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Rubber compounding process

🕙 2023-09-19 👆

Rubber refining, including plastic refining, mixing, hot refining, recycling, etc., is mainly carried out using open mills and internal mixers.

NR and CR usually use narrow roller pitch thin passes in the open mill to obtain the required plasticity (range), meeting the requirements of subsequent processes for the fluidity of the mixed rubber and the physical and mechanical properties of the finished product. The introduction of "M promoting" or "plasticizing agent" in NR, and the injection of "dextrin water turbidity (self-made)" in CR can accelerate the plasticizing process. (Note: CR molding involves a "configuration transformation" process, which is well controlled and facilitates the molding effect.)

Mixing is often said to be the most critical processing step in the production process of rubber products. Textbooks or related monographs have extensively discussed the factors that affect mixing quality and evaluate the processability of mixing in open and internal mixers. For the key, complex, and ever-changing "feeding sequence", textbooks and other materials often simplify and provide general introductions, and literature is also relatively "confusing", Often appearing in the color of "following the crowd" and "copying is acceptable", from a different perspective, or "feeding order" is sometimes exactly the "confidential ingredient".

Related equipment:


Dual hydraulic mixer

In practice, many formulas do not need to be "rigorous" in terms of the "feeding sequence". "There are no tricks to win," and "confused rubber refining" can also obtain "qualified products". There are also many formulas with different refining methods and significant performance differences. If you do not pay attention to the "feeding sequence", you will not be able to produce the required mixed rubber, which is detrimental to the subsequent processing and will not produce qualified products (with a high waste rate!). In fact, it is necessary to carefully consider, consider, and experiment Compare and extract the best "feeding sequence", including feeding timing and time, feeding quantity and batch, etc. To explore, formulate, and confirm the appropriate feeding sequence, it is not only necessary to understand the characteristics of rubber and compounding agents, but also to focus on understanding the mutual (additive, synergistic, antagonistic) effects, affinity, physical and chemical reaction ability and degree between rubber (mixed rubber), rubber/compounding agents, and different types and varieties of compounding agents.

People not only need to strengthen their theoretical literacy in these aspects, but more importantly, they need to practice, compare, speculate, and verify more. Here, we mainly provide some examples for supporting and referencing the operation of mixer mixing. As for the "reasons" or "reasons", it is not appropriate or desirable to comment too much. It is better for people to "experience and think". Firstly, "use its nature" and temporarily ignore "its reason and nature". If conditions permit, gradually strive to "know its reason and nature", so as to handle problems more rationally, solve problems, and move towards the realm of "using the ingenuity, keeping in mind". If we must "know its reason and nature", we may have to wait for a long time in history

Example 1: "NR/brass valve" is directly bonded.

Method a: NR is first molded to reach the required plasticity A, followed by mixing. The plasticity of the mixed rubber reaches AA, and after bonding, it meets the standard with adhesive adhesion greater than 97% -100%.

Method B: NR is first molded to only reach plasticity B.

Example 2: CR/brass direct bonding (using imported adhesive SW).

If the CR thin pass is appropriate, the non-uniform tear off bonding strength shall not be less than 6MPa (or even up to 10MPa), and 100% adhesive adhesion shall be achieved; If the thin pass is insufficient, it is only about 4MPa.

Example 3: NR hollow tire (inner inflated aircraft model small tire) (With the help of an adhesive layer, the semi-finished product with two sides reaching a certain vulcanization state (such as about TC70) is bonded into a complete tire in the presence of an "inflating agent".)

If NR cannot reach the appropriate plasticity during mixing, it is difficult to obtain a finished product with uniform thickness, smooth surface without bubbles or scars, and when combined, it is easy to cause poor adhesion and scrap. The molding and mixing of "adhesive for bonding" are relatively more focused on rubber mixing technology, with a reasonable formula and appropriate molding and mixing. Even if the vulcanization state of the "two halves" far exceeds TC100, they will stick very well!

NBR, SBR, BR, EPDM, IIR, usually have different brands of "Mooney viscosity" to choose from, making it difficult to obtain or increase "plasticity" with the help of thin pass. However, before mixing, it is appropriate to thin pass or roll and pound for a period of time to improve its uniformity, reduce "agglomeration", and also improve the quality of the mixed rubber and finished product.

Example 4: First refine NBR base rubber, then fill with NBR (finished product with A45-50 degrees)

General party: NBR 100, "active/anti" 7, filler 47, resin 11, plasticizer 27, sulfur/promoter 3.6

Mixer formula: NBR 100, "active/anti" 14, filler 94, resin 22, plasticizer 54 (total 284)

Finished mixed rubber formula: internal mixing mother rubber 142, NBR50, sulfur/catalyst 3.6.

Our company has a relatively large amount of "plasticizer". In this way, we first "mix the base rubber" and store it for a period of time. When in use, we use a mixer to fill the other half (50 parts) of NBR and add (sulfur/promoter). Not only does the mixture disperse well, but the product surface is smooth and free of strings or beads, with stable quality, and the adhesion with aluminum castings is very reliable.

If the NBR adopts Jinhu 35LM and Bayer 3445, it can be filled into thin sheets through the opening mill roll; If JSR230S is used, it is best to thin it several times before filling, otherwise the quality (especially for bonding) will be difficult to stabilize.

Example 5: Using glue for NR/BR and other similar compounds

For the NR/BR blend formula, although NR is first thinned to a certain degree of plasticity, it takes a considerable amount of time for NR/BR to blend to the "no hole in the roll" state. It is better to mix the molded NR with a certain amount of filler to form a strong roll adhesive and then fill it with BR, which is not only easy to operate but also greatly saves time.

For the combination of NR/SBR/BR, BR/SBR has good compatibility and is easy to form a "hole free" roll wrap adhesive. After mixing to a certain level, it will be relatively time-saving and easy to operate by filling with NR that meets the standard of thin pass.

The combination of CR/NR, CR/BR, and the like often results in the addition of thin NR and BR in the later stage of CR mixing (NR and BR do not need to participate in the CR thin pass process), which not only saves time but also ensures good physical and mechanical performance and quality of the finished product.

As for the internal mixer for molding NR, after the internal mixer is completed, further thinning of the mixer for a period of time will achieve more uniform and better physical properties.

For method b in Example 1, the premature "binding" of NR and carbon black masterbatch results in a "weaker" shear force on the NR molecular chain, making it more difficult to break, damaging NR's "wetting" of carbon black, damaging its "fluidity", and ultimately damaging the "wetting ability of the rubber material to copper", ultimately damaging adhesion.

For Example 2, in addition to the plasticity of CR, the timing of adding "direct adhesive SW" is crucial. Method a: After adding carbon black/white carbon black and mixing for a period of time, "SW" is added, with a non-uniform tear off adhesion strength of ≥ 6MPa (even up to 10MPa) and 100% adhesive adhesion; If "SW" is added first and then carbon black/white carbon black is added, SW will be "adsorbed" or even "reacted" and "consumed", with a bonding strength of only 1MPa to 0MPa.

Example 6: Low hardness (high oil consumption) products

For rubber products that often rely on "high oil consumption" to obtain "low hardness" (such as 30-40 degrees Shore A), the method of adding fillers first and then (sometimes after adding vulcanizing agent) slowly adding oil (referred to as "post refueling method") is often more uniform than other methods (such as mixing fillers/oil and adding fillers and oil in batches), with good physical and mechanical properties and small dispersion of test data, and the apparent condition of the product, And the condition of "rotten edges" at the "closing line" will be greatly improved. NR (black) products with low hardness (Sho A32 ± 3 degrees) and high tear strength (≥ 45KN/m) are mixed with 45 parts of N220 carbon black first, and then slowly filled with 72 parts of aromatic oil to meet the standard. The test performance data is stable (attached: the vulcanization system should be carefully selected). The "back filling method" used in Example 4 is also effective in this regard.

For rubber products with high content of "filler and oil", first stir the "filler/oil" evenly at high speed, and then mix, which will undoubtedly save time and be more uniform than adding filler and oil separately in batches. For example, EPDM automotive special-shaped hot air hose contains 90 to 100 parts of carbon black (such as N330, N550, N660, etc.) and 45 to 50 parts of naphthenic oil or paraffin oil. When mixing, the "carbon black/naphthenic oil" is evenly mixed and added. Even if the mixing time is extended and the number of thin passes is increased, the surface of the vulcanized rubber test piece still cannot eliminate the "bead bubbles". However, this mixed rubber also undergoes processes such as "re refining", "hot refining", and "extrusion", and its vulcanized rubber tube reaches a "smooth surface without bubbles". Undoubtedly, the confirmation of the mixing method still needs to refer to the "production process history" of its products.

Example 7: Rubber products with high Mooney viscosity EPDM as the main material

EPDM with high Mooney viscosity, such as American DSM8340A, with an ML of 80 at 125 ℃; Bayer EPT9600125 ℃ ML is 94. The mixing roller temperature of the mixer should be high, and the roller pitch should be small. First (all or an appropriate amount), add oil to reduce the viscosity, and then slowly add carbon black in small batches. This moderately reduces the viscosity compared to the common "carbon black/oil mixing and adding" or "interval batch adding", which is conducive to the wetting, mixing, and dispersion of fillers (such as carbon black). The physical and mechanical performance is relatively superior and relatively stable.

Example 8: "NR/BR/carbon black compound" and "carbon black mother compound"

For the carbon black mixing method of "NR50/BR50" with adhesive. For example, "mixing first and then mixing", "mother compound mixing", "NR diluting method", "BR diluting method", the processing performance and finished product performance of mixed rubber are very different, each with its own unique characteristics, and there are quite a few domestic and foreign literature. NR and BR have different affinity for carbon black, and carbon black tends to combine with BR more. By using different methods of adding carbon black, adjusting the distribution of carbon black in different colloidal phases and achieving uniform dispersion, different performance levels or performance balances can be achieved. For example, in the "BR post dilution method", most or even all of the carbon black is first mixed into NR before adding BR for mixing, which moderately reduces the "concentration" of carbon black in the BR phase, significantly improves fatigue resistance, and saves mixing time.

This reminds us of the concept of "carbon black mother rubber refining", especially factories that do not produce carbon black rubber, and we should pay careful attention to this. There are factories purchasing "carbon black masterbatch", which is composed of (SBR 2/carbon black 6/oil 2), which is precisely the culprit for the "skin cracking" of the produced "small car tires" during storage. For example, NBR 70/N330 45/Gumarone 5/DBP 10-15 parts are first externally refined to form a master rubber, and NBR 30 is filled during use and a "vulcanization system" is added. The product's performance meets the standard and is stable. If NBR 50 parts are used to first refine the master rubber, and NBR 50 parts are filled and a "vulcanization system" is added during use, the hardness of the vulcanized rubber is relatively high, brittle and hard, which can easily break when folded. (Regarding this, 2 parts of the self-made XT-307 regenerant were used to disperse the excessively agglomerated carbon black inside and effectively combine with the 50 parts of NBR after filling, restoring its performance and making it more flexible.)

It is not difficult to understand that the amount of carbon black and the corresponding resin and oil used in the "carbon black mother rubber" should be carefully adjusted and determined. This once again reminds people that the "wetting" of fillers by rubber (as well as resin/oil) plays a significant role in the mixing process. The "agglomeration" of carbon black (or filler) has a significant "destructive ability" on subsequent processing and finished product performance.

Example 9: Low hardness (Shore A40) IIR products containing "white carbon black/coupling agent/carbon black"

After adjusting the roller pitch and temperature, IIR wrapping and moderate kneading, try the following three mixing methods:

A method (white carbon black/coupling agent) involves adding a small amount slowly, gradually increasing the "amount added each time". After each addition, it is refined for an additional period of time. After adding, the left and right rubber are moderately recycled, and then a small amount of carbon black is added multiple times. The resulting mixed rubber has a "smooth and glossy section", is flexible and strong, and has good fluidity. The surface of the product is smooth and free of small bead bubbles, and the "broken edges" at the parting line can be eliminated;

At the beginning of method B (silica/coupling agent), the material was added too quickly and too much;

C method first adds carbon black, followed by adding (silica/coupling agent);

Using methods b and c, it is difficult to achieve the effect of method a, which may indicate that the "strength" of IIR on the physical and chemical interactions between different fillers is not the same, as is the case with various rubbers. Therefore, the "feeding sequence" during mixing should be considered.

It is said that only 50% of the addition amount of coupling agent SI-69 participates in the reaction with silica, and often requires about 24 hours of parking (reaction) time. People should consider this when using it.

Example 10: Other examples

For CR, MgO should be added separately first, and it is not advisable to add St simultaneously to prevent "agglomeration" and poor dispersion. Domestic MgO generally does not stick to rollers, while Japanese MgO will stick to rollers when used in large quantities and added quickly. In fact, adding domestic CC together will greatly improve.

Adding "terpene resin" to NR, appropriately increasing the shear force on the cold roller to break and disperse, and then increasing the roller temperature to melt and further evenly disperse, will greatly improve its tensile strength.

Mixing "sepiolite" (100 parts) in SBR will cause the cold roller to stick to the roller and cannot be pulled down; If the roller temperature is higher than 70 ℃, the roller will not stick, which is extremely rare in other fillers.

In short, the "feeding sequence" of the mixing process is infinitely variable, and its complexity is difficult to describe. People can only practice and speculate more, comprehensively summarize relatively reasonable methods, and avoid "following the crowd"