An outstanding alternative
The cationic photoinduced polymerization of epoxides and oxetanes is an alternative technology to the well-established free-radical UV/EB curing of acrylates. UV/EB cationic curing offers several advantagescompared with traditional radiation curing:
• Better adhesion on difficult substrates (e.g. metals,plastics, glass and ceramics) thanks to low shrinkage and possible chemical bonding with the substrate.
• A living polymerization: shape forming is easily achievable for expandable or shrinkable systems after radiation exposure. Then full cure can be obtained, which is beneficial for low-migrating systems.
• High curing speed under air (no air inhibition)
• Low odor
• Greater toughness
• Lower viscosity
• Withstands sterilization
• Good barrier and electrical properties

Growing application areas

The advantages of cationic UV/EB systems make them mostly suitable for varnishes and inks for rigid packaging application like metal can ends and rims, flexible packaging like plastic tubes, shrink sleeves and stand-up pouches just to mention a few growing end-use applications.

Dedicated products from Perstorp
Perstorp products are key components for UV/EB cationic systems. The three product groups we offer are:
• TMPO as reactive diluent
• Boltorn® dendritic polyesters polyols as chaintransfer crosslinkers and flexibilizers
• Alkoxylated polyols as chain-transfer crosslinkers and flexibilizers

TMPO – the cationic reactive diluent
The main components of UV/EB cationic formulations are resins, diluents and photoinitiators, as exemplified by 3,4- epoxy cyclohexyl methyl-3,4 epoxy cyclohexane carboxylate as the main resin, and trimethylolpropane oxetane (TMPO) as the reactive diluent.

TMPO is a clear colorless, non skin irritating reactive diluent. It combines increased reactivity with very good diluting power of epoxy resins, and contributes also to increased toughness.

Crosslinking and fine-tuning with polyols
The ring opening of the epoxide and/or the oxetane can be accompanied by crosslinking with hydroxyl functional compounds like polyether polyols and Boltorn® dendritic polymer polyols. These polyols act as chain-transfer crosslinkers and flexibilizers, which can be used up to 30 wt% of the formulation.

Overprint varnishes and coatings
TMPO is a very effective, non-irritant and reactive diluent for cycloaliphatic epoxides. It improves the curing speed by up to 2-times for the same photoinitiator concentration, thereby offering economical advantages. Furthermore, TMPO improves the through cure and increases chemical resistance. It can be used up to 50 % in a formulation. 

TMPO accelerates curing speed
Controlling the properties of coatings and adhesives can be readily achieved by choosing the right combination of polyols. The functionality and nature of the polyols determine end-product properties like flexibility and chemical resistance. Perstorp´s wide range of alkoxylated polyols and Boltorn® aliphatic hyperbranched polyols offers the versatility to control these properties. This ability to obtain the desired end-product properties helps formulators to meet highly demanding requirements like excellent formability for applications such as rim and metal can-end coatings.

Unique UV laminating adhesives
Cationic dark cure offers novel advantages such as laminating opaque substrates under UV radiation, something not achievable by other UV chemistries. A correctly formulated cationic adhesive can be exposed to UV light after being spread on a web immediately prior to nipping two films together.

Flexographic and screen printing inks
Since the early 1990’s, Perstorp has taken the lead in bringing cost-effective dendritic polymers to the market for a number of applications. Our dendritic polymers are sold under the Boltorn® trademark. Now we have started to modify the structure of our dendritic polymers to meet the needs of cationic radiation curing, thereby offering new opportunities to formulators and end users.

Boltorn® H2004 improves ink flow
One example is Boltorn® H2004 dendritic polyol, which not only offers the general advantages of polyols in cationic systems like improved flexibility, but also further improves the chemical resistance and rheological behavior of flexographic inks while maintaining high curing speed. The ink transfer can be improved at high speed thanks to the better Newtonion behaviour of printing inks containing Boltorn® H2004.

Boltorn® H2003 for higher crosslinking density
Using Boltorn® H2003 is another possibility which enables excellent flow and provides a higher crosslinking density than Boltorn® H2004. Highly pigmented screen printing inks can easier be obtained while maintaining very good flow rate and high gloss.

For more details – take a look at our new application brochure on Cationic radiation curing >>