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Polypropylene compounds for Automobile applications
Transcript of Polypropylene compounds for Automobile applications
Reasons for increasing use of plastics
Economy in terms of :
• Raw material price (as compared to specially treated sheet metals)
• Tooling cost
• Conversion costs
• Integration of parts
• Costs associated with sub-assembly and assembly
Weight reduction leading to decrease in fuel consumption
Styling potential- various shapes are possible to accommodate different child parts and also to take care of needs of customers.
Functional design- Parts can be designed in intricate shapes depending on space available
Useful properties of PP compounds
• Low specific gravity- Light weight contributing to weight reduction. 3% weight reduction in automobiles results in 1% saving in fuel consumption.
• Low cost and easy processability
• Availability of PP in various grades having a range of properties- PP is available as homopolymers, random copolymers and impact copolymers with a range of impact stiffness values.
• Ability to get compounded with a number of mineral fillers and fibres.
• Total recyclability
• Adequate stability to variation in temperature and humidity changes.
• Ability to attain desired light stability by incorporation of additives.
• PP and PP compounds can have desired scratch and stain resistance
Twin-screw extruders are most commonly used for the production of PP compounds for automotive applications.It is essential that the twin-screw extruder has high productivity as well as the ability to disperse the additives such as elastomers and fillers within PP and prevent the degradation of the material during the manufacturing process.
When using talcum at a minute particle size, the discharge rate significantly decreases at the same screw rotational speed. This is because when supplying the granulator with talcum having a minute particle size, the air between the particles is also brought into the machine, which causes the backflow of air toward the upper stream of the screws
Engineering plastics and metals have been extensively replaced by polypropylene (PP) based materials in automotive parts in order to achieve weight reductions and cost savings. To accomplish this, PP compounds which are made from PP and other components are under intense investigation.
Because polypropylene (PP) is low in cost but has outstanding mechanical properties and moldability, it accounts for more than half of all the plastic materials used in automobiles.
In recent years the environmental adaptability of PP has drawn significant attention.
To address this issue, screws having the design shown below are used. In this design the screws maintain the resin in the semi-molten state in the primary mixing section, maintaining the space between the cylinder and the resin passing through the section, and then discharge the air that was brought in along with the talcum through a vent port located on the extruder’s downstream side.
Improving Impact strength
Of all the fillers, glass fiber—which has the high rigidity and aspect ratio—shows the greatest stiffening effect, followed by the tabular-shaped talcum. Given the considerations of cost, performance and processability, talcum and glass fiber are mostly used for PP compounds for automotive applications.
Effect of talcum size on izod impact strength and flexural modulus is shown below :
To improve the rigidity of PP, material development has been conducted using short-length glass fiber. By adding short-length glass fiber to PP, the threshold of heat resistance increases to a point near the melting point of PP. This type of PP is therefore used in automotive parts for areas to be exposed to severe heat, such as the engine compartment.Furthermore, in order to add even more rigidity, long glass fiber reinforced PP is currently under development.
The long glass fiber reinforced PP is produced according to the following procedures: Glass fiber is continuously supplied from glass-fiber rolls, in a manner called roving, to the molten resin in the impregnating die.
The pellet length and glass fiber length are equal in the long glass fiber reinforced PP. Using these pellets, automotive parts are produced through injection molding. However, if the glass fiber breaks dur ing the injection-molding process, the rigidity of PP will deteriorate. Therefore, special screws having a minimal mixing effect are used.
For example, the long-glass-fiber reinforced PP is used for front-end modules, back door panels and door interior panels. In order to satisfy the performance requirements, it is important to have the technology that enhances the strength of interface between the glass fiber and PP.
Most of the PP compounds for automotive applications are made into automotive parts through injection molding. In the molding process the material is supplied to the cylinder of the injection-molding machine, melted at a temperature of approximately 200°C and injected into the die. However, various defects such as weld lines, flow marks, mold flashes, sink marks, warpage, thread formation and uneven coloring can occur during the molding process.
By optimizing the EP copolymer structure design so that a high swell ratio can be obtained, a material that can be formed through injection molding at a low pressure, and that does not readily produce mold flashes, can be obtained.
Improving Impact Resistance
The impact PP containing EP copolymer, as described previously, is used for PP compounds in automotive applications.
It is observed that the glass-transition temperature shows the minimum value when the ethylene content is approximately 60 wt%, and the performance as an impact modifier is good in this zone.
However, the compatibility between EP copolymer and PP decreases as the ethylene content increases.For this reason EP copolymer with ethylene content of 30 – 40 wt% is often employed, considering a balance between the EP copolymer performance and the size of the dispersed particles.
Adding more Functionality
In recent years, even more functions such as easy paintability, conductivity, scratch-resistant property and low coefficient of linear thermal expansion are in demand. Automotive parts such as bumper facias and side mouldings are often painted prior to installation.
To achieve weight reductions in automobiles, resin applications for outer panels such as fenders and doors are currently being investigated. Thus far, polyphenylene ether/polyamide alloy and polycarbonate/ABS alloy outer panels have been put to practical use.
It can be expected that, from this point forward, PP compounds will continue to play the primary role in resin materials for automotive applications due to their high cost-performance, outstanding moldability and environmental suitability.
Moldability of PP compounds