Common rubber mixing processes
The mixing of rubber is to use the mechanical force of a rubber mixer to evenly disperse various additives in the rubber, forming a mixture of rubber and certain compatible components (additives, other polymers) as the medium, The process of a multiphase colloidal dispersion system using incompatible rubber additives such as powder fillers, zinc oxide, pigments, etc. as dispersion phases. The specific technical requirements for the mixing process are: the mixing agent is evenly dispersed to achieve the best dispersion of the mixing agent, especially reinforcing agents such as carbon black, to ensure consistent rubber performance. The rubber obtained after mixing is called "mixed rubber", and its quality has a significant impact on further processing and product quality.
1. Mixing of natural rubber
Natural rubber is the main type of rubber used for raw rubber molding, and good results can be achieved through both open and internal mixing machines. When using the open mill for molding, the low temperature (40-50 ℃) thin pass (roller pitch 0.5-1 mm) molding method and segmented molding method are usually used to achieve the best results. When using an internal mixer for molding, the temperature should be below 155 ℃ and the time should be about 13 minutes. As the molding time increases, the plasticity of the plastic compound also increases. But do not over refine, otherwise the plasticity will become too high and the physical and mechanical properties will decrease.
Accelerator M is often added as a plasticizer in natural rubber molding to improve the molding effect. Accelerator M is suitable for both open mixer and internal mixer molding. After natural rubber is molded, in order to achieve relaxation (commonly known as fatigue recovery) and uniform plasticity of the rubber molecular chains, it is necessary to park for a certain period of time (4-8 hours) before it can be used in the next process. At present, the main varieties of natural rubber used in China include domestic cigarette adhesive and standard adhesive, imported cigarette adhesive and Malaysian standard adhesive, etc.
Due to the different initial Mooney viscosities of the aforementioned adhesives, the required molding time to achieve the same plasticity is naturally different. The order of its molding time in terms of length is: imported tobacco adhesive>domestic tobacco adhesive>domestic standard adhesive>Malaysian standard adhesive. The initial Mooney viscosity of constant viscosity and low viscosity standard Malaysian rubber, oil extended natural rubber, tire rubber, and easy to operate rubber is relatively low (generally less than 65), and can be directly mixed without plasticizing.
Natural rubber blend (NR) has comprehensive physical and mechanical properties, high elasticity at room temperature, and a rebound rate of over 50% to 85%. Natural rubber also has high mechanical strength, good resistance to bending fatigue, small hysteresis loss, low heat generation under multiple deformations, high tear strength, good wear and cold resistance, as well as good airtightness, waterproofing, electrical insulation, and thermal insulation. Natural rubber has good alkali resistance and resistance to general acids, but it is not resistant to concentrated strong acids, and natural rubber also has good processing performance. Natural rubber has good mixing performance, good roll wrapping, high strength and initial viscosity of raw rubber, good plasticity, compatibility, and wettability to the blending agent. The plasticity of the rubber material increases quickly and the heat generation is low. Therefore, the wettability of the compounding agent is good, the powder is eaten quickly, and the dispersion is relatively easy; The mixing time is short, and the mixing operation process is easy to master. However, natural rubber is sensitive to mixing time. If the mixing time is too short, the surface of the mixed rubber will appear granular, causing difficulties in rolling and extrusion; Long mixing time can lead to over mixing. Open the mixer for mixing, with a roller temperature between 45 and 55 ℃, and the front roller being 5 ℃ higher than the rear roller. The internal mixer often uses a one-stage mixing method for mixing, with a discharge temperature below 120 ℃.
2. Mixing of styrene butadiene rubber
Styrene butadiene rubber (SBR), also known as polystyrene butadiene copolymer. Its physical structure performance, processing performance, and product usage performance are similar to natural rubber. Some properties such as wear resistance, heat resistance, aging resistance, and vulcanization speed are better than natural rubber, and can be used in combination with natural rubber and various synthetic rubbers.
When mixing styrene butadiene rubber, it is difficult to disperse the mixing agent, consume a large amount of electrical energy, generate a large amount of heat, and the quality of the rubber material is greatly affected by temperature. Therefore, when manufacturing styrene butadiene rubber material, mixing conditions should be specified according to its characteristics. When mixing in the mixer, a more suitable method is to directly add the carbon black in the formula into the master rubber, and mix the rubber material with styrene butadiene rubber. Before adding the admixture, it is required to mix the styrene butadiene rubber and natural rubber evenly before adding them. Due to the poor wettability of the blending agent of styrene butadiene rubber compared to natural rubber, the mixing time is extended compared to natural rubber. Taking the 22 inch (55.9 cm) open mill for mixing tread rubber as an example, if carbon black masterbatch rubber is used, the mixing time is 25-30 minutes. If carbon black masterbatch rubber is not used, the mixing time can be appropriately extended. The roller temperature should not be too high, generally below 55 ℃ for the front roller and below 50 ℃ for the rear roller. The temperature of the adhesive is around 55-60 ℃. The selection of capacity depends on the dosage of styrene butadiene rubber, with 30% styrene butadiene rubber added, the capacity is the same as that of natural rubber; Adding 50% styrene butadiene rubber can reduce its capacity appropriately.
When mixing styrene-butadiene rubber, it generates a lot of heat and heats up quickly, so the mixing temperature should be appropriately lower than that of natural rubber. The compounding agent is difficult to mix and disperse in styrene butadiene rubber, so the mixing time is longer than that of natural rubber. Mixing the rolls with a mixer has good performance, but it is easy to wrap the cold rolls. Therefore, the temperature of the front roll should be 5-10 ℃ lower than that of the rear roll. It is necessary to increase the number of thin passes and carry out supplementary processing to facilitate the uniform dispersion of the mixed agent. Mixing with an internal mixer should adopt two-stage mixing, with a smaller capacity. If the mixing time is too long, the plasticity changes little, but gel will be produced, affecting the physical and mechanical properties. The discharge temperature should be lower than 130 ℃.
3. Blending of polybutadiene rubber
The cohesive strength of cis-1,4-polybutadiene rubber is low, and its adhesive and self-adhesive properties are weak. During the mixing process, the raw rubber is in a fragmented state, and the dispersant of the compounding agent is poor, making it easy to roll off. Blending of cis-1,4-polybutadiene rubber on the open mill is not easy to compress into sheets and is easy to roll off. Therefore, it is advisable to use small roller moments and low roller temperatures (40-50 ℃) for mixing. In order to achieve uniform dispersion of the compounding agent, supplementary processing is required. When mixing with an internal mixer, the capacity can be increased by 10%, and the mixing temperature can also be slightly higher to facilitate the dispersion of the compounding agent. The discharge temperature is generally between 130 and 140 ℃, and two-stage mixing is beneficial for uniform dispersion. Reverse mixing method can also be used for mixing. This can save 40% of rubber mixing time. When the carbon black content is high or high structure fine particle carbon black is used, two stages of mixing must be used to disperse evenly, or reverse mixing method can be used for mixing.
The process performance of cis-1,4-polybutadiene rubber is greatly different from other synthetic rubbers. It does not undergo mechanochemical degradation or thermal oxidation degradation within the general processing temperature range (50-140 ℃), and when mixed with carbon black, it forms a hard structure. The cohesive strength of the raw rubber and rubber material is low, and the adhesion and self-adhesion are weak. During the mixing process, the raw rubber is fragmented, and poor dispersion of the compounding agent can easily cause roll off, So it generally needs to be used in combination with natural rubber and styrene butadiene rubber. Different types of polybutadiene rubber have different mixing characteristics. The high cis type has better mixing characteristics, including good roll wrapping and dispersibility. The roller temperature at the beginning of the mixing process should be lower than that of natural rubber, with a temperature range of 40-50 ℃ being ideal. The Zhongshun style requires a lower temperature. Low cis polybutadiene rubber has good processing performance due to its inability to crystallize. The sulfur content of butadiene rubber is lower than that of other diene rubber. This is because sulfur migrates quickly in polybutadiene rubber and is prone to frost spraying. The main method for blending cis-1,4-polybutadiene rubber is two-stage mixing. The use of two-stage mixing can significantly improve the processing performance of the rubber material and the physical and mechanical properties of the vulcanized rubber. When using a stage of mixing, the plasticity of the conveyor belt cover rubber is 0.33; When using two-stage mixing, it is 0.38, and when the plasticity is 0.2-0.25, roll detachment phenomenon will occur. The tensile strength of the vulcanized rubber prepared by the two-stage method has also been improved. Softeners should be added after carbon black dispersion to improve the tensile strength and elongation stress of the vulcanizate. Mixing cis-1,4-polybutadiene rubber with styrene butadiene rubber in an internal mixer and then adding other additives does not significantly improve the properties of the rubber compound and vulcanizate. If raw rubber and carbon black are mixed together, and then softeners are added, the tensile stress and tensile strength can be increased.
4. Mixing of butyl rubber
When butyl rubber is mixed in production, the feeding method (i.e. adding raw rubber and compounding agent after the feeding material is rolled) and the thin pass method (i.e. using a cold roller and a small roller pitch to repeatedly thin pass one half of the raw rubber in the formula, and then adding another half of the raw rubber after it is rolled) are generally used. The temperature of the mixed rubber is generally controlled at 40-60 ℃ (the temperature of the front roller should be 10-15 ℃ lower than the temperature of the rear roller); The speed ratio should not exceed 1:1.25, otherwise air can easily get sucked into the rubber material and cause foaming of the product. The compounding agent should be added in small quantities in batches, and can only be cut after the compounding agent is completely eaten and mixed in. When butyl rubber is mixed in an internal mixer, one-stage mixing, two-stage mixing, and reverse mixing methods can be used. The loading capacity can be slightly larger than that of natural rubber (5% to 10% larger); During the mixing process, reinforcement fillers should be added as early as possible to produce maximum shear force and better mixing effect; The mixing time is 30% to 50% longer than that of natural rubber; The temperature of mixing is usually controlled below 121 ℃ for the first stage of mixing and discharging, and around 155 ℃ for the second stage of mixing and discharging. High filling rubber materials are prone to compression and dispersion, i.e. granulation, during mixing in an internal mixer. The treatment method is to increase the adhesive capacity or use reverse mixing method. In order to improve the mixing effect of butyl rubber and increase the proportion of bonded rubber, the mixed rubber material can be subjected to "heat treatment", which involves adding 1.0-1.5 parts of the "heat treatment" agent (such as p-dinitrobenzene) to the butyl rubber and making it uniform, and then treating it at high temperature. The "heat treatment" of butyl rubber blends can be divided into two types: dynamic and static. The former is carried out simultaneously with the first stage of mixing on an internal mixer, with a process temperature of 120-200 ℃; The latter is directly placed in steam or hot air for 2-4 hours.
Butyl rubber has low cohesive strength, poor self-adhesion, and poor roll wrapping. In addition, its high saturation makes it technologically incompatible with other rubbers. In the absence of dedicated mixing equipment, the machine must be thoroughly cleaned before and after mixing processing to avoid mixing other raw rubber and affecting the quality of the rubber material. When mixing butyl rubber, it is difficult to disperse the compounding agent. When starting the mixer for mixing, the feeding method (i.e. adding raw rubber and additives after the feeding rubber rolls) or the thinning method is used. One half of the raw rubber in the formula is repeatedly thinned with a small roller moment, and the other half is added after the rolls are rolled. When using an internal mixer for mixing, the rubber loading capacity is 10% to 20% larger than that of natural rubber, and the mixing temperature is 150 ℃. When there are many fillers, the two-stage mixing method and reverse mixing method can also be used. Butyl rubber is easy to wrap around cold rolls, so the temperature of the rear roll should be about 10 ℃ higher than that of the front roll. Butyl rubber has a high cold fluidity, and the dispersal of the compounding agent is difficult, making it difficult to roll. When mixing rubber products in the mixer, the feeding method can be used (that is, taking a small piece of the same rubber material from the previous mixing and wrapping it on the roller, and then adding butyl rubber and the compounding agent) or the thin pass method (that is, half of the raw rubber in the formula is repeatedly thinned with a small roller pitch, and the other half is added after the rolling is wrapped).
5. Mixing of chloroprene rubber
Chloroprene rubber is produced by lotion polymerization, and the production process is mostly single kettle batch polymerization. The polymerization temperature is usually controlled between 40-60 ℃, and the conversion rate is around 90%. Excessive polymerization temperature, high final conversion rate, or air entering the polymerization process can all lead to a decrease in product quality. The relative molecular weight is adjusted using the sulfur thiuram (tetraalkylmethylaminothiocarbonyl disulfide) system in production. The main drawback of the sulfur thiuram system is that the sulfur bonds are not stable enough, which is one of the important reasons affecting storage performance. If thiols are used to adjust the relative molecular weight, this performance can be improved. Chloroprene rubber is different from general synthetic rubber in that it does not vulcanize with sulfur, but rather with zinc oxide, magnesium oxide, etc.
The processing performance of chloroprene rubber depends on the viscoelastic behavior of unvulcanized rubber, which is related to the variety and temperature of the chloroprene rubber. Due to the fact that mixing is generally carried out in an elastic state to utilize the shear force of the elastic state of the rubber material to disperse the filler well. Therefore, in order to avoid the influence of high temperature during the mixing of chloroprene rubber, fillers should be added as soon as possible to achieve a certain degree of mixing in the elastic state. When using a mixer for mixing, G-type chloroprene rubber is sensitive to temperature changes. When the roller temperature exceeds 70 ℃, it severely sticks to the roller and appears in a viscous flow state. The compounding agent is not easily dispersed. When using an internal mixer for mixing, its capacity should be appropriately reduced. Generally, the filling coefficient should be 0.6, and the mixing should be divided into two stages to minimize the mixing temperature. The discharge temperature should be lower than 100 ℃.
The disadvantages of chloroprene rubber when mixed in a mixer are high heat generation, easy adhesion to the rollers, easy burning, and slow dispersion of the compounding agent. Therefore, the mixing temperature should not be too high, the capacity should be small, and the roller speed ratio should not be large. Due to its strong sensitivity to temperature, general-purpose chloroprene rubber presents a granular state at room temperature to 71 ℃. At this time, the cohesion of the raw rubber is weakened, which not only causes severe adhesion to the roller, but also makes it difficult to disperse the compounding agent. The elastic state temperature of non sulfur regulated chloroprene rubber is below 79 ℃, so the mixing process performance is better than that of sulfur regulated chloroprene rubber, with less tendency to stick to rollers and scorch. When using a mixer for mixing, in order to avoid roller sticking, the roller temperature is generally controlled below 40-50 ℃ (the temperature of the front roller is 5-10 ℃ lower than the temperature of the rear roller), and during raw rubber kneading, the roller distance should be gradually adjusted from large to small. During mixing, the acid absorbent magnesium oxide is first added to prevent scorching, and finally zinc oxide is added. To reduce heat generation during mixing, carbon black and liquid softener can be added alternately in batches. Operating aids such as stearic acid and paraffin can be gradually added in a dispersed manner, which not only helps with dispersion but also prevents roller sticking. The mixing time of sulfur regulated chloroprene rubber in the opening mill is generally 30%~50% longer than that of natural rubber, and the mixing time of non sulfur regulated chloroprene rubber can be about 20% shorter than that of sulfur regulated chloroprene rubber. In order to avoid the rapid heating of chloroprene rubber during mixing in the mixer, if the speed ratio is less than 1:1.2, the cooling effect will be better. Reducing the refining capacity is also a way to ensure operational safety and good dispersion. At present, the refining capacity of sulfur regulated chloroprene rubber in China should be 20% to 30% less than that of natural rubber in order to operate normally. Due to the tendency of chloroprene rubber to scorch, the two-stage mixing method is usually used when using an internal mixer for mixing. The mixing temperature should be relatively low (the discharge temperature is generally controlled below 100 ℃), and the loading capacity of the rubber should be lower than that of natural rubber (the capacity coefficient is generally taken as 0.50-0.55). Zinc oxide should be added to the tablet press during the second stage of mixing. In response to the problem of easy coking and difficult dispersion of chloroprene rubber during mixing, the Lina internal mixer adopts the most advanced four diamond dual rotor synchronous operation in China, combined with the top bolt of the mixer moving in an "X" curve, which has good dispersion effect and short time, and can effectively reduce the coking phenomenon of chloroprene rubber during mixing and processing.
6. Mixing of ethylene propylene rubber
Ethylene propylene rubber can also be processed using ordinary rubber refining equipment, but due to its poor plasticizing effect and lack of viscosity, the rubber is not easy to wrap around the rollers. Generally, narrow roller spacing is used first, and after forming a continuous sheet shape, the roller spacing is relaxed for mixing processing. The roller temperature should be 50-60 ℃ for the front roller and 60-70 ℃ for the rear roller. The feeding sequence of EPDM rubber 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 - thin pass, and lower slice. Ethylene-propylene rubber is not prone to over melting during mixing, and the compounding agent is evenly dispersed, but the self viscosity is poor. Ethylene-propylene rubber is mixed in an open mill. Generally, a small roller moment is used to continuously wrap the rubber, and then the roller moment is gradually relaxed and a mixture is added. The roller temperature is between 60 and 70 ℃. The use of an internal mixer for mixing should be done at high temperatures, with mixing temperatures ranging from 150 to 160 ℃ helping to improve the dispersion and mechanical properties of fillers and softeners. The adhesive capacity can be 10% to 15% higher than other adhesive materials.
7. Mixing of nitrile rubber
The mixing process performance of nitrile rubber is poor, and the heat generated during mixing is high, which makes it easy to roll off. The wetting ability of the powdered mixture is poor, and the mixture is difficult to disperse. When the amount of carbon black is large, it will cause the rubber material to heat up quickly and be prone to scorching. When mixing nitrile rubber on the open mill, the operation method of low temperature, small capacity, small roller pitch, and slow feeding should be adopted to promote the uniform dispersion of the mixture. If using an internal mixer for mixing, the cooling in the mixing room should be strengthened. Carbon black and ester softeners should be added alternately in batches, and the discharge temperature should not exceed 130 ℃. The solubility of sulfur in nitrile rubber is relatively low, making it difficult to mix and disperse. It should be added at the beginning of mixing. There are five types of acrylonitrile content in nitrile rubber: 42% -46%, 36% -41%, 31% -35%, 25% -30%, and 18% -24%. The higher the content of acrylonitrile, the better the oil resistance, but the corresponding decrease in cold resistance.
Rubber mixing equipment: