[0001] This invention relates to improved latices and more particularly to improved synthetic
latices which show amphoteric properties. There have previously been proposed various
synthetic latices which are produced by polymerisation of monomers such as styrene,
butadiene, methyl methacrylate, ethyl acrylate, vinyl acetate, vinyl chloride, vinylidene
chloride and certain mixtures of these mononers.
[0002] Convertionally, latices have been formed in one of two ways. These arc that the stabilisation
is due to electrostatic repulsion between surfaces with adsorbent surfactants and/or
due to steric replusion (steric stabilisation) due to the presence of polymer chains
with or without fixation or anchoring moietes at the latex surfaces.
[0003] Sterically stabilised latices may also have functional groups as part of or attached
to the particles which groups are normally of the same charge and thus there is also
a charge stabilisation acting to prevent coagulation. Electrostatically and/or sterically
stabilised latices normally have an overall negative charge although positively charged
latices are known. Although latices are widely used in industry, they can be unsatisfactory
in that the surfactant is normally charged and can often be preferentially absorbed
onto a surface or can compete for charged particles on a surface with the latex particles.
[0004] The other major form of latices are the so-called surfactant free latices which may
in fact contain a quantity of surfactant stabilised latex and in this case the main
mechanism for stabilisation lies in the repulsion because of the similar charges on
the various particles. These latices have conventionally had an overall negative charge
although positively charged latices are known. To the best of our knowledge,.there
has been no disclosure in the literature of amphoteric latices and certainly we know
of no disclosure of surfactant free amphoteric latices.
[0005] It is a principal object of the present invention to provide latices which minimise
or overcome the disadvantages of previously known latices.
[0006] The invention includes an amphoteric latex characterised in being formed by co-polymerising
monomeric polymerisable compounds including a base monomer and two further monomers
one including an amine group (R
3NH
+) and the other a carboxyl group (RCOOH) by the use of a polymerisation catalyst,
the latex having ionisable amine and carboxyl groups on the surfaces thereof.
[0007] Preferably the polymerisable monomers including an amine group is selected from:-
POLYMERISABLE AMINO COMPOUNDS
[0008]

[0009] The monomer containing the carboxyl group may be selected from:-
CARBOXYLIC ACID MONOMERS
[0010] Methacrylic Acid Acrylic Acid Itaconic Acid Fumaric Acid Glutaric Acid Crotonic Acid
Citraconic Acid
[0011] In one aspect of the invention the latices may be formed using a styrene monomer
as the base monomer but, alternatively, they may be formed using other monomers such
as methyl methacrylate, ethyl acrylate, vinyl acetate, vinyl chloride, vinylidene
chloride and certain mixtures of these and other monomers, for example a mixture of
styrene and butadiene monomers.
[0012] In much of our experimental work the amine used was N,N-Diethylaminoethyl methacrylate
and the carboxyl was methacrylic acid. The polymerisation catalyst can satisfactorily
be potassium persulphate. Alternatively, other initiators, including y-radiation can
equally well be used. We have found that the optimum pH for minimum coagulation is
pH 1.2.
[0013] We have found that the particle size of the latex can be controlled by controlling
the concentration of monomers in the solution.
[0014] Latices which have the same properties, particularly iso-electric point and surface
charge but which are of different sizes can be made. It is necessary to increase the
concentration of the monomers in the solution with a further increase in the proportion
of monomers which have functional groups to compensate for the increase in surface
area to maintain a required charge density.
[0015] Latices made in accordance with the invention can be of one of two forms, either
hard latices or soft latices. Hard latices are used for a number of applications,
particularly in coating fine papers and in certain medical applications where they
can be carriers for radioactive isotopes and, because of the differences in cell structures,
so the isotopes can selectively be delivered to such areas. Once delivered a scan
or the like can be made and the distribution of the isotopes determined.
[0016] Soft latices are used in paints and when the paint is drying the latices tend to
form a hard transparent film incorporating pigment and filler, thereby providing the
outer surface of the paint. In many applications the latices are to be placed on a
surface which is negatively charged and it has often been necessary to use an intermediate
so that the latex is not repelled by the material on which it has been located.
[0017] Using latices made in accordance with the invention adjustment of the pH can cause
variations in the surface charge and the latex may selectively exhibit positive or
negative characteristics depending on which side of the iso-electric point it is located
at the particular pH value. The actual iso-electric point can be varied by variation
of the proportion of carboxyl to amine and thus where one is constrained to operate
at a certain pH the surface charge of the latices can still, within limits, be controlled.
In the formation of the latex conditions are selected so that one of the functional
groups, the amine or the carboxyl acid is deactivated so the growing particles all
exhibit the same charge and, as such tend to repel so that there is little coagulation
during growth. At the end of polymerisation, by altering the conditions, the deactivated
functional group can be reactivated and the required properties of the latex are revealed.
[0018] Wetshall describe one method of making the latices of the invention, together with
the various properties of the latices in relation to the accompanying drawings in
which:-
Fig. lshows the electrophoretic mobility of latices as a function of pII at 10-2ionic strength. The latices were prepared with various molar ratios of acid to amine
(R).
Fig. 2 shows the electrophoretic mobility of latex B as a function of pH and conditioning
at pH3

,pH6.3 (O) and pH10.3 (O). Ionic strength is fixed by 5 x 10-2M NaCl.
Fig. 3 shows the zeta potential of latex B.as a function of pH at various constant
ionic strengths.
Fig. 4 illustrates the conductometric and potentiometric titration of amphoteric latex
B (R = 1.09) with excess added HC1 by base KOH.
Fig. 5 illustrates the variation of surface charge on the amphoteric polystyrene latex
as function of pH. The pHpzc is assumed to be the pHiep.
[0019] In making the latex we treated the components as follows:-
The styrene monomer used was purified by vacuum distillation at a temperature of about
35°C and under reduced pressure.
[0020] The amine used was N,N-Diethylaminoethyl methacrylate (DEAM molec.wt. = 185) and
was vacuum distilled at 80°C at 10 mm Hg. The carboxyl was methacrylic acid (MA molec.wt.
= 86) and this was shaken with sodium chloride, the aqueous layer separated and the
acid dehydrated over calcium chloride. It was then vacuum distilled (73°C at 20 mm)
under nitrogen. All purified monomers were stored at 0°C.
[0021] Potassium persulphate (KPS) was used as the polymerisation catalyst and was of an
analytical grade, it was used without further purification.
[0022] Water used was triply distilled from an all-Pyrex apparatus.
[0023] Preparation of amphoteric latices
[0024] The basic recipe used consisted of :-

[0025] Prior to mixing the reactants, the pH of a mixture of DEAM, MA and water was adjusted
to the desired value selected, as will be described hereinafter, as 1.2 with concentrated
HCl.
[0026] The materials were poured into a 273 cc capacity container purged with nitrogen (-
10 min) sealed and tumbled end-over-end (- 50 rpm) in a water bath at 70°C for a specified
time. At the end of the reaction time the latex was decanted through a filter packed
with glass wool in order to remove any coagulum formed.
[0027] Alternatively, any other form of mixing vessel in which an inert atmosphere can be
maintained can be used.
Purification
[0028] After the polymerisation, the latex suspension contained (in addition to copolymerised
amine and carboxylate groups) some potassium sulphate, sulphuric acid, hydrochloric
acid, unreacted monomer, and possibly some soluble copolymers. The latex was dialysed
against distilled water, using well- boiled Visking dialysis tubing, until the specific
conductivity of the diaylsate was lower then 2 x 10
-6mho can
-3; thus usually required about 10 changes of water over a period of two weeks. The
ratio of dialysate to latex used was -50 to 1.
[0029] In order to ascertain both quantitatively and qualitatively that the dialysis efficiently
removed all soluble components, the latex particles were sedimented by centrifugaticn
at 2.5 x 10
4g for one hour. The supernatant was then discarded and the latex cake was redispersed
with slightly acidified distilled water (pH3). This procedure was repeated 10 times.
[0030] The latices purified by these methods, i.e. dialysis and centrifugation, were subsequently
compared by measuring the electrophoretic mobility at various pHs. No significant
differences between the samples were noted, thus suggesting that the charge groups
are integral parts of the surface.
Particle size determination
[0031] Electron microscopy was used to determine both particle diameters and size uniformity.
Copper grids of 300 mesh were covered with a film of Formvar and a light carbon layer
was deposited by vacuum evaporation. Grids were dipped into a dilute latex suspension
and allowed to dry. The electron microscope used was a HS-9 (Hitachi Ltd.). Particle
diameters were measured on the negatives using a magnifying glass. In order to calibrate
the electron microscope a micrograph of a carbon replica (2160 lines/mm) was taken
at the same magnification as that used to examine the latex particles. In order to
compare the degree of uniformity of the latex particles it is desirable to have a
single number that may be defined as the uniformity ratio U. Where U = D
W/D
N D
W, the weight average diameter of n
i particles is

and D
N, the number average diameter, is
D
N = (Σ
in
iD
i)/(Σ
in
i)
[0032] We consider that latices having U < 1.01 are monodisperse (that is having a unitorm
particle size), but in many cases a uniformity ratio which is higher than this would
be satisfactory.
Diectrophoresis
[0033] The electrophoretic nobilities of the latices were measured by the microelectrophoretic
technique. The apparatus employed is manufactured by Rank Bros. The mobility values
were converted into zeta potentials, following the known Wiersema et al treatment.
Titration of surface groaps
Equipment
[0034] The pH was measured using a combined glass-calomel electrode and a Radiometer (Copenhagen)
pM 26 meter. The electrodes were calibrated with Merck Titrisol buffer solutions of
pH 4.00 ± 0.02 and 9.00 ± 0.02 at 20°C. The conductance was measured by a Wayne-Kerr
conductance bridge assembly. The titrations were carried out on 100 cm
3 diluted samples of purified latex in a thermostatted glass container at 20°C. The
latex was stirred with a magnetic stirrer and carbon dioxide was excluded by passing
a stream of nitrogen over the latex surface. The titration cell had a tight- fitting
cap with sockets to accept the ground glass cones on the combined electrode, the conductivity
cell, the micrometer syringe tip and nitrogen inlet.
Conductometric and potentiometric titration
[0035] A weighed amount of latex (~1 g) was diluted to 100 cm
3 with distilled water in a thermostatted glass container. Both sets of electrodes
of the pH meter and the conductance bridge were placed in the diluted latex dispersion.
Mechanical stirring of the latex was started and the pH of the latex sample was adjusted
to a pH of 2.5 ± 0.1 with dilute HCl solution. The sample was then titrated with standard
1 N/l KOH in 0.02 ml increments by means of a micrometer syringe fitted with a glass
needle.
[0036] Both conductance and pH were studied as a function of volume of standard base consumed
until a pH of about 11 was reached.
[0037] Following the procedures set out hereinbefore we obtained the following results.
(a) Stability and uniformity of latex
[0038] During the initial preparative stages of polymerisation it was found that pH was
a critical influence on the stability and particle uniformity of the latex. Consequently,
pH was varied to optimise the preparation with respect to stability and particle size.
[0039] The effect of changing pH over the range of 1 to 3.9 on the monodispersity (D
W/D
N) and stability (% coagulum). was examined at constant composition of reactants, temperature
and time. The polymerisation time was set at 24 hours to assure a conversion close
to 100%.
[0040] The data shown in Table 1, indicates a decrease in the stability and monodispersity
with pH increase in excess of 1.2.

[0041] The trend parallels an increase in the degree of ionisation of methacrylic acid with
pH that subsequently results in a decrease in the net positive charge. This apparently
effects the stability of the system and also monodispersity by permitting homocoagulation
of particles as the charge on the particles is not sufficient to keep these separate.
Lower stability, at pH 1.0, may be attributed to the increase in ionic strength of
the medium. The latex suspension B, prepared at pH 1.2, has an excellent stability,
with the axception of the pH region near the iso-electric point, and consists of highly
monodisperse particles with the average diameter of 185 nm. Samples A, C, D and E
varied in a degree of uniformity, as shown in the last column of Table l; however,
no significant deviations from the average particle size of the latex E were noted.
[0042] An attempt was also made to prepare an amphoteric -latex under conditions having
a higher pH but with a larger ratio of the methacrylic acid to the amine than used
in the basic recipe. The pH was adjusted to 6.5 with a phosphate buffer (final concentration
0.02 M) and the content of DEAM was reduced to 0.2 g. The produced material showed
good monodispersity (U = 1.005) with an average particle size of 504 nm. However,
the stability was poorer than that of the latex B. The iso-electric point was determined
using the microelectrophoretic technique to be at pH 4.6, and the electrophoretic
mobility vs pH dependence is shown in Fig. 1.
(b) Control of iso-electric point and zero-point-of- charge
[0043] In the next stage, an attempt was made to produce an amphoteric latex characterised
by different iso-electric point. Polymerisations were carried out with the various
amine to methacrylic acid combinations in the same manner as for the case of latex
B preparation.
[0044] Firstly, the acid content was kept constant, at 0.5 g, and the quantity of amine
was varied. The results are shown in Table 2.

[0045] Thus it appears that, similarly to the effect of pH, a decrease in the net positive
charge due to the decrease in the number of aminium groups yields a poor stability
and low uniformity.
[0046] In order to stabilise the system adequately, the level of amine was set at 1.0 g
and the molar ratio of acid to amine was increased approximately from zero to 2. The
results are listed ii, Table 3.

[0047] Predictably, the location of the iso-electric point shifted toward the higher pH
(Fig. 1) as the acid/amine ratio was decreased. The average particle size (185 nm)
was found to remain basically unchanged although monodispersity was affected and was
the highest for equimolar acid and amine monomers, (J).
(c) Chemical stability of surface groups
[0048] It has been reported that where pure amine is used it is found to be prone to rapid
hydrolysis at high pH values. No evidence though is available on the rate of hydrolysis
of amine polymer or amine/acid copolymer.
[0049] In an attempt to assess the effect of pH on hydrolysis of amine surface groups, the
following conditioning experiment was carried out. Dilute (0.01 % w/v) samples of
latex, in 5 x 10
-2 mol NaCl, were adjusted to a desired pH and allowed to equilibrate in a shaking water
bath (25°C) for about 72 hours before making measurements. At the end of this time,
the pH was measured again and the zeta potential was determined. The data shown in
Fig. 2 strongly suggest that amine groups, at least under the conditions of the experiment,
are resistant to hydrolysis, since the electrophoretic mobility depends only on the
pH and not on the ageing under different conditions used. Further substantiation for
this conclusion has been obtained from an examination of the effect of salt concentration
on the zeta potential of particles. The results are summarised in Fig. 3. The iso-electric
point, as indicated by the reversal of sign, occurs at pH 6.7, in good agreement with
the value estimated from the conditioning experiment. The location of the iso-electric
point is unaffected by the salt concentration thus supplying an additional proof of
a covalent nature of bonding on the surface groups to the interface.
Characterisation of Amphoteric Latex
(a) Electrokinetic behaviour of amphoteric latex
[0050] The electrophoretic mobility of latex B was measured in various constant ionic strength
solutions, i.e. 10
-3, 10
-2 and 10
-1 mol dm
-3 NaCl, as a function of pH. The electrophoretic mobility was converted to zeta potential
using interpolation of the graphical results published by Wiersma et al for particle
radius a = 185 nm and the Debye-Huckel parameter K = 0.329 x 10
10 
giving Ka as 190, 60 and 19. The results are shown in Fig. 3. The iso-electric point
is located at the mutual intersection of the three ε-pH curves and the ε = 0 axis
at pH 6.7 ± 0.1. The zeta potential-pH values are fairly symmetrical about the pH
iep, suggesting similar magnitude of surface charge on the cationic and anionic surfaces.
The data shown in Fig. 2 at 5 x 10
-2 mol dm
-3 HaCl show the same pH
iep and the magnitude of the zeta potential lies between the values at 10
-2 and 10
-1 mol dm
-3 NaCl. It is interesting to note that latex, in the pH range 6-7.5, coagulated readily
but it was subsequently redispersed fully when pH was changed to lower or higher values.
Also, a sample of latex left coagulated for several days showed the same trend. This
may well be because of a very high charge residing on the particle surface.
(b) Conducometric and potentiometric titration
[0051] Much of our knowledge of ionogenic polymer latices has been based on experimental
observation of electrokinetic potentials and total charge by conductometric titration
as used, for example, by Vanderhoff and Van den Hul. Recently, increased emphasis
has been placed on potentiometric titration of the surface charge to assess the effects
of pH and ionic strength on the surface charge.
[0052] Commonly the functional groups responsible for the surface charge are sulphate and
carboxylate. Organic sulphates approximate strong acid behaviour with intrinsic acidity
constants characterised by pK
a ~2.
Latices with sulphate surface groups are essentially completely.ionised in slightly acidic
and neutral solutions. In this case, conductometric titration is more satisfactory
than potentiometric titration as potentionetric titration ehdpoint is rather difficult
to detect with high accuracy. Latices with carboxylate surface groups may be usefully
characterised by both conductometric and potentiometric titration. The methods are
really complementary. The conductometric titration endpoints indicate the amounts
of excess strong acid or base and the total surface charge without detailing the pH
dependence of the surface charge or the surface dissociation constants. If the endpoints
are known from either conductometric or potentiometric titration, then the potentiometric
titration yields information on the fractional ionisation and dissociation constants
as a function of pH. Where conductometric titration is not carried out simultaneously,
the endpoints may be found using Gran's method.
[0053] In the case of amphoteric latices with different surface groups, neither of these
techniques alone is capable of yielding sufficient information about the sign and
magnitude of the surface charge.
[0054] The titration data for amphoteric latex B are shown in Fig. 4. As in the case of
the copolymer, the cationic and anionic sites are not distinguished by endpoints in
the conductometric titration. Two endpoints are observed, at 0.460 and 0.680 ml corresponding
to the titration of excess strong mineral acid and then the total ionisable protons
from weak acid groups. Further addition of strong base serves merely to increase the
OH" concentration and the conductance increases more rapidly. The conductometric titrations
give no clue as to the relative amounts of -RCOOH and -R
3NH
+ groups: The total number of ionisable surface sites is given by 0.220 x 10
-3 moles/0.8277 g of latex. Using the specific surface area 30.9 m
2/g this corresponds to 83.2 µCoul/cm
2 of protonic charge.
[0055] The potentiometric titration shows only one clear inflexion at about .56 ml and pH
6.8. If, however, Gran's method is applied to the pH-volume results, endpoints are
obtained at 0.464 and 0.670 ml in reasonable agreement with the conductometric results.
On the other hand, when Gran linearisation plots are attempted for data points between
the excess strong acid and excess strong base endpoints, the Gran functions are non-linear
with volume. Thus, the endpoint for the titration of carboxylic acid groups alone
cannot be determined.
[0056] However, from independent electrophoretic mobility of zeta potential measurements
as a function of pH at various constant ionic strengths, the iso-electric point, pH
iep, has been established at pH
iep = 6.7 ± 0.1. In the absence of specific adsorption of anions or cations this also
corresponds to the point-of-zero-change, pH
pzc. At these conditions, the number of ionised RCOO groups is equal to the number of
-R
3NH
+ groups. Hence we may use this data to locate the endpoint at pH 6.7 and 0.555 ml,
in close agreement with the main inflexion in the titration curve. Taking the difference
between the conductonetric endpoints and the iso-electric endpoint yields the maximum
number of positive and negative sites. These correspond to values +40 and -43 µCoul/cm
2 at pH 3.7 and pH 10.4 respectively. Using the potentiometric titration the magnitude
of the surface charge shown in Fig. 5 appears to be slightly greater than the result
from the conductometric titration. This may be due to the error in endpoint detomination
in both methods. For example, in the determinaton of the charge from the difference
between blank and latex titrations, errors tend to become larger at high and low pH
where OH
- and H
+ have significant buffer capacity compared to surface charge.
[0057] In comparison to previously prepared polystyrene latices this latex is unique in
two respects, (1) the sign of the surface charge is controlled by pH, and (2) the
magnitude of the charge that can be developed, ca±40 µC/cm
2, is higher than negatively charged surfactant free sulphonated or carboxylate latices.
[0058] This latex also has the unusual property that it is easily redispersed after coagulation
by simply altering the solution pH so as to charge the surface. The high magnitude
of the charge and consequently high surface potential must cause sufficient electrostatic
repulsion to overcome the van der Waals attractive forces.
1. An amphoteric latex characterised in being formed by co-polymerising monomeric
polymerisable compounds including a base monomer and two further monomers one including
an amine group (R3NH+) and the other a carboxyl group (RCOOH) by the use of a polymerisation catalyst,
the latex having ionisable amine and carboxyl groups on the surfaces thereof.
2. An amphoteric latex as claimed in claim 1 wherein the monomer including the amine
group is selected from the group comprising, t-Putylaminoethyl Acrylate, t-Butylaminoethyl
Methacrylate, N,N-Diethylaninoethyl Acrylate, N,N-Diethylaminoethyl Methacrylate,
N.N-Diethylaninoethyl Methacrylamide, N,N-Dimethylaminocthyl Acrylamide, N,N-Dimethylaminocthyl
Methacrylate, H,H-Dimethylamincethyl Vinyl Ether and Allyamine.
3. An amphoteric latex as claimed in claim 1 wherein the monomer including the carboxyl
group is selected from the group comprising Methacry'ic Acid, Acrylic Acid, Itaconic
Acid, Fumaric Acid, Glutaric Acid, Crotonic Acid and Citraconic Acid.
4. An amphoteric latex wherein the base monomer is selected from the group comprising
styrene, methyl methacrylate, ethyl acrylate, vinyl acetate, vinyl chloride, vinylidene
chloride, mixtures of these and other monomers.
5. An amphoteric latex as claimed in claim 1 wherein the base monomer is styrene,
the monomer including an amine group is N,N-Diethylaminoethyl methacrylate and the
monomer including a carboxyl group is Methacrylic Acid.
6. An amphoteric latex as claimed in claim 1 wherein the latex coagulates readily
in a restricted pH range but redisperses when the pH is changed to be outside the
range.
7. An amphoteric latex as claimed in claim 6 where coagulation occurs in the pH range
6 to 7.5.
8. A method of making an amphoetric latex comprising mixing a base monomer with two
further monomers one including an amine group, the other a carboxyl group, with water,
adjusting the pH of the mixture, adding a base monomer, and a polymerisation catalyst,
permitting polymerisation to take place, filtering the latex to remove coagulum and
dialysing the latex against distilled water.
9. A method as claimed in claim 8 wherein the base monomer is selected from the group
comprising styrene, methyl methacrylate, ethyl acrylate, vinyl acetate, vinyl chloride,
vinylidene chloride, mixtures of these and other monomers, the mononer including the
amine group being selected fromthe group comprising t-Butylaminoethyl Acrylate, t-Butylaminoethyl
Methacrylate, N,N-Diethylaminoethyl Acrylate, N,N-Diethylaminoethyl Methacrylate,
N,N-Diethylaminoethyl Methacrylamide, N,N-Dimethylaminoethyl Acrylamide, N,N-Dimethylaminoethyl
Methacrylate, N,N-Dimethylaminoethyl Vinyl Ether and Allyamine, the monomer including
the carboxyl group being selected from the group comprising Methacrylic Acid, Acrylic
Acid, Itaconic Acid, Fumaris Acid, Glutaric Acid, Crotonic Acid and Citraconic Acid,
the pH being adjusted to 1.2 with concentrated acid.
10. A method as claimed in claim 9 wherein the polymerisation catalyst is selected
from the group comprising potassium persulphate and y-radiation.