Common problems in rubber factories, their causes, and solutions

The difference between rubber and other engineering materials is that its processing process usually faces many unique problems that are different from common problems in the processing of other non rubber materials. This article provides some opinions on the main problems, possible causes, and solutions that may arise in the production process of rubber production enterprises.

Firstly, any change (or change) in the rubber material and processing process will inevitably affect the other properties of the (unvulcanized or vulcanized) rubber, which may improve or deteriorate. We need to have a full understanding of this, which is very important. Therefore, the impact of any change on the rubber formula and processing process should be studied in advance. We should first conduct experiments in the laboratory to determine the impact of these changes on the overall processing process and the physical properties of the vulcanized rubber, including the special properties of the rubber compound. Similarly, small-scale experiments should also be conducted in the factory to gain a better understanding of the potential impact of such changes (or changes) on the processing process and rubber products. In addition, small-scale research should be conducted on the product to ensure that these changes do not pose long-term risks to the product, and attention should be paid to all appropriate health and safety precautions. "These changes to the rubber formula can only be implemented after advanced scientific experiments.

The following is a list of the problems that rubber production enterprises face during the production process. This is only the author's personal viewpoint and not all the problems encountered during rubber processing. Of course, the frequency of these problems varies greatly depending on the actual production situation of each enterprise.

(1) Raw material storage area

-Cold deformation

-Stability of pre dried powder mixture

(2) Mixing

-Mixing quality

-Unvulcanized rubber elasticity (resilience)

-Viscosity

-Dispersibility

-Rubber spray frost

-Raw rubber strength

-Self-adhesive

-Adhesion

-Tumorigenesis

-Roll off

-Wrap the rear roller

(3) Extrusion

-Extrusion expansion rate

-Extrusion speed (melting failure)

-Appearance (smoothness of extruded adhesive surface)

-Shear thinning

(4) Rolling

-Bubbles

-Rolling relaxation

(5) Molded vulcanization

-Take off chess

-Model pollution

-Underglue

-Stomata

-Shrinkage of finished products

-Mold shrinkage crack

1 Raw material storage area

The following are several common issues related to raw material storage areas.

1. Cold deformation

Cold deformation refers to the gradual deformation of the raw rubber package under the action of gravity (or other weight of the package). Different varieties of rubber exhibit varying degrees of cold deformation. For example, a pack of TSR 10 natural rubber may have a lower degree of cold deformation than a pack of highly linear synthetic rubber. In fact, the same type of rubber with different specifications will also exhibit varying degrees of cold deformation. For example, the degree of cold deformation of highly linear cis butadiene rubber is greater than that of long branched cis butadiene rubber. Similarly, NBR with a narrower molecular weight distribution typically has better cold deformation resistance than NBR with a wider molecular weight distribution.

Modern freight containers and film packaging can reduce or eliminate most of these cold deformation problems, but cold deformation problems still exist within a specific range.

2. Stability of pre dried powder mixture

In the past 20 years, the trend in the rubber industry has promoted the use of pre weighed mixed powder mixtures, which are packaged in special bags that can be divided into multiple bags. Refining is carried out indoors or directly provided by external suppliers after mixing, in order to reduce factory dust while improving productivity and accuracy.

In the process of developing pre weighed mixed powder mixtures used in factories, it is necessary to consider the particle size of powder additives, the density differences of these additives (to eliminate layering), and the possible chemical reactions between different chemical additives after powder mixing and storage. Before being used in factory mixing and processing, certain chemical reactions between these powdered additives strictly limit the effective storage period of pre weighed powdered mixtures. Examples of chemical reactions occurring under dry conditions are when DPC (diphenylguanidine) is mixed with a sulfenamide accelerator, or when CTP (N-cyclohexylthiophthalimide) reacts with a sulfenamide accelerator.

2 Mixing

The following are some common questions in the mixing process:

1. The best mixing quality

Maintaining stable mixing quality (or mixing state) is crucial for avoiding different quality issues in subsequent processes. It is important to control the entire processing process of the mixing process (including the energy consumed during rubber discharge) to maintain the stability of batch by batch rubber quality. Similarly, the order of adding admixtures, mixing methods (such as one-stage mixing, multi-stage mixing, reverse sequence mixing, positive sequence coal mixing, etc.), type of mixer, rotor structure, rotational speed and/or barrel speed ratio, water temperature, cooling system type, tooth tip clearance, and filling coefficient are all very important for maintaining mixing quality.

2.2 Unvulcanized rubber elasticity (resilience)

During the mixing process, the elasticity of unvulcanized rubber usually decreases with increasing mixing time. In rubber factories, different batches of mixed rubber have varying degrees of unvulcanized rubber elasticity (rebound), which can affect the quality of the mixed rubber in the next process. If the mixing time of the mixed rubber is short (the mixing period is short), it may have high elasticity, high extrusion expansion rate during the extrusion process, or cause underfill during injection molding.

During the development process of the formula, different elastomer substrates can affect the deformation resistance of specific rubber compounds. On the other hand, different elastomer substrates can also cause the rubber to have different plasticizing speeds during mixing. If the rubber material is molded too quickly during the mixing process, the resilience of the final rubber will decrease. Finally, rubber compounds using high filled carbon black and other fillers will have lower resilience and extrusion expansion rate.

3. Viscosity

Simply put, the viscosity of the rubber material refers to its resistance to flow. The viscosity of the adhesive material is high, and it has high resistance to flow in subsequent processes. The viscosity of rubber materials is usually measured using a rotational viscometer (Mooney viscosity), capillary viscometer, or sinusoidal vibration viscometer (usually RPA).

Like the unvulcanized rubber elasticity mentioned earlier, the viscosity of the final rubber will decrease with increasing mixing time during mixing. However, the viscosity of rubber materials usually does not decrease as quickly as the elasticity of unvulcanized rubber decreases with the increase of mixing time and processing time. Similarly, a high viscosity of the adhesive material can lead to the formation of

There are various problems, such as increasing the frequency of underfill during injection or extrusion processes.

In addition to improving various mixing methods, there are also many formula techniques commonly used to reduce the viscosity of rubber materials. For example, selecting an elastomer with a lower average molecular weight as the base material, using a small amount of liquid rubber, using more operating oil, using carbon black with lower filling content or larger particle size, and carbon black with lower specific surface area can reduce the viscosity of the rubber material. Similarly, by correctly using lubricating fillers, organic silanes with white carbon black, and surface treated fillers, and selecting certain active additives for peroxide vulcanization, the viscosity of the rubber compound can also be effectively reduced.

4. Dispersibility

Blending agents with good dispersibility can usually be used for effective mixing processing. Obtaining good dispersion can effectively improve important vulcanizate properties, such as wear resistance.

Secondly, when adding carbon black to the rubber blend, it is very important for carbon black to have good dispersibility in order to achieve a high degree of carbon black dispersion. In fact, the type of carbon black used (such as high or low structure, fine or large particle size, high or low filling amount) often largely determines its final dispersion. Even properties such as particle hardness can have a significant impact on dispersion. Sometimes it is necessary to obtain the necessary carbon black dispersion through multi-stage mixing. The average molecular weight and molecular weight distribution of the elastomer matrix can also affect the dispersion of carbon black. Sometimes, the rational use of carbon black masterbatch compounds (such as SBR1600 series) can be beneficial for improving the dispersion of carbon black.

White carbon black is particularly difficult to disperse in many rubber compounds, and sometimes the dispersion of white carbon black can be improved by correctly using certain additives.

Due to the fact that vulcanizing agents are usually added in the late stage of mixing to avoid excessive heat generation in the rubber material, it is often not possible to obtain sufficient mixing time to achieve good dispersion of the vulcanizing agent, which can reduce the physical properties of the rubber material after vulcanization and cause quality problems.

5. Spray frost

The term frost spray is commonly used to describe the appearance problems caused by the precipitation of surface exudates or certain coordination agents during the unvulcanized or vulcanized stage. It is generally believed that due to the differences in chemical properties of many admixtures, they cannot completely dissolve in the elastic matrix and will precipitate over time (frost spraying). In the factory, frost spraying during unvulcanized storage can lead to problems such as adhesion of semi-finished products in the next process. Spraying frost on vulcanized products will cause customer dissatisfaction.

To solve the spray box problem, the first step is to determine which type or combination of agents caused the frost (appearance alone may not be sufficient to make this determination). Once a determination is made, various corrective measures can be taken. The sources of frost spraying can come from many places, including the formation of zinc on site, the type and amount of sulfur, the type and amount of accelerators and vulcanizing agents, the type and amount of antioxidant used, the type and amount of oil used, and the mixing method and steps.

6. Raw rubber strength

This term only describes the strength of the rubber material in the unvulcanized stage. Sometimes it is important for the rubber material to have a certain raw rubber strength. This can prevent a complex shape of unvulcanized extruded rubber from being flattened due to the influence of gravity, or it can prevent premature "bursting" of the tire on the secondary tire forming machine.

Sometimes, excessive plasticization of the rubber matrix during the mixing process can lead to poor strength of the raw rubber. Sometimes using special phase mixing techniques can improve the raw rubber strength of the rubber compound.

Most rubber materials with high raw rubber strength are based on natural rubber (as they are strain crystalline elastomers). However, usually for elastomers, if the elastomer matrix has a higher average molecular weight or sometimes a narrower molecular weight distribution, then higher raw rubber strength will be obtained. Similarly, for EPDM, the length of the branch chain and the percentage content of ethylene have a significant impact on the raw rubber strength of the rubber compound. The star like structure of molecules can also improve the strength of raw rubber. Sometimes, block polymers can also improve the strength of raw rubber. There are also reports that unstable cross-linking formed through post polymerization treatment can improve the strength of raw rubber.

Blending agents can also be used to improve the hardness of raw rubber. It is already known that adding a small amount of additives such as trans polyoctene rubber (TOR) can significantly improve the strength of the rubber compound. Similarly, there are reports that adding chemical accelerators to carbon black filled rubber can improve the raw rubber strength of the rubber. It is worth noting that the use of fully reinforced carbon black with a higher filling amount can particularly improve the strength of the raw rubber.

Finally, an effective way to improve the main adhesive strength of the adhesive may be to expose the adhesive to an electron beam.

7. Self-adhesive

Excellent semi-finished self-adhesion refers to the ability of two layers of unvulcanized rubber cord fabric to adhere to each other under appropriate pressure during a very short contact parking time. Excellent self-adhesive performance is crucial for forming embryos or manufacturing conveyor belts. However, having self-adhesive semi-finished products may require some methods, which is not necessary or desirable to apply molding operations, as it may cause problems due to strong adhesion.

The self-adhesiveness of semi-finished rubber materials based on natural rubber is usually stronger than that of rubber materials based on other elastomers. In fact, for blends based on natural rubber, the higher the natural rubber content, the stronger the self-adhesive performance of the semi-finished product. Similarly, the factory environment (temperature and humidity) also has a significant impact on the self adhesion of semi-finished products. Sometimes, using a longer mixing time during the mixing stage can also improve self viscosity. In addition, appropriate selection and use of the correct tackifier can also improve self adhesion.

8. Associativity

The adhesion performance between the adhesive and the metal surface is very important for predicting processability. "Adhesion" is different from "self adhesion", which has been explained above. A highly self-adhesive adhesive does not necessarily indicate strong or weak adhesion to metal surfaces. Sometimes, a certain degree of adhesion is essential for good processing performance. For example, if the rubber material does not stick to the barrel of the extruder during the processing, it cannot be extruded. On the other hand, the strong adhesion between the adhesive and the metal surface can make processing very difficult.

Many times, simply adjusting the temperature of the production equipment during the processing can improve the adhesion performance. However, for one type of adhesive, the optimal temperature may not necessarily be the optimal temperature for another type of adhesive. The optimal processing temperature depends on the adhesive material.

Generally speaking, compounds with higher viscosity values are not as good as compounds with lower viscosity values. Many times, it is advisable to consider using calender isolators and refiner isolators. Similarly, when using certain elastomer substrates in combination, such as chloroprene rubber and other synthetic rubbers, it can reduce adhesion.

9. Tumorigenesis

There are various reasons for the occurrence of lumps in the batch of adhesive. Sometimes the buildup is left over from the previous batch of rubber, followed by very incompatible elastomers, especially when used with very different Mooney viscosity values. Furthermore, the occurrence of tumors may be due to poor chemical compatibility of fillers, or a combination of these factors and poor mixing conditions (as mentioned above). Sometimes the tumor contains undispersed aggregates. Sometimes it can be seen that the nodulation after vulcanization is caused by the dispersion of the vulcanizing agent, which is due to the excessive concentration of undispersed vulcanizing agent ultimately leading to nodulation.

10. Roll off

Roll detachment on a two roll open mill refers to the inability of the mixed rubber to form a horseshoe pattern in the distance between the open mill rolls and to hang down from the rollers, showing little or no adhesion to the rollers. In order to improve the rolling resistance of tire tread, there is currently a trend towards using solution polymerized styrene butadiene rubber instead of emulsion polymerized styrene butadiene rubber. The rubber material based on solution polymerized styrene butadiene rubber has better roll off performance on the open mill because of its high molecular weight and narrow average molecular weight distribution. There have also been reports of high filling content of cis-1,4-polybutadiene rubber material demolding. Previously, there were reports that milk powder was used on hot open mill rollers, causing temporary stickiness on the surface of the rollers.

There are also ways to reduce or eliminate the problem of roller detachment on the open mill by adjusting the roller pitch, surface temperature of the roller, and/or roller speed ratio appropriately.

11. Wrap the rear roller

Like the roll off discussed above, sometimes the mixed rubber will also be transferred to the back roll of the two roll open mill. Usually, this problem can be solved by adjusting the temperature of the rollers, the distance between the rollers, the speed ratio of the rollers, or changing the model of the mixer.