For FAST curing
PHENOL-FURFURAL RESINS
...
check fhese variables
I
Manufacturing
LLOYD H. BROWN and
Procedure
Melting point
DAVID D. WATSON
Amount and type of catalyst
PH
The Quaker Oats Co.,
Reaction temperature
Unreacted materials
Barrington, 111.
Ratio of furfural to phenol
N
SLOWhas limited use of phenolCURIKG
furfural resins to the making of longflow compounds. These resins have an economic advantage over similar materials from phenol and formaldehyde, resulting from the high combining weight of furfural and the relatively high cost of phenol. Because rapid cures are desirable in many applications, a method of accelerating cure of resins from furfural is important industrially. The effects of variables such as melting point, reaction temperature and catalyst, mole ratio, and unreacted materials on rate of cure of phenol-furfural resins are discussed. Also, properties of products molded from a rapid-curing phenol-furfural composition are described. Experimental
Resin Preparation. Methods described by Brown (2) lvere used. Unless otherwise noted the catalyst was 1% sodium hydroxide, based on phenol ; reaction temperature was 135 ' C. ; and furfural-phenol ratio \vas 0.75 to 1.
CUF'ING
2 4 T I M E - MINUTES A T 3 1 5 . F .
6
Figure 1. Curing rates of resins from two aldehydes are compared. A blend of equal parts of the two resins cures almost as rapidly as the more rapid-curing one
Molding Compound Preparation. Molding compounds were made according to this formulation: Resin, lb. Wood flour, lb. Shell flour, Ib. Mineral filler, g. I\-igrosine,, g. Stearic acid, g. Water, g.
4.5 3.75 1.2 136 77 45 45
The resin in the formulation included 15 p.h.r. (parts per hundred parts resin) of hexamethylenetetramine, or 10 p.h.r. of hexamethylenetetramine and 5 of boric acid. Compounds were milled a t 210" to 230" F., to identical cup flow values (ASTM D 731-55T). Specimens were cured for 5 minutes at 330" F. and 2000 p.s.i. except for flexural bars, which were cured for 10 minutes. The compounds used for water absorption and shrinkage specimens were made from an identical formulation to that given above, excluding water. These were milled a t 205' to 245" F. Test Methods. Free phenol was determined by steam distillation of the resin in ethylene glycol ( 5 ) . Tribromophenol was precipitated from the distillate and weighed (7). Free furfural was determined by dissolving the resin in alcohol-benzene and then using the laurylamine method of Siggia (9). Values obtained were about 9797, of the theoretical, on known samples. Duplicates agreed within less than 1%. Two methods were used for rate of cure comparisons. One ( 3 ) involved determination of acetone-extractable material in sheets of resin-impregnated paper, which were cured under pressure for varying lengths of time. The second method consisted of determining the deflection of 4-inch diameter by '/*-inch molded disks loaded at the center 15 seconds after removal from the mold.
Discussion
Effect of Various Factors on Rate of Cure. The resins in Figure 1 are commercial products with approximaEely equal melting points, The slow curt: of the phenol-furfural resin may be caused by its lower functionality. I t has a molecular weight of 482, which is close to trimer, whereas the phenol-formaldehyde resin has a molecular weight of 552, which is close to pentamer (8). Imoto ( 4 ) has said that the curing speed of phenol-furfural resins is a function of their molecular weight, and work in this laboratory confirms his observation. Bender ( 7 ) has said that ortho substitution is favored by use of mild catalysts and high temperatures for the reaction of phenol and formaldehyde. The resulting resins, with the more reactive para positions open, cure more
30
-
W
d25-
2 4 0
a IX
weoI2 0 W
a
L
15-
10-
2 CURING TIME
-
4
MINUTES AT
5
6
335' F.
Figure 2. Curing rates of resins indicate that strong catalysts and lower reaction temperatures are preferable VOL. 51, NO. 5
MAY 1959
683
Table 1.
Melting Point Is Increased as the Ratio of Furfural to Phenol Is Increased
Mole Furfural/ Mole Phenol
Melting Point,
Free Furfural, %
Free Phenol, %
102 116 141
0.2 0.8 2.2
0.7
c.
0.50 0.75 0.85
rapidly than conventional phenol-formaldehyde resins. Figure 2 shows this is not the case with phenol-furfural resins. Strong catalysts and low reaction temperatures produce the most rapid-curing resins. Figure 3 shows that speed of cure increased with increasing ratio of furfural to phenol. This is probably a function of increasing molecular weight as evidenced by melting points. Some phenol-furfural resins are high in unreacted phenol and furfural. These materials were added in varying quantities to the low-aldehyde resin in Table 25
2c -1 W
m t-
u
2
IX
I5
W
IW 0
IO
P 4. W
5
I 3
I 2
I
I
4 CURING T I M E - M I N U T E S AT
5 3 3 5 - F.
Figure 3. Rates of cure of furfuralphenol resins are increased as the amount of furfural is increased
Table II. Boric Acid-Hexamethylenetetramine Catalyzed Phenol-Furfural Compound Cures as Rapidly as a Commercial Phenol-Formaldehyde Formulation Hot DeflecCure, tion, Resin Hexa HaBOa Sec. Mils Phenolfurfural 10 5 45 101 10 10
Phenol-formaldehydeb 10 10 10
Phenol-formaldehydeb 15 15 15
5 5
60 90
74 49
5 5 5
45 60 90
151 128 102
0 0
0
45 60 90
684
119 95 51
Com-
Yield,
0.0 1.0 0.3
71 95 98
%
I. Furfural had no effect at normal levels on rate of cure, but phenol may have a slight accelerating effect (Figure
4)* Rapid-Curing Phenol-Furfural Compositions. Pearce (6) disclosed that boric acid used with hexamethylenetetramine improves rate of cure of furfural-phenolic molding compounds. H e indicated improvement was also obtained with phenol-formaldehyde compounds. When Pearce’s observation was checked using a phenol-furfural resin and curves similar to Figures 1 through 4, no effect could be seen. The solutions used for impregnating contained water; this may have been responsible for the anomoly, as hexamethylenetetramine borate may have formed. The effect was further checked by hot deflection tests on molded disks. Table I1 shows the boric acid-hexamethylenetetramine catalyzed phenol-furfural compound cures as rapidly as a commercial phenol- formaldehyde formulation. Boric acid did not accelerate the cure of the phenol-formaldehyde resin (a conventional novolak), and this combination gave considerable blooming. Apparently boric acid, or some reaction product thereof, was incompatible with the phenol-formaldehyde resin tested. In Table I11 physical properties of molded specimens from the phenol-
Table Ill. Physical Properties of Molded Specimens from Phenol-Furfural Resin Are Comparable to Standard Specifications and Other Commercial Compounds
Test Dielectric strength, volts/
Spec.n
Value
S/T (100’ C.)
milb
79
s/s
(1000
C.)
57
Impact, ft. Ib./ inch notche Flexural p.s.i. Shrinkage,”mils/ inch Specific gravity’ Water absorption, %Q
0.32
0.24 min.
8500
9000 min.
7 1.375
8
0.8
0.8
mas.
1.45 max.
max.
ASTM D 700-55T, Type 2 (General purpose wood-flour filled). ASTM D 14955T. c A S T M D 256-54T. Method A (Notched Izod). d ASTM D 790-49T. e ASTM D 551-41. f ASTM D 792-50. Method A. ASTM D 570-54T. 0
Parts per hundred parts of resin. mercial resin. 5
9.3
8.0
Moisture, %
INDUSTRIAL AND ENGINEERING CHEMISTRY
2 CURING TIME
Figure 4. cure
- MINUTES
Free
AT 3 3 5 . F .
phenol accelerates
furfural compound in Table I1 are comparable to commercial general purpose compounds. A similar resin from furfural and natural (92%) phenol gave similar properties, when it was cured at temperatures above 325 F.
Conclusions The rate of cure of phenol-furfural resins is improved by an increase in melting point or by an increase in ratio of furfural to phenol. Strong alkaline catalysts and low reaction temperatures produce the most rapid-curing resins. Free phenol has a slight accelerating effect upon the rate of cure and free furfural has no effect. Phenol-furfural resins compounded with boric acid and hexamethylenetetramine cure as rapidly as conventional general purpose phenol-formaldehyde compounds, and give cured products with comparable propertie.. literature Cited
(1) Bender, H. L., Modern Plastics 31 (NO.7 ) , 115-18, 200 (1954). (2) Brown, L. H., IND. ENG. CHEhf. 44, 2674 (1952). (3) Brown, L. H., Huett, P. G., Division of Paint, Plastics and Printing Ink Chemistry, Preprint Booklet 14, No. 2 , 49-64 (1954). (4) Imoto, M., Sumitome, H., Matoba, K., Chem. High Polymers ( T o k y o ) 6,244-60 (1 949). (5) Kline, G. M., “Chemical Analysis of Plastics,” to be published. ( 6 ) Pearce, S. F. (to Imperial Chemical Industries, Ltd.), U. S. Patent 2,606,887 (Aug. 12, 1952). (7) Petrov, G. S., Shmidt, Y . , Org. Chem. Ind. (U.S.S.R.), 2, 102-4 (1936). (8) Rast, K., Ber. 5 5 , 1051, 3727 (1922). (9) Siggia, S., Segal, E., Anal. Chem. 25, 830-1 (1952).
RECEIVED for review May 26, 1958 ACCEPTED December 29, 1958
