Plasticizing and mixing of rubber

With the deepening of reform and opening up, all industries in China have developed rapidly, and the rubber industry is no exception. However, some enterprises often limit their development due to the lack of necessary technical personnel and skilled operators. Therefore, it is imperative to quickly train the technical personnel of these enterprises and improve the production skills of the operators.
As we all know, the rubber processing industry is a highly specialized technical work. It not only requires technical personnel to have a high level of business, but also should have a broad knowledge of other disciplines. Constantly digest and absorb new technologies, new processes and new raw materials of rubber industry at home and abroad. Only in this way can a more economical formula with the best comprehensive balance performance be designed. 
However, this is only half of the rubber processing, and the other half is done by the operators. If there is a good formula, there is no operator to successfully complete a series of processes such as batching, mixing, extrusion, calendering and vulcanization. Then no matter how good the formula is, it can't produce a good product. Therefore, improving the operational level of operators is a strong guarantee for improving product quality. Only the perfect combination of technicians and operators can make my country's rubber processing industry rise to a new level.
Through more than ten years of practical work experience, the author focuses on the use of open mills for plasticizing and mixing of rubber.
1.Plasticizing of rubber
For rubber products, raw rubber is required to have a certain degree of plasticity. For example, the plasticity of raw rubber is required to be around 0.25 to 0.35 for the rubber of molded industrial products. The plasticity requirements for calendering, extrusion, sponge glue, mucilage and other rubber materials are about 0.4 to 0.6. If the plastic lift does not reach the required plasticity, it will bring great difficulties to the mixing, and at the same time, the quality of the product cannot be guaranteed. Therefore, the raw rubber with high Mooney viscosity must be masticated to obtain the required plasticity to ensure the smooth progress of the subsequent processes. If the plasticity is too high or too low, what are the adverse effects on rubber processing and products? If the raw rubber is not mixed enough and the plasticity does not meet the requirements, the mixing will be difficult, and the phenomenon of roll off will occur; in addition, the shrinkage of the rubber compound will occur. rate increased. If the raw rubber is excessively plasticated, the hardness and tensile strength of the mixed rubber will decrease, and the resistance to aging of the medium will decrease. Therefore, it is very important to do a good job in the plasticization of raw rubber according to the requirements of different products.
Not every raw rubber needs to be masticated. This depends on the rubber mastication characteristics, Mooney viscosity, etc. Under normal circumstances, the Mooney viscosity (ML100℃1+4) below 80 does not need to be masticated except for special-purpose rubber compounds, and it can be mixed with several thin passes before mixing. The raw rubber with Mooney viscosity (ML100℃1+4) above 80 needs to be masticated, but this is not entirely the case. For example, although the Mooney viscosity of fluororubber is 65-180, its molecular chain is rigid and stable in nature. After a long time of plasticizing, the viscosity of raw rubber not only decreases very little, but sometimes increases. Therefore, fluororubber can be directly mixed without mastication.
From the perspective of the domestic rubber processing industry, the main plasticizing products are unsaturated natural rubber and hard nitrile rubber. For EPDM, butyl, neoprene, cis-butadiene and acrylate adhesives, if they are used for molded products, they do not need to be masticated and can be directly mixed. But for calendered, extruded and sponge products, in order to ensure that the shrinkage rate of the product is small and the surface of the product is smooth, even if the raw rubber can be mixed without mastication, multi-stage mastication should be carried out to improve the plasticity of the raw rubber. Produce qualified products. In recent years, soft butadiene rubber imported from abroad, such as Japanese nitrile 240S, Russia's CKH-26, etc., has a low Mooney viscosity value. Generally, there is no need to masticate, but direct mixing.
The following will speed up the plasticizing methods and precautions of natural rubber and nitrile rubber.
Due to its high Mooney viscosity and high mechanical strength, natural rubber is very difficult to mix if it is not masticated (here mainly refers to 1# ~ 5# smoked sheet rubber). Therefore, natural rubber must be masticated before mixing in order to obtain suitable plasticity.
Because natural rubber is crystalline rubber, especially in winter, if the crystallization is not removed, it will bring great difficulties to cutting rubber and damage the equipment. Therefore, before cutting the glue, it is necessary to bake the glue. Baking glue is generally carried out in the glue drying room, the temperature is about 60 ℃, and the time is 48 hours.
After the raw rubber is taken out from the drying room, first remove the impurities on the surface of the raw rubber, and then use a rubber cutting machine to cut into small pieces of about 5 kg for use. It is best to separate the glue blocks with a release agent to avoid sticking to each other and keep them clean. The cut rubber block needs to be broken, and the factory is generally carried out on the rubber mixing machine. Taking a 14-inch rubber mixer as an example, first narrow the baffle (the working surface accounts for about 2/3 of the length of the roller), and adjust the roller distance (0.5mm), in order to prevent the rubber mixer from being overloaded and causing " trip” and damage the equipment. Then put the rubber blocks into the rubber mixing machine one by one by one end of the big cone, and the amount of glue is about 20 kg. When loading glue, you should face the mixing machine sideways or stand on the side of the mixing machine to prevent the glue from popping out and hurting people, and at the same time turn on the cooling water. The broken raw rubber should be masticated in time, and should not be parked for a long time. Because of the long time of parking, the raw rubber will re-stick into a ball. It is more troublesome to cut and break the glue again.
When plasticizing, put the raw rubber into the plasticizer, and let it fall directly into the material tray through the roll distance, without wrapping the roll. It is repeated for about 30 minutes, and the roll temperature is controlled below 50°C. If the temperature is too high, the phenomenon of false plasticity will occur (once the plasticized rubber has cooled, the phenomenon of recovery will occur). In order to avoid this phenomenon, the lower the temperature of the roller, the better, and the increase of the mechanical shear force is conducive to the degradation of rubber macromolecules, so as to obtain the plasticizing effect.
In the first ten minutes after the start of mastication, the natural rubber mastication effect is very obvious. The increase in plasticity slows down over time. If the mixing is stopped at this time, it will be quickly reflected during the mixing: the plastic compound is not easy to wrap the roll, even if the roll is wrapped, the surface is not smooth, and the powder cannot be added. At this time, the plasticized rubber should be re-thinned and plasticized. until a satisfactory result is obtained.
From the above situation, despite the continuous extension of time, the increase in the plasticity of natural rubber is not as significant as in the first ten minutes. But it does not mean that only ten minutes of plasticizing will do. On the contrary, subsequent plasticizing is necessary. Otherwise, the labor intensity will be increased and the mixing time will be prolonged during mixing, and the quality of the mixed rubber cannot be guaranteed.
The level of plasticization and the quality of it can be observed by those with operating experience through the naked eye. The plasticized raw rubber surface is smooth and translucent. At this time, the raw rubber has basically reached its plasticity. If you are inexperienced, you can also relax the roll distance (2 to 3 mm) to wrap it around the front roll. If the surface of the raw rubber is not smooth, it means that the mastication is not enough. If the surface is smooth, it means that the mastication is good. Of course, if the plasticizing is not enough, it must be re-plasticized until a satisfactory plasticizing effect is obtained.
The roll distance of plasticized raw rubber is relaxed to 2-3mm for the lower sheet. The film is 80 cm long, 40 cm wide and 0.4 cm thick. Cool naturally, and then use a barrier agent. You can also directly use the scale to weigh the required amount of the mixed rubber. The plasticized raw rubber is generally used after being parked for 8 hours.
Nitrile rubber, here mainly refers to hard nitrile rubber, the initial Mooney viscosity is 90 to 120, and the plasticity and process performance are extremely poor. If it is not masticated, the mixing cannot be carried out. Its raw rubber has high toughness and high heat generation in plasticizing, so plasticizing is particularly difficult. Compared with natural rubber, the plasticizing temperature of nitrile rubber is lower than that of natural rubber (about 40°C), and the loading capacity is about 15 kg less (70% of natural rubber). As the acrylonitrile content of nitrile rubber increases, the mixing becomes easier. Under normal circumstances, nitrile-26, nitrile-40, these nitrile rubber compounds used for molded products, can be plasticized for a period of about 40 minutes. The nitrile rubber with special requirements needs to undergo multi-stage mastication in order to obtain satisfactory plasticity. For nitrile-18, after a period of mastication, the plasticity is only about 0.18, which is too low for rubber products, and multi-stage mastication must be carried out. After 35 minutes of plasticizing, the nitrile rubber-18 has a plasticity of about 0.23, which basically meets the processing requirements.
The methods of plasticizing nitrile rubber and plasticizing natural rubber are basically the same. The low-temperature thin-pass method and the segmented plasticizing method are usually used. The surface of the plasticized raw rubber is smooth and shiny, and there are no large or small holes on the surface after wrapping.
In the process of plasticizing, there are many factors that affect the plasticizing effect of raw rubber. But for the specific operators, it is nothing more than the influence of the temperature, roll distance, time, rubber loading capacity and operation proficiency of the rubber mixing machine.
Practice has proved that the lower the roll temperature, the better the plasticizing effect. However, in actual operation, the cooling of the roller is limited by various objective reasons, and it is impossible to fully achieve the desired effect. Therefore, the segmented plasticizing method is often used to make up for its deficiencies. The cold air circulation climbing frame device can also be used to speed up the cooling of the film and improve the plasticizing effect.
The smaller the roll distance, the better the plasticizing effect. The raw rubber is passed through the rollers with a small roll distance. On the one hand, the shearing force of raw rubber is large, which makes the rubber macromolecules degrade rapidly; on the other hand, the thinner the film, the faster the heat dissipation. This is of great benefit to the plasticizing effect. We have done a comparative test. Compared with the roll distance of 0.5mm and the roll distance of 1mm, the larger the roll distance, the longer the plasticizing time, about 10 minutes. And the plasticizing quality without small roll distance is good.
Similarly, the amount of glue loaded also directly affects the plasticizing effect of raw rubber. It can be imagined that the raw rubber with a large amount of glue will pass through the gap for a long time. In the same plasticizing time, the plasticizing effect with a small amount of glue is better than that with a large amount of glue. Some operators are greedy for speed during plasticizing, and the amount of glue loaded is much higher than the specified requirements, which is very wrong. If the amount of glue loaded is too much, firstly the equipment cannot bear it. Under overload operation, it is very easy to "trip" and also increase the risk of damage to the equipment; not good too. Therefore, the amount of glue loaded depends on the equipment and type of glue, and the amount of glue loaded cannot be arbitrarily increased. Generally, the loading amount of synthetic rubber is about 20% less than that of natural rubber.
Some synthetic rubbers are unstable in quality during synthesis, and the quality of each piece of rubber varies greatly. A person with operating experience can see at a glance which piece of rubber takes a relatively short time for plasticization, and which piece of rubber takes longer to plasticize. Experienced operators often pay less attention to the concept of time when plasticating, and rely on the effect of plasticating. For raw rubber with poor quality, the plasticizing time is often longer than the specified time. We have a less mature experience. Taking nitrile-26 as an example, if the raw rubber block is black, then the rubber is easy to be masticated. If the raw rubber is whitish, it is not easy to masticate. As for whether the content of acrylonitrile in the above two blocks is different? Or other problems during synthesis, we have not done any tests in this regard, so we cannot jump to conclusions. During mastication, the whitish raw rubber only feels that the "moisture" is large, and it is difficult for the raw rubber to pass through the roll gap, which increases the mixing time. If the same mixing time is used, the quality of the rubber compound is definitely not good. The domestic EPDM rubber also has the above problems. The black raw rubber is easy to wrap the roll during mixing, and the white raw rubber is more difficult to wrap the roll. For some raw rubber, it takes about 20 minutes to wrap the roll, and the mixing time is prolonged invisibly.
2.Mixing of rubber
Mixing is one of the most important and complex steps in rubber processing. It is also one of the most prone to quality fluctuations. The quality of the rubber compound directly affects the quality of the product. Therefore, it is very important to do a good job of rubber mixing.
As a rubber mixer, how to do a good job of rubber mixing? I think in addition to strictly mastering the necessary knowledge of each rubber type, such as the mixing characteristics and dosing sequence, it is necessary to work hard, think hard, and mix rubber with heart. Only in this way is a more qualified rubber smelter.
To ensure the quality of the mixed rubber during the mixing process, the following points should be done:
1. All kinds of compounding agents with small dosage but great effect should be fully mixed and evenly mixed, otherwise the rubber compound will be scorched or vulcanized undercooked.
2. Mixing should be carried out in strict accordance with the mixing process regulations and the feeding sequence.
3. The mixing time should be strictly controlled, and the time should not be too long or too short. Only in this way can the plasticity of the mixed rubber be guaranteed.
4. Do not throw away large amounts of carbon black and fillers at will, and be sure to use them up. And clean the tray.
Of course, there are many factors that affect the quality of compound rubber. However, the specific manifestations are uneven dispersion of the compounding agent, frost spray, scorch, etc., which can be observed visually.
Uneven dispersion of compounding agent In addition to the particles of compounding agent on the surface of the rubber compound, the film is cut with a knife, and there are also compounding agent particles of different sizes on the cross-section of the rubber compound. The compound is mixed evenly, and the section is smooth. If the phenomenon of uneven dispersion of the compounding agent cannot be solved after repeated refining, the roller rubber will be scrapped. Therefore, the rubber mixer must strictly abide by the process regulations during the operation, and from time to time, take the film from both ends and the middle of the roller to observe whether the compounding agent is evenly dispersed.
Frosting, if it is not a problem of formula design, then it is caused by the improper order of dosing during the mixing process, or the uneven mixing and the agglomeration of the compounding agent. Therefore, it is necessary to strictly control the mixing process to avoid the occurrence of such phenomena.
Scorch is one of the most important problems in the mixing process. After the rubber material is scorched, the surface or the internal part has elastic cooked rubber particles. If the scorch is slight, it can be solved by the thin pass method. If the scorch is serious, the rubber material will be scrapped. From the perspective of process factors, the scorch of the rubber compound is mainly affected by the temperature. If the temperature of the rubber compound is too high, the raw rubber, vulcanizing agent and accelerator will react during the mixing process, that is, scorch. Under normal circumstances, if the amount of rubber during mixing is too large and the temperature of the roller is too high, the temperature of the rubber will increase, resulting in scorch. Of course, if the feeding sequence is not correct, the simultaneous addition of vulcanizing agent and accelerator will also easily cause scorch.
Fluctuations in hardness are also an important factor affecting the quality of the compound. Compounds of the same hardness are often mixed with different hardnesses, and some are even far apart. This is mainly due to the uneven mixing of the rubber compound and the poor dispersion of the compounding agent. At the same time, adding less or more carbon black will also cause fluctuations in the hardness of the rubber compound. On the other hand, the inaccurate weighing of the compounding agent will also cause fluctuations in the hardness of the rubber compound. Such as the addition of vulcanizing agent and accelerator carbon black, the hardness of the rubber compound will increase. The softener and raw rubber are weighed more, and the carbon black is less, and the hardness of the rubber compound becomes smaller. If the mixing time is too long, the hardness of the rubber compound will decrease. If the mixing time is too short, the compound will harden. Therefore, the mixing time should not be too long or too short. If the mixing is too long, in addition to the decrease in hardness of the rubber, the tensile strength of the rubber will decrease, the elongation at break will increase, and the aging resistance will decrease. At the same time, it also increases the labor intensity of operators and consumes energy.
Therefore, the mixing only needs to be able to fully disperse various compounding agents in the rubber compound, and to ensure the required physical and mechanical properties and the requirements of calendering, extrusion and other process operations.
As a qualified rubber mixer, not only has a strong sense of responsibility, but also must be familiar with various raw rubbers and raw materials. That is, not only to understand their functions and properties, but also to be able to accurately name their names without labels, especially for compounds with similar appearances. For example, magnesium oxide, nitric oxide and calcium hydroxide, high wear-resistant carbon black, fast-extrusion carbon black and semi-reinforced carbon black, as well as domestic nitrile-18, nitrile-26, nitrile-40 and so on.
Mixing of rubber. Most units and factories use open rubber mixers. Its biggest feature is that it has great flexibility and mobility, and is especially suitable for the mixing of frequent rubber variants, hard rubber, sponge rubber, etc.
When mixing with an open mill, the order of dosing is particularly important. Under normal circumstances, the raw rubber is put into the roll gap along one end of the pressing wheel, and the roll distance is controlled at about 2mm (take a 14-inch rubber mixer as an example) and roll for 5 minutes. The raw glue is formed into a smooth and gapless film, which is wrapped on the front roller, and there is a certain amount of accumulated glue on the roller. The accumulated rubber accounts for about 1/4 of the total amount of raw rubber, and then anti-aging agents and accelerators are added, and the rubber is tamped several times. The purpose of this is to make the antioxidant and accelerator evenly dispersed in the glue. At the same time, the first addition of the antioxidant can prevent the thermal aging phenomenon that occurs during high temperature rubber mixing. And some accelerators have a plasticizing effect on the rubber compound. Zinc oxide is then added. When adding carbon black, a very small amount should be added at the beginning, because some raw rubbers will come off the roll as soon as carbon black is added. If there is any sign of off-roll, stop adding carbon black, and then add carbon black after the rubber is wrapped around the roller smoothly again. There are many ways to add carbon black. Mainly include: 1. Add carbon black along the working length of the roller; 2. Add carbon black to the middle of the roller; 3. Add it close to one end of the baffle. In my opinion, the latter two methods of adding carbon black are preferable, that is, only a part of the degumming is removed from the roller, and it is impossible to remove the entire roller. After the rubber compound is taken off the roll, the carbon black is easily pressed into flakes, and it is not easy to disperse after being rolled again. Especially when kneading hard rubber, the sulfur is pressed into flakes, which is particularly difficult to disperse in the rubber. Neither refinishing nor thin pass can change the yellow "pocket" spot that exists in the film. In short, when adding carbon black, add less and more frequently. Don't take the trouble to pour all the carbon black on the roller. The initial stage of adding carbon black is the fastest time to "eat". Do not add softener at this time. After adding half of the carbon black, add half of the softener, which can speed up the "feeding". The other half of the softener is added with the remaining carbon black. In the process of adding powder, the roller distance should be gradually relaxed to keep the embedded rubber within an appropriate range, so that the powder naturally enters the rubber and can be mixed with the rubber to the maximum extent. At this stage, it is strictly forbidden to cut the knife, so as not to affect the quality of the rubber compound. In the case of too much softener, carbon black and softener can also be added in paste form. Stearic acid should not be added too early, it is easy to cause roll off, it is best to add it when there is still some carbon black in the roll, and the vulcanizing agent should also be added at a later stage. Some vulcanizing agents are also added when there is still a little carbon black on the roller. Such as vulcanizing agent DCP. If all the carbon black is eaten, the DCP will be heated and melted into a liquid, which will fall into the tray. In this way, the number of vulcanizing agents in the compound will be reduced. As a result, the quality of the rubber compound is affected, and it is likely to cause undercooked vulcanization. Therefore, the vulcanizing agent should be added at the appropriate time, depending on the variety. After all kinds of compounding agents are added, it is necessary to further turn to make the rubber compound evenly mixed. Usually, there are "eight knives", "triangle bags", "rolling", "thin tongs" and other methods of turning.
"Eight knives" are cutting knives at a 45° angle along the parallel direction of the roller, four times on each side. The remaining glue is twisted 90° and added to the roller. The purpose is that the rubber material is rolled in the vertical and horizontal directions, which is conducive to uniform mixing. "Triangle bag" is a plastic bag that is made into a triangle by the power of the roller. "Rolling" is to cut the knife with one hand, roll the rubber material into a cylinder with the other hand, and then put it into the roller. The purpose of this is to make the rubber compound evenly mixed. However, "triangle bag" and "rolling" are not conducive to the heat dissipation of the rubber material, which is easy to cause scorch, and is labor-intensive, so these two methods should not be advocated. Turning time 5 to 6 minutes.
After the rubber compound is smelted, it is necessary to thin the rubber compound. Practice has proved that the compound thin pass is very effective for the dispersion of the compounding agent in the compound. The thin-pass method is to adjust the roller distance to 0.1-0.5 mm, put the rubber material into the roller, and let it fall into the feeding tray naturally. After it falls, turn the rubber material by 90° on the upper roller. This is repeated 5 to 6 times. If the temperature of the rubber material is too high, stop the thin pass, and wait for the rubber material to cool before thinning to prevent the rubber material from scorching.
After the thin pass is completed, relax the roll distance to 4-5mm. Before the rubber material is loaded into the car, a small piece of the rubber material is torn off and put into the rollers. The purpose is to punch out the roll distance, so as to prevent the rubber mixing machine from being violently subjected to a large force and damaging the equipment after a large amount of rubber material is fed into the roller. After the rubber material is loaded on the car, it must pass through the roll gap once, and then wrap it on the front roll, continue to turn it for 2 to 3 minutes, and unload and cool it in time. The film is 80 cm long, 40 cm wide and 0.4 cm thick. Cooling methods include natural cooling and cold water tank cooling, depending on the conditions of each unit. At the same time, it is necessary to avoid contact between the film and soil, sand and other dirt, so as not to affect the quality of the rubber compound.
In the mixing process, the roll distance should be strictly controlled. The temperature required for the mixing of different raw rubbers and the mixing of various hardness compounds is different, so the temperature of the roller should be mastered according to the specific situation.
Some rubber mixing workers have the following two incorrect ideas: 1. They think that the longer the mixing time, the higher the quality of the rubber. This is not the case in practice, for the reasons described above. 2. It is believed that the faster the amount of glue accumulated above the roller is added, the faster the mixing speed will be. In fact, if there is no accumulated glue between the rollers or the accumulated glue is too small, the powder will easily be pressed into flakes and fall into the feeding tray. In this way, in addition to affecting the quality of the mixed rubber, the feeding tray must be cleaned again. The falling powder is added between the rollers, which is repeated many times, which greatly prolongs the mixing time and increases the labor intensity. Of course, if the accumulation of glue is too much, the mixing speed of the powder will be slowed down. It can be seen that too much or too little accumulation of glue is unfavorable for mixing. Therefore, there must be a certain amount of accumulated glue between the rollers during mixing. During kneading, on the one hand, the powder is squeezed into the glue by the action of mechanical force. As a result, the mixing time is shortened, the labor intensity is reduced, and the quality of the rubber compound is good.

The above two points, I hope to attract the attention of the rubber refining personnel.