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How to verify the rubber calendering effect through tensile test ISO 37?

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I. What is the rubber calendering effect?

The calendering effect of rubber refers to the fact that during the calendering process, the rubber molecular chain and the compound particles such as needles and sheets are arranged in the direction of calendering, resulting in the anisotropy of physical and mechanical properties of the calendered film in the longitudinal (calendering direction) and transverse (vertical direction).

- Longitudinal (MD, calendering direction/roller parallel): high tensile strength, small elongation, and large shrinkage.

- Transverse (TD, vertical calendering direction/vertical roller vertical): low tensile strength, high elongation, and low shrinkage.

- Essential cause: Rubber macromolecular chains and needle/sheet fillers are oriented in the direction of calendering under calendering shear force, resulting in anisotropy.

How to verify the rubber calendering effect through tensile test ISO 37?cid=24


II. How to verify the calendering effect through experiments?

The core of the experiment is to verify anisotropy by comparing the tensile properties of specimens in different directions.

1. Purpose of the experiment

The differences in tensile strength, elongation at break and shrinkage rate in the vertical and horizontal directions of the calendered compound were measured, and the calendering effect size was evaluated

2. Equipment and materials

- Equipment: Laboratory small calender/three-roll calender, refiner, vulcanizing machine, tensile testing machine, caliper, ruler, oven.

- Specimen: compound rubber (natural rubber/synthetic rubber + filler), dumbbell-shaped tensile specimen (GB/T 528 Type 1A).

3. Experimental steps

(1) Preparation of rubber materials

- Mix according to the recipe to control Mooney's viscosity and plasticity consistently.

- The refiner is thin and ejected, and the temperature (e.g. 60–80°C) and roller distance (e.g. 2–3mm) are controlled.

(2) Calendering (key)

- Set the calender parameters: roll temperature (e.g. 70–90°C), roll spacing (e.g. 1.0–2.0mm), roll speed (e.g. 10–20r/min), speed ratio (e.g. 1:1.2).

- The compound is passed through the calender to make a uniform film and mark the calendering direction (MD).

- Leave at room temperature for 24 hours to eliminate internal stress.

(3) Specimen cutting

- Cut dumbbell specimens (5–10) along the calendering direction (MD).

- Cut dumbbell specimens (5–10) along the vertical calendering direction (TD).

- Simultaneous cutting of shrinkage specimens: 100 mm × 100 mm squares, marked MD/TD.

(4) Vulcanization

- Vulcanization according to the vulcanization curve of the compound under standard vulcanization conditions (e.g. 150°C×t90).

(5) Performance testing

- Tensile performance: Tensile testing machine measures the tensile strength and elongation at break of MD/TD (GB/T 528).

- Shrinkage: Measured MD/TD size after vulcanization, calculated:

Shrinkage rate (%) = [(original length − current length)/original length] × 100%

(6) Data processing

- Calculate the average and standard deviation of each performance of MD/TD.

- Degree of calendering effect = | MD Performance − TD Performance | / MD Performance × 100%.

4. Judgment criteria

- The greater the difference in longitudinal and horizontal performance, the more significant the calendering effect.

- General: MD tensile strength is 10%–30% higher than TD for obvious calendering effect.

5. Influencing factors:

Understanding these factors can help you control variables or analyze results in your experiments:

* Formulation factors: the higher the glue content, the greater the calendering effect; The more needle or sheet fillers (such as calcium carbonate and talc), the more significant the effect; Increasing the amount of softener can reduce the effect.

* Process factors: Increasing the calendering temperature, reducing the calendering speed, increasing the plasticity of the compound, and extending the film parking time can help reduce the calendering effect.

Related products and device

rubber tensile testing machine ISO 37, ASTM D412

WDW Series Computer Control Electronic Universal Testing Machine made by UNITEDTEST range from 100N to 600KN load capacity with various models like single columns, table type, door frame type etc., is used to perform tension, compression, flexure/bending, shearing, peeling etc., test for metal and nonmetal specimens.

Vice type Rubber tensile test fixture

Tensile test fixture for plastics is based on the test standatd ASTM D638, determination of the tensile properties of unreinforced and reinforced plastics in the form of standard dumbbell-shaped test specimens when tested under defined conditions of pretreatment, temperature, speed.

Related Standard

ISO 37 Tensile Test on Rubber, vulcanized or thermoplastic

ISO 37 and ASTM D412 are both widely recognized tensile test methods designed to evaluate the stress-strain characteristics of various rubber materials, including natural rubber, synthetic rubber, silicone rubber, and thermoplastic elastomers (TPEs). While both standards aim to determine the tensile properties of rubber and elastomers, they differ in their specific methodologies and applications.

ASTM D412 Tensile Strength of Rubber and Elastomers

ASTM D412 test methods cover procedures used to evaluate the tensile (tension) properties of vulcanized thermoset rubbers and thermoplastic elastomers. These methods are not applicable to ebonite and similar hard, low elongation materials. 

The methods appear as follows:

Test Method A—Dumbbell and Straight Section Specimens

Test Method B—Cut Ring Specimens

ASTM D2137 Rubber Brittleness Point Test of Flexible Polymers and Coated Fabrics

ASTM D2137: Standard Test Methods for Rubber Property—Brittleness Point of Flexible Polymers and Coated Fabrics


Summary

ASTM D2137 test method detailed in this standard is used to evaluate the brittleness of rubber materials, or rubber coated fabrics, when exposed to low-temperature flex with an impact under specified conditions of striker speed. ASTM D2137 tests performed will be used to determine the lowest temperature at which rubber compounds will not show fractures or coating cracks when exposed to specified impact conditions.

ASTM D2663 Carbon black dispersion test in Rubber

ASTM D2663: Standard Test Methods for Carbon Black—Dispersion in Rubber


ASTM D2663 test methods cover the degree of dispersion of carbon black in rubber. Visual dispersion ratings correlate with certain important physical properties of the compound. 

ISO 812 Rubber, vulcanized or thermoplastic — Determination of low-temperature brittleness

ISO 812:2017 specifies a method for determining the lowest temperature at which rubber materials do not exhibit brittle failure or the temperature at which half of the test pieces used in a test fail when impacted under specified conditions.

The temperatures thus determined do not necessarily relate to the lowest temperature at which the material can be used since the brittleness will be affected by the conditions of test and especially by the rate of impact. Data obtained by this method are, therefore, intended to be used to predict the behaviour of rubbers at low temperatures only in applications in which the conditions of deformation are similar to those specified in the test.


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