Preparation process
1. Rubber baking
NR rubber baking temperature is generally 50~60℃, time is 24~36h, winter heating time is 36~72h.
CR rubber baking temperature is generally 24~40℃, time is 4~6h.
The rubber baking temperature should not be too high, otherwise it will affect the physical and mechanical properties of rubber.
2. Rubber cutting
Use a rubber cutting machine to cut the raw rubber into small pieces. The weight of each piece varies depending on the type of rubber. NR is 10~20kg per piece, and CR is no more than 10kg per piece.
3. Rubber breaking
The rubber block needs to be broken by a rubber breaking machine for plasticizing. The rubber breaking roller distance is generally 2~3mm, and the roller temperature is controlled below 45℃.
Open mill plastication process
(I) Principle of open mill plastication
The two rollers of the open mill rotate relative to each other at different speeds. The rubber is placed above the two rollers and is brought into the roller gap by the rollers under the action of friction. Due to the different rotational linear speeds of the roller surface, the rubber passes through the roller gap at different speeds and is subjected to friction shearing and extrusion. The rubber passes through the roller gap repeatedly and is plasticized.
(II) Process method of open mill plastication
1. Roller plastication method
Wrap the rubber sheet on the front roller and let it pass through the rollers repeatedly and plasticize naturally until the specified plasticity requirements are met.
Disadvantages: long plastication time, low efficiency, and low plasticity in the end.
It is divided into: one-stage plasticization: long plasticization time, low efficiency, not suitable for plasticization of raw rubber with high plasticity requirements.
Segmented plasticizing: plasticize with rollers for 10-15 minutes, unroll, cool, and park for 4-8 hours before plasticizing again until the required plasticity is reached. It is usually divided into two-stage plasticizing and three-stage plasticizing, depending on the plasticity requirements.
2. Thin-pass plasticizing method
When the roller gap is below 1 mm, the rubber material does not wrap around the roller after passing through the roller gap, but falls directly to the rubber receiving plate, allowing the rubber material to return to the top of the roller gap and pass through the roller gap again, and repeat this several times.
Advantages: The rubber material dissipates heat quickly, the cooling effect is good, the plasticity of the plasticized rubber is uniform, the quality is high, and any degree of plasticization can be achieved.
3. Chemical plasticizing plasticizing method
Use chemical plasticizers to increase the plasticizing effect, improve plasticizing production efficiency and save energy. Chemical plasticizers should be used in the form of masterbatch, and the roller temperature of the open mill should be appropriately increased.
(III) Factors affecting plasticizing on the open mill
1. Glue loading capacity
Glue loading capacity depends on the specifications of the open mill. A large capacity makes it difficult to dissipate heat, which increases the temperature of the rubber and reduces the plasticizing effect. A too small capacity reduces production efficiency.
Reasonable capacity is calculated according to the empirical formula:
Q—Plasticizing capacity, L; K—Empirical coefficient, the value is generally 0.0065~0.0085. L/cm3; D—Roller diameter, cm; L—Roller working part length, cm
Synthetic rubber generates a lot of heat during plasticizing, and the glue loading capacity should be less than NR.
2. Roller spacing
The smaller the roller spacing, the more obvious the mechanical plasticizing effect. The actual roller spacing used during thin pass is generally 0.5~1mm.
3. Roller speed and speed ratio
When the roller spacing is constant, increasing the roller speed or speed ratio of the open mill will increase the mechanical shearing effect of the rubber, thereby improving the mechanical plasticizing effect. The speed ratio of the open mill is generally between 1.15~1.27. If the speed ratio is too large, the temperature rises faster.
4. Roller temperature
The lower the roller temperature, the better the plasticizing effect. Too low roller temperature can easily cause equipment overload and damage. The plasticizing temperature is related to the type of raw rubber. NR usually controls the front roller temperature at 45~55℃ and the rear roller temperature at 40~50℃.
5. Plasticizing time
In the first 10~15min of the plasticizing process, the Mooney viscosity of the rubber material decreases rapidly, and then gradually slows down.
6. Chemical plasticizer
The use of plasticizer can improve the plasticizing effect, shorten the plasticizing time, and reduce elastic recovery. When using chemical plasticizers, appropriately increasing the temperature will improve the plasticizing effect. The plasticizing temperature is generally 70~75℃.
The amount of plasticizer used in NR is generally 0.1~0.3% of the weight of the raw rubber, and synthetic rubber should be increased to 2~3%.
Plasticizing process of internal mixer
Advantages: high degree of automation, high production efficiency, energy saving, low labor intensity;
Disadvantages: high temperature, difficult cooling, easy to over-refine, the material is amorphous, and it needs to be equipped with a corresponding tablet press.
(I) Working principle of internal mixer
After the material is added to the internal mixing chamber from the feeding hopper, the feeding door is closed, and the upper push bolt of the pressing device drops to pressurize the material. Under the action of the pressure and friction of the upper push bolt, the material is brought into the gap between two rotors with spiral edges, speed ratio, and relative rotation, causing the material to be subjected to the strong kneading action of constantly changing and repeated shearing, tearing, stirring and friction in the kneading system composed of rotors and rotors, rotors and internal mixing chamber walls, upper push bolts, and lower push bolts, so as to achieve the purpose of plasticizing.
The material is mainly subjected to several actions in the internal mixing chamber:
1. The action between the rotors and between the rotor and the inner wall of the mixing chamber;
2. The stirring action between the rotor edges;
3. The reciprocating cutting action of the rotor axial direction
(II) Processing methods of internal mixer plasticizing
There are three processing methods of internal mixer plasticizing: one-time plasticizing, segmented plasticizing and chemical plasticizing.
(III) Factors affecting internal mixer plasticizing
1. Temperature
Internal mixer plasticizing is high-temperature plasticizing. The raw rubber is subjected to high temperature and severe mechanical shearing in the internal mixer, mainly high-temperature oxidation, and the required plasticity can be obtained in a short time. The plasticizing temperature of the general internal mixer is above 120℃, and some can even reach 160℃, but too high temperature will lead to the decrease of physical and mechanical properties of rubber.
Generally, the temperature of NR plasticizing should not exceed 155℃. Use internal mixer to plasticize synthetic rubber to avoid gelation. The temperature range depends on the specific characteristics of the rubber. For SBR, the temperature should be controlled below 155℃ to avoid gelation. When using a plasticizer, the plasticizing temperature can be controlled at 160℃.
2. Speed
Fast speed and high plasticizing efficiency. The speed increases from 25 rpm to 75 rpm, and the plasticizing time is shortened from 30 min to 10 min. The increase in speed will inevitably accelerate the heat generation and temperature rise of the rubber compound, so cooling must be strengthened.
3. Time
When plasticizing with an internal mixer, the plasticity of the rubber compound increases with the increase in plasticizing time. When plasticizing with a plasticizer, the plasticizing effect will be improved, and the plasticizing time can be shortened by 30%~50%.
4. Upper push bolt pressure
The upper push bolt must be pressurized to increase the shearing effect of the rotor on the rubber compound. If the pressure is too small, the rubber compound cannot be compacted, but if the pressure is too large, the equipment load will be too large. The upper push bolt pressure is generally 0.5~0.8MPa.
5. Glue loading capacity
The glue loading capacity of internal mixers of various specifications is 48%~62% of the volume of the internal mixing chamber.
6. Chemical plasticizer
The plasticizing temperature of the internal mixer is high. The use of chemical plasticizer plasticizing method is reasonable and effective. It can not only fully exert the plasticizing effect of the plasticizer, but also reduce the discharge temperature under the same conditions and improve the quality of plasticized rubber.
Plasticizing process of screw plasticizer
(I) Working principle of screw plasticizer
In the screw plasticizer, the raw rubber is subjected to the strong mechanical stirring effect of the screw on the one hand, and on the other hand, due to the friction between the screw and the inner wall of the machine body, a large amount of heat is generated, and the temperature is as high as 150~180℃, thereby accelerating oxidation and cracking to obtain the plasticizing effect. Rubber Technology Network
Advantages: continuous production, large production capacity, suitable for large tire factories.
Disadvantages: high discharge temperature, large thermoplasticity of plasticized rubber, uneven quality, and irregular discharge.
(II) Factors affecting the plasticizing of screw plasticizer
1. Plasticizing temperature
If the temperature is too low, the equipment load is too large, the plasticity of the plasticized rubber is low and uneven. If the temperature is too high, it is easy to cause excessive oxidation and degradation of the macromolecular chain and damage the quality of the rubber. Therefore, the plasticizing temperature must be controlled within an appropriate range. The plasticizing temperature of NR is generally controlled below 60°C at the tail, 80~90°C at the body, 90~100°C at the head, and the discharge temperature is below 180°C.
2. Feeding speed
The feeding speed should be appropriate and uniform. If the speed is too fast, the residence time of the rubber in the barrel is short, the plasticization is uneven, and the phenomenon of clamping occurs. If the speed is too slow, it will not only reduce production efficiency.
3. Size of the discharge pores
The size of the discharge pores depends on the requirements of the plasticization degree of the rubber. If the pores are small, the discharge speed and discharge amount are reduced, the plasticity of the rubber is too large, and the production efficiency is reduced. On the contrary, the discharge pores are enlarged, the discharge amount is large, and the production capacity is improved, but the plasticity of the plasticized rubber is low and uneven.
Supplementary processing after plasticizing
1. Tableting or granulation
2. Cooling and drying
3. Parking
The dried film must be stacked for more than 4~8 hours according to regulations before it can be used in the next process.
4. Quality inspection
