August 10, 1919 



Hardwood Record — Veneer & Panel Section 



23 



Water Proofing Panels 



Factors Affecting the Water Resistance of Plywood 



By O. L. SPONSLER* 



N THE CONSTRUCTION of the modem airplane 

 many parts are made from built-up wood, be- 

 cause great strength combined with light weight 

 is required. The extremes of weather to which 

 the plane is subjected demand a plywood which will not 

 come apart when thoroughly wet, or when dried after 

 repeated wettings. Such a water-proof plywood has been 

 manufactured for many years, but it had attained no spe- 

 cial prominence until the War Department suddenly asked 

 the manufacturers to furnish several million feet. 



It was required that the plywood should withstand con- 

 tinuous soaking in cold water for ten days, or continuous 

 boiling for eight hours, without separation of the plies. 

 Such specifications fairly dazed most of the manufacturers 

 of panels, for it seemed that the requirements were too 

 severe. 



Demand for Water-Resistant Glue 



Naturally the first demand of the manufacturer was 

 for a glue which would not dissolve after it had once 

 "set" between the plies of wood. Many kinds of glue were 

 tried but only two proved sufficiently insoluble and were 

 at the same time sufficiently cheap and available to be- 

 come extensively used. These two glues have casein and 

 blood albumin for their bases. Both react quite differ- 

 ently from animal and vegetable glue which are so com- 

 monly used by panel makers, and both required the use 

 of new machinery and the development of new methods. 



Efurly Manufacturing Difficulties 



The Government placed enormous orders and de- 

 manded immediate production. Manufacturers worked 

 day and night to fill these orders but were constantly 

 beset with difficulties and troubles which taxed them to 

 the utmost notwithstanding their optimistic determination 

 to produce results. Experiment and production had to 

 go hand in hand, and naturally there was considerable 

 loss in time and material. The troubles often seemed 

 mysterious, erratic, and discouraging. About nine times 

 out of ten the trouble was charged against the glue, and 

 the manufacturer of it was called to account. At other 

 times some particular ingredient used in mixing the glue 

 was suspected and other brands were tried in its stead. 

 Often the method of mixing the glue was altered, or the 

 treatment or "curing" of the panels was suspected and 

 changed, with the hope of eliminating the trouble. The 

 greatest troubles came when thick plies — 1/12", 1/10", 

 and J/g" — were being glued, particularly when casein 

 glues were being used. The strength tests of the glued 



joints were usually very satisfactory, but the boiling and 

 soaking tests showed a high percentage of failures. 



Observing Effect of Moisture in Veneer 



Early in April of 1918, experiments were in progress 

 at the Forest Products Laboratory dealing with the effect 

 of the moisture content of veneer upon the appearance 

 and the degree of water resistance of the finished panel, 

 in an eastern plant about 10,000 feet of thin plywood 

 were made by the writer with the use of a blood glue 

 formula that originated at the Forest Products Laboratory. 

 The moisture content of the veneer was between 1 5 and 

 20 per cent at the time of gluing. A little later in the 

 summer, another lot of plywood with about the same 

 moisture content was made in a middle western factory. 

 In both cases the water resistance of the plywood was 

 greater when the moisture content of the veneer was high. 



In November, more extensive work was carried out to 

 determine the minimum amount of moisture in the veneer 

 at which I 00 per cent of the plywood would be sufficiently 

 resistant to withstand the severe tests of boiling and soak- 

 ing required by the government. This work was done 

 in veneer plant in one of the north central states, where 

 the veneer was cut and the plywood glued up in the same 

 plant. There had been considerable trouble at this plant 

 in getting the plywood to withstand the boiling test, 

 whenever the face plies were more than 1/16 in thick. 

 Results of 1 30 tests showed a uniform increase in failures 

 as the thickness of the plies increased. Failures consisted 

 of a separation of the plies; sometimes only a square inch 

 or less would be loose, while in other cases one or both 

 face plies would fall away from the core. 



In tabular form this is brought out more clearly: 



THICKNESS OF PLIES 



1 - 1 2 in 



50% 



I-lOin. 



25% 



•Wood Technologist, Forest Products Laboratory, Madison, Wis. 



Results of 8 hours boiling 1-20 in. I - 1 6 in 



No separation of plies. . . 85',"( 60% 

 Separation varying from 



slight to complete 15% 40% 50% 75% 



These test panels were taken from the regular day's 

 run and were made under the shop conditions as ordi- 

 narily found in plants of this character. The cause of these 

 failures was finally located in the moisture condition of 

 the veneers at the time of gluing. A large number of 

 tests were then made to ascertain the proper moisture 

 content at gluing. The results were obtained entirely 

 from panels glued with casein glues. 



Study of General Mzmufacturing Conditions 



In locating the trouble, however, a systematic search 

 was made for the cause, and practically all of the opera- 



