It is known to use an energy absorbing interlayer of plasticized polyvinyl butyral (PVB) with one or more rigid layers such as glass in a safety glazing. Such a glazing is usually prepared by interposing the PVB layer between glass sheets while eliminating air from between the engaging surfaces and then subjecting the assembly to elevated temperature and pressure in an autoclave to fusion bond the PVB and glass and form an optically clear structure. These glazings are used in windows such as the front, side and rear windows in motor vehicles, particularly windshields, where the interlayer can absorb a blow from the head of an occupant without penetration of the windshield.
Automobile and homeowners, especially those who have experienced a vehicle break-in, theft or hurricane damage, are increasingly interested in intrusion resistant glazings. Glass has been identified as the weak link in the overall intrusion resistance of vehicles and buildings. For instance, some police reports estimate that broken glass is the entry route for at least 60% of unauthorized entry into passenger vehicles. There is also a belief that a 30-second delay in entry is sufficient to discourage many spontaneous thieves because the increased break-in time and additional noise call attention to the thief. In response, carmakers, at least, are replacing traditional tempered glass with laminated glass for enhanced security. Common automotive laminated glass, though, can be defeated by determined thieves who use more sophisticated tools to puncture by impact and pull laminated glass out from its frame.
Many attempts to improve the performance of glass laminates have been made, including modifying the stiffness and/or impact resistance of the PVB interlayer. For example, U.S. Pat. No. 4,814,529, to Cartier et al., discloses lightly cross-linking PVB resin to selectively increase molecular weight of the PVB and the modulus of a plasticized sheet formed therefrom for use in laminated safety glass assemblies. U.S. Pat. No. 5,246,764, to LaPorte et al., discloses laminated glazing with improved impact strength where mean break height for a dropped mass of a glass laminate increased by dispersing adhesion resistant means on the surface of PVB sheet. U.S. Pat. No. 5,482,767, to Karagiannis et al., discloses glass laminates of improved impact resistance comprising a PVB interlayer having discrete particles of crosslinked polyvinyl butyral integrally randomly dispersed throughout a matrix of PVB.
In recent years additional sophisticated features are appearing in such windows to enhance performance. These include special, layered metal/dielectric stacks for solar radiation control which may also be electrically conductive for defrosting, defogging, etc; holographic layers as solar reflecting mirrors and in head-up displays to facilitate viewing instruments on the vehicle dashboard while looking straight ahead; photochromic and electrochromic layers which controllably change color and/or visible transmission upon exposure to solar radiation or application of a voltage; layered protective antilacerative structures on the inboard side of a conventional three layer glass/PVB sheet/glass laminate to minimize lacerations from sharp edges of broken glass; special plastic layers in bilayer structures replacing one glass layer of such a three layer glass laminate, and similar, functional performance-enhancing layers and coatings. These performance layers are usually deposited on or adhered to a carrier layer which is different from the low modulus, elastomeric PVB which is unsuitable as a carrier. For use in safety glazings a carrier layer should have good clarity, be relatively uniform in thickness and strong having high modulus to facilitate ease of handling and processing during association with the performance layer(s). Frequently biaxially oriented polyethylene terephthalate is used as noted, for example, in U.S. Pat. No. 4,465,736.
Using PET in glass laminates offers many advantages. As U.S. Pat. No. 5,024,895, to Kavanagh et al., and U.S. Pat. No. 5,091,258, to Moran, disclose, PET can be biaxially stretched to improve strength and can be heat stabilized to provide low shrinkage characteristics when subjected to elevated temperatures. The tensile modulus, a desired property of glass laminate interlayers and an indication of the stiffness of the interlayer, at 21° C.-25° C. of PET is about 1010 Pa as compared with about 107 Pa for PVB of the type used in safety glazings. This increased stiffness of PET is a desirable property for use in glass laminates.
Many applications of PET involve a layer of PET being used as a carrier for the aforementioned functional layers, such as solar radiation blockers, antennas or heat strips. U.S. Pat. No. 5,979,932, to Jourdaine et al., U.S. Pat. No. 5,091,258, to Moran, and U.S. Pat. No. 5,932,329, to Frost et al., disclose a PET layer between two PVB layers wherein the PET layer is provided with an infrared-reflective coating. U.S. Pat. No. 4,017,661, to Gillery, discloses a composite interlayer wherein a PET sheet is coated with an optically clear, electrically conductive, transparent coating used as a carrier for metal layers which can be electrically heated for defrosting the glass laminate. U.S. Pat. No. 5,024,895, to Kavanagh et al., discloses a PET layer between two PVB layers that includes an integrated infrared-reflective and an electrically conductive coating.
Although most prior art laminates generally provide acceptable resistance to shattering when struck with a blunt object, there is often unacceptable resistance to penetration and pullout. For example, in the case of automobile break-ins, prior art glass laminates often will not shatter, but will break. After being broken, though, the prior art glass laminates usually are significantly weakened and are thus susceptible to being pulled out of their frames. This lack of stiffness in prior art glass laminates can effectively offset the otherwise acceptable shatter-resistant nature of the glass laminates, particularly in automotive break-in situations. Many prior art glass laminates do not exhibit acceptable strength properties after being broken.
Furthermore, although simply increasing the PVB thickness can improve resistance to penetration, this solution does not alleviate the poor resistance to pullout problem. Increased thickness laminates can also be limited by optical clarity. It is of utmost importance that glass laminates used for automotive safety glazings exhibit a high degree of optical clarity; that is, exhibit high level of visible transmission and low levels of optical haze or light scattering. Prior art laminates do not provide a means to achieve a significant increase in laminate stiffness without compromising optical clarity.
It would thus be desirable to develop a glass laminate that exhibited superior penetration resistant qualities, as well as an increased stiffness so as to improve the glass laminate's resistant to pullout after the glass is broken, for example, for use in security glazing applications such as architectural and automotive glazing. Furthermore, it would be desirable to improve the stiffness of a laminate without sacrificing its optical clarity.
SUMMARY OF THE INVENTION
This invention provides a composite laminate interlayer for use in manufacturing intrusion resistant glass laminates having a higher laminate stiffness. Such laminates of the present invention comprise an interlayer having two outside layers of PVB, and two inside layers of bonded optical grade PET. To increase the stiffness in the laminate, it has been found that using two pieces of optical grade PET improves the stiffness of the laminate but does not significantly reduce the optical clarity of the laminate. This contrasts common practice which recognizes that as the thickness of a single layer of PET increases, its optical clarity is reduced. Optical grade PET with haze levels less than ˜1.0% is limited to about 0.175 mm (7 mil) or less. Combining two pieces of optical grade PET, for example two 7-mil pieces, improves the laminate's resistance to breaking and pullout/push-in as a single layer of PET of the same combined thickness would, without significantly sacrificing the optical clarity of the laminate.
Another aspect of this invention provides a composite laminate interlayer comprising two layers of PET between layers of plasticized PVB adhesive where at least one of the PVB layers exhibits enhanced stiffness. In one aspect of the invention, high stiffness in plasticized PVB is provided by reducing the level of plasticizer in the PVB. This is indicated by a rise in glass transition temperature (Tg) of the plasticized PVB, e.g. about 2 to 3° C. above typical values for a commercial application. For instance, for automotive applications, where plasticized PVB sheet typically has a Tg of about 30-33° C., high stiffness PVB of this invention will have a Tg of at least 35° C.
Another aspect of this invention provides interlayer composites with other functional layers, e.g. sound dampening elastomer layers or radiation blocking layers.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
As used herein glass transition temperature (Tg) of plasticized PVB is determined by rheometric dynamic analysis using the following procedure. Plasticized PVB sheet is molded into a sample disc of 25 millimeters (mm) in diameter. The PVB sample disc is placed between two 25-mm diameter parallel plate test fixtures of a Rheometrics Dynamic Spectrometer II. The PVB sample disc is tested in shear mode at an oscillation frequency of 1 Hertz as the temperature of the PVB sample is increased from −20 to 70° C. at a rate of 2° C./minute. Experience indicates that the method is reproducible to within +/−1° C. Commercially used PVB typically has a Tg in the range of 30-33° C.
Visual haze for both the PET film components and the glass laminate are measured according to ASTM D1003 using Illuminant C and an observer angle of 2°.
As used herein, the units of “mils, “mm” and “inch” will be used in describing the thickness of a glass laminate interlayer. The relationship of these units is illustrated as such: 0.25 mm is roughly equivalent to 10 mils and 0.01 inch.
The preferred embodiment of the present invention is a glass laminate in which two bonded PET sheets are placed between two standard sheets of PVB. It has been found that increasing the thickness of the PET layer greatly enhances the stiffness of the glass laminate, thus improving the strength of the glass laminate after breaking. Optical quality of PET film, particularly visual haze, though, is a problem above thicknesses of about 7 mil, whereas PET of about 7 mil or less exhibits acceptable haze of 1% or less. For example, typical haze levels for 10 mil Mylar film produced by E. I. du Pont de Nemours is 2% and above that thickness the haze levels are much higher. The high haze of the PET layers greater than about 7 mil prohibits the use of these thick layers in combination with PVB for use as automotive glazings, where optical clarity is essential.
A key advantage of the present invention involves the inherent nature of layers of PET over 10 mils having unacceptable haze characteristics. It has been found that using two layers of PET with acceptable haze characteristics (less than about 1% haze) in combination have the desired and required optical clarity for use in glass laminates. Thus, for instance, providing two 7 mil layers of PET for a total PET layer thickness of 14 mil gives the laminate improved stiffness qualities, without significantly diminishing the optical clarity of the laminate. Providing two layers of PET with acceptable optical clarity between two layers of PVB provides many advantages, including a laminate with increased stiffness (when compared to a conventional laminate) without sacrificing the optical clarity of the laminate.
In the preferred embodiment, two optical grade PET sheets are used in place of a thicker PET sheet. The most preferred embodiment employs two PET sheets of about 7-mil thickness each bonded together between two PVB layers. The application of the PET in this manner significantly reduces the haze problem otherwise associated when a single, 14 mil thick PET layer is used.
Preferably, the two PET layers are bonded with an adhesive. A layer of plasticized PVB, polyurethane 2-component reactive cure, pressure sensitive acrylic resins, and other known adhesives can all be used so long as the chosen adhesive does not substantially increase the haze of glass laminate of this invention. The chosen adhesive used with the composite interlayer of the present invention preferably should not substantially differ in sound attenuating performance from that of a laminated glass made with standard PVB. Laminated glass having high sound attenuation will most likely have decreased intrusion resistance, which is not a desired property of the present invention.
One preferred adhesive is a pressure sensitive acrylic resin. This class of adhesives exhibits the preferred clarity that is a desired component of this invention. The acrylic resin preferably comprises polyacrylate esters of varying molecular weights and ester functionalities. The acrylic resin may also contain functional monomers which are used to provide a means for crosslinking of polymer molecules to increase the creep resistance or shear strength. The most preferred adhesive is commercially available from Solutia, Inc. as Gelva multi-polymer solution adhesive (GMS 263).
PVB resin is produced by known aqueous or solvent acetalization processes reacting PVOH with butyraldehyde in the presence of acid catalyst, followed by neutralization of the catalyst, separation, stabilization and drying of the resin. It is commercially available from Solutia, Inc. as Butvar® resin. PVB resin typically has a weight average molecular weight greater than 70,000, preferably about 100,000 to 250,000, as measured by size exclusion chromatography using low angle laser light scattering. On a weight basis PVB typically comprises less than 22%, preferably about 17 to 19% hydroxyl groups calculated as polyvinyl alcohol (PVOH); up to 10%, preferably 0 to 3% residual ester groups, calculated as polyvinyl ester, e.g. acetate, with the balance being acetal, preferably butyraldehyde acetal, but optionally including a minor amount of acetal groups other than butyral, for example 2-ethyl hexanal as disclosed in U.S. Pat. No. 5,137,954.
The PVB resin of the sheet is typically plasticized with about 20 to 80 and more commonly 25 to 45 parts plasticizer per hundred parts of resin. Plasticizers commonly employed are esters of a polybasic acid or a polyhydric alcohol. Suitable plasticizers are triethylene glycol di-(2-ethylbutyrate), triethyleneglycol di-(2-ethylhexanoate), tetraethyleneglycol diheptanoate, dihexyl adipate, dioctyl adipate, mixtures of heptyl and nonyl adipates, dibutyl sebacate, polymeric plasticizers such as the oil-modified sebacic alkyds, and mixtures of phosphates and adipates such as disclosed in U.S. Pat. No. 3,841,890 and adipates and alkyl benzyl phthalates as disclosed in U.S. Pat. No. 4,144,217. Also mixed adipates made from C4 to C9 alkyl alcohols and cyclo C4 to C10 alcohols as disclosed in U.S. Pat. No. 5,013,779. C6 to C8 adipate esters such as hexyl adipate are preferred plasticizers. A more preferred plasticizer is triethylene glycol di-(2-ethylhexanoate).
In an alternative embodiment, the amount of plasticizer used is a convenient means to modifying and controlling the stiffness of the PVB. A useful surrogate property for stiffness is Tg which is directly related to the level of plasticizer. The plasticized PVB sheet used in the laminates of the preferred embodiment, though, have normal Tg values of about 33° C. or less. Some embodiments of this invention can have a Tg of at least 35° C. or higher by modifying the plasticizer content of the PVB.
As disclosed in U.S. Pat. No. 5,618,863 it is also often useful or desirable to incorporate a UV absorber in PVB. In addition to plasticizer, optional UV absorber and adhesion control agent, PVB sheet may contain other performance-enhancing additives such as pigments or dyes for coloring all or part of the sheet, antioxidants and the like. PVB sheet is prepared by mixing combined plasticizer and other additives (e.g. UV-absorber, adhesion control agent and the like) with PVB resin and forcing the mixture under pressure through a die opening to form a sheet. Thickness of the PVB sheet can range from about 0.1 to 2 mm (about 4-80 mil) thick, typically about 0.375 to about 1.5 mm (about 15-60 mils) thick to provide the desired performance in the glass laminate.
The PET sheets for use in the composite interlayer of this invention are preferably biaxially stretched to improve strength and been heat stabilized to provide low shrinkage characteristics when subjected to elevated temperatures (i.e. less than 2% shrinkage in both directions after 30 min. exposure at 150° C.). The tensile modulus (at 21-25° C.) of polyethylene terephthalate is about 1010 Pa as compared with about 107 Pa for a plasticized polyvinyl butyral of the type used in safety glazings. To facilitate bonding of PVB to PET, any known coating and/or surface treatment may be applied to the PET. Preferably, plasma treatment of biaxially stretched polyester to improve adhesion as disclosed in European Patent 157030 B1, and U.S. Pat. No. 4,732,814, incorporated herein by reference, is used. Another method for treating the surface of PET film includes deposition of a thin layer of carbon by vacuum sputtering as disclosed by Kittler in U.S. Pat. No. 4,865,711 (incorporated herein by reference).
Composite interlayers according to this invention are prepared by known procedures. See for instance, U.S. Pat. Nos. 4,973,511, 5,024,895 and 5,091,258 (incorporated herein by reference) for methods for laminating plasticized PVB to surface-treated PET sheet. Because final bonding conditions will be experienced when the interlayer is bonded to glass, the degree of bonding of PVB to PET in the composite interlayer is not critical. If a layer of plasticized PVB in the composite interlayers of this invention has a Tg higher than convention, a person of ordinary skill in the art will appreciate a need to raise the processing temperature of the PVB sheet by an amount corresponding to the elevation in Tg. Suitable temperatures for bonding PVB to PET will typically be in the range of 50 to 120° C. The PET/PVB composites can be pressed, e.g. in a nip roller, to improve adhesion.
Glass laminates using sheets of the invention are prepared by known procedures, e.g. as disclosed in U.S. Pat. Nos. 5,024,895; 5,091,258; 5,145,744; 5,189,551; 5,264,058 and 5,529,654 (all of which are incorporated herein by reference). In one conventional method, the composite interlayer is placed between two sheets of glass and heated under vacuum to a temperature in the range of about 85 to 120° C. for about 10 to 30 minutes, depending on temperature, to remove air from between the layers of the laminate and to seal the laminate edge prior to autoclave lamination. After de-airing and edge sealing are completed, the laminate is preferably heated in an autoclave at elevated temperature (about 90 to 165° C.) and pressure (about 1000 to 2000 kPa) for sufficient time to firmly bond the layers of the laminate. Non-autoclave methods as disclosed in U.S. Pat. No. 5,536,347 may are also useful, e.g. for plasticized PVB having only moderate increases in Tg.
A typical glass laminate of the current invention contains glass sheets that are usually float glass, heat strengthened glass or tempered glass typically in the range of 1 to 10 millimeters (mm) thick, preferably in the range of 1-5 mm thick. Between and bonded to the glass sheets is a composite interlayer comprising two PET sheets between layers of plasticized PVB adhesive. Each PVB sheet is preferably about 0.76 mm thick, and each PET sheet is biaxially-oriented and can be in the range of about 0.025 to 0.25 mm thick (1 to 10 mils), preferably about 0.175 mm thick (7 mils) thick, for a total preferred PET thickness of 0.350 mm (14 mils). The optical clarity of each PET layer must exhibit acceptable optical clarity characteristics, preferably below 1% haze. The PET layers are bonded together, preferably with a pressure sensitive acrylic resin.
For desired adhesion to the PVB, the surface of the PET is preferably modified, such as by roughening of the surface or by chemical modification of the material at the surface. Such modification can be effected by flame treatment, chemical oxidation, corona discharge, carbon sputtering, plasma treatment in vacuum or in air or other treatments well known to those of ordinary skill in the art.
The PVB is plasticized, most preferably with triethylene glycol di-(2-ethylhexanoate). The plasticizer can be added, alternatively, in an amount to provide a Tg of at least 35° C. In the laminates of this invention, the PVB layers can be similar, e.g. both PVB layers having a Tg of at least 35° C. or, having a conventional Tg value, with the preferred embodiment encompassing similar layers of conventional PVB. Alternatively, the two layers can be different, e.g. one plasticized PVB layer having a conventional Tg of about 30 to 33° C. and the other PVB layer being stiffened by having a Tg of at least 35° C.
A further embodiment of the present invention is a glass laminate containing an additional functional layer. This layer can be, e.g., a sound attenuating elastomer layer as disclosed in U.S. Pat. No. 5,796,055 (incorporated herein by reference) or a radiation blocking layer, e.g. comprising a composite of one or more layers of metal or metal oxide deposited onto either PET layer as disclosed in any of U.S. Pat. Nos. 5,024,895, 5,091,258 or 5,932,329 (the disclosures of all four patents being incorporated herein by reference). If a sound attenuating layer is used, it should be added to the glass laminate of the present invention by replacing one of the two PVB layers.
An alternative embodiment to the present invention is the use of multiple layers of optical grade PET, limited only by the haze characteristics of the final glass laminate. For instance, using two 5-mil sheets and one 2-mil sheet of optical grade PET could be used to improve the stiffness of the glass laminate without reducing its optical clarity. It is within the scope of this invention to use multiple thinner, optical grade PET sheets in place of an equivalent thicker sheet of PET, the total number of sheets used being limited only to having an acceptable optical clarity.