When using low-hardness rubber compound, it
is often encountered that colorful patterns are formed on the surface of the
rubber compound. Although this phenomenon will not affect the performance of
the rubber compound, it will seriously affect the appearance of the rubber
compound and cause trouble for customers.
Cause Analysis:
(1) Formula design
1. In general, the smaller the particle
size of carbon black, the larger the specific surface area, the more
by-products carried on the particles, and the greater the possibility that some
by-products will be precipitated from the rubber compound due to the difference
in solubility, resulting in overcolor. Like N220, N330 and other highly
reinforced carbon blacks, the possibility of overcoloring is much greater than
that of N660, N774, N990 and other carbon blacks. In addition, N550 and N774
appear reddish, and N330 is bluish.Inert fillers are added to the raw materials
of EPDM rubber, and the inert fillers are preferably silicon powder. It can
effectively absorb the number of functional groups on the surface of carbon
black, and can react with hydroxyl, carbonyl and carboxyl groups on the surface
of carbon black, and at the same time these functional groups are bound, thus
preventing these functional groups from coming into contact with oxygen in the
air and risking overtone formation under sunlight scattering.The carbon black
of the raw material of EPDM rubber is selected as medium and low reinforced
carbon black. Its function is that the number of functional groups carried on
the surface of the medium and low reinforced carbon black is higher and the
number of reinforced carbon black is less, that is, the number of surface
hydroxyl groups, carboxyl groups and carbonyl groups At the same time, it is
easy to mix with rubber during the production process, so as to avoid the risk
of overcolor caused by functional groups and by-products on the surface of
carbon black.In the preparation process, the medium and low reinforcing carbon
black and the oil are added in stages, the purpose is to avoid the phenomenon
of slippage during the mixing process, resulting in the rubber material not
heating up normally.
2. The use of higher quality paraffin oil
will reduce the risk of overcolor, such as high flash point and higher purity
paraffin oil. In addition, oil-extended ethylene-propylene rubber can be used
in formula design to relatively reduce the use of paraffin oil;
3. 4010NA (EPDM is generally not added,
natural rubber is more), and additives such as petroleum resin will also have
the problem of overcolor.
4. Paraffin oil, carbon black and some
additives are not uniformly dispersed during rubber mixing.
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