Foreword

With the deepening of reform and opening up, China's various industries have been rapid development, the rubber industry is no exception. However, some enterprises due to the lack of necessary technical personnel and skilled operators often limit their development. Therefore, it is urgent 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 in addition to a high level of business, but also should have a broad knowledge of other disciplines. Continuously digest and absorb new technologies, new processes and new raw materials of the rubber industry at home and abroad. Only in this way can we design a more economical, the best comprehensive balance of performance of the formula. However, this is only half of the rubber processing, and the other half is done by the operator. If there is a good formula, and there is no operator to successfully complete the ingredients, mixing, pressing, calendering and vulcanization and so on a series of processes. Then no matter how good the formula is, it cannot produce a good product. Therefore, improving the operational level of operators is a powerful guarantee for improving product quality. Only the perfect combination of technicians and operators can make China'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 mill for rubber plastication, mixing and other aspects of the problem.

Plastication of 1. rubber

For rubber products, the raw rubber is required to have a certain degree of plasticity. For example, for the rubber of molded industrial products, the plastic degree of raw rubber is required to be about 0.25 to 0.35. For calendering, extrusion, sponge, mucilage and other plastic requirements in the degree of 0.4~0.6. If the glue does not reach the required plasticity, it will bring great difficulties to mixing, and the quality of the product cannot be guaranteed. Therefore, for the higher Mooney viscosity of raw rubber must be plasticized, and obtain the required plasticity, to ensure the smooth progress of the subsequent processes. Can be too high or too low on the rubber processing and products will have any adverse effects? If the raw rubber mixing is insufficient, the plasticity can not meet the requirements, the mixing difficulties, will occur off the roller and other phenomena; In addition, the shrinkage of the rubber material increases. If the raw rubber is over-plasticized, the hardness and tensile strength of the mixed rubber will decrease, and the aging resistance of the medium will decrease. Therefore, it is very important to do a good job in plasticizing raw rubber according to different product requirements.

Not every raw rubber is masticated. This depends on the rubber mastication characteristics, Mooney viscosity, etc. Under normal circumstances, the Mooney viscosity (ML100 ℃ 14) below 80 can be mixed without mastication except for special-purpose rubber, and can be mixed through several times before mixing. Mooney viscosity (ML100 ℃ 1 4) above 80 raw rubber is to be masticated, but this is not entirely the case. Such as fluororubber despite its Mooney viscosity of 65~180, but its molecular chain rigid, stable nature, raw rubber after a long period of plastication, not only the viscosity decreased very little, and sometimes increased. Therefore, the fluororubber can be directly kneaded without being plasticized.

From the domestic rubber processing industry, the main plasticizing is unsaturated natural rubber and hard nitrile rubber. For EPDM, butyl rubber, neoprene, cis-butyl rubber and acrylate rubber, if used for molded products, there is no need for mastication, can be directly mixed. However, for calendering, extrusion and sponge products, in order to ensure that the shrinkage rate of the products is small and the surface of the products is smooth, even the raw rubber that can be mixed without plasticizing should be subjected to multi-stage plasticizing to improve the plasticity of the raw rubber and produce qualified products. In recent years, soft butadiene rubber imported from abroad, such as Japan's nitrile 240S, Russia's CKH-26, etc., because of its low Mooney viscosity value. Generally, it is not necessary to carry out mastication, but to carry out mixing directly.

The following is a slight increase in the plasticizing methods and precautions of natural rubber and nitrile rubber.

Natural rubber because of its raw rubber Mooney viscosity value, high mechanical strength, if not plastic mixing is very difficult (here mainly refers to 1#~ 5# cigarette glue). Therefore, the natural rubber must be plasticized before mixing in order to obtain a suitable degree of plasticity.

Because natural rubber is crystalline rubber, especially in winter, if the crystallization is not lifted, it will bring great difficulties to the rubber cutting and damage the equipment. Therefore, before cutting the glue, it is necessary to bake the glue. Baking glue is generally carried out in the baking glue room, the temperature is about 60 ℃, and the time is 48 hours.

After the raw rubber is taken out from the drying room, the impurities on the surface of the raw rubber are first removed, and then the rubber cutter is cut into small pieces of about 5kg for standby. It is best to separate the glue blocks with a separating agent to avoid mutual adhesion and keep them clean. The cut rubber blocks need to be broken, and the general factory is carried out on the rubber mixing machine. Taking a 14-inch rubber mixer as an example, first narrow the baffle (the working face accounts for about 2/3 of the length of the roller) and reduce the roller distance (0.5 ㎜) in order to prevent the rubber mixer from "tripping" and damaging the equipment due to excessive load. Then put the rubber blocks into the rubber mixing machine one by one against one end of the big tooth wheel, and the amount of rubber loaded is about 20kg. When loading glue, you should face the glue mixing machine sideways or stand on the side of the glue mixing machine to prevent the glue from ejecting and hurting people. At the same time, you should open the cooling water. Broken raw rubber should be plasticized in time and not parked for a long time. Because the parking time is long, the raw rubber will stick into a ball again. It is more troublesome to cut and break glue again.

When masticating, put the raw rubber into the masticator and let it fall directly into the tray through the roller distance, not the roller. Repeatedly, the time is about 30 minutes, the roller temperature is controlled below 50 degrees. If the temperature is too high, false plasticity will occur (once the plastic is cooled, it will recover). In order to avoid this phenomenon, the lower the roller temperature, the better. At the same time, the increase of mechanical shear force is beneficial to the degradation of rubber macromolecules, so as to obtain the mastication effect.

In the first ten minutes after the start of mastication, the mastication effect of natural rubber is very obvious. The increase in plasticity slows down over time. If the mixing is stopped at this time, it will be reflected quickly during mixing: plastic rubber is not easy to roll, even if it is roll, the surface is not smooth, and powder cannot be added. At this time, the plasticizing glue should be re-plasticized. Until satisfactory results are obtained.

From the above situation, despite the continuous extension of time, the growth of natural rubber plasticity is not as effective as the first ten minutes. But it doesn't mean that it's only 10 minutes of mastication. On the contrary, later mastication is very necessary. Otherwise, the mixing will increase the labor intensity and prolong the mixing time, and the quality of the mixing rubber cannot be guaranteed.

The degree of mastication and the quality can be observed by people with operational experience with the naked eye. The surface of raw rubber made of plastic is smooth and translucent. At this time, the raw rubber has basically reached the plasticity. If the lack of experience, can also relax the roller distance (2~3mm), so that it is wrapped in the front roller. If the surface of raw rubber is not smooth, it means that plasticizing is not enough. If the surface is smooth, it means that plasticizing is done. Of course, if the plasticizing is not enough, it must be re-plasticized until a satisfactory plasticizing effect is obtained.

The distance between the plasticized raw rubber rollers is relaxed to 2~3mm for the lower piece. The film is 80cm long, 40cm wide and 0.4cm thick. Cool naturally, then beat the spacer for later use. You can also directly use the scale to weigh the amount of the mixture. Plastic raw rubber is generally used after 8 hours of parking.

Nitrile rubber, here mainly refers to hard nitrile rubber, the initial Mooney viscosity of 90~120, plasticity and process performance is very poor, if not plasticized, mixing can not be carried out. Its raw rubber toughness, plastic heat, so the plastic is particularly difficult. Compared with natural rubber, the plasticizing temperature of nitrile rubber is lower than that of natural rubber (about 40 ℃), and the rubber loading capacity is about 15kg less (70% of natural rubber). As the acrylonitrile content of the nitrile rubber increases, the mixing is easier. Under normal circumstances, nitrile -26 and nitrile -40, which are used for molding products, can be used for a period of plasticizing raw rubber for about 40 minutes. The special requirements of the nitrile rubber to carry out multi-stage mastication, in order to obtain a satisfactory degree of plasticity. For nitrile -18, after a period of plastication, the plasticity is only about 0.18, which is too low for rubber products, and multi-stage plastication must be carried out. After 35 minutes of plasticizing nitrile rubber -18, the plasticity can reach 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 plastic raw rubber is smooth and shiny, and there are no large or small holes in the surface after the roller is wrapped.

In the process of plasticizing, there are many factors that affect the effect of raw rubber plasticizing. But for the specific operator, nothing more than the temperature of the mixing machine, roller distance, time, loading capacity and operation proficiency.

Practice has proved that the lower the roller temperature, the better the effect of mastication. 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 use of segmented plastic refining method to make up for its shortcomings. 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 mastication effect. The raw rubber passes through the roller at a small roller pitch. On the one hand, the shear 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 is emitted. This is of great benefit to the mastication effect. We have done comparative tests. Compared with the roller pitch of 0.5 ㎜ and the roller pitch of 1 ㎜, the larger the roller pitch, the longer the plasticizing time, about 10 minutes. And there is no small roll distance of the plasticizing quality is good.

Similarly, the amount of glue is also directly affected by the plasticizing effect of raw rubber. It can be imagined that it takes a long time for raw rubber to pass through the roller gap with more glue loading. In the same mastication time, the mastication effect with less glue loading is better than that with more glue loading. Some operators are greedy for speed when plasticizing, and the amount of glue is much higher than the specified requirements, which is very wrong. Too much glue, first of all, the equipment can not bear, overload operation, prone to "trip" at the same time also increased the risk of damage to the equipment; Secondly, the plasticizing time is prolonged, that is, the labor intensity is increased, and the quality of plasticized rubber is not good. Therefore, the amount of glue loading depends on the equipment and the type of glue, and the amount of glue loading cannot be arbitrarily increased. Under normal circumstances, the amount of synthetic glue is about 20% less than that of natural glue.

Some synthetic glue in the synthesis of unstable quality, the quality of each piece of glue is relatively large. People with operation experience can see at a glance which piece of plastic takes a relatively short time and which piece of plastic takes a longer time. Experienced operators tend not to pay much attention to the concept of time when plasticizing, but based on the plasticizing effect. The synthetic quality of raw rubber is not good, and the mastication time is often greater than the specified time. We have a less mature experience. Take nitrile -26 as an example. If the raw rubber is black, then the rubber is easy to plasticize. If the raw glue is white, it is not easy to plasticize. As for the above two kinds of rubber block is different in acrylonitrile content? Or other problems during synthesis, we have not done this test, can not jump to conclusions. When plasticizing, I only feel that the "moisture" of the white raw rubber is relatively large, and it is difficult for the raw rubber to pass through the roller seam, thus increasing the mixing time. If the same mixing time is used, the quality of the mixture is definitely not good. Domestic EPDM rubber also has the above problems. Raw rubber that is black when mixed is easy to roll, while white raw rubber is more difficult to roll. Some raw rubber to go through nearly 20 minutes or so to wrap the roller, virtually prolonged mixing time.

Mixing of 2. rubber

Mixing is one of the most important and complex processes in rubber processing. It is also one of the processes that are most prone to quality fluctuations. The quality of the compound directly affects the quality of the product. Therefore, it is very important to do a good job in the mixing of rubber.

As a rubber refiner, how to do a good job in rubber mixing? I think that in addition to strictly mastering the mixing characteristics of each rubber, dosing sequence and other necessary knowledge, we should also use our brains, think hard and refine the rubber with our heart. Only in this way is a more qualified rubber refiner.

In the mixing process to ensure the quality of the compound should do the following:

1. All kinds of compounding agents with small dosage but large effect should be fully mixed and evenly, otherwise it will cause scorching or undercuring of the rubber.

2, in strict accordance with the mixing process procedures, feeding sequence for mixing.

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 mixture be guaranteed.

4. Do not throw away the large amount of carbon black and filler at will, but use it up. And clean the tray.

There are many factors that affect the quality of the compound. But the specific performance in the combination of uneven dispersion, spray frost, scorch, etc., these are visible.

In addition to the particles with compounding agent on the surface of the compound, if the film is cut with a knife, the compound particles with different sizes will also be found on the cross section of the compound. Mixing uniform compound, the section is smooth. If the uneven dispersion of the compounding agent cannot be solved after repeated re-refining, the roller glue will be scrapped. Therefore, the rubber refiner must strictly abide by the process regulations during the operation, and take films from both ends and the middle of the roller from time to time to observe whether the compounding agent is evenly dispersed.

Spraying frost, if it is not a problem of formula design, is caused by improper dosing sequence during mixing, or uneven mixing and agglomeration of compounding agents. So in the mixing process to strictly check, to avoid the occurrence of this kind of phenomenon.

Scorching is one of the most important problems in the mixing process. After the rubber material is scorched, the surface or internal part has elastic cooked rubber particles. If the scorch is slight, it can be solved by a thin pass. If the scorch is serious, the rubber material is scrapped. From the perspective of process factors, the scorching of the rubber compound is mainly affected by 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, scorching. Under normal circumstances, the amount of glue during mixing is too large and the roller temperature is too high, which will increase the temperature of the glue and cause scorching. Of course, improper addition sequence, vulcanizing agent and accelerator added at the same time also easily cause scorch.

The fluctuation of hardness is also an important factor affecting the quality of the mixture. The same hardness of the rubber, often mixed out of the hardness is not the same, some even far. This is mainly due to the uneven mixing of the rubber and the poor dispersion of the compounding agent. At the same time, less or more carbon black will also cause fluctuations in the hardness of the rubber. On the other hand, the inaccurate weighing of the compounding agent will also cause fluctuations in the hardness of the compound. Such as vulcanizing agent, carbon black accelerator, rubber hardness becomes larger. More softeners and raw rubber are weighed, while less carbon black is added, and the hardness of the rubber material becomes smaller. If the mixing time is too long, the hardness of the rubber material becomes small. If the mixing time is too short, the compound will harden. Therefore, the mixing time should not be too long and too short. Mixing is too long, in addition to the hardness of the rubber material becomes small, the tensile strength of the rubber material decreases, the elongation at break increases, the aging resistance decreases, and so on. At the same time, it also increases the labor intensity of the operator and consumes energy.

Therefore, mixing as long as the various compounding agents can be fully dispersed in the rubber, and to ensure that the required physical and mechanical properties and calendering, extrusion and other process operation requirements on the line.

As a qualified rubber refiner that is not only have a strong sense of responsibility, at the same time, must also be familiar with all kinds of raw rubber and raw materials. That is, not only to understand their functions and characteristics, but also to be able to accurately say their names without labels, especially those with similar appearance. For example, magnesium oxide, nitric oxide and calcium hydroxide, high wear-resistant carbon black, fast-pressing carbon black and semi-reinforcing carbon black, as well as domestic nitrile -18, nitrile -26, nitrile -40 and so on.

Mixing of rubber. Most of the general units and factories use open type rubber mixing machine. Its biggest feature is that it is flexible and mobile, and it is especially suitable for the mixing of frequent rubber variants, hard rubber, sponge rubber, etc.

Mixing with an open mill, the dosing sequence is particularly important. Under normal circumstances, the raw rubber is put into the roller gap along one end of the roller, and the roller distance is controlled at about 2 ㎜ (taking a 14-inch rubber mixing machine as an example) and rolled for 5 minutes. The raw glue is wrapped on the front roller into a smooth non-gap film, and there is a certain amount of accumulated glue on the roller. Accumulated glue accounts for about 1/4 of the total amount of raw glue, then antioxidant and accelerator are added, and glue is pounded several times. The purpose of this is to make the antioxidant and accelerator evenly dispersed in the glue. At the same time, the anti-aging agent is added first to prevent the thermal aging phenomenon that occurs when the rubber is mixed at high temperature. Some accelerators have a plasticizing effect on the rubber. Zinc oxide was then added. When adding carbon black, a very small amount should be added, because some raw rubber will take off the roller as soon as carbon black is added. If there is any sign of roll off, stop adding carbon black, and then add carbon black after the rubber material is smoothly wrapped on the roller again. There are many ways to add carbon black. The main points are: 1. Add carbon black along the working length of the roller; 2. Add carbon black to the middle part 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, the roller is only partially degummed, and it is impossible to completely remove the roller. After the rubber is taken off the roller, the carbon black is easy to be pressed into a sheet, and it is not easy to disperse after the roller. In particular, when a hard rubber is kneaded, sulfur is pressed into a sheet shape and is particularly difficult to be dispersed in the rubber. Neither return refining nor thin passage can change the yellow "hemp" spots existing in the film. In short, when adding carbon black, add less and more frequently. Don't try to save trouble by pouring all the carbon black on the roller. The initial stage of adding carbon black is the fastest time to eat. At this time, do not add softener. After adding half of carbon black, add half of softener, which can speed up the "eating. The other half of the softener is added along with the rest of the 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 mixes 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 mixture. In the case of too much softener, carbon black and softener can also be added as a paste. Stearic acid cannot be added too early, which is easy to cause roll off. It is better to add it when there is some carbon black in the roller, and 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, DCP will be melted into liquid form and fall into the tray. In this way, the amount of vulcanizing agent in the compound is reduced. Thus affecting the quality of the mixture, it is likely to cause the vulcanization is not ripe. Therefore, the vulcanizing agent should be added at the right time, depending on different varieties. After all kinds of compounding agents are added, it is necessary to further turn over and mix the rubber material evenly. Usually turning operation methods have "eight knives", "triangle bag", "roll", "thin pass" and other methods.

The "eight knives" are cut at an angle of 45 ° along the parallel direction of the roller, four times on the left and right. The remaining glue is added to the roller after twisting 90 degrees. The purpose is that the rubber material is rolled in the longitudinal and transverse directions to facilitate uniform mixing. "Triangle bag" is a rubber bag that is made into a triangle by means of the force of the roller. "Rolling" is a cutter with one hand, and the rubber material is rolled into a cylindrical shape with the other hand, and then put into the roller. The purpose of this is to mix the rubber evenly. However, "triangle bag" and "roll" are not conducive to heat dissipation of rubber materials, easy to cause scorch, and labor intensity is high, so these two methods should not be advocated. Turned time 5~6 minutes.

After the rubber material is turned over, the rubber material shall be thinly passed. Practice has proved that the dispersion of the compound in the compound is very effective. The thin pass method is to adjust the roller distance to 0.1~0.5 ㎜, put the rubber into the roller, and let it fall into the tray naturally. After it falls, twist the rubber material 90 degrees to the upper roller. This is repeated 5 to 6 times. If the temperature of the compound is too high, stop the thin pass, and then thin pass after the compound is cooled to prevent the compound from scorching.

After the thin pass is completed, relax the roll distance to 4~5 ㎜. Before the rubber material is loaded on the car, a small piece of rubber material is torn off and put into the roller, so as to break the roller distance and prevent a large amount of rubber material from entering the roller, which will cause the rubber mill to be violently subjected to greater force and damage the equipment. After the rubber material is loaded on the car, it should pass through the roller seam once, then wrap it on the front roller, continue to turn for 2-3 minutes, and then release and cool it in time. The film is 80cm long, 40cm wide and 0.4cm thick. Cooling methods include natural cooling and cold water tank cooling, which depends on the conditions of each unit. At the same time, to avoid the film and soil, sand and other dirt contact, so as not to affect the quality of the mixture.

The roll distance should be strictly controlled during the mixing process. The temperature required for the mixing of different raw rubber and the mixing of various hardness rubber materials are different, so it is necessary to master the roller temperature according to the specific situation.

Some rubber refining workers have the following two incorrect ideas: 1. The longer the mixing time, the higher the quality of the glue. This is not the case, for reasons already described above. 2. It is believed that the mixing speed will be accelerated if the accumulated glue above the roller is less and the powder is added faster. In fact, if there is no accumulated glue between the rollers or the accumulated glue is too small, the powder is easily pressed into sheets and falls into the material tray. In addition to affecting the quality of the mixed glue, the material tray should be cleaned again and the fallen powder should be added into the roller room again. This is repeated many times, which greatly prolongs the mixing time and increases the labor intensity. Of course, if the accumulation of glue too much, powder mixing speed will slow down. It can be seen that too much or too little accumulation of glue is disadvantageous to mixing. Therefore, there should be a certain amount of accumulated glue between the rollers during mixing. When mixing, on the one hand, the powder is squeezed into the glue by mechanical force, on the other hand, the rubber is easy to form a fold under the action of mechanical force, and the powder is easily involved in the glue with the fold. As a result, the mixing time is shortened, the labor intensity is reduced, and the quality of the compound is good.

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

from: rubber technology network, rubber industry portal!