Note: Descriptions are shown in the official language in which they were submitted.
1~3~j~5 06-12-0410
PROCESS FOR STABILIZING THE VISCOSITY OF POLYVINYL
ACETALS AND A LAMINATE SAFETY GLASS USING SAME.
The present invention relates to a method for stabilizing
the viscosity of polyvinyl acetal resins and the use of such resins
in laminated safety glass. More particularly, the present inven-
tion is directed to a method for stabilizing the viscosity of the
polyvinyl butyral resins used as interlayers for laminated safety
glass.
Polyvinyl acetal interlayers are well known in the prior art.
These materials are used to prepare laminated safety glass which
is used in various vehicle and architectural applications. The
most common application for the laminated safety glass is the
windshields in automobiles.
The polyvinyl acetal interlayers are subjected to elevated
temperatures during extrusion, seasoning of printed sheet and
fabrication of the sheet into a laminate. The elevated tempera-
tures cause degradation of the polyvinyl acetal which is evidenced
by decreased viscosity and development of yellow color. Some
decrease in viscosity may also occur upon aging.
A need exists in the art for a method which will provide
polyvinyl acetal interlayers with improved viscosity stability
especially for use in bilayer safety glass.
Laminated safety glass prepared by laminating a thermoplastic
interlayer between two sheets of glass is well known in the prior
art. Such laminates are widely used in automobile windshields
and as windows and doors in architectural applications.
In recent years there has been a growing interest in laminated
safety glass wherein only one sheet of glass is used, usually on
the outboard side, that is, the side of the laminate that faces
the outside of the vehicle or architectural structure. The in-
board glass sheet, that is, the glass sheet that faces the interior
of the vehicle or the architectural structure, is not used. The
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06-1~-0410
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result is a glass/plastic laminate which is sometimes referred to
as a bilayer or a bimodal laminate. The plastic component of the
bilayer may be a single sheet or a composite of two or more com-
ponents. These include a plastic component which is coated with
another material, two or more plastic components laminated together,
with various combinations of the above. Examples of such bilayer
or bimodal construction are taught in U.S. Patents 3,625,792,
3,652,379, 3,762,981, 3,781,184, 3,806,387 and Belgium Patent
803,902.
Bilayer or bimodal laminated safety glass constructions have
received widespread interest because of the theory that a person
who collided with the laminate from the inboard side would tend
to suffer less serious injuries by hitting a plastic component
rather than a glass component. However, the dropping of the in-
board glass sheet in laminated safety glass has given rise to new
problems. The thermoplastic interlayer, which is no longer encased
by the protective glass components, is now more susceptible to de-
gradation which may adversely affect the properties of the laminate.
Moreover, plastic sheets which are plasticized in order to obtain
optimum physical properties may suffer frcm loss of plasticizer and
a deterioration of properties. It has been proposed in the prior
art to cover the plastic sheet with a protective coating or a pro-
tective layer. Many of the suggested coverings or coatincsdo not
provide the desired level of protection to the plastic sheet.
The present invention provides a method for stabilizing poly-
vinyl acetal resins and sheets made therefrom against viscosity de-
gradation which occurs during the thermal processing of these
materials. More particularly, the present invention provides a
method for minimizing the decrease in resin viscosity which occurs
when the resin is exposed to elevated temperatures. The present
process comprises incorporating a pH buffer into the polyvinyl
acetal resin. Polyvinyl acetal resins which are buffered in
accordance with the teachings of the present invention exhibit a
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smaller decrease in resin viscosity during the thermal processing
and aging of these materials as compared to similar resins which
do not contain a buffer.
An improved laminated safety glass is obtained utilizing the
buffered polyvinyl acetal sheet.
DESCRIPTION OF THE DRAWINGS
Figure I is a graph of percentage viscosity drop versus the
pH of the buffer as determined by the plastic oxidation test which
is described below. The resin and buffer systems used are those
set forth in Examples 1 to 20. The arrows on the graph show the
range in the experimental data at a given pH when using various
buffers.
Figure II is a plot showing various components present in a
citrate buffering system at various pH levels. The species present
include citric acid, monosodium citrate, disodium citrate and tri-
sodium citrate. Curve A represents the amount of citric species;
Curve B re~presents the amount of monosodium citrate species; Curve
C represents the amount of disodium citrate species; and Curve
D represents the amount of the trisodium citrate species.
The Polyvinyl Acetal Resins
The polyvinyl acetal resins which are employed in the present
invention are well known in the art. These resins and processes
for their preparation are described at length in Morrison et al.
U. S. Patent No. Re 20,430, dated June 29, 1937, and Lavin et al.
U.S. Patent No. 2,496,480. In general, polyvinyl acetal resins
made from saturated lower unsubstituted aliphatic aldehydes are
the most suitable with polyvinyl acetal resins made from butyral- 7
dehyde being preferred for safety glass interlayers.
In general, the polyvinyl acetal resins employed have Staud-
inger molecular weights ranging from about 50,000 to 600,000 and
preferably from 150,000 to 270,000 and may be considered to be made
up, on a weight basis, of from 5 to 30 percent hydroxyl groups, cal-
culated as polyvinyl alcohol; 0 to 40 percent ester groups,
preferably acetate groups, and the balance substantially acetal.
When the acetal is butylaldehyde acetal, the poly-
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vinyl acetal resin will preferably contain, on a weight basis, from 12 to 25
percent hydroxyl groups, calculated as polyvinyl alcohol and from O to 3 per-
cent by ester, e.g., acetate groups, calculated as polyvinyl ester, the
balance being substantially butyraldehyde acetal.
The polyvinyl acetal resin may be plasticized to the extent of
about 20 to 80 parts plasticizer per hundred parts resin and more commonly
between 20 and 50 parts for normal windshield use. This latter concentra-
tion is generally used with polyvinyl butyrals containing 12 to 23 percent
vinyl alcohol by wei~ht. In general, the plastici~ers which are commonly
employed are e9ters of a polybasic acid or a polyhydric alcohol. Particu-
larly suitable are triethylene glycol di(2-ethyl butyrate), dibutyl sebacate,
and dihexyl adipate and combinations of dihexyl adipate and a phosphate
plasticizer.
The pH Buffer Systems
The pH buffer8 ù9ed in the present invention are systems which re-
slst changes in pH when acids or bases are added to the system. The buffers
; u8ed are complex systems which comprise at least two components. At least
one of the components of the buffer is at least a bidentate ligand with the
ability to lnteract with a metal to form a chelated r$ng structure. In
addition, the buffer should be substantially free of hydrochloric, nitric
and Yulfuric acids which may cause resin degradation. The preferred bidentate
ligands are those which form 5 or 6 member ring structures, which structures
u8ually have greater stability.
A further description of pH buffers, which are well known to those
9killed in the art, may be found in Bates, R. G., EL~CTROMETRIC pH DETERMINA-
TIONS, John Wiley & Sons, Inc., New York {1954) and in Lange's Handbook of
Chemlstry, 9th Edition, pages 951-9S2 as ~ell as in other well known
references. Bidentate ligands with the ability to interact with a metal to
form a chelated ring structure are also well known to those skilled in the
art. Such bidentates are described in Martell, A. E. & Calvin, M.,
~ ~ 7 3 5~5
C-06-12-0410
CHEMISTRY OF THE METAL CHELATE COMPOUNDS, Prentice Hall, New York (1952)
as well as in other well known references.
Examples o~ buffer components which are at least bidentate with the
ability tv interact with a metal to form a chelated ring structure include
phosphates such as sodium acid phosphate, disodium phosphate, dipotassium
phosphate, ethyl acid phosphate; and the corresponding pyrophosphates,
tripolyphosphates and tetraphosphates citrates such as sodium acid citrate,
dlsodium citrate, dipotassium citrate, tripotassium citrate, disodium mono-
potassium citrate, ethyl acid citrate and dimethyl citrate; ortho phthalates
such as sodium acid~-phthalate, disodium Or-phthalate, potassium acid d~-
phthalate, dipotassium F~phthalate and ethyl acid Cr-phthalate; borates such
as sodium acid borate, disodium borate, dipotassium borate, ethyl acid
borate and the corresponding meta and tetra-borates; potassium tetroxalate,
potasslum hydrogen tartrate, potassium acid succinate, potassium succinate,
potassium acid phenyl succinate, potassium salicylate, potassium sulfosali-
cylate, potassium acld 1,2-cyclohexane dicarboxylate, potassium acid 2,3-
naphthalene dlcarboxylate, dipotassium diacid 1,4,5,8-naphthalene tetra-
carboxylate, ~onoesters of phosphoric acid, partially neutralized amine
salts, etc.
20 . Examples of the preferred buffer systems used in the present inven-
tion include the alkali metal and alkaline earth metal phosphates, citrates,
phthalates, borates and cltrate/phosphate combinations used below in the
worklng examples.
The buffer systems are usually complex systems comprising more
than one species of chemical compound at any given pH. The nature of the
species present will vary with the pH as is illustrated in Figure II where
at a pH of 4, four different species are present. These species and the
amount of each is set forth below.
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Species % ~y ~eight
citric acid 8.5
- monosodium citrate 77.0
disodium citrate 14.0
trisodium citrate 0.5
100 . O
The buffers used in the present invention have a pH in the range of
from 3 to 7. Lower p~ values should be avoided in order to avoid resin
degradation. At pH values above 7 the viscosity stabilization becomes less
pronounced. The amount of buffer used is at least 20 parts (dry basis) per
million parts (ppm) of polyvinyl acetal resin. The upper limit, which should
not exceed 20,000 ppm is determined by the particular buffer, the polyvinyl
acetal resln and the amount of stability required. ~hen using larger amounts
of buffer there is a danger of losing the desired physical or optical pro-
perties in the interlayer and a lamlnated safety glass made from this inter-
lsyer. Preferably, 5Q to 1300 parts per million (ppm) of buffer are used and
more preferably from 50 to 800 ppm are used.
The particular buffer selected from any given appllcation should be
compatible with the polyvinyl acetal resin used and not cause any adverse
effects on the physical and optical properties of laminated safety glass
prepared from the polyvinyl acetal resin.
The buffers may be added to the polyvinyl acetal resin during any ;~
of the normal resin processing operatlons such as washing, drying, etc.
The buffers may also be added to the plasticizer used for the resin and
thereby incorporated into the resin during the plasticization step. Thi3
methot is especlally effective if the buffer is soluble in the plasticizer.
The buPfer may also be added to the resin during a shaping or forming opera-
; tion, 8uch as during extrusion of the resin into a sheet. Preferably, the
buf~ers are incorporatet into the resin prior to the resin being fabricated
into a shaped article.
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In the preferred method, the buffer is prepared in aqueous solu~ion
so as to obtain a pH in the range of from 3 to 7. The buffer solution is then
incorporated into the polyvinyl acetal resin.
~ uring extrusion or seasoning, thermally unstable resins will under-
go a significant decrease in resin viscosity and will exhibit an increasein yellow color indicating that the resin is being degraded. Resins buffered
according to the teachings of the present invention retain a major amount of
their original viscosity and, in many instances, do not exhibit as much
increase in yellow color as do the unbuffered resins. The test methods used
to measure viscosity retention and yellow color are described below.
TEST METHODS
Pls6tic Oxidation Test
One hundred parts of resin, a plasticizer for the resin and the
buffers being evaluated are blended in a flask ~o form a homogeneous system.
The plasticized resin sample is then placed in a 130C. air circulating oven
for 45 minutes. A control sample i8 held at room temperature. The vis- ;
cosities of the control sample and the heated sample are measured in a
methanol solution containing 7-1/2% resin solids. The viscosity measurements
are carried out at 20C. using an Ostwald-Fenske Viscometer and the results
reported in centipoises. The percent of viscosity retention on the heated
sample is then cslculated to give a measure of the thermal oxidative sta-
b~lity of he buffered resin. The viscosity change ls calcul~ted as follows:
original viscositY - viscosity after treating X 100% % h
original viscosity
It ~hould be noted that the plastic oxidation test (POT) is mainly
for screening purposes. The test contitions make no attempt to exclude air.
Consequently, viscosity changes and color development are more severe than
compa~able measurements made under actual extrusion conditions.
Percent Yellow Determination
The percent yellow values are determined using a 7.5% resin solu-
tlon in methanol and a Klett-Summerson Photoelectric Colorimeter. The
:
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absorption is measured at 420 millimicrons with a blue filter and at.660
millimicrons with a red filter and the readings converted to percent trans-
mission. Subtraction of the 420 millimicron reading from the 660 millimicron
reading gives the percent yellow. These values indicate the amount of yellow
color in the resin.
Brabender Compounding
The desired amount of plasticizer is added to 40.0 g. of the poly-
vinyl acetal resin and the mixture is stirred manually to obtain a uniform
blend. The plasticized resin is then placed in the mlxing chamber of a
Brabender Plastograph (Model No. 537) equipped with sigma blades. The ~-
Plastograph is made by Brabender Corp., Rochelle Park, New Jersey. The sample
ls then mixed for 7 minutes at 150C. and 50 RPM (blade speed). The sample
iB then tested for viscosity reduction and percent yellow color.
~he following examples are set forth in illustration of the present
invention and should not be construed as a limitation thereof. All parts and
percentages given are by weight unless otherwise indicated.
EXAMPLES 1 to 20
In these Examples various aqueous buffer solutions are prepared
according to the teaching set forth in Lange's Handbook of Chemlstry, 9th
Edition, pageæ 951 to 953. The buffers used in Example~ 2 to 12 are Clark
and Lub9 buffer mixtures while the citrate/phosphate buffers used in Example~
14 to 20 are MacIlvaine buffer mixtures. The buffer solutions are then added
to a conventional polyvinyl butyral resin so as to provide 1000 parts of
buffer (dry basis) per million parts of resin. The butyral resin has a
polyvinyl alcohol content of 18-21%, a residual acetate content of less than
2.5% by weight with the balance of the resin being substantially butyral
groups. The resin has a titer level of 106 cc. due to the presence of
potassium acetate. The resin and buffer mixture is then mixed with 42 parts
of triethylene glycol dit2-ethyl butyrate) plasticizer per hundred parts
of resin (phr). The resln, buffer and plasticizer are thoroughly mixed
. .
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C-~6-12-0410
and the resulting plasticized blend is then subjected to the plastic oxidation
test. After the plasti~ oxidation test, viscosity measurements are made on
the exposed samples. The results of these tests are tabulated in Table I
below.
TABLE I
SUMMARY OF EXAMPLES 1 to 20
Viscosity
ExamPle Buffer pHDecrease
1 Control None - 60.5
2 phosphate 6 26.9
3 " 6 36.9
4 " 7 36.9
" 8 42.6
6 phthalate 4 30.4
7 " 4 37.4
8 " 5 35.2
9 ~' 6 39.6
borate 8 45.6
11 ~' 8 57.4
12 ~' 9 58.7
13 potassium scetate/
acetic acid 4.869.5
14 citrate/pho~phate 2.2 34.8
" " 3 24.4
16 " " 4 24.8
17 " " 5 24.4
18 " " 6 34.8
19 " " 7 43.9
" " 8 46.9
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The data in Table I above (except for Example 13) show that poly-
vinyl butyral resin~buffered in accordance with the teaching of the present
invention exhibit less viscosity change than the control example which is
not buffered. The examples further show that the preferred pH range of the
buffer is from 3 to 7, as is further illustrated in Figure I. Example 13
illustrates that an acetate buffer, ~hich does not contain a component which
i8 at least a bidentate, is not effective in minimizing the percent viscosity
change of the polyvinyl butyral resin.
EXAMPLES 21 to 26
Examples 21 to 26 iilustrate that in certain instances the buffer
compounds of the present invention will also reduce the yellow color developed
in polyvinyl butyral resin during thermal processing. The polyvinyl butyral
re~in used in these examples is plasticized wlth 41 parts per hundred parts
of resln o a plastlclzer mlxture which is a 60/40 welght ratlo of dihexyl
adipate/octyl diphenyl phosphate. Examples 21 to 23 are control examples
which do not contain any buffer. Examples 24 to 26 contain a Clark and Lubs
phosphate buffer having a pH of 6. The samples are processed in a Brabender
mixing device using the conditions set forth above. The samples are then
tested for yellow color as described above. The test results on these
20 ' samples are set forth ln Table II below.
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TABLE II
SUMMARY OF EY~MPLES 21 to 26
Example Buffer Amount(l) % Scrap(2) % Yellow
21Control None 0 14.8
22 " " 20 22.4
23 " " 50 35.8
24Phosphate 1300 0 14.6
" 130020 lS.7
26 " 60050 29.0
(1) Parts of buffer (dry basis) per million parts of
resin.
(2) Examples 22 and 25 use self generated scrap while
Examples 23 and 26 use 20% by weight of self
generated scrap and 30% by weight of commercial scrap
all of which contains a triethylene glycol di(2-ethyl
butyrate) plasticizer.
The data in Table II above illustrates the increase in color in poly-
vinyl butyral compositions which contain scrap material. A comparison of
Examples 25 and 26 with Examples 22 and 23, respectively, illustrates that
there i8 less yellow color developed in the polyvinyl butyral compositions
which are buffered according to the teachings of the present invention than
ln the corresponding unbuffered samples.
EXAMPLRS`27 to 30
The following examples illustrate the viscosity and color reduction
that i8 obtained when extruding plasticized polyvinyl butyral resin, which
has been buffered accorting to the present invention, into sheets. The resin
u8et is conventional polyvinyl butyral resin which has been plasticized with
42 parts per hundred parts of resin of a triethylene glycol di(2-ethyl
butyrate) plasticlzer. The sheet is buffered with a thousand parts per
million of a MacIlvaine~s citrate/phosphate buffer system having a pH of 3.
The extruded sheet is tested for viscosity retention and yellow color according
to the procedures outlined above. The results of these tests are tabulated
ln Table III below.
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TABLE III
SUMMARY OF EXAMPLES 27 to 30
Resin
Viscosity
Example % Scrap(cps) % Yellow
27 Control - no buffer None 178 25.5
28 buffered None 198 21.~
29 Control - no buffer 50 169 27.7
buffered 50 189 23.2
A review of the data in Table III above illustrates that sheet
materlal extruded from polyvinyl butyral resin which is buffered according
to the present invention has a higher sheet viscosity and less yellow color
than the control samples which do not contain a buffer compound.
The data in the following Table IV illustrates the effect of the
amount of buffer (pH of 3) on the viscosity change when using the plastic
oxidation test. The resin and buffer system is the same as that used in
Example 28 above.
TABLE IV
BUFFER CONCENTRATION VERSUS RESIN VISCOSITY
Amount of Buffer (PPM)Resin Viscosity (cps)
None 87
100
100 121
200 134
300 142
500 150
600 152
700 153
800 :.155
gao 156
1000 157
1300 160
unheated control 230
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EXAMPLES 31 to 36
In these examples a polyvinyl butyral resin which is plasticized
with 33 parts of dihexyl adipafe per hundred parts of resin (phr) is sub-
Jected to the plastic oxidation test described above. Three different lots
of this material from two different sources are used in these examples. The
plasticized resin is buffered with one hundred parts per million ~dry basis)
of a MacIlvaine's citrate/phosphate buffer system having a pH of 6. The
samples are then tested for percent viscosity loss according to the pro-
cedure outlined above. The results of these tests are tabulated in Table V
below.
TABLE V
SUMMARY OF EXAMPLES 31 to 36
Example X Viscosity Decrease
31 Control - no buffer 63
32 buffered 34
33 Control - no buffer 43
34 buffered 27
Control - no buffer 60
36 buffered 50
The above data indicate that tl-e plasticized resin which is buffered
accordlng to the present invention exhibits less of a viscoslty change during
the plastic oxidation test.
EXAMPLE 37
In thi~ Example a bilayer glazing unit (a single sheet of glass
laminated to a 30 mil (762 microns) sheet of plasticized polyvinyl butyral)
is tested for stability in an accelerated aging test. The polyvinyl butyral
resin, which is plasticized with 42 parts of triethylene glycol di(2-ethyl
butyrate), is buffered with 100 ppm of a MacIlvaine'a citrate/phosphate
buffer system having a pH of 5.5. A control is used wherein the polyvinyl
butyral sheet contains 38 parts of plasticizer. The samples are exposed $n
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an air oven at 150~F. (65.6C.). At periodic intervals, samples of the poly-
vinyl butyral resins are analyzed for resin viscosity, plasticizer content and
alkalinity titer levels.
The alkalinity titer is the number of milliliters of 0.01 normal
hydrochloric acid required to neutralize lO0 grams of the polyvinyl acetal
resin. This is an arbitrary standard used to designate the alkalinity of the
resin. The alkalinity titer is determined by dissolving 7 grams of the poly-
vinyl acetal resin in 250 cc. of preneutralized ethyl alcohol and titrating
j with 0.005 normal hydrochloric acid to the endpoint using bromphenol blue
; 10 indicator and calculating from the result obtained to determine the milli-
liters of 0.01 normal acid required for 100 grams resin. The results of these
tests are tabula~ed in Table VI below.
TABLE VI
SUMMARY OF TESTS ON BILAYER
7 days 14 21 28 35
Initial (at 150) days days days days
s CONTROL
Resin Viæcosity tcps) 160 -- 9062 40* --
Plasticizer Content
(phr) 38 - 30 2318
Titer (cc) 16 ~ 161 --
20 BUFFERED SYSTEM
Resin Viscosity (cp8) 200 180 158 -- 142 148
. . .
Plasticizer Content
(phr) 42 39 34 -- 27 24
Titer (cc) 39 -- 39 -- 34 23
*partlal insolubility.
The data in Table VI above illustrates the improved resin stability
that is achieved when the resin is buffered in accordance with the teaching6
of the present invention.
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EXAMPLE 38
Exa~ple 37 is rep~ated he~e e~cept that the polyvinyl bu~yral re~ins
~n the b~layers are c~vered on tne inboard side with a polyethylene tere-
phthalate film. The samples are exposed and evaluated as in Example 37 above.
No change was observed in the plasticizer content because the polyethylene
terephtllalate film prevented plasticizer loss. However, changes were observed
in visc~ity and titer levels. Test results on these samples are tabulated
in Table VII below.
TABLE VII
SUMMARY OF TESTS ON BILAYER COVERED WITH PET
7 days 14 21 28 35 42
Initial (at 150) days days days days ~y~
CO~T~OL
Re~in Viscosity 195 125 117107 102 96 93
(Cp6~
Titer (cc) 60 -- 52 44 45 28 40
BUPFERED SYSTEN
Resin Viscosity 200 174 178 -- 185 194 190
(cps)
Titer (cc) 68 -- 64 -- 50 48 64
The data in Table VII above further illustrate that resins buffered
according to the present invention exhibit improved viscosity stability.
The present invention also contemplates covering the poly-
vinyl acetal sheet with other plastic or resinous components such
as polyvinyl chloride, polyvinyl fluoride, cellulose acetate, poly-
vinylidene chloride, polycarbonate, polymethyl methacrylate, cellu-
lose aceto-butyrate, cellulose tripropionate, silicon polymers,
polyurethanes, etc. The plastic cover sheet which is placed over
the polyvinyl acetal may optionally be coated with adhesion pro-
moters and/or abrasion resistant layers which materials are well
known to those skilled in the art.
3n The buffered polyvinyl acetals of the present invention may
also be used as the interlayer in conventional laminated safety
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10~3575
glass where the polyvinyl acetal sheet is sandwiched between
two layers of glass. The buffered polyvinyl acetals may
also be used as a covering on the inboard sheet of convention-
al three ply sandwich construction as is described in Belgium
Patent 803,902. This covering is used to obtain better
impact properties and to reduce injuries which may occur when
a person hits the glass.
The titer levels referred to above are usually
due to salts added during the manufacture of the resin as is
described in U.S.P. 2,496,480 to Lavin et al. In other
instances the salts are added in order to obtain a titer
within a certain range in order to obtain a desired level
of adhesion of the interlayer to the glass sheet. This is
done so as to obtain optimum impact properties in the
resulting laminated safety glass as is described in U. S.
Patents 3,249,489, 3,249,490, 3,402,099, 3,271,234, 3,271,235
and other references. The buffers of the present invention
may cause a decrease or an increase in adhesion of the inter-
layer to the glass sheet depending on the particular buffer
used. Those skilled in the art, upon reading the presen~
specification, will be able to make any necessary adjustments
in the level of titer control agents in order to obtain the
desired impact strength.
The present invention also contemplates the use of
conventional processing aids and additives in the resin.
These include dyes, pigments, antioxidants, ultraviolet light
stabilizers, etc. It is apparent from the foregoing that
many changes and modifications can be made without departing
from the spirit and scope of the present invention.
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