Low Surface Energy Bonding - A Guide

Table of Contents

Introduction
The Challenge
Super Strength
Gunning for the Right Answer

Introduction

Recently, Advanced Adhesives has been working with various manufacturers on a variety of applications to develop a solution not only for bonding the impossible, but bonding the impossible structurally.

Advanced Adhesives was asked to solve the issue of bonding low surface energy materials (LSEs) structurally for various high-profile applications where the need was for a structural bond with no additional fixings and without any surface pre-treatments.

We like a challenge and to meet the brief, set to work developing an innovative adhesive technology, based on a unique chemistry - the upshot was the development of our PP3000 bonding solution.

Graham Crozier, Managing Director, Advanced Adhesives

The PP3000 bonding solution is formulated to specifically bond LSE materials to each other structurally. However, the adhesive can bond LSEs to other composites, glass, metals, plastics and substrates. It has the ability to bond to E-coat, enabling different materials to be freely combined. The 2-part MM A based, equal mix ratio material is supplied in 50 ml and 400 ml dual mix cartridges for easy application, which self-mixes the material, and hence, the application is of a pre-mixed adhesive.

Crozier adds that "Many designers and manufacturers within the manufacturing industry are turning to the use of low surface energy plastics such as thermoplastic polyolefin, TPO. polypropylene. PP and polyethylencs like HDPE, due to the many benefits they have over conventional plastics: low cost, durability, ease of processing and having superior chemical resistance and temperature stability. However, these are among the most difficult-to-bond materials, due to their non-polar, 'wax-like' surfaces, and can lead to issues when manufacturers need to assemble and join these plastics."

Upon application of a liquid adhesive to the surface of an LSE plastic, PE, PP, HDPE or TPO, the adhesive beads up rather than 'wetting out' on the surface - much like when it rains just after polishing a car and the rain beads up on the car body. Crozier said, "This poor wetting out on LSE plastics where the liquid adhesive will not spread and make intimate contact with the surfaces to be bonded results in poor adhesion, thus resulting in poor bond strength.”

Crozier added, "Conversely, with a higher surface energy substrate where the surface energy of the adhesive is less than that of the substrate, the adhesive will spread and 'wet out' the surfaces, giving the intimate contact required and thus good adhesion."

The Challenge

Traditionally, LSE surfaces would have to be solvent welded or mechanically attached, as it is not possible to achieve true adhesion. In the molding process, mechanical attachments such as fixings and screws etc need additional requirements to create features for the fixings to be employed and, by using fasteners, potentially forming stress concentration areas where the plastic can fail prematurely, while also leading to unsightly surfaces.

Crozier said, "In addition to fixings/fasteners the several techniques that are used to increase the surface energy of an LSE plastic, making it easier to bond - plasma, corona, flame treatment or solvent-based adhesion promoters. Once these treatment have been applied, it increases the surface energy and allows the liquid adhesive to wet out and give an increased bond level; but rarely to a genuine structural strength. These additional treatments have a knock-on effect in the increased production cost and production process time, adding cost to the assembly of a component."

The PP3000 has already been successfully used in some large-scale applications. Two UK vehicle manufacturers use the adhesive to bond dashboard assemblies, instrument panels, with truly impressive results obtained from their testing.

"Testing was carried out alongside four other adhesives, at different temperatures, environmental conditions, and environmental cycles," states Crozier.

PP3000 was the only adhesive that passed the complete test and gave total substrate failure throughout the testing procedure, regardless of the environmental conditions or test procedures where other adhesives failed. PP3000 was tested with plasma-treated components, alongside non-plasma treated components and the PP3000 proved to perform equally as well, proving a structural bond achievement with 110 need additional surface treatments.

Graham Crozier, Managing Director, Advanced Adhesives

plastic stretching with no adhesive failure

PP3000 bonded components on load/tensile testing, showing plastic stretching with no adhesive failure.

Super Strength

Crozier asserts that, “The PP3000 was also tested for an application for bonding LGF polypropylene against other adhesives and gave substrate failure at the peak temperature requirement of +95 °C with a 1mm and 2 mm bond line thickness. It also yielded strengths greater than seven times that of the next best adhesive in these tests.”

PP3000 components were then bonded and again tested, but to an increased temperature of +120 °C to test for a greater resistance capability of the adhesive and the components. Again, the adhesive gave substrate failure, proving the adhesive was stronger than the substrates, even at this increased temperature. The PP3000 is being used very successfully in this automotive application.

Graham Crozier, Managing Director, Advanced Adhesives

The PP3000 is also used for bonding automotive bumper components, motorbike fairings, motorhome fairings and panels, sports equipment, molded components etc. as well as for bonding PP baby bath molding for one customer.

Besides Advanced Adhesives' own in-depth trials, the PP3000 has also been subjected to extensive testing by prospective manufacturers looking to use the adhesive. In addition to strength testing, chemical resistance has also been performed, with proven environmental stability and retained strength resistance against many substances, including diesel, petrol, motor oil, xylene, IPA, salt spray conditioning and water at 60 °C.

"The adhesive strength is tested to ISO 4587, which is an overlap shear test where the substrates are bonded together with a pre-determined overlap, then allowed to cure," explains Crozier. "After curing, the adhesive bonds are pulled in the shear mode at a constant speed, and the maximum force to failure is recorded. By achieving substrate failure on many components, the adhesive was proved to be stronger than most of the varying test materials themselves, thus delivering genuine structural bond strength on LSE substrates.”

Within the range now, Advanced Adhesives offers other adhesives of the same chemistry of the PP3000 to compliment this original material, developed for specific applications, but all based on the original proven chemistry of the PP3000. Advanced Adhesives offers a low viscosity grade PP3000LV for applications requiring a thin bond line and an ultra-high viscosity version for bonding larger surface areas. PP3000UHV is another variant specifically designed for applications requiring application of the adhesive to vertical surfaces or to fill a larger gap. Black variants are also offered by Advanced Adhesives for applications requiring a black bond line for aesthetic purposes.

Gunning for the Right Answer

Advanced Adhesives was approached by a customer in the plastic molding industry to provide an adhesive solution to permanently bond a low surface energy material. Here, the customer wanted to bond the polypropylene body of a spray gun it produces to a polypropylene spout assembly - and a nickel-coated nut into the polypropylene body.

"We needed a solution to permanently bond our universal spray gun assemblies together," says the customer. "We have an issue with permanently bonding PP and resisting the 15 bar pressure, along with resisting the pressurized water/air within, with no deterioration of the adhesive. The adhesive has to operate in very cold climates, sometimes -300c, with the water freezing, and we need the joint not to fail. Assembly time is important, as other adhesives trialed, i.e., cyanoacrylates, were too instant, but also did not resist water and the cold temperatures very well - and so ended up failing."

Crozier went on to say that, "On receipt of the component, assessment of requirements and the component parts themselves, we established that one adhesive shone out from our range of products that would bond the components, but also cope with the arduous low temperature application the adhesive had to survive in. That adhesive was PP3000, specifically developed for bonding PP, along with other dissimilar substrates. We knew it would cope with bonding permanently the difficult-to-bond PP, together with the nickel coated nut, but also cope with the environmental factors and not be broken down by the pressurized water within the components."

During tests carried out in-house and by the customer, the PP3000 showed incredible performance and was duly adopted. Simple application was performed using hand-held bi-mixer guns to dispense the pre-mixed adhesive direct to the components. The 4-6 minute open time was sufficient for the operators to apply and assemble the components before the adhesive cured.

Benefits to customers:

  • Manufacturing process efficiency
  • Long term dependability
  • Cost effectiveness
  • Reliability of adhesive system employed in its working environment

Advanced Adhesives Ltd

This information has been sourced, reviewed and adapted from materials provided by Advanced Adhesives Ltd.

For more information on this source, please visit Advanced Adhesives Ltd.

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