38 questions about rubber mixing process?


1. Why should rubber be plasticized?

  The purpose of rubber mastication is to shorten the macromolecular chain of rubber under the action of machinery, heat, chemical, etc., so that the rubber temporarily loses its elasticity and increases its plasticity to meet the technological requirements in the manufacturing process. Such as: make the compounding agent easy to mix in, easy to calender and extrude, the molded pattern is clear, the shape is stable, and the fluidity of the pressed and injection molded rubber is increased, so that the rubber can easily penetrate into the fiber, and can improve the solubility and adhesion of the rubber . Of course, some low-viscosity and constant-viscosity rubbers are sometimes not necessarily masticated, such as domestic standard granular rubber and standard Malaysian rubber ( SMR ).

2. What factors affect the plasticization of rubber in the internal mixer

  The mastication of raw rubber in the internal mixer is a high-temperature mastication, with the lowest temperature above 120 , generally between 155 ℃ and 165 . The raw rubber is subjected to high temperature and strong mechanical action in the internal mixer cavity to produce violent oxidation, which can obtain the ideal plasticity in a short time. Therefore, the main factors that affect the raw rubber mastication of the internal mixer are:

  ( 1 ) Equipment technical performance, such as speed, etc.,

  ( 2 ) Process conditions, such as time, temperature, wind pressure and capacity.

3. Why the mastication characteristics of various rubbers are different

  The mastication of rubber is closely related to its chemical composition, molecular structure, molecular weight and molecular weight distribution. Natural rubber and synthetic rubber have different characteristics in structure and performance. Generally speaking, natural rubber is easier to masticate, while synthetic rubber is more difficult to masticate. As far as synthetic rubber is concerned, isoamyl rubber, neoprene rubber is close to natural rubber , styrene butadiene rubber, butyl rubber followed by nitrile rubber is the most difficult.

4. Why use raw rubber plasticity as the main quality standard of plasticized rubber

  The plasticity of raw rubber is related to the difficulty of the entire manufacturing process of the product, and directly affects the physical and mechanical properties of the vulcanized rubber and the important properties of the product performance. If the plasticity of the raw rubber is too high, the physical and mechanical properties of the vulcanized rubber will be reduced. If the plasticity of the raw rubber is too low, it will cause difficulties in the next process, making it difficult for the rubber to be evenly mixed, and the surface of the semi-finished product is not smooth during calendering and extrusion, and the shrinkage rate is large. It is difficult to grasp the size of the semi-finished product. It is difficult to wipe into the fabric, causing the rubber curtain to peel off, which greatly reduces the adhesion between the layers of the fabric. Uneven plasticity will cause inconsistencies in the process performance and physical and mechanical properties of the rubber compound, and even affect the inconsistent use performance of the product. Therefore, correctly grasping the plasticity of raw rubber is a problem that cannot be ignored.

5. What is the purpose of mixing

  Mixing is to mix the raw rubber and various compounding agents together through the rubber equipment according to the proportion of the compounding agent specified in the rubber formula, and make the various compounding agents uniformly dispersed in the raw rubber. The purpose of compound mixing is to obtain uniform physical and mechanical performance indicators and meet the compound performance indicators specified by the formula, so as to facilitate the next process operation and ensure the quality of the finished product.

6. Why does the compounding agent clump

  The reasons for the agglomeration of the compounding agent are: insufficient plasticization of raw rubber, too large roller spacing, too high roller temperature, too large rubber filling capacity, powder compounding agent containing coarse particles or agglomerates, gel, etc. The improvement method is to take specific conditions: fully masticate, appropriately adjust the roll distance, lower the roll temperature, pay attention to the feeding method; dry and screen the powder; cut properly when mixing.

7. Why does too much carbon black in the rubber produce " dilution effect "

  The so-called " dilution effect " is due to the excessive amount of carbon black in the rubber compound formulation, and the relative decrease in the amount of rubber, resulting in close contact between the carbon black particles, which cannot be well dispersed in the rubber compound. It is the " dilution effect ". In this way, because there are many large carbon black particles, rubber molecules cannot penetrate into the carbon black particles, and the interaction between rubber and carbon black is reduced, and the strength decreases and the expected reinforcement is not achieved. effect.

8. What effect does the structure of carbon black have on the performance of the compound?

  Carbon black is produced by thermal decomposition of hydrocarbon compounds. When the raw material is natural gas (its main component is aliphatic hydrocarbons), a six-membered carbon ring is formed; when the raw material is heavy oil (higher aromatic hydrocarbon content), the six-membered ring of carbon already contains carbon, and further dehydrogenation Condensation forms a polycyclic aromatic compound, thereby generating a hexagonal network structure layer of carbon atoms, and 3-5 overlapping layers of this layer become crystals. The spherical particles of carbon black are amorphous crystals composed of several groups of such crystals without a certain standard orientation. There are unsaturated free bonds around the crystals, which make the carbon black particles agglomerate with each other to form small branches of varying numbers, which is called the structure of carbon black.

  The structure of carbon black varies with different preparation methods. Generally, the structure of furnace carbon black is higher than that of channel carbon black. Acetylene carbon black has the highest structure. In addition, the structure of carbon black is also affected by the raw material, which contains high aromatic hydrocarbon content. , The structure of carbon black is high, and the yield is also high; on the contrary, the structure is low, and the yield is also low. The smaller the diameter of the carbon black particles, the higher the structure. Within the same particle size range, the higher the structure, the easier it is to extrude, and the extruded product has a smooth surface and less shrinkage. The structure of carbon black can be measured by the oil absorption value. In the case of the same particle size, a large oil absorption value indicates a high structure, and vice versa, a low structure. High-structure carbon black is difficult to disperse in synthetic rubber, but soft synthetic rubber requires carbon black with a higher modulus to increase its strength. Fine-particle high-structure carbon black can improve the wear resistance of the tread rubber. The advantages of low-structure carbon black are high breaking strength, high elongation, low tensile strength, low hardness, soft rubber, and low heat generation, but the wear resistance is worse than that of high-structure carbon black with the same particle size.

9. Why does carbon black affect the scorch performance of rubber?

  The structure of carbon black affects the scorch time of the rubber compound: the structure is high and the scorch time is short; the smaller the particle size of carbon black, the shorter the scorch time. The surface properties of carbon black particles are affected by scorching: mainly refers to the oxygen content on the surface of carbon black, high oxygen content, low PH value, acidic, such as channel black, scorching time is longer. The amount of carbon black affects the scorch time: The large amount of carbon black can significantly shorten the scorch time because the increase in carbon black produces a tendency to promote scorch. Carbon black has different effects on the Mooney scorch time of rubber compounds in different curing systems.

10. What is one stage mixing and what is two stage mixing

  One-stage mixing is to add plasticized rubber and various compounding agents (for some compounding agents that are not easy to disperse or use a small amount of compounding agents can be made into masterbatch in advance) one by one according to the requirements of the process regulations, that is, the masterbatch is mixed in the internal mixer After mixing, add sulfur or other vulcanizing agents on the tablet press and some super accelerators that are not suitable to be added in the internal mixer. In short, a stage of mixing is a mixing that is completed at once without stopping in the middle.

  The two-stage mixing is to first mix various compounding agents except vulcanizing agent and super accelerator uniformly with the raw rubber, make a masterbatch, cool down the next piece, park for a certain period of time, and then proceed on the internal mixer or open mixer Add vulcanizing agent for supplement processing.

11. Why should the film be cooled before storage

  The temperature of the film cut by the tablet press is very high. If it is not cooled immediately, it is easy to produce early vulcanization and glue, which will cause trouble to the next process. In our factory, the tablet press is taken down, the release agent is dipped in the film cooling device, blow-dried, and sliced for this purpose. The general cooling requirement is to cool the film temperature below 45 , and the rubber material should not be parked for too long, otherwise it will cause the rubber material to bloom.

12. Why should the sulphur adding temperature be controlled below 100

  This is because when sulfur and accelerators are added to the compound, if the temperature exceeds 100 , it will easily cause early vulcanization (ie scorch) of the compound. In addition, sulfur is dissolved in the rubber at high temperatures, and after cooling, the sulfur agglomerates on the surface of the rubber compound, causing blooming and uneven dispersion of sulfur. Rubber . Technology Network

13. Why should the mixed film be parked for a certain period of time before it can be used?

  The mixed rubber film has four purposes for storage after cooling: ( 1 ) to restore fatigue to the rubber and relax the mechanical stress experienced during mixing; ( 2 ) to reduce the shrinkage of the rubber; ( 3 ) to make the compounding Continue to diffuse during the parking process to promote uniform dispersion; ( 4 ) to further generate bonded rubber between the rubber and carbon black to improve the reinforcement effect.

14. Why do we need to strictly implement the stepwise dosing and pressurization time

  Dosing sequence and pressurization time are important factors that affect the mixing quality. Dosing in sections can improve mixing efficiency and increase its uniformity, and there are special regulations for the dosing order of certain drugs, such as: liquid softeners must not be dosed with carbon black at the same time to avoid agglomeration. Therefore, the segmented dosing must be strictly implemented. If the pressurization time is too short, the rubber and the drug material cannot be sufficiently rubbed and kneaded, resulting in uneven mixing; if the pressurization time is too long, the temperature of the mixing chamber is too high, which affects quality and reduces efficiency. Therefore, the pressurization time must be strictly enforced.

15. What is the impact of the filling capacity on the quality of the mixed and plasticized rubber

  The filling capacity refers to the actual mixing capacity of the internal mixer, which usually only accounts for 50-60% of the total capacity of the mixing chamber of the internal mixer . If the capacity is too large, there is no sufficient gap in the mixing, and sufficient mixing cannot be performed, resulting in mixing Uneven refining; elevated temperature will easily cause self-sulfurization of the rubber; it will also cause motor overload. If the capacity is too small, there is no sufficient friction resistance between the rotors, and idling occurs, which also causes uneven mixing, which affects the quality of the rubber compound and reduces the utilization rate of the equipment.

16. Why should the liquid softener be added last when mixing the rubber?

  During rubber mixing, if liquid softener is added first, it will over-swell the raw rubber and affect the mechanical friction between rubber molecules and fillers, reduce the mixing speed of the rubber, and cause uneven powder dispersion. , Even clumping. Therefore, the liquid softener is generally added last during mixing.

17. Why does the compounded rubber material " self-vulcanize " after being placed for a long time

  The main reasons for " self-sulfurization " during the placement of the compounded rubber are: ( 1 ) too much vulcanizing agent is used, and too much accelerator; ( 2 ) large rubber loading capacity, high temperature of the rubber mixer, and insufficient film cooling; ( 3) ( 4 ) Improper parking, that is, the temperature of the parking place is too high, and the air is not circulated.

18. Why mixer rubber compound to have a certain pressure

  During mixing, in addition to the raw rubber and chemical materials in the mixing chamber of the internal mixer, there are still a considerable amount of voids. If the pressure is insufficient, the raw rubber and chemical materials will not get enough friction and kneading, resulting in poor mixing. After increasing the pressure, the rubber will be subjected to strong friction and kneading up and down, left and right, so that the raw rubber and compounding agents will be quickly mixed evenly. In theory, the higher the pressure, the better, but due to the limitations of equipment, the pressure cannot be unrestricted in practice. Generally speaking, the wind pressure is about 6Kg/cm2 .

19. Why do the two rollers of the open rubber mixer have a certain speed ratio?

  The purpose of the design speed ratio of the open rubber mixer is to strengthen the shearing effect, produce mechanical friction and molecular chain breakage on the rubber material, and promote the dispersion of the compounding agent. In addition, the slow forward roll speed is also conducive to operation and safe production.

20. Why does the internal mixer produce thallium phenomenon

  Generally speaking, there are three reasons for the inclusion of thallium in the internal mixer: ( 1 ) The equipment itself has defects, such as air leakage from the top, ( 2 ) insufficient wind pressure, ( 3 ) improper operation, such as not paying attention when adding softeners , Often make the top bolt and the internal mixer cavity wall stick to the rubber, if it is not cleaned in time, it will be affected over time.

21. Why is the compound film crushed?

  Due to carelessness during mixing, it is often crushed, and the reasons are many. The main reasons are: ( 1 ) Violation of the dosing sequence specified in the process regulations or excessive dosing; ( 2 ) The mixing room temperature is too low during mixing; ( 3 ) It is possible that the amount of filler in the formula is too large. Due to poor mixing, the rubber material was crushed. After crushing, add plasticized rubber or masterbatch of the same rubber number to the crushed compound, and then perform technical treatment after pressing and discharging. Rubber - Technology Network

22. Why is the order of dosing required?

  The purpose of the dosing sequence is to improve the mixing efficiency and ensure the quality of the mixed rubber. Generally speaking, its dosing sequence is as follows: ( 1 ) Adding plasticized rubber to soften it to facilitate mixing with the compounding agent. ( 2 ) Add small medicines: zinc oxide, stearic acid, accelerators, anti-aging agents, etc., these are all important components of the rubber, first add them so that they can be evenly dispersed in the rubber. ( 3 ) Carbon black or other fillers such as clay, calcium carbonate, etc. ( 4 ) Liquid softener, rubber swelling makes carbon black and rubber easy to mix. If it is not carried out in this order of addition (except for formulas with special requirements), it will seriously affect the quality of the compound.

23. Why are several raw rubbers used in the same formula?

  With the development of raw materials in the rubber industry, there are increasing varieties of synthetic rubber. In order to improve the rubber and improve the physical and mechanical properties of vulcanized rubber, improve the processing performance of the rubber, and reduce the cost of rubber products, there are often several in the same formula. Use raw rubber together.

24. Why does the rubber produce too high or too low plasticity?

  The main reasons for this situation are the improper plasticity of the plasticized rubber; the mixing time is too long or too short; the mixing temperature is improper; the combined rubber is not blended well; the plasticizer is added or less added; the carbon black is less and more Or the wrong variety will occur. The improvement method is to properly grasp the plasticity of the plasticized rubber, control the mixing time and temperature, and mix the rubber evenly, and the compounding agent should be accurately weighed and strengthened inspection.

25. Why is the specific gravity of the compounded rubber too large and too small?

  The causes are inaccurate weighing of compounding agents, missing and mismatched ingredients. Carbon black, zinc oxide, calcium carbonate more than the prescribed amount and raw rubber, oil plasticizer, etc. are less than the prescribed amount, the proportion of the rubber compound may exceed the prescribed amount. On the contrary, the result is also the opposite. In addition, during rubber mixing, too much powder flying or sticking to the wall of the container (such as a small medicine box), and the filling is not clean, all of which can cause the specific gravity of the rubber to be too large or too small. The improvement method is to check the matching time Whether there is any error in the weighing, strengthen the operation to prevent the powder from flying and pay attention to the uniform mixing of the rubber.

26. Why is the hardness of the compounded rubber too high and too low?

  The main reason for the hardness of the rubber compound is the inaccurate weighing of the compounding agent. For example, the weighing of vulcanizing agent, reinforcing agent, and accelerator is higher than the formula amount, resulting in super high hardness of the vulcanized rubber; on the contrary, the weighing of rubber and plasticizer is too much. If the dosage specified in the formula or the weight of the reinforcing agent, vulcanizing agent, and accelerator is less than the dosage specified in the formula, the hardness of the vulcanized compound will inevitably be low. The improvement measures are the same as overcoming the fluctuation factors of plasticity. In addition, after adding sulfur, uneven smashing will also cause hardness fluctuations (locally too large or too small).

27. Why does the rubber have a slow start of vulcanization?

  The main reason for the slow starting point of rubber vulcanization is that the weighing of the accelerator is less than the specified amount, or the mixing of zinc oxide or stearic acid is omitted; secondly, the wrong type of carbon black can sometimes cause the vulcanization speed of the rubber to be delayed . The improvement measures are to strengthen the three inspections and weigh the medicine accurately.

28. Why does the rubber produce insufficient sulfur?

  The lack of sulfur in the rubber compound is mainly caused by the missing or under-mixing of accelerators, vulcanizing agents and zinc oxide. However, improper mixing operations and excessive powder flying can also lead to insufficient sulfur in the rubber. The improvement measures are: in addition to accurate weighing, strengthening the three inspections, and not missing or mismatched medicines, the mixing process should also be strengthened, and a large amount of powder should be prevented from flying away.

29. Why are the physical and mechanical properties of the compounded rubber inconsistent?

  Inaccurate weighing of compounding agents, mainly due to missing and mismatched reinforcing agents, vulcanizing agents and accelerators, will seriously affect the physical and mechanical properties of the compound after vulcanization. Secondly, if the mixing time is too long, the dosing sequence is unreasonable, and the mixing is uneven, it can also cause the physical and mechanical properties of the rubber to be unqualified after vulcanization. Take measures first to strengthen precision work, implement the three-inspection system, and prevent mismatching and missing ingredients. However, for poor quality rubber materials, supplementary processing must be carried out or used in qualified rubber materials. Rubber Technology . Network

30. Why does the rubber produce scorch?

  The reasons for the scorching of the rubber compound can be summarized as follows: unreasonable formulation design, such as vulcanizing agent, too much accelerator; too large filling capacity, improper mixing operation, such as high temperature of the rubber mixing machine, unloading the film After that, it is not fully cooled, premature addition of sulfur or uneven dispersion, resulting in a high concentration of vulcanizing agent and accelerator; storage without thin heat dissipation, excessive rolling or storage time, etc., will cause The rubber is scorched.

31. How to prevent rubber scorching

  Prevention of scorch is mainly to take corresponding measures for the causes of scorch.

  ( 1 ) prevent scorching, such as kneading temperature strictly controlled, in particular the vulcanization temperature, cooling condition improved, following the same procedures feeding a predetermined order, plus a strong plastic material management.

  ( 2 ) Adjust the vulcanization system in the formula and add an anti-scorch agent appropriately.

32. Why do you need to add 1-1.5% stearic acid or oil when dealing with rubber with heavier scorch?

  For the rubber with lighter scorch, pass it on the open mill (roll distance 1-1.5mm, roll temperature below 45 ) 4-6 times, park for 24 hours, mix it with the good material. The dosage is controlled below 20% . But for the rubber with heavier scorch, there are more vulcanized bonds in the rubber. Adding 1-1.5% of stearin will make the rubber swell and accelerate the destruction of the cross-linked structure. The mixing ratio of good rubber materials should not be higher than 10%. Of course, for some rubber materials with severe scorch degree, in addition to stearic acid, add 2-3% oil softener to help swelling. After treatment Can only be degraded to use. As for the more severely scorched rubber, it cannot be directly processed and used as raw materials for reclaimed rubber.

33. Why should the rubber be stored on the iron plate

  Plastics and rubber compounds are very soft. If they are placed on the ground at will, sand, gravel, soil, wood chips and other debris will stick to the rubber and will not be found easily. After mixing, it will seriously reduce the quality of the product, especially for some thin products. It is even more fatal. If metal debris is mixed in, it will cause machine equipment accidents. Therefore, the rubber must be stored on a special iron plate and stored in a designated place.

34. Why does the plasticity of rubber compounds sometimes vary greatly

  The plasticity change of the rubber compound has many influencing factors, including: ( 1 ) The sampling of the plastic compound is not uniform; ( 2 ) The plastic compound is improperly pressurized during mixing; ( 3 ) The amount of softener is incorrect; ( 4) ) Changes in raw materials, especially changes in raw rubber and carbon black, the main measure to solve the above problems is to strictly implement the process regulations and pay attention to the technical notices of raw material changes at any time.

35. After the rubber compound is discharged from the internal mixer, why is it necessary to carry out thin-through pour mixing?

  The temperature of the compound discharged from the internal mixer is generally above 125 , and the temperature of sulfur addition should be below 100 . In order to quickly reduce the temperature of the compound, it is necessary to repeatedly pour the compound, and then add sulfur and add sulfur. Accelerator operation.

36. What problems should be paid attention to during processing when using insoluble sulfur rubber

  Insoluble sulfur is unstable and can be converted into general soluble sulfur. The conversion is slower at room temperature, but it accelerates as the temperature rises. When the temperature is above 110 , it can be converted into ordinary sulfur within 10-20 minutes. Therefore, this sulfur should be stored at the lowest possible temperature. During the processing of ingredients, care should be taken to maintain a low temperature (below 100 ) to prevent it from turning into ordinary sulfur. Due to its insolubility in rubber, insoluble sulfur is often difficult to disperse uniformly, so full attention should be paid to the process. Insoluble sulfur is only used to replace general soluble sulfur and does not change the vulcanization process and the properties of the vulcanizate. Therefore, if the temperature is too high during the process, or stored for a long time at a higher temperature, it is meaningless to use it.

37. Why should the sodium oleate used in the film cooling device be recycled?

  Due to continuous operation of the release agent sodium oleate used in the cold water tank of the film cooling device, the film coming from the tablet press keeps heat in the sodium oleate, which will cause its temperature to rise rapidly and fail to achieve the purpose of cooling the film. In order to reduce its temperature, it is necessary to carry out circulating cooling, only in this way can the cooling effect and isolation effect of the film cooling device be more effective.

38. Why the film cooling device uses a mechanical hob better than an electric hob

  The film cooling device has tried electric hobs at the beginning, which is complicated in structure and difficult to maintain. The rubber at the knife edge is easy to vulcanize early and is unsafe. Later, mechanical hobs are used, which is convenient for maintenance and repair, ensuring product quality and safe production.

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Reprinted from: Rubber Technology Li Xiuquan Studio