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Several practices in the production of rubber products

🕙 2023-08-08 👆

1、 The "feeding sequence" of the mixing process

As is well known, mixing is an important processing process for rubber products. Surprisingly, the order of adding additives during the mixing process is relatively "vague" in the literature, usually showing a situation of "following the crowd" and "copying is acceptable". Perhaps, because sometimes it is also precisely 'confidential content'. In practice, many formulas do not need to be strict with the "feeding sequence", and "there are no tricks to win" and "muddled rubber refining" are also "normal production"; However, it is undeniable that there are many formulas that, without paying attention to the "feeding sequence", cannot produce the required mixed rubber, which is detrimental to the subsequent processing and cannot produce "qualified products". Below are just a few examples of operating a mixer for mixing. As for the "reason" or "reason", I do not want to comment more, leaving it for readers to "experience and think".

Example 1 (NR carbon black adhesive)/(valve nozzle) brass direct bonding

a) NR thin tube reaches a certain plasticity of plastic compound, then carbon black is added and mixed step by step. The adhesive strength and adhesive amount meet the standard, and the test data is stable.

b) NR thin pass, when the required plasticity of the plastic compound is not achieved, carbon black is added for mixing. Even if the plasticity of the a) method mixed compound is first achieved through thin pass mother compound (the total energy consumption is nearly twice that of the a) method), and then mixed step by step, it is difficult to achieve the bonding effect of the a) method, and the data is also quite scattered.

b) In the law, NR combines with carbon black too early through mixing, making it more difficult for macromolecular chains to "break", damaging fluidity and bonding effect.

Example 2 (CR/Carbon Black) Adhesive/Brass Direct Bonding

CR has reached the required plasticity through thin pass, and then

Note: If a small amount of "transformation agent FH 210" is added during thin pass molding, it will promote the transformation of CR configuration, resulting in faster molding and better results.

a) After adding fillers such as carbon black/white carbon black and kneading appropriately, the "direct adhesive SW" is added. The non-uniform tear off adhesive strength exceeds 6MPa (up to or exceeding 10MPa), and the adhesive adhesion is 100%.

b) First add "adhesive SW" and then add (carbon black/white carbon black). In extreme cases, the non-uniform tear off bonding strength is less than 1MPa, and the adhesive adhesion is close to 0%.

b) In the law, carbon black and white carbon black adsorb the "adhesive SW", or come into contact or react with each other, causing premature consumption and greatly losing the bonding effect.

Example 3 Low hardness rubber products

For rubber products that often rely on "high oil content" to obtain "low hardness" (such as Shore A30 °), the method of first adding filler/finally slowly adding oil (later refueling method) is often more uniform than other methods (such as adding filler/oil mixture at the same time, spacing between filler and oil or batch addition), with good physical and mechanical performance and relatively concentrated data. At least, the apparent condition of the product and the occurrence of "rotten edges" at the clamping line have greatly improved. NR products with low hardness (32 ° ± 3 °)/high tear strength (greater than 45kN/m) require this mixing method to meet the standards and have stable test data; Practice has proven that the "post refueling method" is also an effective rubber mixing method for "low hardness/uniform, fine, and microporous" sponges.

The "post filling method" mentioned at the 2005 annual meeting as an example of "reverse thinking" is also an effective mixing method for such "high oil content/low hardness" rubber products. High viscosity is beneficial for obtaining high shear force and helps to disperse the filler.

Example 4 Rubber products with high Mooney viscosity EPDM as the main material

For the mixing of high Mooney viscosity EPDM (such as Dutch DSM's 8340A, ML 125 ℃ value of 80; Bayer EPT 9650, ML 12 ℃ value of 94) as the main material, in addition to high roller temperature, first (all or an appropriate amount) oil is added, and then the viscosity is lowered before adding carbon black in small batches. Compared to the common mixing method (carbon black/oil mixed and added together, or added in intervals or batches), relatively superior and relatively stable mechanical properties can be obtained.

Moderately reducing viscosity first is beneficial for wetting the filler and for mixing and uniform dispersion of the filler.

Example 5 Low hardness (40 °) IIR products containing (silica/coupling agent) and carbon black

After adjusting the roll spacing and temperature, IIR roll wrapping and moderate kneading:

a) (White carbon black/coupling agent) A small amount is slowly added, gradually increasing the "addition amount" each time. After each batch is added, an appropriate amount of time should be added. After adding, the left and right rubber should be cut back for refining, and then carbon black should be added. The resulting mixed rubber not only has a "smooth and shiny section", is flexible and strong, and has good flowability, but also has a smooth surface of the vulcanized product without "small bubbles", and the "rotten edges" at the mold closing line are reduced or even eliminated.

b) The method is the same as a), but (silica/coupling agent) is added too quickly and in excessive amounts at the beginning, and even, such as

c) Adding carbon black first, followed by adding (white carbon black/coupling agent), it is difficult to achieve the effect of method a) using both b) and c) methods.

It should be noted that the interaction strength between different types of fillers in IIR varies, which inevitably affects the mixing effect.

Example 6 (NR/BR/carbon black) mixed rubber

The addition method of carbon black in the mixed rubber (such as blending first and then mixing, NR thinning method, BR thinning method, and master rubber blending method) is different, and the processing performance and finished product performance of the obtained mixed rubber will vary greatly. There are quite a few domestic and foreign literature on this aspect. NR and BR have different affinity for carbon black, and appropriate "carbon black addition method" should be selected based on the required (main) performance (combination). By using different carbon black addition methods, the distribution of carbon black in different colloidal phases can be adjusted to achieve uniform dispersion and achieve varying levels of performance (or balance of performance).

For example, in order to achieve fatigue resistance, most or all of the carbon black is first added to NR, and then added to BR for subsequent mixing after moderate rubber mixing (BR dilution method). The fatigue resistance is significantly better than the other three methods, and the mixing time is relatively saved.

In short, understanding and mastering the interaction between rubber and compounding agents, as well as their effects on processing performance, physical and mechanical performance, and product quality, can be confirmed and adjusted in practice, which helps to accurately confirm the "feeding sequence" of the mixing process.

2、 T90 of vulcanization instrument

At the 2002 annual meeting, Teacher Wu had already discussed T90.

Many articles in the magazine assume that the vulcanization condition is t90, which is confusing.

a) T90, the "appropriate vulcanization time" in statistical significance, may not necessarily be the "optimal vulcanization time" for a specific performance. Given that different types of products have different specific performance groups, it is necessary to have an understanding and mastery of the "appropriate vulcanization time" corresponding to various properties (such as fatigue resistance, wear resistance, tear strength, etc.), in order to more effectively select the "vulcanization time".

b) The "statistical significance" of t90 is relatively obvious for diene rubber, such as NR, BR, SBR, and NBR; Some rubbers (such as IIR, ECO), as well as certain vulcanization systems, have less clear statistical significance of t90. Sometimes, it is necessary to double the t90 time to cure (such as no internal bubbles), which is a common fact in factories and laboratories.

c) Due to the artificial setting of the testing time for the vulcanizer, the size of t90 for the vulcanizer curve with "slightly inclined upward" or "inclined upward" depends significantly on the "set testing time", resulting in "uncertainty" in the calculated t90.

d) As an instrument for (online) testing the quality of rubber materials (determining trade-offs), the vulcanizer has transitioned from "with rotor" to "without rotor", with the aim of accelerating the testing process. The differences in sample thickness, presence or absence of rotors, and operating history between these two types of vulcanizers result in different "heat transfer effects". Although there is a similar (shape) "vulcanization history curve", the calculated t90 is very different, and the "rotor free" t90 is much smaller! It should be remembered that the "heat transfer conditions" of the vulcanization equipment (and molds) are different from those of the vulcanization instrument, and the differences are even greater. Many times, in addition to referring to the T90, the actual vulcanization conditions of the product also need to be adjusted wisely using the vulcanization effect integrator.

e) The "vulcanization curve" and the calculated t90 based on it, especially for the "combined combination" with different vulcanization mechanisms or different types of vulcanization systems, are sometimes just "false" and have situations such as immature vulcanization and internal foaming, which are meaningless for product production.

Generally speaking, t90 only has the meaning of "qualitative" or even "semi quantitative". The actual vulcanization conditions should be adjusted and confirmed based on the actual working conditions of t90 to ensure product quality.

3、 "Broken edge" of mold closing line of molded rubber products

"Rotten edges" ("shrunk edges", "concave edges", back riding) are a combination of formula, process, and mold design. As for the specific situation, which factor or combination of factors is at work, it is often difficult to determine and "prescribe the right medicine"! Usually, low hardness adhesives with high oil content, adhesives with high recycled rubber (rubber powder) content, and adhesives with short scorch time (or rapid vulcanization) are relatively prone to "rotten edges". Example 3 and Example 4 show the effect of rubber mixing process on "rotten edges". As is well known, appropriately lowering the vulcanization temperature, relatively strict control of the amount of rubber filled and its appropriate distribution are commonly used process methods in factories to eliminate "rotten edges". As for the "effect of mold design", the following are some examples that may be helpful and helpful in solving "rotten edges".

Example 1 EPDM (Shore A75-80 °) conductive adhesive

In 2000, the author conducted a trial production of EPDM conductive rubber products using the molding method and the "plastic injection mold type wide flow rubber groove", which caused serious "edge damage". Subsequently, it was modified to a thin line (width and depth of about 1.0-1.5mm) glue flow groove, with the formula and vulcanization process unchanged, and the "rotten edges" no longer appeared. This method has been successfully applied to various rubber products.

Example 2

Four cavity the first mock examination is adopted for the small tire of four-wheel drive (remote control) model racing car. Each cavity is relatively independent. The products in each cavity are "broken edges" near the center of the mold. It is estimated that the pressure difference in different areas of the mold is caused. The single cavity mold is used to greatly improve the "broken edges" so as to eliminate them. The effect will be better if the "single cavity injection mold" is used instead.

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