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Description  |
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BACKGROUND OF THE INVENTION
Field of the Invention: This invention relates to laminated windows and a
method for maintaining the clarity of laminated windows, particularly
laminated windows used in motor vehicle glazing containing a polyurethane
interlayer which has one major surface exposed to the atmosphere. More
particularly, this invention relates to retarding bloom in such laminates
when the polyurethane interlayer normally blooms upon exposure to the
atmosphere.
Description of the Prior Art: Safety glass for motor vehicle glazing, such
as windshields, has for some time been made in a trilayer configuration;
namely, two outer layers of glass bonded together with an interlayer of
polyvinyl butyral. Although the trilayer safety glass windshield is safer
than the older monolithic glass windshield, the trilayer is,
unfortunately, a principal source of both lacerative and concussive injury
in automobile accidents. During the course of an accident, an impacting
passenger's head may shatter the inner glass ply of the windshield to form
jagged slivers of glass which can severely lacerate the face and forehead
of the impacting passenger.
In attempts to improve the safety performance of the trilayer safety glass
windshield, the prior art has disclosed various bilayer windshields
composed of an outer ply of glass and an inner ply of flexible plastic.
The exposed inner plastic ply serves as a spall shield, shielding the
passengers from any jagged slivers of glass which may result during the
course of an accident.
Typical of the prior art relating to bilayer windshields is the following:
U.S. Pat. Nos. 1,128,094 to Benedictus; 1,342,267 to Mascart; 2,047,253 to
Brooks; 2,143,482 to Herrmann et al.; 2,184,876 to Sherts; 2,234,829 to
Neher et al.; 2,454,886 to Saprio; 2,489,026 to Gilbert; 2,526,728 to Bert
et al.; 3,157,563 to Baum; 3,532,590 to Priddle; 3,575,790 to Fleck;
3,580,796 to Hick et al.; 3,626,023 to Brizgys; 3,781,184 to Domicone et
al.; 3,805,985 to Hagiwara et al.; 3,806,387 to Peetz et al.; and
3,808,077 to Rieser et al.; and South African Pat. No. 73.3884.
Some of the most promising work in the development of safety glass involves
the use of polyurethane interlayers. See, for example, U.S. Pat. Nos.
3,388,032 to Saunders; 3,509,015 to Wismer et al. and 3,620,905 to
Ahramjian, and Belgian Pat. No. 785,125, all of which disclose the use of
polyurethane interlayers in fabricating safety glass. The polyurethanes
have optical clarity and have excellent impact resistance over a wide
temperature range.
There is, however, a disadvantage associated with certain of the
above-mentioned polyurethanes. A particular disadvantage is that certain
polyurethane formulations develop "bloom", characterized by a hazy surface
appearance, upon exposure to the ambient atmosphere. The bloom is a
surface deposit which is believed to be derived from low molecular weight
material leaching to the surface of the polyurethane sheeting. This bloom
scatters light and reduces the transparency of the interlayer and of any
laminates prepared therefrom with the surface of the polyurethane exposed.
Although the bloom can be removed by dry wiping or solvent wiping, the
bloom often recurs after removal. With bilayer windshields, bloom is a
problem because one surface of the polyurethane is exposed to the
atmosphere.
SUMMARY OF THE INVENTION
Surprisingly, the present invention has determined that blooming can be
retarded significantly if a layer of wax is applied to the exposed surface
of the polyurethane interlayer. Initially the wax was applied to protect
the surface of the polyurethane and to improve the cleanability. However,
the present invention has discovered that the layer of wax not only makes
the surface of the polyurethane easier to clean, but also significantly
retards blooming.
More specifically, the present invention provides a method of retarding
bloom in a transparent laminated window in which a layer of normally
blooming polyurethane is laminated to a rigid transparent sheet in such a
manner that a major surface of the polyurethane is exposed. The method of
the invention involves applying a wax to the exposed surface of the
polyurethane to retard blooming. The invention also provides the article
that results from said method, the article comprising a rigid transparent
sheet such as glass laminated to a first major surface of a layer of
polyurethane and a layer of wax covering the second major surface of the
polyurethane layer.
DETAILED DESCRIPTION
The polyurethanes which have been found to be the most troublesome with
regards to blooming are those prepared with high molecular weight
polyester polyols, that is, those having a molecular weight of about 1000,
particularly within the range of about 1500 to 5000 and higher. These
polyester polyols are prepared by condensing organic polycarboxylic acids
or anhydrides thereof with organic polyols. Examples of such polyurethanes
are described by U.S. Pat. Nos. 2,871,218 to Schollenberger and 3,214,411
to Saunders et al. Belgian Pat. No. 794,206 describes such polyurethanes
in sheet or interlayer form for use in motor vehicle glazing applications.
In general, the polyurethanes are prepared by reacting an organic
polyisocyanate, a polyester polyol as described above, and a chain
extender or curing agent which is a compound having at least two active
hydrogens and a molecular weight below 250. The reaction is conducted by
techniques well known in the art, such as the "one-shot" or bulk
polymerization method and the prepolymer method. The polyurethanes can be
shaped into sheet form by extruding or calendering thermoplastic
polyurethanes such as described in the aforementioned U.S. Pat. No.
3,214,411 to Saunders et al. After the polyurethane has been prepared in
sheet form, the sheet is cut to size and laminated with other sheeting
material. For motor vehicle glazing applications, the sheeting material to
which the polyurethane is laminated is usually a rigid transparent sheet
such as glass, polycarbonate or acrylic, with glass being preferred. A
detailed description of fabricating bilayer glass-polyurethane laminates
is found in U.S. Pat. No. 3,808,077 to Rieser et al., which description is
hereby incorporated by reference.
Besides extruding or calendering, the polyurethane can be shaped into sheet
form by casting and curing in place, such as described in U.S. Pat. No.
3,509,015 to Wismer et al. Casting and curing in place is applicable to
both thermoplastic and thermosetting polyurethanes and avoids any
additional laminating operations since the polyurethane can be cast and
cured in place adjacent to the desired substrate such as glass and be
strongly bonded thereto. Thus, casting and curing in place is not only a
method of shaping the polyurethane sheet but is simultaneously the
technique for laminating the polyurethane to another material.
Bilayer laminates of the present invention are prepared such that one major
surface of the polyurethane is exposed to the atmosphere. Usually this
involves laminating a bilayer structure comprising a layer of glass or
other rigid transparent material secured to a layer of polyurethane such
as described in the aforementioned U.S. Pat. No. 3,808,077. However, other
laminate configurations are obviously possible in which the exposed layer
of glass or other rigid transparent material is laminated to other layers
in such a fashion that the polyurethane layer remains exposed.
The bloom which develops on the exposed surface of the normally blooming
polyurethane interlayer develops as a surface haze and increases in
intensity as a function of time. The bloom initially is light and can be
removed from the surface of the polyurethane by gentle wiping with clean
dry cloths. However, the bloom will keep recurring upon continued exposure
of the interlayer to the atmosphere. Besides being unsightly and reducing
visible light transmittance, the bloom, if not removed early in its
development while it is light, will quickly become so heavy it will
practically be impossible to remove entirely.
Somewhat surprisingly, the present invention has determined that the
application of a layer of wax to the exposed surface of the polyurethane
interlayer will significantly retard blooming. For example, a freshly
laminated glass-polyurethane bilayer laminate will often show bloom on its
exposed surface within a few weeks after lamination. However, applying a
layer of wax to the exposed surface of the polyurethane immediately after
lamination will normally retard blooming for at least 6 months, after
which time the polyurethane merely has to be rewaxed to provide further
long-lasting protection. The phenomena of blooming is not only dependent
on the composition of the polyurethane, as mentioned above, but also may
be affected by the relative humidity. For example, when glass-polyurethane
bilayer windshields are exposed during a period of high relative humidity,
i.e., above 60 percent, the incidence of bloom is usually high. On the
other hand, when glass-polyurethane bilayer windshields are prepared
during a period of low relative humidity, i.e., below 40 percent, the
incidence of bloom is usually low.
The waxes which can be used in the practice of the invention are those
which can be deposited as a film which does not significantly reduce the
luminous light transmittance of the laminate, i.e., not more than about
one percent using Illuminant "C" as the light source and W. Beckman Quartz
Spectrophotometer, Model DK-2A, as the measuring instrument. The wax can
be either a natural or synthetic wax. The natural waxes include mineral
waxes, vegetable waxes and animal waxes. Examples of suitable natural
waxes include paraffin wax, decolorized monitan wax, double-bleached
ozoccenite wax, ceresin, carnauba wax and beeswax.
In addition to the natural waxes mentioned above, synthetic waxes are also
suitable in the practice of the invention. Examples of synthetic waxes are
fatty acids, such as myristic acid; fatty acid esters, such as
polyethylene glycol monostearate; aliphatic ketones, such as stearone and
laurone; amines and amides, such as octadecyl-hexadecyl amine and
octadecyl-hexadecyl amide; chloronaphthalene waxes such as a halowax;
synthetic mineral waxes such as Duroxin wax; synthetic animal waxes such
as synthetic beeswax. For a thorough description of waxes which may be
suitable in the practice of the invention, reference is made to Industrial
Waxes, H. Bennett; Volumes 1 and 2, Chemical Publishing Company, Inc., New
York, N.Y.
To apply the wax to the exposed surface of the polyurethane, the waxes are
usually emulsified in water or dissolved in solvents recited hereafter
before application. The water or solvent evaporates after application to
leave a waxy film which is then rubbed to give a polish or luster. Usually
one or more waxes are blended with one another or sometimes with resins to
provide the best results.
Wax emulsions are not emulsions in the strict sense of the word because an
emulsion is one immiscible liquid dispersed in another. Wax emulsions used
in the practice of the present invention are dispersions of a solid waxy
phase in an aqueous phase, which also may contain an organic solvent for
the wax. Flocculation of the particles is prevented by emulsifiers, soaps
and electrolytes in the aqueous phase. In addition to emulsifiers, soaps
and electrolytes, the wax emulsion usually contains various other
ingredients such as light stabilizers, anti-oxidants, cleaners to remove
old wax deposits and various other resins, such as polysiloxane, which
facilitate the smoothness and gloss of the surface film.
When a wax is to be emulsified, it is first melted and then combined with
water and the other ingredients present in the emulsion. The temperature
of the water should be above the melting point of the wax, and combination
with the wax should be accomplished with vigorous agitation. To assist in
emulsifying the wax, one or more of the waxes may be first saponified. It
is not necessary to effect complete saponification, but only to an extent
that will produce a good emulsification with the water.
Besides water emulsions, wax solutions can also be employed in the practice
of the present invention. However, waxes are only slightly soluble in most
commercial solvents and the selection of a solvent for the wax must be
governed in part by how the solvent affects the polyurethane. many of the
commercially available solvents, such as chlorinated solvents, ethers and
ketones, solubilize the polyurethanes, thus disqualifying these solvents
for use in the practice of this invention. Other solvents such as lower
aliphatic alcohols and esters are not particularly suitable in the
practice of the invention because they soften the surface of the
polyurethane, thereby lessening its abrasion resistance.
Solvents which can be used are those which are essentially harmless to
polyurethane, for example, aliphatic petroleum derivatives such as
alkanes, ligroin, petroleum ether, naphtha and mineral spirits. Since the
waxes are only at best moderately soluble in these solvents, a solvent for
the waxes can be considered as a substance which can put the wax in a
fluid state. The prime purpose of waxed solvents is to disperse and make
the wax more liquid so as to give it more suitable application properties.
When the wax is to be solubilized, it is first melted and then combined
with the solvent and other ingredients such as dispersion agents or
adjuvant resins present in the solution. When adding a solvent to a melted
wax composition, the solvent should be heated to a temperature above the
melting point of the wax to avoid crystallization of the wax upon cooling.
The solvent should be combined thoroughly with the wax at a sufficiently
slow rate of mixing so that the mixing does not splash the contents
unnecessarily or introduce air bubbles into the mixture. As with the wax
emulsions, the waxes can be mixed with one another or with various resins
to provide for optimum properties.
Wax emulsions and solutions are commercially available from many
manufacturers. These preparations usually contain mixtures of waxes
blended for economy and optimum results. They also usually contain light
stabilizers, anti-oxidants, cleaners to remove old waxes and any of a
number of resins, particularly polysiloxanes, to facilitate the smoothness
and gloss of the surface film. Examples of preferred commercially
available wax preparations are Turtle Wax Vinyl Top Wax, a mixture of
beeswax and paraffin wax dissolved in mineral spirits, and Westley's Vinyl
Top Lustre, a similar wax mixture emulsified in water. These commercial
wax preparations can be used as received from the manufacturer or combined
with additional organic solvent or water as desired. For example, a 0.1 to
4 percent by weight solution of Turtle Wax Vinyl Top Wax in ligroin makes
an excellent wax preparation for retarding bloom of glass-polyurethane
bilayer laminates.
To apply the wax to the exposed surface of the polyurethane, a wax emulsion
or a wax solution is first applied to a soft cotton pad so as not to
scratch the polyurethane and then applied to the polyurethane with a
swirling motion. There should be a fairly heavy wax deposit over the
entire area as indicated by a milky appearance to the windshield. At that
time, a clean soft polishing cloth is taken and the excess wax removed and
a thin hazy film with a swirl pattern but of approximately uniform
thickness throughout should be evidenced. When the film is being thinned
down, it begins to dry and after the film is totally dry, another clean
cotton cloth is used to buff or polish the film.
The resulting wax film is relatively thin, has a thickness preferably in
the range of at least approximately 0.25 microns, and not exceeding
approximately 25 microns. The minimum thickness is dictated by the need to
provide sufficient protection from blooming for the otherwise exposed
polyurethane surface, while the maximum thickness is limited to one which
does not significantly affect the optical quality of the laminate, that
is, the wax coating should not significantly reduce the luminous
transmittance of the laminate nor create objectionable optical distortion,
such as streaks, fuzziness or the like, in the laminate.
The wax preparations as described above are preferably applied directly to
the exposed surface of the polyurethane immediately after the laminate is
prepared. It has been found that windshields which have been waxed
immediately after a preparation will usually not show any evidence of
bloom for periods of at least 6 months. On the other hand, laminates in
which the exposed surface of the polyurethane has not been waxed often
show evidence of bloom within 2 or 3 weeks after their preparation.
Besides being applied to the exposed surface of the polyurethane
immediately after the laminate is prepared, wax emulsion-containing
cleaners or wax solutions can be applied to polyurethane laminates which
also show evidence of bloom. If the bloom is not too heavy, the wax
solutions will remove what bloom there is and provide a protective coating
to retard the development of any further bloom. If the windshield is
heavily bloomed, as much of the bloom as possible should be removed by dry
wiping and then applying the wax composition to the polyurethane surface.
If the first waxing does not remove the bloom, a second waxing usually
will be effective in removing the remaining bloom. However, if the bloom
is stubborn and will not be removed by rewaxing, the polyurethane laminate
should be heated to a temperature of approximately 125.degree. to
200.degree.F. (which approximates 51.67.degree. to 93.33.degree.C.) before
rewaxing. This heat treatment followed by waxing usually removes the most
stubborn bloom.
In the fabrication of laminates with exposed polyurethane surfaces,
so-called delamination lines sometimes form due to the working involved in
removing the release plate from the exposed surface of the polyurethane.
These delamination lines occur where the release plate does not readily
separate from the polyurethane, thus necessitating some prying action.
Along these delamination lines, bloom develops quite quickly and heavily.
To retard this type of bloom, it has been found beneficial to heat the
polyurethane to a temperature of approximately 125.degree. to
200.degree.F. (which approximates 51.67.degree. to 93.33.degree.C.) before
waxing. The prior heat treatment before the application of the wax assists
in retarding this type of bloom, whereas the same samples which were not
heat treated will show evidence of bloom more rapidly in spite of the
waxing application.
The invention can be used in the preparation of bilayer laminated windows
which can be used in such applications as vehicular closures, particularly
windshields, side lights and back lights for motor vehicles and aircraft.
The bilayer laminates can also be used for architectural glazing such as
windows, transparent walls and the like. The bilayer laminates comprise a
rigid transparent layer as glass laminated to a major surface of
polyurethane and a layer of wax covering the second major surface of the
polyurethane layer. Besides glass, other rigid transparent sheets could be
used such as polycarbonate or acrylic, but glass is preferred. The
thickness of the individual layer varies with the use intended but it
generally varies somewhat as follows. The rigid transparent sheet should
have a thickness of about 75 to 500 mils (which approximates 1.9 to 12.7
millimeters). For vehicular closure use, the thickness should range from
about 90 to 250 mils (which approximates 2.3 to 6.4 millimeters). The
polyurethane interlayer thickness in general should range in thickness
from about 1 to 75 mils (which approximates 0.025 to 1.9 millimeters).
For vehicular closure use, the thickness should range from about 2 to 50
mils (which approximates 0.5 to 1.3 millimeters). The thickness of the wax
layer has been discussed above.
EXAMPLE I
Six 12 inch by 12 inch (which approximates 300 millimeters by 300
millimeters) bilayer glass-polyurethane laminates were prepared in which
the glass was 1/8 inch (which approximates 3 millimeters) thick float
glass and the polyurethane layer was a polycondensate prepared from
4,4'-methylene-bis(cyclohexyl isocyanate), a hydroxy-terminated
polybutylene adipate diol having a molecular weight of about 2000 and
1,4-butanediol in a mole ratio of 3.3:1:2.4. The thickness of the
polyurethane was about 30 mils (which approximates 0.76 millimeters).
Three of the samples were waxed on their exposed polyurethane surfaces
with an aqueous wax emulsion commercially available as Westley's Vinyl Top
Lustre. The waxy phase contained about 5-10 percent paraffin wax,
approximately 15 percent beeswax, and about 75-80 percent
dimethylpolysiloxane, the percentages by weight being based on total
weight of paraffin wax, beeswax and dimethylpolysiloxane. The wax was
applied with soft cotton pads in a swirling motion as described above. The
three remaining samples were not waxed.
After a period of about two days, the samples which were not waxed bloomed,
that is, the exposed surface of the polyurethane had a hazy deposit,
whereas the waxed samples showed essentially no bloom, although in the
corner areas where the exposed surfaces of the polyurethane was
fingerprinted, there was a slight detection of bloom. The waxed samples
after one month showed no further evidence of bloom.
EXAMPLE II
Six 23 inch by 61 inch (which approximates 580 millimeters by 1550
millimeters) curved bilayer windshields prepared by the method described
in U.S. Pat. No. 3,808,077 to Rieser et al. mentioned above were prepared.
Each windshield comprised a layer of 1/8 inch thick (which approximates
3.2 millimeters) float glass and a 30 mil thick (which aproximates 0.76
millimeters) layer of polyurethane such as described in Example I above.
Two of the bilayer windshields were heat treated at a temperature of about
175.degree.F. for 30 minutes immediately after fabrication. Two of the
windshields were waxed immediately after fabrication on their exposed
polyurethane surfaces with a commercially available paraffin wax and
beeswax mixture emulsified in mineral spirits sold commercially as Turtle
Wax Vinyl Top Wax. The waxing procedure was that generally described in
Example I. The wax contained about 50 percent by weight beeswax, about
40-45 percent by weight paraffin wax and 5-10 percent by weight
dimethylpolysiloxane, the percentages by weight being based on total
weight of beeswax, paraffin wax and dimethylpolysiloxane. The two
remaining windshields were both heat treated, cooled and then waxed as
described immediately above. The six windshields were then exposed indoors
at normal atmospheric conditions. The windshields which were only heat
treated and not filmed with wax developed bloom on the exposed surface of
the polyurethane after a period of about two to three weeks, whereas the
other windshields showed no evidence of bloom. After about 3 to 4 weeks,
the windshields which were only filmed with wax showed a slight evidence
of bloom where the exposed surface of the polyurethane was handled during
lamination. The windshields which were both heat treated and immediately
thereafter filmed with wax showed no evidence of bloom anywhere on the
exposed surface of the polyurethane after a period of eight months.
EXAMPLE III
Five 12 inch by 12 inch (which approximates 300 millimeters by 300
millimeters) glass-polyurethane laminates were prepared such as described
in Example I. The five samples were then placed outdoors without any
treatment and after two weeks showed no evidence of bloom. Four of said
five laminates were treated and the exposure continued. The first laminate
was a control and was given no treatment whatsoever. The second laminate
was merely given a dry wiping with a cotton cloth. The third laminate was
washed with a one percent detergent solution (JOY detergent). The fourth
laminate was washed with commercially available WINDEX solution and a
fifth laminate was polished with the Turtle Wax Vinyl Top Wax of Example
II dissolved in ligroin (4 percent by weight Turtle Wax solution based on
weight of solvent).
After one additional week of outdoor exposure, all the samples appeared
clear with no evidence of bloom. The samples were then treated as
described above.
After a further exposure period of one week, the samples were again
investigated. The first laminate was still clear with no evidence of
bloom. The second laminate which had been dry wiped had now developed a
heavy bloom, which could be mostly wiped off by dry wiping. The third
laminate which had been washed with the aqueous detergent solution also
had developed bloom, but this bloom could be mostly removed with an
additional washing. The fourth laminate which had been washed with WINDEX
bloomed but the bloom could mostly be removed with an additional washing
with WINDEX. The fifty laminate which had been waxed with a ligroin
solution of Turtle Wax was completely clear and was then again filmed with
wax. After an additional 10 days, the first laminate had developed a heavy
dirt stain. The second, third and fourth laminates had developed a heavy
bloom which could not be completely removed by the retreating. The fifth
laminate which had been rewaxed with ligroin solution of Turtle Wax was
perfectly clear. The laminates were then retreated as described above.
Three months later the samples were then again investigated. The first
laminate had a heavy dirt stain. The second, third and fourth laminates
all developed a heavy bloom which could not be removed by retreating. The
fifth laminate was perfectly clear.
The form of the invention shown and described herein represents an
illustrative embodiment thereof and various changes other than those
enumerated previously may be made without departing from the gist of the
invention as defined in the claimed subject matter which follows.
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