/^-fc-* U.S. DEPARTMENT OF AGRICULTURE LCW-RESIN-CCNTENT AND PESIN%^ 9 ]945 * ATLANTA PULP PIASTICS May 1945 TLANTA BRANCH ATLANTA, GEORGIA No. R1483 UNITED STATES DEPARTMENT OF AGRICULTURE FOREST SERVICE FOREST PRODUCTS LABORATORY Madison, Wisconsin In Cooperation with the University o< Wucorwin LP W-KSS IN-CONTENT AT RSS m FULP PLASTICS By S. '.. SCHWARTZ, Engineer J. C. FEW, Engineer and H. ?.. MEYER, Engineer SUM IARY Experiments at the Forest Products Laboratory have demonstrated that molded plastics can be made from high-yield v;ood pulps in which little or no phenolic resin has "been incorporated. A comparison of plastics so made with pulp plastics containing as much as 40 percent of phenolic resin by weight indicated some loss of ultimate tensile and compressive strength and lowered resistance to water absorption, but improved toughness, The most promising of the plastics containing no resin appeared tc be those produced from pulps obtained by milling water-cooked chips. By coating the surfaces of the pulp mats with resin before molding them, their water resistance was considerably improved. Plastics of moderate strength and good water resis- tance were also produced by the addition of small amounts of water-soluble phenolic resin or nonphenolic rer. ins. INTRODUCTION In previous work on pulp-reinforced phenolic plastics,- it wac found that the resin content of groundwood plastics could be greatly reduced and yet produce a pulp plastic comparable, except for water resistance, to a pulp plastic containing 40 percent of resin. The study of low-resin-content mixtures was therefore extended to include a variety of hi - ield pulps. MATERIALS A variety of aspen and spruce pulps and a neutral sulfite semi- chemical pulp from a mixture cf black tunelo and aveetgum were used in these experiments. The asren groun consisted of a neutral sulfite scmiche~.ical pulp, a chlorinated and extracted modification of the same pulp, a ground- wood pulp, and a water-cock pulp. The soruce group consisted of an acid sulfite semi chemical pulp, a water-cook pulp and a groundwood pulp. -"Pulp P.o-inforced Plastic;;" , Forest Products Laboratory Mimeogl ' . P-1461 Report No, R1483 The chlorinated and extracted pulp was obtained by treating aspen neutral sulfito somichemical pulp with 16 percent chlorine and a 7 percent caustic soda solution. This pulp was included as an example of a completely delignifiod, yet relatively high-yield product. The groundwood pulps were made in the Forest Products Laboratory experimental grinder and the others by milling the treated wood chips in a double-rotating disk mill. All pulps were evaluated and used as prepared. No further processing was used or re- quired. Two powdered phenolic resins, one of high and one of medium fluidity, were used. The high-fluidity type worked well in the plastics with a low resin content, but was found not to be generally applicable for plastics with a high resin content. The resin of medium fluidity was therefore used in the plastics with a high resin content which are included in this study for comparison purposes. Water-soluble phenolic resins, a resinous byproduct from a wood rosin refining operation, and a soda spent-liquor lignin were used in some plastics. The powdered and water-soluble phenolic resins used for some of the plastics and the resinous byproduct from the wood rosin re- fining operation used in others are commercially available materials. The lignin used was an experimental material recovered from the spent liquor produced in the soda process for making paper pulp. EQUIPMENT AND PROCEDURE Except as otherwise noted, the plastics tested were made by sus- pending the pulp, powdered phenolic resin, and zinc stearate (1 percent of the mixture) in water at about 1 percent consistency. A wetting agent was used to aid in dispersing the powdered material. The stuff was formed into a mat g-l/2 inches in diameter in a pressure-forming apparatus. The resin content was calculated on the basis of the oven-dry weight of resin retained in the mixture. The wet mat was dried in a forced circulation oven at 35° to 1+0° C (95° to 10l+° F.) for 2k hours and conditioned at 2k° C. (75° F.) and 50 percent relative humidity for at least 2h hours before being molded. The materials with a high resin content were molded between cauls for 13 minutes at l60° C- (320° F. ) and drawn hot. The products containing little or no resin were also molded between cauls for 13 minutes, but the molding temperature was increased to 180° C. (356° F#), pressures of U.000 pounds per square inch were used, and the panels "were chilled before the pressure was released to prevent blistering. Although the higher molding temperature favored the moldability and improved the water resistance of the product, it is probable that lower pressures might have been adequate with some compositions. The method used to make these plastics is described in greater detail elsewherei.. Testing of Plastics Strength properties were determined on specimens taken from one to four nominal 1/ 8- inch- thick panels of each pulp plastic. In general, the num- ber and type of specimens taken from each panel were as follows: two tensile Report No. Rll+83 -2- specimens, two compression specimens to determine the elastic properties, four compression specimens to determine ultimate strength, five toughness specimens, and two water-absorption specimens. Tests other than toughness were made according to the methods outlined in "Federal Specifications for Flastics, Organic: General Specification (Methods of Tests) L-P-U06, December 9> 19^2." The toughness tests were made on the Forest Froducts Laboratory intermediate capacity toughness testing machine. RESULTS The properties of the pulps and the corresponding plastics are given in tables 1 and 2. A comparison between the phenolic plastics contr ing about Uo percent of resin and the corresponding resin-free pulp plastics shows that the latter products had decidedly lower ultimate tensile and compressive strengths and, usually, considerably lower proportional-limit stress values in tension. On the other hand, the toughness values increased. The plastic flow of the resin-frec materials, as measured, was practically nil, and the resultant plastics had relatively poor water resistance. By the addition of 10 to 15 percent of resin, plastics intermediate in ultimate strength, toughness, and water resistance were obtained. Com- pressive-yield stress and proportional-limit stress values obtained were, on the other hand, almost invariably higher for the plastics containing little resin than with those of high resin contents. In a few cases the proportional limit stress in tension was also higher. The high-fluidity resin used in the lower-resin products may have been a contributing factor in these effects. Groundwood plastics Nos. 113 and 160 (table 2) are strong but lack toughness and water resistance when compared with the average properties of the respective, aspen and spruce groups. Neutral sulfite semichemic.il plastics Nos. I5." and 153, on the other hand, have both strength and tough- ness but again have somewhat high water absorption. Flastics Nos. 211 and 212, in which pulps produced from water-cooked chips were used, have good water resistance and moderate strength. The neutral sulfite semichemical pulp vhich had been delignified by chlorination and caustic soda extraction produced a plastic (No. 157) which, though not water resistant, has very high toughness and exceptional elongation in tension before rupture. Pulps moldod without resin produced plastics (Nos. I69, 167, 229, 215, 216, 185 ?- n(i 21U-E in table 2) which were high in toughness and rela- tively low in tensile and compressive strength but varied from extremely poor to fair in water resistance. With respect to water resistance, plasties Nos. 215 end 2lUs, prepared from milled water-cooked chips, were decidedly the best of the resin-free plastics. The water resistance of all the resin-free pulp plastics may be greatly improved by surfacing them with small amounts of resin. Coating the pulp mats with water-soluble phenolic while wet, as in the cases of plastics Nos. 19S and 1S9B, or surfacing them with paper impregnated with phenolic resin before they were molded, as in the cases of plastics Nos. 213 ^nd 214C , Report No. RlUg3 -3- was found to bo effective. The latter method also improved the finish and permitted colored surfacos to be readily obtained. Resinous materials other than powdered phenolic resin were added to certain high-yield pulps to aid in water resistance and moldability. By using as little as lU percent of water-soluble phenolic resin, as with plastic No. 171 i excellent water resistance was obtained in a neutral sul- fite semichemical plastic. Sixteen percent of a resinous byproduct from wood rosin refining was used in plastic No. 225- In plastics Nos. 186, 226, and 187 approximately 15 percent of lignin was used. In general, these non- phenolic materials are less effective than equivalent percentages of phenolic resin, but have the advantage of lower cost. In some instances, these compositions may be laminated instead of being molded from a pulp mat. Thus, a resin-filled paper was made from the neutral sulfite semichemical pulp (No. 5083 N in table 1), which con- tained 11 percent of powdered phenolic resin. The powdered resin used in this pulp was added in the beater before the stock was run on the paper machine. The resin-filled paper was molded at 180° C (356° I.) and U,000 pounds per square inch to form cross-laminated flat panels. The properties of the plastic thus obtained were: Property Value Tensile strength Maximum 18,900 p.s.i. Proportional-limit stress 6,900 p.s.i. Modulus of elasticity 2,000,000 p.s.i. Edgewise compressive strength Maximum 17,200 p.s.i. Proportional-limit stress 3>300 p.s.i. Modulus of elasticity 1,800,000 p.s.i. Toughness (FPL) 16 inch-pounds per inch of width Water absorption 7*3 percent The reason why high-yield pulps, with or without small amounts of resin, can be readily molded into plastic-like substances has not been de- termined. For example, the chlorinated and extracted neutral sulfite semi- chemical aspen pulp, which is practically devoid of lignin, molded to a fairly dense, coherent product (plastic No. 167, table 2). The moldability of this pulp may possibly be due to the fact that its hemicellulose content is high in comparison with that of ordinary chemical pulps. It would not be expected that hemicellulose, or any carbohydrate, as a bonding agent would contribute in large measure to water resistance. In fact, the plastic has the high water-ab sorption value of 135 percent. Plastics with a low water-absorption value were obtained when aspen groundwood pulps were employed, even though the lignin in wood is con- sidered chemically combined with the hemicellulose and not available as a Report No. RlU83 -U- plastic binder. If rood is pressed under the same conditions used with these pulps, horever, sonc transformation, accompanied "by considerable darkening in color, takes plrco, and a plastic-like product is formed—. It is logical to suppose a similar effect occurs when the irood is reduced to the form of a mechanical pulp. It is conceivable that a partial separation of tho lignin from the homicellulose occurs during the molding operation and that this "activated" lignin contributes to the moldability of the product Still further improvement is obtained with pulps from water-cooked chips, since considerable lignin is liberated during cooking, and the weakening of the "bond between the fibers allows better defibering than is possible with groundwood. More drastic cooking, it is believed, probr.bly would not give an improved product because of the degradation of the cellulose that would be caused and the possible polymerization of the separated lignin to a more infusible state. CONCLUSIONS The experiments described, while exploratory in nature, indicate that high-yield pulps have definite possibilities as plastic bases, partic- ularly for low-cost products. Prom a ro."-nntcri.~l standpoint, at least, considerable reduction in cost over high-resin-content plastics is indicated. Further investigations of cooking procedures, pulp processing, and molding compositions are, of course, needed to bring about desired im- provements. The necessity of cooling the molds before drawing the plastic would in some cases be a disadvantage, especially with shaped articles. Also, since flow is generally limited, accurato and uniform preforms would be necessary. 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