Porous epoxy was fabricated using natural rubber latex (NRL) as the void template. In this study, two mixing sequences were selected: epoxy, hardener, and then latex (EHL) and epoxy, latex, and then hardener (ELH). The extraction process was carried out to extract the latex particles from the cured epoxy sample using toluene as extraction medium with ultrasonic technique for 1 hour. The formation of porous structure in epoxy system is dependent essentially on the amount of latex removed from the epoxy matrix. As expected, the density results showed lower values in the porous epoxy in ELH mixing sequences. More porous structure in epoxy was obtained which was proven by the increasing in porosity % which has led to lowering of the value in dielectric constant which is preferred for electronic packaging application. However, it also caused a decrease in flexural strength and modulus.
Recently, porous materials have received increasing attentions in the fields of electronics, photonics, and life science [
Epoxy DER 331 and polyamide A062 were supplied by Euro Chemo-Pharma Sdn. Bhd. Natural rubber latex was supplied by Grtah Hindus Sdn. Bhd. Epoxy DER 331 has an epoxide equivalent weight of 182–192 g/eq. Polyamide A062 as a curing agent has 110 equivalent per H active. Toluene 2,4-diisocyanate was obtained from Merck Schuchardt OHG, Germany, with density of 1.22 g/cm3.
The epoxy resin, hardener, and latex were added according to 100 phr, 60 phr, and 0.5 phr, respectively. The latex content was fixed from 0.5 to 2.0 phr. The first series was followed by the sequences of epoxy, hardener, and then latex (EHL). The second series was followed by the sequences of epoxy, latex, and then hardener (ELH). Epoxy resin and hardener were stirred using mechanical stirrer and then latex was added using dropper until homogenous. After that, the mixture was put in container and placed in oven for curing at 100°C for 1 hour.
The samples were immersed in toluene using ultrasonic technique for 60 minutes. Porous structure formed once the latex was extracted out.
The morphology of the fracture surface was observed using scanning electron microscope (SEM) model JSM-6460LA. The cross section of the SEM specimen was sputter coated with a thin layer of palladiums using the auto fine coater, model JEOL JFC 1600.
The relative density and total pore volume of the porous structure were determined using ACCUPYC II 1340 Gas Displacement Pycnometer according to ASTM D6226.
Based on density data, the porosity was determined using
The flexural test was done according to ASTM D790 using a universal testing machine Instron 5560. The test was done at a crosshead speed of 5 mm/min. The specimen was cut to the required dimension of 60 mm × 13 mm × 3 mm.
The dielectric measurement was performed using impedance gain/phase analyzer (Solartron 1260) in the frequency range of 1 kHz to 1 MHz at room temperature:
Figure
Porosity % of porous epoxy extracted by toluene using mixing sequence of EHL and ELH, respectively.
Figure
Density of porous epoxy extracted by toluene using mixing sequence of EHL and ELH, respectively.
Figure
The flexural strength of porous epoxy extracted by toluene using mixing sequence of EHL and ELH, respectively.
Figure
SEM micrograph showing the flexural fracture surface morphology of porous epoxy at 2.0 phr latex in mixing sequences: (a) epoxy, latex, and then hardener (ELH) and (b) epoxy, hardener, and then latex (EHL), respectively.
The flexural modulus of porous epoxy is shown in Figure
The flexural modulus of porous epoxy extracted by toluene using mixing sequence of EHL and ELH, respectively.
Figures
Dielectric constant value of porous epoxy at 103 and 106.
Mixing sequence | 103 Hz | 106 Hz |
---|---|---|
ELH | 36.75 | 5.01 |
EHL | 48.56 | 8.59 |
The dielectric constant of porous epoxy for epoxy, latex, and hardener (ELH) mixing by ultrasonic technique.
The value of dielectric constant of porous epoxy for epoxy, hardener, and latex (EHL) mixing by ultrasonic technique.
Porous epoxy thermosets were successfully prepared and compared between mixing sequences of epoxy/latex/hardener (ELH) and epoxy/hardener/latex (EHL) using toluene as extraction medium with ultrasonic technique. Overall ELH mixing sequences showed the higher porosity as compared to EHL mixing sequences and it can be concluded that ELH is the better mixing sequence in forming the porous epoxy. As the latex content increased from 0 phr to 2.0 phr, the microstructure of the epoxy systems changed from dense to porous. Both the flexural strength and modulus for porous epoxy decreased with increasing the latex content. This was due to the increase in porosity % in the porous epoxy and correlated well with morphological observations obtained by SEM. Lastly, dielectric constant value for porous epoxy was found to decrease with decrease in the density of material.
The authors declare that there is no conflict of interests regarding the publication of this paper.
The financial support of Fundamental Research Grant Scheme (FRGS) Grant no. 9003-00472 is gratefully acknowledged.