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Coating tests that evaluate five types of durability

September 1st, 2016 / By: / Business, Technical

Matthew Logan (left), product application engineer at APV Engineered Coatings, closes a gallon pail with a mallet, while Gregory Marty, chemist and QC manager, tests Pencil Hardness on a sample of coated material using graphite pencils. The test allows him to evaluate the value of hardness of a particular coating formulation. Photo: APV Engineered Coatings.
Matthew Logan (left), product application engineer at APV Engineered Coatings, closes a gallon pail with a mallet, while Gregory Marty, chemist and QC manager, tests Pencil Hardness on a sample of coated material using graphite pencils. The test allows him to evaluate the value of hardness of a particular coating formulation. Photo: APV Engineered Coatings.

The right coating chemistry can make the difference between a fabric that has long-lasting durability and one that fades, degrades and fails before its expected lifecycle. This is especially true of fabrics used in awnings, tents, flexible decking, outdoor furniture, marine upholstery and automotive seating, where exposure to harsh environments can hasten the product lifecycle.

Many complex parameters surround the engineering process of a new coating formulation. The success of the coating depends on testing and evaluating the integrity and performance of the formulation on every substrate for its individual application.

Coating engineers rely on a number of industry-standard tests that allow customers to certify the products to meet the performance needs of the end user. These tests are conducted in controlled laboratory settings and are designed to be more rigorous and aggressive than a real-world environment, to give a good indication of performance and lifecycle.

Every member of the supply chain is involved in securing the success of the product in the field; this includes the material manufacturer, the coating applicator and the fabricator. It’s essential to understand the testing processes and how these tests are used to evaluate coatings for specific applications and materials.

Adhesion

Whether working with flexible vinyl or woven fabrics, achieving adhesion is imperative to the success of the coating chemistry. ASTM D751-06 covers adhesion testing for coated fabrics such as tarpaulins and rainwear. It evaluates:

  •  Breaking strength
  •  Elongation
  • Bursting strength
  • Puncture resistance
  • Tearing strength
  • Hydrostatic resistance
  • Strength
  • Tack-tear resistance
  • Low-temperature bend, impact and crack resistance
  • Seam strength (including dead load seam strength)
  • Accelerated heat aging
  • Blocking resistance at elevated temperatures
  • Crush resistance
  • Wicking of coated cloth

ISO 2409 is a good qualitative test method for assessing the resistance of coatings to separation from substrates when a right-angle lattice pattern is cut into the coating, penetrating through to the surface of the material. The method may be used either as a pass-fail test, or when circumstances are appropriate, as a six-step classification test that can be conducted in the laboratory or on field samples.

Abrasion

Coatings on substrates can be damaged by abrasion during manufacturing and service. With woven materials, resistance to abrasion is affected by factors such as the inherent mechanical properties of the fibers; the dimensions of the fibers; the structure of the yarns; the construction of the fabrics; and the type, kind and amount of finishing material added to each of those components.

While “abrasion resistance” (often stated in terms of the number of cycles on a specified machine, using a specified technique to produce a degree or amount of abrasion) and “durability” (defined as the ability to withstand degradation or wearing out in use, including the effects of abrasion) are frequently related, the relationship varies by the end use of the material or the environment to which it is exposed.

The ASTM D4157 Standard Test Method for Abrasion Resistance of Textile Fabrics (Oscillatory Cylinder Method, also known as the Wyzenbeek Method) is a widely-used test for woven textiles such as furniture fabric and clothing. A sample size of fabric is pulled taut into a frame and a Wyzenbeek machine is used to rub the material using a #10 cotton duck fabric as the abradant, or a wire screen. The test is finalized when two yarn breaks occur, when appreciable wear is determined, or when 100,000 double rubs are reached.

Another abrasion test is the ASTM D4966-98 Standard Test Method for Abrasion Resistance of Textile Fabrics (also known as the Martindale Abrasion Tester Method). It’s used on flexible vinyl as well as woven fabrics such as contract seating, flexible roofing membrane, automotive seating, marine upholstery, wall coverings and rolled decking membrane. A coating is applied to the substrate, and the specimen is mounted to a Martindale Abrasion Tester. The material is then subjected to rubbing in a figure-eight motion until failure or after reaching the manufacturer’s specified number of rubs, after which the substrate is evaluated.

Accelerated light aging

Sunlight, moisture and heat can induce changes in the physical properties of coated materials, which can lead to reduced overall performance, color fading and adverse alterations in the flexibility of the material. Accelerated light aging (sometimes referred to as QUV testing or accelerated weathering) is a critical test for products used outdoors.

In the coatings industry, ASTM G155 Standard Practice for Operating Xenon Arc Light Apparatus for Exposure of Non-Metallic Materials is a commonly used test to learn the effects of light aging during the development process. Typically, the test exposes a similar material of known performance (a control) with the test specimen to provide a comparison. In this test, xenon arc light and a water apparatus are used to reproduce the weathering effects that occur when materials are exposed to sunlight, either directly or through window glass, and moisture such as rain or dew.

The American Association of Textile Chemists and Colorists (AATCC) has a similar test method, TM16.3, to test colorfastness to light with a xenon arc light source. Another related standard, ASTM D 4329, addresses fluorescent ultraviolet (UV) lamp exposure of plastics (including polymer coatings). Determining the appropriate accelerated light aging test is specified by the application of the material or the preference of the customer.

Stain resistance

As a coated material acquires dirt, water repellency can decrease. Chemistries can be custom-formulated to help withstand this by pulling water into the fabric to lift out stains, then pushing water back out of the substrate.

Stain resistance can be tested with AATCC TM 8, Colorfastness to Crocking. This method is designed to determine the amount of color transferred from the surface of colored textile materials to other surfaces by rubbing. A colored test specimen is rubbed ten times with a white crock test cloth in both a wet and dry state. Color transferred to the white test cloth is assessed by comparing it with the Gray Scale for Staining or the Chromatic Transference Scale, and a grade is assigned.

The automotive industry has a specific test for evaluating crocking and the stain performance of a coating on upholstery or seating. The industry follows the ASTM D4966-98 (or the Martindale Abrasion Test) and incorporates unwashed denim with the upholstery substrate. Each manufacturer has a variance to this test and differs in its interpretation of an acceptable degree of blue dye transfer.

Stain resistance often correlates with chemical resistance; another important stain resistance test is covered under the ASTM D1308 Standard Test Method for Effect of Household Chemicals on Clear and Pigmented Organic Finishes. This test evaluates criteria such as discoloration, change in gloss, blistering, softening, swelling, loss of adhesion or special phenomena. It is conducted by introducing droplets of the contaminant onto the substrate for an interval of time specified by the customer or chemist. The droplet is either covered with a watch glass for a more stringent test or left uncovered. It is then wiped clean and the area is examined immediately for effects.

AATCC TM 130 Soil Release: Oily Stain Release Method is designed to measure the material’s ability to release oily stains during laundering. It is primarily used by fabric finishers to evaluate the likely performance of soil release finishes in actual use. In the test, a stain (corn oil or other stain of a customer’s choosing) is applied to a test specimen. An amount of the staining substance is forced into the fabric using a specified weight. The test fabric is then laundered and the residual stain is rated on a scale of 5 to 1 by comparison to a control sample showing a graduated series of stains.

Water repellency

There are many ways to test the water-repellent properties of coated fabrics. Determining the appropriate test depends entirely on the end use of the material. The most widely used method, AATCC TM 22 Water Repellency: Spray Test, is applicable to any textile, and is especially suitable for measuring the water-repellent efficacy of finishes applied to fabrics. Water is sprayed against the surface of a tightly-stretched specimen, producing a wetted pattern whose size depends on the relative repellency of the fabric. The wetted pattern is then compared with pictures on a standard chart.

Two other AATCC tests that can be performed are Water Immersion TM 70 and Hydrostatic Pressure TM 127, which indicate coating performance in regard to water repellency and durability. Understanding the environmental conditions of the coated material over its lifecycle is critical to determining which test is appropriate. For more information, or to purchase copies of the standard test methods mentioned in this article, visit www.astm.org, www.iso.org and www.aatcc.org.

Ryan Scott is a research and development chemist at APV Engineered Coatings, a high-performance coating manufacturer. Within the past two years, he has developed and commercialized more than
20 unique coatings with multiple functionalities including intumescence, anti-graffiti, oleophobicity
and thermal conductivity.

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