BORATED RESINS ERNEST P. IRA-NY, Shawinigan Chemicals Ltd., Shawinigan Falls, Quebec, Canada
T
HE reactions of boric acid with monohydric alcohols follow the normal course of esterification. Combination takes place slowly in the absence of catalysts but is markedly accelerated by mineral acids. I n the case of polyhydric alcohols, however, esters form readily, even without acid catalysts. This trend is still more pronounced in the action of boric acid upon ~nacromolecularsubstances which carry multiple numbers of hydroxyl groups on every molecule. This description applies to many technically important plastics such as polyvinyl alcohol, partially hydrolyzed polyvinyl esters and acetals, and cellulose esters and ethers. Solutions of these resins are viscous fluids. Upon addition of small amounts of boric acid these solutions change quickly into elastic gels which are insoluble, nonadherent soft soIids, containing the original solvent as dispersed phase. The presence of water or alcohols prevents this phenomenon entirely, and their addition even in small amounts causes the gelled body to revert to the condition of a viscous fluid solution. When the solvents are evaporated, the resins are obtained in the form of insoluble and infusible masses from which, however, they can be recovered in their original condition by contact with water or alcohols. The same effect is observed if boric acid is added t o the resins on hot mixing rolls: they lose their plasticity and are converted into loose infusible masses which, upon repeated passage, disintegrate into powder. These observations are readily explained, considering that boric acid is tribasic and capable of forming spatially interlinked polyesters with the macromolecular polyalcohols. The resulting structures are a continuous phase whose formation forces solvents and plasticizers present t o disperse within its network of fixed and bridged macromolecules. The whole system assumes the character of an insoluble and infusible gel. That the bridging is brought about by the formation of boric acid esters or some type of more loosely bound complexes of analogous structure is supported by the following facts: Polyvinyl alcohol which is entirely insoluble in the lower aliphatic alcohols dissolves readily in warm methanol in the presence of one mole of boric acid, and this solution tolerates the addition of considerable quantities of benzene. This clearly indicates that polyvinyl alcohol is not present as such but in combination, evidently with boric acid. All insolubilized borated resins release boric acid and are recovered unchanged if treated with water. For example, a polyvinyl acetal resin (Alvar) rendered entirely insoluble by treatment with 2 to 5 per cent boric acid and drying, remains entirely unaffected by dioxane which is normally an excellent solvent for it; upon addition of a few per cent of water, it dissolves immediately. The same borated resin, suspended in water and heated, coalesces a t exactly the same temperature as the untreated resin but fails t o do so in a saturated aqueous solution of boric acid. The same observations can be made with plasticized celluloseacetate, using acetone. That the available alcohol groups of the borated resin are actually engaged is indicated by the fact that efficient esterification agents, such as acetic and phthalic anhydrides and benzoyl, phthalyl, and phosphorus chlorides, are almost without effect. Acetalization also fails. Quantitative experiments were carried out by placing a resin solution of known concentration into a miniature mixing machine and adding successive small amounts of boric acid. Incipient gelation could be observed clearly by the motion, adhesion, and cohesion of the material. Beyond a certain minimum of boric acid sufficient to effect the phase inversion, further addi-
Boric acid reacts readily with macromolecular substances such as cellulose esters and ethers and partially hydrolyzed polyvinyl esters and acetals, which contain a plurality of hydroxyl groups per molecule. The products formed are spatially linked, insoluble, infusible boric acid esters or esterlike association compounds. These compounds are hydrolyzed by mere contact with water or alcohols and regenerate the original thermoplastic resin without change. Various uses can be made of this effect-for example, in the production of thermoplastic and rubberlike materials in the form of fine powders or in temporary protective coatings ; marginal additions of acid boric provide a means of sensitive control of molding materials and in film casting.
tions caused little change. Agents capable of restoring fluidity, such as water, methanol, cetyl alcohol, ethylene glycol, etc., seem to be effective in a stoichiometric ratio relative to the macromolecular alcohol but not to the boric acid present. The minimum of boric acid required to cause gelation depends on the chain length of the macromolecules. Evidently, if at least two points of every chain molecule must be h e d in order to produce a continuous lattice, the effective amount of boric acid should be inversely proportional to the molecular weight of otherwise similarly constituted macromolecules. For example, two analytically comparable polyvinyl acetal resins made from different polyvinyl acetates (solutionviscosities,l5 and 95 centipoises, respectively) required 0.30 and 0.15 per cent boric acid to become completely insoluble. The smallness of these amounts shows that only a few bridges are required for effective lattice formation and that the yield per boric acid molecule must be high. All polyvinyl resins are mixtures of polymer homologs varying greatly in molecular weight. If an insufficient amount of boric acid is added to a solution, the longest and most susceptible molecules are precipitated while the shorter ones remain in solution. By this means sharp fractionations can be effected without need of the exacting control of conditions on which the usual precipitation methods depend. For example, a polyvinyl acetal resin (Alvar 7-90, containing 8.2 per cent polyvinyl alcohol) in toluene solution was treated with 0.6 per cent boric acid which precipitated a firm gel and left an approximately equal quantity of resin in solution. The two fractions were boiled with water, dried, and compared (Table I, A ) . Fractionation of the same resin with the same amount of boric acid but in a different solvent (80 toluene-20 xylene) gave almost the same result. This is in accord with the fact that the insolubilized portion is generally insoluble because of its structure (Table I, B ) . The precipitated portion of A was redissolved, treated twice again under the same
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December, 1943
INDUSTRIAL AND ENGINEERING CHEMISTRY
conditions, and yielded only negligible amounts of soluble materials whose viscosities fell within less than 5 per cent of that of the first crop. The data under C refer to another resin (Alvar 15-70, 5.6 per cent polyvinyl alcohol), similarly treated. The constancy of the analytical values shows that only the molecular weight is subject t o fractionation. The action of boric acid on macromolecular substances containing hydroxyl groups is essentially one of vulcani~ationor curing. However, unlike the permanent change brought about in rubber or thermosetting plastics, this effect is immediately and completely reversible, merely by contact with water. It is possible, therefore, to convert thermoplastic resins into inert, insoluble, and infusible masses for the convenience of some operation, and afterwards to recover them unchanged by washing or leaching with water.
-
TABLE I. ANALYEJIS OB POLYVINYL RESIN FRACTIONS Fraction Original Soluble Insoluble
Visaosity in 5% Toluene S o h , Centipoises A B C 5.83 3.56 7.48
11.15 3.36 5.62 6.95 13.47
Polyvinyl Aoetate, % Aloohol, yo A B C A B C
14.8
ii:d
40.0
ii:i
::::
8.20 5.60 7.35 8.22 8.25
.... .... ....
FINELY POWDERED THERMOPLASTICS
The task of comminuting thermoplastic materials is limited by their nature. Devices based on attrition are useless because of the difficulty encountered in dissipating the generated heat. Crushers using the impact principle cannot reduce tough plastics below a particle whose mass is insufficient to provide the energy required for further shattering a t any safe speed of the machine. This limit is usually between 30 and 50 mesh. If, however, the thermoplastic material has been converted into a heat-insensitive form by treatment with boric acid, it can be ground or crushed in this condition by any effective means and to any required fineness (patents assigned to Shawinigan Chemicals Ltd.). This actually has been done with thermoplastic resins which are among the toughest types known: high-polymer polyvinyl formals (Formvars), acetals (Alvars), butyrals (Butvars, used in safety glass and rubber substitutes), as such or combined with large amounts of plasticizers; plasticized cellulose acetates and acetate-butyrates (Tenites) ; high-molecular cellulose ethers, etc. All these materials are converted into brittle infusible masses upon admixture of 2 to 5 per cent boric acid and can be reduced to fine powders with such ease that a single pass through a small impact crusher connected with an air-floating train yielded about 60 per cent fines passing 100 mesh, the remainder containing little material coarser than 80 mesh. These fine powders can be leached free from boric acid by stirring with a few changes of water. They are then again thermoplastic and must be dried at a low enough temperature to avoid coalescence. The dry powders are dense, free flowing, and stable; they are in substance identical with the original materials. Plasticizers do not interfere with the process even if present in high amounts. They are converted into dispersed phases and accompany the resin through all operations without imparting plasticity but assume again their original function in the recovered material. Their amount is limited only by considerations of stability of the powder under storage conditions. These fine thermoplastic powders cannot be obtained by any other known methods and represent a new type of product. It is not improbable that their availability in quantity and variety m a y stimulate entirely novel applications in many fields. Promising trials have been conducted with unplasticized, high-polymer, polyvinyl formal (Formvar 16-95)powders in the Schori
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gun. The powder is sprayed through the flame of a gas torch upon metal surfaces where it forms a substantial, highly resistant coating. Borated or recovered resin powders of this type are of interest in the making of abrasive tools. MOLDING
In making resin powders by the above method, enough boric acid must be used to assure the formation of rigid molecular lattices. By employing smaller or marginal amounts, various physical properties can be sensitively adjusted, For example, the usefulness of thermoplastic resins for injection molding depends not only on their respective fluidities under given temperature conditions, but also on some constitutional factor which may be described as the ability to transmit a directed force as such, without dissipating it in form of hydrostatic pressure. It appears that this quasi solid quality is closely connected with the presence of a lattice structure which, in more or less effective disposition and continuity, is superimposed upon the essentially fluid substance of the resin. If it is lacking, proper performance in the injection machine cannot be assured by either higher temperatures or increased admixture of plasticizers. Thus, most plasticized cellulose acetates (Tenites) inject more easily into intricate molds than certain polyvinyl acetal (Alvar) compounds, in spite of the fact that their absolute fluidities (measured by outflow through a heated steel capillary under high pressures) are much lower. I n Table I1 temperatures of equal fluidity are given together with temperatures a t which molded articles begin to deform or “unmold”. It appears that the more easily injectable Tenites are generally more rigid than the Alvar compounds. Addition of boric acid to the latter tends to narrow the differences among the recorded temperatures; a t the same time it actually improves the injectability, form stability, tolerance for plasticizers, and resistance against solvent actiou of the Alvars.
TABLE 11. FLUIDITY AND SOFTENINQ TEMPERATURES Resin Tenjte H a Tenite M a a Alvar 11-906 5% plasticizer5 boho acid boria acid Alvar 11-80,10% plasticizer + O 5 borio soid i 0 P borio acid a Celluloae acetate Eastman). a Polyvinyl acetal ( L w i n i g a n ) . c Dibutyl phthalate.
$y:;p +
Equal Flmdity Point, a C. 125 125 93 100 108 90
95 100
Softe,n@, Point 125 120 64 70 74 50 54 65
1
Mottled moldings are made from mixed granulated plastics of different colors and flowing properties. The latter must be carefully adjusted in order to produce precisely repeatable effects; this is very conveniently done by addition of boric acid within a range of a few tenths of one per cent. INSOLUBLE FILMS FROM SOLUTIONS
Where insoluble films are required, present practice employs solutions of heat-reactive resins which, after evaporation and baking, are rendered insoluble by an irreversible chemical process. Borated thermoplastic resins can be deposited from solutions which are maintained in fluid form by small additions of water or alcohol to the solvent. When evaporated to dryness, the coatings are insoluble except in solvents containing water or alcohol; they are also light colored and unaffected by heat. Films of this kind may find many practical applications as protective coatings for temporary use.
INDUSTRIAL AND ENGINEERING CHEMISTRY
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OF BORICACIDON SPEEDOF FILM CASTING TABLE 111. EFFECT
Film
Blank Borated Blank Borated Blank Borated Blank Borated Blank Borated
Min. after Casting
Load,
Lb./
Sq. In.
6 6
15 15
9 9
20 20 30 30 30 30 30 30
11
11
15 15 18 18
Elongation, %
Seo. under Load TOO TOQ
10 30
5 60 30
Permanent 60 Permanent
soft Soft Broke 150 Broke 100 Broke 50 200 Nil
For example, 25 parts of a polyvinyl acetal resin (Alvar 15-80) and 5 parts dibutyl phthalate are dissolved in 70 parts ethyl acetate containing 2 per cent water. To this solution are added 0.5 part boric acid, enough to cause complete insolubility of the evaporated film but not enough to make it brittle. I n this condition it is neither dissolved nor softened by dry ethyl acetate or any other anhydrous nonalcoholic solvent but dissolves at once if small amounts of water or alcohol are added to the latter. The capacity of borated resins to hold plasticizers or solvents dispersed as an internal phase is remarkable, amounting to proportions as high as 1:15 or more. These compounds are nontacky nonsweating gels which can be used in the preparation of hectograph masses, coatings for printing rolls, etc. As an illustration, the above example of an insoluble coating can be modified to a mixture of 20 parts resin and 80 parts dibutyl phthalate, dissolved in 40 parts ethyl acetate containing 2 per cent water and 0.4 part boric acid. When cast, the viscous but entirely fluid solution leaves a firm nontacky gel, which absorbs and retains printing inks, stains, and dyes, and is little affected by tempemture changes. I t is generally insoluble but can be readily dissolved and recast using aqueous solvents. FILM CASTING
An important factor in casting films from solutions by the usual methods is the rate at which the solvent is released in the final stage when the film is supposed to become self-supporting. In
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many cases small amounts of solvent are retained so tenaciously that subsequent operations are seriously delayed; in nearly all cases faster and more economical casting would result if the tensile strength of the "green" film could be improved without resorting to enforced drying methods. This is readily achieved by the phase inversion caused by small amounts of boric acid added to responsive film-forming media, such as all cellulose derivatives and polyvinyl acetal resins. Although the last remnants of solvent are not removed, these films gel upon reaching the stationary retentive condition and assume full physical strength which allows subsequent handling without delay. The data in Table I11 show this effect in films cast from a solution of 100 parts polyvinyl acetal resin (Alvar 15-80) in 250 parts ethyl acetate containing 5 parts water and 2 parts boric acid. The films were cast on a glass plate, lifted after a given number of minutes, and immediately placed under load. Blank tests were carried out without boric acid.
TABLEIV. Boric Acid, yo None 0.2 0.5 1.0 2.0
EFFECT OF BORICACID ON SINTERING TEMPERATURE
% Elongation a t 80 5 min. Infinite 300 25
d;
25 min.
In'iiit e 100 20 4
C. after: 60 min.
..
€%?,%.
.. ..
25 6
135
175
180 195
205
Entirely analogous results were obtained with the same resin plasticized with 15 per cent triethylene glycol hexoate, using the same solvent, and also with a solution of 60 parts cellulose acetate, 40 parts triacetin, and 2 parts boric acid in 400 parts methyl ethyl ketone containing 2 per cent water. I n amounts up to 2 per cent, boric acid remains dissolved or homogeneously dispersed in the films and does not interfere with their transparency. It contributes markedly to their heat resistance as shown in Table IV. Films of the polyvinyl acetal resin referred to in Table I11 (Alvar 15-80) were completely dried at 60' C. and suspended at 80" C. under a load of 50 pounds per square inch. The sintering temperatures were observed by slowly heating without load.
High-Temperature Heat Content of CALCIUM CARBIDE G. E. MOORE Pacific Experiment Station, U. S. Bureau of Mines, Berkeley, Calif.
P
ART of the program of thermodynamic investigation of metallurgically important substances being conducted a t this station involves measurement of heat contents above 25" C. by the so-called drop method; this paper presents some data obtained on a high-grade commercial sample of calcium carbide in the range 200° to 1000° C. This sample was part of that used by Kelley (2) in low-temperature specific-heat measurements, and i t is probably of the highest purity available at present (91 per cent). Although there is considerable uncertainty in correcting for the impurities, i t nevertheless seemed desirable to obtain and report these data on this important substance.
b T H E method and apparatus were described previously (6). The sample was contained in a sealed platinum-rhodium alloy capsule; there was some action of the carbide on the container after the measurements a t 1000" C., but it was insufficient to cause any appreciable error. The error in the measurements is certainly not more than one per cent throughout the entire range studied, and the results are in general reproducible t o a few tenths per cent. Correction was made for the major impurities reported by Kelley (%)-namely, 6.47 per cent CaO, 1.15 per cent SiOn, and 0.77 per cent ALO,-using data from his tables (3). The mate-
