Stock material or miscellaneous articles – Composite – Of silicon containing
Reexamination Certificate
2001-10-09
2003-07-29
Moore, Margaret G. (Department: 1712)
Stock material or miscellaneous articles
Composite
Of silicon containing
C528S034000, C524S091000, C524S588000, C524S715000, C524S863000
Reexamination Certificate
active
06599633
ABSTRACT:
This invention is concerned with organosiloxane compositions which are curable to elastomers and which have good resistance to yellowing and other ageing effects. These compositions are useful for example as sealing materials.
Organosiloxane compositions which cure to elastomeric solids are well known. Typically such compositions are obtained by mixing a polydiorganosiloxane (for example a polydimethylsiloxane) having reactive terminal groups, generally silanol groups, with a silane cross-linking agent for the polydiorganosiloxane, for example an alkoxy silane, an acetoxy silane or oximo silane, a filler and usually also a curing catalyst. These compositions are frequently single part compositions curable upon exposure to atmospheric moisture at room temperature or two part compositions curable upon mixing under room conditions.
One important application of the above-described curable compositions is their use as sealants. In use as a sealant, it is important that the composition has a blend of properties which render it capable of application as a paste to a joint between substrate surfaces where it can be worked, prior to curing, to provide a smooth surfaced mass which will remain in its allotted position until it has become cured to an elastomeric body adherent to the adjacent substrate surfaces. It is frequently desirable that the composition cures quickly enough to provide a sound seal within several hours but not so quickly that the surfaces cannot be tooled to desired configuration shortly after application. The cured composition should have a strength and elasticity appropriate for the particular joint concerned.
Within the group of pigmented silicone sealants, various inactive or reinforcing fillers, such as calcium carbonates, talc, glass microspheres or PVC powder may be included. These can act as reinforcing fillers and reduce the formulation cost. However, where a clear or transparent formulation is required, these fillers are not suitable and fine particle size silicas such as fumed silicas may be employed as the reinforcing filler.
Generally speaking, those compositions which are required to demonstrate thixotropic properties prior to cure and higher tensile and tear strength properties when cured employ larger quantities of the reinforcing fillers. The silicas are generally employed in those compositions which are intended to be transparent or at least substantially so whereas the calcium carbonates are often employed in compositions where transparency is less required and pigment is used instead.
It is also a common practice in the formulation of silicone based sealants to incorporate a material which serves as a plasticiser for the composition. Commonly the trimethylsilyl terminated polydiorganosiloxanes (for example trimethylsilyl terminated polydimethyl siloxane (PDMS)) are used for this purpose. Whilst they are effective at the time of application of the sealant and subsequently, at least for a time, sometimes they exude from the sealant over time and may give rise to staining of the surrounding substrates onto which the material bleeds.
A number of organic materials have been considered in the past as extenders or fillers with a view to reducing the cost of the silicone sealant compositions. These materials are generally classified into two groups. The first group are high volatility solvents and the resulting sealants are generally referred to as diluted sealants. Examples of suitable solvents are toluene or xylene. The high volatility of these solvents causes a number of disadvantages in diluted sealants, such as high shrinkage (high volume loss due to evaporation of the solvent), flammability, VOC (volatile organic content), hazardous component labelling, health and safety issues, etc.
The second group are those low volatility solvents or lower molecular weight plasticizers, which have good compatibility with the polydiorganosiloxane having reactive groups. The resulting sealants are termed “extended” sealants and generally show much more acceptable properties than diluted sealants. The solvents or lower molecular weight plasticizers can completely or partially replace the PDMS plasticizer in the formulation.
A number of extenders are well known in the literature. Examples are low molecular weight polyisobutylene (PIB), phosphate esters, mineral oil fractions (e.g. isoparaffines), polyalkylbenzenes and in particular, heavy alkylates. The expression “heavy alkylates” refers to the alkylated aromatic materials remaining after distillation of oil in a refinery. The alkylbenzene extenders such as the heavy alkylates offer an especially attractive combination of properties, i.e. high boiling points, excellent compatibility with the polydiorganosiloxane polymer matrix (resulting in cured silicone sealants of good to excellent transparency), low environmental impact, low vapour pressure (and therefore low shrinkage), positive effect on the rheological properties (reduced stringing), etc.
The fact that these extenders give rise to highly transparent (i.e. glass clear) sealants is particularly useful. Silicone sealants which are highly transparent previously required careful matching of the refractive indices of the polymer and fumed silica used as a filler. This had generally been achieved by inclusion of phenyl substituted siloxanes in the formulation, or, more effectively, by using phenyl-substituted copolymers in the formulation. Both of these options however, increase the cost of the formulation. Thus the use of an alkyl benzene extender as described above, provides a much cheaper way of obtaining transparency.
However, when exposed to artificial or natural weathering, alkyl benzene extended sealants tend to yellow rather rapidly. Over prolonged weathering, these extended sealants continue to yellow, and also lose their transparency. This problem does not occur with other extenders, such as phosphate esters or polyisobutylene.
The applicants have found a way to minimise these problems with the sealants using these advantageous extenders.
The present invention provides silicone composition comprising a polymer having siloxane units and two or more silicon bonded hydroxyl or hydrolysable groups per molecule, a crosslinker, an extender material which comprises either an alkyl substituted aryl compound or an alkylcycloaliphatic compound, and a u.v. light stabiliser comprising a benzotriazole moiety.
Using the formulation of the invention, the problem of discoloration of extended silicone sealants upon weathering has been alleviated, and where transparent compositions are produced, a substantially stable transparency with ageing has been achieved.
The retention of transparency noted with certain compositions of the invention is particularly surprising since it would be expected that loss of clarity with weathering is due to the evaporation of the alkylbenzene extender, and indeed, this loss can be monitored by measuring weight loss of samples with time. The addition of the benzotriazole has no impact on the evaporation rate and yet the advantageous property (transparency) arising from the addition of alkylbenzene is retained nonetheless.
The concept of “comprising” where used herein is used in its widest sense to mean and to encompass the notions of “include” and “consist of”.
U.V light stabilising compounds which include a benzotriazole group are known in the art. Examples of suitable u.v. light stabilisers are u.v. light stablilising compounds of general formula (I)
where R
1
is hydrogen or a substituent group, R
2
and R
3
are independently selected from hydrogen or a substituent.
Examples of suitable substituents for R
1
include a lower alkyl ester of a carboxylic acid, an ethyl sulphonyl group, chloro, lower alkoxy. In particular R
1
is hydrogen or chloro.
Where R
1
is other than hydrogen, it is suitably located at the 5-position of the ring.
Examples of suitable substituents for R
2
and R
3
include optionally substituted alkyl, lower alkoxy, lower carbalkoxy, cyclohexyl, phenyl and chlorine. Suitable optional substituents for alkyl groups R
2
an
Dandois Robert
Stammer Andreas
Wolf Andreas
Dow Corning S.A.
McKellar Robert L.
McKellar Stevens
Moore Margaret G.
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