Method of treating interlayer material and laminated windows comprising interlayer material so treated

This invention relates to making continuous ribbons of clear, flexible interlayer material into a maximum number of interlayers having a coating of graded intensity along a longitudinal edge portion thereof for use in curved laminated safety glass windshields. The method avoids rubbing or differentially stretching the flexible interlayer material during processing, which includes cutting a continuous ribbon of clear, flexible interlayer material into successive interfitting flexible sheets of trapezoidal shape, applying a shade band, preferably by electrostatic spraying, to a predetermined portion of each sheet and laminating the sheet so treated to one or more rigid transparent sheets of glass or a recognized plastic substitute for glass. When the flexible sheets so treated are laminated to one or more relatively rigid transparent bent sheets of glass or rigid transparent plastic, the resulting laminates are safer than those produced by the prior art method that requires differentially stretching the partially shaded interlayer and have better optical properties than prior art shaded laminates comprising interlayers produced by pervious prior art techniques that incorporated rubbing the sheet while applying a coating.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to curved laminated transparencies comprising a 
flexible interlayer having a shade band of uniform tone but a pattern of 
graded intensity laminated to one or more rigid, transparent sheets of 
glass or rigid transparent plastic, and to preparing a thermoplastic 
interlayer for a curved laminated glass assembly having a colored, 
light-absorbing band in a marginal area of its thermoplastic interlayer. 
The present invention more especially relates to a method for the 
preparation of individual interlayers from a continuous ribbon of clear 
flexible interlayer material and to the use of such interlayers in 
laminated windshields having more acceptable optical properties and/or 
improved resistance to impact than those containing interlayers produced 
by other methods developed in the prior art. 
Windshields of automobiles utilize laminated glass assemblies comprising 
two sheets of glass and a thermoplastic interlayer such as a sheet of 
plasticized polyvinyl butyral resin or polyurethane. In the current models 
of automobiles these glass assemblies are bent along the longitudinal 
axis. In the manufacture of these curved laminated glass assemblies, a 
thermoplastic interlayer is placed between two matched curved glass sheets 
and the resulting sandwich is subjected to a laminating process to form a 
transparent laminated glass assembly. The thermoplastic interlayer is 
provided with a marginal rectilinear or straight band of a light-absorbing 
material in order to reduce glare from the sun when the laminated glass 
assembly is mounted as a windshield in an automobile. The marginal band is 
preferably graduated in concentration of light-absorbing material with the 
greatest concentration of light-absorbing dye being present nearest the 
upper peripheral portion of the thermoplastic interlayer and the dye 
concentration in the band diminishes gradually toward the lower edge of 
the band until the amount at the lower edge is almost imperceptible. When 
such a graduated dyed band in the plastic interlayer is laminated between 
a pair of curved glass sheets and the resulting laminated windshield is 
mounted in a tilting or non-vertical fashion, the cut-off point between 
the dyed band and the undyed portion appears to be curved to a person 
inside the car. In order to provide an apparent horizontal cut-off line 
between the dyed band and the non-dyed portion, it is necessary to distort 
the flat thermoplastic interlayer sheet to curve the boundary between the 
dyed and the undyed portion in such a manner as to compensate for the 
curvature that results from differential distortion of the interlayer 
sheet. 
Flat thermoplastic interlayer sheets having a curved colored band suitable 
for use as interlayers in curved laminated glass assemblies have been 
prepared from a continuous thermoplastic sheet having a straight colored 
band by differential stretching of the sheet until the colored band is 
curved and then heating the sheet while in the stretched condition in 
order to relieve stresses. The sheets are then cooled to room temperature 
while still in a stretched condition. Such a method is limited with 
respect to the amount of stretch that can be used as well as with respect 
to the degree and type of curvature obtainable. 
Thermoplastic sheets stretched differentially by the method described have 
a non-uniform thickness even when cut from a continuous ribbon of 
interlayer material of uniform thickness. Furthermore, the heating of the 
differentially stretched sheet does not eliminate the stresses entirely. 
The combination of the residual differential stresses and the difference in 
thickness resulting from the differential stretching of the thermoplastic 
sheets (which usually vary about 5 mils (0.13 mm) in interlayer sheets of 
30 mils (0.762 mm) nominal thickness) makes it difficult to obtain a 
uniform bonding between the thermoplastic interlayer sheets and a glass 
sheet or a pair of glass sheets to produce a laminated safety glass 
windshield. Laminating assemblies having interlayers of non-uniform 
thickness in an oil autoclave may result in some oil penetration at the 
interface between the thinner portion of the interlayer and the glass or 
other rigid transparent sheet. Furthermore, differential stretching causes 
certain uneveness in the gradation or texture of the band of shading that 
is applied to the interlayer material. The art of shaded laminated 
automobile windshields required some improvement to provide tinted 
windshields that were not susceptible to delamination and had better 
optical properties than those available from the prior art. 
2. Description of the Prior Art 
U.S. Pat. No. 3,455,197 to Richardson and U.S. Pat. No. 3,467,332 to 
Bachman disclose different types of apparatus for cutting a continuous 
sheet or ribbon of clear plastic interlayer material into interfitting 
trapezoidal shapes. Such apparatus are designed for ultimate use of the 
clear plastic in windshields that do not have dyed portions. 
U.S. Pat. No. 3,113,034 to Fix discloses an axially displaceable printing 
roller whose axial reciprocating movement is correlated with the movement 
of a continuous ribbon of plastic interlayer material to provide a graded 
pattern of dye that is repeated along the length of the continuous ribbon. 
Such ribbon must necessarily be cut with triangular end portions that do 
not interfit. Furthermore, the axial reciprocation of the printing roller 
forms rub marks that impair the optical properties of the printed portion 
of the interlayer. 
U.S. Pat. No. 3,593,405 to Beckham discloses a typical prior art technique 
involving the application of a shade band of graded intensity onto a 
rectangular sheet of plastic interlayer material. The interlayer material 
containing the shaded portion of graded intensity is differentially 
stretched to provide a curved cut-off line so as to appear substantially 
horizontal when laminated between curved glass sheets and installed in a 
tilted position in a frame of an automobile. 
U.S. Pat. No. 2,933,759 to Startzell discloses an umbrella-type apparatus 
for simultaneously differentially stretching the opposite side edges of a 
plurality of layers of a continuous ribbon of plastic interlayer material 
so that the plurality of layers may be simultaneously differentially 
stretched when the umbrella-type of plastic stretching apparatus is 
opened. Stretch marks usually remain in the interlayer sheeting. When the 
shape requires severe differential distortion, these can extend into the 
shaded region. 
U.S. Pat. No. 3,696,186 to Stark et al discloses apparatus for 
differentially stretching a continuous ribbon of dyed interlayer material 
by passing the latter while heated to deformation temperature over a 
portion of a conical roll and then rapidly chilling the differentially 
stretched plastic interlayer material. Non-uniform stretching results in 
non-uniform thickness. 
The teachings in the patents enumerated and described previously fail to 
provide a method of treating clear interlayer material so that a maximum 
number of interfitting trapezoidal shapes can be provided with a graded 
shade band along a specific longitudinal edge portion thereof for 
lamination with curved glass to result in a laminated glass-plastic 
windshield having a plastic layer of substantially uniform thickness, of 
substantially uniform tension of a low magnitude merely sufficient to 
avoid wrinkling and of substantial uniformity of adhesion to the glass 
throughout the entire extent of the laminated windshield. 
SUMMARY OF THE INVENTION 
According to the present invention, a continuous ribbon of plastic 
interlayer material of substantially uniform overall thickness is cut into 
successive lengths of interfitting trapezoidal shapes. The ribbon is 
preferably embossed to avoid entrapping air in the resulting laminate. The 
individual sheets so formed are oriented into a predetermined orientation. 
This may be accomplished after stacking alternate trapezoidal sheets in 
separate layers and orienting each stack to facilitate removing each sheet 
from its associated stack in its desired predetermined orientation. 
Each sheet has a graded coating applied to a portion thereof while so 
oriented. The sheet is supported in unwrinkled condition with a minimum of 
stress applied thereto during the application of the coating. Each sheet 
of trapezoidal shape of said plastic interlayer material is supported in 
such a manner as to avoid wrinkling and differential stretching during the 
step of applying the coating. A coating of graded intensity is applied to 
a preselected longitudinal portion only of each trapezoidal sheet so that 
the resulting coated interlayer sheet has a substantially uniform 
thickness (except for an embossment pattern) and has a substantially 
uniform stress of small magnitude sufficient to avoid wrinkling throughout 
its entire extent. The non-uniform application of the graded coating does 
not appreciably affect the uniform thickness and substantially uniform 
stress of the trapezoidal sheets. The uniform thickness improves the 
ability of the interlayer to be adhered or bonded uniformly to a curved 
glass sheet with which it forms a laminated safety glass windshield with 
minimum chance for oil penetration at the portion of the interface between 
a thinner portion of a differentially stretched interlayer and the 
adjacent glass sheet during fabrication of the laminated windshield and 
minimizes the chance for delamination due to local points of high stress 
which serve as origins of delamination in the laminated windshield. These 
points of high stress may be the result of differential stretching during 
the distortion of the plastic interlayer in order to convert a straight 
line cut-off between the shaded and clear portions of plastic or may be 
due to the inability of the glass to laminate successfully against the 
thinner portions of the plastic that result from the differential 
stretching of the plastic. 
The present invention will be better understood in the light of a 
description of a specific embodiment thereof that follows.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
According to the present invention, a continuous roll of flexible plastic 
interlayer material, such as polyurethane or plasticized polyvinyl 
butyral, is unrolled from a roll 10 to form a continuous ribbon 12. The 
ribbon 12 is cut into interfitting alternate sheets of trapezoidal shape 
14 and 16 while the ribbon 12 is smoothly supported in an unwrinkled 
condition. A typical apparatus for treating a continuous ribbon of clear 
plastic is disclosed in U.S. Pat. No. 3,455,197 to Richardson. It is also 
feasible to wind the continuous ribbon about a special mandrel such as 
disclosed in U.S. Pat. No. 3,467,322 to Bachman. In the latter apparatus, 
a plurality of layers are wound in unwrinkled condition around the 
periphery of the mandrel and each circumference of the ribbon is cut at 
approximately two diamatrically opposite locations to form a pair of 
interfitting trapezoidal sheets from each layer. 
After the sheets 14 and 16 of trapezoidal shape are cut and separated from 
the continuous ribbon 12, the alternate sheets 14 are stacked in one stack 
15 in one common orientation and the alternate sheets 16 are stacked in 
another stack 17 oriented in another common orientation. Then each sheet 
16 in turn is removed from stack 17 and applied in a desired orientation 
while at a uniformly but lightly stressed condition merely sufficient to 
avoid wrinkling on a supporting table, which may be a vacuum supporting 
table (not shown). Each sheet 14 in the stack 15 is oriented to the same 
orientation as stack 17 before each sheet 14 is treated similarly after 
the sheets 16 are depleted from stack 17. FIG. 2 shows how both stacks are 
oriented for sheet removal. 
Each plastic sheet 14 or 16 of trapezoidal shape is brought to a dye 
applying station where a band of dye is applied to an area 20 that is 
exposed to a spray composition while the remaining areas 22 and 24 of the 
trapezoidally shaped sheets are shielded from contact with the spray 
composition. 
The dyed area 20 of the substrate has a convexly curved bottom margin 26 of 
low intensity and an upper concavely curved margin of high intensity. The 
upper concavely shaped high intensity dyed boundary 28 conforms to the 
shape of the upper edge of one or more glass sheets to be laminated to the 
dyed interlayer material to form a bent laminated windshield having a 
graduated shade band that is graduated in intensity transverse to the 
longitudinal dimension of the windshield. The convexly shaped lower border 
26 of the shade band area 20 is barely noticeable as the intensity of the 
dye applied decreases in a direction transvese to the longitudinal 
dimension of each sheet with a maximum intensity at its concave upper 
border and a minimum intensity along its convex lower border. 
The curvature of the lower border 26 is so chosen that when the windshield 
comprising inner and outer glass sheets 30 and 32, respectively, is 
mounted in tilted relation in a windshield frame, as shown in FIG. 5, the 
convexly curved lower border 26 between the shaded area and the unshaded 
area 22 that is curved from the application of the dye appears as a 
horizontal line. 
One of the important features of this invention is that during the handling 
of the plastic continuous ribbon or the individual sheets of trapezoidal 
shape that are cut from the plastic that it is held in smooth, slightly 
stretched relationship where the stretching is uniform throughout the 
entire extent of each plastic sheet that is cut from the continuous 
ribbon. It is also essential that the ribbon itself be maintained in a 
smooth, unwrinkled, slightly but uniformly stretched condition during the 
cutting operation. In this manner, the uniformity of thickness of the 
ribbon as received from the manufacturer of the ribbon (which approximates 
30 mils (0.76 mm).+-.5 mils (0.13 mm) with embossments approximately 1 mil 
(0.025 mm) thick) is not distorted. The adhesion throughout the entire 
extent of the trapezoidal sheets of interlayer material to the glass is 
substantially uniform throughout the entire extent by virtue of the 
relative uniformity of thickness of the interlayer material of the 
laminated assembly. 
During coating, the properly oriented sheet of flexible interlayer 
material, such as polyurethane of plasticized polyvinyl butyral, is 
supported directly on an apertured vacuum platen having a flat upper 
surface and vacuum is applied to hold the flexible sheet in a fixed 
position in an unwrinkled condition on the platen. The dye is applied 
obliquely and the nozzle of the spray gun may have its orifice modified to 
ensure a desired pattern of non-uniform intensity transverse to the 
longitudinal dimension of the substrate. Preferably, an electrostatic 
spray gun is used to provide finely divided particles of dye composition. 
The coating station is usually enclosed and provided with a conventional 
exhaust hood (not shown) to remove any portion of the spray that does not 
adhere to the substrate. If desired, heat lamps may be provided to 
irradiate the coated portion of the substrate to help volatilize the 
volatile component of the dye composition applied. 
It is noted that the boundary between the coated portion and the remainder 
of the flexible plastic sheet that is uncoated is curved. This curvature 
may be obtained by curving the edge of the masking means that is 
interposed between the spray gun and the portion of the substrate desired 
to be free of coating. If desired, a curved cam may be used to guide the 
reciprocating movement of one or more spray guns to provide the curved 
cut-off line between the coated and uncoated portions of the flexible 
interlayer sheet. 
Suitable dye solutions comprise a mixture of organic dye components blended 
to yield a desirable color. A preferred dye mixture is a blend of blue, 
yellow and red-violet dye components. A preferred blue dye component 
comprises an anthraquinone derivative, such as 
1,4-diethylamino-anthraquinone. A preferred yellow dye component is a 
monoazo compound with a molecular formula of C.sub.17 H.sub.16 O.sub.2 
N.sub.4. A preferred red-violet dye component appears by infrared analysis 
to be an anthraquinone derivative comprising an amine functionality. 
However, positive identification of the latter component is not 
obtainable. A preferred blend of the preferred dye component, 37 parts by 
weight of Solvaperm Red-Violet R, available from American Hoescht 
Corporation, 37 parts by weight of Calco Oil Blue N and 26 parts by weight 
of Calco Yellow G Concentrate available from American Cyanamid 
Corporation, yields a relatively colorfast blue-green shade band. This dye 
mixture or blend of dye components is dissolved in a concentration 
preferably about 1 to 2% by weight of dye components in a solvent system, 
which is preferably a combination of tetrahydrofuran and N-lower 
alkyl-pyrrolidone, preferably containing about 75 to 85% by volume of 
tetrahydrofuran and about 25 to 15% by volume of N-methyl-pyrrolidone. 
This solvent system satisfies the requirement of high dye solubility, 
preferably greater than 2%, and proper volatility to assure optical 
uniformity in the shade band. Non-uniformity in the shade band is caused 
by both too little volatility, which results in a mottled texture, and too 
high volatility, which results in undissolved dye particles being 
physically bonded to the surface of the substrate. The solvent system 
suggested is also an acceptable solvent for antioxidants and ultraviolet 
stabilizers which are preferably added to the dye component. 
A typical dye composition comprising 38.5% by weight Solvaperm Red-Violet 
R, 38.5% Calco Oil Blue N and 23.0% Calco Oil Yellow G Concentrate (both 
Calco dyes available from American Cyanamid) is dissolved in a hundred 
milliliters of solvent for each 1.5 grams of the above dye composition. 
The solvent consists of 85% by volume of tetrahydrofuran and 15% of 
N-methyl-pyrrolidone. The solution thus formed is electrostatically 
sprayed on an unmasked portion of a sheet of polyurethane or plasticized 
polyvinyl butyral at an oblique angle to yield an optically uniform 
blue-green shade band in the lengthwise direction of the shade band and a 
graded intensity of shade band from a maximum intensity at one edge of the 
shade band to a relatively low intensity adjacent a convexly curved 
cut-off line with an uncoated portion beneath a mask during the coating. 
Another suitable dye composition comprises 1.2 grams of the previous dye 
composition in 100 milliliters of a solvent system consisting of 80% by 
volume of tetrahydrofuran and 20% by volume of N-methyl-pyrrolidone and 
further comprising in addition to the solvent, 0.12 grams of an 
antioxidant, Irganox 1035 and 2.4 grams to an ultraviolet light 
stabilizer, Tinuvin 770, both available from Ciba-Geigy Corporation. The 
latter solution is also electrostatically sprayed to yield a relatively 
colorfast shade band having the desired gradation in one direction and 
uniform intensity in the other direction. 
After the sheet of interlayer material is provided with the elongated shade 
band area, it is removed from the coating area while avoiding wrinkling 
and differentially stretching said interlayer material during its 
handling, maintained in a room of controlled temperature and humidity, 
preferably about 68.degree. F. (20.degree. C.) and 20% relative humidity 
and assembled as an interlayer between a pair of glass sheets of matching 
curvature with the boundary of the shaded area having relatively high 
intensity aligned with an edge of the curved glass sheet and the cut-off 
line of relatively less intensity aligned with marks previously made in 
the bent glass sheet. Excess plastic is trimmed and cut away from the 
margin of the assembly. The assembly of dyed interlayer material and two 
glass sheets is then prepressed by passing the assembly between a pair of 
prepressing rolls of the type depicted in U.S. Pat. No. 3,351,001 to 
Achkio, while at a temperature of 150.degree. F.-225.degree. F. 
(65.degree. C.-107.degree. C.) to remove air from between the interfaces 
of the assembly and to smooth the embossed pattern and to seal at least 
the peripheral edge portion of the interfaces between the components of 
the assembly. The rolled, prepressed assembly is then laminated in an oil 
autoclave under standard conditions, which involve temperatures of 
225.degree. F. to 300.degree. F. (107.degree. C. to 149.degree. C.) and a 
simultaneous pressure of 175 psi to 225 psi (12.3 to 15.8 Kg/cm.sup.2) for 
15 to 60 minutes depending on the other parameters and the severity of 
shape of the assembly. 
The tinted laminated windshields that result from the aforesaid operation 
have superior optical properties and superior resistance to oil 
penetration during fabrication and to delamination during subsequent use 
than windshields containing partially dyed interlayers that are 
differentially stretched prior to their assembly. The assembly produced 
according to this invention need not be edge rolled according to the 
method taught in U.S. Pat. No. 2,999,779 to Morris, which thickens the 
margin of the interlayer to improve the resistance to oil penetration at 
the interfaces during exposure to a pressurized oil autoclave. When the 
glass-plastic assembly containing an unwrinkled interlayer prepared 
according to the present invention is edge rolled, the marginal portion of 
the interlayer so treated develops a frame of different color intensity 
than the remainder of the shade band. The uniformity of thickness of the 
interlayer sprayed with a dye composition along an edge portion while in 
an unwrinkled condition avoids the need to thicken the peripheral portion 
preferentially. 
If desired, the partially dyed sheet of flexible interlayer material may be 
laminated to a single glass sheet utilizing a parting material and a 
second glass sheet which is separated from the interlayer after lamination 
according to the technique disclosed in U.S. Pat. No. 3,808,077 to Rieser 
and Chabal. The description of the method of forming a bilayer windshield 
disclosed in this latter patent is incorporated herein by reference for 
the details of such an operation. Fabricating a bent bilayer windshield 
having an outer glass sheet with an exposed convex major surface is 
similar to fabricating a bent trilayer windshield with two outer bent 
glass plies except for interposing a layer of parting material between the 
interlayer sheet and the inner convex major surface of the inner glass 
sheet having an exposed concave major surface. The parting material 
facilitates removal of the inner glass sheet after lamination. 
The resulting laminated windshields produced according to the present 
invention are characterized by better adhesion between the shaded plastic 
and the glass. It is believed that this is the result of the more uniform 
tensioning of the interlayer material with a small tension sufficient only 
to prevent wrinkling and of substantially uniform stress throughout the 
entire interlayer. The interlayer sheets, which have a substantially 
uniform thickness in the roll from which they are cut, are not distorted 
so that different portions thereof have different thicknesses, which makes 
the problem of adhering to the other elements of the laminated windshield 
exceedingly difficult. In other words, having a plastic interlayer of more 
nearly uniform thickness throughout provides an improved adhesion 
throughout the entire extent of the laminated windshield that results. 
While the present invention is not so limited, typical thicknesses for the 
outer glass sheet of a bilayer windshield and for the inner and outer 
glass sheets of a trilayer windshield range between 0.040 inch (1 mm) and 
0.250 inch (6 mm) and said layer of flexible plastic interlayer material 
is of substantially uniform thickness between 0.015 inch (0.4 mm) and 
0.060 inch (1.5 mm). 
The following test was performed to evaluate the relative impact resistance 
of laminates containing undistorted polyvinyl butyral interlayers between 
two glass sheets such as result from the present invention compared with 
that of similar laminates containing differentially stressed polyvinyl 
butyral interlayers. A first set of control laminates 12 inches (30 cm) 
square containing 2 sheets of float glass and a layer of plasticized 
polyvinyl butyral taken from a continuous roll having a nominal thickness 
of 30 mils (0.76 mm) and differentially stretched in the manner of 
stretching a continuous roll of dyed polyvinyl butyral required to develop 
a curved cut-off line between dyed and undyed portions of a shaded 
windshield interlayer were fabricated. The fabrication included standard 
laminating technique for sample fabrication involving prepressing followed 
by a final lamination in an oil autoclave at 200 psi and 275.degree. F. 
for 45 minutes to produce 25 control specimens simulating specimens cut 
from the bottom portion of a standard production shaded windshield. 
A set of 17 test specimens 12 inches (30 cm) square were also produced 
using 2 plies of float glass and 30 mils (0.76 mm) nominal thickness 
polyvinyl butyral to simulate the clear portion of a shaded windshield 
incorporating unstretched polyvinyl butyral interlayers using the same 
laminating procedure as for the control specimens. All the glass sheets in 
the specimens had the same nominal thickness. 
The control specimens and the test specimens were subjected to impact tests 
to determine the mean break height from which a steel ball of 5 pounds 
(2.27 kg.) mass would penetrate a specimen. The ANSI Code Z 26.1, 1977, 
American National Standard Safety Code for Safety Glazing Materials for 
Glazing Motor Vehicles Operating on Land Highways, requires that 10 out of 
12 specimens hold a 5 pound steel ball from a drop height of 12 feet. From 
the results obtained in these tests, the 42 specimens tested would be 
expected to pass the ANSI Code requirement. The results of the impact 
tests are as follows. 
______________________________________ 
MEAN HEIGHT PENETRATION TEST RESULTS 
Types of Number of Mean Penetration 
Specimen Specimens Height Sigma 
______________________________________ 
Control 25 18.18 feet 1.74 
Test 17 16.66 feet 1.22 
______________________________________ 
The results of the penetration test conclusively proves the superiority of 
test specimens comprising unstretched interlayers to penetration compared 
to control specimens comprising differentially stretched interlayers. For 
laminated windshields having shade bands, the clear area of the prior art 
windshields that require differential stretching is less resistant to 
penetration on impact than the equivalent area of shaded laminated 
windshields produced with interlayers processed according to the present 
invention. 
The form of the invention shown and described in this disclosure represents 
an illustrative preferred embodiment thereof. It is understood that 
various changes may be made without departing from the gist of the 
invention as defined in the claimed subject matter which follows.