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Vinyl esters get radical in composite markets


Vicki P. McConnell

Delving into the polymerisation of vinyl esters via reactive free radicals, Vicki P. McConnell reports on the range of performance and processing options deliverable by manipulating these matrix molecules.

Within the spectrum of fibre reinforced plastic (FRP) resin choices, vinyl esters may be viewed as the 'Malcolm in the Middle' resin type, positioned between unsaturated polyester resins and epoxies. Die-hards tout unsaturated polyesters for low price and ease of processing; epoxy advocates point to superior mechanical properties for structural applications, albeit at a higher price and more complex processability. Currently vinyl esters are 50-100% higher in cost than unsaturated polyester resins, while epoxies begin at 3-4 times the cost of unsaturated polyesters. Yet after five decades competing with unsaturated polyesters and epoxies, vinyl esters have yet to reach their zenith in end-use markets.

These versatile resins continue to show promise in FRP components with traditional glass, aramid and carbon fibres as well as vapour-grown carbon nanofibres, and even sawdust. Vinyl esters are processable by the full range of moulding methods, notably compression moulding of sheet moulding compounds (SMC) as well as infusion techniques such as resin transfer moulding (RTM), vacuum-assisted RTM, and the Seemann Composites Resin Infusion Molding Process (RTM, VARTM, SCRIMP). In large marine, corrosion resistant, industrial, infrastructure, automotive and wind energy components, reinforced vinyl esters offer high strength, toughness, tensile elongation, and heat and chemical resistance properties, along with Class A surface potential, high hydrophobicity and resistance to osmotic blistering.

Reactive free radicals

Vinyl esters actually draw upon the best of unsaturated polyester and epoxy chemistries. The polymer backbone is formed by reacting unsaturated monocarboxylic acids (such as methacrylic acid) with high molecular weight bisphenol A and novolac epoxides. Unsaturated ester functional groups or linkages at the end of each vinyl ester linear molecular chain serve as sites for chain growth and crosslinking.

Polymerisation of vinyl esters basically occurs via the catalytic mechanism of free radicals. These are unstable molecular intermediates that become reactive and mobile during decomposition of a peroxide initiator, and the addition of coreactant monomers such as styrene in the resin formulation. The initial free radicals combine with the functional ester linkages in a chain reaction until all the radicals have formed stable covalent bonds and terminate, ending the reaction by forming the final three-dimensional, crosslinked polymer. Heat is generated during this exothermic chemical process. The goal of optimised composite fabrication is to achieve a resin’s peak exotherm with as little added external heat or pressure as possible, since these factors contribute to processing time and cost. For room temperature initiation of vinyl ester composite curing, metal salts such as cobalt or amines are added to help lower initiation temperature, which achieves a lower-temperature cure cycle and helps avoid stress shrinkage in the composite component.

According to Dr Charles Pittman, research professor in the Department of Chemistry at Mississippi State University (MSU) in Starkville, Mississippi, USA: “An important aspect of vinyl ester resin behaviour is viscosity, or resistance to flow. The formulation of low-viscosity vinyl esters, which are highly flowable, allows them to be pumped in or drawn in by vacuum to infuse the smallest spaces in woven fibre preforms. This tailorable viscosity, together with initiation temperature manipulation, provides control over gel time and cure cycles, and extends processing flexibility by eliminating the need to use an autoclave in FRP fabrication.”

He goes on to explain that epoxies don’t use the free radicals polymerisation mechanism or employ diluent solvents for processing.

“Styrene serves as a diluent or thinner and coreactant in vinyl ester curing, is economical and induces hydrophobicity or resistance to water absorption during polymerisation.”

MSU participated on the prototyping team for the US Navy’s first vinyl ester/carbon fibre composite mast, the 28 m tall by 10.6 m diameter (93 ft tall, 35 ft) hexagonal Advanced Enclosed Mast/Sensor or AEMS for the Spruance-class destroyer.

Current MSU research is focused upon the processability of low-styrene vinyl ester (33%) composite reinforced with vapour-grown carbon nanofibres. The hollow fibres are supplied by Applied Sciences, Cedarville, Ohio, USA.

“We’re studying the degree to which varying nanofibre load levels and amounts of styrene diluent affect fibre dispersion and agglomeration," says Pittman. "The research encompasses initial fibre nanodispersion, wet out behaviour, overall matrix/fibre adhesion, bonding of vinyl ester chain ends during cure, and ultimately, interlaminar shear strength between resin and fibre.”

Pittman and MSU colleagues Dr Tom Lacy and Dr Hossein Toghiani are also applying dynamic and quasi-static mechanical property data from tensile, bending and compression tests to model the microdynamics of these nanocomposites.

“This work is aimed at the Holy Grail within multiscale computational engineering, namely the use of such modelling to predict the mechanical life history of a single composite part, from atomistic and molecular dynamic computations through micromechanics and on to continuum mechanics. We believe the understanding being gained could ultimately be applied to significantly increase the mechanical properties and optimise processability of vinyl ester resins and vinyl ester composites.”

Moving beyond middle ground

International resin suppliers rely on in-house laboratory analyses to improve commercial vinyl ester formulations. From Reichhold Inc, Durham, North Carolina, USA, DION and HYDREX vinyl ester resins answer a full range of global FRP end uses through formulations with extended catalysed stability, low exotherm and shrinkage, low styrene and VOC, high elasticity, and UV-curability. Al Naser, Vice President, Commercial North American Composites, observes that “while vinyl esters have their own market space, benefits in cost, processing and performance are moving these resins into the space held by epoxy.”

To which Steve Hardebeck, Technical Director/North America, adds “we’re finding vinyl esters gaining ground in markets such as wind energy, where new players and new facilities are coming online.” He points geographically to China and India for significant growth in wind energy applications, and believes that as carbon fibre replaces glass in longer but lighter composite wind blades, vinyl ester resins offer an opportunistic matrix option.

“Compared to epoxies, vinyl esters can bring similar mechanical properties but offer tremendous advantages by decreasing processing time, such as eliminating the need for post cure,” he says.

In order to further expand its global footprint, Reichhold expects to open new 100 000 ft2 production facilities for its base resins, including vinyl esters, in Tianjin, China, by the end of 2011. This initial phase will utilise less than 25% of the overall site, enabling future expansion.

The company formulated its urethane-modified DION 9800 vinyl ester specifically to compete with high-performance resins such as epoxy by offering enhanced toughness and wet out properties in carbon fibre composites processed through infusion and other processes. Naser estimates products of this type can provide up to a 50% reduction in cycle time.

High-performance anti-corrosion applications form another strong market area for vinyl ester resins, and Reichhold confirms this as a growth market, despite a down year in 2010.

“Corrosion projects have followed the macro indicators for larger markets, with some of the capital projects being delayed or cancelled due to the state of the global economy,” Naser says. "However, regulatory-driven projects such as those for power plant FGD [flue gas desulphurisation] must be completed. In addition, corrosion repair or refurbishment costs can be significantly lower than complete replacement costs for corrosion-damaged components such as FGD stacks, potable and waste water pipes and pipe lining, ducting and chemical storage tanks."

Reichhold’s DION 9500 rubber-modified vinyl ester promotes surface adhesion in glass fibre composites for these applications, along with high strength, toughness and elongation, and high viscosity at the end of the cure cycle. To meet demands from customers in the marine and automotive markets, Reichhold offers vinyl ester resins that include low viscosity, high heat distortion or thickenable for SMC, and low styrene, suitable for products that require high quality surface finish and resistance to osmotic blistering.

Versatility plus

Whether fighting corrosion in pipes that transport fresh water or protecting people riding within flame-resistant bus and train transport interiors, vinyl esters are finding a fit in an ever-widening array of FRP parts. This is certainly the case for EPOVIA vinyl esters from Cray Valley, France, and US sister company Cook Composites, Kansas City, Missouri. First commercialised in 1979, EPOVIA formulations now encompass some 25 different products serving the supplier’s top three markets: anti-corrosion, marine and industrial. Sebastien Taillemite, Composites Marketing Manager/Europe, reports a 20% production growth rate this year in its vinyl ester resins, at several thousand tonnes.

In Asia, isocyanate-modified EPOVIA RF 5000 vinyl ester is seeing use in automotive SMC parts such as rocker covers and oil sumps that require high heat resistance. Asia’s FM approval for fire, smoke and toxicity (FST) performance in ducting for the semiconductor industry is achieved with EPOVIA RF 2000 SEHA, which also provices good mechanical properties in sections that are typically 12 m long and 10-30 cm in diameter (39 ft, 4-12 inches). Applications that need high levels of toughness, elongation and fatigue properties, such as impact-sensitive parts, adhesives and ballistic products, draw upon EPOVIA KRF-3200. Taillemite reports toughness on the order of G1c of 2750 J/m2, which he says is unique among available vinyl ester resins.

EPOVIA KRF 3202 also delivers toughness properties for wind energy and military applications, plus viscosity at around 100 cps with high elongation, suitable for rapid infusion. Assessing the dominance of infusion processes in blade production, Cray Valley believes low viscosity, high fatigue versions of EPOVIA VE can compete well with epoxies.

“This is especially true for offshore composite blades,” Taillemite states, “where the epoxy can be four times more hydrolytic than vinyl ester.”

He adds that Cray Valley is investigating reinforcement options such as nano particles and macro core shell particles.

Other top markets for EPOVIA vinyl esters include marine, industrial, and anti-corrosion equipment in the power, chemical, semiconductor, oil and mining industries. In Korea, Cray Valley is working in partnership with Hankuk Fibre Glass Ltd (HFG) to provide abrasion and corrosion resistant composite pipes as replacement for cast-iron at thermoelectric and nucleur power plants. In ash transfer piping, EPOVIA KAYAK KRF-1001 vinyl ester with alumina/silicone carbide filler is filament wound on the inner layer of the pipe, and with glass fibre in the outer layer. To date, the vinyl ester composite has demonstrated twice the expected lifetime for the piping, giving the US and Korean companies the positive impetus to work together on large diameter FGD chimney pipes (for up to 200 m/656 ft tall chimneys in 10 m/33 ft long vinyl ester/glass fibre pipe sections with 5.4 m/18 ft diameter). The first trial run for this application is expected as this article goes to print.

in the USA, Cray Valley and Cook Composites are working with the Maine Department of Transportation to re-line failing steel culverts near Amherst that provide lake access to spawning fish. The rusted-out culverts (43 m long by 3 m diameter) are about 14 m down from the road surface, and would have required them to be completely dug up to be replaced. A special infusion grade of EPOVIA vinyl ester resin allowed fabrication of some 14 glass-reinforced liner panels to re-line the bottom half of the culverts, which were then back-grouted for stability. The dimensions of each panel: 3 m (10 ft) long and 2.6 m (8.6 ft) in diameter, 7 mm (0.28 inch) thick and weighing 116 kg (256 lbs).

Green growth

DSM Composite Resins, headquartered in Switzerland, produces at least 25 vinyl ester formulations, including hybrids with polyurethane, within its Atlac, Daron and Palatal resin systems. All feature high temperature and chemical resistance properties, with formulation variability for cure time/temperature control and a variety of reinforcing fibres. As with other suppliers mentioned here, DSM’s key FRP markets include anti-corrosion, infrastructure, automotive, and wind energy, and the company is on a competitive track to establish a presence for its resins in all these end uses in China. DSM recently opened a new R&D Centre at its facility in Shanghai, with capabilities ranging from resin formulation to composite application developments.

Green resins remain important in any geography or application, and DSM first offered a styrene-free formulation of its Atlac vinyl ester for cure-in-place pipe (CIPP) relining in 2008. No-styrene resins are also produced by NoVOC Performance Resins LLC, Sheboygan, Wisconsin, USA, for CIPP and other infrastructure rehabilitation applications in both potable water and sewage piping. Reichhold is evaluating bio-based and styrene-free unsaturated polyester and vinyl ester, and also started an evaluation project this year with the US Soybean Board for infusion-moulded panels and small parts.

No-styrene CIPP formulations are produced within the Vipel line of vinyl ester resins available from AOC, Collierville, Tennessee, USA. Dr Thomas Folda, Vice President of Technology, points out that the company “works with individual customers to provide the chemistries that meet ‘fitness for use’ needs. Standard formulations are often modified to reduce air entrapment and promote wet out, as well as accommodate specific fillers and fibres.”

Earlier this year, AOC introduced its EcoTek Green Technologies for composites and cast polymers, including L040-TNVG, a styrene-free, filled vinyl ester, and L040-LCVG, a UV-curable, styrene-free vinyl ester.

A natural extension of AOC’s green initiatives comes in the form of its new Wind Energy Laboratory. Though not designed to produce full-scale blades, the lab is fully equipped with processing equipment and analytic tools and is capable of mimicking the critical elements of the blade manufacturing process. Further, the lab supports customers who are fabricating nacelles and rotor nose cones for wind turbine systems. Vipel bisphenol A epoxy-based vinyl esters are also formulated to meet corrosion-resistance performance requirements in power plant and waste-to-energy incinerator exhaust system stack liners, water/waste water infrastructure, chemical and industrial equipment, and in fire-retardant applications.

Derakane and Hetron vinyl ester resins from Ashland Performance Materials, Dublin, Ohio, USA, have made significant inroads in anti-corrosion applications over the past 50 years, including in some of Europe’s largest glass-reinforced vinyl ester absorber vessels within jet bubbling reactors. Other components for this market fabricated with Ashland’s vinyl esters: stack liners, ducting, limestone slurry piping, spray headers and carbon capture and sequestration systems. The company also formulates its Derakane Momentum vinyl esters for pultrusion processing of reinforced products such as gratings, ladder rails, cable trays, and structural profiles for sewer plants. For infusion processes, Bruce Colley, global product manager, state that “the chemical structure of our vinyl ester formulations yields a balance of damage tolerance and stiffness, especially in thinner parts, as compared to other more rigid materials in highly fibre-loaded applications.”

Preliminary testing of infusion moulded turbine blades by the Germanischer Lloyd indicate a 25% cycle time improvement with Derakane 601-200 vinyl ester as compared to epoxy.

Ashland entered the wind energy market with its Aropol unsaturated polyester resin and Maxguard gel-coat products. More recently, Derakane is carving out space in this application with preliminary testing of infusion moulded turbine blades by the Germanischer Lloyd, indicating a 25% cycle time improvement with Derakane 601-200 vinyl ester as compared to epoxy. Kevin Lambrych, Ashland’s global wind energy technology leader, believes this could translate to a cost savings of 25-30% per blade.

Ashland has a joint effort with Amtech LLC and Endurance Wind Power to produce small to mid size wind blades in Washington state. The company is also supplying vinyl ester resin for wind energy projects with the Ohio Third Frontier Advanced Energy Program and the US Department of Energy-funded offshore floating wind turbine platforms proposed by the University of Maine.

Getting hands on

For all the emphasis on the benefits of vinyl esters in huge FRP power plant chimneys and larger wind turbine blades, there’s still an important place for these resins in hand lay up of small-quantity production parts and prototypes. Steve and Maureen Hassett, owner/founders of Custom Composites Technologies Inc (CCTI), Bath, Maine, USA, can certainly give 10 years of testimony to this fact at their fabrication facility, though Steve’s composites experience goes back to the 1980s and boat building along Maine’s coast. CCTI’s capabilities include prepreg layup, VARTM, post-cure to 121°C (250°F), custom tooling and custom oven builds. Among the unique and demanding one-offs built by CCTI: seats for a human centrifuge, arms for a medieval catapult, and carbon and glass fibre reinforced boat hulls and masts.

Several recent projects at CCTI illustrate the fabricator’s utilisation of reinforced vinyl ester resin. For an underwater tidal energy generator using composite turbine blades, US Windblade (also located in Bath) contracted with CCTI for vacuum infusion of helical foils and rings for this generator. Steve Hassett explains that the 6 m (20 ft) long, 41 cm (16 inch) deep, 8 cm (3 inch) thick foils are made as upper and lower sections bonded at the leading and trailing edges. The 1.22 m (4 ft) diameter rings, which hold the foils in position, require build-up areas at four key locations to accommodate bolts or bushings.

Hassett says he selected Ashland’s rubber toughened DERAKANE 8084 vinyl ester for its low water absorption properties and high damage tolerance, relevant since the turbine blade parts encounter added fatigue from the force of the water and impact from debris in the water. CTTI fabricated the components in 75 days, with no post cure required.

Another Ashland resin, HYDREX 100-HF, solved a problem for Premier Custom Trough, Blue Earth, Minnesota, USA, which builds composite channels that are buried in railroad track ballast and enclose cables used for power, signaling and communications. Ron Fox, Premier’s CEO, tells Reinforced Plastics that he ordered nearly US$200 000 worth of pultruded glass/epoxy troughs that failed to meet the required geometric and performance needs for placement in tunnels in New York City’s light rail system. He turned to CCTI to manufacture 198 trough adapters that would accommodate 45/90° bends and allow connections between different trough end sizes. Fox worked with CCTI in specifying the Ashland vinyl ester because it offered tailored gel and cure time, met mandated fire, smoke and toxicity (FST) properties and provided sufficient strength properties.

Each adapter was hand laid up and infused, with Hassett estimating that it took his crew about 20 days to produce all the parts, using two sets of moulds. Resin to glass fibre ratio in the trough adapters is 60/40. Fox says 198 adapters represent about 3 to 4 miles of track, and provided connection between different trough section sizes, which avoided a break in the overall trough run for that length of track.

Expanding vinyl ester universe

The world of vinyl esters is clearly expanding, with new resin formulations produced by the suppliers mentioned here as well as others: Interplastic and IDI Composites in the US, Poliya in Turkey, and Huachang Polymer Co in China. All are following the signs of positive market potential. In one of the strongest markets for composite products, anti-corrosion, both the new build and retrofit segments look promising.

According to forecasts from the McIlvaine Co, Northfield, Illinois, USA, orders globally for power plant scrubbers throughout the next decade will remain far above pre-2000 levels. The FGD market will grow to $14 billion in 2011, especially in India and China.

“China, with the most coal-fired boilers and FGD systems of any country, also has the largest repairs market, and is moving along the same path traveled by the US, which found it necessary to renovate many early FGD systems,” states McIlvaine’s 2010 FGD Markets and Strategies report.

In the company’s biweekly North American Municipal Wastewater Treatment Facilities & People report this August, spending of over $80 billion in the US and Canada is predicted for upgrade and expansion of municipal wastewater treatment facilities in the next five years. Certainly composites have a share in this significant funding opportunity.

Vinyl esters truly bridge the price performance gap between general-purpose polyesters and epoxies.

Ultimately, maybe being the 'Malcolm in the Middle' resin choice isn’t a negative, especially as end users look for options in both materials and fabrication methods. At Quantum Composites, a wholly owned subsidiary of Premix and based in Bay City, Michigan USA, fully a third of the SMC compounder’s product line is in vinyl esters. Matt Douglas, product manager, states that “the successes of our customers convince us that vinyl esters truly bridge the price performance gap between general-purpose polyesters and epoxies. In the next several years, we expect to see new products introduced that will be real game changers. Our biggest challenge is not material performance, but rather the mindset of engineers and designers who feel they need to use epoxy resins. When the ease of handling and wide processing parameters are added to the cost advantage, choosing a vinyl ester structural sheet moulding compound creates advocates across all decision-making functions.”

Getting radical – beyond the unique free radical polymerisation chemistry of vinyl esters – means shaking up the status quo, which is just what Kenway Corp and Arkema Inc did last year in the vacuum infusion of a 540 kg (1200 lb), 13 cm (5 and 3/8 inch) thick FGD blind flange. To achieve low-stress cure of this large part, Kenway, of Augusta, Maine, USA, applied the patented BlocBuilder nitroxide-mediated, controlled radical polymerisation technology developed by Arkema Inc, Philadelphia, Pennsylvania, USA, to a Cray Valley/Cook Composites EPOVIA vinyl ester reinforced with nano-additives. This combination resulted in unprecedented room-temperature cure control over a flow and gel period of several hours with exotherm at less than 150°C (310°F).

Cray Valley’s Taillemite reports that his company continues working with Kenway and Arkema to take the next step in fabricating such large components. This will involve infusion moulding of more complex shapes, and the moulds to make such large composite components. He expects a progress announcement in this regard relative to Cray Valley resins in early 2011.
 


This feature was published in the November/December 2010 issue of Reinforced Plastics magazine.

 

 

 

 

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