Polymers are comprised of molecules that have long, repeating chains. Their properties depend on the type of molecules that are bonded and how they are bonded. The range of polymers and properties is enormous. For example, some polymers, like rubber and fabric, are flexible and others, like epoxy, can be inflexible.

A common misconception is that polymers are always synthetic (plastic); however, some polymers, such as wood and rubber, are actually natural. Common polymers used in manufacturing processes include polypropylene and polyethylene (1).

Compared to metals and ceramics, polymer surfaces are more susceptible to surface damage (see Polymer Surface/Interface Consortium). The ability to retain surface quality in the long run is one of the critical property considerations for selecting polymers in many engineering applications. Scratch, which is a form of surface damage, is one of the primary causes for deteriorating surface quality in polymers. 

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Polymer surface/interface consortium (2)
Using indentation, optical scattering and chemical spectroscopy, the objective of the Polymer Surface/Interface Consortium is to develop and refine new scratch-related measurement and testing methodologies for the coatings and plastics industries. These plastics include multifunctional coatings, nano-composites and certain thermoplastics.

Recent discoveries include:

• Time-dependent measurement methods of scratch behavior in complex polymer systems.
• Recommendations for selecting correct parameters to evaluate scratch resistance.
• Test methods or characterizing surface behavior and scratch resistance in coatings as they relate to curing time.
• The development of a standardized nano-indenter scratch test method to predict optical performance after field scratch tests for multi-layer composites.

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STLE-member Dr. Mohammad Motaher Hossain, assistant professor at the department of mechanical and industrial engineering, Texas A&M University-Kingsville, defines scratch resistance as “the ability of a material to resist surface deformation due to sliding indentation of an asperity under the application of a prescribed normal load (3).”

Surface quality for polymers falls into three categories (4):

• Surface aesthetics. Even scratches that don’t diminish function have the potential to reduce value. 
• Structural integrity. This is especially important when it comes to food packaging, which can affect both quality and safety. Scratches can lead to tearing, which affects barrier properties. Scratches also can reduce load bearing capacity.
• Durability. Scratches on coating surfaces can result in corrosion and/or other damage underneath the coating. For some products, scratches affect performance and can even cause failure of the component and ultimately the entire device.

Dr. Hung-Jue Sue, professor at the department of materials science & engineering, Texas A&M University, explains, “By definition, scratch can be considered as a branch of abrasive wear. So indeed scratch behavior can be related to wear and abrasion. In fact, we have shown good correlation between scratch behavior and wear performance for some high performance polymers.” 

Scratch is a single-pass, single-asperity branch of tribology as the surface damage during scratching arises due to sliding indentation of a rigid asperity. 

At Texas A&M University’s Scratch Behavior of Polymers Consortium, the research emphasis is on mechanistic and material science aspects of scratch-induced deformation in polymeric materials. At the industrially funded consortium, they use numerical analysis that includes finite element method (FEM), to produce correlations between material properties of a range of polymers and scratch mechanisms. 

In addition, through numerical simulation and experiments, the center looks at how well polymer coatings protect against scratch. The thinking is that the synergy between FEM simulation and experiments offers a more comprehensive understanding of polymer scratch behavior than either method would have in isolation.

A key aspect of this research is scratch visibility—determining the critical point of visibility and the factors that affect it, such as observation angle. By using parameters based on human physiology, customized machine-vision software, called Tribometrics® (see Tribometrics®), evaluates the onset of scratch visibility via pattern recognition.

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Tribometrics® (5)
Surface Machine Systems was founded in 2006 to commercialize and promote the polymer scratch research of Texas A&M Polymer Technology Center’s SCRATCH Consortium.

Formerly called Automatic Scratch Visibility software or ASV, Tribometrics is the company’s machine-based scratch assessment and analysis tool.

Tribometrics, designed to imitate human visual systems, determines the onset of the visibility of scratch and the apparent visibility of mars in a manner that closely matches assessments made by humans of the same damage. 

However, the software is much more reliable than even trained human observers. It does not vary with fatigue or lighting conditions and creates a permanent documentation of steps to reproduce the analysis. It provides a real time assessment of scratch, on a material specimen, correlated with the load information.

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Another area of concern for polymer surface damage is “mar,” which in the past was only defined as “subtle surface damage.” At the Polymer Technology Center, researchers were able to show that plastic deformation exists before the critical point of visibility. Thus, the definition of mar now extends to surface damage that is below the critical level of visibility. As a result, the center is designing experiments to address mar behavior of polymers based on this definition (6).

Effects of scratch
Visible scratches can reduce the value of automotive exteriors and interiors, housing for electronic products, etc. While this affects aesthetics, the functionality may be unaffected. However, scratches on food packaging films can cause them to tear prematurely or compromise barrier properties, which may ruin the contents. 

In coating applications, damaged coating layers due to scratches may lead to corrosion or damage of the underlying substrate. Scratches on technology components can cause permanent loss of data. Scratches also can act as stress concentration points leading to reduction in load-bearing capacity and ultimately result in premature fracture of a structural component.

Dr. Hoang Pham, principal scientist, Avery Dennison Corp., explains how scratch can affect the product label. “Surface damage, such as scratches, mars and scuffing, tend to devalue the aesthetics of the package,” he says. “It’s an aesthetic phenomenon, and in some limited cases deep scratches can be an initiation site for long-term plastic part failure. In our labeling industry, surface damages on labels are not tolerated (see How Avery Dennison Addresses Scratch). 

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How Avery Dennison addresses scratch
To develop scratch-resistant or surface-damage-resistant polymers, establishing relationships between material and surface properties and surface damages incurred during scratching is necessary. Understanding the underlying mechanics responsible for the development of different surface damage features during scratching also is needed. By gaining this knowledge, scratch-resistant polymers can be produced, fabricated and deployed in various applications, such as label production at Avery Dennison Corporation. 

Hoang Pham, principal scientist, Avery Dennison Corp., explains, “There are two types of surface damage that are seen in our decorating industry. The first, that is seen directly from consumers, is decoration damage. For this type of damage, several technologies are used for convertors and printers to prevent the damages. Typically, the printed surface of the decoration label is coated with a varnish, which is a hard coating that prevents scratches and scuffing. However, other methods can be used, such as over lamination of the printed surface with a film such as a biaxially-oriented polypropylene film (BoPP) or a polyethylene terephthalate film (PET). Others also may use a slip additive in their ink to lower the coefficient of friction and prevent damage to the decoration.

“The second type of surface damage is the actual scratch, mar and scuff observed on the label face stock. If this is severe, the damage can show up even after printing. To prevent this type of surface damage, we will spec in scratch-resistant polymers to prevent these types of surface damages on our polymer film face stock. Since we do not manufacture all of our face stock, we specify suitable surface damage resistant polymer resin for our face stock.”

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“It affects the aesthetic of the decoration on the product label. It is the first thing that a customer sees and therefore impacts whether or not a customer will buy the product. The label not only acts as a decoration to attract consumers’ eyes, it also acts as an information panel for the consumer. Surface damage also can affect the readability of such information. Hence, scratch, mar and scuffing resistance are important properties for label decoration.”

The complexity of surface scratch deformation
Polymeric materials have been used extensively for exterior parts, where the appearance is as important a characteristic as the products themselves. However, scratch is complex in terms of both loading condition and damage process. Therefore, the mechanisms have been very difficult to understand completely—clear methodologies have not been fully established in order to improve the properties, Hossain explains. 

Surface deformation during scratching is a complex mechanical process since it involves large-scale plastic deformation, varying strain rate along the scratch path, frictional heat dissipation and complex stress field development. The time-dependent, temperature-dependent and pressure-dependent behaviors of polymers and surface characteristics of the interacting bodies add to this complexity. 

During the scratching process, surface interaction between the rigid scratch tip and polymer substrate produces friction and heat. To understand the polymer scratch behavior using computer modeling, an appropriate frictional model should be included in the computer model, which considers the adhesive friction as well as friction due to sub-surface material deformation. 

Hossain defines factors that need to be considered in assessing polymer scratch phenomena. “Molecular chain orientation of the polymer and roughness of the interacting surfaces are important since they can affect the frictional behavior,” he says. “The scratch tip geometry, scratching speed and applied normal load also influence the scratch behavior of polymers. Furthermore, strain softening-strain hardening phenomena, mechanical response under tensile and compressive loadings, rate-, temperature- and pressure-dependent material response and visco-elasticity also play important roles in determining the scratch behavior of polymers. Also, appropriate boundary conditions need to be applied in the model to understand the polymer scratch behavior using computer simulation."

Hossain adds: “To some extent, we can relate material and surface properties of polymers with the surface damages incurred during scratching. Our research has demonstrated that material properties such as yield stress and strain-hardening slope significantly affect the scratch-induced surface damages. Material properties in tension influence certain types of surface damage whereas material properties in compression affect other surface damage features. We also have shown that surface properties, such as coefficient of friction, influence the polymer scratch behavior significantly. Furthermore, by including appropriate material and surface properties in the computer model, we were able to predict the scratch behavior of amorphous polymers, thus showing the relationship between these properties/behaviors.”

Testing
ASTM International, formerly the American Society for Testing and Materials, is an international standards organization that creates and makes available technical standards for a spectrum of systems, services, products and materials. Currently ASTM has two standards that address scratch testing and evaluation. 

ASTM D7187 (7). This test addresses the nano-scratch method to determine the resistance to scratch and mar of paint coatings. The first step is to find the correlation between damage characteristics and external factors such as deformation. The second step is to correlate damage characteristics to visual luster reduction. Plastic deformation and fracture both make a significant contribution to mar. ASTM D7187 evaluates scratch/mar based on these mechanisms.

ASTM D7027 (8). This test covers a laboratory procedure using a scratch machine to produce and characterize surface damage in a control environment for polymeric coatings as well as plastics. The test can characterize polymer mar and scratch resistance through the measurement of a range of relevant material properties. The entire scratch-inducing and data acquisition procedure is automated to prevent user-influenced results.

During scratch testing, according to the ASTM D7027 standard, Sue explains, a linearly increasing normal load is applied on a 1-mm diameter spherical stainless-steel tip, which traverses on a polymer surface at a prescribed scratching speed. Due to the application of progressive normal load, the ASTM D7027 standard test generates a continuous progression of surface damage on the scratch path. Thus, various surface damage transitions can be located on the material in a single simple test, depending on the severity of applied loading and polymer type. This reduces the number of tests required to fully characterize polymer scratch behavior. 

Sue adds that this test also allows for an in-depth analysis of polymer scratch behavior in a straightforward and quantitative manner. “The combined usage of scratch-visualization software, such as Surface Machine System’s Tribometrics®, and the ASTM D7027 scratch test standard enables quantitative characterization of scratch visibility, which is used to evaluate scratch resistance from the aesthetic point of view,” he says. 

Computer modeling for assessing scratch deformation
Computer modeling has been used to establish the relationship between material properties and scratch damage mechanisms and to understand the underlying mechanics. Computer simulation also has been used to predict the surface damage caused by scratching so that it can be used to design scratch resistant polymers; this has the potential to save a significant amount of time and money.

Surface deformation induced during scratching is very complicated due to the development of complex stress scenarios and the unique material behavior of polymers. Experimental study alone is not adequate to understand various aspects of scratch damage mechanisms. 

Computer modeling, such as FEM modeling, has been used in recent years to facilitate the understanding of polymer scratch behavior and to develop surface damage-resistant polymers. 

Using computer modeling, influences of various material and surface properties on surface damage features have been investigated by researchers to establish a correlation between mechanical properties and surface characteristics, and the surface damage that occurs during scratching. Hossain observes that if such a relationship could be established, it would pave the way to design surface-damage-resistant polymers through computer simulation. Researchers also have been working to predict the evolution of surface damage during scratching in order to provide specific guidelines for designing scratch resistant polymers.

Dr. Han Jiang, professor at the school of mechanics and engineering, Southwest Jiaotong University, Chengdu, China, says, “The highly non-linear material characteristics and finite deformation in an extremely localized area significantly complicate the analysis of polymer scratch. Consequently, no analytical solutions are available for polymer scratch behavior. Numerical modeling, such as the finite element method, has been shown to be an effective research approach since it possesses the mathematical framework allowing for integration of the physical phenomena and material response together. However, to guarantee the modeling accuracy, long computational time and tremendous computational resources are required.”

Jiang goes on to explain that there are many issues that need to be considered when numerically modeling polymer scratch behavior: These include: 

1. A thorough mechanistic understanding, with the help of experiment data and microscopic observation, of the polymer scratch deformation/damage mechanisms such as yield, crazing, crack initiation, chip formation, etc.
2. A material-constitutive model capable of describing the inherent non-linear visco-plastic as well as rate/temperature-sensitive and stress/strain status-dependent, finite deformation behavior of polymer and its composites.
3. Integration of validated constitutive relationships into the numerical model.
4. An optimal algorithm for better computation efficiency. Jiang concludes this is a must-have because of the complexity from both scratch deformation/damage mechanisms and the constitutive model.

“Because of the inherent complexity from both scratch deformation/damage mechanisms and the constitutive model, as well as the variety of polymer catalogues, the coupled effect of material and surface properties on surface scratch damage is unavoidable,” Jiang says. “Although certain general guidance for polymer scratch can be made qualitatively, a definitive correlation between material and surface properties with scratch damage, if it exists, only can be achieved by a thorough understanding and careful investigation for the specified polymer and scratch condition.”

Sue also is convinced the computer modeling is the key to analyzing polymer scratch behavior. “The biggest advantage, in my view, is that we can study the fundamental mechanics of polymer scratch behavior using computer modeling,” he says. “We can parametrically study how various material and surface properties affect the scratch behavior using computer modeling without performing the actual experiments, which saves time and money. Ultimately, computer modeling enables us to design better scratch resistant polymers for various applications.”

Hossain agrees. “By including appropriate material constitutive behavior and frictional relationship in the computer model, we can predict certain types of surface damages incurred during scratching. Our research (at Texas A&M University’s Polymer Technology Center) has successfully used computer modeling to predict the scratch-induced surface damages in amorphous polymers. This capability has huge implications as we can use computer modeling to design surface damage resistant polymers. However, for semi-crystalline polymers, predicting scratch behavior using computer modeling could be rather complicated because of the skin-core morphology.”

Computer modeling for designing scratch-resistant polymers
The sheer number of polymer materials complicates not only the analysis but also the development of scratch-resistant polymers. “Considering the wide range of polymers, which may be classified as thermoplastic, thermoset, rubbery, etc., the establishment of an appropriate constitutive model and damage criterion, as well as efficient simulation strategy, is not straightforward,” Jiang says. “But the collective impact of those efforts on the academic and industrial research on polymer material will be significant.” 

Jiang goes on to explain that a physics-based mechanical analysis model will be constructed and the mechanisms behind complicated polymer scratch damage modes and their relationship with material properties can be obtained. “The key scratch-related parameters and their influence on scratch performance of polymers will provide comprehensive and versatile guidelines for the design of material with good scratch resistance,” he says. 

According to Pham, computer simulation using finite element analysis allows developers, especially in the automotive industry, to design scratch resistant surfaces—but computational methods still play a role. 

“For some parts surface topographic design, such as texturing, can be used to reduce the scratch and mar,” Pham says. “However, these complicated surfaces are not easily analyzed for stresses applied that may cause surface damage. For these systems, computational methods are best used for analysis.

“Additionally, the applied stresses may not affect the surface alone but also affect the sub-surface and the zone close to the scratch. As an example, for multi-layer systems, such as coatings or multi-layer films, understanding the stresses developed at the vicinity of the scratch zone also is important. Through this type of computation analysis, one can engineer a plastic part to be damage resistant.”

Sue clarifies that computer modeling allows researchers to understand how various material and surface properties affect the scratch behavior of polymers without performing the actual experiments. “However, we need to validate the computer model with some experimental evidence to make sure that the results obtained are reliable and accurate,” he says.

“Once the model is validated, it can be used to study the fundamental scratch mechanics and how, by changing one material or surface parameter, the surface damage during scratching varies. By doing so, the key properties can be identified, which can be modified to design better scratch-resistant polymers.”

At Kaneka US Materials Research Center, workers are using all possible methods to improve scratch properties and develop anti-scratch materials, including modifying surface and material properties. “Computer modeling is an essential technique to develop high-performance materials, not only for anti-scratch but also other purposes,” says Dr. Masaya Kotaki, general manager for Kaneka US Materials Research Center Kaneka Americas Holding, Inc. “We can accelerate the materials development by minimizing the actual lab work. The effectiveness is much more significant in anti-scratch materials development than other applications as it is very complex in both loading manner and damage process.”

Sue observes that the property of scratch is gaining increasing attention because of its importance in many durable consumer goods and that it will continue to be a required property for many applications.

The ability to analyze the relationship between scratch properties, specific materials and conditions using computer modeling and ultimately design scratch-resistant polymers based on the data produced has major implications for polymer manufacturers and the industries that use those materials. 

“We can use computer modeling to understand the scratch behavior of films, coatings and laminates,” Hossain says. “We can use computer modeling to design coating thicknesses and material properties needed to improve the overall scratch resistance of multi-layer coatings. We also can use computer modeling to understand the delamination failure that occurs during scratching. Furthermore, tearing of polymer films due to scratching can be studied using computer modeling.” 

Kotaki concludes, “We have been using computer simulation for new product development in electronics applications, but the possible fields for using computer simulation for product development are unlimited.”

REFERENCES
1. From What is a Polymer? by Alina Bradford for Live Science. Available here.
2. Available here.
3. Mohammad Motaher Hossain, doctoral dissertation, Quantitative Modeling of Polymer Scratch Behavior, Dec. 2013.
4. Ibid.
5. Available here.
6. From Texas A&M University’s Polymer Technology Center: Scratch Behavior of Polymers. Available here.
7. Available here.
8. Available here.