Rubber processing Q&A
- Why does rubber need to be molded
The purpose of rubber plasticization is to shorten the large molecular chains of rubber under mechanical, thermal, chemical and other actions, causing the rubber to temporarily lose its elasticity and increase its plasticity, in order to meet the process requirements in manufacturing. For example, making the compounding agent easy to mix, facilitating rolling and extrusion, with clear molded patterns and stable shapes, increasing the flowability of molded and injection molded rubber materials, making it easy for the rubber material to penetrate fibers, and improving the solubility and adhesion of the rubber material. Of course, some low viscosity and constant viscosity rubbers may not necessarily be plasticized. Domestic standard particle rubber, standard Malaysian rubber (SMR).
- What factors affect the plasticization of rubber in an internal mixer
The mixing of raw rubber in an internal mixer belongs to high-temperature mixing, with a minimum temperature of 120 ℃ or above, generally between 155 ℃ and 165 ℃. Raw rubber is subjected to high temperature and strong mechanical action in the chamber of the mixer, resulting in severe oxidation and achieving ideal plasticity in a relatively short period of time. Therefore, the main factors affecting the mixing of raw rubber and plastic in the internal mixer are:
(1) Equipment technical performance, such as speed, etc,
(2) Process conditions, such as time, temperature, wind pressure, and capacity.
- Why do various rubbers have different plasticizing properties
The plasticity of rubber is closely related to its chemical composition, molecular structure, molecular weight, and molecular weight distribution. Due to their different structures and properties, natural rubber and synthetic rubber are generally easier to plastic than synthetic rubber. In terms of synthetic rubber, isoprene rubber and chloroprene rubber are close to natural rubber, followed by styrene butadiene rubber and butyl rubber, while nitrile rubber is the most difficult.
- Why is the plasticity of raw rubber used as the main quality standard for plastic compound
The plasticity of raw rubber is related to the difficulty of the entire manufacturing process of the product, and directly affects the important properties of the physical and mechanical properties of vulcanized rubber and the usability of the product. If the plasticity of raw rubber is too high, it will reduce the physical and mechanical properties of vulcanized rubber. If the plasticity of raw rubber is too low, it will cause difficulties in the next process, making it difficult to mix the rubber material evenly. During rolling, the surface of the semi-finished product is not smooth and the shrinkage rate is large, making it difficult to grasp the size of the semi-finished product. During rolling, the rubber material is also difficult to rub into the fabric, causing phenomena such as peeling of the hanging rubber curtain fabric, greatly reducing the adhesion between the layers of the fabric. Uneven plasticity can lead to inconsistent process and physical mechanical properties of the rubber material, and even affect the inconsistent performance of the product. Therefore, mastering the plasticity of raw rubber correctly is an issue that cannot be ignored.
5. What is the purpose of mixing
Mixing is the process of mixing raw rubber and various additives together through rubber equipment according to the proportion of additives specified in the rubber material formula, and ensuring that all additives are evenly dispersed in the raw rubber. The purpose of mixing rubber materials is to obtain uniform and consistent physical and mechanical performance indicators that meet the prescribed formula, in order to facilitate process operations and ensure the quality requirements of finished products.
6. Why do admixtures clump together
The reasons for the caking of the compounding agent are: insufficient plastic mixing of raw rubber, too large roll spacing, too high roll temperature, too large glue loading capacity, coarse particles or caking substances contained in powder compounding agent, gel, etc. The improvement method is to adopt specific measures based on the specific situation: fully plasticizing, appropriately adjusting the roller spacing, reducing the roller temperature, and paying attention to the feeding method; Drying and screening of powders; Cutting should be appropriate during mixing.
- Why does an excessive amount of carbon black in the rubber material produce a “dilution effect”
The so-called “dilution effect” is due to the excessive amount of carbon black in the rubber formulation, which leads to a relative decrease in the quantity of rubber, resulting in close contact between carbon black particles and inability to disperse well in the rubber material. This is called the “dilution effect”. Due to the presence of many large carbon black particle clusters, rubber molecules cannot penetrate into the carbon black particle clusters, and the interaction between rubber and carbon black is reduced, resulting in a decrease in strength and the expected reinforcement effect cannot be achieved.
8. What is the impact of the structure of carbon black on the properties of rubber materials
Carbon black is generated by the thermal decomposition of hydrocarbon compounds. When the raw material is natural gas (which is mainly composed of fatty hydrocarbons), a six membered carbon ring is formed; When the raw material is heavy oil (with a high content of aromatic hydrocarbons), the six membered ring containing carbon is further dehydrogenated and condensed to form a polycyclic aromatic compound, thereby forming a hexagonal network structure layer of carbon atoms. This layer overlaps 3-5 times and becomes a crystal. The spherical particles of carbon black are amorphous crystals composed of several sets of crystals with no specific standard orientation. There are unsaturated free bonds around the crystal, which cause carbon black particles to condense with each other, forming small branching chains of varying numbers, which is called the structure of carbon black.
The structure of carbon black varies with different production methods. Generally, the structure of furnace process carbon black is higher than that of tank process carbon black, and the structure of acetylene carbon black is the highest. In addition, the structure of carbon black is also affected by the raw materials. If the aromatic hydrocarbon content of the raw materials is high, the structure of carbon black is higher, and the yield is also higher; On the contrary, the structure is low and the yield is also low. The smaller the diameter of 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 surface of the extruded product is smooth with less shrinkage. The structure of carbon black can be measured by its oil absorption value. When the particle size is the same, a high oil absorption value indicates a high structure, while the opposite indicates a low structure. High structured carbon black is difficult to disperse in synthetic rubber, but soft synthetic rubber requires high modulus carbon black to improve its strength. Fine particle high structured carbon black can improve the wear resistance of tread rubber. The advantages of low structure carbon black are high tensile strength, high elongation, low tensile strength, low hardness, soft rubber material, and low heat generation. However, its wear resistance is worse than that of high structure carbon black with the same particle size.
- Why does carbon black affect the scorching performance of rubber materials
The influence of the structure of carbon black on the scorching time of rubber materials: high structural and short scorching time; The smaller the particle size of carbon black, the shorter the coking time. The effect of surface properties of carbon black particles on coking: mainly refers to the oxygen content on the surface of carbon black, which is high in oxygen content, low in pH value, and acidic, such as slot black, which has a longer coking time. The effect of the amount of carbon black on the scorching time: a large amount can significantly shorten the scorching time because the increase in carbon black generates bound rubber, which has a tendency to promote scorching. The effect of carbon black on the Mooney scorch time of rubber materials varies in different vulcanization systems.
10. What is a first stage mixing and what is a second stage mixing
One stage mixing is the process of adding plastic compound and various additives (for some additives that are not easily dispersed or used in small amounts, they can be pre made into masterbatch) one by one according to the process requirements. That is, the masterbatch is mixed in an internal mixer, and then sulfur or other vulcanizing agents, as well as some super accelerators that are not suitable to be added in the internal mixer, are added to the tablet press. In short, a mixing process is completed in one go without stopping in the middle.
Second stage mixing refers to the process of uniformly mixing various additives, except for vulcanizing agents and super accelerators, with raw rubber to produce a base rubber. The lower part is cooled and parked for a certain period of time, and then supplementary processing is carried out on the internal mixer or open mill to add vulcanizing agents.
11. Why do films need to be cooled before they can be stored
The temperature of the film cut off by the tablet press is very high. If it is not immediately cooled, it is easy to produce early vulcanization and adhesive, causing trouble for the next process. Our factory comes down from the tablet press, and through the film cooling device, it is immersed in isolation agent, blown dry, and sliced for this purpose. The general cooling requirement is to cool the film temperature to below 45 ℃, and the storage time of the adhesive should not be too long, otherwise it may cause the adhesive to spray frost.
- Why control the temperature of sulfur addition below 100 ℃
This is because when sulfur and accelerator are added to the mixed rubber material, if the temperature exceeds 100 ℃, it is easy to cause early vulcanization (i.e. scorching) of the rubber material. In addition, sulfur dissolves in rubber at high temperatures, and after cooling, sulfur condenses on the surface of the rubber material, causing frost and uneven dispersion of sulfur.
- Why do mixed films need to be parked for a certain period of time before they can be used
The purpose of storing mixed rubber films after cooling is twofold: (1) to restore fatigue of the rubber material and relax the mechanical stress experienced during mixing; (2) Reduce the shrinkage of the adhesive material; (3) Continue to diffuse the compounding agent during the parking process, promoting uniform dispersion; (4) Further generate bonding rubber between rubber and carbon black to improve reinforcement effect.
14. Why is it necessary to strictly implement segmented dosing and pressurization time
The dosing sequence and pressurization time are important factors affecting the mixing quality. Segmented dosing can improve mixing efficiency and increase uniformity, and there are special regulations for the dosing sequence of certain chemicals, such as: liquid softeners should not be added at the same time as carbon black to avoid agglomeration. Therefore, it is necessary to strictly implement segmented dosing. If the pressure time is too short, the rubber and medicine cannot be fully rubbed and kneaded, resulting in uneven mixing; If the pressurization time is too long and the mixing room temperature is too high, it will affect the quality and also reduce efficiency. Therefore, the pressurization time must be strictly enforced.
15. What is the impact of filling capacity on the quality of mixed and plastic rubber
The filling capacity refers to the actual mixing capacity of the internal mixer, which often only accounts for 50-60% of the total mixing chamber capacity of the internal mixer. If the capacity is too large, there is no sufficient gap in the mixing, and sufficient mixing cannot be carried out, resulting in uneven mixing; An increase in temperature can easily cause self vulcanization of the rubber material; It can also cause motor overload. If the capacity is too small, there is not enough friction resistance between the rotors, resulting in idling and uneven mixing, which affects the quality of the mixed rubber and also reduces equipment utilization.
- Why do liquid softeners need to be added last when mixing rubber materials
When mixing rubber materials, if liquid softeners are added first, it will cause excessive expansion of the raw rubber and affect the mechanical friction between the rubber molecules and fillers, reduce the mixing speed of the rubber materials, and also cause uneven dispersion and even agglomeration of the powder. So during mixing, liquid softeners are usually added last.
17. Why does the mixed rubber material “self sulfurize” after being left for a long time
The main reasons for the occurrence of “self sulfur” during the placement of mixed rubber materials are: (1) too many vulcanizing agents and accelerators are used; (2) Large rubber loading capacity, high temperature of the rubber refining machine, insufficient film cooling; (3) Or adding sulfur too early, uneven dispersion of the medicine materials causes local concentration of accelerators and sulfur; (4) Improper parking, such as excessive temperature and poor air circulation in the parking area.
18. Why does the mixing rubber material in the mixer need to have a certain air pressure
During mixing, in addition to the presence of raw rubber and medicinal materials in the mixing chamber of the internal mixer, there are also a considerable number of gaps. If the pressure is insufficient, the raw rubber and medicinal materials cannot be rubbed and kneaded enough, resulting in uneven mixing; After increasing the pressure, the rubber material will be subjected to strong friction and kneading up, down, left, and right, making the raw rubber and compounding agent quickly and evenly mixed. In theory, the higher the pressure, the better. However, due to limitations in equipment and other aspects, the actual pressure cannot be unlimited. Generally speaking, a wind pressure of around 6Kg/cm2 is better.
- Why do the two rollers of an open rubber mixing machine need to have a certain speed ratio
The purpose of designing a speed ratio for an open rubber refining machine is to enhance the shear effect, generate mechanical friction and molecular chain breakage on the rubber material, and promote the dispersion of the blending agent. In addition, the slow forward rolling speed is beneficial for operation and safety production.
- Why does the internal mixer produce thallium inclusion phenomenon
There are generally three reasons for the inclusion of thallium in the mixer: (1) there are problems with the equipment itself, such as air leakage from the top bolt, (2) insufficient air pressure, and (3) improper operation, such as not paying attention when adding softeners, often causing adhesive to stick to the top bolt and the wall of the mixer chamber. If not cleaned in time, it will eventually affect.
21. Why does the mixed film compress and disperse
Due to carelessness during mixing, it often disperses due to various reasons, mainly including: (1) violating the dosing sequence specified in the process regulations or adding too quickly; (2) The temperature in the mixing room is too low during mixing; (3) Excessive dosage of fillers in the formula is possible. Due to poor mixing, the rubber material was crushed and dispersed. The dispersed rubber material should be added with the same grade of plastic compound or mother rubber, and then subjected to technical treatment after being compressed and discharged.
22. Why is it necessary to specify the order of dosing
The purpose of the dosing sequence is to improve the efficiency of rubber compounding and ensure the quality of the mixed rubber material. Generally speaking, the order of adding chemicals is as follows: (1) Adding plastic to soften the rubber, making it easy to mix with the compounding agent. (2) Add small medicines such as zinc oxide, stearic acid, accelerators, anti-aging agents, etc. These are important components of the adhesive material. First, add them so that they can be evenly dispersed in the adhesive material. (3) Carbon black or other fillers such as clay, calcium carbonate, etc. (4) Liquid softener and rubber swelling make carbon black and rubber easy to mix. If the dosing sequence is not followed (except for formulas with special requirements), it will seriously affect the quality of the mixed rubber material.
23. Why are there several types of raw rubber used together in the same formula
With the development of raw materials in the rubber industry, the variety of synthetic rubber is increasing. In order to improve the physical and mechanical properties of rubber and vulcanized rubber, improve the processing performance of rubber, and reduce the cost of rubber products, several types of raw rubber are often used in the same formula.
24. Why does the rubber material produce high or low plasticity
The main reason for this situation is that the plasticity of the plastic compound is not appropriate; Mixing time is too long or too short; Improper mixing temperature; And the glue is not well mixed; Excessive or insufficient addition of plasticizers; Carbon black can be produced by adding too little or using the wrong variety. The improvement method is to appropriately grasp the plasticity of the plastic compound, control the mixing time and temperature, and mix the rubber evenly. The mixing agent should be accurately weighed and inspected.
25. Why does the mixed rubber material produce a specific gravity that is too large or too small
The reasons for this include inaccurate weighing of the compound, omissions, and mismatches. If the amount of carbon black, zinc oxide, and calcium carbonate exceeds the specified amount while the amount of raw rubber, oil plasticizers, etc. is less than the specified amount, there will be situations where the specific gravity of the rubber material exceeds the specified amount. On the contrary, the result is also the opposite. In addition, during the mixing of rubber materials, excessive powder flying or sticking to the container wall (such as on a small medicine box), and failure to pour out the added material completely can cause the specific gravity of the rubber material to be too high or too low. The improvement method is to check whether there are any errors in the weighing during the mixing, strengthen the operation, and prevent powder flying and ensure even mixing of the rubber material.
26. Why does the hardness of mixed rubber materials become too high or too low
The main reason for the high or low hardness of the rubber material is the inaccurate weighing of the compounding agent, such as the weight of the vulcanizing agent, reinforcing agent, and accelerator being higher than the dosage of the formula, resulting in the ultra-high hardness of the vulcanized rubber; On the contrary, if the weight of rubber and plasticizers exceeds the prescribed amount in the formula, or the weight of reinforcing agents, vulcanizing agents, and accelerators is less than the prescribed amount in the formula, it will inevitably lead to low hardness of the vulcanized rubber material. Its improvement measures are the same as overcoming the factor of plasticity fluctuations. In addition, after adding sulfur, uneven grinding can also cause fluctuations in hardness (locally too large or too small).
27. Why does rubber material have a slow vulcanization starting point
The main reason for the slow vulcanization starting point of rubber materials is due to the less than specified amount of accelerator being weighed, or the omission of zinc oxide or stearic acid during mixing; Secondly, the wrong type of carbon black can sometimes cause a delay in the vulcanization rate of the rubber material. The improvement measures include strengthening the three inspections and accurately weighing the medicine materials.
28. Why does the rubber material produce sulfur deficiency
The occurrence of sulfur deficiency in rubber materials is mainly caused by missing or insufficient combinations of accelerators, vulcanizing agents, and zinc oxide. However, improper mixing operations and excessive powder flying can also lead to sulfur deficiency in rubber materials. The improvement measures are: in addition to achieving accurate weighing, strengthening the three inspections, and avoiding missing or mismatched ingredients, it is also necessary to strengthen the mixing process operation and prevent a large amount of powder from flying and losing.
29. Why are the physical and mechanical properties of mixed rubber materials inconsistent
Inaccurate weighing of the compounding agent is mainly due to missing or mismatched reinforcing agents, vulcanizing agents, and accelerators, which can seriously affect the physical and mechanical properties of the vulcanized rubber compound. 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 vulcanized rubber to be unqualified. Firstly, measures should be taken to strengthen precision craftsmanship, implement the three inspection system, and prevent wrong or missed dispensing of pharmaceutical materials. However, for rubber materials with poor quality, supplementary processing or incorporation into qualified rubber materials is necessary.
30. Why does the rubber material produce scorching
The reasons for the burning of rubber materials can be summarized as follows: unreasonable formula design, such as excessive use of vulcanizing agents and accelerators; Excessive rubber loading capacity, improper rubber mixing operation, such as high temperature of the rubber mixing machine, insufficient cooling after unloading, premature addition of sulfur or uneven dispersion, resulting in high concentration of vulcanizing agents and accelerators; Storage without thin cooling, excessive rolling or prolonged storage time can cause burning of the adhesive material.
31. How to prevent scorch of rubber materials
Preventing coking mainly involves taking corresponding measures to address the causes of coking.
(1) To prevent scorching, such as strictly controlling the mixing temperature, especially the sulfur addition temperature, improving cooling conditions, adding materials in the order specified in the process specifications, and strengthening rubber material management.
(2) Adjust the vulcanization system in the formula and add appropriate anti coking agents.
32. Why add 1-1.5% stearic acid or oil when dealing with rubber materials with high degree of burning
For rubber materials with relatively light burning degree, thin pass (roller pitch 1-1.5mm, roller temperature below 45 ℃) 4-6 times on the open mill, park for 24 hours, and mix them into the good material for use. The dosage should be controlled below 20%. However, for rubber materials with high degree of scorching, there are more vulcanization bonds in the rubber material. Adding 1-1.5% stearic acid can cause the rubber material to swell and accelerate the destruction of cross-linking structure. Even after treatment, the proportion of this type of rubber added to the good rubber material should not exceed 10% Of course, for some severely burnt rubber materials, in addition to adding stearic acid, 2-3% oil softeners should be added appropriately to aid in swelling. After treatment, they can only be downgraded for use. As for the rubber material with more severe scorching, it cannot be directly processed and can only be used as raw material for recycled rubber.
33. Why do rubber materials need to be stored on iron plates
Plastic and mixed rubber are very soft. If placed on the ground casually, debris such as sand, gravel, soil, and wood chips can easily stick to the rubber material, making it difficult to detect. Mixing them can seriously reduce the quality of the product, especially for some thin products, which is fatal. If metal debris is mixed in, it can cause mechanical equipment accidents. So the adhesive material must be stored on specially made iron plates and stored in designated locations.
34. Why does the plasticity of mixed rubber sometimes vary greatly
There are many factors that affect the plasticity changes of mixed rubber, mainly including: (1) inconsistent sampling of plastic rubber; (2) Improper pressurization of plastic compound during mixing; (3) The quantity of softeners is incorrect; (4) The main measure to solve the above problems is to strictly follow the process regulations and pay attention to technical notifications of raw material changes, especially the changes in raw rubber and carbon black.
35. Why is thin pass reverse mixing necessary after the mixed rubber is discharged from the internal mixer
The temperature of the discharged rubber material from the internal mixer is generally above 125 ℃, while the temperature for adding sulfur should be below 100 ℃. In order to quickly reduce the temperature of the rubber material, it is necessary to repeatedly pour the rubber material and then carry out the operation of adding sulfur and accelerator.
36. What issues should be noted during the processing of using insoluble sulfur adhesive
Insoluble sulfur is unstable and can be converted into general soluble sulfur. The conversion is slower at room temperature, but accelerates with increasing temperature. When it reaches above 110 ℃, it can be converted to ordinary sulfur within 10-20 minutes. Therefore, this sulfur should be stored at the lowest possible temperature. During the ingredient processing, care should also be taken to maintain a lower temperature (below 100 ℃) to prevent it from being converted into ordinary sulfur. Insoluble sulfur, due to its insolubility in rubber, is often difficult to uniformly disperse, and should also be given sufficient attention in the process. Insoluble sulfur is only used to replace general soluble sulfur, without changing the vulcanization process and the properties of the vulcanized rubber. Therefore, if the temperature is too high during the process, or if it is stored for a long time at a higher temperature, then using it is meaningless.
37. Why does sodium oleate used in the film cooling device need to be circulated
The isolation agent sodium oleate used in the cold water tank of the film cooling device, due to continuous operation, the film coming down from the tablet press continuously retains heat in the sodium oleate, which will cause its temperature to rapidly rise and fail to achieve the purpose of cooling the film. In order to reduce its temperature, it is necessary to carry out cyclic cooling, only in this way can the cooling and isolation effects of the film cooling device be more effectively exerted.
38. Why is a mechanical roller better than an electric roller for film cooling devices
The film cooling device was initially tested with an electric heating roller, which had a complex structure and difficult maintenance. The rubber material at the cutting edge was prone to early vulcanization, making it unsafe. Later, mechanical rollers were used for easy maintenance and repair, ensuring product quality and safe production.
Post time: Apr-12-2024
