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Effective Particle Sizing of Powder Coatings through Wet-Dispersion
R. Siva
Marpol Pvt Ltd
Margao, Goa
siva@marpolind.com
 

Introduction

The determination of particle size of powder coatings and its distribution within the bulk of a powder are matters of great importance. Particle size distribution of the powder coatings is critical to both the final product performance and the application properties. Two factors dominate when considering the significance of particle size. The first is the presence of larger particles which will protrude above the surface and affect the finish of the cured coatings and it can even clog the spray nozzles while spraying the powder. The second is to control the excess of fine particles (less than 10 microns) which influence the ability of powder to flow properly through the application equipments it can lead to problems like powder spitting at the gun head etc. Presence of excess fines can lead to poor coverage & agglomeration of powder particles etc. So full distribution of particle size is important to maintain optimum performance of the powder application as well as powder manufacturing process.
There are many methods are available to determine the particle size distribution of powder coatings (e.g.) air-jet sieve, coulter counter and laser diffraction method etc. The laser diffraction method has become the accepted technique (Ref: IS0 13320 particle size distribution, Guide to laser diffraction) for determining the particle size distribution of powder coatings. This technique offers speed, performance, ease of use and reliable results. The advantages of laser diffraction method are

 a. Measurement is based on fundamental scientific principle.
 b. Entire sample is being measured which gives better accuracy.
 c. The method is rapid and highly reproducible etc.

Particle size distribution of powder coatings can be measured by laser diffraction method in two ways. One is wet media measurement in which
Particles are dispersed in liquid and the other one is dry media measurement in which particles are dispersed in air. This article describes about the critical factors which need to be considered while choosing the method i.e. wet or dry for determination of particle size of powder coatings and support the particle size measurement based on wet-dispersion method. Factors which influence the measurement of particle size distribution and effective methods for establishing SOP for particle size measurement are also discussed.


Laser diffraction

Fig 1



The lay out of a laser diffraction particle sizer is shown in fig1.A small Helium-Neon laser provides a parallel monochromatic laser beam which is expanded in diameter to ensure large number of particles will be in the measuring zone at any one time. Powder sample is dispersed at an adequate concentration in a suitable media either liquid (wet media) or gas (dry media) and passed through the beam of monochromatic laser light. As the sample passes through the beam each particles scatter light at various angles related to their respective sizes; smaller particle scatter light at large angles and vice versa. The scattered light is collected by a Fourier transform lens and sent onto detectors. The scattered light is measured by detectors and numerical values relating to their scattering pattern are recorded.

Wet or Dry

Particle size distribution of powder coatings can be measured either by wet or dry analysis. Wet analysis involves the use of suitable dispersant to separate the particles followed by stabilization with surfactants to prevent the recombination of particles during measurement. Vigorous stirring & ultrasonication provides better dispersion and de-agglomeration of particles. In dry analysis clean compressed air is used to draw the sample into the measurement zone.

Which dispersion method is suitable for powder coatings? Wet or dry!!

Which method will give a better accuracy is a question which takes longer to answer. So while discussing about the suitable dispersion method for powder coatings consideration should be given to correct terminology including accuracy, precision, reproducibility and repeatability.

Generally particle size analyser is used in various industries such as pharmaceutical, mines & coatings etc. The selection of correct dispersion method depends on the physical/chemical characteristics of the sample taken for measurement.
In general dry analysis is preferred for the samples which are absolutely free flow nature and non-hygroscopic nature. Dry measurement is used in the following cases where sample particles can’t be measured or measurement purpose can’t be accomplished by wet measurement.

a. Samples which dissolve in water or the liquid medium (e.g.) foods and some pharma drugs.
b. Particles that agglomerate easily in liquid medium such as magnetized particles.
c. Particles used dispersed in the air

Wet analysis is preferred for the samples which are hygroscopic nature or can pick up moisture when exposed to atmosphere. Environment conditions also to be looked at; at high humid environment primary particles tend to agglomerate which can lead to erratic result in dry measurement. In table 1, important key points are highlighted; it helps to understand the significance of wet method for powder coatings.

Table 1
 

 Ideally particle size distribution of powder coatings in wet and dry method should be same. But in practice it is not so because of the nature of powder coatings; we need to select a method which is highly repeatable it means that running the same sample on the same instrument should produce very similar results. The particle size distribution of one powder sample was tested using wet dispersion method as well as dry dispersion method at different laboratories and at different operating conditions. The results were tabulated in table 2.

Table 2
 

The same powder sample shows coarser PSD in dry media measurement (Refer Lab 1A to Lab 2C). The results of Lab 2D and Lab 2E are similar to wet analysis results. The wet and dry results should be compared with agreement confirming that the material is in the same state of dispersion under the measurement conditions. It is important to establish standard operating procedure (SOP) for both wet and dry methods.
A useful aid in establishing a SOP for dry method is the “size-pressure curve”, which is a graph with size (D10, D50, D90) plotted on the y –axis and dry-powder feeder air pressure plotted on the x-axis, measurements usually being done at 1,2,3 &4 bar. The resulting graph can be split into three zones

Zone 1 - Starting at 0 bar. Size decreases with increasing air pressure. That is
“Dispersion is occurring”
Zone 2- A plateau with little change of size with increasing air pressure. That is
“Stability”
Zone 3- Size again decreases with increasing air pressure.
“Milling is occurring”
A similar graph can be obtained for wet method when ultrasonication power is varied. But the degree of variation in wet method is very less at different operating conditions compare to dry method.

Wet method development

The particle size distribution of powder coatings can be effectively analysed using wet method which is one of the wide spread method for obtaining reproducible results using laser diffraction method. However end users need to consider some important factors which play a vital role in particle size measurement using wet dispersion method. Some of them are highlighted here;

a. Sampling

Sampling of powder coatings is the important aspect of particle size measurement. The sample taken for PSD measurement should be a representative sample of the entire quantity. Alan Rawale of Malvern Instruments has established broad guidelines as to the relative magnitude of source of error

Variable Estimated error

Sampling 5-20%
Dispersion 3-10%
Instruments < 2.0%

He refers the golden rule of sampling

a. Powder should be sampled when in motion i.e. sample should be collected for measurement when the pulverisation is going on.
b. The whole sample stream should be taken over many short time increments rather than part of the stream being taken for the whole of the time. For e.g. if 2000 kgs is pulverized then powder needs to be sampled at different intervals in between.

Specially designed sample collectors can also be used for sampling. A point to be kept in mind about sampling is; if there is material > 75 microns then sampling will be the biggest potential error in the measurement because it is the number of particles in the largest size in band that is important; because large particles occupy larger volume than smaller particles. One 100 micron particle equals 1,000,000 one micron particles. However, if there is significant material < 10 microns then repeatability will depend on de-agglomeration of the sample. In such a case wet-analysis provides reproducible results.

Role of refractive index in powder coatings & Selection of appropriate optical model

In laser diffraction method it is important to enter the appropriate refractive index (RI) value of the sample and the dispersion medium. An optical property of the sample such as refractive index etc plays a significant role in particle size measurement. Small difference in the refractive index may cause significant difference in the resulting particle size distribution. It is difficult to find the refractive index (RI) value of all the powder coatings samples.
Let us see whether this statement is applicable for powder coatings or not!

There are two basic models are used in PSD measurement of powder coatings.
One is Fraunhofer approximation and the second one is Mie theory.
Fraunhofer approximation does not make use of any knowledge of the optical properties of the sample and only scattering at the contour of the particles is considered. So refractive index of the powder sample has no meaning whereas in Mie theory refractive index of the powder sample is considered.

Fraunhofer approximation is not suitable for the samples which are transparent or semi transparent nature and also not suitable for particles which are smaller size i.e. D(v,100) less than 50 um where as Mie theory offers the best solution for particles smaller than 50 um.
Powder coatings are opaque nature not transparent and the particle size of powder coatings is distributed from < 10 um to in the range of 100 um. So both Fraunhofer approximation and Mie theory gives more accurate result.
Particle size distribution of one powder coatings sample is tested as per fraunhofer approximation as well as Mie theory and the results are tabulated (Ref table 3).

Table: 3
 

The same experiments are repeated for few more samples of different nature (e.g.) clear powder and highly pigmented powder etc. and there is no significant difference noticed in PSD using both the optical models.

Dispersion of powder sample

Dispersion of powder sample is at the heart of this subject. Powder sample taken for PSD measurement need to be dispersed well in the transporting medium to get reproducible results. Selection of right dispersant solution, addition of surfactant, ultrasonic application and speed of pump etc play a significant role in dispersion. Surfactants are often required to wet the powder sample and prevent agglomeration of fine particles. Surfactants lower the surface tension of dispersant thereby contact angle between the dispersant and the powder particle is reduced. Generally 5% or 10% concentration diluted surfactant is adequate to wet the powder particles. The concentration of surfactant in the dispersant need to be controlled; at higher concentration level surfactant can produce bubbles within the dispersion unit which will lead to erratic PSD results.

Standardization

It is important to establish standard operating procedure (SOP) for both wet and dry laser diffraction method. The following points are important;

a. Speed of the stirrer
The powder sample is dispersed well in the media with the help of high speed stirrer; large particles require higher pump speed. However at higher speed, formation of bubbles in the dispersant solution is inevitable which can alter the test results. The optimum level of pump/stirrer rate can be determined by way of monitoring the changes in the particle size as the pump speed is increased.

b. Ultrasonication
Ultrasonication helps to de-agglomerate the powder particles and ensure better dispersion; however there are possibilities ultrasonic application can affect the particle size at high frequency and longer time. This can be accomplished by exposing the powder sample to ultra sound for varying lengths of time.

c. Validation &Repeatability check
Laser diffraction systems are based on scientific principles thus calibration in the strict sense is not required; however it is still necessary and desirable to confirm the correct operation of the instrument by validation procedure. Primary validation can be made with any certified or reference standard material and generally spherical reference material is preferred. End users can also keep some reference powder sample of known PSD for parallel validation and confirmation.ISO 13320 standard establish reproducible goals at D(v,0.1),D(v,0.5) and D(v,0.9) based on coefficient of variation. The PSD of the same powder sample needs to be repeatedly checked for 3 or 5 times. The permissible limits are given in table 4.

Table: 4

 

                                                     Coefficient of variation

d. Sample concentration
The concentration of powder sample used for wet measurement must be set so as to allow reproducible scattering data to be obtained without observing multiple scattering. When concentration of powder particles are low enough then turbidity is linear with concentrations. At low obscuration level reported particle size will be constant. But at higher concentration particles are so close together that the scattered radiation is re-scattered by other particles which can lead to erratic PSD results. The concentration at which multiple scattering occurs can be assed by measuring the particle size of stable dispersion as a function of measurement obscuration.

Conclusion

In the present study it can be concluded that the particle size distribution of powder coatings using wet-dispersion laser diffraction method provides reproducible results and ideal for powder coatings.

Reference

1. Particle size analysis-The key to quality of powder coatings by McFadyen
2. Particle size Analysis-Guide to laser Diffraction Methods
ISO/DIS 13320.
3. The Technology, Formulation and Application of Powder Coatings by
David M Howell.
4. Powder coatings Chemistry & Technology by Pieter Gillis De Lange