TENT or
ANALYSIS [CP tILF.EIP UREA-FORMALDEHYDE CLUES
September 1944
JUL,
190
lriOs Veport Is One of a Series
Issued In Cooperation with the
41141Y.4410*-0111 COMMIntr
on
Alva?Art 'DESIGN CVITERIA,
Under the Super*Ision of Mc
AtVONAUTICAll. 'IBIDAPID
Sic. 1333
UNITED STATES DEPARTMENT OF AGRICULTURE
OREST SERVICE
FOREST RODIICTS LABORATORY
Madison, Wisconsin
Coo0iiatiow with -Ake fvfrsity of \Wielmoin
ANALYSIS FOXI FILUR CONTENT OF UP.t,A-FORMALDEHYDE GLUES 1
By
A. E. GABRIEL, Chemist
and
LEAH E. COHODAS, Chemist
ons•ries 00.
It is generally agroA that; there is an. upper limit to the amount of
filler that it is desirable to have incorporated in a urea-formaldehyde
adhesive because unlimited Amounts could adversely affect strength and other
properties. After consultation with representatives of the glue manufacturers, the upper limit in Army-Nav Aeronautical Specification AN-G-8 was
set at 20 percent of the weight of the dry resin. Unfortunately, "filler"
is not adequately defined in this specification • nor is there any prescribed
method for determination of filler. It is important in the revision of
Specification AN-G-8 and in the proposed Federal Specification for urea
formaldehyde glue that this deficiency be overcome. To this end the Forest
Products _laboratory has recently given considerable attention to • methods of
filler analysis.
The ideal method for filler analysis should permit complete separation
of such substances as wood flour. walnut, shell flour, set urea-formaldehyde
rosin, and any other inert insoluble constituent from the active adhesive
ingredient. This ideal is difficult to attain, however, inasmuch as walnut
shell flour, wood flour, and set resin haw some solubility in all solvents;
and the urea-resin ingredient is never in one form only but varies in degree
of polymerization from sirplo molecules that may be truly soluble through
micelles in the form of a colloidal sol, to higher polymers that appear to be
in neither true nor colloidal solution but in gross suspension. It seems
probabl(, that thi insoluble suspended urea rosin would not be ;Idhesivt., like
t1.1e rosin in colloidal form end should therefore, in the ideal method, be
separated out with
int,,redints acknowledged as filler. The amount of
rosin in this form is usually only a part of one percent but may run as high
as several percent.
Six methods have been suggested by various individuals and companies
for the determination of fill4r La urea-formaldehyde rosin glues. They all
consist of simple separation ofthe filler by solvents to dissolve the
1
This mimeograph is one of a series of progress reports prepared by the
Forest Products Laboratory to further the Nation's war effort. Results
hero reported aro preliminary and may be revised as additional data
become available.
Mimeo. No.
1333
-1-
adhesive resin, leaving the filler insoluble and capable of being filtered,
dried, and weighed. These methods depend on the use of the following
solvents:
(1)
(2)
(3)
(4)
(5)
(6)
50 percent ethyl alcohol in water
hot water
20 percent acetic acid
90 percent formic acid
boiling in water made slightly alkaline with NaOH
20 percent acetic acid, 20 percent alcohol,
60 percent water
Detailed directions for these six different methods and also a method
for determination of nonvolatile content are included at the and of this
report. The original wording has in some places boon changed, but the m
= ethods
are essentially as suggested.
It may be noted that all those methods express filler as a percentage
of the nonvolatile content of thc, glue instead of the resin content as in
Specification AV-G-. This is a little more lenient and raises slightly the
amount of filler p ermitted by any sot figure, such as "20 percent." This
method of expression seems desirable, however, as the nonvolatile content is
relatively easy to determine and avoids th,: complicated procedure that would
be necessary to determine strictly rosin content.
In method 3, 20 percent acetic acid is employed. In paragraph . F-3b
'(3) of the draft of the proposed Federal Specification of July •21, 1943 the
use of 10 percent acetic acid was prescribed. Following the issuance of this
draft, several glue manufacturers are said to have stated that 10 percent
acetic acid would not dissolve their resins. Urea resins are probably not
more soluble in 20 percent acetic acid than in 10 percent; but the higher
concentration is a better stabilizinF agent for the urea rosin colloidal sol,
and thu objection may have arisen from. the occurrence of flocculation before
filtration was complete. For this reason. 20 percent acetic acid is employed
• and a filtering aid to speed the rate of filtration introduced into method 3.
In order to draw a comparison as to the ease with which these methods
may be used and the accuracy and precision of the results that can be obtained thereby, 11 commercial glues, consisting of t urea, 2 fortified urea,
and 1 melamine glue, wen examined independently by 2 different analysts
using the first 4 method previously listed. The data obtained on the
commercial glues arc presented in. table 1.
From table 1, nom of the methods of analysis appears to be as precise
as most methods of inorganic quantitative chemical analysis. The average
differences in percentages of insoluble between duplicate determinations on
the same sample varied from 0.3 percent for the formic acid to 0.8 percent
for the hot water. This would indicate poorest precision by the hot water
method. The differonc,:' in precision by the other three methods was not
statistically significant.
No.
1333
-2
0
_ In no sample was the quantity of the insoluble actually present inthe
11 glues of table 1 known. In the 6 glues, 1 to 4 and 10 and 11, there was
no vegetable filler present bit always a small but Unknown amount of insoluble,
probably nonadhesive rosin. In glues 5 to 9, vegetable fill e r was present
in addition to some insoluble rosin, and perhaps a small percentage of inorganic insolubles resulting from the catalyst-buffer system. The manufacturers will undoubtedly know accurately the amount of vegetable filler incorporated in their glues but probably not the proportion of the resin that is
insoluble.
On the unfortifiod urea-formaldehyde glues, Nos. 1 to 8 in table 1,
50 percent alcohol gave higher values for insoluble residue than any of the
other three solvents. On the melamine and fortified ureas the values for the
hot water method were the highest. In practically all instances, the formic
acid gave the lowest values for insoluble content.
In addition to these comparative determinations on different glues,
the solvents employed in five of the methods, among others, were used on
walnut shell flour, maple wood flour, and powdered set urea resin, all of
which are materials in common 1130 as filler constituents, to determine their
solubility in various solvents. The data obtained from the tests on filler
materials are given in table 2. It may be seen from this table that maple
wood flour, walnut shell flour, and sot resin show some solubility in all the
liquids triad as solvents. Between the first six solvents there appears to
be little choice in this respect.
Sot urca-formaldehyde.rosin is decomposed by or is soluble in hot
water and is practically all dissolved by long-continued boiling. This,
along with the length of time and equipment required for method 5, is a reasonable basis for objection to that method, in which filler and resin are
separated by prolonged refluxing in water made v e ry slightly alkaline with
NaOH.
Concentrated formic acid has a strong solvent action on all three
fillers, particularly when hot. The facility with which it dissolves sot
rosin in urea-formaldehyde glue may be a chief objection to method 4.
Another objection to the use of 90 percent formic acid is the disagreeable
action of this chemical on the skin of the analysts. Unless rubber gloves
aro worn or other strict precautions taken, formic acid is insidiously
absorbed by the skin causing soreness and large blisters.
A circumstance that may haw a bearing upon the choice of solvents for
separation of filler from soluble urea resins is the observation that the
English glues, Aerolite 300F, Aerolite 303, and Beetle Cement AF, could not
13:: filtered when treated with 90 percent formic acid, although filtration
was not difficult, when the treatment was with 20 percent acetic acid or with
20 percent acetic plus 20 percent alcohol.
Method 1 using 50 percent alcohol gave values for total insoluble content higher than generally expected for each particular glue. It is not
certain that it gives too high results, for how high they should: be, as said
No. 1333
-3-
bofore, is not known. Noverth(Aoss, for a glue specification that places a
maximum limit on the amount of filler that may be incorporated in the glue,
the adoption of a method that would give a maximum amount of insoluble to be
classed as filler might, under certain circumstances, work a hardship on the
glue manufacturer. From this point of view a method that would place the
insoluble content somewhat lower would be preferable in that it would benefit
the manufacturer without lowering quality standards so far as is known at
this time.
In the matter of choice between the methods using hot water and 20
percent acetic acid, the former offers freedom from the use of filtr-aid
and the extra operations and error this may entail. It also has the advantaco of more rapid filtration of the straight urea-formaldehyde resins. In
the melamine and fortified urea resins on the other hand, hot water fails to
dissolve a residue that is of gummy consistency and slow to filter. Although
melamino and fortified urea resins Are not included in the proposed Federal
Specification for cold-sottdng urea-formaldehyde glues, typical examples
(Nos. 9, 10, and 11 in table 1) ha-k, , e been included among those glues studied,
inasmuch as it would seem desirable to have a method for filler analysis
chosen Car urea-formaldehyde glues that would, if possible, be suitable also
for glues of other types. Though the hot-water method (No. 2) seems quite
satisfctory for '1,-;he unfortified urea glues, it has not proved sufficiently
applicable to all urea-rosin typ o s.
It may be noted from table 1 that 90 percent formic acid usually gave
lower percentages of insoluble than did 20 percent acetic acid. To ascertain
if precipitation of soluble resin by acetic acid may be the explanation of
the greater amount of matter insoluble in 20 percent acetic acid than in 90
• percent formic acid, as reported in table 1, three uroa-rosin solutions wore
analyzed for insoluble content by 20 percent acetic acid and by centrifuging.
The results are-aS shown in table 3.
The findings that the quantity of insoluble matt_r thrown down by
centrifuging was in no case significantly less than the quantity insoluble
in 20 percent acetic acid would reasonably lead to the conclusion that the
acetic acid was not, precipitatin;: rein in these instances. The data would
rather Indicate that acetic acid has some solubilizing action similar to
that of formic acid upon set urea rosin although less pronounced. Additional
evidence for this conclusion lay in the observation that, with all three
glues, when a small quantity of opalescent supornntrInt layer after centrifuging was poured into 20 percent acetic acid, the oplscencc; gradually
cleared up upon standing for a fen minutes.
Precipitation upon the addition of 20 percent acetic acid as in method
found in -a. single instance This occurred with Bakelite BC -U-16257, a
clear, water-white, resin solution of 69 percent solids, which became cloudy
upon addition to. 20 percent acetic acid with coagulation of a small proportion
3
Was
of the resin previously in solution. Although this was the only case of
precipitation observed, it indicated that 20 percent acetic acid was not
universally suitable as a solvent for filler analysis of urca-resin glues.
No. 1333
Precipitation did not occur from Bakelite BCU-16257 upon the addition
of a solution of the following composition: 20 percent acetic acid, 20
percent ethyl alcohol, 60 percent water. This 20-20-60 mixed solvent may
therefore be more generally acceptable than acetic acid alone. The effect
of this solvent upon fillers commonly used in urea-resin glues is shown in
table 2 to be of a reasonable order of magnitude. It was tried upon various
urea resins, fortified areas, and melamine glues, and in all of these its
behavior was almost identical to that of 20 percent acetic acid. Quantitative data obtained by method 3 with 20 percent acetic acid and by method 6
with 20-20-60 solvent upon several urea resins arc shown in table 4.
The results of filler analysis of the six proposed methods may be
summarized as follows: IlOthod 1 with 50 percent alcohol gave insoluble contents for the straight urea resins somewhat higher than appeared reasonable.
Method 2 with hot water gave results too high for the melamine and fortified
urea glues. Mothod 3 with 20 percent acetic acid gave results of a reasonable order of magnitude except in the single instance of Bakelite BCU-16257
with which some immediate precipitation occurred. Method 4 with 90 percent
formic acid, and Method 5 with boiling dilute NaOH solution, had too great a
solvent action on sot urea resin employed as a filler. Method 6 using
20-20-60 mixed acetic acid and alcohol solvent gave results of a reasonable
order of magnitude -Ind reasonable precision, including the ono glue with
which 20 percent acetic acid failed.
For a method generally applicable to all types or urea-formaldehyde
resin slues, the use of 20-20-60 mixed solvent is to be preferred to any of
the othr solvents examined in this study. It has worked satisfactorily on
straight urea, fortified urea, and melamine-resin glues, giving results of
about the expected order of magnitude in instances where there is some basis
for a prior prediction. Method 6 is precise, rapid, easy to employ, and
without hazard to the analyst. In consideration of the advantages of method
6 over the other methods, this procedure is recommended for the proposed
Federal Specification on urea-formaldehyde glues and for Specification AN-G-8
whom next revised.
No. 1333
-5-
APPENDIX
Method 1 for insoluble filler b r 50percent ethyl alcohol
At least 100 grams of glue shall be mixed according to the manufacturer's directions, the weighing being done with an accuracy of at least 1/10
of ono percent. To approximately 2 grams of this mixed glue, accurately
weighed to the nearest millicrtm in a 100-cubic centimeter beaker or
Erl=eyer, flosk, shall be added 50 cubic centimeters of 50 p;rcent (by
volum) ethyl alcohol solution in water and the rnixturc stirred c)i . 1 minute.
Ther shall be added 0.500 gram of acid-washed diatomaceous ,,arth.t.thz.-.t has
been previously oven dried overnight at about 110° C. and cooled in a
desiccator. The stirring shall b continued for 2 more minutes and the suspension immediately filtered through a previously welched sintor , x1 glass
crucible of medium porosity. All the solid material remaining in the beaker
shall be washed carefully with a total of 50 cubic centimeters of 50 percent
ethyl alcohol solution onto thc, filter after which the crucible shall be
dried at about 110° C. for 16 hours, cooled in a desiccator and weighed. The
rDsiduc minus 0.500 gram, the 11,..iht of the diatomaceous earth, shall 1).; considered as the filler content of the glue, and its amount shall be calculated
as a percentage of the amount of nonvolatile content of the glue as determined in method 7.
Method 2 for insoluble filler by hot water
At least 100 grams of glue shall be mixed, according to the manufacturer's directions, the weighing being done with an accuracy of at least
1/10 of ono percent. Approximatel y 2 grams of this nixed glue shall be
accurately weighed to the nearest milligram into a 100-oubic centimeter
beaker or Erlenmeyer flask. Fifty cubic centimeters cf water shall be brought
to a boil, added at once to the glue, the mixture vigorously stirred for 1/2
minute, th,3n immediato1:7 poured through a sinte,rod glass crucible of knoan
weight that has just been heated by another portion of boiling water poured
trough it. Thu mixture of mixed glue in hot water shall he rapidly filtered
with suction and th,_1 residue washed with 50 cubic c:;ntimetors of boiling
water. The crucible shall then be dried for 16 hours in an oven at about
110* C., cooled in a desiccator, and weighed. Thc, residue shall ho consider.:, d as the filler content of the Llue, and its amount shall be calculated
as a percentage of th,.: amount of nonvolatile content of the glue as determined in method 7.
-Jilter-cel and Colite Analytical Filter-aid have been found suitable for
this purpose.
No.
1333
Method
3 f2E.12222.2121E112.11EIL12 .2ercent acetic acid
At least 100 grams of glue' shall be mixed according to the manufacturor's directions, the weighing being done with an accuracy of at least
00 of one percent. To approximately 2 grams of this mixed glue, accurately
weighed to the nearest milligram ma 100-cubic centimeter beaker or
Erlenmeyer flask shall be added 50 cubic centimeters of a cold solution made
of 20 parts of glacial acetic acid and 80 parts of distilled wator by volume.
This mixture shall be thoroughly stirred for 1 minute. Then shall be added
0.500 gram of acid-washod diatomaceous earth that has been previously oven
dried overnight at 110° C. and cooled in a desiccator'. The stirring shall
be continued for two more minutes and the suspension immediately filtered
through a previously weighed sintered glass crudible of medium porosity. All
the solid material remaining in the beaker shall be washed out carefully with
20 percent acetic acid solution onto the filter. The residue on the fil t er
shall be washed with a total of at least 50 cubic centimeters of 20 percent
acetic acid, then with cold distilled water to remove the acid, after which
the crucible shall be dried at about 110° C. for about 16 hours, cooled in a
desiccator and weighed. The residue minus 0.500 gram, the weight of the
diatomaceous earth, shall be considered as the filler content of the glue,
and its amount shall be calculated as a percentage of the amount of nonvolatile content of . the glue as determined in method 7.
Method 4 for insoluble filler by 90 ercent formic acid
At least 100 grams of glue shall be mixed according to the manufacturer's direction, the weighing being done with an accuracy of at least 1/10
of one percent. To approximately 2 grams of this mixed glue, accurately
weighed to the nearest milligram in a 100-cubic centimeter beaker or
Erlenmeyer flask shall be added 30 cubic centimeters of 90 percent formic
acid. The mixture shall be swirled around gently at intervals, then
allowed to stand . overnight, after which it shall be filtered through a pre.
viously weighed sintered glass crucible of medium porosity using suction.
The flask and residue shall be washed with two 20 cubic centimeter washes
of distilled water and then with 20 cubic centimeters of denatured alcohol.
After preliminary drying by suction the crucible shall be placed in an oven
at about 110° C. for 30 minutes, then cooled in a desiccator and reweighod.
The residue shall be considered as the filler content of the glue and its
amount shall be calculated as a percentage of the amount of nonvolatile
content (ff . -the glue as determined in method 7.
Method r for insoluble filler by rolonTed boilin in
slightly alkaline water
S
Apt least 100 grams of glue shall be mixed acoording to the manufacturer's direction, the weighing being done with an accuracy of at least 1/10
of one percent. One gram of this mixed glue, accurately weighed to the
nearest milligram in a large flask shall be added to 2 liters of water treated
wi th 3 oubic centimeters of 20 percent sodium hydroxide solution, boiled
4 hours under reflux to prevent loss of water, filtered through a previously
for
No. 1333
-7-
weighed Gooch or sintered glass crucible and dried at about 110° C. for 16
hours, cooled in a desiccator and weighed. The residue shall be considered
as . the filler content of the glue, and its amount shall be calculated as a
percentage of the amount of nonvolatile content of the glue as determined
in method 7..
Method .6 for insoluble filler by 20 percent acetic acid
plus 20 percent ethyl al ohol.
At least 100 grams of glue shall be nixed according to the manufacturer's directions, the weighing being done with an accuracy of at least
1/10 of one percent. To approximately 2 grams of this mixed glue, accurately
weighed to the nearest milligram in a 100-cubic centimeter beaker or
Erlenmeyer flask shall be added 50 cubic centimeters of a cold solution made
of 20 parts of glacial acetic acid, 20 parts ethyl alcohol, and 60 parts of
distilled water by volume. This mixture shall be thoroughly stirred for 1
minut, and then shall be added 0.500 gram of acid-washed diatomaceous
earth— that has been previously oven dried overnight at 110° C. and cooled
in a desiccator. The stirring shall be continued for 2 minutes more and the
suspension immediately filtered through a previously weighed sintered glass
crucible of medium porosity. All the solid material remaining in the beaker
shall be washed out carefully with the same acetic acid-ethyl alcohol
solution onto the filter. The residue on the filter shall be w a shed with a
total of at least 50 cubic centimeters of the acetic acid-alcohol solution,
then with 95 percent ethyl alcohol to remove the acid, after which the
crucible shall be dried at about 110° C. for 2 hours, cooled in a desiccator,
and weighed. The residue minus 0.500 gram, the weight of the diatomaceous
• earth, shall be considered as the filler content of the glue, and its amount
shall be calaulated as a percentage of the amount of nonvolatile content of
the glue as determined in method. 7.
Method 7 for nonvolatile content
About 1 gram, accurately weighed, of the mixed glue shall be transferred from a suitable weighing bottle onto a previously weighed glass
microscope slide or small petri dish and distributed evenly over most of the
surface. The slide or dish with its thin layer of glue shall be allowed to
dry in warm air at from 110 0 to 140 0 F. , as on top of an oven or over a hot
radiator, for 1 to 2 hours. The slide shall then be put in a drying oven at
about 110° C. for 24 hours, after which time it is cooled in a desiccator
and reweighed. The residue after 24 hours drying, calculated as a percentage
of the wet mixture, shall be considered as the nonvolatile content of the
glue and shall serve as a 'basis for calculating the filler content as determined in one of the preceding methods.
/To. 1333
-8-
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Table 2.--Solubility in various solvents of three materials used as
fillers in urea-formaldehyde glues
Percentage loss in weight
: Approximate
time
Set
:
Walnut
for
Maple :
g :urea-resin–
1
2
solution– :wood flaur–:shell flour
:
Solvent
Cold water .....
minutes
3.0
Hot water.........: 1 minute
50 percent alcohol; 4 minutes
3.8
:
3.5
2.7
5.1
4.4
8.2
3.9
6.4
10 percent acetic
acid ...: 4 minutes
3.9
20 percent acetic :
acid : 4 minutes
39
20 percent acetic :
:
acid plus 20
percent alcohol: 4 minutes
5.3
5.2
4.4
90 percent formic
:16 hours
acid.... . 6.1
4.7
24.5
37.6
37.4
91.8
Hot 90 percent
formic acid...,: 5 minutes
Hot dilute NaCH
4 horirs
7.9
;
:
4.0
:
7.9
100
•
-These times are approximately the periods of contact between glue and
solvent in the several methods 1 to 6.
-Dried at 110° C. for 1 hour and kept in a desiccator before use.
.gCommercial product, approximately 100 mesh, dried over calcium chloridein a des ccator before use.
i
No. 1333
Table 3.;-Percentage of nonvolatile content of three urea-resin
solutions determined by 20 percent acetic acid and
by92ELE112.di.r&
Resin syrup
(undiluted and
without
hardener)
Bakelite BOU-12772
Nonvolatile content
Thrown down' ,
:Insoluble in 20 per-:
: cent acetic acid : by centrifugingA
Percent
Percent
4.1
0.9
Perkins L-100 UTormite 430 1.4
8.2
4.0
7.6
1
– Calculated from the nonvolatile contents before and after
centrifuging.
Table 4.--Percent insolubles in five urea-resin glues when analyzed
with 20 ercent acetic acid and with 20-20-60 solvent
Percentage of the nonvolatile content
Urea-resin
glue
:Insoluble in 20 percent:Insoluble in 20-20-60
solvent
acetic acid
1
Bakelite BO-12772–w:
Bakelite
nil 49
o•
Beetle Cement A....
• •
1
Perkins L-100– Uformite 4o0
1
•
•.:
0.9
0.9
15.8
15.4
17.5
17.5
1.4
.7
8.2
8.4
IThese glues contain no wood flour or walnut shell flour.
No, ,1333