Curing (chemistry)

The crosslink density increases until the system reaches the end of the chemical reaction.

[3] Curing can be induced by heat, radiation, electron beams, or chemical additives.

An intermediate case involves a mixture of resin and additives that requires external stimulus (light, heat, radiation) to induce curing.

Sulfur breaks down to form polysulfide cross-links (bridges) between sections of the polymer chains.

Oxygen atoms serve as the crosslinks, analogous to the role played by sulfur in the vulcanization of rubber.

[citation needed] In the case of concrete, curing entails the formation of silicate crosslinks.

In many cases, the resin is provided as a solution or mixture with a thermally-activated catalyst, which induces crosslinking but only upon heating.

Upon heating the mixture, the peroxide converts to a free radical, which adds to an acrylate, initiating crosslinking.

As heat is applied, the viscosity of the resin drops before the onset of crosslinking, whereupon it increases as the constituent oligomers interconnect.

This process continues until a tridimensional network of oligomer chains is created – this stage is termed gelation.

In terms of processability of the resin this marks an important stage: before gelation the system is relatively mobile, after it the mobility is very limited, the micro-structure of the resin and the composite material is fixed and severe diffusion limitations to further cure are created.

Thus, in order to achieve vitrification in the resin, it is usually necessary to increase the process temperature after gelation.

[6] Cure monitoring is, for example, an essential component for the control of the manufacturing process of composite materials.

A simple way to monitor the change in viscosity, and thus, the extent of the reaction, in a curing process is to measure the variation of the elastic modulus.

[7] As shown in Figure 4, after an "induction time", G' and G" start to increase, with an abrupt change in slope.

Then the reaction continues and the system starts to react more like a solid: the storage modulus increases.

[9] Assuming that each bond formed during the crosslinking releases the same amount of energy, the degree of curing,

[9] Conventional dielectrometry is carried out typically in a parallel plate configuration of the dielectric sensor (capacitance probe) and has the capability of monitoring the resin cure throughout the entire cycle, from the liquid to the rubber to the solid state.

It is capable of monitoring phase separation in complex resin blends curing also within a fibrous perform.

The most suitable format for use in cure monitoring applications are the flat interdigital capacitive structures bearing a sensing grid on their surface.

The curing process can be monitored by measuring changes in various parameters: Ultrasonic cure monitoring methods are based on the relationships between changes in the characteristics of propagating ultrasound and the real-time mechanical properties of a component, by measuring:

Figure 1: Structure of a cured epoxy glue. The triamine hardener is shown in red, the resin in black. The resin's epoxide groups have reacted with the hardener. The material is highly crosslinked and contains many OH groups, which confer adhesive properties.
Figure 2: General representation of the chemical structure of vulcanized natural rubber showing the crosslinking of two polymer chains ( blue and green ) with sulfur (n = 0, 1, 2, 3 …).
Figure 3: Simplified chemical reactions associated with curing of a drying oil. In the first step, the diene undergoes autoxidation to give a hydroperoxide . In the second step, the hydroperoxide combines with another unsaturated side chain to generate a crosslink. [ 4 ]
Figure 4: Evolution in time of storage modulus G' and loss modulus G" during a curing reaction.