Resin degassing - P166

 

Resin degassing
Practice document
Document Type	Practice
Document Identifier	166
Themes	Materials deposition and consolidation management
Tags	Moisture effects Vacuum application Gas flow
Prerequisites	Systems Knowledge

Introduction

Resin degassing, which involves holding liquid resin under vacuum to remove entrapped volatiles is a common practice for liquid composite moulding. Entrapped volatiles can become bubbles within resin during processing. The removal of volatiles ultimately leads to a reduction in porosity within the part.

Significance

Degassing is an important step for any composite manufacturing process that involves liquid resin. For example, degassing is recommended prior to wet layup, resin infusion, resin transfer moulding, resin injection moulding, spray up and filament winding. Resin degassing is also a critical step before impregnating prepreg during prepreg manufacturing. Effective degassing before depositing the resin is critical for meeting porosity requirements and quality specifications.

Scope

Successful degassing involves using vacuum to remove volatiles, entrapped air and moisture from the liquid resin. It is important to differentiate degassing from de-bulking, which implies drawing vacuum on the un-cured layup to conform/consolidate the plies to the tool and remove entrapped air or gases. De-bulking is typically performed on dry reinforcements or pre-preg whereas degassing is conventionally used with liquid resin.

On this page, we will discuss what the composition of the volatiles and the mechanisms for removing them during degassing (i.e. how does resin degas). Parameters for degassing such as time, temperature and hydrostatic resin pressure will be discussed. Equipment needed and common practice for degassing will also be covered.

Analysis

What is degassing intended for?

Parts made with liquid composite moulding (LCM) processes may contain porosity. One of the sources for porosity is resin outgassing - volatiles coming out of the resin and remaining present in the gas state during processing. Under vacuum conditions, the effect is exaggerated such that a small amount of outgassed elements can result in a large amount of porosity. Degassing aims to remove those elements before the resin cures with the goal of reducing porosity.

Other sources for porosity in liquid composite moulded parts include: tool/bag leakage, air entrapment at the flow front, inadequate resin flow rate, etc., as explained here.

What elements typically degas and where do they come from?

Air, moisture and volatiles can outgas during processing and cure.

Over the life of the liquid resin, although very slowly, air and water molecules can saturate the surface of the resin, which creates a concentration gradient. The gradient then drives the diffusion through the resin volume towards the bottom of the container. Air and moisture can also be entrapped during the resin manufacturing process and the mixing process.

At 100%RH, polyester resin can absorb up to 0.5wt% to 1wt% water. Although it does not seem like much, under vacuum conditions, water can vaporize into ~8X the volume of liquid water in the resin. Volatiles can also be the low molecular weight compound that is part of the resin formulation such as solvents or by-products from resin curing.

How does resin degas?

The common method for resin degassing involves applying vacuum to an air-tight vessel that holds the liquid resin to be degassed. Typically, bubbling will be observed during degassing. If we look closely, the bubbles will go through a nucleation and growth stage before they rise to the resin surface and escape from the resin.

The second gas transport mechanism is diffusion. Just like concentration gradient, which drove the air and water molecules into the resin, when the resin is under vacuum, the molecules can diffuse outwards. However, gas and moisture removal via diffusion is very slow under vacuum. It is not practical for typical part production. This leaves bubbling as the main mechanism.

What affects the resin degassing rate?

To facilitate/initiate the bubble nucleation, a porous media such as a piece of Scotch-brite can be placed in the the resin. The larger surface area of the porous media provides a substrate for the bubbles to nucleate (heterogeneous nucleation) as opposed to bubbles nucleating from the resin volume (homogeneous nucleation). Homogeneous nucleation requires more energy compared to heterogeneous nucleation. Using a container that has a rough surface (plastic) is more effective than a container with a smooth surface such as glass, for the same reason.

Temperature and resin hydrostatic pressure are two important parameters that can affect the degassing rate. An increase in temperature can lower the resin viscosity, making bubbles rise to the surface faster. Because the rate of bubble growth is dependent on the diffusion rate, at higher temperature, molecules can diffuse through the resin faster and form into a bubble. Higher temperature comes with higher bubble growth rates.

For the bubbles to grow, the pressure inside the bubble (vapor pressure) needs to be higher than the pressure of the resin around it (resin hydrostatic pressure). The easiest way to decrease hydrostatic pressure is by applying vacuum. In some situations, even with vacuum, resin can still have significant levels of hydrostatic pressure. For example, consider a deep barrel of resin. Due to the weight of resin, the pressure at the bottom of the barrel is considerably higher than near the surface, which means the resin at the bottom cannot be effectively degassed. Thus, whenever possible, resin should be degassed in large, shallow volumes.

To further speed up the degassing process, an agitator/mixer can be used to stir the resin. The mixer can also bring the resin from the bottom of the container to the surface where the hydrostatic pressure is less, so the bubbles can form and escape more rapidly.

When to degas the resin and when not to?

During composite manufacturing, degassing is performed before the liquid resin is deposited into the mould. Resin components (part A and part B) can be degassed either before mixing, after mixing or both. Degassing before mixing usually aims to remove the air, moisture and volatiles absorbed into the resin during storage and resin manufacturing. Degassing after mixing can further remove the air introduced during mixing and volatiles generated from the chemical reaction. During mixing, air can be easily mixed into the resin which can later serve as nucleate sites for porosity and voids. Caution needs to be taken when degassing after mixing to make sure the resin's pot life (gel time) is longer than the degassing time plus the time needed for deposition. Typically, resin is degassed for 10 - 15 minutes under full vacuum but this is a function of the variables discussed above.

Depending on the application, some polymers do not benefit from degassing (such as some thick two part epoxy, two part urethane, silicone etc.). Those polymers typically have very high viscosity. Although bubbles can still nucleate within those materials but the bubbles can not float/migrate to the top surface and escape. Sometimes the material will not have enough flow to collapse the bubbles formed by degassing and eventually trapping more bubbles. In those cases, the material is better off left un-degassed. Whenever possible, follow manufacturer's instruction for degassing and processing materials.

Tips for degassing

• A see-through vacuum chamber can provide insight into what is going on with the resin under vacuum
• Because resin "boils" under vacuum, a large container inside the vacuum chamber may be used to prevent resin overflow. The analogy of pouring a carbonated drink into a class, where it bubbles up and overflows is apt.
• Release the vacuum slowly after degassing is finished. Depending on where the vacuum chamber inlet valve is, sometimes the air gushing in can blow the resin all over the vacuum chamber
• A typical degassing process would go as follows:
  • Liquid resin (part A, part B, or mixed) in a container (cup/bucket) is placed in a vacuum chamber with consideration for resin bubble over (ie. either an oversized container for the volume of resin or a secondary container to catch the overflow)
  • Porous media (Scotch-brite pad) may be added to promote nucleation
  • The vacuum chamber with resin may or may not be placed in an oven to increase temperature
  • Vacuum is pulled on the chamber and held for ~10-15 minutes depending on the variables discussed above
  • Vacuum is slowly vented such that incoming air does not adversely affect the resin (ie. blow the resin out of the container)
  • Resin is removed from the chamber and processed

Equipment for degassing

Typical equipment needed for degassing are:

• Vacuum pump
• Vacuum chamber
• Specialty degassing chamber
• Oven
• Porous media (Scotch-brite pad)

Maturity

Degassing is a very common practice, hence the knowledge is mature. The underlying scientific knowledge regarding gas transport mechanisms, bubble nucleation and growth and hydrostatic pressure is well established.

Related pages

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Introduction to Composites Articles
Foundational Knowledge Articles
Foundational Knowledge Method Documents
Foundational Knowledge Worked Examples
Systems Knowledge Articles
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
Practice Documents	Resin degassing - P166
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Perspectives Articles

References

About	Help

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.

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.

Pre-impregnated (prepreg) material refers to fibre that is already combined with resin. It is the most common material form used in aerospace.

During prepreg production, (e.g. fibres are run through a resin bath), prepreg is heated and partially cured to B Stage (< 5 % degree of cure). Thermoset prepregs (e.g. epoxy prepreg) have to be kept in a freezer at around -20 °C. At room temperature, the epoxy starts to cure.

Flow front refers to the leading edge of the resin as it is flowing into and through the fibre preform.

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.