For all kinds of articles in life, our feelings about them are intuitive, such as the concepts of hardness, elasticity, toughness, brittleness, etc. This information obtained through intuitive feeling and perception is generally called qualitative judgment. Qualitative judgment is usually vague, imprecise and not conducive to comparison. Just as we know that iron is hard and steel is hard, which one is harder? Is there any other standard to measure the hardness of the two besides hitting each other? Is there a common set of standards to measure the properties of different materials for easy description and dissemination?
In fact, each industry has a set of mature standard measurement tests for product performance in various fields（ Row mark ）China has also formulated standard tests for measuring various indicators of various products（ national standard ， GB ）。 By comparing the performance values of various materials measured in standard tests, we can have a clearer understanding of materials in life. The following will be introduced Polyurethane products（ PU products ）Performance and test involved in daily use 。
1、 Tensile strength and elongation
definition
Tensile strength: in the tensile test, the maximum tensile stress per unit area of the sample until it breaks.
Elongation (elongation at break): the ratio between the increase in the distance between the markings and the initial gauge length when the sample breaks, expressed as a percentage.
explain
In life, tension is one of the most common forces on products Tensile action The tensile strength and elongation of the product must be measured. All kinds of polyurethane products, whether polyurethane foam (PU foam), elastomer, adhesive, synthetic leather resin film, etc., shall be subject to tensile test. The tensile strength index indicates the maximum tensile force that the product can withstand, and the elongation rate indicates the elongation of the product in the process of breaking.
In addition, the elastomer sometimes has a constant tensile strength (modulus) index, which refers to the tensile stress at a specific elongation.
[Refer to GB/T 1040-2006, GB/T 528-2009 for details]
2、 Tear strength
definition
In the direction perpendicular to the rising direction of the foam, the sample is torn for more than 50 mm under the effect of tension and a certain tensile rate. The maximum tear force in the process of tearing divided by the thickness is the tear strength.
explain
In real life, the tearing force generally appears at the same time as the pulling force. When an object is pulled, the pulling force and the tearing force often act on the object together. Soft foamed plastics (PU soft foam) and elastomer materials have tear strength index (N/m or N/cm), which describes the tear resistance of materials.
[Refer to GB/T 10808-2006, GB/T 529-2008 for details]
3、 Compressive strength, indentation hardness and compression set
definition
Compressive strength (for PU hard foam) and indentation hardness (for PU soft foam): stress generated per unit area when the sample is loaded and deformed to a certain extent.
Compression permanent deformation (for PU soft foam): under the specified temperature, make the sample maintain a constant deformation for a certain period of time. After the sample recovers for a period of time, measure the difference between the initial thickness and the final thickness of the sample, and calculate its ratio to the initial thickness. The percentage represents the compression permanent deformation of the material.
explain
PU foam is often used as cushion material in life, such as cushion, mattress, pillow core, etc. We often evaluate that a certain cushion is too soft or a certain pillow is too hard, and the compression strength and indentation hardness are quantitative and accurate, so that there are evidence and standards to be followed in production and quality inspection.
Usually, when we say that a pillow can not be used, it is mostly because the pillow is hard to expand after being compressed, and loses its elasticity. This situation is the life version of the compression set test. We can describe the service life of PU soft foam by the performance of compression set. Obviously, the lower the compression set, the longer the life of PU soft foam.
[Refer to GB/T 8813-2008, GB/T 10807-2006, GB/T 6669-2008 for details]
4、 Bending strength
definition (For rigid and semi rigid polyurethane materials)
Place the sample on two supports, apply load at a certain rate in the center of the sample, and the maximum bending stress (or failure load) of the sample at the specified deformation (deflection equal to 1.5 times of the sample thickness).
explain
PU rigid foam products can also be used as structural parts, such as beams and tabletops of simple plank houses. At this time, the mechanical properties of PU rigid foam become a property that must be considered. The bending strength is a performance index indicating whether the material is easy to bend and break.
[Refer to GB/T 9341-2008 for details]
5、 Impact strength
definition
Cantilever beam and simply supported beam are used for testing, and the impact strength of the material is expressed by the energy absorbed per unit area when the sample is damaged.
explain
PU rigid foam is also used as automobile bumper or other shell structure products. Impact strength (also called impact strength) is the key attribute of PU bumper and other products that may withstand impact. Generally speaking, materials with higher impact strength are less likely to be damaged and knocked down.
[Refer to GB/T 1043.1-2008, GB/T 1843-2008 for details]
6、 Rebound rate
definition
Soft bubble rebound rate: the percentage between the rebound height and the original drop height of a small steel ball with a specified weight falling freely from the specified height to the sample.
Elastomer resilience rate (standard resilience): when a compact and freely raised flat sample is impacted by a spherical object at the end, the ratio of output energy to input energy.
explain
Whether it is the rebound rate of soft foam or the rebound rate of elastomer, the test is the performance of energy returned by the product. For PU products used for buffering, such as protective products such as cushion air cushion and airbag, the lower the rebound rate, the better, so as to achieve the best buffering effect, and the buffering product with zero rebound rate is the best; However, for products that rely on reaction force, such as soles, runway glue surfaces, basketball court glue, etc., the closer the rebound rate is to 1, the better, so that there is not too much energy loss.
[Refer to GB/T 6670-2008, GB/T 1681-2009 for details]
7、 Shear strength
definition
The maximum average shear force that can be borne by the unit bonding area when the standard adhesive sample prepared under specified conditions applies a force parallel to the adhesive layer on the substrate under certain test conditions to cause shear failure of the bonding surface.
explain
For the evaluation of polyurethane adhesive, the adhesive layer shear strength is an important technical indicator, which represents the adhesive strength of the adhesive layer parallel to the adhesive surface. This indicator is one of the criteria for us to evaluate the merits of this adhesive. Next, we will introduce another key technical index of adhesive adhesion - peel strength.
[Refer to GB/T 7124-2008 for details]
8、 Peel strength
definition
The maximum failure load per unit bonding area is the force required per unit width of the specimen when the bonding surface is peeled off. Generally, the unit is N/cm, N/m or kN/m, and the unit can be N/1.5cm, N/2.5cm for special occasions. For different adhesive joints (rigid&rigid, rigid&flexible, flexible&flexible), there are many testing methods, the main ones are 180 ° peel, T-shaped peel, roller peel and 90 ° peel.
explain
Peel strength is an important index to evaluate the bonding performance of adhesives. It represents the maximum bonding force of adhesives in the vertical direction of the bonding surface, which is the most important attribute of adhesives. When we usually say that a certain adhesive is tight or not, we actually mean peel strength and shear strength, of which peel strength plays a major role.
[Refer to GB/T 2791-1995, GB/T 2790-1995, GB/T 7122-1996 for details]
9、 Thermal conductivity
definition
Heat passing through unit time when unit area, unit thickness and temperature difference are 1 ℃. [Unit W/(m · K), used unit kcal/(cm · h · K)]
explain
For polyurethane foam, especially rigid polyurethane foam (PU rigid foam), thermal conductivity is an important performance index. Because a large part of the application of PU rigid foam is related to thermal insulation, its excellent thermal insulation performance makes PU rigid foam have its shadow in almost any industry. The thermal conductivity is a direct characterization of the thermal insulation performance of the product. The lower the thermal conductivity of the product, the better its thermal insulation performance.
[Refer to GB/T 10294-2008, GB/T 3399-1982, GB/T 10801.1-2002 for details]
10、 Flame retardancy
definition
Oxygen index: under the specified test conditions, measure the minimum oxygen concentration just required to maintain the combustion of the sample in the mixed flow of oxygen and nitrogen.
Horizontal combustion method and vertical combustion method: ignite the horizontal and vertical samples with a small flame ignition source, and then grade their combustion performance.
explain
Polyurethane itself is a combustible material, but many applications have certain requirements for the flame retardancy of polyurethane. Therefore, various flame retardants are added in production to achieve the corresponding flame retardant level, so that the combustible polyurethane can become a refractory and incombustible material. The highest flame retardant level of polyurethane can achieve self extinguishing in case of fire. The oxygen index, horizontal combustion and vertical combustion grades mentioned above are common combustion performance indexes of polyurethane. The higher the oxygen index, the more difficult it is to burn. Horizontal combustion grades include HB, HB40 and HB75, of which HB has the lowest flammability and HB75 has the highest flammability; Vertical combustion is classified as V-0, V-1 and V-2, of which V-0 has the lowest flammability and V-2 has the highest flammability.
[Refer to GB/T 2406-2008, GB/T 2408-2008, GB/T 8332-2008, GB/T 8333-2008 for details]
The above is just a brief introduction to the performance of various indicators of polyurethane products. The specific test method needs to refer to the measurement test formulated by the relevant national standards. I hope that by understanding these indicators, we can have a more accurate and detailed understanding of common materials and products in life, and make our consumption more rational.
