THE FUNDAMENTALS OF ACRYLIC
Trouble Shooting Problems within the Laboratory
Vocabulary:
Polymerization: is the "start to finish" thermo/chemical process through which acrylic resin ascends
during the curing (hardening) process. When acrylic ascends through this process, heat is generated to
facilitate the molecular reconfiguring and structuring necessary for the transformation from a powder
and liquid to a hardened acrylic plastic.
Thermo/chemical process: a molecular coming together of two or more chemicals that interact with
each other to produce heat. This heat is a by-product of the on going molecular reaction that is taking
place between each atom within each molecule of each contributing chemical.
Thermo/catalytic process: the molecular coming together of two or more specific chemicals that
aggressively interact with each other to hasten the production of heat, to accelerate the thermo/chemical
process between other molecules of inherent ingredients of particular compounds.
Cold Cure acrylic: acrylic resin that is formulated so that the curing (polymerization) process is
activated/initiated by special chemicals within the two components. When the cold cure resins are
combined, a thermo/catalytic reaction begins, thus allowing the acrylic to polymerize without the
induction of an outside heat source.
Heat curing and Heat Cured Acrylic: Acrylic powder and liquid that does not cure by itself. The
induction of an outside heat source is necessary to activate (start) the thermo/chemical reaction needed
for the polymerization of the resin.
Using Cold Cure resin with heated pressure vessels-Using today’s standard Orthodontic cold cure resins with the standard, heated pressure pot or vessel,
makes for an accelerated polymerization cycle, which is conducive of today's laboratory procedures.
This accelerated processing allows for quicker fabrication of dental appliances that are hard, dense, and
free from porosity.
By stacking or pouring an appliance with cold cure resin, and inserting said case into a heated pressure
vessel, the heat and pressure accelerates the thermo/chemical process, so that this process starts and
stops quicker and with complete polymerization of all the molecules of resin present.
Salt & Pepper technique: the alternating application of the acrylic monomer and polymer to "stack
up" the dimension (thickness) of acrylic resin on the plaster model/cast.
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Stack: to "build up" the layers of acrylic to the desired depth or thickness for each specific appliance.
Dough Technique: To gently mix the powder and the liquid together in a silicone or none vinyl
container at the 3:1 ratio (ratio could be slightly less) and allowed to gel to a soft cream cheese
consistency. This soft mass can be rolled, shaped, spread onto the prepped cast as a means of
Acrylic application. Do not incorporate air pockets into this mass; otherwise you may get air bubbles
that DO NOT cure out during the pressurization process.
What is crazing?
Crazing is the fracturing of the molecular infrastructure between the resin molecules. When acrylic
starts to polymerize, it links with other like molecules and establishes building blocks. Crazing happens
when the short building blocks, break away from each other and create a visual fracture in the structure
of the resin. Non cross-linked acrylic will craze when liquid monomer is put on the acrylic during a
patch or repair.
What is cross-linking?
Cross-linking is the chemically initiated process that allows all the molecules of resin to establish long
chains of interlocking building blocks of cured resin. With cross-linked resin, you get extremely
strong, dense, visually clear and crack and craze resistant acrylic resin. Not all acrylics are supplied to
the orthodontic industry cross-linked. Non cross-linked acrylic will craze when liquid monomer is put
on the acrylic during a patch or repair and will be less costly.
What is Porosity?
Porosity or tiny bubbles inside the cured acrylic is what is referred to as porosity. There may be several
reasons for porosity. The most often cause of porosity is the improper ratio of powder to liquid or
forgetting to add air to the pressure pot.
1. When not enough monomer is applied to a stack of polymer, thus not allowing for the proper
hydration of the polymer.
3 parts of powder to 1 part liquid
If not enough liquid is used to totally wet all the little balls of resins (wet, not drench and flood away),
the inherent chemical reaction of the curing process is compromised if not enough liquid is used. The
compromising of the exothermic stages of polymerization can exhibit porosity. When we see tiny air
bubbles, what we are actually seeing are pockets of unprocessed or incomplete polymerized molecules
of resin.
2. Accidentally moving the acrylic around after stacking and/or after the (gelation) polymerization
process has started; usually this is evident in one location of the finished appliance, i.e., the first portion
that was stacked/poured.
3. Taking too long to pour or stack--if polymerization has started before case is inserted into the curing
pot, porosity “may” develop.
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4. Forgetting to activate/turn on the air to the pressure vessel. If model/appliance is placed into the
very warm pressure vessel and the NO air is not injected into the pot. No person may ever notice or
remember that the air wasn't put into the pot. Sometimes in a rush, routines become so automated, that
a simple step could be missed, and not noticed because each step is being so unconscientiously
performed.
A minute delay in getting the pot closed and air into pressure vessel could cause porosity.
The ambient temperature of your environment could accelerate the speed at which the polymer starts to
cure. If powder, liquid, model, and air temperature are above 75F, this elevation in temperature starts
the polymerization process quicker than if the ambient temperatures of all of the above are below 68F.
Soaking models in warm or hot water, warming models under heat lamps, also accelerate the “on the
model” curing rate.
So, if a technician is slow at complete the pour up process, and for reasons undetected, the
thermo/chemical process begins and is underway, prior to going into the curing pot, porosity will
develop. Once acrylic has gotten to the rubbery state, curing in the pot will not prevent porosity. You
are ¾ of the way home, and going doing hill. Pressure pot at this point, makes no difference.
Almost all porosity can be eliminated by learning and being consistent with the pouring/stacking
process and curing in a hot pressure pot for the appropriate amount of curing time, without interruptions
of the “inside the pot curing process”. The pressure/curing vessel should not be opened within this first
5 minutes of the curing process for each case.
To open the pot within the first 5 minutes WILL interrupt the curing process. By doing so, you release
the air pressure, you release the heat contained within the pot, and then immediately add another case
that is room temperature, and you inject cold pressurized air into the pot. All of these can stress your
original case and create problems. If you find that your work cycle for putting cases into the curing pot
is sooner then the 5 minutes, then consider getting a second curing pot.
About Curing and Your Pressure Pot:
1. Using your Auto Cure Pressure Pot, you will need a rack in the bottom of your pot. The rack
should be taller than 1/2”. You will put and keep enough HOT distilled water in your curing pot
to remain just below your rack (rack can be two inches high, but keep water below rack). (Most
pots come with bottom racks of 1/2” to 3/4”, but you can use a shallow dish) Try to achieve a
water temperature range of 105 to 110 F for best results.
2. Place the model with the newly poured uncured acrylic into the pressure pot. Close the pot.
Apply the normal amount of pressure into the pot. 20 to 22 psi. You will have the tightest
fitting appliance after approximately 5 minutes of curing in the pot.
3. Allow cases to cool before removing from hot models. Work case is ready for trimming and
finishing. For your information, acrylic resin is cured enough for this finishing procedure, but
acrylic actually takes 24 hours for the resin to fully complete the polymerization curing out
gassing process.
Processing in this manner will yield super hard, super dense, clear and properly fitting
appliances.
LAS VEGAS, NEVADA
2/16/16
89113
702-914-8842
Fax 702-914-8847
LAS VEGAS, NEVADA
2/16/16
89113
702-914-8842
Fax 702-914-8847