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{{Document
|Increment=162
|Page order=112
|Document title=Cure of thermosetting polymers
|Document type=A
|Document template=Foundational knowledge article
|Image=CrosslinkPolymerChainsIllustration-x5kjRhw3MXQJ.png
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|Parent document=A232
|Themes=Thermal management
|Class2=Material
|Tags=Cure and crystallization, Matrix, Thermoset polymers
|RelatedPageTags=Cure and crystallization, Matrix, Thermoset polymers
|Prerequisites=A105, A104, A138
}}
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==Introduction==
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For thermosetting polymers, the manufacturing process step of curing is necessary to transform the viscous polymer resin into a rigid solid. During this process, chemical reactions take place that result in the formation of molecular bonds that set the polymer into shape.

==Scope==
This page provides an overview of the curing process for thermosetting polymers. It covers a brief overview of the chemical curing reaction, physical changes experienced during the curing process, and measurement and monitoring techniques.

==Significance==
The curing process for thermoset polymers is responsible for their polymerization and the formation of the molecular crosslink network of bonds that give thermosets their desirable mechanical properties. This process solidifies the polymer matrix, thereby locking the composite part into its desired shape. Completion of the curing process (complete chemical reaction) is critical for obtaining the full mechanical properties of the polymer.

==Prerequisites==
Recommended documents to review before, or in parallel with this document:

*[[A105|Thermoset polymers]]
*[[A104|Degree of cure]]
*[[A138|Systems Catalogue: Matrix materials]]

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==Overview==
Thermoset polymers are synthesized during the curing process. Polymer chains link together by means of the short crosslink structures, creating a connected rigid network of linked polymer chains. It is this resulting structure that give thermoset polymers their unique mechanical characteristic traits, making thermosets such as epoxy or polyester desirable for use as the matrix component in composite materials.

[[File:CureCycleThermosetCuring-SHTqy4wpeCjc.png|alt=|left|thumb|Example of a 1-step cure cycle temperature-time profile used to cure a thermoset polymer.]]
In the curing process, the thermoset polymer transforms from a flowing liquid resin into a rigid polymer. The timing and control of the curing process is a critical process step within the composite manufacturing process, as once the polymer takes on its rigid form, the composite part shape is locked into place and cannot be reshaped. During this transformation process, the thermoset polymer undergoes a series of chemical reactions and physical changes. Full completion of the chemical reactions is necessary for the polymer to acquire its desired properties, particularly its mechanical properties. 

For some thermoset polymers, the curing reaction can occur to completion at room temperature, while many other thermoset polymers require a substantial amount of energy in the form of external heat input (e.g. oven or press process) to initiate and complete the process. An initiator (catalyst) may also be involved to initiate and accelerate the polymerization reactions.

==Chemical Reaction (Polymerization)==
[[File:PrePolymerChainsIllustration-xbNhA6RYp2HE.png|alt=|right|thumb|Simplified schematic of monomers pre-polymerization, prior to their formation of long linear polymer chain structures.]]
During the curing process, the thermoset polymer undergoes polymerization and transitions from a flowing liquid resin into a rigid polymer. 

[[File:CrosslinkPolymerChainsIllustration-x5kjRhw3MXQJ.png|alt=|right|thumb|Simplified schematic of crosslinked polymer chains. Crosslinks (shown in red) bridge between the main linear polymer chains.]]
Two polymerization reactions are taking place in this process: 
#Linear polymer chains form and grow with the joining together of short monomers
#Crosslinks form between the linear polymer chains


With passing reaction time, this occurs throughout the polymer forming a linked rigid network of polymer molecules. The specific chemical reactions that take place are dependent on the thermoset polymer system resin and hardener.

For some specific examples of these reactions, please see:
*[[A113|Epoxy page - Systems Catalogue]]
*[[A103|Polyester page - Systems Catalogue]]


===Polymer Chain Growth===
The initial resin state prior to curing consists of short unreacted monomers. These are short repeating molecule units that when reacted together form longer linear polymer chains. This occurs in a variety of methods depending on the thermoset polymer. It can happen by homopolymerization with the input of heat, or through addition reactions where polymer chains are formed by joining the resin monomers to the curing agent molecule to form a co-polymer chain.

The short monomer lengths prior to curing provides the thermoset polymer resin with a low viscosity state as a liquid at room temperature, or at only moderately elevated temperatures (e.g. 100°C). This allows for relative ease of processing into composite materials when compared to thermoplastics {{Ref|D620bb9d-07ee-37f8-a593-96ba618d6067}}. The longer polymer chain length present with thermoplastics, requires high temperature processing for resin flow and processing into composite materials.

===Crosslink Reaction===
Thermoset polymers used in composites are most often are synthesized by a chemical reaction between two substances: resin and the hardener (crosslinking agent). In these cases, crosslinks between the resin polymer chains are created by addition reaction of the crosslinking agent. The crosslinking agent itself is a short molecule that has reactive sites that react and covalently bond to reactive sites on adjacent polymer chains. In a simple analogy, these short crosslinks bridge the linear polymer chains together. As the crosslink reaction continues, an interconnected three-dimensional network of linked polymer chains is formed that give the thermoset polymer its rigidity. 

The frequency of the crosslinks along the linear polymer chain is known as the crosslink density. Increased crosslink density provides the thermoset polymer added rigidity, higher temperature stability (through a higher [[A210|glass transition temperature]]), improved resistance to chemical attack, but with increased brittleness.

==Physical Changes During the Curing Process==
During the curing process as polymerization and crosslinking formation takes place, the thermoset polymer goes through several distinct transformation phases.

[[File:ThermosetCurePhaseTransformation-N4HaaJH3ZzZy.png|thumb|500px|Thermoset polymer phase transformations during the curing process.|alt=|none]]
Behavioural stages of a thermoset polymer as it cures:
#Initially the polymer is a flowing viscous liquid
#At gelation, the polymer changes from a viscous liquid to a rubbery gel
#As the curing advances, the rubbery gel transforms into glassy solid at vitrification


===Zone 1 - Viscous Liquid Zone, before gelation=== 
At room temperature, the resin is highly viscous. As the temperature is increased, the viscosity drops drastically. The reduction of viscosity with temperature continues until curing starts due to the external heat, which causes an increase in molecular weight and thereby viscosity. The viscosity increases rapidly with increasing degree of cure until the resin gels and the resin effectively transforms from a liquid to a solid.

===Zone 2 - Rubbery Gel Zone, after gelation and before vitrification=== 
As the curing process advances with time and temperature, at a certain degree of cure, a giant macromolecule is formed which spans the material and the material gels. For typical epoxies, gelation occurs at a degree of cure of 0.5 to 0.6. At gelation, the viscosity increases from a finite value to infinity and an insoluble macromolecule (gel) appears in the system. The resin now starts to develop modulus and exhibit viscoelastic properties. As the degree of cure advances, modulus is increased and viscoelastic properties evolved. During this stage, with the advancement of polymerization, the relaxation time and storage modulus of the material increases while loss modulus of the material decreases.

===Zone 3 - Glass Solid Zone, after vitrification=== 
As the cure advances, the glass transition temperature also advances. Further curing also reduces the free volume in the material (cure shrinkage) and hence the polymer chain mobility. Over a critical free volume range, the chain mobility is hindered and the polymer transforms from a rubbery to a glassy state. This transition, does not occur at a specific temperature but over a temperature range. Glass transition (Tg) + 25℃ is usually mentioned in the literature as the threshold for this transition. Past this transition to the glassy state, due to the low chain mobility, the curing rate drops significantly and consequently modulus development slows down. As a simplification, the material in this stage is usually assumed to behave like an elastic material because the relaxation times are sufficiently long. 
Most resins never achieve a degree of cure of one (i.e. DOC =1). The final degree of cure is generally 0.85-0.95 for most commercial epoxy systems.

==Measuring and Monitoring Cure==
The crosslinking reaction is exothermic (heat releasing) allowing it to be measured in the laboratory using different methods at various degrees of accuracy to indirectly assess the curing progress. Numerically, the extent of the curing reaction that has occurred in a thermoset polymer can be represented by the [[A104|degree of cure (DOC) index]]. It indicates what percentage of the crosslinking reaction has taken place in the polymer.
*[[A104|See here to learn more about the degree of cure]]
*[[M100|See here to learn how to measure the degree of cure]] 


Other methods that may be used to monitor curing, depending on the thermoset system include:
*Rheological methods
*Dielectric methods
*Spectroscopy (e.g. FTIR) methods


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