Two-Component Polyurethane Systems

Innovative Processing Methods

Chris Defonseka

• Publisher: De Gruyter
• Year: 2019
• Print ISBN: 9783110639575
• eBook ISBN: 9783110639797
• Edition: 1
• Topic: Tecnología e ingeniería
• Subtopic: Química

Thanks to their unique physico-chemical nature, two-component polyurethane (PU) systems have found widespread industrial application. This book gives practical guidance on the selection of raw materials and machinery, as well as the calculations and formulations necessary for the successful production and processing of two-component PUs. Readers will benefit from the troubleshooting advice based on the author's extensive industry experience.

1 Introduction to polyurethanes

1.1 An overview

Polyurethanes(PUs) are an important branch of plastics belonging to the thermosetting group unlike other polymers such as polyethylene (PE), polystyrene (PS), polypropylene (PP), polyvinyl chloride and many others which belong to the thermoforming group, meaning these polymers can be re-used for production. PU materials having a wide range of versatile properties, including pleasing aesthetic values, find many essential applications in important sectors such as comfort, automobile, building construction, footwear, coating, space travel, industrial, consumer products, packaging, adhesives and others.

Unlike the thermoforming polymers which are derived from their monomers, PUs do not have a basic monomer as such to start with and the urethane monomer is produced by reacting a polyol (an alcohol with more than two reactive hydroxyl groups per molecule) and a diisocyanate or polymeric isocyanate in the presence of suitable catalysts and additives with water as the primary blowing agent to form a foam. Since a wide variety of diisocyanates and a wide range of polyols can be used to produce PUs, a broad spectrum of materials can be produced to meet the needs of specific applications.

PUs can be broadly categorised as follows: flexible, semi-rigid, rigid, microcellular, viscoelastic or thermoplastic urethanes. When considering comfort applications such as mattresses, cushions, sheets, slab-moulded automobile and aircraft seats and viscoelastic products and the medical applications, one may conclude that the flexible foams have the highest number of demand.

Viscoelastic foams, also known as memory foams, are PU foams with special ultra-comfort properties. Standard flexible PU foams used for comfort applications are two-dimensional, while viscoelastic foams with four-dimensional properties such as density, hardness, temperature and time that can be varied, gives much superior comfort. When they first arrived on the market, it was called a ‘miracle foam’, their speciality properties being ultra-comfort and great therapeutic values, although they were heavier and also cost more than standard PU foams. Viscoelastic foams are generally graded by their density and thicknesses and foams selected by most people for a mattress will range from about 4 inches (10.2 cms) to 8 inches (20.3 cms), although a thinner topper for an existing bed, is also an option. The current availability of custom-ordered two-component systems for their productions is a great boost for these foam producers.

Years back, the production of PU foams involved the mixing of multiple components needing careful formulating and weighing each chemical accurately which had to be precise to prevent foam wastes. Now, most PU systems, except for the very large volume foam producers are available as two components and producers have the options of using standard systems available in the market or custom made to meet individual end application needs. However, they are not without disadvantages, as each system can produce a foam of one density only with predetermined properties. More advanced systems are also available, where the densities can be adjusted by variations in the mixing ratios.

Large volume PU foam producers with continuous systems will opt for purchases of raw material components in bulk, storing them in large tanks connected to a central mixing head and draw from them on pre-set quantities. These systems will allow a foam producer products of varying densities and properties but the foam wastes can be higher. However, for other applications like coatings, mouldings, integral skin moulding, in-situ building insulation, appliance insulation, sound proofing, footwear mouldings and others, the use of two-component systems would be more practical and effective with lesser costs.

The emergence of two-component PU systems, although slow at the beginning, has been a great boost in foaming. Because of rapid advances in technology, most of these two-component systems can be ‘hand-mixed’ in simple proportions of component A and component B or processed using simple mixing machines. Some are even available in small cans, containers or small packs for instant processing and applications.

The presentation of identifying and use of ideal biomass fillers and methodologies for increasing thermal conductivity in PU foams should be of interest to foam producers and in particular to the automotive industry. Detailed information of both solvent borne and water borne two-component PU systems which have emerged in the last few years, also should be of interest.

To impart valuable knowledge and encouragement to readers, small foam producers and would-be entrepreneurs to realise the exciting possibilities of two-component systems, the author presents a complete cost-effective design of a foaming system and a simple cutting machine suitable for making large blocks of flexible, high resilience and viscoelastic foams using two-component PU systems, based on an actual project designed, fabricated and implemented by the author. Also presented are information on formulations, processing techniques, a two-component raw material system for large foam blocks, mould designs, handling and safety factors. The highlighting of specialty two-component systems developed and made available by well-known companies, along with lists of key raw material suppliers and machinery suppliers should provide useful additional information.

1.2 Chemistry of polyurethanes in brief

The polymeric material known as PU forms a family of polymers which are essentially different from most of the other plastics in that there is no urethane monomer and this polymer is invariably created during the manufacture of a PU product.

PUs are made by the exothermic (heat emitting) reactions between alcohols with two or more reactive hydroxyl (OH) groups per molecule (diols, triols and polyols) and isocyanates that have more than one reactive isocyanate group (NCO) per molecule (diisocyanates and polyisocyanates). For example, when a diisocyanate reacts with a polyol, the group formed by the reaction between the two molecules is known as the ‘urethane linkage’. This is the essential part of a PU molecule.

The physical properties, as well as the chemical structure of a PU depends on the structures of the original reactants. The characteristics of the polyol/polyols used such as relative mass, the number of reactive functional groups per molecule and the molecular structures will influence the final properties of the polymer. There is a fundamental difference between the manufacture of most PU products and the manufacture of many other plastics products.

Polymers such as PE, PP, PS and others are produced in chemical plants in the form of granules, powders or others. Products are made from these basic raw materials by heating, shaping under pressure and cooling by different processing methods. The properties of these end products will almost completely depend on the original polymer. On the other hand, raw material components for PUs are liquids and usually made directly into products using mixing processes, with options for adjusting for desired properties at the time of formulating.

1.3 Some important polyurethanes

PUs are very versatile materials and can be made with textures from soft, rigid to elastic with varying densities and properties. With the advent of waterborne systems and PU composites made up of non-traditional fillers like biomass flours/powders, bamboo fibres and so on, applications are increasing rapidly. Some of the important areas of foams are provided in the following sections:

1.3.1 Flexible foams

Flexible PU foams account for about 30% by volume being used for bedding, furniture and in the automotive and aircraft industries. Viscoelastic foams, which can also be classified as flexible foams are largely used for bedding. Flexible foams can be made in almost any variety of firmness. They are durable, supportive and comfortable, with viscoelastic foams providing ultra-comfort.

1.3.2 Rigid polyurethane foams

Rigid PU and polyisocyanurate foams create one of the most popular, energy-efficient temperature and lower noise levels in homes and commercial properties, builders use rigid PU and polyisocyanurate foams. These foams are effective insulation materials that can be used in roof and wall insulations, insulated windows, doors and as air barrier sealants.

1.3.3 Coatings, adhesives, sealants and elastomers

The use of PUs in coatings, adhesives, sealants and elastomers offer a broad and growing spectrum of applications and benefits. PU coatings can enhance a product’s appearance and also lengthen its lifespan. PU adhesives can provide strong bonding advantages, while PU sealants provide tighter seals. PU elastomers can be moulded into any shape, are lighter than metal, offer superior stress recovery and can be resistant to many environmental factors.

1.3.4 Thermoplastic polyurethane

Thermoplastic polyurethanes (TPU) offer a myriad of combinations of physical property and processing applications. They are highly elastic, flexible and resistant to abrasion, impact and weather. TPUs can be coloured or fabricated in a wide variety of methods with product durability as a special feature. TPU is an elastomer that is fully thermoplastic. Like all thermoplastic elastomers, TPUs are melt-processable. In addition, they can be processed on extrusion, injection, blow moulding and compression moulding equipment. They can also be vacuum-formed or solution-coated and well suited for a variety of fabrication methodologies. They have many uses in applications in the building construction, automobile and footwear industries.

1.3.5 Reaction injection moulding

Car bumpers, electrical housing panels and computer telecommunication equipment enclosures are some of the parts that are produced with PUs using reaction injection moulding (RIM) techniques. RIM process produces parts that are not achievable using standard injection moulding processes. In addition to high strength and low weight, PU RIM parts can exhibit heat resistance, thermal insulation, dimensional stability and a high level of dynamic properties. Automotive, construction, appliances, furniture, recreation and sporting goods are a few of the markets using RIM technologies.

1.3.6 Binders

PU binders are used to adhere numerous types of particles and fibres to each other. Their primary areas of use are in the manufacturing of wood panels, rubber or elastomeric flooring surfaces and sand casting for the foundry industry. PU binders are widely used in the manufacture of oriented strand board (OSB). These wood panels are used in structural sheathing and flooring, pre-fabricated housing and so on. Re-bonded foam underlay uses PU binders to adhere scrap foam pieces, which are often flexible PU foams.

1.3.7 Waterborne polyurethane dispersions

Waterborne polyurethane dispersions (PUDs) are coatings and adhesives that use water as the primary solvent. With increasing environmental concerns and stricter regulations on the amount of volatile organic compounds (VOCs) that can be emitted into the atmosphere, PUDs are being used in more industrial and commercial applications.

1.4 Some polyurethane applications

1.4.1 Apparel

When researchers discovered that PUs can be made into fine threads, they were combined with other polymers to make lightweight, stretchable materials ideal for garments. Over the years, PUs have been improved into what is known as spandex fibres. Because of today’s constant advances in PUs and processing techniques, manufacturers can make a broad range of apparel for garments, sports clothes and variety of accessories.

1.4.2 Appliances

PUs are an important component in major appliances that are used by consumers every day. The most common are rigid foams as insulation for refrigerators and freezer thermal insulation sys...

Table of contents

• Cover
• Title Page
• Copyright
• Preface
• Contents
• 1 Introduction to polyurethanes
• 2 Types of polyurethanes
• 3 Two-component systems
• 4 Basic raw materials for polyurethanes
• 5 Mould designs
• 6 Selection of machinery
• 7 Calculations, formulating and formulations for polyurethanes
• 8 Processing, moulding methods and troubleshooting
• 9 Project set-up for small volume foam producers
• 10 A case study: reduction of excessive waste, improvement of foam quality and process efficiency
• Appendix 1: Some suppliers of two-component polyurethane systems, dispensing and cutting machines
• Appendix 2: Recommended solutions for deficiencies in PU parts moulding and block moulding
• Appendix 3: Glossary of PU terms
• Abbreviations
• Index