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Cure system effect on low temperature dynamic shear modulus of natural rubber.

Article Excerpt
Natural rubber (NR) is used in many dynamic applications. Its ability to strain crystallize imparts high physical properties. These properties can develop at light levels of loading, which along with polymer structure, can result in low levels of damping, ideal for rubber isolators.

Cure systems are known to affect low temperature properties. In natural rubber, modulus increases with progressive levels of crystallization. The degree of crosslinking is known to affect the amount of crystallization (ref. 1).

Service temperature of natural rubber is limited at the high end to approximately 80[degrees]C, depending on cure system. At the lower end, natural rubber has a glass transition temperature of -72[degrees]C, but it becomes effectively non-compliant much before this.

A designer desires constant spring rates for critical applications. Unfortunately, rubber properties, including modulus, are affected by time, temperature, strain history, frequency, etc. The rapid increase in modulus at lower temperatures may be avoided, to a degree, in formulation development by using polymers with very low Tgs such as polybutadiene (BR) or silicone (MVQ). However, these polymers do not possess the strength and fatigue resistance properties of NR. Lower temperature properties of NR can be improved by polymer blending, use of plasticizing oils, or by varying the cure system.

The methodology for measuring lower temperature shear modulus was explored on a servohydraulic dynamic tester. Also, a variety of cure systems was studied in lightly filled natural lubber. The shear modulus changes can be applied in engineering applications where time and temperature dependent changes in modulus are important. A 48 hour soak (exposure at temperature) is the primary evaluation period, simulating a weekend of exposure at temperature without use.

Large crosslinks, sulfur or various longer crosslinks, show a marked delay in modulus increase as compared to short crosslinks formed with lightly crosslinked peroxide and EV sulfur cure systems. High crosslink density cure systems are still best at minimizing the crystallization induced modulus increase at low temperatures (ref. 2).

Several alternate cure systems and variations on traditional sulfur cure systems exist today. These systems ale used for a variety of intended purposes, including heat resistance, fatigue and reversion resistance. An evaluation of the low temperature vs. room temperature modulus change was made for these crosslink systems. Some of these introduce large molecules into the crosslink. These large molecules could interfere with some crystallization and thereby minimize modulus increase. Most of the studied systems are shown in table 1.

* Sulfur cured systems at various sulfur levels;

* peroxide cure system (DCP);

* peroxide cure system with coagent TMPTMA;

* low sulfur systems with Na-HMT;

* low sulfur systems with CIMB;

* low sulfur systems with BDBzTH; and

* low sulfur systems with NPDI or urethane.

The cure system components CIMB, Na-HMT, BDBZTH and in some cases urethane are primarily intended to reduce reversion during cure in high sulfur systems. They become part of the crosslink system. Since the molecules are so bulky, it was decided to examine their influence on lower sulfur cure systems, and their influence on crystallization.

The urethane cure system introduces a very large molecule into the crosslink. Similarly, during peroxide vulcanization, the coagent TMPTMA can become part of the crosslink system.

Experimental

Simple base formulations were used for the evaluation and are shown in table 2. Formulations were mixed in two passes on a laboratory internal mixer.

Modulus measurements were made using dual lap shear samples on a servohydraulic dynamic test machine (ref. 3). Samples were cured and bonded 30 minutes at 160[degrees]C....