Fire resistant thermoplastic silicone vulcanizates

Details Fire resistant thermoplastic silicone vulcanizates Fire resistant thermoplastic silicone vulcanizates

2001-06-13

2002-08-13

Mulcahy, Peter D. (Department: 1713)

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...

C524S265000, C524S269000, C524S456000, C524S506000

Reexamination Certificate

active

06433049

ABSTRACT:

BACKGROUND OF THE INVENTION
Thermoplastic elastomers (TPEs) are polymeric materials, which possess both plastic and rubbery properties. They have elastomeric mechanical properties but, unlike conventional rubbers, they can be re-processed at elevated temperatures. This re-processability is a major advantage of TPEs over chemically crosslinked rubbers since it allows faster cycle times, recycling of fabricated parts, and results in a considerable reduction of scrap.
In general, two main types of thermoplastic elastomers are known. Block copolymer thermoplastic elastomers contain “hard” plastic segments, which have a melting point or glass transition temperature above ambient as well as “soft” polymeric segments, which have a glass transition or melt point considerably below room temperature. In these systems, the hard segments aggregate to form distinct microphases and act as physical crosslinks for the soft phase, thereby imparting a rubbery character at room temperature. At elevated temperatures, the hard segments melt or soften and allow the copolymer to flow and to be processed like an ordinary thermoplastic resin.
A second type of thermoplastic elastomer is referred to as a simple blend (physical blend) and can be obtained by uniformly mixing an elastomeric component with a thermoplastic resin.
When the elastomeric component is also cross-linked during mixing, a thermoplastic elastomer known in the art as a thermoplastic vulcanizate (TPV) results. Since the crosslinked elastomeric phase of a TPV is insoluble and non-flowable at elevated temperature, TPVs generally exhibit improved oil and solvent resistance as well as reduced compression set relative to the simple blends.
Typically, a TPV is formed by a process known as dynamic vulcanization, wherein the elastomer and the thermoplastic matrix are mixed and the elastomer is cured with the aid of a crosslinking agent and/or catalyst during the mixing process. A number of such TPVs are known in the art, including some wherein the crosslinked elastomeric component can be a silicone polymer while the thermoplastic component is an organic, non-silicone polymer (i.e., a thermoplastic silicone vulcanizate or TPSiV). In such a material, the elastomeric component can be cured by various mechanisms, but it has been shown that the use of a non-specific catalyst, such as an organic peroxide, can also result in at least a partial cure of the thermoplastic resin itself, thereby reducing or completely destroying ability to re-process the composition (i.e., it no longer is a thermoplastic elastomer). In other cases, the peroxide can lead to the partial degradation of the thermoplastic resin. To address these problems, elastomer-specific crosslinkers, such as organohydrido silicon compounds, can be used to cure alkenyl-functional elastomers.
Arkles, in U.S. Pat. No. 4,500,688, discloses semi-interpenetrating networks (IPN) wherein a vinyl-containing silicone fluid having a viscosity of 500 to 100,000 mPa·s is dispersed in a conventional thermoplastic resin. Arkles only illustrates these IPNs at relatively low levels of silicone. The vinyl-containing silicone is vulcanized in the thermoplastic during melt mixing according to a chain extension or crosslinking mechanism, which employs a silicon hydride-containing silicone component. This disclosure states that the chain extension procedure results in a thermoplastic composition when the vinyl-containing silicone has 2 to 4 vinyl groups and the hydride-containing silicone has 1 to 2 times the equivalent of the vinyl functionality. On the other hand, silicones which predominantly undergo crosslinking reaction result in thermoset compositions when the vinyl-containing silicone has 2 to 30 vinyl groups and the hydride-containing silicone has 2 to 10 times the equivalent of the vinyl functionality. Typical thermoplastics mentioned include polyamides, polyurethanes, styrenics, polyacetals and polycarbonates. This disclosure is expanded by Arkles in U.S. Pat. No. 4,714,739 to include the use of hybrid silicones which contain unsaturated groups and are prepared by reacting a hydride-containing silicone with an organic polymer having unsaturated functionality.
In WO 96/01291 to Advanced Elastomer Systems, thermoplastic elastomers having improved resistance to oil and compression set are disclosed. These systems are prepared by first forming a cured rubber concentrate wherein a curable elastomeric copolymer is dispersed in a polymeric carrier not miscible therewith, the curable copolymer being dynamically vulcanized while this combination is mixed. The resulting rubber concentrate is, in turn, blended with an engineering thermoplastic to provide the desired TPE. Silicone rubber is disclosed as a possible elastomeric component, but no examples utilizing such a silicone are provided. Further, this publication specifically teaches that the polymeric carrier must not react with the cure agent for the curable copolymer.
Flame resistance is an important property in many applications for thermoplastics and thermoplastic elastomers. For instance flame resistance is of particular importance for materials used in coating cable used for transmission in plenums and risers in buildings. Plastic materials with insufficient flame retardant characteristics in such cable coatings can contribute to the spread of fire within a building. Furthermore, when fire burns through cable jacket and insulation the result can be the loss of the ability of the wire or optical fiber to communicate.
There have been numerous attempts in the prior art to provide flame retardant thermoplastics. Typically it has been necessary to heavily fill the thermoplastic material with additives such as inorganic fillers until the desired degree of flame retardancy has been achieved. However, this results in several disadvantages, as large proportions of additives could normally be expected to detract from the physical properties of the base.
In wire and cable jackets another approach to reduce to flame spread and smoke evolution is the use of fluoropolymers. These, together with layers of other materials, have been used to control char development, jacket integrity and air permeability to minimize restriction on choices of materials for insulation within the core. Commercially available fluorine-containing polymer materials have been accepted as the primary insulative coating for conductors and as a jacketing material for plenum cable without the use of metal conduit. However, fluoropolymer materials are somewhat difficult to process. Also, some of the fluorine-containing materials have relatively high dielectric constant, which makes them unattractive for communication media.
Further, a fluoropolymer is a halogenated material. There has been a desire to overcome some problems, which exist with respect to the use of halogenated materials such as fluoropolymers and polyvinyl chloride (PVC). These materials promote undesired levels of corrosion in fires. If a fluoropolymer is used, hydrogen fluoride forms under the influence of heat, causing corrosion. For PVC, hydrogen chloride is formed.
The use of silicones as additives to non-halogenated thermoplastics has been proposed for improving fire retardant characteristics.
Frye in U.S. Pat. No. 4,387,176 proposed a flame retardant thermoplastic composition comprised of 50 to 97 percent by weight of a thermoplastic, 1 to 40 percent of a silicone base such as linear silicone fluid or gum, 1 to 20 percent of a metal organic compound such as magnesium stearate, and 1 to 20 percent of a silicone resin such as MQ resin.
Cui et. al in
Proc. Beijing Int. Symp. Exhib. Flame Retard
, (1993) pp 138-44, describes the use of silicone oil in EPDM rubber filled with aluminum trihydrate (ATH). EPDM is a terpolymer composed of ethylenic, propylenic moieties and a ethylidene norborene monomer. The silicone oil was not compatible with EPDM rubber, making it necessary to premix the oil with the ATH before addition to the EPDM. A crosslinking agent, referred to as DCP but not described, was found to be important to improved

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