Emulsions based on acrylic polymers

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).

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 400-450°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 <b>USP patents 4,217,015</b> and <b>4,350,514</b>.

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 <b>US patent 5,145,511</b>. 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.

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: <ul id="ul0001" list-style="dash" compact="compact"><li>the film can be volatilized simultaneously during the sealing of the panel to the CRT funnel;</li><li>the film volatilizes by depolymerization without leaving organic residues or in any case lower than 0.05% by weight;</li><li>the film is volatilized in absence of oxygen allowing to eliminate the additional equipments for the introduction of air rich in oxygen;</li><li>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;</li><li>the film decomposes without forming bubbles which would cause the separation of the aliminium film (the so called blistering phenomenon);</li><li>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.</li></ul>

Object of the invention is an aqueous emulsion based on uncured and uncurable acrylic polymers and having filming temperatures between 20 and 40°C during the forming process of the cathode ray tube, having the following composition: <ul id="ul0002" list-style="none" compact="compact"><li>A) butylmethacrylate from 45-70 parts by weight</li><li>B) methylmethacrylate from 20-45 parts by weight</li><li>C) methacrylic acid from 1-4 parts by weight</li><li>D) ethylacrylate from 5-9 parts by weight.</li></ul>

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. Insead 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 and 40°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 and 40°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 propylene-glycolmonomethylethylether, 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 to 65°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 <maths id="math0001" num=""><math display="inline"><mrow><mover accent="true"><mrow><mtext>M</mtext></mrow><mo>¯</mo></mover></mrow></math><img file="EP0735008A2_D0001.tif" /></maths>w/<maths id="math0002" num=""><math display="inline"><mrow><mover accent="true"><mrow><mtext>M</mtext></mrow><mo>¯</mo></mover></mrow></math><img file="EP0735008A2_D0002.tif" /></maths>n (<maths id="math0003" num=""><math display="inline"><mrow><mover accent="true"><mrow><mtext>M</mtext></mrow><mo>¯</mo></mover></mrow></math><img file="EP0735008A2_D0003.tif" /></maths>n 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, alkylbenzene-sulphonates, 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: <dl id="dl0001" compact="compact"><dt>1° scanning:</dt><dd>from 25°C to 375°C at 11°C/min.</dd><dt>2° scanning:</dt><dd>from 375°C to 440°C at 5°C/min.</dd><dt>3° scanning:</dt><dd>isotherm at 440°C for 45 minutes.</dd></dl>

For illustrative but not limitative purposes, some exemplifying examples of the present invention are reported herein.

<b>EXAMPLE 1</b>

Preparation of the emulsion

An emulsion having the following composition in % by weight is prepared: <tables id="tabl0001" num="0001"><table frame="all"><tgroup cols="2" colsep="1" rowsep="0"><colspec colnum="1" colname="col1" colwidth="78.75mm" /><colspec colnum="2" colname="col2" colwidth="78.75mm" /><tbody valign="top"><row><entry namest="col1" nameend="col1" align="left">Butylmethacrylate (MABU)</entry><entry namest="col2" nameend="col2" align="right">50</entry></row><row><entry namest="col1" nameend="col1" align="left">Methylmethacrylate (MMA)</entry><entry namest="col2" nameend="col2" align="right">41</entry></row><row><entry namest="col1" nameend="col1" align="left">Ethylacrylate (AE)</entry><entry namest="col2" nameend="col2" align="right">7</entry></row><row rowsep="1"><entry namest="col1" nameend="col1" align="left">Methacrylic acid (AMA)</entry><entry namest="col2" nameend="col2" align="right">2</entry></row></tbody></tgroup></table></tables> In a 12 m³ 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°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°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°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 minuts. At the end, at the temperature of about 40°C, DBGA (dibutylglycolacetate) is added, 5 parts by weight per 100 parts by weight of the total weight of the <maths id="math0004" num=""><math display="inline"><mrow><mtext>A+B+C+D</mtext></mrow></math><img file="EP0735008A2_D0004.tif" /></maths> monomers forming the emulsion copolymer. Then the temperature is brought to 35°C adding NH₃ in a 28% aqueous solution until a pH 7.5-8 is reached, finally an antimicrobic Kathon LXE of Röhm &amp; Haas dissolved in demineralized water is added. <tables id="tabl0002" num="0002"><img file="EP0735008A2_D0005.tif" /></tables><tables id="tabl0003" num="0003"><img file="EP0735008A2_D0006.tif" /></tables>

<b>Emulsion characterization</b>

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 µg/ml, depending on the particles size.

Considering the temperature (20°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°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°C per min. starting from -100°C up to 100°C. The test is carried out at least twice on different samples until the onset Tg has a maximum variation within 2°C, preferably 1°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 <dl id="dl0002" compact="compact"><dt>1° scanning:</dt><dd>from 25°C to 375°C at 11°C/min</dd><dt>2° scanning:</dt><dd>from 375°C to 440°C at 5°C/min</dd><dt>3° scanning:</dt><dd>isotherm at 440°C for 45 minutes.</dd></dl> 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 CHCl₃. 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 CHCl₃, 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.

<b>EXAMPLE 2</b>

The emulsion is prepared according to the procedure of Example 1 but using the following composition in % by weight: <tables id="tabl0004" num="0004"><table frame="all"><tgroup cols="2" colsep="1" rowsep="0"><colspec colnum="1" colname="col1" colwidth="78.75mm" /><colspec colnum="2" colname="col2" colwidth="78.75mm" /><tbody valign="top"><row><entry namest="col1" nameend="col1" align="left">Butylmethacrylate (MABU)</entry><entry namest="col2" nameend="col2" align="char" char=".">65.3</entry></row><row><entry namest="col1" nameend="col1" align="left">Methylmethacrylate (MMA)</entry><entry namest="col2" nameend="col2" align="char" char=".">25.7</entry></row><row><entry namest="col1" nameend="col1" align="left">Ethylacrylate (AE)</entry><entry namest="col2" nameend="col2" align="char" char=".">7</entry></row><row rowsep="1"><entry namest="col1" nameend="col1" align="left">Methacrylic acid (AMA)</entry><entry namest="col2" nameend="col2" align="char" char=".">2.</entry></row></tbody></tgroup></table></tables> Moreover after point 6 of Example 1 no coalescent is added. The characterization data are reported in Table 2.

<b>EXAMPLE 3 comparative</b>

The emulsion is prepared according to the procedure of Example 1 but using the following composition in % by weight: <tables id="tabl0005" num="0005"><table frame="all"><tgroup cols="2" colsep="1" rowsep="0"><colspec colnum="1" colname="col1" colwidth="78.75mm" /><colspec colnum="2" colname="col2" colwidth="78.75mm" /><tbody valign="top"><row><entry namest="col1" nameend="col1" align="left">Butylmethacrylate (MABU)</entry><entry namest="col2" nameend="col2" align="char" char=".">30</entry></row><row><entry namest="col1" nameend="col1" align="left">Methylmethacrylate (MMA)</entry><entry namest="col2" nameend="col2" align="char" char=".">61</entry></row><row><entry namest="col1" nameend="col1" align="left">Ethylacrylate (AE)</entry><entry namest="col2" nameend="col2" align="char" char=".">7</entry></row><row rowsep="1"><entry namest="col1" nameend="col1" align="left">Methacrylic acid (AMA)</entry><entry namest="col2" nameend="col2" align="char" char=".">2.</entry></row></tbody></tgroup></table></tables> 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.

<b>EXAMPLE 4 comparative</b>

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 <b>USP patent 5,145,511</b>, from line 61, col. 3, is utilized. The organic residue remaining after the thermal cycle defined above is about 1.5% by weight.

<b>EXAMPLE 5 comparative</b>

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 <b>USP patent 5,145,511</b>, of Example 4 at col. 3 from line 67, is utilized. The minimum filming temperture is about 41°C. The organic residue remaining after the thermal cycle defined above is of the same order as in Example 4.

<b>EXAMPLE 6 comparative</b>

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. <tables id="tabl0006" num="0006"><img file="EP0735008A2_D0007.tif" /></tables>