Stock material or miscellaneous articles – Web or sheet containing structurally defined element or... – Including a second component containing structurally defined...
Reexamination Certificate
1997-07-11
2002-03-19
Kiliman, Leszek (Department: 1773)
Stock material or miscellaneous articles
Web or sheet containing structurally defined element or...
Including a second component containing structurally defined...
C428S331000, C428S411100, C428S446000, C428S447000, C428S500000, C428S515000, C428S688000, C428S689000, C428S702000, C523S212000
Reexamination Certificate
active
06358601
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to an antistatic protective coating and method for making the same. More particularly, the invention relates to an antistatic protective coating incorporating negatively charged inorganic oxide particles, a salt of a perfluorinated anion, and a surfactant.
BACKGROUND OF THE INVENTION
Thermoplastic and thermosetting polymers are used to form a wide variety of structures for which optical clarity, i.e., good light transmittance, is a desired characteristic. Examples of such structures include camera lenses, eyeglass lenses, binocular lenses, retroreflective sheeting, automobile windows, building windows, train windows, boat windows, aircraft windows, vehicle headlamps and taillights, display cases, eyeglasses, watercraft hulls, road pavement markings, overhead projectors, stereo cabinet doors, stereo covers, furniture, bus station plastic, television screens, computer screens, watch covers, instrument gauge covers, optical and magneto-optical recording disks, and the like. Examples of polymer materials used to form these structures include thermosetting or thermoplastic polycarbonate, poly(meth)acrylate, polyurethane, polyester, polyamide, polyimide, phenoxy, phenolic resin, cellulosic resin, polystyrene, styrene copolymer, epoxy, and the like.
Many of these thermoplastic and thermosetting polymers have excellent rigidity, dimensional stability, transparency, and impact resistance, but unfortunately have poor abrasion resistance. Consequently, structures formed from these materials are susceptible to scratches, abrasion, and similar damage.
To protect these structures from physical damage, a tough, abrasion resistant “hardcoat” layer may be coated onto the structure. Many previously known hardcoat layers have been formed from radiation curable prepolymers such as (meth)acrylate functional monomers. Such hardcoat compositions have been described, for example, in Japanese patent publication JP02-260145, U.S. Pat. No. 5,541,049, and U.S. Pat. No. 5,176,943. One particularly excellent hardcoat composition is described in WO 96/36669 A1. This publication describes a hardcoat formed from a “ceramer” used, in one application, to protect the surfaces of retroreflective sheeting from abrasion. As defined in this publication, a ceramer is a composite (preferably transparent) having inorganic oxide particles, e.g., silica, of nanometer dimensions dispersed in a crosslinkable mixture.
Thermoplastic polymers and thermosetting polymers, as well as hardcoats formed from such polymers, are poor electrical conductors. As a consequence, static charge can build-up on structures and coatings formed from these materials. Static charge attracts dust, which impairs optical clarity and appearance. Even the ceramer hardcoat compositions of WO 96/36669 A1 are susceptible to static charge build up. It has been desirable, therefore, to use antistatic agents to help prevent and/or dissipate static charges.
There are two common approaches for using antistatic agents with a hardcoat. One approach involves coating an antistatic agent directly onto the hardcoat layer. This approach, however, generally does not provide long-lasting antistatic protection in that such coatings tend to be easy to wipe or wear away. Another approach involves pre-mixing antistatic agents with the uncured hardcoat composition before the hardcoat is coated onto its substrate and cured. Generally, effective antistatic agents incorporated into hardcoat compositions have included one or more components in which at least one of the components is ionic. For example, U.S. Pat. No. 5,176,943 describes an antistatic agent including a combination of ionic and nonionic perfluorinated compounds.
Unfortunately, ceramers tend to be incompatible with many ionic compounds, including ionic antistatic compounds. The incompatibility arises from the preferred way in which ceramers are formed. Many ceramers are derived from aqueous sols of inorganic colloids in which the colloids are extremely charge sensitive. Adding ionic ingredients to such sols tends to destabilize the colloids, causing the colloids to flocculate and precipitate out of the dispersion. Flocculation is not conducive to forming high quality antistatic coatings. Firstly, flocculation results in local accumulations of particles, and these accumulations are large enough to scatter light. This detracts from optical clarity. Additionally, the accumulations of particles may cause nibs and/or other defects in the resultant coatings. Abrasion resistance may also be impaired. It would be desirable, therefore, to provide an approach in which antistatic agents could be incorporated into ceramer hardcoat compositions without causing the inorganic colloids to flocculate.
Relative humidity (RH) can affect the performance of antistatic coatings. For example, some antistatic systems might perform adequately at relatively high relative humidity, e.g. RH≧40%, yet perform dismally at relatively low relative humidity, e.g., RH<40%. Accordingly, there is also a need for antistatic systems that retain antistatic properties over wider ranges of relative humidity, particularly for conditions below 40% RH.
SUMMARY OF THE INVENTION
The present invention relates to an approach for improving the compatibility of antistatic agents and ceramer hardcoats so that such hardcoats can be provided with excellent antistatic characteristics. In particular, it has been discovered that the antistatic agents of this invention can be combined with aqueous sols of surface treated, colloidal inorganic oxide particles having a negative surface charge without destabilizing the negatively charged colloids. It has further been discovered that the combination of the antistatic agent and negatively charged, surface treated colloids of this invention interspersed in a cured ceramer matrix unexpectedly provides dramatic improvements in antistatic protection. The antistatic approach of this invention also provides excellent antistatic protection over a wide range of relative humidity, including conditions in which RH is below 40%, especially at 15% to 40% RH, preferably 20% to 40% RH.
In one aspect, the present invention relates to a cured, anti-static ceramer coating, comprising:
(a) a polymeric matrix;
(b) a plurality of surface treated, colloidal, inorganic oxide particles interspersed in the polymeric matrix, wherein the colloidal inorganic oxide particles have a negative surface charge;
(c) an antistatic amount of an antistatic agent dispersed in the polymeric matrix, wherein the antistatic agent comprises a perfluorinated anion having at least one perfluorinated moiety; and
(d) an amount of a nonionic surfactant effective to bias the perfluorinated moiety to migrate to a surface of the polymeric matrix.
In another aspect, the present invention relates to a substrate bearing an antistatic ceramer coating of the type described above.
In another aspect, the present invention relates to a coatable, radiation curable, antistatic, ceramer composition comprising:
(a) a curable binder component which is preferably a plurality of copolymerizable, free radically curable monomers and/or one or more silane monomers;
(b) a plurality of surface treated, colloidal, inorganic oxide particles having a negative surface charge;
(c) an antistatic amount of an antistatic agent, wherein the antistatic agent comprises a salt of a perfluorinated anion having at least one perfluorinated moiety; and
(d) an amount of a nonionic surfactant effective to bias the perfluorinated moiety to migrate to a surface of the composition.
In another aspect, the present invention relates to a method of providing a polymeric substrate with a protective, cured, antistatic ceramer coating comprising the steps of:
(a) coating an admixture onto the polymeric substrate, wherein the admixture is a coatable ceramer composition comprising a plurality of surface treated, colloidal, inorganic oxide particles having a negative surface charge, a curable binder component which is preferably a plurality of copolymerizable, free radically
3M Innovative Properties Company
Kiliman Leszek
Pribnow Scott R.
LandOfFree
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