Lignin for Reinforcing Rubber J. J. KEILEN
AND
ARTHUR POLL.4II
West Virginia Pulp and Paper Company, Charleston, S. C .
Lignin, as made from the waste black liquor of the sulfate wood pulp process, is an effective reinforcing agent for synthetic or natural rubbers when incorporated into latex by the coprecipitation or master batching procedure. GR-S, reinforced in this manner with 38.5 volumes of lignin, yields vulcanizates having a tensile strength of 2900 pounds per square inch and a tear resistance of 380 pounds per inch, values exceeded only with channel black. With 77 volumes loading the tensile strength of 2800 pounds per square inch and tear resistance of 550 pounds per inch are higher than those for any other pigment tested. I n abrasion resistance lignin vulcanizates are be-
tween carbon blacks and the common inorganic fillers. Shore hardness values are close to those for channel black. Lignin-reinforced rubbers weigh appreciably less per unit volume owing to the low specific gravity, 1.3, of lignin. The brown color of lignin permits a wide color range, without sacrifice of mechanical properties, by blending with white pigments. Lignin coprecipitates require only a brief milling time for complete mastication. Details are given on the preferred procedure for preparing master batches of lignin and GR-S. Other suggested incorporation procedures made possible by the colloidal properties of lignin are reviewed.
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be attained. The rigidity of the government program in supplying the large quantities of synthetic rubber for essential military purposes precluded the investigation of the minor changes required in the large rubber manufacturing plants. Compounding and curing of the lignin GR-S coprecipitates also required some study because of slightly varying characteristics. With the progress that has been made in all directions, the use of lignin for reinforcing synthetic rubber is now assured. Lignin is a reinforcing pigment for rubber which is particularly adapted to the newly developed processes of coprecipitation of the pigment with synthetic or natural rubber. Because of the
HE lignin used in this work comes from pine wood where it
occurs as one of the incrusting substances associated with cellulose fibers. The chemical structure of lignin as found in nature is not yet established, and it is probable that the product is further modified by the recovery processes used. Abundant evidence has accumulated, however, in the course of nmnufacturing hundreds of tons of the material that the product is reproducible and uniform in grade. The lignin so made reacts as if it always had the same fundamental chemical structure with minor substituents. The prevailing opinion today is that this structure is cyclic and consists of various polymers of a unit of C41H1206, containing in addition about eight to ten hydroxyl or methoxyl groups. The lignin used averaged about four hydroxyl and four methoxyl groups for C41H3206 comprising a unit molecular weight of about 840. This particular lignin contained about 1 to 2% of sulfur which appeared to exist principally in a stable thio form. One of the hydroxyl groups appears to be phenolic, reacting lyith alkalies and many organic compounds. The lignin applied for reinforcing rubber was made from the black liquor available a t a large southern sulfate pulp mill. I n the process used, the black liquor, which contains the lignin dissolved as a sodium compound, is acidified. The lignin precipitates and is filtered and washed free of occluded impurities. On drying, a brown free-flowing powder is obtained which is easily handled in bags, barrels, drums, or hopper cars. The qpecific gravity of lignin is 1.3, and it has a packed density of 23-30 pounds per cubic foot. This lignin from the sulfate process is representative of more than a million tons potentially available annually from sulfate and soda pulp mills in the United States. LIGNIN AND RUBBER
Lignin as a reinforcing agent for synthetic rubber is a development of wartime research to augment the supplies of carbon black and other fillers for rubber. Although the completion of the research came too late t o be of commercial value during the war, it is being carried over into peacetlme work, where i t will enhance the properties of rubber products using other pigments. It shows promise of strengthening the position of synthetic rubber with respect t o natural rubber, and thus making the industry more independent of foreign natural rubber supplies. The obstacles preventing the prompt war application of lignin for reinforcing rubber have now been largely overcome. The process for the manufacture of lignin itself was the subject of con, siderable development work before commerical production could
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LATEX WATER
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1eii\iie stre~igt,l~ comparable with t,lio>eobtained using carboil blacks a t lox loadings, and higher tensile strength and tear resistmces a t high loadings. Loner specific gravity than other c o ~ i i ~ n u1,einforcing ii agents since it i.5 7OC; as heavy as carbon black, 5 0 5 as heavy as clay OT calcium carbonate, 3 3 7 as heavy as titanium dioxide, and 23% as heavy as zinc oxide. \Vide color possibilities, retaining high teiikile strength. Loweririg of processing time by reducing the breakdowi t inic a n d rliminating addition of the filler or reinforcing agent. ACKNOWLEDGhlEh-T r .
1l i ( , aiitllorb n.i.4i to Pspresq their appreciation of the technical :isiiztuncc of \{-alter I
