Further investigations of the prevulcanization of natural rubber latex
A comparison has been made of the vulcanization of
NR in the latex form and in the dry form. In latex form,
NR wa s found to vu Lcani ze at a much f asterra t ethan in the
dry form. The faster rate of reaction in latex prevulcanization
is attributed to the presence of the aqueous
phase which acts as a medium for the dissolution of ZDBC
and sulphur, and provides a suitable environment for the
two vulcanizing ingredients to react to form the active
A comparison has also been made of the prevulcanization
behaviour of NR latex and synthetic cis-polyisoprene latex.
It has been suggested that the indigenous vulcanization
accelerators and activators present in NR latex act as
additional vulcanizing accelerators and therefore increase
the rate of prevulcanization.
Experiments have been carried out to determine the
solubilities of sulphur and ZDBC in the aqueous phase of
the latex during prevulcanization. Sulphur was found to
dissolve to an appreciable extent; ZDBC was found to have
a solubility of ca. 0.011%.
The effects of varying the levels of sulphur and
ZDBC upon NR latex prevulcanization was investigated. The
variations of the following properties with time of
prevulcanization were investigated: free and network-combined
sulphur, disappearance of ZDBC, crosslink insertion,
and tensile strengths of cast films. Information was also
obtained concerning the polymer-solvent interacticn
parameter for the cast films swollen in n-decane at 23°C.
Investigations of the different types of sulphidic
crosslinks as a function of prevulcanization time for
vulcanizate obtained from prevulcanized latex have also
been carried out. Tne crosslinks were found to be
mainly of the polysulphidic type.
The effect of varying the dialkyl chain-length and
metal counterion of the dithiocarbamate accelerator upon
NR latex prevulcanization have been studied. The molal
effectiveness of the zinc dialkyl dithiocarbamates appears
to pass through an optimum as tbe dialkyl chain-length of
the accelerator is increased. PbDEC was found to be of
very low activity as an accelerator and the water soluble
SDEC has poor. accelerating properties when compared to
ZDEC. Plausible hypotheses to account for the
observations have been postulated.