Cationic Epoxies – Advantages

Overview

 As discussed in our previous article, cationic and free-radical are the two most popular mechanisms for UV adhesives. Approximately 93% of the UV market is in free radical chemistry  while cationic has about 7%. Although a minority of the market, cationics provide strategic advantages over traditional free-radical acrylate chemistries. These advantages include:

  1. Shadow Cure
  2. Delay Cure
  3. Reduced Shrinkage
  4. No oxygen inhibition
  5. Higher operating temperatures

Shadow Cure

When energy from light is introduced into a cationic system the photoinitiator releases a strong acid. This acid acts as a catalyst starting the polymerization. It is important to note, for the polymerization to occur only the acid is necessary, not the light energy producing the acid. This strong acid has two benefits over a free-radical. The first advantage is that the active life of the acid catalyst is much longer than a free radical. Free radicals’ active life is measured in seconds while the acid can survive for days. The second benefit is that acid catalysts are not consumed in the polymerization reaction whereas free radicals are. These two differences allow cationic reaction to continue curing after UV exposure unlike free-radical reactions.  This is the shadow cure effect. As long as the system has had UV exposure it will continue to cure after the light source is removed.

Delay Cure

The delay cure is a functional use of the shadow cure phenomenon. There are techniques, such as incorporating polyols into epoxy cationic reactions, which play on the shadow cure effect and delay the polymerization of the species. By delaying the polymerization there is a lag time between light exposure and the substance hardening.

Delay cure cationics can provide a unique strategic advantage to manufacturing processes. For example:  a process that requires a one part adhesive to bond two, light blocking substrates. Historically a UV PSA could be used. These free radical adhesives cure instantaneously but when polymerized are partially sticky in nature (think Gluedot or the clear sticky material on the back of a new credit card). Due to the tackiness of these products, they will mate well between two substrates after polymerization. The problem is the PSA will never be a tough polymer. This limits the upper boundary of the adherence strength and may eventually lead to creep. Cationics can solve this problem as the substrates can be mated similarly to a PSA before the adhesive is completely polymerized. Strong controls, however, on UV exposure is paramount so the reaction rate of the cationic is consistent.

Reduced Shrinkage

Large percentages of shrinking can induce stress between bonded substrates. Cationics shrink less than typical free-radical systems. The mechanism behind polymerization causes shrinkage for both free radical and cationic reactions. Unique to cationic reactions, however, is the epoxy ring opening step before molecule to molecule interaction. This step actually lengthens the molecule and can offset the shrinkage caused by polymerization. The ring opening step is unique to cationic epoxy and does not occur in free radical acrylate reactions.

No Oxygen Inhibition

Free radical UV polymerization can be terminated early by oxygen in the air. The oxygen itself will react with the free radical stopping chain initiation and also preventing further chain extension through premature chain termination. This leaves the surface of the material unfinished and tacky. Cationics will not be inhibited in this fashion. This allows them to cure in ambient conditions with better ease than free radical UVs.  It is important to know that humidity can slow the cationic reaction. Relative humidity must be 70% or higher to see this effect. Since the cationic reaction is initiated by an acid, basic molecules or moisture can lessen the effect of the catalyst.  If the moisture level is decreased, however, the reaction will proceed as normal.

Higher Operating Temperatures

Typical free radical acrylate products will not survive temperatures above 120°C and, even if they do, the product could possibly yellow and diminish in physical properties. Cationic epoxies, however, can survive temperatures as high as 200°C. Resin Designs’ Vividcure 86011 is capable of handling extreme operating conditions. Contact us if you have a unique problem that needs a unique solution. Next blog we will discuss UV systems that utilize on a secondary cure mechanism for those applications where UV chemistry is just not enough.

Understanding UV Curing Adhesives

Often referred to as UV Curing, Ultraviolet Curing utilizes high-intensity ultraviolet light to generate a photochemical reaction which instantly polymerizes adhesives, inks and coatings. The main two types of photochemical reactions used are acrylate free radical and cationic epoxy. UV curing differentiates itself from other traditional drying methods due to its increased production speed, reduced reject rates, enhanced scratch and solvent resistance and expedited superior bonding.

Originally, UV curable inks and coatings were used as a substitute to solvent-based products. Traditional heat and air drying methods work by solvent evaporation. This particular method shrinks the application of coatings by 50% and causes environmental pollutants. However, in UV curing, there are no environmental pollutants, no loss of volume or coating thickness, and reduced stress on the substrate due to shrinkage. This means that there is a higher productivity in a shortened period of time with a reduction in waste, energy, use and pollutants.

What Industries Use UV Curing

UV curing was first introduced in the 1960’s and became increasingly popular among several industries including electronics, graphic arts, automotive, telecommunications and more. This process has since transitioned into a multi-billion dollar worldwide industry, containing about 4% of the industrial coatings market. UV curing has increased over 10% per year, expelling traditional water and solvent-based thermal drying processes. Why has UV curing seen such dramatic growth? The UV curing process causes an astounding increase in productivity, enhancement of product quality, as well as several environmentally-friendly traits as no evaporation of product occurs unlike solvent-based products. Our UV products are 100% solids.

How UV Curing Works

UV adhesives are an all-inclusive, single component adhesive solution. The base polymer and photosensitive initiator are all contained in one package. To make the product, liquid monomers and oligomers are mixed with photoinitiators.

There are two different types of photochemical reactions, free radical and cationic. In free radical reaction, energy from light radiation is absorbed by the photoinitiator. The excited photoinitiator releases a free radical that initiates the polymerization. Cationic reactions are slightly different as the energy absorbed releases a positively charged ion instead of a free radical. During the polymerization of a cationic reaction more positively charged ions are released. This allows the cure to occur after light exposure unlike free radical polymerization. This phenomenon is often called “dark curing”.

The Benefits of UV Curing

UV curing offers an array of benefits including the following:

  • Physical properties are improved
  • Easy to automate
  • Production speeds and capacity are much faster
  • Reduced clean-up labor and set-up
  • Minimal emissions; ensuring a safer work place
  • Less floor space required
  • Reduced scrap

Improved Physical Properties

Manufacturers typically consider UV chemistry due to the improved gloss, abrasion resistance, enhanced chemical resistance, and unique control of properties such as hardness, elasticity and adhesion.

Quick Production Speeds

Since the process requires less space, this allows for higher production speeds as well as less direct labor. The decreased down time and higher throughput increases machine utilization and directly impacts plant capacity. Essentially, UV curing provides increased productivity and enhanced plant and equipment efficiency.

Reduced Clean-Up Labor and Set-Up

Since UV chemistries will not cure without UV energy exposure, they can be put out overnight without concern for the ink or coating drying in the machine. This is typically a huge advantage for graphic arts printing presses, plastic decorating machines and coding machines.

Not as Much Floor Space Needed

UV curing processes generally require considerably less floor space than drying ovens.  Several drying processes require longer cure times which require large amount of floor space. For instance, adhesives and potting applications using two-part adhesives have cure times which is measured in days.  When the number of parts is large, more floor space is necessary.

Reduced Scrap

Curing issues are detected instantly because UV polymerization only takes a matter of seconds, increasing inspection and scrap removal efficiency. Regarding painting and coating applications, the reduced time it takes to UV cure removes the chance for dust and particles to pollute the part surface.  During some processes, there may need to be an inspection of the ink, coating or adhesive application before beginning the UV curing process. If there are issues, the ink, coating or adhesive can be removed easily.

Choose Resin Designs Products

The chemists at Resin Designs offer an array of products, often used for encapsulation or bonding. Two of our most popular urethane acrylate products include the Vivid Cure 71151 and Vivid Cure 71141.

The Vivid Cure 71151 is a water white, non-yellowing adhesive used as an encapsulate for optical applications. The Vivid Cure 71141 is another high-performance adhesive featuring PET & RPET Bonding. Both products are RoHS & REACH Compliant.

Resin Designs also specializes in cationic epoxy formulations. These homopolymers polymerize due to acid generation during UV initiation instead of free radical initiation. This allows for high glass transition temperature polymers and shadow curing effects. Vivid Cure 86011 is an example of a cationic epoxy with an operating temperature beyond 120°C. Our next blog will discuss the benefits of using cationic chemistry verses traditional acrylate chemistry for UV adhesives.

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Unlike thermoplastic alternatives such as hot melt adhesives, cured thermosetting epoxies will not re-flow or melt when heated. Instead, epoxies will undergo a transition from a hard rigid state to a more pliable, rubbery state. The temperature range during which this transition takes place is known as the glass transition temperature, Tg.

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At Resin Designs, we help customers create new connectors with low wire insertion forces using silicone gel grommet seals. Along with this, existing connectors can be modified to accept gel seals with slight tooling changes. The goal is to ensure proper initial compression on the gel seal and held in place with latching caps to retain internal pressure within the connector. Read more

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The design of gaskets comes in a nearly unlimited number of component arrangements and material combinations from which they inherit their properties. As a result, gaskets can be classified in various ways ranging from the category of the utilized materials to the function and application of the resulting gaskets.

Gaskets can also be classified according to their manufacturing/ application processes. Specifically, they can fall in one of two categories: Read more

Optimizing Rear Grommet Sealing of Electrical Connectors with Resin Designs GT Gel Pads

Introduction

Resin Designs brand GelTek™ Silicone GT gels are perfect for sealing automotive electrical connectors. However, in regards to sealing applications, the item being sealed and the seal itself are one system which cannot be treated independently. This is also true for silicone gel seals for electrical connectors. In order to achieve sufficient sealing, the seal and the connector must be compatible.

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