Eraser with thermoplastic binder

An eraser composition composed of conventional fillers and additives wherein the binder is a styrene-butadiene-styrene block polymer having a butadiene portion of 10 to 40%. The eraser composition can be easily extruded using simple commercial machines presently available and thus is inexpensive to manufacture. Furthermore, the use of this particular material as a binder produces a superior eraser composition.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The invention relates to an eraser, composed of fillers, a binder of a 
thermoplastic material based on synthetic rubber as the binder, and 
possibly other additions. 
2. Description of the Prior Art 
In a known eraser of this type, described in Belgian Pat. No. 673,229, 
materials, such as, polychloroprene, chlorosulfonated polyethylene, 
polybutadiene, polyacrylonitrile or polyisobutylene are used as binders. 
Moreover, factice can also be used in this case. 
However, this eraser is disadvantageous primarily because of either an 
additional vulcanization process or a peroxidic crosslinking is required 
for the particular binders. In addition, as a rule, large amounts of 
softeners, such as, for example, chloroparaffins, which have a great 
tendency to migrate, are required. When factice (chlorosulfur rape oil) is 
used which is a natural product whose composition cannot be exactly 
defined, it is not possible to obtain consistency in the final products 
without continuous adjustments to the process, particularly, with respect 
to the hardness, abrasion properties, or the absorption of softeners. 
In accordance with another proposal in U.S. Pat. No. 3,738,951, a 
butadiene-styrene block polymer with a butadiene to styrene ratio of 4:1 
to 1:1 (80:20 to 50:50 percent by weight) is to be used as the binder. 
Furthermore, in this case, the use of factice and of a naphthenic oil as 
the softeners is suggested. 
Also, there are the disadvantages resulting from the problems created by 
the use of factice, and the aromatic substances in the naphthenic oils may 
be physiologically harmful. 
SUMMARY OF THE INVENTION 
I have discovered an eraser which does not have the above-mentioned 
disadvantages. More specifically, the eraser of the present invention does 
not require an additional production process, does not have the tendency 
to migrate, and can produce consistent products which are physiologically 
harmless. In addition, the present eraser can be extruded using simple, 
commercially available machines or can be produced extremely 
inexpensively, without complicated manufacturing devices and methods, by 
injection molding. 
More specifically, the present invention relates to an eraser composition 
composed of a styrene-butadiene-styrene block polymer (sometimes referred 
to as SBS block polymer) with a butadiene portion of 10 to 40% by weight 
as a binder and conventional fillers and additives. 
The disadvantages of the presently known erasers, in which the butadiene 
portion is equal to or substantially greater than the styrene portion, can 
be avoided and especially inexpensive and qualitatively excellent erasers 
can be produced with a styrene-butadiene-styrene block polymer in which 
the styrene portion preponderates as in the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
It is particularly advantageous with the present invention that 5 to 25% by 
weight of stereospecific, liquid, low-viscosity polybutadiene is admixed 
with the binder of the invention. The admixed polybutadiene preferably has 
a molecular weight of 1000 to 4000 and a viscosity of 700 to 3000 mPa.s. 
Furthermore, it is advantageous when the polybutadiene has 70 to 80% 1,4 
cis-double bonds, 15 to 30% 1,4 trans-double bonds and 1 to 5% vinyl 
double bonds. 
It has also been found advantageous to add 1 to 20% by weight 
polynorbornene (a polymer of di-cyclopentadiene) as an additional binder. 
This addition serves to improve and increase the abrasion which is 
particularly important for satisfactory functioning of the eraser. 
Since polynorbornene has a relatively large specific surface and an 
extremely good capability to absorb fats and oils, its presence makes it 
possible to achieve an especially fine and absorptive abrasion and to 
significantly improve the erasing efficiency. 
Suitable fillers for use in the present invention include those 
conventionally known, such as, chalk, kaolin, amorphous silicic acid, 
pumice stone, corundum, powdered quartz and glass. 
The following examples illustrate the invention. The quantities are given 
in parts by weight, and since they are with reference to a total of 100 
parts, are simultaneously given in percent by weight. 
EXAMPLE 1 
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Total Parts 
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Styrene-butadiene-styrene block polymer 
13.0 
with 60% styrene portion 
Polybutadiene MG 1500 12.0 
Chalk (calcium carbonate) 1-6.mu. 
74.9 
2,2'-isobutylidene-bis-(4,6-dimethylphenol) 
0.1 
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EXAMPLE 2 
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Styrene-butadiene-styrene block polymer 
11.0 
with 80% styrene portion 
Polybutadiene MG 1500 13.0 
Chalk (calcium carbonate) 1-6.mu. 
70.9 
Polynorbornene 5.0 
2,2,'-methylene-bis-(4-methyl-6-tert.-butyl- 
0.1 
phenol) 
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EXAMPLE 3 
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Styrene-butadiene-styrene block polymer 
15.0 
with 70% styrene portion 
Polybutadiene MG 3000 5.0 
Chalk (calcium carbonate) 1-10.mu. 
74.0 
Aliphatic hydrocarbon with 18-36 carbon 
6.0 
atoms, TP 50-60.degree. C., MG 360-420 
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EXAMPLE 4 
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Styrene-butadiene-styrene block polymer 
18.0 
with 80% styrene portion 
Polybutadiene 12.0 
Chalk (calcium carbonate) 10-40.mu. 
35.0 
Pumice powder 34.9 
Di-lauryl-3,3'-thiodipropionate 
0.1 
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EXAMPLE 5 
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Styrene-butadiene-styrene block polymer 
12.0 
with 60% styrene portion 
Polybutadiene MG 3000 4.0 
Polynorbornene 8.0 
Aliphatic hydrocarbon with 30-40 carbon atoms, 
8.0 
TP 62-92.degree. C., MG 580-700 
Chalk (calcium carbonate) 1-5.mu. 
67.9 
Di-lauryl-3,3'-thiodipropionate 
0.1 
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EXAMPLE 6 
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Styrene-butadiene-styrene block polymer 
20.0 
with 60% styrene portion 
Aliphatic hydrocarbon (as in Example 3) 
15.0 
Amorphous silicic acid 10.0 
Chalk 54.7 
Antioxidants 0.2 
Dye 0.1 
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EXAMPLE 7 
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Styrene-butadiene-styrene block polymer 
20.0 
with 80% styrene portion 
Polybutadiene MG 1500 15.0 
Kaolin 59.7 
Antioxidants 0.2 
Polynorbornene 5.0 
Dye 0.1 
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EXAMPLE 8 
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Styrene-butadiene-styrene block polymer 
25.0 
with 70% styrene portion 
Polynorbornene 15.0 
Powdered quartz 50.0 
Aliphatic hydrocarbon (as in Example 5) 
10.0 
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