3.1 Reactivity based on Aging Time of Curing System
Figure 2 compares reactivity based on the aging time of the curing system. The test result indicated that as the aging time of the curing system increased, the liner viscosity build-up was accelerated. To confirm an accurate liner-reaction speed, gel time was compared as shown in Fig. 3. The gel time was 2.30 h if the liner was manufactured after 3.0 h of the aging of the curing system, and 1.16 h if it was manufactured after 216 h (9 d) of aging time. It can be seen that gel time was reduced by approximately 50%. Through the test result, the aging time of the curing system was confirmed as the factor that adjusts the liner’s hardening speed.
3.2 Factors that effect reaction speed in curing system
To confirm raw materials that effect reaction speed in the curing system, the test was conducted using three cases as shown in Table 3. Each case created a curing system and 168 h (7 d) of aging was performed. After the aging, the liner was manufactured to confirm viscosity build-up. When manufacturing the liner, TPB excluded from case 2 and MA excluded from case 3 were directly injected as solid state. As shown in Fig. 4, cases 1 and 3 had the reaction speed accelerated at the same level. However, case 2 showed that the reaction speed slightly increased compared with the case of manufacturing the liner without going through the aging of the curing system. This did not have a major impact on the reaction speed in the curing system. On the other hand, cases 1 and 3 both contain TPB, and it is determined that TPB dissociated in the IPDI solution is the raw material that has major impact on a liner’s hardening reactivity. Mixing IPDI and TPB can form activated complex as shown in Fig. 5. The activated complex reduces activation energy, and facilitates the urethane reaction of Fig. 6 [16,17].
Table 3 Effect of Aging time on Viscosity Build-up.
Curing system
|
Composition
|
Aging time
|
Case 1
|
IPDI
|
168 h (7 d)
|
TPB
|
MA
|
Case 2
|
IPDI
|
MA
|
Case 3
|
IPDI
TPB
|
3.3 Activation of curing catalyst
The raw material that has a major impact on reaction speed in the curing system is the curing catalyst of TPB. Color changes of TPB can be confirmed during the aging of the curing system after it is manufactured. It is determined that such changes in color are related to the activation of TPB. Figure 7 indicates color changes of the curing system based on aging, and it is confirmed that the color has changed to dark yellow as the aging proceeds. Through this, it can be seen that the TPB catalyst is activated. The color of bismuth changes from faint-yellow to dark-yellow if it is oxidated [18]. As shown in Fig. 5, if TPB is ligand bound with isocyanate, it forms TPB-isocyanate complex. This is an oxide of TPB, and through the color changes, the formation of activated complex can be confirmed indirectly.
3.4 Relationship between liner hardening speed and mechanical properties
It was confirmed that the liner’s hardening reaction time is changed depending on the aging time of the curing system. Hardening reaction speed and the liner’s mechanical properties are compared in Table 4. Aging times of the curing system were divided into two cases for cases 4 and 5. The test result indicated that hardening reaction speed is not related to the mechanical properties of the liner. Since the catalyst’s role is not to be directly involved in the reaction but only affects reaction speed, it does not affect the properties of the product [19].
Table 4 Effect of Aging time on Mechanical properties of liner.
Curing system
|
Aging time, h
|
Sm, MPa
|
Em,
%
|
E,
MPa
|
Hs
|
Case 4
|
24
|
6.77
|
320.90
|
5.06
|
63
|
Case 5
|
3
|
6.54
|
327.60
|
5.41
|
61
|
3.5 Comparison of adhesive strength between liner and propellant based on the liner’s hardening speed
The adhesive strength between the liner and propellant based on the liner’s hardening speed was compared in Table 5. Aging times of the curing system were divided into two cases of cases 6 and 7. The adhesive strength test was conducted as shown in Table 2. The test result indicated that the curing system in the cubic tension mode had the adhesive strength increased by approximately 66% in case 7 compared with case 6. The shorter the aging time of the curing system, the slower the reaction speed of the liner. Thus, in the case of curing the liner at a temperature of 70℃ for 18 h as shown in Fig. 8, the liner that applied the curing system of case 7 was not completely hardened. The liner has the most excellent adhesive strength when it is less hardened compared with when it is completely hardened. In addition, it is generally known that the lower the hardness of the liner, the higher the adhesive strength [20,21]. To achieve a strong adhesive strength after the adhesion is completed, it is more advantageous when the adhesive strength at interface between the adhesive and substrate molecules is greater than the cohesiveness between the adhesive’s own molecules [3]. The shorter the aging time of the curing system, the slower the hardening speed of the liner. This leads to functional groups remaining on the liner’s surface reacting with functional groups of the propellant. Thus, it is determined that adhesive strength between the liner and propellant was improved.
Table 5 Effect of Aging time on the adhesion.
Curing system
|
Aging time, h
|
Peel, daN/cm
|
Shear,
MPa
|
Cubic tesion,
MPa
|
Case 6
|
24
|
1.18
|
0.44
|
0.53
|
Case 7
|
3
|
1.06
|
0.65
|
0.89
|