Most technical data sheets include cure instructions that detail the time and conditions of a curing reaction. Broadly speaking, chemical curing is a state the material reaches when it does what you need it to do. From a scientific perspective, however, this definition seems lacking.
A more objective definition of polymer curing is the transition where the polymer becomes a solid structure. In this article, we discuss polymer curing, examining the reactions that occur and techniques used to measure curing kinetics.
How is Polymer Curing Started?
Many mechanisms of polymer curing exist, depending on the material in use. The curing of acrylics or latex materials provides a simple foundation for understanding polymer curing of simple systems. Latex is in many indoor paints while acrylic polymers find use as nail polish and other cosmetics.
The curing mechanisms for both polymers are essentially the same. These polymers are solid materials either dispersed or dissolved in a carrier (water for latex and solvents for acrylics). Once applied onto a surface, the carrier material – which is volatile – evaporates leaving only the dried polymer material.
Figure 1: Polymers like acrylics cure by physical drying and coalesce into solid films
No new chemical bonds form, instead the non-solids evaporate and what remains is the dissolved/dispersed polymer film. The polymers coalesce into a solid film but do not undergo any structural change from liquid to solid state. We will later examine methods that measure the degree of resin cure.
Two-Part Thermoset Polymer Curing
Materials like 2-part epoxy resin by contrast to acrylics undergo structural changes during the curing process. This process in known as crosslinking where linking of polymer units occur and new chemical bonds form. Here, a resin mixes with curing agents and undergoes an acid base chemical reaction. Once all resin chains react with all catalyst chains in the mixture, a polymer solid results.
With epoxy, an oxirane resin reacts with a hydrogen donor such as an amine or amide. With polyurethane systems, the reaction occurs between isocyanate and a hydrogen donor like polyols.
Figure 2: Depiction of epoxy crosslinking reaction using an amine catalyst
With both acrylics and the thermoset polymers, users can heat the materials to speed up the curing process. With acrylics, the heat increases the volatility of the carrier, helping it evaporate faster. For 2-part thermosets, the thermal energy provided by heat helps the chains move within the mixture which speeds up the reaction.
One-Part Thermoset Polymer Curing
Several thermoset materials are single component, reacting via various mechanisms to form chemical crosslinks. Type II polyurethanes and moisture curing silicones are examples of single-component systems that crosslink through atmospheric moisture.
Alkyds are popular resins for protective metal coatings and road paints. These are single-part systems that cure upon exposure to the atmosphere as well. For this resin; however, it is the oxygen and not moisture that catalyzes the chemical crosslinking.
Other popular one-part thermoset polymer systems include 1-part epoxy which reacts once exposed to a minimum temperature. As the system reaches the threshold temperature, a blocking group releases, and the epoxy reacts like a 2-part system. Many polyurethane adhesives are 1-part systems containing excess isocyanate that reacts with atmospheric moisture upon exposure.
Polymer Curing of UV Systems
UV curable systems react upon exposure to specific wavelengths of UV light and subsequently crosslink to form solid structures. In these formulations, a photo initiator breaks down upon light exposure and forms a free radical. This free radical is highly reactive and links together short chain units into a cohesive polymer network.
Most UV curable systems contain acrylate or methacrylate resins. A UV system contains oligomers which are long chain resins and monomers which are short chain. Most cured properties derive from the oligomers while the monomers serve mainly to lower viscosity.
Techniques Measuring Degree of Cure
The concept of a cured chemical system is as much relative as absolute. In relative terms, a system may not reach full cure, yet users may consider its performance adequate. A cure schedule posted in a technical data sheet however contains the absolute state of cure. This means under the conditions specified, the system will fully react.
For most 1-part systems, absolute cure is a measure of hardness. For acrylic coatings, pencil hardness determines when the system reaches final cure under certain conditions. To determine the cure time at a temperature, the system remains at that temperature for several days to reach full hardness. Shorter interval measurements then monitor approximately how long it takes to reach that hardness. The tables below illustrate how this experiment would go.
|
|
1 Hour |
2 Hours |
3 Hours |
6 Hours |
12 Hours |
|
Pencil Hardness |
5B |
2B |
HB |
F |
2H |
|
|
|
|
|
|
|
|
|
7 Hours |
8 Hours |
9 Hours |
10 Hours |
11 Hours |
|
Pencil Hardness |
F |
F |
H |
H |
2H |
Table 1: Example of measuring cure time of an acrylic at a fixed temperature using pencil hardness
For 2-part thermoset systems, we use more sophisticated techniques to monitor curing kinetics. Specifically, thermosets like epoxies and polyurethane produce heat as a by-product of crosslinking. We can measure this heat using techniques like differential scanning calorimetry (DSC).
DSC is a technique that measures the heat flow through a sample at pre-defined temperature ranges. Scientists apply special equations to the data produced to measure the enthalpy of the sample. The enthalpy is a measure of the energy released from the sample as it’s heated. This energy correlates to the relative amount of unreacted material remaining after a given reaction time and temperature.
The idea here is that as the system cures for longer, the enthalpy measured by the DSC diminishes. The signal loss is because the instrument only measures the energy produced from any unreacted polymer segments. The degree of cure is a measure of the enthalpy measured relative to the total enthalpy of the system. To calculate the degree of cure requires the below equations.
% Cure = (1-(ΔHcure/ΔHuncured))*100%
ΔHcure – is the heat energy measured at a given time interval
ΔHuncured – is the total heat energy produced from all bonds reacting
The figure below illustrates the heat signature from a typical experiment
Figure 3: Typical DSC scans of unreacted epoxy. More area under the curve equates to a lower % cure
The experiment includes several scans at each temperature so that % Cure vs. Time can be plotted. The curves from these plots then allow for computation of full cure (100% cure).
By combining DSC with a UV light source, similar measurements for UV curable materials are possible. A more common approach, however, is Fourier Transform Infrared (FTIR) spectroscopy to determine the cure kinetics of these systems.
With this method, a UV source couples with an FTIR. Scientists look for characteristic bands found only in the uncured material to diminish over time. As more data collects, plots help extrapolate when the system reaches full crosslinking.
