Self-Reinforced PP: A Rising Star for New-age Cars

There has been a rise in interest within the automotive industry for self-reinforced polypropylene (PP). One of the main reasons has been low cost of the raw material, which sets it apart from other self-reinforced plastics (SRP).

Self-reinforced PP bridges the gap between isotropic polymers and glass-reinforced materials and offers a unique combination of processing and performance features. This has led to continued exploration of its use in emerging and new applications as well.


Let’s begin by getting to know the basics i.e. structure, production process and properties of self-reinforced PP.

Self-reinforced Polypropylene : Structure and Properties

Self-reinforced plastics are also called SRP’s. These consist of a polymer matrix reinforced with high tenacity fibers or tapes of the same polymer family.

An example of such a material that is gaining popularity in several different industries is polypropylene-reinforced polypropylene. Among self-reinforced plastics, self-reinforced PP has gathered the most attention due to the low cost of the raw material.

Production Process

Compaction of high modulus polypropylene (PP) tapes or fibers under carefully controlled conditions of temperature and pressure, forms self-reinforced PP.

In a typical production process, a small part of the tape or fiber surface is melted and recrystallized upon cooling to bind the surface together. The rest maintains its high level of molecular orientation. Thus, the final product sheet is able keep a high proportion of the original tape or fibers’ physical properties.

Unreinforced PP	Self- reinforced PP
Lesser strength	Better mechanical properties than unreinforced PP
More material used to achieve optimum performance	Less material usage to achieve the same level of performance and properties

Comparison between Unreinforced PP and Self- reinforced PP

Glass-reinforced PP	Self- reinforced PP
Contaminated with mineral fibers	Not contaminated with high levels of mineral fibers – advantages with handling & tool wear
Recyclability not at par with SRPP	Improved recyclability  & reduced weight than glass-reinforced

See the table below for typical tensile properties of self-reinforced PP.

Property	Value
Tensile Strength, MPa	207
Modulus, GPa	6.4
Strain at Failure, %	5.7

Table: Typical Tensile Properties of Self-Reinforced PP

In these tests, the tensile properties were measured on dog-bone specimens prepared from sheets. For these studies, each sheet consisted of ten layers of fabric and the thickness of the sheet was 0.05 inches. Note that there is little difference in the tensile properties in the two perpendicular directions.

Besides the exceptional tensile properties noted in the table above, the low density of the self-reinforced PP translates into:

• Weight savings 
• Good impact strength
• Unique combination of high strain-to-failure and
• High tensile strength
• Impact resistance is retained at low temperatures, even at temperatures as low as - 40° C
• High levels of abrasion resistance

Self-reinforced PP Applications

1. Self-reinforced PP has several applications in the automotive industry, both as exterior and interior components. 
2. Self-reinforced PP’s high resistance to impact and abuse, even at low temperatures, makes it ideal for automotive components. 
3. Self-reinforced PP has significant advantages in underbody shield applications. Particularly improved mechanical properties, reduced weight and enhanced recyclability.
4. Load floors are a major opportunity in the interior of automobiles for self-reinforced PP. Applications with either foam or honeycomb.
5. Applications in automotive interiors, such as knee bolsters and pillar covers use the outstanding energy management properties of self-reinforced PP. As safety requirements continue to evolve, these applications are expected to grow significantly. 
6. Applications of self-reinforced PP outside the automotive industry include military vehicles and body armor.
7. Commercial applications cover various sporting goods such as kayaks, snowboards and surfboards. Other industries are utilizing self-reinforced PP for luggage, packaging and several other cold-temperature applications.

Challenges with Self-reinforced PP

The main challenge in using self-reinforced PP is defining a suitable manufacturing scenario

The primary concern in the production of a self-reinforced PP structure is combining the fiber and matrix into a single product. Various fabrication methods to produce self-reinforced PP are:

• Hot compaction
• Overheating
• Co-extrusion
• Film stacking and
• Traditional melting or powder impregnation

In most cases, the hot compaction process is utilized. This process occurs through the partial melting of the fibers so that the molten outer surface of the fibers becomes the matrix after cooling. But, the issue with this approach is that the processing window for hot compaction is small, about 5°C.

The need for a wide processing window is to ensure good forming of the material while also guaranteeing that the reinforcement fibers remain unaffected by the consolidation temperature. Thus, excessive heating results in the fiber losing molecular orientation which affects the mechanical properties of the final part. On the other hand, insufficient heating leads to a poor interfacial adhesion between the fiber and the matrix.

How to Address the Problem of Narrow Processing Window?

	The concern of a narrow processing window can be addressed using a combination of constraint and co-extrusion. Using that approach, the processing window can be enlarged to about 20-40°C, which allows for the creation of excellent bonding between the co-extruded PP types with different melting temperatures. Several commercial products are presently produced using the co-extrusion technique.
	As another approach to deal with the small processing window of the hot compaction process, the film stacking method has been explored for the manufacture of self-reinforced PP. In that technology, after the reinforced textile structure is produced, it is sandwiched between films. Thus, the reinforcing and matrix-forming layers and films alternately lie on top of each other before the hot pressed structure is produced. The advantages that are provided by the film stacking method include a wide processing window and no expensive pre-production.
	A final approach for addressing the narrow processing window for the production of a self-reinforced PP structure is to utilize a random copolymer that has a lower melting temperature than homopolymer PP. This methodology has allowed for an increase in the processing window of self-reinforced PP materials to 40°C. This technology, along with the other suggested approaches, have contributed to the commercialization of self-reinforced PP.

From an economic perspective, the high price of self-reinforced PP is a major restraint on further market development, particularly in the automotive industry. Yet, self-reinforced PP deserves a premium price because of the benefits it offers in terms of mechanical properties. Material suppliers must address strategies to reduce their cost structure to levels that permit competitive pricing with other materials that are utilized in automobiles. In addition, markets outside of the automotive industry in which price pressure is not so severe must continue to be pursued. Particularly intriguing in this regard are emerging applications in sports fields.

Conclusion

To summarize, self-reinforced PP is best suited for applications where features of the process and performance are valued and can be balanced.

Processing advantages offered by self-reinforced PP include :

1. Thermoformability, which enables low cost tooling with low pressure and moderate temperature processing
2. Ease of handling and low wear on tools, due to the all PP construction and
3. Recyclability, due the all PP structure

Performance benefits include :

• Weight savings, resulting from PP’s low density
• High impact resistance, even at low temperatures, due to unique internal construction and
• Resistance to abrasion, since the structure is tough

This combination of processing and performance features has led to a rise in interest in self-reinforced PP within the automotive industry. It will also lead to continued exploration of its use in emerging and new applications as well.