The use of polymer matrix composites (PMCs) to replace metal components is providing the Department of Defense (DoD) with a cost effective path to create lighter, more fuel efficient, and faster vehicles (land, sea, and air). In addition, PMCs do not suffer from the effects of corrosion like their metal counterparts so there is significant savings in maintenance over a component lifetime and DoD can avoid the use of corrosion inhibiting coatings that contain toxic chemicals (e.g., those that contain hexavalent chromium). One negative aspect to PMC production and repair is that volatile organic compounds (VOCs) are often produced during processing and cure of the PMC. Elimination of VOCs is a critical objective and is essential for increasing PMC applications while decreasing their environmental footprint. Since repair work is often done in the field where environmental control is difficult, VOC elimination is especially important.
The primary objective of this project was to eliminate VOCs during the repair of composite structures at DoD maintenance facilities. This project was intended to demonstrate the feasibility of using phenyl ether-based reactive diluents with extremely low volatility to eliminate or drastically reduce VOCs by replacing the reactive diluent styrene in vinyl polyester (VE) resins. In addition, this project sought to develop a photo-initiated curing system as a first step in increasing DoD’s flexibility in composite repair strategies.
The approach for this project was to develop reactant diluents starting from the readily available and inexpensive phenyl ether. This compound can be quickly and efficiently converted to styryloxy derivatives that have the potential to serve as reactive diluents for vinyl ester resins, thus replacing styrene altogether. Experiments were conducted to determine how well the new compounds will “dissolve” the vinyl ester resins. By systematic changes in the R-group, viscosity, solubility, and vapor pressure of the new reactive diluents can be tailored.
Candidates that show the best combination of efficient chemical synthesis, significantly low vapor pressure, and low viscosity for the polyester blends were selected. Composite lay-ups were fabricated using the polyester blends and traditional initiators. These test panels were evaluated using standard American Society for Testing and Materials (ASTM) methods for compression and shear strength. Concurrently, virgin composite lay-ups using store-bought polyester resins were prepared in 1/8-inch thick (1-inch wide) test samples for evaluation. Engineered defects consisting of 1/2-inch holes were made and repaired using styrene-based polyester blends and the new phenyl ether blends. The "repaired" samples were tested for mechanical strength and repair appearance.
This project was successful in preparing three new reactive diluents for use with vinyl ester resins. The new reactive diluents (RDs), 4-(4’-tributylsilylphenoxy)styrene (S1), (3-phenoxyphenyl)methyl methacrylate (A1), and 4-phenoxyphenyl methacrylate (A2) were prepared in quantities ranging from 12 to 100 g and each RD was fully characterized by analytical and spectroscopic methods. Each of the RDs prepared in this study possessed viscosities at ~100 cP and vapor emissions were reduced by 10,000+ times when compared to styrene. This project found that S1 did not react well with the VE and this led to phase separation. In addition, S1 was found to be unreactive when exposed to UV light in the presence of 1 wt-% Ph3S(CF3SO3). In contrast, both A1 and A2 were found to cure well with the VE resin utilizing common free-radical initiators.
Mixtures of A1 and A2 from 30 to 60 wt-% with the VE resin were prepared and yielded a homogeneous solution. A mixture of ~50 wt-% was selected as the most suitable for repair work. Characterization of the resin mixtures and a composite layup indicated that cured A1 suffers from a low glass transition temperature (Tg). Thermogravimetric analysis data indicate this may be due to a small amount of unreacted A1. At this point, A2 appeared as the top candidate to pursue for repairing the engineered defects. It is noteworthy to mention that this reactive diluent VE repair resin contains no styrene.
Repair of 1” holes were performed using A2 and styrene as the reactive diluent. Each RD afforded void free repairs as well as providing ample wet-out and layup workability. Mechanical testing for the repaired samples showed no statistical difference in failure mode/strength; however, in both cases the repair recovered only a small portion of the materials original strength.
This program demonstrated that a styrene free VE repair resin can be developed based on phenyl ether derivatives. Further testing at a larger scale is needed to more fully evaluate the strength and mechanical properties for this new class of reactive diluents.