Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Processes of preparing a desired or intentional composition...
Reexamination Certificate
2000-06-23
2002-09-24
Moore, Margaret G. (Department: 1712)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Processes of preparing a desired or intentional composition...
C524S102000, C524S106000, C524S147000, C524S303000, C524S304000, C524S305000, C524S392000, C524S710000, C524S720000, C524S750000, C524S588000
Reexamination Certificate
active
06455617
ABSTRACT:
The present invention relates to the use of additives for the stabilization of the constituent silicone elastomers of moulds obtained by the crosslinking of polyaddition or 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 moulds thus obtained for the reproduction by moulding of decorative and industrial objects.
Another subject-matter of the invention is a process for the preparation of silicone moulds.
Silicone compositions, in particular silicone polycondensation compositions, are used for the reproduction by moulding 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 being prepared from silicone elastomer and being named “membrane”. After crosslinking the silicone, the membrane is separated from the starting object. This membrane constitutes the mould which will be used for the reproduction of the object to be copied.
This type of mould is widely used for the reproduction of objects made of resin, such as polyester resin, capable of faithfully reproducing the finest details of the object to be copied. Numerous reproductions can thus be manufactured. However, during this use, the mould is subjected to gradual modifications: the constituents of the polyester resins, in particular styrene, diffuse into the membrane and are polymerized. At the same time, the physicochemical structure of the mould 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 mould being torn off when the polyester item is removed from the mould. At this stage, the mould 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 moulding conditions. For example, the use of resins with a high content of styrene or of peroxide is an aggravating factor as it increases the possibilities of diffusion of the styrene or of the peroxide. The exothermic nature of the polymerization of the resin is also an aggravating factor. The variety of the factors which can influence the degradation of the silicone mould means that, until now, the solutions provided have never been entirely satisfactory.
European Patent Application EP-A-787 766 provides an improvement to the longevity of silicone moulds 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.
The need still exists for an improvement in the lifetime of silicone moulds and thus for additives which make it possible to achieve this objective.
An objective of the present invention is thus to provide additives which make it possible to protect the elastomer constituting the silicone mould with respect to the resin which is used to produce the item to be moulded, in particular polyester resin, so as to make it possible to carry out a greater number of mouldings than that which was achievable previously, while maintaining, indeed even improving, the surface appearance of the moulded items.
The Applicant Company has found that additives comprising R—S—R groups in their structure make it possible to increase the longevity of silicone moulds in unexpected proportions, this being when the silicone mould originates from the crosslinking of a polyaddition composition or of a polycondensation composition.
The Applicant Company has also found that the combination of these additives or of phosphites with an additive capable of generating
groups also makes it possible to increase the longevity of the moulds, these additives behaving synergistically.
The Applicant Company has also found, surprisingly, that the additives capable of generating
groups are effective by themselves alone, including the 1-alkyl sebacates with a terminal —NOR group, whereas the same 1-alkyl sebacates with an —NR group as disclosed in EP-A-787 766 are less effective and do not behave synergistically with the other additives according to the invention.
The incorporation of thioesters in a silicone composition has already been disclosed.
EP-A-076,630 relates to the specific case of silicone polycondensation compositions catalysed by a catalyst based on chelated titanium. The addition of a thioester or other additives, such as di(tert-butyl)phenol, prevents the elastomer from turning yellow in colour over time under the effect of the chelated titanium catalyst.
EP-A-654,497 relates to the specific case of silicone polyaddition compositions comprising an agent for controlling the crosslinking, intended to prevent premature gelling at room temperature, which makes it possible to stabilize the composition, which will be crosslinked at the appropriate moment by heating. This application provides, as agent for controlling the crosslinking, triazines having at least one peroxy group on a carbon atom of the triazine nucleus and/or an organosulphur compound of formula R
6
—(S)
m
—R
7
, in which R
6
and R
7
are monovalent groups having at least 3 carbon atoms or a monovalent hydrocarbonaceous group exhibiting an ester bond or R
6
and R
7
, taken together, form a ring, m being an integer from 1 to 3.
A subject-matter of the present invention is the use, for increasing the longevity of moulds produced from silicone elastomer crosslinking by a polyaddition or polycondensation reaction, of an additive capable of stabilizing the silicone elastomer constituting the mould with respect to materials to be moulded, the additive being 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 moulding, of generating at least one group:
synergistic combination of (a)+(b),
synergistic combination of (a) and/or (b) with phosphites (c),
with the exception of the use of bis(1-octyl-oxy-2,2,6,6-tetramethyl-4-piperidyl) sebacate as sole additive in a mould made of silicone elastomer crosslinked by a polycondensation reaction.
More particularly, a subject-matter of the invention is such a use for the stabilization of the constituent silicone elastomers of moulds intended for the moulding of polyester items, in order in particular to prevent, within the silicone elastomer, the polymerization of the styrene resulting from the polyester resin, without interfering with the polymerization at the heart and at the surface of the polyester.
In a particularly preferred way, the use is targeted at obtaining a number of mouldings per mould increased by more than 10%, preferably by more than 20%, more particularly by more than 30%.
Another subject-matter of the invention is a process for the preparation of silicone moulds or of silicone elastomer capable of being used for the preparation of such moulds, in which process one or more additives in accordance with that which is disclosed hereinabove is/are added to a conventional elastomer-precursor silicone composition.
All the characteristics given hereinbelow apply to both these subject-matters.
Preference is given, among the additives (a), to:
(i) thiopropionates and in particular those 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,
among which thiopropion
Gay Michel
Howe Fabienne
Pusineri Christian
Viennet Joëlle
Moore Margaret G.
Rhodia Chimie
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