Elastic cover of an elastomeric material

An elastic covering for closing off a nozzle outlet area containing at least one nozzle orifice of an ink printing head filled with an aqueous liquid comprising a polar elastomer, a hydrophilic additive, and at least one ingredient selected from the group consisting of cross-linking agents, activators, and accelerators.

BACKGROUND OF THE INVENTION 
The present invention relates to elastic coverings made of an elastomeric 
material and the use of these coverings to close off or seal the nozzle 
outlet area on the ink printing head of an ink ejection system used in ink 
jet printing. 
The ink printing head of an ink ejection system is filled with ink for 
printing purposes and filled with ink or another fluid, such as a neutral, 
aqueous liquid for purposes of transportation and storage. The nozzle 
outlet area must be closed off or sealed from the atmosphere during 
relatively long periods of inactivity, during transportation, and during 
storage. For closing off the nozzle outlet area and the nozzle orifices 
emanating therefrom, use should be made not only of the elastic 
characteristic of elastomeric materials, but also, particularly, the 
viscoelastic (flow) behavior of these elastomers for filling up 
exceedingly small cavities and interstices in the zone of the closed 
nozzle outlet area. The nozzle zone of ink printing heads is sensitive to 
contamination caused, for example, by paper fibers and dust accumulations. 
Contamination in the nozzle zone leads, with secondary effects, to an 
altered nozzle geometry, so that droplets emitted from the nozzle are 
greatly affected in size, shape, direction, and velocity. In the case of a 
relatively long inactivity between periods of printing, during transport 
and storage of the ink printing heads or of the printing units equipped 
with such printing heads, it is, furthermore, necessary to avoid an 
excessive thickening of the ink in the aperture regions of the nozzles and 
leakage of the ink or corresponding fluid by shock during transportation, 
or the drawing of air into the printing head together with the leakage of 
ink or fluid. 
German Application DOS 2,702,663, discloses an elastic covering for the 
closing of nozzles of an ink printing head filled with an aqueous liquid, 
in which the covering consists of an elastomeric material, especially of a 
silicone rubber. In this arrangement, the viscoelastic (flow) behavior of 
these elastomeric materials is used to fill up exceedingly small cavities 
and interstices in the region of the closed nozzle outlet area. In 
addition to these intentionally emphasized properties, however, these 
silicone elastomeric materials also exhibit disadvantages due to their 
inherent properties caused by their chemical structure because silicone 
rubbers of this type are hydrophobic, i.e. they are water-repellant. 
Monomers and oligomers separate from these rubber types and remain behind 
in the nozzle outlet area of the printing head after removal of the 
sealing material form the area. The nozzle outlet area or portion thereof 
exhibiting these deposits can, thereafter, no longer be wetted by the ink 
spread on this area. At the nozzle rim and in the proximity of the nozzle, 
the symmetry and thus the uniformity of the droplet formation is 
disturbed. These disturbances lead to an altered breakup characteristic of 
the droplets and thus to an impairment of the image reproduction on the 
recording medium. It has been found experimentally that it is impossible 
to completely suppress these deposits, and the addition of hydrophilic 
additives to the covering material is not feasible due to the hydrophobic 
character of the silicone. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide a sealing material for 
the nozzle outlet area of an ink printing head which leaves deposits with 
improved properties on the area to be covered by sealing material. 
It is a further object of the present invention to provide a sealing 
material for the nozzle outlet area of an ink printing head which improves 
the wetting characteristics of the outlet area after the sealing material 
has been removed. 
Additional objects and advantages of the present invention will be set 
forth in part in the description which follows and in part will be obvious 
from the description or can be learned by practice of the invention. The 
objects and advantages are achieved by means of the compositions, 
instrumentalities, and combinations particularly pointed out in the 
appended claims. 
To achieve these objects and in accordance with its purpose, the present 
invention provides an elastic covering for closing off a nozzle outlet 
area containing at least one nozzle orifice of an ink printing head filled 
with an aqueous liquid, the covering comprising an elastomeric material 
having viscoelastic properties for filling exceedingly small cavities and 
interstices in the area to be closed off, wherein the elastomeric material 
comprises a mixture of a polar elastomer, a hydrophilic additive, and at 
least one ingredient selected from the group consisting of cross-linking 
agents, cross-linking activators, and cross-linking accelerators. 
It is to be understood that both the foregoing general description and the 
following detailed description are exemplary, but are not restrictive of 
the invention. 
DETAILED DESCRIPTION OF THE INVENTION 
In accordance with the present invention, it has been found that mixtures 
containing a polar elastomer and other defined components are especially 
well suited for providing a covering for sealing the nozzle outlet area of 
ink printing heads. Elastomers are high polymer substances having the 
property of extensibility with elastic recovery. 
Two classes of polar elastomers are especially preferred for use in the 
present invention, namely butadiene-acrylonitrile copolymers and 
ethylene-vinyl acetate copolymers. Butadiene-acrylonitrile copolymers 
(NBR) are nitrile type rubbers which are formed from the reaction of 
butadiene and acrylonitrile. The ratio of butadiene to acrylonitrile 
generally varies from 82:18 to 55:45. These copolymers may be cured with 
both sulfur and non-sulfur containing vulcanizing or cross-linking agents, 
with the possible addition of a cross-linking activator or accelerator. 
Fillers are also used with NBR to improve properties. 
The ethylene-vinyl acetate copolymers (EVA) which are the second class of 
elastomers preferred for the covering compositions of the present 
invention have a ratio of ethylene to vinylacetate from 75:25 to 40:60 and 
are similarly used with cross-linking agents, activators, and fillers. 
Combinations of certain polar elastomers may also be useful in the present 
invention. 
The mixture of the present invention contains a hydrophilic additive. The 
hydrophilic additive is, in general, a surfactant. Ethylene oxide adducts 
are especially suitable as a hydrophilic additive for use in the present 
invention. Examplary of suitable ethylene oxide adducts are adducts or 
ethylene oxide with low molecular weight or fatty alcohols, adducts of 
ethylene oxide with fatty acids, adducts of ethylene oxide with amines, 
and low molecular weight ethylene oxide adducts with end-positioned 
alcoholic OH-groups (polyethylene glycols). 
The adducts of ethylene oxide with fatty alcohols which can be used are, 
for example, nonionic surfactants, generally of the formula R--(OCH.sub.2 
CH.sub.2).sub.n --OH where R is an alkyl group with about 8 to 18 carbon 
atoms, and n is about 2 to 25. They are sold under trade names such as 
Neodol (Shell Chemical), Tergitol (Union Carbide) and Genapol (Hoechst). 
Also, adducts of ethylene oxide with propyl or butyl alcohol are suitable 
hydrophilic additives according to this invention. 
The adducts of ethylene oxide with fatty acids, which can be used are, for 
example, surfactants generally of the formula R--CO (OCH.sub.2 
CH.sub.2).sub.n OH wherein R is an alkyl group having about 8 to 18 carbon 
atoms, n is about 2 to 25. These are sold under the trade name of Genagen 
by Hoechst. 
The adducts of ethylene oxide with amines which can be used are, for 
example, generally of the formula R--CH.sub.2 --N=[(CH.sub.2 CH.sub.2 
O).sub.n H].sub.2 wherein R is an alkyl group having about 8 to 18 carbon 
atoms, and n is about 2 to 25. They are sold under the trade name Genamin 
by Hoechst. 
The low molecular weight polyethylene glycols, which are ethylene oxide 
adducts with end-positioned alcoholic OH-groups, which can be used are, 
for example, compounds which have a molecular weight of about 200 to 800 
and are of the general formula H (OCH.sub.2 CH.sub.2).sub.n OH and are 
sold under the trade name Carbowax by Union Carbide Corporation and 
Genoplast or Polyethylenglycol resp. by Hoechst. 
Mixtures of the above surfactants are also suitable. Further it is possible 
to additionally include minor amounts (0.001-0.5 weight %) of certain 
specific wetting agents such as those based on fluorocarbons. One such 
type of wetting agents is a sulfonamide ethylene oxide adduct with 
perfluoroalkyl residue. 
In accordance with the present invention, at least one ingredient selected 
from the group consisting of cross-linking agents, cross-linking 
activators, cross-linking accelerators, and mixtures thereof, is combined 
with the polar elastomer and the hydrophilic additive. Exactly which of 
these substances will be used and in what proportions will depend on the 
particular elastomer and the property desired. 
Typically, compounds such as tetramethylthiuram disulfide (thiuram) 
##STR1## 
are used as vulcanizing agents for NBR. Organic peroxides are also well 
known cross-linking agents. Typical peroxide accelerators include 
di-tert-butyl peroxide and dicumylperoxide. A typical cross-linking 
activator would be triallylcyanurate. 
Additives well known to the polymer technique may also be used in 
conjunction with the present compositions. These include dispersing 
agents, softening agents, pigments, fillers, plasticizers, antioxidants, 
aging retardants, and hydrolysis protection agents. Typical fillers known 
to the polymer art which can be used include, for example, various forms 
of silica, clay and talc. 
The use of polar elastomers, in particular NBR and EVA, in conjunction with 
the recited hydrophilic additives and wetting agents, is especially 
advantageous in that an elastic covering material is achieved which is 
compatible with the aqueous ink or aqueous fluid which fills the ink 
printing head. The compositions of the present invention dissolve and 
absorb undesirable, partially thickened ink residues which contact the 
elastic covering, and thus provide a self-cleaning action. Further, the 
nonionic, water soluble wetting additive has a surfactant effect on the 
nozzle outlet area. Surprisingly, the elastic covering material of the 
present invention has the further advantage that the additives which 
impart the hydrophilic properties of the composition, such as polyethylene 
glycol, reinforce the viscoelastic flow behavior of the elastomer, so that 
the force required for the sealing effect can be considerably reduced. 
It has also been discovered that portions of the hydrophilic additive 
transferred to the nozzle outlet area after the covering is removed are 
replenished in the surface of the covering material by migration from the 
center of the material towards the surface. The interface of the covering 
material with the ink printing head thus retains its hydrophilic character 
even after frequent usage. 
The use of the hydrophilic additive in the sealing material thus promotes 
its wettability by ink, so that after removal of the covering, ink will 
remain on the surface of the covering and impurities from the surroundings 
will increasingly be incorporated into the surface of the covering 
material.

The following examples are given by way of illustration to further explain 
the principles of the invention. These examples are merely illustrative 
and are not to be understood as limiting the scope and underlying 
principles of the invention in any way. All percentages referred to herein 
are by weight unless otherwise indicated. 
The following examples show compositions which are elastomeric mixtures 
suitable for use as a covering material according to the present 
invention. The compounds are introduced into the compositions essentially 
in the indicated sequence, and are incorporated by kneading or milling. 
Where trade names for compounds are used, the manufacturer is indicated. 
EXAMPLE 1 
An elastic covering is made from the following composition: 
______________________________________ 
100 Parts by weight 
Butadiene-acrylonitrile rubber 
with 38% acrylonitrile 
1.0-3.0 
" Stearic acid additional 
cross-linking 
2.0-5.0 
" Zinc oxide, active 
accelerator 
0.5-2.0 
" Methylene bis (tert-butyl-para- 
cresol (Phenolic antioxidant) 
15.0-25.0 
" Precipitated silica (filler) 
20.0-40.0 
" Natural clay (filler) 
10.0-25.0 
" Phenol/cresol alkanesulfonic 
acid ester (plasticizer) 
1.5-2.0 
" "Vulkacit Thiuram" (vulcanizing 
agent) (Bayer) 
1.0-1.5 
" "Vulkacit J" (vulcanizing agent) 
(Bayer) 
0.5-1.0 
" "Struktol SU 108" (sulfur as 
vulcanizing agent) (Schill 
and Seilacher) 
2.0-15.0 
" "Genoplast 200" (Polyethylene 
glycol m.w. 200) (Hoechst AG.) 
______________________________________ 
The vulcanizing agents used serve to cross-link the composition. 
EXAMPLE 2 
An elastomeric covering is made from the following composition: 
______________________________________ 
100 Parts by weight 
Ethylene-vinyl acetate copolymer 
in a ratio of 40:60 
20.0-40.0 
" Talc (mineral filler) 
0.5-1.5 
" Polycarbodiimide (hydrolysis 
protection agent) 
0.2-1.0 
" 4,4'-Thio-bis-(6-tert.butyl- 
metacresol) (Monsanto) 
(antioxidant) 
0.5-2.0 
" Triallyl cyanurate (cross-linking 
activator) 
3.0-5.0 
" 1,1-Bis (tert-butylperoxy)- 
3,3,5-trimethylcyclohexane 
(cross-linking agent) 
2.0-10.0 
" "Genoplast 200" (polyethylene 
glycol m.w. 200) (Hoechst AG.) 
______________________________________ 
It will be understood that the above description of the present invention 
is susceptible to various modifications, changes and adaptations, and the 
same are intended to be comprehended within the meaning and range of 
equivalents of the appended claims.