Dr. F. Parodi  -  Industrial R&D Expert

technical web papers:  # 1

Sizing Compositions for Glass Fibers Affording Polypropylene 

Matrix Composites with High Mechanical Properties:  A Case Story

Fabrizio Parodi

Sizing compositions for glass fibers for polypropylene reinforcement were typically based for years on a hydrolyzed aminoalkyltrialkoxysilane as the coupling agent, and an aqueous, colloidal dispersion of the alkali metal salt (namely, the potassium salt) of a carboxylated, low-molecular-weight crystalline polypropylene.  These polypropylene dispersions are typically attainable by dissolving at about 160°C under pressure a maleic anhydride-grafted polypropylene (of average Mn = 3500-4000) in water, containing a stoichiometric amount of caustic potash, and an appropriate polyoxyethylene isooctylphenyl ether as the emulsifying agent.  The resulting coatings were currently providing fiber surfaces with an acceptable wettability by molten, and a reasonable compatibility with crystallized, polypropylene matrices, thus affording composites with a good, overall spectrum of mechanical properties.

Novel fiber sizing compositions were developed in 1984, leading to, at that time unparalleled, enhancements of mechanical properties of glass fiber-reinforced homo-polypropylene:  + 15% of tensile and flexural strength, + 40% of impact strength, and - 30% of flexural compliance at 120°C, in composites containing 30 wt % of short fibers. Yet, the analysis of the IZOD fracture mechanics of such composites showed 7% and 30% gains in the maximum (peak) stress and total fracture energy, respectively.  Principally, it was made possible through a minimization of the fiber surface polarity (the surface free energy of the new dry sizing films being lowered of 2-4 erg cm^-2 with respect to conventional ones, as calculated from contact angle measurements), implying in turn a better thermodynamic compatibility at the interface between the so-coated fibers and polypropylene.  This was also well demonstrated by marked decreases of the critical fiber length, and correspondingly 35-40% higher values of the fiber/matrix shear strength, in these composites [1,2].   The SEM observation of their fracture micromorphology gave a visual confirmation of said enhancements of the bond strength between fibers and polymer matrix. Figures 1a and 1b show in fact the (tensile) fracture surfaces of short glass fiber/homo-polypropylene composites, with the conventional and novel fiber sizing compositions, respectively (both with a 30 wt % of glass fibers of average diameter = 13 µm).

Figure 1a

Figure 1b

Even greater improvements of mechanical properties were achieved with slightly chemically-modified polypropylene matrices, i.e. containing modest amounts of maleic anhydride- or acrylic acid-grafted polypropylene.  The matching between the thermodynamic characteristics of such (slightly carboxylated) polypropylene matrices and those of the novel fiber sizing compositions was so good, indeed, that outstanding levels of fiber-to-matrix adhesion were reached.  This is well evidenced by the SEM micrographs of Figures 2a and 2b, for the (tensile) fracture surfaces of composites from a homo-polypropylene + acrylic acid-grafted polypropylene blend (with an overall content of 0.3 wt % of acrylic acid) as the matrix, and 30 wt % of glass fibers (average diameter: 13 µm) with the conventional and novel fiber sizing compositions, respectively.

Figure 2a

Figure 2b

Such strong enhancements of the fiber-to-matrix bonding capacity caused by the novel sizing compositions resulted in the following enhancements of the mechanical performance of these composites (novel vs. conventionally-sized fibers):  + 24% of tensile and flexural strength, + 55% of impact strength, and - 40% of flexural compliance at 120°C.

The "secret" behind these accomplishments was the replacement of the alkali metal hydroxide as the neutralizing agent in the aforementioned aqueous dispersions of maleic anhydride-grafted polypropylene, employed as the main fiber sizing component, with totally different bases.  Such bases were selected in order to leave crystalline, weakly acidic polypropylene films to coat the fiber surfaces after their drying at 130-160°C, in place of the corresponding potassium carboxylate ionomer given by the previous, conventional sizing compositions.  Such acidic, crystalline polypropylene films possess a far better melt-miscibility, and a fairly good attitude to co-crystallize, with pure, high-molecular-weight homo-polypropylene matrices.  Moreover, the coating film itself can be tightly linked to fibers via ionic and/or covalent amide bonds formed by its free -COOH groupings with the -NH2 and -NH- ones of the amino-silane, chemically bonded to glass fibers [3].

Further improvements of mechanical properties of homo-polypropylene matrix composites endowed by the new fiber-sizing strategy (i.e., additional + 12-18% of tensile, flexural and impact strengths) were subsequently achieved in 1987, by increasing (doubling or tripling) the average molecular weight of the carboxylated polypropylene, dispersed in water as a colloid, and used as the fiber coating material, as said above [4].  This showed the important role played by the toughness of a co-crystallized interphase between fibers and the polypropylene matrix, made strongly adhering to the fibers themselves.  Suitable carboxylated polymers were obtained by controlled thermal degradation of standard isotactic homo-polypropylene (injection-molding grade) at 300°C under nitrogen, followed by peroxide-promoted grafting of suitable, high-boiling, maleic acid esters [5].

The three development steps of fiber sizing compositions described above are well outlined in the table below, through a comparison of the typical mechanical properties of a homo-polypropylene matrix reinforced with 30% by weight of short glass fibers with the three types of surface treatment.  Composites were obtained by single-screw extrusion of dry blends of 4.5 mm-chopped glass fibers and homo-polypropylene chips (average fiber diameter = 13 µm;  volume-average length of fibers within the resulting composites = 580-610 µm).

Property

Units

Fiber Sizing Composition

Conventional

1984

1987

Tensile Strength

MPa

70

80

90

Tensile Modulus

GPa

5.6

5.8

6.1

Tensile Elongation at Break

%

2.0

2.2

2.2

Flexural Strength

MPa

105

120

135

Flexural Modulus

GPa

5.8

6.0

6.2

IZOD Impact Strength

un-notched

kJ m^-2

23

31

36

notched

J m^-1

70

80

94

F. Parodi and C. Belgiovine, "Coupling Glass Fibers for Polypropylene Reinforcement", Proceedings of the 6th National Meeting on Composite Materials (Milan, Italy, May 8-9, 1985), Pitagora Editrice, Bologna, Italy, 1985, pp. 105-114 (in Italian);  Macplas, 84, 95-97 (1987) (in Italian);  Macplas International, Jul. 1987, pp. 119-120.

F. Parodi, "Fillers and Reinforcing Agents", Proceedings of the 24th AIM Meeting-School "Additives for Polymeric Materials" (Gargnano, Garda Lake, Italy, May 27-31, 2002), Pacini Publ., Pisa, Italy, 2002, pp. 223-286 (in Italian).

F. Parodi, unpublished work.

F. Parodi , "Development of High-Performance Glass Fibers for Polypropylene Reinforcement", Plastic & Kauçuk (Istanbul, Turkey), March-April 1989, pp. 4-5 (in Turkish).

F. Parodi and C. Belgiovine, unpublished results.

fparodi@iol.it