Flexible Polyurethane Foams

A Practical Guide

Chris Defonseka

• Publisher: De Gruyter
• Year: 2019
• ISBN:
  9783110639834
• Edition: 1
• Topic: Technology & Engineering
• Subtopic: Materials Science
• Index: Technology & Engineering

Flexible and viscoelastic polyurethane foams have enormous potential as viable business ventures and have replaced many traditional materials used in everyday life.

This book describes the chemistry of flexible and viscoelastic polyurethane foams as well as calculations and formulating methodology for quality production. The author presents detailed information on foam manufacturing, based on over 45 years of hands-on industry experience.

1 Introduction

1.1 Brief history

Polyurethane (PUR) foams were first introduced into the market in the 1950s. They are commonly known as ‘polyurethane foams’, or ‘PU foams’. With rapid development and growth over the years, these are available under three main categories – (1) flexible, (2) semi-rigid and (3) rigid – of which the most popular one is the flexible foam.

1.1.1 Flexible foams

This new material gained great interest and common acceptance initially in the mattress sector as a superior replacement for traditional materials such as rubber, fibre and cotton because of its unique properties. Compared with rubber latex foam, flexible PUR foams are superior in many ways. They can be produced in densities ranging from 12 to 50 kg/m³. Flexible PUR foams have maximum tear strength, better resistance to oxidation, aging, as well as fire, and its open cell structure enables it to absorb and emit humidity (e.g. body heat).

In the latter years, a new PUR foam with special properties has been developed and introduced into the market. This is viscoelastic foam and is commonly called ‘memory foam’. This foam soon became very popular because of its medicinal properties, in addition to the superb comfort it provided and some even called it ‘miracle foam’. The viscoelastic foam was initially too expensive for widespread use; however, over the years, it became cheaper and more affordable.

1.1.2 Rigid polyurethane foams

Rigid PUR foam is a key insulating material used in residential and commercial construction industries, such as for spray PUR foam roofing, PUR laminated panels, insulated sheathing and single-skin and foamed panels for garage doors. These foams are also used as binder applications such as particle board, laminated stranded lumber, moisture-resistant rim board and moisture-resistant flooring. Additionally, there are many speciality applications such as wheels and core binders.

1.1.3 Automotives

Automotive applications include interior and exterior parts. Some are energy-absorbing PUR foams, dashboard panels, steering wheels, interior and exterior trims, head liners and many others.

1.1.4 Elastomers

For customers who are looking for an elastic, durable and wear-resistant material for demanding applications, the PUR cast elastomers are ideal. These systems are used to produce rollers, wheels, vibration-damping parts and encapsulants. Spray PUR elastomer systems (known as ‘thick-film coatings’) coat and protect a wide range of wood, metal and concrete surfaces.

1.1.5 Insulation

Many versatile energy-efficient PUR systems are available to the appliance industry, especially the makers of household refrigerators and commercial refrigeration systems, as well as water-heater manufacturers.

1.1.6 Moulded parts

For applications of design flexibility, low production volumes and product durability, these PUR systems can be used in reaction injection moulding for a wide variety of non-automotive parts and components. From agricultural equipment body panels, to the snowboard cores to the trim for heavy-duty trucks, rigid PUR foams, elastomeric PURs and solid PUR can be an ideal solution to produce lightweight, durable moulded parts. Newer technologies are also expanding the use of the PUR system-I composites.

1.1.7 Spray roofing systems

Spray PUR foam roofing systems, high-performance coatings, sprayed insulation and adhesives offer a range of commercial construction roofing solutions to contractors and building owners. These products provide durable, trouble-free performance in any climate and under any weather condition.

1.1.8 Speciality urethane systems

Diphenylmethane based PUR and elastomer systems are available for filters, footwear, integral skin foams, carpet underlay, turf backing, flexible moulded foams and rigid foams.

These are some of the main categories of PUR. Flexible foams and viscoelastic foams are the most versatile and utilised foams, and have the widest range of applications.

This incredible material initially dominated the domestic market with end uses such as mattresses, pillows, cushions and mattress toppers. It has also spread into other sectors such as industrial and automotive applications, medical applications and space travel. The basic raw material components for flexible PUR are polyols. Polyols are petroleum-based compounds and, in keeping with global environmental concerns, continuous research has produced polyols from soya and other sources, making these foams more cost-effective.

1.2 Types of foams

Many types of foams are on the market. The popular ones are: polystyrene, polyethylene, polyester-based PUR and polyether-based PUR. All these foams are synthetic foams belonging to the plastic family, with two very desirable properties: (1) being easily malleable or moulded, and (ii) capable of ‘softening’ and regaining its original shape (especially flexible PUR foams). Where comfort is concerned, polyether-based PUR foams give the best results because of their extra-soft texture and they also cost less than polyester-based foams (which are rough and give a ‘scratchy’ surface).

Polyether PUR foams have wide applications in furniture, bedding, pillows, padding and carpet underlay. Polyester-based PUR foams are used for textiles, shoulder pads, noise reduction and other applications. Both are used singly or in combination for applications in automotive, aircraft, household and footwear industries. These foams are made from base chemicals (polyols) derived from liquefied ethylene gas (which is obtained during the refining of crude oil). Polyols called ‘ecopolyols’ that are based on vegetable oils and other sources have been developed. With rapid development programmes, these new polyols may replace the traditional ones quite soon.

During recent years, mattresses in the bedding sector have been made from materials such as coir, cotton, latex foam and flexible PUR foams. The use of steel springs and coils are being fast replaced with PUR foams to maximise comfort. Flexible PUR foams are very important in our day-to-day life. The discovery and introduction of viscoelastic foam have escalated the demand for PUR flexible foams and, according to reports, it is a billion-dollar market in North America alone.

In the 1960s, Swedish scientists, at the request of NASA, developed a special PUR foam to counter the G-forces their astronauts experienced during take-off. They produced a new type of PUR foam called viscoelastic foam. This incredible foam material has achieved a very high profile in the bedding market (especially in the mattress sector) because of its unique properties of proven maximum comfort and medical benefits.

Viscoelastic foams have very low resilience and damping effects. They continually adjust to the shape of the human body and its weight, thereby dramatically reducing pressure points, creating a sensation of ‘floating on a cloud’ and ensuring maximum comfort with improved blood circulation, guaranteeing a tossing-free, deep sleep. This material has been thoroughly tested commercially and medically by researchers, and is highly recommended as the best ‘sleep’ material on the market. Unlike other foams, viscoelastic foams are four dimensional in that they have density, hardness, temperature and time effects, properties that make them truly special foams.

1.3 Basic foam grades

Cheap grades are foams where quality is not a priority. These include soft or coarse foams used for gymnasium mats or camping sleep pads and carpet underlay (where the foams are heavily loaded with fillers). Cheaper base polymers called ‘graft’ polymers are also used to reduce costs. Foam blocks made from re-bonded foam scrap and cut to desired sizes are used for carpet underlay and thick slabs as bases for mattresses.

Medium soft grades are good-quality foams used for comfort (e.g. back cushions on a couch or a window seat). Here, the density and firmness of the foam are important factors. They are also used for low-end inexpensive foam mattresses.

Firm quality gradesare foams used for high-end or special applications such as bedding, aircraft seats, furniture and auto seats. Here, the important properties are firmness, the ‘spring effect’, anti-fungal, ultraviolet (UV)-protected, fire-retardant and any other property as desired by a customer or on market demand.

High-end grades are called ‘high-resiliency (HR) foams’ and are used for special applications depending on the end use or customer preference. These foams can have some or all of the properties mentioned above.

Viscoelastic foam is a quality foam, like no other. It is used for luxury, comfort and special applications such as medical uses and space travel. It is a four-dimensional foam that is much heavier and costlier than standard PUR foams, with densities ranging up to ≥96 kg/m³. It is used as luxury mattresses with thicknesses ranging from 4 inches to 8 inches. This speciality foam is also used in pillows, pads and many other applications.

1.4 Viscoelastic foam mattresses: Marketing hype or molecular miracle?

In the last decade, a new type of flexible foam burst onto the foam market and came to be known as a viscoelastic or ‘slow’ foam. It quickly became popular and was also called memory foam because of its property of full recovery.

Its immediate impact and marketing led some people to believe that this was a ‘magic’ material because it had quite amazing and unusual properties. In a comparatively short time, it was apparent that the claims of its ‘magical’ properties were not marketing ‘hype’ by the manufacturers. The marketplace was excited by the fact that it was suitable for ‘space travel’ (which is a story in itself) but also had many benefits. Foam-market experts realised that anybody can promote a well-made product, but that a different situation arose if the product itself had major advantages over competing materials for comfort and health features with proven therapeutic values.

A key feature of viscoelastic foam is that it is ‘lazy’ or slow with high damping. One of the main characteristics is very low resilience, meaning that there is no ‘spring’ in the foam. Another important aspect is the temperature sensitivity of the foam (which is a key active property of viscoelastic foams). If one were to ‘sink’ a hand into viscoelastic foam, it leaves an indentation that disappears as the foam recovers slowly. Unlike conventional flexible foams, which are two dimensional (density and hardness can be varied), viscoelastic foams are four dimensional (hardness, density, temperature and time can be varied). Most flexible foams are highly resilient and ‘bouncy’, whereas a viscoelastic foam allows moulding to the body shape. The low indentation force deflection (IFD) of these foams allows a body to sink rather deep into the foam without resistance while maintaining the ‘firm’ feel of good-quality foam.

Because of their special medical benefits, many hospitals are now using viscoelastic foam mattresses for their beds. Conventional flexible foams (especially HR foams) cause pressure points on a body but viscoelastic foams, under the influence of body heat, soften and allow the body to sink in, while firmly cushioning it. The soft foam flows around the pressure points, thus spreading the load over a larger area and creating negative pressure. For patients who have to lie in bed for prolonged periods, this property prevents the creation of ulcers (‘bed sores’). Another beneficial aspect of viscoelastic foam is the promotion of free-flowing blood circulation for a body lying prone for long periods, such as during surgery. This foam cushions the body without pressure, so there are many benefits for people with pain in the back or joints, and this has created a market demand.

Viscoelastic foam mattresses have been replacing conventional mattresses for some time and, although these mattresses are more expensive, people have been willing to pay the extra cost, which they think is justified as compared with the benefits of using one. Another way to avoid paying high prices is to use a 2ʺ or 3ʺ ‘mattress topper’ on top of the conventional mattress, which is very effective. There are other products of viscoelastic foam products now available on the market: pads, pillows, footwear and medical cushions.

1.5 Flexible slabstock foams

There are four main types of slabstock foams: (1) conventional, (2) high-resilience, (3) filled foam and (4) high load-bearing. Each type is formulated differently to provide varying characteristics depending on the intended market.

Conventional slabstock –Standard formulations can be modified readily to produce a wide range of foam properties. These foams can be broadly classified into four basic general hardness or softness grades. In general, the hardness of a foam is defined by the IFD, which can be manipulated by changing or increasing/decreasing some of the components of a foam formulation.

As a general guide, foams having a density ≈16 kg/m³ and an IFD of 10–20 are ‘super soft’, whereas foams with densities of 32–45 kg/m³ and an IFD>40 are considered to be ‘hard’. Anything in between can be considered to be ‘soft’ to ‘intermediate’.

HR foams are made in a wide range of densities and hardness levels, and are designed primarily to offer enhanced support. These foams are more ‘elastic’ and feel heavier than standard foams. They are more expensive than other slabstock foams, so their uses are usually limited to high-performance products.

Filled foams – Filled slabstock foams use inorganic fillers to increase the foam density and also to improve the load-bearing characteristics. These foams often lead to the perception that they are higher-quality products. Filled foams decrease quality by reducing the tensile strength, elongation and tear strength, and perhaps causing a long-term decrease in the fatigue resistance of the foam. In general, calcium carbonate is used as the filler but melamine fillers can also be used for improved flammabilit...