Investigation of the Influence of Different Filler Contents of Wine
Biobased polybutylene succinate (PBS) represents a promising alternative to petrochemical-based polymers. However, the use of this biopolymer is limited in many areas by its low resilience against environmental influences. With the help of bio-based stabilizers, the thermo-oxidative degradation process can be slowed down. Suitable stabilizing additives are natural antioxidants present in plant extracts with a high flavonoid content, which can be found in grapes, wine, and wine by-products.
In this study, PBS was modified with two different bio-stabilizers based on wine grape pomace (WGP). The highest filler content tested was 20 wt.-%. In addition to improving stability, the additives also impact the polymer’s mechanics. The influence of these functional fillers on the fracture mechanical properties was examined in a quasi-static test. The crack growth was recorded using integrated video monitoring. Based on the results, the corresponding crack resistance curve and tearing modulus were determined depending on filler type and content.
The two bio-stabilizers based on red (RWP) and white wine pomace (WWP) differ distinctly in terms of their influence on fracture mechanical properties. The influence of RWP on the fracture toughness is significantly higher than that of WWP. Especially at lower filler contents with RWP, there is a strong increase in the fracture mechanics parameter tearing modulus (TJ) and an increase in the slope of the R-curve. With 5 wt.% RWP DOM, the TJ is 13.64 x 102, whereas with WWP Silv a value of only 6.39 x 102 can be achieved. This difference is also reflected in the increase in the R-curves. With 5 wt.% a slope of the fitted R-curve of 265.59 (RWP DOM) and 121.02 (WWP Silv) could be determined with the first derivative.
In the optical analysis, it was noticeable that the RWP particles were more homogeneously dispersed in the matrix while the WWP filler tended to agglomerate. The inhomogeneous distribution and strong agglomeration tendency can be attributed to a higher sugar content of WWP and a higher particle size distribution. The top cut (D97) of WWP Silv is 62.37 ± 0.05 µm and that of RWP DOM is 51.97 ± 0.09 µm.
Biobased polyesters such as poly(butylene succinate) (PBS) show great potential to be used as biobased alternatives to conventional petroleum-based polyolefins. To access technical applications, biobased materials are mostly stabilized using conventional additives, which impair their biobased character. WGP, a largely unused, low-value by-product of winemaking, shows great potential to improve the thermo-oxidative stability. Since WGP is a natural material, annual variations must be considered for its use as stabilizing bio-filler on an industrial scale.
This study investigates the impact of annual variations of WGP on the stabilizing effects in PBS. WGP of two different varieties and three vintages were studied. The composition and properties of the native by-products were analyzed, and WGP-based functional fillers were prepared by industrial mill-drying. The bio-fillers obtained were analyzed regarding their physical, thermal, biochemical, and antioxidant properties and blended into PBS with filler contents up to 20 wt.-% by twin-screw extrusion. The biocomposites’ thermal and thermo-oxidative properties were investigated subsequently.
All WGP varieties and vintages increased the thermo-oxidative stability of PBS by at least 24% at a filler content of 3 wt.-%, demonstrating the potential of WGP as a reliable stabilizer. However, the maximum stabilization effect achieved varied slightly. The results of this study showed that minor differences in the bio-filler properties can be related to meteorological data, while the antioxidant activity, pH, and fat content could be used as bioanalytical indicators to evaluate the thermo-oxidative stabilization effects of WGP-based functional fillers to enable reliable industrial applications of WGP as a polymer stabilizer.
Constant demand for plastics, finite availability of fossil resources, and the associated environmental impact have driven the development of materials from alternative resources such as biomass. The developments of the recent decades have led to commercially available biobased polymers such as PBS. PBS offers attractive mechanical properties comparable to those of its petrochemical counterparts, such as polyethylene (PE) and polypropylene (PP).
Like polyolefins, PBS is prone to thermo-oxidative degradation mechanisms. Therefore, biobased polyesters such as PBS must be stabilized to minimize or prevent degradation during processing. A well-established method to stabilize polymers against thermo-oxidative degradation is the addition of additives or functional fillers as stabilizers. Currently, most of the stabilizers used in biobased polymers are the same additive-systems which are used in conventional plastics.
However, apart from their stabilizing effect, conventional additives are subject to criticism. Zimmermann et al. determined and compared the toxicity of conventional and biobased plastics and found comparable results for both types of plastics, identifying not the matrix polymer, but the chemicals they contain (e.g., additives) as the cause of the hazardous effects. Consequently, conventional antioxidants are suspected to be harmful to human health and the environment.
Biobased plastics do not necessarily have toxicological potential by nature, but the use of conventional additives can change this potential for the worse. Therefore, not only the matrix polymers, but also the additives should be based on non-toxic and biogenic resources to ensure sustainable materials. A strategy to prevent radical-based chain scissions, leading to thermo-oxidative degradation, is the addition of antioxidants as stabilizers to the polymer matrix material.
Antioxidants act as radical scavengers, inactivating free radicals and stabilizing highly reactive intermediates through hydrogen-atom or single-electron transfer. Most of the antioxidants established in the polymer industry are phenolic compounds. In addition to artificial synthesis of such reactive compounds, a wide range of natural polyphenols are present in biogenic materials.
For sourcing natural polyphenolic compounds, existing biomass with poor utilization such as agricultural by-products are favorable to avoid competition with food production. Wine grape pomace (WGP) is a by-product that is produced in high volumes around the world. Apart from residual stems, WGP mainly consists of wine grape skins and seeds, each accounting for around 50% of the total mass.
Depending on the wine grape variety, WGP is directly separated from the grape juice (white wine) or pressed after fermentation (red wine). The winemaking by-product represents in total about 20 to 30% of the grape’s initial mass and is rich in functional substances such as natural polyphenols.
The main component in both, wine grape skins and seeds, are fibers, with the seeds generally having a slightly higher proportion. While the protein contents of wine grape skins and seeds are comparable, skins may contain more sugars. However, the sugar content varies in the range of 1 to 78% depending on the variety, environmental factors, and winemaking processing methods.
The lipid fractions and fats of wine grapes in forms of oils and fatty acids are mainly present in the seeds with shares up to 21%. Regarding the content of phenolic compounds present in wine grape skins and seeds, contradictory statements can be found in the literature. While some studies reported wine grape skins to be higher in phenolic compounds, others found the seeds to be the main source of phenols.
Apart from the total share of phenols, the main polyphenols in wine grape skins and seeds differ. While wine grape skins contain more phenolic acids and tannins, wine grape seeds have a higher share of flavan-3-ols such as catechin. In general, the proanthocyanidins in wine grape seeds show a lower degree of polymerization than those contained in the wine grape skins. Moreover, anthocyanins (polyphenolic pigments) are exclusively found in red wine grape skins.
In addition to wine grape skins and seeds, multiple studies have been conducted to analyze the biochemical composition of WGP in terms of its macromolecules such as proteins, fats, and sugars, as well as its antioxidant properties in terms of total phenolic content (TPC) and antioxidant activity (AA).
The sugar content in WGP of different varieties and from different regions varies in a range of 0.8 to 27.6%. In general, white WGP (WWP) tends to have a higher sugar content due to the differences in the winemaking process compared to red WGP (RWP). Furthermore, the sugar content of RWP is influenced by the maceration times. With longer maceration times, more sugars are converted into alcohol by fermentation, reducing the overall sugar content in the RWP.
Other main components in WGP are proteins with a content of 6.33 to 15.50% dry-weight-basis (dwb) and fats or lipids in a range of 4.62 to 12.50% dwb. The antioxidant properties of WGP are usually evaluated by the Folin-Ciocâlteu (FC) assay to determine TPC, while AA is investigated by 2,2-diphenyl-1-picrylhydrazyl (DPPH) as well as 2,2′-azinobis-(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS) assays.
For the TPC of WGP, a broad range of 10 to 79 mg gallic acid equivalents (GAE) per g dwb is reported in the literature. The AA of WGP determined by the ABTS assay ranges from 19 to approx. 70 mmol Trolox equivalents (TE) per 100 g dwb and for the DPPH assays an AA of about 12 to 51 mmol TE/100 g dwb is reported.
It is well known that the exact biochemical composition of grape skins, seeds, and accordingly, WGP depends on the variety, the wine processing, as well as the region and the weather conditions of cultivation. Hosu et al. investigated the influence of variety, vineyard, and vintage on the quality of Romanian white wines. The study revealed that all factors had an impact on the composition of the wine.
Grapes of the same variety but from different vineyards differed in TPC. The results also showed that warm and sunny weather promotes the biosynthesis of polyphenols during cultivation. Accordingly, wines cultivated in an area with fewer hours of sunshine have a lower polyphenol content.
Bock et al. conducted a long-term study on the phenology, acidity, and sugar content of the white grape varieties Müller-Thurgau, Riesling, and Silvaner from Würzburg (Germany). Changing climate conditions had an impact on the sugar content and acidity of wine grapes. The sugar content increased with longer ripening periods, while the amount of acid components decreased.
Van Leeuwen et al. reviewed the effects of climate change on viticulture and the required adaptations in wine production. The review focused on the consequences of climate change such as higher temperatures and droughts, leading to advanced harvest times. Rising temperatures and less precipitation affected the composition of grapes. Higher alcohol content and lower acidity were found for wines cultivated at high temperatures, while a persistent water deficit led to lower sugar contents.
Ky et al. analyzed the TPC and the AA of WGP skins and seeds of various varieties from two different vintages cultivated in France. In addition to variety related differences, a significant impact of the vintage on the polyphenols in wine grape skins and seeds was found. Higher TPC values were obtained for the vintage with a slightly higher average temperature and less accumulated precipitation.
The authors assumed that the resulting water deficit was the reason for increased polyphenol biosynthesis in the wine grapes, which led to higher TPC of the corresponding WGP. These results highlight the influence of the grape variety, the winemaking, and the conditions of cultivation on the biochemical composition of WGP.
Apart from the analysis of WGP itself, several studies were conducted regarding the utilization of wine by-products as biobased additives or fillers in polymers. The addition of wine by-products and their extracts increased the stability of conventional plastics (PE and PP) against thermo-oxidative degradation and (UV-)aging.
For instance, Nanni et al. found an increase in dynamic oxidation induction temperature (OIT) of PP by approx. 14 to 18% by adding wine grape seeds, skins, and stems at a filler content of 6 wt.-%. Biopolymers were also investigated in combination with wine by-products, focusing on using the wine by-products as cost-effective bio-fillers.
The use of Canadian WGP in PBS showed its potential as low-cost filler. While the mechanical properties of the biocomposites were strongly affected (increase in tensile modulus, decrease in tensile strength and elongation at break), their thermal properties were found to be like those of neat PBS.
Previous studies by the authors have demonstrated that WGP can be used as a functional filler for biopolymers without additional extraction steps, suppressing thermo-oxidative degradation in polyhydroxyalkanoates (PHA) and PBS. In an initial study, filler contents of 5 to 20 wt.-% were blended into PBS to determine whether WGP-fillers act as antioxidants in addition to their use as cost-effective fillers.
Although all filled materials showed a higher OIT compared to neat PBS, the lowest filler content of 5 wt.-% resulted in the highest thermo-oxidative stability. In a second study, WGP was blended into PHA as a matrix material using filler contents of 1, 2, 3, 5, and 10 wt.-%. The results showed that WGP also stabilizes PHA and that the desired stabilization effects can already be achieved at filler contents below 5 wt.-%.
Literature reports great potential for the use of WGP as thermo-oxidative stabilizer in polymers. However, the above-mentioned bioanalytical analysis revealed differences in the composition of WGP and its functional compounds such as polyphenols with respect to annual variations. None of the previous studies on the use of WGP as a polymer filler have considered the influence of annual variations by analyzing WGP of different vintages.
Consequently, the aim of this study is to investigate the identified research gap by addressing the following three scientific questions: What are the differences in the biochemical composition of WGP-fillers of different vintages? Is WGP a reliable thermo-oxidative stabilizer for biopolymers, independent of vintage and variety? Can bioanalytical indicators be identified to evaluate the stabilization efficiency of the WGP-fillers?
Accordingly, the results of this study provide an in-depth evaluation of the potential of WGP as an alternative biobased stabilizer for biopolyesters such as PBS. Furthermore, these interdisciplinary investigations will not only support future research on polymer stabilizers based on natural by-products but will also form the basis for an industrial application of this alternative approach.