The Crosslinked Network in Sulfur and Peroxide Cures

An effort was initiated to fundamentally characterize crosslink networks and thereby to better understand the effect of the network on physical properties.  The work compared two natural rubber model (unfilled) compounds; namely, an efficient sulfur cured compound was compared to a peroxide cured compound.  The characterization included tensile properties at room temperature and 100°C, crosslink density by solvent swelling, time domain NMR, DMA strain sweep, creep at room temperature, crystallization kinetics, creep at 100°C, and FTIR.  The crosslink densities were designed to be similar.

 The scope includes preparation of model (unfilled) compounds with the same crosslink density and an examination of the compounds for the following explanations [potential mechanism(s)] for improved performance:

1. provides network uniformity (as measured by time domain NMR)
2. increases hysteresis (as measured by DMA strain sweeps)
3. provides the ability to have bonds break and reform to relieve stress (as measured by room temperature creep)
4. promotes stress-induced crystallization (as measured by crystallization kinetics)
5. promotes reversion (as measured by hot (100°C) creep)
6. avoids loss of stereo-regularity (cis to trans rearrangement) during cure (as measured by FTIR). 

The sulfur cured compound had narrower distribution of molecular weights between crosslinks.  This enabled the compound to crystallize faster.  The ability to crystallize rapidly upon stretching provided higher ultimate properties, higher resistance to deformation at room temperature, higher modulus, and higher hysteresis.

The conclusions of the preliminary study are:

1)     Network structures which enable (promote) crystallization during stretching provided better performance.

2)     Network structures with a narrow molecular weight distrution between crosslinks may promote crystallization during stretching.