Copolymers, Polyblends, and Composites

7 Compatibilization Concepts in Polymer Applications

Downloaded by UNIV OF MISSOURI COLUMBIA on September 20, 2013 | http://pubs.acs.org Publication Date: June 1, 1975 | doi: 10.1021/ba-1975-0142.ch007

NORMAN G . GAYLORD G a y l o r d Research Institute, Inc., 273 Ferry St., Newark, N. J. 07105

The general incompatibility of polymers may be overcome by suitable compatibilizing agents, i.e. block or graft copolymers having segments of similar structure or solubility parameter as the polymers being mixed. Presence of a compatibilizing agent influences the properties of polymer blends and alloys as well as polymer melts and solutions. Solution of incompatible polymers in a common solvent as well as dispersion of a polymer in a non-solvent is promoted by the presence of a compatibilizing agent. Dispersion of additives such as fillers and reinforcing agents (including cellulose, clay, glass fibers, and powdered metals) in a polymer matrix is enhanced by a compatibilizing agent at the additive-polymer interface; interfacial interaction promotes adhesion of solid polymers to polymeric or inorganic substrates.

T

he general incompatibility of polymers often prevents preparation of useful blends. Introduction of a compatibilizing agent permits the blending of otherwise incompatible polymers to yield compositions w i t h unique properties generally not attainable from either of the components of the polyblend. The presence of a compatibilizing agent influences the properties of polymer melts and solutions as w e l l as polymer blends and alloys. Solution of incompatible polymers i n a common solvent as w e l l as dispersion of a polymer in a nonsolvent is promoted by the presence of a compatibilizing agent. Dispersion of additives such as fillers and reinforcing agents i n a polymer matrix is en hanced by the presence of a compatibilizing agent at the polymer-additive interface. Adhesion of polymers to surfaces is similarly dependent on com patibilization at the interface. A qualitative overlook of the role of the compatibilizing agent i n these polymer applications indicates common denomi nators and continually expanding potential. Polyblends and Alloys F o r useful polyblends, the term compatibilization refers to the absence of separation or stratification of the components of the polymeric alloy during the expected useful lifetime of the product. O p t i c a l clarity of a polyblend is related to the particle size of the dispersed phase and/or the difference i n the 76

In Copolymers, Polyblends, and Composites; Platzer, N.; Advances in Chemistry; American Chemical Society: Washington, DC, 1975.

7.

GAYLORD

77

Compatibilization

refractive indexes of the dispersed and matrix phases. Absence of transparency is not indicative of incompatibility i n accordance with the broad definition given above and used hereafter. Block and graft copolymers possessing segments with chemical structures which are the same as those of the polymers to be blended are effective com patibilizing agents. Thus, an A B block or graft copolymer compatibilizes polymers A and Β (see Diagram 1 ) . A


A

Β

(1)

Since polymer properties are derived from finite length segments of a particular structure, random copolymers do not compatibilize homopolymers and, i n many cases, copolymers based on one ratio of comonomers are not com patible w i t h copolymers based on a different ratio of the same comonomers. Block copolymers are usually superior to graft copolymers as compati bilizing agents since the latter may have restricted accessibility to the backbone segment of the copolymer, particularly when numerous branches are located on a single backbone polymer. Block and graft copolymers are generally prepared b y polymerization processes, e.g. polymerization of a monomer i n the presence of a nonpropagating polymer chain or propagation of a polymer chain in the presence of a monomer differing from that polymerized initially ( I , 2, 3). Block and/or graft copolymers may be preformed and then added to the mix ture undergoing compatibilization, or they may be generated in situ. T h e latter approach permits preparation of compatibilizing agents b y the post re action between two polymers, e.g. a compatible polymer blend is prepared from incompatible ethylene-methacrylic acid and methyl methacrylate-methacrylic acid copolymers b y milling in the presence of zinc acetate w h i c h results i n the formation of ionic bonds between the incompatible polymers (4) (Diagram 2 ) . PE COOH ΡΕ

PMMA

I COO"

(2)

I +Zn+ - O O C COOH -

PMMA

A compatibilizing agent is also formed in situ when incompatible polymers are subjected to shearing forces which rupture polymer chains to generate radicals which undergo coupling. The molecular weight of the segments i n a block or graft copolymer plays an important role i n compatibilizing efficiency. A copolymer having a segment with molecular weight greater than 150,000 is generally a poor compatibilizing agent since intra- and intermolecular interactions such as chain entanglements may reduce the accessibility of such a segment to the homopolymer whose compatibilization is desired. T h e minimum molecular weight of a segment for effective compatibilization w i l l vary with the polymer structure. However, as a qualitative rule-of-thumb, since an oligomer containing 10-15 monomer units has characteristics reasonably similar to those of the higher molecular weight



78

COPOLYMERS,

POLYBLENDS,

A N D COMPOSITES

polymer, a segment of a block or graft copolymer containing 10-15 monomer units is an effective compatibilizing agent for the corresponding higher molecular weight homopolymer. Notwithstanding the general incompatibility of polymers and copolymers, polymers w i t h solubility parameters that differ b y 0.5 unit are compatible al though their structures may differ. Thus, poly (methyl methacrylate) ( P M M A ) , poly (ethyl acrylate) ( P E A ) , poly (vinyl chloride) ( P V C ) , and poly (butadieneco-acrylonitrile) (90/10-60/40) form useful compatible blends because their solubility parameters are i n the range 9.2-9.4. T h e difference i n solubility parameters resulting i n compatibility m a y be as m u c h as one unit when the polymers are of relatively l o w molecular weight. Extrapolation of the concept of the compatibility of polymers with different structures but similar solubility parameters to the construction of effective compatibilizing agents leads to the use of block and graft copolymers having segments with suitable solubility parameters to compatibilize polymers w h i c h differ both i n structure a n d solubility parameter. Thus, an A B block or graft copolymer compatibilizes polymers A and C when C has a solubility parameter similar to that of B, polymers D and Β when D has a solubility parameter similar to that of A , and polymers C and D when the solubility parameter of C is similar to that of B, and D has a solubility parameter similar to that of A . Modification of M e c h a n i c a l Properties. Application of these concepts is illustrated b y impact polystyrene: polystyrene ( P S ) grafted onto polybutadiene ( P B D ) permits as much as 4 0 % P B D i n PS to be incorporated whereas, i n the absence of the graft copolymer, incompatibility is detectable b y stratification when more than 1 0 % elastomer is blended w i t h P S (Diagram 3 ) . A c r y l o n i t r i l e PBD

PBD PBD

PBD

(3)

Ρ (S-AN)

PS

Ρ (S-AN)

PS

butadiene-styrene resin ( A B S ) consists of the compatibilized blend of poly(styrene-co-acrylonitrile) [ P ( S - A N ) ] , P B D , and Ρ ( S - A N ) grafted onto P B D (Diagram 3 ) . T h e properties may be modified b y adding copolymer to the polyblend generated b y graft copolymerization. A B S - t y p e resins are used as impact modifiers for P V C , but the resultant blend has insufficient transparency for application i n clear bottles. Transparency can be obtained b y grafting P M M A onto crosslinked P B D (5) or poly (butyl acrylate) (6) w h i c h has been previously grafted onto P S (Diagram 4 ) . I n this case the P M M A branch is compatible w i t h P V C by virtue of its solubility pa rameter, and optical clarity results from suitable component ratios i n the graft copolymer so that the refractive index matches that of P V C . T h e desired results are not obtained if a copolymer of methyl methacrylate and styrene is PBD PS

PS-PMMA

PMMA

PVC


(4)

7.

GAYLORD

79

Compatibilization PVC PMMA PMMA

(5)


PEHA

grafted onto the elastomer since the solubility parameter of the grafted co polymer branch differs from that of P V C . A n impact resistant P V C - a c r y l i c polymer alloy is formed by grafting P M M A onto poly (ethylhexyl acrylate) ( P E H A ) and then blending the resultant mixture of graft copolymer and homopolymers with P V C (7) (Diagram 5). Modification of Melt and Solution Properties. In addition to modifying mechanical properties resulting from polymer blending i n the presence of a suitable compatibilizing agent, the melt and solution viscosities of the compatibilized blend differ from those of the blend i n the absence of the agent because the compatibilized blend is not truly compatible; it is only partially or marginally so. The solution of a polymer in a good solvent, e.g. PS i n toluene or P E A i n methyl ethyl ketone ( M E K ) , has a high viscosity because the chains are ex tended and undergo intermolecular entanglement. Solution i n a poor solvent, e.g. P S i n M E K or P E A i n toluene, has a lower viscosity because the chains are coiled and undergo intramolecular rather than intermolecular entanglement. The marginally compatible polyblend is analogous to the poor solvent case in that presence of the discrete dispersed phase results i n less interaction be tween phases and, therefore, lower melt viscosities. Styrene-butadiene block copolymers have lower melt viscosities than random copolymers of the same composition and yield solutions of lower viscosity at the same concentration. This is because of the incompatibility but inseparability of the segments of the chain. The P V C - a c r y l i c polymer alloy previously described is characterized by high speed injection moldability and, i n sheet form, by deep drawing capa bility, both of w h i c h are the result of low melt viscosity. A cellulose-ethyl acrylate graft copolymer compatibilizes cellulose and P E A , cellulose and P V C , as well as starch and P V C ( 8 ). These compositions containing P V C obviously have better flow properties than non-thermoplastic cellulose or starch. H o w ever, they also have better flow properties than P V C alone. Polymer Dispersions and Solutions The effect of the presence of compatibilized incompatible components is apparent i n P V C plastisols. Monomeric and polymeric esters are good plasticizers for P V C because they have suitable solubility parameters. A good plasticizer is one w h i c h , in sufficient quantity, w o u l d almost be a solvent for the polymer. However, a good plasticizer, i.e. solvent, i n a plastisol results in a high viscosity composition. This may be unsuitable for slush molding or other applications when l o w viscosity is desirable. T h e latter is obtained b y adding a secondary plasticizer such as a hydrocarbon oil. In reality, the latter is not a plasticizer but actually a non-solvent w h i c h converts the good solvent plasticizer to a poor solvent mixture with resultant decrease i n plastisol vis-



80

COPOLYMERS,

POLYBLENDS,

A N D COMPOSITES

cosity. A similar result is achieved by using a plasticizer with poor solvent characteristics by virtue of the presence of a long hydrocarbon side chain, e.g. dihexadecyl and dioctadecyl phthalate and trimellitate. Solutions of incompatible polymers i n a common solvent cannot be mixed without early demixing on standing w h i c h is analogous to the inability to maintain i n the solid state the homogeneity of a blend of incompatible polymers undergoing mixing i n the molten state. Thus, two layers result when a solution of P S i n toluene is mixed with a solution of P E A i n toluene. W h i l e the PS chains are extended i n good-solvent toluene, the acrylic polymer chains are coiled i n poor-solvent toluene. Absence of polymer mixing results i n demixing. Similarly, when M E K is used as the common solvent, demixing results since M E K is a poor solvent for P S and a good solvent for the acrylic polymer. H o w ever, long term solution stability is achieved i n the presence of PS-grafted P E A since the latter has segments w h i c h are compatible with and mix with the incompatible homopolymers. This effect has been termed oil-in-oil disper sion (9, 10). A PS-grafted poly (ethylene oxide) as emulsifier permits preparation of a water-in-oil emulsion of P S . Solubility of the poly(ethylene oxide) segment in water results i n solubilization of the latter i n the PS matrix ( I I ) . In situ generation of a compatibilizing agent permits preparation of non aqueous dispersions of polymers i n non-solvents. Polymerization of methyl methacrylate i n the presence of linseed oil generates a P M M A - g r a f t e d linseed oil w h i c h promotes the dispersion of P M M A i n mineral spirits. T h e appearance of the high solids dispersion resembles that of an aqueous dispersion or latex, i.e. translucent blue-white when polymer particle size is smaller than 0.05μ, white when 0.1-2μ, and chalky white when larger than 2μ (12). Stable dispersions of very uniform particle size and solids contents below 8 0 % can be prepared by using a graft copolymer of natural rubber as the compatibilizing or dispersing agent ( 1 3 ) . Products based on hydroxystearic acid are even better dispersing agents (14). H i g h solids dispersions of P M M A , P V C , and poly (vinyl acetate) i n hydrocarbon solvents have been prepared by these techniques. Stability of the dispersions apparently results from the steric barrier formed by the solvent-soluble segments i n the graft copolymer and, as has been shown, the repulsion forces are 10 times greater than the attraction forces ( 1 5 ) . Reinforced and Filled Polymer Composites Use of wood flour, cotton, and other cellulosic products as fillers i n phenolic resin and urea resin compositions is based on the reaction of these methylolcontaining resins with the hydroxyl functionality of the cellulose, i.e. formation of a cellulose-phenolic resin or cellulose-urea resin compatibilizing agent. A cellulose-ethyl acrylate graft copolymer functions as a compatibilizing agent for incorporating non-thermoplastic, unreacted cellulose as a filler i n P E A , P M M A , or P V C (8) (Diagram 6 ) . T h e presence of an inorganic p o l y m e r organic polymer graft copolymer or compatibilizing agent changes the role of cellulose cellulose

PEA PVC


(6)

7.

GAYLORD

81

Compatibilization


the inorganic polymer from a filler to a reinforcing agent, i.e. the filler crosslinks the organic polymer. The interaction of glass fibers containing surface silanol groups with an unsaturated silane couping agent, e.g. vinyl triethoxysilane or methacrylatopropyltrialkoxysilane results in the appendage of reactive unsaturation on the fiber surface. When the latter reacts with a styrene-unsaturated (maleate or fumarate) polyester mixture, styrene copolymerizes with the unsaturation in the polyester and on the glass fiber surface. Thus, a glass-x-(styrene-polyester) graft copolymer serves to compatibilize the glass and the crosslinked polyester (see Reaction 7 ) . When the silane coupling agent contains an amino group,

glass—Si—OH + I

(RO) Si—X—CH=CH 8

I

I

glass—Si—O—Si—X—CH=CH I 1

2

-CH=CH

2

2

(7) H H polyester—OOC—C=C—COO-

glass glass—χ—polyester-S polyester-S

reaction with the glass surface results in formation of surface amine groups which react with and become incorporated into a cured epoxy resin. Thus, a glass-x-epoxy resin graft copolymer compatibilizes the glass and the cured epoxy resin ( Reaction 8 ). glass—Si—OH + ( R O ) S i — X — X H 3

2

ss—Si—Ο—Si—X—NH I

I

glass

2

I

epoxy resin

(8)

glass—χ—epoxy resin epoxy resin Silane coupling agents may also be used to generate compatibilizing agents from inorganic fillers and thermoplastic polymers. When a mixture of poly ethylene, clay, an alkyl- or allyltrialkoxysilane, and dicumyl peroxide is pre pared at 1 2 0 ° C , polymer and peroxide are unchanged. However, the hydroxyl functionality on the clay surface reacts with the silane coupling agent. When the compounded mixture is molded at 1 6 5 ° C , peroxide decomposes to generate free radicals which react with the polyethylene and the alkyl or allyl group on the silane resulting in coupling of the polymer and the silane. The in situ generated polyethylene-x-clay graft copolymer serves as compatibilizing agent for polymer and filler, the latter actually acting as a crosslinking agent for the polymer (16) (Reaction 9 ) . In situ polymerization of maleic anhydride in the presence of a polyolefin, a free radical catalyst, and a filler such as wood pulp, clay, or glass fibers results


82

COPOLYMERS,

polymer

polymer

XCH Si(OR) 2

filler

3

XCH Si-, 2

250°F *

peroxide

filler

POLYBLENDS,

A N D COMPOSITES

r polymer L

|350°C

-J

>

X

C

H

filler

2

S

i

—I

(9)

peroxide polymer polymer-z-filler


filler in formation of a polyolefin-x-poly ( maleic anhydride ) - x - f i l l e r compatibiliz ing agent. T h e poly (maleic anhydride) is grafted onto the polyolefin and interacts w i t h the filler through covalency, i.e. reaction of the anhydride group w i t h filler hydroxyl groups, and/or hydrogen bonding between carboxyl and hydroxyl groups (17). Fillers, fibrous reinforcing agents, powdered metals, and other materials having surface functionality are readily dispersed i n polymers containing carboxyl functionality. T h e latter may be random copolymers such as poly(ethylene-co-methacrylic acid) or graft copolymers such as (acrylic acid-g-polyethylene) and (maleic anhydride-g-polypropylene). As i n the pre vious cases, the polymer-x-filler compatibilizing agent is generated at the polymer-filler interface. In lieu of covalent or hydrogen bonds, the compatibilizing agent m a y contain ionic bonds. Thus, interaction of clay with a long chain unsaturated quaternary ammonium compound results i n ionic bonding between the clay and the cationic ends of the organic molecule. Reaction of treated clay w i t h a polyunsaturated monomer results i n copolymerization of the surface unsatura tion with the monomer. O n blending the resultant encapsulated clay containing residual surface unsaturation b y virtue of the presence of unreacted monomer unsaturation with polyethylene, radical sites generated on the latter react w i t h unsaturation on the clay. A clay N " ^ ^ ^ - x - p o l y e t h y l e n e compatibilizing agent is thus generated in situ (18, 19). +

Adhesives Improved interfacial interaction promoted b y the presence of a compati bilizing agent i n composites containing fillers or reinforcing agents is used i n adhesive formulation. Adhesive for application of P V C floor tile to a concrete surface contains P M M A grafted onto natural rubber (20). T h e graft copolymer undergoes stratification when applied to the back of the tile because of the solubility parameter match between the P V C tile and the grafted P M M A PVC PMMA I NR NR concrete


(10)

7.

GAYLORD

83

Compatibilization

branches. A conventional natural rubber ( N R ) adhesive is then applied to the floor before placing the tile down (Diagram 1 0 ) .


The compatibility of P M M A and P V C is also used in adhesives for applying P V C decorative film to wooden or gypsum surfaces; a mixture of P M M A a n d an epoxy resin makes a useful adhesive. T h e less than 1 0 % compatibility of P M M A with the epoxy resin limits the level of adhesion. However, b y polymeri zation of methyl methacrylate in the presence of epoxy resin, up to 3 0 % P M M A in the composition is compatibilized because of in situ generation of the graft copolymer (Diagram 1 1 ) . PVC PMMA ^ resin epoxy

(11) '

v

w w w w w wood or gypsum Application of solubility parameter match is also demonstrated i n adhesives for polyester tire cord. Whereas resorcinol-formaldehyde resin is used i n con junction with polyvinylpyridine latex as an adhesive for rayon tire cord, this composition is not suitable for poly (ethylene terephthalate) ( P E T ) cord. However, using hexylresorcinol rather than resorcinol results i n a match of the solubility parameters of adhesive and fiber with resultant increased adhesion (21). Similarly, a polyurethane containing polyester segments is an effective adhesive for lamination of P E T film to wood. T h e polyester segment of poly urethane is compatible with the polyester film surface while the urethane linkages provide hydrogen bonding interaction between adhesive and substrate (Diagram 1 2 ) . PET NHCOO-polyester-OCONH

(12)

wood Recapitulation Preparation of polymer blends is limited b y the general incompatibility of polymers. Compatibilization can be achieved b y suitable compatibilizing agents, i.e. block or graft copolymers with segments of structure or solubility parameter similar to those of the polymers being mixed. A compatibilizing agent influences the properties of polymer blends and alloys as well as polymer melts and solutions. Solution of incompatible polymers i n a common solvent as well as dispersion of a polymer i n a non-solvent is promoted b y a compatibilizing agent. Dispersion of additives such as fillers and reinforcing agents (including cellulose, clay, glass fibers, and powdered metals) i n a polymer matrix is en hanced b y the presence of a compatibilizing agent at the polymer-additive


84

COPOLYMERS,

POLYBLENDS, A N D COMPOSITES

interface. Compatibilization of the interface also promotes the adhesion of solid polymers to polymeric or inorganic surfaces.


Literature Cited 1. Ceresa, R. J., "Block and Graft Copolymers," Butterworths, Washington, 1962. 2. Burlant, W . J., Hoffman, A . S., "Block and Graft Copolymers," Reinhold, New York, 1960. 3. Ceresa, R. J., "Block and Graft Copolymers," in "Encyclopedia of Polymer Science and Technology," Mark, H. F., Gaylord, N . G . , Bikales, Ν. M., Eds., 2, 485, Interscience, New York, 1965. 4. Rees, R. W . , Ε. I. duPont de Nemours & Co., U.S. Patent 3,437,718 (1969). 5. Himei, S., Takine, M . , Akita, K., Kanegafuchi Chemical Industry Co., Japanese Patent 866 (1967); Chem. Abstr. (1967) 67, 22418. 6. Ryan, C. F., Crochewski, R. J., Rohm & Haas Co., U.S. Patent 3,426,101 (1969). 7. Souder, L . C., Larsson, Β. E., Rohm and Haas Co., U.S. Patent 3,251,904 (1966). 8. Gaylord, N . G . , U . S. Plywood-Champion Papers, Inc., U.S. Patent 3,485,777 (1969). 9. Molau, G . E., J. Polymer Sci. Part A (1965) 3, 1267; Ibid., 3, 4235. 10. Riess, G . , Periard, J., Banderet, Α., in "Colloidal and Morphological Behavior of Block and Graft Copolymers," Molau, G . E . , E d . , p. 173, Plenum, New York, 1971. 11. Bartl, H . , Bonin, W . V . , Makromol. Chem. (1962) 57, 74. 12. Schmidle, C. J., Brown, G . L . , Rohm & Haas Co., U.S. Patent 3,232,903 (1966). 13. Osmond, D . W . J., Thompson, M . H., Imperial Chemical Industries, L t d . , British Patent 893,429 (1962); U.S. Patent 3,095,388 (1963). 14. Osmond, D . W . J., Waite, F . Α., Walbridge, D . J., Imperial Chemical Industries, Ltd., U.S. Patent 3,514,500 (1970). 15. Dowbenko, R., Hart, D. P., Ind. Eng. Chem. Prod. Res. Develop. (1973) 12, 14 16. Union Carbide Corp., "Silane Adhesion Promoters in Mineral-Filled Composites," Tech. Bull (1973) F-43598. 17. Gaylord, N . G . , U . S. Plywood-Champion Papers, Inc., U.S. Patent 3,645,939 (1972). 18. Amicon Corp., French Patent 1,539,053 (1968). 19. Hausselein, R. W . , Fallick, G . J., Appl. Polym. Symp. (1969) 11, 119. 20. Bevan, A . R., Bloomfield, G . F., Adhes. Age (1964) 7 ( 2 ) , 36; Ibid., 7 ( 3 ) , 34. 21. Iyengar, Y., Erickson, D . E . , J. Appl. Polym. Sci. (1967) 11, 2311. R E C E I V E D April 3, 1974.


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