Instant adhesives: Latest advances
30 November 2016
Most people are aware of superglue, or cyanoacrylate to give its technical name, but how many know that a whole host of industrial applications can benefit from these advanced adhesives? Bob Goss, senior technology specialist with Henkel, considers the question
Cyanoacrylates are versatile and easy to use, can perform multi-substrate bonding and deliver fixture times of a few seconds in most applications. In addition, they provide fast assembly solutions for a variety of materials including plastics, elastomers and metals, and cure at room temperature to achieve very high bond strengths – in most cases the substrate will fail before the adhesive joint breaks.
So, how do they work? Well, it should first be noted that the product is kept liquid inside the bottle by an acidic stabiliser. However, when dispensed, the moisture on the surface of the substrate neutralises the stabiliser and cure is initiated. The best results are obtained in atmospheres with about 40-60% humidity. Full cure will be obtained with less humidity, but a little more time will be required.
Cure in seconds
With a typical bond line thickness of just 0.1mm, cyanoacrylates offer initial cure in seconds. In fact, the joint will achieve about 50% of its total strength within 60 seconds, and 100% in 12-24h. Joints set into a rigid thermoplastic with an operating temperature range from -50 to +80-90°C, although some offer up to 120°C.
Most engineering plastics can be joined, including ABS, LCP (liquid crystal polymer), PBT, polycarbonate, PMMA (acrylic), nylon (polyamide) and polysulfone, as well as nitrile, butyl and neoprene rubbers. Cyanoacrylates are also suitable for wood, leather, fabrics, steel and aluminium.
For plastics and rubbers where the surface energy is below 33mN/m, such as PEEK, POM (acetal/Delrin), polyethylene, polypropylene, silicone rubber, Viton and PTFE (Teflon), a bond can still be achieved by ensuring that the surface is properly wetted. Cyanoacrylates are not suitable for bonding glass.
Despite the resounding attributes of cyanoacrylates, technology developments in this area continue apace. For instance, recent or current development programmes at Henkel include those targeting acidic/porous bonding, toughened grades and flexible grades such as the recently introduced LOCTITE 4092 for medical industry applications.
Further developments include low odour products and cyanoacrylates with gap filling properties. LOCTITE 3090, for example, is a two-part product that offers the capability to fill gaps up to 5mm.
To focus on a particular development area – toughened instant adhesives – these feature rubber particles in an adhesive matrix that act as crack arrestors. This is because, although normal cyanoacrylates can support high tensile loads, under a peel load, joints can break relatively easily due to crack propagation. As a result, for applications where peel strength is required, toughened adhesives such as LOCTITE 480 are required. Although the cure time is a little slower (about 60 seconds), such products offer particularly good adhesion between metals, or between metal and rubber.
Epoxies represent a viable alternative to instant adhesives. These typically comprise a resin and a hardener which are mixed together (manually or automatically) to achieve a cured product. Cure times range from 5-10 minutes to 4h. However, as a rule of thumb, if an epoxy is required with good durability, adhesion and toughness, then a slow-curing epoxy is often the best choice.
The properties of epoxies vary enormously in terms of flexibility/toughness, temperature resistance, viscosity, colour, cure speed and conductivity (electrical and thermal). Typically, epoxies offer temperature resistance to 120°C, although products up to 180°C are available, while in terms of viscosity, these adhesives range from about 5-1000 Pa.s. Epoxies are traditionally strong performers on metals and ceramics, but less good on elastomers and some plastics.
In a direct comparison between cyanoacrylates and epoxies, the former are better for small part bonding in plastics or rubber where an instant cure is required, while the latter are targeted at larger components made from composites or metals where good gap filling properties are needed.
The benefits of both cyanoacrylates and epoxies are combined in LOCTITE 4090. This hybrid instant structural adhesive comprises part-epoxy for high bond strength and impact resistance, and part-cyanoacrylate for speed and substrate versatility.