Propylene-based elastomers for efficient thermoforming

With global demand for beverage cups, containers and lids projected to rise 5% per year to $23.4 billion in 2014, according to The Freedonia Group Inc., manufacturers are looking for ways to improve processing efficiency and product quality. Many are discovering that polypropylene (PP) blended with Vistamaxx? propylene-based elastomers from ExxonMobil Chemical provides a better balance of impact resistance, clarity and stiffness, as well as being more efficient to process. Vistamaxx propylene-based elastomers have an ever-expanding range of possibilities to compounding and polymer modification, films and nonwoven applications. Based on proprietary Exxpol? metallocene technology, Vistamaxx propylene based elastomer can be processed in many ways, including extrusion, injection moulding, blown moulding, thermoforming etc methods.

Manufacturers of beverage cups and containers face a number of challenges, especially when they use homopolymer polypropylene (hPP) in thermoforming. Some of these issues include lack of detail, incomplete forming or cracking during manufacture. This is due to the poor flexibility and low-temperature impact resistance of PP. Because of low impact resistance, products can be damaged during handling and transportation. Another problem is poor clarity, which can result from using a polyethylene-based impact modifier.

Material challenges
Materials typically used by manufacturers include polyethylene terephthalate (PET), polystyrene (PS), homopolymer polypropylene (hPP) or random copolymer PP (RCP). PET provides good clarity but exhibits poor heat resistance. As a result, PET containers cannot be used for hot beverages or used in a microwave, as they may warp. PET is also difficult for masterbatch dispersion and cannot be used long-term, refilled or re-used. PS can either be transparent or coloured and offers good processability, but it has poor heat resistance and inadequate impact and tear resistance, especially at low temperatures.

Whilst hPP offers good heat resistance and broad food contact compliance, it lacks flexibility. This can result in unsatisfactory detail and incomplete forming or cracking during manufacture, leading to higher rejection rates than with other materials. It also exhibits low impact resistance during transportation and can be brittle at low temperatures. RCP can be a more expensive option than hPP. But whilst it provides good clarity and flexibility its impact resistance remains inadequate at low temperatures.
Honghua Machinery Plastic & Rubber Co. Ltd. (Honghua) of China, a manufacturer of extrusion, thermoforming and moulding machines used for the production of food and beverage containers,
has committed to help manufacturers solve these challenges. Their approach is to consider and evaluate the materials being used, and recommend the most viable and cost-effective material solutions to meet customers' requirements.

To improve flexibility, Honghua initially recommended a "three-in-one" polymer formulation. Comprising 15% low density polyethylene (LDPE), 15% high density polyethylene (HDPE) and 70% PP, it improved flexibility during processing, but reduced clarity. Incorporating a fourth ingredient, a nucleating or clarifying agent, can improve clarity but it also can introduce a problem of odour during processing.

New possibilities
Honghua turned to ExxonMobil Chemical to explore the solutions that Vistamaxx propylene-based elastomers could offer for flexibility. Together, the companies developed two solutions. For applications requiring a balance of impact resistance, clarity and stiffness, they developed a dry blend formulation of 5% to 10% Vistamaxx 3020 propylene-based elastomer with PP.

This has been proven to reduce cup breakage duZoom Lebron XI 11