Page 155 - Engineering Plastics Handbook

128 Engineering Plastics

plastics incur deterioration or discoloration upon receiving high-energy
light such as UV rays. The UV has high energy to make the material
chemically unstable, and such an unstable state is readily combined with
oxygen in the air to deteriorate the material, during which various chem-
ical compounds are produced that cause discoloration of the material.
General ABS has excellent moldability and mechanical properties, but
has poor weatherability due to the double bond of the butadiene rubber
component used as the impact reinforcing agent. The double bond requires
less energy to cleave one chain, in comparison to the single bond, and
therefore is easy to discolor by oxidation. Therefore, in the environment
directly receiving UV rays, such as outdoors, ABS resin sees a change in
the appearance and color of the resin and exhibits a rapid deterioration
of mechanical properties.
To prepare the resin having balanced properties by providing weather-
ability while maintaining several excellent properties of ABS resin, meth-
ods including addition of the ultraviolet stabilizer, painting, and metal
plating may be adopted. However, it is not the essential improvement.
Here, various resins have been developed wherein the butadiene compo-
nent is removed as the cause of deterioration and, instead, the rubber
containing no unsaturated bond is replaced.
The representative resin includes ASA resin using acryl rubber
instead of butadiene rubber and AES resin using EPDM rubber. Those
resins are for use outdoors without postprocessing such as painting and
metal plating and have been reported to have comparable properties,
moldability, and weatherability.
However, there are slight differences according to the manufacturing
processes. In general, ASA is made by preparing acrylic rubber latex and
then grafting SAN with emulsion polymerization; AES is prepared by two
methods. One method is to dissolve EPDM rubber in a polar solvent and
then graft SAN thereon to prepare AES. The second method works by
reacting EPDM rubber with SAN during the extrusion procedure to pre-
pare AES.
In comparison to AES prepared in the polymerization, AES prepared by
grafting SAN in the extrusion procedure may reduce the manufacturing
cost by simplification of the procedures, but has the disadvantages that
(1) it is somewhat inferior in view of the properties and weatherability, and
(2) the use of polar solvent makes the procedure complicated and induces
environmental problems. Particularly, due to low gloss of AES, it can be
limited to applications in the electric and electronics fields and therefore
cannot be widely used in comparison to ASA resin.
Therefore, at present the demand for ASA as the weatherable resin is
great all over the world. Moreover, recently ultraweatherable ASA resins
having heat resistance and surface properties, processability, and weather-
ability are being developed.