Field of the Invention

The present invention relates to drawn articles comprising polyolefins which have excellent transparency and mechanical properties. The present invention also relates to methods for producing such articles, and to devices comprising such articles.

Background to the Invention

When it comes to transparent polymeric materials, polycarbonate (PC), poly(methyl methacrylate) (PMMA) and polystyrene (PS) are typically the materials of choice. These plastics have a high optical transparency, with transmittance values of 88-92 % in the visible light regime. Since they are amorphous polymers, the absence of crystalline regions contributes to their optical transparency. However, mechanical properties of these polymers in terms of elastic modulus and tensile strength are relatively unsatisfactory, with values of only 2-3 GPa and 50-70 MPa, respectively[l] [2].

In the case of semi-crystalline polymers like polypropylene (PP) and polyethylene terephthalate (PET), transparent products can be manufactured by controlling polymer morphology, requiring the dimensions of spherulites to be much smaller than the wavelength of visible light. One way to achieve this is by adding nucleating or clarifying agents, as in the case of sorbitol-clarified isotactic PP[3] [4]. Furthermore, transparent amorphous PET products can be acquired by rapid cooling of the melt to below the glass transition temperature (T _g ).

High-density polyethylene (HDPE), another type of semi-crystalline polymers, is one of the most commonly used plastics. Unfortunately, however, approaches for making transparent PP and PET are not suitable for making polyethylene products transparent because of a too rapid crystallization rate and low T _g . Typically, HDPE is processed via injection moulding or extrusion-based processes, leading to isotropic or near isotropic materials and a relatively low elastic modulus (< 1.1 GPa) and tensile strength (< 35 MPa)[5]

Uniaxial solid-state drawing at temperatures close to but below the melting temperature is a simple, effective and low-cost post-processing method to increase the stiffness and tensile strength of polymeric materials, notably polyolefms[6]. For instance, the Young’s modulus and tensile strength of ultra-drawn HDPE fibres can be as high as 70 GPa and 1.5 GPa, respectively[7]-[10]. In the case of ultra-high molecular weight polyethylene (UHMWPE), solid-state drawing is also widely used in combination with solution-processing or solvent-free techniques [1 1]-[14]. Here, Young’s modulus and tensile strength have been reported of 100-180 GPa and 2-5 GPa, respectively[1 ] [16]. Uniaxial drawing in the solid- state is effective in improving mechanical properties of polymers like HDPE because chain relaxation phenomena are limited or absent and hence a high degree of chain orientation and chain extension is generated[17] [18].

Similarly to isotropic HDPE products, solid-state drawn HDPE fibres or films are normally not transparent. The dimensions of the crystals, being larger than the wavelength of visible light, and the high degree of crystallinity partially account for the poor

transparency[19]. Additionally, the introduction of internal voiding and defect structures on the surface and in the bulk of the fibres or films after ultra-drawing will induce light scattering, resulting in a poor transparency in the visible light regime[20]. Hence, resultant opaque oriented polymer products have limited applicability in fields like built environment and automotive glazing where both high transparency and mechanical properties are needed.

Transparent and oriented HDPE films have recently been obtained by adding small amounts of specific additives like 2-(2H-benzotriazol-2-yl)-4,6-ditertpentylphenol (BZT) in Shen et n/[21]-[23]. It was argued that these additives predominantly reduced interfibrillar scattering, resulting in an optical transmittance after drawing of around 90 %.

However, the cost of using such additives in polymers constitutes a barrier to their use in practical applications. Moreover, in some cases the additives are expelled from the drawn polymers which reduces their transparency.

In summary, there is a need for a highly transparent material which can be easily and cheaply produced, and which shows excellent mechanical properties such as Young’s modulus and tensile strength. Further, there is a need for an easy way to make drawn polymeric articles which have high transparencies, without the need for expensive additives.

Summary of the Invention

The present invention is based on the unexpected discovery that polyolefin based articles having high transparency and high mechanical performance can be produced simply by regulating the drawing conditions without the need to incorporate additives. The beneficial effects of regulating the drawing conditions, especially drawing temperature, on optical and mechanical performance of solid-state drawn articles, particularly HDPE films, is described herein.

The present invention therefore provides a drawn article comprising polyolefin having a high transmittance without the need for further additives to improve the transmittance.

In one embodiment the present invention provides a drawn article comprising polyolefin, wherein the article has a transmittance of at least 70% when measured with a film thickness of 80 mih at a sample-to-detector distance of 40 cm and at a wavelength of 550 nm, wherein the melt flow index (MFI) of the polyolefin is 2.0 g/10 min or higher at 190 °C/21.6 kg, and wherein no other component having a refractive index which is higher than the isotropic refractive index of the polyolefin is present in an amount equal to or greater than 0.25 % by mass relative to the mass of polyolefin. The Young’s modulus of the drawn article in at least one direction of the drawn article is preferably higher than 5 GPa,

In another embodiment the present invention provides a drawn article comprising polyolefin, wherein the article has a transmittance of at least 70% when measured with a film thickness of 80 pm at a sample-to-detector distance of 40 cm and at a wavelength of 550 nm, and wherein the crystallinity of the polyolefin (Xc) is greater than or equal to 80%.

In an aspect of the above embodiment, the melt flow index of the polyolefin is 2.0 g/10 min or higher at 190 °C/21.6 kg.

In a further embodiment the present invention provides a drawn article comprising polyolefin produced via a method comprising the steps of providing a polyolefin and uniaxially drawing the polyolefin at a draw ratio of 10 or more at a temperature from 45 °C below the isotropic melting temperature of the polymer to 25 °C below the isotropic melting temperature of the polymer, wherein the article has a transmittance of at least 70% when measured with a film thickness of 80 pm at a sample-to-detector distance of 40 cm and at a wavelength of 550 nm, wherein the melt flow index (MFI) of the polyolefin is 2.0 g/10 min or higher at 190 °C/21.6 kg, and the Young’s modulus in at least one direction of the drawn article is higher than 22 GPa.

In some aspects of the above embodiments no other component having a refractive index which is higher than the isotropic refractive index of the polyolefin is present in an amount equal to or greater than 0.25 % by mass relative to the mass of polyolefin. In some embodiments the density of the polyolefin is greater than 0.955 g/cm ³ .

In some embodiments the drawn article comprises more than 95 wt.% polyolefin, preferably comprising more than 97.5 wt.% polyolefin, more preferably more than 99.75 wt.% polyolefin.

In some embodiments the polyolefin is polyethylene.

In some embodiments the polyolefin is polyethylene which is linear low density polyethylene (LLDPE), low density polyethylene (LDPE), high density polyethylene (HDPE), or any combination thereof; preferably the polyethylene is HDPE.

In some embodiments the drawn article has a transmittance of at least 75%, preferably at least 85%, when measured at a film thickness of 80 pm at a sample-to-detector distance of 40 cm and at a wavelength of 550 nm.

In some embodiments the Young’s modulus of the drawn article in at least one direction of the drawn article is higher than 10 GPa, preferably higher than 12.5 GPa.

In some embodiments the tensile strength of the drawn article in at least one direction of the article is higher than 0.3 GPa, preferably higher than 0.4 GPa.

In some embodiments the drawn article is a fibre, a tape or a film, preferably a film.

In a further embodiment the present invention provides a method for producing a polyolefin article, comprising the steps of providing a polyolefin and drawing the polyolefin at a draw ratio of 10 or more at a temperature from 45 °C below the melting temperature of the polymer to 25 °C below the melting temperature of the polymer. The polyolefin article is preferably uniaxially drawn. In the polyolefin article the melt flow index (MFI) of the polyolefin is preferably 2.0 g/10 min or higher at 190 °C/21.6 kg.

In some embodiments, the drawn polyolefin may be further extruded.

In some embodiments of the above method the polyolefin is polyethylene, preferably linear polyethylene.

In some embodiments of the above method the polyolefin is polyethylene, which is

HDPE.

In some embodiments of the above method the polyolefin is a linear polyethylene which is drawn at a temperature of 90 °C or higher and 110 °C or less. In some embodiments of the above method the polyolefin is a linear polyethylene which is drawn at a temperature of 100 °C or higher.

In some embodiments of the above method the article is drawn at a draw speed of 500 mm/min or less, preferably at a speed of 100 mm/min or less.

In a further embodiment the present invention provides an article obtainable by any of the methods described above.

In a further embodiment the present invention provides a drawn article as described above, which is produced by any of the above described methods.

In a further embodiment the present invention provides a device comprising a drawn article as described above, wherein the device is a windshield, a window, a visor, an impact resistant article or a display unit. The device is preferably a display unit, more preferably a liquid crystal display (LCD) or an organic light emitting diode (OLED). In a particularly preferred embodiment, the device is a window.

Brief Description of the Figure

Fig. 1(a) shows stress-strain curves at different drawing temperatures (7( _/ ).

Fig. 1(b) shows _max and li,-ans of oriented HDPE films as a function of drawing temperature at a drawing speed of 100 mm/min. The background shade change signifies the transition from homogeneous to inhomogeneous drawing and indicates the processing window for creating highly oriented polymer films.

Fig. 2(a) shows a schematic diagram of the laminated structure consisting of drawn HDPE film sandwiched between glass slides and TPU interlayers.

Fig. 2(b) shows photographs viewed through different layers.

Fig. 2(c) shows transmittance of glass; TPU interlayers sandwiched between two glass slides; and drawn HDPE films sandwiched between two glass slides with or without TPU interlayers versus visible light wavelength tested at a sample-to-detector distance of 40 cm.

Fig. 3 shows photographs of drawn HDPE films (A = 15) drawn at different drawing temperatures when placed directly on top of an object (near field), (Fig. 3(a)), and when placed at a 40 cm distance from an object (far field), (Fig. 3(b)). The HDPE films were sandwiched between glass slides and TPU interlayers. In Fig. 3(b), the films are marked and located between the dashed lines. The thickness of the drawn HDPE films is around 80 pm.

Fig. 4 shows schematic diagrams of the beam path inside the UV-vis machine, corresponding to two different sample-to-detector distances of 5 cm (Fig. 4(a)) and 40 cm (Fig. 4(b)), together with transmittance data versus wavelength tested at a sample-to-detector distance of 5 cm (Fig. 4(c)) and 40 cm (Fig. 4(d)). The HDPE films (2 = 15) were sandwiched between glass slides and TPU interlayers. The thickness of the drawn HDPE films is around 80 pm.

Fig. 5 shows the transmittance of drawn HDPE films (A = 15) at different drawing temperatures versus visible light wavelength, indicating high transparency for T _d ³ 100 °C. Drawn HDPE films were sandwiched between glass slides and TPU interlayers and tested at a sample-to-detector distance of 40 cm. The thickness of the drawn HDPE films is about 80 pm.

Fig. 6 shows the transmittance of drawn HDPE films at draw ratio (2) = 8, 10, 15 and 20 as a function of drawing temperature (Fig. 6(a)), and at different drawing temperatures as a function of draw ratio (Fig. 6(b)), at a wavelength of 550 nm, showing maximum transmittance at T _d ³ 100 °C and 2 = 15. Drawn HDPE films were sandwiched between glass slides and TPU interlayers and tested at a sample-to-detector distance of 40 cm. The thickness of the drawn HDPE films is about 80 pm.

Fig. 7 shows Young’s modulus (Fig. 7(a)) and tensile strength (Fig. 7(b)) of drawn HDPE films at different draw ratios as a function of drawing temperature. The background shade change indicates the transition from opaque to transparent films.

Fig. 8 shows a comparison of specific strength, specific modulus and appearance of common transparent materials together with previous transparent drawn HDPE films with BZT additives[21] and current drawn HDPE films.

Fig. 9(a) shows stress-strain curves of the solid-state drawing process with the influence of drawing speed at 110 °C.

Fig. 9(b) shows X _max and Xm _m as a function of drawing speed at 1 10 °C.

Fig. 10 shows transmittance of HDPE drawn films with different thicknesses at a drawing temperature of 1 10 °C and a draw ratio of 10. Drawn films were sandwiched between glass slides and TPU interlayers and tested at a sample-to-detector distance of 40 cm.

Fig. 11 shows transmittance of HDPE films (A = 15) drawn at different drawing speeds at T _d = 1 10 °C. The films were sandwiched between glass slides and TPU interlayers and tested at a sample-to-detector distance of 40 cm. The thickness of the drawn HDPE films is around 80 pm.

Fig. 12 shows DSC results of T,„ and X _c of drawn HDPE films as a function of drawing temperature at A = 15 (Fig. 12(a)), and draw ratio at T _d = 110 °C (Fig. 12(b)).

Detailed Description of the Invention

The present invention relates to drawn articles comprising polyolefins, which are both highly transparent and have excellent mechanical properties.

Additives for increasing the transmittance of drawn polyolefins, such as BZT, are known in the art[22]. However, such additives are generally expensive, and can affect the mechanical properties of the article. Thus, in one embodiment, the present invention provides drawn articles containing no additives which have a higher isotropic refractive index than that of the polyolefin in an amount equal to or greater than 0.25% by mass relative to the mass of polyolefin. The polymeric articles according to the present invention provide excellent transmittance without the need for such additives.

In some embodiments the highly transparent drawn articles of the present invention comprise less than 0.25% of 2-(2H-benzotriazol-2-yl)-4,6-ditertpentylphenol, cinnamon oil and oligostyrene oil, wherein the oligostyrene has an average M _w of from 400 to 2000 g/mol. The content of the polyolefin (purity) is measured with techniques like Fourier-transform infrared spectroscopy (FTIR), nuclear magnetic resonance (NMR), elemental analysis (EA) and/or energy-dispersive X-ray spectroscopy (EDS) and is calculated by the following equation:

wherein m _po iyoiefm is the mass of polyolefin in the sample, and m _lo iai is the total mass of the sample. As used herein the term isotropic refractive index refers to a dimensionless number expressing the ratio of the speed of light travelling through vacuum to the speed of light travelling through the polymeric material. Refractive indices of polymers are for example reported in the Polymer Data Handbook, Oxford University Press, 1999[33]. The refractive index of a sample may be measured via any suitable means, for instance by identifying the critical angle (0c) and applying Snell’s law, as described in R.K. Rrishnaswamy, Polymer Testing, 24 (2005) 762-765[34]. When the material is anisotropic, e.g. due to drawing, and so has multiple refractive indices, the isotropic refractive index should be obtained by removal of the orientation via heating, then measuring the refractive index as set out above.

In an embodiment, the present invention provides drawn articles which contain no additives which have a higher isotropic refractive index than that of the polyolefin in an amount equal to or greater than 0.25%, preferably 0.15%, more preferably 0.10% by mass relative to the mass of polyolefin. Most preferably the article contains less than 0.05% by mass, relative to the mass of polyolefin, of such additives. In some embodiments, the drawn articles according to the present invention contain no such additives.

The articles according to the present invention comprise at least one polyolefin.

Unless specifically noted otherwise, the polyolefin may be a homopolymer, a copolymer or a terpolymer. Polymers described herein may be straight chain, branched chain, comb, block, or any other structure. The polymers may be homogenous or heterogenous, and may have a gradient distribution of co-monomer units. The polyolefin of the present invention preferably comprises preferably more than 95 weight percent of olefin monomer units. Most preferably, the polyolefin of the present invention comprises only olefin monomer units.

In preferred embodiments, the polyolefin is a homopolymer. In some embodiments the polyolefin according to the present invention is polypropylene, or polyethylene, preferably polyethylene.

Suitable comonomers which may be present in the polyolefin such as polyethylene include propene, 1 -butene, 1-pentene, 4-methylpentene, 1 -hexene, and/or 1-octene.

The polyethylene used herein is preferably polyethylene homopolymer.

When the polyolefin is polyethylene, the type of polyethylene may optionally be selected from linear low density polyethylene (LLDPE), low density polyethylene (LDPE), high density polyethylene (HDPE), high molecular weight polyethylene (HMWPE), ultra- high molecular weight polyethylene (UHMWPE) or any combination thereof; preferably the polyethylene is HDPE or HMWPE or UHMWPE.

As used herein the term high density polyethylene (HDPE) refers to a class of ethylene based polymers having a density of greater than 0.92 g/cm ³ and a molecular chain structure with less than 1 long chain per 1000 carbon atoms and less than 1 short chain per 1000 carbon atoms. Long chain branches are herein defined as side chains with more than 20 carbon atoms such as oligo- or poly- ethylene branches attached to the polyethylene backbone. Short chain branches are herein defined as side chains with at most 20 carbon atoms, such as methyl, ethyl or butyl side chains introduced by copolymerisation. As used herein the term HDPE includes high molecular weight polyethylene (HMWPE) and ultra- high molecular eight polyethylene (UHMWPE) or mixtures thereof.

The polyethylene according to the present invention may be linear or branched. In some embodiments the polyolefin is linear polyethylene, preferably linear HDPE. Linear polyethylene is herein understood to mean a polyethylene with less than 1 side chain per 100 carbon atoms, and preferably less than 1 side chain per 300 carbon atoms. The number of side chains can be determined by ¹³ C nuclear magnetic resonance (NMR) spectroscopy[35] [36].

The drawn article of the present invention preferably comprises greater than 90 weight percent, more preferably more than 95 weight percent of the polyolefin. Most preferably the drawn article comprises more than 97.5 weight percent of polyolefin. In some embodiments, the drawn article consists entirely of polyolefin.

In a preferred embodiment the drawn polyolefin article comprises more than 95 weight percent polyethylene, wherein the polyethylene comprises more than 95 weight percent ethylene monomer units hi a further embodiment the drawn polyolefin article consists of polyethylene, preferably polyethylene homopolymer.

Drawn articles of the present invention have excellent transparency properties. In particular, articles according to the invention have transmittances of at least 70% when measured with a film thickness of 80 pm at a sample-to-detector distance of 40 cm and at a wavelength of 550 nm. The articles preferably have a transmittance of at least 75%, more preferably at least 80%, most preferably at least 85% when measured at a film thickness of 80 pm at a sample-to-detector distance of 40 cm and at a wavelength of 550 nm.

Drawn articles of the present invention preferably have tensile strengths in at least one direction of the drawn article which is greater than 0.3 GPa, more preferably greater than 0.4 GPa, even more preferably greater than 0.6 GPa, most preferably more than 0.7 GPa.

Drawn articles of the present invention preferably have Young’s moduli in at least one direction of the drawn article of greater than 5 GPa, preferably 10 GPa, more preferably greater than 12.5 GPa, even more preferably greater than 15 GPa, yet more preferably greater than 17 GPa, most preferably more than 22 GPa.

In some embodiments of the present invention the polyolefin has a high crystallinity (X _c ). Polyolefins according to the present invention preferably have crystallinities of 80% or greater, more preferably 82% or greater. The percentage crystallinity is measured using differential scanning calorimetry (DSC), as set out in the Examples.

The polyolefin according to the present invention preferably has a melt flow index (MFI) of 2.0 g/10 min or higher at 190 °C/21.6 kg. This allows the polymer to be extruded during processing. Melt flow index can be measured based on ISO 1133. In some embodiments, the polyolefin according to the present invention has a melt flow index of 10 g/10 min or higher at 190 °C/21.6 kg, preferably 20 g/10 min or higher at 190 °C/21.6 kg.

An advantageous feature of articles according to the present invention is that the polyolefin materials used, e.g. polyethylene, generally have low densities. Thus articles according to the invention generally have high values of specific strength (tensile strength divided by density) and specific modulus (elastic modulus divided by density). The polyolefin according to the present invention preferably has a density of greater than 0.94 g/cm ³ . More preferably, the polyolefin according to the present invention has a density of greater than 0.95 g/cm ³ , most preferably more than 0.955 g/cm ³ . The upper limit of the density of the polyolefin according to the present invention may suitably be less than 1.2 g/cm ³ , preferably less than 1.0 g/cm ³ , more preferably less than 0.98 g/cm ³ . In some embodiments the polyolefin according to the present invention is polyethylene, having a density of greater than 0.955 g/cm ³ and less than 1.0 g/cm ³ . The density of the polyolefin may be determined by any suitable method, such as the density-gradient technique according to ASTM D1505. The weight-average molecular weight (M _w ) of the polyolefin according to the invention may optionally be less than 1000 kg/mol, preferably less than 400 kg/mol. M _w as used herein is measured via high-temperature size exclusion chromatography (HT-SEC) at 160 °C[32]

A major advantage of polyethylene over most other solid materials is its low density ip < 1 g/cm ³ ), which leads to high values of specific strength and specific modulus of ultra- drawn polyethylene fibres and films. The specific strength and modulus of transparent HDPE films according to the invention can be as high as 800 MPa/g-cm ³ and 27 GPa/g-cm ³ and are similar to values for high strength glass fibres. The specific modulus of the transparent films is also similar to that of a classic engineering material like aluminium, while its specific strength is about 7 times higher than aluminium. The specific strength and modulus of common transparent materials like sheet glass, PMMA, PC and PS, previous transparent drawn HDPE films with BZT additives produced by Shen et a/. [21] in combination with our current transparent drawn HDPE films are shown in Fig. 8. Sheet glasses, including laminated glass, tempered glass or toughened glass like Gorilla ^® glass, have relatively low specific strengths due to their high densities (p ~ 2.5 g/cm ³ ). In comparison, polymers usually have lower densities around 0.9-1.3 g/cm ³ . Commercial transparent polymeric materials such as PMMA, PC and PS typically possess specific moduli of 2-3 GPa and specific strengths of 40-60 MPa. However, highly transparent drawn HDPE films according to the present invention were shown to have a specific strength which is more than 10 times higher than both traditional sheet glass and transparent polymeric materials, and also about 20 % higher than previous drawn HDPE films where transparency was induced through the addition of additives like BZT[21]. Hence, HDPE films according to the present invention successfully combine high transparency with lightweight, high strength and high stiffness, making them of interest for a wide range of applications.

In an embodiment the drawn article according to the present invention is a fibre, a tape or a film. In a preferred embodiment the drawn article is a film.

The inventors surprisingly discovered that polyolefin articles having excellent transparency can be obtained by varying the conditions under which the sample is drawn without the need for further additives. In particular, by drawing the article at a drawing temperature (T _d ) from 45 °C below the isotropic melting temperature of the of polyolefin used to 25 °C below the isotropic melting temperature of the of polyolefin used, excellent transparency properties are obtained. This can be seen from Figs 3, 5 and 6, and from Table 1 in Example 1. The melting temperature is measured by differential scanning calorimetry (DSC). The isotropic melting temperature is measured in a second heating scan after the polymer was heated first to a temperature of at least 30 °C above its melting temperature.

The methods according to the present invention optionally further comprise an extruding step, wherein the drawn polyolefin is further extruded.

In some embodiments uniaxial drawing is used in in order to provide improved mechanical properties, as demonstrated herein.

In the case of HDPE, as shown in the working examples, drawing at a temperature between 90 °C and 1 10 °C at a draw ratio of 10 or more was shown to yield excellent properties, both optical and mechanical.

In an embodiment, the present invention relates to a method for producing a drawn polyethylene article, comprising the steps of providing a polyethylene and drawing the polyethylene at a draw ratio of 10 or more at a temperature of 90 °C or higher and 110 °C or less and further extruding the drawn polyolefin.

In a further embodiment, the polyolefin can be isotactic polypropylene (i-PP) (having a melting temperature of 165 °C). In a further embodiment of the above method, the polyolefin is isotactic polypropylene, and the sample is drawn at a temperature of 120 °C or higher and 140 °C or less.

In a further embodiment of the method described above the polyolefin is

polyethylene, preferably HDPE, HMWPE or UHMWPE.

The drawing step may be carried out from 45 °C below the isotropic melting temperature of the of polyolefin used to 25 °C below the isotropic melting temperature. In some embodiments the drawing is carried out at a temperature of 35 °C below the isotropic melting temperature of the polyolefin used or higher.

In some embodiments the drawing step is carried out at a temperature of 90 °C or higher and 1 10 °C or less. In a preferred embodiment the drawing is step is carried out at a temperature of 95 °C or greater, more preferably 100 °C or greater. In some embodiments the drawing step is carried out at a temperature in a range of 95 °C to 105 °C. The drawing may be performed at a constant temperature, or the temperature may vary throughout, whilst remaining within the range specified above. The article is drawn at a draw ratio of 10 or more. Preferably, the article is drawn at a draw ratio of 15 or more, even more preferably the article is drawn at a draw ratio of 20 or more. There is no specific upper limit on the draw ratio. In some embodiments the article is drawn at a draw ratio of 10 or more and 30 or less, preferably 15 or more and 25 or less. As used herein, the draw ratio is calculated from the length after drawing divided by the length before drawing using the following equation:

The length before and after drawing can be determined by any suitable method, such as by marking the sample with ink marks before and after drawing.

The inventors further discovered that polyolefin articles according to the present invention exhibited excellent mechanical properties, h particular, it was discovered that by using a suitable draw ratio and drawing temperature an advantageously high Young’s modulus and tensile strength could be obtained, see Figs 7 and 8. It was shown that with increasing draw ratio, elastic modulus and tensile strength are improved. Without wishing to be bound by the theory, these improved characteristics are believed to result from the unfolding of molecular lamellae and high degree of chain orientation induced by solid-state drawing.

It was shown that these excellent mechanical properties could be obtained by using a range of drawing temperatures. However, optimum properties were obtained when a drawing temperature of 1 10 °C or less was used. The mechanical properties were also shown to improve with increasing draw ratio (l). Drawing temperatures of 110 °C and below are preferred to obtain optimal Young’s modulus and tensile strength. The maximum draw ratio is not limited, but should generally not be too high in order to obtain optimal mechanical properties. Thus, transparent and high strength polyolefin films can be achieved within a wide processing window for solid-state drawing from 45 °C below the isotropic melting temperature of the of polyolefin to 25 °C below the isotropic melting temperature of the of polyolefin. In the case of polyethylene, excellent properties can be obtained in the processing window for solid-state drawing between 90 °C and 110 °C.

As illustrated in Fig. 11, transmittance values were shown to gradually increase with decreasing drawing speed at a similar draw ratio. Further, both the yield stress and strain hardening increase with increasing drawing speed (see Fig. 9(a)). As a result of this increase in strain hardening, drawing behaviour becomes more stable, leading to an increase in l,,,a _c and l,,-cms with drawing speed (see Fig. 9(b)). Thus, an optimized combination of drawing temperature and drawing speed should be aimed for to obtain high optical clarity and excellent mechanical properties. The skilled person would readily be able to select a suitable drawing speed. In some embodiments, articles of the present invention are drawn in a range of 20 mm/min to 1000 mm/min, preferably 50 mm/min to 600 mm/min. In an embodiment the articles are drawn in a range of greater than or equal to 100 mm/min and less than or equal to 500 mm/min. In a further embodiment the articles are drawn at a speed of less than 500 mm/min, preferably less than 300 mm/min, more preferably less than 100 mm/min.

An embodiment of the present invention incudes an article obtainable via any of the methods described above. The novel drawn articles described herein may be produced by any of the methods described above.

Drawn articles according to the present invention have potential applications in a wide variety of fields such as automotive glazing, windshields, displays for electronic devices, glazing for buildings, protective windows, visors and so on.

The present invention therefore also provides a device comprising a drawn article according to the present invention, wherein the device is a windshield, a window, a visor, an impact resistant article or a display unit, wherein the device is preferably a display unit, more preferably a liquid crystal display (LCD) or an organic light emitting diode (OLED). In a particularly preferred embodiment, the device is a window.

The publications, patent publications and other patent documents cited herein are entirely incorporated by reference. Herein, any reference to a term in the singular also encompasses its plural. Where the term“comprising”,“comprise” or“comprises” is used, said term may be substituted by“consisting of’,“consist of’ or“consists of’ respectively, or by“consisting essentially of’,“consist essentially of’ or“consists essentially of’ respectively. Any reference to a numerical range or single numerical value also includes values that are about that range or single value. Unless otherwise indicated, any % value is based on the relative weight of the component or components in question.

Examples

The following are Examples that illustrate the present invention. However, these Examples are in no way intended to limit the scope of the invention. Unless otherwise specified, parameters disclosed herein are measured as set out in the Examples below.

Materials: Borealis VS4580 (Borealis AG, Austria) was used as high-density polyethylene (HOPE). This polymer grade has a melting temperature ( T _m ) of 134 °C, a pellet density of 0.958 g/cm ³ and a number-average (M _n ) and weight-average (M _w ) molecular weight of 37 kg/mol and 134 kg/mol, respectively as measured by HT-SEC and a melt flow index (MFI) of 0.6 g/10 min at 190 °C/2.16 kg and 21 g/10 min at 190 °C/21.6 kg measured based on ISO 1 133. Thermoplastic polyurethane (TPU) ST-6050 sheets were provided by Schweitzer-Mauduit International, Inc. (USA).

Preparation of Specimens: Isotropic HDPE sheets with a thickness of 0.2-0.5 mm were manufactured by compression moulding using a Dr. Collin P300E (Germany) hot press at 160 °C for 3 min, followed by cooling down to room temperature (RT). For optical properties, optical microscopy and thermal characterizations, rectangular-shaped samples with dimensions of 20 mm x 10 mm were cut from these hot-pressed sheets. For surface morphology imaging as well as mechanical tests, dumbbell-shaped specimens were cut from these sheets according to ASTM D638 Type V standard with gauge dimensions of 9.5 mm x 3.2 mm. All these samples were then drawn at different drawing temperatures from 70 °C to 125 °C in an Instron 5900R84 (UK) universal tensile tester equipped with an environmental chamber. Drawing speed was varied between 100 mm/min to 500 mm/min, although most of the drawing was performed at 100 mm/min. Unless otherwise stated, drawing was performed at 100 mm/min in the examples. Draw ratio was measured by the length ratios before and after drawing using ink marker lines initially spaced every 1-2 mm. The average thickness (/) of the drawn HDPE samples was calculated by weighing the samples, and using the following equation:

m

t = - - - p x l x w

Where m is the mass of the drawn fdms, p is the density of the drawn HDPE films (assuming a crystal density of 1 g/cm ³ based on previous research[21] [24]), and 1 and w are, respectively, the length and width of the films after solid-state drawing. At least three specimens were used for each test.

Specimens for optical appearance and properties consisted of drawn HDPE films sandwiched between TPU interlayers and two glass slides (see Fig. 2(a)) in order to remove surface scattering from the drawn films. Compression moulding of this laminated structure was performed using a Rondol (UK) hot press at 100 °C for 5-10 min and a pressure of 3 bar.

Characterization Techniques: Transmittance spectra of the HDPE/TPU/Glass laminates were obtained using a PerkinElmer Lambda 950 (USA) UV-vis spectrometer equipped with an integrating sphere with 100 mm diameter in the wavelength range of 400-700 nm at an interval of 1 nm, measured at least three times for each sample. UV-vis tests were carried out at a sample-to-detector distance of 5 cm or 40 cm as shown in Fig. 4(a) and Fig. 4(b).

Differential scanning calorimetry (DSC) of drawn HDPE films was carried out using a TA Instruments (UK) DSC25. Samples of 5-10 mg were placed in aluminum pans with a single heating-cooling cycle performed under a flow of nitrogen gas at a constant heating rate of 10 °C/min. At least three tests were carried out for each condition. The melting point (T _m ) and enthalpy of fusion (AH/) of the drawn films were obtained from the first heating scan.

The crystallinity (X _c ) was calculated using the following equation:

Where DH is the enthalpy of fusion of 100 % crystalline polyethylene crystals, which is equal to 293.0 J/g[25].

The maximum draw ratio which still produced transparent films was judged by visual inspection during the solid-state drawing process. Above a specific draw ratio, whitening occurred in the drawn films. The maximum transparent draw ratio ( n _ans ) was defined at this critical point, which was different from the maximum draw ratio (l,,, _ac ) which was defined as the draw ratio at break. Young’s modulus and tensile strength of drawn oriented HDPE films were measured using an Instron 5566K1071 (UK) universal tensile tester at a crosshead speed of 100 mm/min at RT. Specimens with gauge lengths of 50-100 mm were tested using manual wedge action grips. Young’s modulus was calculated from the tangent of the engineering stress-strain curve at a strain below 0.5 %. The mean and standard deviation of the Young’s modulus and tensile strength were calculated from at least three samples.

Example 1: Optical properties of HDPE films HDPE films according to the present invention were produced as described above, and their transmittance was measured according to the methods described above. Further details of the measurement techniques are set out below.

In order to remove the influence of surface scattering when evaluating the optical properties of oriented HDPE films, these films were sandwiched between two glass slides with TPU interlayers as schematically shown in Fig. 2(a). The chosen TPU interlayers have a refractive index (w = 1.50) similar to glass (n = 1.52)[27] and HDPE (n = 1.54)[5], reducing the degree of light reflections at the interfaces. After being sandwiched between glass and TPU, a more clear appearance with higher transmittance values was observed for the oriented HDPE films (Fig. 2(b) and 2(c)), demonstrating that the TPU interlayers successfully eliminated the light scattering at the surface of the HDPE films[21].

The thickness of the films also affects their optical performance. Thinner films usually possess higher transparency (Fig. 10) since they contain fewer defects or dust particles that can scatter light. Unless otherwise specified film thicknesses of around 80 pm were used in the present examples.

Most studies concerned with optical transparency use photographs of sample appearances by positioning the sample at a very close distance to an object, often involving placing of the transparent sample directly on top of a background image[4] [28] [29].

However, according to ASTM D1746-15, regular transmittance usually refers to the ability of an observer to“see-through” a specimen in order to clearly distinguish a relatively distant object, analogous to the visibility of the distant scenery seen through a window. The optical appearance of the oriented HDPE films (). = 15) drawn at different temperatures was shown when placed close to an object but also at a relatively far distance from an object (Fig. 3).

The drawn films were shown to be completely opaque at T _d = 70 °C and 80 °C. However, when the drawing temperature was increased from 80 °C to 90 °C, the appearance of the drawn HDPE films changed from opaque to transparent. Films drawn above 100 °C had a highly transparent appearance. Moreover, the visibility as seen through opaque films (T _d =

70 °C and 80 °C) or translucent films (T _d = 85 °C) when placed at a far distance from an object (Fig. 3(b)) was less than when placed close to an object (Fig. 3(a)). Hence, transparency should be evaluated not only at a small sample-to-object distance (near field) but also at a large sample-to-object distance (far field) in order to determine a sample’s usefulness as a transparent article. Transmittance spectra for solid materials are customarily measured in the near field using a short sample-to-detector distance (typically below 5 cm) in a UV-VIS spectrometer [30] [31]. Here, optical performance was tested at both a short (near field) and a relatively long sample-to-detector distance (far field). For a sample-to-detector distance of 5 cm, the sample was placed at the entrance port of the integrating sphere (Fig. 4(a)). In the present examples the transmittance spectra contain the light scattered in the forward direction. At a sample-to-detector distance of 40 cm, the specimen was placed further away from the integrating sphere to provide more relevant transmittance data (Fig. 4(b)). Transmittance spectra at both sample-to-detector distances are shown in Figure 4(c) and 4(d).

Transmittance values measured at both distances were around 92.0 % for glass and 90.5 % for a single TPU interlayer sandwiched between two glass slides. For HDPE film drawn at 110 °C sandwiched between two glass slides and TPU interlayers, the transmittance at 40 cm sample-to-detector distance was 1-2 % lower than the value measured at a distance of 5 cm. However, differences in transmittance as high as 16 % or 28 % were obtained at these two distances for HDPE films drawn at 85 °C or 80 °C, respectively. This discrepancy in transmittance values for different sample-to-detector distances is in accordance with the optical appearance at different sample-to-object distances (see Figs 3(a) and 3(b)). Given the potential practical applications for transparent high strength films according to the present invention, subsequent optical tests were all performed at a sample-to-detector distance of 40 cm (far field).

Influence of drawing temperature (T _:i )

The influence of drawing temperature on transmittance in the visible light range is shown in Fig. 5 and Table 1, below.

Table 1. Transmittance values of drawn HDPE films (2 = 15) at different drawing temperatures at a wavelength of 700 nm, 550 nm and 400 nm

Wavelength 70 °C 80 °C 85 °C 90 °C 100 °C 110 °C 120 °C 125 °C

700 nm 9.3 % 17.2 % 58.5 % 80.5 % 87.1 % 88.5 % 88.9 % 89.2 %

550 nm 4.3 % 8.2 % 46.6 % 76.7 % 85.8 % 87.8 % 88.0 % 88.9 %

400 nm 2.5 % 2.8 % 28.8 % 61.5 % 73.1 % 73.8 % 75.5 % 82.0 % At T _d < 80 °C, transmittance values of drawn HDPE films at l = 15 were all below 18 %. Transmittance of the films increased to over 75 % when T _d increased to 90 °C. With further increasing drawing temperatures (T _d > 100 °C), optical transmittance was shown to exceed 89 % at high wavelengths within the visible spectrum, which is only 3 % lower than glass (~ 92 %). This transmittance improvement with increasing drawing temperature corresponds well with the optical appearance change shown in Fig. 3.

It is well known that the most sensitive wavelength to the human eye is around 550 nm within the visible spectrum. Fig. 6(a) shows the change in transmittance with increasing drawing temperature at this wavelength. Transmittance of drawn HDPE films at l = 15 was significantly improved from 8 % at T _d = 80 °C to 77 % at T _d = 90 °C. Transmittance became even better (> 86 %) for T _d > 100 °C, with films showing a consistent tendency of improved optical clarity with increasing drawing temperature.

The above mentioned optical experiments demonstrate the surprising result that by raising the drawing temperature in the solid-state drawing process the transparency of oriented HDPE films can be significantly enhanced.

Influence of draw ratio (X)

Fig. 6(b) demonstrates the influence of draw ratio on optical transparency of drawn HDPE films. It was shown that for all drawing temperatures, transmittance was maximum at around l = 15, with transmittance decreasing slightly at higher draw ratios.

Influence of drawing speed

As illustrated in Fig. 11, transmittance values were shown to gradually decrease with increasing drawing speed at similar draw ratio. Accordingly, an optimized combination of drawing temperature and drawing speed should be aimed for when requiring high optical clarity.

Example 2: Mechanical properties of HDPE films

HDPE films according to the present invention were produced as described above, and their mechanical properties were measured according to the methods described above. Further details of the measurement techniques are set out below.

Influence of temperature Engineering stress-strain curves of HDPE films during solid-state drawing at different drawing temperatures (T _d ) were obtained, and the results are shown in Fig. 1(a). With increasing drawing temperature, the yield stress was shown to drop from 13.8 MPa to 3.6 MPa for drawing temperatures ranging from 70 °C to 120 °C. Strain hardening behaviour also becomes less pronounced with increasing drawing temperatures. At T _d < 110 °C, strain hardening results in stable neck formation and homogeneous deformation even at high draw ratios (l). When T _ci > 1 10 °C the solid-state drawing process becomes inhomogeneous with localized necking at low draw ratios as a result of the weak strain hardening. Thus, films with improved mechanical properties can be obtained by drawing at T _d < 110 °C. Fig. 1(b) shows the maximum draw ratio (k _max ) and the maximum transparent draw ratio (k _trans ) as a function of drawing temperature, where k _max is related to the maximum extensibility of the molecular network above which further drawing would lead to failure i.e., no further orientation would develop, whereas k _trans is the maximum draw ratio before‘whitening’ starts to occur and the film becomes opaque. At T _d = 80 °C, the highest l„, _3c is obtained for this grade of F1DPE, indicating an optimum drawing temperature of 80 °C for ultimate mechanical performance. However, all films were opaque at T _d < 80 °C. Conversely, transparent films were obtained at T _d > 90 °C. Both k _max and k _trans were reduced with further increasing drawing temperatures due to less strain hardening. Hence, transparent oriented HDPE films and homogeneous drawing even at high draw ratios were obtained in a temperature window between 90 °C and 1 10 °C.

Influence of draw ratio and temperature

The Young’s modulus and tensile strength of drawn HDPE films are shown in Fig. 7 at different drawing temperatures and draw ratios. This figure demonstrates that with increased draw ratio, elastic modulus and tensile strength are improved. Young’s modulus was shown to be largely independent of drawing temperature for T < 100 °C and draw ratios between 10 and 20 (Fig. 7(a)), with modulus values of around 19 GPa for drawn HDPE films at the highest transparent draw ratio (l = 15). A high modulus of around 27 GPa was achieved for drawn HDPE films at l = 20 and T _d = 80-100 °C. For drawing temperatures below 100 °C, the tensile strength was about 650 MPa and 800 MPa at l = 15 and l = 20, respectively (Fig. 7(b)). Both modulus and strength were shown to drop slightly with increasing T _d above 100 °C at high draw ratios, with values of around 24 GPa and 700 MPa for l = 20 at T _d = 1 10 °C, respectively. Thus, an optimum combination of optical and mechanical performance can be obtained by carefully tuning the draw ratio and drawing temperature. For instance, if high mechanical performance is preferred, a draw ratio of 20 can be used yielding a higher modulus and strength at similar optical transparency (see Fig. 6(b)). Thus, depending on specific applications, transparent and high strength HDPE films can be achieved within a wide processing window for solid-state drawing between 90 °C and 1 10 °C.

Due to the low density of the polyethylene used in the present Examples, high values of specific strength (tensile strength divided by density) and specific modulus (elastic modulus divided by density) were obtained. The specific strength and modulus of the transparent HDPE films produced according to the invention were as high as 800 MPa/g-cm ³ and 27 GPa/g-cm ³ and are similar to values for high strength glass fibres.

Influence of drawing speed

Besides drawing temperature, the drawing speed was also shown to influences the solid-state drawing process of the HDPE films. Both the yield stress and strain hardening were shown to increase with increasing drawing speed (see Fig. 9(a)). As a result of this increase in strain hardening, drawing behaviour becomes more stable, leading to an increase in X _max and X _trans with drawing speed (see Fig. 9(b)). In accordance with time-temperature equivalence [26] this trend is opposite to that of drawing temperature (Fig. 1(a) and (b)).

Example 3: Thermal properties of HDPE films

In terms of thermal properties, the melting temperatures (T _m ) of drawn HDPE films was shown to fluctuate at around 141 °C for T _d = 70-125 °C and therefore T _m can be regarded as largely independent of drawing temperature for a draw ratio of 15 (see Fig.

12(a)). However, an increase in crystallinity (X _c ) (~ 5 %) was shown with increasing T _d .

With respect to draw ratio, both T _m and X _c were both shown to gradually increase with increasing X. In Fig. 12(b), the increase in T _m and X _c between HDPE films (X = 30) drawn at 110 °C and the original isotropic hot-pressed film is shown to be 5 °C and 12.5 %, respectively.

Thus samples with increased crystallinities may be obtained by drawing at a higher drawing temperature and draw ratio.

Conclusion Drawn polymeric articles having excellent transparency and mechanical properties are provided herein. It was surprisingly discovered that through cautiously controlling the drawing parameters, especially drawing temperature and draw ratio, highly oriented polyolefin articles can be obtained with high levels of transparency combined with mechanical properties which are more than 10 times greater than those of common transparent polymers like PC, PMMA and PS, without the need for additives. Such transparent articles can potentially replace traditional laminated glass as well as commercial transparent polymeric materials, and are therefore of interest for a wide range of applications including windows and glazing, windshields, visors, displays etc.

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