Preparation and properties of natural rubber latex-g-modified cationic polyacrylamide copolymers and its palm oil absorbent

Materials

The main materials used for this work were 60% HA-latex (Chana company, Thailand), potassium persulphate 99%, RFCL Ltd (India) and CPAM (Zhejiang, China). Palm oil was purchased by Local market in Songkhla, Thailand. Terric 16A16 as a non-ionic surfactant was supplied by Lucky Four Co. Ltd. Palm oil was purchased locally in HatYai, in Thailand.

Preparation of natural rubber grafted with MCPAM (NR-g-MCPAM)

A 5 g sample of CPAM was dissolved by stirring in distilled water at 80±3°C for 1 h. After cooling, the CPAM solution was mixed with K₂S₂O₈ solution, and stirred at 60°C for 45 min, to produce MCPAM. Then 17 g of NR latex in the presence of Terric 16A16 was mixed with the MCPAM and stirred at 60°C for 3 h. The influence of MCPAM concentrations of 0, 25, 50, 100 and 150 phr on the properties of the NR-g-MCPAM was investigated. The mixtures were cast on glass plates then left at room temperature for 3–4 days. The ungrafted NR and MCPAM were also extracted by toluene and water. ²⁴ After that, it was dried in an oven at 50°C for 24 h and kept it in desiccator before characterisation in next step.

Characterisation of NR-G-MCPAM

The chemical structure of polymer composite was investigated by using an attenuated total reflection, Fourier Transform Infrared spectrophotometer (ATR-FTIR) (Equinox 55; Bruker) for 100 scans. For determination of the swelling ratios in toluene, the samples were dried at 50°C for 24 h and weighed until a constant weight was obtained. The degree of swelling ratio was estimated from equation (1) (1) where W₁ is the original weight of the dried sample and W₂ is the weight of the swollen sample after immersion for 5 days. For thermal analysis, TGA7 (Perkin Elmer) was used for testing a sample (5–6 mg) under N₂ with a flowrate of 45 mL min⁻¹, and from 50 to 850°C at a heating rate of 10°C min⁻¹. In addition, TGA was heated from 50 to 300°C and used to prepare the NR-g-MCPAM to continue investigations of the chemical structure by ATR-FTIR. For the determination of oil absorption, 0·1 g of product was immersed in palm oil solution (palm oil diluted with toluene, 10% oil). The swollen gels were taken out at regular time intervals, gently blotted with tissue paper, weighed on a balance and replaced into the same bath. This procedure of swelling and weighing was continued until the sample achieved a constant final weight. The equilibrium oil-absorbency of this sample Qeq can be calculated by equation (2) (2) where W₀ is the initial weight of the sample, and W_e is the weight at equilibrium. Experiments were run in triplicate and average values were reported as a mean. The Zeta potential of modified NR particles in latex was determined by using a microelectrophoresis apparatus (Zetasizer 4, Malvern) at 25°C at pH 7.

ATR-FTIR study

Details of the possible mechanisms for forming a graft copolymer from MCPAM using K₂S₂O₈ to generate active radicals are presented in Fig. 1. When heated the K₂S₂O₈ decomposed to persulphate radicals. The persulphate radicals would then abstract the allylic hydrogen from the NR molecules. At the same time, the persulphate radicals would attract the hydroxyl group from MCPAM as shown in Fig. 1. A difficult dissolution of the graft copolymer in an organic solvent was achieved after synthesis of the graft copolymer. The chemical structure of NR-g-MCPAM was then confirmed by ATR-FTIR. Figure 2 shows the chemical structure of NR-g-MCPAM as observed by ATR-FTIR.

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Possible chemical reactions of NR-g-MCPAM using potassium persulphate as catalyst

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2

ATR-FTIR spectra of MCPAM, NR and NR-g-MCPAM having 12 wt-% of MCPAM

A C–H stretching at 2854, 2926 and 2961 cm⁻¹ was observed in the NR. In addition, the C–H deformation of –CH₃, the C-H deformation of –CH₂– and the C–H deformation of cis C = C–H from NR was found at 1448, 1375 and 836 cm⁻¹ respectively. The main wave numbers of MCPAM were found at 1347 (CH₂ wagging), 1450 (CH₂ bending), 1618 (NH₂ wagging), 1660 (C = O), 2935 (CH₂ symmetric stretching), 3199 (NH₂ symmetric stretching) and 3342 cm⁻¹ (NH₂ asymmetric stretching). The peaks at 3300–3500 cm^–1 were due to amine groups, whereas the peak at 1659 and 952 cm^–1 was due to the amide group and quaternary ammonia. These results were confirmed by Lu and co-worker. ²⁰ In the case of the graft copolymer, the main peaks of NR-g-CPAM was located at 3034 and 1663 cm⁻¹ that were referring to NH₂ symmetric stretching and C = O, respectively. Moreover, the main peaks of NR still remained but the peak of NR slightly shifted when compared to the NR alone due to its hydrogen bonding in the graft copolymer.

Thermal behaviour of NR-g-MCPAM

TGA thermograms of plain NR, MCPAM, and NR-g-MCPAM are presented in Fig. 3. The weight loss of all samples started at 100°C, which was due to removal of water. This result was supported by Riyajan and co-worker. ⁶ A three-stage thermal degradation profile was observed for all samples, the first stage of degradation in plain NR, MCPAM and NR-g-MCPAM specimens was negligible. The first weight loss stage varied with no specific trend for different compositions. This may be attributed to some residual water content in the network and the remaining acetate functional groups which are more susceptible to cleavage than the OH functional group. ⁶ The overall temperature range for first stage was up to 200°C; the second stage from 200 to 400°C and the third stage from 400°C up to 600°C. The thermal stability of NR and CPAM altered significantly after modification with MCPAM. The lower and higher thermal stability in NR and CPAM, respectively were mainly due to the presence of the grafted copolymer.

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TGA thermograms of MCPAM, NR and NR-g-MCPAM having 12 wt-% of MCPAM

Influence of MCPAM on swelling ratio of NR-g-CPAM

In water medium, the swelling ratio of the grafted copolymer increased as a function of the MCPAM content as shown in Fig. 4. The highest swelling ratio of the grafted copolymer was found in the presence of 63% MCPAM. This is due to the hydrophilic behaviour of MCPAM in the grafted copolymer. This corresponded to previous work. ⁶ Then it was found that the swelling ratio of NR-g-ST was dramatically increased due to the presence of hydrophilic starch. The highest swelling ratio was found in samples containing 150 phr ST. This was because ST exhibits good water solubility and leads to an excellent swelling ratio in water. Therefore, at 63% MCPAM, the excess MCPAM from the NR-g-CPAM may dissolve in water. In the case of the toluene medium, the lowest swelling ratio of the grafting copolymer was found at 12%MCPAM. However, when more than 12% MCPAM was added, the swelling ratio of this sample increased with increasing MCPAM content due to the positive charge of MCPAM in the grafted copolymer. The highest swelling ratio was observed in samples in the presence of 63% MCPAM. This helps to explain that the excess MCPAM posseses positive charge. The positive charge in the grafted copolymer was directly related to the ability to absorbe toluene. These results agreed with the absorption of oil as shown in the oil absorption study section.

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Influence of MCPAM on swelling ratio of NR-g-MCPAM in water and toluene

Effect of temperatures on swelling ratio of NR-g-MCPAM

The swelling ratio of the NR-g-MCPAC decreased as a function of temperature in water (Fig. 5a). The lowest swelling ratio was observed at 90°C. The swelling ratio of the grafted copolymer at 30, 50, 70 and 90°C was 1·1, 0·9, 0·1 and 0 respectively. In the presence of toluene, the swelling ratio of the graft copolymer decreased with rising temperature (Fig. 5b). However, when the temperature rise increased by 50 to be 70–90°C, the swelling ratio of the sample increased. This is due to chain scission in the graft copolymer. The optimal temperature for the grafted copolymer was 70°C. This result was also supported by Riyajan and co-worker, ⁶ when they studied the preparation of a graft copolymer made from NR and cassava starch.

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Influence of temperature on swelling ratio of NR-g-MCPAM having 12 wt-% MCPAM in water and toluene

Effect of initiator on swelling ratio of NR-g-MCPAM

The influence of K₂S₂O₈ and Na₂S₂O₃ on the swelling ratio of the grafting copolymer in water is displayed in Fig. 6a. After adding K₂S₂O₈ to the grafting copolymer, the swelling ratio of this sample was dramatically decreased due to the grafting of MCPAM onto the NR molecule. The swelling ratio of the grafted copolymer at 0·5 g showed the lowest value when compared to other samples due to the highest chemical interactions occurring between the MCPAM and NR.

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Influence of initiator on NR-g-MCPAM having 12 wt-% MCPAM in water and toluene

In a toluene medium, the swelling ratio of the grafted copolymer decreased as a function of the K₂S₂O₈ as more free radical was formed after adding more K₂S₂O₈. Then, the cross-linked formations were produced and the swelling ratio of the grafted copolymer decreased. ⁶ This result is opposites to the results with Na₂S₂O₃. When Na₂S₂O₃ was added in increasing amounts from 0·1 to 0·3 g, the swelling ratio of the grafted copolymer increased. This result indicated that chain scissions occurred during the copolymer grafting process. Therefore, K₂S₂O₈ is a good initiator for the graft copolymer reaction. Na₂S₂O₃ exhibited a much weaker initiating activity compared to K₂S₂O₈; this is especially typical of the effects of Na₂S₂O₃ concentrations, between 0·1 and 0·3 g and can be explained by differences in the reactivity of the initiator during the grafting reaction.

Effect of DMAMMC on swelling ratio and zeta potential of NR-G-MCPAM

The effect of DMAMMC in toluene on the swelling ratio of the grafted copolymer is presented in Fig. 7. It was clear that the swelling ratio of the graft copolymer decreased with increasing DMAMMC. Owing to the polar characteristics of the DMAMMC, the swelling ratio of this sample decreased. This is due to the greater number of polar groups in the resulting graft copolymer.

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Influence of DMAMMC on swelling ratio of NR-g-MCPAM having 12 wt-% MCPAM in toluene

The effect of DMAMMC on the zeta potential of the graft copolymer is presented in Fig. 8. The zeta potential of this sample increased as a function of the DMAMMC concentration while zeta potential of NR latex was −35 mV due to charge ion from phospholipids and protein in NR latex.^{10, 24} The highest zeta potential of this sample was found at 20 g of DMAMMC. The adsorption of DMAMMC onto the NR-g-MCPAM hydrogel surface was indicated by an increase in the absolute value of the zeta potential when compared to that at lower amount of DMAMMC (Fig. 8). The adsorbed polymer caused alterations to the electrical double layer of the charged particle due to the ion redistribution and, consequent, reduction of the surface charge.

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Influence of DMAMMC on zeta potential of NR-g-MCPAM having 12 wt-% of MCPAM

This result agreed with Yang and co-worker. ²⁴ They found that the zeta potential of the absorbents increased as a function of the cationic groups in the sample.

Palm oil absorption study

The grafted copolymer (NR-g-MCPAM) samples in the presence of 2 g DMAMMC was used to see if it would absorbe palm oil (Fig. 9a and b). The method for evaluating the amount of palm oil absorbed was estimated from equation (2). After the grafted copolymer samples containing different amounts of MCPAM were immersed in toluene or water containing oil for 2 h it was clear that the stability of the NR was much lower than any of the grafted copolymers. From these results it is concluded that the advantages of these copolymer absorbents are that they are highly stable, flexible and easy to use in the environmental. The effect of time on the absorption of palm oil by the graft copolymers is shown in Fig. 9. It is clear that the absorption of palm oil from toluene by all samples increased as a function time, and reached a maximum value of 10 g g⁻¹. From Fig. 9a, it can be seen that the absorbency of oil by the grafted copolymer increased with increasing MCPAM, but there was one exception at 12% MCPAM. The reason was that MCPAM and DMAMMC posses positive charges and these play a big role in the ability to absorbe oil (Fig. 10) large agglomerates of oil were achieved after adding the graft copolymer (Fig. 10) There are many benefits arising from the use of grafted copolymers to absorbe oil The positive charges in the grafted copolymers was directly related to their ability to absorbe oil.

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Oil absorbency of NR and NR-g-MCPAM with different MCPAM contents in presence of 2 g DMAMMC

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Possible palm oil of -g-MCPAM having 12 wt-% MCPAM and 2 g DMAMMC

Because of the described features, this adsorbent may be highly beneficial to extract oil from contaminated toluene and water. The advantages of this absorbent are its low cost, a high efficiency to absorbe oil and easy to use and recover oil. This results agrees with the absorption of oil using cationic organic polymers like P(AM-DMDAAC-BA). ²⁰ They possess a high positive charge and are thus more effective in the coagulation process of oily wastewater. Considering their performances, P(AM-DMDAAC-BA) would prove to be very effective for removing oil from wastewater.

The lowest palm oil absorption was found for a grafted polymer containing 12% MCPAM. Since, the swelling ratio of this sample in toluene was lowest compared to other samples as shown in Fig. 4. The small flocs formed by coagulation could be built up into larger agglomerates by flocculation with the copolymer, with the larger particles formed in this way having accelerated rates of sedimentation. ²⁴ With these performances, P(AM-DMDAAC-BA) was thought to be effective in the removal of oil from wastewater. While the flocculant dosage increased to some extent, the palm oil removal efficiency decreased and restabilisation took place. The reason was that the number of cationic groups obviously increased and the surface of the colloid particles would be positively charged, and this was directly related to the repulsion between the added polymer molecules and the polymer chains that were already adsorbed on the colloid and particle surface. ²⁴ Flocculation experiments of oily wastewater indicated that the degreasing effect of P(AM-DMDAAC-BA) optimised at the dosage of 50 mg L⁻¹, which is a comparatively higher dosage. In order to reduce the operating cost and ensure the treatment efficiency simultaneously, it was very essential to make use of P(AM-DMDAAC-BA) together with some other cheap natural macromolecule flocculant aids or some inorganic flocculants.

In the case of water containing oil, the relation between the palm oil absorption and different MCPAM contents of the graft copolymers is shown in Fig. 9b. These results showed the same trend with diluted palm oil in toluene medium. The highest and lowest in oil absorption was found at 63 and 12% MCPAM in the grafted copolymer respectively.

However, the oil absorption of the sample in water medium was lower than that of in toluene medium. This is due to its lower swelling in the water medium containing oil.