Tack strength

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. 

In the literature, there are several reports that examine the role of conventional fillers like carbon black on the autohesive tack (uncured adhesion between a similar pair of elastomers) [225]. It has been shown that the incorporation of carbon black at very high concentration (>30 phr) can increase the autohesive tack of natural and butyl rubber [225]. Very recently, for the first time, Kumar et al. [164] reported the effect of NA nanoclay (at relatively very low concentration) on the autohesive tack of BIMS rubber by a 180° peel test. XRD and AFM show intercalated morphology of nanoclay in the BIMS rubber matrix. However, the autohesive tack strength dramatically increases with nanoclay concentration up to 8 phr, beyond which it apparently reaches a plateau at 16 phr of nanoclay concentration (see Fig. 36). For example, the tack strength of 16 phr of nanoclay-loaded sample is nearly 158% higher than the tack strength of neat BIMS rubber. The force versus, distance curves from the peel tests for selected samples are shown in Fig. 37. 

Fig. 36 Effect of nanoclay content on tack strength samples containing 0, 2, 4,...

Table 4.1. Tack strengths of uncompounded rubber stocks 173)...

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. 

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. 

The high hot-tack strength of Dowlex allows it to perform in form, fill, and seal sacks.t ... 

Higher hot-tack strength means faster automated packaging line speeds — 16% faster for the octene versus hexene resins in this case. 

Film Gauge 2.8 mil LLDPE-rich commercial film Figure 25-02. Hot-tack strength of octene vs hexene LLDPE resins. ... 

General Description Dow Chemicals Affinity resins, homogenous ethylene alpha-olefin copolymers, contain up to 20% octene comonomer. Affinity polyolefin plastomers (POPs) can be used in any of the extrusion technologies as a sealant, to deliver an unparalleled combination of low seal-initiation temperature, hot-tack strength, toughness, tear resistance, and transparency.f ... 

ExxonMobil Bicor films are coated, sealable OPP films with outstanding optical properties, superior machinability, excellent flavor and aroma barrier properties, broad seal ranges, excellent heat-seal and hot-tack strength, and good moisture and oxygen barrier properties. 

Hot-Tack Strength The force required to separate a molten seal in heat-sealable thermoplastic films. It determines the rate at which the film can be sealed. Also called Ultimate Hot-Tack Strength. 

Figure 7. Hot-tack strength ofAFFINITY POP and EVA resins as a function of seal-bar temperature. Nylon/EAA/Sealant (I/I/1.5 mil) blown co-ex film was used. (Reproduced with permission from reference 25. Copyright 2002 Taylor and Francis Group, LLC.)...

FIGURE 4 Tack strengths imparted to the mbber compound by different tackifiers... 

Figure 4 matches the tack strength performance of palm kernel oil alkyd resins as a tackifier in rabber compound againsta reference (the same ENR compound without alkyd), polyterpene (a 3-pinene resin with softening point of around 30°C) and petroleum resin (C5 type aliphatic hydrocarbon resin withsoftening point around 100°C), two of the very common tackifiers in the tire industry. These... 

With reference to the alkyd eompositions in Table 2, phthalic anhydride was the only petroleum based ehemieaL while the other constituents could be eonsid-ered as natiual materials. As portion of the phthalic anhydride was replaced by fumaric acid, the natural eontent of the alkyd has increased, and the tack strength has also improved. Figure 5 is an empirical fit of the tack strength with the fumaric acid content in the alkyd. The tack strength achieved a maximum for AlkFA25 which contained around 5% by weight of fumaric acid in the alkyd. 

FIGURES Empirical fit of the tack strength with the fumaric acid content. 

The palm oil-based alkyds could be used as tackifiers to replace polyterpene and petroleum resin in ENR mbber compound. They could provide better tack strength, impart better road gripping and reduce rolling resistance in tread rabbet However, they should be apphed at low level to avoid reducing the abrasive resistance of the rabbet... 

Dependence of tack strength (kN m ) upon contact time (h) between nitrile rubber and natural rubber. The dots represent experimental measurements the line is derived from Eqn. 1... 

Hot-tack strength n. In heat sealing, the strength of the seal just at the end of the dwell when the die halves part (clearly more a concept than a measurable property). 

EVAs are used to extrusion coat many substrates including nylon, polyester, cellophane, and polypropylene films. EVAs are excellent heat sealers, have good hot tack strength, and adhere well to a variety of substrates. The tackiness of EVA copolymers can cause problems in extrusion coating but using dusting powders on the film and additives in the polymer can help the process run smoothly [ 19]. 

W. G. Forbes and L. A. McLeod, Dependence of tack strength on molecular properties, Trans. Inst. Rubber Ind. 34, 154 (1958). 

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