Emulsions based on acrylic polymers

This invention relates to aqueous emulsions based on uncured and uncurable acrylic polymers and their use for the forming process of the cathode ray tube comprising as comonomers the following components: PA0 A) butylmethacrylate from 45-70 parts by weight PA0 B) methylmethacrylate from 20-45 parts by weight PA0 C) methacrylic acid from 1-4 parts by weight PA0 D) ethylacrylate from 5-9 parts by weight having a minimum filming temperature such as to produce film between about 20.degree. and 40.degree. C., with or without the use of coalescent agents, the nitrogen thermal cycle which simulates that utilized in the CRT process, being the following: PA0 1.degree. scanning: from 25.degree. C. to 375.degree. C. at 11.degree. C./min. PA0 2.degree. scanning: from 75.degree. C. to 440.degree. C. at 5.degree. C./min. PA0 3.degree. scanning: isotherm at 440.degree. C. for 45 minutes, the organic residue after said thermal treatment being lower than 0.05% by weight, preferably 0.01%.

FIELD OF THE INVENTION 
The present invention relates to emulsions based on acrylic polymers. In 
particular the emulsions of the invention are suitable for preparing 
metallized luminescent screens for cathode ray tubes. 
More specifically the invention relates to the use of special emulsions 
based on acrylic polymers as single layer which can be volatilized by 
depolymerization without combustion with slight organic residue 
concurrently with the panel sealing to the CRT funnel (cathode ray tube). 
BACKGROUND OF THE INVENTION 
Filming methods utilizing solvent-based filming lakes are known, which, 
when deposited on a humid phosphor layer, produce a thin and continuous 
film on which a metal layer condenses by sputtering. 
Such processes produce handmade articles having good characteristics, 
however they have the drawback of requiring the employment of solvent 
which must be subsequently evaporated and recovered to avoid environmental 
pollutions. The production plants need therefore units for the solvent 
recovery. 
For this reason films deriving from drying of aqueous emulsions have been 
studied since long. 
On the panel carrying the screen, the aqueous emulsion deposits, which is 
dried by heating. 
The purpose of this polymeric film is to remove the irregular surface of 
the phosphor particles forming the screen, and to provide a very uniform 
sublayer on which the sputtered metal then deposits in order to form a 
surface like a mirror to reflect the screen light. 
Thanks to this polymeric film the metal does not penetrate into the screen 
below even though the phosphor particles can jut outside the film. 
The screen then undergoes a treatment at high temperature from 
400.degree.-450.degree. C. (the so called baked-out) to remove the 
polymeric film. The gaseous products of the polymeric film decomposition 
come out of the holes of the metal layer. 
A drawback of this process consists in that the carbon residues deriving 
from the film decomposition lead to an insufficient gloss of the screen. 
These processes require therefore an additional process of slow prebaking 
to completely remove all the organic materials. 
Processes wherein sufficient amounts of air are used during baking to 
volatilize the acrylic polymer, are also known. See for instance U.S. Pat. 
Nos. 4,217,015 and 4,350,514. 
This needs process changes for introducing air, leading to more expensive 
and complex processes. 
Aqueous emulsions based on acrylic polymers capable of forming films for 
preparing cathode tubes are described in U.S. Pat. No. 5,145,511. It is a 
process for manufacturing cathode tubes with only one step of film removal 
deriving from the emulsion concurrently with the sealing of the cathode 
tube panel. 
To this purpose it must be noticed that any filming acrylic emulsion cannot 
be employed in the process of the patent indicated from an industrial 
point of view. 
In fact tests carried out by the Applicant (see the examples hereafter) 
have shown that the emulsions based on acrylic esters utilized in said 
patent do not succeed in meeting industrial requirements, i.e., in useful 
times, compared with conventional processes. The necessary times for 
having the film decomposition are high. As a matter of fact said tests 
have shown that the decomposition of said acrylic polymers leaves 
considerable residues, from about 0.5-2% by weight which are detrimental 
for the screens gloss.

DESCRIPTION OF THE INVENTION 
The Applicant has unexpectedly found that it is possible to overcome the 
drawbacks of the processes of the prior art both in terms of film 
decomposition times and in terms of image gloss, utilizing a particular 
aqueous emulsion based on acrylic polymers which allows to carry out a 
film containing phosphors which has the following combination of 
properties: 
the film can be volatilized simultaneously during the sealing of the panel 
to the CRT funnel; 
the film volatilizes by depolymerization without leaving organic residues 
or in any case lower than 0.05% by weight; 
the film is volatilized in absence of oxygen allowing to eliminate the 
additional equipments for the introduction of air rich in oxygen; 
the film decomposes at a temperature lower than the melting temperature of 
the "frit", meaning by "frit" the complex composition formed by the 
emulsion, phosphors and other components necessary for depositing the 
material layer which has to be activated in the cathode tube; 
the film decomposes without forming bubbles which would cause the 
separation of the aluminum film (the so called blistering phenomenon); 
the absence of residue or amounts lower than 0.05%, preferably lower than 
0.01% by weight, produces a considerable improvement of the luminescent 
layer gloss with consequent higher yields in the image definition of the 
cathode tube. 
Object of the invention is an aqueous emulsion based on uncured and 
uncurable acrylic polymers and having filming temperatures between 
20.degree. and 40.degree. C. during the forming process of the cathode ray 
tube, having the following composition: 
A) butylmethacrylate from 45-70 parts by weight 
B) methylmethacrylate from 20-45 parts by weight 
C) methacrylic acid from 1-4 parts by weight 
D) ethylacrylate from 5-9 parts by weight. 
The A, B, C and D percentages are intended as starting comonomers utilized 
in the process for preparing the emulsion. 
These are the preferred monomers. 
As component A) instead of butylmethacrylate, lauroyl-, decyl-, undecyl-, 
ethyl-, propyl-, ethylhexylmethacrylate, etc., can for instance be used. 
As component C) instead of methacrylic acid, other polymerizable carboxylic 
acids, such as acrylic acid, itaconic acid, etc. can be used. Instead of 
D) methylacrylate can be used considering the relative molecular weight 
and the different filming temperature of the polymers containing this 
comonomer. 
The ratio between the component A) and B) must be such as to give a minimum 
filming temperature between about 20.degree. and 40.degree. C. 
The aqueous emulsion object of the patent gives linear uncured and 
uncurable polymers during the thermal cycle, as defined below, to which 
the film is subjected during the application leading to the CRT formation, 
wherefore also under this point of view no carbon residues are produced. 
The minimum filming temperature (TMF) of the emulsion must be such as to 
produce the film between about 20.degree. and 40.degree. C. with or 
without the use of coalescent agents. 
These will be added in case the composition of the monomers is such as to 
produce a higher TMF. 
In this case amounts of the order of some per cent by weight, generally 
from 3 to 7% by weight, as dibutylglycol diacetate (DBGA) can be added 
without substantially changing the emulsion properties. 
Other coalescent agents are for instance 
propyleneglycolmonomethylethylether, etc. 
Just as an indication the glass transition temperature (Tg) can be used in 
the on-set point as filming index. 
The Tg is generally comprised from 40.degree. to 65.degree. C. 
It is preferable that the distribution of the particles diameter is of 
monomodal type, of sizes generally from 100 to 200 nm, excluding coarse 
particles or particles aggregates leading to a bimodal distribution. 
Preferably sizes are from 120 to 170 nm. 
The polydispersion of the particles sizes is narrow, generally lower than 
0.1. 
The weight average molecular weights (Mw) are generally from 100,000 to 
500,000 and the distribution of the molecular, weights Mw/Mn (Mn number 
average molecular weight) is rather wide, generally from 3.5 to 6. 
The process for preparing the emulsion of the invention is a conventional 
process, preferably in semi-batch, which utilizes surfactants and 
initiators known in the art of emulsions. 
As surfactants, sulphonated paraffines, alkylbenzenesulphonates, 
nonylphenols ethoxylates, etc., can be mentioned. 
As initiators ammonium, sodium, or potassium persulphates; tertiary butyl 
hydroperoxide, cumene hydroperoxide can be mentioned. 
The final step of the reaction can be formed by a redox treatment to remove 
the polymerization residues by using the normal catalytic couples, such as 
for instance sodium formaldehyde sulphoxylate and terbutylhydroperoxide. 
The polymers having an acrylic basis obtained from the emulsion are linear 
and uncured. 
A process for preparing the emulsion is described in detail in Example 1. 
The properties, charateristic of the emulsions of the invention are 
measurable by means of chemical-physical measurements as reported in 
Example 1. In particular it is measured the amount of residue at the end 
of the thermal cycle specified below which simulates that utilized for 
preparing CRT. 
The thermal cycle carried out in inert ambient, for instance nitrogen, is 
the following: 
1.degree. scanning: from 25.degree. C. to 375.degree. C. at 11.degree. 
C./min. 
2.degree. scanning: from 375.degree. C. to 440.degree. C. at 5.degree. 
C./min. 
3.degree. scanning: isotherm at 440.degree. C. for 45 minutes. 
EXAMPLES 
For illustrative but not limitative purposes, some exemplifying examples of 
the present invention are reported herein. 
Example 1 
Preparation of the Emulsion 
An emulsion having the following composition in % by weight is prepared: 
______________________________________ 
Butylmethacrylate (MABU) 
50 
Methylmethacrylate (MMA) 
41 
Ethylacrylate (AE) 7 
Methacrylic acid (AMA) 
2 
______________________________________ 
In a 12 m.sup.3 reactor equipped with stirrer, reflux refrigerant connected 
with a vacuum pump and with a nitrogen feeding, the reactor is made inert 
with two vacuum operations and nitrogen flow, then a bottom charge is 
introduced (pt. 2, Tab. 1). 
The internal temperature of the reactor is brought to 78.degree. C. by 
means of a thermostatic bath. 
Then a part of the preemulsion (200 kg) indicated in pt. 1, Tab. 1, is 
started to be added. 
After 5 minutes the solution of initiator is added. 
When the internal temperature reaches 84.degree. C., the second part of the 
preemulsion starts to be fed. 
The feeding of the preemulsion occurs in about two hours keeping the 
temperature constant. 
Finally the preemulsifier is washed with 100 kg of demineralized water (pt. 
4, Tab. 1); it is cooled, the reaction is completed for one hour at the 
same temperature. Then the reactor is cooled to 50.degree. C. (pt. 5, Tab. 
1) and the redox system is added to complete the removal of the residual 
monomers. 
The redox mixture is reported in pt. 6, Tab. 1 and the reaction is carried 
out for 30 minutes. At the end, at the temperature of about 40.degree. C., 
DBGA (dibutylglycolacetate) is added, 5 parts by weight per 100 parts by 
weight of the total weight of the A+B+C+D monomers forming the emulsion 
copolymer. 
Then the temperature is brought to 35.degree. C. adding NH.sub.3 in a 28% 
aqueous solution until a pH 7.5-8 is reached, finally an antimicrobic 
Kathon LXE of Rohm & Haas dissolved in demineralized water is added. 
TABLE 1 
______________________________________ 
LOAD AMOUNT (kg) 8000 
COMPONENT WEIGHT KG 
______________________________________ 
1) PREEMULSION 
Demineralized H.sub.2 O 
1470 
EMULGATOR K30 (40%) (sodic alkyl- 
22 
sulphonate) 
Butylmethacrylate 1850 (50% by wt.) 
Methylmethacrylate 1516 (41% by wt.) 
Ethylacrylate 258 (7% by wt.) 
Methacrylic acid (in 75% solution) 
100 (2% by wt.) 
TOTAL 1 5216 
2) BOTTOM LOAD 
Demineralized H.sub.2 O 
2340 
EMULGATOR K30 (40%) 25 
3) REACTION INITIATION IN NITROGEN FLOW 
The internal temperature of the reactor is brought to 78-79.degree. C., 
then the 
preemulsion is added and 5 minutes later the initiator solution 
(ammonium 
persulphate) 
Preemulsion 200 
Ammonium persulphate 21 
Demineralized H.sub.2 O 
130 
At the reaction peak (towards 83-84.degree. C.), feeding of the 
preemulsion is 
started. 
4) FEEDING IN NITROGEN FLOW 
Duration: 2 hours 
Temperature: 82-84.degree. C. 
After the two feeding hours the preemulsifier and the line with 100 kg 
of 
demineralized water are cleaned 
Preemulsion 5016 (41.8 kg/min) 
H.sub.2 O preemulsifier washing 
100 
5) THERMAL COMPLETION 
Duration: 60 minutes 
Temperature: 82.degree. C. 
The reactor inside is cooled up to 50.degree. C. 
6) REDOX COMPLETION 
Duration: 30 minutes 
Temperature: 50.degree. C. 
FORMOPON (4%) (sulphoxylate of sodium 
75 
formaldehyde) 
7) FINAL ADDITION AT 35.degree. C. 
NH.sub.4 OH (28%) up to pH 7.5-8 
34 
KATHON LXE 8 
Demineralized H.sub.2 O 
8 
______________________________________ 
Emulsion Characterization 
The emulsion was characterized as follows: 
the pH, the particles size and their distribution, the Tg (glass transition 
temperature) were determined. The methods utilized are the following: 
Determination of the Particles Size 
A Counter Nanosizer N4MD was utilized with the following modalities. 
In a 4.5 ml cuvette in acrylic material, having an average transparence of 
70% at 340 nm, about 4 ml of deionized water and an emulsion portion are 
introduced to be examined in such a way as to reach a particles 
concentration changeable from 1 to 2000 .mu.g/ml, depending on the 
particles size. 
Considering the temperature (20.degree. C.), viscosity and refraction index 
parameters of the suspending agent (1.33 of the deionized water), the 
cuvette is positioned in the proper place and it is let thermostate for 
about 10 minutes. 
After thermostating one proceeds to determine the particles diameter and 
standard deviation according to the use manual of the instrument: Manual 
of the Coulter Electronics 1985-1986, edit. Technical Communication. 
Glass Transition (Tg), Organic Residues, Distribution of the Molecular 
Weights 
The other tests for determining the glass transition, the organic residue, 
the distribution of the molecular weights, are not carried out on the 
emulsion but on the dried product working as follows. 
The emulsion is poured into an aluminium support having a diameter of 5 cm 
and depth of 0.7 cm, so that the thickness is of about 2-3 mm, it is put 
in stove at 70.degree. C. for 12 hours to dry the product. A film having 
an 1.5 mm thickness adhering to the support is obtained. The film is 
punched to obtain disks having a diameter of about 4 mm. 
Determination of the Glass Transition 
A DSC (differential scanning calorimeter) of Mettler TA30 type was 
utilized. 
About 20-30 mg of product in the form of a disk, obtained as above, are 
introduced in the proper aluminium container, whose lid is pierced. The 
sample is heated at the rate of 20.degree. C. per min. starting from 
-100.degree. C. up to 100.degree. C. The test is carried out at least 
twice on different samples until the onset Tg has a maximum variation 
within 2.degree. C., preferably 1.degree. C. Also the mid-point and 
end-set Tg were measured. 
Determination of Nitrogen Residues 
A thermobalance Mettler M3 was utilized. 
The thermogravimetric analysis is carried out on 20 mg of product in the 
form of a disk, prepared as above. 
The disk is loaded into an alumina pot, the pot is then introduced in the 
thermobalance stove. A nitrogen flow (20 liters/hour) is maintained in the 
stove; before starting heating one awaits 20 minutes so as to allow the 
complete removal of the oxygen present in the room. 
After this period heating is started according to the following thermal 
profile 
1.degree. scanning: from 25.degree. C. to 375.degree. C. at 11.degree. 
C./min 
2.degree. scanning: from 375.degree. C. to 440.degree. C. at 5.degree. 
C./min 
3.degree. scanning: isotherm at 440.degree. C. for 45 minutes. 
When the thermal cycle is over, the organic residues are determined by 
determination of the weight loss. 
Determination of the Distribution of the Molecular Weights 
An equipment Waters 510 is utilized and it is operated as follows. 
About 80 mg of product in the form of a disk are dissolved in 4 cc of 
CHCl.sub.3. 
The so obtained solution is injected into a liquid chromatograph. 
Test Conditions: 
ULTRASTYRAGEL columns 106 105 104 103 (polystyrene cured with 
divinylbenzol). 
Refraction index detector, mobile step CHCl.sub.3, calibration with 
polystyrene. 
The data of the characterization indicated above are reported in Table 2. 
The minimum filming temperature (TMF) was determined according to ASTM 
D2354 standard. 
Example 2 
The emulsion is prepared according to the procedure of Example 1 but using 
the following composition in % by weight: 
______________________________________ 
Butylmethacrylate (MABU) 
65.3 
Methylmethacrylate (MMA) 
25.7 
Ethylacrylate (AE) 7 
Methacrylic acid (AMA) 
2. 
______________________________________ 
Moreover after point 6 of Example 1 no coalescent is added. The 
characterization data are reported in Table 2. 
Example 3 Comparative 
The emulsion is prepared according to the procedure of Example 1 but using 
the following composition in % by weight: 
______________________________________ 
Butylmethacrylate (MABU) 
30 
Methylmethacrylate (MMA) 
61 
Ethylacrylate (AE) 7 
Methacrylic acid (AMA) 
2. 
______________________________________ 
According to the procedure of Example 1, 6 parts by weight of 
dibutylglycolacetate (DBGA) coalescent on the total weight of the monomers 
introduced as specified above, are added. The characterization is reported 
in Table 2. 
Example 4 Comparative 
An aqueous emulsion containing about 46% by weight of a copolymer having an 
acrylic basis and having a pH between 9 and 10 indicated as Solution A) in 
U.S. Pat. No. 5,145,511, from line 61, col. 3, is utilized. 
The organic residue remaining after the thermal cycle defined above is 
about 1.5% by weight. 
Example 5 Comparative 
An aqueous emulsion containing about 38% by weight of a copolymer having an 
acrylic basis and having a pH between 2 and 5 indicated as Solution B) in 
U.S. Pat. No. 5,145,511, of Example 4 at col. 3 from line 67, is utilized. 
The minimum filming temperture is about 41.degree. C. 
The organic residue remaining after the thermal cycle defined above is of 
the same order as in Example 4. 
Example 6 Comparative 
An emulsion was prepared by mixing the two emulsions of Example 4 and 5 in 
volume ratio 1:1. 
The organic residue remaining after the thermal cycle indicated is of the 
same order as in Example 4. 
Although the invention has been described in conjunction with specific 
embodiments, it is evident that many alternatives and variations will be 
apparent to those skilled in the art in light of the foregoing 
description. Accordingly, the invention is intended to embrace all of the 
alternatives and variations that fall within the spirit and scope of the 
appended claims. The above references are hereby incorporated by 
reference. 
TABLE 2 
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3 4 5 6 
(com- 
(com- 
(com- 
(com- 
para- 
para- 
para- 
para- 
Examples 1 2 tive) 
tive) 
tive) 
tive) 
______________________________________ 
Tg (.degree.C.) 
51 54 -- 38 49 46 
(mid-point) 
Particles 
150 124 146 126 66 -- 
average 
diameter (nm) 
Weight Loss 
99.99 99.99 99 98.5 98 98 
.DELTA.P/P 
(%) 
Molecular 
Weight 
Mw 300,000 113,000 -- -- -- -- 
Mn 60,000 20,000 -- -- -- -- 
Mw/Mn 5 5.4 -- -- -- -- 
TMF (.degree.C.) 
30 36 38 -- 41 -- 
______________________________________