For all kinds of products in life, our feelings about them are intuitive, such as the concepts of hardness, elasticity, toughness, and brittleness. This kind of information obtained through intuitive feelings and perceptions is generally called qualitative judgment. Qualitative judgments are usually vague, inaccurate, and unfavorable for comparison. Just as we know that iron is hard and steel is hard, then which one is harder? Is there any other standard to measure the hardness of the two besides colliding the two? Is there a set of general standards to measure the properties of different materials? To facilitate description and dissemination?

In fact, each industry has a set of mature standard measurement tests (standards) for product performance in various fields. Our country has also formulated standard tests for various indicators of various products for measurement (GB, 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 introduce the performance and testing involved in the daily use of polyurethane products (PU products).

1. Tensile strength and elongation

definition

Tensile strength: In a tensile test, the maximum tensile stress per unit area of ​​the sample until it breaks.

Elongation (elongation at break): The ratio of the increase in the distance between the marking lines when the sample is broken to the initial gauge length, expressed as a percentage.

Description

In life, tensile force is one of the most common forces on products. For any product with tensile force, tensile strength and elongation are technical indicators that must be measured. All kinds of polyurethane products, whether it is polyurethane foam (PU foam), elastomer, adhesive, synthetic leather resin film, etc., need to be tensile tested. 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, elastomers sometimes have a constant elongation strength (constant elongation modulus) index, which refers to the tensile stress at a specific elongation.

[Refer to GB/T 1040-2006, GB/T 528-2009 for details]

Two, tear strength

definition

The sample is torn more than 50mm in the direction perpendicular to the rising direction of the foam under tensile force and a certain tensile rate. The maximum tearing force during the tearing process divided by the thickness is the tearing strength.

Description

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 together on the object. Flexible foam (PU soft foam) and elastomer materials have a tear strength index (N/m or N/cm), which indicates the tear resistance of the material.

[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): the stress generated per unit area when the sample is loaded and deformed to a certain extent.

Compression set (for PU soft foam): Make the sample maintain a constant deformation for a certain period of time at a specified temperature. 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 it Compared with the initial thickness, the percentage represents the compression set of the material.

Description

PU foam is often used as cushion material in daily life, such as cushions, mattresses, pillow cores, etc. We often evaluate that a certain cushion is too soft or a certain pillow is too hard, and the compressive strength and indentation hardness quantify this evaluation , Accurate, so that there are evidences and standards to check in production and quality inspection.

Usually, when we say that a pillow cannot be used, it is mostly because the pillow is squashed and hard to expand and loses its elasticity. This situation is the life version of the compression set test. We can describe the service life of the PU soft foam through the performance of the compression set. Obviously, the lower the compression set value, the longer the life of the PU soft foam.

[Refer to GB/T 8813-2008, GB/T 10807-2006, GB/T 6669-2008 for details]

Fourth, bending strength

definition (for rigid and semi-rigid polyurethane materials)

Place the sample on two supports, and apply a load at the center of the sample at a certain rate. The maximum bending stress (or failure load) of the sample at a specified deformation (deflection equal to 1.5 times the thickness of the sample).

Description

Polyurethane rigid foam products can also be used as structural parts, such as beams of simple board houses, table tops, etc. At this time, the mechanical properties of PU rigid foams have become an attribute that must be considered. Bending strength is a performance index that shows whether the material is easy to bend and break.

[Refer to GB/T 9341-2008 for details]

Five, impact strength

definition

Using cantilever beams and simply supported beams and other methods for testing, the impact strength of the material is expressed by the energy absorbed per unit area when the sample is broken.

Description

PU hard foam is also used as a product for car bumpers or other shell structures. Impact strength (also called impact strength) is the key attribute of PU bumpers and other products that may withstand impact. Generally speaking, the higher the impact strength of the material, the less likely it is to be broken and smashed.

[Refer to GB/T 1043.1-2008, GB/T 1843-2008 for details]

Six, rebound rate

definition

Rebound rate of soft foam: The percentage of the rebound height of a small steel ball of a specified weight from a specified height to the sample.

Elastic body resilience rate (standard resilience): The ratio of the output energy to the input energy when an object with a spherical end point is used to impact a flat sample that is tightened and can be raised freely.

Description

Whether it is the rebound rate of soft foam or elastomer, the test is the performance of the product to return energy. For PU products for cushioning purposes, such as: cushioning air cushions, airbags and other protective products, the lower the rebound rate, the better, so that the cushioning effect is the best, and the cushioning product with 0 rebound rate is the best; but For products that rely on reaction forces such as shoe soles, runway rubber surfaces, and basketball court rubbers, the closer the rebound rate is to 1, the better, so that there is not much energy loss.

[Refer to GB/T 6670-2008, GB/T 1681-2009 for details]

Seven, shear strength

definition

The standard adhesive samples prepared under the specified conditions, under certain test conditions, apply a force parallel to the adhesive layer to the substrate to cause shear failure of the adhesive surface, the maximum that the unit adhesive area can withstand Average shear force.

Description

For the evaluation of polyurethane adhesives, the shear strength of the adhesive layer is an important technical indicator, which characterizes the adhesive strength of the adhesive layer parallel to the adhesive surface. This indicator is our evaluation of the superiority of the adhesive. One of the inferior standards. The following will introduce another key technical indicator of adhesive adhesion-peel strength.

[Refer to GB/T 7124-2008 for details]

8. Peel strength

definition

The maximum breaking load per unit bonding area is the force required within the unit width of the sample when the bonding surface is peeled off. Generally, the unit is N/cm, N/m or kN/m, and N/1.5cm, N/2.5cm can be used in special occasions. For different bonding joints (rigidity & rigidity, rigidity & flexibility, flexibility & flexibility), there are many test methods, the most important ones are 180° peeling, T-shaped peeling, roller peeling, and 90° peeling.

Description

Peel strength is an important index to evaluate the adhesive performance of an adhesive. It characterizes the maximum adhesion of the adhesive in the vertical direction of the bonding surface, which is the most important property of the adhesive. We usually say that a certain kind of glue is not tight or firm, but in fact we are talking about 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]

Nine, thermal conductivity

definition

When the unit area, unit thickness, and temperature difference are 1℃, the amount of heat that passes in a unit of time. 【Unit W/(m·K), previously used unit kcal/(cm·h·K)】

Description

For polyurethane foam, especially rigid polyurethane foam (PU rigid foam), thermal conductivity is an extremely 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 presence in almost any industry. The thermal conductivity directly characterizes the thermal insulation performance of the product. The lower the thermal conductivity of the product, the better the thermal insulation performance.

[Refer to GB/T 10294-2008, GB/T 3399-1982, GB/T 10801.1-2002 for details]

Ten, flame retardant performance

definition

Oxygen Index: Under specified test conditions, in a mixed gas stream of oxygen and nitrogen, measure the minimum oxygen concentration required to just maintain the combustion of the sample.

Horizontal combustion method and vertical combustion method: After the samples placed in the horizontal and vertical directions are ignited with a small flame ignition source, their combustion performance is graded.

Description

Polyurethane itself is a flammable material, but many applications have certain requirements for the flame-retardant properties of polyurethane. Therefore, various flame retardants are added in the production to achieve the corresponding flame-retardant level, so that the flammable polyurethane becomes flame-retardant. The highest flame-retardant grade of polyurethane materials can reach self-extinguishing in case of fire. The oxygen index, horizontal combustion and vertical combustion levels mentioned above are all common combustion performance indicators of polyurethane. The higher the oxygen index, the more difficult it is to burn. The horizontal combustion classifications are HB, HB40, and HB75. Among them, HB is the lowest in flammability and HB75 is flammable. The highest; vertical combustion classification V-0, V-1, V-2, of which V-0 has the lowest flammability and V-2 has the highest flammability.