Sofia Martinez
Plastic is a versatile material with a wide range of applications, from everyday items to specialised equipment. Its impact resistance, or ability to withstand shock without breaking, is a key strength. However, the impact resistance of plastic varies depending on its type and the temperature it is exposed to. At room temperature, most plastics are flexible and strong, but when exposed to low temperatures, they can become stiff, brittle, and more susceptible to breaking. This is an important consideration for manufacturers, especially when selecting materials for products that may be exposed to a wide range of temperatures. Different types of plastics offer varying levels of impact resistance, and understanding these properties is crucial for product design and material selection. While plastic has its advantages, it is essential to consider its limitations and how environmental factors can affect its performance.
| Characteristics | Values |
|---|---|
| Impact resistance | Varies with temperature, type of plastic, and toughness. Plastics can withstand impact without breaking or cracking. |
| Brittleness | Plastics become more brittle at low temperatures. |
| Stiffness | Plastics become stiffer at low temperatures. |
| Thermal expansion | Plastics change in density and size when exposed to temperature changes. |
| Thermal conductivity | Plastics can allow or prevent heat transfer. |
| Wear rate | Plastics change their wear behaviour at low temperatures. |
| Tensile strength | A factor in determining the suitability of a plastic for a particular application. |
| Flexural strength | A factor in determining the suitability of a plastic for a particular application. |
| Corrosion resistance | Important in environments with exposure to moisture, chemicals, or corrosive substances. |
| Coefficient of friction (COF) | Plastics typically range from 0.2 to 0.6, with lower numbers indicating less friction and better wear resistance. |
| Electrical properties | Varies with type of plastic. |
| Mechanical strength | Varies with type of plastic. |
| Chemical resistance | Varies with type of plastic. |
| Temperature resistance | Varies with type of plastic. |
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Impact resistance
The viscoelastic nature of plastic means they respond to impact stress in ways similar to both liquids and solids. Like solids, they can retain their form, strengths, and elasticity, but like liquids, they have some flow that is affected by the environment. This is different from how other materials like metal, glass, or ceramics respond, which can be challenging for manufacturers.
The impact resistance of plastics is measured in controlled environments through tests like the Notched IZOD Impact and the Gardner Impact. These tests do not consider the effect of environmental factors, which can change the actual impact resistance of the material. Service temperature is an important factor in impact resistance. At elevated temperatures, impact resistance tends to be higher, but when the temperature is lowered, plastic products tend to turn stiffer and more brittle.
Some plastics with good impact resistance include LDPE, Lexan, Acrylic plastic, and ABS. LDPE, or Low-density Polyethylene, is an economical plastic with great toughness and flexibility. Lexan is a polycarbonate with high impact strength (100 times greater than glass) and is often used in safety applications. Acrylic plastic is lightweight, resistant to shattering, and naturally transparent, making it a good alternative to glass. ABS is a hard, ridged plastic with good impact resistance, even at low temperatures, due to its small butadiene particles that absorb impact energy.
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Plastic behaviour
At a crystalline scale, plasticity in metals is a consequence of dislocations. Such defects are relatively rare in most crystalline materials but are numerous in some and are part of their crystal structure. In such cases, plastic crystallinity can result. In brittle materials, such as rock, concrete, and bone, plasticity is caused predominantly by slip at microcracks. In cellular materials, such as liquid foams or biological tissues, plasticity is a consequence of bubble or cell rearrangements.
The plasticity of a material is directly proportional to its ductility and malleability. Plasticity in a crystal of pure metal is caused by two modes of deformation in the crystal lattice: slip and twinning. Slip is a shear deformation that moves atoms through many interatomic distances relative to their initial positions. Twinning is plastic deformation that occurs along two planes due to a set of forces applied to a given metal piece.
The temperature is an important factor in the plasticity of a material. Most metals show more plasticity when hot than when cold. Lead, for example, shows sufficient plasticity at room temperature, while cast iron does not possess sufficient plasticity for any forging operation even when hot. When heated, most metals become plastic and can be shaped.
The effects of high temperatures on plastic materials are well-known, but low temperatures can also have a structural impact on most plastics. At room temperature, typical thermoplastics and other plastics are semi-flexible and have a low failure rate under stress. However, when they reach extremely cold temperatures, they tend to harden and become more brittle. At this point, they become similar in structure and function to glass, increasing the risk of fracture or breakage.
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Brittleness
When plastics reach extremely cold temperatures, they tend to harden and become more brittle. At this point, they become similar in structure and function to glass. This tendency can create problems if the plastic is under strain because it increases the risk of fracture or breakage. Cold temperatures can also cause a change in the dimensions of a plastic component, which then changes its wear behaviour, friction, and overall mechanical properties. The glass transition happens over various temperatures, so chemists can’t rate this change with one particular temperature. However, the glass transition premature threshold, the lowest temperature in that range, is the defined low temperature for that particular plastic. After that point, the impact resistance of the plastic lowers while the failure rate due to cracking and breaking increases.
The viscoelastic nature of plastic means they respond to impact stress in ways similar to both liquids and solids. Like solids, they can keep their form, strengths, and elasticity, but like a fluid, they have some flow that is affected by the environment. This is different from how metal, ceramic, and even glass respond, which can create challenges for manufacturers using plastics for the first time. One of the factors that this viscoelastic nature affects is impact resistance. This is the ability of plastic to withstand impact without cracking or shattering, and many types of plastic offer very high impact resistance.
Impact resistance (or impact strength) describes a material’s or product’s ability to absorb shock or impact energy without breaking. Resistance against impacts is one of the key requirements in plastic product design. It is clear that a helmet or a ski boot should have high impact resistance, but almost every plastic product is subjected to impacts at some point in its service life. All products should preferably be impact resistant to a degree for more reasons than one. They may fall from a store shelf before they ever meet the customer. Impact resistance is in fact one of the key strengths of plastics. It is why they are used in helmets, riot shields, and sports gear. They replace glass in many applications, and in some cases, they can be more durable than metals.
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Thermal expansion
Like most materials, plastic expands as the temperature increases. This is known as the coefficient of thermal expansion (CTE). Different plastics have different CTEs, depending on their composition and brand. For example, Teflon (PTFE) has a CTE of 100-160, while UHMW Polyethylene has a CTE of 130-200. Acetal (POM) has a CTE of 80-120, and Nylon (PA) has a CTE of 90-95.
The CTE is an important factor to consider when selecting a plastic for a specific application, especially when the plastic will be exposed to extreme temperatures or mated with another material, such as metal, that has a different thermal expansion rate. If the plastic is constrained or subjected to prolonged exposure to high temperatures, it can experience thermal degradation, leading to a loss of strength and toughness and an increased susceptibility to cracking, chipping, and breaking.
In the context of 3D printing, the thermal expansion of plastics is also crucial. Different areas of a 3D-printed object can have different temperatures, resulting in varying values of thermal expansion or contraction. This can be modelled using finite element analysis and specialised software.
When choosing a plastic material for a project, it is essential to consider the environmental temperature range, the required dimensional and stiffness tolerances, and the expected loads or forces at different temperatures. A low temperature for a prolonged period can sometimes cause similar damage to a high temperature for a short duration. Additionally, part geometry and material thickness will also impact the material's behaviour under extreme temperatures.
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Wear rate
The wear rate of a plastic material is influenced by several factors, including temperature, load, and sliding speed.
At room temperature, most thermoplastics and other plastics are semi-flexible and have a low failure rate under stress. However, when the temperature drops, plastics tend to harden and become more brittle, increasing the risk of fracture or breakage. This change in temperature can also alter the dimensions of a plastic component, which in turn affects its wear behaviour, friction, and overall mechanical properties. Manufacturers often use ultra-low deep freezers to test the performance of plastics at low temperatures.
The wear rate of plastic materials is also influenced by the load applied. Studies have shown that the wear rate increases with an increase in the normal load for all materials. Additionally, the friction coefficient, which measures the resistance to motion between two surfaces in contact, also affects the wear rate. The friction coefficient generally increases with the duration of rubbing and normal load for glass fibre, nylon, and PTFE.
The sliding speed or velocity of the plastic material also impacts its wear rate. As the sliding speed increases, the wear rate tends to increase for all tested materials. However, the specific values of wear rate can vary depending on the sliding speed and the type of material. For example, nylon and PTFE have been found to exhibit the highest wear rates under identical conditions, while glass fibre reinforced plastic has a lower wear rate at certain sliding speeds.
Some specific types of plastics, such as ABS, exhibit good resistance to wear and tear. ABS, or Acrylonitrile Butadiene Styrene, is a hard, ridged plastic that is chemically resistant and holds up well to wear and tear. This makes it suitable for products like football helmets, canoes, and office equipment. Other plastics, such as HDPE (High-Density Polyethylene), offer a wide range of applications due to their high strength-to-density ratio and flexibility.
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Frequently asked questions
Impact resistance (or impact strength) describes a material’s ability to absorb shock or impact energy without breaking.
When the temperature is lowered, plastic products tend to turn stiffer and their impact resistance decreases. At extremely cold temperatures, plastics become more brittle and are more prone to breaking.
LDPE (Low-density polyethylene) is a plastic with great impact resistance. Lexan™ is a polycarbonate with impact strength 100 times greater than glass. ABS is also known for its impact resistance, especially in low temperatures.
In addition to impact resistance, other important properties of plastics include tensile strength, flexural strength, thermal conductivity, wear rate, coefficient of friction, corrosion resistance, and chemical resistance.
