Figure 1: Estimated market for FGD for coal-fired power plant, 1996-2010. (Source: IEA).
Figure 1: Estimated market for FGD for coal-fired power plant, 1996-2010. (Source: IEA).
Figure 2: Kenway's flange. (Photo courtesy of Kenway Corporation.)
Figure 2: Kenway's flange. (Photo courtesy of Kenway Corporation.)
Figure 3: 14 cm thick infused GRP laminate produced during Kenway process development. (Photo courtesy of Andre Cocquyt.)
Figure 3: 14 cm thick infused GRP laminate produced during Kenway process development. (Photo courtesy of Andre Cocquyt.)
Figure 4: Inter-laminar shear results for NuTack BLU.
Figure 4: Inter-laminar shear results for NuTack BLU.
Figure 5: Resin flow illustration: general purpose adhesive (left) and NuTack BLU (right).
Figure 5: Resin flow illustration: general purpose adhesive (left) and NuTack BLU (right).

Vinyl ester resins have a long history in flue gas desulphurisation (FGD) processes as a cost-efficient and reliable alternative to metals to withstand the highly corrosive environment of the FGD process. EPOVIA® vinyl esters, from Cray Valley and CCP, allow fibre reinforced plastic (FRP) fabricators to manufacture difficult-to-infuse parts in an even more cost-efficient way. This new process, developed by Kenway Corporation of Augusta, Maine, USA, won the Award for Technical Innovation for Corrosion Applications at the American Composites Manufacturers Association (ACMA) Composites+Polycon trade show in Tampa, Florida, USA, in January.

Corrosion-resistant resins

Corrosion-resistant materials are widely used in our daily life. They include painted steel, glass containers, stainless steel articles, and a wide variety of plastics to meet the demands of most general purpose applications. In more extreme environments though, such as exposure to salt water, harsher temperatures, abrasive conditions, pressure/vacuum or highly corrosive chemicals, more specialised materials are required.

FRP made of vinyl ester resin is one material that offers considerably higher resistance to attack in aggressive chemical environments, including various kinds of acids and caustic materials. That's why vinyl esters are widely used in fabricating industrial equipment and structures such as absorption towers, process vessels, storage tanks, pipes, hoods, ducts and exhaust stacks.

Air and water pollution treatment is one industrial sector that is increasingly using vinyl ester-based composites to achieve unique and light weight structures that also withstand corrosive attack.

In fact, in order to minimise the adverse effects of sulphur dioxide (SO2) and sulphur trioxide (SO3) on the environment, many power plants and industrial facilities use FGD scrubbers to remove SO2 and SO3 from combustion gases. The global market for FGD plants is likely to exceed €1 billion by 2010. Since FGD plants are mainly required for air pollution control within large coal-fired and oil-fired power generating stations and refineries, the largest markets for FGD scrubbers are located in North America and China (see Figure 1).

The conditions within a scrubber and its accompanying installations are very severe and cause corrosion problems for typical engineering materials such as unprotected carbon steel. Vinyl ester based FRP is a significantly less expensive, and a more easily fabricated, option over rubber-lined carbon steel or nickel alloy-clad carbon steel, and it is thus becoming more widely used in the FGD market.

Corrosive environments

The main reason flue gases are so corrosive is easy to understand. Wet scrubbing is generally used to remove SO2 and SO3 from combustion gases, which in turn results in significant reductions of the flue gas temperature. Whereas in the past, the flue gases were reheated, they are now directly introduced in the stack without reheating to improve the energy efficiency of the power plant. As a result, condensation occurs and the dissolution of SO3 in water creates sulphuric acid, a very corrosive chemical.

This same kind of sulphuric acid is also likely to occur in decomposing matter within water pipes. Well known bacteria such as thiobaccillus concretivorus and thiobaccillus ferrooxidans decompose sewage into hydrogen sulphide gas, which reacts with moisture to form the corrosive sulphuric acid. Through the production of sulphuric acid, these microbes are then mainly responsible for the corrosion of steel and concrete pipe. Although sulphuric acid concentrations in water and sewer pipes are significantly lower than in FGD stacks, glass reinforced plastic (GRP) pipes still prove to be the most reliable and cost efficient solutions in regions where these bacteria are active.

Improved cost efficiency

Even with this competitive advantage over alternative, exotic metal hybrid products, the FRP industry continues to develop improved processes such as the one currently used by Kenway Corporation, of Augusta, Maine, USA. In January 2009, the American Composite Manufacturers Association (ACMA) presented the Award for Technical Innovation for Corrosion Applications to Kenway for its FRP corrosion resistant blind flange (see Figure 2). This flange, which is used in an FGD project, is made through an innovative vacuum infusion process that incorporates alternative cure technology to reduce manufacturing and labour costs associated with producing large components that can often cause warp during open moulding.

Kenway's production blind flange is 13 cm (5 3/8 inch) thick and 1.5 m (66 inch) in diameter, and weighs 540 kg (1200 lb). This very large part is manufactured by controlling the gel time until the full 13 cm laminate has infused, but the real key to the Kenway technology is in managing the cure process to avoid the traditional pitfalls of excessive laminate exotherm of such a thick and sizeable FRP part. The very controlled cure and manufacturing process employed by Kenway for the flange, not only eliminated costly post-machining but also significantly reduced manufacturing labour by over 65%.

This innovation further enhances the advantages of composites for corrosion resistant applications and demonstrates the manufacturing and labour savings of using composites to replace other materials. Through its innovative process controls, Kenway has also demonstrated that it can maintain tolerances of less than 3 mm per meter (0.005 inch per foot). As a result of this technology, Kenway was able to manufacture a part which exceeded the customer's technical specifications while also dramatically reducing manufacturing costs, thereby making the company far more competitive in the marketplace.

This technological advance was achieved with contributions from raw material partners such as Arkema, OCV and Cook Composites and Polymers (CCP). The process development was managed by closed moulding expert Andre Cocquyt, contracted by Kenway to help it with the transition from open to closed moulding. On the resin side, a low viscosity and low exotherm EPOVIA vinyl ester was specifically developed for applications requiring extremely thick composite laminates. Manufactured and marketed by CCP in North America and worldwide by Cray Valley, EPOVIA vinyl ester resins are workhorse resins used within the FRP sector and have more than 30 years of experience in aggressive corrosive environments.

Available technology

In fact, not one but two EPOVIA infusion resins have been developed for such demanding applications. The first is designed for smaller, room temperature cure applications that require longer gel times of around 60 minutes, but need reduced exotherm to manufacture such extra thick laminates (+5 inch/+12.5 cm). These very thick laminates, produced with conventional vinyl ester resin cure packages, would literally explode and crack from the heat of exotherm and resulting out-gassing. These high laminate exotherms if left uncontrolled would reach temperatures in excess of 230°C (450°F) and result in seriously cracked and structurally flawed laminates. In 12.5 cm (5 inch) thick laminates, exotherms can be controlled down to 90-125°C (195-260°F). This reduced exotherm allows void free, thick laminates to be produced vis-à-vis the vacuum infusion process (VIP).

The second resin uses a totally different cure concept from Arkema (BlockBuilder® RC50) that allows for much longer flow times for extremely long gel times (several hours) at ambient temperature. While obtaining very long flow and gel times, the technology does require heating of the mould (after infusion) to 50-65°C (120-150°F) with either electric heat or hot water to initiate gel, cure, and if needed, post cure of the resin. Laminate exotherm is also controlled to less than 150°C (310°F) and provides void free, structural laminates as noted in Figure 3.

Wind turbine blades

The markets for these infusion grades include the rapidly expanding wind energy sector (FRP turbine blades), corrosion applications (such as FGD and chimney stack liners), and in infrastructure markets (for example, large FRP structural I-beams). Many of these applications will be in excess of 5-15 cm (2-6 inch) in thickness, 65-70% glass and/or carbon fibre reinforced laminates, and can reach 50 m (150 ft) or more in length (wind turbine blades).

The benefits of Epovia vinyl ester chemistries for these markets are many. Low infusion viscosities in the 110 cps range are easily obtained, and if slightly heated to 35°C (95°F), the viscosity drops to less than 50 cps for faster infusion and greatly enhanced flow over the typical, 1000 cps viscosity epoxy resins used in the wind energy industry.

Another major benefit of vinyl ester resin is improved cure over epoxy resins, particularly in initial green strength development. This becomes a critical issue with long wind blades or long structural beams made of epoxy resins which require the parts to be post cured on the moulds at temperatures above 120°C. These post cure temperatures require moulds to be built of materials that can be exposed to 120°C for 6-8 hours on a daily basis. Epovia vinyl esters develop rapid green strength, and if post cure is required for maximum cure and property development, only the vinyl ester parts themselves need to be post cured and not the expensive moulds. This results in faster mould turnarounds and improved productivity when vinyl esters are used over infused epoxy resins. Mechanical properties of the high performance Epovia vinyl esters generally exceed the current commercially available wind energy epoxy properties, and are much easier to process, cure and post cure.

Additional infusion products

A recent product breakthrough for closed moulders was launched by CCP at the International Boatbuilders Exhibition (IBEX) in Miami in October 2008. The product was specifically designed as a reactive tackifier for positioning dry glass fabrics and core materials prior to vacuum infusion or resin transfer moulding (RTM).

The aerosol packaged product is called NuTack® BLU. As can be seen in Figure 4, the use of NuTack BLU significantly improves inter-laminar shear results when compared to the non-reactive adhesive commonly used in the industry. Further, once resin infusion begins, NuTack BLU not only reacts with, but also dissolves into the infused resin, and therefore does not interfere with resin flow when compared to general-purpose adhesives, as seen in Figure 5.

Bright future

As discussed, new materials are available for the rapidly emerging structural and corrosion markets. These newer materials should be investigated by FRP fabricators and specifiers when considering composite GRP materials for part production as they offer significant advantages over conventional metal, brick and cement-based materials currently used in the industry. The success of vinyl ester based GRP in FGD power plant applications should allow fabricators to expand the use of GRP materials into additional industrial and structural applications.

In the future, an increase in waste incineration is expected due to an overall increase of waste, as well as the limited availability of landfills. Government regulations are expected that will require the recovery of energy from these wastes, while also reducing the need for new landfills. The chloride and sulphur content of these wastes will lead to highly corrosive flue gases, which in turn will require the use of corrosion resistant GRP materials to protect these incinerator chimneys and stacks. It means a bright future for the composites industry and the further use of vinyl ester resins.