Appending dataframe to existing Excel worksheet using Openpyxl - excel

Question

I'm trying to create a new spreadsheet and worksheet containing the column headings from a dataframe. I then want to append new data to the worksheet every For loop iteration. I am likely to have a large amount of data and therefore thought it would be necessary to write it out to Excel after every iteration rather than writing the whole DF at the end.
The "Append data to existing worksheet" code in the For loop works correctly (ie gives me 3 rows of values) on its own if I am writing to a spreadsheet that already contains the column headings that I have created within Excel. But when I run the code as you see below, I only end up with the column headings and the values from the last For loop iteration. I'm obviously missing something simple but can't seem to work it out. Any help would be much appreciated
import openpyxl as xl
import pandas as pd
import numpy as np
import datetime as dt
fn = '00test101.xlsx'
# Create new workbook
wb = xl.Workbook()
wb.save(fn)
book = xl.load_workbook(fn)
writer = pd.ExcelWriter(fn,engine='openpyxl')
writer.book = book
writer.sheets = dict((ws.title, ws) for ws in book.worksheets)
# Write DF column names to new worksheet
DF = pd.DataFrame(columns=['A','B','C'])
DF.to_excel(writer, 'ABC', header=True, startrow=0)
writer.save()
for i in range(3):
a = np.array([1,3,6]) * i
# Overwrite existing DF and add data
DF = pd.DataFrame(columns=['A','B','C'])
DF.loc[dt.datetime.now()] = a
# Append data to existing worksheet
book = xl.load_workbook(fn)
writer = pd.ExcelWriter(fn,engine='openpyxl')
writer.book = book
writer.sheets = dict((ws.title, ws) for ws in book.worksheets)
DF.to_excel(writer, 'ABC', header=None, startrow=book.active.max_row)
writer.save()
# Remove unwanted default worksheet
wb = xl.load_workbook(fn)
def_sheet = wb.get_sheet_by_name('Sheet')
wb.remove_sheet(def_sheet)
wb.save(fn)