Stock material or miscellaneous articles – Composite – Of silicon containing
Reexamination Certificate
2000-10-13
2004-06-01
Peng, Kuo-Liang (Department: 1712)
Stock material or miscellaneous articles
Composite
Of silicon containing
C528S035000, C528S031000, C528S032000, C528S043000, C528S012000, C528S036000, C427S387000
Reexamination Certificate
active
06743515
ABSTRACT:
TECHNICAL FIELD
The invention relates to self-adhesive addition-crosslinking silicone compositions and addition-crosslinked silicone elastomers and composite materials prepared therefrom.
BACKGROUND ART
It is known that the adhesion of addition-crosslinked silicone elastomers to numerous substrates, such as plastics, metals and glasses, is poor, i.e. if an addition-crosslinking silicone elastomer material is applied to a substrate and then crosslinked, the silicone elastomer formed can, as a rule, be peeled away from the substrate surface without problems. Only small tensile forces; frequently, even spontaneous delamination of the silicone elastomer from the substrate is found. However, since strong and permanent adhesion of the silicone elastomer to the substrate is of primary importance in numerous applications, a large number of special measures have been proposed for achieving a strong bond between substrate and silicone elastomer.
In principle, the adhesive strength of the silicone elastomer/substrate composite can be increased by suitably changing the chemical and/or physical characteristics of the substrate or its surface prior to application of the addition-crosslinking silicone elastomer composition. This can be effected, for example, by pretreating the substrate surface with adhesion-promoting additives (so-called primers), by subjecting the substrate surface to a plasma treatment, by formulating the substrate to contain special additives, by selectively adjusting the morphology of the substrate, by increasing the surface roughness, etc. These measures have, inter alia, the disadvantage that additional process steps are required. As the characteristics of the substrate often have to meet special requirements, use of these methods of increasing adhesive strength is often not possible.
The adhesive strength of the silicone elastomer/substrate composite can also be increased by selectively changing the chemical and/or physical characteristics of the addition-crosslinking silicone elastomer material. Numerous adhesion-promoting additives are known which, when mixed with the uncrosslinked silicone material, give rise to self-adhesion of the resulting silicone elastomer to various substrates. These include compounds which contain highly reactive functional groups, such as alkoxy, epoxy, carboxyl, amino, etc., these groups generally being chosen so that the adhesion promoter is capable of reacting both with the substrate and with a silicone elastomer component. Although such adhesion promoters may make it possible to dispense with a pretreatment of the substrate, the adhesive strength achieved frequently does not meet the desired requirements. In addition, an increase in the adhesive strength by means of higher content of these adhesion promoters is limited, since the highly reactive groups contained exhibit increasingly disadvantageous effects on performance characteristics such as shelf-life, crosslinking characteristics (inhibition), toxicological safety, etc. For these reasons, interest focused on keeping the content of adhesion promoters as low as possible.
EP-A-686 671 describes a self-adhesive adhesion-crosslinking material which employs no special adhesion promoter, because the adhesion-promoting component is either an organohydrogenpolysiloxane which has on average at least two SiH groups per molecule and whose monovalent Si-bonded radicals comprise at least 12 mol % of hydrocarbon radicals having an aromatic ring; or is a compound which has on average at least one SiH group per molecule and contains a group consisting of two aromatic rings, the two aromatic rings being separated from one another by —R
13
R
14
Si—, —R
13
R
14
SiO—, —OR
13
R
14
SiO— or —R
13
R
14
SiOR
13
R
14
Si——, the radicals R
13
and R
14
being monovalent hydrocarbon radicals. The adhesion-promoting component can thus simultaneously function as the crosslinking agent of the silicone elastomer material. Good adhesion to organic plastics (especially ABS) is achieved with this composition, while at the same time the cured or partially cured products exhibit good demoldability from the metallic vulcanization mold (chromium- or nickel-coated steel mold or mold of an aluminum alloy). The high content of greater than 12 mol % of radicals containing aromatic rings in the SiH-containing, adhesion-promoting component results, however, in considerable incompatibility with the other components of the addition-crosslinking silicone elastomer material. This leads to partial separation (exudation) during storage, necessitating repeated homogenization of the ingredient containing this component before use. This incompatibility, which is already evident from a milky turbidity of the uncrosslinked material, also manifests itself in substantially reduced transparency of the silicone elastomer parts produced therefrom. If the adhesion-promoting component simultaneously acts as a crosslinking agent of the silicone elastomer composition, the incompatibility leads to vulcanization problems, which result in inhomogeneous network formation and poor mechanical vulcanization properties. To overcome these vulcanization problems, it is necessary, in addition to the adhesion-promoting SiH-containing component, to use an SiH-containing crosslinking agent completely compatible with the silicone elastomer material, which however results in other disadvantages, for example higher values of the compression set, and higher tendency to exudation of the adhesion-promoting component. The high content of greater than 12 mol % of radicals containing aromatic rings in the SiH-containing, adhesion-promoting component also results in a considerable structural viscosity and thixotropy of the silicone elastomer material, which is undesired in numerous applications, for example, injection molding of liquid silicone rubber. Finally, the adhesion of this composition to metals is also insufficient.
EP-A-875 536 describes a self-adhesive adhesion-crosslinking silicone rubber mixture which is distinguished by the fact that
a) the SiH crosslinking agent contains at least 20 SiH groups, other radicals being aliphatically saturated,
b) an alkoxysilane and/or alkoxysiloxane having epoxy functional groups is/are present, and
c) a peroxide is optionally present.
The use of glycidyloxypropyltrimethoxysilane (Glymo) is particularly preferred. The silicone rubber mixture described in EP-A-875 536 is particularly suitable for the production of composite shaped articles which consist of the silicone elastomer and an organic plastic. However, the composition described in EP-A-875 536 has the disadvantage that sufficient adhesive strength can be achieved only with the use of very SiH-rich crosslinking agents having on average at least 20 SiH groups per molecule. In the examples there, crosslinking agents having 30 SiH groups per molecule are used. The use of such polyfunctional crosslinking agents considerably reduces the shelf-life of addition-crosslinking silicone rubber mixtures, i.e. the flowability is considerably impaired, which may lead to stiffening of the material. As a result, proper processing of the material, for example by injection molding, is no longer possible. In addition, in order to achieve high adhesive strength, it is necessary to use relatively large amounts of alkoxysilane/alkoxysiloxane having epoxy functional groups, with the result that the crosslinking rate is considerably reduced. Although this can be partly compensated by using a peroxide, as described in EP-A-875 536, only peroxides having a low initiation temperature, such as the 2,4-dichlorobenzoyl peroxide described, are suitable for this purpose, due to the necessarily low crosslinking temperature (softening of the organic plastic). These peroxides on the one hand are toxicologically very unsafe owing to the cleavage products and secondary products liberated (PCB problem) and on the other hand further impair the shelf-life of the material.
In summary, it may be said that none of the conventional addition-crosslinking silicone elastomer compositions satisfactorily meet the r
Achenbach Frank
Eberl Georg
Müller Philipp
Brooks & Kushman P.C.
Peng Kuo-Liang
Wacker-Chemie GmbH
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