Unsaturated polyester resins (UP resins) belong to the duroplast family of plastics. They can not be re-formed once they have hardened, are insoluble, cannot be melted or welded and will char at high temperatures. Cured polyester resins are highly transparent but are only UV resistant when they contain the requisite additives – as is the case with those we have on offer. Polyester casting resins can be successfully coloured using dyes for synthetic resins.
Cold curing of the UP resins requires the addition of a peroxide hardener (benzoyl peroxide or ketone peroxide: e.g. MEKP) and small amounts of an accelerator in order to initiate the chemical reaction. The resins that we have on offer have been pre-accelerated at the factory with a cobalt accelerator.
These so-called unsaturated polyester resins (UP) are the right choice anytime transparent bodies need to be created – that means not only for casting seahorses in synthetic blocks but for model making as well. Among the disadvantages associated with this material is the fact that a certain degree of shrinkage (approximately 7 to 10%) must be taken into account when working with it. The most common use of UP resins is in the production of glass fibre reinforced plastic.
Polyester resins must be kept closed when being stored in a cool place protected from light. They can be stored between 3 and 6 months at room temperature (preferably colder) in the closed original packaging.
Because of the styrene that it contains, polyester casting resin is inflammable and hazardous to your health. The MEKP hardeners work as fire accelerators and are caustic. There must be good ventilation at the workstation and contact with skin and eyes should be avoided at all costs. Wearing chemical resistant GLOVES and protective glasses are therefore highly recommended. Furthermore, a barrier cream is recommended for the cleaning and care of your skin. Beyond that, polyester casting resin is hazardous to water and must not be allowed to enter into the plumbing, into bodies of water or the soil.
Please take note of the dangers and the safety information on the package and the information under the title “Safety at the Workplace” found under “Casting Resins”. We will be glad to send you safety data sheets upon request.
Treatment: In the case of polyester resins, the mixing ratio is dependent on the amount being used. The small amount of hardener required is most accurately calculated as a percentage of volume and dispensed by employing a pipette or a disposable syringe. The working time and the de-moulding time are also determined by the amount of hardener used. The relevant data can be found at the individual products. If the resin is worked with in low temperatures (but not less that 16 °C!), the amount of hardener can be slightly increased. All in all, it is advisable not to try to force the curing process through the addition of large amounts of hardener because that can lead to stresses or even cracks in the material and to a yellowing of the material to boot. Amounts of hardener larger than 5% will make the resin soft.
Metallic, inorganic or organic objects can be used for the purposes of embedding as long as they are completely dry. Moisture – or also older MEKP hardener – will cause striations to form in the material. Casting moulds can be made from materials that have smooth, polished surfaces (glass, metal, plastic). Transparent moulds allow one to check on the development of bubbles during the casting process. Moulds that are made from non-releasing materials must definitely be treated with a release agent beforehand. Silicone moulds and PP or PE moulds will release themselves from the moulded piece. But it should be noted that even with these moulds, some adhesive residue could occur. The following can help in these instances:
Large volumes of polyester casting resin can be used at one time but remember that there is the disadvantage that is has much more shrinkage when hardened (7 to 10%!) (as compared to PUR casting resin). Large blocks should not be poured all at once because the resulting heat from the reaction can create a high degree of tension in the material and thereby cause cracks. It is much better to do the work in many stages of 2 cm thick layers.
Embedding of objects is done in the following manner: first a layer of resin should be poured on the bottom of the mould to form a base and then be allowed to begin curing – the resin becomes a gel. The dry object should them be placed upon this layer whereby it will no longer sink into it. In order to be able to better monitor and remove any bubble inclusions, 10 to 20 mm thick layers should then be poured one after the other. Each layer should be added after the heat from the reaction occurring in the previous layer has subsided. As it hardens, the moulded piece usually separates itself from the wall of the mould by virtue of the shrinkage that is typical for polyester. As a result, it is possible that, especially in the case of smaller objects, a small amount of sticky residue may remain on the surface. This is caused by contact with oxygen in the air. This smudgy, incompletely hardened film-like layer can be removed by means of wet sandpaper with 500 to 700 grit or by washing it away with acetone.
Similarly, when a mirror smooth surface is the goal, in any area where the resin is subject to contact with the air it must be covered up before it begins to gel but after all the air bubbles have risen to the top. For this purpose, glossy polypropylene or hostaphan film will serve you well. If the cast piece is not covered as described, the surface can be somewhat gluey after curing but that can be dealt with as described above.
Exhaust gases that have become concentrated in air spaces like bubbles or have simply developed under the covering film during the hardening process of UP resins can have a negative effect on the curing process. Polyester resins should therefore be poured as free of bubbles as possible and de-moulded or, alternatively, have the covering film removed as quickly as possible after the cessation of the reaction heat!
In order to attain a high degree of hardening, objects that were produced at room temperature can be further hardened by exposing them to 80 °C heat (post curing). Both hardened UP resin and casting putty have similar mechanical workability. The usual tools used for wood or metal work will also serve to saw, drill and sand both these materials. acrylic polish paste should be used for polishing work. Your sanding work is best done with wet sanding paper in a sequence starting with 150 grit and continuing with 400, 800 and 1200 grit paper. The final polishing should then be done with the above-mentioned acrylic polish paste and a polishing cloth.