| Wool fabric shrinkage could be divided into three types:(a)dimensional change due to relaxation,(b)dimensional change which takes place when the moisture content of a relaxed fabric is altered,and(c)felting shrinkage.Dimensional change which takes place when the moisture content of a relaxed fabric is altered is reversible,while felting shrinkage,which occurred due to the scale structure on the surface of wool fibers,is irreversible.Felting shrinkage of wool fabrics during tumble drying causes a serious problem for wool garments.However,few studies involved in shrinkage behavior and mechanism of wool fabric in a tumble dryer under heat,moisture and mechanical action.In the current study,using a modified domestic thermoelectric air-vented tumble dryer with adjustable parameters,the influence of fabric temperature,moisture content of wool fabric and mechanical action on wool fabric shrinkage(especially felting shrinkage) was explored.At the same time the mechanism of felting shrinkage in a tumble dryer was performed with in-depth analysis.The main research contents and results are as follows:(1)The shrinkage behavior of wool knitted fabrics with different stitch densities and different shrink-resist–finished wool fabrics during tumble drying process was explored and analyzed.The current study carried out the drying process without the interruption but used different drying time cycles to explore shrinkage behavior of wool fabrics during the process of tumble drying.In addition,the reason why shrinkage of different treated wool fabrics occurred during tumble drying was explored by repeating washing and tumble drying.The result showed that the shrinkage in fabric length and width decreased with increased stitch densities in warp and weft with a linear trend basically.This is because the more loose the fabric is,the more easily for the fibers to move,which promotes felting shrinkage.The rate of decrease in the length dimension of the untreated wool fabric was constantly higher in the first 30 min,while the rate of shrinakge decreased in the last 10 min.Different from untreated wool fabric,for the Chlorine-Hercosett–finished wool fabric,the rate of dimensional change in length increased when the fabric became drier,and there was a noticeable reduction in dimensional size as moisture content fell below 30%.In addition,the shrinkage of untreated wool fabric increased steadily after repeat washing and tumble dry,which means that the dimensional change was mainly caused by felting.However,the dimension of the Chlorine-Hercosett–finished wool fabric can be recovered slightly or remained after the first cycle of washing and drying.The dimensional change was closely related to the altered moisture content of wool fabric.The understanding of the shrinkage behavior of wool fabrics during tumble dry process provided a guide for designing differentiated drying programs for untreated and treated wool fabrics.(2)The influence of temperature and moisture content of fabric on shrinkage was explored and analyzed.The initial moisture content of wool fabric and heater power of the tumble dryer were adjusted to explore the effect of moisture content and temperature of wool fabric.In addition,differential frictional effect(D.F.E.)of dry wool fibers and wet wool fibers was measured,and the mechanism why properties of wool fibers,which varied with altering moisture content,influence felting shrinkage was explained.The result showed that at room temperature,for the untreated wool fabric,the shrinkage in the fabric length increased rapidly with increasing initial moisture content of fabric when initial moisture content was within 15%,but when the initial moisture content was more than 15%,the length shrinkage of wool fabric increased slightly and leveled off.D.F.E.of wet wool fibers is lower than dry fibers.It was concluded that with increasing moisture contents wool fibers showed behavior of glass transition,mechanical properties of wool fibers changed,for example,bending rigidity and initial modulus decreased,facilitated felting shrinkage of untreated wool fabric.Behavior of glass transition occurred when wool absorbed less water with rising temperature,therefore temperature and moisture content influence felting shrinkage of wool fabric jointly.(3)The influence of the mechanical action(both the severity and the type of the mechanical action) on felting shrinkage of untreated wool fabric was explored and analyzed.The movements of wool fabric at different rotation speeds of the rotating drum were investigated.Two-dimensional tracks of the geometric center point of the fabric during tumble rotation were recorded using the fabric movement tracking system.The total mechanical actions at different rotation speeds were determined by measuring the extent of the removed threads at the outer edges of thread assemblies in woven fabric during tumble drying.Finally,the felting shrinkages of wool fabric at different mechanical actions in the tumble dryer were assessed and analyzed.The result showed that in general,the more severe is the mechanical action in the tumble dryer,the greater is the felting shrinkage of wool fabric.According to the track of fabric movement and the empirical formula of the momentum theorem,kinetic energy theorem,impact force applied on the fabric increased with the increasing rotation speed of the drum at projectile movement.According to the size of the circular track area of fabric movement during tumble rotation,rubbing intensity between fabrics increased with the drum rotation speed decreased.Impacting of the fabric to the drum wall appears to have a smaller contribution to felting shrinkage than rubbing between fabrics.Felting shrinkage usually occurs at the direction that mechanical action applied on.In other words,the mechanical action that is perpendicular to the plane of the fabric has little effect on wool fabric shrinkage.(4)Understanding the effect of temperature,moisture content and mechanical action on shrinkage of wool fabric,and taking into account of drying rate,Specific Moisture Extraction Rate(SMER),evenness of drying and appearance smoothness at the same time,drying methods were presented for untreated wool fabric(on behalf of non machine-washable wool fabric)and Chlorine-Hercosett–finished wool fabric(on behalf of machine-washable wool fabrics) from the perspective of fabric movement.For non machine-washable wool fabric,avoiding the tangential mechanical action between fabric and the drum wall or the shelf that it contact is required,while the recommended drying movement for machine-washable wool fabric is projectile motion accompanied by tumbling occasionally (i.e.complex motion).Compared with the existing drying methods (flat drying or rotating with the tumble drum),energy consumption could be reduced considerably.The innovation points of this research are as follows:(1)It is revealed that shrinkage behaviors and mechanisms of untreated wool fabric and Chlorine-Hercosett–finished wool fabric during tumble drying are different due to the different surface structure of wool fibers.It is felting and the decreased moisture content during tumble drying that mainly caused the shrinkage of untreated wool fabric and Chlorine-Hercosett–finished wool fabric,respectively.(2)The mechanism why moisture content of untreated wool fabric influence felting shrinkage was revealed.It was believed that compared with dry wool fibers,the change of bending rigidity and the initial modulus of wool fibers with increased water,made wool fibers entangle and taut more easily,so water absorbed in wool could promote felting shrinkage of wool fabric.(3)Based on the position distribution of the traced fabric sample in the tumble dryer and the percentage of thread removal,it was found that both of the type and severity of mechanical action applied on wool fabric affect felting shrinkage in the tumble dryer.Impacting between the drum wall and wool fabric caused less felting shrinkage than rubbing between fabrics under the same mechanical action severity.Felting shrinkage usually occurs at the direction that mechanical action applied on.(4)The new drying methods of wool fabrics that are not machine-washable and machine-washable were presented respectively.For non machine washable wool fabric,it is important to avoid felting shrinkage of wool fabric,so avoiding the tangential mechanical action between fabric and the drum wall or the shelf that it contact is required,while for machine-washable wool fabric,it is important to improve drying performance and energy effiency,so the recommended drying movement for machine-washable wool fabric is projectile motion accompanied by tumbling occasionally (i.e.complex motion).Compared with the existing drying methods (flat drying or rotating with the tumble drum),the optimized drying methods can reduce the energy consumption considerably.In theory,the study explored the effect of heat,moisture,and mechanical action on wool fabric shrinkage and the reason,which could enrich and deepen the felting shrinkage mechanism of wool fabric.In practice,drying method for wool fabrics was optimized,which could not only decrease the felting shrinkage of untreated wool fabric,but also save much energy. |
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