Properties and Nano-observation of Natural Rubber with Nanomatrix Structure - Natural Polymers, Biopolymers, Biomaterials, and Their Composites, Blends, and IPNs - page 317

table 1. Nitrogencontent and incubation time for HANR, Fresh NR, and Deproteinized NR.

Specimens Incubation time (min) X (wt %)

HA-NR
E-DPNR a
U-DPNR b
EU-DPNR c
Fresh NR
Fresh E-DPNR
Fresh U-DPNR
Fresh EU-DPNR

0
720
60
780
0
720
60
780

0.300
0.017
0.020
0.008
0.450
0.014
0.004
0.005

a E-DPNr; enzymatically deproteinized HA-NR.
b U-DPNR:urea-treated HA-NR.
c EU-DPNR: urea-treated E-DPNR.

A plot of total nitrogen content versus time, t , required for the deproteinization of
HANR and fresh NR with urea at 303 K is shown in Figure 1. The nitrogen content
of HANR and fresh NR decreased suddenly to 0.022 and 0.005 wt% after 10 min,
respectively, after adding urea. The difference in the nitrogen content between HANR
and fresh NR may be attributed to the amount of the proteins that are weakly attracted
to the rubber, as mentioned above. It is quite important to note that the nitrogen content
of fresh NR decreases to and reaches a definite value of 0.004 wt% within 10 min,
expressing an advantage of urea compared to proteolytic enzyme in view of the rapid,
efficient deproteinization. The dependence of the total nitrogen content on temperature
is also shown for fresh NR in Figure 1. The nitrogen content decreased to 0.004 wt%
within 10 min at all temperatures, ranging from 303 to 363 K. This may be in part due
to the ability of urea to form hydrogen bonds with the proteins and detach themselves
from the rubber particles with urea, based upon the previous work (Creighton, 1984).
Consequently, urea is proved to be more effective to remove the proteins from fresh
NR, rather than the proteolytic enzyme.

Figure 1. Nitrogen content of HA-NR at 303 K ( ◊ ), andfresh U-DPNR at 303 K ( ○ ), 333 K ( □ ), and 363
K ( Δ ) versustime for incubation.