International Journal of Adhesion & Adhesives 99 (2020) 102559

Available online 6 February 20200143-7496/© 2020 Elsevier Ltd. All rights reserved.

A comprehensive investigation into the structure-property relationship of wax and how it influences the properties of hot melt adhesives

Divann Robertson a,*, Albert van Reenen a, Heidi Duveskog b

a Department of Chemistry and Polymer Science, University of Stellenbosch, Private Bag X1, 7602, Matieland, South Africa b Contextualize (Pty) Ltd, Wavecrest, Strand, 7140, South Africa

A R T I C L E I N F O

Keywords: Hot melt Thermal analysis Structure property relations Wax Fischer-Tropsch

A B S T R A C T

A detailed study was conducted on seven different waxes used in hot melt adhesive formulations together with conventional resins and tackifiers, to characterize the waxes and investigate the effects of wax structure and morphology on the thermal behaviour and basic properties of the resultant hot melt adhesive formulations (HMAs). The waxes selected included representatives of each of the following types: Fischer-Tropsch wax (FT), fully refined paraffin wax (FRP), by-product polyethylene wax (BPPE), microcrystalline wax (microwax), alpha- olefin wax (AO), carnauba wax and first intention polyethylene wax (FIPE). Fourier-transform infrared spec-troscopy (FTIR), differential scanning calorimetry (DSC), high temperature size exclusion chromatography (HT- SEC), nuclear magnetic resonance spectroscopy (NMR) and confocal laser scanning microscopy (CLSM) were used as analytical tools to characterize the waxes. Molecular chain architecture as determined by solution 13C NMR highlighted the superior chain linearity of FT wax. Methyl, ethyl and butyl short chain branching were detected in other waxes. Solid-state 13C CP-MAS NMR provided information on the semi-crystalline nature of the waxes. FIPE, AO and Microwax showed significant structural mobility at room temperature as observed by 1H Wideline NMR and was attributed to chain branching and mobile crystalline domains respectively. This was supported by CLSM micrographs. All waxes enhanced crystallinity in both metallocene catalysed polyethylene (mPE) and ethylene-vinyl acetate (EVA) based HMAs. This was confirmed by the characteristic splitting in FTIR bands and increased DSC enthalpies observed for the HMA relative to the neat polymers. Of the formulations containing high melting waxes, FT wax resulted in HMAs with narrower crystallization profiles, an important factor in determining HMA set times. HMA viscosities were found to be dependent on the molecular weight of the wax while the HMA melting temperatures and enthalpies were more dependent on the crystalline morphology of the waxes.

1. Introduction

Hot melt adhesives (HMAs) are progressively becoming the preferred type of adhesive in many diverse automated production environments due to the low setting time of HMAs which enables increased production throughput. HMAs are thermoplastic materials comprising three main components: 1) A thermoplastic polymer of high molecular weight which provides mechanical toughness, 2) A tackifying resin or tackifier that facilitates wet-out and provides tackiness, 3) A crystalline wax which facilitates low setting time, lowers melt viscosity and provides heat resistance within the final HMA formulation. Thermal stabilizers or antioxidants are often added to prevent thermal degradation of the HMA upon mixing and application at elevated temperatures. Various

polymers and tackifiers can be used, depending on the specific substrate to be bonded. Some of the widely used polymers for commercial HMAs include ethylene-vinyl acetate (EVA) copolymers, metallocene catalysed polyolefins, styrene-based block copolymers (SBC), polyamides and polyesters with EVA copolymers being the most commonly used. Amongst the tackifiers, naturally derived rosin esters are most common and mainly used with polymers containing polar moieties. Petroleum derived hydrocarbon tackifiers are preferred when using non-polar (polyolefin) polymers to improve HMA blend compatibility. Hot melt adhesives are used in an extensive number of industries, some major applications include: packaging, labels, tapes, automotive and book-binding. Fast setting speeds, low volatile organic compounds (VOC’s) and low costs are key advantages of HMAs when compared to water-

* Corresponding author. E-mail address: ddr@sun.ac.za (D. Robertson).

Contents lists available at ScienceDirect

International Journal of Adhesion and Adhesives

journal homepage: http://www.elsevier.com/locate/ijadhadh

https://doi.org/10.1016/j.ijadhadh.2020.102559 Received 1 July 2019; Accepted 31 January 2020

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based and solvent-based adhesive counterparts. Packaging HMAs are the largest adhesive market for waxes.

Waxes play an integral part in the formulation process and in the performance of the HMA and typically make up between 10 - 30 wt% of the adhesive formulation. The most commonly used waxes are Fischer- Tropsch (FT) waxes, paraffin waxes and microcrystalline waxes. FT waxes are produced from synthesis gas (hydrogen plus carbon monox-ide) using feedstock such as natural gas, coal or biomass. FT waxes are essentially long chain n-alkanes with very minor amounts of methyl branches, olefins and oxygenates. The predominantly linear molecular structures result in low melt viscosities, good thermal stability and high crystallinity, all of which are advantageous in the HMA application. Paraffin and microcrystalline waxes are petroleum derived products obtained from the refining process of crude oils. Of these, paraffin waxes are also predominantly n-alkanes while microcrystalline waxes contain naphthenes and significant amounts of branched material. Therefore, microcrystalline waxes are less crystalline than fully refined paraffin waxes, resulting in a more flexible structure ideal for lower temperature applications. Other waxes such as first intention polyethylene (FIPE) waxes, by-product polyethylene (BPPE) waxes, polypropylene (PP) waxes and alpha-olefin (AO) waxes are also used, but to a lesser extent and for more application specific purposes.

In recent years, multiple studies have been conducted on the per-formance of different HMA formulations [1–17]. There is not a signifi-cant literature dealing with the correlation between different wax types and their microstructures on the properties of HMAs. These relationships are normally inferred in practice by analysing the performance prop-erties of the HMA rather than through chemical characterization. Kalish et al. studied n-alkanes of various carbon lengths within an EVA adhe-sive system and reported that co-crystallization can be facilitated when having a wax and EVA polymer with similar crystallisable poly-methylene segments [2]. Viscoelastic and adhesion properties of EVA-based HMAs were reported by Park and co-workers. Here the effect of two FT waxes (one medium melting wax and one high melting wax) was reported, and it was shown that an increase in wax content from 15 wt% to 30 wt% altered the miscibility between polymer and tackifier [14]. Moyano et al. investigated ethylene/n-butylacrylate (EBA) based HMAs and reported on the influence of FT and microcrystalline waxes on the properties of the polymer/tackifier/wax ternary blends [18]. This report states that the microcrystalline wax resulted in higher tack and a more flexible blend. It was concluded that at a wax concentration of 19.9 wt%, the properties and compatibility of the ternary blends were related to the nature of the wax.

The focus of the present study is to examine and comprehensively characterize a variety of waxes in terms of molecular microstructure, chemical composition, crystallinity and thermal behaviour and investi-gate how these parameters influence the basic properties of the formu-lated hot melt adhesive blends. An extensive array of analytical tools was used including high temperature size exclusion chromatography (HT-SEC), differential scanning calorimetry (DSC), Fourier-transform infrared spectroscopy (FTIR), solution nuclear magnetic resonance spectroscopy (solution NMR), solid-state NMR, viscosity measurements and confocal laser scanning microscopy (CLSM). The impact of struc-tural and morphological differences of these waxes on the performance properties of the ultimate hot melt adhesives will be communicated in a subsequent publication.

2. Experimental

2.1. Materials

Two thermoplastic polymers were used: 1) ethylene-vinyl acetate copolymer (EVA) having a vinyl acetate content of 28 wt% (Evatane 28–420, Arkema Inc., Pennsylvania, USA), 2) metallocene catalysed polyethylene (mPE) (Affinity GA1950, Dow Chemicals, Selangor, Malaysia). A rosin ester tackifier (Sylvalite RE100S, Kraton Chemicals,

Florida, USA) with softening point of 100 �C and Tg of 48 �C was used for the EVA-based HMA formulations and a hydrogenated hydrocarbon tackifier (Eastotac H-130R, Eastman, Tennessee, USA) with softening point of 130 �C and Tg of 70 �C was used for the mPE-based HMA for-mulations. Seven different types of waxes were used in the study. The selection of waxes represents one of each of the following types: Fischer- Tropsch wax (FT, J100, Juniper Specialty Products, Texas, USA), fully refined paraffin wax (FRP, FRP58-60, Loba Chemicals, Mumbai, India), by-product polyethylene wax (BPPE, CWP500E, Trecora Chemicals, Texas, USA), microcrystalline wax (Microwax, W445, Sonneborn, New Jersey, USA), alpha-olefin wax (AO, AlphaPlus C30þ, Chevron Phillips, Texas, USA), carnauba wax (SZ1, Spirzon, North Carolina, USA) and low density polyethylene wax produced from direct synthesis and known as first intention polyethylene wax (FIPE, AC-8, Honeywell, New Jersey, USA). Irganox 1010 (Sigma-Aldrich, South Africa) was added to the blends as thermal stabilizer to prevent degradation during mixing at elevated temperatures. Properties of the polymer and waxes are listed in Tables 1 and 2 respectively.

2.2. Preparation of hot melt adhesive (HMA) formulations

Individual components (polymer, wax and tackifier) were thor-oughly mixed to prepare the final HMA. A thermal stabilizer (Irganox 1010, 0.5 wt%) was added to the mixture to prevent thermal degrada-tion. The raw materials were placed in a glass reactor and lowered into a preheated oil bath at 170 �C and allowed to heat up and melt for 30 min. Thereafter the HMA mixture was stirred for 60 min using an overhead paddle stirrer operating at a stirring speed of 300 rpm. Formulation composition ratio of polymer: wax: tackifier was kept constant at 40 : 20: 40 wt%. A schematic representation of the HMA preparations is shown in Fig. 1.

2.3. Characterization methods

2.3.1. High temperature size exclusion chromatography (HT-SEC) Molecular weight determination was done by high temperature size

exclusion chromatography using a PL-GPC 220 high-temperature chro-matogram system (Polymer Laboratories, Varian Inc., Amherst, MA). The system was equipped with three PL gel columns and a differential refractive index detector. 1,2,4-trichlorobenzene (TCB, distilled and filtered twice) was used as solvent and eluent at a flow rate of 1 ml/min with 0.0125% butylhydroxytoluene (BHT) as stabilizer. BHT was also used as a flow-rate marker. Samples were dissolved at 160 �C in TCB at concentrations of 2 mg/ml. Narrowly dispersed polyethylene standards were used for calibration purposes. Samples were analysed in duplicate.

2.3.2. Differential scanning calorimetry (DSC) Endothermic melting and exothermic crystallization behaviour were

studied using DSC under an inert nitrogen (N2) atmosphere and oper-ating at a gas flow rate of 50 ml/min. Analyses were carried out using a TA Instruments Q100 calorimeter (TA Instruments, Delaware, USA), which was calibrated with an indium metal standard according to

Table 1 Properties of polymers.

Polymer aMn (g/ mol)

bMw (g/ mol)

cPDI (Mn/ Mw)

dΔHm (J/

g)

eTm

(�C)

fTc

(�C)

EVA 6791 23 906 3.5 25 69 49 mPE 8110 19 579 2.4 25 67 49

a Number-average molecular weight. b Weighted-average molecular weight. c Polydispersity ¼ Mw/Mn. d Endothermic melting enthalpy. e Endothermic melting peak temperature. f Exothermic crystallization peak temperature.

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standard procedures. Samples were subjected to three thermal cycles. In the first heating cycle, the specimens were heated up to 150 �C to erase the thermal history. The second cycle consisted of a non-isothermal cooling step from 150 �C down to � 10 �C. During the third cycle, samples were reheated to 150 �C. The second heating cycles were used to calculate melting temperatures and enthalpies. Heating and cooling rates were fixed at 10 �C/min.

2.3.3. Attenuated total reflectance - Fourier transform infrared spectroscopy (ATR-FTIR)

FTIR measurements were carried out on a Thermo Scientific Nicolet iS10 spectrometer (Waltham, USA) operating in attenuated total reflectance (ATR) mode with a spectral resolution of 4 cm-1. The system was equipped with a diamond crystal and 32 scans were recorded per sample between 600 and 4000 cm-1 wavenumbers. A background spectrum was collected (32 scans) before each sample. Thermo Scientific OMNIC (version 9) software was used for processing of the collected data.

2.3.4. Solution nuclear magnetic resonance spectroscopy (NMR) Chemical compositions of the waxes were studied by 13C NMR and

1H NMR spectroscopy. Analyses were done on a 600 MHz Varian Unity Inova NMR spectrometer (Varian Inc. USA supplied by SMM In-struments, South Africa) equipped with an Oxford magnet operating at 600 MHz having a 5 mm inverse detection pulsed field gradient probe. Spectra were recorded at 120 �C and samples were prepared by dis-solving 60 mg of wax in deuterated 1,1,2,2-tetrachloroethane (TCE-d2) in a NMR tube. TCE-d2 was used as internal reference at 74.5 ppm. Samples were analysed in duplicate.

2.3.5. Solid-state nuclear magnetic resonance spectroscopy (SS-NMR) Solid-state NMR spectra were acquired using a Varian VNMRS 500

MHz two-channel spectrometer using 6 mm zirconia rotors and a 6 mm Chemagnetics™ T3 HX MAS probe. The 13C cross-polarization (CP-MAS) spectra were recorded at ambient temperature with proton decoupling, a 3.5 μs 90� pulse and a recycle delay of 5s. Magic-angle-spinning (MAS) was done at 5 kHz and Adamantane was used as an external chemical

shift standard where the downfield peak was referenced to 38.3 ppm. The power parameters were optimised for the Hartmann-Hahn match. The contact time for cross-polarization was 1 ms. Proton (1H) wideline experiments were done on the same probe in static mode. All samples were analysed in a similar manner for comparison purposes.

2.3.6. Brookfield viscosity Melt viscosities of the HMA formulations were measured at 125 �C,

150 �C and 175 �C following a modified ASTM 3236-15 method. Ex-periments were conducted using a Brookfield DV3T rheometer (Massa-chusetts, USA) with RS-232 interface and RheocalcT software. Sample mass were ca. 10 g and measurements were done in duplicate for reproducibility purposes.

2.3.7. Cloud point HMA formulations were heated in a beaker to above their melting

point. This was done in a temperature-controlled oven. A thermometer was dipped into the molten HMA until the mercury column equilibrated and stopped rising. Thereafter the thermometer was removed and a droplet of the HMA was maintained on the bulb of a thermometer by rotation of the thermometer between both hands of the laboratory technician. Upon cooling the HMA started to solidify and went from a clear molten state to a hazy semi-solid state. The temperature at which this change occurred was recorded as the cloud point. Due to the subjectivity of the test, the measurements were repeated several times per sample to determine the average cloud point.

2.3.8. Confocal laser scanning microscopy (CLSM) Micrographs were obtained using a LSM780 confocal microscope

(Zeiss, Germany) using ZEN 2012 software. Images were collected using the EC Plan-Neofluar 10x/0.3 M27 and LD Plan-Neofluar 40x/0.6 Corr M27 objectives. Excitation was performed with a 514 nm Argon multi-line laser and emission was detected in the range 539–753 nm. Laser power, gain and pinhole size were optimised for the most suitable signal intensity.

3. Results and discussion

3.1. Neat materials

3.1.1. HT-SEC Molecular weight distributions (MWD) of the waxes and polymers

are shown in Fig. 2. The chromatograms clearly illustrate significant differences for the various waxes. Apart from the carnauba wax which showed a bimodal distribution, all samples showed a unimodal MWD. FIPE is overlaid with the neat polymers in Fig. 2b rather than the waxes due to its larger molecular weight and polydispersity. FRP appears to be the most homogeneous (lowest PDI and narrowest distribution) in wax chain length. Chromatograms for the two polymers were comparable with EVA having a broader MWD and containing a slightly larger amount of the higher molecular weight fraction than mPE. Table 3

Table 2 Properties of waxes.

Wax aCongealing point (�C) bKinematic viscosity (mm2/s)

FRP 53 4#

AO 72 8#

Microwax 77 19#

Carnauba 82 35#

FT 102 18 * BPPE 103 44 * FIPE 101 901 *

a Onset of solidification from the molten state. ASTM D938 standard test method.

b ASTM D445 standard test method (#100 �C, *120 �C).

Fig. 1. Schematic representation of HMA preparations.

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summarizes the HT-SEC results for all neat materials. Waxes are listed in increasing order of number-average molecular weight (Mn).

3.1.2. Characterization of wax microstructure and chemical composition Infrared spectroscopy was used to study the chemical composition

and molecular interactions while solution 13C NMR was used to inves-tigate the type and degree of branching in the wax samples. Comple-mentary 1H NMR was also performed (Supplementary Information) to further investigate subtle differences in molecular structure or archi-tecture. Fig. 3 represents an overlay of the 13C NMR spectra. Results for FT, FRP and BPPE waxes were indicative of a linear backbone structure containing little to no short chain branching - mainly saturated chain end carbons (1S, 2S, 3S, 4S) and backbone methylene carbons are visible on the spectra. This is in good agreement with FTIR results which showed predominantly the characteristic stretching and bending vi-brations associated with non-polar hydrocarbon molecules, seen in Fig. 6. Additional smaller peaks associated with short chain branching were observed in the NMR spectra for the rest of the waxes. Methyl, ethyl and butyl branches were detected, with mainly methyl branches identified in the Microwax sample. Due to the molecular complexity of AO and carnauba wax, they are represented in Figs. 4 and 5 respectively. Evidence of unsaturated alkene moieties were observed in FTIR for AO at 909 cm-1, 980 cm-1, 1642 cm-1 and 3078 cm-1 due to ¼C–H bending (wagging), C––C stretching and ¼C–H stretching vibrations. These were further assigned with solution 13C NMR to long chain molecules con-taining terminal-, internal double bonds and vinylidene moieties. Polar

functionalities such as hydroxyls, carbonyls and ester C–O groups were observed for the carnauba wax. These FTIR bands were present at 3300 cm-1, 1740 cm-1 and 1170 cm-1 respectively. The proposed structure for the carnauba wax is shown in Fig. 5 and the NMR results supports the functional groups that were identified with FTIR. Structural elucidation of the carnauba wax is in good agreement with reported literature [19].

3.1.3. Characterization of wax morphology FTIR is sensitive to crystal structure and conformational disorder

within materials [20]. The characteristic hydrocarbon bending and stretching regions used to study the structural and conformational changes are the C–H stretching (3000–2800 cm-1), methylene (CH2) scissoring (1460 cm-1) and methylene (CH2) rocking (720 cm-1) vibra-tions. This study focuses on the rocking regions. The presence of methylene rocking vibrations suggests some degree of intermolecular order and the splitting of these bands provides information on the crystalline nature of the sample. Due to intermolecular coupling in the orthorhombic unit cell in ordered structures, these bands appear as doublets [21]. In Fig. 7a, two distinct bands were observed for all waxes at 720 cm-1 and 730 cm-1 indicating an ordered orthorhombic crystal packing and high crystalline nature of these waxes. The splitting was least distinct for FIPE suggesting a less ordered crystal packing that can be attributed to the larger degree of chain branching and broad MWD. Material that are predominantly amorphous or of low crystallinity show a single peak at 720 cm-1 as was the case for both polymers, see Fig. 7b. A faint shoulder at 730 cm-1 indicates a low degree of crystallinity for the polymers.

The morphology was further studied with solid state 13C CP-MAS NMR and results are shown in Fig. 8. Due to the cross-polarization dy-namics associated with this specific type of SS-NMR experiment, the rigid domains (mostly contributed by crystalline regions) of the analysed samples are selectively enhanced. SS-NMR is sensitive, not only to chemically different environments but also to morphological differences and therefore allows for the investigation of the crystalline and amor-phous domains in materials. All spectra displayed a dominant peak centred at δ ¼ 32.5 ppm attributed to an all-trans arrangement of the internal methylene backbone carbons (–CH2–) in rigid domains and in alignment with FTIR results. Furthermore, saturated carbon atoms at the chain ends are visible at δ ¼ ~14 ppm and δ ¼ ~23 ppm as well as a distinct shoulder at δ ¼ 30.5 ppm observed for Microwax and FIPE. The latter is due to internal methylene backbone carbons in amorphous (mobile) domains having more gauche conformers. Only faint mobile methylene shoulders were detected for AO and BPPE. The observed values are in good agreement with the carbon-13 chemical shifts reported in the literature for polyethylene, as well as shorter n-alkanes in the

Fig. 2. HT-SEC chromatograms of a) waxes and b) polymers.

Table 3 Summary of HT-SEC data for all neat materials.

Sample aMp (g/mol) bMn (g/mol) cMw (g/mol) dPDI

Polymers EVA 18 090 6791 23 906 3.5 mPE 17 480 8110 19 579 2.4

Waxes AO 368 333 421 1.3 FRP 404 395 421 1.1 Microwax 606 501 634 1.3 Carnauba 741 533 669 1.3 FT 655 631 900 1.4 BPPE 792 668 922 1.4 FIPE 5328 2448 5144 2.1

a Peak molecular weight. b Number-average molecular weight. c Weighted-average molecular weight. d Polydispersity ¼ Mw/Mn.

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orthorhombic crystalline form. Like the solution NMR results, FIPE showed the smallest chain end carbon signals due to its high molecular weight and increased mobility of the chain ends. No clear evidence of a mobile methylene region was observed for the rest of the waxes (FT, FRP,

carnauba wax) implying that these waxes have higher crystallinity and are more rigid at room temperature compared to the aforementioned two waxes (AO and BPPE). The methylene group close to the polar carbonyl moiety resulted in a peak at 65 ppm in the carnauba wax spectrum.

Fig. 3. Solution 13C NMR spectra of all waxes.

Fig. 4. Solution 13C NMR spectrum of AO wax.

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1H Wideline experiments were done to probe the motional dynamics in the waxes. Fig. 9 shows a comparison of 1H Wideline spectra of all the waxes. Three distinct regions could be identified: a mobile component (sharp narrow section), an intermediate phase (broadened middle sec-tion) and a rigid (broad base section) domain. These different domains could clearly be observed for the FT, FRP and carnauba waxes. These

waxes showed the largest degree of rigidity of which the FT wax have the largest mobile component that may be beneficial for flexibility. The carnauba wax structure appeared to be fairly rigid. BPPE, Microwax, FIPE and AO showed more liquid-like behaviour as seen by the presence of the dominant sharp and narrow peak. Microwax appeared to be most mobile of the waxes and this is consistent with the distinct mobile

Fig. 5. Solution 13C NMR spectrum of carnauba wax.

Fig. 6. Full FTIR spectra of neat waxes.

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methylene peak observed from the 13C CP-MAS results. Only a subtle amorphous methylene shoulder was observed in Fig. 8 for AO yet the mobile component dominates in the 1H Wideline spectrum suggesting possible small, highly mobile crystalline domains.

3.1.4. Thermal behaviour of neat waxes Thermal behaviour of the neat waxes was analysed by DSC with

onset temperatures, peak temperatures and melting enthalpies summa-rized in Table 4. Melting endotherms and crystallization exotherms are presented in Fig. 10a and b respectively. All thermograms, with excep-tion of AO, had bimodal profiles ranging across various temperature ranges. FRP had narrow thermal transitions due to its narrow, more homogeneous molecular weight distribution as seen from HT-SEC re-sults. The FT wax showed characteristic double melting behaviour. FT, BPPE and FIPE showed the highest peak temperature for both melting and crystallization transitions. AO, FRP and Microwax had peak tem-peratures towards the lower temperature region whereas the transitions for carnauba wax were observed at an intermediate temperature relative to the other six waxes. The melt and crystallization temperatures closely follow the molecular weights of the waxes, with lower molecular weight waxes melting sooner and crystallizing at lower temperatures. The very

broad thermograms observed for BPPE can be ascribed to the increased heterogeneity in this wax in terms of chain length, branching frequency and therefore defects in the crystal structure. The effect of chain branching and increased heterogeneity in the system is further demon-strated when comparing FT and BPPE, having similar molecular weights and PDI: the FT wax has an overall higher crystallinity and rigidity than BPPE as shown in the SSNMR section. The lower crystallinity for FIPE, Microwax and AO were evident by the decreased melting enthalpies and correlated well with the mobile methylene peaks and liquid-like signal seen in 13C CP-MAS and 1H Wideline SS-NMR spectra. Enthalpies for the other waxes are all very similar implying that their degree of crystal-linities were also comparable.

3.1.5. CLSM After crystallization from the molten state, the crystal morphology of

the waxes was investigated by CLSM. Fig. 11 displays the images of the waxes at 40x magnification and the scale bar designates a distance of 20 μm. All waxes showed “space filling” crystal morphologies with FRP having the coarsest texture possibly due to the homogeneous molecular weight distribution allowing for the growth of larger and more perfect crystals. AO showed the smoothest crystal morphology as well as a very fine crystal structure. This further validates that the high mobility (seen in 1H Wideline NMR) within the AO structure arises due to the presence of many smaller crystals compared to the larger crystal networks seen for most of the other waxes. Unsaturated moieties could also contribute to a decrease in crystallinity (large mobile region) of the AO wax. Spherulitic structures (yellow squares) were observed for FIPE and are similar to those usually seen in polyethylene polymers. Due to its mo-lecular weight being so much higher than the rest of the waxes, the resultant crystal morphology appears to be very “polymer-like” (large spherulites). These CLSM images indicate that crystal morphologies among the waxes can be distinctly different even though their chemical nature are very alike. These crystal morphologies are important funda-mental properties of the waxes and will directly influence the perfor-mance of the resultant adhesive formulation.

3.2. Hot melt adhesive formulations (HMAs)

3.2.1. Structure and property behaviour of HMAs Fig. 12a and b illustrate the methylene rocking vibrations of mPE-

based HMAs and EVA-based HMAs respectively. Two distinct bands were observed for all HMAs at 720 cm-1 and 730 cm-1 indicating an ordered orthorhombic crystal packing and some degree of crystallinity in these adhesives. As reported previously, the splitting of FTIR

Fig. 7. FTIR spectra of the methylene rocking region for a) waxes and b) polymers.

Fig. 8. 13C CP-MAS NMR spectra of all waxes.

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methylene bands were not observed for the homopolymers and tacki-fiers due to the low crystallinity of the polymers and the lack of long- range order in the smaller tackifier molecules. Therefore, it can conclusively be reported that the wax induced the crystallinity in the HMA formulations and can be referred to as the crystallization- enhancing component. The splitting was less distinct for HMAs formu-lated with FIPE and Microwax in both mPE and EVA series, suggesting a lower crystallinity attributed to a less ordered crystal packing due to the lower crystalline nature of the two neat waxes discussed earlier in the report.

Melt flow behaviour of hot melt adhesives is a key property which directly influences the application process. Fig. 13 illustrates the HMA viscosities as a function of temperature between 125 �C and 175 �C. All formulations showed a decrease in viscosity with an increase in tem-perature due to enhanced mobility of molecular chains at elevated temperatures. The difference in viscosities, especially in the lower temperature range (between 125 �C and 150 �C) is dependent on mo-lecular weight and the melt temperature of the neat waxes but at 175 �C there is a levelling of the viscosity due to sufficient thermal energy and chain mobility to facilitate the flow of the adhesive. However, FIPE still had a higher viscosity at this temperature because its molecular weight was an order of magnitude larger than the rest. Carnauba wax on the

other hand, has an intermediate molecular weight and melting tem-perature, yet its viscosity was higher than expected. This is most likely as a result of the secondary molecular interactions in the wax that was discussed earlier. HMAs formulated with lower molecular weight waxes had lower viscosities due to ease of mobility of these shorter molecular chains upon heating. Larger molecules need more energy (heat) to promote chain mobility and hence facilitate the flow of the adhesive. As a result of the higher molecular weight and its molecular weight het-erogeneity, inclusion of FIPE wax into the HMA formulations resulted in viscosities distinctly higher than the rest of the formulations. This im-plies that the flow behaviour in the molten state, during adhesive application, may vary depending on the application temperature. Furthermore, the EVA HMAs showed higher viscosities compared to the mPE formulations, most likely due to the higher molecular weight fraction of EVA contributing to the resistance to flow.

3.2.2. Thermal behaviour of HMAs Both the melting endotherms and crystallization exotherms of the

HMAs, as seen in Figs. 14 and 15, had profiles similar to those of the neat waxes. This was especially evident for the mPE based series, except that with these the thermal transitions occurred over a broad temperature range resulting in broader thermograms due to the presence of the less crystalline polymers in the formulations. When comparing the contours of the mPE and EVA based HMAs DSC thermograms, slight differences can be observed for certain formulations. This was particularly evident when comparing the FIPE, FRP and FT based HMAs. The mPE based HMA formulations containing the aforementioned three waxes resulted in distinct splitting of the endothermic melting peaks and the crystalli-zation exotherms for FIPE and FT formulations were considerably nar-rower than the EVA based counterparts. It therefore implies that the resultant crystalline morphology within the two different HMA systems differed as a result of differences in interactions between polymer, wax and tackifier. A predominantly non-polar environment existed for the mPE HMAs whereas some polarity existed for the EVA HMAs due to the acetate polymer moieties and rosin ester based tackifier. Onset of crys-tallization shifted to lower temperatures for all HMAs as compared to the neat waxes in Fig. 10. This phenomenon indicates that less wax homo-crystallization occurred due to interactions between HMA components and hints towards compatibility within the formulations.

Despite the variations in DSC profiles, the melting enthalpies of the HMAs, as summarized in Table 5, were very similar. For the mPE HMA series, all enthalpies were within 10% of each other. Enthalpies for the

Fig. 9. 1H Wideline spectra of all waxes.

Table 4 Summary of DSC data for all waxes.

Wax ID aTom bTm1

cTm2 dΔHm

eToc fTc1

gTc2

(�C) (�C) (�C) (J/g) (�C) (�C) (�C)

FRP 20 33 53 244 53 49 30 AO 31 59 81 160 71 68 56 Microwax 22 42 69 136 76 71 42 Carnauba 47 77 83 235 80 78 72 FT 70 94 110 235 104 100 89 BPPE 30 100 114 210 104 99 92 FIPE 40 107 110 170 103 98 60

a Onset of melting. b Lower (1st) endothermic melting peak temperature. c Higher (2nd) endothermic melting peak temperature. d Endothermic melting enthalpy. e Onset of crystallization. f Higher (1st) exothermic crystallization peak temperature. g Lower (2nd) exothermic crystallization peak temperature.

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EVA HMA series were within 12% of each other. Incorporation of Microwax resulted in low crystalline HMAs mainly due to the low crystallinity of the neat wax, ideal for lower temperature applications where flexibility is key. The lowest molecular weight wax, namely FRP resulted in formulations with the highest crystallinity in the EVA HMA series. It is possible that the crystallisable sequences of this wax matched the crystallisable polymethylene domain of the EVA to a greater extent than the longer chain waxes and therefore facilitated co-crystallization [1,2]. Apart from FIPE, the rest of the waxes resulted in crystallinities of mPE HMAs within 3% of their EVA based counterparts indicating that these waxes allowed for similar co-crystallization in both the non-polar mPE and polar EVA HMA systems despite differences in neat wax crystallinities. FIPE showed higher crystallinity within the mPE HMA formulation which may be attributed to the bulky long-chain non-polar FIPE molecule associating better with the non-polar mPE. High melting points and early onset of crystallization in HMAs containing FT and BPPE waxes, are beneficial for thermal stability of the formulations and

will facilitate reduced set times. The melting enthalpy of the final HMA seemed to be more dependent on the crystallinity of the neat wax and the association of wax and polymer rather than the molecular weight. Enthalpies for both neat polymers were calculated as 24 J/g, thus con-firming that the waxes were the main contributors towards the HMA crystallinities. However, the fact that the enthalpies are not too vastly different implies that the two HMA systems exhibit different crystalli-zation rates but still result in similar final HMA crystallinities when comparing mPE and EVA counterparts.

The cloud point is the temperature where the molten adhesive starts to appear turbid upon cooling due to crystallization and is also a mea-sure of compatibility between individual components in the formula-tion. Turbidity arises due to the scattering of light because of the formation of crystallites. Fig. 16 illustrates the cloud point for both the mPE HMA series and EVA HMA series as a function of the wax used within the formulations. The results are in good agreement with trends seen for DSC crystallization temperatures, with HMAs formulated with

Fig. 10. a) DSC melting endotherms and, b) DSC crystallization exotherms of all waxes.

Fig. 11. CLSM images of neat waxes after crystallization from the molten state. (40X magnification, scale bar ¼ 20 μm).

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FT, BPPE and FIPE waxes, showing higher cloud point temperatures followed by formulations containing carnauba wax, AO and Microwax. The cloud point for the EVA based HMA containing carnauba wax was lower than the corresponding mPE system and may be a result of the improved compatibility between EVA, rosin ester tackifier and the polar groups of carnauba wax (also showing slightly higher enthalpy within EVA set). Conversely, incorporation of the FIPE appeared to be more compatible in the mPE formulations due to its long hydrocarbon mo-lecular structure associating better with the entirely non-polar mPE and hydrocarbon tackifier system resulting in more co-crystallization and less wax homo-crystallization. This is consistent with the shifts in exothermic crystallization onset temperatures seen in Fig. 15 where carnauba wax showed a greater shift within the EVA formulation and FIPE showed a greater shift within the mPE formulation. FRP wax resulted in formulations (both mPE and EVA based) with the lowest cloud point temperature.

Results indicate that HMAs (both mPE and EVA based) formulated with the FT, BPPE and FIPE waxes would have comparable setting onset temperatures, however, due to the narrower DSC exotherm, incorpora-tion of the FT wax will result in shorter setting times. The FRP based HMAs would have better hot tack and much longer open and set times. A low solidification temperature may also result in poor tack properties in

certain applications.

4. Conclusions

Comprehensive analyses were carried out on seven different waxes to investigate the chemical, thermal, structural and morphological nature of the materials and to correlate these properties to the properties of mPE and EVA based hot melt adhesive formulations incorporating these waxes. Distinct splitting of characteristic methylene bands in FTIR emphasized the orthorhombic crystal packing and high crystalline na-ture of the waxes (no splitting seen for the polymers). Solution 13C NMR emphasized the superior linearity of the FT wax molecular chain and no clear evidence of short chain branching. Methyl, ethyl and butyl branching were detected among the other waxes. Fine crystal mor-phologies were observed for AO and Microwax during CLSM imaging which correlated with the high mobility seen from 1H Wideline NMR data. Waxes with larger crystallites resulted in higher rigidity and had higher DSC melting enthalpies with FT and BPPE waxes melting and crystallizing at the highest temperatures. Large “polymer-like” spheru-litic structures were visible for FIPE due to its large molecular weight. CLSM images clearly points out that molecular weight, polydispersity and chain linearity are all factors influencing the crystal morphology of

Fig. 12. FTIR spectra of the methylene rocking region for a) mPE HMAs and b) EVA HMAs.

Fig. 13. Brookfield viscosity profile of a) mPE HMAs and b) EVA HMAs.

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the waxes. The carnauba wax is a complex material and trends in its behaviour were more difficult to correlate with its structure. Results obtained from the various analytical techniques discussed within this study are in good agreement with one another and provides a good overview on the fundamental characteristics of the investigated neat materials. This study indicates that molecular architecture, crystallinity and chain functionality of the wax will affect certain HMA properties to different extents. It was found that the characteristic combination of superior molecular chain linearity, enhanced crystallinity and low melt

viscosity of the FT wax could be very beneficial in the formulation of high-performance hot melt adhesives.

An ongoing study is focused on the correlation between the funda-mental characterization reported within this work, and the application- based properties and performance of the resultant HMAs, including bond testing (open times and set times), cleavage testing, shear adhesion failure, peel adhesion failure, thermal stability and viscoelastic behaviour.

Fig. 14. DSC melting endotherms of a) mPE HMAs and b) EVA HMAs.

Fig. 15. DSC crystallization exotherms of a) mPE HMAs and b) EVA HMAs.

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Author contributions

This manuscript was written with contributions of all authors. All authors have given approval to the final version of the manuscript.

Declaration of competing interest

The authors declare no conflict of interest.

Acknowledgments

The authors would like to thank Juniper Specialty Products for funding as well as Duc Huynh, Fran Brady and Megan Matthews for their assistance with the experimental work.

Appendix A. Supplementary data

Supplementary data to this article can be found online at https://doi. org/10.1016/j.ijadhadh.2020.102559.

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Table 5 Summary of DSC melting enthalpies of the HMAs.

Wax used mPE HMAs EVA HMAs

ΔHm (J/g) ΔHm (J/g)

FRP 68 79 AO 46 69 Microwax 42 47 Carnauba 62 69 FT 71 66 BPPE 67 68 FIPE 71 45

Fig. 16. Cloud point temperatures of all HMAs.

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