Fundamental Understanding in Rolling Ball Tack of Tackified Block Copolymer Adhesives

In the pressure sensitive adhesive (PSA) industry, rolling ball tack is a very common tack test, which is simple, inexpensive and easy to operate. This work attempts to search for key parameter(s), which will affect the rolling ball tack of a PSA based on a blend of styrene-isoprene-styrene triblock copolymer(SIS) and hydrocarbon tackifier(s). We want to better understand whether this particular PSA performance is controlled by the surface or bulk properties of the adhesive.

Firstly, to test the contribution from the surface properties, we employ a model system of SIS/aliphatic tackifier in 1/1 wt. ratio as the control. Part of the tackifier in this PSA is then replaced by various amounts of low molecular weight diluents with different surface tensions. The idea is to vary the surface properties of the PSA because these low surface tension and low molecular weight diluents tend to migrate to the PSA surface. It is observed that the incorporation of a lower surface tension and a lower molecular weight diluent in the PSA tends to produce a larger increase in rolling ball tack compared with the unmodified PSA. On the other hand, the incorporation of a higher surface tension and a more compatible diluent tends to produce a larger increase in loop, peel and quick stick. Each diluent lowers the shear adhesion failure temperature (SAFT) of the diluent-modified PSA. These observations are explained in terms of tackifier molecular weight, and surface tension and compatibility of the various components (polyisoprene, tackifier, diluent and oil) in the adhesive formulation.

Secondly, to test the contribution from the bulk properties, we derive an equation for rolling ball tack in terms of the bulk viscoelastic behavior of the block copolymer PSA. However, experimental values of rolling ball tack do not follow this equation. Also, with increasing tackifier concentration in SIS, rolling ball tack has very different behavior compared with loop, peel, quick stick and probe tack. The latter set of performance criteria is known to be related to PSA bulk viscoelastic behavior. Therefore, these suggest that rolling ball tack is related more to the surface properties than to the bulk properties of the adhesive based on these results and those of the diluent-modified PSA systems.     

Rolling Motion of a Ball on Pressure Sensitive Adhesives

Rolling motion of a ball on pressure sensitive adhesives was carefully observed under well-controlled conditions. Rolling distance was measured as a function of time by means of stroboscopic photography, and rollout distance was measured as a function of initial height of a ball. Both rolling distances and rollout distances were analysed according to a unified theory, where rolling friction coefficient (f) of a pressure sensitive adhesive is involved. It is suggested that f depends on viscoelastic properties of the adhesives.     

Rolling Motion of a Ball on Pressure Sensitive Adhesives

Rolling motion of a ball on pressure sensitive adhesives was carefully observed under well-controlled conditions. Rolling distance was measured as a function of time by means of stroboscopic photography, and rollout distance was measured as a function of initial height of a ball. Both rolling distances and rollout distances were analysed according to a unified theory, where rolling friction coefficient (f) of a pressure sensitive adhesive is involved. It is suggested that f depends on viscoelastic properties of the adhesives.     

Pressure Sensitive Adhesives Based on VectorR SIS Polymers I. Rheological Model and Adhesive Design Pathways

Dexco Polymers (a Dow/Exxon partnership) has been manufacturing Vector^R SIS polymers since 1990.¹ This paper describes experiments carried out to study Vector SIS polymers and model pressure sensitive adhesive (PSA) formulations based on Exxon Chemical's Escorez^R 1310LC tackifier. The adhesive behavior of tackified polymers was quantitatively analyzed by applying the rheological principle of time-temperature superposition² and the mapping approach,^3,4 and the pressure sensitive rheological model⁵ developed earlier. This model⁵ was developed by expanding and modifying an equation [adhesive fracture strength = (intrinsic adhesion) × (bulk energy dissipation)] proposed by Gent et al.^6,7 and Andrews et al.^8,9 This study delivers two key results. The first is that the fracture strength of the PSA/steel bond is the multiplication of three terms: the intrinsic (or interfacial) adhesion, the bonding and the debonding terms (Fig. 1). The debonding term is correlated with the logarithm of the loss modulus at the PSA debonding frequency or with the logarithm of the monomeric friction coefficient of the block copolymer/tackifier system. Both the loss modulus and the monomeric friction coefficient measure the energy dissipation in the bulk adhesive. The second is that PSA design pathways can be established by a mapping approach in the rheological space of the plateau modulus versus the loss modulus peak position in the frequency scale (Fig. 2). Plateau modulus is the bonding parameter because it measures the wetting capability of the adhesive with the substrate surface. The loss modulus peak position is the debonding parameter because it corresponds approximately to the time scale (or the frequency scale) in which one deforms the adhesive to maximize energy dissipation. Therefore, the tackifier and oil combination lowers the plateau modulus, but increases the T_g of the polyisoprene phase of the SIS polymer. This increase in T_g is equivalent to the lowering of the rate of local rearrangement (frequency of segment jumps) of the polyisoprene chains of the block copolymer. An optimal “tackification pathway” in this rheological space is achieved by tailoring the tackifier type and T_g, and the amount of oil used in the PSA.

In brief, the PSA rheological model and mapping approach described in this work for Vector SIS polymers give a comprehensive understanding and adhesive design pathways. This concept and approach not only allow raw material suppliers to improve and design better tackifier and polymer products, but also provide PSA formulators a quantitative tool to achieve PSA end property results.     

Bonding and Debonding Processes in Tack of Pressure-Sensitive Adhesives

Rolling friction coefficient of two acrylic pressure sensitive adhesives (PSAs) are measured as a function of velocity at several temperatures. It is found that the time (or velocity)-temperature superposition procedure is applicable for one PSA, while it is not for the other. The authors came to the conclusion that the rate of increase of the true contact area is different for the two adhesives. The time-temperature superposition is possible only in the case where the activation energy of the bonding process is equal to that of the debonding process.     

Terpolymers Composed of Ethyl Acrylate, Methoxypolyethyleneglycol Methacrylate and Methoxypolypropyleneglycol Methacrylate as Pressure Sensitive Adhesives and their Blood Compatibility

Terpolymers composed of ethyl acrylate (EA), methoxypolyethyleneglycol methacrylate (MPEGMA) and methoxypolypropyleneglycol methacrylate (MPPGMA) were synthesized by the photopolymerization technique. The high molecular mobilities for these terpolymers were shown by dynamic contact angle and adhesion tension measurement. The 180-degree peel strength and probe tack for pressure sensitive adhesives (PSA) made of these terpolymers were good but the holding power was not enough to apply them as PSAs. It was found that these terpolymers should be modified to obtain high holding power. The blood compatibility of these terpolymers was also investigated. It was found that they had a significant blood compatibility. Thrombi were not observed on the terpolymer surface after immersion in blood while, on a polystyrene (PSt) surface, many blood clusters were observed. After immersion in platelet-rich plasma (PRP), a few adhered platelets were observed on terpolymer surface but they did not deform and maintained their spherical form, while many platelets were observed on polystyrene.     

Terpolymers Composed of Ethyl Acrylate,Methoxypolyethyleneglycol Methacrylate and Methoxypolypropyleneglycol Methacrylate as Pressure Sensitive Adhesives and their Blood Compatibility

Terpolymers composed of ethyl acrylate (EA), methoxypolyethyleneglycol methacrylate (MPEGMA) and methoxypolypropyleneglycol methacrylate (MPPGMA) were synthesized by the photopolymerization technique. The high molecular mobilities for these terpolymers were shown by dynamic contact angle and adhesion tension measurement. The 180-degree peel strength and probe tack for pressure sensitive adhesives (PSA) made of these terpolymers were good but the holding power was not enough to apply them as PSAs. It was found that these terpolymers should be modified to obtain high holding power. The blood compatibility of these terpolymers was also investigated. It was found that they had a significant blood compatibility. Thrombi were not observed on the terpolymer surface after immersion in blood while, on a polystyrene (PSt) surface, many blood clusters were observed. After immersion in platelet-rich plasma (PRP), a few adhered platelets were observed on terpolymer surface but they did not deform and maintained their spherical form, while many platelets were observed on polystyrene.     

Hot Melt Adhesive Model: Interfacial Adhesion and Polymer/Tackifier/Wax Interactions

This work continues our study of the hot melt adhesive (HMA) model published earlier [1]. This HMA model was developed based on the pressure sensitive adhesive (PSA) tack model established previously [2]:

P = P₀BD (1)

where P is the adhesive bond strength, P₀ is the interfacial (intrinsic) adhesion term, B is the bonding term and D is the debonding term. The previous paper [1] describes the B and D terms in detail. However, only a brief discussion of the P₀ term was given. The present paper will provide a more in-depth but still rather qualitative study of the P₀ term within the framework of the adhesion model described in Eq. (1). HMAs studied are ethylene/vinyl acetate copolymer (EVA)/tackifier/wax blends. Substrates studied are untreated and corona-discharge-treated polyolefins such as polypropylene (PP) and polyethylene (PE). First, it has been found that the tackifier surface tension could be roughly correlated with one of its thermodynamic parameters: the solubility parameter dispersion component. Secondly, except for EVA/tackifier binary blends, the compatibility of any two of these three components, the EVA polymer, the tackifier and the wax, in a HMA can be estimated from surface tension and X-ray photoelectron spectroscopy (XPS) measurements. Thirdly, based on the study of the EVA/mixed aliphatic-aromatic tackifier/wax model HMA system, it has been observed that the HMA/polyolefin substrate interfacial composition depends on the wax/substrate compatibility. The cause of an inferior peel strength of a HMA containing a high wax content to a polyolefin substrate is possibly due to the formation of a weak boundary layer (WBL) of wax at the interface and/or low dissipative properties of the HMA.

Also, the relationship between EVA/tackifier/wax interactions and HMA peel strength will be discussed. A correlation between the EVA/tackifier compatibility measured by cloud point and viscoelastic experiments to the debonding term, D, in Eq. (1) has been found.     

Adhesion and Failure Mechanisms of a Model Hot Melt Adhesive Bonded to Polypropylene

A model hot melt adhesive (HMA) based on an ethylene/vinyl acetate copolymer (EVA), an Escorez® hydrocarbon tackifier, and a wax has been used to bond together polypropylene (PP) films to give equilibrium bonding. Peel strengths were determined over a broad range of peel rates and test temperatures. Contrary to the peel behavior of joints with simple rubbery adhesives [1], peel strengths with this semi-crystalline adhesive are not rate-temperature superposable, and multiple transitions in failure locus occur. The semi-crystalline structure of the HMA also prevents rate-temperature superposition of its dynamic moduli.

At different test temperatures, the dependence of peel strength on peel rate shows some resemblance to the dependence of the loss tangent of the bulk adhesive on frequency. This is consistent with a previous result [2] that the HMA debonding term. D, varies with the loss tangent of a HMA at the T-peel debonding frequency.

This model HMA, similar to block copolymer/tackifier blends [3], consists of two phases: an EVA-rich and a tackifier-rich phase, in its amorphous region. At a low peel rate of 8.33 × 10^-5 m/s, the peel strength shows a maximum at a temperature that corresponds to the transition temperature of the tackifier-rich phase (T₁). At a higher peel rate of 8.33 × 10^-3 m/s, the peel strength rises with increasing test temperature, but becomes essentially constant at temperature T₁'. It is believed that, to optimize the peel strength of a HMA at ambient temperature, it is advantageous to formulate the EVA polymer (or other semi-crystalline polyolefins) with a compatible tackifier that yields a tackifier-rich phase with a transition temperature (T₁') in the vicinity of room temperature.