p-Static capped stretched acrylic transparencies

Abstract

The present invention provides an aerospace or aircraft transparency which, generally, includes a cast acrylic having a conductive acrylic incorporating a conductive ionic polymer is cast thereatop and cured. In a second embodiment hereof, a stretchable acrylic formulation having a conductive polymer admixed therewith along with a minor amount of a cross-linking agent cast atop a ply of unfilled stretchable acrylic. The stack is simultaneously cured and then stretched.

Claims

1. A p-static dissipating acrylic transparency, comprising: a non-conductive stretched transparent cast acrylic sheet comprising poly(methyl methacrylate) and having a smooth surface formed by casting between a glass or polished metal plate; and a transparent cast acrylic cap overlaying the smooth surface of the cast acrylic sheet, the cap comprising poly(methyl methacrylate) and a conductive polymer incorporated therewith for dissipating p-static electricity, wherein the cast acrylic sheet and the cast acrylic cap are cured together and then stretched together to produce the acrylic transparency with the cap, wherein the transparent cast acrylic cap is fused to the non-conductive cast acrylic sheet, and wherein the acrylic cap further comprising a crosslinking agent comprising triallyl isocyanate and at least one of 1,3-butanediol dimethacrylate, 1,4-butanediol dimethacrylate, neopentyl glycol dimethacrylate, or mixtures thereof.

2. The transparency of claim 1 wherein the cast acrylic sheet is a stretched cast unfilled poly(methyl methacrylate).

3. The transparency of claim 1, wherein the cap is a stretched cast filled cross-linked poly(methyl methacrylate).

4. The transparency of claim 1, wherein the conductive polymer is a polythiophene.

5. The transparency of claim 4, wherein the thiophene is poly(3,4-ethylenedioxythiophene-poly(styrenesulfonate).

6. The transparency of claim 1 wherein the conductive polymer is poly(3,4-ethylenedioxythiophene-poly(3-octylthiophene).

7. An aircraft transparency comprising the transparency of claim 1, the cap dissipating p-static electricity.

8. The transparency of claim 1, wherein the non-conductive cast acrylic sheet and the acrylic cap each have a degree of crystallinity resulting from being stretched.

9. The transparency of claim 1, wherein the acrylic cap comprises from about 0.1% to about 10% by weight of the conductive polymer, based upon an entire weight of the cap.

10. The transparency of claim 1, wherein the acrylic cap comprises from about 0.3% to about 5% by weight of the conductive polymer, based upon an entire weight of the cap.

11. The transparency of claim 1, wherein the acrylic cap is formed from an admixture comprising the conductive polymer, the poly(methyl methacrylate) and the cross-linking agents.

12. The transparency of claim 11, wherein the admixture comprises from about one to about ten parts by weight of the cross-linking agents of the acrylic cap, based on 100 parts by weight of the admixture.

13. The transparency of claim 11, wherein the admixture comprises from about three to about seven parts by weight of the cross-linking agents of the acrylic cap, based on 100 parts by weight of the admixture.

14. The transparency of claim 11, wherein the cast acrylic sheet is formed from an admixture comprising the poly(methyl methacrylate) and a cross-linking agent.

Description

DESCRIPTION OF THE INVENTION

(1) As hereinabove noted, the present invention pertains to an acrylic transparency having an outer layer or surface of conductive acrylic.

(2) As is known to those skilled in the art to which the present invention pertains a cast acrylic is, essentially, a poly (methyl methacrylate) prepared by casting, under pressure, methyl methacrylate monomer admixed with a catalyst, between glass cell plates and cauls and subsequently polymerizing the methyl methacrylate into a poly (methyl methacrylate). Under suitable polymerization conductors the resultant cast acrylic has a very smooth surface due to the glass casting cell plate.

(3) A stretchable acrylic is a methyl methacrylate having a small amount of a cross-linker in admixture therewith which is cast in the same manner as the non-stretchable acrylic.

(4) With respect to a stretchable acrylic, the acrylic is demolded from a glass cell and then bi-axially stretched at high temperature which provides a degree of crystallinity to the resulting stretched acrylic.

(5) Stretched acrylics are deployed primarily as aerospace transparencies due to their ability to arrest a crack from propagating after cracking is initiated, since non-stretched acrylic is very susceptible to crack propagation. Thus, because of the ability to arrest such crack propagation stretched acrylic has been widely adopted as transparencies in the aerospace industry for use as cabin windows on jetliners, cockpits, and the like.

(6) Typically, stretched acrylic transparencies are stretched from a blank of acrylic material to a desired fitness in order to improve the physical characteristics. Stretching is generally accomplished by either a pulling technique such as disclosed in U.S. Pat. No. Re. 24,978 or by a pressing technique such as taught in U.S. Pat. No. 3,632,841, the disclosures of which are hereby incorporated by reference.

(7) In both techniques, an acrylic blank is preheated to above its softening temperature, stretched and, thereafter, controllably cooled to below the softening temperature before the stretching forces are relaxed. Controlled cooling is done in order to avoid the effects of plastic memory.

(8) After the stretched acrylic sheet is cooled it is then ground and/or polished to remove any surface deformations to provide the requisite desired optical smoothness.

(9) However, as is noted hereinabove, such stretched acrylic suffers from the p-static issues discussed hereinabove.

(10) Thus, in a first embodiment hereof and, in accordance with the present invention, a transparency is prepared by casting a second or outer layer of a conductive acrylic on top of a ply of non-conductive acrylic.

(11) The second or outer layer comprises a methyl methacrylate monomer or oligomer having a conductive polymeric compound in admixture therewith.

(12) The conductive polymeric impregnant, generally, comprises a conductive polymer which is used to fill the methyl methacrylate monomer.

(13) Amongst the useful conductive polymers are, for example, the polythiophenes, such as, poly(3,4-ethylenedioxythiophene), poly (3-octyl thiophene), a poly(3,4-ethylenedioxythiophene)-(polystyrenesulfonate) and the like and mixtures thereof.

(14) In manufacturing the present transparency the amount of conductive polymer admixed with the methyl methacrylate will range from about 0.1 to about 10% by weight, of conductive material, based upon the entire weight of the outer layer and, preferably, from about 0.3 to about 5%, by weight, based upon the entire weight of the cap.

(15) In preparing the outer layer the acrylate and the polythiophene are admixed under high shear.

(16) Thereafter, the mixture is polymerized under traditional catalytic polymerization conditions.

(17) Among the useful catalysts are, for example, benzoyl peroxide, azobisisobutyronitrile (AIBN) and the like as well as mixtures thereof.

(18) Preferably, the catalyst is azobisisobutyronitrile (AIBN).

(19) Generally, the catalyst is used in an amount of about 0.01 to about 0.2 parts, by weight, per 100 parts, by weight, of acrylic and, preferably, from about 0.02 to about 0.07 parts, by weight, per 100 parts, by weight, of acrylic.

(20) In a second embodiment, the top layer may comprise a filled catalyzed methyl methacrylate having a cross-linking agent in admixture therewith which is cast against or onto a ply of unfilled stretchable acrylic. The two castings are then cured together. Curing, ordinarily, takes place in at a temperature ranging from about 120 F. to about 175 F. and is usually completed in about 2 to about 6 hours. After cure, the stack of the two acrylics are stretched together to give a stretched acrylic with a thin cap of conductive stretched acrylic.

(21) The cross-linker used herein is, for example, triallyl isocyanate, and 1,3-butanediol dimethacrylate, 1,4-butanediol dimethacrylate, neopentyl glycol dimethacrylate and the like as well as mixtures thereof.

(22) Generally, the cross-linker or cross-linking agent is present in an amount ranging from about one to about ten parts, by weight, based on 100 parts, by weight, of the acrylate and conductive polymer admixture and, preferably, from about three to about seven parts, by weight, of the cross-linker per 100 parts, by weight, of the admixture.

(23) Alternatively, the cross-linking agent and a suitable solvent for both the acrylate and conductive ionic polymer may be admixed together therewith. The solvent places both the ionic conductor and the acrylate into solution and the solvent can then be driven off upon cure under the reaction conditions noted above.

(24) A particular preferred solvent is then N-methyl-2-pyrrolidone (NMP).

(25) As is known to those skilled in the art, typically, after casting and stretching some polishing and grinding may be necessary to improve light transmission through the conductive cap.

(26) For a more complete understanding of the present invention reference is made to the following illustrative examples. In the examples, all parts are by weight. It is to be understood that the examples are to be construed as non-limiting and are provided for illustrative purposes only.

EXAMPLE I

(27) Into a suitable vessel is charged 90 parts of methyl methacrylate with 5 parts of poly(3,4-ethyleneclioxythiophene)-poly(stryrenesulfonate) to which is added 5 parts of 1,4-butanediol dimethacrylate.

(28) An additional 0.01 percent AIBN is also charged into the vessel. The reactants are mixed together under high speed with a high shear mixer until a homogeneous mixture is obtained. The mixture is then cast between glass plates and heated to about 175 for a period of four hours. The result is a p-static stretchable acrylic.

EXAMPLE II

(29) Following the procedure of Example I a conductive cast acrylic is prepared by mixing together the above ingredients but deploying 95 parts of methyl methacrylate while eliminating the dimethacrylate cross-linking agent after casting between the glass plates. A conductive cast acrylic is obtained.

EXAMPLE III

(30) Following the procedure of Example I a p-stretchable acrylic is obtained by mixing together 10 parts of N-methyl-2-pyrrolidone, 4.5 parts of the above-identified conductive polymer, 4.5 parts of 1,4-butanediol dimethacrylate and 81 parts of methyl methacrylate. The N-methyl-2-pyrrolidone places the ionic conductor and acrylate into solution which is, then, mixed under high shear. After a homogenous solution is obtained the mixture is then cast between glass plates under the same conditions outlined hereinabove. The above formula and process without the cross-linker yields a conductive cast acrylic.

EXAMPLE IV

(31) Following the procedure of Example II a conductive cast acrylic is prepared from the same compounds, but using 85.5 parts of methyl methacrylate, while eliminating the cross-linking agent to yield a conductive cast acrylic.

(32) It is to be appreciated by the preceding that there is obtained herein a p-static cap which can be cast atop non-conductive cast acrylic to provide an improved transparency for use where dissipation of p-static electricity is desired and, in particular, in aerospace applications. Additionally, a stretchable p-static cap can be cast atop non-conductive stretchable acrylic to provide a billet of conductive stretched acrylic that can subsequently be stretched.