Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Processes of preparing a desired or intentional composition...
Reexamination Certificate
1999-07-01
2001-02-06
Hoke, Veronica P. (Department: 1714)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Processes of preparing a desired or intentional composition...
C524S415000, C524S416000, C524S436000, C524S417000, C524S377000
Reexamination Certificate
active
06184269
ABSTRACT:
TECHNICAL FIELD
This invention pertains to moldable, polyethylene-based intumescent flame retardant materials (PE FRIMs). More specifically, this invention pertains to such moldable, thermoplastic elastomer, intumescent compositions containing high density polyethylene, chlorinated polyethylene and a relatively small amount of a relatively high molecular weight silphenylene siloxane elastomer.
BACKGROUND OF THE INVENTION
U.S. Pat. No. 5,834,535 describes a family of compositions in accordance with its title, “Moldable Intumescent Polyethylene and Chlorinated Polyethylene Compositions.” These compositions are readily moldable by a variety of polymer molding practices into articles that have good physical strength. Moreover, the molded bodies have an additional property of serving as a heat and fire barrier. Upon exposure to a flame, they initially burn but do not melt and drip. Instead, they form a relatively strong foamed barrier of carbonized char and inorganic ceramic-like material. In some embodiments of these high density polyethylene (HDPE) and chlorinated polyethylene (CPE) compositions, a silicone elastomer (specifically, an organosiloxane) is used in addition to, or as a replacement for, some of the CPE. In many formulations the siloxane polymer improved the processibility of the material by reducing the mixing torque requirement. Further, in the molded product the polysiloxane reduced smoke evolution.
It is an object of this invention to provide a family of polyethylene and chlorinated polyethylene-based intumescent flame retardant materials containing silphenylene-siloxane elastomers for improved intumescence properties and improved processing properties.
SUMMARY OF THE INVENTION
In accordance with this invention, certain high molecular weight silphenylene siloxane elastomers are used in combination with thermoplastic high density polyethylene, chlorinated polyethylene elastomer and suitable specified additives to form moldable blends that yield intumescent molded products. Such molded mixtures of HDPE, CPE and silphenylene siloxane elastomer form a thermoplastic elastomer that has high temperature resistance and produces a good foamed char barrier with little smoke when exposed to a flame and burned.
Thermoplastic elastomers are a family of materials that have the properties of elastomers but can be processed as plastics. Being elastomeric, they have the desirable properties of flexibility, impact resistance, energy and shock absorption, and sound and vibration reduction. They can be formed by compression molding, injection molding, extrusion, vacuum forming and blow molding. Thermoplastic elastomers are recyclable, and in plant scrap generated during processing can be chopped up and used again to make parts. If such material can be made intumescent, then a convenient form of fire protection is available that can be introduced in vehicles on the assembly line.
The present invention teaches intumescent thermoplastic elastomer compositions that can be molded into automotive parts. The moldings are very effective as a heat and fire barrier. Upon burning, these materials will not melt and drip but will form a relatively strong foamed barrier of char and inorganic ceramic-like material. The intumescent material could be used to replace existing plastic parts in cars or as a cover or shield to protect other plastic parts. The material is based on high density polyethylene as the hard phase of thermoplastic elastomer and chlorinated polyethylene and a specified silphenylene siloxane silicone rubber as the soft phase. Properties of compression molded and injection molded parts will be presented.
Moreover, moldings produced from the compositions suitably have at least certain minimal physical properties such as a tensile strength (for injection moldings) of 0.5 MPa (72 psi) and an elongation at break of at least six percent at normal room temperature (ASTM D412). Such properties give moldings sufficient strength and durability so as to be useful in architectural applications, automotive applications, marine applications or the like. In addition to being formable and having useful physical properties for such applications, the moldings also have “intumescence efficiency.” When exposed to elevated temperatures, the intumescent additives react or decompose to convert the molded plastic body to a residual insulating foam-like structure that is resistant to burning and insulates the back side of the molding from the high temperatures of the combustion flame.
Suitable constituents of the subject intumescent thermoplastic elastomer molding compositions comprise:
1. Resin Matrix
A high density polyethylene resin is mixed with a chlorinated polyethylene elastomer and silphenylene siloxane elastomer in proportion to obtain desired physical properties in a molded part. Suitable examples of HDPE and CPE are, of course, well known. However, the selection of the subject silicone constituent requires some explanation.
Silphenylene siloxane elastomeric polymers based, for example, on 1,4-phenylene-hexamethyltrisiloxanyl monomer or 1,4-phenylene-1,1,3,5,5-pentamethyl-3-vinyltrisiloxanyl monomer are suitable provided that the polymers are of sufficient molecular weight to provide the desired physical and intumescence properties to the molded composition. For example, Mn (i.e., number average molecular weight) values of 64,000 for poly(1,4-phenylene-hexamethyltrisiloxanyl) and of 109,000 for poly(1,4-phenylene-1,1,3,5,5-pentamethyl-3-vinyltrisiloxanyl) yield suitable elastomers. A further characteristic of the subject silicones is that each monomer moiety contains a silphenyl and a siloxane group. The silphenyl group has two silicon atoms that in the above examples each have two attached methyl groups. One of these methyl groups can be replaced with a phenyl group. Any of the four methyl groups can be replaced with another alkyl group such as ethyl, propyl, isopropyl group or fluorinated analogs of these groups. As illustrated, the siloxane group has a single silicon atom with two methyl groups or a methyl group and a vinyl group. The methyl groups on the siloxane portion of the monomer can be replaced with other alkyl groups such as ethyl, propyl, isopropyl group or fluorinated analogs of these groups. One of the siloxane methyl groups can also be replaced with a phenyl group.
A liquid chlorowax (e.g., Paroil 145) is suitably used as a plasticizer when needed to impart flexibility to the resin matrix at low temperatures.
2. Thermal and Oxidation Stabilizers
Although any suitably compatible stabilizer can be used with HDPE, CPE and the subject silicone for protection against heat and oxygen, it is found that a system consisting of distearylthiodipropionate (DSTDP) and a butylated reaction product of p-cresol and dicyclopentadiene (Wingstay L) is very effective as an antioxidant. In addition to such antioxidant, MgO is used in the formulation to absorb evolved HCL produced during aging of chlorinated polyethylene and thus acts as an effective dehydrochlorination stabilizer.
3. Gas-Generating Foaming Agents
These agents are used in the subject compositions to generate gases in order to foam the polymeric matrix before it is consumed by the fire. The residue that remains after burning of most organic material will have a porous char structure and will thus be an effective thermal barrier. Two preferred gas-generating agents used in this invention are ammonium dihydrogen phosphate, NH
4
H
2
PO
4
, and ammonium polyphosphate, approx. (NH
4
PO
3
)
n
, which emit ammonia when heated. Hydrated alumina and hydrated magnesia are also preferred because they emit water vapor when heated. The ammonium dihydrogen phosphate will also form phosphoric acid which will act as a catalyst to encourage char formation from polyhydroxy compounds.
4. Char Formers
Any suitable compatible starch or other carbohydrate may be used in the moldable compositions to form heavy char when exposed to fire. Polyhydric alcohols also perform the same function. A preferred char former is monopentaerythritol or dipentaerythritol.
5.
Abu-Isa Ismat Ali
Zhu Huide D.
Brown George A.
General Motors Corporation
Hoke Veronica P.
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