Radiation curable wire and cable insulation compositions

Details Radiation curable wire and cable insulation compositions Radiation curable wire and cable insulation compositions

2000-04-03

2001-12-04

Jagannathan, Vasu (Department: 1714)

Synthetic resins or natural rubbers -- part of the class 520 ser

Synthetic resins

Processes of preparing a desired or intentional composition...

C524S261000, C524S264000, C524S560000, C524S563000

Reexamination Certificate

active

06326422

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to polymeric compositions useful as insulation for wire and cable products used for underhood automotive applications or 125° C. rated appliance wire applications. More specifically, the compositions of the invention are ethylene-vinyl ester copolymer formulations with improved irradiation crosslinkability.
2. Detailed Description of the Prior Art
Polymeric compositions exhibiting a balance of physical properties, processability and flame retardance comprised of crosslinkable ethylene-vinyl acetate copolymers, silanes and hydrated inorganic filler(s) are known and have found wide acceptance in the wire and cable industry. Such compositions are disclosed in U.S. Pat. Nos. 3,832,326 and 3,922,442 to North, et al., and U.S. Pat. Nos. 4,349,605 and 4,381,362 to Biggs, et al. Besides the polymer, silane and hydrated filler, the formulations typically also contain other additives such as stabilizers, lubricants, antioxidants and the like.
To achieve useful wire and cable insulation compositions having the necessary balance of physical properties and thermal and chemical resistance, it is necessary that the compositions be crosslinked. This can be accomplished using chemical crosslinking agents, typically organic peroxides, or by exposing the composition to ionizing radiation.
Irradiation crosslinking has had limited commercial acceptance in the wire and cable industry. Contributing to this lack of commercial acceptance is the reduced crosslinking efficiency obtained with stabilized formulations. It is well known that the commonly used phenolic antioxidants reduce radiation crosslinking efficiency to generally unacceptable levels and, if the necessary degree of crosslinking is to be achieved in formulations containing such stabilizers, it is necessary to include a promotor. Useful promotors are typically multifunctional compounds, e.g., multifunctional acrylates and methacrylates.
It would be highly advantageous if wire and cable formulations were available which could be radiation crosslinked to achieve acceptable levels of cure without the use of multifunctional promoters. It would be even more advantageous if these compositions had acceptable balance of properties making them suitable for low tension primary cable applications. These and other objectives are realized with the improved compositions of the invention which incorporate a zinc salt of a mercaptobenzimidazole compound.
Benzimidazole compounds are known stabilizers for thermoplastic resins such as polyethylenes and polypropylene. U.S. Pat. No. 3,218,276 discloses the use of alkyl benzimidazoles to stabilize fiber-forming polyolefins. Polypropylene fiber-forming compositions containing 0.2 to 2.0 percent benzimidazole with other conventional additives are disclosed.
U.S. Pat. No. 2,997,456 teaches the use of metallic mercaptobenzimidazole compounds as stabilizers for polymers of 1-olefins, primarily polypropylene, to protect against molecular degradation under conditions of elevated temperature and/or mechanical working and zinc mercaptobenzimidazole is specifically mentioned.
The combination of hindered phenols with various zinc salts of mercapto compounds to stabilize polyolefins is taught in U.S. Pat. Nos. 4,260,661; 4,693,937; 4,797,323 and 4,824,883. Other references which disclose benzimidazole stabilizers for polymeric materials include U.S. Pat. Nos. 4,459,380; 4,808,643 and 5,196,462.
SUMMARY OF THE INVENTION
The improved radiation curable wire and cable insulation compositions of the invention contain (a) 35 to 60 weight percent of a copolymer of ethylene and a vinyl carboxylate, alkyl acrylate or alkyl methacrylate, (b) 40 to 65 weight percent of a hydrated inorganic filler, (c) 0.25 to 2 weight percent of an alkoxysilane wherein the alkoxy group has from 1 to 6 carbon atoms and (d) 0.5 to 5 weight percent zinc salt of a mercaptobenzimidazole.
In a preferred embodiment (a) is an EVA copolymer, (b) is ATH, (c) is a vinylalkoxysilane and (d) is ZMTI. It is even more advantageous for certain wire and cable applications when the composition contains 40 to 55 weight percent (a) having a melt index from 0.3 to 5 g/10 min, 45 to 65 weight percent (b), 0.5 to 1.5 weight percent (c), and 1 to 4 weight percent (d).
Wire and cables coated with the above-defined compositions and irradiated utilizing e-beam or other conventional radiation sources to effect cure, i.e., crosslinking, are also embodied herein. The compositions are particularly useful for fabricating low tension primary wire and cable as defined by SAE Standard J1128.
DETAILED DESCRIPTION
The present invention relates to improved radiation curable insulating compositions comprising copolymers of ethylene and a vinyl carboxylate or alkyl acrylate or methacrylate, a silane, a hydrated inorganic filler, and a zinc salt of a mercaptobenzimidazole compound. These compositions can be cured to acceptable levels without the use of multifunctional promotors and, as a result, are useful for insulating wire and cable.
The polymeric component used to obtain the radiation crosslinkable compositions of the invention, also referred to as the base resin, is a copolymer of ethylene and a functionalized comonomer which may be a vinyl ester or an alkyl acrylate, the latter being used in the generic sense to encompass alkyl esters of both acrylic and methacrylic acid. The vinyl carboxylate may be a vinyl ester of a C
2
-C
6
aliphatic carboxylic acid, such as vinyl acetate, vinyl propionate, vinyl butyrate, vinyl pentanoate or vinyl hexanoate. The acrylates may be any of the C
1
-C
6
alkyl esters of acrylic or methacrylic acid including, for example, methyl, ethyl, propyl, butyl, pentyl or hexyl acrylate or methacrylate.
Preferred copolymers for the compositions of the invention are ethylene-vinyl acetate (EVA) copolymers containing about 9% to about 45% and, more preferably, 9% to about 30%, vinyl acetate, with the balance being ethylene. Terpolymers of ethylene, vinyl acetate and other known olefinic monomers polymerizable therewith can also be employed. Generally, if a third monomer is present, it will constitute no more than about 15% of the polymer composition.
Copolymers of ethylene and butyl acrylate are another useful type of base resin which can be used to formulate the improved compositions of the invention. Useful ethylene-butyl acrylate (EBA) copolymers contain about 10% to about 45% and, more preferably, 20% to 40% butyl acrylate—the balance being ethylene. n-Butyl acrylate is a preferred comonomer.
Blends of EVA and EBA, particularly where EVA comprises the major component of the blend, can also be used. The EVA will generally constitute greater than 75% of such blends. It is also possible to include minor amounts of other crosslinkable polymers or copolymers in the composition of this invention; however, the ethylene copolymer should comprise at least 50% of the total base resin component. Representative of such minor polymeric components which can be used in such embodiments include polyethylene, polypropylene, ethylene-propylene copolymers and terpolymers, and the like. Low density polyethylene (LDPE) and linear low density polyethylene (LLDPE) having melt indexes from 0.5 to 5 provide particularly desirable blends when used in amounts of 30% or less, based on the total polymers comprising the base resin component.
The ethylene copolymer or ethylene copolymer blend comprising the base resin generally has a melt index (MI) in the range 0.1 to 10 g/10 min and, more preferably, from 0.3 to 5 g/10 min. Melt index figures are equivalent values correlated from the melt flow rates determined in accordance with ASTM D 1238.
Fillers used for the present invention are hydrated inorganic fillers, e.g., hydrated aluminum oxides (Al
2
O
3
3H
2
O or Al(OH)
3
), hydrated magnesia, hydrated calcium silicate, hydrated magnesium carbonates, or the like. Hydrated alumina (ATH) is most commonly employed. Water of hydration chemically bound to these inorganic fillers is released endothermically upon combusti

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