Hot tack

An example of such data is plotted in Fig. 6.10, along with the heat seal data for the same polymer. The hot tack strength is very low compared to the heat seal strength, since the full seal strength develops only after the material is completely solidified and cooled to ambient conditions. 

On most modern horizontal and vertical form-fill-seal equipment (see Section 9.3), seal jaws are not flat bars because these give weaker seal strengths than jaws designed with serrations. If one looks at flexible packages in the marketplace, one will find vertical and horizontal serrations as shown in Fig. 6.11. 

Vertical serration gives the strongest heat seals, but can also suffer from small channel leaks caused by folds that occur in the material in the heat seal area. These are often referred to as serum leakers. If horizontal serrations are used, these cut off the serum leakers, but because the serrations run parallel to the peeling forces, the seals are weaker. 

The serrations can be cut at various included angles, and have various numbers of teeth per inch, and are usually truncated as shown in Fig. 6.11. This approach minimizes the cutting of the structure during heat sealing. There are many different serrated seal jaw designs available from equipment manufacturers and film suppliers. A thorough discussion is beyond the scope of this book. 

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Hot tack strength is the abiUty of a heat-seal layer to hold together while molten, before the seal cools and sets up. This is a technically important property which is difficult to measure reproducibly in the laboratory. Owing to the reinforcing effect of ionic bonding on melt strength, ionomer sealing layers provide superior performance in a wide spectmm of appHcations. 

Metallocene-catalysed very low density polyethylene (m-VLDPE) has become available with densities of as low as 0.903. This is of use for sealing layers of multi-layer films since sealing can commence at lower temperatures than with conventional materials such as LLDPE and EVA (see Section 11.6) with the polymer seal exhibiting both cold strength and hot tack strength. 

Polymer High MW (>10,000) Tg usually < RT Physically crosslinks on cooling Strong Strength Hot tack Viscous loss —> peel force... 

In most adhesives, tackifier is the ingredient present in the highest proportion. Tackifying resins are primarily used to reduce adhesive viscosity and adjust the 7g of the adhesive s amorphous matrix phase. Through their effects on the other ingredients and the overall system they can also dramatically affect wet out, hot tack, open time, set speed, and heat resistance. 

Tackifiers. The tackifiers usually are hydrocarbon resins (aliphatic C5, aromatic C9) or natural resins (polyterpenes, rosin and rosin derivates, tall oil rosin ester). They improve hot tack, wetting characteristics and open time and enhance adhesion. The content on tackifiers in a hot melt can be in the region of 10-25%. 

LLDPE with narrow molecular weight distribution exhibits a lower, a sharper melting point [3], better hot tack and heat seal properties as well as higher clarity and better impact resistance (Fig. 3), tensile strength (Fig. 4) [11], and lower levels of alkane-soluble components. The most distinguishing characteristic of metallocene-based LLDPEs is that they are not restricted by the current immutable property relationships that are... 

The cylindrical-type sealing process for strip packaging does not usually have a cooling cycle or quench surface, hence any pull on the seal ply will tend to weaken the seal while the sealant is still pliable. Therefore, it is desirable that the sealant have good hot tack or a high melt viscosity at processing temperatures. Hot tack is generally accepted to mean the adhesion quality of the sealant from the moment it leaves the seal head and the time it returns to its set or permanent condition. 

D 3706 Test Method for Hot Tack of Wax Polymer Blends by the Flat Spring Test... 

More recently, somewhat incompatible systems with a-methylstyrene copolymers, ethylene-vinylacetate resins, and paraffin wax (13) have shown exceptional hot tack although the blend is clearly incompatible. Table III shows that the adhesion increases with the cloud point of the blend. 

Ionomer (Surlyn) is a methacrylic acid and ethylene modified molecule with a metal ion (sodium, zinc, magnesium). Easy sealing, soft, strong, grease-resistant clear film, and seals well in contact with contaminants. Puncture-resistant, with a high hot tack. Approximately double the price of LDPE, but can be used in thinner gauges. Can be used in the inner ply of laminates, at approx, gauge of LDPE and as a skin pack over sharp or pointed objects. 

Caliper, coating weight, softening, melting point, sealing range, viscosity, hot tack, ability to seal in the presence of contamination. 

Hot tack varies between materials, increasing as follows ... 

Cooling therefore is more critical to the materials showing low hot tack and may be achieved by air (cooled), water or chilled water. 

A certain percentage of resin is almost always incorporated into formulations, with resin content varying from 8 to 25%. Hydrocarbon resins are used most often, but rosin esters, terpenes, and indene resins, which are more heat stable, are also common. Resins provide better flow, hot-tack, adhesion, and wetting characteristics. 

Surlyn 1702. Extrusion coating excellent low-temperature sealability and hot-tack. 

Hot-Tack The highest hot-tack strength and broadest hot-tack range of any sealant, yields higher packaging line speeds, and reduced seal failures and leakers. 

Figure 9-02. Hot-tack DuPont Surlyn 1601 Packforsk method.P ...

Puncture Toughness/ Strength Tear Optics Oigano- leptics Oxygen Barrier Hot Tack Sealing Stiffness... 

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