Abstract:
A coating of atactic polypropylene over a transparent armor substrate improves resistance to penetration while allowing convenient repair of minor abrasions and scratches.

Description:
BACKGROUND 
       [0001]    Drawbacks to conventional transparent armor include the need to use thicker panels to achieve desired levels of protection, thus incurring a weight penalty, and environmental erosion and scratching of the surface, which reduces transparency. A need exists to mitigate these problems. 
       BRIEF SUMMARY 
       [0002]    In a first embodiment, an armor system includes a hard, transparent armor substrate, and a transparent coating of atactic polypropylene bonded to the armor substrate. 
         [0003]    In another embodiment, a vehicle incorporates the armor system of the first embodiment, with the transparent coating configured to face an exterior surface of the vehicle, the armor system configured as a window, windscreen, or viewing port of said vehicle. 
         [0004]    A further embodiment involves treating the armor system of the first embodiment by heating and smoothing the transparent coating, thereby improving optical clarity thereof. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
         [0005]      FIG. 1  shows the increase in velocity required to penetrate armor (V-50) due to the presence of a 19 mm polyurea coating. 
       
    
    
     DETAILED DESCRIPTION 
       [0006]    Definitions 
         [0007]    Before describing the present invention in detail, it is to be understood that the terminology used in the specification is for the purpose of describing particular embodiments, and is not necessarily intended to be limiting. Although many methods, structures and materials similar, modified, or equivalent to those described herein can be used in the practice of the present invention without undue experimentation, the preferred methods, structures and materials are described herein. In describing and claiming the present invention, the following terminology will be used in accordance with the definitions set out below. 
         [0008]    As used in this specification and the appended claims, the singular forms “a”, “an,” and “the” do not preclude plural referents, unless the content clearly dictates otherwise. 
         [0009]    As used herein, the term “and/or” includes any and all combinations of one or more of the associated listed items. 
         [0010]    As used herein, the term “about” when used in conjunction with a stated numerical value or range denotes somewhat more or somewhat less than the stated value or range, to within a range of ±10% of that stated. 
         [0011]    As used herein, the term “armor substrate” refers to new and conventional forms of transparent armor including, without limitation, laminates of soda-lime or borosilicate glass with polycarbonate as well as transparent ceramic armor including aluminum oxynitride (“Alon”), spinel (including nanocrystalline spinel), and the like, and combinations thereof. 
         [0012]    Description 
         [0013]    Elastomeric coatings were found to substantially increase the ballistic limit of underlying steel armor substrates, with a composite array of elastomer-steel panels enjoying increases armor penetration resistance, as reported in Roland et al., “Elastomer-steel laminate armor” Composite Structures 92 (2010) 1059-1064, incorporated herein by reference. 
         [0014]    Various coatings including polyurea and butyl rubber have shown to function well in this application, and the coating itself may include a combination of materials. 
         [0015]      FIG. 1  shows the increase in average velocity required to penetrate armor (V-50) due to the presence of a 19 mm polyurea coating. The coating contribution to penetration resistance systematically increases with increasing substrate hardness. On steel substrates, mass efficiencies exceeding a factor of two have been achieved. 
         [0016]    With regard to conventional transparent armor, thicker panels are required to achieve higher ballistic performance, with a concomitant weight penalty which is especially undesirably in the case of vehicles, adversely impacting performance, fuel economy, and payload, while the bulkier panels impinge on interior space. Furthermore, conventional transparent armor can be prone to environmental abrasion or scratching, reducing transparency and requiring costly and time-consuming repair. 
         [0017]    This armor system may be applied to vehicles including manned or unmanned vehicles suitable for travel on the ground, or in the air, on the surface of water or underwater, and combinations thereof. It may be used in windows, windscreens, viewing ports, and the like. 
         [0018]    As described herein, a transparent armor system includes a polymer coating applied to a transparent armor substrate. The density by area of this transparent armor system can be less than that of conventional armor systems while providing equal or greater protection. 
         [0019]    The protective function of the coating is believed to arise from an impact-induced phase transition with consequent large energy absorption, so that the substrate should be stiff enough to allow rapid compression of the coating. Atactic polypropylene with a glass transition temperature of about −20° C. functions as a suitable coating due to this phenomenon, while providing the desired transparency. 
         [0020]    Armor Substrate 
         [0021]    The armor substrate is preferably transparent and with sufficient rigidity and hardness to support the coating while also itself resisting penetration. Most preferably, the armor substrate has a hardness of at least 150, 200, 300, 400, 500, or more, as measured using the Brinell method with a tungsten ball of 10 mm diameter and 3,000 kg force. 
         [0022]    The armor substrate may be one or more new or conventional forms of transparent armor including, without limitation, laminates of soda-lime or borosilicate glass with polycarbonate and transparent ceramic armor including aluminum oxynitride (“Alon”), spinel (including nanocrystalline spinel), and the like, and combinations thereof. Nanocrystalline ceramic material that might be suitable for use as an armor substrate is described in commonly-owned U.S. Provisional Patent Application No. 61/907,440 filed on Nov. 22, 2013, incorporated herein by reference. 
         [0023]    Traditional bullet-resistant glass is available with coatings under the trade names MARGARD and MAKROLON intended to improve scratch resistance. The present armor system may be used with any such forms of coated transparent substrates, termed secondary coatings to distinguish them from the atactic polypropylene coating of the invention. It is believed that hard coatings may increase the effective hardness of the glass, thus improving performance of the system as seen in  FIG. 1 . The polypropylene coating senses the hardness of the substrate of length-scales commensurate with the wavelength of the longitudinal pressure wave—this may guide the design of the thickness of a secondary coating. 
         [0024]    Coating 
         [0025]    The polymeric coating is preferably atactic polypropylene. It was found that isotactic polypropylene would crystallize and fail to provide the desired ballistic performance. A suitable molecular weight may be from about 40 to about 80 kilograms/mol for an atactic polypropylene polymer. In preparing the polymer, it should be cooled quickly to avoid formation of crystals large enough to scatter visible light. 
         [0026]    The coating thickness may range, for example, from about 0.25 cm to about 2.0 cm. 
         [0027]    The coating may be bonded to the armor substrate using various techniques. It may be in direct contact with the armor substrate or bonded thereto via an intermediate adhesive. It may be cast into place on the armor substrate. Mechanical bonding may be used, for example using a frame, clamps, bolts, or other fasteners. A combination of bonding techniques may be used. 
         [0028]    An advantage of this transparent polymeric coating is its reversible solidification (as opposed to solidification via a practically irreversible chemical change in other polymers). Thus, abrasions and scratches may be removed by heating, optionally while contacting the surface of the polymer with a smooth surface. It was found that a temperature of about 100° C. was sufficient to repair atactic polypropylene. Such repairs could easily be made in the field. 
         [0029]    Concluding Remarks 
         [0030]    All documents mentioned herein are hereby incorporated by reference for the purpose of disclosing and describing the particular materials and methodologies for which the document was cited. 
         [0031]    Although the present invention has been described in connection with preferred embodiments thereof, it will be appreciated by those skilled in the art that additions, deletions, modifications, and substitutions not specifically described may be made without departing from the spirit and scope of the invention. Terminology used herein should not be construed as being “means-plus-function” language unless the term “means” is expressly used in association therewith.