The Effect of Reactives Diluents to the Physical Properties of Acrylated Palm Oil Based Polyurethane Coatings

The development of polyurethane with hydroxyl access in a molecule leads to a new alternative of low toxicity green product. Palm oil is one of the major commodities in Malaysia. The potential of palm oil to be used as coatings raw material such as alkyd is limited due to low unsaturated side on fatty acid chains. To overcome this limitation, palm oil was modified through transesterification process to produce polyol. Acrylated isocyanate (urethane oligomer) was then grafted onto polyol to produce polyurethane with vinylic ends. The polyurethane was formulated with different cross-linkers (reactive diluents) and cured under UV radiation. The effect of three different diluents; monoacrylate, diacrylate and triacrylate on the properties of cured polymer were studied in this research. Fourier Transform Infrared (FTIR), Hydroxyl Value Titration, Gel Content, and Volatile Organic Compound (VOC) were used for characterization. Physical testing performed were Pencil Hardness and Pull-Off Adhesion test. Novel palm oil-based polyurethane coatings have been found to have good properties with mono acrylate functionality.


Introduction
Polyurethanes are versatile class of polymers with variety of applications, because their properties can easily be modified by varying the components from which they are constructed [1].Polyurethanes have been used in adhesive and coating industry for a long time.Since the conventional polyurethanes are petroleum based, the utilization of renewable resources to produce environmental friendly coatings is gaining popularity [2].Oils and fats of vegetables and animals can be utilized as alternative raw materials.They offer huge number of possibilities in chemistry which rarely can be met by petrochemicals [3].Vegetable oils contain unsaturation on fatty acids bond which have potential in synthesis of biopolymers [4].
This research, palm oil was used as the renewable raw material.Unfortunately, its limitation was due to low unsaturation on fatty acid chains [5].To overcome this limitation, palm oil was modified through transesterification process with glycerine to produce polyol (Scheme 1).Glycerine used in this process contributed to primary and secondary hydroxyl groups on the polyol.This is an effort to have alternative hydroxyl functionality similar to castor oil.Isocyanate is one of the basic components a Corresponding author : munirah591@perlis.uitm.edu.my

Instrumentation
FTIR spectroscopic measurements using KBr pellets were carried out using Perkin-Elmer IR spectrometer and the samples were cured under Fusion UV radiation curing machine (80 kW) lamp.

Synthesis
The process began with transesterification reaction of palm oil to produce polyols.The transesterification process was done with the access of one mole triglyceride, 4 mole of glycerine and 6 mole of methanol.5% PTSA was used as catalyst.Palm oil, glycerin and methanol were charged into 500 mL multi-neck reaction flask equipped with a stirrer, thermometer, water cooled condenser, and nitrogen inlet.Catalyst was added after the reaction temperature reached 50°C and stir rate was set to 550 -650 rpm.Hydroxyl value test was done during the reaction to obtain the optimum hydroxyl value of polyol from palm oil.
The formation of urethane oligomer was done by charging one mole IPDI and one mole HEA into multi-neck reaction flask.1% DBTL was used as catalyst and 0.5% hydroquinone was added to inhibit further reaction.FTIR was used to monitor the progress of urethane oligomer formation.Polyol from palm oil was then physically blended with urethane oligomer to form polyurethane resin.The reactions occur at room temperature and the resin was characterized by FTIR. 3 types of reactive diluents were used as cross-linkers; monoacrylate, diacrylate and triacrylate.Polyurethane was mixed well with reactive diluents, 3% of photoinitiator (Irgacure 500) and 1% of film formation agents (NMP).The ratio between polyurethane resin and reactive diluents were varied from 50% to 100%.The coatings were applied on the glass substrates as thin films (40μm) by using hand coater and cured under Fusion UV radiation light conveyer at constant speed 3.2 m/min, 80 kW bulb with 15 cm light intensity with one pass.

Physical testing
Gel content test was done on the cured samples.The cured films were carefully peeled off from the glass substrates and then weighed.A known weight of the cured films was extracted in acetone at 32°C for 24 hours.The residues were dried at 50°C to a constant weight.The gel content of the cured film was then determined by the following equation.

Gel Content
where W t is the weight after extraction and W o is the weight before extraction.Volatile organic compound test was conducted to determine the amount of volatile compounds in cured samples.These compounds have low boiling point and high vapor pressure.The cured samples were weighed and put under 120°C for 2 hours.After 2 hours samples were taken out and weighed.The percent of volatile organic compounds was calculated by following equation : where W i is initial weight and W f is final weight.
The hardness test was done by pencil hardness and referring to ASTM D3363 as standard method.This test uses special pencils with different degrees of hardness to scratch the coating as indication of hardness.The amount of pressure and the angle of the pencil to the substrate affect the results.By referring to ASTM D3363 standard method, pull-off adhesion test was done using a portable Defelsco posi test MP 2571 adhesion tester.A load was increasingly applied to the surface until the dolly was pulled off.The force required to pull the dolly off yielded the tensile strength in MegaPascals (MPa).

Synthesis and characterization test
Three different types of acrylates (reactive diluents/cross-linkers) were used in this research: butyl acrylate, tripropyleneglycol diacrylate (TPGDA), and trimethylolpropane triacrylate (TMPTA) with acrylate functionality of one, two, and three, respectively.In theory, more acrylates should translate to better physical properties due to the higher cross-linking density.Hydroxyl value of polyol obtained from palm oil was calculated to be 1331.12mg NaOH/g using hydroxyl value titration method.Higher hydroxyl values indicate better properties by the polyurethane resin, because more urethane linkages can be formed.FTIR spectra showed the formation of polyol from palm oil.The formation of hydroxyl in polyol was detected at 3307 cm -1 (Figure 1).The formation of urethane oligomer was determined by FTIR spectroscopy.The presence of isocyanate and hydroxyl was detected at 2260 cm -1 and a broad peak at 3200 cm -1 , respectively.The spectrum shows the peak at three time interval (1 st , 2 nd and 3 rd ).By measuring the peak area of NCO peak, it was possible to give an estimate of the degree of urethane formation.Intensity of absorption bands at 1709 cm -1 (C=O) and 1120 (C-O) shows the present of urethane linkage.The hydrogenbonded NH peaks can also be observed at 3200 cm -1 .However this peak overlapped with hydroxyl broad peak as shown in Figure 2. The process was stopped when half of the initial NCO contents were eliminated.The remaining NCO groups were used in the formation process of acrylated polyurethane.Acrylated polyurethane was synthesized by mixing the polyol from palm oil with the urethane oligomer.The full formation of acrylated polyurethane took about 9 hours as indicated by the NCO peak at 2260 cm -1 (Figure 3).The peak intensity for isocyanate decreased during the reaction and totally depleted at the end of the reaction as shown in Figure 4.The formation of acrylated polyurethane was further judged through the reduction of peak intensity of OH at 3300-3400 cm -1 .

Gel content test
Gel content values obtained from the test can be used to predict the degree of cross-linking after UV curing process because the gel content is proportional to degree of cross-linking.The measurements indicate the amount of insoluble material in a cross-linked sample.The higher the percentage of gel content, the higher the degree of cross-linking.Gel content of control sample was found to be 83%.Degree of cross-linking in cured coatings can be varied by adding the different types of reactive diluents; acrylates and varying the ratio between resin and acrylates.The percentages obtained were relatively high, indicating that the coatings were highly cross-linked material.When acrylates were added into the formulation, the acrylates cross-linked the long polyurethane chains together which altered the chemical and physical characteristics.

Volatile organic compound (VOC) test
Volatile organic compound usually refers to solvent content in the system.The lower the amount of VOC in the coating, the safer it is to be used.The present of VOC will increase the degree of flammability of the coating and also increase the health risk associated with application of the coating.Therefore, there is a maximum level of VOC emission fixed for certain applications.According to the result of VOC test, usage of TPGDA gives relatively high percentage of VOC as compared to another two acrylates; butyl acrylate and TMPTA.When it has less functionality, the acrylate was diluted with more solvent to achieve the volume needed.These lead to faster curing due to solvent evaporation and lead to lower solid content.

Pencil hardness
Hardness is the resistance of a coating to indentation or scratching.'H' stands for hardness and 'B' stands for blackness.The hardness of these coatings was found to be between B and 2B.It shows that the coating has a soft feeling surface compared to conventional polyurethane which range from H to 2H.It also been influence by fewer airborne impurities which have time to contact the wet film and lead to softness.

Pull-off adhesion test
Pull-off adhesion test indicates the coating resistance towards the applied perpendicular tensile force when the coatings are forced to be separated from the substrate.According to Figure 6, it shows butyl acrylate gave relatively high coating resistance towards applied force as compared to TPGDA and TMPTA.TPGDA and TMPTA caused higher cross-linking density, which in turn contributed to more rigid coatings.Rigid coatings have higher cohesion between themselves rather than adhesion to the substrates.types of acrylate loading.

Figure 4 . 5 .
Figure 4. Gel content of cured film with Figure 5. VOC of cured films with different different types of acrylate loading.types of acrylate loading.

Table 1 .
Effect of types of acrylates on coating properties.