How to measure gel time - M101

How to measure gel time
Foundational knowledge method
Document Type	Method
Document Identifier	101
Themes	Thermal management Flow and consolidation management
Relevant Class	Material
Tags	Method Chemical properties Deposition Material parameters Resin flow Thermoset polymers

Scope

This page outlines several methods that can be used to measure the gelation time of a thermoset resin. The discussed methods are of varying complexities, with each providing different levels of detail that can be obtained. At the simplest level, resin gel time can be measured manually with a stir stick or probe inserted into a cup of curing resin. For increased precision, an inexpensive tabletop gel timer device can be used to measure the time for gelation to occur. Detailed measurement can be done using a specialized laboratory rheometer that can also measure the resin viscosity, information leading up to the gelation point, in addition to the gelation time.

The choice of method may be influenced by the equipment that the user has available, the time involved, and the extent of detailed information that is required by the user.

Significance

Gelation is defined as the point at which a thermoset resin loses its ability to flow. It occurs during the curing process when the developing cross-links network forms to the extent that it inhibits fluid-like motion, and resin viscosity effectively increases towards infinity.

Determining the reaction time to reach gelation has important implications for composites processing. Beyond the gel point, the thermoset matrix no longer flows, and effectively becomes a gel-solid. From a practical standpoint, this means that the fiber bed impregnation and laminate consolidation should be completed before gelation.

Prerequisites

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

• Viscosity (Coming soon)

Overview

Three test methods are provided that measure the gel time of a thermoset resin. The table briefly summarizes each method with the level of complexity involved, any specialized equipment necessary, and general comments about the test methods. The table is meant to be an initial guide, where it is recommended that each method be reviewed in detail in order to determine the most appropriate method for your particular use.

	Test Method	Necessary Equipment	General Comments:
Simple	Manual Probe Measurement	Sample Cup Stir stick or probe Optional heating device (heated testing)	The method provides the reaction time until gelation is reached. It is very operator dependent and subjective in the determination of the resin gel point. Heated tests limited to isothermal holds may be possible depending on the specific laboratory heating equipment available.
Moderate	Gel Timer Measurement	Gel Timer Optional heating device (heated testing)	The method provides the reaction time until gelation is reached. Heated tests limited to isothermal holds may be possible depending on the specific gel timer equipment used.
Detailed	Rheometer Measurement	Rheometer (rotational shear type) Heater unit (heated testing)	The method provides the reaction time until gelation is reached, and viscosity data prior to this point. Heated tests are possible with specialized heated rheometers. These specialized units can reproduce ramp rates, isothermal holds, and multi-step cure cycles.

Manual Probe Measurement

Gel Timer

Rheometer

Scope

The simplest method to determine gel time is to manually probe the curing resin by handheld applicator stick, physically checking for resin transition from liquid to solid-like state. The method laid out here is based on the previous but no longer active ASTM standard D2471. While this manual measurement is operator dependent, it can yield sufficiently precise and reproducible results .

Measurement can be carried out for room temperature reaction or at elevated temperature by heating the resin and sample containers. This manual probe test can be carried out simultaneously with the exotherm heat measurement test (see: How to measure curing time and degree of cure).

Setup

Equipment

• Suitable sample container
• Stopwatch (if particular gel timer does not include a time keeping device)
• Stick or probe
• Suitable mixing cup and mixing stick
• Optional laboratory heating equipment, e.g. waterbath (for isothermal temperature hold measurements)

Test specimen

From ASTM D2471, the following test volumes are recommended :

• Thin-section applications, adhesives, tool coatings, etc.: 15mL
• Laminating and surface casting resins: 120mL

Note that the above amounts are test volumes. It is best practice to mix a sufficient greater amount of resin than is needed, and then transfer to the sample container the recommended volume amount. This is particularly important for small test volumes to reduce possible stoichiometric errors that may result from the inherent difficulty in measuring resin components in small amounts. As a rough guide, ASTM D2471 recommends a 60mL working volume for a 15mL sample test volume.

Procedure

Sample Conditioning

For heated measurements, preheat all test materials from the resin to the sample containers to the target temperature prior to testing. Preheating can be done using a temperature controlled bath, oven, etc. For large test containers, this preheating process can take hours. To avoid initiating the resin reaction prior to the intended test start, ensure that the individual resin components are heated separately. Periodically check temperatures during the preheat process with a thermocouple or appropriate measurement device, and record the time required for future reference.

Test Procedure

Please refer to ASTM D2471 for further details and recommendations :

1. Transfer the appropriate stoichiometric amounts of the resin components to the appropriate mixing container.
2. Start stopwatch and mix together thoroughly. The reaction time has begun.
3. If the test is to be carried out "heated", the preheated test container can be removed from the heating device and placed on a non-conducting surface for the remainder of the test. If the reaction time is long and sample cooling is occurring; either insulate as best as possible the sample container, or perform the measurement with the sample container placed in the heating equipment if it is safe to do so.
5. Transfer an appropriate amount of mixed resin to the sample container. Record the mass of resin transferred and the height of resin filled in the sample container.
6. Every 15 seconds, probe the centre surface of the reacting resin with the applicator stick. Clean probe and repeat.
7. "gel time" is determined when the reacting material no longer adheres to the end of probe.

Analysis

Sample volume sensitivity

Thermoset cure kinetics are heavily influenced by the volume of reacting resin (both amount and sample dimensions). To obtain representative results, the former ASTM 2471 standard recommended that the resin sample container be chosen so that the filled resin height closely replicates the intended thickness of the part being produced. For laminating resins, approximately 120mL of resin volume was recommended for measurement. Both the resin volume (or corresponding mass) and sample thickness should be recorded for every test measurement.

Limitations

While elevated temperature testing can be performed for isothermal (constant temperature) conditions, representative temperature ramps and multi-step temperature cure cycles are difficult to accurately simulate because of thermal mass differences between the test sample and representative part. To more accurately simulate temperature effects, a heat-controlled rheometer should be considered.

Scope

Gel timers are simple inexpensive equipment setups that time how long it takes for a reactive thermoset resin to gelate. Gel timer setups work by having a motor attached to a wire, hook or plate that is submerged into a sample cup of reacting resin. The motor runs until the resin reaches gelation and stiffens. At which point the motor stops and the machine stops timing, or gives an audible alert. The exact workings vary by the specific device.

Some gel timers have heated bath modules, while others may allow the user to setup their own heated bath setup to perform heated isothermal measurements. Other gel timers may only allow for room temperature measurements.

Please see the catalogue volume for a list of suppliers selling gel times.

Setup

Equipment

• Gel timer
• Stopwatch (if the particular gel timer does not include a time keeping device)
• Gel timer sample cup
• Sample wire, hook, plate, etc. (dependent on particular gel timer unit)
• Suitable resin mixing cup and mixing stick
• Optional laboratory heating equipment, e.g. water bath (for isothermal temperature hold measurements)

Test specimen

• The amount of resin is dependent on the particular gel timer equipment, and its associated sample cup size

Procedure

Sample Conditioning

For heated measurements, preheat all test materials from the resin to the sample containers to the target temperature prior to testing. Preheating can be done using a temperature controlled bath, oven, etc. For large test containers, this preheating process can take a few minutes. To avoid initiating the resin reaction prior to the intended test start, ensure that the individual resin components are heated separately. Periodically check temperatures during the preheat process with a thermocouple or appropriate measurement device, and record the time required for future reference.

Test Procedure

1. Catalyze or mix resin (two part resin system) in an appropriate sample mixing cup, and start timing. Note that the reaction time starts once catalyzation or mixing has commenced.
2. Upon catalyzation or resin mixing, pour the resin into the gel timer sample cup. Record the mass of resin transferred and the height of resin in the sample container.
3. If a heated water bath is used, lower the sample cup into the bath.
4. Refer to the gel timer manufacturer’s manual for specific instructions on how to operate the gel timer.
5. Run the gel timer until the resin gelation point is reached.

Analysis

Result variations

• As was noted in the procedure, the reaction time starts immediately upon catalyzation or two part resin system mixing. Failure to accurately note this time, or start the gel timer equipment immediately after reaction initialization is one source of error.
• Differing sample amounts and container size can lead to gel time variations on the order of minutes.
• On the same gel timer equipment, there may be test-to-test variability on the scale of a few minutes.

Limitations

The scope of the gelation measurements is limited by the gel timer equipment that is accessible by the user. For example, if a controlled temperature bath can be attached, gelation measurements for heated isothermal temperature holds can be performed. If a heated bath cannot be attached, heated testing may be attempted by insulating the sample cup during the measurement duration. However, this may not be appropriate if the resin reaction time is long and significant sample cooling occurs.

While heated bath setups do allow for elevated temperature testing, there are limitations due to the time lag in increasing both the temperature of the bath and the cup of resin sample. Generally, this time-temperature lag means that temperature ramps and multi-step temperature cure cycles cannot be accurately simulated. To more accurately simulate temperature effects, a heat-controlled rheometer should be used instead. These typically test a sample size that is 10mL or less, responding effectively instantaneously to temperature change.

Scope

Laboratory rheometer testing instrument.

A rheometer is a specialized laboratory device that examines the response of a liquid, suspension, or soft viscous solid when subjected to applied forces. In particular, the method described here concerns the use of rotational shear type rheometers, examining a liquid’s response to shear forces.

A shear type rheometer can be used to measure both the viscosity and the gelation time for liquid resin systems through a standard viscosity time sweep measurement. For this measurement, a constant shear rate is held throughout an isothermal temperature hold, and the resin’s response is measured.

Measuring the gel time of a liquid resin system by rheometry provides both the reaction time until gelation is reached and the viscosity data prior to this point. Performing measurements at elevated temperature can also be done using specialized heated rheometers. These specialized units are available by multiple laboratory equipment suppliers and can accurately reproduce ramp rates, isothermal holds, and multi-step cure cycles.

Please see the catalogue volume for a list of suppliers selling rheometer equipment.

Setup

Equipment

• Rheometer
• Rotational shear type measurement setup: parallel plate (disposable), cone and plate, etc.
• Suitable sample mixing cup and mixing stick
• Micro-pipette
• Optional temperature control unit (e.g. Peltier system or convection oven)

Test Specimen

• The resin sample size required is dependent upon the shear measurement setup, size of the test surfaces and the gap spacing between the surfaces. For parallel plate, sample volumes are on the order of 10-15mL or less.
• When mixing resin formulations for small sample sizes – ensure that enough sample is prepared in order to reduce any stoichiometric sensitivity that can result from the inherent precision error when measuring out small volumes. It may be necessary to mix 20, 30, 50, or 100mL of sample depending on the mix ratios that are targeted.

Procedure

Sample Conditioning

For heated tests carried out on a rheometer, preheating the resin components prior to testing is generally not required. Given the small sample volumes tested (typically 10-15mL), resin heating occurs nearly instantaneous once placed into the preheated rheometer.

Test Procedure

1. Setup the rheometer according to the equipment manufacturer’s procedure. For heated measurement, pre-heat the test setup.
2. Catalyze or mix resin (two part resin system) in an appropriate sample mixing cup.
3. Upon resin sample catalyzation or mixing (multi-part resin system), immediately measure and add the sample to rheometer setup and begin the viscosity testing. Disposable micro-pipettes can be used to carefully transfer the resin from the mixing cup to the rheometer sample stage. For more viscous resins, wooden stir sticks work well.
4. Carry out viscosity testing until gelation.
5. To aid in rheometer cleanup and to avoid possible equipment damage, the viscosity measurement may be stopped prematurely once the resin viscosity is observed to increase rapidly. If wanting to carry out testing until gelation, disposable testing plates may be an option and available from the rheometer manufacturer.

Analysis

Testing shear rate

Polymer resins are non-newtonian fluids where the viscosity is shear rate dependent. For viscosity testing, the shear rate should be kept low to avoid shear thinning effects. The low resin shear stress closely represents the relatively slow resin flow found in typical composite infusion manufacturing processes.

Limitations

Rotational vs. oscillating test measurement

Rheometer rotational motions: (left) continuous rotation, (right) rotational oscillation. Reproduced from .

Rheometers can operate with either continuous rotation or rotational oscillation motion of the plate, cone, etc. setup. The information that can be obtained differs between the two motions, and each has its advantages and disadvantages.

Continuous Rotational Motion	Oscillating Rotational Motion
Advantages: More closely simulates fluid flow processes dependent on flow velocity or volume flow rate	Advantages: Can provide information past gelation point - when sample becomes solid
Limitations: Can only measure up to gelation point, can't determine when resin is completely cured	Limitations: Nature of shear forces imposed on resin may not be representative of resin flow process (compared to continuous rotation)

To approximate the resin’s gelation point, both rheometer motions can be used. However, for curing polymers, oscillating motion is generally considered preferable as it can measure and provide information past the gelation point until the sample becomes a solid .

External Links

• Basics of rheology (Anton Paar)
• Time-dependent behavior with gel formation or curing (Anton Paar)

Related pages

Page type	Links
Introduction to Composites Articles
Foundational Knowledge Articles	Basic Definitions of Material Properties - A211 Composite properties - A214 Cure of thermosetting polymers - A162 Degree of cure - A104 Foundational method documents - A153 Glass transition temperature (Tg) - A210 Heat of reaction - A114 Material properties - A150 Polymer properties - A212 Reinforcement properties - A213 Specific heat capacity - A117 Thermal conductivity - A116 Thermal diffusivity - A143 Thermal phase transitions of polymers - A102 Thermoset polymers - A105 Viscosity (resin) - A203
Foundational Knowledge Method Documents	How to measure curing time and degree of cure - M100 How to measure gel time - M101 How to measure reinforcement content (and corresponding matrix content) - M109
Foundational Knowledge Worked Examples
Systems Knowledge Articles	Systems knowledge method documents - A191
Systems Knowledge Method Documents
Systems Knowledge Worked Examples
Systems Catalogue Articles
Systems Catalogue Objects – Material
Systems Catalogue Objects – Shape
Systems Catalogue Objects – Tooling and consumables
Systems Catalogue Objects – Equipment	Rheometer - A357
Practice Documents	Ensuring appropriate resin flow and part consolidation for a new cure cycle - P119 Ensuring appropriate resin flow and part consolidation for a new material - P120 Practice for Developing a Deposition Step - P157
Case Studies	Cost Comparison Study of a GFRP Leisure Boat Hull Manufacturing Method; Spray Up vs Resin Infusion - C115 Optimizing Cost vs Weight for Low Production Volume Parts - C112
Perspectives Articles	Composite materials engineering webinar session 3 - Constituent materials - Resin - A122 Composite materials engineering webinar session 5 - Manufacturing processes - Introduction - A124 Composite materials engineering webinar session 6 - Manufacturing processes - Prepreg processing - A125 Composite materials engineering webinar session 7 - Manufacturing processes - Liquid composite moulding - A126 Fabric Forming: how it affects design and processing, and how simulation can address this - A310 Fibre Architecture: Availability, pros and cons, and selection for my application - A309 Pultrusion of Thermoplastic Composites - A334 Resin Behaviour During Processing: What are the key resin properties to consider when developing a manufacturing process? - A257 Understanding Polyester Resin Processing: The Effect of Ambient Temperature on the Final Part - A286

References

About	Help

Thermosets are a class of polymer that undergo polymerization and crosslinking during curing with the aid of a hardening agent and heating or promoter. Initially they behave like a viscous fluid. During curing, they change from viscous fluid to rubbery gel (viscoelastic material) and finally glassy solid.

If heated after curing, initially they become soft and rubbery at high temperatures. If further heated, they do not melt but decompose (burn)

Comes in two parts: part A (resin) and B (hardener). When mixed, curing reaction starts and is not reversible.

Examples include epoxy or polyester.

For polymer matrix composites (PMCs), resin refers to the matrix; the continuous material phase that binds the reinforcement together, maintains shape, and transfers load. Resins are divided into two main groups: thermosets and thermoplastics.

In composites processing, viscosity is an indicator of how easily the resin matrix will mix with the reinforcement and how well it will stay in place during processing. The lower the viscosity, the more easily resin flows. Resin viscosity ranges considerably across chemistries and formulations.

By scientific definition, viscosity is a measure of a material’s resistance to deformation. For liquids, it is in response to imposed shear stresses.

The continuous material phase that binds the reinforcement together, maintains shape, transfers load, protects the reinforcement from environment and damage, and provides the composite support in compression.

Desirable characteristics:

• Moisture/chemical resistance
• Low density
• Processability

Any manufacturing and/or decision making activity that occurs during any stage of the development design cycle (e.g. conceptual design to production).

In the context of Knowledge in Practice, practice refers to the systematic use of science based knowledge to reduce composites manufacturing risk, cost, and development time.

Engineered materials (designed to have specific properties) made from two or more constituent materials with different physical or chemical properties. The constituents remain separate and distinct on a macroscopic level within the finished structure.