Skip to content

Impact Resistance - tips for plastic material selection

Either I am dumb or the Izod and Charpy values expressed on the datasheets are confusing, but I have never been able to utilize them in practise. Apart from the units being sometimes in J/m and sometimes in J/m2, the tests are conducted in circumstances that are not particularly realistic in terms of load case, product geometry, material orientation and temperature. I understand that the general purpose of the data is to help designers to compare different plastic grades, but in case of impact resistance I haven’t found them useful. If you have been able to make use of them in practise, please tell us how in the comments at the bottom of this page. I have instead opted to divide the plastic materials into three groups:

1 Brittle materials that should be avoided when risk of impacts is high

These are for instance amorphous grades, such as PS, PSU, SAN and PMMA (although it is used on ice-skating rink panels). They all share a lack of overall toughness. Lack of toughness is also a factor that reduces the impact resistance of some reinforced plastics. PPS+40GF, for instance, has very high modulus and therefore low toughness. If you knock the edge of the corresponding Plasticprop sample, you can hear a glassy sound. This indicates low impact strength. The same applies for polypropylene. PP-H (homopolymer) is stronger and stiffer than PP-C (copolymer), but its toughness and therefore impact resistance is smaller.

2 Impact resistant materials

Polycarbonate PC is the mother of all impact resistant plastics. However, its superior properties are not fully explained by high toughness. The molecule structure of PC is very firm and it requires a lot of energy to tear the chains apart. The high impact resistance of polycarbonate is utilized in many alloys like PC/ABS, PC/PBT and PC/PA. Here is an amusing video that illustrates PC impact resistance over PMMA:

ABS is also known for its impact resistance which remains good also in low temperatures. This is based on the small butadiene particles that absorb the impact energy. The same principle applies also for HI-PS and impact modified PP (PP+TPE-O). PPO was earlier quite common choice for TV housing. It is often used as an example of good impact resistance, but my personal experience of the material is limited.

3 The materials in between the two

Practically all engineering plastics like PA, PBT and POM withstand impact relatively well. This again depends on toughness. If you can cause a permanent deformation of a material sample by bending it, the material is probably quite impact resistant.

Other material aspects to take into consideration

Reinforcement

Reinforcements are two-edged swords when it comes to impact resistance. Increased strength is a benefit but high stiffness reduces toughness. Being able to decode the Charpy-V or Izod values might help, but I suggest studying each case with a hammer.

The effect of temperature

Service temperature is an important factor in impact resistance. In elevated temperatures it tends to be higher (to a limit), but when it is lowered, plastic products tend to turn stiffer. As the modulus of the material is increased, some toughness is lost. Amorphous plastics do this gradually but semi-crystalline materials might turn from ductile to brittle very suddenly as the temperature goes below their Tg. A water bucket made of PP (Tg app -10°C), for example, might easily break on a cold winter day while a HD-PE bucket (Tg app. -90°C) is still ductile. Based on this, one might expect PA6 (Tg app. 50°C), for instance, to be brittle in room temperature, but it is not. It is not that simple, in other words; each plastic material has its own characteristics. Finding a stress-strain-temperature curve for each material candidate is therefore helpful and Campusplastics database is worth researching: Type for an example "stress-strain PC" into the search-area to find diagrams available for different PC grades.

Impact fatigue

Resistance against fatigue applies to impacts as well. Some plastic products might withstand single shocks well, but fail with repeated impact, even with smaller energy level. My practical experience on such cases is limited, but my understanding is that amorphous materials are more sensitive to impact fatigue than semi-crystalline, as is the case with normal fatigue. More detailed information on the phenomenon can be found in this article by Dupont.

Geometry

How to increase impact resistance with geometry? Very often material selection is determined by multiple requirements. A compromise in material impact resistance does not necessarily mean loss of ductility for your product, however, while even the toughest material fails if the product is not properly designed. Tips how to increase impact resistance with mechanics/geometry can be found here.

 

Comments

Michelle
Hello Markus, I'm trying to design a dispenser for use at sporting events (such as marathons). I'm relatively inexperienced at working with plastics so most of what I've been doing is guesswork. I thought of using PP or PTT for the outer casing, but I'm not sure either of those is the best choice possible. My apologies for the lack of details, I'm not sure what I should include and what not. Any help at all would be appreciated! Kind regards, Michelle
Markus
Michelle, PP sounds like a good choice. PPT is new to me, are you sure you don't mean PBT? That is an option as well. You should determine if you need transparency, flexibility or integral hinge. Those limit the possibilities quite a lot. But it sounds like you are still in quite early phase with your desing, so perhaps you can come back to this when you know more. Good luck, Markus
Wolfgang Arnezeder
Hi , could anybody give a hand ? I need to increase izod in Abs and does Tpe the job ? Thanks Wolfgang
Markus
Wolfgang, You mean compaunding ABS and TPE, TPE-S or TPE-V for instance? Theoretically, this should increase material toughness, on the other hand material strength is decreased. ABS is a mixture of SAN and EPDM-particles. Shouldn't this combination drive the same purpose as you are proposing. I would assume there is an ABS grade available with maximized impact resistance. Markus

Sivut

Add new comment

I appreciate commenting with your real name