Industrial Polymer Applications

Essential Chemistry and Technology

William R Ashcroft

• Publisher: Royal Society of Chemistry
• Year: 2019
• Print ISBN: 9781782628149
• eBook ISBN: 9781788018111
• Edition: 1
• Topic: Physical Sciences
• Subtopic: Industrial & Technical Chemistry

Industrial Polymer Applications provides a comprehensive overview of the diverse properties and applications of thermoset and thermoplastic polymer technologies used routinely in the modification, protection, repair, restoration and bonding of the main classes of industrial engineering materials such as concrete, masonry, wood, metal, rubber, plastic, glass and advanced ceramics.

The Author, with extensive industrial experience in the design and development of polymeric adhesives, composites, concrete repair and industrial coatings materials, provides a balanced perspective of the essential chemistries and technologies for each of the relevant polymeric solutions. This book includes explanations as to why polymers are needed and the specific problems and key industrial application challenges that can be overcome for each class of engineering material. The use of supplementary information boxes, suggestions for further reading, and supportive appendices including worked examples delivers an easy to understand guide of relevant industrial applications of polymers.

Written in an accessible way, the book provides a supplementary text for undergraduates, postgraduates and industrialists who have studied or are involved in chemistry, polymer chemistry, industrial chemistry, materials science, chemical engineering, mechanical engineering, civil engineering or corrosion engineering, science and technology.

1 Concrete Modification, Protection and Repair

1.1 Introduction to Concrete and the Need for Modification, Repair and Protection

Concrete is the earliest known synthetic engineering composite and is characterised by its ease of use and ability to be formed into a multiplicity of shapes which harden over time into a durable rock-like material. It is the most versatile and widely handled construction material in building and civil engineering projects, since it can be used with or without reinforcement, because it can be pre-stressed or post-tensioned, and as it can be chemically modified in a variety of ways to boost physical and mechanical properties as well as to meet specific application requirements.

Unreinforced concrete is the best-known example of a large-particle reinforced composite and is made from blends of gravel and sand aggregate embedded in a matrix of Portland cement paste – sand is required to pack into the gravel voids so the aggregate blend provides solid reinforcement to the cement matrix to improve mechanical strength; cement acts as the binder which holds the overall mixture together. Combinations of cement, water and only fine sand aggregates, referred to as mortars, perform in exactly the same way as bulk concrete whereby the principal components of the cement react with water (eqn (1.1) and (1.2)) to yield a calcium silicate hydrate (tobermorite) solid gel. The solid tobermorite gel grows in the form of fibrils, which ultimately interlock to form a continuous solid matrix coating around and bonded to the aggregate particles.

(1.1)

(1.2)

There are differences between the two calcium silicate minerals on reaction with water – the dicalcium silicate (2CaO·SiO₂) sets more slowly but produces higher strengths, and the tricalcium silicate (3CaO·SiO₂) sets rapidly but produces low strengths. Both generate heat, which can cause loss by evaporation of water which is required to ensure complete hydration. The unavoidable presence of 3CaO·Al₂O₃ (tricalcium aluminate) created during clinkering gives rise to the phenomenon of “flash set”, generating a large amount of heat of hydration unless additions of CaSO₄·2H₂O (gypsum) and CaO (lime) are made to divert reaction to form an insoluble layer of 3CaO·Al₂O₃·3CaSO₄·32H₂O (ettringite) over the surface of the aluminate crystals and subsequent slow reaction to form the 3CaO·Al₂O₃·CaSO₄·12H₂O hydrate which reduces the overall durability of concrete.

The rate of set and final strength of concrete and cement-based mortars is not only controlled by the water content and the relative amounts of the various slow and fast hydrating minerals in the cement, but also by addition of accelerators which increase the rate of hydration and retarding agents which are used to slow down/control the set time.

Fact File: Portland Cement

Portland cement is produced from finely inter-ground mixtures of the various minerals of calcium which result from clinkering limestone and clay through a rotary kiln in the historical Portland process. Typically this kiln product, called cement clinker, is composed primarily of 2CaO·SiO₂ (dicalcium silicate) and 3CaO·SiO₂ (tricalcium silicate), with lower amounts of 4CaO·Al₂O₃·Fe₂O₃ (tetracalcium aluminoferrite), 3CaO·Al₂O₃ (tricalcium aluminate) and CaSO₄·2H₂O (gypsum). The kiln product is then ground with varying amounts of gypsum to produce the various types of commercial Portland cements covered in the BS EN 197 and ASTM C150 specifications:

• Type I – general use cements when the special properties specified for the other types are not required;
• Type IA – air-entraining cement for the same uses as Type I, where air-entrainment is desired;
• Type II – for general use, more especially when moderate sulfate resistance is desired;
• Type IIA – air-entraining cement for the same uses as Type II, where air-entrainment is desired;
• Type II(MH) – for general use, more especially when moderate heat of hydration and moderate sulfate resistance are desired;
• Type II(MH)A – air-entraining cement for the same uses as Type II(MH), where air-entrainment is desired;
• Type III – for use when high early strength is desired;
• Type IIIA – air-entraining cement for the same use as Type III, where air-entrainment is desired;
• Type IV – for use when a low heat of hydration is desired;
• Type V – for use when high sulfate resistance is desired.

The differences between the various types are rather subtle and have developed over time to meet specific performance requirements in different concrete and cement-based construction applications. In addition, white cements which contain no more than 0.50 wt% of ferric oxide (Fe₂O₃) are available and are used to ensure clean bright consistent colours for aesthetic architectural concrete as well as masonry and cementitious products.

Why the need for polymeric modification, repair and protection? There are a variety of issues inherent to concrete manufacture and deterioration in service which call for repair, maintenance and even overhaul solutions to conserve functional integrity. Problems start with the water : cement ratios and water levels used in manufacture, which have a marked influence on the workability of concrete and mortar mixes – to obtain a workable combination, more water is required than that which is needed for hydration. Unfortunately, any excess water lowers the density and creates voids in the set concrete, which contribute to shrinkage cracking and reduction of overall durability and create an ongoing need for maintenance, repair and overhaul starting from the point of casting. Inclusion of water-reducing agents, air-entraining agents, and polymer modifiers (see Section 1.2) in freshly mixed concrete can, however, permit equal workability at below-normal water levels with relatively little reduction in final strengths.

Fact File: Concrete Reinforcement

When reinforcing bars (rebars) and mesh made from high tensile and compressive strength steel, polymers or alternate composite materials are embedded into concrete before it sets, the resulting reinforced concrete is able to withstand applied stresses (tensile, bending and compression). Incorporation of fibrous materials made of steel, glass, synthetic and even natural fibre also increases structural integrity, depending on their length, distribution and orientation with, for example, lightweight thermoplastic fibres in 1- and 2-component solid premixes providing a simple alternative to embedding light reinforcement mesh.

It is evident that concrete is an inherently complex material to produce and, somewhat frustratingly, it also proves difficult to repair as new concrete, cementitious mortars and grouts will not stick well to old/new concrete without polymer modification or without the use of polymeric bonding, priming or conditioning agents. Concrete is not only prone to shrinkage cracking as it matures, it has essentially low tensile strength and elasticity, which decreases under tension causing cracking within the composite matrix, and even though it has relatively high compressive strength long-term structural loading can cause concrete to deform or creep.

The inherent porosity and alkaline nature of concrete also make it susceptible to liquid ingress as well as biological and chemical attack (even from carbon dioxide from the air – see box) if left unprotected. The following sections provide an insight into the essential chemistries and technologies of the various polymer types used to supplement or substitute cement as a binder in the design and use of concrete replacement, as well as for repair and protection.

Application Challenge – Concrete Carbonation

Carbon dioxide from the air can react with the calcium from calcium hydroxide and calcium silicate hydrate in concrete to form calcite (CaCO₃) in a process known as carbonation. The surface of fresh concrete reacts quickly, and the more porous and permeable the concrete, the faster and deeper the penetration inside into the concrete pore fluid. Although the process of carbonation increases both the compressive and tensile strength of the concrete, there is a resulting decrease in alkalinity which leads to problems of corrosion for embedded steel reinforcement – when the pH falls from the normal 12.5–13.5 range encountered in concrete pore fluid before carbon dioxide penetration, and whilst it remains at pH 10 or above, steel reinforcement is passivated and protected from corrosion; when, however, the pH drops below 10, the steel's thin layer of surface passivation dissolves and corrosion occurs. Pre-coating of steel reinforcement or replacement with non-corroding reinforcements are two options to prevent the corrosion process; sealing of the pores or application of a protective coating to the surface are other options (Sections 1.4 and 2.2). When left unaddressed, the corrosion of the reinforcing steel can lead to spalling of the embedding concrete, which in turn leads to accelerated deterioration of structural integrity – it will then be necessary to make repairs with polymer concrete mortars (Section 1.3).

Recommended Reading

• M. Neville, Properties of Concrete, John Wiley & Sons Inc., 5th edn, 2012, ISBN-13: 9780273755807.
• N. B. Winter, Understanding Cement, WHD Microanalysis Consultants Ltd., 2012, ISBN-13: 9780957104525.
• M. Raupach and T. Büttner, Concrete Repair to EN 1504: Diagnosis, Design, Principles and Practice, CRC Press, 2014, ISBN: 9781466557468.
• Concrete Repair: A Practical Guide, ed. M. G. Grantham, CRC Press, UK, 2014, ISBN: 9780415447348.
• M. H. Irfan, Chemistry and Technology of Thermosetting Polymers in Construction Applications, Springer Link, 1998, ISBN: 9789401060790.

1.2 Polymer-modified Cement Repair and Restoration

This section describes the major industrial repair and restoration applications for polymer-modified cement concretes, mortars and levelling/wearing screeds – other applications such as adhesives, grouts and waterproofing are covered in subsequent sections.

Fact File: Polymer-modified Levelling and Wearing Screeds

BS 8204, ISO 6707 and EN 13813 refer to a levelling screed as “a screed suitably finished to obtain a described level and to receive the final flooring”. A wearing screed is defined as “a screed that serves as a flooring”. A pumpable self-smoothing screed is defined as “screed that is mixed to a fluid consistency, that can be transported by pump to the area where it is to be laid and which will flow sufficiently to give the required accuracy of level and surface regularity”. This is often referred to as a “self-levelling screed” and is also used as a levelling screed or a wearing screed.

1.2.1 Essential Chemistry and Technology

Portland cement mortars and concretes are typically modified with thermoplastic polymer emulsions or powders to improve the workability of mixtures at reduced water cement ratios, thereby increasing the density of the ultimate finished particulate composite and reducing the tendency for shrinkage and cracking. Polymer emulsions, known as gauging liquids, are supplied as part of a two-part system to keep then separate from the solid hydraulic cement component; powdered polymers are supplied pre-mixed in the solid hydraulic cement component as one-part alternatives to which water is added at point of use.

As water is lost during the curing process of a cementitious polymer concrete mix, suspended polymer particles coalesce to form an interpenetr...

Table of contents

• Cover
• Title
• Copyright
• Contents
• 1 Concrete Modification, Protection and Repair
• 2 Masonry and Wood Protection and Repair
• 3 Metal Protection and Repair
• 4 Rubber and Plastic Bonding, Repair and Restoration
• 5 Glass and Ceramics Repair and Bonding
• Appendix 1 Glossary of Resins, Polymers and Plastics
• Appendix 2 Thermoset Resin Stoichiometry Calculations
• Appendix 3 Cited International Standards, Practises, Specifications and Test Methods
• Subject Index