Thermoplastic and biodegradable polymers of cellulose

PII:

so141-3910(97)00151-1

Polymer Degradation and Stobikry 59 (1998) 107-I 15 0 1998 Elsevier Science Limited. All rights reserved Printed in Northern Ireland 0141-3910/98/$19.00

ELSEVIER

Thermoplastic and biodegradable polymers of cellulose J. Simon,“* H. P. Miller,” R. Koch” & V. Miiller* aZentrale Forschung, ZF-MFF, Q 18, Bayer AG, D-51368 Leverkusen, Germany b Wolff Walsrode AG, D-29655 Walsrode. Germany

(Accepted 12 July 1997)

Polymers which are biodegradable currently achieve high interest in materials science since they offer reductions of landfill space during waste management as well as new end-user benefits in various fields of applications. Among these materials, those from renewable resources such as polysaccharides additionally offer COz-neutrality, partial independence from petrochemistry-based products and the exploitation of natures synthesis capabilities via photosynthesis. Cellulose, being a constituent of wood, is regenerated in much larger quantities than starch by natural photosynthesis from COz and water. The very substantial, but so far little exploited category of cellulose-based materials, which has lead to some of the very first industrial polymer-products such as celluloid and cellophane still offers numerous new possibilities for polymeric materials. Basically two main groups of cellulose-materials can be distinguished: regenerated celluloses are suitable only for fibre and film production from conventional and new processes. Secondly, thermoplastically processable cellulose derivatives such as esters can be used for extrusion and moulding. Based on general considerations on the correlation between biodegradability and molecular structure, cellulose derivatives allow both thermoplastic processing and post-consumer waste management via biological decomposition. Ways to realise this demanding new mix of properties considering biodegradability, thermoplastic behaviour and material-properties as well as possible synthetic strategies and their realisation are presented. 0 1998 Elsevier Science Limited. All rights reserved

1 INTRODUCTION

biodegraded. Unfortunately, economic and environmental aspects of production and processing have so far militated against the widespread use of these renewable raw materials. The following gives a historic introduction and shows in greater detail that, already in the past, this has led to interesting products being squeezed out of the market (Fig. 1). Starch is a prime attractive renewable raw material and has reached some significance in biodegradable materials. On a much larger scale than starch, however, cellulose as a component of wood is regenerated by natural photosynthesis from CO2 and water. The total biomass produced on earth is approx. 200 billion tonnes, of which only approximately 6 billion tonnes are used. Cereal/oil, sugar and wood each account for a third of this quantity. A large part of the wood is used as biodegradable materials in its original form as firewood, wood for

There are two main reasons for the interest in biodegradable materials: l

l

the growing problem of waste and the resulting general shortage of landfill availability; and the need for the environmentally responsible use of resources together with the CO2 neutrality aspect.

The second point in particular is closely linked with the significant but to date under-exploited potential of natural molecular synthesis producing rapidly renewable raw materials which can also be *To whom correspondence 61598.

should be addressed. Fax: 0214 30 107

108

J. Simon

chemistry cellulose

Fig. 1. Cellulose:

a renewable

cellulose from wood

resource.

construction and furniture manufacture. A smaller proportion of the wood is processed and supplied as pulp to the chemical and paper industries.’ Conversion of the pulp to fibre and film using regeneration processes does not significantly affect the biodegradability. The same applies basically to pulp refining products used in the paper industry. In essence, however, refining of pulp and its derivatisation means a change in the molecular structure and thus the biodegradability. Cellulose products have a long industrial history and at Bayer’s subsidiary, Wolff Walsrode AG, cellulose nitrate has been produced since 1878. Celluloid was the first thermoplastic polymer material and has developed a number of other trends and applications (Fig. 2).2 Consumer packaging technology were later revolutionised

Nitro-cellulose Collodium

Fig. 2. The first thermoplastic

et al by ‘Cellophanec@’ or ‘TranspariP, the first transparent packaging materials (Fig. 3). The rigid chain structure of cellulose has been utilised since 1920 to produce this dimensionally stable, water vapourpermeable, biodegradable regenerated cellulose with the requisite rigidity. If one ignores the economic problems and those inherent in the production of cellophane, it is clear that, from today’s standpoint, the material properties of ‘Cellophane@’ would make it a highly attractive biodegradable material. The thermoplastic cellulose mixed esters (‘Cellidot-@‘) were discovered by Schutzenberger in 1865 and developed to technical maturity by Bayer AG between 1946 and 1956.3 Virtually no other moulding compound exhibits such interesting property combinations with such an enormous variety of excellent material properties such as high impact strength, outstanding transparency and good insulation properties, and even today, it would be an extremely interesting materialalthough not very economical-from the point of view of renewable raw materials. The beginning of the 1970s saw cellophane being replaced by bulk plastics, primarily for economic and packaging reasons. The market decline for regenerated cellulose film, linked with strong competition and thus a drop in price, together with the high outlay for meeting environmental regulations, no longer permitted economic production of ‘Transparit@’ (Wolff Walsrode) or ‘Cellophan@’

Camphor softener

polymer

material:

nitrocellulose-based

‘Celluloid’.

109

Thermoplastic and biodegradable polymers of cellulose

Properties: -

transparency and gloss stiffness dimensional stability high water vapour permeation low tear-propagation resistance

Fig. 3. Properties

of regenerated

(Kalle, Wiesbaden) for established applications. In the light of this situation, an attempt was made by Bayer to shift the properties of cellophane towards polyolefin plastics-flexible, tear-resistant, waterresistant and machine-friendly but without losing the advantage of biodegradability. By modifying cellophane with an aliphatic polyether urethane as the polymer blend component, it was possible to produce a fully biodegradable film comparable to Polyethylene for commercial-scale use in the hygiene and packaging sectors. Degradation of this product ‘Cellblend’ was comparable under compost conditions to the biodegradation of cellophane. The state after 3, 7 and 10 days in the

0

days

cellulose:

‘Transparit’

and ‘Cellophan’

compost is shown in Fig. 4. But since a price of approximately 15 DM kg-’ for a biodegradable film was not negotiable on the market until 1992, when the waste disposal advantage became a factor, the production of ‘Transparit@’ was terminated at Wolff Walsrode AG and thus ‘Cellblend’ development was not pursued. Work on the subject of biodegradable materials has shown, however, that there is a need for such materials if certain requirements and conditions are met, these being:

3 days

l

a clearly defined regulation of material utilisation, as for example implemented in the meantime in DIN 41900, LAGA M 10;

7 days

10 days

appearance after degradation in compost Fig. 4. Biodegradation

of Transparit-Polyurethane

blend (‘Cellblend’)

in compost.

110

J. Simon et al.

0

nation-wide biowaste collection and composting; 0 acceptance of biodegradable materials in biological waste bins (Waste Management Legislation (German Verpackungsverordnung)).

On the assumption that these conditions were met, development work got under way with the following aims: The development of compostable, extrudable cellulose derivatives which: 0

0

0 0 0

0

could be produced with established cellulose chemistry; could be processed on conventional plant by for example extrusion, injection moulding or blow moulding; had technically utilisable properties; were moisture-resistant; and potentially suitable for food and drug applications; exhibit a cost/performance ratio (including disposal costs) comparable to standard plastics.

1.1 Structure and degradability The base molecule cellulose is structured on B- 1,4glycoside-linked glucose units. The good mechanical properties which make cellulose suitable as a building block in plants derive from the rigidity of the cellulose chains on the basis of inter- and intramolecular hydrogen bonds between the free hydroxyl groups in positions 2, 3 and 6 of the glucose repeat units. The superstructures produced are so stable that they cannot be broken by increasing the temperature. The cellulose degrades below the theoretical melting point (Fig. 5). To obtain a melt processable,

fusible derivative, it is therefore necessary to break these hydrogen bonds. A drop in the melting point below the degradation temperature is achieved by derivatising the hydroxyl groups. Examples of this effect are industrially used polymeric materials such as cellulose acetate, cellulose acetobutyrate (Cellidor@), benzyl cellulose and ethyl cellulose. All these derivatives require virtually complete derivatisation of the OH groups to achieve thermoplasticity. The number of substituted OH groups per anhydroglucqse unit is referred to as the average degree of substitution or DS value. As the level of substitution rises, however, the biodegradability is increasingly reduced because obviously a certain number of non-substituted glucose units must be present for the sterically demanding enzyme attack. 4-6 Thermoplasticity and biodegradability are thus incompatible properties in conventional derivatives (Fig. 6). 1.2 Synthesis strategy and conversion of cellulose Three synthesis strategies were feasible: classical cellulose derivatives with a DS which is adequate for thermoplasticity but whose biodegradability is not too greatly reduced (compromise solution); introduction of substituents forming longer chains and in this way spatially separating the cellulose chains from one another (with lower DS values). The side chains should also be biodegradable; introduction of a small number of substituents with a high volume ratio. Since the first strategy, on the basis of knowledge gained from existing derivatives, could only be

molecular structure

fiber structure Fig. 5. Structure of cellulose raw material.

Thermoplastic and biodegradable polymers of cellulose

111

max.

biodegradation

degree of substitution Fig. 6. Biodegradation

and therrnoplasticity

successful in a very narrow DS range, the number of possibilities available to influence the material properties by the type and level of derivatisation was extremely restricted. Attention therefore turned to the second route. This involved finding a reagent which: could react with cellulose in technically utilisable reactions; was available on an industrial scale; was suitable for chain formation; and was biodegradable both as a monomer and an oligomer.

of substituted

celluloses.

before the main chain, so that products with a DS above 1 were not, as desired, further degraded by cellulase. The synthesis strategy was modified on the basis of these results. Using low-substituted cellulose hydroxyalkyl ethers instead of unmodified cellulose,

cellulose lacton m

In a screening, lactones, especially e-caprolactone, proved to be good reagents which largely meet these requirements. As cyclic esters of hydroxycarboxylic acids, lactones were capable of forming ester side chains on the cellulose backbone (Fig. 7). The biodegradability of poly-hydroxyhexanoic acid esters (poly-&-caprolactone) is known and so the biodegradability of the side chains is apparent. Reaction conditions were developed which allowed cellulose hydroxyhexanoic acid esters with different degrees of substitution to be produced. Examination of these produced the following results (Fig. 8) As the degree of substitution increased, the softening point dropped as expected. The enzyme degradability of the main chain due to cellulases decreased as the DS increased and with a DS of approximately 1, came virtually to a standstill. The side chains were degraded by esterases. Degradation stopped one to two ester units

cellulose-ester Fig. 7. Esterification

of cellulose with lactones.

molar degree of substitution (MS) Fig. 8. Degradation of cellulose-hydroxyhexanoic by cellulases and esterases.

acid esters

J. Simon

112

an ethylene- or propyleneoxide spacer was inserted between the ester side chains and the main chain. This spacer was intended to completely separate the ester units and provide a sterically preferred link point for the lactone units (Fig. 9). A low total DS achieved in this way with fully separable long side chains should provide broad scope for product modifications whilst maintaining biodegradability and thermoplasticity. A positive concurrent effect of this synthesis strategy was the higher graft yield based on the improved accessibility and associated higher reactivity of the cellulose ether. This and the level of hydroxyalkyl units led to a reduction in raw material costs by ‘diluting’ the high costs for a-caprolactone. This synthesis concept led to a series of cellulose ether esters which were correspondingly synthesised by reacting low-substituted biodegradable hydroxyethyl, hydroxypropyl or even methyl cellulose with &-caprolactone and alkali catalysis. It proved useful to carry out the reaction in swelling, relatively dipolar aprotic media such as dimethylsulfoxide, dimethylformamide, dimethylacetamide or even dioxane. As derivatisation continues, the reaction formulation passes from the swollen heterogeneous state to a homogeneous solution. The product can be isolated by precipitating or evaporating the solvent. The ring-opening reaction with the &-caprolactone can however, in principle, produce two different products. 0-acyl splitting of the cyclic ester gives OH terminated oligo ester side chains which are connected to the backbone molecule by an ester bond. Carboxylate-terminated oligo side chains linked by ether bonds are obtained by 0-alkyl splitting. Synthesis can be guided in one

et al. or other direction by the reaction conditions used (Fig. lO).One series each of carboxyl and hydroxyl group terminated products based on Hydroxyethyl-cellulose were produced with different quantities of grafted &-caprolactone and tested for their degradation performance in an enzyme test using cellulases and esterases. The result is shown in Fig. 11. Compared with the hydroxyl-terminated products, the carboxyl-terminated products showed a significant biodegradability towards enzyme attack up to considerably higher degrees of substitution. On the basis of this result and the need to reduce material costs, the third route was ultimately pursued (Fig. 12). To introduce a small quantity of bulky substituents exhibiting carboxyl termination, the proven low-substituted biodegradable alkyl or hydroxyalkyl cellulose were again esterified with different dicarboxylic acid substituents. The ether groups divided at the cellulose chains clearly facilitated the introduction of the dicarboxylic acid substituents by means of the corresponding dicarboxylic anhydrides. At the same level of substitution, this

m

+0 O\\ 0

cellulose

lacton

acyl-cleavage

alkyl-cleavage

s

o(~o)L,, \

cellulose-ester

1

cellulose-ether

Fig. 10. Acyl-cleavage and alkyl-cleavage of lactones to cellulose-ethers and -esters.

cellulose

oxiran

leading

la&on

cellulose-ether-ester Fig. 9. Introduction of a oxiran-spacer: cellulose-ether-esters from lactones and alkyleneoxides.

molar degree of substitution

Fig.

11. Biodegradation of cellulose-ethers cellulases and esterases.

(MS) and

-esters

by

113

Thermoplastic and biodegradable polymers of cellulose

cellulose -derivative

di-carboxylic acid anhydride

’

cellulose-ester

IIEE-

Methylcellulose Methylcellulosepbthalat ~;;;(ellulosc

r,,a

I

1

1

,

cam 3 ‘0

10 20 m

II

yi

release of glucose @g/ml enzymatic testing

degree of substitution

Fig. 13. Biodegradation

of cellulose-ether-phthalates lases and esterases.

Fig. 12. Cellulose-dicarboxylic and thermoplasticity

acid esters: Biodegradation as a function of structure.

reaction yielded cellulose derivatives in descending order of biodegradability: maleic, cyclohexanoic dicarboxylic acid, tetrahydrophthalic and phthalic anhydride. Substitution with Maleic anhydride in particular gave, (not simultaneously), thermoplastic and biodegradable products. Phthalic anhydride in particular has technical and economic potential. It yields products based on cellulose and cellulose derivatives such as hydroxypropyl, hydroxyethyl and methyl cellulose which exhibit the required biodegradability with simultaneous extrudability. The phthalic acid semi-ester of cellulose is thereby split under compost conditions by means of abiotic hydrolysis. Due to the electronic and steric conditions, this effect is understandable and clearly more distinctive than with similar non-aromatic products. Higher substitution of the cellulose backbone clearly above a DS of 1.0 is possible as a result of the potential for abiotic hydrolysis without the biodegradability of the product being lost. For example, this is illustrated on methyl cellulose phthalates in which the phthalic acid substituents were linked more directly to the anhydroglucose unit in comparison to underived cellulose (Fig. 13). Whilst the free carboxyl unit proved good for biodegradation and abiotic degradation, this is not acceptable for the pH stability and water resistance or for the processing stability of these products. The present underived hydroxyl groups of the base molecule can form inter- and intramolecular crosslinking with the free carboxyl group of the relatively acid phthalic acid semi-ester with formation of ester bonds. This is a problem particularly if there are irregular dwell times in the processing machines because both the product properties and the melt viscosity

so 70 II)

by cellu-

during processing are significantly influenced by this effect. One possibility for eliminating crosslinking reactions is to neutralise the free carboxyl groups. These are temperature-stable and exhibit consistent mechanical properties over the whole range of normal service temperatures (Fig. 14). The disadvantage for many applications, however, is that these products, depending on the degree of neutralisation of the free carboxyl groups, can swell with water or are even soluble. Another route was therefore taken to eliminate this problem (Fig. 15). Some of the free carboxyl groups were for example esterified with alkylene oxides. This blocking led to a similar reduction in the cross-linking trend such as neutralisation. The softening point was further reduced compared with the postesterified products. Biodegradability, however, was also slowed down so that this reaction stage limits the extent to which the degrees of substitution could be varied without reducing biodegradability. Degradation under compost conditions in lE+lO

lE+C+

T %

lE+O8

w * .w .

lE+O7 i; +

lE+06

1

lE+05 -100

-50

0

50

loo

150

ml

250

300

temperature [” C] Fig. 14. E-Modulus

and storage propylcellulose-phthalates

modulus of Hydroxy(sodium salt).

J. Simon et al.

114

hydroxypropylcellulose phthalate

hydroxypropylcellulose-phthalate esterified with propylene oxide

Fig. 15. Synthesis of hydroxypropylcellulose-phthalates

the Controlled Cornposting Test (CCT) is shown in Fig. 16. With these products there was a noticeable induction phase of about 15 days before CO*

120..

HPCP-NP salt

IW-

HPCP-PO ester

20

40

esterified with propylene oxide.

started to be evolved. Since this phase did not occur with completely neutralised cellulose phthalates, the cause was presumably the local shift in the pH due to abiotically, hydrolytically liberated phthalic acid. Typical mechanical properties of such materials are given in Fig. 17. The base material was a typical cellulose product with a relatively high rigidity. Using different biodegradable plasticisers made it possible to broadly influence the mechanical profile.

2 PERSPECTIVES

80

time [days] Fig. 16. C02-production from biodegradation of celluloseether-phthalates in the ‘controlled composting test’.

The results and experience available to date show even further potential for biodegradable materials based on the renewable raw material cellulose. Material development in this group of products

Hydroxypropyl-Hydroxypropylcellulosecellulosephthalate phthalate PO-ester

Benzyl- Cellulosecellulose acetate 2.0 2.2

E-modulus [MPa] elongation at break [%]

yield strength [MPaJ

Fig. 17. Mechanical properties of cellulose-derivatives.

Polystyrene

Thermoplastic and biodegradable polymers of cellulose

is therefore currently progressing intensively. Questions regarding the optimisation of synthesis for the systems presented are therefore being tackled, together with modifications to the mechanical properties, particularly with a view to obtaining more flexible materials, suitable low- and highmolecular modifiers and the connection between structure and degradability of cellulose derivatives. The potential scope for application of these products is also actively being investigated.

ACKNOWLEDGEMENTS

We would like to thank our research partners in this project for their help and cooperation, and the Federal Ministry for Food, Agriculture and

115

Forestry and the Fachagentur Nachwachsende Rohstoffe (Agency for Renewable Raw Materials) for their financial support. REFERENCES Nevell, T. P. and Zeronian, S. H. (ed.), Cellulose Chemistry and its Applicntions. Ellis Hotwood, Chichester, 1985. Brydson, J. A., Plastic Materials, 5th edn. Buttetworths, London, 1989, pp. 212-293. Miiller, F. and Leuschke, Ch., in KunststofiHandbuch Vol. 3/l, ed. Becker, Braun. Carl Hanser, Munich, 1992, p. 396. 4. Walker, L. P. and Wilson, D. C. et al., Biotechnol. and Bioeng., 1992,40, 1019-1026. 5. Potts, J. E., Plastics, environmentally degradable. KirkOthmer Encyclopedia of Chemical Technology, 3rd edn. Suppl. Vol., 1984, pp. 626668. 6. Brodmann, B. W. and Devine, M. P., J. of Appl. Pal. Sci., 1981, 26,997-1000.