Multilayered packaging materials for electrostatic applications

Details Multilayered packaging materials for electrostatic applications Multilayered packaging materials for electrostatic applications

2001-07-09

2004-05-04

Zacharia, Ramsey (Department: 1773)

Stock material or miscellaneous articles

Composite

C428S412000, C428S423100, C428S473500, C428S474400, C428S475500, C428S480000, C428S500000, C428S521000, C428S522000, C428S523000, C428S524000, C428S532000, C428S924000, C206S719000

Reexamination Certificate

active

06730401

ABSTRACT:

BACKGROUND OF THE INVENTION
Polyester materials are widely used as extrusion and injection molding resins for applications such as fibers, films, automotive parts, and food and beverage containers. Commonly used polyesters include poly(ethylene terephthalate) (PET), poly(1,4-butylene terephthalate) (PBT), poly(1,4-cyclohexylene-dimethylene terephthalate) (PCT), and poly(ethylene 2,6-naphthalenedicarboxylate) (PEN). These polyesters typically have good heat resistance and high glass transition temperatures. For those applications in which extrusion and molding temperatures must be maintained below about 240° C., these highly crystallizable polyesters are not used because their melting points are too high. In such cases, amorphous or slowly crystallizable copolyesters are used since these materials may be processed at moderate temperatures.
For applications of packaging of static sensitive electronic components such as disk drive heads and integrated circuits, materials that are conductive or static dissipative and processable at moderate temperatures are required. The optimum fitness-for-use criteria for this market include electrostatic dissipation properties, dimensional stability, washability, thermoformability, acceptable slitting characteristics, peelable seal characteristics to cover tapes, and low migration levels of condensable materials from the package to the packaged components. Thermoplastics used for packaging of static sensitive electronic components often consist of blends of non-conductive polymers with inherently dissipative polymers (IDP), inherently conductive polymers (ICP), or conductive fillers. Blends containing IDPs have surface resistivities greater than 10
5
and less than 10
12
ohms/square. Blends containing ICPs or conductive fillers have surface resistivities less than 10
5
ohms/square. While blends utilizing IDP have been preferred, industry trends are toward lower surface and volume resistivities and faster static decay times than provided by IDP blends.
Many patents disclose IDPs and their use as electrostatic dissipating additives for other non-conductive polymers. IDPs based on polyurethane copolymers derived from polyethylene glycol are disclosed in U.S. Pat. Nos. 5,159,053, 5,342,889 and 5,574,104. Such polyurethane copolymers are commercially available from The B. F. Goodrich Company under the tradename Stat-Rite™ and may be blended with other polymers as an electrostatic dissipative agent. Ethylene oxide copolymers used to impart electrostatic dissipating properties to various polymers are disclosed in U.S. Pat. Nos. 4,719,263, 4,931,506, 5,101,139 and 5,237,009. Polyetheresteramide electrostatic dissipating polymers are disclosed in U.S. Pat. Nos. 4,230,838 and 5,604,284, and blends of polyetheresteramide copolymers with other polymers are disclosed in U.S. Pat. Nos. 5,298,558 and 5,886,098. Another source disclosing blends of IDPs with non-conductive matrix polymers is “Electrically Conductive Polymer Composites and Blends,” Polymer Engineering and Science, 32(1), 36 (1992).
As for ICPs blended with non-conductive polymers, WO 91/10237 discloses compositions with electrostatic dissipating properties that contain a non-conductive matrix polymer and at least two additives. In one of the examples, a copolyester of poly(ethylene terephthalate) containing 1,4-cyclohexanedimethanol is combined with polyaniline and another conductive material. The use of polyaniline in imparting electrical conductivity to various polymers including thermoplastic polyesters is also disclosed in U.S. Pat. No. 5,567,355. Another source disclosing blends of inherently conductive polymers and non-conductive matrix polymers for electrostatic dissipating applications is “Processable Intrinsically Conductive Polymer Blends,” Journal of Vinyl Technology, 14, 123 (1992). ICPs alone are disadvantaged in the market place due to outgassing or release of volatiles.
Several references disclose the blending of conductive fillers with non-conductive polymers. U.S. Pat. Nos. 5,643,990 and 6,184,280 disclose the use of carbon fibrils in imparting electrical conductivity to various polymers including thermoplastic polyesters. A commercial product under the tradename Shock Block™, available from Hyperion Catalysis Int'l. of Cambridge, Mass., utilizes a highly-conductive, hollow, graphite fiber to impart conductivity to plastics. Shock Block™ is a mono-layer sheet that is static dissipative on the side in contact with the electronic part and conductive on the other side. Carbon black is another conductive filler used in imparting electrical conductivity to various polymers including thermoplastic polyesters as disclosed in U.S. Pat. Nos. 5,382,384, 5,250,228, and 5,093,036. The use of carbon black and impact modifiers in imparting electrical conductivity and mechanical toughness to amorphous copolyester resins is disclosed in U.S. Pat. No. 5,643,991. Due to the physical nature of conductive fillers, problems often arise with particle contamination even when blended with a polymer.
Multi-layer electrostatic dissipative structures are disclosed in U.S. Pat. No. 5,914,191. The outer layer(s) are comprised of a blend of copolyester and electrostatic dissipating polymer and the core layer is comprised of a polymer having a haze value less than 5 percent.
SUMMARY OF THE INVENTION
A multi-layer structure comprises at least one electrostatic dissipative outer layer and a conductive core layer. The outer layer comprises a material selected from the group consisting of an inherently dissipative polymer, an inherently dissipative polymer blended with a non-conductive matrix polymer, an inherently conductive polymer blended with a non-conductive matrix polymer in an amount sufficient to impart a surface resistivity of greater than 10
5
and less than 10
12
ohms/square, and mixtures thereof. The core layer comprises a material selected from the group consisting of an inherently conductive polymer, an inherently conductive polymer blended with a non-conductive matrix polymer, a conductive filler blended with a non-conductive matrix polymer, and mixtures thereof. The multi-layer structure has unexpected improved electrical properties over prior art structures because the surface resistivity of the outer layer in the multi-layer structure is less than the surface resistivity of the outer layer alone or in another multi-layer structure absent contact with the core layer.
DESCRIPTION OF THE INVENTION
This invention relates to a new class of thermoformable multi-layer structures for applications requiring a thermoplastic material that can dissipate an electrostatic charge. Many applications exist in which the multi-layer structure of the present invention may be used, i.e. packaging for static sensitive electronic components, clean room glazing and multi-wall sheets used as partitions, fabricated boxes and extruded profiles.
The multi-layer structure of the present invention has unexpected improved electrical properties over prior art structures utilized for the same applications. The surface resistivity of the outer layer in the multi-layer structure is less than the surface resistivity of the outer layer alone or in another multi-layer structure absent contact with the core layer. This lowering of the surface resistivity of the outer layer is caused by contact with the conductive core layer. This phenomenon is shown in the Examples below. Additionally, the multi-layer structures exhibit lower volume resistivities and faster static decay times than monolayer electrostatic dissipative structures. The multi-layer structures also provide lower particulate contamination or sloughing compared to monolayer structures utilizing conductive fillers.
The multi-layer structure comprises at least one electrostatic dissipative outer layer and a conductive core layer. Preferably, two outer layers are utilized with the core layer sandwiched therebetween. The outer layer is a dissipative layer having surface resistivity of about 10
5
to about 10
12
ohms/square. The outer layer comprises a material selected

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