Method of fabricating an eraser

A method of fabricating an eraser which contains a main basic material of low crystallizable polyolefin thermoplastic elastomer and normal synthetic rubbers having no hard segment for performing a crosslinking effect, namely, any one or more of chlorinated polyethylene, ethylene-propylene terpolymer, ethylene ethyl acrylate, ethylene-vinyl acetate copolymer, atactic polypropylene, styrene-butadiene rubber, nitrile rubber and the like. The thermoplastic elastomer exhibits similar properties to vulcanized rubber without particular vulcanizing operation and crosslinking reaction. In case that less than 40 parts by weight of the elastomer is contained in the eraser, the composition does not form an eraser, while in case that higher than 100 parts by weight of the elastomer is contained in the eraser, the composition incorporates excessive rubber-like properties. The method comprises the steps of mixing and kneading 50 to 800 parts by weight of factice, 0 to 500 parts by weight of filler and predetermined amount of pigment which 40 to 100 parts by weight of the low crystallizable polyolefin thermoplastic elastomer. Thus, the eraser is remarkably improved in the properties such as resistances to deterioration by aging, exposure to sunlight, migration of plasticizer, and tearing strength by its metallic band.

BACKGROUND OF THE INVENTION 
This invention relates to a method of fabricating an eraser and, more 
particularly, to a method of fabricating an eraser which contains low 
crystallizable polyolefin thermoplastic elastomer of a main basic material 
and normal synthetic rubbers having no hard segment portion for performing 
a crosslinking effect as required, i.e., any one or more of chlorinated 
polyethylene, ethylene-propylene terpolymer, ethylene ethyl acrylate, 
ethylene-vinylacetate copolymer, atactic polypropylene, styrene-butadiene 
rubber (SBR), nitrile rubber (NBR) and the like to not only provide 
superior erasing performance as compared with the conventional plastic 
eraser and to but also remarkably improve its properties such as its 
deterioration by aging, sunlight exposure, migration of plasticizer, weak 
body (low rigidity), weak tearing strength by an eraser retaining metallic 
band of the defects of the conventional eraser. 
The conventional erasers sold at present in the market are largely 
classified into two types, one of which is a natural rubber eraser 
fabricated by mixing a factice and a filler with natural rubber of its 
basic material and vulcanizing the composition with sulfur in a 
semivulcanizing state, and the other of which is a synthetic rubber or 
plastic eraser fabricated by mixing a filler with emulsion polymerization 
type vinyl chloride (paste resin) of its basic material, kneading the 
composition with excess amount of plasticizer and then steam heating the 
composition to thereby gel the composition in semigelled state. 
Although the natural rubber eraser incorporates advantageous properties 
such as capability of free selection of its rigidity, and large crack and 
tear strengths, it contrary has insufficient erasing capacity and 
undesirable using touch. This eraser also accomodates a difficulty in 
controlling its vulcanized state, namely, in case that the neutral rubber 
is insufficiently vulcanized, it becomes tacky, while in case that the 
natural rubber is excessively vulcanized, it loses its erasing properties 
to consequently occur irregular erasing capacity under quality control of 
the eraser and does not exhibit stably vulcanized state even after the 
eraser product is fabricated from the natural rubber, with the result that 
the eraser is adversely affected by exposure to sunlight and high 
temperature to thus progressively vulcanize the eraser, to thereby 
continuously increase its rigidity to cause the eraser to become brittle 
or remarkably hardened on the surface, and to thereby entirely loss its 
erasing function. Further, its excess unreacted sulfur blooms to the 
eraser surface to thereby disadvantageously lower the erasing capacity, 
and yellows the eraser to cause a great loss of its beauty appearance. 
Moreover, its production not only necessitates complicated technique 
requiring high skillfulness at the steps of kneading and vulcanizing crude 
rubber but also adversely affects its cost due to disposal of a great deal 
of chip rubber produced after the vulcanizing step without any 
utilization. 
The synthetic rubber or plastic eraser containing mainly vinyl chloride, on 
the other hand, incorporates very preferable erasing capacity, favorite 
use feeling and outstanding erasing function better than the natural 
rubber eraser, however has a difficulty in employing excess amount of 
plasticizer and controlling the composition in semigelled state upon 
heating of the composition during production of the eraser and thus 
requires high technology to obtain uniform quality of eraser in the same 
manner as in the vulcanizing step of the natural rubber eraser. That is, 
in case that the plastic eraser is insufficiently gelled, it becomes 
brittle to cause the composition to be discretely collapsed and to become 
impossible to erase, while in case that the plastic eraser is excessively 
gelled, it is not worn in use to cause the eraser chips not to separate 
from the eraser and to thereby remarkably lower its erasing capacity. 
Although the plastic eraser product accommodates sufficiently higher 
resistances to exposure to sunlight and aging than the natural rubber 
eraser, it causes, since it is semigelled, its liberated plasticizer to 
bleed on the eraser surface to thereby migrate to other resin such as 
coated surface of a pencil or pencil case to erode the surface and to thus 
cause the surface to disadvantageously become tacky. The elimination of 
the disadvantages is expected, but they are not yet sufficiently 
countermeasured. 
Although a pencil with an eraser or an eraser contained in a mechanical 
pencil is convenient in use, the eraser thus used requires not only 
general performance of the ordinal eraser but also, since it is engaged by 
a metallic band for retaining the eraser and is caulked thereby, its 
pinch-off or tear resistance by the burrs of the periphery and caulked 
portion of the band. The reason why the plastic eraser is not used for 
this application is that the plastic eraser does not have sufficient tear 
resistance and cannot endure this using pressure. Since the natural rubber 
eraser has almost durable tear resistance with certain rigidity and 
resistance to the using pressure, it is mostly used for the present pencil 
with the eraser and the eraser contained in the mechanical pencil. 
However, the natural rubber eraser still incorporates such disadvantages 
that has insufficient weather resistance and severe deterioration by aging 
as was described before, and particularly becomes impossible to erase in 
several days under exposure to sunlight. There have been proposed various 
trials to eliminate these disadvantages by improving vulcanizer or 
plasticizer or the like, however any of these trials has not yet been 
completed. There have also been proposed, as its plastic materials for the 
plastic eraser, employments of one or more of atactic polypropylene, 
ethylene vinylacetate copolymer (EVA), ethylene-propylene (EP) rubber, 
styrene-butadiene rubber (SBR), nitrile rubber (NBR), etc. as synthetic 
rubber and plastic having rubber-like properties in addition to polyvinyl 
chloride (PVC), however they do not have sufficiently satisfied high 
quality totally in view of its erasing capacity, mechanical strength and 
aging stability. 
There has further been proposed a prior invention of an eraser having novel 
block copolymer in addition to natural rubber and polyvinyl chloride (PVC) 
as disclosed in Japanese Patent Publication No. 23450/1971, the 
rubber-like composition of which eraser is monovinyl-substituted aromatic 
hydrocarbon copolymer mainly having conjugated diolefin and styrene of 
basic materials, so-called styrene (SBS block copolymer) thermoplastic 
elastomer. When this composition is used as the eraser basic material, it 
is thermally unstable due to its unsaturated bond, and is difficult to 
continue kneading and molding steps at high shearing force and temperature 
for long time. The composition starts thermally deteriorating at higher 
than 150.degree. C. for long time to finally cause the composition to 
become brittle. The composition accommodates such disadvantages that, in 
case where its styrene content is increased in the eraser to improve its 
workability, it deteriorates its flexibility and exhibits residual strain 
particularly at high temperature to cause deformation of the eraser or 
increase in hardness of the eraser to thereby lack its wearing properties 
so as to undesirably deteriorate its erasing capacity. 
Eraser stock material is, incorporating excessive wearing properties, not 
preferred. In this respect, of the present thermoplastic elastomers, 
urethane series incorporates the most plastic-like properties and high 
hardness, polyester, styrene and polyolefin series sequentially 
incorporate in this order increased rubber-like properties, and the 
polyolefin series accommodates the most suitable wearing properties as its 
basic material. The styrene series deteriorates its oil and solvent 
resistances, and when it is used for the eraser basic material, the resin 
itself solely incorporates insufficient erasing capacity. Although white 
factice is added, in order to increase its rigidity, to the resin or other 
basic material is blended therewith, it is difficult to mix a great deal 
of factice or basic material with the resin composition due to its 
deterioration of workability and to tendency of its embrittlement. When 
this eraser is used for an eraser with a pencil or contained in a 
mechanical pencil, it is readily torn by its metallic band to be thus 
insufficient in use. 
There have also been proposed prior inventions of resin erasers which 
respectively employed ethylene-vinyl acetate copolymer (EVA) and atactic 
polypropylene (APP) as disclosed in Japanese Patent Publication Nos. 
44245/1977 and 112429/1977. These resins are apparently rubber-like 
elastic material, however since they do not incorporate cross-linking 
structure, they tend to occur cold flow at cold working time to thus cause 
the eraser material to adhere onto paper surface and to thereby perform no 
erasing function but to contrary contaminate the paper surface. When the 
eraser is allowed to stand for under static pressure applied thereto, it 
is plastically deformed to cause the eraser to disable to retain its 
configuration. Furthermore, the eraser itself becomes brittle and cannot 
thus endure its tear by its metallic band. 
SUMMARY OF THE INVENTION 
Accordingly, an object of this invention is to provide a method of 
fabricating an eraser which incorporates high erasing capacity, weather 
and aging resistances by eliminating the disadvantages of the conventional 
eraser. 
Another object of this invention is to provide a method of fabricating an 
eraser which does not migrate plasticizer contained therein nor erode the 
coating or resin surface of a pencil or pencil case. 
Yet another object of this invention is to provide a method of fabricating 
an eraser which can adhere its tearing properties by its metallic band and 
incorporates its body durable for its using pressure. 
Still another object of this invention is to provide a method of 
fabricating an eraser which does not require steps of kneading, 
vulcanizing and gelling itself necessary for skillfulness. 
Still another object of the invention is to provide a method of fabricating 
an eraser which can effectively utilize regenerative materials of its 
stock materials. 
In order to perform the aforementioned and other objects, the present 
inventors have discovered low crystallizable polyolefin thermoplastic 
elastomer, as a result of various points of view and studies conducted to 
search plastic material satisfying the eraser material incorporating 
natural rubber-like properties and plastic workability. 
Thermoplastic elastomers of the type developed recently as stock material 
for an eraser exhibiting similar properties to vulcanized rubber without 
particular vulcanizing operation and crosslinking reaction and being 
similarly moldable to the ordinal thermoplastic resin are classified 
largely into styrene, polyurethane and polyester block copolymer, and 
polyolefin graft copolymerization polymer. The most suitable basic 
material for the eraser of this invention is preferably the low 
crystallizable polyolefin graft copolymer which incorporates preferable 
elasticity, wearing properties and mechanical strength as compared with 
other thermoplastic elastomers. 
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
According to the present invention, the eraser fabricated by the method of 
this invention comprises low crystallizable polyolefin thermoplastic 
elastomer which contains the types from hard resin elastomer like graft 
copolymer resin produced by conducting ethylene and .alpha.-olefin of 
stock materials by means of coordinating anionic polymerization and one 
type of graft reaction to soft resin elastomer like rubber having a 
hardness of 50 (by Shore A scale) on the surface thereof. The eraser 
fabricated by this invention preferably employs the elastomer resin having 
relatively low surface hardness like rubber. 
The elastomer resin employed in this invention consists of a hard segment 
portion, namely, a thermoplastic resin layer and a soft segment portion, 
namely, an amorphous elastomer layer in polymer in the same manner as 
other thermoplastic elastomers so that the hard segment performs its 
physical bond and crosslinking structure function corresponding to the 
crosslinking point of rubber. This resin elastomer is reversible so that, 
when it is heated, its hard segment portion is molten to cause its 
crosslinking structure to collapse to thus become fluid state and 
accordingly moldable in any shape while when it is cooled, the hard 
segment portion is hardened to cause the molten hard segment portion to 
become again crosslinking structure. When, therefore, mixing kneading and 
molding the eraser compositions, it can be molded in the same molding 
machine as that for the conventional thermoplastic resin, requiring no 
particular vulcanizing and gelling steps. 
Since the resin elastomer employed in this invention has very high 
flexibility, it does not need mass plasticizer as PVC eraser at all, but 
has no apprehension of occurring failure due to migration of plasticizer 
without any plasticizer. Since the resin elastomer incorporates thermally 
reversible properties to cause no chemical change therof, sprue, runner 
and scraps produced in the molding step can be reused, resulting in 
rational resource-saving and energy-saving in its process and cycle as 
compared with the conventional rubber series and PVC eraser. 
Inasmuch as the resin elastomer employed in this invention incorporates not 
only soft and thermal stability, wide molding temperature range, superior 
weather and aging resistances peculiar for the polyolefin series, and high 
resistance to oil, solvent and medicines, but also weaker crosslinking 
structure than the natural rubber, it accommodates suitable wearing 
properties most adapted for the basic material of the eraser. In addition, 
the resin elastomer has superior elastic properties at low temperature and 
can retain its flexibility up to -70.degree. C. even without plasticizer 
to thereby allow the elastomer to extremely stably continue its erasing 
capacity, though the PVC eraser is hardened at low temperature to cause 
the eraser to become brittle to thereby exhibit its lower erasing 
performance. 
The low crystallizable polyolefin graft copolymer thermoplastic elastomer 
employed in this invention is readily available as a raw material from 
various commercial sources. For example, E. I. DuPont de Nemours & Co. 
sells polyolefin thermoplastic elastomers under the trade designations TPN 
and SOMEL, and Uniroyal sells such elastomers under the trade designation 
TPR. Other sources of such elastomers include Mitsui Petrochemical 
Industries Ltd. selling under the trade designations MILASTOMER and 
TAFMER, and Sumitom Chemical Company Ltd. selling under the trade 
designation ESPREN EPF. The polyolefin elastomer used in the present 
invention is thus well known in the art. 
The present invention has been completed to discover the superior 
properties of the low crystallizable polyolefin graft polymer 
thermoplastic elastomer as a result of severe studies and searches of all 
the single and composite materials by the present inventors. That is, the 
low crystallizable polyolefin graft polymer thermoplastic elastomer forms 
the basic resin material in the plastic eraser composition of this 
invention. In case that less than 40 parts by weight of the polyolefin 
graft polymer thermoplastic elastomer is contained in the eraser, the 
composition does not form an eraser itself, while in case that higher than 
100 parts by weight of the thermoplastic elastomer is contained in the 
eraser, the composition incorporates excessive rubber-like properties, 
resulting in slight wearing properties in use to thereby cause the worn 
scraps not to separate from the eraser to thus disadvantageously lower the 
erasing capacity of the eraser. 
The lower crystallizable polyolefin graft polymer thermoplastic elastomer 
employed in this invention performs solely the erasing properties as a 
basic material, however in order to facilitate ready fabrication of the 
eraser and to improve its smooth using touch and tearing resistance by its 
metallic band, 50 to 100 parts by weight of normal synthetic rubber having 
no crosslinking structure may also be added to 100 parts by weight of the 
low crystallizable polyolefin graft polymer thermoplastic elastomer. The 
synthetic rubber may contain one or more of synthetic resins incorporating 
rubber-like properties such as, for example, chlorinated polyethylene, 
ethylene-propylene-diene terpolymer (EPDM), ethylene ethyl acrylate (EEA), 
ethylene-vinyl acetate copolymer (EVA), atactic polymer polypropylene 
(APP), styrene-butadiene rubber (SBR), nitrile rubber (NBR), etc. These 
synthetic rubbers may impart flexibility to the low crystallizable 
polyolefin graft polymer thermoplastic elastomer eraser composition, and 
provide effects of aiding the wearing properties of the eraser in use and 
increasing the size of erasing scraps to thereby prevent the pulverized 
and scattered scraps from contaminate the environments. Particularly, the 
chlorinated polyethylene of these synthetic rubbers incorporates 
preferable compatibility with the low crystallizable polyolefin graft 
polymer thermoplastic elastomer to enable mixture of a great deal of 
factice essentially for enabling filling of a large quantity of filler in 
the eraser to thereby exhibit high erasing capacity of the eraser. The 
chlorinated polyethylene thus added to the eraser remarkably enhances the 
erasing capacity and use feeling, improves the keading and molding 
properties in its process, increases its bending strength, sufficiently 
endures using pressure, and exhibits tearing resistance by its metallic 
band. The chlorinated polyethylene used for the eraser is a rubber-like 
elastic resin obtained by mixing 30 to 40% by weight of chlorine with 
polyethylene and chlorinating the mixture, and incorporates no 
crosslinking structure, but accommodates superior weather, oil, solvent 
and flame resistances, superior compatibility with various resins, 
non-crystalline state to thus cause preferable fluidity at molding time 
and capability of a great deal of filler mixture. 
The factice employed in the eraser fabricated by this invention is produced 
by vulcanizing vegetable or fish oil with sulfur chloride, and is spongy 
white powder, which is commonly called "white factice". The factice 
permits the eraser to exhibit erasing function so that the more the 
factice is mixed with the eraser, the higher the erasing effect becomes in 
the eraser. However, in case that the factice is excessively added to the 
eraser, the composition becomes difficult to be molded and brittle to thus 
cause the eraser to be disabled for use. The factice is preferably mixed 
in amounts of 50 to 800 parts by weight, preferably more than 500 parts by 
weight. 
The filler may contain, for example, calcium carbonate, magnesium 
carbonate, silica, clay, alumina, titanium oxide, etc., and/or suitably 
pigment as coloring material for presenting color in the eraser. The 
amount of the filler in the eraser depends essentially upon its using 
touch, rigidity, etc. in response to the intended product eraser, and is 
preferably in the range of 0 to 500 parts by weight. 
All the mixing steps used for the normal rubber and plastic may be applied 
to the step of mixing the compositions in the eraser in the method of 
fabricating the eraser of this invention, that is, include a dry blending 
process by Henschel mixer, a melt blending process by Banbury mixer, a 
mill blending process by mixing mill, a blending process by screw kneader, 
etc. The rubber composition uniformly kneaded can be press molded at 
approx. 150.degree. C. or can be extrusion molded by a normal screw 
extruder at approx. 150.degree. C..+-.20.degree. C. The method of 
fabricating an eraser of this invention preferably comprises the steps of 
extrusion molding the composition at 150.degree. C..+-.20.degree. C. under 
resin pressure of 300.+-.50 kg/cm.sup.2 through a die of predetermined 
shape mounted at a screw extruder having a vent hole, cooling the molded 
composition, and cutting it in required size.

The present invention will be hereinafter described further by way of 
typical exemplary process of the preferred embodiments thereof. 
______________________________________ 
Mixing example number 
Mixing composition 
NO. 1 NO. 2 NO. 3 
______________________________________ 
Low crystallizable polyolefin 
90 60 60 
thermoplastic elastomer 
Chlorinated polyethylene 
0 30 30 
Factice 600 400 600 
Heavy calcium carbonate 
300 200 200 
______________________________________ 
EXAMPLE 1 
Stock materials mixed at the ratio as listed above table were uniformly 
kneaded at 130.+-.20.degree. C. of surface temperature of mixing mill to 
thus obtain rubber composition. After this rubber composition was hot 
pressure molded at 110.degree. to 150.degree. C. under the pressure of 
approx. 50 kg/cm.sup.2 for 5 minutes, it was cut in size of 
1.5.times.2.0.times.5.0 mm to thus produce eraser products. 
EXAMPLE 2 
Stock materials mixed at the ratio as listed in the above table in the same 
manner as the Example 1 were uniformly kneaded at 130.+-.20.degree. C. by 
Banbury mixer to thus obtain rubber composition. The composition was 
charged into a pelletizer and was thus formed in granular state. After the 
granular composition was extruded at 130.degree. C. of the screw unit and 
150.degree. C. of die unit under the pressure of 300 kg/cm.sup.2 in a 30 
mm-dia. bend type double-screw extruder, it was cooled and cut in size of 
0.7-dia..times.2.0 mm to thus obtain circular rod products. 
The eraser obtained according to the above process exhibited superior 
erasing capacity of pencil characters and letters to that of the PVC 
eraser sold in the market, no failure on the coating of pencil and pencil 
case due to its migration of plasticizer. Although the PVC eraser is 
hardened at low temperature to thereby deteriorate its use feeling, the 
eraser obtained according to this invention presented the same use feeling 
at low temperature as at room temperature. When the eraser was applied to 
a pencil attached with an eraser and to an eraser contained in a 
mechanical pencil, it performs superior tearing resistance by its metallic 
band. The eraser fabricated by this invention occurred no low erasing 
capacity rate due to exposure to sunlight nor aging deterioration as 
compared with the natural rubber eraser. When the eraser fabricated by 
this invention was teated by exposure to sunlight in a showwindow, it 
retained all the properties as initially incorporated though the natural 
rubber eraser exhibited impossible erasing capacity in three days. 
It should be understood from the foregoing description that since the 
eraser of this invention is thus fabricated, it can incorporate no 
migration of plasticizer to coating of a pencil or pencil case contrary to 
the conventional plastic eraser, suitable rigidity, preferable tearing 
resistance by its metallic band, no deterioration by exposure to sunlight 
and aging as compared with the natural rubber eraser, and retain stable 
properties even at low temperature in high quality. It should also be 
appreciated that the method of fabricating the eraser of this invention 
does not require, as was described before, the gelling and vulcanizing 
steps necessary for difficult skillfulness in the steps as the 
conventional plastic and natural rubber erasers but allows effective use 
of stock materials without loss advantageously.