Reaction and Cure in Melamine Plastics INFLUENCE OF CROSS LINKING ON PROPERTIES H.
P.
WOHNSIEDLER, 1. H. UPDEGRAFF, AND R. H. HUNT, JR.
American Cyanamid Co., Stamford, Conn.
Optimum properties of melamine resins and other plastics can be realized when the relationship
of particular properties to cure i s known. Empirical methods can be used to achieve this but it i s of interest to know what significance attaches to cure when i t i s expressed in terms of degree of reaction (Lm). The measurement of frn over a broad range of cure for resin in the molded state was conducted. Physical properties of the plastic over the same cure range were determined and related to Lm. Heat distortion, flexural strength, and water absorption reach their optimum values a t different degrees of reaction whereas modulus of elasticity undergoes little or no change with degree of reaction. Degree of polymerization and polymer structure are also related to degree of reaction but a number of alternative interpretations of average states are possible. By assigning the methylene bridge as the most plausible linking mechanism i t appears that the best properties of the plastic are associated with significant cross linking.
ELAMINE resins and plastics undergo a marked change M in properties the curing reaction proceeds. One the purposes of this work was to explore this change and to relate the of
as
physical properties of melamine plastic to the degree of reaction. As the curing reaction advances the polymer size of necessity must increase. It, therefore, becomes possible to obtain some idea of the average degree of polymerization for a given degree of reaction. This requires that the reaction mechanism be known, or a plausible mechanism be assigned. When the degree of reaction has reached a critical value where linear polymer growth no longer serves as a satisfactory structural explanation it becomes necessary to interpret the reaction in terms of cross linking. Thus another purpose of this work was to determine what significance experimentally determined degree of reaction values have for polymer structure in this system. Bifunctional condensation systems, or those yielding linear molecules have been studied by Carothers ( 1 ) and Flory ( 8 ) . When equivalent amounts of the reactive groups A and B are present, the degree of reaction ( p ) and the average degree of polymerization (DP,) are related.
DP, = I 1-P where p = fraction of A and B groups reacted. In the condensation of p-cresol dialcohol (2,6-bishydroxymethyl-4-methylphenol) where only linear growth takes place through formation of dimethylene ether linkages, Kammerer (7) determined p from the weight of water discharged on heating. In this case 2 x Moles condensation water = Moles original methylol
*
For 1/1 - p , equal in this case also to the average DP,, values of 83 were obtained by heating 4 hours a t 130' C. I n the phenolformaldehyde system the average DP, of the novolac, composed of linear chains, has been estimated at 7 from determination of the molecular weight by freezing point methods as well as by estimating p (8). 82
As reaction proceeds in polyfunctional condensation systems of higher functionality, beyond the point where the reaction can be interpreted in terms of formation of linear or branched chains, the situation becomes more involved. I n the kinetics and statistics developed for these systems by Flory (8-6) in which monomer functionality is greater than two, the simplifying assumption is made that reactions occur only between functional groups on different molecules. This is admitted to be a possible source of error (6). In a previous paper (9) the degree of reaction was determined for an experimental melamine-formaldehyde resin in the unmolded and molded states. The polymer structure was proposed as being one in which melamine residues are joined by methylene bridges. In the unmolded state the degree of cure corresponded with an average DP, of 2 to 3. In the cured state it was necessary to postulate from the degree of reaction that intramolecular reaction or cross linking of groups within individual molecules had occurred. The polymer was pictured as being largely of a network type in which branched chains were infrequently cross linked with a DP, above 20 to 30. This work has now been extended to include resips in intermediate and more advanced degrees of cure.
Description of Resins Two molding resins were used in this work. An experimental, unfilled resin was made by reacting formaldehyde and melamine under slightly alkaline conditions in the molar ratio of 2 to 1. After reaction in aqueous solution the resin was recovered by vacuum concentration after which its degree of polymerization was advanced by heat treatment to convert it to suitable molding plasticity. The resin in this form is referred t o as the unmolded state of the resin. Test bars, '/z X l / 4 X 5 inches, were molded a t a temperature of 310' F. and a pressure of 7500 pounds per square inch. Moldings were made over a range of cure times from undercure to overcure. A series of identical bars were molded a t each cure time; in each series several bars were ground to
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 48, No. 1
THERMOSETTING RESINS Table 1.
Analysis of Unfilled Melamine Resin at Various Stages of Cure and Estimated Irn Values
(Analysis on dry and true melamine-formaldehyde polymer basis, original results corrected for moisture, methoxyl, ash, buffer ( 9 ) ) Cure Time a t 310: F., Min. U n mo1ded Molded 0.5 1.5 2.5 3.5 4.5 5.5 7.5 10.5 15.5 20.5
Nitrogen, % 49.10
FormaldeMe,
Melamine, % 73.67
49.74 50.26 50.38 50.67 50.97 51.09 51.38 51.60 51.79 52.04
,
34.56
Condensation Water/100 Grama Grams Moles 8.23 0.457
34.89 36.30 35.06 35.20 34.52 35.71 35.58 35.38 36.48 36.38
9.51 10.71 10.65 11.23 10.89 12.37 12.66 12.80 14.17 14.46
%
74.62 75.41 75.59 76.03 76.47 76.66 77.08 77.42 77.69 78.08
powder for analysis and the remaining ones were used for physical tests. Thus, the degree of cure in the bars analyzed and in the corresponding bars tested was the same within the accuracy of reproducing the molding cycle. Since the cycle could be timed precisely, this element of error was considered small for the slow-curing experimental molding material. This molding resin was prepared in similar manner and with substantially the same starting composition as the resins described in the earlier work (9). The analysis of all resins was carried out in an identical manner. A cellulose-filled melamine molding resin was the other type examined; it represents a commercial product used in the manufacture of dinnerware, housings, and kitchen accessories. The complexity of the commercial product made i t impractical t o estimate the degree of reaction by chemical analysis.
0.528 0.595 0.592 0.624 0.611 0.687 0.703 0.711 0.787 0,802
-NHz
I
-NH
+ HOCHz.NH+ HOCH2.NH-
-NH.CHz.NH-
-+
I
+
--NeCH2.NH-
+ H20 + H2O
+ HOCHz-NH-
+
-NH.CHz.N-
N where R =
4
N
\c/ I
HzN.R.NH.CHz0H
+ H2N.R.NH.CHzOH
NH
NH
CHz
CHZ
OH
OH
-,
NH
"
CH2
CH*
NH
OH
1.96 1.96 1.94 1.94 1.89 1.96 1.94 1.91 1.97 1.96
- - - - -NH.R.NH.C&-
-HN.R.NH.CH2NH
NH
CH2
CHz
NH
NH
HOCH2.HN.R.NH.CHz-
- - - - - -N.R.NH.CHzCH2 OH
'
As in the earlier work (9) the degree of reaction will be represented by Lm where Moles condensation water Moles melamine
For straight or branched chains
DP,
1 1 - Lm
=
This relationship ceases to apply when cross l i k i n g begins and Lm > 1.0. Expressions have been derived (9) for the relationship of DP, and Lm when the latter is >1.0. These are for idealized polymer system only. In graphical form the various expressions may be used for visualizing the type of polymer and the various degrees of cross linking corresponding with any Lm value as well as the maximum degree of cross linking which can occur. The relationship of Lm to p' where p' is defined as Moles condensed methylol Moles original methylol
or
Moles condenestion water Moles original CH,O
(methylol = -CHzOH)
-HN.R.NH.CHz.NH.R.NH.CHz-
January 1956
0.99 1.03 1.01 1.13 1.15 1.16 1.28 1.30
0.QQ
and cross linking in the form
The course of the general condensation reaction may be represented structurally in its various phases: the initial formation of linear polymer
/ \ -C Cll
0.89
0,600
0.603 0.606 0.608 0.611 0.613 0.616 0.619
Lm =
+ HzO
0.592 0,598
1.96
-HN.R.NH.CHz.NH.R.NH.CHz-
CIlzOH --NH.CH20H
F M
Lm 0.78
The formation of branched chains, strongly favored by condensation theory (3, 6 )
Degree of Reaction, Cross linking For the present purpose it will be assumed that the condensation mechanism is exclusively that of methylene bridge formation. This is justified to a large extent on the basiv of existing knowledge. No distinction is made between the following mechanisms whereby this linkage may be arrived at:
Melamine Moles/ 100 grams 0.585
NH
NH
CHZ
CHz
OH
OH
is Lm = 2p' or for the more general case
Lm = p' X
F where -.is F M
M
the formaldehyde melamine molar ratio
By reference t o Table I the Lm values determined for un6lled resin for the series of progressive cure times will be noted. I n
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
83
this series, the molding composition is assigned zero cure time. The test bar having the lowest cure time was made by closing the mold in 30 seconds on high pressure and immediately chilling the mold. This is considered as having '/2-minute cure. Several of the next higher cures also had to be chilled in the mold before removing the test piece. They were timed in the same way.
I ' 1 I
em
I
I O
oe 150 1
I
I
200 -
HEAT DISTORTION TEMP O C A
110 15000
HEAT DISTORTION TEMP%
FLEXURAL STRENGTH PS 1
'"i
10000
WATER ABSORPTION n
-
10
1 20
15000
CURE T I M E - MINUTES
Figure 1.
Unfilled melamine resin degree of reaction and properties versus cure time
FLEXURAL STRENGTH PS I 10000
0 5
A transition in polymer structure theoretically occurs when Lm > 1.0 (9). At this point some cross linking must take place. As reaction continues Jyith the resin in the set state and where Lm > 1.0 the network becomes more and more tightly knit. Since 2 moles of formaldehyde are present for each mole of triazine the Lm might be expected to approach a limiting value of 2. Data in Table I show that the cured plastic falls far short of this limiting value. The shape of the curve for Lm against cure time shows that the rate of liberation of water is rapid a t the start but that it soon decreases. -4fter 20 minutes in the mold further change is occurring very slowly. At this point 1.3 molecules of water have been liberated for each triazine ring. Assuming only methylene linkages, 0.7 methylol group for each triazine ring remains unreacted. At the inception of cross linking the molded piece is not boil resistant but it becornes so after a further cure interval and increased cross linking. Such properties as heat distortion temperature and flexural strength attain their maximum values during the cure interval of significant cross linking.
Physical Properties of Resins The physical measurements applied to test bars included the determination of flexural strength, Young's modulus, heat distortion temperature, and weight increase in 30-minute boil test. Flexural strength and Young's modulus in flexure were detera4
WATER ABSORPTION
01 5 IO CURE TIME- MINUTES
Figure
15
2. Filled melamine resin properties versus cure time
Modulus. on the other hand, is only slightly influenced, if at all, by the advancement of cure. The test results are plotted in Figure 1. This result suggests thst the iigidity of melamineformaldehyde condensate at room temperature may be due in part to the strength of secondary forces such as hydrogen bonds. As the temperature is raised, the strength of secondary forcm falls rapidly, and the properties become more dependent on the presence of covalent chemical bonds. The heat distortion temperature roughly measures that condition where the modulus has fallen t o a standard low value. This property is of technological value, since it indicates the ability of the material to sustain loads and maintain its shape in service a t elevated temperatures. As expected, the heat distortion temperature rapidly increases with the build-up of methylene linkages in the plastic. The manner in which the heat distortion temperature increases as the cure proceeds in the unfilled resin is shown in
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 48, No. 1
THERMOSETTING RESINS 1
I
L50
1
I
1
I
1 I
I1
''
1 I
1 P 11
' '
1 1
1 1
*
-
HEAT DISTORTION TEMP. '6.
110
-
qI
15000 FLEXURAL STRENGTH
RS.1.
-
10000 -
-
I I
For any given Lm value a number of alternative, average molecu k r structures can exist as brought out in earlier work. When Lm is >1.0, however, structures can no longer be limited t o straight or branched chain8 as a satisfactory interpretation. Intramolecular linking must then enter the consideration of structure in order for this t o be harmonious with degree of reaction values and the postdated reaction mechanism. In this region of L m values > L O , the cross-linking density or fraction of units cross linked can be determined only for some arbitrarily assigned average size of primary chain according to the scheme of earlier work (9). A t values for Lm of
