FIELD OF INVENTION

This invention generally relates to alkyds. More particularly, the invention relates to a biocompatible palm oil fatty acid-based alkyd, the method of producing the alkyd and use of the alkyd.

BACKGROUND ART

Rubber process oil is derived from petroleum crude which has been distilled. Process oil is used to facilitate the operation of the manufacturing of both synthetic and natural rubber compositions such as mixing, moulding, extruding etc. as the process oil can penetrate the rubber polymer. Process oil is also used to improve the physical properties of rubber products.

To manufacture rubber process oil, a lubricant fraction is obtained by distillation of crude oil under reduced pressure. The process oil typically contains 70 to 99% of aromatics. In recent years, the carcinogenicity of polycyclic aromatic hydrocarbons (PAH) have garnered attention. According to European legislation EU Substance Directive 67/548/EEC, any oil containing more than 3% of PAH content must be labelled as toxic. There has been effort to produce process oils that contain less than 3% of PAH content, however, such products are still toxic due to the presence of PAH.

Also, at present, all rubber process oils are derived from petroleum crude oil - an unrenewable, unsustainable, and polluting resource. The processing of crude oil to produce various petroleum derived products emits vast amounts of greenhouse gas emissions. For example, PAH emissions from rubber tires are significantly higher than those produced from the exhaust of modern passenger vehicles ¹ .

Alkyd resins are polyfunctional vegetable oil-modified polyesters, synthesized via a step- wise polymerization process. Lee ² discloses the synthesis of a series of alkyd resins from palm kernel oil. These alkyds were reported for its use as tackifiers in the rubber composition for tire and pneumatic tire composition applications ³ . However, the biocompatibility of these alkyds were not reported. Hence, it is an aim of the present invention to provide an alternative to petroleum derived rubber process oil that is biocompatible, environmentally friendly, sourced from renewable resources, non-toxic and free of PAH. This invention thus aims to alleviate some or all the problems of the prior art. ¹ H. Klingenberg et al., Nicht Hmitierte AutomobHAbgaskomponenten, Volkswagen AG,

Forschung und Entwicklung 1988.

² Lee, S. Y., Gan, S. N., Hassan, A., Terakawa, K., Hattori, T., Ichikawa, N., & Choong, D. H. (2011). Reactions Between Epoxidized Natural Rubber and Palm Oil-Based Alkyds at Ambient Temperature. Journal of Applied Polymer Science, 120, 1503-1509. ³ Hattori, T.; Terakawa, K.; Ichikawa, N.; Sakaki, T.; Gan, S.N.; Lee, S.Y. Rubber composition for tire, and pneumatic tire using the same, US 8,100,157 B2, 24-Oct-2006.

SUMMARY OF THE INVENTION

In accordance with an aspect of the invention, there is provided a palm oil fatty acid-based alkyd. The alkyd comprises a palm fatty acid comprising stearic acid, and a hydrophilic trifunctional neopentyl monomer.

In an embodiment, the hydrophilic trifunctional neopentyl monomer may be 2,2- bis(hydroxymethyl)propionic acid.

In another embodiment, the alkyd may comprise about 70 wt% to about 80 wt%, preferably about 80 wt% of palm fatty acid; and about 20 wt% to about 30 wt%, preferably about 20 wt% of hydrophilic trifunctional neopentyl monomer.

In another aspect of the present invention, there is provided a method for manufacturing the alkyd. The method comprises the steps of: i) providing a palm fatty acid comprising stearic acid, ii) providing a hydrophilic trifunctional neopentyl monomer; iii) mixing the palm fatty acid and the hydrophilic trifunctional neopentyl monomer together in the presence of a catalyst to form a mixture; and iv) subjecting the mixture to a polyesterification reaction.

In an embodiment, the hydrophilic trifunctional neopentyl monomer may be 2,2- bis(hydroxymethyl)propionic acid.

The amount of the catalyst provided may be about 0.1 wt%. The catalyst used may be sodium hydroxide.

The temperature of the polyesterification reaction of step (iv) may be about 180°C to about 200°C whereas the reaction time is about 10 hours to about 20 hours, preferably about 18 hours In a further aspect of the invention, there is provided a process oil that comprises the alkyd of the present invention that may be used in the manufacture of carbon black-filled rubber compositions. In another aspect of the present invention, there is provided a use of the alkyd as a processing aid in the manufacture of carbon black-filled rubber compositions.

The alkyd of the present invention provides for various beneficial properties when used as a processing aid when compared to the use of conventional petroleum derived process oil in the manufacture of carbon black-filled rubber compositions. The alkyd of the present invention when used as a processing aid provides for comparable physical properties for tensile strength, elongation at break value, tensile modulus and compression set as well as improved ageing properties when compared to a rubber composition manufactured using conventional process oil.

The invention provides for various advantages as outlined above and will be further elaborated in the following pages.

DETAILED DESCRIPTION

The present invention is directed at a biocompatible alkyd derived from palm oil fatty acid, the method of producing the alkyd and its use.

Biocompatible palm oil fattv acid-based alkyd

In an embodiment of the present invention, there is provided a palm oil fatty acid-based alkyd. The alkyd comprises a palm oil fatty acid and a hydrophilic trifunctional neopentyl monomer. The palm fatty acid is palm-derived stearic acid. The hydrophilic trifunctional neopentyl monomer comprises at least one hydroxy and at least one carboxy functional group. The hydrophilic trifunctional neopentyl monomer preferably comprises a combination of two hydroxy and one carboxy functional groups.

In an embodiment of the present invention, palm-derived stearic acid comprising 18 carbon atoms with purity of at least 90% was used. The purity of stearic acid of at least 95% is preferred and a purity of at least 97% is most preferable. Stearic acid is a monosaturated fatty acid having the longest carbon chain length that can be found in palm oil. A monosaturated fatty acid having the longest carbon chain length provides good compatibility and miscibility with non-polar rubber polymers during physical blending. The non-polar rubber polymers used may include but not limited to natural rubber (NR), butadiene rubber (BR), and ethylene propylene rubber (EPDM or EPR).

Stearic acid is commonly used for accelerated sulphur vulcanization of rubber. Stearic acid by itself can cause acute oral toxicity, dermal irritation, and is toxic to aquatic life. However, when stearic acid was used in the manufacture of the alkyd of the present invention, the alkyd was tested and found to be non-toxic. It was determined that, although the monomers such as stearic acid by itself are often toxic, most polymers comprising these monomers are safe and non-toxic.

The hydrophilic trifunctional neopentyl monomer may be 2,2-bis(hydroxymethyl)propionic acid, or also named as 3-hydroxy-2-(hydroxymethyl)-2-methylpropanoic acid. This hydrophilic trifunctional neopentyl monomer hereinafter is referred to as dimethylol propionic acid (DMPA). The purity of DMPA is at least 98%. DMPA behaves as a di- functional monomer rather than a tri-functional monomer due to a difference in reactivity of carboxylic acid groups in monomers. DMPA with secondary carboxylic acid groups are more steric hindered and hence having a lower reactivity relative to stearic acid with primary and unhindered carboxylic acid groups. It was assumed that none of the carboxylic acid groups of DMPA reacted.

The method of producing the alkyd is using a block copolymerization process which resulted in the alkyd of the present invention having esterified stearic acid groups functioning as non-polar moieties and the unreacted carboxylic acid groups functioning as polar moieties. These polar moieties provide good compatibility and miscibility with polar polymers or rubbers during physical blending. The polar rubbers used may include but not limited to (acrylo) nitrile butadiene rubber (NBR) and chloroprene rubber (CR).

The composition of the alkyd comprises palm stearic acid of at least about 70 wt% to at most about 80 wt%, preferably 80 wt% and DMPA of at least about 20 wt% to at most about 30 wt%, preferably 20 wt%.

Stearic acid is derived from palm oil fatty acid while DMPA is essentially a non-toxic compound. Both compounds are not derived from petroleum and provides a biocompatible alternative to petroleum derived products.

The alkyd of the present invention is subjected to several characterization tests to determine the acid values through acid value titration, glass transition temperatures through differential scanning calorimetry and molecular weights through gel permeation chromatography.

The test results confirm that the alkyd has a moderately low number-average molecular weight, M _n of nearly 1000 g/mol and moderately low T _g of about 30°C. The moderately long hydrocarbon chains of the alkyd may improve its miscibility with non-polar polymers or rubbers. The moderately low T _g of the alkyd has a good plasticizing effect during physical mixing with other polymers or rubbers. Also, the alkyd was established to have a higher acid number, indicating it has polar moieties to enhance miscibility with polar polymers or rubbers. The molecular structure of the alkyd described above comprises two opposing ends with different polarities which enables good miscibility with both polar and non-polar polymers or rubbers during physical blending. Details of these tests are shown in Example 1.

A biocompatibility test was carried out by using in vitro succinate dehydrogenase activity (MTT assay) against three cell lines i.e. human keratinocytes, mouse hepatocytes and canine kidney cells. The cytotoxicity level was determined based on the cell viability relative to the control group in accordance with ISO 10993-5:2009. A control group containing cells without alkyd treatment was also provided. Test results show that all three cell lines had more than 80% cell viability which confirms that the alkyd is non-cytotoxic. Details of the test are shown in Example 2.

Method of producing the alkyd

The method for producing the alkyd of the present invention mainly comprises the following steps: i) providing a palm fatty acid and at least one hydrophilic trifunctional neopentyl monomer; ii) mixing the palm fatty acid and the hydrophilic trifunctional neopentyl monomer together in the presence of a catalyst to form a mixture; and iii) reacting the mixture via a polyesterification reaction.

The catalyst used is sodium hydroxide. Sodium hydroxide is a non-toxic compound and is used in the present invention to substitute conventionally used catalysts such as lithium hydroxide or metal oxide catalysts such as tin oxides. These conventional catalysts are highly toxic. The use of sodium hydroxide in the present invention provides for the non- cytotoxic properties of the present invention. The preferred amount of catalyst is about 0.1 wt%.

The palm oil fatty acid, monomers and catalyst are mixed in any suitable vessel. A reaction flask is used in the present invention as it provided effective control of the reaction. The reaction flask is equipped with a mechanical agitator, thermometer, nitrogen gas inlet and a Dean-Stark decanter. The polyesterification reaction is carried out at a temperature of at least about 180°C and at most about 200°C. The reaction time is at least about 10 hours, preferably at least about 18 hours and at most about 20 hours. Any other suitable reaction temperature and time may be applied.

During the polyesterification reaction, heat is applied to the reaction vessel and gradually increased and subsequently maintained for a specific length of time. This allows for a slow polyesterification rate to occur, allowing uniform distribution of monomers to form hydrophobes and hydrophilies along the alkyd's backbone. The reaction is allowed to continue until reaching an acid value of at least about 80 mg KOH/g of resin and at most about 100 mg KOH/g resin.

Use of the alkyds as a processing aid in carbon black-filled rubber compositions

The inventors have discovered that the alkyds are suitable for use as a processing aid in the manufacture of carbon black-filled rubber compositions. Carbon black was selected as it was the most widely available filler used for rubber compounding. A process oil used in the manufacture of rubber compositions enhances the processability of the composition by improving the dispersion of fillers and flow characteristics of the composition.

Conventional process oil used in the manufacture of rubber such as naphthenic, aromatic and paraffinic oils are typically derived from petroleum crude oil. These hydrocarbons are produced through the distillation of crude oil which is highly polluting to the environment. The alkyd of the present invention seeks to provide a biocompatible alternative to petroleum derived process oils which is manufactured in a more environmentally friendly process that uses only renewable, non-toxic materials.

To evaluate the performance of the alkyd of the present invention, a batch of carbon blackfilled rubber composition was prepared.

The rubber composition comprises the following components: i) a rubber host; ii) a carbon black filler; iii) a vulcanising agent; iv) an accelerator; v) a vulcanisation activator; and vi) a process oil.

The rubber host may be derived from solid synthetic rubber or natural rubber. Any suitable grade of synthetic or natural rubber may be used which includes, but not limited to acrylonitrile butadiene rubber (NBR), styrene butadiene (SBR), polybutadiene rubber (BR), ethylene-propylene-diene rubber (EPDM), polyisoperene rubber (IR), natural rubber (NR), epoxidised natural rubber (ENR), deproteinised natural rubber (DPNR) and any mixture thereof. In the present rubber composition, acrylonitrile butadiene rubber (NBR) and deproteinised natural rubber (DPNR) were used. 100.0 p.p.h.r. or rubber host was used in the rubber composition.

Carbon black acts as a filler for the rubber composition. Any suitable type or grade of carbon black may be used such as N 110 Super abrasion furnace (SAF), N220 Intermediate SAF (ISaFE), N326, N330, N375 High Abrasion Furnace (HAF), N300 Easy processing channel (EPC), N550 Fast Extruding Furnace (FEF), N660 High Modulus Furnace (HMF), N762, N770 Semi Reinforcing Furnace (SRF), N880 Fine thermal (FT) and/or N990 Medium thermal (MT). In the present rubber composition, N550 FEF black was used at an amount of about 20.0 p.p.h.r. to about 100.0 p.p.h.r., preferably about 30.0 p.p.h.r. to about 50.0 p.p.h.r..

Any suitable vulcanising agent may be used for vulcanisation of the rubber composition. Preferably, the rubber mixture is vulcanised by adding sulphur at an amount of about 0.1 p.p.h.r. to about 5.0 p.p.h.r., preferably about 1.0 p.p.h.r. to about 2.5 p.p.h.r..

Any suitable accelerator may be used. The accelerator used may include but are not limited to diphenylguanidine (DPG), tetramethylthiuram monosulphide (TMTM), tetramethylthiuram disulphide (TMTD), tetrabutylthiuram disulphide (TBTD), tetraethylthiuram disulphide (TETD), zinc dimethyldithiocarbamate (ZDMC), zinc diethyldithiocarbamate (ZEDC), zinc dibutyldithiocarbamate (ZDBC), 2- mercaptobenzothiazole (MBT), 2,2'-dibenzothiazyl disulphide (MBTS), morpholinylbenzothiazole-2- sulphenamide (MBS), N-t-butylbenzothiazole-2- sulphenamide (TBBS), N-cyclohexylbenzothiazole-2-sulphenamide (CBS) and any mixture thereof. The accelerator may be added in an amount of about 0.3 p.p.h.r. to about 5.0 p.p.h.r., and preferably about 0.5 p.p.h.r. to about 2.0 p.p.h.r.

Any suitable vulcanization activator may be used. The vulcanization activator used may be selected from a group consisting of zinc oxide, stearic acid, and zinc-2-ethylhexanoate which may be used alone or in combination. In the present rubber composition, zinc oxide and stearic acid was used. Zinc oxide may be added in an amount of about 3.0 p.p.h.r. to about 10.0 p.p.h.r., and preferably about 2.0 p.p.h.r. to about 5.0 p.p.h.r while stearic acid may be added in an amount of about 1.0 p.p.h.r. to about 5.0 p.p.h.r., and preferably about 1.0 p.p.h.r. to about 2.0 p.p.h.r..

The alkyd of the present invention was used as a process oil. To evaluate the performance of the alkyd, various amounts of the alkyd was added into the rubber composition, for example, 0.0 p.p.h.r., 2.5 p.p.h.r., 5.0 p.p.h.r., 7.5 p.p.h.r. and 10.0 p.p.h.r..

The method of producing the rubber composition are as follows: i) providing the appropriate amount of a rubber host, a carbon black, a vulcanising agent, an accelerator, a vulcanisation activator and the alkyd of the present invention; ii) mixing all components together at ambient temperature to produce a mixture; and iii) curing the mixture.

Different batches of rubber compositions comprising different proportions of the process oil are prepared. Any suitable mixing device may be used such as a kneader, an internal mixer, an open rubber mill or a continuous mixer. In the present rubber composition, a two-roll mill was used. The mixing of step (ii) may be conducted at room temperature as a start. Any other suitable operating temperature may be used. The operational temperature of the mixing of step (ii) should not exceed about 120 °C, whereas the preferred temperature is about 70 °C to about 100 °C.

In step (iii), the mixture is cured using heat. Any suitable curing device may be used such as heating ovens and hot pressers. In the present rubber composition, a hot presser was used. To determine the suitable curing time and other curing parameters of the rubber composition, the rubber mixture is placed under a rheometer at a curing temperature, preferably about 140 °C to about 180°C. Rheometer results reveal the cure time needed at that particular curing temperature. For the embodiment of the present invention, an about 140 °C to about 150 °C curing temperature was used.

The resulting batches of synthetic and natural rubber compositions having different proportions of the process oil are then subjected to tests to determine the physical properties of the rubber compositions. From the test results, it is shown that the rubber composition comprising the alkyd of the present invention acting as a processing aid has comparable physical properties such as tensile strength elongation at break range, tensile modulus and compression set when compared to a rubber composition manufactured using conventional process oil. After accelerated ageing, the retention of these physical properties was also comparable. In the case of natural rubber, it exhibited superior retention of tensile strength and elongation at break range compared to the rubber composition manufactured using conventional process oil. Details of these tests are in Examples 4 and 5.

EXAMPLES

The following Examples illustrate methods of testing the various physical properties of the rubber composition manufacture using the alkyd of the present invention as a process oil. These Examples do not limit the invention, the scope of which is set out in the appended claims.

Example 1

Physicochemical properties testing of alkyd

Acid number was determined following a procedure adapted from ASTM D1980-87(1998), with a modification in which the alkyd of known weight was dissolved in a mixture of ethanol and toluene in a ratio of 1:2 before being titrated with standardized potassium hydroxide solution.

Glass transition temperatures ( 7^) was measured using a Mettler Toledo DSC 1 differential scanning calorimetry (DSC) analyser and the data was analysed using STARe SW 13.00 software. Temperature calibration was carried out using cyclohexane. Sample was subjected to heat-cooling cycles from -120°C to 80°C at a heating rate of 10°C/min under nitrogen with the flow rate of 50 mL/min. The glass transition temperature ( 7^) was determined as the midpoint of the change in the pre- and post-transition baselines associated with the difference in heat capacity at the glass transition. Result obtained was corrected with cyclohexane and the DSC test was performed following a procedure adapted from ASTM D3418-15 (2015).

Number-average molecular weight was determined using a Malvern gel permeation chromatography (GPC) Viscotek using 3 CLM3005-T5000, Organic GPC/SEC columns. The alkyd was dissolved in tetra hydrofuran (THF) at 0.2% w/v and filtered using polytetrafluoroethylene filter with pore size of 0.45 pm. THF was used as the mobile phase and a flow rate of 0.8 mL/min was employed. Calibration was performed against polyisoprene standards.

The physicochemical properties of alkyd such as acid number, glass transition temperature and number-average molecular weight are indicated in Table 1. _ Table 1: The physiochemical properties of alkyd _ Acid number Glass transition Number-average molecular

(mg KOH/g) temperature, T _g , °C weight, M _n (g/mol)

Alkyd 85.3 31.9 972

The test results confirm that the alkyd has a moderately low number-average molecular weight, M _n of nearly 1000 g/mol and moderately low T _g of about 30°C. The moderately long hydrocarbon chains of the alkyd may improve its miscibility with non-polar polymers or rubbers. The moderately low T _g of the alkyd has a good plasticizing effect during physical mixing with other polymers or rubbers. Also, the alkyd was established to have a higher acid number, indicating it has polar moieties to enhance miscibility with polar polymers or rubbers.

Example 2

Biocompatibility test of the alkyd

The biocompatibility of the alkyd was evaluated using MTT assays against three cell lines, which were human keratinocytes (HaCaT), mouse hepatocytes (H2.35) and canine kidney cells (MDCK). The cytotoxicity level was rated based on the cell viability relatives to control in accordance with the ISO 10993-5:2009 standard. The test was carried out in both dose- and time-dependent manners, with 6 varying concentrations of alkyd (3.125, 6.250, 12.500, 25.000, 50.000 and 100.000 pg/mL), and the cells were treated for 3 prolonged durations (24, 48 and 72 hours). The control group are cells without any alkyd treatment. Table 2 summarized the cytotoxicity results.

Table 2: The Cytotoxicity Properties of Alkyd

Cell viability (%)*

Cell lines Alkyd

HaCaT >80

H2.35 >80

MDCK >80

* In both dose- and time-dependent manners.

In conclusion, it was found that the alkyd yields above 80 % of cell viability over the three cell lines. Thus, alkyds are considered as non-cytotoxic. Example 3

Preparation of carbon black-filled rubber compositions

18 blends of rubber batches using synthetic rubber (9 blends) and natural rubber (9 blends) with varying amounts of the alkyd of the present invention were prepared according to the proportions shown in Table 3 below. A conventional petroleum process oil which is a naphthenic oil was used as comparison in Blends 6 to 9. The components where then mixed using a two-roll mill at ambient temperature. The resulting mixture are then placed under a rheometer at 150°C to obtain curing time and finally cured in a hot presser at 150 °C.

Table 3: Formulation of carbon black-filled rubber compositions

Example 4 Physical properties of the carbon black-filled synthetic rubber composition comprising the alkyd of the present invention

Tensile tests were carried out following BS ISO 37 where tensile strength, elongation at break and tensile modulus at 100% elongation were obtained. Compression set tests using 25% strain was performed in accordance with ISO 815. Ageing condition of tensile tests was set at 100 °C for 72 ± 2 h whereas compression set samples were aged at 70 °C for 22 ± 2 h. The retention percentages of tensile properties were calculated following the equation:

Aged value

Retention (%) = - - - - — x 100%

Unaged value

The results of the tests conducted on the carbon black-filled synthetic rubber composition comprising the alkyd of the present invention are shown in Table 4 below.

Table 4: Comparison of the physical properties of synthetic rubber composition using the alkyd

Example 5

Physical properties of the carbon black-filled natural rubber composition comprising the alkyd of the present invention

Tensile tests were carried out following BS ISO 37 where tensile strength, elongation at break and tensile modulus at 100% elongation were obtained. Compression set tests using 25% strain was performed in accordance with ISO 815. Ageing condition of tensile tests was set at 100 °C for 72 ± 2 h whereas compression set samples were aged at 70 °C for 22 ± 2 h. The retention percentages of tensile properties were calculated following the equation:

Aged value

Retention (%) = - - - - — x 100%

Unaged value The results of the tests conducted on the carbon black-filled natural rubber composition comprising the alkyd of the present invention are shown in Table 5 below.

Table 5: Comparison of the physical properties of natural rubber composition using the alkyd

Conclusion

From Example 2, tests confirm that the alkyd of the present invention is non-cytotoxic where cell viability were above 80% in all tests conducted.

Based on Examples 4 and 5, it is clear that the carbon black-filled rubber composition comprising the alkyd of the present invention used as a processing aid has comparable physical properties when compared to a similar rubber composition that uses petroleum derived process oil. However, it is surprisingly observed that the natural rubber composition in Example 3 possesses superior retention of physical properties, particularly, tensile strength and elongation at break in comparison to the rubber composition using petroleum derived process oil.

Hence, it can be concluded that the alkyd of the present invention is suitable to be used as an alternative to petroleum derived process oil where the alkyd is environmentally friendly, biocompatible, fully derived from renewable resources and non-toxic.

As will be readily apparent to those skilled in the art, the present invention may easily be produced in other specific forms without departing from its scope.