## 5 Wax Chemical Inhibitors
### Pour Point Depressant (PPD)  
PPDs function by co-crystallizing into the paraffin structure via van der Waals forces. This allows the polar end tail (unattached) to form a steric interference with the alignment of other new wax molecules. PPDs decrease the pour point temperatures of the crude oil and then weaken the wax deposition solid structure, thus allowing for the easy removal of the deposition by shear force. It should be noted that the decrease in the crude oil's PPD does not reflect the decrease in the WAT of the crude oil, as PPDs do not inhibit the crystallization of the wax crystals but rather inhibit the growth of the wax crystals [33; 88]. The difference between PPD and wax dispersant is that the wax dispersant adsorbs into the wax crystal and hinders the further sticking of the wax molecules. Examples of PPD are polyethylene vinyl acetate (EVA), methyl methacrylate (MMA), olefin-maleic anhydride copolymer (MAC), and diethanolamine (DEA) [63; 89]. One of the most common PPDs used in the industry is polyethylene vinyl acetate (EVA). EVA is the product of the copolymerization of ethylene and vinyl acetate (VA). There are two processes involved in the wax deposition inhibition mechanism. Firstly, the polar compound, which is VA, comprises methyl and methylene groups that contains two active oxygen atoms--due to this, EVA exhibits strong van der Waals interaction with the long-chain paraffin waves. As a result, the wax solubility is increased, the gel strength of the wax is reduced, and ultimately there is a reduction in the wax deposition [75; 90; 91]. Secondly, the non-polar long alkyl moieties of EVA will interact with the long-chain paraffin waves. This interaction will then alter the wax crystallization process, which leads to a reduction in the pour point value (or a lower WAT) [68]. The shape and growth of the wax crystals will form in different axial directions when different EVA concentrations are utilized [91]. Meanwhile, the concentration of EVA does not directly influence the PPD performance. For example, EVA (with 20 wt % VA content) at 50 ppm and 500 ppm concentrations both reduced the pour point by more than 26 \({}^{\circ}\)C [76]. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Type of Crude Oil** & **Sample Used as Wax Crystal Modifier** & **Year/References** \\ \hline Malaysia & Silane-based surfactants with SiO\({}_{2}\) particles & 2018/[77] \\ \hline China (Xuzhou) & Nano composite of montmorillonite & 2016/[78] \\ \hline Karama & Poly (cinamoyloxy ethyl methacrylate-co-octadecyl arylate) & 2008/[79] \\ \hline Iranian & Ethylene vinyl acetate copolymer & 2008/[80] \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Examples of wax crystal modifier used in was deposition study. Another type of PPD that is commonly used in the industry is the comb polymer, which can be categorised into two different polymer groups: maleic anhydride copolymer (MAC) and poly-acrylate/ methacrylate (PA or PMA) ester polymers. The length of the pendant chain as well as the length of the chain wax molecule should be similar to obtain optimum deposition inhibition performance. The polyvinyl backbone of the polymer has little or no effect on the performance of the inhibitor; in contrast, amending the backbone regularity significantly affects the structure of the polymer which functions as a pour point as well as the degree of the crystallinity reducer [46; 89]. MAC, PA and PMA performed best with longer alkyl chains compared to short alkyl chains by reducing the wax crystal size. This is because the longer the alkyl chain, the higher the solubility of the copolymer in the paraffin wax structure; thus, the pour point is reduced from 27 \({}^{\circ}\)C to \(-3\)\({}^{\circ}\)C (at 10,000 ppm) [84; 92]. [PERSON] et al. (2012) also reported that MAC is not only a good pour point reducer agent but also an excellent reducer of the deposition rate of wax and the yield stress of the wax gel [93].