Member Control Panel
Additive masterbatches to promote degradability in plastics
| By Andrew Barclay, Wells Plastics Ltd Additives to promote degradability in polymers are not a new concept, but have been in the technological arena for at least 30 years following pioneering work in the field by, for example, Scott G. ,Griffin G, Gilead D., and Albertsson A. Currently available commercial products consist of polymeric based masterbatches containing a pro-oxidant (salts of transition metals) and reaction rate modifiers (generally but not exclusively antioxidants). These commercial products are added to polymer feedstocks at levels between 1 and 3% by the polymer converter to impart the controlled oxobiodegradable characteristic to the polymeric product being manufactured. These additives are predominately used in the polyolefin film market for a wide variety of packaging and agricultural mulch applications, but there are other applications in the sheet, closures and bottle industries. Myths Many myths surround the technology, generally attributable to misunderstandings regarding the science behind it. These include :
These can all be shown to be unfounded once the technology is better understood. Technology Initial Polymer Breakdown The high molecular weight of commercial grades of polymers render them too hydrophobic and, therefore, very resistant to direct microbial attack. It is been well known for many years that the presence of certain metal ions in polymers such as polyethylene and polypropylene can effect and accelerate their degradation. In the presence of oxygen the metal ions catalyse oxidative chain scission of the polymer, causing a serial reduction in polymer molecular weight which ultimately results in acute embrittlement and microfragmentation (see Figure 1). |
 (Figure 1) |
| When the polymer molecular weight is sufficiently reduced it becomes available for microbial attack which further breaks down the polymer into carbon dioxide, water and biomass. A reduction of the polymer chain length from its initial value of around 250,000 to a value between 4,000 and 10,000 increases its intrinsic microbial accessibility and enables subsequent biodigestion. This complete process is known as oxobiodegradation. Technology A convenient method of measuring the degree of initial breakdown is through Infra-Red spectroscopy (FTIR). As oxo-degradation causes the formation of a carbonyl group at the point of every scission then the measurement of the onset and level of carbonyl group development in the test sample is a direct measure of its induced degradation by the metal ion pro-degradant. This “carbonyl Index” can be used to ascertain the rate of breakdown of the polymer (see Figure 2). Another convenient and accurate method is the measurement of the polymer’s molecular weight. Attempts to directly measure specific physical properties such as Elongation or Tensile Strength are generally ineffective as the polymer breaks down too rapidly to enable such testing. |
 (Figure 2) |
| Carbonyl Group Development It can be seen from Figure 2 that, following a “dwell time” wherein no breakdown reaction commences, the film containing oxobiodegradable additive has rapidly proceeded to a point of embrittlement (=5% elongation). This accelerated testing was performed at 50oC (chosen to be a “reasonable” temperature to give a balance between an accelerated timescale and real-life values) in a heat / UV ageing cabinet and Arrhenius principles then used to estimate the performance at 20oC. Comparison with the control not containing any oxobiodegradable additive shows that the treated film has a dwell time equivalent to around 4 months at 20oC followed by an induced embrittlement point around 8 months later. Oxobiodegradable technology enables the reaction profile to be adjusted to fit particular technical requirements such as shelf life, subsequent product lifetime and general fit-for-purpose attributes. Molecular Weight reduction Determination of the polymer sample’s molecular weight during the initial breakdown period clearly demonstrates that chain scission is occurring. In the example shown in Figure 3 a rapid fall in molecular weight was measured during ageing at 50oC in a heat/UV ageing cabinet. In fact the film sample’s weight average molecular weight was reduced from its initial value of 170,000 to below 5,000 during the ageing period. It should be borne in mind that the molecular weight reduction is not the result of a simple chain scission resulting in lower molecular weight polyethylene, but that at every point of scission a carbonyl entity is formed. This effectively changes the simple hydrocarbon that is polyethylene into compounds such as short to medium chain carboxylic acids which are far more hydrophilic and readily biodegraded. |
 (Figure 3) |
| Biodegradation The subsequent biodegradation (often referred to as mineralisation) of pre-oxidised polyolefin has been studied many times, sometimes with specific microbial strains, under general composting conditions and within aqueous conditions. These studies, and many others, have demonstrated the ultimate mineralisation of preoxidised polyolefins. A specific result, obtained from polyethylene agricultural mulch film may be found in Figure 4. |
 (Figure 4) |
| It can be seen that when the pre-oxidised film was tested according to ASTM D 5338 (an aerobic biodegradation test method) the biodegradation was very rapid, with over 77% of the carbon in the polymer being converted to CO2 within 45 days. This is an extreme result caused by the thinness of the film and the activity of the specific oxobiodegradable (OBD) formulation utilised and generally OBD products are not recommended for composting applications as their biodegradation rate would normally be much slower, especially in film thicknesses and types used in packaging products. Notwithstanding this, it is evident from this independent testing that very fast mineralisation rates can be achieved with OBD additives. Food Contact Suitability The major suppliers of OBD additives all claim food contact suitability and specific compliance with the European directive 94/62/EC pertaining to the presence of toxic heavy metals and the American equivalent, CONEG. Major food contact specification bodies include the European Community (through EC directive 2002/72/EC and its subsequent amendments), Canada (through their Food Inspection Agency / Bureau of Chemical Safety) and, in America, the FDA (through their positive listings and “Chapter” suitability). Individual OBD additive suppliers can be approached for independent verification of their products’ suitability for food use and their absence of proscribed heavy metals. In addition, studies have been performed that have confirmed that there is no bio-accumulation of products or possible by-products from oxobiodegradable film use in agriculture, which should give confidence in their suitability for these and other purposes. All of these data give confidence in the suitability of oxobiodegradable additives for food use and their general lack of toxicological contraindications. Examples of Use
Summary Oxobiodegradable additives are not a new concept but have been refined over the years to give better control over the kinetics of the reaction and to therefore increase the broad suitability of these additives for use in a large variety of applications. The “myths” regarding toxicity, mere fragmentation into smaller plastic shards and no subsequent biodegradation can be seen to be ill-founded from the data and references contained within this report. In fact, using reasonable accelerated ageing conditions, oxobiodegradable films have been shown to undergo controlled oxidation resulting in serial chain scission giving a rapid reduction in molecular weight, embrittlement and micro-fragmentation of the original product and the inherent denaturing of the polymer to oxidative by-products. The oxidised by-products can be seen to have undergone biodegradation to ASTM 5338 and, in the example given, to have achieved >77% mineralisation within 45 days. OBD products are not recommended for applications that require a composting property to meet such composting specifications as EN 13432, but they do have a strong market niche in many significant areas such as check-out bags, food packaging, agricultural films and many other sectors. Oxobiodegradable additives have become an important product of choice in a broad variety of plastics products and their continuing growth is a testament to their proven efficacy in the right applications. ANDREW BARCLAY Wells Plastics Ltd Tel: +44 (0)1785 817421 Fax: +44 (0)1785 817771 Email: technical@wellsplastics.com Web: www.wellsplastics.com |
