Stock material or miscellaneous articles – Composite
Reexamination Certificate
2002-04-19
2004-05-25
Boykin, Terressa (Department: 1711)
Stock material or miscellaneous articles
Composite
C428S212000, C428S213000
Reexamination Certificate
active
06740410
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a thermoformable polymeric multi-layer article or laminate for dissipating electrostatic charges and comprises an electrostatic dissipating or discharging core layer and at least one overlayer. The overlayer is substantially free of particulation, and sloughing. Preferably, the core layer has better dissipative properties than the overlayer in order to drain electrostatic charges away from the surface of the laminate.
BACKGROUND
Most thermoplastics are electrical insulators. The accumulation and retention of static electrical charges on the surface of most plastics due to their low conductivity is well known. The accumulation of static charge on the surface of a plastic is undesirable for various reasons. Sometimes the static charge on these materials can discharge very quickly and damage sensitive components or articles which come in contact with the plastic. In addition, dust is typically attracted to and accumulates on materials carrying a static charge. Therefore, the electrostatic dissipative properties of plastic materials are of importance in various applications where static charge accumulation must be avoided.
Four major approaches have been used to alleviate the accumulation of static electrical charges in plastics: external chemical treatments, internal chemical additives, conductive fillers and polymeric additives. Each approach while effective in certain specific applications, suffers from deficiencies.
The external chemical treatments, hygroscopic surfactants that can be applied to the surface of the plastic article, suffer due to their reliance on high humidity for effectiveness. Generally, the relative humidity needs to be greater than 30%. Also, permanence is an issue since the external chemical treatment can be rubbed, wiped or washed off resulting in a loss of electrostatic discharge (“ESD”) protection. These types of chemical treatments are particularly undesirable in sensitive electronic handling applications where contamination of the components is an issue. Examples of these chemical treatments include fatty acids and their amines or salts, quaternary ammonium salts, monoalkyl glycerides, alkyl phosphonates and sulfonamides.
Internal chemical additives rely upon additive migration to the surface of the plastics to provide ESD protection to the plastic. Internal chemical additives are generally low molecular weight non-polymeric, hygroscopic surfactants which are compounded into the plastic material itself. While this approach provides more permanence than the external chemical treatments, just like external chemical treatments it is also prone to be rubbed, wiped or washed away. Internal chemical additives migrate to the surface of the plastic because of their limited compatibility with the plastic. When the additive migrates to the surface, it can be wiped, abraded or washed off, leaving the surface without any ESD protection. A static charge can then build on the surface, since the material is not protected. This lack of permanence results in periods of ESD susceptibility in which the plastic cannot dissipate a charge until additional additives can bloom to the surface. In addition, the ESD additive can contaminate sensitive devices that come into contact with the plastic article. The internal chemical additives also depend upon high humidity to be effective.
The use of conductive fillers can also alter the electrical properties of plastics. Examples of conductive fillers include conductive carbon black, carbon fibers, metal-coated substrates, metallic fibers or fillers. Although the conductive fillers are neither humidity dependent or susceptible to being wiped, washed or rubbed away, they do have certain disadvantages. For example, conductive fillers tend to increase the melt viscosity for processing the blend. Also, conductive fillers tend to limit the colorability of the plastic materials. Generally, they also tend to decrease the physical properties of the blend such as impact resistance. Also, contamination concerns often preclude the use of plastics loaded with conductive fillers in environments such as cleanrooms.
Conductive fillers also tend to have steep loading curves. As the percentage of conductive filler in the insulative thermoplastic is increased from zero, the composite material's bulk will remain insulative until a continuous network of the conductive filler is formed. At this point, often called percolation, the resistivity will drop sharply with increasing filler percentage. The steep slope of the loading curve once the network is formed demonstrates the high level of variability that can be inherent in the electrical properties in these heterogeneous materials, particularly after secondary processes such as thermoforming. On a microscopic scale, different domains or regions within the composite will have varying concentrations of filler and hence varying levels of resistivity. Regions with low levels of filler can be electrically insulative and retain voltages that can potentially damage sensitive microelectronic devices. Such regions or domains are commonly called “hot spots.” Conductive regions within the material can discharge voltages too rapidly, releasing current densities that can damage devices. This deficiency is more of a concern as microelectronic devices become more and more sensitive.
Numerous examples of internal polymeric additives exist. An example of the addition of an electrostatic dissipative polymer being added to an insulative polymer can be found in U.S. Pat. No. 3,425,981 to Puletti. The patent to Puletti discloses an olefin polymer composition containing ethylene oxide based polymers and exhibiting enhanced antistatic properties. Additionally, U.S. Pat. No. 5,010,139 to Yu discloses an antistatic polymeric composition consisting of a blend of a polymeric material and antistatic additive which is an ethylene oxide based copolymer or terpolymer material. The polymeric material can be any thermoplastic, thermoplastic elastomer or elastomer including acrylonitrile butadiene styrene (ABS); copolymers of styrene and acrylonitrile modified with acrylic elastomers (ASA); polyamides; polybutylene terephthalate (PBT); polyethylene terephthalate (PET); polyethylene terephthalate glycol (PETG); polymethylmethacrylate (PMMA); polyurethane (TPU); polyvinyl chloride (PVC); chlorinated polyvinyl chloride (CPVC); polycarbonate (PC); polyoxymethylene (POM); polyphenylene oxide (PPO); copolymer of styrene and maleic anhydride SMA; and styrene acrylonitrile copolymer (SAN).
U.S. Pat. No. 5,159,053 discloses a thermoplastic polyurethane which has electrostatic dissipative properties. The thermoplastic polyurethane comprises the reaction product of an ethylene ether oligomer glycol reacted with a non-hindered diisocyanate and an extender glycol. The ethylene ether oligomer intermediate comprises a polyethylene glycol, having an average molecular weight from about 500 to about 5,000. Such materials are commonly called inherently dissipative polymer (IDP).
U.S. Pat. No. 6,140,405 discloses the modification of the aforementioned electrostatic dissipative thermoplastic polyurethane with salts, enhancing the rate of static dissipation in the neat form and when blended with a variety of base polymers.
U.S. Pat. No. 5,342,889 discloses electrostatic dissipative polymeric compositions which are blends of an effective amount of a chain extended polymer and a matrix polymer. The chain extended polymer is formed from low molecular weight polyethers which are reacted with a chain extender and a diisocyanate. The matrix polymers include PVC, CPVC, a terpolymer of styrene, acrylonitrile and diene rubber; a copolymer of styrene and acrylonitrile modified with acrylate elastomers; a copolymer of styrene and acrylonitrile modified with ethylene propylene diene monomer rubber; rubber modified impact polystyrene; thermoplastic polyesters including PBT, PET and polyether-ester block copolymer; polyphenylene oxide; polyacetal; polymethyl methacrylate or mixtures thereof.
The usage of polym
Fahey Timothy Edward
Kim Kyung J.
Ludlow, III James Minor
Boykin Terressa
Dunlap Thoburn T.
Hudak, Shunk & Farine Co. LPA
Noveon IP Holdings Corp.
Powell Joe A.
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