Preparation of monolayer particle coated powder by the dry impact blending process utilizing mechanochemical treatment

Co&ids and Surfaces A: Physicochemical and Engineering Aspects, 82 (1994) 117-128 0921-7757/94/$07.00 0 1994 - Elsevier Science B.V. All rights reserved.

117

Preparation of monolayer particle coated powder by the dry impact blending process utilizing mechanochemical treatment” H. Hondaa,*, M. Kimurab, F. Honda’, T. Matsmod, M. Koishi” aFaculty of Industrial Science and Technology, Science University of Tokyo, 102-1, Tomino Oshamanbe-cho, Yamakoshi-gun, Hokkaido 049-35, Japan bDepartme n t of Electric Engineering, Science University of Tokyo, Suwa College, 5000-1, Toyohira, ChinoCity, Nagano 391-02, Japan “PHD Inc. 3-23-18-803, Tomioka-cho, Hakodate-shi, Hokkaido 041, Japan dSchool of Medicine, Kitasato University, l-15-1, Kitasato Sagamihara-shi, Kanagawa 228, Japan (Received

2 January

1992; accepted

29 July 1993)

Abstract Spherical silica particles 0.3, 0.6 and 0.9 pm in diameter, and spherical polyethylene particles 5.2 and 11.9 pm in diameter were used as coating and core materials respectively. Scanning electron microscopy revealed that the silica particles were fixed on the surface of the polyethylene particles, and a monolayer particle coated powder was formed by a dry impact blending preparation method when the ratios of the coating and core particle sizes were 0.3/11.9, 0.6/11.9, 0.9j11.9 and 0.3/5.2. In the dry impact blending of a binary powder mixture which contains two different sizes, it was shown that the ratio of the core and coating particle sizes, the particle sizes themselves and the action of impulsive forces during the operation were important factors in the effective preparation of the monolayer particle coated powder. The binding energies were calculated on the assumption that the Coulomb and London-van der Waals interactions were the basis of adhesion for these monolayer particle coated polyethylene powders. Consequently, it was found that the Coulomb interaction between particles with different electric charges was not always advantageous for the formation of the monolayer particle coated powder. Key words: Binary powder

mixing; Dry impact blending

method;

The modification of powder surfaces is a method for the preparation of highly functional materials. The importance of the powder surface modification method is recognized from the point of view of being capable of improving and controlling the powder properties, for example, catalytic activity, wettability, electrical properties, electronic properties, optical properties, rheological properties, etc. This concept of microfabrication on powder sur-

SSDI

0927-7757(93)02620-T

force; Mechanochemical

treatment

faces can be used in many

Introduction

*Corresponding author. “Dedicated to the memory

Interparticle

detergents,

dental

materials,

In our laboratory, tion method physicochemical

fields, e.g. in

metallurgy,

the powder

has been studied phenomena

toners, inks,

etc. [ 11.

surface modificaon the basis of the

of a dry mixing

pro-

cess. Travers and White [2] have reported a mixture which demonstrates the adhesion of fine particles mixture more

of K. Meguro.

industrial

cosmetics, pharmaceuticals, foods, copy cements, pigments, ceramics, paints,

formed

on

the surfaces

of these cohesive homogeneous by mixing

than

of coarse particles the

free-flowing

particles.

A

was frequently

random particles.

mixture Hersey

H. Honda et ai.lColloids

118

[3]

has

coined

the term

“ordered

mixture”

to

explain this phenomenon in the mixing of cohesive particles. The mixture consisted of ordered units in which the weight of fine particles adhering on the surface of the coarse particles was constant. The formation of ordered units arises from just a surface modification of large particles by fine particle materials. The powder mixture in which fine components adhered to the surface of the more coarse component has been named “interactive mixture” instead of the term “ordered mixture” as a result of many arguments by Egermann [4-g], Thiel et al. [9], Thiel [ 10,111 and Staniforth [12,13]. Honda et al. [14] and Fan et al. [ 151 have reported the details of all the arguments with respect to powder mixing in some review articles. We developed a dry impact blending method (Koishi et al. [ 161) which has an impulsive force in addition to the interactive powder mixing procedure. It was clearly confirmed in previous works [ 16-191 that the dry impact blending procedure was capable of preparing composite or encapsulated-type particles, namely, if inorganic fine particles were used as coating materials, the particles were fixed and embedded in the surface of the core particles, and if polymer or metallic fine particles were used as coating materials, the particles were partially melted for continuous film formation on the surface of the core particles. Furthermore, the randomized configuration of many fine particles on each core particle in the interactive mixing system was rearranged to an ordered state, giving a monolayer particle coated powder. In this paper, important factors for an effective monolayer particle coated powder were discussed using scanning electron microscopy (SEM) and binding energies calculated on the assumption that the Coulomb interaction and London-van der Waals interaction were the basis for the adhesion between particles.

Surfaces A: Physicochem.

Sumitomoseika)

Eng. Aspects 82 ( 1994) 117-128

and silica (SS-03, SS-06 and SS-09,

Mitsubishikasei) particles were used at various weight ratios as the core and coating materials respectively.

Three kinds of spherical

were used as coating

materials;

silica particles

the particle

diame-

ters were 0.3, 0.6 and 0.9 urn, and the density was 2.01 g cme3. Polyethylene particles were used as core materials, and had diameters and a density of 0.92 g cme3. Preparation

of silica-modtjied

of 5 and 10 urn,

polyethylene

method The dry impact blending preparation comprises a two-step blending process. The first step is a mechanical blending process (named dry blending) of the core and coating materials for the preparation of an interactive mixture formed by the adhesion of the fine coating particles on the surface of the coarse core particles. During this stage, a centrifugal impeller rotatingtype batch mixer was used (MECHANOMILL MM-10 type, Okadaseiko Co., Ltd., Tokyo). A schematic diagram of the batch mixer is shown in Fig. 1. Sample material ( 15 g) was loaded into the mixer, and was blended at room temperature for 10 min. The rotational speed of the vessel was maintained at 1000 rev min-‘. After the interactive

Methods Materials In the dry blending and the dry impact blending experiment, polyethylene (LE-1080 and LE-2080,

Fig. 1. Schematic model of centrifugal impeller rotating-type batch mixer (MECHANOMILL): A, rotating vessel; B, fixed container; C, lid; D, rotor; E, powder.

H. Honda et al./Colloids Surfaces A: Physicochem. Eng. Aspects 82 (1994 J 117-128

mixture was prepared, the following coating process is responsible for the final preparation of the composite

particles.

repeated.

119

The

moving

track

of powders

in the

machine is shown in Fig. 2. Since the powder particle (interactive mixture) repeatedly impacts on

The second step is a mechanical impact blending process (termed dry impact blending) of the inter-

the surface, fine powder particles become attached to and arranged on the surface of the core particles.

active mixture

The time

particles.

for the preparation

An impact-type

of the composite

hybridization

machine

with jacket was used (HYBRIDIZER type-O, Nara Machinery Co., Ltd., Tokyo; the system is now patented). Figure 2 is a schematic diagram of the machine for producing the mechanical impacts. A thermometer was set in the circulation route to measure the inner atmospheric temperature of the machine. The machine is surrounded by a jacket through which heat medium or coolant is circulated. Water was used as the coolant in these experiments. In this machine, powders (the interactive mixture) are guided through a feed chute into the center of the machine and blown off in a peripheral direction by the centrifugal force generated by the high speed rotor. The dispersed powder particles hit striking pins rotating at over 10000 rev min-‘. Consequently, the powder receives mechanical impacts by these collisions on its surface. The powder reaching the periphery of the machine re-enters the circulation route and returns to the center of the machine. This process is continually

required

short; the powder depending

on the revolution

at 300-500

was quite rev min-’

rate of the rotor. This

means that powder particles experience a number of impacts for a short time. In these experiments for the preparation of the composite particles, 15 g of powder prepared by the batch mixer were fed through the machine with striking pins rotating at 16000 rev min-‘, and the dry impact blending was carried out in 10 min. The atmospheric ature in the circulation route eventually 55°C. Measurement

of electrostatic

temperrose to

charge

Figure 3 is a schematic diagram of the system for measuring the electrostatic charge of the dispersed particles. The apparatus is constructed from a mixer, a hopper and a static charge detector connected electrometer (TR-865 1, SANKYO to an POWTECH). Each powder poured into the mixer was dispersed by turbulent air flow and obtained a triboelectrical charge by contact or friction. The

w Fig. 2. Schematic model of the moving track of the powder in the impact-type hybridization machine (HYBRIDIZER): A, feed chute; B, circulation route; C, trace of powder; D, rotor; E, striking pin; F, jacket (heat medium/coolant).

for the circulation circulated

E

I/D

F

Fig. 3. Schematic model of the apparatus for measuring the static charge on powder particles: A, powder; B, the apparatus for dispersing the powder; C, tube; D, Faraday well; E, electrometer; F, vacuum pump.

H. Honda et al./Colloids Surfaces A: Physicochem. Eng. Aspects 82 (1994)

120

I1 7-128

measured by pouring a known weight of powder into the detector through the hopper. The results

the dry blending method. However, many silica particles randomly adhere to the surface of polyethylene particles in the interactive mixture, as shown

are shown in Table 1.

in Fig. 4a. The

geometrical

morphology

of the

formed

of adhered

silica particles

is very

specific charge on the triboelectrified

particles

was

Results and discussion Silica-modijied

polyethylene

similar to that of the fractal structure of colloidal aggregates analyzed by a diffusion-limited aggregation model [20,21] or experimentally verified on some colloids [22,23]. Batch mixing over 60 min

particles

Figure 4 shows typical SEM photographs of coated particles prepared by the dry blending or dy impact blending of silica and polyethylene particles. Many fine silica particles are clearly fixed on the surface of each polyethylene particle and are rearranged for monolayer particle adhesion by the dry impact blending treatment. Figure 5 shows a schematic diagram of the dry treatment processes. Generally, in the batch mixing of particulate matter, characterization of the resultant mixture is generally classified into two major groups: one involves only free-flowing particles and the other contains cohesive particles. Since the free-flowing mixture, the so-called non-interactive mixture, generally does not have a repulsive or an attractive interaction mechanism, the individual particle is able to move independently. However, the interactive mixture consisting of cohesive particles has some interparticulate bonding mechanism, and permits particles to move only with an associated unit of particles. During the first step of our experimental processes, an interactive mixture of silica and polyethylene particles was prepared by

Table 1 Charge of a particle Sample

Polyethylene

Silica

calculated

from mean particle

Mean particle

size

structure

size, density

produces

the fracture

or deformation

of the core

particles and does not contribute to the preparation of a monolayer particle coated state. During the second step, 10 min of dry impact blending enables silica particles randomly adhered on the surface of the polyethylene particles to rearrange into an ordered state with some specific lattice structure. Figure 6 shows typical SEM photographs of the silica-coated polyethylene particles prepared by dry impact blending utilizing mechanochemical treatment. The monolayer particle coated powder was formed by dry impact blending, when the ratios of the particle sizes (coating/core) were 0.3/l 1.9, 0.6/l 1.9,0.9/l 1.9 and 0.3/5.2. For the combinations 0.615.2 and 0.915.2, the arrangement of silica particles on the surface of the polyethylene did not always form a monolayer particle coated powder. Figure 7 shows SEM photographs of the surface of the polyethylene particle after the silica particles were peeled off. The specimen was prepared in the following way. After the composite particles were potted

in epoxy

resin, the dried

resin block

and mean specific charge

Density

Mean specific charge

Value of -Q

@m)

(Mg mm31

(C g-‘1

(C)

5.2 11.9

0.92 0.92

-9.50 -8.79

x lo-’ x lo-’

6.43 x 10-l’ 7.14 x 10-16

0.3 0.6 0.9

2.01 2.01 2.01

+2.86 +2.65 +2.22

x lo-’ x lo-’ x lo-’

8.13 x 10-Z’ 6.02 x lo-” 1.70 x lo-l9

or q

was

H. Honda et al./Colloids Surfaces A: Physicochem. Eng. Aspects 82 (1994 ) 117-l 28

b

a

121

-

C Fig. 4. Typical SEM photographs of the batch mixing (dry blending) and the dry impact blending of silica and polyethylene (all bar scales represent 1 pm): a, batch mixing of 35 wt.% silica (0.3 pm) and 65 wt.% polyethylene (5.2 pm); b, dry impact blending of 35 wt.% silica (0.3 pm) and 65 wt.% polyethylene (5.2 pm); c, dry impact blending of 20 wt.% silica (0.3 pm) and 80 wt.% polyethylene (11.9 pm).

0

0

;;::

_:og+*s .

l*

.

8’1 ‘1 %* l*

l

c

D

B

Fig. 5. Schematic model of microhybridization technology: A, core powders; B, coating powders; C, interactive mixture; D, composite (monolayer particle coated powders).

cut using a microtome to produce fine sections. The fracture surface appearance of the polyethylene was then observed under a microscope. The mean depth penetration into the surface of the core particles could be measured using the SEM photographs. Silica particles 0.3 urn, 0.6 urn and 0.9 urn in diameter were embedded in the surface of the 11.9 urn diameter polyethylene partitles at depths of 0.03 urn, 0.11 urn and 0.19 urn respectively. Silica particles 0.3 urn in diameter were embedded in the surface of the 5.2 urn diameter polyethylene particles at a depth of 0.07 urn. In the system using 11.9 urn polyethylene as core particles,

the depth

of depression

increased

consis-

122

H. Honda et al./Colloids Surfaces A: Physicochem. Eng. Aspects 82 ( 1994) 117-t 28

method Fig .6. Typical SEM photographs of silica-modified polyethylene powder prepared by the dry impact blending preparation (all bar scales represent 5 pm): a, dry impact blending of 20 wt.% silica (0.3 pm) and 80 wt.% polyethylene (11.9 pm); b, dr! I impact (11.9 pm); c, dry impact blending of 45 wt.% silica (0.9 pm) and 55 wt.% ble nding of 35 wt.% silica (0.6 pm) and 65 wt.% polyethylene polyethylene (11.9 pm); d, dry impact blending of 35 wt.% silica (0.3 pm) and 65 wt.% polyethylene (5.2 pm).

tently with the size of the coating particles. The combination of 0.3 urn silica and 5.2 urn polyethylene particles displayed deeper depressions than the combinations of 0.3 urn silica and 11.9 urn polyethylene particles. In these cases, the depth of depression increased with the decrease in the size of the core particle. The surface morphology of the composite particles was influenced by the combination of silica and polyethylene particle sizes. Good composite particles were formed, consistent with a decrease in the ratio of the size of the coating particle to that of the core particle. The particle size ratio of the core and coating particles was an important factor in the preparation of the mono-

layer particle coated powder. More basic discusconcerning the silica particles, deeply sion embedded in the surface of the polyethylene particles, has already been presented in a previous paper [24]. In contrast, the three- or two-dimensional morphologies of colloidal aggregates via Brownian particle trajectories show a fractal-like structure. One of the most prominent features of the surface deposits formed by the diffusion-limited aggregation mechanism is the formation of isolated treelike clusters. Now, in our experiments, the surface morphology of the silica-coated polyethylene composite prepared by the dry impact blending method

H. Honda et al./Colloids Surfaces A: Physicochem. Eng. Aspects 82 (1994)

a

C

I1 7-128

123

b

d

Fig. 7. Typical SEM photographs of a bare polyethylene surface and silica arrangements, embedded in epoxy resin after fracture (all bar scales represent 1 pm): (a) and (c) polyethylene surface (5.2 pm); (b) silica (0.3 pm) arrangement before fracture; (d) detached silica (0.3 pm) arrangement after fracture.

was a monolayer particle coated structure. From considerations, the random thermophysical arrangement of coating particles on the surface of the core particles never spontaneously changes into an ordered arrrangement [25]. Therefore it is

confirmed that the monolayer particle coated structure is formed by the action of impulsive forces during the preparation. These SEM observations show that the particle size itself, the particle size ratio of the core and

H. Honda et al.JColloids

124

Surfaces A: Physicochem.

Eng. Aspects 82 ( 1994) 117-128

coating particles, and the action of impulsive forces are important factors in the effective preparation

when calculating the short-range energy between particles. As an outline of the composite formation

of a monolayer

system, a negatively charged with a radius a is uniformly

Estimation

particle

coated

powder.

of binding energy

The adhesion

forces between

by any particles

classified into five groups: (1) attraction (van der Waals interaction, electrostatic action,

etc.); (2) physicochemical

number

spherical core particle coated on its surface

of positively

charged

spherical

may be

coating particles with radius b that is much smaller than a. When the number of coating particles in

forces inter-

the system is N and the charge of a core and of a coating particle are assumed to be -Q and +q

forces (mechano-

chemical interaction, chemical reaction, sintering, etc.); (3) interfacial force and capillary pressure at the freely movable liquid (liquid bridge); (4) adhesion and cohesion forces not as a freely movable binder (highly viscous bonding); (5) form-closed adhesion (interlocking). It is not necessary to consider the forces in groups (3)-( 5), because spherical particles in dry blending treatments are dealt with in a solvent-free system. The forces in groups (1) and (2) are important factors for particle adhesion mechanisms during treatment. However, a chemical reaction or a sintering between the silica and polyethylene interface does not occur, Therefore adhesive energies are calculated on the limited basis of the London-van der Waals and Coulomb interactions. The energy for the preparation of a monolayer particle coated powder was calculated on the assumption that the pure electrostatic interaction works mainly on adhesion between particles [ 193, and is the work needed to bring a final coating particle from infinity. As a result, if the adhesion mechanism between particles with different sizes is mainly subject to this attraction due to the electrostatic interaction, it was deduced that the increase in the ratio of core particle size to the coating particle size was advantageous for the formation of the monolayer particle coated powder. This calculation result was in good agreement with the SEM observations. In this paper, the energy of the monolayer particle coated system is calculated on the basis of the assumption that the Coulomb and Londonvan der Waals interactions work together. The London-van der Waals interaction is not negligible

respectively,

the total

positive

charge

is given by

Q’ = Nq and the total charge of the system becomes Q, = Q’ - Q. The adhesive energy per one coating particle, utot, is then composed of the sum of four parts, i.e. u,_, u_, uV_ and u,* 2. The term u,_ is a contribution from the Coulomb interaction between a core and each coating particle, and ue22 is that between coating particles. The term from the London-van der uy1 2 is the contribution Waals interaction between a core and each coating particle, and uVZz is that between coating particles. Initially, the interaction energies between a core and one of the coating particles are considered. The system is schematically illustrated in Fig. 8. The energy due to the Coulomb interaction between two particles is given by 1 -Qq G,,, = 471~~ __ D

(1)

where D is the distance between two centers and is given by D = a + b + 1, where 1is the gap between the surfaces of the two spheres and sO is the dielectric constant of the vacuum. However, the energy due to the London-van der Waals interaction calculated by Hamaker [26] is

Fig. 8. Schematic model for the calculation between coating and core pax titles.

of the

energy

H. Honda et al./Colloids Surfaces A: Physicochem. Eng. Aspects 82 ( 1994 ) 117-l 28

based on the microscopic Heitler, and is expressed as

theory

of London-

A 2ab 2ab u“1.2= - L i * D* - (a + b)2 ’ D2 -(a - b)* + In D* - (a + b)* D2 -(a

- b)*

1

(2)

where A,,, is the so-called Hamaker-van der Waals constant for two substances 1 and 2, namely a core and a coating particle. Next the interaction energy between coating particles in a monolayer particle coated system is calculated. The model is graphically shown in Fig. 9. To this end, it is considered that the radius of the coating particles is very small in comparison with that of the core, so that coating particles are distributed on the sphere of a core particle with some two-dimensional lattice structure and monolayer particle. If n is the number of coating particles per unit area for an appropriate lattice structure on the sphere of a core, the total number of coating particles is given by N = 4nD*n, and the charge per unit on the sphere becomes 6, = nq, which in turn gives the total positive charge Q’ = 47cD26, =

125

47rD*nq. Let dS, and dS, then be surface elements at a distance r on the sphere of radius D for either the Coulomb or London-van der Waals interactions. The interaction energy per one coating particle is generally written in the form

s

2X ui(r)dS,dS2

uizz = -

where i denotes e or v, the factor + occurs to avoid double counting for each pair of surface elements, and ui(r) is a kind of energy density depending on the type of interaction. This density usually depends only on the distance Y= 20 sin(8/2), where 0 is the angle subtended by the chord r as viewed from the center. Therefore the integration with respect to the other three angles except for angle i3 is easily performed, and Eq. (3) becomes

%,2 =y

fni(2Dsin$sinOd0

(4)

where a small angle B,, is defined by t?e= D’lD, D’ = 2b + I’ being the distance between centers of two neighboring coating particles, and I’ the gap between their surfaces. It is convenient for ease of performing the integration to change the integral variable from 6’ to t with the transformation t = sin(B/2), which reduces Eq. (4) to a simpler form: (4~0~)~ ui, 2 = ___ N

Ui(2Dt)t dt s

In the case of a Coulomb interaction, the energy density has the form 102

11

u,(r) = 471~ 7 = 8mOD t 0

Inserting Eq. (6) into Eq. (5), and after the integration has been completed, the energy is given by (471D2aJ2

u?2.2 = 8moDN Fig. 9. Schematic model for between coating and particles.

the

calculation

of the

energy

N- (Q')* - 8ne,DN

(7)

To obtain the final form of Eq. (7), B. is set to be approximately zero because of the uniformity of

H. Honda et al./Colloids Surfaces A: Physicochem. Eng Aspects 82 ( I994 ) 1I 7-l 28

126

positive charges on the sphere and the long-range nature of the Coulomb interaction. However, the energy density due to the Londonvan der Waals interaction is given by the extension of Eq. (2) to the system on the sphere n*A,, u,(r) = - __

r

with a particle

0’ -(a

2b2

Y* - (2b)*

2b2

6-‘- IT* - (2b)2 +

y2 +ln

y2

r L

,7

_1

1. +

D* t*

+ 2 In

1

(I-$)ln(l-g)

1

b* +

- b)*

y1 --+I

b* ;n2& 2 D*t*-b* 12

=-

density

+ ln 0’ - (a + b)*

\“‘-,

-6,

u sin* 0 2

b2\.

- 3)

In

(8)

der Waals constant where A2,2 is the Hamaker-van for the coating particle material. By substituting Eq. (8) into Eq. (5) and further calculations, the following result for the interaction energy are derived:

Let us now introduce variables: 1

the following

a

(11)

The final form of the adhesive present assumption is then 1

tot = 87c&~U( 1 + Y + 2X) _p

sin*?

B.

-3

be noticed

q(Nq -

y

+

12 ( x2+xy+x (9)

that the angle

BO cannot

energy

Y x*+.X+y+xy

?+xy+x+y

_A

Y2

l12

NA2

2

L

2(1+

y+2x)2

1

1

xln

il l-

Y2 (l+y+2x)2

1

y + XZ

Y2

l+y+2x-2(1+y+2x)*

-Qq

z-----+

471~ D -2

A

(Q')* -___ N

1

I

y + xz

STCE~D x In 2ab

the

2Q)

be

set to zero in this case (in contrast to the case of Coulomb interaction) because of the singularity due to the short-range nature of the Londonvan der Waals interaction. Therefore the sum of Eqs. (1) (2) (7) and (9) give the total interaction energy per coating particle, namely the adhesive energy, as

within

x2 + x + xy

+ 2 In

1 It should

A 1.2

b*

dimensionless

1’ FJ=1

y=b

x=2a

u

+

1

2ab

i * D2 - (a + b)’ + D2 - (a - b)2

sin2 1 + y + 2x y + xz Sin21+y+2x-(1+y+2X)*

(12)

Y2 i

H. Honda et al./Colloids Surfaces A: Physicochem. Eng. Aspects 82 (1994)

When x <<1, the main contribution comes from the first term in the second line. The first line in Eq. (12) is a contribution from the Coulomb interaction; it can have a repulsive effect only when Q’ = Nq > 2Q, whereas a contribution from the London-van der Waals interaction produces an attraction. The Coulomb interaction between particles with different electrical charges was not always advantageous for forming the monolayer particle-coated powder. However, there is no such condition where Q’ > 2Q, as is clearly seen from Table 1. This means that the Coulomb plus London-van der Waals interaction gives an attraction. A numerical evaluation of the cohesive energy is performed using Eq. (12). The electrostatic charges of a core and a coating particle, Q and q, are listed in Table 1 and are estimated from the specific charge density, the particle size and the particle density. The Hamaker-van der Waals constants are taken from Lyklema [27] and Fowkes and A,,, = [28], and are A,,, = 1.25 x lo-‘eV 0.374 eV for polyethylene and silica respectively. The value of A1,2 has not been measured, so it is approximated by the geometrical mean value of that ?r polyethylene a;d silica, namely A,,, = A,,,A2,2 = 6.83 x lo- eV. The structure of the coating particles on the surface of a core particle has a two-dimensional hexagonal lattice, as is clearly seen from Figs. 4 and 6. This shows that a good approximation for the number of coating particles per unit area is given by n = l/2&?. The variables x and z are not yet estimated, so the cohesive energy is calculated as a function on x on the assumption that I’ equals 1, i.e. z = 1. These results are shown in Fig. 10 for the available experimental data; they are labeled as curves d, e, a, f, b and c, for the ratio of particle sizes y = 0.025, 0.050, 0.057, 0.076, 0.12 and 0.17 respectively. It is clearly seen that when x is fixed at some point, the adhesive energy for the monolayer particle coated state monotonically increases with an increase in y. The slope of this energy curve with

117-128

127

1

C

t -100

’

’

I

I

I111111

I

0.00001

0.0001 x.

I

I11111 a. 001

-

Fig. 10. Calculation results of the total energy of the monolayer particle coated system: curve a, silica (0.3 urn) modified polyethylene (5.2 urn). y = 0.058; curve b, silica (0.6 urn) modified polyethylene (5.2 urn), y = 0.12; curve c, silica (0.9 urn) modified polyethylene (5.2 urn), y = 0.17; curve d, silica (0.3 urn) modified polyethylene (11.9 urn), y=O.O25; curve e, silica (0.6 urn) modified polyethylene (11.9 urn), y = 0.050, curve f, silica (0.9 pm) modified polyethylene (11.9 urn), y = 0.076.

respect to x, which indicates the adhesive force at point x, also becomes greater with an increase in y. The value of the adhesion energy and the depth of a hollow on the core surface due to the adhesion of a coating particle was increased with the increase in the ratio of the coating particle size to core particle size. The potential energies with variable arrangement of the coating particles on the core particle surface by the action of impulsive forces should be calculated to show the mechanism of formation of the monolayer particle coated structure. The potential energy equals one half of the total of the energy due to the Coulomb and van der Waals interactions between all particles. However, it is impossible from SEM photographs to define the separation lengths 1 and 1’ which were obtained to be more than 4 A by Krupp [29]. Thus, the absolute value of the cohesive energy is not calculated. The quantitative argument will be left as the subject of a

H. Honda et al.lColloids Surfaces A: Physicochem. Eng. Aspects 82 ( 1994) 117-l 28

128

future describe

study,

with

a more

the composite

realistic

treatment

to

is capable

of

J.A. Hersey, Powder Technol., 11 (1975) 41. H. Egermann, Powder Technol., 26 (1980) 235. H. Egermann, Powder Technol., 30 (1981) 289. H. Egermann, Powder Technol., 35 (1983) 135. H. Egermann, Powder Technol., 36 (1983) 117. H. Egermann, Powder Technol., 42 (1985) 285. W.J. Thiel, F. Lai and J.A. Hersey, Powder Technol.,

system.

Conclusions The dry impact forming

monolayer

combination solvent-free

blending particle

method

coated particles

of two different system.

observations

that

by the

size particles

It is concluded the coating

and

from core

in a SEM

particle

sizes themselves, the ratio of the core and the coating particles, and the action of an impulsive force during the preparation are important factors in the preparation of the composite particles. The necessity for two of the factors, i.e. the size and the ratio, is qualitatively demonstrated by the results of simulations in which the total adhesive energy of the monolayer particle coated system is calculated on the basis of the London-van der Waals and Coulomb interactions between the coating particles and between the coating and core particles. Finally, we indicated the technical value of this method. It is regarded as a new process for producing microcapsules and composite materials. The procedure has the advantage of being able to be carried out in a solvent-free system. Furthermore, the complex of organics and inorganics can be prepared by the two-step procedure involving dry blending and dry impact blending. It is possible to create various surface properties by the combination of particles

with different

sizes.

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