Mixing is usedRubber mixing machinetakeRaw rubberorPlasticizingRaw rubber and compounding agent refiningRubber mixtureThe process ofRubber processingThe most important production process.Essentially, it is a process in which the compounding agent is evenly dispersed in the raw rubber. The granular compounding agent is a dispersed phase, and the raw rubber iscontinuous phase。
Chinese name
Mixing
Purpose
Rubber processing
Purpose
Improve the physical and mechanical properties of rubber products
Mixing means to improveRubber productsTo improve the processing and molding process and reduce production costs, it is necessary to add various additives, such as fillers, reinforcing agents, acceleratorsVulcanizing agent、antioxidant、Scorch inhibitorThese additives include solid, liquid and other materials. The additives are dispersed evenly to ensure the consistency of the rubber properties.Therefore, the process of evenly dispersing various additives into raw rubber or plastic compound on the rubber mixing machine is called mixing, and the compound made by mixing is called mixed rubber.
Mixing is the process of mixing raw rubber or plastic rubber with various additives through mechanical action.If the mixing is not good, the rubber will have various problems, such asScorch、Frost sprayingWaitCalendering, pressing out, gluingvulcanizationSuch processes are difficult to be carried out normally, and the performance of finished products is reduced.
The mixing process includes four stages: mixing, dispersion, mixingPlasticization。Raw rubberOr plastic glueRubber mixing machineUnder the action of shearing and stretching, rheology, fracture and fragmentation will occur, which will fully contact with the compounding agent to mix it.This process is called wetting stage or powder eating stage.Under the action of mechanical force, the compounding agent mixed with rubber is further broken into fine particles of small size, while increasing the contact area and further improving the mixing uniformity.Therefore, it is also called microscopic dispersion.Various additives in raw rubber or plastic compounduniform distributionProcess.The so-called mixing refers to only increasing the distribution uniformity of the compounding agent in the rubber compound without changing the size of its particles.Therefore, this process is also called macro dispersion or simple mixing.Rubber molecules continue to break under the mechanical and chemical action, so as to reduce the viscosity and achieve uniform mixing.These processes do not occur in isolation, but simultaneously.
Rubber mixtureThe general process flow of preparation is as follows:
carbon blackIn the process of dispersing to raw rubber or plastic rubber, the rubber first infiltrates into the pores of carbon black agglomerate to form carbon black rubber aggregates with high concentration, which are dispersed in raw rubber or plastic rubber without carbon black, and then these carbon black rubber aggregates are rubbed apart under the action of sufficient physical shear or tensile force, and the aggregates gradually reduce and mix until fully dispersed.In the carbon black rubber aggregate agglomerate, the attachment formed by carbon black has aphysical adsorptionOfcohesionOnly when the physical shear or tensile force is greater than this physical adsorption cohesion, can the agglomerate be rubbed apart.becauseRaw rubberOr plastic mixingDeformationThe required shear or tensile force is related to its viscosity. To overcome the physical cohesion of carbon black agglomerate and rub it apart, it is required that the raw rubber or plastic rubber have a certain viscosity.With the development of dispersion process, the raw rubber or plastic compound contained in carbon black gradually decreased, and the viscosity of rubber compound gradually decreased.The dispersion of carbon black and the number of fine particles increased, the contact area between carbon black and raw rubber or plastic compound increasedElastic recoveryThe value increases gradually. When the value does not increase any more, it indicates that the dispersion process is over.
Wetting and dispersion properties of the mixture
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The difficulty of dispersion of powdered and granular admixtures in rubber is related to their surface properties.According to their surface properties, they can be basically divided into two categories:Hydrophilicity, such as carbonate, clayzinc oxide、Lithopone, white carbon blackmagnesium oxideAnd other alkaline inorganic substances;The other is withHydrophobicity, such as various [[carbon black]].The surface properties of the former areRaw rubberDifferent, so it is not easy to be wetted. The surface property of the latter is similar to that of raw rubber, and it is easy to be wetted.In order to improve the dispersion of the compounding agent, the effective method is to addsurface active agent Surfactants are mostly organic compounds, including - OH, - NH2, - COOH, - NO2, - NO, - SH, etcPolar groupIn addition, their molecular structure also contains non-polar long chains orbenzene ringtypeAlkyl group, hydrophobic.Therefore, the surfactant acts as the medium between the coordination agent and the rubber, improving the mixing effect, and at the same time, it can stabilize the dispersion state of the dispersed coordination agent particles in the rubber, thus improving theRubber mixtureStability.
Rubber mixing process
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Mixing
The essence of rubber mixing process is thatRaw rubberIn the process of uniform dispersion, the granular coordination agent is dispersed phase, and the raw rubber iscontinuous phase。During the mixing process, the rubber molecular structure, molecular weight size and its distribution, and the aggregation state of the compounding agent all changed.Through mixing, rubber and compounding agent play a physical and chemical role, forming a new structure.Rubber mixtureIt is a kind of dispersion system with complex structural characteristics.Because the viscosity of raw rubber is very high, in order to make the compounding agent penetrate into the raw rubber and mix and disperse evenly, it is necessary to useRubber mixing machineStrong mechanical shear action.The mixing process varies according to the type of rubber mixer used.Rubber mixing machine mainly includes open type and closed type.NewerMixerIt is a screw transmission device, which can save energy and floor area, reduce labor intensity, and facilitate continuous production.
Mixing process of open rubber mixer
Mixing
Rubber in open typeRubber mixing machine(See color picture) Two rotationsRollerIn the gap of, it is repeatedly sheared and controlled by manual operation.It mainly includes three stages: ①Raw rubberHeat.SoonPlasticizingThe raw rubber ofBlending。②Add mixing agent.Generally add firstSoftener、stearic acidAnd then add powder, carbon black, etc. (carbon black can also be mixed with some raw rubber in advance to form master batch).Sulfur is generally added at the end of mixing, or after the rubber material is parked, during the second refining. ③Evenly mix and slice.When all the ingredients have been added, continue refining to make the mixture even, and then unload the film.
Open millThe mixing is divided into three stages: roll wrapping, powder eating and refining
Open mill mixing stage
① Wrap roll Wrap roll YesOpen millThe premise of mixing.During mixingRaw rubberIn open millRollerThere are four states on the.Zone 1, 2, 3 and 4.
It can be seen that the appropriate temperature should be selected to mix the raw rubber in the roll 2 area.All kinds of rubberGlass transition temperatureThey are different, so the optimal temperature of zone 2 of the wrapping roll of different rubbers is also different.Natural rubberandEmulsion polymerized styrene butadiene rubberOfmolecular weight distributionWide, so the suitable temperature range is wide. Generally, only Zone 1 and Zone 2 appear. Under the general operating temperature, there is no obvious Zone 3, so the roll wrapping and mixing performance are good.Cis-1,4-polybutadiene rubberIf the molecular weight distribution of synthetic rubber is narrow, the suitable temperature range is narrow. The low-temperature roll wrap is in Zone 2. If it is above 323.5K, it will change to Zone 3. At this time, even if the roll pitch is reduced to the minimum, it cannot return to Zone 2.It can be seen from this that the operating conditions must be mastered for various raw rubber or plastic compound, and the appropriate temperature must be selected to mix them in zone 2 of the wrapper roll to prevent the transition to zone 1 or zone 3, while the calendering should be in zone 4.
② After wrapping the roller with powdered rubber, a proper amount of stacking glue shall be reserved above the roll pitch, and then the compounding agent shall be added to the stacking glue.When adding the compounding agent, due to the resistance of the stacked adhesive, it folds up to form ripples, which are constantly overturned and replaced, the compounding agent is brought into the corrugated part of the stacked adhesive, and into the roll pitch, so that the compounding agent is in the radial directionshear deformation Under the action of, not only circumferential mixing occurs, but also mixing and dispersing to the thickness of roll wrap film.The amount of accumulated rubber shall not be too much or too little. Too little will make the mixing agent only scrape into the rubber surface at the shear of the rear roller and rubber, and the mixing effect in the rubber thickness direction (radial) is small, so it is not easy to mix the mixing agent into the interior, and the mixing effect is poor;Too much rubber will make the excess rubber roll over the roll pitch, and cannot enter the roll pitch smoothly without mixing.
③ Due to the high viscosity of rubber during retreading, the circumferential flow only occurs along the rotation direction of the roller during mixing, andlaminar flowThe rubber close to the surface of the roller and accounting for one third of the thickness of the roll wrapping film cannot flow and become a "dead layer". Only two thirds of the rubber is active layerShear forceIt is too small for the compounding agent to enter it.In addition, the accumulated glue at the top of the roll pitch will also form part of the wedge-shaped "backflow zone".Therefore, immediately cut and refine the powder after eating, constantly change the flow direction of the rubber material, so that the rubber material of the dead layer is constantly brought to the top to accumulate the rubber and brought into the active layer, so that the ingredients can be fully dispersed and mixed.
Open millAnalysis of advantages and disadvantages of mixing
The disadvantages of open mill mixing are that the powder flying pollutes the environment, poor safety, high labor intensity, low production efficiency, and small production scale;The advantage is that there are many kinds of rubber suitable for mixing or manufacturing special rubber, and the flexibility is great.
Mixing process of closed rubber mixer
Closed typeRubber mixing machine(See the color picture) The gap between the rotor surface and the mixing chamber wall varies widely with the rotation of the rotor, which has a strong shear effect on the rubber.The mixing effect is much greater than that of the open mixer. Various powder ingredients are mixed with the rubber in the mixing room without flying outside, and the mixing process is controlled by the automatic instrument. For example, during the mixing process, the rubber temperature, the volume of materials in the room and the power consumption have a certain law (see figure).[2]
The mixing process of internal mixer generally includes one-stage mixing method and two-stage mixing method.
One stage mixing method
The one-stage mixing method of internal mixer is completed at one time from feeding, mixing to cooling.Generally, it should be used together with the tablet pressRaw rubberOr plastic rubber and mixing agent shall be put into the mixing room of internal mixer in a certain order, and after mixing,Rubber dischargePress the tablet on the tablet press to reduce the temperature of the rubber, and then addVulcanizing agentAnd the ingredients that need to be added at low temperature, and then through the rubber tamping device or manual rubber tamping repeatedly to mix evenly, and then through the internal mixer and the tablet press to get a uniformRubber mixtureThis method is called one-stage mixing method
(1) Traditional one-stage mixing method
Add plastic compound and various additives gradually and in batches. After each feeding, put down the top bolt, and the degree of pressurization depends on the specific situation.
The mixing procedure of the traditional one-stage mixing method is generally: rubber (raw rubber, plastic rubberReclaimed rubberEtc.) →stearic acid→ Promoters, activatorsantioxidant→ Reinforcing filler → LiquidSoftener→ Rubber discharge → Tablet press processingSulfurAnd overspeed accelerator → unloading → cooling and parking
(2) One stage mixing method of segmented glue feeding
Batch joiningRaw rubberTo strengthen the dispersion of carbon black, improve and enhance the processing properties of rubber compoundVulcanizatePhysical and mechanical properties.
Specific operation: input 60%~80% raw rubber and compounding agent（Vulcanizing agentAnd super accelerator), at the mixing temperature of 70~120 ℃, mix to 70~80% of the total mixing time, then put the rest of the raw rubber together with the vulcanizing agent and super accelerator, and mix again for 1~2 min,Rubber dischargeLower the panel.
Two-stage mixing method
Some rubber materials such asNeopreneMaterial, cis-1,4-polybutadiene rubberInternal mixerAfter mixing, it is cooled down in the tablet press and left for a period of time. After cooling and left for a period of time, the viscosity increases, which makes the rubber have greater shear or tensile force when mixing, making the rubber and ingredients easier to break and mix.Return to the internal mixer for mixing, and then addVulcanizing agent, super accelerator, etc., and make them uniformly dispersed to obtain uniformRubber mixtureThis method is called two-stage mixing method.
(1) Traditional two-stage mixing method
High speed and high pressure internal mixer is used for the first stage of rough mixing, so that the rubber and the compounding agent are mixed to make the master rubber, which is sent to the low speed after being placed for a period of time after unloadingInternal mixerplusSulfurIt is mixed with accelerator, and then processed by tablet press.
(2) Two stage mixing method with segmented glue feeding
In the first stage of mixing, about 80% of the raw rubber is put into the same mixing as the traditional two-stage mixing method to prepare the master rubber.When mixing in the second stage, the remaining 20%Raw rubberPut it into the master rubber for mixing and evenly discharge the rubber.
Analysis of advantages and disadvantages of internal mixer mixing
Internal mixer mixing andOpen millCompared with mixing, the degree of mechanization is high, the labor intensity is small, the mixing time is short, the production efficiency is high, and the flying of powder is reduced.However, the internal mixer has slow heat dissipation and difficult mixing temperature control, so it is not suitable for temperature sensitive compounds,Rubber dischargeIrregular, unfixed shape.
Reverse mixing
First divideSulfurAll other additives except accelerator are filledInternal mixerMedium, then inputRaw rubberConduct mixing.[3]
Key points of low temperature mixing process
Low temperature one-time methodRubber mixingThe process is different from the traditional multi-stage mixing process (using high-temperature oxidation cracking, assisted by mechanical shearing, to realize the fracture of rubber molecular chain) in thatOpen millAdditive investmentsulfur, strengthening the machinery of rubberShear forceIt weakens the effect of high-temperature oxidative cracking. Within the effective time, the greater the shear on the rubber, the better the break of the rubber molecular chain and the dispersion of the compound. Therefore, how to improve the shear of the mixer on the rubber is the most critical factor.
Rubber compoundThe shear force of is proportional to the shear rate, and the shear rate is proportional to the speed ratio of the open mill (machineRoll speed ratioThe main ratio is 1:1.07, the secondary ratio is 1:1.143, the roller speed ratio of the tablet press is 1:1.09, and the tablet output is 1:1.143), which is proportional to the speed of the open mill and inversely proportional to the roller spacing of the open mill.The key factors affecting the low-temperature process - roll temperature, roll pitch, roll speed The most important factor affecting the low-temperature mixing is the roll temperature of the opening roll,Because in order to achieve the so-called low temperature mixing (that is, the rubber material is always mixed at a low temperature between 70 and 90 degrees on the opening roll, the opening roll body temperature must be kept between 20 and 40 degrees, and the maximum roll temperature cannot exceed 60 degrees, otherwise it is very easy to occurScorchAnd impactsulfurThe mother rubber is evenly dispersed.[3]
Preparation before mixing
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Preparations before mixing mainly include: spot check and inspection of raw materials;Supplementary processing of unsuitable ingredients;Weighing of ingredients.
Quality inspection of raw materials
Generally, the inspection contents of the compounding agent mainly include purity, particle size, moisture, mechanical impurity contentash contentandVolatile matterContent, pH, viscosity of liquid mixture, etc.The details vary according to the type of mixture.Raw rubberOr plastic compound, in addition to testing its chemical composition andMooney viscosityIn addition, physical and mechanical properties should also be tested.
Supplementary processing of compounding agent
The supplementary processing of blending agent mainly includes the grinding of solid blending agent;Drying and screening of powder mixture;Preheating, melting and filtering of low melting point mixture;Heating and filtration of liquid mixture;Paste andMaster rubberPreparation, etc.
smash
Lump or coarse-grained mixtures need to be crushed, ground or planed into fine pieces (such as stearic acidparaffin wax, asphalt and turpentine, etc.) to disperse in the rubber.The commonly used equipment for crushing is disc typegrinder、Ball mill、Airflow mill、Hammer crusher、FlakerEtc.
dry
When the compounding agent contains more water or low volatile impurities, the compounding agent is easy to agglomerate, which not only makes screening difficult, but also is easy to be pressed into large agglomerates during mixing and not easy to disperse, which makes it easy to generate bubbles inside the rubber compound or semi-finished products during vulcanization, resulting in waste products.Therefore, it is necessary to remove or reduce the moisture or low volatile impurities.drying equipmentWith drying roomVacuum drying ovenSpiral continuous dryer, etc.
screen
When the particle size andParticle size distributionIt does not meet the standard, or it contains mechanical impurities, such as sand, wood chips, small metal devices, and large particles and agglomerations of the compounding agent itself.Because the mixing of these impurities will reduce the physical and mechanical properties of the vulcanizate and easily damage the equipment.Common equipment areVibration sieveDrum screen, spiral screen, etc.
Melting, filtration and heating
Low melting point solidSoftenerHeating and melting must be carried out first to reachDehydrationAfter reducing the viscosity, remove various mechanical impurities while hot [[filtering]].For the liquid softener with [[viscosity]] too high, it should be heated and dehydrated to reduce its viscosity, so as toThermal filtrationRemove impurities.
Some individual compounding agents (those that are difficult to mix evenly in a short time, with high heat generation and energy consumption) are separately mixed with raw rubber or plastic compound in a relatively large proportion to make a simple, high concentration master batch rubber[1]。Some additives, such as accelerator, zinc oxide, sulfur, etc., are mixed with liquid softener in advance to form a paste mixture, which makes the additives easy to disperse, reduces the flying of powder additives, and improves the working conditions.
Weighing of mixture
The weighing of compounding agent is one of the most important processes for preparation of rubber compound.The accuracy of weighing plays an important role in the processing performance and product quality of the rubber compound.Therefore, we must be meticulous, good and accurate.There are two ways to weigh and feed the mixture: manual weighing and automatic weighing.The former is applicable to small and medium-sized production with low technical level, while the latter is applicable to large-scale production with high technical level.
Mixing can be usedOpen mill、Internal mixerContinuous with screwMixer。Mixing with open mill requires skilled manual operation, high labor intensity and unsafe.Mixing operation with internal mixer is safe and labor intensity is low, which is the most common application at present.Mixing requires that the mixture be evenly dispersed inRaw rubberMedium, formingColloidal stateDispersion to makeVulcanizateBest performance.At the same timeRubber mixtureMaterialPlasticityThere are also certain requirements to make it meet the requirements of subsequent processes.There are one and two sectionsMixing method。The former is to add compounding agent in rubber gradually.The latter is added firstSoftenerAnd powderFillerFor the first stageCoarse mixing，Rubber compoundAfter cooling and placing for a certain time, addSulfurandacceleratorConduct the second mixing.
Combine rubber (raw rubber) with various additives (seeRubber fit）OnRubber mixing machineUniformly mixedRubber processingtechnology.In order to add powdered ingredients to the rubber, the raw rubber must bePlasticizingAnd improve its plasticity and fluidity.The mixing process is the most basic process of rubber processingRubber mixtureThe quality of raw materials has a decisive impact on the process performance of semi-finished products and the quality of finished products.
