By: Terri Shadrix
When you’ve decided to use a hook and loop fastener, also known as a touch fasteners, you’re halfway there. Now you just have to determine which one will provide the closure strength required for your application under normal operating conditions.
This article explores the factors that determine the strength of the bond between components and planning for the forces that are determined to tear them apart, literally.
Hook and Loop Basics
We all know the first component features tiny hooks which provide the “aggressiveness” of the closure. The second component is comprised of loops that appear “hairy” and are much softer than the hook. Pressed together, the hooks catch in the loops and the two pieces fasten or bind temporarily until pulled apart. When a pull or force is applied the sides will separate by pulling or peeling the two surfaces apart, and the strips will make a distinctive "ripping" sound.
Hook-and-loop fasteners have been used for a wide variety of applications where a temporary bond is required including products in the medical, electronics, personal care, automotive and textile industries.
So what makes a strong connection?
The strength of the bond depends on how well the hooks are embedded in the loops, how much surface area is in contact with the hooks, and the nature of the force pulling it apart. This is called peel strength. Understanding those forces and knowing if they’re rolling gradually across the connection or a perpendicular pull straight away helps determine if you can get away with less surface area or lower density hook/loop combinations.
Shear strength tests aim to determine the resistance to separation when forces are applied parallel to the plane of the fastener. The most common test method measures the shear strength of hook and loop touch fasteners using a constant rate of extension tensile testing machine.
Some touch fasteners are strong enough that a two-inch square (5 × 5 cm) piece is enough to support a 175-pound (80 kg) load. (Schwarcz, Joseph A. October 2003). We've also seen an unofficial rule of thumb is that a two square inch piece will "support" one hundred pounds. However, it does depend on how you adhere the pieces to the corresponding substrates or materials. That will be the weakest link in your holding the closure.
The Bottom Line: You have to test the combination of hook/loop density, surface area and attachment method to create the right level of strength.
Ways to maximize the strength of a bond between the two flexible pieces:
• increase the area of the bond by using larger pieces
• ensure that the force is applied parallel to the plane of the fastener surface, such as bending around a corner
• increase the number of hooks and loops per area unit
Finally, be sure the fastener is fastened!
Based on the application your hook and loop must be securely attached through a sew-on process or adhesive backing. In a sew-on application the hook and loop is reliant on the sewing process and strength of thread. However, in an adhesive application there are several factors that come into play in choosing adhesive.
• Rubber Based Adhesive – High, tack quick setting, achieves 90% of bond strength in about 1 hour. Full bond strength in 24 hrs. Temperature operating range 0º-160º F
• Acrylic Adhesive – High temperature use. High Strength, moderate tack medium setup. Full bond strength in 24 hours. Temperature operating range 0º-226º F
• PC Tape – Precoated tape – factory applied adhesive backing activated by special adhesives, solvents, heat, ultrasonic or dielectric
• FR Tape – Flame Retardant Tape – meets FAA 25 853 (b) specification.
• PET Tape – Polyester tape for marine use or for resistance to UV light and Chemicals
Reference: (Schwarcz, Joseph A. (October 2003). Dr. Joe & What You Didn't Know: 99 Fascinating Questions About the Chemistry of Everyday Life. Ecw Press. p. 178. ISBN 978-1-55022-577-8)