What is the mixing process?

The mixing of rubber is a process in which various compounding ingredients are uniformly dispersed in rubber by the mechanical force of rubber mixer, and a multiphase colloid dispersion system with compounding ingredients (such as powder filler, zinc oxide, pigment, etc.) is formed with rubber as the medium or a mixture of rubber and some compatible compounding ingredients (compounding ingredients and other polymers) as the medium. ) is incompatible with rubber as dispersed phase. The specific technical requirements of the mixing process are: the compounding agent is evenly dispersed, so that the compounding agent, especially the reinforcing compounding agent such as carbon black, reaches the best dispersion degree to ensure the consistency of rubber properties. The compound obtained after mixing is called "mixed compound", and its quality has an important influence on further processing and product quality.

1 natural rubber mixture

Natural rubber is the main type of raw rubber plasticization, and both open mill and internal mixer can obtain good results. When plasticating with an open mill, the low-temperature (40 ~ 50℃) thin pass (roll distance 0.5 ~ 1 mm) plasticating method and segmented plasticating method are usually used for the best results. When plasticating with an internal mixer, the temperature should be lower than 65438 055℃ and the time should be about 65438 03 minutes. With the increase of plasticizing time, the plasticity of plasticized rubber increases. But don't overdo it, otherwise the plasticity will become too high and the physical and mechanical properties will decrease.

Accelerator M is often added as a plasticizing agent to improve the plasticizing effect of natural rubber. Accelerator M is suitable for open mill plasticization and internal mixer plasticization. After plasticization of natural rubber, in order to make the rubber molecular chain relax (commonly known as fatigue recovery) and have uniform plasticity, it needs to be parked for a certain time (4 ~ 8 h) before it can be used in the next process. At present, the natural rubber used in China mainly includes domestic tobacco flake rubber and standard rubber, imported tobacco flake rubber and Malaysian standard rubber.

Because the initial Mooney viscosity of the above rubber is different, the plasticizing time required to obtain the same plasticity is of course different. Plasticizing time sequence is: imported cigarette gum > domestic cigarette gum >: domestic standard gum > Malaysian standard gum. Constant viscosity and low viscosity standard Malaysian rubber, oil-filled natural rubber, tire rubber and easy-to-handle rubber have low initial Mooney viscosity (generally less than 65) and can be directly mixed without plasticization.

Natural rubber compound (NR) has good comprehensive physical and mechanical properties, high elasticity at room temperature, and the rebound rate can reach above 50% ~ 85%. Natural rubber also has high mechanical strength, good bending fatigue resistance, small hysteresis loss, low heat generation under repeated deformation, high tear strength, good wear resistance and cold resistance, as well as good air tightness, water resistance, electrical insulation and heat insulation. Natural rubber has good alkali resistance and general acid resistance, but it is not resistant to strong acid, and also has good processability. Natural rubber has good mixing performance, good winding performance, high raw rubber strength and initial viscosity, good plasticity, compatibility with compounding agent and wettability, fast plasticization speed of rubber compound and low heat generation. Therefore, it has good wettability to the compounding agent, fast powder eating and easy dispersion; The mixing time is short, and the mixing operation process is easy to master. However, natural rubber is sensitive to the mixing time. If the mixing time is too short, particles will appear on the rubber surface, making it difficult to calender and extrude. If the mixing time is too long, it will lead to over-thinning. Mixing in open mill, the roller temperature is between 45 ~ 55℃, and the front roller is 5℃ higher than the back roller. The internal mixer usually adopts the first-stage mixing mode, and the glue outlet temperature is below 120℃.

2. Mixing of styrene-butadiene rubber

Styrene butadiene rubber (SBR) is also called polystyrene butadiene polymer. Its physical and mechanical properties, processing properties and product properties are close to those of natural rubber, and some properties, such as wear resistance, heat resistance, aging resistance and vulcanization speed, are superior to those of natural rubber, so it can be used in conjunction with natural rubber and various synthetic rubbers.

When mixing SBR, the compounding agent is not easy to disperse, and the power consumption and calorific value are high, and the quality of rubber compound is greatly affected by temperature. Therefore, when making SBR compound, the mixing conditions should be stipulated according to its characteristics. When mixing in an open mill, a more suitable method is to make the carbon black in the formula into a masterbatch and directly add it to mix with the rubber compound of styrene-butadiene rubber. Before adding the compounding agent, styrene-butadiene rubber and natural rubber should be mixed evenly before adding. Because the wettability of SBR to compounding agent is worse than that of natural rubber, the mixing time is longer than that of natural rubber. Take the 22 in (55.9 cm) open mill for mixing tread rubber as an example. If carbon black masterbatch is used, the mixing time is 25 ~ 30 min. When there is no carbon black masterbatch, the mixing time can be appropriately extended. The roller temperature should not be too high, generally, the front roller is below 55℃ and the rear roller is below 50℃. The temperature of the compound is about 55 ~ 60℃. The selection of capacity depends on the amount of SBR. When 30% SBR is mixed, its capacity is the same as that of natural rubber. When 50% SBR is added, its capacity can be reduced appropriately.

Styrene-butadiene rubber (SBR) produces more heat and heats up quickly during mixing, so the mixing temperature should be lower than that of natural rubber. The compounding agent is difficult to mix and disperse in SBR, so the mixing time is longer than that of natural rubber. It is better to wrap the roll with an open mill, but it is easy to wrap the cold roll, so the temperature of the front roll should be 5 ~ 10℃ lower than that of the back roll. It is necessary to increase the number of thin passes and carry out supplementary processing to promote the uniform dispersion of the ingredients. Two-stage mixing should be used for mixing with internal mixer, and the capacity should be smaller. If the stirring time is too long, the plasticity changes little, but it will produce gel, which will affect the physical and mechanical properties. The glue outlet temperature should be lower than 130℃.

3. Mixing of cis-polybutadiene rubber

Cis-1, 4- polybutadiene rubber has low cohesive strength and weak self-adhesion. During the mixing process, the raw rubber is broken, and the dispersion of the compounding agent is poor and it is easy to fall off. Cis-1,4-polybutadiene rubber is not easy to be crushed into pieces when mixed on an open mill, and it is easy to roll off. Therefore, it is advisable to use smaller roll moment and lower roll temperature (40 ~ 50℃) for mixing. In order to make the ingredients evenly dispersed, supplementary processing is needed. When mixed by internal mixer, the capacity can be increased by 10%, and the mixing temperature can be slightly higher, which is beneficial to the dispersion of compounding ingredients. Generally, the glue-out temperature is 130 ~ 140℃. Two-stage mixing is beneficial to uniform dispersion, and reverse mixing can also be used. This can save 40% of the rubber mixing time. When the carbon black content is high or carbon black with high structure and fine particles is used, it must be mixed in two stages to disperse evenly, or it can be mixed by reverse mixing method.

The technical properties of cis-polybutadiene rubber are quite different from those of other synthetic rubbers. In the general processing temperature range (50 ~ 140℃), there is basically no mechanochemical degradation and thermal oxidation degradation. When mixed with carbon black, it will form a hard structure, and the adhesive strength of raw rubber and mixed rubber is low, and the adhesion and self-adhesion are weak. In the process of mixing, raw rubber is broken, and the compounding agent has poor dispersibility and is easy to fall off. Generally, it needs to be mixed with natural rubber and styrene-butadiene rubber. Different kinds of cis-polybutadiene rubber have different mixing characteristics, and high cis-polybutadiene rubber has better mixing characteristics, including better winding and dispersibility. The roller temperature should be lower than that of natural rubber, and 40 ~ 50℃ is ideal. Cis requires a lower temperature. Low cis-cis-1, 4- polybutadiene rubber has good processability because it does not crystallize. The sulfur content of cis-polybutadiene rubber is lower than that of other diene rubbers. This is because sulfur migrates quickly in cis-1, 4- polybutadiene rubber, which is prone to frosting. Butadiene rubber should be mixed mainly in two stages. Two-stage mixing can obviously improve the technological properties of rubber compound and the physical and mechanical properties of vulcanized rubber. When one-stage mixing is used, the plasticity of conveyor belt covering adhesive is 0.33; When two-stage mixing is used, it is 0.38, and when the plasticity is 0.2 ~ 0.25, the rolling phenomenon will occur. The tensile strength of vulcanizate prepared by two-step method was also improved. Softening agent should be added after carbon black dispersion, which can improve the tensile strength and tensile stress of vulcanizate. The properties of rubber compound and vulcanized rubber can not be improved obviously by mixing polybutadiene rubber and styrene-butadiene rubber in an internal mixer and then adding other additives. If raw rubber and carbon black are added at the same time, and then softener is added, the tensile stress and tensile strength can be improved.

4. Mixing of butyl rubber

When butyl rubber is used to produce mixed rubber, the feeding method (that is, the raw rubber and compounding agent are added after the rubber is wrapped) and the thin pass method (that is, half of the raw rubber in the formula is repeatedly thinned with cold rolls and small rolls, and the other half is added after the raw rubber is wrapped) are generally adopted. The temperature of the compound is generally controlled at 40 ~ 60℃ (the temperature of the front roller should be lower than that of the back roller10 ~15℃); The speed ratio can't exceed 1: 1.25, otherwise the air will be easily involved in the rubber compound, resulting in foaming of the product. The batch should be added in small quantities in batches, and can only be cut after the batch is completely eaten and mixed. When mixing butyl rubber with internal mixer, one-stage mixing, two-stage mixing and reverse mixing can be used. The filling amount is slightly larger than that of natural rubber (5% ~10%); In the process of mixing, reinforcing filler should be added as soon as possible to produce maximum shear force and better mixing effect; The mixing time is 30% ~ 50% longer than that of natural rubber; The first-stage mixing temperature is usually controlled below 65,438 0,265,438 0℃, and the second-stage mixing temperature is usually controlled around 65,438 0.55℃. High-filled rubber compound is easy to disperse when mixing in internal mixer, that is, granulation. The treatment method is to increase the filling quantity or adopt the back mixing method. In order to improve the mixing effect of butyl rubber and increase the proportion of bonded rubber, the rubber compound can be "heat treated", that is, 1.0 ~ 1.5 parts of "heat treated" agent (such as p-dinitrobenzene) is added to butyl rubber to make it uniform, and then high temperature treatment is carried out. The "heat treatment" of butyl rubber compound can be divided into dynamic and static. The former is mixed with the first stage of internal mixer at the same time, and the process temperature is 120 ~ 200℃. The latter is directly placed in steam or hot air for 2 ~ 4 h.

Butyl rubber has low adhesive strength, poor self-adhesion and poor wrapping property. In addition, it has high saturation and is technically incompatible with other rubbers. When there is no special mixing equipment, the table must be thoroughly cleaned before and after mixing to avoid mixing with other raw rubber and affecting the quality of rubber. When butyl rubber is mixed, the compounding agent is difficult to disperse. When mixing in an open mill, the feeding method (that is, raw rubber and compounding agent are added after the feed rubber is wound around the roller) or the dilution method is adopted. Half of the raw rubber in the formula is repeatedly diluted with a small rolling moment, and the other half is added after the roller is wound. When mixed by internal mixer, the rubber loading is 10% ~ 20% higher than that of natural rubber, and the mixing temperature is 150℃. When there is too much filler, two-stage mixing method and reverse mixing method can also be used. Butyl rubber is easy to wrap the cold roll, so the temperature of the back roll should be about 10℃ higher than that of the front roll. Butyl rubber flows cold, and the compounding agent is difficult to disperse and roll. When rubber products are mixed in an open mill, the feeding method (that is, a small piece of the same rubber from the previous rubber mixing is rolled on a roller, and then butyl rubber and compounding agent are added) or the thinning method (that is, half of the raw rubber in the formula is repeatedly diluted with a small roller distance, and the other half is added after rolling) can be adopted.

5. Mixing of chloroprene rubber

Neoprene rubber is produced by emulsion polymerization, and the production process is mostly single-pot batch polymerization. The polymerization temperature is mostly controlled at 40 ~ 60℃, and the conversion rate is about 90%. Too high polymerization temperature, too high final conversion rate or air entering during polymerization will lead to the decline of product quality. In production, sulfur-Qiu Lan system is used to adjust the relative molecular weight. The main disadvantage of sulfur-Qiu Lan system is that sulfur bond is not stable enough, which is one of the important reasons that affect storage. This property can be improved if the relative molecular weight is adjusted by mercaptan. Different from general synthetic rubber, chloroprene rubber is vulcanized with zinc oxide and magnesium oxide instead of sulfur.

The processability of chloroprene rubber depends on the viscoelastic behavior of unvulcanized rubber, which is related to the variety and temperature of chloroprene rubber. Because mixing is generally carried out in an elastic state, the filler can be well dispersed by using the shear force of the elastic state of the rubber compound. Therefore, in order to avoid the influence of high temperature, fillers should be added as soon as possible to achieve a certain degree of mixing in an elastic state. G-type chloroprene rubber is very sensitive to temperature changes when mixing with an open mill. When the temperature of the roller exceeds 70℃, it will stick to the roller seriously and be in a sticky state, so the compounding agent is not easy to disperse. When mixing with an internal mixer, its capacity should be reduced appropriately, and the general filling coefficient is 0.6. Generally speaking, it is mixed in two stages to minimize the mixing temperature. The glue outlet temperature should be lower than 100℃.

The disadvantages of mixing chloroprene rubber in open mill are high calorific value, easy to stick to rollers, easy to scorch and slow dispersion of additives, so the mixing temperature should not be too high, the capacity should be small, and the speed ratio of rollers should not be large. Due to its strong sensitivity to temperature, general-purpose chloroprene rubber will be granular at room temperature to 765438 0℃, at which time the cohesion of raw rubber will be weakened, which will not only seriously stick to the roller, but also make the compounding agent difficult to disperse. The elastic state temperature of non-sulfur chloroprene rubber is below 79℃, so the mixing process performance is better than that of sulfur chloroprene rubber, and the tendency of roller sticking and scorching is small. When mixing with an open mill, the roller temperature is generally controlled below 40 ~ 50℃ (the temperature of the front roller is 5 ~ 10℃ lower than that of the back roller) to avoid roller sticking. When kneading raw rubber, the roller distance should be gradually adjusted from large to small. When stirring, first add acid absorbent magnesium oxide to prevent scorching, and finally add zinc oxide. In order to reduce the heat of mixing, carbon black and liquid softener can be added alternately in batches. Operating AIDS such as stearic acid and paraffin wax can be added gradually in dispersion, which can not only help dispersion, but also prevent roller sticking. In general, the mixing time of sulfur-modified chloroprene rubber in open mill is 30%~50% longer than that of natural rubber, and the mixing time of non-sulfur-modified chloroprene rubber is about 20% shorter than that of sulfur-modified chloroprene rubber. In order to avoid the rapid temperature rise of chloroprene rubber in the mixing process of open mill, the cooling effect will be better when the speed ratio is less than 1∶ 1.2. Reducing rubber mixing capacity is also a method to ensure safe operation and good dispersion. At present, the rubber refining capacity of sulfur-modified chloroprene rubber in China is 20% ~ 30% less than that of natural rubber in order to operate normally. Because chloroprene rubber is easy to burn, two-stage mixing method is usually used when mixing with internal mixer. The mixing temperature should be lower (the discharge temperature is generally controlled below 100℃), and the filling capacity is lower than that of natural rubber (the capacity coefficient is generally 0.50 ~ 0.55). Zinc oxide should be added to the tablet press in the second stage of mixing. In order to solve the problem that chloroprene rubber is easy to scorch and difficult to disperse in the mixing process, Lina mixer adopts the most advanced four-rhombic double rotors in the same direction at present, and the top bolt of the mixer moves in an "X" curve, which has good dispersion effect and short time, and can effectively reduce the scorch phenomenon of chloroprene rubber in the mixing process.

6. Mixing of ethylene-propylene rubber

Ethylene propylene rubber can also be processed by ordinary rubber mixing equipment. However, due to the poor plasticizing effect of ethylene-propylene rubber and lack of viscosity, rubber is not easy to wrap. Generally, a narrow roll gap is adopted to form a continuous sheet, and then the roll gap is relaxed for mixing processing. The temperature of the front roller is 50 ~ 60℃, and the temperature of the rear roller is 60 ~ 70℃. The feeding sequence of EPDM is generally as follows: raw rubber wrapping roller-1/2 carbon black-1/2 carbon black-stearic acid-zinc oxide (or magnesium oxide)-accelerator-crosslinking agent-primary and lower sheet. Ethylene-propylene rubber is not easy to over-refine when mixing, and the compounding agent is evenly dispersed, but its self-viscosity is poor. Ethylene propylene rubber is mixed with an open mill. Generally, it is wound continuously with a small rolling moment, and then the rolling moment is gradually relaxed and the compounding agent is added. The roller temperature is between 60 ~ 70℃. The mixer should be mixed at high temperature, and the mixing temperature is 150 ~ 160℃, which is beneficial to the dispersion of fillers and softeners and the improvement of mechanical properties. The filling capacity is higher than other compounds 10% ~ 15%.

7. Mixing of NBR

NBR has poor mixing process performance, high calorific value during mixing, easy rolling, poor wetting ability to powder compounding agent, and difficult compounding agent dispersion. The amount of carbon black is large, the rubber compound heats up quickly and is easy to scorch. When mixing NBR on the open mill, the operation method of low temperature, small capacity and small roll gap should be adopted to promote the uniform dispersion of compounding ingredients. If the mixer is used for mixing, the cooling of the mixing chamber should be strengthened, carbon black and ester softener should be added alternately in batches, and the glue outlet temperature should not be higher than 130℃. Sulfur has low solubility in NBR, so it is difficult to mix and disperse, so it should be added at the early stage of mixing. The acrylonitrile content in NBR is 42% ~ 46%, 36% ~ 4 1%, 3 1% ~ 35%, 25% ~ 30% and 18% ~ 24%. The higher the acrylonitrile content, the better the oil resistance, but the lower the cold resistance.

8. Mixing of silicone rubber

Silicone rubber can be divided into thermal vulcanization type (high temperature vulcanized silicone HTV) and room temperature vulcanization type (RTV), among which room temperature vulcanization type can be divided into polycondensation reaction type and addition reaction type.

Silicone rubber can be mixed without plasticization, and is generally operated by an open mill. During the mixing process, keep the drum temperature low (not more than 50℃) for a short time. When adding raw rubber, wrap the front (slow) roller first, and subcontract the rear roller when mixing and eating powder. It is advisable to operate on both sides. After coating, a compounding agent can be added. After mixing evenly, the rubber compound is completely coated and the surface is smooth. Silicone rubber is soft, so putty knife should be used for mixing and cutting. After mixing, the compound should be left for a certain time (> > 24 h) to facilitate the dispersion of the compound components, and it must be recovered before use. Rubber mixture should be used together with refining. If the time is too long, the performance of vulcanized rubber will be reduced.

9. Mixing of fluororubber

Fluorine rubber has high Mooney viscosity, high rigidity and high friction heat generation, so it is difficult to mix in general. When mixing fluororubber in an open mill, it is required that the roller distance is small, the capacity is small, and the roller temperature is controlled at 50 ~ 60℃. At the initial stage of mixing, the roller is cooled and raw rubber is added for about 10 times to form uniform coating rubber. Adjust the roller torque, keep a small amount of glue accumulation, and then add the compounding agent. The mixing time is usually not strictly defined, but it is required to be as fast as possible. It is difficult to mix fluororubber with an internal mixer, but the meshing internal mixer has a strong cooling system and can mix fluororubber. The mixed compound should be parked for 24 h before use, and must be recycled before use, so that the compounding agent can be dispersed evenly and the fluidity and self-adhesion of the compound can be improved.

Mixing of 10 acrylate rubber

Polyacrylate rubber has low Mooney viscosity and can be directly mixed without plasticization. The open mill is easy to stick to the rollers when mixing, and it is difficult to operate because of winding the front and rear rollers at the same time. Therefore, lower roller temperature (30 ~ 50℃) is beneficial to the dispersion of anti-sticking roller and compounding agent. The temperature of the front roller is higher because of the fast gluing roller or the high temperature roller. The roller distance is about 2 ~ 3 mm, the thin pass 1 ~ 2 times, and the compounding agent is added, and the compounding agent is well dispersed in the mixing. There is no danger of roller sticking in internal mixer, which can shorten the mixing cycle, but low speed mixing is suitable.

ACM compounds can be mixed directly without plasticization. During the mixing process, it may stick to the roller or fall off from the roller. Excessive roller temperature should be avoided, and the cooling water can be fully opened. The amount of glue should not be too high, about half of the rated amount. Press the roller gap to the minimum position, press stearic acid on the roller, and then adjust the normal roller gap. Adding raw rubber, wrapping and keeping proper amount of rubber; Adding antioxidants and other small materials; Add filler such as carbon black, and add the filler falling into the receiving tray to the roller again until it is completely mixed into the glue. After the filler is added, the glue can be cut, turned over and packaged. If it is difficult to cut off the sticky roller, you can stop and cut it off, and then you won't stick the roller. After cutting the rubber compound, dilute it by 5 ~ 10 times and mix it as evenly as possible. After the thin pass is completed, if the roller temperature and glue temperature are not too high, adjust the roller distance to a suitable position so that there is a proper amount of glue on the roller, and then add vulcanizing agent. After the vulcanizing agent is added, it is packed in a thin tube and mixed as evenly as possible. When vulcanizing agent is added, if the roller temperature is too high, the rubber compound is easy to scorch (self-vulcanizing in advance). However, if the roller temperature is too low, it is not conducive to the melting and dispersion of vulcanizing agents such as sodium stearate and potassium. Generally, the temperature should be controlled at about 40 ~ 70℃. Adding vulcanizing agent, uniformly mixing, and cooling. After storage 1 ~ 24 h, the compound can be used once. The storage period of rubber mixture containing sodium stearate and potassium stearate can be about one month. The rubber compound containing sodium and potassium stearate and 0.5 part of No.3 vulcanizing agent should be used up within 3 days, otherwise it may be burnt. When vulcanizing, the rubber compound should be formed quickly to prevent the rubber compound from vulcanizing in advance in the high-temperature mold, resulting in lack of rubber or bubbles and cracks. Especially the operation of combined die should be paid more attention to. After the mold loading is completed, the pressure can be slower to prevent the rubber from being pressed out of the mold, and it will not fill the mold cavity too quickly, resulting in lack of rubber. After pressurization, exhaust quickly 1 ~ 2 times. When the rubber mixture burns during mixing, storage or vulcanization, it will cause cracks in the overlapping part of vulcanized rubber or leave large bubbles in the middle of rubber. In severe cases, there will be flow marks on the rubber surface, and the surface will be rough and uneven, which will also affect the bonding of the skeleton. When silica is treated with silane coupling agent, silica can be added after mixing, or silane can be added into silica before mixing.

1 1 conclusion

As long as we know more about the unique properties of commonly used typical rubber, make full use of its mixing process characteristics, better solve the problem of recycling waste rubber and reduce the price of raw rubber, the application field and demand of typical rubber will achieve greater development.