Understanding Toughness and Rigidity from the Modification of Plastic Products

Oct 09, 2022

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“Stiffness” refers to the force required for unit deformation of an object; “Flexibility” refers to the deformation of an object under a unit force. It can be seen that the larger the “stiffness” is, the less likely the object is to deform (shown by the small elongation); Objects with greater “flexibility” are more likely to deform (shown by greater elongation). In an ideal state, the stiffness of the object tends to infinity (or the deformation of the object under the force is so small that it can be ignored), so we call the object a rigid body. In mechanical analysis, its own deformation may not be considered. Therefore, rigidity is an attribute reflecting the difficulty of object deformation. The ductile material is relatively soft, and the tensile elongation at break and impact strength of the physical property sheet are relatively large; Hardness, tensile strength and tensile modulus of elasticity are relatively small. The hardness and tensile strength of rigid materials are relatively high; Elongation at break and impact strength may be lower; The tensile modulus of elasticity is large. The bending strength reflects the rigidity of the material. The greater the bending strength is, the greater the rigidity of the material is. On the contrary, the greater the toughness is. According to ASTM D790 Standard Test Methods for Flexural Properties, these test methods are suitable for both rigid and semi-rigid materials. It is not said that it is suitable for ductile materials, so elastomers with great toughness will not be tested for bending strength.

The above mentioned toughness and rigidity are relative to the tested mechanical properties. Accidents may occur. For example, after glass fiber reinforced plastic is used, its rigidity becomes larger, but the tensile strength and impact strength may also increase. Under impact and vibration loads, materials can absorb large energy to produce certain deformation without damage, which is called toughness or impact toughness. Building steel (mild steel), wood, plastic, etc. are typical ductile materials. The toughness of materials shall be considered for pavement, bridge, crane beam and structures with seismic requirements. Rigidity and brittleness are generally connected. Brittleness refers to the property that when the external force reaches a certain limit, the material is suddenly destroyed without warning, and there is no obvious plastic deformation when it is destroyed. The characteristics of the mechanical properties of brittle materials are that the compressive strength is far greater than the tensile strength, and the limit strain value at failure is very small. Bricks, stones, ceramics, glass, concrete, cast iron, etc. are brittle materials. Compared with ductile materials, they are quite unfavorable for resisting impact load and bearing vibration.

As engineering plastics, we hope it has good toughness and rigidity at the same time. In order to improve the toughness of materials, efforts should also be made to improve the rigidity. Generally, adding elastomer can increase toughness, and adding inorganic filler can increase rigidity. The most effective method is to combine the toughening of elastomer with the strengthening of filler. Poor impact resistance is a performance defect of some important plastics in industry. For example, PVC, PS, PP, etc., their application is limited especially at low temperature due to their low impact resistance. However, the impact resistance of thermoplastics can be greatly improved by adding “impact modifier”. There are many kinds of impact modifiers, such as ACR acrylate resin, MBS methyl methacrylate butadiene styrene copolymer, CPE chlorinated polyethylene, ABS, EVA, and EPDM. From the perspective of the modification effect of plastic products, ACR has the best comprehensive performance. MBS is an important impact modifier for transparent products and plays an important role in the global impact modifier market.