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Description  |
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TECHNICAL FIELD
This invention relates to adhesive compositions derived from
anionically-prepared copolymers containing organometallic-substituted
styrene and to sheet materials coated therewith.
BACKGROUND ART
Pressure-Sensitive Adhesive Art
Normally tacky pressure-sensitive adhesive (hereinafter referred to by the
abbreviation "PSA") compositions suitable, for example, for use in
adhesive tapes must have an art-recognized (1952 Fall Symposium, Division
of Paint, Varnish and Plastics Chemistry, American Chemical Society)
four-fold balance of adhesion, cohesion, stretchiness and elasticity. PSA
coated tapes have been produced and sold for at least a half century.
The early PSA tapes relied upon natural rubber for the elastomeric base and
wood rosins as tackifiers to provide adhesive compositions with the
requisite four-fold balance of properties. While tackified natural rubber
provided a PSA composition which was of commercial significance,
improvements in such compositions were sought because of the expanded
expectation level of performance of PSA compositions. Various improved PSA
compositions were thus developed.
Ionic polymerization produced block copolymer elastomers such as linear AB
and ABA block copolymers which were likely candidates for the elastomer
base in the PSA compositions and many were incorporated into such
compositions to produce adhesives having high performance characteristics.
For example, Harlan (U.S. Pat. No. 3,239,478) produced PSA compositions
based on ABA block copolymer, tackifier resin and extender oil,
recognizing that improved tack and cohesive strength could be obtained
despite a heavy loading of extender oil. Miller (U.S. Pat. No. 3,519,585)
produced an improved PSA composition having high peel strength, creep
resistance and tack by blending AB and ABA block copolymers with a
tackifier resin.
Other elastomer candidates for preparing PSA compositions include radial
teleblock copolymers and multiarm star block copolymers. The various
polymer structures described by the terms "branched", "radial" and "star"
are not the same. "Branched" is a generic term indicating a nonlinear
structure which may contain various polymeric subunits appended to various
places on a main polymer chain or backbone. Such structures are typically
complex in nature and may be derived by free radical or cationic
polymerization. The term "radial" generally refers to branched polymer
structures obtained by linking individual polymeric segments to yield a
mixture of polymers having four or fewer arms joined centrally. The term
"star" describes the structure of a multiarm polymer with copolymer arms
which are joined together at a nucleus formed of a linking group which is
virtually a point relative to the overall size of the remainder of the
polymer structure. Non-terminating coupling agents, those in which the
polymerizing anionic structure is retained, are generally preferred as
linking agents for "star" structures.
While several references disclose preparing adhesive compositions or PSA
compositions employing radial teleblock copolymers and multiarm star block
copolymers, none have recognized that novel anionically-prepared
copolymers containing organometallic-substituted styrene may be used to
prepare PSA compositions nor that such compositions exhibit unusual melt
viscosity characteristics as well as excellent adhesive properties. For
example, St. Clair (U.S. Pat. No. 4,444,953) describes asymmetric star
block polymer prepared by terminally linking together a mixture of
styrene-isoprene AB block polymers and isoprene homopolymers. The melt
viscosity of such asymmetric star polymers is generally significantly
higher than their linear counterpart. Marrs et al (U.S. Pat. No.
3,658,740) discloses the preparation of PSA compositions by combining
branched block copolymers with linear block copolymers, tackifiers and
organic solvents. Marrs' PSA formulation requires a solvent as a critical
element to provide an adhesive formulation which bonds to a wide variety
of substrates but fails to address the need for hot melt processability.
Nash (U.S. Pat. No. 4,163,764) discloses the preparation of PSA
compositions employing a two-step process in which a monovinyl-arene
monomer, such as styrene, is first polymerized, followed by a second stage
where diene monomer and additional initiator are added and the resulting
polymerized product linked to give linear or radially-branched polymers.
These polymers, when formulated with tackifiers, exhibited superior tack
and creep resistance. Feeney et al (U.S. Pat. No. 4,288,567) employs a
branched block copolymer described in Prudence (U.S. Pat. No. 3,949,020)
and relies upon a solution preparation process to achieve an adhesive
composition having increased tack, faster molten solution time, and
improved tack retention in hot melt blends.
Copolymer Art
While several references disclose the preparation of various copolymers
which may be suited for use as a rubbery base material for PSA
compositions, none known to applicants discloses the anionically-prepared
copolymers containing organometallic-substituted styrene defined in the
claims or the use of such copolymers in PSA compositions. The following
discussion is intended to assist the reader in understanding related
copolymer art.
According to Odian, Principles of Polymerization, 2nd Ed.,
Wiley-Interscience, p. 18, (1981) polymers fall into three structural
groups: linear, branched and crosslinked. Branched polymer molecules are
those in which there are side branches of linked monomer protruding from
various central branch points along the main polymer chain and that have
several idealized configurations. Branched polymers are known in at least
three configurations. They may be "comb-like" where each branch is of
equal length, "dendritic" where branches occur on branches (series
branching), or "star-like" where all branches radiate from a single point.
Branching often imparts various desirable properties, for example, branched
polymers have been made that have improved melt flow and processability.
Additionally, appropriate branching disrupts long linear polymer backbones
to thereby reduce crystallinity. In free radical and cationic
polymerization processes, for example in the production of polyethylene,
branching is largely uncontrolled and its extent is dependent on
polymerization variables. In some cases branching can be as high as 15-30
branches per 500 monomer units. In contrast, anionic polymerization
processes yield very narrow molecular weight distributions and a unique
structure. Branched polymer structures produced by anionic polymerization
are generally star shaped (arrayed about a central point or nucleus)
although the structure can be varied by coupling together individually
prepared arms of different structure.
Such polymers are described by St. Clair in U.S. Pat. No. 4,391,949 where
"asymmetric" star block copolymers are prepared by mutually linking
together individually prepared living polymers, which may be represented
by (AB)Li and (C)Li, with polyalkenylaromatic linking reagents. The
structural formula describing the resulting polymer is given as
(A--B).sub.x --Y--(C).sub.z, where x plus z is greater than six. A
statistical distribution of polymer products would be obtained from this
process, where the average structure is equal to the mole ratio of the
respective charges. Further chain growth would only be possible through
the linking nucleus Y.
Crossland, U.S. Pat. No. 4,010,226, has also recognized the problem of
preparing block polymers with an asymmetric configuration and, to avoid
the statistical distribution of polymers obtained by St. Clair, first
coupled a set of polymer arms with divinylbenzene, then continued the
polymerization, utilizing the anionic centers that remain on the
divinylbenzene residue, to produce a different set of arms bound to the
same nucleus. The number of new arms grown would thus equal the number of
arms coupled together, since linking with divinylbenzene (DVB) is a
non-terminating process and each newly grown arm would have an anionic
terminus. Fahrbach, U.S. Pat. No. 4,086,298, discloses star-block
copolymers having a mixture of arms where some arms are formed by first
polymerizing styrene with alkyllithium to form living polymer blocks,
represented by (A)Li, and then adding a mixture of styrene and butadiene
to form a graded copolymer represented by A-B.fwdarw.A' where the arrow
represents a graded segment. Other arms are made up of only the
butadiene-styrene graded copolymer segment. These arms are then linked
together with a polyfunctional coupling agent, such as DVB, to give
star-branched polymers. U.S. Pat. Nos. 4,221,884, 4,248,980, 4,248,982,
4,248,983, and 4,248,984, Bi and Milkovich, describe a similar series of
polymers in which more complex polymer arm segments are linked together
using a polyalkenyl aromatic, such as divinylbenzene, to form an
asymmetric star molecule.
Prudence (U.S. Pat. No. 3,949,020) prepares branched block polymers by a
method wherein divinylbenzene is added with the diolefin monomer to a
polystyryllithium initiator. However, according to Bi and Fetters
(Macromolecules 9, 732-742 [1976]), such a method leads to gelation when
the divinylbenzene/initiator ratio is three or greater.
Martin, in U.S. Pat. Nos. 4,080,400, 4,143,089, 4,148,838, and 4,273,896,
describes a composition obtained from the linking together of anionically
active polymers (from, e.g., styrene) with silanes of the formula,
X.sub.4-a-b Si(R).sub.b (CH.dbd.CH.sub.2).sub.a, where X is a displaceable
group, R is alkyl, a is 1 to 4 and b is 1 to 3. One of the stated objects
of these patents is to couple polymeric carbanions with silanes and then
form new carbanions which can be used to initiate the polymerization of
cyclic silicones or "other unsaturated monomers". No disclosure is
provided directed towards the step of using other unsaturated monomers
except for certain unspecified hydrocarbon/siloxane block polymers.
It has been established [Nametkin, Chemical Abstract Nos. 85:47314a (1976),
87:185046g (1977), and 89:110569n (1978)]that vinylsilanes of the type
described by Martin will copolymerize in an anionic fashion, for example
with butadiene; however, reactivity is very low, with up to 300 hours
required for good conversion. Furthermore, copolymers of vinyl silanes
with dienes initiated by butyl lithium are unimodal but exhibit peak
broadening due to the occurrence of chain termination reactions caused by
spontaneous cleavage reactions producing lithium hydride (Nametkin,
Chemical Abstract No. 93:168679x, 1980). Loss of LiH during anionic
homopolymerization of vinyltrimethylsilane has also been observed and has
been used to explain the poor conversion and spread in molecular weight
distribution observed in these polymers [Nametkin, Dokl. Nauk SSSR, 215,
861 (1974)]. Chaumont [Eur. Poly. J. 15, 537 (1979)]prepared vinylsilyl
terminated polystyrenes via anionic polymerization; however, it was
necessary to cap the polymer anion with diphenylethylene in order to
reduce side reactions.
Chlorosilane-substituted styrenes are well-known compounds and have been
used, for example, to prepare polysiloxane macromolecular monomers
[Kawakami, Polymer J., 14, 913 (1982)]. Chromatography gels have been
described based on poly-.alpha.-methylstyrene dianions and
chlorodimethylsilylstyrene [Greber, Angew. Makromol. Chem. 1971, 16/17,
325]. Compositions for the encapsulation of electrical equipment have been
derived from organosilicon monomers having styrenyl groups (Lewis, U.S.
Pat. No. 2,982,757). Hirao et al. (Macromolecules 1987, 20, 242) has
studied the anionic homopolymerization of (4-alkoxysilyl) styrenes and
reaction of the resultant homopolymer with polystyryllithium.
There has been no disclosure, however, of the use of
organometallic-substituted styrenes, e.g., chlorosilanesubstituted
styrenes, in the preparation of condensed phase polymers or of PSA
compositions made therewith.
SUMMARY OF THE INVENTION
The present invention provides pressure-sensitive adhesive compositions
comprising as a rubbery base material elastomeric copolymers and block
copolymers, e.g., based upon styrene/isoprene, having a novel condensed
phase structure wherein polymer branches occur along the polymer backbone,
either at a predetermined location or at random locations. The polymers
are made by a method which comprises the step of reacting, under
polymerization conditions, hydrocarbyl lithium initiator, at least one
anionically polymerizable compound, and an organometallicsubstituted
styrene condensing agent. The reactants may be added simultaneously to
produce a copolymer with polymer branch segments randomly located along
the polymer backbone or sequentially to produce a copolymer with branches
located at the same predetermined location along the polymer backbone. The
resultant polymers may be further reacted with a linking agent to form
multi-arm copolymers. The copolymers and their method of preparation are
respectively claimed in U.S application Ser. Nos. 107,292, and 107,262,
filed 10/9/87, now U.S. Pat. No. 4,857,618, now U.S. Pat. No. 4,857,615.
The resultant elastomeric polymers are compatible with any of a wide
variety of known tackifier resins and plasticizers to produce unique
pressure-sensitive adhesive (PSA) compositions having unexpectedly low
melt viscosities and, thus, excellent melt processability. In addition,
the PSAs of this invention show improved high temperature shear adhesion
relative to their linear counterparts, with the shear strength exhibited
by condensed phase diblock polymer PSAs being particularly surprising in
view of the tensile properties of the base polymers.
Specifically, the method comprises the step of reacting, under
polymerization conditions, the following:
(a) hydrocarbyl lithium initiator;
(b) at least one anionically polymerizable compound; and
(c) a condensing agent having the general formula
CH.sub.2 .dbd.C(R')QY(R).sub.n (X).sub.m I
wherein
Y is tetravalent Si, Ge, Sn or Pb;
X is H, --OR", Cl, Br or F wherein R" is a monovalent lower alkyl group
having from 1 to 6 carbon atoms;
R is hydrogen, a monovalent lower alkyl group having from 1 to 6 carbon
atoms, or phenyl;
Q is phenylene;
R' is hydrogen, a monovalent lower alkyl group having from 1 to 6 carbon
atoms, or phenyl;
m is an integer of 1, 2, or 3; and
n is an integer equal to 3-m, in a mole ratio of (a) to (c) of about (1
+m):1 to form a condensed phase copolymer.
The elastomeric polymers are anionic copolymers comprising at least one
anionically polymerizable monomer and a condensing agent (I) monomer
wherein the mole percentage of condensing agent (I) in each copolymer
segment containing the condensing agent (I) is in the range of about 0.01%
to about 5%.
The polymers are generally copolymers of the condensing agent (I) with
conjugated diene monomer, or are block copolymers of conjugated diene and
vinyl aromatic monomers (wherein at least one block is a copolymer of
condensing agent monomer and either diene or vinyl aromatic monomer). The
monovinyl aromatic monomer yields a hard polymer segment having a high
T.sub.g, i.e., above 25.degree. C. The conjugated diene monomer yields a
soft (generally elastomeric) polymer segment having a low T.sub.g, i.e.,
not greater than about 0.degree. C.
The polymers of the PSA compositions of the invention are preferably
elastomeric anionic polymers comprised of conjugated diene monomer,
typically containing 4 to 12 carbon atoms, monoalkenyl or monovinyl
aromatic monomer and the condensing reagent (I) wherein the mole percent
of condensing reagent in a polymer segment containing such reagent is
about 0.01 to about 5.0, preferably about 0.02 to about 2.0. Typically,
the copolymer contains on a weight basis from about 50% to about 90%
conjugated diene and about 50% to about 10% monoalkenyl or vinyl aromatic
monomer.
In one embodiment, branch points are introduced at predetermined loci in
the polymer chain by addition of condensing agent in a sequential fashion,
i.e., after formation of a living polymer segment via conventional anionic
polymerization techniques. Thus, copolymer is prepared by first forming a
living linear polymer segment, then reacting the living polymer segment
with the condensing reagent to form a condensed living copolymer and next
polymerizing therewith additional polymerizable compound to form a
condensed phase block copolymer. Such a block copolymer may be represented
by the following general formula:
(A).sub.z Z.sub.q --B II
where:
A is a nonelastomeric polymer segment based on a monovinyl aromatic
compound such as styrene, alpha-methylstyrene, para-methylstyrene, and
t-butyl styrene;
B is an elastomeric polymer segment based on a conjugated diene compound,
such as butadiene,isoprene, and piperylene;
Z is the residue of a condensing reagent having the general formula
CH.sub.2 .dbd.C(R')QY(R).sub.n (X).sub.m I
where X, R, Y, Q, R', m and n have been defined above;
q is an integer from 1 to about 10;
x is an integer from 2 to about 10; and
wherein the mole percentage of Z in the segment (A).sub.x Z.sub.q is in the
range of about 0.1% to about 5%.
The method comprises the further step of contacting the resulting condensed
phase block copolymer of Formula II with a multifunctional linking agent
such as a polyalkenyl aromatic linking agent under reactive conditions
thereby forming a multi-arm condensed phase block copolymer. Such a block
copolymer may be represented by the following general formula:
[(A).sub.x Z.sub.q--B].sub.y L.sub.z III
where:
A, Z, B, x, and q have been defined above;
L is the residue of a multifunctional linking agent;
z is an integer from zero to about 10;
y is an integer from 1 to about 50 and, when y is 1, z is zero;
wherein the mole percentage of Z in the segment (A).sub.x Z.sub.q is in the
range of about 0.1% to about 5%.
The method also comprises first forming a living linear polymer segment,
adding a second polymerizable compound to form a living linear block
copolymer segment, then reacting the living linear block copolymer segment
with the condensing reagent to form a condensed living block copolymer,
and next polymerizing therewith additional polymerizable compound to form
a condensed phase block copolymer represented by the following general
formula:
(A--B).sub.x Z.sub.q --B IV
where:
A, B, Z, x and q are defined above and wherein the mole percentage of Z in
the segment (A--B).sub.x Z.sub.q is in the range of from about 0.01% to
about 1%.
The method comprises the further step of contacting resulting block
copolymer IV with a multifunctional linking agent under reactive
conditions thereby forming a multi-arm condensed phase block copolymer
represented by the general formula shown below:
[(A--B).sub.x Z.sub.q --B].sub.y L.sub.z V
wherein:
A, B, Z, L, x, q, y and z are defined above and wherein the mole percentage
of Z in the segment (A--B).sub.x Z.sub.q is in the range of about 0.01% to
about 1%.
Other condensed phase block copolymers besides II and IV are also
contemplated and may be linked to form multi-arm condensed phase block
copolymers other than III and V. Such block copolymers, including II, III,
IV, and V, may be represented by the general formula:
[(W).sub.x Z.sub.q --W'].sub.y L.sub.z VI
wherein:
W is selected from the group consisting of A, B, BA, and AB, W' is selected
from the group consisting of B, BA and AB, and A, B, Z, L, x, q, y and z
are defined above, and wherein the mole percentage of Z in the segment
(W).sub.x Z.sub.q is in the range of from about 0.01% to about 5%.
In a second embodiment, randomly placed branch centers are generated on the
polymer chain by polymerization of a mixture of condensing agent and
anionically polymerizable monomer or monomers. The method involves
simultaneously reacting a hydrocarbyl lithium initiator, polymerizable
compound, and condensing reagent to form a living condensed phase
copolymer having a randomly-branched structure which may be represented by
the following general formula:
B/Z VII
wherein B and Z are defined above, and wherein the mole percentage of Z in
the copolymer is from about 0.01% to about 1%.
Copolymer VII may be further reacted with a multifunctional linking agent,
thereby forming a multi-arm condensed phase copolymer. Such a copolymer
may be represented by the general formula:
(B/Z).sub.y L.sub.z VIII
wherein B, Z, L, y and z are defined above, and wherein the mole percentage
of Z in the unlinked copolymer is from about 0.01% to about 1%.
Monovinyl aromatic monomer may be polymerized with condensing reagent to
form a randomly-branched living copolymer which may be further treated by
adding a different polymerizable compound such as butadiene, isoprene, or
piperylene, after completion of the simultaneous reaction and permitting
the different polymerizable compound to copolymerize with the living
copolymer to form a condensed phase block copolymer. The resultant
copolymer may be further reacted with a multi-functional linking agent
thereby forming a multi-arm condensed phase block copolymer. Such block
copolymer may be represented by the general formula:
[(A/Z)--B].sub.y L.sub.z IX
wherein A, B, Z, L, y and z are defined above, and wherein the mole
percentage of Z in the segment A/Z is in the range of from about 0.1% to
about 5%.
In addition, a randomly-branched living copolymer derived from monovinyl
aromatic monomer may be further treated by adding a mixture of a different
polymerizable compound and additional condensing reagent, after completion
of the simultaneous reaction, and permitting the mixture to copolymerize
with the living copolymer to form a block copolymer having "condensed"
structure randomly placed in both blocks. This block copolymer may be
further reacted with a multifunctional linking agent under reactive
conditions thereby forming a multi-arm condensed phase block copolymer.
Such a block copolymer may be represented by the general formula:
[(A/Z)--(B/Z)].sub.y L.sub.z X
wherein A, B, Z, L, y and z are defined above, and wherein the mole
percentage of Z in the segment A/Z is in the range of from about 0.1% to
about 5% and in the segment B/Z is from about 0.01% to about 1%.
Alternatively, a different condensed phase block copolymer may be prepared
by first forming a living linear polymer segment, adding a mixture of a
second polymerizable compound and the condensing reagent, and then
permitting the mixture to copolymerize with the living linear polymer
segment produced by polymerization of the first polymerizable compound.
The resulting block copolymer may be further modified by contacting it
with a multifunctional linking agent under reactive conditions thereby
forming a multi-arm condensed phase block copolymer. Such a block
copolymer may be represented by the general formula:
[A--(B/Z)].sub.y L.sub.z XI
wherein A, B, Z, L, y and z are defined above, and wherein the mole
percentage of Z in the segment B/Z is in the range of from about 0.01% to
about 1%. The unlinked block copolymer may be alternatively modified to
include an additional linear polymer segment to provide a block copolymer
which may be represented by the general formula
A--(B/Z)--A X | | |
