142
Ind. Eng. Chem. Prod. Res. Dev. 1982, 21, 142-145
Phenolic Thermospheres. Chemical Design and Principles George L. Brode, Peter W. Kopf, and Sui-Wu Chow' Union Carbkie Corporation, Coatings Materials Division, Bound Brook, New Jersey 08805
Particulate heat-reactive phenoVformaklehyde resins were prepared by an aqueous suspension polymerization process. The process, conducted in a low-viscosity medium, affords precise end-point control, and yields products of uniform composition which were isolated as a fresflowing powder directly, without the need for a grinding operation. In contrast, the conventional bulk process for preparing one-step resins depends on cooling of a viscous, molten mass to arrest the polymerization, producing products with large variations in degree of polymerization. Moreover, particulate resdes prepared by this new process display superior sintering resistance and storage stability. The cure characteristics, as indicated by dynamic mechanical analysis, are comparable to resoles obtained by the conventional bulk process.
Introduction The preparation of solid resoles involves the thermal condensation polymerization of various reactive phenol/ formaldehyde products. If uncontrolled, the formation of insoluble and infusible thermoset compositions results. Thus, the conventional preparative process requires exact temperature and time control. Rapid discharge and cooling of the viscous molten resin are necessary to obtain a material within the range of predetermined end point (Brode, 1981; Knop and Scheib, 1979). Moreover, the resoles are low molecular weight materials having poor sintering resistance; this makes it difficult to obtain solid resoles in a finely divided form. In this paper, we wish to describe the preparation of particulate, heat-reactive phenol/ formaldehyde resins by an aqueous suspension polymerization process. Process development and pilot plant studies are discussed elsewhere (Regina-Mazzucaet al., 1981). This unique polymerization process for the manufacture of particulate phenol/formaldehyde resins employs a protective colloid for the formation of a suspension from which the resole can be isolated in a free-flowing, finely divided state. The particle size can be controlled in the 10-100 pm range. The polymerization is conducted in a low-viscosity medium which affords precise end-point control and yields a product of uniform composition. In contrast, the conventional bulk process depends on rapid cooling of a reacting, viscous molten mass to arrest the polymerization which often produces material with a large variation in degree of polymerization, even within a given batch. The particulate resoles produced by the new process display superior sintering resistance and storage stability. This can be attributed to the increased hydrophobicity and higher glass transition temperatures of the products. Nuclear magnetic analysis shows that the particulate resoles contain slightly less methyl01 functionality and increased benzylamine and methylene bridges as compared to the microstructures of conventional bulk resoles. Together with the protective colloid, these subtle but discernible differences account for the improved handling characteristics of the product. Experimental Section Preparation of particulate resoles can be illustrated by the following example based on phenol: phenol, 100 g; 50% aqueous formaldehyde, 72 g; hexamethylenetetramine, 9 g; gum arabic, 1 g; water, 83. The above were charged into a Morton flask equipped with a mechanical stirrer, thermometer, and reflux con0196-432118211221-0 142$0 1.2510
denser and heated to 85 "C. A clear solution was formed initially. After about 5 min, the solution became cloudy (cloud point) and turned into a creamy, tan-colored suspension. An exothermic reaction ensued which could be easily maintained at 85 "C with a water bath in the laboratory. After the exotherm had subsided, heating was resumed to maintain the reaction at 85 "C for 30-100 min. The precise length of reaction time depends on the flow property of the product desired. After the predetermined reaction time had elapsed, the dispersion was cooled to less than 40 "C. On large-scale preparations, cooling can be effectively accomplished by vacuum reflux. The reaction was diluted with about 500 mL of water at ambient temperature and subsequently washed by decantation with 500-mL portions of water. The particulate resole was isolated by filtration and dried at ambient conditions for about 10 min and then at 60 "C for 20-30 min in a Pfaltz and Bauer fluidized bed drier. The particulate resole wgs characterized by 150 "C Hot Plate Gel Time (stroke test) and 125 "C Inclined Plate Flow (P.F.). Inclined Plate Flow and Hot Plate Gel Time are commonly used methods in the phenolic industry to assess the suitability of a product for a given application. Inclined Plate Flow. A 1-g sample of a pulverized product was compressed to a pellet 12-13 mm in diameter. This pellet was placed on a glass plate and heated for three minutes in a 125 "C oven. The plate was then tilted to a 60" angle and heating continued for an additional 20 min. The distance (measured in millimeters) the resin traveled is known as plate flow (P.F.). These values reflect both the melt viscosity and the cure rate of the resin. Hot Plate Gel Time. A 1-g sample was heated at 150 "C on a hot plate while being stroked with a spatula at a rate of about 1 stroke/s. Strands were formed with each stroke as long as the material remained thermoplastic. The time (in seconds) required for the strand formation to cease is defined as the gel time. The method is a measure of the cure rate. Aliquot samples were withdrawn from the reaction mixture in one experiment. The resin and aqueous phase were separated by means of high-speed centrifuge and quantities of each weighed. The unreacted phenol was measured by gel permeation chromatography, while the formaldehyde was determined by the hydroxylamine method. Gel permeation chromatography was performed on a Waters Associates HPLC with a column bank consisting of 1000, 500, 500, 100, and 100 8, p-styragel. Tetrahydrofuran was used as the solvent. Particle size and particle size distribution were determined using Coulter Counter Model TA2. DMA ther0 1982 American Chemical
Society
Ind. Eng. Chem. Prod. Res. Dev., Vol. 21, No. 2, 1982 143
Table I. Preparation of Particulate Resoles by Suspension Polymerization fraction unreacted phenol wt% wt%of resin aqueous resin aqueous phase
phase
phase
51 47 55 56 42
49 53 45 44 58
0.3 0.18 0.15 0.10 0.06
MOO
I
/
A
phase
10 MIN REACTION l!Ml
~
...-..__. 20 MIN ~
-
40 MIN
---- 80 MIN -. - - 100MIN
20
60
40
1 oo
80
TIME, MINUTES
Figure 2. Molecular weight distribution. Table 11. Microstructures of Solid Resole
:
I'
1 _/' 15
1 I
20
"
,
'i
25
WIN.) Figure 1. Gel permeation chromatogram of particulate resole. ELUTION TIME
mograms were obtained using a Du Pont Model No. 981 Dynamic Mechanical Analyzer. Samples were supported on woven glass cloth.
Results and Discussion General Description of the Process. The aqueous suspension polymerization process employs hexamethylenetetramine (hexa) as the catalyst and a protective colloid to promote formation of a suspension which permits the isolation of the solid resole as discrete and finely divided particles. The chemistry of the process can be envisioned as involving the following stages: (1)condensation of phenol/hexa/formaldehyde to yield a low molecular weight, but water-insoluble, intermediate; (2) formation of a resin-in-water suspension; and (3) further polymerization of the initial condensation products, yielding resole resins of the desired degree of polymerization. The appearance of a cloud point within a few minutes at 85 "C and the fact that about 60% of the phenol and 40% of the formaldehyde (Table I) were consumed in the first 10 min indicate that the initial condensation to water-insoluble species is very rapid. In the presence of a protective colloid and agitation, a suspension of resin in water results. The relative amounts of the resin and the aqueous phase reached about 50/50 (by weight) in less than 10 min and remained at this relative ratio throughout the reaction (Table I). The bulk of the unreacted phenol
structure ArCH,OCH,OH ArCH,OH ArCH,OCH,Ar ArCH,-Ar ArCH,N
particulate resole 9parts
hexa 2 25
6 26 41
partic-
ulate
hexa
bulk resole Gparts hexa
4 22 4 34 33
0 30 8 28 32
resole
6parts
(280%) is in the resin phase, suggesting that further condensation polymerization is in the resin phase. The entire polymerization process takes place in contact with a low-viscosity medium and, therefore, eliminates the heat dissipation problem encountered in the conventional bulk process, thus affording precise control of the end point of the polymerization. As shown by GPC at different time intervals, the number average molecular weight advances relatively slowly, while the weight average molecular weight increases rapidly, resulting in a resole of high dispersity in molecular weight distribution (Figures 1and 2). Depending on the length and temperature of polymerization, products with 150 "C gel times ranging from 20 to 120 s and plate flow values of 12 to 100 mm have been obtained. Role of Hexamethylenetetramine (Hexa). Hexa (or ita equivalent) is advantageous for the production of free-flowing, easily isolable, particulate resoles. The hydrolysis of hexa has been shown to yield aminomethylated products so that hexa or an equivalent amount of ammonia and formaldehyde can be used interchangeably (Knop and Scheib, 1979). As expected, when ammonia is used in this process, the particulate resole produced is essentially equivalent to that prepared with hexa a t equivalent concentrations. Low molecular weight amines, such as methylamine and dimethylamine, also yield particulate resoles. However, the products display
144
Ind. Eng. Chem. Prod. Res. Dev., Vol. 21, No. 2, 1982
Table 111. Glass Transition Temperature and Moisture Absorption of Particulate Resole
Tg (DMA), resin
"C
particulate resole
48-54 33-43 58-74
bulk resole Novolac - - .-.._ _ _
I
FREOUENCY
.....- ..., DAMPING
wt % gain,
22 'C, 100% RF. 24 h 2.5-3.3 7.2-8.4 5.7-6.5
FREQUENCY DAMPING
1
1
1
40
1
1
a0
1
1
1
120
1
1
1
160
1
1
200
'C
Figure 4. DMA thermogram of particulate resole.
r-
'I)oz
-
GUM ARA111C
,'
-
40
80
120
160
Ma
'C
1
c
i
6 -
Figure 3. DMA thermogram of bulk resole.
inferior sintering resistance when compared to hexa-catalyzed products. The microstructure of various particulate resoles was determined by I3C NMR and the results are summarized in Table 11. A hexa-catalyzed bulk resole is included for comparison. The data show that the particulate resole contains less methylol structure but increased benzylamine and methylene bridges. The use of hexa for the production of solid resole has been practiced in phenolic resin manufacture (Knop and Scheib, 1979). The formation of benzylamine and azomethine structures in hexa-modified resoles is known and the characterization of these structures from the reaction of hexa with phenol and novolacs by NMR spectroscopy has been reported previously (Kopf and Wagner, 1973). I t is believed that these subtle but discernible differences in microstructure account for the lessened tendency to absorb moisture and increased glass transition temperatures of the particulate resoles. The water absorption data and glass transition temperatures (T's) as determined by Dynamic Mechanical Analyzer (DhA) are summarized in Table 111. Cure of Particulate Resole. The curing behavior of the particulate resole as compared t o the conventional resole was examined by dynamic mechanical analysis (DMA). The dynamic mechanical spectrum measures resonant frequency (proportional to storage modulus) and the damping (proportional to the ratio of loss modulus to storage modulus) as the sample is cured (Hassel, 1978). For these experiments, the resins were supported by woven glass cloth and subjected to a heating rate of 5 "C/min. The spectra are shown in Figures 3 and 4. In the 80-130 "C region, the resin exists as a viscous liquid and the resonant frequency is at a minimum. As the resin cures, the modulus increases, resulting in the increase of the resonant frequency. The onset of cure is considered to occur when this frequency begins to rise. Both the particulate resole
0.32 pm
i I
i I 5
0
1
I 2W
MICROMITIR
Figure 5. Particle size distribution.
prepared by the suspension process and the conventional bulk resole exhibited the onset of cure in the region of about 130-140 "C, reflecting the similarity in the basic structural units of the resoles. Protective Colloids. In the preparation of particulate resoles by suspension polymerization, the protective colloids serve three functions: (1)as an aid in particle formation, (2) as a stabilizer for the particles once formed, and (3) to prevent agglomeration during isolation. A number of colloids were studied and among these gum arabic is particularly effective in yielding easily isolable products having flow properties suitable for a broad range of applications. Gum arabic is a naturally occurring polysaccharide whose constitution has been described (Whisler and Bemiller, 1973). The use of gum arabic for the preparation of stable aqueous phenolic dispersions has been described previously (Harding, 1974). The particle size and particle size distribution depend on type and concentration of the protective colloid, agitator and reactor configuration, and volume fraction of the dispersed phase. For a given reactor design and volume fraction of the dispersed phase, the particle size is a function of the concentration of the protective colloid and, in general, higher concentrations favor the formation of smaller particles. A typical particle size distribution, as measured by Coulter Counter, obtained using gum arabic as the protective colloid is shown in Figure 5. In this
Ind. Eng. Chem. Prod. Res. Dev., Vol. 21, No. 2, 1982 145
Table V. Relative Reactivity-PTS vs. Bulk Resole
Table IV. Sintering of Particulate Resole
P.F., 32"C, resin particulate resole bulk resole/Novolac Novolac
mm 28 61 78
65h
NS NS NS
38"C, 24 h
43 "C, 20 h
NS NS sintered NS
NS sintered fused NS
experiment, the population expressed as weight percent is shown as a function of particle diameter and the weight mean diameter is 31.5 pm. As expected, the weight mean overemphasizes the larger particles and tends to distort the mean diameter toward the larger particles. Measurements based on number populations show that about 90% of the particles are in the range 23-12 pm and the number mean is about 6 pm. Sintering Resistance. The sintering behavior is an important consideration in the handling and storage of a pulverized resole. Sintering can be described as the fusion of particles to form a porous mass caused by viscous flow of the resin. Environmental conditions such as temperature, pressure, humidity, and time all contribute to the sintering of certain pulverized phenolic resins. Laboratory experiments show that the particulate resoles display superior sintering resistance as compared to most of the commercial, pulverized one-step resins. In these experiments, the resins are placed in a test tube to the height of 4 in. and stored in a constant-temperature oven a t 32, 38, and 43 "C for a prescribed length of time. The conditions of the resins were then characterized, by visual observation, as unaffected, sintered, or fused. A sintered sample is one in which there is partial fusion but which can be reparticulated by external pressure. However, the original particulate size distribution cannot be obtained without regrinding. The results (Table IV) show that the sintering resistance of the particulate resoles prepared by the suspension process is intermediate between the conventional heat-reactive products and a novolac. The improved sintering resistance of the particulate resole can be qualitatively correlated with the glass transition temperature of the resins and decreased tendency to absorb moisture from the atmosphere. Storage for 24 h a t 100% RF resulted in about only 3.2 w t % weight gain for the particulate resole, while a model commercial pulverized one-step resin gained 7.2 5%. The results are summarized in Table 111. Cure Rates. The relative reactivity of the particulate resole at 25-60 "C as compared to a bulk resole was
resin
PTS bulkresole
rate constant, day-' 60°C 40°C 25°C
1nA
E, kcal/ mol
0.029 0.097
44.6 38.1
30 25.2
0.67 1.50
0.003 0.018
evaluated by measuring the inclined plate flow values as a function of time and temperature. The data were treated as first-order kinetics according to the equation In ( l / P . F . - 13) = k t (days) + C The cure rate constants (k),activation energies, and the frequency factors were calculated from the usual Arrhenius extrapolations and are summarized in Table V. The results show that the particulate resole cures at about 0.1 times the rate of bulk resole at room temperatures. However, this differential reactivity decreases as temperature is increased; at 60 "C it is about 0.5 times that of a bulk resole. Dynamic mechanical analysis thermograms show that both the particulate resole and the bulk resole exhibit the onset of cure at 130-140 "C region. Therefore, the particulate resole achieves improved shelf stability with no significant loss of cure speed at elevated temperatures. These data also allow the estimate of the expected shelf-life. If we know the storage temperature (2') and the acceptable plate flow range (P.F.i - P.Fef)for a given application, the shelf-life (t,J can be calculated as t,,(T) = (l/k) In [(P.F.i - 13)/(P.F.f - 13)l
Literature Cited Brode, G. L. I n "Encyclopedla of Chemical Technology"; Kirk-Othmer, Ed., 3rd ed.; Wlley: New York; in press, 1981. Hardlng, J. U S . Patents 3944703, 1976 and 3 623 103, 1974 (to Union Carb k 6 Corporation). Hassel, R. L. I d . Res.lDev. Ocl 1978, 160. Knop, A.; Scheib, W. "Chemistry and Applications of Phenolic Resins"; Springer-Verlag: New York, 1979; p 64ff, 38ff. Kopf, P. W.; Wagner, E. R. J . Polym. Sci. 1979, 1 1 , 939. Reglna-Mauuca, A. M.; Ark, W. F.; Jones, T. R. American Chemlcal Society, Dhr. Org. Coat. Plast. Chem. Preprints, l 8 l s t Natlonal Meeting, Atlanta Qa.. Aor 1981. Whisier, R. L.; BeMiiier, J. N. "Industrial Gums"; Academic Press; New York, 1973; Chapter X.
Received for review July 28, 1981 Accepted September 14, 1981
Presented at the 181st National Meeting of the American Chemical Society, Atlanta, Ga., Mar 1981, Division of Organic Coatings and Plastics.
