Photo-curing of Resins (Cure on demand)
Light curing resins can be processed either as pure resins or together with fillers and/or fibrous reinforcing agents. Glass fibers because of their low cost, excellent properties and their transmission of light are the most common reinforcement used. Sunrez light cure technology provides a method to cure resins with ultraviolet (UV-A) light which is a component of natural sunlight. The resin is compounded with a photo-initiator system, which, on exposure to UV-A light, initiates the gelling and curing process. This process takes place much more quickly than conventional peroxide curing, usually with 3-5 minutes.
One of the many advantages of photo-initiated resin curing is the pot life and processing time can be as long or as short as desired, and the curing process can be started at any time by simply switching on the curing lamps. The Sunrez photo-curing technology is based on a one component system which has many advantages over conventional peroxide curing of unsaturated resin depending on the particular application, including:
- no metering and mixing of resins, catalysts, and accelerators required
- no premature gelling of resins as is common in peroxide cure
- can reuse photo-initiated resin which drains off part during fabrication
- greatly reduced clean-up (time and materials) of molds and equipment
- greatly reduced waste generation and disposal cost
- significantly reduced styrene emissions during photo-curing
- no storage and handling of dangerous peroxides
- curing time is not a function of temperature (constant from summer to winter)
Conventional Peroxide Curing
Conventionally, unsaturated polyester resins are converted from a liquid to a three dimensionally cross-linked solid state by adding curing catalysts. As a rule, these are organic peroxides used in conjunction with metal salt solutions, typically cobalt accelerators. By appropriately adjusting the type and amount of curing catalysts and accelerator, the gel time and pot life of the resin can be modified to meet the processing requirements of the part being fabricated. Part thickness can play a significant role in the cure time of the part since it affects the exotherm temperature. Other factors that affect cure when a peroxide catalyst is used are the ambient temperature during processing, and the materials of construction of the mold, since all of these alter the amount of exotherm released and retained during cure. The subsequent curing time or time to demold is several times longer than the gelling time, normally three to four times as long.
Advantages of UV-A Light
The unique light curing technology developed by Sunrez uses a photo-initiator system that absorbs light radiation between 340 and 410 nanometers (nm). UV-A ultraviolet lamps which do not emit the short wave radiation (UV-B and UV-C) can be used. Thus, all the disadvantages resulting from very high energy short wave ultraviolet such as irritation and burning of skin and eyes are prevented. Commercial lamps, such as those for sun tanning, can be used. The, best results so far have been achieved with a Sunrez 400 Watt lamp. Excellent results are also achieved with direct sunlight as a "natural lamp". On the other hand the resin remains processable for several hours in areas lit only by filament lamps or fluorescent lamps and free from direct sunlight.
Photo-curing is possible wherever the material is applied, either thinly on a single surface or on a mold surface as a thick laminate, as long as the light rays are able to reach through the thickness of the resin. The method can be used with laminates made with hand layup, fiber spraying (chop), filament winding, or resin infusion. High glass contents are no obstacle. Aluminum hydroxide can also be added as filler in proportions of up to 180%, depending on the resin type. We have demonstrated on very large parts that laminates of up to about 20 mm (3/4"+) thickness can be readily cured in a single pass.
Light Cure Technology in Your Shop
Sunrez developed photo-initiator systems are added to specially formulated versions of a resin which is then shipped in drums or tanker to your shop. The resin is drawn off and used as received. No additional catalysts are added. A true one part resin system. Parts are fabricated normally and any excess resin is filtered and saved for use on the next part. When the fabricator feels that the part is completed, the part is exposed to UV-A light, and cured for several minutes - very thick parts a little more, thin parts a little less. The finished part is removed from the fabrication area and the next part started. For laminations via hand lay-up, the brushes and rollers need to be cleaned only at the end of the day.