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Dead rubber: Early vulcanization of a compounded rubber compound containing a vulcanizing agent, resulting from improper processing or storage. Essentially, it is localized crosslinking within the rubber. The material thus formed is referred to as “dead rubber.” This phenomenon is known in technical terms as scorch; in industrial settings, it is commonly called “dead rubber,” which simply denotes rubber that has undergone scorch.

Note: For both winter and summer compound formulations, adjust the accelerator dosage as needed to appropriately extend the scorch time while reducing the Mooney viscosity to meet production‑processing requirements.

In actual processing, to prevent scorching and ensure good in-mold flowability of the rubber compound, it is common practice to add a scorch retarder.

Early vulcanization, also known as self‑cure, occurs when the rubber compound undergoes premature crosslinking during processing or while stored prior to vulcanization.

The primary causes of scorching in rubber compounds are improper formulation design—such as the use of an unsuitable type of accelerator or excessive dosage—as well as excessively high temperatures during mixing, extrusion, and calendering, and prolonged exposure to elevated storage temperatures.

In addition to addressing the aforementioned causes by optimizing the formulation and processing conditions, scorch resistance can also be enhanced by incorporating scorch retarders to extend the scorch time of the rubber compound.

Cause:

(1) Improper formulation design, an imbalanced vulcanization system, and excessive use of vulcanizing agents and accelerators.

(2) For certain rubber compounds that require mastication, if the mastication process fails to meet specifications—resulting in excessively low plasticity and overly rigid rubber—the temperature may rise sharply during mixing. Similarly, if the roll temperature of a mixer or other roller‑type equipment (such as a re‑mixer or calender) is too high and cooling is inadequate, scorching may occur on site.

(3) During the discharge of compounded rubber, if the sheet is too thick and heat dissipation is inadequate, or if the material is piled up prematurely without cooling, combined with poor warehouse ventilation and excessively high ambient temperatures, heat can accumulate, which may also lead to scorching.

(4) Poor management during rubber compound storage—leaving the material stacked and unused even after its remaining scorch time has expired—can lead to spontaneous scorching. This results in processing difficulties, degrades the product’s physical properties and the smoothness and flatness of its surface, and may even cause joint failure at the product’s seams.

Remedial measures:

(1) The rubber compound formulation should be appropriate and well‑balanced; for example, accelerators should preferably be used in combination. Scorch retardation is essential. To accommodate high‑temperature, high‑pressure, and high‑speed mixing processes, an appropriate amount (0.3–0.5 phr) of a scorch inhibitor may also be incorporated into the recipe.

(2) Strengthen cooling measures for the rubber compound during the mixing and subsequent processing stages, primarily by strictly controlling machine and roll temperatures and ensuring adequate circulating cooling water, so that the operating temperature does not exceed the scorching threshold.

(3) Emphasize the management of rubber compound semi-finished products: each batch must be accompanied by a production‑line card, the “first‑in, first‑out” storage principle must be strictly observed, and a maximum allowable storage period for each truckload of material must be established and not exceeded. The warehouse shall maintain adequate ventilation.

(4) The vulcanization system was adjusted, and the adoption of a secondary sulfenamide accelerator significantly improved scorch resistance.

(5) Improve the storage and processing conditions of the rubber compound, such as enhancing cooling;

(6) Use a coking inhibitor.

Factors Influencing Scorching and Corresponding Solutions

During processing, storage, and molding, the rubber compound is subjected to heat, which can lead to premature vulcanization (crosslinking) and the loss of its flowability and reprocessability—this phenomenon is known as scorch. In other words, scorch results from the cumulative effects of heat and time on the compound during manufacturing; it may also be referred to as the compound’s thermal history. The longer the thermal history and the higher the temperature, the more rapidly the scorch time of the compound can be shortened. Given a fixed compound formulation, what are the primary factors contributing to scorch, and what are the corresponding solutions?

1. The degassing temperature and the cooling storage temperature are too high.

Solution: The discharge temperature for the masterbatch is typically around 135°C, with a maximum of no more than 150°C; the discharge temperature for the final compounded rubber is generally kept below 105°C, with a maximum of no more than 110°C. Storage temperatures should generally be maintained below 45°C, with a maximum of no more than 50°C.

2. Parking Duration

Remedial measures: If the resting time is too short, the rubber compound’s fatigue recovery is insufficient, leading to a shorter scorch time; typically, the minimum resting period should be no less than 6 hours. If the masterbatch rests for too long, it can cause difficulties in final mixing, prolong the mixing process, increase scorch losses, and shorten the scorch time. Conversely, if the final mix rests for an excessively long time, blooming or self‑vulcanization may occur, also resulting in a shortened scorch time.

3. The mixing uniformity is poor.

Solution: Avoid localized concentration of certain compounding agents and accelerate the vulcanization rate to prevent localized scorching.

4. The rubber mixing process refers to the duration of the mixing time.

Solution: While ensuring the quality (uniformity) of the rubber mixing process, minimize the mixing time as much as possible.

5. Equipment Cooling Performance

Solution: Ensure adequate equipment cooling to reduce the temperature rise of the rubber compound and extend the scorch time. Regularly clean the equipment to enhance its heat‑transfer efficiency.

6. Extrusion or calendering temperature and speed

Solution: While ensuring the production process and capacity, minimize the process temperature and speed as much as possible.

An ideal anti‑scorching agent should possess the following properties: (The anti‑scorching agent CTP is suitable only for sulfur‑vulcanized systems and cannot be used in peroxide‑ or metal‑oxide‑vulcanized systems.)

Anti‑coking agents are mainly classified into three categories: organic acids, nitroso compounds, and thioamide compounds.

1. It exhibits excellent anti‑scorch performance, shows little selectivity toward various accelerators and rubber compounds, and is insensitive to other compounding agents.

2. It has no adverse effect on the vulcanization characteristics of the rubber compound or on the properties of the vulcanizate.

3. Exhibits good storage stability and handling performance; does not cake, dust, or spray frost.

4. Complies with industrial production safety and hygiene requirements;

5. Excellent value for money;

6. Ideally, it should also offer other beneficial features.