Some analytical methods for determining the degree of cure of phenol-resorcinol formaldehyde resin adhesive in a glue-line were investigated. The degree of cure of resin was evaluated by the ultraviolet absorption of water extractable materials from partially cured glue, the retention of potassium bromide or sodium chloride embedded in cured glue, the retention of sodium ion involving as a catalyst and the heat of cure determined by means of differential scanning calorimetry. Furthermore, the relation between the degree of cure of resin in a glue-line calculated by ultraviolet spectrophotometric and potassium bromide embedding methods and glue-joint strength was elucidated.
The results obtained are as follows :
(1) The undercured resin has a major peak at 275 nm with a minor peak at 269 nm in ultraviolet spectra. The relation between the water insoluble materials and absorbance ratio at 269 nm and 275 nm bands is a curve of second degree. The effect of wood extractives on absorbance ratio is negligible.
The amount of potassium bromide and sodium chloride embedded in glue increase exponentially with increase of water insoluble materials in partially cured glue. The effect of curing temperature on potassium bromide retention is negligible. Further, sodium ion used as a catalyst at the synthesis of resin is embedded in cured glue and the amount of that can be applicable to the indication of curing.
The heat of cure decreases with increase of degree of cure, and the differential scanning calorimetric curves give the informations on curing reaction and the optimum curing temperature for wood-adhesive bonding.
The each analytical method can be used to determine the degree of cure of resin in a glue-line, and the equations below are found as a calibration curve.
For absorbance ratio(Y) :
Y= 0.842 - 0.001139 X + 0.000065 X^2
For potassium bromide retention(Y) :
Y= 1.758 x <1.035>^^^x (curing temperature ; 40℃)
For sodium chloride retention(Y) :
Y= 3.667 x <1.027>^^^x
For sodium ion retention(Y) :
Y= 3.856 x <1.038>^^^x
And for heat of cure(Y) :
Y= -0.008 X^2 - 0.30 X + 105.2
Where ; Y= variables corresponding to each analytical method
X=degree of cure
(2) The degree of cure of resin in a glue-line of wood bonding is determined by the absorbance ratio of water soluble materials from cured glue and the amount of potassium bromide embedded in cured glue. The both methods can be available for the index of bond quality. The ultraviolet spectrophotometric method is used without knowing the original sample weight, but on the potassium bromide embedding method the initial concentration of potassium bromide in glue is a dominant factor.
It has become apparent that the sufficient bond quality is obtained at more than 70 percent degree of cure.