Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – At least one aryl ring which is part of a fused or bridged...
Reexamination Certificate
2002-11-20
2004-06-08
Moore, Margaret G. (Department: 1712)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
At least one aryl ring which is part of a fused or bridged...
C524S748000
Reexamination Certificate
active
06747079
ABSTRACT:
The present invention relates to the use of additives for the stabilization of the constituent silicone elastomers of molds resulting from the crosslinking of polyaddition or polycondensation compositions, in particular polycondensation compositions.
Another subject-matter of the invention is polyaddition and polycondensation compositions capable of being used for the preparation of these silicone elastomers.
A further subject-matter of the invention is the molds thus obtained for the reproduction by molding of decorative and industrial objects.
Another subject-matter of the invention is a process for the preparation of silicone molds.
Silicone compositions, in particular silicone polycondensation compositions, are used for the reproduction by molding of decorative and industrial objects. The reproduction of objects consists, in a first step, in manufacturing a negative of the object to be copied. This negative is prepared from silicone elastomer and is named “membrane”. After crosslinking the silicone, the membrane is separated from the starting object. This membrane constitutes the mold which will be used for the reproduction of the object to be copied.
This type of mold is widely used for the reproduction of objects made of resin, such as polyester resin, capable of faithfully reproducing the finest details. However, during this use, the mold is subjected to gradual modifications: the constituents of the polyester resins, in particular styrene, diffuse into the membrane and are polymerized. The physicochemical structure of the mold in contact with the resins changes: it gradually hardens while losing its antiadhesive nature and its tear strength. These modifications finally result in surface fragments of the mold being torn off when the polyester item is removed from the mold. At this stage, the mold is no longer usable.
Various degradation mechanisms are involved. They can depend just as much on criteria related to the silicone elastomers as to the resins or to the molding conditions. It is probable that the polymerization mechanism is a radical mechanism: formation of free radicals, initiation and propagation of the radical polymerization of the styrene. High styrene or peroxide contents, the exothermic nature of the polymerization of the resin and the presence of oxygen are aggravating factors. The variety of the factors which can influence the degradation of the silicone mold means that, until now, the solutions provided have never been entirely satisfactory.
One means of improving the resistance to polyester resins of a silicone mold consists in introducing, into the elastomer, derivatives which may inhibit one or more phases of the radical polymerization.
European Patent Application EP-A-787 766 thus provides an improvement to the longevity of silicone molds by incorporating, in the polycondensation composition, an additive selected from a group composed of sterically hindered phenols, sterically hindered bisphenols, sterically hindered thiobisphenols, zinc dialkyldithiophosphates, zinc diaryldithiophosphates, aromatic amines or sterically hindered amines which can be 1-alkyl sebacates with a terminal NR group.
In FR-A-2 773 165, the additive is chosen from the group consisting of:
(a) antioxidant additives comprising, in their structure, at least one R—S
q
—R′ group in which R and R′ are monovalent hydrocarbonaceous groups having at least 3 carbon atoms or a monovalent hydrocarbonaceous group having an ester bond or R and R′ together form a ring, q being an integer of between 1 and 3 inclusive,
(b) additives which are inhibitors of free radicals and which are capable, under the conditions of the molding, of generating at least one group:
synergistic combination [lacuna] (a)+(b),
synergistic combination [lacuna] (a) and/or (b) with phosphites (c).
The additives of FR-A-2 773 165 include:
thiopropionates of formula:
in which R
5
is an alkyl group having from 1 to 15 carbon atoms inclusive and x is an integer of between 1 and 4 inclusive, and preferably thiodipropionates (x=2) among which compounds may be mentioned:
ditridecyl thiodipropionate (CAS 10595-72-9)
distearyl 3,3′-thiodipropionate (CAS 693-36-7)
dilauryl 3,3′-thiodipropionate (CAS 123-28-4);
compounds comprising several thioether groups R—S
q
—R′ connected to a tetravalent carbon, preferably tetra (thioether)pentaerythritol, for example pentaerythritol tetra (laurylthiopropionate) or PETL (CAS 29598-76-3).
EP-A-854 167 provides several types of additives, including phenols, thiodipropionic acids, polysulfides, phosphonates, and the like, the phenols being sterically hindered.
The additives most often provided are phenols and thiodipropionic acids, the phenols being sterically hindered.
Furthermore, manufacturers seek to introduce these additives in the highest possible amount but come up against phenomena of exudation of the additive at the mold-molded component interface, which leads to inhibition of the polymerization of the resin layer which is in contact with the surface of the mold. It would be advantageous to have available additives which can be incorporated, without risks of exudation, in an amount sufficient to provide a high degree of protection.
The Applicant Company has found, unexpectedly, that compounds of thiomonopropionic type exhibit noteworthy advantages with respect to other known additives, including with respect to thiodipropionates. Thiomonopropionates make it possible to achieve high molar concentrations without, however, resulting in phenomena of exudation and thus of inhibition at the surface of the molded items, even after many consecutive molding operations. These additives can result in a performance in terms of number of molding operations which is unexpected in the light of what is generally obtained with conventional additives, including thiodipropionates.
A subject-matter of the present invention is thus the use, for increasing the longevity of molds made of silicone elastomer which crosslinks by a polyaddition or polycondensation reaction, preferably a polycondensation reaction, of an additive capable of stabilizing the silicone elastomer constituting the mold, in particular with respect to materials to be molded, the additive corresponding to the following formula (1):
in which R
1
and R
2
are linear or branched alkyl radicals or cycloaliphatic radicals having from 1 to 40 carbon atoms.
In particular, but not exclusively, for the molding of objects having reliefs with a depth or thickness of greater than or equal to one millimeter and with a width or diameter of less than 1 to 5 mm, in particular of several millimeters, it is preferable to have R
1
+R
2
>20 carbon atoms. The additive is then a solid additive of waxy type. It is thus possible to have large amounts of additive in the reliefs of the mold, which additive will gradually melt during successive molding operations. The maximum C condensation (C number of R
1
+R
2
) is advantageously such that the melting point of the waxy additive is ≦150° C. Preferably, these waxy additives have a melting point ≧40° C., preferably of between 50 and 120° C. approximately. By way of example, the molding of the polyester is exothermic and the heat thus released makes it possible to provide for the gradual release of the additive.
In particular, but not exclusively, for the molding of objects not exhibiting such reliefs or objects comprising details which are not well marked, it is preferable to have R
1
+R
2
≦20 carbon atoms, in particular 6≦R
1
+R
2
≦20. The additive is liquid and can diffuse over time in a substantially continuous fashion, so as to maintain an appropriate amount of additive in the surface layer of the mold in contact with the component to be molded. More particularly, 6≦R
1
+R
2
≦18, preferably 8≦R
1
+R
2
≦18, more preferably still 10≦R
1
+R
2
≦18. R
1
+R
2
≧8 or 10 is particularly appropriate when, as is the case with polyester resins, the polymerization of the res
Bressieux Jérôme
Deforth Thomas
Pusineri Christian
Burns Doane Swecker & Mathis L.L.P.
Moore Margaret G.
Rhodia Chimie
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