CA2135450A1

CA2135450A1 - Latex medium for a coating composition

Info

Publication number: CA2135450A1
Application number: CA 2135450
Authority: CA
Inventors: Mathieu Joanicot
Original assignee: Individual
Current assignee: Rhodia Chimie SAS
Priority date: 1993-11-10
Filing date: 1994-11-09
Publication date: 1995-05-11
Also published as: NO944282L; NO944282D0; FI945278A; EP0652269A1; FI945278A0; FR2712296A1; FR2712296B1

Abstract

ABSTRACT

LATEX MEDIUM FOR A COATING COMPOSITION

The present invention relates to a medium for a coating composition based on film-forming latices prepared from unsaturated ethylenic monomers, characterized in that the said medium comprises, per 100 parts by weight of latex A (calculated on the basis of the solids content), which has a particle size of between 100 and 500 nm, from 0.1 to 100 parts by weight of latex B (calculated on the basis of the solids content) which has a particle size of between 5 and 40 nm.
Use of the above medium for the preparation of paper-coating compositions, varnishes and paints, adhesives, pressure-sensitive adhesives and the like.

Description

~ 1 3 ~ 4 ~ O

LAT~ N3DI~N FOR A COATING COMæOSITION
The present invention relates to a latex medium for a coating composition.
Coating compositions based on a latex mediu~
are nowadays of a considerable technical and economic importance. Among these coating compositions particular mention may be made of papermaking compositions, that is to say for surface application for paper, and more particularly paper-coating co~positions, paint compositions, in particular varnish and paint compositions for the inside and for the outside, textile compositions, wood-coating compo~itions, adhesive compositions, pres~ure-sensitive adhesive (PSA) compositions, coating compositions for cement ~ ;
mortar, and the like.
Apart from the medium, these coating compositions co~prise at least one additional compo~ent chosen from a filler, a pigment, a surfactant, a thickening agent, an antioxidant, a natural binder ;`~
20 (starch, casein), a dispersing agent, an antifoam -agent, a biocide, and the like. ~ ~
One of the chief difficulties to be re~olved ~ -in developing them lies chiefly in that the compositions of the above type must have surface 25 properties which differ rrom the bulk properties. ~ -Thus, in the case where these compositions comprise ~ `
fillers and/or pigmsnts, the latex must adhesively bind the fillers together and to the ~ubstrate. -;

-' . -- ~'`;'`:`'~'`, ---` 2 1 3 ~ ~ ~ 0 ~ ~;

~ 73863-32 In addltlon, the latex partlcles must migrate to the surface of the coatlng in order that the coating should have a flne smooth appearance wlthollt asperitles, especlally wlth the alm of obtainlng good prlntablllty propertles. -Practlcal experlence shows that obtalnlng correct surface propertles runs counter to obtalnlng good bulk propertles.
The present lnventlon has made lt posslble to solve the above problem by proposlng a partlcular latex medlum maklng lt posslble to obtaln an entlrely advantageous compromlse between the bulk propertles and the surface propertles. ~;
In fact, accordlng to the lnventlon a medlum has been found for a coatlng composltlon based on fllm-formlng latlces prepared ~r from unsaturated ethylenlc monomers, characterized ln that the sald medlum comprlses, per 100 parts by welght of latex A
(calculated on the basls of the sollds content) whlch has a ~;
partlcle slze of between 100 and 500 mn, from 0.1 to 100 parts ~-by welght of latex B (calculated on the basls of the sollds , -..:
content)~ whlch has a partlcle slze of between 5 and 40 nm.
Latex A preferably has a partlcle slze of between 120 and 300 nm.
~,' In addltlon, preferably from 0.5 to 20 parts, or better stlll from 1 to 10 parts of latex B are employed per lO0 parts of latex A.
~ .

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h;~

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- 21354~o 3 73863-32 ;

Preparation of latex A~
The preparatlon of latex A ls descrlbed in many patent documents such as, for example, US 5,194,539 and EP 486,374. In addltlon, latlces A are avallable commerclally.
The polymerlzable monomers conælæt of at least one monomer chosen from styrene, butadlene, acryllc esters and vlnyl nltrlles.
Acryllc esters denote esters of acryllc acld and of methacryllc acld wlth Cl-C12, preferably Cl-C8 alkanolæ such aæ methyl acrylate, ethyl acrylate, propyl acrylate, n-butyl acrylate, -: : . : ...
lsobutyl acrylate, 2-ethylhexyl acrylate, methyl methacrylate, :~ -ethyl methacrylate, n-butyl methacrylate and lsobutyl i`; :- .;
methacrylate.
Vlnyl nltrlles lnclude those contalning from 3 to 12 carbon atoms, ln partlcular acrylonltrlle and methacrylonltrlle.
Styrene may be replaced entlrely or partlally wlth alpha-methylstyrene or vlnyltoluene.
Among the ethylenlcally unsaturated monomers capable ;-of be1ng polymerlzed wlth the above monomers, and the quantlty of whlch can range up to 40% by welght of the total of the monomers, there may be mentloned~
- carboxyllc acld vlnyl esters such as vlnyl acetate, vlnyl versatate, vlnyl proplDnate, =~

~c - ~ 2135~o 4 ~:
- ethylenic unsaturated mono- and dicarboxylic acids 6uch as acrylic acid, methacrylic acid, itaconic acid, maleic acid, fumarie acid and monoalkyl e6ter6 of dicarboxylic acid~ of the type referred to with alkanols which preferably contai~
1 to 4 carbon atoms and their N-substituted derivatives, - amides of unsaturated carboxylic acid~ ~uch as acrylamide, methacrylamide, N~
methalolacrylamide or -methacryl-~;de, - ethylenic monomers containing a sulphonic acid group and its alkali metal or a~monium salts, for example vinylsulphonic acid, -~
vinylbenzenesulphonic acid, alpha-acrylamidomethylpropanesulphonic acid and 2-sulphoethylene methacrylate, -- ethylenically unsaturated monomer6 containing a seaondary,.tertiary or quaternary amino group or a heterocyclic group containing nitrogen, for ;-~
example vinylpyridi3es, vinylimidazole, aminoalkyl : (meth)acrylates and amir.oalkyl (meth)acryla~ides uch as dimethylaminoethyl acrylate or methacrylate, di-tert-butylaminoethyl acrylate or -~ `
- methacrylate, dimethyla~ihomethylacrylamide or -methacrylamide etc., as well ao zwitterionic monomers such as sulphopropyl (dimethyl) ::~-:; e.i , .- . ~,.
aminopropyl acrylate etc., - esters of (meth)acrylic acids with .. , ~.'.' -~ 21354~o ~ :~

alkanediol~ preferably containing 2-8 carbon atoms, such as glycol mono(meth)acrylate, ~ .";;
hydroxypropyl mono(meth)acrylate, 1,4-butanediol `.
mono(meth)acrylate, and monomers containing two polymerizable double bonds, ~uch as ethylene ~-glycol dimethacrylate.
In addition to the abovementioned monomers it is also possible to use, in minor quantities which are also between 0.1 and 5 %, unsaturated ethylenic monomers of a crosslinkable nature, such as glycidyl (meth)acrylate or vinyl and acrylic silane~
3y way of example of compositions for the production of latex media there may be mentioned ~ `~
composition~ containing, on a weight basi~, 25-90 %, preferably 45-75 % of styrene and/or acrylonitrile, 75-10 %, preferably 55-25 %, of butadiene and/or acrylates, 0-20 %, preferably 1-10 % of unsaturated carboxylic acid and 0-40 %, preferably 0-15 %, of other unsaturated ethylenic monomers.
Polymerizat on is carried out in a manner ;~
known per se, in aqueous emulsion, of polymerizable ~-~
monomers in the pre~ence of at least one radical initiator and preferably in the presence of a transfer agent, for example of 'he mercaptan type, with a `'`~ J.
25 concentration of monomers in the reaction mixture which - ~
~ . . . ~.
is generally between 20 and 60 % by weight.
Any kind of free-radical initiator -con~entional in emulsion polymerization can be ~ ~ .

2135~0 suitable. !, Examples of initiators include hydroperoxides such as aqueous hydrogen peroxide, diisopropylbenzene hydroperoxide, Rodium, potassium or ammonium persulphates and cationic initiators such as azobisisobutyronitrile or 4,4'-azobis(4-cyanovaleric acid). ;~
These initiators may be used in combination -`
with a reducing agent such as, for example, bisulphite.
The quantity generally lies between 0.05 and 2 % by weight relative to the quantity of the monomer~.
The polymerization temperature, a function of the initiator used, is generally between 50C and 100C, preferably between 70 and 50C. The stabilization of the particlea is ensured, if need be, by any known colloidal sta~ilization system such as anionic, cationic, amphoteric and nonionic emulsifier~
Anionic emulsifiers are, for example, alkali metal alkyl sulphates, alkylsulphonates, ;~
alkylarylsulphonates and alkyl phosphates, dialkyl sulphosuccinates and sodium, potassium or ammonium salts of fatty acids, ~aturated or otherwise. Examples of cationic emul6ifiers are alkylpyridinium or alkylammonium salts such as N-ethyldodecyla~monium 25 chloride or bromide or cetylammonium chloride or `
bromide. Nonionic emulsi~iers which are mentioned in particular are polyoxyethylenated and/or polyoxypropylenated derivati~es of fatty alcohol~, of ..

., . . - ' ': ~ :~

213S4SO :
, fatty acids or of alkylphenols.
The polymerization may be carried out continuously, noncontinuously or semicontinuously with continuous introduction of a part of the monomers, and may be of the "seeded" or "incremental" type according to any known variant for obtaining particles of homogeneous and heterogeneous structure.
Preparation of latex B~
For convenience in the following description ;~ ;
it will be ~aid that latex B is made up of nanoparticles. Any monomer containing ethylenic ;~
unsaturation may be chosen as starting material and, in particular, the monomers and polymerization i~itiators ',i~'~ ,",~
referred to above in the case of latex A.
Among the polymers which may constitute the `
said latex 3 particles there may be mentioned ;-~
homopolymers or copolymers containing units derived from vinyl, acrylic and vinylaromatic monomers, from vinyl esters, from alkyl esters of a and unsaturated acids, from esters o8 unsaturated carboxylic acid~
from vinyl chloride, -rom vinylidene chloride and/or . ~ .
from dienes.
By way of illustration, the following monomers may be mentioned more particularly~
~ styrene and its derivatives (vinyltoluene, ethylvinylbenzene) - esters, hydroxyesters and amides of `
(meth)acrylic acid, such a~ methyl methacrylate, ;~

'~

213~4~0 ;~

butyl acrylate or (meth)acrylamide - vinyl esters ~vinyl acetate, vinyl ;
propionate) - vinyl and vinylidene chloride~
- vinylpyridines (2-vinylpyridine, 4 vi~ylpyridine, 2-methyl-5-vinylpyridine) - di(ethyl)aminoalkyl (meth)acrylates - ~ b ~-- di(ethyl)aminoalkyl(meth)acrylamides .
- allylamide - ethyleneimine -:
- ~meth)acrylonitrile - N-vinylimidazole - dialkylaminomethylstyrenes - vinylpyrrolidone - divinylbenzene and itC derivati~es - - conjugated dienes (butadiene etc.) - polyallyl derivatives (tetraallylethylene etc.) .~: .
- polyol (meth)acrylates (ethylene glycol dimethacrylate e~c.) - methylenebis(acryl2mide) :~
- bis(acrylamido)acetic acid.
By way OI vinyl and acryli-c monomers which are suitable for the invention there may be mentioned more particularly thoae derived from styrene, from acrylic acid, from acrylic ester of the type of N~
: hydroxysuccinimide acrylic ester~ such as N~
acryloyloxysuccinimide .~nd N-acryloyloxyphthal~mide~

,',~,."'.',',.

r ~35~o -~ -` - .
g ~
methacrylic acid, monobenzyl maleate, 2-vinylpyridine, Fityrene methylsulphonate, chloromethylstyrene, hydroxypropyl methacrylate, hydroxybutyl acrylate, hydroxyethyl acrylate, acrylonitrile and/or acrolein. ~-~
According to a variant, the particles have ion-forming or react ve groups at their surfaces, preferably chosen from OH, SO3H, SO3R, SO4R, COOH, CHO, PhCH2Cl, NH2, NR2, NR3 with R being a Cl-C3 alkyl radical, CONH2, NH-NH2, ;;
CN, CO2(CH2)~0H with n denoting an integer from 1 to 8, and N-hydroxyimide ester~
The functional groups present at the surface of the nanoparticles may also be derived from the ~ ~ -subsequent chemical conversion, for exa~ple by nucleophilic 6ubstitution, of one or a number of 15 reacti~e groups carried by the poly~er chain ~:
constituting the nanoparticles.
The nanoparticles according to the invention have a narrow particle size di6tribution. This monodi6persity of particle size makes it po6sible to have acce6s to the true adsorption surface of the nanoparticles and consequently to the optimum binding capacity for the latter, especially on fillers and pigments and on the substrate6 to be covered.
Any particle size distribution whose 6tandara 25 error is lower than or equal to 30 %, and preferably of ;~
the order of 20 %, is considered to be narrow.
The nanoparticles are capable of being obtained by polymerization of a direct microe~ulsion or `~ P

,~ ` `

-: , 213~o ,~
of the corresponding monomer~. The term microemulsion ~;
i8 intended to denote a thermodynamically stable dispersion in contrast to a di~persion which i8 only -;
kinetically stable and which coalesces at the end of a certain period.
The polymerization of a direct microemul ion ~-~
and the recovery of the said aqueous dispersion at the end of the polymerization are used, the said direct microemulsion being obtained beforehand by titration with the aid of an effective quantity o anionic surfactantt either of a direct emulsion stabilized with at least one ionic surfactant, or of an inverse . : :
emulsion stabilized with at least one nonionic surfactant, the two emulsions, called initial emulsions, consisting of at least one monomer in aqueous dispersion.
The aqueous dispersion of nanoparticles which is thus obtained may furthermore be subjected to a purification operation if appropriate.
In a first -t~ge, `herefore, the process involve~ the formation of this direct microemulsion.
In the case of an inverse initial emulsion the corresponding micrcemulsion i~ prepared by the 80- ~ -called inversion method inspired by the Schulman ~ ~
...... ~.. -.. ...
process (J.Phys.Chem. 1959, 63 p. 1677).
It consists, in a first step, in formulating - -~
an emulsion of the water-in-oil type, the oil --~
consisting of the monomer( 8) in question, with the aid ~ -- ..
- ~. ,...--:

2~ 35 ~ ~ o 11 ~, .~, of a lipophilic nonionic emulsifier, that i8 to ~ay one with a low hydrophile-lipophile balance HLB. The oil forms approximately 70 % by wPight of the mixture. The quantity of this nonionic surfaetant i~ added 80 as to obtain a stabilized emulsion.
In general, a small quantity of an anionic surfactant is also added. Its ~uantity i6 appro~imately 0.5 % by weight relative to the weight of monomer. This emulsion i8 then inverted by addition of an aqueous solution of a hydrophilic nonicnic surfactant, that i8 to 6ay of high HLB until phase inversion is observed.
This phenomenon of inver6ion can be detected macroscopically, since during the titration a change is observed successively from a milky emulsion to a gel ~ ~;
and then to a transparent, staole, direct mieroemulsion of low ~iscosity.
The nonionic emulsifier used may be chosen from polyethoxylated nonylphenol (NP) derivatives. The lipophilic or hydrophilic nature of these co~pounds ~-20 varies as a fu~ction of the number of their ethoxy `;~
units. The pairs NP5/NPl 5, NPl 5/NPI 2, NP7/NPI 2 and NP7/NPI 5 may thus be mentioned as a pair of an ionie emulsifier of low HLB/nonionic emulsifier of high HLB.
According to a preferred embodiment the ~-inverse emulsion additionally and before its titration contains an anionic surfactant. This may be, for :`
exa~ple, sodium dodecylbenzene6ulphonate (DBS), ~odium 1auryl =u1phate (SDS~ or oodlu= dioetyl ~ulpho~eeirat-r~ ~ r~

2135~o :

(aerosol OT~
The nanoparticle dispersions obtained by this inverse emul~ion technique contain a large quantity of surfactants and cannot therefore be employed directly.
The removal of excess surfactants is obtained subsequently by using a purification technique. This removal i8 preferably carried out by dialysis which has -~
the advantage of not altering the particle size. No flocculation phenomenon is observed.
According to another process for the preparation of nanoparticles of latex ~, at least one of the monomers containing ethylenic unsaturation, referred to abo~e and capable of polymerizing in !~
aqueous emulsion, is added incrementally to the reactor and a polymerization initiator and at least one ~urfactant are added incrementally so that nanolatex 4~ ~ J
particles of particle size smaller than ~ nm are :: ..
obtained.
The term "incremental addition" includes all forms of addition of a small part of the monomer and/or of the initiator to the aqueous solution during a period of time necessary for all the monomer and all -the initiator to be added.
This includes cyclic addition~, noncontinuous 25 additions and their combinations. ~ ~
The addition of monomer and initiator is ` ~!"~'"`~' preferably continuous and kept constant during a certain period of time.
, . ' .. .''~
/~;

2135~o i ~ , The preferred monomers employed are alkyl acrylates and methacrylates in which the alkyl portio~
contain~ fro~ 1 to 10 carbon ato~s and carboxylic acids containing ethylenic un~aturation, such as acrylic, methacrylic, maleic, fumaric and itaconic acid and the corresponding amides.
In the caAe of copolymers derived partly from ;~
acrylic monomer~, a quantity of acrylic mono~er which is typically between 30 and 99 % of the total weight of the monomerA, preferably from 50 to 90 %, is employed.

For certain applications it i de irable that ~ ,:
the latex B should be crosslinked. To do this, a crosslinking agent is added to the reaction mixture for aqueous emulsion polymerization, which is chosen ~rom N-alkylol amides such as N-methylolacrylamide, N-methylolmaleimide, certain difunctional monomers such as divinylbenzne, divinylnaphthalene and divinylcyclohexane, as well as various diacrylates Ruch ;~
as ethylene glycol dimethacrylate.
, . ~. ~ .
The quantit e6 of crosslinking agents are generally between 0.05 and 10 %, from 0.1 to 5 % and `
preferably from 0.1 ~o 1.0 %, based on the total weight -of all the monomers added. '.. ".'.
As indicated above, any polymerization 25 initiator can be employed. ` -However, in the case where the initiator is water-soluble, which is the case with peroxides and , ~ ., .,,~i persulphates, it i6 critical to keep the ionicity o ~ . . .;~

'"' "'`"`' .. ..
.. .

. . .

213~4~0 the reaction mixture constant by removing a proportion of the water from the reaction mixture and by adding thi6 same water to the initiator feed stream.
It has been found, in fact, that by keeping a constant ionicity in the reaction mixture the agglomeration of the particles is avoided and particle~
exhibiting a narrower particle ~ize distribution are obtained. By diluting the water-soluble initiator with an appropriate quantity of water the ionic strength of the reacticn mixture is kept constant.
A semiconti~uous or continuous polymerization ;
proces~ may be employed for preparing latices B. This ~ r';!~
makes use of the addition, in an incremental manner, of the monomer, of the crosslinking agent if noed be and ~ fi of the lnitiator solution~ to the reactor, which is heated to a temperature which is generally between approximately 45 and 90C, water and at least one surfacta~t being ircluded during a certain period of ;-;~
time, in contrast to the noncontinuous ~'batch" process. `~
The reactor may optionally contain a small quantity of monomer before the beginning of the incremental polymerization, for the purpose of acting -as a "seed". Such a small quantity of monomer is generally lower than 30 % and preferably lower than ...-. .,,. - , .. . .
10 % of the total weight of the monomer employed.
The rate of addition of the monomer is generally governed by various factors such as the -;"
dimension~ of the reactor, the temperature increase due 2 1 3 5 4 ~ o to the exothermic reaction, the cooling capacity and the like, 80 that the reaction temperature i8 generally ~aintained in a specific range.
. . .:
The reaction mixture i8 maintained at a temperature between the ambient temperature (10 to 25C) and the boiling point of the aqueous golution.
~ -Y.'~
The reaction is generally performed at atmospheric pressure but it can be increased to promote the polymerization.
The monomer feed and the initiator feed must be combined as the ~ame single feed in the case where the initiator is ~oluble.
In addition, if the initiator is water~ -soluble and is an ionic salt such as ammonium 15 persulphate, this initiator is introduced into the ` ~-reactor so that the ionicity in the reaction mixture is maintained at a constant level. This objective is typically obtained by initially tran~ferring a proportion of the water .ro~ the reaction mixture to 20 the initiator feed in order to give ri~e to ion concentrations which are appreciably equal both in the feed and in the reactor. -Feeding the reactor using an incremental method generally makes it pos ible to obtain a a 2S stabilized concen~ration of free radicals, which ~akes it possible to avoid the low radical concentration~
observed during single initiator charges and prolonged `~
.-, .. . : ~
feed period~. It i~ the continuous availability of fre~

,,, 21354~0 ~ ~
~ .
16 73863-3~ ~
radicals that allows the formation of the new chaln and of the ~--particle to cornpete effectlvely with the addltion of monomer to already exlstlng particles, whlch results ln latex B particles ~-exhiblting an improved monodlspersity (that is to say a narrow partlcle slze distribution).
The terms "partlcle size dlstrlbutlon" or "partlcle slze" are lntended according to the invention to mean the ;
: . ~ ".
dlmenslon of the mean dlameter of the volume of the partlcle, . ~ . , .. ~ ..:
measured by photocorrelatlon spectroscopy. Latex B partlcles have a partlcle 9 ize which ls generally between 5 and 40 nm and :~
preferably between lO and 30 nm. ~.
Of course, the propertles of latex B depend to a large extent on the monomers chosen for their preparation. `
The constltuent monomers of latlces B may have a glass transltlon temperature Tg of between -54C and +130C. Of course, these Tg values rnust be chosen as a functlon of the type - i of chosen coatlng composltlons. A slmple rule, but not unl~ue ~ .:
ln the case of the medlum accordlng to the lnventlon, ls that, advantageously, the Tg of latex B is slmllar to the Tg of latex A. Thls rule can be applled ln particular to palnts.
When the medium accordlng to the lnventlon ls employed ;~^
ln papermaking compositlons, the latex A

. ~- ., ~

...... .
., .,~
;
.~

~135~
17 ~ -employed i8 generally a latex A of aa organic copolymer chosen from a styrene/butadiene, styrene/acrylate and styrene/butadiene/acrylate copolymer.
Pigment is added to the medium, such as, for exam~le, a mineral clay such as kaolin, calcium carbonate, titanium oxide, satin white, hydrargillite, talc, barium sulphate, plaster, as well as various adjuvants such as dispersing age~ts, natural binders such as starch, biocides, foa~ suppressor~, slip agents ~ ;
and the like.
A coating composition generally comprises: - ;
- 100 parts by weight of pigment, , ;
- 2 to 25 parts by weight o~ medium, `
- 1 to 10 parts by weight of adjuv~nts 8uch as those referred to above.
The medium content i8 lower (2 to 8 days) in :~
the case of compositions for photogravure (Rotogravure); it is greater (6 to 15 beams) in the `-case of offset compositions. ~ c The coating compositions according to the invention exhibit a better coating ratio, that is to say that, compared with Xnown compositions, the "`''.::.' '' ;'`'.'``?~
compositions according to the invention make it possible to obtain a satisfactory paper coating while employing less composition.
In addition, the medium according to the invention enables the fillers to be hidden better and - `
makes it possible to obtain a better ink uptake, which `~

o ,: , 18 ,:.
makes the papermaking compositions according to the invention more particularly advantageous for photogravure ("Rotogravure").
The compositions according to the invention .~
5 are also aimed at pressure-sensitive adhesive (PSA) .:-~ .,,.i.
compositions comprising a medium according to the invention. The compositions are traditionally based on an acrylic latex A such as, for example, tho~e described in patentfi US-A-5,164,441 and GB-A 1,257,940. .
~atex B is preferably hard with a Tg higher ..
than QC and prepared with the same monomers (generally `.-` , an alkyl (meth)acr,vlate and a (meth)acrylic acid). -~
In accordance with the inYention it has been found that the simultaneous addition of 0.01 to 0.3 % ~". ~:
15 by weight of a water-~oluble polymer such as .
carboxymethyl cellulose to latex B makes it possible to .. `~
obtain an asymmetric tack (adhesiveness) due to the rise of latex B to the surface at the time of drying. ";a~
In addition, the uGe of hard nanolatices B (Tg higher than or equal ~o 0) reduces "blocking" that i8 to say "tack", in other words the adhesiveness. ~ `
The present invention i8 also aimed more .
specifically at paint compositions comprising the : ~-., ~,.~
medium according to the invention in which the latices ,,,~., ";,., A and B are preferably essentially based on acrylic or styrene-acrylate copolymer based on at least one `~
alpha/beta un~aturated monoolefinic carboxylic monomer such as ~meth)acrylic acids and on at least one ~'-~. '..

,~ """,""',''.,, ~

2 ~L 3 ~ 4 5 o ~ :
19' (meth)acrylic acid e~ter of a pri~ary or secondary alcohol containing from 1 to 18 carbon atoms and optionally on styrene. Such latice~ A are, for example, de~cribed in US-A-3,320,198 and in EP-A 5,010,000 which are mentioned as reference.
In paints for the exterior it i8 reco~mended that latex A 6hould ha~e a Tg lower than 0C and that latex B should ha~e a Tg higher than 0C, preferably hisher than 10C.
In fact, in this particular case, the paint employed outside will exhibit a high resistance to soiling due to the migration of the particles of latex B to the surface.
To make an acrylic paint containing styrene/acrylate, the latex mediu~, preferably neutralized with a base (for example agueous a~monia or amine) i8 mixed in a known manner with a pigment dispersion containing usual adjuva~ts as well as pigments and pulverulent filler materials.
It has been found surprisingly and unexpectedly that the medium according to the invention permits wet abrasion resistance without affecting `~
stability.
In addition, as latex B tends to migrate to the external surface of the paint, the "hardness" of this surface can be modified by a suitable choice of the Tg of the constituent (co)polymer of latex B. ~ -In this paint application latex B prefarably -~
. ' 213~0 exhibits a Tg of between -50 and +50C, preferably between -30 and +30C.
The following exa~ples are given by way of ^ -illustration, no limitation whatever being implied.
In all that follows all that precedes, unleo~
the contrary is expressly mentioned, the parts and percentages are by weight. -EXAMP~E 1 Pre~aration of a latex A
The following monomers are introduced into a 25-litre reactor made of stainless steel, fitted with a stirrer and a jacket for circulating water enabling the temperature inside the reactor to be regulated at 80C
during the polymerization:
15 - 83 parts by weight of water, - 9 parts by weight of styrene, - 75.5 parts by weight of butyl acrylate, - 10 parts by weight of 2-ethylhexyl acrylate, ;
- 0.5 parts by weight of ethylene glycol dimethacrylate, - 2.7 parts by weight of acrylic acid, - 0.5 parts by weight of acrylamide, - 0.5 parts by weight of sodium sulphoethyl -~
methacrylate - 0.1 parts by weight of sodium alkylsulphonate, - 1.6 parts by weight of ammonium persulphate.
The polymerization is performed at 80C for ;~

~`','-'.''~

2~3~4~0 10 h.
The neutralized latex Lo has a mea~ particle size of 0.15 ~m, a Tg of -40C, a solids content of --~
53.3 %, a pH of 6 and a Brookfield viscosity, measured ~ `~
at 50 rev/min, of 280 mPa 8.

2-1 Preparation of a latex B~
In a 1-litre reactor made of stainlesE steel fitted with a stirrer and a jacket for circulating ; --- ~q;
10 water enabling the tempe`rature inside the reactor to be -regulated, a reaction stream consisting of the following is introduced into the reactor:
65 parts of butyl ac~ylate, ~ ~
25 parts of methacrylic acid, ~ æ"~".
10 parts of methyl methacrylate.
1 part of Vazo-64(3) (2,2'-azobisisobutyronitrile in solution in methyl methacrylate). :~
This feed solution is introduced in a ~-controlled manner into the reactor which contains 185 parts of water and 3 parts of sodium dodecyl sulphate.
The reactor is kept at 85C with constant stirring.
At the end of the addition of monomer the reactor is still kept at 85C for another hour. The reaction mixture is then taken to an almost complete conversion by treatment with 0.015 parts of t-butyl hydroperoxide and 0.015 parts of sodium metabisulphite at a temperature of 62C.

" """";''~

~' 21354~0 A latex with a solid~ co~tent of 25 % is ~ --obtained, with a particle size of 14 nm.
The latex is light in colour and has a Tg of approximately -5C.
2-2 Pre~aration of latex B (2) 0.15 parts of am~onium per~ulphate ~,'',. '~:,:!~"' predissolved ia 0.3 parts of water are added to a solution containing 2.57 parts of sodium lauryl sulphate a d 241 parts of water. After an initiation -~
period of 5 minutes a mixture of 43 partQ of butyl acrylate and 57 parts of butyl methacrylate i8 W`7-'~
introduced into the solution over 2 hours. The temperature i8 raised to 92C over 30 minutes and then the mixture is cooled and filtered.
A blue-grey latex is obtained which has a solid content of 23.10 %.
This latex has a particle ~ize smaller than 30 nm and a Tg of 0C.
2-3 Preparation of latex B (3) The operating method of example 2-2 is ;~
reproduced exactly except that 47 ~0 of methyl methacrylate, 43 % of butyl acrylate and 10 % of -methacrylic acid are employed.
This latex thus obtained has a particle size ~-smaller than 40 nm and a Tg of ~20C.
2-4 Preparation o$ latex B (4) -,-~: . :: .
The operating method of example 2-2 is reproduced exactly except that 20 % of methyl ;-.. ""' ~'.
, ~

methacrylate, 70 % of butyl acrylate and 10 % of -~ "
methacrylic acid are e~ployed. This latex thus obtainod .;
ha6 a particle size smaller tha~ 40 nm and a Tg of -20C.
EXAMPLE 3:
Pre~aration of the media The media Ll, L2, L3 and L~ are prepared Qimply by mixing, at slightly acidic pH, 100 parts of latex A
and 10 parts of lati~es B~l), B(2), B(3) and B(4) `~
10 respecti~ely. The media Ll, L2, L3 and L~ thu~ obtained .`~
ha~e solid~ contents of approximately 50 %.
Preparation of the ~aint:
The pigment pa6te which ha~ the following composition (in parts by weight) is prepared simply by .
15 mixing in a paint disperser:
Water 3000 - ~.
Tylose MH 6000 XP ~ (thickener)40 Coatex P 50 ~g (acrylic dispersant) 37 ;;
Clérol TPE 714 ~ (antifoam) 20 :~
Proxel BD ~ (biocide) 9 ..
TiO2 RL68 530 Omyalite 90 (calcium carbonate of particle size ~ l~m) 2130 D~rcol 50 (calcium carbonate of particle size smaller than 5 ~m)3200 to 89.66 parts of this pigment paste are added 15 part~
of a modium chosen from ~2~ L3 and L~ and 0.9 parts of water, to obtain a paint P2, P3 and P~, re~pecti~ely, : .- , ~-~ .
- . ~: :;

: ~` :`:

~ 1 3 5 4 5 0 ~ ~
i ., , ~ ~

which has a Pigment Volume Concentration (PVC) of approximately 75 %. - ~`
Characteristics of the media L" L3, L, and of the ~aint8 P2~ P3~ P~
The following are determined on the media L
L3 and L, and the paints P2, P, and P,: ;
- Brookfield viscosity at 50 rev/min (v) - storage stability 28 days at 55C.
To do thi~, a sample of medium or o~ paint i8 `,`:. ,~ ; `
10 heated at 55C for 28 days in a receptacle protected `-from air. The stability (S) of the medium or of the -paint is given by the ratio of the Brookfield viscositieE of the paints, measured at 50 rev/min at 20~ after 30 day~, to the initial visco~ity.
Also measured on the media L2, L3 a d L~ are~
the pH, the solid~ content (%) and the minimum fil~
forming temperature (MFFT in C), that is to say the lowest temperature at which a film is obtained. A wet abrasion resistance (WAR) test is performed on the `~
20 paints P2, P3 and P~, which i~ performed with the aid o~ ;
a C-- abrasiometer according to DIN Standard 53778. ~he WAR is evaluated as the number of cycles at the end of which the paint i8 eroded.
The results obtained are collated in Table 1 below insofar as the media are concerned and in Table 2 below in~ofar a~ the paints are concerned.

'' ', :' ' ''`""

`' ,`'`'',''`.",`'~``'`
`;~
, ' ' '~

` 2 1 3 5 ~ ~ o TABLE 1 .
- ., .;,..........

~-di~= ~ pH SC ~ (=P- ~) 5 Z8 d/ MFFT
._ ,',, ` ',- ~.'`~
L? 7.7 47.9 2080 0.75 0 ; ` ~
.~ ,, _ _ - ___ -', ,. . ' ` ,' -.-.'-.
L3 8 47.8 1200 0.65 0 , . .__ Ls 7.8 48.0 600 1.05 0 , ....... . ,__ ' `,~

,__ "'~

Paint Medi~ V S 28 d/ WAR (~o.
.___. employed (mPa 8)55C of cyrle~) Pl L2 4080 1 3910 ._ p3 L3 4440 1.15 1370 .
_ .~
P~ L~ 4120 1.1 3400 , __ _ ,,_, ,~,__ ~ .. ,.. ,,.,,,,", Po LA 3560 1.05 2550 Paint Po i8 a control paint prepared only with the medium LA.
From Tables 1 and 2 above it appears that paints which have satis$actory viscosities and which remain perfectly stable at the end of the 28 days at 55C are obtained. ~:
It is noted, furthermore, that the addition of binder B with Tg = 0C greatly improves the WAR.

~ ' : ~! . , . ~ .!

Claims

1. Medium for a coating composition based on film-forming latices prepared from unsaturated ethylenic monomers, characterized in that the said medium comprises, per 100 parts by weight of latex A (calculated on the basis of the solids content) which has a particle size of between 100 and 500 nm, from 0.1 to 100 parts by weight of latex B (calculated on the basis of the solids content) which has a particle size of between 5 and 40 nm.

2. Medium according to claim 1, characterized in that, per 100 parts of latex A which has a particle size of between 120 and 300 nm, the said medium comprises 0.5 to 20 parts, preferably from 1 to 10 parts of latex B which has a particle size of between 10 and 30 nm.

3. Medium for a coating composition according to claim 1 or 2, characterized in that it is employed in compositions for paper coating, for paint, for varnish, for textile finishing, for wood coating, in adhesive compositions, in pressure-sensitive adhesive compositions and in coating compositions for cement mortar.