Note: Descriptions are shown in the official language in which they were submitted.
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583-219-0
TITLE OF THE INVENTION
ADHESIVE FOR METHACRYLATE PLASTICS
BACKGROUND OF THE INVENTION
Field of the Invention:
The present invention relates to adhesives for
methacrylate plastics in the form of volatile organic fluids
or in the form of adhesive varnishes consisting of a solution
of a polymer in such a fluid.
Description of the Background'
EP-B 867 application describes an adhesive for
methacrylate plastics which contains nitromethane or
nitroethane as a solvent. Optimum adhesive properties are
found when mixtures of nitromethane and dichloromethane are
employed.
Task and Solution
Dichloromethane and other chlorinated hydrocarbons are
considered toxic and carcinogenic. For this reason, there is
a need for an adhesive for methacrylate plastics that does not
contain chlorinated hydrocarbons, but has properties
comparable to those of nitromethane-dichloromethane mixtures.
Requirements include short superficial dissolving times and
low stress corrosion cracking, as well as low toxicity.
SUMMARY OF THE INVENTION
Accordingly, one object of the present invention is to
provide an adhesive material for methacrylate plastics which
achieves bonding of such plastics of improved strength, which
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adhesive does not contain a chlorinated hydrocarbon component.
Briefly, this object and other objects of the present
invention as hereinafter will become more readily apparent can
be attained in a method of bonding methacrylate plastic to
each other and other plastic materials by applying adhesive of
a low molecular weight alkyl formate or a volatile adhesive
mixture or adhesive varnish containing a low molecular weight
alkyl formate to at least one bonding surface of plastics to
be bonded to each other, at least one of said plastics being a
methacrylate plastic, and bonding the plastic surfaces to each
other.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
The present invention is directed to a low-molecular
weight alkyl formate as an adhesive or as an additive to
volatile adhesive mixtures, or as an additive to adhesive
varnishes for bonding parts of methacrylate plastics to each
other or with other plastics. A preferred aspect of the
invention is the use of a mixture of at least 15 wt.% of a
low-molecular weight alkyl formate and up to 85 wt. % of other
volatile organic fluids free of chlorinated hydrocarbons and
having a boiling point not exceeding 200°C, particularly
nitromethane and/or nitroethane.~
During the superficial dissolving process, the swelling
plastic surface exerts a significant swelling pressure on the
material layer beneath it, possibly resulting in tearing. The
adhesive of the present invention has the surprising effect
that swelling, softening and dissolving processes interact in
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such a way that tearing is largely suppressed, and tears that
do form are largely closed again due to the effect of the
adhesive. Although not all of the advantageous properties of
the known nitromethane-dichloromethane mixtures are obtained,
the adhesives according to the invention, because of their
complete absence of chlorinated hydrocarbons and an improved
strength of the adhesive bonds, represent an important
improvement in the field of methacrylate plastics bonding.
Volatile Adhesives
l0 A volatile adhesive of the present invention is one which
superficially dissolves plastic surfaces and brings about
adhesion, while the adhesive completely evaporates again.
These solvents are suitable for bonding parts with an exact
fit relative to each other. It is advantageous that they have
boiling points from 30 to 150°C (under normal pressure).
Suitable volatile solvents include alkyl formates such as
methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl, n-amyl,
isoamyl, and n-hexyl formates. Because of their high
volatility, methyl fornaate and ethyl formate are particularly
preferred.
Mixtures of at least 15 wt. % of a low-molecular alkyl
formate and up to 85 wt. % of other volatile organic fluids
free of chlorinated hydrocarbons having a boiling point not
exceeding 200°C have various advantages compared to the alkyl
formates by themselves. While methyl and ethyl formate alone
may result in tearing, particularly when bonding plastic parts
with internal tensions, this risk is much lower when using
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mixtures with nitromethane and nitroethane. Mixtures of 25 to
75 wt. % methyl formate and/or ethyl formate and 75 to 25 wt.
% nitromethane and/or nitroethane are preferred. The addition
of other solvents, such as ketones or esters, e.g. ethyl
acetate, are also within the scope of the invention.
By adding 0.5 to 10 wt.% of alcohols having boiling
points between 70 and 280°C, improvements in the quality of
the adhesive joints can be achieved. Because of lower
volatility, they are temporarily concentrated during the
drying of the adhesion joint at their surface. This prevents
flaws in the surface that may result from premature skin
formation or from condensation of water from humid ambient air
because of cooling caused by evaporation. An addition of n-
butanol, e.g. prevents wrinkling; the addition of
phenoxyethanol prevents the clouding of the adhesive joint in
the case of high air humidity.
Adhesive Varnishes
Adhesive varnishes are adhesives with a polymer content
which remains in the adhesive joint after the evaporation of
20. the volatile components and produces the bond between the
bonded parts. Adhesive varnishes are advantageous if the
parts to be bonded cannot be form-fitted together, and if
during the bonding, distances of e.g. 0. 1 to 1 mm must be
bridged.
Like any adhesive resin, the dissolved polymer must have
a sufficient adhesiveness to the material of the parts to be
bonded, and also a sufficient internal strength. These
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conditions are best fulfilled if the dissolved polymer itself
is a methacrylate plastic, e.g., polymethylmethacrylate.
Thermoplastic polymers are preferred. Other suitable adhesive
resins are polyvinyl acetate, polyvinyl ether, or cellulose
5 acetobutyrate. The polymer content has an upward limit based
on the viscosity of the solution. The solution viscosity at
20°C generally should not exceed 10,000 mPa s. It is
advantageous to use approximately 3 to 30 wt. % of a polymer
which results in a viscosity~of the solution ranging from 500
to 2,000- mPa s.
Bonding of Methacrylate Plastics
The adhesives to be used according to the invention
primarily serve to bond parts of methacrylate plastics with
each other or with parts of other plastics such as polyvinyl
chloride, polystyrene, butadiene-styrene plastics;- ABS
plastics, or cellulose esters (CAB). In order to bond
methacrylate plastics to other materials such as wood, paper,
cardboard, leather, metal, ceramic material, glass, porcelain,
the adhesive varnishes of the invention are preferably used.
A suitable methacrylate plastic is in particular acrylic
glass. Examples are poured, extruded, or injection-molded
polymethylmethacrylate or copolymers of the methylmethacrylate
with no more than 10 wt. % of other ethylene-unsaturated
radically polymerizable monomers, particularly low-molecular
alkyl acrylates. The methacrylate plastics also include
methylmethacrylate-acrylonitrile copolymers,
methylmethacrylate-styrene-malefic anhydride copolymers,
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shock-resistant modified polymethylmethacrylate, and the like.
To bond with volatile adhesives, the closely fitting
bonding surfaces are wetted with the adhesive and joined in
the desired position. The adhesive also can be applied to the
edge of the contact surface, so that it is drawn by itself
into the gap between the bonding parts because of capillary
force. When using adhesive varnishes, it is sufficient if one
of the plastic surfaces to be bonded is coated with a
thickness sufficient to fill the joint, and to join the parts
1Q together.
The superficial dissolving time, during which the parts
to be bonded are easily moved relative to each other, usually
lasts approximately 30 to 90 seconds. After this, a
sufficient stability is achieved for holding the parts to be
bonded in the desired position even without support. The
adhesive joint is then left to stabilize, preferably under a
slight pressure, because of evaporation of the volatile
adhesive components. After a drying time of 1 to 4 hours, a
high adhesion strength is obtained. It clearly increases
during the course of 1 to 2 weeks, or preferably by tempering
for 2 to 6 hours at 60 to 90°C.
Having generally described this invention, a further
understanding can be obtained byvreference to certain specific
examples which are provided herein for purposes of
illustration only and are not intended to be limiting unless
otherwise specified.
Determination of Adhesive Strength
The following table contains combined tension and shear
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resistance values measured on bonded parts of extruded and
poured acrylic glass with the following adhesives of the
invention:
A) Volatile adhesive (wt. %), viscosity 500-700 mPa s:
44% ethyl formate, 44% nitroethane, 4 %
phenoxyethanol, 3 % n-butanol, 5 % acetic ester;
B) Adhesive varnish (wt.%), viscosity 750- 1,000 mPa s:
39 % ethyl formate, 39 % nitroethane, 4 %
phenoxyethanol, 3 % n-butanol, 5 % acetic ester, 10%
polymethylmethacrylate.
These examples of the invention are compared with known
adhesives C and D with the compositions:
C) Volatile adhesive (wt. %): 52 % dichloromethane, 40
% nitromethane, 5 % phenoxyethanol, 3% ethanol
15 D) Adhesive varnish (wt.%): 44 % dichloromethane, 36 %
nitromethane, 7 % phenoxyethanol, 3% ethanol, 10%
polymethylmethacrylate.
The combined tension and shear resistance is measured by
20 subjecting a 2 cm2 large adhesive area to tensile stress in the
direction of the adhesive area. The first measurement is
taken after drying at room temperature for 5 days, the second
measurement after tempering over 5 hours at 80°C.
Bonded plastics Comb. shear resistance in mPa
tension
and
25 -adhesive after 5 days after tempering
Cast
-pe~rree~- acrylic glass (PLEXIGLAS GS) Nov. 24
94
- volatile adhesive A 31.3 42.3 ,
- adhesive varnish B 31.0 46.0 ~~"
3
-
0 C 30.3 33.9
volatile adhesive
- adhesive varnish D 27.6 31.7
r
Extruded acrylic glass (PLEXIGLAS XT)
- volatile adhesive A 39.3 51.6 Nov. 24
94
- adhesive varnish B 40.1 54.2 ,
35 - volatile adhesive C 37.9 43.2
-
dh
i
a D 34.9 43.6
es
ve varnish
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Bonding Under Bending Stress
Two strips measuring 85 x 10 x 2 mm of extruded acrylic
glass were placed on top of each other with their surfaces and
connected at the ends with lead adhesive tape. The strips
were then elastically spread apart by way of spacers placed
between them in the center, resulting in a uniform bending
radius of 500-600 mm, corresponding to a bending stress of 6
Mpa. The strips subject to bending stress were glued with
their edges onto a plane plate of extruded acrylic glass.
After drying, the tearing and bubbling in the adhesive area
and on the convex outside surfaces were evaluated.
For bonding, the volatile adhesive A and the adhesive
varnish B according to the invention, and for control purposes
the liquid adhesive C and adhesive varnish D based on
1~ dichloromethane/nitromethane were used. Findings:
A: no tears in bonded area,
many fine tears along edge, few bubbles
B: no tears in bonded area,
many fine tears in outer bonding edge,
_C: no tears in bonded area,
very fine tears, in part closed by dissolving, on
edge
D: no tears, many bubbles.
Having now fully described the invention, it will be
apparent to one of ordinary skill in the art that many changes
and modifications can be made thereto without departing from
the spirit or scope of the invention as set forth herein.
