Abstract:
The present invention is an optically clear impact resistant polyurethane which provides exceptionally high heat distortion temperatures and excellent chemical resistance. The invention is particularly useful for transparency applications that require excellent impact resistance coupled with high heat distortion temperatures, such as glazings for buildings, vehicles, riot shields, aircraft canopies, and face masks.

Description:
This is a CIP of application Ser. No. 08/382,562 filed Feb. 2, 1995, now abandoned. 
    
    
     BACKGROUND 
     1. Field of the Invention 
     The present invention relates to optically transparent heat and impact resistant polyurethane. 
     2. Background of the Invention 
     Currently, the standard material to which all optically transparent plastic materials are compared to for impact resistance is polycarbonate. These materials can be characterized by the temperature and pressure at which the material undergoes distortion. The heat distortion temperature of 9034 series clear polycarbonate is 280° F. at 264 psi. 
     Polycarbonate has a V-50 0.22 cal FSP (fragment simulating projectile) impacting rating of 925 ft./sec. This means that 50% of 22-caliber projectiles fired through a 0.250&#34; polycarbonate sheet penetrate a 20 mils-thick 2024 T6 aluminum sheet (the &#34;witness sheet&#34;) placed 6 inches behind the back surface of the poly carbonate sheet. 
     U.S. Pat. No. 3,866,242, which is incorporated herein by reference, discloses a polyurethane polymer protective shield. The polyurethane is produced by: 
     (a) reacting either a polyether glycol or a polyester glycol having a molecular weight of about 700 to 1,000 with methylenebis(cyclohexyl isocyanate) in an equivalent ratio of about three NCO to each hydroxyl to form a prepolymer, and 
     (b) reacting the prepolymer with an aromatic amine curing agent having a methylene bridge between two aromatic rings, such as 4,4&#39;-methylenebis(2-chloroaniline), in an equivalent ratio of 0.90 to 1.04 NH 2  /1.0 NCO. 
     U.S. Pat. No. 4,808,690, which is incorporated by reference herein, discloses a transparent polyurethane polymer made from a polyol cured prepolymer. The prepolymer is made from a polyisocyanate and at least one multifunctional hydroxy-containing intermediate. 
     SUMMARY OF THE INVENTION 
     The optically clear polyurethane of this invention can be prepared by first producing a prepolymer by reacting one equivalent of a polyester glycol or a polyether glycol having a weight average molecular weight of between about 600 and about 1200 with 4,4&#39;-methylenebis(cyclohexyl isocyanate) in an equivalent ratio of 2.5 to 4.5 NCO for each OH, with a preferable ratio of about 3 to 3.5 NCO for each OH. The prepolymer is then reacted with an aromatic diamine curing agent such as 4,4&#39;-methylenebis(3-chloro-2,6-diethylaniline) in an equivalent ratio of 0.95 to 1.02 NH 2  /1.0 NCO, with the preferred range being 0.96 to 1.0 NH 2  /1.0 NCO. 
     The polyurethane of the present invention is particularly useful for transparency applications that require excellent impact resistance coupled with high heat distortion temperatures, such as glazings for buildings, vehicles, riot shields, aircraft canopies and face masks. 
     One object of this invention is to provide a method for producing optically transparent plastic sheets for use in applications requiring excellent impact resistance, excellent chemical resistance, and high heat distortion temperatures. 
     Another object of this invention is to provide transparent polyurethanes having excellent optical clarity, excellent ballistic properties, and high heat distortion temperatures compared to prior art materials. 
     These and other objects of the present invention are described in greater detail in the detailed description of the invention, the examples and the attached claims. 
     DETAILED DESCRIPTION OF THE INVENTION 
     The polyurethane of the present invention is prepared from aliphatic diisocyanates; OH-containing intermediates selected from polyester glycols, polyether glycols and mixtures thereof; and aromatic diamine curing agents. The following is a detailed description of each of these constituents: 
     OH-Containing Intermediates 
     The OH-containing intermediates which can be used to prepare the polyurethanes of this invention are polyether glycols and polyester glycols having a weight average molecular weight of between about 500 to about 1,200. Polyether glycols and polyester glycols having molecular weights of about 600 to 900 are especially effective. Polyether glycols that can be used include polytetramethylene ether glycols having a weight average molecular weight of between about 500 and about 1000. 
     Representative polyesters which can be used to prepare the compositions of this invention include polycaprolactones and polyesters based on esterification of dicarboxylic acids of four to ten carbon atoms, such as adipic, succinic and sebacic acids, in the presence of low molecular weight glycols of two to ten carbon atoms, such as ethylene glycol, propylene glycol, diethylene glycol, 1.4-butanediol, 1.6-hexanediol and 1,10-decanediol. The polycaprolactones are prepared by condensing caprolactone in the presence of difunctional active hydrogen compounds such as water or the low molecular weight glycols listed above. Polyesters obtained by esterification of caprolactones, dicarboxylic acids and glycols can be derived by well known esterification or transesterification procedures, as described, for example, in the article D. M. Young, F. Hostettler et al., &#34;Polyesters from Lactone,&#34; Union Carbide F-40, p. 147. The preferred OH-containing intermediates are polyester glycols that are the esterification products of adipic acid or caprolactone with glycols of two to ten carbon atoms. The most preferred OH-containing intermediates are polyester glycols prepared from 1,6-hexanediol and adipic acid, 1,10-decanediol and adipic acid, or 1,10-decanediol and caprolactone. 
     Isoanates 
     The isocyanates which can be used to prepare the polyurethanes of this invention include diisocyanato-dicyclohexylmethanes and preferably isomeric mixtures thereof containing from about 20-100 percent of the trans,trans isomer of 4,4&#39;-methylenebis(cyclohexyl isocyanate), hereinafter referred to as &#34;PICM.&#34; Other components usually present in the mixtures of position and/or stereoisomers of the diisocyanato-dicyclohexylmethane used in this invention are the cis,trans and cis,cis isomers of PICM and stereoisomers of 2,4&#39;-methylenebis(cyclohexyl isocyanate). These, as well as the trans,trans PICM isomer, are present in amounts which can be controlled by the procedures used to prepare the diisocyanato-dicyclohexylmethane. Preferred diisocyanates are isomeric PICM mixtures. An especially preferred mixture contains not less than about 50 percent of the trans,trans trans isomer and no more than about 20 percent of the cis,cis isomer of 4,4&#39;-methylenebis(cyclohexyl isocyanate). This preferred mixture, when reacted with a polyester glycol and cured with 4,4&#39;-methylenebis(3-chloro-2,6-diethylaniline) produces a high-quality impact-resistant, transparent polyurethane. 
     The PICM used in this invention is prepared by phosgenating the corresponding 4,4&#39;-methylenebis(cyclohexyl amine) (PACM) by procedures well known in the art, as disclosed in, e.g., U.S. Pat. Nos. 2,644,007, 2,680,127, and 2,908,703, which are incorporated herein by reference. The PACM isomer mixtures, upon phosgenation, yield PICM in a liquid phase, a partially liquid phase, or a solid phase at room temperature. The PACM isomer mixtures can be obtained by the hydrogenation of methylenedianiline and/or by fractional crystallization of PACM isomer mixtures in the presence of water and alcohols such as methanol and ethanol. 
     Diamine Curing Agents 
     The aromatic diamine curing agents which can be used in the preparation of the polyurethanes of this invention have only primary amine groups. The preferred diamine curing agents have the following chemical formula: ##STR1## wherein R 1  and R 2  are each independently selected from methyl, ethyl, propyl, and isopropyl groups, and R 3  is selected from hydrogen and chlorine. Examples of the preferred diamine curing agents are the following compounds, manufactured by Lonza Ltd. (Basel, Switzerland): 
     LONZACURE® M-DIPA: R 1  ═C 3  H 7  ; R 2  ═C 3  H 7  ; R 3  ═H 
     LONZACURE® M-DMA: R 1  ═CH 3  ; R 2  ═CH 3  ; R 3  ═H 
     LONZACURE® M-MEA: R 1  ═CH 3  ; R 2  ═C 2  H 5  ; R 3  ═H 
     LONZACURE® M-DEA: R 1  ═C 2  H 5  ; R 2  ═C 2  H 5  ; R 3  ═H 
     LONZACURE® M-MIPA: R 1  ═CH 3  ; R 2  ═C 3  H 7  ; R 3  ═H 
     LONZACURE® M-CDEA: R 1  ═C 2  H 5  ; R 2  ═C 2  H 5  ; R 3  ═Cl 
     wherein R 1 , R 2  and R 3  refer to the above chemical formula. Lonzacure® M-CDEA is available in the United States from Air Products and Chemical, Inc. (Allentown, Pa.). The preferred diamine curing agent is 4,4&#39;-methylenebis(3-chloro-2,6-diethylaniline). 
     Preparation of the Invention 
     The polyurethanes of this invention can be prepared by one shot, quasi-prepolymer or full prepolymer methods, all of which are well known in the art. The preferred method of preparing the polyurethanes according to the invention is as follows: The methylenebis(cyclohexyl isocyanate) is first mixed with the OH-containing intermediate in an equivalent ratio of 2.5 to 4.5 NCO/1.0 OH and then heated to 190-275° F., preferably 260-275° F. The mixture is then heated under dry nitrogen so that the methylenebis(cyclohexyl isocyanate) reacts with the OH-containing intermediate to form a prepolymer. The heat source is then removed, the prepolymer is cooled to about 160° F., and the percent NCO in the prepolymer is determined. Additional methylenebis(cyclohexyl isocyanate) is then added to achieve an equivalent weight of from 415 to 425. The prepolymer is then reacted with the aromatic diamine curing agent in an equivalent ratio of 0.95 to 1.02 NH 2  /1.0 NCO. The prepolymer is then cured at 240-275° F. for 4 to 16 hours. The curing time is longer for the lower temperatures and shorter for the higher temperatures. 
     Preferred prepolymers for use in the present invention are prepared from adipic acid/hexanediol polyester having a weight average molecular weight of about 1000 and 4-4&#39;-methylenebis(cyclohexyl isocyanate) containing a minimum of 50% of the trans,trans isomer, and a maximum of 20% cis,cis isomer PICM. The equivalent ratio of NCO groups to OH groups is about three to one. 
     The polyurethane polymers of this invention can be cast, compression molded, extruded or injection molded. Casting is the preferred method, because it produces a polyurethane polymer with optimal optical characteristics. 
     The prepolymer may be cast in a mold prior to curing. The polyurethane material according to the invention may also be partially cured, by selecting an appropriate curing time and temperature, and then formed into the desired shape. Using this process, the polyurethane material can be formed into a simple or complex shape and then subsequently fully cured. 
     A triol may be added to the prepolymer in an amount sufficient to produce one percent cross-linking based upon equivalents of reactants, for example 4 to 8% by weight based on total reactants. Triols that are useful in the present invention include trimethylol ethane and trimethylol propane. The addition of a triol to the prepolymer increases the heat distortion temperature and improves the ballastics properties of the cured polyurethane. 
     The impact and crack propagation resistance of the polyurethanes according to the invention may be improved by the addition of an activated polybutene with epoxide functionality at one end in order to promote the formation of micro voids in the polyurethane. An activated polybutene may be added to the prepolymer in an amount ranging from 2.0 to 2.5% by weight based on the prepolymer without adversely affecting the desired optical and heat distortion properties. The addition of amounts greater than 2.5% by weight may result in a translucent or opaque polyurethane. Activated polybutenes that are useful in the present invention include compounds of the following formula: ##STR2## where n has a range of values such that the weight average molecular weight is about 365. One example of such an activated polybutene is Actipol E6, available from Elf Atochem. 
     Various anti-oxidants, ultraviolet stabilizers, and mold release agents may be used in the preparation of the polyurethanes of this invention. For example, one or more anti-oxidants may be added to the prepolymer in an amount of from 1 to 5% by weight based on total reactants. Anti-oxidants that are useful in the present invention include those of the multifunctional hindered phenol type. One example of a multifunctional hindered phenol type anti-oxidant is Irganox 1010, available from Ciba Geigy, which has the following chemical formula: ##STR3## 
     A UV-stabilizer may also be added to the prepolymer, either prior to or during the curing step, in an amount of from 1.5 to 2.0% by weight based on total reactants. UV-stabilizers that are useful in the present invention include benzotriazoles. Examples of benzotriazole UV-stabilizers include Cyasorb 5411 and Tinuvin 328. Cyasorb 5411, available from American Cyanamid, has the following chemical formula: ##STR4## Tinuvin 328, available from Ciba Geigy, has the following chemical formula: ##STR5## Another UV-stabilizer that may be used is Cyasorb 3604, available from American Cyanamid, which has the following chemical formula: ##STR6## 
     The polyurethanes of the present invention may be cast between glass plates, and the glass plates may be advantageously coated with a mold release agent prior to casting. Mold release agents that are useful in the present invention include silanes, for example Repcon rain repellant, available from Unelko Corporation (Chicago, Ill.). 
    
    
     EXAMPLES 
     In the following examples, all parts are by equivalent weight or weight percent unless otherwise indicated. These examples are illustrative and representative of the various embodiments of the present invention described above. 
     The materials used in the following examples were obtained from the following commercial sources: 
     
         ______________________________________4,4&#39;-methylenebis(cyclohexyl            Miles Industrial Chemicalsisocyanate)polyester glycols prepared from            RUCO Polymer Corporation1,6-hexanediol and adipic acid4,4&#39;-methylenebis(3-chloro-2,6-            Air Products and Chemicals, Inc.diethylaniline)trimethylol propane            Hoechst Celanesetrimethylol ethane            AldrichIrganox 1010     Ciba GeigyCyasorb 5411     American CyanamidCyasorb 3604     American CyanamidTinuvin 328      Ciba GeigyActipol E6       Elf AtochemRepcon           Unelko Corporation______________________________________ 
    
     Example I 
     A polymer was prepared by adding 2,187 parts of polyester glycols prepared from 1,6-hexanediol and adipic acid (molecular weight 1,038) to 1,813 parts of 4,4&#39;-methylenebis(cyclohexyl isocyanate) (containing approximately 50% of the trans,trans isomer) at 180-200° F. The mixture was agitated under a dry N 2  blanket and heated to a temperature of 275° F. over a period of approximately 60 minutes, at which time the heat source was removed. The prepolymer was cooled to 160° F. over approximately 30 minutes, and the percent NCO was determined. (The cool down time period is not a critical parameter.) Additional 4,4&#39;-methylenebis(cyclohexyl isocyanate) was added to achieve an equivalent weight of 420. The prepolymer was then reacted with 4,4&#39;-methylenebis(3-chloro-2,6-diethylaniline) in an equivalent ratio of 1.0 NCO to 0.98 NH 2  groups. The polymer was cast between glass plates so as to produce an optical sheet 0.25 inches×12 inches×12 inches, and was then cured at 250° F. for 12 hours. The glass plates were coated with Repcon rain repellent as a mold release. 
     The cured polyurethane sheet exhibited the following properties: 
     
         ______________________________________Luminous transmittance 85%Haze                   3.5%Hardness Shore &#34;D&#34;     80-81Heat Distortion at 264 psi                  302° F.V-50, 0.22 cal FSP     1180 ft./sec.______________________________________ 
    
     The luminous transmittance and haze were measured using a Gardner Laboratory XL 230 calorimeter. 
     Example II 
     To the prepolymer prepared according to Example I, 1.5% Cyasorb 5411 and 0.2% Irganox 1010 were added, based upon the prepolymer weight. Prior to casting the prepolymer, 0.25% Cyasorb 3604 was added based upon the total weight of the reactants (including the weight of the curing agent). The mixture was then cast and cured as described in Example I. The cured polyurethane sheet exhibited the following properties: 
     
         ______________________________________Luminous transmittance              85%Haze               3.5%Hardness Shore &#34;D&#34; 80-81V-50 0.22 cal FSP  1192 ft./sec.1000 hour accelerated ultraviolet              No change in the aboveexposure           properties2000 hour accelerated ultraviolet              No change in the aboveexposure           properties6 month outdoor weathering              Virtually no change in the above(Phoenix, AZ)      properties______________________________________ 
    
     The luminous transmittance and the haze were measured as in Example I. The ballistic properties of the polyurethane material were not affected by either the accelerated ultraviolet exposure or the outdoor weathering. 
     Example III 
     To the prepolymer prepared according to Example II, trimethylol propane (&#34;TMP&#34;) was added so as to produce one percent cross-linking based upon equivalents of reactants. A 0.25×12×12 inch test sheet was cast, as in Example I. The cured polyurethane sheet exhibited the following properties: 
     
         ______________________________________Luminous transmittance 85%Haze                   3.5%Hardness Shore &#34;D&#34;     80-81Heat distortion at 264 psi                  320° F.V-50 0.22 cal FSP      1224 ft./sec.Craze resistance to isopropyl                  &gt;7,000alcohol______________________________________ 
    
     The luminous transmittance and the haze were measured as in Example I. The addition of TMP to the prepolymer improved both the heat distortion temperature and V-50 test as compared with Examples I and II. The V-50 test result shows a 34% improvement over polycarbonate, and the heat distortion temperature is 45° F. higher than the heat distortion temperature of polycarbonate. 
     Example IV 
     To the prepolymer prepared according to Example II, 2.0 to 2.5 percent by weight Actipol E6 was added. The addition of the Actipol E6 further improved the impact and crack propagation resistance of the cured polyurethane sheet. 
     The foregoing disclosure of examples and other embodiments of the present invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise examples and embodiments disclosed. Many variations and modifications and modifications of the examples and embodiments described herein will be obvious to one of ordinary skill in the art in light of the above disclosure. The scope of the invention is to be defined only by the claims appended hereto, and by their equivalents.