Traditional practices of manufacturing are being thrown out in favour of newer ones due to major changes in the industry and a general consciousness of the impact vehicles have on factors like climate change. Here’s a look at how adhesives can help.
The automotive industry is prone to plenty of shifts and changes owing to various factors. This impacts the methods and processes involved in manufacturing vehicles. Traditionally used materials are either no longer used anymore or their use has been limited due to a rise in a stronger, more durable material. One such material is adhesives.
Consisting of four major types, adhesives are fast gaining a stronger hold in the automotive industry due to consumer- or environment-driven trends. Not only that, OEMs are now witnessing the versatile use of the material and its applications. Adhesives are no longer simply used for interior applications like gluing on mirrors; they are also used for body-in-white (BIW) applications in body shops and even under the bonnet, where they are exposed to harsh and volatile environments.
Globally, the advantages of adhesives and even sealants is widely felt. Chemical or adhesive companies are gradually rolling out various adhesive substitutes to replace metal parts or even enable the usage of materials other than metal. Since metal-to-metal joining methods are not suited to join dissimilar materials to each other, adhesives play an important role in such connections as well.
Commonly used adhesives
The US-based Adhesives and Sealants Council (ASC)- where companies like Henkel, 3M, Covestro, Sika, and Dow are members-actively promotes the usage of adhesives, especially in the automotive industry. The ASC has a report and a comprehensive guide meant for OEMs, which details the usage and the difference between each type of adhesives meant for the global automotive industry.
Based on strength, curing time, usage, and elongation properties, according to the ASC, adhesives and sealants should be classified as five major types: Epoxies, polyurethane (PUR), ethylene vinyl acetate (EVA), acrylic (Methacrylate) and rubber-based. The same materials can be used as one component, wherein they are directly applied or as two-component after metering and mixing.
In its OEM guide for adhesives and sealants, ASC details the differences in the compounds in terms of strength and flexibility (elongation). Epoxies are the strongest and least flexible; PUR is not quite as strong as epoxies but offers a greater range of flexibility; acrylic adhesives are a middle-ground, offering both high strength and high flexibility. Rubber-based and EVA products are, however, most commonly used as sealants offering the most flexibility but the least strength.
Adhesive and sealant families are available in one-component, two component or solid preforms. One-component liquid adhesives are manufactured with the adhesive and catalyst in a single component. They are sold in a ready-to-use form and can be directly taken out of their storage tanks and used. With two-component adhesives, the adhesive and catalyst come in separate components that then require metering and mixing. Solid preform adhesives allow the operator greater control over liquid adhesives. These materials are placed in the desired position and they then expand upon heat-curing. EVAs and rubber-based products are available in solid preform.
Usage of Adhesives
Adhesives that are used in OEM body shops need to contend with various factors to retain their functionality. They are also used in various applications such as structural bonding, for anti-flutter purposes, body sealing and expandable reinforcement.
As far as structural bonding is concerned, OEMs’ main focus is safety and durability. The need for adhesives and sealants differs when it comes to light vehicles against heavier vehicles owing to the construction of the two. Light vehicles like passenger vehicles don’t have separate frame-on-frame architecture like the heavier trucks and other MHCVs. They are normally constructed as unibodies which are to then contain the passengers. This is the area which will contain the most weight and will have to be stiffer, safer, more durable, and will also have to bear most amount of impact in the event of a crash.
Among the India-specific solutions is the use of structural adhesives as bonding solutions, which not only optimise but also provide an opportunity to eliminate spot welds and improve stiffness, fatigue and crash performance of vehicles. Structural inserts as 3D parts help improve stiffness and crash of box sections like A, B and C pillars or sections by optimising steel design with weight reduction opportunities.
Primarily, car unibodies have been constructed using steel which are joined by spot welding. In this joining application, adhesives like epoxies have been used in conjunction with the welds. Structural bonding using adhesives has also proven to increase stiffness and to increase the durability of the spot welds. As welding is a time-consuming and costly process, it is better to keep fewer welds, which means that the welds need to be further apart. However, a larger space between welds reduces the stiffness of the unibody. Here, adhesives come in to play, by allowing a larger space to be kept between welds without increasing the amount of stress on each weld.
Structural adhesives can join dissimilar materials, while distributing stress more evenly across the joints compared to fasteners and spot welding, enabling designers to specify thinner, lighter materials while meeting the strength requirements. Materials such as a higher grade of steel, the more expensive aluminium, or even composites are lighter than traditional steel, which contributes to the current industry trend of light-weighting. In this manner, OEMs can ensure vehicle safety along with making vehicles fuel-efficient and light-in-weight. Additionally, liquid sealants make it possible to join dissimilar materials like metal and plastic, while reducing material costs four to five times compared to hard gaskets.
Using different materials can result in certain complications such as galvanic corrosion. In the case of galvanic corrosion, adhesives act as a barrier between dissimilar materials with different electrochemical potential to prevent such corrosion from taking place. Henkel Adhesive’s Bonderite M-NT solutions are next-generation metal pre-treatment for painted metal surfaces that enable the increased use of light metals while maintaining excellent corrosion protection, reducing/eliminating the heavy metals and sludge generation. They also save time, lower costs and reduce energy requirements while cutting down on waste as compared to the traditional phosphate coatings. Developed in India, Bonderite M-NZ is a sustainable low temperature phosphate that provides excellent corrosion protection for steel surfaces, galvanised steel, electrogalvanised coatings of zinc and aluminum, complying with the strict specifications of the automotive industry.
Adhesives are also used for processes like hem flange bonding in places like doors, hoods and decklids in similar manner to structural bonding.
Factors propelling the use of adhesives
In recent years, the growing emphasis by governments and international commitments towards climate change policies such as the Paris Climate Agreement, has made OEMs conscious about the emission rate of their vehicles. While there are various technologies that can reduce the amount of fuel consumed, nothing beats the concept of lightweighting. With lightweighting, OEMs can achieve the desired fuel efficiency goals which resonates well with consumers. At the same time, it can ensure that their vehicles keep to standards that the government wants them to achieve, like BS VI. This is possible with the usage of adhesives as lighter grade steel, lighter material substitutes and aluminium can be joined or bonded with steel.
“Pollution is rampant these days and road traffic accounts for a major part of it. That’s why improving the environmental footprint of cars is an important goal – all the more so because the limits set are going to become stricter in 2020. The aim is to reduce the carbon emissions and make cars run cleaner and greener. To achieve this car manufacturers are turning to lightweight construction to meet the new emissions targets, because lighter cars consume less energy. For example, a medium-sized car weighs 1.4 tonnes. The use of modern adhesives and sealants can already reduce that weight by at least 15 per cent and thereby contribute to lowering the car’s carbon emissions – and it can do so without affecting quality in the slightest. In addition, the multi-material structures and fiber-reinforced composites that are essential to lightweight construction can only be joined together securely with glue,” says Barun Bharadwaj, Business Director – Transport and Metal, Henkel Adhesives Technologies India Pvt Ltd.
The other benefits of adhesives are gasketing and sealing off a portion to prevent corrosion and liquid escaping. It also prevents the accumulation of dirt and dust in sensitive parts of the car. This is where sealants like rubber and EVA are used.
The current trend of lightweighting which is experienced by the industry has propelled adhesives to become one of the major components in the manufacturing of vehicles. This is largely seen as the structure of automobiles has changed over the years. Faced with tighter regulatory demands and growing consumer awareness of better fuel efficiency and reduced emissions, automotive manufacturers and suppliers are increasingly moving from traditional steel sheet or galvanized alloys to mixed and very lightweight metals that offer significant savings. Metal parts are being replaced with composite like substances for the purpose of light-weighting itself. For the most part, lightweighting has played a key role in the advent of adhesives as the new bonding material. However, there are other factors- even though they seemingly came in later.
Making cars quieter is something that adhesives and composites can achieve together. The sound of metal against metal (metal parts are welded down or are fixed with nuts and bolts) is now a non-existent thing in most cars. Additionally, the company offers high dampening foam (HDF) which improves door slamming and roof drumming noise with weight reduction opportunities. Liquid applied sound deadeners (LASD), which are sprayable acoustic solutions, can replace traditional bitumen-based sheets, thus reducing process complexity as well as leading to weight reduction. For example, Henkel’s Teroson Liquid Applied Sound Deadeners, significantly – and safely – reduce the solid-borne noise and vibrations generated by the engine and the wheels during driving.
LASD is really changing the way car manufacturers approach acoustic solutions. Following the megatrend of light-weighting, Henkel’s sustainable LASD solution based on renewable material proofs up to 40 per cent weight reduction compared to traditional pads without compromising the acoustic performance. This directly leads to improved fuel economy and a more comfortable driving experience because the LASD technology limits emissions to levels far lower than the current safety standards. To sum up by eliminating the need for traditional bitumen pads, LASDs also clear the way for weight saving and increased performance without the costly disadvantages.
Another thing influencing the push towards adhesives is the growing disruption of the automotive industry in general. The move towards EVs is fast gaining traction.
“Going forward Electric vehicles are the way to go and the government has been focused on electric vehicles with a focus on climate change and lowering pollution. We believe that there are a whole range of solutions for automotive electric motors and batteries. We, at Henkel, have ensured that with our Loctite, Bondrite and Technomelt range of solutions we offer the electric vehicle manufacturers the complete range of solutions for sealing and gasketing, potting and encapsulation, bonding, low-pressure moulding, as well as adhesive and sealant solutions meant for lithium-ion batteries,” adds Bharadwaj.
In the same vein, Bostik India Private Ltd, a subsidiary of Arkema Chemicals, claims that the company is all for eco-friendly and user friendly solutions. The company’s solvent-based adhesive solutions range consists of synthetic rubber and PUR adhesives. As the company caters mainly to larger and heavier vehicles, these sprayable, heat resistant products can be used for bus interiors and vinyl flooring.
Bostik’s most impressive product range remains its hot-melt adhesive range that are broadly divided into three types based on technology and application: Pressure-sensitive adhesives for one sided application like interiors; polyester or polyamide adhesives with heat resistant qualities for headliner attachments; and web adhesives that are powerful enough to bond various substrates. These find their use in interior laminations and seats.
Issues with using adhesives
While adhesives are in fact a boon to the auto industry and OEMs, they still need to keep in mind several issues that can arise through improper usage of adhesives about their use. For one adhesives have conventionally been used only in interiors. But now their usage has expanded to cover even the parts of vehicles that are exposed to more wear and tear like the carriage and chassis.
Adhesive need to be high viscosity to allow other liquids to wash over them or else the wash out can cause imperfections in painting, which is done after bonding. The heat from the paint curing methods is used to also cure the adhesive components at times. They must be easy to dispense and adhere to oily metals, since the body panels have mill oils and stamping fluids on them, which will not be cleaned until the paint pretreatment process.
Varying temperatures is something that adhesives have to be strong enough to stand. The dynamic temperatures in the engine over time is enough to ruin any chemical substance and cause it to lose its adhesive properties. One of the things to keep in mind during the process of bonding is failure, which can be caused due to dissimilar substrates having a different heat expansion rate. Once adhesives are applied metal parts expand and cool quicker than non-metal or composite parts and even the adhesive or sealant. This can cause the adhesive itself to fail, or the substrate to break off. The substrate can also deform upon cooling.
The ASC guide states that OEMs test the heat resistance of adhesives in between the range of 140 C to 205 C. Adhesives must also be able to tolerate the treatments and services such as e-coat or electrophoretically applied paint which can be rigorous owing to degreasing, rinsing and immersing the BIW in liquids.
How OEMs benefit from using adhesives
Adhesives help manufacturers to build structures that are stronger stiffer, lighter and better performing they make products quieter and more comfortable. They solve difficult joining problems by replacing more time consuming methods like brazing soldering welding and screwing.
Body and paint shop processes have become more efficient with cleaning, forming, cutting, joining, bonding, glazing, vibration damping, seam sealing, automotive sound deadening, pre-paint surface treatments and corrosion protection.
“We offer solutions for cleaning, bonding, gasketing, lubricating, coating or porosity sealing, acoustics challenge, machining fluids for various powertrain applications such as - engine, transmission and driveshaft, and the like. Our range for chassis manufacturing and maintenance has lubricants and cleaners for all metal-forming processes such as stamping and casting. Metal pre-treatments and coatings protect wheels and frames from corrosion. Strong bonding comes with friction bonding for brake pads and rubber-to-metal bonding for suspension components,” says Bharadwaj, while explaining how adhesive solutions can resolve plenty of manufacturing hassles faced by OEMs.
While adhesives themselves are a replacement trend, as technology progresses, certain adhesive and sealant families act as a replacement for some others. According to Bostik India officials, web adhesives, that cover a large base of the substrate are ideal for lamination, in place of hot-melts, as they would prevent the burning of PU cloth that is used in the interior of cars. As far as sealants are concerned, hot-melt adhesives are replacing butyl sealants when it comes to door-sealing like water-shielding and weather strips. This results in reduced weight in vehicle doors. Following this trend, hot-melt adhesives are replacing solvent-based adhesives as they are the best for one sided applications that do not require a pretreatment of the substrate. Unlike solvent-based adhesives they are VOC compliant as well.
Keeping a track of these changes in solutions and technologies when it comes to manufacturing, will help OEMs implement better more cost-effective technologies that are in keeping with policy regulations regarding the environment.
Car manufacturers are turning to lightweight construction to meet the new emissions targets, because lighter cars consume less energy.”
- Barun Bharadwaj,
Business Director – Transport and Metal,
Henkel Adhesives Technologies India Pvt Ltd