Surface modification effect is good, these 10 indicators have the final say!

Release Date:

2022-06-10

In the actual production process, the correct characterization of the surface modification effect is very important for the timely adjustment of the modifier, process and equipment parameters. The characterization method of powder surface modification effect can be divided into direct method and indirect method.

Direct characterization method:Surface wettability, surface energy, surface electrical properties, dispersion in polar or non-polar media, optical and wave-absorbing properties, type of surface modifier (adsorption and chemical reaction type), coating amount, surface structure, morphology and surface chemical composition, etc;
Indirect characterization:Determination of surface modified powder in the field of application of the product performance.

Next, let's take a look at what are the commonly used methods for characterizing the modification effect in the research and production of powder surface modification?

1. Wetting contact angle

Concept:Wetting contact angle is the main criterion of wettability, such as the use of organic surface modifiers for surface modification of inorganic fillers, the more complete the surface coating of the modifier (the greater the coating rate), the greater the wetting contact angle of inorganic fillers in water.

Determination method:Angle measurement method, length measurement method, capillary penetration speed method, etc.

2. Activation index

Concept:After surface modification of the inorganic powder, the surface is non-polar, in the water due to the huge surface tension, will be like the oil film floating does not sink, therefore:

Activation index = mass of floating part in sample (g)/total mass of sample (g)

Without surface activation (I. e. modification) treatment of inorganic powder, activation index = 0; the most thorough activation treatment, activation index = 1.0.

Determination method:Taking the calcium carbonate activation index determination method as an example, the steps include:

Weigh about 5g sample, accurate to 0.01g.
Place in a separatory funnel, add 200mL of water, and shake back and forth at a speed of 120 times/min for 1min. Place gently on the hopper frame and stand for 20min-30min.
After obvious layering, the sinking calcium carbonate is put into a glass sand crucible dried at 105 ℃ ± 2 ℃ in advance until the quality is constant, and the water is removed by suction filtration.
Place in an electrothermal constant temperature drying oven and dry at 105 ℃ ± 2 ℃ until the mass is constant.

3. Oil absorption value

Concept:The oil absorption value is usually expressed in the quality of linseed oil required for 100g samples. Most fillers use the oil absorption value as an indicator to roughly predict the demand for resin.

Determination method:Taking the method for determining the oil absorption of talc powder as an example, the steps include:

Accurately weigh the quality of the bottle containing refined linseed oil (accurate to 0.001g) and record;
Weigh the dried talc powder sample about 5g
The refined linseed oil in the dropper bottle was added to the sample on the plate by spotting. The amount of oil added each time shall not exceed 10 drops. After the oil is added, press and grind with an ink mixing knife to make the oil penetrate into the sample. Continue to add drops to the oil and the sample until it can form clumps. From this point on, after adding a drop of oil, it is necessary to fully press and grind with an ink mixing knife to form a uniform consistency, which just moistens the powder mass, does not crack or break, and can adhere to the plate, which is the end point;
Weigh the mass of the post-endpoint dropper and refined linseed oil to the nearest 0.001g and record.

4. Dispersion stability in solution

Concept:By dispersing the particles and determining the change of turbidity, density and sedimentation at a certain position after standing, it is characterized by the change of turbidity, density and sedimentation. Generally speaking, the slower the change of turbidity, density and sedimentation with time, the better the dispersion stability of the powder in the solution.

Determination method:Turbidity can be measured by a turbidity meter, density can be measured by a density (specific gravity) meter, and sedimentation can be measured by a sedimentation balance.

5. Deposition time

Concept:In general, the better the dispersion, the slower the settling rate and the longer the settling time. Therefore, the settling time can be used to relatively compare or evaluate the surface modification effect of the powder.

Determination method:First, take a certain amount of modified powder and configure it into a suspension with a certain concentration, then move the suspension into a settling tube with a certain scale, and record the time for the particles in the suspension to settle to the specified scale.

6. Adsorption type

Concept:Adsorption types can be divided into physical adsorption and chemical adsorption. Surface modifier molecules chemically adsorbed on the surface of powder particles are stronger than physical adsorption, and are not easily desorbed when strongly stirred or mixed or compounded with other components.

Determination method:It can be determined by fat extractor (three-necked flask with electric stirring and reflux condenser) or hot water washing. The method and determination process of the fat extractor are as follows: the modified powder sample is added into a three-necked flask containing a certain amount of toluene solvent, heated to boiling state, refluxed, stirred, suction filtered, fully washed, then dried to constant weight at 120 ℃, and the surface modifier molecules covered on the particle surface by physical adsorption are extracted by toluene to obtain the surface modified powder with the surface physical adsorption removed. The amount of toluene extraction reflects the amount of surface modifier that is physically adsorbed. In a certain period of time, the greater the amount of toluene extraction, the more physical adsorption, the greater the proportion of the adsorption surface.

7. Coating amount and coating rate

Concept:The coating amount refers to the mass of the surface modifier adsorbed on the surface of a certain mass powder. The coverage ratio is the percentage of the coverage area of the surface modifier molecules on the surface of the powder (particle) to the total surface area of the powder (particle).

Determination method:Infrared spectroscopy, especially diffuse reflectance infrared Fourier transform spectroscopy, can be used to quantitatively determine the amount of coating or adsorption of powder surface modifiers. The surface coating ratio can be calculated from the coating amount and the cross-sectional area of the surface modifier molecules.

8. Particle size distribution

Concept:The change of particle size and distribution after the surface modification of the powder can reflect whether the particles have agglomerated during the surface modification process, especially whether the hard agglomeration has occurred.

Determination method:All kinds of particle size measurement instruments can be detected.

9. Particle morphology

Concept:Direct observation of the morphology of the coating layer on the surface of the powder is valuable for evaluating the effect of surface modification of the powder.

Determination method:Scanning electron microscopy (SEM), transmission electron microscopy (TEM), optical microscopy, etc.

10, other

For other purposes of powder surface modification, such as giving the powder surface electricity, heat, flame retardant, antibacterial, wave absorption, adsorption and other functions or properties, the corresponding performance testing, characterization and evaluation methods can also be taken. For example:

Calcined kaolin used for cable insulation filler, detecting the volume resistivity after surface treatment;
used for filling flame retardant aluminum hydroxide, magnesium hydroxide, brucite powder and other flame retardant fillers, testing the oxygen index after surface modification;
The mica-based titanium oxide pigment used as a pigment, I .e., pearlescent mica, is tested for its refractive index after titanium dioxide coating modification;
Nano-zinc oxide for antibacterial purposes, and its antibacterial properties after surface modification were tested;
Inorganic powder for cosmetics, detection of its modified ultraviolet absorption function;
For environmental protection and biochemical purposes of diatomite, zeolite, etc., detection of its surface modification after the adsorption performance;
Ultrafine or nano-composite powder used for wave-absorbing coating materials, testing the wave-absorbing properties of the modified composite;
Surface modification of pigments for improving the dispersion stability of inorganic pigments in aqueous solutions, and detection of changes in surface potential after surface modification;
Titanium dioxide used as a paint pigment was tested for weatherability and chemical stability after surface coating modification with silica and alumina.

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