Design, preparation and properties of novel flame retardant thermosetting vinyl ester copolymers based on castor oil and industrial dipentene

Abstract A novel bio-based flame-retardant thermosetting vinyl ester resin monomer was synthesized from castor oil. The chemical structures of the monomer was characterized by FTIR and 1H-NMR. In order to improve its rigidity and expand its application in the field of bio-based materials, it was mixed with certain proportions of another reactive bio-based VER monomer, which had rigid and strong polar groups, and then a series of copolymers were prepared with thermal curing method. Then their tensile property, hardness, morphology of fractured surface, flame retardant property, DMA and thermostability were all investigated. The results indicated that the copolymers had relatively high tensile strength of 11.2 MPa, and the limiting oxygen index is above 23% in all prepared copolymers. DMA demonstrates that the glass transition temperature of the cured resins is up to 56.1°C. Thermogravimetric analysis shows that the copolymers have excellent thermal stability.


INTRODUCTION
Thermosetting vinyl ester resins (VERs) are a novel type of excellent resins that play a very important role in the industry due to their high curing degree, high cross-linking density, broad formulation and low energy consumption 1-5 . The cured materials of VERs are superior in mechanical properties, heat resistance, electrical properties and chemical stability than cheaper polyester resins 6-8 . In addition, the VERs are obtained from the reaction of an epoxy resin and an unsaturated carboxylic acid, which endow VERs with the excellent properties of both epoxy resins and unsaturated polyesters 9, 10 . Thus, VERs have gained growing attention.
Recently, because of fossil depletion and environmental pollution, renewable bio-based materials have attracted much attention, owing to high compatibility, low cost, annual renewability, low pollution, high added value and potential biodegradability 11- 16 . Castor oil (CO) is a high-purity light-yellow oily liquid obtained from the seed of castor plant 17 . CO can form many useful derivatives through familiar organic reactions (Fig. 1) 18 . One major derivative is ricinoleic acid (RA), which has many functional groups, such as hydroxyl, carboxyl and unsaturated double bond groups 19-21 .These functional groups can be epoxidized, esterifi ed and hydrogenated, which makes RA a renewable chemical feedstock for preparation of fi ne chemical products, such as plasticizers, elastomer, biolubricant, polyurethane, and coating 17, 22 . As a byproduct from camphor preparation and pulp--paper industry, industrial dipentene is a very promising material with low price and high output 23 . This renewable material has many important and typical components, such as p-cymene, limonene, terpinolene, camphene, α-terpinene and other menthadienes (Fig. 2) 24 . With two reactive unsaturated double bond groups, industrial dipentene is a good candidate to prepare new products via Diels-Alder reaction, glycidylation, epoxy ring-opening reaction and esterifi cation.
In addition, fl ammability becomes a major factor that hinders further development of VERs in some high--fl ame-retardancy applications 25-27 . The best method for improving fl ame retardance is to introduce reactive fl ame groups into the structures of VER monomers 28 . Among numerous fl ame retardants, P-containing fl ame retardants are the most commonly-used and are particularly effective for VERs 29 . In our previous work, retardant diglycidyl ester of maleinized dipentene modifi ed with methacrylic anhydride and dibutyphosphate ( MDDMD) (Fig. 3) VE monomer with rigid and strong polar groups was prepared 30 . Herein, we emphatically report a simple 'controllable esterifi cation' strategy to prepare RA-based VE monomer for coatings. In order to improve the rigidity of RA-based coating, MDDMD was mixed into the RA-based monomer. Then we prepared series of thermosetting copolymers with different rigid-fl exible characters from MDDRA(retardant diglycidyl ester of ricinoleic acid modifi ed with methacrylic anhydride and dibutyphosphate) and MDDMD monomers. Their fl ammability, thermal properties and tensile properties were all investigated. This study can provide guidance for the design and manufacture techniques of this kind of coatings to some degree.

Synthesis of r etardant diglycidyl ester of ricinoleic acid modifi ed with methacrylic anhydride a nd dibutyphosphate (MDDRA)
First, 150.00 g of r icinoleic acid, 470.00 g of epichlorohydrin and 1.16 g of benzyltriethylamine chloride were added into a 1 L four-necked fl ask with a magnetic stirrer, a thermometer, a refl ux condenser and a dropping funnel, and then heated to 115 o C with mechanical stirring for 2 h. Then the mixture was cooled to 60 o C, and 28.00 g of calcium oxide and 20.00 g sodium hydroxide were added to the fl ask with mechanical stirring and reacted for 3 h. Finally, the light yellowish diglycidyl ester of ricinoleic acid (DRA) was obtained by fi ltering and evaporating.
Then 20.00 g of DRA and 0.16 g of triphenylphosphine were weighed and put into a four-necked fl ask, and heated to 40-50 o C with mechanical stirring, and then 11.86 g of dibutyphosphate was added slowly through a dropping funnel, and then heated to 70-75 o C with mechanical stirring for 4 h, fi nally the yellow viscous ester (DDRA) was obtained. After that, 30.00 g of DDRA, 16.20 g of methacr ylic anhydride, 0.24 g of hydroqu inone and 0.48 g of triphenylphosphine were put into a four-necked fl ask, and was heated to 120 o C and kept reacting for 2 h. Finally the dissociative methacrylic acid was removed by washing with distilled water and weak alkali, and then the residual water was removed by vacuum distillation, and then a yellow low-viscosity liquid resin (MDDRA) was obtained. The synthesis route is shown in Figure 4.

FTIR analysis
The purifi ed MDDRA was subjected to IS10 Fourier--transform infra-red (FTIR) spectroscopic analysis, to monitor the disappearance or formation of various functional groups in the wavelength range of 4000-400 cm -1 .

H nuclear magnetic resonance spectroscopy
1 H nuclear magnetic resonance (NMR) spectra of the purifi ed MDDRA was recorded in CDCl 3 on an ARX300 spectrometer, operating at 500 MHz. Chemical shifts are given in ppm.

Tensile property testing of copolymers
Tensile properties of copolymers were measured by a CMT4303 universal test machine at a crosshead speed of 10 mm/min according to ASTM D638-03. And the test region of the samples was 50.0×13.0×3.2 mm 3 . Five sample pieces were prepared for the sake of accuracy and all the samples were tested at 25 o C.

Morphology of the fractured face
Scanning electron microscopy (SEM) observed on a 3400N Hitachi Co was used to investigate the tensile fractured face of copolymers. The specimens were mounted on an aluminum stub and sputter coated with a thin layer of gold to avoid electrostatic charging during examination.

Hardness of cured copolymers
The hardness of all the cured resins was measured by a TH 210 Shore D durometer at 25 o C.

Flame retardant property
The limiting oxygen index (LOI) values were measured by a JF-3 oxygen index meter (Nanjing, China) according to ASTM D2863-97, and the test region of the samples was 130.0×6.5×3.2 m.

Dynamic mechanical thermal analysis (DMA)
The storage modulus and glass transition temperature were determined by a Q800 dynamic mechanical analyzer (DMA) in stretch test mode. All the cured resins had a dimension of 60.0×10.0×4.0 mm and were swept from -60 to120 o C at a heating rate of 3 o C/min as well as a frequency of 1 Hz.
Thermogravimetric analysis (TGA) Thermal degradation characteristics and the percent weight loss of the cured resins were measured by a 409PC thermogravimetric analyzer (Netzsch, Germany). Each of samples was tested from 25 to 800 o C at a rate of 15 o C/ min under a nitrogen atmosphere.

Fourier transform infrared (FTIR) analysis of MDDRA monomer
The FTIR spectra of DRA, DDRA and MDDRA are shown in Figure 5. As for DRA, the peaks at 2927 and 2855 cm -1 correspond to the antisymmetric and symmetric stretching vibrations, respectively, of C-H in -CH 3 and -CH 2 -, and the peak at 1464 cm -1 corresponds to the bending vibration of C-H. In addition, the absorption peak of epoxy functional group at 856 cm -1 indicates the successful synthesis of DRA. On the spectra of DDRA, the strong peak at 3379 cm -1 corresponds to the hydroxyl stretching absorption, and its strong intensity indicates the formation of a hydroxy system among DDRA molecules. Other peaks appear at 1738 cm -1 (strong, cor responding to the stretching vibration of C=O from the ester groups), 1120 and 910 cm -1 (characteristic of P-O-C), 1030 cm -1 (P-O), and 1245 cm -1 (P=O). However, the peak of epoxy functional group at about 856 cm -1 and the characteristic peak of P-OH at 2600 cm -1 disappear. All these changes indicate the successful synthesis of DDRA. As for MDDRA, new absorption peaks of C=O at 1785 cm -1 and C=C at 1636 cm -1 appear, and the peak of O-H at 3410 cm -1 is weakened. All these changes indicate the successful reaction between DDRA and methacrylic anhydride to form MDDRA.

H-nuclear magnetic resonance ( H-NMR) analysis of MDDRA monomer
The 1 H-NMR spectra of DDRA and MDDRA are shown in Figure 6. For DDRA, the peaks at 3.9 and 4.41 ppm correspond to the protons on the epoxy group, indicating RA has reacted with epichlorohydrin. The strong peaks at about 2.0 ppm correspond to the protons of the hydroxyl group, which proves the occurrence of epoxy ring-opening. The peak at about 3.86 ppm corresponding to the methylene protons next to the hydroxyl group also indicates the occurrence of epoxy ring-opening. For MDDRA, the 1 H-NMR signal of the hydroxyl group at about 2.0 ppm disappears, which indicates the successful esterifi cation between hydroxyl groups and methacrylic anhydride. The appearance of new peaks at 5.58 and 6.15 ppm corresponding to the protons of -CH=CH-further confi rms the occurrence of esterifi cation. In addition, the shift of methyne protons from 3.90 to 4.64 ppm and the appearance of new peaks at 1.93 ppm of methyl protons also indicate the occurrence of esterifi cation. All these 1 H-NMR results indicate that the target product has been successfully synthesized. Figure 7 shows the tensile stress-strain curves of the copolymers with different weight ratios. The details of tensile properties are summarized in Table 1. Clearly, the tensile stress-strain curves show that MDDRA70/ PDA30 has the highest elongation at break and the lowest tensile strength, while MDDMD70/PDA30 shows the lowest elongation at break and the highest tensile strength, indicating MDDRA and MDDMD are characteristic of fl exible and rigid materials, respectively. With the decrease of MDDRA content, the tensile strength increases, ranging from 1.9 to 11.2 MPa, while the elongation at break decreases, ranging from 35.5% to 13.1%. It is generally known that the tensile properties of the copolymers are simultaneously determined by the cross-linked state and the chemical structures of the copolymers. Compared with MDDRA, the rigid endocyclic structure of MDDMD monomer can endow the copolymerized system with higher tensile strength and lower elongation at break. With the increasing content of MDDRA monom er in copolymerized system, the content of fl exible aliphatic chains structure increases, and the content of rigid endocyclic structure decreases, the rigidity of copolymerized system also decreases. In addition, the nonterminal double bond located on the endocyclic structure of MDDMD monomer also possess cross-linking curability. Thus the tensile stress-strain curves of the copolymers displayed such a changing trend. Table 1 also shows the shore hardness of copolymers with different weight ratios. With the decrease of MDDRA content, the shore hardness of copolymers increases. This is mainly because the weight proportion of rigid alicylic hexatomic str ucture from MDDMD increases and proportion of long-chain fl exible structure from MDDRA decreases with the increase of MDDMD content. This analysis result is consistent with the changing trend of tensile properties.

Morphology of the fractured surface
The morphologies of tensile fracture surfaces of copolymers were investigated by scanning electron microscopy (SEM). The SEM graphs displayed in Figure 8 show that the MDDMD70/PDA30 copolymerized system displays a brittle fractured character with smooth glass-like fractured surfaces, it is mainly because the MDDMD70/ PDA30 cured system has more rigid alicyclic hexatomic structure. Moreover, the samples are uniformly forced with tensile rupture, so the copolymerized system has the characteristic of obvious fast fracture. On the contrary, with the increase of MDDRA monomer content, the surface fl atness of copolymers decreases, and the shear zone of the crack branch is signifi cantly enlarged. It could be explained by the fact that the cured MDDRA70/ PDA30 system is a fl exible material, so the increase of MDDRA content would reduce the surface fl atness of copolymers.

Flame retardant property
The fl ammability of copolymers characterized by the limiting oxygen index (LOI) test is shown in Figure 9. LOI could determine the minimum oxygen concentration that supports the combustion of polymers. It is clear that LOIs are all above 23.1%, and the LOIs decrease from 27.2% to 23.1% with the increase of MDDRA content in copolymers. Compared with traditional VER    Figure 10 shows the storage modulus (E ' ) and tanδ curves of copolymers with different weight ratios. Dynamic mechanical analysis (DMA) is a versatile thermal analysis technique to quantitatively study the viscoelastic behavior of cured copolymers. Clearly, the E ' of the cured copolymers increases with the decrease of MDDRA content and the E ' curves of all the cured copolymers show a similar trend. Briefl y, at about -80 o C, E ' remains at a high level from 2250 to 3750 MPa, and with the temperature rise from -80 to 40 o C, E ' decreases and eventually all the E ' values are close to a constant level below 100 MPa. The E ' of prepared copolymers increased with the decreasing content of MDDRA monomer in copolymerized system. The cause could be consistent copolymerized materials, the fl ammability of our prepared copolymers get improved to a certain extent. It concluded that the P content in MDDRA monomer is less than that in MDDMD monomer, and the P content rises from 3.12 wt% to 4.71 wt% with the decrease of MDDRA monomer content. As we know, P can both promote carbonization and inhibit combustion, a higher P content in the copolymers decelerates the burning and probably provides better fl ame protection for the copolymers. In addition, the rigid alicyclic hexatomic structure in copolymerized also could improve the fl ammability of copolymers. So the LOIs of copolymers increased with the decreasing content of MDDRA monomer in copolymerized system.   with the affecting factors of tensile properties. With the increase of MDDMD content, more fl exible aliphatic chains from MDDRA are replaced by the rigid endocyclic structure from MDDMD, leading to a reduction of the free volume in the polymeric matrix and bestow the copolymers with certain high rigidity, and in this case chemical structure could be the main infl uence factor on the properties of the copolymerized system, so the MDDMD70/PDA30 system has higher E ' . E ' is related to the crosslink density (ν e ) of cross-linked materials in the rubbery state, which could be used to calculate ν e as follows:

Dynamic mechanical thermal properties of copolymers
where E ' is the storage modulus at T g + 20 o C, R is the gas constant and T is the absolute temperature at T g + 20 o C 31 .
In order to ensure the copolymers are in the rubbery state, we set the temperature, where E′ was taken, at T g + 20 o C for each sample. Table 2 shows the crosslink density of the cured copolymers. Clearly, the ν e of the cured copolymers increases with the increase of MDDMD content. Theoretically, the MDDRA monomer and the MDDMD monomer have two same reactive terminal do-uble bonds, which are relatively structurally symmetrical in the MDDMD monomer and structurally unsymmetrical in the MDDRA monomer. And the relatively symmetrical structure in MDDMD monomer could make for improving the ν e of the cured copolymers. In addition, the nonterminal double bond located on the endocyclic structure of MDDMD monomer also possess cross-linking reactivity. So the ν e value of the cured copolymers increases with the increase of MDDMD content Compared with MDDMD monomer, though MDDRA has a much small molecular structure, MDDRA monomer still has less reactive cross-linking point because MDDMD monomer has another reactive double-bonds located on alicylic hexatomic structure.
The peak temperature of tanδ corresponds to the glass transition temperature (T g ). On the tanδ curves (Fig. 10), each cured copolymer has a very clear single T g , which indicate all the cured copolymers are compatible and have a homogeneously cross-linked structure (Fig. 11). Clearly, all the cured copolymers have a relatively high T g up to 40.9-56.1 o C, and T g increases with the increase of MDDMD content, probably because of the molecular structures and cross-linked state of the copolymerized system, which is similar to the cause of mechanical proper- Figure 11. The possible crosslinking schematic diagram of copolymerized system Table 2. The crosslink densities of copolymers with different weight ra ties. With the increasing content of MDDMD monomer, the introduction of more rigid and strong polar groups from alicyclic groups somehow overcome the action of long fatty acid chains on copolymerized system, so the T g increases with the increase of MDDMD content. In addition, with higher MDDRA content, the molecular chains of the cured system become more mobile, and relaxing process of molecular chains is prolonged, so the tanδ curves show wider and lower peaks.
The thermal stability of the cross-linked networks plays a very important role and is greatly infl uenced by the molecular structure, concentration and type of the remaining polar groups, chemical composition, and molecular chain rigidity. Figure 12 shows the TGA curves of copolymers with different weight ratios. The initial decomposition temperature (T i ), temperature of maximum mass loss (T max ), fi nal decomposition temperature (T f ) and the char yield at 800 o C are all listed in Table 3. Clearly, all the copolymers are thermally stable even up to 200 o C and all the TGA curves display a single-step mass loss decomposition behavior.
Weight loss started at above 273 o C in all the cured copolymers, while the fi nal char yields of the copolymers are all above 13.1%, indicating that all the copolymers have high thermostability. The mass loss might be attributed to the thermal degradation of dibutyphosphate groups on Figure 12. TG analysis curves of copolymers with diff erent weight ratios Table 3. TG analysis data of copolymers with different weight ratios MDDRA and MDDMD, as well as the rupture of ester groups and C-C groups from MDDRA and MDDMD. The thermograms (Table 3) show that the concentrations of MDDMD do not much affect the nature of cross-links formed during curing, so the T i and T max are both close between the two mixed resins with different MDDRA/MDDMD weight ratios. The thermostability of M DDMD70/PDA30 is higher compared with other mixed copolymerized systems. The high thermostability of MDDMD70/PDA30 is probably attributed to the large proportions of rigid alicyclic structures and endocyclic structures, which delay the decompositi on of copolymer and subsequent diffusion of decomposition products. Moreover, all the data do not display a good uptrend, but the reasons are complex. For different curing copolymers, the molecular structure of mixed monomers, curing degree and crosslink density can affect the thermostability of copolymers. Thus, the thermostability of prepared copolymers c hanges irregularly.

CONCLUSIONS
A novel bio-based VER monomers named MDDRA and a series of copolymers containing MDDRA, MDDMD and PDA were successfully prepared. It was proved that MDDRA70/PDA30 was a fl exible material with relatively low tensile strength, modulus and hardness, while MDDMD70/PDA30 was a rigid material with relatively high tensile strength, modulus and thermal stability. The hardness of cured resins ranged from 40.2 to 5.1 HD and the LOIs varied from 27.2 to 23.1 with the increase of MDDRA content. DMA shows that the glass-transition temperatures of the copolymers increased from 40.9 to 56.1 o C with the increase of MDDMD content. TGA demonstrated that the copolymers containing higher content of MDDMD had higher thermal stability, and the thermal initial degradable temperatures of the copolymers were all above 270 o C, demonstrating the high thermal stability of all the copolymers. All the evidence demonstrated that the addition of MDDRA monomer could improve the toughness of copolymerized system. This study presents a perfect combination of rigid alicyclic structure and fl exible aliphatic chain structure. It also has signifi cant implications for the design of fully bio-based novel fl ame retardant thermosetting vinyl ester copolymers with desired properties.