A Review of Synthetic Approaches to Biodegradable Polymeric

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A Review of Synthetic Approaches to Biodegradable Polymeric Carboxylic Acids for Detergent Applications Y. H. Paik, E. S. Simon, and G. Swift Rohm and Haas Company, 727 Norristown Road, Spring House, PA 19477

Polymeric carboxylic acids such as poly(acrylic acid) and copoly (acrylic-maleic acid)s are widely used as cobuilders in detergent powder formulations. They were introduced in the early 1980s in combination with zeolites as partial replacements for polyphosphates, which were removed from detergent formulations because wastewater treatment plants did not remove them from wastewater. The polyphosphates promoted the eutrophication of rivers and lakes and thereby promoted environmental imbalance and the death of certain aquatic lifeforms. Although the nonbiodegradability of the currently used polymeric carboxylic acids eliminates the possibility of eutrophication of the aqueous environment, it results in an increasing accumulation of these acids in the environment. No known toxicity is associated with these polymers, but because negative results are always accompanied by uncertainty, questions as to the fate of the acids and their effects in the environment remain. Therefore the prudent option is to develop biodegradable replacements for the current products, and this goal has been recognized, desirable, and difficult almost since the time polymers were introduced. In this review chapter we trace the major synthetic approaches that have been tried and have failed to meet the requirements for performance and biodegradation, reach some conclusions on the current status of re-

0065-2393/96/0248-0079$12.00/0 © 1996 American Chemical Society

In Hydrophilic Polymers; Glass, J.; Advances in Chemistry; American Chemical Society: Washington, DC, 1996.

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HYDROPHILIC POLYMERS


search, and suggest what may be the most fruitful future directions for research in the quest for biodegradable detergent polymers.

TTHIS CHAPTER REVIEWS SYNTHETIC APPROACHES TO DEVELOPING biodegradable polymeric carboxylic acids (PCAs) for use in detergents, assesses the current status of these approaches, and considers the implications for future research. In order to present a comprehensive review that is easily readable by those not overly familiar with the field of biodegradable PCAs and detergents, this chapter is divided into several sections. These sections cover the background on the need for biodegradable detergent polymers, a perspective on currently used commercial polymers, some important considerations in the development of biodegradable detergent polymers, approaches that have been tried, conclusions, and suggestions for future research directions. PCAs used in detergents represent a large percentage of the volume of all such polymers. PCAs are used in water treatment and as pigment dispersants, oil field applications, thickeners, etc. Other polymers containing water-solubilizing functionalities such as hydroxyl, amino, and ether groups are also widely used in a variety of applications, and ultimately all will probably be expected either to be removed from effluent streams or to be biodegradable; otherwise their concentration in the aqueous environment will continue to increase. Hence the continuing environmental responsibility shown by the detergent industry over many years will eventually have to be accepted by all other industries using water-soluble polymers. For the record, the history of the detergent industry's effort in environmental protection is summarized in Table I.

Background The need for biodegradable detergent polymers in particular and all water-soluble polymers in general is a result of the difficulty in reTable I. Summary of Environmental Responsibility of the Detergent Industry Period

Action by Detergent Industry

1960s

Branched-chain surfactants replaced because of nonbiodegradability Eutrophication problem of polyphosphates recognized Polyphosphates replaced by PCAs PCAs reevaluated

1970s 1980s 1990s


5.

PAIK ET AL. A Review of Synthetic Approaches

Surface water/estuaries/oceans / Use -> Disposal -> Sewage treatment

81 Sediments

\ Sludge

Landfill/compost/incineration


Figure 1. Environmental exposure to water-soluble polymers. c o v e r i n g t h e m after use a n d the fact that they are d i s p o s e d o f either directly into the environment or into p u b l i c and/or industrial sewage treatment plants. T h e polymers are almost always i n very l o w concentration aqueous solutions (in the range o f a f e w parts p e r m i l l i o n ) i n wastewater streams, a n d unless they are biodegradable or are trapped b y adsorption o n sewage sludge, they pass through treatment plants, permeate the aqueous a n d sedimentous environments, a n d s l o w l y i n crease i n concentration. Potential routes into the environment for a nonbiodegradable water-soluble p o l y m e r are s h o w n schematically i n F i g u r e 1. T h e key objectives i n the design of biodegradable detergent P C A s , therefore, are to have them biodegrade i n the sewage treatment plant and thus to have their environmental impact c o n f i n e d . I n order to have a longer exposure to the sewage treatment plant a n d to increase the opportunity for biodégradation, the polymers s h o u l d also be adsorbable o n sewage sludge. T h i s adsorbability w i l l increase their residence time i n the plant from a f e w hours ( 5 - 1 0 h) to several days (6—12 days).

Current Detergent Polymers T h e two major p o l y m e r compositions currently used i n detergent formulations are p o l y a c r y l i c a c i d a n d copolyacrylic—maleic acids. T h e p o l y a c r y l i c acids have molecular masses i n the 4 0 0 0 - 5 0 0 0 - D a range and are used i n the U n i t e d States and E u r o p e , whereas the copolymers have molecular masses i n the 70,000-Da range a n d are u s e d e x c l u sively i n E u r o p e . T h e n e e d for two polymers of different compositions and molecular masses is attributable to differences i n U n i t e d States and E u r o p e a n laundry conditions a n d water hardness levels. B o t h polymers are classed as cobuilders that to some degree control p H o f the wash bath, detergency, dispersion o f soil particles, a n d sequestration o f hardness ions. T h e wash performance of the acrylics is very sensitive to molecular weight, as i n d i c a t e d i n u n p u b l i s h e d work b y F r e e m a n (1) at R o h m and Haas. Detergency performance peaks at a molecular mass of about 5000 D a , a n d the whiteness index peaks closer to a molecular mass


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HYDROPHILIC POLYMERS Table II. Biodegradability and Removability of PCAs Acrylic Acid


Molecular Weight

% C0

% Removal

45 20 9 16

45 21 40 58

2

1,000 2,000 4,500 10,000 12,000 60,000 70,000

Acrylics and Maleics % C0

% Removal

31

83

20

95

2

e

93

°At polymer levels typically found in sewage treatment plants, this value is >80%.

of 10,000 D a . Since detergency, the removal of soil, is a more important function of the p o l y m e r than the whiteness index (dispersion of soil), the polymers i n use i n the U n i t e d States are closer to 5000 D a i n molecular mass. T h e performance of the copolymers peaks at a molecular mass of around 70,000 D a . T h e fate of these polymers i n wastewater streams and the e n v i r o n ment after use i n the laundry was recently r e v i e w e d b y F r e e m a n (2) and is only b r i e f l y touched on here. T a b l e II contains a summary o f the biodegradability (% CO2) a n d sewage sludge adsorption (% Removal) data for a molecular mass range of the two different polymers l a b e l e d w i t h C . T h e concentration of the polymers i n water for this study was 20 p p m . It is clear that the biodegradability of p o l y a c r y l i c acid is inversely related to the molecular mass and that the adsorption on sewage sludge, or bioremovability, is directly related to the m o l e c u lar mass of the p o l y m e r . T h e currently u s e d product (molecular mass o f 4 5 0 0 Da) is not considered biodegradable but is largely bioremovable b y adsorption, especially i n the realistic concentration range o f 1—2 p p m , and hence does not enter the greater aqueous environment. Instead, it remains o n the sewage sludge for l a n d filling, l a n d a p p l i c a tion, or incineration. C o p o l y m e r s are h i g h l y adsorptive i n the molecular mass ranges evaluated, and the l e v e l o f biodégradation appears to b e higher for them than for p o l y a c r y l i c a c i d . H o w e v e r , copolymers o f these two monomers have some low-molecular-mass fractions that are more susceptible to biodégradation than the major high-molecular-mass polymer component. T h e two presently used commercial polymers are therefore similar i n that they are not biodegradable to any extent and are not released into the aqueous e n v i r o n m e n t i n large quantities. H o w e v e r , these qualities do not preclude the n e e d for the d e v e l o p ment o f f u l l y biodegradable replacements. S u c h replacements are 1 4


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n e e d e d because some sludge is contaminated w i t h p o l y m e r a n d because some waste releases do not go through the sewage treatment plants. Some appreciable quantity of nonadsorptive, nonbiodegradable p o l y m e r is also still entering the environment.


Requirements for Biodegradable PCAs H a v i n g discussed the limitations of the current products, w e can n o w define the performance, biodégradation, a n d b i o r e m o v a b i l i t y requirements for biodegradable detergent P C A s . T o be acceptable, performance must be at least e q u i v a l e n t to that of current products. T h e n e w product must p r o v i d e e q u a l performance at the same cost whether it is used at a higher or a l o w e r l e v e l i n the detergent formulation. T h i s restriction, of course, limits the complexity of the chemistry a n d the raw material base that can be used to manufacture P C A s . T h e polymers must also be stable for a time (up to 1 h) i n the basic l a u n d r y m e d i u m ( p H 10), frequently at temperatures as h i g h as 60 °C, a n d must consistently perform the expected functions. T h e biodégradation requirement for detergent polymers is total biodégradation to gaseous products (CO2 i n aerobic environments a n d C O 2 - C H 4 i n anaerobic environments), minerals, a n d biomass such that no organic residue remains i n the environment. T h e rate of b i o degradation is expected to be faster than the rate of disposal into an environment, so that no b u i l d u p is observed. Preferably, the biodégradation rate is faster than the residence time i n a sewage treatment plant, the most c o m m o n disposal environment, thus e n s u r i n g no entry into the greater environment. I n a sewage treatment facility, h y d r a u l i c turnover occurs i n a few hours (5-10 h) a n d sludge turnover occurs w i t h i n days (8-14 days). Therefore i f a P C A is water-soluble a n d n o n adsorbable o n sewage solids, the biodégradation rate s h o u l d be fast enough that biodégradation reaches c o m p l e t i o n i n a f e w hours, whereas an adsorbable p o l y m e r w i l l have several days to biodegrade a n d may e v e n complete its biodégradation after removal from the treatment plant i n , for example, l a n d a p p l i c a t i o n or composting facilities. T h e advantage i n P C A s b e i n g both adsorbable a n d biodegradable is therefore o b v i o u s l y i n the control of their release into the e n v i ronment. F o r P C A s to be accepted as totally biodegradable as d e s c r i b e d above, they must meet severe accountability requirements a n d not just pass the standard tests, such as those for the A m e r i c a n Society of T e s t i n g a n d Materials, the Organization for E c o n o m i c C o o p e r a t i o n a n d D e v e l o p m e n t , the M i n i s t r y of International T r a d e a n d Industry, the F r e n c h Standards, the G e r m a n Standards, a n d the International


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Standards Organization, w h i c h are regulatory recommendations rather than true assessments o f biodegradability. T h e stricter requirements for total biodégradation are best e x e m p l i f i e d b y c o n s i d e r i n g the carbon balance equations for any organic P C A : A e r o b i c biodégradation C + O2 —» CO2 + Cbiomass Downloaded by NORTH CAROLINA STATE UNIV on May 3, 2015 | http://pubs.acs.org Publication Date: January 15, 1996 | doi: 10.1021/ba-1996-0248.ch005

A n a e r o b i c biodégradation C —» CO2 + CH4 + Cbiomass Total accountability o f the carbon is essential i n any biodégradation evaluation, regardless o f the environmental exposure. F o r total, acceptable biodégradation, the carbon i n t r o d u c e d into the e n v i r o n ment, C , must be accounted for as gaseous carbon, that is, carbon dioxide and/or methane, a n d as carbon incorporated into biomass. N o residue s h o u l d b e left. A l l the levels are r e a d i l y measurable b y a variety of techniques, and radiolabels are possible i f it is desirable to w o r k at the P C A environmental exposure levels usually observed (i.e., p p m level). C o r r e c t l y carried out, the experiments w i l l indicate the rate and extent of biodégradation, and these parameters bear o n the rate o f removal from the disposal environment, usually the sewage treatment plant. A d s o r p t i o n o n sludge can b e s t u d i e d b y exposing the P C A s to sewage sludge, d i r e c d y d e t e r m i n i n g the amount adsorbed at different aqueous concentrations, a n d then extrapolating to the environmental concentration. S u c h studies w i t h P C A s that biodegrade r a p i d l y measure adsorbability a n d biodegradability i n c o m b i n a t i o n .

Approaches to Biodegradable PCAs T w o major approaches to the synthesis of P C A s are available a n d have b e e n explored: (1) synthetic polymers are prepared from appropriate monomers, and (2) polymers are formed b y the m o d i f i c a t i o n of renewable resources. P r e p a r i n g polymers from monomers has r e c e i v e d more attention u p to this point, b u t renewable resource chemistry is fast b e c o m i n g fashionable and, as w e shall see, may b e the best approach to obtaining f u l l y biodegradable P C A s that is consistent w i t h the goals of the industry. Synthetic Polymers. Synthetic polymers are o f two types: v i n y l , or a d d i t i o n , polymers and condensation polymers. Because o f their availability, familiarity, a n d ready p o l y m e r i z a t i o n , m u c h effort has been e x p e n d e d i n t r y i n g to u t i l i z e acrylic and m a l e i c a c i d mono-


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mers to synthesize biodegradable detergent polymers. V i n y l p o l y m e r izations are therefore discussed first. Vinyl Polymerizations. Because v i n y l - p o l y m e r i z e d carbon c h a i n P C A s based o n acrylic a n d m a l e i c acids at a molecular mass that allows performance d o not biodegrade, v i n y l p o l y m e r i z a t i o n approaches attempt to take advantage o f the biodegradability o f their oligomers. I n order to pursue these approaches, the d e f i n i t i o n o f the highestmolecular-mass oligomer that w i l l totally biodegrade must b e established. Since oligomers do not have useful properties i n detergents, they must be c h a i n extended to a useful m o l e c u l a r w e i g h t range consistent w i t h detergent performance b y u s i n g bonds susceptible to degradation b y some m e c h a n i s m (biodégradation, h y d r o l y s i s , etc.) after disposal of the p o l y m e r i n a g i v e n environment, l e a v i n g a biodegradable shortc h a i n oligomeric a c i d . T h i s molecular mass b u i l d i n g is represented here schematically, w i t h X as the susceptible c h e m i c a l l i n k b e t w e e n oligomer units o f carboxyl monomers. x

χ

x

X

X

χ

Oligomer Biodégradation. Biodégradation o f acrylic a n d m a l e i c a c i d oligomers has r e c e i v e d considerable attention i n industrial a n d academic laboratories. A major p r o b l e m has b e e n the lack o f consistency i n measuring the extent o f biodégradation. A l l too frequently, o n l y imprecise measurements have b e e n made. T h e s e measurements i n c l u d e b i o c h e m i c a l oxygen d e m a n d ( B O D ) , w h i c h is not a direct measurement o f biodégradation (3) because it does not measure carbon removal or balance, a n d loss o f oligomer content b y g e l permeation chromatography w i t h o u t establishing the oligomer concentration sensitivity range of the method. I n spite of this lack, agreement that oligomers o f acrylic a c i d are not biodegradable above a degree of p o l y m e r ization (dp) o f ca. 6 - 8 is general. I n i n i t i a l work b y L e v e r Brothers (4), w h o u s e d alcohol c h a i n transfer agents to control molecular mass, substantial biodégradation (30-50%) occurred i n B O D testing i f molecular mass was less than ca. 1000 D a (dp ~ 15). T h e s e results indicate that a substantial portion o f low-molecular-mass p o l y m e r or oligomer was present. M a t s u m u r a et a l . (5) c h e m i c a l l y synthesized several acrylic oligomers w i t h different molecular masses a n d d e t e r m i n e d b y B O D that oligomers w i t h a d p o f less than 7 were biodegradable. Subsequently, fractionated oligomers were studied b y L a r s o n et a l . (3), scientists from N i p p o n S h o k u b a i (6), a n d I. K a w a i (7, 8). C a r e f u l b i o degradation studies i n a l l cases substantiated the earlier c o n c l u s i o n that o n l y oligomers w i t h a d p of less than ca. 8 are biodegradable.



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S i m i l a r results w i t h oligomerie maleic acids have b e e n observed i n our laboratories, b u t the d p for complete biodégradation appears to be l o w e r than that for acrylics (that i s , ca. 4) (9). T h i s result appears to indicate that carboxylic a c i d density may play a role i n i n h i b i t i n g the biodégradation of P C A s , since the d p 4 m a l e i c oligomers have the same n u m b e r o f a c i d groups as the d p 8 acrylic oligomers. T h e oligomer results show that for any approaches based o n this chemistry, precise molecular mass control must b e maintained d u r i n g synthesis, and this control is not easy to achieve, because most radicalpromoted oligomerization processes give a molecular mass distribution a n d not a u n i m o d a l oligomer. I n order to c o m p l y w i t h the rule o f d p 8, the molecular masses o f the oligomers must b e considerably b e l o w a d p o f 8 to ensure that the molecular mass envelope does not exceed that upper acceptable l i m i t . Oligomer Chain Extensions. A n u m b e r of patents have b e e n obtained i n the area of oligomer c h a i n extensions, because many attempts have b e e n made to control oligomer c h a i n length i n a p o l y m e r backbone that has b u i l t - i n weak l i n k s . A patent granted to A m e r i c a n C y anamid (10) utilizes the copolymerization o f ketene acetals w i t h acrylic a c i d , a chemistry that is similar to that p i o n e e r e d b y the late W . J. B a i l e y ( J J ) as a route to i n t r o d u c i n g biodegradable ester linkages into polyethylene a n d other polyolefins a n d r e n d e r i n g the products biodegradable. T h e chemistry is o u t l i n e d i n Scheme I. T h e concept is good, but the weaknesses o f the approach are that the ketene acetal is unstable i n aqueous a c i d i c solution a n d the copolymerization must b e conducted b y u s i n g the s o d i u m salt of acrylic a c i d . I n a d d i t i o n , the copolymerization parameters o f the t w o monomers are not k n o w n , a n d control o f oligomer c h a i n length a n d the ester weak l i n k - s p a c e r is therefore u n l i k e l y . T h e s e problems may b e the reasons that no biodégradation data are reported i n the patent. Deutsche G o l d e - u n d - S i l b e r (12) free radical c o p o l y m e r i z e d acrylic a c i d w i t h acrolein i n the presence o f n-dodecylmercaptan as the c h a i n transfer agent to control molecular w e i g h t i n the oligomer range. T h e p o l y m e r i z a t i o n was c l a i m e d to proceed through both the a l d e h y d i c a n d the v i n y l groups o f acrolein. T h i s product was t h e n

ÇO2H

Scheme 1.



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87

Scheme

further reacted i n a C a n n i z z a r o or a cross-Cannizzaro reaction w i t h formaldehyde to produce oligomeric chains containing acrylic a c i d blocks w i t h a l d e h y d i c a n d alcoholic functional groups to promote b i o degradation, as s h o w n i n Scheme II. T h e polymers are c l a i m e d on the basis of B O D determinations to be 5 0 - 6 0 % biodegradable. H o w e v e r , no information o n the l e v e l of low-molecular-mass materials present is g i v e n , a n d the true b i o d e gradability of these oligomers cannot be assessed. C o p o l y m e r i z i n g d i v i n y l monomers w i t h acrylic and m a l e i c acids i n ratios that give statistical oligomer c h a i n lengths that are k n o w n to be biodegradable has b e e n evaluated b y R o h m a n d Haas (13) and B A S F ( 14). T h e object is to have the d i v i n y l m o n o m e r of sufficient c h a i n length a n d l a b i l i t y that the b r a n c h i n g links w i l l be readily biodegradable and the w h o l e network structure s h o w n i n Scheme III w i l l collapse to o l i g o m e r i c carboxylic acids that are themselves biodegradable. Unfortunately, control of the degree of branching, w h i c h is detrimental to biodégradation, a n d the oligomeric a c i d c h a i n length is difficult, a n d at best o n l y partial biodegradability is achieved. T h e polymers are adsorbable and bioremovable i n sewage treatment facilities. T h e y are thus equivalent to current products but offer no major advantages. T w o other attempts to u t i l i z e oligomers of acrylic a c i d are w o r t h noting. A R o h m a n d Haas patent (15) reported that the free radical grafting of acrylic a c i d onto poly(ethylene glycol) p r o d u c e d partially biodegradable detergent polymers. T h e i n a b i l i t y to control the acrylic oligomer side c h a i n molecular mass was the l i m i t a t i o n o n the extent of biodégradation; complete biodégradation c o u l d not be achieved. E c o l a b holds a patent (16) o n a process i n w h i c h molecular mass control b y the use of a mercaptan c h a i n transfer agent was attempted, a n d



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i Scheme III. Branched

oligomers.

the acrylic oligomer chain was attached through esterification of the poly(ethylene glycol) chain ends, in essence to produce a ladder polymer. Biodégradation was claimed, but no supporting evidence was given for biodégradation of the acrylic portion of the polymer. These two similar approaches are outlined in Schemes IV and V. These approaches, based on control of the molecular masses of free radicalinitiated oligomerizations either in grafts or in block copolymerization, are technologically limited, because it is not possible to achieve the narrow molecular mass distribution of oligomers that is necessary for their total biodégradation. Vinyl Polymerizations. Vinyl polymerization attempts to copolymerize the unsaturated acid monomer with another vinyl monomer so that the polymer chain produced will have weak links and a susceptibility to biodégradation. That poly(vinyl alcohol) is the only biode-



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Scheme IV. Poly(ethylene glycol) graft chemistry. gradable carbon c h a i n p o l y m e r is consistent w i t h the failure o f this approach. M a t s u m u r a (17) has essentially c o p o l y m e r i z e d a l l available acrylic esters w i t h acrylic a c i d to no a v a i l ; n o c o m p o s i t i o n w i t h a m o lecular mass i n the range useful i n detergents has significant b i o d e gradability. E v e n copolymers of v i n y l alcohol fail to balance biodégradation a n d detergent properties. Polymers w i t h h i g h v i n y l alcohol content and l o w a c i d monomer, acrylic, maleic, a n d fumaric contents are biodegradable b u t lack useful properties, a n d the reverse is true for polymers containing l o w levels o f v i n y l alcohol (18,19). A f e w years ago, w o r k at Solvay et C i e (20) a n d Sandoz (21 ) i n d i cated that p o l y ( a - h y d r o x y a c r y l i c acid) prepared b y the hydrolysis of poly(a -chloroacrylic acid) was biodegradable a n d h a d good detergent properties. T h e reasoning was that the hydroxy 1 functionality p r o v i d e d a weak l i n k at w h i c h biodégradation c o u l d be initiated. H o w e v e r , subsequent w o r k i n d i c a t e d that the r a p i d carbon loss observed i n testing, w h i c h was considered a n i n d i c a t i o n o f biodégradation, resulted i n large part from adsorption onto sewage sludge a n d removal from the environment rather than true biodégradation. M o r e recently, successful conversion o f p o l y ( v i n y l alcohol) into acid-containing polymers b y reaction w i t h m a l e i c a n h y d r i d e a n d sub-

Scheme V. Voly(ethylene glycol)-oligomer

esters.


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HYDROPHILIC POLYMERS Ο OH

OH

OH

OH


NH

2

COîH COîH

OH

Ο

OH

Q=J

OH

COzH

C0 H

C02H

2

Scheme VI. sequently aspartic acid through a Michael reaction of the amine func tionality to the maleic ester double bond has been reported by Procter and Gamble (22). The products should be biodegradable, but no indi cation is given in the patent. The chemistry is illustrated in Scheme VI. Matsumura et al. (23) reported homopolymers of vinyloxyacetic acid as biodegradable detergent polymers and gave evidence to sub stantiate their claims. The polymer is a derivative of poly(vinyl alco hol) (see structure 1), which is postulated to be the intermediate in the biodégradation pathway formed through ether cleavage. The drawbacks to this elegant approach are the cost and availability of the monomer. C o n d e n s a t i o n P o l y m e r s . Condensation polymers containing a carboxyl functionality are the second class of synthetic polymers that have been evaluated as detergent polymers. They are more difficult

CQ H C 0 H C 0 H 2

2

2

1


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91

A Review of Synthetic Approaches

HO,

COiH C0 H

η

2


Scheme VII. Poly (a-malic) acid. to synthesize than free-radical polymers containing a carboxyl func tionality, but they are more readily controlled as far as functionality placement and molecular mass are concerned. Also, because they con tain chemical bonds that are found in nature, they are expected to be more easily biodegradable. They have the general structure - R - X - R - X - R - X - R - X - R - , where X is the condensation linkage that includes ester [ - C ( = 0 ) - 0 - ] , amide [ - C ( = 0 ) - N H - ] , ether [ - Ο - ] , and acetal-ketal [ O - C - O ] , and R is an alkyl group from C i containing a carboxyl functionality. Polyesters. Polyesters with a carboxyl functionality have only re cently been evaluated in detergents; a recent patent granted to B A S F (24) suggests their potential. The polyesters are based on citric acid, tartaric acid, 1,2,3,4-butanetetracarboxylic acid, and suitable polyols including ethylene glycol and higher homologs. The patent contained no biodégradation data beyond bioremovability on sewage sludge. The polyesters appear to have laundry performance, but because the butane polyacid is required to boost acidity, the cost of the polymers is probably unrealistic for commercial use. Earlier, Lenz and Vert (25) prepared poly(a-malic acid), and Matsumura et al. (26) prepared the random poly(a//3-malic acid). Matsumura demonstrated that his polymers had poor laundry performance and rapid biodegradability. Both polymer types are very unstable and hydrolyze rapidly in aqueous basic solution (Schemes VII and VIII). They are therefore unlikely to be useful in the highly alkaline detergent bath. Polyamides. A plethora of recent patents has been based on poly(amino acids). These acids are predominantly based on poly(aspartic acid) and poly(glutamic acid) (2 and 3). This activity probably stems in large part from the observation by Sikes and Wheeler (27)

Scheme VIII. Poly(alfi-malic) acid.


92



2, poly(aspartic acid)

3, poly(g!utamic acid)

that m i x e d poly(amino acids) isolated from crustaceans are good at m i n e r a l dispersal, w h i c h is a k e y property for detergent polymers. C o m p a n i e s active i n this research i n c l u d e D o n l a r (28), C y g n u s (29), R h o n e - P o u l e n c (30), and E n i c h e m (31), w h i c h focus o n poly(aspartic acid), a n d Procter and G a m b l e (32), w h i c h has an interest i n p o l y ( g l u tamic acid). T h e poly(aspartic acids) are prepared b y two different methods: the condensation p o l y m e r i z a t i o n o f L-aspartic a c i d a n d the thermal a d d i t i o n - c o n d e n s a t i o n p o l y m e r i z a t i o n of a m m o n i a a n d m a l e i c a n h y d r i d e - m a l e i c acid. T h e molecular masses of the products are different: masses of polymers from L-aspartic a c i d range from 1,000 to 70,000 D a , whereas the maleic process gives polymers w i t h molecular masses ranging from 2000 to 4000 D a . Results to date indicate the incomplete biodégradation o f polymers from either process; a n u n i d e n t i f i e d residue is always present (33,34) at the e n d of testing. L a u n dry performance appears to be satisfactory. Polyacetak and Polyketals. Polyacetals (4) a n d polyketals containing a c i d functionality represent a very elegant approach to b i o d e gradable detergent polymers. Monsanto scientists (35) d e v e l o p e d the salts of polyacetal and polyketal carboxylates as a c o m b i n a t i o n h y d r o l ysis a n d biodégradation approach to biodegradable detergent p o l y mers. Esters of g l y o x y l i c a c i d and p y r u v i c a c i d are readily p o l y m e r i z e d anionically through their carbonyl functionality, a n d the products are stable i n basic m e d i a ; after the esters have b e e n used as detergent polymers at h i g h p H , the p H falls r a p i d l y as they are discarded into a wastewater stream, and hydrolysis to their m o n o m e l i c salts is rapid. Biodégradation of the monomers is r a p i d a n d complete. T h e polymers have never b e e n used c o m m e r c i a l l y because the cost per performance is too h i g h . H o w e v e r , they have b e e n the w e l l s p r i n g o f several similar approaches, a l l based o n the f o l l o w i n g general structure:

C 0 H C02H C 0 H C 0 H 2

2

2

4, polyacetals


5.

93

PAIK ET AL. A Review of Synthetic Approaches ÇO2H HOH-

CO2H

H

OH-

OH—Ι—H

OH -H

OH-

H

H—I—OH +

°

H

C

-

C

H

O

+

O H + H OH

C0 H 2

H C0 H 2


5, polyethercarboxylic acids B A S F recently extended this ketal-acetal chemistry and patented two variants on the Monsanto theme. The first variant is based on using glutaconic acid and dialdehydes such as glyoxal and glutaraldehyde to form acetals (36) (5); the other variant uses long-chain keto acids and polyfunctional alcohols to form ketals (37). The biodegradability of poly(ethylene glycol) is well established (37), and several attempts to prepare carboxylated polyethers as biode gradable detergent polymers have been made. The initial reports on this approach were from Monsanto. Crutchfield et al. (38) evaluated the anionic polymerization of epoxysuccinic acid. Matsumura et al. (39) thoroughly researched this chemistry, establishing the molecular weight and structural relationships with biodégradation. They prepared and cationically polymerized several epoxy esters (represented by the general structures in Scheme IX) to molecular masses in the range of a few hundred to several thousand daltons. Not unexpectedly, after hydrolysis, bulky substituents were detrimental to biodégradation and molecular mass was also important.

where X and Y are both carboxyl groups or one is an alkyl group.

Renewable Resource Chemistry: Natural Polymers.

Re-

newable resource chemistry or chemistry based on the use of natural polymers will become increasingly important over the next few years, particularly in the area of biodegradable polymers. The advantages are that the polymers are generally accepted as biodegradable or at least not harmful to the environment. They are available, and they are in some cases, for example, starch, very inexpensive. Their disadvan-


94



6

7

tages are that (1) i n n o case is a natural p o l y m e r usable as isolated i n the detergent applications u n d e r consideration; (2) i n t r o d u c i n g the carboxyl functionality is often not easy a n d is a m o d i f i c a t i o n that c a n add cost; a n d (3) the effects o n biodégradation m a y b e negative, d e p e n d i n g o n the l e v e l o f modification r e q u i r e d . T h e three approaches that have r e c e i v e d the most attention are grafting v i n y l m o n o m e l i c carboxylic acids onto suitable substrates; oxidation of polysaccharides, particularly starch, because o f its availability a n d l o w cost; and c h e m i cal derivatization, for example, b y monoaeylation w i t h p o l y c a r b o x y l i c acids. These approaches are a l l b r i e f l y discussed i n this section. Graft Chemistry. Several attempts (40-44) at free-radical grafting o f acrylic a c i d onto starch have b e e n made. T h e major p r o b l e m encountered i n every case was the lack o f control o f the molecular mass o f the acrylic p o l y m e r . A s p r e v i o u s l y stated, such control is very difficult to achieve b y free-radical p o l y m e r i z a t i o n . A t t a c h i n g the " r i g h t " n u m b e r o f oligomer chains to the substrate is also essential to ensure that the graft w i l l b e active i n detergency a n d w i l l retain its biodegradability. N o t surprisingly, structures 6 a n d 7 have not m e t these stringent requirements. B A S F (45) substituted proteins such as casein a n d soy proteins for starch b u t also were able to demonstrate biodegradability o n l y for the natural substrate. T h e p r o b l e m that is seen i n starch grafts, that is, molecular mass control o f the acrylic p o l y m e r i z a t i o n , also pertains here. Starch Oxidation Chemistry. T h e inexpensive nature a n d the biodegradability o f starch a n d other polysaccharides have attracted m u c h activity i n the area of oxidation i n a n effort to introduce carboxyl functionality at either the 6 p o s i t i o n from the primary h y d r o x y l or at the 2 a n d 3 positions b y ring cleavage. T h e s e oxidations have b e e n a c h i e v e d p r e d o m i n a n t l y b y c h e m i c a l means, w i t h p e r i o d a t e - c h l o r i t e


5.


95 OH

OH l.Periodate 2. Chlorite

A)-


Scheme X. a n d hypohalite particularly favored (Scheme X ) , a n d o n l y recently has evaluation of catalytic oxidation b e g u n . C h e m i c a l oxidation has b e e n reported b y several academicians (46-49) and i n d u s t r i a l laboratories (50-52). N o P C A that combines the properties r e q u i r e d for detergent performance a n d biodégradation has yet b e e n p r o d u c e d . M a t s u m u r a (48) has the most balanced perspective on this aspect of the o x i d i z e d polysaccharides. H e s h o w e d that as the carboxyl content increases i n the polysaccharide, biodégradation decreases a n d detergent performance increases. W h e t h e r the two effects can ever be balanced to produce a biodegradable detergent p o l y m e r is not clear. F r o m the c o m m e r c i a l standpoint, other potential problems w i t h the c h e m i c a l oxidation of starches are that the reactions are done i n very d i l u t e solution a n d that large quantities of inorganic salt byproducts are generated. A n obvious solution to the p r o d u c t i o n of inorganic salts is to d e v e l o p catalytic processes, a n d this effort is n o w r e c e i v i n g attention. Catalytic processes u s i n g noble metal catalysts a n d air (Scheme X I ) appear to be p r o m i s i n g , but little control over the rates a n d degree of oxidation has b e e n a c h i e v e d . H o w e v e r , i t is b e c o m i n g clear that such processes are an excellent way to d e v e l o p e c o n o m i c a l l y feasible c o m m e r c i a l processes, a n d recent patent activity underscores this poss i b i l i t y (53, 54). Chemical Derivatization. F r o m the very early days of the search for biodegradable detergent polymers, the monoacylation of polysaccharides w i t h p o l y f u n c t i o n a l carboxylic acids, e s p e c i a l l y as the anhydrides o f m a l e i c a n d s u c c i n i c acids, has b e e n a favored route. T h e acylation reaction is very w e l l understood, a n d it seems to be an easy and c o m m e r c i a l l y attractive process. H o w e v e r , the difficulties are that ,OH(C02H)

OH

Scheme XL



96


Scheme XII. Acylation of starch. starch, for example, is not readily soluble in organic solvents and that in the neat reaction, starch has some tendency to cross-link through esterification of two carboxyl functionalities in the same monomer, especially as the degree of substitution increases to the level of carboxyl that is needed for performance. The approaches patented by F M C (55) and Procter and Gamble (56) and shown schematically in Scheme XII do not indicate that the products are fully biodegradable.

Conclusions The need for biodegradable detergent polymers is very real, and this need is supported not only by regulatory expectations but by the detergent industry itself as a self-policing body that has always been concerned about and responsive to environmental pollution. Because of this concern for the environment, the industry expects a totally biodegradable polymer that meets exacting standards of degradation achievable in a short time while maintaining a cost-per-performance balance consistent with that of current products. The number of approaches that have been tried is very large, and no polymer to date meets the industry requirements. Free-radical polymerized vinyl carboxylates are unlikely to meet the goal because of their molecular mass limitation for biodegradabil-



5.

PAIK ET AL.

A Review of Synthetic

Approaches

97

ity: performance requires a h i g h e r m o l e c u l a r mass, a n d b i o d é g r a d a t i o n occurs only at a l o w m o l e c u l a r mass. C o m b i n a t i o n s o f renewable resources a n d v i n y l monomers also seem to be p r e c l u d e d because o f the same strict m o l e c u l a r mass requirements o f the graft arms from the v i n y l monomer. C u r r e n t polymers based o n acrylic a c i d a n d m a l e i c a c i d , b e i n g bioremovable i n wastewater treatment plants, are a better i n t e r i m sol u t i o n to the potential e n v i r o n m e n t a l problems than are some of the i n d i c t e d partially biodegradable polymers. S w i t c h i n g to the partially biodegradable polymers w o u l d introduce uncertainty about the residues that w o u l d require extensive a n d expensive fate a n d effects evaluation. Future

Directions

As the foregoing discussion makes clear, the most reasonable approaches to totally biodegradable water-soluble polymers for detergents are modification of renewable resources such as starch to balance cost per performance a n d d e v e l o p m e n t of condensation polymers w i t h carboxyl functionality. I n order to be acceptable, such polymers must be totally biodegradable, as d e s c r i b e d i n this chapter, that is, from the standpoint of accountability i n test protocols that are accepted w i d e l y , and not m e r e l y able to pass some o f the current test protocols, such as that o f the O r g a n i z a t i o n for E c o n o m i c C o o p e r a t i o n a n d D e v e l o p ment, that w e r e d e v e l o p e d for legislative rather than scientific purposes.

References 1. Freeman, M. B. Presented at the 80th American Oil Chemists Meeting, Cincinnati, OH, May 1994. 2. Freeman, M. B. Environ. Technol. 1993, 14, 101-112. 3. Larson, R. J.; Swift, G.; Williams, R. Polym. Mater. Sci. Eng. Washington, Fall 1992. 4. Lever Brothers. U.S. Patents 3 922 230, 4 095 035, and 4 132 735. 5. Matsumura, S.; Maeda, S.; Takahashi, J.; Yoshikowa, S. Kobunshi Ronbunshu 1988, 45(4), 317-324. 6. Hayashi, T.; Mukou, M.; Tani, Y. Appl. Environ. Microbiol. 1993, 59, 1555-1559. 7. Kawai, F., Kobe Commercial University, Kobe, Japan, personal communication. 8. Kawai, F. Appl. Microbiol. Biotechnol. 1993, 39, 382-385. 9. Swift, G.; Yocom, K., Rohm and Haas, unpublished work. 10. American Cyanamid. U.S. Patent 4 923 941, 1990. 11. Bailey, W.; et al. Contemp. Top. Polym. Sci. 1979, 3, 29. 12. Deutsche Gold-und-Silber. U.S. Patents 3 896 086 and 3 923 742. 13. Rohm and Haas. Eur. Patent 430 574.



98 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30. 31. 32. 33. 34. 35. 36. 37. 38. 39. 40. 41. 42. 43. 44. 45. 46. 47. 48. 49. 50. 51. 52. 53. 54. 55. 56.


BASF. D E 3 818 426. Rohm and Haas. U.S. Patent pending. Ecolab. U.S. Patent 4 874 540. Matsumura, S.; et al. Yukagaku 1986, 35, 167-175 and 937-944. Matsumura, S.; et al. J. Am. Oil Chem. Soc. 1993, 70(7), 659-665. Rohm and Haas. U.S. Patent 5 191 048. Solvay et Cie. U.S. Patents 4 107 411 and 4 182 806. Sandoz. D E 37 111 304. Procter and Gamble. U.S. Patent 5 221 711. Matsumura, S.; et al. Macromol. Chem. Rapid Commun. 1988, 9, 1-5. BASF. U.S. Patent 5 217 642. Lenz, R. W.; Vert, M. Polym. Prepr. (Am. Chem. Soc., Div. Polym. Chem.) 1972, 20, 608. Matsumura, S.; et al. Yukagaku 1986, 35, 937-944. Sikes and Wheeler to U.S. Alabama. U.S. Patents 4 534 881, 4 866 161, and 50 511 401. Donlar Corporation. U.S. Patents 5 057 597, 5 116 513, and 5 221 733. Cygnus. U.S. Patent 5 219 986. Rhone-Poulenc. Eur. Patent 511 037. Enichem. Eur. Patent 454 126. Procter and Gamble. WO 9 306 202. Pettigrew,C.;Wheeler, A. P. Abstracts, Bio/Environmental Polymer Society Annual Meeting, Chicago, IL; August 1993. Lowe, K. C.; Koskan, L. Polym. Mater. Sci. Eng. 1993. Monsanto. U.S. Patents 4 146 495, 4 114 226, 4 887 033, etc. (acetals) and 4 146 934 (ketals). BASF. Eur. Patents 280 223-A1 (glutaconic acid) and D E 4 106 354-A1 (keto acids). Kawai, F. Crit. Rev. Biotechnol. 1987, 6(3), 273-307. Crutchfield, M. M. J. Am. Oil Chem. Soc. 1978, 55, 58. Matsumura, S.; et al. Yukagaku 1987, 36(110), 874-881. Sanyo, J. P. U.S. Patents 6 131 497 and 6 131 498, 1986. Grillo Werke. U.S. Patent 496 329 and D E 3 834 237, 1988. Mitsubishi Petrochemicals. Jpn. Patent 4 028 709. Rhone-Poulenc. Eur. Patents 465 286 and 465 287. Taechung Moolsan. WO 9 302 118. BASF. D E 4 016 002. van Bekkum, H.; et al. Prog. Biotechnol. 1987, 3, 157-161. van Bekkum, H.; et al. Starch 1985, 37, 192. Matsumura, S. Angew. Macromol. Chem. 1993, 205, 17-29. Besemer, A. Ph.D. Thesis, University of Delft, 1993. Procter and Gamble. Eur. Patent 542 496. Ferruzi. Eur. Patent 472 042. Zielke, R. Tenside Deterg. 1977,14(5), 250-256. Novamont. WO 9 218 542 and Eur. Patent 385 252. Roquette Freres. Eur. Patents 511 081 and 455 522 and U.S. Patent 4 985 553. F M C Corporation. U.S. Patents 3 941 771 and 4 029 590. Procter and Gamble. U.S. Patent 3 919 107.

RECEIVED for review November 18, 1993. ACCEPTED revised manuscript September 20, 1994.


A Review of Synthetic Approaches to Biodegradable Polymeric

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