Roles of Additives in Scratch Resistance of High Crystallinity Polypropylene Copolymers.
Scratch behavior in neat and talc-filled high crystallinity polypropylene (PP) copolymers containing various additives is investigated using a constant load scratch test and two types of indentation tests (Rockwell and Shore D). The talc-filled PP copolymer exhibits high scratch visibility and scratch depth. The addition of a nucleating agent or lubricant improves the scratch resistance of the talc-filled PP copolymer. Differential scanning calorimetry, scanning electron microscopy and attenuated total reflectance Fourier transform infrared spectroscopy are used to characterize crystallinity, morphology and scratch mechanisms in PP systems. It is found that the scratch resistance of the PP copolymer systems investigated, in terms of scratch depth and scratch visibility, depends mainly on the fracture features generated during the scratch process. The influences of talc, nucleating agent and lubricant on the scratch behavior of PP are discussed.
Themorplastic olefins (TPOs), based on polypropylene (PP) copolymer, have been established as the materials of choice for interior automobile parts and other component applications. However, PP copolymer is relatively soft and is easily damaged by surface scratches, thus limiting its acceptability for automotive applications. As a result, improving the scratch resistance of TPOs is critical for maximizing the use of PP copolymers. Research on scratch resistance in TPOs is gaining more attention in recent years [1-6]. Typical scratch patterns and scratch mechanisms in TPOs were investigated by Hutchings et al, [2, 3], Wu et aL  and Chu et aL [5, 6]. Many factors, such as filler type, additive, lubricant, impact modifier and surface morphology, are believed to affect the scratch behavior in TPOs. These factors and their effects need to be investigated.
Owing to the rising attention to scratch resistance in polymers, several scratch testers are being developed to quantitatively evaluate the scratch resistance of bulk polymers, coatings and films [5-10]. Scanning electron microscope (SEM), surface interferometer, atomic force microscope (AFM)  and acoustic emission are being used to characterize the fine scratch patterns on polymer surface. Of these, the constant load scratch test developed by Ford Motor Company (Ford Lab Test Method) is being considered as a standard for the automotive industry. The scratch behavior of a broad range of polymers under the constant load scratch test has been studied previously . A scratch model proposed by Hamilton and Goodman has been applied to interpret the mechanics of the constant load scratch process [11, 12]. From the above efforts, it has been shown that a certain level of rigidity and hardness in polymers is essential to improve scratch resistance . The rigidity of PP copolymer depends on the degree of cr ystallinity and can be improved by adding rigid fillers. It is for this purpose that a high crystallinity PP copolymer and talc were used in this study. However, the stress analysis based on the scratch model  indicates that a higher magnitude of tensile stress will be generated just behind the scratch head if the rigidity of the polymer is increased. This will likely promote cracking, crazing, and debonding in the presence of the particulate inhomogeneities in filled polymers, such as TPOs. These fracture features may lead to intense scattering of light from surface and, in turn, increase scratch visibility. Thus, a delicate balance between rigidity and toughness has to be achieved to obtain scratch resistant TPOs. A better understanding of the roles of additives and fillers in the scratch behavior of TPOs is needed for the maximum utilization of TPOs for automotive applications.
The current study attempts to investigate whether or not the scratch resistance, in terms of scratch depth and scratch visibility, can be correlated with the conventional material characteristics, such as indentation hardness and crystallinity, in TPOs. Then, the main cause for the high crystallinity TPOs to exhibit different scratch resistance characteristic is discussed. The effect of filler and additives on the scratch behavior of high crystallinity PP copolymer is investigated. It is hoped that the findings obtained from this study will provide guidelines for improving the scratch resistance of TPOs.
High crystallinity PP specimens for the present study were obtained from Color and Composite Technologies, Inc., and are listed in Table 1. The PP copolymer, containing 20% polyethylene (PE) comonomer, has a melt flow rate (MFR) of 35 dg/min (ISO 1133). Talc, which has a median particle size of 3 [micro]m (9603, Polar Minerals, Inc.) and a Mohs hardness of 1, was utilized for the present work. Sodium 2.2'-methylene bis-(4,6-di-tert-butyl phenyl) phosphate (NA-11, Amfine Chemical Corp.) was used as the nucleating agent. The specimens used for the scratch test were injection molded. The surfaces of the molded plaques were smooth on both sides. Medium dark graphite color concentrate was added at 4% by weight to give a dark color background for better scratch visibility analysis. The lubricant (MB50-001, Dow Corning Corp.) used was a pelletized formulation, containing 50% ultra-high molecular weight siloxane polymer dispersed in PP copolymer.
Scratch Test and Indentation Tests
The constant load scratch test was performed according to the Ford Lab Test Method (FLTM) BN108-13 in this study. Detailed instrumentation and experimental procedure of the scratch test were described in the literature . Each plaque was scratched using highly polished steel balls (1 mm diameter) with applied dead weight loads of 7N, 6N, 3N, 2N, and 0.6N, respectively. The sliding velocity used was approximately 100 mm/s. All tests were performed at room temperature. All test plaques were conditioned at 25[degrees]C for more than 24 hours prior to testing.
After the plaques were scratched, they were evaluated with a reflected-light polarizing microscope having a Xenon light source. An analyzer with image analysis software was used to measure the "gray scale mass which is equal to the total gray scale value of the object. The objective lens of the camera was positioned directly facing the scratch surface. The objective lens registers a portion of the scratch about 1 mm long. Electronic signals from each scratch line were integrated and recorded. The optical mass of an object, M, is the sum of the gray level values, GL, of all pixels in the image. The individual gray level values are assigned by the image analysis software program in units ranging from 0-255, where 0 = black and 255 = white.
M = [[[sigma].sup.n].sub.t=1] [GL.sub.i] (1)
where n is the number of pixels. The brightness of the object, B, is defined as:
B = M/A (2)
where A is the area of the object. The scratch visibility, [delta]B, is defined as the percent change of the brightness between the scratch and the background,
[delta]B = [B.sub.scratch] - [B.sub.background]/[B.sub.background] X 100% (3)
The depth of the scratch was measured with an interferometer (WYKO NT-2000), equipped with the WYKO Vision-32 analysis software. Magnification was set at 5X and the depth measurement was made using the depth histogram of the scanned area. The scratch mechanisms were examined using the JSM-6400 SEM operated at an accelerating voltage of 15 kV.
The Rockwell and Durometer hardness tests were performed on the TPOs samples, following the ASTM D785 and the ASTM D2240 (type D) test procedures, respectively. Details of the indentation tests are described in the literature [5, 6].
Differential Scanning Calorimetry (DSC) Analysis
The heat of fusion of the specimens was measured using DSC (Perkin-Elmer, Pyris-1). Samples were scraped from the skin of the plaque, which is relevant to scratch performance for DSC analysis. Samples were first heated from 30[degrees]C to 200[degrees]C at a rate of 10[degrees]C/min and cooled by decreasing the temperature to 30[degrees]C at a rate of 30[degrees]C/min. After the samples were held at 30[degrees]C for 2 min, they were heated again to 200[degrees]C at a rate of 10[degrees]C/min for a second scan. The reported heat of fusion for the first and second heating runs were calculated based on PP content alone.
Attenuated Total Reflectance (ATR) Fourier Transform Infrared (FTIR) Spectroscopy
The scratch process occurs on the polymer surface. Scratch damage is limited to within a few micrometers underneath the surface of the sample. Therefore, only the additives on or near the surface can affect the scratch behavior of polymers. ATR-FTIR spectroscopy (Nicolet Avatar 360), which is a convenient surface-sensitive analytical tool for probing functionality of polymer structure, was used to detect the presence of additives on the sample surface.
The results of scratch visibility, scratch depth and indentation hardness of various PP copolymers are listed in Table 2. Scratch visibility and scratch depth are measured from the 7N load scratch test.
In terms of scratch visibility, three unfilled PP copolymers (A1, A2 and A3) show low scratch visibility. Scratches are more visible in specimens filled with 25% talc (B1). The addition of the nucleating agent or lubricant reduces the scratch visibility of the filled specimens significantly (B2, B3). These specimens show comparable scratch visibility. However, the scratches are still more visible than those from the unified specimens.
Scratch depth, defined as the distance between the plaque surface and the bottom of the groove, is measured based on the depth histogram by an interferometer. The recorded scratch depth is the average of at least three measurements along the scratch. The unfilled PP copolymers (A1-A3) have relatively shallow scratch depths. No significant difference in scratch depths is found among these specimens. In the case of the talc-filled PP copolymer (B1), it shows a much deeper scratch depth. A significant reduction in scratch depth is observed in filled specimens with the addition of a nucleating agent (B2) or lubricant (B3), although their scratch depths are still higher than that of the neat PP copolymer (A1).
However, the results from both the Rockwell R and Shore D hardness tests show all six PP samples have almost the same indentation hardness value regardless of the use of talc or additives. Neither of the indentation hardness measurements can accurately predict the differences in the scratch performance of the various PP materials studied here.
Typical surface profiles of PP copolymers using an interferometer are shown in Fig. 1. In general, ridges of the deformed material are produced on both sides of the scratch groove surface. Three unfilled PP copolymers (A1-A3) show similar surface profiles with a smooth groove (Fig. 1a). Extensive debris, giving a more rugged surface around the scratch groove (Fig. 1b-d), is found in three talc-filled PP copolymers (B1-B3). Without any additive, deep subsurface cracks are clearly shown in the talc-filled PP (El) (Fig. 1b). This indicates that the scratching process generates significant fracture damage in talc-filled PP.
SEM micrographs of the 7N load scratches on PP copolymers are shown in Figs. 2-6. For unfilled PP (A1), it shows an "ironing" scratch morphology , which is a less severe plastic deformation scratch pattern (Fig. 2). No detectable crazes or cracks are found on the scratch surface. However, there is matrix debris in the middle of the scratch. The boundary between the scratch and plaque surface is not distinct, suggesting that the scratch is shallow. A similar morphology with less matrix debris in the scratch is observed in the specimens with nucleating agent (A2) and lubricant (A3).
Significantly different scratch patterns are found in talc-filled PP copolymers (B1-B3), as shown in Figs. 3-5. For the sample with 25% talc (B1), the scratches become more visible. Many fracture features, such as matrix debris, cracks, debonding and voids, are observed in the middle of the scratch track. Many ripple-like crazes/cracks form a parabola and extend beyond the scratch track. In addition, talc/resin debonding is found using high magnification SEM (Fig. 6). These fracture features contribute to the whitening on the scratch surface, making the scratches on the surface of the talc-filled PP (B1) much more visible. Similar fracture features, without ripple-like crazes/cracks, are found in the specimens having nucleating agent (B2) (Fig. 4). With an addition of lubricant (B3), the level of fracture features reduces even more (Fig. 5).
DSC is a common tool for gaining useful information about the characteristics and stability of the crystalline structure, as well as the thermal history of the sample . Figure 7 shows the heat of fusion results of PP copolymers on the surface in the first and second heating runs using DSC. All samples show a similar shape in the first melting curve with one peak temperature at about 166[degrees]C. This indicates that all the tested PP copolymers have similar crystalline characteristics. It is noted that the chemical nature of the PP copolymer is not known, it is difficult to estimate the heat of fusion for the 100% crystallinity PP copolymer. As a result, only the heat of fusion for PP systems is reported. An increase in the heat of fusion (crystallinity) for specimens with additives is found in both unfilled and talc-filled PP copolymers based on the first heating run. The lubricant appears to be more effective in increasing crystallinity than the nucleating agent. This indicates that crystallinity of P P copolymers on the surface is affected by both processing conditions and the presence of additives.
Figure 8 illustrates the ATR-FTIR spectra of PP copolymers with and without lubricant. The peaks shown within the wavenumber range of 750-1500 [cm.sup.-1] indicate the characteristic bands for lubricant. The ATR spectrum of the as-received lubricant is also plotted as a reference. The characteristic bands ascribing to lubricant are detected at 1260, 1091-1081 and 801 [cm.sup.-1] in unfilled PP with lubricant (A3) and in the talc-filled PP with lubricant (B3). This clearly indicates the presence of lubricant on the surface of the PP copolymer. It is noted that the samples containing nucleating agent were also investigated using ATR-FTIR spectroscopy. No significant evidence of nucleating agent was found on the sample surface possibly due to the small concentration of the nucleating agent in PP.
Scratch behavior of a wide range of polymers under the constant load scratch test has been previously studied (11). There appears to be a power-law relationship between the scratch depth and the scratch hardness (Fig. 9). The scratch hardness is defined as the applied normal load divided by the moving projected contact area (2, 11). That is, for a spherical scratcher, the scratch hardness, [H.sub.s], can be written as:
[H.sub.s] = P/A = 4P/[pi][[omega].sup.2] (4)
where P is the applied normal load, A is the moving projected contact area, and [omega] is the residual scratch width. It is noted that the power-law relationship is related to the geometry effect between the scratch depth and the scratch hardness (11). It is found that neat PP and TPOs fall within a transitional region where scratch depth is sensitive to scratch hardness.
Obviously, a certain degree of hardness and rigidity in TPOs is essential for resisting deformation during scratch (Fig. 9). In this study, a high degree of crystallinity in terms of heat of fusion was found in the skin of all tested PP copolymers (Fig. 7). Although there is as much as a 20% change in heat of fusion among the talc-filled PP systems (B1-B3) based on the DSC first heating run, no apparent changes in indentation hardness are observed. This may be because the crystallinity of the tested samples is already high and the hardness values depend on factors other than the local surface crystallinity alone. Consequently, the indentation hardness of the high crystallinity PP systems remains the same regardless of the presence of filler or additives.
It has been reported that, providing that the deformation and damage modes are similar, the mechanical response in the indentation process is similar to that of the scratch process (15, 16). However, as seen in Table 2, the indentation hardness study cannot reveal the differences in the scratch resistance of PP systems with or without additives. The talc-filled PP copolymer (B1) exhibits a much higher scratch visibility and scratch depth (Table 2). Thus, no direct correlation is found between indentation hardness and the scratch resistance of high crystallinity PP copolymers. This implies that indentation hardness cannot be considered a measurement of scratch resistance in TPOs because significantly different damage mechanisms and mechanics are involved in the indentation and scratch processes. It will be shown in the following discussion that different scratch mechanisms are observed in different PP systems, resulting in different scratch visibility.
Based on the stress analysis of the scratch model proposed by Hamilton and Goodman (12), a schematic showing the distribution of surface stress, [[sigma].sub.xx], along the center line of the scratch track is given in Fig. 10. Obviously, a much higher maximum tensile stress is generated just behind the scratch head during scratch (F [neq] 0) than that caused by the indentation process (F = 0) alone. The maximum tensile stress increases when the rigidity of polymers or coefficient of friction involved in the scratch process increases. When the tensile stress exceeds the surface tensile strength or the surface crazing stress of polymers, tensile tear will occur on the scratch surface. As a result, cracks and/or crazes will be generated on the polymer surface. For polymer blends or filled polymers with weak interfacial adhesion, it is likely to promote cracking, crazing, and debonding during scratch. These fracture features can drastically increase the ability of the surface to scatter light, which results in w hitening and leads to an increase in scratch visibility. A similar trend is found between scratch depth measured by interferometer and scratch visibility measured by image analysis (Table 2). This implies that fracture features are related to the scratch depth for polymers if similar fracture patterns are operative among the samples. A deeper scratch always results in a more complex fracture surface and lead to more fracture features, thus increasing scratch visibility. Scratch surface feature analyses by SEM and by interferometer provide a correlation between fracture feature and scratch visibility in TPOs. This correlation is discussed in terms of the role of additives and filler in the scratch behavior of TPOs in what follows.
(1) Effect of Talc Filler
Talc is used as a rigid filler to improve the rigidity of polymers. Unfortunately, talc-filled PP copolymers always show poor scratch resistance. The above findings can be rationalized below. An increase in rigidity of the talc-filled PP generates a higher tensile stress on the surface during scratch, thus increasing the chance for PP matrix/talc debonding and for crazing to form in the matrix. Once the initial crack or craze forms, poor adhesion between the matrix and the talc can create more debonding and growth of crack/craze during scratch. Clear evidence can be seen in the severe subsurface cracks detected in the surface profile analysis (Fig. 1b). SEM micrographs also show evidence of debris, voids and successive parabolic cracks extending to the unscratched region on the surface (Fig. 3). The smooth surface of the talc seen in high magnification SEM indicates poor bonding between the talc and the matrix (Fig. 6). The exposed natural white color talc fillers will further contribute to the whitening of the scratched surface, especially for dark colored samples. These fracture features and mechanisms are the main reasons why talc-filled PP exhibits poor scratch resistance.
(2) Effect of Lubricant
Lubricant, used to decrease the coefficient of friction during the scratch process, plays an important role in improving the scratch resistance of TPOs. The primary effects of decreasing the coefficient of friction include: 1) the shifting of the location of maximum von Mises stress from the surface to subsurface, thus reducing the size of plastic zone on the surface (11, 12), and 2) a drastic decrease in the magnitude of the maximum tensile stress induced by scratch (Fig. 10), thus reducing fracture features on the scratch surface. It is the latter effect that contributes to the improvement of scratch resistance in talc-filled PP systems. In this study, siloxane lubricant was chosen as an additive in PP systems to improve the lubricity and the flow of PP copolymers and to modify the surface characteristics of the sample. Detection of the presence of lubricant on the surface of PP copolymers by ATRFTIR spectroscopy (Fig. 8) suggests that the lubricant flows to the surface because of its low surface tension. For talc-filled PP copolymers, the lubricant helps reduce the scratch depth and visibility significantly. This finding is further confirmed by comparing the SEM micrographs (Fig. 3 vs. Fig. 5). Fewer fracture features are observed on the scratch surface of the lubricated PP (B3). However, the same effect is not found in the unfilled specimen (A3). This is because the unfilled PP (A1) itself has already had a good scratch resistance. No severe scratch damage is observed.
It should be noted that surface crystallinity is increased with the addition of lubricant (AS, B3) (Fig. 7). An increase In crystallinity should improve the rigidity of the PP surface. However, it also increases the possibility of fracture scratch pattern caused by a higher tensile stress as discussed in the previous section. In this study, no severe fracture features can be observed in lubricated PP (A3, B3). Again, this implies that the lubricant, which helps decrease coefficient of friction during scratch, plays a significant role in reducing the magnitude of tensile stress on the scratch surface. Without the presence of scratch fracture patterns, an increase in surface crystallinity can also help resist plastic deformation on the surface of the scratched sample. As a result, a better control of surface crystallinity without inducing fracture is desirable for improving scratch resistance of TPOs. This may be another reason why the talc-filled lubricated PP (B3) exhibits a slightly better scratch resistanc e than that of the talc-filled PP with the nucleating agent (B2).
(3) Effect of Nucleating Agent
The addition of nucleating agent reduces scratch depth and whitening in talc-filled PP copolymer (B2). No significant evidence of nucleating agent was found on the surface of the samples using ATR-FTIR because of the small amount of nucleating agent used in the PP systems. However, it is possible that the injection mold process causes the low molecular weight nucleating agent, which also has low surface tension, to migrate to the surface. This speculation still awaits further verification.
For high crystallinity PP copolymers, it is found that the scratch resistance depends mainly on the fracture features generated on the scratch surface, which is confirmed by SEM and interferometer surface analyses. The traditional indentation hardness tests cannot be used to determine scratch behavior of high crystallinity PP systems because different damage mechanisms and mechanics are involved between the scratch and indention processes. The scratch depth by interferometer surface analysis is found to be consistent with the scratch visibility by image analysis. This suggests that the scratch depth can also be used to characterize the scratch behavior in high crystallinity PP copolymers, provided that the damage features among the samples are the same. Talc is found to be ineffective for improving the scratch resistance of TPOs. The addition of nucleating agent or lubricant reduces scratch depth and scratch visibility of the talc-filled PP since the additives are likely to flow to the surface and reduce scr atch fracture features during scratch.
The authors thank Clark Thomas and Vald Beltran for their support and assistance in this study. Research grant from the State of Texas (Grant #APR-98-32191-72380) is also greatly appreciated.
B = brightness.
[delta]B = scratch visibility.
M = optical mass of an object.
GL = sum of the gray level value of all pixels.
[H.sub.s] = scratch hardness.
P = applied normal load.
A = projected area.
[omega] = residual scratch width.
(*.) Corresponding author.
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Materials Chosen for This Study. Code Materials Unfilled PP: A1 PP copolymer A2 PP copolymer with 0.5 wt% nucleating agent A3 PP copolymer with 5 wt% lubricant Talc-Filled PP: B1 PP copolymer with 25 wt% talc B2 PP copolymer with 25 wt% talc and 0.5 wt% nucleating agent B3 PP copolymer with 25 wt% talc and 5 wt% lubricant Scratch Visibility, Scratch Depth and Identation Hardness of Various PP Systems. Materials Scratch Visibility (%) Scratch Depth ([micro]m) Rockwell Hardness A1 0.00 0.8 103 A2 0.00 0.8 104 A3 0.08 0.7 99 B1 2.12 4 100 B2 0.47 2.1 98 B3 0.43 2 97 Materials Shore D Hardness A1 76 A2 76 A3 74 B1 78 B2 78 B3 77
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|Author:||XIANG, C.; SUE, H.-J.; CHU, J.; MASUDA, K.|
|Publication:||Polymer Engineering and Science|
|Date:||Jan 1, 2001|
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