Studies on the Influence of Monomers on the Performance Properties of Epoxy Acrylate Resin

Twelve blend samples were prepared by physical mixing of epoxy acrylate resins with various monomers viz. ethoxylated phenol monoacrylate (EOPA), tripropylene glycol diacrylate (TPGDA) and trimethylol propane tri acrylate(TMPTA), having weight ratio of epoxy acrylate resin and monomers are 50:50, 60:40, 70:30, 80:20. These samples were cured under UV radiation using 5% photo initiator by weight. These blends were evaluated for mechanical, chemical & thermal properties. It was found that the sample having mono & tri functional monomers shows better properties than the samples having di functional monomer.


Introduction
Photo-initiated curing reactions are powerful processes to quickly create highly cross-linked polymer networks by supplying an appropriate form of energy, generally ultraviolet (UV) light, to a thermosetting material like epoxy, unsaturated polyester, acrylic resins etc., containing photo initiating species [1][2][3][4][5] In recent years, epoxy acrylate prepolymer suitable for use in ultraviolet (UV) curing systems, command the largest use in the market and are typically used in applications ranging from paper and card over-print varnishes, wood coatings, screen and lithographic inks and solder resist inks for printed circuit boards 6-9.Other areas such as vacuum metallizing base coatings, adhesive laminates, release coatings and video disc coatings are also becoming more important 10.Epoxy acrylates are noted for adhesive properties, flexibility, nonyellowing, hardness, and chemical resistance 11-13.The epoxy backbone promotes tough-ness and flexibility to cured films whilst the carbon-carbon and ether bonds of the same improve the chemical resistance.The epoxy acrylate resins have the merits of: (a) high reactivity (b) possibility of structural variation in backbone (e.g.bisphenol A or epoxy novolacs) [14][15][16] which provides good coating properties after curing and (c) excellent adhesion performance (due to the presence of pendant hydroxyl groups).However, the UV cured films based on epoxy diacrylate as oligomer are very brittle and the relative elongation at break is very low 17-19.The epoxy acrylate resin can further be cross linked with different functionality monomers to change or modify its performance.These acrylic monomers are the essential part of every UV/EB formulation.Nevertheless, there are only a few studies published explaining the underlying logic of explaining the use of acrylic monomers.In the past experimental works, trimethylolpropane triacrylate (TMPTA) has been used by various researchers 20,21 .
Therefore, for the first time we tried to prepare a blend by physical mixing of epoxy acrylate resin with mono, di & tri functional monomers and studied the effect by using different concentration of these monomers.

Preparation of epoxy acrylate resin
A general purpose, bisphenol a based epoxy resin (EEW: 180), acrylic acid and triethyl amine were used for the synthesis of epoxy acrylate resin.The epoxy acrylate resin was synthesized by reacting epoxy resin and acrylic acid in 1:1 mole ratio.Tri ethylamine was used as a catalyst.The acid value of prepared resin was 3.0 mg KOH/gm solid as shown in Scheme 1.

Preparation of blends of epoxy acrylate & monomers
Epoxy acrylate resin was physically mixed with different monomers of different concentration with an interval of 10 wt.%.All the samples were designated as shown in Table 1.

Curing of blend samples
The blend samples with 5 wt.% photo initiator [Darocure 1173] were cured in a UV Oven at a speed of 10 meter per minute in atmospheric air with 80 watts /cm mercury vapor bulb.

Preparation of panels
The blend samples were applied on sand blasted steel panels of size 150 × 100 × 1.25 mm with a bar applicator (M/s Sheen Instruments Ltd., U.K.).A dry film thickness of about 10-15 µ was maintained on all the panels.The films were cured as per the cure schedule discussed earlier.

Mechanical properties Adhesion
The adhesion properties of the cured films were evaluated by cross -cut adhesion test method.In this test, parallel cut were made on the coating in two directions to form a series of small squares, originally one hundred of them, 1.25 mm in size.An adhesive tape was applied to the cross cuts, the tape was rolled at the place to assure good adhesion, and then removed with a force perpendicular to the coated substrate.The number of squares removed, gave the numerical value of its adhesiveness.

Impact resistance of cured film
The impact resistance of the cured film samples was evaluated by dropping a hemispherical shaped 2-Lb weight from different heights, ranging from 25 to 15inches (DEF1053 specification).The tests were carried out on the uncoated side of the panels facing the falling weight.

Pencil hardness
As per the specification in ASTM D3363, pencil hardness of the cured films was determined by using a calibrated set of drawing leads ranging from 6B, the softest, to 6H, the hardest.The first pencil that scratches out the coating from the substrate was reported as the coating's hardness.

Chemical properties
The panels were also examined for a visual change in the film by conducting spot test with different chemicals like H 2 SO 4 (10% Sol.), NaOH (10% Sol.) & Toluene etc. for 24h at ambient temperature.

Thermal Properties
The thermal stability of the blend samples was determined by comparing the onset degradation temperature (up to 5% wt.loss) of the cured samples with Thermogravimetric analyzer (TGA) of Universal V3.9A TA instrument, at a heating rate of 20 0 C/min. in nitrogen atmosphere from 50 to 600 0 C. The blend samples with EOPA & TMPTA shows improved adhesion & pencil hardness which is due to the very bulky side group present in EOPA, directly linked to the polymeric network, limiting the freedom of movement of this network.Whereas in TMPTA, the polarity of the linkage group gives more degree of dipolar interaction which leads to better mechanical properties.The TPGDA shows inferior mechanical properties except impact resistance as compared to EOPA & TMPTA.This is due to the higher freedom of movement around the linkage, enabling the stretch with the polarity, giving dipolar interaction which is responsible for mechanical properties.Table 2.The surface properties of the cured film of different blends samples.

Comparative acid, alkali & solvent resistance of epoxy acrylate modified with different monomers & their different concentrations cured under UV -Radiation
Table 3 shows the comparative acid, alkali & solvent resistance of cured film blends.A quick perusal of Table 3 signifies that the coating film prepared with TMPTA offered maximum resistance towards acid, alkali & solvent, as compared to the film of other monomers.This behavior is due to the high reactivity of TMPTA, which results in a more complex structure, resulting better resistance as compared to other samples.

Thermal properties
The system was evaluated for thermal stability in nitrogen atmosphere by thermogravimetric analysis.The thermogravimetric (TG) traces were obtained for samples

Conclusion
From the preceding results & discussions it can be concluded that the films of blend sample containing EOPA & TMPTA shows better adhesion, hardness, chemical resistance & thermal resistance properties as compared to TPGDA.Epoxy acrylate resin containing EOPA & TMPTA can be used in harsh environment such as automotives and it is also suitable for metal coating at ambient temperature.As Epoxy acrylate resin containing TPGDA has high impact resistance so it can be used as an oligomer in UV-cured coatings for plastic instead of flexible urethane acrylate.

Table 2
shows the surface properties of the cured film of different blends samples.The table indicates that except R 1 D 1 , R 1 D 2 , R 1 D 3, and R 1 D 4 , all the blend samples pass through adhesion test.

Table 3 .
The comparative acid, alkali & solvent resistance of cured film blends