Method of preparing prelaminate using rough-surfaced interlayer

A method of preparing a prelaminate for a safety glazing which involves providing a thermoplastic interlayer having a first air removal surface which includes a multiplicity of microscopic embossments of substantially identical shape integrally projecting from the plane of one side of the interlayer in a regular pattern of rows and a second air removal surface on the other side of the interlayer which is different from the first air removal surface and includes a multiplicity of microscopic peaks and valleys of varying heights and depths arranged in an irregular, non-linear pattern, interposing the interlayer between two layers of glass and deairing the interfaces with the glass layers during which the first air removal surface is partially transferred to and imposed on the second air removal surface to provide deair paths of reduced obstruction.

BACKGROUND OF THE INVENTION 
This invention relates to thermoplastic interlayer having rough surfaces 
and more particularly to a particular form of rough surface for optimum 
deairing in a prelaminate with glass. 
Plastic sheet, typically of polyvinyl butyral (PVB), is known as an 
interlayer for use with optically transparent glass in laminated safety 
glass assemblies used, for example, in vehicle windshields, building 
windows and the like. 
It is further known, (see, for example, U.S. Pat. No. 4,035,549, to Kennar) 
to make the surface of the sheet rough to facilitate deairing, i.e. 
evacuating air from an interface between the sheet and a glass layer 
during preparation of a prelaminate of the sheet with glass. More 
specifically, minute channels between the smooth surface of the glass and 
the rough surface of the opposing contiguous sheet form routes for air to 
escape from between the two members when pressure or vacuum is applied 
with heat during preparation of the prelaminate. The deaired prelaminate 
is then subjected to elevated temperature and pressure bonding conditions, 
usually in a downstream autoclave, to form the finished safety glass 
assembly. 
Inadequate deairing results in visual defects in the finished safety glass 
assembly in the form of undesirable bubbles or local unlaminated regions. 
Deair completeness is conveniently measured by light transmission through 
the prelaminate before final laminating in the autoclave. The greater such 
transmission, the greater the quality of deairing provided by A particular 
profile of rough surface. 
Optimum deairing is a continuing need in the laminated safety glass art. 
SUMMARY OF THE INVENTION 
Now, improvements have been made in interlayer surface roughness which 
increase the quality of prelaminates formed therewith. 
Accordingly, a principal object of this invention is to provide an 
interlayer with a particular roughness profile which optimizes deairing 
during preparation of a prelaminate for use in a safety glass assembly. 
Another object is to provide a prelaminate of high quality as determined by 
a remarkably high degree of light transmission therethrough. 
Other objects will in part be obvious and will in part appear from the 
following detailed description and claims. 
These and other objects are accomplished by a thermoplastic interlayer 
having a first air removal surface on one side comprising a multiplicity 
of microscopic embossments of substantially identical shape integrally 
projecting from the plane of one side of the interlayer in a regular 
pattern of mutually perpendicular rows and a second air removal surface on 
the other side which is different from the first air removal surface, 
comprising a multiplicity of microscopic peaks and valleys of varying 
heights and depths arranged in an irregular, non-linear pattern, the 
average roughness height, R.sub.z, of the embossments and peaks being less 
than about fifty microns and the average distance, S.sub.m, between the 
embossments and the peaks being less than about 300 microns, such 
interlayer in a prelaminate with each side press-bonded bonded to a layer 
of glass being capable of transmitting at least 85% of incident light. 
Also provided in the method of preparing a prelaminate for a safety glazing 
by deairing the interface with glass on each rough-surfaced side of a 
thermoplastic interlayer and heating the interlayer and glass to collapse 
the rough surfaces, is the improvement facilitating deairing wherein a 
regular roughness pattern on one side of the interlayer is partially 
transferred to and imposed on a random roughness pattern on the other side 
of the interlayer to provide deair paths which are less obstructed than 
those of the unmodified random pattern. 
In a more specific aspect, a method is provided for preparing a prelaminate 
for a safety glazing which comprises: 
a) providing a thermoplastic interlayer having: 
(i) a first air removal surface on one side comprising a multiplicity of 
microscopic embossments of substantially identical shape integrally 
projecting from the plane of one side of the interlayer in a regular 
pattern of mutually perpendicular rows; and 
(ii) a second air removal surface on the other side which is different from 
the first air removal surface comprising a multiplicity of microscopic 
peaks and valleys of varying heights and depths arranged in an irregular, 
non-linear pattern; 
b) interposing the interlayer between two layers of glass; 
c) increasing the temperature of the interlayer between the glass layers to 
above the glass transition temperature of the thermoplastic; 
d) developing a negative pressure on the assembly of c) to draw air from 
the interface between one glass sheet and the first air removal surface at 
a rate greater than occurring at the interface between the other glass 
sheet and the second air removal surface; 
e) partially collapsing embossments of the first air removal surface to 
cause the regular pattern to at least partially transfer to and modify the 
second air removal surface thereby facilitating air removal from the 
interface of the modified second air removal surface through channels 
formed by the transferred regular pattern; and then 
f) increasing the temperature of the assembly of e) to substantially 
completely collapse the embossments and peaks of the first and modified 
second air removal surfaces to provide the prelaminate. 
Further provided is a prelaminate producible by the methods noted above 
which is capable of transmitting at least 85%, and even as much as 95% or 
more, of light incident thereon.

DETAILED DESCRIPTION OF THE INVENTION 
Thermoplastic interlayer usable in the invention must be capable of 
strongly bonding to a rigid panel such as glass to form an 
impact-dissipating layer in a laminated safety glass assembly. Exemplary 
thermoplastics include poly(ethylene-vinyl acetate), poly(ethylene-vinyl 
acetate-vinyl alcohol), poly(ethylene-methyl methacrylate-acrylic acid), 
polyurethane, plasticized polyvinyl chloride, etc. Polyvinyl butyral (PVB) 
and more particularly partial PVB containing about 10 to 30 weight % 
hydroxyl groups expressed as polyvinyl alcohol is preferred. Such partial 
PVB further comprises about 0 to 2.5 weight % acetate expressed as 
polyvinyl acetate with the balance being butyral expressed as polyvinyl 
butyral. The non-critical thickness of the thermoplastic sheet can vary 
and is typically about 0.25 to 1.5, preferably about 0.35 to 0.75 mm. PVB 
sheet is commercially available from Monsanto company as Saflex.RTM. sheet 
and E.I. duPont de Nemours and Co. as Butacite.RTM. polyvinyl butyral 
resin sheeting. 
PVB sheet is plasticized with about 20 to 80, preferably 25 to 45 parts of 
plasticizer per 100 parts of PVB resin. Such plasticizers are known to 
those skilled in the art and are typically disclosed in U.S. Pat. No. 
4,654,179, col. 5, lines 56-65, the content of which is incorporated 
herein by reference. Dihexyl adipate is preferred. 
In addition to plasticizer(s), sheet of the invention may optionally 
contain additives to improve performance such as dyes, pigment colorants, 
light stabilizers, antioxidants, glass adhesion control agents and the 
like. The sheet may be provided with a colored anti-glare gradient band 
extending along one side adjacent its edge which may be incorporated into 
the sheet according to the method and system disclosed in U.S. Pat. No. 
4,316,868, the content of which is incorporated herein by reference. 
Referring to the drawings, thermoplastic interlayer sheet 10 is depicted in 
FIG. 1. Interlayer 10 differs from the prior art in having different 
surface roughness profiles on its opposite major sides. More particularly, 
interlayer 10 includes a first air removal surface 12 comprising a 
multiplicity of microscopic embossments 14 substantially identical in 
shape which, in the illustrated embodiment, are V-shaped in vertical cross 
section when surface 12 faces downwardly. As illustrated, each embossment 
14 projects from the plane of the sheet (schematically identified as 16) 
and forms an integral continuation of one side of interlayer 10. 
Embossments 14 are arranged in the regular pattern shown in FIG. 1 in the 
form of linear rows extending in mutually perpendicular directions on one 
side of interlayer 10. 
Embossments 14 may differ in shape from the preferred pyramidal 
configuration shown. Conical, cylindrical, frusto-conical and the like may 
alternatively be used. 
Second air removal surface 18 on the other side of interlayer 10 is 
different from first air removal surface 12 and comprises a multiplicity 
of microscopic peaks 20, the surfaces of which define valleys 22, such 
peaks and valleys being of varying heights and depths. In contrast to 
first air removal surface 12, peaks 20 and valleys 22 are in an irregular, 
random, non-linear pattern on such other side of interlayer 10. This 
irregularity is schematically highlighted as 24 in FIG. 1 representing the 
outline of peaks of differing heights behind the plane of the cross 
section defining peaks 20. 
The inverted V-shaped open spaces 26 between immediately adjacent 
embossments 14 collectively provide straight, unobstructed, relatively 
short, linear, uniform deair corridors to the open-ended edges of 
interlayer 10 through which air relatively quickly exhausts during 
formation of a prelaminate in a manner to be presently described. In 
contrast, the randomized pattern of peaks 20 and associated valleys 22 of 
unmodified (FIG. 1) second air removal surface 18 can be expected to clog 
the paths through which air passes in moving to the edges of the 
interlayer. More particularly, with the random pattern the exhausting air 
collides with the randomly oriented peaks as it moves to the periphery of 
the interlayer tending to lengthen the exhaust paths and incrementally 
extend the time for air removal in comparison with that of regular first 
air removal surface 12. 
Surface roughness is defined by the frequency which is the number of 
embossments 14 or peaks 20 per unit of distance in a given direction 
(usually the direction of extrusion (MD) and 90.degree. thereto (CMD)) and 
the amplitude or height of embossments 14 and peaks 20 which is the 
distance from the lowest valley to the highest peak. Techniques and 
systems for measuring these parameters are known to those skilled in the 
art and are disclosed in: U.S. Pat. No. 2,904,844, col. 3, lines 15-18; 
U.S. Pat. No. 3,591,406, col. 3, lines 49-53; U.S. Pat. No. 4,035,549, 
col. 2, lines 5-28; U.S. Pat. No. 4,925,725, col. 2, line 40 to col 3, 
line 47; U.S. Pat. No. 5,091,258, col. 7, lines 34-53. 
The system herein to characterize roughness is a model S8P Perthomoeter 
from Mahr Corporation, Cincinnati, Ohio which used a tracing stylus to 
measure actual roughness. In this regard, R.sub.1 (in microns, .mu.), 
defined according to DIN 4768 (May 1990), is the average peak to valley 
height which is the arithmetic mean of the individual measurement lengths 
l.sub.e aligned together. l.sub.e can be set as desired and is 2.5 mm 
herein. Frequency is characterized in this system according to DIN 4762 in 
terms of the average distance between profile irregularities (S.sub.m) 
(.mu.) within a reference length l.sub.m wherein l.sub.m can be set as 
desired and is 12.5 mm herein. 
Prelaminate light transmission is measured with a photometer from Tokyo 
Denshoku Co., or equivalent. A light transmission measurement is relative 
to a clear laminate obtained after autoclave bonding which is taken to be 
100%. 
Conventional techniques known in the art are used to produce first and 
second air removal surface 12, 18. First air removal surface 12 is roll 
molded downstream of a sheet-shaping die, not shown, by passage through a 
nip between two rotating rolls, i.e. an embossing roll having indentations 
formed in its surface which are complementarily-shaped negatives of the 
embossments 14 and a cooperating backup roll. An equipment system capable 
of modification to provide surface 12 is described in and shown in FIG. 1 
of U.S. Pat. No. 4,671,913. Second air removal surface 18 is formed by 
controlling one or more of the following during shaping of the interlayer, 
usually by extrusion: polymer molecular weight distribution, water content 
of polymer melt, melt and die exit temperature, die exit geometry etc. 
Systems describing such techniques are disclosed in U.S. Pat. Nos. 
2,904,844; 2,909,810; 3,994,654; 4,575,540; 5,151,234 and European 
Application No. 0185,863, published Jul. 2, 1986. 
The different roughness profiles on the two sides of interlayer 10 provide 
an unexpected improvement in deair performance brought about by 
cooperative interaction of the different surface profiles. FIGS. 2 and 3 
illustrate the postulated behavior of the surfaces of interlayer 10 during 
preparation of a prelaminate for a safety glazing. More particularly, 
interlayer 10 is interposed between two layers of preheated glass 27, 28 
(FIG. 2). Contact with such glass conductively increases the temperature 
to that of the glass--i.e. to a temperature adequate to facilitate 
collapsible deformation of the surface, which temperature is typically 
above the glass transition temperature of the thermoplastic of the 
interlayer. The glass/interlayer/glass trilayer assembly of FIG. 2 is then 
placed in a flexible rubber bag or equivalent, having an opening 
communicating with a source of negative pressure. A vacuum ring 
encompassing the periphery of the trilayer assembly is an acceptable 
equivalent. The negative pressure draws air from interface 30 (FIG. 2) 
between glass sheet 28 and first air removal surface 12 at a removal rate 
considered greater than occurring along interface 32 between glass sheet 
27 and second air removal surface 18. This difference is due to the 
previously described unobstructed paths for air passage between the 
embossments of air removal surface 12 as compared with the tortuous paths 
delimited by the random peaks and valleys of surface 18. Such preferential 
rush of air from interface 30 creates a slight negative pressure along 
surface 12 which partially collapses embossments 14 of first air removal 
surface 12 as illustrated by flattened cross section 34 in FIG. 3. This 
flattening is aided by the momentary greater pressure on the opposite side 
along interface 32 containing the randomized second air removal surface 
18. Since interlayer 10 is at elevated deformation temperature and is 
trapped between glass layers 27, 28, the downwardly collapsing embossments 
14 deflect the thermoplastic on the opposite (upper in FIG. 2) side 
upwardly to at least partially transfer the regular pattern to and modify 
the randomized pattern of second air removal surface 18. This is brought 
out in FIG. 3. A downwardly collapsing regular embossment 14 on the lower 
surface will cause upward deflection of a particular random peak on the 
upper surface, which peak is of sufficient initial height such that on 
deflecting it becomes high enough to abut against the glass surface. This 
is depicted by modified peak 35 in the plane of FIG. 3 abutting glass 
surface 37. Other modified peaks behind the plane of FIG. 3 generated the 
same way as 35 are shown as 39 and this represents the regular pattern 
superimposed on the initial random surface. However, random peaks of upper 
surface 18 of lesser initial height than just described continue to 
provide a random pattern in modified surface 18 and these are identified 
in the plane of FIG. 3 as 41. 43 in FIG. 3 schematically represents 
modified peaks of such initial lesser height behind the plane of FIG. 3. 
Paths 38 of the thus modified second air removal surface 18 formed by the 
transferred regular pattern from the opposite side are considered to 
facilitate flow of air from interface 32 of the modified second air 
removal surface. This effect of introducing an array of regular peaks from 
the regular pattern to a previously entirely random surface results in 
shorter, less tortuous air escape paths. Air flow through the modified 
random pattern is similar to that occurring on the side with the regular 
pattern and is considered less obstructed than that occurring through the 
unmodified random pattern. 
Subsequently increasing the temperature of the glass/interlayer/glass 
assembly of FIG. 3 in an oven or equivalent substantially completely 
collapses the embossments and peaks of the first and modified second air 
removal surfaces of interlayer 10 and press bonds the interlayer to the 
glass to provide the prelaminate. Subjecting such prelaminate to 
conventional bonding conditions of elevated temperature and pressure, in a 
manner known to those in the art usually in a downstream autoclave, firmly 
bonds the opposing surfaces of the interlayer 10 to the contiguous glass 
layers to form the finished safety glass assembly. 
The invention is further described in the following examples which are for 
illustration only and not to limit or restrict the invention. 
EXAMPLE C 1 
This is not according to the invention. 
Thirty mils (0.76 mm) thick Saflex.RTM. sheet containing a nominal level of 
32 parts plasticizer per 100 parts PVB resin is obtained from Monsanto 
Company having a surface roughness on each side characterized by values of 
R.sub.z of 30-35 microns and S.sub.m of 450-500 microns. The pattern is 
random as illustrated at 18 in FIG. 1. Such roughness profile is generated 
by melt fracture, typically by passage of the thermoplastic forming the 
sheet through a rectangular sheeting die opening formed by lands which are 
at a temperature less than that of the bulk of the extruding melt. This is 
achieved by flow of an appropriate temperature conditioning fluid through 
channels just below the land surfaces. 
40.times.60 in (102.times.154 cm) sections of such sheet are placed on a 
heating surface at 80-180.degree. F. (26 to 82.degree. C.) to heat the 
sections to this temperature. An embossing station comprises a 4 inch (10 
cm) diameter embossing roll pressing against a 4 inch (10 cm) diameter 
rubber-faced backup roll at a pressure of 30 pounds per lineal inch. The 
entire surface of the embossing roll has sharply profiled abutting 
pyramidal cavities forming a retiform surface thereon at a frequency of 88 
cavities per inch. The face of the back up roll is covered with a high 
extensibility, temperature resistant rubber chosen to be capable of 
stretching without fracturing. An anti-stick release coating is on the 
surface of the embossing roll which is at 177.degree. C. The embossing 
roll rotates at a surface speed of about 10-20 fpm (3-6 m per min.). A 
vacuum roll adjacent the nip formed by the embossing and back up rolls 
pulls the embossed sheet from the embossing roll surfaces The precut sheet 
sections are passed through the nip, removed by the vacuum roll beyond the 
nip and placed on a table at room temperature. Each sheet section is then 
turned over and the process repeated to emboss the other side. 
Sheet sections sharply embossed on each side with a regular pattern in the 
manner just described are visually displeasing to the eye and therefore 
considered commercially unacceptable. The appearance is described as a 
moire pattern and is caused by the frequency of the embossments not being 
in exact accurate register on each side of the sheet--i.e. the slightly 
different frequency on each side results in interference patterns which 
are not in phase with each other. Sheet with such moire effect presents an 
undulating pattern similar to a wood grain appearance. 
Using the roughness measuring system previously described, the sheet was 
characterized by an R.sub.z of 30 microns and S.sub.m of 280 microns. 
EXAMPLE C 2 
This likewise is not according to the invention. 
Unembossed Saflex sheet as initially described above having a random 
roughness surface profile on each side (R.sub.z =30-35 microns and S.sub.m 
=450-500 microns) is cut into sections as in Example C 1. Sheet sections 
at about 15-18.degree. C. are placed between two layers of similarly 
dimensioned float glass at about 30.degree. C. to raise the sheet to this 
temperature which is above the glass transition temperature of the 
plasticized PVB. The three layer glass/sheet/glass assembly is placed in a 
flexible rubber bag connected to a negative pressure source to develop a 
pressure of 1/3 atmosphere (33.5 k Pa) within the bag thereby drawing air 
from the two interfaces of the sheet with the glass. The three layer 
assembly is then passed through an oven to raise the temperature of the 
three layers to about 100.degree. C. and then removed from the oven and 
cooled to room temperature. Percent light transmission of the prelaminate 
thus formed is measured as 69-70%. Such a laminate is industrially 
unacceptable since at least 85% is required to be commercially viable. 
This control Example illustrates unacceptable performance of a surface 
roughness profile in the form of a random pattern on each side of the 
sheet. 
EXAMPLE C 3 
A commercially available plasticized PVB sheet different from that of the 
invention has a random pattern on each side similar to that described in 
Example C 2 except R.sub.z is 34 microns and S.sub.m 291 microns. A 
prelaminate formed using the procedure described in Example C 2 has a 
light transmission of 85-87%. 
EXAMPLE 1 
This is according to the invention. 
Sheet sections as described in. Example C 1 are embossed using the 
procedure of C 1 on only one side--i.e. the second side is not embossed 
and has the initially present randomized pattern having R.sub.z of 30-35 
microns and S.sub.m of 450-500 microns. As in C 1, the regular roughness 
profile of the embossed side has R.sub.z of 30 and S.sub.m of 280 microns. 
Measured light transmission in (4-6 samples) prelaminates formed therewith 
is as high as 95% and is generally in the 87-95% range. This unexpectedly 
high light transmission obtained using interlayer having different 
roughness profiles on each side is the result of the mechanism previously 
described with reference to FIGS. 2 and 3. This is contrasted with the 
lower light transmission in Examples C 2 and C 3 where the interlayer 
sheet has a random roughness pattern on each side. 
Interlayer of the present invention preferably has an R.sub.z on each side 
of between 20-40 microns and a frequency on each side as characterized by 
S.sub.m of less than about 300 microns, preferably between 80 and 300 
microns. 
The preceding description is for illustration and should not be taken as 
limiting. Various modifications and alterations will be readily suggested 
to persons skilled in the art. It is intended, therefore, that the 
foregoing be considered as exemplary only and that the scope of the 
invention be ascertained from the following claims.