RF curable Type I wood adhesive composition comprising vinyl acetate/NMA copolymer emulsions containing tetramethylol glycoluril

An aqueous vinyl acetate/N-methylolacrylate copolymer emulsion for RF curable Type I wood adhesive compositions prepared by the polymerization of vinyl acetate and N-methylolacrylamide in the presence of 1 to 5 wt% tetramethylol glycoluril and a stabilizing system consisting essentially of 3 to 5 wt% polyvinyl alcohol, the weight percentages being based upon vinyl acetate monomer.

TECHNICAL FIELD 
The present invention relates to aqueous vinyl 
acetate/-N-methylolacrylamide (VAc/NMA) copolymer emulsions and RF curable 
Type I wood adhesive compositions. 
BACKGROUND OF THE INVENTION 
Previous aqueous VAc/NMA copolymer emulsions prepared in the absence of any 
protective colloid or surfactant required the addition of a functional 
monomer, for example, acrylic acid, to obtain emulsion stability and 
cleanliness. These emulsions, however, had undesirably high emulsion 
viscosities and were unsuitable as RF curable wood adhesives. To be 
suitable as an RF curable wood adhesive a VAc/NMA copolymer emulsion must 
exhibit manufacturing stability and long term viscosity stability and the 
polymer must be of high molecular weight and demonstrate rapid cure. To 
provide a wood adhesive having acceptable flow properties for machine 
application, the VAc/NMA copolymer emulsion should have a viscosity of 
5,000 to 6,000 cps which will drop to a suitable viscosity when the acidic 
metal salt catalyst (aluminum chloride) is added. 
Relatively stable, but difficult to prepare and reproduce, aqueous VAc/NMA 
copolymer emulsions were prepared using the protective colloid, polyvinyl 
alcohol (PVOH). The addition of a small amount of an ethoxylated 
nonylphenol nonionic surfactant led to emulsions that were more 
reproducible and approached all of the desired properties of an RF curable 
Type I wood adhesive but did not achieve them. 
SUMMARY OF THE INVENTION 
The present invention provides PVOH-stabilized aqueous emulsions containing 
a vinyl acetate/N-methylolacrylamide (VAc/NMA) copolymer and tetramethylol 
glycoluril (TMGU) demonstrating an improved combination of properties as 
an RF curable Type I wood adhesive. The manufacture of these emulsions is 
reproducible. The VAc/NMA copolymer emulsions have low grit values, 
excellent viscosity stability and good catalyzed pot life, and give 
unexpectedly high green strengths after RF curing. These emulsions also 
pass Type I and Type II wood adhesive performance testing. 
The TMGU is added to the VAc/NMA copolymer emulsion polymerization recipe, 
i.e. it is present during the polymerization reaction, rather than 
post-added. Having the TMGU present during the polymerization results in 
better incorporation of the TMGU into the polymer and gives an emulsion 
viscosity that is desired. Doing so also allows for the use of lower 
levels of NMA in the copolymer and lower levels of PVOH in the stablizing 
system while still obtaining accepatble emulsion viscosity, manufacturing 
stablility and emulsion storage stability. Post-adding the TMGU (using the 
same stabilizing system) would yield an unstable and low viscosity 
emulsion. 
The emulsions are aqueous colloidal dispersions containing 40 to 60% solids 
of an emulsion copolymer consisting essentially of 90 to 98 wt % vinyl 
acetate and 2 to 10 wt % NMA prepared in the presence of 1 to 5 wt % TMGU 
and a stabilizing system consisting essentially of 3 to 5 wt % PVOH, the 
wt % of TMGU and PVOH being based upon vinyl acetate monomer. 
Wood adhesive compositions would comprise the VAc/NMA/TMGU emulsion and 
optionally, other components typically used in the art including fillers, 
crosslinking resins and acidic metal salt catalysts.

DETAILED DESCRIPTION 
The PVOH-stabilized aqueous dispersion, or emulsion, of a VAc/NMA copolymer 
and TMGU is prepared by polymerizing vinyl acetate and NMA in the presence 
of TMGU and a PVOH stabilizing system. The emulsions are prepared by 
incorporating during the polymerization process 1 to 5 wt %, preferably 1 
to 3 wt %, TMGU into an aqueous colloidal dispersion containing 40 to 60% 
solids of a copolymer prepared by the emulsion polymerization of a monomer 
mixture sufficient to provide a copolymer consisting essentially of 90 to 
98 wt % vinyl acetate and 2 to 1 wt % NMA. The copolymer is prepared in 
the presence of a stabilizing system consisting essentially of 3 to 5 wt % 
PVOH which is preferably 70 to 91 mole % hydrolyzed. 
The preferred emulsions are prepared by incorporating 1 to 3 wt % TMGU into 
an aqueous colloidal dispersion containing 50 to 55% solids of a copolymer 
consisting essentially of 94 to 98 wt % vinyl acetate and 2 to 6 wt % NMA 
prepared in the presence of a stabilizing systems consisting essentially 
of 3 to 5 wt % PVOH which is 85 to 89 mole % hydrolyzed. Suitable PVOH's 
would have a degree of polymerization (DPn) ranging from 150 to 2000, 
preferably 220 to 1400. It is also preferred to use a stabilizing system 
comprising two 85-89 mole % hydrolyzed PVOH's, one having a DPn of 150 to 
610 (low mol wt) and the other having a DPn of 1000 to 1800 (medium mol 
wt), in a wt ratio ranging from 20:80 to 50:50, preferably 35:65 to 40:60. 
The VAc/NMA copolymers may optionally contain up to 5 wt %, preferably 
about 1 to 3 wt %, of suitable comonomers such as acrylamide, 
hydroxethylacrylate, hydroxypropylacrylate and carboxylate-containing 
monomers such as acrylic acid. 
Tetramethylol glycoluril (TMGU) is added in-situ during the polymerization 
of the vinyl acetate and NMA, preferably by the delay addition of a 
mixture of NMA and TMGU. A method for preparing TMGU is disclosed in U.S. 
Pat. No. 4,064,191. TMGU is also sold commercially by American Cyanamid 
Company under the trademark CYMEL.RTM. 1172 resin and is represented by 
the formula: 
##STR1## 
A lower level of NMA can be used when TMGU is added to the emulsion 
polymerization recipe without sacrificing emulsion stability and 
performance and, in fact, these areas may be improved. Previous attempts 
to prepare emulsions at NMA levels below 6% gave emulsions that were 
gritty, difficult to prepare and marginal in adhesive performance testing. 
The TMGU, although not a monomer, improves the emulsion stability when 
added to the polymerization recipe and improves the emulsion performance 
properties. Further, being a tetramethylol functional compound, it reacts 
with the NMA for a tighter crosslinked polymer network as evidenced by 
improved performance properties. 
Suitable VAc/NMA copolymer emulsions are prepared by copolymerizing the 
monomers in the presence of the PVOH stabilizing system. Substantially all 
of the PVOH and a portion of the vinyl acetate monomer is initially 
charged to the polymerization vessel. Most advantageously, at least about 
10 to 30 wt % and preferably, at least about 15 wt % of the total vinyl 
acetate to be polymerized is initially charged to the reactor. The 
remainder of the vinyl acetate is added, desirably at a substantially 
uniform rate over a period of time. 
The polymerization reaction is performed at temperature ranging from 
55.degree. to 75.degree. C., preferably about 65.degree. C., and at 
essentially atmospheric pressure. 
Various free-radical generating materials can be used in carrying out the 
polymerization of the monomers, such as peroxide compounds. Combination 
systems employing both reducing agents and oxidizing agents can also be 
used, i.e., a redox system and are preferred. The oxidizing agent is 
generally employed in an amount of 0.01 to 1%, preferably 0.05 to 0.5%, 
based on the weight of vinyl acetate monomer introduced into the 
polymerization system. The reducing agent is ordinarily added in the 
necessary equivalent amount. 
When reference is made to incremental addition, whether of vinyl acetate, 
any comonomer, or free-radical source, substantially uniform additions, 
both with respect to quantity and time, and intermittent additions are 
contemplated. Such additions are also referred to a "delay" additions. 
It is preferred that the NMA and the TMGU be added to the polymerization 
reaction as delay additions. 
The RF curable Type I wood adhesive compositions containing the VAc/NMA 
copolymer emulsions of the invention would also contain other components 
well known in the art, for example, crosslinking resins such as 
melamineformaldehyde resins and phenolic resins, fillers such as starch 
and nut shell flour, and acidic metal salt catalysts such as aluminum 
chloride, chromium nitrate, magnesium chloride and the like, all in 
amounts well known in the art. 
In Examples 1 and 2 the following PVOH's were used: 
______________________________________ 
AIRVOL .RTM. PVOH 
MOLE % HYDROLYSIS 
DPn 
______________________________________ 
203 87-89 220 
205 87-89 550 
523 87-89 1400 
540 87-89 2000 
______________________________________ 
EXAMPLE 1 
To a one gallon glass reactor equipped with a thermometer, reflux 
condenser, electronic temperature controller, and metering pumps rather 
the following ingredients: 
______________________________________ 
Vinyl acetate 395 g 
Disodium phosphate 4.2 g 
Ferrous ammonium sulfate (5% aq soln) 
12 g 
Airvol 523 (10% aq soln) 604 g 
Airvol 203 (10% aq soln) 369 g 
Water 490 g 
______________________________________ 
The reactor contents were heated to 65.degree. C. under a nitrogen blanket 
and 3 g sodium metabisulfite and 15 g water were then added. The 
polymerization was initiated with approximately 0.7ml of a 
t-butylhydroperoxide (t-BHP) solution consisting of 0.55 g t-BHP (70%) and 
87.4 g water. The initiator system was added over 5 hours 40 minutes. 
At polymerization initiation there were added (simultaneously with the 
initiator and subsurface to the reaction medium) three separate feed 
delays: 
______________________________________ 
Delay I 
Vinyl acetate 1810 g 
Delay II 
N-methylolacrylamide (48%) 
183 g 
Cymel 1172 TMGU (45%) 100 g 
Water 383 g 
Delay III 
Sodium metabisulfite 1.4 g 
Disodium phosphate 1.02 g 
Water 76.6 g 
______________________________________ 
Delay I was added over 4 hours; Delay II was added over 5 hours; and Delay 
III was added over 4 hours. 
The reaction temperature was maintained at 65.degree. C. with a jacket 
temperature of 55.degree.-65.degree. C. The vinyl acetate free monomer was 
12% at the end of the first hour and the vinyl acetate free monomer 
averaged between 6-10% for the remainder of Delay I. The vinyl acetate 
free monomer content was controlled by the appropriate addition of the 
initiator. 191 g of the NMA/Cymel 1172 Delay II solution was added over 
the first hour with the remainder uniformly added over the remaining 4 
hours. At the end of 4 hour 40 minutes the vinyl acetate free-monomer was 
below 0.5% and the emulsion pH was 4.8. The pH of the emulsion was 
adjusted to 5.6 with 20% ammonium hydroxide. The resulting emulsion (Run 
15) was 52% solids. Approximately 300 ppm of total grits were obtained 
after filtering through both a 100 and 325 mesh screen. Emulsion viscosity 
was 6320 cps, Brookfield viscosity at 20 rpm. 
EXAMPLE 2 
Numerous vinyl acetate polymer emulsions (Runs 1-14) were prepared in the 
presence of PVOH's, surfactants and TMGU as shown in following Table 1 
following the general procedure of Example 1 (Run 15). 
TABLE 1 
__________________________________________________________________________ 
Total PVOH 
Airvol 
Surfactant.sup.b 
NMA TMGU Emulsion 
Run 
on VAM (%) 
PVOH.sup.a 
on VAM (%) 
on VAM (%) 
on VAM (%) 
Solids (%) 
__________________________________________________________________________ 
1 4.4 523/205 
-- 6 -- 53 
(44/56) 
2 3.1 523/205 
CO 997 6 -- 53 
(75/25) 
(0.5) 
3 4.4 523/205 
-- 6 -- 53 
(60/40) 
4 4.4 523/205 
CO 997 6 -- 53 
(43/57) 
(0.5) 
5 4.4 523/203 
CO 997 6 -- 53 
(43/57) 
(0.5) 
6 4.4 523/203 
CO 997 6 -- 53 
(43/57) 
(0.5) 
7 4.4 523/203 
CO 997 6 -- 53 
(57/43) 
(0.5) 
8 4.4 523/203 
CO 997 6 -- 52 
(43/57) 
(0.5) 
9 4.4 523/203 
-- 4 3 53 
(78/22) 
10 4.4 523/203 
-- 4 2 52 
(78/22) 
11 4.4 523/203 
-- 4 2 52 
(57/43) 
12 4.4 523/203 
-- 4 2 52 
(67/33) 
13 4.4 523/203 
-- 4 2 52 
(62/38) 
14 4.4 523/203 
-- 4 2 52 
(62/38) 
15 4.4 523/203 
-- 4 2 52 
(62/38) 
16 4.4 523/540 
-- 3 3 52 
(78/22) 
17 4.4 523/203 
-- 4 2 52 
(62/38) 
18 3.4 523/203 
-- 4 2 53 
(62/38) 
19 4.4 523/540 
-- 2 2 52 
(62/38) 
__________________________________________________________________________ 
.sup.a (wt/wt) 
.sup.b Igepal CO 997 nonylphenoxy poly(ethyleneoxy) ethanol 
EXAMPLE 3 
The vinyl acetate emulsions of Runs 1-15 were tested for radio frequency 
green strength, boiling water (Type I) and cold water (Type II) strengths 
according to the following procedures: 
Radio Frequency Green Strength Procedure: 200 g of emulsion are catalyzed 
with 10 g of 32.degree. Baume aluminum chloride solution. The viscosity 
and pH of the catalyzed emulsion are recorded. Six pieces of Hard Rock 
maple are selected to be bonded in the 5 KW L&L radio frequency GluAll 
laminator. The maple is 12.times.2.5.times.0.75 in. 
(30.5.times.6.4.times.1.9 cm). Three pieces are face glued with a 6 mil 
coating of catalyzed adhesive. Three bonds are formed quickly by hand to 
minimize the open time of the adhesive. The three bonds are placed in the 
radio frequency machine and joined. The cure cycle can vary but 30 seconds 
is the standard condition. After the cure is affected, 2 of the 3 bonded 
assemblies are selected and cut into test constructions which satisfy the 
standard method of ASTM D905. Twelve ASTM D905 blocks are cut from the two 
bonded assemblies and are broken in an Instron tensile tester at 0.5 
in/min (1.3 cm/min). The time lapse since the boards were removed from the 
RF press and the tensile strength are recorded. The bond should be totally 
tested within a fifteen minute period from removal from the RF press. The 
tensile strength numbers are averaged and recorded. 
Boiling Water (Type I) Testing: This test is usually run simultaneously 
with the cold water testing. 200 g of emulsion are catalyzed with 10 g of 
32.degree. Baume aluminum chloride solution. The viscosity and pH of the 
catalyzed emulsion are recorded. Two pieces of Douglas fir are selected to 
be bonded in the 5 KW L&L radio frequency GluAll laminator. The fir is 
12.times.2.5.times.0.75 in. A piece is face glued with a 6 mil coating of 
catalyzed adhesive. The bond is formed quickly by hand to minimize the 
open time of the adhesive. The bond is placed in the radio frequency 
machine and joined. The cure cycle can vary but 2 minutes is the standard 
condition. After the cure is affected, the bonded assemblies are allowed 
to age for 7 days at 25.degree. C. and 50% relative humidity. The 
assemblies are then selected and cut into test constructions which satisfy 
the standard method of ASTM D905. Twelve ASTM D905 blocks are cut from 
each bonded assembly. The specimens are subjected to two cycles of 
immersion in boiling water for 4 hours immersion followed by 16 hours of 
drying at 120.degree. F. (49.degree. C.). After the second boiling water 
cycle the specimens are immersed in cool water for 30 minutes to cool them 
and broken in an Instron tensile tester at 0.5 in/min (1.3 cm/min). The 
tensile strength numbers are averaged and recorded along with the degree 
of wood fiber tear. 
Cold Water (Type II) Testing: 200 g of emulsion are catalyzed with 10 g of 
32.degree. Baume aluminum chloride solution. The viscosity and pH of the 
catalyzed emulsion are recorded. Two pieces of Douglas fir are selected to 
be bonded in the 5 KW L&L radio frequency GluAll laminator. The fir is 
12.times.2.5.times.0.75 in. (30.5.times.6.4.times.1.9 cm). A piece is face 
glued with a 6 mil coating of catalyzed adhesive. The bond is formed 
quickly by hand to minimize the open time of the adhesive. The bond is 
placed in the radio frequency machine and joined. The cure cycle can vary 
but 2 minutes is the standard condition. After the cure is affected, the 
bonded assemblies are allowed to age for 7 days at 25.degree. C. and 50% 
relative humidity. The assemblies are then selected and cut into test 
constructions which satisfy the standard method of ASTM D905. Twelve ASTM 
D905 blocks are cut from each bonded assembly. The specimens are subjected 
to three cycles of exposure to a vacuum soak in room temperature water of 
4 hours immersion followed by 16 hours of drying at 120.degree. F. 
(49.degree. C.). After the third water soak cycle the specimens are broken 
in an Instron tensile tester at 0.5 in/min (1.3 cm/min). The tensile 
strength numbers are averaged and recorded along with the degree of wood 
fiber tear. 
TABLE 2 
______________________________________ 
RF Green 
RUN Strength Type I Type 2 
______________________________________ 
1 2254 191 371 
2 1941 282 412 
3 1767 302 151 
4 2115 245 458 
5 2021 168 183 
6 1924 123 205 
7 1856 139 135 
8 1846 325 517 
9 2070 265 462 
10 2446 343 444 
11 2758 195 276 
12 2488 343 444 
13 2414 243 201 
14 2446 245 444 
15 2710 -- -- 
______________________________________ 
It can be seen from the data in the above tables that the VAc/NMA copolymer 
emulsions prepared in the presence of TMGU provided for unexpectedly 
improved RF green strength while maintaining good Type I and Type II bond 
strengths. 
STATEMENT OF INDUSTRIAL APPLICATION 
The present invention provides an aqueous vinyl 
acetate/N-methylolacrylamide copolymer emulsion showing high green 
strength after radio frequency curing and acceptable Type I and Type II 
performance testing as a wood adhesive.