Most surface coating application begins with the structure or article to be coated. The surface needs to be prepared so as to remove contaminants that would detract the adhesion of the coating. However, there are one type of surface coating system that is used in quite opposite manner. These coatings are called gelcoats. They are used in such a way that substrate is built up onto the coating so that the gelcoat becomes an integral part of the article to be produced. The optimal dispersion of the nanofillers in the resin is a key factor for achieving the aforementioned properties.
- A gelcoat is a material used to provide a high quality finish on the visible surface of a fibre-reinforced composite material. The most common gelcoats are based on epoxy or unsaturated polyester resin chemistry. Gelcoats are modified resins which are applied to moulds in the liquid state. They are cured to form crosslinked polymers and are subsequently backed up with composite polymer matrices, often mixtures of polyester resin and fibreglass or epoxy resin with glass, kevlar and/or carbon fibres. The manufactured component, when sufficiently cured and removed from the mould, presents the gelcoated surface. This is usually pigmented to provide a coloured, glossy surface which improves the aesthetic appearance of the component. Gelcoats are designed to be durable, providing resistance to ultraviolet degradation and hydrolysis. These require very high levels of durability to overcome the mechanical and thermal stresses encountered during the curing and demoulding processes.
- Gelcoat is a polyester resin specially formulated with thixotropic ingredients for increased viscosity and non-sag properties. It incorporates pigments for desired colour and contains additives for controlling flow-out, gel and cure times. Polyester resin is one part of the two-pack system other being Methyl ethyl Ketone Peroxide (MEKP). Most high quality marine gelcoats use ISO/NPG (Isophthalic/Neopentyl Glycol) resins, which are higher quality polyesters.
Standard polyester resin should not be used as a gelcoat, because it is not a coating. Standard resin does not have the same ingredients as gelcoat and has very little strength on its own. Standard polyester resin must be used in conjunction with a fiberglass material to reinforce the resin for structural integrity.
Composition of Gelcoats
Gelcoat is the outer most resin rich layer over FRP moulding. It has the main function of protecting fibre and laminate from harsh environment and gives freedom to choose various colours. The actual end use decides the composition of gelcoats.
In general, polyester resins result from the reaction between a diprotic acid and a polyhydric alcohol. The cured product occurs due to both type of polymerization which are:
Condensation polymerization: In early part of the reaction monomer concentration decreases rapidly, the molecular weight increases in an en masse fashion with time with the formation of by product.
Addition polymerization: Polymerization occurs in a fast way reaction occur over whole period unlike condensation reaction. No by-product is formed.
Main component of unsaturated polyester are:
Unsaturated polyester: Fumaric acid and maleic anhydride, these compounds forms polymer backbone which have sites for cross linking.
Saturated acid: Degree of spacing or concentration of unsaturated acid is determined by it, e.g. phthalic anhydride or isophthalic acid.
Glycol for bridging of acid, e.g. propylene glycol or neopentyl glycol.
Unsaturated monomers such as styrene, at end of reaction they react with backbone unsaturation to form cured product, until then it act as a solvent.
Condensation polymer occur with first three of the above components and linear chain is formed which is cross linked by the last component.
Any change in type or proportion of reactants results in a change in properties of the polymeric product so an infinite number of polyesters are theoretically possible. There are three important materials which lend their name to generic groups of polyester used in gelcoats:
Orthophthalic: Phthalic anhydride is one of the major raw materials used in polyester resin and give rise to a group of resins called orthophathlic. These resins are used for general purpose gelcoat where weather, water, chemical resistance is not required in finished coat.
Isophthalic: Gelcoat made from this resin has better weatherability, improved water and chemical resistance, and is more resilient and tougher. Therefore suited for manufacture of small boats, transportation, and outdoor furniture and building facades.
Neopentyl glycol: These when coupled with isophthalic impart excellent water and chemical resistant and weather properties. These are used where high quality and performance is required which are swimming pools, yachts, larger boats, spas and sanitary ware.
These act as a solvent for unsaturated polyester and copolymerize with the unsaturation in polymer chain. In gelcoat, styrene is the most common monomer used. It is so because:
It is good solvent.
Boiling point is high.
It is capable of curing unsaturation present in polymer backbone.
It is economical.
These are incorporated to provide sufficient thixotropy to gelcoat so that it can be applied on vertical surface without defect. Thixotropy plays important role as if it is too high then it may result in orange peel, proper levelling may not occur and if too low then result in sag and run. Fumed silica, organoclays, hydrogenated castor oil and precipitated silicas are used as thixotropes.
These have very fine size and impart minimal colour. To reduce cost, reduce curing shrinkage and impart special properties such as flame retardance. Commonly used fillers are talc, clays, silica's, and alumina.
Pigments are added in dry powder or in form of dispersion in polyester vehicle. Selection is done such that they have good light and heat stability and be chemically inert in presence of organic peroxide. It should not have any effect on curing of gelcoat. Basic function is to give colour and hence impart high degree of opacity.
These are the chemical species which react with the catalyst to decompose into free radical and in turn react with unsaturation in styrene and the fumarate in the polyester backbone, resulting in crosslinking. Cobalt octoate is generally used. Other auxiliary catalyst such as amine types can be used but they have strong tendency to discolour on exposure.
It is the second part of the two component system of gelcoat. It is the second part of the two component system of gelcoat. The catalyst used most exclusively in gelcoat is methyl ethyl ketone peroxide (MEKP). The normally recommended level of catalyst is 1-3% depending on temperature condition with 2% optimum.
Fiber Reinforcements: To give high strength composites as in laminates, SMC, and BMC.
Wetting Agents: It facilitates wetout of fillers and reinforcements.
Bubble Release Agents: It is added to enhance air bubble release in laminating or casting.
Curing of gelcoat
During curing, thermoset resins evolve from liquids of low molecular weight to solids with fully developed three dimension cross linked networks. Cross-links can be formed by chemical reactions that are initiated by curing agents, temperature, pressure or radiation. The cross-linking and branching action results in a loss of polymers ability to move as individual polymer chains, consequently resulting an increase in viscosity. Initially the resin viscosity drops upon the application of maximum heat flow and then begins to increase again as the chemical reactions commence between average length and degree of cross-linking. This point is known as the gelation point and is characterized by the material transition from a viscous liquid to a rubbery solid exhibiting visco-elastic type behaviour.
Consequently, an increase in stiffness is experienced after the onset of gelation allowing the material to be able to sustain strains and stresses. The curing behaviour of unsaturated polyester is due to different concentration of styrene monomer which is measured through viscosity, gel time, and maximum exothermal temperature. The curing reaction is a very complicated process that is affected by many different factors, such as weather, humidity, resin uniformity, conditions of ingredients as they are stored and equipment conditions.
The process of cure of thermosets consists of two main stages that are the heating period of liquid resin either pure or in the form of composites with fillers and the cure reaction in the mould. The primary structure of the master model is formed by the energy balance which considers several factors which are the accumulation of heat in the composite, the heat generated by the chemical reaction, the heat conduction in the material, and the heat dissipation at the composite skin.
Gelcoat has a shorter pot life of around 15 - 20 minutes which is much less than conventional 2-K system. Film wet time gives sufficiently long 'Wet edge' but has less cure time. The viscosity of gelcoat is adjusted to take account of temperature variation.
Brush and roller: Brush is the easiest method to apply gelcoats. Brush method has the advantage in very good air release and low emission of styrene. The pigmentation of gelcoat should be adjusted to make brush strokes not visible. Ideally, each two layers of 12 mil are applied. The second layer is applied after the first has initially cured and does not open when the second is brushed on. However, it is not easy to maintain an even layer thickness over the whole piece with this application method. A special hand application rolling method is mainly used for the production of large moulded articles with large surfaces to achieve a relatively short coating time. However, not every gelcoat in brush consistency is suitable for rolling and special formulations must often be selected for large surface objects.
Spray Application: Spray application is categorized on the basis of introducing catalyst to the gelcoat and gelcoat atomization. It is build up by 3 to 4 passes applied wet on wet each having thickness of 100-200 um. This technique is useful since it allows the air from the thick film to escape which would have been induced due to spray fan. It is the most preferred method of applying gelcoat and different types of guns are available.
'Hot pot' guns: In this system, a measured amount of catalyst is stirred by hand directly into a container (pressure pot) which is then sprayed from a pressure feed tank within the allowable gel time period. This is currently the most accurate system but uses a large quantity of clean-up solvent and any unforeseen delays can lead to lost hoses and a difficult cleaning job. This system should not be used for continuous production due to waste and safety reasons.
Catalyst Injection: Many Gelcoat applications today use the catalyst injection system, especially for high production fabrication. It permits uninterrupted spraying. In this system, the liquid catalyst is injected into the atomizing air supply by an adjustable venturi device with a flow meter.
The ball settings of the catalyst flow meter are translatable into cubic centimetres or grams per minute so the catalyst feed can be matched to the measured weight output of gelcoat per minute and ratio established. As the output of the gun changes, e.g., change of tip or change of pump pressure, the catalyst delivery rate must also be adjusted to maintain the proper ratio and assure the scheduled performance of the gelcoat in gel and cure time.
Gelcoat film thickness is a very important control point in the process since the thick gelcoat may cause cracking and thin may lead to undercured film when exposed to flexing forces. Generally, the thickness of gelcoat film varies from 0.4 mm to 0.6 mm. Variation of film thickness occurs with specially formulated products. However, there is a specific optimum thickness range for each formulation of gelcoat required by the manufacturer of the product. Another critical view to consider is the average thickness of gelcoat on a part may not prevent cracking. For example, if a part averages 0.45 mm thick, but the corner areas are 0.66 mm, localized cracking may occur over thick areas. It is important to achieve the proper thickness in the most highly stressed areas of a part because thickness is a critical control point for crack prevention, so the spray process are temporary rated as the best method for gel coat application.
Occasionally, gel coats are hand-applied to surfaces with a brush. Specialized gelcoats with high levels of durability are sometimes used to manufacture moulds which in turn are used to fabricate composite products. Such gelcoats must resist mechanical and thermal stresses encountered during the curing and de-moulding processes. A primer gel coat is a specialized gel coat designed to protect the exterior of a composite product and is painted after the product is removed from the mould.
Manufacturing of FRP or composite products
FRP is developed over mould which can be of two types :
Male, onto which the part is moulded.
Female, into which the part is moulded.
There should not be any variation or non-required indentation on surface of mould as any defect on the surface of the mould is directly transferred to the gelcoat which is applied over it.
Firstly, mould is treated with release system so that finished product can be removed from it.
Mold Releasing Mechanism: Silicone type releasing agents form a thin oily film layer of thickness 1-20 um with no soil-repellent or stainproofing properties. The layer separation takes place during the mold releasing. These have excellent mold releasing capability and workability but a limitation is that these often penetrate into molded articles so that secondary processing of molded articles is difficult.
The two components of gelcoat is mixed which is to be applied over mould as shown in figure 2. Then gelcoat is applied which is allowed to cure for 40-60 minutes and the exposed surface will be dry to touch but still uncured due to air inhibition and styrene evaporation (Fig. 3).
After sufficient curing, fabrication of structure of article is commenced. Over the gelcoating, glass fibre and polyester laminating resin are rolled so that the air can be removed and intimate contact between resin and glass with gelcoat can be made (Fig. 4).
Styrene from the laminated resin migrates into uncured, air exposed surface of the gelcoat and cross-links with the gelcoat, thus creating a chemical bond between the laminate and gelcoat. This is allowed to gel and cure. When the part is cured it is released and removed from the mould. Trimming and finishing operation is carried out, and article is complete.
Effect of nanofillers in gelcoat
Nanofillers are supposed to be additives in solid form, where on dimension is in nanoscale and differ from the polymer matrix in terms of their composition and structure. They generally comprise inorganic materials or organic materials. Inactive fillers or extenders raise the quantity and lower the prices, whereas active fillers bring about specific improvements in certain mechanical or physical properties. The activity of active fillers may have a variety of causes, such as the formation of a chemical bond or filling of a certain volume and disruption of the conformational position of a polymer matrix, and also the immobilization of adjacent molecule groups and possible orientation of the polymer material.
The active fillers can be electrically conducting or non-conducting. The conductive fillers used were multi-wall carbon nanotubes and exfoliated nanographite. The non-conductive ones were nanoclay and nano-titanium dioxide. The way that the nanofillers were introduced into the gelcoat was via their 2% inclusion by weight of the resin in the plain resin itself, forming thus a masterbatch. These fillers require high shear force which is obtained by mixer, dissolver, 3 roll millers, etc via rotating and sometime turbulent flow to break the agglomerates and disperse it uniformly in resin. Other fillers are added one at time in the masterbatch. The duration of the mixing process was around 5 hours, the discs employed were impellers of 70 mm diameter with rotational speed around 200 rpm and, the temperature was set at 80°C.
The DC volume conductivity measurements for the gelcoats are depicted in diagram. One may highlight that the inclusion of the conductive fillers i.e. CNTs and nanographite has further increased the conductivity of the reference gelcoat between 2 and 1.5 orders of magnitude. Extensive researches on carbon nanotubes and their polymers have established that CNTs can be conductive, semi-conductive or even metallic depending on their structure and that only a small weigh fraction is needed to form a percolating network in a polymer or in a composite. The above is directly related to their graphene structure and their high aspect ratio and surface area which in turn is translated into electrical conductivity values of 106 to 107 S/m shown in fig. 6.
If five sample of gelcoat are prepared one as a reference and others with having one of nanofillers which are CNTs, the exfoliated nanonographite, the nanoclay and the titanium dioxide in it. Following tests are performed on these sample to check the properties on the coating film.
Viscosity: It is connected with the structure, morphology, density, volume fraction and the aspect ratio of the fillers.So, the carbon nanotubes by having a larger aspect ratio (thinner and longer) and higher volume fraction than the exfoliated nanographite have a bigger impact on the resin's viscosity. The nano-doped ones are thixotropic and therefore no re-agglomeration on segregation was observed after storing them at room temperature for more than 90 days (Fig. 7).
Tensile tests: The beneficiary effect of all the nanofillers on the tensile properties is obvious. For all the cases the Elastic modulus, along with the maximum stress and strain at failure were significantly increased. The increase of the maximum stress values was in the range of 35–65% and for the maximum strain at failure was between 26–97%. Higher aspect ratio and surface area and excellent mechanical properties result in a high tensile strength as in case of CNTs. The restriction in the mobility of the polymeric chain is brought about by the reinforcing mechanism of titanium dioxide (Fig. 8).
Fracture test: Reinforcing mechanisms and claim that the significantly large aspect ratio of the fillers which allows them to act as nano-bridges between the notch edges. Extra energy is needed in order to pull them out from the matrix or break them and then initiate the crack propagation. This extra energy is then translated into improved fracture toughness properties. Thus CNT and nanographite having aspect ratio of around 500 (sometimes 1000) have better fracture properties than the inorganic fillers i.e. the nanoclay and the titanium dioxide have aspect ratios between 10–100.
Coefficient of Linear Thermal Expansion (CLTE) tests: The glass transition temperature, Tg and the storage modulus G! Remained practically unaffected by the presence of the fillers. The tests is performed at below and above the glass transition temperature, Tg, of the reference gelcoat i.e. 200°C. The effect at the CLTE value of all the nanofillers below Tg seems to be of no practical use. Nevertheless, the CLTE values for all the nano-enhanced epoxy gelcoats are reduced. In other words the nano-enhanced gelcoats will expand less than the reference material. Materials expand because an increase in temperature leads to greater thermal vibration of the atoms in a material, and hence to an increase in the average separation distance of adjacent atoms or polymer molecules in the case of thermosets. The size, the high surface area and the aspect ratio of the fillers are responsible for the aforementioned improvement. The nano - fillers occupy certain space among the polymer macromolecules making thus their vibration and their separation more difficult to occur.
Weatherometer test: Carbon based products ranging from coatings to wood and plastics form free radicals when exposed to Ultra-violet (UV) radiations. These free radicals chemically react with oxygen to yield a photo oxidized product which modifies the product appearance and also its mechanical properties. The Weatherometer (WOM) device uses a combination of carbon arc, UV radiation and water spray to simulate destructive weather conditions in an accelerated manner. Although precise equivalents are impossible to determine, a practical calculation is: 300 hours in the weatherometer equals one year real time (Fig. 9).
The data from WOM tests concern discolouration and colour fading of the samples and weight loss. In case of gelcoat if it is black then it will remain black after the addition of the different nanofillers. As a consequence the discolouration colour fading data are of no-practical value with the differences among the five types of specimens to be trivial. Therefore only the weight loss percentage data are of significance. The weight loss for the nano-filled gelcoats is less than the reference gelcoat, meaning that the reference samples were more prone to environmental degradation of their molecules making thus their vibration and their separation more difficult to occur.
The tensile and fracture properties as well as, the electrical and thermal are improved with induction of nanofillers. The presence of the nanofillers also decreased the coefficient of linear thermal expansion and made the nanodoped epoxy gelcoats more resistant to UV degradation. The addition of carbon nanotubes and the exfoliated nanographite lead to an all around enhancement of properties, while the addition of nanoclay and nano-titanium dioxide is beneficial only for the tensile and thermal conductivity properties. However the inclusion of the nanofillers had no practical impact on glass transition temperature. One of the drawbacks of the introduction of the nanofillers into the resin was the increase of viscosity which can be an issue of process ability of those materials when composite manufacturing is considered.
Advantages of gelcoats
The gelcoat applied is to guarantee a smooth external surface and for the assurance of fibers from immediate exposure to the environment.
The gelcoat also enhances fire protection of the beam and provides an additional barrier against moisture.
Gelcoats are stronger and more durable than standard polyester resins (orthophthalic).
Gelcoat serves as a release agent to prevent the piece part from sticking to the mould in which it is being made. Without gelcoat the liquid resin would eat through the wax applied to the surface of the mould and would bond to the mould. Gelcoat cures too quickly to attack the wax layer so the finished moulded door will not adhere in the mould.
The gelcoat becomes the outer surface of the part, UV stabilizers must be added to the gel coat formulation to protect the underlying resin. Pigments are added to the gel coat to impart the desired finish color.
Defects in gelcoat
Boat Painting: A boat may be painted with anything, but some materials are better than others. Paint is applied more easily than gelcoat and usually has a better shine because it goes on more smoothly, but it is frequently more expensive and does not go on in thick layers. The polyurethane paint gives a shiny finish and will last longer in the sun than any other finish but is expensive. The inside of the boat can be painted with any of these materials but the interior is rarely sanded smooth so the polyurethane is wasted. Gelcoat has the advantage of being a thicker coating for longer wearability as well as the fact that is will stick to a tacky resin surface without sanding. A thicker coating is useful on the bottom of the hull if it is frequently dragged across sand, and on the floor where there is a lot of foot travel.
Mold Use: Gelcoat is normally used in a mold for making fiberglass parts. It provides both the colour for the part and a barrier against pinholes on the surface of the fiberglass laminate. If brushed, two separate thick coats should be laid in different directions to cover thin areas and brush steaks. A rule of thumb is to brush a heavy coat in one direction, let it tack up for approxi-mately 10 minutes, and then brush a second heavy coat across the brush marks of the first coat. If you wait too long between coats the 2nd brush coat will dissolve the 1st (thin) coat and wrinkle the surface. When sprayed, it will produce an “orange-peel” surface for best resin bonding. For mold use, a thickness of at least 20-25 mils is desirable. Let the gel coat cure long enough so that it will not transfer to your fingertips, before lami¬nating the fiberglass.
Splatter Finish: If a splatter or webbed finish is desired, colored gelcoat is mixed with clear webbing solution (Webbing solution is a lacquer-based material and can be used with lacquer paint, polyester-based resin and gelcoat, and polyester-based urethane. It will not work with acrylic enamels, water-based paints, or acrylic-based urethanes.) In different ratio for different effects. Pour approximately 1-2 inches into a standard spray gun; add the appropriate amount of catalyst, and spray. The coverage depends on how fast you move the gun. Since the webbing solution will make the gelcoat dry tack free there is no need of adding surfacing agent. For best results, it should be sprayed while the base coat previously laid down is still tacky.
After the detailed study of gelcaot formulation, manufacturing, properties, and its variation on addition of nano fillers following points can be concluded:
A strong bonding between gelcoat and fiberglass layer is achieved if proper thickness of gelcoat is applied over the composite substrate.
Before mixing of components, the role of the catalyst in curing behavior and its performance should be understood.
The knowledge on role of gelcoat used in laminated system need to be proved experimentally in order to develop a new idea by control the gelcoat thickness to enhance composite product with using standard amount of catalyst to design a better product.
The dependency to use less corrosive catalyst material that suitable to environment and enhance the performance of gelcoat on laminated composite strength such as polyether ether ketone, acetyl- acetone peroxide, vinyl polymerization peroxide, benzoyl peroxide that are commonly used in the gelcoat.
Gelcoats are already filled with filler so nanofillers are required to provide extra properties for good performance and hence there requirement is essential.
Surface Coatings Raw materials and usage- Vol. 1 by Chapman & Hall.
Coating Technology Handbook 3rd Ed Edited by Arthur A. Tracton.