Peel the difference: Turning citrus peels into organic coatings

In recent years, there has been growing interest in exploring new and innovative ways to create sustainable and environmentally friendly paints. In collaboration with the Eindhoven University of Technology (TU/e) and a major player in the coating industry, PTG/e supported the development of an environmentally friendly way of generating polycarbonate coating resins by copolymerization of limonene oxide with carbon dioxide (CO2).

To explore more about this sustainable coating, let us first discuss some important aspects of powder coatings. Unlike conventional coatings like paints, which are films that are formed via the evaporation of a solvent, powder coatings typically are dry powders that are applied electrostatically and cured via temperature or with ultraviolet light and require no volatile components. Nowadays, these coatings are intensively used in industry to provide protection against aggressive environmental conditions and/or for decorative purposes. Some examples of powder-coated products can be seen in Figure 1. The binder or resin is the main constituent of a typical thermoset powder coating (TPC) and is the film-forming element of the product. It provides adhesion to a substrate, binds pigments and other additives together, and determines important properties such as durability, flexibility, and hardness. In addition, colors, additives, and fillers can be added to modify certain properties of the coating like gloss, opacity, and stability.

Figure 1: Examples of powder-coated applications which are applied for decorative and protective purposes, such as parts for the automotive industry and powder-coated metal objects like pipelines.

Several types of thermoset powders, derived from epoxies, acrylics, hydroxyls (polyester), and carboxyl(polyurethane) groups can be used in the synthesis of TPCs. Some of these types suffer from poor exterior durability or moderate chemical resistance. Therefore, an alternative could be a polycarbonate-based resin which is typically amorphous, and usually exhibits properties like high transparency and low UV absorption, particularly suitable for outdoor use. However, the downside of commercially synthesized polycarbonate is the use of phosgene (Cl2C=O), which is a highly toxic gas that requires serious environmental and safety considerations. The co-monomer bisphenol A (BPA) is also debated for its potential adverse health effects.

Limonene oxide is a potential biobased epoxide derived from limonene and could be a good alternative to phosgene / BPA. Limonene is a major component that can be found in the oil of citrus peels, like oranges. It is a colorless liquid that is often used as a flavoring agent in food manufacturing. Its abundance and its multiple functionalities make it an attractive, renewable building block for polymer synthesis. Via a chemical reaction that involves carbon dioxide (CO2), a fully recyclable poly(limonene carbonate) (PLC) can be synthesized, carrying functional groups that can be modified or crosslinked to introduce new functionalities such as antibacterial activity, hydrophilicity, and water solubility. This makes CO2, also known as the primary driver of climate change, a useful molecule that can be used as a monomer to create a sustainable polycarbonate powder coating. This pathway is shown schematically in Figure 2. As a result, this fully limonene-derived PLC has great potential as a TPC binder, which can deliver good exterior durability and chemical resistance. These two properties make this type of renewable binder a great alternative for the manufacturing of powder coatings, avoiding any concerning reactants.

Graphic about the oil from a citrus fruit can turn into a coating for organic paints.

Figure 2: Simplified overview that involves the reaction mechanism of a limonene-based coating. Here, first limonene is epoxidized to limonene oxide. The latter is used for the synthesis of poly(limonene carbonate) (PLC) in the presence of carbon dioxide. With the use of a crosslinking molecule (thiol-based) and radical initiator, under the influence of ultraviolet light, a crosslinked network (TEN) is formed via a thiol-ene reaction with the pendant isoprenyl groups of PLC and results in the formation of a thin layer on a particular surface.

Whether you’re looking to enhance material sustainability, develop new materials, or material research, our team has the knowledge, expertise, and state-of-the-art infrastructure to help you achieve your goals. And, with a large network within the TU/e, we’re able to tap into a wealth of knowledge and resources. Contact us to learn more about how we can help you!

For additional technical details about this topic, feel free to visit the main article via the following link: Limonene-derived polycarbonates as biobased UV-curable (powder) coating resins

Sustainable coating from citrus peels. PTG Eindhoven contributed in this research to create organic coatings.

Confocal Raman spectroscopy

Whenever a foreign material is confined between two extruded transparent films or a tiny particle is trapped within a coating matrix, it may ruin products with high optical requirements. It is not uncommon that our customers, often multi-layer film producers or coating companies, contact us to help find the root cause of such contaminations.

Confocal_Raman_spectroscopy_contaminationReaching the unreachable!
Confocal Raman spectroscopy is a highly suitable technique for this, as the laser beam can be focused on a particular spot beneath a material surface, shown in the schematic. We discuss the capabilities of this technique here in 2 demonstration examples.


Confocal_Raman_spectroscopy_eps1. Polyethylene bag
As a first example we have placed a closed polyethylene (LDPE) bag containing poly(ethylene terephthalate) (PET) granules directly under the confocal Raman apparatus. We performed a depth analysis through the bag into the granules, while continuously analyzing the material composition. A clear transition can be seen going from LDPE towards PET in the spectra below. This example also demonstrates another use case for this technique, in which a bag of unknown material could be analyzed without needing to risk opening the bag, as confocal Raman spectroscopy can be used to analyze the material right through the packaging.


Confocal Raman Aceton2. Glass vial with acetone
The same principle applies to unknown liquids inside a glass vial. In this example we placed a glass vial with acetone directly underneath the Raman microscope. The measurements can be performed through the glass barrier, while continuously analyzing the chemical composition of both the glass vial and the acetone. The spectra below clearly show the transition between the different materials


Confocal Raman spectroscopy can be used to identify unknown substances, particularly when very detailed and local sample analysis is required. With this technique the chemical composition of particles with a particle size down to 1 μm can be analyzed. Moreover, these particles can be analyzed even if they are fully enclosed inside a matrix, as we have demonstrated in the examples.

Have you ever encountered small particulate matter trapped in your product without knowing its origin? Confocal Raman spectroscopy may be the answer. Please feel free to contact us to discuss the possibilities!





Confocal Raman Aceton

Measuring thermal expansion by thermomechanical analysis (TMA)

When a material is heated or cooled, its size changes proportional to the original size and the change in temperature. This thermal expansion (or shrinkage) of materials needs to be considered in numerous applications.

We highlight this importance using two common examples, shown in Figure 1, and briefly discuss how we can measure as well as influence this property

Thermal expansion analysis on an antenna by PTG Eindhoven. Measuring thermal expension in car reflectors by using TMA.

Figure 1. Examples of applications where control over thermal expansion is critical. Left: Antenna tower; right: reflector of a car headlight assembly, visible behind the transparent cover.

The first example comprises 5G antennae, which are increasingly important in our daily lives as they transmit data from our phones, cars, and many other devices. In any antenna, data transfer is most efficient when transmitter and receiver wavelengths are matched. Since 5G operates at high frequency (up to 54 GHz), these antennae can be quite small, as the length of an antenna is inversely proportional to the frequency. Such antenna modules are often densely constructed, in which heat from the integrated circuits can build up. Moreover, many of these antennae are exposed to the everyday weather and fluctuating temperatures. As such, the antenna can undergo thermal expansion, which can lead to reduced efficiency or even damage to the antenna due to material warpage. Therefore, the thermal expansion coefficient is a crucial parameter in material selection for this application.

The other example is a car headlight assembly, particularly focusing on the reflector part (behind the transparent cover), which has a main function to direct the light towards the road. These reflectors are molded plastic parts, coated with an aluminum reflective layer. In such an assembly the temperature can vary greatly, not just from ambient conditions but also from heat generated by the lightbulb itself (although with modern LEDs this is less of an issue). However, the plastic, often polycarbonate (PC), expands much more under increasing temperature than the metal coating. It is not hard to imagine how this can result in delamination of the coating from the plastic, causing the reflector to malfunction. This example also demonstrates the importance of considering the thermal expansion behavior of materials, especially when combining them for any application.

A material’s expansion behavior is captured in the coefficient of thermal expansion (CTE), which can be measured using thermomechanical analysis (TMA). TMA is a technique that accurately measures dimensional changes in a sample, as a function of temperature (or time). Expansion can be measured using quartz compression or tensile probes, but other quartz probes (3-point bending, penetration) can also be used to measure heat deflection temperatures or softening points. Our TMA sample holder and quartz compression probe can be seen in Figure 2 below.

TMA graph image

Figure 2. Left: Aluminum sample in the TMA holder. The quartz compression probe accurately measures any length changes. The thermocouple on the right side monitors the temperature. Right: TMA measurement data, with CTE values for each material.

To demonstrate differences in thermal expansion for the materials used in a car headlight assembly, we have measured several materials using our PerkinElmer Diamond TMA: aluminum, PC, and glass. The measurement data is also shown in Figure 2.

We can calculate the CTE of the materials from the slope of the curves in Figure 2, as shown, which are in close agreement with values from literature. Additionally, TMA data can also be used to measure the glass transition temperature (Tg) of a polymer sample; for this PC sample Tg was found to be 148 °C.

We clearly see a large difference in CTE for PC and aluminum, demonstrating the issue for the reflector part of our car headlight assembly, and allowing us to think of a solution. A common method to decrease the CTE of polymers is to add fillers with a low CTE, like glass. Therefore, one possible solution would be to make a PC composite with glass fibers, which is cheap and additionally strengthens the polymer. For example, the CTE of a 30 % glass-filled PC was determined as 22 · 10-6 K-1, which is a close match to the CTE of aluminum and would therefore be a suitable composite material for the headlight reflector application.

However, when compounding such composite materials more aspects need to be considered, such as fiber dispersion during compounding or fiber orientation during processing of the parts. This can result in anisotropy in the material properties, which can be expressed as greatly differing CTEs in the machine or transverse direction. As such, careful analysis of CTE is crucial in many aspects of material and product development.

We are happy to support you with TMA measurements for any of your thermal expansion challenges, so please feel free to contact us for further information.


TMA analysis done by PTG Eindhoven - The Material Innovators.

Improving polymer properties

PTG/e’s unique multipurpose drawing line is capable of stretching tapes and fibers from polyolefins to high-temperature polymers such as polyamides or ketone polymers.  By drawing these materials in one direction, improved mechanical properties (stiffness and strength) can be obtained.

As an example, the higher stiffness of artificial grass is the reason why it stands upright. And higher tensile strength is the reason debris/construction bags and woven shoppers are so strong. In addition to these everyday applications, the composite industry is very interested in the combination of low-density (light-weight) materials and their improved mechanical properties upon drawing. You can find composites based on drawn thermoplastic materials in all kinds of extreme applications, such as aerospace engineering, ballistic protection, oil & gas transport and sports.

Drawing line PTG Eindhoven

The stiffness (tensile modulus) of thermoplastic polymers, like polyolefins, polyamides or ketone polymers, can be controlled by orientating polymer chains. This orientation can be achieved during stretching of the material between the glass transition and the melting temperature of the polymer, which is also called solid-state drawing. See the image below.

cross-section drawing line PTG Eindhoven

How does a drawing line works?
The drawing line consists of a draw unit with 5 godet rolls, a draw unit with 3 godet rolls separated by 2 heating zones in an oven, another draw unit with 3 godet rolls, and finally a winder. The drawing speed can be adjusted per unit, or even per roll on the oven unit. The temperature of each induction-heated roll and oven plate can be controlled separately, up to 280 °C. Afterwards, the materials are collected on a metal or cardboard tube for further analysis either at PTG/e or at the customer.


Drawing line PTG Eindhoven research brainport

Advantages PTG/e drawing line

Since we are able to draw small lengths of tapes and fibers (starting from 20 meters) on our drawing line, less material is needed for your tests. Also, your own production line can keep running, since you are performing your tests in the PTG/e laboratory.
If you like to learn more about our capabilities, please contact us.

An image of the Drawing line of PTG Eindhoven located in Brainport Eindhoven, the Netherlands.

Especially for polymers size matters

Important properties of polymeric materials like tensile strength and viscosity critically depend on the size, or rather, the chain length of the macromolecules that they consist of. In other words, these properties are defined by the molecular weight distribution and the average molecular weight of the polymer.

Measuring the molecular weight of a polymer therefore provides crucial information for understanding many aspects related to the behaviour of polymeric materials.

Thus, chemically identical polymers can show different tensile properties as a result of differing molecular weight. Such differences frequently result from polymer degradation and are especially relevant in the context of recycling. Also in polymer production, the molecular weight is a key parameter in quality control. While often only the melt flow index (MFI) is measured, knowledge of the actual molecular weight (distribution) provides a more detailed picture. Finally, in the development of new polymer materials, assessment of the molecular weight is a key factor for optimizing synthesis conditions.

The molecular weight of many conventional polymers can conveniently be assessed by a technique called Size Exclusion Chromatography (SEC). This technique separates dissolved polymer molecules according to their size by passing them through a column packed with porous particles. While the larger molecules cannot enter the pores and therefore elute from the column relatively rapidly, the smaller ones do enter the pores of the column material and thus experience a net retardation. The final result is a chromatogram, showing the amount of material eluting from the column versus the elution time, with the elution time being inversely related to the molecular weight.

One key issue with SEC is the fact that polymeric materials need to be dissolved in a suitable solvent. However, not all polymers are the same and their solubility heavily depends on their chemical nature and molecular weight. While many common polymers (e.g. perspex or polystyrene) are easily dissolved in tetrahydrofuran (THF), more polar materials like polyamides or some polyesters, require hexafluoroisopropanol (HFIP) for complete dissolution. Even water (H2O) may be the only suitable solvent for certain polymers. On the other hand, the industrially important class of polyolefins (e.g. polyethylene, polypropylene) can only be dissolved in a chlorinated solvent and the complete SEC analysis is performed at 160 °C!

It is clear that every type of material needs specific measurement conditions to assess its molecular weight. At PTG/e, we have many years of experience with polymers of widely varying nature. Our state-of-the-art SEC equipment, running on different solvents, enables us to cover molecular weight determinations of almost any polymeric material, including those that are notoriously ‘difficult’ to dissolve.

Please contact us if you would like to find out whether molecular weight determination by SEC can provide a breakthrough insight into your material of interest!

This pictures show the Size Exclusion Chromatography. Which we use at PTG/e for Characterization.

Surface structure analysis by profilometry

The analysis of surface structures is of great importance in many industries, such as chip/sensor manufacturing, inkjet printing or membrane production. In these industries surface analyses are used for instance as quality control, checking surface roughness or finding the root cause of defects.


As an example, one of our customers had approached us to help solve an issue with a curable resin product. The application of this product requires a very flat surface. At first, the resin was cast on a Teflon film, in order to ‘copy’ the flat surface of these films onto the resin product. By the naked eye, such a film indeed appears very flat, but surface profiling revealed that the film has depth differences of 1-1,5 micrometers (see Figure 1). A silicon wafer, a known flat substrate material, shows depth differences of just 30 nanometers (see Figure 2). Using this silicon wafer as substrate for the curable resin did result in the desired smoothness of the final product. Therefore, surface profiling enabled our customer to choose the right substrate for their product.

PTG Eindhoven Surface structure analysis by profilometry
Surface profiling

Figure 1: Surface profile of Teflon foil.

Profilometry is a great way to analyse a surface of a product. There are two ways optical profilometry and stylus profilometry, For a soft surface optical profilometry will gif the best results.

Graph of surface profilometry. When a surface seems flat is doesn't mean it is. With this analysis technique we at PTG Eindhoven can analyse the surface of the material.

Figure 2: Surface profile of the silicon (Si) wafer with a line profile analysis, indicated by the pink raster. The line profile is represented in the graph.

Another example below shows a microchip, which can be found in everyday devices like laptops or smartphones. A detailed image of its complex surface profile can be used to inspect the chip for any damages or incorrect assembly. The surface images in Figure 3 were obtained by an optical surface profiling technique.

2D image of a surface structure analysis by profilometry.

3D structure analysis by profilometry, in this result it's easy to see the surface is not flat.

Figure 3: Surface profile in 2D and 3D of a microchip in common electronics.

Using this technique, surfaces can be analysed quickly and accurately. Surface profiling can be done optically (optical profilometry), in which case light is used to illuminate a surface. The reflected light is detected and translated into a 2D/3D profile image. However, profiling can also be performed physically (stylus profilometry), where a stylus is used to probe a surface. Both techniques are extremely sensitive, capable of measuring depth differences of less than 1 nanometer. The choice of which technique is preferred mostly depends on the sample surface. For a very soft surface, you want to choose optical profilometry, so the surface is not changed as a result of the measurement. If a surface is absorbing (almost) all light, stylus profilometry is preferred.

At PTG/e, we offer both optical and stylus profilometry, as each technique has its pros and cons (which can often be compensated by the other technique). As such, we will always decide together with our customers which technique is best suited for their samples.

Interested in optical and physical surface profiling? Please contact us, it’s our pleasure to discuss the possibilities.


Surface analysis one of the analysis techniques from PTG Eindhoven.