TAGS: Thermoplastic Composites
This article was first published in 2017 and is revised in 2022.
Polyolefin composites or POC’s are generally defined as materials that contain two or more phases and have a distinct boundary between the two phases. The careful combination of different and unique systems can result in improved structures and functions over the individual components alone.
The properties of POC’s can be enhanced through the appropriate engineering techniques.
Some of the current applications for POC’s include their use in automotive parts. Specifically, one area where there has been a great deal of interest is in automotive bumper beams. In addition, polyolefin composites are also presently used in construction applications. For many of the described uses, natural-filled polyolefin composites are the desired material of choice.
Despite of multiple applications of POC, there’s something that restricts its performance. Let’s see what that is and how could it be overcome…
What Limits the Use of Polyolefin Composites?
One of the primary issues with POC’s that has limited their use in more applications is the
incompatibility between the polyolefin matrix polymer and the reinforcing fiber or filler. That incompatibility at the interface results in reduced adhesion between the two materials in the composite structure.
This factor limits the stress transfer via the interface, meaning that the reinforcement potential of the fibers or particles cannot be exploited to the full extent, especially for short fibers and particles.
The incompatibility can also lead to the
fibers or particles forming aggregates rather than being uniformly dispersed in the polymer matrix. This fact leads to either an optimum property profile not being realized or the presence of very non-uniform properties in the final composite.
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NOTE:
Incompatibility between the polyolefin matrix polymer and the reinforcing fibers or fillers results in reduced adhesion between the two materials & aggregation of fillers inside the polymer matrix
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Related Read: Solutions to Optimize Matrix Fiber Interface in Thermoplastic Composites »
Overcoming Polymer Composites Incompatibilities
In composites that are based on polymers other than polyolefins, there are several well established approaches for addressing these incompatibility issues. Among the techniques that have been utilized are:
Each of these technologies has had varying degrees of success when used with a variety of matrix polymer / reinforcement combinations.
In general, these approaches are based on the establishment of an interaction, either chemical or physical, between the matrix polymer and the reinforcing material in the composite. That interaction leads to an improved interface being present compared to the material that does not have such an interaction.
The interaction is often due to the presence of specific chemical functionalities or groups that exist in the structure of both the polymer matrix and the reinforcing material. The chemical group in the matrix polymer has an interaction with the chemical group in the reinforcement and this leads to a favorable interface in the resulting composite structure.
However, the issue with the use of polyolefins as the matrix polymer is that there are limited possibilities for interactions with the reinforcement in the composite. With there are no polar chemical groups in the polyolefin polymers generally present. It is the presence of such polar functional groups that usually leads to the type of interactions that are required to have
good interfacial properties in composites.
Ways to Improve the Interface in Polyolefin Composites
There are several ways that this lack of chemical functionality in the polyolefins can be overcome.
#1 - Plasma Treatment of Polyolefins
One approach that has been utilized with some success is the surface treatment of polyolefins, using a variety of techniques, including plasma treatment. This approach can include the use of various materials as additives as well as the treatment itself.
The technology definitely adds chemical functionality to the non-polar polyolefin polymers. However, the chemical species that are produced are often varied and it is not always possible to obtain reproducible results. In addition, since the technology involves the use of additional processing, there is added cost to the polyolefin materials. This impacts one of the positive features of the use of polyolefins, the relatively low cost of the polymers themselves.
#2 - Use of Coupling Agents
Often times, the
coupling agents that are used are functionalized resins that are polyolefins that have been chemically modified. One of the common chemical functionalities that are utilized is an anhydride chemical group on the polymer backbone. The polar functionality of the resin allows it to function as a coupling agent in mixtures of dissimilar materials, such as are present in polyolefin composites.
The polyolefin portion of the coupling agent is compatible with the polyolefin matrix polymer and the anhydride functionality promotes interaction with the reinforcement in the composite. The resins are used in a variety of applications, including filled / reinforced polyolefin formulations as well as additives for polyolefin / fiber composites in glass-filled and mineral-filled applications.
But do look for the compatibility, Otherwise...
This general procedure has been successfully utilized with several polyolefin composites that contain natural fibers
in the composite. In these types of composites, the hydrophobic polyolefin matrices can protect the reinforcement from humidity. However, a gap between the matrix and reinforcement, as can often be observed in uncompatibilized products, acts as a pathway for humidity into the material. The
absorption of moisture leads to dimensional changes in the composite. This is an issue for almost all applications. In addition, absorbed moisture can cause a reduction of the mechanical performance and of the product lifetime.
#3 - Modifying Polyolefin Surface via Fiber
Like coupling agents, the interaction can be enhanced via the fiber, usually by modifying its surface. The fiber-based methods usually depend on the modification of the fiber in either organic or aqueous solvents. The most common treatments are silane treatments followed in popularity by maleated polyolefins.
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The basic interaction between organic and inorganic material via silane takes place as:
Interaction between Organic and Inorganic Material via Silane
In this molecule, the functional group R is the part of what is meant to be compatible with the polyolefin matrix polymer. On the other hand, the X groups in this molecule interact with the inorganic portion of the polyolefin composite. By choosing a silane moiety with the proper R and X groups, compatibility with both the polyolefin matrix polymer as well as the inorganic reinforcement can be tailored as needed.
- Maleated polyolefins are efficient in conventional composites, like PP-glass fiber materials. Pretreatment of the fiber or filler is usually performed by employing an organic solution of the maleated polymer. The solvent that is used in these cases is usually either boiling xylene or hot toluene. In either case, the solvent needs to be removed from the fiber or filler before additional processing of the composite is performed. In addition, melt-based pretreatments can be done in either a roll-mill or in a thermokinetic mixer.
With both of these approaches, tensile and flexural strength can be improved. However, the effects of increasing the product stiffness are generally smaller in magnitude. For both silane and maleated polyolefin pre-treatment of the fibers and fillers results on impact strength and water absorption are presently too few to draw definite conclusions. Also, in the case of impact strength testing, the interpretations of test results are often clouded by the large variety of methods (Charpy, Izod, etc.) that have been utilized for the determinations.
#4 - Modified Montmorillonite Clay for Reinforcing Polyolefin Nanocomposites
A general discussion of polyolefin composites would not be complete without at least a brief mention of nanocomposites. Recently,
polyolefin nanocomposites have attracted significant research interest. The research activities have mainly focused on the use of chemically modified montmorillonite clay as the reinforcement material. Improved gas barrier and mechanical properties have been achieved at low nanoclay loadings of from 4-6 wt. %. Ways to continue to improve the interface in these composites using the techniques that have already been outlined are being investigated.
Concluding Best Ways to Improve Polyolefin Composites
Summarizing, then, there are two ways to improve the interface in polyolefin composites.
- It is possible to modify the polyolefin matrix polymer through the use of coupling agents. These coupling agents can be either introduced as chemical functionalities onto the polyolefin matrix or through the use of a compatibilizing agent that is added to the polymer / filler combination.
- The fiber of filler themselves can be modified to induce compatibility with the polyolefin matrix polymer. Different chemical groups can be added to the fiber or filler surface through this approach. The physical properties that are affected the most are the tensile and flexural strength. The conclusions about other properties require additional investigations to define the effect that is observed.
Finally, it is, of course possible to combine both of these approaches in a polyolefin composite material. The final goal of the various approaches is to
enhance the interfacial adhesion in these materials. Through that enhancement, the property profile of the final composite can be improved. That improvement will lead to expanded utilization of polyolefin composites in new and unique applications.
Optimize Thermoplastic Composite Interface for High Heat Applications
Talk to Mark DeMeuse where he will share how to reach higher heat resistance with your thermoplastic composite materials by learning to
improve your filler/polymer interface properties. He will also help you tailor surface properties (fiber sizing, use of coupling agents, reactive functionalization…) for high-temperature applications (automotive, aircraft, electronics…) thereby improving HDT, strength & fracture toughness.
