this function. In particular, auxiliaries with large relative molecular masses can penetrate into the short chains of rubber macromolecules and can effectively adsorb the easy-ejection aids.
(4) Grasp the upper limit of the amount of easy-to-spray auxiliaries, and ensure that the amount of various spray auxiliaries in the formulation is within its solubility range.
(5) Mixing rubber parking: The rubber material must be parked for not less than 8 hours before the molding out, so that all kinds of compounding agent can be fully dispersed, and at the same time, it helps the internal stress of the rubber material to relax, and the force is even, balanced and stable. The dispersion can inhibit the spray of particles.
(6) Ensure that the vulcanizates are in the state of vulcanization
(7) Strengthen the management of finished products, packaging, and storage conditions. The finished products should be sealed in plastic bags to prevent aging during contact with sunlight, oxygen, etc. during storage. Avoid storage at low temperatures. Causes of blooming of various substances: Sulfur yellow spray: Excessive amount of sulfur in rubber compound Insoluble sulfur in poor quality Sulfenamide accelerator poor quality Mixing or processing temperature is too high Reaction products: Excessive amount of anti-aging agent and its reaction product in the rubber compound: Excessive amount of parts in the rubber compound is stored for a long time Anti-ozone agent and its reaction product: Excessive quantities in the rubber compound The storage time is too long Fatty acid: Too much zinc stearate in rubber: Stearic acid and zinc oxide are added early and the mixing or processing temperature is higher. Calcium stearate: Free stearic acid in the sizing mix with cooling water tank Hard water reaction wax (components from the formulation or oil): Excessive mass fraction of wax in oil Excessively high reaction product of anti-tar agent: Delayed action due to excessive mixing or processing temperatures Advance Resin: Parts are stored too long. Adhesive: Parts are too long to store