REACTIVE MODIFICATION OF HDPE
5.1 Characterisation of Raw Materials
In this work, the selection of reactive components for the blends is vital in controlling the degree of grafting reactions. As highlighted by Chang [22], excessive grafting may result in a highly branched comb-like structure or even a crosslinked network and thus a lightly grafted copolymer is believed to be a more efficient compatibiliser than an excessively grafted one.
For the above reasons, a HDPE-g-MAH with a low level of grafted MAH (1 wt%) and a low molecular weight di-functional solid DGEBA type epoxy resin were selected for this investigation. Being solid at room temperature enables good physical mixing characteristics of the epoxy resin with other thermoplastic resins and additives in both pellets and powder forms before the extrusion (compounding) process. Because of the low molecular weight nature of the epoxy resin, effective diffusion of its functional groups to the blend interfaces for reaction can be expected.
As the reactive grafting process could cause a reduction in flow rates of HDPE resin due to possibilities of extensive chain branching and gelation as discussed earlier, a high flow rate injection moulding grade HDPE resin was selected for this study to ensure sufficient flowability of the final ternary blends.
It is known that cyclic maleic anhydride undergoes hydrolysis in the presence of water producing dicarboxylic acids while heating reverts these dicarboxylic acids back to the five-membered cyclic anhydride form [198]. These reversible reactions can be represented by Reaction Scheme 1. The dicarboxylic acids and five-membered cyclic anhydride can be identified through FTIR analysis and are represented by peaks located around 1714 cm-1 (due to C=O stretching of carboxylic acid) and 1866 cm-1 / 1790cm-1 (due to asymmetric / symmetric C=O stretching of cyclic anhydride) respectively [58, 198]. The presence of another characteristic band around 916 cm-1 is due to the symmetric COC stretching of the cyclic ethers.
Reaction Scheme 1: Hydrolysis and dehydration of maleic anhydride moiety of HDPE-g-MAH
The HDPE-g-MAH used in this work contains both dicarboxylic acids and cyclic anhydride as shown in the FTIR spectrum in Figure 5.1. Some of the cyclic maleic anhydride moieties appeared to be hydrolysed by the atmospheric moisture during storage and also possibly by moisture that was picked up during the manufacturing (grafting) process. The sharp doublet at
Figure 5.1 FTIR spectrum of HDPE-g-MAH
Heating the HDPE-g-MAH pellets in an oven at 110°C converts the dicarboxylic acid back to their cyclic anhydride form as shown in Figure 5.2.
The FTIR spectra indicate that it is possible to convert almost all dicarboxylic acid into its cyclic form after a prolonged heating time (above 40 hours).
Figure 5.2 FTIR spectra of HDPE-g-MAH with various drying time
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and epoxy through reaction between maleic anhydride and epoxy moieties.
However, as shown in Reaction Scheme 2, it is known that the cyclic anhydride groups do not react directly with epoxy groups [199].
Reaction Scheme 2: Reaction between epoxy and cyclic anhydride moieties
Thus it is essential to convert the anhydride rings to dicarboxylic acid through the hydrolysis reaction shown in Reaction Scheme 1 before esterification with epoxy ring of the DGEBA can take place as illustrated in Reaction Scheme 3.
One of the carboxylic acid groups reacts with the epoxy ring to form a half-ester and a hydroxyl group. This is followed by another reaction of the remaining carboxylic acid moiety of the half-ester with another epoxy moiety to form an ester linkage and another hydroxyl group. Therefore as illustrated in Reaction Scheme 3, the DGEBA can be anchored onto the backbone of the HDPE molecule through esterification between the carboxylic acids and the epoxy ring.
Reaction Scheme 3: Expected esterification reaction between anhydride and epoxy group catalysed by hydrated zinc acetate
Bayram and his co-workers [200, 201] reported the effectiveness of using hydrated zinc acetate as an esterification catalyst in reactions involving styrene maleic anhydride/GMA [200] and styrene maleic anhydride/polyol [201] blends. Hydrated zinc acetate has been selected in this study as a potential catalyst for catalysing the esterification reaction between the anhydride and epoxy groups as it is expected to be able to liberate water molecules during processing via dehydration as shown in Reaction Scheme 4 [202].
Reaction Scheme 4: Dehydration of zinc acetate dihydrate [202]
Thermogravimetric analysis (TGA) performed on the hydrated zinc acetate as shown in Figure 5.3 indicates that the dehydration process started at around 72°C and ends at around 114°C. About 16wt% of water was librated during the dehydration process and this is in good agreement with the molecular formula of the zinc dihydrate. Thus the liberation of water molecules from the hydrated zinc acetate can be expected during the compounding processes.
Figure 5.3 TGA thermogram of zinc acetate dihydrate Water ↑
Acetate ↑
Zinc Oxide
The FTIR spectrum of the DGEBA resin used in this study is presented in Figure 5.4. The spectrum reveals the presence of characteristic absorption bands of the para disubstituted aromatic ring at 829 cm-1, epoxy group at 915 cm-1, the geminal dimethyl groups of bisphenol A as a doublet at 1384 cm-1 and 1362 cm-1, the aromatic ether group at 1039 cm-1 and 1245 cm-1, and the hydroxyl group can be seen at 3446 cm-1. The detailed peak assignments of this unreacted epoxy resin are listed in Table 5.1.
Figure 5.4 FTIR spectrum of epoxy resin
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Table 5.1 DGEBA Infrared Spectral Peak Assignments [203-205]
Wavenumber, cm-1 Assignment
3446 O-H stretching
3055 -CH-(O-CH2) epoxy stretching
3037 Aromatic C-H stretching
2966, 2931, 2872 Aliphatic C-H stretching 2070, 1887 Disubstituted aromatic rings 1607, 1582, 1509 Aromatic C=C stretching
1462 Methylene C-H bend
1413 (C-H ) epoxy deformation
1384 & 1362 (doublet band) CH3 bending of the geminal dimethly groups of bisphenol A
1245 Phenyl-oxygen stretching
1183, 1085 In-plane aromatic C-H bending
1039 Aromatic ether alkyl C-O stretching
1012 In-plane aromatic C-H bending
945 Out-of-plane aromatic C-H bending
915 Epoxy ring (CH-O-CH2) deformation
830, 573, 559 Out-of-plane bending of 2 adjacent H of para-disubstituted aromatic rings
proposed to be the characteristic bands for the epoxide group [203]. As mentioned by Nishikida and Coates [203], for bisphenol A type epoxides, the first of these bands is expected to occur at around 1233 cm-1. The overlapping of this band with the aromatic ether absorption band at 1245 cm-1, leaves the 915 cm-1 absorption of the terminal epoxy group,
as the main band for characterization, and for monitoring the kinetics of curing and the determination of unreacted epoxide groups. However in this study, the 915 cm-1 region of the epoxy spectrum cannot be utilised for the monitoring of epoxy reaction as it is overlapped by components of the HDPE and HDPE-g-MAH as illustrated in Figure 5.5. The 908 cm-1 region of the HDPE spectrum is due to the presence of vinyl unsaturation (−CH = CH2) while the 916 cm-1 of the HDPE-g-MAH as discussed earlier is due to the symmetric COC stretching of the cyclic ether. Due to the overlapping of major FTIR absorption peaks of the terminal epoxy groups of the DGEBA, the carbonyl region of the blends were evaluated for possible reactions through monitoring of the maleic anhydride / dicarboxylic acid content and also formation of new ester groups due to esterification reactions.
O CH2 CH
O CH2 CH
Figure 5.5 FTIR spectra of HDPE, HDPE-g-MAH and Epoxy