1. Why should rubber be plasticized?
The purpose of rubber plasticization is to shorten the macromolecular chain of rubber under mechanical, thermal, chemical and other effects, so that the rubber temporarily loses its elasticity and increases its plasticity to meet the process requirements in the manufacturing process. For example: to make the compounding agent easy to mix, easy to calender and extrude, the molded pattern is clear, the shape is stable, increase the fluidity of the rubber compound for molding and injection, make the rubber compound easy to penetrate into the fiber, and improve the solubility and adhesion of the rubber compound. Of course, some low-viscosity and constant-viscosity rubbers are sometimes not necessarily plasticized, domestic standard granular rubber, standard Malaysian rubber (SMR).
2. What factors affect the plasticization of rubber in an internal mixer?
The plasticization of raw rubber in an internal mixer belongs to high-temperature plasticization, with a minimum temperature of more than 120°C, generally between 155°C and 165°C. The raw rubber is subjected to high temperature and strong mechanical action in the internal mixer cavity, resulting in severe oxidation, and can obtain ideal plasticity in a relatively short time. Therefore, the factors that affect the plasticization of raw rubber in the internal mixer are mainly:
(1) Equipment technical performance, such as rotation speed, etc.
(2) Process conditions, such as time, temperature, wind pressure and capacity, etc.
3. Why do various rubbers have different plasticization characteristics?
The plasticization of rubber is closely related to its chemical composition, molecular structure, molecular weight and molecular weight distribution. Due to the different characteristics of natural rubber and synthetic rubber in structure and performance, it is generally easier to plasticize natural rubber and more difficult to plasticize synthetic rubber. As for synthetic rubber, isoprene rubber and chloroprene rubber are close to natural rubber, followed by styrene-butadiene rubber and butyl rubber, and nitrile rubber is the most difficult.
4. Why is the plasticity of raw rubber used as the main quality standard for 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 performance of the product. If the plasticity of 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 subsequent processing, making it difficult to mix the rubber evenly, and the semi-finished product surface is not smooth when calendering and extruding, and the shrinkage rate is large. It is difficult to grasp the size of the semi-finished product. During calendering, the rubber is also difficult to rub into the fabric, causing the rubber curtain fabric to peel off, etc., greatly reducing the adhesion between the fabric layers. Uneven plasticity will cause inconsistency between the process performance and physical and mechanical properties of the rubber, and even affect the inconsistent performance of the product. Therefore, correctly mastering the plasticity of raw rubber is an issue that cannot be ignored.
5. What is the purpose of mixing?
Mixing is to mix the raw rubber and various compounding agents through rubber equipment according to the proportion of compounding agents specified in the rubber formula, and make the various compounding agents evenly dispersed in the raw rubber. The purpose of mixing the rubber is to obtain uniform physical and mechanical performance indicators that meet the rubber performance indicators specified in the formula, so as to facilitate the subsequent process operation and ensure the quality requirements of the finished product.
6. Why do compounding agents clump?
The reasons for the agglomeration of compounding agents are: insufficient plasticization of raw rubber, too large roller gap, too high roller temperature, too large rubber loading capacity, coarse particles or agglomerates in powder compounding agents, gel, etc. The improvement method is to take the following measures according to the specific situation: sufficient plasticization, appropriate reduction of roller gap, lower roller temperature, pay attention to the feeding method; dry and screen the powder; cut appropriately during mixing.
7. Why does excessive carbon black use in rubber produce a "dilution effect"?
The so-called "dilution effect" is that in the rubber formula, the amount of carbon black is too much, and the amount of rubber is relatively reduced, resulting in close contact between carbon black particles, and they cannot be well dispersed in the rubber. This is the "dilution effect". In this way, because there are many
large carbon black particles, rubber molecules cannot penetrate into the carbon black particles, the interaction between rubber and carbon black is reduced, the strength is reduced, and the expected reinforcement effect cannot be achieved.
8. What effect does the structure of carbon black have on the performance of rubber?
Carbon black is generated by thermal decomposition of hydrocarbon compounds. When the raw material is natural gas (whose components are mainly aliphatic hydrocarbons), a six-membered ring of carbon is formed; when the raw material is heavy oil (with a high content of aromatic hydrocarbons), because it already contains a six-membered ring of carbon, it is further dehydrogenated and condensed to form polycyclic aromatic compounds, thereby generating a hexagonal network structure layer of carbon atoms. 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. The crystals contain unsaturated free bonds around them. This bond makes the carbon black particles condense with each other to form small branched chains of varying numbers, which is called the structure of carbon black. The structure of carbon black varies depending on the preparation method. Generally, the structure of furnace carbon black is higher than that of channel 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 material is high, the structure of carbon black is high and the yield is also high; otherwise, the structure is low and the yield is also low. The smaller the diameter of the carbon black particle, the higher the structure. In the same particle size range, the higher the structure, the easier it is to extrude, and the surface of the extruded product is smooth and the shrinkage is small. The structure of carbon black can be measured by the oil absorption value. Under the same particle size, a large oil absorption value indicates a high structure, and vice versa. High-structure carbon black is difficult to disperse in synthetic rubber, but soft synthetic rubber requires carbon black with a higher modulus to improve its strength. Fine-particle high-structure carbon black can improve the wear resistance of tread rubber. The advantages of low-structure carbon black are high tear 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 rubber: the higher the structure, the shorter the scorch time; the smaller the particle size of carbon black, the shorter the scorch time. The effect of carbon black particle surface properties on scorch: mainly refers to the oxygen content on the surface of carbon black. High oxygen content, low pH value, acidic, such as channel black, has a longer scorch time. The effect of carbon black dosage on scorch time: large dosage can significantly shorten the scorch time because the increase in carbon black generates more bonded rubber and tends to promote scorch. The effect of carbon black on the Mooney scorch time of rubber in different vulcanization systems is different.
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 used in small amounts, master batches can be made in advance) one by one according to the requirements of the process regulations, that is, to mix the master batch in the internal mixer, and then add sulfur or other vulcanizing agents and some super accelerators that are not suitable for adding to the internal mixer on the tablet press. In short, one-stage mixing is mixing that is completed in one go without stopping in the middle.
The two-stage mixing is to first mix various compounding agents except the vulcanizing agent and super accelerator with the raw rubber evenly to make a masterbatch, cool it down, leave it for a certain period of time, and then carry out additional processing on an internal mixer or an open mixer to add the vulcanizing agent.
