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BLD-WIND70-06 · Lesson 5 · 16 clock hours · lab

Resin Systems: Mixing, Pot Life, and Cure Monitoring

Objective. Students will mix a two-part epoxy and a two-part polyester resin to the manufacturer's specified mix ratio by weight, monitor gel time using a wooden stick test, and record ambient temperature and humidity data to predict and verify cure behavior.

⚠ Pending professional review

This lesson, its worked examples and its practice questions has not yet been verified by a licensed professional engineer or master tradesperson or by a certification or approval body. It is a study draft and may contain errors. Do not use it as instructions for work on a job site or on live equipment. Always follow your instructor, your supervising professional, and current official references and regulations.

VoTechWorks program approval and accreditation are still in progress. Credentials are issued only by the certifying bodies named in each program, after their own exams.

Why Resin Systems Matter in Blade Repair

Wind turbine blades are built from fiber-reinforced polymer composites — layers of fiberglass or carbon fiber held together by a cured resin matrix. When a blade is damaged by lightning, erosion, or impact, technicians must restore that matrix using the same class of resin the manufacturer specified. Getting the chemistry right is not optional: an under-mixed or out-of-ratio repair will never fully cure, will be weak, and can fail catastrophically under the cyclic loads a blade sees in service. This lab focuses on two resin families used in blade repair: epoxy and polyester.

Two-Part Resin Systems: How They Work

Both systems use a chemical reaction between two components. When you combine them, molecules cross-link to form a rigid, three-dimensional polymer network.

In both cases the Technical Data Sheet (TDS) is your primary reference. It tells you the exact mix ratio, pot life, gel time, and full-cure schedule at specified temperatures.

Mix Ratio and Why Accuracy Is Critical

The mix ratio is a chemical stoichiometric requirement, not a suggestion. Too much or too little hardener (or catalyst) leaves unreacted molecules in the cured part, reducing strength, causing surface tackiness, or preventing cure entirely. The ±2 % tolerance used in this lab is tight on purpose: real repair specifications are equally demanding.

Always mix by weight using a calibrated digital scale unless the TDS specifically states by volume. Weight is more repeatable and removes error from different component densities.

Mixing procedure:

1. Tare the scale with the empty mixing cup.

2. Add Part A to the target weight.

3. Add Part B to reach the combined target weight that satisfies the specified ratio.

4. Mix thoroughly — scrape the sides and bottom of the cup — for the time the TDS specifies, typically 2–3 minutes.

Pot Life and Gel Time

Pot life is the time after mixing during which the resin remains workable (low enough viscosity to apply). Gel time is the moment the resin transitions from liquid to a gel state — it will no longer flow. Both are measured from the moment mixing begins.

Temperature is the single biggest variable. Warmer ambient conditions accelerate the cross-linking reaction, shortening pot life. Cooler conditions slow it. This is why blade repair specifications always state the temperature range within which the repair must be performed, and why you must measure and record ambient temperature and relative humidity before mixing every batch.

The wooden stick test is a simple field method: insert a clean wooden stir stick into the mixed resin every minute and lift it slowly. When the resin strings or resists and the stick leaves a visible track that does not flow back, gel has occurred. Record the elapsed time — that is your measured gel time.

High humidity can interfere with some resin systems (especially moisture-sensitive epoxies), slow surface cure, or cause blushing. Always record relative humidity and compare it against TDS limits before starting a repair.

Cure Monitoring with a Shore D Durometer

Gel is not full cure. After gelation the resin continues to cross-link and harden over hours or days (depending on system and temperature). The Shore D durometer measures surface hardness on a 0–100 scale; harder materials read higher. The TDS specifies the target Shore D value at full cure.

By taking durometer readings at regular intervals and plotting them, you can verify whether a repair is progressing on schedule and predict when it will reach handling strength and full post-cure hardness. Recording the initial weight of the plaque and comparing it at intervals can also reveal whether excessive solvent or volatile components are off-gassing, which would indicate a mixing or materials problem.

Safety — Non-Negotiable Points

Key terms8

Mix ratioThe manufacturer-specified proportion of resin to hardener or catalyst, usually expressed by weight, that must be achieved for the resin to cure correctly.
Pot lifeThe usable working time after mixing during which the resin remains fluid enough to apply; it decreases as temperature increases.
Gel timeThe elapsed time from mixing until the resin transitions from a liquid to a gel state and can no longer be worked; detected in the field with the wooden stick test.
ExothermThe heat released by the cross-linking chemical reaction as a resin cures; a large, deep batch can reach dangerous temperatures.
Shore D durometerAn instrument that measures the surface hardness of rigid or semi-rigid polymers on a 0–100 scale; used to track cure progress and verify full-cure hardness against TDS values.
Technical Data Sheet (TDS)The manufacturer's document specifying mix ratio, pot life, gel time, cure schedule, and hardness values for a specific resin product; the technician's primary reference during repair.
MEKPMethyl ethyl ketone peroxide; the organic peroxide catalyst used to initiate cure in polyester resin systems; a strong oxidizer and serious eye and skin hazard.
Post-cureThe period after initial hardening during which cross-linking continues, bringing the resin to its full design hardness and mechanical properties.

Worked examples4

1. A TDS specifies a 2:1 mix ratio by weight (Part A : Part B). You want to mix a 90 g total batch. How many grams of each part do you weigh out?
  1. A 2:1 ratio means 2 parts A for every 1 part B, so there are 2 + 1 = 3 total parts.
  2. Weight of Part A = (2/3) × 90 g = 60 g.
  3. Weight of Part B = (1/3) × 90 g = 30 g.
  4. Check: 60 g + 30 g = 90 g total. Ratio = 60/30 = 2.0 — correct.
Answer: Weigh out 60 g of Part A and 30 g of Part B.
2. You mixed 60 g Part A and 31.5 g Part B (targeting a 2:1 ratio by weight). Is this batch within the ±2 % tolerance?
  1. Calculate the actual ratio: 60 g ÷ 31.5 g = 1.905 (Part A : Part B).
  2. Target ratio = 2.000. Deviation = |2.000 − 1.905| = 0.095.
  3. Percent deviation = (0.095 / 2.000) × 100 = 4.75 %.
  4. 4.75 % is greater than the ±2 % tolerance limit.
Answer: The batch is outside tolerance (≈4.75 % off) and must be remixed.
3. A polyester resin TDS calls for 1.5 % MEKP catalyst by weight of resin. You are mixing 80 g of resin. How many grams of MEKP do you add?
  1. Convert percentage to decimal: 1.5 % = 0.015.
  2. MEKP mass = 0.015 × 80 g = 1.2 g.
  3. Tare the scale with the mixing cup containing 80 g of resin, then add MEKP until the scale reads an additional 1.2 g.
Answer: Add 1.2 g of MEKP to 80 g of polyester resin.
4. At 65 °F your gel time was 28 minutes. At 85 °F your gel time was 14 minutes. Approximately how many minutes of pot life do you have remaining if you started applying the 85 °F batch and 10 minutes have already elapsed?
  1. Gel time at 85 °F = 14 minutes (this is also a practical estimate of maximum pot life).
  2. Elapsed time = 10 minutes.
  3. Remaining pot life ≈ 14 − 10 = 4 minutes.
  4. Note: pot life is typically slightly less than gel time, so you should plan to finish application before the 4-minute mark.
Answer: Approximately 4 minutes of pot life remain — work quickly or discard the batch.

The math in this lesson1

Learners split a 90 g batch at a 2:1 ratio into 60 g Part A and 30 g Part B.
6.RP.3
In class: Ratio mixing drill: students use a digital postal scale to mix five successive batches of epoxy resin (small quantities, 50–100 g per batch) at manufacturer-specified ratio, recording actual ratio achieved; any batch outside ±2% of target ratio is remixed under instructor guidance. · Pot-life experiment: students mix same-size batches at two different ambient temperatures (simulate with water bath at approximately 65 °F and 85 °F), insert wooden stir sticks at one-minute intervals, and plot gel time vs. temperature on a graph, comparing to the manufacturer's technical data sheet curve. · Cure monitoring: students cast a small test plaque, record initial weight, check at four-hour intervals with a Shore D durometer, and document findings in a cure log; they predict when full post-cure hardness will be reached based on TDS data.
Materials: Two-part epoxy resin system with TDS, two-part polyester resin system with TDS, digital postal scale (0.1 g resolution), mixing cups and sticks, thermometer, hygrometer, Shore D durometer, graphing paper or tablet spreadsheet, cure log template, water bath containers
Check: Instructor checks three scored items: mix-ratio accuracy (all five batches within ±2% = pass), completed pot-life graph with at least four data points at each temperature (complete = pass), cure log with all entries and correct prediction within one cure cycle = pass; all three must pass.

Practice check8

Answer each question, then check it. Nothing is recorded; this is your own practice.

NCCER Wind Turbine Tech — Blade and Composites Repair
1. Why must a two-part epoxy resin be mixed at the manufacturer's specified ratio rather than estimated by eye?
NCCER Wind Turbine Tech — Blade and Composites Repair
2. A technician mixes a 100 g batch targeting a 4:1 ratio by weight (Part A : Part B). What is the correct weight of Part B?
NCCER Wind Turbine Tech — Blade and Composites Repair
3. Which instrument is used in this lab to track the progression of resin cure hardness over time?
GWO Basic Safety Training — Hazard Awareness
4. A technician performing an outdoor blade repair notices that the temperature has increased from 65 °F to 85 °F since she mixed the resin. What is the most likely effect on pot life?
OSHA 10 — Hazard Communication / GHS
5. MEKP, the catalyst used with polyester resins, is classified as which type of chemical hazard?
NCCER Wind Turbine Tech — Blade and Composites Repair
6. When performing the wooden stick test to detect gel time, how does a technician know the resin has gelled?
OSHA 10 — Fire Prevention and Chemical Safety
7. A student mixes a batch of epoxy in a deep, narrow cup using a much larger quantity than the lab specifies. What safety hazard does this create?
NCCER Wind Turbine Tech — Blade and Composites Repair
8. According to good practice in composites repair, why should you record both ambient temperature AND relative humidity before mixing resin for a blade repair?
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