by Ravi Mehra, PSD Rotoworx P/L & 4Roto LLC
Breaking Out of the Polyethylene Dependancy
As far back as the 1980’s there has been a very clear desire from rotational moulders and designers: “We need more variety of resins to work with, other than “Polyethylenes”. As time has gone by the request for more variety of resins has continued. The intensity of this message has not waned; the industry just kept hoping and asking for more.
Over the years resin suppliers have made presentations at conferences introducing different materials. Some of them were modified grades of Polyethylene, formulated to address a specific need, but there were other non-Polyethylene resins promoted as well. These included: Nylon 6, Nylon 11 & 12, PVDF, Polycarbonate, Polyurethane, Polybutylene, and Polypropylene, just to name a few. Why then does rotomoulding continue to be defined by Polyethylenes? Not because that is the only material available, but because it is the material that is most widely used. Here are some reasons why:
- It readily sinters and so is easy to mould.
- It is widely available.
- It has good physical properties and can be adapted to a variety of applications.
- Tradition - moulders are very familiar with it and therefore confident to use it.
- Market Identification – end users know rotomoulding is a Polyethylene based industry and therefore design products to suit the capabilities of the material.
- Cost – it is a lower cost material offering a cost competitive solution to end users.
These are all compelling reasons to stick with the tried and tested material. However, Polyethylenes do have some limitations and continued requests for new resins suggest that there are opportunities that are being missed. As previously mentioned, there are other rotomoulding options to Polyethylene. In this article we are going to focus on Polypropylene.
Why Consider Polypropylene?
- Where strength is concerned (stiffness, tensile strength, resistance to creep), it is superior to Polyethylene.
- It has good impact at ambient temperatures and reasonable impact at temperatures down to 0oC.
- It has significantly higher operating temperature than Polyethylene.
- It has excellent chemical resistance.
- It has better abrasion resistance with harder surface and high gloss.
- It has lower shrinkage than Polyethylene which combined with higher stiffness yields flatter parts, with minimal warpage and tighter tolerances.
- It is not difficult to mould, it just requires higher temperatures.
- It brings added value and is cost competitive.
If we as an industry desire to expand our horizons, we must move beyond the Polyethylene based identity. Polypropylene is the next best addition to a rotomoulder’s product offering. Why? Primarily because it is not difficult to mould and existing moulds and equipment can be used. With a few slight modifications to operating parameters, you are able to mould Polypropylene as readily as Polyethylene.
In addition to being easy to mould, it is a logical choice because under certain product performance requirements, Polypropylene offers performance characteristics significantly superior to Polyethylene. It will open up new markets for those willing to bring Polypropylene to their moulding capabilities.
The table below provides a comparison of typical properties between LMDPE Polyethylene and Polypropylene (PP).
|Density||g/cm3||0.937 to 0.941||0.895 to 0.905|
|Melt Index||g/10min||3.5 to 5.5||10 to 35|
|Tensile Strength||MPa||16 to 20||20 to 27|
|Flexural Modulus||MPa||650 to 850||1,000 to 1,300|
|HDT (@ 0.455MPa)||Degrees C||55 to 65||80 to 120|
|ESCR 2% Igepal||Hours||<<100||>1000|
|Impact Strength, ARM||J/mm||20 to 30 |
|5 to 25 |
Table 1 : Material Property Comparison of Polyethylene v Polypropylene – clearly shows significant advantages that Polypropylene has over Polyethylene.
Figure 1 - Manholes - Underground
Figure 1 illustrates an underground manhole that benefits greatly from Polypropylene's increased strength and resistance to creep. This is especially true when forces are applied over a long period of time, as in underground applications, and it is easy to install.
Following are a few basic processing modifications when moulding Polypropylene v Polyethylene:
- Needs to be moulded hotter: Polypropylene requires a higher Peak Internal Air Temperature (PIAT) which is achieved by using a higher oven temperature and/or extending the oven residency time.
- Can be demoulded hotter: Polypropylene has higher crystallization temperature, the product solidifies earlier, and the part can be demoulded sooner during the cooling phase.
- Polypropylene exhibits a natural lubricity compared to Polyethylene. This allows Polypropylene to run without use of mould release or a minimal amount if absolutely necessary.
- Polypropylene has lower shrinkage than Polyethylene, i.e., 1.5% to 2.5% compared with 3% to 3.5%.
Applications and Opportunities
There are many opportunities that exist for rotational moulding that exceed the performance capabilities of Polyethylene. Polypropylene’s higher mechanical strength, superior resistance to long term creep, superior environmental stress crack resistance, excellent chemical resistance, significantly higher thermal capability, higher scuff resistance, and gloss and smoother flatter surfaces, allows for its use in underground/infrastructure applications, chemical storage, large tanks, under hood automotive parts, hydraulic and hot water tanks, hot fill applications, and furniture & display items.
|Figure 2: Chemical Storage Tanks - 25Kl||Figure 3: Hot Fill Hoppers|
Other special applications in the Health Industry, Life Sciences, Pharmaceutical & Biotech industries, Research Labs, Food and Beverage industry applications where autoclaving /sterilization is required. Basically, Polypropylene provides a solution for moulders where product requirements limit the use of Polyethylene resins in a multitude of applications.
|Figure 4: School Furniture||Figure 5: Under Hood Applications|
We live in a changing world. New products with new requirements are being developed more rapidly than ever. If the rotational moulding industry is to keep up with this changing world, a one dimensional resin approach will not work. The good news is that learning how to design, process, and promote Polypropylene is not a difficult step for rotomoulders. The product is available and technical specifications and processing guides will make the change easy.
New grades of Polypropylene are evolving, with new products being developed specifically for rotational moulding. The recent introduction of “RMPP141” rotomouldable Polypropylene by PSD Rotoworx, Australia, is a unique new entrant. Most existing grades of Polypropylene have to trade off between stiffness and toughness. RMPP141 offers a very attractive balance between both stiffness and impact, while still maintaining or enhancing other performance. Another major achievement with this new Polypropylene “RMPP141” is that it can be pulverized at ambient conditions, eliminating the necessity for expensive cryogenic grinding, and thereby reducing costs. Further this means the moulder can process any scrap generated.
It is hoped that this article has demonstrated the many reasons to consider Polypropylene as part of your rotomoulding capabilities and product offering. There is very little standing in the way of expanding your business through diversified offerings except your decision to do so.
Compiled by PSD Rotoworx P/L in conjunction with 4Roto LLC (distributors in the Americas for RMPP141).
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