Materials Selection Criteria and Methods

Flame-Resistant Suitcase

We want our suitcase not to burn when close to heat sources. That's why we need it to be flame-resistant. Remember that requirements always concern the final product (which characteristics it has to have) while specifications are about the material, that's why they are connected to phenomena and material properties. Combustion is a chemical reaction between a material and oxygen. It produces heat and light under the form of a flame.

Limiting Oxygen Index (LOI)

The limiting oxygen index (LOI) is the minimum concentration of oxygen, expressed as a percentage (%), that will support combustion of a polymer. It is measured as the minimum oxygen level for which the material, ignited by a burner, keeps on burning for three minutes or over a distance of 50 mm. Let's keep in mind that the atmosphere is made of around 21% of oxygen. If a material, at room temperature, has LOI>28% we are talking about self-extinguishing materials (optimal flame-resistant materials), if LOI

20-26% slow-burning/flammable materials. There are many tests and methods to measure flammability but most of them require a ridiculous amount of closed conditions (very complex methods) and they are not even accepted worldwide: HB, V-0, V-1, V-2, 5V, 5VB and 5VA. So the properties linked to this specification are oxygen index and flammability.

Specification: scratch resistant

The suitcase should be scratch resistant. Scratch resistance refers to the hardness of a material in terms of resistance to scratches and abrasion by a harder material forcefully drawn over its surface. We want the suitcase to be scratch resistant enough to remain transparent and maintain its aesthetics. The property linked to this specification is hardness. Despite this, scratch resistance is not so easy to define scientifically and associate to a specific property. Scratches are aesthetically undesirable (important for design), can impair the component's functionality (DVD) and can threaten its structural integrity.

They can be possibly caused by: assembling on a production line, transportation, polishing, cleaning, maintenance operations and accidental events during service life. It is important to have a reliable way of evaluating the material scratch resistance. Unfortunately, scratch resistance cannot be definitely associated with a single material property making it hard to find a precise and reliable method to measure it.

It is possible to test the "hardness" of a material also at home. Some devices were designed in order to simplify its testing, but the majority of them do not give reliable results (example: manually activated devices). Several devices for measuring the scratch resistance are commercially available, technical datasheets seldom report these data.

Scratch testing: Methods to measure scratch resistance:

1. Surface typography
2. Scratch maps

They are based on the classification according to the observed morphology. It considers the depth, the width and the length of the penetration.

To calculate scratch resistance we could use: 1. Optical microscopy 2. Scanning electron microscopy 3. Atomic force microscopy 4. Wide angle X-ray diffraction 5. Scanning probe microscopy Optical properties: Approach focused on the perception of scratch using psychophysical tests, very interesting in the automotive sector and for coatings applications. Critical load is defined as the minimum load at which the coating begins to fail and continues to fail at higher loads. It is determined by means of: visual inspection (penetration depth comparable with visible wavelength roughness λ), measurements and scratch profiles. Actually scratch visibility depends on factors such as: lighting (angle, wavelength), scratch geometry, roughness, colour etc. but, in physics, quantitative evaluations are needed (example: gloss measurements, the amount of light reflected by a sample in specular direction). Gloss.

Measurements have their own limitations too: they are relatively insensitive to single large scratches, dependent on surface roughness.

Importance of non-specularly reflected light fraction: it is better than gloss since it allows amore precise characterization of the relationship between damage and appearance.

Non-specular measurements can be used on different surfaces too. Scratch damage can also bedetected by colour variations, provoked by two indenter shapes (for area or line contact), twotypes of load (constant or linearly increasing) or two test modes (single ormultiple damage).

Scratch damage index (SDI): luminance variation between scratch and unscratched surfaces under constant load.

Scratch visibility index (SVI): onset of scratch visibility as perceived by the naked eye, determined as a set of S I values.

However, SVI measurements performed by human subjects are not consistent enough to quantify scratch damage. Alternative methods based on optical imaging tools are more

successful: contrast, size and continuity are evaluated. To simulate human perception both chromatic (artificial natural-like light source) and geometric (setup geometry) parameters need to be carefully considered. Findings suggest that scratch damage is rather insensitive to changes in colour, gloss and texture of the surface, because of its inherent high contrast. On the other hand, subtle mar damage is more dependent on surface perceptual properties. Mar visibility is greater for green, glossy and smooth surfaces. Finally, scratch damage measurement is still a very open research topic that needs to be deepened.

Scratch hardness: It is commonly defined as a force to area ratio (very much like indentation hardness). Standard test methods exist, such as ASTM G171; unfortunately, the so-determined scratch hardness number is dependent on applied normal load. Example: for polymers Hs is proportional to the compressive yield stress and inversely correlated with residual depth. Scratch hardness

Hardness is often correlated with indentation hardness, which is typically reported on technical datasheets using various definitions.

Mohs hardness is the first characterization of scratch resistance, proposed by German geologist Mohs in 1812. It is a qualitative ordinal scale based on the ability of a material to visibly scratch reference minerals. Since then, several quantitative definitions of hardness have been developed in material science. Mohs proposed a method to measure scratch resistance by scratching rocks and minerals against each other (from a geologist's perspective). For example, talc has the lowest hardness (1) while diamond has the highest (10). However, this method is considered obsolete as it lacks scientific knowledge.

Other methods can be used to test material hardness, such as Rockwell, Brinell, and Vickers. These methods use varying loads and are independent of the choice of indenters D and F. The hardness is measured in dimensionless scale units of pressure (Pa).

materialsHardness Vickers Hv: Hardness Brinell Hb: Hardness Rockwell Hr: It is the most useful and It shows that hard materials reliable. scratch less easily. Koop: similar to Vickers with different pyramidal shapes. Shore: indentation of a steel rod, typical for rubbers and plastics. Different scales exist, A and D are the most common. Barcol: indentation of a sharp point with a flat tip Typical for thermoset composites. In conclusion, what we learned from this part is that the material property strictly related to scratch resistance is the hardness.

Specification: soft touch Softness (to the touch) can be evaluated by a combination of stiffness and hardness. S represents the softness index and it can be calculated by using the formula on the right side, where E stands for the Young's modulus. The higher the index is, the higher is the material's hardness (high indexes: hard materials, low indexes: soft materials). There are different graphs in which there is a ranking of materials.

depending on the stiffness ⁽¹⁾, the softness index ⁽²⁾ and softness index vs stiffness ⁽³⁾. Warmth (to the touch): it is a property of materials in which heat slowly flows away upon contact. It is related to how the person perceives the temperature of the material when he touches it. The human body is around 35-36°C while the atmosphere is 20-23°C. The warmth index W can be calculated by using the formula on the right side, while the heat flux Q from the contacting surface is equal to: High heat capacity materials take a lot of heat from our body when we touch them. Materials with: small heat capacity and small conductivity (feel warm), while materials with high heat capacity, high conductivity and high hardness (feel cold). So, as we wanted our suitcase to be easy to handle, we defined as a specification the soft touch. The property related to soft touch is the Young's modulus and the material hardness. Specification: resist detergents/mild solvents or UV When put in contact with

Chemical substances can cause degradation in materials. When the material is washed, it comes into contact with liquids such as detergent and solvents. When the material is exposed to outdoor conditions, it is exposed to solar radiation, which can adversely affect its properties.

There are two types of degradation depending on durability:

• Irreversible degradation: This involves chemical changes in the material, such as thermal liquid interaction (when the temperature is so high that it changes the chemical properties of the material), oxidation (when oxygen in the atmosphere attacks the material and creates a layer), and environmental stress cracking (ESC) (when the material is put in contact with certain substances and a mechanical stress is applied, causing it to crack).
• Reversible degradation: This involves physical changes in the material, such as radiation damage (when the material is exposed to sunlight) and liquid interaction (when substances interact with the material, such as solvents).

Physical ageing is the irreversible degradation of materials. This degradation can be caused by high temperature, exposure to oxygen and ozone, electromagnetic radiation (such as UV and X-rays), aggressive liquids, and gamma radiation. Often, a combination of these factors leads to changes in the molecular and macromolecular structure of the material. This can result in chain scission (reduction of molecular weight) or crosslinking (increase of molecular weight). These changes affect both the aesthetic features (such as color and smoothness) and the mechanical properties (such as embrittlement) of the material.

For example, after 5000 hours of artificial aging, a running track may become stiffer and more difficult to deform. The original material can be stretched almost twice its length (which is why stadium chairs can be deformed with a flame), but the aged material cannot. Another effect of aging is yellowing, which is caused by chemical degradation (oxidation/crosslinking) of polymers. There is a scale that measures the degree of yellowing, ranging from 0 to...