CA1043683A

CA1043683A - Polyvinyl butyral composite interlayer for laminated safety glass and process for the preparation thereof

Info

Publication number: CA1043683A
Application number: CA198,265A
Authority: CA
Inventors: Robert H. Fariss; Andrew D. Skibo
Original assignee: Monsanto Co
Current assignee: Monsanto Co
Priority date: 1973-04-27
Filing date: 1974-04-26
Publication date: 1978-12-05
Also published as: DE2420355A1; JPS5053427A; GB1470844A; JPS5650711B2

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

10~36~3 C-06-12~0364 POLYVINYL BUTYRAL COMPOSITE INT~RLAY~R
FOR LAMINAT~D SAF~TY GLASS
BACKGROUND OF T~E INVERTION
1. Field of the Invention The present invention relates to a polyvinyl butyral composite interlayer for laminated safety glass and a process for preparation ~, thereof. Morespecifically, the present in-vention relates to a composite interlayer for laminated safety glass which contains a color gradient.

2. Description of the Prior Art Polyvinyl butyral sheet material is well known as an interlayer for the laminated safety glass used in vehicles, 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 automobile windshields. In these applications the interlayer i~ tinted with a color gradient which is positioned 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 wind-shields~ 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 interlayer, the sodium bicarbonate dust is washed off the sheet which is then dried and laminated to the glass. Printing the interlayer material gives rise to the need to dust and then wash the sheet. Noreover, the adhesion of the printed portion of the sheet to glass may be adversely effected by the printing step.
A definite need exists in the art for a polyvinyl butyral inter-layer material having a color gradient with desired adhesion to glass and which does not re~uire dusting and washing prior to use.

~L
q~

-: ,- - - : . .

10436~93 SUMMARY OF THE INVE~TION
The present invention solves the aforementioned pro-blems heretofore known in the prior art by providing a process for preparing a composite polyvinyl butyral interlayer 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 sheet which is in face-to-face contact with the other sheet of polyvinyl butyral.
In a preferred embodiment of the present invention there is provided a process for preparing a composite inter-layer for laminated safety glass which comprises:
A. passing a first continuous sheet of thermoplastic material between nip rolls to impart machine direction, sheet tension and drawdown into the sheet;
B. adjusting the temperature of the sheet to a.temperature in the ranqe of from 32 to 82C;
C. removing any wrinkles from the sheet; and then D. combining the first continuous sheet.with a second con-tinuous sheet which is tinted wherein the second sheet has received substantially the same treatment as out-lined in process steps A to C above;
E. cooling the resulting composite.
In a further preferred embodiment of the present invention, there is provided a process for preparing a com-posite interlayer for laminated safety glass which comprises:
A. (1) passing a first continuous sheet of polyvinyl butyral between a first set of nip rolls to impart machine direction, sheet tension and drawdown into the sheet;

-3-r~

10~;~133 (2) adjusting the temperature of the first sheet to a temperature in the range of from 32 to 82C while removing any wrinkles in the sheet;
~3) allowing the first sheet to droop in a catenary loop between a second and third set of nip rolls;

(4) adjusting the temperature of the first sheet to a temperature in the range of from 32 to 82C; and simultaneous with steps 1 to 4 above;
~. (1) passing a second continuous sheet of polyvinyl butyral which is printed on one side with a color gradient, between a set of nip rolls and (2) allowing the second sheet to droop in a catenary loop between the nip rolls;
(3) adjusting the temperature of the second sheet to a temperature in the range of from 32 to 82C; and C. (1) passing the-first and second sheets into a combining nip where they are combined wherein the printed surface of the second sheet of polyvinyl butyral is combined to the first sheet so as to provide a composite wherein the bond strength between the first and second sheet is at least 0.Z95 Kg/cm;
(2) cooling the resulting composite;
wherein the process steps outlined in A and B above are con-trolled to provide substantially the same line speed, sheet tension and temperature in each of the first and second sheets of polyvinyl butyral being fed into the combining nip.
~ESCRIPTION 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.

L ~ ~ - 3a -~0436~33 Figure II illustrates two sheets of polyvinyl butyral 11, each of which is printed with an ink 12, wherein the printed sides are combined.
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 14, which is sandwiched between two sheets of polyvinyl butyral 13.
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.

- 3b -^;~

C-06~12-0364 10~3 Figure VII is a schematic diagram illustrating another more de-tailed process for combining tuo s-heets of polyvinyl buty~al, one of uhich has been printed with a gradient ~herein the gradient i8 at the interface of the resulting composite.
DESCRIPTION OF THE PREPERRED EHRODIMENTS
In general the polyvinyl butyral resins employed in the present invention have Staudinger molecuIar 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 weig~t ~asis, o~ from 5 to 30% 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 preferably contain, on a weight basis, from 9 to 25Z hydroxyl groups, cal-culated as polyvinyl alcohol and from O to 3% acetate groups, calculated as polyvinyl acetate, the ~alance being substantially butyraldehyde.
It is conventional to ad~ust the alkaliniq titer of the polyvinyl butyral resin so as to optimize the impact strength of the resulting laminate.
Methods for adjusting the alkalin$ty titer are d$scussed 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 res$n and more commonly between 30 and 50 parts for normal uindshield use. This latter concentration i8 generally used ~;with polyvinyl butyrals containing 18 to 23% vinyl alcohol by weigh~. 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 di(betabutoxyethyl~ adipate.
The resulting plasticized resin mixture 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 ~nvention is in the range of from 5 to .

" - :

C~06-12-0364 10~83 1.~24 Dun wlth a range Or from 0.127 to 0.762 n~n bel~ ~rer~rr~
for use in auto-mobile windshields. The choice of thickness of the individual sheets will ultimately depend on the thickness desired in the composite sheet and those skilled in the art can readily select sheets o~ appropriate thickness for any given application.
In one embodiment of th~ present invention the surface of the sheet to be printed, which is the surface which will ultlmately be in laminated con-tact with another sheet of polyvinyl butyral, is relatively smooth so as to provide a better surface for printing. On the other hand, the surface of the polyviuyl 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 wintshields 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 20~, respectively. Preferably, the light transmission in the darkest portion of the gradient is in the range of 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.

,

- 5 - ~

~, .

~043683 Referring to the drawings, ~igure y illustrate~ ~ne embod~ment of the present invention wherein molten pol~vinyl butyral is extruded onto the printed surface of the polyvinyl ~utyral 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 is 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 gurface of the polyvinyl sheet. The resulting composite is passed over the die roll 57, past a stripper roll 54 and a take-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, polyvinyl butyral sheet material mayoptionally 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.
~ igure 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 ls relatively smooth while the opposite side of the sheet has a conventional rough surface.
The clear sheet 60 enters the top section of the combining unit through the upper tension rolls 62. These tension rolls serve two purposes:
(1) they act as a positive, constant speed driving section; and (2) they are driven at a slower line speed than the combining rolls 65 to impart a desired amount of sheet tenæion and drawdown in the clear sheet 60 to prevent cross machine direction wrinkles and to control curl. This sheet tension is re-.

~-06-12-0364 quired to elimlnate wrinkles in the c~mbined 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 deter-mined by measuring the width change of the gheet. If the drawdo~n in the upper sheet does not ~atch that of the luwer 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 1~% slower heing typical values.
The clear sheet 60 then passes under a ~ank of electric infra-red pre-heaters which adjust the temperature of the incoming sheet to a temperature in the range Or from 21 to 82C. The p~eferred temper~ture is 43 to 71C
as measured w~th an infra-red pyrometer ~ust 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, eliminating any wrinkles which may have formed due to sheet growth during heating.
The clear sheet 60 is then carried to the upper lay-on roll 64 which transfers the sheet to the upper com~ining rolls 65. The lay-on roll 64 is operated at anywhere from zero nip pressure Cnip open) to a nip pressure of 2.95 Kg/cm dependin4 on the parttculz~r 6heet used as w~l]. ~6 other operating conditions. The clear sheet 60 wraps the upper combining roll 65 from 15 to 154C until lt reaches the comblnin~ nip 69. The combinlr~
rolls are heated at from 32C to 82C wlth 43 to 71c belng preferred.
The comblning nip pressure run~ rrom ~.48 ~g/c~ to 8,85 K~/c~ ~1th 2.95 to 5.90Kg/cm being preferred. The upper combining roll 65 surface is slightly less tacky than the bottom combining roll 65 surface 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 .

.. . , ~ .. . . . , , . ~ . . -.
' 1~436~3 relax between the relaxing nip 66 and the lo~er tension ni~ 62 by running the lower tension nip 3% to 20% slower than the relaxing 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 li~e 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 dire~tion sheet tension to prevent cross-machine d~rection wrinkles and to control curl. The lower tension roll speed is from lZ 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 ~he upper lay-on roll. The sheet wraps the lower combining roll 65 from about 15 to 154C\ Which lS heated to the same temPerature as the upper combinlnF roll.
After passing through the combining nip 69, the combined sheet (0.762 nun 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.
While Figure VI refers to the combining of two Q03~hnnsheets to form a 0.762nncomposite, 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 regard to the latter, one or both sheets may be printed with any desired pateerns, designs or colors.
One of the ma;or 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 ' " '' ' '' . . .: ' :
,~ . , , :

-C-06-12-0364 ~0~3 on the roll and by vapars released from the surface 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 com~ining roll to less than 17oC 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 com6ining rolls. ~y preh ating within IlC of the sheet combining ~emperature, the sheet growth on the com6ining rolls i9 limited to an amount insufficient to cause wrin~les. A com6ination 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 possible.
Figure VII illustrates a s~eet com6ining process similar to that shown in Figure VI above, except that several tension, catenary loop and temperature controls have been added. These process controls are designed to match the tension, temperature and line speed of the sheets to be combi~ed 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. This 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 minimum level by a first upper tension sensor 83.
A first upper temperature sensor 84 controls the pre-conditioning heaters to obtain a sheet temperature in the range of 32C to 82C with ~3C to 71C being preferred.
The clear sheet then passes through a second upper tension nip 85 into a rela~ing section where the sheet is allowed to droop in a controlled catenary loop 6etween the second upper tension nip 85 and the third upper tenslon nip 88. Tension nips 85 and 88 control the tension and catenary loop _ g _ : -10~;~683 in the relaxing section. ~le speed of these ~ension rolls, w~ich form tension nips 85 and 88, is controlled ~y the catenary loop sensor 86. The catenary loop is maintained in the relaxing section to relax out as much inherent sheet stress as possible so that the clear sheet, which enters this section with a shrink level o~ from 4% to 24%, leaves with a shrink level of less t~an 4%. 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 1%
to 30~ wLth 4Z to 16% being preferred. This nip is controlled by a second upper tension sensor 89.
After the relaxing section the sheet passes through a heating section where the sheet temperature is again adjusted 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 adjust the sheet temperature to the desired co~b~n~ng temperature. T~e temperature of the sheet just before combining is measured and controlled by a third temperature sensor 93, which is located just before the com~ining 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 gradient 81 enters the lower section of the sheet combining unit which is essentially the same as that described above.
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 dewTinkled just prior to printing and .. . .
~. , - .
. ~ , ..

C-06-12-03~4 then fed directly int~ the combining un~t. Al80 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 pasges into a relaxing section 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 second 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 fl.exible bowed roll 105 into the combining nip 94. A third lower temperature sensor 106 is located -between the flexible bowed roll lOS 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 ln the upper and lower section and to ad~ust 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 opti~um properties in the laminate.
In the combining nip 94 the clear sheet ôO 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 a temperature of less than 21C being preferred. The composite sheet is 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 description of Figure VII refers to an upper and lower .. . . . ..

7 C_06-l2-0364 10~83 section. However, it is understood that the operation can be carried out side by side in which case the upper and lower sections would become the left and right ~ections or first and second sections of the operation.
It is also possible to com~ine 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 ~he art upon reading the present specification.
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 Kg/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 pressure used to make the laminate also serve to increase the band strength of the composite interlayer.
Preferably, the co~posite sheet has a curl value of less than 20.
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 :: : . - . .

10436~3 , ~

10 minutes and placed on a flat tahle with the ~urface of the sheet that was ~;
supported in the oven placed face down on the table. Tfie sample is examined ~;
and if the ends of the sample curl bac~ from the table surface, the distance from the end of the sample to the point that is still in contact with the table i8 measured in millimeters. T~e 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) X 100 = X curl ;-A curl value greater than 20% 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 composlte 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 Vl, having a bond strength of Q2 95 K~/om and a curl value of 3% is laminated between two sheets of glass of the type conventionally used ln the preparation of automob~le 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~polyvinyl butyral inter- ~ -layer prepared by conventional methods. The properties of the glass laminates 2~ prepared using the camposite interlayer of the present invention are compar-able to the properties of those laminates prepared using a conventional single ply polyvinyl butyral interlayer.
While the foregoing description has been dlrected 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 couid be used aH interlayers for laminated safety glass. Examples of other suitable interlayer material include polyurethanes, polyamides, poly(ethylene-vinyl acetate), poly(ethylene-vinyl alcohol), etc.

,, .. . , , ~ . ... . .... .. ... .... .. ... .. . . .. . .. .
. ::: , . - ~ : ~ , C-06-12-0364 104~83 From the foregoin~ description of the present invention, it will become apparent that many variations and modifications are possible without departing from the spirit and scope thereof.

: . . - . -.. . . .

Claims

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

1. A process for preparing a composite interlayer for laminated safety glass which comprises:
A. passing a first continuous sheet of thermoplastic material between nip rolls to impart machine direction, sheet tension and drawdown into the sheet;
B. adjusting the temperature of the sheet to a temperature in the range of from 32 to 82°C;
C. removing any wrinkles from the sheet; and then D. combining the first continuous sheet with a second con-tinuous sheet which is tinted wherein the second sheet has received substantially the same treatment as out-lined in process steps A to C above;
E. cooling the resulting composite.

2. A process as in claim 1 wherein the temperature in step (B) is adjusted to between 38 to 66°C.

3. A process as in claim 1 wherein the thermoplastic sheets are polyvinyl butyral.

4. A process as in claim 1 or 3, wherein the second continuous sheet is printed with a color gradient.

5. A process for preparing a composite interlayer for laminated safety glass which comprises:
A. (1) passing a first continuous sheet of polyvinyl butyral between a first set of nip rolls to impart machine direction, sheet tension and drawdown into the sheet;
(2) adjusting the temperature of the first sheet to a temperature in the range of from 32 to 82°C while removing any wrinkles in the sheet;

(3) allowing the first sheet to droop in a catenary loop between a second and third set of nip rolls;
(4) adjusting the temperature of the first sheet to a temperature in the range of from 32 to 82°C; and simultaneous with steps 1 to 4 above;
B. (1) passing a second continuous sheet of polyvinyl butyral which is printed on one side with a color gradient, between a set of nip rolls and (2) allowing the second sheet to droop in a catenary loop between the nip rolls;
(3) adjusting the temperature of the second sheet to a temperature in the range of from 32 to 82°C; and C. (1) passing the first and second sheets into a combining nip where they are combined wherein the printed surface of the second sheet of polyvinyl butyral is combined to the first sheet so as to provide a composite wherein the bond strength between the first and second sheet is at least 0.295 Kg/cm;
(2) cooling the resulting composite;
wherein the process steps outlined in A and B above are con-trolled to provide substantially the same line speed, sheet tension and temperature in each of the first and second sheets of polyvinyl butyral being fed into the combining nip.

6. A process as in claim 5, wherein the shrink level of the first and second sheets is reduced to less than 4%
prior to entering the combining nip.

7. A process as in claim 5 or 6, wherein the temperature rise of the first and second sheets during the combining step is less than 17°C.

8. A process as in claim 5 wherein the sheet tension, temperature and line speed for each of the first and second sheets is being constantly monitored and synchronized.