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BACKGROUND OF THE INVENTION
The glue and cement art is a mature technology. Many and varied glues and
cements have been described and much speculative disclosure has been made
about such compositions. In spite of, or perhaps because of the abundance
of information available about glues and cements, needs for improved
compositions continue to exist. It has become extremely difficult to find
the necessary information about suitable cements for specific purposes.
The need for safer cements has become more apparent as industry and the
consumer have become more aware of the potential toxic liabilities of
various glue and cement solvents, but these cements must effectively bond.
In view of this need for increased safety, certain cements have utilized
cyclic 5- or 6-membered ring lactam solvents such as
N-methyl-2-pyrrolidone.
U.S. Pat. No. 3,404,117 of Uffner describes an adhesive film-forming
composition that, when in the dry state, is insoluble in either water or
dry cleaning solvents. The adhesive comprises a blend of one of a certain
class of fluorinated copolymers and a plasticizer in a solvent. Some of
the solvents claimed are N-methyl-2-pyrrolidone and aliphatic alcohols.
G.B. Pat. No. 1,572,481 describes an ". . . adhesive containing polyvinyl
alcohol or ethylene/vinyl alcohol copolymer, a crystalline solvent for the
polymer and a viscosity-reducing diluent." The viscosity-reducing diluents
include polyhydric alcohols.
U.S. Pat. No. 2,616,868 of Heisenberg and Kleine describes liquid
compositions for solutions of polymers and copolymers of vinyl chloride
which are used to spin threads. In their description of the prior art,
Heisenberg and Kleine point out that there are a series of lactones and
lactams which, according to the literature, are " . . . suitable as
solvents for polymers and copolymers of vinyl compounds."
Fogle, et al, U.S. Pat. No. 4,152,313, describe an adhesive composition of
" . . . vinyl acetate-ethylene emulsion and an admixture of toluene and
N-lower alkyl substituted pyrrolidone."
Still other adhesives for use in joining plastics, such as plastic pipes or
the like, are known and generally comprised of polymers such as
poly(acrylonitrile-butadiene-styrene) (ABS) or poly(vinylchloride) (PVC)
in a solvent or a blend of solvents such as methyl ethyl ketone, toluene
and/or tetrahydrofuran.
Still other adhesives for use in pipes or the like are known which comprise
a suitable polymer such as ABS or PVC and a solvent mixture consisting
only of lactones or lactams, and in particular, preferred lactams such as
N-methyl-2-pyrrolidone (NMP). These formulations perform well in warm
weather and at elevated temperatures, but their properties as bonding
agents are less acceptable under low temperature conditions.
Generally, the requirements for solvent cements for use in the plastic
industry require that certain strength criteria are met and that the
solvents be suitable for use without causing undue user health problems.
The standards for solvent cements for use in the United States are
specified in Bulletin ASTM D2564-80, published by the American Society for
Testing Materials, 1916 Race Street, Phila. Pa. The ASTM Specification for
"Solvent Cements for Polyvinyl Chloride Plastic Pipe and Fittings"
specifies that the solvent cement should have certain minimum resin
content and meet certain minimum requirements for viscosity and strength.
For example, the specified minimum resin content for cement for PVC pipe
is 10% by weight of the solution and the lap shear strength when tested in
accordance with the test procedures requires that the joint between two
surfaces have a strength of at least 250 psi after a 2 hour curing time,
at least 500 psi after a 16 hour curing time, and at least 900 psi after a
72 hour curing time.
Since, generally, these solvent cements are made from flammable liquids,
the specification requires that the solvent should be kept away from
sources of ignition and, in addition, ventilation should be maintained not
only to minimize fire risk, but to minimize breathing of solvent vapors.
In addition, one should avoid direct contact of the solvent cement with
the user.
One of the preferred adhesive solvents is N-methyl-2-pyrrolidone. While it
is flammable, it has a relatively high flashpoint of 204.degree. F. and,
according to the manufacturer, it has a low order of oral toxicity and
only produces mild topical effects. Because of these characteristics,
N-methyl-2-pyrrolidone is one of the better or safer solvents to use in
the solvent cements. However, when N-methyl-2-pyrrolidone alone is used in
combination with resins such as ABS or PVC a specified by the ASTM Testing
Standards, the solvent adhesive may not pass the lap shear strength
requirements.
SUMMARY OF THE INVENTION
The present invention provides novel solvent cements which provide and
combine acceptable characteristics in their properties of flammability,
safety to the user, environmental acceptability, and bonding strength.
The invention relates to cements comprising a combination of a solvent
which is a 5- or 6-membered ring lactam and a cosolvent which is one or
more lower alkanols and/or one or more lower alkyl esters of lower
alkanoic acids and, optionally, include a water-insoluble polymer such as
poly(vinylchloride) or poly(acrylonitrile-butadiene-styrene).
The invention also relates to a method for forming a cemented juncture
between surfaces of articles comprising primarily water-insoluble polymers
such as PVC and/or ABS which comprises applying a cement of the invention
to at least one of the surfaces to be joined, mating the surfaces to be
joined, and allowing the juncture to form.
This invention also relates to the use of thixotropic, e.g.,
viscosity-adjusting agents, in combination with the solvent adhesives of
the invention.
DESCRIPTION OF THE PREFERRED EMBODIMENT
In my solvent cement, I have found that although certain solvents when used
alone have insufficient adhesive properties to produce a solvent adhesive
of sufficient strength to meet the ASTM lapshear test requirements, when
combined and added to a polymer resin, these combinations produce a
solvent adhesive which exceeds ASTM lapshear strength requirements. More
particularly, I have found that the combinations of cyclic 5- or
6-membered ring lactams such as N-methyl-2-pyrrolidone and either lower
alkanols such as methyl alcohol or lower alkyl esters of lower alkanoic
acids such as ethyl acetate, alone or together with resins such as ABS or
PVC, produce a solvent cement which meets or exceeds the lapshear strength
requirements specified in the ASTM Test Standards. This is particularly
surprising in view of the fact that methyl alcohol or ethyl acetate alone
or together with ABC or PVC do not provide good solvent cements.
Suitable 5- or 6-membered ring lactams for use in the cements of the
present invention include N-phenyl-2-pyrrolidone, 2-piperidone,
2-pyrrolidone, N-ethyl-2-pyrrolidone, N-methyl-2-pyrrolidone, and
equivalents thereof. N-Alkylated lactams, particularly N-methylated
lactams, are preferred. Presently preferred is N-methyl-2-pyrrolidone
because of its ready availability and relatively low cost.
Suitable lower alkanols for use in the cements of the invention are
preferably alkanols of four carbon atoms or less such as methanol,
ethanol, isopropanol and n-butanol. Methanol is presently preferred
because of its ready availability and low cost.
Suitable lower alkyl esters of lower alkanoic acids are preferably esters
of lower alkanols as defined above and lower alkanoic acids of one to four
carbon atoms such as methyl acetate, ethyl acetate, ethyl formate, ethyl
propionate and the like. Ethyl acetate is presently preferred.
The polymer or polymer mixture dissolved in the solvent mixture to form the
cement of the invention may be any water-insoluble synthetic organic
polymer which is soluble in the solvents to provide an effective cement.
Suitable polymers include PVC, ABS, polycarbonate, polystyrene,
polyacrylates, cellulose acetate, polyacrylamide, polyamide. and the like.
Of these, PVC and ABS are of most interest due to their widespread use.
The polymer or polymer mixture dissolved in the solvent mixture to form
the cement of the invention may be polymer waste or freshly prepared
polymer. Generally, the polymer or polymers in a cement are identical, or
at least chemically similar, to the polymer(s) to be cemented.
In general, I have found that if methyl alcohol is used, the alcohol
maximum percentage should not exceed 20% by weight or jelling occurs, and
minimum percentage rate of alcohol should be approximately 3% by weight or
more to provide a workable range for the adhesive when the resin content
is about 12%. As the percentage of alcohol decreases in the mixture, it is
still usable; however, the lowering of the alcohol content has a
diminishing effect on the effectiveness of bonding by the solvent
adhesive. It is presently preferred to use methanol-based cements of about
78% NMP, 10% methanol, and 12% resin.
When ethyl acetate is used in the solvent adhesive, the workable range of
ethyl acetate ranges from a maximum of 51% by weight of ethyl acetate to a
minimum of approximately 3% of ethyl acetate when the resin content of the
cement is about 13%. As a general rule, the lesser the amount of ethyl
acetate, the lesser the effectiveness of the strength of bonding by the
solvent adhesive.
When the cements of the invention are a combination of NMP, ethyl acetate
and a resin such as PVC or ABS, it has been found that gellation of the
cements, particularly upon aging, can occur when the percent of resin is
10 to 15% and the amount of ethyl acetate significantly exceeds 50%, for
example, when it is 60% or more. When the resins are omitted from the
formulations, amounts of ethyl acetate can easily be 75%, and amounts as
high as 90% have been successfully used.
In general, the solvents such as methyl alcohol and ethyl acetate have been
found to decrease cure time of the cement so that one can obtain higher
structural strengths at lower cure times.
In a further preferred embodiment of the invention small amounts of a
thixotropic agent, e.g., colloidal silica, which provides viscosity
control, and optionally, a small amount of an inert metal salt are added
to the cement of the invention.
Suitable thixotropic agents are added in relatively small amounts, for
example, from about one to five percent by weight of the total
formulation. Preferably, they are added in amounts of two to three
percent. These agents have been found to be useful for viscosity control
in the cements of the invention, although they also increase the
complexity and the cost of the cements and are, therefore, most useful in
specialized situations where the incremental cost differential is
justified by the improved properties.
It has been found that the cements of the present invention improve in
quality, i.e., they provide improved bonding, as they age. By aging, I
mean standing on the shelf before use.
In general, small amounts, i.e., generally less than five percent, of inert
diluents such as metal salts can be added to the cements of the invention
without deleterious effect. For example, from one to two percent of sodium
chloride was added to several formulations of the invention and these
cements had excellent bonding properties.
The following are typical examples of mixtures of the present invention.
EXAMPLE 1
Approximately 78.3% by weight of N-methyl-2-pyrrolidone was added to 5.7%
methyl alcohol with the balance being ABS resins. After mixture, the
solvent cement was applied to test samples and cured in accordance with
the ASTM Standards and subjected to shear tests at 2 hour, 16 hour, and 72
hour intervals. Two samples of each were tested and all of the samples
exceeded the test requirements.
EXAMPLE 2
Example 1 was repeated except PVC resins were used and, again, all the test
results for the 2, 16, and 72 hour cure exceeded the ASTM Standards.
EXAMPLE 3
A batch of solvent adhesive was formed by mixing approximately 36% by
weight of N-methyl-2-pyrrolidone with approximately 51% by weight ethyl
acetate and approximately 13% by weight ABS resin. The solvent cement was
applied to test samples and allowed to cure and was tested at 2, 16, and
72 hour intervals. Each of the samples exceeded the ASTM Standards.
EXAMPLE 4
The solvent adhesive of Example 3 was prepared except that PVC resins were
used in place of the ABS resins. The samples were, again, found to exceed
the ASTM test requirements.
The following Table I is illustrative of the test results obtained with
different amounts of NMP and methyl alcohol (Me) on bonding of PVC to PVC.
In order to meet ASTM lap shear requirements, the average (two samples)
bond strength at 2 hours should exceed 250 psi, at 16 hours should exceed
500 psi, and at 72 hours should exceed 900 psi.
TABLE I
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Avg. Lap Shear
Ex. Composition Strength [PVC
Test
No. Solvents Resin/Adjuvant
to PVC (psi)]
Hours
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5 78.3% NMP 12% PVC/2.9% 366 2
5.75% Me Aerosil*, 1.2%
876 16
NaCl 1150 72
6 78.3% NMP 12% PVC/2.9% 341 2
5.75% Me Aerosil, 1.2%
744 16
NaCl 966 72
7 74% NMP 11.2% PVC/2.7%
483 2
10.9% Me Aerosil, 1.1%
792 16
NaCl 1016 72
8 76.1% NMP 11.5% PVC/2.8%
416 2
8.4% Me Aerosil, 1.1%
804 16
NaCl 1000 72
9 71.8% NMP 10.9% PVC/2.6%
525 2
13.6% Me Aerosil, 1.1%
684 16
NaCl 1075 72
10 71% NMP 11% PVC/2% 408 2
Aerosil
15% Me 1% NaCl 852 16
11 74.6% NMP 11.3% PVC 322 2
14.1% Me
12 81.6% NMP 12.4% PVC 314 2
6% Me
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*Aerosil is colloidal silica available from Degussa
Table II illustrates the test results for average (two samples) lap shear
strength (given in psi) obtained with varying amounts of ethyl acetate
(EA) and added resins in combination with NMP on bonding of ABS to ABS,
ABS to PVC, and PVC to PVC.
TABLE II
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Ex.
Composition Average Lap Shear Strength
No.
Solvents
Resins
Adjuvant
ABS to ABS
ABS to PVC
PVC to PVC
Hours
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13 65.5% NMP
2.1% ABS
2.2% -- 508 558 2
21.8% EA
8.4% PVC
Aerosil
-- 789 851 16
14 71.4% NMP
2.1% ABS
none -- 458 516 2
17.8% EA
8.6% PVC
15 36% NMP
2.1% ABS
2.6% -- 462 558 2
51% EA 8.3% PVC
Aerosil
850 1030 1100 16
-- 1150 1436 72
16 65.5% NMP
2.1% ABS
2.1% -- 450 500 2
21.8% EA
8.4% PVC
Aerosil
-- 1119 1256 72
17 36% NMP
2.1% ABS
2.6% -- 458 465 2
51% EA 8.35% PVC
Aerosil
-- 870 1024 72
18 36% NMP
4.9% ABS
none 250 250 275 2
49% EA 9.8% PVC
19 40% NMP
3% ABS
none 362 312 425 2
48% EA 9% PVC
20 43.4% NMP
2.2% ABS
none 312 325 350 2
43.4% EA
11% PVC
21 40% NMP
12% PVC
none 325 275 412 2
48% EA
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EXAMPLE 22
In order to evaluate the necessity for dissolved resin in the cements the
invention, a cement comprising 25% NMP and 75% EA was used to bond ABS to
PVC and PVC to PVC, and the average (two samples) ASTM lap shear strength
for each formulation was measured at two hours. The results showed 300 psi
for ABS to PVC and 462 psi for PVC to PVC. Both results are well above the
ASTM Standards.
EXAMPLE 23
A cement comprising 10% NMP and 90% EA was used to bund ABS to ABS, ABS to
PVC, and PVC to PVC, and the average (two samples) ASTM lap shear strength
for each formulation was measured at two hours. The results showed 232 psi
for ABS to ABS, 265 psi for ABS to PVC, and 288 psi for PVC to PVC. The
ASTM Standard is not met for bonding of ABS to ABS, indicating that the
use of this formulation would be limited to bonding of ABS to PVC and PVC
to PVC.
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