3.1 Polymer types of plastic film in beer cans
In this study, polymer type of coated part of cans including inside and outside layer for both body and lid were identified. FT-IR results showed that the polymer types used for coating were varied even within the same can. Fig. 1 illustrates that in one of samples, SWE-B-1 can, three different polymeric coating were used. Polyethylene terephthalate (PET) and epoxy resin were used in inside and outside coating of lid, whereas for inside and outside coating of body, epoxy resin and poly(1,2-butanediol isophthalate) were used respectively. Apart from the country origin of cans, epoxy resin was the prevailing polymeric coating used in both body and lid in all cans analyzed (Fig. 2, Table S4). Especially, inside layer of body was coated by epoxy resin in all Asian beer cans (n=12). Epoxy resin is commonly used in can coating as it features firmness to heat condition, adhesion, formability, chemical resistance under many conditions17. Not to mention it also flexible and adhere well to different metal surfaces26. Epoxy resin was also frequently used in inside layer of North American and European beers bodies, followed by poly(ethyl methacrylate) and poly(ethylacrylate-co-styrene) (Fig. 1, Table S4). As some printing exhibited in almost all surface of the outside coatings of can body, the polymeric coating used were more varied with poly(1,2-butanediol isophthalate) being the most frequently employed in European beer cans (Fig. 1).
The lid part used different type of coating materials with epoxy resin was the commonly used especially for outside layer (Fig. 1). While the inside layer employed not only epoxy resin but also PET in European and North American cans and phenoxy resin in Asia and European cans (Fig. 1, Table S4). Due to various concern to BPA, other polymeric coating materials have been introduced to the market, such as polyester and acrylic-phenolic materials27. Phenolic resins made from phenols and aldehydes are highly corrosion resistant and have less flexibility properties26.
3.2 Chemical additives contained in beer cans
Both body and lid part of all cans were subjected to analyzed. In general, the concentrations of additives in body part were higher than those in lid part (Fig. 3). Additive’s composition in both body and lid were dominated by phthalate additives (PAEs) followed by BHT. DEHP was used as the major PAE additive used in can coatings (Fig. 4), occupying for 71% and 89% both body and lid respectively and followed by DBP as the second frequently used PAE additive. At least an average of 450 ng/g of DEHP (Table 2) was detected beer can with the highest concentration of DEHP was detected in lid part of Norwegian beer for 5,300 ng/g (Table S5). Both body and lid parts utilized the same average concentration of DMP and DEP. Whereas for DiBP, DBP, and BHT, body parts utilized one order magnitude higher in concentration than lid part. DEHP, DBP, DiBP, and BBP are actually the four phthalates candidate for substance of very high concern (SVHC) that required authorization prior using21. It was interesting to know that all analytes except for DEHP and BHT were not detected in UK-B-2 can, although this can utilized the same polymer coating with UK-B-1.
An antioxidant of BHT was also detected in almost all samples (ranged from 6.0 – 520 ng/g) with highest average concentration was detected in North American beer for 298 ng/g (Table 2). During manufacturing process, antioxidant is commonly added to protect polymer from undergo oxidation mechanism22 due to illumination and mechanical stress28. On the other hand, DOA was only frequently detected in the body part (Table 2) and in lid part of THA-B-2 samples (Table S5). Moreover, Body part of BEL-B-2 sample was detected to contain the highest concentration of DOA for 9,600 ng/g (Table S5). Adipates have actually been demonstrated to have greater solubility in polar solvents such as 3% acetic acid and 10-35% ethanol29. DAP and BBP were only detected in few samples (Table 2).
A principal component analysis of 5 PAEs, DOA and BHT was performed for exploring the similarities or differences between samples. There were 2 principal components extracted which explaining 31% and 21% of the total variance for PC 1 and PC 2 respectively (Fig. 5). The dominant eigenvalues were DEHP, BHT, and DEP for PC1 and DOA and DEP for PC2 (Fig. 5, Table S6). The graphic distribution showed that there were at least 3 groups of beer can having the same characteristic in additives concentration. Some European, North American and two Chinese beer cans (marked in red color) have same characteristic with high concentration of both DEHP and BHT. Whereas Indonesian and Mexican beer cans (marked in green color) shared the same characteristic having high concentration of DOA. One intriguing result was that all Japanese beer analyzed in this study, along with Myanmar beer can and one Belgian beer can (marked in blue color) shared similarities in containing high concentration of DEP (Fig. 5). However, there were no particular pattern and correlation between additives concentration and beer can origin observed in this study. This result suggests that the manufacturing country of beer cans might be different from the country where these products were being marketed.
3.3 Investigation on deteriorated sample
This study suggested that wide variety of plastic polymers are used in films of inside/outside and body/lid of beer cans. As described earlier, metal debris including cans have been found in marine debris in both coastal and deep-sea environment, but previous studies categorized can as ‘metal’, not plastic. Recently, Nurlatifah et al.11 analyzed plastic film of a Chinese beer can from the western North Pacific (depth: 5,813 m) which were found remained intact. The types of polymer are epoxy resin in inside and poly(triethyleneglycol isophthalate) in outside of the can body.
On the other hand, deterioration process was observed in one field sample found in the beach in Ariake bay (Fig. 6a). This field sample was found with only half of can body remained (Fig. 6b). Although the outside coating film was hardly remained, the inner coating was still well preserved and being peeled out (Fig. 6c). This suggest that the outside coating is more prone to breakdown and release microplastic to environment. FT-IR results proved that degradation was hardly taken place for inner coating of field sample as there were no changes in FT-IR spectra compared to the new one (Fig. S1). In contrast, the outside coating seemed to undergo degradation process, proven by changes in FT-IR spectra of field sample which was far different from the new one.
In pursuance of understanding its toxicological risk, additives analysis was performed for both field and new can samples. An intriguing result showed that aging can contained a higher concentration of additives for more than one order of magnitude (Table 3). The new can only contained DEHP and DOA as plasticizer and BHT as its antioxidant at the levels of 1,600 ng/g and 61 ng/g, respectively. Whereas the can retrieved from the environment contained other PAEs additives such as DMP, DEP, DiBP, and DBP. Liu et al.30 also reported that ionic strength such as NaCl and CaCl2 can promote the sorption of DBP and DEP in microplastics of PS, PE, and PVC due to the salting out effect. This result shows that during the weathering process, additives may not only leach to the environment, but also adsorb to the surface of the polymer.
In contrast, a metal can, having the same brand with this study but retrieved from the sea floor of the West Pacific, only contained DMP and BHT with lower concentration of 9.0 ng/g and 18 ng/g respectively11. The migration rate or leaching ability of polymeric coatings may vary according to polymer thickness22, but the possibility of chemical additives to leach to surrounding environment has gained attention due to its weak bond to the polymer24. Paluselli et al.25 reported the ability of phthalates to leach to seawater from plastic products as much as 120 ng/g, 83 ng/g, 69 ng/g, and 9.5 ng/g for DBP, DiBP, DEP, and DMP respectively from polyethylene bags and polyvinyl chloride cables. Other study reports how the leachate from plastics may inhibit marine microbes31.