Ultraviolet Protection Factor of Hemp / Filament Hybrid Yarn Knitted

It is generally accepted that synthetic fi bres provide high UV protection capability of textile products, while cellulose fi bres have low UV absorption capacity. However, textile materials made of synthetic fi bres are not considered to be comfortable for wear in warm weather, whereas cellulose-based textile materials are considered to be much more comfortable to wear, especially in the summer months. For this research, the idea was to produce specially blended knitted fabrics from staple/fi lament hybrid yarns in order to provide clothing materials with eff ective UV protection properties, keeping satisfactory comfort properties as well. To obtain hybrid yarns, folding technique was used. The Ultraviolet Protection Factor (UPF), as a quantitative measure of the eff ectiveness of the material to protect human skin against UV radiation, was determined for the rib knitted fabrics by in vitro test method according to the European standard. The physical properties of the knitted fabrics related to their UV protection properties: bulk density, porosity and air permeability were also determined. The hybrid yarns, from which the rib knitted fabrics were produced, having always hemp yarn as a staple component, caused the diff erences in UV protection capability of the rib knitted fabrics through the infl uence of a fi lament spun component. The hybrid yarn aggregation within knitted fabrics also infl uenced their UPF values. Even though the pure hemp knitted fabric was marked as “UPF>50” (according to European standard) due to natural pigments and lignin in hemp fi bres which act as UV absorbers, most of the hemp/fi lament hybrid yarn knitted fabrics exhibited even higher UPF. Therefore, the engineering approach proposed in this research was confi rmed as an eff ective way to design clothing materials with optimal UV protection capability and other wear comfort parameters.


Introduction
In the past few decades, sun protection campaigns have been initiated in Australia, the United States and Europe to educate the population about sensible behaviour of the sun, since the rising incidence of skin cancer has been observed for many years.In addition, the decrease of the age at which fi rst symptoms appear is frightening [1].Because of the long interval between the sun exposure and the development of skin cancer, a higher increase may be expected in the future.Th erefore, adequate sun protection is becoming even more necessary.Th e World Health Organization (WHO) mainly recommends photoprotection by means of clothing for children.Although the textile fabrics themselves undergo limited UV-induced degradation, they still provide excellent UV protection [2].Various investigations have pointed out the infl uence of clothing design [3,4], fabric construction (open porosity, weight, and thickness) [5][6][7][8][9][10][11][12][13], dyes, chemical agents or UV stabilizers applied to the fabric [10,[14][15][16][17][18], and type of fi bre [19][20][21] on UV protection properties.However, the porosity or tightness of the fabric is the dominant parameter in obtaining UV radiation protection, followed by the ability of fi bres to absorb UV radiation (UVR).Nowadays there are some eco-friendly but expensive and complex methods for imparting UV protection capabilities to textile materials.However, in practical view, these technologies are time-consuming, complicated and expensive.In this research, an attempt was made to approach the UV protection properties of knitted fabrics from the viewpoint of the interaction of fi bre, yarn and fabric in order to obtain high and permanent UV protection properties.Growing demand for comfort clothing and environmentally friendly fi bres caused the increase in production and use of natural fi bres.On the other hand, some of desired garment properties, such as high UV protection capability, shape stability, durability and easy care can be easily achieved by using manmade fi bres.Natural cellulose fi bres (cotton, fl ax, hemp) have excellent hygienic properties but low UV absorption capacity, except those grey (virgin state) or dark coloured [5,22].Having this in mind, the novel blended knitted fabrics were produced from specially designed staple/fi lament hybrid yarns in order to provide clothing materials with eff ective UV protection properties, keeping satisfactory comfort properties as well.

Materials and methods
In this research, folding technique was used to obtain staple/fi lament hybrid yarns.Eco-friendly non-bleached hemp yarn was chosen to fold with a fi lament component.Th is hemp (Cs) yarn with the linear density of 50 tex was combined with viscose (Cv) fi lament yarn with the linear density of 30 tex, or textured polyamide (PA) fi lament (Tactel ® , Du Pont) with the linear density of 16 tex.Th e staple hemp component was mainly two-folded (Cs/Cs, Cs/Cv, Cs/PA ) with one of the fi lament component, except the three-folded sample composed of the hemp yarn and two ends of the Tactel component (Cs/PA/PA) in order to produce complex yarns with similar linear density.To investigate the infl uence of the unique internal structure of these specially designed staple/fi lament hybrid yarns on UV protection properties, homogeneous two-folded hemp yarn was also produced.Th e folding twist intensity of all the hybrid/fi lament yarns was 310 turns per meter in S direction.In order to release the yarns from tension, aft er folding they were subjected to the following treatments: steaming 20 min at 80 ºC in an autoclave and storage for at least 72 hr in a conditioned room (65% relative humidity ± 2% and 20 ºC ± 2 ºC).Measured characteristics of the yarns used in this investigation are presented in Table 1.
zaščite rebrastih pletiv zaradi vpliva fi lamentne komponente.Na vrednosti UZF je vplivala tudi združitev hibridne preje v pletivu.Četudi je pletivo iz čiste konoplje uvrščeno v kategorijo materialov z "UZF>50" (v skladu z evropskim standardom) zaradi naravnih pigmentov in lignina v konopljinih vlaknih, ki absorbirajo UV žarke, pa je imela večina izdelanih pletiv iz hibridnih prej s kombiniranjem konopljine in fi lamentne preje celo višji UZF.Naš pristop, predlagan v tej raziskavi, se je torej izkazal kot učinkovit način za načrtovanje oblačilnih materialov z optimalno UV zaščito in drugimi parametri udobja pri nošenju.Ključne besede: ultravijolični zaščitni faktor, konopljeno-fi lamentna hibridna preja, pletivo, zračna prepustnost Th e rib knitted fabrics (1:1) were knitted from these yarns on the V-bed fl at machines diff ering in gauge settings (E 10 and E 12).In this way, two series of knits were produced diff ering in stitch density, designed as L (lower density) and H (higher density).Th e two-folded hemp yarn could not be processed on a 12 gauge machine due to stiff ness of the yarn.Th erefore, the hemp/hemp rib knitted fabric with higher stitch density was missing from the experimental material.Th e knit samples were kept without tension on a fl at surface in a conditioned room (65% R.H. ± 2% and 20 °C ± 2 °C) for the relaxation before testing.Th ese commercially produced knitted fabrics in their existing state aft er last commercial treatment were used for all tests.Stitch density and length of loop were determined according to standard procedure ISO 4921: 2000 (Knitting -Basic concepts -Vocabulary).Areal density (mass per unit area) of the knitted fabrics was determined according to ISO 3801: 1977 (Textiles -Woven fabrics -Determination of mass per unit length and mass per unit area).Th ickness of the knits was measured in accordance with ISO 5084:1996 (Textiles -Determination of thickness of textiles and textile products).Structural properties of the knitted fabrics are presented in Table 2. Th e Ultraviolet Protection Factor (UPF), as a quantitative measure of the eff ectiveness of the material to protect human skin against UV radiation, was determined for the rib knitted fabrics by in vitro test method according to the European standard EN 13758-1:2001+A1:2006 (Textiles -Solar Ultraviolet Protective Properties -Part I: Method of test for apparel fabrics).Th is method is a widely accepted laboratory-based test method according to which the UV transmission through textile materials is  (1), where E(λ) [Wm -2 nm -1 ] is spectral irradiation for appropriate solar radiation spectrum, ε(λ) is the spectral weighting function of erythemal action spectra, T(λ) is the spectral transmittance through specimen, and Δλ [nm] is appropriate wavelength measuring interval.Physical properties of the knitted fabrics investigated in this study, in order to explain their UV protection capabilities, included: bulk density, porosity and air permeability.Bulk density of the knitted fabrics, ρ k [g cm -3 ], was calculated by dividing their areal density (mass per unit area) by thickness.Porosity of the knitted fabrics, P [%], defi ned as the portion of all air spaces in knitted fabrics both between yarns and inside them, was calculated using the equation 2.
where ρ f [g cm -3 ] is the fi bre density.Air permeability of the knitted fabrics, defi ned as the volume of air measured in cubic meters passed per minute through a square meter of fabric at a constant pressure [m 3 m -2 min -1 ], was measured according to standard procedure (ISO 9273:1995, Textiles -Determination of the permeability of fabrics to air).A Textest FX-3300 air permeability tester was used.Th e air pressure diff erential between the two surfaces of a sample (20 cm 2 test area) was 100 Pa.Five tests per each knitted fabric were conducted.

Results and discussion
Th e folding technique provides a complex yarn differing from its components in geometry as a consequence of the changes in fi bre orientation.If complex folded yarn is composed of various fi bre and yarn types (staple, fi lament, textured fi lament), the hybrid structure is expected to take advantage of both the fi bre properties and intrinsic characteristics of component yarns.As a result of the linear density and geometry of the complex yarns, both series of the rib knitted fabrics (L and H designed) were characterized by some variations of structural properties (thickness and areal density, Table 2) and physical properties (bulk density and porosity) which are presented in Table 3.Generally, rib knitted fabrics have high stitch density structure.However, the rib knitted fabrics made from staple/fi lament hybrid yarns were characterized by relatively low bulk density and high porosity, despite their relatively high thickness, making them suitable for summer clothing.Bending rigidity of the hemp yarn may account for the exhibited physical properties of the knitted fabrics.Hemp fibres are coarse and do not have much elasticity and subsequently hemp yarn has the increased bending stiff ness which must have resulted in a decrease of a knitting barrier manifested itself in limited stitch density.During a row formation in a weft knitted fabric, the yarn takes the shape of the Greek latter "Ω" which requires suffi cient elasticity; otherwise the knit cannot be produced on the chosen gauge setting machine.Th e porosity of the L-designed Cs/ Cv knitted fabric was similar to that of the pure hemp rib knit, whereas the porosity increased for Cs/PA knit and slightly reduced for Cs/PA/PA knit.However, visual examination of the L-designed knitted fabrics indicated some diff erences in pore distribution between them, as presented in Figure 1.
As a consequence of increased bending rigidity or reduced fl exibility of the hemp yarn, the all-hemp rib knit with higher stitch density could not be produced.However, it seems that the introduction of a fi lament component into complex yarn caused the improved elasticity of the hybrid structure which made it possible to produce the hemp/fi lament knitted fabrics with higher stitch density.
Th e H-designed rib knits made from hybrid yarns, with the exception of hemp/Tactel knit, were characterized by reduced porosity in relation to the corresponding lower stitch density variants (Table 3).
Visual examination of the H-designed knitted fabrics indicated also the changes in their open porosity, as can be noted in Figure 2.An increase in the surface stitch density of the rib knits led to a reduction in the inter-yarn pore size.Th is is particularly observed in Cs/PA/PA rib knit where changes in loop confi guration noted not only in relation to the L-designed counterpart but also in relation to the other knitted fabrics (Table 2).Th e changes in loop confi guration, manifested in higher course density of the Cs/PA/PA rib knit, can be explained by the improved elasticity of the Cs/PA/PA hybrid yarn.
Since the open porosity of a textile material is a governing factor which infl uences its transmission behaviour, the air permeability of the rib knitted fabrics was determined as an indication of their open porosity.Air permeability was chosen because of the known fact that the amount of airfl ow through fabric depends mostly on the openness of the fabric structure.Th e results of the air permeability of the rib knitted fabrics are given in Figure 3. Within the L-designed rib knitted fabrics, the hemp/viscose knit was most permeable followed by hemp/Tactel, all-hemp and hemp/Tactel/Tactel knits.Since the rib knitted fabrics were characterized by the same stitch density, the spacing of yarn appeared to be the main parameter infl uencing the openness of fabric structure and hence air permeability.In the case of more compact yarn, the air in the yarn is reduced but the space between yarn segments is larger, so the fabric has a more open, permeable structure.In addition to linear density of the viscose fi lament which is lower than that of the single hemp yarn, regular cross section, smooth surface and close packing of monofi laments in viscose yarn caused a reduction in hemp/viscose hybrid yarn spacing in relation to that of the twofolded hemp yarn.As a result, Cs/PA knit exhibited increased air permeability.Monofi laments accommodation into a polyamide textured fi lament caused a high bulk structure with a large content of air space in hemp/Tactel hybrid yarn.Although Cs/ PA knit looks like having quite close structure due to durable crimps and loops introduced into a Tactel component (Figure 1c), its air permeability which was higher than that of all-hemp knitted fabric can be explained by the lowest linear density of Cs/PA yarn and the lowest thickness of the corresponding knit.Th e Cs/PA/PA knitted fabric was thinner than pure hemp knit but exhibited lower air permeability due to high covering power or the spacing of the Cs/PA/PA hybrid yarn coming from its confi guration.Th e H-designed hemp/fi lament knitted fabrics exhibited, as expected, lower air permeability then that of the L-designed counterparts due to the increased stitch density and consequently reduced size of inter-yarn pores.Th e air permeability of the H-designed Cs/Cv and Cs/PA knits was similar to that of the pure hemp knit, whereas the Cs/PA/PA knit exhibited even lower air permeability as a consequence of its increased stitch density.
According to the spectrophotometric measurement, given in Table 4, the rib knitted fabrics were characterised by low UVB and UVA transmittance.In comparison with the pure hemp knitted fabric, the hemp/viscose, (Cs/Cv)L, knit was characterised by higher transmittance and consequently lower UPF (Figure 4), which can be explained by higher open porosity of knit.UPF values (Figure 4) indicated the excellent protection capability of all other hemp based knitted fabrics (UPF>50) according to European standard (Textiles -Solar UV protective properties -Part 2: Classifi cation and marking of apparel (EN 13758-2:2003+A1:2006. Th e excellent capability of all-hemp knit to protect against UVR can be attributed to the fact that hemp single yarn was in the gray-state, and thus natural pigments and lignin in hemp fi bres could act as UV absorbers having a favourable eff ect on UV protection.Th e introduction of the textured component into hemp/textured fi lament hybrid yarns caused the reduction in the UVR transmission through the knitted fabrics and hence UV protection increased.On account of durable crimps and loops introduced into the textured polyamide fi lament, the hemp/textured fi lament hybrid yarns were characterized by large spacing within the rib knits.Th e Cs/PA/PA knit having the lowest air permeability and consequently the lowest openness exhibited the lowest UVR transmittance and the highest UPF.However, Cs/PA knit exhibited the UVR transmittance lower than that of the pure hemp knit (Table 4) despite its higher air permeability.Bearing in mind the fact that fabric porosity is a static property unlike air permeability which includes dynamic force (air pressure diff erence), it seems that in this case the openness -air permeability relation is not simple as expected.On the other hand, by visual examination of the samples, it seems that both Cs/PA and Cs/PA/

Conclusion
In the scope of this study, the rib knitted fabrics composed of hemp/fi lament hybrid yarns were compared to the pure rib hemp knit in terms of UV radiation transmittance in order to bring to light the potential of the formation of novel hybrid structures for UV protection.Th e folding technique applied for a combination of staple and fi lament yarns made it possible not only to combine diff erent fi bre properties, but to provide the complex yarn with the unique internal structure.Th e results confi rmed that folding operation could be an eff ective way to obtain "new quality" textile structures with specifi c properties.In addition, it was proved that blended yarns obtained by combining the staple yarn with other yarn types in complex hybrid yarns could be the way to take advantage of both, the fi bre properties and the intrinsic characteristics of plied components (fi lament, textured).Th is complex interaction can also be modifi ed by knitted fabric design through the infl uence on yarn aggregation within fabrics.
Although the pure hemp knitted fabric was placed in the "excellent UV protection category" (according to the Australian/New Zealand standard AS/ NZS 4399) due to natural pigments and lignin in hemp fi bres which act as UV absorbers, most of the hemp/fi lament knitted fabrics exhibited even higher UPF.Th erefore, this engineering approach merits further consideration as an eff ective means for design of clothing materials with optimal UV protection properties and other wear comfort parameters.
Although the results obtained cannot be explicitly extrapolated to textile materials other than those tested in this study, it could be said with certainty that the confi guration of complex yarn can be a key determinant of the UV protection capability of textile materials.

Figure 4 :
Figure 4: Ultraviolet protection factor of the hemp/filament hybrid yarn knitted fabrics

Table 1 :
Characteristics of the yarns

Table 2 :
Structural characteristics of the hemp/fi lament hybrid yarn rib knitted fabrics a) L indicates a rib knitted fabric with lower stitch density b) H indicates a rib knitted fabric with higher stitch density

Table 3 :
Physical properties of the hemp/fi lament hybrid yarn rib knitted fabrics

Table 4 and
Figure 4).Th is can be explained by considering the interaction between UV radiation and fi bres.When UV light reaches a textile fabric, UVR can be transmitted through the pores, and refl ected or absorbed by fi bres.Fibres can absorb or refl ect UVR not only from the surface of the fabric but also from the specifi c surface of the open pores.It is assumed that the absorption of UVR by fi bres inside the channels was hastened with a decrease in the size of open pores in the H-designed knitted fabrics.It may be expected that hemp fi bres were responsible for it in Cs/Cv knitted fabric.Since both hemp and textured polyamide fi lament may have contributed to the increased UVR absorption in Cs/ PA knit, this can be a reason for its highest UV protection capability among the knitted fabrics having similar air permeability.