CA1052682A - Polyvinyl butyral composite interlayer for laminated safety glass and process for the preparation thereof - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

The present invention relates to a process for the preparation of a polyvinyl butyral composite interlayer for laminated safety glass. A color gradient is printed onto a polyvinyl butyral sheet and the printed surface is then combined to a second sheet of polyvinyl butyral.
The composite sheet does not require any dusting to prevent ink strike off and the problem of undesirable ink transfer in rolled sheet material is minimized.

Description

_ This application is a Divisional of Canadian Patent - Application 198,265 filed April 26, 1974.
The present invention relates to a polyvinyl butyral composite interlayer for laminated safety glass and a process -for preparation thereof. More specifically, the present inven-tion relates to a composite interlayer for laminated safety glass which contains a color gradient. ;
Polyvinyl butyral sheet material is well known as an interlayer for the laminated safety glass used in vehicles, -~
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especially windshields, and in architectural applications. In many applications the interlayer is tinted with an ink so as to provide a colored laminate. One of the major uses for tinted laminates is in automobil~ windshields. In these applications the interlayer is tinted with a color gradient which is positi-, . ...
oned so as to form a glare-reducing color gradient band at the top of the laminated windshield.
The gradient printing operation for the interlayers, used in windshields, usually involves printing ink in the form of dots on the surface of the polyvinyl butyral sheet. The sheet is then dusted with a material such as sodium bicarbonate :.::
to minimize undesirable ink transfer. Prior to use as an inter-,"3 layer, the sodium bicarbonate dust is washed off the sheet which is then dried and laminated to the glass. Printing the inter-layer material gives rise to the need to dust and then wash the i: ~
1 sheet. Moreover, the adhesion of the prin-ted portion of the '~J sheet to glass may be adversely affected by the printing step.
:., , A definite need exists in the art for a polyvinyl butyral interlayer material having a color gradient with desired adhesion to glass and which does not require dusting and washing i 30 prior to use.
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105'~2 SUMMARY OF THE INVENTION
- The present invention solves the aforementioned proble~ls heretofore known in the prior art by providing a process; for preparing a composite polyvinyl butyral inter-layer which does not require dusting and washing prior to use.
The interlayer prepared by this process comprises combining at least two sheets of polyvinyl butyral wherein at least a portion of at-least one of the sheets of polyvinyl ~
`~ butyral is tinted with an ink applied to the side of the `
:10 sheet which is in face-to-face contact with the other sheet -, . . -, . .:
of polyvinyl butyral.
In accordance with a preferred embodiment of the present invention, there is provided a polyvinyl butyral interlayer for laminated safety glass wherein the polyvinyl Y, butyral interlayer comprises at least two sheets of poly-vinyl butyral combined together wherein at least a portion of one of the sheets of polyvinyl butyral is printed with an ink wherein the printed side of the sheet is in face-to-face contact with another sheet of polyvinyl butyral.
In accordance with a further preferred embodiment :
of the present invention, there is provided a polyvinyl butyral interlayer for laminated safety glass wherein the polyvinyl butyral interlayer comprises two sheets of polyvinyl butyral combined together wherein at least a portion of one of the sheets of polyvinyl butyral is printed with a gradient ~' color band wherein the printed side of the sheet is in face-to-- face contact with the other sheet of polyvinyl butyral.
~; In accordance with a still further preferred embodiment of the present invention, there is provided 30 a laminated safety glass comprising a polyvinyl butyral inter-layer interposed between two sheets of glass wherein the , ~ - 3 -105'~682 `
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polyvinyl butyral interlayer comprises at least two sheets of polyvinyl butyral combined together wherein at least a portion of one of the sheets of polyvinyl butyral is printed ,l with an ink wherein the printed side of the sheet is in face-to-face contact with another sheet of polyvinyl butyral.
Y In accordance with a still further preferred ` embodiment of the present invention, there is provided a process for preparing a composite gradient polyvinyl butyral interlayer for laminated safety glass which comprises extrud-ing a layer of molten polyvinyl butyral onto the surface of a moving web of polyvinyl butyral which has been previously printed with a gradient color band wherein the molten polyvinyl butyral is extruded on the printed surface of the sheet.
. 'i '' ,J DESCRIPTION OF THE DRAWINGS
Figures I to IV inclusive, illustrate cross sections of the composite interlayers of the present invention. -' Figure I illustrates a sheet of polyvinyl butyral 11 printed with an ink 12 wherein the printed side is in face-to-face contact with a second sheet of polyvinyl butyral 13.
Figure II illustrates two sheets of polyvinyl butyral 11, each of which is printed with an ink 12, wherein ~ the printed sides are combined.
.~..'1 ; Figure III illustrates a sheet of polyvinyl butyral 11, printed on both sides with an ink 12, wherein the printed sides are combined to two other sheets of polyvinyl butyral 13.
Figure IV illustrates a colored wedge shaped sheet of polyvinyl butyral 1~, which is sandwiched between two ` sheets of polyvinyl butyral 13.

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Other variations on the above composite configura-tions will become apparent to those skilled in the art upon reading the present specification.
Figure V is a schematic diagram illustrating the extrusion of molten polyvinyl butyral onto the surface of a sheet of polyvinyl butyral which has been printed with a -gradient.
,Figure VI is a schematic diagram illustrating :.
-,the combining of two sheets of polyvinyl butyral, one of which has been printed with a gradient wherein the gradient is at the interface of the resulting composite.

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C-~6-12-0364 ; Figure VII is a schematic diagram illustrating another more de-taLled process for combining two sheets of polyvinyl butyral, one of which has been printed with a gradient wherein the gradient is at the interface of the resulting composite.
; 5 D~SCRIPTION OF T~E PREFERRED ~MBODDMENTS
In general the polyvinyl 6utyral resins employed in the present invention have Staudinger molecular weights ranging from about 50,000 to 600,000 and preferably from 150,0aO to 270,000 and may be considered to be made up, on a weight basis, of from 5 to 30Z hydroxyl groups, calculated as polyvinyl alcohol, O to 10% ester groups, calculated as polyvinyl ester, and the balance substantially butyraldehyde. The polyvinyl butyral resin will ;1 preferably contain, on a weight basis, from 9 to 25% hydroxyl groups, cal-Ji culated as polyvinyl alcohol and from O to 3X acetate groups, calculated as polyvinyl acetate, the balance being substantially butyraldehyde.
It is conventional to ad~ust the alkalinity titer of the polyvinyl i butyral resin so as to optimize the impact strength of the resulting laminate.
Methods for adjusting the alkalinity titer are discussed in U.S.P. 3,262,835, 3,294,490, 3,396,074, 3,271,235 and 3,231,461 as well as in other patents.
The polyvinyl butyral resin is plasticized with from about 20 to 80 parts plasticizer per 100 parts resin and more commonly betweeD 30 and 50 :
parts for normal windshield use. This latter concentration is generally used with polyvinyl butyrals cootaining 18 to 23% vinyl alcohol by weight. In general, the plasticizers which are commonly employed are esters of a poly-basic acid or a polyhydric alcohol. Particularly suitable are triethylene glycol di~2-ethylbutyrate), dibutyl sebacate, and dl(betabutoxyethyl) adipate.
The resulting plasticized resin miyture is then generally extruded in the form of sheets which are then printed as described below.
The thickness of the individual polyvinyl butyral sheets used to prepare the composites of the present invention is in the range of from 5 to 1~:)5;Z68Z

1.'i24 mm wlth a ran~e Or from 0.127 to 0.762 mm bein~ Drer~rr~A
for use in auto-mobLle windshields. The choice of thicknes~ of the individual sheets will -ultLmately depend on the thickness desired in the composlte sheet and those skilled in the art can readily select sheets of appropriate thickness for any given application.
ID one embodiment of th~ present invention the surface of the sheet to be printed, which is the surface which will ultimately be in laminated con-tact with another sheet of polyvinyl butyral, is relatively s~ooth so as to provide a better surface for printing. On the other hand, the surface of the polyvinyl butyral sheet which will ultimately be in laminated contact with the glass member of the laminated safety glass will preferably have a con-ventionally rough surface so as to prevent blocking of the rolled sheet, and to facilitate deairing of the laminate. The above mentioned surfaces are well known in the art and need no further discussion here.
The processes for tinting automobile windshield interlayer material are usually designed to give a color gradient. In the gradient printing process the ink is printed in a pattern such that there is a dark section at the top of the interlayer which gradually and uniformly fades off into a clear section at the bottom of the interlayer. In the resulting laminated wind-shield the dark portion of the gradient is at the top of the windshield. In ~-- automobile windshields the darkest portion of the color gradient usually has ;~' an optical density in the range of from 1.70 to 0.70 which corresponds to a percent light transmission of from 2 to 20Z, respectively. Preferably, the light transmission in the darkest portion of the gradient is in the range of ` 25 from 4 to 10%.
The present invention is also applicable to those polyvinyl butyral printing operations which do not require a color gradient. These usually involve non-windshield application wherein the sheet is uniformly tinted.

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, ~C~SZ6~2 Referring to the dra~ingæ, Figure y illustrates ~ne embodiment of the present invention wherein ~olten polyvinyl butyral is extruded onto the printed surface of t~e polyvinyl butyral sheet. In Figure V polyvinyl butyral sheet material 50 is passed over an idler roll 51 through a printing station 52 wherein one side of the sheet is printed. The printed sheet i9 then passed over a series of drive rolls 53 and idler rolls 51 to a nip formed by the outlet of the extruder 56 and a die roll 57 where molten polyvinyl butyral is extruded onto the printed surface of the polyvinyl sheet. The resulting composite is passed over the die roll 57, past a stripper roll 54 and a take-10 off roll 55 and an idler roll 51 to a winder 58. The resulting gradient , composite 59 is then transferred to storage or shipping.
-~, Referring again to Figure V, polyvi~yl butyral sheet material may optionally be fed to the printing station directly from an extruder or other sheet making apparatus thereby eliminating the necessity of winding and unwinding the sheet material prior to feeding it to the printing station.
Figure VI illustrates a sheet combining operation wherein two 15 mil sheets of polyvinyl butyral are combined to form a 30 mil composite having a printed gradient at the interface of the composite sheet. The first ' sheet 60 is clear material having conventional surfaces on both sides while the second sheet 61 is printed on one side with a gradient which is designed ~, to appear as the tinted color band at the top of a windshield in the resulting `~ laminate. Preferably, the printed side is relatively smooth while the opposite side of the sheet has a conventional rough surface.
The clear sheet 60 enters the top section of the com~ining unit through the upper tension rolls 62. These tension rolls serve two pusposes:
; (1) they act as a positive, constant speed driving section; and (2) they are driven a~ a slower line speed than the com~ining rolls 65 to impart a desired amount of sheet tension and drawdown in the clear sheet 60 to prevent cross machine direction wrinkles and to control curl. This sheet tension is re-, .

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quired to eliminate wrinkles in the combined sheet due to sheet growth during heating and to match the amount of drawdown in the upper clear sheet 60 to the drawdown in the lower printed sheet 61. The drawdown is usually determined by measuring the width change of the sheet. If the drawdown in the upper sheet does not match that of the lower sheet, the final product will have excessive curl. The upper tension rolls are run 1% to 30~ slower than the combining rolls 65 with 4% to 16% slower being typical values.
0 The clear sheet 60 then passes under a bank of electric infra-red pre-heaters which adjust the temperature of the incoming sheet to a temperature in the range of from 21 to 82C. The preferred temperature is 43 to 71C., as mea-sured with an infra-red pyrometer just after the bowed flexible rolls 63. After being pre-heated, the sheet passes over a flexible bowed roll 63 (Mt. Hope Vari-Bow Roll) which serves to spread the sheet in a cross-machine direction, ,.~ I
i eliminating any wrinkles which may have formed due to sheet growth during heating.
0 The clear sheet 60 is then carried to the upper lay-on roll 64 which transfers the sheet to the upper combining rolls 65. The lay-on roll 64 is operated at anywhere from zero ~ nip pressure (nip open) to a nip pressure of 2.95 Kg/cm depend-- ing on the particular sheet used as well as other operating conditions. The clear sheet 60 wraps the upper combining roll 65 from 15 to 154C until it reaches the combining nip 69.
The combining rolls are heated at from 32C to 82C with 43 to 71C being preferred. The combining nip pressure runs from 1.48 Kg/cm to 8.85 Kg/cm with 2.95 to 5.90 Kg/cm being preferred. The upper combining roll 65 surface is slightly less tacky than the bottom combining roll 65 surface ~,, ~05'~6~Z ~ .
to facilitate sheet transfer to the bottom roll.
Polyvinyl butyral sheet printed with a gradient sheet 61 enters the unit through the bottom relaxing nip .
66, generally at a temperature of from 21 to 49C with a drawdown of from 10% to 35%. The sheet is allowed to , ~

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105'~682 C-46-12~Z364 ..
relax between the rel~ing nip 66 and the lower tensiQn ni~ 62 by running the lower tension nip 3% to 20X slower than the rela~ing nip. The printed sheet 61 goes through the lower tension rolls 62, which are run at a line speed equal to or less than the combining roll line speed and preferably at the same line speed as the upper clear sheet 60. The lower tension rolls serve the same purpose as the upper tension rolls, namely they build in machine direction sheet tension to prevent cross-machine direction wrinkles and to control curl. The lower tension roll speed is from 1% to 30% less than the combining roll speed with from 4% to 16% less being preferred.
The gradient printed sheet 61 then passes under electric infra-red preheaters which are set to give temperatures equal to the temperature achieved in the top section. The printed sheet 61 is then transferred to the lower combining roll 65 by the lower lay-on roll 64, operating in a manner similar to the upper lay-on roll. The sheet wraps the lower combining roll 65 from 15 about 15 to 151Pc~ which ls heated to the same temperature as the upper combinin~ roll.
After passing through the combining nip 69, the combined sheet (0.762 . mm j is transferred to a cooling roll 67, which it wraps from 15 to 154C. The cooling roll surface is usually kept below 21C. After leaving the cooling roll 67, the sheet is wound and packaged.
~ Zhile Figure VI refers to the combining of two ~038hnnsheets to form a 0.762m~.composite, it is apparent that other sheet thicknesses can be `~ used. In addition, wide variation in sheet surfaces and sheet composition ., .
is permissible as well as wide variations in the printing of the sheets. In ` 25 regard to the latter, one or both sheets may be printed with any desired patterns, designs or colors.
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One of the major problems in sheet combining is wrinkling of the - sheet in both the machine direction and cross-machine direction. These wrinkles are caused by sheet growth on the combining rolls during heating 105'~82 an the roll and by vapors released from the surfàce of the sheet which are trapped between the sheet and the roll surface 6ehind the combining nip. The wrinkles caused by sheet growth during heating can be controlled by restricting the temperature rise of ~the sheet on the combining roll to less than 17qC by preheating the sheet before it gets to the combining roll or by introducing machine direction and cross~machine direction stresses into the sheet before it reaches the com~ining rolls. ~y preheating within 11C of the sheet combining temperature, the sheet growth on the combining rolls is limited to an amount insuf~icient to cause wrin~les. A combination of the two techniques is preferred.
In addition, the tension, both in the machine and cross-machine directions, the temperature and the line speed of the sheets to be combined should be as evenly matched as pos~ible.
Figure VII illustrates a sheet co~bining process similar to that shown in ~igure VI above, except that several tension, catenary loop and temperature ~ontrols have been added. These process controls are designed to match the tension, temperature and line speed of the sheets to be combined so as to provide optimum properties in the resulting composite interlayer.

:., Clear sheet 80 enters the upper section of the combining unit through a first upper tension nip 82. T~is tension nip controls the sheet tension through the pre-conditioning section comprising pre-conditioning heaters and a dewrinkling section where the sheet is dewrinkled using con-:~, ventional means. Sheet tension in the pre-conditioning section is controlled at a mi~imum leveI by a first upper tension sensor 83.
A first upper te~perature sensor 84 controls the pre-conditioning heaters to obtain a sheet temperature in the range of 32C to 82C with 43C to 71c being preferred.
The clear sheet then passes through a second upper tension nip 85 into a relaxing section where the sheet is allowed to droop in a controlled i 30 catenary loop between the second upper tension nip 85 and the third upper tension nip 88. Tension nips 85 and 88 control the tenslon and catenary loop 9 105'~;8Z .

C~6-12-0364 in the relaxing section. The speed of these tension rolls, which form tension nips 85 snd 88, is controlled by the catenary loop sensor 86. The catenary loop is maintained in the relaYing section to relax out as much inherent sheet stress as possible so that the clear sheet, which enters this section with a shrink level of from 4~ to 24Z, leaves with a shrink level of less than 4Z. Also included in the relaxing section is a second upper temperature sensor 87.
The third upper tension nip 88 controls the tension through the sheet stressing section to achieve drawdown levels in the range of from lX
to 30% with 4% to 16% being preferred. This nip is controlled by a second upper tension sensor 89.
After the relaxing sec~ion the sheet passes through a heating ; section where the sheet temperature is again ad~usted in the range of from 32C to 82C, wlth 43C to 71C being preferred.

The clear sheet 80 then passes over a lay-on roll 90 onto a temperature control roll 91 in order to ad~ust the sheet temperature to the , .
desired combin~ng temperature. T~e temperature of the sheet iust before ~ combining is measured and controlled by a third temperature sensor 93, ':
- ~` which is located iust before the combining nip 94. After leaving the temperature control roll91, the clear sheet 80 passes over an upper flexible ::: .
bowed roll 92 for dewrinkling and then passes into the combining nip 94.

Meanwhile, polyvinyl butyral sheet which has been printed with a ~-.,: ::
~j gradient 81 enters the lower section of the sheet combining unit which is , . .
essentially the same as that described above.

- 25 The printed sheet 81 enters the lower section of the combining unit through a first lower tension nip 95 and then through a pre-conditioning ~: .
" section which comprises pre-conditioning heaters and a dewrinkling unit.
This section may not be needed in an inline operation where the sheet is con-ditioned to a certain temperature and dewrinkled ~ust prior to printing and ~~ .

': ' 1~5'~;82 C-06-12-03~4 then fed directly into the combining unit. Also located in the pre-conditioning unit is a first lower tension sensor 96 and a first lower temperature sensor 97. The printed sheet 81 then passes into a relaxing sectioD formed by a second lower tension nip 98 and a third lower tension nip 101. In the relaxing section the sheet is allowed to droop in a controlled catenary loop where a lower catenary loop sensor 99 and a ~econd lower tem-perature sensor 100 measure the droop and the temperature of the sheet, respectively.
Upon leaving the third lower tension nip 101, the sheet passes into a preheating section similar to that discussed above. The sheet which is preheated as in the upper section, passes over a lay-on roll 103 onto a lower temperature control roll 104 and then over a lower flexible bowed roll 105 into the combining nip 94. A third lower temperature sensor 106 is located between the flexible bowed roll 105 and the combining nip 94.
Referring again to Figure VII, a control section 109 is provided to receive tension, temperature and catenary loop data from the sensors in the upper and lower section and to adjust these variables so that the tension, temperature and degree of relaxation in the catenary loop is matched for the upper and lower sections so as to provide optimum properties in the laminate.
In the combining nip 94 the clear sheet 80 is press-tacked to the gradient sheet 81 so as to form a composite where the printed surface is at the interface of the two sheets.
After combining, the composite sheet 108 passes over a number of cooling rolls 107, which reduce the sheet temperature to less than 27C with ;~, 25 a temperature of less than 21C being preferred. The composite sheet is , ':': i then wound and transferred to shipping or storage.
The roll speeds, nip pressures and temperatures used in reference to the process illustrated in Figure VII are the same as those given in re-ference to the process illustrated by Figure VI unless otherwise specified.
The above de~cription of Figure VII refers to an upper and lower ' .

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~,-06-12-0364 ~Jection. However, it is understood that the operation,can be carried out side by side ln which case the upper and lower sections would become the left and right sections or first and second sections of the operation.
It is also possible to combine two printed sheets using the above processes.
In the case of a three ply composite the middle layer can be printed on both sides and then laminated between two clear sheets, using modifications of the processes outlined above. Other such modifications will become apparent to those skilled in the art upon reading the present specification.
; 10 The heating steps referred to throughout the specification may be carried out by those means that are well known in the art. The sheet may be . . .
heated by any of those various means which include electrical heaters, IR
heaters, hot air devices, steam heated devices, hot water heated devices, etc. Conversely, any cooling steps which are needed to adjust sheet tempera-ture would use conventional means such as cool air, chill rolls, etc. Like-- wise, the degree of wrap around any given roll may be varied within broad limits.
The composites of the present invention are press-tacked to provide a bond strength, as measured by a 180 degree pull apart test, of at least 20 0.295 K~/cm. The bond should be sufficient to maintain the unitary nature ,, ~
of the composite during shipping, storage, handling and fabrication into laminated safety glass. During the glass laminating step the heat and 1~~
pressure used to make the laminate also serve to increase the bond strength i of ~he composite interlayer.

~; 25 Preferably, the composite sheet has a curl value of less than 20.
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~;~ The curl value is measured by cutting a sample approximately 508 cm X 2 cm from the machine direction of composite. The test sample is then placed in a circulating air oven maintained at 43C. ~ 1C. for ten minutes. In the oven one-half of the test sample is supported on a flat shelf while the other half hangs down unsupported. The sample is then removed from the oven after . . .
' - 12 -105'~;82 C-~6-12-0364 10 minutes and placed on a ~lat table with the surface of the sheet that was supported in the oven placed face down on thP table. T~e sample is examined and if the ends of the sample curl back from the ta61e surface, the distance from the end of the sample to the point that is still in contact with the table is measured in millimeters. The measurements for each end of the sample are averaged and the percent curl is determined by the following formula:
mm of curled sheet/original length ~millimeters) ~ 100 = X curl A curl value greater than 20Z indicates that there are residual stresses in the sheet which will cause the sheet to curl thereby causing processing difficulties in the glass laminating operations. More preferably, the composite sheet has a curl value of less than 15% and most preferably, less than 10%.
A 30 ml. composite gradient sheet prepared by the process illustrated in ~igure VI, having a bond strength of 02 95 R~/om and a curl value of 3X is laminated between two sheets of glass of the type conventionally used in the :
preparation of automobile windshields. The resulting laminated safety glass ; is evaluated for optical density, sheet beauty and adhesion and compared to glass laminates prepared using a single ply Q762~nPolYvinYl butyral inter-layer prepared by conventional methods. The properties of the glass laminates

2~ prepared using the composite interlayer of ~he present invention are compar-able to the proper~ies of those laminates prepared using a conventional single ; ply polyvinyl butyral interlayer.
While the foregoing description has been directed to polyvinyl butyral interlayers, one skilled in the art will readily recognize that the present invention is readily adaptable to preparing composites of other thermoplastic materials which could be used as interlayers for laminated safety glass. Examples of other suitable interlayer material include polyurethanes, polyamides, poly(ethylene-vinyl acetate), poly(ethylene-vinyl alcohol), etc.

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C--06-12-03~4 From the foregoing descrl~tion of t~e present i~vention~ it will become apparent that many variations and ~odifications are possible without departing frum the spirit and scope thereof.

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Claims (16)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A polyvinyl butyral interlayer for laminated safety glass wherein the polyvinyl butyral interlayer comprises at least two sheets of polyvinyl butyral combined together wherein at least a portion of one of the sheets of polyvinyl butyral is printed with an ink wherein the printed side of the sheet is in face-to-face contact with another sheet of polyvinyl butyral.

2. An interlayer as in Claim 1 wherein one of the sheets of polyvinyl butyral is printed with a gradient color band.

3. An interlayer as in Claim 1 wherein the total thickness of the combined interlayer is in the range of from 0.0051 to 0.191 cm.

4. An interlayer as in Claim 1 wherein two sheets of polyvinyl butyral are combined.

5. An interlayer as in Claim 3 wherein three sheets of polyvinyl butyral are combined.

6. An interlayer as in Claim 5 wherein the middle sheet of poly-vinyl butyral is printed on both sides.

7. A polyvinyl butyral interlayer for laminated safety glass wherein the polyvinyl butyral interlayer comprises two sheets of polyvinyl butyral combined together wherein at least a portion of one of the sheets of polyvinyl butyral is printed with a gradient color band wherein the printed side of the sheet is in face-to-face contact with the other sheet of polyvinyl butyral.

8. A laminated safety glass comprising a polyvinyl butyral interlayer interposed between two sheets of glass wherein the polyvinyl butyral interlayer comprises at least two sheets of polyvinyl butyral combined together wherein at least a portion of one of the sheets of polyvinyl butyral is printed with an ink wherein the printed side of the sheet is in face-to-face contact with another sheet of polyvinyl butyral.

9. Laminated safety glass as in Claim 8 wherein one of the sheets of polyvinyl butyral is printed with a gradient color band.

10. Laminated safety glass as in Claim 8 wherein the total thickness of the combined interlayer is in the range of from 20 to 75 mils.

11. Laminated safety glass as in Claim 8 wherein two sheets of poly-vinyl butyral are combined.

12. Laminated safety glass as in Claim 8 wherein three sheets of polyvinyl butyral are combined.

13. Laminated safety glass as in Claim 12 wherein the middle sheet of polyvinyl butyral is printed on both sides.

14. Laminated safety glass as in Claim 8 wherein the polyvinyl butyral interlayer comprises two sheets of polyvinyl butyral combined together wherein at least a portion of one of the sheets of polyvinyl butyral is printed with a gradient color band wherein the printed side of the sheet is in face-to-face contact with the other sheet of polyvinyl butyral.

15. A process for preparing a composite gradient polyvinyl butyral interlayer for laminated safety glass which comprises extruding a layer of molten polyvinyl butyral onto the surface of a moving web of polyvinyl butyral which has been previously printed with a gradient color band wherein the molten polyvinyl butyral is extruded on the printed sur-face of the sheet.

16. A composite interlayer as in claim 1 wherein the plasticized polyvinyl butyral sheets contain from 20 to 80 parts of plasticizer per hundred parts of polyvinyl butyral.