Understanding coupling agents: Analytical and mechanical methods for an optimized use of graft copolymers (HAWC)

Recyclable coupling agents for interfaces between different types of thermoplastics

Plastics will take on even greater significance in the future across a wide range of applications, such as packaging, tubes, films for the construction sector, and structural materials. In this context, desired property profiles (weight lightweight, high toughness, short cycle times during production) can often only be achieved by using composites and multilayer films/components in which different thermoplastics are equipped with functional additives. At the interface between the different thermoplastics, coupling agents play a key role. For this reason, graft co-polymers are frequently used as coupling agents for the application-oriented design of interfaces, which can be adapted to a wide variety of requirements in various ways. For example, graft co-polymers are used to ensure adhesion in multilayer films. Another key application is the bonding of fibers (natural-, glass- and carbon fibers) to polyolefin matrices. Graft copolymers are also used to achieve adhesion between polyolefin compounds and metallic or mineral surfaces. Completely new needs for coupling agents have emerged because of the transition to a circular plastics economy. Here, coupling agents with new property profiles are required, which should also meet requirements in terms of long-term resistance and hazard potential.

To produce coupling agents and compatibilizers, polyolefins (polypropylene, polyethylene, elastomers) are often used in reactive extrusion processes with unsaturated (polar) monomers. These can be, for example, cyclic unsaturated acid anhydrides (especially maleic anhydride). Another possibility is the partial oxidation of polyolefins during processing, which leads the to in situ formation of polar functional groups.

However, the understanding of the molecular structure of the coupling agents obtained in this way is currently incomplete, as there are no suitable analytical measurement protocols for them. The average molecular weights are often determined by intrinsic viscosity measurement or high-temperature gel permeation chromatography (HT-GPC), as it is state-of-the-art for semi-crystalline polyolefins. In the case of maleic anhydride-grafted (MA) polyolefins, the total MA content can be determined by titration techniques or with spectroscopic analysis such as infrared spectroscopy.

Analytical challenges and solutions

In daily practice, mechanical parameters are used to assess the adhesion behavior of coupling agents. For this purpose, the T-peel test, the 180° peel test, and the climbing drum peel test are widely used. However, their results mainly reflect the properties of the tested system, which, beyond the adhesion behavior of the coupling agents, also strongly depend on the bonding partners, their molding process, and the sample preparation (conditioning). Therefore, the mechanical tests at advanced stages of development bring the most benefits when structures made of composites are to be designed using strength considerations. In contrast, the chromatographic and spectroscopic tests mentioned above, are possible at earlier stages of development, but they provide information on the chemical composition of the coupling agents themselves, while transferability to the system behavior of a composite sample is difficult.

For novel developments aimed at the application areas of plastics recycling, composites, and multilayer films or, for example, photovoltaics, the situation is further complicated by the fact that many different mechanical loads must be taken into account, which may occur in combination. This significantly widens the gap between analytical materials testing and mechanical materials testing. An inherent problem is that tests and current molecular characterization methods do not adequately represent the long-term behavior of coupling agents within these applications.

However, these proposed analytical approaches do not take into account:

(1)   the proportion of actually grafted polyolefin,

(2)   the distribution of the graft monomer along the molecular weight distribution,

(3)   the distribution of the graft monomer between the chains,

(4)   the proportion of free graft monomer.

The analytical state of the art leaves the following questions unresolved:

(1)   to what extent are mechanical properties and performance influenced by the chemical properties?

(2)   Can the mechanical properties be derived from the analytical data?

The information in (1) - (3) is of considerable importance for the determination of structure-property relationships, for the optimization of the grafting process, and for the optimization of processing conditions. The fraction of unused free graft monomers (4) is essential for regulatory purposes. Clarification of the issues (5 and 6) can shorten testing times and accelerate developments.

The goals of the joint project are:

1)    Development of analytical methods to determine the molecular structure of graft co-polymers for coupling agents, addressing the contents of items 1 to 4 mentioned above.

2)    Measurement data from peel tests shall be systematically compared with analytical data. By identifying a correlation between peel tests and chemical analysis, an estimation of the expected peel behavior from the analytical data should be made possible, according to points 5 and  

In addition, the joint project is also intended to offer interested parties along the entire value chain an interdisciplinary platform for developing targeted solutions to the technical problems arising around the topic of coupling agents.

Project focus & Approach

In the project, initially, the state-of-the-art (open and patent literature) for the characterization of graft co-polymers will be compiled. Based on this, technological gaps will be identified, the need will be concretized and approaches to solutions will be developed.

The planned analytics should make it possible to analyze polar-modified polyolefins by means of grafting regarding molecular parameters that were previously inaccessible but are relevant to the application properties. The focus is on:

• distribution of the graft monomer along the molecular weight,
• proportion of grafted material or proportion of non-grafted polyolefin,
• molecular weights of the grafted and non-grafted fractions,
• residual monomer content.

These methods will be elaborated on well-characterized samples in close coordination with the participants. The focus here is on (coupled) HPLC methods and their coupling/extension with multispectral detection. For this purpose, comprehensive equipment for high-temperature HPLC is available at the LBF. In terms of detection capabilities, for example, an infrared or UV detector can be used, as well as NMR spectroscopy, FT-IR spectroscopy, or Raman microscopy, depending on the molecular information sought. Subsequently, the developed techniques will be applied to specific samples of the participating project partners. The detailed planning and selection of the experiments will be done in close coordination with the participants and considering the respective input. Special attention will be paid to a possible application of the analytical methods to be developed in the laboratory environment of a routine laboratory.

At the same time, the mechanical tests commonly used today will be compiled and evaluated. In particular, the focus is on different variants of peel tests. High-performance test methods are available at the LBF for this purpose: Starting with quasi-static test methods (tensile test, three-point bending), through test equipment for creep and relaxation tests (also under the simultaneous influence of water or cooling liquids), to dynamic-mechanical test rigs that also allow control of the ambient humidity and temperature (environmental DMTA). Building on this preliminary work and the available instrumentation, mechanical tests based on peel tests are to be combined with chemical-analytical experiments in order to be able to investigate the bonding of grafted polyolefins in detail (points 5 and 6 above).

Currently, a sample provision by the participants is anticipated, exceptions can be discussed in the kickoff meeting.

Depending on the number of participants and the project budget, further investigations can be carried out on coupling agents that have undergone accelerated aging in laboratory tests. For this purpose, extensive state-of-the-art equipment for weathering is available at the LBF, with which the influence of temperature, humidity/water, UV radiation, etc. on the chemical and morphological material changes can be specifically simulated. With regard to aging-related material failure, such changes occur both in the material volume and at the interface, which is why infrared and Raman microscopy, and electron microscopy are available for their investigation with high spatial resolution.

Finally, the results of physical-chemical material analysis will be combined with those of mechanical testing in order to develop structure-property relationships that can be used for future applications, for example in the field of renewable energy and circular economy.

This offers the following advantages for the participants in the project:

•  Optimization of their own processes in terms of economy and product quality.
• Contribution to strengthening the supply chain/resilience.
• Significantly improved material selection adapted to long-term properties.