"3-47-2. 2,720,501 United States Patent Office Patented Oct
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Oct. 11, 1955 R. T. VAN NESS 2,720,501 AQUEOUS CONDENSATION PROCESS FOR THE PREPARATION OF POLY WINYL ACETAL RESINS Filed Aug. 10, 1954 ALDEHYDE-f f 27 INVENTOR Robert Terry Van Ness "3-47-2. 2,720,501 United States Patent Office Patented Oct. 11, 1955 2 alcohol groups, and the remainder is polyvinyl butyral. In the attached drawing there is illustrated in a sche 2,720,501 matic fashion a flow sheet representing the process of AQUEOUS CONDENSATIGN PROCESS FOR THE this invention. A solution of polyvinyl alcohol in water PREPARATION OF POLYWNYLACETAL RESNS is prepared in vessel which is fitted with a means for agitating and a means for heating the contents of the Robert Terry Van Ness, Wilmington, Del, assignor to vessel. Polyvinyl alcohol from a storage vessel 3 is E. I. du Pont de Nemours and Company, Wimington, added to vessel 1 along with sufficient cold water at a Del, a corporation of Delaware temperature from about 20° C. to 30° C. to form a sus Application August 10, 1954, Serial No. 448,876 10 pension containing about 7 to about 15% solid polyvinyl alcohol. Polyvinyl alcohol is prepared by hydrolyzing 4 Claims. (CI. 260-73) polyvinyl acetate in the presence of an alkaline catalyst or an acid catalyst. Depending upon the polyvinyl ace tate used, the type of hydrolysis catalyst, and the de This invention relates to the preparation of polyvinyl 5 gree to which the hydrolysis is completed, many varieties acetal resins and, more particularly, it relates to a proc of polyvinyl alcohol may be prepared. These polyvinyl ess for the preparation of Such a resin in an aqueous me alcohols may vary in viscosity, in purity, in the amount dium and in the absence of a solvent for the resin. of residual unhydrolyzed acetate groups on the polymer It is well known that polyvinyl acetal resins may be chain, and other features. Any of these varieties of prepared by condensation of polyvinyl alcohol with any 20 polyvinyl alcohol may be employed in the process of this of a group of aldehydes. It has been the general prac invention to prepare a polyvinyl acetal resin, although tice in the past to carry out this reaction in the presence selected varieties are preferred when it is desired to pre of a large amount of a solvent such as alcohol and there pare a resin having good enough optical quality to be by to recover the polyvinyl acetal resin in the form of a used in the manufacture of safety-glass interlayer. For thick viscous solution. in order to transform the viscous 25 the latter purpose it is preferred to employ pure polyvinyl solution of polyvinyl acetal into a flake material the so alcohol containing not more than about 3% by weight lution was subjected to a series of water treatments by of vinyl acetate units and having a viscosity from about means of which the alcohol solvent was removed and 20 to about 60 or more centipoises when measured as a the resin precipitated in the form of a flake material. 4% aqueous solution. The alcohol solvent was then purified by means of ex 30 The mixture of polyvinyl alcohol and cold water is tensive distillation steps and recirculated to the begin agitated in vessel for about 30 minutes in order to ning of the process. break up any large lumps of the polyvinyl alcohol. This it has now been found that polyvinyl alcohol may be aqueous suspension is then heated to a temperature of condensed with an aldehyde in the absence of a solvent about 95 C. and agitated until a clear solution is ob for the condensed resin thereby eliminating the costly tained. The solution is then cooled to a temperature of process of recovering an alcohol solvent. about 75 C, It is an object of the present invention to provide a An acid catalyst is added from storage vessel 2 to the process for the condensing of polyvinyl alcohol with an polyvinyl alcohol solution in an amount sufficient to lower aldehyde in the presence of a reaction medium which its pH to about 2. The catalyst may be added to the is not a solvent for the condensed resin. It is a specific 40 polyvinyl alcohol at any step in the above-described prep object of this invention to provide a process for the prep aration of the polyvinyl alcohol solution. Almost any aration of polyvinyl butyral resin which does not in strong acid may be used as a catalyst Such as hydro clude the necessity of recovering large amounts of an chloric, formic, phosphoric, sulfuric, methylolsulfonic or expensive organic solvent. Other objects will be appar toluenesulfonic, the last of these, particularly the para ent from the more detailed description of the invention isomer, being preferred primarily because it produces which follows. more uniform resin particles at a higher rate of reac The above objects are accomplished in the process tion and with fewer adhesions to the reactor walls. Be of this invention by the steps of (1) preparing a mixture cause of the high rate of reaction associated with the use of an aqueous polyvinyl alcohol Solution, a strong acid of p-toluenesulfonic acid in this process, it may be de catalyst, and an aliphatic aldehyde having 2-6 carbon 50 sirable in some instances to employ a mixture of sulfuric atoms, said aldehyde being present in an amount which acid and p-toluenesulfonic acid as the catalyst of this is 5% to 40% in excess over the theoretical amount process. It appears that there is an optimum pH of the which will condense with the desired number of hy solution at this point since pH values of about 2.5 or droxyl groups in the polyvinyl alcohols, (2) pumping higher cause the condensation reaction, which will be de the resultant mixture at 50° C. to 95 C. in a recircu 55 scribed later, to be too slow for commercial attractive lating loop from which a continuous side stream is drawn ness. On the other hand, pH values below about 1.5 off, the flow rate of the side stream being not more than tend to cause excessive corrosion of the reaction vessel. about Ao of the flow rate in the recirculating loop and Polyvinyl alcohol, aldehyde and acid catalyst may be the linear velocity of the fluid in the recirculating loop combined in any order of addition, for example, the poly being not less than about 5 feet per second, (3) collect 60 vinyl alcohol and the acid catalyst may be combined ing the material flowing from the side stream and main prior to the addition of aldehyde, the polyvinyl alcohol taining it at a temperature of about 60° C. to 100° C. and the aldehyde may be combined prior to addition of until the desired degree of acetalization is essentially the acid catalyst, or all three constituents may be added completed, (4) raising the pH of the resulting polyvinyl simultaneously. The preferred order of combination is acetal slurry to some value from about 5 to about 7, (5) 65 that shown in the drawing. removing excess aldehyde, (6) stabilizing the resulting The solution of polyvinyl alcohol, water, and acid cata polyvinyl acetal resin slurry by raising the pH to about lyst is discharged from vessel through junction 6 to the 8 or more and (7) recovering a polyvinyl acetal resin inlet of pump 7. At the same time, aldehyde from storage flake. In the preferred embodiment of this invention the vessel 5 is added through junction 6 to the inlet of pump aldehyde is butyraldehyde and the polyvinylbutyral resin 70 7. The rate of aldehyde addition is carefully controlled product contains not more than about 2% by weight of with relation to the amount of solution being added from vinyl acetate groups, 18% to 21% by weight of vinyl vessel 1. The amount of aldehyde which is added should 2,720,501 3. 4 be in excess of the theoretical amount required to pro 10 minutes for the material flowing in the prereactor loop. duce the desired final acetal product and it is preferred This hold-up time is sufficient to allow the resin which that from about 10% to about 40% excess aldehyde be is forming to pass the initial sticky stage and to be intro employed. High quality polyvinyl butyral resin, which is duced into the condensation reaction 12 without fear of used for an interlayer in safety glass production, is pref forming agglomerates. The temperature employed in the erably acetalized to the extent that approximately 18% to prereactor loop is from about 50° C. to about 95 C. and 21% by weight of the product resin is vinyl alcohol which the pressure employed in the loop should be high enough means that the acetalization reaction is from about 69% to prevent vaporization of the materials flowing in the to about 75% complete. The theoretical amount of bu loop thus preventing pump cavitation, loss of circula tyraldehyde required to produce such partially acetalized 10 tion, flow and so forth. products may be expressed as a weight ratio of butyralde The partially condensed substance flowing through hyde to polyvinyl alcohol, a ratio of 0.60 producing a sidestream 11 is collected in reactor 12 and maintained 18% vinyl alcohol resin and a ratio of 0.57 producing a for about two hours at a temperature of about 60 C. 21% vinyl alcohol resin. Since these ratios are for the to 100° C. in order to complete the condensation reac theoretical butyraldehyde requirements, it is pointed out 5 tion.