Treated knitted fabric for hip protective pads for elderly women. Part II. Performance relevant to thermal comfort

Abstract

This paper describes Part II of a study of hip protective pads made from spacer fabric treated with shear thickening fluid (STF). Hip fractures are a substantial contributor to morbidity and mortality in the elderly, and incidence is rising worldwide. Hip protective pads reduce fractures, but wearing adherence is poor. Pads made from breathable and comfortable fabrics can increase adherence and prevent more hip fractures. In this research, warp-knitted spacer fabrics were treated with STF using one of two methods. Treatment I involved STF and room temperature vulcanizing (RTV) silicone, while treatment II consisted of STF and a mixture of isopropyl alcohol and tetraisopropyl titanate. In Part I, STF-treated knitted spacer fabric was shown to have significantly greater force attenuation capacity than untreated fabric and comparable to force attenuation capacity of closed cell foam. In Part II, the effects on performance relevant to the thermal comfort of the wearer of each treatment method, the amount of STF used, and layering were assessed using a sweating guarded hot plate, and comparisons made between treated fabrics and with closed cell foam. Treatments I and II produced pads with significantly different permeability index and mass compared to closed cell foam. Treatment II produced pads with high force attenuation capacity, suitable permeability indexes, and low mass – preferred characteristics in hip protective pads. The study concluded that knitted spacer fabrics treated with STF, isopropyl alcohol, and tetraisopropyl titanate are potentially useful materials for hip protective pads.

Keywords

hip protective pad knitted spacer fabric shear thickening fluid thermal comfort

The causes, costs, and epidemiology of hip fractures were covered in Part I of this paper.¹ One method of preventing hip fractures in elderly people at high risk of falling is to use hip protective garments.² Hip protective garments consist of protective pads, either hard (plastic or resin) shields or soft (closed cell foam) pads to be placed bilaterally on the hip to cover the greater trochanter, attached to a protective garment.³

As outlined in Part I,¹ acceptance of and adherence to wearing hip protective garments by elderly people is low. Even among the highly selected group of individuals who initially find the idea of wearing hip protective garments acceptable, adherence declined rapidly after initiation in most studies. For example, in one study by Hubacher and Wettstein,⁴ 38% of all initial wearers had stopped wearing the hip protective garments by the end of the first month; by the end of the second month more than half had stopped (59%), and by the end of the third month more than two-thirds of initial wearers had stopped. Adherence generally decreased over time, with much of the decline occurring in the first few months of the study.^5–7

The most important reasons for refusal are problems of discomfort and fit, the perceived extra effort needed to wear them, inconvenience (including when toileting), their appearance, and the personal belief that they are not at risk for hip fractures.^8–10

To increase wearers' adherence to hip protective garments, their negative aspects must be addressed but balanced against their primary attribute of protective effectiveness.

People wear clothing for many reasons, but primarily to protect their bodies from the environment. As clothing is worn, the human body interacts dynamically with it and the adjacent environment in physical, sensory, psychological and informational terms.¹¹ Four processes interact simultaneously to regulate the comfort status of the wearer. They are physical processes in clothing and surrounding environments, physiological processes in the body, and neurophysiological and psychological processes.¹¹ Physical processes in the environment and clothing, which regulate the physical conditions for the survival and comfort of the body, follow the laws of physics. The thermoregulatory responses of the body and the sensory responses of skin nerve endings follow the laws of physiology. The body's thermoregulatory and sensory systems inform the brain of the physical conditions affecting comfort status and respond to physical stimuli from clothing and the environment to produce physiological conditions that promote the survival of the body.¹¹

According to Li and Wong,¹¹ comfort depends on the individual's perception of numerous sensations. It encompasses many facets of human senses, including vision (aesthetic comfort), thermal perception (comfort and warmth), pain (prickling and itching), and touch (smooth, rough, soft, and stiff). Comfort also has less measurable but nonetheless important inputs from social and cultural sources. Subjective perceptions imply a psychological process in which all relevant sensory perceptions are formulated, weighed, combined and evaluated against past experiences and present desires to form an overall assessment of comfort status. Body–clothing interactions (thermal and mechanical) and the outer environment (physical, social, and cultural) substantially affect the comfort status of the wearer.

Thermophysiological wear comfort is one aspect of wear comfort. It is related with the feeling of hot and cold when the person wears the clothing. This type of comfort is reached if the person doesn't need to remove or add clothing with the purpose to adjust with the temperature. This thermal comfort is affected by the changes in physiological factors of the body, such as temperature of skin and core, level of activity of the wearer along with fabric thermal resistance and moisture vapor transmission. Environmental factors, for instance temperature and humidity, also considerably influence the thermal comfort.¹² Heat generated by metabolism can be life-saving or fatal depending on the surrounding atmosphere and existing circumstances.¹² In normal condition, human bodies are comfortable in a very narrow temperature range of 28–30℃.¹³

According to Adanur,¹⁴ protective clothing refers to garments and other fabric-related items designed to protect the wearer from harsh environmental effects that may result in injuries or death. Protective clothing is needed to protect against physical hazards, impact, abrasion, and against toxic hazards.

Protective clothing is constructed from materials classed as protective textiles. Protective textiles are a division of technical textiles that are defined consist of all those textile-based products which are used mainly for their functional features or performance rather than their aesthetic or decorative features.¹⁵

Personal protective textiles can be classified as industrial, agricultural, military, civilian, medical, sports and space protective textiles, depending on the end use.¹⁶ Many types of protective clothing are uncomfortable to wear as they are resistant to water vapor.

When the body cannot dissipate its excess heat to the environment, the heat stress occurs. It is a serious problem especially during physical working.¹⁷ Heat stress and discomfort, resulting from hot, bulky protective garments, are some of the disadvantages of current protective clothing. While providing protection, it is essential for advanced protective ensembles to minimize heat stress. The effect of protective clothing on heat stress is determined by the level to which the clothing influences the heat transfer between the wearer and the environment. The breathability, or moisture vapor permeability of the clothing, can affect the evaporation of moisture from the body and heat exchange. Clothing weight, stiffness, and bulkiness add additional burden that can increase metabolic heat production in the stressful conditions.¹² Poor comfort leads to poor compliance and therefore to ineffective protection.

As described in Part I,¹ spacer fabric and shear thickening fluid (STF) are materials that hold promise for hip protective pads in terms of their protective and comfort properties. The research outlined in this paper (Part II of our study) aimed to determine the comfort performance (thermal resistance, evaporative resistance, and mass) of protective pads made of warp-knitted spacer fabric impregnated with STF. The effects of the two treatments, the amount of STF used and of layering on performance relevant to the thermal comfort of the wearer were investigated.

Materials and methods

Warp-knitted spacer fabric constructed from 100% polyester yarns was used as the material for an experimental hip protective pad. The fabric thickness was 5 mm and the mass of fabric was 330 g/m². This fabric had a net-like open structure created with interconnected wales. The structure was chosen because of its openness, which previous work has shown to be useful in terms of thermal comfort properties (Figure 1). The yarn count for back and face structure was 20 tex and the spacer yarn was monofilament of 15 tex.

Knitted fabric of 92.35%:7.65% cotton elastane was made for pocket assembly. The mean wale density value 15 wales/cm and mean course density was 25 courses/cm. The fabric mass per m² was 252.62 ± 1.14 g. The fabric thickness was 0.68 ± 0.008 mm.

Polyurethane closed cell foam that claimed to be suitable for impact protective pad by the manufacturer was selected for comparison. Warp-knitted spacer fabrics were treated with either STF and room temperature vulcanizing (RTV) silicone (treatment I, in three combinations) or STF and a mixture of isopropyl alcohol and tetraisopropyl titanate (treatment II, in three combinations – see Part I¹ for details). P1, P2, and P3 were the pad treated with treatment 1, while Pad A, Pad B, and Pad C were the pad treated with treatment II. The thicknesses of closed cell foam and treated fabric can be seen in Table 1.

Table 1.

Thickness of the experimental pads¹

No.	Code	Thickness (mm)	Material	Treatment
1	CF	15.89	Polyurethane viscoelastic closed cell foam	–
2	SR	15.00	Polyester knitted spacer fabrics (3 layers)	–
3	P1	14.52	Treated polyester knitted spacer fabrics (3 layers)	Treatment I
4	P2	14.67	Treated polyester knitted spacer fabrics (3 layers)	Treatment I
5	P3	14.59	Treated polyester knitted spacer fabrics (3 layers)	Treatment I
6	PA	15.53	Treated polyester knitted spacer fabrics (3 layers)	Treatment II
7	PB	15.80	Treated polyester knitted spacer fabrics (3 layers)	Treatment II
8	PC	15.97	Treated polyester knitted spacer fabrics (3 layers)	Treatment II

Test methods and data analysis

The thermal and moisture resistance of the pad inserted into pocket was tested using Measurement Technology Northwest's integrated sweating guarded hotplate (iSGHP). The instrument measures the thermal resistance (R_ct) and water vapor resistance (R_et) properties of textile materials. It provides simple, fully automated testing in compliance with the ISO 11092 and ASTM F1868 standards.

The iSGHP was operated by automatic control software ThermDAC8, which performs numerous automatic data analysis functions, including calculation of thermal resistance and area-weighted averages. All calculations were performed on data logged to a test-generated .csv file based on a user-defined logging interval.¹⁸

The equations used in calculations are as follows:¹⁸

Dry thermal resistance, R_ct, was calculated in SI units for each zone by the formula

R_{ct} = \frac{(T_{skin} - T_{amb})}{Q / A}

(1)

where R_ct = thermal resistance (m²·℃/W), T_skin = zone average temperature (℃), T_amb = ambient temperature (℃), and Q/A = area-weighted heat flux (W/m²).

Evaporative resistance, R_et, was calculated in SI units for each zone by the formula

R_{et} = \frac{(P_{sat} - P_{amb})}{Q / A}

(2)

where R_et = evaporative resistance (m²· Pa/W), P_sat = saturation vapor pressure at skin temperature (Pa), and P_amb = ambient vapor pressure at ambient temperature (Pa).

The permeability index (I_m), which characterizes the ability of water vapor to move through clothing and affects the amount of evaporative cooling that can occur as a result, was calculated as follows:

I_{m} = \frac{K \cdot R_{ct}}{R_{et}}

(3)

where I_m = permeability index (dimensionless), R_ct = thermal resistance, R_et = evaporative resistance, and K is a constant (60.6515 Pa/℃)

The permeability index (I_m), developed by Woodcock,¹⁹ is an indicator of the evaporative performance of a fabric. The permeability index is measured on a sweating hot plate or with a sweating thermal manikin and expresses the fraction of evaporation that takes place with the sample compared with evaporation through the air layer only. The permeability index is dimensionless and has value between 0 and 1. A value of 0 implies that the fabric is water vapor impermeable that is, it has an infinite water vapor resistance and a fabric with value of 1 has both thermal resistance and water vapor resistance of an air layer of the same thickness.

Material for the hip protective pad was cut into 13.5 × 13.5 cm squares. Knitted fabric suitable for a hip protection garment of 30 × 30 cm was used as a fabric layer, containing a pocket of 21 × 21 cm sewn in its middle. The pad was inserted into the pocket (Figure 2). The pocket was made to mimic the real wear situation where the wearer used the garment and the pad was inserted into pocket in the garment. The pad size was the same as that of the existing pad, and smaller than the plate on the SGHP. Therefore to completely cover the SGHP plate, the fabric and the pocket were made to the required size.

The fabric layer sample was carefully positioned over the plate so that there was no trapped air under the sample. Once the sample was in place, the sample hold-down flap was installed. Then the height of the hotplate was adjusted until the air velocity sensor tip was even with the 7 mm spacer block. It was important that the surface of the sample is 15 mm below the air velocity sensor probe for accurate measurements.

Three specimens from each experimental pad were tested. The average dry thermal resistance and evaporative resistance values of the three specimens was calculated.

To determine the significance of the difference between experimental pads, statistical analysis was carried out. One-way analysis of variance (ANOVA) tests were conducted to determine the equality variance. Pearson's correlation analysis was conducted to measure significance of relationships between each data.

Results and discussion

Pad performance relevant to thermal comfort

Effect of amount of STF used

Figure 3(a) presents the mean dry thermal resistances of experimental pads. It can be seen that the dry thermal resistances of three layers of knitted spacer fabric samples P1, P2, and P3 treated with the three versions of treatment I (1.0, 1.5, and 2.0 kg/m² of STF, respectively), show similar value. No significant differences were detected (p > 0.05), meaning that the amount of STF used did not affect the dry thermal resistance of the pads. This was probably because the treatment I treated knitted spacer fabrics thicknesses were similar.

The dry thermal resistance and evaporative resistance of spacer fabric samples treated with treatment II PA, PB, PC were not significantly different (p > 0.05). Different amounts of tetraisopropyl titanate (125, 250, and 125 mL/m², respectively) did not influence dry thermal resistance of the knitted spacer fabrics. It is also due to the treatment II treated knitted spacer fabrics thicknesses were similar.

When all the experimental pads were compared, it could be seen that the dry thermal resistance value of the knitted spacer fabrics treated with treatment I and the knitted spacer fabrics treated with treatment II were different. The ANOVA test confirmed that the dry thermal resistance value of the treatment I and treatment II treated knitted spacer fabrics were significantly different (p < 0.05), the dry resistance value of treatment II treated knitted spacer fabric and closed cell foam also different, while post hoc test confirmed that the closed cell foam had the same dry thermal resistance value as treatment I treated knitted spacer fabric.

This was most likely because the treatment I treated knitted spacer fabrics thickness were thinner than those of treatment II since the weight of the STF and RTV silicone overlaid the spacer fabric so that the fabric thickness was slightly decreased. The thickness of closed cell foam was similar to those of treatment I treated knitted spacer fabrics, so that their dry thermal resistance value were the similar.

Treatment II treated knitted spacer fabrics had the highest dry thermal resistance value since the knitted spacer fabrics themselves had the greatest thickness among all the experimental pads.

Figure 3(b) displays the mean evaporative resistance of experimental pads. It can be seen that the evaporative resistance value of knitted spacer fabric samples treated using treatment I (P1, P2, P3) were similar (p > 0.05). Evaporative resistance of spacer fabric samples treated with treatment II were not significantly different (p > 0.05). Different amounts of tetraisopropyl titanate (125, 250, and 125 mL/m², respectively) did not influence evaporative resistance. However knitted spacer fabrics treated with treatment I had lower evaporative resistance value than those of knitted spacer fabrics treated with treatment II. This was because the STF reduced the size of openings in the fabric macro structure, with openings becoming smaller as the amount of STF increased, and filled the voids between the spacer yarns.

When the wearer is performing an activity such as walking the evaporative and thermal resistance of clothing decreases, and the presence of openings reduces them further still.^20,21 As the treated knitted spacer fabrics retained openings (albeit smaller), it is reasonable to expect that treated spacer fabric in a protective pad worn on a moving human body will have lower evaporative and thermal resistance than a pad without openings.

When all experimental pads were compared, it could be seen that the dry thermal resistance value of knitted spacer fabrics treated with treatment I and knitted spacer fabrics treated with treatment II were different. The ANOVA test confirmed that the dry thermal resistance value of the treatment I and treatment II treated knitted spacer fabrics were different p < 0.05. Post hoc test confirmed that the closed cell foam, treatment I, and treatment II treated knitted spacer fabrics had a different evaporative resistance. The closed cell foam had the highest evaporative resistance value. This was likely because the morphology of knitted spacer fabric allows for easy moisture transfer from the wearer's skin to the environment, whereas closed cell foam has a closed structure that does not permit moisture transfer.

Treating knitted spacer fabric with STF using two different methods produced pads with similar permeability indexes (Figure 4). Treatment I produced treated fabrics with slightly lower permeability indexes than those of Treatment II. This was due to treatment I involving RTV silicone and larger volumes of STF than treatment II. This reduced the size of pad openings and hence the permeability index.

Figure 4 shows that treated knitted spacer fabrics had a significantly higher permeability index than closed cell foam (p < 0.05). Body movement and the presence of wind increase the permeability index,²⁰ so in practical use treated knitted spacer fabric is expected to have a considerably higher permeability index than demonstrated in Figure 4.

Effect of treatment and layering

The dry thermal resistance of three layers of treated knitted spacer fabric (P1) is lower than that of three layers of untreated fabric (SR) (Figure 5(a)). This was because the thickness of the fabrics decreased due to treatment. The heavy mass of treatment material that applied to the surface of the fabric caused the fabric thickness to decrease due to gravity.

The dry thermal resistance values of different treated knitted spacer fabric assemblies were significantly different (p < 0.05). The number of layers of treated knitted spacer fabrics in the assembly was significantly and positively correlated with dry thermal resistance (r = 0.998) (Figure 5(a)). This was because the thicknesses were increased.

Figure 5(b) shows that the evaporative resistance of treated knitted spacer fabric is higher than that of untreated fabric. It also shows that the evaporative resistance values of different numbers of layers of treated knitted spacer fabric are significantly different (p < 0.05). The number of layers of treated knitted spacer fabrics in the assembly was significantly and positively correlated with evaporative resistance (r = 0.998) (Figure 5(b)).

Treatment II produced pads with lower mass than treatment I (Figure 6). This was because treatment II involved application of considerably less tetraisopropyl titanate per square millimeter than the mass of RTV silicone used in treatment I.

Summary

The application of treatments produced pads with different characteristics. Application of treatment I produced pads with higher force attenuation capacity (Part I) and higher mass than those of pads with application of treatment II, while application of treatment II produced pads with similar or lower force attenuation capacity and lower mass than those of pads with application of treatment I. For example, the P1 and PC pads had similar force attenuation, but the PC pad had lower mass. Low mass is a highly desirable characteristic for hip protective pads for elderly women, as it improves comfort and therefore is likely to increase adherence. The PC pad also had a higher permeability index than closed cell foam, which means greater heat and moisture transfer, so less sweating and more comfort for the wearer. Treatment I could be used to create pads for other protective applications where high force attenuation capacity and moderate permeability index are desired and the mass of the pad is not critical.

Conclusion

Treated knitted spacer fabric had a higher permeability index than closed cell foam and greater ability to transfer moisture to the environment.

The two treatment methods produced pads with significantly different force attenuation characteristics (see Part I of the study),¹ permeability indexes, and mass. Treatment II produced pads with high force attenuation capacity, suitable permeability indexes and low mass, characteristics that are highly desirable in hip protective pads. The study demonstrated that knitted spacer fabric treated with STF and a mixture of isopropyl alcohol and tetraisopropyl titanate is a suitable material for hip protective pads.

Footnotes

Declaration of conflicting interests

The authors declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The authors received no financial support for the research, authorship, and/or publication of this article.

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