Exploring Reactive Extrusion: Advancing Polymer Production & Functionalization

Reactive extrusion (REX) is a process that enables the production or functionalization of polymers. Here, production refers to a polymer synthesized from its most basic building blocks via polymerization, whereas functionalization refers to a polymer that undergoes post-reactor chemical modifications.

Examples of polymers obtained via REX polymerization include thermoplastic polyurethanes and polyamide (nylon) 6; those obtained via REX functionalization include grafting monomers onto polyolefins. In general, twin–screw extruders play a key role in these REX processes because of their capability to achieve high levels of mixing and ability to handle materials exhibiting high viscosity. Accordingly, the scope of this article is the modification of polyolefins via REX functionalization using corotating, twin-screw extruders.

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Figure 1: Chemical structures of polyethylene and polypropylene. The structures in brackets represent the basic repeating unit of each polymer; n represents the number of repeating units forming the polymer backbone/chain. Source (all): C. Escobar

Why Are Compounders Interested in Functionalization?

Generally, polyolefins such as polyethylene and polypropylene (Figure 1) exhibit a non-polar nature, i.e., the electrical charge along their backbone is evenly distributed, which makes them relatively inert. In contrast, functionalized polyolefins (Figure 2) exhibit a polar nature, i.e., the electrical charge along their backbone is unevenly distributed. This characteristic affords new functionality to the polyolefins including reactivity, which in turn helps expand their applications. In other words, reactive extrusion enhances the value of polyolefins.

Figure 2: Chemical structures of polyethylene functionalized with monomers such as maleic anhydride (MAH) and vinyltrimethoxysilane (VTMS).

Safety First       

In general, polyolefin extrusion processes have inherent physical risks such as high operating temperatures and pressures. In addition to such physical risks, REX brings chemical risks that need to be considered and addressed prior to the implementation of a functionalization process. Figure 3 shows a few examples of such risks. The latter type of risk will depend on the chemical nature of the compound, also known as a monomer, that will be grafted onto the polyolefin backbone. 

Figure 3: Examples of physical and chemical risks present in some reactive extrusion processes used to functionalize polyolefins.

For example, in some cases there may be a need to dissolve the monomer in a specific solvent to feed it into the REX process, and such solvent could be flammable. In other instances, the monomer itself may be flammable, toxic, corrosive or all of the above. Furthermore, depending on the type of chemistry/functionalization desired, there may be a chance for high-energy releases. To ensure safe implementation and operation of a REX process, due diligence must be taken to fully understand these risks, both from a raw materials and process perspective. 

Methodologies such as the management of change (MoC) can help mitigate such risks. MoCs help identify and implement appropriate precautions such as engineering controls, testing and characterization, and personal protective equipment that help minimize risks. Examples of such precautions include adequate ventilation, inert atmospheres, equipment with the appropriate electrical classification, differential scanning calorimetry to understand the thermal properties and behaviors of the materials used in the process, heat of mixing to assess any increases in energy/temperature as raw materials are mixed, thermal screening unit to assess any thermal and pressure hazards, heat-resistance gloves, goggles, fire-resistant lab coats, respirators, etc.  Overall, it’s crucial to approach the functionalization of polyolefins with a safety-first mindset.

Advantages and Disadvantages of REX

The advantages of using REX to functionalize polyolefins include, among others, the economics of a continuous process, no need (or limited amounts) for solvents, the ability to handle materials with a higher and wider range of viscosities, relatively low investment costs, and the flexibility offered by the modular nature of corotating, twin-screw extruders.

Some of the disadvantages include, among others:

• the potential variation in reaction kinetics (i.e., how fast the raw materials react), which is dependent on the chemistry of the targeted process
• limited residence time
• potential for polymer degradation and crosslinking
• low grafting yields
• sometimes high volatility of monomers.

Generally, REX offers benefits for polyolefin functionalization, but potential limitations exist.

Figure 4: Select examples of monomers used for the functionalization of polyolefins via REX: maleic anhydride (MAH), glycidyl methacrylate (GMA) and vinyltrimethoxysilane (VTMS).

Influencing Factors: What To Consider

Process parameters, the physicochemical properties of raw materials and the equipment configuration are all factors that influence the outcome of a reactive extrusion process for polyolefin functionalization. For example, higher temperatures may promote the thermal degradation of the raw materials, impact the viscosity of the molten polyolefin, and change the speed of reaction of the different chemical species. Higher pressures may improve the solubility and diffusion of the chemical species in the molten polyolefin; the type, molecular weight and chemical structure of the polyolefin determine its rheology, and this may have an effect on how fast the chemical species diffuse through the melt, and thus, influencing the grafting yield.

Equally important, the screw configuration plays a significant role in how intimately the reactive species mix with each other, i.e., ensures the homogenous distribution and dispersion of the different chemical species within the polyolefin melt. Ultimately, it is important to understand that all these factors are interrelated and compounders will need to strike a balance between most of them to achieve a desired grafting yield.

The Role of Chemistry in REX

Typically the functionalization of polyolefins via a REX process will include the use of monomers and initiators. The former are the chemical species that will graft onto the polyolefin backbone. The latter are the chemical species that will generate the reactive sites along the polyolefin backbone onto which the monomers will graft.

Figure 5: Select examples of initiators used for the functionalization of polyolefins via REX: 2,5-di(tert-butylperoxy)-2,5-dimethylhexane (DTBH), dicumyl peroxide (DCP), and OO-t-butyl O-(2-ethylhexyl) monoperoxycarbonate (TBEC).

In most cases, the type of monomers used to functionalize polyolefins are those that exhibit a reactive double bond in their structure. The initiators are, in general, free radical generators known as peroxides that contain oxygen-oxygen (O-O) bonds in their structure and that are thermally activated. Figures 4 and 5 show select examples of monomers and peroxides, respectively.

The mechanism by which the monomer grafts onto the polyolefin can be summarized, in general, as follows: In the polyolefin melt state, at the appropriate temperature, the initiator will decompose (become activated) by dissociating at the O-O bonds, creating chemical species called radicals. Subsequently, these radicals will abstract a hydrogen from the polyolefin backbone and, in turn, create a reactive site. Depending on the type of polyolefin (polyethylene vs. polypropylene) being functionalized, the presence of such reactive sites could result in grafting, crosslinking or chain scission.

For example, in the case of polyethylene, if a monomer is present and near the reactive site, then the monomer will likely graft onto the backbone. However, if the monomer is absent or not reactive enough and another polymer chain with a reactive site on it is present and near, then these two chains will react with each other to form a crosslink. In the worst-case scenario, this may lead to gels if the chemistry and process parameters are not optimized.

For polypropylene, the monomer grafting mechanism is similar to that of polyethylene.

Figures 6A (top) and 6B: High-level description of the mechanism by which polyolefins are functionalized with maleic anhydride by means of REX.

However, in the case in which a monomer is absent or does not graft immediately onto a reactive site, the polypropylene backbone will undergo chain scission (break, also known as β-scission) much more readily than polyethylene and generate a shorter polymer chain with a lower molecular weight. This is an undesirable outcome because it will have a detrimental effect on the mechanical properties of the resulting grafted polypropylene.

Figure 6 shows a high-level description of the mechanisms just described. Altogether, a wide variety of monomers and initiators are available for the functionalization of polyolefins. Furthermore, the type of polyolefin and desired chemistry will dictate the degree of functionalization and level of undesired reactions or byproducts.

REX Applications

The introduction of functionality widens the range of applications for polyolefins. For example, MAH-grafted polyethylene could be used as an impact modifier for polyamides, a coupling agent between polyethylene and cellulose, and a compatibilizer between polyethylene and ethylene vinyl alcohol layers in packaging films. In addition, a potential key application of functionalized polyolefins includes the compatibilization of waste streams in plastics recycling.

Reactive extrusion is a versatile process that enables modification and enhances the value of polyolefins, but it’s also a process that entails inherent risks that require safety precautions. The functionalization of polyolefins via reactive extrusion is a process with many interrelated factors (physical, chemical, equipment) that influence the grafting yield. Furthermore, it can provide polyolefins with a variety of functional groups and chemistry that result in a wider range of applications.

ABOUT THE AUTHOR: Carlos Escobar is a research scientist in core R&D at The Dow Chemical Co. in Midland, Michigan. In this role, he leads projects focused on extrusion-based technologies such as reactive extrusion, mechanical dispersion, compounding and specialty processing. His 11 years of experience at Dow include process design, research and development, troubleshooting, process scaleup, external manufacturing and commercial qualification of many extrusion-based products. Contact: 989-636-6442; EscobarMarin@dow.com; dow.com.