2.1 Materials

Dipentaerythritol (DPE), 2,2-bis(hydroxymethyl) propionic acid (DMPA), isophorone diisocyanate (IPDI), p-toluene sulfonic acid (PTSA) and dibutyltin dilaurate (DBTDL) were supplied by Merck- Schuchardt Germany. 2-hydroxyethyl acrylate (2-HEA) was supplied by Aldrich Chemicals Co., USA. Trimethylolpropane triacrylate (TMPTA) was purchased from UCB Chemicals, Belgium and oleic acid (palm oil source) was obtained from Acid Chemical Co., Malaysia. All chemicals were used as received without further purification.

2.2 Synthesis of hyperbranched polyester polyol (HBP)

The first generation hyperbranched polyester polyol (HBP-1) was synthesized by melt polycondensation technique. DPE was used as the core and DMPA as the chain extender. The synthesis was derived from established methods described elsewhere^[2,3,8,9]. The reaction was carried out at 130‒150^oC under nitrogen gas atmosphere in the presence of p-TSA as the catalyst (0.4wt% of DMPA). The reaction was monitored periodically by checking the acid value until an endpoint of below 10 mg KOH/g was achieved. The HBP-1 was then reacted with the palm oil oleic acid at 190‒220°C and labeled as second -generation hyperbranched polyester polyol (HBP-2). The progress of the reaction was monitored by the determination of the hydroxyl values (OHV).

2.3 Synthesis of hyperbranched polyurethane acrylates (HBPUA)

The acrylate terminated urethane hyperbranched prepolymers/resins were prepared by carrying out the reaction in a 250 mL five necked flask equipped with a thermometer, dropping funnel, mechanical stirrer and nitrogen gas inlet. Required amount of IPDI with NCO: OH ratio of 9:1 was reacted with HBP-2 at 40‒75°C for 3 h with the presence of 0.1%‒2.0% DBTDL as the catalyst, as described in our earlier work^[1] NCO terminated hyperbranched polyurethane (PU) prepolymers from HBP-2 was produced. 2-HEA was dropped into the flask with the same ratio of NCO. The synthesis procedure followed was derived from established methods described elsewhere in literature^[5]. Dioxane was added to the flask in order to control the viscosity. The progress of the reaction was monitored by observing the consumption or disappearance of –N=C=O (IPDI) and –OH (HBP-2) groups at approximately 2250‒2270 cm^‒1 and 3400‒3500 cm^‒1 respectively^[1,6,7,8].

2.4 Electron beam (EB) curing of HBPUA – based formulations

The prepolymers (HBPUA) were dissolved in acrylate monomer such as TMPTA to reduce the viscosity. The coating formulations were prepared at least one day prior to curing. The formulated resins were coated on to glass plates using a manual bar coater (RDS 30, Japan), and then irradiated with a 200 keV electron beam machine, Curetron type EBC-200-AA2 (supplied by Nissin High Voltage, Japan) with accelerating voltage of 200 keV, beam current at 2 mA and dose rate of 10 kGy/pass.

2.5 Characterization of the resin

FT IR spectroscopy analysis was carried out using a spectrometer model Perkin Elmer Spectrum 2000. A drop of each sample was spread into a thin layer on the NaCl cells. The samples were scanned at wave numbers ranging from 4000 cm^‒1 to 400 cm^‒1. The molecular weight (M_w) distributions were determined using Polymer Laboratories GPC, Model PL-GPC 50 Plus with PLgel 10 µm Mixed-B column. Tetrahydrofuran, 1.0 mL/min, was used as the mobile phase. Calibration was carried out by linear polystyrene of known molecular weight and dispersity. The viscosity of the synthesized resins was measured at 25^oC with spindle #42 using a Brookfield Viscometer Model RVTDV-IICP. The ¹³C-NMR spectra were recorded with a JEOLECP 400 MHz NMR using dimethylsulfoxide (DMSO-d₆) as a solvent. Standard method ASTM D4274-99 and MPOB Test Method c2.7: 2004 were used to determine hydroxyl values (OHV) and the acid values (AV) of the HBP, respectively.

2.6 Characterization of the cured resins

3.1 HBP-1, HBP-2 and HBPUA characterization

The FTIR spectra of oleic acid, HBP-1, HBP-2 and HBPUA are shown in Fig.1.

HBP-1 resin: FTIR (KBr, cm^‒1): 3200-3600 cm^‒1 (-OH), 2943 cm^‒1 (-CH₃), 2872 cm^‒1 (-CH₂), 1717 cm^‒1 (C=O), 1464 cm^‒1 (aliphatic, -CH₂), 1371 cm^‒1 (aliphatic, -CH₃-), 1240 cm^‒1 (C-O). ¹³C-NMR (300 Hz, DMSO-d₆): δ174.75 ppm (-C-CO-O-), δ70.9 ppm (-C-CH₂-O-), δ65.43 ppm (-CH₂-OH), δ39.5 ppm (-C-C-C-). The presence of these bands indicates the successful reaction between DPE and DMPA to the production of HBP-1. These results confirmed the formation of hyperbranched polyester polyol resin (HBP-1)

HBP-2 resin: FTIR (KBr, cm^‒1): 3200-3600 cm^‒1 (-OH), 2936 cm^‒1 and 2843 cm^‒1 (CH₃-, -CH₂-); 1744 cm^‒1 (-C=O-); 1468 cm^‒1 (-CH₂-, aliphatic) and 1240 cm^‒1 (-COO-, ester). ¹³C-NMR (300 Hz, CDCl₃): δ174.75 ppm (-C-CO-O-), δ70.9 ppm (-C-CH₂-O-), δ65.43 ppm (-CH₂-OH), δ39.5 ppm (-C-C-C-). The hydroxyl value (OHV) of the HBP-2 was 700 mg KOH/g. The presence of these bands indicated the success of the reaction between HBP-1 and oleic acid to produce HBP-2 resin.

HBPUA: FTIR (KBr, cm^‒1): 3300-3360 cm^‒1 (-NH- bending), 1562 cm^‒1 (-CN stretching), 1635 and 810 cm^‒1 (double bonds of acrylate groups). These suggested that both acrylate and amine groups have been successfully bonded to HBP-2 resin to form.

Fig.1Full-screen View

FT IR spectra of oleic acid, HBP-1, HBP-2 and HBPUA.

Figure 2 shows the ¹H-NMR spectra of HBPUA. A peak at 6.4 ppm was assigned to the acrylic group in HBPUA (-NHCO-). The presence of δ5.1‒5.2 ppm peaks were assigned to the hydrogen atom attach to the unsaturated carbon (-CH=CH-). The peaks at 3.8‒4.2 ppm were assigned to the hydrogen atom of methylene connected to ester group (-CH-COO-). These suggested that both acrylate and amine groups have been successfully bonded to HBP-2 resin to form HBPUA. The FT IR and ¹H NMR analyses, confirmed that both acrylate and amine groups have been successfully bonded to HBP-2 resin to form HBPUA.

Fig.2Full-screen View

¹H NMR spectrum of HBPUA.

Table 1 summarized data obtained from gel permeation chromatography analysis, viscosity and hydroxyl value (OHV) tests of HBP-2 and HBPUA.

The low viscosity implied that HBP-2 and HBPUA molecules were less entangled due to the spherical shape. One of the most interesting properties of hyperbranched polymers was their high M_w and low viscosity characteristic in comparison to linear analogue^[3,9]. Low polydispersity index obtained justified these findings.

3.2 Characterization of the cured resins

EB curing of HBPUA: HBPUA contained unsaturated sites which were capable to undergo polymerization through the double bond. A thin layer of HBPUA was applied on a glass plate and polymerized to a solid film upon exposure to EB radiation. The prepolymer HBPUA cured at 100 kGy. Fig.3 showed the FTIR spectra of the cured HBPUA sample and did not show any peak representing acrylate group at 1634 cm^‒1 and 810 cm^‒1 since it has undergone polymerization on exposure to EB radiation.

Fig.3Full-screen View

FT IR spectra of cured HBPUA and uncured HBPUA.

Properties of the cured film: The formulations of HBPUA were developed based on the radiation dose and its effect on the hardness and gel content. These were summarized in Table 2. An increase in the amount of TMPTA from 40% to 60% resulted in an increase in pencil hardness and gel content of the cured films. TMPTA having multifunctional acrylic group formed cross linking with HBPUA producing hard films. Higher reactivity of the acrylates in TMPTA contributing high degree of cross linking and made it possible to use less radiation dose for curing.

The TGA and DTG thermograms of the HBPUA formulations were as shown in Fig.4. The thermal decomposition of HBPUA started at 200^oC with 98% weight loss. The weight loss of HBPUA-1: TMPTA at the ratio of 6:4 was 95% at temperature of 200^oC. Fig.5 showed that thermal stability of HBPUA increased in the presence of TMPTA, i.e. T_max of 450^oC was shifted to a higher temperature i.e. T_max 455^oC. This indicated that the thermal stability of HBPUA improved by TMPTA monomer. The effect was believed to be due to the induced chemical interaction in the form of cross linking.

Figure 6 exhibited the interfacial adhesion between HBPUA/TMPTA and MDF as shown in the scanning electron micrographs (SEM). The morphology showed good interaction between cured film and substract.

Fig.4Full-screen View

TGA and DTG thermograms of HBPUA.

Fig.5Full-screen View

TGA and DTG thermograms of HBPUA/TMPTA.