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Introduction
During the polymerization process, polymer chains grow in length, and the mechanical properties of the resin improve linearly (one-dimensionally) until the desired molecular weight is reached. Only when the main chain has side branches can a two-dimensional growth be considered. In many polymers, the intermolecular forces that align chains next to each other are so strong that the polymer inherently possesses the required mechanical strength. In other types of polymers, intermolecular bonds and the arrangement of chains and side groups lead to crystallization, which enhances the physical properties of the polymer.
However, in thermosetting resins, neither the intermolecular energies are high enough, nor is crystallization possible to ensure a solid physical state at room temperature. Therefore, to create three-dimensional cross-links and enhance their resistance, compounds are used that form bidirectional bonds with the chains, leading to their chemical stabilization and solidification.
Cross-Linking Density
Cross-linking density refers to the number of cross-links formed in a given volume of polymer. Naturally, the more complete the curing process or the more reactive the hardener used, the higher the cross-link density becomes, and the polymer chains between two cross-links become shorter.
Effect of Curing on the Viscosity of Epoxy Resin
During the curing process, the resin transitions from a viscous liquid to a rigid glassy solid. This transformation is one of the most tangible changes in the resin.
The graph above shows the viscosity changes of epoxy resin during curing at different curing conditions and temperatures.
It is evident from the image that the curing process occurs much faster at higher temperatures compared to lower ones. The primary reason for this difference is the variation in the curing kinetics under different thermal conditions, which can even lead to structural differences in the cured resin.
Effect of Curing Temperature on Epoxy Resin Kinetics
In the diagram, model a represents the curing of epoxy resin at room temperature, while model b shows curing at high temperature.
The lower the curing temperature, the longer the curing time. This allows the resin to release the stresses caused by mixing with the hardener and application on the surface before the molecular structure is stabilized. Eventually, this results in lower residual stress in the system, which improves long-term durability.
Effect of Curing on Physical Properties of Epoxy Resin
A critical point about the change in physical properties during curing is that a fully cured resin will never melt.
As seen in the chart below, the flat line represents the modulus change of a non-cross-linked polymer with increasing temperature. Because the cured resin is not stabilized by strong intermolecular crystalline bonds, its cross-links do not break until thermal degradation (burning) occurs. Moreover, these cross-links cause the Young's modulus to drop significantly less when transitioning from the glassy region to the rubbery plateau compared to uncross-linked systems.
Glass Transition Region Changes During Epoxy Resin Curing
The graph below shows the changes in the glass transition temperature (Tg) during the curing process. Naturally, this change increases over time and is not directly affected by curing temperature.
The following diagram is drawn to better understand the physical state of the resin during curing. As shown, the resin's physical state depends on curing time and temperature. The lower glassy region of the diagram refers to the uncured resin below its initial Tg, and the upper red glassy region (on the gelation diagram) refers to the cured resin's glassy state, which is also below the cured resin's Tg.
The direction of the blue curve in the middle of the diagram shows that the gelation and completion of curing cause the resin’s Tg to rise from very low temperatures to higher levels. The more complete the curing process, the higher the polymer's glass transition temperature becomes. This trend was also seen in the previous graph.
The fact that uncured resin is fluid at room temperature and becomes glassy after curing is due to this very effect of the curing process on Tg.
At higher temperatures and over time, the resin enters the polymer degradation region, where thermal energy overcomes the polymer's intermolecular bonds. As shown in the diagram, due to the cross-links between polymer chains, no molten state exists between the Tg and degradation temperature.
Modulus Changes During Curing
The graph below shows the changes in storage modulus (G') and loss modulus (G") during the curing process. Initially, G" is higher than G', indicating a liquid state. As the curing process progresses, both moduli increase until they intersect. This intersection point is considered the gel point (noting that it depends on the frequency of measurement and requires precision).
At this gel point, the loss modulus reaches its maximum and then begins to decrease as the curing continues. This decrease occurs because the polymer enters the glass transition region, and the glassy freezing of some polymer segments reduces the contribution of the loss modulus.
If the curing temperature is significantly higher than the glass transition temperature, G" will not decrease during curing.
Abadgaran Company’s Hardeners:
Abadgaran Chemical Industries produces phenalkamine hardeners, which enable epoxy resin curing at low temperatures and under humid conditions. Additionally, the company manufactures a modified aromatic amine hardener that significantly enhances the chemical and mechanical resistance of the cured resin.
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