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Plastics & Elastomers
Thermoplastic Composites : Optimizing Matrix Fiber Interface

Thermoplastic Composites: Solutions to Optimize Matrix Fiber Interface

Thermoplastic composite materials often suffer from a lack of fiber-matrix adhesion. A weak interface in these materials results in low stiffness and strength but a high resistance to fracture. On the other hand, a strong interface produces high strength and stiffness but generally low fracture resistance.

So, how to overcome this lack of fiber-matrix adhesion? Can fiber surface modification help? Find out all your answers here! Explore the ultimate solutions to optimize matrix / fiber interface in thermoplastic composites and achieve desired strength & stiffness along with fracture resistance in your product.

Overview

Achieving High Performance Thermoplastic Composites

Achieving High Performance Thermoplastic Composites

Polymer composites are playing an increasing role in a wide variety of applications. In particular, thermoplastic composites are under increased scrutiny. They are easier to recycle & reuse compared to thermosetting matrix composites.

Uses for thermoplastic composites are currently being envisioned for the automotive industry. In the automotive industry, for example, they have to offer a unique combination of:

  • High thermal and oxidative stability
  • Toughness
  • Solvent resistance

Yet thermoplastic composite materials often suffer from a lack of fiber­-matrix adhesion. This low stiffness and strength but a high resistance to fracture. Optimizing the matrix/fibers interface enables to achieve high strength and stiffness but generally low fracture resistance. Other properties of composites, affected by interface characteristics include resistance to:
This is remedied using fiber surface modification. Usual surface treatments for epoxy­-based thermoset composites do not work well for use in thermoplastic composites.

Thermoplastic Composites
Thermoplastic Composites with Excellent properties
Matrix Fiber Interface Optimization Methods

Matrix Fiber Interface Optimization Methods

Chemical modification/addition of a third, compatibilizing phase bridges the fiber and matrix phase. It improves the interfacial characteristics of many polymer composites systems. It is important since the mechanical properties and performance not only depends on the properties of constituent fiber and matrix but also on the quality of interfacial bond.

This fact is exemplified by the data contained in the table below. The modulus data is given for Nylon 6,6/carbon fiber composites that use unsized carbon fibers & carbon fibers containing polyurethane (PU) on the fiber surface. It shows that the presence of the PU largely affects the measured modulus values. An increase of greater than 50% is observed. The increase in modulus is due to an improvement in the interfacial bond in the composite.

Carbon Fiber Modulus (MPa)
 Unsized 8870
 PU Sizing 13600
Modulus of Nylon 6,6/ Carbon Fiber Composites


Depending on matrix nature, which functional group you should try to obtain?


Unlike polyamides, polyolefins need different functional groups on the reinforcing fiber surface to promote chemical interactions. There is no surface functionality universally applicable to any thermoplastic polymer material. Instead, the most effective functional groups are different for a particular thermoplastic polymer utilized as matrix material in the thermoplastic composite.

Thus, while selecting an appropriate surface treatment of the reinforcing fiber, consider the matrix polymer used in the composite. Otherwise, you won't get the most effective chemical functionality. This can lead to a situation where fiber­/matrix interface is not optimized and the desirable physical properties are not achieved. This will affect the performance of the composite in the end­-use application.

Non-polar matrix (Polyolefins)

  • Surface groups that are primarily non-polar in nature are most desirable.
  • This can often be done by the use of silane materials that contain alkyl groups.
  • Those surface functionalities are compatible with the polyolefin matrix polymer in the thermoplastic composite and, thus, will provide compatibility.

Polar matrix

(Nylon, PET)

  • It is most effective to produce functionalities such as carbonyl and hydroxyl groups on the fiber surface.
  • Those chemical groups are capable of chemical interaction or even chemical reaction with the matrix polymer.
  • Those interactions will lead to an improved interface compared to the use of fibers that do not have carbonyl or hydroxyl groups.


In our exclusive guide, find out the solutions for an easier matrix or resin selection.

Resin Selection For Thermoplastic Composites


Matrix/Fiber interface optimization can be done by following methods:

  1. Chemical modification of the fiber surface
  2. Using sizing agents/compatibilizing phase


Chemical Modification of the Fiber Surface

Chemical Modification of the Fiber Surface

Oxidation methods that provide surface functionality consist of performing oxidizing reactions in a liquid or gas environment. These form oxygen-­containing functional groups, such as carboxyl, carbonyl, lactone and/or hydroxyl groups on the surface of the reinforcing fiber. At the same time, these oxidation methods also increase the surface area of the fiber. These improve the stress transfer from the weak and compliant matrix polymer to the strong and stiff reinforcing fibers.

Chemical Modification
Benefits  Limitations
  • One of the primary advantages of the chemical modification approach is that many different functional groups, including carbonyl, hydroxyl, carboxyl and other groups can be added to the fiber surface.
  • It can be used to introduce chemical interactions with any matrix polymer in the thermoplastic composite.
  • Majority chemical modification approaches do not allow effective control of concentration of specific functional groups. Instead, several different functional groups are added at the same time.
  • Some of the functional groups added will not be effective at promoting interaction with the matrix polymer. Thus, the efficiency of the modification approach is lower than other methods. 

Chemical Modification Approaches


Chemical modification approaches investigated to treat fiber surfaces used in thermoplastic composites include:

  • Wet chemical etching
  • Flame oxidation
  • UV/ozone treatment
  • Corona and oxygen plasma treatment (very effective)

Of these, chemical etching is often able to provide specific chemical groups onto the fiber surface. So, controlling the chemistry of the fiber surface is possible. Other approaches often generate of several different chemical groups onto the fiber surface. Hence, it can be difficult to produce a specific concentration of desired chemical functionality.

Chemical Etching
Chemical Etching
(Source: Precision Micro)

Oxygen plasma treatment is the most effective. It increases the interfacial bond in the composite. But, one limitation of oxygen plasma is that they are usually generated in a vacuum. To receive proper treatment, insert parts into a sealed chamber and pump the gas away before discharge. These factors lead to vacuum operation requiring expensive equipment that must be carefully maintained. Also, it is more expensive and time consuming than other processes that are utilized.


Sizing Agents/Compatibilizing Phase

Sizing Agents/Compatibilizing Phase

Sizing process is one of the most used and efficient optimization methods. It provides the needed chemical functionality to the reinforcing fiber. Sizing agents are also known as matrix compatibilizing agents.

When using sizing agents, a thin layer of polymer adheres to the fiber surface and protects the fiber from damage during both processing and handling. This function is in addition to the chemical interaction with the matrix polymer that the sizing agent provides. Several methods have been developed to improve the fiber surface wettability or to increase the quantity of surface functional groups.

The interfacial bond between the reinforcing fibers and the matrix resin can be enhanced by enlarging the surface area, which provides more points of contact/anchorage between the fiber and the matrix polymer. The other way that the interfacial bond between the reinforcing fiber and the matrix can be increased is through an increase in the physiochemical interaction between the component materials in the composite.

The polymer layer is normally applied at a concentration of less than 1% of the total composite mass. The most used coatings for thermoset composites are epoxy and phenolic resins, among others. However, these sizing agents don’t generally own a temperature stability of greater than 300° C, which is near the processing temperatures of most thermoplastic materials. Polyurethanes are available for use with Nylon matrix polymers and can effectively improve the interface in composites that use those polymers. That effect has already been shown with the data in Table 1.

Aqueous Suspension Prepregging


Polyimides can be used in structural thermoplastic composites by employing a pre­impregnation and composite manufacturing technique called aqueous suspension prepregging.

1st phase – This technique utilizes a polyimide precursor, polyamic acid, allowing the polymer matrix to be applied to the reinforcement together with the polyamic acid during a single pre­impregnation step. The approach uses the polyamic acid neutralized with a base to produce a salt. The polymer matrix, in powder form, is, then, dispersed in an aqueous solution of the polyamic acid salt, which acts as a dispersant.

2nd phase – In a second phase of the process, the polyamic acid is thermally converted into a polyimide that acts as an effective sizing agent for the thermoplastic matrix polymer. That approach has proven to be effective using both PEI and PEEK as the matrix material in the thermoplastic composite. The usefulness of the concept has been shown to be due to a chemical interaction between the polyimide and the matrix polymer that is being utilized.

Following the technical line of reasoning just discussed, it is desirable to have chemical functional groups linked with the reinforcing fiber that can interact with chemical groups that are contained in the thermoplastic matrix polymer. But, in the case of some of the matrix Polymers such as polyethylene or polypropylene, there are no chemical groups that are highly reactive. In that case, a slightly different approach to improve the interface can also be utilized.

Carbon fabrics impregnated with PEEK polymer suspensions
Carbon fabrics impregnated with PEEK polymer suspensions: a) with PAA BTDA/ODA interphase and b) PMDA/ODA interphase.
(Source: Research Gate)


Why to use a third component?


The most common approach in the case discussed above is to use functionalized polyolefins as a third component in the thermoplastic composite. Typical functional groups that can be added to polyolefins include:

  • Maleic anhydride, and
  • Acrylic acid

Those functional groups are usually added to the base polyolefin at amounts of less than 5% by weight. Functionalized polymers that contain these groups are usually added to the composite at levels of 1-2% by weight.

The general concept behind the use of such materials in thermoplastic composites is that the polyolefin-­based functionalized polymer will be compatible with the thermoplastic matrix polymer.

Pros of Adding a Third Component


On the other hand, the presence of the functional groups will allow for potential interactions with corresponding surface chemical groups on the reinforcing fiber. In that sense, the presence of the third component acts as a bridge between the reinforcing fiber and the matrix polymer. Its role is essentially to act as a compatibilizing agent between the two chemically dissimilar materials in the composite. Most important in that regard is the introduction for the potential for chemical interactions between the two components in the thermoplastic composite.

This approach is typically a less expensive way to improve the composite interface than are the other methods that have been discussed. It does not involve additional processing steps but rather simply includes the addition of a third material in the fabrication of the thermoplastic composite. As such, the production process is identical to the process that has been developed for the initial composite that does not contain the third component material. Due to this fact, the implementation of the technology will be relatively easy and straightforward.

Cons of Adding a Third Component


  • The largest issue with the use of a third component is that in order for it to be most effective, the compatibilizing agent should migrate and diffuse to the interface between the matrix and the polymer. 
  • To guarantee migration effect, the compatibilizing material needs to have a low molecular weight. 
  • To be certain of an adequate concentration of the third material getting to the interface, it may be necessary to use more than 1-2 % as optimum concentration.
  • Both low molecular weight and increasing concentration will lower the mechanical strength of the final composite. So, there will be some loss of properties that the compatibilizing agent is expected to provide.

As thermoplastic composites continue to expand in their use, optimization of the interface will continue to be a challenge. The ideas presented in this guide can be used as guidelines for understanding the factors that need to be considered. As additional applications are developed, continuing efforts on this topic will be required. Those efforts will lead to the highest values of the composite properties being realized and that will lead to the thermoplastic composites being effectively utilized in new applications. In addition, as different polymers are considered for use as the matrix material in thermoplastic composites, it will be necessary to develop effective ways to optimize their interface with the reinforcing fiber. Such optimization will allow for the most effective utilization of thermoplastic composites in new and emerging developments.

Sizing Agent Chemistries for Thermoplastic Composites


As thermoplastic composites continue to expand in terms of their use temperatures, additional chemistries are being developed to deal with the thermal stability needs for the sizing agent. Polyimides are gaining additional investigations and interest, due to their high solvent resistance and high service temperature.

The use of the polyimides as sizing agents are becoming more extensive as use temperature of thermoplastic composites continues to increase to 300°C and higher. For example, using both PEEK and PEI as matrix polymer requires processing temperatures above 300°C and, thus, high thermal stability for the appropriate sizing agent is a need.

The following table summarizes the various chemistries that are available for use as sizing agents. It will and also summarize some thermoplastic polymers for which each chemistry is most effective. Also, some of the pro’s and con’s of each chemistry are summarized. For completeness purposes, epoxies are included in this Table to show how they compare with these other chemistries.

Chemistry

Matrix Polymer

Pros

Cons

Epoxy

None - Doesn’t interact with most thermoplastics

  • Inexpensive
  • Well-known
  • Poor thermal stability
  • Doesn’t interact with most
  • Thermoplastics

Silane

PP, PBT
  • Versatile
  • Inexpensive
  • Sensitive to Water
  • Low Thermal Stability

Polyurethane

PA

  • Solvent Resistance
  • Thermal Stability
  • Sensitive to Water

Polyimide

PEEK, PEI

  • Thermal Stability
  • Chemical Resistance
  • Very Expensive
Common Sizing Agent Chemistries for Thermoplastic Composites

As can be seen from this table, all sizing agents have certain advantages and disadvantages. Thus, for the commodity thermoplastic polymer matrix materials, PP and PBT, the recommended silane sizing agents are relatively inexpensive but generally suffer from thermal stability issues.

However, those thermal stability issues are not very serious because both PP and PBT are normally not used at high temperatures. On the other hand, the recommended sizing agents for the more specialized thermoplastics, like PEI and PEEK, have very good thermal stability but are quite costly. In general, this is not a primary concern since thermoplastic composites that utilize these specialized polymers as the matrix polymer generally have applications for which price is not a primary concern.


Learn More on Thermoplastic Composites!


Reach faster higher heat resistance with your thermoplastic composite materials by learning to improve your filler/polymer interface properties in practice. Take the course: Thermoplastics Composites Interface Optimization for High Heat Applications today!

Thermoplastics Composites Interface Optimization for High Heat Applications



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