Note: Descriptions are shown in the official language in which they were submitted.
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The present invention relates to a method of bonding
polymethyl methacrylate resins.
In order to bond methacrylate ester resins to one
another or to other resins, volatile organic solvents
which can dissolve these resins may be used as adhesives.
The solvents swell or dissolve the boundary layers of
the resin articles to be bonded together and give a
visually essentially homogeneous bond under favourable
conditions after evaporation.
There are many organic solvents for polymethacrylates,
but only a few of them can be used as adhesives.
Desirably the solvent is intended quickly to dissolve
the resin faces to be bonded together, rapidly to
provide sufficient stability under load in order to
exclude the inadvertent displacement of the bonded parts
and to harden into a visually clear and bubble-free
bond having tensile strength. Most organic solvents
fulfil only some of these requirements.
Most solvents rapidly generate a high swelling
pressure in the region near the surface. This high
pressure, especially in the case of highly stressed,
for example3 curved parts, causes so-called stress
cracking which originates at right angles to the
adhesive face. Only a few solvents, e.g. some aliphatic-
chlorinated hydrocarbons, produce no stress cracking.
However, most of these solvents evaporate so quickly in
the air that as a result of the drop in temperature due
to evaporation of the solvent, atmospheric moisture is
precipitated in the adhesive joint and causes permanent
clouding~ Air bubbles also often arise as a result of
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the shrinkage of the swollen plastic surface during
gradual evaporation of the solvent.
In all cases, the tensile strength of the adhesive
joint is lower than that of the bonded resins themselves,
so that the adhesive joint ruptures under loading.
In view of these various factors, methylene chloride
(dichloromethane) has proved hitherto to be the
adhesive best suited to methacrylic ester resins. It
dissolves the resin quickly and results within 10 seconds
in a bond stable under load, so that frequently even
desired corrections of the relative position of the
bonded resin articles are no longer possible. After 2
days the final strength of the bond is reached, but the
margin of the adhesive face may be slightly cloudy,
which can generally be avoided only if the operation
is carried out in an atmosphere having low absolute
atmospheric moisture.
It is an object of the invention to provide a
method of bonding polymethyl methacrylate resins using
materials having advantageous adhesive properties.
According to the present inven~ion we provide a
method of bonding a surface of an article made of a
polymethyl methacrylate resin to another surface which
comprises treating at least one of said surfaces with an
adhesive composition comprising at least one nitroalkane
selected from nitromethane and nitroethane, contacting
the said surfaces and drying the said surfaces to form
a bond between the said surfaces.
From experiments which we have carried out in
accordance with the present invention, we have found
~ 4 -
that the use of nitromethane or nitroethane tends to
obviate the disadvantages observed with previous solvents,
the resulting bonds being free of clouding and bubbles
and having increased tensile strength.
The method according to the present invention may be
employed for example either to bond together two articles
both made from a polymethyl methacrylate resin or to bond
one such article to an article made from another material.
In the method according to the present invention, the
adhesive composition is advantageously applied to the
article made from a polymethy~ methacrylate resin in the
case when the second-mentioned article is made from a
different material.
The term "polymethyl methacrylate resin'~ is used
herein to denote not only homopolymersof methyl
methacrylate but also copolymers containing a predominant
proportion, especially more than 80% by weight, of units
of methyl methacrylate and a smaller portion of one or
more unsaturated comonomers copolymerisable therewith,
such as other methacryla~e esters, esters of acrylic
acid, styrene, acrylo- or methacrylonitrile, vinyI
chloride, vinylidene chloride or vinyl esters. The
resin to be bonded should be soluble or at least strongly
swellable in suitable organic solvents, i.e. they should
be substantially non-cross-linked. The molecular weight
of the resin is only of minor importance for a good
- bonding effect. Consequently, non-meltable bul~
polymers can be bonded just as well as extruded or
injection-moulded resins. The bonding of injection
mouldings is one of the special advantages of the use of
L7~5~i
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nitromethane or nitroethane as adhesives according to
the invention.
Nitroethane acts on polymethyl methacrylate resins
rather more slowly than nitromethane. Thus, the
period during which the relative position of the articles
to be bonded can be corrected after joining is thereby
` extended. Due to its higher boiling point nitroethane
evaporates more slowly than nitromethane, so that the
drop in temperature due to evaporation is reduced and
the risk that the adhesive joint will be clouded by
condensation water in the case of high atmospheric
moisture is reduced still further. This is especially
true in the case where nitroethane is used in
admixture wi-th other solvents such as methylene chloride
as adhesive or adhesive additive. Nitroethane can be
employed for all the applications described in any
desired mixture ratio with nitromethane. The above-
mentioned differences regarding the speed of action and
of evaporation are then manifested in proportion to the
mixture ratio
Although nitromethane or nitroethane can each be
used alone as adhesive, it is preferred to use mixtures
of nitromethane and/or nitroethane with other volatile
organic liquids with boiling points of not more than
200, preferably below 150. These other liquids can
themselves be solvents for the resins to be bonded and
include for example ketones, esters, ethers and
aliphatic chlorinated hydrocarbons, e.g. acetone,
cyclohexanone, methyl acetate, butyl acetate, tetrahydro-
furan, chloroform, carbon tetrachloride, dichloromethane,
dichloroethane, etc. These solvents ~ se have the
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above-described disadvantages as adhesives, but in
admixture with at least 30% by weight of nitromethane
or nitroethane (calculated on the weight of the mixture)
these disadvantages are considera~ly reduced. Thus,
stress cracking clearly decreases and adhesive strength
is increased. The following table sets out the adhesive
tensile strength of bonded articles of extruded polymethyl
methacrylate resin 14 days after bonding:-
Solvent Adhesive Mixture with Adhesive
tensile nitromethane, tensile
strength weight ratio strength
N/mm2 of solvent to N/mm2
nitromethane
. _ . . _ . .
Acetone 23.9 40 : 60 34O0
Cyclo- 1605 20 : 80 35.2
hexanone
Toluene 19.8 40 : 60 32.4
Tetra- 22.8 60 : 40 33.3
hydro-
furan
Dichloro- 35O9 60 : 40 42.4
methane
(methylene
chloride)
1,2-di- 34.2 60 : 40 36.4
chloro-
ethane
. . .
Under the same conditions nitromethane alone provides
an adhesive tensile strength of 36.6 N/mm . The
chlorinated hydrocar~ons which alone give similarly high
strengths ena~le, in a mixture with nitromethane or
7B.~
nitroethane, higher strength values to be achieved than
with each of the components alone. Mixtures of nitro-
methane or nitroethane and dichloromethane in a weight
ratio between 30:70 to 80:20 are preferably used in the
method accorcling to the invention.
Nitromethane or nitroethane can also be used in
admixture with organic liquids which are not solvents
for the resins to be bonded and which do not develop
their own adhesive effect. They can be selected in
order to extend the swelling time of the adhesive
composition or to prolong the period after which stability
under load is reached, which is sometimes desirable in
the case of difficult bonds. These additives can be
less volatile than nitromethane or nitroethane, in
which case the lower speed of evaporation prevents the
formation of bubbles in the adhesive joint and clouding
due to condensation of atmospheric moisture.
Examples of such mixtures and the strengths obtained
in the bonding of polymethyl methacrylate resin are
given in the following table:
Solvent Additi~e Weight Adhesive
ratio (solvent strengt~
- - - -to additive) in N/mm
Nitromethane Ethanol 80:20 31.5
Nitromethane Trichlorethane 60:40 27.8
Nitromethane Dimethylformamide 70:30 31.3
Nitromethane + Phenoxyethanol 95: 5 43.0
dichloromethane
(40:55 parts
by weight)
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When nitromethane or nitroethane or a mixture
thereof with volatile organic liquids are used as
adhesive, the adhesive bond is formed of the material
of the resin to be bonded. It is, however, also
possible to use in the adhesive composition so-called
adhesive lacquers which contain a dissolved polymer in
the solvent. After the solvent has evaporated, the
polymer forms the adhesive bond. Such polymer additives
include e.g. polymethyl methacrylate, polyvinyl acetate,
polyvinyl ether or cellulose acetobutyrate. In practice,
an upper limit is set on the proportion of polymer by
the viscosityof the solution. The solution viscosity
at 20 should generally not lie above 10,000 mPa.s.
Advantageously, solutions containing approximately 3
to 30% by weight of a poly~er are employed, such
` polymer concentrations providing viscosities in the
region of 500 to 2,000 mPa.s for the solution,
The method according to the invention is of
particular application for bonding two articles each made
of a polymethyl methacrylate resin. In order to bond
a polymethyl methacrylate article to an article made
of another material such as wood, paper, cardboard,
leather, metal, ceramic material, glass, porcelain or
resins of other kinds, e.gO phenol resin or aminoplast
resin moùldings, polycarbonate, PVC, urethane foam
materials and the like, adhesive lacquers are advanta-
geously used, if nitromethane or nitroethane alone or
mixtures thereof with volatile organic liquids, cannot
give the desired adhesive strengthO
The bonding of extruded polymethacrylate resin
11~7~
g
articles to one another is a preferred area of
application of the method of the invention. Cast
-polymethyl methacrylate articles can similarly be
bonded together. Such articles are preferably prepared
from homopolymers of methyl methacrylate or copolymers
of rnethyl methacrylate with at most 10% by weight of other
unsaturated monomers. The use of nitromethane or
nitroethane in a mixture with dichloromethane is
preferred for this purpose.
The second table set out above refers to the use of
a mixture consisting of 40 parts by weight of
nitromethane and 55 parts by weight of dichloromethane
to which 5 parts by weight of phenoxyethanol have been
added. The use of such a combination of ingredients with
the optional substitution of nitroethane and/or the
optional variation of the relative amounts of the
ingredients is advantageous since, when it is applied,
clear adhesive joints are obtained even when working in
an atmosphere with a high relative humidity. The same
effect can be achieved i;f phenoxyethanol is replaced
by other glycol ethers such as methyl glycol or
alkyldiglycol ether. To obtain a high adhesive strength,
these additives should be employed in quantities up to
10% by weight relative to the total solvent mixture.
The following Examples which illustrate the invention
describe a bonding operation using nitromethane and
another bonding operation using nitroethane under
otherwise identical conditions. The strength result with
nitromethane is given first, followed by the value
obtained with nitroethane.
~1~L7~35~
- 10 -
E~
The longitudinal sides of two test-pieces (10 mm x
270 mm x 60 mm) of polymethyl methacrylate are dipped
for approximately 60 seconds in nitromethane or nitro-
ethane and joined together. Within 120 seconds the bondis stable under load, i.e. it is no longer displaceable.
By the loading of a joining part with a weight of 150
g/cm the adhesive faces are brought into intimate
contact~ The load is maintained for 24 hours, After
storage at room temperature for 13 days without loading
10 test-pieces (8 mm x 20 mm x 120 mm) are produced from
the bonded article and the tensile strength is determined
by reference to DIN 53455. Result: 36~6 N/mm and
34~8 N/mm respectively.
Example 2
The bonding operation described in Example l is
carried out except that a mixture of methylene chloride
and either nitromethane or nitroethane (60/40) is used
as solvent adhesive. The test-piece is stable under
load within 20 seconds. The tensile strength after
storage for 14 days is 43.7 N/mm and 36.4 N/mm
respectively.
Example 3
The bonding operation described in Example 1 is
carried out except that a solvent mixture of n~ethylene
chloride/nitromethane or nitroethane/phenoxyethanol
(55/40/5) is used for the operation. The adhesive joint
is completely clear and free of bubbles. After storage
for 14 days at room temperature a strength of 43 N/mm2
and 33.7 N/mm2 respectively is achieved.
Ex~e~
S The test is conducted as described in Example 1
except that the solution applied is one of 30 parts of
polymethyl methacrylate in 70 parts of a solvent mixture
consisting of methylene chloride/nitromethane or
nitroethane/phenoxyethanol (57/40/3). The bond is stable
under load after 3 minutes. After storage for 14 days a
strength of 37 N/mm and 31.8 N/mm respectively is
obtained.
