Silicone elastomers

Details Silicone elastomers Silicone elastomers

1999-02-10

2001-02-27

Dawson, Robert (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...

C524S497000, C524S448000, C524S430000, C528S024000, C264S299000

Reexamination Certificate

active

06194508

ABSTRACT:

TECHNICAL FIELD
The invention relates to peroxidically crosslinkable silicone rubber compositions, their preparation and articles in which they are present.
BACKGROUND OF THE INVENTION
Peroxidically crosslinked silicone elastomers, generally referred to as HTV (high temperature vulcanizing) silicone rubber have a certain, undesirable yellow coloration absent from addition-crosslinked silicone elastomers. This yellowing is very well-known, and numerous attempts have been made to prevent or reduce this undesirable coloration. It is known that yellowing depends on numerous influencing factors which explains why the degree of yellowing varies greatly. For example, peroxidically crosslinked silicone elastomers are only slightly yellowish immediately after preparation, i.e. in the untempered state. However, after thermal post-treatment, for example for 4 hours at 200° C., which is indispensable for many applications, yellowing is particularly pronounced. It is also known that the type of peroxide used has, for an otherwise unchanged composition, a considerable influence on the degree of yellowing. Thus, silicone elastomers which have been crosslinked by means of dicumyl peroxide are significantly more yellow than those which have been crosslinked using bis(2,4-dichlorobenzoyl) peroxide.
JP 04161458 (S ETSU CHEM IND CO LTD) describes the use of specific peroxides, namely dialkyl peroxides, in combination with an inorganic or organic alkali metal base such as sodium hydroxide, sodium silanolate or sodium methoxide, with the latter being reacted with the filler-containing organopolysiloxane composition at elevated temperature before the addition of peroxide. A disadvantage of this process is, inter alia, that it is generally restricted to certain peroxides because of the different methods of preparing the silicone elastomers (extrusion or compression molding). In other words, there is no free choice of peroxide. In addition, the basic alkali metal compounds added leads to a series of undesirable secondary reactions, results in disadvantages in the process, can increase the tendency of the compound to stick to the rolls, can cause difficulties in removal of the crosslinked silicone elastomer from the mold, and can have an adverse effect on the final properties of the silicone elastomers, e.g. compression set and thermal stability.
A further factor which causes and/or influences yellowing is the presence of nitrogen-containig residues in the HTV polymer or filler used. These residues can originate from the PNCl
2
catalyst used in the preparation of the polymer or from silazane residues or products formed from them. Factually, yellowing can in many cases be reduced a limited amount by intensive kneading at elevated temperatures i.e. careful elimination of such residues, but this is associated with additional process steps which are not economically acceptable.
It would be desirable to provide additives which can be added to the uncrosslinked HTV silicone rubber composition and reduce yellowing. According to the prior art, the additives disclosed by the following references are known to be useful for this purpose.
DE 3726010 (WACKER-CHEMIE GMBH) describes peroxidically crosslinkable silicone rubber compositions to which organopolysilanes are added to reduce yellowing. Although a significant reduction in yellowing can be achieved in this way, considerable disadvantages have to be accepted; these relate, in particular, to significantly higher compression set values and an increase in the surface tack and adhesion, which can be associated with processing disadvantages (tendency to stick to the rolls, poorer demoldability) and undesirable final properties (surface tack).
DE 3243194 (DEGUSSA AG) teaches the use of polymethylhydrogensiloxane (H-siloxane) which is added to the peroxidically crosslinkable silicone rubber composition in amounts of from 0.1 to 2% by weight (based on the total composition) as an additive to reduce yellowing. In numerous cases, yellowing can be significantly reduced or even eliminated completely in this way. However, it has been noted that frequently, significantly higher amounts of H-siloxane than the amounts disclosed as useful must be used to achieve a satisfactory reduction in yellowing. In these cases in particular, but also in other cases, the use of the relatively low molecular weight polymethylhydrogensiloxanes creates disadvantages in processing which result, first in an increased tendency of the compoumd to stick to the rolls, and second, in difficulty in removing the crosslinked silicone elastomers from the mold (also referred to as mold adhesion). This is understandable since H-siloxane is known to be used as a coupling agent.
It is known that the tendency to yellowing can be reduced by intensively kneading the silicone rubber composition at high temperatures (>150° C.) for a number of hours. The occurrence of yellowing is then a particular disadvantage if the process for preparing the silicone rubber composition does not include such a (time-consuming and energy-intensive) baking-out step. In these cases in particular, relatively high amounts of H-siloxane are necessary to reduce yellowing and the associated disadvantages become particularly serious.
DISCLOSURE OF THIE INVENTION
It is an object of the invention to remedy the disadvantages of the prior art and, in particular, to provide peroxidically crosslinkable silicone rubber compositions which have reduced yellowing.
DESCRIPON OF THE PREFERRED EMBODIMENTS
The invention provides peroxidically crosslinkable silicone rubber compositions containing a filler which has covalently bound Si—H-containing groups.
The peroxidically crosslinkable silicone rubber compositions of the invention preferably comprise
(A) organosiloxanes comprising units of the formula
R
1
a
SiO
(4-a)/2
  (I),
where the radicals R
1
can be identical or different and are each a substituted or unsubstituted hydrocarbon radical having from 1 to 20 carbon atoms or are H or OH and a is 0, 1, 2 or 3,
(B) a filler which has covalently bound SiH-containing groups,
(C) if desired, other reinforcing and/or nonreinforcing fillers which are SiH-free,
(D) peroxide(s) which promote the crosslinking of the silicone rubber composition, preferably organic peroxides,
(E) other additives such as heat stabilizers, plasticizers, mold release agents, coupling agents, hydrophobicizing agents, flame retardants, UV absorbers, silicone resins, etc.
With reference to component A, preferred examples of substituted or unsubstituted hydrocarbon radicals R
1
are alkyl radicals such as methyl, ethyl, propyl, isopropyl, tert-butyl, n-octyl, 2-ethylhexyl and octadecyl radicals; alkenyl radicals such as vinyl, allyl, hexenyl and undecenyl radicals; cycloalkyl radicals such as the cyclopentyl or cyclohexyl radicals; cycloalkenyl radicals such as the cyclopentenyl or cyclohexenyl radicals; aryl radicals such as the phenyl, tolyl, xylyl or naphthyl radicals; aralkyl radicals such as the benzyl or phenylethyl radicals; and also halogenated and/or organofuccconalized derivatives of the above radicals, e.g. the 3,3,3-trifluoropropyl or cyanomethyl radicals.
Preferred radicals R
1
are methyl, vinyl, phenyl and 3,3,3-trifluoropropyl radicals.
Alkyl radicals, in particular methyl radicals, are preferably bound to at least 70 mol % of the Si atoms present in the organopolysiloxane comprising units of the formula (I). If Si-bonded vinyl and/or phenyl radicals are present in the organopolysiloxanes (A) in addition to Si-bonded methyl and/or 3,3,3-trifluoropropyl radicals, then the former are present in amounts of from 0.001 to 30 mol %.
The organosiloxanes (A) preferably consist predominantly of diorganosiloxane units. The end groups of the organopolysiloxanes (A) are preferably trialkylsiloxy groups, in particular the trimethylsiloxy radical or the dimethylvinylsiloxy radical; however, one or more of the alkyl radicals present in the chain and/or terminal siloxane units can also be replaced by hydrogen, hydroxy groups or alkoxy groups such as methoxy and ethoxy r

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