Suspension polymerization in an organic medium

Inherently tacky infusible pressure-sensitive adhesive microspheres are prepared by polymerizing at least one monomer which when polymerized will form a pressure-sensitive adhesive having a glass transition temperature less than abotu -20.degree. C. in an organic medium in which the monomer is substantially insoluble and in the presence of a surfactant and a suspension stabilizer which are soluble in the organic medium and substantially insoluble in the monomer under conditions of shear sufficient to form suspended particles of a size less than 200 microns. A monomer soluble initiator is preferably employed. The preferred media are ethylene glycol and glycerol and the preferred monomers are 2-ethyl hexyl acrylate and methacrylic acid.

BACKGROUND OF THE INVENTION 
The present invention relates to normally tacky pressure-sensitive adhesive 
microspheres useful in the production of removable and repositionable note 
paper tape and label products. 
The mid-1970's saw the introduction of removable and repositionable note 
papers which have found broad acceptance in the marketplace. The adhesives 
utilized in some of the products were infusible, inherently tacky, 
elastomeric microspheres prepared by an aqueous suspension polymerization 
process. Aqueous suspension polymerization processes are described, for 
instance, in U.S. Pat. Nos. 3,691,140 to Silver, 4,166,152 to Baker et 
al., 4,495,318 to Howard, and 4,598,212 also to Howard. 
The problem with adhesive microspheres produced by an aqueous suspension 
polymerization process is that there is a need to recover the microspheres 
from the medium in which they are prepared, then redisperse them in a 
organic solvent for application to paper or another substrate. This 
procedure is required in the instance of paper because the presence of 
water will cause paper to which the adhesive microspheres are applied to 
curl. Aqueous suspension polymerization is, therefore, a costly means of 
note paper manufacture. 
It has also been stated in connection with the production of removable and 
repositionable products that if the adhesive force provided is too small, 
the product will literally fall off the surface to which it is applied, 
and if too great, will tear or delaminate paper surfaces at normal removal 
rates. Application of the adhesive coatings so as to achieve peel values 
within a desired range requires therefore considerable know-how. 
A considerable reduction in manufacturing costs and better control over the 
product performance can result if the adhesive is prepared where the 
medium can be effectively used to control particle size and therefore 
adhesion and also where there is eliminated the need to recover the 
adhesive from the medium in which it is prepared by a tedious and 
expensive step such as coagulation. This is the object of the instant 
invention. 
SUMMARY OF THE INVENTION 
According to the present invention there is provided a suspension 
polymerization process which utilizes an organic medium in which infusible 
adhesive microspheres are prepared with shear being employed to control 
microsphere diameter. 
Preparation of the adhesive microspheres in an organic medium enables 
production of a microsphere adhesive product which can be directly applied 
from the medium to a substrate, such as paper without curl, or the medium 
substantially eliminated to enable adhesive transfer to a more volatile 
organic medium for application to a paper substrate again without curl, 
and enable production of the useful removable and repositionable product 
at a substantially reduced cost. 
The process of the instant invention involves the polymerization of one or 
more monomers to form an inherently tacky, infusible, pressure-sensitive 
adhesive microspheres having a glass transition temperature of less than 
about -20 C. in an organic medium in which at least one of the principal 
monomers is substantially insoluble. Polymerization preferably occurs in 
the presence of an initiator which is soluble in the monomers and 
substantially insoluble in the medium and in the presence of a surfactant 
and one or more suspension stabilizers either of which may be nonanionic, 
anionic, cationic or amphoteric in nature, and both of which are soluble 
in the organic medium and substantially insoluble in the monomers. The 
presently preferred medium is ethylene glycol or glycerol and the 
principal or bulk monomer is preferably at least one alkyl acrylate ester 
which is insoluble in the organic medium, preferably 2-ethyl hexyl 
acrylate which is either homopolymerized or preferably copolymerized with 
a minor amount of an unsaturated carboxylic acid preferably acrylic and/or 
methacrylic acid. The monomers are selected and/or proportioned to produce 
an inherently tacky, pressure-sensitive adhesive microspheres having a 
glass transition temperature less than about -20 C. Polymerization 
preferably occurs under autogenous conditions in the presence of from 0.15 
to about 0.5 percent by weight of the monomers of a monomer-soluble 
initiator, preferably benzoyl peroxide. It is presently preferred that the 
suspension stabilizer be an anionic and/or a nonionic suspension 
stabilizer, and the surfactant an anionic surfactant. Polymerization 
occurs with agitation at a shear rate sufficient to form particles having 
a particle size smaller than about 200 microns, preferably smaller than 
about 90 microns. 
The formed adhesive particles may be applied to a substrate directly from 
the medium in which they are prepared. Because ethylene glycol and 
glycerol have low vapor pressures, however, it is expedient to employ a 
more volatile organic medium, as the carrier from which the adhesive 
particles are applied to a substrate. Uniquely, it is unnecessary to 
recover the adhesive particles from the organic medium in which they are 
prepared. Ideally, since they are insoluble in the medium, the particles 
can be allowed to separate from the medium, the medium drained off and 
replaced by the more volatile medium. If it is desired to minimize the 
amount of polymerization medium present in the volatile medium, the 
mixture of adhesive polymer particles and polymerization medium can be 
centrifuged to achieve an even greater separation of the medium from the 
adhesive polymer particles. The organic medium is then drained off and a 
concentrate of adhesive polymer particles dispersed in the more volatile 
organic carrier for application to a substrate, such as paper, for the 
production of removable labels, note paper products and tapes, or to a 
self-supporting non-paper substrate such as mylar and aluminum foil for 
production of removable products such as tapes and labels. If desired, the 
dispersion of adhesive polymer particles in the volatile organic medium 
can be subjected to a second centrifuging to completely eliminate the 
medium in which the adhesive polymer particles were prepared. 
DETAILED DESCRIPTION 
According to the present invention, there is provided a suspension 
polymerization process for the production of inherently tacky, infusible, 
pressure-sensitive adhesive microsphere polymers suitable for the 
production of removable and repositionable products such as labels, note 
paper, tapes and the like without the need for recovery of the polymers 
from the medium in which they were prepared. 
The tacky microspheres are prepared in an organic medium in which at least 
the principal or bulk of the monomers are substantially insoluble under 
conditions of shear in the presence of a suitable suspension stabilizer 
and surfactant. 
The organic medium which is useful in accordance with the present invention 
includes polyols such as diols, triols and other media in which the 
principal monomer or monomers to be polymerized are insoluble. The 
presently preferred medium is ethylene glycol and/or glycerol. 
The principal monomers which can be homopolymerized or copolymerized are 
those which are insoluble in the organic medium and constitute the bulk of 
the monomers present in the suspension. They include alkyl acrylate esters 
such as isooctyl acrylate, 2-ethyl hexyl acrylate, butyl acrylate, 
sec-butyl acrylate, methyl butyl acrylate, 4-methyl 2-pentyl acrylate and 
the like. Comonomers which can be used are unsaturated mono and 
dicarboxylic acids such as methacrylic acid, acrylic acid, fumaric acid 
and the like. Other comonomers include dibutyl fumarate and the like, 
methacrylates such as methyl methacrylate, isodecyl methacrylate and the 
like, styrene, vinyl acetate and the like. All that is required is that 
the principal monomers be substantially insoluble in the medium and be 
homopolymerizable or copolymerizable in suspension droplet form to form an 
infusible product which is an inherently tacky pressure-sensitive adhesive 
having a glass-transition temperature of less than about -20 C. It is 
presently preferred to employ as monomers 2-ethyl hexyl acrylate and 
methacrylic acid in which the methacrylic acid content is from 0 to about 
5 percent by weight of the monomers. Methacrylic acid is particularly 
preferred when the medium is ethylene glycol since it is relatively more 
soluble in 2-ethyl hexyl acrylate compared to the other acids, helps speed 
up the reaction, and a substantial portion copolymerizes with 2-ethyl 
hexyl acrylate. 
Polymerization preferably occurs in the presence of a monomer soluble 
initiator such as benzoyl peroxide, chloro methyl benzoyl peroxide, 
lauroyl peroxide, decanoyl peroxide and the like. The concentration of the 
initiator is from about 0.15 to about 0.5 percent by weight of the 
monomers preferably about 0.25 percent by weight of the monomers. Benzoyl 
peroxide is presently preferred. Autogenous reaction conditions are 
required for peroxide initiators. 
Actinic radiation and electron beam radiation may also be used to initiate 
the polymerization process. 
A suspension stabilizer is required to prevent coalescence of the polymer 
particles formed during polymerization. A suspension stabilizer may be an 
anionic suspension stabilizer such as polyacrylic acid, polymethacrylic 
acid, copolymers of acrylic or methacrylic acids with acrylamide, vinyl 
pyrrolidone or dimethyl amino ethyl methacrylate and the like, and 
nonionic suspension stabilizer such as polyvinyl pyrrolidone, partially 
hydrolyzed polyvinyl acetate, and cationic stabilizers such as quaternized 
polydimethyl aminoethyl methacrylate and the like, and amphoteric 
stabilizers such as quaternized copolymers of acrylic acid and 
dimethylaminoethyl methacrylate and the like, as well as mixtures thereof. 
The suspension stabilizer must be soluble in the organic medium but 
substantially insoluble in the monomers. Concentration of suspension 
stabilizer typically employed is from about 3 to about 15 percent by 
weight of the monomers preferably from about 7 to 8 percent by weight of 
the monomer. 
A surfactant is required in addition to the stabilizer for optimum 
stability in a concentration which may or may not be above its critical 
micelle concentration. Typical concentrations range from about 0.25 to 
about 3 percent by weight preferably about 1 percent by weight of the 
monomers when the medium is ethylene glycol. Anionic surfactants are 
preferred for anionic and nonionic stabilizers and cationic surfactants 
for cationic suspension stabilizers. 
Typical anionic surfactants that can be used are sulfosuccinates and alkyl 
aryl polyether sulfonates. Sulfosuccinates include sodium dioctyl 
sulfosuccinate (Aerosol OT, manufactured by American Cyanamid) and sodium 
dihexyl sulfosuccinate (Aerosol MA, manufactured by American Cyanamid), 
sodium alkyl aryl polyether sulfonates (Triton X-200, manufactured by Rohm 
and Haas) and sodium alkyl benzene sulfonate such as sodium dodecyl 
benzene sulfonate (Siponate DS-10, manufactured by Alcolac). Nonionic 
surfactants that can be used are alkyl arylpolyether alcohols (Triton 
N-111, manufactured by Rohm & Haas) and the like, and these are preferably 
used in combination with anionic surfactants. Cationic surfactants of the 
type cetyl trimethyl ammonium bromide can be used instead of anionic 
surfactants in combination with cationic stabilizers. 
pH will range from about 4 to about 7.5 for anionic stabilizers or 
combinations with nonionics and a pH range of 4 to 5 is preferred for 
nonionic suspension stabilizers. 
Some degree of internal polymer cross-linking is required for cohesive 
strength and to achieve infusibility. One way to achieve this is by 
hydrogen abstraction using a peroxide initiator. Another way is to employ 
a multifunctional additive such as multifunctional acrylate, trialyl 
cyanurate and the like during polymerization to allow cross-linking 
reactions to occur to control gel content. As the gel content is 
increased, the modulus of the polymer increases as well. A low modulus is 
desired to get quick wetting and bond formation on surfaces on which the 
label is applied. Hence, an optimum balance between gel content and 
modulus is necessary to get good performance characteristics. Cationic 
and/or amphoteric comonomers can be used to change the specific adhesion 
characteristics to certain substrates. Amphoteric monomers include 
betaines such as 1-(3-sulphopropyl)-2-vinyl pyridinium betaine and the 
like. 
Water need not be present but can be tolerated so long as the water content 
does not cause curl upon application of the product to a paper substrate. 
Low levels are desirable. 
Shear, as induced by agitation, is required and is used effectively to 
control particle size. It is presently preferred that sufficient shear be 
induced to provide a particle size smaller than about 200 microns, 
preferably smaller than about 90 microns. When the level of shear is too 
high, there is tendency for the formed particles to be so fine that on 
application to a substrate at moderate coat weights it will perform like a 
continuous film. Such a film would show low adhesion to rough surfaces 
which is not desirable. If shear is too low, particles of too great a size 
will be formed and tend to be too aggressive due to the high peel force 
per point of contact and increse the probability, for products removable 
from paper, of inducing fiber pick or paper tear. Preferably shear rates 
sufficient to provide particles smaller than about 200 microns should be 
used. These particles when applied to a substrate will give a 
discontinuous pattern having peel values such as indicated in Table I 
below. 
Salts such as sodium chloride and lithium chloride, which are soluble in 
the continuous organic phase, can be effectively used to reduce solubility 
of monomers in the medium, to control viscosity of the medium and in 
combination with shear also control particle size. Salts are typically 
present in levels 0.5 to 10% by weight of the monomers. 
Gel content, as determined by extraction with tetrahydrofuran, can range 
from 60 to 80 percent by weight of the polymer preferably about 65 to 
about 75 percent. 
Polymerization in the organic medium eliminates the need to recover the 
microspheres from the medium particularly when the microspheres are 
applied to a paper substrate. It is because the organic medium will not 
cause curl of a paper substrate. When ethylene glycol or glycerol is the 
medium, their boiling points are too high for rapid evaporation. 
Accordingly, it is desirable for the production of removable and 
repositionable products having a paper substrate to transfer the 
microspheres to a more volatile medium such as heptane for coating onto 
paper. Uniquely, there is no need to recover the particles from the medium 
in which they are formed. Instead, gravity separation or centrifugation 
can be used to remove the bulk of the medium in which the particles are 
prepared and the particles and residue of the medium simply incorporated 
into a more volatile organic medium. There is no need to recover the 
stabilizer or emulsifier and the amount of volatile medium added is 
sufficient to insure that upon evaporation a discontinuous coat will 
result on the substrate. Further centrifuging of the microspheres 
dispersed in the volatile organic medium will substantially eliminate the 
ethylene glycol or glycerol, thus facilitating the drying at low 
temperatures. A discontinuous coating can be achieved in part by swelling 
of the polymer particles in the medium in which they are prepared and/or 
dispersed which insures a separation of particles from each other upon 
evaporation of the medium and shrinkage of the particles. In any event, 
particles will remain discrete and will not fuse even at higher solids 
concentrations due to the fact that each particle is internally 
cross-linked and is incapable of continuous film formation. 
To achieve a removable and repositionable product, it is preferably 
desirable to have a solids content in the medium in which the particles 
are dispersed, which is inversely proportional to particle size so as to 
limit the area of the particles available for contact with a surface to 
which the net product is applied. Larger particles show a higher tendency 
to transfer onto substrates after substantial dwell times compared to 
smaller particles due to the fact that contact area and specific adhesion 
per particle is higher. 
The adhesive particles may be applied directly to paper and other 
substrates. Priming may be desirably used to improve anchorage to the 
paper and minimize transfer of microspheres to substrates. For 
microspheres prepared from a copolymer of 2-ethyl hexyl acrylate and 
methacrylic acid, a particularly useful primer is zinc oxide in an acrylic 
base polymer. This is coated on the paper and dried before coating the 
dispersion of microspheres. The particles will adhere better to such a 
primed surface due to the interaction between the acid groups present in 
the polymer and Zn.sup.+2 present in the primer. The reduction in transfer 
of the microspheres coated on both unprimed and primed paper onto a glass 
mirror and polished stainless steel plate is shown in Table II below. 
Products produced in accordance with the present invention generally 
comprise discontinuous coat of adhesive microspheres on at least a portion 
of at least one side of the carrier material and present in an amount to 
provide, in the zone bounded or defined by the adhesive, from about 10% to 
about 30%, preferably from about 15% to about 25%, of the adhesive present 
in the pattern, available for contact with a smooth substrate, such as 
stainless steel or glass, relative to the amount of adhesive which would 
have been present if the adhesive were applied as a continuous film. To 
achieve this level for effective contact, from about 30% to about 75% of 
the zone which would have been occupied by a continuous film, is covered 
by pressure-sensitive microspheres. The segments have an average height of 
at least from about 15 microns, preferably at least about 20 microns to 
account for the roughness of the face material and the surface to which 
the product is to be applied.

The following examples are illustrative but no wise limiting of the instant 
invention. 
EXAMPLE 1 
One gram of lithium chloride was dissolved in 156 grams of ethylene glycol 
(over 99% pure) contained in a reactor having a diameter of 4.25 inches 
and equipped with a stirrer having two pitched turbine blades, one at the 
bottom being 3.5 inches in diameter and the other 3 inches in diameter 
placed 0.5 inch above the first blade which was 0.75 inch from the bottom 
of the reactor. To this mixture there was added 4.14 grams of solid 
polyacrylic acid provided by Monomer Polymer Labs and designated as 
Product No. 8878. The polyacrylic acid was dissolved with good agitation 
while a 25 percent solution of sodium hydroxide was added to maintain the 
pH at 6.5. There were separately mixed 49 grams of 2-ethyl hexyl acrylate, 
1 gram of methacrylic acid and 0.125 gram of benzoyl peroxide. This 
mixture was slowly added to the reactor. The reactor was evacuated and 
repressurized with nitrogen. There was then added to the reactor 1.79 
grams of a 28 percent solution of Triton X-200 in 5 grams of ethylene 
glycol with agitation (400 rpm). The reactor was heated to 65.degree. C. 
and maintained at this temperature with agitation (400 rpm) for 4.5 hours 
from the onset of an exotherm and then allowed to cool. There were formed 
polymeric microspheres having particles less than 90 microns in diameter. 
The mixture was allowed to settle with the polymer microspheres floating 
to the top. The gel content of the microspheres as determined by 
tetrahydrofuran extraction for 48 hours was found to be 70.5%. The bulk of 
the ethylene glycol was drained off and replaced by heptane to a solids 
content of about 11% and applied to paper as a coating. The paper did not 
curl and there was a left a removable and repositionable product free of 
paper curl. The typical adhesion values of microspheres is shown in Table 
I. 
EXAMPLE 2 
Following the procedure of Example 1 there was formed a mixture of 10 grams 
sodium chloride, 215 grams ethylene glycol, and 72 grams Acrysol HV-1 (a 
ten percent solution of a polyacrylic acid in water). And to this there 
was added with agitation a solution of 99 grams of 2-ethyl hexyl acrylate, 
1 gram methacrylic acid and 0.25 gram benzoyl peroxide, the reaction 
mixture was agitated at 400 rpm under nitrogen purge after the addition of 
the mixture of 2-ethyl hexyl acrylate, methacrylic acid and benzoyl 
peroxide. There was then added to the reactor 3.57 g of 28% solution of 
Triton X-200 in 8.0 grams of ethylene glycol. After 4.5 hours of reaction 
at 65.degree. C., there were formed tacky microsphere particles having a 
particle size of less than about 90 microns. The gel content of the 
microspheres as determined by extraction with tetrahydrofuran for 48 hours 
was found to be 74%. The mixture was allowed settle, the bulk of the 
ethylene glycol drained off and the residue transferred heptane to a 
solids content of about 10%. When applied to canary yellow paper, there 
was formed without curl a repositionable note paper product. The typical 
adhesion values of these microspheres is shown in Table I. 
Table I compares the performance of the polymers of Example 1 to Example 2 
as a function of solids content in heptane. The product of Example 1 has 
better adhesion to the paper face stock. Table II compares microsphere 
transfer of the product of Example 2 as a function of primer used. 
TABLE 1 
______________________________________ 
% SOLIDS COAT TLMI 
POLY- PA- (IN WEIGHT PEEL 
MER PER HEPTANE) g/sq. m N/M COMMENT 
______________________________________ 
EX. 1 UP 8 7.3 57.6 VLPS,NT 
EX. 1 UP 11 9.5 63.0 VLPS,NT 
EX. 1 UP 11 15.4 85.7 VLPS,NT 
EX. 1 UP 11 18.9 92.0 VLPS,NT 
EX. 2 UP 10 9.5 85.1 T 
EX. 2 UP 10 13.4 135.1 T 
______________________________________ 
UP = Unprimed paper 
VLPS = Very light panel stain 
NT = No transfer of microspheres onto SS panel 
T = Transfer of microspheres onto SS panel 
SS = Stainless steel 
TLMI PEEL = 4.5 lb roll, 20' dwell, 300"/min.peel rate, SS 
Example 1 particles were &lt;90 micron diameter as polymerized 
Example 2 particles were &gt;90 micron diameter as polymerized 
TABLE II 
______________________________________ 
% TRANSFER % TRANSFER 
POLY- TO MIRROR TO SS 
MER PAPER 24 HR. 10 DAYS 24 HR. 
10 DAYS 
______________________________________ 
EX. 2 UP 5-7 8-9 3-6 7-8 
EX. 2 P-1 0.6-1.8 1.5-3.0 0.6-1.8 
0.9-1.5 
EX. 2 P-2 0.03 0.12 0.05 0.05 
______________________________________ 
UP = Unprimed paper 
P-1 = Primed paper (Acryloid C10LV resin only. No ZnO in primer) 
P-2 = Primed paper (80% ZnO in Acryloid C10LV resin) 
SS = Stainless steel 
10% dispersion of microspheres in heptane coated onto paper. Particle 
population count per square cm. was determined under low magnification. 
Slips of the coated paper were applied on polished stainless steel plate 
and on glass mirror. The transfer of the particles on the two substrates 
was determined as percentage of the original population, after 24 Hrs. an 
10 days 
EXAMPLE 3 
The procedure of Example 1 was repeated except that agitation was increased 
to 620 rpm. The formed microspheres had a particle size less than about 70 
microns. 
EXAMPLE 4 
The procedure of Example 1 was repeated except the agitation was reduced to 
150 rpm. Microsphere size was less than about 180 microns. 
EXAMPLE 5 
Following the procedure of Example 1, 4.14 grams of polyacrylic acid was 
dissolved in 195 grams of glycerol contained in a reactor. A 25 percent 
solution of sodium hydroxide was added to maintain a pH at 6.7. There was 
separately mixed 1 gram of lithium chloride and 2 grams of deionized water 
in 9 grams of glycerol, and 49 grams of 2-ethyl hexyl acrylate, 2 grams of 
methacrylic acid and 0.125 gram of benzoyl peroxide. They were slowly 
added to the reactor. The reactor was evacuated and repressurized with 
nitrogen. There was then added to the reactor 1.79 grams of a 28% solution 
of Triton X-200 in 5 grams of glycerol. The reactor was heated to 
65.degree. C. and maintained this temperatue with agitation (400 rpm) for 
4.5 hours from the onset of an exotherm and then allowed to cool. There 
were formed polymeric microspheres having a particle size less than 50 
microns.