Understanding how monofunctional, difunctional and multifunctional monomers influence the properties of various oligomers is crucial to successful formulating. Monomers greatly affect both performance and physical properties - including viscosity, surface cure, chemical resistance, pendulum hardness, flexibility, and stain resistance. With careful monomer selection, formulators can manipulate the final properties of their coatings, inks and adhesives for optimal performance. A recent study was conducted to determine how different monomer types affect the key properties of several oligomers.

Test Results: Key Performance Properties Surface Cure

Surface cure was determined by evaluating 12-micron films cured on a glass substrate at 4, 8, 16 and 25 meters per minute. Cure was determined by rubbing a paper towel-covered finger over the film surface and looking for surface marring or fibers remaining on the film.

As expected, test results revealed that higher functionality monomers yield better surface cure rates regardless of the oligomer tested. The lowest functionality oligomer tested did not cure well even at 5 meters per minute, regardless of the monomer. The 3-mole ethoxylated trimethylolpropane triacrylate yielded the fastest cure rates out of all monomers tested, regardless of the oligomer because ethoxylation helps to overcome surface oxygen inhibition. The di-trimethylolpropane tetraacrylate and TMPTA also yield fast cure speeds; although their effectiveness varies with the oligomer.

The tripropylene glycol diacrylate slightly outperforms the propoxylated neopentyl glycol diacrylate in cure speed. Both difunctional monomers have oxygen groups in the backbone to help overcome surface cure inhibition from the presence of oxygen.

Chemical Resistance

Acetone swab testing was performed on films cured at 8 meters/minute/lamp. Acetone is applied to a cloth and rubbed on the films in a circular motion for a minimum of 5 seconds to a maximum of 300 seconds. The number of seconds required to cause wear of the film through to the surface of the substrate is recorded.

Acetone resistance is a function of the crosslink density of the film and the chemical structure of the film's components.

Highly functional monomers exhibit the best acetone resistance regardless of the oligomer. When comparing the trifunctional monomers, the TMPTA outperforms the 3-mole ethoxylated trimethylolpropane triacrylate as a result of its lower molecular weight and resulting higher crosslink density. Also, the 3-mole ethoxylated trimethylolpropane triacrylate is a more polar monomer as a result of the ethoxylation, and is less resistant to the acetone. The difunctional monomers (the tripropylene glycol diacrylate and propoxylated neopentyl glycol diacrylate) produce very similar results, with values lower than more highly functional monomers.

The best performance in comparing the oligomers is seen with the faster curing oligomers. These include: the bisphenol-A-based epoxy diacrylate oligomer, which contains OH groups, cures rapidly, and produces a hard film; the amine modified polyester acrylate oligomer, which contains amine groups and cures very rapidly; and the hexafunctional, aromatic urethane acrylate oligomer. The oligomer with the lowest cure speed and lowest crosslink density upon curing, the difunctional urethane acrylate oligomer, also exhibits the poorest chemical resistance.

Pendulum Hardness

The pendulum hardness of the formulations was measured on 100 micron films cured at 8 meters/min/lamp according to test method ISO 1522 (PERSOZ). The films were exposed to an atmosphere of 50% relative humidity at 23degC for 24 hours.

High functionality monomers yield better pendulum hardness results throughout the test series. The tripropylene glycol diacrylate produces films with slightly higher pendulum hardness values than its difunctional counterpart, propoxylated neopentyl glycol diacrylate.

The bisphenol-A-based epoxy diacrylate oligomer, the epoxy acrylate oligomer, and the hexafunctional, aromatic urethane acrylate oligomer exhibit the best pendulum hardness results.

The results with the ethoxy ethoxy ethyl acrylate monomer in the amine modified polyester acrylate oligomer are better than for any of the other oligomers.

Flexibility

A mandrel bend cylinder using test method ISO 1519 determined the flexibility of the oligomers. One hundred-micron films were cured onto plaques at a speed of 8 meters/minute/lamp using a lamp of 120 watts/centimeter. The films were conditioned for 24 hours in an atmosphere of 50% relative humidity at 23degC.

Flexibility of the hexafunctional, aromatic urethane acrylate oligomer series and the amine modified polyester acrylate oligomer series containing the di-trimethylolpropane tetraacrylate and the TMPTA could not be measured, as these films are too brittle. The flexibility of the films within an oligomer series is inversely proportionate to the monomer functionality. Even though the TMPTA and the 3-mole ethoxylated trimethylolpropane triacrylate are both trifunctional, the 3-mole ethoxylated trimethylolpropane triacrylate, which is ethoxylated, produces more flexible films.

Stain Resistance

In accordance with ISO 4211, glass plaques were coated with 12-micron films and cured at 8 meters/minute/ lamp and subsequently conditioned for 24 hours in an atmosphere of 50% relative humidity and 23degC. Droplets of tea, coffee, water and iodine tincture were pipetted onto the films and covered with a glass cup for 16 hours to prevent evaporation. The films were carefully cleaned with water to remove the droplets and surface discoloration. Discoloration within the film, not on the film surface, is analyzed. Results are recorded as follows.

5. No mark, no surface aspect modification
4. Very slightly marked
3. Slightly marked
2. Highly marked, no surface aspect modification
1. Highly marked, surface aspect modification
It was found that the higher the functionality, the better the stain resistance with a few exceptions. The formulations containing the bisphenol A based epoxy diacrylate oligomer and the amine modified polyester acrylate oligomer exhibited the best stain resistance.

Guidelines for Effective Monomer Selection

Based on the results of this study, coatings, inks and adhesives formulators should no longer limit product selection to oligomers only. Instead, they should also give major consideration to monomer selection. Selecting the appropriate monomer for their particular application enables them to achieve the desired performance and physical properties. Following are a few key findings that formulators can use as guidelines during monomer selection.

Although functionality plays a large role in determining the performance properties of monomers and oligomers, monomers with the same functionality often perform differently.

Hardness, chemical resistance, cure speed, and stain resistance are generally directly related to functionality.

Flexibility is inversely related to functionality.

The 3-mole ethoxylated trimethylolpropane triacrylate, as a result of its ethoxylation, allows fast cure; even faster cure than the tetrafunctional monomer the ditrimethylolpropane tetraacrylate in the oligomers tested.

Acetone resistance and pendulum hardness are maximized with the use of the TMPTA and the ditrimethylolpropane tetraacrylate.

Monomers and Oligomers Tested

Bisphenol-A-based epoxy diacrylate oligomer - Provides a good balance of water properties and high reactivity.

Amine modified polyester acrylate oligomer - For use in UV/EB curing systems. Exhibits good chemical resistance and film hardness, making it suitable for wood and paper coatings.

Hexafunctional, aromatic urethane acrylate oligomer - With excellent cure response, low viscosity, and very high crosslink density.

Difunctional urethane acrylate oligomer - Offers excellent weathering properties coupled with an inherently low viscosity. Additional benefits include an excellent balance of high tensile strength and high elongation, along with a fast cure response.

Ethoxy ethoxy ethyl acrylate - Slightly water dispersible, monofunctional monomer which acts as a reactive diluent.

Propoxylated neopentyl glycol diacrylate - A low viscosity, low skin irritation monomer for use in free radical polymerization.

Tripropylene glycol diacrylate - A low-volatility, low-viscosity monomer for use in free radical polymerization.

Trimethylolpropane triacrylate (TMPTA) - A low viscosity liquid monomer, which offers fast cure response and low volatility during free-radical polymerization.

3-mole ethoxylated trimethylolpropane triacrylate - A low skin irritation, fast curing monomer for use in free radical polymerization.

Di-trimethylolpropane tetraacrylate - A low skin irritation, tetrafunctional monomer offering fast cure response and a high crosslink density upon curing.

NOTE: Formulations tested contain 48% oligomer, 48% monomer and 4% photoinitiator.

For more information on oligomers, contact Sartomer Co., Oaklands Corporate Center, 502 Thomas Jones Way, Exton, PA 19341; phone 800/SARTOMER or 610/363.4100; visit www.sartomer.com; or Circle Number 60.

Links

• Sartomer Co.