Part One: The Rotational Molding Industry

Common Plastic Materials for Rotational Molding

Part One: The Rotational Molding Industry

Common Plastic Materials for Rotational Molding

An unusual feature of the rotomolding process, compared to other molding techniques, is that it is conducted without imposing shear on the polymer melt. This means that not all plastic materials are able to be rotationally molded. Choice is limited to relatively few options; the key alternatives are described below.

The most common material used in rotomolding is a commodity thermoplastic, polyethylene (PE), which softens, becomes tacky and ultimately melts at practical temperatures.

PE fulfils the three primary requirements for a material to be able to be rotationally molded. Firstly, it must be able to flow adequately to coat the mold cavity evenly as the mold is rotated. Secondly, individual particles of material must be able to sinter together when heated. Thirdly, the material must have sufficient thermally stabilty to survive at elevated temperature and a relatively long cook time.

Most plastic materials for rotational molding are special formulations which have been developed to achieve specific performance characteristics. Whilst grades designed for other processes (eg injection molding and cast film extrusion) may, in principle, be rotomoldable, it pays to use specific “rotomolding” grades.

Rotomolding grades should have relatively high melt flow, superior thermal stability, and an acceptable balance of physical properties. For PE, melt flow is commonly measured by Melt Index (MI). General purpose rotomolding grades have a MI between 4-7 g/10 minutes, grades for tanks and large products often have a lower MI (1.5 to 4.0 g/10 minutes). Generally grades for rotomolding need to be delivered to the mold as a fine powder (35 mesh); this may come directly from a supplier, or powder may be produced in-house by the rotomolder. Other product formats are used occasionally, but powder is the norm for most rotomoldable plastics.

The following sub-sections describe the more commonly used plastic materials and their general characteristics.

Polyethylene (PE)

This material, the most commonly used in rotomolding, has flexible to medium stiffness, excellent impact, chemical, and environmental stress crack resistance, is relatively easy to process and is mostly available with UV stabilizer already incorporated (for improved weatherability). Most rotomolding grades in the natural, unpigmented form will meet the requirements of FDA, USDA, NSF and UL (Horizontal Burn).

Applications include tanks, containers, toys and playground equipment.

The stiffness of PE grades is primarily influenced by its density, which is measured in g/cm3. Rotomolding grades are commonly designated as LLDPE (Linear Low Density - normally <0.940 g/cm3) or HDPE (High Density - normally >0.940 g/cm3). The term LMDPE (Linear Medium Density Polyethylene) is also sometimes used to describe grades in these ranges.

PE grades with a significantly lower density have also been used in rotomolding, for applications requiring extreme flexibility and good recovery characteristics. Such material has been described variously as VLDPE (very low density), PE plastomer or PE elastomer. Density range is generally 0.880 - 0.900 g/cm3.

Crosslink Polyethylene (XLPE)

XLPE contains an additive formulation which reacts with the PE base material during the molding cycle, forming a crosslinked molecular (networked) structure which is similar to a thermoset. Properly crosslinked resins provide excellent impact, outstanding environmental stress crack resistance, and other long-term properties. In addition, XLPE parts that undergo fire testing will soften but will not drip and flow, unlike conventional PE grades.

Applications include gas and oil storage tanks, trash containers, as well as parts requiring maximum toughness.

XLPE is also very suitable for parts being used in cold temperatures. Most XLPE grades do not meet FDA, USDA or NSF requirements. Because of its thermoset nature, XLPE cannot be recycled by extrusion, like conventional PEs.

Specialty Polyethylenes

Polyethylenes can be modified to provide specific properties, where the application so demands.

Flame retardant additives can be incorporated into polyethylene to allow parts to pass the stringent UL vertical burn test. The inclusion of FR additive packages is usually accompanied by a significant loss in physical properties, especially impact strength. Applications can include hospital, airline and military containers.

Foaming or chemical blowing agents (CBAs) can be incorporated by dry blending with powder or by extrusion compounding. This creates a polymer system that, if correctly formulated, will release gas into the molten material and create a polymer foam. This foam can be used to impart additional stiffness to a structure, when combined with an outer layer of solid PE skin, or even as part of a three layer “sandwich” structure (solid skin / foam / solid skin). Applications requiring buoyancy or flotation properties, heat insulation or sound reduction can be molded with this resin system.

Polyethylenes can be chemically modified to allow the resin to adhere to metals or other resins; normally PE will not adhere permanently to other substrates. These resins, commonly known as “rotolining” resins, can be used to line metal vessels to provide corrosion and chemical resistance. A rotolining resin can also serve as a bonding or tie layer for co-rotational molding in which two dissimilar resins are joined.
Flexible polyethylenes have very low density, compared to regular rotomolding grades. These polymers exhibit highly elastic properties and rotomolded products are able to recover from major deformations without structural failure. Typical applications for these PE’s include road barriers, pet toys and fenders for speciality vehicles.

Polyvinyl chloride (PVC)

In the early days of rotomolding, the use of PVC compounds was common; today PVC rotomolding tends to be viewed as a speciality technique. PVC grades are mostly supplied in liquid form (known as a plastisol) or, less commonly, as a powder. The liquid plastisols are fluid suspensions of fine particle size resins in a plasticizing liquid.

PVC compounds can present some challenges in processing and are of moderate cost. They can be formulated to produce parts ranging from flexible to semi-rigid.

Applications include balls, doll heads, teething rings, planters, novelty items and flexible bellows.

Polyamide (PA), also known as “Nylon”

Polyamides form a family of thermoplastics, which exhibit similar physical properties. They are often referred to colloquially as “nylon”, although this is a specific trade name from one particular supplier.

PA 6 – This material has excellent tensile strength, stiffness and impact strength (at temperatures above 32 degF). It also exhibits high heat resistance, with a substantial retention of physical properties at elevated temperatures. Resistance to most types of chemical attack is excellent and PA6 has significant barrier properties to hydrocarbons.

As a rotomolding material it is significantly more expensive than PE, but it costs less than other types (PA 11 & PA 12).

PA 6 is prone to moisture absorption from the atmosphere, which can initiate degradation (by hydrolysis) during molding, so it is often rotomolded under an inert dry atmosphere (eg nitrogen gas or dry ice). Applications include military fuel tanks, hydraulic oil and solvent tanks, grain buckets, and air ducts.

PA 11 & PA 12 – Mechanical and barrier properties for these grades are marginally lower than for PA 6, but they can be rotomolded without the need for an inert atmosphere. As raw materials in powder form, they are still prone to some moisture absorption, albeit to a much lesser extent than PA 6, so powder is supplied in moisture-proof packaging. Applications include heating and air conditioning ducts, gasoline tanks and chemical tanks.

Polycarbonate (PC)

This engineering polymer has excellent mechanical properties including stiffness, tensile strength, impact strength, creep resistance and heat resistance. In an unpigmented form, the polymer is fully transparent, but the presence of gas bubbles in the wall of a rotomolded product may detract from this appearance. For success, PC powder may require drying. Applications include light fixture globes, snowmobile engine hoods, shipping containers and other applications where excellent physical properties are required.

Polypropylene (PP)

This family of commodity polymers is widely used in injection molding, but less so in rotomolding. Stiffness and heat resistance is superior to PE, but toughness is inferior, especially below freezing temperatures. Stiffness can be manipulated by incorporating copolymers into the basic homopolymer backbone; this is usually done to improve impact strength. Most grades of PP will require cryogenic grinding to be used to produce a powder. Applications include bio-chemical vessels, solar panels, automotive components, underground chambers and medical storage containers.