Volitalization and the vacuum infusion process

If you've produced a part with vacuum infusion at some point you've probably noticed what appears to be air bubbles in the finished composite. Most likely, what you've seen isn't simply air from the surrounding atmosphere due to a leak in the bag...it's vaporized resin! Understanding the relationship between resin selection, ambient temperature, and vacuum pressure is critical to producing a properly cured part via vacuum infusion.

Recently, Polynova Composites participated in the evaluation of two polyester infusion resins. The test plans called for the production and testing of prototypical solid laminate test coupons wherein the resin system was the only variant. While the resin chemistries were thought to be similar, remarkably one system volatilized to the point of boiling at 24” Hg gauge vacuum (“HgV), whereas the other did so at 27” HgV at room temperature. Since boiling of the resin would result in an unacceptable void content in the final part, and since vacuum pressure plays a critical roll in the infusion process, an investigation was launched to better understand the factors involved.

We turned our attention to improving our understanding of the boiling behavior in the context of known characteristics of the resin systems. Thermodynamically, the boiling point of a liquid can be thought of as the disruption of equilibrium between thermal saturation and corresponding pressure saturation where an increase in thermal energy or a decrease in pressure results in a phase change to the vapor state.

We can visualize the phase change as a function of pressure and temperature with the aid of a phase diagram. Figure 1 is a portion of a hypothetical phase diagram, wherein the phase line separates the liquid and vapor phases. The phases are in equilibrium and co-exist with each other at any point on the line. Phase transitions occur at constant temperature and pressure at

the points on this line. If the pressure is reduced at a given temperature, in this example from p to p’, the phase transition occurs and the liquid boils. Like-wise, an increase in temperature for a given pressure would result in a similar phase transition.

If a single point along the phase line for a given liquid is known, such as the absolute boiling point (a.k.a. the boiling point temperature at atmospheric pressure) the curve can be reasonably extrapolated using thermodynamic equations.

Turning now to the resin system evaluation, plugging the potential bad actor compounds that might cause the resin to boil into the proper thermodynamic equations paints an interesting phase co-existence picture.

While we don’t know the specifics of each resin’s backbone, both were believed to be based on isophathalic acid, and both systems contain a fair amount of styrene (~35%). The resin systems differ in the type of initiator specified by the manufacturer, where one specifies methyl ethyl ketone peroxide (MEKP) as the free radical initiator, and the other specifies a 60/40 blend of cumene hydroperoxide (CHP) and MEKP as the free radical initiator. The system that volatilized at the higher pressure (~24” HgV) specified the MEKP, whereas the system with the lower volatilization pressure (~27” HgV) specified the CHP/MEKP blend.

The standard boiling point of styrene is 293 F. MEKP’s boiling point and decomposition point are the same at 154.4 F. The standard boiling point of CHP may be derived from the literature boiling point of 213.8 F at 8 mm HgV as being ~503 F. Phase co-existence lines for each compound are plotted in Figure 2.

The boiling point pressures at 77 F are of particular interest, where MEKP boils at 23.184” HgV and styrene and CHP boil at 29.451” HgV and 29.916” HgV, respectively. One can fairly conclude from the plots that the use of MEKP significantly influences the onset of boiling.

When we take a closer look at the pressures/temperatures of interest for vacuum infusion processing, as shown in Figure 3, the affect becomes more apparent as the boiling pressure increases to 20” HgV at 95 F, a temperature that is well within the range of exothermic onset for many resin systems. The implication here is that while volatilization may not be visibly evident during the infusion, it will likely become prevalent just prior to resin gel.

Just how much gas could be evolved? Reviewing the Material Safety Data Sheet for the subject MEKP solution reveals that the actual weight percent methyl ethyl ketone peroxide in the solution is 34%, the rest of the solution being high boiling point components. If we assume a standard 1.5 weight percent addition of the MEKP solution to the resin, our actual MEKP content is 0.51 weight percent. For a 1,000-gram mass of resin, we’re adding 5.1 grams of MEKP. The formula weight of MEKP is ~210, so the mole fraction of the 5.1 grams MEKP added to the 1,000 grams of resin is ~ 0.0242. Using this and the 77 F boiling point pressure for MEKP from Figure 2 (23.184” HgV (gauge)) the ideal gas laws yields a potential of ~ 2.63 liters (160 in³) of evolved gas.

So, does this mean MEKP cannot be used in the infusion process? No. Just how prevalent the evolution of gas is in any given system depends on a number of factors ranging from the resin chemistry and, like the glycol mix in your car’s radiator, its affect on the boiling point of the MEKP, to free radical species evolution and cross-linking during the reaction. Another factor is the selected laminae and its ability to nucleate bubble formation, like Mentos to diet Pepsi. The point, however, remains the same: it doesn’t take very much of a low boiling compound to create a huge void content problem.

A word of caution, while we moved to “pure” CHP as the initiator of choice for the application of interest, as the peroxide plays a critical role in the formation of the thermoset and ultimately it’s physical and mechanical properties, the decision to move to CHP was made under the guidance of the resin manufacturer. Therefore, as you’re thinking about designing and producing your next part, although you’ll clearly benefit from details of this article, don’t be afraid to seek out the wealth of knowledge available from your supply chain.

References for further reading:

1.) W.R. Salzman Website, www.chem.arizona.edu/~salzmanr/, Department of Chemistry, University of Arizona

2.) Modified Trouton’s Rule for Predicting the Entropy of Boiling, Ind. Eng. Chem. Res. (1996), 35, 1788-1792

3.) Some calculations for organic chemists: boiling point variation, Boltzmann factors and the Eyring equation, Tetrahedron Letters 41 (2000) 9879-9882

4.) Ulicky, G., Kemp, T.J., Comprehensive Dictionary of Physical Chemistry, Ellis Horwood Limited (1992)

What is Vacuum?

Atmospheric pressure, the force per unit area exerted against a surface by the weight of the air molecules above that surface, can be measured and expressed in a number of ways. At sea level the standard pressure is 14.7 psia or 29.92" of mercury (Hg) or 760 mm of mercury (Torr). Because the atmospheric pressure varies with weather and altitude, the sea level pressures are used as a reference point. The term "vacuum" describes pressure that is below atmospheric pressure.

nches of mercury ("Hg) are a common measure of vacuum and are expressed in two different ways. One way is in "Hg gauge ("HgV), where the scale starts at 0"Hg (atmospheric pressure) and goes to 29.92" Hg, or full vacuum. The other way is in "Hg absolute ("HgA), where the scale is reversed such that the gauge reads 29.92" Hg at atmospheric pressure and 0" Hg at full vacuum. To relate the two methods 24" Hg gauge pressure at sea level would be 29.92 - 24 = 5.92"Hg absolute pressure.

Polybeam enabled cabin top infusion

On February 5^th 2008 the Landing School of Boat Building and Design infused a cabin top for a small powerboat. Polybeam703 was utilized as an infusion flow reinforcement to allow rapid and uniform resin flow. Polybeam703 was layed directly against the plain balsa core to act as both a structural reinforcement and flow media. The z axis fibers improve damage tolerance and shear strength, while the smooth surface profile allows for excellent bond strength between the core and skins.

The laminate schedule consisted of the following materials and resin.

Vectorply E-LTM 1808
Polynova Polybeam703
1/2" Balsa
Polynova Polybeam703
Vectorply E-LTM 1808

Resin: Pro-Set epoxy 500-700 cps viscosity

Seven resin feeds were setup using Enkafusion strips as the resin carrier. Rope was used to facilitate the vacuum around the perimeter of the tool. The total infusion time was approximately 30 minutes.

Part prior to infusion

Finished part

Have a question about composites? Ask here

Predictive Analysis and Flex Testing

Recently, Liquid Access approached Polynova Composites with the task of engineering a stiffer version of their Rocket competition slalom ski. While the current version performed well for skiers up to 135lbs., heavier skiers were requesting an increase in the ski’s stiffness for improved turning responsiveness. Since stiffness of a panel is dependent not only on the material’s flexural modulus, but is also a function of the cube of the thickness of the panel, increasing the core thickness would be the simplest solution. However, the desire to utilize the current tooling and manufacturing method excluded this option. To solve the problem, Polynova selected predictive finite elemental analysis as an aid in developing a new laminate. The analysis was then verified with three point bend testing of skis built with both the original and the proposed laminates.

The original Rocket Sit-Ski is infused with a vinyl ester resin and is comprised of a gel coat on the ski bottom, unidirectional carbon, a plain sheet of ½” foam core enveloped with Polybeam® 703 – a spacer fabric based IFR™, biaxial fiberglass, and a gel coat on the ski top. Vinyl ester resin was chosen because of its increased mechanical properties, particularly elongation to failure.

In manufacturing the gel coat is first sprayed into a standard female mold with a two-inch flange. Unidirectional carbon fiber is laid into the mold, followed by the Polybeam®. A plain sheet of ½” foam core precut to fit the mold is then laid in, followed by another ply of the Polybeam®. The biaxial fiberglass lamina and a semi rigid gel coated floating counter tool (float tool) are placed over the ply stack. The term floating here refers to the counter tools free positioning within the vacuum envelope. The Polybeam® extends beyond the float tool to accommodate the resin feed and vacuum port lines. The resin feed and vacuum ports are then appropriately placed, and the infusion proceeds. The ski is released at the end of the cycle, and trimmed for delivery.

Liquid Access set a flexural modulus multiple of two to three over the original ski as the design parameter for the new ski. Theoretical analysis of the original and candidate laminate schedules was made possible by VectorLam™ and Strand7 software. VectorLam™ accurately predicts mechanical properties of laminates built to various processes

including vacuum infusion. VectorLam’s™ ability to rapidly build and compare multiple laminate scenarios significantly decreased the time required in the initial design phase. The individual ply data from VectorLam™ was then imported into Strand7 FEA where the laminates were reconstructed. Strand7 is a general-purpose finite element analysis system consisting of pre-processor, solvers and post-processor. The laminated composites module provides fully interactive analysis of both symmetric and unsymmetrical composite laminates. The laminate engineering properties and the characteristic matrices are calculated based on standard laminate theory.

A major challenge was to accurately simulate the core under loading in conjunction with the laminate skins. Plate elements representing the laminate skins and brick elements representing the core accurately portrayed displacement but there was a gray area in the first mode of failure. Building the entire laminate utilizing Quad8 plate elements, which contain 8 equally spaced nodes around the perimeter of the plate, allowed for accurate representation of the interaction between core and skins without compromising displacement prediction. The core was assigned on the mid-plane and the laminate skins were offset by half the thickness of the core to accurately replicate the relationship between core and skins.

Utilizing additional unidirectional carbon on the top and bottom of the ski proved to be the most effective solution to meeting the design goal (2X to 3X flexural modulus). The added carbon unidirectional along the length of the ski increased the modulus which in turn decreased overall displacement of the ski under load.

To verify the predictive model, flexural testing of skis built with the original and proposed laminate was performed at the National Composites Center.

To learn more and read the complete article, please visit our website.

National Renewable Energy Laboratory study

The following statistics represent failures of wind turbine prototypes.

National Renewable Energy Laboratory study;

45 failures of prototypes;

20% lightning related;

16% foreign object impact;

13% tip deflection/tower contact;

20% adhesive bond failure;

18% voids in skin core;

13% improper cure of materials.

Above is an excerpt from North American Windpower, May 2006, p24.

It’s clear there are still major processing issues in composite wind turbine manufacturing. The Polybeam703 and HiFlux90 can help provide answers to those issues.

Polybeam703 is a three dimensional reinforcement which also acts as a resin flow channel. The z axis fiber increases overall laminate shear strength and provides uniform resin flow. The smooth surface profile of the x and y axis fibers provide excellent bond strength.

Key Benefits Include:

Reduces waste associated with other infusion techniques;

Improves infusion rates and uniformity;

Improves damage tolerance, core interface bond, and shear properties;

Highly conformable with an excellent surface profile;

HiFlux90 is a bilateral infusion flow reinforcement designed to maximize permeability and minimize set up time. From woven roving and chopped strand mat to high yield carbon the HiFlux90 provides uniform laminate wet out and superior infusion speed.

Key Benefits Include:

Enhanced mechanical properties;

Improve infusion rates and uniformity;

Reduce waste associated with other infusion techniques;

Highly conformable;

Permitted Foolishness

The Federal Clean Air Act mandates permitting as a Title V source for any stationary source that emits (i) more than 100 tons of any pollutant per year, (ii) more than 10 tons per year of any hazardous pollutant, or (iii) more than 25 tons per year of a combination of hazardous pollutants.

Progressive technologies, such as vacuum infusion, offer the opportunity to dramatically reduce emissions of cancer-causing agents like styrene, while also positively impacting production by reducing labor costs, reducing cost associated with waste disposal, and increasing unit through-put per production hour. Considering the relative inefficiency of open-mold processing, the exposure risk it poses to laborers, and the resulting environmental emissions, its unfortunate that manufacturers continue to file for Title V status as their production capacity increases, rather than choosing to adopt advanced processes that pose long-term economic gains and reduce pollution.