JPH02208664A - Developer and production thereof - Google Patents

Abstract

PURPOSE:To prevent the degradation in image density by charge-up even under low humidity by making a triboelectrostatic charge series to toner particles>fine powder particles>carrier particles or toner particles<fine powder particles< carrier particles. CONSTITUTION:The fine powder particles of a pigment, etc., having <=0.2 grain size ratio with respect to the carrier coated with a resin are passed through an impact section adjusted to 0.5 to 7mm shortest spacing between a rotating piece and a stationary piece or between two kinds of the rotating pieces under the conditions of 10 to 100 deg.C atmosphere temp. The carrier particles stuck on the carrier surface in the state of holding the fine powder thereon are obtd. and the carrier particles and the toner particles are mixed so that the triboelectrostatic charge series thereof is made to the toner particles>the fine powder particles>the carrier particles or the toner particles<the fine powder particles<the carrier particles. The charge-up hot only at and under ordinary temp. and ordinary humidity but under the low humidity as well is prevented in this way.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to a developer mainly used in electrophotography,
In particular, it relates to a developer containing an improved carrier and a method for producing the same.

[Problems to be solved by conventional techniques and inventions] Conventionally,
As disclosed in JP-A-47-17434, JP-A-47-13954, and JP-A-56-13946, carriers in which iron powder, glass powder, etc. are coated with resin are used to adhere toner powder to the carrier surface. Its main purpose was to prevent spent from impairing its durability, and to improve the charging properties of the manner powder.

However, resin-coated carriers have high electrical resistance, and are prone to carrier charge-up (a sudden increase in the amount of charge), especially under low humidity conditions, and strongly adsorb toner, resulting in poor development characteristics and a decrease in image density. There were some drawbacks, such as being visible.

To prevent this, it has been proposed to add conductive powder internally or externally to the toner itself, but in the case of internal addition, unless a large amount of conductive powder is added, there is no charge-up prevention effect. In this case, the electrical resistance of the toner itself is also reduced, which is the same as in the case of external addition, so that the transfer characteristics are deteriorated and favorable results cannot be obtained, especially under high humidity conditions.

In addition, the following proposals have been made in an attempt to improve the results by mixing fine powders such as pigments into the resin of the resin-coated carrier.

JP-A-53-100242 discloses that by incorporating nigrosine into a carrier coating resin, the negative chargeability of the toner is improved and peeling of the resin film and electrostatic aggregation are prevented.

JP-A-56-75659 discloses a method of increasing the electrical conductivity of the resin-coated carrier and the durability of the carrier by incorporating porous carbon black into the carrier-coating resin.

JP-A No. 60-57362 discloses a technique for preventing the edge phenomenon by incorporating a conductive magnetic fine powder treated with a coupling agent into a carrier coating resin.

Furthermore, JP-A-60-73631 discloses a silicone resin-coated carrier containing inorganic filler particles and a silane coupling agent in the resin to improve the strength of the resin coating.

However, dispersing fine powders such as pigments in resin has limitations due to compatibility and dispersibility between the resin and fine powders, and all resins and all fine powders cannot be well dispersed. Therefore, when a fine powder is combined with a resin that has poor compatibility and dispersibility with the powder, poor dispersion is likely to occur, and as a result, the chargeability imparted to the toner becomes uneven, resulting in fogging, etc. Image defects and spent images are more likely to occur.

This tends to occur when the amount of resin coating is small, specifically when the amount of resin is 0.1 to 5% by weight relative to the carrier, and in severe cases, the carrier powder and fine powder coated only with resin are dispersed. There are even cases where carrier powder coated with a resin is mixed with the powder.

The present invention provides a developer that solves the above-mentioned drawbacks and a method for producing the same.

In other words, the purpose of the present invention is to reduce carrier charge-up under low humidity, which is considered to be the cause of not being able to obtain good images.
A developer containing a new resin-coated carrier that solves problems such as in-machine contamination due to toner scattering or scratches caused by carrier adhesion on the photoreceptor, etc., due to charge-up even in low humidity environments, and its developer. A manufacturing method is provided.

[Means for Solving the Problems] The present invention provides carrier particles having fine powder particles such as pigments held on the surface of a carrier coated with a resin, and a developer containing at least toner particles. is a developer characterized in that its triboelectrification series is toner particles>fine powder particles>carrier particles or toner particles<fine powder particles<carrier particles, and its manufacturing method includes resin-coated developer. Fine powder particles such as pigments having a particle size ratio of 0.2 or less to the carrier are placed at an ambient temperature of 10 to 100°C, and the shortest gap between the rotating piece and the fixed piece or between the two types of rotating pieces is The carrier particles are passed through an impact part having a diameter of 0.5 to 7 mm to obtain the carrier particles with the fine powder held on the surface of the carrier, and the carrier particles are mixed with at least toner particles, and their friction This is a method for producing a developer, characterized in that the charging series is toner particles>fine powder particles>carrier particles or toner particles>fine powder particles>carrier particles.

The developer configured according to the present invention does not cause a decrease in image density due to charge-up not only under low temperature and low humidity conditions but also under normal temperature and normal humidity conditions.

The mechanism behind this effect is not clear, but it is probably due to the strong electrostatic attraction between the toner and carrier during charge-up being relaxed by the presence of fine powder particles located between the toner and carrier in the charging series. It is considered that

Incidentally, JP-A-61-966 proposes a microcarrier in which a fluidity improver is added to a magnetic powder-dispersed carrier. A fluidity improver is added to the carrier and stirred, and the carrier particles in the present invention are completely new, unlike the carrier particles in which fine particles are attached as in the present application.

In the present invention, applying fine powder particles (hereinafter abbreviated as particles A) to the surface of a resin-coated carrier means applying impact energy (mechanical and Thermal energy) is applied to mechanically bond a part of the resin on the surface of the carrier, which has been melted or softened by the impact energy, to the particles, and the particles are cooled and fixed in this state, and the shape of the particles A remains unchanged. It does not need to be retained on the carrier, nor does it need to be completely buried.

Further, the shape of the carrier may be spherical or other irregular shapes, but preferably the particles A are uniformly attached to the surface of the carrier, and the particle size of the particles A is equal to the particle size 1 of the carrier. 0.2 or less is preferable. Note that the above structure can be seen using an electron microscope.

Each particle constituting the present invention will be explained below.

In the developer of the present invention, carrier core materials that can be used include magnetic materials such as iron powder, oxidized iron powder, ferrite, and nickel, as well as glass beads, silica particles, sodium chloride, ammonium chloride, and Rochelle salt. etc. can be used. The particle size of the core material is suitably about 20 to 500 μm, preferably about 30 to 300 μm.

Covering resins include silicone resin, fluorine resin,
Styrene resin, acrylic resin, styrene-acrylic resin, polyvinyl acetate, cellulose derivative, maleic acid resin, epoxy resin, polyvinyl chloride, polyvinylidene chloride, polyvinylidene bromide, polycarbonate, polyester, polyethylene, polypropylene, phenol Resin, PVA, fumaric acid ester resin, polyacinolonitrile, polyvinyl ether, chloroprene rubber,
Acetal resin, ketone resin, xylene resin, butadiene rubber, styrene-butadiene copolymer, etc. can be used. The resin coating thickness for the carrier core material is approximately 0.1 to 3
The amount of resin in the carrier is preferably about 0.1 to 1 μm.
It is used in an amount of about 20% by weight, and especially in the present invention, it is used in an amount of about 0.1 to 0.1% by weight.
This effect is exhibited when the coating is as thin as 5% by weight.

The fine powder particles (particles A) used include metal oxides such as hydrophilic silica, hydrophobic silica, cerium oxide, alumina, zirconia, silicon carbide, boron carbide, clay, talc, and magnesium oxide. In addition, charge control agents such as nigrosine, quaternary ammonium salts, triphenylmethane dyes, and metal monoazo dyes and pigments may also be used. Among these, high-resistance fine powder is particularly preferable because it reduces fluctuations in development characteristics due to changes in environmental humidity. The term "high resistance" as used herein means that the powder is compressed into a tablet and the volume resistance is 10'Ωcm or more when 10V is applied.

The content of the particles A is based on 100 parts by weight of the coating resin,
1 to 80 parts by weight, particularly 2 to 50 parts by weight are preferred.

Further, the fine particles preferably have a particle size of 0.2 or less when the particle size of the resin-coated carrier is 1 in order to obtain a uniform coating of the fine powder, and furthermore, the particle size distribution of the resin-coated carrier is also sharp. In order to obtain a uniform coating of particles A, it is preferable that the resin-coated carrier particles contain 40% or more, more preferably 50% or more of the particle size distribution in R±logm when the particle size of the resin-coated carrier is 8 μm. It is preferable that the

The particle size and particle size distribution referred to here are measured by Luzex (trade name) measured by optical reading, or by dispersing the powder in a liquid medium and determining the particle size from the electrical signal when it passes through the orifice of an aperture. The volume average particle diameter and volume particle size distribution were measured using a Coulter Counter (manufactured by Coulter Inc.), an Elzone Particle Counter (manufactured by Particle Data Inc., USA), and the like.

Note that the resin-coated carrier preferably has a spherical shape in order to uniformly cover the particles A.

In the present invention, the charging series of particles A is located between resin-coated carrier particles (hereinafter referred to as carrier particles B) to which particles A are attached and toner particles (hereinafter referred to as toner particles C). However, the triboelectricity of each particle can be set in accordance with the present invention by selecting particles with a triboelectric charge amount determined by the triboelectricity measurement method described later. result,
It becomes possible to achieve the charging series in the present invention,
This makes it possible to prevent a decrease in image density due to charge-up under low humidity, which is the object of the present invention. If the charging series is not in the above order, the charge-up suppressing effect under low humidity will not be sufficient.

The charging series is determined using the apparatus shown in FIG. 5 by the method described below.

That is, in the apparatus shown in FIG.
Toner particles C and carrier particles B whose triboelectric charge amount is to be measured are placed in a metal measuring container 62 equipped with a wire screen 63 with a mesh size (usually 400 meshes) that allows toner particles C1 and particles A to pass through. Alternatively, about 4 g of a well-mixed mixture of particles A and carrier particles B at a weight ratio of 1:9 (carrier particles B is 9) is added, and metallic phthalo 4 is applied. The weight of the entire measurement container 62 at this time is W
t(g) Next, the suction device 61 (the part in contact with the measurement container 62 is at least an insulator) draws suction from the suction port 67, adjusts the air volume adjustment valve 66, and adjusts the pressure of the vacuum gauge 65 to 700 mmHg. Make it. In this state, suction is performed sufficiently (for about 1 minute or until there is almost no change in the potential of the electrometer 69) to remove the toner particles C or the fine powder particles A. The potential of the electrometer 69 when removed is V (volt), where 68 is a capacitor, and the capacitance C(
μF), and the weight of the entire measurement container after suction is W
2 (g).

Through the above operations, the triboelectric charge amount T (μC/g) of the toner particles C1 or particles A is calculated as shown in the following formula.

However, the measurement conditions are 23° C. and 60% RH.

In the present invention, the fact that the charging series of particle A is located between carrier particle B and toner particle C means that the triboelectric charge amount T (Particle) of particle A' with respect to carrier particle B is
and the polarity of the triboelectric charge amount T (Toner) of the toner particle C with respect to the carrier particle B is the same, and its absolute value is l T (Toner) l > l T (Par
This means that there is a relationship of 1 (ticle) l, and as a result, carrier particle B is the reference, that is, O, so the triboelectric charging series is toner particle C x particle A x carrier particle B.
Or, toner particles C>particles A>carrier particles B.

Note that known toner particles C can be used as the toner particles C mixed with the carrier particles B according to the present invention, and preferably,
For styrene resin, polyester resin, epoxy resin,
A toner in which internal additives such as a colorant and a charge control agent are dispersed, a capsule toner, and a polymerized toner in which toner particles are suspended polymerized particles can be used.

Further, other external additives, preferably silica, alumina, Ti
Fluidizers, abrasives such as E, etc. can be added.

The carrier particles B according to the developer of the present invention may be produced by any production method as long as it can form a structure in which the particles C are attached to the resin-coated carrier as described above, but the following preferred method is used. The manufacturing method is described below.

This adhesion method consists of two steps: a pretreatment step in which particles A, such as pigments, are dispersed and uniformly adhered to the resin-coated carrier, and a step in which the adhered particles A are immobilized by impact force.

In the pretreatment, particles A were dispersed and rubbed against the resin-coated carrier to adhere to the resin-coated carrier by electrostatic force (and van der Waals force).

Generally, a mixer with high-speed stirring blades is used, but any mixer with mixing and dispersing functions may be used.

Figure 1 shows an example of a mixer with high-speed stirring blades, and the pretreatment requires good dispersion of both particles A and the resin-coated carrier, and that the respective particles are not substantially crushed. .

Therefore, although it is determined by the physical properties of this material, for toner materials, the processing temperature is preferably 0 to 50°C, the rotation speed around the blade is 5 to 50 m/sec, and the processing time is 1 minute to 60 minutes. Also, when performing such a treatment, since the temperature increases due to stirring, it is preferable to cool the inside of the tank by cooling the jacket or by introducing cooling air.

This pretreatment equipment does not need to be a mixer with high-speed stirring blades, as long as it has dispersion and mixing functions and has a sufficiently long residence time, and a crusher or vibratory mill that satisfies the above conditions is sufficient. In addition to 0 or more particles, which can be used with a reduced impact force such as
may be dispersed, filtered and dried, and then fixed.

In such pretreatment, fluidity and dispersibility of the particles A are important for uniformly adhering the particles A to the resin-coated carrier. That is, if the particles A exhibit strong aggregation, they cannot be separated into individual particles in the pretreatment step, and uniform adhesion tends to be difficult. Similarly, if the fluidity is extremely poor, it is similarly difficult to form individual particles and uniform adhesion is similarly difficult. In the case of particles A other than silica that have poor fluidity and dispersibility, use a method in which fine silica powder is added and mixed to the particles A in advance to improve fluidity and dispersibility and to make them adhere uniformly to the resin-coated carrier. is preferable, and the amount of silica fine powder added is 0.0% based on the weight of particles A.
It is used in an amount of 1 to 10% by weight, preferably 0.1 to 5% by weight. As such fine silica powder, hydrophobic fine silica powder treated with a positively or negatively charged silane coupling agent, a hydrophobic treatment agent, silicone oil, etc. is preferable.

The silica fine powder preferably has a specific surface area of 40 to 400 m2/g as measured by a nitrogen gas adsorption method, and is treated with a degree of hydrophobicity of 30 to 80% as measured by a methanol titration test. Particularly preferred is silica fine powder.

The "methanol titration test" specified herein for evaluating the degree of hydrophobicity of the treated silica fine powder is carried out as follows.
Add to 50 ml of water in a ml Erlenmeyer flask. Methanol is titrated from the burette until all of the silica is wetted. At this time, the solution in the flask is constantly stirred with a magnetic stirrer. The end point is observed when the entire amount of fine silica powder is suspended in the liquid, and the degree of hydrophobization is expressed as the percentage of methanol in the liquid mixture of methanol and water when the end point is reached.

Next is the method of immobilization, but it is undesirable for particles A such as crushed pieces of the resin-coated carrier or pigment to be released, or for the particles A that have been attached to be released again. preferable.

For this purpose, it is preferable to control the impact force within a range that does not crush the resin-coated carrier and the temperature within a range that does not cause fusion and agglomeration. A bin mill with a large number of rotating bins (see Figure 4-1) and a crusher (secondary mill) with a recycling mechanism that generates an impact between rotating blades or hammers (rotating pieces) and liners (fixed pieces) −
(See Figure 1 and Figure 3-1) is valid.

The circumferential speed of the tip of the rotating piece in this device is 30 to 130 m
/sec, the temperature is preferably between the resin-coated carrier and the particles A
Although it varies depending on the physical properties of
~90℃ is good, and the residence time in the shock part is 0.02 sec.
~12 seconds is preferable; in the case of a bottle mill, it is necessary to increase the concentration of the powder. In the type of equipment shown in Figure 2-1 or Figure 3-1, the powder to be treated is collected near the liner by centrifugal force, so the latitude of the powder concentration is wide, and between the bottle mills or between the blade or hammer and the liner. The shortest gap between them is preferably about 0.5 to 7 mm, more preferably! Good results can be obtained when adjusting to ++un to 5 cities.

To explain in more detail with reference to FIG. 2-1, the resin-coated carrier and particles A are introduced from the inlet 24, pass through the inlet chamber 20, and pass along the rotating dispersion blade 14 between the rotating blade 15 and the liner 18. Impact part 19 (No. 2-2
(shown in the partially enlarged view of the figure), passes through the outlet chamber 21, passes through the return passage 22 and the blower 25, and circulates through the same circuit again. After the immobilization process is completed, the product is taken out from the product takeout port 23.

Here, the powder consisting of the resin-coated carrier and particles A receives an impact between the blade 15 and the liner 18 in the impact section 19 due to mechanical energy and thermal energy in which a part of the mechanical energy is converted into heat. A part of the resin-coated carrier and particles A are melted or softened to undergo a fixation treatment. Here, if necessary, it is preferable to flow cooling water through the jacket 26 to adjust the ambient temperature. In FIG. 2-2, the gap a between the blade 15 and the liner 18 is the shortest gap, and The space corresponding to is the impact part.

FIG. 3-3 shows the positional relationship between the liner 29 of the immobilization device and the rotating rotor 31, and the shortest gap between the liner 29 and the rotor 31 is defined as the tip of the protrusion toward the inner periphery of the liner 29. The same applies to the case where a blade or a hammer is used instead of the rotor 31, which refers to the difference in the radius of two circles, the circumference 51 obtained by connecting the two circles, and the locus 52 of the protrusion of the rotor 31.

FIG. 4-2 is a schematic view of a bottle in a bottle mill type fixing device as seen from the front of the device, and the gap 55 between the fixed bottle 39 and the rotating bottle 54 is the shortest gap. In addition, 57
indicates the maximum gap, and 56 indicates the trajectory of the rotating bin 54.

[Example] Example 1 Spherical ferrite particles with a particle size of 50 μm were coated with PVDF (polyvinylidene fluoride) and polyethylene tetrafluoride,
The particle size distribution obtained by obtaining a spherical resin-coated carrier with a particle size of 51 μm (resin content 3% by weight) is a volume particle size distribution of 51±
ioμm contained 52% resin-coated carrier.

To 100 parts by weight of this coated carrier, aluminum oxide particles A (electrical resistance 6 x 106 Ωcm) with a particle size of 0.5 μm were added.
5 parts by weight were added, and the mixture was processed at 35 m/sac for 7 minutes using a mixer as shown in FIG. 1 to uniformly mix the particles A and the resin-coated carrier.

Next, using the apparatus shown in Figure 2-1, the minimum gap was 1 mm, the speed was 70 m/sec, the ambient temperature was 28°C,
Processed for 9 minutes. When observed with an electron microscope, it was observed that the particles A were uniformly and firmly held on the resin-coated carrier.

To 100 parts by weight of the carrier particles B thus obtained, 3 parts by weight of styrene resin toner particles 0 containing nigrosine and carbon black and 0.5 parts by weight of silica were uniformly mixed to obtain a developer.

When the developer was used to produce an image using a modified machine called NP-2015 (manufactured by Canon), it was found that the image was produced at low temperature and low humidity (15℃, 2
0%), a good image was obtained without a decrease in density due to charge-up. Also, normal temperature and humidity (23℃60%)
Good images were also obtained.

Further, when the amount of triboelectric charge was measured using the measuring device shown in FIG. 5, it was found that carrier particles B of aluminum oxide particles A
The amount of triboelectric charge T (Particle) is +7
μc/g, and the amount of triboelectric charge of toner particles C with respect to carrier particles B was +21 μc/g. In other words, the triboelectric charging series is: toner particle C>particle A>carrier particle B
Met.

Example 2 Spherical ferrite particles with a particle size of 45 μm were coated with silicone resin to obtain a spherical resin-coated carrier with a particle size of 46 μm (resin content: 3% by weight). The particle size distribution was a volume particle size distribution of 46±10 μm, which contained 56% of the resin-coated carrier. 100 parts by weight of this coated carrier was added with a particle size of 0.
Add 3 parts by weight of 5 μm cerium oxide particles A, and mix at 75 m/sec, 8 using a mixer as shown in Figure 1.
Processed for minutes.

Next, using the apparatus shown in FIG. 2-1, the process was performed for 8 minutes at 75 m/sec with a minimum gap of 1 mm, and the temperature inside the machine (ambient temperature) was 70°C.

When observed under an electron microscope, it was observed that the particles were uniformly and firmly held on the resin-coated carrier.

A charge control agent and carbon black were internally added to 8,100 parts by weight of the carrier particles thus obtained.

3 parts by weight of polyester resin toner particles and 0.3 parts by weight of silica.
4 parts by weight were uniformly mixed to obtain a developer.

When the developer was used to produce an image using a modified machine called NP-7650 (manufactured by Canon), it was found that the image was produced at low temperature and low humidity (15°C).
20%), good images were obtained without a decrease in density due to charge-up.

Further, when the amount of triboelectric charge was measured using the measuring device shown in FIG. 5, the amount of triboelectricity T (Particle) of cerium oxide particles A with respect to carrier particles B was -6 μc/
g, and the amount of triboelectric charge of toner particles C with respect to carrier particles C was -24 μc/g. That is, the triboelectrification series was toner particles C, particles A, and carrier particles B.

[Effects of the Invention] As explained above, the surface of the carrier coated with resin is such that the triboelectrification series is toner particles C>particles A>carrier particles B or toner particles C>particles A>carrier particles B. By adhering particles A to carrier particles B and then mixing them with toner particles C to form a developer, charge-up can be prevented not only at room temperature and humidity but also at low humidity. When the developer is used in photography, etc., high-quality images with stable image density can be obtained regardless of environmental conditions.

Further, the carrier particles B of the developer can be easily produced under predetermined conditions using impact force from a rotating piece or the like.

[Brief explanation of the drawing]

FIG. 1 is a diagram schematically showing an example of a stirring device for mixing resin-coated carrier and particles A, and FIG.
The figure is a diagram schematically showing an example of an apparatus for immobilizing particles A on the surface of a resin-coated carrier, and FIG. 2-2 is a partially enlarged view of the apparatus in FIG. Figure-1 is a diagram schematically showing another example of an apparatus for immobilizing particles A on a resin-coated carrier, and Figures 3-2 and 3-3.
The figure is a partial view of the apparatus shown in Fig. 3-1, and Fig. 4-1 is a diagram schematically showing an example of a bottle mill type apparatus for immobilizing particles A on a resin-coated carrier. FIG. 4-2 shows a partial view of the device shown in FIG. 4-1, and FIG. 5 is a diagram schematically showing an example of the device for measuring the amount of triboelectric charge. 1... Jacket 21i1 Stirring blade 3...
Motor 4...Lid 5...Base 6...Control board 7...Cylinder
8... Lid lock 9... Cylinder 1O... Direction control unit 11... Discharge port 12... Rotating shaft 13
... Rotor 14 ... Dispersion blade 15.
...Rotating piece (blade) 16...Partition disk 17.
...Casing 18...Liner 19...
- Impact part 20... Inlet chamber 21... Outlet chamber 22... Return path 23... Product take-out valve 24... Raw material input valve 25... Blower 26... Jacket 27... Axis of rotation
28...Casing 29...Liner
30...Blower blade 31...Rotor (with blade) 32...Outlet 33...Raw material input port 3
4...Return path 35...Product outlet
36... Entrance 37... Jacket 38.
...Casing 39...Fixed bin 41...Raw material input port 43...Return path 45...Outlet 47...Rotating shaft 51...Day 54...Rotating pin 57...Maximum gap 62...Measuring container 63...Gold steel screen 65...Vacuum gauge 67...Suction port 69...Electrometer 40...Inlet 42...Circulation blower 44...Product extraction port 46...Rotor 48...Jacket 52.56...Trajectory 55...Shortest gap 61...Absorber 64...Metal lid 66...Air volume adjustment valve 68...Condenser

Claims (1)

[Claims] 1. A developer containing at least toner particles and carrier particles formed by holding fine powder particles on the surface of a carrier coated with a resin, the triboelectric charging system of which is a toner. A developer characterized in that particles>fine powder particles>carrier particles or toner particles<fine powder particles<carrier particles. 2. A resin-coated carrier and fine powder particles having a particle size ratio of 0.2 or less to the carrier are mixed into a rotating piece, a fixed piece, or two at an ambient temperature of 10 to 100°C. The seeds are passed through an impact section with a minimum gap of 0.5 to 7 mm to obtain carrier particles with the fine powder held on the surface of the carrier, and the carrier particles and at least toner are A method for producing a developer, characterized in that particles are mixed and their triboelectrification series is set as toner particles>fine powder particles>carrier particles or toner particles<fine powder particles<carrier particles.