Vinyl ester – the midway boat building resin

Resins for building reinforced plastic boats have become a largely bipolar affair. Polyester remains the general, affordable medium for ‘commodity’ boat building while epoxy, usually carbon reinforced, has become the resin of choice for high-specification performance craft. Consequently, many builders overlook the fact that there is a third resin family that is intermediate in terms of properties and price and offers advantages of its own.

Brodrene AS in Norway uses Reichhold's Dion vinyl ester infusion resin in its fast passenger ferries.
Brodrene AS in Norway uses Reichhold's Dion vinyl ester infusion resin in its fast passenger ferries.
Hatteras Yachts of North Carolina, USA, uses Hydropel vinyl ester from AOC in its latest motor yacht, which is fabricated by the SCRIMP process (Reinforced Plastics, June 2007, page 14).
Hatteras Yachts of North Carolina, USA, uses Hydropel vinyl ester from AOC in its latest motor yacht, which is fabricated by the SCRIMP process (Reinforced Plastics, June 2007, page 14).

Vinyl ester resins are somewhere between polyesters and epoxies in terms of mechanical properties and price. One of their major advantages is that they offer better resistance to moisture absorption and hydrolytic attack than polyester. A vinyl ester skin between the gel-coat and the glass/polyester laminate, or over the gel-coat, therefore tends to prevent the hydrolysis-induced osmotic blistering that fibreglass boat owners fear. Boat builders, or owners retrospectively, can incorporate this protection without having to go to the expense of using epoxy – though the latter constitutes an even more effective moisture barrier. Alternatively, vinyl ester can be used for the entire laminate.

Cured vinyl ester is more flexible and has greater toughness than polyester. This means it can better resist the effects of fatigue which can cause cracking in hard-worked hull and deck structures. Vinyl ester can also withstand temperatures of up to around 200°C without distorting. (The resin may benefit from an elevated temperature post-cure to maximise these properties.) On the downside, vinyl ester is less easy to process than polyester, requiring thorough surface preparation and particular ambient conditions during cure if good resin-fibre adhesion is to be achieved. Vinyl esters typically adhere more readily to glass than to aramid and carbon, though (subject to appropriate measures) they can be used with all three reinforcements. Vinyl ester is not a volatile organic compound (VOC)-free option since it is dispersed (like polyester) in an unsaturated monomer, typically styrene – much of which is released during cure. It exotherms when curing, just like polyester.

In terms of its molecular structure, vinyl ester combines elements of both polyesters and epoxies. The presence of ester groups gives them an affinity with polyester, but they can also be regarded as a modified form of epoxy since most are produced by reacting an acrylic or methacrylic acid with an epoxy (typically a bisphenol-A) resin. Like polyesters, they are dissolved in styrene, to levels of up to 50%, to achieve a viscosity suitable for thorough wetting out and lamination. This monomer also promotes cross-linking between resin molecules so that the liquid resin can ‘set’ to a solid. As with polyester, a catalyst and accelerator are required to promote the cure process at room temperature.

The reason vinyl esters are less susceptible to water degradation than polyesters is that they have fewer ester groups. Esters are readily hydrolised, and this can lead to significant moisture uptake by the cured composite. The tendency of vinyl esters to resist this makes them an attractive choice for the below-waterline areas of boat hulls that will remain afloat for long periods, as with most yachts and workboats. Vinyl esters also resist chemical corrosion because they have fewer reactive sites than polyesters. (This property also accounts for the widespread use of vinyl esters in such applications as pipelines and chemical storage tanks.)

Marine products

As with polyesters, vinyl ester producers use molecular manipulation and additives to fine tune resin properties to suit specific applications. Boat builders require a resin that is easy to laminate, cures at room temperature, is compatible with preferred reinforcements and is suitable for hand lay-up, spray-up or closed moulding processes. Good cosmetics, thermal tolerance and toughness are also important, as well as the all-important hydrolytic stability. These days, reduced styrene content has become a significant requirement in the drive to comply with the latest industrial emissions requirements.

The mix of qualities targeted is best illustrated by reference to typical systems on the market and their usage.

Interplastic Corp, which claims to have invented epoxy-based vinyl esters some 30 years ago, emphasises the material's hydrolytic stability by citing a laboratory study that it conducted over a period of 15-plus years. According to the company, test panels coated with vinyl ester laminating resin and skin coat showed no signs of osmotic blistering even after 15 years of immersion in water at ambient temperature. Analysis after an additional year showed that the panels had suffered little impairment of physical properties – flexural strength and modulus, impact strength and so on.

The company's CoREZYN® vinyl esters are available in a number of variants. VE8110, a popular general laminating product for boat builders, is complemented by the low-VOC VE8121, both products being suitable for hand lay-up and spray-up. Cure is rapid, with peak exotherm being reached in as little as 15 minutes and gel times of between 15 and 40 minutes. The low-VOC product has a maximum of 35% styrene by weight. Corvex 169–826 is a low-viscosity formulation optimised for vacuum infusion. Marine classification society Det Norske Veritas has certified this resin, along with COR C-1036-NCU clear iso/NPG® gel-coat, for infusion processes. Additional products include a low-VOC vinyl ester resin modified for extra toughness, Corvex 201–307, plus a number of gel-coats and a series of resins and gel-coats for the marine toolmaker.

Powerboat producers Advantage Boats, Cigarette Racing and Fountain Powerboats have all adopted Interplastic vinyl esters. Advantage found that substituting CoREZYN for polyesters solved problems it had experienced with pre-release. The new material also surpassed its expectations for laminate strength and flow. Use of vinyl ester helped the company achieve a weight of under 8000 lbs for its 100 mph-capable 40 ft Poker Run craft. Cigarette Racing uses Interplastic's vinyl esters on all its production boats, including its F-2 Series race boat, the 38 ft Top Gun model. In this case, CoREZYN VE8121 has helped it produce an exceptionally popular high-performance craft featuring a strong, light hull based on end grain balsa-cored, carbon-skinned sandwich construction. Fountain Powerboats found that adopting CoREZYN VE8155 vinyl ester tooling resin enabled it to produce tougher and truer glass composite mould tools at competitive cost, and that the resin withstands well the high temperatures present when curing a hull.

Performance sailboat builder J Boats uses Interplastic vinyl esters in conjunction with TPI's Seemann Composites Resin Infusion Moulding Process (SCRIMP) to produce tough, enduring hull structures. Luxury motor yacht producer Lazzara Yachts used to keep osmotic blistering at bay with a double skin coat of polyester, but now uses a single CoREZYN VE8123 barrier coat and says it has never had a blister or a hull failure in the several years since making the transition.

Reichhold Inc offers several Dion® and Hydrex® vinyl ester formulations to suit different marine fabrication approaches. Mike Papamichael, business manager Europe and the Middle East for vinyl ester, says that a good example is Dion 9100-710, a pre-promoted and thixotroped vinyl ester resin which is used in skin laminates by leading boatbuilders such as Azimut Benetti, Italy. This grade combines high structural performance and fast wetting properties for hand lay-up or spray-up applications. Hydrex 100-HF, a low VOC resin offered in pre-promoted and non-promoted forms, is suitable for fabricating FRP structures using vacuum infusion processes at room temperature. Hydrex 100-LV (3600) is a low-styrene vinyl ester for boat builders seeking compliance with the latest emission regulations. With a styrene level of under 35%, it helps fabricators comply with the onerous Maximum Achievable Control Technology (MACT) regulations in the United States. Despite the reduced styrene, the resin still flows easily and provides good physical properties when cured. It is described as being suitable for high-rate production at room temperature, lending itself equally to marine skin coats or to complete laminates.

Reichhold also produces Norpol® barrier coats aimed at improving exterior finishes of boats. A film of unreinforced vinyl ester is applied to the back surface of the gel-coat. Norpol VBC further improves hydrolysis and blistering resistance as well as enhancing surface finish.

Florida's Velocity Powerboats has used Hydrex 100 in several of its models including the 41 ft Super Sport, a 100 mph power-boat. In this case, the resin is used with S-glass, Kevlar and carbon reinforcements as appropriate to particular areas of the craft's sandwich monocoque structure. This vinyl ester's low water absorption rate leads to high blister resistance, while high strength and toughness result in superior resistance to impact, thermal and de-moulding cracking. High heat distortion temperature means laminates retain their physical properties at high temperatures and in dark, heat absorbing colours. Minimal print-through enables Velocity to achieve good finishes with a minimum of post-mould filling and sanding.

When West Bay SonShip Yachts of British Columbia decided to transition its composites production from spray-up to a closed mould vacuum infusion process, it chose vinyl ester resins to largely replace the isophthalic polyester resins it had been using for its semi-custom luxury GRP motor yachts. A live demonstration of the vacuum infusion process observed at an IBEX boat building trade show convinced West Bay's fibreglass manager at the time, Mike Kluftinger, to make the changes.

“We saw infusion as a clean way to make better parts while also making material and labour savings,” said Kluftinger.

Working closely with distributor Composites One, West Bay selected Hydrex 100-HF low-styrene resin for the infusion process it had developed in house.

“The viscosity of this resin is low so it permeates well, and the exotherm is very low,” reported Kluftinger. “We couldn't find a resin with an exotherm lower than Reichhold's.”

Low exotherm means less shrinkage and therefore less print-through. Sanding and polishing are virtually eliminated.

A powerful vacuum pump is used to assist infusion. West Bay SonShip gained experience by infusing small items at first, working up to larger items such as complete bulkheads and stringers, refining the process all the while. Weight savings of up to 30% and stronger parts with higher glass-to-resin ratios were among the benefits of the progressive shift to infusion and new materials.

Atlac 580 ACT vinyl ester resin supplied by DSM Composite Resins and Euroresins is providing Windy Boats AS with the benefits of high osmosis resistance and low shrinkage for the vacuum-infused structures of its Windy 52 ft series motor yachts. Atlac-580 ACT is a pre-accelerated thixotropic vinyl ester-urethane resin and is used to impregnate the glass skinned/polyvinyl chloride (PVC) foam cored sandwich laminates. When the first hull was fabricated, a good cure was achieved in thin layers, and peak exotherm did not exceed 70°C in the thick keel section. DSM reports that the hull was infused in less than two hours. Earlier this year DSM launched the Atlac E-coat 65 vinyl ester barrier coat. This has a styrene content of less than 35% and can be sprayed onto the gel-coated mould. The company expects that it will increase productivity by replacing the time-consuming hand application of traditional tie coats. Surface quality is also improved.

AME 6000 vinyl ester resin from Ashland Composite Polymers is used by European luxury motor yacht builder Sunseeker, notably in its 124 ft Sunseeker 37M, one of the larger composite production yachts on the market. The product suits Sunseeker's open mould, hand lay-up fabrication process. According to Olli Piiroinen, European technical service representative for Ashland, advantages of AME 6000 include low styrene content and good surface cosmetics. The resin has received Lloyds Register approval for use in marine applications. Ashland also produces the Hetron range of epoxy vinyl esters.

Hatteras Yachts of North Carolina, USA, uses Hydropel vinyl ester from resin specialist AOC in its latest motor yacht, which is fabricated using the SCRIMP process. The vinyl ester infused hull also has a vinyl ester skin coat behind the gel-coat. According to Chris Walker, manager of structural and composite engineering at Hatteras Yachts, AOC was on hand for the initial infusions to ensure that the yacht builder's requirements were met.

“We needed optimum viscosity and open times for filling the 60 ft hull in one shot,” said Walker. “We also needed to keep the exotherm low for cosmetics while retaining the structural properties we required in the cured part.”

Larger vessels

Vinyl ester is increasingly specified for larger vessels, including military craft where it is favoured for its resilience (typically 5% elongation to breaking). Intermarine SpA, a member of the Rodriquez Cantieri Navali ship and boat building group, selected vinyl ester for use in its latest patrol craft, the composite 13.2 m V2000 Class. Meanwhile, Sweden's radical Visby class corvettes have sandwich-constructed hulls with vinyl ester/carbon skins enclosing PVC core. In this case, a vinyl ester further toughened with rubber additives is used and special sizing is applied to the carbon reinforcing fibres to promote fibre-resin adhesion. A spokesman for Visby ship builder Kockums AB has claimed that the new sizing increases the delamination strength of the laminate by up to a quarter compared with a previous standard sizing, and that the combination of the fibre so treated with the rubber-modified vinyl ester resin yields mechanical properties almost equal to those of epoxy laminates.

Norway's smaller Skjold class air cushion patrol craft similarly utilise vinyl esters, along with polyesters, in fibreglass skins over polymethacrylimide (PMI) foam core. Vinyl esters were also specified for the hull laminate of the Mirabella V megayacht built by VT Shipbuilders in the UK. Glass and Kevlar reinforcements are used in the sandwich structure of this 244 ft long vessel. A vinyl ester primer formed the gel-coat, avoiding the possibility of pinholing that has marred conventional gel-coats when standard wax releases are used. The upper deck has carbon/vinyl ester skins over a foam core. All internal decks, tanks and bulkheads were vacuum-infused as flat glass/vinyl ester, foam-cored sandwich panels.

Reichhold's Mike Papamichael says that more and more vinyl ester resins are being used with infusion processes and carbon fibre in the production of light, high-performance craft. For example, Brodrene AS in Norway manufactures 27 m passenger ferries using Dion 9500-501, a pre-promoted, toughened vinyl ester with good adhesion to carbon fibre. The resulting laminate has high strength, stiffness and shear strength. Fabricating such vessels in vinyl ester/carbon results in much lower weight than if they were to be built in aluminium. This is reflected in the high performance of these 30 kt-capable ferries.

Power train

Finally, the resilience and versatility of vinyl ester resin are illustrated by its innovative use in a marine vessel power train, showing that it is not restricted to hulls and decks.

In an effort to beat the corrosion that can occur in vessel stern drives when dissimilar metals (aluminium housings, stainless steel shafts, bearings etc) are immersed in salt water, an electrolyte, propulsion specialist Volvo Penta used composites in engineering its Ocean OSI stern drive series. A stern drive is a permanently attached outboard drive unit mated with an inboard engine. For its Ocean Series, Volvo Penta replaced the die-cast aluminium external parts of the drive which are normally exposed to salt water with fairings made from sheet moulding compound (SMC) incorporating vinyl ester resin. This secured high corrosion resistance and hence durability, and also enabled the company to provide a product with improved aesthetics, hydrodynamic profile and performance.

Volvo Penta wanted an ‘aluminium-like’ material that could be compression moulded in order to achieve the required rate of series production. After a prolonged search and evaluation process, it chose QC-8700 Engineered Structural Composite Moulding Compound from Quantum Composites. This is a vinyl ester SMC reinforced with 63% 1 inch glass fibres and was designed as a metal replacement. The material retains more than 80% of its room temperature stiffness at 66°C, an important factor given the heat generated by the gears inside the drive. Part thickness for the moulded access cover, cowling and gimbal housing averages 0.2 inch (5 mm), not dissimilar to that of the predecessor aluminium parts. The highly loaded gimbal ring, which serves to attach the drive to the boat transom, is up to 1.5 inches thick. A minimal internal aluminium frame is retained for the drive leg but, since this is entirely enclosed by composite, it is well protected from sea water.

The Ocean Series has proved popular with owners, particularly sea fishermen and recreational boaters. The unit weighs a fifth less than an equivalent aluminium stern drive. A single access panel facilitates owner maintenance. An improved exhaust path moulded into the two-piece SMC cowling has reduced back pressure, thereby improving engine performance and fuel economy.