PP (polypropylene) sheets are widely used in automotive interiors, packaging materials, electronic devices and other fields due to their light weight, chemical corrosion resistance and easy processing. However, different application scenarios have significant differences in the mechanical properties of PP sheets, especially the core indicator of tensile strength. This article will systematically analyze the key points of the tensile strength of PP sheets from three levels: international standard system, test operation specifications, and multi-dimensional influencing factors, and provide technical support for material research and development, quality control and engineering applications.
What international or national standards are followed for tensile strength testing of PP sheets?
In the field of plastic performance testing, the ISO 527 series standards form a basic testing framework. The standard system mainly includes three components: the first part (general principle) mainly regulates the basic concepts of tensile tests, such as the definition of professional terms such as yield strength and elongation of breakage, and also makes requirements for the morphological classification of the sample, including two types: common dumbbell structure and rectangular structure. In terms of setting test conditions, the key parameters such as ambient temperature, air humidity and tensile rate that the laboratory needs to control are focused on. Especially for the grading setting of stretching speed, for example, for thin film materials, slow stretching of 1 mm/min is usually used, while sheet-like materials may be used to detect faster speeds of 50 mm/min.
Regarding the sample preparation rules, the ISO 527-2 standard details the specific specifications of the two samples. Taking the 1BA/1BB model in the dumbbell type sample as an example, the width of the narrow part is set to 4 mm and the gauge length is fixed at 50 mm. This design is more effective for discovering local defects in the material. The width of the rectangular specimen is expanded to 10 mm. Although the test data is more stable, it may not accurately reflect the local performance problems of the material. In the third part of the standard for special materials, such as polypropylene sheets with a thickness of no more than 1 mm, the standard recommends that different rates be selected according to the test purpose - a slow speed of 1 mm/min is suitable for simulating long-term static load conditions, and a fast speed of 50 mm/min is closer to the actual situation of impact loads.
The ASTM D638 standard commonly used in North America has its characteristics in sample design. For example, although the width and gauge parameters of Type I rectangular specimens are the same as those of the ISO standard, the structure of the fixture fixture is particularly optimized, making it easier to clamp in actual operation. In contrast, dumbbell specimens with a width of 4 mm in ISO standard have a higher sensitivity to detect internal defects due to their small contact area. This characteristic makes it widely used in factory quality inspection. There are also differences in the test rate classification method of the two standard systems, and it is necessary to select and adapt according to the usage scenarios of the specific materials.
When conducting material performance testing, it is often necessary to distinguish test speeds. For example, slow test (a speed of 5 mm per minute), this solution is more suitable for materials like engineering plastics (such as plastic parts around the dashboard of a car), and is mainly used to observe the maximum strength value that the material itself can achieve. Quick tests (setting of 500 mm per minute) are more used to simulate actual usage scenarios, such as the impact situations that electronic product packaging films may encounter during transportation. In this case, the data obtained can better reflect the real usage status.
Regarding the selection of test standards, the main reference in China is the standard document GB/T 1040.2. The thickness range of the specimen specified in it can be from 0.1 mm to 14 mm, basically covering all specifications of PP sheets produced by injection molding or extrusion processes. It is particularly important to note that the standards clearly require that when conducting comparative tests, the environmental conditions must be controlled to be controlled to the temperature of 23 degrees and the humidity must not exceed 2 degrees, and the humidity must be kept in the range of 50%±5%, and the samples must be placed in advance for at least 40 hours - these regulations are mainly to ensure that the data from different laboratories can be compared with each other.
The EU standard (DIN EN ISO 527) emphasizes measurement accuracy, for example, the gauge distance error must be controlled within the range of plus or minus 0.2 mm. This is mainly because if the gauge distance deviation exceeds 1%, the final calculated intensity data may have an error of about 0.5% to 1%. For PP materials used in the medical field, additional biosafety testing is required as specified in ISO 10993, which is to confirm that the precipitated substances after soaking the material will not affect its tensile performance.
The industry internal standard QB/T 2471-2000 has special requirements for extruded PP sheets. For example, pure PP materials require a tensile strength of at least 28 megapas, and the elongation at break cannot be less than 150%. If a modified material with 30% talc powder is added, although the tensile strength can be increased to more than 40 MPa, the elongation standard will be relaxed to 50% - this setting is actually to find a balance point between the rigidity and toughness of the material.
When choosing a test plan, pay special attention to the differences between standards. For example, in terms of sample shape, the dumbbell-type sample (4 mm wide in the middle narrow part) adopted by the ISO standard is prone to expose local defects of the material, which is more suitable for the selection of formulas in the R&D stage. The rectangular specimens (10 mm width) used by the ASTM standard have better stability during mass production inspection. The selection of test speed should also be combined with actual application scenarios. For example, automotive interior parts are suitable for using slow tests of ISO 527 to evaluate long-term stress performance. However, if it is a packaging material for electronic products, ASTM rapid tests should be used to simulate transportation shocks.
What are the precautions for tensile testing of PP sheets?
About sample preparation and processing
Pay special attention to the operation method during the sample preparation stage. For example, mechanical cutting may cause stress concentration at the edge of the material, that is, it is easy to crack the edge. At this time, it is recommended to use laser cutting, so that the area affected by heat can be basically controlled within 0.1 mm. For extruded sheets, samples need to be cut in the direction of material flow and vertical direction respectively. For example, the tensile strength in the machine direction is usually 15% to 20% higher than that in the transverse direction.
In terms of environmental control, simply put, the temperature and humidity conditions need to be controlled. It is particularly important to note that the pretreatment temperature needs to be maintained within a range of 23 degrees Celsius plus or minus 1 degree to avoid changes in the crystallinity of the material due to temperature fluctuations. For PP materials containing ingredients such as PE wax, they must be placed in a dryer for about two days to balance. The purpose of this is to prevent moisture absorption from causing the molecular structure to loosen.
Key points for calibration of test equipment
The accuracy level of the extensometer used to measure strain should be selected according to the situation. When the gauge length exceeds 25 mm, a high-precision model with an error of no more than 0.5% should be used. If you want to do a low-temperature test at minus 40 degrees, you need to use liquid nitrogen to cool the fixture parts, otherwise the strength of stainless steel parts will drop by about 40% at low temperatures.
The selection of load sensors is also particular. When dealing with thick sheets, you should choose a sensor with a range of 20 kN, and the accuracy error should be controlled within one thousandth. If you are measuring thin film materials, it would be more appropriate to change to a 5 kN range, which can avoid measurement deviations caused by sensor overload. For example, if the overload is 10%, the error may reach two percentage points.
Key controls during the test
The clamping method will directly affect the experimental results. It is recommended to use a hydraulic wedge clamp to apply a clamping pressure of 5 to 10 MPa, so that the probability of sample slippage can be reduced from 15% to less than 3%. The clamping length should be more than 50 mm. If only 30 mm is clamped, the end is prone to crushing, resulting in a quarter increase in the risk of fracture.
Pay attention to the identification of data anomaly processing. If the material breaks before yielding, this may be caused by defects in the sample or excessive clamping. Such data should be discarded. Each set of experiments must obtain at least five valid data points, and the calculated coefficient of variation must not exceed 5% to indicate that the results are reliable.
What are the factors affecting the tensile strength of PP sheets?
Properties of the material itself
· Molecular composition
For example, for materials such as homopolymer polypropylene (HPP), due to the relatively regular arrangement of molecular chains, the crystallinity can usually reach the range of 50%-60%, and the corresponding tensile strength is about 25-35 MPa. However, because of the dispersed structure of the rubber phase inside copolymer polypropylene (CPP), although the impact resistance can be increased by 50% or even doubled, the tensile strength will be reduced to 20-30 MPa.
· Influence of degree of crystallinity
In short, the higher the crystallinity, the higher the tensile strength. For example, for every 10% increase in crystallinity, the strength can be increased by about 3-5 MPa. Pay special attention to the impact of differences in crystal forms. For example, the strength of β-crystal polypropylene can be about 15% higher than that of α-crystal polypropylene, which is mainly due to the thicker lamellar structure of β crystals. In order to improve the crystallization effect, adding nucleating agents is a common method. For example, the NA-11 nucleating agent can speed up the crystallization rate by about three times and increase the tensile strength by 8%-12%.
Technical means of material modification
· Filler reinforcement method
The more common one is glass fiber filling. When the addition amount reaches 30%, the material strength can rise to 60-80 MPa. Here we should pay attention to the arrangement direction of the fiber. For example, when the arrangement direction of the glass fiber reaches more than 80%, the effect is best. If it is less than 60%, the strength improvement may be less than half. Another method is to use nano-calcium carbonate. If this filler is added by 5%, the tensile strength can be increased by about 10%. However, if it is added too much, more than 10%, it will agglomerate, especially if the particles are too large. If it exceeds 200 nanometers, it is easy to cause stress concentration problems.
· Balance of toughening treatment
When the amount of toughening agents such as EPDM rubber is added to 20%, although the tensile strength will drop to 18-22 MPa, the ductility of the material will be greatly improved, and the elongation at break can be increased from the original 10% to 300%. This modified material is particularly suitable for packaging materials that need to be bent repeatedly, such as the sealing gasket in the beverage bottle cap, which is an application scenario that requires both toughness and deformation resistance.
· POE elastomer: This material can increase the notched impact strength from 4 kJ/m2 to 18 kJ/m2 while maintaining a strength of 25 MPa. For example, parts such as car bumpers that require high safety are more suitable for this material.
Processing parameters
· Extrusion process
· Melt temperature: For example, when the temperature is controlled at 200 to 210 degrees Celsius, the tensile strength of the material is the best, but if the temperature rises above 230 degrees, the internal structure of the material is easy to decompose, and the tensile strength will drop by about 15%.
· Cooling rate: For example, the tensile strength of the product is 5% to 8% higher when treated with water cooling at 5 degrees than when treated with air cooling at 25 degrees. This is mainly because the material crystallizes more fully when the cooling is fast, for example, the crystal particle size can be reduced from 50 microns to 20 microns.
· Injection molding process
· Holding pressure: When the pressure increases from 50 MPa to 100 MPa, the density of the material will increase from 0.90 to 0.92 grams per cubic centimeter. At this time, the tensile strength can increase by 8% to 12%. This is mainly because high pressure can reduce the voids inside the material.
· Mold temperature: For example, using a low-temperature mold at 40 degrees, although the material can be crystallized quickly and the strength is increased by 5%, the surface is prone to flow marks; if a high-temperature mold at 80 degrees is used, although the surface is smoother, the strength will decrease by about 3%.
Environment and use conditions
· Temperature effect
· Low temperature embrittlement: At -20 degrees, the strength of unmodified PP material will drop to 15 MPa, which is half of that at room temperature. At this time, the material will become particularly brittle, for example, the impact strength will directly drop by 80%.
· High temperature softening: When the temperature rises to 80 degrees, the material strength will drop to 18 MPa, but the elongation can be doubled, which means that the material has undergone thermal softening. At this time, it is necessary to improve the performance by mixing other materials.
Under the mechanical test conditions of conventional speed (the loading speed is controlled at about 1 mm per minute), the yield strength value measured is roughly in the range of 28 MPa. This value can actually be considered as the strength characteristic of the material itself. When the experimental conditions are adjusted to high-speed impact mode (for example, the loading speed is increased to 500 mm per minute), the yield strength of the material will be significantly improved, reaching about 35 MPa. This change is mainly due to the strain hardening phenomenon of the material during high-speed deformation. However, it should be noted that in this case, the elongation at break of the material will be sharply reduced from the original 300% level to about 50%. At this time, it may be necessary to adjust the material formula, such as using copolymer polypropylene materials with better impact resistance to improve.
In terms of chemical resistance, for example, after being immersed in a 5% sodium hydroxide liquid for three days, the tensile strength index of the material will drop significantly by about 40%. The main reason for this situation is related to the hydrolysis reaction of the material molecular chain in an alkaline environment. It is possible to consider replacing copolymer polypropylene materials or coating the surface of the material for protection. When investigating the effects of ultraviolet aging (for example, continuous irradiation with UVB light source for 1000 hours), the material strength index will drop by about a quarter. To address this problem, for example, if a HALS type light-stabilized material with a concentration of 0.5% is added, the strength loss can be effectively controlled within 10%.
