How to Solve Fiber-reinforced Plastics Recycling Issues?

TAGS:  Thermoplastic Composites    Sustainability and Bioplastics    

This article was first published in 2016 and is revised in 2022.

Fiber-reinforced plastics or FRPs are lightweight, have high specific mechanical properties, are corrosion resistant, have long life cycles, and are easy to manufacture in different shapes.

This combination of properties has made FRP very attractive materials for use in the automotive industry and other transportation fields.

Recycling of thermoset FRPs present an especially difficult challenge due to the fact that once the thermoset matrix resin molecules are crosslinked, they cannot be melted or reformed. In comparison to others, thermoplastic-based fiber reinforced plastics are inherently recyclable.

However, FRPs are difficult to recycle due to their multiphasic nature. They can contain three or more components, including fiber reinforcement, resin matrix and fillers.

Here, let's focus on the effects of recycling on thermoplastic-based fiber reinforced materials and how to overcome the related issues.

Effects of Recycling on Fiber-reinforced Plastics

Several methods are currently used for the recycling of fiber-reinforced plastics. These include:

• Mechanical breakdown
• Thermal recycling
• Chemical recycling

These techniques are primarily used for thermoset matrix composites but these approaches could also be used for thermoplastic matrix composites as well.

FRP Recycling Issues: Fiber Breakage, Polymer Degradation, Poor Fiber Dispersion

Energy considerations suggest that remolding is a much more desirable option than these ideas for such materials. The biggest issue with the remolding of thermoplastic composites is that they show a degradation of the mechanical properties, the extent of which depends on both the recycling process as well as on the service conditions history.

• The loss in properties is often due to fiber breakage that occurs during the recycling of the composite material itself. 
• The observed fiber breakage means that the reinforcing fibers in the recycled material will be shorter than in the initial product, hence, the reinforcing effect that is provided by the fibers will be reduced. 

This is because the length of the reinforcing fiber is a primary factor in determining the properties of the FRP.

It should be noted that the fiber breakage during reprocessing has a smaller impact on the reinforcing properties in short-fiber FRPs than is observed for long fiber-reinforced plastics. This means that mechanical recycling has a great potential for these materials. In addition, the simplicity of mechanical recycling and its relatively low cost make it an obvious choice for the recycling of fiber-reinforced thermoplastics.

In addition to reductions of the fiber length during recycling, degradation of the matrix polymer is also an issue. That degradation can also lead to changes in the fiber/matrix interface bonding in recycled thermoplastic composites. Since the interfacial bonding is known to be a primary factor that affects the mechanical properties of the final product, changes in it can significantly impact the properties of the composite.

Related to the interfacial bonding is the level of dispersion of the fiber reinforcement in the matrix polymer. The ideal scenario is to have uniform dispersion of the fiber throughout the composite.

Though, it is known that the processing conditions can have a significant impact on the presence of agglomerates, the additional processing that is involved with recycling of the fiber-reinforced plastic can lead to the potential for increased regions of high levels of fiber, if the processing is not done properly. This can have an effect on the properties of the final product, particularly on the uniformity of the properties in different regions.

The properties that are affected by the recycling of fiber-reinforced plastics include:

• Tensile strength and 
• Young’s modulus

These properties of the FRP are related to the length of the reinforcing fiber with higher values being observed with longer fibers. As previously explained, recycling of the FRPs has the effect of causing fiber breakage and this fiber breakage leads to a reduction in both the tensile strength and Young’s modulus of the FRP. Reductions of up to 25% of both parameters with multiple recycling steps have been reported in the polymer literature.


All of these effects will influence the mechanical properties of the FRP.

• Fiber breakage will be dependent on the length of the fiber in the initial FRP. 
• Polymer degradation will occur regardless of what the nature is of the reinforcing fiber. 
• Dispersion of long-fiber reinforcements is quite difficult in general and the dispersion may actually improve upon recycling due to fiber breakage. 

However, it must be noted that the reinforcing effect will also lessen with the corresponding decrease in fiber length. Thus, there will be a balance that will need to be achieved between the dispersion and the reinforcing effect in the FRP.

It should be noted that many of the discussed effects are observed with both carbon fiber and glass fiber as the reinforcing material in the FRP. Currently, carbon fiber composites have found their way into the manufacturing of high-end items such as aircraft and spacecraft parts, golf club shafts and racing car bodies. Presently, most waste goes to the landfill. This presents a unique issue due to the relatively high cost of the carbon fiber.

Related Read: Adding Carbon Fiber to the Designer’s Palette »

Overcome Problems Related to Recycling Issues

In this section, suggestions will be provided for ways to address the issues that have been previously discussed. In some cases, the recommendations have already been proven to be successful in certain situations. In other instances, the approaches have not yet been completely verified and additional research work is required to validate that the proposals are effective.

#1 - Solution to Fiber Breakage

It needs to be established that with any type of mechanical processing of the FRP that some level of fiber breakage is likely to occur. Thus, necessary action is needed to limit the observed fiber breakdown. Or, conversely, if it is accepted that some amount of fiber damage will occur during the recycling of FRPs it must be accepted that the recycled product may not be useful in certain applications.

The accompanying Figure is a graph of the properties of glass-reinforced polypropylene vs. fiber length. The data that are shown below clearly indicate that there is a dependence of the various mechanical properties on the length of the glass reinforcing fiber.

FRP Properties vs. Fiber Length of Reinforcement

Still, it also needs to be observed that all of the properties, including stiffness, toughness and strength, reach their maximum values at some fiber length value. Beyond that fiber length, there is little or no additional reinforcing effect that is observed.

There are two primary points to be gathered about recycling of fiber-reinforced plastics from the information in Figure.

1. First, depending on the desired application for the FRP material, the optimum fiber length needs to be determined for the fiber reinforcement. As long as that fiber length is maintained during the various recycling steps, the FRP product will continue to be useful in the desired application.
   
   Conversely, processing conditions during the recycling steps will need to be monitored and adjusted to guarantee that minimum fiber length is always obtained. For example, during pelletizing operations that are associated with the re-melting and remolding process, the reprocessed pellets will need to be produced to such a size that the desired fiber length is always maintained.


2. The second point that can be determined from Figure is that if the initial FRP product contained long-fiber reinforcement and significant fiber breakage occurs during recycling, the product may still have use in certain applications for which the desired properties are not so high.
   
   This can be rationalized due to the fact the fiber breakage will lead to a lower reinforcing effect and, hence, lower properties. There are likely applications, however, for which the obtained properties are still adequate. Thus, it should be possible to use the recycled RFP’s for other products for which a slightly lower mechanical property profile is adequate.

#2 - Solution to Polymer Degradation

Turning now to the issue of polymer degradation, this is, of course, a common problem that is associated with many polymer processing operations. Though, the presence of the reinforcing fiber provides another level of complexity to the polymer degradation concern that is not encountered in unreinforced polymers.

This is due to the fact that the presence of the reinforcing material can lead to additional polymer degradation beyond what is usually observed during recycling. In other words, the reinforcing fiber can in fact promote degradation of the matrix polymer. This can be a chemical effect or it can be a mechanical breakdown of the polymer during recycling.

Due to this increased tendency for polymer degradation in FRPs, higher levels of stabilizing agents that are normally used should be utilized with recycling FRP’s. The stabilizers that should be used are well known for most polymers and these same materials can be used in FRPs, simply in higher concentrations. It does need to be pointed out, however, that for high temperature polymers that are seeing an increased usage in FRPs, the stabilizer that is used needs to possess high temperature performance features itself. Otherwise, the polymer degradation that is observed will not be reduced significantly.

#3 - Solution to Dispersion of the Reinforcing Fiber

The final issue that needs to be addressed for the effective recycling of FRPs is the dispersion of the reinforcing fiber. With each reprocessing cycle of the FRP, the potential for poor dispersion increases. This is because the FRP is a multiphasic material and such materials are known to separate into regions of the component materials under certain processing conditions such as elevated temperature.

The dispersion of the reinforcing fiber in recycled FRPs can be enhanced through the use of compatibilizing agents between the fiber and the matrix polymer.

• Compatibilizing agents are often used during the production of the initial FRP and it is recommended that they should be added to the mixture every time that a reprocessing step is utilized. 
• This should help guarantee uniform dispersion of the fiber in the FRP. 
• The chemical nature of the appropriate compatibilizing agent to use is dependent on the nature of the matrix polymer. 
• The compatibilizing agent should be able to interact with the chemical agents on the reinforcing fiber to enhance dispersion. 
• For example, for PP/glass fiber materials, PP that is functionalized with maleic anhydride is often an effective compatibilizing agent to utilize.

As already briefly discussed, carbon fiber reinforced composites present an additional challenge with their recycling. Due to the high value of the carbon fiber, much research has been conducted to extract the fiber from end-of-life components and from manufacturing scrap. The goal of that research is to use the materials for creating other carbon-fiber composites.

Recycled carbon fibers can be used in bulk molding compounds for smaller, non-load-bearing components and as recycled materials in load-bearing shell structures. The recycled carbon fiber is also finding use in phone cases, laptop shells and even water bottle cages for bicycles. Such use can preserve the relatively expensive carbon fibers by utilizing them in other applications than for which they were initially targeted.

Conclusion

In this article, technical issues that are involved with the recycling of thermoplastic fiber-reinforced plastics have been highlighted and addressed. The issues that have been discussed primarily affect the final mechanical properties of the material. That effect can impact the utilization of the fiber-reinforced plastic in the desired application, depending on the required product performance.

Several solutions for addressing the concerns have been suggested. On the other hand, additional research is needed to further validate their usefulness. As FRPs continue to be utilized in industries such as automotive and other transportation fields, such research will be critical for the establishment of the effective economic utility of FRPs in these developing applications.

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