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Scratch Resistance of Mineral-Filled Polypropylene Materials.

Pigmented mineral-filled polypropylene (PP-PMF) is marketed as a potential alternative to acrylonitrile-butyldiene-styrene (ABS) and Polycarbonate/ABS for automotive interior components. PP-PMF is more easily damaged by surface scratch/mar, thus limiting its acceptance for such applications. This study focuses on investigating the scratch/mar mechanisms of PP-PMF having different mineral fillers and additives. A new method is introduced to characterize the scratch visibility by image analysis. A correlation is found between scratch visibility and scratch hardness measured by interferometer. It is found that Wollastonite filler imparts higher scratch/mar resistance. Addition of an interface modifier and a lubricant can further improve scratch/mar resistance. It is also found that Talc-filled PP-PMF has poor scratch/mar resistance, irrespective of the addition of an interface modifier or a lubricant.

INTRODUCTION

P-PMF is replacing PC/ABS and ABS in many automotive applications due to its good recyclability, good weatherability and low cost. However, PP-PMF is relatively soft compared to PC/ABS or ABS and is susceptible to scratch and mar during the manufacturing processes and lifetime of consumer use. It is critical to improve the scratch/mar resistance of PPPMF for future applications.

The problem of scratch/mar is a complicated mechanical process [1, 2]. Scratch performance of polymers is determined both by the scratch stress field associated with the indenter geometry and by material properties. The geometry of the indenter has significant effects on scratch resistance of polymer [3]. Sharp indenters create deeper and more brittle failure modes. Here, we focus on the failure created by blunt indenters, which is closer to the failure we see in practice. In this study. a scratch indenter with a 1 mm diameter ball was used to characterize the scratch/ mar behavior of PP blends.

Scratching of a flat solid surface under a spherical indenter was modeled by Hamilton and Goodman [4]. Based on this model, the stress fields during scratch clearly show a singularity due to the maximum tensile stress just behind the indenter, which is a probable location for the initiation of craze/crack on the surface of polymers. The yield stress determines the extent of the plastic zone on the surface and the subsurface of the scratched samples. Indentation hardness and elastic recovery are also found to relate to the residual scratch depth [5].

In general, for low modulus polymers, an increase in modulus and a decrease in friction coefficient can reduce the scratch depth and size of the plastic zone on the scratch surface [5]. Typically, three methods are used to improve the tribological properties of a given plastic material, namely, 1) modifying the polymer molecular structure, e.g. crystallinity, 2) blending with scratch resistant polymers, and 3) producing polymer composites with various fillers and additives. In the present study, the effects of different filler types and additives are discussed. Also, a new lubricant and an interface modifier, which are known to be able to decrease the coefficient of friction and enhance the rigid filler effectiveness, are evaluated.

It is well known that the hardness of PP is dependent on the degree of crystallinity and can also be affected by various factors such as the type of fillers or additives used [6, 7]. Here, we study the degree of crystallinity of PP prepared with various formulations. Polymers usually have a skin-core structure after injection molding [8]. In injection molded polymers, the scratch performance is most likely affected by the skin instead of the core. Therefore, the parameters for describing the scratch resistance in this work are all associated with the surface properties of PP-PMF. The scratch direction is chosen to be along the melt flow direction. In addition to the scratch depth, scratch hardness which considers indenter geometry, applied normal load and the resulting scratch depth are also used to quantify the scratch resistance of PP-PMF.

Pigments affect scratch visibility. Scratches usually show up more obviously in dark color plaques than in light color plaques. A new method, which employs image analysis to characterize scratch visibility, is introduced. Results of the image analysis performed on plaques with the same color were compared in this study. The failure modes of filled PP were also investigated and their effects on scratch visibility were discussed.

EXPERIMENTAL

Materials

Specimens of PP-PMF compounds with different mineral fillers were obtained from Eastman Chemical Company and are listed in Table 1. The PP copolymer (SG-702) was received from Montell Polyolefins. Two types of fillers were selected for this study: Talc with a median particle size of 3 [mu]m (Luzenac 8230), and wollastonite with a medium particle size of 8 [mu]m (Nylos8). The Mohs hardness is 1.0 for the talc and 4.5 for the wollastonite. Maleated PP (Epolene G-3003), obtained from Eastman Chemical, was used in this study. 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 by weight of 4% was added. Myvaplex 600 was used as a lubricant and was obtained from Eastman Chemical.

Scratch Tests and Measurements

The scratch test was done using Ford Lab Test Method (FLTM) BNl08-13. This apparatus consists of a movable platform connected to five beams with 250 mm in length. A scratch pin is attached to one end of each beam. A highly polished hardened steel ball (1.0 [pm] 0.1 mm diameter) is placed on the tip of each pin. Each pin is loaded with a weight that exerts a force of 7N, 6N, 3N, 2N, and 0.6N, respectively. Driven by compressed air, the beams draw the pins across the polymer surface and generate scratches. The scratch is made at a sliding velocity of approximately 100 mm/s. All tests were performed at room temperature. Although the test method requires that grained surfaces be evaluated, only the smooth surfaces of the specimens were tested in this study.

After the plaques were scratched, they were evaluated with a reflected light polarizing microscope incorporating a Xenon light source. An image analyzer with Image Analysis Software was used to measure the "gray scale mass," which is the total gray scale value of the object. The camera objective lens is positioned at an angle of 90[degrees] from the scratch. The objective lens then registers a portion of the scratch about 1 mm long. The electron signal for each scratch line is then integrated and recorded. The optical mass of an object, M, is the sum of the gray level values, GL, of all pixels in the object. The individual gray level values are assigned by the image analysis program in unit steps in the range of 0-255, where 0 = black and 255 = white. The optical mass, M, can be computed from:

M = [[[sum].sup.n].sub.t=1] [GL.sub.i] (1)

Where n is the number of pixels. The brightness of the object, B, is

B = [frac{M}{A}] (2)

Where A represents the area of the object. The percentage change in the brightness between the scratch and the background is the scratch visibility, [Delta]B, given by,

[Delta]B = [frac{[B.sub.scratch] - [B.sub.background]}{[B.sub.background]}] x 100% (3)

The depth of the scratch was measured using an interferometer (WYKO NT-2000, WYCO Corp., using WYKO Vision-32 Analysis Software). The magnification was set at 5X. Depth measurements were made from the depth histogram of the scanned area. The scratches were also examined using a scanning electron microscope (SEM).

Indentation Hardness Tests

For the Rockwell Hardness test, the ASTM D 785 test procedure was followed. The Tru-Blue Hardness Tester made by United Testing Systems, Inc. was used. The indenter was a round steel ball with 12.5 mm in diameter (Rockwell R scale). The Rockwell hardness number is a measure of the non-recoverable indentation after a heavy load of 588N for a period of 15s, and subsequently reduced to a minor load of 98N for another duration of 15 s. Normal hardness is then defined as the load divided by the projected area.

The durometer hardness test was performed according to ASTM D 2240 (type D). A portable testing unit was used. The tester is spring loaded with a protruding hardened steel indenter. The indenter has a 30[degrees] spherecone with a 0.1 mm tip radius. The depth of penetration was measured using a dial gauge. The scale (the hardness number) is graduated from 0 to 100, where each number represents 0.025 mm indentation.

Differential Scanning Calorimeter Analysis

The heat of fusion of the specimens was measured using the differential scanning calorimeter analysis (DuPont DSC 10) technique. In this study, the material from the skin that is relevant to characterizing the scratch performance was scraped from the surface of the plaque. The heat change in each specimen was measured twice. In the first run, the specimen was equilibrated at 30[degrees]C and then scanned at 10[degrees]C/min from 30[degrees]C to 250[degrees]C. After the first run, the specimen was cooled to room temperature at the same rate. The second run was used to determine the crystallinity of specimens with the same thermal history. The reported values of the heat of fusion were calculated based on the polypropylene content alone.

RESULTS

Scratch visibility was measured using an image analyzer and results from the 7N load scratch test are shown in Fig. 1. Both grades of unfilled PP (El, E2) showed similar scratch visibility with slightly lower visibility in the PP with additional maleated PP (E2). Scratches are more visible in specimens filled with 20% talc (T1, T2, T3). The presence of additional maleated PP and lubricant do not reduce their scratch visibility. Wollastonite-filled PP (W1) has a scratch visibility comparable to that of the talc-filled PP. Scratch visibility decreases with addition of maleated PP (W2) and becomes even lower with the addition of lubricant (W3). Specimen W3 has a scratch visibility comparable to that of the unfilled PP.

The depth of the scratch is measured using an interferometer. A typical surface profile of a scratch is shown in Fig. 2. Ridges of deformed material are produced on both sides of the scratch (groove). Extensive debris and cracking are observed at the bottom of the scratches. The distance between the plaque surface and the bottom of the groove, defined as the "scratch depth" is calculated from the depth histogram. The computed scratch depth is the average of at least three measurements along the scratch and the results are shown in Fig. 3. As can be seen from Fig. 3, the unfilled PP's (El, E2) have a relatively shallow scratch depth. Talc-filled PP (T1, T2, T3) have deeper scratches with a large data scattering regardless of the additives used. The reason for the high variation of depth in the talc-filled specimens is that the indenter "skips" on the surface during the test. The scratch depths of the wollastonite-filled PP (W1, W2, W3) are significantly lower than those in talc-filled PP. A slight reduction i n scratch depth was also observed in specimens with additional maleated PP (W2) and lubricant (W3). Specimens W2 and W3 show scratch depths comparable to those in unfilled PP's (El, E2), similar to the scratch visibility.

Figures 4 to 10 show the scanning electron micro-graphs of the 7 N scratch test specimens. For unfilled PP (El), the scratch is shallow, with fine ripple marks visible on the bottom of the scratch (Fig. 4). The ripples form a parabolic shape with several visible fine cracks between them. It is believed that the marks are created by a continuous material flow around the indenter resulting in the formation of a uniform groove. In addition, matrix debris is found in the middle of the scratch. The boundary between the scratch and plaque surface is not clear, suggesting that the scratch is shallow and that the ridges formed at the sides of the scratch are not well defined. A similar morphology with a less amount of matrix debris in the scratch is also observed in PP with maleated PP (E2).

Significant matrix plastic deformation is found in the scratch of PP with 20% talc (T1), as shown in Fig. 5. Multiple cracks are found at the boundary between the scratched and the unscratched areas. In addition to matrix plastic deformation and voids, some debonding between the talc and matrix is also observed (Fig. 6). Fragmentation and smooth surface of the talc suggest the presence of delamination within some of the talc fillers. More cracks and extensive plastic deformation are found in specimens with maleated PP (T2) and with lubricant (T3). Fine cracks extend from both sides of the scratch at an angle of about 30[degrees] to the scratch in specimen T3 (Fig. 7). In the middle of the scratch, a significant but randomly oriented matrix plastic zone is observed, with the presence of some voids. In addition, talc/resin debonding and delamination between talc layers was also observed. The scratched specimen has more distinct boundaries between the scratch and the unscratched area. This suggests that the scr atch is deeper and plastic deformation is significant.

In PP with wollastonite (W1), a very different morphology is observed. The plastically deformed matrix also contains parabolic marks with fine cracks extending from both sides of the scratch at an angle of about 30[degrees] to the scratch (Fig. 8). The width of the cracks is larger and the distance between successive parabolic marks seems to be longer than that in unfilled PP. In the middle of the scratch, a significant amount of matrix debris, voids, and filler/resin debonding are also observed (Fig. 9). In the specimen with maleated PP (W2), a similar morphology is observed with much less filler/matrix debonding and a smaller plastic deformation in the matrix. With the addition of a lubricant (W3), the extent of cracking and matrix plastic deformation reduces even more (Fig. 10). This may explain the decrease in scratch visibility in the wollastonite-filled specimens (W2, W3) as shown in Fig. 1.

The results from both Rockwell R and Shore D hardness tests are shown in Fig. 11. The Shore D hardness of the filled PP increases slightly with the addition of fillers, regardless of the type of filler used. The addition of maleated PP or lubricant does not affect the hardness. Like the Shore D hardness, Rockwell R hardness of the filled PP is higher than the unfilled PP regardless of the filler. Additional maleated PP and lubricant seems to have a positive effect on the Rockwell hardness. None of the hardness measurements can accurately predict the differences in scratch performance of the various PP materials.

All three talc-filled specimens show similar morphologies, in that they exhibit significant matrix plastic deformation, voids and talc delamination. These fracture features contribute to the whitening observed in the scratch. Addition of maleated PP or lubricant does not change failure features. Wollastonite-filled PP shows a smaller extent of matrix plastic deformation as compared with talc-filled PP. Bonding between filler and resin is also improved with maleated PP, and the void fraction in the matrix is reduced to the same level as that in the unfilled PP. The addition of lubricant further reduces the amount of fracture features. The scratch hardness due to the presence of wollastonite can reduce the scratch depth and the accompanying damage (matrix plastic deformation, void) in the resin. A lower coefficient of friction and better bonding strength between filler and resin are also important factors that determine the amount of fracture features (scratch visibility) in shallow scratches. It is found that neither a lubricant nor an interface modifier could reduce the scratch visibility of deep scratches in talc-filled PP.

The DSC results from both 1st heating scan and 2nd heating scan are plotted in Fig. 12. The specimens are taken from the skin of the plaques and no significant difference between the results of unfilled and filled PP is found. This is due to the high nucleating tendency of PP. The heat of fusion of PP remains the same in both filled and unfilled PP.

DISCUSSION

Hardness defines the ease with which a material can be indented, drilled, or abraded. It is a complex property, which is related to the test methods and the mechanical properties of the material, such as Young's modulus and yield stress, etc. The relationship between hardness and mechanical properties is not usually straightforward. However, there is a tendency for materials with a high modulus and strength to exhibit higher hardness. Both indentation hardness and scratch hardness are generally used to quantify scratch resistance of materials [9]. A correlation between these two types of hardness seems to exist when the deformation is simply plastic deformation induced by larger included angle indenters. But it has also been shown that the above two types of hardness measurements are not in good agreement with each other when the deformation modes differ from each other.

Indentation hardness can be measured by a quasistatic penetration of the material surface with an indenter exerting a known force. In this study, two indentation hardness methods were used to investigate the scratch resistance of plastic materials. As shown in Fig. 11, these indentation hardness measurements cannot be used to differentiate between various materials and no correlation is found between the hardness and the scratch/mar resistance of PP. This is due to the difference in the nature of the two tests: indentation test is primarily a quasi-static test while the scratch test is generally considered as a dynamic process dependent on the dynamic friction coefficient during the sliding process [9]. The indentation hardness tests are obtained from static tests and the results obtained by such methods should not be considered a measure of the scratch/mar resistance of the plastics in question.

Scratch hardness is generally used to parameterize material resistance to scratch/mar [1]. The original definition of the scratch hardness, in analogy with indentation hardness, is given by:

[H.sub.s] = [frac{L}{[A.sub.s]}] (4)

Where L is the normal load and [A.sub.s] is the sliding contact area. For a spherical indenter, the sliding contact area is circular and can be written as [10]:

[A.sub.s] = [frac{[pi][(w).sup.2]}{4}] (5)

where w is the measured scratch width. Generally, polymers are viscoelastic-plastic in nature. A parameter, q, is taken into account in the definition of the scratch hardness [H.sub.s]

[H.sub.s] = q [frac{L}{A}] = q [frac{4L}{[pi][(w).sup.2]}] (6)

as given by Briscoe et al. (2, 3); where L is the load on the indenter, and q = 2, for a plastic deformation, and 1 [less than] q [less than] 2 for a viscoelastic-plastic deformation. The value of q is dependent on the viscoelasticity of the material, the sliding velocity, and indenter geometry.

In this study, the same sliding velocity and indenter are used in the tests. By assuming q to be a constant between 1 and 2, no significant difference is found between the scratch hardness of PP and PP-PMF materials. This is due to the fact that the scratch width is very difficult to be determined accurately in a shallow scratch.

A relatively high elastic recovery is found in polymers during scratch [5, 11]. For a spherical blunt indenter, we assume an even elastic recovery in the scratch deformation region. Similar to previous studies [7], we introduce the geometry of the indenter to calculate the relationship between scratch depth, d, and scratch width w

w = 2 [sqrt{2rd - [d.sup.2]}] (7)

where r is indenter radius. The scratch hardness, [H.sub.d], is then estimated by

[H.sub.d] = [frac{L}{[pi](2rd - [d.sup.2]}] (8)

As shown in the Fig. 13, based on the definition given by Eq 8, a good correlation is found between scratch visibility and the scratch hardness. Materials with deeper scratches show a lower scratch hardness and a higher scratch visibility, provided that they exhibit similar scratch damage mechanisms

Studies have been made to investigate the mechanisms for whitening during scratch [6, 7, 11]. Stress whitening, such as crazing, cracking, voids, etc., can result in significant enhancement of scratch visibility. A deeper scratch always results in a more complex fracture surface and with more fracture features. This increases scratch visibility. As shown in Fig. 5, additional talc fillers (T1) increase the chance for resin! filler debonding and crazing, thus creating scratch whitening. Exposed white talc fillers also contribute to whitening of the scratch. Wollastonite-filled PP (W1) shows similar fracture features on the specimen surface and thus the scratch visibility (Figs. 8 and 10). But, the amount of plastic deformation, which leads to stress whitening, is smaller in the wollastonite-filled PP (W1) than in the talc-filled PP (T1). Debonding between wollastonite and the resin is overshadowed by the plastic deformation in the resin phase. For specimens with improved interfacial bonding (wollastonite-fill ed PP (W2, W3)), the scratch depth (or scratch visibility) can be reduced significantly.

Addition of maleated PP, an interface modifier, is not as effective as in Wollastonite-filled PP (W2) in improving both scratch depth and scratch visibility in the talc-filled PP (T2). This is probably due to the fact that Wollastonite does not have a layered structure and therefore, no delamination is observed in Wollastonite. Also, the Mohs hardness of Talc is 4 times lower than that of wollastonite. Even when the surface of talc is treated with an interface modifier, supposedly to provide a good bonding with the resin, the talc delaminates and creates other defects during scratch, which is responsible for scratch visibility.

Lubricant is important in reducing scratch depth and the amount of scratch whitening in filled PP. It has been shown that an increase in friction coefficient leads to shifting the plastic yield zone to the surface and increases the yielded zone size on the surface, resulting in an increase in scratch visibility [4, 5]. In this study, a typical glycol monostearate lubricant is used. The lubricant flows to the surface due to its low surface tension without affecting the physical properties of the original system. The effects of the lubricant are not evident in the results of normal indentation hardness tests (Rockwell and Shore D Hardness tests) as shown in Fig. 11. Figures 1 and 2 show the effects of lubricant on the measured scratch depth and scratch visibility, respectively. For talc-filled PP, the lubricant does not change either the scratch depth or scratch visibility. This is probably because the scratch is too deep to be affected by the lubricant on the surface. The scratch depth and scratch visibility of wollastonite-filled PP are reduced by the addition of lubricant.

An increase in crystallinity on the surface may result in a higher scratch hardness PP, but an increase in crystallinity can also embrittle the material. A higher crystallinity in the skin may contribute to the higher amount of debonding, cracking, and scratch whitening observed in this specimen. DSC results show that there is no difference in crystallinity between the specimens with or without fillers, probably due to the already high crystallinity of PP.

CONCLUSIONS

A good correlation is found between scratch visibility and scratch hardness based on scratch depth measurements. PP-PMF material with higher scratch hardness exhibits lower scratch visibility. Material with higher scratch hardness is more resistant to scratch, resulting in less damage to the material. Amongst the materials studied, the PP-PMF specimen with 20 wt% wollastonite (W3) is the best in terms of scratch visibility. Wollastonite is found to be able to reduce scratch depth and scratch visibility. The addition of an interface modifier and a lubricant can further reduce scratch depth and whitening. The PP-PMF specimens with 20 wt% talc (T1, T2, T3) are the poorest in terms of scratch visibility in these specimen. Talc is found to be ineffective in reducing scratch depth and scratch visibility. The addition of an interface modifier and lubricant does not reduce scratch depth or whitening in talc-filled PP.

ACKNOWLEDGMENT

The authors thank Gary Phillip, Jeff Crist, Clark Thomas, and Vald Beltran for their support and assistance in this study. This study would not have been completed without their help.

NOMENCLATURE

L = Applied normal load.

d = Scratch depth.

[A.sub.s] = Sliding contact area.

w = Scratch width.

[H.sub.s] = Scratch hardness by width.

[H.sub.d] = Scratch hardness by depth.

r = Radius of the indenter.

B = Brightness.

[Delta]B = Scratch visibility.

M = Optical mass of an object.

[GL.sub.n] = Sum of the gray level value of all pixels.

REFERENCES

(1.) J. A. Williams, Trib. Int., 29, 675 (1996).

(2.) B. J. Briscoe, E. Pelillo, and S. Sinha, Polym. Eng. Sci., 36, 2996 (1996).

(3.) B. J. Briscoe, P. D. Evans. E. Pelillo, and S. K. Sinha, Wear, 200 (1996).

(4.) G. M. Hamitonm and L. E. Goodman, J. Appl. Mech., 33, 371 (1966).

(5.) C. Xiang. H.-J. Sue, and J. Chu, ANTEC 99, 3463 (1999).

(6.) J. Chu, L. Rumao, and B. Colman, SAE Tech. Paper, 970659 (1997).

(7.) J. Chu, L. Rumao, and B. Colman, Polym. Eng. Sci., 38, 1906 (1998).

(8.) S. Xavier, J. M. Schultz, and K. Friedrich, J. Mater. Sci., 25, 2411 (1990).

(9.) B. J. Briscoe, P. D. Evans, S. K. Biswas, and S. K. Sinha, Trib. Int., 29, 93 (1996).

(10.) M. J. Pickles and I. M. Hutchings, Automotice Automation Limited, 381 (1997).

(11.) V. Jardret, H. Zahouani, J. L. Loubt, and T. G. Mathia, Wear, 218, 8 (1998).

(12.) R. Kody and D. Martin, Polym. Eng. Sci., 36, 298 (1996).
 Materials Used in This Study.
Code Materials
E1 Unfilled PP copolymer
E2 PP copolymer with 3 wt% maleated PP
T1 PP copolymer with 20 wt% talc
T2 PP copolymer with 20 wt% talc, 3 wt% maleated PP
T3 PP copolymer with 20 wt% talc, 3 wt% maleated
 PP, 0.25 wt% lubricant
W1 PP copolymer with 20 wt% wollastonite
W2 PP copolymer with 20 wt% wollastonite,
 3% maleated PP
W3 PP copolymer with 20% wollastonite,
 3 wt% maleated PP, 0.25 wt% lubricant
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Article Details
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Author:CHU, J.; XIANG, C.; SUE, H.-J.; HOLLIS, R. DAMON
Publication:Polymer Engineering and Science
Article Type:Statistical Data Included
Geographic Code:1USA
Date:Apr 1, 2000
Words:4340
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