Maya Ellison
Impact strength is a critical measurement of a product's resistance to breaking when dropped or struck by an external force. This property is especially important in plastic product design, where impact resistance is a key requirement. To improve the impact strength of plastics, manufacturers employ various techniques, including copolymer modification, polymer blending, and the use of impact modifiers or reinforcing fillers. The addition of glass fibers, for instance, can significantly enhance impact strength, while the choice of polymer type and design features also play a crucial role in optimizing impact resistance.
| Characteristics | Values |
|---|---|
| Impact strength | A measure of whether a product will break when dropped or struck by an external force |
| Impact resistance | Can be improved by using impact modifiers like elastomers (POE, SEBS, MBS, EPDM) or maleic anhydride grafting on PP, POE, SEBS, and EPDM |
| Copolymer modification | Introducing alkane monomer with long carbon chains during synthesis of plastic polymers to achieve impact resistance |
| Polymer blending | Adding impact modifiers like rubber or thermoplastic elastomers to plastic polymers |
| Glass fibers | Adding glass fibers can increase impact strength, but too much can decrease it; glass fibers are usually coated with silanes surface coating to increase bonding force with plastic polymers |
| Mineral fillers | Incorporating low aspect ratio mineral fillers like talc and mica can increase stiffness without affecting strength |
| Geometry | Replacing sharp corners with fillets and radii can improve material flow and reduce residual stresses |
| Fiber reinforcement | Using fiber reinforcement and design features can increase stiffness and strength while maintaining lightweight and cost-effectiveness |
| Temperature | Impact strength is affected by temperature; it tends to be higher at elevated temperatures and lower at sub-zero temperatures |
| Brittleness | Greater strength often denotes higher brittleness, which is the opposite of impact resistance |
| Flexibility | More flexible polymers like polypropylene and polyethylene tend to have better impact strength |
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What You'll Learn
Copolymer modification during plastic polymer synthesis
One example of copolymer modification is the use of SEPTON™, a high-performance TPE compound, to improve the impact strength of polycarbonate (PC) plastic. PC is a tough material, but its thicker sections can become brittle due to lower notch impact strength. By adding SEPTON™ during the synthesis process, the impact strength of PC can be significantly increased while maintaining its processability.
Another example of copolymer modification is the use of HYBRAR™ and SEPTON™ to modify ABS (acrylonitrile butadiene styrene) plastic. ABS is a durable thermoplastic with excellent impact strength, but its impact strength can be further enhanced by adding these compounds. Additionally, varying the ratios of acrylonitrile, butadiene rubber, and styrene during ABS production can also improve impact resistance. Higher butadiene content results in better impact resistance but reduced rigidity.
Copolymer modification can also be achieved by blending polymers with other materials, such as polycarbonate or other polymers, to produce ABS/PC blends with improved impact resistance over a wider temperature range. Reinforcing fillers, such as micronized rubber, glass/carbon fibers, or minerals, can also be added to strengthen the ABS plastic and enhance its impact resistance, especially at low temperatures.
While copolymer modification during plastic polymer synthesis can effectively improve impact strength, it is important to consider the potential trade-offs. For example, increasing impact resistance by adding long carbon chains or ether bonds to the polymer structure may sacrifice the plastic's tensile strength and modulus. Additionally, the choice of additives and modifiers depends on the specific polymer, the desired effect, and processing conditions. As such, copolymer modification allows for the creation of plastics with tailored properties, making it an economically attractive option for improving impact strength.
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Adding impact modifiers to plastic polymers
Plastic polymers are often modified during synthesis to improve their impact strength. This process, known as copolymer modification, involves introducing long carbon chains or materials with ether bonds to the main chains of the polymer. This lowers the Tg temperature of the plastic, increasing impact resistance at room temperature and below but compromising tensile strength.
Impact modifiers are compounds added to plastic resins to enhance their toughness and durability. They are particularly effective for applications requiring resilience at low temperatures and high speeds. The amount of impact modifier added depends on the desired level of toughness, with higher levels of reactive impact modifiers needed for super toughness.
The most common impact modifiers on the market include elastomers like POE, SEBS, MBS, and EPDM. Maleic anhydride grafting on PP, POE, SEBS, and EPDM can also increase compatibility with polar polymers. Other frequently used impact modifiers include methyl methacrylate-butadiene-styrene copolymer (MBS), ternary Ethylene propylene rubber (EPT), acrylonitrile butadiene styrene (ABS), acrylate resin (ACR), chlorinated polyethylene (CPE), and ethylene vinyl acetate (EVA). These modifiers not only improve impact resistance but also enhance processing performance and reduce embrittlement at low temperatures.
Glass fibers are another effective additive to improve the impact strength of plastic polymers. Coating the glass fibers with silanes increases the bonding force between the glass fiber surface and the plastic polymer, ensuring that the reinforcement is maintained even when subjected to external forces. The addition of glass fibers can significantly enhance the impact strength of nylon 6, as demonstrated in various tests.
In addition to impact modifiers and glass fibers, mineral fillers such as talc and mica can be incorporated into plastic polymers to increase stiffness without affecting overall strength. This method avoids the anisotropic behavior associated with fiber-reinforced plastics while maintaining cost-effectiveness and lightweight properties.
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Using stiffening ribs in plastic design
Improving the impact strength of plastics is a critical aspect of product design, especially when considering the potential for external forces or drops. One effective method to enhance impact strength is through the use of stiffening ribs in plastic design.
Understanding Stiffening Ribs
Stiffening ribs, also known as reinforcing ribs, are structural elements incorporated into plastic designs to improve rigidity and strength. These ribs increase the bending stiffness of a part, allowing for better impact resistance. By adding ribs, the moment of inertia increases, resulting in enhanced bending stiffness. This relationship can be understood through the formula: Bending Stiffness = E (Young's Modulus) x I (Moment of Inertia).
Benefits of Using Stiffening Ribs
Stiffening ribs offer several advantages in plastic design:
- Enhanced Strength and Stiffness: Stiffening ribs improve the overall strength and stiffness of the plastic product, making it more resistant to deformation and external forces.
- Material and Weight Savings: The use of stiffening ribs can reduce the need for thicker walls or increased material usage. By optimizing the rib design, manufacturers can save on material costs and weight, making the final product more lightweight and cost-effective.
- Molding Benefits: Stiffening ribs can help eliminate heavy cross-section areas that may cause molding problems. They also facilitate the activity of plastic melt and provide channels for melt filling in thin-walled injection-molded parts.
- Design Flexibility: Stiffening ribs can be added or omitted based on the specific structural requirements of the part. This flexibility allows designers to optimize the impact strength while minimizing warpage and stress concentration issues.
Considerations for Using Stiffening Ribs
When incorporating stiffening ribs in plastic design, several key considerations should be made:
- Rib Thickness: The thickness of the stiffening ribs should be less than the wall thickness of the strengthened product. This helps prevent joint sag and minimizes sinking effects on the opposite side.
- Rib Height: While increasing the number of stiffening ribs is recommended to enhance stiffness, the height of each rib should be limited. Excessive rib height can lead to damage, reduced rigidity, and potential molding challenges.
- Orientation and Arrangement: The orientation of stiffening ribs is crucial for maximizing bending stiffness. Ribs oriented in one direction may increase stiffness, while the wrong orientation may provide no benefit. Proper arrangement and distribution of ribs are essential for optimal performance.
- Surface Treatment: To enhance the aesthetics of the final product, designers can employ decorative reinforcing rib structures to cover any surface depression or shrinkage marks that may occur during the cooling process.
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Increasing the percentage of butadiene in ABS plastic blends
Improving the impact strength of plastics is a critical aspect of manufacturing, especially when products are susceptible to being dropped or struck by external forces. ABS (acrylonitrile butadiene styrene) plastic is a widely used engineering thermoplastic known for its strength, stiffness, and impact resistance.
When it comes to increasing the percentage of butadiene in ABS plastic blends, it is important to understand the resulting changes in material properties. Butadiene is a rubbery material that enhances the impact resistance of ABS blends. By increasing the butadiene content, the impact resistance of the blend can be improved. This is particularly beneficial for applications where the plastic needs to withstand impacts at freezing temperatures, such as in the automotive, electronics, and appliance industries.
However, it is essential to consider the trade-off between impact resistance and rigidity. As the percentage of butadiene increases, the rigidity of the ABS blend decreases. This reduction in rigidity may become a concern for specific applications that require structural stability. Therefore, optimizing the composition based on the specific requirements of the application is crucial.
To further enhance the impact strength of ABS blends with higher butadiene content, additional techniques can be employed. One approach is to blend ABS with polycarbonate or other polymers, resulting in ABS/PC blends with improved temperature impact resistance. This technique can be particularly useful for applications where the plastic needs to withstand a wide range of temperatures.
Moreover, reinforcing fillers can be added to strengthen ABS blends with higher butadiene content. Micronized rubber, glass/carbon fibers, or minerals can be incorporated to enhance the overall impact strength. Additionally, optimizing the mechanical design features, such as ribbing, bosses, and radii, can further improve the impact resistance of the blend.
By carefully adjusting the percentage of butadiene and utilizing these supplementary techniques, manufacturers can produce ABS plastic blends with improved impact strength, making them more resilient to bumps, shocks, and drops across various temperature ranges.
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Incorporating low aspect ratio mineral fillers
The impact strength of plastics can be improved by incorporating low aspect ratio mineral fillers. Mineral fillers are particles added to binders (resin, thermoplastics, cement) to create a composite material with improved properties or reduced cost.
Mineral fillers with length-to-width ratios below 10:1 (low aspect ratios) contribute to uniform shrinkage, reducing warping in thermoplastic composites. Low aspect ratio fillers also help to avoid the non-uniform properties that can result from the orientation of high-aspect-ratio fillers during mould filling.
Talc and mica are commonly used low aspect ratio mineral fillers that can significantly increase the stiffness of plastics without affecting their strength. Talc is a soft, stable mineral derived from layering sheets of magnesium hydroxide with silica. It is often used in packaging and food applications. Mica provides a good balance of increased tensile strength, uniform properties, and reduced mould shrinkage. It is particularly effective at increasing the flexural modulus of nylon, boosting it by 400%.
Other mineral fillers that can improve the impact strength of plastics include silica, Franklin fibre, and wollastonite. Silica is unique among fillers in that it increases the impact strength of plastics and nylon, while most other fillers decrease it. Wollastonite can improve moisture content, wear resistance, thermal stability, and dielectric strength.
In addition to mineral fillers, other types of fillers such as glass beads, glass fibres, and coal and shale oil fly ash can also be used to improve the impact strength of plastics.
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Frequently asked questions
Impact strength describes a material’s ability to withstand shock or impact without breaking.
There are several ways to improve the impact strength of plastics, including:
- Copolymer modification: Modifying the plastic during synthesis by introducing long carbon chains or materials with ether bonds to the main chains of the polymer.
- Polymer blending: Adding impact modifiers such as rubber or thermoplastic elastomers to the plastic polymers.
- Adding fillers: Incorporating reinforcing fillers like glass fibers, carbon fibers, or minerals can increase impact strength.
- Design modifications: Optimizing the design of the plastic part by adding ribs, bosses, and radii can improve impact resistance.
Common impact-resistant plastics include polyphenylsulfone (PPSU), PEEK, TPE (thermoplastic elastomer), nylons, and acetals. Polycarbonate is also known for its excellent impact resistance properties.
Several factors can influence the impact strength of plastics, including temperature, composition, molding method, and design. The application temperature is particularly important, as impact resistance is often mitigated by tensile strength and other factors. Sharp corners and notches tend to increase the chance of breakage, while rounded corners make parts more durable.
