U.S. patent application number 12/844055 was filed with the patent office on 2011-02-17 for process for preparing polyacrylates.
This patent application is currently assigned to tesa SE. Invention is credited to Jennifer Beschmann, Alexander Prenzel.
Application Number | 20110040050 12/844055 |
Document ID | / |
Family ID | 43425990 |
Filed Date | 2011-02-17 |
United States Patent
Application |
20110040050 |
Kind Code |
A1 |
Prenzel; Alexander ; et
al. |
February 17, 2011 |
PROCESS FOR PREPARING POLYACRYLATES
Abstract
Use of amino acids as regulator substances in the free-radical
polymerization of monomer mixtures comprising at least 70% by
weight of at least one acrylic and/or methacrylic ester and
reaction solution comprising one or more solvents and one or more
monomers, at least 50% by weight of the solvents being organic
solvents, and at least 70% by weight of the monomers being acrylic
and/or methacrylic esters, with at least one amino acid is being
present.
Inventors: |
Prenzel; Alexander;
(Hamburg, DE) ; Beschmann; Jennifer; (Hamburg,
DE) |
Correspondence
Address: |
GERSTENZANG, WILLIAM C.;NORRIS MCLAUGHLIN & MARCUS, PA
875 THIRD AVE, 8TH FLOOR
NEW YORK
NY
10022
US
|
Assignee: |
tesa SE
Hamburg
DE
|
Family ID: |
43425990 |
Appl. No.: |
12/844055 |
Filed: |
July 27, 2010 |
Current U.S.
Class: |
526/70 ;
252/182.14; 252/182.17; 252/182.23; 526/310; 526/329.7 |
Current CPC
Class: |
C08F 20/18 20130101;
C08K 5/36 20130101; C08F 2/38 20130101; C08F 2/38 20130101; C08F
20/18 20130101 |
Class at
Publication: |
526/70 ;
252/182.23; 252/182.14; 252/182.17; 526/329.7; 526/310 |
International
Class: |
C08F 2/38 20060101
C08F002/38; C09K 3/00 20060101 C09K003/00; C08F 120/18 20060101
C08F120/18 |
Foreign Application Data
Date |
Code |
Application Number |
Aug 12, 2009 |
DE |
10 2009 036 967.8 |
Claims
1. Reaction solution comprising one or more solvents and one or
more monomers, at least 50% by weight of the solvents being organic
solvents, and at least 70% by weight of the monomers being acrylic
and/or methacrylic esters, wherein at least one amino acid is
additionally present.
2. Reaction solution according to claim 1, wherein at least 90% by
weight of the solvents are organic solvents.
3. Reaction solution according to claim 1, wherein the reaction
solution is a homogeneous solution.
4. Reaction solution according to claim 1, wherein the at least one
amino acid has at least one sulphanyl, selanyl and/or hydroxyl
group.
5. Reaction solution according to claim 4, wherein the at least one
sulphanyl, selanyl and/or hydroxyl group is terminal.
6. Reaction solution according to claim 1, wherein said at least
one amino acid is at least one amino acid of a natural or
non-natural .alpha.-amino acid of the following structural formula
##STR00003## where R.sup.1, R.sup.2 and R.sup.3 are selected
independently of one another, with R.sup.1 selected from the group
(i), consisting of (i) linear and branched C.sub.1 to C.sub.18
alkyl radicals, linear and branched C.sub.2 to C.sub.18 alkenyl
radicals, linear and branched C.sub.2 to C.sub.18 alkynyl radicals,
aryl radicals, cycloaliphatic, aliphatic and aromatic heterocycles,
and hydrogen, and R.sup.2 and R.sup.3 are selected from the group
consisting of group (i) or are selected from the from group
consisting of group (ii), (ii) acyl radicals, (--C(O)--R'),
alkoxycarbonyl radicals (--C(O)--O--R'), carbamoyl radicals
(--C(O)--NR'R''), and sulphonyl radicals (--SO.sub.2R'), where R'
and R'' are radicals from group i) that are selected independently
of one another, and X is oxygen (O), sulphur (S) or selenium (Se),
and n is a natural number including zero.
7. Reaction solution according to claim 6, wherein n is in the
range of 0.ltoreq.n.ltoreq.18.
8. Reaction solution according to claim 1, wherein the acrylic
and/or methacrylic ester or esters satisfy the formula ##STR00004##
where R.sub.1=H or CH.sub.3 and R.sub.2=H or an unbranched or
branched, aliphatic, alicyclic or aromatic, unsubstituted or
substituted hydrocarbon radical having 1 to 20 carbon atoms without
C.dbd.C double bonds.
9. A polyacrylate obtained from a reaction solution of claim 1 by
free-radical polymerization of the monomers present therein.
10. Process for preparing a polyacrylate from the reaction solution
of claim 1 by free-radical polymerization, wherein the solvent is
removed from the polymerization product after the polymerization
down to a residual solvent amount of not more than 2% by
weight.
11. Process according to claim 10, wherein the amino acid or acids
remain in the polymerization product when the solvent is
removed.
12. Process according to claim 10 wherein the solvent removed is
supplied to a recycling operation.
13. Process according to claim 10, wherein at least one of the
organic solvents used is obtained from a recycling operation.
14. A process comprising the free-radical polymerization of monomer
mixtures comprising at least 70% by weight of at least one acrylic
and/or methacrylic ester wherein amino acids are present as
regulator substances.
Description
[0001] The invention relates to a process for preparing
polyacrylates by means of free radical polymerization, using
natural or non-natural amino acids containing thiol and/or hydroxyl
groups as non-toxic and non-volatile chain-transfer regulators for
regulating the molar masses and molar mass distribution, allowing
the solvent used in the polymerization, following its removal, to
be reused for further polymerizations in the case of a drying
operation or in the case of the preparation of a polyacrylate
hotmelt, without a change in the molar masses and molar mass
distribution of the polyacrylates as the number of recycling steps
goes up.
BACKGROUND OF THE INVENTION
[0002] 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 also rising.
Hotmelt processes are already state of the art for SIS adhesives.
In contrast, acrylate 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,
and so acrylate hotmelts with an average molecular weight of >1
000 000 g/mol are difficult to process from the melt.
[0003] On the other hand, low molecular weight acrylate 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 an acrylated
photoinitiator into the acrylate polymer chain and are then
crosslinked with UV radiation [U.S. Pat. No. 5,073,611].
Nevertheless, the achievable shear strength for systems of this
kind is still not satisfactory, although, as a result of the low
average molecular weight (.apprxeq.250 000 g/mol), the flow
viscosity is relatively low.
[0004] The preparation of relatively high molecular weight acrylate
PSAs (average molecular weight between 250 000 g/mol and 1 000 000
g/mol) necessitates 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.
[0005] The residual monomers remaining would disrupt the hotmelt
operation. Consequently, acrylate monomers are polymerized
conventionally in solution and then concentrated in a concentrating
extruder [EP 0621 326 B1].
[0006] Nevertheless, the concentration of this acrylate PSA causes
problems, since environmental considerations frequently dictate the
use of solvent mixtures, such as special-boiling-point spirit and
acetone (state of the art). Toluene is suspected of being
carcinogenic, and is therefore no longer used. In a concentrating
operation, a solvent mixture means that there is no continuous
boiling point, and so it is very difficult to remove the solvent
down to a fraction of below 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 fulfils the
functions of avoiding gelling, lowering the average molecular
weight, absorbing the heat given off in the initiation phase,
reducing the molecular weight distribution, and yet ensuring a high
conversion.
[0007] The regulators used are generally thiols, alcohols or
halides, such as carbon tetrabromide, for example [cf., for
example, H.-G. Elias, "Makromolekule", Huthig & Wepf Verlag
Basel, 5th edition, 1990]. The use of halide regulators, as
described in U.S. Pat. No. 7,034,085 B2, for example, is decreasing
persistently on environmental grounds. Thiols and alcohols are
suitable as regulators and, depending on concentration, greatly
reduce the average molecular weight of the polymer. Often, however,
they have the disadvantage of being volatile and are therefore
located in the distillate following the removal of the solvent
mixture. This results in the disadvantages that reusing the solvent
mixture removed by distillation for further polymerizations leads
to an accumulation of the compound with chain-transfer regulator
activity, and hence that a reproducible molar mass distribution is
not ensured as the recycling rate goes up.
[0008] US 20070299226 A1 describes various chain transfer
regulators based on a thiol functionality. They do remain partly in
the polymer, but have the disadvantage that they are partly toxic
and, on account of their strong and unpleasant odour, are not
suitable when the polymer is used for a product, such as a
pressure-sensitive adhesive tape, for example, with which points of
contact in everyday use are frequent. Moreover, the same
specification describes polyfunctional thiols for use as
regulators, but such thiols may even lead to crosslinking of the
polymer and hence to high melt viscosities, thereby making it no
longer possible to carry out processing from the melt. The
mercapto-functionalized photoinitiators that are described in EP 1
311 554 B1 likewise result in crosslinking or make it necessary to
ensure that the polymer prior to processing is not exposed to light
or any other electromagnetic radiation, since otherwise there may
be crosslinking.
[0009] It is an object of the invention to optimize the preparation
of polyacrylates in respect of recovery of the solvent.
[0010] It should advantageously be possible to obtain polyacrylate
compositions having sufficiently high average molecular weights
that they can be used, for example, as pressure sensitive
adhesives. The solvent mixture or solvent used in preparing the
polyacrylates ought preferably to be able to be reused without
purification steps following distillative removal; and the capacity
for the polyacrylate to be processed in a hotmelt operation ought
to be ensured.
SUMMARY OF THE INVENTION
[0011] It has been possible to achieve this object to outstanding
effect through the use of amino acids as regulator substances in
the free-radical polymerization of acrylic and/or methacrylic
monomers in the preparation of polyacrylates, more particularly of
monomer mixtures comprising at least 70% by weight of at least one
acrylic and/or methacrylic ester.
[0012] The invention relates accordingly to a reaction solution,
especially for free-radical polymerization, comprising one or more
solvents and one or more monomers, at least 50% by weight of the
solvents being organic solvents, and at least 70% by weight of the
monomers being acrylic and/or methacrylic esters, the reaction
solution additionally being admixed with at least one amino
acid.
[0013] The amino acids used may be natural or non-natural amino
acids.
DETAILED DESCRIPTION
[0014] It is possible to use a single organic solvent or a mixture
of two or more organic solvents, possibly with water as well. The
weight fraction of organic solvents when water is present in the
solvent mixture employed is at least 50% by weight, advantageously
at least 90% by weight.
[0015] It is very advantageous for the reaction solution to be a
homogeneous solution.
[0016] The amino acid or acids serve as polymerization regulators
in the free-radical polymerization. Regulators, polymerization
regulators or regulator substances are identified synonymously in
the context of this specification as compounds having high transfer
constants, which are used in free-radical polymerizations in order
to limit the degree of polymerization of the macromolecules that
form. They do not destroy the radical functionalities that are
capable of propagation, but instead take over these functionalities
from the ends of the growing macromolecules, in order then
themselves to initiate the propagation of a new chain. In this way,
for each radical produced in the system, two or more relatively
short polymer chains are formed instead of one long one. Since the
regulator substance does not destroy the radicals, there is also no
change in the overall polymerization rate (see ROMPP online 2009,
document code RD-18-00666).
[0017] In one very preferred procedure, the at least one amino acid
has at least one sulphanyl group (also referred to below as thiol
groups or --SH group), at least one selanyl group (also referred to
below as --SeH group) and/or a hydroxyl group (also referred to
below as --OH group). In one advantageous embodiment of the
invention, the aforementioned groups in the amino acid are
terminal.
[0018] The reaction solution comprises at least one acrylic- or
methacrylic-based monomer, a monomer mixture comprising at least
70% by weight of an acrylic or methacrylic ester, or, preferably, a
monomer mixture comprising at least 70% by weight of acrylic and/or
methacrylic esters.
[0019] In the text below, acrylic esters and methacrylic esters are
also referred to collectively as acrylic monomers.
[0020] The acrylic monomers are selected at least partly,
preferably completely, from the group of the monomers encompassing
compounds of the general formula
##STR00001##
where
R.sub.1=H or CH.sub.3
[0021] and R.sub.2=H or an unbranched or branched, aliphatic,
alicyclic or aromatic, unsubstituted or substituted hydrocarbon
radical having 1 to 20 carbon atoms. The alcohol residue of acrylic
acid monomers very preferably has no C.dbd.C double bonds.
[0022] In the presence of at least one free-radical initiator, the
monomers can be polymerized via a free radical polymerization, with
the at least one natural or non-natural amino acid containing at
least one thiol and/or hydroxy group acting as polymerization
regulator.
[0023] The invention further provides polyacrylates obtainable from
a reaction solution as described above, particularly by
free-radical polymerization of the monomers present therein.
[0024] Further encompassed by the invention is a process for
preparing a polyacrylate from such a reaction solution by
free-radical polymerization. For the free-radical polymerization it
is possible to use one or more typical initiators, the initiators
selected being more particularly those which do not enter into any
unwanted secondary reactions with the amino acids. After the
polymerisation the procedure is preferably such that the solvent is
removed from the polymerization product. The residual solvent
fraction is to be lowered to a fraction of not more than 5%, more
particularly not more than 2%, especially not more than 0.5%, by
weight, based on the mixture that remains following removal of the
solvent (polymerization product, residual solvent, amino acids,
possibly further constituents). It is preferred to aim for a
solvent-free system. In particular the polymerization product is
processed further from the melt (known as a "hotmelt").
[0025] In a preferred procedure, the solvent removed is supplied to
a recycling operation. The solvent used for preparing the reaction
solution may be taken wholly or partly from a recycling operation.
With particular advantage the solvent is wholly or partly
circulated--that is, following a polymerization, some or all of the
solvent removed is used for preparing a reaction solution for a
further polymerization.
[0026] In one preferred procedure the amino acid, or, where the
reaction solution had two or more amino acids added to it, the
amino acids, remain in the polymerization product when the solvent
is removed. This may be accomplished more particularly through
precipitation in the form of a solid. It is therefore particularly
preferred for there to no longer be any amino acid radicals present
(or, if any at all, in a negligible amount) in the solvent removed,
particularly in the solvent supplied to the recycling
operation.
[0027] All of the observations below refer, unless specifically
described otherwise, to all aspects of the invention (reaction
solution, preparation process, resultant polyacrylate and use of
the amino acid as a regulator substance).
[0028] The monomer mixture is preferably selected such that
plasticizing monomers in particular, and also monomers with
functional groups that are capable of entering into reactions,
especially addition reactions and/or substitution reactions, and
also, optionally, further copolymerizable comonomers, more
particularly hardening monomers are chosen. The nature of the
polyacrylate to be prepared (more particularly PSA; however, also
possible for use as heat-sealing compound, non-tacky viscoelastic
material, and the like) may be influenced more particularly by
varying the glass transition temperature of the polymer through
different weight fractions of the individual monomers.
[0029] For purely crystalline systems there is a thermal
equilibrium between crystal and liquid at the melting point
T.sub.m. Amorphous or partly crystalline systems, in contrast, are
characterized by the conversion of the more or less hard amorphous
or partly crystalline phase into a softer (rubber-like to viscous)
phase. At the glass transition point, particularly in the case of
polymeric systems, there is a "thawing" (or "freezing" on cooling)
of the Brownian molecular motion of relatively long chain
segments.
[0030] The transition from the melting point T.sub.m (also "melting
temperature"; actually defined only for purely crystalline systems;
"polymer crystals") to the glass transition point T.sub.g (also
"glass transition temperature") can therefore be considered as a
fluid transition, depending on the proportion of partial
crystallinity in the sample under analysis.
[0031] In accordance with its measurements, the glass transition
temperature can be reported as a dynamic temperature or as a static
temperature. Figures for dynamic glass transition temperatures are
based on the determination by means of dynamic mechanical analysis
(DMA) at low frequencies (temperature sweep; measurement frequency:
10 rad/s; temperature range: -40.degree. C. to max. 130.degree. C.;
heating rate: 2.5.degree. C./min; Rheometric Scientific RDA III;
parallel-plate arrangement, sample thickness 1 mm: sample diameter
25 mm: pre-tensioning with a load of 3N; sample stress for all
measurements 2500 Pa), while those for the static glass transition
temperature and for the melting points relate to determination by
means of differential scanning calorimetry (DSC) in accordance with
DIN 53765:1994-03.
[0032] In order to obtain polymers, such as PSAs or heat-sealing
compounds, for example, having desired glass transition
temperatures, the quantitative composition of the monomer mixture
is advantageously selected such that the desired T.sub.g for the
polymer results in accordance with an equation (E1) in analogy to
the Fox equation (cf. T. G. Fox, Bull. Am. Phys. Soc. 1 (1956)
123).
1 T g = n w n T g , n ( E 1 ) ##EQU00001##
[0033] In this equation, n represents the serial number of the
monomers used, w.sub.n the mass fraction of the respective monomer
n (% by weight) and T.sub.g,n the respective static glass
transition temperature of the homopolymer of each of the monomers
n, in K (kelvins).
[0034] For the application of the polyacrylate as a PSA, the
fractions of the corresponding components are preferably selected
such that the polymerization product exhibits more particularly a
dynamic glass transition temperature .ltoreq.15.degree. C.
(DMA).
[0035] As softening and/or apolar monomers it is preferred to use
acrylic and methacrylic esters with hydrocarbon radicals comprising
4 to 14 C atoms, preferably 4 to 9 C atoms. Examples of monomers of
this kind are n-butyl acrylate, n-butyl methacrylate, n-pentyl
acrylate, n-pentyl methacrylate, n-amyl acrylate, n-hexyl acrylate,
hexyl methacrylate, n-heptyl acrylate, n-octyl acrylate, n-octyl
methacrylate, n-nonyl acrylate, isobutyl acrylate, isooctyl
acrylate, isooctyl methacrylate, and their branched isomers, such
as 2-ethylhexyl acrylate and 2-ethylhexyl methacrylate, for
example.
[0036] As monomers with functional groups it is preferred to use
those selected from the following listing of functionalities:
carboxyl, sulphonic acid or phosphonic acid groups, phenols, thiols
or amines.
[0037] Particularly preferred examples of monomers are acrylic
acid, methacrylic acid, itaconic acid, maleic acid, fumaric acid,
crotonic acid, aconitic acid, dimethylacrylic acid,
.beta.-acryloyloxypropionic acid, trichloroacrylic acid,
vinylacetic acid, vinylphosphonic acid, itaconic acid,
diethylaminoethyl acrylate, diethylaminoethyl methacrylate,
dimethylaminoethyl acrylate and dimethylaminoethyl
methacrylate.
[0038] Additionally it is possible in principle for the purposes of
the invention to use all compounds with vinylic functionality that
are copolymerizable with the monomers identified above, and that
may also be used to adjust the properties of the resultant PSA.
[0039] Monomers identified by way of example are as follows: methyl
acrylate, ethyl acrylate, propyl acrylate, methyl methacrylate,
ethyl methacrylate, benzyl acrylate, benzyl methacrylate, sec-butyl
acrylate, tert-butyl acrylate, phenyl acrylate, phenyl
methacrylate, isobornyl acrylate, isobornyl methacrylate,
t-butylphenyl acrylate, t-butylphenyl methacrylate, dodecyl
methacrylate, isodecyl acrylate, lauryl acrylate, n-undecyl
acrylate, stearyl acrylate, tridecyl acrylate, behenyl acrylate,
cyclohexyl methacrylate, cyclopentyl methacrylate, phenoxyethyl
acrylate, phenoxyethyl methacrylate, 2-butoxyethyl methacrylate,
2-butoxyethyl acrylate, 3,3,5-tri-methylcyclohexyl acrylate,
3,5-dimethyladamantyl acrylate, 4-cumylphenyl methacrylate,
cyanoethyl acrylate, cyanoethyl methacrylate, 4-biphenylyl
acrylate, 4-biphenylyl methacrylate, 2-naphthyl acrylate,
2-naphthyl methacrylate, tetrahydrofurfuryl acrylate, maleic
anhydride, hydroxyethyl acrylate, hydroxypropyl acrylate,
hydroxyethyl methacrylate, hydroxypropyl methacrylate,
6-hydroxyhexyl methacrylate, allyl alcohol, glycidyl acrylate,
glycidyl methacrylate, 2-butoxyethyl acrylate, 2-butoxyethyl
methacrylate, methyl 3-methoxyacrylate, 3-methoxybutyl acrylate,
phenoxyethyl acrylate, phenoxyethyl methacrylate, 2-phenoxyethyl
methacrylate, butyldiglycol methacrylate, ethylene glycol acrylate,
ethylene glycol monomethylacrylate, methoxy-polyethylene glycol
methacrylate 350, methoxy-polyethylene glycol methacrylate 500,
propylene glycol monomethacrylate, butoxydiethylene glycol
methacrylate, ethoxytriethylene glycol methacrylate,
octafluoropentyl acrylate, octafluoropentyl methacrylate,
2,2,2-trifluoroethyl methacrylate, 1,1,1,3,3,3-hexafluoroisopropyl
acrylate, 1,1,1,3,3,3-hexafluoroisopropyl methacrylate,
2,2,3,3,3-pentafluoropropyl methacrylate,
2,2,3,4,4,4-hexafluorobutyl methacrylate,
2,2,3,3,4,4,4-heptafluorobutyl acrylate,
2,2,3,3,4,4,4-heptafluorobutyl methacrylate,
2,2,3,3,4,4,5,5,6,6,7,7,8,8,8-pentadecafluorooctyl methacrylate,
dimethylaminopropylacrylamide, dimethylaminopropylmethacrylamide,
N-(1-methylundecyl)acrylamide, N-(n-butoxymethyl)acrylamide,
N-(butoxy-methyl)methacrylamide, N-(ethoxymethyl)acrylamide,
N-(n-octadecyl)acrylamide, and also N,N-dialkyl-substituted amides,
such as, for example, N,N-dimethylacrylamide,
N,N-dimethylmethacrylamide, N-benzylacrylamides,
N-isopropylacrylamide, N-tert-butylacrylamide,
N-tert-octylacrylamide, N-methylolacrylamide,
N-methylolmethacrylamide, acrylonitrile, methacrylonitrile, vinyl
ethers, such as vinyl methyl ether, ethyl vinyl ether, vinyl
isobutyl ether, vinyl esters, such as vinyl acetate, vinyl
chloride, vinyl halides, vinylidene chloride, Vinylidene halides,
vinylpyridine, 4-vinylpyridine, N-vinylphthalimide, N-vinyllactam,
N-vinylpyrrolidone, styrene, o- and p-methylstyrene,
o-butylstyrene, 4-n-butylstyrene, 4-n-decylstyrene,
3,4-dimethoxystyrene. Macromonomers such as 2-polystyrene-ethyl
methacrylate (molecular weight M.sub.w from 4000 to 13 000 g/mol),
poly(methyl methacrylate)ethyl methacrylate (M.sub.w from 2000 to
8000 g/mol).
[0040] The monomers may advantageously also be selected such that
they contain functional groups which support subsequent radiation
crosslinking (by electron beams, UV, for example). Suitable
copolymerizable photoinitiators are, for example, benzoin acrylate
and acrylate-functionalized benzophenone derivatives; monomers
which support crosslinking by electron beams are, for example,
tetrahydrofurfuryl acrylate, N-tert-butylacrylamide, and allyl
acrylate, this list not being conclusive.
[0041] Free-radical initiators that can be used for the free
radical polymerization are all typical initiators known for such
purpose in respect of acrylates. The production of C-centred
radicals is described in Houben Weyl, Methoden der Organischen
Chemie, Vol. E 19a, pp. 60-147. These methods can be employed in
analogy. Examples of radical sources are peroxides, hydroperoxides
and azo compounds; certain non-exclusive examples of typical
free-radical initiators include potassium peroxodisulphate,
dibenzoyl peroxide, cumene hydroperoxide, cyclohexanone peroxide,
di-tert-butyl peroxide, azodiisobutyronitrile, cyclohexylsulphonyl
acetyl peroxide, diisopropyl percarbonate, tert-butyl peroctoate
and benzpinacol. In one very preferred variant the initiators are
added in two or more stages, and so the conversion is raised to
above 90%. The residual monomer content remaining in the polymer
may in this way be lowered to below 10% by weight; a low residual
monomer content considerably enhances the properties of the
polyacrylate with respect to its further processing in a hotmelt
operation.
[0042] The initiators used to initiate the polymerization are
preferably selected such that they have a low tendency to form side
chains in the polymers; their grafting activity is preferably below
a value of .epsilon.<5 at the temperature of the reaction
mixture when the initiator is added.
[0043] The initiators may advantageously be added before or at the
beginning of the polymerization to the monomer solution, and it is
possible for the mixture to be topped up with initiators during the
polymerization. The initiators are preferably used in the reaction
solution in a fraction of 0.001% to 1% by weight, more preferably
of 0.025% to 0.1% by weight, based on the monomer mixture.
[0044] The absolute grafting activity (crosslinking efficiency) is
defined as the number of chemical side-chain formations per 100 mol
units of decomposed initiator. In analogy to van Drumpt and
Oosterwijk [Journal of Polymer Science, Polymer Chemistry Edition
1976, 14 1495-1511], a value for this number can be specified by
determining the dimers in a defined solution of the initiator; see
also DE 43 40 297 A1:
[0045] An exact 0.1 molar solution of the initiator in
n-pentadecane is decomposed under an He atmosphere. The reaction
time is selected such that it corresponds to ten times the
half-life of the respective initiator at the temperature selected.
This ensures virtually complete decomposition of the initiator.
Next, the fraction of dimeric pentadecane formed is measured by
means of GLC. The percentage fraction .epsilon. is reported as a
measure of the grafting activity. The reaction temperature is
typically selected such that the half-life of the test initiator at
that temperature is 15 minutes.
[0046] High .epsilon. values for the grafting activity denote a
high tendency on the part of the initiator to form side chains
during the polymerization, whereas small .epsilon. values result in
preferentially linear polymers.
[0047] In one preferred procedure for the process, the sequence of
the process is as follows: [0048] the reaction solution used is an
at least 50% strength solution of the monomers, with the initiator
or initiators and the amino acid or amino acids containing at least
one thiol, selanyl and/or hydroxy functionality added, [0049] the
free-radical polymerization is carried out in a temperature range
from 50.degree. C. to 90.degree. C., [0050] during the
polymerization, initiation is repeated at least one using an
initiator for free-radical polymerizations that has a low tendency
to form side chains (grafting activity .epsilon.<5 at the
prevailing reaction temperature), [0051] if desired, the reaction
is controlled by dilution of the reaction solution, depending on
the viscosity of the polymer, [0052] a controlled re-initiation
takes place with up to 2% by weight, based on the monomer mixture,
of an initiator with an increased tendency to form side chains
(grafting activity .epsilon.>10 at the prevailing reaction
temperature), [0053] the polymerization is carried through to a
conversion >90%, preferably >95%.
[0054] Preferred initiators having a low .epsilon. value
(.epsilon.<5) are those whose free radicals cause no or very
infrequent abstraction of hydrogen on the polymer chains, on
account of their low energy content. Preference here is given for
example to the use of azo initiators such as azoisobutyrodinitrile
or derivatives thereof, an example being
2,2-azobis(2-methylbutyronitrile) (Vazo.RTM. 67, DuPont).
[0055] Initiators having a high tendency to form side chains (high
.epsilon. value >10) produce high grafting yields even at
relatively low temperatures. Particular preference here is given to
using bis(4-tert-butylcyclohexyl) peroxydicarbonate (Perkadox.RTM.
16, Akzo Chemie), dibenzoyl peroxide or the like.
[0056] The polymerization can be carried out in the presence of an
organic solvent or in the presence of water, or in mixtures of
organic solvents and/or water. Solvents used for the polymerization
may be all solvents that are suitable or used typically for
free-radical polymerizations, with acetone, ethyl acetate, benzine,
toluene or any desired mixtures of these solvents being
particularly appropriate.
[0057] The solvent or solvent mixture is present within the
reaction solution in customary quantity ranges; advantageously it
is present at 20% to 99% by weight, based on the reaction solution;
very preferably, however, as little as solvent as possible is used.
The polymerization time--depending on conversion, temperature and
initiation--is between 6 and 48 h.
[0058] The polyacrylates prepared, especially adhesive
polyacrylates, have an average molecular weight of between 250 000
and 1 000 000 g/mol, the average molecular weight being measured by
SEC or GPC (for measurement see experimental section). The
(co)polymers prepared generally possess the same or slightly
narrower molecular weight distributions as the polymerizations
carried out analogously with conventional regulators. The
polydispersity may be lowered to a value of less than 6. As a
result of the relatively narrow molecular weight distribution,
there is a fall in the flow viscosity of the polyacrylate
(especially of the PSA), and the polyacrylate is much simpler to
process as a hotmelt (lower temperature needed for melting, higher
throughput for the concentration procedure).
[0059] One particularly preferred variant of the inventive process
uses polymerization regulators comprising natural or non-natural
.alpha.-amino acids containing thiol, selanyl and/or hydroxyl
groups, advantageously compounds of the following general
structural formula
##STR00002##
where R.sup.1, R.sup.2 and R.sup.3 are selected independently of
one another.
[0060] R.sup.1 is preferably selected from the group (i),
encompassing
(i) linear and branched C.sub.1 to C.sub.18 alkyl radicals, linear
and branched C.sub.2 to C.sub.18 alkenyl radicals, linear and
branched C.sub.2 to C.sub.18 alkynyl radicals, aryl radicals,
cycloaliphatic, aliphatic and aromatic heterocycles, and
hydrogen.
[0061] R.sup.2 and R.sup.3 are preferably selected from group (i)
above or from group (ii), encompassing:
(ii) acyl radicals, more particularly alkanoyl radicals, and
cycloalkanecarbonyl radicals and arenecarbonyl radicals
(--C(O)--R''), alkoxycarbonyl radicals (--C(O)--O--R''), carbamoyl
radicals (--C(O)--NR'R''), and sulphonyl radicals (--SO.sub.2R'),
where R' and R'' are radicals from group i) that are selected
independently of one another;
[0062] X is selected more particularly to be oxygen (O), sulphur
(S) or selenium (Se).
[0063] The letter n in the FIGURE indicates the length of the
hydrocarbon chain; n is a natural number including zero, and
particularly 0.ltoreq.n.ltoreq.18.
[0064] The .alpha.-amino acids have a stereogenic centre (labelled
by the star * in the FIGURE) and are therefore chiral. In a further
advantageous embodiment of the invention, the chain-transfer
regulators are used as the optically cure substance in the (D) or
(L) configuration or are employed in the form of a racemic
mixture.
[0065] As regulators it is additionally possible to use
.beta.-amino acids, aromatic amino acids and other non-natural
amino acids familiar to the skilled person.
[0066] The process selected in this way makes it possible very
effectively to prepare polyacrylates, especially PSAs, having the
desired adhesive properties.
[0067] In another advantageous variant of the inventive process,
the natural or non-natural amino acids containing thiol, sulphanyl
and/or hydroxyl groups are used with a weight fraction of 0.001% to
5%, more particularly of 0.005% to 0.25%.
[0068] For the use of the polyacrylates prepared by the inventive
process as PSAs, the polyacrylates are optimized optionally by
blending with at least one resin. Tackifying resins for addition
that can be used include, without exception, all tackifier resins
that are already known and are described in the literature.
Representatives that may be cited include the pinene and indene
resins, rosins, their disproportionated, hydrogenated, polymerized
and esterified derivatives and salts, the aliphatic and aromatic
hydrocarbon resins, terpene resins and terpene-phenol resins, and
also C5, C9 and other hydrocarbon resins. Any desired combinations
of these and further resins may be used in order to adjust the
properties of the resultant adhesive in accordance with
requirements. Generally speaking, it is possible to use all resins
which are compatible (soluble) with the corresponding polyacrylate,
reference being made more particularly to all aliphatic, aromatic
and alkylaromatic hydrocarbon resins, hydrocarbon resins based on
pure monomers, hydrogenated hydrocarbon resins, functional
hydrocarbon resins and natural resins. Reference may be made
expressly to the depiction of the state of knowledge in the
"Handbook of Pressure Sensitive Adhesive Technology", by Donatas
Satas (van Nostrand, 1989).
[0069] In another advantageous development, one or more
plasticizers are metered into the PSA, examples being low molecular
mass polyacrylates, phthalates, whale oil plasticizers
(water-soluble plasticizers) or plasticizing resins. In one
preferred development, phosphates/polyphosphates are used for
acrylate hotmelts.
[0070] The acrylate hotmelts may additionally be blended with one
or more additives such as ageing inhibitors, light stabilizers,
ozone protectants, fatty acids, resins, nucleating agents,
expandants, compounding agents and/or accelerants.
[0071] They may also be admixed with one or more fillers such as
fibres, carbon black, zinc oxide, titanium dioxide, solid or hollow
glass (micro)spheres, microspheres made of other materials, silica,
silicates and chalk.
[0072] For use as a PSA in particular it is advantageous for the
inventive process if the polyacrylate is applied as a layer
preferably from solution to a support or to a carrier material.
[0073] One advantageous onward development of the inventive process
is when the polyacrylates prepared as described above are
concentrated to give a polyacrylate composition whose solvent
content is .ltoreq.5% by weight, more particularly .ltoreq.2% by
weight. This operation takes place preferably in a concentrating
extruder. In that case, in an advantageous variant of the process,
the polyacrylate composition is applied as a hotmelt composition,
in the form of a layer, to a support or to a carrier material.
[0074] The invention therefore further provides adhesive tapes
coated on one or both sides with adhesive.
[0075] In one particularly advantageous variant of the two
processes set out above, the solvent or solvent mixture is used
again in the polymerization, and the polymers thus prepared, even
after a very high recycling rate of the solvent or solvent mixture,
exhibit no change in molar masses or in molar mass
distribution.
[0076] For the two variants of the inventive process that have just
been set out, it is preferred as carrier materials, for adhesive
tapes, for example, to use the materials that are customary and
familiar to the skilled person, such as films (polyester, PET, PE,
PP, BOPP, PVC), nonwovens, foams, woven fabrics and woven sheets,
and also release paper (glassine, HDPE, LDPE). This list is not
conclusive.
[0077] For PSA use it is particularly advantageous to crosslink the
polyacrylates following coating onto the support or onto the
carrier material. For the production of the pressure-sensitive
adhesive tapes, the polymers described above are for this purpose
blended optionally with crosslinkers. Crosslinking may
advantageously be induced thermally or by means of high-energy
radiation, in the latter case more particularly by electron beams
or, following addition of suitable photoinitiators, by ultraviolet
radiation.
[0078] Preferred radiation-crosslinking substances according to the
inventive process include, for example, difunctional or
polyfunctional acrylates or difunctional or polyfunctional urethane
acrylates, difunctional or polyfunctional isocyanates or
difunctional or polyfunctional epoxides. Very advantageously it is
likewise possible to use metal chelate compounds. However, use may
also be made here of any other difunctional or polyfunctional
compounds that are familiar to the skilled person and are capable
of crosslinking polyacrylates.
[0079] Suitable photoinitiators include preferably Norrish type I
and type II cleavers, with some possible examples of both classes
being benzophenone derivatives, acetophenone derivatives, benzyl
derivatives, benzoin derivatives, hydroxyalkylphenone derivatives,
phenyl cyclohexyl ketone derivatives, anthraquinone derivatives,
thioxanthone derivatives, triazine derivatives or fluorenone
derivatives, this list making no claim to completeness.
[0080] Also claimed is the use of the polyacrylate prepared by the
inventive process as a pressure sensitive adhesive.
[0081] Particularly advantageous is the use of the polyacrylate
PSA, prepared as described, for an adhesive tape, in which case the
polyacrylate PSA may be applied to one or both sides of a
carrier.
EXAMPLES
Test Methods
[0082] The following test methods were employed to evaluate the
adhesive properties and general properties of the PSAs
prepared.
180.degree. Bond Strength Test (Test A)
[0083] A 20 mm wide strip of an acrylate PSA applied as a layer to
polyester was applied to steel plates. The PSA strip was pressed
onto the substrate twice using a 2 kg weight. The adhesive tape was
then immediately pulled from the substrate at 300 mm/min and at an
angle of 180.degree.. The steel plates were washed twice with
acetone and once with isopropanol. The results are reported in N/cm
and are averaged from three measurements. All measurements were
conducted at room temperature.
Shear Strength (Test B)
[0084] A 13 mm wide strip of the adhesive tape was applied to a
smooth steel surface which had been cleaned three times with
acetone and once with isopropanol. The application area was 20 mm13
mm (lengthwidth). The adhesive tape was then pressed onto the steel
support four times under an applied pressure of 2 kg. At 80.degree.
C. a 1 kg weight was affixed to the adhesive tape, at room
temperature a 1 kg or 2 kg weight. The holding powers measured are
reported in minutes and correspond to the average from three
measurements.
Residual Monomer Content (Test C)
[0085] The residual monomer content was determined analytically by
liquid extraction of the PSAs used, followed by capillary gas
chromatography.
Rheology (Test D)
[0086] The measurements were carried out using the Dynamic Stress
Rheometer instrument from Rheometrics. The frequency range from 0.1
to 100 rad/s was scanned at 25.degree. C. The temperature sweep was
measured 10 rad/s in a temperature range from -25.degree. C. to
130.degree. C. All experiments were conducted with the parallel
plate arrangement.
Gel Permeation Chromatography GPC (Test E)
[0087] The average molecular weight M.sub.w and the polydispersity
PD were determined by the following techniques: the eluent used was
THF containing 0.1% by volume trifluoroacetic acid. Measurement was
carried out at 25.degree. C. The preliminary column used was
PSS-SDV, 5 .mu.m, 10.sup.3 .ANG.(10.sup.2 nm), ID 8.0 mm50 mm.
Separation was effected using the columns PSS-SDV, 5 .mu.m,
10.sup.3 and also 10.sup.5 and 10.sup.6, each with ID 8.0
mm.times.300 mm. The sample concentration was 4 g/l, the flow rate
1.0 ml per minute. Measurement was made against PMMA standards.
K Value (According to FIKENTSCHER) (Test F):
[0088] The K value is a measure of the average size of molecules of
high polymers. It is measured by preparing one percent strength (1
g/100 ml) toluenic polymer solutions and determining their
kinematic viscosities with the aid of a VOGEL-OSSAG viscometer.
Following standardization to the viscosity of toluene, the relative
viscosity is obtained, and the K value can be calculated from this
FIGURE by the method of FIKENTSCHER (Polymer 1967, 8, 381 ff.)
Preparation of the Examples
Example 1a-c
[0089] A 2 l glass reactor conventional for free-radical
polymerizations was charged with 4 g of acrylic acid, 4 g of maleic
anhydride, 32 g of N-tert-butylacrylamide, 180 g of 2-ethylhexyl
acrylate, 180 g of n-butyl acrylate, 150 g of acetone and [0090] a)
0.2 g of N-acetyl-(L)-cysteine [0091] b) 0.4 g of
N-acetyl-(L)-cysteine [0092] c) 4.0 g of N-acetyl-(L)-cysteine.
[0093] After nitrogen gas had been passed through the reactor for
45 minutes, with stirring, the reactor was heated to 58.degree. C.,
and then 0.2 g of azoisobutyronitrile (AIBN) was added. After a
reaction time of an hour, 0.2 g of Vazo.RTM. 52 (DuPont), after
1.30 h 0.4 g of Vazo.RTM. 52, and after 2 h 0.6 g Vazo.RTM. 52 were
added. After 4 h, dilution was carried out with 150 g of acetone.
After 24 h and again after 36 h, Perkadox.RTM. 16 (Akzo) was added,
at 0.4 g each time. After a reaction time of 48 h, the
polymerization was discontinued and the reaction mixture was cooled
to room temperature. The polymer was analysed using test methods C,
E and F.
Example 2 (Reference)
[0094] A 2 l glass reactor conventional for free-radical
polymerizations was charged with 4 g of acrylic acid, 4 g of maleic
anhydride, 32 g of N-tert-butylacrylamide, 180 g of 2-ethylhexyl
acrylate, 180 g of n-butyl acrylate and 150 g of
acetone/isopropanol (97:3). After nitrogen gas had been passed
through the reactor for 45 minutes, with stirring, the reactor was
heated to 58.degree. C., and then 0.2 g of azoisobutyronitrile
(AIBN) was added. After a reaction time of an hour, 0.2 g of
Vazo.RTM. 52 (DuPont), after 1.30 h 0.4 g of Vazo.RTM. 52, and
after 2 h 0.6 g Vazo.RTM. 52 were added. After 4 h, dilution was
carried out with 150 g of acetone/isopropanol (97:3). After 6 h and
again after 8 h, Perkadox.RTM. 16 (Akzo) was added, at 0.4 g each
time. After a reaction time of 12 h, the polymerization was
discontinued and the reaction mixture was cooled to room
temperature. The polymer was analysed using test methods C, E and
F.
Example 3 (Reference)
[0095] A 2 l glass reactor conventional for free-radical
polymerizations was charged with 4 g of acrylic acid, 4 g of maleic
anhydride, 32 g of N-tert-butylacrylamide, 180 g of 2-ethylhexyl
acrylate, 180 g of n-butyl acrylate and 150 g of acetone. After
nitrogen gas had been passed through the reactor for 45 minutes,
with stirring, the reactor was heated to 58.degree. C., and then
0.2 g of azoisobutyronitrile (AIBN) was added. After a reaction
time of an hour, 0.2 g of Vazo.RTM. 52 (DuPont), after 1.30 h 0.4 g
of Vazo.RTM. 52, and after 2 h 0.6 g Vazo.RTM. 52 were added. After
4 h, the batch underwent gelling and the polymerization was
discontinued.
Example 4a-c
[0096] A 2 l glass reactor conventional for free-radical
polymerizations was charged with 4 g of acrylic acid, 4 g of maleic
anhydride, 32 g of N-tert-butylacrylamide, 180 g of 2-ethylhexyl
acrylate, 180 g of n-butyl acrylate, 150 g of acetone and [0097] a)
0.2 g of dodecanethiol [0098] b) 0.4 g of dodecanethiol [0099] c)
4.0 g of dodecanethiol.
[0100] After nitrogen gas had been passed through the reactor for
45 minutes, with stirring, the reactor was heated to 58.degree. C.,
and then 0.2 g of azoisobutyronitrile (AIBN) was added. After a
reaction time of an hour, 0.2 g of Vazo.RTM. 52 (DuPont), after
1.30 h 0.4 g of Vazo.RTM. 52, and after 2 h 0.6 g Vazo.RTM. 52 were
added. After 4 h, dilution was carried out with 150 g of acetone.
After 24 h and again after 36 h, Perkadox.RTM. 16 (Akzo) was added,
at 0.4 g each time. After a reaction time of 48 h, the
polymerization was discontinued and the reaction mixture was cooled
to room temperature. The polymer was analysed using test methods C,
E and F.
Example 5a-c
[0101] A 2 l glass reactor conventional for free-radical
polymerizations was charged with 4 g of acrylic acid, 4 g of maleic
anhydride, 32 g of N-tert-butylacrylamide, 180 g of 2-ethylhexyl
acrylate, 180 g of n-butyl acrylate, 150 g of acetone and [0102] a)
0.2 g of Thiocure.RTM. PETMP [0103] b) 0.4 g of Thiocure.RTM. PETMP
[0104] c) 4.0 g of Thiocure.RTM. PETMP.
[0105] (Thiocure.RTM. PETMP: pentaerythritol
tetra(3-mercaptopropionate), from Bruno Buch Thio chemicals). After
nitrogen gas had been passed through the reactor for 45 minutes,
with stirring, the reactor was heated to 58.degree. C., and then
0.2 g of azoisobutyronitrile (AIBN) was added. After a reaction
time of an hour, 0.2 g of Vazo.RTM. 52 (DuPont), after 1.30 h 0.4 g
of Vazo.RTM. 52, and after 2 h 0.6 g Vazo.RTM. 52 were added. After
4 h, dilution was carried out with 150 g of acetone. After 24 h and
again after 36 h, Perkadox.RTM. 16 (Akzo) was added, at 0.4 g each
time. After a reaction time of 48 h, the polymerization was
discontinued and the reaction mixture was cooled to room
temperature. The polymer was analysed using test methods C, E and
F.
Results
[0106] In order to allow the activities of the regulator to be
compared, the chain-transfer constant was determined with the aid
of the Mayo equation (E2)
1 P n = 1 P n , 0 + C tr [ regulator ] [ monomer ] ( E 2 )
##EQU00002##
where P.sub.n is the degree of polymerization, P.sub.n,0 is the
degree of polymerization of the unregulated reaction, [regulator]
is the concentration of the chain-transfer regulator, [monomer] is
the monomer concentration, and C.sub.tr is the chain-transfer
constant.
[0107] The results of the polymerizations are listed in Table
1.
TABLE-US-00001 TABLE 1 Example M.sub.w PD K value C.sub.tr
Conversion 1a 115 000 5.35 62.3 0.90 99.1% 1b 120 000 4.36 53.1
99.2% 1c 25 700 2.12 22.0 99.1% 2 486 000 7.60 72.2 1.5 10.sup.-3
99.4% 3 -- -- -- -- -- 4a 88 200 8.68 64.6 0.49 99.2% 4b 133 000
4.76 57.1 99.4% 4c 31 300 2.69 26.8 98.9% 5a 103 000 20.26 89.9
0.78 99.3% 5b 171 000 6.57 67.1 99.0% 5c 49 400 2.12 30.2 98.2%
M.sub.w: average molecular weight (weight average) [g/mol] PD:
polydispersity, C.sub.tr: chain-transfer constant
[0108] Table 1 shows that a polymerization in pure solvent (Example
3) is attended by problems. Achieving a high conversion requires a
relatively large quantity of initiator, in which case the batch has
undergone gelling after a reaction time of just 4 h, and it is
necessary to discontinue the polymerization. The polymerizations of
Examples 1, 2, 4 and 5 show that regulators prevent the gelling
whilst still enabling high conversions of >98% to be achieved,
through initiation in two or more stages and with two or more
initiators. Yet alcohols, such as isopropanol, are of only limited
suitability, since the latter regulator cannot be incorporated into
the PSA and hence remains in the solvent. For concentration to the
hotmelt, it is necessary to carry out distillative removal of a
solvent mixture at low pressure. The throughput in that case is
greatly reduced as result of a change in boiling point.
[0109] Thiols (Examples 1, 4 and 5), in contrast, are incorporated
into the polymer during the polymerization process and do not
adversely affect the concentration process. In addition to these
criteria, the achievable average molecular weight and the molecular
weight distribution (dispersity) are critically important for the
technical adhesive properties. Examples 1a-c show that amino acids
containing thiol groups, such as N-acetyl-(L)-cysteine, are the
most efficient regulators and therefore achieve the lowest
polydispersity for the polyacrylate PSA in question. Moreover, in
contrast to dodecanethiol, such amino acids are not toxic, thereby
removing any controversy from operation with such compounds and
from the residual presence of trace amounts in the polyacrylate
material. The tetrafunctional thiol PETMP causes high crosslinking
of the polymer, which would likewise give rise to disadvantages in
the concentration process.
Preparation of the Starting Polymers for PSA Examples B1 to B4
[0110] Described below is the preparation of the starting polymers.
The polymers investigated are prepared conventionally via a free
radical polymerization in solution. For the first polymerization of
the respective base polymers P1 to P4, fresh solvent is used; all
further polymerizations are carried out with the solvent or solvent
mixture recovered by method 1.
Base Polymer P1 (Comparative example)
[0111] A reactor conventional for free-radical polymerizations was
charged with 27 kg of 2-ethylhexyl acrylate, 67 kg of n-butyl
acrylate, 3 kg of methyl acrylate, 3 kg of acrylic acid and 66 kg
of acetone/isopropanol (95:5). After nitrogen gas had been passed
through the reactor for 45 minutes, with stirring, the reactor was
heated to 58.degree. C. and 50 g of AIBN were added. Thereafter the
external heating bath was heated to 75.degree. C. and the reaction
was carried out constantly at this external temperature. After 1 h
a further 50 g of AIBN were added, and after 4 h dilution took
place with 20 kg of acetone/isopropanol mixture. After 5 h 30 min
and again after 7 h, bis(4-tert-butylcyclohexyl) peroxydicarbonate
was added for re-initiation--150 g each time. After a reaction time
of 22 h, the polymerization was discontinued and the reaction
mixture was cooled to room temperature. The polyacrylate has a
conversion of 99.6%, a K value of 76.0, a solids content of 54%, an
average molecular weight of Mw=760 000 g/mol, polydispersity PD
(Mw/Mn)=9.6.
Base Polymer P2 (N-acetyl-(L)-cysteine)
[0112] A reactor conventional for free-radical polymerizations was
charged with 27 kg of 2-ethylhexyl acrylate, 67 kg of n-butyl
acrylate, 3 kg of methyl acrylate, 3 kg of acrylic acid, 50 g of
N-acetyl-(L)-cysteine and 66 kg of acetone. After nitrogen gas had
been passed through the reactor for 45 minutes, with stirring, the
reactor was heated to 58.degree. C. and 50 g of AIBN were added.
Thereafter the external heating bath was heated to 75.degree. C.
and the reaction was carried out constantly at this external
temperature. After 1 h a further 50 g of AIBN were added, and after
4 h dilution took place with 20 kg of acetone. After 5 h 30 min and
again after 7 h, bis(4-tert-butylcyclohexyl) peroxydicarbonate was
added for re-initiation--150 g each time. After a reaction time of
22 h, the polymerization was discontinued and the reaction mixture
was cooled to room temperature. The polyacrylate has a conversion
of 99.7%, a K value of 75.8, a solids content of 54%, an average
molecular weight of Mw=755 000 g/mol, polydispersity PD
(Mw/Mn)=4.7.
Base Polymer P3 (rac-N-acetylcysteine methyl ester)
[0113] A reactor conventional for free-radical polymerizations was
charged with 27 kg of 2-ethylhexyl acrylate, 67 kg of n-butyl
acrylate, 3 kg of methyl acrylate, 3 kg of acrylic acid, 55 g of
rac-N-acetylcysteine methyl ester and 66 kg of acetone. After
nitrogen gas had been passed through the reactor for 45 minutes,
with stirring, the reactor was heated to 58.degree. C. and 50 g of
AIBN were added. Thereafter the external heating bath was heated to
75.degree. C. and the reaction was carried out constantly at this
external temperature. After 1 h a further 50 g of AIBN were added,
and after 4 h dilution took place with 20 kg of acetone. After 5 h
30 min and again after 7 h, bis(4-tert-butylcyclohexyl)
peroxydicarbonate was added for re-initiation--150 g each time.
After a reaction time of 22 h, the polymerization was discontinued
and the reaction mixture was cooled to room temperature. The
polyacrylate has a conversion of 99.5%, a K value of 75.7, a solids
content of 54%, an average molecular weight of Mw=751 000 g/mol,
polydispersity PD (Mw/Mn)=5.1.
Base Polymer P4 (Dodecanethiol)
[0114] A reactor conventional for free-radical polymerizations was
charged with 27 kg of 2-ethylhexyl acrylate, 67 kg of n-butyl
acrylate, 3 kg of methyl acrylate, 3 kg of acrylic acid, 62 g of
dodecanethiol and 66 kg of acetone. After nitrogen gas had been
passed through the reactor for 45 minutes, with stirring, the
reactor was heated to 58.degree. C. and 50 g of AIBN were added.
Thereafter the external heating bath was heated to 75.degree. C.
and the reaction was carried out constantly at this external
temperature. After 1 h a further 50 g of AIBN were added, and after
4 h dilution took place with 20 kg of acetone. After 5 h 30 min and
again after 7 h, bis(4-tert-butylcyclohexyl) peroxydicarbonate was
added for re-initiation--150 g each time. After a reaction time of
22 h, the polymerization was discontinued and the reaction mixture
was cooled to room temperature. The polyacrylate has a conversion
of 99.6%, a K value of 76.3, a solids content of 54%, an average
molecular weight of Mw=768 000 g/mol, polydispersity PD
(Mw/Mn)=5.2.
Method 1: Concentration/Preparation of the Hotmelt PSAs:
[0115] The acrylate copolymers (base polymers P1 to P4) are very
largely freed from the solvent (residual solvent content
.ltoreq.0.3% by weight; cf. the individual examples) by means of a
single-screw extruder (concentrating extruder, BERSTORFF GmbH,
Germany). The solvent is condensed and used again without further
purification for the polymerization of P1 to P4, with a check being
made by determination of the K value, using test method F, after
each polymerization, in order to ensure that PSAs having the same
properties are prepared. The screw speed was 150 rpm, the motor
current 15 A, and a throughput of 58.0 kg liquid/h was realized.
For concentration, a vacuum was applied at three different domes.
The reduced pressures were, respectively, between 20 mbar and 300
mbar. The exit temperature of the concentrated hotmelt is
approximately 115.degree. C. The solids content after this
concentration step was 99.8%.
Method 2: Preparation of the Modified Hotmelt PSAs:
[0116] The acrylate hotmelt PSA prepared by method 1 as elucidated
above was conveyed directly into a downstream WELDING twin-screw
extruder (WELDING Engineers, Orlando, USA; Model 30 MM DWD; screw
diameter 30 mm, screw 1 length=1258 mm; screw 2 length=1081 mm; 3
zones). Via a solids metering system, the resin Dertophene.RTM.
T110 (DRT RESINS, France) was metered in zone 1 and mixed in
homogeneously. The rotary speed was 451 rpm, the motor current 42
A, and a throughput of 30.1 kg/h was realized. The temperatures in
zones 1 and 2 were each 105.degree. C., the melt temperature in
zone 1 was 117.degree. C., and the composition temperature on exit
(zone 3) was 100.degree. C.
Method 3: Production of the Inventive Adhesive Tapes, Blending with
the Crosslinker System for Thermal Crosslinking, and Coating:
[0117] The acrylate hotmelt PSA prepared by methods 1-2 were melted
in a feeder extruder (single-screw conveying extruder from TROESTER
GmbH & Co KG, Germany) and using this extruder were conveyed as
a polymer melt into a twin-screw extruder 1.3 (cf. FIG. 1)
(LEISTRITZ, Germany, ref. LSM 30/34). The assembly is heated
electrically from the outside and is air-cooled via a number of
fans, and is designed such that, with the effective distribution of
the crosslinker Polypox R16 (pentaerythritol polyglycidyl ether,
CAS No.: 3126-63-4, tetrafunctional epoxide from UPPC AG, Germany)
and of the accelerant Polypox H205 (polyoxypropylenediamine, CAS
No.: 9046-10-0, amine hardener from UPPC AG, Germany) in the
polymer matrix, there is at the same time a short residence time
ensured for the adhesive in the extruder. For this purpose, the
mixing shafts of the twin-screw extruder were arranged in such a
way that conveying elements are in alternation with mixing
elements. The addition of the respective crosslinkers and
accelerants is made with suitable metering equipment, where
appropriate at two or more locations (cf. FIG. 1: metering
locations 1.1 and 1.2) and, where appropriate, with the use of
metering assistants into the unpressurized conveying zones of the
twin-screw extruder.
[0118] Following exit of the ready-compounded adhesive, i.e. of the
adhesive blended with the crosslinker and accelerant, from the
twin-screw extruder (exit: circular die, 5 mm diameter), coating
takes place in accordance with FIG. 1 onto a carrier material in
web form onto the coating roll (BW). The time between metered
addition of the crosslinker-accelerant system and the shaping or
coating procedure is termed the processing life. This processing
life indicates the period within which the adhesive, blended with
the crosslinker or crosslinker-accelerant system, can be coated
with the visually good appearance (gel-free, speck-free). Coating
takes place with web speeds between 1 m/min and 20 m/min; the
doctor roll (RW) of the 2-roll applicator is not driven.
Examples B1-B4
[0119] The base polymers P1-P4 are polymerized in accordance with
the polymerization process described, and are concentrated using
method 1 (solids content 99.8%), and a sample of the
non-crosslinked polymer is analysed for its K value, in order to
show that there are no residues in the solvent recyclate that have
a molar mass regulator function. Subsequently, using method 2, the
compositions were blended with Dertophene.RTM. T110 resin, and
these resin-modified acrylate hotmelts were then compounded
continuously by method 3 with the crosslinker-accelerant
system.
[0120] Detailed description given by way of example: in the
twin-screw extruder described in method 3, a total mass flow
consisting of 70 parts of polymer P1 and 30 parts of
Dertophene.RTM. T110 resin of 533.3 g/min (corresponding to 373
grams of pure polymer per minute) was blended with 0.45% by weight
(based on polymer solids) of the epoxide crosslinker Polypox R16
and with 0.72% by weight (based on polymer solids) of the
accelerant Polypox H205. The accelerant was metered via a
peristaltic pump at metering location 1.1 (see FIG. 1), while the
crosslinker was metered in likewise via a peristaltic pump at
metering location 1.2.
[0121] The processing life of the completed compound formulation
was more than 10 minutes at an average adhesive temperature of
125.degree. C. after leaving the LEISTRITZ twin-screw extruder.
Coating takes place on a 2-roll applicator as per FIG. 1, with roll
surface temperatures of 100.degree. C. in each case and at a coat
weight of 110 g/m.sup.2 onto 23 .mu.m PET film.
Results
[0122] Examples B1 to B4 were carried out a total of ten times, the
polymerization being carried out in each case using the solvent
recovered by distillation, by method 1, in order to demonstrate
that there are no residues of the regulator in the solvent anymore.
After each concentration of the polymer, the K values were
determined by test method F (K1 to K10 for each respective
polymerization mixture). For technical adhesive testing of Examples
1 to 4, test methods A and B were carried out.
[0123] The results of the polymerizations are listed in Table
2.
TABLE-US-00002 TABLE 2 Example K1 K2 K3 K4 K5 K6 K7 K8 K9 K10 B1
76.0 76.1 76.3 76.5 76.8 77.0 77.4 77.7 78.0 78.4 B2 75.8 75.8 75.9
75.8 75.8 75.9 75.7 75.8 75.7 75.8 B3 75.7 75.8 75.7 75.7 75.7 75.8
75.9 75.7 75.7 75.7 B4 76.3 76.3 76.2 76.1 76.1 75.9 75.9 75.8 75.9
75.7
[0124] Table 2 demonstrates that reusing the solvent distillate
consisting of acetone and isopropanol (Example 1) is attended by
problems. The isopropanol acts as a chain-transfer regulator, but
because of its lower vapour pressure as compared with acetone it is
depleted with each distillation cycle, hence leading, in the
polymerization carried out with the solvent recyclate, following
distillation, to increases in the molar masses and hence in the K
value. This would mean that following each distillation it will be
necessary to check the proportion of isopropanol to acetone and,
where necessary, re-adjust it once more, in order to continue to
obtain polymers with the same molar mass distribution.
[0125] The polymerizations of Examples 2 and 3 demonstrate that
amino acids which are solids are suitable regulators and that even
after multiple re-use of the solvent recyclate there is no change
in K value, within the bounds of measurement accuracy.
[0126] Similar results were also found with dodecanethiol as the
regulator, although with a tendency in evidence for the K values to
drop over time, indicating a reduction in the molar mass. In spite
of the low vapour pressure it is likely that a certain fraction of
dodecanethiol, which is a liquid, is entrained together with the
solvent and hence gradually accumulates in the recyclate.
[0127] The regulator concept of the invention for improving the
recycling of solvent for the polymerization has therefore been
proven.
[0128] Table 3 shows the bond strengths to steel and also the
holding powers for the first mixture in each case.
TABLE-US-00003 TABLE 3 Example HP RT 10 N BS steel [N/cm] 1.1 4150
6.6 2.1 3180 6.8 3.1 3100 6.8 4.1 3150 6.9 Coat weight: 110
g/m.sup.2. HP: Holding powers [min] BS: Bond strength to steel
[0129] The composition regulated using isopropanol gave the highest
shear strength as compared with the other polymerization
regulators, which, however, are not substantially poorer. In
general it was possible to show that similar technical adhesive
properties were achievable with all the regulators.
[0130] The advantage of the regulators of the invention will be
underscored below by means of further examples.
[0131] The further technical adhesive tests of Examples 1.10-4.10
were carried out again by test methods A and B, using the samples
whose polymers had been polymerized with the solvent re-used ten
times. In addition, the flow viscosity was measured, using test
method D. The results of the polymerizations are listed in Table
4.
TABLE-US-00004 TABLE 4 .eta.[Pa * s] at 130.degree. C. Example HP
(RT, 10 N) BS steel [N/cm] and 1 rad/s 1.10 4620 5.9 5800 2.10 3200
6.9 4800 3.10 3150 6.8 4800 4.10 3010 6.9 4600 Coat weight: 110
g/m.sup.2. HP: Holding powers [min] BS: Bond strength to steel
.eta.: Flow viscosity by test D
[0132] This example shows very clearly the advantage of the
inventive regulator system. As a result of the change in molar mass
in Example 1.10, there is a very significant change in the
technical adhesive properties as compared with those of Example
1.1. The other regulators produced samples with reproducible
technical adhesive data; in Example 4.10 there is a slight fall in
the shear strength, owing to the reduction in molar mass, and the
viscosity as well is lower as compared to the compositions
regulated with amino acids.
* * * * *