This technical information has been contributed by
Holden Plastics Corporation
PLASTICS: THERMOSET & POLY
Thermosets Engineering Plastics For Demanding Applications
Thermosets are a group of engineering plastics which are particularly
well suited to demanding requirements. For the engineer, they are
materials offering outstanding performance which is well understood and
proven through decades of success. For the manager, they represent
substantial cost savings as a replacement for metals, and as an
alternative to other plastics offering similar performance, but at greater
cost.
The hallmark of these materials is the ability to withstand heat and
pressure for long periods of time, without failure. Their dimensional
stability, creep resistance, chemical resistance, stiffness, and high
temperature capability make them excellent choices where reliability
under adverse conditions is important. A basic understanding of
thermosets, and their strengths, can open the door to new ideas, and
prevent the costly error of using the wrong material when designing in
plastic.
Thermosets are different from other plastics. All plastics are composed
of long strings of molecules connected end to end. In most plastics,
these strings are independent of each other. These materials are like
tangled masses of spaghetti, and are called thermoplastics.
Thermoplastics may be melted over and over again like a wax candle,
figure 1. Because of this property, thermoplastics are melted at a high
temperature, and then formed in a cool mold where they solidify.
Thermosets are different because their molecules become
interconnected in a chemical reaction called crosslinking. Crosslinking is
an irreversible process which requires heat. For this reason, thermosets
are "cured" in a hot mold. Once crosslinked, all the molecules are tied
together so the thermoset cannot be re-melted. Take cement for
example. Once the sand, water, and concrete have been mixed together
and cured, it is impossible to soften the mixture again, figure 1. The
principal members of the thermoset group are Phenolic, Epoxy, Diallyl
Phthalate (DAP), and Polyester. Other's include Polyimide, Alkyd,
Melamine, and Silicone materials.
Theory:
This lab will use the `Alumalite' two part polymer system. The two parts are measured out in
equal volumes and mixed together. Once mixed these parts form long polymer chains. By
themselves this would leave a rubbery solid. Crosslinking also occurs which makes these
materials stiff. During this process at least some heat is released as bonds are broken and
made. In the presence of heat this reaction will occur faster, and polymerization will be more
complete. when fully cured the Alumalite will have a beige, or creamy appearance. When
not fully cured the result will be a semi-transparent brown, that may still be sticky. This
material will tend to gel within a minute, and be solid within two, so speed is required as
soon as the two components have been mixed.
Procedure:
1. Place the mold in a freezer to chill it for an hour before use.
2. Estimate the volume of the mold, using a previous part.
3. put on apron, safety goggles, and gloves. If the ventilation is poor, or the work prolonged,
use a breathing mask.
4. Taking care to use the proper cylinder for the proper Alumalite component, measure out
two equal volumes or parts A and B to each cylinder. The volume should be just over half
the required total.
5. Make sure that all mixing, and measuring facilities are ready.
6. Remove the mold from the freezer. Clean the mold, and spray with release agent.
7. Hook up and verify the operation of the thermocouples.
8. Pour the two components into the mixing cup, and mix thoroughly - a brownish mixture
will appear in a few second.
9. Pour this into the mold, and start the stopwatch, and start measuring temperature. Use an
air jet to keep the surface of the part cooler.
10. At regular intervals (10 seconds) measure temperature, and touch the surface lightly to
see if it has solidified. Record the time when the surface becomes `solid but tacky' as the gel
time. Record the time when the surface becomes hard as the set time.
11. When satisfied that the process is complete, remove the part and make observations.
12. Repeat the process, but this time at normal room temperature.
13. Repeat the process again, but this time use a an oven to preheat the mold, and a heat
gun/oven to speed the process
I liked this portion of the lab. I wanted to make some molds and do more chess pieces. But, we spent so much time fighting with the plastic injection machine. We could have spent more time investigating a process that produces usable parts.