Why Hot Cast Polyurethane?
Polyurethanes play a vital role in many industries- from shipbuilding to footwear, construction to cars.
The biggest demand for polyurethane products is in the form of foams and these constitute some 90% by weight of the total market for polyurethanes.
Multotec specialises in solid polyurethane elastomers, tailored to meet the needs of specific applications. In mining applications where abrasion resistance, impact and tear strength are so vitally important, the integrity of the polyurethane is critical. The name polyurethane has been used to describe a great number of polymers. In order to simplify the detail of polyurethanes
we will provide a background to the principle components and describe the formation of the polyurethane molecule.
A polyurethane molecule is comprised of three principle components.
- A Resin This forms the backbone of the polyurethane molecule and is more correctly referred to as the polyol. Multotec uses three main types of polyol- polyesters, polyethers and polycaprolactones. An Isocyanate- in our case more specifically a diisocyanate. Multotec predominately uses various Toluene Diisocyanates (TDI) and Methylene Diisocyanates (MDI).
- The resin or polyol and the diisocyanate are chemically reacted together to form a prepolymer.These two components can be greatly varied to give maximisation of different properties to the polyurethane molecule and the final polyurethane product.
- A Curative This is the final ingredient in the process and in chemical terms is known more correctly as a chain extender. This is used in a further chemical reaction to convert the liquid prepolymer into a solid elastomeric product. This process is also called crosslinking and it is the binding together of the various atoms to form the polyurethane molecule. Again different curatives give different properties to the final polyurethane product because of the way they crosslink the various atoms in the polyurethane molecule. The curative is a critical factor in a high performance polyurethane.
Cast Polyurethane Elastomers (CPU)
As prefaced previously, cast polyurethane elastomers are made by mixing a reactive liquid prepolymer with a curative and pouring the mix into an open mould. There are a large number and variety of cast polyurethanes with a wide range of physical properties and cost. We will limit this detail to the high performance polyurethanes used by Multotec in mining products.
The best known and most commonly used high performance CPU are Polyether /TDI prepolymers reacted with a curative 3,3’-dichloro-4,4’-diaminodiphenylmethane which is commonly known as MOCA. This curative or diamine chain extender gives the highest physical crosslink density to the final poyurethane molecule of any curative on the market today.
The above mixture is degassed and cast into moulds and this process is undertaken at a temperature of about 100 C. In order to ensure the highst performing product the casting and curing conditions are very important. All of our products are cast under critical temperature control and after an initial mould cure there is a final post cure to ensure that the chain extender fully cross links and ensures optimun properties in the finished product.
Injection Moulded Thermoplastic Polyurethane Elastomers (TPU)
Thermoplastic polyurethane elastomers are supplied as granules or pellets for processing by techniques such as injection moulding or extrusion.TPU’s are generally classified by their hardness and are supplied by a number of major chemical companies.
As such the processor accepts what he is supplied and cannot vary the chemistry of the product.
TPU is usually made from a MDI (diioscyanate) reacted with a polyether or polyester
and also a chain extender usually 1,4-butanediol. These ingredients are set in the supplied pellets.
In the injection moulding process the components are constituted in the pellet and they are then melted together and injected into a mould.This process takes around five minutes. The TPU is constituted so that it can be melted and reformed. In other words it has a different molecular structure to CPU and does not have the crosslink molecular strength that a CPU has. The injection moulding process also limits product applications of TPU.
The Difference?
Well its all in the formation of the polyurethane molecule and how it is accomplished.
Firstly, the ingredients in the prepolymers used by Multotec are varied to provide the best
properties in the polyurethane that forms the finished product . They are not an off the shelf generic product. The science of this variation is called stoichometry and Multotec’s technical staff have developed formulae variations to fit a range of applications. The use of polyols such as polycaprolactones provides a much wider range of application, life and performance.
Secondly, the curative or chain extender used by Multotec which is MOCA provides a
crosslinked polyurethane molecule that is superior to any other in either a cast or thermoplastic process. MOCA is not available as a chain extender in thermoplastic systems.
Thirdly, the hot cast process provides a long slow cure and post cure that enhances the crosslinking of the polyurethane molecule and ensures that it is complete and that there is no unreacted components.This cannot be accomplished in the relatively short processing time in injection moulding.
The Result?
Multotec cast polyurethane products are manufactured from only the best materials available using techniques that ensure both quality and technical superiority. Head to head trials have regularly demonstrated that a cast polyurethane will outperform an injection moulded polyurethane in a wide range of mining applications - from Granny Smith Gold Mine , Ok Tedi, Boral Quarrying Operations and Hanson, the proof from the field categorically supports the chemistry of cast polyurethane technology.