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Hardwood Record — Veneer & Panel Section 



June 10, 1921 



The Madison Laboratory on Casein Glues 



Manufacture, Preparation and Application of These Glues Are Described in the 

 Light of the Latest Experience and Scientific Research 



One of the many direct resulta of the war has been the intro- 

 duction of casein glues into the wood wol^king industries of this 

 country. Previously they had been used in Europe to a limited 

 extent in book binding and cabinet work, but their production on 

 a large scale was unknown. In the United States the use of casein 

 glues is more recent and their development has been more rapid 

 than in Europe. At the beginning of 1918 only one plant in this 

 country was manufacturing a water-resistant casein glue. With 

 our entrance into the war, the demand of the Army and Navy for 

 w^ate.proof plywood enormously increased the production so that 

 by the latter part of 1918 four different companies were manu- 

 facturing casein w^ater-resistant glues and the Forest Products 

 Laboratory had developed several formulas for making them from 

 the casein. 



Types of Casein Glues 



There are in general two types of casein glues which for con- 

 venience may be designated as prepared glues and wet mix glues. 

 In the former, the glue components are mixed dry; while in the 

 latter, the ingredients are added separately and at different times 

 during the mixing process. Casein glues on the market are of the 

 dry mix type; that is, they are prepared and shipped ready to 

 add to water. The Forest Products Laboratory has done a large 

 amount of work in developing casein glues and some of the best 

 formulas are of the wet mix type. 



Manufacture of Casein 



The principal constituent of all casein glues is casein, a product 

 obtained from milk. When milk sours naturally, casein is precipi- 

 tated and appears as "curd." When produced in this way it is 

 known as self-soured or natural soured casein. Other acids may be 

 added to the milk to precipitate the casein, the principal ones be- 

 ing used are hydrochloric (muriatic) and sulphuric. Rennet is 

 also used to some extent as a precipitating agent. 



The usual steps in the production of casein are: (1) removal 

 of fat in the form of cream from the milk whicb is usually ac- 

 complished by means of a separator, (2) precipitation of the 

 casein, (3) washing to remove the acid and other impurities, (4) 

 drying, and (5) grinding to a powder. The care used in these 

 various steps of manufacture has a large effect upon the glue 

 making properties of the casein. 



The principal requirements of casein to make it desirable for 

 glue making are (1) that the percentage of impurities such as 

 acid, moisture and fat must be small, (2) freedom from sour odors, 

 (3) clearness and uniformity of color; and, (4) fineness of parti- 

 cles. By grinding a casein to fine particles a minimum of time 

 is required in the mixing and a more uniform glue is obtained. 

 A casein ground to pass through a screen of from 50 to 60 

 meshes to the inch will give good results. 



Manufacture of Glue 



To produce glue, casein is mixed with other ingredients, the 

 chief of which are lime and water. These three constituents alone 

 will give a glue of good water-resistant properties but of short 

 life. Other ingredients are, therefore, added to increase the 

 working life and the water resistance and improve the other quali- 

 ties of the glue. Caustic soda, sodium fluoride and sodium silicate 

 are used in patented formulas. They all lengthen the life of the 

 glue and sodium fluoride probably gives the glue antiseptic prop- 

 erties. Still other ingredients are added to give the glue some 

 particularly desired property. Oils are usually added to the dry 

 mix glue to prevent disting in handling the glue. Formulas vary 

 therefore, not only with reference to materials used, but as to the 

 proportions of different ingredients. 



Several formulas for mixing casein glues have been developed 



at the Forest Products Laboratory. One of the best of these, with 

 a discussion of the method of mixing and a description of mate- 

 rials required, follows: 



GLUE No. 4-A> 

 Formula 

 I 00 parts casein 

 I 30 to 280 parts water 

 soak I 5 minutes 



15-22 parts hydrated powdered lime 

 90 parts water 



70 parts silicate of soda 



Method of Mixing 



The proper quantity of water is introduced into the glue pot 

 and the mixing blade is brought into action at a speed correspond- 

 ing to about 50 or 60 revolutions per minute. The stirring is 

 allowed to continue during the addition of the casein to the water 

 and for a few minutes thereafter until the mixture becomes mush- 

 like in consistency, through the absorption of the free water by 

 the casein; the blade is then stopped and the mixture allow^ed to 

 soak. 



After a period of fifteen minutes the soaking is considered com- 

 plete and the mixing blade is again brought into action. The lime 

 water mixture is now added and two or three minutes later the 

 liquid silicate of soda is introduced. 



The mixing is allowed to continue for twenty minutes to one- 

 half hour after the addition of the silicate of soda, whereupon a 

 smooth, freely flowing mixture of uniform texture and free from 

 lumps should obtain. 



Usually some actual experience in the mixing of this glue is 

 necessary before satisfactory results can be expected, and it is 

 rather advisable to have a new operator witness an actual demon- 

 stration. This is due to the fact that no precise quantity of water 

 can be prescribed, because of the variation in the water absorbing 

 qualities of different caseins. The criterion of whether or not the 

 proper quantity of soaking water has been added is the viscosity 

 of the finished (mixed) glue. If its consistency is too thin an 

 excess of water beyond that required has been used, and it is best 

 to reject the batch and try again. Similarly, if the consistency is 

 too thick and heavy an insufficient quantity of water has been 

 used. The water required for various types of casein lies in the 

 following ranges : 



Lactic acid casein 1 30 to 1 70 parts water 



Sulphuric acid casein 



Hydrochloric acid casein! 



170 to 220 parts water 



Rennet casein 280 parts water 



Description of Materials 



Casein. — It is essential that the casein be of a reasonably pure 

 grade, carefully manufactured, free from offensive odors, objec- 

 tionable color, such as dark yellow^ or dirty color, and low^ in fat» 

 and free acid. It should be fine enough to pass through a 60- 

 mesh sieve. 



Lime. — .\ high calcium lime gives satisfactory results but limes 

 containing a relatively large amount of magnesia can be used if a 

 sufficient quantity is taken to give the required calcium hydroxide 

 content. It can be prepared for use by adding just enough water 



' U. S. Patent Ko. l,2r>l,.irKi on this pmrrss; ha.t been fjrnntfd to F!nmuc1 

 fttittrrmnn of the Fo7TSt Protlucfs Lnhoratorjf and assigned to the United 

 ^tafefi Government, 



(Continued on page 4S) 



