Thermoset polymers - A105

Thermoset polymers
Foundational knowledge article
Document Type	Article
Document Identifier	105
Themes	Thermal management Materials deposition and consolidation management Residual stress and dimensional control management Machining and assembly management Quality/inspection management
Relevant Class	Material
Tags	Cure and crystallization Matrix Physical properties Thermoset polymers Thermal transformation
Prerequisites	Matrix

Introduction

Thermoset or thermosetting polymers are one of the two groups of polymers (the other being thermoplastics). They are characterized by the molecular crosslink covalent bonds that are formed between adjacent primary polymer chains. Thermosets undergo polymerization and crosslinking during a curing stage, which in many cases occurs with the aid of a hardening agent and heating or promoter.

Thermoset precursor materials may be of low molecular weight, and some after mixing will flow and crosslink at room temperature – allowing for relative ease to process. However, many thermosets require the input of heat to complete the curing (crosslinking) process.

Scope

This page provides a brief overview to the family of polymers classified as thermoset or thermosetting polymers. It briefly covers their molecular structure, their material forms, and an introduction to the thermoset curing process and their post-cured properties.

Significance

Thermosets are a popular polymer matrix material for fibre reinforced composites. Common examples include polyesters and epoxies. They are the most predominant type of matrix system, particularly in the aerospace market segment. Thermoset matrix composites are more widely used than thermoplastic matrix composites due to their lower viscosity at relatively low temperatures (less than 100°C), which better facilitates the resin/fibre wetting process .

Prerequisites

Recommended documents to review before, or in parallel with this document:

• Systems Catalogue: Matrix materials

Overview

Thermoset or thermosetting polymers are one of the two groups of polymers (the other being thermoplastics). Thermosets undergo polymerization and crosslinking during a curing stage, which in many cases occurs with the aid of a hardening agent and heating or promoter. Initially, they behave like a viscous fluid. During curing, they change from viscous fluid to rubbery gel (viscoelastic material) and finally glassy solid. If heated after curing, initially they become soft and rubbery at high temperatures. If further heated, they do not melt but decompose (burn). As a result, thermoset polymers are considered single use materials as they cannot be reprocessed.

Crosslink Structure

Thermosets are characterized by their molecular crosslink structure, where adjacent linear polymer chains are joined together by covalent bonds. Often, the crosslink structure is accomplished by additive atoms and molecules that are covalently bonded to the chains . It is these bonds that form between the chains that give the crosslink structure its name.

Illustration of the molecular crosslink formation for a thermoset polymer.

Thermoset resins often come in two parts: part A (resin) and B (crosslinking curing agent, or sometimes referred to as hardener). The ‘curing reaction’ (curing process) is the physical synthesis between these two substances by a chemical reaction.

Material Forms

Uncured thermoset polymers often come in the form of a liquid resin that is to be combined with reinforcement fibres in either a hand lay-up or liquid composite moulding composite manufacturing process. They can also be provided in a partially-cured and tacky almost solid-like state in the form of a resin film that is to be used in resin film infusion processes. Extending the latter form, another popular handling form for thermosets is to be pre-combined with reinforcement fibres as a B-stage prepreg material.

Please see the systems catalogue for more information regarding thermoset resin material forms (Thermoset Polymers - Systems Catalogue page).

Curing and Final Properties

Link to main Curing of Thermosetting Polymers page

A curing process is necessary to polymerize the thermoset polymer and form the molecular crosslink structure responsible for their desired properties. During this process, the thermoset polymer transforms from a flowing liquid resin into a rigid polymer. Curing can be initiated by various means depending on the particular polymer system; heat, radiation, chemical additives, etc., or a combination of these.

Heat is often employed as part of this process to fully cure the polymer. An example of a cure process temperature profile and the evolution of the crosslinking process (represented as degree of cure) is shown below.

Example of a thermoset polymer heated cure cycle process showing the crosslink development (degree of cure). .

Once curing is complete, fully cured thermosets exhibit the following :

• Excellent environmental resistance
• Excellent solvent resistance
• High mechanical strength and stiffness
• Low toughness (downside)

After curing, if heated to very high temperatures, thermosets chemically breakdown.

Thermoset polymer breakdown if subjected to high thermal breakdown temperatures.

1. Initially softening
2. After time, transforming into char

Example Thermoset Materials

Examples of popular thermosets used as polymers matrices include:

• Polyesters
• Vinyl esters
• Epoxies
• Phenolics
• Bismaleimides

For more information on popular thermoset matrix materials, see the catalogue volume thermoset matrix page.

Related pages

Page type	Links
Introduction to Composites Articles
Foundational Knowledge Articles	Basic Definitions of Material Properties - A211 Composite properties - A214 Cure of thermosetting polymers - A162 Degree of cure - A104 Glass transition temperature (Tg) - A210 Heat of reaction - A114 Material properties - A150 Outgassing of Polymer Resins - A343 Polymer properties - A212 Reinforcement properties - A213 Thermal phase transitions of polymers - A102 Thermoplastic polymers - A161 Thermoset polymers - A105 Viscosity (resin) - A203
Foundational Knowledge Method Documents	How to measure curing time and degree of cure - M100 How to measure gel time - M101
Foundational Knowledge Worked Examples
Systems Knowledge Articles
Systems Knowledge Method Documents
Systems Knowledge Worked Examples
Systems Catalogue Articles
Systems Catalogue Objects – Material
Systems Catalogue Objects – Shape
Systems Catalogue Objects – Tooling and consumables
Systems Catalogue Objects – Equipment	Rheometer - A357
Practice Documents	Decreasing Cure Cycle Time - P121 Practice for developing a thermal transformation process step - P105 Troubleshooting scale-up issues from thin to thick parts - P140 Troubleshooting scaling-up issues from coupons to parts - P134
Case Studies	Troubleshooting of room temperature processes for large recreational and industrial parts - C100
Perspectives Articles	Composite materials engineering webinar session 1 - Introduction - A120 Composite materials engineering webinar session 3 - Constituent materials - Resin - A122 Composite materials engineering webinar session 4 - Thermal management and resin cure - A123 Composite materials engineering webinar session 5 - Manufacturing processes - Introduction - A124 Composites Process Simulation: A Review of the State of the Art for Product Development - A283 Dr. Anoush Poursartip's cdmHUB global composites expert webinar - A136 Effect of cure on mechanical properties of a composite (Part 1 of 2) - A319 Effect of cure on mechanical properties of a composite (Part 2 of 2) - A320 Heat Transfer in Composites Processing - A321 Introduction to the processing of thermoplastic composites - A322 Resin Behaviour During Processing: What are the key resin properties to consider when developing a manufacturing process? - A257 Understanding Polyester Resin Processing: The Effect of Ambient Temperature on the Final Part - A286

References

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Thermosets are a class of polymer that undergo polymerization and crosslinking during curing with the aid of a hardening agent and heating or promoter. Initially they behave like a viscous fluid. During curing, they change from viscous fluid to rubbery gel (viscoelastic material) and finally glassy solid.

If heated after curing, initially they become soft and rubbery at high temperatures. If further heated, they do not melt but decompose (burn)

Comes in two parts: part A (resin) and B (hardener). When mixed, curing reaction starts and is not reversible.

Examples include epoxy or polyester.

The continuous material phase that binds the reinforcement together, maintains shape, transfers load, protects the reinforcement from environment and damage, and provides the composite support in compression.

Desirable characteristics:

• Moisture/chemical resistance
• Low density
• Processability

A class of polymer, some common examples include polypropylene and polyethylene.

They soften and melt upon heating (i.e. potentially recyclable), high viscosity when melted, therefore difficult to saturate fibres. Usually needs a lot of pressure and heat to process.

In composites processing, viscosity is an indicator of how easily the resin matrix will mix with the reinforcement and how well it will stay in place during processing. The lower the viscosity, the more easily resin flows. Resin viscosity ranges considerably across chemistries and formulations.

By scientific definition, viscosity is a measure of a material’s resistance to deformation. For liquids, it is in response to imposed shear stresses.

For polymer matrix composites (PMCs), resin refers to the matrix; the continuous material phase that binds the reinforcement together, maintains shape, and transfers load. Resins are divided into two main groups: thermosets and thermoplastics.

Engineered materials (designed to have specific properties) made from two or more constituent materials with different physical or chemical properties. The constituents remain separate and distinct on a macroscopic level within the finished structure.

Pre-impregnated (prepreg) material refers to fibre that is already combined with resin. It is the most common material form used in aerospace.

During prepreg production, (e.g. fibres are run through a resin bath), prepreg is heated and partially cured to B Stage (< 5 % degree of cure). Thermoset prepregs (e.g. epoxy prepreg) have to be kept in a freezer at around -20 °C. At room temperature, the epoxy starts to cure.

Degree of cure (DOC) is an indication of how far the chemical curing reaction (crosslinking process) has advanced in a thermoset resin.

DOC is defined with a number between 0 and 1 (or 0% and 100%) where 100% is a fully cured resin. It does not have to fully reach 100% for the resin to become solid or the part to be used. In some aerospace applications, resins are only cured to about 90%. Higher the degree of cure, higher the mechanical properties.