Stock material or miscellaneous articles – Composite – Of silicon containing
Reexamination Certificate
2001-12-12
2004-01-06
Dawson, Robert (Department: 1712)
Stock material or miscellaneous articles
Composite
Of silicon containing
C427S387000, C522S025000, C522S027000, C522S028000, C522S029000, C528S023000, C528S025000, C528S031000, C528S033000
Reexamination Certificate
active
06673457
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a novel organopolysiloxane composition curable by irradiation with actinic rays such as ultraviolet light, X-rays and electron beams to give a cured coating film of the composition on a substrate surface exhibiting good adhesion to the substrate material and excellent releasability so as to be useful as a release coating agent.
It is an established technology to employ a curable organopolysiloxane composition as a coating agent on a surface required to have surface releasability such as releasable sheets for temporary protection of the adhesive surface of a pressure-sensitive adhesive label, processing sheets for use in a molding process of plastic resins and back surface of a pressure-sensitive adhesive tape in a rolled form.
A curable organopolysiloxane composition widely employed for this purpose in the prior art is prepared by uniformly dissolving, in an organic solvent such as toluene, an alkenyl group-containing organopolysiloxane of a high degree of polymerization and an organohydrogenpolysiloxane together with a catalytic amount of a platinum compound as a catalyst for promoting the hydrosilation reaction or so-called addition reaction between the silicon-bonded alkenyl groups and the silicon-bonded hydrogen atoms forming a crosslinked structure. Although such a composition can readily be cured by a heating treatment to give a cured coating film having good adhesion to the surface of a substrate such as plastic films exhibiting excellent surface releasability and slipperiness, a serious problem accompanying use of the organopolysiloxane composition of this type is the environmental pollution caused by the solvent vapor emitted from the composition during processing.
A curable organopolysiloxane composition of this type but free from any organic solvents can of course be prepared if the alkenyl group-containing organopolysiloxane has a sufficiently low degree of polymerization. Such a curable organopolysiloxane composition, however, is practically disadvantageous because the cured coating film obtained therefrom is so fragile and exhibits no adhesion to the substrate surface on which the coating layer of the composition is cured.
Alternatively, a radiation-curable organopolysiloxane composition free from any organic solvents can be prepared by using an acrylate-modified organopolysiloxane. Although the curable organopolysiloxane composition of this type gives a cured coating film having good adhesion to the surface of plastic films, a disadvantage therein is that the curing treatment thereof requires use of an expensive radiation source such as special ultraviolet lamps and electron beam accelerators necessarily leading to an increase of the cost for the treatment. In addition, the irradiation treatment of the coating layer thereof must be performed in an atmosphere of an inert gas such as nitrogen resulting in a further increase of the cost for the curing treatment.
Further alternatively, there are known curable organopolysiloxane compositions comprising an epoxy-modified, vinyl ether-modified or oxetane-modified organopolysiloxane and a photopolymerization initiator which is typically an iodonium salt compound.
Epoxy compounds in general are known to have advantages of very little curing shrinkage by polymerization and excellent adhesion to the surface of a variety of substrates including metals and plastics so that epoxy resin-based coating compositions are widely employed for coating surfaces of a tinplate. When a curable composition comprising an epoxy-modified organopolysiloxane is used for the purpose of release coating, however, excellent releasability can hardly be obtained unless the epoxy equivalent of the composition is unduly decreased necessarily leading to a decrease in the mechanical strength of the cured coating film and adhesion of the cured coating film to the substrate material although this problem of a decrease in the adhesive bonding strength can at least partly be solved by undertaking a pretreatment of the substrate material with a primer composition assuming that an increase in the cost for the pretreatment can be disregarded.
SUMMARY OF THE INVENTION
The present invention accordingly has an object, in view of the above described problems and disadvantages in the prior art, to provide a novel and improved radiation-curable organopolysiloxane composition free from any organic solvents, which is curable by irradiation with ultraviolet light, electron beams or X-rays to give a cured coating film exhibiting good adhesion to the surface of a substrate such as plastic films and exhibiting excellent releasability against a variety of adhesive surfaces.
Thus, the radiation-curable organopolysiloxane composition provided by the present invention comprises, as a uniform mixture without any organic solvents;
(A) 100 parts by weight of an aromatic compound having, in a molecule, at least two cationically polymerizable organopolysiloxanyl groups; and
(B) from 0.3 to 5 parts by weight of an onium salt compound as a photopolymerization initiator of the component (A).
In particular, the component (A) is a compound represented by the general formula [X—Y]
n
Z, in which the subscript n is a positive integer of 2 or larger, X is an organopolysiloxanyl group having a cationically polymerizable group, Y is a divalent group linking X and Z and Z is an n valent organic residue having an aromatic ring. The group X is preferably an organopolysiloxanyl group represented by the average unit formula
[(R
1
)
p
(R
2
)
q
SiO
(4−p−q/2
]
b
[(R
1
)
r
(R
2
)
s
SiO
(3−r−s)/2
], (I)
in which R
1
is a monovalent hydrocarbon group free from a cationically polymerizable group, R
2
is a monovalent hydrocarbon group substituted by a cationically polymerizable group, the subscript b is 0 or a positive number not exceeding 10000 and the subscripts p, q, r and s are each 0 or a positive integer not exceeding 3 with the proviso that p+q and r+s are each 0, 1, 2 or 3, all of p, q, r and s being not simultaneously equal to 0, and that, when b is not 0, r, s and r+s are each 0, 1 or 2. The above mentioned cationically polymerizable group is preferably an epoxy group, oxetanyl group or vinyloxy group.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
As is described above, the essential ingredients in the inventive radiation-curable organopolysiloxane composition are the components (A) and (B). The base ingredient of the composition is the component (A) which is an aromatic compound having at least two specific organopolyasiloxanyl groups in a molecule each containing a cationically polymerizable group to give a cured film of the composition by irradiation with actinic rays on the surface of a substrate exhibiting good adhesion thereto.
The component (A) is an aromatic compound having a general structure represented by the formula [X—Y]
n
Z, in which X is an organopolysiloxanyl group having one or more of cationically polymerizable groups, Y is a divalent organic group linking X and Z and Z is an n-valent organic residue containing a monocyclic or polycyclic aromatic ring, the subscript n being a positive integer not smaller than 2. The group X is essential in order to effect radiation-induced curing of the composition and also to impart releasability to the cured film of the composition on a substrate surface.
The organopolysiloxanyl group denoted by X in the formula showing the general structure of the component (A) is represented by the average unit formula:
[(R
1
)
p
(R
2
)
q
SiO
(4−p−q)/2
]
b
[(R
1
)
r
(R
2
)
s
SiO
(3−r−s)/2
], (I)
in which each symbol has the meaning defined before.
The molecular structure of the organopolysiloxanyl group is not particularly limitative including straightly linear and branched structures optionally with a cyclic or partially cyclized structure.
The group denoted by R
1
in the above given formula (I) is, each independently from the others, an unsubst
Ogawa Masahiko
Takahashi Masatoshi
Dawson Robert
Keehan Christopher M
Millen White Zelano & Branigan P.C.
Shin-Etsu Chemical Co. , Ltd.
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