Scratch Damage and Recovery of Controlled Epoxy Networks
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Scratch Damage and Recovery of Controlled Epoxy Networks Hamed Lakrout1, Aaron M. Forster2, Lipiin Sung2, Chris A. Michaels3, and Deborah Wang2 1
The Dow Chemical Company, New Products, Midland, MI 48674 Building and Fire Research Laboratory, NIST, MD 20899, U.S.A 3 Chemical Science and Technology Laboratory, NIST, MD 20899, U.S.A. 2
ABSTRACT Surface scratches in a series of controlled epoxy networks (CEN) were measured using a combination of instrumented indentation protocols and laser scanning confocal microscopy. Identical epoxy chemistry with increasing molecular weight between crosslinks provided different viscoelastic relaxation behaviors with the same modulus at ambient conditions. The glass transition temperatures ranged between 70 °C and 117 °C. The high Tg CEN exhibited the lowest penetration depth and the highest elastic recovery. The results are analyzed with respect to the macroscale bulk properties and underlying molecular architecture of the CEN materials. EXPERIMENTAL† A series of five controlled Epoxy Networks with identical chemistry with increasing molecular weight between crosslinks have been provided by The Dow Chemical Company. Bulk characterization included DSC, DMA, tensile testing, compression testing, and thermal conductivity measurements. Surface mechanical properties and scratch measurements were performed using a commercial nanoindenter. Scratch topographic features were measured using a laser scanning confocal microscope (LSCM). RESULTS AND DISCUSSION The bulk and surface properties of the different epoxies have been compiled into Table 1. All of the epoxies have a glass transition temperature that is at least 50 °C above ambient conditions. Measurements of the molecular weight between crosslinks show that the glass transition temperature, Tg, of the epoxy increases as the molecular weight decreases. The impact of the polymer architecture on bulk thermal properties is further confirmed from thermal conductivity measurements, which exhibits a similar increase with the glass transition temperature. The room temperature quasi-static properties of these materials from tension and indentation measurements do not change significantly with network architecture. The tensile modulus, Etensile, and indentation modulus, EDSI, were consistent for all of the epoxies tested. Similarly, the yield stress, σy, determined from compression tests did not significantly vary with network architecture. The hardness was highest for materials A and B and lower for C, D, and E. Table 1 Compilation of physical and surface property data †
Certain commercial products or equipment are described in this paper in order to specify adequately the experimental procedure. In no case does such identification imply recommendation or endorsement by the National Institute of Standards and Technology, nor does it imply that it is necessarily the best available for the purpose.
Sample ID A B C D E
Mc g/mol 630 920 1500 2920 5290
Tg °C
k W/(m⋅K)
117 88 80 74 71
0.323 0.326 0.329 0.342 0.347
σy, compressi
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