Depending
on
their
origin
,
plastics
can
be
classified
in
____________________
plastics
and
synthetic
plastics
.
a
)
Natural
plastics
are
obtained
directly
from
raw
vegetable
materials
-
for
example
,
cellulose
or
latex
,
or
from
animal
proteins
.
An
example
of
a
natural
plastic
is
rubber
.
b
)
Synthetic
plastics
are
made
from
compounds
extracted
from
____________________
.
The
majority
of
plastics
belong
to
this
group
.
Synthetic
plastics
are
obtained
in
two
stages
:
1
)
First
,
petroleum
is
transformed
in
____________________
into
different
substances
with
molecules
called
____________________
.
Examples
of
monomers
are
the
ethylene
(
etileno
)
or
the
propylene
(
propileno
)
.
2
)
Then
,
these
monomers
go
through
a
chemical
reaction
called
____________________
,
where
the
monomers
____________________
with
each
other
until
a
huge
molecule
(
macromolecule
)
is
created
.
This
macromolecule
is
called
polymer
or
plastic
.
Examples
of
polymers
are
the
polyethylene
(
polietileno
,
____________________
)
or
the
polypropylene
(
polipropileno
,
____________________
)
During
the
polymerisation
reaction
,
we
can
add
some
additives
to
obtain
plastics
with
better
____________________
,
and
we
can
also
add
pigments
to
give
them
____________________
.
