Method for producing polyacrylates

Details Method for producing polyacrylates Method for producing polyacrylates

2002-05-10

2004-07-20

Zalukaeva, Tatyana (Department: 1713)

Synthetic resins or natural rubbers -- part of the class 520 ser

Synthetic resins

Polymers from only ethylenic monomers or processes of...

C526S219600, C526S227000, C526S229000, C526S309000, C526S323200, C526S328500

Reexamination Certificate

active

06765078

ABSTRACT:

The invention relates to a process for preparing polyacrylates having an average molecular weight of between 250,000 g/mol and 1,000,000 g/mol and a narrow molecular weight distribution.
Owing to ongoing technological developments in the coating process, there is a continuing demand for new developments in the field of pressure sensitive adhesives (PSAs). In industry, hotmelt processes with solvent-free coating technology for preparing PSAs are of growing significance, since the environmental strictures are becoming ever greater and the prices of solvents are rising. Hotmelt processes are already state of the art for SIS adhesives. In contrast, acrylic PSAs are still processed largely from solution. In this respect, an excessive average molecular weight continues to present problems, since, although it is essential for high shear strength, it causes a sharp rise in the flow viscosity, so that acrylic hotmelts with an average molecular weight of >1,000,000 g/mol are difficult to process from the melt.
On the other hand, low molecular weight acrylic hotmelts have already been successfully implemented as hotmelt PSAs (BASF AG, e.g. UV 203 AC resins). Here, benzophenone derivatives or acetophenone derivatives are incorporated as acrylated photoinitiators into the acrylic polymer chain and then crosslinked with UV radiation [U.S. Pat. No. 5,073,611]. Nevertheless, the obtainable shear strength with such systems is still not satisfactory, although as a result of the low average molecular weight (≈250,000 g/mol) the flow viscosity is relatively low.
The preparation of relatively high molecular weight acrylic PSAs (average molecular weight between 250,000 g/mol and 1,000,000 g/mol) requires specific polymerization processes. Polymerization cannot be carried out without solvent, since at a certain point in time the flow viscosity becomes too high and the conversion of the reaction is very low. The residual monomers would disrupt the hotmelt process. Consequently, acrylic monomers are polymerized conventionally in solution and then concentrated in a concentrating extruder [EP 0621 326 B1].
Nevertheless, the concentration of this acrylic PSA causes problems, since for environmental reasons solvent mixtures, such as special boiling point spirits and acetone, for example, are frequently used (state of the art). Toluene is suspected of being carcinogenic and is therefore no longer utilized. A mixture of solvents leads to the absence of a continuous boiling point in the concentration process, with the consequence that it is very difficult to remove the solvent to a fraction of less than 0.5% (percent by weight based on the polymer). Attempts are therefore made to polymerize acrylates in only one solvent and with one regulator. The regulator meets the functions of avoiding gelling, lowering the average molecular weight, absorbing the heat given off in the initiation phase, lowering the molecular weight distribution, and yet ensuring a high conversion.
The regulators used are generally thiols, alcohols or halides, such as carbon tetrabromide, for example [cf., for example, H.-G. Elias, “Makromoleküle”, Hüthig & Wepf Verlag, Basle, 5th Edition, 1990]. The use of halide regulators is decreasing persistently, however, on environmental grounds. Thiols and alcohols are suitable as regulators and, depending on concentration, greatly reduce the average molecular weight of the polymer but lead to a marked broadening of the molecular weight distribution. This is undesirable for acrylic PSAs, since polyacrylates with too low a molecular weight sharply reduce the cohesion and polyacrylates with a very high molecular weight make the melt viscosity a hindering factor in processing as a hotmelt.
In recent years, in contrast, a new polymerization process has been developed which makes it possible to prepare a large number of polymers with a very narrow molecular weight distribution (Macromolecules, 1999, 32, 5457-5459; WO 98/01478). The polymers described therein, however, all have a low average molecular weight (<200,000 g/mol). Moreover, in all cases the conversion is well below 90%. Both the residual monomer fraction and the low average molecular weight rule out use in the hotmelt process and use as PSAs.
It is an object of the invention to provide a process for preparing polyacrylate compositions of sufficiently high average molecular weights to be used as pressure sensitive adhesives, yet retaining the capacity for the processing of the hotmelt process, by achieving high conversion in the polymerization with a narrow molecular weight distribution.
This object is achieved, surprisingly and unforeseeably for the skilled worker, by a process as described in the main claim. The subclaims describe further developments of said process and applications of the polyacrylates prepared by said process.
Claim
1
relates accordingly to a process for preparing polyacrylates wherein the monomer mixture for preparing the polyacrylates is composed of at least 70% by weight of at least one acrylic monomer of the general formula
where R
1
is H or CH
3
and R
2
is H or an alkyl chain having 1-20 carbon atoms, the monomers are polymerized in the presence of at least one free-radical initiator by free-radical polymerization with at least one thioester as polymerization regulator, and the polymerization is conducted such that the average molecular weight of the polyacrylates is in the range from 250,000 g/mol to 1,000,000 g/mol, and the molecular weight distribution, M
w
/M
n
, is <4.
As free-radical initiators for the free-radical polymerization it is possible to use any customary initiators known for this purpose for acrylates. The preparation of C-centered radicals is described in Houben Weyl, Methoden der Organischen Chemie, Vol. E 19a, pp. 60-147. These methods may be employed analogously. Examples of radical sources are peroxides, hydroperoxides, and azo compounds; as nonexclusive examples of typical radical initiators mention may be made here of potassium peroxodisulfate, dibenzoyl peroxide, cumin hydroperoxide, cyclohexanone peroxide, di-t-butyl peroxide, azodiisobutyronitrile, cyclohexylsulfonyl acetyl peroxide, diisopropyl percarbonate, t-butyl peroctoate, and benzpinacol. In one very preferred variant, the initiators are added in a number of stages, so that the conversion is increased to more than 90%. The residual monomer content of the polymer can in this way be decreased to below 10% by weight; by virtue of a low residual monomer content, the properties of the polyacrylate are considerably improved in respect of its further processing by the hotmelt process.
The initiators added at the beginning are preferably chosen for their low propensity to form side chains in the polymers; their grafting activity is preferably below a level of &egr;<5 at the temperature of the reaction mixture when the initiator is added.
The absolute grafting activity (crosslink efficiency) is defined as the number of chemical side chains formed per 100 mol units of decomposed initiator. In analogy to van Drumpt and Oosterwijk [Journal of Polymer Science, Polymer Chemistry Edition 14 (1976) 1495-1511], it is possible to specify a value for this number by determining the dimers in a defined solution of the initiator; see also DE 43 40 297 A1:
A precisely 0.1 molar solution of the initiator is decomposed in n-pentadecane under an He atmosphere. The reaction time is chosen to correspond to ten times the half life of the respective initiator at the chosen temperature. This ensures virtually complete decomposition of the initiator. Subsequently, the fraction of dimeric pentadecane produced is measured by means of GLC. The percentage fraction &egr; is stated as a measure of the grafting activity. The reaction temperature is normally chosen so that the half life of the test initiator at this temperature is 15 minutes.
High &egr; values for the grafting activity imply a high propensity of the initiator to form side chains in the polymerization, whereas small &egr; values result in prefere

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