Plasticity of Phenol-Formaldehyde Resins and Molding Powders LAWRENCE M. DEBING AND SAMUEL H. SILBERKRAUS
.
Monsanto Chemical Company, Plastics Division, Springfield, Mass.
A brief description of the principal methods ties and still have different HE complex nature of used in this country for determining the closing times (6). phenol-aldehyde r e s i n s and the Of plasticity of phenolic molding powders is of Figure a simple 2 illustrates form of extrusion the use molding powder formulations presented* one-stage and twomold for measuring plasticity. limit this paper to a presenstage resins are compounded with and The powder charge is placed tation and discussion of the without various standard fillers to illustrate in mold cavity A , and presmethods commonly used for plasticity measurements in this the inadequacy of any one method to sure on the powder is exerted country; some additional inclassify these materials properly as to plasmaterial through force B. through The plastic orifice formation is included on factors influencing the flow charticity. C, and by measuring the exacteristic of a molding powder. The factors to be considered in the plastruded length a t definite time All of the methods comticity determination are discussed on the intervals, the plasticity of the monly employed for deterbasis of experimental data presented. The powder can be graphically represented as shown in Figure 3. mining f l o ~ characteristics are comp~exities of this problem are such that dependent upon maintaining By studying plots of this type, test methods appear be the it is possible to compare variboth temperature and presonly practical methods available. Standous types of molding powders sure a t a c o n s t a n t value, ardization of one method would be desirby noting the following points: since the plasticity of a pheable, but no one test method appears to be the time required for fluxing nolic molding compound is the powder, the rate of flow as closely related to both factors. adequate. indicated by the slope of the The relation of the plasticity flow line, the over-all time for of the molding powder to temperature and pressure is a fundamental characteristic of flow to cease which may be regarded to some extent as the curing rate of the powder, and the over-all extent of flow the molding poader ( 6 ) . One of the first methods used for determining the plasticity which may be termed its “fluidity” or “plasticity”. or fluxing property of phenolic molding powders was the use Figure 4 is a diagram of Tinius Olsen’s Bakelite flow tester, of a simple flash-type cup mold as shovm in Figure 1. The as originally described by Peakes ( 6 , 7 ) . The principle of the mold is placed in a hydraulic press equipped with electrically operation is the same as that for the extrusion method. The procedure is t o place a preform or tablet of the molding powor steam-heated platens. The pressure is supplied preferder on ram surface A and, by releasing stop B , permit the ably by an accumulator system. The procedure is to charge t h e powder to be tested into the mold cavity and record the preform to be carried upward into mold cavity C. Weights D are the source of pressure which causes the plastic material time required for the mold to close completely. The mold is of the flash type so that excess material can be forced out to flow upward in the orifice of split cone E. The flow of the material forces follower rod F to move and record this moveof the mold at A. A large variety of molds could be used inment on chart G. -4clockwork causes the chart to move a t a stead of the cup mold illustrated; a multiple-cavity cup or tube-base mold of the flash constant rate, and by type is widely used. I n combining these two mo-
T
could both properly fill the same number of cavi-
FIQURE1 (left) FLASH-TYPE CUPMOLD,AND FIGURE 2 (right) EXTRUSION-TYPE MOLD 972
Figufe 6 also iilustrates the use of the extrusion
INDUSTRIAL AND ENGINEERING CHEMISTRY
August, 1941
973
It has been recommended that the plasticity of a thermosetting material be determined with the effect of cure reduced to a minimum @),but the relation of plasticity to cure is a matter of practical interest and cannot be ignored if the test is to be of practical value (3).
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TABLEI. RESINFORMULATIONS"
One-Stage Resin Two-Btage Resin Pure phenol 100 100 Formaldehyde (37.5% by weight) 110 72 Barium hydroxide 4 Oxalic acid 2 a The reactants were refluxed to the point of approximately complete consumption of formaldehyde and then dehydrated under a pressure of 50 mm. mercury.
W E I G H T EXTRUDED
FIGURE3. TYPICAL FLOW CURVES FOR MOLD
THE
TABLE11. MOLDING POWDER FORMULATIONS IN P E R CENT'
EXTRUSION
Rloldina Dowder A
B
C
49.5
49.5
b: 5
principle for determining plasticity. This test method was developed in the Bell Telephone Laboratories (1). The powder charge is placed in mold cavity A , and pressure on the powder is exerted through force B. The plastic material is forced to flow into a series of orifices, C, varying in diameter from 0.093 to 0.015 inch. The object of this method is to measure not only the plasticity of a material, but also its over-all moldability in various commercial molds. The moldability of a material is arbitrarily designated by measuring the length of flow in each orifice and multiplying the length by an orifice number. The sum of these products is referred to as the plasticity-set index of the material.
Experimental For comparative purposes typical one-stage and two-stage resins were selected. The details of resin and molding powder formulations are shown in Tables I and 11. Molding powders A and E, which differ only in resin type, were tested by the various plasticity methods described, and the data show that the recorded plasticity is a function of the test method as well as the material: Molding powder Bakelite flow tester Plasticity-set index Tube-base test (cup-type mold)
...
...
A
E
50 22.3 22
75 26.4 14
...
b15
D . E
49.5
0:5
.. .. .. .... 56:O .. .. 5010 ..
F
G
H
1
45:O 0.5
4k:O 0.5
4510 0.5
4k:O 0.5
4k:O 0.5
4.5 6O.C
j.5
2 . 2 5 1.0 50.0 50.0
50:O
.
.. ..
..
.. ..
..
The effect of cure on the plasticity of a molding powder is illustrated by the test results on molding powders E, G, H, and I, in which the hexamethylene tetramine content of a molding compound based on a two-stage resin is varied from 10 to 0 parts per 100 parts of resin: Molding
Olsen Bakelite
Run Tube-Base at Reduced Test
Plasticity-Set
Powder E
Tester
Pressure
Index
75 85 70 70
10 15 16
26.4 21.1 23.3
0 H
I
15
.. Off-s'cale a Compounded and processed same as A except that the resin is based on p-cresol instead of pure phenol. Ja
(60)
These data indicate that the orifice type of plasticity test method under the test conditions used does not demonstrate the effect of cure on plasticity to the same extent that the tube-base or pressure-distribution method does. B
The inconsistencies of these data indicate that a determination of plasticity in any one specific mold type will not give information from which can be predicted the behavior of a molding compound in a variety of commercial molds, and the desirability of a test method to express plasticity in absolute units is obvious. The relation of the various empirical test methods can be varied merely by changing the over-all dimensions of the test mold. This fact is demonstrated by the effect of varying the diameter of the orifices in the extrusion methods. For example, molding powder A differs from molding powder E by 50 per cent in the Bakelite flow tester, which has an orifice of 0.125 inch, but the same powder differs by 8 per cent as measured by orifice 2 in the Bell tester, which has an orifice of 0.078 inch. If the material were thermoplastic, then the force required t o produce flow would be that required t o overcome the internal resistance within the powder itself and the resistance due t o the mold surfaces. In the case of thermosetting molding powders, however, these two factors do not have a constant value, owing to the chemical changes occurring within the material during the test period.
..
Asbestos .. Cotton flock . .. .. 4 All molding powder formulations were processed on differential rolls of fixed temperatures and for the same length of time The ratio of resin to wopd flour has been held constant, and the lower percentage of two-stage resin is offset by the addition of hexamethylene tetramine.
FIQURE 4. BAKELITE FLOWTESTEROF TINIUSOLSEN
INDUSTRIAL AND ENGINEERING CHEMISTRY
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It should be noted, however, that under another set of test conditions-namely, a different mold temperature and pressure-the effect of cure could be more pronounced in either test. To avoid drawing erroneous conclusions regarding the similarity in plasticity of various molding powd e r s , t h e plasticity tests should be run a t more than one temperature to bring out the differences due to cure. T h e e f f e c t of h e x a methylene tetramine on cure was definitely shown by the p r o g r e s sively i n c r e a s e d rigidity of molded pieces containing t h e h i g h e r percentage of hexamethylene tetramine. These effects 0 .2 .4 .b .8 LO L2 /$ /.6 TVould i n d i c a t e t h a t t h e r a t e of INCHES change of resistFIQURE5. TYPICAL FLOW CURVES ance to flow FOR THE OLSENFLOW TESTER proceed a t a slower rate as the hexamethylene tetramine content of the molding powder is reduced, However, this tendency was only partially detected by the test methods used, and the results of plasticity determinations by these methods would point to the fact that variations in cure rate produce no differences in plasticity.
U
J
FIGURE 6. PLASTICITY-SET INDEX MOLD Resin reactivity or rate of cure can be studied in numerous ways in the case of resins without fillers. The rate of change of viscosity, solubility, melting point, refractive index, or chemical reactivity have all been used and are cited in the references listed. However, information obtained on the characteristics of the resin is not generally applicable to the moldability of a molding powder based upon the resin in question.
Vol. 33, No. 8
T h e effect on plasticity of the fillers used in compounding a molding powder is as great as that produced by the resin used. Little i n f o r m a t i o n is available to account for the effects produced by various fillers, but it is probable that the wetting or impregnation of the filler, as well as particle size and shape, are important. Figure 7 demonstrates the INCH effect of varying the fillers used. FIGURE 7. PLASTICITY-SET INDEX Flow Tester Molding powders Pressure, Plastioity-4, B,C, and D are Lb/Sa. Set Tube Filler In. Index Base based on the same A Wood flour 700 22 3 22 o n e - s t a g e resin, ~1 Asbestos 700 08-scale Off-scale ... and contain wood $ ~ ~ ~ ~ ~ f i o c ki i . 5 12 flour, asbestos, D* None 700 9.9 3-4 Processed 30 minutes, cotton flock, and no filler, respectively. Various grades of wood flour will, in themselves, produce variations in plasticity covering a wide range. The effect of the filler as demonstrated by these data is striking. Although the compounds with the unfilled resin were processed identically, and presumably the resins were advanced to practically the same degree in each case, the plasticities of the molding compounds are widely different. The inconsistencies of the test results, especially in the case of material C, can be explained as follows: d and C are not materially different as tested by the plasticity-set index method, but on the Bakelite flow tester the difference is that between a material of a very stiff flow and one of average flow. The high plasticity-set index of compound C may be explained by the fact that the resin apparently flowed away from the filler into the small orifices. By the method of calculating plasticity-set index, this unusual effect gave a high index. The result is that the Bakelite flow tester checked the data obtained in the large orifices by the plasticity-set index method. However, material C is a cotton-flock-filled material, and it is evident that testing this compound in an orifice of small dimensions would cause complications, since the fiber length of the filler is sufficiently great to restrict flow into the orifice. The wood-flour-filled compound A is checked reasonably well by the three methods. Material B was quite soft, and all three tests indicated that fact. I n comparing the unfilled material D with the wood-flourfilled material A, it should be noted that the two materials have approximately the same plasticity, even though material D is processed thirty times as long as material A. This is surprising, since i t would be expected that material D would be considerably more advanced than material A; this is shown to be the case by the tube-base method. These two materials were checked on a tumbler mold under molding conditions that gave a satisfactory piece from material A, but under the same conditions material D produced only a partially filled out tumbler.
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August, 1941
INDUSTRIAL AND ENGINEERING CHEMISTRY
The factor of heat conductivity in both the processing and the testing of a material has been discussed (4). It should be of definite interest to process and test two like materials differing only in the heat conductivity of the filler used. The particle shape and size of these two fillers should be the same. Preliminary tests on a bronze powder and a clay indicate that the difference in heat conductivity between fillers is of minor importance.
Conclusion 1. The inability to obtain a consistent correlation in the rating of a series of molding powders as t o plasticity by using various test methods has been shown.
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2. The effect of cure on the plasticity of a molding powder depends on the test method used. 3. The fillers used in compounding have a marked effect on the plasticity of the resultant molding powder.
Literature Cited (1) Burns, R.,Proc. Am. SOC.Testing Materials, 40, 1283 (1940).
(2) Farrer, M., Brit. Plastics, 4, 19, 51 (1932). (3) Krahl, M., Ibid., 6, 235 (1934). (4) Leokvitzky, A. N., Plasticheskie M a s s y , 3, 43 (1934). (5) Norton, A. J., Plastics & Molded Products, 7, 271 (1931). (6) Peakes, G . L., Plastic Products, 10, 53, 93, 132 (1934). (7) Rossi, L. M., and Peakes, G. L. (to Bakelite Corp.), U.S. Patent 2,066,016 (Deo. 29, 1936).
Phenolic Resins for Plywood LOUIS KLEIN The historical development of phenolic resins for use in plywood is traced for the period from 1900 to the present day. The various physical forms in which the resin has been used as a plywood adhesive are described-i. e., as a powder, in organic and aqueous solution, as a dispersion, and finally in film form. New products, watersoluble phenolic powders of good stability and rapid curing propertie#, are discussed. Development work covering the use of phenolic resins as impregnants to produce products such as Improved Wood and wood with better dimensional stability are covered. The fields of application for resinbonded plywood with special emphasis to aircraft construction are touched briefly.
URING the last decade, one of the outstanding developments in the resin field, which witnessed many important advances during this period, was the growth of resin-bonded plywood ( I , 9, 16, 17, 18, 81). This development, because it took place in somewhat unspectacular fashion, has not perhaps been appreciated by those not directly interested in plywood. Moreover, resin-bonded plywood is not readily identifiable as such, and there are many applications where it is employed without the fabricator or ultimate user being aware of its identity. There are in the United States today approximately 150 hot presses. If this figure is compared with less than a dopen in 1934, some idea of the rapid growth during the last five or six years may be obtained. Compared t o the total of approximately 7000 hot presses in the molding industry, the figure of 150 is not particularly impressive. However, the capacity of a plywood hot press is generally high. Practically all of them have multiple openings; they may have as many as twenty platens, and they range in size up to 100 X 150 inches. This permits ii productive capacity %measuredin many thousands of square feet per day for a single press. Unfortunately it is not pos-
D
The Resinous Products & Chemical Company, Philadelphia, Penna.
sible to obtain an accurate figure on the total volume of resinbonded plywood, but it runs to many million square feet per month. The actual total of resin used in the manufacture of plywood is not accurately known, but it is estimated to be between 5 and 10 million pounds per year, a figure that is relatively small compared to the use of phenolic resins in such fields as molding powders, laminating varnishes, and oleoresinous varnishes. However, again it must be pointed out that in the manufacture of plywood, the phenolic resin is a relatively small proportion of the total weight, generally less than 10 par cent. The growth of phenolic resin bonding from the beginning of the century t o the present time will be reviewed here, first, from 1901 to 1934, and secondly, from 1934 to the present. 1934 is chosen as the division point because Tego resin film was first manufactured in the United States in that year. From that point on, the growth of resin bonding proceeded rapidly. I n referring t o phenolic resins, we mean not only iesins manufactured from phenol itself, but also products made from cresols, xylenols, and other substituted phenols. For practical purposes, however, the resins used commercially are prepared almost exclusively by reacting formaldehyde with phenol or commercial cresylic acid. No attempt will be made to cover the chemical reactions involved in the preparation of the resins or their behavior during the process of hot pressing. These are similar to those involved in the manufacture and curing of the thermosetting phenolic resins used in molding and are fully described elsewhere ( I 1,83). There are two general theories concerning the nature of the adhesive forces acting between resin and wood. One postulates that the action is entirely mechanical and that the resin is embedded in the pores of the wood, the strength of the bond depending in part on the degree of penetration and the cohesive strength of the cured resin. The second or polar theory emphasizes the inhence of secondary valence forces, and there is considerable experimental evidence to substantiate its conclusions (8,?21).
