RESINS

1. Introduction: Unsaturated Polyester Resins (UPER) represent the commonest P in FRP, because they offer a host of advantages, including ease of processing, the ability to “cure” without evolving any volatiles (even affording ambient temperature cures), excellent mechanical properties, good resistance to corrosion, good electrical properties, good dimensional stability etc. Other resins used in FRP include Epoxies and Phenolics. Thermoplastic resins are perhaps beyond the scope of this presentation, as are “exotic” reinforcing materials like carbon fiber, Kevlar™ etc. We shall deal with UPER in Fiberglass Reinforced materials for the most part

2. Basic Chemistry: UPER’s are made by the condensation polymerisation of dihydric alcohols (e.g. Propylene Glycol, DiEthylene Glycol, NeoPentylene Glycol etc) and dibasic acids or anhydrides (e.g. Phthalic Anhydride, Isophthalic Acid, Adipic Acid, Maleic Anhydride, Fumaric Acid etc). The unsaturation is usually incorporated via a dibasic acid / anhydride like Fumaric Acid or Maleic Anhydride. These resins are dissolved in solvents which are also monomers, Styrene being the most common, followed by Methyl Methacrylate (MMA)

The polymer chains of these resins can be crosslinked or cured through the monomer(s) with the help of catalysts that generate active free radicals. These catalysts include peroxides like MEK-Peroxide, Cumene HydroPeroxide and Benzoyl Peroxide. Some of these catalysts may be activated even at ambient temperatures, by accelerators like oil soluble heavy metal salts, e. g. Cobalt Octoate, Copper Napthenate etc; the curing may be further expedited by using promoters like DiMethyl Aniline

3. Grades: A large number of the basic types of ingredients required to make UPER’s is commercially available. Moreover, there is a wide variety of additives that can be compounded with UPER’s, in order to impart special performance characteristics (or even processing parameters); such additives include Antimony Trioxide and Chlorinated Wax (for imparting Flame Retardance), Fumed Silica (for imparting thixotropic character), MMA (for modulating the Refractive Index so as to match that of the Fiberglass reinforcement), Low Profile Additives for appearance parts in SMC / BMC / DMC, UV stabilisers for weather resistance etc

Such possible diversities in the UPER chemistry make it attractive to the FRP technologist. The resin can be virtually tailor-made to suit any process or any product. In fact many of the FRP processes have been made possible because of UPER’s, and there has been a lot of synergy in developments in polyester resin technology and in FRP technology

4. Specifications & their Significance:

a. Viscosity: In a way, the viscosity of a resin indicates its degree of polymerisation. Viscosity is measured usually in terms of centipoise or Pascal-second, and can be easily measured by the user with a Ford Cup, which is inexpensive, albeit not very accurate. A hand lay-up resin will have a viscosity ~ 600 – 950 cps (140 – 220 sec in Ford Cup #4), while a resin for Roof Light Sheets will have a viscosity ~ 250 cps, the lower viscosity being essential to ensure adequate wet out. SMC / BMC / DMC resins may have somewhat higher viscosity, while spray-up resins have lower viscosity.
b. Thixotropic behaviour: Resins may be designed to have shear dependent viscosity; thixotropic behaviour is higher viscosity (resistance to flow) at a lower shear rate (speed of stirring); this is useful when laying up on a vertical or an inclined surface, so that the resin is easy to brush or spray, and at they same time they tend to stay put and do not drip so easily. A gelcoat is an extreme illustration of thixotropic behaviour.
c. Monomer content: The monomer content should be optimum; there should be sufficient monomer available for a complete and adequate cure; on the other hand, an excess of monomer will result in homopolymerisation, at the cost of mechanical and other properties. Checking volatiles content in an oven is a simple exercise.
d. Gel time: This indicates the bench life available to the moulder. Even for elevated temperature curing, the gel time is indicative of the speed of cure initiation. The overall speed of cure also depends on the peak exotherm (ibid). Gel time is also indicative of the storage life of the resin. There should be an optimum balance between speed of cure and safe storage life. Gel time tests should be standardised, since it depends on sample size, shape of the gel cup and its material of construction (steel / aluminium / glass).
e. Peak exotherm: This is a measure of the reactivity of the resin. A higher peak exotherm will afford faster cures, but may be unsuitable for large parts (especially for castings) which may tend to crack. The huge heat sink available in SMC / BMC / DMC materials (in the form of the fillers).
f. Acid number: This is a somewhat indirect measure of the molecular weight of the resin (to get a more accurate picture, the hydroxyl number should also be checked, but this is rather a cumbersome procedure). For example, a good isophthalic resin should have a low acid number (~ 6 is ideal) for adequate corrosion resistance. Acid number is critical for SMC / BMC / DMC, because of its direct relationship with the rate of thickening
g. MgO thickening: The residual acid in the UPER reacts with magnesium oxide to produce an apparent increase in molecular weight, resulting in an exponential rise in viscosity. This process is central to the concept of SMC / BMC. The thickening process is influenced by a number of factors including the type of thickening agent, moisture content etc.
h. Refractive index: This is a critical property for Roof Light applications, as well as for clear castings.
i. Mechanical properties: For the structural engineer or the chemical engineer, the critical design parameter for FRP is its flexural modulus. The ultimate tensile elongation is very critical for pultrusion and filament winding.

5. Speciality Grades:

a. Flame Retardant grades can be designed to conform to UL-94 norms (the most widely used set of specifications). There is a growing tendency to use low smoke / zero smoke flame retardant materials. Most flame retardant resins are opaque, but specially formulated resins can produce sheets with significant translucence
b. Roof Light resin is designed in such a way that its refractive index in the cured state matches the resultant refractive index of the fiberglass reinforcement. A good roof light resin should have an UV stabiliser
c. SMC / BMC / DMC resins generally have high peak exotherm, high tensile and flexural modulus.
d. Low Profile grades are designed so that the mouldings do not show the fiber profile, and have acceptable (even paintable) appearance surface
e. Pultrusion resins combine resilience (as indicated by high elongation) with rigidity (high flexural strength), fast cure (high peak exotherm) and heat stability (high HDT)
f. Filament Winding grades have high resilience
g. Putty resins combine resilience with high HDT. They also have high storage life at reasonable curing speed (moderate peak exotherm, long gel time)
h. Isophthalic resins offer good resistance to many corrosive environments, including some acids. They also have good electrical properties
i. Bisphenol A based resins have superior resistance to corrosion, especially alkalies. These resins approach epoxy resins in corrosion resistance
j. Vinyl Esters combine the processing ease of polyesters with the superior toughness, adhesion and corrosion resistance of epoxy resins. In fact the vinyl ester resin chemistry superimposes a polyester character on an epoxy backbone
k. Bonding resins are designed for adhesion to specific substrates like PVC (in chemical process equipment) or Acrylic (in bathtubs and sanitaryware)
l. Gelcoats are special resins with thixotropic property; it is possible to lay up or spray a thick (up to 400 GSM) layer without any reinforcement
m. Low emission resins are designed to reduce styrene emission during the curing process
n. A topcoat is used as the final, off mould layer in chemical process equipment

6. Curing Systems: Polyester resins need a free radical generator for the final crosslinking reaction (curing).

a. MEK Peroxide (~8 – 9% active oxygen) is the popular choice, combined with an accelerator like Cobalt Octoate. It is preferable to use a strength of 1% Cobalt (compared to 3%), since it will reduce dosing errors. It is strongly advised that styrene be used as the diluent for the Cobalt Octoate, since the MTO (mineral turpentine) that is commonly used, is not compatible with the polyester matrix, and will bloom to the surface, creating dirty patches, loss of gloss and reduction in mechanical strength and weatherability
b. Benzoyl Peroxide is a common catalyst at elevated temperatures, for SMC / BMC / DMC, as well as for pultruded sections. BPO + DiMethyl Aniline can be an ambient temperature curing agent, as utilised in putties
c. Bisphenol A resins and Vinyl Esters have sluggish cure rates, and their curing is promoted by DiMethyl Aniline

7. Other Accessories:

a. Pigment pastes must be based on a polyester resin carrier, and not on a phthalate plasticiser like DBP or DOP (same reason as 6a above)
b. The fillers used need to be carefully selected, on the basis of compatibility, wettability, effect on ultimate properties, processability and cost. Average particle size, particle size distribution and moisture content are important specs for fillers
c. PVA (polyvinyl alcohol) is a common release agent for hand laid up parts. Zinc Stearate is used in compression moulding as the mould lubricant
d. Magnesium Oxide is the most common thickening agent
e. It is not advisable to make gelcoats in situ, with ordinary fillers like chalk powder

8. Useful Tips:

a. Postcuring brings out the best in the cured laminate (or even in castings), specially for chemical process equipment and flame retardant applications
b. Resins and accessories should be stored in a cool, dry place. Resins and accessories are flammable, and Peroxides can be explosive
c. Storage of resins in PE containers is not a good idea; its viscosity and gel time will increase

9. Other Resins:

a. Epoxy Resins are quite common in filament winding and pultrusion, where the enhanced performance merits the higher cost                                                                                                   b. Phenolic Resins for even hand lay-up applications are now commercially available. They have exceptional resistance to specific corrosive environments