EuroWire September 2020

Technical Article

Run

1

2

3

4

5

6

UA-1

70

70

70

0

0

0

UA-2

0

0

0

70

70

70

Acrylates-2

24

22.5

21.2

24

22.5

21.2

N-vinyl monomer

6

5.6

5.3

6

5.6

5.3

Di-functional monomer

0

1.9

3.5

0

1.9

3.5

▲ ▲ Table 1 : UV curable resins with various amounts of di-functional monomer

2 Results and discussion

and tensile strength. It is necessary to examine the nanoscale morphologies in order to elucidate the phenomenon. However, as far as we know, the morphologies of the films had not been examined. In general, UV curable urethane acrylates are known as they are composed of rigid polyurethane and acrylates sequences (hard segment) and flexible polyether segments (soft segment). Therefore, it might be possible to confirm the nanoscale morphology of the UV curable urethane acrylates films by measuring nanoscale modulus and adhesion maps. In this study, the AFM technology had been used to investigate the phase structure of the Run5 film. Figure 3 shows an AFM phase image of modulus and adhesion maps of the Run5 film. The characteristic phase separated morphologies consisting of bright and dark nanophasic domains were observed in both maps. Bright domains (higher modulus domains) in modulus maps corresponded with dark domains (lower adhesion domains). This result yields that it is assumed that these domains were composed of rigid polyurethane and acrylate sequences. On the contrary, dark domains (lower modulus domains) in modulus maps corresponded with bright domains (higher adhesion domains). It is assumed that these domains were composed of flexible polyether segments. 3 Conclusions It was found that the tensile strength of low modulus UV curable urethane acrylate resin was greatly affected not only by di-functional acrylic monomer but also by urethane acrylate oligomer.

they had similar tensile strength in spite of their different Young’s modulus. This experiment yields that using UA-1 is more suitable to achieve high mechanical strength with low Young’s modulus than using UA-2. Dynamic mechanical analyses of the Run2 and Run5 films were carried out and the results are plotted in Figure 2 . Both tanδ curves of Run2 and Run5 showed bimodal shapes. This result implied the induction of the phase separation of lower Tg polyether diol and higher Tg acrylate polymer to Run2 and Run5 films. Run2 showed inflection points at -40°C and -4°C. Run5 showed inflection points at -54°C and 5°C. In light of the evidence, the phase separation status between Run2 and Run5 was different, and this different phase separation status might cause the different relationship between Young’s modulus

The addition of the di-functional monomer enhanced the crosslink density, and Young’s modulus was controlled by the amounts of di-functional monomer. Table 1 lists the tested resins. The di-functional monomer to the resin was varied from 0 to 3.5 wt % to the formulations containing 70 wt % of UA-1 (Run1, 2 and 3) and UA-2 (Run4, 5 and 6). Figure 1 shows the relationship between Young’s modulus and tensile strength. The addition of di-functional monomer enhanced not only Young’s modulus but also tensile strength for both UA-1 and UA-2 formulations. As compared with Run1 and Run4 or Run2 and Run5 (the same formulation except for urethane acrylate oligomers),

▼ ▼ Figure 1 : Relationship between Young’s modulus and tensile strength

▼ ▼ Figure 3 : AFM phase images (Run5). Upper figure: modulus map, lower figure: adhesion map

Tensile strength [MPa]

Young’s modulus [MPa]

▼ ▼ Figure 2 : Loss tangent (tanδ) of Run2 and Run5 films

tanδ

Temperature [°C]

71

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September 2020