Hot melt adhesive

from Wikipedia, the free encyclopedia
Replacement cartridges for hot glue guns

Hot melt adhesives , including hot adhesives , hot melt adhesives , hot melt or (Switzerland) hotmelt mentioned are solvent or water-free and at room temperature more or less solid products present and in the hot state as a viscous liquid to be applied to the adhesive surface. When they cool down, they solidify reversibly and create a firm bond. This group of adhesives are thermoplastic polymers based on various chemical raw materials.

history

Probably the first systematically produced adhesive in human history was a hot-melt adhesive: Neanderthals as well as modern humans ( Homo sapiens of the Cro-Magnon era ) used birch pitch to bond the stone and wood of their weapons and tools together at least 45,000 years ago . The oldest evidence of the use of birch pitch comes from Campitello in Italy and is about 200,000 years old. Birch pitch was obtained from birch bark by dry distillation.

The Stone Age man found in 1991 as a glacier mummy - called Ötzi - who lived around 3340 BC. Chr. On the Similaun died, the tips of its fixed arrows from flint on the shafts of the wood of the wooly snowball means vegetable fibers and birch tar.

Classification

The hot melt adhesives can be divided into physically and chemically setting adhesives. The physically setting adhesives are thermoplastics , the chemically setting ones form thermosetting plastics , ie three-dimensional networks. Another type of classification is with regard to the solidification mechanism. The hotmelt adhesives are divided into amorphous polymers, partially crystalline polymers, and high molecular weight polymers, with the first two having a glass transition temperature above room temperature and the latter having a glass transition temperature below room temperature.

The most important polymers used for physically setting hot melt adhesives are polyamide resins, saturated polyesters, ethylene-vinyl acetate copolymers (EVA), polyolefins, block copolymers (styrene-butadiene-styrene or styrene-isoprene-styrene) and polyimides. Polyamides, polyesters and polyimides are used in so-called high-performance hot-melt adhesives, ethylene-vinyl acetate copolymers and polyolefins in so-called mass-produced hot melt adhesives.

ingredients

The actual properties of the adhesive layer in terms of adhesion, cohesion and temperature behavior are determined by the base polymers, with various additives being required to ensure functionality or to achieve further special properties. Resins serve to increase the tack of the melt at the processing temperature. Long-chain (dibutyl or nonyl) phthalic acid esters serve as plasticizers for insufficiently flexible polymers. Aromatic amines or phenols serve as stabilizers or antioxidants during the processing of the melt under the influence of oxygen. Chalk , barite or titanium dioxide serve to increase strength and as an extender to reduce costs and waxes serve as viscosity regulators.

Base polymers

designation Processing temperature Remarks
PA Polyamides > 200 ° C
PE Polyethylene 140 to 200 ° C
APAO amorphous polyalphaolefins ~ 170 ° C
EVAC Ethylene - copolymers ~ 150 ° C
TPE -E Polyester - elastomers
TPE-U Polyurethane elastomers
TPE-A Copolyamide elastomers
Vinyl pyrrolidone / vinyl acetate copolymers ~ 130 ° C water soluble

resin

Stabilizers

  1. primary ("radical scavengers", e.g. phenols)
  2. secondary (peroxide decomposer)

wax

  • natural ( beeswax )
  • synthetic (fully, partially synthetic, chemically modified)

Nucleating agents

Nucleating agents can be added to modify partially crystalline plastics. They ensure that crystal formation occurs at a higher temperature. With polypropylene , for example, cycle times can be reduced by up to 30% and the crystal structure can be optimized. A side effect is greater shrinkage .

properties

Naturally, a connection through a hot melt adhesive without post-crosslinking can only be heat-resistant up to a maximum of its solidification temperature. In most cases, the heat resistance of such a connection is still below its softening temperature range . The processing temperatures depend on the type of base material and are in the range between 120 and 240 ° C. High-melting hot melt adhesives based on polyimides have a processing temperature of over 260 ° C. Typical viscosities at the processing temperatures of hot melt adhesives are between 20 Pas (polyamide) and 10,000 Pas (ethylene-vinyl acetate copolymers).

The strength and long-term stability of hotmelt adhesives can be increased if these are given the property of continuing to crosslink after the melting temperature has been reached through additives or a special structure of the polymers. This is achieved both with epoxy resin systems and with the polyurethane hot melt adhesives with post-crosslinking, which are the most widely used today, in which the subsequent chemical reaction is initiated by external moisture. These post-crosslinking hot melt adhesives can be applied at relatively low temperatures (approx. 60–80 ° C) and withstand temperatures of up to 120–150 ° C when cured.

application

The materials are selected depending on the area of ​​application with regard to the adhesion properties on the substrates, the processing temperature, the heat resistance, the chemical resistance and the hardness.

delivery

Hot melt adhesives are offered as granules , as powder, as foil, as hobbock , as fleece, as barrels or as sticks ("candles"). Pressure-sensitive hot-melt adhesives, as they are u. a. found on envelopes are delivered in block form. Here the adhesive is surrounded by a film for better handling until processing, which mixes with the adhesive during melting. This film has almost no influence on the processing or the adhesive properties.

Polyester and polyester amide hot melt adhesives are partly made from renewable raw materials and are in principle compostable.

processing

The processing temperatures are mostly in the range between 120 and 240 ° C., the viscosity necessary for optimal wetting being present but no thermal or oxidative damage to the melt yet occurring.

Different supply systems (e.g. extruder , tank, barrel melting systems) are used depending on the form of delivery or packaging of the adhesive .

In industry, the application is carried out by temperature-controlled hot glue devices with heated hoses and application heads with nozzles.

For occasional use and DIY sector will hot glue guns available. These are simple hand-held devices with temperature control using PTC thermistors , which process hot-melt glue cartridges in stick form (also called "candles" or "sticks").

Particularly when bonding metals, it is important to ensure that, due to the good thermal conductivity of the parts to be joined, the hotmelt adhesive does not solidify too quickly in the boundary layer area, thus preventing complete wetting of the surfaces.

Advantages and disadvantages

advantages

  • Solvent free
  • Almost unlimited shelf life
  • Low material price
  • Fast processing (only cooling the bond instead of slow hardening in the case of solvent-based adhesive systems, or the chemical reaction of two-component adhesives)
  • Adhesive residues can be liquefied by heating and removed from most surfaces without leaving any residue (for cleaning tools or for separating old bonds).
  • Bonding of different materials possible, particularly suitable for slightly porous or fibrous material surfaces
  • Can level out unevenness in the bonded surfaces
  • Elasticity of the glue line
  • No dosing and mixing processes necessary before processing

disadvantage

  • Special equipment is required for processing, these typically need a power connection.
  • Hazard potential when handling the hot device and the hot mass
  • Mechanically not very dimensionally stable, they tend to creep even with relatively low static loads and higher ambient temperatures
  • relatively low heat resistance
  • Chemically not very resistant
  • Adhesive gap cannot be chosen as small as desired → higher adhesive thickness than with many other adhesives, whereby foils made of hot melt adhesives can have a thickness of 10–200 µm.
  • high viscosity and heat load on the parts to be joined

Typical areas of application

  • Packaging industry, e.g. B. Gluing cardboard boxes, envelopes, bags
  • Clothing, e.g. B. Gluing shoulder pads in jackets (easy to clean)
  • Oil filters in vehicles, e.g. B. Gluing the paper roll in the housing
  • Electrical engineering, e.g. B. partial or complete casting of assemblies ("housing replacement") or components for mechanical stabilization, isolation or, especially in the case of inductances, to reduce mechanical vibrations of the copper coil and the corresponding noise development.
  • Cable bushings, sealing sleeves, e.g. B. for lines in motor vehicles
  • Furniture and wood industry, laminating technology, covers and upholstery for seats in vehicles
  • Shoe industry
  • Diapers, e.g. B. Gluing the absorbent nonwovens into the envelope
  • Carpet industry, glue carpet backing to the wear layer
  • DIY enthusiasts ( hot melt glue guns with glue candles)
  • Bookbinding (attaching the cover to the book, as an alternative to dispersion adhesives) or joining all pages together (spine gluing)
  • Sanitary industry (bonding ESG glass panes in aluminum shower door profiles)

Since the beginning of the 2000s, hot melt adhesives have been used in the paper and packaging sectors, which can be processed at temperatures of around 100 ° C and thus reduce thermal stress. Further special applications are UV-reactive hotmelt adhesive systems made from UV-curing polyacrylates for the production of labels and adhesive tapes, in which very different adhesion and cohesion properties can be achieved by regulating the intensity of the UV radiation. Water-soluble hot melt adhesives have been developed for labels for reusable plastic bottles.

Trivia

Also, sealing wax , rosin , amber and shellac are expected to melt adhesives. Hot melt adhesives (mostly ethylene-vinyl acetate copolymers) can also be used as heat-seal adhesives.

See also

Individual evidence

  1. ↑ a b c d e f Bodo Müller, Walter Rath: Formulation of adhesives and sealants the competent textbook for study and practice . Vincentz Network GmbH & Co KG, 2004, ISBN 978-3-87870-791-2 , p. 83 ( limited preview in Google Book search).
  2. ↑ a b Jürgen Klingen: Joining Technology Gluing: Instructions for the modern and safe gluing process in industry and craft . John Wiley & Sons, 2019, ISBN 978-3-527-81606-4 , pp. 1 ( limited preview in Google Book search).
  3. ↑ a b c d e f g h i j k l m n o p q Walter Brockmann, Paul Ludwig Geiß, Jürgen Klingen, K. Bernhard Schröder: Adhesive Bonding Technology, Adhesives, Applications and Processes . John Wiley & Sons, 2012, ISBN 978-3-527-66050-6 , pp. 35, 50 ( limited preview in Google Book search).
  4. ↑ Document DE102007027801A1: Reactive Polyurethane Hotmelts - Document DE102007027801A1 , accessed on November 25, 2019
  5. ↑ a b c d Gerd habenicht: Gluing basics, technologies, applications . Springer Science & Business Media, 2008, ISBN 978-3-540-85264-3 , p. 73,147 ( limited preview in Google Book Search).
  6. ↑ Manfred Rasche: Manual Bonding Technology . Carl Hanser Verlag GmbH Co KG, 2012, ISBN 978-3-446-43198-0 , p. 392 ( limited preview in Google Book search).