SIST ISO 14887:2002
(Main)Sample preparation -- Dispersing procedures for powders in liquids
Sample preparation -- Dispersing procedures for powders in liquids
Préparation de l'échantillon -- Procédures pour la dispersion des poudres dans les liquides
La présente Norme internationale a été élaborée pour aider les techniciens pratiquant des analyses granulométriques à obtenir une bonne dispersion des composants poudre/liquide dans des combinaisons pour lesquelles ils n'ont aucune expérience. Elle fournit des modes opératoires permettant de : - mouiller une poudre dans un liquide; - désagglomérer les grumeaux mouillés; - déterminer si la composition de la solution peut être ajustée afin d'éviter la formation de grumeaux; - sélectionner des agents dispersants afin d'éviter que les grumeaux ne se reforment; - évaluer la stabilité de la dispersion par rapport à l'éventualité d'une nouvelle formation de grumeaux. La présente Norme internationale est applicable aux particules dont les dimensions sont comprises entre 0,05 µm et 100 µm. Elle fournit un questionnaire sur la nature de la poudre et du liquide impliqués. Les réponses sont utilisées sous forme de diagrammes destinés à guider l'utilisateur dans son choix d'un agent dispersant à même de convenir pour la dispersion de la poudre dans le liquide. La présente Norme internationale n'est applicable qu'à la préparation de dispersions simples, diluées (moins de 1 % en volume de solides) destinées à des analyses granulométriques. Elle ne traite pas de mélanges complexes et à forte concentration de solides tels que peintures, encres, produits pharmaceutiques, herbicides ou matières plastiques composites vendus dans le commerce.
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INTERNATIONAL ISO
STANDARD 14887
First edition
2000-09-01
Sample preparation — Dispersing
procedures for powders in liquids
Préparation de l'échantillon — Procédures pour la dispersion des poudres
dans les liquides
Reference number
ISO 14887:2000(E)
©
ISO 2000
---------------------- Page: 1 ----------------------
ISO 14887:2000(E)
PDF disclaimer
This PDF file may contain embedded typefaces. In accordance with Adobe's licensing policy, this file may be printed or viewed but shall not
be edited unless the typefaces which are embedded are licensed to and installed on the computer performing the editing. In downloading this
file, parties accept therein the responsibility of not infringing Adobe's licensing policy. The ISO Central Secretariat accepts no liability in this
area.
Adobe is a trademark of Adobe Systems Incorporated.
Details of the software products used to create this PDF file can be found in the General Info relative to the file; the PDF-creation parameters
were optimized for printing. Every care has been taken to ensure that the file is suitable for use by ISO member bodies. In the unlikely event
that a problem relating to it is found, please inform the Central Secretariat at the address given below.
© ISO 2000
All rights reserved. Unless otherwise specified, no part of this publication may be reproduced or utilized in any form or by any means, electronic
or mechanical, including photocopying and microfilm, without permission in writing from either ISO at the address below or ISO's member body
in the country of the requester.
ISO copyright office
Case postale 56 � CH-1211 Geneva 20
Tel. + 41 22 749 01 11
Fax + 41 22 749 09 47
E-mail copyright@iso.ch
Web www.iso.ch
Printed in Switzerland
ii © ISO 2000 – All rights reserved
---------------------- Page: 2 ----------------------
ISO 14887:2000(E)
Contents Page
Foreword.iv
Introduction.v
1 Scope .1
2 Normative reference .1
3 Terms and definitions .1
4 Symbols and abbreviated terms .2
5 Examination of the dry powder .3
6 Selection of a liquid and trial dispersion .3
7 Examination of the dispersion .4
8 Identification of possible dispersing agents .6
9 Optimization of the dispersion method.12
10 Maintenance of dispersion stability during sample handling.15
Annex A (informative) Alternative dispersion-stability tests.16
Annex B (informative) Commercial dispersing agents in the various dispersing agent categories.18
Bibliography.23
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ISO 14887:2000(E)
Foreword
ISO (the International Organization for Standardization) is a worldwide federation of national standards bodies (ISO
member bodies). The work of preparing International Standards is normally carried out through ISO technical
committees. Each member body interested in a subject for which a technical committee has been established has
the right to be represented on that committee. International organizations, governmental and non-governmental, in
liaison with ISO, also take part in the work. ISO collaborates closely with the International Electrotechnical
Commission (IEC) on all matters of electrotechnical standardization.
International Standards are drafted in accordance with the rules given in the ISO/IEC Directives, Part 3.
Draft International Standards adopted by the technical committees are circulated to the member bodies for voting.
Publication as an International Standard requires approval by at least 75 % of the member bodies casting a vote.
Attention is drawn to the possibility that some of the elements of this International Standard may be the subject of
patent rights. ISO shall not be held responsible for identifying any or all such patent rights.
International Standard ISO 14887 was prepared by Technical Committee ISO/TC 24, Sieves, sieving and other
sizing methods, Subcommittee SC 4, Sizing by methods other than sieving.
Annexes A and B of this International Standard are for information only.
iv © ISO 2000 – All rights reserved
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ISO 14887:2000(E)
Introduction
The evaluation of particle size distribution is of crucial importance for research projects, product development,
process control, quality control, and other technical activities where particle size effects are important. Paints, inks,
filled plastics, ore processing, pharmaceuticals, agricultural and cosmetic products depend on accurate particle size
analysis for their commercial production.
A typical powder is composed of clumps of “primary" particles that are held together by weak or strong forces. The
size of clumps remaining after the powder has been wetted into a liquid depends in part on how much energy has
been expended in breaking up these clumps. Since a clump responds to most particle sizing methods as a large
particle would, the presence of clumps in incompletely dispersed samples skews the reported particle size
distribution to larger sizes than if all the clumps were broken up. A particle size analysis is useful only if the sample
is prepared so that the particles are in a well-defined degree of dispersion, preferably one in which most clumps are
fully deagglomerated and in which the particles do not reagglomerate or adhere to the walls of the sample
container during the time required for analysis.
While "complete" dispersion to primary particles is often desired, it is important to remember that in many cases the
most useful information is obtained when the sample is not fully dispersed. For example, if a customer blends the
powder into a liquid using a low-shear process that does not break moderately strong bonds in the clumps, the
quality control tests for powder intended for that customer should use similarly low shear during sample preparation
and analysis.
Because of the impurities present, the equipment available for breaking up clumps, the methods used for particle
size analysis, and the dispersing agents available for testing may vary from one site to another, the procedure
developed at one site by applying the guidelines in this International Standard may differ from (but be as valid and
as useful as) that developed at another site for the same powder.
A list of references for further study, including standards for evaluation of some of these more complex systems, is
given in the bibliography.
Annex A discusses some of the complications that arise
� when the powder has a surface treatment or soluble components;
� when the liquid contains ionic or polymeric solutes;
� when the dispersing agent contains minor ingredients.
Annex B covers the classification of commercial dispersing agents in the various dispersing agent categories.
© ISO 2000 – All rights reserved v
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INTERNATIONAL STANDARD ISO 14887:2000(E)
Sample preparation — Dispersing procedures for powders in
liquids
1 Scope
This International Standard was developed to help particle size analysts make good dispersions from powder/liquid
combinations with which they are not experienced. It provides procedures for
� wetting a powder into a liquid;
� deagglomerating the wetted clumps;
� determining if solution composition can be adjusted to prevent reagglomeration;
� selecting dispersing agents to prevent reagglomeration;
� evaluating the stability of the dispersion against reagglomeration.
This International Standard is applicable to particles ranging in size from approximately 0,05 to 100µm. It provides
a series of questions on the nature of the powder and liquid involved. The answers are used with charts that guide
the user to generic dispersing agents that are likely to be suitable for dispersing the powder in the liquid.
This International Standard applies only to the preparation of simple, dilute dispersions (less than 1 % by volume
solids) for particle size analysis. It does not deal with the formulation of complex and commercial mixtures highly
loaded with solids, such as paints, inks, pharmaceuticals, herbicides and composite plastics.
2 Normative reference
The following normative document contains provisions which, through reference in this text, constitute provisions of
this International Standard. For dated references, subsequent amendments to, or revisions of, any of these
publications do not apply. However, parties to agreements based on this International Standard are encouraged to
investigate the possibility of applying the most recent edition of the normative document indicated below. For
undated references, the latest edition of the normative document referred to applies. Members of ISO and IEC
maintain registers of currently valid International Standards.
ISO 8213:1986, Chemical products for industrial use — Sampling techniques — Solid chemical products in the
form of particles varying from powders to coarse lumps.
3 Terms and definitions
For the purposes of this International Standard, the following terms and definitions apply.
3.1
agglomerate
assemblage of particles which are loosely coherent
SEE floc (3.5)
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ISO 14887:2000(E)
3.2
aggregate
assemblage of particles rigidly joined together
NOTE Because of the confusion which exists in the use of the above terms they are used sparingly throughout the text.
3.3
clump
assemblage of particles which are either rigidly joined or loosely coherent
3.4
critical micelle concentration
CMC
concentration of dispersing agent above which micelles will form
3.5
floc
assemblage of particles which are very loosely coherent
SEE agglomerate (3.1)
3.6
primary particles
units that are to be measured in the particle size analysis, in general harder to break than clumps
3.7
Tyndall effect
light scattered perpendicular to a beam of light passing through a liquid that contains particles
4 Symbols and abbreviated terms
For the purposes of this International Standard, the following symbols and abbreviations apply.
2
S Volume-specific surface area (m /kg)
V
3
CMC Critical micelle concentration (mol/m )
3
IS Ionic strength (mol/m )
M Complete –1-th moment of the density distribution of particle volume
–1,3
PEO Polyethoxy = (-CH -CH -O-)
2 2 n
PPO Polyisopropoxy = (-CH -CH(CH )-O-)
2 3 n
pH pH at which the zeta potential is zero for an amphoteric surface (which is positively charged at lower
iso
pH and negatively charged at higher pH)
pK pH at which half the hydrogen ions from acid groups are ionized
a
pK pH at which half the hydroxide ions from base groups are ionized
b
q Density distribution of particle volume
3,i
x Upper particle size of the i-th particle size interval (m)
i
µm Micrometer
� Zeta potential [V]
® Registered trade name.
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ISO 14887:2000(E)
5 Examination of the dry powder
5.1 Sampling
Sampling shall comply with the requirements specified in ISO 8213, unless a method specified in a national
standard or mutually agreed upon by the analyst and client takes precedence. Sample preparation shall always be
done consistently so that repeated preparations based on replicate samples of a batch of powder (which was
carefully mixed before being sampled or subdivided into samples) give closely comparable results.
5.2 Clump size range and particle size range
Sprinkle the dry powder on a microscope slide and examine it using an optical microscope at � 200 magnification
or other suitable magnification. Put a cover glass over the powder on the microscope slide and tap the cover glass
lightly with a spatula (take care to avoid breaking the cover glass) to see how easy it is to crush the clumps. Note
the approximate size range of the clumps that are not broken up by such crushing. If the majority of the particles
are smaller than 1µm, use a transmission or scanning electron microscope to observe and characterize the
particles.
5.3 Shape and surface roughness; their variation with size
Note whether the surfaces of the fundamental particles are spherical or crystalline, smooth or rough, porous or
nonporous. Determine whether all the sizes of particle have the same morphology. If the particles are very rough or
2
porous, obtain an experimental measure of the volume-specific surface area (m /kg). If this value is large
compared to the area computed for spheres with the powder's particle size distribution then an unusually large
amount of dispersing agent (compared to a similar size distribution of spherical nonporous particles) may be
required to stabilize the dispersion.
NOTE The volume-specific surface area of spheres may be calculated from
SM� 6 (equation 35 in ISO 9276-2)
V �1,3
where
n
x
i
Mq� ln (equation 31 in ISO 9276-2)
�1,3 � 3,i
x
i�1
i�1
6 Selection of a liquid and trial dispersion
6.1 Selection of a liquid
The analyst shall list the liquids that are commonly used for dispersing the solids for the selected method of particle
size analysis and shall strike from the list any that fail to satisfy the following criteria.
� If the method is sedimentation, the liquid shall have a specific gravity that differs sufficiently from that of the
powder to permit the use of this method.
� If the method is light scattering, the liquid shall have a refractive index (at the analytical wavelengths) that
differs sufficiently from that of the powder to permit the use of this method.
� The liquid shall have negligible reactivity with the powder.
� The liquid shall not swell or shrink the particles by more than 5 % in diameter.
� The liquid shall provide a solubility of less than 5 g of powder per 1 kg of liquid.
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ISO 14887:2000(E)
NOTE This is to minimize Ostwald ripening that could cause the particle size distribution to change during the
measurement time.
� The liquid shall have a change in the solubility (for the powder) with temperature of less than 0,1 mg/l per
kelvin, or the temperature shall be controlled throughout the preparation and analysis to keep the solubility
from changing by more than 0,5 mg/l.
NOTE If the particle size analysis method requires 10 mg of powder dispersed in 1 litre of liquid, a temperature rise of 5 K
(from an ultrasonic probe or particle-analysis instrument warmth) would cause the dissolution of 1 mg or 10 % of the powder.
6.2 Preparation of a test paste of the powder
Put two drops (or 0,1 g) of the liquid on an etch-roughened glass plate ("frosted" glass). Blend in a roughly equal
amount of powder by sprinkling powder on the liquid surface and rubbing it into the liquid using a circular motion of
a 10 mm wide spatula, applying a moderate amount of pressure (sufficient to read 1 kg on the scale of a balance).
The objective is to wet all the powder surfaces and to break up all clumps of powder into primary particles. The high
concentration of solids provides crowded conditions that favour collision between clumps and breakup into primary
particles. These crowded conditions will also favour flocculation unless the particles repel one another.
6.3 Preparation of a dilute dispersion of the powder
Make a dilute dispersion (4 % by mass) from the concentrated paste by adding a few drops at a time of the liquid
and blending in with the spatula until 50 drops (about 2,5 g) of liquid have been added. This quantity should be
sufficient for examination with a microscope. If a larger quantity is required for other types of test, the analyst shall
follow the instructions given in 7.2.
7 Examination of the dispersion
7.1 Evaluate for under- or over-grinding
Examine the dilute dispersion using an optical microscope (for particles larger than 1µm in diameter) or an electron
microscope (for particles smaller than 1µm in diameter). Use � 200 magnification with the optical microscope and
view the particles by transmitted light.
Note whether the clumps originally seen in the dry powder have completely broken up during the procedure for
making the paste and diluting it. If not, the analyst shall make a new dispersion using ultrasonic treatment (see 9.2).
The analyst shall evaluate this new dispersion and increase, as needed, the energy put in to breakup clumps until
full dispersion is attained.
Note what fraction of primary particles have become broken during the procedure for making the paste and diluting
it. If the fraction of particles broken is over 5 %, the analyst shall make a new dispersion by simply stirring the
powder into the liquid. The analyst shall evaluate this new dispersion and increase the energy put in to breakup
clumps as needed until full dispersion is attained with less than 5 % breakage of primary particles (see 9.2).
Record the conditions that avoid under- or over-grinding and use these to prepare dispersions for evaluation until
the clump breakup process is optimized according to the procedures in 7.2.
7.2 Evaluation of stability
7.2.1 Introduction
If the suspending liquid has a viscosity below 10 mPa�s and the particles are well-dispersed, very small particles
will appear to move randomly in the microscope's field of view. Particles in the 1µm to 5µm range are best for
observing this effect. Note that, even if the powder consists mostly of larger-size particles, there are likely to be a
few particles inside the 1µm to 5µm range that can indicate whether or not the dispersion is stable. If the particles
are smaller than 1µm some other form of evaluation shall be used, such as measuring the rheological stress-strain
4 © ISO 2000 – All rights reserved
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ISO 14887:2000(E)
�1 �1
cycle of a 10 % by volume solids dispersion from 0,1 s to 100 s to see if it exhibits hysteresis (indicative of
structure formation and breakage) or not (in which case the dispersion is stable).
Observe the dilute dispersion using the optical microscope. Note what happens when two particles come close
together. Rate the stability as good if the particles repel each other rather than coming into contact. Rate the
stability as marginal if the particles collide and stay together briefly before separating again. Rate the stability as
poor if the particles collide and remain in contact to form a permanent floc. If the stability is good, no added
dispersing agent is required to form a stable dispersion. If the stability is marginal or poor then either solution
conditions (such as pH) shall be changed or a dispersing agent shall be added to provide stability.
Other methods for evaluating dispersions are noted in annex A. If microscopy and the other techniques are not
feasible then particle size analysis may be used to evaluate stability. If a series of analyses separated by several
hours lie within the reproducibility of the instrument (determined using a dispersion that is known to be stable) then
thesampledispersionmaybeconsideredtobestable.
7.2.2 Notes on optical microscopy
Optical microscopy is the simplest and most effective way of evaluating the degree of deagglomeration and the
stability of dispersions containing particles that are above 1µm in size. Note that particles whose refractive index is
close to that of the liquid will not provide enough contrast to be viewed with the optical microscope. At a solids
concentration of a few percent, well-dispersed particles will appear to behave as separate entities. As the cover
glass is moved sideways over the surface of the slide, note whether the particles in the dispersion move individually
and not as a bonded group. Note whether particles that are below about 5µm in size may exhibit "Brownian
motion", as particles move about erratically due to unbalanced collisions of the particle with molecules of the
surrounding fluid.
Particles smaller than the limit of optical resolution (about 0,3µm) appear as bright spots when they are illuminated
from the side with a dark field behind them ("ultramicroscopy"). Although the width of the spot is indeterminate, the
size of the particle responsible for the spot may be estimated by its Brownian motion: the more actively a spot
moves, the smaller the particle creating the spot. The size of the smallest detectable particle using this technique
depends on the scattering power of the particles. Particles of titanium dioxide or of a metal as small as about
0,02µm may be observed using this technique, but for oil droplets the limit of observation is about 0,1µm.
Dispersion stability is destroyed if the particles stick to the glass microscope slide. This is a particular problem for
positively charged particles, since glass is normally negatively charged. Such adhesion can also invalidate the
measurement process, especially for light-scattering methods where the amount of solid circulating for analysis
may be so small that it is completely removed by adsorption on the walls of the sample circulation system. In such
cases, one can chemically treat the glass (with a cationic adsorbate such as dodecyl trimethyl ammonium bromide)
so that it becomes positively charged and thus prevents deposition of the particles being analysed.
7.2.3 Notes on electron microscopy
Evaluation by electron microscopy requires that the dispersion be spread out on a thin support film and dried. As
the liquid evaporates and the liquid surface shrinks between two particles, surface tension can pull previously well-
dispersed particles into contact to form a clump. This problem can be minimized if the analyst can use a liquid with
a low surface tension. Dispersion stability shall be judged as good if the particles are well spread out on the grid
and bad if they are found mostly in clumps.
7.3 Evaluation of any flocs formed
If flocs have formed, put a cover glass over the dispersion on the microscope slide. Use a spatula to push the cover
glass gently from the edge to slide it over the dispersion and apply shear force to the flocs. Note whether the flocs
break up and how rapidly they reform. Flocs are reversible if they break up under shear and then reform similar
flocs. Flocs are unstable if shear causes large, loose flocs to roll up into small, tight flocs. Flocs are strong if they do
not break up with gentle sliding. In the last case, the addition of dispersing agent may not be effective unless a high
shear force can be applied to break the floc in the presence of dispersing agent.
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ISO 14887:2000(E)
8 Identification of possible dispersing agents
8.1 Wetting of the solid particle by the liquid
The control of the wetting process allows the adhesion forces to be modified between the particles and the binding
forces produced by liquids in the intermediate capillaries to be partially modified.
The general aim for particles size analysis is a spontaneous wetting as complete as possible. This can be searched
by two ways:
� low-interfacial tension liquid/gaseous by wetting agents;
� low-interfacial tension solid/liquid by hydrophilizing agents.
In the case of insufficient wetting, a simultaneously mechanical treatment can be recommended (highly intensive
ultrasonic treatment of the suspension, kneading of the system as a plastic mixture with a spatula).
8.2 General principles
Subclauses 8.2 to 8.4 explain the principles used in developing the decision charts in 8.5. Complete dispersion of a
powder in a liquid occurs when the individual particles that made up the original clumps have become separated,
move independently of each other, and remain separated from one another. This requires that there be no
attractive force between the particles as they approach one another. If there is an attraction then the
solid/dispersion will exhibit non-Newtonian flow and have a yield stress (i.e. the dispersion will be able to support a
finite shear stress without any flow occurring.) Most of the indirect tests of dispersion rely on this effect. For
example, a dispersion with a yield stress enables settling particles to form an open structure which does not
collapse under the force of gravity. Such a dispersion will settle to form a higher sedimentation volume (lower
sediment density) than a completely dispersed system would.
Highly anisotropic particles form a more or less rigid gel at very low concentrations of solids when there is a net
attractive force between the particles.
8.3 Charge stabilization
8.3.1 Introduction
Particles which bear a surface charge will repel each other if the electrostatic repulsion is larger than the
polarizability attraction (also called the Hamaker or Van der Waals attraction). A surface charge corresponding to a
zeta potential greater than 30 mV is generally sufficient to provide a stable dispersion. Charge stabilization is the
best way to stabilize dispersions in which the liquid has a relative dielectric permittivity greater than 30 (methanol at
room temperature has a relative dielectric permittivity of 33, water of about 80) and an ionic strength less than
0,1 mol/l (i.e. a low concentration of ions in solution).
8.3.2 Surface ionization
The charge on the particle may arise from ionization of surface groups (influenced by the pH of the solution). For
example, surface amine groups will adsorb a hydrogen ion from solution and become positively charged if the pH is
below the pK for the powder. Surface carboxyl groups will lose a hydrogen ion and become negatively charged if
b
the pH is above the pK for the powder. Amphoteric surface groups, such as the OH groups found on a metal oxide
a
or hydroxide, will adsorb a hydrogen ion and become positive if the pH is below the pH for that oxide and will
iso
lose a hydrogen ion and become negative if the pH is above the pH . The dependence of hydrogen ion adsorption
iso
on pH is such that (when the ionic strength is below 0,1 mol/l) the zeta potential generally becomes large enough to
stabilize a dispersion if the pH is two or more units away from pH .
iso
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ISO 14887:2000(E)
8.3.3 Differential dissolution of lattice ions
If the powder is ionic and does not dissolve significantly in the liquid, the presence of a soluble salt of one of the
ions making up the powder may result in the adsorption of the common ion. For example, a sodium bromide
solution will disperse silver bromide in water and a potassium hydrogen phosphate solution will disperse calcium
hydroxyapatite in water. Since these soluble salts do not reduce the surface tension of water significantly, they are
not classified as surfactants.
8.3.4 Adsorption of multiply charged ions
If the powder is ionic or has highly polar bonds and the liquid is water, multiply charged ions which are not part of
the crystal lattice may be adsorbed to form a charged surface of soluble salts. Examples are the polyphosphate,
hexametaphosphate, pyrophosphate and polysilicate ions. Since the salts involving these ions do not reduce the
surface tension of liquids in which they are dissolved, they are not classified as surfactants.
If the powder is a nonpolar organic material and the liquid is a polar organic material, surface ions can be created
by adding a neutral ion-pair to the system. The ion-pair adsorbs on the particle surface and then dissociates into
ions, one of which desorbs to leave a charged particle. For example, the additive trimethyldodecylamine
hydroxybenzoate dissociates to form an aliphatic quaternary amine (cationic) and a polar organic acid (anionic).
8.3.5 Adsorption of surfactant ions
Organic powders suspended in water can become charged by adsorbing the organic ion of a surfactant, leaving the
inorganic counter-ion in solution. Organic amines adsorb a hydrogen ion to become positively charged if the pH is
below the pK ; organic acids lose a hydrogen ion to become negatively charged if the pH is above the pK .
b a
Zwitterionics and amino acids have a more complex dependence of charge on pH. The dependence of ionization
on pH is such that (when the ionic strength is below 0,1 mol/l) the zeta potential generally becomes large enough to
stabilize a dispersion if the pH is more than two units above the pK of a hydrogen-ion-releasing material or if the
a
pH is more than two units below the pK of a hydrogen-ion-accepti
...
SLOVENSKI STANDARD
SIST ISO 14887:2002
01-januar-2002
3ULSUDYDY]RUFHY3RVWRSNL]DGLVSHU]LMRSUDãNDVWLKVQRYLYWHNRþLQDK
Sample preparation -- Dispersing procedures for powders in liquids
Préparation de l'échantillon -- Procédures pour la dispersion des poudres dans les
liquides
Ta slovenski standard je istoveten z: ISO 14887:2000
ICS:
19.120 Analiza velikosti delcev. Particle size analysis. Sieving
Sejanje
SIST ISO 14887:2002 en
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.
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SIST ISO 14887:2002
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SIST ISO 14887:2002
INTERNATIONAL ISO
STANDARD 14887
First edition
2000-09-01
Sample preparation — Dispersing
procedures for powders in liquids
Préparation de l'échantillon — Procédures pour la dispersion des poudres
dans les liquides
Reference number
ISO 14887:2000(E)
©
ISO 2000
---------------------- Page: 3 ----------------------
SIST ISO 14887:2002
ISO 14887:2000(E)
PDF disclaimer
This PDF file may contain embedded typefaces. In accordance with Adobe's licensing policy, this file may be printed or viewed but shall not
be edited unless the typefaces which are embedded are licensed to and installed on the computer performing the editing. In downloading this
file, parties accept therein the responsibility of not infringing Adobe's licensing policy. The ISO Central Secretariat accepts no liability in this
area.
Adobe is a trademark of Adobe Systems Incorporated.
Details of the software products used to create this PDF file can be found in the General Info relative to the file; the PDF-creation parameters
were optimized for printing. Every care has been taken to ensure that the file is suitable for use by ISO member bodies. In the unlikely event
that a problem relating to it is found, please inform the Central Secretariat at the address given below.
© ISO 2000
All rights reserved. Unless otherwise specified, no part of this publication may be reproduced or utilized in any form or by any means, electronic
or mechanical, including photocopying and microfilm, without permission in writing from either ISO at the address below or ISO's member body
in the country of the requester.
ISO copyright office
Case postale 56 � CH-1211 Geneva 20
Tel. + 41 22 749 01 11
Fax + 41 22 749 09 47
E-mail copyright@iso.ch
Web www.iso.ch
Printed in Switzerland
ii © ISO 2000 – All rights reserved
---------------------- Page: 4 ----------------------
SIST ISO 14887:2002
ISO 14887:2000(E)
Contents Page
Foreword.iv
Introduction.v
1 Scope .1
2 Normative reference .1
3 Terms and definitions .1
4 Symbols and abbreviated terms .2
5 Examination of the dry powder .3
6 Selection of a liquid and trial dispersion .3
7 Examination of the dispersion .4
8 Identification of possible dispersing agents .6
9 Optimization of the dispersion method.12
10 Maintenance of dispersion stability during sample handling.15
Annex A (informative) Alternative dispersion-stability tests.16
Annex B (informative) Commercial dispersing agents in the various dispersing agent categories.18
Bibliography.23
© ISO 2000 – All rights reserved iii
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SIST ISO 14887:2002
ISO 14887:2000(E)
Foreword
ISO (the International Organization for Standardization) is a worldwide federation of national standards bodies (ISO
member bodies). The work of preparing International Standards is normally carried out through ISO technical
committees. Each member body interested in a subject for which a technical committee has been established has
the right to be represented on that committee. International organizations, governmental and non-governmental, in
liaison with ISO, also take part in the work. ISO collaborates closely with the International Electrotechnical
Commission (IEC) on all matters of electrotechnical standardization.
International Standards are drafted in accordance with the rules given in the ISO/IEC Directives, Part 3.
Draft International Standards adopted by the technical committees are circulated to the member bodies for voting.
Publication as an International Standard requires approval by at least 75 % of the member bodies casting a vote.
Attention is drawn to the possibility that some of the elements of this International Standard may be the subject of
patent rights. ISO shall not be held responsible for identifying any or all such patent rights.
International Standard ISO 14887 was prepared by Technical Committee ISO/TC 24, Sieves, sieving and other
sizing methods, Subcommittee SC 4, Sizing by methods other than sieving.
Annexes A and B of this International Standard are for information only.
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SIST ISO 14887:2002
ISO 14887:2000(E)
Introduction
The evaluation of particle size distribution is of crucial importance for research projects, product development,
process control, quality control, and other technical activities where particle size effects are important. Paints, inks,
filled plastics, ore processing, pharmaceuticals, agricultural and cosmetic products depend on accurate particle size
analysis for their commercial production.
A typical powder is composed of clumps of “primary" particles that are held together by weak or strong forces. The
size of clumps remaining after the powder has been wetted into a liquid depends in part on how much energy has
been expended in breaking up these clumps. Since a clump responds to most particle sizing methods as a large
particle would, the presence of clumps in incompletely dispersed samples skews the reported particle size
distribution to larger sizes than if all the clumps were broken up. A particle size analysis is useful only if the sample
is prepared so that the particles are in a well-defined degree of dispersion, preferably one in which most clumps are
fully deagglomerated and in which the particles do not reagglomerate or adhere to the walls of the sample
container during the time required for analysis.
While "complete" dispersion to primary particles is often desired, it is important to remember that in many cases the
most useful information is obtained when the sample is not fully dispersed. For example, if a customer blends the
powder into a liquid using a low-shear process that does not break moderately strong bonds in the clumps, the
quality control tests for powder intended for that customer should use similarly low shear during sample preparation
and analysis.
Because of the impurities present, the equipment available for breaking up clumps, the methods used for particle
size analysis, and the dispersing agents available for testing may vary from one site to another, the procedure
developed at one site by applying the guidelines in this International Standard may differ from (but be as valid and
as useful as) that developed at another site for the same powder.
A list of references for further study, including standards for evaluation of some of these more complex systems, is
given in the bibliography.
Annex A discusses some of the complications that arise
� when the powder has a surface treatment or soluble components;
� when the liquid contains ionic or polymeric solutes;
� when the dispersing agent contains minor ingredients.
Annex B covers the classification of commercial dispersing agents in the various dispersing agent categories.
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SIST ISO 14887:2002
INTERNATIONAL STANDARD ISO 14887:2000(E)
Sample preparation — Dispersing procedures for powders in
liquids
1 Scope
This International Standard was developed to help particle size analysts make good dispersions from powder/liquid
combinations with which they are not experienced. It provides procedures for
� wetting a powder into a liquid;
� deagglomerating the wetted clumps;
� determining if solution composition can be adjusted to prevent reagglomeration;
� selecting dispersing agents to prevent reagglomeration;
� evaluating the stability of the dispersion against reagglomeration.
This International Standard is applicable to particles ranging in size from approximately 0,05 to 100µm. It provides
a series of questions on the nature of the powder and liquid involved. The answers are used with charts that guide
the user to generic dispersing agents that are likely to be suitable for dispersing the powder in the liquid.
This International Standard applies only to the preparation of simple, dilute dispersions (less than 1 % by volume
solids) for particle size analysis. It does not deal with the formulation of complex and commercial mixtures highly
loaded with solids, such as paints, inks, pharmaceuticals, herbicides and composite plastics.
2 Normative reference
The following normative document contains provisions which, through reference in this text, constitute provisions of
this International Standard. For dated references, subsequent amendments to, or revisions of, any of these
publications do not apply. However, parties to agreements based on this International Standard are encouraged to
investigate the possibility of applying the most recent edition of the normative document indicated below. For
undated references, the latest edition of the normative document referred to applies. Members of ISO and IEC
maintain registers of currently valid International Standards.
ISO 8213:1986, Chemical products for industrial use — Sampling techniques — Solid chemical products in the
form of particles varying from powders to coarse lumps.
3 Terms and definitions
For the purposes of this International Standard, the following terms and definitions apply.
3.1
agglomerate
assemblage of particles which are loosely coherent
SEE floc (3.5)
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3.2
aggregate
assemblage of particles rigidly joined together
NOTE Because of the confusion which exists in the use of the above terms they are used sparingly throughout the text.
3.3
clump
assemblage of particles which are either rigidly joined or loosely coherent
3.4
critical micelle concentration
CMC
concentration of dispersing agent above which micelles will form
3.5
floc
assemblage of particles which are very loosely coherent
SEE agglomerate (3.1)
3.6
primary particles
units that are to be measured in the particle size analysis, in general harder to break than clumps
3.7
Tyndall effect
light scattered perpendicular to a beam of light passing through a liquid that contains particles
4 Symbols and abbreviated terms
For the purposes of this International Standard, the following symbols and abbreviations apply.
2
S Volume-specific surface area (m /kg)
V
3
CMC Critical micelle concentration (mol/m )
3
IS Ionic strength (mol/m )
M Complete –1-th moment of the density distribution of particle volume
–1,3
PEO Polyethoxy = (-CH -CH -O-)
2 2 n
PPO Polyisopropoxy = (-CH -CH(CH )-O-)
2 3 n
pH pH at which the zeta potential is zero for an amphoteric surface (which is positively charged at lower
iso
pH and negatively charged at higher pH)
pK pH at which half the hydrogen ions from acid groups are ionized
a
pK pH at which half the hydroxide ions from base groups are ionized
b
q Density distribution of particle volume
3,i
x Upper particle size of the i-th particle size interval (m)
i
µm Micrometer
� Zeta potential [V]
® Registered trade name.
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ISO 14887:2000(E)
5 Examination of the dry powder
5.1 Sampling
Sampling shall comply with the requirements specified in ISO 8213, unless a method specified in a national
standard or mutually agreed upon by the analyst and client takes precedence. Sample preparation shall always be
done consistently so that repeated preparations based on replicate samples of a batch of powder (which was
carefully mixed before being sampled or subdivided into samples) give closely comparable results.
5.2 Clump size range and particle size range
Sprinkle the dry powder on a microscope slide and examine it using an optical microscope at � 200 magnification
or other suitable magnification. Put a cover glass over the powder on the microscope slide and tap the cover glass
lightly with a spatula (take care to avoid breaking the cover glass) to see how easy it is to crush the clumps. Note
the approximate size range of the clumps that are not broken up by such crushing. If the majority of the particles
are smaller than 1µm, use a transmission or scanning electron microscope to observe and characterize the
particles.
5.3 Shape and surface roughness; their variation with size
Note whether the surfaces of the fundamental particles are spherical or crystalline, smooth or rough, porous or
nonporous. Determine whether all the sizes of particle have the same morphology. If the particles are very rough or
2
porous, obtain an experimental measure of the volume-specific surface area (m /kg). If this value is large
compared to the area computed for spheres with the powder's particle size distribution then an unusually large
amount of dispersing agent (compared to a similar size distribution of spherical nonporous particles) may be
required to stabilize the dispersion.
NOTE The volume-specific surface area of spheres may be calculated from
SM� 6 (equation 35 in ISO 9276-2)
V �1,3
where
n
x
i
Mq� ln (equation 31 in ISO 9276-2)
�1,3 � 3,i
x
i�1
i�1
6 Selection of a liquid and trial dispersion
6.1 Selection of a liquid
The analyst shall list the liquids that are commonly used for dispersing the solids for the selected method of particle
size analysis and shall strike from the list any that fail to satisfy the following criteria.
� If the method is sedimentation, the liquid shall have a specific gravity that differs sufficiently from that of the
powder to permit the use of this method.
� If the method is light scattering, the liquid shall have a refractive index (at the analytical wavelengths) that
differs sufficiently from that of the powder to permit the use of this method.
� The liquid shall have negligible reactivity with the powder.
� The liquid shall not swell or shrink the particles by more than 5 % in diameter.
� The liquid shall provide a solubility of less than 5 g of powder per 1 kg of liquid.
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NOTE This is to minimize Ostwald ripening that could cause the particle size distribution to change during the
measurement time.
� The liquid shall have a change in the solubility (for the powder) with temperature of less than 0,1 mg/l per
kelvin, or the temperature shall be controlled throughout the preparation and analysis to keep the solubility
from changing by more than 0,5 mg/l.
NOTE If the particle size analysis method requires 10 mg of powder dispersed in 1 litre of liquid, a temperature rise of 5 K
(from an ultrasonic probe or particle-analysis instrument warmth) would cause the dissolution of 1 mg or 10 % of the powder.
6.2 Preparation of a test paste of the powder
Put two drops (or 0,1 g) of the liquid on an etch-roughened glass plate ("frosted" glass). Blend in a roughly equal
amount of powder by sprinkling powder on the liquid surface and rubbing it into the liquid using a circular motion of
a 10 mm wide spatula, applying a moderate amount of pressure (sufficient to read 1 kg on the scale of a balance).
The objective is to wet all the powder surfaces and to break up all clumps of powder into primary particles. The high
concentration of solids provides crowded conditions that favour collision between clumps and breakup into primary
particles. These crowded conditions will also favour flocculation unless the particles repel one another.
6.3 Preparation of a dilute dispersion of the powder
Make a dilute dispersion (4 % by mass) from the concentrated paste by adding a few drops at a time of the liquid
and blending in with the spatula until 50 drops (about 2,5 g) of liquid have been added. This quantity should be
sufficient for examination with a microscope. If a larger quantity is required for other types of test, the analyst shall
follow the instructions given in 7.2.
7 Examination of the dispersion
7.1 Evaluate for under- or over-grinding
Examine the dilute dispersion using an optical microscope (for particles larger than 1µm in diameter) or an electron
microscope (for particles smaller than 1µm in diameter). Use � 200 magnification with the optical microscope and
view the particles by transmitted light.
Note whether the clumps originally seen in the dry powder have completely broken up during the procedure for
making the paste and diluting it. If not, the analyst shall make a new dispersion using ultrasonic treatment (see 9.2).
The analyst shall evaluate this new dispersion and increase, as needed, the energy put in to breakup clumps until
full dispersion is attained.
Note what fraction of primary particles have become broken during the procedure for making the paste and diluting
it. If the fraction of particles broken is over 5 %, the analyst shall make a new dispersion by simply stirring the
powder into the liquid. The analyst shall evaluate this new dispersion and increase the energy put in to breakup
clumps as needed until full dispersion is attained with less than 5 % breakage of primary particles (see 9.2).
Record the conditions that avoid under- or over-grinding and use these to prepare dispersions for evaluation until
the clump breakup process is optimized according to the procedures in 7.2.
7.2 Evaluation of stability
7.2.1 Introduction
If the suspending liquid has a viscosity below 10 mPa�s and the particles are well-dispersed, very small particles
will appear to move randomly in the microscope's field of view. Particles in the 1µm to 5µm range are best for
observing this effect. Note that, even if the powder consists mostly of larger-size particles, there are likely to be a
few particles inside the 1µm to 5µm range that can indicate whether or not the dispersion is stable. If the particles
are smaller than 1µm some other form of evaluation shall be used, such as measuring the rheological stress-strain
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ISO 14887:2000(E)
�1 �1
cycle of a 10 % by volume solids dispersion from 0,1 s to 100 s to see if it exhibits hysteresis (indicative of
structure formation and breakage) or not (in which case the dispersion is stable).
Observe the dilute dispersion using the optical microscope. Note what happens when two particles come close
together. Rate the stability as good if the particles repel each other rather than coming into contact. Rate the
stability as marginal if the particles collide and stay together briefly before separating again. Rate the stability as
poor if the particles collide and remain in contact to form a permanent floc. If the stability is good, no added
dispersing agent is required to form a stable dispersion. If the stability is marginal or poor then either solution
conditions (such as pH) shall be changed or a dispersing agent shall be added to provide stability.
Other methods for evaluating dispersions are noted in annex A. If microscopy and the other techniques are not
feasible then particle size analysis may be used to evaluate stability. If a series of analyses separated by several
hours lie within the reproducibility of the instrument (determined using a dispersion that is known to be stable) then
thesampledispersionmaybeconsideredtobestable.
7.2.2 Notes on optical microscopy
Optical microscopy is the simplest and most effective way of evaluating the degree of deagglomeration and the
stability of dispersions containing particles that are above 1µm in size. Note that particles whose refractive index is
close to that of the liquid will not provide enough contrast to be viewed with the optical microscope. At a solids
concentration of a few percent, well-dispersed particles will appear to behave as separate entities. As the cover
glass is moved sideways over the surface of the slide, note whether the particles in the dispersion move individually
and not as a bonded group. Note whether particles that are below about 5µm in size may exhibit "Brownian
motion", as particles move about erratically due to unbalanced collisions of the particle with molecules of the
surrounding fluid.
Particles smaller than the limit of optical resolution (about 0,3µm) appear as bright spots when they are illuminated
from the side with a dark field behind them ("ultramicroscopy"). Although the width of the spot is indeterminate, the
size of the particle responsible for the spot may be estimated by its Brownian motion: the more actively a spot
moves, the smaller the particle creating the spot. The size of the smallest detectable particle using this technique
depends on the scattering power of the particles. Particles of titanium dioxide or of a metal as small as about
0,02µm may be observed using this technique, but for oil droplets the limit of observation is about 0,1µm.
Dispersion stability is destroyed if the particles stick to the glass microscope slide. This is a particular problem for
positively charged particles, since glass is normally negatively charged. Such adhesion can also invalidate the
measurement process, especially for light-scattering methods where the amount of solid circulating for analysis
may be so small that it is completely removed by adsorption on the walls of the sample circulation system. In such
cases, one can chemically treat the glass (with a cationic adsorbate such as dodecyl trimethyl ammonium bromide)
so that it becomes positively charged and thus prevents deposition of the particles being analysed.
7.2.3 Notes on electron microscopy
Evaluation by electron microscopy requires that the dispersion be spread out on a thin support film and dried. As
the liquid evaporates and the liquid surface shrinks between two particles, surface tension can pull previously well-
dispersed particles into contact to form a clump. This problem can be minimized if the analyst can use a liquid with
a low surface tension. Dispersion stability shall be judged as good if the particles are well spread out on the grid
and bad if they are found mostly in clumps.
7.3 Evaluation of any flocs formed
If flocs have formed, put a cover glass over the dispersion on the microscope slide. Use a spatula to push the cover
glass gently from the edge to slide it over the dispersion and apply shear force to the flocs. Note whether the flocs
break up and how rapidly they reform. Flocs are reversible if they break up under shear and then reform similar
flocs. Flocs are unstable if shear causes large, loose flocs to roll up into small, tight flocs. Flocs are strong if they do
not break up with gentle sliding. In the last case, the addition of dispersing agent may not be effective unless a high
shear force can be applied to break the floc in the presence of dispersing agent.
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ISO 14887:2000(E)
8 Identification of possible dispersing agents
8.1 Wetting of the solid particle by the liquid
The control of the wetting process allows the adhesion forces to be modified between the particles and the binding
forces produced by liquids in the intermediate capillaries to be partially modified.
The general aim for particles size analysis is a spontaneous wetting as complete as possible. This can be searched
by two ways:
� low-interfacial tension liquid/gaseous by wetting agents;
� low-interfacial tension solid/liquid by hydrophilizing agents.
In the case of insufficient wetting, a simultaneously mechanical treatment can be recommended (highly intensive
ultrasonic treatment of the suspension, kneading of the system as a plastic mixture with a spatula).
8.2 General principles
Subclauses 8.2 to 8.4 explain the principles used in developing the decision charts in 8.5. Complete dispersion of a
powder in a liquid occurs when the individual particles that made up the original clumps have become separated,
move independently of each other, and remain separated from one another. This requires that there be no
attractive force between the particles as they approach one another. If there is an attraction then the
solid/dispersion will exhibit non-Newtonian flow and have a yield stress (i.e. the dispersion will be able to support a
finite shear stress without any flow occurring.) Most of the indirect tests of dispersion rely on this effect. For
example, a dispersion with a yield stress enables settling particles to form an open structure which does not
collapse under the force of gravity. Such a dispersion will settle to form a higher sedimentation volume (lower
sediment density) than a completely dispersed system would.
Highly anisotropic particles form a more or less rigid gel at very low concentrations of solids when there is a net
attractive force between the particles.
8.3 Charge stabilization
8.3.1 Introduction
Particles which bear a surface charge will repel each other if the electrostatic repulsion is larger than the
polarizability attraction (also called the Hamaker or Van der Waals attraction). A surface charge corresponding to a
zeta potential greater than 30 mV is generally sufficient to provide a stable dispersion. Charge stabilization is the
best way to stabilize dispersions in which the liquid has a relative dielectric permittivity greater than 30 (methanol at
room temperature has a relative dielectric permittivity of 33, water of about 80) and an ionic strength less than
0,1 mol/l (i.e. a low concentration of ions in solution).
8.3.2 Surface ionization
The charge on the particle may arise from ionization of surface groups (influenced by the pH of the solution). For
example, surface amine groups will adsorb a hydrogen ion from solution and become positively charged if the pH is
below the pK for the powder. Surface carboxyl groups will lose a hydrogen ion and become negatively charged if
b
the pH is above the pK for the powder. Amphoteric surface groups, such as the OH groups found on a metal oxide
a
or hydroxide, will adsorb a hydrogen ion and become positive if the pH is below the pH for that oxide and will
iso
lose a hydrogen ion and become negative if the pH is above the pH . The dependence of hydrogen ion adsorption
iso
on pH is such that (when the ionic strength is below 0,1 mol/l) the zeta potential generally becomes large enough to
stabilize a dispersion if the pH is two or more units away from pH .
iso
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ISO 14887:2000(E)
8.3.3 Differential dissolution of lattice ions
If the powder is ionic and does not dissolve significantly in the liquid, the presence of a soluble salt of one of the
ions making up the powder may result in the adsorption of the common ion. For example, a sodium bromide
solution will disperse silver bromide in water and a potassium hydrogen phosphate solution will disperse calcium
hydroxyapatite in water. Since these soluble salts do not reduce the surface tension of water significantly, they are
not classified as surfactants.
8.3.4 Adsorption of multiply charged ions
If the powder is ionic or has highly polar bonds and the liquid is water, multiply charged ions which are not part of
the crystal lattice may be adsorbed to form a charged surface of soluble salts. Examples are the polyphosphate,
hexametaphosphate, pyrophosphate and polysilicate ions. Since the salts involving these ions do not reduce the
surface tension of liquids in which they are dissolved, they are not classified as surfactants.
If the powder is a nonpolar organic material and the liquid is a polar organic material, surface ions can be created
by adding a neutral ion-pair to the system. The ion-pair adsorbs on the partic
...
NORME ISO
INTERNATIONALE 14887
Première édition
2000-09-01
Préparation de l'échantillon — Procédures
pour la dispersion des poudres dans les
liquides
Sample preparation — Dispersing procedures for powders in liquids
Numéro de référence
ISO 14887:2000(F)
©
ISO 2000
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ISO 14887:2000(F)
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ii © ISO 2000 – Tous droits réservés
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ISO 14887:2000(F)
Sommaire Page
Avant-propos.iv
Introduction.v
1 Domaine d'application.1
2Référence normative .1
3Termesetdéfinitions.1
4 Symboles et abréviations .2
5 Examen de la poudre sèche .3
6Sélection d'un liquide et essai de dispersion.3
7 Examen de la dispersion.4
8 Identification des agents dispersants adaptés.6
9 Optimisation de la méthode de dispersion .12
10 Maintien de la stabilité de la dispersion pendant la manipulation de l'échantillon.15
Annexe A (informative) Autres essais de stabilité de dispersion .17
Annexe B (informative) Agents dispersants industriels pour les différentes catégories d'agents
dispersants.19
Bibliographie .24
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ISO 14887:2000(F)
Avant-propos
L'ISO (Organisation internationale de normalisation) est une fédération mondiale d'organismes nationaux de
normalisation (comités membres de l'ISO). L'élaboration des Normes internationales est en général confiéeaux
comités techniques de l'ISO. Chaque comité membre intéressé par une étude aledroit de faire partie ducomité
technique créé à cet effet. Les organisations internationales, gouvernementales et non gouvernementales, en
liaison avec l'ISO participent également aux travaux. L'ISO collabore étroitement avec la Commission
électrotechnique internationale (CEI) en ce qui concerne la normalisation électrotechnique.
Les Normes internationales sont rédigées conformément aux règles données dans les Directives ISO/CEI,
Partie 3.
Les projets de Normes internationales adoptés par les comités techniques sont soumis aux comités membres pour
vote. Leur publication comme Normes internationales requiert l'approbation de 75 % au moins des comités
membres votants.
L’attention est appelée sur le fait que certains des éléments de la présente Norme internationale peuvent faire
l’objet de droits de propriété intellectuelle ou de droits analogues. L’ISO ne saurait être tenue pour responsable de
ne pas avoir identifié de tels droits de propriété et averti de leur existence.
La Norme internationale ISO 14887 a étéélaboréepar le comité technique ISO/TC 24, Tamis, tamisage et autres
méthodes de séparation granulométrique, sous-comité SC 4, Granulométrie par procédésautresquetamisage.
Les annexes A et B de la présente Norme internationale sont données uniquement à titre d’information.
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ISO 14887:2000(F)
Introduction
L’estimation de la distribution granulométrique est d’une importance primordiale pour les projets de recherche, le
développement de produits, la maîtrise des procédés, le contrôle de la qualité,etd’autres activités techniques pour
lesquelles les effets granulométriques sont importants. La production commerciale des produits de peinture,
d’encres, de plastiques garnis, de traitement des minerais, pharmaceutiques, agricoles et cosmétiques dépendent
d’une analyse granulométrique précise.
Une poudre type est constituéed’aggrégation de blocs de particules «primaires» qui sont maintenues ensemble
par de fortes ou de faibles forces. La dimension des blocs qui restent après que la poudre a été mouilléedépend
en partie de la quantité d’énergie dépensée à casser ces blocs. Étant donné qu’un bloc réagit de la même manière
aux analyses granulométriques qu’une grosse particule, la présence de blocs dans des échantillons d’essai
dispersés de manière incomplète fait dériver la distribution granulométrique enregistrée vers des tailles de
particules plus grandes que si tous les blocs étaient entièrement cassés. Une analyse granulométrique n’est utile
que si l’échantillon d’essai est préparé de telle sorte que les particules sont dispersées d’une manière bien définie
— de préférence de façon que la majorité des blocs soient entièrement désagglomérés et que les particules ne se
réagglomèrent pas ou n’adhèrent pas aux parois du récipient d’échantillon au cours de l’analyse.
Bien qu’une dispersion «complète» des particules primaires soit souvent souhaitée, il est important de se rappeler
que dans de nombreux cas les informations les plus utiles sont obtenues avec des échantillons d’essai dont la
dispersion n’est pas totale. Si un client mélange, par exemple, la poudre dans un liquide en employant un procédé
à faible cisaillement qui ne casse pas des liaisons moyennement fortes au sein des blocs, les essais de contrôle de
la qualité de poudre réalisés pour ce client doivent utiliser un cisaillement faible similaire pendant la préparation et
l’analyse de l’échantillon d’essai.
En raison de la présence d’impuretés, l’équipement disponible pour casser les blocs, les méthodes utilisées pour
l’analyse granulométrique et les agents dispersants disponibles pour les essais peuvent varier d’un site à l’autre, la
procédure mise au point sur un site en appliquant les lignes directrices de la présente Norme internationale peut
être différente (mais aussi valide et utile) de celle mise au point sur un autre site pour la même poudre.
Une liste de références permettant d’approfondir le sujet, qui comprend également des normes d'évaluation
applicables à certains de ces systèmes plus complexes, est donnée dans la bibliographie.
L'annexe A traite de certaines des complications qui se produisent
� avec des poudres ayant reçu un traitement de surface ou contenant des composantes solubles;
� avec des liquides contenant des solutés ioniques ou polymériques;
� lorsque l'agent dispersant contient des ingrédients mineurs.
L’annexe B comporte des classifications des agents dispersants industriels pour les différentes catégories d’agents
dispersants.
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NORME INTERNATIONALE ISO 14887:2000(F)
Préparation de l'échantillon — Procédures pour la dispersion des
poudres dans les liquides
1 Domaine d'application
La présente Norme internationale a étéélaborée pour aider les techniciens pratiquant des analyses
granulométriques à obtenir une bonne dispersion des composants poudre/liquide dans des combinaisons pour
lesquelles ils n’ont aucune expérience. Elle fournit des modes opératoires permettant de
� mouiller une poudre dans un liquide;
� désagglomérer les grumeaux mouillés;
� déterminer si la composition de la solution peut être ajustéeafin d'éviter la formation de grumeaux;
� sélectionner des agents dispersants afin d'éviter que les grumeaux ne se reforment;
� évaluer la stabilité de la dispersion par rapport à l’éventualité d’une nouvelle formation de grumeaux.
La présente Norme internationale est applicable aux particules dont les dimensions sont comprises entre 0,05µm
et 100µm. Elle fournit un questionnaire sur la nature de la poudre et du liquide impliqu és. Les réponses sont
utilisées sous forme de diagrammes destinés à guider l'utilisateur dans son choix d’un agent dispersant à même de
convenir pour la dispersion de la poudre dans le liquide.
La présente Norme internationale n'est applicable qu'à la préparation de dispersions simples, diluées (moins de
1 % en volume de solides) destinées à des analyses granulométriques. Elle ne traite pas de mélanges complexes
et à forte concentration de solides tels que peintures, encres, produits pharmaceutiques, herbicides ou matières
plastiques composites vendus dans le commerce.
2Référence normative
Le document normatif suivant contient des dispositions qui, par suite de la référence qui y est faite, constituent des
dispositions valables pour la présente Norme internationale. Pour les références datées, les amendements
ultérieurs ou les révisions de ces publications ne s’appliquent pas. Toutefois, les parties prenantes aux accords
fondés sur la présente Norme internationale sont invitées à rechercher la possibilité d'appliquer l’édition la plus
récentes du document normatif indiqué ci-après. Pour les références non datées, la dernière édition du document
normatif en référence s’applique. Les membres de l'ISO et de la CEI possèdent le registre des Normes
internationales en vigueur.
ISO 8213:1986, Produits chimiques à usage industriel — Techniques de l'échantillonnage — Produits chimiques
solides de petite granulométrie et agglomérats grossiers.
3 Termes et définitions
Pour les besoins de la présente Norme internationale, les termes et définitions suivants s'appliquent.
3.1
agglomérat
assemblage de particules faiblement liées les unes aux autres
VOIR flocon (3.5)
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ISO 14887:2000(F)
3.2
agrégat
assemblage de particules fortement liées les unes aux autres
NOTE En raison de la confusion qui existe entre les termes de 3.1 et 3.2, ceux-ci sont peu utilisés dans le reste du texte.
3.3
grumeau
assemblage de particules présentant une cohésion plus ou moins forte
3.4
concentration micellaire critique
CMC
concentration d'agent dispersant au-delà de laquelle des micelles se forment
3.5
flocon
assemblage de particules présentant une cohésion relativement faible
VOIR agglomérat (3.1)
3.6
particules individuelles
unités à mesurer dans une analyse granulométrique, généralement plus difficiles à casser que les grumeaux
3.7
effet Tyndall
lumière diffusée perpendiculairement à un rayon lumineux traversant un liquide contenant des particules
4 Symboles et abréviations
Pour les besoins de la présente Norme internationale, les symboles et abréviations suivants s'appliquent.
2
S Aire volumique spécifiquede lapoudre(m /kg)
V
3
CMC Concentration micellaire critique (mol/m )
3
IS Force ionique (mol/m )
M Moment d’ordre moins un complet de la distribution de densité du volume de particules
–1,3
PEO Polyéthoxy = (-CH -CH -O-)
2 2 n
PPO Polyisopropoxy = (-CH -CH(CH )-O-)
2 3 n
pH pH auquel le potentiel dzêta est égal à zéro pour une surface amphotère (chargée positivement à un
iso
pH inférieur et négativement à un pH supérieur)
pK pH auquel la moitié des ions hydrogène des groupes acides sont ionisés
a
pK pH auquel la moitié des ions hydroxyde des groupes basiques sont ionisés
b
q Distribution de densité du volume de particules
3,i
ème
x Granulométrie supérieure du i intervalle de taille de particules (m)
i
µm Microm ètre
� Potentiel dzêta [V]
® Marque déposée
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ISO 14887:2000(F)
5 Examen de la poudre sèche
5.1 Échantillonnage
L’échantillonnage doit être conforme aux exigences spécifiées dans l’ISO 8213, sauf si une méthode spécifiéepar
une norme nationale ou acceptéed’un commun accord par l’analyste et le client est préférée. La préparation des
échantillons d’essai doit toujours être réaliséedemanière cohérente de sorte que des préparations répétées sur la
base d’échantillons subdivisésd’un même lot de poudre (soigneusement mélangéeavant le prélèvement
d’échantillons ou la subdivision en échantillons) donnent des résultats comparables.
5.2 Plage granulométrique des grumeaux et des particules
Étaler la poudre sèche sur une lame porte-objet et l'examiner à l'aide d'un microscope permettant un
grossissement de � 200 ou tout autre grossissement adapté. Recouvrir la lame d'une lamelle couvre-objet et
tapoter légèrement dessus avec une spatule (en prenant garde de ne pas briser la lamelle), pour voir avec quelle
facilité les grumeaux peuvent être écrasés. Noter approximativement la plage granulométrique dans laquelle se
situent les grumeaux qui ne sont pas cassés. Si la plupart des particules sont inférieures à 1µm, utiliser un
microscope électronique à transmission ou à balayage pour les observer et les caractériser.
5.3 Forme et état de surface; variations en fonction de la dimension
Noter si les surfaces des particules élémentaires sont sphériques ou cristallines, polies ou rugueuses, poreuses ou
non poreuses. Déterminer si toutes les dimensions granulométriques ont la même morphologie. Si les particules
2
sont très rugueuses ou poreuses, réaliser une mesure expérimentale de l'aire volumique spécifique (m /kg). Si
cette valeur est élevée par rapport à la valeur calculéede l’aire volumique spécifique des sphères avec la
distribution granulométrique de la poudre, alors une quantité d'agent dispersant plus importante (par rapport à celle
nécessaire pour une distribution granulométrique similaire de particules sphériques non poreuses) peut être
nécessaire pour stabiliser la dispersion.
NOTE L’aire volumique spécifique des sphères peut être calculée à partir de la formule suivante:
SM� 6 (équation 35 dans l'ISO 9276-2)
V �1,3
où
n
x
i
Mq� ln (équation 31 dans l'ISO 9276-2)
�1,3 3,i
�
x
i�1
i�1
6Sélection d'un liquide et essai de dispersion
6.1 Sélection d'un liquide
L'analyste doit établir une liste de correspondances entre les liquides couramment utilisés pour disperser les
solides, d’une part, et la méthode d'analyse granulométrique sélectionnée, d’autre part, et éliminer de la liste tout
liquide ne satisfaisant pas aux critères suivants:
� s'il s'agit d'une méthodedesédimentation, la masse volumique du liquide doit être suffisamment différente de
la masse volumique des grains constitutifs de la poudre, pour que cette méthode puisse être utilisée;
� s'il s'agit d'une méthode de diffusion de la lumière, l’indice de réfraction du liquide (aux longueurs d'onde
analytiques) doit être suffisamment différent de celui de la poudre, pour que cette méthode puisse être utilisée;
� la réactivité du liquide avec la poudre doit être négligeable;
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ISO 14887:2000(F)
� le rétrécissement ou le gonflement des particules, éventuellement provoqué par le liquide, ne doit pas être
supérieur à 5 % de leur diamètre;
� la solubilité assurée par le liquide doit être inférieure à 5 g de poudre pour 1 kg de liquide;
NOTE Ceci a pour but de minimiser la maturation d'Ostwald qui pourrait faire varier la distribution granulométrique pendant
la mesure.
� la variation de solubilité du liquide (pour la poudre) en fonction de la température doit être inférieure à 0,1 mg/l
par kelvin, sinon la température doit être contrôlée pendant toutes les phases de préparation et d'analyse pour
maintenir ces variations de solubilité en dessous de 0,5 mg/l.
NOTE Si la méthode d'analyse granulométrique implique la dispersion de 10 mg de poudre dans un litre de liquide, la
dissolution de 1 mg ou 10 % de la poudre peut se faire par un simple échauffement de 5 K (par la chaleur d'une sonde à
ultrasons ou d'un appareil d'analyse granulométrique).
6.2 Préparation d'une pâte témoin de la poudre
Verser deux gouttes (ou 0,1 g) du liquide sur une plaque de verre dépoli (verre «givré»). Y mélanger une quantitéà
peu près égale de poudre en la saupoudrant à la surface du liquide et en l'incorporant par un mouvement circulaire
d'une spatule d'environ 10 mm de large, en exerçant une pression modérée (suffisante pour inscrire 1 kg sur le
cadran d'une balance). Le but est de mouiller toutes les surfaces de poudre et de casser tous les grumeaux en
particules élémentaires. Une forte concentration en solides crée les conditions d'encombrement nécessaires pour
favoriser les collisions entre les grumeaux et leur désagglomération en particules élémentaires. Ces conditions
d'encombrement favorisent également la floculation, à moins que les particules ne se repoussent mutuellement.
6.3 Préparation d'une dispersion diluée de la poudre
Préparer une dispersion diluée(4% en masse) delapâte concentrée en ajoutant très progressivement 50 gouttes
(environ 2,5 g) de liquide, tout en mélangeant avec la spatule. En général, cette quantité suffit pour une
observation au microscope. Si une quantité plus importante est nécessaire pour d'autres types d'essai, l'analyste
doit suivre les instructions données en 7.2.
7 Examen de la dispersion
7.1 Évaluation d'un broyage insuffisant ou excessif
Examiner la dispersion diluée à l'aide d'un microscope (pour les particules dont le diamètre est supérieur à 1µm)
ou d'un microscope électronique (pour les particules dont le diamètre est inférieur à 1µm). Utiliser, pour le
microscope, un grossissement de� 200 et observer les particules par lumière transmise.
Noter si les grumeaux observés à l'origine dans la poudre sèche sont entièrement cassés par la procédure de
préparation etdedilutiondelapâte. Si ce n'est pas le cas, l'analyste doit réaliser une nouvelle dispersion par
traitement aux ultrasons (voir 9.2). L'analyste doit évaluer cette nouvelle dispersion et augmenter, si besoin est,
l'énergie ultrasonore nécessaire jusqu'à désagglomération de tous les grumeaux et obtention d'une dispersion
totale.
Noter la proportion de particules élémentaires qui ont été cassées pendant la préparation etdedilutiondelapâte.
Si les particules cassées représentent plus de 5 %, l'analyste doit réaliser une nouvelle dispersion en mélangeant
simplement la poudre au liquide. L'analyste doit évaluer cette nouvelle dispersion et augmenter, si besoin, l'énergie
nécessaire jusqu'à désagglomération de tous les grumeaux et obtention d'une dispersion totale en cassant moins
de 5 % de particules élémentaires (voir 9.2).
Consigner les conditions qui permettent d’éviter un broyage insuffisant ou excessif et les reproduire pour préparer
des dispersions destinées àêtre évaluées jusqu'à ce queleprocédé de désagglomération des grumeaux soit
optimisé conformément aux procédures de 7.2.
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ISO 14887:2000(F)
7.2 Évaluation de la stabilité
7.2.1 Introduction
Si la viscosité du liquide de suspension est inférieure à 10 mPa�s et si les particules sont bien dispersées, de très
petites particules se déplacent de manière aléatoire dans le champ observé au microscope. Ce phénomène est
plus facile à observer lorsque la dimension des particules est comprise entre 1µm et 5µm. Noter que, m ême si la
poudre est principalement constituée de particules de dimensions plus importantes, elle contient probablement
quelques particules comprises entre 1µm et 5µm qui permettent d ’évaluer le degré de stabilité de la dispersion. Si
la taille des particules est inférieure à 1µm, utiliser une autre m éthode d'évaluation, telle que la mesure d'un cycle
�1 �1
contrainte-déformation rhéologique d'une dispersion de 10 % en volume entre 0,1 s et 100 s , pour détecter
l'existence (indiquant la formation et la rupture de structures) ou l'absence (indiquant la stabilité de la dispersion)
d’un phénomène d'hystérésis.
Observer la dispersion diluée à l'aide d'un microscope. Noter ce qui se produit lorsque deux particules se
rapprochent l'une de l'autre. La stabilité est bonne si les particules se repoussent mutuellement plutôt que d'entrer
en contact. La stabilité est moyenne si les particules entrent en collision et restent brièvement en contact les unes
avec les autres avant de se séparer de nouveau. La stabilité est mauvaise si les particules entrent en collision et
restent en contact pour former un flocon permanent. Si la stabilité est bonne, il n'est pas nécessaire d’ajouter une
quantité supplémentaire d’agent dispersant pour assurer une dispersion stable. Si la stabilité est moyenne ou
mauvaise, on peut soit agir sur les conditions dans la solution (telles que le pH) pour les modifier, soit ajouter un
agent dispersant afin d'améliorer la stabilité.
D'autres méthodes d'évaluation des dispersions sont présentées dans l'annexe A. S’il n’est pas possible de le faire
par microscopie ou par une autre technique, il est possible d’avoir recours à une analyse granulométrique pour
évaluer la stabilité.Siles résultats d’une série d'analyses, effectuées à plusieurs heures d’intervalle, s'inscrivent
dans les limites de reproductibilité de l'instrument (déterminées à l'aide d'une dispersion reconnue pour sa bonne
stabilité), la dispersion échantillon peut être considérée comme stable.
7.2.2 Remarques sur la microscopie optique
La microscopie optique est le moyen le plus simple et le plus efficace pour évaluer le degré de désagglomération
et la stabilité d'une dispersion contenant des particules de dimensions supérieures à 1µm. Noter que les particules
dont l'indice de réfraction est proche de celui du liquide ne présentent pas un contraste suffisant pour pouvoir être
observées à l'aide d'un microscope optique. À une concentration en solides de quelques pour cent, des particules
bien dispersées se comportent comme des entitésséparées. En déplaçant latéralement la lamelle couvre-objet sur
la surface de la lame porte-objet, observer le déplacement des particules et noter s’il est individuel ou groupé.
Noter si des particules de moins de 5µm peuvent avoir un mouvement brownien (c ’est-à-dire irrégulier, en zigzag),
en raison de collisions déséquilibrées entre les particules et les molécules du liquide environnant.
Lorsqu'elles sont éclairées latéralement et observées sur un arrière-plan sombre, les particules plus petites que la
limite de résolution optique (environ 0,3µm) apparaissent comme des taches claires ( «ultramicroscopie»). Bien
que la largeur de la tache soit indéterminée, la taille de la particule à l'origine de cette tache peut être évaluée
grâce à son mouvement brownien: plus une tache se déplace, plus la particule à l'origine de cette tache est petite.
La taille de la plus petite particule pouvant être décelée à l'aide de cette technique dépend du pouvoir de diffusion
des particules. L’observation de particules de dioxyde de titane ou de métal aussi petites que 0,02µm est possible
à l'aide de cette technique, alors que celle de gouttelettes d'huile est limitée à environ 0,1µm.
La stabilité de la dispersion est rompue si les particules adhèrent au verre de la lame porte-objet. Ceci représente
un véritable problème pour les particules à charge positive, le verre étant généralement chargé négativement. Une
telle adhérence peut également invalider le procédé de mesure, notamment pour les méthodes de diffusion de la
lumière où la quantité de solide circulant pour l'analyse peut être si faible qu'elle est entièrement adsorbée par les
parois du système de circulation de l'échantillon. Dans ce cas, il est possible de traiter chimiquement le verre (avec
un tensio-actif cationique tel que le bromure de dodécyl triméthyl ammonium) pour le charger positivement et éviter
ainsi le dépôt des particules analysées.
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ISO 14887:2000(F)
7.2.3 Remarques sur la microscopie électronique
Pour l'évaluation par microscopie électronique, étaler la dispersion sur un film fin et la laisser sécher. Lorsque le
liquide s'évapore et que la surface du liquide entre deux particules se rétrécit, une tension superficielle peut être à
l'origine de la formation de grumeaux constituésde particules précédemment bien dispersées. Ce problème peut
être résolu en utilisant un liquide faiblement tensioactif. La stabilité de la dispersion doit être considéréecomme
bonne si les particules sont bien réparties sur la grille et comme mauvaise si la plupart d’entre elles forment des
grumeaux.
7.3 Évaluation des formations de flocons
En cas de formation de flocons, poser une lamelle couvre-objet sur la dispersion. À l’aide d’une spatule, faire
glisser doucement la lamelle couvre-objet d’un bord à l’autre de la lame porte-objet, pour exercer sur les flocons
une force de cisaillement. Noter si les flocons se cassent et la vitesse à laquelle ils se reforment. S'ils se cassent
sous l'effet du cisaillement pour se reformer ensuite, les flocons sont dits réversibles. Si, sous l'effet du
cisaillement, on observe la formation de grands flocons lâches qui s'enroulent pour former de petits flocons serrés,
les flocons sont dits instables. S'ils résistent au passage de la lamelle couvre-objet sur la dispersion, les flocons
sont dits résistants. Dans ce dernier cas, l'ajout d'agent dispersant peut rester sans effet, à moins de pouvoir
exercer un cisaillement important sur les flocons pour les casser en présence de l'agent dispersant.
8 Identification des agents dispersants adaptés
8.1 Mouillage des particules solides par le liquide
Le contrôle du procédé de mouillage permet de modifier les forces d’adhésion entre les particules, et de modifier
partiellement les forces de liaison créées par les liquides dans les capillaires intermédiaires.
L’objectif général de l’analyse granulométrique est un mouillage spontané aussi complet que possible. Ceci peut
être réalisé moyennant deux conditions:
� faible tension interfaciale liquide/gaz due aux agents de mouillage;
� faible tension interfaciale solides/liquides due aux agents hydrophilisants.
En cas de mouillage insuffisant, un traitement mécanique simultané peut être recommandé (traitement par
ultrasons hautement intensif de la suspension, malaxage du système comme un mélange plastique avec une
spatule).
8.2 Principes généraux
Les paragraphes 8.2 à 8.4 expliquent les principes appliquéspour élaborer les diagrammes décisionnels en 8.5. La
dispersion complète d'une poudre dans un liquide a lieu lorsque les particules individuelles qui constituent le bloc
d'origine sont séparées définitivement et qu’elles se déplacent indépendamment les unes des autres. Ceci implique
l'absence de forces d'attraction entre les particules lorsqu'elles se rapprochent les unes des autres. En présence
d’une attraction, le solide/la dispersion présente un écoulement non newtonien ainsi qu’une limite apparente
d'élasticité (c'est-à-dire que la dispersion peut être soumise à une contrainte de cisaillement donnée sans
présenter de signe d'écoulement). La plupart des essais indirects de dispersion se basent sur ce phénomène. Une
dispersion qui présente une limite apparente d'élasticité permet, par exemple, la sédimentation des particules et la
formation d'une structure ouverte qui ne s'écrase pas sous l'effet de la pesanteur. Une telle dispersion est à
l'origine d'un volume de sédiments plus important (densité de sédiments inférieure) que dans le cas d'un système
entièrement dispersé.
En présence de forces attractives nettes entre les particule
...
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