After the silicone and curing agent are mixed, the viscosity will experience a dynamic change process of first being stable for a short time and then gradually increasing, and finally curing into an elastomer. This process is mainly driven by chemical reactions (cross-linking reactions). The specific change rules and influencing factors are as follows:
I. Typical stages of viscosity change
Initial mixing stage (0-10 minutes)
Viscosity is stable or slightly decreased: After the silicone and curing agent (such as platinum catalyst, peroxide, etc.) are mixed, the molecular chains begin to disperse, but large-scale cross-linking has not yet occurred. At this time, the viscosity may drop temporarily due to the shear thinning effect (stirring action), or remain close to the viscosity of the original silicone (such as 10,000-50,000 mPa·s).
Induction period (10 minutes-several hours)
Viscosity rises slowly: The curing agent begins to catalyze the cross-linking reaction between the silicone molecular chains (such as the reaction of Si-H and vinyl in addition-type silicone) to form a three-dimensional network structure. At this time, the viscosity rises slowly, and the material is still operable (such as coating and injection molding).
Gelation stage (critical point)
Viscosity increases sharply: When the cross-linking point density reaches the critical value, the silicone changes from liquid to gel (i.e., "non-flowing" state). At this time, the viscosity approaches infinity, and the material loses fluidity, but it has not yet fully cured.
Curing completion stage (several hours to several days)
Viscosity tends to be stable: The cross-linking reaction continues, and the hardness, elasticity and other properties of the material gradually improve, and finally reach a fully cured state. At this time, the viscosity no longer changes, forming a stable elastomer (such as Shore A hardness 20-80).
2. Key factors affecting viscosity change
Curing agent type and dosage
Addition type silicone: Using platinum catalyst, the curing speed is proportional to the catalyst concentration. For example, 1% platinum catalyst can make silicone gel in 1 hour, while 0.1% may take 6 hours.
Condensation type silicone: Depends on moisture curing, and the amount of curing agent (such as organotin) affects the reaction rate. Excessive curing agent may cause early gelation and shorten the operation time.
Peroxide-cured silicone: The curing agent decomposes to produce free radicals, which initiate cross-linking. The reaction is violent at high temperature, and the viscosity rises very quickly (such as gelation within a few minutes).
Temperature
High temperatures accelerate curing: For every 10°C increase in temperature, the curing rate generally doubles. For example, an addition silicone that takes 4 hours to cure at 25°C may only take 30 minutes at 80°C.
Low temperatures delay curing: Low temperatures (e.g., below 5°C) may result in incomplete curing or persistently high viscosity.
Mixing ratio
Ratio deviation: Too much curing agent can cause local overheating, bubbles, or embrittlement; insufficient curing agent can result in incomplete curing and chronically low viscosity. For example, the standard ratio of addition silicone is 10:1 (silicone: catalyst), and deviations of more than 5% may affect performance.
Additives
Plasticizers: Reduce initial viscosity (e.g., adding 5% silicone oil can reduce viscosity from 30,000 mPa·s to 15,000 mPa·s), but may slow down the curing rate.
Fillers: For example, fumed silica can significantly increase viscosity (adding 10% can increase viscosity from 20,000 mPa·s to 100,000 mPa·s) while improving mechanical properties.
3. Practical application significance of viscosity change
Operation time control
Determine the "pot life" (Pot Life) according to the viscosity rise curve, that is, the time from mixing to gelation. For example, the pot life of mold silicone is usually 30-60 minutes, and pouring must be completed during this period.
Process optimization
Coating process: Coating must be completed when the viscosity is low (such as <50,000 mPa·s) to avoid leveling defects after gelation.
3D printing: Adjust the printing speed and layer thickness by real-time monitoring of viscosity changes to prevent interlayer peeling.
Quality control
Abnormal viscosity (such as too fast gelation or long-term non-curing) may indicate curing agent failure, uneven mixing or environmental temperature and humidity problems, which need to be checked in time.
4. Typical case analysis
Case 1: Addition silicone (10:1 ratio)
After mixing at 25°C: initial viscosity 25,000 mPa·s, rising to 50,000 mPa·s after 1 hour (still operable), and gelation after 2 hours.
After mixing at 80°C: initial viscosity 25,000 mPa·s, rising to 50,000 mPa·s after 15 minutes, and gelling after 30 minutes.
Case 2: Condensation type silica gel (containing organic tin curing agent)
Under 50% humidity environment: initial viscosity after mixing 40,000 mPa·s, rising to 200,000 mPa·s after 4 hours (gelling).
Under 90% humidity environment: initial viscosity after mixing 40,000 mPa·s, rising to 200,000 mPa·s after 1 hour (reaction acceleration).
5. Advanced suggestions
Real-time monitoring: Use a rotational viscometer or rheometer to track viscosity changes and draw a curing curve.
Simulation software: Use CAE software (such as POLYFLOW) to predict viscosity changes over time and temperature and optimize process parameters.
Low temperature storage: Unused mixed silica gel can be refrigerated (5-10°C) to extend the operating time, but moisture condensation should be avoided.
By understanding the viscosity change pattern after mixing silicone and curing agent, the curing process can be accurately controlled to ensure stable product performance. It is suitable for high-precision fields such as mold manufacturing, electronic packaging, and medical devices.