Chapter 4
Miscibility in Polymer Recycling
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Richard S. Stein University of Massachusetts, Amherst, MA 01003
Thermodynamic principles governing miscibility are discussed, and the reason for most mixtures encountered in recycle feedstock being immiscible are explained. The relationship between miscibility, interface diffuseness and mechanical strength are discussed as are means for their modification. Factors governing the crystallization of both miscible and immiscible polymer mixtures are analyzed. It is demonstrated that the properties of polymers prepared from mixed feedstocks can be affected by both composition and processing conditions. Polymers which are recycled may be classified into categories of those which are: 1. Commingled 2. Separated Commingled polymers are those where different species are mixed together, whereas separated ones have these separated. Separated plastics are most valuable for recycling since their properties are not degraded as a result of mixing them with other plastics. The problem is that sources of separated plastics are limited. These arise from: 1. Plastic waste arisingfrommanufacturing processes. 2. Plastic articles separated by the consumer 3. Plastic articles separated after collectionfromthe consumer An appreciable amount of "in house" recycling takes place in Category (1), where scrap resultingfromthe fabrication of plastic articles is collected and reused by the manufacturer. This is a process which has been taking place for many years and it will undoubtedly continue to increase. It is cost effective for the manufacturer and processor. For Category (2), it is necessary that the consumer be able to recognize plastics of different types and conveniently return these to some collector. It has been successful, for example, for polyethylene terephthalate (PET) soft drink bottles, where return can be encouraged (in some localities) by imposing a deposit, and redemption machines have been installed in supermarkets which read bar codes on bottles, refund the deposit, and then grind them up for compactness in shipping. Another readily identifiable polymer is high density polyethylene (HDPE) used for
0097-6156/92/0513-0039$06.00/0 © 1992 American Chemical Society
In Emerging Technologies in Plastics Recycling; Andrews, G., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1992.
Downloaded by UNIV OF MASSACHUSETTS AMHERST on September 15, 2015 | http://pubs.acs.org Publication Date: November 13, 1992 | doi: 10.1021/bk-1992-0513.ch004
40
EMERGING TECHNOLOGIES IN PLASTICS RECYCLING
milk and water jugs. It has been reported that 25% of the plastic jugs which Exxon uses for motor oil come from this source (1). Styrofoam is also readily recognizable and is used for coffee cups, hamburger "shells" and cafeteria trays. These are often used in centralized locations such as school cafeterias where collection is feasible. Separated plastics can also come from larger scale users. For example, polyethylene from agricultural mulch and polypropylene from bailing straps. A problem that occurs with even separated plastics is that they can become mixed with small amounts of foreign plastics. For example, PET soda bottles often have polyethylene bases and polypropylene caps. It would be desirable to encourage manufacturing practices which avoid such mixing, as the necessary separation adds to recycling costs. Styrofoam products are often contaminated with paper and food wastes, but these can easily be removed by washing with water and detergent and floatation. There are not too many examples in current use of where plastics are separated after collection from the consumer. Manual separation is labor intensive and expensive, and automatic means for this are not readily available. In principle, separation might be carried out by a device that could detect some property of the plastic, and separate it accordingly. The property could be a bar code, a spectrographic indicator, or an NMR signal. While there is need for research to develop such devices, it is uncertain whether such automatic separation can be accomplished economically. Other than the above, a large fraction of the plastic waste stream consists of mixed plastics which cannot be easily or economically separated. One must then consider how to use these in forms where there properties will not seriously suffer. Properties of Commingled Plastics
As will be discussed in the next section, most plastic mixtures occurring in commingled plastics will be "immiscible". That is, they will not form a single phase but will separate into two or more phases. These will be separated by boundaries which may be sharp or diffuse. With a sharp boundary between two different polymeric regions, there is often little molecular interpénétration, so that there is a region of mechanical weakness. Thus, failure is likely at these low adhesion boundaries, so the physical properties of such an immiscible commingled mixture will generally be poorer than those of the individual components. To improve the properties of phase separated systems, it is therefore desirable to increase the strength of the interface, this may be accomplished by 1. Modifying one or both components by techniques such as grafting or copolymerization so as to render them more miscible. 2. Carrying out a chemical reaction so as to bind components together at the interface. The binding may be through chemical reaction, such as grafting, or else through secondary interactions such as hydrogen bonding or charge transfer. 3. Adding an interfacial agent which binds the two phases together. An example is a diblock coplymer, one block of which is miscible with one of the phases and the other block with the other phase. Such materials act like emulsifying agents to stabilize immiscible suspensions. Principles of Miscibility
As mentioned, chemically different polymers are usually immiscible. It may be understood why immiscibility is more common for polymer pairs than for low molecular weight species in terms of thermodynamic considerations. For a process to occur spontaneously at constant temperature, T, and pressure, P, it is necessary that the Gibbs free energy, G, decrease. For two
In Emerging Technologies in Plastics Recycling; Andrews, G., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1992.
4. STEIN
41
Miscibility in Polymer Recycling
polymers to dissolve in each other, this free energy change is the free energy of mixing, A G i , which may be resolved into enthalpy, A H i , and entropy, m
x
m
x
AS ix, components according to m
AG ix = AH i m
m x
- Τ AS ix m
Downloaded by UNIV OF MASSACHUSETTS AMHERST on September 15, 2015 | http://pubs.acs.org Publication Date: November 13, 1992 | doi: 10.1021/bk-1992-0513.ch004
The entropy of mixing of an ideal solution is given by AS ix = - R [ nj Inxj + n2lnX2 ] m
where ni and r\2 are the numbers of moles of the two components and xi and X2 are their mole fractions. Since the xi terms are 1 or less, their logarithms will be zero or negative, so A S i is generally positive. This is reasonable since entropy is associated with disorder, and a solution is more disordered than the separated m
x
components. The increase in A S i contributes to a decrease in A G i and is a principal driving force for materials being soluble. The above equation for ideal solutions presumes that molecules of both components are of equal size. This is usually not true for polymer mixtures, so refinements by Flory (2), and others lead to the modification, m
AS^
=-R[
x
m
nj Ιηφι + νΙη
x
ή>2]
Here, the 's are the volume fractions which become equal to the mole fractions when the molecules are of equal size. This modification also leads to zero or positive A S j . The number of moles of component i is given by m
x
ni = w; / Mi where W J is its weight and M\ is its molecular weight. For polymer mixtures, the M's are large so the n's are small. Thus, A S i will be less for polymer mixtures than for low molecular weight mixtures. The physical significance of this is that if monomer units are linked together to form a polymer chain, their are fewer ways m
x
for them to mix than if they are not. A consequence is that A S i is a smaller driving force for miscibility of polymers than for low molecular weight species so that miscible polymer pairs are less frequent. Consequently, polymer miscibility is m
x
more controlled by A H i than is the case for low molecular weight mixtures. m
x
The Enthalpy of Mixing of Polymers Theories of A H j based on nearest neighbor pair interactions have been proposed by Scatchard and Hildebrand (3). These lead to equations of the sort m
x
AH x = RTxni
