Activation index is a fast, widely‑used laboratory indicator to evaluate surface‑modification effect of organic‑treated kaolin.
Raw unmodified kaolin is hydrophilic; it fully wets and sinks in water. After successful coupling‑agent modification, kaolin particle surfaces turn hydrophobic. Hydrophobic particles cannot be wetted by water and float on the water surface, while incompletely‑modified hydrophilic fractions sink to the bottom. The activation index equals the mass fraction of floating hydrophobic kaolin in the total sample, expressed as a percentage.
- Unmodified raw kaolin: Activation index ≈ 0 %
- Well‑modified high‑quality kaolin filler: Activation index ≥90 %
- Over‑modified or unevenly‑coated samples may not reach theoretical maximum value.
Instruments and Reagents
- Analytical balance (precision 0.001 g)
- Electric stirrer with adjustable speed
- 250 mL‑300 mL beaker / separatory funnel
- Slow‑speed qualitative filter paper (pre‑dried to constant weight)
- Constant‑temperature drying oven (105 °C ±2 °C)
- Deionized water
- Vacuum filter device (optional, improves efficiency)
Standard Test Procedure
- Sample pre‑treatment
Dry the kaolin powder sample at 105 °C for 2 h, cool to room temperature inside a desiccator to remove residual surface moisture. Weigh 5.000 g of prepared kaolin sample, record total mass as W₀. - Dispersion and phase separation
Transfer weighed kaolin into a 250 mL beaker, add 150‑200 mL deionized water. Start stirring at 500‑600 rpm for 30 min to fully disperse powder into water phase.
Stop stirring, let suspension stand undisturbed for 60 min until obvious liquid‑solid stratification forms. Fully‑activated hydrophobic kaolin floats on top; un‑activated hydrophilic kaolin settles at the bottom.
Alternative separatory‑funnel method: Load sample and water, shake vigorously 30 times, stand 30 min, drain the lower sediment layer directly, keep floating fraction inside funnel for collection.
- Collect floating activated fraction
Carefully scoop or siphon all floating suspended kaolin on water surface, avoid disturbing settled sediment at bottom. Filter floating material using pre‑weighed filter paper (filter paper mass = Wₚ). - Dry and weigh
Place filter paper together with collected floating kaolin into 105 °C oven, dry to constant weight. Record total mass of filter paper plus floating powder as Wₜ. - Parallel test
Run 3 parallel replicates for each kaolin sample, discard abnormal data and calculate average value.
Calculation Formula
[\text{Activation index } H(%)=\frac{W_t-W_p}{W_0}\times100%]
- H: Activation index, %
- W₀: Total mass of original kaolin sample, g
- Wₚ: Constant‑weight mass of filter paper, g
- Wₜ: Mass of filter paper plus dried floating kaolin powder, g
Round final result to one decimal place.
Key Factors Affecting Test Accuracy
- Stirring speed & time
Too high stirring speed will break hydrophobic particle layers and cause floating powder to sink; insufficient stirring leaves particle agglomerates, resulting in falsely low activation index. Keep speed stable at 500‑600 rpm with fixed stirring duration. - Standing time
Insufficient standing causes incomplete sediment‑float separation; over‑long standing leads partial hydrophobic particles gradually sink under gravity. Strictly follow standardized standing duration. - Sample moisture
Undried kaolin carries residual moisture, particles partly pre‑wetted, directly lowers measured activation index, so pre‑drying step cannot be skipped. - Particle size of kaolin
Ultra‑fine kaolin with high specific surface area has weak buoyancy; partial fine hydrophobic particles may suspend in water rather than fully float, which brings minor deviation to test results. - Avoid disturbing sediment layer
When collecting floating powder, do not stir or draw bottom sediment into filter, otherwise measured value will be artificially high.
Limitations of Activation Index Test
- Activation index only reflects surface hydrophobic degree, cannot directly represent grafting rate of coupling agents. High activation index does not absolutely guarantee excellent compatibility with polymer matrix.
- For partially amphiphilic modified kaolin, this method yields ambiguous floating‑sediment results; combine with supplementary characterization: water contact‑angle measurement, FT‑IR, TGA thermal‑gravimetric analysis to evaluate real modification quality.
Reference Qualification Range for Modified Kaolin Filler
| Kaolin Grade | Target Activation Index |
|---|---|
| General‑purpose filling kaolin | 80 %‑88 % |
| Plastic & rubber‑grade modified kaolin | ≥90 % |
| High‑performance coating‑grade hydrophobic kaolin | ≥92 % |
