Frequently and Non-Frequently Asked Questions:

Q?What is a thermoset resin?
A.

A thermoset resin is a polymeric material which is initially in an unreacted state and is capable of crosslinking and forming a 3 dimensional network when reactive components are mixed, catalysts are added, and/or heated to elevated temperature. Thermosetting resin systems may consist as two part reactive systems, such as often found in laminating, compression molding, or VARTM resins, or may be one component systems where the reactive components are combined but latent enough for processing, such as prepreg matrices. The properties of a thermoset resin depend on the formulation as well as how the resins are processed including the cure cycle (i.e. time and temperature).

Q?What is the shelf-life of a thermosetting resin?
A.

The shelf-life of a thermosetting resin is how long the individual parts (if Part A / B) or single component resin system can be stored in a controlled environment and still meet the product specifications. The shelf-life depends on the storage conditions and to a large extent on the temperature of storage. In most two component resin systems, it is usually not necessary to freeze the epoxy side (Part A) or the curing agent side (Part B). However, sub ambient temperature storage for single component reactive systems are necessary and lower temperature will lead to a longer shelf-life. Keeping the lids or caps tight on the resin and hardener containers is always necessary to prevent moisture ingression. Please refer to the data sheet or contact API for storage conditions of specific resins.

Q?What is pot-life of thermoset resin system?
A.

Pot-life refers to the time in which a thermoset resin system can be effectively used in a process at a specific temperature. For example, for a two component system, this is after Part B is added to Part A. Pot-life is usually measured by characterizing the viscosity relationship with time at a specific temperature. A common measure of pot-life in two part resin systems is the time it takes for the mixed resin viscosity to double, however some processes may allow for a much larger increase in viscosity before processing and part quality is reduced. As an example, an epoxy VARTM resin may have an initial mixed viscosity of 400 cps at 21 ⁰C, and the time it takes to increase to 800 cps may be considered the pot-life. Usually epoxy resins with a longer pot-life also take longer time to gel and cure at room temperature.

Q?What can I do to increase the pot-life of my resin system?
A.

Pot-life can be extended by lowering the temperature of the environment and therefore that of the container and resin. This however also increases the viscosity. Pot-life is also influenced by the mass of mixed resin and the surface area to volume of the container. Also, type of container will have an influence on the pot-life, with those containers having higher thermal conductivity (i.e. metal) better for longer pot-life. This helps dissipate internal heat that is generated by the reacting components.

Q?What is the working time of thermoset resin system?
A.

Commonly pot-life and working time are used interchangeable but the difference can be understood in analysis of a specific process. In wet-laminating, the working time would refer to how long the resin can be effectively used once it is out of the container that it is mixed in. In the container (or pot), the mixed resin will increase in viscosity much more rapidly than when spread out in a thin film. All thermosetting resins are good insulators and also evolve heat during the curing process (exothermic reactions). Therefore, when the mixed resin is in a larger mass/volume, the heat cannot be dissipated as fast as it would when applied in a thin film or an application with high surface area to volume. Hence, the additional heat increases the rate of reaction which increases the viscosity.

Q?Why do larger masses/volumes of mixed resin cure faster than thin films?
A.

This is related to the exothermic heat of reaction combined with the insulating effect of thermosetting resins (thermal conductivity of epoxy resins commonly range from 0.17 to 0.26 W/m K). Thicker resin castings and laminates generate greater internal heat which increases the reaction rate. This reduces the pot-life/working time, and gelation and cure time.

Q?Why did the mixed resin in the container generate excessive heat and start smoking?
A.

This most likely occurred because the quantity of mixed resin mass/volume was too large or left in the container too long and the heat of reaction could not be dissipated before uncontrolled reaction. Heating the mixed curing agent and epoxy resin will exacerbate this problem. All epoxy resin systems generate heat when they are curing and if it is excessive enough to cause degradation, the resin system may smoke. This is due to the heat generated during the reaction coupled with the low thermal conductivity of polymers. Therefore, larger mixed masses/volumes will have a greater tendency to have uncontrolled exotherms, resulting is excessive heat, degradation, and smoke. It is advised to pour out all mixed resins after use to thickness less than 1/8 inch to prevent uncontrolled exotherms. It is important to note that smoke generated from exotherming resins is toxic and must be handled accordingly.

Q?What would cause my resin and/or hardener look cloudy?
A.

For some high performance resins systems, the resin or hardener may crystallize when exposed to cold conditions or after long storage times. This may cause a cloudy appearance and could also affect viscosity. While this is not common, it may be observed when the resin system has been shipped during the winter and has seen very low temperature in transit or has been placed in a cold warehouse. The resin system should not be used in this condition. It is best to contact API as to the time and temperature required to reverse the crystallization.

Q?Why is the surface of my part cloudy and/or sticky?
A.

Most likely it is amine blush – Please see Technical Note #1. However, a cloudy appearance on the surface of a composite part may be due to entrapped air in the form of small air bubbles. This may be due to a bag leak after VARTM processing or one of many issues when wet-laminating.

Q?What is amine blush?
A.

Please see Technical Note #1

Q?Can I add more hardener (Part B) to increase the cure speed?
A.

No. This is not recommended since it will alter cure time and may severely affect mechanical properties. Please contact API if a lower viscosity resin system is necessary.

Q?Can I add solvents to lower the viscosity (i.e. thin-out) API epoxy resins?
A.

No. This is not recommended since it will alter cure time and may severely affect mechanical properties. Please contact API if a lower viscosity resin system is necessary.

Q?Can I add fillers to API epoxy resins?
A.

This is not recommended without contacting API as to which fillers and concentrations.

Q?Why did my curing agent darken over time?
A.

It is common for amine curing agents to darken with age. This is due to oxidation and is oftentimes observed after the second time the curing agent container has been opened. The extent of color depends on the curing agent type, type of container, length of time it was left open to the atmosphere, and age. Usually, unless the curing agent has been left open for a long time or has exceeded its shelf life, it is fine to use. If this has not been observed before with this product, please contact API.

Q?What safety measures should be taken when working with epoxy?
A.

It is always recommended that eye and skin protection should be worn when working with any thermosetting resin system. Severe skin and eye irritation and sensitization can occur. It is recommended that adequate ventilation be used for all resin systems during use. A respirator should be used where there is inadequate ventilation. Even after cure, it is not recommend to be exposed to any dust during machining or cutting without adequate protection. Please contact API for more information and the MSD relating to the product you are using.

It is also important to minimize the thickness of excess mixed resin to reduce the possibility of uncontrolled reaction and exotherms. Pour fast reacting mixed resin systems out to less than 1/8 inch thickness and allow to harden and fully cure before disposal.

Q?Are all epoxy resin systems damaged by UV degradation?
A.

Yes. Even non-amine cured type epoxy systems that have the good UV stability will darken slightly over time and with high intensity exposure.

Q?Why has the color changed on the surface of my epoxy composite parts while inside our building?
A.

Most likely you are getting photochemical oxidation of the surface due to your overhead lights. This is more common if you are using tungsten halogen or high intensity discharge lighting. It is best to contact the lighting manufacturer and check the UV output. You can check the problem by covering some parts with a UV resistant cover or tarp. This may be the solution. If inside UV lighting is an issue, filters may also be added to solve the problem.

Q?Why is my epoxy resin system taking longer to cure than usual?
A.

There are many possibilities including:

A.) The mix ratio was not correct for the resin (Part A) and curing agent (Part B). This could be due to simply reversing the Part A and Part B. If using pumps, it could arise from using pumps that were not primed, clogged, or not calibrated.

B.) The temperature of the resin or environment was reduced. This is oftentimes observed in the winter months and in uncontrolled manufacturing environments. In general, the reaction rate doubles for every 10 C increase in temperature. Therefore, reductions in the temperature of the part or incoming resin can significantly reduce the time for curing.

C.) The resin was used to make a thinner part than the original part. In general, thinner parts (i.e. higher surface area to volume) take longer to cure because the heat generated from the reaction is minimized and therefore does not accelerate the cure speed.

D.) The resin was not mixed adequately. Lower mixing temperature and therefore higher viscosity during mixing can lead to inadequate mixing if the time is not extended. It is necessary to ensure adequate mixing by scraping down the sides and bottom of a container if mixed by hand or in a batch operation. Unmixed or “dead zones” will lead to incomplete cure. Also, very small batches of mixed resin may also have greater opportunity for incomplete mixing and also measuring.

E.) Fillers or solvents were added to the system. This is never recommended unless discussed with API personnel. Almost all fillers except microballoons increase the thermal conductivity and therefore reduce the heat build-up in the part. This will therefore have a similar effect as a thin film. Also, solvents may reduce cure time by one of many mechanisms including reacting with amine curing agents.

F.) The curing agent (Part B) and/or epoxy resin (Part A) crystallized. If a Part A or Part B is found to have crystallized, it cannot be used until it is back to its de-crystallized and back to its original state. Crystallization should be check for both Part A and Part B components if they have been subjected to cold conditions or long storage.

G.) The resin system was subjected to high humidity and/or carbon monoxide. See Technical Note#1.

H.) The wrong products (Part A & Part B) were combined. If using multiple epoxy systems or the same Part A with a fast and slow hardener option, the slower curing agent may have been used. Likewise, a different product from a different vendor all together may have been used.

I.) You are used to working with polyesters/vinylesters. Polyesters and vinyl ester resins cure by a different mechanism than epoxy resin systems. These systems maintain low viscosity until they “snap cure” through an addition double bond reaction, generating high conversion and therefore hardness in a much shorter time.

Q?What is the gelation point (time) of a thermoset resin?
A.

The definition of gel point of a thermosetting resin system is the time at which the molecular weight (wt avg.) tends to infinity. Before this (gel) point, the reacting resin may have increased in viscosity and molecular weight but can be dissolved in an appropriate solvent. After the gel point, a part of the resin is not soluble in any solvent (except possibly under excessive heat and pressure resulting in bond breaking). Physically, the gelation point (or time) of a thermosetting resin is the time required at specific temperature where the resin transitions from a free flowing liquid to an elastic rubber (or semi-solid). This is often measured on a hot-plate where a stick is placed intermittently in contact with the resin until the time at which the resin does not form any more strings upon contact. This is then recorded as the gel time. There are various ASTMs for this type of measurement with ASTM D3532 used often for prepreg matrix resins. It is useful to know the gel time during processing since after this time the resin will no longer be able to flow, shaped, or processed. The best method to measure gel time is using a rheometer where the point at which G” (the loss modulus) increases is usually called the gel point. Oftentimes, the gel point is taken as the point where G’ and G” crossover, but this can be often be fundamentally in error and it is actually before this point. This crossover, however, often is indicative of a hard gel. After a resin matrix has gelled, cure induced stresses are now generated in fiber reinforced composite systems.

Q?What is vitrification and why is it important?
A.

Vitrification is defined as the point (or time) where the glass transition temperature of the curing thermosetting system reaches the cure temperature. In regards to kinetics, this is where the reaction transforms from reaction control to diffusion control and therefore the reaction rate is drastically reduced. From a physical form, this is where the resin transforms from a hard rubbery like material to a glassy solid (i.e. note that some resins are formulated to never get glassy and therefore do not fit this definition). As measured by a rheometer, vitrification is usually associated with a maximum in the loss modulus (G”) with holding at an isothermal temperature. Vitrification is important as it relates to thermosetting resins since after it is vitrified more heat will be required to further increase the conversion (degree of cure) at an appreciable rate if it is capable of further reaction. For example, a room temperature cure laminating resin at 70 ⁰F may become vitrified within 24 hours but will take 7 to 14 days to achieve maximum cure and optimum properties at this temperature. This is because the cure rate as slowed so much after vitrification. However, further conversion is usually possible even after a long cure at room temperature if the temperature is increased above the glass transition temperature of the resin system. This is usually quite small for most room temperature cure resins that have been formulated appropriately. Instead of waiting the full 7 days plus to achieve ultimate properties for a room temperature cure resin, the temperature can usually be raised after the resin system has gelled to a temperature higher than ambient, and the same effect may be accomplished in less than 2 hours. These are generalizations and the data sheets should be referenced for each API resin system.

Q?What is the glass transition temperature?
A.

The glass transition temperature (Tg) is the temperature at which a polymer transitions from rigid, glassy materials to a more complaint, rubbery material. On a molecular scale, the glass transition temperature is the temperature associated with free rotation (crank shaft motion) of major chain segments which were previously hindered at lower temperature due to intramolecular energy barriers. The glass transition temperature is not a discrete thermodynamic transition but one that occurs over a temperature range and therefore subject to limited interpretation. Complicating the reported Tg value for polymers is the measurement technique (DSC, DMA, TMA…), parameters used in the analysis technique (ramp rate, frequency, probe, etc.), sample size, and fiber orientation, to name a few. Therefore, it is necessary to understand how the Tg was determined for the polymer sample. The glass transition temperature is necessary for understanding and defining the upper temperature limits where the polymer or matrix resin can be utilized. While the glass transition temperature provides some insight for determining an upper use temperature limit of a polymer, the modulus and strength retention of the material with increasing temperature is often more applicable. The comfortable limit of upper use temperature of the polymer is therefore application and user dependent and relies on the design and also other factors such as the environment. If the polymer is going to be used in a wet environment, then the “wet” glass transition temperature (i.e. glass transition temperature after saturation in water) is an important design parameter. The “wet” glass transition temperature is even more difficult to get an accurate value since in most thermal analysis techniques the polymer is being dried out during the test. Another complication is often observed in making test plaques for measuring the Tg of thermoset systems. This is due to the mass to volume ratio of the sample that is cured. For example, as a result of the exothermic reaction of epoxy resins, thicker castings made at room temperature may result in much higher glass transition temperatures than thin castings. Also, the glass transition temperature is very much dependent on the cure cycle including ramp rate and temperature.

Q?How is the glass transition temperature measured?
A.

Measurement of the glass transition temperature (Tg) is usually performed using one of many thermal analysis techniques. This is because at the glass transition temperature, the heat capacity, free volume, modulus, and dielectric constant drastically change for polymeric materials. The most common methods for determining Tg of polymers is differential scanning calorimetry (DSC), dynamic mechanical analysis (DMA), and thermomechanical analysis (TMA). When performing a dynamic DSC experiment it is found that in the vicinity of the glass transition temperature, there is a baseline shift, corresponding to a change in the heat capacity. Since the Tg is not due to an enthalpy change, and therefore is a secondary transition, the shift may be difficult to observe with DSC. Oftentimes, faster heating rates are therefore used but this can artificially increase the Tg value. In any case, the glass transition temperature from the DSC thermogram is taken as the midpoint of the inflexion of the change in the baseline. For fiber reinforced composite materials, the use of dynamic mechanical analysis (DMA) is often the best method of analysis to determine the glass transition temperature since it is a much more sensitive technique. Furthermore, the stiffness and damping as a function of temperature can be determined by using DMA, which applies an oscillatory force at a controlled frequency in its basic operation. Also, this technique allows the change in modulus to be determined up to and after the Tg which is important for understanding composite performance. It is important to note that all thermal analysis techniques measure Tg differently and can even be reported differently from the same instrument. For example, the reported Tg as determined by tan delta from a DMA experiment may be over 20 ⁰C higher than that as determined by the storage modulus, E’, from the same DMA experiment. Also, large differences in the reported Tg value may be found between techniques and further complicated by the use of different parameters. Accordingly, the Tg as reported by tan delta by DMA may be 20 ⁰C higher than that determined by DSC. Another thermal analysis technique often used to measure the glass transition temperature of polymers is thermomechanical analysis (TMA). Different from DMA, TMA applies a static force and measures the dimensional change as a function of temperature/time. The dimensional change is a result of a change in the free volume of the polymer and is related to the coefficient of thermal expansion (CTE). At the glass transition temperature, the free volume of the polymer is greatly increased resulting from greater polymer chain mobility, and therefore a slope change in thermal expansion. Oftentimes, in the vicinity of the glass transition temperature, the slope change is quite broad. While this can lead to difficulty in determination of an exact value for Tg, the breadth of the transition is an important feature of the polymer or composite material. While not as sensitive as a technique as DMA for measuring Tg, it offers additional data as does DSC, all of which complement the understanding of these viscoelastic materials.