Curing

Atlac high performance resins can be cured with a wide variety of peroxides and accelerators. Cobalt and/or amine accelerators have to be added by the fabricator for curing at room temperature of non pre-accelerated Atlac resin types. The proper choice of the curing system depends to a large extent on the application technique and the requirements of the final product. Gel times varying from 2 to 200 minutes can be obtained with the correct choice of curing system.

All types of polyester, vinyl ester and bisphenol A resins can be cured with similar systems, using readily available materials. Catalysts (peroxides / hardeners) for un-saturated polyesters are unstable, energy rich molecules which decompose into highly reactive molecule fractions - defined as radicals - under the influence of heat, metal salts and amines (accelerators), or ultra violet light. These radicals are capable of reacting with the polyester or styrene molecule, forming new radicals, and starting of a chain reaction.

Polyester, vinyl ester and similar resin systems can be, and often are, cured at ambient temperature. The chemical reaction provoked by the catalysts and accelerators creates an exothermic (heat generating) reaction that promotes optimum cure. However control of the temperature is critical to avoid de-lamination caused by stress and shrinkage from excessive temperatures.

Thick laminates
Fabrication of thick laminates easily results in overheating during cure and a tendency to warpage due to thermally induced stresses and strains. But exotherm temperatures are very much resin dependant.

Sometimes it is necessary to control the heat build-up in laminates, for example when flat sheets (minimum warpage) or very thick sections are laid up in one operation. The fabrication of flanges, in particular, can be critical when a combination of relatively short gel times and low exotherm (minimum shrinkage) are required. Compared with the traditional Atlacs, Atlac E-Nova FW 1045 and FW 2045 can be more easily cured fulfilling these conditions. For specific resin related curing systems, the expertise centre should be contacted.

Post-curing
Post-curing is necessary to obtain the optimum heat and chemical resistance of the Atlac high performance resins. Recommended post-cure conditions are 3 to 6 hours at 90 to 100°C - longer times and adjusted post-cure schedules being required for thicker laminates and/ or more complex shapes. Lower temperatures are ineffective; higher temperatures can lead to embrittlement. Laminates must be at least 24 hours old before post-curing. Laminates up to one year old can be post-cured successfully. It is strongly recommended for glass fibre reinforced parts that are exposed to a chemical environment. Chemical resistance of parts or laminates from Atlac resins that have been post-cured at temperatures below 90-100°C have to be tested according to the requirements of the specific application. The glass transition temperature (Tg) strongly depends on the temperature at which post-curing is carried out.

Curing reagents and additives
The gel time can be changed by varying peroxide levels, cobalt additions or the use of inhibitors. If cobalt levels are too low, this can lead to poor cure at low workshop temperatures. Different cure systems for the Atlac resins are given for the required gel times at various ambient processing temperatures (see resin specific technical information brochure). When faster gel and cycle times are needed or thin sections have to be cured, the methylethyl ketone peroxide types (MEKP) may be substituted for acetyl acetone peroxide (AAP). Unsaturated polyester and vinyl ester urethane resins can be cured with general purpose medium active MEKP, resulting in a wide range of gel times. The addition of MEKP to vinyl ester resins result in an initial foaming and in thick laminates MEKP cure systems may lead to overheating during cure and warpage of the laminate. However, the MEKP systems are particularly effective at lower temperatures. For longer gel times, MEKP systems can easily be inhibited. Cumene hydroperoxide (CuHP) systems for vinyl esters are preferred for many applications due to the absence of foaming after the addition of peroxide. CuHP systems allow a wide choice of gel times followed by well-controlled cure. This enables relatively thick laminates to be made in one go, reducing the risk of overheating and warpage.

Peroxides

Methyl Ethyl Ketone Peroxide (MEKP) – Medium Activity
This is a colorless liquid, usually supplied at a 50% concentration in a phlegmatizing solution. This is the most common peroxide and the levels added to the resin normally range between 1,0% and 2,5%. For Atlac resins, it should be used together with cobalt salts and, when necessary with amines and or inhibitors.

Formula: MEKP – medium active

MEKP is the most widely used catalyst system. It is used with promoters, usually 6% cobalt naphthenate or 6% or 12% cobalt octoate. The MEKP used most often is supplied at 9% active oxygen. Water in the catalyst will adversely affect resin cure, but MEKP can be checked for excessive water content by mixing small amounts with equal parts of styrene. A haze in the mixture indicates excessive water. For optimum results, it is important to maintain the recommended ratio of MEKP to CoNap in the cure system.

Methyl Ethyl Ketone Peroxide (MEKP) – Low Activity
This is a colorless liquid, usually supplied at a 50% concentration in a phlegmatizing solution. It is often used when long gel times are required or when the ambient temperature is high. This MEKP-peroxide is especially recommended for the cure of vinyl ester resins, because it gives less foaming. The gassing is observed starting immediately after the peroxide and the accelerator has been mixed in. This “gassing” is oxygen evolved by the decomposition of the H2O2 present in the peroxide formulation. Low Active MEKP contains less hydrogen peroxide than the medium Active MEKP, and hence gives less oxygen.

Formula: MEKP - low active

It should be noted that this decomposition of hydrogen peroxide without resulting in a gelation of the resin is also the reason why Acetyl Aceton Peroxide (AAP) cannot be used for the curing of standard vinylester resins.  The gassing can be a real problem as no time is available for degassing. This will result in oxygen inclusions and micro porosity in the molding. The E-Nova Technology combines the easy processing of polyester with the chemical resistance of vinyl esters and low foam curing is possible with standard MEKP peroxides.

Cumene Hydroperoxide (CHP)
CHP is a clear liquid. The use of Cumene Hydroperoxide (CHP) can eliminate the foaming experienced with traditional epoxy vinyl ester resins (Atlac 430 and 590) catalyzed with MEKP/cobalt naphthenate catalyzed systems. Another advantage of these systems is that peak exotherms are lowered resulting in less shrinkage, and less warpage. In cool weather, a small amount of dimethylaniline may be used to accelerate cure. Care must be taken to ensure that a thorough cure is obtained, particularly at ambient temperatures. A post cure is recommended to ensure a thorough cure.

Formula Cumene Hydroperoxide

Benzoyl Peroxide (BPO)
Dibenzoyl Peroxide is available on the market in powder, emulsion and paste forms. In combination with amine accelerators it shows a very fast cure, which is hardly influenced by humidity and fillers. Even at low temperatures a relatively good cure will be obtained.

Formula BPO

BPO – amine systems may cause higher exotherm temperatures, and are more difficult to fully post cure. However, in applications were hypochlorite or peroxides are present, BPO/amine curing is recommended. In these cases cobalt (metals) do have a detrimental effect on the chemical resistance performance.

NOTE: The promoter should never be mixed directly with a peroxide catalyst (such as MEKP). Mixing would cause a violent reaction, and a fire or explosion could result.

Accelerators and Promoters
Promoters and accelerators are used to speed up and enhance the cure.

Cobalt Octoate / naphthenate
Cobalt solutions are blue or purple liquids and are available on the market with different percentages of active cobalt that can be used with MEKP and CHP curing systems. Dilution in styrene will prevent small particles of cobalt from forming and will facilitate uniform mixing. Other cobalt accelerators can be used, such as Naphtenate and Versatate, but these both have low reactivity. Furthermore when stored for a long periods in unsatisfactory conditions, they lose their reactivity. Please note that selected grades of Cobalt Naphthenate are acceptable according to FDA regulations.

Dimethylaniline (DMA)
Dimethylaniline is a yellow amine liquid with a strong odor. DMA can be used with MEKP, BPO (ambient cure), and CHP catalyst systems. The addition of DMA is not required with MEKP and CHP systems. However, small amounts of DMA may be used in conjunction with cobalt to improve Barcol development and/or shorten the cure time. With ambient temperature BPO systems, the addition of DMA is required.

Inhibitors
Inhibitors are used to lengthen the gel time of vinyl ester and polyester resins to give a controllable cure. The most widely available is a 10% solution of tertiary butyl catechol (TBC). Inhibitors should be used with care as additions above 0,25% can lead to undercure, low Barcol, or reduced corrosion resistance. Recommended inhibitor levels vary from type to type and from resin to resin. A general guide for addition levels is up to 0.30% of a 10% solution. Some common inhibitors include tertiary butyl catechol (TBC), hydroquinone (HQ), and toluhydroquinone (THQ). Tert-butyl catechol is not effective with cumene hydroperoxyde systems.

UV stabilizers
A five-year study conducted on laminates made from Atlac resins, showed that little or no degradation occurred. If an ultraviolet absorber is deemed to be necessary, either an additional level of 0.2% throughout the laminating resin or 0.2% to 0.5% in the topcoat is effective. Recommended UV absorbers are Tinuvin 320 and Cyanosorb UV5411 (Tinuvin - Ciba Geigy, Cyanosorb - Cyanamid).

Electrically conductive materials
The creation or improvement of the electrical conductivity of a composite is generally achieved by the introduction of carbon, in some form, into the laminate. This can be achieved by incorporating a carbon based veil - one or more carbon fibres into the band of reinforcement on a filament winding machine - or by incorporating carbon (or graphite) in powder form into a resin before lamination. Once the level of conductivity required has been met, the pipe system or other equipment must be satisfactorily earthed.

Abrasion resistant additives
Abrasion resistance in corrosion resistant composite material is generally required for equipment handling slurries or solids in suspension, which would generally erode or abrade standard corrosion resistant materials.

Typical additives that may be included within internal and/or external barriers, or throughout the total thickness of a laminate, are usually based upon various forms of aluminium oxide or silicon carbide (SiC). Secondary fillers or additives are generally required to achieve a satisfactory material dispersion and resin viscosity. The characteristics of abrasion vary immensely from one application to another.

Flame or fire retardant additives
Antimony pentoxide may be used with some resin systems, and alumina trihydrate will improve the fire resistance of both halogenated and non-halogenated resins. In the case of alumina trihydrate, the high level of filler required may have negative effects on corrosion resistance, mechanical properties and general handling properties of the resin.