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Coatings Ingredients

Impact Resistance: An Important Mechanical Property in Coatings

Impact Resistance
  1. Optimizing Mechanical Properties of Coatings
  2. Coating’s Resistance to Mechanical Impact
  3. Obtaining Good Impact Resistance in Coatings
  4. Test Methods to Determine Impact Resistance of Coatings

Optimizing Mechanical Properties of Coatings

The performance of a coating largely depends on its mechanical properties that optimize the coating structure to achieve the desired performance level when needed. Coated substrates are regularly subjected to different types of mechanical or physical damages during a lifetime.

By exhibiting the desired balance of mechanical properties, such as impact resistance, flexibility, hardness, toughness, a coating can meet its service requirements for a specific application as well as withstand adverse effects of damages.

The mechanical properties of coatings are defined as below:

  1. Impact resistance is the ability of a coating to resist crack or break caused by high mechanical loads and stress levels. These mechanical loads are produced either by shrinking or swelling, mechanical abuse, and weathering. Impact resistance is one of the important mechanical properties of a coating to be considered while formulating coating for protective purposes (corrosion protection, microbial protection, etc.). 
  2. Flexibility showcases the ability of a coating to bent or flexed without getting cracked or undergoing other failures.
  3. Hardness quantifies the resistance to penetration of a coating layer by a harder body. It is the measure of resistance of the paint film to scratch, deformation, and indentation.
  4. Toughness is the ability of a coating to resist both fracture and deformation. It is the strength and resilience of a coating layer.

These mechanical properties determine the coating’s ability to withstand strain imposed in a short time, such as an impact without tearing, breaking, or rupturing of the layer.

Let's explore how to obtain good impact resistance in coatings by different test methods in detail...

Coating’s Resistance to Mechanical Impact

Impact resistance measures the material’s resistance to mechanical impact getting without undergoing any physical changes. It is a measure of a coating’s ability to withstand a shock. Impact resistance of a paint film can be considered as energy dissipation by vibration or rotation of various molecular segments so that at no time there is enough energy to cause a fracture. It is an important property to obtain information about the degree of cross-linking and cure in the coating layer.

  • An under cured coating may not have enough crosslinking during cross-linking and exhibit a low impact value, and
  • An over cured coating may be brittle and have a low impact value.

As cross-link density increases, the impact values improve for coating.

Under cured coatings
Under Cured Coatings by IFS Coatings

Obtaining Good Impact Resistance in Coatings

To obtain good impact resistance in coatings, the paint film must consist of a polymer that has a sufficiently high molecular weight to have strong intermolecular entanglement (and therefore, high tensile strength). At the same time, paint should have sufficiently low viscosity (by choice of proper molecular constituents and limiting molecular weight) that flow, and accompanying energy dissipation will take place.

Role of Plasticizers

Plasticizers are additives often used to improve flexibility, formability, and impact resistance of coating films. Some applications require flexibility and impact resistance at low temperatures. Impact resistance can be significantly improved at increased plasticizer levels, as well as being a function of the plasticizer type.

Commercial coatings require the optimum choice of plasticizer type and concentration to meet required costs, hardness or modulus, permanence, and low-temperature properties. For plasticizers to be effective, they must be compatible with the resin system and must not impact undesirable properties, such as color formation. There are a wide variety of plasticizers available. Common types include phthalates, such as dioctyl phthalate, adipates, epoxies, chlorinated paraffins, and others.

Plasticizer Selection

Fillers and Extenders

The mineral fillers play an important role in enhancing mechanical properties of paints and coatings. Filler materials are added to polymeric coatings and these additions are known to significantly improve coatings properties, such as tensile strength, wear resistance and impact resistance.

Fillers and Extenders Selection

Temperature Plays an Important Role

The flexibility and toughness of coatings are also dependent on temperature. High temperatures may cause the coating to bake or cure excessively. It causes the coating to become brittle with decreased impact resistance, or it may become more resistant to the environment than would occur if it were only air-dried under ambient conditions.

Also, coatings at a temperature below glass transition temperature (Tg) are hard and brittle with poor flexibility and impact resistance unless there is an auxiliary loss mechanism below Tg or below the temperature at which the coating is used.

Test Methods to Determine Impact Resistance of Coatings

Impact tests signify toughness, or impact strength, of a coating to absorb energy under mechanical load. The drop impact test (or falling-weight impact test) is a commonly used test method to determine the impact resistance.

The test standards used for impact resistance testing include:

ASTM D2794 – Test Method for Resistance of Organic Coatings to the Effects of Rapid Deformation (Impact)

This standard is described as a test procedure and offers three procedures to determine the degree of cracking produced in an impact deformation:

  • Visual inspection with a magnifier,
  • Visual inspection after application of an acidified copper sulfate solution, and
  • Use of a pin-hole detector.

During the test, a flat-coated panel is placed under a weighted spherical ball assembly. Then the weighted ball is dropped onto the panel from different heights. The impact causes a dimple to form in the test panel, and it is examined visually or with a ten-power lens to determine the extent of cracking or other failures.

The measurements are carried out by dropping the ball directly on the coated surface or
on the reverse side with the results being reported as direct or reverse impact, respectively.

This is a simple test and widely recognized in the coatings industry providing useful information about the performance characteristics of the coating.

Source: BYK

ASTM G14 – Standard Test Method for Impact Resistance of Pipeline Coatings (Falling Weight Test)

This test method covers the determination of the energy required to rupture coatings applied to pipe under specified conditions of impact from a falling weight. It uses a falling fixed weight having a specified diameter impact surface. The fixed weight is restrained vertically and dropped from varying heights to produce impact energies over the required range.

An electrical inspection is carried out to detect resultant breaks in the coating. Impact resistance is determined as the amount of energy required to cause penetration into the coating film.

The higher the amount of energy required to cause cracks or penetration into the coating film,
the higher the impact resistance and durability of the coating.

This test method holds a great significance in the pipeline coating industry to screen pipe coatings and determine their ability to resist mechanical damage during shipping, handling, and installation.

Impact Testing Instruments by Presto Group
Impact Testing Instruments
Source: Presto Group
Drop Weight Tester by Polylabs
Drop Weight Impact Tester
Source: PolyLab LLC

ISO 6272 – Paints and Varnishes - Rapid-deformation (Impact Resistance) Tests

This test method evaluates the resistance of a dry film of paint, varnish or related product to crack or peel from a substrate when it is subjected to a deformation caused by a falling weight or dropped under standard conditions. The test method is divided into two parts:

Part 1: Falling-weight test, large-area spherical indenter using a 1 kg weight with the 20 mm diameter spherical indenter attached.

Part 2: Falling-weight test, small-area spherical indenter using either a 12.7 mm or 15.9 mm indenter.

The method described can be applied as:

  • A pass/fail test – The test is carried out from one drop height and with a specified mass, to test compliance with a particular specification.
  • Q classification test – The test determines the minimum mass and/or drop height for which the coating cracks or peels from its substrate by gradually increasing the drop height and/or the mass.

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