Production Steps and Requirements of Phenolic Honeycomb Cores for Aviation Parts
If they are to be used in aviation internal parts, Aramid Honeycomb Cores are required to have low fire smoke toxicity, to be flame-retardant and to comply with FAR 25.853 FST. For internal composite parts/sandwich panels to be used in aviation, where low FST properties are required, the material of choice is phenolic resins.
Traditionally, phenolic resins have been solvent-based, and the most common organic solvents that are used for this purpose are ethanol, methanol, isopropyl alcohol and acetone, or combinations of these solvents with a low boiling point like acetone, isopropyl alcohol, methanol and ethanol cause high Volatile Organic Content (“VOC”) emissions. For example, most of the solvent-based phenolic resins for honeycomb production contain 35–40% ethanol, a mixture of 35–40% methanol and ethanol, 35–40% isopropyl alcohol, or other mixtures of these solvents, all of which lead to high VOC emissions during phenolic honeycomb production.
Source: https://www.fpl.fs.fed.us/documnts/fplmisc/rpt1796.pdf
The standard methodology for manufacturing phenolic honeycomb cores is explained in the following source: https://patents.google.com/patent/EP0440871B1/en
1. Epoxy-based adhesive lines, which are carbon black-colored heat-curing adhesives and which determine cell size, are printed onto substrate material in a continuous printing line and are automatically cut in the desired sheet length on printing lines.
2. The adhesive line-printed sheets are stacked on top of each other to the desired number of levels and made to temporarily adhere to each other along adhesive lines on a stacking table.
3. Blocks of the stacked sheets, which are temporarily adhered to one another by their edges are placed between platens in a press and subjected to heat and pressure to obtain permanent sheet adhesion along the printed lines of adhesive. Then, a backing material is adhered to the block ends for use in expansion.
4. The block of sheets in which the adhesive has been fully cured is then stretched to the desired length and width dimensions, thus creating the honeycomb structure with the desired cell size and geometry.
5. The expanded block is then coated in phenolic resin in a resin bath. Following this, the coated expanded block is weighed in order to establish whether the required level of coating has been reached. If necessary, it is dipped again in the resin bath to attain the desired level of coating.
6. The phenolic resin-coated expanded block is then cured in an oven (in two temperature dwells), in order that the honeycomb core reaches the desired strength and density.
7. Finally, the cured resin-coated block is sliced into sheets with the desired thickness and then trimmed.
The oven-drying and curing temperatures used in phenolic honeycomb core are at the drying step 70°C to 110 °C and at the curing step 120 to 150 °C respectively. These two steps are where the solvent evaporation, curing and curing exotherm takes place. These temperatures are higher than the boiling point and ignition temperature of the solvents used in phenolic resins, which is why most fire accidents and toxic solvent emissions (VOC) occur in solvent-based phenolic honeycomb production undertaken in curing ovens.
Harmful Emissions and Flammability Risks of Solvent Usage in the Production Process of Phenolic Honeycombs.
The concentration range of a gas or vapor in the air where it is possible that the air-gas or air-vapor mixture is flammable or explosive when exposed to an ignition source is called the Explosive Limit or Flammable Limit. For an explosion or fire to happen, three elements are required:
1. A flammable substance (most of the volatile organic compounds),
2. An oxidizer (oxygen or air),
3. A source of ignition (spark or high heat).
Below the explosive or flammable range, the mixture is unable to burn since it is too diluted, while above the upper explosive or flammable limit, the mixture is too concentrated to burn. The limits are called the “Lower Explosive or Flammable Limit” (LEL/LFL) and the “Upper Explosive or Flammable Limit” (UEL/UFL) (Table 2.).
The density of isopropyl alcohol (relative vapor density) = 2.1 (air=1) (source: http://www.ilo.org/dyn/icsc/showcard.display?p_version=2&p_card_id=0554) and the density of other solvents in the vapor phase, like most of the organic solvents, is higher than air, so evaporated solvents can accumulate at the bottom of the oven or at any place in the oven where air circulation is limited. During the manufacturing of honeycomb materials using solvent-based phenolic resins, flammable substances evaporate and are absorbed into the air of the production facility. During the cure process for honeycomb materials, a high level of heat is applied, usually at temperatures near 150°C and beyond, thus confirming the presence of all the three requirements for an explosion. Using water as a solvent, instead of the volatile flammable organic solvents, eliminates most of the fire risks involved in the honeycomb production process.
Future SHE Trends in the European Union (REACH (EC 1907/2006))
The potential environmental and health risks in the production of solvent-based phenolic honeycombs were investigated in a paper given at the 38th International SAMPE Symposium by S. P. Qureshi et al. (source: Advanced Materials: Performance Through Technology Insertion: 38th International SAMPE Symposium and Exhibition, Anaheim, Convention Center, Anaheim, CA, May 10–13, 1993, pp. 16–27). Qureshi et al. discuss the suitability of water-based phenolic resins in the production of phenolic honeycombs and propose that using water-based phenolic resins brings reduced health and environmental risks compared to organic solvent-based phenolic honeycomb production.
The European Commission on Clean Air for Europe has estimated the value of the impacts for releasing one ton of PM2.5 of NH3, SO2, NOx and VOCs in different Member States (Table 3.).
At first, only organic solvent-based phenolic resins were used in phenolic honeycomb production. However, as manufacturers seek to find formulations that reduce the amount of volatile organic components, solvent-based phenolic resins have fallen out of favor. Governmental regulations often mandate a significant reduction or even the elimination of volatile organic solvents from such formulations, particularly for products with selected uses. The reduction or elimination of volatile organic solvent emissions has provided new commercial opportunities for the application of resins with low levels of volatile organic compounds. Due to damage to the economy of the EU, VOC emissions (Table 4.) are expected to be more strictly controlled and reduced with REACH (EC 1907/2006).
Possible Positive Effects of the Use of Water-based Phenolic Resins in the Production of Honeycomb Cores in the European Union
Due to current and upcoming obligatory restrictions in the EU with respect to occupational safety, health and the environment (Table 3 and Table 4), it is expected that avoiding solvents in production and switching to water-based phenolic resins will make it a lot easier to open and maintain new workplaces.
Choice of Solvent-based and Water-based Phenolic in Relation to the End Product Performance of Honeycomb Cores
The the choice of to use organic solvent or water as the base for phenolic resins do not affect the performance of the phenolic honeycomb end product. Despite all the drawbacks of organic solvents, the main reason for not switching from solvent-based phenolic to water-based phenolic resins was believed to be the increasing production time for honeycomb core producers. Since the organic solvents that are used with phenolic resins have a lower boiling point than water, they can be removed faster and more easily during core production. Thus, using using a solvent in the honeycomb core production increases production speed and productivity, which was the main original motivation for some producers to use solvent-based phenolic cores.
Source: https://doi.org/10.1002/app.27776
Possible Positive Effects of the Use of Water-based Phenolic Resins in the Production of Honeycomb Cores in the USA and European Union
Due to the reduced fire risk and the low occupational safety risk of using water-based phenolic instead of solvent-based phenolic in the production of phenolic honeycombs, Kordsa-AHT enjoys the advantage of more sustainable production than its counterparts in the USA and EU.
