Blooming, also known as blooming, is a common quality problem during rubber processing. It refers to the phenomenon that the compounding agents contained in unvulcanized rubber or vulcanized rubber migrate to the surface and precipitate. Sometimes, this spray appears as frost-like crystals, so it is customarily called "spray frost". The most common blooming substance is sulfur, because sulfur is the most widely used vulcanizing agent in general rubber, and its solubility in rubber is low, so blooming is easy to occur. In fact, judging from the appearance of the sprayed materials, they may not all be in the form of frost. There are also substances sprayed out in the form of oil (softeners, plasticizers) or powdery particles (mostly fillers, antioxidants, accelerators, etc.), and even carbon. Black spurts are also seen.
Cause analysis
1.1 Formula design
1.1.1 Sulfur spray
Sulfur powder is the most commonly used vulcanizing agent for rubber products. It is also an easy-to-spray compounding agent. The solubility of sulfur in different rubbers is different. The solubility in CR, BR, SBR and NR is higher, while the solubility in saturated rubbers such as EPDM, IIR and CIIR is low.
When the amount of sulfur is less than the solubility in the glue, the sulfur is in a stable dissolved state and will not migrate and emit. When the amount of sulfur is greater than the saturated solubility, which is supersaturated, the sulfur is in an unstable dissolution state in the rubber compound. At this time, the temperature decrease will cause the sulfur crystals to precipitate and migrate to the rubber surface to form sulfur spray. The solubility of sulfur in rubber is affected by its crystal form. Ordinary sulfur has an S8 ring structure at room temperature and is an orthorhombic α-sulfur crystal. It is soluble in rubber. When heated to 96°C and then cooled, it becomes β. - Sulfur crystals, whose solubility in rubber decreases. When β-sulfur crystals are heated to 120°C, they turn into liquid γ-sulfur. When γ-sulfur is heated to boiling and cooled rapidly, they turn into elastic sulfur, which is insoluble sulfur. Insoluble sulfur has the characteristics of not dissolving, not crystallizing, and not migrating in rubber, so it does not migrate and emit. Even so, insoluble sulfur itself is still a metastable substance, and the processing temperature still needs to be reduced as much as possible. If heated at 100 to 130°C for a long time, the insoluble sulfur can undergo crystal transformation until it is transformed into α-sulfur crystals, that is, soluble sulfur. Therefore, even if insoluble sulfur is used, attention must be paid to the entire heating history during rubber processing. In addition, amine accelerators, vulcanizing agents DTDM and alkali will promote the conversion of insoluble sulfur into soluble sulfur, so attention should also be paid to the formula design. The relative molecular mass of the sulfur-containing molecule in the newly developed addition polymerization vulcanizing agent PAS-80 is only a few hundred. It has a short chain structure and good stability. In particular, it has the characteristics of a hydrocarbon chemical bond end-capped structure and is not susceptible to thermal excitation or It is broken by the attack of external free radicals, but at the vulcanization temperature, active sulfur is easily released, and an effective vulcanization reaction occurs [1].
In order to speed up the vulcanization speed in formula design, some super-fast accelerators are often used, such as thiuram-type TMTD, which can also decompose and release active sulfur at the vulcanization temperature, so it is also called a sulfur donor. In addition to thiurams, there are also new sulfur donor vulcanizing agents such as DTDM and DTDC, which can also release active sulfur at vulcanization temperature, thereby replacing part of the sulfur. The amount of released active sulfur is related to its effective sulfur content, specific formula and vulcanization reaction. Since the use of sulfur donors as vulcanizing agents or accelerators participates in the cross-linking reaction, it can offset part of the sulfur. Therefore, when designing the formula, not only must be considered The amount of sulfur, or the amount of insoluble sulfur, the effective content of sulfur in the insoluble sulfur, and the effective sulfur content in the sulfur donor, that is, the total sulfur content, must also be considered. When using sulfur donors, the amount of sulfur should be reduced accordingly.
1.1.2 Spraying of accelerator and antioxidant
Since the polarity and structure of accelerator and antioxidant molecules are quite different from those of rubber molecules, accelerators and antioxidants The compatibility between the agent and rubber is poor. When its dosage exceeds its solubility in rubber or temperature changes, blooming will easily occur. The accelerator has high solubility in rubbers with high polarity and low saturation, such as CR and SBR, and low solubility in rubbers with low polarity and high saturation, such as IIR and EPDM. When using only one accelerator and antioxidant, in order to achieve the desired effect, the dosage of accelerator and antioxidant must be large, and the spraying phenomenon is prone to occur [2]. In addition, the structure of the accelerator also has an important impact on its migration in rubber. The closer the accelerator is to the rubber molecular structure, the closer the solubility parameters will be, the better the compatibility, and the higher the solubility, but the resistance to migration in rubber will be The smaller it is too, the easier it is to migrate. Some accelerators contain different functional groups, such as anti-scorch groups, promotion groups, active groups, vulcanization groups, etc., each with different functions. For example, the thiuram accelerator DPTT (bispentamethylthiuram disulfide), compared with TMTD (tetramethylthiuram disulfide), its active group piperidinyl group has more space than the dimethylamino group. The steric hindrance is much greater, so the activity effect is poor and the reaction speed is slow, but it also increases the migration resistance and greatly improves the bloom resistance. Anti-aging agent 4010 does not have side groups, antioxidant D does have side groups, and antioxidant MB has a large diameter, so antioxidant 4010 is easier to migrate and eject than antioxidants D and MB [3].
Therefore, when designing the formula, it is best to use a combination of accelerators and antioxidants. The minimum dosage of each accelerator and antioxidant is determined through theoretical calculations, so that the dosage is controlled within the solubility range and meets the physical and mechanical properties of the rubber. If the processing technology requires it, the selection and dosage of the accelerator and antioxidant should be determined based on their structure and mechanism of action. Including vulcanization speed, vulcanization flatness, cross-linking density, formed molecular network and blooming, etc., should be comprehensively considered. For example, when replacing the accelerator TMTD with DPTT, in addition to taking into account the differences in migration and blooming properties in rubber, the differences in vulcanization speed and vulcanization flatness between the two should also be considered. In order to maintain the same cross-linking density and similar The performance of vulcanized rubber also needs to consider the difference in effective sulfur content, which can be converted into the same total sulfur content through conversion.
1.1.3 Filler ejection
The fillers mainly include white carbon black, calcium carbonate, magnesium carbonate, talc powder, clay, mica powder, graphite powder, aluminum hydroxide powder, wood Su et al. Its shape and physical properties are completely different from rubber, and its compatibility with rubber is poor. When the amount of filler is large, the rubber network will have poor binding ability and may be ejected from the surface of the product. The microstructure of fillers includes needle-like, spherical and flake-like. Mica powder and graphite powder themselves are flake-like. Talcum powder, aluminum hydroxide powder, calcium carbonate, etc. are also available in flake-like form. The smaller the diameter and the larger the diameter-to-thickness ratio of the filler with a flaky structure, the longer the migration distance of the easy-to-bloom compounding agent through its surface, the greater the resistance and the slower the migration and ejection speed. Compared with needle-shaped and spherical fillers of the same type. Fillers, flake fillers make it more difficult for other ingredients to migrate and eject. Moreover, the resistance to migration and ejection is also large, which is beneficial to improving the anti-ejection effect, and the dosage can be increased appropriately. However, flaky fillers will increase the shear strain energy consumption of the material under dynamic use conditions, increase the strain heat generation of the product under dynamic stress, and accelerate the aging of rubber products. Using surface treatment agents such as coupling agents to modify inorganic fillers allows the filler particles to chemically combine with rubber molecules and form a network structure, which can reduce filler migration [2].
1.1.4 Spraying of softeners, plasticizers, and lubricants
Softeners and plasticizers are distributed between rubber macromolecules to reduce the force between rubber molecules. , when the solubility parameters of these compounding agents are close to those of rubber and their compatibility is good, the dosage can be increased appropriately and they will not spray out. Generally speaking, those with large molecular weight, high viscosity, good compatibility with rubber, and those with large steric hindrance in the molecular structure are not easy to spray out, such as pine tar, asphalt, liquid coumaron resin, etc. Polymer plasticizers with low relative molecular weight generally do not spray when used in polymer rubbers that have the same molecular structure and contain more structural units that are the same or similar to its molecular structure. For example, liquid chloroprene is used in CR, and liquid chloroprene is used in CR. Polyisoprene is used for NR and IR, liquid polyisobutylene is used for IIR and CIIR, and liquid nitrile is used for NBR. Low molecular weight polyethylene (also known as polyethylene wax) and chlorinated polyethylene (also known as chlorinated paraffin or chlorinated paraffin oil) are not easy to spray out when used in EPDM and EPM.
1.2 Process and other factors
1.2.1 Rubber mixing and other processing techniques
Rubber mixing and other processing techniques should be carried out strictly in accordance with the requirements of the process regulations. Rubber plasticity If it is too low, the compounding agent will be dispersed unevenly. Appropriate control of plasticity can improve the solubility and adsorption capacity of the compounding agent, which is beneficial to the dispersion of the compounding agent in the rubber material and reduces the probability of blooming. During the mixing process, materials are added successively and turned over according to the prescribed procedures of the process. The rubber compound is parked after mixing to allow natural migration, dispersion and stress relaxation process for the compounding agents, and then heat refining, calendering and extrusion are carried out. As well as molding and vulcanization, it can improve the dispersion of compounding agents and reduce the internal stress of the rubber material, which can reduce blooming.
1.2.2 Vulcanization process
The solubility of the easy-to-bloom compounding agent in the rubber varies with the degree of vulcanization of the product. Generally, when the product reaches normal vulcanization, the compounding agent reaches Maximum solubility, this is because of the rationally designed formula, there is no supersaturation of the compounding agent, and the cross-linking during the vulcanization process forms chemical bonds (C—Sx—C, C—S—C, C—C, C—O—C etc.), strengthens the chemical or physical bonding process between the compounding agent and the raw rubber molecules and between the compounding agents, which is beneficial to the dissolution of the compounding agent in the rubber. First, the three-dimensional cross-linked network formed by normal vulcanization has a high cross-link density and solid The locking ability is strong, and the resistance to shuttle movement of compounding agents in the three-dimensional network increases. However, the three-dimensional cross-linked network formed when sulfur is lacking has low cross-link density and sparse network, and the locking ability of compounding agents is reduced, and the shuttle movement of compounding agents in the three-dimensional network resistance is reduced. For example, when a hard rubber is formed with a very high cross-linking density, the amount of sulfur exceeds 40 parts by mass, and the accelerator TMTD exceeds 4 parts by mass. Since the network is extremely dense and the mesh is extremely small, the resistance is extremely high, and the sulfur and accelerator will not eject; Secondly, during normal vulcanization, the compounding agent participates in the formation of chemical bonds or other side reactions, properly consuming the original compounding agent, and efficiently forming an ideal cross-linked network. For example, the sulfur contained in sulfur and sulfur donors is relatively sufficient. It is converted into cross-linked bound sulfur, reducing free sulfur, making effective and full use of the compounding agent, reducing the content of unreacted compounding agent, and reducing the concentration of the compounding agent, so that the product maintains good spray resistance. Frost performance. Although the dosage of various ingredients in the designed formula is reasonable, if the product is deficient in sulfur, the ingredients that should be fully reacted will not be fully reacted and will remain in their original state, resulting in oversaturation of the ingredients and blooming.
If the product is over-sulfurized, causing vulcanization to return to its original state, cross-linking bonds are broken, and the vulcanization network is damaged. Like the aging effect, it also destroys the chemical or physical combination between various compounding agents, raw rubber molecules, and compounding agents in the rubber system. The solubility of the compounding agent in the rubber system is reduced, and those compounding agents that are partially in a supersaturated state will migrate and precipitate from the rubber, forming bloom.
1.2.3 Effect of temperature changes
The solubility of a substance refers to the amount of solute that 100 grams of solvent can dissolve under a certain temperature and pressure. When the temperature and pressure change, the solubility will change. Change, rubber products are generally used under normal pressure, and the impact of pressure can be ignored. However, the temperature difference from processing temperature to use temperature is large, and the impact of temperature on performance is also large. When the temperature is high, the solubility of the compounding agent is large, and when the temperature is low, the solubility of the compounding agent decreases. The processing temperature of rubber is generally higher than room temperature. At this time, the solubility of the compounding agent is high and generally in a dissolved state. However, when it is stored and used at room temperature or lower external environment temperature, the solubility decreases, causing migration and precipitation, forming Spray frost. Therefore, when designing the rubber formula, the use temperature conditions of the product must be considered, and the design must be combined with the solubility of the compounding agent under the conditions of storage, transportation and use of the product.
1.2.4 Effect of Aging
The aging of rubber products is that the three-dimensional network structure of the vulcanized mesh is partially damaged due to bond breakage, thereby weakening the network's adsorption and locking of the compounding agent. Ability to cause migration and precipitation of compounding agents, resulting in blooming.
1.2.5 Stress concentration
Stress concentration is also an aging phenomenon. When rubber products are acted upon by external forces, it is easy to cause stress concentration and cause surface rupture, causing the original supersaturated state. The compounding agent precipitates on the surface of the crack and spreads to the surroundings.
2 Preventive measures
In view of the above reasons for blooming of rubber products, firstly avoid them from the perspective of formula design, secondly take measures from the perspective of process, and then consider packaging, storage, Conditions such as transportation and use must be avoided. To sum up, the following measures can be taken:
(1) Use rubber together, especially rubber types that have good compatibility with it and have high solubility of the compounding agent in it, so that the compounding agent is fully dissolved in the rubber. Unsaturated state;
(2) According to the long-term minimum temperature conditions of packaging, storage, transportation and use, the amount of easy-to-spray compounding agent should be controlled within the solubility range during formula design;
(3) Choose accelerators and antioxidants that have good compatibility with rubber and have synergistic effects. Use accelerators with large steric hindrance such as DPTT instead of accelerators with small steric hindrance such as TMTD. Pay attention to the effective sulfur content after replacement. equal. Choose to use polymeric antioxidant RD and antioxidants with large steric hindrance to replace antioxidants with small steric hindrance. Choose a low molecular weight plasticizer that has good compatibility with the main polymer rubber to replace or partially replace the small molecular plasticizer, such as using liquid polymer to replace general oils. Use insoluble sulfur or polymerized sulfur to completely or partially replace ordinary sulfur; use fillers with a flaky microstructure, small particle size, and large diameter-thickness ratio to completely or partially replace fillers with large particle sizes and spherical or needle-like microstructure;
(4) Strictly control the rubber mixing process. The dispersion of the easy-to-bloom compounding agent can be enhanced by pre-preparing the masterbatch of the easy-to-blooming compounding agent. If rubber and the easy-to-blooming compounding agent are used together, the easy-to-blooming compounding agent should be added in advance to a location with relatively low compatibility. Prepare the masterbatch from the poor rubber, and then mix it with another component to prepare the mixed rubber compound. The mixed rubber compound should be passed through with a small roller pitch and crossed with triangular bales and flat bales, and in Before refining, the mixed rubber material should be parked for more than 8 hours to ensure the diffusion, migration and even distribution of the compound in the rubber material, and to relax the internal stress of the rubber material and improve the bonding and adsorption with the rubber;
(5) Ensure that vulcanization conditions are controlled in the normal vulcanization state;
(6) Prevent semi-finished products and finished products from unnecessary concentrated stress during production, transportation, and storage, and prevent possible damage to finished products The influence of aging factors.