Volume 2, Number 1, Fall 2001

The Effects of Some Organic and Inorganic Pigments on the Tensile and
Impact Properties of Injection-molded Polypropylene

ABSTRACT

Color pigments are used in the plastics industry to enhance the aesthetic appeal of plastics products and in some cases, to color-code some plastics products used in specific applications. There are three types of colorants. These are dyes, organic, and inorganic pigments. Recently, there has been a trend to shift from the use of inorganic pigments to organic pigments since the former are perceived as health hazards This is because inorganic pigments contain heavy metals such as cadmium, lead, and chromium, which are considered as health hazards. This work was undertaken to study the effects of the change from inorganic to organic pigments on the tensile and impact properties of injection-molded polypropylene (PP). Since PP is a semi-crystalline plastics, it is supposed that the change in pigments might affect the crystallization process during processing, and therefore, its tensile and impact properties.

INTRODUCTION

Plastic materials have played, and continue to play, a significant role in improving our standard of living. A cursory survey of the use of plastics in the transportation industry, the building industry, the electrical and electronic industry, the agricultural industry, the medical industry, etc. would attest to this claim. Indeed, the data on the world production of plastic materials support this assertion; world production of plastic materials have grown from 300,000 tons in 1939 to 91,237,000 tons in 1992¹ presenting an annual growth of 11 percent during the 53-year period. Probe Economics, Inc.² and Modern Plastics³ estimate that the United States consumed 23.3 and 34.9 million tons of selected thermoplastics (high-density polyethylene, low-density polyethylene, linear-low-density polyethylene, polystyrene, polypropylene, and polyvinyl chloride) in 1994 and 1999, respectively. These data indicate an annual consumption growth rate of 8.4%, which is quite impressive for an industry given that the U.S. gross domestic product (GDP)⁴ grew at 3.8% annually for the same time period. Given the successful history of plastic materials in enriching our lives, it would seem pertinent to continue enhancing our fundamental understanding of plastic materials to maximize its benefits to society.

It is general knowledge within the plastics industry that most plastics resins would have limited use but for the additives that are added to them. These additives usually enhance the properties of plastics materials such as their strength, durability, aesthetic appeal, and processability. Because of the importance of additives to plastics materials, many studies^5,6 have been undertaken to understand their effects on the performance of plastics, especially since it has been shown that incorporating additives into plastics materials during fabrication often affects its rheological^7,8, mechanical^9,10, and optical properties¹¹ in an unpredictable, and sometimes, harmful fashion. A recent trend in the plastics industry has been to replace inorganic pigments used in plastic products with organic pigments because of health and environmental concerns^{12, 13}. Kaul¹³ suggested that the health concerns emanate from

· environmental problems in manufacturing (i.e., management of waste water, waste residues, waste solvents, etc.)

· traces of potentially harmful impurities such as dioxins, and heavy metals such as cadmium, lead, chromium, mercury, etc. in final products

· environmental fate at disposal stage

· toxicological and environmental toxicological safety of final products in use.

In addition to the environmental concerns, color pigments are required to have the following features^{13, 14}:

· Ease of incorporation and dispersion

· Color fastness: a measure of its inherent ability to resist the chemical and physical influences to which it is exposed during and after its incorporation into a material.

· Compatibility with other additives

· Heat stability: the ability to resist any change of shade through chemical degradation that come about from the drastic processing conditions of most plastics

· Permanence: the ability to avoid the tendency to bleed or migrate out of the host plastics.

While all color pigments must meet these requirements, inorganic pigments are generally favored for plastics applications because of their ability to provide excellent resistance to heat, light, weathering, migration and chemicals at relatively low cost. Organic pigments, however, are known for their high color strength, brightness, and good transparency¹³. In this study, the authors want to examine if switching from inorganic to organic pigments (additives) would have any significant effects on the mechanical properties of injection-molded polypropylene.

Polypropylene was chosen for this study because of the availability of research on the characteristics of the material. Furthermore, although both polyethylene and polyethylene are commodity resins and semi-crystalline in nature, it is easier to work with polypropylene than polyethylene in terms of their crystallization rates when compared with the time scale of the injection molding process. The crystallization rate of polyethylene is several times faster than that of polypropylene¹⁶, thus making it difficult to control the crystallization process with polyethylene. As Lenz ¹⁷ noted, "the crystallization rate varies widely among the different polymers. The rate in polyethylene and Teflon is so high that crystallization cannot be prevented or significantly reduced by quenching the melt in liquid nitrogen". Polypropylene is used for making products such as carbonated-beverage bottle caps, automobile bumper facia and grills, disposable diapers, disposable syringes, portable coolers, stadium seats, multilayered blow-molded bottles, and outdoor furniture.

EXPERIMENTAL

The polypropylene used in this study was Pro-fax 6323â . It was manufactured by Montell Polyolefins. Pro-fax 6323 resin is a general-purpose polypropylene homopolymer with a melt flow rate (MFI) of 12 g /10 min (230 ° C / 2.16 kg). This PP was used because it did not contain titanium dioxide, which is known to have a nucleating effect on PP¹⁸. Therefore, any nucleating effects caused by the addition of pigments to PP will be primarily attributed to the pigments. Pigments used are listed in Table 1.

Masterbatches were prepared by melt blending with a C.W. Brabender counter-rotating twin-screw extruder (Model 2003). The polypropylene resin was used as a carrier for the pigments. Before introducing the resin and the pigments into the extruder, the pigments were dispersed over the resin by placing the mixture of the resin and the pigments in a gallon paint can and shaking the can in a paint-shaker for about 30 minutes for each mixture. The extruder barrel temperature profile was

Zone 1 (rear zone) 329 ° F

Zone 2 (front zone) 347 ° F

Zone 3 (Die) 365 ° F

Melt temperature 385 ° F

The screw speed was set at 22 rpm. The extrudates were run through a cooling (water) trough, an air-dryer, and a pelletizer. All masterbatches were prepared with pigment concentrations of five percent by weight.

Polypropylene impact and tensile test specimens were prepared at 0.5, 1.0, and 2.0 wt% pigmentations using a 60-ton Sandretto injection-molding machine. The mold temperature was 100 ° F and the barrel temperature profile was

Rear zone 360 ° F

Middle zone 390 ° F

Front zone 405 ° F

Nozzle 410 ° F

Injection pressure was set at 1450 psi, the holding pressure was set at 800 psi, holding pressure time was 8 seconds and total injection time was 10 seconds. Screw rotation speed was 65 rpm and screw backpressure was 100 psi. Cooling time was 10 seconds.

Tensile testing was done according to ASTM D-638 with an Instron Universal Testing Instrument, Model 1011. A strain rate of 2 in/min was used for testing all the tensile specimens. All the specimens were conditioned at room temperature ~74 ° F for about a year before testing and were not exposed to ultraviolet (UV) light.

Notched impact testing was performed according to ASTM D-256 with a Tinius Olsen Model 92T Impact Tester. The impact specimens were conditioned in the same fashion as the tensile specimens. In addition, the specimens were conditioned for at least 36 hours after notching and before impact testing them.

RESULTS AND DISCUSSION

Table 2 show the effect of the pigments on the modulus of elasticity, the tensile strength at yield, and the tensile strength at break of PP. Table 3 shows the influence of the pigments on the tensile strain at yield and break, respectively. Based on these results, we suggest that the incorporation of organic and inorganic pigments into PP did not adversely affect its modulus of elasticity, but rather increased it. The inorganic pigments increased the modulus of elasticity by 6.8 percent while the organic pigments increased it by 21 percent on the average. With respect to tensile strength at yield and break, the inorganic pigments showed a slight decreasing effect on these properties while the organic pigments showed 10 percent and 98 percent increases, respectively, on these properties. Table 3 shows that incorporation of the pigments into PP decreased the tensile strain at break by 69 percent while the inorganic pigments showed, on the average, a 9 percent increase. These results agree with the findings of Kenig at el.¹⁹ and Krisher and Marshall²⁰, which showed that incorporating pigments into PP affected its mechanical properties. Krisher and Marshall²⁰ observed that the addition of certain pigments to PP decreased its mechanical properties. The authors, however, did not state if the pigments they used in their study were organic, or inorganic or a combination of organic and inorganic pigments. Thus, it is not possible at this time to compare the results of this study with theirs. Kenig et al.¹⁹ who used only organic pigments for their work observed increases in the mechanical properties of PP. Our results showed that neither the organic nor the inorganic pigments decreased the mechanical properties of PP, but that organic pigments had a greater positive impact on the properties of PP than inorganic pigments. We believe that the reason for this difference was due to the nucleating effects of these pigments, which Wunderlich²¹ attributed to the level of the nucleating activity of the pigments. He suggested that organic pigments are more active nucleating agents than inorganic pigments. Therefore, incorporating organic pigments into PP resulted in a higher degree of crystallization of PP than did the inorganic pigments. This in turn led to the higher values in the modulus of elasticity and tensile strength at yield and break, respectively.

This finding has some design implications for the plastics engineer. When a plastics component is stressed, it responds by deforming to an extent governed by its stiffness, which can be quantified by its modulus of elasticity. Consequently, it could be suggested that by incorporating organic pigments into natural PP, its resistance to deformation will be increased by about 21 percent. This benefit to the structural integrity of PP is in addition to the pigments’ primary function of coloring it.

 

Table 3. Tensile strain of injection-molded neat and pigmented polypropylene

Table 4 shows that impact strength increased by 23 to 52 percent with the addition of the pigments to neat PP. Although the results showed general increase in the impact strength of neat PP, it is not clear why the organic pigments had a higher effect (35 percent increase) on the impact strength than the inorganic pigments (26 percent increase). These results are perplexing since impact strength is supposed to increase with increasing tensile strain at break²². Based on this supposition and the data in Table 2, the organic pigments were expected to adversely affect the impact strength of neat PP. However, our findings were contrary to expectations, but are similar to those of Kenig et al.¹⁹ and Krisher and Marshall²⁰. These findings suggest that more studies are needed to understand this phenomenon.

Table 4. Izod notched impact strength of injection-molded neat and pigmented polypropylene

Tables 5 and 6 show the effect of time on the tensile strength and impact strength of the neat and pigmented PP. These tables show a considerable change in values of these properties with time, which suggests that crystallization may continue to take indefinitely, although very gradually, at ambient temperatures. The authors are currently studying the effects of these changes on the optical properties of pigmented PP.

Table 5.  Effect of time (aging) on Tensile Strength at yield of injection-molded neat and pigmented polypropylene

CONCLUSION

This work showed that incorporating organic and inorganic pigments into neat PP did not adversely affect its mechanical properties. On the contrary, these properties increased with the addition of these pigments. The results show that the increases were higher with organic pigments than with the inorganic pigments. Furthermore, our findings suggest that more work may be needed to shed light on the relationship between elongation at break and impact strength of neat and pigmented PP.

ACKNOWLEDGEMENT

We would like to thank Montell for donating the polypropylene, and BASF, CIBA, and Cerdec for donating the color pigments.

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