Global demand for polyolefins is expected to grow to 132 million tonnes by 2013 according to Noru Tsalic, Consultant at Applied Market Information. Polypropylene constitutes 53 million tonnes and polyethylene 79 million tonnes of this. From 1983, when the market was only 24 million tons, demand has risen fastest in Asia (excluding Japan) with an annual rate of 10%, with Central and South America at around 6.5% and Europe and NAFTA at 4%.
The increase has been double that of the gross domestic product (GDP). The rapid rise in some economies has been due to increased investment, exports and domestic consumption. Tsalic was speaking at the AMI conference on Polyolefin Additives, which took place in Cologne in 2010.
REACH compliance for polyolefin additives has been studied by Steptoe & Johnson. Polymers are currently exempt from registration but not authorisation, however monomers/substances in the finished polymer must be registered if they are in the finished polymer at 2% w/w or more and 1 tonne or more per year. Regulations are complex and include a Candidate List of substances of very high concern (SVHC), which comes into play even if the chemical is present as a contaminant.
In most cases, the aim is to ensure long-term stability of polyolefins against heat, UV light and stresses such as shear, to prevent yellowing, embrittlement and loss of mechanical properties. Sachtleben Chemie has reviewed the additives used as stabilisers:
- Processing aids, e.g. hindered phenols, phosphites and hydroxylamines
- Long-term stabilisers, e.g. hindered amines, phenols and thio synergists
- UV stabilisers, e.g. HALS, benzophenone and benzotriazole compounds
- Metal deactivators, e.g. hindered phenols and amines, and thio synergists.
Migrating metal ions can catalyse oxidation and are removed by combining them into a stable complex. Ultrafine particles of zinc sulphide were tested and found to be an effective co-stabiliser in combination with organic metal deactivators in thermal stabilisation of talc-filled PP.
Norner Innovation has looked at the importance of process stabilisation on long term heat stability of polyolefins. Combining a phenolic antioxidant and a phosphite gives a synergistic effect; the phenolic contributes to yellowing and the phosphite improves initial colour and colour stability. High performance phosphites aid long term heat stability, as can vitamin E and hydroxylamine based antioxidants in certain combinations of additives. There is a new hybrid antioxidant from Sumitomo Chemical with both phosphorus and phenolic groups. It improved processing and heat stability of polypropylene, resists hydrolysis and migration. In rotomoulding tests it showed excellent colour stability, good impact strength and a long oxidation induction time (OIT).
NOR-HALS is an effective UV stabiliser for applications such as agricultural films, as supplied by Clariant Produkte (Deutschland). The company has studied the effects of substituting different groups such as octyloxy, propoxy and cyclohexyloxy. HALS was tested with 3 different waxes and PE based waxes were more effective. The backbone was also tested for effectiveness. The final product was named HALS NOW, based on the final chemical formula of N-O and wax in combination. It has been tested in the laboratory and in Almeria in EVA films including pesticide exposure conditions.
Chemtura supplies innovative phosphite antioxidants. For 40 years the most cost-effective phosphite in use in polyolefins was trisnonylphenyl phosphite (TNPP), which was favoured by ExxonMobil and SABIC. The increase in production output of polyolefins and the rise in use of metallocene-catalysed PE is increasing demand for phosphite for processing. The next generation product is Weston 705, which is in liquid form and has synergistic effects with other antioxidants. It is said to increase polymer solubility and minimise blooming and plate-out problems. This new chemical is currently undergoing regulatory testing across the world, with an aim to get approval by the end of 2010.
Beta nucleation of PP improves toughness and ductility, gives microvoiding during stretching to produce microporous films, and improves thermoformability. There are only a few beta nucleants. Mayzo has a beta nucleant masterbatch that can be used with alpha nucleated PP resins. In films it can give white/opaque effects without the use of pigments, and in BOPP it can give a breathable low density film. It increases impact strength in moulded items and allows lightweighting in thermoforming.
Nova Chemicals has an interpolymer of polyethylene and polystyrene that can be used to modify polyolefins. In tests it gave higher melt strength, which can give lower density foams or improved properties in LDPE. It increased the strength of PP sheet, permitting downgauging. The effect was similar with HDPE and it improved cycle times and properties of a thermoformed part.
There is a lot of debate about the role of prodegradant additives in making polyolefins biodegradable for short-term use such as packaging, and this has been studied by Dr Marek Koutny at Tomas Bata University. Degradation is affected by the soil condition, availability of oxygen, etc. Prooxidant additives are transition metal ions in organic complexes added in masterbatch and they promote abiotic oxidation. This breaks down the polymer into low molecular weight components, which in turn might be degraded by bacteria. This theory was tested in soil at temperatures of 25C and 58C, and in compost, after pre-treatment to activate thermo- and/or photo-oxidation. Conditions were more favourable in compost leading to 18% of LDPE carbon mineralisation after 200 days, and about 25% of LLDPE with manganese or iron based agents. There were lower results of around 3-5% with HDPE samples.
Loughborough University has been working with SABIC on PE blends for pipe applications. A twin screw extruder produced uniform blends of HD/HDPE, mLLD/HDPE and LD/HDPE. The level of branching of components affected compatibility. HDPE/HDPE blends had better strength and stiffness.
SABIC is leading new developments in polypropylene compounds. PP is very susceptible to radical-initiated degradation and may require primary and secondary stabilisation to break the cycle of radical generation. Crystallisation can be a slow process and this can be speeded up using nucleating agents, either soluble (e.g. sorbitol) or insoluble (e.g. organic salts like sodium benzoate or minerals such as talc). Recently, SABIC has generated a transparent impact material for applications such as houseware, containers and food packaging. It has also developed PP grades for automotive to avoid problems such as tiger stripes, which arise due to flow instability at the flow front during moulding.
In Sweden, Polykemi has been working on the next generation of glass fibre reinforced PP compounds. The company has 21 compounding lines and generates around 38,000 tonnes per year: 60% is dedicated to the automotive industry and the company has a second production facility in China (5 extruders). The automotive industry is under pressure to reduce environmental impact across the globe and one solution is to reduce the weight and thus reduce the fuel requirements. The glass reinforced PP compounds include chopped glass strands and a coupling agent. Polykemi has looked at glass down gauging, resulting in a lighter weight compound, which is stiffer and only slightly less impact resistant. There is potential for use as a PA and PBT replacement.
Milliken has produced a new high performance reinforcement that might be used in automotive applications such as mineral-filled or TPO products: bumper, rocker panel, fascia, wheel lip moulding, roof ditch moulding, grill, air deflectors, side cowl, knee bolster, pillar trim, body side moulding and even floor mats. The new reinforcement is a synthetic mineral-based fibre.
Packaging films often require an anti-block additive. Imerys Performance Minerals supplies a range of products: calcined clays are adequate, low cost anti-block for commodity films; diatomaceous earths give very good properties at high loadings; and amorphous silicates are required for ultrahigh clarity films. Grace has also developed a silica for this application: it has excellent optical properties, is relatively pure and amorphous. It works by imparting a microroughness to the film surface.
Biaxially oriented polypropylene (BOPP) can be stabilised by additive blends to enable increased film output, for example by maintaining melt viscosity during conversion. Albemarle has been testing additive combinations to achieve this in combination with colour stability. Baker Hughes Polymers makes polyethylene wax and is about to launch polypropylene waxes for the compounding industry.
One of the largest suppliers of additives for polyolefins, BASF (combined with the former Ciba), has conducted extensive research into full formulations for different applications. For example, the company has studied the effect of colorants and rubber on light stability in PP and LLDPE in artificial turf including the performance to Norm standards compared with long term real life behaviour.
The AMI conference, Polyolefin Additives 2010 provided an effective forum for compounders, end users and industry suppliers to look at the current challenges in formulating effective materials for a wide range of applications. The next conference is scheduled for 18-20 October 2011 at the Maritim Hotel in Cologne, Germany.