Supplier of compounded rubber: Quality issues of compounded rubber and their causes

       In the previous article, we introduced the quality issues of the mixed rubber and their causes. Next, we will continue to introduce other issues and their corresponding causes.

　　1. Quality issue: The relative density of the mixed rubber is too high, too low, or uneven.

　　Causes:

　　Inaccurate weighing of additives, missing or incorrect mixing; when the amounts of carbon black, zinc oxide, calcium carbonate, or oil-based plasticizers are less than the specified amount, the relative density of the rubber material will exceed the specified amount; excessive loss of dust during mixing, excessive adhesion to the container wall, or the feeding container not being emptied properly; uneven mixing.

　　2. Quality issue: The scorch time of the mixed rubber is too long or too short.

　　Causes:

　　A. Extended scorch time: Incorrect type of accelerator, insufficient addition; missing zinc oxide; incorrect type of carbon black, such as using coal tar instead of machine oil; excessive addition of asphalt or clay.

　　B. Shortened scorch time: Excessive addition of accelerator or incorrect type; incorrect type of carbon black; excessive calcium carbonate.

　　3. Quality issue: The hardness of the mixed rubber is too high, too low, or uneven.

　　Causes:

　　Inaccurate weighing of additives, such as excessive reinforcing agents, vulcanizing agents, or accelerators, will result in higher vulcanized rubber hardness; conversely, lower hardness; excessive weighing of plasticizers and rubber will lead to lower hardness; uneven mixing will result in inconsistent vulcanization hardness.

　　4. Quality issue: Frosting of the mixed rubber.

　　Causes:

　　Insufficient mixing of the rubber material, uneven; inaccurate weighing of additives; sulfur agglomeration or exceeding its solubility in rubber at room temperature; excessive rubber temperature during sulfur addition; excessive use of softeners; prolonged storage time of the rubber material; insufficient sulfur in the product, etc.

　　5. Quality issue: Slow vulcanization onset of the mixed rubber.

　　Causes:

　　Inaccurate weighing of accelerators (too little), or missing zinc oxide or stearic acid; incorrect type of carbon black.

　　6. Quality issue: Insufficient sulfur in the mixed rubber.

　　Causes:

　　Missing or insufficient mixing of accelerators, vulcanizing agents, and zinc oxide; improper mixing operation, excessive loss of vulcanizing agents or accelerators.

　　7. Quality issue: Layering.

　　Causes:

　　Butyl rubber mixed into natural rubber or vice versa.

　　8. Quality issue: Roll sticking or roll release.

　　Causes:

　　A. Roll sticking: Excessive roll temperature or too small roll gap; excessive plasticity; excessive softeners; excessive mixing time or violation of the addition order, such as adding asphalt rosin later.

　　B. Roll release: Too low rubber content; hard rubber quality; large roll gap during mixing; certain synthetic rubbers.

　　9. Quality issue: Contamination of the mixed rubber.

　　Causes:

　　Physical contamination of elastomers and rubber products caused by dust, dirt, sand, and other substances; chemical contamination of butyl rubber and ethylene-propylene rubber caused by other elastomers (such as natural rubber and nitrile rubber); not using separate scoops for different additives; using inappropriate additives; residual additives in previously used trays; oil leakage from the oil seal of the internal mixer; residual rubber sticking to the rotor, discharge door, feed hopper, and top bolt; regular cleaning with clean rubber can reduce such problems; residual rubber sticking to the discharge trough, receiving tray, guiding trough, and overhead flipping device; residual rubber accumulating at the sealing ring; untidy areas around the internal mixer and tablet press.

　　10. Quality issue: Physical and mechanical properties are unqualified or inconsistent.

　　Causes:

　　Inaccurate weighing of additives, especially reinforcing agents; mismatching or missing vulcanizing agents and accelerators; excessive mixing; unreasonable feeding order and uneven mixing can easily lead to inconsistent performance.

　　11. Quality issue: Performance differences between batches of mixed rubber.

　　Causes:

　　Differences in initial feeding temperature; differences in cooling water flow or temperature; differences in top bolt pressure; errors in weighing additives; differences in additives between different batch numbers; use of substitute additives; differences in discharge time and temperature; different operators using different methods to process rubber on the tablet press; variations in kneading time; different degrees of dispersion.

　　12. Quality issue: Poor calendering performance of the mixed rubber.

　　Causes:

　　Improper selection of roll temperature; failure in controlling the ratio of roll temperature to roll speed and roll cylinder speed; too low Mooney viscosity of the rubber material; excessive addition of tackifiers; excessive filling of viscous fillers (such as clay); lack of appropriate processing aids in the formulation; insufficient or excessive loading; incorrect selection of elastomer viscosity; poor dispersion; rubber material prone to scorching; rubber material staying on the open mill for too long.