A stain resistant no-mix orthodontic adhesive is disclosed. The adhesive comprises: PA0 (a) a first component having a relatively low viscosity and which contains an acrylic or methacrylic diester of ethoxylated bisphenol-A, an acrylic or methacrylic diester of an alkanediol, and benzyl acrylate or methacrylate; and PA0 (b) a second component containing a filler and an acrylic or methacrylic diester of ethoxylated bisphenol-A, an acrylic or methacrylic diester of an alkanediol, and benzyl methacrylate or acrylate, said filler being present in an amount within the range of from about 50 to about 80 weight percent of said second component, PA0 wherein, one of said first and second components contains a perioxide catalyst and the other of said components contains a tertiary amine activator for said peroxide catalyst.

The invention relates to a stain-resistant no-mix orthodontic adhesive. 
BACKGROUND OF THE INVENTION 
The term "no-mix orthodontic adhesives" is used to refer to two-package 
adhesive systems that are not mixed prior to applying to the tooth to bond 
an orthodontic bracket thereto. A typical example is an acrylic resin 
system consisting of two packages; one part contains an acrylic resin and 
an accelerator and the other contains an acrylic resin and a catalyst. 
Typically, a "primer" coating of unfilled resin and, e.g., the 
accelerator, is applied to the surface of the tooth and to the bonding 
surface of the bracket, and the other part of the two-package system (a 
filled resin system containing the catalyst) is then interposed between 
the two primer coats. The accelerator then activates the catalyst when the 
two parts of the system contact each other, and the resin is thereby 
cured. Examples of such no-mix systems are described by Lee et al. in U.S. 
Pat. No. 4,340,529 and by Johnston in U.S. Pat. Nos. 4,200,980 and 
4,363,624. 
No-mix adhesives seldom achieve the ultimate bond strength achieved by the 
more conventional two package chemically cured adhesives that are mixed 
prior to use. However, no-mix adhesives are gaining favor in the 
orthodontic profession because the no-mix adhesives are easier to use and 
do provide certain functional advantages. First, a conventional two-part 
mix adhesive has a finite working time after it has been mixed, usually of 
the order of 2-3 minutes. Thus, after such an adhesive has been mixed, the 
dentist must hurry to use it before it sets up. In contrast, the no-mix 
adhesive is not "mixed" until the paste portion is placed in contact with 
the liquid ("primer") portion, just as the bracket is being placed. 
Therefore, the dentist can work with one bracket at a time at a 
non-hurried pace. In many cases, with mix adhesives, a significant 
proportion of the mixed formulation sets up before it is used, with 
resulting waste of material. Also, once the no-mix adhesive has been 
applied, the setting time is quite short (e.g., 20-45 seconds); therefore, 
there is less chance that the bracket will drift out of place once it has 
been positioned on the tooth than is the case with mix adhesives wherein 
the setting time is longer. 
Since the advent of orthodontic brackets made of clear materials such as 
polycarbonate, and more recently single crystal alumina or clear sapphire, 
there has been a need to employ bracket adhesives that resist staining 
from foods and beverages. Obviously the aesthetic advantage of using a 
clear bracket would be somewhat diminished if the bracket adhesive were to 
discolor. This invention provides a no-mix orthodontic bracket adhesive 
that achieves a bond strength equal to or better than current no-mix 
adhesives that are commercially available, and which resists staining from 
beverages and foodstuffs significantly better than any of the currently 
available commercial no-mix orthodontic adhesives. 
BRIEF SUMMARY OF THE INVENTION 
The invention provides an orthodontic adhesive containing two parts that 
become active and cure to a stain resistant material when a layer of one 
of the parts is placed in contact with a layer of the other part, the two 
parts comprising: 
(a) a first component having a relatively low viscosity and which contains 
an acrylic or methacrylic diester of ethoxylated bisphenol-A or equivalent 
diester that is free of hydroxyl groups, an acrylic or methacrylic diester 
of an alkanediol, and benzyl acrylate or methacrylate; and 
(b) a second component containing essentially the same resin mixture found 
in the first component and sufficient filler to make a paste, preferably 
wherein the filler loading is relatively high, 
wherein one of said first or second components contains a peroxide catalyst 
such as benzoyl peroxide, and the other of said first or second components 
contains a tertiary amine activator for said peroxide catalyst. 
THE PRIOR ART 
In addition to the Lee et al. and the Johnston patents cited above, the 
relevant prior art includes Ying, U.S. Pat. No. 4,540,723, who discloses a 
resin system similar to the one used in the adhesive of the invention 
which contains a monofunctional monomer such as benzyl methacrylate 
(Ying's resin system is used in a dental restorative composition), and 
Kumar, U.S. Pat. No. 4,500,657, who discloses resin systems wherein the 
monomers are free of hydroxyl groups or other functional groups that are 
relatively hydrophilic. The Kumar patent also relates to dental 
restorative compositions.

DETAILED DESCRIPTION OF THE INVENTION 
The no-mix adhesive of the invention contains two components, a liquid 
component having a relatively low viscosity and a paste component. Both 
components can use essentially the same resin formulation, although 
proportions of the individual monomers of the resin formulation may vary 
from one component to the other. The first monomer in the resin 
formulation is preferably ethoxylated bisphenol-A dimethacrylate ("EBDM") 
or similar diacrylate or dimethacrylate ester that is free of functional 
groups such as hydroxyl, amino, carboxyl, or the like that impart 
water-sensitivity. Illustrative examples of such other diesters that can 
be used include ethoxylated bisphenol-A diacrylate or other alkoxylated 
bisphenol-A diacrylate or dimethacrylate. The resin formulation also 
includes a reactive diluent monomer such as an alkanediol diacrylate or 
dimethacrylate. Illustrative examples of such reactive diluents include 
1,6-hexanediol dimethacrylate, 1,8-octanediol dimethacrylate, and the 
equivalent diacrylate esters. The final monomer in the resin formulation 
is benzyl methacrylate or acrylate. 
The proportion of the monomers in the resin is not narrowly critical. 
Broadly, the proportions of the monomers in the resin will usually be 
within the ranges given in the following table, which also displays the 
viscosities (at 23.degree. C.) of the two components: 
______________________________________ 
PERCENTAGES, BASED ON RESIN WEIGHT 
BROAD PREFERRED 
______________________________________ 
PRIMER 
EBDM (or equivalent) 
40-90 70-80 
Reactive Diluent 
5-30 5-10 
Benzyl acrylate or 
5-30 15-20 
methacrylate 
Viscosity, centipoises 
100-2000 200-600 
PASTE 
EBDM (or equivalent) 
40-90 70-80 
Reactive Diluent 
5-30 5-10 
Benzyl acrylate or 
5-30 15-20 
methacrylate 
Viscosity, centipoises 
50,000-500,000 
100,000-300,000 
______________________________________ 
The polymerization catalyst is added to one of the components, and the 
catalyst activator or accelerator is added to the other. Preferably, the 
activator is added to the primer formulation. The activator is usually an 
aromatic tertiary amine such as 
N,N-bis(.beta.-hydroxyethyl)-2-methyl-p-toluidine, 
N,N-diethyl-p-toluidine, N,N-bis(.beta.-hydroxyethyl)-p-toluidine, ethyl 
4-(N,N-dimethylamino)benzoate, or the like. The accelerator is used in 
relatively large proportions (compared to the amount usually used in 
two-package mix formulations), usually in the range of from about 2 to 
about 10 weight percent of the total resin weight in the component to 
which it is added. To ensure that the accelerator does not precipitate 
during storage, it is useful to warm the resin slightly while adding the 
accelerator (e.g., to about 60.degree.-70.degree. C., which is above the 
melting point of the accelerator), to mix the heated resin/accelerator 
mixture for about 15-30 minutes, and to then filter the resin containing 
the accelerator through a 400-mesh screen to make sure that no nucleation 
particles are present in the resin. 
The polymerization catalyst is added to the other component, usually the 
past component. Usually, the catalyst will be a peroxide such as benzoyl 
peroxide, methyl ethyl ketone peroxide, or the like. The peroxide is used 
in catalytically effective amounts, e.g., from about 1 to about 5 weight 
percent, based on the weight of the resin in the component to which it is 
added. A stabilizer such as an alkylphenol is included in the component 
that contains the peroxide. 
The conventional fillers can be used in the paste component. These include 
colloidal silica, finely divided glass (preferably acid washed, heat 
treated barium or strontium glass, as described by Denton et al. in U.S. 
Pat. No. 4,492,777), finely divided quartz, and the like. The acid washed 
and heat treated glass powders described in the cited Denton et al. patent 
are heated at an elevated temperature (below the sintering temperature of 
the glass) sufficient to cause a significant reduction in the specific 
surface area of the glass powder. The Denton et al. patent is incorporated 
herein by reference. Preferably, the filler is silane-treated to promote 
bonding to the resin, as is conventional. The filler is preferably used in 
relatively high proportions, such as from about 100 to about 400 parts by 
weight per hundred parts by weight of resin in the paste. 
The preferred way to use the no-mix adhesive of the invention is to apply 
the primer to the clean bonding surface of the orthodontic bracket and to 
the etched surface of the tooth to which the bracket is to be applied, and 
to interpose a layer of the paste between the two layers of primer as the 
bracket is applied. The resin will set up in a rather short period of time 
(15-30 seconds), so the bracket need only be held in place by the dentist 
for this short period of time. In fact, the viscosity of the paste 
component is such that the bracket will not ordinarily drift or shift its 
position after placement, even prior to setting of the adhesive. 
The experimental section below illustrates a specific formulation of the 
invention, and compares its mechanical properties and stain resistance 
with a number of commercially available no-mix orthodontic adhesives. 
The no-mix adhesive of the invention used in the experimental section had 
the following formulation: 
______________________________________ 
Parts, by weight 
______________________________________ 
PRIMER Components 
EBDM 70 
1,6-hexanediol dimethacrylate 
10 
Benzyl methacrylate 20 
N,N--bis(.beta.-hydroxyethyl)-2-methyl- -p-toluidine 
7.5 
PASTE COMPONENTS 
Resin portion of paste 
EBDM 80 
1,6-hexanediol dimethacrylate 
5 
Benzyl methacrylate 15 
Butylated hydroxy-toluene (stabilizer) 
0.03 
Benzoyl peroxide 2.0 
PASTE Formulation 
Resin, described above 100 
Glass filler.sup.(1) 285 
Colloidal silica filler.sup.(2) 
15 
______________________________________ 
.sup.(1) Finely divided (0-13 microns) acid washed, heat treated barium 
glass treated with 1.5 weight percent 
gammamethacryloxypropyltrimethoxysilane (A174 Silane) 
.sup.(2) Aerosil OX50, treated with 3.0 weight percent A174 Silane. OX50 
has an average particle size of 0.05 micron. 
The bond strength of the no-mix formulation described above (Example 1) was 
evaluated, and was compared with the bond strengths of four commercial 
no-mix adhesives (Controls A-D). 
The primer components of three of the four commercial no-mix adhesives were 
analyzed for their components by HPLC, with the following results: 
______________________________________ 
AREA UNDER PEAK, PERCENT 
CONTROL CONTROL CONTROL 
COMPONENT A C D 
______________________________________ 
N,N--bis(.beta.-hydro- 
69.5 47.6 40.5 
xyethyl)-p-toluidine 
Unknown 1 -- 8.3 10.5 
Triethylene glycol 
15.0 2.5 3.0 
dimethacrylate 
Unknown 2 -- 2.6 -- 
Bis-GMA 4.1 12.6 19.5 
Bisphenol-A -- 5.9 8.4 
dimethacrylate 
______________________________________ 
The following was also determined for the paste components: 
______________________________________ 
FILLER TYPE Quartz Silica Quartz 
______________________________________ 
Percent Solids 
40.6 64.0 69.1 
______________________________________ 
The bond strength testing was carried out as follows: 
Metal brackets were used. They were bonded to phosphoric acid etched bovine 
enamel by coating the enamel and the bonding surface of the bracket with 
the primer portion of the formulation, and then the paste portion was 
applied to the primed enamel and the primed bracket was immediately placed 
on the tooth and held in place for 5 seconds. Each bovine enamel piece was 
embedded in one end of an acrylic cylinder whose dimensions were 11/2 cm 
in diameter by 2.8 cm long. The enamel surfaces were polished with 600 
grit silicone carbide grinding paper prior to etching with phosphoric acid 
for 60 seconds. The surfaces were rinsed with tap water for 60 seconds 
after the etching. Bond strengths were measured after 5 minutes, 10 
minutes, 60 minutes, and 24 hours. 
The 5, 10, and 60 minute tests were carried out on dry samples, the 24 hour 
tests were carried out on samples that were immersed in water at 
37.degree. C. about one hour after placement and kept there for the 
remainder of the 24 hours. The bond strength was tested in shear as 
follows: 
The acrylic cylinders containing the bracket bonded to enamel were clamped 
in the test piece holder of an Instron Laboratory tester so that a flat 
striker rod impinged upon the tie wings of the bracket at right angles. 
The Instron was turned on (a crosshead speed of 0.5 mm per minute was 
used), and the force required to break the bracket away from the substrate 
was recorded. Bond strength equals force at break divided by bonding area 
of the bracket, and is expressed in MPa's. 
The table below displays the results of the bond strength testing. Each 
number is the average of 12 samples. 
TABLE I 
__________________________________________________________________________ 
BOND STRENGTH RESULTS 
5 min. 10 Min. 
60 Min. 
24 Hrs. 
No-Mix Adhesives 
Mean 
S.D..sup.1 
Mean 
S.D. 
Mean 
S.D. 
Mean 
S.D. 
__________________________________________________________________________ 
Example 1 8.3 1.3 10.8 
0.9 
12.3 
1.5 
12.6 
1.5 
Control A 9.0 1.3 10.9 
1.1 
12.9 
1.2 
12.5 
1.3 
Control B 8.6 1.3 10.0 
1.6 
11.5 
1.4 
11.6 
1.6 
Control C 7.7 2.1 10.5 
1.5 
11.0 
1.2 
11.2 
2.1 
Control D 6.4 1.0 9.3 
1.0 
11.4 
1.5 
11.0 
0.9 
__________________________________________________________________________ 
.sup.1 S.D. = Standard Deviation 
Each of the no-mix adhesives that were tested for bond strength were also 
tested for stain resistance by the following procedure: 
Each formulation was mixed (in proportions of about 1:1 primer-paste, by 
volume) and immediately molded into a small disc-shaped article 
(dimensions were 1 mm thick, 10 mm diameter). Two samples of each test 
adhesive were then immersed in each of water, coffee, or tea for four 
weeks at 50.degree. C. The test specimens were rated 1 (least staining) 
through 5 (most staining) for each test liquid. The ratings are 
comparisons with a control of the same adhesive stored in water at 
5.degree. C. for the same period of time. (None of the controls exhibited 
any staining when stored in water at 5.degree. C.) The results were as 
follows: 
TABLE II 
______________________________________ 
STAIN RATING IN 
WATER COFFEE TEA 
______________________________________ 
Example 1 1 1.3 1.5 
Control A 3.3 2.5 2.2 
Control B 2.7 2.5 2.7 
Control C 3.3 4.0 4.0 
Control D 4.7 4.7 4.7 
______________________________________ 
Each number is the average of six ratings that were made by six members of 
a color panel.