Vinyl Ester Resins are becoming increasingly popular in GRP fabrication, not only as of the resins of choice for corrosion equipment but also for the fabrication of composite materials for structural, automotive, marine, and military applications.
In this blog, we have tried to summarize the characteristics, curing behavior, and new emerging applications of these very useful products. Crest Speciality Resins (P) Ltd. is one of the pioneers in the development of Vinyl Ester Resins and Terephthalate Resins in the country.
Our range of Vinyl ester Resin is the most comprehensive and the successfully used in prominent chemical industries for several years. The company’s ever-increasing investments in R&D, managed by researchers with degrees from IIT’s and abroad have aided the quality of the products manufactured at our plant. We follow a strong quality system and all our products are made in ISO 9000 compliance. Our current range of Vinyl Ester resins comprises the following products:
The Industry’s Ideal Vinyl Ester Resins
Chemically Vinyl ester resins offer a bridge between lower cost, rapid cure, easily processable unsaturated polyesters, and high-performance epoxy resins. The following characteristics of vinyl esters make them a valuable matrix resin:
- Vinyl ester resins have high tensile elongation and still possess high Heat distortion temperature.
- They have high cohesive strength, wettability, and adhesion to glass fiber and many other fibrous reinforcements.
- They process easily with conventional catalysts that have relatively low toxicity, easy dispersibility, and from the Indian context easy availability.
- And needless to say, they have exceptionally high chemical resistance.
As a result of the above, unique features vinyl esters are not only the resins of choice for corrosion equipment but also the fabrication of composite materials for structural, automotive, marine, and military applications.
Advantages over Bisphenol resins and conventional resins
Common unsaturated polyester resins including isophthalic and terephthalic resins are formed by reacting an aromatic diacid and an unsaturated carboxylic diacid with dihydric alcohol called diol and dissolving the resultant product in styrene. These resins suffer from three major drawbacks:
1. Poor hydrolytic resistance arises due to the distribution of ester linkages all over the molecule. The resistance to alkalis is especially poor.
2. Poor chemical resistance to chlorinated chemicals due to hindered unsaturation in the molecule which remains poorly reacted even under the toughest curing conditions and
3. These resins also suffer from poor chain flexibility which leads to poor crack resistance, poor glass-resin adhesion, and low burst pressures. An increase in flexibility by incorporating saturated aliphatic dicarboxylic acid or an oxygenated diol such as diethylene glycol in the formulation results in still poorer chemical resistance and lower HDT. Vinyl ester resins, on the other hand, are prepared by reacting an epoxy resin with an unsaturated monocarboxylic acid, such as methacrylic acid, and dissolving the resultant intermediate product in a monomeric solvent such as styrene. The reaction is often tricky and is carried out in presence of catalysts, inhibitors, etc.
General-purpose vinyl ester resins are prepared from bisphenol epoxies and the high-performance vinyl ester resins are prepared from novolac epoxies. As a result of this unique chemistry, all vinyl ester resins have terminal unsaturation i.e. the vinyl group is situated at the end of the molecule which provides them high reactivity, low residual unsaturation, and high tensile elongation. The ester linkage is also terminal and protected by pendant methyl groups which make them less susceptible to chemical attack. Vinyl ester resins also have an unreacted hydroxyl group in their structure, which is partly responsible for their excellent adhesion to glass and metal surfaces.
Vinyl ester resins beat their nearest competitor ‘the Bisphenol Resins’ in organic chemical resistance, mechanical properties, and curing characteristics. A typical bisphenol resin has more or less the same chemical resistance as an isophthalic resin unless it is post-cured at above room temperature. The stochiometric dependence on styrene concentration is also high, and the styrene emission during cure poses serious health hazards.
Bisphenol resins have poor adhesion characteristics because of the lack of a secondary bonding mechanism in their structure. Vinyl ester resins provide laminates with excellent adhesive strength, reverse impact, and burst pressure compared to bisphenol and other conventional resins. See Appendix for more details.
Types of Vinyl Ester Resins
within the two broad categories of vinyl ester resins, one based on bisphenol epoxy and the other on epoxy novolac, there are several variants caused by the molecular weight of epoxy resins, co-reactant monomers, modifier acids, additives, and processing conditions. The following grades of vinyl ester resin are internationally recognized:
1. General purpose vinyl ester resin based on bisphenol epoxy
2. Reduced styrene, High HDT, General purpose vinyl ester resin
3. Vinyl ester resin primer
4. Fire retardant vinyl ester resin based on brominated epoxy
5. Pultrusion grade vinyl ester resin
6. Rubber modified vinyl ester resin
7. Superior vinyl ester resin based on epoxy novolac
8. High HDT, Low styrene, vinyl ester resin based on epoxy novolac
Epoxy novolac based vinyl ester resins provide high cross-link density and hence excellent retention of mechanical properties at elevated temperatures. These resins are particularly useful in applications where resistance to organic solvents and chlorine or chlorine environment is required. They offer an excellent alternative to stainless steel and exotic alloys. Pultrusion grade vinyl ester resin provides faster line speed, better gloss, and reduced shrinkage besides high chemical resistance.
Advantages over Rubber Lined Steel
Vinyl ester resin-based composites offer several advantages over rubber-lined steel vessels.
1. Lightweight of composite, lighter support structure, and easy handling
2. Ease of assembly on site
3. Easy repair
4. Integrated liner and
5. Proven chemical and abrasion resistance in continuous service over a long period.
Compatibility with Thermoplastic liners
Vinyl ester resins have been successfully used with thermoplastic liners for enhanced chemical resistance. Polypropylene and PVC liners are recommended for corrosive applications up to a working temperature of 80oC. Fluoropolymer liners such as PVDF, ECTFE, FEP, and PFA are recommended for extremely aggressive applications and can provide a working temperature of up to 145oC.
Curing of Vinyl Ester Resins
The excellent chemical resistance and mechanical performance of vinyl ester resins result from two complex chemistries independently performed by the manufacturers of the resin and the GRP equipment at two different locations.
A mistake at either end can result in premature failure of the GRP equipment in-service conditions. Our experience, however, suggests that the majority of problems with vinyl esters arise due to inadequacy in curing the resin at the fabricator end. Vinyl esters are typically cured by 3 component systems: a catalyst, an accelerator, and a promoter.
While catalyst, usually an organic peroxide performs the major task through a free radical reaction, it needs a transition metal ion to produce a free radical of its own for initiating the reaction. Thus the role of the accelerator, which supplies the transition metal ion, is equally crucial.
Vinyl esters need a minimum of 1% (v/w) of a full-strength (9% active oxygen) MEKP catalyst to do the job, irrespective of the weather condition i.e. even at 50 deg C ambient temperature. A higher concentration (say 1.5%) is generally used, which can go up to 2.5 % in winter months or high humid conditions.
Higher catalyst concentrations are also recommended for tooling applications where resin is invariably sprayed. It also helps in improving the gloss. Irrespective of the amount used, it is important to know the active oxygen content of the catalyst as most peroxides degrade on storage and active oxygen content of 7.5% may not sound very low but it can make the curing reaction very sluggish.
MEKP is usually supplied as a 50% solution in a phthalate plasticizer and the purity of the plasticizer especially its free acid constant can have a major influence on its stability and shelf life. Other factors that greatly contribute to the effectiveness and stability of MEKP is:
1. Free hydrogen peroxide content and
2. Dimer content
Both the above contaminants, which are invariably present in MEKP as the bi-product of the reaction can adversely affect the curing characteristics of MEKP without affecting the active oxygen content.
A careful selection of the source of MEKP is therefore vital for the successful application of vinyl ester resins. In addition to the composition, the storage conditions and material of construction of storage container can also have a major influence on the activity of MEKP, which is usually supplied in white PE containers, as traces of metal contaminants such as copper and zinc can deactivate the catalyst. Similarly, storage of MEKP at above 30 deg C can lead to a decrease in the activity of the catalyst as a result of the degradation of the hydrogen peroxide component of the catalyst.
Like MEKP, one needs a minimum of 0.5% (v/w) cobalt accelerator (2% cobalt) for carrying out the curing reaction. The cobalt accelerator is stable but has reduced activity in presence of certain pigments and fire retardant additives. Also, the cobalt accelerator is often difficult to mix and may require good agitation. An overdose of cobalt accelerator ( say above 1.5%) should be avoided as it can negatively influence the curing reaction.
The promoter (usually an amine) is supposed to be the only optional component in the vinyl ester curing reaction, although due to the high inhibitor levels of most Indian vinyl ester formulations, it is invariably recommended for speeding up the reaction. If catalyst and accelerator levels are kept fixed, the promoter, which can be varied from 0-2% (v/w) at 10% active content, is typically used to offset the change in weather conditions or to vary the gel time and exotherm. Amine promoters, however, have a very prominent and essential role in vinyl ester resins catalyzed by BPO catalysts at room temperature. In this case, one has to maintain the ratio of BPO/Amine is strictly in the 10-20 range, and both under and overdose can lead to incomplete curing. Also, remember BPO is never 100% active.
Post-curing is often necessary to complete a radical polymerization reaction although vinyl esters are relatively forgiving to the temperature used for the post-curing reaction. Nevertheless, if the resin matrix is not completely cross-linked, it is vulnerable to chemical attack, especially to the solvent attack.
It is therefore strongly recommended to post-cure FRP equipment at above 80oc when exposure to the solvent environment is required. Post curing can often lead to a dramatic increase in the HDT of the vinyl ester resins by as much as 15oc and a drop in residual styrene content to less than 1% level.
Of all the resins used in GRP, vinyl esters perhaps have the shortest shelf life. They should be stored at temperatures below 25 deg C but above 15 deg C. Ideally storage for more than 15 days at the users’ end should be avoided.
The resin drums should be kept fully closed even during usage. Condensation of moisture can occur even in air-conditioned workshops during the monsoon months and moisture is one of the biggest deterrents of vinyl esters curing reaction.
An extra dose of MEKP in the top layer is generally recommended in monsoon months to overcome the inhibition posed by the condensed moisture.
Newer Applications of Vinyl Ester Resins
The excellent combination of chemical resistant properties, mechanical strength, high HDT, and dry temperature tolerance as well as tolerance to organic chemicals and oxidizing agents of novolac base vinyl esters have made them a product of choice for the fabrication of chemical handling equipment in industries.
Vinyl esters find special applications in bleaching plants of paper and pulp mills where a great deal of active chlorine is used.
They are also used in flue gas desulfurization plants using limestone and gypsum slurries.
Similarly, the high reactivity of vinyl ester resins provides an unmatched pultrusion speed and is the reason for their increasing use in this industry.
A few new applications of vinyl ester resins are listed below:
Glass Flake linings
A glass flake lining system based on vinyl ester resins can provide exceptional long-term corrosion protection to concrete and steel surfaces under severe conditions of immersion, corrosive fluids, high temperature, and abrasion.
Typical applications include chemical and fuel storage tanks, pipes, salt and fresh water circulating equipment, sewage plants, structural parts, scrubbers, fans, ducting, clarifiers, and other aggressive environments where a standard resin coat or paint would generally fail.
Glass flake lining can also be safely used in swimming pools, fish ponds, dams, and waterways for aquaculture where permanent waterproofing is often desired.
The lining system is usually 1 mm thick and comprises a vinyl ester primer, a glass flake vinyl ester coat, and a resin-rich vinyl ester top coat. Vinyl ester-based glass flake lining system, because of its strong adhesion to the substrate and exceptional chemical resistance, resists osmosis, which is the main cause of failure of immersed coatings.
Vinyl ester-based fiberglass grating is an ideal product when one requires a product that is highly resistant to corrosive environment besides being fire resistant, non conducting, lightweight, maintenance-free, and architecturally attractive.
These gratings can be made by pultrusion or open molding, although pultrusion provides products of increased load capacity due to high glass content and a quality finish. Specially formulated vinyl ester resins such as C’POL 741 or Derakane 640-900 are recommended for this application.
Industrial Flooring and Waterproofing Membranes
Vinyl ester resins can be filled with a variety of graded quartz powders, silica, ceramic fillers, or 1/4 inch aggregates to produce a heavy-duty flooring system that can protect the substrate from extreme chemical exposure, impact, and severe thermal shocks.
Glass flake-filled Vinyl ester coatings are used as waterproofing membranes for lining swimming pools and water tanks. Glass flakes reduce the permeability of the membrane by 20 times compared to glass-reinforced or unreinforced resin.
Vinyl ester slurries are typically used in metal finishing plants, chemical processing facilities, electronic units, and power and water treatment plants. They are applied as 1/16 to 1/8 inch thick coatings over a vinyl ester primer.
Vinyl ester troweled surfaces are applied at a thickness of 3/16 to 1/4 inch and are generally heavily filled. They are ideal for use in chemical processing plants and hazardous material storage areas where heavy movements are expected. Vinyl ester flooring systems can be put to light use within 24 hours of application and can gain full strength in 72 hours.
General-purpose vinyl ester resins, because of their high strength and durability are increasingly being used in marine applications for the production of racing boats, yachts, and hulls.
Resistance to water permeation and abrasion combined with lightweight and outstanding fatigue resistance are the main determinants.
Ace The Business With Leading Vinyl Ester Manufacturers
Crest’s range of Vinyl Esters is famous globally with an impeccable record of robust performance in highly corrosive environments in some of the biggest chemical processing industries all over. Its strong performance and high shelf life, have enabled CREST to become the largest manufacturer of Vinyl Esters in India and win over the trust of the composites industry.