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Glass Transition Temperature: An Important Thermal Property of Coatings

Coating's Glass Transition Temperature
  1. Resins and Coatings Properties
  2. Basics of Glass Transition Temperature
  3. What Factors Govern Glass Transition Temperature of Coatings?
  4. Glass Transition Temperature for Specific Applications
  5. Popular Methods to Measure Glass Transition Temperature

Resins and Coatings Properties

Resins are the most important components of paints and coatings. The resin type, chemistry of the polymer chain, crosslinking, etc. have a key role in defining the final properties (physical, mechanical, thermal...) and performance of coatings.

Understanding some of the basic properties of the polymers used in coatings is essential for the formulator while developing a high-performance product. Many of the basic properties of polymers are predictable based on their structural features i.e. linear, branched, rigid, or flexible.

One such property of resins that holds key importance is “Glass Transition Temperature”. Each coating’s glass transition temperature (Tg) is a net result of the effects of its constituents as well as the composition and ratio of the resin used.

For any coating chemist, the glass transition temperature is an important thermal property of coatings which contributes to determining their application area and service life properties.

Let's review the basics of glass transition temperature and the factors that govern it.

Basics of Glass Transition Temperature

Glass Transition Temperature, Tg, is defined as a temperature at which resin changes from a rigid glassy material to a soft material. It is not a melting point but involves the material becoming “softer”. This temperature (measured in °C or °F) depends on the chemical structure of the resin and can therefore be used to select the right resin depending upon the end-use of coatings.

Physically, the glass transition temperature is the point at which polymer chains gain enough energy to increase their mobility within the polymer matrix. The transition from the glass to the rubber-like state is an important feature of polymer behavior, marking a region of dramatic changes in the physical properties, such as hardness and elasticity of the coating.

The Tg is a very intricate concept and is not represented by a single value, even though it is an excellent starting point for understanding coating dynamics.

Free Volume Theory

The glass transition temperature of a polymer can be understood in terms of the Free Volume Theory. The free volume is a measure of the internal space available within a polymer matrix. When the free volume increases, so do the freedom of movement of polymer chains.

  • A polymer chain that can move around easily will have an extremely low Tg, while one that doesn't move so well will have a high one. 
  • A polymer at Tg or below its Tg has no free volume for the polymer chains to move. Heating a polymer above its Tg increases the free volume and makes the mobility of the polymer chains possible.

The free volume theory states that a consistent value of 2.5% free volume is required to achieve segmental motion.

Free Volume Theory
Free Volume Theory: Volume Changes in Polymers as Temperature Increases
(Source: Research Gate)

Physical chemistry for coatings

What Factors Govern Glass Transition Temperature of Coatings?

There are several factors that govern the glass transition temperature of coating. It is imperative to be aware of these factors to find out how coatings perform in a given service conditions and how Tg can vary with changes in formulation, application, cure and crosslinking, and environmental parameters.

Type of Resin and Its Chemical Structure

The most common factors that determines Tg of coating is the resin use. Resins are broadly classified into three groups – thermoplastics, thermosets, and elastomers. They can be further divided into two types according to their molecular arrangements i.e. amorphous and crystalline.

  • Amorphous polymers have a random molecular structure that does not have a sharp melting point. Instead, amorphous material softens gradually as temperature rises. Amorphous polymers only exhibit a Tg.
  • Crystalline or Semi-crystalline polymers have a highly ordered molecular structure. These do not soften as the temperature rises but have a defined and narrow melting point. Crystalline polymers exhibit a Tm (melt temperature) and typically a Tg since there is usually an amorphous portion as well (“semi”-crystalline).

A Heat Versus Temperature Plot for a Crystalline Polymer (L) and Amorphous Polymer (R)
A Heat Versus Temperature Plot for a Crystalline Polymer (L) and Amorphous Polymer (R)
(Source: PSLC)

Key Facts  KEY FACTS 
  1. As crystallinity influences the Tg, it is important to differentiate between amorphous and semi-crystalline polymers. 
  2. Many synthetic polymers used in coatings are amorphous rather than crystalline, so they do not have a distinct melting point.

Thermosets and Thermoplastics

Typical thermoset polymer will exhibit high Tg due to restricted molecular mobility driven by higher density and higher quantity of crosslinks (e.g. multifunctional epoxies). While thermoplastic polymer chains are capable of a wider range of movement in terms of flow or translational motion. But thermoplastics with rigid backbone structures tend to have higher Tg because of:

  • Bulky side groups that restrict/hinder rotation around the primary chain, and
  • Intermolecular interactions

For example, linear aliphatic amorphous polymers are more likely to have low Tg (such as high-density polyethylene, Tg = -120 to -130°C), in contrast to polymers with more rigid backbone structures, e.g., poly(benzimidazole), with a Tg of ~ 490-500°C.

Glass Transition Temperature of Various Polymers
Glass Transition Temperature of Various Polymers

The chemical building blocks of a polymer, i.e., backbone, side chains, and chain-to-chain interactions, and how all the chemistry is connected plays a distinctively significant role in finding the glass transition temperature.

To sum-up, following structural features of resin impact Tg:

Molecular Weight In straight chain polymers, increase in molecular weight leads to decrease in chain end concentration resulting in decreases free volume at end group region and hence, increases Tg.
Molecular Structure & Monomer Composition Insertion of bulky, inflexible side group increases Tg of material due to decrease in mobility. Also, changing monomer composition varies Tg of the polymer. For example, if the ratio of hard and soft monomers has mostly the hard monomer, then the Tg of the final polymer will be higher. In a styrene-butadiene latex emulsion, the styrene has a Tg of ~100°C while that of butadiene is ~-85°C. Changing the ratio of styrene to butadiene will change the Tg.
Chemical cross-linking and Modulus Crosslinking of a polymer changes many other characteristics of a polymer. Crosslinking reduces the mobility of polymer chains and decreases the free volume. This reduction in mobility shows in form of increased stress in a polymer on extension. Coating Tg will increase as the crosslink reaction continues. The higher the cross-linked density, the higher the Tg & the modulus. The build-up of coating Tg is also dependent on the catalyst concentration and cure temperature.
Polar groups Presence of polar groups increases intermolecular forces; inter chain attraction and cohesion leading to decrease in free volume resulting in increase in Tg.

Other factors that have a significant impact on glass transition temperature are:

  • Branching
  • Alkyl chain length
  • Bond interaction
  • Flexibility of polymer chain
  • Film thickness, etc.

When focused on coatings, we also need to consider the polymer’s interactions with pigments, additives, plasticizers, residual solvents, the substrate, and the influence of environmental conditions.

Pigments and Extenders

Pigments for Paints, Coatings and Inks Pigments and extenders are important constituents of coatings and their physical properties play a key role in defining the final properties of coatings. Pigment Volume Concentration, PVC, indicates the volumetric concentration of extenders (or pigments) in the coating.

CPVC (Critical Pigment Volume Concentration) stands for PVC corresponding to the random tightest possible packing of the extender (or pigment) particles and the minimum amount of binder necessary to fill the interstices between extender particles. Pigment volume concentration (PVC) influences mechanical behavior of the coatings, thus influences glass transition temperature of coatings.

Addition of Plasticizers

Addition of plasticizer increases the free volume in polymer structure. Plasticizers not only increase elasticity but also lower the glass transition temperature, which adversely affects blocking stability and can lead to sticky surfaces.

This results in polymer chains sliding past each other more easily. As a result, the polymer chains can move around at lower temperatures resulting in decrease in Tg of a polymer.

» Get Tips to Select the Right Plasticizer for Your Coatings Application!

Coalescing Agents

Coalescing agents function as temporary plasticizers for the polymer particle and thereby reduce the minimum film forming temperature* (MFFT). The MFFT of the emulsion relates directly to the glass-transition temperature (Tg) of the polymer.

The higher the Tg, the higher the MFFT. 

Coalescing agents allow the formation of polymeric films at ambient temperature conditions of film application, even of polymers showing a MFFT of above these temperature conditions. Coalescing agents typically lowers Tg of the polymer.

*The minimum film-forming temperature is the lowest temperature required to coalesce a polymer and applied to a substrate into a thin film.

» Understand the Role of Coalescing Agents in Coating’s Film Formation!

Water or Moisture Content

Increase in moisture content leads formation of hydrogen bonds with polymeric chains increasing the distance between polymeric chains. And, hence increases the free volume and decreases Tg.

Effect of Entropy and Enthalpy

The value of entropy for amorphous material is higher and low for crystalline material. If value of entropy is high, then value of Tg is also high.

Pressure and Free Volume

Increase in pressure of surrounding leads to decrease in free volume and ultimately high Tg. Overall, it is the resin’s structure, molecular weight, crosslinking, functional groups, interaction with other components of the formulation and application conditions, all have direct impact on glass transition temperature.

It is also important to note that Tg is an important property while studying viscosity at a given temperature, physical and chemical properties of the coating formulation.

Glass Transition Temperature for Specific Applications

Now we are aware that understanding the glass transition temperature can be critical for determining coating suitability for specific applications. Tg is an especially important consideration in the selection of the proper resin for a for desired coating performance.

Here are the few examples (not an exhaustive list yet) explaining:

Strategies to Achieve High Tg End Application Requirements
Compared to conventional coatings, the UV variants are comprised of lower molecular weight materials that react to make denser, highly cross-linked networks. They also often have higher Tg and make harder films with good abrasion and chemical resistance. The corrosion coating must have a glass transition temperature, Tg, above the operating temperature of the pipeline to prevent damage from pipe movement.
Multifunctional epoxy resins have commonly been used to produce highly crosslinked coatings with high glass transition temperatures. The Tg of an epoxy is affected not only by the choice of epoxy, curing agent and filler used, but also by the curing conditions. Epoxies demonstrate a wide range of Tg, from as low as 50°C to upwards of 250°C. In addition to improving its performance properties, adding heat while curing the epoxy helps improve its glass transition temperature (Tg). Industrial paints and coatings need a higher Tg because high hardness and the lack of tack are requirements at the service temperatures, meaning the coating will be more rigid and less susceptible to dirt pick-up.
Acrylonitrile-butadiene rubber toughened vinyl ester resins have been found to produce high glass transition temperature thermosetting materials. Vinyl ester precursors ranging in number average molecular weight from 3600 to 3800 have been made into toughened coatings having glass transition temperatures a few degrees above 140°C. Glass transition temperature has direct impact on the hardness and flexibility of the coating. For example, a coating technology used for making floor polish or house paint would both make extremely poor choices for an elastomeric roof coating. Thus, Tg is an important property to consider while selecting resin for a given application.
Another approach to making high temperature polymeric resins for protective coatings is to graft amino groups on polyetherimides. Coatings with glass transition temperatures ranging from 150 to 210°C were produced by this method.
Polycyanurates, as products of a cyclotrimerization reaction of cyanate ester monomers, are another route to high glass transition temperatures.

Popular Methods to Measure Glass Transition Temperature

Several methods are available to measure glass transition temperature that typically provide a range of values with subtle to significant differences. These test methods include:

  1. Differential Scanning Calorimetry (DSC)
  2. Differential thermal analysis (DTA)
  3. Dynamic mechanical analysis (DMA)
  4. Thermo mechanical analysis
  5. Thermal expansion measurement
  6. Micro-heat-transfer measurement
  7. Isothermal compressibility
  8. Heat capacity determination, etc.

Among these methods, DSC, DTA and DMA are by far the most dominant techniques used for glass transition temperature measurements.

Let’s discuss these techniques in detail...

Differential Scanning Calorimetry (DSC)

Differential Scanning Calorimetry (DSC) is a thermo-analytical technique used to study the thermal properties of the polymer using a differential scanning calorimeter. It is widely used to determine the glass transition temperatures of resins. This test method applies to amorphous materials or to partially crystalline materials containing amorphous regions, that are stable and do not undergo decomposition or sublimation in the glass transition region.

DSC Practical Interpretation with Examples for Polymer Development

In this method, a differential scanning calorimeter measures the difference in heat flow to a sample and to a reference against time or temperature while the temperature change of the sample, in a specified atmosphere, is programmed.

Glass Transition Temp. Measurements of Different Polymers Using DSC
Glass Transition Temp. Measurements of Different Polymers Using DSC
(Source: Mettler-Toledo Analytical)

The test standards used to find Glass Transition Temperature of resins via DSC include:

  • ASTM E1356-08 (2014) – Standard Test Method for Assignment of the Glass Transition Temperatures by Differential Scanning Calorimetry
  • ISO 16805:2003 – Binders for paints and varnishes — Determination of glass transition temperature
  • ASTM D3418-15 – Standard Test Method for Transition Temperatures and Enthalpies of Fusion and Crystallization of Polymers by Differential Scanning Calorimetry
  • ASTM D6604-00(2017) – Standard Practice for Glass Transition Temperatures of Hydrocarbon Resins by Differential Scanning Calorimetry
  • ISO 11357-1:2016 – Plastics — Differential scanning calorimetry (DSC)
    • Part 1: General principles
    • Part 2: Determination of glass transition temperature and step height

Differential Thermal Analysis (DTA)

Differential Thermal Analysis (DTA) is a popular thermal analysis technique and is often employed for measuring Tg of the material. In principle, differential thermal analysis is a technique which is similar to differential scanning calorimetry (DSC).

  • The technique involves an inert reference material.
  • The material being studied in DTA undergoes various heating and cooling (thermal) cycles where the temperature difference between the reference and the material under analysis is determined. 
  • Both the reference and sample materials are kept at the same temperatures throughout the heating cycles to ensure that the testing environment is uniform.

Measurement Principles of DTA
Measurement Principles of DTA (Source: Hitachi High-Tech Corporation)
Where Graph (a) Shows the Temperature Change of the Furnace, Reference and Sample Against Time
Graph (b) Shows the Temperature Difference (ΔT) Against Time Detected with the Differential Thermocouple

The test standards used to determine Glass Transition Temperature of resins via DTA include:

  • ASTM E794 - 06(2018) – Standard Test Method for Melting and Crystallization Temperatures by Thermal Analysis

Dynamic Mechanical Analysis (DMA)

Dynamic Mechanical Analyzer measures the stiffness of materials as a function of temperature, humidity, dissolution media or frequency.

Typical DMA Analysis Graph
Typical DMA Analysis Graph

In this technique, a mechanical stress is applied to the sample and the resultant strain is measured by the instrument. These parameters are used to evaluate glass transitions, degree of crystallinity and stiffness behavior of the sample.

When dealing with films and coatings, DMA is applied in 3 different options:

Application of DMA
Application of DMA

The test standards used to determine Glass Transition Temperature of resins via DMA include:

  • ASTM E1640 - 13 – Standard Test Method for Assignment of the Glass Transition Temperature by Dynamic Mechanical Analysis

Note: It is imperative to understand when, how, and why coatings perform as they do from varying application methods, differing cure profiles, and a wide array of environmental conditions.

Get Inspired: DMA Practical Interpretation with Case studies for Polymer Development

2 Comments on "Glass Transition Temperature: An Important Thermal Property of Coatings"
Jordi F Nov 15, 2020
Great article for those of us who barely knows about Tg & MFFT although we would like to learn more about DSC technics and how modifications on coatings formulation could affect directly to Tg properties
Peter G Nov 4, 2020
Very nice article for those of us who are just learning about coating formulation and failure analysis.

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