Manufacturer of blended rubber: Quality issues of blended rubber and their causes

　　1. Quality issue: Poor dispersion of mixed rubber

　　Causes:

　　A. Mixing process

　　Insufficient mixing time, discharge temperature too high or too low; simultaneously adding acidic and basic additives (such as adding stearic acid and anti-scorch agent ESEN with zinc oxide and magnesium oxide); insufficient plasticization; improper order of additive addition; filler added too late during the mixing cycle; simultaneously adding small particle size carbon black and resin or viscous oil; insufficient dispersion time for metal oxides; adding liquid accelerator after the rubber has already torn or cracked; batch size of rubber too large or too small.

　　B. Process

　　Not following the established mixing procedure; aggregates of oily and dry materials sticking to the top plug and feed hopper; inappropriate rotor speed; rubber being discharged from the press too quickly; not properly using the turning device on the press.

　　C. Equipment

　　Failure of temperature control in the internal mixer; insufficient pressure on the top plug; excessive wear at the weld layer in the mixing chamber; failure of roller temperature control on the press; malfunction of the overhead turning device on the press.

　　D. Materials

　　Expired rubber and partial coagulation; ethylene-propylene rubber or butyl rubber too cold; frozen natural rubber; insufficient pre-plasticization of natural rubber; excessive moisture in fillers (caking); adding viscous additives below the pouring point temperature; inappropriate use of additives.

　　E. Formulation design

　　The difference in Mooney viscosity of the elastomers used is too large; inappropriate selection of plasticizers and rubber; too many hard particle additives; excessive small particle size fillers; using resins with too high a melting point; insufficient liquid plasticizer; excessive fillers and plasticizers.

　　2. Quality issue: Scorching of mixed rubber

　　Causes:

　　A. Formulation

　　Excessive amounts of curing agents and accelerators; curing system acting too quickly; incorrect weighing of additives; excessive small particle size fillers; insufficient liquid plasticizer.

　　B. Mixing operation

　　Excessive filling capacity; insufficient cooling of the internal mixer; excessive rotor speed; initial feeding temperature too high; discharge temperature too high; incorrect timing for adding accelerators to the internal mixer; or adding sulfur too early, or uneven dispersion causing local high concentration of curing agents and accelerators; poor dispersion of accelerators and/or curing agents; resin accumulation on the rotor; failure to add anti-scorch agent; rolling too early without sufficient thin cooling, or rolls too large, or insufficient cooling after discharging.

　　C. Storage

　　Accumulating rubber while it is still hot and wet; rubber stored for too long; storage area temperature too high; lack of air circulation.

　　D. Measures to prevent scorching

　　Strictly control roller temperature, improve cooling conditions, add materials according to operating regulations, strengthen rubber management, etc.; adjust the curing system, add anti-scorch agents. Common anti-scorch agents are organic acid anhydrides (such as phthalic anhydride), generally not exceeding 0.4 parts; new high-efficiency anti-scorch agents, 0.1-0.2 parts, such as anti-scorch agent CTP (N-cyclohexylthio-phthalimide); the order of adding anti-scorch agents should be before curing agents and accelerators.

　　3. Quality issue: Clumping of mixed rubber additives

　　Causes:

　　Insufficient plasticization of raw rubber, excessive roller gap, excessive roller temperature, powder falling onto the roller surface; pressed into sheets; excessive filling capacity; coarse particles or clumps of powdered additives; too much gel.

　　4. Quality issue: Large shrinkage of mixed rubber

　　Causes:

　　A. Non-sulfur rubber: Plasticity too low; mixing time too short or internal mixing time too long, leading to aggregation.

　　B. Sulfur-added rubber: Rubber begins to scorch.

　　5. Quality issue: Rough surface of mixed rubber

　　Causes:

　　A. Non-sulfur rubber: Mixing time in the internal mixer too long, excessive carbon black gel.

　　B. Sulfur-added rubber: High rubber temperature and roller temperature causing scorching, mixing in some already reduced rubber.

　　6. Quality issue: Plasticity of mixed rubber too high, too low, or uneven.

　　Causes:

　　Plasticity of plasticized rubber inappropriate; mixing time too long or too short; improper mixing temperature; blending not well mixed; excessive or insufficient plasticizer; excessive or insufficient carbon black or wrong type used.