Hot or not: systematic review and laboratory evaluation of the hot needle test for microplastic identification

Microplastics and Nanoplastics

Table 3 Fibers used in the present study with their melting (T_m) or burning points and observed behavior towards the hot point of a soldering iron set at 350 °C. Glass transition temperature (T_g) of polymers is provided from the literature

Synthetic textile fibers	T_g / T_m	Observed Response to Hot Point	Pass( +) or Fail(-) HNT
Polypropylene	-13°C^c / 165–175 °C	Melting/curling	+
Acrylic	80–95°C^d; 240–250°C^a	Melting/curling	+
Polyester	70°C^a / 240–260 °C	Melting/curling	+
Polyester-coated cotton	NA	Partial melting/shrinking	+
Nylon	50°C^c / 270 °C	Melting/curling	+
Natural or Semi-synthetic	Decomposition^b
Cellulose acetate cigarette filter	190–200 °C (lit T_g) / 240–243 °C (lit T_m) 220–240 °C (shrinks); 250–300 °C (blackens); 270–280 °C (liquifies)	Melting/softening or possible wavering/shrinking/curling	+
Viscose	175–200 °C	No reaction	–
Silk	260–285 °C	No reaction	–
Cotton	210 °C	Wavering; movement away from heat; no curling nor melting	–
Spartina grass	NA		–
Digested, dead		Slight shrinking, burning
Digested, live		No reaction
Undigested, live		Slight curl or no reaction

^a A wide range of melting or softening points (~ 135–290 °C) for different types of acrylic fibers are found in reference guides [53]. We observed significant softening of the test material (sweater fiber) in the melting point analyzer at 240–250 °C prior to burning. Decomposition temperature of polyacrylonitrile is about 300^oC^a. Amorphous acrylics will not have a T_m
^b Degradation of cellulose acetate from cigarette butt filters is observational data from a melting point analysis (ramp 10 °C/min) and a literature glass transition [5] for unplasticized cellulose diacetate) and melting point (Benavente et al. 2018) temperatures. Thermal decomposition temperature is reported in peer-reviewed and industry grey literature for viscose [51], silk [74] and cotton [49]
NA: Data not available
^c Harper (2006) [27] for polypropylene, polyethylene terephthalate and Nylon (6,6)
^d Richards [62] for polyacrylonitrile