Comparative Study of Aromatic and Cycloaliphatic Isocyanate Effects On

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Article Comparative Study of Aromatic and Cycloaliphatic Isocyanate Eﬀects on Physico-Chemical Properties of Bio-Based Polyurethane Acrylate Coatings

Nurul Huda Mudri 1,2,*, Luqman Chuah Abdullah 1,3,* , Min Min Aung 3,4 , Mek Zah Salleh 2, Dayang Radiah Awang Biak 1,5 and Marwah Rayung 3,6

1 Department of Chemical and Environmental Engineering, Faculty of Engineering, Universiti Putra Malaysia, Serdang 43400, Selangor, Malaysia; [email protected] 2 Radiation Processing Technology Division, Malaysian Nuclear Agency, Kajang 43000, Selangor, Malaysia; [email protected] 3 Institute of Tropical Forestry and Forest Products (INTROP), Universiti Putra Malaysia, Serdang 43400, Selangor, Malaysia; [email protected] (M.M.A.); [email protected] (M.R.) 4 Centre of Foundation Studies for Agricultural Science, Universiti Putra Malaysia, Serdang 43400, Selangor, Malaysia 5 Institute of Advanced Technology, Universiti Putra Malaysia, Serdang 43000, Selangor, Malaysia 6 Department of Chemistry, Faculty of Science and Technology, Universiti Putra Malaysia, Serdang 43400, Selangor, Malaysia * Correspondence: [email protected] (N.H.M); [email protected] (L.C.A.); Tel.: +60-3-8946-6288 (L.C.A.) Received: 15 May 2020; Accepted: 4 June 2020; Published: 3 July 2020

Abstract: Crude jatropha oil (JO) was modified to form jatropha oil-based polyol (JOL) via two steps in a chemical reaction known as epoxidation and hydroxylation. JOL was then reacted with isocyanates to produce JO-based polyurethane resin. In this study, two types of isocyanates, 2,4-toluene diisocyanate (2,4-TDI) and isophorone diisocyanate (IPDI) were introduced to produce JPUA-TDI and JPUA-IPDI respectively. 2,4-TDI is categorised as an aromatic isocyanate whilst IPDI is known as a cycloaliphatic isocyanate. Both JPUA-TDI and JPUA-IPDI were then end-capped by the acrylate functional group of 2-hydroxyethyl methacrylate (HEMA). The effects of that isocyanate structure were investigated for their physico, chemical and thermal properties. The changes of the functional groups during each synthesis step were monitored by FTIR analysis. The appearance of 1 1 1 urethane peaks was observed at 1532 cm− , 1718 cm− and 3369 cm− while acrylate peaks were 1 1 detected at 815 cm− and 1663 cm− indicating that JPUA was successfully synthesised. It was found that the molar mass of JPUA-TDI was doubled compared to JPUA-IPDI. Each resin showed a similar degradation pattern analysed by thermal gravimetric analysis (TGA). For the mechanical properties, the JPUA-IPDI-based coating formulation exhibited a higher hardness value but poor adhesion compared to the JPUA-TDI-based coating formulation. Both types of jatropha-based polyurethane acrylate may potentially be used in an ultraviolet (UV) curing system specifically for clear coat surface applications to replace dependency on petroleum-based chemicals.

Keywords: jatropha oil; polyurethane acrylate; 2,4-toluene diisocyanate (2,4-TDI); isophorone diisocyanate (IPDI); UV curing

1. Introduction Polyurethane (PUR) is a versatile resin that can be used for various applications such as coatings, adhesives, sealants and elastomers. Basically, PUR is synthesised by reaction of polyols with isocyanates

Polymers 2020, 12, 1494; doi:10.3390/polym12071494 www.mdpi.com/journal/polymers Polymers 2020, 12, 1494 2 of 17 at a specific controlled temperature and reaction speed in a nitrogen purged system. Currently, most of the polyols that are used in synthesising PUR are derived from petroleum and natural gas sources [1,2]. As non-renewable sources, petrochemical resins are facing issues with supply shortages and worldwide price fluctuations. Moreover, the processing of these chemicals may release greenhouse gases that are harmful to the environment, thus leading to global warming. Alternatively, usage of low toxic natural-based polymeric materials has been proposed to replace the dependency on petroleum-based chemicals in order to achieve sustainable development [3,4]. Vegetable oil, cellulose and sugar are among the bio-based feedstock that have been used in producing PUR [1,5]. Between these feedstocks, vegetable oil serves as the most promising option to function as the polyol starting material because of its availability, simple processing method and being cost effective. Moreover, it has a unique characteristic whereby the unsaturated part of the backbone of the triglycerides enables it to undergo chemical modification [3,4]. Several bio-based oil such as soy [5], palm [6], castor [2], canola [2], neem [7], etc., have been studied for PU application. However, these vegetable oils are classified as edible oils and the utilisation of food supplies for a non-food purpose has attracted a negative perception from society. The Jatropha curcas tree is one of the main industrial crops in Malaysia with a total coverage area of 809 ha [8]. It is traditionally used in soap, fertilisers and as an alternative medicine by the local community. The oil is extracted from the seed of the fruit of jatropha with a yield of around 30–40% of oil content. It has a high unsaturated fatty acid content (78–84%) given by oleic acid (C18:1) and linoleic acid (C18:2) which reflects its sensitivity to chemical reaction [9]. Jatropha oil (JO) is a non-edible oil because of its phorbic ester compound that is toxic for oral consumption [10]. Therefore, converting jatropha oil into polyols for the purpose of PU production has no critic related to the edible oil source issue. Previous researchers have investigated the optimum parameters to synthesis jatropha oil-based epoxy [11,12] and polyols resin [13] for coating applications. Several studies have been conducted related to Jatropha oil-based polyurethane acrylate (JPUA) but only limited to Toluene diisocyanates (TDI)-based isocyanates for different applications such as wood coating [14], and a polymer electrolyte [15,16]. A few of isocyanates are commonly used in the coating industry such as 2,4-TDI, 4,40-methylenediphenyl diisocyanate (MDI), 1,6-hexamethylene diisocyanate (HDI), and isophorone diisocyanate (IPDI). Among these isocyanates, TDI is the most reactive because of the existence of its aromatic ring [17]. Diisocyanate is a volatile compound that may affect human health via the inhalation route. Studies of cancer risk have revealed that TDI and MDI are linked to cancer risk with the ability to bind the DNA and lead to genotoxicity [18]. Meanwhile, HDI and IPDI are safer and have been testified as not associated with carcinogenic effects [19,20]. In this study, a jatropha oil-based polyurethane (JPUA) was synthesised based on the reaction between jatropha oil-based polyol (JOL) with 2,4-TDI and IPDI. The effects of the linear chain and aromatic ring of the isocyanates were investigated related to the physicochemical properties of the JPUA coating.

2. Materials and Methods

2.1. Materials Crude jatropha oil was purchased from Biofuel Bionas Malaysia Sdn. Bhd., Malaysia. Hydrogen peroxide (30%), formic acid (98%), sulfuric acid (95%) and methanol (99.8%) were supplied by R&M Chemicals, India. 2,4-TDI (IUPAC name: 2,4-Diisocyanato-1-methylbenzene) (95%), IPDI (IUPAC name: 5-isocyanato-1-(isocyanatomethyl)-1,3,3-trimethylcyclohexane) (98%), dibutyltin dilaurate (IUPAC name: [dibutyl(dodecanoyloxy)stannyl] dodecanoate) (DBTDL) (95%), HEMA (IUPAC name: 2-Hydroxyethyl 2-methylprop-2-enoate) and trimethylolpropane triacrylate (TMPTA) were supplied by Sigma-Aldrich, Germany. N,N-Dimethylformamide (DMF) (98%) and benzophenone (IUPAC Polymers 2020, 12, 1494 3 of 17 name: Diphenylmethanone) were purchased from Fisher Scientiﬁc (USA) and Acros Organic (Belgium) respectively. All chemicals were used as supplied without further puriﬁcation.

2.2. Synthesis of JOL The bio-based JOL was prepared in two steps namely epoxidation and hydroxylation. This method was adopted from [13] with slight modiﬁcation. For epoxidation, the reaction was performed based on a molar ratio of 1:0.6:1.7 for JO: formic acid: hydrogen peroxide. This experimental set-up was designed based on an amount of 400 g of JO per batch. Initially, JO was mixed with formic acid at 300 rpm at 40 ◦C until it became homogenous. Then, hydrogen peroxide was added slowly using a dropping funnel and the temperature was raised to 60 ◦C. In each hour, an oxygen oxirane content test (OOC) was performed based on the ASTM D1652-97 Method A standard. The reaction continued for 4.5 h based on previous literature in order to achieve the maximum value of OOC. The mixture was then cooled and transferred into a separating funnel. The aqueous layer was discarded and the epoxidised jatropha oil (EJO) was washed with distilled water until it achieved a neutral pH level. In the second reaction, a stoichiometric of EJO (10 mol) was reacted with a mixture of methanol (9 mol) and 0.07% diluted sulfuric acid (1 mol). At ﬁrst, methanol in the presence of the acid catalyst was activated by a heating and stirring process under conditions of 40 ◦C, 300 rpm for 15 min. Afterwards, the EJO was added into the solution and continuously mixed for 30 min at 65 ◦C. OOC was conducted to check the disappearance of the oxirane ring during the synthesis process by the titration method. The JOL mixture was cooled and then moved into a separating funnel to remove the aqueous layer. The JOL was washed with distilled water until it was free from acid. The excess water and methanol were removed using a rotary evaporator at a temperature of 60 ◦C. The JOL was then tested for hydroxyl value (OHV) referring to ASTM D4274-99 Test Method C prior to the isocyanation reaction.

2.3. Synthesis of JPUA A corresponding amount of JOL: 2,4-TDI: HEMA was calculated based on a molar ratio of 1:1:1. The reaction was accomplished in a four-neck ﬂask complete with a stirrer and condenser in a nitrogen purged system. First, JOL was mixed with DMF at 60 ◦C with a stirring rate of 350 rpm. Next, 2,4-TDI was added in a dropwise manner within 15 min and the reaction was mixed continuously for 2 h. Then, the temperature was cooled down to 40 ◦C and HEMA was added into the mixture. Later, the temperature was raised up to 70 ◦C for 1 h upon complete termination of the reaction of JPUA-TDI. During the reaction, DMF was added into the mixture to control the viscosity of the solution to avoid the gelling phenomena. The amount of DMF was set to be 50% (w/w) of the total amount of JPUA-TDI. The same procedure was repeated for the IPDI-based JPUA with the same NCO/OH ratio. However, 1% (w/w) of DBTDL was added into the mixture of JOL as a catalyst to initiate the reaction with the IPDI-based isocyanate.

2.4. Formulation and Curing of JPUA In the JPUA-TDI and JPUA-IPDI-coating formulation, TMPTA and benzophenone were selected as a trifunctional acrylate monomer and photoinitiator respectively. The quantity of benzophenone was set at 4% (w/w) of the total amount of the formulation. The amount of each component of the formulation is listed in Tables1 and2 respectively.

2.5. Preparation of UV-Cured Film The coating formulation was coated on a glass substrate (10 cm 10 cm 0.3 cm) using a bar × × applicator with a thickness of 50 µm. The wet film was exposed under UV radiation (UV-IST, Nürtingen, Germany) at an energy level of 200 W/cm at a speed of 5 m/min. The film was considered fully cured after nine passes under UV light based on non-tackiness test using a finger (ASTM D1640). Polymers 2020, 12, 1494 4 of 17

Table 1. Jatropha oil-based polyurethane-toluene diisocyanate (JPUA-TDI)-based coating formulation.

Code TMPTA (%) JPUA-TDI (%) Benzophenone (%) TDI 25 25 75 4 TDI 30 30 70 4 TDI 35 35 65 4 TDI 40 40 60 4 TDI 45 45 45 4 TDI 50 50 50 4

Table 2. Components of jatropha oil-based polyurethane-isophorone diisocyanate (JPUA-IPDI)- based coating.

Code TMPTA (%) JPUA-IPDI (%) Benzophenone (%) IPDI 25 25 75 4 IPDI 30 30 70 4 IPDI 35 35 65 4 IPDI 40 40 60 4 IPDI 45 45 45 4 IPDI 50 50 50 4

2.6. Characterisation

2.6.1. Viscosity The viscosity of the JO, EJO, JOL, JPUA-TDI and JPUA-IPDI were determined by using a Haake Mars Rheometer (Thermo Scientiﬁc, Waltham, MA, USA) at 25 ◦C. Approximately 1 g of resin was placed on the sample platform for testing. A ﬂat plate spindle (PP 60) was moved on the sample in rotation mode with a gap of 1 mm. The viscosity was measured using plate-plate geometry mode and analysed by Rheo Win software.

2.6.2. Tintometer The colour measurement was performed using a Lovibond Colour Tintometer (Model F, Amesbury, UK). The colour recognition was achieved based on matching the similarity of the resin colour with the combination colour ﬁlter of red, yellow, blue and neutral.

2.6.3. Fourier Transform Infra-Red (FTIR) Spectroscopy The conversion of the functional group during the chemical reactions was analysed using FTIR (Bruker Tensor II, Ettlingen, Germany). It was equipped with an attenuated total reﬂectance (ATR) 1 accessory with a pure diamond crystal. The reading was in the spectral range of 500–4000 cm− with a 1 resolution of 4 cm− . This instrument ran an average of 32 scans per sample.

2.6.4. Gel Permeation Chromatography (GPC) The molar mass of the polymer and its polydispersity index (PDI) were determined by using GPC (Waters Corporation, MA, USA). Total of 5 mg of sample was diluted in 5 mL THF and injected into three types of column; Styragel HR1, Styragel HR 3 and Styragel HR 5 respectively.

2.6.5. Thermal Analysis Thermal gravimetric analysis (TGA) of the JPUA-TDI and JPUA-IPDI-cured film was conducted using a Netzsch (TG209 F3, Selb, Germany) tester based on ASTM E1131. The samples were run under conditions from room temperature up to 700 ◦C with a heating rate of 10 ◦C/min under a nitrogen-purged atmosphere. Polymers 2020,, 12,, xx FORFOR PEERPEER REVIEWREVIEW 5 of 19 Polymers 2020, 12, x FOR PEER REVIEW 5 of 19 Polymers 2020,, 12,, xx FORFOR PEERPEER REVIEWREVIEW 5 of 19 conditions from room temperature up to 700 °C with a heating rate of 10 °C/min under a nitrogen- conditions from room temperature up to 700 °C with a heating rate of 10 °C/min under a nitrogen- conditionspurged atmosphere. from room temperature up to 700 °C with a heating rate of 10 °C/min under a nitrogen- purged atmosphere. purged atmosphere. 2.6.6.Polymers Pendulum2020, 12, 1494 Hardness 5 of 17 2.6.6. Pendulum Hardness 2.6.6. Pendulum Hardness The hardness of the UV-cured film was measured using a Pendulum Hardness tester (TQC, The hardness of the UV-cured film was measured using a Pendulum Hardness tester (TQC, CapelleThe aan hardness den Ijssel, of theNetherlands). UV-cured Thefilm test was was measured conducted using referring a Pendulum to ASTM Hardness D4366 with tester the (TQC,König Capelle2.6.6. Pendulum aan den Ijssel, Hardness Netherlands). The test was conducted referring to ASTM D4366 with the König Capelledamping aan mode. den Ijssel,The data Netherlands). was counted The in test seconds was conducted during the referring damping to ASTM time of D4366 the pendulum with the König till it dampingThe hardnessmode. The of thedata UV-cured was counted film was in seconds measured duri usingng the a Pendulum damping Hardness time of the tester pendulum (TQC, Capelle till it dampingtotally stopped. mode. TheReadings data was were counted taken inin secondstriplicate duri forng each the dampingsample and time the of theaverage pendulum value tillwas it totallyaan den stopped. Ijssel, The Readings Netherlands). were taken The test in wastriplicate conducted for each referring sample to ASTMand the D4366 average with value the König was totallycalculated. stopped. Readings were taken in triplicate for each sample and the average value was calculated.damping mode. The data was counted in seconds during the damping time of the pendulum till it calculated. 2.6.7.totally Adhesion stopped. Test Readings were taken in triplicate for each sample and the average value was calculated. 2.6.7. Adhesion Test 2.6.7. Adhesion Test 2.6.7.A Adhesion cross hatch Test adhesion test was done according to ASTM D3359-09 (Biuged, Guangzhou, A cross hatch adhesion test was done according to ASTM D3359-09 (Biuged, Guangzhou, China).A crossThe kit hatch came adhesion complete test with was a cutter done blade, according tape,tape, brushbrushto ASTM andand magnifyingmagnifyingD3359-09 (Biuged, glass.glass. AA Guangzhou, 1-mm1-mm spacespace China).A crossThe kit hatch came adhesion complete test with was a done cutter according blade, tape, to ASTM brush D3359-09 and magnifying (Biuged, glass. Guangzhou, A 1-mm China). space China).cutter blade The kitwas came chosen complete based withon the a cuttercoating blade, thickn tape,tape,ess tobrushbrush prepare andand themagnifyingmagnifying lattice pattern. glass.glass. AAA 1-mm1-mmpiece ofspacespace 3M cutterThe kit blade came was complete chosen with based a cutteron the blade, coating tape, thickn brushess andto prepare magnifying the lattice glass. pattern. A 1-mm A spacepiece of cutter 3M cutterScotch blade transparent was chosen tape wasbased applied on the oncoating the lattice thickn andess topulled prepare off atthe approximately lattice pattern. at A a piece 180° angle.of 3M Scotchblade wastransparent chosen based tape was on the applied coating on thickness the lattice to and prepare pulled the off lattice at approximately pattern. A piece at a of 180° 3M Scotchangle. ScotchAny dirt transparent was gently tape removed was applied using onthe the brush lattice and and the pulled surface off was at approximatelyexamined using at aa magnifying180° angle. Anytransparent dirt was tape gently was appliedremoved on using the lattice the andbrush pulled and otheff at surface approximately was examined at a 180 ◦usingangle. aAny magnifying dirt was Anyglass. dirt The was adhesion gently score removed was givenusing based the brush on the and remo theved surface area ofwas the examined lattice as describedusing a magnifying in Table 3. glass.gently The removed adhesion using score the brushwas given and thebased surface on the was remo examinedved area using of the a magnifying lattice as described glass. The in adhesion Table 3. glass. The adhesion score was given based on the removed area of the lattice as described in Table 3. score was given based on the removed area of the lattice as described in Table3. Table 3. Explanation of adhesion grading. Table 3. Explanation of adhesion grading. Table 3. Explanation of adhesion grading. Grade Table Removed 3. Explanation Area (%) of adhesion Appearance grading. Grade Removed Area (%) Appearance Grade Removed Area (%) Appearance GradeGrade Removed Removed Area Area (%) (%) Appearance Appearance

5B None 5B None 5B5B None None

4B Less than 5% 4B Less than 5% 4B4B Less Less than than 5% 5%

3B 5-15% 3B3B 5-15% 5–15% 3B 5-15%

2B 15-35% 2B2B 15-35% 15–35% 2B 15-35%

1B1B 35 35–65% - 65% 1B 35 - 65% 1B 35 - 65%

0B0B Greater Greater than than 65% 65% 0B Greater than 65% 0B Greater than 65%

2.6.8. Contact Contact Angle Angle 2.6.8. Contact Angle 2.6.8. Contact Angle Static contact angles were examined on the surface of the cured film ﬁlm of JPUA-TDI JPUA-TDI and JPUA IPDI Static contact angles were examined on the surface of the cured film of JPUA-TDI and JPUA IPDI at 25 Static °CC using contact an anglesAttension were Theta examined Optical on Contact Contactthe surface Angle Angle of the tester tester cured (Biolin (Biolin film ofScie Scientiﬁc, JPUA-TDIntific, Manchester, Manchester, and JPUA IPDIUK). UK). at 25 °C◦ using an Attension Theta Optical Contact Angle tester (Biolin Scientific, Manchester, UK). atApproximately 25 °C using an 77 µµL AttensionL of deionised Theta water Optical was Contact applied Angle on the testercoating (Biolin surface Scie using ntific, the the Manchester, sessile sessile dropping dropping UK). Approximately 7 µL of deionised water was applied on the coating surface using the sessile dropping Approximatelymethod. The The contact contact 7 µL ofangle angle deionised readings readings water were were was measured applied onba based sedthe coatingon its intersection surface using with the the sessile surface dropping within method. The contact angle readings were measured based on its intersection with the surface within method.one minute The after contact the deionised angle readings water were was measured dropped. based on its intersection with the surface within one minute after the deionised water was dropped. one minute after the deionised water was dropped. 2.6.9. Transmittance Transmittance and Haze Tests 2.6.9. Transmittance and Haze Tests 2.6.9. Transmittance and Haze Tests 2.6.9.The Transmittance transmittance and (ASTM Haze Tests E1348) and haze tests (ASTM D1003) of the JPUA-TDI and JPUA-IPDI

cured ﬁlm were performed using a Haze Illuminant tester (BYK-Gardner, Geretsried, Germany).

This machine measured both properties simultaneously as a percentage value. All readings were taken three times and the average value was recorded. Polymers 2020, 12, x FOR PEER REVIEW 7 of 20 Polymers 2020, 12, x FOR PEER REVIEW 7 of 20 3. Results and Discussion Polymers 2020, 12, x FOR PEER REVIEW 7 of 20 Polymers3. Results2020, and12, 1494 Discussion 6 of 17 Polymers3.1. Synthesis 2020, 12 of, x JPUAFOR PEER REVIEW 7 of 20 3. Results and Discussion 3.1.Polymers Synthesis 2020, 12 of, x JPUAFOR PEER REVIEW 7 of 20 3. ResultsTwo seriesand Discussion of JPUA namely JPUA-TDI and JPUA-IPDI were successfully synthesised. The 3.1. Synthesis of JPUA 3.physical3. Results ResultsTwo properties and seriesand DiscussionDiscussion of ofJPUA the products namely JPUA-TDIduring stepwi andse JP synthesisUA-IPDI reaction were successfully are reported synthesised. in Table 4. The physical3.1. SynthesisTwo properties series of JPUA of ofJPUA the productsnamely JPUA-TDIduring stepwi andse JPsynthesisUA-IPDI reaction were successfully are reported synthesised. in Table 4. The 3.1. Synthesis ofTable JPUA 4. Properties of jatropha oil-based resins intermediate to produced JPUA. physical3.1. SynthesisTwo properties series of JPUA of ofJPUA the productsnamely JPUA-TDIduring stepwi andse JPsynthesisUA-IPDI reaction were successfully are reported synthesised. in Table 4. The Table 4. Properties of jatropha oil-based resins intermediate to produced JPUA. physical properties of the products during stepwise synthesis reaction are reportedOOC in Table OHV4. TwoTwo series seriesChemical of of JPUA JPUA Reaction namely namely JPUA-TDI JPUA-TDI and and JPUA-IPDI JPUA-IPDI werewere successfullysuccessfully synthesised. The The physical Sample Table 4. Properties of jatrophaAppearance oil-based Tintometer resins intermediate State at Room to Tempproduced JPUA. physical properties of the products during stepwise synthesis reaction are reported(% perOOC mole) in Table (mg OHV 4.KOH/g) propertiesSample of theChemical products Reaction duringAppearance stepwise synthesis Tintometer reaction State at Room are Temp reported in Table4. Table 4. Properties of jatropha oil-basedRed: resins 3.9 intermediate to produced JPUA. (% perOOC mole) (mgOHV KOH/g) Sample Chemical Reaction Appearance Tintometer State at Room Temp Table 4. Properties of jatropha oil-basedRed: resins 3.9 intermediate to produced JPUA. Table 4. Properties of jatropha oil-basedYellow: resins1.0 intermediate to(% produced perOOC mole) JPUA.(mgOHV KOH/g) Sample Chemical Reaction Appearance Tintometer State at Room Temp JO No reaction Red: 3.9 Liquid - - Yellow: 1.0 (% perOOC mole) (mgOHV KOH/g) Sample Chemical Reaction Appearance Blue: Tintometer 0 State at Room Temp JO ChemicalNo reaction Red: 3.9 LiquidState at - OOC - OHV Sample AppearanceYellow: Tintometer 1.0 (% per mole) (mg KOH/g) Reaction Blue: 0 Room Temp (% per mole) (mg KOH/g) JO No reaction Neutral: 0.2 Liquid - - Yellow:Red: 3.9 1.0 Blue: 0 Neutral: 0.2 JO No reaction Red:Red: 1.2 3.9 Liquid - - Blue:Yellow: 0 1.0 Neutral:Yellow: 0.2 1.0 JOJO NoNo reaction reaction Red: 1.2 LiquidLiquid - -- - Yellow:Blue: 4.4 0 Neutral:Blue: 0 0.2 EJO Epoxidation Red:Neutral: 1.2 0.2 Liquid 4.25 ± 0.08 - Yellow: 4.4 Blue: 0 EJO Epoxidation Red:Neutral: 1.2 0.2 Liquid 4.25 ± 0.08 - Yellow: 4.4 Blue: 0 EJO Epoxidation Neutral:Red: 1.20 Liquid 4.25 ± 0.08 - Yellow:Red: 1.2 4.4 Blue:Yellow: 0 4.4 EJOEJO EpoxidationEpoxidation Neutral: 0 LiquidLiquid 4.25 ± 4.25 0.08 0.08- - Red:Blue: 2.2 0 Blue:Yellow: 0 4.4 ± Neutral: 0 EJO Epoxidation Red:Neutral: 2.2 0 Liquid 4.25 ± 0.08 - Yellow: 9.0 Neutral:Blue: 0 0 JOL Hydroxylation Red: 2.2 Liquid 0.2 ± 0.03 149.44 ± 0.23 Yellow: 9.0 Blue: 0 JOL Hydroxylation Red:Neutral:Red: 2.2 2.20 Liquid 0.2 ± 0.03 149.44 ± 0.23 Yellow: 9.0 Blue:Yellow: 0 9.0 JOLJOL HydroxylationHydroxylation Neutral: 0 LiquidLiquid 0.2 ± 0.03 0.2 0.03 149.44 ± 0.23 149.44 0.23 Yellow:Red:Blue: 2.2 9.0 0 Blue: 0 ± ± Neutral: 0 JOL Hydroxylation Red:Neutral: 2.9 0 Liquid 0.2 ± 0.03 149.44 ± 0.23 Blue:Yellow: 0 9.0 Neutral: 0 JOL Hydroxylation Red: 2.9 Liquid 0.2 ± 0.03 149.44 ± 0.23 Yellow: 0 Neutral:Blue: 0 0 JPUA-TDI Isocyanation (aromatic) Red:Red: 2.9 2.9 Semi liquid - - Yellow: 0 Isocyanation Blue:Yellow: 0 0 JPUA-TDIJPUA-TDI Isocyanation (aromatic) Red:Neutral: 2.9 0 SemiSemi liquid liquid - - - - (aromatic) Yellow:Blue: 0 0 Blue: 0 JPUA-TDI Isocyanation (aromatic) Neutral:Neutral: 2.0 2.0 Semi liquid - - Yellow:Red: 2.9 0 Blue: 0 Neutral: 2.0 JPUA-TDI Isocyanation (aromatic) Red: 1.6 Semi liquid - - Blue:Yellow: 0 0 Neutral: 2.0 JPUA-TDI Isocyanation (aromatic) Red:Red: 1.6 1.6 Semi liquid - - IsocyanationIsocyanation Yellow:Yellow: 0 0 Neutral:Blue: 0 2.0 JPUA-IPDIJPUA-IPDI Red: 1.6 LiquidLiquid - -- - (cycloaliphatic)Isocyanation Yellow:Blue: 0 0 (cycloaliphatic) Blue: 0 JPUA-IPDI Red:Neutral:Neutral: 1.6 2.0 3.0 Liquid - - Isocyanation Yellow: 0 (cycloaliphatic) Blue: 0 JPUA-IPDI Neutral: 3.0 Liquid - - Isocyanation Yellow:Red: 1.6 0 (cycloaliphatic) Blue: 0 JPUA-IPDI Neutral: 3.0 Liquid - - (cycloaliphatic)Isocyanation Blue:Yellow: 0 0 JPUA-IPDIColourColour measurementmeasurement is is one one of ofthe the physical physical Neutral:properties properties 3.0 for oilLiquid for and oil its and derivatives. its- derivatives. Any -colour Any colour changeschangesColour provide provide measurement(cycloaliphatic) information is onone on production of production the physical output output Neutral:Blue:properties during 0 3.0 during physical for physicaloil treatmentand its treatment derivatives. and chemical and Any chemicalreaction. colour reaction.

InchangesIn this thisColour study, study, provide measurement the the information colourcolour of of isthe the onone JO-based production JO-based of the physicalresin output resin differed Neutral:properties di duringffered in 3.0 eachphysical infor eachreaction oil treatmentand reaction step.its derivatives. Theand step. colourchemical The Anyof colour reaction.JO colour in its of JO in its original form was brownish and then changed to light yellow and yellow after reaction of epoxidation originalchangesIn thisColour formstudy, provide measurement was the information colour brownish of isthe andonone JO-based production thenof the changed physicalresin output differed toproperties during light in each yellowphysical for reaction oil and treatmentand yellow step.its derivatives. Theand after colourchemical reaction Anyof reaction.JO colour ofin its epoxidation originaland hydroxylation. form was brownish For the isocyanationand then changed stage, to the light colour yellow of JPUA-TDIand yellow was after slightly reaction brownish of epoxidation while andInchanges hydroxylation.thisColour study, provide measurement the information colour For theof isthe isocyanationonone JO-based production of the physicalresin output stage, differed properties theduring in colour each physical for reaction of oil JPUA-TDI treatmentand step.its derivatives. Theand was colourchemical slightly Anyof reaction.JO brownishcolour in its while andJPUA-IPDI hydroxylation. was visually For the seen isocyanation as light yellow. stage, This the colourobservation of JPUA-TDI was comparable was slightly as reportedbrownish by while [15] JPUA-IPDIoriginalInchanges this study, formprovide was wasthe visually information colourbrownish seenof theand on asJO-based thenproduction light changed yellow. resin output to differed Thislight during yellow observation in each physical and reaction yellow wastreatment afterstep. comparable reaction Theand colourchemical of as epoxidation of reported reaction.JO in its by [15] for JPUA-IPDIfor JPUA-TDI. was The visually colour seen difference as light yellow.between This JPUA-TDI observation and JPUA-IPDIwas comparable may asaffect reported the aesthetic by [15] JPUA-TDI.andoriginalIn this hydroxylation. study, form The wasthe colour colourbrownish For di the offference isocyanationtheand JO-based then between changed stage,resin JPUA-TDI to differedthe light colour yellow in and ofeach JPUA-TDIand JPUA-IPDI reaction yellow wasafterstep. may slightly reactionThe a ffcolourect brownish of the epoxidation of aestheticJO whilein its property forproperty JPUA-TDI. of the The end-product colour difference [21]. To betweenavoid human JPUA-TDI bias during and JPUA-IPDI visual assessment, may affect specific the aesthetic colour JPUA-IPDIandoriginal hydroxylation. form was was visually brownish For the seen isocyanationand as then light changed yellow. stage, toThis the light colourobservation yellow of JPUA-TDIand was yellow comparable was after slightly reaction as reportedbrownish of epoxidation by while [15] ofpropertyrecognition the end-product of wasthe end-productconducted [21]. To using avoid[21]. a Totintometer human avoid biashuman as list during edbias in duringTable visual 4. visual Under assessment, assessment, room conditions, specific specific colour all colourresins recognition forJPUA-IPDIand JPUA-TDI. hydroxylation. was The visually Forcolour the seen differenceisocyanation as light yellow.between stage, This theJPUA-TDI colourobservation of and JPUA-TDI JPUA-IPDIwas comparable was mayslightly asaffect reportedbrownish the aesthetic by while [15] wasrecognitionwere conducted in liquid was form using conducted except a tintometer JPUA-TDIusing a tintometer as which listed was in as Table inlist aed viscous4 in. Under Table semi-liquid 4. room Under conditions, roomform. conditions, all resins all resins were in liquid propertyforJPUA-IPDI JPUA-TDI. of wasthe The visuallyend-product colour seen difference [21].as light To yellow.betweenavoid human This JPUA-TDI observation bias during and JPUA-IPDIwas visual comparable assessment, may asaffect reported specific the aesthetic bycolour [15] formwererecognitionproperty except in liquid of JPUA-TDI wasthe form end-productconducted except which JPUA-TDI using [21]. was a Totintometer in whichavoid a viscous washuman as inlist semi-liquid a ed biasviscous in duringTable semi-liquid form.4. visual Under assessment, roomform. conditions, specific all colourresins 3.2.for JPUA-TDI.FTIR Analysis The colour difference between JPUA-TDI and JPUA-IPDI may affect the aesthetic wererecognitionproperty in liquid of wasthe form end-productconducted except JPUA-TDI using [21]. a Totintometer whichavoid washuman as inlist a ed biasviscous in duringTable semi-liquid 4. visual Under assessment, roomform. conditions, specific all colourresins 3.2.3.2. FTIR FTIR Analysis Analysis wererecognition in liquid was form conducted except JPUA-TDI using a tintometer which was as inlist aed viscous in Table semi-liquid 4. Under roomform. conditions, all resins 3.2. FTIR Analysis wereThe in comparativeliquid form except FTIR JPUA-TDI spectra which in preparation was in a viscous of natural-based semi-liquid form. polyols of jatropha oil is shown 3.2. FTIR Analysis in Figure1. In its original state, a signal of the carbon double bond of the crude JO was detected 3.2. FTIR Analysis at 3012 cm 1. A similar trend was observed in the other types of natural oil such as palm [22] and − canola oil [2]. This peak then disappeared during the epoxidation reaction where the carbon double bond was converted into an oxirane ring (C-O-C) upon reaction with hydrogen peroxide which was observed at 824 cm 1. Based on the titration method, the OOC obtained was 4.25 0.08% (Table4). − ± Next, the oxirane ring broke during hydroxylation which was indicated by the disappearance of the 1 1 oxirane peak (824 cm− ) and a new broad peak of the OH group was formed at 3435 cm− . The OHV Polymers 2020, 12, 1494 7 of 17 Polymers 2020, 12, x FOR PEER REVIEW 8 of 19 achieved forfor JOLJOL waswas 149.44 149.44 ±0.23 0.23 mg mg KOH KOH/g/g (Table (Table4). 4). The The predicted predicted stepwise stepwise chemical chemical reactions reactions of ± JOLof JOL are are depicted depicted in Figurein Figure2. 2.

a) JO

-HC=CH-

b)EJO Intensity (a.u) Intensity

c) JOL

-OH

4000 3500 3000 2500 2000 1500 1000 500 Wavenumber (cm-1)

Figure 1. Fourier transform infra-red (FTIR) spectra during epoxidation and hydroxylation of jatropha oil. Figure 1. Fourier transform infra-red (FTIR) spectra during epoxidation and hydroxylation of The FTIR spectrum during isocyanationjatropha of JOL oil. is depicted in Figure3. The absorption peak 1 1 at 1532 cm− and 1718 cm− corresponded to amide (-NH) bending and amide (-NH) stretching 1 respectively. The peak of carbonyl (C=O) was observed at 3369 cm− [23]. The absence of the (N=C=O) peak reﬂected that all isocyanates either from 2,4-TDI or IPDI were consumed during the reaction [15,16]. 1 The characteristics of the acrylate group were determined by the absorption band of 1663 and 815 cm− which related to the acrylate double bond (-CH=CH2) and vinyl functionality of (CH2=CH-COO-) respectively [24]. Both JPUA-TDI and JPUA-IPDI indicated a similar trend under FTIR because of possessing the same functional groups. Generally, the reactivity of the reaction depends on the structure of isocyanate being used. The electrophilic carbon of isocyanate (N = C = O) was attacked by nucleophilic of oxygen (O-H) from alcohol during the isocyanation process. In 2,4-TDI, the benzene ring acted as an electron withdrawing group via an inductive eﬀect. The negative charge in benzene was then delocalised in the π ring system. This enhanced the positive charge on the carbon of isocyanate (N = C = O) to further undergo a nucleophilic addition reaction with oxygen (O-H) derived from JOL. Therefore, this raised the reactivity and speeded up the reaction during isocyanation [25].

Polymers 2020, 12, 1494 8 of 17 Polymers 2020, 12, x FOR PEER REVIEW 9 of 19

Figure 2. Proposed chemical reactions to produce jatropha oil-based polyol (JOL).

Apart from that, the steric hindrance eﬀect formed in IPDI because of the existence of three Figure 2. Proposed chemical reactions to produce jatropha oil-based polyol (JOL). methylene (-CH3) groups which bonded to the cyclic ring. This eﬀect reduced the reactivity of the IPDI and slowed down the isocyanation reaction [25]. The chemical structures explain the reason for The FTIR spectrum during isocyanation of JOL is depicted in Figure 3. The absorption peak at the high reactivity of 2,4-TDI compared to IPDI. The proposed chemical reactions of JPUA-TDI and 1532 cm−1 and 1718 cm−1 corresponded to amide (-NH) bending and amide (-NH) stretching JPUA-IPDI are illustrated in Figure4. respectively. The peak of carbonyl (C=O) was observed at 3369 cm−1 [23]. The absence of the (N=C=O) peak reflected that all isocyanates either from 2,4-TDI or IPDI were consumed during the reaction [15,16]. The characteristics of the acrylate group were determined by the absorption band of 1663 and 815 cm−1 which related to the acrylate double bond (-CH=CH2) and vinyl functionality of (CH2=CH- COO-) respectively [24]. Both JPUA-TDI and JPUA-IPDI indicated a similar trend under FTIR because of possessing the same functional groups.

Polymers 2020, 12, x FOR PEER REVIEW 10 of 19 Polymers 2020, 12, 1494 9 of 17

a) JOL

3435 -OH

b) JPUA Intensity (a.u) Intensity

2270 3369 -N=C=O- -NH 815 1532 H2C=CH-COO- 1718 -NH -C=O 1663 JOL JPUA-TDI JPUA-IPDI HC=CH2

4000 3500 3000 2500 2000 1500 1000 500 Figure 3. Conversion of (a) JOL into (b) JPUA-TDIWavenumber and(cm-1) JPUA-IPDI via the isocyanation reaction.

Figure 3. Conversion of (a) JOL into (b) JPUA-TDI and JPUA-IPDI via the isocyanation reaction.

Generally, the reactivity of the reaction depends on the structure of isocyanate being used. The electrophilic carbon of isocyanate (N=C=O) was attacked by nucleophilic of oxygen (O-H) from alcohol during the isocyanation process. In 2,4-TDI, the benzene ring acted as an electron JOLwithdrawing group via an inductive effect. The negative charge in benzene was then delocalised in the π ring system. This enhanced the positive charge on the carbon of isocyanate (N=C=O) to further undergo a nucleophilic addition reaction with oxygen (O-H) derived from JOL. Therefore, this raised the reactivity and speeded up the reaction during isocyanation [25]. Apart from that, the steric hindrance effect formed in IPDI because of the existence of three a) 2,4-TDImethylene (-CH3) groups which bonded to the cyclic ring. This effectb) reducedIPDI the reactivity of the IPDI and slowed down the isocyanation reactionIsocyanation [25]. The chemical structures explain the reason for the high reactivity of 2,4-TDI compared to IPDI. The proposed chemical reactions of JPUA-TDI and JPUA-IPDI are illustrated in Figure 4. 60 °C, 300 rpm, 2hr

70 °C, 300 rpm, 70 °C, 300 rpm, 1 hr 1 hr

HEMA HEMA

Acrylate group

Side chain Side chain JPUA-TDI JPUA-IPDI

FigureFigure 4. Isocyanation4. Isocyanation ofof JOL JOL to to produc producee (a) (JPUA-TDIa) JPUA-TDI and (b) and JPUA-IPDI. (b) JPUA-IPDI.

Polymers 2020, 12, 1494 10 of 17

3.3. Viscoelasticity Property The molar mass and viscosity of all the resins in this study are presented in Table5. It can be observed that the molar mass of the JO-based products increased at each stage of the synthesis. This indicated that additional chains were formed via grafting of functional groups such as epoxy, hydroxyl and isocyanates onto the triglyceride backbone. The polydispersity index (PDI) of all jatropha oil-based resins except JPUA-TDI was near to 1.00. This reﬂected a good distribution of the molar mass within a narrow bell shape curve [26].

Table 5. Molar mass of jatropha-oil based resin.

Sample Mw Mn PDI (Mw/Mn) Viscosity (Pas) JO 1278 948 1.35 0.056 EJO 1452 1041 1.39 0.342 JOL 1768 1426 1.23 2.336 JPUA-TDI 6871 3192 2.15 10.820 JPUA-IPDI 3151 2457 1.28 0.096

For the viscosity property, the higher crosslinking in each reaction step reﬂected the longer polymer chain which led to higher viscosity [6]. This trend agreed with the current ﬁnding for JO, EJO and JOL. However, the trend slightly changed for JPUA-TDI and JPUA-IPDI because of a calculated amount of 50% (w/w) DMF added during the isocyanation reaction to avoid gelation. The additional solvent decreased the viscosity of the JPUA-TDI and JPUA-IPDI from their actual values. Nevertheless, to compare the viscosity only between JPUA-TDI and JPUA-IPDI resins would be considered valid as the success of the reaction was supported by FTIR and GPC data. From Table5, the viscosity value of JPUA-TDI was far higher (10.82 Pas) compared to JPUA-IPDI (0.095 Pas). The fast reaction between the reactive 2,4-TDI with JOL produced a bulk molar mass (Mw) compared to the average molecular number (Mn). This led to a broad distribution of molar mass (PDI = 2.15) with a high viscosity JPUA-TDI resin. Thus, JPUA-TDI required an appropriate reactive diluent to achieve the intended viscosity during the coating process. For JPUA-IPDI, a good distribution of molecular (PDI = 1.28) may maintain low viscosity resin [26].

3.4. Thermal Analysis TGA analysis is useful in providing information regarding the purity of a compound and its stability temperature during processing. The TGA thermogram and weight loss derivative curves in Figure5 revealed that both the JPUA series were degraded in three stages. At temperatures below 100 ◦C, trapped water was removed during the degradation process. The first stage was observed in the range of 200–300 ◦C. This 18–25% weight loss corresponded to the decomposition of urethane linkage into primary or secondary amine, olefin and dioxide. The second stage of degradation occurred between 300–400 ◦C which was attributed to the cleavage of the backbone of the oil mainly from the aliphatic and alkyl groups of the fatty acid. Major degradation took place at this temperature range with a weight loss of up to 73%. Finally, the thermal degradation beyond a temperature of 400 ◦C was contributed by further thermo-oxidation of the JPUA films. These findings were comparable with [27,28] for other types of polyurethane-based coatings. Even though JPUA-TDI (6871 g/mol) had a higher molar mass compared to JPUA-IPDI (3151 g/mol), both showed a similar thermal stability pattern with a final residue of 2.8% and 1.2% respectively.

3.5. UV Curing of the Coating In a UV curing system, acrylate is the functional group that is responsible to react with radical species during polymerisation. The radical species was generated by a photoinitiator after being 1 exposed to UV light. Based on the FTIR spectra in Figure6, the acrylate absorption peak at 1663 cm− Polymers 2020, 12, x FOR PEER REVIEW 12 of 19

fast reaction between the reactive 2,4-TDI with JOL produced a bulk molar mass (Mw) compared to the average molecular number (Mn). This led to a broad distribution of molar mass (PDI = 2.15) with a high viscosity JPUA-TDI resin. Thus, JPUA-TDI required an appropriate reactive diluent to achieve the intended viscosity during the coating process. For JPUA-IPDI, a good distribution of molecular (PDI=1.28) may maintain low viscosity resin [26].

Table 5. Molar mass of jatropha-oil based resin.

Sample Mw Mn PDI (Mw/Mn) Viscosity (Pas) JO 1278 948 1.35 0.056 EJO 1452 1041 1.39 0.342 JOL 1768 1426 1.23 2.336 JPUA-TDI 6871 3192 2.15 10.820 JPUA-IPDI 3151 2457 1.28 0.096

3.4. Thermal Analysis TGA analysis is useful in providing information regarding the purity of a compound and its stability temperature during processing. The TGA thermogram and weight loss derivative curves in Figure 5 revealed that both the JPUA series were degraded in three stages. At temperatures below 100 °C, trapped water was removed during the degradation process. The first stage was observed in Polymersthe range2020 ,of12 200–300, 1494 °C. This 18%–25% weight loss corresponded to the decomposition of urethane 11 of 17 linkage into primary or secondary amine, olefin and dioxide. The second stage of degradation occurred between 300–400 °C which was attributed to the cleavage of the backbone of the oil mainly 1 1 (-CHfrom= CHthe aliphatic2), 815 cmand− alkyl(CH groups2=CH-COO-) of the fatty 660 acid cm. Major− (C degradation=C bending) took was place wiped at this temperature out after exposure underrange UVwith irradiation. a weight loss of This up to data 73%. indicated Finally, the that thermal the acrylatedegradation group beyond had a beentemperature utilised of 400 during the radical°C was polymerisation contributed by further reaction. thermo-oxidation This ﬁnding of matched the JPUA with films. other These types findings of acrylatewere comparable system such as epoxidisedwith [27,28] soy for beanother acrylatetypes of polyurethane-based [29], epoxidised palm coatings. acrylate Even [22 though], epoxidised JPUA-TDI Jatropha (6871 g/mol) acrylate had [24] and a higher molar mass compared to JPUA-IPDI (3151 g/mol), both showed a similar thermal stability a waterborne acrylate system [28]. pattern with a final residue of 2.8% and 1.2% respectively.

0.0 JPUA-TDI 100 -1.0 JPUA-IPDI 80 -2.0 -3.0 60 -4.0 Mass (%) Mass JPUA-TDI 40 -5.0 JPUA-IPDI I II III -6.0 20

Derivative Mass (%/min) -7.0 I II III Polymers0 2020, 12, x FOR PEER REVIEW -8.0 13 of 19 0 100 200 300 400 500 600 700 800 0 100 200 300 400 500 600 700 800 o o (-CH=CHa) 2), 815 cm−1Temperature (CH2=CH-COO-) ( C) 660 cm−1 (C=C bending)b) was wiped outTemperature after exposure ( C) under

UV irradiation. This data indicated that the acrylate group had been utilised during the radical polymerisation reaction. This finding matched with other types of acrylate system such as epoxidised Figure 5. (a) Thermal gravimetric analysis (TGA) thermogram and (b) derivative curve of JPUA-TDI Figure 5. (a) Thermal gravimetric analysis (TGA) thermogram and (b) derivative curve of JPUA-TDI soy bean acrylate [29], epoxidised palmand acrylate JPUA-IPDI. [22], epoxidised Jatropha acrylate [24] and a waterborneand JPUA-IPDI. acrylate system [30]. 3.5. UV Curing of the Coating In a UV curinga) JPUA-TDIsystem, acrylate (uncured) is the functional group that is responsible to react with radical species during polymerisation. The radical species was generated by a photoinitiator after being exposed to UV light. Based on the FTIR spectra in Figure 6, the acrylate absorption peak at 1663 cm−1

b) JPUA-TDI (cured)

Intensity (a.u) Intensity c) JPUA-IPDI (uncured)

d) JPUA-IPDI (cured)

1663 815 660

4000 3500 3000 2500 2000 1500 1000 500

Wavenumber (cm-1)

FigureFigure 6. 6. FTIRFTIR curves curves of JPUA-TDI of JPUA-TDI and andJPUA-IPDI JPUA-IPDI beforebefore and after and UV after irradiation UV irradiation respectively. respectively.

3.6. Mechanical Properties of Cured Film The mechanical property tests were conducted on a cured film containing either JPUA-TDI or JPUA-IPDI with corresponding ratio of a TMPTA monomer and benzophenone as a photoinitiator. Initially, benzophenone induced the curing reaction by absorbing the energy from the UV light and became excited to a higher energy level. Next, benzophenone became unstable and experienced a photocleavage reaction and turned into a radical species known as benzoyl radical. Benzoyl radical attacked the carbon double bond (C=C) on the acrylate part which was given by JPUA-TDI, JPUA- IPDI (Figure 3) and TMPTA for propagation of the polymer chain. This reaction continued until all the acrylate was used up, thus reaching the termination reaction of the polymerisation [31].

3.6.1. Hardness Test The hardness of JPUA-TDI and JPUA-IPDI films is presented in Figure 7. The hardness property reflects the coating toughness and its durability. From the result, JPUA-IPDI had a higher hardness value compared to JPUA-TDI-based film. The hardness characteristic was linked with crosslinking

Polymers 2020, 12, 1494 12 of 17

3.6. Mechanical Properties of Cured Film The mechanical property tests were conducted on a cured ﬁlm containing either JPUA-TDI or JPUA-IPDI with corresponding ratio of a TMPTA monomer and benzophenone as a photoinitiator. Initially, benzophenone induced the curing reaction by absorbing the energy from the UV light and became excited to a higher energy level. Next, benzophenone became unstable and experienced a photocleavage reaction and turned into a radical species known as benzoyl radical. Benzoyl radical attacked the carbon double bond (C = C) on the acrylate part which was given by JPUA-TDI, JPUA-IPDI (Figure3) and TMPTA for propagation of the polymer chain. This reaction continued until all the acrylate was used up, thus reaching the termination reaction of the polymerisation [30].

3.6.1. Hardness Test The hardness of JPUA-TDI and JPUA-IPDI films is presented in Figure7. The hardness property reflects the coating toughness and its durability. From the result, JPUA-IPDI had a higher hardness valuePolymers compared 2020, 12, x FOR to JPUA-TDI-basedPEER REVIEW film. The hardness characteristic was linked with crosslinking14 of 19 density that was highly dependent on the JPUA-TDI and JPUA-IPDI molecular structure as illustrated density that was highly dependent on the JPUA-TDI and JPUA-IPDI molecular structure as in Figure4. illustrated in Figure 4.

140

120

100

60 JPUA-TDI JPUA-IPDI 40 Konig Hardness (sec) Hardness Konig 20

0 25:75 30:70 35:65 40:60 45:55 50:50 Ratio of TMPTA to JPUA

Figure 7. Hardness measurement of JPUA-TDI and JPUA IPDI-based coating. Figure 7. Hardness measurement of JPUA-TDI and JPUA IPDI-based coating. The symmetrical rigid structure of the side chain of 2,4-TDI on the JPUA-TDI backbone caused stericThe hindrance. symmetrical This erigidffect structure generated of constraints the side chai forn the of 2,4-TDI benzoyl on radical the JPUA-TDI to attack the backbone carbon doublecaused bondsteric (Chindrance.= C) of the This acrylate effect gene chainrated during constraints the propagation for the benzoyl state of radicalradical polymerisation.to attack the carbon Meanwhile, double JPUA-IPDIbond (C=C) oofffered the acrylate a more flexible,chain during asymmetrical the propagation aliphatic state structure of radical of IPDI polymerisation. side chain with Meanwhile, less steric hindranceJPUA-IPDI [offered31]. This a more made flexible, JPUA-IPDI asymmetrical more favourable aliphatic duringstructure the of UVIPDI curing side chain reaction with compared less steric tohindrance JPUA-TDI. [32]. This made JPUA-IPDI more favourable during the UV curing reaction compared to JPUA-TDI.The hardness property of the film increased as the ratio of monomer increased because of the high crosslinkingThe hardness polymerisation property of via the hydrogen film increased bonding as [ 6the,22 ]ratio between of monomer JPUA and increased TMPTA. because In the case of the of JPUA-IPDI,high crosslinking the hardness polymerisation dropped via gradually hydrogen starting bonding at a ratio[6,22] of between 40:60 because JPUA ofand maximum TMPTA. limit In the of thecase reactive of JPUA-IPDI, site of polymerisation the hardness dropped as the trifunctional gradually starting acrylate at continued a ratio of to40:60 be added. because This of maximum may have causedlimit of thethe degradationreactive siteof of chain polymerisation scission of theas the polymer trifunctional network acrylate as well continued as leading to to be the added. decrease This of themay hardness have caused value. the degradation of chain scission of the polymer network as well as leading to the decrease of the hardness value.

3.6.2. Adhesion Test The chemical structure of the polymer and its flexibility highly affects the adhesion property. From the result in Table 6, the increasing amount of trifunctional acrylate given by TMPTA led to a higher degree of crosslinking network after being cured under UV radiation. A high density of crosslinking caused the surface to become brittle [6,22].

Table6. Adhesion score of JPUA films.

JPUA Resin Sample Adhesion Score TDI 25 4B TDI 30 4B TDI 35 4B JPUA-TDI TDI 40 1B TDI 45 1B TDI 50 0B IPDI 25 1B IPDI 30 1B JPUA-IPDI IPDI 35 1B IPDI 40 0B IPDI 45 0B

Polymers 2020, 12, 1494 13 of 17

3.6.2. Adhesion Test The chemical structure of the polymer and its ﬂexibility highly aﬀects the adhesion property. From the result in Table6, the increasing amount of trifunctional acrylate given by TMPTA led to a higher degree of crosslinking network after being cured under UV radiation. A high density of crosslinking caused the surface to become brittle [6,22].

Table 6. Adhesion score of JPUA films. PolymersPolymersPolymersPolymers 2020Polymers 2020Polymers ,2020 12, 2020,12 x2020, ,12FOR x , 2020 ,FOR12 ,x 12 ,FORPEER x, PEERx12FOR FOR ,PEER xREVIEW FORPEER REVIEW PEER REVIEW PEER REVIEW REVIEW REVIEW 15 15of 15 of20 15of 2015 of20 of15 20 20of 20 PolymersPolymersPolymersPolymers 2020Polymers 2020Polymers, 202012, ,202012 x , 2020 ,FOR12 x , 2020 ,FOR12 x, PEER,12FOR x, ,PEER12FOR x ,FORPEER xREVIEW FORPEER REVIEW PEER REVIEW PEER REVIEW JPUA REVIEW REVIEW Resin Sample Adhesion Score 15 15of 15of20 15 of20 15 of20 15 of 20 of 20 20 IPDIIPDIIPDI TDI50IPDI 50IPDI 50 25IPDI 50 50 50 0B 0B 0B 0B 4B 0B 0B IPDIIPDIIPDI TDI50IPDI 50 IPDI 3050 IPDI 50 50 50 0B 0B 0B 0B 4B 0B 0B TheThe TheJPUA-TDI TheJPUA-TDIThe JPUA-TDI TheJPUA-TDI JPUA-TDI JPUA-TDI based based based basedfilm based film basedfilm had filmhad film hada film hadbettera had bettera hadbettera abetteradhesion better adhesiona betteradhesionTDI adhesion adhesion 35 propertyadhesion property property property property than property than 4Bthan the than thethan JPUA-IPDIthe than JPUA-IPDIthe theJPUA-IPDI JPUA-IPDIthe JPUA-IPDI JPUA-IPDI which which which which matchedwhich matched which matched matched matched matched JPUA-TDI itsits lowerits Thelowerits Theitslower TheJPUA-TDI lowerits hardnessThelowerJPUA-TDI hardnessTheJPUA-TDI lowerhardness TheJPUA-TDI hardnessJPUA-TDIhardness JPUA-TDI basedhardnessvalue basedvalue basedvalue basedfilm value (Section based valuefilm (Section based filmvaluehad(Section filmhad (Sectionfilm (Sectionhada film3.6.2). betterhad a(Section 3.6.2). hadbetter a 3.6.2). hadbetter a 3.6.2).betteraadhesionThis3.6.2). better aadhesionThis 3.6.2). betteradhesionThis TDI indicated adhesionThis Thisindicatedadhesion 40propertyindicatedadhesionThis property indicated indicatedproperty indicatedpropertythat propertythat than property thatthan JPUA-TDI that 1Bthan JPUA-TDIthatthe than JPUA-TDIthe thatthan JPUA-TDIJPUA-IPDIthe JPUA-TDIthan JPUA-IPDI the JPUA-TDI JPUA-IPDIthehad JPUA-IPDIthehad JPUA-IPDI had JPUA-IPDIa had whichahadlower which alower had which alower awhichmatched lower whichdensity lower amatched densitywhich matchedlowerdensity matched densitymatched density matched density itscrosslinkingcrosslinkingits lowercrosslinkingits lowercrosslinkingits crosslinkinglowerits lowercrosslinkingits hardness lower networkhardness lower networkhardness networkhardness hardnessnetwork networkhardnessvalue and network valueand value andinversely value(Section andinversely andvalue (Sectioninversely value and(Sectioninversely inversely(Section owned(Sectioninversely 3.6.2).owned (Section 3.6.2).owned 3.6.2). owned ownedgood3.6.2). This good3.6.2). owned This good3.6.2). This flexibilityTDI goodindicated goodThis flexibility indicated This flexibility good 45indicated This flexibility flexibilityindicated indicated comparedflexibility indicated thatcompared thatcompared thatcompared JPUA-TDIcompared that1B JPUA-TDI that compared toJPUA-TDI thatto JPUA-IPDI.JPUA-TDI toJPUA-IPDI.JPUA-TDI to JPUA-TDIJPUA-IPDI. tohad JPUA-IPDI. JPUA-IPDI.hadto had JPUA-IPDI. a hadThe alower had The alower had Theadditionalower Theaddition a lower The densityadditiona lower densityTheaddition lower additiondensity of densityaddition ofdensity ofdensity of of of crosslinkingTMPTATMPTAcrosslinkingTMPTAcrosslinkingTMPTAcrosslinkingTMPTA crosslinkingin TMPTAcrosslinkingin both network inboth network in both in thenetwork both thebothinnetwork JPUA-TDIthenetwork andboth JPUA-TDIthenetwork and theJPUA-TDI andinversely JPUA-TDIthe JPUA-TDIandinversely and inverselyJPUA-TDI and andinversely and inversely and ownedJPUA-IPDI-basedinversely andownedJPUA-IPDI-based and ownedJPUA-IPDI-based andownedJPUA-IPDI-based good JPUA-IPDI-basedowned good ownedJPUA-IPDI-based good flexibility TDIgood flexibilitygood flexibilitygood 50re flexibility resins flexibility sinsre comparedflexibility sins reof comparedre sins of comparedmoresins re ofcomparedmore sins ofcompared 0Bmoreof comparedthantomore moreof than toJPUA-IPDI. than moreto JPUA-IPDI.35% than to JPUA-IPDI.35%than to JPUA-IPDI.35% causedthan to JPUA-IPDI. 35% caused 35%JPUA-IPDI. caused The35% caused The causeda Theadditionbrittle acaused Theadditionbrittlea The additionbrittlea aThe additionbrittle film brittle addition afilmof addition brittle filmof offilm film of filmof of TMPTAsurfacesurfaceTMPTAsurfaceTMPTAsurfaceTMPTA surfaceand TMPTAin and TMPTAsurface in both andreduced in bothandreduced inandboth reduced thein both and reducedthe inreducedboth JPUA-TDIbondedthe both reducedJPUA-TDIbondedthe JPUA-TDIbondedthe JPUA-TDIthebonded bonded JPUA-TDIattachme JPUA-TDIattachmebondedand attachmeand attachme and JPUA-IPDI-basedattachme andntJPUA-IPDI-based attachme andnt JPUA-IPDI-basedto and ntJPUA-IPDI-basedto the JPUA-IPDI-basednt tothent JPUA-IPDI-based substrate.tothe to ntsubstrate. IPDIthe thesubstrate.to substrate.the 25substrate.re sinsre Thissubstrate. sinsreThis sins reofThis result resins of Thismoreresult Thissinsre of moreresult sins Thisofed 1Bmoreresult of resultedthanmore in of edthanmore resultin athanedmore indecreased35%aed than decreasedin35% a thanedin decreased 35% acausedthan aindecreased 35% causeddecreased 35% acaused decreased35%adhesion caused adhesionacaused brittle adhesionacaused brittleaadhesion adhesion brittlea score adhesionabrittlefilm score brittleafilm score brittle film score filmscore film score film surfacephenomenaphenomenasurfacephenomenasurfacephenomenasurface phenomenaandsurface andphenomenasurface and reducedin andreducedin JPUA-TDI and reducedinJPUA-TDI and reducedinJPUA-TDI in reduced bondedJPUA-TDI JPUA-TDI reducedinbonded JPUA-TDIbonded from bondedfrom attachmebonded from 4Battachmebonded from 4Bfromattachme to 4B attachmetofrom 1B 4Battachme to1Bnt4B andattachme to 1Bnt toand4Bto nt1B to andthe 0B. 1Bto nt to0B.theand nt1Band substrate.JPUA- to0B.the ntJPUA- tosubstrate.IPDI andthe0B. 0B. JPUA-tosubstrate.the JPUA-IPDIsubstrate.the0B. 30JPUA- substrate.IPDI ThisJPUA-substrate.IPDI also ThisIPDI alsoIPDI This result alsoshowed ThisIPDI result alsoshowed This alsoresult showed Thised 1B resultalso showed ed resultshowed ina ed result similarinashoweded similar decreasedinaed similar decreasedinaed asimilarindecreased a trendsimilar indecreaseda trend similar decreaseda trend decreasedadhesion withtrend trendadhesionwith adhesion withtrend declining adhesionwith declining withadhesion scoredeclining adhesionwith scoredeclining declining score decliningscore score score phenomenaphenomenaphenomenaphenomenaphenomenaphenomena in inJPUA-TDI inJPUA-TDI inJPUA-TDI in JPUA-TDI inJPUA-TDI JPUA-TDIfrom from from 4B from 4B fromto 4B fromto1B 4B to1B 4Band to1B 4Band to 1B and0B. to1B 0B.and 1BJPUA- and0B. JPUA- IPDI and0B. JPUA- 0B. IPDIJPUA- 0B. 35 JPUA-IPDI JPUA-IPDI alsoIPDI alsoIPDI alsoshowedIPDI alsoshowed alsoshowed 1B also showed showeda similar showeda similar a similar a similara trend similar a trend similar trend with trend withtrend withtrend declining with declining with declining with declining declining declining adhesionadhesionadhesionadhesionadhesion performanceadhesion performance performance performance performance performance from from from 1B from 1Bfrom to JPUA-IPDI1B fromto 0B. 1B to1B0B. to 0B. 1Bto 0B. 0B.to 0B. adhesionadhesionadhesionadhesionadhesion performanceadhesion performance performance performance performance performance from from from 1B from 1B fromto 1B from to0B. 1B to 0B.1B to 0B.1B to 0B. to0B. 0B. IPDI 40 0B 3.6.3.3.6.3.3.6.3. Water3.6.3. 3.6.3.Water Water3.6.3. WaterContact Water Contact WaterContact Contact Contact Angle ContactAngle Angle Angle Angle Angle IPDI 45 0B 3.6.3.3.6.3.3.6.3. Water3.6.3. Water3.6.3. Water3.6.3. WaterContact WaterContact Water Contact Contact ContactAngle ContactAngle Angle Angle Angle Angle IPDI 50 0B TheThe Thewettability ThewettabilityThe wettability Thewettability wettability wettability of of JPUA-TDI ofJPUA-TDI of JPUA-TDIof JPUA-TDI JPUA-TDIof JPUA-TDI and and andJPUA-IPDI-base andJPUA-IPDI-base and JPUA-IPDI-base andJPUA-IPDI-base JPUA-IPDI-base JPUA-IPDI-based dcoating coatingd coatingdd coating coatingwered werecoating were investigatedwere wereinvestigated investigatedwere investigated investigated investigated using using using usingthe using the waterusingthe waterthe thewater waterthe water water contactcontactcontactThecontactThe contactangle The wettabilityanglecontact The wettabilityangle The wettabilityasangle The anglewettabilityas presented wettability asanglepresented wettability aspresented ofas presented of JPUA-TDIpresentedas ofJPUA-TDI presentedin ofJPUA-TDI in ofTableJPUA-TDI inofTableJPUA-TDI inTable JPUA-TDIinand 7.Table and Tablein 7.It andJPUA-IPDI-base 7.TableItwas andJPUA-IPDI-base 7.wasIt and7.JPUA-IPDI-base wasItfound and ItJPUA-IPDI-base 7. wasfound JPUA-IPDI-basewas Itfound JPUA-IPDI-base was foundthat found that found thatdall thatcoatingdall thatcoating dcoatingall coating thatcoatingdall coatingall dcoating coating were dcoatingall coatingformulations werecoating formulationscoating were formulationsinvestigated were formulations investigated wereformulations investigatedwere formulations investigated investigated were investigatedwere wereusing in usingwere werein theusing in theweretheusing inrangetheusing thein range watertheusing the inrange waterthe rangeofthewaterthe range of waterthe rangeofwater ofwater of of contact7575contact - 75contact The89°- 75contact 89° -75angle contact which89°JPUA-TDI - angle 75contactwhich-89° angle89° which - as angle 89°which was asanglewhichpresented was asanglepresented which was asnearlypresented based wasas nearlypresentedwas as nearlypresented was nearly presentedin hydropnearly film inhydropTable nearly inhydropTable hadinTablehydrop hydrophobicin 7.Table hobic in 7.TablehydropIt ahobic better7.TablewasIt hobic(90°). 7.wasIt hobic(90°). 7. wasItfound hobic(90°). 7. Itwas found adhesionA(90°). wasIt found(90°). A hydrophobicwas found thatA(90°).hydrophobic found thatAhydrophobic A found that allhydrophobic hydrophobicpropertythatAall coating thatallhydrophobic coating thatall coating surfacall coating surfacall formulations coatingsurfac than formulationscoating esurfac formulations surfacindicatese formulations theindicatesesurfac formulations indicatese formulations e JPUA-IPDIindicates indicates ewere that indicateswere that were thatin itwere that in itthecanwerethat init whichcanthewere that rangeincanittheprevent it in rangepreventcanthe can inrangeitthepreventmatched ofrangecantheprevent prevent rangeof rangeofprevent of of its of lower75itselfitself75 - itself75 89° - fromhardnessitself75 89° -itself from 75which 89° - from itself75 which 89° -contamination from which89°-fromcontamination 89°which wascontamination from valuewhich wascontamination which contamination wasnearly wascontaminationnearly (Section wasnearly was nearlybyhydrop nearly byhydrop nearly anybyhydrop any3.6.2 by hydrop by hobicanymoisturehydrop hobicany moisturebyhydrop).any hobicmoisture This any(90°).hobicmoisture moisturehobic(90°). hobic(90°).condition indicatedmoisture Acondition(90°). A(90°).conditionhydrophobic A(90°).conditionhydrophobic condition Ahydrophobic Aconditionandhydrophobic that Ahydrophobicand hydrophobicandincreases JPUA-TDIand increasessurfacand increases surfac and increases surfacincreases esurfac theincreasesindicatesesurfac theindicatessurface had thelifetimeindicatese lifetimethe eindicates the alifetimeindicatese lower thatthelifetime indicates lifetime thatof thatlifetimeofit the that densityofitcan the that itofcan thesubstrateofthat itcanpreventsubstrate the itofthepreventcansubstrate it canprevent crosslinkingthesubstrate cansubstrateprevent preventsubstrate prevent itselfitselfitself from from from contamination contamination contamination by by anyby any anymoisture moisture moisture condition condition condition and and andincreases increases increases the the thelifetime lifetime lifetime of of theof the thesubstrate substrate substrate networkitselfespeciallyespeciallyitselfespecially fromespecially especiallyfrom and especiallyitselffor contamination for contaminationmetal inverselyfor metalfrom for formetal and metalfor metal andcontamination metalandwood owned andwood byand woodby andanywood[32]. wood [32].any good wood[32]. moisture [32].bymoisture [32]. flexibility [32].any conditionmoisture condition compared conditionand and increases increases to and JPUA-IPDI. theincreases the lifetime lifetime Thethe of additionlifetime ofthe the substrate substrate of of the TMPTA substrate in especiallyespeciallyespeciallyespeciallyespecially especiallyfor for metal for metal for metal for and metalfor metaland metalandwood andwood andwood and[32].wood wood[32]. wood[32]. [32]. [32]. [32]. both theTheThe JPUA-TDIThecrosslinking ThecrosslinkingThe crosslinking The crosslinkingcrosslinking crosslinking and density density JPUA-IPDI-baseddensity density densityand densityand and flexibility andflexibilityand flexibility and flexibilityflexibility resins flexibilityof of the of the of of theofmolecular more themolecularof themolecular the molecular thanmolecular molecularchain 35%chain chain chainalso caused chainalso chain also affected alsoaffectedalso a affectedalso brittle affectedaffected the affectedthe filmthesurface thesurface thesurface the surface surface surface and wettabilitywettabilityTheThe Thecrosslinking Thepropertycrosslinking Thecrosslinking Thepropertycrosslinking crosslinking crosslinking [33,34]. density [33,34].density density Thedensity densityand The JPUA-TDI-baseddensityand and JPUA-TDI-based flexibility and flexibility and flexibility andflexibility flexibility flexibilityof ofco the ofating the ofco themolecularof ating the molecular ofshowed themolecular the molecularshowed molecular molecular anchain chain increasing anchain chainalsoincreasing chainalso chain also affected also contactaffected also affectedalso contactaffected affected theangle affectedthe anglethesurface valuethesurface thesurface valuethesurface surface surface reducedwettabilitywettabilitywettability bondedwettability property attachmentproperty property property [33,34]. [33,34]. [33,34]. toThe[33,34]. the TheJPUA-TDI-based The substrate. TheJPUA-TDI-based JPUA-TDI-based JPUA-TDI-based This co resultedating co coating atingshowedcoating inshowed showed a decreasedshowedan increasing an an increasing anincreasing increasing adhesion contact contact contact angle scorecontact angle anglevalue phenomena angle value value value wettabilityfromfromwettabilityfromwettability 75.84°fromwettability from75.84°wettability 75.84°fromwettability property 75.84° to 75.84°property to 87.13°.property 75.84° to 87.13°.property to 87.13°.propertyto [33,34]. 87.13°.property Thisto87.13°.[33,34]. This [33,34].87.13°. This [33,34]. trend The[33,34]. This trend This The[33,34]. trend TheJPUA-TDI-basedThis wastrend TheJPUA-TDI-based trendwas TheJPUA-TDI-based trend was TheproportionalJPUA-TDI-based was proportionalJPUA-TDI-basedwas proportionalJPUA-TDI-based was proportional proportional proportional co cotoating to coatingthe tocoatingthe showed cotoating additionthe to showedco atingadditionthe showed totheatingaddition showed additionthe showedadditionan showedof an addition increasing of an the increasing ofanthe increasing ofan TMPTAthe ofincreasing anTMPTA theincreasing oftheTMPTA increasingcontact TMPTAthe contactTMPTA monomer contact TMPTAmonomer contact monomeranglecontact monomerangle contactmonomer angle valuemonomerangleratio. valueangleratio. valueangleratio. valueratio. ratio.value valueratio. in JPUA-TDI from 4B to 1B and 0B. JPUA-IPDI also showed a similar trend with declining adhesion fromMeanwhile,Meanwhile,fromMeanwhile,from 75.84°Meanwhile,from Meanwhile,75.84°from 75.84°Meanwhile,from 75.84°tothe 75.84° tothe 87.13°. 75.84° JPUA-IPDI-basedtothe87.13°. JPUA-IPDI-based tothe 87.13°. thetoJPUA-IPDI-based 87.13°. This totheJPUA-IPDI-based 87.13°. JPUA-IPDI-based This87.13°. ThisJPUA-IPDI-based trend This trend This trend This wastrend coating trendwas coating trendwas proportionalcoating was proportionalcoating wascoatingproportionaldisplayed was displayedproportionalcoating proportionaldisplayed proportionaldisplayed displayed to displayed an to the an to inversethe an toadditioninversethe an to additioninverseanthe toaddition theinverse an inversetrendtheaddition trendadditioninverse of trendaddition of thetrendof oftrendtheof contact ofTMPTAthe trendof contact of TMPTAthe of contact ofofTMPTAthe contact the TMPTAofcontactangle TMPTA monomeranglecontact TMPTA monomerangle monomeranglevalue anglemonomervalue monomeranglevalue monomerratio.valuefrom valueratio.from ratio.valuefrom ratio.from fromratio. ratio.from performanceMeanwhile,88.35°88.35°Meanwhile,88.35°Meanwhile, 88.35°Meanwhile,to88.35° Meanwhile,to 79.87° 88.35°Meanwhile, to79.87° the to79.87° from tothe 79.87° when JPUA-IPDI-based79.87°theto when JPUA-IPDI-based the 1B79.87° whenJPUA-IPDI-basedthe whenthe thetoJPUA-IPDI-basedwhen theJPUA-IPDI-based 0B. percentagewhentheJPUA-IPDI-based percentagethe thepercentage percentagethe percentage coating percentage coatingof coatingof TMPTA coating ofTMPTA displayedcoating ofTMPTA ofdisplayedcoating TMPTA displayedTMPTAof addeddisplayed TMPTAaddeddisplayed addeddisplayedan added aninto addedinverse intoan inverseadded intoan the inverse anintothe inverseinto an coatingthetrendinverse intocoating thetrendinverse thecoating trend coatingthe oftrend coatingformulation trend of contactformulation coatingtrend of contactformulation of contactformulation of formulationcontact of anglecontactformulation angle contactwas anglewas valueangle was increased.anglevalue wasincreased. anglewasvalue increased. fromvaluewas increased. valuefromincreased. valuefrom increased. from from from 88.35°TheThe88.35° The88.35°higher higherThe88.35°toThe 88.35°higher to79.87° The88.35°higher tocrosslinking 79.87°higher tocrosslinking79.87° higher to crosslinking79.87°when to 79.87°crosslinkingwhen crosslinking 79.87°when crosslinking thewhen dewhenthe depercentagewhennsitythe de percentagensitythe depercentagensitythe dein percentagethensity innsitydethepercentage inthepercentagensity ofJPUA-IPDI-based inthe inJPUA-IPDI-based of TMPTAthe theJPUA-IPDI-based ofinTMPTA JPUA-IPDI-basedoftheTMPTA JPUA-IPDI-based of TMPTA JPUA-IPDI-basedof TMPTAadded TMPTAadded added addedcoatinginto added coatinginto added coating intothe coating into the coatingtended intocoatingthe tended coatinginto coatingthe tended coatingthe tended coatingtotendedthe formulationcoatingto increasetended formulationcoating toincrease formulationtoincrease toformulation increase formulationincreaseto the formulation increasethe wassurface the wassurface the thewassurfaceincreased. was surfaceincreased.the surfaceenergy wasincreased. energy was surfaceincreased. energy increased. energy increased.energy energy 3.6.3. Water Contact Angle TheandandThe andThehigherits itsandhigherThe interfacialand Thehigherits interfacial andThehigherits crosslinkinginterfacial its higher crosslinkinginterfacial higheritsinterfacial crosslinking interfacialcrosslinkingtension. crosslinkingtension. crosslinkingtension. detension. tension. deConsequently,nsity tension.Consequently,density Consequently,density in deConsequently,nsity Consequently, inthedensity intheConsequently,nsity JPUA-IPDI-basedinthe JPUA-IPDI-basedin the it JPUA-IPDI-basedinthe decreasedit JPUA-IPDI-based the decreasedit JPUA-IPDI-based decreasedit JPUA-IPDI-basedit decreased decreased it decreasedthe thecoating contact thecoating contact thecoating thecontact coating contacttended the coatingcontactangle tendedcoating angle contact tended angle tended value.angleto tended anglevalue. toincrease tended value. angle toincrease value. Contrariwise,to increasevalue. Contrariwise,to increase value.Contrariwise, to increasethe Contrariwise, increase theContrariwise, surface the Contrariwise,surface the surfacethe the surfacethe theenergy surface sterictheenergy surface steric energythe thesteric energy sterictheenergy steric energy steric andhindrancehindranceand hindranceanditsThe hindranceandits interfacialhindrance andits interfacial wettability hindranceandits interfacialeffect its effectinterfacial its interfacialeffect interfacialtension. effectled effect tension.led effecttension. ofledto tension. ledto JPUA-TDI ledatension.Consequently, to atension.Consequently,reducedled to atoreducedConsequently, areducedConsequently,to a Consequently, reduced reduced aConsequently, andcrosslinking reducedcrosslinking itcrosslinking JPUA-IPDI-based decreasedit crosslinking decreaseditcrosslinking decreaseditcrosslinking itdecreased decreasednetworkit networkdecreasedthe networkthe contact networkthe networkcontact the in contactnetwork thein coating contactthe theangle incontact the angle incontactthe inJPUA-TDI-basedangle theJPUA-TDI-basedvalue.anglein the were JPUA-TDI-basedvalue.angle thevalue. JPUA-TDI-basedangleJPUA-TDI-based value.Contrariwise, investigated value.JPUA-TDI-basedContrariwise, value.Contrariwise, Contrariwise, Contrariwise, coating. Contrariwise,coating. coating.the using coating.thecoating. steric the This stericcoating. theThis stericthe This stericthe This stericThis water stericThis hindrancehindrancehindrancehindrancehindrance hindranceeffect effect effect effectled effectled effectledto ledto aled to areducedled to areducedto areduced to areduced a reduced crosslinkingreduced crosslinking crosslinking crosslinking crosslinking crosslinking network network network network network innetwork in the in the in theJPUA-TDI-basedin theJPUA-TDI-based in theJPUA-TDI-based theJPUA-TDI-based JPUA-TDI-based JPUA-TDI-based coating. coating. coating. coating. coating. This coating.This This This This This contactcontributedcontributedcontributedcontributedcontributed anglecontributed to asto more to presentedmore to moreto chainmore tomorechain chainmore chainmobility inchainmobility Tablechainmobility mobility mobility in 7mobility .in the It inthe was in JPUA-TDI-basedthein JPUA-TDI-basedthe inthe foundJPUA-TDI-based theJPUA-TDI-based JPUA-TDI-based JPUA-TDI-based that all coating coating coating coating coatingstructure structurecoating formulations structure structure structure andstructure and anddirected and directedand were directed anddirected directed into directedto thelow tolow to rangelowto low tolow low of contributedsurfacesurfacecontributedsurfacecontributedsurfacecontributed surfaceenergy.contributed energy.surfacecontributed energy.to energy. to energy. moreHence, to more Hence,energy. to moreHence, to chain more Hence, toHence,thechainmore the chainmoreHence, trendthemobilitychain trend thechainmobility the trendchainmobility of trendthe mobility trendof themobility inof trendthemobility inofcontactthethe of contactinthe the ofcontactinJPUA-TDI-basedthe incontacttheJPUA-TDI-based the contactan inJPUA-TDI-basedthe an glecontacttheJPUA-TDI-based an gleJPUA-TDI-based valuean gle JPUA-TDI-based anvaluegle glevaluean invaluegle value in coatingthe invaluethecoating JPUA-TDI-basedinthecoating in JPUA-TDI-basedthecoating theinJPUA-TDI-basedstructurecoating JPUA-TDI-basedthecoatingstructure JPUA-TDI-based structure JPUA-TDI-basedstructure structure andstructure and coating and directedcoating and directedcoating and directedcoating andcoatingincreaseddirected increaseddirectedcoatingto directedincreased tolow increased toincreasedlow to low increasedto low to low low 75–89◦ which was nearly hydrophobic (90◦). A hydrophobic surface indicates that it can prevent itself surfacewithwithsurfacewithsurface thewithsurface thewithenergy.surface increasing theenergy.withsurface increasing theenergy. theincreasing energy. increasingtheHence, energy.increasing Hence,energy. increasingHence,amount amountHence, the Hence,amount the Hence, amount trendtheamount oftrend the amountof trendTMPTA.the of TMPTA.trendthe of trendTMPTA. theof oftrendTMPTA.the TMPTA.of contactofthe This ofcontact TMPTA. theThis ofcontactthe This contactresultthe Thisan contactresultThis anglecontactresult Thisan gleresultwas valueresult anglewas valuean gleresult was comparablevalueangle was comparable invaluewasgle comparable invaluethe was comparable invalue thecomparable JPUA-TDI-basedinthe comparableinJPUA-TDI-based the with inJPUA-TDI-basedthe withJPUA-TDI-basedthe withJPUA-TDI-based [33] withJPUA-TDI-based [33]with [33] whowith [33]who [33] coatingwho investigated [33]coatingwho investigated whocoating investigated coatingwho investigated increased coatinginvestigated increasedcoating investigatedincreased increasedthe increasedthe increasedthe the the the from contamination by any moisture condition and increases the lifetime of the substrate especially for withinfluenceinfluencewithinfluencewith theinfluencewith theinfluencewith increasing theofinfluencewith increasing theof reactive increasing the ofreactive increasing the ofreactive ofincreasing reactive increasing reactiveamountof diluent amount reactivediluent amount diluent amount diluent amountdiluentofon amount onofdiluent TMPTA.the onof TMPTA.the on of TMPTA.water the onof water TMPTA.the ofon theTMPTA.water This TMPTA.watercontact the Thiswater contact This resultwatercontact This resultcontact This contactangleresult Thisangle wasresultcontact angleresultwas property.angleresult was comparable angleproperty. wascomparable property.angle wascomparable property.was property.comparable comparable property. comparable with with with [33] with [33] with [33] whowith [33]who [33]who investigated [33]who investigated who investigated who investigated investigated investigated the the the the the the metalinfluenceinfluenceinfluence andinfluenceinfluence ofinfluence wood ofreactive ofreactive ofreactive [of 32reactive of reactivediluent]. reactivediluent diluent diluent ondiluent on diluentthe on the on waterthe on water the on waterthe contactwaterthe watercontact watercontact contact anglecontact anglecontact angle property.angle property.angle property.angle property. property. property. TableTableTable 7.Table Table7.Contact Contact7.Table Contact7. 7. Contact Contactangle 7. angle Contact angle measurement angle measurementangle measurement angle measurement measurement measurement of ofJPUA-T ofJPUA-T ofJPUA-T of JPUA-T DI JPUA-TofDI andJPUA-TDI and DIJPUA-IPDI-basedandDI JPUA-IPDI-basedand DIand JPUA-IPDI-based JPUA-IPDI-basedand JPUA-IPDI-based JPUA-IPDI-based coatings. coatings. coatings. coatings. coatings. coatings. TableTableTableTable 7.Table 7. Contact7.TableContact Contact7.Table Contact7. 7.Contact angle Contact7. angle Contact angle measurement angle measurement measurementangle measurement angle measurement measurement measurement of ofJPUA-T ofJPUA-T ofJPUA-T JPUA-TDI ofJPUA-TDI ofJPUA-TDI and JPUA-TDI and DI JPUA-IPDI-basedand andDI JPUA-IPDI-basedandDI JPUA-IPDI-basedand JPUA-IPDI-basedandJPUA-IPDI-based JPUA-IPDI-based JPUA-IPDI-based coatings. coatings. coatings. coatings. coatings. coatings. coatings.

TDI 25 TDI 30 TDI 35 TDI 40 TDI 45 TDI 50 TDITDI TDI25 TDI25 TDI25 TDI25 25 25 TDITDI TDI30 TDI30 TDI30 TDI30 30 30 TDITDI TDI35 TDI35 TDI35 TDI35 35 35 TDITDI TDI40 TDI40 TDI40 TDI40 40 40 TDITDI TDI45 TDI45 TDI45 TDI45 45 45 TDI TDI TDI50 TDI50 TDI50 TDI50 50 50

IPDI 25 IPDI 30 IPDI 35 IPDI 40 IPDI 45 IPDI 50 IPDIIPDIIPDI 25IPDI 25IPDI 25 IPDI 25 25 25 IPDIIPDIIPDI 30IPDI 30IPDI 30 IPDI 30 30 30 IPDIIPDIIPDI 35IPDI 35IPDI 35 IPDI 35 35 35 IPDI IPDI IPDI 40 IPDI 40 IPDI 40 IPDI 40 40 40 IPDI IPDI IPDI 45 IPDI 45 IPDI 45 IPDI 45 45 45 IPDI IPDI IPDI 50 IPDI 50 IPDI 50 IPDI 50 50 50

3.6.4.3.6.4.3.6.4. Transmittance3.6.4. 3.6.4.Transmittance Transmittance3.6.4. Transmittance Transmittance Transmittance Test Test Test Test Test Test 3.6.4.3.6.4.3.6.4. Transmittance3.6.4. Transmittance3.6.4. Transmittance3.6.4. Transmittance Transmittance Transmittance Test Test Test Test Test Test TheThe Theoptical TheopticalThe optical Theoptical opticalproperty propertyoptical property property property ofproperty of the ofthe of thefilmsof filmsthe ofthefilms wasthefilms filmswas wasfilmscharacterised was characterisedwas characterised was characterised characterised characterised by by aby aTransmittanceby byTransmittancea Transmittancea by aTransmittance Transmittancea Transmittance and and andHaze and Hazeand Haze andtest.Haze Hazetest. Hazetest. The test. Thetest. The test. The The The transmittancetransmittancetransmittanceThetransmittanceThetransmittance Theopticaltransmittance Theoptical Theoptical measures The optical measures propertyoptical measures propertyoptical measures propertymeasures property measurestheproperty theofproperty amountthe of the amountthe of the amount offilms theamountthe ofamountfilms theof thefilms amount of lightwasthefilms oflightwasfilms of lightwasfilms ofcharacterisedthat lightwas characterisedthat lightof was characterised thatpasseslightwas characterisedthatpasses thatcharacterised passes characterised thatpasses throughpasses bythrough passes bythrough a bythrough Transmittancethrougha by a aTransmittance bythrough material.a Transmittancea by material. aaTransmittance material.a aTransmittance a material. Transmittancematerial. a Basedmaterial. Basedand Basedand Basedand onHazeBased onand Haze FigureBased andon Haze Figure andon test.HazeFigureon Hazetest. Figure on8, HazeFiguretest.The 8, test.allFigureThe 8,test. all The 8,test. all 8,The all The all8, The all transmittanceJPUA-TDIJPUA-TDItransmittanceJPUA-TDItransmittanceJPUA-TDItransmittanceJPUA-TDItransmittancetransmittanceJPUA-TDI and and measures andJPUA-IPDI-based measures andJPUA-IPDI-based andmeasures JPUA-IPDI-based measures andJPUA-IPDI-based measuresJPUA-IPDI-based themeasures JPUA-IPDI-based the amountthe amountthe amountthe coating amountthe coating amountof coating amount of lightcoating coating offormulationslight offormulationscoatinglight thatof formulationslight thatofformulationslight formulations thatpasseslight thatformulationspasses thatpasses were thatpasses throughwere passes throughwere passes categorisedthroughwere were categorisedthrough categorised throughwerea categorised through material.acategorised material.a categorised material. aas amaterial.as transparentmaterial.a as Basedtransparentmaterial. asBased transparentas Basedtransparent transparent as Basedon Basedtransparenton Figure Basedmaterialson Figurematerials on Figurematerialson Figure materialson 8, materialsFigure 8, all asFigurematerials 8,allas 8,all as 8, all as 8,asall all as JPUA-TDIthetheJPUA-TDI theJPUA-TDItransmittance transmittancetheJPUA-TDI theJPUA-TDItransmittance JPUA-TDIthetransmittance andtransmittance and transmittance andJPUA-IPDI-based andJPUA-IPDI-based values andJPUA-IPDI-based values andJPUA-IPDI-based values JPUA-IPDI-based values JPUA-IPDI-basedvalueswere werevalues were maintainedwere coatingweremaintained coatingmaintainedwere coatingmaintained maintainedcoating formulationscoatingmaintained formulationscoatingabove formulationsabove formulationsabove formulations above 85%formulationsabove 85% abovewere85% [35]. were 85% [35].85%were categorised [35]. were85% The categorised [35]. wereThe [35]. categorised were The incorporation[35].categorised The incorporationcategorisedThe categorisedincorporation Theasincorporation incorporation astransparent asincorporationtransparent astransparent as oftransparent as of transparent TMPTA oftransparentTMPTA materialsof TMPTAof materials TMPTA ofmaterialsTMPTA in materials TMPTA in materialsthe asinmaterialsthe asin thein as the inasthe as the as thethe transmittancethe transmittancethe thetransmittance thetransmittance transmittance transmittance values values values values werevalues werevalues were maintainedwere maintainedwere maintainedwere maintained maintained maintained above above above above85% above85% above 85%[35]. 85% [35].85% [35]. 85%The [35]. The[35]. Theincorporation[35]. Theincorporation Theincorporation Theincorporation incorporation incorporation of ofTMPTA ofTMPTA of TMPTAof TMPTA of TMPTAin TMPTA inthe inthe in thein the inthe the Polymers 2020, 12, 1494 14 of 17

The crosslinking density and flexibility of the molecular chain also affected the surface wettability property [33,34]. The JPUA-TDI-based coating showed an increasing contact angle value from 75.84◦ to 87.13◦. This trend was proportional to the addition of the TMPTA monomer ratio. Meanwhile, the JPUA-IPDI-based coating displayed an inverse trend of contact angle value from 88.35◦ to 79.87◦ when the percentage of TMPTA added into the coating formulation was increased. The higher crosslinking density in the JPUA-IPDI-based coating tended to increase the surface energy and its interfacial tension. Consequently, it decreased the contact angle value. Contrariwise, the steric hindrance effect led to a reduced crosslinking network in the JPUA-TDI-based coating. This contributed to more chain mobility in the JPUA-TDI-based coating structure and directed to low surface energy. Hence, the trend of the contact angle value in the JPUA-TDI-based coating increased with the increasing amount of TMPTA. This result was comparable with [33] who investigated the influence of reactive diluent on the water contact angle property.

3.6.4. Transmittance Test The optical property of the ﬁlms was characterised by a Transmittance and Haze test. The transmittance measures the amount of light that passes through a material. Based on Figure8, all JPUA-TDI and JPUA-IPDI-based coating formulations were categorised as transparent materials as the transmittance values were maintained above 85% [35]. The incorporation of TMPTA in the formulation did not show any trend in terms of transmittance value for both the JPUA-TDI and JPUA-IPDI-basedPolymers 2020, 12, x FOR coatings. PEER REVIEW 16 of 19

88 JPUA-TDI JPUA-IPDI

Transmittance (%) Transmittance 87

86 25:75 30:70 35:65 40:60 45:55 50:50 Ratio of TMPTA to JPUA

Figure 8. Transmittance value for variation of JPUA-TDI and JPUA-IPDI films. Figure 8. Transmittance value for variation of JPUA-TDI and JPUA-IPDI films. 3.6.5. Haze Test 3.6.5. Haze Test Haze is defined as the cloudy appearance of a material because of the light scattering of an imperfect surface.Haze Several is defined factors as the have cloudy been identifiedappearance to of aff aect material the haze because property of the such light as poorscattering dispersion, of an theimperfect coating surface. technique, Several curing factors method, have types been of materialsidentified andto affect additives the andhaze surface property treatment. such as poor dispersion,From Figure the coating9, JPUA-IPDI technique, maintained curing a lowmethod haze, fortypes all TMPTAof materials ratios and with additives haze values and of aroundsurface 3%treatment. whichcomplies with the standard for clear materials in industry. However, JPUA-TDI at an initial ratioFrom of 25:75 Figure showed 9, JPUA-IPDI a higher haze maintained value of 22.3%.a low haze The increasingfor all TMPTA amount ratios of TMPTAwith haze as avalues reactant of diluentaround into3% which the JPUA-TDI complies coating with the formulation standard for improved clear materials its viscosity. in industry. Hence, aHowever, good dispersion JPUA-TDI of the at coatingan initial formulation ratio of 25:75 reduced showed the a higher haze value haze of valu thee JPUA-TDI-basedof 22.3%. The increasing film [36 ].amount The JPUA-TDI-based of TMPTA as a filmreactant started diluent to achieve into thethe industryJPUA-TDI requirement coating form rangeulation at ratio improved of 45:55 at aits haze viscosity. reading Hence, of 2.81 a 0.01%.good dispersion of the coating formulation reduced the haze value of the JPUA-TDI-based film [37].± The JPUA-TDI-based film started to achieve the industry requirement range at ratio of 45:55 at a haze reading of 2.81 ± 0.01%.

JPUA-TDI

Haze (%) 10 JPUA-IPDI

0 25:75 30:70 35:65 40:60 45:55 50:50 Ratio of TMPTA to JPUA

Figure 9. Haze data of JPUA-TDI and JPUA-IPDI-based films.

Polymers 2020, 12, x FOR PEER REVIEW 16 of 19

88 JPUA-TDI JPUA-IPDI

Transmittance (%) Transmittance 87

86 25:75 30:70 35:65 40:60 45:55 50:50 Ratio of TMPTA to JPUA

Figure 8. Transmittance value for variation of JPUA-TDI and JPUA-IPDI films.

3.6.5. Haze Test Haze is defined as the cloudy appearance of a material because of the light scattering of an imperfect surface. Several factors have been identified to affect the haze property such as poor dispersion, the coating technique, curing method, types of materials and additives and surface treatment. From Figure 9, JPUA-IPDI maintained a low haze for all TMPTA ratios with haze values of around 3% which complies with the standard for clear materials in industry. However, JPUA-TDI at an initial ratio of 25:75 showed a higher haze value of 22.3%. The increasing amount of TMPTA as a reactant diluent into the JPUA-TDI coating formulation improved its viscosity. Hence, a good dispersion of the coating formulation reduced the haze value of the JPUA-TDI-based film [37]. The PolymersJPUA-TDI-based2020, 12, 1494 film started to achieve the industry requirement range at ratio of 45:55 at a15 haze of 17 reading of 2.81 ± 0.01%.

JPUA-TDI

Haze (%) 10 JPUA-IPDI

0 25:75 30:70 35:65 40:60 45:55 50:50 Ratio of TMPTA to JPUA

Figure 9. Haze data of JPUA-TDI and JPUA-IPDI-based ﬁlms. Figure 9. Haze data of JPUA-TDI and JPUA-IPDI-based films. 4. Conclusions

Two varieties of JPUA based on TDI and IPDI isocyanates have been successfully synthesised. JPUA-TDI had a higher viscosity and molar mass with a broad PDI value compared to JPUA-IPDI resin. Both JPUA-TDI and JPUA-IPDI showed a similar degradation trend under TGA analysis. The JPUA-TDI-based formulation had low hardness but good adhesion compared to the JPUA-IPDI-based formulation. The best adhesion for both JPUA-TDI and JPUA-TDI-based ﬁlms were at a ratio of 35:65. Both JPUA-TDI and JPUA-IPDI coating formulations owned a good transparency property. It is suggested that both JPUA-TDI and JPUA-IPDI can be used for clear coat coatings such as for windows and for the automotive and construction industry.

Author Contributions: Writing—original draft preparation, N.H.M.; writing—review and editing, L.C.A. and M.M.A.; methodology, M.Z.S. and M.R.; supervision, D.R.A.B. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by FUNDAMENTAL RESEARCH GRANT SCHEME (FRGS/1/2018/ STG01/MOSTI/02/1). Acknowledgments: The authors acknowledge the Department of Public Service Malaysia for the scholarship to complete this work. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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