Silane-functionalized polyolefins are often used to make compositions and articles that can be crosslinked by exposure to moisture. In certain cases, fillers that are immiscible with the polymer are incorporated in the compositions to impart functional properties (such as flame-retardancy). Examples of immiscible flame-retardant fillers are metal hydrates, carbon black and halogenated (also known as organo-halogen) compounds. The latter include various halogenated polymers and halogenated monomers, chlorinated paraffins, etc. In order to achieve good dispersion of these immiscible fillers with silane-functionalized polyolefins, the melt mixing and fabrication steps need to be done at conditions where crosslinking (due to even parts per million (ppm) quantities of water that are inevitably present) is retarded. An effective means of assessing the efficacy of moisture-cure control additives (such as scorch-retardants or cure-accelerators) is to make melt-mixed compositions, that are subsequently molded and aged in a water bath (with the degree of crosslinking, by hot-creep measurements, assessed at various time intervals).
US 2013/0174416 discloses a process for manufacturing a multilayer article, the article comprising two crosslinked semiconductive layers separated by and bonded to an insulation layer, the semiconductive layers formed from a peroxide-crosslinkable olefin elastomer and the insulation layer comprising composition comprising a silane-grafted olefinic elastomer, the process comprises the steps of: (A) injecting the silane-grafted olefinic elastomer between the two crosslinked semiconductive layers so as to have direct contact with each semiconductive layer, and (B) crosslinking the silane-grafted olefinic elastomer in the absence of a peroxide catalyst.
US 2012/0178868 discloses a crosslinkable mixture comprising a polyolefin, an alkoxysilane, an organopolysiloxane, a free radical initiator and a liquid polymer modifier. The organopolysiloxane contains two or more hydroxyl end groups. When the crosslinkable mixture is melt-shaped, a unique crosslinked composition is formed. The liquid polymer modifier improves flexibility of the melt-shaped article without decreasing dielectric strength.
US 2012/0178867 discloses that crosslinked, melt-shaped articles are manufactured by a process that does not require the use of post-shaping external heat or moisture, the process comprising the steps of: (A) forming a crosslinkable mixture of a (1) organopolysiloxane containing one or more functional end groups; and (2) silane-grafted or silane-copolymerized polyolefin; (B) melt-shaping and partially crosslinking the mixture; and (C) cooling and continuing crosslinking the melt-shaped article. Crosslinking is promoted by the addition of a catalyst to the mixture before or during melt-shaping or to the melt-shaped article.
US 2010/0159206 discloses that tapes comprising (A) a first ribbon comprising (1)(a) a polyolefin containing at least one silane functionality, and (1)(b) a hydroxy-terminated silicone polymer, or (2) a blend of a vinyl silane, polyolefin, organic initiator, e.g., peroxide, and a hydroxy-terminated silicone polymer, and (B) a second ribbon comprising a catalyst masterbatch, the second ribbon carried on the first ribbon, allow for the delivery of a consistent mixture of resin and catalyst to an injection molding machine.
These disclosures teach the use of hydroxyl-terminated PDMS for accelerating silane crosslinking of polyolefins. Although a wide range of hydroxyl-terminated PDMS are identified by these disclosures, the working examples use hydroxyl-terminated polydimethylsiloxane DMS-15 (Mn of 2,000-3,500 g/mol, viscosity of 45-85 centistokes (0.000045-0.000085 square meters per gram (m2/g)), hydroxyl group (OH) level of 0.9-1.2%), and did not contain flame retardants (although the use of the latter are disclosed generally). DMS-15 grade of hydroxyl-terminated PDMS does accelerate silane crosslinking in the absence of flame-retardant fillers.
However, these disclosures do not teach the present invention. While they disclose a wide range of hydroxyl-terminated PDMS for use in crosslinking of silane-functionalized polyolefins, they do not convey that when flame-retardant fillers are present in the compositions, hydroxyl-terminated PDMS with an Mn≥4,000 g/mol, viscosity≥90 cP and hydroxyl group content of 0.9% or less, surprisingly yield more scorch-resistant compositions and a higher degree of crosslinking after moisture cure than those with an Mn≤4,000 g/mol, viscosity≤90 cP and a hydroxyl group content of more than 0.9%. That is, when a flame-retardant filler is present in the formulation, the molecular weight (viscosity) of the hydroxyl-terminated PDMS affects melt processability and moisture-induced crosslinking.
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