Simultaneous improvements in the
Cryogenic Tensile Strength, Ductility and Impact Strength of Epoxy
resins by a Hyperbranched polymer
by Jiao-Ping Yang, Zhen-Kun Chen, Guo Yang, Shao-Yun Fu, Lin
Ye
Ramesh Ramachandran
Geevarghese George
Structures of Components
Pure Epoxy Resin Vs Modified Epoxy
Experimental
2a. Experimental
2a.1 MaterialsEpoxy Resin: bisphenol A diglycidyl ether (DGEBA)
Curing Agent: methyltetrahydrophthalic anhydride (MeTHPA)
Accelerator:
benzyl dimethylamine (BDMA)
Hyperbranched Polymer:
polyester Boltorn H3032 hydroxyl groups
Amorphous solid at RT
2a.2 Sample Preparationsolution of H30 in acetone + epoxy is heated to 60 deg, and a homogenous mix is formed
Acetone evaporates during heating
H30 content varied from 0% to 15 %
stoichiometric amount of curing agent was added to the mix
Homogenous solution was casted into mold at 80 deg, then cured at 130 deg for 3 h, 150 for 10h, and post cured at 170C for 5 h
2b. Testing
Tensile TestingASTM D638
Loading Speed:
2 mm/min
Load: 10 kN
Cryogenic Temperature:Dipped samples and clamps in LN
Impact TestingASTM D256
Specimens dipped in a liquid nitrogen for 10 mins
Cryogenic thermocouples embedded inside That's how we know its 77K
PALS (Positron annihilation lifetime spectroscopy)Used to study the voids and defects on solids
Non-destructive spectroscopy
The positronium lifetime 3 and intensity I3 of the long-lived components were analyzed
DSCDifferent rates of heating 5, 10, 15 and 20 deg C/min
Tg was also studied30 to 200 deg celcius
Heating rate 10 degC/min
SEMTo study fracture surfaces
Fracture surfaces were fracture surfaces were cleaned with alcohol and spray coated with a thin layer of evaporated gold to improve conductivity.
3. Resuts and Discussion
Tensile Properties
Impact Strength and SEM
Thermal Properties
3.1 Tensile Properties
Reminder.. One unit of the epoxy network
Things to note (in modified epoxy)Free Volume
Free Space
More Hydrogen BondsFormed bcos of large no of unreacted Hydroxyl (-OH) groups
Bcos MeTHPA was added stoichiometrically to cure epoxy resin
PALS Results confirm the presence of free volume
Positronium lifetime 3 = shows average size of free volumes
Intensity I3 = shows the concentration of free volumes
DSC Scan
Displays peak exotherm at the same temperatures
H30 has little effect on the cure kinetics
Increase in TS @RT:Due to large no of HB
Flexible H30 reduces internal residual stresses
Decrease in TS @RT: Flexible H30 decrease rigidity of polymer chain
More free spaces and free volumes reduces XL density
At less than 10% H30, the latter factors are overshadowed by the former.
So, a net increase is seen
Beyond this conc., the factors that decrease the TS are predominant.Too much free volume and free spaces
Too much flexibilty
epoxy resins cured at high temperature, followed by bulk shrinkage during cooling
TS in Cryogenated sample (77K):Thermal shrinkage causes Stronger HB than in RT case
Reduction in free volume and free space
Residual stresses made more severe in bulk epoxyBut H30 helps reduce the effect of residual stresses
Too much H30 can reduce the TS like in the RT samples
Table 1: Modulus is decreasedBecause of incorporation of H30 (read soft, flexible)
But compared to RT samples, it is highBecause Thermal shrinkageMore HB
Table 1: Faliure strainLower in cryogenic samplesMacromolecules are frozen up
Free volumes and free spaces are reduced at 77KReduction in ductility
Ductility is increased as H30 increasesBcos easier mobility for macromol. as H30 (flexible, soft) is added H30 added free volume and free space increase mobility
3.2 Impact Strength
3.2 SEM Analysis (RT Vs 77K)
3.3 Thermal Properties
High Tg Restriction of Mobility:Large no of HB
Hyperbranched structure of H30 (like in PS) // Steric Effect
But H30 can also make the mobility easier! (soft, flexible)
Decrease in XL density (PALS Result corresponds to inc in free volume and space) should also reduce Tg
ALL these decreasing Tg effects is observed only above 15% conc of H30
Conclusion
Addition of H30 improvements in the tensile strength, failure strain and impact strength at 77 KMaximum tensile strength at 77 K is obtained for the modified system with 10 wt% H30
The elongation at break at 77 K is consistently increased by the addition of H30
The maximum cryogenic impact strength is obtained at 10 wt% H30.
The observed protonema by SEM on fractured surfaces is used to explain the RT and cryogenic impact strength for the H30-modified epoxy resins.
Appropriate amount of H30 does not affect Tg.
Cryogenic engineering applications!!
Thank You.
Click to edit the title text formatClick to edit Master title style
Click to edit the outline text formatSecond Outline LevelThird Outline LevelFourth Outline LevelFifth Outline LevelSixth Outline Level
Seventh Outline LevelClick to edit Master text styles
Second level
Third level
Fourth level
Fifth level
11/24/15