Polyvinyl Alcohol

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Expert overviews covering the

science and technology of rubber

Volume 16, Number 12, 2005

E. Ogur

Polyvinyl Alcohol:

Materials, Processing

and Applications

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1. A commissioned expert review, discussing a key topic of current interest, and referring to the References and

Abstracts section. Reference numbers in brackets refer to item numbers from the References and Abstracts section. Where it has been necessary for completeness to cite sources outside the scope of the Rapra Abstracts database, these are listed at the end of the review, and cited in the text as a.1, a.2, etc.

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records on the database to aid retrieval.

Item 1

Macromolecules

33, No.6, 21st March 2000, p.2171-83

EFFECT OF THERMAL HISTORY ON THE RHEOLOGICAL BEHAVIOR OF THERMOPLASTIC POLYURETHANES Pil Joong Yoon; Chang Dae Han

Akron,University

The effect of thermal history on the rheological behaviour of ester- and ether-based commercial thermoplastic PUs (Estane 5701, 5707 and 5714 from B.F.Goodrich) was investigated. It was found that the injection moulding temp. used for specimen preparation had a marked effect on the variations of dynamic storage and loss moduli of specimens with time observed during isothermal annealing. Analysis of FTIR spectra indicated that variations in hydrogen bonding with time during isothermal annealing very much resembled variations of dynamic storage modulus with time during isothermal annealing. Isochronal dynamic temp. sweep experiments indicated that the thermoplastic PUs exhibited a hysteresis effect in the heating and cooling processes. It was concluded that the microphase separation transition or order-disorder transition in thermoplastic PUs could not be determined from the isochronal dynamic temp. sweep experiment. The plots of log dynamic storage modulus versus log loss modulus varied with temp. over the entire range of temps. (110-190C) investigated. 57 refs.

GOODRICH B.F.

USA

Accession no.771897

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Location Companies or organisations mentioned Abstract Authors and affiliation Source of original article Title

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Report 1 Conductive Polymers, W.J. Feast

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Report 40 Internal Mixing of Rubber, J.C. Lupton Report 41 Failure of Plastics, S. Turner, Queen Mary College. Report 42 Polycarbonates, R. Pakull, U. Grigo, D. Freitag, Bayer AG. Report 43 Polymeric Materials from Renewable Resources,

J.M. Methven, UMIST.

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Report 54 Health and Safety in the Rubber Industry, A.R. Nutt, Arnold Nutt & Co. and J. Wade.

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Volume 12

Report 133 Advances in Automation for Plastics Injection Moulding, J. Mallon, Yushin Inc.

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Report 154 Polymers in Building and Construction, S.M. Halliwell, BRE.

Report 155 Styrenic Copolymers, Andreas Chrisochoou and Daniel Dufour, Bayer AG.

Report 156 Life Cycle Assessment and Environmental Impact of Polymeric Products, T.J. O’Neill, Polymeron Consultancy Network.

Volume 14

Report 157 Developments in Colorants for Plastics, Ian N. Christensen.

Report 158 Geosynthetics, David I. Cook.

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Report 166 Developments in Thermoplastic Elastomers, K.E. Kear

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Volume 15

Report 169 Engineering and Structural Adhesives, David J. Dunn, FLD Enterprises Inc.

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Report 176 Polymer Processing with Supercritical Fluids, V. Goodship and E.O. Ogur.

Report 177 Bonding Elastomers: A Review of Adhesives & Processes, G. Polaski, J. Means, B. Stull, P. Warren, K. Allen, D. Mowrey and B. Carney.

Report 178 Mixing of Vulcanisable Rubbers and Thermoplastic Elastomers, P.R. Wood.

Report 179 Polymers in Asphalt, H.L. Robinson, Tarmac Ltd, UK. Report 180 Biocides in Plastics, D. Nichols, Thor Overseas Limited.

Volume 16

Report 181 New EU Regulation of Chemicals: REACH, D.J. Knight, SafePharm Laboratories Ltd.

Report 182 Food Contact Rubbers 2 - Products, Migration and Regulation, M.J. Forrest.

Report 183 Adhesion to Fluoropolymers, D.M. Brewis and R.H. Dahm, IPTME, Loughborough University.

Report 184 Fluoroplastics, J.G. Drobny.

Report 185 Epoxy Composites: Impact Resistance and Flame Retardancy, Debdatta Ratna.

Report 186 Coatings and Inks for Food Contact Materials, Martin Forrest, Smithers Rapra.

Report 187 Nucleating Agents, Stuart Fairgrieve, SPF Polymer Consultants.

Report 188 Silicone Products for Food Contact Applications, Martin Forrest, Smithers Rapra.

Report 189 Degradation and Stabilisation of Polymers, Stuart Fairgrieve, SPF Polymer Consultants Report 190 Electrospinning

Jon Stanger, New Zealand Institute for Plant and Food Research

Nick Tucker, New Zealand Institute for Plant and Food Research

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ISBN: 978-1-84735-095-4

Processing and Applications

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Contents

1. Introduction

...3

2. Application

Scope

...4

3. Manufacture

...6

4. Polymer

Chemistry

...7

4.1

Viscosity in Solution ...7

5. Water

Solubility

...8

6.

PVAL Film Properties ...9

6.1 Barrier Proporties ...9

6.2 Other Film Properties ...10

6.2.1 Mechanical

Properties

...10

6.2.2 Plasticiser

Effects

...10

6.2.3

Oil and Solvent Resistance ...10

6.2.4

Water Sensitivity and Hygroscopy ...10

6.2.5 Adhesion

Characteristics...10

6.2.6 Antistatic

...10

7. Solution

Processing

...10

8. Melt

Processing

...11

8.1 Extrusion

...12

8.2 Injection

Moulding

...12

8.3 Film

Moulding

...12

8.4 Blow

Moulding

...13

9. Packaging

...13

9.1 Active

Packaging

...13

10. PVAL in Medicine: Hydrogels ...13

11. Biopolymers

...14

12. Blends

...15

13. Blends of PVAL ...15

13.1 PVAL-Starch

Blends

...15

13.2 Other

Blends

...16

13.2.1 PVAL/Gelatin

Blends

...16

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The views and opinions expressed by authors in Rapra Review Reports do not necessarily reflect those

of Smithers Rapra Technology or the editor. The series is published on the basis that no responsibility

or liability of any nature shall attach to Smithers Rapra Technology arising out of or in connection with

any utilisation in any form of any material contained therein.

Author contact details: Vannessa Goodship

WMG, Univeristy of Warwick, Coventry, CV4 7AL

Daniel

Jacobs

WMG, Univeristy of Warwick, Coventry, CV4 7AL

14. Degradability

...17

14.1 Biodegradable

Plastics

...18

14.2 Oil-Based Naturally Degradable Polymers ...18

14.3 Photo-degradable

Plastics

...18

15. Plastics and Biodegradable Plastics Disposal ...19

15.1 Composting

...20

15.2 Incineration

...21

15.3 Landfilling

...21

16. Conclusion

...21

References ...22

Abbreviations and Acronyms ...22

Subject Index ...00

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Poly(vinyl alcohol):

Properties, Applications and Disposal

This review report is produced to provide a concise introduction to poly(vinyl alcohol); the material itself, processing, applications and disposal. It will also consider comparable properties of similar materials with which this material is likely to compete. The definitions and requirements of materials that wish to operate in the ‘green’ arena will also be introduced.

Poly(vinyl alcohol) is the most commercially important water soluble plastic in use. It is tasteless, odourless, it will biodegrade and is biocompatible. As well as being soluble in water, it is slightly soluble in ethanol, but insoluble in other organic solvents. As environmental concerns over the disposal of plastic wastes have grown and focus has switched towards product life cycle and disposal, poly(vinyl alcohol) has a readymade and viable disposal route. In light of increasing environmental legislation, this will allow it to be utilised in non-traditional areas of use. With increasing interest in the use of biodegradable and sustainable alternatives for mass applications, poly(vinyl alcohol) provides an interesting point of comparative study on a oil-based synthetic material with the properties of a ‘green polymer’.

It is also be readily blended with a number of natural materials and can exhibit properties that are compatible with a range of applications. The inclusion of natural fibres and fillers can give further improvements in mechanical properties without compromising overall degradability. Therefore, the potential benefits of this material given its water soluble characteristics are huge, but this must be offset against practical considerations of its long term life cycle in changeable environmental conditions.

In the future biodegradable materials could be used within a variety of product applications, but there are, at present, significant penalties in terms of performance, cost and availability. The predominant benefit which can be attributed to biopolymer usage is that the materials themselves can be biodegradable. This of course makes them an increasingly attractive proposition for manufacturers who are faced with increasingly stringent EU legislative requirements when it comes to the environmental performance of their products, or those who wish to tap the market of the environmentally conscious consumer. This review will try to capture current and future trends in this area.

1. Introduction

Like all common polymers poly(vinyl alcohol) can be abbreviated. However, unlike polymers such as polypropylene (PP), in which the abbreviations are universally used, poly(vinyl alcohol) does not have a singularly recognised abbreviation. Therefore according to various literature sources it is referred to as PVAL, PVOH or PVA and therefore all these terms, although confusing, are correct in the context of the standards to which they were used.

The terminology that shall be used here is PVAL in accordance to British Standard BS EN ISO 15023-1:2006 (a.1).

Poly(vinyl alcohol) and its properties have been known to scientists for a long time. It was first prepared by Hermann and Haehnel, Germany, in 1924. It is made by hydrolysing polyvinyl acetate (PVA or PVAc – dependent on where you look).

Worldwide consumption of PVAL had reached several hundred thousand tonnes annually in 2006; this is widely predicted to increase by about 2.5% annually between 2006 and 2011. There are a number of worldwide producers of this material with the majority of production based in Asian countries. China has the largest share of the market as of commercial trading figures for 2006 with 45% of the entire market with this figure likely to expand. Japan and the United States are also both large producers and consumers of PVAL materials. Kuraray (Japan) is the largest single manufacturer of PVAL accounting for 16% of all world capacity and offering over 50 different grades of this material.

Trade names for PVAL (PVOH or PVA) include Elvanol (DuPont), Mowiol (Kuraray) and Poval (Kuraray).

The material exists in the marketplace as fully hydrolysed, medium hydrolysed or partially hydrolysed copolymers or custom blends. Most of the market is dominated by sales of the fully hydrolysed grade of material. This difference in hydrolysis will be explained in Section 4. In line with international standards, grades of this material are presented in terms of the degree of hydrolysis and the viscosity in water at specified conditions. An indication of the range of hydrolysis levels available in the marketplace is shown in Table 1.

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Using 2008 commercial prices as a general guide, PVAL materials are priced according to viscosity. Low viscosity materials (classed as having a viscosity less than 0.01 Pa-s), were trading for half the price per tonne (150 Euros), than high viscosity materials (>0.03 Pa-s) which were trading at 300 Euros per tonne. Medium viscosity materials (0.01-0.03 Pa-s) were trading at 200 Euros. However, like all oil-based plastics prices are highly variable.

2. Application Scope

The major uses of PVAL depend on region. In the United States and Western Europe the majority of PVAL is consumed in the production of poly(vinyl butyral) (PVB). This is used in the inner layer of laminated safety glass for automobile window screens. It is made by reacting PVAL with butyraldehyde [CH3(CH2)2CHO] and this market is one which is

expected to grow in the future.

PVAL finds other uses in a broad range of products produced for example as a protective colloid in the manufacture of polymer emulsions. Other applications include: the binding of pigments and fibres, dip coated articles, protective strippable coatings, the manufacture of detergents and cleansing agents, adhesives, emulsion paints, and solution cast film. These applications all involve the use of the PVAL in solution as its thermal degradation at about 150 °C (crystalline melting point of PVAL ranges from 180 °C to 240 °C), limits its ability to be used in conventional thermoplastic processing equipment without modification. When PVAL is plasticised, it

is possible to avoid thermal dehydration however, some properties are sacrificed.

In China the largest market is as a polymerisation aid in emulsified polyvinyl acetate and polyvinyl chloride (PVC). Therefore, much of the PVAL produced today is used as a protective colloid in the manufacture of polymer emulsions.

The use of emulsifying agents improves the quality of synthesis of PVC and helps properties such as the ability to absorb plasticisers and the production of uniform grain sizes. Other materials commonly used as emulsifying agents are methyl cellulose derivatives or combinations. Two PVAL materials of differing molecular weight may be used together synergistically improving the process.

In the formation of emulsified polyvinyl acetate, PVAL acts as a protective colloid. PVAL has excellent properties in emulsion such as ‘wet tackiness’, high strength and it is highly resistant to creep. As with the formation of PVC, the structure of the PVAL affects the emulsion performance of the polyvinyl acetate. For this process, PVAL with an active surface is also required, therefore partially hydrolysed grades are utilised which have residual acetate groups which are hydrophobic in nature.

In Japan Vinylon spun fibre is the biggest use of PVAL. It is used in a variety of applications from the manufacture of traditional Japanese dress, canvas, cement reinforcement, agriculture, fishing nets, ropes and paper making (which is the largest market). It can be produced by either dry or wet spinning. This is a different process to spinning from a melt, which is called melt spinning and used for production of fibres

Table 1 Selection of grades of PVAL offered by Kuraray

Grade Viscosity

DIN 53015 (a.2) (mPa-s)*

Degree of hydrolysis (saponification) (mol. - %)

Partially hydrolysed Mowiol 3-85 3.4-4.0 84.2-86.2

Partially hydrolysed PVA 220 27.0-33.0 87.0-89.0

Partially hydrolysed PVA 225 45.0-52.0 86.5-89.0

Medium Hydrolysed PVA CST 24.0-30.0 95.5-96.5

Medium Hydrolysed PVA 613 14.5-18.5 92.5-94.5

Fully hydrolysed PVA 117 25.0-30.0 98.0-99.0

Fully hydrolysed Mowiol 4-98 4.0-5.0 98.0-98.8

Fully hydrolysed Mowiol 56-98 52.0-60.0 98.0-98.8

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such as polyethylene (PE), PP, polyamide (PA) and polyester. In the case of vinylon a spinning solution is used with most production then carried out by wet spinning. The product is staple fibre. The solution is spun into fibres which pass through a water bath before being drawn and cut into fibres. Dry spinning uses hot air instead of water, drawing and then collection of the filament yarn onto spindles. Vinylon is characterised by a lighter weight (specific gravity of 1.3) than natural fibres such as wool or cotton. It is abrasion resistant, durable and resistance to weathering, chemicals and rotting and has properties similar to cotton in fibre form.

Demand is also increasing for PVAL fibres as a safer replacement for asbestos fibres and in the use of polarising optical films for liquid crystal displays (LCD) (a.3). The PVAL material is used for optical applications which effectively prevent reflection of light at the surface of image display devices such as plasma display panels (PDP), cathode ray tubes (CRT) and LCD, and exhibits excellent scratch resistance. The film, used for preventing reflection of light has excellent scratch resistance and can be produced at a low cost. In a typical application, the film is formed by successively laminating a hard coat layer, a first high refractivity layer, a second high refractivity layer, and a low refractivity layer.

The water solubility and biodegradability of this material are the key properties for its usage. This has also attracted the recent attention of a packaging industry keen to address environmentally friendly solutions to the issue of plastic waste. This will be discussed in Section 5.

Other important areas where PVAL water solubility is used include textile sizing agents – the PVAL is applied to protect the textile fibres during production (such as knitting or weaving and so on), then once made the garment can be washed to remove the PVAL.

PVAL adhesives are used widely with cellulose materials as they give excellent adhesion to them. They are, therefore, used in a range of applications from postage stamps, labels, paper bags to book binding and make an ideal glue for papercraft activities.

Commercially, PVAL is the most important plastic for production of water soluble film. It is characterised by excellent film-forming, emulsifying, and adhesive properties. It has a high tensile strength, good flexibility, good oxygen barrier properties and good solvent resistance properties.

All the applications so far involve the use of the PVAL in solution as its thermal degradation is about 150 °C (crystalline melting point of PVAL ranges from 180 °C to 240 °C). Therefore until recently this limited its ability to be used in conventional thermoplastic processing equipment. However, when PVAL is plasticised, it is possible to avoid thermal dehydration. Melt processing applications are discussed in Section 8.

Therefore, PVAL finds use in variety of market sectors such as (a.3): s s s s s s s s

PVAL was investigated in the 1970s for use in controlled release of agrochemicals using combinations with pesticides. It is also used in current production of seed tapes. For these kind of applications the polymer must degrade without impact (be it chemical or biological) to the surrounding environment. Therefore, degradation and disposal issues will also be discussed later in the review.

PVAL is used in the manufacture of laundry bags and hamper liners for use in health care facilities. The filled bag is sealed shut with an attached adhesive strip. When placed in the washer, the adhesive and bag break down completely during the hot washing and disinfection. The bags are impervious to bacteria and viruses during normal use, as well as resistant to gases, solvents, and cool liquids, reducing the risk of contamination and thereby protecting hospital staff.

In food industries, PVAL is used as a binding and coating agent. The film coating is used in applications where moisture barrier protection properties are required. PVAL protects active components and other ingredients from oxygen, e.g., tablet coating formulations intended for products such as food supplement tablets.

Other more unusual products include the biodegradable PVAL pet waste bag for picking up pet waste and eco-disposal. It can be placed in the bin or flushed down the toilet. For lazy (or inept) golfers, there are water soluble golf balls made from PVAL. For golfers not willing to look for their balls, these can be abandoned on courses

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Figure 1 Molecule of PVAL (left) and chemical representations (right and centre) to biodegrade naturally. For those who wish to practise

their swings while at sea, they can be hit straight into the sea to dissolve.

This variety of sectors and applications areas are possible because poly(vinyl alcohol) has a number of useful and tailorable properties. It is odourless and non-toxic, it has excellent film forming, emulsifying, and adhesive properties. It also has good resistance to grease, oils and a large number of solvent materials. It has good mechanical performance with high tensile strength and flexibility, it also has high oxygen and aroma barrier properties. However, these properties are all dependent on the level of humidity. Water, which acts as a plasticiser, causes a reduction in tensile strength, but also increases the elongation and tear strength of the material.

The sheer range of applications for this material make it an interesting material of study. However, first a more fundamental introduction to PVAL manufacture and material properties will be given.

3. Manufacture

The first commercial production of PVAL was by the company Wacker (Germany) with the trademark Synthofil, which was used as medical sutures. In 1930, Japanese researchers Sakurada, Yazawa and Tomanari successfully wet spun PVAL. In 1950 this was realised commercially as Vinylon fibre (see Section 2). This fibre has high heat resistance amongst other interesting properties.

Unlike other members of the vinyl group, PVAL is not polymerised from a monomer in direct polymerisation, but prepared by hydrolysis of poly(vinyl acetate) (PVAc) in alcohol solution. This is a rubbery, synthetic polymer and partial or complete hydrolysis of the PVAc is used to prepare PVAL. The reaction proceeds via free radical mechanism or suspension polymerisation. Partial or complete hydrolysis removes acetate groups from the PVAc. The resultant material PVAL has the same degree of polymerisation as the original PVAc material. Therefore by varying the degree of polymerisation of the original PVAc a number of different grades of material with different properties can be produced. The reaction can be controlled to produce any degree of replacement of acetate groups. Other materials can be used to produce co-polymer materials which also replace the acetate group. Monomers such as ethylene and acrylate esters are commercially used in this way.

Once converted from PVAc the hydrolysed alcohol product is typically in the 87% to 99% range. If it is above 98% it can be considered to be fully hydrolysed with an expected crystallinity value of 40-50% Highly but not fully hydrolysed PVAL has a degree of crystallinity of 30-40% and a melting point of 225 °C. The reduced crystallinty in partially hydrolysed grades is due to the residual acetate groups. Other factors affecting crystallinity are the amount of plasticiser and water present in the compound. The production process used (acid or base catalysed) also has an effect. A lower crystallinity means lower strength but increased water solubility compared to fully hydrolysed (higher crystallinity) grades which can be considered as homopolymers as opposed to mixed vinyl acetate and vinyl alcohol co- polymer. Chemical representations commonly used to represent PVAL are shown in Figure 1.

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4. Polymer Chemistry

The physical characteristics and specific functional uses depend on the both the degree of polymerisation and hydrolysis produced. PVAL is classified into classes namely: partially hydrolysed and fully hydrolysed. PVAL is crystalline in nature, this is unusual since it is an atactic linear polymer, meaning the positioning of the side chains of secondary alcohol groups is random. This is generally accompanied by an inability to crystallise. In fact PVAc is not crystalline, but PVAL which contains smaller hydroxyl groups, is typically crystalline regardless of stereoregularity. This is because hydroxyl groups do not disrupt the crystalline lattice structure as their small size allows them to ‘slot’ in. However, in contrast, the presence of residual acetate groups greatly diminishes the crystal formation and the degree of hydrogen bonding because of their more bulky nature.

Therefore, polymers that are highly hydrolysed have a high tendency to crystallise and to undergo hydrogen bonding. As the degree of hydrolysis increases, the molecules will very readily crystallise, and hydrogen bonds will keep them associated if they are not fully dispersed prior to dissolution. At degrees of hydrolysis above 98%, manufacturers of PVAL advocate a minimum temperature of 96 °C to ensure that the highest molecular weight components have enough thermal energy to go into solution.

Increasing levels of hydrolysis leads to increasing

crystallinity

This also allows a level of control to be exerted over the crystallinity produced and the physical characteristics and specific functional uses of PVAL vary with this property. The crystallinity affects the water solubility, strength, gas permeability and thermal characteristics. Since the crystallinity is dependent on both the degree of hydrolysis and the average molecular weight of the polymer it also depends on the degree of polymerisation that occurs.

The level of hydrolysis will therefore have a direct effect on the properties and it is necessary to distinguish these materials into levels (partially, medium, fully hydrolysed). The actual degree of hydrolysis depends on how many percentage mol of residual acetate groups remain, as PVAL is characterised by the presence of the polar OH groups in the structure. For ISO 15023 (a.1), the level must be equal to or above 70%.

Because PVAL refers to the material that can be both fully and partly hydrolysed, it can actually be considered in partially hydrolysed cases to be a co-polymer of vinyl acetate and vinyl alcohol monomer (which does not actually exist). The monomer components may be distinguished if necessary using the convention V-AL and V-OAc.

Therefore PVAL is characterised by the presence of polar alcohol (OH-groups), and these groups give rise to reduced water solubility and also allow other reactions to take place.

Increasing hydrolysis leads to: s s s

s

In many cases, however, a partially or fully hydrolysed material may be used for the same application. For example in adhesives, where water resistance is required, a fully hydrolysed material may be used and where water resistance is not an issue, a partially or fully hydrolysed material may be used.

It is a water-soluble synthetic polymer that is it is made from petrochemicals. So although it is biodegradable, it is not a biopolymer. A biopolymer is a material made from a biomaterial such as plant starch. This distinction will be discussed in Section 10.

4.1 Viscosity in Solution

In water solution partially hydrolysed PVAL (88% and below) has a stable viscosity which does not change over time. However, the viscosity of a solution of highly hydrolysed material will show a gradual increase in viscosity over time, and if left long enough may even gel. This tendency to increase in viscosity over time, increases fairly linearly with concentration.

The films produced from partially hydrolysed solutions are characterised with poor water and heat resistance. Those from fully hydrolysed grades have the opposite properties being water and creep resistant properties. Through the use of additives it is possible to stabilise or destabilise those changes in viscosity which occur.

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5. Water Solubility

The main use of PVAL is when it is dissolved in water. Poly(vinyl alcohol) is among a group of water-soluble plastics that have a long history of use in niche applications. Many of these polymers are biologically stable when they are in the solid state but will biodegrade readily once they are dissolved. As PVAL is formed by hydrolysis of PVAc, by controlling the degree of hydrolysis, its solubility can be modified, which results in grades that will dissolve only in hot or cold water. Some grades can be mass produced by extrusion, injection moulding or film blowing (to produce potentially readily disposable components), but others must be cast from solution.

Single dose laundry blocks or liquids have become commonplace in many households. These are water soluble films used for the packaging (as you will find if you accidentally get them wet, they become very sticky.) Surprisingly the first use of water soluble packaging was in the 1960s, this was for carrying harmful chemicals in agriculture. It is relatively more recently that they have become such a commonplace item for household detergents and cleaning items. As already mentioned, the solubility depends on a number of factors such as the degree of polymerisation and degree of hydrolysis. The degree of hydrolysis is particularly important because of the inherent chemical structure. The many hydroxyl groups mean it has a high affinity for water and the hydrogen bonding between the hydroxyl groups impedes the overall solubility. In contrast, partly hydrolysed PVAL contains acetate groups. This acts in the opposite way to weaken the hydrogen bonding and increase solubility and there is a relationship between temperature of solubility and presence of acetate groups. Therefore, the behaviour of PVAL in solution is quite complex.

However, generally a partially hydrolysed version of the same material will be more soluble at a lower temperature with solubility increasing as hydrolysis levels increase.

Commercial water-soluble films can be tailored to dissolve in water at very specific temperatures. PVAL is not the only water soluble polymer on the market. Other water soluble polymers are shown in Table 2.

Table 2 Water-soluble polymers

Type Production Source

PVAL Synthetic* Non-renewable*

Ethylene vinyl alcohol (EVOH) Synthetic* Non-renewable* Cellulose acetate Natural Renewable (cellulose)

Chitosan Natural Renewable

(invertebrates) * made from fossil fuels

The term non-renewable source refers to the use of fossil fuels, which are finite in nature. PVAL and EVOH are highly unusual, synthetic crude oil-based polymers in that they dissolve in water. EVOH is most commonly used as an oxygen barrier layer in multi-layer film packaging (see Section 7.1) although it is expensive to use and the high price limits its applications elsewhere. PVAL is also the only polymer consisting of a carbon-carbon backbone that biodegrades. Polymers made from renewable resources include plant-based materials such as plant starch (which can be renewed by re-planting crops) and animals sources such as chitosan (found in insect exo-skeletons). Chitosan also has good gas barrier properties and is used as a coating for other biodegradable polymers and edible coatings. Both cellulose acetate and chitosan are water-soluble. The water solubility of polymer materials means that they can end up as pollutants in water ecosystems (river, ocean) and affect aquatic life if they are not monitored and removed. Removal is done by microorgasms for PVAL and other similar potentially water soluable pollutants include polyacylics, polycarboxylates, polyethers and polyglutamic. For PVAL, research as far back as 1936 (a.6) found that PVAL in waste water was biodegraded by a phytopathogenic organism of fungal origin, Fusarium lini. The by-products being water and carbon dioxide. Since then many studies have been carried out on PVAL degradation in both aerobic conditions and soil samples with satisfactory results. The actual biodegradation mechanisms are beyond the scope of this review however the reader is referred to a survey carried out by Chiellini and co-workers (a.7) in 2003 for an overview of this subject.

The solubility of PVAL makes a big difference to the final application. For example consider three alternative PVAL materials that can be considered as very soluble, moderately soluble and slightly

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soluble. A guide to potential applications based on this property is shown in Table 3.

Table 3 Solubility as application guide

Very soluble Moderately

soluble Slightly soluble Pharmaceutical capsules which dissolve to release drugs Loose fill packaging Food packaging

Medical sutures Agricultural films

Diaper films Detergent single

dose blocks

Single use food service items

Medical containers Life span short Life span:

medium

Life span: high – must survive until disposal

As we have seen, the solubility of PVAL depends on both the molecular weight and the level of hydrolysis. To add to this, PVAL that is partly hydrolysed is less dependent on temperature to be soluble. For highly hydrolysed grades however, it may be necessary for solutions to reach temperatures just under 100 °C before dissolution occurs. For disposal of items such as food packaging, diaper films and medical containers, left to degrade in the environment, it is unlikely that they would ever reach high enough temperatures to dissolve in rain water for example. It is therefore, necessary to consider whether degradation by environmental conditions or degradation by dissolving into water is the required outcome. Methods of degradation will be covered later in Section 14.

Partially hydrolysed PVAL is easier to dissolve than fully hydrolysed PVAL

6. PVAL Film Properties

PVAL is the most commercially important water soluble film, therefore, it is worth taking a look at the major properties of these materials.

Given their water soluble nature it is probably not surprising that the first consideration is that the resultant mechanical properties depend on humidity. High humidity produces a film which is soft and tough. Low humidity gives a hard and brittle material. These depend initially on the degree of polymerisation since tensile strength and elongation increase with degree of polymerisation

and partly hydrolysed materials are weaker than fully hydrolysed materials.

A higher degree of polymerisation gives increasing strength, but humidity has the effect of a plasticiser,

swelling the structure to make it softer and tough.

6.1 Barrier Proporties

PVAL materials have excellent oxygen barrier properties (and also act as a barrier to carbon dioxide and nitrogen). These values are comparable to the performance of EVOH and much better than the common commodity packaging materials listed in Table 4.

Table 4 Oxygen permeability of

common materials and PVAL

(30 μm film, 50% humidity)

Material Oxygen Permeability

(ml/m2/day)

Hot water soluble PVOH 0.24

Warm water soluble PVOH 0.36

Cold water soluble PVOH 1.85

EVOH 0.29-2.4 Nylon 6 26-38 PET 40-80 PVC 50-390 High-density polyethylene (HDPE) 1700-2400 PP 2,000-10,000 Low-density polyethylene (LDPE) 12,000

An increase in dissolution temperature (and crystallinity) improves the performance of the oxygen barrier. However, the downside is the affinity to water. Water vapour will permeate through the films and therefore PVAL is a poor barrier to water vapour. Interestingly this is the opposite of polyethylene which allows oxygen to pass through its structure but not water vapour. Wet PVAL film has little strength but a dry film has good tensile properties.

PVAL has found application as a barrier layer in flexible packaging but not in rigid packaging (such as bottles) where EVOH and PA are utilised. Whilst aluminium of micrometer thickness is traditionally used in flexible packaging, creating a virtually perfect barrier, using plastics by lamination or co-extrusion gives high (but not perfect) barrier performance but at a better cost

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performance. Other flexible packaging barrier materials used in this way include poly(vinylidene chloride) (PVDC), EVOH or PA.

However, it needs to be considered that PVAL, EVOH and PA are only effective barriers in a dry state. Therefore, sandwich constructions in which they are protected by materials with water barrier properties are necessary to maintain the oxygen barrier properties of these materials.

6.2 Other Film Properties

6.2.1 Mechanical Properties

Degree of polymerisation affects both the tensile strength and elongation film properties, so that as the degree of polymerisation increases so does the strength. The tensile strength also increases with a rise in the degree of hydrolysis.

However, as well as these factors, the humidity also has a marked effect on physical properties.

At low humidities, PVAL film is hard and brittle, whilst at high humidity it is soft and flexible due to the plasticising effect of the water vapour. This dependence is greater for partially hydrolysed grades than for those that are fully hydrolysed.

Humidity leads to increased flexibility and softness.

6.2.2 Plasticiser Effects

Water acts as a plasticiser for PVAL whether by the action of humidity or as a solution. At low humidities the same effect can be achieved by glycols which serve to prevent the PVAL film from becoming hard and brittle. For thermal processing techniques such as injection moulding it is necessary to add plasticiser to prevent premature degradation. These will be discussed further in the thermal processing section.

6.2.3 Oil and Solvent Resistance

PVAL is highly resistant to oils (animal, vegetable or mineral origin) and organic solvents (aromatic and aliphatic hydrocarbons, esters, ethers and ketones). This resistance like many of the other properties of PVAL is

affected by the degree of polymerisation and the degree of hydrolysis.

6.2.4 Water Sensitivity and Hygroscopy

PVAL films will swell and take up water if immersed. Partially hydrolysed grades have higher levels of hygroscopy and solubility to water vapour than fully hydrolysed grades.

These factors and the rate of swelling can all be decreased by heat treatment which increases the crystallinity of the film.

6.2.5 Adhesion Characteristics

As the degree of hydrolysis increases there is a change in the relative number of acetate and hydroxyl groups present. This in turn creates changes in the adhesion character of the film.

When used as an adhesive agent, low degrees of hydrolysis give adhesion to hydrophobic surfaces, higher degrees of hydrolysis lead to adhesion to hydrophilic (strong affinity for water) surfaces.

6.2.6 Antistatic

PVAL is resistant to static build up and this property can be useful in packaging applications where atmospheric dust is present.

7. Solution Processing

The spinning of Vinylon fibres from solution, primarily of interest only to the Japanese market has already been discussed. Another process using PVAL in solution to spin fibres is electrospinning. In this case very thin, nano-sized fibres can be produced, allowing the resultant fibres to be used in a range of medical and filtration applications. The major drawback with electrospinning is the volumes that can be produced. If large volume equipment comes onto the market, this type of manufacture could be applicable to more than just niche markets.

Since many applications described here are carried out in solution, it is worth considering how these

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solutions are made. These methods can also be used to differentiate between a fully hydrolysed and partially hydrolysed grade.

It has been shown that the solubility is dependent on two factors, degree of polymerisation and hydrolysis. Fully hydrolysed grades are not really soluble at room temperature so a stirred water/PVAL mixture will need to be heated to nearly boiling to ensure full dissolution.

For partially hydrolysed grades, the plasticiser effect of the water and solubility means the solute (PVAL in this case) tends to form lumps if prepared in the same way as fully hydrolysed grades, therefore in this case, the PVAL should be added slowly to the solution whilst stirring. The temperature can be raised if necessary to speed this process up.

Whilst solution concentrations obviously vary from application to application, a 10% solution is a very common dilution rate for PVAL and can therefore be considered as an indicator of the levels of PVAL used in solution for the purposes here.

8. Melt Processing

PVAL has a decomposition temperature of 180 °C. However it does not melt below 180 °C, and has a melting point range of between 180 and 240 °C. Some properties are shown in Table 5. Therefore thermal processing of PVAL presented a considerable challenge worldwide.

Table 5 Properties of PVAL

Property Typical value

Density 1.19-1.31 g/cm³

Melting point 180-240 °C (dependent

on degree of hydrolysis)

Boiling point 228 °C

Degradation temperature (unplasticised)

180 °C

Unplasticised PVAL degrades at temperatures above 180 °C due to water elimination from the chemical structure. Further degradation can cause discoloration and crosslinking to occur as the hydroxyl and acetate side groups are eliminated. Whilst the melting point of PVAL can vary between 180-240 °C depending on the level of hydrolysis, the effect of the relatively

low degradation reaction temperature means that melt processing applications a limited to only materials with low hydrolysis levels or materials that a heavily plasticised (which generally meant sacrificing the useful properties inherent in the PVAL materials in the first place).

Therefore, until the last decade, the unwelcome onset of thermal and shear degradation during melt processing of PVAL limited the use of conventional thermoplastic melt technology. Application of PVAL was limited to aqueous solutions and cast films. However, advances in PVAL polymer formulation mean that the properties of PVAL can now be retained during thermoplastics processing and it is possible to produce water-soluble, non-toxic, biodegradable plastics by conventional thermoplastics forming techniques. A brief description of some of these techniques follows.

Extrusion

Process for producing either pellets for other heat forming processes such as injection moulding, sheets (which can also be used for thermoforming) or profiles such as tubes, pipes and rods.

Film blowing

Process for producing thin films products such as shrink wraps and carrier bag film.

Injection Moulding

High shear and pressure process for production of complex and mass produced components such as mobile phone covers, lids, automotive door handles, motorcycle helmets.

Thermoforming

Low pressure process which deforms sheet material into relatively simple shapes such as trays, cups and shallow tubs.

Blow Moulding

Inflation production process which produces hollow articles such as bottles, containers and tanks.

Melt spinning

Production of thin fibre strand produced from molten plastic passed through a multi-holed profile. (This is not to be confused with spinning from a solution, as in the production of Vinylon fibre or electrospinning which produces PVAL nano-fibres and is also a solution process.)

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PVAL melt processing temperatures tend to range between 180 and 240 °C. This depends on the level of hydrolysis and specific formulation and additives used. Generally fully hydrolysed grades, which have higher melting points, require higher temperatures to be used. They also tend to be harder to process. To improve the processibility of PVAL it is necessary to use a plasticiser and lubricants which lower melt temperature and viscosity and therefore allow lower processing temperatures to be utilised. The degree of degradation is strongly influenced by acetate groups which liberate acetic acid during processing which catalyses further degradation to occur. Higher levels of hydrolysis 98 mol% are more resistant to degradation that 88 mol%. Temperature rather than shear appears to be a dominant factor in this.

In such applications, the basic PVAL must be plasticised so as to reduce its melting point, and obtain a thermal processing window for processing. In general plasticisers, such as glycerol, polyethylene glycol, sorbitol and other compounds have been used.

Mowiflex TC is an example of a thermoplastic melt processable material currently sold by Kuraray. This can be used to produce blown and sheet film as well as injection moulded components. As expected of PVAL, it is moisture sensitive and sold in moisture resistant bags, however, it can be dried if needed at 60-80 °C for 6-8 hours using conventional air drying equipment. Therefore, it can be treated like other moisture sensitive materials such as Nylon routinely used in processing.

8.1 Extrusion

Tubing and hoses produced by extrusion retain many of the properties recognisable from solution processing. They are highly flexible and retain excellent solvent resistance to oils, greases and other chemicals. Processing is carried out on standard extruders with low residence times. Keeping residence times as low as possible is one key factor in successful processing of PVAL materials. Thermal damage produces discoloration of PVAL products, as it does with many other thermoplastic components.

8.2 Injection Moulding

For injection moulding of PVAL, recommended mould tool temperatures are in the 70-80 °C range with melt temperatures ranging from 190-220 °C. This is similar

to parameters used to process polyethylenes. This can be performed on conventional equipment using hot or cold runners to produce a variety of moulded components. With the fast cycling time possible with injection moulding it is possible to mass produce a variety of components and products very rapidly and economically.

An example component is exhibited in the Science Museum, London, UK. This is a biodegradable mobile phone casing developed by researchers at the University of Warwick's, Warwick Manufacturing Group, in conjunction with PVAXX Research & Development Ltd. A water soluble mobile phone case was the component. However, this idea had a twist. By imagining the components end-of-life and burying the case after use with a sunflower seed beneath it, the phone biodegraded and the sunflower seed began to grow and eventually flowered. This was timed with the growing awareness of the problem of plastic waste and sustainability, as well as the newly introduced European Union WEEE Directive (waste electrical and electronic equipment) (a.8) which covered the ever growing problem of electrical waste. Therefore, discarded mobile phones were a hot topic and this application captured public attention. However, problems of water proofing the product for use (in rain or wet environments for instance), mean as yet this application is not in commercial production.

8.3 Film Moulding

An extrudable PVAL material for blown film application was marketed by A. Schulman, Akron, USA under the name Aquasol. It is reported to have the same physical properties as the cast film and could be used in conventional polyolefin melt equipment. The target market to be biodegradable replacement for LDPE (a.9). Most of the market in these film applications is for disposable packaging.

Table 6 shows a typical range of properties shown by

various grades of PVAL in comparison to other film materials. Most marked here is the tear strength and tensile strength which is actually comparable to that produced by a polypropylene material. PVAL materials also show good puncture resistance and impact strength. However performance is markedly affected by the degree of hydrolysis and therefore the crystallinity of the base material as well as the molecular weight, distribution and branch structure.

Figure

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References

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