Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Mixing of two or more solid polymers; mixing of solid...
Reexamination Certificate
2001-09-18
2003-07-15
Teskin, Fred (Department: 1713)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Mixing of two or more solid polymers; mixing of solid...
C525S271000, C525S272000, C526S079000, C526S086000, C526S174000
Reexamination Certificate
active
06593430
ABSTRACT:
The invention relates to block copolymers comprising at least two hard blocks S
1
and S
2
made from vinylaromatic monomers and, between these, at least one random soft block B/S made from vinylaromatic monomers and from dienes, where the proportion of the hard blocks is above 40% by weight, based on the total block copolymer.
The invention further relates to a process for preparing the block copolymers, and also to their use.
U.S. Pat. No. 4,939,208 describes linear, transparent styrene-butadiene block copolymers of the structure S
1
—B
1
—B/S—S
2
. The polymerisation of styrene and butadiene in the presence of a Lewis base, in particular tetrahydrofuran (randomizer), gives the random copolymer block B/S. The length of the B/S segment depends on the amount of Lewis base.
EP-A 0 654 488 describes polymodal coupled styrene-butadiene block copolymers. The blocks B/S contain a styrene gradient (tapered block). By adding polar compounds, such as tetrahydrofuran, as randomizers, the random proportion in the blocks can be increased.
Polymerisation of styrene and butadiene in the presence of small amounts of tetrahydrofuran as randomizer gives a high proportion of homopolybutadiene blocks and a tapered transition to the polystyrene block. If the amount of tetrahydrofuran is increased, this gives butadiene-styrene copolymer blocks with some degree of random character, but at the same time there is also a sharp increase in the relative proportion of 1,2 linkages in the polydiene (1,2-vinyl content). The high 1,2-vinyl content, however, impairs the thermal stability of the corresponding block copolymers and increases the glass transition temperature.
DE-A 19615533 describes an elastomeric styrene-butadiene block copolymer in which the relative proportion of 1,2 linkages in the polydiene is 15% and the proportion of the hard phase is from 1 to 40% by volume. The polymerization of the soft phase is undertaken in the presence of a soluble potassium salt.
The use of potassium alcoholates or potassium hydroxide and of organolithium polymerization initiators is described in U.S. Pat. No. 3,767,632, U.S. Pat. No. 3,872,177, U.S. Pat. No. 3,944,528 and by C. W. Wolfford et al. in J. Polym. Sci, Part. A-1, Vol. 7 (1969), pp. 461-469.
Random copolymerisation of styrene and butadiene in cyclohexane in the presence of soluble potassium salts is described by S. D. Smith, A. Ashraf in Polymer Preprints 34 (2), 672 (1993) and 35 (2), 466 (1994). The soluble potassium salts mentioned comprise potassium 2,3-dimethyl-3-pentanolate and potassium 3-ethyl-3-pentanolate.
It is an object of the present invention to provide a glass-clear impact-modified polystyrene which has a balanced toughness/stiffness ratio and does not have the abovementioned disadvantages. In particular, the impact-modified polystyrene should have high intrinsic thermal stability and reduced thixotropy. It should also be compatible with styrene polymers, so that transparent mixtures are obtained. Efficiency in impact-modification of styrene polymers, in particular standard polystyrene, should be increased.
We have found this object is achieved by means of block copolymers comprising at least two hard blocks S
1
and S
2
made from vinylaromatic monomers and, between these, at least one random soft block B/S made from vinylaromatic monomers and from dienes, where the proportion of the hard blocks is above 40% by weight, based on the total block copolymer. In preferred block copolymers, the 1,2-vinyl content in the soft block B/S is less than 20%.
For the purposes of the present invention, vinyl content is the relative proportion of 1,2 linkages of the diene units, based on the total of 1,2, 1,4-cis and 1,4-trans linkages. The 1,2-vinyl content of the soft blocks is preferably from 10 to 20%, in particular from 12 to 16%.
vinylaromatic monomers which may be used for the hard blocks S
1
and S
2
or else for the soft blocks B/S are styrene, &agr;-methylstyrene, p-methylstyrene, ethylstyrene, tert-butylstyrene, vinyltoluene or mixtures of these, preferably styrene.
Preferred dienes for the soft block B/S are butadiene, isoprene, 2,3-dimethylbutadiene, 1,3-pentadiene, 1,3-hexadienes or piperylene or mixtures of these, particularly preferably 1,3-butadiene.
It is preferable for the block copolymer to be composed exclusively of hard blocks S
1
and S
2
and also of at least one random soft block B/S, and not to contain any homopolydiene blocks B. Preferred block copolymers contain external hard blocks S
1
and S
2
having different block lengths. The molar mass of S
1
is preferably from 5000 to 30000 g/mol, in particular from 10,000 to 20,000 g/mol. The molar mass of S
2
is preferably above 35,000 g/mol. Preferred molar masses of S
2
are from 50,000 to 150,000 g/mol.
Between the hard blocks S
1
and S
2
there may also be more than one random soft block B/S. Preference is given to at least 2 random soft blocks (B/S)
1
and (B/S)
2
having different proportions of vinylaromatic monomers and therefore different glass transition temperatures.
The block copolymers may have a linear or star-shaped structure.
The linear block copolymer used preferably has the structure S
1
—(B/S)
1
—(B/S)
2
—S
2
. The molar ratio of vinylaromatic monomer to diene S/B in the block (B/S)
1
is preferably below 0.25 and in the block (B/S)
2
is preferably from 0.5 to 2.
The star-shaped block copolymers used are preferably those which have a structure in which at least one arm of the star has a block sequence S
1
—(B/S) and one arm of the star has the block sequence S
2
—(B/S), or those in which at least one arm of the star has the block sequence S
1
—(B/S)—S
3
and at least one arm of the star has the block sequence S
2
—(B/S)—S
3
. S
3
here is another hard block made from the vinylaromatic monomers mentioned.
Particular preference is given to star-shaped block copolymers having structures in which the star has at least one branch having the block sequence S
1
—(B/S)
1
—(B/S)
2
and at least one branch having the block sequence S
2
—(B/S)
1
—(B/S)
2
, or in which the star has at least one branch with the block sequence S
1
—(B/S)
1
—(B/S)
2
—S
3
and at least one branch with the block sequence S
2
—(B/S)
1
—(B/S)
2
—S
3
. The molar ratio of vinylaromatic monomers to diene, S/B, is preferably in the range from 0.5 to 2 in the outer block (B/S)
1
and preferably below 0.5 in the inner block (B/S)
2
. The higher content of vinylaromatic monomer in the outer random block (B/S)
1
makes the block copolymer more ductile for unchanged total butadiene content, and this proves to be particularly advantageous in blends with standard polystyrene.
The star-shaped block copolymers with the additional inner block S
3
have higher stiffness at comparable ductility. The block S
3
therefore acts as a filler within the soft phase, without changing the ratio of hard phase to soft phase. The molar mass of the blocks S
3
is generally substantially lower than that of the blocks S
1
and S
2
. The molar mass of S
3
is preferably in the range from 500 to 5 000 g/mol.
The novel block copolymers may, for example, be formed by sequential anionic polymerisation, where at least the polymerisation of the blocks (B/S) takes place in the presence of a randomizer. The presence of randomizers brings about the random distribution of the dienes and vinylaromatic units in the soft block (B/S). Suitable randomizers are donor solvents, such as ethers, e.g. tetrahydrofuran, or tert-amines, or soluble potassium salts. In the case of tetrahydrofuran, the amounts used for ideal random distribution are generally above 0.25 percent by volume, based on the solvent. At low concentrations, “tapered” blocks are obtained with a gradient in comonomer makeup.
At the same time, the larger amounts specified of tetrahydrofuran are used, the proportion of 1,2 linkages of the diene units increases to from about 30 to 35%.
In contrast, if potassium salts are used there is only an insignificant increase in the 1,2-vinyl content in the soft blocks. The resultant block copolymers are therefore less susceptible to crossli
Fischer Wolfgang
Gausepohl Hermann
Knoll Konrad
Koch Jürgen
Naegele Paul
BASF - Aktiengesellschaft
Keil & Weinkauf
Teskin Fred
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