Bio-based lightweight packaging materials
Paul Fowler, Qiuyun Liu, Radek Braganca
BC, Bangor University, Gwynedd, UK, LL57 2UW
1 Introduction
Plastic is the least recycled packaging material compared with metals, glass and paper. In the UK the total consumption of plastics amounted to 2,414m tonnes in 2003 [1], of which approximately 60% was packaging that was discarded as soon as the product was opened. Many of these plastics end up in landfill sites, where they will remain for centuries. The prices of all synthetic polymers are a linear function of the price of petroleum, and the latter has been increasing in recent years. Thus, a continued rise in petrochemical production requires an increased exploitation of non-conventional sources, suggesting market opportunities for new, renewable-based polymers.
In recent years, a number of companies have introduced bio-based polymers made from agricultural resources as substitutes for petroleum-based thermoplastics. Most of those polymers are designed for packaging or with potential to be used in packaging in the near future. Theose-bio based polymers are expected to lessen the environmental impacts of packaging by shrinking landfill requirements, whilst also reducing dependence on non-renewable resources.
Why should packaging materials be lightweight? Because lightweight means a reduction in materials use; means reduced cost of materials and transport; means reduced waste and energy demand. Lightweighting can be achieved by using low density materials, by developing novel multilayer thin film or foamed sandwich structures. Successful exploitation of bio-based lightweight packaging materials will result in a reduction in costs and also contribute to a reduction in landfill requirements and carbon dioxide emissions.
2 Overview of packaging materials
The five volume polymers currently used in packaging are polyethylene (PE), polypropylene (PP), polyvinyl chloride (PVC), polystyrene (PS, or expanded polystyrene (EPS)) and polyethylene terephthalate (PET). Packaging is the major use for PE and PP. High density polyethylene (HDPE) is used in applications such as containers, milk and detergent bottles, bags and industrial wrapping. Low density polyethylene (LDPE) is used for pallet and agricultural film, bags, coatings and containers. PP is employed in film, crates and microwavable containers. PS finds uses in CD/DVD cases, trays and foam insulation, while PET is used in bottles, film and other food packaging applications. Table 1 shows the prices of thermoplastic polymers.
Table 1: Price of thermoplastic polymers [2]
Material LDPE HDPE PP PVC PS & EPS PET Average
price (€/kg) (2005)
Recently, many bio-based polymers have been developed. According to the materials they are manufactured from, bio-based polymers can be classified as aliphatic polyesters (for example: polylactic acid (PLA)), starch polymers (e.g. Mater-bi), cellulosic and whole-crop polymers (e.g. Cellophane) and microbially synthesised polyhydroxy alkanoates (e.g. poly-3-hydroxybutyrate (PHB)). The examples given are designed for packaging or with the potential to be used in packaging applications in the near future. Table 2 lists their main producers and production capacities.
Table 2: Key polymers and their producers
Polymers Producer 2006
Price, €/Kg
2006
Capacity, Kt pa PLA NatureWorks (Cargill), USA 1.8-2.4 140
Mater-bi Novamont, Italy 1.5-4.5 20
Cellophane Innovia, UK ~3.3 N/a
PHB Metabolix, USA 10-12 5
Compared to petroleum-based polymers, the current prices for bio-based polymers are higher, as shown in Tables 1 and 2. However, the prices of all synthetic polymers are a linear function of the price of petroleum, and the latter has been increasing in recent years; meanwhile the prices of bio-based polymers are expected to decrease once higher sales volumes are achieved. For example, the price for starch polymer could decrease to €0.7~0.9/Kg, which is cost-competitive with petroleum- based polymers. Potentially the price for bio- based packaging material could be further reduced by making lightweight structures. 3 Potential methods for weight reduction and new packaging formats
The main purpose of packaging is not only to protect the packaged product from its surroundings, but also to maintain the quality of the product for its entire shelf life, while addressing communication, legal and commercial demands. Adequate mechanical and good barrier properties are necessary throughout the service life of a packaging application.
3.1 Mechanical properties of bio-based polymers
Different packaging application types have different mechanical property requirements. For flexible materials such as films, elongation at break is the most critical property. For the production of rigid materials (trays), the most important mechanical property is the stiffness measured by Young’s modulus.
Young’s modulus is defined as the ratio, for small strains, of the rate of change of stress with strain. Young’s modulus can be experimentally determined from the slope of a stress-strain curve created during tensile tests conducted on a sample of the material.
Starch polymers normally have low modulus and high elongation, while PLA shows high stiffness and low elongation. Table 3 shows some mechanical properties of bio-based polymers. As a comparison, Table 3 also shows some of the mechanical properties of petroleum-based polymers. It may be seen that bio-based polymers (PLA, PHB and Cellophane) are stiffer than petrochemical-bio-based alternatives (HDPE, LDPE, PP, PS, and PVC), while other bio-based materials
Mixtures of these polymers could theoretically have a range of intermediate properties.
Table 3: Mechanical properties of polymers
3.2 Influence of additives on stiffness
Recent literature shows that the addition of natural fibres/fillers into bio-based polymers may not only improve the mechanical properties but also provide major savings in cost and carbon dioxide emissions (up to 44%) [3]. Potential reinforcements of bio-based polymers are summarised in Figure 1.
Figure 1: Potential reinforcements for bio based polymers
Polymers Modulus, MPa Elongation, %
PLA 3500 50 2500 20 500 300 Mater-bi 350 500 PHB 3250 20 Cellophane 3000 22 1500 70 500 450 PBS 750 750 HDPE 750 500 LDPE 100 650 PP 1500 600 PVC 1500 50 PS 3150 10
REINFORCING NATURAL FIBRES
NON WOOD NATURAL/BIOFIBRES WOOD
FIBRES BAST GRASS FIBRES SEED/FRUIT LEAF STRAW FIBRES Soft and Hard Woods Cotton, Coir Kenaf, Flax, Jute, Hemp Corn/Wheat Rice Straws Sisal,
Pineapple Leaf Fibre
Bamboo Fibre, Elephant Grass
The effect of these fibres will depend on a range of factors such as the interfacial adhesion between the fibres and the matrix polymer, the dimensions and the morphology of the fibres. To illustrate the potential of the fillers to significantly increase the modulus, some examples from recent literature are provided in
Table 4.
Thermoplastic starch (TPS) shows the lowest modulus in the range 47–90MPa, but upon addition of fibres, a 2-3 times increase in modulus may be achieved. The modulus of PLA is increased to 8000MPa from 3500 (~2.5 times increase) by addition of 30% flax fibres.
Table 4: Influence of fillers on the modulus
Matrix polymer Modulus (MPa) Reinforcement Volume fraction (%) Modulus composite (MPa) Reference TPS a) 47.2 Ethanolamine activated montmorillonite 5 145.5 [4]
TPS 87 Medium size fibres b) 5 190 TPS 87 Medium size fibres 12 522 c)
[5]
TPS 80 Cellulose microfibrils 10 3000 [6] PLA 2700 Recycled newspaper 10 5300 [7,8]
PLA 3500 Flax 30 8000 [9]
TPS 81.5 Montmorillonite 20 850 [10]
TPS 45 Waxy maize starch crystals/
5 d) 298 [11]
a) Thermoplastic starch b) 2-3mm in length c) as a comparison the author also reported that HDPE modulus only increased two times with 12% fibres, d) 20 % glycerol as a plasticiser
3.3 Potential methods for weight reduction
Packaging materials are used to maintain the quality of the product for its entire shelf life. Lightweight packaging materials should also impart the same performance functionality, but in a material that is correspondingly lighter weight. It is the production of rigid materials (trays) that contributes most of the percent weight of a package rather than flexible film. Therefore, lightweight packaging products may be achieved by reducing the weight of rigid materials.
Mass is equivalent to density multiplied by volume (or thickness multiplied by area). Therefore, any operations that may lead to a reduction in density or volume (thickness) will help to bring about a weight reduction in a packaging product. 3.3.1 Low density materials
Table 5 shows the density of some bio-based polymers. Densities for non-wood
fibres are in the range of 1.0–1.4g/cm3. Densities of wood fibres range from 0.5g/cm3 up to 1.3g/cm3 due to significant variations for different types of woods. Normally, balsa (cork-like) has very low density (0.1–0.2g/cm3) while ironwood
(guaiac) has very high density (1.1–1.3g/cm3). The same type of trees grown in different places may possess different densities, for example the density of teakwood (African) is 0.98 g/cm3, and teakwood (Indian) in 0.66–0.88 g/cm3 [12].
Where the density of a composite structure, such as a packaging tray, is Dc and Dp
is density of polymers, Vp is the volume fraction of polymers and Df is the density of fibre,
Vf is the volume fraction of fibre, then according to mixture rule:
Dc = VpDp+VfDf
Clearly, the density of the composite will decrease upon increasing the volume fraction of low density fibres. Thus, the addition of fibres to bio-based polymers could make composite tray structures that are both stronger (as shown in Table 4) and lighter.
It should be noted that the price of natural fibres is lower compared to bio-based polymers. Waste fibres from the wood processing industry could be zero cost, while bast fibres which are amongst the most expensive of natural fibres still cost only in the region of 0.2–0.4 €/Kg. Therefore, the addition of fibres also reduces the cost of the raw materials’ inputs for a composite tray.
Table 5: Density of bio-based polymers
PLA Mater-bi PHB PBS
Density, g/cm3 1.25-1.3 1.3 1.2 1.1
The density of biopolymer based packaging trays may also be decreased by making foamed structures. The density of a thermoformed foam tray made from PLA has been reduced by about 75% from its original 1.24 g/cm3 to around 0.30 g/cm3 [13]. The density of foamed materials strongly depends on the volume of air bubbles.
3.3.2 Volume (thickness)
The minimum thickness of a packaging material is inversely proportional to the modulus of materials, namely, the stiffer the materials, the thinner the product may be. As shown in Table 4, the addition of fibres leads to a significant increase in modulus, a feature which allows us to make a thinner film or rigid tray.
Where M is the mass of a material, D is the density of such a material, T is the thickness of the material, A is the area of the material and E is the modulus of the material, we can describe the opportunity to reduce weight when using stiff PLA instead of PP to produce trays with similar area for similar products:
C•WPLA = WPP,
where C is a coefficient indicating the weight reduction C•TPLA•A•DPLA = TPP•A•dPP
C•TPLA•DPLA = TPP•DPP
C = (TPP/TPLA )•(DPP / DPLA)
Assuming that the tray would have the same characteristics and properties as a beam, it may be deduced from the classical engineering bending equation that the
minimum thickness of the tray is inversely proportional to the cubic root of the modulus, therefore:
C = (EPLA/EPP)0.33•(DPP/DPLA)
C = (3500/1500)0.33•(1/1.3) = 1.01
However if we consider that a PLA tray is reinforced with 10% cellulose microfibrils, the modulus of composite is 8000MPa (see Table 4). In this case the proportionality coefficient would be:
C = (8000/1500)0.33•(1/1.3) = 1.37.
This predicts that a fibre-reinforced PLA may offer up to 37% weight savings. If we accept that there may be errors in the assumptions, such as: (a) the tray may not behave totally like a beam; (b) the increase of the modulus may not be the expected one; and (c) the density of cellulose microfibrils is similar to that of PLA, it is safe to consider that a reduction in weight of at least 15% is possible.
3.4 New packaging formats
On the basis of the above information it is acceptable to consider that savings in the weight of packaging can be obtained by using stiff materials; by adding reinforcement; by making multilayer structure/sandwich foamed materials or by a combination of those methods.
Foamed sandwich materials have been developed in both starch polymers and PLA. A possible method of reducing weight and raw materials’ costs is the development of sandwich foamed material, with a core in starch and a skin in more expensive PLA, as shown in Figure 2.
Figure 2: Scheme of a sandwich foamed material 3.5 Proposed manufacturing methods
3.5.1 Rigid thin skin film
Mater-bi film extrusion technology does not differ greatly from that of conventional extrusion technology and can use the same machine designed for PE film. However, PLA extrusion technology is quite different from the conventional technique. PLA needs to be dried to less than 250 ppm of moisture before making film, therefore, it is difficult to convert a conventional blown film line to a line capable of producing PLA film. The extruder for PLA film needs to be more
3.5.2 Foam core
Foamed materials have been developed in both starch polymers and PLA. The first foamed PLA trays were introduced by Coopbox SpA, Italy, in 2005 with a reduction of PLA's density by about 75% [13].
A special grade of Mater-bi starch-based biodegradable resin from Novamont in Italy is also being used for foamed food trays. Foamed Mater-bi trays have a density around 0.10 g/cm3 vs. 1.28 g/cm3 for the solid resin [13].
Foamed bio-based polymers could be made by tandem extruder with conventional gases such as nitrogen, carbon dioxide or butane.
3.5.3 Film lamination
Multilayer technology allows several materials to be combined to give a wide range of properties. This is particularly useful in packaging applications, where wrappings must not affect contents and at the same time protect from damage and exposure to contaminants. This technology has been used to make multilayer films with petroleum-based polymers.
New multilayer films from bio-based polymers become available with the development of new equipment. Automated Packaging Systems has recently launched a blown film extrusion system for creating multi-layer films [14]. This new co-extruder produces film in rolls that are 40 percent larger than standard rolls, which may result in more efficient production with fewer splices and less scrap.
Technically, there is no difficulty in making a multilayer foamed sandwich structure. The key point is how to balance the materials and properties to achieve low weight (cost) and high performance products.
4 Conclusions
In recent years, many bio-based polymers have been introduced as substitutes for petroleum-based thermoplastics. Though the price of these polymers is high at the moment, it may come down as their market share and hence volume of sales increases.
Analysis of the properties of the novel bio-based polymers indicates that they can be stiffer or with higher elongation than conventional plastics used in packaging. This suggests that it might be possible to reduce the weight of food packaging by developing ultra-thin materials reinforced with fibres/microfibrils, or by using foamed sandwich materials.
In the short term, using bio-based lightweight materials in packaging could lead to significant reduction in waste. In the long term it is expected that technologies would be developed that allow a more systematic use of such materials in both packaging (bottles for soft drinks) and other uses (such as foam for house insulation). These applications will further reduce the amount of materials ending up in landfill.
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