*3.4. Water-Vapor Transmission Rate (WVTR)*

The water-vapor transmission rate is another important barrier property of packaging materials. Through the transmission of water vapor into a package, not only could the freshness of the packed food be affected, but the growth of microorganisms could also increase. In order to reduce and improve the water-vapor transmission rate (WVTR) of a fiber-based material, the coating barrier material should manifest resistance toward polar water vapor, and be able to close as many pores and voids as possible, preventing the interaction between the polar groups of cellulosic fibers and water vapor [45]. The WVTR was measured gravimetrically, and was expressed as an amount of water vapor in one gram able to pass through a material, usually within 24 h, in our study at 23 ◦C and 50% relative humidity. Raw untreated paper, as a hydrophilic and porous material, is known to be a poor barrier against water vapor. The WVTR values for uncoated PF and SF substrates were 690 g·m−<sup>2</sup> × 24 h and 609 g·m−<sup>2</sup> × 24 h, respectively (Figure 3). The coated samples showed significantly improved WVTR values, with chitosan coated on SF paper showing approximately 60% lower values when compared with uncoated paper, and an even better performance on PF substrate. The behavior of the

alginate led to very similar WVTR values for both substrates, corresponding to a 35% reduction for alginate-coated SF and a 44% reduction for PF paper. Taking into account the different thicknesses of the samples, the water-vapor permeation coefficients (WVPCs) were calculated by multiplying the water-vapor transmission rate and the thickness of the sample (Figure 3). A significant reduction in the permeation coefficients was achieved with chitosan, where the WVPC values for both coated substrates were at least 50% lower than those of uncoated SF and PF paper. The same trend was observed and quantified for alginate-coated PF and SF samples (reductions of 35% and 42% for SF and PF, respectively). Both materials partially met the criteria stated above for the reduction in water-vapor transmission rate. After coating, the paper sheet was densified, and fibers were partially or totally covered with the coating material, resulting in the reduced interaction between cellulosic fibers and water vapor, and the reduced diffusion of water vapor. Thus, both materials, despite their hydrophilic characteristics, contributed to a reduction in water-vapor permeability.

**Figure 3.** Water-vapor transmission rates (**a**) and water-vapor permeation coefficients (**b**) for uncoated and coated paper samples from primary fiber (PF) and secondary fiber (SF) (*n* = 6).

#### *3.5. Wettability and Water Absorptiveness*

The wettability of the uncoated and coated samples was assessed by a contact-angle (CA) measurement using deionized water (Figure 4). The water absorptiveness was characterized by performing Cobb measurements for 60 s, where the Cobb value was the amount of deionized water per area which could be absorbed by the substrate during the given period of time (Figure 5).

A contact angle below 90◦ is characteristic for hydrophilic surfaces. The uncoated PF substrate showed the highest CA, since this paper was already industrial-sized using ASA and starch. By applying chitosan and alginate onto the PF paper's surface, the initial contact angle decreased to ~80◦ (chitosan) and ~35◦ (alginate). On the other hand, the SF uncoated paper had very low CA, which was only measurable for eight seconds. The SF coated with chitosan exhibited a stable and higher CA (70◦ for 30 s) when compared with the uncoated SF paper. By coating the SF paper with alginate, the initial contact angle was lowered to 30◦, but the time-dependent wettability was impacted, resulting in it being stable over the 30 s testing time.

Alginate-coated samples of both SF and PF were in a comparable range, when it came to surface hydrophobicity and water resistance.

**Figure 4.** Contact angle of uncoated and coated samples measured with deionized water for 30 s (*n* = 6).

According to the contact-angle measurements, the PF uncoated substrate appeared to be a hydrophobic material (CA ≥ <sup>90</sup>◦). The Cobb 60 s value for the PF uncoated substrate was 25 g·m−2. In contrast, the SF uncoated substrate reached saturation with water at 60 s, resulting in a higher water uptake (Cobb 60 s = 155 g·m<sup>−</sup>2), and complete water penetration.

Chitosan- and alginate-coated PF samples were able to absorb at least 50% more water for 60 s when compared with the PF uncoated substrate. Consequently, the PF coated with chitosan or alginate became more hydrophilic, which coped very well with the CA measurements for PF samples.

On the other hand, the chitosan coating enormously affected the water absorptiveness of the SF paper, where a reduction of at least 80% was achieved. The Cobb value obtained with alginate-coated SF paper (149 g·m−<sup>2</sup> for 60 s) was only slightly lower (<4%) when compared with uncoated SF paper, and no significant reduction was observed. SF paper is an unsized raw paper, which is considered to be very hydrophilic. By coating it with sodium alginate, which is also a hydrophilic material, the water uptake was not significantly reduced. On the other hand, the positively charged chitosan used for the coating of both substrates interacted very intensively with the negatively charged cellulosic fibers. The chitosan solution used for coating was only water-soluble in the presence of acetic acid at pH 4. Above this pH, chitosan was not water-soluble, and could be considered as "hydrophobic" [44]. Due to the fact that the SF paper was not treated with coating chemicals, and the fibers were fully available for positively charged chitosan, the interaction between the fibers and chitosan obviously took place. On that note, the pH could be shifted to the neutral or alkaline region, thus changing the paper's water absorptiveness, and making chitosan-coated SF paper water-repellent. The PF paper, which was mass- and surface-sized, manifested a very low water uptake, and interacted differently with alginate and chitosan when compared with the SF coated samples. The Cobb values of the PF coated samples were higher when compared with the uncoated PF paper. According to these measurements, different trends could be observed for alginate- and chitosan-coated samples. Irrespective of the paper substrate, alginate caused a hydrophilization effect, while the influence of chitosan on water uptake depended strongly on the paper substrate and its composition.

**Figure 5.** Cobb values for uncoated and coated samples measured for 60 s with deionized water (*n* = 6).

#### *3.6. Migration Experiments*

Since food-contact materials should not release any substances that cause unacceptable changes in the composition of the food, the overall migration needs to be kept as low as possible [41]. The two raw papers were of different qualities in this respect. A paper produced from secondary fiber is considered to be the worst case, especially in terms of contamination with mineral oil hydrocarbons, while a paper produced from clean primary fiber is preferable.

Table 4 shows the results of the migration tests performed in triplicate. Alginate and chitosan exhibited a good barrier performance for the SF paper. Setting the total migration of the uncoated SF paper to 100%, 63.8% ± 0.1% of the observed migration was accounted for as mineral oil, which, in turn, consisted of 57.8% ± 0.1% MOSH and 6.02% ± 0.16% MOAH. Using the alginate coating, the overall migration could be reduced to 16.3% ± 1.0%, of which 7.9% ± 0.25% were mineral oil hydrocarbons (MOH), consisting of 5.49% ± 0.18% MOSH and 2.41% ± 0.42% MOAH. The chitosan coating reduced the overall migration to 29.5% ± 1.6%, which consisted of 9.16% ± 0.3% MOH, divided into 8.43% ± 0.2% MOSH and 0.73% ± 0.34% MOAH.

The migration of the PF sample was naturally low, and coatings to reduce migration were not actually necessary. As expected, the values of the samples coated with alginate and chitosan were below the detection limit, and are, therefore, not given in Table 4. Unlike other barrier properties, which depended on the quality of the coating layer, densification, pores, voids and surface chemistry, it seems that MOSH and MOAH migration primarily depended on the change in surface chemistry rather than the other factors mentioned in our work. An explanation for such a low migration could be the hydrophilic and polar characteristics of these two materials, resulting in a high resistance toward organic non-polar compounds. As such, alginate performed better than chitosan most probably due to its higher polarity and slightly higher densification of the paper.

**Table 4.** Overall migration of mineral oil hydrocarbons (MOH), mineral oil saturated hydrocarbons and mineral oil aromatic hydrocarbons (MOSH/MOAH), in uncoated and coated paper samples from primary fiber (PF) and secondary fiber (SF) (%, *n* = 3).


**\*** Remnants consisted of substances with a retention time outside the range of C16–C35, and substances subtracted from the MOH (e.g., Diisopropylnaphthalene-DIPN).

#### *3.7. Permeation Experiments*

The use of deuterated n-alkanes allowed the performance of two-sided tests in the migration cells. This meant the determination of migration and permeation was possible in one experimental set-up without any interferences. This saved a lot of time and resources, and gave a quick and easy screening method for the barrier behavior of the natural polymers.

Gas-phase migration into dry food is limited by volatility. It was shown that it is relevant up to a chain length of C24, and not detectable beyond a chain length of C28, as substances with higher boiling points remain in the packaging material, and do not migrate [5]. According to the theory, the highest levels for permeation were found for d-C16 and d-C18. An interesting observation was the behavior of the added aromatic active substances. Although all four compounds had a volatility and boiling points in the C12–C16 range (Table 5), only menthol permeated through the papers (coated and uncoated). Apparently, the polar groups of the aromatic compounds interacted strongly with the polar groups of the paper.



From the two tested biopolymers, the alginate-coated samples showed better barrier properties against permeation than the chitosan-coated samples, as shown in Table 6. Under the given test conditions, the permeation rates of d-C14 to d-C20 were between 9.43 <sup>μ</sup>g·dm<sup>−</sup>2/day and 13.7 <sup>μ</sup>g·dm<sup>−</sup>2/day for the uncoated recycled fiber, and was reduced with the chitosan coating by 37%–50%, and was reduced with the alginate coating by 18%–50%. We observed similar permeation rates of the deuterated n-alkanes through the uncoated SF and PF papers, but lower permeation rates for primary fiber after the coating process, especially for the alginate coating. An explanation might be the difference between the two papers in terms of the three-dimensional structures, as well as the chemistry of the fiber surfaces, and a higher pressure on the SF barriers, due to the high load of possible migrants [46].

**Table 6.** Comparison of the permeation rates of deuterated n-alkanes of various chain lengths, and menthol through coated and uncoated paper samples from primary fiber (PF) and secondary fiber (SF) (μg·dm<sup>−</sup>2/day; *<sup>n</sup>* = 2, data given individually).


#### **4. Conclusions**

Biomaterials, such as alginate and chitosan, are biopolymers with higher degrees of complexity when compared with conventional synthetic surface-treatment chemicals. Therefore, the interactions between these two materials and the substrate are variable, and could bring about a comparative advantage for paper-based packaging producers when compared with synthetic barrier materials. Applying those two bio-based materials could improve the barrier properties of paperboard for

food-packaging applications. Even with a pick-up weight of 6 g·m<sup>−</sup>2, the permeability, migration, and transmission were significantly reduced. Depending on the paper substrate, specific barrier properties were differently affected, and could be selectively optimized and adjusted for the consumer's needs, thus giving the packaging producers certain flexibility for some specific applications. One of the most interesting findings resulted from the combination of SEM imaging, and the tests of migration and permeation. It was shown that a continuous surface layer of the biopolymeric materials was not necessary to substantially improve the barrier properties. This is an interesting aspect not only for future research, but also for coating and packaging technologists in the industry. Irrespective of the use of substrates made from primary or secondary fibers, medium-to-high grease resistance was accomplished. The water-vapor transmission rate was reduced by at least 35%. The water resistance or absorptiveness was clearly substrate-dependent, and optimal values were conditioned through the further utilization of packaging materials. The overall migration of organic volatile compounds was successfully reduced by 70% and 84% upon coating the SF substrate with chitosan or alginate, respectively. Migration for the PF sample was naturally low, and coatings were not actually necessary to reduce migration. The permeation of deuterated n-alkanes through both papers was reduced by up to 50%.

Summarizing all results, alginate and chitosan showed excellent barrier behavior.

**Author Contributions:** S.K. designed and performed the coating experiments, physical measurements, and barrier characterization. A.W. performed the migration and permeation experiments for the samples, including the determination of MOH levels using HPLC–GC–FID. A.Z. performed the SEM experiments. S.K., A.W., E.L. and W.B. analyzed the data, and wrote the paper.

**Funding:** This research was funded by the Austrian Research Promotion Agency (FFG), Austropapier–Vereinigung der Österreichischen Papierindustrie and Austrian Pulp, Paper and Packaging Industry (No. 855640).

**Acknowledgments:** The authors acknowledge the industrial partners, Mondi Group, Zellstoff Pöls AG, Delfort Group, W. Hamburger GmbH, Smurfit Kappa Nettingsdorf, the Austrian Research Promotion Agency (FFG), and Austropapier–Vereinigung der Österreichischen Papierindustrie, for their technical and financial support.

**Conflicts of Interest:** The authors declare no conflict of interest.
