Compatibilization of Lignocellulosics with Plastics - American

Chapter 5

Compatibilization of Lignocellulosics with Plastics Ramani Narayan

Downloaded by COLUMBIA UNIV on August 6, 2012 | http://pubs.acs.org Publication Date: December 4, 1992 | doi: 10.1021/bk-1992-0476.ch005

Michigan Biotechnology Institute and Michigan State University, Lansing, M I 48909

The blending of lignocellulosic polymers with synthetic polymers leads to immiscible blends whose properties tend to be poor and undesirable. Tailor-made cellulose-polystyrene graft copolymers have been used as compatibilizers/interfacial agents to prepare cellulosic -polystyrene alloys and wood-plastic alloys. The graft copolymers function as emulsifying agents and provide for a stabilized, fine dispersion of the polystyrene phase in the continuous phase of the cellulosic matrix. Transmission electron microscopy and thermal analyses was used as evidence for formation of these compatibilized cellulosic blends (alloys).

An important aspect of today's polymeric materials industry involves the blending of commercially available materials. The objective is to prepare new materials by blending two or more unique polymers to obtain desirable combinations of characteristics imparted by its components, while maintaining an optimized relationship of cost to performance. The blending/alloying of two polymers can produce a homogeneous one-phase material, or result in a two-phase morphology. The presence of a stabilized two-phase morphology is often desirable because it can be organized into a variety of structures. Variations in morphology, imparted by varying domain structures, generally lead to significant changes in physical properties and subsequent end-uses. The driving force for blends and alloys comes from: 1. Poor economics of new polymer and polymer production, and 2. The need for new materials whose performance-cost ratios can be closely matched to specific applications. It has been predicted that the worldwide market for polymer blends and alloys will reach a level of 2.4 billion lbs by 1994, at an annual growth rate of 8.3% from 1.6 billion lb in 1989 (1). From being relatively unknown on a broad commercial basis in 1980, it is reported that for every five pounds of resin sold about one pound is a blend or alloy (2). As many as 1000 technical papers are published and another 0097-6156/92/0476-0057$06.00/0 © 1992 American Chemical Society

In Emerging Technologies for Materials and Chemicals from Biomass; Rowell, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1992.

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MATERIALS AND CHEMICALS FROM BIOMASS

1000 patents awarded on the subject each year. Several excellent books and reviews have been written (3 - 5). It is, therefore, rather surprising that lignocellulosic and other biomass materials have not seen much use as one of the components of the blend/alloy systems. This paper reviews the use of lignocellulosic biopolymers in blends and alloys with synthetic polymers, with emphasis on our work in compatibilization of lignocellulosics with plastics.


Blends and Alloys The terms blends and alloys are often used interchangeably possibly because of the convenience of semantics equating the two concepts. While the term "blend" is a general term for the mixture of two or more polymers, the term "alloy" is generally used to describe a specific type of blend, namely a "compatibilized blend" that offers a unique combination or enhancement in properties. Miscible Blends. Completely miscible blends are relatively uncommon. They exhibit single phase behavior, are thermodynamically compatible, form a molecular solution, give rise to a single glass transition, and are generally optically transparent. There is a smooth variation of properties with composition. A n example of a miscible polymer blend system includes the well-known poly(phenylene oxide) (PPO) - polystyrene (PS) blends commercialized by General Electric (GE) under the trade name N O R Y L . In these blends PS imparts processability to a relatively difficult-to-process PPO. Other examples include polyvinyl chloride) (PVC) poly(methylmethacrylate) (PMMA) blends, and poly(styrene-co-acrylonitrile) (SAN) - P V C blends. Miscible polymer blends are useful to overcome specific problems such as processability, as in the case of the PPO-PS blend. Heat distortion, hardness, tensile, creep are some of the additional properties that can be added by blending in another polymer. The goal is to fine tune die properties of a particular polymer to meet specific application needs. Miscible Cellulosic Blends. In the lignocellulosic area, cellulose acetate is reported (6) to yield miscible blends with polystyrene phophonate esters, and polyvinyl pyridine) (PVP). A wholly amorphous blend with a single Tg is obtained from CA-polystyrene phosphonate ester systems when annealed above the glass transition temperature of C A (7). Another cellulose derivative, namely nitrocellulose, is miscible with poly(caprolactone) over the full composition range and clearly shows a single Tg at any composition (8). It is more difficult to prepare cellulose-polymer blends than polymer-polymer blends due to the small number of common solvents available (blends cannot be prepared from the melt because cellulose degrades before softening). Total miscibility has never been observed, but miscibility at certain compositions has been shown to occur in many instances(911). Immiscible Blends. Most polymer blends fall in to this category. The phases undergo gross segregation with minimal interfacial contact between the two phases resulting in poor mechanical properties. Strength and toughness values are minimal and are lower for the blend than for any of the pure components (12-14). This is a direct consequence of their incompatibility arising from negligible entropy of mixing and typical positive heat of mixing. Such blends generally have little value for end-


5. NARAYAN

Compatibilization of Lignocellulosics with Plastics

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use applications. Examples of such blends are PS/ΡΕ, PPO/PE, Nylon/PPO, and Cellulosics/Synthetic polymers. Immiscible but Compatibilized Blends - Alloys. Immiscible blends which have been "compatibilized" generally exhibit a two-phase stabilized morphology, in which one of the phases is either finely dispersed or co-interpenetrating on a microscopic scale. These alloys show two glass transition temperatures and are opaque if the phases are large enough. The improved compatibility can lead to a dramatic improvement in properties usually because of control and stabilization of the morphology. Ultimately the nature of the domain structures dictates the physical properties and performance of these systems. The extent of interfacial adhesion between the dispersed and continuous phases also contributes to the properties. In the field of polymer blends, such compatibilized mixtures of polymers are referred to as polymer alloys. Concerning the original meaning of the term "alloy" it must be noted for the puritans amongst us that in the field of metallurgy, from which it was borrowed, this usually refers to miscible blends showing a smooth variation of properties with composition. The key concept in preparing polymer alloys with improved properties is the use of compatibilizers or interfacial agents. Block and graft copolymers of the form A - B have been used by the polymer industry as compatibilizers or interfacial agents to improve interfacial adhesion, reduce interfacial tension, and provide for a stabilized and ordered morphology. Understanding the role compatibilization plays is key to conceptualizing not only the performance of the materials, but also how they differ from blends. Examples of commercial polymer alloys include (15): - ABS/nylon containing alloys which are about 10 times stronger than the simple blend - PC/ABS containing grades of alloys for exterior auto parts - PC/PBT containing alloys for engine-rack cradles - Poly(phenylene ether)/polyamide containing alloys which combine the chemical resistance of nylon and the creep resistance and toughness of PPE, with an especially high heat deflection temperature. - ABS/PVC containing alloys for applications ranging from seat components for mass transit systems to providing materials with improved melt flow and thermal stability. Block and Graft Copolymers Pure block and graft copolymers by themselves are also two-phase systems in which gross segregation of the two phases is prevented because the component polymers are chemically linked. This results in microphase separation in which the microphases exist in a unique and ordered domain morphology, conferring unique and altered physical properties to the product. In particular, studies of extensive morphology and physical properties have been reported for block copolymer systems (16). Considerable amount of work, pioneered by Stannett (17) and Arthur (18) on the synthesis & characterization of cellulosic graft copolymers, has been done. However large scale industrial applications have eluded these polymers, and the problems


60



preventing large scale use have been discussed by Stannett (12,19). Cellulosic block copolymers have been prepared by Gilbert's group (20,21). D. N.-S. Hon discusses in detail new developments in cellulosic derivatives and copolymers in another chapter in this book. In this paper the discussion will be restricted to cellulosic blends and alloys as defined earlier and illustrated below. Figure 1 illustrates the concept of blends and alloys. Thus, when A and Β polymers are mixed in unmodified form, the resultant product does not maintain the property characteristics of either A or Β homopolymers and there is a loss in property as shown by the curve 1 in Figure 1 (Immiscible blend). If the A and Β polymers give rise to a miscible polymer blend, the properties of the blend will be composition dependant and follow the line 2. Curve 3 represents the synergism of a compatibilized blend or A/B-alloy, in which enhancement and a unique balance of properties is achieved. As discussed earlier the synergism observed by using the compatibilization approach is due to improved interfacial adhesion and reduced interfacial tension between the phases, as well as the formation of a stabilized and finely dispersed phase morphology. Very little work has focussed on alloys in which one of the blend components is a lignocellulosic polymer. Glasser and coworkers (22) have investigated polymer blends of hydroxypropyl lignin (HPL) with polyethylene(PE), ethylene vinyl acetate copolymer (EVA), poly(methyl methacrylate (PMMA), and with polyvinyl alcohol) (PVA). The blends produced a two-phase morphology, with the PE-HPL system behaving as a immiscible blend system. However, incorporation of vinyl acetate groups (ethylene-vinyl acetate copolymer ) resulted in a compatibilized blend (alloy) with improved tensile strength. Stannett and coworkers pioneered some work on cellulosic block copolymers and its use in cellulosic alloys (23), and recently, we have been working on cellulosic alloys (24, 25) which is discussed in detail in this paper. Cellulosic Alloys The blending of lignocellulosic polymers with synthetic polymers generally leads to immiscible blends. As discussed earlier, the properties of such blend tend to be poor and undesirable. We have prepared cellulosic-PS alloys using tailor-made cellulose acetate (CA) - PS graft copolymers as compatibilizers. The graft copolymers function as emulsifying agents and provide for a stabilized, fine dispersion of the PS phase in the continuous phase of the C A matrix. Synthesis of Tailor-Made Graft Copolymers. We have reported the synthesis of cellulose-PS graft copolymers with precise control over the molecular weight of the PS graft, the degree of substitution, and the backbone-graft linkage (26 - 28). The key step in the synthesis is the preparation of the carboxylate polymer anion, and its nucleophilic displacement reaction with mesylated cellulose acetate. Figure 2 outlines the synthetic scheme which was adopted. Recently, we have simplified the synthesis as outlined in Figure 3 by preparing anhydride-terminated PS instead of carboxylate-terminated PS which reacts directly with the -OH groups in C A , thereby



NARAYAN

Compatibilization of Lignocellulosics with Plastics

Figure 1. Illustration of the concept of blends and alloys.



n CH=CH

n BuLi 2

I

-Li

2

+

I

R

R Downloaded by COLUMBIA UNIV on August 6, 2012 | http://pubs.acs.org Publication Date: December 4, 1992 | doi: 10.1021/bk-1992-0476.ch005

Bu-J-CH —CH-

THF Anionic Polymerization

C0

2

end capping

Bu-L-CH —CH-4-COO"

T

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Chain end functionaiized anion

l

Bu

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Cellulose acetate mesylate

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Cell

Compatibilization of Lignocellulosics with Plastics - American

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